Comprehensive Inorganic Chemistry III [Volume 8. Inorganic Photochemistry] 9780128231449

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COMPREHENSIVE INORGANIC CHEMISTRY III

COMPREHENSIVE INORGANIC CHEMISTRY III EDITORS IN CHIEF

Jan Reedijk Leiden Institute of Chemistry, Leiden University, Leiden, the Netherlands

Kenneth R. Poeppelmeier Department of Chemistry, Northwestern University, Evanston, IL, United States

VOLUME 8

Inorganic Photochemistry VOLUME EDITOR

Vivian W.W. Yam Department of Chemistry, The University of Hong Kong, Hong Kong, China

Amsterdam • Boston • Heidelberg • London • New York • Oxford Paris • San Diego • San Francisco • Singapore • Sydney • Tokyo

Elsevier Radarweg 29, PO Box 211, 1000 AE Amsterdam, Netherlands The Boulevard, Langford Lane, Kidlington, Oxford OX5 1GB, United Kingdom 50 Hampshire Street, 5th Floor, Cambridge MA 02139, United States Copyright Ó 2023 Elsevier Ltd. All rights reserved No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Details on how to seek permission, further information about the Publisher’s permissions policies and our arrangements with organizations such as the Copyright Clearance Center and the Copyright Licensing Agency, can be found at our website: www.elsevier.com/permissions. This book and the individual contributions contained in it are protected under copyright by the Publisher (other than as may be noted herein). Notices Knowledge and best practice in this field are constantly changing. As new research and experience broaden our understanding, changes in research methods, professional practices, or medical treatment may become necessary. Practitioners and researchers may always rely on their own experience and knowledge in evaluating and using any information, methods, compounds, or experiments described herein. In using such information or methods they should be mindful of their own safety and the safety of others, including parties for whom they have a professional responsibility. To the fullest extent of the law, neither the Publisher nor the authors, contributors, or editors, assume any liability for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions, or ideas contained in the material herein. Library of Congress Cataloging-in-Publication Data A catalog record for this book is available from the Library of Congress British Library Cataloguing-in-Publication Data A catalogue record for this book is available from the British Library ISBN 978-0-12-823144-9

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CONTENTS OF VOLUME 8 Editor Biographies

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Volume Editors

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Contributors to Volume 8

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Preface

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8.01

Introduction: Inorganic photochemistry Vivian WW Yam

1

8.02

Luminescent transition-metal complexes and their applications in electroluminescence Peng Tao and Wai-Yeung Wong

2

8.03

Non-sacrificial photocatalysis Qiang Liu and Li-Zhu Wu

8.04

Redox photocatalysis Stefan Bernhard and Husain N Kagalwala

103

8.05

Luminescence chemosensors, biological probes, and imaging reagents Lawrence Cho-Cheung Lee and Kenneth Kam-Wing Lo

152

8.06

Photoactivated metal complexes for drug delivery Peter C Ford, John V Garcia, Camilo Guzman, Sheila Kulkarni, and Emily Wein

254

8.07

Photochemical CO2 reduction Yusuke Tamaki and Osamu Ishitani

298

8.08

Water oxidation catalysis in natural and artificial photosynthesis Yu Guo, Alexander Kravberg, and Licheng Sun

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8.09

Photochromic materials H Maeda, M Nishikawa, R Sakamoto, and H Nishihara

356

8.10

Photochemically driven molecular machines based on coordination compounds Alberto Credi, Serena Silvi, Massimo Baroncini, Leonardo Andreoni, and Chiara Taticchi

417

8.11

Lanthanide-doped upconversion nanomaterials Liangliang Liang, Jiaye Chen, and Xiaogang Liu

439

8.12

Lanthanides as luminescence imaging reagents Laura Francés-Soriano, Niko Hildebrandt, and Loïc J Charbonnière

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Contents of Volume 8

8.13

Ultrafast dynamics of photoinduced processes in coordination compounds Fernandez-Teran Ricardo J Fernández-Terán and Julia A Weinstein

511

8.14

Luminescent supramolecular assemblies Vonika Ka-Man Au, Michael Ho-Yeung Chan, and Vivian Wing-Wah Yam

574

8.15

Multicomponent supramolecular photochemistry Fausto Puntoriero, Francesco Nastasi, Giuseppina La Ganga, Ambra M Cancelliere, Giuliana Lazzaro, and Sebastiano Campagna

628

8.16

Recent progress and application of computational chemistry to understand inorganic photochemistry Thomas Penfold, Conor Rankine, and Julien Eng

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Structural characterization of excited state transition metal complexes by x-ray transient absorption spectroscopies Lin X Chen

679

8.17

8.18

d-d and charge transfer photochemistry of 3d metal complexes Matthias Dorn, Nathan Roy East, Christoph Förster, Winald Robert Kitzmann, Johannes Moll, Florian Reichenauer, Thomas Reuter, Laura Stein, and Katja Heinze

707

8.19

Luminescence properties of the actinides and actinyls Laura Lopez-Odriozola, Lauren Walker, and Louise S Natrajan

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EDITOR BIOGRAPHIES Editors in Chief Jan Reedijk Jan Reedijk (1943) studied chemistry at Leiden University where he completed his Ph.D. (1968). After a few years in a junior lecturer position at Leiden University, he accepted a readership at Delft University of Technology in 1972. In 1979 he accepted a call for Professor of Chemistry at Leiden University. After 30 years of service, he retired from teaching in 2009 and remained as an emeritus research professor at Leiden University. In Leiden he has acted as Chair of the Department of Chemistry, and in 1993 he became the Founding Director of the Leiden Institute of Chemistry. His major research activities have been in Coordination Chemistry and Bioinorganic Chemistry, focusing on biomimetic catalysis, molecular materials, and medicinal inorganic chemistry. Jan Reedijk was elected member of the Royal Netherlands Academy of Sciences in 1996 and he was knighted by the Queen of the Netherlands to the order of the Dutch Lion (2008). He is also lifetime member of the Finnish Academy of Sciences and Letters and of Academia Europaea. He has held visiting professorships in Cambridge (UK), Strasbourg (France), Münster (Germany), Riyadh (Saudi Arabia), Louvain-la-Neuve (Belgium), Dunedin (New Zealand), and Torun (Poland). In 1990 he served as President of the Royal Netherlands Chemical Society. He has acted as the Executive Secretary of the International Conferences of Coordination Chemistry (1988–2012) and served IUPAC in the Division of Inorganic Chemistry, first as a member and later as (vice/past) president between 2005 and 2018. After his university retirement he remained active as research consultant and in IUPAC activities, as well as in several editorial jobs. For Elsevier, he acted as Editor-in-Chief of the Reference Collection in Chemistry (2013–2019), and together with Kenneth R. Poeppelmeier for Comprehensive Inorganic Chemistry II (2008–2013) and Comprehensive Inorganic Chemistry III (2019-present). From 2018 to 2020, he co-chaired the worldwide celebrations of the International Year of the Periodic Table 2019. Jan Reedijk has published over 1200 papers (1965–2022; cited over 58000 times; h ¼ 96). He has supervised 90 Ph.D. students, over 100 postdocs, and over 250 MSc research students. Kenneth R. Poeppelmeier Kenneth R. Poeppelmeier (1949) completed his undergraduate studies in chemistry at the University of Missouri (1971) and then volunteered as an instructor at Samoa CollegedUnited States Peace Corps in Western Samoa (1971–1974). He completed his Ph.D. (1978) in Inorganic Chemistry with John Corbett at Iowa State University (1978). He joined the catalysis research group headed by John Longo at Exxon Research and Engineering Company, Corporate Research–Science Laboratories (1978–1984), where he collaborated with the reforming science group and Exxon Chemicals to develop the first zeolite-based light naphtha reforming catalyst. In 1984 he joined the Chemistry Department at Northwestern University and the recently formed Center for Catalysis and Surface Science (CCSS). He is the Charles E. and Emma H. Morrison Professor of Chemistry at Northwestern University and a NAISE Fellow joint with Northwestern University and the Chemical Sciences and Engineering Division, Argonne National Laboratory. Leadership positions held include Director, CCSS, Northwestern University; Associate Division Director for Science, Chemical Sciences and Engineering Division, Argonne National Laboratory; President of the Chicago Area Catalysis Club; Associate Director, NSF Science and Technology Center for Superconductivity; and Chairman of the ACS Solid State Subdivision of the Division of Inorganic Chemistry. His major research activities have been in Solid State and Inorganic Materials Chemistry focusing on heterogeneous catalysis, solid state chemistry, and materials chemistry. His awards include National Science Council of Taiwan Lecturer (1991); Dow Professor of Chemistry (1992–1994); AAAS Fellow, the American Association for the Advancement of Science (1993); JSPS Fellow, Japan Society for the Promotion of Science (1997); Natural Science Foundation of China Lecturer (1999); National Science Foundation Creativity Extension Award (2000

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and 2022); Institut Universitaire de France Professor (2003); Chemistry Week in China Lecturer (2004); Lecturer in Solid State Chemistry, China (2005); Visitantes Distinguidos, Universid Complutenses Madrid (2008); Visiting Professor, Chinese Academy of Sciences (2011); 20 years of Service and Dedication Award to Inorganic Chemistry (2013); Elected foreign member of Spanish National Academy: Real Academia de Ciencia, Exactas, Fisicas y Naturales (2017); Elected Honorary Member of the Royal Society of Chemistry of Spain (RSEQ) (2018); and the TianShan Award, Xinjiang Uygur Autonomous Region of China (2021). He has organized and was Chairman of the Chicago Great Lakes Regional ACS Symposium on Synthesis and Processing of Advanced Solid State Materials (1987), the New Orleans National ACS Symposium on Solid State Chemistry of Heterogeneous Oxide Catalysis, Including New Microporous Solids (1987), the Gordon Conference on Solid State Chemistry (1994) and the First European Gordon Conference on Solid State Chemistry (1995), the Spring Materials Research Society Symposium on Environmental Chemistry (1995), the Advisory Committee of Intense Pulsed Neutron Source (IPNS) Program (1996–1998), the Spring Materials Research Society Symposium on Perovskite Materials (2003), the 4th International Conference on Inorganic Materials, University of Antwerp (2004), and the Philadelphia National ACS Symposium on Homogeneous and Heterogeneous Oxidation Catalysis (2004). He has served on numerous Editorial Boards, including Chemistry of Materials, Journal of Alloys and Compounds, Solid State Sciences, Solid State Chemistry, and Science China Materials, and has been a co-Editor for Structure and Bonding, an Associate Editor for Inorganic Chemistry, and co-Editor-in-Chief with Jan Reedijk for Comprehensive Inorganic Chemistry II (published 2013) and Comprehensive Inorganic Chemistry III (to be published in 2023). In addition, he has served on various Scientific Advisory Boards including for the World Premier International Research Center Initiative and Institute for Integrated Cell-Material Sciences Kyoto University, the European Center SOPRANO on Functional Electronic Metal Oxides, the Kyoto University Mixed-Anion Project, and the Dresden Max Planck Institute for Chemistry and Physics. Kenneth Poeppelmeier has published over 500 papers (1971–2022) and cited over 28000 times (h-index ¼ 84). He has supervised over 200 undergraduates, Ph.D. students, postdocs, and visiting scholars in research.

VOLUME EDITORS Risto S. Laitinen Risto S. Laitinen is Professor Emeritus of Chemistry at the University of Oulu, Finland. He received the M.Sc and Ph.D. degrees from Helsinki University of Technology (currently Aalto University). His research interests are directed to synthetic, structural, and computational chemistry of chalcogen compounds focusing on selenium and tellurium. He has published 250 peer-reviewed articles and 15 book chapters and has edited 2 books: Selenium and Tellurium Reagents: In Chemistry and Materials Science with Raija Oilunkaniemi (Walther de Gruyter, 2019) and Selenium and Tellurium Chemistry: From Small Molecules to Biomolecules and Materials with Derek Woollins (Springer, 2011). He has also written 30 professional and popular articles in chemistry. He is the Secretary of the Division of Chemical Nomenclature and Structure Representation, International Union of Pure and Applied Chemistry, for the term 2016–2023. He served as Chair of the Board of Union of Finnish University Professors in 2007–2010. In 2017, Finnish Cultural Foundation (North Ostrobothnia regional fund) gave him an award for excellence in his activities for science and music. He has been a member of Finnish Academy of Science and Letters since 2003.

Vincent L. Pecoraro Professor Vincent L. Pecoraro is a major contributor in the fields of inorganic, bioinorganic, and supramolecular chemistries. He has risen to the upper echelons of these disciplines with over 300 publications (an h-index of 92), 4 book editorships, and 5 patents. He has served the community in many ways including as an Associate Editor of Inorganic Chemistry for 20 years and now is President of the Society of Biological Inorganic Chemistry. Internationally, he has received a Le Studium Professorship, Blaise Pascal International Chair for Research, the Alexander von Humboldt Stiftung, and an Honorary PhD from Aix-Maseille University. His many US distinctions include the 2016 ACS Award for Distinguished Service in the Advancement of Inorganic Chemistry, the 2021 ACS/SCF FrancoAmerican Lectureship Prize, and being elected a Fellow of the ACS and AAAS. He also recently cofounded a Biomedical Imaging company, VIEWaves. In 2022, he was ranked as one of the world’s top 1000 most influential chemists.

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Zijian Guo Professor Zijian Guo received his Ph.D. from the University of Padova and worked as a postdoctoral research fellow at Birkbeck College, the University of London. He also worked as a research associate at the University of Edinburgh. His research focuses on the chemical biology of metals and metallodrugs and has authored or co-authored more than 400 peer-reviewed articles on basic and applied research topics. He was awarded the First Prize in Natural Sciences from Ministry of Education of China in 2015, the Luigi Sacconi Medal from the Italian Chemical Society in 2016, and the Outstanding Achievement Award from the Society of the Asian Biological Inorganic Chemistry in 2020. He founded Chemistry and Biomedicine Innovation Center (ChemBIC) in Nanjing University in 2019, and is serving as the Director of ChemBIC since then. He was elected to the Fellow of the Chinese Academy of Sciences in 2017. He served as Associated Editor of Coord. Chem. Rev and an editorial board member of several other journals.

Daniel C. Fredrickson Daniel C. Fredrickson is a Professor in the Department of Chemistry at the University of WisconsinMadison. He completed his BS in Biochemistry at the University of Washington in 2000, where he gained his first experiences with research and crystals in the laboratory of Prof. Bart Kahr. At Cornell University, he then pursued theoretical investigations of bonding in intermetallic compounds, the vast family of solid state compounds based on metallic elements, under the mentorship of Profs. Stephen Lee and Roald Hoffmann, earning his Ph.D. in 2005. Interested in the experimental and crystallographic aspects of complex intermetallics, he then carried out postdoctoral research from 2005 to 2008 with Prof. Sven Lidin at Stockholm University. Since starting at UW-Madison in 2009, his research group has created theory-driven approaches to the synthesis and discovery of new intermetallic phases and understanding the origins of their structural features. Some of his key research contributions are the development of the DFT-Chemical Pressure Method, the discovery of isolobal bonds for the generalization of the 18 electron rule to intermetallic phases, models for the emergence of incommensurate modulations in these compounds, and various design strategies for guiding complexity in solid state structures.

Patrick M. Woodward Professor Patrick M. Woodward received BS degrees in Chemistry and General Engineering from Idaho State University in 1991, an MS in Materials Science, and a Ph.D. in Chemistry from Oregon State University (1996) under the supervision of Art Sleight. After a postdoctoral appointment in the Physics Department at Brookhaven National Laboratory (1996–1998), he joined the faculty at Ohio State University in 1998, where he holds the rank of Professor in the Department of Chemistry and Biochemistry. He is a Fellow of the American Chemical Society (2020) and a recipient of an Alfred P. Sloan Fellowship (2004) and an NSF Career Award (2001). He has co-authored two textbooks: Solid State Materials Chemistry and the popular general chemistry textbook, Chemistry: The Central Science. His research interests revolve around the discovery of new materials and understanding links between the composition, structure, and properties of extended inorganic and hybrid solids.

Volume Editors

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P. Shiv Halasyamani Professor P. Shiv Halasyamani earned his BS in Chemistry from the University of Chicago (1992) and his Ph.D. in Chemistry under the supervision of Prof. Kenneth R. Poeppelmeier at Northwestern University (1996). He was a Postdoctoral Fellow and Junior Research Fellow at Christ Church College, Oxford University, from 1997 to 1999. He began his independent academic career in the Department of Chemistry at the University of Houston in 1999 and has been a Full Professor since 2010. He was elected as a Fellow of the American Association for the Advancement of Science (AAAS) in 2019 and is currently an Associate Editor of the ACS journals Inorganic Chemistry and ACS Organic & Inorganic Au. His research interests involve the design, synthesis, crystal growth, and characterization of new functional inorganic materials.

Ram Seshadri Ram Seshadri received his Ph.D. in Solid State Chemistry from the Indian Institute of Science (IISc), Bangalore, working under the guidance of Professor C. N. R. Rao FRS. After some years as a Postdoctoral Fellow in Europe, he returned to IISc as an Assistant Professor in 1999. He moved to the Materials Department (College of Engineering) at UC Santa Barbara in 2002. He was recently promoted to the rank of Distinguished Professor in the Materials Department and the Department of Chemistry and Biochemistry in 2020. He is also the Fred and Linda R. Wudl Professor of Materials Science and Director of the Materials Research Laboratory: A National Science Foundation Materials Research Science and Engineering Center (NSF-MRSEC). His work broadly addresses the topic of structure–composition– property relations in crystalline inorganic and hybrid materials, with a focus on magnetic materials and materials for energy conversion and storage. He is Fellow of the Royal Society of Chemistry, the American Physical Society, and the American Association for the Advancement of Science. He serves as Associate Editor of the journals, Annual Reviews of Materials Research and Chemistry of Materials.

Serena Cussen Serena Cussen née Corr studied chemistry at Trinity College Dublin, completing her doctoral work under Yurii Gun’ko. She then joined the University of California, Santa Barbara, working with Ram Seshadri as a postdoctoral researcher. She joined the University of Kent as a lecturer in 2009. She moved to the University of Glasgow in 2013 and was made Professor in 2018. She moved to the University of Sheffield as a Chair in Functional Materials and Professor in Chemical and Biological Engineering in 2018, where she now serves as Department Head. She works on next-generation battery materials and advanced characterization techniques for the structure and properties of nanomaterials. Serena is the recipient of several awards including the Journal of Materials Chemistry Lectureship of the Royal Society of Chemistry. She previously served as Associate Editor of Royal Society of Chemistry journal Nanoscale and currently serves as Associate Editor for the Institute of Physics journal Progress in Energy.

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Rutger A. van Santen Rutger A. van Santen received his Ph.D. in 1971 in Theoretical Chemistry from the University of Leiden, The Netherlands. In the period 1972–1988, he became involved with catalysis research when employed by Shell Research in Amsterdam and Shell Development Company in Houston. In 1988, he became Full Professor of Catalysis at the Technical University Eindhoven. From 2010 till now he is Emeritus Professor and Honorary Institute Professor at Technical University Eindhoven. He is a member of Royal Dutch Academy of Sciences and Arts and Foreign Associate of the United States National Academy of Engineering (NAE). He has received several prestigious awards: the 1981 golden medal of the Royal Dutch Chemical Society; in 1992, the F.G. Chiappetta award of the North American Catalysis Society; in 1997, the Spinoza Award from the Dutch Foundation for Pure and Applied Research; and in 2001, the Alwin Mittasch Medal Dechema, Germany, among others. His main research interests are computational heterogeneous catalysis and complex chemical systems theory. He has published over 700 papers, 16 books, and 22 patents.

Emiel J. M. Hensen Emiel J. M. Hensen received his Ph.D. in Catalysis in 2000 from Eindhoven University of Technology, The Netherlands. Between 2000 and 2008, he worked at the University of Amsterdam, Shell Research in Amsterdam, and Eindhoven University of Technology on several topics in the field of heterogeneous catalysis. Since July 2009, he is Full Professor of Inorganic Materials and Catalysis at TU/e. He was a visiting professor at the Katholieke Universiteit Leuven (Belgium, 2001–2016) and at Hokkaido University (Japan, 2016). He is principal investigator and management team member of the gravitation program Multiscale Catalytic Energy Conversion, elected member of the Advanced Research Center Chemical Building Blocks Consortium, and chairman of the Netherlands Institute for Catalysis Research (NIOK). Hensen was Head of the Department of Chemical Engineering and Chemistry of Eindhoven University of Technology from 2016 to 2020. Hensen received Veni, Vidi, Vici, and Casmir grant awards from the Netherlands Organisation for Scientific Research. His main interests are in mechanism of heterogeneous catalysis combining experimental and computation studies. He has published over 600 papers, 20 book chapters, and 7 patents.

Artem M. Abakumov Artem M. Abakumov graduated from the Department of Chemistry at Moscow State University in 1993, received his Ph.D. in Chemistry from the same University in 1997, and then continued working as a Researcher and Vice-Chair of Inorganic Chemistry Department. He spent about 3 years as a postdoctoral fellow and invited professor in the Electron Microscopy for Materials Research (EMAT) laboratory at the University of Antwerp and joined EMAT as a research leader in 2008. Since 2015 he holds a Full Professor position at Skolkovo Institute of Science and Technology (Skoltech) in Moscow, leading Skoltech Center for Energy Science and Technology as a Director. His research interests span over a wide range of subjects, from inorganic chemistry, solid state chemistry, and crystallography to battery materials and transmission electron microscopy. He has extended experience in characterization of metal-ion battery electrodes and electrocatalysts with advanced TEM techniques that has led to a better understanding of charge–discharge mechanisms, redox reactions, and associated structural transformations in various classes of cathode materials on different spatial scales. He has published over 350 papers, 5 book chapters, and 12 patents.

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Keith J. Stevenson Keith J. Stevenson received his Ph.D. in 1997 from the University of Utah under the supervision of Prof. Henry White. Subsequently, he held a postdoctoral appointment at Northwestern University (1997– 2000) and a tenured faculty appointment (2000–2015) at the University of Texas at Austin. At present, he is leading the development of a new graduate level university (Skolkovo Institute for Science and Technology) in Moscow, Russia, where he is Provost and the former Director of the Center for Energy Science and Technology (CEST). To date he has published over 325 peer-reviewed publications, 14 patents, and 6 book chapters in this field. He is a recipient of Society of Electroanalytical Chemistry Charles N. Reilley Award (2021).

Evgeny V. Antipov Evgeny V. Antipov graduated from the Department of Chemistry at Moscow State University in 1981, received his Ph.D. in Chemistry in 1986, DSc degree in Chemistry in 1998, and then continued working at the same University as a Researcher, Head of the Laboratory of Inorganic Crystal Chemistry, Professor, Head of Laboratory of fundamental research on aluminum production, and Head of the Department of Electrochemistry. Since 2018 he also holds a professor position at Skolkovo Institute of Science and Technology (Skoltech) in Moscow. Currently his research interests are mainly focused on inorganic materials for application in batteries and fuel cells. He has published more than 400 scientific articles and 14 patents.

Vivian W.W. Yam Professor Vivian W.W. Yam is the Chair Professor of Chemistry and Philip Wong Wilson Wong Professor in Chemistry and Energy at The University of Hong Kong. She received both her B.Sc (Hons) and Ph.D. from The University of Hong Kong. She was elected to Member of Chinese Academy of Sciences, International Member (Foreign Associate) of US National Academy of Sciences, Foreign Member of Academia Europaea, Fellow of TWAS, and Founding Member of Hong Kong Academy of Sciences. She was Laureate of 2011 L’Oréal-UNESCO For Women in Science Award. Her research interests include inorganic and organometallic chemistry, supramolecular chemistry, photophysics and photochemistry, and metal-based molecular functional materials for sensing, organic optoelectronics, and energy research. Also see: https://chemistry.hku.hk/wwyam.

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David L. Bryce David L. Bryce (B.Sc (Hons), 1998, Queen’s University; Ph.D., 2002, Dalhousie University; postdoctoral fellow, 2003–04, NIDDK/NIH) is Full Professor and University Research Chair in Nuclear Magnetic Resonance at the University of Ottawa in Canada. He is the past Chair of the Department of Chemistry and Biomolecular Sciences, a Fellow of the Royal Society of Chemistry, and an elected Fellow of the Royal Society of Canada. His research interests include solid-state NMR of low-frequency quadrupolar nuclei, NMR studies of materials, NMR crystallography, halogen bonding, mechanochemistry, and quantum chemical interpretation of NMR interaction tensors. He is the author of approximately 200 scientific publications and co-author of 1 book. He is the Editor-in-Chief of Solid State Nuclear Magnetic Resonance and Section Editor (Magnetic Resonance and Molecular Spectroscopy) for the Canadian Journal of Chemistry. He has served as the Chair of Canada’s National Ultrahigh-Field NMR Facility for Solids and is a past co-chair of the International Society for Magnetic Resonance conference and of the Rocky Mountain Conference on Magnetic Resonance Solid-State NMR Symposium. His work has been recognized with the Canadian Society for Chemistry Keith Laidler Award and with the Gerhard Herzberg Award of the Canadian Association of Analytical Sciences and Spectroscopy.

Paul R. Raithby Paul R. Raithby obtained his B.Sc (1973) and Ph.D. (1976) from Queen Mary College, University of London, working for his Ph.D. in structural inorganic chemistry. He moved to the University of Cambridge in 1976, initially as a postdoctoral researcher and then as a faculty member. In 2000, he moved to the University of Bath to take up the Chair of Inorganic Chemistry when he remains to the present day, having been awarded an Emeritus Professorship in 2022. His research interests have spanned the chemistry of transition metal cluster compounds, platinum acetylide complexes and oligomers, and lanthanide complexes, and he uses laboratory and synchrotron-based X-ray crystallographic techniques to determine the structures of the complexes and to study their dynamics using time-resolved photocrystallographic methods.

Angus P. Wilkinson

Angus P. Wilkinson completed his bachelors (1988) and doctoral (1992) degrees in chemistry at Oxford University in the United Kingdom. He spent a postdoctoral period in the Materials Research Laboratory, University of California, Santa Barbara, prior to joining the faculty at the Georgia Institute of Technology as an assistant professor in 1993. He is currently a Professor in both the Schools of Chemistry and Biochemistry, and Materials Science and Engineering, at the Georgia Institute of Technology. His research in the general area of inorganic materials chemistry makes use of synchrotron X-ray and neutron scattering to better understand materials synthesis and properties.

CONTRIBUTORS TO VOLUME 8 Leonardo Andreoni Dipartimento di Chimica “G. Ciamician”, Università di Bologna, Bologna, Italy; and CLAN-Center for Light Activated Nanostructures, Bologna, Italy Vonika Ka-Man Au Department of Science and Environmental Studies, The Education University of Hong Kong, Hong Kong, P R China Massimo Baroncini Dipartimento di Scienze e Tecnologie Agro-alimentari, Università di Bologna, Bologna, Italy; and CLANCenter for Light Activated Nanostructures, Bologna, Italy

Lin X Chen Department of Chemistry, Northwestern University, Evanston, IL, United States; and Division of Chemical Science and Engineering, Argonne National Laboratory, Lemont, IL, United States Alberto Credi Dipartimento di Chimica Industriale “Toso Montanari”, Università di Bologna, Bologna, Italy; and CLANCenter for Light Activated Nanostructures, Bologna, Italy Matthias Dorn Department of Chemistry, Johannes Gutenberg University Mainz, Mainz, Germany

Stefan Bernhard Department of Chemistry, Carnegie Mellon University, Pittsburgh, PA, United States

Nathan Roy East Department of Chemistry, Johannes Gutenberg University Mainz, Mainz, Germany

Sebastiano Campagna Dipartimento di Scienze Chimiche, Biologiche, Farmaceutiche ed Ambientali, University of Messina, Messina, Italy

Julien Eng Chemistry, School of Natural and Environmental Sciences, Newcastle University, Newcastle upon Tyne, United Kingdom

Ambra M Cancelliere Dipartimento di Scienze Chimiche, Biologiche, Farmaceutiche ed Ambientali, University of Messina, Messina, Italy

Fernandez-Teran Ricardo J Fernández-Terán Department of Chemistry, University of Sheffield, Sheffield, United Kingdom

Michael Ho-Yeung Chan Institute of Molecular Functional Materials and Department of Chemistry, The University of Hong Kong, Hong Kong, P R China Loïc J Charbonnière Équipe de synthèse pour l’analyse (SynPA), Institut Pluridisciplinaire Hubert Curien (IPHC), UMR 7178, CNRS/Université de Strasbourg, ECPM, Strasbourg, France Jiaye Chen National University of Singapore, Singapore, Singapore

Peter C Ford Department of Chemistry and Biochemistry, University of California, Santa Barbara, CA, United States Christoph Förster Department of Chemistry, Johannes Gutenberg University Mainz, Mainz, Germany Laura Francés-Soriano nanoFRET.com, Laboratoire COBRA (Chimie Organique, Bioorganique, Réactivité et Analyse UMR6014 & FR3038), Université de Rouen Normandie, CNRS, INSA, Normandie Université, Rouen, France

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John V Garcia Department of Chemistry and Biochemistry, University of California, Santa Barbara, CA, United States Yu Guo Institute of Natural Sciences, Westlake Institute for Advanced Study, Hangzhou, China; and Center of Artificial Photosynthesis for Solar Fuels, School of Science, Westlake University, Hangzhou, China Camilo Guzman Department of Chemistry and Biochemistry, University of California, Santa Barbara, CA, United States Katja Heinze Department of Chemistry, Johannes Gutenberg University Mainz, Mainz, Germany Niko Hildebrandt nanoFRET.com, Laboratoire COBRA (Chimie Organique, Bioorganique, Réactivité et Analyse UMR6014 & FR3038), Université de Rouen Normandie, CNRS, INSA, Normandie Université, Rouen, France; Université Paris-Saclay, Saint-Aubin, France; and Department of Chemistry, Seoul National University, Seoul, South Korea Osamu Ishitani Tokyo Institute of Technology, Meguro, Tokyo, Japan Husain N Kagalwala Department of Chemistry, Southern Methodist University, Dallas, TX, United States Winald Robert Kitzmann Department of Chemistry, Johannes Gutenberg University Mainz, Mainz, Germany Alexander Kravberg Department of Chemistry, School of Engineering Sciences in Chemistry, Biotechnology and Health, KTH Royal Institute of Technology, Stockholm, Sweden Sheila Kulkarni Department of Chemistry and Biochemistry, University of California, Santa Barbara, CA, United States Giuseppina La Ganga Dipartimento di Scienze Chimiche, Biologiche, Farmaceutiche ed Ambientali, University of Messina, Messina, Italy Giuliana Lazzaro Dipartimento di Scienze Chimiche, Biologiche, Farmaceutiche ed Ambientali, University of Messina, Messina, Italy

Lawrence Cho-Cheung Lee Department of Chemistry, City University of Hong Kong, Kowloon, Hong Kong, P R China; and Laboratory for Synthetic Chemistry and Chemical Biology Limited, New Territories, Hong Kong, P R China Liangliang Liang National University of Singapore, Singapore, Singapore Qiang Liu Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry & School of Future Technology, University of the Chinese Academy of Sciences, Beijing, P R China; and State Key Laboratory of Applied Organic Chemistry, College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou, P R China Xiaogang Liu National University of Singapore, Singapore, Singapore Kenneth Kam-Wing Lo Department of Chemistry, City University of Hong Kong, Kowloon, Hong Kong, P R China; and State Key Laboratory of Terahertz and Millimeter Waves, City University of Hong Kong, Kowloon, Hong Kong, P R China Laura Lopez-Odriozola Centre for Radiochemistry Research, Department of Chemistry, School of Natural Sciences, The University of Manchester, Manchester, United Kingdom; and The Photon Science Institute, The University of Manchester, Manchester, United Kingdom H Maeda Tokyo University of Science, Noda, Chiba, Japan Johannes Moll Department of Chemistry, Johannes Gutenberg University Mainz, Mainz, Germany Francesco Nastasi Dipartimento di Scienze Chimiche, Biologiche, Farmaceutiche ed Ambientali, University of Messina, Messina, Italy Louise S Natrajan Centre for Radiochemistry Research, Department of Chemistry, School of Natural Sciences, The University of Manchester, Manchester, United Kingdom; and The Photon Science Institute, The University of Manchester, Manchester, United Kingdom

Contributors to Volume 8

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H Nishihara Tokyo University of Science, Noda, Chiba, Japan

Yusuke Tamaki Tokyo Institute of Technology, Meguro, Tokyo, Japan

M Nishikawa Tokyo University of Science, Noda, Chiba, Japan

Peng Tao Department of Applied Biology and Chemical Technology and Research Institute for Smart Energy, The Hong Kong Polytechnic University, Hung Hom, Hong Kong, P R China

Thomas Penfold Chemistry, School of Natural and Environmental Sciences, Newcastle University, Newcastle upon Tyne, United Kingdom Fausto Puntoriero Dipartimento di Scienze Chimiche, Biologiche, Farmaceutiche ed Ambientali, University of Messina, Messina, Italy Conor Rankine Chemistry, School of Natural and Environmental Sciences, Newcastle University, Newcastle upon Tyne, United Kingdom Florian Reichenauer Department of Chemistry, Johannes Gutenberg University Mainz, Mainz, Germany Thomas Reuter Department of Chemistry, Johannes Gutenberg University Mainz, Mainz, Germany R Sakamoto Tohoku University, Sendai, Miyagi, Japan Serena Silvi Dipartimento di Chimica “G. Ciamician”, Università di Bologna, Bologna, Italy; and CLAN-Center for Light Activated Nanostructures, Bologna, Italy Laura Stein Department of Chemistry, Johannes Gutenberg University Mainz, Mainz, Germany Licheng Sun Institute of Natural Sciences, Westlake Institute for Advanced Study, Hangzhou, China; Center of Artificial Photosynthesis for Solar Fuels, School of Science, Westlake University, Hangzhou, China; and Department of Chemistry, School of Engineering Sciences in Chemistry, Biotechnology and Health, KTH Royal Institute of Technology, Stockholm, Sweden

Chiara Taticchi Dipartimento di Chimica Industriale “Toso Montanari”, Università di Bologna, Bologna, Italy; and CLANCenter for Light Activated Nanostructures, Bologna, Italy Lauren Walker Centre for Radiochemistry Research, Department of Chemistry, School of Natural Sciences, The University of Manchester, Manchester, United Kingdom Emily Wein Department of Chemistry and Biochemistry, University of California, Santa Barbara, CA, United States Julia A Weinstein Department of Chemistry, University of Sheffield, Sheffield, United Kingdom Wai-Yeung Wong Department of Applied Biology and Chemical Technology and Research Institute for Smart Energy, The Hong Kong Polytechnic University, Hung Hom, Hong Kong, P R China Li-Zhu Wu Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry & School of Future Technology, University of the Chinese Academy of Sciences, Beijing, P R China Vivian WW Yam The University of Hong Kong, Hong Kong, P R China Vivian Wing-Wah Yam Institute of Molecular Functional Materials and Department of Chemistry, The University of Hong Kong, Hong Kong, P R China

PREFACE Comprehensive Inorganic Chemistry III is a new multi-reference work covering the broad area of Inorganic Chemistry. The work is available both in print and in electronic format. The 10 Volumes review significant advances and examines topics of relevance to today’s inorganic chemists with a focus on topics and results after 2012. The work is focusing on new developments, including interdisciplinary and high-impact areas. Comprehensive Inorganic Chemistry III, specifically focuses on main group chemistry, biological inorganic chemistry, solid state and materials chemistry, catalysis and new developments in electrochemistry and photochemistry, as well as on NMR methods and diffractions methods to study inorganic compounds. The work continues our 2013 work Comprehensive Inorganic Chemistry II, but at the same time adds new volumes on emerging research areas and techniques used to study inorganic compounds. The new work is also highly complementary to other recent Elsevier works in Coordination Chemistry and Organometallic Chemistry thereby forming a trio of works covering the whole of modern inorganic chemistry, most recently COMC-4 and CCC-3. The rapid pace of developments in recent years in all areas of chemistry, particularly inorganic chemistry, has again created many challenges to provide a contemporary up-to-date series. As is typically the challenge for Multireference Works (MRWs), the chapters are designed to provide a valuable long-standing scientific resource for both advanced students new to an area as well as researchers who need further background or answers to a particular problem on the elements, their compounds, or applications. Chapters are written by teams of leading experts, under the guidance of the Volume Editors and the Editors-inChief. The articles are written at a level that allows undergraduate students to understand the material, while providing active researchers with a ready reference resource for information in the field. The chapters are not intended to provide basic data on the elements, which are available from many sources including the original CIC-I, over 50-years-old by now, but instead concentrate on applications of the elements and their compounds and on high-level techniques to study inorganic compounds. Vol. 1: Synthesis, Structure, and Bonding in Inorganic Molecular Systems; Risto S. Laitinen In this Volume the editor presents an historic overview of Inorganic Chemistry starting with the birth of inorganic chemistry after Berzelius, and a focus on the 20th century including an overview of “inorganic” Nobel Prizes and major discoveries, like inert gas compounds. The most important trends in the field are discussed in an historic context. The bulk of the Volume consists of 3 parts, i.e., (1) Structure, bonding, and reactivity in inorganic molecular systems; (2) Intermolecular interactions, and (3) Inorganic Chains, rings, and cages. The volume contains 23 chapters. Part 1 contains chapters dealing with compounds in which the heavy p-block atom acts as a central atom. Some chapters deal with the rich synthetic and structural chemistry of noble gas compounds, low-coordinate p-block elements, biradicals, iron-only hydrogenase mimics, and macrocyclic selenoethers. Finally, the chemistry and application of weakly coordinating anions, the synthesis, structures, and reactivity of carbenes containing non-innocent ligands, frustrated Lewis pairs in metal-free catalysis are discussed. Part 2 discusses secondary bonding interactions that play an important role in the properties of bulk materials. It includes a chapter on the general theoretical considerations of secondary bonding interactions, including halogen and chalcogen bonding. This section is concluded by the update of the host-guest chemistry of the molecules of p-block elements and by a comprehensive review of closed-shell metallophilic interactions. The third part of the Volume is dedicated to chain, ring and cage (or cluster) compounds in molecular inorganic chemistry. Separate

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chapters describe the recent chemistry of boron clusters, as well as the chain, ring, and cage compounds of Group13 and 15, and 16 elements. Also, aromatic compounds bearing heavy Group 14 atoms, polyhalogenide anions and Zintl-clusters are presented. Vol. 2: Bioinorganic Chemistry and Homogeneous Biomimetic Inorganic Catalysis; Vincent L. Pecoraro and Zijian Guo In this Volume, the editors have brought together 26 chapters providing a broad coverage of many of the important areas involving metal compounds in biology and medicine. Readers interested in fundamental biochemistry that is assisted by metal ion catalysis, or in uncovering the latest developments in diagnostics or therapeutics using metal-based probes or agents, will find high-level contributions from top scientists. In the first part of the Volume topics dealing with metals interacting with proteins and nucleic acids are presented (e.g., siderophores, metallophores, homeostasis, biomineralization, metal-DNA and metal-RNA interactions, but also with zinc and cobalt enzymes). Topics dealing with iron-sulfur clusters and heme-containing proteins, enzymes dealing with dinitrogen fixation, dihydrogen and dioxygen production by photosynthesis will also be discussed, including bioinspired model systems. In the second part of the Volume the focus is on applications of inorganic chemistry in the field of medicine: e.g., clinical diagnosis, curing diseases and drug targeting. Platinum, gold and other metal compounds and their mechanism of action will be discussed in several chapters. Supramolecular coordination compounds, metal organic frameworks and targeted modifications of higher molecular weight will also be shown to be important for current and future therapy and diagnosis. Vol. 3: Theory and Bonding of Inorganic Non-molecular Systems; Daniel C. Fredrickson This volume consists of 15 chapters that build on symmetry-based expressions for the wavefunctions of extended structures toward models for bonding in solid state materials and their surfaces, algorithms for the prediction of crystal structures, tools for the analysis of bonding, and theories for the unique properties and phenomena that arise in these systems. The volume is divided into four parts along these lines, based on major themes in each of the chapters. These are: Part 1: Models for extended inorganic structures, Part 2: Tools for electronic structure analysis, Part 3: Predictive exploration of new structures, and Part 4: Properties and phenomena. Vol. 4: Solid State Inorganic Chemistry; P. Shiv Halasyamani and Patrick M. Woodward In a broad sense the field of inorganic chemistry can be broken down into substances that are based on molecules and those that are based on extended arrays linked by metallic, covalent, polar covalent, or ionic bonds (i.e., extended solids). The field of solid-state inorganic chemistry is largely concerned with elements and compounds that fall into the latter group. This volume contains nineteen chapters covering a wide variety of solid-state inorganic materials. These chapters largely focus on materials with properties that underpin modern technology. Smart phones, solid state lighting, batteries, computers, and many other devices that we take for granted would not be possible without these materials. Improvements in the performance of these and many other technologies are closely tied to the discovery of new materials or advances in our ability to synthesize high quality samples. The organization of most chapters is purposefully designed to emphasize how the exceptional physical properties of modern materials arise from the interplay of composition, structure, and bonding. Not surprisingly this volume has considerable overlap with both Volume 3 (Theory and Bonding of Inorganic NonMolecular Systems) and Volume 5 (Inorganic Materials Chemistry). We anticipate that readers who are interested in this volume will find much of interest in those volumes and vice versa Vol. 5: Inorganic Materials Chemistry; Ram Seshadri and Serena Cussen This volume has adopted the broad title of Inorganic Materials Chemistry, but as readers would note, the title could readily befit articles in other volumes as well. In order to distinguish contributions in this volume from

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those in other volumes, the editors have chosen to use as the organizing principle, the role of synthesis in developing materials, reflected by several of the contributions carrying the terms “synthesis” or “preparation” in the title. It should also be noted that the subset of inorganic materials that are the focus of this volume are what are generally referred to as functional materials, i.e., materials that carry out a function usually through the way they respond to an external stimulus such as light, or thermal gradients, or a magnetic field.

Vol. 6: Heterogeneous Inorganic Catalysis; Rutger A. van Santen and Emiel J. M. Hensen This Volume starts with an introductory chapter providing an excellent discussion of single sites in metal catalysis. This chapter is followed by 18 chapters covering a large part of the field. These chapters have been written with a focus on the synthesis and characterization of catalytic complexity and its relationship with the molecular chemistry of the catalytic reaction. In the 1950s with the growth of molecular inorganic chemistry, coordination chemistry and organometallic chemistry started to influence the development of heterogeneous catalysis. A host of new reactions and processes originate from that time. In this Volume chapters on major topics, like promoted Fischer-Tropsch catalysts, structure sensitivity of well-defined alloy surfaces in the context of oxidation catalysis and electrocatalytic reactions, illustrate the broadness of the field. Molecular heterogeneous catalysts rapidly grew after high-surface synthetic of zeolites were introduced; so, synthesis, structure and nanopore chemistry in zeolites is presented in a number of chapters. Also, topics like nanocluster activation of zeolites and supported zeolites are discussed. Mechanistically important chapters deal with imaging of single atom catalysts. An important development is the use of reducible supports, such as CeO2 or Fe2O3 where the interaction between the metal and support is playing a crucial role.

Vol. 7: Inorganic Electrochemistry; Keith J. Stevenson, Evgeny V. Antipov and Artem M. Abakumov This volume bridges several fields across chemistry, physics and material science. Perhaps this topic is best associated with the book “Inorganic Electrochemistry: Theory, Practice and Applications” by Piero Zanello that was intended to introduce inorganic chemists to electrochemical methods for study of primarily molecular systems, including metallocenes, organometallic and coordination complexes, metal complexes of redox active ligands, metal-carbonyl clusters, and proteins. The emphasis in this Volume of CIC III is on the impact of inorganic chemistry on the field of material science, which has opened the gateway for inorganic chemists to use more applied methods to the broad areas of electrochemical energy storage and conversion, electrocatalysis, electroanalysis, and electrosynthesis. In recognition of this decisive impact, the Nobel Prize in Chemistry of 2019 was awarded to John B. Goodenough, M. Stanley Whittingham, and Akira Yoshino for the development of the lithium-ion battery.

Vol. 8: Inorganic Photochemistry; Vivian W. W. Yam In this Volume the editor has compiled 19 chapters discussing recent developments in a variety of developments in the field. The introductory chapter overviews the several topics, including photoactivation and imaging reagents. The first chapters include a discussion of using luminescent coordination and organometallic compounds for organic light-emitting diodes (OLEDs) and applications to highlight the importance of developing future highly efficient luminescent transition metal compounds. The use of metal compounds in photo-induced bond activation and catalysis is highlighted by non-sacrificial photocatalysis and redox photocatalysis, which is another fundamental area of immense research interest and development. This work facilitates applications like biological probes, drug delivery and imaging reagents. Photochemical CO2 reduction and water oxidation catalysis has been addressed in several chapters. Use of such inorganic compounds in solar fuels and photocatalysis remains crucial for a sustainable environment. Finally, the photophysics and photochemistry of lanthanoid compounds is discussed, with their potential use of doped lanthanoids in luminescence imaging reagents.

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Vol. 9: NMR of Inorganic Nuclei; David L. Bryce Nuclear magnetic resonance (NMR) spectroscopy has long been established as one of the most important analytical tools at the disposal of the experimental chemist. The isotope-specific nature of the technique can provide unparalleled insights into local structure and dynamics. As seen in the various contributions to this Volume, applications of NMR spectroscopy to inorganic systems span the gas phase, liquid phase, and solid state. The nature of the systems discussed covers a very wide range, including glasses, single-molecule magnets, energy storage materials, bioinorganic systems, nanoparticles, catalysts, and more. The focus is largely on isotopes other than 1H and 13C, although there are clearly many applications of NMR of these nuclides to the study of inorganic compounds and materials. The value of solid-state NMR in studying the large percentage of nuclides which are quadrupolar (spin I > ½) is apparent in the various contributions. This is perhaps to be expected given that rapid quadrupolar relaxation can often obfuscate the observation of these resonances in solution. Vol. 10: X-ray, Neutron and Electron Scattering Methods in Inorganic Chemistry; Angus P. Wilkinson and Paul R. Raithby In this Volume the editors start with an introduction on the recent history and improvements of the instrumentation, source technology and user accessibility of synchrotron and neutron facilities worldwide, and they explain how these techniques work. The modern facilities now allow inorganic chemists to carry out a wide variety of complex experiments, almost on a day-to-day basis, that were not possible in the recent past. Past editions of Comprehensive Inorganic Chemistry have included many examples of successful synchrotron or neutron studies, but the increased importance of such experiments to inorganic chemists motivated us to produce a separate volume in CIC III dedicated to the methodology developed and the results obtained. The introduction chapter is followed by 15 chapters describing the developments in the field. Several chapters are presented covering recent examples of state-of-the-art experiments and refer to some of the pioneering work leading to the current state of the science in this exciting area. The editors have recognized the importance of complementary techniques by including chapters on electron crystallography and synchrotron radiation sources. Chapters are present on applications of the techniques in e.g., spin-crossover materials and catalytic materials, and in the use of time-resolved studies on molecular materials. A chapter on the worldwide frequently used structure visualization of crystal structures, using PLATON/PLUTON, is also included. Finally, some more specialized studies, like Panoramic (in beam) studies of materials synthesis and high-pressure synthesis are present. Direct observation of transient species and chemical reactions in a pore observed by synchrotron radiation and X-ray transient absorption spectroscopies in the study of excited state structures, and ab initio structure solution using synchrotron powder diffraction, as well as local structure determination using total scattering data, are impossible and unthinkable without these modern diffraction techniques. Jan Reedijk, Leiden, The Netherlands Kenneth R. Poeppelmeier, Illinois, United States March 2023

8.01

Introduction: Inorganic photochemistry

Vivian W.W. Yam, The University of Hong Kong, Hong Kong, P R China © 2023 Elsevier Ltd. All rights reserved.

Inorganic Photochemistry constitutes a major branch of inorganic chemistry. Apart from the structural diversity and the versatility of the coordination and bonding modes of the ligands as well as the diversity of the metal centers, each with its own unique electronic configuration, redox properties, coordination geometries, photochemistry, and reactivities, inorganic photochemistry has expanded its scope and horizon to focus not only on the importance of novel structures and bonding but also on the many functional properties and aspects of the coordination and organometallic compounds. In the 18 chapters of this volume, we will introduce the many versatile facets of coordination and organometallic compounds, spanning areas that are at the interfaces of inorganic photochemistry, other branches of chemistry, supramolecular science, biology, medicine, catalysis, materials, energy, and environment and sustainability; all of which form an indispensable part of our daily lives. Topics all the way from the structure and bonding, reactivity to coordination and supramolecular assembly and noncovalent interactions, spectroscopy, excited state characterization, excited state chemistry, and photophysics, to a number of application studies, including light harvesting, photoactivation, electroluminescence, photocatalysis, chemosensors, photochromic materials, photoactuation and photomachines, and imaging reagents, have been included. The volume starts with a discussion on the use of luminescent coordination and organometallic complexes for organic lightemitting diodes (OLEDs) applications to highlight the importance of developing high efficiency Luminescent Transition-Metal Complexes and Their Applications in Electroluminescence as an energy-efficient and saving measure. This is followed by the utilization of metal complexes in photo-induced bond activation and catalysis, highlighted by Non-Sacrificial Photocatalysis, and Redox Photocatalysis, which is another fundamental area of immense research interest and development. The rich photophysical properties of coordination and organometallic complexes facilitate their applications in Luminescence Chemosensors, Biological Probes, and Imaging Reagents, as well as in Photoactivated Metal Complexes for Drug Delivery. The utilization of coordination and organometallic complexes in Photochemical CO2 Reduction, and Water Oxidation Catalysis in Natural and Artificial Photosynthesis will be described to highlight the importance of these metal-mediated or catalyzed processes that are closely related to the development of solar fuels and photocatalysis, which is highly important for a sustainable environment. The photoinduced reactivities and photochemistry of these complexes have rendered them promising candidates for the applications in Photochromic Materials, and Photochemically Driven Molecular Machines. The volume will move on to discuss the photophysics and photochemistry of lanthanide compounds, in which their utilization in Lanthanide-doped Upconversion Nanomaterials and in Luminescence Imaging Reagents will be explored. Despite the importance in the development of functional materials for a sustainable environment, the fundamental principles of the underlying photophysical and photochemical events are of paramount importance. The Ultrafast Dynamics of Photoinduced Processes in Coordination Compounds will be discussed to highlight the important features of the excited states for those photophysical and photochemical events. On the other hand, Luminescent Supramolecular Assemblies and Multicomponent Supramolecular Photochemistry of the coordination and organometallic compounds will be discussed. In particular, non-covalent metal– metal interactions present in coordination complexes have been found to play important roles in the stabilization of many of these hierarchical assemblies, and such noncovalent interactions, which have strengths comparable to that of strong hydrogen bonds, are also associated with a number of intriguing properties in metal complexes, which also provide an additional dimension for the tuning of the excited state properties. The Recent Progress and Application of Computational Chemistry to Understand Inorganic Photochemistry, and Structural Characterization of Excited State Transition Metal Complexes by X-ray Transient Absorption Spectroscopies will be discussed in the latter part of the volume. The Photochemistry of 3d Metal Complexes, in particular the earthabundant metal complexes that have important implications to sustainability, will also be explored, with the volume ended with a discussion on Luminescence Properties of the Actinides and Actinyls. We believe that we have assembled a rather comprehensive, broad, and encompassing coverage of topics and chapters with contributions from authoritative chemists in the community to showcase the significance and impact that inorganic chemistry has made in the current state-of-the-art developments not only in the field of chemistry but also in the fields of materials science, energy and sustainable environment, biomedicine, atom economy, catalysis, and green science. We anticipate that with the creativity and wisdom of the chemists as well as the versatile state-of-the-art methods and techniques, this is just the beginning of the many adventures and exciting journeys and developments that an inorganic chemist would take. There will be many more scientific developments and a wealth of knowledge and opportunities lying in front of us which are beyond our imagination.

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8.02 Luminescent transition-metal complexes and their applications in electroluminescence Peng Tao and Wai-Yeung Wong, Department of Applied Biology and Chemical Technology and Research Institute for Smart Energy, The Hong Kong Polytechnic University, Hung Hom, Hong Kong, P R China © 2023 Elsevier Ltd. All rights reserved.

8.02.1 8.02.2 8.02.2.1 8.02.2.2 8.02.3 8.02.3.1 8.02.3.1.1 8.02.3.1.2 8.02.3.2 8.02.3.2.1 8.02.3.2.2 8.02.3.2.3 8.02.3.3 8.02.3.3.1 8.02.3.3.2 8.02.3.3.3 8.02.3.4 8.02.3.4.1 8.02.3.4.2 8.02.3.4.3 8.02.3.5 8.02.3.6 8.02.3.7 8.02.3.8 8.02.3.9 8.02.3.10 8.02.3.10.1 8.02.3.10.2 8.02.3.11 8.02.3.11.1 8.02.3.11.2 8.02.3.11.3 8.02.3.11.4 8.02.3.11.5 8.02.3.11.6 8.02.4 Acknowledgments References

Introduction Device architectures and working mechanisms of electroluminescence Organic light-emitting diodes Light-emitting electrochemical cells Luminescent transition-metal complexes for electroluminescence Iridium(III) complexes Iridium(III) complexes with bidentate ligand Iridium(III) complexes with tridentate/tetradentate ligand Platinum(II) complexes Platinum(II) complexes with bidentate ligand Platinum(II) complexes with tridentate ligand Platinum(II) complexes with tetradentate ligand Gold(I/III) complexes Gold(I) complexes with carbene ligand Gold(III) complexes with tridentate ligand Gold(III) complexes with tetradentate ligand Copper(I) complexes Carbene ligand-based copper(I) complexes Three-coordinate copper(I) complexes Four-coordinate copper(I) complexes Ruthenium(II) complexes Rhenium(I) complexes Osmium(II) complexes Rhodium(III) complexes Palladium(II) complexes Silver(I) complexes Two-coordinate silver(I) complexes Four-coordinate silver(I) complexes Others Tungsten(VI) complexes Manganese(I/II) complexes Iron(II/III) complexes Nickel(0/II) complexes Zirconium(IV) complex Chromium(III) complexes Conclusion

3 4 4 4 4 5 5 27 29 29 34 39 42 42 44 46 48 49 51 52 55 56 57 59 59 63 63 64 66 66 67 69 71 72 73 73 74 74

Abstract By taking the advantages of phosphorescence, thermally activated delayed fluorescence, and metal-assisted delayed fluorescence, luminescent transition-metal complexes (LTMCs) are a promising class of photofunctional materials for the electroluminescence, which can make full use of their triplet and singlet states and then remarkably boost the efficiency of devices. In this chapter, the recent research progress on the LTMCs (including iridium(III), platinum(II), gold(I/III), copper(I), ruthenium(II), rhenium(I), osmium(II), rhodium(III), palladium(II), silver(I), tungsten(VI), manganese(I/II), iron(II/III), nickel(0/II), zirconium(IV), and chromium(III) complexes, etc.) as the emitters for electroluminescence will be summarized and discussed. The design strategies on the molecular structures, and photophysics of these LTMCs and their electroluminescence applications are presented in detail. In addition to the most popular iridium(III) and platinum(II) complexes involved in the design of organic electroluminescence (organic light-emitting diodes, light-emitting

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electrochemical cells, etc.), the relatively less explored LTMCs and their emerging applications in organic electroluminescence are also included.

8.02.1

Introduction

In recent years, luminescent transition-metal complexes (LTMCs) play important roles in developing high-performance photofunctional materials.1–5 Different from the classical fluorescent materials showing emission from the singlet states, most of the LTMCs can use their singlet and triplet states simultaneously, which could greatly enhance the efficiency of organic light-emitting diodes (OLEDs) and light-emitting electrochemical cells (LECs).1–5 Since the first phosphorescent OLED was successfully realized by S. R. Forrest and coworkers in 1998, luminescent transition-metal complexes, particularly for those based on noble metals (e.g., iridium(III), platinum(II), etc.) as the triplet emitters toward highly efficient organic electroluminescence (EL), have drawn an increasing attention.4–6 Besides the iridium(III) or platinum(II)-based complexes, many other LTMCs also gained much attention.7 Owing to the presence of d orbitals in the transition-metal ions, the interaction of d orbitals with organic ligands will contribute to the rich excited states of LTMCs, including metal-to-ligand charge-transfer (MLCT), metal-metal-to-ligand charge transfer (MMLCT), ligandto-metal charge-transfer (LMCT), ligand-to-ligand charge-transfer (LLCT), intraligand charge transfer (ILCT), metal-centered (MC), ligand-centered (LC) excited states, etc. Moreover, the triplet-emitting complexes could make full use of both singlet and triplet excitons by efficient spin-orbit coupling (SOC) effect to remarkably boost the efficiency of electroluminescence, exceeding the conventional upper limit of efficiency of the device based on the traditional fluorescent emitters.8 Recently, it is also found that some transition-metal ions can also induce efficient delayed fluorescence (e.g., thermally activated delayed fluorescence (TADF), metal-assisted delayed fluorescence (MADF)) of LTMCs, making them promising candidates in the device design of thermally activated delayed fluorescent electroluminescence.9 The photophysical properties of LTMCs play significant roles in the performance of electroluminescence. Thus, the rich excited states of LTMCs, together with their structural diversities, would provide more opportunities in future applications.10 In this chapter, we will summarize and discuss the recent research progress (especially from the year 2013) on the use of LTMCs as the emitters (e.g., phosphorescent or delayed fluorescent materials, etc.) for electroluminescence (Fig. 1). The transition-metal complexes will include both organometallic and coordination complexes of iridium(III), platinum(II), gold(I/III), copper(I), ruthenium(II), rhenium(I), osmium(II), rhodium(III), palladium(II), silver(I), tungsten(VI), manganese(I/II), iron(II/III), nickel(0/II), zirconium(IV), and chromium(III), etc. The molecular design, photophysical properties of these complexes, and their applications

Fig. 1

Luminescent transition-metal complexes and their applications in electroluminescence.

4

Luminescent transition-metal complexes and their applications in electroluminescence

in electroluminescence are described in detail. Besides the iridium(III) and platinum(II) complexes, the most popular phosphors involved in the design of electroluminescence, we also focus on the relatively less explored LTMCs and their emerging applications in electroluminescence. Before the discussion of these luminescent transition-metal complexes, we will also provide a brief introduction of the working principles of electroluminescence (OLEDs and LECs), including the device architectures and related working mechanisms.

8.02.2

Device architectures and working mechanisms of electroluminescence

8.02.2.1

Organic light-emitting diodes

In addition to the functional materials (e.g., emitters, host materials, carrier transport materials, etc.) involved in OLEDs, the structure of the device will be another critical factor governing the performance of organic electroluminescence. Since the first thinlayered OLED showing high device efficiency was realized in 1987, various device structures were designed for achieving good carrier injection and balanced carrier transportation. Meanwhile, accompanying with the technological developments, the deeper understanding of the working mechanisms of OLEDs was also realized.11,12 In general, OLEDs are composed of multiple functional layers ranging from several nanometers to tens of nanometers. The functional layers include transparent metal oxide anodes (e.g., indium tin oxide (ITO)), hole transport layer (HTL), light-emitting layer (EML), electron transport layer (ETL), and metal cathode (e.g., aluminum (Al)) (Fig. 1). The working principle of the typical OLED can be outlined in the following process. Under an applied voltage, holes and electrons are injected through the anode and cathode, respectively. Then the carriers are transported in the HTL and ETL, and holes and electrons are injected into the EML to form excitons, and finally the electroluminescence was realized by the radiative deexcitation of the emitter within EML.13 For most phosphorescent or delayed fluorescent OLEDs, to reduce the probability of the triplet-triplet annihilation of the phosphorescent or delayed fluorescent emitters, those light-emitting materials are usually used as the dopant and dispersed in a suitable host material.13 The host material not only assumes the role of carrier transport but also acts as the energy donor of the emitter.13 So, the performance of OLED is closely related to the device structure and its functional materials. Thus, the light-emitting material as the core functional material is of particular importance.

8.02.2.2

Light-emitting electrochemical cells

Compared to the OLEDs, LECs have much simpler sandwich structures composed of three functional layers (anode, EML, cathode) (Fig. 1). The EML containing the charged materials is inserted into an anode (e.g. ITO, etc.) and an air-stable cathode (e.g. aluminum, silver, gold, etc.). In a typical LEC, the EML not only emits light but also transports carriers simultaneously.14 There are two physical models (electrochemical doping model and electrodynamic model) for the description of the working mechanism in LECs. Upon applying a voltage to an LEC, the electrochemical doping model is based on the hypothesis that the redistribution of the charged species forms n- and p-doped regions slowly, resulting in the gradual appearance of a n-i-p junction between the electrodes. For the electrodynamic model, the charged species in the EML will begin to migrate, leading to the reduction of the carrier injection barrier. The accumulation of the charged species at the interfaces of the electrodes and the EMLs will induce the electric double layers, which gives rise to the generation of electric field that offsets the interfacial potential and thus facilitates the carrier injection. In the LECs, the sharp decline in the potential within the LECs is supposed to be across the n-i-p junction according to the electrochemical doping model or close to the interfaces according to the electrodynamic model. Similar to the OLED, the holes and electrons from the electrodes are injected into the EML, and then the combination of them will generate the light emission of LECs.14

8.02.3

Luminescent transition-metal complexes for electroluminescence

As a new generation of light-emitting materials in OLEDs, transition-metal complex-based luminescent materials boosting the efficiency greatly through the full utilization of the excitons in the devices have become the most important emitters.1–6 One of the well-known functions of transition-metal ions (e.g., iridium(III), platinum(II), etc.) is the efficient spin-orbit coupling effect, which can help complexes harness both singlet and triplet states to remarkably improve the device efficiency.1–6 Recently, for iridium(III) and platinum(II) complexes, more attention was still paid on the color tuning (especially in highly efficient deep-blue or near infrared (NIR) emission), and a variety of iridium(III) and platinum(II) complexes with novel structural features have also been well developed, which is quite different from the classical bidentate ligands (e.g., 2-phenylpyridine-type cyclometalating ligand). It is also reported that many transition-metal ions (e.g., copper(I), gold(I/III), palladium(II), tungsten(VI), zirconium(IV), etc.) can also induce the efficient delayed fluorescence of LTMCs, making them promising candidates for efficient electroluminescence. In the following sections, the representative LTMCs developed recently (especially from the year 2013) as the light-emitting materials for OLEDs will be discussed in detail.

Luminescent transition-metal complexes and their applications in electroluminescence 8.02.3.1

5

Iridium(III) complexes

Iridium(III) complexes possess an octahedral geometry with a d6 electronic configuration. So far, the phosphorescent iridium(III) complexes represent a class of the most successful triplet emitters due to their excellent and rich photophysical properties, for instance, highly efficient photoluminescence (PL), controllable emission colors covering ultraviolet to near-infrared, and tunable lifetime of excited states.6 Moreover, the iridium(III) complexes also show high chemical stability, which is in favor of the device fabrication and operation. So far, there are various kinds of iridium(III) emitters which have been exploited. According to the coordination mode of ligands, they can be classified as bidentate ligand-based complexes and tridentate ligand-based complexes. Thus, the iridium(III) complexes will be discussed as follows.

8.02.3.1.1

Iridium(III) complexes with bidentate ligand

The majority of iridium(III) complexes developed are the bidentate ligand-based ones.6 These complexes containing three ligands could be divided into homoleptic and heteroleptic complexes. The homoleptic iridium(III) complexes consist of the same ligand, whereas the heteroleptic ones consist of different ligands. The photophysical properties of this class of iridium(III) complexes are highly dependent on the ligand structures. By rational structural design, the emission color of those complexes can range from ultraviolet to near-infrared. 8.02.3.1.1.1 Blue emissive iridium(III) complexes Blue emitters are the necessary component in the application of OLEDs, and lots of efforts have been made on the molecular design of blue emissive iridium(III) complexes.14–16 The general molecular design principle of blue iridium(III) complex is based on the 2phenylpyridine ligand. The introduction of electron-withdrawing groups into the phenyl moiety or incorporation of electrondonating groups into pyridine moiety can remarkably increase the energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), thereby resulting in the blue-shift of the emission.14

The well-known blue emissive iridium(III) complex bis(2-(4,6-difluorophenyl)pyridinato-C2,N)(picolinato)iridium(III) (FIrpic) (1) is based on the ligands of 2-(2,4-difluorophenyl)pyridine and picolinic acid.17–19 Complex 1 shows sky-blue phosphorescence in the wavelength of 470 nm in dichloromethane. By replacing the auxiliary ligand picolinic acid with 2-(3-(trifluoromethyl)-1H-1,2,4-triazol-5-yl)pyridine and 2-(1H-tetrazol-5-yl)pyridine, the emission wavelengths of complexes 2 and 3 are 459 and 460 nm, indicating the efficient blue-shifting induced by those two auxiliary ligands. Recently, by incorporating an electrontransporting layer with the low refractive index, Shin et al. successfully designed a series of blue phosphorescent OLEDs based on complex 1 with the peak external quantum efficiency (EQE) of 34.1%.20

6

Luminescent transition-metal complexes and their applications in electroluminescence

The systematic functionalization of complex 1 was further carried out by Kozhevnikov et al.21 Different kinds of alkyl groups (complexes 4–6) or mesityl moieties (complexes 7 and 8) were introduced into complex 1 to investigate their effects on the photophysical properties and electroluminescence. Owing to the excellent solubility of those functionalized complexes in commonly used solvents, solution-processed phosphorescent OLEDs based on complexes 4–8 were fabricated. It is found that complex 7 showed the significantly enhanced device performance over complex 1. OLEDs with 7 doped into poly(vinylcarbazole) (PVK): 1,3-bis(5-(4-(tert-butyl)phenyl)-1,3,4-oxadiazol-2-yl)benzene (OXD-7) gave a maximum current efficiency (CE) of 19.1 cd A
1 at a brightness of over 5000 cd m
2 with an EQE of 8.7%. Optimized multilayer devices gave a maximum CE of 23.7 cd A
1 and EQE of 10.4%. Those results show that the systematic functionalization of complex 1 could effectively improve the performance of the solution-processed device.21

The blue-shift of emission wavelengths of the iridium(III) complexes can be realized by decreasing the energy level of HOMO. As shown in complexes 9–12, the strong electron-withdrawing substituents (heptafluoropropyl carbonyl or trifluoromethyl carbonyl) were incorporated into the 30 position of the cyclometalating ligand.22 At 298 K, the emission maxima in chloroform are at 453 nm for 9, 454 nm for 10, 447 nm for 11, and 447 nm for 12 (Fig. 2A). In 9-(3-(9H-carbazole-9-yl)phenyl)-3-(dibromophenylphosphoryl)-9H-carbazole films doped with those complexes in 10 wt%, the photoluminescent quantum efficiencies (PLQYs) of 9– 12 are determined to be 74%, 52%, 63%, and 42%, respectively. Phosphorescent OLEDs based on 9 and 10 show maximum EQEs of 17.1% and 12.6% and Commission Internationale de l0 Eclairage (CIE) coordinates of (0.14, 0.16) and (0.14, 0.17), respectively (Fig. 2B).22

Ancillary ligands also have remarkable influences on the excited states of iridium(III) complexes. In the aspect of the design of the novel ancillary ligands, Zheng and coworkers designed two novel ancillary ligands (phenyl(pyridin-2-yl)phosphinate and dipyridinylphosphinate) with good electron transport property for efficient blue phosphorescent iridium(III) emitters (13 and 14).23 The OLEDs based on new complexes show excellent EL performances with a peak CE of 58.78 cd A
1, a maximum EQE of 28.3%, a peak power efficiency (PE) of 52.74 lm W
1 and negligible efficiency roll-off. The results implied that the pyridinylphosphinate ligands have potential use in designing novel high-performance blue iridium(III) emitters.23

Luminescent transition-metal complexes and their applications in electroluminescence

7

Fig. 2 (A) UV
vis absorption and PL spectra of the complexes 9–12 in chloroform. (B) EQE versus current density curves of OLEDs based on 9– 12. Adapted from Lee, S.; Kim, S.-O.; Shin, H.; Yun, H.-J.; Yang, K.; Kwon, S.-K.; Kim, J.-J.; Kim, Y.-H. J. Am. Chem. Soc. 2013, 135, 14321, with permission from American Chemical Society. Copyright 2013.

In addition to the neutral blue iridium(III) emitters, the ionic ones are another important class of blue iridium(III) emitters, which have already been used widely in LECs and OLEDs.24–27 Recently, Henwood et al. designed four biimidazole-type N^N ancillary ligands, and by using 2-(2,4-difluorophenyl)pyridine) or its derivative as the cyclometalating ligands, a series of deep-blue cationic iridium(III) complexes (15–18) were well prepared by a strategy of tethering the biimidazole to rigidify the complexes and dramatically boost their PLQYs.28,29 Compared to the PLQY of 2% for complex 16, a remarkable increase in the PLQY (as high as 68%) is realized for complex 17, which is among the brightest deep-blue cationic iridium(III) emitters reported.29

The counterion is an essential component for the ionic iridium(III) complex, making it possible to manipulate the photophysical properties of those emitters. Complex 19 with PF6
as the counterion emits blue phosphorescence at 475 nm in CH3CN with very low PLQY (0.03) in neat film, which is the result of the severe triplet-triplet annihilation induced by intermolecular interaction.31 To solve this problem, Ma et al. used three new bulky tetraarylborate anions to improve the PLQYs of cationic iridium(III) emitters (20
22) (Fig. 3).30 The PLQYs of the prepared iridium(III) emitters 20–22 are remarkably enhanced in the solid state. In the neat film, complex 22 with the largest counter anion exhibits the highest PLQY (0.39), which is about 12 times higher than that

8

Luminescent transition-metal complexes and their applications in electroluminescence

of complex 19. These results imply that the selection of bulky counter anions will be an effective method to boost the PLQY of cationic transition-metal complex and also has a negligible effect on the emission wavelength.30

Another important effect of the bulky counter anions of the iridium(III) emitters is the tunable volatility of those materials. In general, because of the inherent ionic nature and low vapor pressure of the ionic complexes, those complexes are seldom sublimable, thereby limiting their further applications in vapor-processed OLEDs. However, recently, there are new insights into the tunable volatility of those ionic emitters.32 It is found that the volatility of those ionic iridium(III) complexes is closely related to their counter anions. A facile, versatile and feasible strategy was proposed by Duan and coworkers, in which a suitable control of the counterions can be an effective approach to tune the volatility of ionic iridium(III) complexes.32 The sublimable cationic blue iridium(III) complexes 24 and 25 are designed (complex 23 is a control complex) by incorporating counterions with large steric hindrance. The vapor-processed blue OLEDs based on complexes 24 and 25 show small efficiency roll-off and high brightness.32 This molecular design strategy may open a new avenue for ionic functional materials in the applications of vapor-processed OLEDs.

Fig. 3

Oak ridge thermal-ellipsoid plot (ORTEP) diagrams of complexes 19–21. Redrawn from data in ref. 30.

Luminescent transition-metal complexes and their applications in electroluminescence

9

So far, the most popular strategy for designing blue emissive iridium(III) complexes based on 2-phenylpyridine is to introduce the fluorine atoms into the ligand. The introduction of fluorine atoms into the phenyl moiety of 2-phenylpyridine can effectively reduce the energy level of HOMO, thereby increasing the energy gap of emitters. However, due to the weakened aromatic carbonfluorine (Caryl-F) bond, the stability of iridium(III) complexes could be decreased by the increasing number of fluorine atoms under the operation of the device, resulting in short device lifetimes.33–36 Tao et al. selected a chemically stable trifluoromethyl group to replace one fluorine atom on the ligand to decrease the number of fluorine atoms of the complexes (26–31).37 Those complexes not only show good chemical stability but also exhibit intense blue emissions with high PLQYs of up to 0.98 in degassed dichloromethane (CH2Cl2) and narrow full width at half maximum (FWHM) of 52 nm. Compared to those of the FIrpic-based OLED, the EL peak, FWHM, and 1931 CIE coordinates of the OLEDs based on complex 29 are all superior. In addition, under the same condition, the maximum efficiencies of the OLED based on complex 29 (12.07 lm W
1, 21.15 cd A
1) also exceed those of the FIrpic-based OLED (11.26 lm W
1, 18.89 cd A
1).37

As mentioned above, the presence of Caryl-F bond within iridium(III) complexes could have a negative effect on the chemical stability and device lifespan of the complexes.33–36 As the fluorine analog, the chlorine shares similar electronic property, which has been used in the molecular design of organic photovoltaic materials.38 Because of the lower electronegativity of chlorine,

10

Luminescent transition-metal complexes and their applications in electroluminescence

the aromatic carbon-chlorine (Caryl-Cl) bond could be more stable than that of the Caryl-F bond. Zhao and his team proposed a chlorine-functionalization strategy to develop a series of chlorine-bearing blue iridium(III) emitters with various auxiliary ligands (32– 36).39,40 These complexes 32–36 exhibit intense blue/sky-blue emissions with PL peaks of 476–468 nm and high PLQYs of 0.70– 0.85 in CH2Cl2. By selecting complex 35 as dopant, the fabricated blue and blue/yellow-based white OLEDs show impressive EQEs of 21.41% and 20.17%, respectively.39,40

The replacement of carbon atom of the phenyl moiety in 2-phenylpyridine derivatives by nitrogen atom is also a powerful method to provide iridium(III) complexes with short emission wavelengths.41,42 For example, complexes 37–39 based on 20 ,60 difluoro-2,30 -bipyridine derivatives were developed by Feng et al., in which the carbon atom at the para position on the phenyl moiety was replaced by a nitrogen atom.41,42 The introduced nitrogen atom has a similar or even stronger effect in lowering the HOMO of complexes. Complexes 37 and 38 with triazole as the auxiliary ligand almost show the same deep-blue emission (lPL ¼ 430 nm with 470 nm as the shoulder) with high PLQYs (0.65 for 37, 0.70 for 38) and short excited state lifetimes (2.97 ms for 37, 3.01 ms for 38) in t-BuCPO doped films at room temperature. The devices based on both emitters show very high EL performance (e.g., EQE of 12.6%, PE of 8.8 lm$W
1 at 100 cd$m
2 for 38) with low efficiency roll-off at high luminance.41 Using 20 ,60 -difluoro-2,30 -bipyridine as the cyclometalating ligand, Liu and coworkers designed a novel sky-blue iridium(III) complex 39 (lPL ¼ 488 nm) with bipolar charge-transport ability by incorporating N^N-type amidinate ligand attaching to an electron-donating carbazole.42 This sky-blue iridium(III) complex can not only be used as an emitter but also can serve as an excellent bipolar host material (T1  2.5 eV).

Recently, Wong and coworkers developed a novel phenyl-pyrimidine-type cyclometalating ligand by just replacing the pyridine moiety in the cyclometalated ligand of FIrpic with pyrimidine.43 Three complexes 40–42 were synthesized by choosing various auxiliary ligands. Those complexes show sky-blue phosphorescence (lPL ¼ 475 nm for 40, lPL ¼ 462 nm for 41, lPL ¼ 463 nm for 42) in toluene (Fig. 4A). Complex 40 shows a slight red-shifted emission with a shorter lifetime in the excited state (0.8 ms for 40, 1.1 ms for FIrpic) and the capability to realize a high-performance blue phosphorescent OLED with a long lifespan (T50 > 2,200 h). Notably, OLEDs based on 41 and 42 as dopants exhibit blue emission with extremely high EQE exceeding 31% (Fig. 4B). This molecular design strategy provides an alternative way to develop long-lasting blue phosphorescent OLED.43

Luminescent transition-metal complexes and their applications in electroluminescence

11

Fig. 4 (A) UV
vis absorption and PL spectra of the complexes 40–42 in toluene. (B) EL spectra and 1931 CIE (x, y) coordinates of OLEDs based on 40–42. Adapted from Sarma, M.; Tsai, W.-L.; Lee, W.-K.; Chi, Y.; Wu, C.-C.; Liu, S.-H.; Chou, P.-T.; Wong, K.-T. Chem 2017, 3, 461, with permission from Elsevier Inc. Copyright 2017.

As discussed above, the most common strategy for the blue-shifting of the emission of iridium(III) phosphors (e.g., FIrpic) is to introduce the fluorine atoms into the phenyl ring.17–20 It has been proved that the Caryl-F bonds on the iridium(III) phosphors are inherently unstable during the device operation.33–36 Replacing the phenyl ring of the cyclometalating ligand by the electrondeficient heterocycle could be a promising approach to solve this problem.41,42,44 Henwood et al. prepared a library of electronpoor heterocycle-based sky-blue/blue ionic iridium(III) complexes bearing methoxy groups (43–46).44 Interestingly, the methoxy groups can show inductively electron-withdrawing effect when it is meta-substituted to the iridium center. Those complexes show highly efficient sky-blue/blue emission (lPL ¼ 515 nm for 43, lPL ¼ 454 nm for 44, lPL ¼ 446 nm for 45, lPL ¼ 457 nm for 46) at room temperature, and the PLQYs of 0.73–0.81.44

Phenylimidazole-based cyclometalating ligands are an important class of C^N ligands which are popularly used for designing highly efficient blue iridium(III) emitters.46–48 A library of 2-phenyl-1H-imidazole-based homoleptic cyclometalated iridium(III) complexes (47–53) functionalized by various substituents (e.g., methyl, substituted phenyl, cyano, fluorine) were rationally

12

Luminescent transition-metal complexes and their applications in electroluminescence

designed for highly efficient blue OLEDs.46–48 The introduction of substituted phenyl groups into the imidazole moiety is intended to tune the emission dipole orientations of the complexes 47–51. For complex 51, the biphenyl group is incorporated into the imidazole moiety to obtain simultaneously improved device efficiency and operation lifetime. The blue OLED based on 51 gave an emission dipole orientation of 91%, maximum EQE of 26.3%, LT80 lifetime of 169 h at 1,000 cd m
2, and color coordinates of (0.17, 0.30).46 Diisopropylphenyl group was selected by Lee and coworkers to modify the phenylimidazole-based blue emissive iridium(III) complexes (52 and 53).47,48 At room temperature, those two complexes exhibit blue emission (lPL ¼ 462 nm for 52, lPL ¼ 454 nm for 53) with extremely high PLQYs of 0.99 for 52 and 0.87 for 53 in 1,3-bis(N-carbazolyl)benzene (mCP) doped films. OLEDs based on 52 and 53 show high EQEs of 22.5% for 52 and 18.9% for 53 in blue phosphorescence with CIEy of 0.3. Moreover, at the luminance of 200 cd m
2, the 52-based OLED exhibits long lifetime (> 550 h), much longer than that of the reported blue OLEDs based on phenylpyridine-type iridium(III) emitters.48 Phenyltriazole-based cyclometalating ligands in iridium(III) emitters were successfully demonstrated to realize pure blue phosphorescence.45 Liu and coworkers prepared two iridium(III) emitters based on phenyltriazole ligands (54 and 55) (Fig. 5A).45 The complex 54 exhibits blue phosphorescence at the wavelength of 458 nm with PLQY of 0.64 in CH2Cl2 at room temperature. Further fluorination on the phenyl ring of complex 54 gives rise to the complex 55 showing deep-blue emission (lPL ¼ 431 nm) with PLQY of 0.41 in CH2Cl2 at room temperature (Fig. 5B). Notably, the PLQY of complex 55 (0.54) is much higher than that of complex 54 in the solid state, demonstrating that the fluorine atom has a great effect on the improvement in the PLQY of the complex. Highperformance pure blue OLEDs with a maximum EQE of 22.5% and CIE coordinates of (0.15, 0.11) were obtained, and the extremely high EQE is induced by a preferred horizontal dipole orientation of the iridium(III) emitter which is in favor of the light extraction from OLED.45

(A)

N3

C2

C3

N1 C1 C2 Ir

Ir N2 N1 C1

Normalized Intensity

(B)

N3

N2

C3

Abs.

1.00

Abs. Em. Em.

0.75

54 55 54 55

0.50 0.25 0.00 300

400

500

600

Wavelength (nm) Fig. 5 (A) ORTEP diagrams of complexes 54 (left) and 55 (right). Redrawn from data in ref. 45. (B) UV
vis absorption and PL spectra of the complexes 54 and 55 in CH2Cl2. Adapted from Li, X.; Zhang, J.; Zhao, Z.; Wang, L.; Yang, H.; Chang, Q.; Jiang, N.; Liu, Z.; Bian, Z.; Liu, W.; Lu, Z.; Huang, C. Adv. Mater. 2018, 30, 1705005, with permission from WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. Copyright 2018.

Luminescent transition-metal complexes and their applications in electroluminescence

13

In the aspect of the ionic blue iridium(III) emitters based on the phenylpyrazole ligand, Duan and his team developed a series of efficient sky-blue and green cationic iridium(III) emitters based on the phenylpyrazole ligand (56–59) recently.49 Owing to the presence of the bulky tetraphenylborate counterions, those complexes not only show the high PLQYs (up to 0.93) but also display excellent sublimability under heating suitable for the vacuum-deposited OLEDs. Green OLEDs based on sublimable complex 59 showed high EQE of 11.3%.49

14

Luminescent transition-metal complexes and their applications in electroluminescence

Compared to the C^N ligands, the use of ligands with the high-field strength (e.g., carbenes) in iridium(III) complexes could increase the emission energy gap and give rise to an increase of the blue phosphorescent efficiency.50–52 Wu and his team designed two kinds of the carbene derivatives (phenyl and benzyl) as the cyclometalating ligands to prepare three carbene-based iridium(III) complexes (60–62).52 Complex 60 shows violet phosphorescence (lPL ¼ 392 nm) with a very low PLQY of 5  10
4, while benzylcarbene derivative has the interrupted p-conjugation of the heterocyclic ligands on the iridium(III) complexes (61 and 62) (lPL ¼ 460 nm for 61, lPL ¼ 458 nm for 62), which can greatly improve the PLQYs of those complexes (0.22 for 61, 0.73 for 62). This molecular design method has been demonstrated to be powerful for the design of deep-blue iridium(III) emitters.52

Phosphorescent OLEDs displaying both deep-blue emission and very high brightness are essential for both solid-state lighting and display applications.12 Based on the previously reported violet emitter 63 (lPL ¼ 380 nm), Lee et al. developed two N-heterocyclic carbene-based iridium(III) complexes (64 and 65) in 2016, which could act as both pure blue phosphors and efficient holeconducting electron/exciton blocking layers.53 Because both the facial (64) and the meridional (65) isomers of the complex possess a strong ligand-metal bond destabilizing the non-radiative metal-centered ligand-field states, the two complexes show high PLQYs (0.76 for 64, 0.78 for 65). The emission energy of the facial complex 64 is much higher than that of the meridional complex 65 in 2methyltetrahydrofuran (Fig. 6). The deep-blue OLEDs with graded emission layer exhibited extremely high luminance (> 7800 cd$m
2) with an EQE of 10.1% and 1931 CIE coordinates of (0.16, 0.09).53

Fig. 6 (A) UV
vis absorption spectra of the complexes 63–65 in CH2Cl2. (B) PL spectra of the complexes 64 and 65 in 2-methyltetrahydrofuran. Adapted from Lee, J.; Chen, H.-F.; Batagoda, T.; Coburn, C.; Djurovich, P. I.; Thompson, M. E.; Forrest, S. R. Nat. Mater. 2016, 15, 92, with permission from Macmillan Publishers Limited. Copyright 2015.

Luminescent transition-metal complexes and their applications in electroluminescence

15

Wong and coworkers developed a series of N-heterocyclic carbene-based sky-blue/blue emissive iridium(III) complexes (66–70) containing various functional groups (e.g., F, methyl, t-butyl, and CF3).54 These five complexes show blue phosphorescence ranging from 420 nm to 450 nm with high PLQYs of as high as 0.99. Very recently, by using N-methyl/aryl-pyrazinoimidazol-2-yl carbene ligands, another library of N-heterocyclic carbene-based sky-blue iridium(III) complexes (71–76) were successfully prepared.55–57 These complexes exhibit sky-blue emission and high PLQYs of 0.78–0.92. Phosphorescent OLED based on 74 realizes excellent EQE of 18.1%, and high brightness of 29,000 cd m
2.55 Carbene-based complexes (77 and 78) were prepared by Kim et al. to investigate the degradation of blue phosphorescent OLEDs involving the formation of polaron pairs induced by exciton within the emitting layers.57

Solution-processable blue emissive iridium(III) complex is an essential component for the preparation of low-cost OLEDs.12 In 2019, Müllen and coworkers selected a facial carbene-based blue emissive iridium(III) complex as the core, and introduced polyphenylene derivatives into the core by Diels-Alder reaction to prepare carbene-based iridium(III) dendrimers (79 and 80).58 However, the iridium(III) dendrimers (79 and 80) display intense sky-blue luminescence at 77 K but have no luminescence at room temperature. It is believed that the Ir-Ccarbene bonds within the dendrimers are elongated by the bulky polyphenylene

16

Luminescent transition-metal complexes and their applications in electroluminescence

dendrons, which makes the T1 states of iridium(III) dendrimers easily accessible to the non-emissive 3MC state.58 This result will provide a useful guide for developing emissive iridium(III) dendrimers in the future. 8.02.3.1.1.2 Green/yellow emissive iridium(III) complexes Green light-emitting iridium(III) complexes are essential to the high-contrast display applications.6,12,13 For lighting, the highquality solid-state lighting can be realized by a combination of blue, green and red emitters (three-color method) or by incorporation of blue, green, yellow, and red emitters (four-color method). Although the multicolor (three-color or four-color) methods could realize high-quality white light in the spectrum, the white OLEDs with complicated device structures are required, thereby limiting their practical applications.12,13 In fact, the complementary color (blue and yellow) method is another promising approach for realizing a white light. For this method, in order to cover more regions in the visible spectrum, there will be requirements for yellow emitters (e.g., broad emission bandwidth).6,8 Thus, from this point of view, developing highly efficient yellow light-emitting iridium(III) complexes is necessary.

Pyridine moiety is the most commonly used building block for designing highly efficient green or yellow emissive iridium(III) complexes.59–61 2-Phenylpyridine is a typical cyclometalating ligand for green iridium(III) complexes. Further modification of 2phenylpyridine ligand can not only realize the fine-tuning of emission color but also provide other excellent properties (e.g., charge-transport ability, excited state lifetime, PLQY, etc.) of the iridium(III) complexes.59–69 In 2015, Zhao et al. developed a series of phosphorescent iridium(III) complexes modified by fluorinated aromatic sulfonyl group (81–84).62 These complexes show extremely high PLQYs of up to 100%. The presence of two cyclometalating ligands of the iridium(III) complexes makes it possible

Luminescent transition-metal complexes and their applications in electroluminescence

17

to manipulate their photophysical properties by modifying individual ligand independently. Wong and his team designed a library of tris-heteroleptic iridium(III) complexes bearing two different cyclometalating ligands (85–90).63 By tuning the electronic properties of individual cyclometalating ligands, both emission energies and charge-transport abilities of these complexes can be well tuned. Owing to the enhanced charge-transport abilities, these complexes show high device performance with a maximum EQE of 20.20%, PE of 35.15 lm W
1, and CE of 69.41 cd A
1.63

Later, Duan and his team prepared sublimable ionic greenish yellow iridium(III) complexes bearing 2-phenylpyridine ligand with the bulky counterions (91 and 92).65 Two complexes show the improved PLQYs (0.59 for 91, 0.52 for 92) in the neat film compared to those in solution (0.34 for 91, 0.33 for 92). The device gives the record-high efficiency (EQE of 14.8%) among the reported ionic iridium(III) emitters.65 Zhou et al. designed a pair of 2-phenylpyridine-based iridium(III) emitters incorporated with the sterically hindered chiral pinene substituents (93 and 94).66 Due to the presence of chiral pinene substituent, those complexes show circularly polarized (CP) green phosphorescence (lPL ¼ 513 nm) with negligible decreased PLQYs at the concentration from 5  10
4 to 5  10
3 mol L
1 in CH2Cl2. OLEDs based on those complexes display excellent performance with a luminance of 135,676 cd m
2, peak EQE of 28%, PE of 88.29 lm W
1, and CE of 103.50 cd A
1. However, the CP phosphorescence signals of OLEDs are too weak, which is the result of the possible chirality transfer from the chiral dopant to the host in the emissive layer.66 Spiro(fluorene-9,90 -xanthene) (SFX) is an interesting bulky building block, which can provide steric hinderance.67 In 2017, using this bulky SFX moiety, Liu and coworkers developed a yellow homoleptic iridium(III) complex based on 2-(spiro(fluorene9,90 -xanthen)-2-yl)pyridine ligand (95).68 In the crystal of complex 95, the negligible intermolecular p-p interactions between the spiro ligands can be observed, and the Ir-Ir distance is enlarged. The incorporation of the spiro ligand into complex 95 could not only facilitate the carrier injection and transport but also suppress the concentration quenching in the emissive layer. The dry- and wet-processed OLEDs based on 95 show high EQEs of 12.1% and 11.3%, PEs of 36.3 and 22.1 lm W
1, CEs of 46.2 and 32.2 cd A
1, respectively (Fig. 7). Notably, at the high luminance of 1000 cd m
2, the roll-off of EQE is only 1.7%, proving that the SFX moiety is the perfect building block for designing stable emitters.68

18

Luminescent transition-metal complexes and their applications in electroluminescence

Fig. 7 Multifunctional spiro ligand-based iridium(III) complex 95 for the dry- and wet-processed OLEDs. Reproduced from Ren, B.-Y.; Guo, R.-D.; Zhong, D.-K.; Ou, C.-J.; Xiong, G.; Zhao, X.-H.; Sun, Y.-G.; Jurow, M.; Kang, J.; Zhao, Y.; Li, S.-B.; You, L.-X.; Wang, L.-W.; Liu, Y.; Huang, W. Inorg. Chem. 2017, 56, 8397, with permission from American Chemical Society. Copyright 2017.

The replacement of the carbon atom adjacent to the nitrogen atom of pyridine by nitrogen atom will give rise to pyridazine, which has been involved in the molecular design of yellow iridium(III) complexes.70–74 Recently, several kinds of 3-phenylpyridazine derivatives were designed for synthesizing yellow homoleptic iridium(III) complexes (96–102).70–72 Complex 96 bearing 2,6dimethylphenol moiety shows yellow emission (lPL ¼ 552 nm).70 Compared to complex 96, complexes 97 and 98 show a blueshift in emission (lPL ¼ 524 nm for 97, lPL ¼ 508 nm for 98) owing to the electron-withdrawing effect of F atoms.71 While complex 99 bearing CF3 shows yellow emission at 566 nm,72 Tong et al. selected sterically hindered bicyclo[2.2.2]oct-2-ene to modify the pyridazine core, and three green emissive iridium(III) complexes (100
102) were obtained by using this hindered ligand. Green OLED based on complex 102 shows a peak EQE of 25.2% and CE of 64.1 cd A
1 even at a high doping concentration.72

Luminescent transition-metal complexes and their applications in electroluminescence

19

For heteroleptic ones based on pyridazine moiety, very recently, Yang and coworkers selected 3-(2,4-difluorophenyl)-6methylpyridazine as the cyclometalating ligand to prepare four green/yellow emissive heteroleptic iridium(III) emitters by changing their auxiliary ligands (103–106).73 The auxiliary ligand has a remarkable influence on the excited states of those complexes, and the emission wavelengths of those complexes are 544 nm for 103, 554 nm for 104, 515 nm for 105, and 499 nm for 106, respectively. These complexes all exhibit high PLQYs of 0.72–0.89. Green OLED based on complex 105 shows an extremely high maximum EQE of 28.7% with a quite low efficiency roll-off (2%) at the high brightness of 1000 cd m
2, and the device performance is superior to the classical green emitter bis(phenylpyridinyl)(acetylacetonate)iridium(III).73 Very recently, by incorporating the well-known chiral moiety of R-camphor, Yang and his team designed two green chiral iridium(III) emitters of 107 and 108 with dual stereogenic centers at ancillary ligand (R) and iridium center (L or D) (Fig. 8A). Two complexes show high PLQYs of up to 93% and intense CP phosphorescence with dissymmetry factors in the order of magnitude of 10
3. The CP OLEDs based on 107 and 108 display excellent performances with the peak EQE of over 30% (Fig. 8B and C). Furthermore, the dissymmetry factors of OLEDs with novel device design are remarkably enhanced (as high as 7.70  10
3). The results clearly proved that CP OLEDs could realize high efficiency and large dissymmetry factor simultaneously via rational molecular and device design.74

Fig. 8 (A) ORTEP diagrams of complexes 107 (left) and 108 (right). (B) EQE-luminance curves and EL spectra of OLEDs based on complexes 107 and 108. (C) Circularly polarized EL (CPEL) spectra of OLEDs based on complexes 107 and 108. Adapted from Lu, G.; Wu, Z.-G.; Wu, R.; Cao, X.; Zhou, L.; Zheng, Y.-X.; Yang, C. Adv. Funct. Mater. 2021, 31, 2102898, with permission from Wiley-VCH GmbH. Copyright 2021.

20

Luminescent transition-metal complexes and their applications in electroluminescence

2-Phenylquinoline is a popular cyclometalating ligand commonly used in red emissive iridium(III) complexes.6,8 Introducing the electron-withdrawing group into the phenyl moiety of the complex can increase its energy gap between HOMO and LUMO of iridium(III) complex, thereby resulting in the blue-shift of the emission wavelength.75–77 Tao et al. designed a series of trifluoromethyl and halogen-containing 2-phenylquinoline-based neutral iridium(III) complexes (109–112) (Fig. 9A).75,76 The involved halogen atoms are fluorine or chlorine. The use of chlorine atom is intended to enhance the chemical stability of chlorine-based yellow iridium(III) emitters because of the more stable Caryl-Cl than that of Caryl-F. Compared to the unmodified acetylacetonebased complex showing an orange emission with PLQY of 0.21, these complexes display intense yellow phosphorescence (lPL ¼ 560 nm for 109, lPL ¼ 574 nm for 110, lPL ¼ 549 nm for 111, and lPL ¼ 550 nm for 112) with remarkably improved PLQYs (0.80 for 109, 0.62 for 110, 0.73 for 111, and 0.49 for 112). The yellow OLEDs based on these complexes display a high peak EQE of over 24% (Fig. 9B). The most notable feature of these yellow complexes is their extremely broad emission band with FWHMs of 103 nm for 109, 101 nm for 110, 94 nm for 111, and 104 nm for 112, the extremely broad yellow emission is quite suitable for preparing complementary-color-based white OLEDs. Due to the broad yellow emission band, the white OLED based on complex 111 gives high color rendering index (CRI) of 74 (Fig. 9C), which is very close to the requirement for practical use.75,76 It should be noted that the yellow iridium(III) complexes based on 2-phenylquinoline derivatives usually possess broader FWHMs than that of the complexes using other types of cyclometalating ligand.75 By using 4,40 -di-tert-butyl-2,20 -bipyridine as N^N ancillary ligands, the ionic iridium(III) complexes (113 and 114) can be realized.65,77 As discussed previously, the bulky tetrakis[3,5-bis(trifluoromethyl)phenyl]borate used as the counterion can regulate the sublimability and photophysical properties of complexes 113 and 114. The dry-processed yellow OLEDs showing the peak EQEs of 15.8% for 113, 11.6% for 114 were successfully realized.65,77

Luminescent transition-metal complexes and their applications in electroluminescence

21

Other types of ligands can also serve as the cyclometalating ligand for designing yellow emissive iridium(III) complexes.78–81 For instance, 7,8-benzoquinoline ligand was used by Dumur, Sun and coworkers to prepare two emitters (115 and 116) for efficient yellow OLEDs with an EQE over 20%.78,79 Benzothiazole moiety is another popular building block for this purpose.80,81 Together with the electron-rich units, such as dibenzo[b,d]thiophene, dibenzo[b,d]furan, and carbazole, several novel cyclometalating ligands bearing benzothiazole were designed. Based on those ligands, complexes (117–119) were synthesized. In CH2Cl2, those complexes show efficient yellow emission ranging from 551 nm to 562 nm with moderate PLQYs (0.10 for 117, 0.11 for 118, 0.34 for 119). However, due to the lower intensity in the shoulder emission, these complexes show narrow FWHMs (less than 80 nm) compared to the 2-phenylquinoline-based ones.80,81 8.02.3.1.1.3 Red/deep-red/near infra-red emissive iridium(III) complexes Phosphorescent iridium(III) complexes showing low emission energies (red, deep-red, near infra-red phosphorescence) are quite important for display, lighting and biological applications.3,5,6 The extension of the p-conjugation or the use of the electronrich moiety is the most efficient strategy for designing iridium(III) complexes with low emission energies so far. The majority of reported class of iridium(III) complexes are based on quinoline, isoquinoline, or their derivatives. The most common challenge for developing long-wavelength emissive iridium(III) complex is on how to realize long emission wavelength and high PLQY simultaneously.6

The electron-rich moieties (e.g., carbazole, thiophene, and benzo[b]thiophene) were involved in designing many of the quinoline-based red/deep-red/near infra-red iridium(III) complexes (e.g., 120–124).82–87 Huang and coworkers prepared a carbazole-containing facial homoleptic red iridium(III) complex 120 with excellent solution processability. The carbazole

22

Luminescent transition-metal complexes and their applications in electroluminescence

Fig. 9 (A) ORTEP diagrams of complexes 110 (left) and 111 (right). (B) The EL spectra and the photographs of OLEDs based on complexes 110– 112. (C) The EL spectra and the photographs of white OLEDs based on complex 111. Adapted from Tao, P.; Zheng, X.-K.; Wei, X.-Z.; Lau, M.-T.; Lee, Y.-K.; Li, Z.-K.; Guo, Z.-L.; Zhao, F.-Q.; Liu, X.; Liu, S.-J.; Zhao, Q.; Miao, Y.-Q.; Wong, W.-Y. Mater. Today Energy 2021, 21, 100773, with permission from Elsevier Ltd. Copyright 2021.

moiety provides the complex with balanced charge-transport property. This complex shows the emission wavelength at 615 nm with PLQY of 0.4 in solution.82 Later, they also use a thiophene unit to realize a series of highly efficient lepidine-based red iridium(III) emitters (121–124). The various auxiliary ligands have a great influence on the fine-tuning of their excited states. Compared to the acetylacetone-based analog (lPL ¼ 611 nm, FWHM ¼ 48 nm), complexes 121–124 show a blue-shift in emission (lPL ¼ 595 nm for 121, lPL ¼ 601 nm for 122, lPL ¼ 595 nm for 123, lPL ¼ 595 nm for 124) and broader FWHMs ranging from 77 nm to 84 nm, implying that the auxiliary ligand could effectively control the intensity of emission shoulder of the 4methyl-2-(thiophen-2-yl)quinoline-based iridium(III) emitters.83–86 Complexes 123 and 124 were further used as dopants for the preparation of efficient warm white OLEDs with EQEs up to 22.74%.85 The further extension of the p-conjugation of thiophene may generate benzo[b]thiophene, which can effectively decrease the energy gap between HOMO and LUMO of iridium(III) emitters. Two efficient picolinate-based deep-red iridium(III) emitters (125 and 126) were designed by Bejoymohandas and coworkers. The two complexes show intense deep-red phosphorescence (lPL ¼ 660 nm for 125, lPL ¼ 651 nm for 126) with high PLQYs of 0.37 for 125 and 0.49 for 126. The OLED based on 126 with 15 wt% doping concentration gives the EQE of about 5%.87

Luminescent transition-metal complexes and their applications in electroluminescence

23

The combination of isoquinoline and benzo[b]thiophene moieties will generate the cyclometalating ligand popular for designing near infra-red iridium(III) emitters.89–91 Bossi and coworkers prepared three near infra-red iridium(III) emitters (127–129) with diketone ancillary ligands.90 The effects of various ancillary ligands on their photophysical properties were investigated. In CH2Cl2, it is found that the ancillary ligands have little influence on the emission energies (lPL ¼ 710 nm for 127, lPL ¼ 704 nm for 128, lPL ¼ 707 nm for 129) but show a remarkable effect on their PLQYs (0.16 for 127, 0.07 for 128, 0.14 for 129). Complex 128 gives the lowest PLQY due to the larger nonradiative rate. The solution-processed NIR OLED using 127 as the emitter realizes a high EQE of about 3% with negligible efficiency roll-off, which is higher than most of the reported NIR emitters.90 Fu et al. developed a BF2-bearing NIR iridium(III) complex 130 with a modified picolinate ancillary ligand. This complex emits at the wavelength of 692 nm with PLQY of 0.08.91 Later, Fu and coworkers designed a series of tris-heteroleptic NIR iridium(III) complexes with the preferentially horizontal orientation (131
133) (Fig. 10).88 Because of the presence of 1-(benzo[b]thiophen-2-yl)isoquinoline with the low triplet energy, these complexes show highly efficient NIR phosphorescence at 715 nm (PLQY ¼ 0.26) for 131, 703 nm (PLQY ¼ 0.18) for 132, 707 nm (PLQY ¼ 0.28) for 133, and the high PLQYs of these complexes could be attributed to the strengthened triplet MLCT. The solution-processed NIR OLED based on complex 131 displays a high EQE of 5.30% with negligible efficiency roll-off.88 In fact, it is possible to further decrease the energy gap between HOMO and LUMO of iridium(III) complexes through the substituent group control on the ligand with small p-conjugation.92 By introducing an electron-withdrawing cyano group into the isoquinoline moiety of 1-phenylisoquinoline and 1-(thiophen-2-yl)isoquinoline ligands, Chen et al. realized highly efficient NIR phosphorescence of iridium(III) complexes based on small p-conjugation (134 and 135).92 Compared to the cyano-free complex bis(1phenylisoquinoline)(acetylacetonate)iridium(III) (Ir(piq)2(acac)), these new complexes displays remarkable red-shifts in emission (lPL ¼ 696 nm for 134, lPL ¼ 708 nm for 135, lPL ¼ 625 nm for (Ir(piq)2(acac)) with acceptable PLQYs (0.16 for 134, 0.06 for 135) in solution (Fig. 11A). Interestingly, the PLQYs of these complexes in the 4,40 -bis(N-carbazolyl)-1,10 -biphenyl (CBP) doped film are greatly enhanced (0.45 for 134, 0.44 for 135). The red-shift of these complexes is the result of the increase of the energy levels of LUMO by incorporating the cyano group into the isoquinoline moiety of the complexes. Owing to the high PLQY of the complexes, the NIR OLED based on 134 shows an extremely high EQE of 10.62% (Fig. 11B).92 This work clearly demonstrates that the substituent control on the ligand with small p-conjugation is also a powerful method to design highly efficient NIR iridium(III) emitters.

Fig. 10

ORTEP diagram of complex 131. Redrawn from data in ref. 88.

24

Luminescent transition-metal complexes and their applications in electroluminescence

In 2019, the asymmetric deep-red emissive iridium(III) complexes based on thianthrene 5,5,10,10-tetraoxide modified isoquinoline (136 and 137) were rationally designed (Fig. 12).93 The phenoxyl or dimesitylboryl group was incorporated into the second cyclometalating ligand of the complex to finely tune the photophysical properties. At room temperature, the emission wavelength of complex 137 (lPL ¼ 660 nm) is slightly longer than that of complex 136 (lPL ¼ 652 nm) in tetrahydrofuran. The PLQYs of both complexes are high (0.48 for 136, 0.50 for 137). Moreover, a solution-processed deep-red OLED based on complex 136 shows an extremely high efficiency with a high peak EQE of 25.8%, which is among the highest EQEs for the reported deep-red OLEDs via the solution-processed method.93 This result implies that the thianthrene 5,5,10,10-tetraoxide moiety is a promising building block for designing efficient iridium(III) phosphors.

Replacing the carbon atom in 4-position of the quinoline moiety of 2-phenylquinoline by nitrogen atom will further decrease the triplet energy of the ligand; that is to say, the 2-phenylquinoxaline derivatives are also the useful cyclometalating ligands for designing deep-red or near infra-red iridium(III) emitters.94–98 In general, the 2-phenylquinoxaline derivatives are prepared by the condensation reaction of symmetric benzene-1,2-diamine derivative and symmetric benzil derivative.96 Thus, the resulting quinoxaline derivatives are also the symmetric cyclometalating ligands, which is beneficial to prepare iridium(III)

Luminescent transition-metal complexes and their applications in electroluminescence

25

Fig. 11 (A) UV
vis absorption spectra of the complexes 134 and 135 in tetrahydrofuran. (B) The EL spectra and the photograph of OLED based on complex 134 at different concentrations. Adapted from Chen, Z.; Zhang, H.; Wen, D.; Wu, W.; Zeng, Q.; Chen, S.; Wong, W.-Y. Chem. Sci. 2020, 11, 2342, with permission from The Royal Society of Chemistry. Copyright 2020.

2.003(8) Å 2.026(7) Å

O5 B1

O6

Ir1 N1

N2

1.978(7) Å

C15

O3

C22 O2 S2

2.036(6) Å

O1 S1

O4

O6 N1 Ir1 C22 N2

O5 B1

C15

O3

S2

139.5°

O4

S1

O1

O2

Fig. 12 ORTEP diagram of complex 137. Adapted from Sun, Y.; Yang, X.; Feng, Z.; Liu, B.; Zhong, D.; Zhang, J.; Zhou, G.; Wu, Z. ACS Appl. Mater. Interfaces 2019, 11, 26152, with permission from American Chemical Society. Copyright 2019.

26

Luminescent transition-metal complexes and their applications in electroluminescence

complexes in the pure form. So far, many 2-phenylquinoxaline-based deep-red or near infra-red iridium(III) emitters including neutral and ionic ones (e.g., 138–144) have been developed.94–98 Yang and coworkers designed two acetylacetone-based near infra-red iridium(III) emitters based on fluorenyl or thienyl modified 2-phenylquinoxaline ligands (138 and 139). The long alkyl and alkoxyl chains on the ligand provide the complexes with excellent solubility for solution-processed OLEDs. Two complexes show efficient near infra-red phosphorescence (lPL ¼ 695 nm for 138, lPL ¼ 713 nm for 139) with moderate PLQYs (0.19 for 138, 0.15 for 139) in CH2Cl2. The solution-processed OLEDs based on two phosphors display the peak EQEs of 5.7% for 138 (lEL ¼ 690 nm) and 3.4% for 139 (lEL ¼ 702 nm).96 Later, Jing et al. used tetraphenylimidodiphosphinate auxiliary ligand to construct a series of fluorine-bearing 2-phenylquinoxaline-based iridium(III) emitters (140–143).97 Compared to complex 138, owing to the presence of fluorine atoms and tetraphenylimidodiphosphinate ligand, these complexes show the blue-shifted emission at the wavelength of 662 nm for 140, 669 nm for 141, 639 nm for 142, and 642 nm for 143 in degassed CH2Cl2. These complexes exhibit high PLQYs ranging from 0.60 to 0.98 (0.68 for 140, 0.60 for 141, 0.79 for 142, and 0.98 for 143). An EQE of 19.9% was realized for the deep-red OLED based on complex 143.97 A sublimable cationic iridium(III) emitter based on 2,3-diphenylquinoxaline (144) was also developed by Duan and coworkers recently.98 In degassed acetonitrile, complex 144 shows highly efficient deep-red phosphorescence with a peak wavelength of 650 nm (PLQY ¼ 0.82). The dry-processed deep-red OLED based on complex 144 realizes a peak EQE of 10.3% with 1931 CIE coordinates of (0.67, 0.33).98

Other isomers (e.g., quinazoline, phthalazine, etc.) of quinoxaline were also successfully involved for the deep-red iridium(III) emitters.99–104 In 2019, Lu et al. designed three deep-red/red iridium(III) complexes based on 4-(4-(trifluoromethyl) phenyl)quinazoline (145–147) by using dithiocarbamate derivatives to provide a unique four-membered Ir–S–C–S ring in the complex. Because the lone pairs of electrons on the nitrogen atoms of dithiocarbamate ligands can remarkably favor the chelation. These complexes could be rapidly prepared in very high yields within several minutes at room temperature. Importantly, the new auxiliary ligands provide these complexes with excellent bipolar charge-transporting properties. The intense red to deep-red phosphorescence of these complexes (lPL ¼ 641 nm for 145, lPL ¼ 628 nm for 146, lPL ¼ 611 nm for 147) can be observed with high PLQYs (0.58 for 145, 0.84 for 146, 0.93 for 147). It should be noted that the emission energies are very sensitive to the substituent on the dithiocarbamate ligand. The red OLED based on complex 147 shows an extremely high peak EQE over 30%.101 In 2013, based on the phthalazine moiety, two bipolar deep-red iridium(III) complexes (148 and 149) were designed rationally by Huang and coworkers. Those iridium(III) complexes emit at the wavelength of 640 nm for 148 and 654 nm for 149.103

Extension of the p-conjugation of 2-phenylbenzo[d]thiazole or incorporation of N^N amidinate auxiliary ligands can also decrease the emission energies of iridium(III) complexes to result in red/deep-red emission.42,81,104 2-(9-Phenyl-9H-carbazol-2yl)benzo[d]thiazole was used to design two deep-red iridium(III) complexes 150 and 151. Because of the nitrogen atom on carbazole para to the metalated carbon, the triplet energies of iridium(III) complexes are remarkably lowered. In degassed CH2Cl2, the emission wavelength of picolinato-based complex 151 (lPL ¼ 624 nm) is slightly shorter than that of acetylacetone-based one 150 (lPL ¼ 630 nm), and these two complexes share similar PLQYs (0.18 for 150, 0.16 for 151).81,104 Replacing the acetylacetone auxiliary ligand of 2-phenylbenzo[d]thiazole-based iridium(III) complex by N^N amidinate ones will not only afford efficient red/deepred phosphorescence (lPL ¼ 630 nm for 152, lPL ¼ 610 nm for 153) but also provide them with excellent bipolar charge-transport ability. The PLQYs of 0.52 for 152 and 0.61 for 153 can be achieved, and a peak EQE of 26.3% was realized for the red OLED by using complex 153 as the emitter, indicating that the N^N amidinate ligands have great promise for designing highly efficient red/ deep-red iridium(III) emitters.42,105

Luminescent transition-metal complexes and their applications in electroluminescence

27

The classical iridium(III) complexes with bidentate ligand usually have three monoanionic ligands. In 2021, Shi et al. proposed a novel molecular design strategy to realize a series of dianionic biphenyl ligand-based near infra-red iridium(III) emitters (154– 157) (Fig. 13).106 These emitters feature three kinds of ligands (2,20 -bipyridyl or 1,10-phenanthroline with no charge, 2phenylpyridine or acetylacetone with
1 charge, biphenyl with
2 charges). Compared to tris(2-phenylpyridinato-C2,N)iridium(III) and bis(2-phenylpyridinato-C2,N) (acetylacetonate)iridium(III) analogs, low-energy absorption band extending to near 600 nm were observed for these complexes, and they show efficient near infra-red phosphorescence (lPL ¼ 706 nm, PLQY ¼ 0.05 for 154; lPL ¼ 695 nm, PLQY ¼ 0.12 for 155; lPL ¼ 704 nm, PLQY ¼ 0.15 for 156; lPL ¼ 688 nm, PLQY ¼ 0.18 for 157) with slightly shorter lifetimes (0.06 ms for 154, 0.15 ms for 155, 0.21 ms for 156, 0.74 ms for 157), indicating that the dianionic biphenyl ligand plays an essential role in the excited state tuning of these complexes. The OLEDs based on complexes 156 and 157 show EQEs of about 2.5%. These results may open a significant avenue for further designing novel phosphorescent iridium(III) emitters with tunable photophysical properties.106

8.02.3.1.2

Iridium(III) complexes with tridentate/tetradentate ligand

To date, the majority of phosphorescent iridium(III) complexes usually consist of three bidentate ligands, and these bidentate ligands may be the same or different from each other.5,6,8,10,107 In fact, the trivalent iridium(III) ion can also coordinate with two tridentate ligands or with one tetradentate ligand and one bidentate ligand theoretically. However, compared to the bidentate ligand-based ones, the phosphorescent iridium(III) complexes bearing tridentate or tetradentate ligands are quite limited and less explored.108–127

Owing to the excellent structural and photochemical stabilities of bis-tridentate iridium(III) phosphors, they are expected to have great potential in electroluminescence. Unfortunately, the lack of an appropriate synthetic method and low synthetic yield of bis-tridentate iridium(III) emitters have been hampering their further exploration for a long time.108,116 Recently, the first bistridentate homoleptic iridium(III) complex 158 was synthesized with a high isolated yield (78%) by Chi and coworkers

28

Luminescent transition-metal complexes and their applications in electroluminescence

ligand (–2)

(A)

ligand (–1)

ligand (–1)

Ir

Ir

ligand (–1)

ligand (0)

ligand (–1)

three monoanionic ligands system (–1, –1, –1)

three types of charged ligands system (0, –1, –2)

(B)

3.8° O1 N1

C1 N1

Ir1 N2

Ir1

O2 C1

C2

C2

N2 N3

C3

4.6° Fig. 13 (A) The comparison of the classical iridium(III) complexes with bidentate ligand (left) and the new concept ones (right). (B) ORTEP diagrams of complexes 154 (right) and 156 (left). Adapted from Shi, C.; Huang, H.; Li, Q.; Yao, J.; Wu, C.; Cao, Y.; Sun, F.; Ma, D.; Yan, H.; Yang, C.; Yuan, A. Adv. Opt. Mater. 2021, 9, 2,002,060, with permission from Wiley-VCH GmbH. Copyright 2021.

(Fig. 14). This complex was obtained by refluxing the mixture of IrCl3$3H2O, free ligand and sodium acetate in acetic acid. Further methylation of complex 158 will afford complex 159. Using the same synthetic protocols, complex 160 was also obtained, proving the generality of the proposed synthetic method. These complexes show intense greenish yellow phosphorescence (lPL ¼ 567 nm, PLQY ¼ 0.16 for 158; lPL ¼ 545 nm, PLQY ¼ 0.57 for 159; lPL ¼ 540 nm, PLQY ¼ 0.49 for 160) with emission lifetimes ranging from 0.75 ms to 2.07 ms. Especially, these emitters show superior photostability than that of tris(2-phenylpyridinato-C2,N)iridium(III) and bis(2-phenylpyridinato-C2,N) (acetylacetonate)iridium(III) in toluene under argon. OLED based on 160 gives a very small efficiency roll-off, and even at high luminance of 1000 cd$m
2, the OLED still shows excellent efficiencies (up to 52.8 lm$W
1, 66.8 cd$A
1, and 20.7%).116 Later, highly efficient red bis-tridentate iridium(III) emitters (161 and 162) were designed by two different tridentate ligands (Fig. 15). These two emitters exhibit extremely high PLQYs of 0.98 (lPL ¼ 593 nm) for 161 and 1.00 (lPL ¼ 595 nm) for 162. Red OLED based on complex 162 exhibits superior EQE of as high as 28.17%.117 In addition to the 1H-pyrazole moiety used to construct the bis-tridentate iridium(III) complexes, the carbene or carbazole moiety also serves as a potential candidate to design novel efficient bis-tridentate iridium(III) complexes (e.g., 163–167).118–120

Phosphorescent tetradentate ligand-based iridium(III) complexes are quite rare, and only a few examples of this class of iridium(III) complexes have been reported so far.125–127 In 2017, Li et al. designed and prepared two tetradentate iridium(III) complexes

Luminescent transition-metal complexes and their applications in electroluminescence

29

possessing one tetradentate ligand and one bidentate ligand (168 and 169) (Fig. 16).127 The selected tetradentate ligands show a nonplanar geometrical configuration with a pyrazole moiety and tripodal arranged terpyridine. In CH2Cl2, complexes 168 and 169 emit in the sky-blue region with emission wavelengths of 490 nm (PLQY ¼ 0.018) for 168 and 484 nm (PLQY ¼ 0.014) for 169. The PLQYs (0.51 for 168 and 0.47 for 169) can be remarkably enhanced in the mCP doped films. Using these two emitters, sky-blue OLEDs with a peak EQE of 10.1% were successfully realized, highlighting the promising application of the tetradentate iridium(III) complexes in highly efficient electroluminescence.127

8.02.3.2

Platinum(II) complexes

Phosphorescent platinum(II) complexes are one of the most popular noble metal-based phosphors owing to their excellent photophysical properties (e.g., tunable emission wavelength from blue to near-infrared, high photoluminescence efficiency, p-p/Pt-Pt interaction-sensitive emission, etc.) and rich structural diversities, which have drawn much attention, especially in electroluminescence.1,2,4,10,128–183 The platinum(II) complexes may also involve the MMLCT transition in their excited states (Fig. 17).1,153 According to the ligand structure, the phosphorescent platinum(II) phosphors can be classified as bidentate, tridentate, and tetradentate complexes.4,10,128–183 Herein, some representative examples of these phosphorescent platinum(II) complexes developed recently will be discussed in detail.

8.02.3.2.1

Platinum(II) complexes with bidentate ligand

Fig. 14 ORTEP diagram of complex 158. Reproduced from Lin, J.; Wang, Y.; Gnanasekaran, P.; Chiang, Y.-C.; Yang, C.-C.; Chang, C.-H.; Liu, S.-H.; Lee, G.-H.; Chou, P.-T.; Chi, Y.; Liu, S.-W. Adv. Funct. Mater. 2017, 27, 1702856, with permission from WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. Copyright 2017.

30

Luminescent transition-metal complexes and their applications in electroluminescence

Fig. 15 ORTEP diagram of complex 161. Reproduced from Gnanasekaran, P.; Yuan, Y.; Lee, C.-S.; Zhou, X.; Jen, A. K.-Y.; Chi, Y. Inorg. Chem. 2019, 58, 10944, with permission from American Chemical Society. Copyright 2019.

Bidentate platinum(II) phosphors are an important class of phosphorescent platinum(II) complexes. These platinum(II) phosphors consist of two different or same bidentate ligands.129–157 The simplest bidentate cyclometalating ligand is 2-phenylpyridine. Based on 2-phenylpyridine, various cyclometalating ligands can be designed by substituent modification or change of aromatic ring for phosphorescent platinum(II) complexes. Zysman-Colman and coworkers introduced the electron-withdrawing pentafluorosulfanyl (SF5) moiety into the different positions of the phenyl moiety of the diketone-based platinum(II) complexes (170–172).131 In degassed acetonitrile, compared to the parent complex 170 (lPL ¼ 477 nm, PLQY ¼ 0.226), the SF5 meta to the metalated carbon red-shifts the emission (lPL ¼ 488 nm, PLQY ¼ 0.075 for 171), while the SF5 para to the metalated carbon blue-shifts the emission (lPL ¼ 468 nm, PLQY ¼ 0.084 for 172). It should be noted that the incorporation of SF5 group remarkably decreases the PLQYs of the complexes.131 Tetrahydroquinoline-based cyclometalating ligands were designed by Meerholz and coworkers to prepare a library of diketone-based platinum(II) complexes (173–175).134 The fluorine atom or trifluoromethyl group were also

Fig. 16 ORTEP diagram of complex 168. Reproduced from Li, Y.-S.; Liao, J.-L.; Lin, K.-T.; Hung, W.-Y.; Liu, S.-H.; Lee, G.-H.; Chou, P.-T.; Chi, Y. Inorg. Chem. 2017, 56, 10054, with permission from American Chemical Society. Copyright 2017.

Luminescent transition-metal complexes and their applications in electroluminescence

Pt

Pt

Monomer

31

Pt

Aggregate

p* IL

MLCT

MMLCT

ds*

p dz2 ds Fig. 17 Molecular orbital diagram illustrating the p-p/Pt-Pt interaction and formation of MMLCT transition. Adapted from Chen, W.-C.; Sukpattanacharoen, C.; Chan, W.-H.; Huang, C.-C.; Hsu, H.-F.; Shen, D.; Hung, W.-Y.; Kungwan, N.; Escudero, D.; Lee, C.-S.; Chi, Y. Adv. Funct. Mater. 2020, 30, 2002494, with permission from WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. Copyright 2020.

incorporated to tune their emission properties. These complexes show sky-blue emission (lPL ¼ 498 nm for 173, lPL ¼ 489 nm for 174, lPL ¼ 501 nm for 175) with quite low PLQYs (0.08 for 173, 0.05 for 174, 0.07 for 175).134 Later, Zheng and coworkers designed a novel heterocycle 4-phenyl-4H-1,2,4-triazole derivative as the ancillary ligand to synthesize a couple of platinum(II) phosphors (176 and 177).135 The two complexes show intense green phosphorescence (lPL ¼ 502 nm, PLQY ¼ 0.71 for 176; lPL ¼ 532 nm, PLQY ¼ 0.65 for 177). The OLED based on complex 176 achieves an extremely high EQE of 26.9% with low efficiency roll-off.135

32

Luminescent transition-metal complexes and their applications in electroluminescence

Recently, chiral phosphorescent platinum(II) complexes have aroused considerable interest.155 In general, there are two methods to design the chiral platinum(II) phosphors. The first method is to introduce the chiral group into the ligand of the complex, and the second one is to use chiral helicene ligand to construct the complex.155 In 2017, Zheng and coworkers selected a pair of chiral (R/S)-4-pinene-2-phenylpyridine ligands to prepare two chiral platinum(II) phosphors (178 and 179).137 The enantiomers show the same emission wavelength (lPL ¼ 526 nm) with high PLQY (0.52) in CH2Cl2. Unfortunately, the circularly polarized phosphorescence of the enantiomer is very weak to observe in CH2Cl2.137 Fuchter and coworkers used a chiral 2-(4-methoxybenzo[c]phenanthren-1-yl)pyridine helicene cyclometalating ligand to prepare a chiral red platinum(II) emitter (180) with high solution dissymmetry factor value of about 10
2.138 Furthermore, complex 180 was doped into the mixtures of 1,3-bis(2-(4-tert-butylphenyl)-1,3,4-oxadiazo-5-yl)benzene and poly(9-vinylcarbazole). This blend film containing complex 180 exhibits a remarkably enhanced dissymmetry factor value of
0.22 at 625 nm. The CP OLED based on complex 180 also realizes an extremely high dissymmetry factor value of
0.38.138 Later, Zheng and coworkers selected the similar helicene cyclometalating ligand 2-(4-fluorobenzo[c]phenanthren-1-yl)-5-(trifluoromethyl)pyridine to prepare a pair of platinum(II) enantiomers (181 and 182) similar to complex 180.139 The two complexes also show the same red phosphorescence (lPL ¼ 612 nm with PLQY of 0.27) in CH2Cl2. However, they found that the dissymmetry factor values of 181 and 182 doped films were extremely low (4.9  10
3 for 181,
4.1  10
3 for 182). In addition, they also measured the dissymmetry factor value for complex 180 under the same test condition. Surprisingly, the dissymmetry factor value for complex 180 in the doped film is also extremely low (4.5  10
3 for 180).139 Thus, the inconsistency in dissymmetry factor value of complex 180 should be further examined.138,139

Pyrazine containing two nitrogen atoms is a useful building block for designing bidentate cyclometalating ligands, which have two potential metalated carbons, and can be used as mononuclear or dinuclear platinum(II) phosphors.143,144 Dibenzo[f,h]quinoxaline was chosen by Kozhevnikov and coworkers to construct two diketone-based platinum(II) phosphors (183 and 184).144 The emission properties are quite different between mononuclear platinum(II) complex 183 and dinuclear one 184. At room temperature, in degassed CH2Cl2, the emission energy of the binuclear complex 184 (lPL ¼ 628 nm, PLQY ¼ 0.20) is significantly lower than that of the mononuclear complex 183 (lPL ¼ 568 nm, PLQY ¼ 0.15), which is the result of the significant decrease in the energy level of LUMO by incorporating the second platinum(II) ion into the complex.144

In 2014, a quinoline-based donor-acceptor-acceptor cyclometalating ligand was designed by Zhu and coworkers for solutionprocessable NIR platinum(II) phosphor (185).146 Attributed to the donor-acceptor-acceptor ligand, complex 185 shows a NIR electroluminescence at 759 nm in CH2Cl2 at 298 K. The polymer OLED based on this phosphor shows a NIR emission at 760 nm with a peak EQE of 0.12%.146

Luminescent transition-metal complexes and their applications in electroluminescence

33

Bidentate N^N ligand-based platinum(II) complexes usually show unique aggregation states, which is highly sensitive to the substituent groups and ligand skeletons and can result in highly efficient solid state phosphorescence from blue to near-infrared. Thus, they have become a very important class of high-performance platinum(II) phosphors.147–152 Currently, the frequently used nitrogencontaining heterocycles for bidentate N^N ligand-based platinum(II) complexes include pyridine, pyrazine, pyrimidine, 1H-pyrazole, 1H-imidazole, isoquinoline, etc. Recently, a library of bidentate N^N ligand-based efficient platinum(II) complexes (186–194) emitting blue to near-infrared were developed by Chi and coworkers.147–152 Complex 186 shows sky-blue emission (lPL ¼ 501 nm) with almost unity PLQY in the neat powder at room temperature. In crystals, the adjacent Pt–Pt distances are 4.32 and 3.48 Å. Interestingly, the remarkable triboluminescence can be observed upon cracking or grinding of the solid of 186.147 For complex 187 as powder, it shows a blue emission at 440 nm with 463 nm as the shoulder (PLQY ¼ 0.28) due to the absence of Pt–Pt interaction (7.22 Å).148 Complex 188 shows the short intermolecular interaction, and the distance of Pt–Pt interaction is 3.63 Å. Notably, the intramolecular hydrogen bond (N$$$H) between two ligands can also be observed (2.27 Å) (Fig. 18). Complex 188 exhibits interesting stimuliresponsive phosphorescence, and the similar deep-red emission can be observed from the sublimed powder and the vacuumdeposited thin film of complex 188, which is probably deduced by the relatively stronger Pt–Pt interaction. The sublimed red powder can turn yellow after acetone rinse and shows a sky-blue emission (Fig. 19). The deep-red OLED based on complex 188 exhibits a very high peak EQE of 23.8% with 1931 CIE coordinates of (0.61, 0.39).149 Later, 5-(pyridin-2-yl)-2-(trifluoromethyl)pyrimidine derivatives and 2-(3-(trifluoromethyl)-1H-pyrazol-5-yl)pyrazine were respectively used as the cyclometalating ligand and auxiliary ligand to prepare two efficient near-infrared phosphorescent platinum(II) complexes (189 and 190). Owing to the strong intermolecular interactions in the solid state, the emission wavelengths of the two complexes are 832 nm (PLQY ¼ 0.38, lifetime ¼ 0.20 ms) for 189, and 820 nm (PLQY ¼ 0.28, lifetime ¼ 0.20 ms) for 190. The NIR OLEDs based on these two emitters realize extremely high peak EQEs of 10.61% (lEL ¼ 794 nm) for 189 and 9.58% (lEL ¼ 803 nm) for 189.150

Chi and coworkers also developed a series of bidentate N^N ligand-based near-infrared platinum(II) phosphors (191–194).151,152 Through the fine-tuning of functional groups and ligand skeletons, the emission energies of these emitters (lPL ¼ 703 nm, PLQY ¼ 0.55 for 191; lPL ¼ 673 nm, PLQY ¼ 0.82 for 192; lPL ¼ 740 nm, PLQY ¼ 0.81 for 193; lPL ¼ 552 nm, PLQY ¼ 0.20 for 194) can be dramatically tuned in thin films (Fig. 20A). The highly ordered aggregations of those complexes are clearly confirmed by the synchrotron X-ray diffraction in the vacuum-evaporated thin films (Fig. 20B). The NIR OLED based on complex 193 with emission wavelength of 740 nm achieves a record high peak EQE of 24% and high radiance of 3.6  105 mW sr
1 m
2 (Fig. 21).151,152

34

Luminescent transition-metal complexes and their applications in electroluminescence

3.625 Å 102.81°

Pt N3 N1

N4

C1 H1 N2

Fig. 18 The intermolecular interaction of complex 188 in crystals. Reproduced from Ganesan, P.; Hung, W.-Y.; Tso, J.-Y.; Ko, C.-L.; Wang, T.-H.; Chen, P.-T.; Hsu, H.-F.; Liu, S.-H.; Lee, G.-H.; Chou, P.-T.; Jen, A. K.-Y.; Chi, Y. Adv. Funct. Mater. 2019, 29, 1900923, with permission from WILEYVCH Verlag GmbH & Co. KGaA, Weinheim. Copyright 2019.

In 2020, Chi and coworkers reported the excited state manipulation of isoquinolinyl pyrazolate-based platinum(II) phosphors (195–199) by steric substituents.153 From this study, it was found that the Pt–Pt interactions could be favored by bulky substituents. The p-p stacking among complexes also has a remarkable effect on the MMLCT transition. These complexes show efficient deepred/NIR phosphorescence (lPL ¼ 672 nm, PLQY ¼ 0.81 for 195; lPL ¼ 673 nm, PLQY ¼ 0.67 for 196; lPL ¼ 691 nm, PLQY ¼ 0.97 for 197; lPL ¼ 678 nm, PLQY ¼ 0.88 for 198; lPL ¼ 687 nm, PLQY ¼ 0.75 for 199) in thin films (Fig. 22). Based on these emitters, OLEDs realize unprecedentedly high EQE over 30% with bright deep-red/NIR emission (about 670 nm) (Fig. 23).153 This study provides a deep understanding of the formation of MMLCT transition of this class of phosphors and future molecular design toward efficient deep-red or near-infrared electroluminescence.

8.02.3.2.2

Platinum(II) complexes with tridentate ligand

Luminescent transition-metal complexes and their applications in electroluminescence

35

Fig. 19 (A) Photographs in various states of complex 188: (i) vacuum-deposited thin film; (ii) sublimed powder; (iii) after acetone rinse, taken under ambient temperature and UV excitation at lmax ¼ 365 nm. (B) Photoluminescence of complex 188 as deposited thin film, sublimed powder, and after acetone rinse. Reproduced from Ganesan, P.; Hung, W.-Y.; Tso, J.-Y.; Ko, C.-L.; Wang, T.-H.; Chen, P.-T.; Hsu, H.-F.; Liu, S.-H.; Lee, G.-H.; Chou, P.-T.; Jen, A. K.-Y.; Chi, Y. Adv. Funct. Mater. 2019, 29, 1900923, with permission from WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. Copyright 2019.

Fig. 20 (A) The UV–Vis absorption spectra in tetrahydrofuran (THF) and emission spectra in solid film of complexes 191–193. (B) Schematic illustration of the 3D ordered packing of complex 193 on the substrate surface. Adapted from Ly, K. T.; Chen-Cheng, R.-W.; Lin, H.-W.; Shiau, Y.-J.; Liu, S.-H.; Chou, P.-T.; Tsao, C.-S.; Huang, Y.-C.; Chi, Y. Nat. Photonics 2017, 11, 63, with permission from Nature Publishing Group. Copyright 2016.

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Luminescent transition-metal complexes and their applications in electroluminescence

Fig. 21 (A) EQE and power conversion efficiencies (PCE) as a function of brightness. (B) Photograph of NIR OLED using complex 193. Adapted from Ly, K. T.; Chen-Cheng, R.-W.; Lin, H.-W.; Shiau, Y.-J.; Liu, S.-H.; Chou, P.-T.; Tsao, C.-S.; Huang, Y.-C.; Chi, Y. Nat. Photonics 2017, 11, 63, with permission from Nature Publishing Group. Copyright 2016.

Fig. 22 Absorption and PL spectra of complexes 195–199 (Insets show the corresponding powder samples under ambient (left) and 365 nm UV irradiation (right). Adapted from Chen, W.-C.; Sukpattanacharoen, C.; Chan, W.-H.; Huang, C.-C.; Hsu, H.-F.; Shen, D.; Hung, W.-Y.; Kungwan, N.; Escudero, D.; Lee, C.-S.; Chi, Y. Adv. Funct. Mater. 2020, 30, 2002494, with permission from WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. Copyright 2020.

Luminescent transition-metal complexes and their applications in electroluminescence

37

Platinum(II) complexes bearing the tridentate cyclometalating ligands usually show bright luminescence and rich emissive excited states (e.g., ILCT (p
p*) excited states, halogen-to-ligand charge transfer (XLCT) excited states, the excimeric excited states, etc.), which have drawn an increasing attention.1,4,159–167 Li and coworkers developed two tridentate platinum(II) chloride complexes (200 and 201).161,162 1,3-Difluoro-4,6-di(2-pyridinyl)benzene is used for complex 200, which shows an efficient blue emission (lPL ¼ 465 nm, PLQY ¼ 0.46) with a narrow FWHM in CH2Cl2. The OLED based on complex 200 exhibits blue electroluminescence with a peak EQE of 16% at 2 wt% doping concentration. Because of the formation of excimer in the singledoped emissive layer with the high doping concentration (8 wt%), a perfect white electroluminescence with CIE coordinates of (0.33, 0.36) and a peak EQE of 9.3% was realized.161 Furthermore, platinum(II) complex based on bis(N-methyl-imidazolyl)benzene (201) as emitter was used to construct an efficient white OLED based on excimer with high peak EQE of 20.1%, high CRI of 80, and perfect CIE coordinates of (0.33, 0.33).162 In 2013, Lam et al. designed tridentate 1,3-bis(N-alkylbenzimidazol-20 -yl)benzene cyclometalating ligands to prepare platinum(II) chloride complexes (202 and 203). In CH2Cl2, the two complexes show intense green phosphorescence with lifetimes in several microseconds (lPL ¼ 507 nm, PLQY ¼ 0.25 for 202; lPL ¼ 530 nm, PLQY ¼ 0.15 for 203). A high EQE of 11.8% was achieved for 202-based green OLED with dual EMLs.163 Later, carbazolyl groups were selected to replace the chloride ligand in tridentate platinum(II) complexes by the same group. They successfully prepared novel platinum(II) carbazolyl complexes (204 and 205) (Fig. 24).164 The two complexes show weak deep-red phosphorescence (lPL ¼ 629 nm, PLQY ¼ 0.03 for 204; lPL ¼ 656 nm, PLQY ¼ 0.008 for 205) in CH2Cl2. Interestingly, in the mCP doped thin film, the emissions of both complexes show intense yellow phosphorescence (lPL ¼ 550 nm, PLQY ¼ 0.64 for 204; lPL ¼ 570 nm, PLQY ¼ 0.52 for 205) with the boosted PLQYs. The OLED doped with 20 wt% of 204 affords CE of 24.0 cd A
1 and EQE of 7.2%.164

Fig. 23 (A) EQE-Luminance plots and (B) EL spectra of OLEDs based on complexes 195–199. Adapted from Chen, W.-C.; Sukpattanacharoen, C.; Chan, W.-H.; Huang, C.-C.; Hsu, H.-F.; Shen, D.; Hung, W.-Y.; Kungwan, N.; Escudero, D.; Lee, C.-S.; Chi, Y. Adv. Funct. Mater. 2020, 30, 2002494, with permission from WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. Copyright 2020.

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Luminescent transition-metal complexes and their applications in electroluminescence

Furthermore, the N^N^C-type tridentate cyclometalating ligands were also involved in preparing efficient platinum(II) complexes.165,166 In 2015, Che and coworkers reported a library of highly efficient platinum(II) complexes with extended p-conjugation of ligands (206–208).165 They show intense phosphorescence (lPL ¼ 530 nm, PLQY ¼ 0.78 for 206; lPL ¼ 521 nm, PLQY ¼ 0.99 for 207; lPL ¼ 588 nm, PLQY ¼ 0.25 for 208) in CH2Cl2 at room temperature, which originate from excited states with mixed 3MLCT/XLCT/ILCT components. Based on these complexes, electroluminescence with the peak EQE of 22.8% was realized.165 Additionally, 1H-pyrazole was incorporated to design N^N^C-type tridentate ligands as well. Huo and coworkers developed two 1H-pyrazole-based tridentate platinum(II) complexes (209 and 210). In complex 209, there is a six-membered metallacycle giving the molecular structure closer to square geometry. Both complexes exhibit bright green phosphorescence with lifetimes in several microseconds (lPL ¼ 521 nm, PLQY ¼ 0.43 for 209; lPL ¼ 511 nm, PLQY ¼ 0.62 for 210) in CH2Cl2 at room temperature.166

In 2013, a N^N^N ligand with an asymmetric structure was designed by De Cola and coworkers to prepare neutral platinum(II) complexes (211 and 212) for solution-processed electroluminescence.167 This asymmetric ligand contains tetrazole and adamantylbearing triazole rings. Intense sky-blue phosphorescence (lPL ¼ 508 nm, PLQY ¼ 0.13 for 211; lPL ¼ 573 nm, PLQY ¼ 0.60 for 212) can be observed for two complexes in neat films. The solution-processed OLED using 212 realizes a peak EQE of 5.6% with peak CE of 15.5 cd$A
1 and PE of 16.4 lm$W
1.167

Luminescent transition-metal complexes and their applications in electroluminescence 8.02.3.2.3

39

Platinum(II) complexes with tetradentate ligand

Recently, tetradentate platinum(II) complexes have also received much attention because of their unique photophysical properties (e.g., narrow FWHM) and rigid structures. The most interesting point for some tetradentate platinum(II) complexes is their narrow emission bandwidth originated from remarkably suppressed vibronic sidebands, which will provide the opportunity for the future molecular design of the emitters with high color purity.168–183 Fleetham et al. developed rigid tetradentate cyclometalating ligands to design a series of novel tetradentate platinum(II) phosphors (213–216) showing efficient deep-blue emission with CIEy less than 0.1. For example, the emitter 214 shows a deep-blue emission peaking at 444 nm with a narrow FWHM of about 20 nm, which is the result of the extremely reduced vibronic sidebands. The OLED based on emitter 216 realizes a deep-blue electroluminescence at 451 nm with a high peak EQE of 24.8%, CIE coordinates of (0.148, 0.079), and extremely narrowed FWHM of 29 nm.168 Later, a phosphorescent tetradentate platinum(II) complex bearing [1,2,4]triazolo[4,3-a]pyridine moiety (217) was prepared by She and coworkers.169 This complex shows an intense green emission (lPL ¼ 501 nm, PLQY ¼ 0.50) in CH2Cl2 at room temperature, enabling efficient green electroluminescence with a peak EQE of 8.7%.169 In 2015, an efficient and stable red platinum(II) phosphor bearing carbazole moiety within the ligand skeleton (218) was synthesized.170 At room temperature,

Fig. 24

ORTEP diagram of complex 204. Redrawn from the data in ref. 164.

40

Luminescent transition-metal complexes and their applications in electroluminescence

complex 218 exhibits a bright red phosphorescence (lPL ¼ 602 nm, PLQY ¼ 0.34) in CH2Cl2. Notably, by using this complex as the emitter, the red electroluminescence with high EQE of 21.5% and long operational lifetimes (LT97 ¼ 600 h, the device operational lifetime to 97% of the initial luminance) at the brightness of 1000 cd$m
2 was achieved.170

1H-Imidazole moiety is usually employed to design tetradentate platinum(II) phosphors. Two imidazole-based tetradentate platinum(II) phosphors (219 and 220) were developed by Li and coworkers for excimer-based white electroluminescence.171 Complex 219 exhibits blue luminescence with a peak wavelength of 470 nm and broad emission bandwidth. However, complex 220 shows a slightly red-shifted emission at 490 nm with a narrowed FWHM. Interestingly, because of the formation of excimers of complex 220, a red-shifted and broad emission band can be obtained in the EL spectra upon increasing the doping concentration of complex 220. By using complex 220 as a single emitter, a perfect white electroluminescence with CRI of 80, peak EQE of 12.5%, and LT80 of over 200 h at 1000 cd m
2 can be achieved successfully.171 In 2017, the macrocyclic tetradentate cyclometalating ligands were well designed by Wang et al. to realize the first examples of efficient deep-blue platinum(II) phosphors (211 and 222) featuring non-distorted flat geometry (Fig. 25).172 In CH2Cl2, owing to the incorporation of the macrocycle, both complexes show intense deep-blue emissions (lPL ¼ 448 nm, PLQY ¼ 0.58 for 211; lPL ¼ 449 nm, PLQY ¼ 0.62 for 212) with high stability to UV light and structural distortion in the excited states. The OLED based on complex 211 exhibits a bright deep-blue electroluminescence (lEL ¼ 451 nm) with a maximum EQE of 15.4%, and a luminance of over 10,000 cd m
2.172 This work highlighted the important role of the macrocycle moiety of complexes in manipulating a photophysical properties and stability.

Fig. 25

ORTEP diagrams of complexes 221 (left) and 222 (right). Redrawn from data in ref. 172.

Luminescent transition-metal complexes and their applications in electroluminescence

41

Very recently, novel tetradentate platinum(II) emitters bearing unique fused 6/5/6 metallocycles (223 and 224) were rationally designed by She and coworkers (Fig. 26).173 9,9-Dimethyl-9,10-dihydroacridine and 9H-pyrido[2,3-b]indole moieties were employed to construct the tetradentate cyclometalating ligands. Due to the incorporation of electron-rich moieties, the prepared platinum(II) complexes possess the bipolar charge-transporting ability. In CH2Cl2, both complexes show bright phosphorescence with lifetimes in several microseconds (lPL ¼ 512 nm, PLQY ¼ 0.43 for 223; lPL ¼ 517 nm, PLQY ¼ 0.27 for 224), which is similar to the emission properties in the poly(methylmethacrylate) (PMMA) films. Under the UV light of 375 nm at 500 W$m
2, complex 223 doped in polystyrene (PS) film shows a higher photostability with LT80 of 190 min. (Fig. 26).173

In addition to the carbon or nitrogen coordinating atom on the tetradentate cyclometalating ligands for platinum(II) phosphors, the oxygen atom can also serve as a coordinating site for this purpose.174–181 In 2013, Che and coworkers designed a series of O^N^C^N ligands to realize a new class of neutral, intense luminescent cyclometalated platinum(II) phosphors (225–228).177,178 The carbazole, 10H-phenoxazine, 10H-phenothiazine, and 5H-indeno[1,2-b]pyridine were selected as the building blocks for the construction of the platinum(II) phosphors. These emitters show bright luminescence (lPL ¼ 553 nm, PLQY ¼ 0.86 for 225; lPL ¼ 526 nm, PLQY ¼ 0.47 for 226; lPL ¼ 527 nm, PLQY ¼ 0.49 for 227; lPL ¼ 515 nm, PLQY ¼ 0.99 for 228). By selecting complex 225 as a yellow emitter, the power efficiency of 61 lm$W
1 can be obtained for the two-color-based white electroluminescence.177,178

Fig. 26 PL spectra of complex 223 under various conditions, the photostability of complex 223 in polystyrene film, and an ORTEP diagram of complex 224 (Inset shows the photograph of the complex 223 in dichloromethane under UV irradiation). Adapted from Li, G.; Zhan, F.; Zheng, J.; Yang, Y.-F.; Wang, Q.; Chen, Q.; Shen, G.; She, Y. Inorg. Chem. 2020, 59, 3718, with permission from American Chemical Society. Copyright 2020.

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Luminescent transition-metal complexes and their applications in electroluminescence

A green platinum(II) phosphor 229 was further prepared by selecting a picolinate-bearing unsymmetric tetradentate cyclometalating ligand. This complex emits at 504 nm with a PLQY of 0.27 in CH2Cl2 and realizes an electroluminescence with a peak EQE of 13.8%.180 Di(1H-3l4-imidazol-3-yl)methane was used by Che and coworkers to develop a library of O^C^C^O-type carbene platinum(II) phosphors (230
233). At room temperature, these complexes show blue phosphorescence (lPL ¼ 457 nm, PLQY ¼ 0.03 for 230; lPL ¼ 443 nm, PLQY ¼ 0.18 for 231; lPL ¼ 460 nm, PLQY ¼ 0.07 for 232; lPL ¼ 461 nm, PLQY ¼ 0.08 for 233) with short lifetimes (0.5–3.5 ms) in solution. By using complex 233 as the emitter, the deep-blue and white electroluminescence are observed with peak CEs of 24 and 88 cd$A
1, respectively.181

The above mentioned tetradentate platinum(II) emitters are mainly supported by cyclometalating ligands. N^N^N^N-type bis(pyridylazolate) chelates were further introduced by Robertson and coworkers to design efficient tetradentate platinum(II) complexes (234 and 235).183 These complexes feature the unique spiro arranged fluorene or acridine structures, resulting in the interesting packing mode in crystals. Notably, at room temperature, the spiro structures have a great influence on the emission energies of the two complexes in CH2Cl2. Bright sky-blue luminescence (lPL ¼ 461 nm, PLQY ¼ 0.82) with a long lifetime of 9.3 ms can be observed for 234, while the complex 235 shows a large red-shift with an emission peak of 520 nm (PLQY ¼ 0.88) and remarkably reduced lifetime (2.9 ms). Interestingly, spiro-acridine-based complex 235 also shows the solvent-dependent emission properties ranging from blue emission at 469 nm (PLQY ¼ 0.13) in cyclohexane to green emission at 537 nm (PLQY ¼ 0.52) in ethanol, which is induced by the charge transfer character in the T1 state of complex 235. In addition, a blue OLED based on complex 235 with CIE coordinates of (0.19, 0.39) was obtained with a peak EQE of 15.3%.183

8.02.3.3

Gold(I/III) complexes

Since the first phosphorescent OLED by using a platinum(II) complex was reported by Forrest and coworkers, many works have been made on the platinum(II) complexes and iridium(III) complexes due to their attractive photophysical properties and utilizable triplet states.1,4,6,8,10,128 Recently, gold(I/III) complexes have also been proved to be the promising candidates for highly efficient emitters. Gold(I/III) complexes usually show rich emissive excited states with emission wavelengths ranging from sky-blue to near-infrared. According to the emission mechanism of gold(I/III) complexes, they can be classified as phosphorescent emitters and TADF emitters, which have been involved in the design of highly efficient OLEDs.2,184–188

8.02.3.3.1

Gold(I) complexes with carbene ligand

Carbene-based linear two-coordinate gold(I) complexes represent the first class of emissive gold(I/III) emitters.189–194 In 2017, Credgington and coworkers developed a novel category of linear gold(I) complexes with cyclic alkyl/amino-carbene (CAAC) ligand

Luminescent transition-metal complexes and their applications in electroluminescence

43

(236–238).192 At room temperature, these complexes show the emission via triplet excited states occurring within 350 ns, after reverse intersystem crossing to singlets. It is found that molecular geometries exist at which the singlet-triplet energy gap is close to zero, such that fast interconversion is possible (Fig. 27). Owing to their excellent emission properties, the green OLED based on complex 237 shows extremely high performance (peak EQE of 27.5%, PE of 75.1 lm$W
1, CE of 87.1 cd$A
1, and luminance of 73,100 cd$m
2).192 Later, a benzimidazolyl carbene with a sterically hindered structure was designed by Thompson and coworkers to prepare a new gold(I) complex (239) with TADF property. In a polystyrene doped film, complex 239 exhibits a deep-blue emission (lPL ¼ 432 nm) with a narrow FWHM (44 nm) and unity PLQY at room temperature. By selecting 239 as an emitter, the deep-blue OLED by vapor-deposition method displays CIE coordinates of (0.16, 0.06) and a high peak EQE of 12%, showing a great potential for deep-blue electroluminescence.193

In 2014, a family of phosphorescent gold(I) carbene-based polymeric materials (242–245) were designed by Chen et al.194 These polymeric materials are assembled by metallophilic interactions among gold(I) carbene complexes (240 and 241) and [M(CN)2]
(M ¼ Auþ or Agþ). In the solid state, except for 245, these polymeric materials show intense pure blue phosphorescence (lPL ¼ 448 nm, PLQY ¼ 0.90 for 242; lPL ¼ 465 nm, PLQY ¼ 0.67 for 243; lPL ¼ 446 nm, PLQY ¼ 0.11 for 244) with short

Fig. 27 (A) Optimized geometries of complex 236 in S0, S1 and T1 states. (B) Schematic of the rotationally accessed spin-state inversion mechanism based on time dependent density functional theory (TD-DFT) calculations, illustrating the stabilization of S1 and destabilization of T1 as the dihedral angle between donor and acceptor increases from co-planar (P) to rotated (R) geometries. Oscillator strength for the S1 to the ground state varies approximately sinusoidally with dihedral angle. S1-T1 degeneracy occurs at angle q0. Trajectories for photogenerated singlets (dot) and electrically-generated triplets (dash) are indicated. Adapted from Di, D.; Romanov, A. S.; Yang, L.; Richter, J. M.; Rivett, J. P. H.; Jones, S.; Thomas, T. H.; Jalebi, M. A.; Friend, R. H.; Linnolahti, M.; Bochmann, M.; Credgington, D. Science 2017, 356, 159, with permission from American Association for the Advancement of Science. Copyright 2017.

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Luminescent transition-metal complexes and their applications in electroluminescence

lifetimes ranging from 0.2 ms to 0.7 ms. Moreover, phosphor 243 is selected as an emitter to explore its potential application in blue OLED, which shows a peak EQE of 1.45%.194

8.02.3.3.2

Gold(III) complexes with tridentate ligand

Besides linear two-coordinate gold(I) complexes, the tridentate luminescent cyclometalated gold(III) complexes also show excellent device performance and have drawn much attention.2 The tridentate C^C^N- or C^N^C-type cyclometalating ligands are frequently used for designing this family of luminescent gold(III) complexes.2,195–199 A 2-(9,9-dihexyl-9H-fluorene-2-yl)-4,6diphenylpyridine cyclometalating ligand is designed to synthesize two strongly emissive alkynyl gold(III) emitters (246 and 247).196 In CH2Cl2, two complexes show intense greenish yellow luminescence (lPL ¼ 540 nm, PLQY ¼ 0.61 for 246; lPL ¼ 539 nm, PLQY ¼ 0.46 for 247) with long lifetimes (217 ms for 246, 179 ms for 247). The long lifetimes imply that there is less metal character in their emissive 3IL excited states. Interestingly, by increasing the applied voltage, the color-tunable OLED based on yellow emissive complex 246 and blue emissive FIrpic was realized with a peak EQE of 13.16% and wide EL spectra ranging from yellow to blue emission, induced by the different energy transfer efficiencies taking place between the two emitters at various applied voltages (Fig. 28).196

Fig. 28 (A) Schematic diagram of the energy transfer takes place between FIrpic (blue emissive) and complex 246 (yellow emissive) at low voltage whereas it is blocked at high voltage due to the saturation of the low-energy emitter for the color tunable OLED. (B) Normalized EL spectra for the color tunable OLED based on yellow complex 246 and blue FIrpic. Adapted from Cheng, G.; Chan, K. T.; To, W.-P.; Che, C.-M. Adv. Mater. 2014, 26, 2540, with permission from WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. Copyright 2014.

Luminescent transition-metal complexes and their applications in electroluminescence

45

2,6-Bis(2,4-difluorophenyl)pyridine and its derivative were used as the tridentate ligands for molecular design. In doped films, complexes 248 and 249 with alkynyl groups show strongly emissive yellow luminescence (lPL ¼ 567 nm, PLQY ¼ 0.65 for 248; lPL ¼ 568 nm, PLQY ¼ 0.80 for 249) with much shorter lifetimes (1.46 ms for 248, 1.19 ms for 249) suitable for efficient OLEDs. An extremely high peak EQE of 23.4% was achieved for OLED based on complex 249.197 Replacing the alkynyl ligands with the aryl ligands gives the highly efficient TADF complexes 250 and 251.198 Compared to the alkynyl-based ones (248 and 249), the complexes in PMMA films show remarkable blue-shifted emissions (lPL ¼ 517 nm, PLQY ¼ 0.84 for 250; lPL ¼ 513 nm, PLQY ¼ 0.78 for 251) and short lifetimes (0.72 ms for 250, 0.85 ms for 251). Owing to the high PLQY and short lifetime, the green OLED based on complex 250 is fabricated with a very high peak EQE (23.8%), clearly demonstrating the bright future of this class of TADF gold(III) emitters.198

Very recently, Yam and coworkers made a great breakthrough in designing highly efficient gold(III) complexes toward highperformance and long-lived OLEDs.199 A family of carbazolyl-based gold(III) complexes (252–254) bearing C^C^N-type cyclometalating ligands are well designed and prepared (Fig. 29A). Owing to the incorporation of a strong s-donating carbazolyl ligand, the excimeric emission originating from the p-p interaction of the tridentate ligand could be suppressed, resulting in a remarkable

Fig. 29 (A) ORTEP diagram of complex 253. Redrawn from data in ref. 199. (B) Emission spectra of complexes 252–254 in toluene at 298 K. Adapted from Li, L.-K.; Tang, M.-C.; Lai, S.-L.; Ng, M.; Kwok, W.-K.; Chan, M.-Y.; Yam, V. W.-W. Nat. Photonics 2019, 13, 185, with permission from Springer Nature Limited. Copyright 2019.

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Luminescent transition-metal complexes and their applications in electroluminescence

improvement in the color purity of complexes. In toluene, these complexes show intense structureless luminescence ranging from red to near-infrared (Fig. 29B), which can be attributed to the triplet LLCT (p[carbazole] / p*[C^C^N]) excited state. Notably, by using these emitters, a dry-processed green OLED based on complex 252 shows an extraordinarily high peak EQE of 21.6%, PE of 64.4 lm W
1 and CE of 71.9 cd A
1, and an extremely long half lifetime of 83,000 h at 100 cd m
2 can be realized for the red OLED based on complex 254. The results highlight the great potentials of this family of gold(III) complexes in developing high-performance OLEDs in the future.199

8.02.3.3.3

Gold(III) complexes with tetradentate ligand

As discussed above, tetradentate metal complexes (e.g., platinum(II) complexes) usually show robust stability because of their rigid ligand skeleton.168–183 This powerful molecular design strategy is also suitable for luminescent gold(III) complexes, and some tetradentate gold(III) emitters have been explored.200–203 Very recently, Che and coworkers proposed an effective microwaveassisted synthetic method to obtain a library of novel TADF tetradentate gold(III) emitters with C^C^N^C-type structures (255– 260) (Fig. 30).200 These complexes feature the 5/5/6 membered rings, and the 6 membered rings were realized by microwaveassisted C–H bond activation. Due to the rigid ligand structure, they exhibit high thermal stability. By changing the substituents on the ligand, these gold(III) emitters show efficient luminescence originating from 3ILCT and 3IL states as well as TADF. An intense green TADF (lPL ¼ 533 nm, PLQY ¼ 0.94) can be observed with a short lifetime (1.61 ms) for complex 258 in toluene at room temperature.200

Fig. 30

ORTEP diagram of complex 260. Redrawn from data in ref. 200.

Luminescent transition-metal complexes and their applications in electroluminescence

47

In 2020, Yam and coworkers also developed a simple one-pot two bond-forming synthetic method to construct a series of luminescent tetradentate gold(III) complexes (261–264) with the fully p-conjugated ligand.201 The key to synthesizing these gold(III) complexes is the coupling of the tridentate gold(III) precursor with carbazolyl boric ester. Interestingly, the coupling reaction can form two regioisomers with either C- or N-coordination simultaneously (Fig. 31). At room temperature, in CH2Cl2, complexes 261–263 with N-coordinated carbazole unit show almost similar deep-red emission (lPL ¼ 660 nm), while complex 264 with C-coordinated carbazole unit exhibits remarkably blue-shifted greenish yellow emission (lPL ¼ 548 nm) (Fig. 32A). The totally different emission properties could be attributed to the distinct excited state characters in complexes 261 and 264. Notably, compared to the emission in CH2Cl2, the emission wavelengths of these complexes show blue-shifts in the mCP-doped films (orange emission for complexes 261–263, green emission for complex 264) (Fig. 32B). Using complex 264 as the emitter, a green electroluminescence with the peak EQE of 6.6% can be realized.201

In the same year, a library of C^C^N^N-type tetradentate gold(III) emitters (265–269) possessing preferential horizontal orientation were also prepared via the one-pot method by Tang et al.202 These complexes contain an N-coordinated carbazole unit. The t-butyl

Fig. 31

ORTEP diagrams of complexes 261 (left) and 264 (right). Redrawn from data in ref. 201.

48

Luminescent transition-metal complexes and their applications in electroluminescence

Fig. 32 Normalized emission spectra of complexes 261–264 in dichloromethane (A) and in mCP-doped thin films (B) at room temperature. (Inset shows the photograph of the complexes 261 and 264 in dichloromethane under UV irradiation). Adapted from Lee, C.-H.; Tang, M.-C.; Kong, F. K.W.; Cheung, W.-L.; Ng, M.; Chan, M.-Y.; Yam, V. W.-W. J. Am. Chem. Soc. 2020, 142, 520, with permission from American Chemical Society. Copyright 2019.

group introduced into the complex not only increases the solubility but also suppresses the p–p stacking, leading to high emission color purity. Except for complex 265 (poorly soluble), complexes 266–268 exhibit greenish yellow emissions (lPL ¼ 560 nm for 266 and 268, lPL ¼ 530 nm for 267) in the mCP doped films. Notably, the high PLQY of 70% can be achieved for complex 268 in the mCP doped film. A peak EQE of 20.6% and half-lifetime of 37,500 h at 100 cd$m
2 can be observed for devices based on these emitters, demonstrating that these novel tetradentate gold(III) emitters will be competitive for future electroluminescence study.202

In 2017, four alkynyl-based tetradentate gold(III) complexes (270–273) were synthesized in high yields by the intramolecular cyclization of tridentate gold(III) precursor via Buchwald-Hartwig coupling reaction.203 Interestingly, the obtained complexes possess unusual, stable 12-membered metallocyclic ring containing alkynyl segment (Fig. 33). In CH2Cl2, these complexes exhibit bright structureless emission (lPL ¼ 584–615 nm) and emission lifetimes in the microseconds, suggesting the triplet emission origin. Notably, owing to the rigidification of the formed tetradentate C^N^C^C ligands that slows down the nonradiative decay pathway, the PLQYs of these gold(III) complexes (0.41–0.49) were greatly improved by two orders of magnitude higher than the corresponding tridentate gold(III) precursors. Red OLED with a EQE of 11.1% was achieved by selecting complex 273 as a phosphor.203

8.02.3.4

Copper(I) complexes

Luminescent copper(I) complexes usually show transitions of distinct MLCT character, which may result in the phosphorescence or thermally activated delayed fluorescence with a small energy gap between the lowest singlet and triplet states. Thus, full utilization of both singlet and triplet excitons makes them promising candidates in highly efficient electroluminescence.10,204–208 To date,

Luminescent transition-metal complexes and their applications in electroluminescence

Fig. 33

49

ORTEP diagram of complex 270. Redrawn from data in ref. 203.

many kinds of luminescent copper(I) complexes have been well explored. The copper(I) ion has a rich coordination mode that can generate diverse emissive excited states. According to the coordination number of copper center, they can be divided into twocoordinate, three-coordinate, and four-coordinate species.10,204–208

8.02.3.4.1

Carbene ligand-based copper(I) complexes

N-Heterocyclic carbene ligands are useful in developing luminescent copper(I) complexes, and the ligand structures may play an essential role in determining their emission nature (e.g., thermally activated delayed fluorescence, phosphorescence, etc.).209–214 In 2014, Yersin and coworkers prepared two luminescent neutral copper(I) complexes (274 and 277) based on different N-heterocyclic carbene ligands and di(2-pyridyl)dimethylborate.210 The two complexes show extremely high PLQYs of over 0.70 with long lifetimes (11 ms for 274, 18 ms for 277) in the solid state. However, these two complexes exhibit distinct emission properties. Complex 274 shows TADF, while complex 277 displays yellow phosphorescence. Together with complexes 275 and 276 as their analogs as investigated by theoretical calculations, the distinct emission properties can be attributed to the different torsion angles between two ligands, which determines the energy gap between the first excited triplet state and singlet state (Fig. 34). Moreover, the SOC effect is very effective in both complexes, providing complex 274 with two thermally equilibrated radiative decay paths (TADF in 62%, phosphorescence in 38%).210 Later, Gaillard and coworkers used a series of N^N ligands with various bridging groups (e.g., CH2, NH, PPh, C(CH3)2) to prepare N-heterocyclic carbene-based ionic copper(I) emitters (278–281).211 All these complexes show efficient TADF (lPL ¼ 463–503 nm) with long lifetimes (6–14 ms) at room temperature. The bridging groups play significant roles in the molecular conformation and PLQY. Complexes 278 and 279 possess planar-like structures, whereas complexes 280 and 281 show boat-like structures. The PLQYs (0.73 for 280, 0.89 for 281) of complexes 280 and 281 are much higher than those (0.22 for 278, 0.15 for 279) of complexes 278 and 279.211 The fast degradation of these ionic copper(I) complexes is one of the main reasons for the low device performance in LECs.209 In 2018, Costa and coworkers demonstrated the degradation mechanism of an N-heterocyclic carbene-based luminescent copper(I) complex (similar to complex 278) used as an emitter in LECs. It is found that the copper(I) complex with two N-heterocyclic carbenes (282) is the by-product of the degradation during the device operation.209,212

50

Luminescent transition-metal complexes and their applications in electroluminescence

Fig. 34 Dependence of the singlet
triplet splitting DE(S1-T1) and the torsion angle N–C–Cu–N (marked by the green line) for carbene ligand-based copper(I) complex as obtained from density functional theory (DFT) and TD-DFT calculations. Reproduced from Leitl, M. J.; Krylova, V. A.; Djurovich, P. I.; Thompson, M. E.; Yersin, H. J. Am. Chem. Soc. 2014, 136, 16032, with permission from American Chemical Society. Copyright 2014.

The use of carbazole derivatives can generate a new family of two-coordinate linear carbene-based copper(I) complexes with neutral charge. Several types of carbene ligands (e.g., benzimidazolyl carbene, monoamido-aminocarbene, diamidocarbene, cyclic alkylamino carbene, etc.) were successfully involved in the molecular design of luminescent copper(I) complexes.192,193,213,214 Complex 283 developed by Thompson and coworkers shows highly efficient deep-blue luminescence (lPL ¼ 428–458 nm, PLQY ¼ 0.35–0.86) in various media.193 In addition, by incorporating the unusual monoamide-aminocarbene and

Fig. 35 (A) PL spectra of complexes (284–289) in PS films at room temperature (RT) and 77 K. (B) Energy gap law plot of complexes (284–289) in PS films at 298 K. (Insets show the photographs of the complexes (284–289) (from left to right) in PS films under UV irradiation). Adapted from Shi, S.; Jung, M. C.; Coburn, C.; Tadle, A.; Sylvinson M. R., D.; Djurovich, P. I.; Forrest, S. R.; Thompson, M. E. J. Am. Chem. Soc. 2019, 141, 3576, with permission from American Chemical Society. Copyright 2019.

Luminescent transition-metal complexes and their applications in electroluminescence

51

diamidocarbene ligands and cyano-substituted carbazole derivatives, a library of N-heterocyclic carbene-based copper(I) emitters (284–289) with efficient TADF were explored.213 The rational selection of the donor (carbazole derivative) and acceptor (carbene) enables the color-tuning from deep-blue to deep-red/near-infrared with short lifetimes in different media (e.g., in doped polystyrene film, lPL ¼ 432 nm, PLQY ¼ 0.80 for 284; lPL ¼ 468 nm, PLQY ¼ 1.00 for 285; lPL ¼ 506 nm, PLQY ¼ 0.90 for 286; lPL ¼ 548 nm, PLQY ¼ 0.78 for 287; lPL ¼ 616 nm, PLQY ¼ 0.30 for 288; lPL ¼ 704 nm, PLQY ¼ 0.03 for 289) (Fig. 35). For 285–289, a linear fit is given by a plot of log(knr) (where knr refers to the non-radiative decay rate) versus DE. However, complex 284 falls well off the line, indicating the different orbital composition in the excited state for this complex (Fig. 35B). The green OLED based on complex 286 shows a high peak EQE of 19.4% and a luminance of 54,000 cd m
2, demonstrating the great promise of this class of two-coordinate copper(I) emitters in the future development of OLEDs.213

In order to realize high-performance for copper(I) emitters that are comparable to iridium(III) phosphors, it not only needs to overcome the weak spin-orbit coupling of copper but also limits the high reorganization energies.214 In 2019, Thompson and coworkers developed a series of amide-based two-coordinate copper(I) emitters (290–297) using the cyclic alkyl/amino-carbene with various degrees of steric hindrance.214 Through the ligand design, these Cu(I) complexes show sufficient orbital overlaps for efficient charge transfer, reduced structural reorganization, and suppressed nonradiative decay. Extremely high PLQYs of over 99% and short lifetimes in several microseconds are realized. Both experiments and theoretical calculations demonstrate the interaction among the emissive singlet charge-transfer states, triplet charge-transfer states, and amide-localized triplet states. A dryprocessed blue OLED based on complex 290 gives a peak EQE of 9.0%, which is the highest efficiency for the copper(I)-based blue OLEDs.214

8.02.3.4.2

Three-coordinate copper(I) complexes

Arylphosphine derivatives are another important type of ligands for designing efficient copper(I) emitters.215–224 A series of monohalogen-based three-coordinate copper(I) emitters bearing arylphosphine ligands (298–305) were explored by Yersin, Osawa and coworkers.218,219 Complexes 298–302 supported by 1,2-bis(bisphenylphosphino)benzene derivatives show intense sky-blue to green TADF (lPL ¼ 473–517 nm) with high PLQYs (0.38–0.95) at room temperature in the solid state. Based on these complexes, a green electroluminescence with a high peak EQE of 22.5% and CE of 69.4 cd$A
1 was realized.218 Yersin and coworkers reported three-coordinate binuclear copper(I) emitters (303–305) with one halogen ligand. At room temperature, emitters show bright skyblue luminescence (lPL ¼ 485 nm, PLQY ¼ 0.92 for 303; lPL ¼ 501 nm, PLQY ¼ 0.52 for 304; lPL ¼ 484 nm, PLQY ¼ 0.76 for 305) with short lifetimes (7.3–12.4 ms). Interestingly, the detailed investigations reveal that the emissions of these emitters originate from both the lowest excited triplet state and singlet state (80% of the emission intensity from TADF, 20% of the emission intensity from phosphorescence).219

52

Luminescent transition-metal complexes and their applications in electroluminescence

8.02.3.4.3

Four-coordinate copper(I) complexes

Later, a new N^P^P-type tridentate ligand with a rigid structure was used to construct the four-coordinate copper(I) emitters (306– 309) with additional halogen or thiophenolate ligand.223 The small energy gaps between the lowest triplet state and singlet state of these complexes result in TADF. The intense green emissions (lPL ¼ 541 nm, PLQY ¼ 0.83 for 307; lPL ¼ 530 nm, PLQY ¼ 0.82 for 308; lPL ¼ 540 nm, PLQY ¼ 0.90 for 309) with short lifetimes (9 ms for 307, 7 ms for 308, 5 ms for 309) are observed at room temperature in the powder state. Also, a green OLED based on 308 with a high luminance of over 9000 cd$m
2 and a high EQE of 16.4% was realized.223 Xu and coworkers designed a series of four-coordinate copper(I) emitters (310
312) bearing tridentate phosphine ligand (Fig. 36).224 The chloride, bromide, and iodide ions were used as another ligand. At room temperature, these complexes show bright green luminescence (lPL ¼ 530 nm, PLQY ¼ 0.76 for 310; lPL ¼ 523 nm, PLQY ¼ 0.79 for 311; lPL ¼ 521 nm, PLQY ¼ 0.83 for 312) originated from both TADF and phosphorescence in films (Fig. 37A). It should be noted that the decay lifetimes decrease from 19 ms for complex 310 to 11 ms for complex 312, which is probably the result of the enhanced SOC effect due to the atomic weight increasing from chlorine to iodine. Using complex 312 as an emitter, the OLED with peak EQE of 16.3%, PE of 35.9 lm W
1, and CE of 40.8 cd A
1 was successfully obtained by full utilization of singlet and triplet excitons (Fig. 37B).224

Fig. 36

ORTEP diagrams of complexes 310 (left) and 311 (right). Redrawn from data in ref. 224.

Luminescent transition-metal complexes and their applications in electroluminescence

53

Fig. 37 (A) Absorption, PL, and phosphorescence (PH) spectra of complexes 310–312 in films. (B) EL mechanisms with various exciton harvesting processes (Em. refers to emission). Adapted from Zhang, J.; Duan, C.; Han, C.; Yang, H.; Wei, Y.; Xu, H. Adv. Mater. 2016, 28, 5975, with permission from WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. Copyright 2016.

The halogen-bridged copper(I) dimers are a unique category of binuclear luminescent transition-metal complexes.220,225,226 A pair of bis(diphenylphosphino)thiophene-based iodide-bridged sky-blue copper(I) emitters (313 and 314) were prepared by Li et al. Two independent radiative decay pathways (TADF and phosphorescence) are also observed in both emitters. The proportion of TADF and phosphorescence is highly dependent on the temperature. The two emitters share similar sky-blue luminescence (lPL ¼ 487 nm, PLQY ¼ 0.69 for 313; lPL ¼ 483 nm, PLQY ¼ 0.86 for 314) with relatively short lifetimes (9.46 ms for 313, 7.62 ms for 314) in powder at room temperature. A peak EQE of 14.5% was realized for the OLED based on complex 314.225 In 2013, the aminophosphane ligands were also used to prepare a series of halogen-bridged copper(I) emitters (315–318).226 The temperature-dependent emission properties can be also observed in these emitters. At room temperature, the emission of all complexes is almost originated from TADF (lPL ¼ 506 nm, PLQY ¼ 0.45 for 315; lPL ¼ 490 nm, PLQY ¼ 0.65 for 316; lPL ¼ 464 nm, PLQY ¼ 0.65 for 317; lPL ¼ 465 nm, PLQY ¼ 0.65 for 318) with short lifetimes (4.1–6.6 ms).226 These superior photophysical properties (e.g., both emissive singlet and triplet state, high PLQY, short decay lifetime, etc.) make the halogen-bridged copper(I) dimers excellent candidates for low-cost and highly efficient OLEDs.

54

Luminescent transition-metal complexes and their applications in electroluminescence

Four-coordinate ionic copper(I) emitters are also designed by employing the combination of N^N ligand and arylphosphine ligand.227–238 In 2019, three deep-red/near-infrared ionic copper(I) emitters (319–321) constructed by diethyl (2,20 -biquinoline)-4,40 -dicarboxylate with extended conjugation and various arylphosphine ligand were well designed by Costa and coworkers.235 At room temperature, these emitters show deep-red/near-infrared emissions (lPL ¼ 704 nm, PLQY ¼ 0.01 for 319; lPL ¼ 709 nm, PLQY ¼ 0.13 for 320; lPL ¼ 701 nm, PLQY ¼ 0.23 for 321) in CH2Cl2, while the emission energies (lPL ¼ 669 nm for 319, lPL ¼ 676 nm for 320, lPL ¼ 671 nm for 321) of these emitters in the solid state were considerably blue-shifted, and the PLQYs were significantly improved (PLQY ¼ 0.26 for 319, PLQY ¼ 0.35 for 320, PLQY ¼ 0.56 for 321). The blue-shift in emission may originate from the J-type aggregates in powder. Moreover, the PLQYs also increase as the arylphosphine ligand becomes more rigid. The first example of deep-red LECs based on complex 321 was realized with high irradiance (130 mW cm
2) and excellent color stability over days.235

A family of ionic copper(I) emitters based on 2-(1H-pyrazol-1-yl)pyridine derivatives (322–326) exhibiting efficient roomtemperature TADF were developed by Lu and coworkers.236,237 Through the ligand modification of the N^N ligand, the photophysical properties of these emitters can be finely tuned (lPL ¼ 490 nm, PLQY ¼ 0.56 for 322; lPL ¼ 465 nm, PLQY ¼ 0.87 for 323; lPL ¼ 492 nm, PLQY ¼ 0.75 for 324; lPL ¼ 495 nm, PLQY ¼ 0.45 for 325; lPL ¼ 518 nm, PLQY ¼ 0.98 for 326) in the solid state at room temperature. For the complexes bearing bis(2-diphenylphosphinophenyl)ether ligand, the decay lifetimes (20.4 ms for 322, 12.2 ms for 323, 22.8 ms for 324, 23 ms for 326) are much shorter than that (134 ms) of complex 325 based on triphenylphosphine ligand at room temperature. Using the emitter 326 with short decay lifetime as the dopant, a green OLED with peak EQE of 6.36% and CE of 17.44 cd m
2 was obtained.236,237

Luminescent transition-metal complexes and their applications in electroluminescence

55

Compared to the cyclometalated iridium(III) or platinum(II) phosphors, the luminescent copper(I) emitters usually suffer from poor structural and photochemical stabilities, especially in solution, limiting their applications in the stable OLEDs.10,204–208 To solve this problem, Armaroli and coworkers proposed a molecular design strategy for designing luminescent copper(I) emitters (328–330 with 327 as the model emitter) with higher stability by using macrocyclic phenanthroline ligands with various ring sizes.238 The complexes 328–330 can be prepared directly from the corresponding macrocyclic ligands in high yield. The incorporation of the macrocycle into the complexes may improve the stability of the complexes by suppressing the decomplexation process. These emitters show intense green luminescence (lPL ¼ 520–532 nm, PLQYs ¼ 0.39–0.44) in the solid state, whereas their emissions (lPL ¼ 560–568 nm, PLQYs ¼ 0.197–0.284) are red-shifted in degassed CH2Cl2. Compared with the model emitter 327, at room temperature, three macrocycle-bearing emitters all show excellent stabilities for some hours under daylight in degassed CH2Cl2. The temperature-dependent emission spectra and decay lifetimes suggest the TADF nature of emission for these emitters in the solid state. For electroluminescence, both luminance and device stability of the green OLED based on complex 330 are improved compared to the macrocycle-free complex 327.238 These results reveal that the structural factors may play important roles in the stability of copper(I)-based electroluminescence.

8.02.3.5

Ruthenium(II) complexes

The research interest toward less expensive luminescent ruthenium(II) complexes is the result of the following desirable aspects: easy synthesis, long-lived excited states, bright luminescence from the triplet state, considerable PLQY, and reversible electrochemical behavior, etc.3,5,10 A large number of octahedral ionic or neutral Ru(II) emitters have been designed and used in catalysis, solar cells, chemosensing and diagnostics, electroluminescence including LECs and OLEDs, etc. The ongoing extensive research efforts in the further molecular design are motivated by their promising performances achieved in these applications.3,5,10,239–248

The applications of the ionic luminescent 2,20 -bipyridine-based ruthenium(II) complexes (331
333) with various alkyl chains in the light-emitting electrochemical cells were explored by Rudmann et al.242 The introduction of the alkyl chain into the complex is to suppress the self-quenching of the excited state. Compared to the unmodified complex 331, a higher photoluminescence yield and red electroluminescence with a high EQE of 5.5% can be obtained for the alkyl-bearing complexes 332 and 333.242 Later, Bonaccorso and coworkers developed an ionic tetrazole-based ruthenium(II) complex (334) with boron tetrafluoride as the

56

Luminescent transition-metal complexes and their applications in electroluminescence

counterion. The LEC based on complex 334 shows a low turn-on voltage, improved and stable luminance.243 An ionic red emissive ruthenium(II) complex (335) was prepared using phenanthroimidazole derivative with the extended p-conjugation. Complex 335 exhibits intense red luminescence in both acetonitrile (lPL ¼ 605 nm, PLQY ¼ 0.119) and film (lPL ¼ 632 nm, PLQY ¼ 0.162). The LEC based on complex 335 showing a red electroluminescence (lEL ¼ 635 nm) with a peak EQE of 0.689% was realized.245

For application in OLEDs, the ionic ruthenium(II) complexes are not suitable for the fabrication of high-efficiency OLEDs using the vacuum-deposition method. The neutral ruthenium(II) complexes are more attractive for OLEDs owing to their volatility. A library of luminescent neutral ruthenium(II) emitters (336–342) with 2-(1H-pyrazol-5-yl)pyridine or 1-(1H-pyrazol-5-yl)isoquinoline ligand were rationally designed and prepared by Tung et al. (Fig. 38).246 The photophysical properties of these complexes are strongly dependent on the ligands. All complexes show weak luminescence or are non-emissive in CH2Cl2, and only complexes 340 and 341 give the intense red luminescence (lPL ¼ 632 nm, PLQY ¼ 0.24 for 340; lPL ¼ 609 nm, PLQY ¼ 0.21 for 341) in the solid state. Complex 340 affords a red OLED with a high peak EQE of 7.03% and an extremely high brightness of 11,638 cd m
2.246 Although the device performance of ruthenium(II) complex is far behind that of the iridium(III)-based one, we believe that the less expensive ruthenium(II) complexes still have a great potential in electroluminescence through the rational molecular design.

8.02.3.6

Rhenium(I) complexes

Rhenium(I) ion possesses a d6 electronic configuration similar to that of the iridium(III) ion. As another class of phosphorescent emitters, rhenium(I) complexes have many merits, for instance, room temperature phosphorescence, high PLQY, relatively short lifetime of excited state, high thermal and structural stabilities.249–254 Some attempts have been made on the electroluminescence application for these rhenium(I) complexes.249–254

The typical phosphorescent rhenium(I) complexes consist of one N^N ligand, three carbon monoxide ligands, and one halogen (e.g., Br, Cl).249–254 So the photophysical properties of the rhenium(I) complex can be manipulated by either selection of appropriate N^N ligand or further modification. The [1,2,5]thiadiazolo[3,4-f][1,10]phenanthroline ligand was selected to prepare a novel tricarbonyl rhenium(I) complex (343) by Hu et al.253 This complex exhibits weak red phosphorescence (lPL ¼ 621 nm, PLQY ¼ 0.04) with very short decay lifetime of 0.04 ms in degassed CH2Cl2 at room temperature. An orange OLED based on complex 343 shows electroluminescence at 590 nm with a peak brightness of 4002 cd m
2 and maximum EQE of 5.4%.253 By modification of the 1,2-diphenyl-1H-imidazo[4,5-f][1,10]phenanthroline, a series of highly efficient phosphorescent tricarbonyl rhenium(I) complexes (344–346) were developed by the same group.254 The introduction of functional group has a great influence

Luminescent transition-metal complexes and their applications in electroluminescence

57

Fig. 38 ORTEP diagrams of complexes 338 (left) and 341 (right). Reproduced from Tung, Y.-L.; Chen, L.-S.; Chi, Y.; Chou, P.-T.; Cheng, Y.-M.; Li, E. Y.; Lee, G.-H.; Shu, C.-F.; Wu, F.-I.; Carty, A. J. Adv. Funct. Mater. 2006, 16, 1615, with permission from WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. Copyright 2006.

on the excited states of these complexes. These complexes show intense phosphorescence (lPL ¼ 540 nm, PLQY ¼ 0.15 for 344; lPL ¼ 540 nm, PLQY ¼ 0.21 for 345; lPL ¼ 580 nm, PLQY ¼ 0.17 for 346) with short lifetimes (0.182 ms for 344, 0.165 ms for 345, 0.183 ms for 346) in degassed CH2Cl2 at room temperature. A yellow OLED based on complex 345 with the maximum CE of 21.1 cd A
1 can be obtained.254

8.02.3.7

Osmium(II) complexes

As with iridium(III) emitters, osmium(II) emitters also have an octahedral geometry with a d6 electronic configuration. In general, the osmium(II) emitters show high PLQYs, but quite long decay lifetimes of excited states originated from the emissive intraligand 3 p-p* transition, limiting their further application in electroluminescence. Thus, the decrease in decay lifetime of emission by increasing the MLCT character in their excited state could be an effective approach to design osmium(II) emitters with short lifetime suitable for efficient electroluminescence.10,255–260

1-(1H-1,2,4-Triazol-5-yl)isoquinoline derivatives as the N^N ligand and arylphosphine ligands were selected by Chi and coworkers for designing efficient near-infrared osmium(II) emitters (347–350) with neutral charge.257 These complexes exhibit efficient near-infrared phosphorescence (lPL ¼ 805 nm, PLQY ¼ 0.002 for 347; lPL ¼ 692 nm, PLQY ¼ 0.15 for 348; lPL ¼ 695 nm, PLQY ¼ 0.08 for 349; lPL ¼ 731 nm, PLQY ¼ 0.04 for 350) with remarkably reduced emission lifetimes (0.04 ms for 347, 0.76 ms for 348, 0.68 ms for 349, 0.16 ms for 350) in CH2Cl2 at room temperature, originating from the triplet MLCT transition. The dry-processed OLED based on complex 350 gives a near-infrared electroluminescence peaking at 718 nm with a maximum EQE of 2.7% and a radiance of 93.26 mW cm
2.257

58

Luminescent transition-metal complexes and their applications in electroluminescence

The emission energies of the phosphorescent osmium(II) emitters can be realized by wisely selecting the N^N ligands. Recently, the remarkable color-tuning of phosphorescent osmium(II) emitters (351–355) was demonstrated by Chi and coworkers.258 They used 3-(benzothiazol-2-yl), 3-(thiazol-2-yl), 3-(benzimidazol-2-yl), and 3-(imidazol-2-yl) azole ligands to construct these osmium(II) emitters. Except for the non-emissive complex 355, the prepared osmium(II) emitters 351–354 exhibit intense phosphorescence (lPL ¼ 622 nm, PLQY ¼ 0.49 for 351; lPL ¼ 642 nm, PLQY ¼ 0.65 for 352; lPL ¼ 580 nm, PLQY ¼ 0.11 for 353; lPL ¼ 538 nm, PLQY ¼ 0.003 for 354) ranging from green to deep-red in CH2Cl2 at room temperature, and the decay lifetimes of emission are also very short (1.77 ms for 351, 2.58 ms for 352, 0.49 ms for 353, 0.024 ms for 354). The high efficiency OLEDs based on these emitters are also demonstrated, and the extremely high EQEs of 17.7% for 351, 15.6% for 352, 15.7% for 353, and 15.6% for 354 were achieved.258 The obtained performances of these osmium(II)-based devices are comparable to the iridium(III)-based OLEDs.

Compared to the iridium(III) emitters, the osmium(II) emitters are more competitive for highly efficient NIR OLEDs.8,259,260 Very recently, some extremely high-performance NIR OLEDs based on novel neutral osmium(II) emitters (356–362) were successfully achieved by Chi et al. (Fig. 39).259,260 The 2-(1H-pyrazol-5-yl)pyrazine, 2-(1H-1,2,4-triazol-5-yl)pyrazine, 1-(1H-pyrazol-5-yl) isoquinoline, and 1-(1H-1,2,4-triazol-5-yl)isoquinoline derivatives were designed for realizing these efficient NIR osmium(II) emitters. In the CBP doped films, emitters 356–359 show intense NIR phosphorescence (lPL ¼ 750 nm, PLQY ¼ 0.14 for 356; lPL ¼ 750 nm, PLQY ¼ 0.18 for 357; lPL ¼ 747 nm, PLQY ¼ 0.39 for 358; lPL ¼ 737 nm, PLQY ¼ 0.40 for 359) with short decay lifetimes (0.132 ms for 356, 0.136 ms for 357, 0.234 ms for 358, 0.308 ms for 359). Based on emitter 359, an extremely high peak EQE of 11.5% was achieved for NIR OLED with the electroluminescence peaking at 710 nm.259 The further extension of the N^N ligands used in emitters (360–362) will generate slightly red-shifted NIR phosphorescence with improved PLQYs (lPL ¼ 759 nm, PLQY ¼ 0.28 for 360; lPL ¼ 745 nm, PLQY ¼ 0.48 for 361; lPL ¼ 750 nm, PLQY ¼ 0.41 for 362) in the doped films (Fig. 40), and these emitters also show short lifetimes ranging from 0.131 ms to 0.169 ms. Notably, by incorporating emitter 361, a record high EQE of 10.32% can be obtained for NIR electroluminescence of 750 nm.260 The extremely high device performances of the NIR OLEDs clearly indicate that the osmium(II) emitters will be a very promising class of NIR phosphors.

Luminescent transition-metal complexes and their applications in electroluminescence

59

P1

N1 N3A

Os

N3 N1A

P1A

Fig. 39 ORTEP diagram of complex 356. Reproduced from Yuan, Y.; Liao, J.-L.; Ni, S.-F.; Jen, A. K.-Y.; Lee, C.-S.; Chi, Y. Adv. Funct. Mater. 2020, 30, 1906738, with permission from WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. Copyright 2019.

8.02.3.8

Rhodium(III) complexes

Due to the existence of thermally accessible non-emissive d-d ligand field excited state, the cyclometalated rhodium(III) complexes are usually emissive at low temperature, but non-emissive at room temperature, thereby limiting their photofunctional applications.261–265 At the same time, the energy level of the d-d ligand field excited states are comparable to those of the emissive excited states of MLCT and/or charge transfer (CT) characters; thus it is quite challenging to develop room-temperature luminescent rhodium(III) complexes, and the examples of these rhodium(III) complexes are extremely rare.261–272

In 2019, Wong and coworkers developed a series of room-temperature luminescent cyclometalated rhodium(III) complexes (363–365) by using 2,3-diphenylquinoxaline ligand.272 The 2,3-diphenylquinoxaline ligand shows a strong s-donor ability and lower-lying intraligand state, which will introduce the lower-lying emissive intraligand excited state and increase the d-d excited state. The various diketone auxiliary ligands were used to finely tune the photophysical properties. At room temperature, these rhodium(III) complexes exhibit intense red luminescence (lPL ¼ 612 nm, PLQY ¼ 0.11 for 363; lPL ¼ 598 nm, PLQY ¼ 0.98 for 364; lPL ¼ 603 nm, PLQY ¼ 0.31 for 365) with short lifetimes (0.79 ms for 363, 1.64 ms for 364, 0.81 ms for 365) in CH2Cl2 (Fig. 41A). In the mCP-doped films, they also show highly efficient room-temperature luminescence (lPL ¼ 603 nm, PLQY ¼ 0.49 for 363; lPL ¼ 597 nm, PLQY ¼ 0.45 for 364; lPL ¼ 602 nm, PLQY ¼ 0.65 for 365) (Fig. 41B), but the decay lifetimes are significantly prolonged (23 ms for 363, 32 ms for 364, 25 ms for 365). The OLED based on complex 365 shows a high peak EQE of 12.2% and a long device half-lifetime of over 3000 h at 100 cd m
2.272

8.02.3.9

Palladium(II) complexes

Metal-assisted delayed fluorescence is a route for utilizing excitons generated by the electric field. The MADF process needs the heavy metal ion to induce both efficient delayed fluorescence and phosphorescence. Some luminescent palladium(II) complexes can

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Luminescent transition-metal complexes and their applications in electroluminescence

Fig. 40 PL spectra of complexes 360–362 in doped films. Adapted from Zhu, Z.-L.; Tan, J.-H.; Chen, W.-C.; Yuan, Y.; Fu, L.-W.; Cao, C.; You, C.-J.; Ni, S.-F.; Chi, Y.; Lee, C.-S. Adv. Funct. Mater. 2021, 31, 2,102,787, with permission from Wiley-VCH GmbH. Copyright 2021.

exhibit efficient metal-assisted delayed fluorescence and/or phosphorescence, which have been involved in the fabrication of highperformance OLEDs.9,169,273–276

A series of luminescent palladium(II) complexes (366–370) constructed by the carbazole-based tetradentate ligands were developed recently by Li and coworkers.9,273 Pyridine-bearing complexes 366 and 367 show intense green luminescence (lPL ¼ 534 nm,

Fig. 41 (A) UV–Vis absorption and PL spectra of complexes 363–365 in CH2Cl2. (B) Normalized PL spectra and PLQYs of complexes 363–365 in the mCP-doped films at different excitation wavelengths. (Inset shows the photograph of thin-film PL of complex 365 under UV irradiation). Adapted from Wei, F.; Lai, S.-L.; Zhao, S.; Ng, M.; Chan, M.-Y.; Yam, V. W.-W.; Wong, K. M.-C. J. Am. Chem. Soc. 2019, 141, 12,863, with permission from American Chemical Society. Copyright 2019.

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61

PLQY ¼ 0.72 for 366; lPL ¼ 530 nm, PLQY ¼ 0.76 for 367) originating from both metal-assisted delayed fluorescence and phosphorescence at room temperature. For both complexes, the intensity of metal-assisted delayed fluorescence in the range of short wavelength is enhanced upon increasing the temperature. The green OLED shows a high peak EQE of 21% and long operational stability with LT90 of more than 20,000 h at 100 cd m
2.9 Complexes 368–370 were generated by replacing one of the pyridine moieties in complex 366 with 1H-pyrazole unit. The obtained complexes show intense sky-blue metal-assisted delayed fluorescence (lPL ¼ 472 nm, PLQY ¼ 0.70 for 368; lPL ¼ 470 nm, PLQY ¼ 0.77 for 369; lPL ¼ 476 nm, PLQY ¼ 0.59 for 370) at room temperature. The broad sidebands (from 410 nm to 450 nm) with low intensities originate from metal-assisted delayed fluorescence of three complexes. Using complex 369 as an emitter, a sky-blue electroluminescence at 476 nm with a maximum EQE of 25.1% was obtained.273

In 2019, She and coworkers used [1,2,4]triazolo[4,3-a]pyridine as a building block to prepare a new family of luminescent tetradentate palladium(II) complexes (371–375). At room temperature, all these emitters show both phosphorescence and metalassisted delayed fluorescence (lPL ¼ 498 nm, PLQY ¼ 0.21% for 371; lPL ¼ 498 nm, PLQY ¼ 0.52% for 372; lPL ¼ 513 nm, PLQY ¼ 1.34% for 373; lPL ¼ 513 nm, PLQY ¼ 1.72% for 374; lPL ¼ 499 nm, PLQY ¼ 2.88% for 375) with long lifetimes (26.7 ms to 152.9 ms) in CH2Cl2.169 Very recently, a breakthrough was made on the highly efficient and extremely stable OLED by using the aggregation of molecular palladium(II) complex 377.274 Complex 377 shows almost unity PLQY and short lifetime (0.62 ms) of the excited state in the aggregation state at room temperature (Figs. 42 and 43), which affords a dopant-free yellow electroluminescence at 588 nm (FWHM ¼ 84 nm) with an extremely high peak EQE of 34.8% and low efficiency roll-off. Notably, this device also shows an extremely long lifespan (LT50 ¼ 9.59 million hours at 1000 cd m
2).274 Later, further replacing the CH moiety of complex 377 by nitrogen atom will generate complexes 378 and 379, and complex 376 was also prepared for comparison.275 In the films, the photophysical properties of complexes 376, 378, and 379 are close to complex 377. The emission wavelengths of these complexes lie between 578 and 590 nm, and the PLQYs are between 0.79 and 0.88. All complexes share the shortlived excited states (1.65 ms for 376, 0.70 ms for 378, and 0.67 ms for 379). Notably, both horizontal emitting dipole ratios and carrier-transport properties of the nitrogen-modified complexes 378 and 379 can be tuned in films, which affords electroluminescence with an extremely high peak EQE (37.3%) and a long device lifespan (LT95 of more than 500 h) at a high initial luminance of about 17,000 cd m
2.275

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Luminescent transition-metal complexes and their applications in electroluminescence

Fig. 42 (A) UV–Vis absorption and PL spectra of complex 377 under different conditions. (B) The room-temperature absorption spectrum (dashdotted line) of complex 377 in neat film, and PL spectra of complex 377 in neat film at room temperature (solid lines) and 77 K (dashed lines). Adapted from Cao, L.; Klimes, K.; Ji, Y.; Fleetham, T.; Li, J. Nat. Photonics 2021, 15, 230, with permission from Springer Nature Limited. Copyright 2020.

Fig. 43 Simplified molecular orbital and schematic energy level diagrams for complex 377, featuring monomer and M-M dimer emissions. Adapted from Cao, L.; Klimes, K.; Ji, Y.; Fleetham, T.; Li, J. Nat. Photonics 2021, 15, 230, with permission from Springer Nature Limited. Copyright 2020.

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Other types of luminescent tetradentate palladium(II) complexes have also been explored by the rational ligand design.276,277 For instance, a series of N^C^C^N-type cyclometalating ligand with oxygen-bridge and/or spirofluorene structure were developed by Che and his team to prepare luminescent tetradentate palladium(II) complexes (380–383). These emitters show intense phosphorescence (lPL ¼ 466 nm, PLQY ¼ 0.39 for 380; lPL ¼ 466 nm, PLQY ¼ 0.47 for 381; lPL ¼ 456 nm, PLQY ¼ 0.20 for 382; lPL ¼ 599 nm, PLQY ¼ 0.07 for 383) in CH2Cl2 at room temperature. A peak EQE of 16.48% was achieved for OLED based on emitter 381.276 Moreover, a O^N^C^N-type cyclometalated tetradentate palladium(II) complex (384) was also explored in the application of OLED by Chow et al.277 Complex 384 shows sky-blue phosphorescence peaking at 498 nm with PLQY of 0.20 in CH2Cl2 at room temperature, and the lifetime is quite long (121 ms), affording electroluminescence with peak EQE of 7.4%.277

8.02.3.10 Silver(I) complexes The coordination chemistry of silver(I) ion is quite similar to that of gold(I) or copper(I) ions.185,193,223 It can form two-coordinate or four-coordinate silver(I) complexes possessing TADF at room temperature, and their attempts on the fabrication of efficient electroluminescence were also made recently.185,193,223,278–283

8.02.3.10.1

Two-coordinate silver(I) complexes

A pair of two-coordinate luminescent silver(I) complexes based on cyclic alkyl/amino-carbene bearing bulky adamantyl moiety (385 and 386) were developed by Romanov et al.278 In crystals, both complexes show coplanar conformation of their amide and

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Luminescent transition-metal complexes and their applications in electroluminescence

carbene ligands (Fig. 44). Two complexes show efficient room-temperature TADF under various conditions. Compared to the emission (lPL ¼ 496 nm, PLQY ¼ 0.19 for 385; lPL ¼ 514 nm, PLQY ¼ 0.45 for 386) of the complexes in films, they show red-shifted emission (lPL ¼ 521 nm, PLQY ¼ 0.74 for 385; lPL ¼ 546 nm, PLQY ¼ 0.55 for 386) with improved PLQYs in toluene. Especially, the relatively short lifetime of the excited state (0.31–0.46 ms) for the two complexes is suitable for OLEDs, and a high peak EQE of 13.7% for dry-processed OLED based on complex 386 was obtained.278 Later, Thompson and coworkers used benzimidazolyl ligand to synthesize a new carbene-based silver(I) complex (387) showing intense room-temperature TADF.193 Different from their gold(I) and copper(I) analogs, two conformations (coplanar and orthogonal) of its amide and carbene ligand in complex 387 were found in its crystals (Fig. 45). This complex also shows solvent-dependent emissions (lPL ¼ 430 nm, PLQY ¼ 0.58 in methylcyclohexane; lPL ¼ 458 nm, PLQY ¼ 0.50 in toluene; lPL ¼ 476 nm, PLQY ¼ 0.19 in 2-methyltetrahydrofuran; lPL ¼ 482 nm, PLQY ¼ 0.03 in CH2Cl2), and the bright deep-blue luminescence (438 nm) with a high PLQY of 0.85 for complex 387 can be observed in its PS film.193

8.02.3.10.2

Four-coordinate silver(I) complexes

In 2019, a series of four-coordinate ionic silver(I) emitters (388–390) supported by arylphosphine and 4,40 -dimethoxy-2,20 bipyridine ligands were designed with various counterions to evaluate their performance in LECs.279 The comprehensive investigation of the degradation mechanism of the LECs based on complex 388 suggests that the irreversible formation of silver nanoclusters under operation limits the device performance. Through controlling the counterions (e.g., PF6
and ClO4
) and optimizing the device architecture, the optimized LECs exhibited an extremely improved stability of 4 orders of magnitude (over 80 h vs 30 s).279 Bis(diphenylphosphine)-nido-carborane was used as the ligand to design a library of neutral silver(I) complexes (391– 393) showing intense TADF.280,281 Interestingly, the alkyl group on the 1,10-phenanthroline has a significant influence on the excited states of silver(I) complexes. At room temperature, upon increasing the length of the alkyl group, the complexes show blue-shifted emission (575 nm for 391, 537 nm for 392, 526 nm for 393) with remarkable enhanced PLQYs (0.36 for 391, 0.78 for 392, 1.00 for 393) and short lifetimes (2.0 ms for 391, 2.8 ms for 392, 1.4 ms for 393) in powder. Theoretical calculations revealed that the alkyl groups could rigidify the molecular geometry in the lowest excited states and regulate the radiative rate of the S1 / S0 transition.280

Fig. 44

ORTEP diagram of complex 385. Redrawn from data in ref. 278.

Luminescent transition-metal complexes and their applications in electroluminescence

Fig. 45

65

ORTEP diagrams of complex 387 with coplanar (left) and orthogonal (right) conformations. Redrawn from data in ref. 193.

The luminescent dinuclear silver(I) complexes were also explored recently.282,283 1,2,4,5-Tetrakis(diphenylphosphino)benzene served as the ligand for preparing such dinuclear neutral silver(I) complexes (394–397) by Lu, Yersin and coworkers.282,283 The coordination of complexes 394–396 were saturated by additional triphenylphosphine and halide ion, whereas complex 397 was terminated by two bis(diphenylphosphine)-nido-carborane ligands. All complexes show intense sky-blue/yellow TADF (lPL ¼ 475 nm, PLQY ¼ 0.98 for 394; lPL ¼ 471 nm, PLQY ¼ 0.91 for 395; lPL ¼ 495 nm, PLQY ¼ 0.74 for 396; lPL ¼ 555 nm, PLQY ¼ 0.70 for 397) and short lifetimes (3.0 ms for 394, 2.9 ms for 395, 2.5 ms for 397, 1.9 ms for 397) in powder, which made them excellent TADF emitters for highly efficient electroluminescence.282,283

In addition to the above mentioned carbene, N^N, and P^P ligands, other types of ligands were also involved in preparing luminescent four-coordinate silver(I) complexes. A luminescent four-coordinate silver(I) complex (398) supported by 2diphenylphosphinobenzenethiolate and 1,2-bis(diphenylphosphino)benzene ligand was prepared.185 Both calculations and photophysical investigations suggest that the TADF (lPL ¼ 505 nm, PLQY ¼ 0.32) originated from LLCT is responsible for the solidstate emission of complex 398 at room temperature. The strong electron-donating thiolate ligand decrease the contribution from silver(I) orbitals to the HOMO of the complexes, thereby reducing the MLCT character of the excited states of the complex.185 Very recently, Yersin and coworkers synthesized an N^P^P-type ligand-based silver(I) complex (399) showing intense sky-blue TADF (lPL ¼ 479 nm, PLQY ¼ 0.70) with a short lifetime of 13 ms at room temperature.223 It is believed that these results will provide a promising option for the development of silver(I)-based emitters in the future.

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Luminescent transition-metal complexes and their applications in electroluminescence

8.02.3.11 Others Although the noble-metal based complexes show attractive photophysical properties and fully utilizable excitons in electroluminescence, these noble-metals are of low abundance and high-cost in nature, limiting the development of low-cost, environmentalfriendly OLEDs in the future.1,2,4,6,8,10,12,13 Recently, other kinds of transition-metal-based emitters (especially based on cheapmetals) have also drawn much attention, and some of them have been proved to be excellent candidates for electroluminescence.284–323

8.02.3.11.1

Tungsten(VI) complexes

Tungsten metal has been used in electric incandescent lamp for more than one century, however, the discovery of luminescent tungsten(VI) complexes appeared just several years ago.286,287 In 2017, a series of air-stable luminescent tungsten(VI) emitters (400–406) were reported by Yeung et al. (Fig. 46).286 These complexes are supported by one O^N^N^O-type tetradentate ligands or two O^N-type bidentate ligands, and the remaining coordination sites are saturated by two oxygen atoms. Most of the complexes show the weak phosphorescence ranging from sky-blue to deep-red with PLQYs below 3%, and a long lifetime of excited state at room temperature. Complex 404 is an exception, and an intense orange room-temperature phosphorescence (lPL ¼ 580 nm, PLQY ¼ 0.22) can be observed in the mCP doped film. Based on this emitter, a solution-processed orange OLED with the peak EQE of 4.79% was achieved successfully (Fig. 47).286 Later, through the ligand modification, another series of the tetradentate tungsten(VI) complexes (407–409) were also developed by the same group. These show intense room-temperature TADF (lPL ¼ 471 nm, PLQY ¼ 0.14 for 407; lPL ¼ 567 nm, PLQY ¼ 0.49 for 408; lPL ¼ 554 nm, PLQY ¼ 0.84 for 409) in the mCP doped films. These complexes show long lifetimes (51.0 ms for 407, 227.8 ms for 408) in the mCP doped films, whereas complex 409 exhibits a very short lifetime of 2.0 ms suitable for electroluminescence. Complex 409 with a short lifetime affords a high peak EQE of 15.56% and brightness of 16,890 cd m
2 for the yellow OLEDs.287 It is believed that the device performance can be further improved by the design of the tungsten(VI) complex and device architecture.

Luminescent transition-metal complexes and their applications in electroluminescence

Fig. 46

67

ORTEP diagram of complex 403. Redrawn from data in ref. 286.

8.02.3.11.2

Manganese(I/II) complexes

(B)

10

EQE / %

Normalized EL intensity

(A)

1

0.1

400

500 600 700 Wavelength / nm

0.01 0.1

1

10

100

1000

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Fig. 47 Normalized EL spectra (A) and EQE-luminance characteristic (B) of OLEDs based on complex 404 with different dopant concentrations (2 wt % (in black), 4 wt% (in red), and 6 wt% (in blue). Adapted from Yeung, K.-T.; To, W.-P.; Sun, C.; Cheng, G.; Ma, C.; Tong, G. S. M.; Yang, C.; Che, C.-M. Angew. Chem. Int. Ed. 2017, 56, 133, with permission from Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. Copyright 2017.

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Luminescent transition-metal complexes and their applications in electroluminescence

Phosphorescent manganese(II) complexes including ionic and neutral ones usually show highly efficient luminescence with tunable emission energies.7,288–311 The exploration of luminescent manganese(II) phosphors is just in its infancy but rapidly flourishing, and their potential application is also emerging in electroluminescence.308,309,311 The first example of OLED based on complex 410 was reported by Chen and coworkers in 2017. The crystal of complex 410 shows an intense green phosphorescence at 516 nm with a PLQY of 0.98, and the lifetime is 355 ms originating from the d-d 4T1 / 6A1 transition of the manganese(II) ion. A green OLED based on complex 410 realizes a high peak EQE of 9.6% by the solution-method.308 Very recently, the first perovskite light-emitting device based on complex 411 was also realized by Yan et al. Complex 411 shows a bright red luminescence at 629 nm with a high PLQY of 0.80, and the lifetime is in millisecond from the triplet excited states. A luminance of 4700 cd m
2 and peak EQE of 9.8% can be achieved by using complex 411 as the emitter.309 In addition to the ionic manganese(II) complexes, the neutral ones were also reported.310,311 In 2015, a bidentate phosphine oxide ligand was used by Zheng and coworkers to prepare three halogen-based tetradentate manganese(II) phosphors (412–414) (Fig. 48).310 At room temperature, these phosphors exhibit bright green luminescence (lPL ¼ 507 nm, PLQY ¼ 0.32 for 412; lPL ¼ 502 nm, PLQY ¼ 0.70 for 413; lPL ¼ 528 nm, PLQY ¼ 0.64 for 414) with long lifetimes (2.2 ms for 412, 0.5 ms for 413, 0.1 ms for 414) in the solid state (Fig. 49). The dramatic decrease of lifetime from Cl to I is the result of the heavy atom effects.310 Later, Huang and his team selected more rigid dibenzo[b,d]furan-4,6diylbis(diphenylphosphine oxide) ligand to design a pair of neutral manganese(II) phosphors (415 and 416) (Fig. 50).311 The two phosphors also show intense greenish yellow luminescence (lPL ¼ 532 nm, PLQY ¼ 0.33 for 415; lPL ¼ 550 nm, PLQY ¼ 0.81 for 416) with the lifetimes in several milliseconds (Fig. 51A). Using phosphor 416 as the dopant, a green electroluminescence with peak EQE of 10.49%, CE of 35.47 cd A
1, and PE of 34.35 lm W
1 was fabricated (Fig. 51B).311 The recent works highlighted the great potential of the manganese(II) phosphors toward low-cost and efficient electroluminescence.

In general, the long-lived MLCT excited states can be observed from many metal complexes with the electron configuration of 4d6 and 5d6.1–6,8,10 However, such excited states were almost not found in the metal complexes with the 3d6 electron configuration. Very recently, Wenger and coworkers presented the first air-stable and emissive manganese(I) complexes (417 and 418) with MLCT luminescence at room temperature in solution.312 These complexes with the low oxidation state are stabilized by bidentate or tridentate isocyanide ligands. In CH2Cl2, these two complexes show a yellow solution because of the absorption band near 400 nm,

Fig. 48 ORTEP diagrams of complexes 412–414. Adapted from Chen, J.; Zhang, Q.; Zheng, F.-K.; Liu, Z.-F.; Wang, S.-H.; Wu, A-Q.; Guo, G.-C. Dalton Trans. 2015, 44, 3289, with permission from The Royal Society of Chemistry. Copyright 2015.

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Fig. 49 (A) Photographs of crystals of complexes 412–414 under ambient light (up) and 365 nm UV light (down). (B) Solid-state excitation spectra (left) and emission spectra (right) of complexes 412–414 at 298 K. Adapted from Chen, J.; Zhang, Q.; Zheng, F.-K.; Liu, Z.-F.; Wang, S.-H.; Wu, A-Q.; Guo, G.-C. Dalton Trans. 2015, 44, 3289, with permission from The Royal Society of Chemistry. Copyright 2015.

Fig. 50

ORTEP diagrams of complexes 415 (left) and 416 (right). Redrawn from data in ref. 311.

and this absorption band is not present in the free ligands (Figs. 52 and 53). Under the excitation of this absorption band, two complexes exhibit the solvatochromic luminescence with the unstructured emission bands (Figs. 52 and 53). The PLQYs in degassed acetonitrile (CH3CN) are quite low at 20  C (5  10
4 for 417, 3  10
4 for 418). They were successfully used as photosensitizers for energy- and electron-transfer reactions. The observed triplet energy-transfer photoreactivity occurred from the ligandcentered 3p–p* state, whereas the electron-transfer photoreactivity is originated from the emissive MLCT state.312

8.02.3.11.3

Iron(II/III) complexes

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Luminescent transition-metal complexes and their applications in electroluminescence

Fig. 51 (A) Solid state emission spectra of complexes 415 and 416 at room temperature. (B) Normalized EL spectra of devices based on complex 416 (Inset shows the photograph of the device). Adapted from Qin, Y.; Tao, P.; Gao, L.; She, P.; Liu, S.; Li, X.; Li, F.; Wang, H.; Zhao, Q.; Miao, Y.; Huang, W. Adv. Opt. Mater. 2019, 7, 1801160, with permission from WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. Copyright 2018.

120

UV-Vis absorption

luminescence

e / 103 M–1 cm–1

417 80 lexc

CH 3C C tol H2 CTHF N ue l2 ne

40

Lbi

*

*

@ 20°C

*

0 300

400 500 wavelength / nm

600

700

Fig. 52 UV–Vis absorption of complex 417 and free bidentate isocyanide ligand (Lbi) in CH2Cl2, PL spectra of complex 417 at 20  C in different solvents. Asterisks indicate the Raman-scattered excitation light. Adapted from Herr, P.; Kerzig, C.; Larsen, C. B.; Häussinger, D.; Wenger, O. S. Nat. Chem. 2021, 13, 956, with permission from Springer Nature Limited. Copyright 2021.

Fig. 53 UV–Vis absorption of complex 418 and free tridentate isocyanide ligand (Ltri) in CH2Cl2, PL spectra of complex 418 at 20  C in different solvents. Asterisks indicate the Raman-scattered excitation light. (Inset shows the photograph of the complex 418 in solution under UV light). Adapted from Herr, P.; Kerzig, C.; Larsen, C. B.; Häussinger, D.; Wenger, O. S. Nat. Chem. 2021, 13, 956, with permission from Springer Nature Limited. Copyright 2021.

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71

The rich coordination chemistry and abundance of iron are also attractive for the photofunctional applications. However, in general, it is a big challenge to realize iron(II/III) complexes with long-lived charge-transfer excited states, and the iron(II/III) complexes are regarded as the non-emissive species at room temperature because of their rapid deactivation of the chargetransfer excited states in subpicosecond induced by the weak ligand field splitting. The breakthroughs made recently by Wärnmark and coworkers will provide us more opportunities to access the luminescent iron(II/III) complexes.313–317 A tris-bidentate carbene ligand-based iron(III) complex (419) was reported by Chábera et al.315 Owing to the better p-acceptor and s-donor electron properties of the carbene ligand used, the lifetime of LMCT excited state of complex 419 is significantly extended, and an unprecedented room-temperature photoluminescence (lPL ¼ 600 nm, PLQY ¼ 3  10
4) is realized with long charge-transfer lifetime of 100 ps in CH3CN, which originates from the long-lived doublet LMCT state of the complex (Fig. 54). The investigation revealed that the carbene ligand can effectively stabilize the excited state to prolong the charge-transfer lifetime.315 Further reduction of complex 419 can generate divalent complex 420 which exhibits intense MLCT absorption bands covering the whole visible region. Moreover, at room temperature, a triplet MLCT excited state with a long lifetime of 528 ps can be formed in CH3CN.316 Tris-carbene phenyl(tris(3methylimidazol-1-ylidene) ligand was also successfully realized to afford the room-temperature luminescent iron(III) complex 421 with an extremely long lifetime of 1.96 ns (Fig. 55). Complex 421 shows a deep-red luminescence (lPL ¼ 655 nm) from a doublet LMCT state with a higher PLQY of 0.021 in CH3CN (Fig. 56).317 These results demonstrated that the long-lived and emissive LMCT and MLCT excited states can be achieved for the iron complexes through the rational ligand design.

8.02.3.11.4

Nickel(0/II) complexes

So far, a large number of copper(I) complexes are luminescent owing to their emissive MLCT states.206,207 However, as an isoelectronic species of copper(I) with 3d10 valence electron configuration, only extremely few examples of luminescent nickel(0) complexes were reported. Nickel(0) can be stabilized by the p-acceptor ligands, such as, isocyanides, phosphines or carbonyls.318–324 In 2003, a carbonyl luminescent nickel(0) complex (422) supported by N-heterocyclic carbene ligand was prepared by Kunkely et al.322 This complex shows a green room-temperature luminescence peaking at 510 nm in argon-saturated CH3CN, which is believed to arise

Fig. 54 (A) Schematic potential energy diagram for complex 419 displaying potential energy curves for key states as a function of an effective nuclear coordinate, QFe-C, for the variation in Fe–C bond lengths, based on computationally optimized geometries of the lowest state of each spin multiplicity (blue doublet, green quartet and red hextet, with optimized geometry points for each spin marked by solid data points) together with the corresponding cross-energies of the alternative spin multiplicities (open data points). The doublet energy curves include the calculated ground state (GS), complemented by the experimental information about the excited doublet state (blue dashed line), assigned to be doublet LMCT, based on the measured experimental absorption and emission wavelengths, lexp-abs and lexp-em, respectively. (B) Steady-state absorption and emission spectra of complex 419 in CH3CN at room temperature. The green line shows the transient absorption spectrum at about 1 ps with ground-state bleach at 528 and 558 nm. Reproduced from Chábera, P.; Liu, Y.; Prakash, O.; Thyrhaug, E.; Nahhas, A. E.; Honarfar, A.; Essén, S.; Fredin, L. A.; Harlang, T. C. B.; Kjær, K. S.; Handrup, K.; Ericson, F.; Tatsuno, H.; Morgan, K.; Schnadt, J.; Häggström, L.; Ericsson, T.; Sobkowiak, A.; Lidin, S.; Huang, P.; Styring, S.; Uhlig, J.; Bendix, J.; Lomoth, R.; Sundström, V.; Persson, P.; Wärnmark, K. Nature 2017, 543, 695, with permission from Macmillan Publishers Limited, part of Springer Nature. Copyright 2017.

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Fig. 55

Luminescent transition-metal complexes and their applications in electroluminescence

ORTEP diagram of the cation of complex 421. Redrawn from data in ref. 317.

from the lowest triplet MLCT of the complex.322 Later, it was found that isocyanide derivatives could also be used as the ligands for realizing nickel(0) complexes 423 and 424 with the long-lived triplet MLCT states.323 At 77 K in frozen toluene, luminescence peaks (lPL ¼ 511 nm for 423, lPL ¼ 554 nm for 424) with lifetimes in microseconds can be observed for both complexes. Unfortunately, these two complexes are non-emissive at room temperature.323 Very recently, the first room-temperature luminescent cyclometalated nickel(II) complex 425 was reported by Wong et al.324 This complex contains 1,3-di(pyridin-2-yl)benzene and carbazole ligands and shows a broad luminescence in the sub-microsecond in both the frozen glass matrix (0.43 ms) and solid state (0.11 ms) at 77 K. The long lifetime of the complexes suggested the triplet emissive states with MLCT character. Detailed investigations revealed that the incorporation of carbazolyl ligand with strong s-donating ability into the nickel(II) center could increase the electron density on the metal center, thereby raising the energy level of the d-d states and leading to the luminescence enhancement.324

8.02.3.11.5

Zirconium(IV) complex

Fig. 56 (A) Absorption (left black curve), normalized PL (right black curve), and normalized excitation spectra (red circles) of complex 421 (inset shows orange PL of 50 mM 421 in dry, air-saturated acetonitrile upon 532 nm excitation). (B) Time-correlated single-photon counting data (black, left y axis), transient absorption data at 390 nm (green circles, right y axis), and monoexponential fit of 1.96  0.04 ns (red). (O.D. refers to optical density). Adapted from Kjær, K. S.; Kaul, N.; Prakash, O.; Chábera, P.; Rosemann, N. W.; Honarfar, A.; Gordivska, O.; Fredin, L. A.; Bergquist, K.-E.; Häggström, L.; Ericsson, T.; Lindh, L.; Yartsev, A.; Styring, S.; Huang, P.; Uhlig, J.; Bendix, J.; Strand, D.; Sundström, V.; Persson, P.; Lomoth, R.; Wärnmark, K. Science 2019, 363, 249, with permission from American Association for the Advancement of Science. Copyright 2019.

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Fig. 57 (A) Absorption and emission spectra of complex 426 in THF under a nitrogen atmosphere. The insets show THF solutions of 426 under ultraviolet (UV; 254 nm), green (LED; 530 nm) and ambient light irradiation (left) and photoluminescence decay obtained via time-correlated single photon counting (right). (B) Kinetic model for the TADF process summarizing parameters determined via temperature-dependent emission and transient absorption spectroscopic studies. Adapted from Zhang, Y.; Lee, T. S.; Favale, J. M.; Leary, D. C.; Petersen, J. L.; Scholes, G. D.; Castellano, F. N.; Milsmann, C. Nat. Chem. 2020, 12, 345, with permission from Springer Nature Limited. Copyright 2020.

In 2020, a luminescent zirconium(IV) complex (426) constructed by two tridentate 2,6-bis(5-mesityl-3-phenyl-1H-pyrrol-2-yl) pyridine ligands was reported by Milsmann and coworkers.325 This complex shows excellent chemical stability to moisture and air. In nitrogen-saturated THF at room temperature, complex 426 exhibits an intense TADF (lPL ¼ 581 nm, PLQY ¼ 0.45) from the higher-lying singlet configuration with the dominant LMCT character. Meanwhile, an extremely long triplet LMCT excited state (350 ms) was observed as well (Fig. 57).325 The attractive photophysical properties of this complex will have a great potential in future electroluminescent applications.

8.02.3.11.6

Chromium(III) complexes

Jiménez and coworkers prepared three highly efficient NIR emissive chromium(III) complexes (427–429) showing dual emissive circularly polarized luminescence (lPL ¼ 725 and 759 nm for 427, lPL ¼ 725 and 751 nm for 428, lPL ¼ 727 and 751 nm for 429) with long lifetimes (around 1 millisecond) in aqueous solution at room temperature.326 The two NIR emission bands can be attributed to the transitions of 2T1 / 4A2 and 2E / 4A2 of the metal center. Using the chiral high-performance liquid chromatography, the racemic mixtures of these complexes were separated as their enantiomers. The obtained enantiomers exhibit high dissymmetry factors (0.18 for 427, 0.19 for 428, 0.17 for 429) and high PLQY (0.17 for 427, 0.14 for 428, 0.15 for 429).326 The earthabundant luminescent chromium(III) complexes will provide more research opportunities in the future.

8.02.4

Conclusion

This chapter summarizes the recent research progress of various LTMCs on their molecular design, photophysical properties, and applications in electroluminescence. Many kinds of new emissive transition-metal complexes, including iridium(III), platinum(II), gold(I/III), copper(I), ruthenium(II), rhenium(I), osmium(II), rhodium(III), palladium(II), silver(I), tungsten(VI), manganese(I/ II), iron(II/III), nickel(0/II), zirconium(IV), and chromium(III) complexes, have been developed and explored comprehensively.

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The photophysical properties (e.g., emission wavelength, PLQY, lifetime of the excited state, etc.) of these complexes can be well manipulated through the rational design of ligands coordinated to the metal center. Most of the luminescent transition-metal complexes possess efficient phosphorescence, metal-assisted delayed fluorescence, and/or thermally activated delayed fluorescence, which could help to make full use of the singlet and triplet excitons generated in OLEDs, thereby remarkably increasing the device performance. The emitters play a significant role in the lifespan of the OLEDs, and some of the transition-metal complexes have been proved to be excellent candidates for the long-lived electroluminescence. The future research focus should be paid on the less explored, cheap-metal-based emitters toward low-cost, environmental-friendly OLEDs. We believe that the diverse chemical structures and emissive properties of luminescent transition-metal complexes will continue to contribute to the future developments of electroluminescence and other photofunctional applications.

Acknowledgments We acknowledge the National Key R&D Program of China (2022YFE0104100), National Natural Science Foundation of China (61905120, 51873176), Start-up Fund for RAPs under the Strategic Hiring Scheme (P0035922), ITC Guangdong-Hong Kong Technology Cooperation Funding Scheme (TCFS) (GHP/038/19GD), CAS-Croucher Funding Scheme for Joint Laboratories, the Hong Kong Research Grants Council (PolyU 15305320), the Hong Kong Polytechnic University, Ms. Clarea Au for the Endowed Professorship in Energy (847S), and Research Institute for Smart Energy (CDAQ) for financial support.

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Mater. 2015, 47, 173. Lin, C.-H.; Hsu, C.-W.; Liao, J.-L.; Cheng, Y.-M.; Chi, Y.; Lin, T.-Y.; Chung, M.-W.; Chou, P.-T.; Lee, G.-H.; Chang, C.-H.; Shih, C.-Y.; Ho, C.-L. J. Mater. Chem. 2012, 22, 10684. Liao, J.-L.; Chi, Y.; Yeh, C.-C.; Kao, H.-C.; Chang, C.-H.; Fox, M. A.; Low, P. J.; Lee, G.-H. J. Mater. Chem. C 2015, 3, 4910. Lee, T.-C.; Hung, J.-Y.; Chi, Y.; Cheng, Y.-M.; Lee, G.-H.; Chou, P.-T.; Chen, C.-C.; Chang, C.-H.; Wu, C.-C. Adv. Funct. Mater. 2009, 19, 2639. Du, B.-S.; Liao, J.-L.; Huang, M.-H.; Lin, C.-H.; Lin, H.-W.; Chi, Y.; Pan, H.-A.; Fan, G.-L.; Wong, K.-T.; Lee, G.-H.; Chou, P.-T. Adv. Funct. Mater. 2012, 22, 3491. Yuan, Y.; Liao, J.-L.; Ni, S.-F.; Jen, A. K.-Y.; Lee, C.-S.; Chi, Y. Adv. Funct. Mater. 2020, 30, 1906738. Zhu, Z.-L.; Tan, J.-H.; Chen, W.-C.; Yuan, Y.; Fu, L.-W.; Cao, C.; You, C.-J.; Ni, S.-F.; Chi, Y.; Lee, C.-S. Adv. Funct. Mater. 2021, 31, 2102787. Indelli, M. T.; Chiorboli, C.; Scandola, F. Photochemistry and Photophysics of Coordination Compounds: Rhodium. In Photochemistry and Photophysics of Coordination Compounds I; Balzani, V., Campagna, S., Eds.; vol. 280; Springer Berlin Heidelberg: Berlin, Heidelberg, 2007; pp 215–255. Didier, P.; Ortmans, I.; Kirsch-De Mesmaeker, A.; Watts, R. J. Inorg. Chem. 1993, 32, 5239. Balzani, V.; Juris, A.; Venturi, M.; Campagna, S.; Serroni, S. Chem. Rev. 1996, 96, 759. Calogero, G.; Giuffrida, G.; Serroni, S.; Ricevuto, V.; Campagna, S. Inorg. Chem. 1995, 34, 541. Humbs, W.; Yersin, H. Inorg. Chem. 1996, 35, 2220. Hannon, M. J. Coord. Chem. Rev. 1997, 162, 477. Lo, K. K.-W.; Li, C.-K.; Lau, K.-W.; Zhu, N. Dalton Trans. 2003, 4682. Yam, V. W.-W.; Wong, K. M.-C. Chem. Commun. 2011, 47, 11579. Gildea, L. F.; Batsanov, A. S.; Williams, J. A. G. Dalton Trans. 2013, 42, 10388. Leung, S.-K.; Kwok, K. Y.; Zhang, K. Y.; Lo, K. K.-W. Inorg. Chem. 2010, 49, 4984. Sieck, C.; Tay, M. G.; Thibault, M.-H.; Edkins, R. M.; Costuas, K.; Halet, J.-F.; Batsanov, A. S.; Haehnel, M.; Edkins, K.; Lorbach, A.; Steffen, A.; Marder, T. B. Chem. Eur. J. 2016, 22, 10523. Wei, F.; Lai, S.-L.; Zhao, S.; Ng, M.; Chan, M.-Y.; Yam, V. W.-W.; Wong, K. M.-C. J. Am. Chem. Soc. 2019, 141, 12863. Zhu, Z.-Q.; Park, C.-D.; Klimes, K.; Li, J. Adv. Opt. Mater. 2019, 7, 1801518. Cao, L.; Klimes, K.; Ji, Y.; Fleetham, T.; Li, J. Nat. Photonics 2021, 15, 230. Cao, L.; Zhu, Z.-Q.; Klimes, K.; Li, J. Adv. Mater. 2021, 33, 2101423. Chow, P.-K.; Cheng, G.; Tong, G. S. M.; Ma, C.; Kwok, W.-M.; Ang, W.-H.; Chung, C. Y.-S.; Yang, C.; Wang, F.; Che, C.-M. Chem. Sci. 2016, 7, 6083. Chow, P. K.; Ma, C.; To, W.-P.; Tong, G. S. M.; Lai, S.-L.; Kui, S. C. F.; Kwok, W.-M.; Che, C.-M. Angew. Chem. Int. Ed. 2013, 52, 11775. Romanov, A. S.; Jones, S. T. E.; Yang, L.; Conaghan, P. J.; Di, D.; Linnolahti, M.; Credgington, D.; Bochmann, M. Adv. Opt. Mater. 2018, 6, 1801347. Fresta, E.; Carbonell-Vilar, J. 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Soc. 1961, 3735. Wrighton, M.; Ginley, D. Chem. Phys. 1974, 4, 295. Tanabe, Y.; Sugano, S. J. Phys. Soc. Jpn. 1954, 9, 766. Cotton, F. A.; Goodgame, D. M. L.; Goodgame, M. J. Am. Chem. Soc. 1962, 84, 167. Lawson, K. E. J. Chem. Phys. 1967, 47, 3627. Hardy, G. E.; Zink, J. I. Inorg. Chem. 1976, 15, 3061. Cotton, F. A.; Daniels, L. M.; Huang, P. Inorg. Chem. 2001, 40, 3576. Qin, Y.; She, P.; Huang, X.; Huang, W.; Zhao, Q. Coord. Chem. Rev. 2020, 416, 213331. Bortoluzzi, M.; Castro, J. J. Coord. Chem. 2019, 72, 309. Bortoluzzi, M.; Castro, J.; Enrichi, F.; Vomiero, A.; Busato, M.; Huang, W. Inorg. Chem. Commun. 2018, 92, 145. Jin, Z. M.; Tu, B.; Li, Y. Q.; Li, M. C. Acta Cryst. 2005, E61, m2510. Berezin, A. S.; Vinogradova, K. A.; Nadolinny, V. A.; Sukhikh, T. S.; Krivopalov, V. P.; Nikolaenkovac, E. B.; Bushuev, M. B. Dalton Trans. 2018, 47, 1657. Zhang, Y.; Liao, W.-Q.; Fu, D.-W.; Ye, H.-Y.; Liu, C.-M.; Chen, Z.-N.; Xiong, R.-G. Adv. Mater. 2015, 27, 3942. Zhang, Y.; Liao, W.-Q.; Fu, D.-W.; Ye, H.-Y.; Chen, Z.-N.; Xiong, R.-G. J. Am. Chem. Soc. 2015, 137, 4928. Sun, M.-E.; Li, Y.; Dong, X.-Y.; Zang, S.-Q. Chem. Sci. 2019, 10, 3836. Balsamy, S.; Natarajan, P.; Vedalakshmi, R.; Muralidharan, S. Inorg. Chem. 2014, 53, 6054. Wu, Y.; Zhang, X.; Xu, L.-J.; Yang, M.; Chen, Z.-N. Inorg. Chem. 2018, 57, 9175. Hu, G.; Xu, B.; Wang, A.; Guo, Y.; Wu, J.; Muhammad, F.; Meng, W.; Wang, C.; Sui, S.; Liu, Y.; Li, Y.; Zhang, Y.; Zhou, Y.; Deng, Z. Adv. Funct. Mater. 2021, 31, 2011191. Jiang, T.; Ma, W.; Zhang, H.; Tian, Y.; Lin, G.; Xiao, W.; Yu, X.; Qiu, J.; Xu, X.; Yang, Y. M.; Ju, D. Adv. Funct. Mater. 2021, 31, 2009973. Li, J.-Y.; Wang, C.-F.; Wu, H.; Liu, L.; Xu, Q.-L.; Ye, S.-Y.; Tong, L.; Chen, X.; Gao, Q.; Hou, Y.-L.; Wang, F.-M.; Tang, J.; Chen, L.-Z.; Zhang, Y. Adv. Funct. Mater. 2021, 31, 2102848. Xu, L.-J.; Sun, C.-Z.; Xiao, H.; Wu, Y.; Chen, Z.-N. Adv. Mater. 2017, 29, 1605739. Yan, S.; Tian, W.; Chen, H.; Tang, K.; Lin, T.; Zhong, G.; Qiu, L.; Pan, X.; Wang, W. Adv. Funct. Mater. 2021, 31, 2100855. Chen, J.; Zhang, Q.; Zheng, F.-K.; Liu, Z.-F.; Wang, S.-H.; Wu, A.-Q.; Guo, G.-C. Dalton Trans. 2015, 44, 3289. Qin, Y.; Tao, P.; Gao, L.; She, P.; Liu, S.; Li, X.; Li, F.; Wang, H.; Zhao, Q.; Miao, Y.; Huang, W. Adv. Opt. Mater. 2019, 7, 1801160. Herr, P.; Kerzig, C.; Larsen, C. B.; Häussinger, D.; Wenger, O. S. Nat. Chem. 2021, 13, 956. Lochenie, C.; Schötz, K.; Panzer, F.; Kurz, H.; Maier, B.; Puchtler, F.; Agarwal, S.; Köhler, A.; Weber, B. J. Am. Chem. Soc. 2018, 140, 700.

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314. Chen, J.; Browne, W. R. Coord. Chem. Rev. 2018, 374, 15. 315. Chábera, P.; Liu, Y.; Prakash, O.; Thyrhaug, E.; Nahhas, A. E.; Honarfar, A.; Essén, S.; Fredin, L. A.; Harlang, T. C. B.; Kjær, K. S.; Handrup, K.; Ericson, F.; Tatsuno, H.; Morgan, K.; Schnadt, J.; Häggström, L.; Ericsson, T.; Sobkowiak, A.; Lidin, S.; Huang, P.; Styring, S.; Uhlig, J.; Bendix, J.; Lomoth, R.; Sundström, V.; Persson, P.; Wärnmark, K. Nature 2017, 543, 695. 316. Chábera, P.; Kjaer, K. S.; Prakash, O.; Honarfar, A.; Liu, Y.; Fredin, L. A.; Harlang, T. C. B.; Lidin, S.; Uhlig, J.; Sundström, V.; Lomoth, R.; Persson, P.; Wärnmark, K. J. Phys. Chem. Lett. 2018, 9, 459. 317. Kjær, K. S.; Kaul, N.; Prakash, O.; Chábera, P.; Rosemann, N. W.; Honarfar, A.; Gordivska, O.; Fredin, L. A.; Bergquist, K.-E.; Häggström, L.; Ericsson, T.; Lindh, L.; Yartsev, A.; Styring, S.; Huang, P.; Uhlig, J.; Bendix, J.; Strand, D.; Sundström, V.; Persson, P.; Lomoth, R.; Wärnmark, K. Science 2019, 363, 249. 318. Malzkuhn, S.; Wenger, O. S. Coord. Chem. Rev. 2018, 359, 52. 319. Ziolo, R. F.; Lipton, S.; Dori, Z. J. Chem. Soc., Chem. Commun. 1970, 1124. 320. Caspar, J. V. J. Am. Chem. Soc. 1985, 107, 6718. 321. Frem, R. C. G.; Massabni, A. C.; Massabni, A. M. G.; Mauro, A. E. Inorg. Chim. Acta 1997, 255, 53. 322. Kunkely, H.; Vogler, A. J. Organomet. Chem. 2003, 684, 113. 323. Büldt, L. A.; Larsen, C. B.; Wenger, O. S. Chem. Eur. J. 2017, 23, 8577. 324. Wong, Y.-S.; Tang, M.-C.; Ng, M.; Yam, V. W.-W. J. Am. Chem. Soc. 2020, 142, 7638. 325. Zhang, Y.; Lee, T. S.; Favale, J. M.; Leary, D. C.; Petersen, J. L.; Scholes, G. D.; Castellano, F. N.; Milsmann, C. Nat. Chem. 2020, 12, 345. 326. Jiménez, J.-R.; Poncet, M.; Míguez-Lago, S.; Grass, S.; Lacour, J.; Besnard, C.; Cuerva, J. M.; Campaña, A. G.; Piguet, C. Angew. Chem. Int. Ed. 2021, 60, 10095.

8.03

Non-sacrificial photocatalysis

Qiang Liu and Li-Zhu Wua, a Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry & School of Future Technology, University of the Chinese Academy of Sciences, Beijing, P R China; and b State Key Laboratory of Applied Organic Chemistry, College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou, P R China a,b

© 2023 Elsevier Ltd. All rights reserved.

8.03.1 8.03.2 8.03.2.1 8.03.2.2 8.03.2.3 8.03.2.4 8.03.2.5 8.03.3 8.03.3.1 8.03.3.2 8.03.3.3 8.03.3.4 8.03.3.5 8.03.4 8.03.4.1 8.03.4.2 8.03.4.3 8.03.5 8.03.6 References

Introduction Non-sacrificial photocatalysis systems Introduction Merging photocatalysts and cobaloximes Platinum complexes Coupling photoredox catalysis with electrocatalysis Particle-based scavenger-free photocatalytic systems Non-sacrificial photocatalytic reactions CeC bond formation CeO bond formation CeN bond formation CeP bond formation Miscellaneous Acceptorless intramolecular dehydrogenation Dehydrogenative oxonation of alcohols Dehydrogenative olefination Dehydrogenative aromatization In situ applications of hydrogen in dehydrogenation reaction Conclusion

80 81 81 81 83 83 84 85 85 90 92 93 95 95 95 97 98 98 100 101

Abstract Photocatalytic CeH bond functionalization has become one of the most promising strategies to prepare complex molecules from readily available starting materials. However, the utilization of stoichiometric oxidants in the oxidative CeH bond functionalization reactions could deliver side reactions and/or stoichiometric wastes, thereby reducing the appeal of these transformations. One general strategy that has emerged to overcome this constraint is non-sacrificial photocatalysis which allows electrons and protons released from the photoredox cycle to produce hydrogen gas. The non-sacrificial strategy offers a convenient protocol for clean and highly efficient bond formations from a variety of ReH bonds. It allows oxidant-sensitive compounds that would otherwise be very difficult to synthesize. In this chapter, recent dramatic developments of nonsacrificial photocatalysis for dehydrogenations and dehydrogenative coupling reactions are discussed via the established scenarios, the types of bond formations, and specific reaction classes. We focus on reactions, the most prominent mechanistic pathways, and the representative products under non-sacrificial photocatalytic conditions. The goal of the chapter is to highlight recent advances in this field and offer an up-to-date reference for chemists exploring greener synthetic strategies in this interesting area.

8.03.1

Introduction

Photocatalysis has emerged in the past decade as a powerful tool for constructing new bonds that are inefficient or thermodynamically unfavorable in modern organic chemistry.1 Whereas many organic compounds are transparent to visible (400–700 nm) and near-ultraviolet (300–400 nm) radiation, photocatalysts absorbing photons in these ranges are always essential to the photocatalysis of organic reactions.2 In most cases, substrates are activated by the photocatalyst-dominated photoredox cycles, in which the photocatalyst undergoes both ‘quenching’ and ‘regenerative’ electron transfer.3 The radical ion intermediates produced from the photoredox cycles can undergo a multitude of pathways to form new bonds under remarkably mild conditions. Among the variety of photoredox strategies for bond formations, the redistribution of ReH bonds from readily available starting materials into more sophisticated value-added architectures is sustainable due to its high step and atom economy.4 Mechanistically, the generation of radical or radical cation species from the inert ReH usually requires extra electron acceptors to be added to complete the photoredox cycle. As a result, stoichiometric oxidants, such as dioxygen, peroxides, hypervalent iodonium salts, and fluorinating agents, have

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Fig. 1 General scheme of a non-sacrificial photocatalytic strategy. Cross-Coupling Hydrogen Evolution Reactions. Dong, K.; Liu, Q.; Wu, L.-Z. Acta Chim. Sinica 2020, 78, 299–310.

been used in this process, which could deliver side reactions and/or stoichiometric wastes, reducing the appeal of these transformations. One general strategy that has emerged to overcome this constraint is non-sacrificial photocatalysis which allows electrons and protons released from the photoredox cycle to produce hydrogen gas.5 Recent developments in this area are mainly focused on acceptorless intramolecular dehydrogenations (AID) and cross-coupling hydrogen evolution (CCHE) reactions. In these transformations, various classes of organic compounds are produced with H2 evolution (Fig. 1). The non-sacrificial strategy offers a convenient protocol for clean and highly efficient bond formations from a variety of ReH bonds. It allows oxidant-sensitive compounds that would otherwise be very difficult to synthesize. In addition, hydrogen production becomes a great added value once the reaction is employed in the mass production of chemicals. This chapter presents and discusses recent achievements regarding the development of non-sacrificial photocatalysis for dehydrogenations and dehydrogenative coupling reactions. The first part of the chapter addresses the established scenarios for nonsacrificial photocatalysis, which are listed as parallels to make the comparison as easy as possible. In the second part (Section 8.03.3), the emphasis is put on the types of bond formations and specific reaction classes arising from scavenger-free photochemical strategies. Our goal is to highlight recent advances in this field and offer an up-to-date reference for chemists interested in inventing greener methods for organic transformations.

8.03.2

Non-sacrificial photocatalysis systems

8.03.2.1

Introduction

The inspiration for the non-sacrificial dehydrogenative strategies arises from a plethora of research on the development of hydrogen evolution catalysts (HECs) for the light-induced splitting of water.6 The non-sacrificial photocatalytic systems may be quite different, but most of them contain a HEC and a photocatalyst, in which substrates are activated by the photocatalystdominated photoredox cycles and hydrogen gas is produced via HEC-involved hydrogen evolution cycles.

8.03.2.2

Merging photocatalysts and cobaloximes

Cobaloxime complexes6c have been successfully employed as HECs for electrocatalytic and photocatalytic H2 production. In 2014, Wu and co-workers reported visible-light-driven CCHE reaction between N-phenyl-1,2,3,4-tetrahydroisoquinolines (THIQs) and indoles, in which cobaloxime Co(dmgH)(dmgH2)Cl2 was used as a catalyst to capture the electrons and protons released during the coupling reaction (Fig. 2).7 The dual photoredox catalyst system, containing organic dye eosin Y as a photocatalyst and Co(dmgH)(dmgH2)Cl2 as a HEC, gave good to excellent yields of cross-coupling products and an equivalent amount of H2. The ligand biacetyl oxime is inexpensive, and cobalt is earth-abundant. In addition, tunable axial ligands of cobaloxime enable the enhancement of hydrogen evolution rate and TONs (turnover numbers), thereby providing distinct possibilities for an ideal HEC. As a result, a series of cobaloximes (Fig. 3) have been successfully paired with various photocatalysts (Fig. 4).8 Regardless of the event of single-electron oxidation of the substrate in the photoredox cycle, the HEC cobaloxime [Co(III)] could be reduced stepwise to [Co(I)] species by either the excited photocatalyst or the reduced photocatalyst. Next, the low-valence [Co(I)]

Fig. 2

CCHE between THIQs and indoles using eosin Y/Co(dmgH)(dmgH2)Cl2 system.

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Non-sacrificial photocatalysis

Fig. 3

Cobaloximes commonly used in non-sacrificial photocatalysis.

Fig. 4

Photocatalysts commonly used in non-sacrificial photocatalysis.

species can be protonated to form [Co(III)
H] species. From this key [Co(III)
H] intermediate, H2 production will be facilitated through the protonation of the hydride (Fig. 5).8 Due to the efficiency of hydrogen evolution could be regulated easily by ligand modification, the photocatalyst/cobaloxime systems have been the most extensively used non-sacrificial photocatalytic protocol for dehydrogenations up to now.

Non-sacrificial photocatalysis

Fig. 5

83

General mechanism paradigm of non-sacrificial photocatalysis with cobaloxime.

8.03.2.3

Platinum complexes

Square-planar platinum(II) complexes are unique from a photochemical perspective because they can be converted to long-lived triplet excited states by visible-light irradiation. In addition, the vacant coordination sites at the platinum center are sensitive to environmental changes and capable of associating with substrates or intermediates.9 Excitation of these complexes with visible light can form more reactive species that have enough lifetime to proceed with electron transfer or hydrogen abstraction event, thereby enabling binding of hydrogen atoms or protons and efficient hydrogen production from scavenger-free systems. In earlier reports, binuclear platinum(II) diphosphite complexes Pt2(m-P2O5H2)44
(Fig. 6) established the first photocatalytic system of platinum complexes that converts isopropyl alcohol into acetone and hydrogen.10 In the current century, platinum (II) terpyridyl complexes (Fig. 6) were used as the sole catalyst for the visible-light-driven scavenger-free synthesis of heteroaromatics from corresponding dihydro-compounds.11

8.03.2.4

Coupling photoredox catalysis with electrocatalysis

The combination of photochemistry with electrochemistry, known as photoelectrocatalysis (PEC), is a swiftly emerging research field that provides new opportunities to green synthesis.12 Particularly, a PEC is a device capable of converting the inert R–H species into more sophisticated value-added molecules and does not require an external oxidant.13 Differing from the use of HECs such as cobaloximes, a cathode with low H2 evolution over potential, usually Pt, acts as an acceptor for electrons and protons to generate H2. PEC for scavenger-free reactions can be broadly classified into the following three categories: (i) electrochemical hydrogen evolution coupling with a PC-involved photoredox cycle (Fig. 7A), (ii) PEC using a photoactive anode (photoanode) (Fig. 7B), (iii) photoredox cycles employing electrochemically generated intermediates as the PCs (Fig. 7C). Typically, PEC for non-sacrificial reactions is composed of an anode, a cathode, and a photocatalyst, as shown in Fig. 7A. After reductive quenching of the excited photocatalyst with the substrate, the reduced photocatalyst is then regenerated by anodic oxidation. The radical cation of the substrate undergoes subsequent reactions to provide a meaningful dehydrogenative product. Meanwhile, the cathode could replace HEC as the hydrogen production center, where protons are reduced to H2.14 Scavenger-free reactions can also be processed in photoelectrosynthesis cells (Fig. 7B).15 In the photoelectrosynthesis cell, the photoanode is coated in a photoresponsive (typically, semiconductor) material whose bandgap corresponds to the energy of a visible-light photon. Under applied potential, the photoanode displays high oxidization capability upon light irradiation, which facilitates substrate activation via electron transfer. At the same time, the cathode proceeds reduction of proton to H2. For nonsacrificial PEC employing electrochemically generated intermediates as the PCs, the identification of an electromediated

Fig. 6

Platinum(II) complexes as the sole catalyst for non-sacrificial photocatalysis.

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Non-sacrificial photocatalysis

(A)

(B)

(C)

Fig. 7

General categories and mechanism paradigms of non-sacrificial PEC.

photocatalyst (e-PC) is curial.16 The pre-photocatalyst should undergo a facile one-electron oxidation event to yield a stable, lightharvesting intermediate at the cathode under applied potential. In addition, the electrochemically generated intermediate should have a sufficiently long-lived excited state to interact with a substrate. The structure of the reported e-PCs and corresponding electrochemical generated intermediates (e-PCþ$) is described and collected in Fig. 7C. Here, an e-PC could be facilely anode-oxidized to its cation radical form e-PCþ$, which is excited to produce *e-PCþ$ as an ultra-strong oxidant capable of various substrates.

8.03.2.5

Particle-based scavenger-free photocatalytic systems

Recently heterogeneous photocatalysts,17 represented by nano-scaled semiconductors and plasmonic-metal nanoparticles, have drawn much attention due to their nature of easy recycling and simple chemical workup. As known to all, the reaction products from heterogeneous photocatalysis can be separated more conveniently with respect to the homogeneous catalysts, and so it

Non-sacrificial photocatalysis

85

provides a greener way for organic chemical transformation. Different from molecular photocatalyst, the bandgap, surface active sites, and charge-carrier dynamics of semiconductor nanoparticles can be rationally tuned by tailoring size, composition, and morphology, thus regulating the activity of heterogeneous photocatalysis.18 Immense effort has been devoted to the application of heterogeneous nanoparticles in the field of solar hydrogen production due to its unique advantages facilitating both light harvest and hydrogen evolution. As excellent H2 evolution photocatalysts, the photogenerated holes of the particles enable substrate oxidation, thereby consuming holes and producing corresponding radical cations that react to give products (Fig. 8A). In addition, the electron-hole recombination can be retarded by decorating semiconductors with metal nanoparticles (Fig. 8B). For the decorated nanoparticles, a redshift and enhancement of the absorption may occur due to the localized surface plasmon resonance (LSPR) effect.19 Thus, heterogeneous nanoparticles can produce an intramolecular dehydrogenation or cross-coupling dehydrogenative reaction without any external oxidant under photocatalytic conditions. We summarize some representative reports of acceptorless photocatalytic reactions by taking particles as the catalyst. As shown in Table 1, the particles can catalyze both photo-induced substrate oxidation and hydrogen evolution in most cases if the wavelength of the light resource is short enough to excite electrons across the bandgap of the semiconductors. More details about the reactions will be discussed in the next section.

8.03.3

Non-sacrificial photocatalytic reactions

8.03.3.1

CeC bond formation

The formation of a carbon-carbon (CeC) bond is paramount in the synthesis of biologically relevant molecules, new materials, and fine chemicals. The oxidative coupling of inert CeH bonds to form new CeC bonds are particularly valuable as stoichiometric external oxidants can be avoided, and dihydrogen is the sole by-product. The photocatalytic CCHE between various tetrahydroisoquinolines and indoles were achieved by Wu and co-workers, who explored an efficient catalytic system consisting of eosin Y as the photocatalyst and graphene-supported RuO2 nanocomposite (G-RuO2) as the HEC (Fig. 9A).20 Under visible light irradiation at room temperature, the catalytic system gave good to excellent yields of cross-coupling products and an equivalent amount of H2. Compared with noble-metal HEC G-RuO2, cobaloxime is inexpensive, and cobalt is earth-abundant. The replacement of GRuO2 with cobaloximes enables CCHE reaction in a low-cost, highly efficient manner. Indeed, Co(dmgH)(dmgH2)Cl2 shows higher efficiency to capture the electrons and protons eliminated from the same reaction, thus providing a noble-metal-free homogeneous strategy for the CCHE reaction (Fig. 9A).7 The combination of photocatalyst and cobaloxime complex is the most widely used catalytic system for visible-light-driven CCHE reactions. For example, visible-light-driven site-specific functionalization of glycine esters with b-keto esters or indole derivatives could be realized successfully by merging Ru(bpy)3(PF6)2 with Co(dmgH)2PyCl (Fig. 9B).41 The direct alkylation of a-CeH of ethers with alkanes is harder to achieve than functionalizing a-CeH of amines due to their higher oxidation potentials. As a further demonstration of the utility of CCHE, the Wu group reported alkylation of isochromans with b-keto esters using a more strongly oxidative 9-mesityl-10-methylacridinium perchlorate (Acrþ-Mes ClO4
) as the photocatalyst, cobaloxime Co(dmgBF2)2(MeCN)2 as the HEC and Cu(OTf)2 as the activator of b-keto esters (Fig. 9C).42 Photocatalytic hydrogen-atom transfer (HAT) processes provide the driving force for the activation of strong CeH bonds and enlarge the versatility of CCHE reactions. Tetra-n-butylammonium decatungstate (TBADT) in the excited state has recently emerged as a promising photocatalyst capable of abstracting a hydrogen atom from inert Csp3eH bonds to form carbon radicals. The synergistic combination of HAT photocatalyst TBADT with HEC Co(dmgH)(dmgH2)Cl2 delivered a photocatalytic CCHE strategy for site-selective alkenylation of alkanes using aryl alkenes (Fig. 9D).43 Subsequently, a photoinduced dehydrogenative coupling reaction of benzylic and aldehydic CeH bonds was reported by Murakami and co-workers (Fig. 9E).44 The reaction straightforward

(A)

(B)

Fig. 8 The mechanism for scavenger-free reactions using particle-based photocatalytic systems. Semiconducting quantum dots for artificial photosynthesis. Li, X.-B.; Tung, C.-H.; Wu, L.-Z. Nat. Rev. Chem. 2018, 2, 160–173.

86 Table 1

Non-sacrificial photocatalysis Typical scavenger-free photocatalytic reactions using particles.

Entry

PS & HEC

Light source

Reaction type

References

1

Eosin Y & G-RuO2

Csp2eCsp3 coupling

20

2

Eosin Y & 2D-MoS2

Csp2eCsp3 coupling

21

3

Eosin Y & Ni-MPA

Alcohols dehydrogenation

22

4

Pt-TiO2

Csp3eCsp3 coupling

23

5

Pt-TiO2

dehydrogenative lactonization

24

6

Pt-TiO2

Alcohols dehydrogenation

25

7 8

Pt-TiO2 Pt-TiO2

Aromatic-ring amination Dehydrogenative aromatization

26

9

Pd-TiO2

Csp2eCsp3 coupling

23b

10

Pd-TiO2

Csp3eCsp3 coupling

28

11

Rh-TiO2

Csp2eCsp3 coupling

29

12

Rh-TiO2

Dehydrogenative aromatization

30

13

CdS

Csp3eCsp3 coupling

31

14

CdS-Ti3C2Tx

Alcohols dehydrogenation

32

15 16

Ni-CdS Ni-CdS

Alcohols dehydrogenation Amines dehydrogenation

33

17

Ni5CdS-MIL-101

Benzylamines dehydrogenation

35

18

MoS2-CdS

Methanol Csp3eCsp3 coupling

36

19

CdSe QDs

SeS coupling

37

20

CdSe QDs, Ni2þ

Alcohols dehydrogenation

38

21

Ru-ZnIn2S4

Methylfurans Csp3eCsp3 coupling

39

22

Pt-Cage-Based LHS

Hg lamp (l > 450 nm) Green LEDs (l ¼ 525 nm) Green LEDs (l ¼ 525 nm) Xe lamp (l > 350 nm) Xe lamp (l > 350 nm) Xe lamp (l > 300 nm) Xe lamp Xe lamp l > 300 nm Xe lamp l ¼ 365 nm LEDs (l ¼ 368 nm) Hg lamp (l ¼ 365 nm) Blue LEDs (l ¼ 453 nm) Blue LEDs (l ¼ 440 nm) Xe lamp (l > 420 nm) Xe lamp (l > 420 nm), blue LEDs Xe lamp (l  420 nm) Blue LEDs (l ¼ 470 nm) Xe lamp (l > 420 nm) Hg lamp (l > 400 nm) Purple LEDs (l ¼ 410 nm) Blue LEDs (l ¼ 455 nm) Xe lamp

Csp2eP coupling

40

27

34

synthesized a-aryl ketones with visible light irradiation in the presence of Ir[dF(CF3)ppy]2(dtbbpy)PF6 and nickel catalyst NiBr2(dtbbpy). In the reaction, a bromine radical is produced from the excited iridium bromide complex (the iridium bromide complex is formed by an exchange between Ir[dF(CF3)ppy]2(dtbbpy)PF6 and NiBr2(dtbbpy). The bromine radical abstracts hydrogen atoms from benzylic and aldehydic CeH bonds to furnish corresponding radical species. The nickel-involved catalytic cycle is responsible for both radical crossing-coupling and hydrogen evolution. These efficient and clean methods for the construction of Csp3eCsp2 and Csp3eCsp3 bonds fully demonstrated the superiority of the CCHE reactions. Also, they attracted more attempts of heterogeneous materials to catalyze these reactions recently. For instance, the results of De showed that the organic dye eosin, combined with mixed-phase 2D-MoS2 nanosheets, could achieve the CCHE reaction between tetrahydroisoquinoline and indoles under the driving of visible light.21 The CCHE system could also be used for the visible-light-mediated coupling reaction of lignocellulose-derived methylfurans. Wang’s group successfully realized the coupling reaction of methyl furan with photocatalyst Ru-ZnIn2S4 and obtained a series of coupling products that can become diesel fuel precursors by further reactions.39 Additionally, Yoshida studied the CCHE reaction between olefins and tetrahydrofuran under near-UV irradiation using platinum-loaded TiO2 as the catalyst,23a and also developed selective acetonylation of toluene with acetone using metal-nanoparticle-deposited TiO2 catalysts (Fig. 9F).23b It was found that the nature of the metal cocatalysts deposited on the TiO2 surface significantly affected the selectivity of the CCHE products. Pd allowed the CCHE between the aromatic ring

Non-sacrificial photocatalysis

(A)

(B)

(C)

(D)

(E)

(F)

(G)

(H)

(I)

(J)

Fig. 9

Photocatalytic CCHE reactions for the construction of Csp3eCsp2 and Csp3eCsp3 bonds.

87

88

Non-sacrificial photocatalysis

of toluene and acetone with high regioselectivity. Pt nanoparticles, on the contrary, promoted the CCHE between the methyl group of toluene and acetone selectively. Photoelectrochemical cells have also been explored as effective CCHE protocols for Csp3eCsp2 bond formations. Xu and coworkers achieved a chloridion-mediated photoelectrochemical CCHE reaction of heteroaromatic Csp2eH and aliphatic Csp3eH bonds without the addition of metal catalysts (Fig. 9G).45 Mechanistically, chlorine atoms are produced through light irradiation of anodically generated Cl2 from Cl
, and then abstract hydrogen atoms from Csp3eH bonds to afford C-radicals. The latter undergo Minisci alkylation to afford the final alkylated heteroarene products. Furthermore, a-CeH functionalization of ethers with isoquinolines and other azoles has been developed with PEC strategy by the Lambert group (Fig. 9H).46 In this system, the trisaminocyclopropenium ion (TACþ, Fig. 7C) is electrochemically oxidized to form a cyclopropenium radical dication intermediate which proceeds HAT from the substrate ethers, thus demonstrating a new reactivity mode for this electromediated photocatalyst. In addition, a PEC method for the CeH alkylation of heteroarenes with organotrifluoroborates has been realized without any external oxidant (Fig. 9I).14 Under the PEC conditions, organotrifluoroborates are oxidized to alkyl radicals by photoexcited Mes-Acrþ which is regenerated by anodic oxidation at a reticulated vitreous carbon (RVC) anode. The alkyl radicals react with the heteroarenes to afford the final products. At the cathode, protons are reduced to generate H2, obviating the demand for external oxidants. In a study by Ackermann and coworkers, the coactions of electrosynthesis and photoredox catalysis provided a chemical oxidantfree approach for the generation of the CF3 radical, which readily participates in intermolecular CeH trifluoromethylations of unactivated arenes and heteroarenes (Fig. 9J).47 Csp2eCsp2 bond formations with CCHE strategies have also undergone considerable developments in recent years. Wu and coworkers reported the earliest example of this approach in 2016, who combined photocatalyst Ir(ppy)3 and HEC Co(dmgH)2(4CO2MePy)Cl to accomplish the intramolecular cyclization of N-aryl enamine to generate indoles and release molecular hydrogen (Fig. 10A).48 Most recently, they offered a new solution to construct phenanthrene skeletons via Csp3eH bond functionalization in a photoredox/cobalt-catalyzed manner (Fig. 10B).49 This reaction relies on the keto-enol tautomerism of 1,3-dicarbonyl moiety with a relatively lower oxidation potential that could be activated by the excited photocatalyst. The critical intermediate a-carbonyl radical, generated from the aryl-3-oxopropanoate through one-electron oxidation-deprotonation sequence, is able to occur an intramolecular homolytic aromatic substitution to afford cyclized radical intermediate for highly substituted 10-phenanthrenols in good to excellent yields. Notably, The CCHE protocol for intramolecular cyclization could proceed without an external photocatalyst or HEC. For example, the Dong team adopted this strategy to realize an efficient and general dehydrogenative cyclization of o-teraryls without additional oxidants under UV light conditions (Fig. 10C).50 Herein, the cobaloxime complex Co(dmg(BF2))2(H2O)2 could play two roles as photocatalyst and hydrogen-producing active center. Even in the absence of a photocatalyst and a HEC, the Zhang group realized the dehydrogenation coupling reaction of o-phenylfuranyl/thienylpyridines/pyrimidines and successfully obtained a variety of benzofuranoquinolines and its analogues with UV light at room temperature under an argon atmosphere (Fig. 10D).51 Intermolecularly, a self-dehydrogenation coupling of styrene compounds to 1,2-dihydro-1-aryl naphthalene was achieved by using Acrþ-Mes as a photocatalyst and cobaloxime Co(dmgH)2PyCl as a HEC under visible light irradiation (Fig. 10E).52 In the proposed mechanism, the benzylic radical gets oxidized to a cation by the Co(II) to provide Co(I), which is then proposed to perform the deprotonation to establish an olefin. Moutet and Reverdy described a similar transformation through the combination of photochemistry with electrochemistry (Fig. 10F).53 The electrochemically-generated phenothiazine (PTZ) radical cation is excited to realize the oxidation of 1,1-diphenylethylene. The obtained 1,1-diphenylethylene radical cation undergoes [4 þ 2] cycloaddition, ultimately furnishing the product. Moreover, the Lei group also achieved the intermolecular dehydrogenative cyclizations between styrenes and alkyne derivatives with a dual catalytic system containing a photocatalyst and a cobalt-based HEC (Fig. 10G).54 The visible-light-driven CCHE strategy also has emerged as a powerful tool for the dehydrogenative alkenylation of aromatic compounds in an oxidant-free manner, thereby precluding byproducts arising from external oxidants. For instance, Lei and coworkers achieved the photo-induced CCHE reactions between styrene compounds and electron-rich arenes (Fig. 10H)55 with a dual catalytic system containing a Acrþ-Mes photocatalyst and a cobaloxime HEC. In addition, a similar dual catalytic system was applied to synthesize substituted 1,3-diene compounds from vinylarenes and ketene dithioacetals by the Li team (Fig. 10I).56 Following these studies, Wu reported a photocatalytic method for the site-selective alkenylation of aldehydes with aryl alkenes combining TBADT, Co(dmgH)(dmgH2)Cl2, and 2,6-lutidine (Fig. 10J).43 As the instability of starting materials or products to the oxidants in these coupling reactions, the use of external oxidant would lead to the increase of byproducts or even the failure of the expected reaction. Therefore, highly efficient and specific chemical transformation can be realized by using a visible-light-driven CCHE scheme under mild conditions. Carboxylic acids are available alkyl radical precursors for photocatalytic coupling reactions, wherein alkyl radicals can be accessed from carboxylic acids via single electron-transfer oxidation and CO2 evolution. The research results of Li, Xu, Wu et al. showed that carboxylic acid derivatives could remove CO2 from the dehydrogenation coupling system via photocatalysis and then generate intermolecular coupling reactions or intramolecular elimination reactions and released H2 (Fig. 11). Li and coworkers reported a near-ultraviolet-driven decarboxylative CeH trifluoromethylation of (hetero)arenes with trifluoroacetic acid as a trifluoromethyl source in the presence of a catalytic amount of Na2S2O8 using Rh-modified TiO2 nanoparticle as a photocatalyst and HEC, in which H2 release is an important driving force for the reaction (Fig. 11A).29 Later, Xu and co-workers reported an electrophotocatalytic method for the direct decarboxylative CeH carbamoylation of N-heteroarenes with oxamic acids by using photocatalyst 4CzIPN (Fig. 11B).57 In the transformation, the requisite carbamoyl radical for the carbamoylation reaction is generated through the thermodynamically favorable single-electron transfer (SET) oxidation of the oxamate substrate by the excited-state

Non-sacrificial photocatalysis

(A)

(B)

(C)

(D)

(E)

(F)

(G)

(H)

(I)

(J)

Fig. 10

Photo-mediated CCHE reactions for the construction of Csp2eCsp2 bonds.

89

90

Non-sacrificial photocatalysis

(A)

(B)

(C)

(D)

Fig. 11

Photo-induced decarboxylative reactions of carboxylic acids.

photocatalyst, followed by decarboxylation. Notedly, the direct decarboxylative CeH alkylation of N-heteroarenes with carboxylic acids could be catalyzed by CeCl3 under PEC conditions (Fig. 11C).57 The alkylation reaction commences with the anodic oxidation of Ce3þ to Ce4þ, which coordinates with the carboxylic acid and then undergoes photoinduced ligand-to-metal charge transfer (LMCT) to revert to Ce3þ and afford a carboxyl radical. Following these researches, the Li group provided a valuable method for alkylation of heteroarenes with the decarboxylative coupling of carboxylic acids using a dual catalytic system of photoredox/hydrogen evolution.58 Furthermore, Wu and coworkers reported a similar CCHE strategy for Heck-type coupling of alkenes (e.g., vinyl arenes, vinyl heteroarenes, vinyl silanes, and boronates) with various alkyl carboxylic acids (Fig. 11D).59 The significant advancement in the decarboxylative Heck coupling protocol was further illustrated by the highly selective three-component coupling of aliphatic acids, acrylates, and vinyl arenes. The key to the success relies on the unique proton/electron-accepting ability of the cobaloxime catalyst, which is beyond the capacity of conventional oxidants. The above transformations required higher reaction temperature and equivalent or excessive oxidant, as well as considering the separation of oxidant residue and pollution-free treatment if the traditional oxidative decarboxylation method is adopted. However, the decarboxylative coupling of carboxylic acids using the CCHE strategy could be highly efficient in an oxidantfree manner and only release H2 and CO2 as the byproducts.

8.03.3.2

CeO bond formation

Phenols are essential fine chemicals widely used in industrial chemicals. Their production often requires multi-step energyconsuming procedures or severe reaction conditions that generate large amounts of toxic waste. The scavenger-free strategy might provide a highly feasible and atom-efficient pathway for phenols synthesis and produces H2 as the only byproduct. Wu and coworkers achieved this by using the dual catalyst system to prepare phenols from benzene and water by selecting a robust oxidative 3-cyano-1-methylquinolinum ion (QuCNþ) as the photocatalyst to activate the inert CeH bond of benzene under UV light irradiation (Fig. 12A).60 In the same way, the synthesis of aryl ethers from the direct coupling of aromatics with alcohol was realized by the dual catalyst system consisting of photocatalyst QuCNþ and HEC Co(dmgBF2)2(CH3CN)2 (Fig. 12B).61 Particularly, intramolecular alkoxylation of 3-phenylpropanols was also performed well to form chromanes under mild conditions. Besides, Lei and co-workers utilized a visible-light-driven CCHE strategy to realize the anti-Markovnikov selective oxidation of b-alkyl styrenes with water and alcohols, affording the corresponding carbonyl compounds (Fig. 12C)62 and enol esters (Fig. 12D)63 respectively. Mechanistically, water and alcohols served as nucleophiles to trap the crucial alkene radical cation intermediates in an Anti-Markovnikov manner. Several groups like Zhu,64 Lei,65 and Luo66 et al. successively developed visible-light-promoted lactonization of 2-aryl benzoic acid to construct benzo-3,4-coumarins by the CCHE pathway combining photoredox catalysis with cobalt catalysis (Fig. 12E). Inspired by the initial success, many chemists are focused on these oxidant-free systems under environmentally benign conditions beyond traditional catalysis using stoichiometric or excess amounts of oxidants. In 2019, Xiao and Shi also developed

Non-sacrificial photocatalysis

91

(A)

(B)

(C)

(D)

(E)

(F)

(G)

(H)

Fig. 12

Photocatalytic scavenger-free reactions for the construction of CeO bonds.

a photoredox/cobalt-catalyzed phosphinyloxy radical addition/cyclization cascade reaction to synthesize phosphaisocoumarins between arylphosphinic acids or arylphosphonic acid monoesters and alkynes with concomitant formation of H2 under mild conditions (Fig. 12F).67 This strategy promoted the application of phosphinyloxy-radical-mediated synthetic transformations via visible light irradiation to produce structurally diverse phosphorus heterocycles. Site-selective chemical modification of unactivated aliphatic CeH bonds represents a tremendous challenge in organic chemistry. The use of dual organic photoredox and cobalt catalysis to access site-selective homobenzylic oxygenation has not been approached until recently. The study of Nicewicz group demonstrated that leveraging the reactivity of benzylic Csp3eH bonds to achieve reactivity at the homobenzylic position can be accomplished using the photocatalytic dehydrogenative method (Fig. 12G).68 In the catalytic system, alkyl arenes undergo dehydrogenation to vinyl benzenes which are followed by anti-Markovnikov Wacker-type oxidation to grant benzyl ketone products. Furthermore, Sayama and co-workers achieved the photoelectrochemical dimethoxylation of furan in excellent faradaic efficiency up to 99% on the BiVO4/WO3 photoanode with moderate oxidation potential (Fig. 12H).69 Mechanically, oxidation of bromide anions by the photoanode affords a pool of bromine cations. By using Brþ/Br
as a mediator, the dimethoxylated product is obtained when furan in MeOH has been added.

92

Non-sacrificial photocatalysis

8.03.3.3

CeN bond formation

The photocatalytic CCHE strategy also expanded to the synthetically valuable CeH/NeH cross-coupling reaction without external oxidant under mild conditions. For instance, Yoshida and co-workers reported a direct amination of benzenes with aqueous ammonia by using Pt-loaded TiO2 as photocatalyst and HEC, wherein anilines are produced with dihydrogen evolution (Fig. 13A).26 In the reaction, ammonia is oxidized by the photo-induced hole on the TiO2 surface to yield an amide radical (,NH2), which directly reacts with the aromatic ring to yield the corresponding aniline. At the same time, the dihydrogen was liberated through simultaneous proton reduction by the photo-formed electron.70 Later on, photocatalytic amination of benzene was performed using an organic photocatalyst QuHþ in conjunction with Co(dmgBF2)2(CH3CN)2 as a HEC (Fig. 13B).60 According to their proposal, a benzene radical cation is formed through single-electron oxidization by the excited-state photocatalyst, and ammonia is activated through the formation of BF3$NH3. Additionally, direct cross-coupling of simple arenes with heterocyclic amines under mild conditions is undoubtedly important for CeN bonds construction. This transformation was achieved by Lei and co-workers, who developed an oxidant-free and selective Csp2eH amination between electron-rich arenes and heterocyclic azoles utilizing an acridinium photoredox catalyst and a cobaltbased HEC in a CCHE manner (Fig. 13C).71 This system works without any sacrificial oxidant and is highly selective for Csp2eH activation, whereas Csp3eH bonds are unaffected. Following this work, the Lambert group delivered a PEC strategy for nonsacrificial coupling of simple arenes with azoles by using TACþ as an e-PC (Fig. 7C), which could be electrochemically oxidized to form a cyclopropenium radical dication intermediate. The radical dication undergoes photoexcitation with visible light to produce an excited state species with oxidizing power sufficient to oxidize benzenes via single electron transfer, resulting in CeH/NeH coupling with azoles (Fig. 13D).72 Soon after, Hu and co-workers also reported a PEC method for similar arene CeH amination by employing hematite (a-Fe2O3) photoanode, which was rendered highly oxidizing under blue LED irradiation (Fig. 13E).73 In addition, the Lei team employed the combination of photocatalyst Acrþ-Mes and HEC Co(dmgH)(dmgH2)Cl2 to (A)

(B)

(C)

(D)

(E)

(F)

(G)

Fig. 13

Visible-light-driven CCHE reactions of NeH with aromatics.

Non-sacrificial photocatalysis

93

achieve a site-selective amination of 2-arylimidazoheterocycles on the C3 position with 1H-pyrazole compounds (Fig. 13F).74 Later, Zhou and co-workers described a similar cooperative catalyst system for the visible-light-driven amination of arenes. The scope of amines was extended to common primary amine and even unstable benzophenone imines (Fig. 13G).75 Xiao expanded the CCHE strategy to a 5-exo-cyclization/addition/aromatization cascade of b,g-unsaturated hydrazones by cooperative photocatalysis and cobalt catalysis (Fig. 14A).76 The catalytic system enabled a facile synthesis of various dihydropyrazolefused benzosultams without an external oxidant and generated the sole byproduct H2. Additionally, under the dual catalytic system of the photocatalyst Acrþ-Mes ClO4
and the HEC Co(dmgH)2PyCl, N-vinylazoles could be successfully constructed from the direct coupling of styrenes with N-heterocyclic nucleophiles such as pyrroles, pyrazoles, benzotriazoles, and indazoles (Fig. 14B).63 Similar CeN cross-coupling reactivity was later demonstrated to form multi-substituted 3,4-dihydroisoquinoline derivatives by the dehydrogenative [4 þ 2] annulation between aromatic ketimine derivatives and styrenes with high regioselectivity and trans diastereoselectivity even if the Z/E mixture of alkenes were employed (Fig. 14C).77 Meanwhile, Li utilized the photoredox/cobaloxime catalytic system to generate iminyl radical, which initiates the cascade CeN/CeC bonds formation to synthesize various isoquinoline-based polyaromatics (Fig. 14D).78 Besides, Shiraishi and co-workers achieved a one-pot synthesis of imines from alcohols and amines with TiO2 loading Pt nanoparticles under UV irradiation at room temperature (Fig. 15A).25 This process proceeds via the Pt-assisted photocatalytic oxidation of alcohols with concomitant liberation of H2 and a catalytic condensation of the generated aldehydes with amines on the TiO2 surface. To further demonstrate the utility of platinum-assisted photocatalytic dehydrogenation on TiO2, the same group extended this strategy to enable the synthesis of benzimidazoles in high yields from 1,2-diaminobenzene and various alcohols at room temperature (Fig. 15B).27 Recently, the Che group developed visible-light-driven dehydrogenation coupling of primary alcohols with o-aminobenzamides to furnish quinazolin-4(3H)-ones in high product yields (Fig. 15C).79 Significantly, binuclear platinum(II) diphosphite complexes are the sole catalysts for the scavenger-free transformation.

8.03.3.4

CeP bond formation

Most approaches for CeP bond formation were associated with one or more limitations such as low selectivity, overoxidation, toxic or stoichiometric transition metals, additives/oxidants, elevated temperatures, etc. In this regard, the photochemical oxidant-free strategy showed good chemical selectivity in the phosphorylation of the Csp3eH bond and Csp2eH under mild conditions. For instance, Lei and co-workers described an acceptorless a-phosphorylation of N-aryl tertiary amines with phosphites under visible light irradiation (Fig. 16A).80 Mechanically, N,N-dialkylanilines are transformed into corresponding iminium ions, which can be intercepted with the phosphorus nucleophiles to furnish the desired aminophosphonates in the dual catalytic system containing the ruthenium-involved photoredox cycle and the cobaloxime-based hydrogen evolution cycle. In addition, Wu and co-workers developed direct CeH phosphorylation of thiazole derivatives with diarylphosphine oxides via eosin B catalyzed hydrogen evolution protocol (Fig. 16B).81 To mimic photosynthesis process, the Zhang team used an emissive poly(ethylene glycol)-decorated tetragonal prismatic platinum(II) cage as a light-harvesting system (LHS) to achieve the same CeH phosphorylation reaction

(A)

(B)

(C)

(D)

Fig. 14

Visible-light-driven CCHE reactions of NeH with alkenes or alkynes.

94

Non-sacrificial photocatalysis

(A)

(B)

(C)

Fig. 15

Miscellaneous oxidant-free photoreactions involving NeH bond formations.

(A)

(B)

(C)

(D)

(E)

(F)

Fig. 16

Photochemical oxidant-free protocols for the construction of CeP bonds.

(Fig. 16C).40 Subsequently, Wu realized the direct phosphorylation of enamine and enamide via cobaloxime catalysis under visiblelight irradiation (Fig. 16D).82 Moreover, the same group achieved a radical addition with internal alkynes generating cyclic benzophosphine oxides rather than linear hydrophosphorylation product by using cobaloxime complex as the sole catalyst (Fig. 16E).83 Furthermore, an energy-saving photoelectrochemical CCHE strategy for the CeP bond construction between tetrahydroisoquinoline and diphenyl-phosphine oxide was described with the BiVO4 photoelectrode as the working electrode, Pt plate as the counter electrode, and Ag/AgCl as the reference electrode (Fig. 16F).84

Non-sacrificial photocatalysis 8.03.3.5

95

Miscellaneous

The developed non-sacrificial photocatalytic cross-coupling has also advanced the construction of CeS, SeS, and even CeSi bonds toward sustainable synthesis. An oxidant-free CeH functionalization/CeS bond formation reaction to prepare benzothiazoles has been developed efficiently and selectively by visible-light photoredox cobalt catalysis (Fig. 17A).85 In this system, stoichiometric oxidation reagents could be avoided, and dihydrogen is the only byproduct. Conversely, the undesirable oxidation byproducts amides were often generated in the traditional oxidative cyclization of thiobenzanilides. Besides, an oxidant-free dehydrogenative sulfonylation of a-methyl-styrene derivatives with sulfinic acids was developed for the construction of allylic sulfones by using a dual-catalyst system containing TBA2-eosin Y and Co(dmgH)2PyCl (Fig. 17B).86 The selective coupling of thiols to produce disulfides is not a simple synthetic procedure. Most of these procedures suffered from low selectivity and overoxidation of the thiols to give undesired products such as sulfoxides and sulfones. Prominently, Wu and co-workers reported that the visible-light irradiation of CdSe Quantum dots (QDs) in water resulted in the conversion of a variety of thiols into disulfides and H2 without external oxidants (Fig. 18).37 In this system, photoinduced QDs holes oxidize thiolate anions to produce thiyl radicals, which couple to form disulfides. Meanwhile, the protons released from thiols are reduced to H2 by the conduction band of QDs. Particularly, the conversion of thiols and the hydrogen emission rate improved dramatically upon the addition of nickel(II) salt as a co-catalyst to provide more sites on the QDs surface for proton reduction. Compared with the dual-catalyst system containing photocatalyst and cobaloxime catalyst, QDs have emerged as promising materials for the application as recyclable photocatalysts in photochemical transformations because of their quantum confinement effects, rich surface-binding properties, and intense absorption in the visible region. Allylsilanes are vital building blocks in synthesizing small molecules and polymers due to their high stability and low toxicity. The Xu group developed a synergistic combination of photoredox catalysis, hydrogen-atom transfer, and cobalt catalysis to accomplish dehydrogenative silylation of alkenes (Fig. 19).87 From the proposed mechanism, the generated photocatalyst oxidizes quinuclidine to form the highly electrophilic quinuclidinium radical cation which selectively abstracts the hydrogen atom from the SieH bond of tris(trimethylsilyl)silane to produce the corresponding silyl radical. Subsequent addition of the radical to the electron-deficient olefin forms a carbon-centered radical intermediate which is captured by Co(II), and then the new species proceed CoeC bond cleavage and b-hydride elimination to furnish the final allylsilane. This dehydrogenative transformation featured high regioselectivity, excellent tolerance of functional groups, a wide scope of substrates, and oxidant-free reaction conditions, beyond traditional catalysis which requires stoichiometric or excess amounts of oxidants.

8.03.4

Acceptorless intramolecular dehydrogenation

8.03.4.1

Dehydrogenative oxonation of alcohols

Non-sacrificial photocatalysis has also expanded to small-molecule functionalization via AID. Among the various types of AIDs, those splitting alcohols into hydrogen and corresponding carbonyl compounds under ambient conditions have attracted particular interest. This is due to (i) the ready accessibility and usually low cost of alcohols, (ii) the clean and sustainable method for carbonyl preparation, and (iii) the potential future hydrogen economy using alcohols as an energy carrier. In 1982, the results of Moutet and Reverdy demonstrated the electrogenerated N,N,N0 ,N0 -tetraphenyl-p-phenylenediamine (TPPD) radical cations (Fig. 7C) and their photoexcitation with UV light enabled oxidation of benzyl alcohol to benzaldehyde (Fig. 20A).88 Later, a more direct protocol was achieved using a photoresponsive electrode or sensitized photoanode. For example, Meyer and co-workers used co-loading of a ruthenium polypyridyl chromophore and the catalyst Ru(II) OH22þ on a core/shell structured material consisting of a core of tin-doped In2O3 nanoparticles and a TiO2 shell to promote the photoelectrochemical dehydrogenation of benzyl alcohol.15b In the proposed photoelectrochemical mechanism, 2e oxidation of benzyl alcohol to benzaldehyde occurs at the surface of the photoanode with hydrogen generated at the Pt cathode. In 2016, Xiao and Xu employed Ni-modified CdS nanoparticles as heterogeneous photocatalysts, efficiently splitting alcohols into hydrogen and corresponding carbonyl compounds under visible light irradiation (Fig. 20B).33

(A)

(B)

Fig. 17

CCHE reactions for the construction of CeS bonds.

96

Non-sacrificial photocatalysis

Fig. 18

Visible-light-driven CCHE reaction for the construction of SeS bond.

Fig. 19

Visible-light-driven CCHE reaction for the construction of CeSi bond.

(A)

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Fig. 20

Hydrogen evolution reactions of alcohols.

The next significant advancement in dehydrogenative oxidation of alcohols was reported by Wu and co-workers, who utilized 3mercaptopropionic acid (MPA)-capped CdSe quantum dot (MPA-CdSe QD) as a photocatalyst, Ni2þ ions as a cocatalyst, and MPA and water as relay reagents (Fig. 20C).38,89 In particular, this method showed selective oxidation of benzyl alcohols to corresponding aldehydes/ketones in the presence of an aliphatic alcohol, due to the difference in bond dissociation energies of the benzylic and

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aliphatic CeH bonds. Soon after, the same group accomplished a similar visible-light-catalytic dehydrogenation via a dual catalyst system merging an eosin Y and a nickel-thiolate complex (Fig. 20D).22 Recently, as a contribution of binuclear platinum(II) diphosphite complexes to acceptorless oxidation, Che team demonstrated that various aliphatic alcohols could be efficiently dehydrogenated to corresponding carbonyl compounds by using Pt2(m-P2O5H2)44
as the sole catalyst under mild photocatalytic conditions (Fig. 20E).79 More recently, Kanai and co-workers reported visible-light-driven acceptorless dehydrogenation of aliphatic secondary alcohols to ketones by a ternary hybrid catalyst system including photoredox catalyst Acrþ-Mes, organocatalyst thiophosphoric acid (TPA), and nickel catalyst Ni(NTf2)2$xH2O (Fig. 20F).90 Mechanistically, a thiyl radical formed by photo-oxidation of TPA abstracts a hydrogen atom from the a-CeH bond of an alcohol substrate, and then nickel catalyst combines with the generated carboncentered radical. Finally, the organonickel species proceeds b-hydride elimination to afford enol, which tautomerizes to product ketone. Moreover, Yoshida and coworkers have developed dehydrogenative lactonization of 1,2-benzenedimethanol with a platinum-loaded titanium oxide photocatalyst at room temperature via UV irradiation (Fig. 20G).24 Accordingly, the reaction pathway was proposed to start with the dehydrogenation of the alcohol to form an aldehyde on the Pt-loaded rutile TiO2, which exhibits a high photocatalytic activity with high selectivity.

8.03.4.2

Dehydrogenative olefination

An elegant example to achieve dehydrogenations of unactivated alkanes using an acceptorless photocatalytic strategy was realized by combining TBADT and cobaloxime Co(dmgH)2PyCl under near-UV irradiation at room temperature (Fig. 21A).91 The protocol employed a photocatalyst TBADT to provide aliphatic radical intermediates from alkanes via HAT. The CeH bond adjacent to the radical is weakened and thus activated for cobaloxime-facilitated HAT resulting in the desired olefin and two metal species capable of combining to release dihydrogen. Following this work, Sorensen reported a similar tungsten/cobaloxime dual catalytic strategy featuring successive HATs for the dehydroformylation of aldehydes (Fig. 21B).92 Similarly, the photocatalyst TBADT initiates the process by HAT of the aldehyde followed by loss of CO to provide the radical intermediate. In 2018, Tunge reported a direct decarboxylative elimination of readily available amino acids to synthesize enamides and enecarbamates by merging an organic photoredox catalyst and a cobaloxime catalyst without stoichiometric oxidants or harsh conditions previously employed in similar eliminations (Fig. 21C).93 In addition, with cobalt-based HEC and an iridium photocatalyst to mediate oxidative decarboxylation, alkyl carboxylic acids could also eliminate carbon dioxide and dihydrogen to form olefins (Fig. 21D).94 Besides oxidant-free photocatalytic dehydrogenations of alcohols and alkanes described above, visible-light-mediated acceptorless dehydrogenation of primary amine has also been developed by Kempe and co-workers. The reaction involves benzylamine oxidation toward an imine combined with the liberation of one equivalent of H2 via a metal-organic framework-based photocatalyst system (Ni5CdS-MIL-101). The intermediate imine is condensated with a second equivalent of benzylamine to form the homocoupled product N-benzyl-1-phenylmethanimine (Fig. 22A).35 Following this work, Zhang and co-workers reported photocatalytic dehydrogenation of primary and secondary amines to corresponding imines by in situ photodeposited Ni clusters on CdS under a 300 W Xe lamp irradiation.34 In addition, Balaraman introduced a catalytic acceptorless dehydrogenation of diarylhydrazine derivatives to access the corresponding azobenzenes with the liberation of H2 by merging the visible-light photoredox catalysis with cobaloxime-based HEC (Fig. 22B).95

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Fig. 21

Formation of olefins through non-sacrificial photocatalysis.

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8.03.4.3

Photocatalytic acceptorless dehydrogenation of amines.

Dehydrogenative aromatization

The acceptorless dehydrogenative aromatization is a fundamentally crucial organic transformation, and its potential applications for reversible dehydrogenation-hydrogenation are demonstrated in energy chemistry like hydrogen-storage materials. As the previous dehydrogenation protocols required relatively harsh conditions, many chemists paid more attention to the convenient visible-light-driven dehydrogenation methods which can avoid stoichiometric oxidants under mild conditions at ambient temperature. For example, Tung and co-workers accomplished photocatalytic hydrogen production from Hantzsch dihydropyridine derivatives in quantitative yields with square-planar platinum(II) terpyridyl complexes (Fig. 23A).11a To further demonstrate the utility of dehydrogenative aromatization of platinum(II) terpyridyl complexes, Tung and Wu extended the scope of the photocatalyst and the substrate, which can produce hydrogen in high quantum yield with large catalytic turnover. In 2009, they employed similar platinum(II) terpyridyl complex to access 3,4-diarylpyrroles with H2 evolution via visible-light irradiation (Fig. 23C).11b Moreover, photoinduced electron transfer from 3,4-diaryl-2,5-dihydrothiophenes to platinum(II) complex led to the formation of 3,4diarylthiophenes in a green manner (Fig. 23D).11c The Liu group also developed visible-light-catalytic dehydrogenative aromatization based on the synergistic application of eosin Y with nickel (II) complex (Fig. 23B).96 Without any external oxidants, Hantzsch 1,4-dihydropyridines, 1,4-dihydropyrimidines, 2,5-dihydrothiophenes, and 2,5-dihydropyrroles were transformed into corresponding aromatic compounds in excellent yield and chemo-selectivity under mild conditions via hydrogen evolution. Soon after, the Li group reported a photocatalytic acceptorless dehydrogenation of tetrahydroquinolines, indolines, and other related N-heterocycles utilizing a [Ru(bpy)3]Cl2$6H2O/Co(dmgH)2PyCl dual catalytic system (Fig. 23E).97 Mechanistically, the proposed catalytic cycle begins with a single electron reduction of Co(III)/Co(II) by the excited state photocatalyst. The amine is then oxidized to N-radical cation, thus returning the photocatalyst to its ground state, and the N-radical cation intermediate is transformed into imine via HAT. Isomerization of the imine allows a second facile dehydrogenation to form the aromatic system. In addition, Kanai also achieved catalytic acceptorless dehydrogenations of tetrahydroquinolines and indolines by hybrid catalysis comprising an acridinium photoredox catalyst and a Pd(BF4)2(MeCN)4 metal catalyst (Fig. 23F).98 They expanded the substrate scope of acceptorless dehydrogenations to tetrahydronaphthalenes by adding a thiophosphoric imide organocatalyst in the catalytic system. Recently, Bahnemann discovered that Rh-photodeposited TiO2 nanoparticles could dehydrogenate N-heterocyclic amines with dihydrogen evolution in an inert atmosphere under visible light illumination.30 Similar acceptorless dehydrogenative sequences of N-heterocyclics have been described by Che and co-workers using binuclear platinum(II) diphosphite complexes as versatile photocatalysts.79 In 2019, Luo developed a visible-light-promoted unsymmetric coupling and controllable aromatization of tetrahydroquinolines enabled by merging Ru(bpy)32 þ/Co(dmgH)2PyCl mediated photoredox dehydrogenation and acid promoted enamine/iminium tautomerization (Fig. 23G).99 The reaction proceeded via acceptorless enamine-iminium coupling, which led to C2eC3 connection in a highly chemo- and regioselective manner. Impressively, Leonori and co-workers exploited an external oxidant-free crosscoupling approach for the construction of anilines using saturated cyclohexanones and amines by a photoredox- and cobaltbased catalysis (Fig. 23H).100 Condensation between amines and carbonyls provides enamine intermediates, which undergo acceptorless dehydrogenative aromatization to afford anilines under a [Ir(ppy)2(dtbbpy)]PF6/Co(dmgH)2(DMAP)Cl dual catalytic system.

8.03.5

In situ applications of hydrogen in dehydrogenation reaction

As described in Section 8.03.2.2, both Co(I) and Co(III)eH species produced in the dehydrogenation processes are active intermediates with strong reducibilities. Therefore, cobaloximes generated in situ might also act as hydrogenation catalysts if appropriate substrates are introduced to the catalytic dehydrogenative system. In 2014, this was achieved by adding nitrobenzene into the photocatalytic CCHE reaction between tetrahydroisoquinolines and indoles, and the oxidative cross-coupling products and reductive aniline were produced with synergistic photocatalyst eosin Y and cobaloxime catalyst Co(dmgH)(dmgH2)Cl2 (Fig. 24A).101 Lately, the combination of hydrogenation of nitrobenzenes and external oxidant-free cyclization of thiobenzanilides was achieved in one pot (Fig. 24B).85 Various substituted nitrobenzenes could be reduced to the corresponding anilines by the in situ generated reductants, while other easily reducible groups like bromo or carbonyl substituents in the nitrobenzenes would not change. To further

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Fig. 23

Photo-mediated aromatization hydrogen evolution.

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Cross-coupling and in situ hydrogenation reactions by visible light photocatalysis.

demonstrate the utility of cross-coupling and in situ hydrogenation reactions, Luo and Wu extended this strategy to enable a photocatalytic asymmetric cross-dehydrogenative coupling of the CeH bonds adjacent to tertiary amines with the a-CeH bonds of ketones. A simple chiral primary amine enabled the transformation through the coupling of a catalytic enamine intermediate and an iminium cation intermediate in situ generated from tetrahydroisoquinoline derivatives by Ru/Co catalysis (Fig. 24C).102 Moreover, under a typical non-sacrificial photocatalytic condition containing a Ru(bpy)32 þ photocatalyst and a cobaloxime catalyst, an oxidative synthesis of tetrahydroquinolines was realized in conjunction with reductive hydrogenation of maleimides. Herein, the electron and proton eliminated from the anilines in the oxidative cyclization sequence are captured by the cobaloxime catalyst to achieve reductive hydrogenation of maleimides (Fig. 24D).103 A similar strategy was also utilized for the external oxidantfree benzannulation reaction, wherein reductive hydrogenation of maleimides occurred in a one-pot (Fig. 24E).49 Interestingly, the in situ hydrogenation process was even observed in visible-light-driven cobaloxime-mediated cross-coupling between phosphine oxides and terminal alkenes, which was performed without any external photosensitizer and oxidant (Fig. 24F).83 In the reaction, the cobaloxime performs a dual role: (i) as a photocatalyst to activate H-phosphine oxide and (ii) as a proton and electron relay to facilitate hydrogenation of terminal alkenes, thus providing a straightforward non-sacrificial strategy for the synthesis of E-alkenylphosphine oxide in excellent chemo- and stereoselectivity.

8.03.6

Conclusion

Undeniably, the recent dramatic development in non-sacrificial photocatalysis gave access to many important scaffolds through acceptorless intramolecular dehydrogenations and cross-coupling hydrogen evolution reactions. In contrast to the more

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conventional strategies with sacrificial reagents, non-sacrificial photocatalysis offers a potential solution for clean, energy-efficient, and atom-economic bond formations. Most notably, the external oxidant-free conditions allow oxidant-sensitive compounds to survive and open the pathway achieving more sophisticated value-added architectures from readily available starting materials under otherwise unattainable mild conditions. Despite this tremendous success, there still remain several major challenges and unexplored directions. For instance, direct CeH activation of inert hydrocarbon compounds is still to be further investigated. As such, there remains the opportunity for new transformations to be discovered. Additionally, more efforts would be preferably guided to the exploration of heterogeneous scavenger-free photocatalytic strategies, as most of the current methods rely on homogenous catalysts. The scalability of non-sacrificial photochemical protocols in industrial applications should also be taken into account. Furthermore, enantioselective syntheses using non-sacrificial photocatalytic strategies are still rare. From the perspective of these long-term goals, the concept of non-sacrificial photocatalysis still has a long way to go but will undoubtedly witness inconceivable improvement in the future.

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Zheng, Y.-W.; Ye, P.; Chen, B.; Meng, Q.-Y.; Feng, K.; Wang, W.; Wu, L.-Z.; Tung, C.-H. Org. Lett. 2017, 19, 2206–2209. Zhang, G.; Hu, X.; Chiang, C. W.; Yi, H.; Pei, P.; Singh, A. K.; Lei, A. J. Am. Chem. Soc. 2016, 138, 12037–12040. Yi, H.; Niu, L.; Song, C.; Li, Y.; Dou, B.; Singh, A. K.; Lei, A. Angew. Chem. Int. Ed. 2017, 56, 1120–1124. Zhang, M.; Ruzi, R.; Li, N.; Xie, J.; Zhu, C. Org. Chem. Front. 2018, 5, 749–752. Shao, A.; Zhan, J.; Li, N.; Chiang, C. W.; Lei, A. J. Org. Chem. 2018, 83, 3582–3589. Yang, Q.; Jia, Z.; Li, L.; Zhang, L.; Luo, S. Org. Chem. Front. 2018, 5, 237–241. Qiao, M.-M.; Liu, Y.-Y.; Yao, S.; Ma, T.-C.; Tang, Z.-L.; Shi, D.-Q.; Xiao, W.-J. J. Org. Chem. 2019, 84, 6798–6806. McManus, J. B.; Griffin, J. D.; White, A. R.; Nicewicz, D. A. J. Am. Chem. Soc. 2020, 142, 10325–10330. Tateno, H.; Miseki, Y.; Sayama, K. Chem. Commun. 2017, 53, 4378–4381. Yuzawa, H.; Kumagai, J.; Yoshida, H. J. Phys. Chem. C 2013, 117, 11047–11058. Niu, L.; Yi, H.; Wang, S.; Liu, T.; Liu, J.; Lei, A. Nat. Commun. 2017, 8, 14226. Huang, H.; Strater, Z. M.; Rauch, M.; Shee, J.; Sisto, T. J.; Nuckolls, C.; Lambert, T. H. Angew. Chem. Int. Ed. 2019, 58, 13318–13322. Zhang, L.; Liardet, L.; Luo, J.; Ren, D.; Grätzel, M.; Hu, X. Nat. Catal. 2019, 2, 366–373. Chen, H.; Yi, H.; Tang, Z.; Bian, C.; Zhang, H.; Lei, A. Adv. Synth. Catal. 2018, 360, 3220–3227. Zhao, F.; Yang, Q.; Zhang, J.; Shi, W.; Hu, H.; Liang, F.; Wei, W.; Zhou, S. Org. Lett. 2018, 20, 7753–7757. Zhao, Q.-Q.; Hu, X.-Q.; Yang, M.-N.; Chen, J.-R.; Xiao, W.-J. Chem. Commun. 2016, 52, 12749–12752. Hu, X.; Zhang, G.; Bu, F.; Lei, A. Angew. Chem. Int. Ed. 2018, 57, 1286–1290. Tian, W. F.; Wang, D. P.; Wang, S. F.; He, K. H.; Cao, X. P.; Li, Y. Org. Lett. 2018, 20, 1421–1425. Zhong, J.-J.; To, W.-P.; Liu, Y.; Lu, W.; Che, C.-M. Chem. Sci. 2019, 10, 4883–4889. Niu, L.; Wang, S.; Liu, J.; Yi, H.; Liang, X.-A.; Liu, T.; Lei, A. Chem. Commun. 2018, 54, 1659–1662. Luo, K.; Chen, Y.-Z.; Yang, W. C.; Zhu, J.; Wu, L. Org. Lett. 2016, 18, 452–455. Lei, T.; Liang, G.; Cheng, Y.-Y.; Chen, B.; Tung, C.-H.; Wu, L.-Z. Org. Lett. 2020, 22, 5385–5389. Liu, W.-Q.; Lei, T.; Zhou, S.; Yang, X.-L.; Li, J.; Chen, B.; Sivaguru, J.; Tung, C.-H.; Wu, L.-Z. J. Am. Chem. Soc. 2019, 141, 13941–13947. Wang, J. H.; Li, X.-B.; Li, J.; Lei, T.; Wu, H.-L.; Nan, X.-L.; Tung, C.-H.; Wu, L.-Z. Chem. Commun. 2019, 55, 10376–10379. Zhang, G.; Liu, C.; Yi, H.; Meng, Q.; Bian, C.; Chen, H.; Jian, J.-X.; Wu, L.-Z.; Lei, A. J. Am. Chem. Soc. 2015, 137, 9273–9280. Zhang, G.; Zhang, L.; Yi, H.; Luo, Y.; Qi, X.; Tung, C.-H.; Wu, L.-Z.; Lei, A. Chem. Commun. 2016, 52, 10407–10410. Yu, W.-L.; Luo, Y.-C.; Yan, L.; Liu, D.; Wang, Z.-Y.; Xu, P.-F. Angew. Chem. Int. Ed. 2019, 58, 10941–10945. Moutet, J.-C.; Reverdy, G. J. Chem. Soc. Chem. Commun. 1982, 654–655. Huang, C.; Li, X.-B.; Tung, C.-H.; Wu, L.-Z. Chem. A Eur. J. 2018, 24, 11530–11534. Fuse, H.; Mitsunuma, H.; Kanai, M. J. Am. Chem. Soc. 2020, 142, 4493–4499. West, J. G.; Huang, D.; Sorensen, E. J. Nat. Commun. 2015, 6, 10093. Abrams, D. J.; West, J. G.; Sorensen, E. J. Chem. Sci. 2017, 8, 1954–1959. Cartwright, K. C.; Tunge, J. A. ACS Catal. 2018, 8, 11801–11806. Sun, X.; Chen, J.; Ritter, T. Nat. Chem. 2018, 10, 1229–1233. Sahoo, M. K.; Saravanakumar, K.; Jaiswal, G.; Balaraman, E. ACS Catal. 2018, 8, 7727–7733. Wang, X.; Dong, K.; Liu, Q. Acta Chim. Sinica 2017, 75, 119–122. He, K.-H.; Tan, F.-F.; Zhou, C.-Z.; Zhou, G.-J.; Yang, X.-L.; Li, Y. Angew. Chem. Int. Ed. 2017, 56, 3080–3084. Kato, S.; Saga, Y.; Kojima, M.; Fuse, H.; Matsunaga, S.; Fukatsu, A.; Kondo, M.; Masaoka, S.; Kanai, M. J. Am. Chem. Soc. 2017, 139, 2204–2207. Jia, Z.; Yang, Q.; Zhang, L.; Luo, S. ACS Catal. 2019, 9, 3589–3594. Dighe, S. U.; Juliá, F.; Luridiana, A.; Douglas, J. J.; Leonori, D. Nature 2020, 584, 75–81. Zhong, J.-J.; Wu, C.-J.; Meng, Q.-Y.; Gao, X.-W.; Lei, T.; Tung, C.-H.; Wu, L.-Z. Adv. Synth. Catal. 2014, 356, 2846–2852. Yang, Q.; Zhang, L.; Ye, C.; Luo, S.; Wu, L.-Z.; Tung, C.-H. Angew. Chem. Int. Ed. 2017, 56, 3694–3698. Yang, X.-L.; Guo, J.-D.; Lei, T.; Chen, B.; Tung, C.-H.; Wu, L.-Z. Org. Lett. 2018, 20, 2916–2920.

8.04

Redox photocatalysis

Stefan Bernharda and Husain N. Kagalwalab, a Department of Chemistry, Carnegie Mellon University, Pittsburgh, PA, United States; and b Department of Chemistry, Southern Methodist University, Dallas, TX, United States © 2023 Elsevier Ltd. All rights reserved.

8.04.1 8.04.2 8.04.2.1 8.04.2.2 8.04.2.2.1 8.04.2.2.2 8.04.2.2.3 8.04.2.3 8.04.3 8.04.3.1 8.04.3.1.1 8.04.3.1.2 8.04.3.1.3 8.04.3.2 8.04.4 Acknowledgments References

Introduction and scope Photophysical and mechanistic aspects of molecular photocatalysis Characteristics of a successful photocatalyst Overview of photocatalyst electronic structures PCs with MLCT excited states PCs with LMCT excited states PCs with MC spin-flipped states Mechanisms of photocatalysis Recent developments in photoredox organic transformations using molecular inorganic catalysts Mononuclear photocatalysts Second and third row transition metals First row transition metals Other metals Multinuclear or cooperative photoredox catalysts Summary and outlook

103 104 105 105 105 107 107 107 108 108 108 125 141 142 142 145 146

Abstract The past decade has seen a massive resurgence of photoredox catalysis-themed research, a field that uses photon-induced electron and energy transfer processes for driving synthetic transformations. Transition metal-based photocatalysts have played a key role in this revival, given their photochemical and electrochemical robustness, and, most importantly, their ability to form charge-transfer excited states and consequently act as strong photooxidants and/or photoreductants. In addition, some of these photocatalysts can participate in inner-sphere processes, leading to previously unattainable mechanistic pathways and products. While the initial exploration phase involved the utilization of ruthenium(II) polypyridine and cyclometalated iridium(III)-based photocatalysts, recent years have witnessed an influx of earth-abundant metal-based compounds, with tremendous progress being made for copper(I/II), chromium(III), iron(II/III), zirconium(IV) and tungsten(VI)-based chromophores. This chapter assesses the advances made in the field of transition metal-based photoredox catalysis, with emphasis on mononuclear photocatalysts. An overview of the photophysical and mechanistic aspects of transition metal photocatalysts is provided, followed by a discussion of current examples of photo-driven synthetic reactions. Multinuclear or cooperative transition-metal based catalytic systems are also briefly reviewed. This new era of photoredox catalysis has embraced emerging concepts like the use of multiphoton excitation, enantioselective photocatalysts, highthroughput photocatalyst synthesis and screening, and has also been used for the development of pharmaceutically relevant molecules. Examples from literature utilizing these concepts will also be presented.

8.04.1

Introduction and scope

Redox photocatalysis, photoredox catalysis or simply photocatalysis can be defined as the utilization of light energy to drive a chemical reaction.1 Utilizing such processes can be advantageous since it allows the generation of reactive species using mild conditions.2–4 While the majority of industrial chemical transformations have been propelled thermally, the use of photons to carry out similar reactions is thus an exciting prospect for accessing unusual molecules and materials.5 This option becomes more relevant from an energy sustainability viewpoint considering that the earth receives approximately 4.3
1020 J of solar energy in an hour, enough to power the energy needs of mankind for a year.6 Exploiting this unlimited energy resource could reduce our day-to-day dependence on fossil fuel-based technologies and contribute to environmental remediation.7,8 These factors have contributed to the resurgence of this relatively old concept, and consequently the last few decades have seen a drastic increase in reports related to photocatalytic synthesis9 and renewable energy conversion.10,11 An important prerequisite for photocatalysis is a catalyst which can absorb the photons upon irradiation. These photosensitizers (PSs) or photocatalysts (PCs) have been vigorously researched, with highly photostable transition metal-based complexes at the forefront.12 Besides variable oxidation states, a primary advantage of a transition metal-based, light-absorbing complex is the charge transfer character of the excited state, often a metal-to-ligand charge transfer (MLCT) or ligand-to-metal charge transfer (LMCT)

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state, which allows the complex to act as a strong electron donor (photoreductant) or acceptor (photooxidant). Another benefit is the photophysical/redox tunability accessible via judicious ligand design strategies. Transition metal-based chromophores also exhibit superior electrochemical and photochemical robustness compared to their purely organic cousins. In order to maximize the photon conversion efficiency, it would be ideal for the complex to absorb in the red regions of the solar spectrum. Of the numerous examples available, ruthenium(II) polypyridine and cyclometalated iridium(III) complexes of the type [Ir(C^N)3] and [Ir(CLN)2(NLN)]þ have been ubiquitously used. Apart from the well-publicized Pschorr reaction reported in 1984,13 and few other infrequent organic transformations (mainly photodriven by [Ru(bpy)3]2þ, where bpy ¼ 2,20 -bipyridine),14 previous photocatalytic applications of these complexes were almost exclusively limited to solar fuel generation,15–17 dye sensitized solar cells (DSSCs),18 and organic light emitting diodes (OLEDs).19–22 Only recently there has been a drastic spike of photoredox organic synthetic transformations being carried out by Ru(II) and Ir(III)-based complexes.23,24 Given the intrinsic cost and rare nature of these noble metals, one current focus has been to adopt earth-abundant transition metals.25,26 Among these, Cu(I)-based chromophoric complexes have been the most widely researched, with smaller but significant progress being achieved with other metals such as chromium(III), iron(II), zirconium(IV) and tungsten(VI). Moving away from molecular transition metal catalysts, substantial advances have been made with respect to heterogeneous systems,27,28 and there has also been a push towards ‘metal-free’ systems, using organic dyes as photocatalysts.29–33 This chapter attempts to review contemporary examples of photoredox catalysis and small molecule activation (Fig. 1). Given that the book focusses on coordination chemistry, and due to the plethora of information available for the different catalytic types, this chapter will be limited to only transition metal-based molecular photocatalytic systems. The following sections will address the photophysical and mechanistic aspects of chromophoric metal complexes and will highlight recent examples of complexes based on second/third row transition metals such as Ru2þ Ir3þ, Zr4þ, Mo0 and W6þ, as well as first row transition metals, specifically Cu1þ/2þ, Ti4þ, V5þ, Cr3þ, Fe2þ/3þ, Co3þ and Ni2þ which have been used for driving photoredox reactions. A brief discussion regarding multinuclear or cooperative transition metal-based photoredox catalysis will also be provided. Strictly speaking, the term ‘photocatalyst’ is used for a light-absorber directly involved in catalysis, while ‘photosensitizer’ refers to a light-absorber which in its excited state interacts with one or more catalysts. However, for the sake of brevity all photoactive molecules will be referred to as PCs, and their roles in direct or indirect photocatalysis will be emphasized.

8.04.2

Photophysical and mechanistic aspects of molecular photocatalysis

The key step during photocatalysis is photon absorption, resulting in the formation of the excited state PC* which is characterized by significant changes to the electronic structure of the molecule. Such an excited state can be designated as metal-to-ligand charge transfer (MLCT), ligand-to-metal charge transfer (LMCT), or metal centered (MC), depending on the type of orbitals predominantly involved in the transition. MC states are usually non-emissive, mainly due to distortions associated with the occupation of antibonding orbitals (for example t2g to eg* promotion in an octahedral field). Recently a few exceptions to these findings have been published and will be discussed later in this chapter. Other common transitions include pure ligand centered (LC) transitions as well as more complicated intra-ligand charge transfer (ILCT) or ligand to ligand charge transfer (LLCT) processes. Often, a transition metal PCs’ light absorption results in the population of a singlet excited state, which can undergo relaxation via intersystem crossing (ISC, kISC) to form a triplet excited state. Both the singlet and the triplet excited states can be emissive and return to the ground state via fluorescence (kf, spin-allowed) or phosphorescence (kp, spin-forbidden) respectively. The involvement of heavy metals such as Ir and Ru greatly facilitate the population of long-lived triplet excited states. These transients are instrumental for

Fig. 1

Overview of photoredox catalysis and its applications discussed in this chapter.

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driving the efficient electron transfer processes that are at the origin of many photocatalytic reaction cycles. There is also the possibility of relaxation through unproductive non-radiative (knr) processes. Consequently, the emissive quantum yield of the sample, Fs and the radiative/non-radiative rate constants for a phosphorescent can be determined by the following equations:

Fs ¼ Fstd ðIs =As ÞðAstd =Istd Þðhs =hstd Þ kr ¼ Fs =ss knr ¼ ð1=ss Þ  kr Where Fstd is the quantum yield of a standard luminophore, Is and Istd are the maximum emission intensities for the sample and the standard, As and Astd are the absorbances of the sample and reference at the excitation wavelength, hs and hstd are the refractive indices of the solvents and ss is the excited state lifetime of the sample. It is desirable that the long-lived excited state can presumably take part in chemical reactions by interacting with the substrate or a reagent via formation of an ‘encounter or precursor complex.’

8.04.2.1

Characteristics of a successful photocatalyst

The ideal photocatalyst needs to meet the following criteria34:

•

•

Visible light absorption - A major proportion of the solar spectrum consists of the visible and near infrared regions. Thus, for practical applications a broad absorption in this range is preferable, with moderate extinction coefficients. For example, [Ru(bpy)3]2þ (in acetonitrile) exhibits an intense MLCT absorption at 452 nm with an extinction coefficient of 13,000 M1 cm1. It is important to note that the absorption edge is more critical for energy conversion than the peak maxima and positions.6 High quantum yield of formation of long-lived excited states - This quantum yield represents the efficiency by which the excited state is formed and can be represented as

FEx:state ¼ Excited state formed=incident photons: Long-lived excited state lifetimes are necessary to allow sufficient interactions with the targeted substrate. Both Ru(II) and Ir(III) based compounds exhibit 100% quantum efficiency of formation and have long-lived excited states (hundreds of nanoseconds to several microseconds range) mainly due to the heavy atom effect facilitating the ISC and, as a consequence, the phosphorescent emission.

• •

Photostability, reversibility and tunability - It is important that the PC displays robustness under illumination and does not undergo decomposition in the presence of reagents. To ensure reversible electron transfer(s) and catalysis, the molecule should exhibit full electrochemical reversibility. Lastly, to improve the versatility of the PC in terms of applications, the excited state properties should be tunable through straightforward synthetic modifications. Excited and ground state potentials matching the substrate redox potentials - Excited state potentials can be determined from the ground state using a Hess cycle involving the luminescence energy and the ground state potentials of a PC,35 E ox ¼ Eox  E00 E red ¼ Ered þ E00

Where E*ox and Eox ¼ excited and ground state oxidation potentials respectively, E*red and Ered ¼ excited and ground state reduction potentials respectively and E0–0 ¼ energy between the zeroth vibrational states of the ground and excited states. E0–0 is generally estimated either from the onset of the emission band or from the emission maximum. Generally, PCs with a more negative E*ox are better photoreductants while those with a more positive E*red are stronger photooxidants. To enable catalyst regeneration, the corresponding ground state potentials must be highly reversible. It is also essential that the redox potentials are aligned with those of the target reaction to ensure that the overall process is exothermic following the initial absorption of a photon.

8.04.2.2

Overview of photocatalyst electronic structures

Generally, transition metal PCs can access three different types of excited states depending upon their inherent electronic structures. Fig. 2 depicts some commonly used transition metal PCs. This section outlines the different types of electronic structures that can be used to classify and understand known PCs. A more detailed explanation can be found in the tutorial review by Förster, C. and Heinze, K.36

8.04.2.2.1

PCs with MLCT excited states

Ru(II) polypyridine complexes and some cyclometalated Ir(III) complexes belong to this class. Fig. 3A shows the Jablonski diagram for [Ru(bpy)3]2þ. Upon irradiation, an electron from the Ru-based highest occupied molecular orbital (HOMO) is promoted to the bpy-based lowest unoccupied molecular orbital, LUMO (1MLCT). Alternatively, this excited state can also be reached by relaxation

106

Fig. 2

Redox photocatalysis

Chemical structures of representative transition metal photoactive complexes.

Fig. 3 Simplified Jablonski diagrams used to represent some commonly used transition metal PCs A) [Ru(bpy)3]2þ B) [Ir(CLN)2(NLN)]þ C) [Fe(bpy)3]2þ D) [Cu(dap)2]þ E) Zr(MePDP)2 and F) [Cr(ddpd)2]3þ.

from a higher energy singlet excited state. This state can further relax through an ISC process to a long-lived 3MLCT state which can emit light through phosphorescence. However, due to the moderate ligand field splitting of Ru, the non-emissive 3MC state becomes accessible via internal conversion (IC) processes. These low-lying states greatly limit the tuning of the excited state by ligand modification.37,38 The advantages of using Ir(III) cyclometalated complexes of the type [Ir(CLN)2(NLN)]þ becomes apparent upon inspection of their electronic structure (Fig. 3B). Firstly, the stronger crystal field splitting of Ir(III) pushes the 3MC state to higher energies. Additionally, the heteroleptic nature results in a HOMO with contributions from the metal and the cyclometalating ligand (CLN), while the LUMO is chiefly located on the ancillary ligand (NLN). Consequently, the HOMO and LUMO can be tuned individually through synthetic modifications.39 While the singlet excited state consists of an 1MLCT state that sits lower than the 1LC state, the triplet state can exhibit a mixed 3MLCT-3LC character, mainly due to greater stabilization of the triplet LC state.40,41 Like [Ru(bpy)3]2þ, low-spin Fe(II) polypyridine complexes like [Fe(bpy)3]2þ, also have low lying 1MLCT states that can relax to the corresponding triplet state. However, unlike its heavier 4d6 counterpart, [Fe(bpy)3]2þ has a weaker field splitting, resulting in 3 MC states at energies lower than the 1/3MLCT states (Fig. 3C). As a consequence of the non-radiative rapid deactivation pathways through these 3MC states, [Fe(bpy)3]2þ is non-luminescent even at 77 K. This example highlights some of the challenges of using

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107

a first-row transition metal as a PC and warrants the use of strong field ligands to create PCs with optimal photophysical properties. For example, replacing bpy with stronger carbene ligands like btz (btz ¼ 3,30 -dimethyl-1,10 -bis(p-tolyl)-4,40 -bis(1,2,3-triazol-5ylidene)) improved the 3MLCT lifetime of [Fe(btz)3]2þ to 528 ps.,42,43 which is around four orders of magnitude slower than the parent [Fe(bpy)3]2þ (50 fs). Such non-emissive MC states can also be avoided when using metals with closed shell d10 configuration. Cu(I)-based polypyridine, phosphine and mixed ligand complexes fall into this category. As shown by Fig. 3D, the absence of d-d transitions ensures an 3 MLCT phosphorescent state without the drawbacks of MC deactivation. Cu(I) complexes however often display a smaller 3 MLCT-1MLCT gap, resulting in a back ISC and thermal repopulation of the 1MLCT state. As a consequence, Cu(I) complexes are known to exhibit phosphorescence at low temperatures while at higher temperatures a process known as thermally activated delayed fluorescence (TADF, kTADF) can dominate.44 Cu(I) complexes can undergo excited state flattening distortion, i.e. a geometry change from the preferred pseudotetrahedral orientation (Cu(I), d10) to a square planar alignment (Cu(II), d9). This distortion results in diminished quantum yields. Common strategies to prevent this distortion include increased chelation as well as rigidification with steric bulk, as seen, for instance, in the complex [Cu(dap)2]þ (dap ¼ 2,9-bis(4-methoxyphenyl)-1,10-phenanthroline).

8.04.2.2.2

PCs with LMCT excited states

Transition metals ions such as Zr(IV) have an empty d shell that can circumvent the deactivating MC states, especially when the nature of the excited state involves LMCT states (Fig. 3E). One of the best known examples, Zr(MePDPPh)2 (MePDPPh ¼ 2,6bis(5-methyl-3-phenyl-1H-pyrrol-2-yl)-pyridine) has two strong tridentate, electron rich pyridine dipyrrolide ligands, which result in a long-lived mixed 3LMCT-3ILCT excited state (325 ms).45 Similarly, low-spin 3d5 Fe(III) complexes with very strong s-donor ligands can also exhibit luminescent LMCT states by deactivating the MC states. For instance, the rigidness of the anionic ligand system phenyl[tris(3-methylimidazol-1-ylidene)]borate, phtmeimb affords an almost octahedral environment around the metal in [Fe(phtmeimb)2]þ, which further increases the energy of the MC states.46 Intriguingly, this complex exhibits fluorescence (2LMCT to 2GS) and is unaffected by O2.

8.04.2.2.3

PCs with MC spin-flipped states

As seen from the previous examples, population of MC states leads to rapid excited-state deactivation and consequently reduced emissive and photocatalytic yields. However, MC spin-flip states which involve a mere rearrangement of electrons within the d orbitals do not undergo distortion and can be long-lived/emissive. High-spin 3d3 transition metal complexes like [Cr(ddpd)2]3þ (ddpd: N,N0 -dimethyl-N,N0 -dipyridine-2-yl-pyridine-2,6-diamine)47 can access such spin-flip states which are lower in energy compared to the deactivating 4MC state (Fig. 3F). In fact, dual emission can be observed from the two available 2MC states and this property has been exploited for use as a potential ratiometric optical thermometer.48

8.04.2.3

Mechanisms of photocatalysis

As previously stated, three general types of mechanisms have been predominantly proposed for photocatalytic processes: outer sphere or electron/energy transfer, atom transfer and inner sphere. Fig. 4 summarizes the different mechanisms.

•

Outer sphere: Outer sphere processes involve either photoinduced electron transfer (PeT) or energy transfer (PET) between the PC and the substrate. Upon excitation, an excited PC* can act as an energy donor, electron donor or electron acceptor. PET occurs when the excess energy from the PC* is transferred to the substrate or reagent. This, in turn, can occur via two pathways: Förster (through-space) and Dexter (through-bond) mechanisms. On the other hand, PeT involves an electron transfer between the PC* and the substrate. There are two ways by which this can be achieved; oxidative quenching if PC* donates an electron to the

Fig. 4

Summary of different mechanisms of photocatalysis.

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substrate, or reductive quenching if PC* accepts an electron from the substrate (the term ‘quenching’ signifying the forced relaxation of the PC* excited state with a loss of emission intensity). Quenching can also be ‘dynamic’ (diffusion-controlled mechanism) or ‘static’ (association between the PC and the quencher before excitation in viscous environments). Diffusioncontrolled outer sphere processes can be detected by Stern-Volmer analysis, which involves monitoring the excited state emission intensity or lifetime of the PC* in the presence of the reagent/substrate, known as quencher, Q. The rate by which these interactions occur can be quantified by using the Stern-Volmer equation I0 =I ¼ 1 þ k q s0 ½Q or s0 =s ¼ 1 þ k q s0 ½Q Where I0/I ¼ ratio of the emission intensities without and in presence of quencher, kq ¼ quenching rate constant, s0 ¼ the excited state lifetime in the absence of the quencher and [Q] ¼ quencher concentration. It should be noted here that Stern-Volmer analysis cannot be used to distinguish between PET and PeT. Reliance on other techniques like transient absorption spectroscopy is necessary to distinguish between the two pathways.34,49

• •

Atom Transfer Radical Addition (ATRA): This mechanism involves the photogeneration of radical intermediates from reagents or substrates via coupled atom/electron transfer interactions with the PC*. These radicals can then take part in either propagation or addition reactions. ATRA is intermediate between inner and outer sphere reactions, since the radicals can be generated either by oxidative/reductive quenching of PC*, or by a direct atom transfer from the substrate to the PC*. Inner sphere: These reactions proceed through a pathway that involves the formation of a bond between the excited PC* and a substrate. Inner sphere reactions can be advantageous since transient radicals/intermediates can potentially be stabilized, which, in turn, can lead to pathways not accessible by other mechanisms.

8.04.3

Recent developments in photoredox organic transformations using molecular inorganic catalysts

8.04.3.1

Mononuclear photocatalysts

This section describes redox photocatalytic reactions that are driven by a photocatalyst composed of a single metal center. Generally, photocatalysts with saturated coordination like Ru polypyridine and Ir cyclometalated complexes take part in outer sphere processes while those with the ability to expand their coordination sphere (or with a tendency to lose ligands, especially in an electronically excited state) such as Cu(I) complexes participate in inner sphere processes.

8.04.3.1.1

Second and third row transition metals

Second and third row transition metal complexes have been the most widely explored chromophores/photocatalysts for photoredox catalysis. These coordination compounds often exhibit long-lived triplet excited states, that, compared to organic dyes and 3d metal complexes, are readily accessible due to the large spin-orbit coupling of 4d and 5d central ions. The generally larger ligand field splitting also renders dissociative MC states less accessible. Depending upon the ligand environment and metal oxidation state, the excited states can have pure 3MLCT, 3LMCT, intraligand (3IL) or mixed (3MLCT-3LC or 3LMCT-3LC) character. These complexes often exhibit highly reversible redox behaviors and can be potent photooxidants and reductants. 8.04.3.1.1.1 Ruthenium(II) polypyridine complexes These compounds were until the 2000s the most employed PCs for photoredox organic transformations.14,50 In what was one of the earliest examples of transition metal complex-based light-driven synthesis, Deronzier and co-workers demonstrated the use of [Ru(bpy)3]2þ to drive the Pschorr reaction in 1984.13 Irradiation of stilbenediazonium tetrafluoroborate salts in the presence of the irradiated PC gave the corresponding phenanthrene product in a quantitative yield. Quenching and flash photolysis experiments revealed that the reaction proceeds via oxidative quenching of the PC* and consequent formation of an aryl radical (Scheme 1). This is followed by an intramolecular radical addition, oxidation by [Ru(bpy)3]3þ and deprotonation to yield the product. Since then, [Ru(bpy)3]2þ and related compounds have been used to catalyze a wide range of reactions, including dehalogenations, cycloadditions, trifluoromethylations and biaryl couplings.51,52 The Yoon group, among several others has been instrumental in utilizing Ru PCs for light-driven organic transformations. In a report which demonstrates the efficiency of these compounds, Ru(II)-based PCs were employed to drive three mechanistically different reactions.53 A [2 þ 2] cycloaddition of enones, as well as a-alkylation of aldehydes were photoinitiated by [Ru(bpy)3]2þ, while [Ru(bpz)3]2þ (bpz ¼ 2,20 -bipyrazine) was used to carry out a [4 þ 2] Diels-Alder cycloaddition. These reactions proceed via reductive quenching of the corresponding PC*, and each involved the formation of a different radical intermediate: anion, neutral and cation, respectively. Interestingly, quantum yield measurements and quenching experiments indicate the dominance of radical chain processes in product formation, rather than the normally expected closed catalytic cycle (Scheme 2). Considering the most common cationic nature of the photocatalysts, it is essential to understand the role of counterions in such reactions. To document their impact, a radical cation Diels-Alder cycloaddition between anethole and isoprene was photo-driven using a series of [Ru(btfmb)3]2þ salts (btfmb ¼ 4,40 -bis(trifluoromethyl)-2,20 -bipyridine) bearing six different counter anions with varying coordinating ability.54 Unexpectedly, under ideal conditions, a drastic change in reaction rates was observed upon changing the character of the counterion (Scheme 2). A systematic investigation revealed that the coulombic ion-pair formation ability of the

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Scheme 1

109

Proposed mechanism of the photocatalytic Pschorr reaction, which proceeds via oxidative quenching of [Ru(bpy)3]2þ*.

Scheme 2 Proposed mechanism of a photocatalytic radical cation Diels-Alder cycloaddition, which proceeds via reductive quenching of the corresponding [Ru]2þ*. Mechanistic studies reveal that the chain propagation pathway dominates over the generally expected closed catalytic cycle pathway.

counterions affects multiple aspects of the reaction mechanism. For instance, the counterion [BArF4] (tetrakis[3,5bis(trifluoromethyl)phenyl]borate) which has the least ability to form ion-pairs with the cationic photocatalyst as well as the anethole radical cation intermediate, gave rise to a stronger photooxidant (E*red ¼ þ1.52 V), longer radical cation chain lengths and shorter reaction times compared to the ArFSO3 bearing photocatalyst (E*red ¼ þ1.08 V). The sluggish catalytic performance of the latter could be improved by inclusion of a sulfonate-binding co-catalyst, which assists in removing detrimental ion-pair interactions. Relevantly, these coulombic effects are more profound in the non-polar solvents that are generally used for photoredox catalysis. These examples illustrate the advantage of having a strong photooxidant/reductant to drive a synthetic reaction, especially when the criteria of redox potential matching with those of the substrate/s is met. However, certain transformations like the oxidation of

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mono and disubstituted aliphatic alkenes are difficult to achieve, given their high oxidation potentials (>2.5 V vs. standard calomel electrode, SCE). To overcome this challenge, a redox auxiliary strategy was adopted, where aryl vinyl sulfides were used instead.55 These modified alkenes have accessible oxidation potentials (1.1 V to 1.4 V vs SCE), and the aryl sulfide moiety can be easily reductively cleaved post-reaction. Using this approach, a radical cation Diels-Alder cycloaddition was successfully carried out using [Ru(bpz)3]2þ, producing cycloadducts which were previously thermodynamically unattainable. In addition to the PeT dominated processes discussed above, Ru(II)-based photocatalysts have also been shown to participate in energy transfer reactions. One such study was documented by Wenger, Bach and co-workers, when they employed [Ru(bpz)3]2þ or [Ru(bpy)3]2þ along with chiral ene-iminium ions to drive [2 þ 2] photocycloaddition reactions.56 Chiral ene-iminium ions are known to be strong photooxidants (in their singlet excited state) and have played an important role in photochemical organocatalysis.57–61 Their triplet excited state chemistry, however, was virtually unexplored.62 To allow triplet sensitization by the Ru(II) complexes, the authors used chiral ene-iminium ions derived from cinnamic aldehydes (triplet energies, ET  200 kJ mol1). Ensuing reactions with olefins gave enantiomerically refined cyclobutanecarbaldehydes (49%–74% yields), a class of compounds that were never obtained previously by direct [2 þ 2] photocycloadditions (Scheme 3). Laser flash photolysis revealed that [Ru(bpz)3]2þ operates almost exclusively by a triplet energy transfer mechanism, while [Ru(bpy)3]2þ undergoes both energy and electron transfer, with only the former pathway yielding the desired cycloaddition products. These results were further corroborated by using fac-Ir(ppy)3, (ppy ¼ 2-phenylpyridine) which is rapidly quenched by the ene-iminium ions via electron transfer, resulting in poor product yields ( 370 nm) and in the presence of O2, both complexes were found to catalyze the photocyanation of tertiary amines (W1: 77%–88%, W2: 78%–95%), and W2 could drive the hydroxylation of aryl boronic acids (57%–82%). More recently, the same group further modified the Schiff base ligand scaffold to improve the visible-light absorption and alter the excited state reduction potentials.111 Again, the prepared compounds were emissive (3IL), with lifetimes in the microsecond range. One of these complexes, W3 (Fem ¼ 0.11, s ¼ 4.6 ms, E*ox ¼ 1.29 V vs SCE, E*red ¼ þ1.1 V vs SCE) was found to be a highly efficient photocatalyst, driving several CeC and CeB bond forming reactions such as borylation of aryl halides (50%–94%), homocoupling of benzylic halides (38%–96%) and arylacylbromides (73%–94%), decarboxylative coupling of redox-active esters (41%–95%), decarboxylative cyanation of redox active esters (40%–88%), dehalogenation of aryl halides (75%–98%) and homocoupling of silyl enol ethers (52%–94%).

Scheme 20 Tungsten(VI) oxo complexes based on Schiff base or quinolinate ligands. Select synthetic transformations and structures of the corresponding photocatalysts are depicted.

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Comparison with fac-Ir(ppy)3 and Ru(bpy)32þ revealed that W3 was either on par or outperformed the conventional photocatalysts, emphasizing the use of W(VI)-based complexes as potential replacements for their rarer and more expensive Ir(III) and Ru(II) central ions.

8.04.3.1.2

First row transition metals

These metals have been less explored as catalysts for photoredox transformations compared to their more ligand substitution inert second and third row counterparts. The corresponding metal complexes generally exhibit short-lived excited states that often are efficiently deactivated by low-lying metal-centered states. Strategies to prevent this population of the dissociative MC states include using strong s-donating ligands, as well as incorporation of metals with completely empty or filled d-orbitals. 8.04.3.1.2.1 Copper homoleptic and heteroleptic complexes Copper(I) complexes are unquestionably the frontrunners among earth-abundant photoredox catalysts.112 Initial studies focused on using homoleptic complexes bearing sterically rigid 1,10-phenanthroline (phen) ligands. As early as 1977, a pioneering report by McMillin et al. shed light on photoinduced intermolecular electron transfer between [Cu(dmp)2]þ and cis-Co(IDA)2(dmp ¼ 2,9dimethyl-1,10-phenanthroline, also known as neocuproine and IDA ¼ iminodiacetato).113 A decade later, Sauvage and co-workers shared their groundbreaking work on using [Cu(dap)2]þ (dap ¼ 2,9-bis(4-methoxyphenyl)-1,10-phenanthroline) to drive reductive CeC coupling of p-nitrobenzyl bromide.114 The rigid ligand environment produces a lifetime (270 ns) suitable for dynamic quenching purposes and makes [Cu(dap)2]þ a strong photoreductant (E*ox ¼ 1.43 V), exceeding the strength of [Ru(bpy)3]2þ. Despite this rich background, it is only recently that the properties of Cu-based homoleptic and heteroleptic complexes have gained wider attention and are being used to photocatalyze a myriad of reactions.115,116 [Cu(dap)2]þ has arguably been the most prolifically used homoleptic Cu(I) photocatalyst, and has been employed for ATRA, reduction and oxidation reactions. In 2015, the Reiser group demonstrated a unique addition of CF3SO2Cl across unactivated alkenes using [Cu(dap)2]Cl as the photocatalyst.117 Remarkably, [Cu(dap)2]Cl displays high selectivity for trifluoromethylchlorosulfonylation, while [Ru(bpy)3]2þ, [Ir(ppy)2(dtbbpy)]þ and Eosin Y show a higher propensity for trifluoromethylchlorination. Mechanistic considerations disclosed that unlike the other PCs used in the study, [Cu(dap)2]Cl acts as both photoreductant and an SO2Cl binding/transfer agent via a concerted inner-sphere process (Scheme 21A). More recently, they used Cu(dap)2Cl to photochemically generate highly iodinated products by an ATRA of iodoform to different olefins, demonstrating both regio- and chemoselectivity.118 Here too, the catalyst displays an ability to bind/stabilize radical intermediates, albeit via loss of a dap ligand, and in the process gains access to elusive reaction pathways (ligand transfer or rebound cycle, Scheme 21B). In an effort to establish structure-activity correlations in Cu(I) homoleptic complexes, Bissember and co-workers synthesized a series of bis(2,9-diaryl1,10-phenanthroline)Cu(I) complexes with sterically and electronically varying substituents.119 These complexes bearing extended p-conjugation were found to be weakly emissive, and as a result, the excited state redox potentials were not calculated. The emissions were predicted to be occurring from 1MLCT excited states due to TADF. Using photochemical ATRA of CF3SO2Cl to olefins as the case study, they demonstrated that steric bulk of the ligands and/or the olefins promotes the chlorination pathway. Interestingly, the more economical and easier to handle Cu(II) salts can also play the role of photocatalyst precursors, or in some instances, the actual photocatalyst. For example, the ligand dap calls for a 4-step synthesis and using Cu(dap)Cl2 as a catalyst would be more cost-effective since only half the amount of ligand is used. In 2018, Reiser, Rehbein and co-workers demonstrated visiblelight mediated oxo-azidation of vinyl arenes in the presence of oxygen, which could be promoted by both [Cu(dap)2]Cl and Cu(dap)Cl2.120 Further analyses using techniques such as EPR, NMR, ATR-IR and radical-trapping experiments revealed intimate interactions between the Cu catalysts and the reagents. Importantly, [Cu(dap)2]Cl enters the proposed Cu(II) catalytic cycle upon excitation, followed by oxidation and loss of a dap ligand (Scheme 22A). This further explains why Cu(dap)Cl2 can catalyze the reaction almost as efficiently. The actual photoactive species was found to be a Cu(II) azide-bridged dimer that forms upon azide coordination. The dimer then undergoes a proposed visible-light induced homolysis (VLIH) to generate a Cu(I) intermediate and an azide radical which adds to the substrate. The Cu(I) intermediate further helps stabilize an oxygen-centered radical and helps eliminate the azidoketone product to regenerate the starting Cu(II) mononuclear species. In another report, the same group carried out chlorosulfonylation of a wide range of olefins, where Cu(dap)Cl2 was used for the first time to carry out ATRA reactions and was actually found to be a more effective photocatalyst than its Cu(I) cousin.121 Here too, Cu(dap)Cl2 is proposed to undergo visible light induced homolysis (VLIH) of the Cu(II)eCl bond, resulting in the formation of a Cu(I) intermediate which may or may not contain a second dap ligand (Scheme 22B). Upon irradiation, this intermediate reduces sulfonyl chlorides to sulfonyl radicals, regenerating Cu(dap)Cl2 in the process. The sulfonyl radical can add to the olefin, forming a C-centered radical which can yield the final product by directly abstracting a chloride from Cu(dap)Cl2. The radical can also directly bind to Cu(dap)Cl2 forming a Cu(III) species, which forms the product by reductive elimination, releasing the Cu(I) intermediate. Utilizing a Cu(II)-based dye with a commercially available ligand system like [Cu(dmp)2Cl]Cl as a precatalyst renders Cu photocatalysis even more accessible and cost-effective.122 This precatalyst was found to be extremely efficient in promoting a number of visible light-driven ATRA reactions, covering a range of mechanistically different reactions such as decarboxylative coupling as well as an Appel reaction (Scheme 23). Additionally, gram-scale functionalization of styrene was achievable with these catalyst systems. Unlike previous reports, this study provided preliminary experimental evidence via UV–visible spectroscopy that Cu(II) catalysts undergo photoinitiation via VLIH. This was backed by a more extensive mechanistic study using intricate techniques such as EPR spin trapping and ultrafast transient absorption spectroscopy.123 Evidence supports a blue-light (lex ¼ 427 or 470 nm)

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Redox photocatalysis

Scheme 21 Concerted, inner-sphere pathways proposed for (A) photochemical trifluoromethylchlorosulfonylation of unactivated alkenes and (B) photo-driven ATRA of iodoform to olefins, using [Cu(dap)2]Cl as the photocatalyst.

mediated homolysis of the Cu(II)eCl bond, which proceeds via a Cl to Cu(II) LMCT, resulting in the formation of a Cu(I) PC and a chloride radical (Scheme 23). These findings further justify the use of Cu(II) complexes as cheap and effective photocatalyst precursors. Apart from substituted phenanthrolines, other much simpler ligand systems have also been successfully employed for creating homoleptic Cu PCs. In 2019, Novák, Peelen and co-workers developed a family of (bis)imino Cu(I) complexes, where the diimine ligands were conveniently synthesized from ethylene diamine and 20 different benzaldehydes.124 These complexes were used to carry out photochemical ATRA of CBr4 to unsubstituted and substituted styrene, as well as addition of ICF2COOEt to styrene generating fluoroalkylated benzyl iodides (Scheme 24). Structure-activity relationships were established, where catalyst activity increased in the order of ortho > meta > para substituted phenyl rings for the investigated diimine ligands. Heteroleptic Cu complexes hold certain advantages over their homoleptic counterparts. Analogous to heteroleptic iridium complexes, the presence of independently tunable ligands can impart desirable photophysical properties to the tailor-made

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127

Scheme 22 Concerted, inner-sphere pathways proposed for (A) photochemical oxo-azidation of vinyl arenes and (B) photo-driven chlorosulfonylation of olefins, using either [Cu(dap)2]Cl or Cu(dap)Cl2 as the photocatalyst. In both examples, a Cu(II) photocatalyst is proposed to undergo a visible light induced homolysis (VLIH) step to generate activated intermediates.

Scheme 23 Photoredox transformations carried out using [Cu(dmp)2Cl]Cl as the precatalyst. The Cu(II) complex undergoes visible light induced homolysis (VLIH) to generate the Cu(I) photocatalyst and a chloride radical.

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Redox photocatalysis

Scheme 24 ATRA reactions carried out by Cu(I) photocatalysts bearing (bis)imine ligands, conveniently prepared from ethylene diamine and 20 different benzaldehydes.

complexes. The synthetic ease of obtaining such complexes is an additional advantage.125 For these reasons, substantial research has been directed towards development of new heteroleptic Cu(I) photocatalysts.126 As with homoleptic Cu(I) complexes, the general strategy to synthesize mixed ligand Cu(I) complexes benefits from structurally rigid ligands with wider bite angles and steric bulk to prevent excited state flattening distortion. Commonly used ligands include diphosphines,127,128 binaphthols, bis-isonitriles, PN and PNP pincers mixed with polypyridine-based ligands. In 2015, Chen and co-workers prepared and characterized four zwitterionic heteroleptic Cu(I) complexes containing 1,10phenanthroline derivatives and a nido-carborane-diphosphine ligand (NCDP).129 The complex equipped with 2,9-dimethyl-4,7diphenyl-1,10-phenanthroline (bcp), Cu(bcp)(NCDP), exhibited an excited state lifetime of 1.4 ms at ambient temperature and E*red ¼ þ1.01 V, indicating a strong photooxidant. Upon visible light irradiation, the complex showed high activity for oxidative cross dehydrogenative coupling reactions between tetrahydroisoquinolines and nitroalkanes. The proposed catalytic mechanism (Scheme 25) includes reductive quenching of the PC*, leading to a tetrahydroisoquinoline radical cation and a Cu radical anion. The latter transfers an electron to molecular oxygen, forming a superoxide radical anion O2-. and regenerating the resting state of the catalyst. Interaction between these radical ions generated by the Cu(I) photocatalyst, followed by further radical propagation steps, lead to an iminium cation, which finally undergoes a nucleophilic attack by the nitroalkane to yield the product. The authors also carried out EPR studies to confirm that superoxide, not singlet oxygen is formed during the photocatalytic cycle and has a major role to play. In the same year Reiser and co-workers reported a series of [Cu(I)(phenanthroline)(bis-isonitrile)]þ complexes for visible lightdriven ATRA and allylation reactions.130 The complex [Cu(dpp)(binc)]BF4 (dpp ¼ 2,9-diphenyl-1,10-phenanthroline, binc ¼ bis(2-isocyanophenyl)phenylphosphonate) especially displayed intriguing properties. With a lifetime of 17 ms (measured in poly(methyl methacrylate), PMMA) and an excited state oxidation potential E*ox ¼ 1.88 V, [Cu(dpp)(binc)]þ is potentially a stronger photoreductant than the widely used [Cu(dap)2]Cl. Indeed, [Cu(dpp)(binc)]þ exhibits superior activity in the ATRA of diethylbromo-malonate to N-boc-allylamine (boc ¼ tert-butyloxycarbonyl) when irradiated at 455 nm. This reactivity trend is contrasted by the relatively higher extinction coefficient of [Cu(dap)2]Cl at 455 nm (and 530 nm), suggesting that the longerlived excited state of the bis-isonitrile complex has a defining role to play in its catalytic activity. This was further corroborated by TD-DFT calculations, which predict [Cu(dpp)(binc)]þ to exhibit phosphorescence rather than TADF at ambient temperature. The catalyst was proposed to promote the reaction through oxidative quenching by the organic halide generating a C-centered radical species which adds to the substrate to form another C-centered radical intermediate. This regenerates the starting Cu(I) complex via reduction, forming a radical cation which is then trapped by the bromide to yield the product (Scheme 26). In addition, the complex was also confirmed to perform as an efficient photocatalyst for allylation of trimethylallylsilane with organic halides. To speed up the discovery of new heteroleptic Cu(I) photocatalysts, Collins and coworkers prepared a combinatorial library of 50 complexes using different diimines and diphosphines (Scheme 27A).131 The library also included homoleptic diimine complexes for comparative purposes. All chromophores were then evaluated as photocatalysts for three mechanistically distinct reactions; decarboxylative coupling of N-(acyloxy)phthalimide and bromoalkyne (photoredox), homolytic activation of ketones

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129

Scheme 25 Proposed mechanism for the oxidative cross dehydrogenative coupling reaction between tetrahydroisoquinolines and nitroalkanes, photocatalyzed by Cu(I) heteroleptic complexes bearing a nido-carborane-diphosphine ligand and substituted 1,10-phenanthrolines.

Scheme 26 Proposed mechanism of the ATRA reaction between diethylbromo-malonate and N-boc-allylamine, promoted by Cu(I) heteroleptic photocatalysts bearing a bis-isonitrile ligand (binc) and substituted 1,10-phenanthrolines. The complex, [Cu(dpp)(binc)]þ, was found to be the optimal photocatalyst.

(proton coupled electron transfer, PCET) and sensitization of vinyl azides (energy transfer). In general, Cu(I) heteroleptic complexes with BINAP (2,20 -bis(diphenylphosphino)-1,10 -binaphthyl) as the diphosphine were found to be efficient across all three reactions, with triazole-based diimine complexes performing optimally in the PCET process (Scheme 27B). While trends regarding catalyst efficiency could be explained for one process, it proved difficult to extrapolate these structure-activity relationships to other

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Scheme 27 (A) Structures of diimine and bisphosphine ligands employed in the combinatorial synthesis of heteroleptic Cu(I) photocatalysts. (B) Photochemical transformations carried out using the library of heteroleptic Cu(I) photocatalysts.

reactions. For instance, while the catalyst [Cu(dq)(BINAP)]BF4 (dq ¼ 2,20 -diisoquinoyl) (E*ox ¼ 1.87 V, s ¼ 4 ns) was efficient for the photoredox and PCET processes, modification of the diimine was necessary to obtain a similar reactivity for energy transferdriven reactions ([Cu(dmp)(BINAP)]BF4, E*ox ¼ 2.04 V, s ¼ 2188 ns). This study emphasized that prediction of catalyst activity is not always straightforward for these complex, light-driven chemistries and having a catalyst with structural variability is highly advantageous. To further support this finding, the same group developed a new bifunctional photocatalyst [Cu(pypzs)(BINAP)] BF4 which consists of sulfonamide moiety appended pyridine-pyrazole diimine ligand (pypzs ¼ 5-(4-fluorosulfonyl)amino-3(2-pyridyl)-pyrazole).132 This catalyst, in the presence of Hantzsch ester (HEH) was found to photochemically drive a reductive pinacol-type coupling reaction (Scheme 28). A wide variety of aldehydes and ketones were used as substrates and as a result synthesis of valuable diols could be achieved under mild conditions. Interestingly, for certain substrates [Cu(pypzs)(BINAP)]þ even outperforms a previously established Ir(III) photocatalyst [Ir(dF(CF3)ppy)2(bpy)]PF6. Further studies using 1H NMR, solvent studies and alternate H-atom donors shed some light on the mechanism. The excited state of the photocatalyst (E*red ¼ 1.96 V,

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131

Scheme 28 Photochemical synthesis of valuable diols via a reductive pinacol-type coupling reaction. The bifunctional photocatalyst, [Cu(pypzs)(BINAP)]þ, upon a concerted PCET step, can participate in hydrogen bonding with the carbonyl substrate through the sulfonamide NH as well as the pyrazole NH. ox s ¼ 8.95 ns) is reductively quenched by HEH (E1/2 ¼ 0.404 V), producing the oxidized HEHþ.. The reduced catalyst can then participate in hydrogen bonding with the carbonyl substrate via the sulfonamide NH as well as the pyrazole NH and forms a ketyl radical through a concerted PCET process. The ketyl radical forms the pinacol-type intermediate upon interaction with another aldehyde or ketone that transforms into the final diol upon receiving an H-atom from HEHþ., which also regenerates the starting photocatalyst via proton transfer. More recently, the Collins group reported 12 new [Cu(N^N)(P^P)]BF4 complexes bearing p-extended diimine ligands in an attempt to further expand the library of heteroleptic complexes.133 Parent heteroleptic complexes containing a dmp ligand, along with the corresponding homoleptic complexes were also synthesized for comparison. A direct consequence of the p-extension was the red-shifted absorptions and larger extinction coefficients of the new heteroleptic complexes than the parent dmp complexes. Overall, the new heteroleptic complexes displayed similar photophysical properties. When tested for an oxidative Appel-type photochemical reaction, the p-extended diimine heteroleptic complexes were found to be active, although barely more than their parent complexes. Interestingly, the p-extended diimine homoleptic complexes were found to be much more active than [Cu(dmp)2]BF4. Moreover, the new complexes (both homoleptic and heteroleptic) were inactive for photochemical PCET, reflecting their inability to undergo reductive quenching in the excited state. While this strategy gave new complexes that are photochemically active for oxidative transformation processes, it does not allow for tuning of photophysical properties. A theme which has gained the attention of researchers in recent years is the introduction of an ‘entatic state’ in Cu(I) photocatalysts. Prevalent in Cu-containing enzymes, this state is essentially a constrained geometry which lowers the reorganizational energy of the catalyst during redox processes and in turn makes them more efficient. Intriguingly, guanidine-quinoline-based ligands have been successfully employed to synthesize Cu(I) and Cu(II) homoleptic complexes with geometries intermediate between tetrahedral and square planar.134,135 Such complexes, however, have not been utilized for photochemical applications. Thus, in 2019, Poisson and co-workers developed a new guanidine-quinoline ligated heteroleptic Cu(I) catalyst [Cu(DPEPhos)(DMEGqu)]PF6 (where DPEPhos ¼ Bis[(2-diphenylphosphino)phenyl] ether and DMEGqu ¼ N-(1,3-dimethylimidazolidin-2yliden)quinolin-8-amine) which was used to photocatalyze borylation of organic halides (Scheme 29).136 Catalyst screening alongside well-known Cu(I) heteroleptic photocatalysts, using DIPEA as a base and blue light irradiation revealed the new complex to be the best catalyst. Substrate scope included iodo and bromoarenes, heteroarenes, as well as vinyl and alkyl iodides, all yielding

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Scheme 29

Photochemical borylation of various substrates driven by the ‘entatic’ photocatalyst, [Cu(DPEPhos)(DMEGqu)].

good to excellent yields. Moreover, the reaction could be extended to a continuous flow process, with yields of 68%–81% on a 5 g scale. Based on mechanistic studies which included a radical clock experiment, and electrochemical analyses, a Cu(I)/Cu(I)*/Cu(0) reaction pathway was proposed. Upon excitation, the photocatalyst undergoes reductive quenching (E*red ¼ þ0.77 V) by the base (E1/2 ¼ þ0.77 V) to yield a Cu(0) species, which in turn transfers an electron to the aryl iodide, returning to its resting Cu(I) state. The in-situ generated aryl iodide radical anion loses the iodide and forms a C-centered radical. Trapping with B2Pin2 finally leads to the borylated product. 8.04.3.1.2.2 Titanocenes Titanium is one of the most earth-abundant metals that is well-known for its applications in semiconductors (as TiO2) and homogeneous catalysis, but has now become a catalyst option in tandem photoredox schemes.137 Surprisingly, titanium(IV) based photoactive complexes are virtually unknown. The only example of such a catalyst was reported in 2020 by the Gansäuer and Flowers groups,138 where titanocene, Cp2TiCl2 (Cp ¼ cyclopentadienyl anion) was used to promote the reductive ring opening of epoxides, as well as 5-exo cyclization of unsaturated epoxides under green LED irradiation. Cp2TiCl2 exhibits an extremely short-lived LMCT excited state and is known to suffer from photodecomposition via loss of the Cp ligand.139 However, this short lived excited state still undergoes reductive quenching in presence of a base like DIPEA, leading to the formation of Cp2TiIIICl following the loss of a chloride (Scheme 30). This event creates a vacant site at the metal center, allowing the epoxide substrate to bind through ring

Scheme 30

Cp2TiCl2 promoted reductive ring-opening of epoxides in the presence of a thioglycolate HAT catalyst and DIPEA as the base.

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opening and consequently yields a b-titanoxy radical. The latter is reduced by a thioglycolate HAT catalyst (methyl thioglycolate, MTG) which in turn forms an S-centered radical. The HAT catalyst is regenerated via another HAT by the DIPEA radical cation, and the resulting cationic species yields the product as a hemiaminal, releasing back Cp2TiCl2 to the catalytic cycle. This work reveals the potential of Ti(IV)-based complexes in photoredox catalysis, although further research is required for improved catalyst stability and excited state lifetimes. 8.04.3.1.2.3 Vanadium(V) oxo complexes Vanadium(V) oxo complexes have found recent use as photocatalysts in selective CeC bond cleavage reactions. One such example was presented by Soo, Hirao and co-workers, where they used a vanadium(V) compound bearing a redox non-innocent hydrazone imidate ligand to selectively cleave CeC bonds in lignin model compounds.140 The vanadium complex V1 was found to catalyze these reactions under aerobic conditions and visible light irradiation (AM1.5 solar simulator, l > 420 nm), giving moderate to good yields of synthetically important building blocks like aryl aldehydes and formates (Scheme 31). Interestingly, the formate by-product was found to hydrolyze slowly and form a phenol, guaiacol, which can poison the catalyst. Mechanistic studies using isotope labeling, radical trapping and DFT calculations suggested a unique LMCT driven inner-sphere CeC bond cleavage process. This work was an advancement for renewable energy conversion processes since it introduced a benign method featuring a relatively earth-abundant metal photocatalyst to potentially generate fine chemicals from an abundant, inexpensive non-food biomass. The only previous example employing a photochemical method was reported by Stephenson and co-workers, who used a two-step procedure involving a tandem TEMPO-oxidation (TEMPO ¼ (2,2,6,6-tetramethylpiperadin-1-yl)oxyl) and CeO cleavage using [Ir(ppy)2(dtbbpy)]PF6 as the photosensitizer.141 In continuation of their efforts to discover more effective vanadium catalysts for CeC bond cleavage, the same group expanded the catalyst library via ligand modification.142 To prevent the previously observed catalyst inhibition, the authors prepared a substrate which upon degradation and hydrolysis would produce a benzyl alcohol instead of a phenol (Scheme 31). Experimental and theoretical studies showed an increase in catalyst activity (V5 > V4 > V6 > V1  V2 > V3) with increasing electron withdrawing groups placed at select positions on the ligand, which could generally be correlated with their stronger oxidizing power (V6 > V5 > V4 > V1  V2 > V3). The lower activity of V6, the complex with the largest number of electron-withdrawing substituents could be due to hindered regeneration of the reduced V(IV) catalyst. Thus, vanadium(V) oxo complexes were found to be a promising class of photocatalysts, although further ligand modulation is necessary to improve their visible light absorption, as well as tune the redox properties. 8.04.3.1.2.4 Chromium(III) polypyridines The photoactivity and potential photooxidation properties of Cr(III)-based polypyridine complexes have been known for decades.143,144 These complexes exhibit long-lifetimes (up to 425 ms), attributed to the intersystem crossing from the quartet excited state to the ‘spin-flipped’ 2[E] state, as well as high excited state reduction potentials (E*red ¼ þ1.40 to þ1.84 V vs SCE). Surprisingly, it was only in 2015 that the first example of Cr(III)-based photoredox catalysis was reported.145 This seminal work by Shores and Ferreira demonstrated the ability of four Cr(III) bipyridine and phenanthroline-based complexes to photocatalyze radical cation mediated intermolecular and intramolecular [4 þ 2] Diels-Alder cycloadditions of a large variety of dienes and dienophiles in good to excellent yields. Importantly, these complexes were found to be more active than the conventionally used Ru(II) polypyridines. The reaction was proposed to be initiated by the reductive quenching of the Cr(III) excited state by the dienophile, generating the corresponding radical cation. Interestingly, oxygen was necessary for the reaction to proceed efficiently and was speculated to play a stabilizing role for the radical intermediates. To gain further insight, detailed analyses were carried out by the Shores and Rappé groups using [Cr(Ph2phen)3]3þ and its reduced form [Cr(Ph2phen)2(Ph2phen)$-]2þ.146 For this investigation, experimental techniques such as catalysis screening, electrochemistry and spectroscopy were combined with theoretical calculations of reaction

Scheme 31 Vanadium(V) oxo complexes used as photocatalysts for selective CeC bond cleavage of lignin models. The presence of electronwithdrawing or electron-donating groups has a profound effect on catalyst activity.

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intermediates. These studies revealed that while the reaction is indeed initiated by the reductive quenching of [Cr]3þ* (yielding the reduced photocatalyst [Cr]2þ), 3O2 plays a crucial role of protecting the photocatalyst from degradation by quenching the excited state, forming 1O2 and the ground state [Cr]3þ (Scheme 32A). 1O2 also serves to regenerate [Cr]3þ by oxidizing [Cr]2þ, forming superoxide 2O2$-. Finally, the latter can reduce the radical cation of the cycloaddition adduct, yielding the product and reforming 3O2. This hypothesis was partly supported by an independent DFT study carried out by Dang, Yu and co-workers, where it was proposed that oxygen operates via an inner-sphere process rather than the outer sphere energy transfer or superoxide suggested previously.147 Through theoretical calculations, the authors suggested that 3O2 quenches [Cr]3þ* to [Cr]2þ via spin modulation brought about by spatial interactions with the ligand p-orbitals. 3O2 also stabilizes [Cr]2þ by forming a quintet complex, which can then reduce the cycloaddition adduct radical cation (Scheme 32B).

Scheme 32 [Cr(Ph2phen)3]3þ photocatalyzed radical cation mediated intermolecular and intramolecular [4 þ 2] Diels-Alder cycloadditions. Molecular O2 plays a crucial role in the catalyst regeneration and protection either by an outer sphere process (A) or an inner sphere process (B). The photocatalyst can also promote the reaction using electron-poor dienophiles via three converging pathways (C).

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Interestingly, Ferreira and Shores could extend their established photocatalytic protocol to electron poor dienophiles as well.148 The reactions were found to proceed efficiently to the final cyclohexene adducts with high yields of products with reversed regioox selectivity, preferably yielding the meta adducts. Using 4-methoxychalcone (E1/2 ¼ þ2.00 V vs SCE) and isoprene as representative examples, three converging pathways were proposed which show the formation of accessible intermediates for [Cr]3þ*(Scheme 32C). A cascade pathway was described where a photochemical [2 þ 2] cycloaddition, followed by a photoinduced one electron ox oxidative vinylcyclobutane rearrangement (E1/2 ¼ þ1.68 V vs SCE) yields the product. An alternate pathway involved the formation ox ¼ þ1.40 V vs SCE) by [Cr]3þ*. The third pathway includes of an enone radical cation, via photooxidation of the enone dimer (E1/2 the formation of an excited state enone which can be oxidized (E*red ¼ 0.80 V vs SCE) by [Cr]3þ* to give the enone radical cation. The latter can the undergo a direct [4 þ 2] cycloaddition or a [2 þ 2] cycloaddition, followed by rearrangement to yield the final product. O2 also plays a role in these reactions, although not as defining as in the previous cases where electron rich alkenes were employed. These studies show that Cr(III) complexes can follow completely different mechanistic pathways for cycloadditions compared to previously studied Ru(II) polypyridines. In another instance of Cr(III)-based photoredox catalysis, Ohkuma and Arai carried out aza-Diels-Alder type reactions investigating a broad range of N-arylamines and functionalized alkenes while using [Cr(bpy)3](OTf)3 (OTf ¼ triflate anion, CF3SO3) under blue light irradiation.149 The corresponding 1,2,3,4-tetrahydroquinoline products were obtained in moderate to high yields (55%–97%), while Ru(bpy)32þ and [Ir(dF(CF)3ppy)2(bpy)]PF6 were not able to catalyze these reactions. Excellent diastereoselectivity was also observed. The high excited state reduction potential of Cr(III) complex (E*red ¼ þ1.45 V vs SCE) is probably responsible for the activity, although no mechanistic details were provided. Unlike the Cr(III) photocatalysts discussed above, [Cr(ddpd)2]3þ, which bears electron rich ligands, displays a more metalcentered redox activity.47 For instance, reduction of [Cr(ddpd)2]3þ results in a labile Jahn-Teller distorted Cr(II) complex, rather than a Cr(III) center with a radical anion ligand observed in [Cr(Ph2phen)3]3þ and [Cr(bpy)3]3þ. [Cr(ddpd)2]3þ displays properties red ¼ 0.73 V, E*red ¼ þ0.87 V vs SCE) which would mark it as a weaker photooxidant but make it (Fem > 0.1, s ¼ 899 ms, E1/2 a better sensitizer for the generation of 1O2. Hence, to help differentiate between PET vs PeT mechanistic pathways, Heinze and co-workers reported the photoredox activity of [Cr(ddpd)2]3þ towards the formation of a-aminonitriles from tertiary amines and trimethylsilylcyanide (TMSCN) in the presence of air.150 Under optimal conditions and blue LED irradiation (462 nm), Nphenyl-1,2,3,4-tetrahydroisoquinoline (Ph-isoq) underwent a 79% conversion in 30 min, while using a 100 W compact fluorescent lamp (CFL) along with 10 equiv. of the cyanation reagent resulted in full conversion in 2 h. Detailed experimental and computational analyses revealed that 2[Cr(ddpd)2]3 þ * is quenched via PET by 3O2, relaxing the excited state to the ground state 4 [Cr(ddpd)2]3þ and forming 1O2. The latter is proposed to form a transient charge transfer complex with the amine, followed by a hydride transfer which produces an iminium cation and hydroperoxide. The iminium ion is then trapped by CN to yield the photocyanation product (Scheme 33). Collectively, these studies highlight the efficacy of photoactive Cr(III) polypyridine catalysts alongside the advantages of using O2 as a sustainable reagent in photocatalysis. 8.04.3.1.2.5 Iron complexes Iron is the most common transition metal in the earth’s crust and using Fe-based catalysts to drive photoredox transformations would make such processes highly sustainable and inexpensive. Unfortunately, in contrast to its isoelectronic congeners Ru(II) and Ir(III), far fewer articles on Fe(II)-based photoredox have been published. This is mainly due to their low-lying deactivating 3 MC states and consequent non-emissive short-lived nature, making them either unreactive or mechanistically difficult to assess. Nevertheless, encouraging studies in the past have demonstrated ultrafast electron injection by Fe(II)-polypyridines into TiO2,151,152 and as a consequence recent years have seen a renewed interest in exploring these complexes, as well as developing new iron-based catalysts for photoredox catalysis.153,154 In 2015, the Ceroni and Cozzi groups reported the first example of [Fe(bpy)3]Br2-driven stereoselective alkylation of aldehydes with a-bromo carbonyls.155 In combination with the imidazolidone organocatalyst developed by the MacMillan group,156 [Fe(bpy)3]2þ was found to be an efficient photocatalyst for the reaction using a wide range of aldehydes and a-bromo carbonyls, with high enantioselectivities and reaction yields comparable to conventional photocatalysts employed under same conditions. Using this photoredox strategy, the authors were also able to generate enantioenriched lactones, which could lead to biologically relevant compounds such as the natural product ()-Isodehydroxypodophyllotoxin. [Fe(bpy)3]2þ was proposed to operate by the same chain-initiating mechanism as the [Ru(bpy)3]2þ-driven reaction,156 where the chromophore acts as a photoreductant, generating an alkyl radical from the a-bromo carbonyl substrate (Scheme 34). Mechanistic studies using EPR, a radical trap, as well as a radical clock experiment provided further support to the chain-initiation and enamine addition processes. Another example of Fe(II) polypyridine-based photoredox was presented by the Collins group in 2016,157 where they used [Fe(phen)3](NTf2)2 (NTf2 ¼ bis(trifluoromethane)sulfonamide anion) in the presence of O2 to photocatalyze the transformation of di-and triarylamines into the corresponding carbazoles. This study aimed to improve their previously established continuous flow process, which used an in-situ formed heteroleptic Cu(I) complex [Cu(Xantphos)(dmp)]BF4 as the photocatalyst (Xantphos ¼ 4,5Bis(diphenylphosphino)-9,9-dimethylxanthene) and I2 as an oxidant.127 [Fe(phen)3]2þ was found to be a more efficient photocatalyst than the previously studied Cu(I) catalyst, and advances were made in the overall methodology, including use of a benign oxidant, decreased residence times due to the use of a tube-in-tube type reactor, and gram scale preparations with improved yields (90% yield vs 50%–95% for the Cu(I) complex). The oxidation of the amines to carbazoles was proposed to proceed via formation of a superoxide radical, although conclusive mechanistic evidence was not provided. There have been few recent instances of Fe(II) and Fe(III)-based photoredox catalysis, where the photocatalyst is generated insitu when Fe salt precursors are coordinated with added ligands or the substrate itself. In 2019, the Jin group reported a mild and

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Scheme 33 Photochemical synthesis of a-aminonitriles from tertiary amines and trimethylsilylcyanide (TMSCN) catalyzed by [Cr(ddpd)2]3þ via an energy transfer pathway.

effective photoredox protocol for decarboxylative alkylation of heteroarenes, using FeSO4.7H2O and 2-picolinic acid as the ligand.158 In the presence of an oxidant, NaBrO3, and LED irradiation (456 nm), alkylation of a broad range of heteroarenes could be carried out successfully. A large number of carboxylic acid derivatives could also be transformed efficiently. Using the same methodology, the authors carried out a visible light (427 nm) driven CeH oxygenation of 2-biphenylcarboxylic acids using an Fe(III) precursor, Fe(NO3)3.9H2O, NaBrO3 as the oxidant and several pyridine-based stabilizing ligands.159 To avoid the use of the terminal oxidant and excess carboxylic acid, the same group employed a redox-neutral strategy to carry out photodriven decarboxylative CeC and CeN bond formations.160 Starting with an Fe(III) precursor, Fe2(SO4)3 and di-(2-picolyl)amine as the ligand, the authors were effectively able to alkylate the corresponding Michael acceptors or azodicarboxylates, producing the final adducts in moderate to high yields. For all three examples, a similar mechanistic pathway was proposed (Scheme 35). First, deprotonation and coordination of the carboxylic acid to the Fe(III) center forms an Fe(III)-carboxylate complex. This complex is speculated to be photoactive, with an LMCT excited state that leads to a dissociative formation of an Fe(II) species and a carboxyl radical. Further radical propagation steps initiated by nucleophilic addition of the carboxyl radical lead to the final product. Fe(III) is regenerated either by the oxidant or a radical intermediate. The role of the added ligand/s was crucial since no reaction occurred in its absence. UV–visible spectroscopy demonstrated significant absorption of the metal-ligand mixtures in the visible range, suggesting the ligand/s assists in the light absorption, and help modulate the redox potentials of the Fe salts. Mechanistic evaluation using a radical trap (2,2,6,6tetramethylpiperidine 1-oxyl, TEMPO) shuts down the reaction, confirming the radical nature of these reactions. Another example of ligand-assisted Fe-photoredox was demonstrated by Noël, Alcázar and co-workers in 2019, when they utilized an N-heterocyclic carbene (NHC) ligand (3-bis(2,6-diisopropylphenyl)-imidazolinium chloride, SIPr$HCl) and Fe(acac)3 to carry out a Kumada-Corriu cross-coupling reaction between aryl halides and Gringard reagents in a continuous flow process.161 Under blue LED irradiation and optimized conditions, n-propylbenzene could be obtained in high yields (98%) from chlorobenzene and n-propylmagnesium bromide. The scope of the reaction could be extended to a wide variety of aryl chlorides (homo and hetero, electron rich and neutral), as well as Gringard reagents, with short residence times of 5–20 min. Using this protocol, multigram scale synthesis of an unprotected indole, 5-cyclohexyl-1H-indole could also be obtained with a 95% yield. Kinetic measurements in the absence and presence of light revealed that light is needed throughout the process. Furthermore, in-line UV–visible analysis of the reaction mixtures suggest the formation of new Fe(I) and Fe(III) species upon the addition of the reagents under irradiation. Based on this evidence, the authors proposed an initial reduction of Fe(III) to Fe(I) by the Grignard reagent, followed by a light-promoted oxidative addition of the aryl chloride. Transmetalation/metathesis followed by reductive elimination releases

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Scheme 34 Stereoselective alkylation of aldehydes with a-bromo carbonyls photocatalyzed by [Fe(bpy)3]2þ, in presence of an imidazolidone organocatalyst.

the product and completes the cycle (Scheme 36). This study shows that Fe-based photocatalysts can be used as a more sustainable alternative to Ni and Pd-based catalysts for cross-coupling reactions. Fe precursor-based photoredox catalysis can be made even more sustainable and atom-efficient if the substrate plays an active role in the formation of the photoactive complex. This was demonstrated by Xia, Yang and co-workers in a study that used such a ligand-free and base-free approach for the difunctionalization (aminoselenation) of alkenes, using FeBr3 as the precursor.162 Under blue LED irradiation, styrene could be efficiently transformed into N-(1-phenyl-2-(phenylselanyl)ethyl)aniline (82% yield) in an air atmosphere. Moreover, this catalyst system outperformed the conventional photocatalysts [Ru(bpy)3]2þ, [Ir(ppy)2(dtbbpy)]þ and Eosin Y which provided negligible conversions. The optimized protocol could be extended to a large scope of alkenes, amines and diselenides, showcasing the effectiveness of this photocatalytic system. Mechanistic studies were carried out to determine the reaction pathway. While treatment with TEMPO led to diminished yields, UV–visible analyses revealed the formation of a blue-light absorbing species in a mixture containing FeBr3 and the amine. Additionally, this mixture was found to emit strong fluorescence at 332 nm which could be quenched by the diselenides, but not the alkene. These results suggest the formation of a photoactivable Fe(III)-amine complex, which undergoes reductive quenching via an single electron transfer (SET) from the diselenide, generating a selenide radical and a selenide cation upon cleavage (Scheme 37). These activated species add to the alkene, forming the corresponding benzylic intermediates. As a consequence, both selenide radicals are converted to their respective cations via oxidation. The benzylic cation forms the product either by nucleophilic addition of the amine, or via a reaction with the reduced Fe(II)-amine complex, releasing the Fe(II) catalyst that can then be oxidized back to Fe(III) by air, closing the cycle. 8.04.3.1.2.6 Cobalt(III) complexes The photophysical and photoredox properties of cobalt-based complexes are relatively unexplored compared to other first row transition metals. In a recent report employing the strategy of using electron-rich ligands to access the emissive states of first-row transition metals, Zysman-Colman and co-workers presented the first examples of photoactive Co(III) complexes that show blue emission at RT.163 The chelating ligands consisted of two strongly s-donating hpp moieties (hpp ¼ 1,3,4,6,7,8hexahydropyrimido[1,2-a]pyrimidine) attached to a central pyridine or pyrazine ring. The resulting homoleptic.

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Scheme 35 Photoredox transformations using photocatalysts generated in-situ from Fe salt precursors, stabilizing pyridine-based ligands and carboxylate substrates.

Scheme 36 Photochemical Kumada-Corriu cross-coupling reaction catalyzed by a Fe-based photocatalyst generated in-situ from Fe(acac)3 and an NHC ligand, SIPr$HCl.

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139

Scheme 37 Ligand-free and base-free photochemical aminoselenation of alkenes using FeBr3. Coordination of the amine substrate to the Fe precursor results in the formation of the photoactive catalyst.

Co(III) complexes Co1 and Co2 were found to have emissive 3LMCT states, exhibiting luminescence lifetimes in the nanosecond range, as well as extremely strong excited state reduction potentials (Co1: Fem ¼ 0.007, s ¼ 5.07 ns, E*red ¼ þ2.26 V vs SCE; Co2: Fem ¼ 0.004, s ¼ 3.21, 8.69 ns, biexponential, E*red ¼ þ2.75 V vs SCE). In fact, their E*red values are much higher than those reported for conventional photocatalysts, making these Co(III) complexes one of the most powerful photooxidants to be synthesized till date. To test the photoredox abilities of Co1 and Co2, the authors chose the relatively unexplored regioselective mono(trifluoromethylation) of polycyclic aromatic hydrocarbons (PAHs). This synthetic transformation has applications in drugs, agro-industry, and materials chemistry, with prior art involving harsh conditions and a lack of chemoselectivity. Under optimized experimental conditions (Scheme 38), selective mono-trifluoromethylation of different PAHs was successfully carried out with 40%–60% yields. For the reaction to work, the substrate had to be photoactive as well, indicating energy transfer as the driving photochemical step. Using pyrene as the substrate, the proposed mechanism involves triplet-triplet energy transfer (TTET) from 3[Co]3þ* to pyrene, resulting in the formation of 3[pyrene]* and regenerating the ground state [Co]3þ. 3[pyrene]* (E*ox ¼ 1.26 V vs SCE) undergoes red PeT via interaction with CF3SO2Cl (E1/2 ¼ 0.18 V vs SCE), forming a pyrene radical cation and a CF3SO2Cl$-, which yields a reactive. CF3 upon fragmentation. The latter can then add selectively to the most electron rich position on pyrene, forming a CF3-pyrene radical. This radical undergoes another PeT, this time with 3[Co]3þ*, leading to the reduced [Co]2þ and a CF3-pyrenyl cation. This cation is deprotonated by the base to yield the final product. The reaction cycle is closed by a single electron transfer between 3 [Co]2þ and the pyrene radical cation, regenerating the ground state photocatalysts. Electrochemical analyses, Stern-Volmer studies and in-situ UV–visible spectroscopy provided further support to the mechanism. Using [Ir(dF(CF3)ppy)2(bpy)][PF6] (E*red ¼ 1.32 V vs SCE) as the photocatalyst instead gave much poorer chemoselectivity, resulting in a mixture of inseparable products. Thus, these novel Co(III) photocatalysts prove to be promising first-row transition metal candidates, and given their powerful redox potentials need to be explored as catalysts for other photoredox transformations. 8.04.3.1.2.7 Nickel(II) multidentate complexes Although nickel-based catalysts have been utilized extensively in tandem photoredox catalysis schemes,164 examples of photoactive nickel complexes are rare. However, recent investigations into tailored ligand design have yielded Ni-based complexes that are luminescent165–167 or more relevantly, display photoredox activity. In 2018, the Hess and Bach groups reported a Ni(II) macrocyclic compound [Ni(Mabiq)]OTf (Mabiq ¼ macrocyclic biquinazoline) that was found to be photoactive for a CeC bond forming reaction.168 Although the Ni(II) complex was found to be non-luminescent, it undergoes a one-electron reduction by NEt3 under visible light irradiation, yielding the reduced species [Ni(Mabiq$-)]. This was confirmed by in-situ UV–visible spectroscopy, as well as by EPR analysis of the independently synthesized reduced complex. Using optimal reaction conditions, [Ni(Mabiq)]þ was found to effectively photocatalyze the cyclization of bromo-alkyl substituted indole (86% yield), with activity and chemoselectivity comparable to Ru(bpy)32þ. The reaction was proposed to occur via reductive quenching of [Ni(Mabiq)]þ* by the amine, followed by

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Scheme 38 (PAHs).

Proposed mechanism for the Co(III) photocatalyzed regioselective mono(trifluoromethylation) of polycyclic aromatic hydrocarbons

subsequent electron transfer to the substrate. The radical species then undergoes cyclization to yield the product (Scheme 39). Based on the results obtained upon using amines with different oxidation potentials (0.78–1.59 V vs SCE), an excited state reduction potential of þ1.25 V (vs SCE) was estimated for [Ni(Mabiq)]þ, making it a stronger photooxidant than Ru(bpy)32þ and comparable to [Ir(dF(CF3)ppy)2(dtbbpy)]þ. The same year, the Gong group reported their findings on using a chiral ligand, DBFOX (4,6-bis((R)-4-phenyl-4,5dihydrooxazol-2-yl)dibenzo[b,d]furan) along with Ni(ClO4)2.6H2O to promote photochemical asymmetric radical addition

Scheme 39

Photochemical cyclization of bromo-alkyl substituted indole driven by [Ni(Mabiq)]þ.

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141

reactions.169 The in-situ generated Ni(II)-DBFOX catalyst exhibited blue-green emission in solution (THF) and a lifetime of 1– 10 ns. Moreover, it catalyzes the addition of tertiary and secondary a-silylalkyl amines to a,b-unsaturated N-acyl pyrazoles with high enantioselectivities (80%–99% ee) under blue LED irradiation. Based on evidence obtained by electrochemical analysis, Stern-Volmer quenching and radical trapping experiments, the Ni(II)-DBFOX catalyst plays a dual role; as a photocatalyst it generates the a-silylamine radical species via reductive quenching (E*red ¼ þ1.53 V vs SCE), and as a chiral Lewis acid it provides the environment for the radical addition by forming an active intermediate with the unsaturated a,b-unsaturated N-acyl pyrazole (Scheme 40). Another well-known macrocyclic ligand, meso-tetraphenylporphyrin (TPP) has found recent use in developing photoactive Ni(II) complexes. In 2019, De Sarkar and co-workers reported the dual photoredox properties of Ni(TPP), i.e. synthetic transformations by oxidative or reductive quenching.170 As expected for porphyrin complexes, Ni(TPP) displayed intense absorptions in the visible region and is emissive with an excited state lifetime, s ¼ 12.9 ms. Electrochemical analysis revealed this complex to a potent photooxidant (E*red ¼ þ1.17 V vs SCE) and reductant (E*ox ¼ 1.57 V vs SCE). These properties were reflected in the ability of Ni(TPP) to photocatalyze CeC bond forming reactions between N,N-dimethylanilines and various maleimides (via reductive quenching), as well as for selenylation and thiolation of anilines (via oxidative quenching). The proposed mechanisms for both reaction types are shown in Scheme 41. For the reductive quenching process, O2 is needed to regenerate the ground state Ni(TPP), as well as for re-aromatization (via peroxy radical) to yield the product. In 2020, Esfandiar and co-workers developed a series of b-brominated TPPs and their Ni(II) and Mn(II) complexes which can promote photooxidation of styrene. Although the free base porphyrins were found to be more active than the corresponding metal complexes, overall stability of the photoredox system improved significantly upon Ni(II) inclusion.171 From these studies, it is evident that Ni-based photoactive complexes are yet another class of promising earth-abundant candidates, and judicious ligand design is required to further expand their applications.

8.04.3.1.3

Other metals

Besides the transition metals mentioned in the previous sections, other transition metal-based complexes such as manganese,176 palladium,177,178 platinum,179,180 rhodium,181,182 rhenium183,184 and gold,185–187 lanthanides like cerium188–193 and gadolinium,194 as well as post-transition metals like aluminum195 have also been demonstrated as photoredox catalysts. A detailed discussion of these examples is beyond the scope of this chapter, but it should be noted that the photoactive complexes based on these

Scheme 40 Photochemical addition of tertiary and secondary a-silylalkyl amines to a,b-unsaturated N-acyl pyrazoles. The photocatalyst is formed in-situ from Ni(ClO4)2.6H2O and a chiral ligand, DBFOX, and plays the dual role of a photocatalyst and a Lewis acid catalyst.

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Scheme 41

Dual photoredox properties of Ni(TPP), catalyzing synthetic transformations via oxidative and reductive quenching pathways.

metals include both traditionally synthesized coordination compounds (Scheme 42), as well as in-situ formed compounds from the corresponding metal salt precursors and additional ligands and/or substrate.196

8.04.3.2

Multinuclear or cooperative photoredox catalysts

The unification of photoredox catalysis with traditional transition metal catalysis has been discovered as another viable and practical strategy for preparative photochemistry.197 For decades, transition metal catalysts have been utilized to promote a wide variety of CeC and C-heteroatom bond forming reactions, with applications in the fields as diverse as process chemistry, materials science, and pharmaceuticals.198 The overall reactivity of the metal catalyst is governed by the rates of the elementary steps such as oxidative addition, transmetalation, and reductive elimination, which, in turn, can be altered and often controlled by judicious ligand design.199–204 Inclusion of a photoactive complex serves as a benign yet powerful alternative to ligand modification. The excited state photocatalyst can directly activate the second transition metal catalyst via oxidative/reductive quenching or generate reactive radical intermediates which can readily stimulate the catalyst (Scheme 43). In some cases, the excited state of the metal catalyst can also be accessed via PET.205 The improved versatility of these coupled systems have increased their use in a vast range of chemical syntheses, including direct cross-couplings involving common functional groups like alcohols, carboxylic acids and CeH bonds.206,207 Several groups have been credited with seminal contributions to cooperative catalytic systems, employing Ir(III) and Ru(II)-based photocatalysts along with transition metals such as nickel (MacMillan,208–211 Doyle,208,212 Molander,213–215 Nishibayashi216,217), palladium (Osawa,218 Sanford,219 Rueping,220 Tunge221,222), gold (Glorius,223,224 Toste,225–227 Shin228,229), copper (Sanford,230 Kobayashi,231,232 Yoon233), rhodium (Rueping234) scandium (Yoon,235) and cobalt (Wu and Lei,236 Li237).

8.04.4

Summary and outlook

The last decade has witnessed an almost exponential growth in the field of photoredox catalysis, which can be attributed to the exceptional photophysical and electrochemical properties exhibited by transition metal-based chromophores (Table 1). Specifically, their capacity to strongly absorb visible light, undergo oxidative/reductive quenching, and in some cases, access the excited

Redox photocatalysis

Scheme 42 catalysts.

143

Structures of additional well-characterized 4d, 5d, lanthanide and post-transition metal complexes that have been utilized as photoredox

Scheme 43 Simplified mechanistic scenarios of cooperative photoredox catalysis, where (A) PC* generates a reactive species which can interact with the transition metal catalyst, or (B) where PC* generates an excited state of the transition metal-substrate complex through an energy transfer process.

state of substrates via energy transfer has resulted in a broad range of light-driven synthetic transformations. This field was initially dominated by Ru(II) polypyridine and Ir(III) cyclometalated photocatalysts, given their superior photophysical robustness. While Ru(II) and Ir(III) complexes continue to be utilized for photoredox catalysis, recent years have seen an influx of new photocatalysts based on earth-abundant transition metals. Among these, rapid advances have been made for copper-based photocatalysts, and their ability to participate in inner-sphere mechanisms can stabilize reactive intermediates, creating opportunities to explore unique reaction pathways. Zirconium(IV)-based pincer complexes are another class of compounds that have displayed fascinating photophysical and photocatalytic properties and are an exciting prospect. Other earth-abundant metals have met with relatively limited success, although careful ligand design has led to promising outcomes and potential photocatalyst candidates. A detained knowledge of the inherent electronic structure of the metal in question can facilitate the chemical modifications required to augment their photoredox properties. This was illustrated recently by the McCusker laboratory, where they studied the excited state quenching dynamics of the non-emissive [Fe(tren(py)3)]2þ (where tren(py)3 ¼ tris(2-pyridyl-methylimino-ethyl)amine) and provided direct evidence that Fe(II) polypyridines engage in electron transfer processes that are mechanistically different than its heavier cousins.238 The recent progress with iron-carbene photoactive complexes further demonstrates the importance of employing strong s-donor ligands that can yield relatively long-lived 3MLCT states and even access 2LMCT states upon variation of the metal oxidation state.46,239,240 Similar headways have been made for chelating isocyanide complexes of Cr(0),241 Ni(0),166 Mo(0),242 and

144

Redox photocatalysis

Table 1

Photophysical and electrochemical properties of catalysts discussed in this chapter.

FPL a (%) Eox a,b (V) Ered a,b (V) E*ox b,c (V) E*red b,c (V) References

Complex

lem a (nm)

Excited State

s

[Ru(bpy)3](PF6)2

615

3

1.1

9.5

þ1.29

1.33

0.81

þ0.77

51

[[Ru(bpz)3](PF6)2

591

3

0.74

N/A

þ1.86

0.80

0.26

þ1.45

51

[Ru(btfmb)3](BArF4)2

573d

3

0.52d

N/A

NA

0.65d

NA

þ1.52

54

[Ru(cpmp)2](PF6)2

709

3

0.48

1.3

þ1.32

0.89

0.47

þ0.90

70

[Ru(cpmp)(ddpd)](PF6)2

755

3

0.056

0.04

þ1.07

0.97

0.61

þ0.71

70

fac-Ir(ppy)3

509

1.38

73

þ0.77f

f

2.24

1.73

þ0.31

172

[Ir(ppy)2(dtbbpy)]PF6

573

3

0.62

17.2

þ1.21

1.51

0.96

þ0.66

173

[Ir(F-mppy)2(dtbbpy)]PF6

547

3

1.22

26.3

þ1.33

1.50

0.94

þ0.77

173

[Ir(dF(CF3)ppy)2(dtbbpy)]PF6

481

3

2.3

68

þ1.69

1.37

0.89

þ1.21

173

[Ir(tF(CF3)ppy)2(pypz)]BArF4

480

d

3

N/A

N/A

þ1.79d

1.42

0.78

þ1.27

80

Na3[Ir(sppy)3]

508g

3

1.6g

73g

þ0.77h

N/A

1.89

N/A

82,84

634

3

0.76

16

þ0.14

N/A

2.2

N/A

91

661

3

0.35

4

þ0.01

N/A

[Cp*Ir(phen)H]Cl

700

3

0.148

N/A

[Ir(dphppy)2(dtbbpy)]PF6

535

3

2.1

[Ir(ttbutpy)(F-mppy)Cl]PF6

525

3

[Ir(ttbutpy)(F-mppy)CN]PF6

488

3

Ph

NMe2

Ir(ppy)2( NacNac

)

Ir(ppy)2(CyNacNacMe)

Me

Ph

MLCT MLCT MLCT MLCT MLCT

e

3

3

MLCT- LC MLCT-3ILCT 3

MLCT- ILCT MLCT-3ILCT 3

MLCT- ILCT MLCT 3

MLCT- ILCT MLCT-3ILCT MLCT MLCT-3ILCT 3

MLCT- ILCT MLCT-3ILCT

(ms)

e

e

d

N/A

91

þ0.88i

i

2.0

1.42

i

1.29

þ0.75i

88

N/A

þ1.24

1.46

1.08

þ0.86

94

3.17

64.6

þ1.70

1.17

0.66

þ1.19

97

3.35

55.8

þ1.95

1.17

0.59

þ1.37

97

325

8.0

þ1.46j

1.60

0.63

þ0.49

45,100

77, 296k

3k

þ0.41l

2.27l

1.80 l

0.06l

101

k

412

k

18

þ0.51l

l

2.26

l

1.68

l

0.07

101

Zr( PDP )2

594

j

3

Zr(HCNN)2

562k

3

Zr( CNN)2

565

3

Zr(MesPDPPh)2

582, 640j

1/3

LMCT-ILCT 350j

45j

þ1.09j

1.69j

0.85

þ0.25

102

Zr( PDP )2

587

1/3

LMCT-ILCT 576

22

N/A

1.86

N/A

þ0.25

103

Mo(MeL)3 Mo(tBuL)3 W1

596m 590m 516,598d

3

MLCT MLCT 3 ILCT

0.225m 4.5m 1.04, 2.37m 19m 62d 2.1d

0.02j 0.08j þ1.47

N/A N/A 1.37

2.2 2.3 0.89

N/A N/A þ0.99

106,107 107 110

W2

515,614d

1/3

ILCT

42d

1.2d

þ1.55

1.42

0.67

þ0.80

110

W3

608d

1/3

ILCT

4.6d

11d

þ1.10f

1.29f

1.29

þ1.10

111

[Cu(dap)2]Cl

670

3

MLCT

d

0.27

N/A

þ0.62

N/A

1.43

N/A

114

[Cu(dmp)2]Cl

700

3

0.09d

N/A

þ0.67

N/A

1.54

N/A

122,174

Cu(bcp)(NCDP)

602d

3

1.4d

4.9d

N/A

1.44d

N/A

þ0.96

129

[Cu(dpp)(binc)]BF4

560n

3

17n

3n

þ0.69

N/A

1.88

N/A

130

[Cu(dq)(BINAP)]BF4

520d

3

0.004d

N/A

þ0.51

N/A

1.87

N/A

131

[Cu(dmp)(BINAP)]BF4,

520d

3

2.19d

N/A

þ0.75

N/A

1.64

N/A

131

[Cu(pypzs)(BINAP)]BF4

620d

3

0.009d

N/A

þ1.11

0.04

0.89

þ1.96

132

Me

Me

Me

k

k

d

3

a

LMCT- ILCT LMCT-3ILCT 3

LMCT- ILCT

j

j

k

3

MLCT MLCT LC MLCT MLCT MLCT

k

j

j

Redox photocatalysis Table 1

145

Photophysical and electrochemical properties of catalysts discussed in this chapter.dcont'd lem a (nm)

Excited State

s

[Cu(DPEPhos)(DMEGqu)]PF6 520d

3

N/A

V5

510

3

N/A

[Cr(Ph2phen)3](BF4)3

744

2

[Cr(bpy)3](ClO4)3

695, 727g

[Cr(ddpd)2](BF4)3

775

Co1

Complex

a

(ms)

FPL a (%) Eox a,b (V) Ered a,b (V) E*ox b,c (V) E*red b,c (V) References N/A

þ0.80

1.96

1.92

þ0.77

136

N/A

þ0.72

N/A

1.71

N/A

142

425

3

N/A

0.27

N/A

þ1.40

145

2

63g

N/A

N/A

0.26

N/A

þ1.45

175

2

898

11

N/A

N/A

0.73

þ0.87

47,150

440, 467, 504

3

0.005

0.7

þ1.75

0.58

1.09

þ2.26

163

Co2

412

3

0.0045

0.4

þ1.98

0.39

1.16

þ2.75

163

[Ni(Mabiq)]OTf

N/A

N/A

N/A

N/A

N/A

0.65

N/A

þ1.25p

168

Ni(II)-DBFOX

430

N/A

N/A

N/A

N/A

1.35

N/A

þ1.53

169

Ni(TPP)

650q

3

12.9q

N/A

þ1.04f

1.44f

1.57

þ1.17

170

MLCT LMCT

o

g

E E E LMCT LMCT

LC

o

g

o

g

a

Measured in MeCN, unless noted otherwise. Versus SCE unless noted otherwise. c Determined from the equations E*ox ¼ Eox - E0–0 and E*red ¼ Ered þ E0–0. d In CH2Cl2. e In toluene. f In DMF. g In H2O. h In aqueous buffer. i Estimated from the potentials of [Cp*Ir(bpy)H]þ. j In THF. k In C6H6. l In dF-C6H4, vs Fcþ/Fc. m In n-hexane. n In PMMA. o In aqueous HCl. p Estimated from studies using different sacrificial amine donors. q In CHCl3. b

W(0),243 as well as for Cr(III) polypyridines244 and Co(III)-hexacarbenes.245 The merger of photoredox with transition metal catalysis is another avenue that has increasingly become popular and has yielded diverse synthetic transformations.197,246 Several new and very promising concepts have come to light during this exploration phase of photoredox catalysis.

• • • •

The use of chiral metal centers along with non-innocent ligands which can enable enantioselectivity.247 The inclusion of photocatalysts which allow operation of inner-sphere mechanisms.248 This can be achieved by installing labile ligands which can be replaced by the substrate during the photocatalytic process. A related, more attractive theme is the in-situ generation of the photoactive complex from metal precursors and added ligands or the substrate itself, making the process more economical.249 The utilization of multiphoton excitation for lab-scale photoredox preparations.250 The adoption of high-throughput methods for synthesis and screening of potential photocatalysts.94,251

Finally, photoredox catalysis is finding applications in a wide variety of fields, for instance, in the synthesis of pharmaceutically and biologically relevant molecules, as well as in materials chemistry in the form of photopolymerizations.252–255 The future for photoredox catalysis seems bright and one can envision some of the above mentioned concepts blending together to produce generalized protocols for photocatalyst fabrication and reaction screening.

Acknowledgments The writing of this chapter was supported by Carnegie Mellon’s Scott Energy Center and the US National Science Foundation (CHE-2102460).

146

Redox photocatalysis

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Ed. 2021, 60, 5370–5376. 192. Shirase, S.; Tamaki, S.; Shinohara, K.; et al. Cerium(IV) Carboxylate Photocatalyst for Catalytic Radical Formation from Carboxylic Acids: Decarboxylative Oxygenation of Aliphatic Carboxylic Acids and Lactonization of Aromatic Carboxylic Acids. J. Am. Chem. Soc. 2020, 142, 5668–5675. 193. Wadekar, K.; Aswale, S.; Yatham, V. R. Cerium Photocatalyzed Dehydrogenative Lactonization of 2-arylbenzoic acids. Org. Biomol. Chem. 2020, 18, 983–987. 194. Ma, J.; Schäfers, F.; Daniliuc, C.; et al. Gadolinium Photocatalysis: Dearomative [2þ2] Cycloaddition/Ring-Expansion Sequence with Indoles. Angew. Chem. Int. Ed. 2020, 59, 9639–9645. 195. Gualandi, A.; Marchini, M.; Mengozzi, L.; et al. Aluminum(III) Salen Complexes as Active Photoredox Catalysts. Eur. J. Org. Chem. 2020, 2020, 1486–1490. 196. Cheng, W.-M.; Shang, R. Transition Metal-Catalyzed Organic Reactions under Visible Light: Recent Developments and Future Perspectives. ACS Catal. 2020, 10, 9170–9196. 197. 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Olsen, E. P. K.; Arrechea, P. L.; Buchwald, S. L. Mechanistic Insight Leads to a Ligand Which Facilitates the Palladium-Catalyzed Formation of 2-(Hetero)Arylaminooxazoles and 4-(Hetero)Arylaminothiazoles. Angew. Chem. Int. Ed. 2017, 56, 10569–10572. 204. Dennis, J. M.; White, N. A.; Liu, R. Y.; Buchwald, S. L. Pd-Catalyzed C–N Coupling Reactions Facilitated by Organic Bases: Mechanistic Investigation Leads to Enhanced Reactivity in the Arylation of Weakly Binding Amines. ACS Catal. 2019, 9, 3822–3830. 205. Welin, E. R.; Le, C.; Arias-Rotondo, D. M.; McCusker, J. K.; MacMillan, D. W. C. Photosensitized, Energy Transfer-mediated Organometallic Catalysis through Electronically Excited Nickel(II). Science 2017, 355, 380–385. 206. McAtee, R. C.; McClain, E. J.; Stephenson, C. R. J. Illuminating Photoredox Catalysis. Trends Chem. 2019, 1, 111–125. 207. Zhang, G.; Cheng, Y.; Beller, M.; Chen, F. 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Visible-Light-Mediated Aromatic Substitution Reactions of Cyanoarenes with 4-Alkyl-1,4-dihydropyridines through Double Carbon–Carbon Bond Cleavage. ChemCatChem 2016, 8, 1028–1032. 217. Nakajima, K.; Nojima, S.; Nishibayashi, Y. Nickel- and Photoredox-Catalyzed Cross-Coupling Reactions of Aryl Halides with 4-Alkyl-1,4-dihydropyridines as Formal Nucleophilic Alkylation Reagents. Angew. Chem. Int. Ed. 2016, 55, 14106–14110. 218. Osawa, M.; Nagai, H.; Akita, M. Photo-activation of Pd-catalyzed Sonogashira Coupling Using a Ru/bipyridine Complex as Energy Transfer Agent. Dalton Trans. 2007, 8, 827–829. 219. Kalyani, D.; McMurtrey, K. B.; Neufeldt, S. R.; Sanford, M. S. Room-Temperature C–H Arylation: Merger of Pd-Catalyzed C–H Functionalization and Visible-Light Photocatalysis. J. Am. Chem. Soc. 2011, 133, 18566–18569. 220. Zoller, J.; Fabry, D. C.; Ronge, M. A.; Rueping, M. Synthesis of Indoles Using Visible Light: Photoredox Catalysis for Palladium-Catalyzed C-H Activation. Angew. Chem. Int. Ed. 2014, 53, 13264–13268. 221. Lang, S. B.; O’Nele, K. M.; Tunge, J. A. Decarboxylative Allylation of Amino Alkanoic Acids and Esters via Dual Catalysis. J. Am. Chem. Soc. 2014, 136, 13606–13609. 222. Lang, S. B.; O’Nele, K. M.; Douglas, J. T.; Tunge, J. A. Dual Catalytic Decarboxylative Allylations of a-Amino Acids and Their Divergent Mechanisms. Chem. A Eur. J. 2015, 21, 18589–18593. 223. Sahoo, B.; Hopkinson, M. N.; Glorius, F. Combining Gold and Photoredox Catalysis: Visible Light-Mediated Oxy- and Aminoarylation of Alkenes. J. Am. Chem. Soc. 2013, 135, 5505–5508. 224. Hopkinson, M. N.; Tlahuext-Aca, A.; Glorius, F. Merging Visible Light Photoredox and Gold Catalysis. Acc. Chem. Res. 2016, 49, 2261–2272. 225. Shu, X.-Z.; Zhang, M.; He, Y.; Frei, H.; Toste, F. D. Dual Visible Light Photoredox and Gold-Catalyzed Arylative Ring Expansion. J. Am. Chem. Soc. 2014, 136, 5844–5847. 226. Kim, S.; Rojas-Martin, J.; Toste, F. D. Visible Light-mediated Gold-catalysed Carbon(sp2)–Carbon(sp) Cross-coupling. Chem. Sci. 2016, 7, 85–88. 227. He, Y.; Wu, H.; Toste, F. D. A Dual Catalytic Strategy for Carbon–Phosphorus Cross-Coupling via Gold and Photoredox Catalysis. Chem. Sci. 2015, 6, 1194–1198.

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228. Patil, D. V.; Yun, H.; Shin, S. Catalytic Cross-Coupling of Vinyl Golds with Diazonium Salts under Photoredox and Thermal Conditions. Adv. Synth. Catal. 2015, 357, 2622–2628. 229. Um, J.; Yun, H.; Shin, S. Cross-Coupling of Meyer–Schuster Intermediates under Dual Gold–Photoredox Catalysis. Org. Lett. 2016, 18, 484–487. 230. Ye, Y.; Sanford, M. S. Merging Visible-Light Photocatalysis and Transition-Metal Catalysis in the Copper-Catalyzed Trifluoromethylation of Boronic Acids with CF3I. J. Am. Chem. Soc. 2012, 134, 9034–9037. 231. Yoo, W.-J.; Tsukamoto, T.; Kobayashi, S. Visible Light-Mediated Ullmann-Type C–N Coupling Reactions of Carbazole Derivatives and Aryl Iodides. Org. Lett. 2015, 17, 3640–3642. 232. Yoo, W.-J.; Tsukamoto, T.; Kobayashi, S. Visible-Light-Mediated Chan–Lam Coupling Reactions of Aryl Boronic Acids and Aniline Derivatives. Angew. Chem. Int. Ed. 2015, 54, 6587–6590. 233. Lee, B. J.; DeGlopper, K. S.; Yoon, T. P. Site-Selective Alkoxylation of Benzylic CH Bonds by Photoredox Catalysis. Angew. Chem. Int. Ed. 2020, 59, 197–202. 234. Fabry, D. C.; Zoller, J.; Raja, S.; Rueping, M. Combining Rhodium and Photoredox Catalysis for C-H Functionalizations of Arenes: Oxidative Heck Reactions with Visible Light. Angew. Chem. Int. Ed. 2014, 53, 10228–10231. 235. Yoon, T. P. Photochemical Stereocontrol Using Tandem Photoredox-Chiral Lewis Acid Catalysis. Acc. Chem. Res. 2016, 49, 2307–2315. 236. Zhang, G.; Liu, C.; Yi, H.; et al. External Oxidant-Free Oxidative Cross-Coupling: A Photoredox Cobalt-Catalyzed Aromatic C–H Thiolation for Constructing C–S Bonds. J. Am. Chem. Soc. 2015, 137, 9273–9280. 237. He, K.-H.; Tan, F.-F.; Zhou, C.-Z.; et al. Acceptorless Dehydrogenation of N-Heterocycles by Merging Visible-Light Photoredox Catalysis and Cobalt Catalysis. Angew. Chem. Int. Ed. 2017, 56, 3080–3084. 238. Woodhouse, M. D.; McCusker, J. K. 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8.05

Luminescence chemosensors, biological probes, and imaging reagents

Lawrence Cho-Cheung Leea,b and Kenneth Kam-Wing Loa,c, a Department of Chemistry, City University of Hong Kong, Kowloon, Hong Kong, P R China; b Laboratory for Synthetic Chemistry and Chemical Biology Limited, New Territories, Hong Kong, P R China; and c State Key Laboratory of Terahertz and Millimeter Waves, City University of Hong Kong, Kowloon, Hong Kong, P R China © 2023 Elsevier Ltd. All rights reserved.

8.05.1 8.05.2 8.05.2.1 8.05.2.2 8.05.2.3 8.05.2.4 8.05.2.5 8.05.2.6 8.05.2.7 8.05.3 8.05.3.1 8.05.3.2 8.05.3.3 8.05.3.4 8.05.3.5 8.05.3.5.1 8.05.3.5.2 8.05.3.5.3 8.05.3.5.4 8.05.3.6 8.05.3.7 8.05.4 8.05.4.1 8.05.4.2 8.05.4.3 8.05.4.3.1 8.05.4.3.2 8.05.4.3.3 8.05.4.3.4 8.05.4.4 8.05.4.5 8.05.4.6 8.05.4.7 8.05.4.8 8.05.5 8.05.5.1 8.05.5.2 8.05.5.3 8.05.5.4 8.05.5.5 8.05.5.6 8.05.5.7 8.05.5.8 8.05.6 Acknowledgment References

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Introduction Structure–property relationships Lipophilicity Formal charge Molecular size Chirality Counterion Receptor-mediated uptake Biocompatibility Chemosensors Metal cation sensors Inorganic anion sensors Amino acid sensors Thiol sensors Reactive oxygen, nitrogen, carbonyl, and sulfur species sensors Reactive oxygen species Reactive nitrogen species Reactive carbonyl species Reactive sulfur species Nucleic acid sensors Enzyme sensors Biological probes Carbohydrate probes Protein probes Bioorthogonal probes Copper(I)-catalyzed azide–alkyne cycloaddition and strain-promoted azide–alkyne cycloaddition Strain-promoted alkyne–nitrone cycloaddition Strain-promoted sydnone–alkyne cycloaddition Inverse electron-demand Diels–Alder reaction Oxygen probes pH probes Polarity probes Viscosity probes Temperature probes Imaging reagents Nucleus and nucleolus stains Endoplasmic reticulum stains Golgi apparatus stains Mitochondria stains Endosome and lysosome stains Lipid droplet stains Microtubule stains Plasma membrane stains Conclusion

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Abstract In the last decade, there has been fast-growing interest in the development of photofunctional transition metal complexes for various biological applications due to their rich photophysical and photochemical properties. In this Chapter, we summarize the recent exploitation of luminescent complexes of transition metal centers having a d6, d8, and d10 electronic configuration in living systems, with a focus on their use as chemosensors, biological probes, and imaging reagents. The structure–property relationships of transition metal complexes are described to shed light on how the lipophilicity, formal charge, molecular size, chirality, and counterion of the complexes affect their cellular uptake and localization properties. Methods to manipulate the cellular uptake and biocompatibility of the complexes are also discussed. The design strategies of transition metal complexes as chemosensors for analytes including ions (e.g., metal cations and inorganic anions), small molecules (e.g., amino acids and biothiols), reactive species (e.g., reactive oxygen, nitrogen, carbonyl, and sulfur species), nucleic acids, and enzymes are reviewed. Additionally, the development of transition metal complexes as biological probes for visualizing specific biomolecules and monitoring the intracellular microenvironment (e.g., oxygen level, pH, polarity, viscosity, and temperature) are summarized. Furthermore, the strategic design of transition metal complexes as imaging reagents of specific cellular structures are described.

8.05.1

Introduction

Luminescent molecular probes have been developed as an important and indispensable tool in life science for the visualization of biological molecules in their native settings, which provides valuable information to the understanding of complex biochemical processes.1 The extensive development of organic fluorophores has made significant contribution to the design of fluorescent probes for bioimaging and biosensing applications.2–6 In recent years, there has been fast-growing interest in the exploitation of the photophysical and photochemical properties of photofunctional transition metal complexes in biological applications. In particular, luminescent complexes of transition metal centers having a d6 (e.g., rhenium(I), ruthenium(II), osmium(II), rhodium(III), and iridium(III)), d8 (e.g., platinum(II) and gold(III)), and d10 (e.g., gold(I) and zinc(II)) electronic configuration have received considerable attention.7–11 The high structural diversity and synthetic versatility of these complexes allow facile manipulation of their photophysical and photochemical behavior through the use of ligands with various structural, electronic, and coordinating properties. Phosphorescent transition metal complexes are particularly attractive as they usually display intense, long-lived, and environment-sensitive emission with large Stokes’ shifts, which minimize possible self-quenching effects. Also, the long emission lifetimes (in the sub-microsecond time regime) eliminate interference caused by the short-lived autofluorescence (on the nanosecond timescale) and thus improve the sensitivity by the use of time-resolved techniques such as phosphorescence lifetime imaging microscopy (PLIM).12 The high photostability of these complexes enables their continuous exposure to irradiation and allows realtime monitoring in biological systems. Additionally, many of these complexes have been identified as efficient photosensitizers for singlet oxygen (1O2), which renders them attractive candidates for photodynamic therapy.13 Furthermore, the absence of many of these metals in living systems enables the cellular uptake of the complexes to be quantitatively determined by inductively coupled plasma-mass spectrometry (ICP-MS).14 These remarkable features have led to the successful development of luminescent transition metal complexes for diagnostic and therapeutic applications. In this Chapter, we summarize the applications of luminescent transition metal complexes with a d6, d8, and d10 electronic configuration as molecular probes in living systems, with a focus on their use as chemosensors, biological probes, and imaging reagents. The structure–property relationships of transition metal complexes are described to shed light on how the lipophilicity, formal charge, molecular size, chirality, and counterion of the complexes affect their cellular uptake and localization properties. Methods to manipulate the cellular uptake and biocompatibility of the complexes are also discussed. The design strategies of transition metal complexes as chemosensors for analytes including ions (e.g., metal cations and inorganic anions), small molecules (e.g., amino acids and biothiols), reactive species (e.g., reactive oxygen, nitrogen, carbonyl, and sulfur species), nucleic acids, and enzymes are reviewed. Additionally, the development of transition metal complexes as biological probes for visualizing specific biomolecules and monitoring the intracellular microenvironment (e.g., oxygen level, pH, polarity, viscosity, and temperature) are summarized. Furthermore, the strategic design of transition metal complexes as imaging reagents of specific cellular structures are described.

8.05.2

Structure–property relationships

There has been extensive research on the structure–property relationships of transition metal complexes to gain insights into their cellular uptake and localization properties, which have found to be strongly associated with the lipophilicity (log Po/w values) and formal charge of the complexes. The molecular size, chirality, and counterion of the complexes also play an important role in determining their cellular uptake efficiency and intracellular localization. Additionally, the cellular uptake of the complexes can be mediated by specific membrane-bound receptor proteins after conjugation with a biological substrate. Furthermore, the biocompatibility of the complexes can be enhanced through structural manipulation, which is critical to their applications as biological probes and imaging reagents in living systems. These properties are discussed with selected examples in the following sections.

8.05.2.1

Lipophilicity

In a pioneering study, the cellular uptake of ruthenium(II) dipyrido[3,2-a:20 ,30 -c]phenazine (dppz) complexes 1–5 (log Po/w ¼ 2.50– 1.30) has been examined by laser-scanning confocal microscopy (LSCM), which reveals that only the most lipophilic complex 5 (log

154

Luminescence chemosensors, biological probes, and imaging reagents

Po/w ¼ 1.30) is efficiently taken up by human cervical adenocarcinoma HeLa cells, giving rise to intense emission in the cytoplasmic region.15 Flow cytometric analyses on the cellular uptake mechanism of complex 5 show that the complex is internalized into the cells through passive diffusion.16 In another study, ICP-MS measurements indicate that the cellular uptake efficiency of iridium(III) complexes appended with an alkyl chain (6–8) follows the order: 7 (log Po/w ¼ 5.34) > 6 (log Po/w ¼ 2.01) > 8 (log Po/w ¼ 9.89).17 The lowest cellular uptake of the most lipophilic complex 8 is possibly due to its largest molecular size and/or self-aggregation in aqueous solution. Investigation on the subcellular distribution of zinc(II) complexes with a salen- or salophen-derived ligand (9– 17) reveals that the cellular localization properties of the complexes are strongly associated with their lipophilicity, which depends on the substituents at the 4-position of the salicylaldehyde scaffold.18 Complexes bearing hydrophilic groups such as pyridinium (9), N-alkylated piperidine (10) and piperazine (11), morpholine (12 and 13), and glucose moieties (14) (log Po/w ¼ 0.48– 1.77) are accumulated in the endosomal/lysosomal compartments, whereas complexes carrying hydrophobic pendants (15–17) (log Po/w > 2) are enriched in the endoplasmic reticulum (ER) of the cells. Notably, the lipophilicity of transition metal complexes is not only determined by the hydrophobicity of the ancillary ligands, but also the nature of the metal centers. As exemplified by a pair of ruthenium(II) and osmium(II) complexes that possess the same charge and ligand scaffold, the ruthenium(II) complex 18 (log Po/w ¼ 2.23) is localized in the mitochondria, while its osmium(II) counterpart 19 (log Po/w ¼ 1.24) is accumulated in the lysosomes of the cells.19

n N

N

Ru

N

N

N

N

N

N

O N

=

N

n

N

_

O

N

N

=

O O

N

+2 1

+2 2

N

N

N

N

N

N

0 3

+2 4

+2 5

+ O C C

N

NH

N

Ir

n

N

N

n=1

(6)

9

(7)

17 (8)

NC N O + N

2+

CN Zn

N O

N

N 9

+ N

Luminescence chemosensors, biological probes, and imaging reagents NC

CN

N O

Zn

N O

R R=

155

R

N

(10)

N

N N

(11)

(12)

N

O

OH

N

N

O

N

(13)

O OH

N

O

NN N

OH (14) OH

HO

(15)

N

(16)

N

NC N O

CN Zn

N O

N

N 17

2+

N N

N

N

M

N

N O

N

OH

M = Ru (18) Os (19)

8.05.2.2

Formal charge

Cellular studies on iridium(III) complexes functionalized with polar ester or carboxylate groups (20–23) show that the cationic complexes 20 and 21 are readily taken up by HeLa cells and enriched in the endosomes and mitochondria, respectively, whereas the anionic complex 22 is localized in the cell surface and complex 23 is incapable of penetrating the plasma membrane.20 The less efficient cellular uptake of the anionic complexes is most likely due to the charge repulsion between the complex and the negativelycharged phospholipid bilayers of the cells. The neutral trinuclear rhenium(I) complex 24 is membrane-impermeable.21 However, upon silver(I) ion sequestration, the resultant cationic adduct is efficiently internalized into human breast adenocarcinoma MCF-7 cells and accumulated in the nucleoli of the cells. These findings demonstrate that cationic transition metal complexes display more efficient cellular uptake than anionic and neutral complexes. The formal charge of the complexes also plays a pivotal role in determining the intracellular localization of the complexes; for example, the neutral iridium(III) complexes 25–27 are enriched in the ER and lipid droplets (LDs) of rat embryonic cardiomyoblast H9c2 cells, while their cationic counterparts 28–30 are localized in the mitochondrial region.22

156

Luminescence chemosensors, biological probes, and imaging reagents

n C C

N Ir N

H2 N N H2

O N

N

=

_

N

_

C

O

_

O

N

N

_

_

C

O

C

_

C

C _

O =

n

O

O

+1 20

OC

_

+1 21

CO

O N

N

CO Re

CO CO

N

N

N

O

Re OC

1 23

N

O N

N

_

1 22

N

Re

OC

O

CO

CO 24 CN

F

CN C

F F

C

N Ir N

N

C

N N NN

C

N Ir N

N

C

N N NN

C

N

N

Ir

N N NN

N

F 25

26

27 CN

+

+ F C

F F

+

CN

C

N Ir N

F 28

N

C

N N NN +

C

N Ir N

29

N

C

N N NN +

C

N Ir N

N N N NN + 30

Luminescence chemosensors, biological probes, and imaging reagents 8.05.2.3

157

Molecular size

As revealed by ICP-MS measurements, the cellular uptake efficiencies of the more lipophilic, octanuclear iridium(III) complexes 31 and 32 (log Po/w ¼ 1.66 and 2.61) are 5.8- and 10.8-fold lower than those of their less lipophilic, monomeric counterpart complexes 33 and 34 (log Po/w ¼ 0.44 and 2.01), respectively, which is ascribed to the higher cationic charge and much larger molecular size of the dendritic skeletons.23 Temperature dependence and chemical inhibition experiments indicate that the dendritic complexes are taken up by HeLa cells via an energy-requiring process (e.g., endocytosis), whereas the monomeric complexes are internalized into the cells through an energy-independent pathway (e.g., passive diffusion). Additionally, the cellular localization properties of the complexes are different; the dendritic complexes 31 and 32 are specifically accumulated in the Golgi apparatus, while the monomeric complexes 33 and 34 are enriched in the perinuclear region of the cells. Polyhedral oligomeric silsesquioxanes (POSSs) that are modified with eight iridium(III) complexes (35–37) also exhibit less efficient cellular uptake than the mononuclear, POSS-free complexes (38–40).24 The POSS complexes 35–37 are diffusely distributed in the cytoplasm of HeLa cells, whereas their POSS-free counterparts 38–40 are localized in the mitochondria. Structure–property relationship analyses on iridium(III) solvato complexes (41–45) show that an increase in the length of the carbon side chain and branched chains in the cyclometalating ligands leads to the translocation of the complexes from the nucleus (41 and 42) to the cytoplasm (44 and 45) (Fig. 1), which appears to be controlled by the nuclear pore complexes in the nuclear envelope.25

8+ C C

C

Ir

N

N

N

C

N

Ir

N

N

N

O HN

N

N

NH

O C C

N Ir N

N

NH

NH

O

O O

N

NH

HN

N

NH

N

N HN

N

NH O

HN

Ir C

NH

O

NH

HN N

NH

O

O

O

N N

N

N _

C

=

N

N

N

Ir

C

C

N _

C 31

_

C 32

N

N Ir C

N

NH

O NH

C

N

HN

N

N

O

O

O

O

N

N

N

O

N

N

C

O

NH HN

Ir

NH

O O

N

O

C

N C

N Ir N

C C

158

Luminescence chemosensors, biological probes, and imaging reagents

Fig. 1 LSCM images of live HeLa cells incubated with complexes 41–45 (10 mM, 10 min, 37  C). Reproduced from Li, C.; Liu, Y.; Wu, Y.; Sun, Y.; Li, F. Biomaterials 2013, 34, 1223–1234, with permission from Elsevier. Copyright 2013.

+

O N

C

Ir

C

N

N

N C

_

NH

N

=

N C 33

N _

C 34

_

Luminescence chemosensors, biological probes, and imaging reagents

159

+ R O Si R Si O O O R O Si Si O R

N _

R

O

Si Si O C R O R= O C Si R O O Si R

N

N

=

Ir N

N _

C

N

N N

N _

_

C

C 35

C

36

37

+ N

C C

N

=

_

N

Ir

N

N

N

N

N _

C

C

N _

_

C

38

C

39

40 +

R R

C C

N Ir N

NCCH3 NCCH3

R=H

(41)

CH3

(42)

CH2CH3

(43)

CH(CH3)2 (44) C(CH3)3

8.05.2.4

(45)

Chirality

It is well known that the chirality of transition metal complexes plays a critical role in their interactions with biomolecules. As revealed by LSCM, the DD-isomers of the binuclear ruthenium(II) complexes 46 and 47 show more prominent nuclear staining than the LL-isomers, which are accumulated in the nucleoli of fixed Chinese hamster ovary (CHO)-K1 cells.26 However, no significant difference is observed in the subcellular distribution of the complexes in live cells, probably due to the entrapment of the complexes in the endosomes of the cells after endocytic uptake. The enantiomers of ruthenium(II) complex 48 display distinctive staining in live cells; the L- and D-isomers are enriched in the nucleus and cytoplasm of human breast adenocarcinoma MDA-MB231 cells, respectively (Fig. 2).27 Temperature dependence and chemical inhibition experiments indicate that the L-isomer is readily taken up by the cells via endocytosis and transported to the nucleus through an adenosine triphosphate (ATP)-dependent active transport mechanism.

160

Luminescence chemosensors, biological probes, and imaging reagents

4+ H N N N

N

N Ru

N

H N N

N

N

N

N

N Ru

N

N N

46 4+

N N

N Ru N

N

H N

H N

N

N

N

N

N

N Ru N

N N

47 2+

N N

N Ru N

N

H N

N

N

48

8.05.2.5

Counterion

The cellular uptake, and more importantly the nuclear uptake of the cell-impermeable ruthenium(II) complexes 1 and 4 are significantly enhanced in the presence of pentachlorophenol (PCP), which is due to the formation of neutral, lipophilic, and relatively stable ion-pair complexes that are efficiently internalized into the cells via passive diffusion.28 The nuclear uptake of the ion-pair complexes is correlated positively with their binding affinity but inversely with their lipophilicity; a high binding affinity and low lipophilicity are essential to minimize non-specific interactions of the ion-pair complexes with proteins and membrane structures in the cytoplasm of the cells.29,30 The complexes are readily effluxed to the cytoplasm and localized in the mitochondria after incubation of the cells in a fresh culture medium; the mode of efflux is energy-dependent and associated with the ATP-binding cassette (ABC) transporter proteins including ABCB1, ABCC1, and ABCG2.31 The nuclear efflux of the L-isomers of the complexes is faster than that of the D-isomers, possibly due to the weaker binding of the L-isomers to DNA. The addition of 2,3,4,5tetrachlorophenol (TeCP) to the culture medium redirects the complexes back to the nucleus, highlighting that chlorophenolate anion serves as a shuttle carrier and modulates the cellular uptake of cationic transition metal complexes.

8.05.2.6

Receptor-mediated uptake

The incorporation of a biological substrate such as carbohydrate, hormone, and vitamin into transition metal complexes renders them to enter the cells through a receptor-mediated pathway. Glucose, which is the most important carbohydrate in cellular metabolism and an energy source for the growth of cells, is transported into the cells through the membrane-bound glucose transporters (GLUTs).32 The high rate of glucose uptake and metabolism in cancer cells is associated with the overexpression of GLUTs.33 ICPMS measurements on iridium(III) complexes modified with a D-glucose (49), D-galactose (50), D-lactose (51), or D-maltose moiety (52) reveal that the cellular uptake efficiency of conjugate 49 is 2.0- to 3.3-fold higher than the other sugar conjugates.34 The cellular uptake of an iridium(III) complex appended with a D-glucose pendant (53; log Po/w ¼ 2.44) is also found to be ca. 4 times higher than its D-galactose counterpart (54; log Po/w ¼ 2.30) regardless of their similar lipophilicity.35 Temperature dependence and chemical inhibition experiments indicate the involvement of GLUTs in the uptake of conjugate 53, which is further supported by its reduced uptake in the presence of D-glucose and 2-deoxy-D-glucose but not L-glucose. A related D-glucose-functionalized rhenium(I)

Luminescence chemosensors, biological probes, and imaging reagents

161

Fig. 2 LSCM images of MDA-MB-231 cells incubated with complexes L-48 (top) and D-48 (bottom) (5 mM, 6 h, 37  C; red) and stained with DAPI (0.5 mg mL1, 5 min, 37  C; blue). The emission intensity profiles correspond to the yellow lines shown in the overlaid images. Reproduced from Zeng, Z.-P.; Wu, Q.; Sun, F.-Y.; Zheng, K.-D.; Mei, W.-J. Inorg. Chem. 2016, 55, 5710–5718, with permission from American Chemical Society. Copyright 2016.

complex (55) also exhibits similar GLUT-mediated uptake.36 Additionally, the glucose conjugates show more efficient uptake by transformed cell lines (HeLa and MCF-7) than non-transformed cell lines (human embryonic kidney HEK293T and mouse embryonic fibroblast NIH/3T3) due to the overexpression of GLUTs in transformed cells.35,36 The substrates of GLUTs are not limited to glucose; for example, GLUT5 is known to show selective uptake of fructose.37 This transporter is overexpressed in breast cancer tissues but its expression in other cancer cells and normal breast tissues is limited.38 The rhenium(I) complex bearing a fructose unit (56) displays more efficient uptake by human breast adenocarcinoma cell lines (MCF-7 and MDA-MB-231) over non-breast cancer (human lung carcinoma A549 and human hepatocellular carcinoma HepG2) and non-transformed cell lines (HEK293T and NIH/3T3).39 The addition of exogenous fructose significantly inhibits the cellular uptake of the conjugate; however, treatment of the cells with glucose or glucose-uptake inhibitors has negligible effects on the uptake. Thus, the cellular uptake of conjugate 56 is mediated by fructose transporters but independent of glucose-specific transporters. Related iridium(III) fructose complexes (57 and 58) also exhibit more efficient uptake by MCF-7 cells than HEK293T cells.40 +

C C

R=

HO

HO

HO

HO

N

O

N

Ir

O 3

N

R

N

N

OH

O OH

(49)

OH OH

O

(50)

OH OH OH

O OH

O HO

OH

O

(51) OH OH

OH

O OH

O HO

OH

O OH

OH

(52)

162

Luminescence chemosensors, biological probes, and imaging reagents

+ S N C

N

Ir

C

O

O

3

N

R

N

N S

R=

HO

HO

OH

O

OH

OH

(53)

OH

O

OH OH

(54)

+

OC OC NH HO HO

NH

NH

Re N

N N

NH

3

S

O

HO

S

CO

O HO 55

+

OC HO

OC OH O OHNH OH

CO Re

O 56

N

N N

Luminescence chemosensors, biological probes, and imaging reagents

+

O C C

N

NH HO

N

Ir

163

O

OH OH

OH

N

N

N

N

=

_

N _

C

_

C

C

57

58

Estrogens play a crucial role in the development and maintenance of normal sexual and reproductive function, and they exert their actions by binding to estrogen receptors.41 The cellular uptake of a ruthenium(II) complex carrying a 17a-ethynylestradiol moiety (59) is significantly reduced at 4  C, indicating that it occurs predominantly via an energy-requiring process.42 Vitamins are essential for cell growth and differentiation. In particular, cancer cells express higher levels of receptors for these nutrients to support their rapid cell proliferation.43 The inhibited cellular uptake of a biotin (vitamin B7)-modified rhenium(I) complex (60) upon incubation at 4  C reveals that the conjugate is internalized into HeLa cells through an energy-dependent pathway.44 Cellular uptake studies on a rhenium(I) complex that is functionalized with cobalamin (vitamin B12) (61) show that the conjugate is taken up by human placental choriocarcinoma BeWo cells via a cubilin receptor-mediated pathway with the assistance of intrinsic factor.45 The uptake is reduced when an excess of cobalamin is added to the culture medium or when the cubilin receptor is not expressed. Cubilin receptor-mediated uptake has also been found for a related rhenium(I) cobalamin conjugate (62), which has been applied to detect cubilin receptors in A549 cells.46

H

OH

H H

OH N N

N Ru N

N

NH

NH 3

O

N

59 +

OC OC

H HN

H NH

Re

N N

N

O

S

CO

NH

NH O

O 60

S NH

NH

2+

164

Luminescence chemosensors, biological probes, and imaging reagents

OC OC

CO Re N

S

+ N

N

N

N

N

N =

NR

NR

N

N

N S

61 H2N

O

NH2

O

O

H2N

NH2

CN N Co N N N

O O H R = H2N

O O NH

HO O _ P O O O

8.05.2.7

62

NH2

N N O

O

O

NH

Biocompatibility

High biocompatibility is essential for a biological probe or an imaging reagent to be employed in living systems. However, many transition metal complexes are considerably cytotoxic toward eukaryotic cells at high incubation concentration and/or long incubation time. Modification of transition metal complexes with a poly(ethylene glycol) (PEG) pendant is a promising approach to enhance the water solubility and biocompatibility of the complexes. Rhenium(I) (63–66)47 and iridium(III) complexes (67– 76)48,49 carrying a PEG unit show substantially reduced cytotoxicity and in vivo toxicity with respect to their PEG-free counterparts. For example, the half maximal inhibitory concentration (IC50) and the median lethal dose (LD50) values of complex 74 toward HeLa cells and zebrafish embryos are determined to be > 300 (12 h) and > 20 mM (96 h post fertilization), respectively, which are much larger than those of its PEG-free counterpart (IC50 ¼ 5.5 mM; LD50 ¼ 1.5 mM).49 Additionally, the incorporation of a cleavable linker that is responsive to light (77 and 78),50 redox (79 and 80),51 and enzymes (81)52 between the complex and the PEG moiety endows a high degree of control of the cytotoxic activity of the complexes, which render them activatable anticancer drugs.

OC n = ca. 112 O

N N

=

On

OC NH O

CO Re N

+ N N NH2

N

N

N

N

N

N

63

64

65

Luminescence chemosensors, biological probes, and imaging reagents

+

O OC OC

CO

N

Re

O

NH

n

n = ca. 112

N

N

O

NH2 66

+

O N

C

N

Ir

C

_

O

23

N

N

N

O

NH

N

=

N _

C

N _

C

_

C

C CHO

67

68

69

+

O

O

NH O C C

N Ir N

N

NH

O

NH

O

4

O

NH

O

O

N O

N C

_

=

N C 70

O

11

O

NH

N _

C 71

_

O

O

O

11

O 11

165

166

Luminescence chemosensors, biological probes, and imaging reagents

+

O N

C

N _

=

C

N

Ir

C

O

NH

n = ca. 112

N

N

N

N _

S

N _

C

O

n

N

N

_

C

_

C

_

C

C

CHO

72

73

74

75

76 +

O C C

N Ir N

O

O

NH

N

O2N

O

O

N NN

N

n

n = ca. 110 (77) ca. 220 (78) +

O C C

N Ir N

N

NH

S

O

O

S

NH

O

O

O n

n = ca. 112

N

O N _

O

N

=

N

_

C

_

C 79

C 80

+

N

N

O

N

O

Au N

O n

n = ca. 113

81

Luminescence chemosensors, biological probes, and imaging reagents

8.05.3

167

Chemosensors

The sensitive and selective detection of a specific cellular species from a plethora of biomolecules is important to the understanding of their roles and functions in different physiological and pathological processes. This requires a probe that is capable of displaying photophysical changes (i.e., changes in emission wavelength, intensity, and/or lifetime) upon the specific binding event or reaction, which can be readily detected by LSCM and PLIM. Compared with those that exhibit emission turn-on at a single wavelength in response to the target analyte, chemosensors that show ratiometric response provide a built-in correction for environmental effects and thus allow a more accurate and precise detection of the analyte both in vitro and in vivo. Chemosensors that display a change in the emission lifetimes are also attractive as the detection is independent of the probe concentration, the power of laser source, and photobleaching, thus improving the sensitivity and reliability of the detection. The design strategies of transition metal complexes as chemosensors for specific analytes are discussed below with selected examples.

8.05.3.1

Metal cation sensors

Zinc(II) ion is the second most abundant transition metal ion in living organisms. It is typically found in proteins as a catalytic, coactive, or structural component and plays an important role in many physiological processes including gene expression, neurotransmission, and immune function.53 Aberrant level of Zn2þ is implicated in various neurological diseases such as epilepsy, Alzheimer’s disease, and Parkinson’s disease.54 Cadmium(II) ion, a congener of Zn2þ that shares similar binding properties, has been reported to be associated with kidney and bone diseases and cancer.55 Di-(2-picolyl)amine (DPA) and its derivatives are well known for their metal cation-binding properties that originate from the strong interaction between the pyridyl nitrogen atoms and various metal cations. Rhenium(I) complexes have been functionalized with a tyraminederived DPA (DPAT) moiety (82–84) for Zn2þ and Cd2þ sensing in live cells.56 These complexes exhibit weak emission due to efficient photoinduced electron transfer (PET) from the DPAT moiety to the excited rhenium(I) polypyridine unit. Upon the addition of Zn2þ and Cd2þ, the complexes show emission enhancement and lifetime extension, as the specific binding of the ions to the DPAT unit substantially suppresses the PET process. The addition of ZnCl2/2-mercaptopyridine-N-oxide (MPO) and CdCl2/MPO to complex 84-loaded HeLa cells leads to enhanced intracellular emission, which is not observed in the cells that are incubated with MPO alone. Iridium(III) DPA complexes (85–91) that display emission enhancement and lifetime extension upon Zn2þ binding have also been developed to image intracellular Zn2þ by LSCM and PLIM.57–59 A luminescent iridium(III) complex appended with a DPA and a 2-pyridylmethylamine moiety on the diimine ligand (92) has been designed as a ratiometric sensor for Zn2þ.60 In the presence of Zn2þ, the complex exhibits a decrease in the intensity of the emission band at 435 nm concomitant with the formation of a new band at 495 nm, resulting in a ca. 6-fold increase in the emission intensity ratio I495nm/I435nm. The complex has been applied to visualize the distribution of Zn2þ in live zebrafish.

OC

N

OC

HO

S

N

NH

CO Re N

+ N N

NH

N

N N

=

N

N

N

N

N

N

82

83

84

168

Luminescence chemosensors, biological probes, and imaging reagents

+

C C

N C

N Ir N

N

N

N

N

=

_

N

C

N

N _

C

N _

C

N _

C

_

CHO 85

86

87

88 +

C C

N Ir N

N N N

N N

N C

_

=

N F

C

N _

F3C

C

F

N _

CF3

89

90

C

_

F 91 +

N C C

Ir N

NH N

N

N

NH

NH

N N N

92 Copper is a redox-active metal that serves as an important catalytic cofactor for many proteins and enzymes such as cytochrome c oxidase, superoxide dismutase, and tyrosinase.61 Elevated levels of copper(II) ion can cause severe oxidative stress, which is implicated in cancer and neurodegenerative disorders. However, the development of chemosensors for Cu2þ is a challenging task as paramagnetic Cu2þ usually quenches the emission of the luminophore by electron and/or energy transfer. The modification of a neutral iridium(III) complex with a DPA moiety affords a ratiometric sensor (93) for Cu2þ.62 The complex shows a short-lived emission band at 486 nm (0.53 ms) and a longer-lived emission band at 648 nm (4.09 ms). Complexation of Cu2þ to the DPA unit leads to quenching of the red emission and thereby a ca. 4-fold increase in the emission intensity ratio I486nm/I648nm, accompanied by a reduction of the lifetime of the red emission to 2.48 ms. Job’s plot analysis of the emission data indicates a 2:1 binding

Luminescence chemosensors, biological probes, and imaging reagents

169

stoichiometry for the complex with Cu2þ. Additionally, the complex displays high selectivity toward Cu2þ over a wide range of biologically relevant metal ions (e.g., Naþ, Kþ, and Mg2þ) and transition metal ions (e.g., Fe2þ, Ni2þ, and Zn2þ). Pretreatment of complex-stained HeLa cells with CuCl2 selectively reduces the red emission (lem > 600 nm), whereas the intensity of the green emission (lem ¼ 535  25 nm) remains relatively unchanged, demonstrating the ratiometric detection of Cu2þ in an intracellular environment.

N

C

Ir

N

C

C

S

N N

N

N

93 Aluminum(III) ion is a competitive inhibitor of several essential metal ions such as magnesium(II), calcium(II), and iron(III) ions that are of similar atomic size and electric charge.63 High concentration of Al3þ can cause damage to the central nervous system (CNS) and thus lead to neurodegenerative disorders including dementia and Alzheimer’s disease. An iridium(III) complex bearing an o-phenolsalicylimine (PSI) moiety (94) has been designed as a chemosensor for Al3þ in live cells.64 The complex is weakly emissive (Fem ¼ 0.0082) and exhibits 13.5-fold emission enhancement at 573 nm in CH3CN in the presence of Al3þ, which is ascribed to the perturbation of the triplet metal-to-ligand charge transfer (3MLCT) state upon the coordination of Al3þ to the PSI unit. The addition of ethylenediaminetetraacetic acid (EDTA) to the Al3þ-coordinated complex decreases the emission intensity due to the sequestration of Al3þ by EDTA. Intense intracellular emission is observed upon exposure of the complextreated HepG2 cells to Al3þ.

+

C C

N Ir N

N

N

N

N

OH N

OH

94 Mercury is one of the most hazardous heavy metals in nature as it has a high binding affinity to sulfhydryl groups and thus it can strongly interfere with cellular processes in living organisms.65 A platinum(II) complex has been conjugated with a rhodamine derivative (95) for the detection of mercury(II) ion in live cells.66 In the presence of Hg2þ, the complex shows 23-fold emission enhancement at 545 nm as a consequence of the Hg2þ-induced desulfurization followed by the opening of the spirolactam ring of the rhodamine moiety. Incubation of the complex-loaded HeLa cells with Hg2þ enhances the emission intensity in the cytoplasmic region. A ruthenium(II) complex carrying a thiourea unit (96) has been developed as a chemosensor for Hg2þ in live cells.67 The design is based on the irreversible Hg2þ-promoted desulfurization and intramolecular cyclization of the thiourea moiety that generates an imidazoline pendant. Upon reaction with Hg2þ in aqueous solution, the complex displays ca. 9-fold emission enhancement with a small hypsochromic shift in its emission maximum, which is attributed to the difference in the excitedstate characteristics between the complex and its reaction product. Treatment of the complex-stained human hepatocellular carcinoma SMMC-7721 cells with Hg2þ results in intense emission throughout the whole cell. Interestingly, the analogous iridium(III) complex 97 exhibits ratiometric response toward Hg2þ.68 The complex shows a decrease in the emission intensity at 560 nm concomitant with an increase in the emission intensity at 620 nm upon reaction with Hg2þ. Exposure of the complex-treated cells to Hg2þ increases the intracellular emission intensity ratio I61515nm/I55010nm from ca. 0.5 to 1.0.

170

Luminescence chemosensors, biological probes, and imaging reagents

C

N

N

Pt

S O

NH NH

N NH

O

NH

95

2+

N N

N Ru N

N N NH

NH

NH S

96

+

C C

N Ir N

N N NH

NH

NH S

97

8.05.3.2

Inorganic anion sensors

Inorganic anions such as halide ions are prevalent in biological systems and they can be both beneficial and harmful to health depending on their concentrations. Chloride is the most abundant anion in living organisms and is associated with many physiological processes including regulation of ion homeostasis, cell volume, and membrane potential.69 Dysregulation of Cl homeostasis is implicated in diseases such as cystic fibrosis, myotonia, and osteopetrosis.70 A platinum(II) complex (98) has been developed for imaging Cl in live cells.71 The complex is non-emissive in water; however, the addition of halide ions (i.e., Cl, Br, and I) to the solution results in the formation of a new emission band at 655 nm, which originates from a triplet metal-metal-to-ligand charge transfer (3MMLCT) emissive state that is associated with halide ion-induced self-assembly of the complex molecules through Pt(II)$$$Pt(II) and p–p interactions. Incubation of the complex-loaded MCF-7 cells in a chloridedeficient buffer reduces the intensity of intracellular emission, indicating the capability of the complex to monitor Cl variations in live cells.

Luminescence chemosensors, biological probes, and imaging reagents

171

+ C

Pt

N

N N

98 Sulfite and bisulfite are endogenously generated from sulfur-containing amino acids in mammals,72 and excessive exposure to SO32 and HSO3 is associated with gastrointestinal diseases.73 Binuclear iridium(III) complexes featuring an electron-withdrawing azo group as a bridge (99–102) have been designed for imaging SO32 and HSO3 in live cells.74,75 The complexes are weakly emissive in aqueous solution (Fem ¼ 0.0018–0.015) due to PET and display substantial emission enhancement (I/Io ¼ 8–27) and lifetime extension in the presence of SO32 and HSO3. Complex 102, which possesses a large two-photon absorption (TPA) cross-section (d750nm ¼ 141.0 GM) in the presence of SO32, has been employed to monitor the levels and distribution of SO32 and HSO3 in the mitochondria of HepG2 and murine macrophage RAW264.7 cells, Caenorhabditis elegans (C. elegans), and rat hippocampal tissue slices upon two-photon excitation.75 An iridium(III) complex containing two aldehyde moieties (103) has been developed for imaging of HSO3 in live normal human liver LO2 cells and zebrafish.76 Upon nucleophilic addition reaction with HSO3, the complex exhibits a 4-fold increase in the emission intensity at 600 nm due to inhibition of the PET from the excited iridium(III) polypyridine unit to the electron-deficient aldehyde group in the resultant aldehyde–HSO3 adduct.

2+ N

C

Ir

C

N

N N N

N

N

N N

N _

C

=

HN

N

N

_

O

100

N C

C

99

C

N

_

C

C

Ir

101

S

N

_

_

C 102

+

C C

N Ir N

CHO N N CHO 103

Cyanide is one of the most toxic anions to living organisms due to its strong binding affinity to the heme unit in cytochrome c oxidase in the mitochondria of the cells, which inhibits the mitochondrial electron transport chain and thus cellular respiration.77 An iridium(III) complex has been conjugated with a copper(II) DPA complex to give a chemosensor (104) for monitoring the uptake of CN by live HeLa cells.78 The weak emission of the complex in aqueous solution (Fem ¼ 0.0047) is due to efficient quenching by the paramagnetic Cu2þ. However, in the presence of CN, the copper(II) DPA complex is destabilized and new copper(II) cyanide complexes (e.g., Cu(CN)2 and [Cu(CN)4]2) are formed due to the strong affinity of CN to Cu2þ. Thus, reaction of the complex with CN gives rise to strong emission enhancement.

172

Luminescence chemosensors, biological probes, and imaging reagents

3+

C C

N

N

Ir

N

N

N

N

N

Cu

OH2

104 Perchlorate is one of the major environmental contaminants due to its widespread industrial use. It poses a threat to human health as it can substitute iodide due to their similar charge and ionic radius, leading to the disruption of thyroid iodine uptake in human body.79 A binuclear iridium(III) complex (105) has been exploited to detect ClO4 in live HeLa cells.80 The complex is non-emissive in solution due to structural distortions that enable facile excited-state relaxation, but shows strong emission in solid state as the intermolecular p–p or C–H$$$p interactions between the phenyl rings can restrict intramolecular relaxation. Upon treatment with ClO4, the complex displays 430-fold enhancement in the emission quantum yield with a hypsochromic shift of 25 nm in the emission maximum. The observed photophysical changes are ascribed to the specific exchange of the counter anion (PF6) with ClO4, which induces trans- to cis- conformational isomerization and the subsequent aggregation of the cis-isomer due to its lower water solubility.

2+

Cl

NN C

Cl Cl

C Cl

Ir

N N

NN N

Cl

NN Ir

N NN

C

Cl Cl

C Cl

105

8.05.3.3

Amino acid sensors

Histidine is one of the essential amino acids for human growth.81 It also plays a pivotal role as a neurotransmitter in mammalian CNS.82 Abnormal levels of histidine and histidine-rich proteins are implicated in pathological conditions such as liver cirrhosis, renal disease, and thrombotic disorders.83 The iridium(III) solvato complex 106 has been reported as the first nuclear stain that is based on the specific metal coordination with histidine and histidine-containing proteins.84 The complex is nonemissive but exhibits significant emission enhancement (I/Io > 300) upon the addition of histidine and histidine-rich proteins such as bovine serum albumin (BSA). Exposure of the cells to the complex results in intense emission in the cell nuclei within minutes, which is attributed to the facile cellular uptake and nuclear accumulation of the complex and the subsequent reaction with histidine and/or histidine-containing proteins inside the nucleus to yield a strongly emissive product. Its structural derivatives (41–45) show similar emission enhancement toward histidine and histidine-rich proteins in the nucleus and/or cytoplasm.25 An iridium(III) complex (107) has been used for two-photon imaging of nuclear histidine.85 Incubation of the complex with histidine increases the intensity of the emission band at 493 nm by 27 fold, which is ascribed to the secondary bonding interactions between the carboxyl group of the complex and the imidazole group of histidine that restrict the nonradiative decay process. Pretreatment of the complex-stained HepG2 cells with histidine decarboxylase (HDC) results in diminished nuclear emission, which is regenerated when the HDC-pretreated cells are incubated with histidine, confirming that the intense nuclear emission is due to the specific histidine recognition. The complex has been utilized to image histidine in the cell nucleus at a higher signal-to-noise (S/N) ratio using stimulated emission depletion (STED) microscopy.

Luminescence chemosensors, biological probes, and imaging reagents

173

+

C C

N

DMSO

Ir

DMSO

N

106

+

O N C C

N

OH N

Ir

N

N

107

Glutamine is a conditionally essential amino acid in living organisms. It is actively taken up by cancer cells for energy production, macromolecular synthesis, redox homeostasis, and signaling.86 An iridium(III) complex (108) has been designed for selective imaging of glutamine in the mitochondria of live HeLa cells.87 Upon the addition of glutamine, the emission of the complex at 557 nm gradually decreases concomitant with the formation of a new band at 475 nm. The complex shows high selectivity toward glutamine over other amino acids and glutamine-containing peptides such as glutathione (GSH). Density functional theory calculations suggest that the selectivity originates from: (1) the coordination of glutamine to the cationic iridium(III) unit through its carboxylate and amine groups, and (2) the formation of hydrogen bonding between the aldehyde group of the complex and the amide group of glutamine, which synergistically stabilize the resultant iridium(III)–glutamine complex. The formation of the iridium(III)–glutamine complex modulates the p–p conjugation between the aldehyde group and the phenyl ring of the cyclometalating ligand, which accounts for the observed photophysical changes.

+

OHC OHC

C C

N Ir N

DMSO DMSO

108

8.05.3.4

Thiol sensors

Biological thiols, such as cysteine (Cys), homocysteine (Hcy), and GSH, play a critical role in maintaining intracellular redox homeostasis. Aberrant levels of biothiols are indicative of different pathological conditions including liver damage, cardiovascular diseases, neurodegenerative diseases, and cancer.88–90 The design of transition metal complexes as chemosensors for biothiols is mainly based on four different reaction mechanisms: (1) nucleophilic aromatic substitution reaction, (2) 1,4-addition reaction of a,b-unsaturated ketones and nitroolefins, (3) redox reaction of azo and disulfide linkages, and (4) cyclization reaction of aldehydes with thiols. Ruthenium(II) and iridium(III) complexes have been functionalized with an electron-withdrawing 2,4-dinitrophenyl (DNP) moiety via a thiol-responsive sulfonamide (109),91 sulfonate ester (110 and 111),92,93 or ether linkage (112–114)94 for the detection of biothiols in live cells. These complexes are weakly emissive due to efficient PET from the metal center to the DNP unit. Upon treatment with thiols, the complexes display substantially enhanced emission due to the departure of the quenching DNP moiety through the nucleophilic aromatic substitution with thiols. Incubation of the cells with the complexes gives rise to intense intracellular emission, which is substantially reduced in intensity when the cells have been pretreated with the thiol scavenger N-ethylmaleimide (NEM), indicating that the complexes are capable of sensing endogenous biothiols in an intracellular environment. Taking advantage of the significant increase in the emission lifetime (from 6 to 225 ns) upon reaction with GSH, complex 110 has been applied to monitor cellular oxidative stress in hepatocytes in response to acetaminophen (APAP)-induced liver injury and hepatic ischemia-reperfusion (I/R) injury.95 The visualization of thiols in specific organelles has been achieved through modification of the

174

Luminescence chemosensors, biological probes, and imaging reagents

complexes with an organelle-targeting unit; for example, complex 111, which contains two DNP moieties and a lysosome-targeting morpholine unit, is capable of sensing biothiols in the lysosomes of HeLa cells.93 Additionally, it has been employed to visualize biothiols in Daphnia magna (D. magna). Strong emission is detected in the regions of esophagus and guts, which should be a result of the transportation of the complex to the body of D. magna through their feeding process and the subsequent reaction with the biothiols distributed in the digestive system. The conjugation of a ruthenium(II) complex with an electron-withdrawing 7-nitro2,1,3-benzoxadiazole (NBD) moiety via a thiol-responsive ether linkage affords complex 115 that is capable of detecting total biothiols and discriminating Cys and Hcy from GSH both in vitro and in vivo.96 In this design, the emission of the ruthenium(II) polypyridine unit is quenched by PET, while the fluorescence of NBD is quenched by the O-substitution at the 4-position. Reaction of the complex with thiols leads to the release of complex 115a with red emission and an NBD derivative NBD-SR (R ¼ Cys and Hcy) that rearranges to give NBD-NHR with intense green fluorescence. Thus, incubation of complex 115 with Cys and Hcy results in an increase in the intensity of the emission band at 628 nm concomitant with the formation of a new emission band at 540 nm (Fig. 3). However, the structural rearrangement is not available for NBD-S(GSH) and thus treatment of the complex with GSH only enhances the emission at 628 nm. The long emission lifetime of the ruthenium(II) complex (> 300 ns) allows the detection of changes in total biothiols in live cells through the application of a time delay to remove the short-lived background autofluorescence and green fluorescence of NBD-NHR (0.80 ns). The two different emission colors of the reaction products enable the discrimination of Cys and Hcy from GSH in live cells, zebrafish, and mice.

2+

N N

N Ru N

N

NH

N

O S O

NO2 NO2

109

O N N

N Ru N

O S O

NO2

2+

NO2

N N

110

O2N

2+ O O S NO2O N N

N Ru N

N N

O O S O NO2 NO2 111

NH

N

O

Luminescence chemosensors, biological probes, and imaging reagents

175

+ O C C

N

NH

N

Ir

NO2

O O

N

N

NO2

112 NO2 N

C

Ir

C

O

N

+

NO2

N

N

113

O C C

N Ir N

+

NO2 NO2

N N

114

OH

Cys /Hcy

N N

O N N

N Ru N

NO N

2+

N Ru N

N

GSH

N

H2N

N

Ru N

N N

115a

NO2 N O N

rearrangem ent

S

N

n

HS

OH

NH n

O

O

n = 1, NBD-S(Cys ) n = 2, NBD-S(Hcy)

OH N

N O N

+

N

115

NO2

115a

NO2

N

2+

2+

OH

n = 1, NBD-NH(Cys ) n = 2, NBD-NH(Hcy)

NO2 N O N

+

O HO

S

O NH2

NH

NH O

O OH

NBD-S(GSH)

Another rationale for the design of thiol sensors is based on the 1,4-addition reaction of a,b-unsaturated ketones with thiols, where the addition of the sulfhydryl group of thiols to the C]C bond will perturb the p–p conjugation between the ketone group and the aromatic ring of the ligand and thus inhibit the intramolecular charge transfer process. An iridium(III) complex appended with two a,b-unsaturated ketone moieties (116) has been prepared for Cys and Hcy sensing in live zebrafish.97 Reaction of the complex with Cys and Hcy causes a ca. 6.5-fold increase in the emission intensity at 580 nm. Selective intracellular Cys sensing has been realized for an iridium(III) complex bearing two a,b-unsaturated ketone units (117).98 Upon reaction with Cys, the complex exhibits an increase in the intensity and lifetime of the emission band at 650 nm, which is due to the inhibition of

176

Luminescence chemosensors, biological probes, and imaging reagents

Fig. 3 (A) Absorption and (B) emission spectra of complex 115 (10 mM) in the absence and presence of GSH, Cys, and Hcy in Tris-HCl buffer (50 mM, pH 7.4). Inset in (B) shows the photo of the emission color of complex 115 in the absence and presence of GSH, Cys, and Hcy. (C and D) Emission intensity of complex 115 (10 mM) at (C) 540 and (D) 628 nm in the presence of different amino acids (200 mM) in Tris-HCl buffer (50 mM, pH 7.4). Amino acids: a) blank, b) GSH, c) Cys, d) Hcy, e) tryptophan, f) threonine, g) glycine, h) valine, i) leucine, j) histidine, k) proline, l) serine, m) tyrosine, n) alanine, and o) aspartic acid. Reproduced from Liu, C.; Liu, J.; Zhang, W.; Wang, Y.-L.; Liu, Q.; Song, B.; Yuan, J.; Zhang, R. Adv. Sci. 2020, 7, 2000458, with permission from John Wiley & Sons, Inc. Copyright 2020.

non-radiative deactivation from the triplet ligand-to-ligand charge transfer (3LLCT) (p(eC(O)C6H5) / p*(2,20 -biquinoline)) state. Incubation of the complex with Hcy and GSH gives negligible photophysical changes. The complex has been exploited to image Cys in human cervical carcinoma KB cells using LSCM and PLIM. An iridium(III) complex that carries a nitroolefin moiety (118) has been reported for selective Cys sensing.99 A substantial increase in the emission intensity at 590 nm is observed upon treatment of the complex with Cys for 30 min, as the quenching nitro group is removed after the 1,4-addition reaction. In the cases of Hcy and GSH, only a small emission enhancement is detected after incubation for 2 h. The fast response of the complex toward Cys (< 30 min) over Hcy and GSH (> 2 h) allows it to distinguish Cys from the other two biothiols. The complex has been used to visualize mitochondrial Cys in live cells, D. magna, zebrafish larvae, and mice. Additionally, the complex has been utilized to monitor Cys-mediated mitochondrial redox activities in inflammatory disorders in murine macrophage J774A.1 cells.

+ O C C

N Ir N

N N O 116

Luminescence chemosensors, biological probes, and imaging reagents

177

+

C O O

N Ir

C

N N

N

117

+

C C

N Ir N

NO2 N N

118 It has been well established that azo and disulfide linkages are prone to reduction by thiols. A non-emissive binuclear iridium(III) complex (119) where the two iridium(III) units are bridged through an azo group has been synthesized for biothiol sensing.100 The complex shows a substantial increase in the emission intensity at 564 nm (I/Io ¼ 30–38) in the presence of thiols, as the azo group is reduced by thiols via a two-electron transfer process to yield an azo anion. Pretreatment of the complex-loaded HeLa cells with NEM significantly reduces the intensity of intracellular emission. A rhenium(I) complex has been conjugated with a perylene diimide (PDI) moiety through a disulfide bond (120) for thiol sensing in the mitochondria of live HeLa cells.101 The complex is weakly emissive in aqueous solution with peak maxima at 557 and 661 nm, which are attributed to the emission of the rhenium(I) polypyridine unit and the aggregated PDI moiety, respectively. The weak emission is due to: (1) efficient Förster resonance energy transfer (FRET) from the rhenium(I) polypyridine unit to the PDI moiety and (2) aggregation-induced quenching of PDI. Treatment of the complex with thiols brings substantial emission enhancement at 557 nm (I/Io ¼ 14.7–29.2) and lifetime extension (from < 10 ns to 1.08 ms), which is due to the reductive cleavage of the disulfide bond and the subsequent release of a highly emissive rhenium(I) complex. Incubation of the complex-treated cells with GSH ethyl ester increases the emission intensity in the mitochondrial region. Negligible emission is observed when the cells have been pretreated with NEM.

2+

C C

N Ir N

N N N

N N N

119

N Ir N

C C

178

Luminescence chemosensors, biological probes, and imaging reagents

+

OC OC O

S

N

O

O

Re N

N N

NH

S

O

O

O

CO

N 3

O 120

Aldehydes are well known for their selective cyclization reaction with the b,g-aminothiol group of Cys and Hcy to form thiazolidine and thiazinane derivatives, respectively. An iridium(III) complex modified with two aldehyde moieties (121) has been designed for two-photon imaging of Cys and Hcy in the mitochondria of the cells.102 The complex is weakly emissive in aqueous solution (Fem ¼ 0.001) but displays 12- and 18-fold emission enhancement in the presence of Cys and Hcy, respectively, which is ascribed to a change in the emissive state from a triplet intraligand charge transfer (3ILCT) (p(phenylamine) / p*(terpyridine)) state to a mixed 3MLCT (dp(Ir) / p*(terpyridine))/3LLCT (p(N^C) / p*(terpyridine)) state. It has been applied to detect mitochondrial Cys and Hcy in live HepG2 cells, multicellular tumor spheroids (MCTSs), zebrafish, and mouse tissue slices. A watersoluble, aldehyde-containing iridium(III) complex (122) has been developed as a chemosensor for Cys and Hcy in live KB cells.103 The weak emission of the complex in aqueous solution (Fem ¼ 0.002) is due to efficient PET. Reaction of the complex with Cys and Hcy substantially increases the emission intensity at 565 nm due to the disappearance of the quenching aldehyde unit. Interestingly, changing the diimine ligand to 4,40 -diamino-2,20 -bipyridine (bpy-(NH2)2) affords a selective ratiometric sensor (123) for Hcy in live HeLa cells.104 Incubation of the complex with Hcy results in quenching of the emission at ca. 585 nm concomitant with the formation of a new band at ca. 495 nm, which is due to a switch of the 3MLCT (dp(Ir) / p*(bpy-(NH2)2) state to a triplet intraligand (3IL) (p / p*) (N^C) state. Treatment of the complex with Cys only leads to a moderate decrease in the emission intensity at 585 nm with a hypsochromic shift of ca. 10 nm in the emission maximum. An iridium(III) complex that carries one aldehyde moiety (124) has also been reported for selective imaging of Hcy in live HeLa cells.105 Emission enhancement is only observed upon incubation of the complex with Hcy but not Cys. The selectivity is attributed to the larger pKa of Hcy-derived thiazinane (> 8.0) than Cys-derived thiazolidine (> 5.0), which inhibits the PET process from the amino group of thiazinane at physiological pH and thus results in emission enhancement. Treatment of HeLa cells with the complex gives rise to intense emission in the perinuclear region. The emission intensity of the cells that are pretreated with NEM and then Cys or lipoic acid remains as weak as those incubated with NEM alone, indicating that the complex exhibits selective imaging of Hcy over Cys and GSH in live cells.

+ O O N OHC OHC

C C

N Ir N

N N

121

N

O O

Luminescence chemosensors, biological probes, and imaging reagents

179

2+

OHC OHC

N

C

N

Ir

C

O

N

N

+ N

NH

122

+

C

OHC OHC

C

N Ir N

NH2 N N NH2

123 + S N C

Ir

C

N

CHO

N N S 124

8.05.3.5 8.05.3.5.1

Reactive oxygen, nitrogen, carbonyl, and sulfur species sensors Reactive oxygen species

Reactive oxygen species (ROS) including superoxide anion radical (O2 –), hydroxyl radical, hydrogen peroxide (H2O2), hypochlorite (OCl), and 1O2 are involved in many physiological processes such as cell proliferation, intracellular signaling, and apoptosis.106 Elevated levels of intracellular ROS can cause severe oxidative stress and damage to nucleic acids, proteins, and lipids, leading to pathological conditions such as inflammation, diabetes, cardiovascular diseases, neurodegenerative disorders, and cancer.107 The first design approach for transition metal complexes as chemosensors for OCl is based on the OCl-induced oxidation of C¼N group to an aldehyde or carboxylate derivative. The photoinduced C¼N isomerization is known to provide a facile nonradiative deactivation pathway for luminescent compounds. Once the isomerization of the C¼N group is inhibited, these compounds are expected to show enhanced emission. An iridium(III) complex has been functionalized with an oxime moiety (125) for sensing and distinguishing endogenous and exogenous OCl in live HeLa cells due to its relatively similar distribution in the cell membrane and the cytoplasm.108 An analogous iridium(III) complex (126) has been prepared for two-photon imaging of OCl in the mitochondria of RAW264.7 cells and in tissue slices from an inflamed mouse model.109 The complex is capable of distinguishing RAW264.7 cells from co-cultured cancerous A549 cells, where both cell types are stimulated to generate OCl. An oxime-modified iridium(III) complex (127) has been reported to display not only emission enhancement and lifetime extension but also strong chemiluminescence upon oxidation of the oxime unit to a carboxyl group by OCl.110 The complex has been employed to image OCl in murine mammary adenocarcinoma LM-3 cells and tissues by LSCM and PLIM and to visualize OCl in live mice by monitoring the chemiluminescence signal, which offers a high S/N ratio as chemiluminescence has negligible background interference. Iridium(III) complexes have also been functionalized with a diaminomaleonitrile moiety (128–133) for imaging OCl in the mitochondria of live cells.111–113 The complexes exhibit a substantial increase in emission intensity upon reaction with OCl, as the C¼N group is oxidized to a carboxylate derivative and thus emission quenching associated with l

180

Luminescence chemosensors, biological probes, and imaging reagents

photoinduced C¼N isomerization is inhibited. The substantial emission enhancement and large TPA cross-sections of complexes 132 (I/Io ¼ ca. 43 at 590 nm; d750nm ¼ 78.1 GM)112 and 133 (I/Io ¼ ca. 80 at 663 nm; d800nm ¼ 97 GM)113 in the presence of OCl render them capable of two-photon imaging of endogenous OCl in lipopolysaccharide (LPS)-stimulated zebrafish and mice, respectively. The strong emission detected in the liver of LPS-treated zebrafish is consistent with the fact that LPS is an endotoxin that can cause liver injury.112 +

NH

N

C

NH

C

7

N

Ir

7

N

OH

N

N

125

+

OH N S N C

Ir

C

N

N

N

N

N S N OH 126

+ S N C

Ir

C

N

N

OH

N N S 127

CN C C

N _

C

=

N

N Ir N

C

N

CN

NH2

N

N _

F

N

+

N

N _

C

_

C

C

N

N

N

_

_

C

S

_

C

F 128

129

130

131

132

133

Luminescence chemosensors, biological probes, and imaging reagents

181

Another design strategy is based on the conjugation of a luminophore with an emission quencher via an OCl-responsive linker. Ferrocene is an electron donor and quenches the emission of the appended luminophore through a PET process. Thus, ruthenium(II) (134)114 and iridium(III) complexes (135 and 136)115,116 that are modified with a ferrocene moiety via an OCl-sensitive hydrazine linker show substantial emission enhancement and lifetime extension within seconds upon incubation with OCl, as the oxidation reaction of the hydrazine linkage with OCl results in the departure of the quenching ferrocene unit. These complexes have been exploited to image OCl in live cells, D. magna, and zebrafish. Additionally, complexes 134114 and 135115 are capable of selectively imaging lysosomal and mitochondrial OCl, respectively. Complex 135 has been used to monitor the OCl generation in mouse liver tissue during I/R injury using two-photon microscopy.115 Ruthenium(II) complexes have been functionalized with a DNP moiety through an OCl-responsive amide linkage (137 and 138) for imaging OCl in live HeLa and RAW264.7 cells.117 Oxidation of the amide linker leads to the departure of the quenching DNP unit accompanied by significant emission enhancement (I/Io ¼ 190 and 1100, respectively). The remarkable 1100-fold emission enhancement and fast response time of complex 138 enable the visualization of OCl generation in the phagosomes of RAW264.7 cells during phagocytosis of zymosan particles. The conjugation of a ruthenium(II) complex with a gadolinium(III) complex via an OCl-responsive 4-amino-3-nitrophenoxy linker gives a heterobimetallic complex (139) for bimodal imaging of OCl in vitro and in vivo.118 The 4-amino-3-nitrophenyl moiety acts as an emission quencher for the ruthenium(II) polypyridine unit; upon reaction with OCl, it is converted to an unstable benzofurazan-1-oxide (BFO) derivative which is then cleaved by the second OCl molecule. Thus, reaction of complex 139 with OCl triggers the release of an emissive ruthenium(II) complex and induces a change in the coordination environment of Gd3þ as the inner sphere water molecule of the gadolinium(III) complex is displaced by the BFO moiety, resulting in an increase in the emission intensity at 612 nm concomitant with a decrease in proton relaxivity. The complex is capable of visualizing endogenous OCl in the lysosomes of stimulated RAW264.7 cells and monitoring OCl generation in a mouse model with lcarrageenan-induced rheumatoid arthritis (Fig. 4). It has also been utilized for magnetic resonance imaging of OCl in mice with inflammation-mediated acute liver and kidney injury.

2+ O N N

N

NH

N

Ru

NH Fe

O

N

N

134

+

O C C

N

NH

N

Ir

NH Fe

O

N

N

N _

C

=

S

N

N

_

C 135

_

C 136

182

Luminescence chemosensors, biological probes, and imaging reagents

NO2

O N N

N Ru N

NH

n

N

2+

NO2

N

n = 1 (137) 2 (138)

NO2

N N

N Ru N

NH

O

N

N O

N

O

N

Gd

O

2+

O

O N N

O O

OH2

139 An OCl-responsive zinc(II) salen complex (140) has been encapsulated into myeloperoxidase for the detection of lysosomal H2O2 in live cells.119 The design rationale is based on the fact that the complex does not display any emission response toward OCl at pH < 8. However, in the presence of myeloperoxidase which generates OCl from H2O2 and Cl, the complex exhibits 81-fold emission enhancement in H2O2- and Cl-containing buffer at pH 4.5–6.0 with negligible interference from other biologically relevant ROS. LSCM reveals that the myeloperoxidase-encapsulated complex is internalized into HeLa cells through endocytosis and transported from early endosomes to late endosomes and lysosomes, and is responsive to both exogenous and endogenous H2O2 in the lysosomes of the cells.

NC N O N

CN Zn

N O

S

S

N

140

Fig. 4 (A) Luminescence imaging of endogenous OCl in mice with rheumatoid arthritis. The mice were injected with phosphate-buffered saline (PBS) containing l-carrageenan (5.0 mg mL1; 100 mL) at the right ankles and PBS (100 mL) at the left ankles. After incubation for 4 h, the mice were injected with complex 139 (100 mM in PBS; 100 mL) at the same area and the emission images were collected at different time points. (B) Time-dependent increase in the mean emission intensities of the right and left ankles of the treated mice (*P < 0.05, **P < 0.01). Reproduced from Shi, W.; Song, B.; Liu, Z.; Zhang, W.; Tan, M.; Song, F.; Yuan, J Yuan Anal. Chem. 2020, 92, 11145–11154, with permission from American Chemical Society. Copyright 2020.

Luminescence chemosensors, biological probes, and imaging reagents 8.05.3.5.2

183

Reactive nitrogen species

Reactive nitrogen species (RNS) refer to nitric oxide (NO) and various NO-derived molecules. NO is an important cellular signaling molecule.120 The diffusion-controlled reaction of NO with O2 – generates peroxynitrite (ONOO). High levels of NO and ONOO are associated with numerous pathological conditions such as inflammation, neurodegenerative disorders, and cancer. The design of transition metal complexes for NO sensing mainly relies on the attachment of an electron-rich o-diaminophenyl moiety to the luminophore to quench the emission via PET. Upon reaction with NO under aerobic conditions, the quenching o-diaminophenyl unit is converted to an electron-deficient triazole ring that blocks the PET process, giving rise to substantial emission enhancement. Rhenium(I) (141–147)121,122 and iridium(III) complexes (148–151)123 have been modified with a diaminoaromatic moiety for the detection of both exogenous and endogenous NO in live cells. Ruthenium(II) (152)124 and iridium(III) complexes (153 and 154)125,126 that feature a 5,6-diamino-1,10-phenanthroline-derived ligand have also been developed as chemosensors for NO. The mitochondria-targeting iridium(III) complex 153 has been applied to visualize endogenous NO in live HeLa cells, MCTSs, and zebrafish using two-photon microscopy.125 Similarly, the iridium(III) complex 154, which shows a significant increase in the intensity (I/Io ¼ 350) and lifetime (from 200.1 to 619.6 ns) of the emission at 587 nm and a large TPA cross-section (d730nm ¼ 60 GM) upon reaction with NO, has been employed to monitor NO generation in LPS-stimulated RAW264.7 cells and zebrafish by LSCM and PLIM upon two-photon excitation.126 The substitution of one of the amino groups of the 5,6-diamino-1,10phenanthroline ligand with a benzyl amine unit endows the complex with high resistance to changes in pH in the range of 4 to 10, and the incorporation of a lysosome-targeting morpholine moiety into the cyclometalating ligands renders the complex capable of specifically visualizing NO in the lysosomes of the cells. l

CO

OC

Re

OC

N N

=

N N

N

H2N

+

NH2

N

N

N

N

N

N

N

N

141

142

143

OC OC

CO Re N

144 +

N N

NH

O

NH2

N N

=

N

N

N

N

N

N

145

146

147

184

Luminescence chemosensors, biological probes, and imaging reagents

+ O N

C

Ir

C

NH

N

NH2

N

N

O N _

N

=

_

F

C

N _

C

O

N

C

N

_

_

C

C

F 148

149

150

151

2+

N N

N Ru N

N

NH2

N

NH2

152 +

C C

N Ir N

N

NH2

N

NH2

153 + O

O

N

C

N

C

N Ir N

N

NH

N

NH2

154 An iridium(III) complex has been functionalized with a DNP moiety through an amide linkage (155) for the detection of endogenous ONOO in stimulated MCF-7 cells and inflamed mice.127 Reaction of the complex with ONOO leads to the oxidative cleavage of the amide bond and the subsequent departure of the quenching DNP unit, which results in 12-fold emission enhancement in the near-infrared (NIR) region (lem ¼ 660 nm). The complex displays high selectivity toward ONOO over other ROS and

Luminescence chemosensors, biological probes, and imaging reagents

185

RNS, particularly O2  and OCl (I/Io < 2) that could trigger the cleavage of the amide bond. Another NIR-emitting iridium(III) complex (156), which exhibits a ca. 100-fold increase in the emission intensity at 702 nm within 300 s upon reaction with ONOO, has been reported for selective imaging of ONOO in the lysosomes of live cells.128 The complex is capable of visualizing the NO/ O2  crosstalk in HepG2 cells, as evidenced by the intense intracellular emission upon treatment of the complex-stained cells with both sodium nitroprusside (SNP; a NO donor) and 2-methoxy-b-estradiol (2-ME; a O2  donor), which is not observed when the cells are pretreated with the ONOO scavenger ebselen or when the cells are incubated with SNP or 2-ME alone (Fig. 5). The complex has been exploited to monitor the ONOO level in APAP-treated LO2 cells and mice, revealing the generation of ONOO in response to liver damage. l

l

l

O

S C

N

C

NH

N

Ir S

NO2

N

+

NO2

OH

N O 155

+ N

N

C

N

F

N

F

Ir N

C

NH

N N

N

F

N

F

N 156 The incorporation of an NIR-emitting, ONOO-responsive iridium(III) complex and a red-emitting, ONOO-insensitive iridium(III) complex into a hydrophilic poly(N-vinyl-2-pyrrolidone) (PVP) backbone yields a dual-emissive polymeric probe 157 for the ratiometric detection of ONOO in stimulated RAW264.7 cells and inflamed mice.129 The introduction of an electron-rich N-(4hydroxyphenyl)amino group to the diimine ligand quenches the emission of the complex mainly via (1) PET and (2) intramolecular motion that facilitates non-radiative deactivation. Upon reaction with ONOO, the complex undergoes an N-dearylation reaction, which leads to the cleavage of the 4-hydroxyphenyl unit concomitant with emission enhancement. Thus, incubation of the polymeric probe with ONOO increases the intensity and lifetime of the emission at 680 nm, whereas those of the emission at 605 nm remain almost unchanged. A substantial increase in the intracellular emission intensity ratio and average emission lifetime (lem  650 nm) is observed in HepG2 cells and in the liver of mice that are pretreated with APAP or ketoconazole, which is suppressed when the cells or mice are incubated with the antioxidant N-acetylcysteine (NAC), indicative of the elevated intracellular ONOO level in drug-induced liver injury and the protective and alleviative effects of NAC on hepatocytes. A ruthenium(II) complex has been conjugated with a cyanine 5 (Cy5) moiety to afford a ratiometric sensor (158) for ONOO in live cells.130 The complex shows strong fluorescence at 660 nm upon photoexcitation at 450 nm, which is due to efficient FRET from the ruthenium(II) polypyridine unit (lem ¼ 610 nm) to the Cy5 moiety (labs ¼ 630 nm). In the presence of ONOO, the absorption of the Cy5 unit is significantly reduced due to the ONOO-mediated oxidative cleavage of the polymethine bridge. Thus, reaction of the complex with ONOO leads to a decrease in the Cy5 fluorescence at 660 nm concomitant with an increase in the ruthenium(II) emission at 610 nm.

186

Luminescence chemosensors, biological probes, and imaging reagents

OH 0.51

O

HN N

N

C

Ir

N

100

+

0.38

O

N

N

N

N

C

+

N

Ir C

N N

O

S

C

N S

N

157 3+ + N O N N

N Ru N

N N

N

N O

N

158

8.05.3.5.3

Reactive carbonyl species

Reactive carbonyl species (RCS) is a class of highly reactive aldehydes including formaldehyde, methylglyoxal, and glyoxal. Formaldehyde is the simplest RCS that has long known been a neurotoxin and carcinogen.131 It can be endogenously generated through various enzymatic reactions or absorbed from the environment. Methylglyoxal and glyoxal are dicarbonyl compounds that are generated as by-products during glycolysis.132 Methylglyoxal is known as the major precursor of advanced glycation endproducts that are implicated in aging, diabetes, and neurodegenerative diseases. A ruthenium(II) complex (159) has been designed for selective imaging of lysosomal formaldehyde using a “dual-key-and-lock” approach.133 The ruthenium(II) polypyridine unit is connected to the quenching DNP moiety through a formaldehyde-responsive linker. The complex is weakly emissive in aqueous solution (Fem ¼ 0.0010) due to efficient PET but displays significant emission enhancement at 644 nm upon reaction with formaldehyde under acidic conditions (pH 3.0–6.0). The emission enhancement is a consequence of the cleavage of the DNP moiety at acidic pH after the formaldehyde-mediated 2-aza-Cope rearrangement. The complex has been used to trace tumor-derived lysosomal formaldehyde and monitor HSO3-induced formaldehyde scavenging in vivo. The aforementioned ruthenium(II) complex (152) and its iridium(III) counterpart (160) have been used for the rapid and effective detection of methylglyoxal in live RAW264.7 cells and D. magna.134 Reaction of the complexes with methylglyoxal substantially increases the emission intensity, as the PET quenching from the amino groups to the metal center is inhibited. The complexes are not responsive toward other RCS and ROS, and they only show a small increase in emission intensity toward glyoxal and NO that are also reactive toward the diamino group.

Luminescence chemosensors, biological probes, and imaging reagents

187

Fig. 5 LSCM images of live HepG2 cells incubated with (A) complex 156 (5 mM); (B) complex 156 (5 mM) and 2-ME (0.5 mg mL1); (C) complex 156 (5 mM) and SNP (200 mM); (D) complex 156 (5 mM), 2-ME (0.5 mg mL1), and SNP (200 mM); and (E) ebselen (100 mM), complex 156 (5 mM), 2-ME (0.5 mg mL1), and SNP (200 mM). Scale bar ¼ 20 mm. Reproduced from Wu, W.; Zhang, C.; Rees, T. W.; Liao, X.; Yan, X.; Chen, Y.; Ji, L.; Chao, H Anal. Chem. 2020, 92, 6003–6009, with permission from American Chemical Society. Copyright 2020.

2+

NO2 NO2 N N

N Ru N

NH

N N

159 +

C C

N Ir N

N

NH2

N

NH2

160

188

Luminescence chemosensors, biological probes, and imaging reagents

8.05.3.5.4

Reactive sulfur species

Reactive sulfur species (RSS) including hydrogen sulfide (H2S), persulfides, polysulfides, and hydrogen polysulfides play many important roles in biological systems. Similar to carbon monoxide and NO, H2S is an important endogenous gasotransmitter that modulates cell signaling.135 Abnormal levels of H2S are associated with pathological conditions such as inflammation, hypertension, diabetes, cirrhosis, and neurodegenerative diseases.136 The iridium(III) complex 161 has been reported for imaging H2S in live cells.137 The complex displays 120-fold emission enhancement at 500 nm in aqueous solution upon the addition of NaHS, which is due to the H2S-induced cleavage of the two ester bonds and the subsequent removal of the acryloyl groups. Common biothiols such as Cys, Hcy, and GSH only cause a small increase in the emission intensity, most likely due to the lower pKa and smaller size of H2S than biothiols. An iridium(III) complex featuring a 5-nitro-1,10-phenanthroline ligand (162) has also been developed for imaging H2S in live HeLa cells and zebrafish.138 The complex exhibits ca. 38-fold emission enhancement at 541 nm in the presence of H2S, as the PET process is inhibited when the electron-withdrawing nitro unit is reduced by H2S to an electron-donating amino group. The aforementioned thiol-responsive iridium(III) complexes 112–114 show a substantial increase in emission intensity in the presence of Na2S over other RSS (e.g., SO32, S2O32, and SCN), which renders them capable of sensing H2S in live HeLa cells.94 Ruthenium(II) complexes have been modified with an electron-withdrawing DNP (163)139 or NBD moiety (164)140 via a H2S-responsive linker for the detection of H2S in live HeLa cells, D. magna, and zebrafish. Complexes 163 and 164 are weakly emissive due to efficient PET quenching but display ca. 86and 65-fold emission enhancement upon reaction with H2S, respectively. Additionally, complex 163 is capable of specifically visualizing H2S in the lysosomes of J774A.1 cells.139 It has been utilized for H2S imaging in live mice, and the increase in emission intensity in response to the LPS and D-Cys treatment demonstrates the capability of the complex in monitoring endogenous H2S level in vivo under different drug stimulation.

+

C C

N Ir N

O N

O

N

O O

161 +

C

F F

C

N Ir N

N

NO2

N

162

NO2 O N N

N Ru N

N

O

N

163

S

NO2

2+

Luminescence chemosensors, biological probes, and imaging reagents

189

2+ O N N

N Ru N

N

N

N

NO N NO2

N

164

8.05.3.6

Nucleic acid sensors

Deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) are important biomolecules that store and transmit genetic information through replication, transcription, and translation, which are important to the growth, development, and reproduction of all living organisms. In eukaryotic cells, most of the DNA is accumulated in the nucleus with some localized in the mitochondria. Since a DNA molecule consists of two polynucleotide chains that form a double helix structure, the most common sensing approach for DNA is based on intercalation, which is the insertion of molecules between the base-pairs of the DNA molecule. The ruthenium(II) dppz complexes 1 and 4 are typical examples of molecular “light switches” for DNA.141,142 This type of complexes are nonemissive in aqueous solution but exhibit intense emission when bound to DNA due to the shielding of the phenazine nitrogen atoms of the dppz ligand from interacting with surrounding water molecules. However, the low cell permeability of complexes 1 and 4 is the major obstacle to their applications in intracellular DNA sensing.15 Upon ion-pairing with PCP, these complexes show significantly enhanced cellular uptake and nuclear accumulation, which allows the visualization of nuclear DNA in a wide range of cell lines by LSCM and structured illumination microscopy (SIM).28 Similarly, the cell-impermeable osmium(II) dppz complex (165) has been delivered to the cells through ion-pairing with TeCP for imaging nuclear DNA.143 Further investigations demonstrate the complex as a nuclear DNA imaging reagent for correlative light and electron microscopy studies in both live and fixed cells, which show the characteristic structural changes in nuclear DNA such as chromosome condensation and decondensation during mitosis. Interestingly, cells that are incubated with the D-isomers of these complexes display more intense emission in the nucleus than those treated with the L-isomers, which is due to the stronger binding of the D-isomers to the right-handed DNA duplex as the nuclear uptake of both enantiomers is similar. Similar enantioselective staining has been observed for the ruthenium(II) complex 166; LSCM and PLIM show that the D-isomer exhibits more apparent nuclear staining in CHO-K1 cells than its L-isomer, which is ascribed to the higher emission quantum yield and longer emission lifetime of the DNA-bound D-isomer.144

2+

N N

N Os N

N

N

N

N

165

2+

N N

N Ru N

N

N

N

N

O O

166 The conjugation of a cell-impermeable ruthenium(II) dppz complex (167) with a nuclear localizing sequence (NLS; VQRKRQKLMP) and a mitochondria penetrating peptide (MPP; FrFKFrFK) (r ¼ D-arginine) gives peptide conjugates 168 and 169 with enhanced cellular uptake for specific imaging of DNA in the nucleus and mitochondria of HeLa cells, respectively.145,146

190

Luminescence chemosensors, biological probes, and imaging reagents

The conjugates show a substantial increase in emission intensity upon binding to calf thymus DNA (ctDNA) with a binding constant (Kb) of 3.6 and 2.8  107 M1, respectively, which are an order of magnitude higher than that of their peptide-free counterpart complex 167 (Kb ¼ 5.04  106 M1).146 The stronger binding is attributed to the additional electrostatic interaction between the cationic peptide and the polyanionic DNA backbone. The staining of DNA in the two specific organelles with the conjugates is confirmed by LSCM, PLIM, and resonance Raman spectroscopy. The high DNA binding affinity of conjugate 168 enables highresolution imaging of chromosomal DNA in the nucleus of HeLa cells and facile identification of different stages of cell division using STED microscopy (Fig. 6).145 Heterobinuclear rhenium(I)–ruthenium(II) dppz complexes (170 and 171) also display substantial emission enhancement upon the addition of ctDNA.147 While complex 170 exhibits predominant nuclear enrichment at all incubation concentrations, the intracellular localization of complex 171 is concentration-dependent; the complex is distributed in the mitochondria and lysosomes of MCF-7 cells at low concentration (6 mM) but accumulated in the nucleus of the cells at high concentration (500 mM). The intracellular dynamics of complex 171 has been monitored by STED microscopy at subdiffraction limits, which reveals that the complex is enriched in both the mitochondria and the lysosomes after uptake and readily translocated to the nuclear region.

2+

O OH N N N

N Ru N

N N

N

167

n+

O

R

NH N N N

O

R=

NH

Ru N

NH

N N

N

+ NH3

NH2

O NH

N

O NH

O

O

+ H2N

NH

O

O NH

NH2

O

H2N +

O NH

O NH

O

NH O

O

O N

O

NH2 , n = 6 (168)

S

+ H2N

NH2

HN O

NH O NH

NH

NH3 +

NH2

+ NH3

NH2

NH

NH

NH

NH NH

NH2

O

NH

NH H2N +

O

NH O

O NH

NH2 O

,n=5

(169)

Luminescence chemosensors, biological probes, and imaging reagents

191

3+ OC OC

CO Re N

N

N

N

N

N

N

N

N

R

NN NN

N Ru N

N

(170)

R = (CH2)5

CH2NH(CH2)6NHCH2 (171) Homobinuclear ruthenium(II) complexes (172 and 173), where the tetrapyrido[3,2-a:20 ,30 -c:300 ,200 -h:2000 ,3000 -j]phenazine (tpphz) ligand acts as a bridging ligand for the two ruthenium(II) polypyridine units, bind to DNA with higher affinities (Kb > 107 M1) through a non-intercalative mechanism.148 While complex 173 is able to stain DNA in the nucleus of both live and fixed cells, complex 172 is only capable of imaging nuclear DNA in fixed cells due to its lower cellular uptake efficiency. The intense nuclear emission is associated with the binding of the complexes to DNA, as supported by two-photon excitation PLIM (TP-PLIM) which shows a long-lived emission signal (> 160 ns) in the nucleus and a shorter-lived cytoplasmic emission in the complex-treated cells.149 Thus, complex 173 has been applied to track the structural changes in nuclear DNA as cells progress through the cell cycle.148 It has also been employed to image chromatin DNA in live cells with improved resolution using SIM and STED microscopy and to generate 3D images of nuclear chromatin by 3D STED microscopy.150

4+ N N

N Ru N

N

N

N

N

N

N

N N

=

N Ru N

N

N

N

N

172

N N

173

In addition to the well-established dppz and tpphz systems, many other transition metal complexes have been reported for DNA sensing in live cells. For example, a ruthenium(II) complex bearing an N-hexyl phenothiazine moiety (174; d760nm ¼ 236.5 GM) has been exploited for selective two-photon imaging of mitochondrial DNA (mtDNA) in live cells.151 The strong reduction in mitochondrial staining upon treatment of the complex-loaded HepG2 cells with deoxyribonuclease (DNase) confirms the specific binding of the complex to mtDNA. A zinc(II) complex (175) that shows emission enhancement upon intercalation to mtDNA has been used for the visualization of mtDNA in live HepG2 cells using STED nanoscopy.152 The negligible change in the emission intensity of the complex-stained cells upon carbonyl cyanide m-chlorophenyl hydrazone (CCCP; a potent mitochondrial oxidative phosphorylation uncoupler) and DNase treatment further supports that the mitochondrial staining is mitochondrial membrane potential-independent and is associated with the intercalative binding of the complex to mtDNA. Super-resolution imaging of the complex-treated cells reveals that mtDNA is distributed within the cristae and inner matrix of the mitochondria. A binuclear platinum(II) complex (176) has been designed as a selective chemosensor for DNA mismatches in live cells.153 The complex displays up to 13.5-fold higher emission enhancement upon binding to mismatched DNA over matched DNA, as the larger pockets of the mismatched sites provide a better shielding of the complex from the aqueous environment and/or organize the complex molecule into a more ordered structure. The complex is also able to detect DNA abasic sites, in which a purine or a pyrimidine base is missing in the nucleotide. The intracellular emission intensity of the complex-loaded, mismatch repair-deficient human

192

Luminescence chemosensors, biological probes, and imaging reagents

Fig. 6 (A) (Top) LSCM and STED microscopy images of chromosomal DNA in the nucleus of complex 168-stained HeLa cells during metaphase. (Bottom) Enlarged view of a single chromosome selected in the LSCM and STED microscopy images and the normalized emission intensity profiles across the white line in the LSCM (black) and STED microscopy images (red). (B) STED microscopy images of HeLa cells incubated with complex 168 (40 mM, 24 h, 37  C). Reproduced from Byrne, A.; Burke, C. S.; Keyes, T. E. Chem. Sci. 2016, 7, 6551–6562, with permission from Royal Society of Chemistry. Copyright 2016.

colorectal carcinoma HCT116 cells is higher than that of the cells that are transfected to restore the mismatch repair activity. Additionally, the complex can differentiate tumor tissues from normal tissues based on different levels of mismatched DNA, as shown by the more intense emission of the complex upon incubation with the DNA samples extracted from the colorectal adenocarcinoma tissue than those from adjacent normal tissue of the same patient.

2+

N N

N Ru N

N

H N

N

N

N S

174

2+

N

S

N N

N N

Zn N

N 175

S

N

Luminescence chemosensors, biological probes, and imaging reagents

193

2+ N

C

N

N

N

Pt

Pt

P

P

C

176

In contrast to DNA, RNA consists of only one polynucleotide chain and it is mainly distributed in the nucleoli and cytoplasm of eukaryotic cells. The nucleolus is the major site where ribosomal RNA (rRNA), which constitutes about 70–80% of total RNA of the cell, is synthesized and assembled. A ruthenium(II) complex carrying a phenanthridine moiety (177) has been designed to visualize RNA in live cells.154 The complex is weakly emissive in aqueous solution (Fem ¼ 0.016) due to: (1) spin-forbidden resonance energy transfer from the 3MLCT state of the ruthenium(II) polypyridine unit to the singlet excited state of phenanthridine, and (2) efficient quenching of the emissive state of phenanthridine in aqueous solution. A 10-fold increase in the emission quantum yield of the complex is detected in the presence of RNA (Fem ¼ 0.16), as the phenanthridine moiety is protected from the aqueous environment upon binding to RNA molecules. Incubation of MDA-MB-231 cells with the complex gives rise to intense emission in the nucleoli and cytoplasm. The conjugation of a green-emitting iridium(III) complex and a red-emitting ruthenium(II) complex via a decyl linker yields a dual-emissive heterobinuclear complex (178) that exhibits two emission bands at 523 and 615 nm.155 The emission intensity ratio I523nm/I615nm increases upon the addition of RNA but not DNA, and the increase is attributable to the association of the complex with the secondary and tertiary structures of RNA. Staining of MCF-7 cells with the complex leads to intense emission in the nucleoli with weaker emission in the cytoplasm. The disappearance of the nucleoli staining after treatment of the cells with ribonuclease (RNase) but not with DNase offers support to the specific interactions of the complex with RNA in the nucleoli. An oxime-containing zinc(II) complex (179) has been identified as an RNA-selective probe.156 Binding of the complex to RNA increases the emission intensity at 570 nm by 2.74 fold, which is associated with: (1) the inhibition of the twisted intramolecular charge transfer state as the complex is protected from the aqueous environment, and (2) the restriction of the torsional motion of the CeC bond and the triphenylamine group through the formation of hydrogen bonding between the oxime moiety of the complex and the base-pairs of RNA. Treatment of the cells with the complex results in intense emission in the nucleoli and cytoplasm.

3+

H2N N N

N Ru N

N

NH

+

NH

N

NH2

S

N

177 3+ F N N

N Ru N

N

10

N

N N

N Ir N

C

F F

C F

178

194

Luminescence chemosensors, biological probes, and imaging reagents

N

OH

N

N

N

N

Zn Cl

Cl

179 Transition metal complexes have been developed for RNA sensing based on RNA-induced aggregation. A ruthenium(II) complex (180) has been applied to detect and image rRNA in the nucleoli of live cells.157 The enhanced emission of the complex at 619 nm in the presence of rRNA is ascribed to the formation of aggregates, possibly via: (1) p–p stacking with the purine and pyrimidine bases, and (2) CeCl,,,O halogen bonding interaction between the chlorine atoms of the complex and oxygen atom of cytosine, guanine, and uracil in RNA molecules. A guanidinium-modified platinum(II) complex (181) has been designed for RNA detection.158 Binding of the complex to RNA molecules through electrostatic interactions and hydrogen bonding promotes the selfassembly of the non-emissive complex molecules via non-covalent Pt(II),,,Pt(II) and p–p interactions, resulting in intense 3 MMLCT emission at 670 nm. Incubation of fixed HeLa and CHO-K1 cells with the complex leads to intense emission in the nucleoli with weaker emission in the cytoplasm, which is due to the binding of the complex to RNA as confirmed by the enzyme digestion experiments involving RNase and DNase. The complex has been employed to monitor the dynamics of nucleolus induced by the addition of an RNA synthesis inhibitor, actinomycin D. A zinc(II) complex (182) has been identified to bind to mitochondrial RNA in HeLa cells.159 The complex displays emission enhancement in the presence of RNA, probably due to electrostatic interactions between the cationic complex and anionic RNA molecules. The specific mitochondrial staining of the complex in both live and fixed HeLa cells indicates that the complex is localized in the mitochondria in a mitochondrial membrane potentialindependent manner, which further supports that the targeting behavior of the complex originates from its binding to mitochondrial RNA.

2+

N N

Cl

N Ru N

N N Cl

180

2+ N N

N

N

N

Pt

+ H2N

NH NH2 181

Luminescence chemosensors, biological probes, and imaging reagents

195

2+ N

N

S

N

N

N

S

N

Zn

N

N 182

Guanine-rich DNA and RNA molecules can fold into higher-order structures such as G-quadruplexes, which are four-stranded helical structures that originate from the stacking of guanine tetrads formed from the association of four guanine bases through hydrogen bonding. These non-canonical structures are involved in a plethora of biological processes such as replication, transcription, translation, and telomere maintenance.160 The aforementioned binuclear ruthenium(II) complex 173 exhibits not only enhanced emission but also a hypsochromic shift in the emission maximum (from > 650 to ca. 630 nm) upon binding to DNA quadruplex, which allows the imaging of DNA quadruplex in live murine thymic lymphoma L5178Y-R cells that are rich in extended G-rich telomeric DNA.148 Two platinum(II) complexes (183 and 184) have been developed to visualize RNA Gquadruplex in live cells.161 The complexes show substantial emission enhancement upon specific binding to RNA G-quadruplex structures due to: (1) shielding of the complexes from interactions with surrounding solvent molecules and O2, and (2) enhanced rigidity of the local environment. No similar increase is observed in the presence of single-stranded RNA, transfer RNA, and HeLa cell total RNA. Treatment of HeLa cells with complex 183 results in luminescent foci in the cytoplasm, and the number and intensity of these luminescent foci gradually decline upon increasing the concentration of the G-quadruplex stabilizers BRACO19 and PhenDC3, indicating that the complex displays the same binding mode as these drug candidates. Complex 183 has been exploited to monitor the quantity and folding dynamics of intracellular RNA G-quadruplex in different cancerous (e.g., HeLa) and normal cell lines (e.g., CHO) in real time, which indicates that RNA G-quadruplex is more abundant and unfolds less frequently in cancer cells than in normal cells.

2+

H N

R N

Pt

N

R N N

R

R R=H

(183)

CH3 (184)

8.05.3.7

Enzyme sensors

Carboxylesterases (CEs) are members of the serine hydrolase superfamily that catalyze the hydrolysis of a variety of endogenous and exogenous substrates containing ester, thioester, carbamate, and amide groups.162 Human carboxylesterase 1 (hCE1) and human carboxylesterase 2 (hCE2) are the two major CEs distributed in human tissues and play a crucial role in lipid homeostasis and xenobiotic metabolism. Aberrant levels of hCE1 and hCE2 are implicated in diseases such as obesity, diabetes, and non-alcoholic steatohepatitis, liver steatosis, and cancer. An NIR-emitting iridium(III) complex (185) has been designed for imaging hCE2 in live cells by PLIM.163 Incubation of the complex with hCE2 leads to enzymatic cleavage of the ester bond accompanied by an increase in the emission lifetime at 639 nm from 338 to 625 ns. This phenomenon is attributed to: (1) the lower proportion of 3MLCT character in the excited state in the cleavage product, and (2) reduced non-radiative decay pathways that arise from large intramolecular motion. Treatment of the complex with other enzymes (e.g., hCE1, acetylcholinesterase, and butyrylcholinesterase) and proteins (e.g., BSA and human serum albumin) only causes subtle changes in the emission lifetime. PLIM reveals that incubation of HepG2 cells with complex 185 gives rise to a longer-lived signal compared with the cells that are pretreated with the hCE inhibitor bis(p-nitrophenyl) phosphate.

196

Luminescence chemosensors, biological probes, and imaging reagents

+

N

C

O

N

Ir

C

O

NN N

N

N

185

8.05.4

Biological probes

The visualization of biomolecules in their native environments is important to gain insights into their roles and functions in various physiological and pathological processes. The physical and chemical behaviors of the biomolecules such as their diffusion, transportation, and intermolecular interactions are strongly associated with the intracellular microenvironment including O2 level, pH, polarity, viscosity, and temperature. Aberrant expression of specific biomolecules and abnormal changes in the intracellular microenvironment are often associated with the occurrence of severe diseases. Many of these features have been recognized as important biomarkers for diagnosis and treatment of diseases. The development of transition metal complexes as biological probes for visualizing specific biomolecules and monitoring intracellular microenvironment are discussed with selected examples in the following sections.

8.05.4.1

Carbohydrate probes

Sialic acids are a family of negatively-charged, nine-carbon monosaccharides derived from neuraminic acid, with the most common member being N-acetylneuraminic acid (Neu5Ac).164 As a terminal component of the glycan chains of glycoproteins and glycolipids, sialic acids are involved in many physiological processes such as cell–cell interaction, signaling, and development. Importantly, the overexpressed sialylated antigens in cancer cells contribute to tumor growth and metastasis.165 Three iridium(III) complexes appended with a phenylboronic acid (PBA) moiety (186–188) have been designed to label sialic acids on cell surface.166 Spectrophotometric titrations indicate that these complexes display higher binding affinity to Neu5Ac (log Kb ¼ 3.61–3.85) over other sugars such as D-glucose, D-galactose, and D-mannose (log Kb ¼ 1.68–3.34) that are commonly found on glycoproteins, which is due to the cooperative two-site binding through: (1) PBA ester formation with the vicinal diol of Neu5Ac, and (2) electrostatic interaction between the ammonium group of the complexes and carboxylate group of Neu5Ac under physiological conditions. Incubation of HepG2 cells with complex 188 results in intense cell membrane staining with much weaker emission in the cytoplasm, which is not observed in the cases of complexes 186 and 187 due to their much more efficient cellular uptake. As revealed by LSCM and ICP-MS measurements, complex 188 is capable of recognizing cell-surface sialic acid residues and distinguishing cancerous HepG2 cells from non-cancerous HEK293T cells.

+

O C C

N Ir N

O

N

NH

O

3

NH HO

N

B

O N _

C

=

N _

C 186

HO

N

N

_

_

C 187

C 188

OH

Luminescence chemosensors, biological probes, and imaging reagents 8.05.4.2

197

Protein probes

Proteins are an important class of biomolecules that play important roles in a wide range of cellular processes. Typically, transition metal complexes are functionalized with a protein-binding ligand for the specific recognition of target proteins in a cellular context. Integrins are a family of heterodimeric cell-surface receptors that facilitate cell–cell and cell–extracellular matrix adhesion.167 Integrin avb3 recognizes ligands that contain the tripeptide RGD sequence, and it is overexpressed in tumor cells and plays a pivotal role in tumor angiogenesis and metastasis. A RGD peptide bearing two histidine units has been cyclized with an iridium(III) solvato complex based on iridium(III)–histidine coordination.168 The resultant peptide conjugate 189 exhibits much more efficient cellular uptake than its linear counterpart by avb3-positive A549 cells. No similar uptake is observed in MCF-7 cells that express avb3 at low levels. The modification of conjugate 189 with an apoptosis-inducing peptide KLA ((KLAKLAK)2) at the C-terminus affords a conjugate that shows higher cytotoxic activity toward A549 cells (IC50 ¼ 4.5 mM; 24 h) over MCF-7 cells (IC50 ¼ 37.7 mM; 24 h) due to avmediated selective uptake.

2+ C N

HN

C Ir

N

N N

NH O H2N

NH NH2 +

O

NH

NH

O

O

NH O HN NH

NH2 O O OH

189

G-Protein-coupled receptors (GPCRs) constitute the largest family of membrane proteins that mediate cellular response to hormones, neurotransmitters, and environmental stimuli in eukaryotic cells. Formyl peptide receptor 2 (FPR2) is a GPCR involved in host defense and inflammation.169 An iridium(III) complex has been conjugated with the agonistic hexapeptide WKYMVm (m ¼ D-methionine) to give conjugate 190 for visualizing FPR2 in live cells.170 Intense intracellular emission is observed in FPR2-expressing human umbilical vein endothelial cells (HUVECs), which is reduced when the FPR2-selective antagonist WRW4 is added to the culture medium or when FPR2 is not expressed. There has been emerging evidence on the critical roles of GPCRs in tumor growth and metastasis.171 Gastrin-releasing peptide has been recognized as a growth factor for cancer cells and its receptor gastrin-releasing peptide receptor (GRPR), a member of GPCRs, is overexpressed in various types of tumor tissues but has limited expression in normal tissues.172 An iridium(III) complex has been modified with a GRPR antagonist, JMV594 (191) for selective imaging of GRPR in GRPR-positive A549 cells.173 The specific binding of the complex to GRPR on the cell surface is confirmed by the reduced emission upon pretreatment of the cells with competitive GRPR antagonists (e.g., JMV594 and RC-3095) or GRPR small interfering RNA (siRNA). The much stronger emission of the complex-loaded A549 cells than LO2 cells that display a much lower expression level of GRPR indicates the capability of the complex in discriminating GRPR-positive cancer cells from normal cells. Dopamine receptor is a member of GPCRs that is upregulated in many types of cancer;174,175 for example, the overexpression of dopamine D1 receptor in breast cancer tissues is associated with a higher tumor stage and grade and a higher number of nodal metastasis.174 Iridium(III) complexes carrying a dopamine moiety (192 and 193) have been developed to image intracellular dopamine receptors.176 Incubation of dopamine receptor-expressing A549 cells with the complexes leads to intense intracellular emission, which is associated with their interaction with intracellular dopamine receptors, as confirmed by the dopamine competition and siRNA knockdown experiments. Complex 193 has been applied to track the internalization of dopamine receptors in live cells.

198

Luminescence chemosensors, biological probes, and imaging reagents

2+ OH C C

N Ir N

N N

O

NH O

O

NH

NH

NH

O

O

NH O

S

NH O

NH

NH

NH

O

O

S NH3 + 190

O O C C

N Ir N

N

NH

NH

O

NH

NH

O

+

NH2 O NH

O

NH

O

NH

NH

NH

O

O NH

O N

N

NH

NH OH O

O NH2

191

C C

N Ir N

OH N

NH

N _

C

OH

O

N

=

N

N _

C 192

+

_

C 193

In addition to agonists or antagonists that bind to receptor proteins, transition metal complexes have been modified with enzyme inhibitors to target specific proteins in cells. Carbonic anhydrase (CA) is a family of enzymes that catalyze the reversible hydration of carbon dioxide. The membrane-bound isoforms CA-IX and CA-XII are overexpressed in tumor cells in response to hypoxia to promote tumor growth by counteracting acidosis through the regulation of intracellular pH.177 A rhenium(I) complex that contains a CA inhibitor, benzenesulfonamide via a cleavable N-acyl imidazole linker (194) has been designed to label endogenous CA-IX and CA-XII on A549 cells through ligand-directed acyl imidazole chemistry.178 Upon binding of the complex to the specific protein via substrate recognition, the cleavable linker reacts with the nucleophilic residues that are proximal to the substratebinding site of the protein, resulting in the covalent attachment of the complex to the protein. As shown by synchrotron-based X-ray fluorescence microspectroscopy that allows qualitative and quantitative detection of trace elements in biological samples, the rhenium signal is detected over the whole cell with a stronger signal in the nucleus upon treatment of the CA-overexpressing A549 cells with the complex, which is not observed in the cells that are incubated with ethoxyzolamide, a competitive inhibitor of CA. Cyclooxygenase (COX) is a membrane-bound, heme-containing enzyme that is involved in the conversion of arachidonic acid to proinflammatory prostaglandins. The expression of the inducible isoform COX-2 is usually upregulated at inflammatory sites and in tumor tissues.179 An iridium(III) complex has been conjugated with a structural analog of the COX-2 inhibitor indomethacin (195) to monitor the COX-2 expression level in live cells.180 Incubation of COX-2-overexpressing HeLa cells with the complex gives rise to intense intracellular emission, which is not observed when HeLa cells have been pretreated with curcumin to inhibit the expression of COX-2 or when LO2 cells are used. Epidermal growth factor receptor (EGFR) is a member of the

Luminescence chemosensors, biological probes, and imaging reagents

199

ErbB family of receptor tyrosine kinases that is overexpressed in many types of cancer.181 An iridium(III) complex that is modified with a EGFR kinase inhibitor, tyrphostin 1 (196) has been identified to be capable of simultaneous visualizing and inhibiting EGFR in live cancer cells.182 Incubation of EGFR-overexpressing human epidermoid carcinoma A431 cells with the complex leads to strong intracellular emission, which is significantly reduced in intensity when the cells have been pretreated with tyrphostin 1 or siRNA to inhibit EGFR activity and knockdown EGFR expression, respectively. Immunofluorescence staining confirms the specific binding of the complex to EGFR in the cells. More intense intracellular emission is observed in A431 cells than in LO2 cells that express less EGFR.

OC OC

CO Re Cl

N N NN

O

O

O

NH

O

N O

N

O NH

NH

O S NH2 O

194 +

C C

N Ir N

N

NH

N

O

N

O

Cl

195

+

O O

C C

N Ir N

CN N

NH O

N

O

CN

196 The accumulation of misfolded or aggregated peptides and proteins is implicated in the occurrence of many human diseases such as neurodegenerative diseases and Type II diabetes.183 Amyloid b (Ab) peptides are cleavage products of amyloid precursor protein by b- and g-secretases.184 The imprecise cleavage by g-secretase at the C-terminus of the Ab sequence produces two major isoforms, Ab40 and Ab42. The self-accumulation of Ab in the brain parenchyma as plaques is the major hallmark of Alzheimer’s disease. A ruthenium(II) complex 197 has been developed to monitor the self-aggregation of Ab peptides in real time.185 The complex exhibits stronger binding to all the forms of Ab (Kd ¼ 341, 365, and 242 mM and 619, 306, and 300 mM for the monomer, oligomer, and fibril of Ab40 and Ab42, respectively) than the well-known Ab probe thioflavin T (ThT; Kd ¼ 644 mM for Ab40 fibrils). Co-staining of Ab aggregates with complex 197 and ThT indicates that in addition to fibrils, the complex can stain oligomers, which are thought to be the major causes of neurotoxicity and cannot be visualized by ThT. The complex has been employed for real-time monitoring of fibril growth in vitro, which reveals that Cu2þ accelerates the aggregate formation, but the morphology of the aggregates is highly dependent on the concentrations of Ab and Cu2þ; substoichiometric levels of Cu2þ promote the fiber formation at low Ab concentrations (< 10 mM), whereas high Ab and Cu2þ concentrations cause the formation of amorphous oligomers. Incubation of Ab40-pretreated rat adrenal pheochromocytoma PC12 cells with complex 197 gives intense emission that is not observed in the untreated cells. Immunostaining confirms that the complex stains intracellular Ab40. Histological staining of brain slices from a transgenic mouse model of Alzheimer’s disease shows that the complex is capable of distinguishing Ab plaques in brain slices. a-Synuclein (aS) is a protein that is abundant in the brain and its assembly and aggregation are associated with the development of Parkinson’s disease.184 The ruthenium(II) complex 4 has been exploited to monitor the aS fibrillization in vitro and detect aS

200

Luminescence chemosensors, biological probes, and imaging reagents

aggregation in live human neuroglioma H4 cells.186 The complex shows an 18-fold increase in the emission intensity at 640 nm in the presence of fibrillar aS, which is 8-fold higher than that in the presence of an equal amount of BSA. The emission enhancement is ascribed to the strong binding of the complex to fibrillar aS, which dramatically changes the local surrounding environment of the complex. This enables the complex to monitor the formation of aS fibrils in real time. Incubation of H4 cells that are transfected to overexpress green fluorescent protein (GFP)-fused aS (aS-GFP) with complex 4 results in much higher intracellular emission than the untransfected cells. A more dramatic increase is observed when the cells have been pretreated with a proteasomal degradation inhibitor, MG-132 to accumulate aS-GFP aggregates. The strong co-localization of the fluorescence of GFP and emission of complex 4 indicates that the enhanced intracellular emission is associated with aS aggregation inside the cells.

F N N

N Ru N

N

NH O

N

2+

N H

197

8.05.4.3

Bioorthogonal probes

Bioorthogonal reactions refer to any chemoselective reactions that can occur under physiological conditions without interfering with the native biological processes.187 In this context, the two complementary functionalities react with one another readily and specifically to generate a stable product under biocompatible conditions. Meanwhile, they should also be absent in living systems and remain inert to the myriad of functionalities that are abundant in natural biomolecules. The rapid development of bioorthogonal chemistry has offered a promising and versatile platform for the detection and visualization of various biomolecules including glycans, lipids, nucleic acids, proteins, and other small metabolites in their native environments, which facilitates the study of the structures, dynamics, and functions of target biomolecules in live cells and organisms.

8.05.4.3.1

Copper(I)-catalyzed azide–alkyne cycloaddition and strain-promoted azide–alkyne cycloaddition

Azide is the most prominent bioorthogonal chemical reporter due to its small size, non-native nature, and inertness to biomolecules. As a 1,3-dipole, it undergoes copper(I)-catalyzed azide–alkyne cycloaddition188 and strain-promoted azide–alkyne cycloaddition (SPAAC) reactions189 with terminal alkynes and cyclooctynes, respectively, to yield substituted triazoles under physiological conditions. The first phosphorescent bioorthogonal probes are derived from iridium(III) complexes appended with a dibenzocyclooctyne (DIBO) moiety (198–200).190 As 1,3,4,6-tetra-O-acetyl-N-azidoacetyl-D-mannosamine (Ac4ManNAz) is metabolically converted by the cells to N-azidoacetyl sialic acids and incorporated into cell-surface glycans, pretreatment of CHO cells with Ac4ManNAz leads to the enrichment of the complex in the plasma membrane. In contrast to complexes 198 and 199 that are localized in the mitochondria of both Ac4ManNAz-pretreated and untreated cells due to their cationic and highly lipophilic character, incubation of Ac4ManNAz-pretreated CHO cells with complex 200 results in intense membrane and granular cytoplasmic staining, as the ionized carboxylate groups in aqueous solution suppress the internalization of the complex and assist in the SPAAC reaction with membrane-bound azide groups. Similar results have been observed for related rhenium(I) (201203)191 and ruthenium(II) DIBO complexes (204 and 205).192 Complex 204 has been utilized to monitor the dynamic behavior of glycans, which reveals that the endosomes, Golgi apparatus, and lysosomes are actively involved in the intracellular trafficking of membrane-bound glycans.192 An iridium(III) DIBO complex (206) has been applied to image the catalytic activity of caspase-3 in apoptotic cells using a “labeling after recognition” sensing approach.193 The caspase-3-cleavable peptide substrate DEVD is functionalized with an azide and a norbornene (Nor) group at the N- and C-terminus, respectively, to afford N3-DEVD-Nor. Compared with the traditional “labeling before recognition” sensing approach where the peptide substrate is modified with two luminophores to give a FRETbased probe, the substrate N3-DEVD-Nor is more easily recognized by the enzyme due to reduced steric hindrance and/or nonspecific interaction. Subsequent bioorthogonal labeling of the cleaved peptide with complex 206 and a rhodamine–tetrazine conjugate (Rh-Tz) results in much faster and more significant lifetime response than the traditional “labeling before recognition” approach (Fig. 7), which allows the visualization of intracellular caspase-3 activity with spatial resolution. An NIR-emitting iridium(III) complex that bears a terminal alkyne moiety (207) has been employed for wash-free imaging of newly synthesized proteins in live cells.194 Reaction of the complex with L-azidohomoalanine (AHA) in the presence of Cu2O nanoparticles in aqueous solution decreases the emission lifetime of the complex from 530 to 116 ns. The changes in the emission lifetime enable the complex to monitor the dynamic process of protein synthesis in AHA-pretreated HeLa cells in real time using PLIM. Iridium(III) complexes featuring a 5-azido-1,10-phenanthroline ligand (208–210) have been developed as phosphorogenic bioorthogonal probes.195

Luminescence chemosensors, biological probes, and imaging reagents

201

These complexes display weak emission (Fem ¼ 0.0013 –0.0018) but exhibit substantial emission enhancement upon reaction with alkyne derivatives such as (1R,8S,9s)-bicyclo[6.1.0]non-4-yne (BCN). Incubation of BCN-modified human osteosarcoma U2OS cells with the complexes results in more intense cytoplasmic staining than the cells without the pretreatment due to the specific SPAAC reaction of the complexes with the BCN units.

+ O N

C

N

Ir

C

NH

NH

3

O O

N

N

N

N

=

_

N _

C

_

C

C

198

199

+

C

HOOC HOOC

C

N Ir

NH

O

N

3

O O

N

N

200

OC OC O O

N N

=

NH

CO Re N

N N

NH 3

O

N

N

N

N

N

N

201

+

202

203

202

Luminescence chemosensors, biological probes, and imaging reagents

2+

O N N

N Ru N

O

N

O NH

O

NH

3

O

N

N

=

N

N

N

N

N

204

205

+ O C C

N

NH

N

Ir

O

N

N

206 +

N

C

N

Ir

C

N

N

207

n N

C C

N C

_

=

N

Ir

N

N

N C

N _

C O

n

=

+1 208

N3

N _

OH +1 209

C O

_

O

NH _

1 210

_

SO3

4

Luminescence chemosensors, biological probes, and imaging reagents

203

Fig. 7 (A) PLIM images of HeLa cells stimulated with cisplatin at different concentrations (1, 5, 10, 20, and 40 mg mL1) for 4 h in two different sensing approaches. In the “labeling before recognition” approach, the cells were incubated with N3-DEVD-Nor (5 mM, 30 min) and then simultaneously treated with complex 206 (5 mM, 30 min) and Rh-Tz (5 mM, 30 min) before stimulation with cisplatin. In the “labeling after recognition” approach, the cells were incubated with N3-DEVD-Nor (5 mM, 30 min) followed by stimulation with cisplatin. Complex 206 (5 mM) and Rh-Tz (5 mM) were added to the culture medium after 3 h and incubated for another 1 h. Scale bar ¼ 20 mm. (B) Relative occurrence of long-lived and short-lived signals during PLIM. (C) Averaged lifetime values of the PLIM images in (A). Error bars represent the standard deviations of three independent measurements. (D) Enlarged views of the selected area in (A) and photoluminescence decay curves of the circled area. Reproduced from Wu, Q.; Zhang, K. Y.; Dai, P.; Zhu, H.; Wang, Y.; Song, L.; Wang, L.; Liu, S.; Zhao, Q.; Huang, W. J. Am. Chem. Soc. 2020, 142, 1057–1064, with permission from American Chemical Society. Copyright 2020.

8.05.4.3.2

Strain-promoted alkyne–nitrone cycloaddition

Nitrones are one of the most common 1,3-dipoles and they undergo strain-promoted alkyne–nitrone cycloaddition(SPANC) with cyclooctynes to yield isoxazolines under physiological conditions.196 A remarkable feature of nitrones over azides is that there is more than one modification site on nitrones, which allows facile tuning of their electronic properties and incorporation of functional handles. The functionalization of iridium(III) complexes with a nitrone moiety affords a series of novel phosphorogenic probes (211– 217) for bioorthogonal labeling and imaging.197,198 These complexes are weakly or non-emissive in solution (Fem  0.023) due to efficient quenching associated with photoinduced isomerization of the nitrone unit. Upon the SPANC reaction with (1R,8S,9s)-

204

Luminescence chemosensors, biological probes, and imaging reagents

bicyclo[6.1.0]non-4-yn-9-ylmethanol (BCN-OH) in aqueous solution, the complexes show up to 135-fold emission enhancement due to the conversion of the quenching nitrone moiety to a non-quenching isoxazoline derivative. Additionally, the direct coordination of the nitrone ligand (k2 ¼ 4.0  102 M1 s1) to the cationic iridium(III) polypyridine unit enhances its reactivity toward BCN-OH (k2 ¼ 11.6–27.0  102 M1 s1). The complexes have been used to label BCN-modified proteins. The emission of the complexes is significantly enhanced by 82.9 to 795.1 fold upon reaction with the BCN-modified proteins, which is attributed to the increased hydrophobicity and enhanced rigidity of the local environment of the complexes after the reaction. Incubation of the complexes with unmodified proteins only causes a small increase in the emission intensity (I/Io ¼ 0.9–5.9). The high phosphorogenicity and bioorthogonality of the complexes enable them to label BCN-modified decane molecule (BCN-C10)197 and SNAP-tag proteins in live cells.198 Related phosphorogenic ruthenium(II) nitrone complexes (218–222) have been prepared for regioselective bioorthogonal labeling of live cells.199 The reactivity of the N-phenyl nitrone complex 222 toward BCN-OH (k2 ¼ 3.06 M1 s1) is about two orders of magnitude higher than its N-methyl nitrone counterpart complexes 218–221 (k2 ¼ 7.8–8.6  102 M1 s1), which is due to the electron-withdrawing phenyl group at the N position of the nitrone moiety. Incubation of BCN-C10-pretreated HeLa cells with the complexes results in more intense emission than the cells without BCN-C10 pretreatment and distinct subcellular staining; for example, punctate cell surface staining with complexes 218 and 220, exclusive plasma membrane staining with complexes 219 and 222, and cytoplasmic staining with complex 221. The regioselective accumulation of the complexes in the membrane or cytoplasm of BCN-C10-treated cells is associated with: (1) the subcellular distribution of BCN-C10, and (2) the lipophilicity and bioorthogonal reactivity of the complexes, as shown by LSCM and ICP-MS measurements. Additionally, the (photo) cytotoxicity of the iridium(III) (214–217)198 and ruthenium(II) nitrone complexes (221)199 can be readily modulated through a bioorthogonal approach, which renders them activatable cytotoxic agents for cancer therapy.

C C

N

N

Ir

+

+ N O_

N

N

O N C

_

N

=

C

O

N _

C

211

S

N

_

N

N

C

N _

N Ru N

N

N C

216

+ N O_

_

214

N

_

N C

213

215

N

C

212

C

S

N

_

_

217 2+

N

O N N

=

N N

NH

NH

N

N

N

N

N

N

O 218

219

220

221

Luminescence chemosensors, biological probes, and imaging reagents

205

2+

N N

N

+ N O_

N

Ru

N

N

222

8.05.4.3.3

Strain-promoted sydnone–alkyne cycloaddition

Sydnones are a class of stable mesoionic 5-membered heterocyclic compounds and they undergo strain-promoted sydnone–alkyne cycloaddition (SPSAC) reaction with cyclooctynes to give pyrazoles under physiological conditions.200 The main advantage of sydnones over azides is that their reactivity can be readily modulated by changing the substituents at the N-3 and C-4 positions of the mesoionic ring, while the stability of the compounds can be maintained in biological media. Since the polarity of sydnone and pyrazole is drastically different, iridium(III) complexes with environment-sensitive emission have been functionalized with a sydnone moiety to afford new bioorthogonal probes (223–225) that can report the SPSAC reaction by photophysical changes.201 Upon reaction with BCN-OH, the complexes display enhanced emission intensities and extended lifetimes (I/Io ¼ 1.1–7.9; s ¼ 0.10–0.25 ms) with a hypsochromic shift in their emission maxima, which is due to the transformation of the dipolar mesoionic sydnone ring to a less polar pyrazole derivative that results in a more hydrophobic pendant. Complex 224 exhibits 24.9- and 44.6-fold increase in emission intensity upon reaction with BCN-modified BSA (BCN-BSA) and ceramide (BCN-Cer), respectively, as these substrates offer a more hydrophobic and rigid local environment for the complex after the reaction. Thus, it has been applied to image BCN-Cer in live HeLa cells. The iridium(III)–ceramide adduct is enriched in the mitochondria of the cells as a consequence of its cationic and lipophilic character. The photocytotoxic activity of complex 224 has been manipulated through the use of a bioorthogonal reagent in view of the emission enhancement and lifetime extension brought about by the SPSAC reaction. Related rhenium(I) sydnone complexes (226–228) have been employed for phosphorogenic labeling of lysosomes using a BCN-modified morpholine molecule.202 These complexes are weakly or non-emissive in solution (Fem  0.0017), as the p-conjugation between the pyridine ring and the sydnone moiety allows more efficient emission quenching and thus larger emission enhancement upon the SPSAC reaction with BCN-OH (I/Io ¼ 8.8–17.3) and BCN-BSA (I/Io ¼ 9.6–38.9).

O O C C

N Ir N

+

+ N NO

NH

N

_

N

O N _

C

=

N

N _

F

C

O

N

_

_

C

C

F 223

224

225

206

Luminescence chemosensors, biological probes, and imaging reagents

CO

OC

Re

OC

N

+ N N

+ O N NO

N N

=

N

N

N

N

N

N

226

8.05.4.3.4

_

227

228

Inverse electron-demand Diels–Alder reaction

The inverse electron-demand Diels–Alder (IEDDA) reaction of 1,2,4,5-tetrazine with dienophiles such as strained alkenes and alkynes, which yields dihydropyridazine and pyridazine derivatives, respectively, is another commonly adopted bioorthogonal reaction because of its very fast reaction kinetics, high reaction selectivity, and high compatibility with water.203 The incorporation of a tetrazine moiety into luminophores usually quenches their emission through FRET or through-bond energy transfer (TBET), which enables the resultant luminophore–tetrazine conjugates to show emission turn-on upon the IEDDA reaction. Thus, tetrazines have been modified with one (229–232) or two rhenium(I) units (233 and 234) to afford phosphorogenic bioorthogonal probes for biomolecular labeling and imaging.204 These complexes are weakly or non-emissive in solution (Fem  0.049) but display substantial increase in emission intensity upon reaction with BCN-OH (I/Io ¼ 8.0–181.1), which is more pronounced than with 5-norbornen-2-ol (I/Io ¼ 1.1–8.0) as PET quenching is still possible in the dihydropyridazine products. The reactivity of the binuclear complexes 233 and 234 toward BCN-OH (k2 ¼ 407.2 and 537.0 M1 s1, respectively) is much higher than that of the mononuclear complexes 229–232 (k2 ¼ 52.4–69.7 M 1 s 1) and the free ligands (k2 ¼ 20.1– 50.3 M1 s1), indicating that the direct coordination of the tetrazine moiety to the positively-charged rhenium(I) polypyridine units accelerates the IEDDA reaction kinetics. Complex 234 has been exploited to track the intracellular distribution of BCN-BSA in live HeLa cells.

OC OC N

N

=

Re N

+ N N

N N

N

CO

N

N

N

N

N

229

230

Luminescence chemosensors, biological probes, and imaging reagents

CO

OC

Re

OC

N

N N

N =

N

+ N N

N N

N

N

N

N

231

OC OC N

N

N Re CO

N N

=

232

CO Re N

2+ N N

N N

N

207

N

CO CO

N

N

N

N

233

234

Since the design of efficient FRET-based probes requires a large spectral overlap of the emission of the luminophore with the absorption of the tetrazine unit (labs ¼ ca. 520 nm), the installation of an additional quenching pathway can facilitate the development of luminogenic bioorthogonal probes with emission in the red and NIR region. Iridium(III) complexes have been functionalized with a 3-chloro-1,2,4,5-tetrazine moiety through a non-conjugated linker (235–238).205 The lower excited-state redox potentials of the complexes (E [Ir2þ/þ*] ¼ 1.02 to –0.75 V versus SCE) with respect to tetrazine (ca. –0.66 V versus SCE) indicate that PET from the excited iridium(III) polypyridine unit to the appended tetrazine moiety is thermodynamically favorable. Thus, the weak emission of the complexes in solution (Fem  0.010) should be a result of efficient FRET and/or PET. Reaction of the complexes with BCN-OH results in substantial emission enhancement (I/Io ¼ 19.5–121.9), which is ascribed to the conversion of the quenching tetrazine moiety into a non-quenching pyridazine derivative. The emission of the complexes is further enhanced (I/Io ¼ 140.8–1133.7) upon reaction with BCN-BSA. Additionally, the reactivity of the tetrazine complexes (k2 ¼ 3.32– 4.27  101 M1 s1) is higher than that of the free ligand (k2 ¼ 1.03  101 M1 s1) due to the indirect electron-withdrawing effect of the cationic iridium(III) polypyridine unit. Complex 237 has been used to label azido sialic acid on the cell surface of Ac4ManNAz-pretreated CHO-K1 cells using a homobifunctional crosslinker that carries two BCN moieties. Iridium(III) complexes featuring a tetrazine-conjugated bipyridine ligand (239–242) are weakly or non-emissive in solution (Fem  0.0202) due to efficient FRET and PET.206 The reactivity of most of the complexes toward BCN-OH (k2 ¼ 20.9–83.1 M1 s1) and trans-cyclooct-4en-1-ol (TCO-OH; k2 ¼ 634.6–1471.3 M1 s1) is higher than that of the free ligand (k2 ¼ 29.1 and 188.0 M 1 s 1, respectively) due to the cationic metal center. The complexes exhibit higher emission intensities and longer emission lifetimes (I/Io ¼ 51.7– 105.3; s ¼ 0.05–0.33 ms) upon reaction with BCN-OH than with TCO-OH (I/Io ¼ 1.8–13.5; s ¼ 0.04–0.28 ms), which is associated with the PET quenching from the resulting dihydropyridazine unit in the reaction products with TCO-OH. Complex 240 has been

208

Luminescence chemosensors, biological probes, and imaging reagents

utilized for phosphorogenic imaging of the lysosomes and ER of HeLa cells using a BCN-modified morpholine and phospholipid molecule, respectively. Using the HaloTag technology that relies on the enzymatic ligation of a HaloTag protein with a chloroalkane substrate, complex 240 has been applied to visualize HaloTag proteins that are endogenously generated in the cytoplasm of the transfected HeLa cells and labeled with a BCN-modified chloroalkane substrate. Additionally, the 1O2-photosensitization efficiency and photocytotoxic activity of the complexes are modulated through reaction with BCN- or TCO-modified substrates. These findings indicate that both the photophysical and photochemical behavior of the complexes can be controlled by a judicious choice of their bioorthogonal reaction partners.

N N

C

N

Ir

C

NH

N

+

Cl N

N

N

N

O N

S

N

=

_

C

N

_

F

S

N

_

C

O

N

_

C

_

C

C

F 235

236

237

238

+ N N

C C

N

N

Ir

N N

N

N

O N _

C

=

N

N _

F

C

_

C

O

N

N

_

_

C

C

F 239

240

241

242

In the aforementioned bichromophoric systems where the luminophore is linked to a tetrazine moiety, the emission quenching efficiency is strongly limited by multiple factors such as spectral overlap, interchromophoric distance, and transition-dipole alignment for the FRET and TBET systems, and thermodynamics of electron transfer for the PET system. A monochromophoric system comprising a single luminophore with a built-in tetrazine motif is believed to be a more efficient and direct approach to develop luminogenic bioorthogonal probes. Thus, iridium(III) complexes that feature a 3-methyl-6-(2-pyridinyl)-1,2,4,5-tetrazine ligand (243–245) have been designed.207 These complexes show very weak emission in solution (Fem  0.0084) and display significant increase in emission intensity upon reaction with BCN-OH (I/Io ¼ 23.1–96.1) and BCN-BSA (I/Io ¼ 128.1–1372.2). The reactivity of the complexes (k2 ¼ 23.0–780.4 M1 s1) is much higher than that of the uncoordinated tetrazine ligand (k2 ¼ 12.9 M1 s1), highlighting the metal-coordination effects of tetrazine on the IEDDA reaction kinetics. Complex 245 has been employed to image BCN-modified HaloTag proteins in transfected HeLa cells.

Luminescence chemosensors, biological probes, and imaging reagents

209

+ N

C

Ir

C

N

N N

N

N

N

=

_

N N

C

N _

F

N _

C

_

C

C

F 243

8.05.4.4

244

245

Oxygen probes

While O2 is an essential molecule for sustaining life, insufficient and excessive O2 supply (i.e., hypoxia and hyperoxia, respectively) are implicated in pathological conditions; for example, hypoxia is a remarkable feature of many diseases including solid tumors,208 whereas hyperoxia leads to the generation of excessive ROS that causes oxidative damage to cells and tissues and dysfunction of organs.209 Phosphorescent transition metal complexes are ideal candidates for monitoring O2 level in a real-time, non-destructive, and reversible manner, as the diffusion-controlled collisional interaction between the long-lived triplet excited state of the complex and the triplet ground state of O2 significantly reduces the emission intensity and lifetime of the complex. Thus, these complexes usually exhibit more intense and longer-lived emission under hypoxia. Phosphorescent platinum(II) and palladium(II) porphyrins represent an important class of transition metal complex-based O2 sensors due to their intense red to NIR emission with long emission lifetimes (in the range of 10–1000 ms). A glucose-decorated platinum(II) porphyrin (246) has been exploited to detect changes in intracellular O2 level in response to metabolic stimulation.210 It has also been used to visualize the O2 distribution in live neurospheres and brain tissue slices. Ruthenium(II) polypyridine complexes have been demonstrated as promising candidates for O2 sensing. A binuclear ruthenium(II) complex where the two ruthenium(II) polypyridine units are bridged with the MPP sequence FrFKFrFK (247) has been developed to monitor the variation in the O2 level in the mitochondria of HeLa cells.211

R

F F F

F

F F

N

R

N

F

F

Pt F

N

F F F

R=

HO S

R N

F

F R

F OH

O 246

OH OH

F

210

Luminescence chemosensors, biological probes, and imaging reagents

7+

N

N Ru

N

+ H2N

NH O N N

N Ru N

N

NH

NH

O NH

O

O

NH

NH

NH

O

+ H2N

+ NH3

NH2

NH2

HN

NH O NH

O

NH O

O NH

N

N N

O

NH2 O

N

247

Iridium(III) polypyridine complexes have emerged as new O2 sensors; for example, complexes 248–252 have been utilized for intracellular O2 sensing and hypoxia imaging.212–214 Through structural modifications, iridium(III) complexes have been directed to the nucleus (253),215 ER (254),216 mitochondria (255),217 and lysosomes (256–258)218 for organelle-specific O2 sensing. Complex 254 has been applied to track the changes in intracellular O2 levels in real time,216,219–221 which reveals that the mitochondrial respiratory function determines the induction of intracellular hypoxia.219 Complex 257 has been employed to visualize the intracellular O2 distribution in the hepatic lobules222 and on the renal surface of live mice223 by PLIM with high spatiotemporal resolution. Quantitative measurements of the intracellular O2 status in the renal tissues in vivo show a decrease in the intracellular O2 concentration in response to acute ischemia, hypoxemia, and anemia.224 Additionally, a lower O2 level is detected in the I/R injured kidney than in the contralateral kidney in live mice. Complexes 248, 254, and 257 have been exploited to image tumor hypoxia in live mice.212,216,218 Intense emission is observed in the tumor region after intravenous administration of the complexes. The longer emission lifetime detected in the tumor region over the extratumor tissues further supports that the observed emission is due to the hypoxic nature of tumor tissues instead of the accumulation of the complexes in tumor tissues. In contrast to complex 254 (lem ¼ 616 nm), the NIR-emitting complex 259 (lem ¼ 720 nm) is capable of imaging tumors that are transplanted 6–7 mm below the abdominal skin surface as NIR emission offers deeper penetration.216 Its structural variant complex 248, which is functionalized with a hydrophilic PVP polymer to increase the tumor retention by the enhanced permeability and retention (EPR) effect, has been used to detect lymph node metastasis and to monitor cancer cell proliferation in vivo.212

O S C

N Ir

C S

O O n = ca. 120

N O

S

O

n

O 248

N

O S

Luminescence chemosensors, biological probes, and imaging reagents

+

F

F

F

F N

C

F F

N

Ir

C

N

N

F

N

249

_

N

C C

N

=

_

C

CN

Ir

CN

N

N F

N

N _

_

C

C

S

C

_

F 250

251

252

+

C C

N Ir N

N

N

N

N

N

N

253

S C

N Ir

C S

N

254

O O

211

212

Luminescence chemosensors, biological probes, and imaging reagents

+ S

N

C

O

Ir C

O

N

S

O

HN + P

255

S

N

C

Ir C

N

O

S

O

N

S

N

C

HN

Ir

O

C

O

N

S

N

NR R R=H

O

258

(256)

CH3 (257)

S C

N Ir

C S

O O

N HO

O

259 Iridium(III) complexes 260225 and 261226 have been rationally designed to simultaneously and synergistically respond to both acidity and hypoxia, the two hallmarks of solid tumor microenvironment (extracellular pH ¼ 6.5–6.9; [O2] ¼ 0–3%). In the former example, the presence of a phenol group renders the emission of complex 260 pH-sensitive; the emission intensity at 685 nm increases by ca. 5 fold upon decreasing the pH from 10.0 to 3.5.225 Thus, the intracellular emission of the complex-stained MCF-7 cells that are incubated under an acidic (pH 6.4) and hypoxic (0% O2) environment is ca. 18-fold higher in intensity than those under normal physiological conditions (pH 7.4; 21% O2), concomitant with an increase in the average emission lifetime from 26 to 102 ns. PLIM reveals that the complex-treated MCTSs show a longer emission lifetime at the center (ca. 120 ns) than the edge of the spheroids (ca. 90 ns), and the emission lifetime of the whole spheroids is extended to ca. 120–140 ns upon changing the pH of the culture medium from 7.4 to 6.4. In the second example, exposure of complex 261 to acidic conditions shifts both the absorption and emission bands to longer wavelengths, which is due to the cleavage of the imine bond and the subsequent formation of a more electron-withdrawing aldehyde moiety.226 Thus, when the pH of the solution decreases from 7.4 to 5.5, there is a gradual decrease in the emission intensity at 610 nm concomitant with the appearance of a new band at 705 nm upon photoexcitation at 450 nm. When the complex is irradiated at 580 nm under acidic conditions, only the emission at 705 nm is detected and the intensity becomes remarkably higher upon decreasing the O2 level from 20 to 0%. In addition to imaging primary tumors with

Luminescence chemosensors, biological probes, and imaging reagents

213

a high tumor-to-normal tissue ratio, the complex is capable of detecting metastatic liver lesions as small as 1 mm in mice bearing orthotopic breast tumors and monitoring cancer cell metabolism in vivo.

+ OH

S C

N Ir

C S

N N

N

260

C N

N Ir C

C

S

N N

n = ca. 113

O

O

O n

261 The red-emitting iridium(III) complex 254 has been conjugated with a blue-emitting, O2-insensitive coumarin derivative (262) to realize ratiometric O2 sensing in live cells.227 Upon decreasing the O2 concentration, there is a remarkable increase in the intensity and lifetime of the red phosphorescence, whereas those of the blue fluorescence remain almost the same. Similarly, the aforementioned iridium(III) complex 248 has been modified with a rhodamine (263) and a cyanine derivative (264) for hypoxia imaging in cancer cells and in tumor-bearing mice, respectively.212 The anionic, red-emitting iridium(III) complex 252 has been paired with a cationic, blue-emitting, and O2-insensitive polyfluorene-based conjugated polyelectrolyte (CPE) (265) for ratiometric and lifetime imaging of O2 in live cells.214 Also, CPE has been modified with red-emitting iridium(III) (266)228 or platinum(II) complexes (267 and 268)229,230 to develop amphiphilic dual-emissive polymers that can self-assemble into ultrasmall polymer dots (size ¼ ca. 10 nm) in aqueous solution for ratiometric and lifetime imaging of intracellular O2 level. Probe 267 has been utilized to image tumor hypoxia in live mice.229 The incorporation of a hydrophobic platinum(II) tetraphenylporphyrin into the hydrophilic PVP backbone gives an unprecedented monochromophoric ratiometric probe (269) for hypoxia imaging.231 The amphiphilic polymer self-assembles to form micelle-like nanoparticles with an average diameter of ca. 118 nm in aqueous solution. In addition to the characteristic long-lived phosphorescence (lem ¼ 760 nm; so ¼ ca. 40 ms) from the platinum(II) tetraphenylporphyrin unit, the polymeric probe displays a short-lived fluorescence band at 660 nm (so ¼ ca. 7 ns) that originates from the assembled clusters of the PVP chain. The probe is enriched in the tumor region of tumor-bearing mice due to the EPR effect, which allows the visualization of the hypoxic tumor tissues with high spatiotemporal resolution.

S C

N Ir

C S

O O

O

N O

N

N

N O

N O

262

N O

N O

O

O

N

214

Luminescence chemosensors, biological probes, and imaging reagents

+ O S

N

C

O

Ir C

O

N

N

S

O

O

O

n

N

O

NH

O

S HO

O N

+

n = ca. 120 263

_

O S C

N Ir

C S

_

SO3

O O

N

N O

O

O

n

O

NH

S

S

N

+ N _

SO3

n = ca. 120 264

_

A

_

_

N +

N

+

A

S _

A

C

=

C n

265

S

N Ir N

CN CN

Luminescence chemosensors, biological probes, and imaging reagents

N +

N O

O

+

O

O

O

0.96

N S

O Ir

C

0.04

N C

S

266

F

F F

F F

N

N Pt

N

0.93

+ N

+ N

N

F F

0.07

F F

F

b

267

R R

RR

F

a

F F

R R

RR

F F

c

N

N Pt N

d

N

R R

F + R = (CH2)6N(CH3)3 268

F

F F

F

e

215

216

Luminescence chemosensors, biological probes, and imaging reagents

N

O

N Pt

N

N

O

S

O

430

O

N

S

269

Functionalization of transition metal complexes with a hypoxia-sensitive unit is another approach to design hypoxia sensors, by rendering them responsive to cellular reductases that are usually overexpressed in hypoxic tumors. The aforementioned H2Sresponsive iridium(III) complex 162 has been applied to image hypoxic HeLa cells as the quenching nitro group can be reduced by nitroreductase to a non-quenching amine derivative.138 It has also been employed to monitor hypoxia induction in zebrafish upon CoCl2 stimulation. Ruthenium(II) complexes carrying a redox-active anthraquinone moiety (270–272) have been prepared for hypoxia imaging.232 These complexes are non-emissive due to efficient PET from the excited ruthenium(II) polypyridine unit to the anthraquinone moiety but exhibit intense emission upon reduction of the quenching anthraquinone unit to a non-quenching dihydroxyanthracene derivative by reductases in the presence of NADPH, the reduced form of nicotinamide adenine dinucleotide phosphate (NADPþ), under hypoxic conditions. The large TPA cross-sections of the complexes (d800nm ¼ 147–196 GM) under hypoxia render them capable of two-photon imaging of hypoxic mitochondria in A549-derived MCTSs with a depth of ca. 130 mm and in zebrafish due to deeper penetration of two-photon excitation. Incubation of the complex-loaded zebrafish with 2,3-butanedione monoxime (BDM) leads to intense emission in the brain, as BDM abolishes cardiac contractility and thus induces cerebral anoxia. Since the emission response of these complexes toward hypoxia is reversible and repeatable, complex 272 has been exploited to image cycling hypoxia (i.e., cycles of hypoxia followed by reoxygenation) in live A549 cells and zebrafish.

2+

R

N N

N Ru N

O N

N

N

N

R=H CH3

O

(270) (271)

C(CH3)3 (272)

Rational tuning of the internal conversion efficiency of iridium(III) complexes through structural manipulation has identified complex 273 as the first molecular probe that is capable of differentiating hypoxia, normoxia, and hyperoxia.233 The complex shows dual emission bands at ca. 520 and 620 nm with comparable intensities in aqueous solution under ambient conditions, which is attributed to the presence of non-conjugated amino groups that results in additional electronic transitions from the amine lone pairs to the p* orbital of the diimine ligand and thus a new triplet amine-to-ligand charge transfer (n / p*) state that inhibits the communication between the 3IL and 3CT states. A higher O2 concentration increases the intensity of the low-energy (LE) band and decreases the intensity of the high-energy (HE) band, while an opposite phenomenon is observed at a lower O2 level (Fig. 8). This spectral response gives rise to naked-eye distinguishable green, orange, and red emission in aqueous solution under an atmosphere of 0, 21, and 100% O2, respectively. Additionally, the lifetime of the HE emission is significantly reduced from 1523 to 94 ns upon increasing the O2 concentration from 0 to 100%, whereas that of the LE emission is only slightly decreased from 38.5 to 29.6 ns. Similarly sensitive spectral response is also observed in live cells, zebrafish, and mice, which enables the detection of both hypoxia and hyperoxia in living systems.

Luminescence chemosensors, biological probes, and imaging reagents

217

Fig. 8 (A) Emission spectra and (B) phosphorescence lifetimes of complex 273 in PBS/CH3OH (9:1, v/v) under an atmosphere of 0 to 100% O2. (C) Stern–Volmer plots of the HE (green) and LE (red) emission toward O2 quenching. Reproduced from Zhang, K. Y.; Gao, P.; Sun, G.; Zhang, T.; Li, X.; Liu, S.; Zhao, Q.; Lo, K. K.-W.; Huang, W. J. Am. Chem. Soc. 2018, 140, 7827–7834, with permission from American Chemical Society. Copyright 2018.

+ O N

C

N

C

N Ir N

NH

N N

NH O

273

8.05.4.5

pH probes

The intracellular pH in normal mammalian cells varies among different organelles, ranging from ca. 4.7 in lysosomes to 8.0 in mitochondria.234 It is associated with cell proliferation235 and metabolic regulation.236 An aberrant pH is implicated in several common diseases such as cancer237 and neurodegenerative diseases.238 Mitochondria-targeting iridium(III) complexes have been functionalized with two morpholine moieties on the diimine ligand (274 and 275) to monitor mitochondrial pH fluctuations during cell apoptosis.239 The complexes display substantial emission enhancement (I/Io ¼ 40 and 50, respectively) upon increasing the pH of the solution from 3.0 to 9.0, as the protonated morpholine unit serves as an electron acceptor and thus allows efficient emission quenching through PET under acidic conditions. The morphology of the mitochondria in the complex-stained HeLa cells changes from thread-like structures to solid granules upon CCCP treatment and the mitochondrial pH decreases from ca. 8.0 to 7.1, indicative of acidification of the mitochondria during apoptosis. An NIR-emitting ruthenium(II) complex (276) has been developed to monitor the fluctuations of pH in the lysosomes.240 The complex is non-emissive in neutral and basic media but exhibits a ca. 400-fold increase in the emission intensity at 680 nm upon decreasing the pH from 7.16 to 1.88. Pretreatment of the complex-loaded human glioblastoma U251 cells with bafilomycin A1, a selective inhibitor of the vacuolar-type Hþ-ATPase to inhibit lysosomal acidification, reduces the intensity of intracellular emission. A lysosome-targeting, pH-responsive iridium(III) complex (277) has also been used to track lysosomal pH changes.241 The complex shows a decrease in emission intensity upon increasing the pH from 4.2 to 7.3. The long emission

218

Luminescence chemosensors, biological probes, and imaging reagents

lifetime of the complex (ca. 1.1 ms) enables the use of time-resolved emission spectroscopy to eliminate the interference caused by background fluorescence (< 100 ns). Thus, the complex has been utilized to develop a platform for time-resolved high-throughput screening of 400 compounds in live human malignant melanoma A375 cells. Mitoxantrone has been identified as the most potent lysosomotropic compound that causes an increase in lysosomal pH.

+ O

N N

C

N

Ir

C

N

N

N

S

N

N

=

_

O

N _

C

_

C

C 274

2+

H N

H N

N N

Ru

N N H

275

N

H N

N

N

N N H 276

+

C C

N Ir N

N

H N

N

N

277 The conjugation of an NIR-emitting, pH-insensitive iridium(III) complex with a green-emitting, pH-sensitive fluorescein moiety affords a ratiometric sensor (278) for monitoring changes in intracellular pH.242 Upon decreasing the pH, the emission intensity at 520 nm decreases while that at 710 nm remains almost unchanged, resulting in a decrease in the emission intensity ratio I520nm/ I710nm. A soft salt-based probe (279), which is composed of: (1) a cationic, pH-sensitive complex with two pyridyl moieties (lem ¼ 625 nm), and (2) an anionic, pH-insensitive complex (lem ¼ 451 and 475 nm) by electrostatic interaction, has been prepared for ratiometric and lifetime imaging of intracellular pH variations in real time.243 Since protonation of the pyridyl units quenches the emission of the cationic complex, the emission intensity ratio I625nm/I451nm of complex 279 increases from 0.18 to 2.86 upon increasing the pH from 2.03 to 7.94, concomitant with an increase in the lifetime of the emission at 625 nm from 73 (at pH 3.99) to 328 ns (at pH 7.94). The pH-dependent photophysical changes of the complex allow quantitative measurements of intracellular pH fluctuations caused by oxidative stress (Fig. 9).

Luminescence chemosensors, biological probes, and imaging reagents

219

Fig. 9 (A) Intracellular pH calibration curve of complex 279 in HepG2 cells. (B) LSCM and (C) PLIM images of HepG2 cells incubated with complex 279 (10 mM, 1 h, 37  C) with or without treatment with H2O2 (100 mM), NEM (100 mM), and NAC (100 mM). Reproduced from Ma, Y.; Liang, H.; Zeng, Y.; Yang, H.; Ho, C.-L.; Xu, W.; Zhao, Q.; Huang, W.; Wong, W.-Y. Chem. Sci. 2016, 7, 3338–3346, with permission from Royal Society of Chemistry. Copyright 2016.

+

O

O

O S C

Ir C S

S

NH

N N

NH

3

O

O O

O

NH

O

N

N

278 _

+ N C C

N Ir N

N

F C

F F

N N

C

N Ir N

CN CN

F

279

8.05.4.6

Polarity probes

Cellular polarity is an important environmental factor that controls various cellular processes associated with protein activities and membrane structures. Also, each organelle possesses an optimal polarity for its specific roles and functions. Abnormal changes in polarity are implicated in dysfunctions of subcellular organelles and various pathological states. An iridium(III) complex bearing an o-carborane moiety (280) has been applied to track the changes in mitochondrial polarity during apoptosis.244 The complex displays a large bathochromic shift in the emission maximum from 535 to 577 nm upon changing the solvent from less polar toluene (0.36 D) to more polar DMSO (3.96 D), accompanied by a decrease in the emission quantum yield and lifetime from 0.80 to 0.21 and from 794.5 to 185.2 ns, respectively, which is due to the introduction of an electron-withdrawing o-carborane unit to the diimine ligand that enhances the transition dipole moment of the complex. LSCM shows that the complex is accumulated in the mitochondria of both cancerous HepG2 and non-cancerous human liver

220

Luminescence chemosensors, biological probes, and imaging reagents

HL-7702 cells. Both the higher intracellular emission intensity and longer emission lifetime of the complex-treated HepG2 cells with respect to HL-7702 cells indicate a less polar intracellular environment in cancer cells. The shorter emission lifetime of the complex in live cells than in apoptotic and dead cells as revealed by PLIM indicates a decrease in the polarity in mitochondria during apoptosis and thus the complex is capable of differentiating live, apoptotic, and dead cells (Fig. 10). An iridium(III) complex (281) has been employed to monitor the changes in ER polarity in live cells in real time.245 The complex is weakly emissive in polar media (Fem ¼ 0.001 in water) but exhibits intense red emission in apolar solvents (Fem ¼ 0.243 in CHCl3). This complex is enriched in the ER in both cancerous (A549 and HepG2) and normal (human embryonic lung fibroblast MRC-5 and HL-7702) cells. The decrease in intracellular emission intensity after incubation of the complex-stained cells with the ER stress inducer tunicamycin indicates that the polarity of the ER is increased under ER stress. Additionally, the complex has been exploited to detect the ER polarity difference in fresh blood in normal and diabetic mice, which reveals that the blood polarity in diabetic mice is lower than that in normal mice.

+ F C

F F

C

N Ir N

N N C

F

C

280

Fig. 10 LSCM and PLIM images of live (top), apoptotic (middle), and dead (bottom) HepG2 cells incubated with complex 280 (10 mM, 37  C), Annexin V–FITC, and propidium iodide (PI). Reproduced from Li, X.; Tong, X.; Yin, Y.; Yan, H.; Lu, C.; Huang, W.; Zhao, Q. Chem. Sci. 2017, 8, 5930–5940, with permission from Royal Society of Chemistry. Copyright 2017.

Luminescence chemosensors, biological probes, and imaging reagents

221

_

O C C

N Ir N

O

N N

O

_

_

O 281

8.05.4.7

Viscosity probes

Intracellular viscosity determines diffusion-mediated processes such as interactions between biomacromolecules, transduction of chemical signals, and transportation of metabolites within a cell.246 Abnormal intracellular viscosity is implicated in various disorders and diseases. The design of transition metal complexes as viscosity probes mainly relies on the incorporation of a rotatable group into the complexes to provide a non-radiative deactivation pathway for the complexes, which leads to efficient emission quenching. Thus, the restriction of the free rotation in viscous media will result in emission enhancement and lifetime extension. Iridium(III) complexes featuring diphosphine ligands (282–287) have been used to track the changes in mitochondrial viscosity.247 The introduction of rotatable phenyl rings to the diphosphine ligand enables the complexes to show higher emission intensities and longer lifetimes upon increasing the viscosity of the solution from 0.55 to 259 cP. Since complex 287 possesses a large TPA cross-section (d750nm ¼ 444 GM) and is localized in the mitochondria of the cells, it has been utilized to monitor the changes in mitochondrial viscosity in a real-time and quantitative manner using TP-PLIM, which shows that the viscosity of dysfunctional mitochondria is significantly increased in a heterogeneous fashion (Fig. 11). A binuclear iridium(III) complex (288) has been applied to measure lysosomal microviscosity in live cells.248 The emission enhancement and lifetime extension of the complex in response to the increase in viscosity of the medium is ascribed to the restriction of the free rotation of the CeC bonds between the two iridium(III) polypyridine units in highly viscous media. The intracellular emission intensity and lifetime of the complex-treated cancerous (A549 and HepG2) cells are higher and longer than those of normal HL-7702 cells, indicative of a more viscous intracellular environment in cancer cells. The complex has been employed to detect the viscosity differences between fresh blood from normal and diabetic mice, which shows a higher viscosity in the blood of diabetic mice. An iridium(III) complex (289) has been exploited to track the changes in viscosity in ER by LSCM and PLIM.249 The complex displays 22.6-fold emission enhancement at 550 nm and lifetime extension (from 380 to 1120 ns) upon increasing the viscosity of the medium from 1.01 to 76.78 cP. These changes are probably associated with the variation in torsion angles between the phenyl and purine rings in the cyclometalating ligand and those between the pyridine and b-carboline rings in the diimine ligand. Treatment of the complex-loaded A549 cells with

Fig. 11 (A) PLIM images of A549 cells incubated with complex 287 (20 mM) at different time intervals. (B) Enlarged views of selected areas in A549 cells incubated with complex 287 (20 mM, 4 h). The lifetime and viscosity were calculated from the circled area. Reproduced from Hao, L.; Li, Z.-W.; Zhang, D.-Y.; He, L.; Liu, W.; Yang, J.; Tan, C.-P.; Ji, L.-N.; Mao, Z.-W. Chem. Sci. 2019, 10, 1285–1293, with permission from Royal Society of Chemistry. Copyright 2019.

222

Luminescence chemosensors, biological probes, and imaging reagents

tunicamycin increases intracellular emission intensity and lifetime in a dose- and time-dependent manner, which is due to the generation of unfolded proteins and thus enhanced viscosity under ER stress. A neutral iridium(III) complex with two rotatable aldehyde groups (290) has been developed for quantitative tracking of ER viscosity.250 The complex exhibits an increase in the intensity (I/Io ¼ 9) and lifetime (from 473 to 1101 ns) of the emission at ca. 530 nm upon increasing the viscosity of the medium as the rotation of the aldehyde groups is restricted. TP-PLIM reveals that the viscosity in ER of MCF-7 cells increases during erastininduced ferroptosis, probably associated with the accumulation of lipid peroxides.

+

P

=

P

N

C

R R

P

Ir

C

P

N

P

P

P

P

P

P

R=H CHO

282

283

284

285

286

287

2+

C C

N Ir N

2

N N

N

N

Ir

N

N

C C

2

288

+

Cl N

N N

C

N N

Ir

N C

N N

N

N Cl 289

N H

Luminescence chemosensors, biological probes, and imaging reagents

N

C

OHC OHC

223

O

Ir

C

O

N

290 A dual-emissive binuclear iridium(III) complex (291) has been developed for ratiometric monitoring of changes in intracellular viscosity.251 Increasing the viscosity of the solution from 1.1 (water) to 950 cP (99% glycerol) enhances the intensity of the two emission bands at 521 and 711 nm by 16.9 and 2.5 fold, respectively, which is attributed to the inhibition of relative position adjustment of the two luminophores in highly viscous media. Incubation of co-cultured cancerous (HepG2) and normal liver (HL-7702) cells with the complex results in a higher intracellular viscosity in cancer cells. The complex has been used to track the viscosity changes in MCF-7 cells during apoptosis in real time, which reveals that the intracellular viscosity increases in apoptotic events. An iridium(III) complex that carries a rotatable methoxy group (292) has been utilized to monitor the changes in viscosity during liquid–liquid phase separation in live cells using PLIM.252 The complex shows an increase in emission lifetime from 163.39 to 1023.90 ns upon increasing the viscosity of the solution from 1.45 to 165.47 cP. As shown by PLIM, continuous irradiation increases the emission lifetime of the complex-stained NIH/3T3 cells, which are pretransfected to express modified fused in sarcoma protein that undergoes light-activated phase transitions. This indicates that the photogenerated protein droplets become larger and more viscous and form solidified aggregates after intense stimulation, revealing the dynamics of phase-separated biomolecular condensates inside the cells with high spatiotemporal resolution.

2+ F C

F F

C

N Ir N

N 8

N

N

N

S C

Ir

N

N

F

C S

291

+

C C

N Ir N

O N N

292

8.05.4.8

Temperature probes

Temperature is a fundamental physical parameter that regulates cellular processes in living systems. Intracellular temperature mapping is pertinent to provide a better understanding of cellular events to develop new strategies for diagnosis and therapy, which is a challenging task as it requires a thermoresponsive probe with high temperature resolution and high spatial resolution.

224

Luminescence chemosensors, biological probes, and imaging reagents

Transition metal complexes have been designed as thermoresponsive probes through the incorporation of the complexes into a water-soluble, thermosensitive polymer backbone. The conformational changes of the polymer chains in response to temperature will result in a change in the hydrophobicity and rigidity of the local environment of the complexes and thereby influence their photophysical properties. A platinum(II) complex has been introduced into the backbone of poly(N-isopropylacrylamide) to yield a thermoresponsive probe (293) for intracellular temperature mapping.253 Upon increasing the temperature, the hydrodynamic diameter of the probe increases accompanied by emission enhancement due to the conformational change of the polymer from an extended structure to an aggregated state, which increases the hydrophobicity and rigidity of the microenvironment of the complex molecules. The incorporation of a blue-emitting iridium(III) complex and a red-emitting iridium(III) complex (294)254 or europium(III) complex (295)255 into the polymer backbone affords dual-emissive probes for in vitro and in vivo temperature sensing by LSCM and PLIM. The intensity and lifetime of the HE emission remarkably increase upon increasing the temperature, while those of the LE emission only moderately change. a

O

O

c

b

HN

O

HN + N

O N

O

C CF3

Pt O

N

293

100

0.89

O O

N F

Ir

C F

C

F

HN

O

O

2.32

HN

O

O

+ N

N

N

N N

F

+

0.11

HN

N

N

Ir

C N

C N

294

0.2

O N F

C F

F

O Ir C

O

96.9

HN

O

2.0

HN

O

0.8

O

HN

+ N

N N

F3C

N O

S 295

O

Eu

O

F

S

N

O F3C

O

CF3

O S

Luminescence chemosensors, biological probes, and imaging reagents

8.05.5

225

Imaging reagents

The cell is the basic structural and functional unit of all living organisms. For eukaryotic cells, they are embedded with a plasma membrane and comprised of a variety of organelles including the nucleus, ER, Golgi apparatus, mitochondria, lysosomes, and LDs surrounded by the cytosol, all of which play an important role in cellular processes. Abnormal activities and dysfunctions of these organelles are usually associated with various disorders and diseases.

8.05.5.1

Nucleus and nucleolus stains

In eukaryotic cells, the nucleus is separated from the cytosol by the nuclear envelope, a double lipid bilayer decorated with nuclear pore complexes that control the transport of substances between the nucleus and the cytoplasm.256,257 The nucleus contains most of the genetic material (i.e., DNA) of the cell and regulates gene expression to control cell activities.256 Nucleolus is the largest substructure in the nucleus and is well known to be the site of ribosome biogenesis.258 It also plays a pivotal role in the regulation of mitosis, cell cycle progression, stress response, and biogenesis of ribonucleoproteins. The most common approach to deliver transition metal complexes to the cell nucleus or nucleolus is to target biomolecules that are abundant in these organelles. The most prominent target in the nucleus is DNA, as exemplified by the aforementioned complexes 166144 and 170–173147–150 that are capable of specifically staining the cell nucleus due to the substantial photophysical changes brought about by the binding to nuclear DNA. The cell-impermeable ruthenium(II) (1 and 4)28 and osmium(II) dppz complexes (165)143 have been delivered to the cell nucleus through ion-pairing with TeCP, which leads to the formation of neutral, lipophilic, and relatively stable ion-pair complexes that facilitate the cellular and nuclear uptake. The L- and D-isomers of the osmium(II) complex 165 are able to differentiate the nucleolus from the nucleus in live and fixed cells, respectively, using PLIM, based on the difference in the emission lifetimes between the nucleoli and others parts of the nucleus.259 Monitoring of the lifetime changes of the two enantiomers reveals the alteration of the cellular microenvironment upon incubation with metabolic inhibitors for nuclear protein synthesis and the dynamic changes in the architecture of the nucleolus, chromosome, and spindle apparatus during the cell cycle process. Ruthenium(II) (296)260 and platinum(II) complexes (297 and 298),261,262 which are entrapped in the endosomal/(auto)lysosomal compartments of the cells after uptake through an energy-dependent pathway, are readily translocated to the nucleus upon irradiation. The translocation is associated with the photogenerated ROS that causes oxidative damage to the endosomes/(auto)lysosomes, allowing the complexes to escape from these organelles and interact with nuclear DNA. In addition to nuclear DNA, many transition metal complexes such as complexes 41 and 42,25 106,84 and 10785 that display specific interactions with histidine and/or histidine-containing proteins are accumulated in the cell nucleus. O

O N

N

O N

N

O N

9.5

N

N

0.5

2+

N N

296

+ O

O

_ N NB + F F

N

N

N

Pt Cl 297

N Ru N

N N

226

Luminescence chemosensors, biological probes, and imaging reagents

5+ HN

H2N

NH2 Pt

N H2

Pt N

N

NH N H2

N

+ N

298 Compared with the nucleus that contains most of the DNA of the cell, the nucleolus is rich in RNA as it is the site where rRNA, the most abundant form of RNA, is synthesized and assembled. Thus, transition metal complexes have been directed to the nucleolus through specific interactions with RNA, as illustrated by the aforementioned complexes 177–181 that are enriched in the nucleoli of the cells with negligible emission in the nucleoplasm due to their selective binding to RNA.154–158 The nucleolar localization of the iridium(III) complexes 299–304 in Madin-Darby canine kidney (MDCK) cells has been ascribed to their binding to nucleolar proteins instead of RNA, given that: (1) the complexes do not show any noticeable change in their emission profiles in the presence of RNA, and (2) pretreatment of the cells with RNase does not significantly alter the nucleolar staining of the complexes.263

+ N

C

Ir

C

N

N _

=

C R=H CONH(CH2)3CH3

N

N

N

N

N

R

N C

N _

C

N _

C

299

300

301

302

303

304

_

Conjugation of transition metal complexes with an NLS peptide is another efficient means to promote the nuclear uptake of the complexes. Ruthenium(II) complexes have been conjugated with a peptide sequence VQRKRQKLMP (168, 305, and 306), which is derived from the transcription factor NF-kB to enhance their cellular and nuclear uptake.145,146,264 In contrast to the peptide conjugate 305 that displays apparent nuclear staining, the more lipophilic conjugate 306 is preferentially accumulated in the nucleoli of CHO cells.264 An iridium(III) complex has been modified with a PAAKRVKLD sequence derived from the human c-Myc protein (307) for staining the nucleus of human fibroblast cells.265 A concentration-dependent nuclear staining has been observed for a ruthenium(II) complex conjugated with a cell-penetrating peptide, D-octaarginine (308); the conjugate exhibits punctate cytoplasmic staining in HeLa cells at low concentrations ( 10 mM) but is enriched in the nucleus, particularly the nucleoli in 60% of the cells at 20 mM.266 Interestingly, the modification of conjugate 308 with a fluorescein moiety at the C-terminus of the peptide (309) facilitates the accumulation of the conjugate in the nucleoli in 91% of the cells at a much lower concentration (5 mM). This indicates that the lipophilic fluorescein unit enhances the interactions with the cell membrane, which facilitates the cellular uptake of the conjugate via a non-endocytic pathway and thus promotes the access to the nuclear import machinery in the cytosol.

Luminescence chemosensors, biological probes, and imaging reagents

+ H2N N N

N Ru N

N

H N

O

N

N

NH

O

O

NH

NH

NH2

NH

NH

O

N

O

N

NH2

NH

O

N

N

N

N

NH

O NH

O

N O

NH2 O

NH2

O

NH3 +

305

S O

NH

NH

O

=

+ NH3

NH

NH2

O

6+

+ H2N

NH

306

4+

+ NH3

C

N

F

N

FO N

Ir C

O

NH

NH

O NH

O

N

NH

O NH

O

NH

O

O

N

F

N

F

H2N +

NH3 +

NH2

307

10+

N N N

N Ru N

NH N

O OH

8

O

N

NH H2N +

N

308

NH

NH O

O NH2 O OH

NH

N

227

NH2

228

Luminescence chemosensors, biological probes, and imaging reagents

OH

10+

S NH

N N N

N Ru N

N

NH

8

O

N

O HO

O

NH

NH

OH

O O

O

NH H2N +

NH2

N

309 Many other transition metal complexes have been reported to show specific nuclear or nucleolar staining but the underlying mechanisms have not been established. For example, the neutral binuclear rhenium(I) complex (310) and its peptide nucleic acid conjugate (311) are localized in the nucleus of HEK293 cells and their nuclear emission is blue-shifted with respect to that in the cytoplasm, which is attributed to the reduced mobility and increased hydrophobicity of the nuclear environment compared to the cytoplasm.267 The cationic adduct formed from the coordination of Agþ to the uncoordinated pyridine rings of the neutral rhenium(I) complex 24 is efficiently taken up by MCF-7 cells and enriched in the nucleoli of the cells.21 The L-isomer of ruthenium(II) complex 48 specifically stains the nucleus of MDA-MB-231 cells, in a sharp contrast to its D-isomer that is localized in the cytoplasmic region.27 The iridium(III) tpphz complex 253 displays specific staining of the nucleus of a wide range of cell lines and has been applied to monitor the O2 level in the nucleus.215 The platinum(II) complexes 312268 and 313269 are accumulated in the nucleus, particularly the nucleoli of live CHO and human dermal fibroblast cells. The zinc(II) complexes 314 and 315 are enriched in the nucleoli and nucleus of HepG2 cells following their uptake through endocytosis and active transport, respectively, which is associated with the different sizes of the nanoparticles formed in aqueous solution (ca. 30 and 50 nm, respectively).270 The zinc(II) complex 316 stains both the plasma membrane and nucleus in a wide range of cancerous cell lines (A549, HCT116, human large-cell lung carcinoma H460, and MCF-7) but not normal human embryonic liver fibroblast cells, as the complex interacts with specific transport proteins located on the plasma membrane and enters the cells via endocytosis and localized in the nuclear region.271 Additionally, in vivo and ex vivo imaging reveals that the complex is accumulated in the nuclear region in the brain cells of larval zebrafish and in the brain endothelium and CNS cells of mice.

OH O

NN OC OC

Re CO

Cl

Re

Cl

CO

CO CO

310 NH O

NN OC OC

Re CO

Cl Cl

Re CO

CO CO 311

NH2

N O

O N

10

O NH

O

Luminescence chemosensors, biological probes, and imaging reagents

n

R C

N

229

N

Pt Cl

(312) R = H, n = 0 + CH2NH3, n = +1 (313)

O

N

O

O O

N

N

N

Zn X

X

X = Cl (314) Br (315) 2+ N N O

N Zn

N N

N

N N

O

O

O 316

The exclusive staining of the nucleus or nucleolus of dead cells over live cells has led to the development of novel cell viability assays. The iridium(III) complex 317 exhibits selective staining of the nucleus of dead KB cells due to its different cellular uptake between live and dead cells.272 ICP-MS measurements show that the intracellular amount of iridium in dead cells is more than 13fold higher than that in live cells. The difference in the cellular and nuclear uptake is associated with the enhanced plasma and nuclear membrane permeability in dead cells. This enables the complex to distinguish live, early apoptotic, and dead cells and to detect cell viability using LSCM and flow cytometry (Fig. 12).

O C C

N

NH OH2

Ir N

OH2 NH O

317

+

230

Luminescence chemosensors, biological probes, and imaging reagents

Fig. 12 LSCM images of live (top), early apoptotic (middle), and dead (bottom) KB cells incubated with Annexin V-FITC (green) and then complex 317 (10 mM, 10 min, 37  C; red). The emission intensity profiles correspond to the white lines shown in the luminescence images. Reproduced from Chen, M.; Wu, Y.; Liu, Y.; Yang, H.; Zhao, Q.; Li, F. Biomaterials 2014, 35, 8748–8755, with permission from Elsevier. Copyright 2014.

8.05.5.2

Endoplasmic reticulum stains

The ER is an interconnected network of membrane-enclosed sacs (i.e., cisternae) and tubules that extends from the nuclear membrane throughout the cytoplasm.273 It is involved in protein synthesis, folding, modification, and transportation to the Golgi apparatus. It also plays a critical role in calcium homeostasis, lipid and steroid hormone synthesis, and xenobiotic metabolism. Cationic rhenium(I) complexes featuring a DPA-derived tridentate ligand (318–320) have been reported to enrich in the ER of a wide range of cell lines including human pancreatic ductal adenocarcinoma (IMIM-PC2 and PT45), human colorectal adenocarcinoma HT-29, HepG2, and MCF-7 cells.274 In contrast to the aforementioned binuclear ruthenium(II) 1,10-phenanthroline complex 173 (log Po/w ¼ 0.96) that is localized in the cell nucleus due to binding to nuclear DNA,148 the more lipophilic 4,7diphenyl-1,10-phenanthroline complex (321; log Po/w ¼ 1.52) shows specific ER staining, which is associated with its interaction with the lipophilic membrane structures that gives rise to intense intracellular emission.275 Iridium(III) complexes comprising a tetrazole-derived (25–27 and 322–324)22 or an N-heterocyclic carbene-based ancillary ligand (325–328)276 are accumulated in the ER of the cells. The zinc(II) salen and salophen complexes appended with hydrophobic pendants (15–17 and 329–340) are enriched in the ER of HeLa cells.18,277 Specific ER staining has also been observed for the zinc(II) terpyridine complexes 341278 and 342279 in HepG2 cells.

OC OC

CO Re N

+ N N

O N

N N

N

=

O O

N

N

O O

N

N

N

N

318

O N

N

319

320

Luminescence chemosensors, biological probes, and imaging reagents

4+

N

N

Ru

N

N

N

N

N

N

N

N

N

N

Ru

N

N

321

+ CN C C

N Ir N

N

C

N N NN

C

322

N Ir N

N

C

N N NN

C

N

C

N _

=

C

324

N RN CN Ir CN N RN

+

N

S

N _

N

_

C

C

_

C

325

R = CH3 (CH2)3CH3

326

N N NN +

N

323

C

N

Ir

327

328

231

232

Luminescence chemosensors, biological probes, and imaging reagents

R N O

N

Zn

O

N

N CN NC

CN

S

R=

329 N

334

F

F

F 330

331

332 Cl

N N 335

N

336

N O

333 Br

N

N

337

NC

F

338

CN Zn

N O

R1 N R2

N R1 R2

R1 = CH3, R2 =

Cl (339) O

R1 = R2 =

(340)

O

2+ N R

N

Zn N

R=

N N

R

N

(341)

S O

(342)

N

O The conjugation of a ruthenium(II) complex with the cell-penetrating peptide Penetratin (RQIKIWFQNRRMKWKK), which corresponds to the third helix of the DNA-binding domain of Antennapedia, directs the resultant peptide conjugate 343 to the ER.145 The selective ER localization of the conjugate allows the visualization of the hollow nature of the tubule of the smooth ER with high resolution using STED microscopy. An iridium(III) complex bearing a perfluorobiphenyl moiety has been conjugated with the peptide sequence KDEL, a specific signal that causes the retention of proteins in the ER, through the p-clamp-mediated cysteine conjugation to afford a luminescent peptide conjugate (344) that displays specific ER staining in HeLa cells.280

Luminescence chemosensors, biological probes, and imaging reagents

NH2

O O N N

N Ru N

NH

NH

N

O NH

O

N

NH O

+ H2N

+ NH3 O NH

NH2

O NH

O

NH

NH

NH

O

O

NH

NH

O

O

NH O

NH

S O NH

O

NH2

NH H2N +

9+ + NH3

NH2 NH

O

NH

O

NH

NH

O

NH H2N +

NH2

233

NH O

NH3 +

NH2

O NH

NH2 O

NH3 +

343

2+

C

Ir C

O

N N

O

N

NH

S F

F F

FF F NH

N

+ NH3

F S

FO

NH

O

N

NH

O

O

O NH

NH O

O

OH

O NH O

NH O

O OH

OH 344

8.05.5.3

Golgi apparatus stains

The Golgi apparatus, which is a stack of flattened, membrane-bound cisternae, is responsible for the post-translational modification of newly synthesized proteins exported from the ER and packaging of the modified proteins into vesicles for the delivery to targeted destinations.281 The rhenium(I) complex carrying a biotin moiety (60) is enriched in the perinuclear region of HeLa cells after uptake, which appears to be the Golgi apparatus.44 The characteristic infrared (IR) signature of rhenium(I) tricarbonyl complexes in the IR transparent window of biological media (i.e., 1800–2200 cm1) renders them attractive candidates for cellular mapping using vibrational techniques such as IR and Raman spectroscopy. Correlative imaging studies that involve the use of fluorescence microscopy and synchrotron radiation Fourier-transform IR (FTIR) spectromicroscopy show that the rhenium(I) complex 345 is localized in the Golgi apparatus of MDA-MB-231 cells.282 The neutral iridium(III) complex 346 is accumulated in the Golgi apparatus of A549 and HeLa cells for at least 6 h without migration to other cellular organelles.283 Specific Golgi apparatus staining has also been observed for the dendritic iridium(III) complexes 31 and 32 in HeLa cells after their uptake via an energy-requiring process.23

OC OC

CO Re Cl

N N NN

N3

6

345

N

C

B

N C

Ir N

N

C N

346

234 8.05.5.4

Luminescence chemosensors, biological probes, and imaging reagents Mitochondria stains

Mitochondria serve as the powerhouses of the cell to generate energy through oxidation of biomolecules for various cellular processes such as biomolecular synthesis, cell proliferation and differentiation, and metabolism.284 They also play a crucial role in programmed cell death (i.e., apoptosis).285 Biscyclometalated iridium(III) complexes are an important class of transition metal-based mitochondria stains due to their intrinsic cationic and lipophilic character, which facilitates the transport and enrichment of the complexes in the mitochondria that possess a membrane potential of ca. 180 mV (interior negative). Specific mitochondrial staining has been observed for cationic biscyclometalated iridium(III) complexes with an ancillary ligand derived from ethylenediamine (21 and 347),20,286 2iminopyridine (38–40),24 2,2-bipyridine (348 and 349),287 1,10-phenanthroline (350–365),287–291 and 5-(2-pyridyl)tetrazole (28–30),22 as revealed by the co-staining experiments with commercial mitochondrial stains and ICP-MS measurements of the iridium content in isolated organelles from the complex-loaded cells. The functionalization of the complexes with various moieties including glucose (53),35 fructose (57 and 58),40 PEG (74, 77, 78, and 80),49–51 DIBO (198 and 199),190 perfluorobiphenyl (366– 370),280 biotin (371),287 and lanthanide chelate (372)292 does not significantly affect their mitochondrial enrichment properties. Complex 29 has been demonstrated to specifically stain the mitochondria in both live and fixed cardiac and skeletal muscle tissue samples.293 NIR-emitting iridium(III) complexes with large TPA cross-sections (373–378; d730nm ¼ 69.4–279.6 GM) have been developed for mitochondria imaging in live cells, MCTSs, and hippocampus slices by LSCM and PLIM.294 Notably, complexes 364 and 365 exhibit good three-photon absorption properties (d980nm ¼ 1.88 and 2.63  1078 cm6 s2 photon2, respectively), which enable the use of three-photon microscopy that offers deeper penetration; the maximum imaging depth for zebrafish and hippocampus slices is significantly enhanced from ca. 200 and 250 mm upon two-photon excitation (lex ¼ 750 nm) to ca. 1000 and 500 mm upon three-photon excitation (lex ¼ 980 nm), respectively.291 These features allow in vivo three-photon imaging of the mouse brain vasculature with deeper penetration and higher resolution.

+ S N C

Ir

C

N

H2 N N H2

S 347

+ N

C

Ir C

N N

N

O N N

=

N

N

N

N

348

NH

349

N

N

N

N

350

351

Luminescence chemosensors, biological probes, and imaging reagents

+ N

C C

N _

N

Ir

N

N

N

=

N _

C

N _

C

_

C

C

NH NH 352

353

354

+ C C

N

_

=

S

C

_

C

N Se N

N

N

N

N

N

Ir

N

N _

F

_

C

N

N

C

_

C

C

F 355

356

357

358

359

+

R F C

F F

C

N Ir N

O N

N

N

N

N

F R=H CH3

(360) (361)

C(CH3)3 (362) OC6H5

(363)

N

_

235

236

Luminescence chemosensors, biological probes, and imaging reagents

+

R N

C

Ir

C

N

N

N

N

N

R = C(CH3)3 (364) OC6H5

(365)

+

O N

C

Ir

C

N _

N

O

N

S

NH

F

N

N

=

N _

C

C

F F

N _

C

FF F

F

F F

N _

N _

C

_

C

C

OH

OH OH

366

367

368

369

370

+

O

C

Ir C

O

N N

NH

N

N

371

HN H

NH 3

O

NH H S

Luminescence chemosensors, biological probes, and imaging reagents

237

+

C C

N Ir N

O

O N

NH

N O

N

Gd

N

O

N N

O O

O

372

+

N N

C

N

Ir

C

N

N N

N S N N

=

N

N

N

N

N

N

N

N

N

N

373

374

375

376

N N Se N N

377

378

Complex 375 has been employed to image the ultrastructures of mitochondria in live HeLa cells using SIM; the length and width of mitochondria are found to be ca. 2.0 and 0.7 mm, respectively, with the lamellar cristae being clearly visible with a thickness of ca. 105 nm.295 The complex has also been exploited to monitor the mitochondrial dynamics including the mitochondrial fusion and fission and the mitochondria–lysosome contacts in live cells, which shows that while the fusion of mitochondria and lysosomes is more pronounced during mitophagy (i.e., an autophagic process that degrades dysfunctional mitochondria in the lysosomes) for recycling damaged mitochondria, mitochondria–lysosome contacts and mitophagy are two independent processes because five mitophagy-associated proteins (i.e., p62, NDP52, OPTN, NBR1, and TAX1BP1) are not involved in the formation of mitochondria–lysosome contacts. The iridium(III) complex 379 has been used to specifically monitor mitophagy in A549 cells in real time, which reveals that the damaged mitochondria are engulfed in autophagosomes, which are then swallowed by lysosomes and transformed into autolysosomes where the damaged mitochondria are digested.296 Complexes 355–363 have been utilized for real-time monitoring of the morphological changes of mitochondria, which shows the gradual transformation of the mitochondria from the reticular structure into small and dispersed fragments during early apoptosis.288–290

+

C C

N

N

N

N

N

Ir N

379

238

Luminescence chemosensors, biological probes, and imaging reagents

Mitochondria-specific staining has been observed for cationic iridium(III) complexes (380–384) that comprise three different types of coordinating ligands.297 The large TPA cross-sections of the complexes (d750nm ¼ 84–450 GM) render them capable of staining the mitochondria of HeLa-derived MCTSs with a penetration depth of up to ca. 120 mm from the surface upon two-photon excitation. Complex 381 has been employed for two-photon imaging of mitochondrial fission and fusion in live cells and C. elegans in real time. Many cationic transition metal complexes such as rhenium(I) (55, 65, 385, and 386),36,47,298,299 ruthenium(II) (18),19 and platinum(II) complexes (387 and 388)300 are also localized in the mitochondria of live cells. A photoactivatable zinc(II) complex (389), which is almost non-emissive in aerated DMSO (Fem  0.01) but shows 114-fold emission enhancement upon photooxidation of the thioethers to sulfoxides, has been developed for superresolution imaging of mitochondria in live cells using stochastic optical reconstruction microscopy that requires multiple cycles of photoactivation and photobleaching.301 The aforementioned mtDNA-binding zinc(II) complex 175 has been exploited for super-resolution imaging of the mitochondria in live cells, which enables clear visualization of the folded crista structures in a single mitochondrion using STED nanoscopy that is not observed in the case of the commercial stain MitoTrackerÒ Deep Red FM.152

+ N

N

Ir

N

N _

C

=

N _

Cl

N F

C

_

N C

N _

C

N

N _

C

_

C

S

C

F 380

381

382

OC OC F F

F FF FF FF F

NH

F F FF FF F

O

N N

=

N

N

N

N

385

383

CO Re N

384

+ N N S NH

386

NH

Luminescence chemosensors, biological probes, and imaging reagents

239

+

X

N

O

N

C

N

Pt

4

P

X = CH2 (387) (388)

O

NC N O N

CN Zn

N O

S

S

N

389 Transition metal complexes have been directed to the mitochondria through other strategies. One common approach is to modify complexes with a cationic and lipophilic triphenylphosphonium (TPP) moiety; for example, the neutral iridium(III) complex 254 is accumulated in the mitochondria after modification with a TPP unit (255) and has been used to monitor the O2 level in the mitochondria.217 Ruthenium(II) complexes have been functionalized with the MPP sequence FrFKFrFK to target the mitochondria for imaging the mtDNA (169)146 and monitoring the mitochondrial O2 level (247).211 In addition to the aforementioned examples that are based on non-covalent, electrostatic interactions, transition metal complexes have been designed to be immobilized in the mitochondria in a reaction-based manner. Taking the amine-reactive iridium(III) complex 390 as an example, proteomic analyses reveal that the mitochondrial immobilization of the complex is associated with its covalent conjugation with mitochondrial proteins such as voltage-dependent anion channel 1 (a protein located on the outer mitochondrial membrane), 143-3 protein 3, and peroxiredoxin 1, as facilitated by the alkaline mitochondrial environment (pH z 8.0).302

+

C C

N Ir N

N

N C S

N

390

8.05.5.5

Endosome and lysosome stains

Endosomes and lysosomes are membrane-bound organelles that play an important role in the endocytic pathways. Endosomes are responsible for the transportation of substances into and out of a cell.303 Lysosomes contain multiple acid hydrolases for decomposing internalized macromolecules and elimination of intracellular disabled biomacromolecules and organelles.304 Although many transition metal complexes are internalized into the cells through an endocytic pathway, there are only a few reports on the development of transition metal complexes as endosome stains. One of the examples is the iridium(III) complex 20, which is taken up by HeLa cells via an energy-requiring process and accumulated as sharp vesicles throughout the cytoplasm.20 The granular staining becomes more evident with increasing incubation time and concentration of the complex. Co-staining experiments involving Alexa Fluor 633-conjugated transferrin confirm that the complex is enriched in the endosomes of the cells. Another example is the zinc(II) complexes that are modified with two pyridinium (391) or TPP moieties (392).277 These complexes display

240

Luminescence chemosensors, biological probes, and imaging reagents

punctate cytoplasmic staining in HeLa cells and their strong co-localization with the enhanced GFP-tagged FYVE domain confirms that the complexes are enriched in the early endosomes.

NC N O

2+

CN Zn

N O

N

N R

R R=

+ N

(391)

+ P

(392)

Compared with endosome stains, significant effort has been devoted to the development of transition metal complexes as lysosome stains. The most common approach is to modify complexes with a lysosome-targeting morpholine moiety (pKa ¼ 5–6), as the protonation of morpholine in acidic lysosomes ( pH 4–5) facilitates the retention of the complexes inside these organelles. For example, iridium(III) (393 and 394)305,306 and zinc(II) complexes (12, 13, and 395)18,277 carrying a morpholine unit exhibit specific lysosomal staining. Complex 394 is localized in the lysosomes for more than 4 days, allowing the monitoring of the dynamic changes of lysosomes during cell migration and apoptosis.306 The aforementioned chemosensors 11193 and 154126 are also modified with a morpholine moiety for selective imaging of biothiols and NO in the lysosomes, respectively. The morpholine unit can serve as an electron donor and quench the emission of the complexes through PET; for instance, the emission of the iridium(III) complexes 393305 and 394306 is pH-sensitive. The emission intensity of the complexes increases upon decreasing the pH due to the inhibition of PET from the morpholine moiety to the excited iridium(III) polypyridine unit after protonation under acidic conditions. Similarly, ruthenium(II) (276),240 iridium(III) (277, 396, and 397),241,307 and platinum(II) complexes (398 and 399)308 that comprise an imidazole ring on the coordination ligands show pH-sensitive emission and lysosomal staining in live cells. The protonation/deprotonation process of the imidazole rings modulates the emissive states of the complexes and thereby changes the emission wavelength and/or intensity of the complexes. Thus, complexes 276240 and 277241 have been utilized to monitor the pH fluctuations in the lysosomes; the latter has been applied to high-throughput screening of potent lysosomotropic compounds.241 The two-photon-active platinum(II) complex 398 has been employed for real-time tracking of the rapid transportation of lysosomes between the cell body and axons in live primary adult mouse dorsal root ganglion neurons and acute brain slices.308 Specific lysosomal staining has also been observed for protonatable transition metal complexes such as the rhenium(I) complex containing an uncoordinated pyridine ring (400),309 iridium(III) complexes carrying an aminoalkyl group (256– 258),218 and zinc(II) complex bearing two sulfur atoms (401).310

+

O N C C

N Ir N

N

N

N

N

393

OH

Luminescence chemosensors, biological probes, and imaging reagents

O

N O

N

C N

N

C

Ir

N

C

N O 394

NC N O

CN Zn

N O

N

N N

N

O

O 395

+

C C

N Ir N

N

N

C

N

C

396

N Ir N

N

NH

N

397 2+

C

N

H N

H N

N

N

N

Pt

Pt

P

P

C

398 2+

C

N

N

NH HN

N

N

Pt

Pt

P

P

399

C

241

242

Luminescence chemosensors, biological probes, and imaging reagents

CO

OC

Re

OC

N

N

N N

N N

N 400

N

N

N

N

Zn S

S 401

The modulation of the self-assembly/disassembly process of transition metal complexes in response to pH has been demonstrated as an effective means for lysosomal imaging. Platinum(II) complexes 402–404 undergo efficient self-assembly via Pt(II),,,Pt(II) and p–p interactions in aqueous solution at low pH, giving rise to intense 3MMLCT emission in the NIR region.311,312 However, upon increasing the pH, the intensity of the 3MMLCT emission gradually decreases due to the lower degree of aggregation of the complexes at high pH, probably associated with the deprotonation of the phenolic or carboxylic proton that: (1) increases the hydrophilicity of the complexes, and (2) enables efficient emission quenching through PET. Treatment of fixed cells with the complexes followed by incubation at different pHs shows that the complexes are capable of forming self-assemblies and thus exhibit more intense intracellular emission at lower pH. The complexes show specific lysosomal staining in live cells. The TPP-modified zinc(II) salen complex (405) undergoes efficient intermolecular Zn,,,O interaction-driven self-assembly in aqueous solution to form large aggregates (size ¼ ca. 1250 nm) that are internalized into HeLa cells via endocytosis and accumulated in the endosomal/lysosomal compartments (Fig. 13A).313 However, in the presence of pyridine (i.e., a coordinating solvent), the large aggregates are dissociated into much smaller particles (size ¼ ca. 31 nm), which are taken up by the cells through passive diffusion and enriched in the mitochondrial region (Fig. 13B). An iridium(III) complex has been conjugated with two self-assembling peptides (406) for long-term tracking of lysosomes.314 The complex self-assembles to form nanoparticles in aqueous solution and the morphology of the self-assemblies transforms from large nanoparticles (at pH 7–8) to smaller particles (at pH 6) and finally jointed networks (at pH 4–5). The pHresponsive self-assembly is associated with the pH-dependent lipophilicity of the complex; the log Po/w value of the complex increases from ca. –0.8 to 0.5 upon decreasing the pH from 8 to 4. The complex has been exploited to monitor the lysosome dynamics using SIM, which reveals the contact of lysosomes with the mitochondria under starvation conditions.

Luminescence chemosensors, biological probes, and imaging reagents

243

Fig. 13 LSCM images of HeLa cells incubated with complex 405 (2 mM; red) in the (A) absence and (B) presence of pyridine and then (A) LysoTracker® Green DND-26 (75 nM; green) or (B) MitoTracker Green® FM (100 nM; green). (1) Luminescence images of complex 405. (2) Luminescence images of organelle stains. (3) Overlaid images of (1) and (2). (4) Differential interference contrast (DIC) images. Scale bar ¼ 10 mm. Reproduced from Tang, J.; Cai, Y.-B.; Jing, J.; Zhang, J.-L. Chem. Sci. 2015, 6, 2389–2397, with permission from Royal Society of Chemistry. Copyright 2015.

n+ R1

N

N

N

Pt

R2

R3

R2

+ (402) R1 = N(CH3)3, R2 = OH, R3 = H; n = 2 + R1 = N(CH3)3, R2 = H, R3 = COOH; n = 2 (403) R1 = N(CH3)2, R2 = H, R3 = COOH; n = 1 (404)

244

Luminescence chemosensors, biological probes, and imaging reagents

NC N O

2+

CN Zn

N O

N + P

N

N

N N

N

N

+ P

N

405

+

R C C

N Ir N

N

N

N

N

N

R

NH R=

O

O NH

NH O

O

SO3H

NH

HN

O

406

8.05.5.6

Lipid droplet stains

LDs are dynamic subcellular organelles that are composed of a surface phospholipid monolayer and a lipid core filled with neutral lipids.315 Apart from serving as intracellular lipid reservoirs, the functions of LDs also include providing substrates for energy metabolism and building blocks for membrane synthesis.316 The aforementioned neutral iridium(III) complexes (25–27) and their derivatives (322–324) have been reported to stain the LDs of H9c2 cells, as revealed by the co-localization experiments with the commercial LD stain BODIPYÒ 500/510 C1, C12.22 The neutral iridium(III) complexes 407 and 408 also exhibit specific enrichment in the LDs in a wide range of cell lines including A549, HeLa, HepG2, and LO2 cells, which is ascribed to their neutral charge and lipophilic character (log Po/w ¼ 2.15 and 2.19, respectively).317 The large TPA cross-sections of the complexes (d810nm ¼ 232 and 220 GM, respectively) allow the visualization of LDs by LSCM and PLIM upon two-photon excitation. Complex 407 has been used to monitor the accumulation of LDs in live cells upon treatment with oleic acid and visualize lipid metabolism in live zebrafish larvae. The rhenium(I) complex that contains a 4-cyanophenyl ring (409) is localized in the LDs of live 3T3-L1 cells and insect Drosophila larval adipose tissues.309 FTIR microspectroscopy shows that the complex has a higher affinity to polar lipids (e.g., phosphatidylcholine, phosphatidylethanolamine, sphingomyelin, sphingosine, and lysophosphatidic acid) than neutral lipids (e.g., triglycerides) and thus it is preferentially accumulated in regions of high concentration of polar lipids such as the membrane of LDs.318 Under complete starvation conditions and during larval to adult metamorphosis, the localization of the complex in Drosophila larval adipose tissues is shifted from LDs to autophagic compartments, revealing the trafficking of polar lipids under these conditions.319 A two-photon-absorbing zinc(II) complex (410) has been developed for labeling and tracking of LDs in adipocytes.320 The complex shows weak emission in aqueous solution but emits strongly when it is localized in the hydrophobic interior of LDs. Co-staining experiments with immunolabeled perilipin-1 that coats LDs exclusively indicate the specific accumulation of the complex in the hydrophobic core of LDs in adipose cells. In contrast to the commercial LD stain BODIPYÒ 493/503, which is internalized into the cells via passive diffusion and localized in the LDs and other hydrophobic intracellular structures, complex 410 is taken up by HeLa cells through clathrin-

Luminescence chemosensors, biological probes, and imaging reagents

245

mediated endocytosis and specifically enriched in the LDs of the cells. The high specificity of the complex toward LDs enables the visualization of LD growth during adipogenesis in rat preadipocytes and two-photon imaging of LDs in adipose tissue slides.

S

N

C

O

Ir C

O

N

S

R

R

(407)

R = CH3

CH2CH3 (408)

OC OC

CO Re

N

N

N

N

N N

NC 409

NC N O

CN Zn

N O

N

N 410

8.05.5.7

Microtubule stains

Microtubules, which are assembled from the dimers of a- and b-tubulin, are the major components of the cytoskeleton of all eukaryotic cells. They are essential in providing structural support to the cell and play a pivotal role in mitosis, intracellular transport, and cell motility.321 The iridium(III) complex 411 has been designed to image the microtubules by correlative light and electron microscopy.322 Molecular docking suggests that the complex binds to both tubulin ab and ba sites, similar to the well-known tubulin-targeting anticancer drug vinblastine. The complex displays ca. 8-fold emission enhancement upon binding to tubulin without responding to other biomolecules such as BSA, which allows the monitoring of the polymerization of tubulin into microtubules without affecting the microtubule growth. The complex stains the microtubules in both live and fixed HepG2 cells, and transmission electron microscopy shows that the distribution of iridium element along the microtubule structures is periodic with an average interspace of 4.1 nm. The specificity of the complex toward microtubule is confirmed by co-staining experiments with the commercial microtubule stain SiR-Tubulin and an a-tubulin antibody in live and fixed cells, respectively. The complex has been utilized to monitor the dynamics of microtubules during mitosis, which reveals the microtubule polymerization through the centriole (in prophase) to mitotic spindles (in metaphase). It has also been applied to visualize the elongated fibril structures throughout the hippocampus region in microtubule-rich brain tissues using STED microscopy, which enables the visualization of the sophisticated

246

Luminescence chemosensors, biological probes, and imaging reagents

microtubule networks in 3D with neuronal subunits including the growth cone, nascent neuritis, and spines being imaged with high spatial resolution.

+ S

O HO HO O

N C

N

Ir

C

N N S 411

8.05.5.8

Plasma membrane stains

The plasma membrane is a phospholipid bilayer embedded with proteins.323 It forms a barrier between the extra- and intracellular environments and regulates the selective transport of substances into and out of the cells.324 It is also involved in cell recognition and signal transduction.324,325 Functionalization of cationic transition metal complexes with two long alkyl chains is an efficient method to accumulate the complexes in the plasma membrane of the cells; for example, iridium(III) complexes appended with two alkyl chains (412– 415) have been reported to stain the cell membrane to different extents.108 The membrane localization progressively increases with the length of the carbon chains, indicating that the alkyl chains partially inhibit the internalization of the complexes into the cells due to the lipophilic interaction with the lipid bilayer and thus complexes with longer chains exhibit a stronger affinity to the cell membrane. Complex 414 has been modified to sense and distinguish between exogenous and endogenous analytes due to its relatively similar distribution in the cell membrane and the cytoplasm. Modification of transition metal complexes with a polar pendant is also an effective means for membrane enrichment. A ruthenium(II) complex bearing a polar D-fructose moiety (416) shows versatile membrane staining of MCF-7 and HeLa cells, which is probably associated with binding to GLUT5.326 Iridium(III) complexes featuring a polar biotin-substituted bipyridine ligand (417 and 418) are also accumulated in the plasma membrane of HeLa cells.287 Intense membrane staining has been observed in KB cells incubated with a polyfluorene-based CPE that is modified with iridium(III) complexes (419).327 The anionic iridium(III) complex 22 is enriched in the plasma membrane of HeLa cells as the charge repulsion between the carboxylate groups of the complex and the phospholipid bilayers of the cells restrains the complex from further internalization.20 A luminogenic probe (420) has been developed for membrane staining through the conjugation of a triscyclometalated iridium(III) complex with a Cy5 moiety.328 The complex is non-emissive in aqueous buffer solution due to a dual quenching mechanism: (1) the iridium(III) emission is quenched by efficient FRET from the iridium(III) polypyridine unit to the Cy5 moiety, and (2) the Cy5 fluorescence is quenched by energy transfer from the singlet excited state of Cy5 to a non-emissive triplet excited state of Cy5, which is facilitated by the spin–orbit coupling effect induced by the iridium atom. Interaction of the complex with the plasma membrane of murine mammary carcinoma 4T1 cells induces a conformational change that makes spin–orbit coupling less efficient, resulting in an increase in the Cy5 fluorescence while the iridium(III) emission remains quenched.

+

NH

C

n

NH

n

C

N Ir N

n = 3 (412) 5 (413) 7 (414) 9 (415)

N N

Luminescence chemosensors, biological probes, and imaging reagents

2+

O N N

N

NH HO

N

Ru

O

OH OH

OH

N

N

416

+

O O C C

N

N

Ir

NH

NH

3

NH H S

O

N

N

N _

C

HN H

=

N

N _

_

C

C

NH NH 417

N +

418

N

+

O

0.96

N S 419

O Ir

C

0.04

N C

S

247

248

Luminescence chemosensors, biological probes, and imaging reagents

_

C N

N Ir C

2

_

O3S

+ N

C

_

SO3

N O O

NH

NH

NH O

N _

SO3 420

8.05.6

Conclusion

In this Chapter, we summarize the recent advances in the development of photofunctional transition metal complexes as chemosensors, biological probes, and imaging reagents. The presence of d-block metal centers allows the complexes to establish new electronic transitions and emissive states such as MLCT, LLCT, and MMLCT states that are lacking in traditional organic fluorophores. As such, transition metal complexes often display very rich photophysical and photochemical properties. The remarkable characteristics including large Stokes’ shifts, long emission lifetimes, and high photostability render them attractive candidates for various biological applications that involve optical detection. The advancement in microscopy techniques continues to push the resolution barrier to the nanoscale, which enables the visualization of spatiotemporal dynamics of cellular structures with unprecedented detail that cannot be achieved by conventional confocal microscopy. One limitation of super-resolution imaging is the intense laser irradiation, which results in rapid photobleaching of common organic fluorophores. In this context, the high photostability of transition metal complexes means that they are very promising alternatives for super-resolution microscopy. Additionally, the translation from live cells to animal models requires probes that are compatible with excitation and detection at longer wavelengths, particularly in the red to NIR region, as long-wavelength light offers several advantages such as deeper penetration, minimal autofluorescence background, and lower phototoxicity. To this end, one of the future directions is to develop complexes that display intense absorption and emission in the red to NIR region. The presence of metal centers also offers new and unique sensing mechanisms that are based on a change in the metal coordination environment and metal–metal and ligand–ligand interactions. Since the contents of biological species are highly dynamic in living systems, it is important to develop complexes that exhibit reversible binding to the target analytes for continuous detection. Remarkably, bioorthogonal chemistry offers a new paradigm for the design of biological probes for imaging specific biomolecules. The modification of transition metal complexes with a bioorthogonal functionality not only confers excellent target specificity and selectivity on the complexes, but also modulates the photophysical properties of the complexes, which will lead to the development of luminogenic probes for the detection of target biomolecules in living systems. In addition to their intriguing photophysical properties, another remarkable feature of photofunctional transition metal complexes is their capability to efficiently sensitize the generation of ROS, which renders them very promising candidates as photosensitizers for photodynamic therapy. The functionalization of complexes with cancer- and organelle-targeting moieties will direct the complexes to specific cell types and organelles for targeted therapy, respectively. Additionally, the incorporation of a stimuliresponsive linker into the complexes will render them activatable (photo)cytotoxic agents. In conclusion, we believe that photofunctional transition metal complexes are important and valuable biological tools and will continue to play an important role in the development of novel reagents for diagnostic and therapeutic applications.

Acknowledgment We thank the Hong Kong Research Grants Council (Project No. CityU 11300019, CityU 11302820, CityU 11301121, T42-103/16-N, C6014-20W, C7075-21GF, and N_CityU104/21) for financial support. We also acknowledge the funding support from “Laboratory for Synthetic Chemistry and Chemical Biology” under the Health@InnoHK Program launched by Innovation and Technology Commission, The Government of Hong Kong SAR, PR China.

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249

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8.06

Photoactivated metal complexes for drug delivery

Peter C. Ford, John V. Garcia, Camilo Guzman, Sheila Kulkarni, and Emily Wein, Department of Chemistry and Biochemistry, University of California, Santa Barbara, CA, United States © 2023 Elsevier Ltd. All rights reserved.

8.06.1 8.06.1.1 8.06.1.2 8.06.1.3 8.06.1.4 8.06.1.5 8.06.2 8.06.3 8.06.4 8.06.4.1 8.06.4.2 8.06.4.3 8.06.5 8.06.5.1 8.06.5.1.1 8.06.5.1.2 8.06.5.1.3 8.06.5.1.4 8.06.5.2 8.06.5.2.1 8.06.5.2.2 8.06.5.2.3 8.06.5.3 8.06.5.4 8.06.6 Acknowledgment References

Introduction Fundamental issues in photo-uncaging Photochemical kinetics Transmission of light through tissue Targeting Reactive excited states Photodynamic therapy (PDT) Uncaging neurotransmitters Uncaging of chemotherapeutic drugs and photoactivated chemotherapy (PACT) Photoactivated chemotherapy (PACT) Photo-uncaging of cancer therapeutics Dual-action complexes Small molecule bioregulators (gasotransmitters) Nitric oxide PhotoNORMs based on iron Manganese photoNORMs Chromium photoNORMs Ruthenium photoNORMs PhotoCORMs Group 6 photoCORMs: Cr, Mo and W Group 7 photoCORMs Group 8 photoCORMs Fe and Ru Photorelease of H2S Nanocarriers and other delivery mechanisms Summary

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Abbreviations and acronyms 5CNU 5-Cyanouracil 5HT Serotonin acac Acetylacetonate biq 2,20 -Biquinoline bpy 2,20 Bipyridine BODIPY Boron-dipyrromethene CYP Cytochrome P450 CORM Carbon monoxide releasing moiety CTSB Cathespin B dmbpy 4,40 -dimethyl-2,20 -bipyridine. dppn 3,6-bis(20 -pyridyl)pyridazine dppz dipyrido-[3,2-a:20 ,30 -c]phenazine ES Excited state F Fluorescence FRET Förster (or fluorescence) resonance energy transfer GABA Gamma amino butyric acid GS Ground state IC Internal conversion ISC Intersystem crossing

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LF Ligand field Me2bpy 6,60 -dimethyl-2,20 -bipyridine Me2dppn 3,6-dimethylbenzo[i]dipyrido[3,2-a:20 ,30 -c]phenazine MLCT Metal to ligand charge transfer mtmp 2-methylthiomethylpyridine NADH 1,4-dihydronicotinamide adenine dinucleotide NAMPT Nicotinamide phosphoribosyltransferase NIR Near-infrared P Phosphorescence PACT Photoactivated chemotherapy PEG Polyethyleneglycol PDT Photodynamic therapy Phen 1,10-phenanthroline Ph2phen 4,7-diphenyl-1,10-phenanthroline photoCORM photoactivated CORM photoNORM Photoactivated NO releasing moiety py Pyridine pydppn 3-(pyrid-20 -yl)-4,5,9,16-tetraaza-dibenzo[a,c]naphthacene RBS Roussin’s black salt ROS Reactive oxygen species RRS Roussin’s red salt RSE Roussin’s red salt ester S Singlet SPE Single photon excitation T Triplet TPE Two photon excitation UCNP Upconverting nanoparticle UV/vis Ultraviolet/visible lirr Wavelength of irradiation or excitation FNO Quantum yield for NO release

Abstract Photochemical release (uncaging) of small bioregulatory or therapeutic molecules at physiological sites offers exquisite control of timing, location and dosage. However, photo-uncaging faces two major problems that challenge its therapeutic applications: the relatively poor transmission of visible light through tissue and the need to deliver the appropriate precursors to the desired targets. In this chapter we provide an overview of the metal complexes and delivery platforms designed to provide spatial-temporal control of such delivery.

8.06.1

Introduction

This chapter discusses the application of transition metal complex photochemistry for the targeted release of small bioactive molecules at specific physiological sites. Precise spatial-temporal control of such drug delivery is highly desirable, especially for therapeutic applications, and the photochemical technique provides such control. When the photochemical precursor is benign, it is defined as “caged.” Electronic excitation that releases or transforms the precursor into a bioactive form can be termed photouncaging. The external signal (light) determines the location and timing of release, while the quantity of light absorbed controls the extent of photoreaction (i.e., dosage). Thus, photo-uncaging can define the location, timing and dosage of delivery. The precision from such delivery has value as an investigative tool and in the potential therapy of disease states. Furthermore, photophysical methods are well suited for imaging if the caged compound itself and/or the photoproduct formed after uncaging are luminescent. The combination of therapeutic release and imaging is sometimes given the adjective “theranostic” although this term is perhaps more commonly used in nuclear medicine. Developing these applications requires understanding the fundamental photochemical mechanisms of effective precursors and defining methods for transporting the caged drug to the physiological targets. In this chapter, we will discuss photoactive metal

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complexes that have been used to deliver compounds such as neurotransmitters and cancer chemotherapeutic drugs as well as small molecule bioregulators (“gasotransmitters”) like nitric oxide (NO) or carbon monoxide (CO) to physiological sites. Metal complexes are particularly attractive for such applications, since one can tune the spectroscopic and excited state reactivities; however, it should be noted that nonmetal chromophores have also been applied for analogous purposes.

8.06.1.1

Fundamental issues in photo-uncaging

A key question that needs to be addressed is the function of the bioregulatory or therapeutic molecule to be uncaged. Cancer chemotherapy is a common purpose, but other issues such as the treatment of antibiotic-resistant infections are receiving significant attention. In developing a compound for a specific task, there are a set of criteria that should be addressed in addition to whether that compound uncages the desired bioactive species when irradiated with light. These include the following challenges:

• • • • •

Is the photochemical precursor compatible with the biological environment to which it will be subjected? Is this precursor soluble in media (solutions, vesicles, polymer carriers, etc.) consistent for administration via injection, transfusion or implanting. Can this precursor be targeted to the desired physiological site rather than be administered systemically? Is the caged compound responsive to the photolysis wavelengths that can be delivered to the target? Does it release the payload only when so excited? Lastly, is the residual photoproduct toxic, benign or therapeutic? and/or is it readily excreted after release of the desired bioactive agent?

Realistically, much of the research in this area has been exploratory, and only for few select cases have most of these issues been fully addressed. In this context, the present chapter can be considered a progress report on the development of new compounds and photo-uncaging strategies as well as an attempt to provide guidelines for the challenges ahead. When faced with questions about which compound or method of delivery might be most desirable, one must consider the task proposed. In addition, there need to be standardized tests to which new compounds and delivery methods are subjected, preferably by an independent laboratory.

8.06.1.2

Photochemical kinetics

Since the uncaged agent may diffuse away from a targeted site or be consumed by unproductive pathways, the rate of the desired photochemical reaction is a crucial consideration. For single photon excitation (SPE), the rate of the photoreaction of interest (Ri) can be defined by Eq. (1), where Fi is the quantum yield (that is, the efficiency by which a photoreaction occurs along specific pathway) and Iabs is the intensity of light absorbed by the photochemical precursor. R i ¼ Fi x Iabs

(1)

Fi is unit-less and can be simply defined as the number of molecules undergoing process i divided by the number of photons absorbed by the photoactive compound of interest. The quantum yield is determined by how excited states initially formed by photoexcitation partition between various deactivation pathways including emission, nonradiative decay to the ground state and reaction to products, under the specific conditions. Iabs can be defined for solution phase studies in terms of the Einsteins of light absorbed by the photoreactant per unit time (an Einstein being Avogadro’s number, i.e., a mole, of photons.). It is a function both of the incident intensity I0 of the excitation light and of the absorbance Abs(l) by the photochemical precursor at the irradiation wavelength(s) lirr. If the photoreactant is the only absorbing species, Iabs is defined by Eq. 2. If there are other absorbing species present, then Iabs will need to be corrected for inner-filter effects.a   (2) Iabs ¼ I0 1  10AbsðlÞ Although such equations are simple, one must keep in mind that Fi may be dependent on the excitation wavelength as well as upon experimental conditions such as the reaction medium and even temperature. As a consequence, Fi values are typically measured in homogeneous solutions at the specific excitation wavelengths and conditions of interest. In selected cases described in this chapter, photoactivation of a chemical process such as the uncaging of a therapeutic agent may be triggered by multiphoton excitation rather than the more typical SPE for which Eqs. (1) and (2) are relevant. In very general terms, such multiphoton processes may occur either simultaneously or sequentially.1 An example of the former is two-photon excitation (TPE) where the excited state initially formed is twice the energy of the incident light energy. The selection rules for TPE are different than those for SPE, and it is important to note that the rate of excited state (ES) formation via TPE is proportional to the incident intensity squared (I02) rather than the linear dependence on I0 implied by Eqs. (1) and (2) for SPE. Furthermore, given that I0 will be highest at the focal point of the excitation beam, TPE and other multiphoton processes offer the potential for threea In discussing photochemical rates, there have been the occasional reports of a first order rate constant for photochemical uncaging from precursor X (i.e., rate ¼ kp[X]). The “rate constants” kp were obtained from apparent exponential plots of optical absorbance changes during a photoreaction. Such “exponential” decay curves derive from the relationship (1–10–Abs(l)) y 2.3Abs(l) when Abs(lirr) < < 1. However, the calculated kp is an artifact that ignores the relationship between Ri and I0 as indicated in Eqs. (1) and (2).

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dimensional spatial resolution in the excitation profile. Such 3D resolution is extensively used in imaging and offers interesting possibilities in photochemical therapy. In contrast, sequential excitation involves first populating one excited state by SPE, then a second one by photons of the same or different frequencies. Lanthanide ion doped upconverting nanoparticles (UCNPs) that can generate ultraviolet/visible (UV/vis) emissions upon near-infrared (NIR) excitation operate in this manner. The kinetics of higher energy excited state formation depend on a number of factors, but generally are non-linear with respect to I0. These issues will be discussed in greater detail below.

8.06.1.3

Transmission of light through tissue

A complication affecting in vivo therapeutic photo-uncaging is that the transmission of light through the heterogeneous environment of an organism will be strongly affected by scattering, as well as the other chromophores naturally present. Any biomedical application, other than subcutaneous, must be responsive to excitation wavelengths for which such transmission is optimal. Tissue penetration is shallow for UV light, and absorption of these shorter wavelengths may also lead to collateral damage. Transmission improves for longer visible wavelengths and is optimal for the near-infrared.2 One strategy to address this issue lies in molecular engineering that modifies the optical properties of the photochemical precursor to shift the effective excitation wavelengths to lower energies. Another approach to enhancing uncaging rates is to increase the molecular absorbance by designing conjugate systems with strongly absorbing antennas (Scheme 1). However, such antennas will work as photosensitizers only if there are acceptor states on the precursor with the appropriate energies to stimulate the desired photoreactions. In this context, we will discuss some molecular conjugates that have been designed to harvest multiple NIR photons in order to access the higher energy reactive states needed to activate the desired uncaging.

8.06.1.4

Targeting

While photo-uncaging can provide spatial-temporal control of the release of the bioactive agent, it is clear that targeting delivery of the caged compound to the tissue of interest would be more effective and safer than simply flooding the system. Thus, targeting is an important consideration in the design of new compounds or conjugates for photo-uncaging bioactive molecules. There are a number of strategies that might be effective. Physical targeting may be achieved by using an implant and/or an injection. Alternatively, biological targeting might be achieved by using antibodies or proteins that seek out specific tissue types. Another biological strategy would be to recruit immune cells, such as macrophages, loaded as a “Trojan horse” with a photochemical precursor of the desired bioactive agent and to use this mechanism to carry the desired conjugate to sites of inflammation such as tumors, infections or wounds.3 How such methodologies have been applied will be summarized below as we review specific applications of photouncaging and present targeting strategies applicable to the uncaging of any bioactive small molecule.

8.06.1.5

Reactive excited states

The electronic spectra of metal complexes display a number of electronic transitions, the most common ones being metal-centered d-d or ligand field (LF) absorptions, metal-to-ligand (d-pL*) charge transfer (MLCT) bands, ligand-to-metal charge transfer (LMCT)

Scheme 1 Cartoon showing how attaching an antenna A to a photochemical precursor carrying a caged bioactive molecule can enhance the rate of uncaging by photosensitizing the reaction at the precursor. Such antennas may be activated either by single photon excitation or in some cases by multi-photon excitation with longer wavelength light.

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bands and intraligand (IL, e.g., pL-pL*) absorptions. However, when assessing the photochemistry of metal complex excited states (ES), it is important to recognize that the ES reached by the initial act of light absorption often decays by non-radiative processes (internal conversion and intersystem crossing (ISC)) to lower energy ES, from which reactions and/or radiative decay back to the ground state occur. We will not provide a general discussion of such photochemistry/photophysics in this venue, but will use Ru(II) bis(diimine) complexes to illustrate some issues to consider. Ruthenium(II) complexes of the type RuIIL2(X)Yn þ, L ¼ a derivative of 2,20 -bipyridine (bpy) or 1,10-phenanthroline (phen), play prominent roles in many photo-uncaging studies.4 The visible absorption spectra of these complexes, like tris(diimine) complexes RuL32 þ, such as Ru(bpy)32 þ and Ru(phen)32 þ, are dominated by strong, spin allowed MLCT (Ru / L) absorption bands. However, unlike the RuL32 þ complexes, the RuIIL2(X)Yn þ complexes are often labile toward photo-induced ligand substitution reactions. One can rationalize this difference using the excited state tuning model described for Ru(II) complexes many years ago.5 For a MLCT ES, charge is transferred from the metal center to the p-unsaturated ligand L to give (formally) a Ru(III) center with a low-spin d5 electronic configuration. Although there is a decrease in metal to ligand p-backbonding that weakens metal bonding with a strong p-acceptor such as CO, for most ligands such MLCT states of the Ru(II) complexes are not very substitution labile. Initial excitation into the singlet 1MLCT state is followed by rapid internal conversion/intersystem crossing to other lower energy states. For the RuL32 þ and for many RuIIL2(X)Yn þ complexes, the lowest energy ES is the triplet 3MLCT, and it is from this state that emission is commonly seen.6 While, the substitution lability of the 3MLCT ES should also be low, we need to take into account the triplet metal-centered, ligand field excited state(s) 3LF that occur at comparable energies (Fig. 1). The presence of such LF states is often not obvious in the absorption spectra of diimine Ru(II) complexes given the much stronger MLCT bands. The 3LF states have a t2g5eg1 electronic configuration (approximating the hexacoordinate Ru(II) center as octahedral) and are subject to strong distortions along metalligand coordination bonds. By comparison, the formal configuration of the 3MLCT ES is t2g5(pL*)1, which is relatively undistorted. As a result, the 3LF excited states provide channels both for non-radiative deactivation to the ground state (GS) and for reaction via ligand substitution. With complexes like Ru(bpy)32 þ, the 3LF state plays a role in nonradiative decay,6 but the photo-substitution pathway is generally exceedingly small. In contrast, for RuIIL2(X)Yn þ ligand substitution is much more common, in part because of the more energetic accessible ligand field ES and because of the monodentate natures of X and Y. This reactivity has been exploited in the delivery in a number of cases for the photo-uncaging of bio-active molecular species.

8.06.2

Photodynamic therapy (PDT)

PDT is not a major theme of this chapter given that it rarely involves uncaging; however, the two topics are closely linked given the common issues regarding tissue transmission of light, targeting of specific physiological sites, and disease states to be treated. The large majority of PDT chromophores are organic dyes, and those that Federal Drug Administration approved for clinical use are porphyrin derivatives.7 There are others that are in clinical trials, including at least one ruthenium complex.8

Fig. 1 Qualitative energy level diagram for a Ru(II) complex such as [RuII(bpy)2(X)Y]n þ. The optical spectrum is dominated by intense MLCT absorptions, while emission (if any) occurs from the lower energy 3MLCT ES. While nonradiative deactivation to the GS can occur from the 3MLCT state, the thermally accessible 3LF state(s) are more distorted and provide pathway for nonradiative deactivation and for decay along a ligand substitution channel.

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In PDT, the principal goal is to photo-sensitize the generation of singlet oxygen (1O2) at the targeted site by energy transfer to triplet oxygen (3O2) or to reduce the latter to the superoxide ion O2 by excited state electron transfer. The singlet oxygen and other resulting reactive oxygen species (ROS) are thus cytotoxic, directly killing the diseased cells or triggering immune responses such as apoptosis that accomplish the same purpose (Fig. 2). The ground states of typical organic dyes are singlets (1GS), so the spinallowed electronic transitions generate short-lived singlet excited states (1ES) that decay by fluorescence, non-radiative decay to the GS, or intersystem crossing (ISC) to longer lived triplet excited states (3ES). Since energy transfer sensitization of the 3 O2 /1O2 conversion by the 3ES/1GS transition is allowed by spin selection rules, dyes with high ISC efficiencies are desirable. Heavy atoms, including metal centers, enhance intersystem crossing; however, they may also enhance the non-radiative deactivation to the ground state. These two properties need to be balanced in order to provide the most effective photosensitizers. An important advantage of the PDT method is that the dye can be repeatedly excited, since it is a photosensitizer and not the bioactive agent itself. In addition, PDT is minimally invasive compared to surgery or chemotherapy, but there are drawbacks. These include low aqueous solubility of currently approved photosensitizers and the same difficulty found with most forms of chemotherapy in delivering the appropriate doses to the specific tissues targeted. The light used for PDT helps with localizing the treatment, but delivering appropriate wavelengths to deep and/or metastatic tumors is challenging. Additionally, PDT agents that do not eliminate from the body may easily lead to prolonged photosensitivity in patients, even after treatment. Lastly, the requirement that O2 needs to be present in the targeted tissues also poses a problem given that tumors tend to be hypoxic owing to high metabolism rates and poor vascular systems.9 Notably, there is parallel interest in the use of PDT for the treatment of infections by antibiotic resistant bacteria.10,11 In the latter application there may be fewer concerns about hypoxia. There is an active interest in transition metal containing dyes for PDT.8,12 In this context, a promising direction for PDT research is in “dual-action” agents, that are capable of both photodynamic action as well as photo-uncaging chemotherapeutic drugs, examples of which will be described below (see Section 8.06.4). The transition metal complexes that have received significant attention are the usual suspects, such as the low spin d6 ruthenium(II) and iridium(III) diimine complexes and some platinum complexes.13– 20 With such heavy metals, intersystem crossing to the desired lowest energy 3ES is very fast, owing in part to the high spin-orbit coupling. In addition, certain metal complexes are effective for multi-photon excitation for PDT, as this method allows one to utilize the longer wavelengths having deeper penetration into tissue.21 As described below, some diimine complexes can be directly excited using intense 800 nm laser pulses.17 Another approach involves conjugating chromophores with a high TPE cross section to a PDT photosensitizer in analogy to Scheme 1. TPE would be followed by energy transfer to the photosensitizer moiety via Förster (or fluorescence) resonance energy transfer (FRET). Examples are metalloporphyrins, decorated with pendant tetrapyrido[3,2a:20 ,30 -c:300 ,20 -h:2000 ,3000 -j]acridine (TPAC) units (Fig. 3) or other porphyrin chromophores (Fig. 4) that are reported to undergo TPE of the pendant groups followed by energy transfer to the metalloporphyrin moiety, intersystem crossing to the triplet state (3ES)22,23 and subsequent photosensitization of 3O2 to 1O2. Notably, these can be activated by wavelengths within the phototherapeutic window.22 Such design principles can be exploited to aid in the rational design of other TPE-PDT agents. Earlier work exploring the application of TPE to PDT include studies by Ishii et al. of Ru(II) (naph)phthalocyanine complexes24 and by Boca et al. of a Ru(II) tris(5-fluorene-1,10-phenanthroline) complex decorated with hydrophilic triethylene glycol (TEG) units to impart water-solubility.25 In this context, Coe has outlined the nonlinear optical (NLO) activity of various Ru(II) complexes and other organometallic systems and shown extensive correlations between charge-transfer, electrochemical and NLO properties.26 In order to transfer such concepts from theoretical and laboratory proofs-of-principle to clinical applications, the biocompatibility and bioaccumulation of these compounds must be addressed. Huang et al. have reported a series of lysosome-targeting Ru(II) tris(bpy) bpyb) complexes decorated with pendant alkylamine groups of various lengths. The authors note that lysosomes (the

Fig. 2 Jablonski energy level diagram for PDT. Abbreviations used: S, singlet; T, triplet; F, fluorescence; P, phosphorescence; IC, internal conversion; ISC, intersystem crossing. Taken with permission from Ref. Lan et al. Adv. Healthcare Materials (2019), 8, 1900132.

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Fig. 3 Metalloporphyrin complexes functionalized by TPACs. TPE of the pendant group leads to excitation of the metalloporphyrin functionality which photosensitizes singlet oxygen formation. Redrawn with permission from Ref. Oar et al. Chem. Mater. 2006, 18, 3682–3692. Copyright 2006 American Chemical Society.

organelle responsible for cellular waste), rather than mitochondria or nuclei, are favorable targets for cytotoxic agents.27 The tertiary alkylamine groups improved the complexes’ water solubility as well as their luminescence lifetimes and TPE cross-sections. Upon staining HeLa cells with these complexes, cell morphology changes consistent with ROS damage were observed upon irradiation by an 800 nm laser. Another example recently reported by Gasser and coworkers is the Ru(II) diimine photosensitizer pictured in Fig. 5a.17 This and a series of similar complexes were designed with the goal of improving the penetration of such PDT dyes into larger tumors where hypoxia is a particular challenge; these able to fully penetrate such tumor models in mice. Complex A was found to have redshifted visible range, single-photon absorption bands, allowing excitation at 595 nm and longer wavelengths. Furthermore, it showed strong two-photon absorption when excited with an ultrafast laser at 800 nm, thereby enabling the phototherapeutic application of PDT at that wavelength. Compound A displayed no dark toxicity but proved phototoxic upon clinically relevant SPE (595 nm) or TPE (800 nm), leading to eradication of the hypoxic centers in larger adenocarcinomic human alveolar basal epithelial tumors implanted in mice. Two other motifs utilizing Ru(II) diimine complexes as photosensitizers in PDT applications involve non-covalent attachment to a nanostructure. In one, Zhang et al.28 prepared assemblies of a Ru(II) complex with single-walled carbon nanotubes (Ru@SWCNTs), for the purpose of bimodal photothermal and TPE PDT. Excitation with an 808 nm diode laser both releases the complex from the SWCNTs and photosensitizes 1O2 formation via TPE of the Ru(II) complexes. The combination of the photothermal effect and the TPE PDT with the Ru@SWCNTs was shown to more effective toward cancer cell cultures and tumor spheroids as well as toward tumors implanted in mice than either the Ru(II) complexes or SWCNTs alone. In the second, porous silicon

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Fig. 4 Water-solubilizing carboxylates with linked unmetallated porphyrins22 capable of multi-photon excitation. Redrawn with permission from Ref. Ogawa et al. Org. Biomol. Chem. 2009, 7, 2241–2246. Published by the Royal Society of Chemistry.

nanoparticles (pSiNPs) were functionalized with PEG for water solubility and with mannose for cancer targeting and were then loaded with the Ru(II) complex shown in Fig. 5b. Irradiation with 800 nm light of the targeted cancer cells incubated with these

Fig. 5 Left: Ruthenium PDT dye A displaying both SPE and TPE activation in large hypoxic tumors. Adapted with permission from Ref. Karges et al. Chem. Eur. J. 2021, 59, 362–370. Right: The photosensitizer Ru(5-Fluo-Phen)2(5-E-Phen)2 þ B used in the pSiNP-Ru-PEG-Man conjugate. Figure adapted with permission from Ref. Le Gall et al. Chem. Med. Chem (2018), 13, 2239–2249.

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pSiNP-Ru-PEG-Man nanoparticles allowed for targeted PDT as well as simultaneous imaging of the cells. The application of the pSiNP-Ru-PEG-Man conjugate decreases cell viability upon treatment to a far greater extent than each component alone.29 Since the singlet oxygen formed by PDT generates ROS, the resulting cellular toxicity has also drawn attention for the treatment of multi-drug resistant infections.30 The dyes under consideration for such therapeutic applications are much the same as for the PDT approaches to anti-cancer treatments. Again, among transition metal complexes, the ones that have received the most attention are derivatives of ruthenium(II) and iridium(III) diimine complexes.31–33

8.06.3

Uncaging neurotransmitters

Neurotransmitters are small molecules responsible for the transmission of chemical signals across the synapse from one neuron to another or from a neuron to a muscle or gland cell.34 Typically these are relatively small molecules such as amino acid derivatives or short sequences of amino acids (neuropeptides) that are stored in synaptic vesicles along with the enzymes required for their synthesis in the neuron. Even simpler molecules such as nitric oxide (NO) and hydrogen sulfide (H2S) have neurotransmission roles.35 Dysfunction of neurotransmitter production or regulation may contribute to neuropsychiatric and neurodegenerative diseases and conditions. Elucidating underlying mechanisms behind how neurotransmitters modulate neurological functions requires tools for studying their regulatory processes. One such tool is photo-uncaging, since this technique allows excellent spatial-temporal control for delivering targeted local concentrations of neurotransmitters in the specific region of a neuron and is proving of value for understanding neurochemical regulatory mechanisms.36–38 Although the detailed neurochemistry involved is beyond the scope of this article, we will summarize some applications of neurotransmitter uncaging using photo-active transition metal complexes. Serotonin (5HT) is a very important neurotransmitter responsible for the modulation of basic organismal functioning. It has a strong regulatory impact on basic behaviors such as feeding and mating, and overall can affect the general mood and wellbeing of a vast and wide variety of organisms throughout the animal kingdom.39 Dysfunction of serotonergic systems in humans can lead to debilitating psychiatric conditions such as depression, anxiety and obsessive compulsive disorders.40 For this reason, the human serotonergic system is a main target of a variety of psychiatric pharmaceuticals used to treat such conditions.41 Currently there exists two commercially available caged serotonin compounds, NPEC-5HT and BHQ-O-5HT (Scheme 2). These compounds tether the hydroxyl and amine substituents of serotonin to the photoactive chemical structure which also acts as a protecting group. However, the organic groups used as the photo-triggers for serotonin release require UV excitation (365 nm) although it has been reported that BHQ-O-5HT can be activated by TPE at 740 nm.42 The photo-uncaging lirr can be red-shifted using as photo-triggers the ruthenium(II) bis(diimine) systems such as those described in Section 8.06.1.5, thereby allowing photoactivation with visible light instead of damaging UV wavelengths. Etchenique and coworkers have reported the one-pot synthesis and photochemical properties (Scheme 3) of the Ru(II)-caged serotonin complex [Ru(bpy)2(PMe3)(5HT)]2 þ.43 This complex is yellow-orange and water-soluble at concentrations well above the tens of millimolar range. It is stable over a period of months when stored as a solid or solution in the dark. Aqueous solutions of this complex display a MLCT band at lmax 447 nm with a molar extinction coefficient of 6.7  103 M 1 cm 1. The photoactivated release of serotonin from [Ru(bpy)2(PMe3)(5HT)]2 þ was observed using both 1H NMR and UV-VIS spectroscopy.43 The 1H NMR spectrum of an irradiated samples (525 nm green LED) showed resonances characteristic of free serotonin that were absent in the spectrum before LED irradiation. The progression of the UV-Vis spectra during photolysis demonstrated an isosbestic point at 434 nm, thus it was concluded that the photoreaction yields only two products, the uncaged 5HT and the aquo substituted starting material (Scheme 3). The reaction was reported as complete after only 12 min of photolysis using a 405 nm diode laser. Plotting the amount of photo-released 5HT vs the total number of photons absorbed yields a plot in which the initial slope of the curve represents the quantum efficiency of the photo-uncaging of serotonin. Using this method, the quantum yield for 5HT release was determined to be 0.034.

Scheme 2

Chemical structures of the commercially available caged serotonin compounds. BHQ-O-5HT and NPEC-5HT.

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Scheme 3 One pot synthesis of [Ru(bpy)2(PMe3)(5HT)]2 þ and the photolysis reaction uncaging serotonin to produce two photoproducts, serotonin and the aquo substituted version of the complex. Reprinted with permission from Ref. ACS Chem. Neurosci. 2017, 8, 1036–1042. Copyright 2017 American Chemical Society.

This strategy represents a reliable method for the delivery of serotonin to specific cells without the use of invasive methods such as picosyringes. Using this system, Etchenique and coworkers demonstrated modulation of the excitability of mouse prefrontal principal cells in response to the uncaged 5HT.43 The electrophysiological studies conducted show that the sensitivity of this system is high enough to yield clean and fast uncaging of 5HT in biological models. This was shown by the neuronal characterization using a 450 nm light source. Like serotonin, dopamine is a very important neurotransmitter responsible for the basic functioning of organismal behavior. It is active in the prefrontal cortex, where it plays a central role in executive functions.44 In addition, there is evidence of dopamine’s involvement in working memory, decision-making, and inhibitory control.45–47 Dysfunction of dopaminergic systems in the human central nervous system (CNS) can lead to a variety of neurophysiological and neuropsychiatric diseases and disorders including Parkinson’s disease, schizophrenia and others.48–55 First-generation antipsychotics such as phenothiazine, thioxanthene, diphenylbutylpiperidine and butyrophenone derivatives and second-generation antipsychotics such as clozapine and risperidone developed for treatment of various neuropsychiatric disorders exert their pharmacological activity on the dopaminergic systems of the CNS.56 The commercially available caged dopamine compounds carboxynitroveratryl (CNV) and (N)-1-(2-nitrophenyl)ethyl (NPEC) (Scheme 4) tether the hydroxyl and amine substituents of dopamine to the photoactive protecting group. Like the caged serotonin compounds, the caged dopamines require activation at ultraviolet wavelengths. Again, the Ru(II)(bpy)2 platform can be used to shift the photo-activation to longer wavelengths. Etchenique and coworkers reported the synthesis and photochemical properties of a Ru(II) dopamine complex [Ru(bpy)2(PMe3)(Dopa)](PF6) (RuBiDopa).57 It is an orange solid soluble up to 5 mM in pH 7 water and stable in solution at 37  C and pH up to 8 (dopamine is oxidized by air at higher pH). The UV-VIS spectrum of RuBi-Dopa shows the typical MLCT absorption band for Ru(II)(bpy)2 complexes, excitation of which leads to release of dopamine as monitored by 1H NMR spectrometry and UV-Vis spectroscopy, with a quantum yield of 0.12 upon excitation with a 405 nm diode laser. The single isosbestic point observed in the UV-Vis spectrum and changes in the 1H NMR spectrum indicate that the only photoproducts are dopamine and Ru(bpy)2(PMe3)(H2O)2þ (Eq. 3). These workers demonstrated that using such uncaging from RuBi-Dopa in living brain slices that dendritic spines express functional dopamine receptors. RuðbpyÞ2 ðPMe3 ÞðDopaÞþ ! RuðbpyÞ2 ðPMe3 ÞðH2 OÞ2þ þ dopamine hv

(3)

Several complexes based on the Ru(II)(bpy)2 platform also demonstrate uncaging when subjected to TPE in the nearinfrared.38,57–60 For example, 800 nm excitation of aqueous RuBi-Dopa with a Ti-Sapphire pulsed laser operating at 80 MHz,

Scheme 4 Chemical structures of dopamine and two commercially available caged dopamine compounds Carboxynitroveratryl-caged dopamine and ((N)-1-(2-nitrophenyl)ethyl)-caged dopamine.

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with a pulse width of 100 fs and an average power of 460 mW, exhibited a weak emission at  630 nm, but only at the focal point of the laser.57 Emission at a wavelength shorter than the excitation is indicative of a nonlinear optical (NLO) process. Varying the laser power between 730 mW and 360 mW showed the photolysis rate to be dependent on the square of the average power as described in Section 8.06.1.2 for TPE. In this context, Etchenique and coworkers used TPE of RuBi-Dopa in the optical manipulation of the neuronal cell somata of mice.57 The cells were placed in a bath with a 300 mM RuBi-Dopa solution and pulsed with a Ti-Sapphire laser at 725 nm to image and determine the morphological characteristics of the spines. The laser wavelength was then switched over to 800 nm to uncage RuBi-Dopa proximal to the spine head and to monitor Ca2þ concentrations before and after uncaging. These experiments suggest that the dendritic spines are not affected by the presence of the caged compound in the bath prior to photo-uncaging. Furthermore, a 4 ms 800 nm pulse proximal to the spine head elicited the dopamine release from RuBi-Dopa and showed a rapid increase in the concentration of Ca2þ within the dendritic spine indicating the presence of dopamine receptors. Endogenous release of dopamine may also be elicited by photo-uncaging nitric oxide. Humphrey and coworkers studied the effects of the photosensitive NO donor Roussin’s Black Salt (Na[Fe4S3(NO)7], RBS, see Section 8.06.5), on basal and electrically stimulated dopamine efflux from the rat striatum.61 The accumulation of nitrite and nitrate, the products of NO autoxidation, was used to assess whether NO production or release has occurred.62,63 Irradiation of rat striatum slices using a cold fiber optic light source had no effect on nitrite accumulation. Furthermore, slices treated with RBS showed low rates of accumulation when kept in the dark, but upon irradiation, the rate increased dramatically. At the peak, nitrite accumulation was equivalent to  20 picomoles per cubic millimeter per min on irradiated slices of striatum treated with RBS. After returning the slices to the dark, nitrite accumulation subsided.61 Dopamine efflux was measured using voltammetry measurements and compared to the signal from introducing exogenous dopamine. Exposing slices to 30 mM RBS in the dark had no significant effect over 30 min. Irradiation of the brain slices results in a light intensity dependent increase in basal dopamine efflux. Digital subtraction of current traces taken before and after irradiation produced voltammograms comparable to those of electrically stimulated efflux and exogenous dopamine. This response was reproducible after a 30 min rest interval. Gamma amino butyric acid (GABA) is another important inhibitory neurotransmitter that is found in both vertebrates and invertebrates.64,65 Like the neurotransmitters discussed earlier, the most popular commercially available GABA photo-triggers have organic photo-protective groups that require UV wavelengths for activation. An alternative GABA phototrigger is Ru(bpy)2(PPh3)(GABA)2 þ (RuBiGABA, Scheme 5), which is synthesized from the reaction of aqueous [Ru(bpy)2(PPh3)Cl]Cl with GABA at 80  C.59 The optical spectrum of RuBiGABA (Fig. 6) displays a MLCT band centered at 425 nm. Over the course of photolysis at 450 nm, the MLCT band shifts to 430 nm corresponding to formation of the aquo complex, Ru(bpy)2(PPh3)(H2O)2þ, and from these spectral changes, the quantum yield was measured to be 0.20. To determine physiological activity, membrane ionic currents of frog oocytes expressing GABA receptors were measured in a solution containing 30 mM RuBiGABA. Repeated light pulses resulted in oocyte membrane current steps comparable to those observed upon direct exogenous application of increasing concentrations of free GABA. After photolysis, the oocytes were washed and the membrane response was returned to the initial baseline with no sign of cytotoxicity. Although RuBiGABA is water-soluble (10 mM), it interacts with lipid bilayers owing to the lipophilicity of the PPh3 ligand. Side effects include decreasing the cell membrane resistance. To address this problem, Etchenique and coworkers prepared the trimethylphosphine analog Ru(bpy)2(PMe3)(GABA)2 þ (RuBiGABA-2, Scheme 5).66 This modification increases hydrophilicity and decreases the cell membrane interactions and side effects. RuBiGABA-2 has high water solubility, and its solutions are stable for months in the dark at room temperature. The photolysis of RuBiGABA-2 monitored by changes in the 1H NMR and UV–Vis spectra gave a single metal containing photoproduct, Ru(bpy)2(PMe3)(H2O)2þ with a quantum yield for GABA release of 0.09.60 Physiological studies with isolated leech ganglion motor neurons showed responses to GABA uncaging from RuBiGABA-2 upon a 800 ms pulse of 405 nm laser light.66 Preliminary experiments with anesthetized mice found no deleterious effects to the mouse brain cortex from direct topical applications of RuBiGABA-2 solutions up to 1 mM. Overall, both RuBiGABA and RuBiGABA-2 provide robust, reliable options for GABA photo-uncaging using visible light. Glutamate is another mammalian neurotransmitter found in the CNS. Glutamate receptor activation via endogenously produced glutamate may contribute to the brain damage that occurs after epileptic episodes, cerebral ischemia, traumatic brain injuries and other neurodegenerative processes.67 Disease states such as epilepsy, amnesia, anxiety, hyperalgesia, and psychosis

Scheme 5 The structures of both ruthenium bis(bipyridine) caged GABA complexes. RuBiGABA with its triphenylphosphine ligand is shown on the left. RuBiGABA-2 with its trimethylphosphine ligand is shown on the right.

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Fig. 6 UV/Vis spectra of RuBiGABA in aqueous solution at pH 7, irradiation at 450 nm. The inset shows the progression of the photoreaction. Reprinted with permission from Ref. Etchenique et al. ChemBioChem (2007), 8, 2035–2038. Published by John Wiley and Sons.

may respond to drugs altering glutamatergic activity,67 so studies of the underlying mechanisms need reliable tools for targeted glutamate delivery. The commonly used organic photo-trigger platforms require UV excitation; however, glutamate can also be photo-uncaged using the RuII(bpy)2 platform.58,68 The optical spectrum of aqueous Ru(bpy)2(PMe3)(glutamate) þ (RuBi-Glutamate), which is prepared in a manner similar to other ruthenium caged systems, shows a characteristic MLCT band centered at 450 nm. Monitoring the spectral changes upon 450 nm photolysis showed the ruthenium photoproduct to be Ru(bpy)2(PMe3)(H2O)2þ, while the 1HNMR spectrum displayed the characteristic resonances of free glutamate. The quantum yield for glutamate photo-release was 0.13 under these conditions.58 The TPE of RuBi-Glutamate was studied in biological and electro-physiological experiments.68 Neocortical mouse brain slices were incubated in a 350 mM solution of RuBi-Glutamate, and the neurons monitored for morphological and electrophysiological changes using microscopy and whole cell patch recordings. After several hours of incubation, no negative effects were detected, and the neurons remained healthy. Two-photon photo-uncaging generates free glutamate which binds to receptors on the surface of neurons and opens ion channels. Thus, RuBi-Glutamate is a reliable tool for photo-uncaging glutamate responsive to visible light and to TPE with NIR light. This provides the opportunity to study underlying mechanisms behind the action of glutamate and possibly the conditions and diseases that amount from unwanted effects of the glutamatergic system.

8.06.4

Uncaging of chemotherapeutic drugs and photoactivated chemotherapy (PACT)

A number of photoactive transition metal complexes have been studied with the goal of uncaging a bioactive substance at the specific target of therapeutic interest. This technique is ideal when there are identified targets, such as a tumor or a localized infection, where one can draw upon the advantages of localization or timing provided by using a light source. Described in this section are studies involving this technique to activate the release of therapeutic drugs. A closely related topic is photo-activated chemotherapy (PACT) a term introduced by Sadler and coworkers.69 We will also describe dual-action metal complexes that under photoactivation uncage a therapeutic agent then serve as a dye for PDT. The uncaging of gasotransmitters is treated as a separate topic in Section 8.06.5.

8.06.4.1

Photoactivated chemotherapy (PACT)

While PACT itself is not a major theme of this chapter, like PDT, therapeutic applications of photolysis techniques face common issues including the attenuation of light transmission and targeting of specific sites. Although in some broader interpretations, PACT includes photo-uncaging, we will use a narrower definition here focusing on photoreactions leading directly to cellular toxicity rather than on the release of a bioactive substance. Among the PACT reactions interest are electron transfer, free radical generation and the internal transformation of a benign metal complex into a therapeutic complex such as a cisplatin analog. The interactions of various polypyridine complexes of Ru(II), Rh(III) and Re(I) complexes with nucleic acids was pioneered by Barton and coworkers.70 While this specific topic is outside the scope of this chapter, it is notable from a photochemical perspective that these complexes have been used as luminophores to report intercalation into DNA71 and that photoinduced redox reactions of

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the intercalated complexes lead to nicking and cleavage of nucleic acids.72,73 There are continuing efforts to exploit these properties in new theranostic anticancer agents.74–77 As noted above, hypoxia at a tumor’s interior is a major issue in the applications of PDT. In this regard Lo and coworkers78 have introduced several Ir(III) polypyridine and o-metalated phenylpyridine (ppy) complexes to which they have attached a pendant PEG derivative to enhance aqueous solubility and improve pharmacokinetics. However, the luminescent PEGylated Ir(III) complexes are considerably less cytotoxic than the PEG-free analogs. By inserting a photolabile nitroveratryl linker between the iridium(III) polypyridine complex and the PEG, these workers were able to use a photoreaction trigger to control the cytotoxic activity in cell cultures. Irradiation at 365 nm released the pendant PEG and markedly enhanced the cellular toxicity of the remaining Ir(III) core, which localizes in the mitochondria. These same workers have developed PEGylated derivatives of luminescent Re(I) and Ru(II) complexes primarily for use as diagnostic imaging agents.79,80 In order to address the need for oxygen-free phototoxicity in tumors, Huang et al.81 have demonstrated that the Ir(III) complex [Ir(ttpy)(pq)Cl]PF6 (ttpy ¼ 40 -(p-tolyl)-2,20 :60 ,200 -terpyridine; pq ¼ 3-phenyl-isoquinoline) is phototoxic both to hypoxic and to normoxic cells. Phototoxicity apparently occurs via one-electron oxidation of NADH, thereby disrupting the electron transport chain resulting in intracellular redox imbalance and apoptotic cell death. However, it is notable that this complex absorbs at fairly short wavelengths (350–480 nm). The Sadler team has long had an interest in developing platinum(IV) compounds that can be converted in situ by photoreduction into active anticancer drugs.82 One goal is to circumvent built-up resistance to anti-cancer drug cisplatin. Diazido Pt(IV) complexes that undergo photoreduction when irradiated with UVA and visible green light (Fig. 7) complexes show good dark stability. Additionally, variation of the ligands at the other four coordination sites provides the opportunity to tune the biological activity of the Pt(II) photoproduct. The resulting Pt(II) complexes display novel cytotoxicity mechanisms that may override the cisplatin resistance. However, targeted delivery of the Pt(IV) precursor and photoactivation with longer wavelengths remain challenges. Besides generating the potentially cytotoxic 4-coordinate Pt(II) complexes, the photochemistry displayed in Fig. 7 also releases the azido radical N3. Although this figure illustrates one fate of the N3 (reaction with another N3 to form N2), N3 is a reactive radical with an aqueous solution standard reduction potential of þ 1.33 V.83 Therefore, it is not surprising that this strong oneelectron oxidant may itself play a physiological role when generated photochemically. The azido radical’s potential toxicity is demonstrated for the visible light photolysis of the Pt(IV) complex trans,trans,trans-[Pt(py)2(N3)2(OH)2] (py ¼ pyridine) in the presence of L-tryptophan (Trp) or melatonin (MLT).84 The resulting indole radicals were characterized using an EPR spin trap. The same study implied that this Pt(IV) may also generate hydroxyl radicals (OH) suggesting that there are multiple photodecomposition pathways, as well as several mechanisms responsible for the phototoxicity of this platinum(IV) complex. The PACT approach may also be effective against certain antibiotic-resistant infections.

8.06.4.2

Photo-uncaging of cancer therapeutics

The precise spatial and temporal control afforded by photo-uncaging extends to the release of chemotherapeutic molecules. In the mode of cisplatin, many inorganic photo-therapeutics depend on the metallated fragment for cytotoxicity. The low selectivity of platinum cancer drugs can also result in difficult side effects for patients. Photo-uncaging of cancer therapeutics from otherwise inactive inorganic platforms, such as those described here, can address this issue.85 Thermally inert, but photochemically active d3 and d6 electronic configurations are common in inorganic photocages. Ligand photosubstitution activity can often be attributed to the presence of relatively low energy 3LF metal-centered excited states (Fig. 1).6 In this section, we will discuss the photo-uncaging of cancer therapeutic molecules by the release of ligands from the metal center. Several factors must be taken into account when designing or evaluating inorganic photocages including lipophilicity, cellular uptake and localization, the toxicity of the residual metal center and any resulting ROS generation. In clear contrast to the cisplatin mode of action, the Bonnet group demonstrated that the complex [Ru(bpy)2(dmbpy)]2 þ (dmbpy ¼ 6,60 -dimethyl-2,20 -bipyridine) photo-uncaged the bidentate dmbpy ligand (Fig. 8). The free ligand, rather than the ruthenium fragment, was shown to be cytotoxic to an A549 lung cancer cell line.86 The authors compared the toxicity of [Ru(bpy)2(dmbpy)]2 þ to that of a similar complex, [Ru(Ph2phen)2(mtmp)]2 þ (Ph2phen ¼ 4,7-diphenyl-1,10-phenanthroline,

Fig. 7 The proposed photoreaction of diazido platinum(IV) complexes leading to the formation of azido radicals plus a platinum(II) species. Adapted with permission from Ref. Shi et al. Inorg. Chem. Front. 2019, 6, 1623–1638. Published by The Royal Society of Chemistry.

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Fig. 8 Structures of select Ru(II) polypyridyl complexes studied by the Bonnet and Glazer groups: [Ru(bpy)2(dmbpy)]2 þ, [Ru(Ph2phen)2(mtmp)]2 þ, [Ru(phen)2(biq)]2 þ, and [Ru(phen)(biq)2]2 þ, respectively.

mtmp ¼ 2-methylthiomethylpyridine). The Ru photoproduct of the latter complex yielded the greater EC50 while demonstrating increased toxicity. In the above context, the Glazer group examined [Ru(phen)2(biq)]2 þ (biq ¼ 2,20 -biquinoline) and [Ru(phen)(biq)2]2 þ (Fig. 8), which are strained complexes that selectively dissociate a biq ligand when excited with visible light. These are noteworthy as the light-activated metal complexes that demonstrate cytotoxicity upon irradiation with red and NIR light (within the phototherapeutic window). However, it is not clear whether the released biq ligand is responsible for the observed cytotoxicity of these complexes.87 In a similar vein, Khnayzer and coworkers have performed detailed in vitro studies of many photo-dissociating Ru(II) complexes, including [Ru(bpy)2(BC)]Cl2 (BC ¼ bathocuproine),88 [Ru(bpy)2(dmphen)]Cl2 (dmphen ¼ 2,9-dimethyl1,10-phenanthroline),89 and [Ru(bpy)2(dpphen)]Cl2 (dpphen ¼ 2,9-diphenyl-1,10-phenanthroline).90 While the focus in these compounds is primarily on the DNA-intercalative properties of the Ru(II) aqua photoproduct and ligand dissociation, it is clear that lipophilicity of the ancillary ligands such as dpphen as well as the high cellular uptake of Ru(II) are important to their cytotoxicity on top of ligand photodissociation, especially for [Ru(bpy)2(dmphen)]Cl2.89 Overall, these represent useful design principles for Ru(II) photocages.

Fig. 9

Structures of several drugs used in photo-uncaging experiments. Left to right: bicalutamide, histamine, and econazole.

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Simple nitriles and pyridyls are useful analogs for anticancer or antimicrobial drugs that contain those moieties.91,92 Metal complexes coordinating these functional groups are especially useful photocages because conjugation of nitriles and heterocycles can be difficult with organic photocages.93 As discussed above, Ru(II) polypyridyls and diimine complexes, with their photophysical and chemical tunability, are capable of these transformations. In particular, manipulating the steric distortion around the Ru center, thereby stabilizing the higher-lying metal-centered orbitals, is a common way to lower the energies and to increase the population of the 3LF excited states to favor photodissociation. White et al.94 have discussed the photophysical properties of Ru(II) imine complexes toward the end of photosubstitution at length. Zhao et al. 95 studied a series of Ru(II) polypyridyl complexes of bicalutamide (Fig. 9), a nitrile-containing antagonist to androgen receptors, which are implicated in prostate cancer. In theoretical calculations of bicalutamide complexes of [Ru(bpy)2]2 þ, [Ru(phen)2]2 þ, and [Ru(biq)2]2 þ, they found that the 3LF excited states lay 0.28–0.45 eV below the 3MLCT states, making ligand dissociation overwhelmingly favorable for these complexes.95 However, this can have negative impacts on the dark-stability of the system, as ligand substitution may occur thermally without light excitation. Renfrew, Alessio and co-workers note that among a series of Ru(II) [9]ane ([9]ane ¼ 1,4,7-trithiacyclononane) diimine complexes, the extended p-conjugated system in [Ru([9]aneS3)(biq)(py)]Cl2 red-shifts its MLCT absorption compared to other [9]ane diimine complexes (such as bpy, phen, and dppz) without sacrificing photostability. The authors attributed this phenomenon to steric hindrance of pyridine photorelease from the deformed Ru center due to biq coordination twisting the equatorial axis by over 35 degrees.96 Imidazoles also present useful models for photo-uncaging bioactive molecules, such as econazole, histamine, or histidine. Carlos and co-workers report that hist (hist ¼ histamine, 2-(1H-imidazol-4-yl)ethanamine), is effectively photo-uncaged from [Ru(phen)2(hist)2]2 þ in aqueous solution.97 This complex is also emissive, presenting the interesting combination of imaging and photo-uncaging. Another example of imidazole photo-uncaging is reported by Karaoun and Renfrew with the luminescent complex [Ru(phen)2(Ec)2]Cl2 (Ec ¼ econazole, 1-(2,4-dichloro-beta-((p-chlorobenzyl)oxy)phenethyl)imidazole).98 Imidazoles are more electron-donating than pyridyls, which are likely contributing to these complexes’ luminescent properties. Extensive work has been done, especially in collaborations between the Podgorski, Turro, and Kodanko groups, regarding photorelease of cysteine protease inhibitors as chemotherapeutic treatment. Cysteine cathepsins are overexpressed in many cancers, which makes them attractive targets for anticancer drugs. One such target is cathepsin K, whose inhibitors can be easily uncaged from metal centers when coordinated through their nitrile moieties.99 In 2011, these workers introduced the caging, and subsequently photouncaging, of nitrile protease inhibitors coordinated to RuII centers.100 Upon photolysis by visible light (> 395 nm), the complex [RuII(bpy)2(Ac-Phe-HCH2CN)2](PF6)2 releases 2 equivalents of the nitrile-based cathepsin inhibitor Ac-Phe-HCH2CN

Fig. 10 Structures of select nitrile-containing cathepsin K and pyridyl-containing cathepsin L inhibitors. On the left, Ac-Phe-HCH2CN, Cbz-LeuNHCH2CN, and Cbz-Leu-Ser(OBn)-CN; on the right, epoxysuccinyl inhibitors (2S,3S)-3-((1-(dimethylamino)-1-oxo-3-phenylpropan-2-yl)carbamoyl) oxirane-2- carboxylate and (2S,3S)-N -(4-acetamidophenethyl)-N -((S)-1-(dimethylamino)-1-oxo-3-(pyridin-4- yl)propan-2-yl)oxirane-2,3dicarboxamide.

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(Na-Acetyl-N-(cyanomethyl)-L-phenylalaninamide, Fig. 10), which effectively inhibits the cysteine protease papain and to a lesser extent, cathepsins B, K, and L. This strategy demonstrated the viability of caging nitrile-containing anticancer agents to photoactive metal complexes. In further studies of inhibitor photorelease from a [Ru(bpy)2]2 þ core, these research groups synthesized, characterized, and demonstrated inhibition of cathepsin activity by [Ru(bpy)2L2] complexes of the cathepsin K inhibitors CbzLeu-NHCH2CN (N2-[(Benzyloxy)carbonyl]-N-(cyanomethyl)-L-leucinamide) and Cbz-Leu-Ser(OBn)-CN (N2-[(benzyloxy) carbonyl]-N-[(1R)-2-(benzyloxy-1-cyanoethyl)]-L-leucinamide, Fig. 10) in cell assays. The RuII-bound inhibitor showed little activity in the dark, which is necessary for effective medical application of these caged drugs. The authors of this 2014 study noted that these data “strongly support the translation of this technology to other enzyme targets and live cell systems.”101 Indeed, in a later study involving cathepsin K inhibitor Cbz-Leu-NHCH2CN, these workers followed the same model, except using the tetradentate TPA ligand (TPA ¼ tris(2-pyridylmethyl)amine) rather than two bipyridines.102 Comparison of these two studies shows that, while the TPA complex demonstrates greater overall cytotoxicity, the bis(bpy) complex undergoes photosubstitution of both caged inhibitors, rather than just one. The difference may be due to the greater steric bulk of TPA physically blocking the release of the second ligand even when the photodissociation is electronically favorable. In another study of Cbz-Leu-Ser(OBn)-CN photorelease from the [Ru(bpy)2]2 þ platform, Herroon et al. report that the use of this photocage in a 3D proteolysis assay is hindered by its low stability in cell growth media, and that nitrile release may occur after prolonged exposure to cell cultures. In this report, 3D proteolysis assays are presented as a viable way of “screening” photocages for biocompatibility.103 Pyridine photo-uncaging can model release of cathepsin L inhibitors. For example, Huisman et al. demonstrated that epoxysuccinyl inhibitors of cathepsin L featuring pyridyl motifs are easily chargeable by a Ru(II) center (Fig. 10).104 Habtemariam et al. also described the release of methyl p- and nicotinate from a ruthenium arene framework. While their focus was on the DNA intercalative properties of the ruthenium photoproduct, this system does exemplify pyridyl photorelease.105

Fig. 11 [Ru(tpy)(biq)(L)]2 þ photoreleases L, the NAMPT inhibitor and anticancer agent CHS-828 (N-[6-(4-chlorophenoxy)hexyl]-N0 -cyano-N00 -4pyridylguanidine) upon irradiation with visible light. This leads to increased cytotoxicity, ROS levels, and damage to mitochondria. The ruthenium(II) prodrug is stable in the dark and accumulates in tumor cells at high concentrations. Confocal fluorescence microscopy using the dye 20 , 70 dichlorofluorescein shows a marked increase in fluorescence upon irradiation owing to the formation of ROS upon CHS-828 release. Graphic used with permission from Ref. Wei et al. J. Inorg. Biochem. 2018, 179, 146–153.

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The enzyme nicotinamide phosphoribosyl transferase (NAMPT) also presents an attractive target for photo-uncaged cancer drugs. Like cysteine cathepsins, NAMPT is overexpressed in several cancers and is implicated in cancer cell metabolism. As a result, NAMPT inhibition can be an effective treatment of cancers.106 In 2017, Lameijer et al. reported photo-activated release of the cytotoxic organic compound STF-31, which binds to [Ru(tpy)(biq)]2 þ through its pyridine functionality. This strategy is particularly of note for targeting hypoxic tumors in which NAMPT is expressed.107 Shortly afterwards, Wei and Renfrew described uncaging the highly cytotoxic anticancer agent and NAMPT inhibitor CHS-828 (N-(6-(4-chlorophenoxy)hexyl)-N0 -cyano-N00 -4pyridylguanidine) from a [Ru(tpy)(bpy)]2 þ framework. Interestingly, the authors note that CHS-828 has undergone Phase I clinical trials as an antitumor agent and has known side effects affiliated with its high cytotoxicity and poor bioaccumulation in tumor tissue. However, its conjugation with ruthenium solves these issues and allows for reversible photoactivation (Fig. 11).108 Imatinib is another anticancer inhibitor, targeting tyrosine kinases, whose photoactivated delivery by the [Ru(tpy)(L)]2 þ (L ¼ Me2bpy, biq, Me2dppn) fragment addresses many of the imatinib side effects. Kodanko, White, Turro, and collaborators also covalently conjugated complexes to an antibody to CD117, another protein related to tyrosine kinases that is overexpressed in cancers. Since the antibody did not diminish the photo-uncaging properties, this is a promising strategy for selective targeting photo-therapeutics.109 The heme-containing cytochrome P450 enzymes (CYPs) are ubiquitous in living systems. CYPs are also overexpressed in many tumors and contribute heavily to drug resistance. As a result, untargeted application of CYP inhibitors can lead to unpleasant side effects, making their photo-release an attractive, alternative method. The Glazer group has reported photo-uncaging of the CYP inhibitors metyrapone, etomidate, and a related imidazole-containing compound (Fig. 12) from a [Ru(bpy)2]2 þ platform.110 In addition, Li et al. examined the photo-uncaging of abiraterone (Fig. 12), another pyridine-containing CYP inhibitor, from [Ru(tpy)(Me2bpy)]2 þ.111 The topic remains underexplored, but these studies demonstrate the suitability of the Ru(II) polypyridyl platform for CYP inhibitor photo-release. Not all photoreleased ligands are N-donor ligands. Curcumin (Fig. 12), the chromophore present in turmeric, is a diketone that functions as a bidentate ligand and itself is photocytotoxic.112 The Chakravarty group has developed numerous examples of prodrugs that photorelease curcumin from a diverse set of metal centers, including Cu(II),113 Co(III),114 Pt(II),115 La(III) and Gd(III).116

8.06.4.3

Dual-action complexes

In this section we will discuss “dual-action” complexes that are capable of photodynamic action and of photo-uncaging chemotherapeutic drugs. The rich photochemistry of Ru(II) complexes has led to the development of many systems capable of multimodal anti-tumor action.117 These may even extend to generating PACT effects in environments free of photo-sensitizable oxygen.118,119 In 2011, Garner et al. reported photorelease of 5-cyanouracil (5CNU), a nitrile-containing compound which exhibits cellular activity, from a [Ru(bpy)2]2 þ fragment which can bind to DNA. While this is not a PDT agent, this work was foundational to establishing 2 modes of action for a RuII-based drug.120 While not within the scope of this chapter, many other studies have explored “dual action” from the perspective of the metal-containing fragment being cytotoxic (similar to cisplatin).121–123 RuII polypyridyls have been studied extensively in the past decade as effective photosensitizers for PDT (see Section 8.06.2) and for related dual-action therapies.69,124 In these studies, ligand design and steric control around the metal center are shown as key to tuning the photochemical properties of the dual-action complexes to favor both photodissociation and ROS generation. One

Fig. 12 Structures of CYP inhibitors studied by the Zamora et al.110 (top row), Li et al.111 (bottom left) and Chakravarty and coworkers112–116 (bottom right).

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Fig. 13 The (tris)heteroleptic complex [Ru(bpy)(dppn)(CH3CN)2]2þ is capable of photo-sensitizing singlet oxygen as well as photo-releasing two acetonitrile ligands upon irradiation with visible light. Reprinted with permission from Ref. Albani et al. J. Am. Chem. Soc. 2014, 136, 17095–17101. Copyright 2014 American Chemical Society.

significant example is the dual-action complex [Ru(bpy)(dppn)(CH3CN)2]2þ (dppn ¼ benzo[i]dipyrido[3,2-a;20 ,30 -c]phenazine), studied by Dunbar, Turro and coworkers (Fig. 13).125 The extended conjugated p system of dppn gives rise to an energetically lowlying, intraligand pp* transition that competitively lowers the population of the dissociative 3LF state. Although the quantum yields for acetonitrile dissociation and ROS generation of [Ru(bpy)(dppn)(CH3CN)2]2þ are lower than those of [(Ru(bpy)2)(CH3CN)2]2þ and [Ru(bpy)2(dppn)]2 þ, respectively, its overall photocytotoxicity index is greater than either of these. This result demonstrates the potential efficacy of therapeutics that show more than one mode of cell-killing upon photoactivation. In 2015, Turro and coworkers presented [Ru(tpy)(Me2dppn)(py)]2 þ (Me2dppn ¼ 3,6-dimethylbenzo[i]dipyrido[3,2-a:20 ,30 -c] phenazine) and [Ru(pydppn)(biq)(py)]2 þ (pydppn ¼ 3-(pyrid-2-yl)benzo[i]dipyrido[3,2-a:20 ,30 -c]phenazine; biq ¼ 2,20 -biquinoline) as dual-action agents capable of photoreleasing a pyridine molecule as well as generating ROS.126,127 The Me2dppn ligand features 2 methyl groups close to the Ru center upon coordination, as well as an extended p-conjugated system (Fig. 14). As with the parent ligand dppn, the extended p system in Me2dppn gives rise to a long-lived 3pp* excited state, that can undergo energy transfer to O2 to form ROS. The 2 methyl groups on Me2dppn lead to longer RueN bonds, and therefore stabilize the related metal-centered antibonding orbitals and lower the energy of the related dissociative 3LF excited state. The competitive population of these excited states due to their relative closeness in energy leads to this dual-action capability upon photoexcitation.126 The pydppn ligand also features an extended p-conjugation and is the tridentate analog of dppn. Similar to Me2dppn (Fig. 14), the extended conjugation in pydppn gives rise to the 3pp* ES responsible for ROS generation. Within the compound [Ru(pydppn)(biq)(py)]2 þ, the biq ligand exerts strain around the ruthenium center so that the 3LF state is comparable in energy to the 3pp* state.127 These studies provide a “proof of concept” that the sterics and electronics of RuII polypyridyls can be tuned in many ways to modify the properties of a dual-action PDT platform. For example, the electron-donating capabilities of the ancillary (do not undergo photorelease) ligands on RuII polypyridyls can strongly influence the wavelengths of light absorbed by these complexes. In a study of a series of 12 Ru(tpy) complexes featuring diverse pyridyl (py), biimidazole (bim), and acetylacetonate (acac) ligands, Rack and Turro indicate that the complex series with the highly electron-donating acac ligands have the lowest-energy 3MLCT transition at 550 nm as well as the highest quantum yields of acetonitrile photorelease (F ¼ 0.014) (Fig. 15). Computational methods indicate that these properties correspond to a low Ru d-

Fig. 14

Structures of phen, dppz, dppn ligands (top row) and Me2dppn and pydppn (bottom row).

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Fig. 15 Experimental and theoretical experiments on three sets of Ruthenium(II) terpyridine complexes indicate that increasingly electron-donating ancillary ligands red-shift the complex’s absorbance and increase the quantum yield of ligand dissociation. Reprinted with permission from Ref. Loftus et al. J. Phys. Chem. C. 2019, 123, 10291–10299. Copyright 2019 American Chemical Society.

orbital character in its ground-state HOMO. These observations suggest that high Ru-L orbital mixing/overlap or high ligand character of the 3MLCT transition is instrumental in related photorelease.128 In a 2020 study, Turro, Kodanko, and coworkers characterized the photochemical and biological properties of RuII polypyridyls featuring aryl derivatives of the Me2dppn ligand, capable of dual-action PCT/PDT behavior.129 These authors note that the flanking aryl groups, introduced in the distal positions of Me2dppn, lie perpendicularly to the plane of the ligand due to allylic strain. This prevents overlap between the aryl substituent’s and the dppn-centered p orbitals, as demonstrated by the small differences between the electronic absorption spectra of the parent complex and those of the aryl-decorated complexes in the visible region. Quantum yields for ligand release and production of 1O2 upon irradiation with visible light are also similar to the parent complex, with a greater dissociation quantum yield and slightly lower quantum yield for production of singlet oxygen due to competitive population of the 3LF state, which leads to dissociation. Several cathepsin inhibitor photo-cages were discussed in the previous section.100,101 In this section is highlighted the effective use of the dppn ligand in these Ru(II) dual-action agents. Podgorski, Turro, and Kodanko describe an Ru(II) photosensitizer [Ru(tpy)(Me2dppn)]2 þ conjugated to a cathepsin B (CTSB) inhibitor, which is released upon irradiation around 470 nm. Protease-inhibitor interaction increases the cellular uptake of the photosensitizers, increasing the efficacy of PDT generation of ROS leading to cell death.130 In a similar fashion, photolysis of [Ru(dppn)(tpy)(Cbz-Leu-NHCH2CN)]2þ effectively releases a cathepsin K inhibitor and generates ROS to cytotoxic effect.131 In summary, the field of photo-uncaging of chemotherapeutics and simultaneous photodynamic action has advanced quickly over the past decade. One limitation is that the majority of these systems are activated by photolysis at visible wavelengths that are somewhat out of the phototherapeutic window. However, given the TPE of similar compounds described above in Section 8.06.3, it appears likely that many of these systems may be active toward TPE at NIR wavelengths. Looking ahead, Bonnet writes about the need to translate between the laboratory and the clinic with respect to many of these drugs; as well as how the behaviors of many of these phototherapeutics vary in vitro and in vivo.132

8.06.5

Small molecule bioregulators (gasotransmitters)

The remarkable discoveries over 30 years ago that nitric oxide is synthesized endogenously and plays key roles in mammalian physiology have led to an astonishing research output relevant to the chemical biology of NO.133 This research required methods to deliver exogenous NO to specific physiological targets. For this purpose, various compounds to release this small molecule bioregulator (SMB) thermally at different rates were developed both for laboratory study and for potential therapeutic applications.134,135 Photochemical uncaging of nitric oxide has also long drawn interest with the goal of delivering NO to therapeutic targets.136 As we will see in Section 8.06.5.1, much of this research effort has involved transition metal complexes as the photoactivated NO releasing moieties (photoNORMs). Carbon monoxide has long been known to be generated endogenously by the enzyme heme oxygenase.137 Subsequently, CO as well as hydrogen sulfide were identified as having bioregulatory properties,138 and these SMBs plus NO are sometimes designated collectively as “gasotransmitters”.139 There has been considerable interest in the potential therapeutic aspects of CO and H2S delivery to biomedical targets, and as a consequence, a number of CO releasing moieties (CORMs)140 and H2S releasing compounds141 have been developed for thermochemical release at physiological targets. Notably, most CORMs are transition metal carbonyls, one example being Ru(CO)3(gly)Cl (CORM-3, gly ¼ glycinato).142 Photo-activated CORMs (photoCORMs) have also drawn considerable research interest owing to the precise control of location, timing and dosage characteristic of photo-uncaging.

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Section 8.06.5.2 will describe photochemical methodologies for the targeted CO release, while Section 8.06.5.3 will describe recent efforts to employ photo-uncaging to deliver H2S and other potentially therapeutic SMBs.

8.06.5.1

Nitric oxide

For NO, the biomedical rationales for controlled delivery include NO’s potent cardiovascular and antimicrobial properties as well as its potential roles in cancer therapy.143 The use of NO to directly kill tumor cells has been a major focus of several laboratories, but one should proceed cautiously in its activation. High localized NO levels (> 800 nM) will induce nitrosative stress and cell apoptosis, but lower NO levels may have the undesirable outcome of stimulating angiogenesis and inducing tumor growth.144 A safer approach is to couple NO delivery to other treatments, such as chemo- or radio-therapy.145,146 NO is a radiation sensitizer, and this was confirmed in early studies with Roussin’s red salt Na2[Fe2S2(NO)4] (RRS) as a photoNORM to deliver NO to hypoxic V79 cell cultures.147 Much like PDT, radio-therapy is more effective under normoxic condition, so one effect of the added NO was to sensitize these cells to g-radiation damage. This effect might prove even more efficient in a living animal, since NO is a potent vasodilator at nanomolar concentrations.148 Therefore, NO release at a hypoxic tumor should increase the localized tissue oxygenation and further enhance the radiation killing of tumor tissue. It should be noted that the effect of NO photo-uncaging should be relatively localized. Although NO diffuses readily in biological media, it is readily consumed by various mechanisms,149 including autoxidation.150 As a result, its physiological lifetime is relatively short (seconds). Various transition metal nitrosyl and O-nitrito complexes have served as photoNORMs, several examples of which are illustrated in Fig. 16. Some of the earliest published photoNORMs were non-heme iron nitrosyl complexes such as the Roussin’s black salt anion [Fe4S3(NO)7] (RBS) and red salt anion [Fe2S2(NO)4]2 (RRS). RBS, which has received attention owing to its very potent bacteriostatic properties,151 was the subject of an early qualitative study where rat tail artery relaxation was used to demonstrate NO release upon RBS photolysis in buffer solutions.152 Complexes of iron, ruthenium, manganese and chromium have received the most attention, although the photoreactions of derivatized gold nanoshells indicate a potentially different approach.153 Quantifying NO release from a photoNORM is also a challenge, especially in vitro in cell cultures and in vivo. In the laboratory, NO release can be measured and quantified using NO specific electrodes,154 or by entraining the NO from a reaction solution and measuring with a Sievers Nitric Oxide Analyzer (NOA) or chemically using the Griess reaction, which actually measures nitrite, the product of NO autoxidation.155 What one sometimes finds in the literature is the monitoring of absorption spectra changes upon photoNORM irradiation with the often (but not always) valid assumption that these reflect the quantitative release of NO. In cell cultures, various markers are used to indicate NO release qualitatively; an example is the conversion of 4,5-diaminofluorescein (DAF-2) to a fluorescent triazole coupled to NO autoxidation (Eq. 4).156

H2N

N NH

NH2 N COO–

–

O

O DAF-2

8.06.5.1.1

O

+ NO O2

COO– –

O

O

(4)

O

DAF-2T

PhotoNORMs based on iron

A number of iron-nitrosyl complexes are photochemically active toward NO dissociation. Among these is sodium nitroprusside Na2[Fe(CN)5NO] (Fig. 16) (SNP), which has long been used as a vasodilator during hypertensive medical emergencies.157 SNP is also photoactive with a quantum yield for NO release (FNO) of 0.18 at an irradiation wavelength (lirr) of 436 nm.154 However, SNP spontaneously releases NO thermally upon contact with tissue containing reducing species,157b thereby limiting its

Fig. 16

PhotoNORM examples.

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Fig. 17

Structures of the iron nitrosyl cluster anions of Roussin’s black salt (RBS) and Roussin’s red salt (RRS).

applicability for photochemical uncaging of NO. It has also been long known that heme nitrosyl complexes undergo NO labilization when photolyzed in solution.158–160 Nonetheless, the non-heme iron sulfur nitrosyl clusters such as RBS and RRS were the first exploited for the photo-uncaging of NO in biological systems. Matthews et al.64 demonstrated in vitro that the smooth muscle of guinea-pig taenia coli infused with RBS underwent reversible relaxation when exposed to visible light (> 400 nm) and that this effect correlated with NO release, as measured by the Griess reaction. In 1997, Bourassa et al. reported the quantitative photochemistry of RRS and RBS (Fig. 17).147 The FNO values (0.07) for RRS in aerobic aqueous media were essentially independent of lirr over the wavelength range 313–546 nm. The initial photo reaction gave RBS as the principal iron containing product plus a yield of 0.5 NO per RRS consumed. The black salt anion is less photoreactive, but gives  6 NO’s plus ferric precipitates under these conditions with a low FNO value of  0.007. These workers also showed that the NO released by visible wavelength irradiation of RRS sensitized g-radiation killing of hypoxic V-79 (Chinese hamster fibroblast) cells. This proof-of-concept experiment showed that simultaneous delivery of NO and radiation to a tumor site is a promising methodology to enhance the effectiveness of cancer radiotherapy. Laser photolysis studies showed that NO release is reversible for both RBS and RRS in deoxygenated solutions, especially in the presence of added NO.161 However, photolysis in oxygenated media led to more permanent changes owing to the trapping of reactive intermediates by O2. The dynamics of these transformations for RRS as determined by laser flash photolysis are summarized in Scheme 6. Computational studies using density functional theory (DFT) and time-dependent DFT (TD-DFT) of Fe/S/NO clusters have attributed NO photolability to excited states displaying mixed d(metal) / p*(NO) charge transfer and d / d metal-centered character.136b Roussin’s red esters Fe2(m-RS)2(NO)4 (RSE) are prepared by derivatizing the bridging sulfides of the RRS (Fig. 18). The RS- group can be as simple as an alkyl sulfide or an amino acid such as cysteine162 or be as elaborate as a protoporphyrin IX (PPIX) center.163 As photoNORMs, these esters offer several advantages over the RRS anion including better thermal stability in aerobic solution and greater photolability toward NO release. More importantly, the RSE present an opportunity to tune the physical and chemical properties including solubility164,165 and the optical spectra using different RS- derivatives. Notably, laser flash photolysis studies have shown that there is a fast back reaction of the initial RSE photoproduct with NO in analogy to the reactions shown for RRS (Scheme 6). As a result, the highest FNO values are found in aerated media where the Fe-containing intermediate is trapped by oxygen.162 By attaching a red light absorbing antenna such as PPIX to the RRS core, the resulting PPIX-RSE displayed greatly enhanced absorbances at longer visible wavelengths due to the porphyrin Q bands.163 Thus, photolysis at those wavelengths led to greater rates of NO release according to Eq. (1), owing to the much higher Iabs at low photoNORM concentration. Furthermore, although fluorescence from the PPIX antenna is  85% quenched by conjugation to the Fe/S/NO cluster, the presence of PPIX-RSE could be monitored with this feature. Similarly, derivatizing the RRS core with fluorescein antennas to give the water-soluble ester Fluor-RSE (Fig. 19) also dramatically enhances the visible range absorbance while retaining some residual fluorescence.164 Again, energy

Scheme 6

Roussin’s red salt photochemistry observed in flash photolysis.

Photoactivated metal complexes for drug delivery

Fig. 18

275

Representative Roussin’s red salt esters.

transfer from this chromophore to the core is demonstrated by the photosensitized NO release and by  90% attenuation of the fluorescence relative to free fluorescein. In 2004 Wecksler et al.166 demonstrated, for the first time, the activation of a photoNORM by two photon excitation using the NIR light needed for tissue transmission. Photolysis of PPIX-RSE by 800 nm light led both to weak fluorescence at  630 nm from the PPIX chromophore and NO release detected using a nitric oxide specific electrode. A more quantitative investigation of the TPE photoreactions of Fluor-RSE (Fig. 19) measured the TPE cross-sections at 800 nm of this conjugate system in solution and showed that NO release was a function of the excitation intensity squared I02 as discussed in Section 8.06.1.2.167 These TPE studies of PPIXRSE and Fluor-RSE have been followed by several other examples167,168 based on the RSE platform including one demonstrating that such photoNORM conjugates can be used to deliver NO to in vitro cell cultures.169 Another approach to multiphoton NIR excitation for NO photo-uncaging is to use upconverting nanoparticles (UCNPs) as antennas that generate visible light upon excitation with a NIR laser. See Section 8.06.5.4 where the focus is on the use of nanocarriers. A different platform for iron-based photoNORMs was reported by Mascharak and co-workers.170 These were low spin ironnitrosyl complexes with carboxamide-containing pentadentate ligands such as [Fe(PaPy3)(NO)](ClO4)2 (PaPy3H ¼ N,N-bis(2pyridylmethyl)amine-N-ethyl-2-pyridine-2-carboxamide). These compounds released NO upon 500 nm excitation with a FNO of 0.185 in acetonitrile (Eq. 5). The ligand PaPy3 contains a carboxamide group with the s-donating anionic nitrogen atom positioned trans to the NO to enhance photolability. However, the stability of this complex in biological media is poor. Computational studies (DFT) interpreted the NO photolabilities in terms of the influence by the various carboxamido-N donor ligands on metal nitrosyl bond strengths.171 2+

NO N

N

Fe

N N

N O

[Fe(PaPy3)(NO)]2+

8.06.5.1.2

Manganese photoNORMs

hn (CH3CN)

CH3CN N N Fe N N N O

2+ + NO

(5)

Mascharak and coworkers have further used pentadentate carboxamido-N donors such as PaPy3 and analogous ligands to prepare an impressive series of electronically analogous manganese nitrosyl complexes such as [Mn(PaPy3)(NO)]ClO4.172 These complexes show strong absorptions in the visible spectrum with Mn(PaPy3)(NO), displaying a MLCT lmax at 635 nm in acetonitrile. Visible light photolysis irreversibly releases NO with high quantum yields in solution and in a sol-gel matrix.173 NO photolability is very likely the result of

Fig. 19

TPE of Fluor-RSE.

276

Photoactivated metal complexes for drug delivery

Fig. 20

Three manganese photoNORMs that absorb strongly in the red.

internal conversion/internal conversion from initially formed charge transfer excited states to reactive states having considerable LF character that are antibonding with respect to MneNO bonds.174 When the PaPy3H ligand was modified by replacing one pyridine with a quinoline to give PaPy2QH (N,N-bis(2-pyridylmethyl)amine-N-ethyl-2-quinoline-2-carbox-amide), the MLCT of the complex ion Mn(PaPy2Q)(NO)þ (Fig. 20) was shifted further to the red.175 Aqueous solutions of Mn(PaPy3)(NO)þ and Mn(PaPy2Q)(NO)þ gave FNO values of 0.39 and 0.69 at lirr 550 nm and, while the quantum yield decreased at longer wavelengths, the latter complex displayed photoactivity upon 810 nm excitation. Other manganese-based photoNORMs described by Mascharak and coworkers replaced the carboxamide group with an imine to give Mn(SBPy3)(NO)2þ (SBPy3 ¼ N,N-bis(2-pyridylmethyl)amine-N-ethyl-2-pyridine-2aldimine) and Mn(SBPy2Q)(NO) (SBPy2Q ¼ N,N-bis(2-pyridyl methyl)amine-N-ethyl-2-quinoline-2-aldimine) that absorb strongly at longer wavelengths, are photoactive toward NO release upon NIR irradiation but are unstable in aqueous media.176 Fig. 20 also illustrates the structure of a similar manganese nitrosyl complex prepared by Hitomi et al. from the pentacoordinate ligand H-dpaq (2-[N,N-bis(pyridine-2-ylmethyl)]-amino-N0 -quinoline-8-yl-acetamido) modified by adding the substituents (OMe, Cl, and NO2, shown is the nitro form Mn(dpaqNO2)(NO)þ).177 The nitro functionalized photoNORM demonstrated a FNO of 0.78 upon irradiation at 650 nm. Direct excitation of this complex ion with a 794 nm diode laser also demonstrated NO release with a FNO of 0.18 in acetonitrile solution.178 This photoNORM was incorporated into polymer microcarriers that combined with macrophages for the deep penetration into tumor spheroids (Section 8.06.5.4).178

8.06.5.1.3

Chromium photoNORMs

A different approach to NO uncaging has been exploited with Cr(III) platforms, namely the photofragmentation of the OeNO bond of an O-coordinated nitrite (Eq. 6) One extensively studied photoNORM is the dinitrito complex transCrIII(cyclam)(ONO)2þ (cyclam ¼ 1,4,8,11 tetraazacyclotetradecane),179 also known as “CrONO.” This strategy was inspired by the NO photo-extrusion reported for the coordinated nitrite of MnIII(TPP)(ONO) (TPP2 ¼ tetraphenylporphyrinato),180 although this pathway was later shown to be minor.180b Subsequently, Yamaji et al. demonstrated irreversible photo-induced release of NO from CrIII(TPP)(ONO),181 the greater efficiency of CrO-NO photo-cleavage being attributable to the more oxophilic character of Cr(III).

ONO N N CrIII N N ONO CrONO

+

O hQ kNO

+ N

N CrIV N N ONO

+ NO

(6)

Aqueous solutions of CrONO are stable (in the dark) at physiological temperatures and proved nontoxic in cell culture studies. NO labilization was shown by flash photolysis studies to be reversible (kNO ¼ 3.1  10 6 M 1 s 1).179 As a result, long-term 436 nm photolysis of a deaerated CrONO solution in a closed vessel gave only the minor photoaquation product transCrIII(cyclam)(H2O)(ONO)2þ with a quantum yield of 0.009. However, photolysis in aerated solutions led to net NO release at much higher quantum yields owing to the trapping of the reactive trans-CrIV(cyclam)(O)(ONO)þ intermediate (Eq. 6). Alternatively, when CrONO photolysis was carried out by entraining with helium to sweep the gaseous products the solution, two moles of NO per mole of CrONO were released.182 With no oxygen present to trap the CrIV intermediate, this species apparently undergoes secondary photolysis to release another NO and to generate a CrV species. However, attempts to isolate and quantitatively characterize the CrV product were unsuccessful.182 Sensitizer and quenching studies suggest that the excited state (ES) responsible for homolytic cleavage of the CrOeNO bond in CrONO is the doublet ligand field excited state 2LF that is typically the lowest energy ES of Cr(III) complexes. Density functional computation results were consistent with this conclusion.182 Reductive trapping of the CrIV intermediate occurred when glutathione (GSH), a common biological antioxidant, was added to the solution. Under those conditions, direct measurement of NO release with an NOA gave FNO ¼ 0.25. The NO photo-uncaged from CrONO in biological media was shown to trigger vasorelaxation in porcine arteries by activating the enzyme soluble guanylyl cyclase.183 Thus, its lack of toxicity as well as the relative stability of CrONO at 37  C in aqueous media points to CrONO as a promising photoNORM.

Photoactivated metal complexes for drug delivery

277

Fig. 21 Representations of Anth-CrONO (left) and of a supramolecular assembly of the CrONO cation with an anion terminated quantum dot (right). Adapted from Ref. 135d, 135d, Coord. Chem. Rev. 2018, 376, 548–564.

However, although CrONO displays a relatively high quantum yield under visible photolysis, the low extinction coefficients of the metal-centered, LF absorption bands characteristic of its optical spectrum are not ideal for therapeutic applications. To address these issues, several new systems were prepared involving more strongly absorbing antennas attached to the CrONO platform in order to enhance the rates of NO release under photolysis. Two of these are illustrated in Fig. 21. In one case, a CrONO analog Anth-CrONO was prepared by covalent attachment of an anthracene chromophore to the cyclam ring.184 While Anth-CrONO does not display desired longer visible absorptions, it demonstrated that a pendant chromophore can serve as an antenna to photosensitize cleavage of the CrIIIOeNO bond. Furthermore, while fluorescence from the anthracenyl chromophore of Anth-CrONO was largely attenuated owing to energy transfer to the Cr(III) center, a residual blue emission remained. This residual emission allowed tracking the Anth-CrONO when its salt was incorporated into L-a-phosphatidylcholine liposomes, a potential carrier for delivery to biological targets (Section 8.06.5.4).185 The second example in Fig. 21 shows an ionic bonded supramolecular assembly of the cationic complex CrONO with a CdSe quantum dot (QD) made water soluble by decorating the surface with dihydrolipoate anions. Visible excitation of the strongly absorbing QDs followed by energy transfer to CrONO results in strongly enhanced NO release.186,187

Fig. 22

Examples of ruthenium photoNORMs.

278

Photoactivated metal complexes for drug delivery

8.06.5.1.4

Ruthenium photoNORMs

Ruthenium nitrosyl complexes are typically very stable, and this feature and the photolability of many such complexes have made these systems extensively studied as photoNORMs.188–211 However, it should be noted that certain ruthenium nitrosyls will spontaneously release NO in biological media,212 so it is essential to define carefully the system in order to accomplish the desired task. Several representative ruthenium nitrosyl complexes are illustrated in Fig. 22. Both thermal and photochemical NO release from the simple ruthenium ammine nitrosyl complexes trans-Ru(NH3)4(L)(NO)n þ (Fig. 22a) were studied extensively by Franco, Tfouni, da Silva and coworkers.192,195,198,210 These complexes and other Ru nitrosyls provide an interesting conundrum with regard to assigning oxidation states. Since the RueNeO angle is typically about 180o, the such complexes are often written as RuII(NOþ).b Certain chemical properties, such as the propensity of the coordinated NO to react with nucleophiles support this qualitative representation. The RueNO bonding is highly delocalized in such complexes, so assigning an oxidation state is rather problematic. Indeed, the chemistry of other ligands coordinated to this {(RuNO)}6 moiety suggests a much higher net charge on the Ru center than implied by the RuII(NOþ) representation.201 Furthermore, photolysis of such complexes typically release the uncharged NO molecule leaving the metal containing residue in the þ 3 oxidation state. For example, irradiation of trans-Ru(NO)(Cl)(cyclam)2 þ with wavelengths in the near-UV led to the formation of trans-Ru(H2O)(Cl)(cyclam)2 þ, a complex for which there is much less ambiguity regarding the Ru(III) formal oxidation state of the metal center.198 An interesting combination of these thermal and photochemical properties is the report by Roveda et al.210 where they added the nucleophile SO32 to the coordinated nitrosyl of trans-Ru(NH3)4(isn)(NO)3þ to give trans-Ru(NH3)4(isn)(N(O) SO3)þ (isn ¼ isonicotinamide). The latter ion is quite stable in aqueous solution at 298 K but laser photolysis (lirr ¼ 355 or 410 nm) releases the N(O)SO3 anion (Eq. 7, F ¼ 0.12), which undergoes homolysis to NO and the sulfite radical SO3. trans  RuðNH3 Þ4ðisnÞðNðOÞSO3 Þþ ! trans  RuðNH3 Þ4ðisnÞðH2 OÞ2þ þ NðOÞSO3 – hv

(7)

Early studies by Miranda and co-workers probed the synthesis and photochemistry of various ruthenium porphyrin nitrosyl complexes Ru(Por)(X)(NO) (X ¼ Cl or ONO, Por2  ¼ e.g., TPP2, tetraphenylporphyrinato, or OEP2, octaethylporphyrinato).188 Upon 355 nm flash excitation, these complexes reversibly release NO to give a RuIII(Por)(X) intermediate (Eq. 8), the back reaction displaying rate constants kNO ¼ 3–5  108 M 1 s 1 in benzene. When X is ONO, flash photolysis also leads to photodissociation of nitrogen dioxide (NO2) to give the reactive {RueNO}7 species RuII(Por)(NO). The analogous phthalocyanine complex Ru(pc)(NO)(NO2), is photoactive toward NO release with low quantum yields at lirr 660 nm.200

O N+ RuII

hQ

RuIII

Cl

+ NO

(8)

Cl kNO

= TPP or OEP

Another ruthenium nitrosyl platform involves the salen complexes Ru(salen)(X)(NO) (salen ¼ N,N0 -ethylenebis(salicylideneiminato) dianion, X ¼ Cl, H2O, ONO) and the analogous salophen complexes Ru(salophen)(X)(NO) (salophen ¼ N,N0 -1,2phenylene-bis(salicylidene-iminato) dianion) (Fig. 22b). Works et al.189,190 showed that photolysis of these photoNORMs releases NO to give the corresponding solvento species RuIII(salen)(X)(Sol). Like the porphyrin analogs, this reaction is reversible (Eq. 9). The rate constant for the back reaction is markedly sensitive to solvent (Sol) with kNO values ranging from 10 2 to 108 M 1 s 1, the stronger electron-donor Sols like acetonitrile or tetrahydrofuran giving the slow rates. In such solvents, NO photolabilization is effectively irreversible and FNO values of 0.13 and 0.07 were measured for lirr at 365 and 546 nm, respectively. Subsequent studies demonstrated that aqueous solutions of the aquo complex [Ru(salen)(H2O)(NO)]NO3 is stable in the dark for a period of weeks even when aerated.201 It is photoactive toward NO release but the optical spectrum showed relatively little absorption at visible wavelengths. Crisalli et al.208 prepared a different water soluble salen complex with the salen aromatic rings modified by carboxylate functionalities; however, this complex displayed the same limitations with regard to absorption in the visible spectrum. O N O

Ru

Sol O

hQ

N C

C N Cl

O

Ru

kNO

O N C

C N

+ NO

(9)

Cl

b In the Enemark-Feltham nomenclature this is considered a {MeNO}6 complex where the 6 is the sum of the d-electrons of the metal plus the electron in the NO p* orbital (Ref.213).

Photoactivated metal complexes for drug delivery

279

Mascharak and coworkers have synthesized Ru(NO) complexes of the PaPy3  and PaPy2Q anions described in Sections 8.06.5.1.1 and 8.06.5.1.2.193 These ligands position a s-donating negatively charged base trans to the RueNO bond, thus stabilizing this moiety. However, Ru(PaPy3)(NO)2þ and Ru(PaPy2Q)(NO)2þ both required irradiation at relatively short wavelengths to labilize NO (FNO ¼ 0.12 at l irr 355 nm; FNO ¼ 0.17 at lirr 410 nm). These researchers extended their studies of {RuNO}6 complexes by preparing ligands with multiple carboxamide groups. These modifications shifted the absorption bands to longer wavelengths and gave higher quantum yields for NO release under physiological conditions.203 An example is the tetradentate anionic ligand bpb2  containing two carboxamide groups (H2bpb ¼ 1,2-bis(pyridine-2-carboxamido)benzene) from which they prepared the ruthenium complexes Ru(bpb)(NO)(X) (X ¼ Cl, py, Im, OH or Resf; Resf ¼ resorufin). Fig. 22c is Ru(bpb)(NO)(Resf). Replacing the pyridyls of the bpb2  ligand with quinoline or isoquinoline extended the absorption spectra further to the red. Notably, the nitrosyl complexes tethered to resorufin are luminescent and hence can be used as trackable donors in cellular studies.202 Among other ruthenium nitrosyl complexes that have received recent attention are terpyridine complexes211 (terpy is 2,20 :60 ,200 terpyridine), an example being the Ru(terpy)(bpy)(NO)3þ cation (Fig. 22d). The spectroscopic properties of these can be tuned by the addition of various substituents on the bipyridine or terpyridine. These gave modest quantum yields for NO release when irradiated at 365 or 436 nm. A more exciting result is that complexes with carbazole or fluorene substituents on the terpyridine ligand, namely [RuII(CzT)(bpy)(NO)](PF6)3 (CzT ¼ 40 -(N-ethylcarbazol-3-yl)-2,20 :60 ,200 -terpyridine) and [RuII(FT)(bpy)(NO)](PF6)3 (FT ¼ 40 -(9,9-dihexyl-9H-fluoren-2-yl)-2,20 :60 ,200 -terpyridine) demonstrated two photon absorption (TPA) properties with moderate TPA cross-sections when excited at 800 nm.211a

8.06.5.2

PhotoCORMs

As noted above, there is considerable interest in developing CO releasing moieties (CORMs) as potential therapeutics for disease states including vascular- and immuno-related dysfunctions,140 antibiotic resistant microbial infections214 and cancer.215 Carbon monoxide is an inhibitor of cytochrome C oxidase in the mitochondria; thus, direct delivery of CO can cause cell death through interruption of cellular respiration. Various triggers for CO release from CORMs have been discussed; for example, reactions of metal carbonyls with H2O2 and other reactive oxidative species (ROS) may serve this purpose.216 Another such trigger is photoexcitation, and this was demonstrated qualitatively in a biological setting by Motterlini and coworkers with the simple carbonyls Mn2(CO)10 and Fe(CO)5, although these have poor aqueous solubility.217 The mechanistic photochemistry of metal carbonyls was studied long before the interest in photoCORMs,218 but most such investigations were in anaerobic, anhydrous media, conditions that are incompatible with therapeutic applications. Schatzschneider and coworkers219 were perhaps the first to design a complex, fac-[Mn(CO)3(tpm)] PF6 (tpm ¼ tris(pyrazolyl)methane), specifically for this purpose. When dimethyl sulfoxide (DMSO) solutions of this salt were delivered to HT29 colon cancer cells, significant photoinduced toxicity was achieved upon 365 nm excitation. The term photoCORM to describe photoactivated CO-releasing moieties was coined subsequently.220 The general design goals that apply to photoCORMs are similar to those one applies to photoNORMs and other caged bioactive substance, whether or not they are transition metal based: High stability in the dark and in aerobic aqueous media. Low toxicity and/or knowledge of the biological effects of photo products aside from CO. Photoactivation at a therapeutically suitable wavelength. Biocompatibility and specificity of delivery. This section will address photochemical principles that may guide photoCORM design, a topic which has been the subject of several reviews,221,222 as well as methods to quantify CO release. In addition, we will describe potential strategies to improve bioavailability and some therapeutic applications with emphasis on anticancer and antimicrobial goals. Although there have been several non-metal systems that can serve as photoCORMs,223 the bulk of such studies have focused on metal carbonyl complexes. Generally, stable carbonyl complexes involve relatively low valent, electron-rich metals owing to strong metal-to-CO p-backbonding. For this reason, metal centers, such as W(0), Mn(I), Re(I), Fe(II) and Ru(II), with d6 low spin configurations have been platforms for photoCORMs owing to their relative inertness to thermal substitutions. Indeed, an analogy can be drawn between the {MNO}6 electron configurations described for several photoNORMS above and d6 metal carbonyls. However, the nature of the excited states (ES) leading to labilization are different. With photoNORMs, NO labilization is often attributed to excitation leading to an ES with metal to NO charge transfer character that weakens MeNO bonding.136b With metal-CO complexes, analogous charge transfer bands are much higher in energy, so lower energy charge MLCT ES involving ligands other than CO or metal-centered LF ES are more likely to be responsible in most photolabilization events. Both of these ES types would be expected to have weaker MeCO bonds relative to the ground state (GS) of a low spin d6 complex. In a MLCT ES the metal center is formally oxidized, and the p-symmetry d-orbitals that participate in p-backbonding are depleted. For a LF ES, in addition to depletion of p-bonding d-orbitals, electron density has been transferred to d-orbitals that have s-antibonding character with respect to metal ligand bonds. Both ES should be more labile that the GS,224 but, based on these qualitative arguments, one would expect the latter would be much more susceptible to CO labilization. The spin-allowed MLCT transitions have the much higher extinction coefficients, so for a complex where both ES types are present, initial excitation likely occurs through a MLCT transition. However, the eventual photoreactivity may depend upon which lower energy ES is populated by internal conversion/intersystem crossing.

280

Photoactivated metal complexes for drug delivery

There are exceptions given that some initially formed excited states are dissociative in character, in which cases the reaction trajectory leading to products is competitive with the non-radiative processes leading to lower ES. In terms of design, a desirable feature of a photoCORM is strong absorption at the longer visible or NIR wavelengths of phototherapeutic window, since the photo-uncaging rate is a function of the intensity of light absorbed (Iabs, Eq. 1). This is one of the desirable features of the bis(bpy) Ru(II) complexes described in Sections 8.06.3 and 8.06.4, namely, that strong, spin allowed MLCT absorptions dominate the visible spectrum. Furthermore, modifying the diimine ligands with electron-donating or -withdrawing substituents allows one to tune the MLCT transitions to higher or lower energies. This property allows one to shift the spectrum so that the compound of interest absorbs light at longer wavelengths. However, it should be emphasized that the ES responsible for the luminescent or chemical response to initial MLCT excitation are not the ones obvious in the absorption spectrum. Instead, as illustrated in Fig. 1, rapid internal conversion and ISC to lower energy ES generally define the photophysical/ photochemical properties, and the natures and relative energies of these ES must be considered in the design of a photoCORM. With these general photochemical properties in mind, the design of many photoCORMs has centered around complexes of the type M(CO)n(L)(X). Typically, the number (n) of carbonyls is between 2 and 4, L is either a mono-, bi- or tri-dentate ligand, in most cases an aromatic nitrogen heterocycle, and X is an auxiliary ligand providing coordinative saturation among other functions. Much of the research has focused on strategies to redshift absorbances that initiate CO labilization to therapeutically suitable longer visible and NIR wavelengths, to increase biocompatibility and to improve targeting. An important question to consider in photo-uncaging research and/or therapy is how does one detect and quantify CO photouncaging. Since many of the metal carbonyl complexes have characteristic spectra, one quantitative method of studying a photoCORMs reactivity in solution is to record the optical spectral changes and assume that the changes observed are due to CO release. This technique is valid if the reactants and products have been well characterized; however, there may be intermediates or sequential photoreactions that could affect how quantitative such measurements may be. Given the goal of photoCORM applications is to deliver CO, then the most important measurement is to detect and quantify CO release. One common method is the myoglobin (Mb) assay.225 This takes advantage of the high affinity of CO for Mb and of the shift of the strong Q-band absorptions upon coordination of CO to the protein heme group (Fig. 23). The absorbance at 540 nm is monitored, and given the known extinction coefficient of Mb and Mb-CO at this wavelength, one is able to directly quantify the amount of CO released. However, while the myoglobin assay has been extensively used for the qualitative determination of CO release, it suffers from several disadvantages, a major one being that it only functions in an anaerobic environment. Another, more quantitative approach is to carry out the reaction in a closed vessel such as illustrated in Fig. 24.226 During a photolysis experiment and at the end, the gas phase can be sampled using a gas-tight syringe and analyzed using a programmable gas chromatograph with a thermal conductivity detector (GC-TCD).220,226 By calibrating the GC-TCD, this technique is very accurate. One needs to correct for amount of CO that remains in the liquid phase, but since the partition coefficient strongly favors the gas phase, this correction is typically small. Another analytical technique is to record the infrared spectrum of the gas phase from such a cell,220,227 but the IR method, while quantitative, is less convenient. Detection of CO release in living cells offers a different challenge. To address this need, several researchers have developed “turnon” sensors that become photoluminescent upon reacting with CO. The several strategies taken have been reviewed by Mukhopadhyay et al.228 A widely used method involves the CO reduction of an aromatic nitro group on the sensor molecule to give the corresponding amine with concomitant changes in the reactivity and spectroscopy. Another involves the CO reaction with a cyclic palladium complex to trigger the release of a fluorophore from coordination where luminescence was largely quenched by the Pd center. For example, Michel et al.229 prepared a dinuclear palladium complex with boron dipyrromethane difluoride (BODIPY) fluorescent dye as a ligand (COP-1). Reaction with CO in aqueous media releases the BODIPY fluorophore resulting in a strong

Fig. 23 The myoglobin assay showing chanz showing changes in the absorbance bands upon coordination of CO. Reprinted with permission from Ref. Atkin et al. Dalton Trans. 2011, 40, 5755. Published by The Royal Society of Chemistry.

Photoactivated metal complexes for drug delivery

281

Fig. 24 Schlenk cell for anaerobic photolysis experiments. The sample is located in the quartz fluorimeter cell. Gas samples are withdrawn through the syringe port at the top. Photo by Zhi Li. Figure taken with permission from Ref. Li, Ph.D. Dissertation. University of California: Santa Barbara, (2017).

fluorescence enhancement (Scheme 7). The reported detection limit is  1 mM, and this sensor was specific to CO, showing little sensitivity to H2O2, NO or H2S. COP-1 serves as a non-toxic, highly specific sensor in vitro when incubated with cells.

Scheme 7 Reaction of COP-1 with CO to give turn-on luminescent detection. Adapted with permission from Ref. Ohata et al. Acc. Chem. Res. 2019, 52, 2841–2848, Copyright 2019 American Chemical Society.

Scheme 8 Photoreaction of W(CO)5(TPPTS)3 showing that initial photos-uncaging of one CO is followed by slow oxidation of the W(0) center to an unidentified product and the release of additional COs.

282

Photoactivated metal complexes for drug delivery

8.06.5.2.1

Group 6 photoCORMs: Cr, Mo and W

Rimmer et al.220 were among the first to specifically design an air and aqueous stable metal carbonyl complex with the goal of photochemical delivery of CO. They studied the water-soluble complex ions M(CO)5(TPPTS)3 (M ¼ Cr and W, TPPTS ¼ tris(sulfonatophenyl)-phosphine trianion), first prepared by Darensbourg and Bischoff.230 These complex anions are stable in aerated buffer solutions even at elevated temperatures, but undergo photosubstitution when irradiated in the near-UV. For W(CO)5(TPPTS)3, the reaction quantum yields of CO release FCO were 0.90 for lirr 313 nm and 0.6 for lirr 405 nm in pH 7.4 buffer solution. Photolysis of either complex ion in a deaerated solution led to release of only one CO equivalent. However, in aerated solutions, analogous photolyses led to subsequent spectroscopic changes indicating that, unlike the initial M(CO)5(TPPTS)3 ions, the photoproducts are air sensitive. For the W(0) system, quantitative analytical studies showed that 1.2–1.6 additional equivalents of CO were released concomitant with oxidation of the initially formed W(CO)4(H2O)(TPPTS)3 (Scheme 8). An important lesson from this study is that the initial photoreaction may lead to products that react further under physiological conditions. Pryce and coworkers231 reported the photochemical, thermal and electrochemical release of CO from a related system, the amino carbene complex Cr(CO)5(C(NC4H8)CH3). Ultrafast flash photolysis studies with time resolved infrared (TRIR) detection combined with DFT calculations concluded that CO photolabilization upon excitation at 400 nm occurred from an initially formed singlet metal to carbene MLCT ES within 3 ps and with a quantum yield of 0.65. Similar results were reported for the analogous tungsten complex [(CO)5WC(NC4H8)CH3].232 In an effort to promote stronger absorptions at longer wavelengths than seen with W(CO)5(TPPTS)3, Rimmer and coworkers221 studied the photochemistry of the group 6 complexes M(CO)4(LL) (M ¼ Cr, Mo, W) where (LL ¼ BPSA2  or DPPQ) (Fig. 25). These complexes display strong visible range MLCT absorption bands in aqueous solution. Complexes in the M(CO)4(BPSA)2  series were marginally stable in aerated aqueous solutions but deaerated solutions were thermally stable in the dark. Photolysis of aqueous Mo(CO)4(BPSA)2  at 355, 366, 436 or 532 nm led to spectral changes and chemical behavior consistent with the uncaging of CO, but this system was not studied extensively owing to instability toward oxygen. The instability of such group 6 M(CO)4(diimine) complexes was exploited by Pfeiffer et al.233 in the design of Mo(CO)4(bpyCH3, CHO ) complex with an aldehyde group in the peripheral position on the bpy ligand. This was coupled to a peptide through the aldehyde function. As noted for the related BPSA complex, this conjugate underwent slow CO loss in aq. solution and thus could be considered a CORM for thermal CO delivery. Photoactivation with 468 nm light using an LED source led to a marked acceleration of such release. The M(CO)4(DPPQ) series first reported by Angelici and coworkers234 was found to be much more robust in aerobic media. The Cr, Mo & W complexes displayed strong MLCT bands in methanolic solutions at lmax 480 nm, 452 nm and 441 nm, respectively, and shift as much as 80 nm to longer wavelengths in nonpolar solvent.221 The hydrophobic DPPQ ligand makes these complexes water-insoluble, although they are soluble in alcohols as well as dimethylsulfoxide, which is commonly used as a drug delivery agent. Adding hydrophilic substituents should provide better aqueous solubility. The strong visible absorbances allow for longer wavelength excitation and all three are photoactive toward CO release. For example, photolysis of Cr(CO)4(DPPQ) in aerobic methanol at 355, 366, 436 or 532 nm resulted in decrease of the MLCT bands in the UV-vis region and the net release of all four COs. The first step is the photo-dissociation of one CO (Fapp ¼ 0.10 at lirr ¼ 436 nm), followed by oxidation in aerated solution to release the full complement of COs. However, this system was not investigated extensively owing to preliminary cell culture experiments suggesting that Cr(CO)4(DPPQ) may be too toxic to use as a photoCORM.221 Another early study of a Group 6 photoCORM by Zhang et al.235described Mo(II) alkynyl complexes (h5-C5H5)Mo(CO)3(CCCH2OR), one of which has a fructopyranose sugar as R to enhance water solubility. This complex undergoes slow release of CO in aqueous solution; CO release was greatly accelerated when the solutions were irradiated at UV wavelengths.

Fig. 25 Left: representative structure of M(CO)4(DPPQ) (M ¼ Cr, Mo or W). DPPQ is diphenylphosphinoquinoline. Right: Structure of BPSA2  (4,7bis(p-sulfonatophenyl)-1,10-phenanthroline also called bathophenanthroline 40 ,400 -disulfonate).

Photoactivated metal complexes for drug delivery

Fig. 26

283

Examples of photoCORMs using the fac-(CO)3Mn or fac-(CO)3Re platforms.

8.06.5.2.2

Group 7 photoCORMs

Beginning with the fac-[Mn(CO)3(tpm)]PF6 complex (Fig. 26a) described by Schatzschneider and coworkers,219 many photoCORMs featuring the fac-(CO)3Mn(I) or fac-(CO)3Re(I) platforms have been reported. Several examples are illustrated in Fig. 26; and Table 1 lists related representative photoCORMs that have been used with in vitro studies of different cancer cell lines. In most cased, cytotoxicity was the primary result observed.219,236–243 Mascharak, Schatzschneider, Westerhausen, Wilson, Zobi and others with their coworkers have drawn upon this platform235–258 and utilized the remaining three coordination sites to tune spectroscopy and bioavailability. Coordination to various heterocyclic aromatic ligands such as the 2,20 -azopyridines and the diimines bpy and phen introduce visible range MLCT bands in the optical spectra with much higher absorbances than for metal centered LF bands. A correlation of Mn and Re photoCORMs by Kottelat et al. shows how the MLCT bands of Re complexes typically occur at higher energy than those of analogous Mn complexes when both are Table 1

Group 7 photoCORMs based on fac-M(CO)3 platform and in vitro studies cancer cell lines studied.

PhotoCORM a

lirr

Cell line

Ref.

[Mn(tpm)(CO)3]PF6 [Mn(CO)3(phen)(PTA)](CF3SO3)

350 nm Visible light

219 236

[Re(CO)3(pbt) (PPh3)](CF3SO3)

305 nm

[Re(CO)3(phen)(pyAl)](CF3SO3) [Re(H2O)(CO)3(pbt)](CF3SO3) [Mn(CO)3(pbt)(PTA)](CF3SO3) [Mn(CO)3(phen)(PTA)](CF3SO3) [Mn(CO)3(S[(CH2)2NH2]2)]Br [Mn(CO)3(cycloS[(CH2)2NH(CH2-)])]Br [Mn(CO)3(L)]Br with the tridentate ligands L ¼ S [(CH2)2NH2]2 (CORM-EDE3) (1) L ¼ cycloS[(CH2)2NH(CH2-)]2 (CORM-EDE4)(2) (OC)3Mn{(PzMe2)22CH–CH2OH} (CORM-OMN1) (3) Re(CO)3(NN)(PR3) (NN ¼ 2,9-Me2phen; bpy; 4,40 (Me)2bpy, or 4,40 -(MeO)2bpy).(PR3 ¼ PTA, THP, or DAPTA)

Low power excitation 365 nm Low-power visible light

HT-29 colorectal cancer cells OVCAR-5 & SKOV-3 ovarian cancer. Cisplatin resistant and susceptible strains MDA-MB-231 Breast cancer cells HT-29 MDA-MB-231 MDA-MB-231

365 and 405 nm

LX-2 Human hepatic cell line

241

White light

HepaRG, LX-2

242

365 nm

HeLa cells wildtype A2780 and ovarian cancer cisplatin resistant strain CA28180CP70

243

237 238 239 240

Ligands: pbt ¼ 2-(2-pyridyl)-benzothiazole; PTA00 ¼ 1,3,5-Triaza-7-phosphaadamantane; pyAl ¼ pyridine-4-carboxaldehye; THP ¼ tris(hydroxymethyl)phosphine; DAPTA ¼ 1,4diacetyl-1,3,7-triaza-5-phosphabicylco[3.3.1]nonane; PzMe2 ¼ bis(3,5-dimethyl-1-pyrazolyl)methane.

a

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in analogous coordination environments.245b Since the LF ES are typically much more labile toward CO uncaging, there is a balance between tuning the MLCT energies by adding substituents to the bis(diimine) ligands and the resulting photolability of the complex. Given that the d-d splitting for Re(I) is generally larger than for Mn(I), this is another feature that must be taken into account in designing such photoCORMS. Notably, some Re(I) complexes, such as that illustrated in Fig. 26b, are luminescent, a feature that provides imaging of the cellular location of these photoCORMs during in vitro experiments.239,243,249 For several luminescent Re(I) carbonyls, Wilson and coworkers have observed quenching by O2, suggesting that these might have bimodal action as photoCORMs and PDT sensitizers.243 For the Mn(I) analogs, there are numerous examples of CO labilization at visible wavelengths,245 but pushing lirr into the NIR has been an ongoing challenge. Electron-withdrawing substituents on the aromatic heterocycle ligand shift the MLCT bands of these Mn(I) and Re(I) photoCORM complexes to lower energy, but often at the cost of much lower quantum efficiencies for CO photo-release. Another approach is to design complexes that have the appropriate functionalities to undergo multi-photo excitation, in analogy to studies described above for certain photoNORMs and PDT agents. This has been accomplished by Jiang et al.255 using TPE with the complex illustrated in Fig. 26c, for which the aromatic heterocycle ligand is the bidentate coordinated terpyridine derivative 40 -p-N,N-bis(2hydroxyethyl)-aminobenzyl-2,20 :60 ,200 -terpyridine (TPYOH). Solutions of Mn(CO)3(TPYOH)X (X ¼ Br or CF3SO3) complexes are very photoactive toward release of multiple COs under visible light excitation (405 nm, 451 nm). Similar response was also triggered by multi-photon excitation at 750 nm and 800 nm using the intense pulses from an ultrafast laser (Eq. 10). Ramu et al.256 have described similar single- (visible) and two-photon (NIR) uncaging of CO from Mn(I) tricarbonyl complexes of ligands bearing 1,8-naphthalimide units.

R

R

N X N Mnl OC OC CO

hn (vis) or

N 2 hn (NIR)

+ CO

N X N OC

Mnl

(10)

N

CO

Another approach to multiphoton NIR excitation for the photo-uncaging of CO is to use upconverting nanoparticles (UCNPs) that generate visible light upon excitation with a NIR laser.1 This approach is discussed further for the photoCORM transMn(bpy)(PPh3)2(CO)2 in Section 8.06.5.4, where the focus is on nanocarriers. Li et al.257 used a different strategy to accomplish CO photo-uncaging from stable metal carbonyls with longer visible and NIR activation of CO. They prepared the dinuclear Re-Mn complexes (CO)5ReMn(CO)3(LL) (LL ¼ phen, bpy, biquinoline (biq) or phenanthroline-carboxaldehyde (phen-CHO)), which introduce a different electronic transition, namely, metal-metal bond to ligand (sMM / pL*) charge transfer (MMLCT) absorption bands. These strong, broad MMLCT bands occur at longer wavelengths and are tunable by modifying the bidentate heterocycle LL. For LL ¼ biq or phen-CHO, these bands extend into the NIR (Fig. 27).

Fig. 27 Absorption spectra of (CO)5ReMn(CO)3(phen) (blue, lmax ¼ 550 nm), (CO)5ReMn(CO)3(bpy) (black, 550 nm), (CO)5ReMn(CO)3(biq) (red, 719 nm) and (CO)5ReMn(CO)3(phen-CHO) (orange, 652 nm) in acetonitrile showing the strong MMLCT absorption bands. From Ref. Li et al. Inorg. Chem. 2017, 56, 6094 6104. Copyright 2017 American Chemical Society.

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285

Photolysis at deep red (659 nm) or NIR (794 nm for LL ¼ biq or phen-CHO) lirr leads to homolytic ReeMn bond cleavage to give metal radicals (Eq. 11). These radicals are trapped by dioxygen to form intermediates that are labile toward CO release via secondary reactions. Exhaustive photolysis released two equivalents of CO, while no CO release was detected in deoxygenated solutions. More work is needed to make such complexes compatible with biological media.

(11)

The choice of ligands for transition metal based photoCORMs is not simply to define the spectroscopic and photolytic behaviors of the resulting complexes. Another property conveyed by ligands is solubility. For example, ligands such as tris(hydroxymethyl) phosphine (P(CH2OH)3) and 1,3,5-triaza-7-phosphaadamantane (PTA) increase aqueous solubility. Other ligands like dansylimidazole are luminophores that allow imaging photoCORM location in cells.247a Ligand design is also crucial in defining bioavailability and for targeting specific tissues for therapeutics. With photoCORMS, this has been accomplished by conjugating with biomolecules like antibodies. Since antigens expressed on the surface of cells and bacteria are very specific, conjugating a photoCORM to the appropriate antibody may allow specificity for certain bacteria or cancer cells. For example, Kawahara et al.248 demonstrated the successful linkage of a photoCORM to a mouse antibody specific to an antigen overexpressed in ovarian cancer cells. This was done by coordinating the fac-Mn(CO)3(phen) chromophore through a pyridine derivative to biotin and linking the resulting complex (Fig. 26d) to an antibody via the strong streptavidin-biotin interaction. These antibodies, specific to overexpressed ovarian cancer cell receptors, were then shown to bind specifically where CO release from the delivered photoCORM was able to kill the cancer cells. Fujita et al.251 demonstrated coordination of a Mn photoCORM to ferritin, the iron storage protein that can be taken up via a receptor mediated endocytic pathway, through a cysteine residue. It was further demonstrated that this conjugate was efficiently taken up into HEK293/kB-Fluc cells and the release of CO upon exposure to light was demonstrated to activate NF-kB which are inducible transcription factors that play important roles in DNA transcription, cytokine production, cell survival and other cellular events (Fig. 28). Fig. 29 shows the fac-Mn(CO)3 tricarbonyl photoCORM coordinated to a modified N,N- 2,2-bis(pyrazolyl)ethylamine (bpea) ligand modified with the peptide -LPLGNSH-OH as developed by Pai et al.252b This complex is stable in aqueous solution and demonstrates CO photo-uncaging. The assembly and stability of such metal complex-polypeptide conjugates opens the possibility of using a targeting peptide153,259 to direct photoCORMs selectively to a subset of cell types with high precision and efficient uptake. Several groups have demonstrated that cells preferentially uptake photoCORMs with more lipophilic ligands242,247b owing to the lipid character of cell membranes. These studies reinforce the necessity of choosing ligands with specific physical properties to facilitate entry into cells. In this context, Mede et al.254 prepared complexes of the bidentate ligands 4-(acetoxymethoxycarbonyl)phenyl-bis(3,5-dimethylpyrazolyl)methane and 4-(acetoxymethoxy)phenyl-bis(3,5-dimethylpyrazolyl)methane with fac-Mn(CO)3Br. Once these complexes penetrate into a cell, the acetoxy groups are cleaved by esterases. This step converts the photoCORM from membrane-permeable to membrane-impermeable, trapping it inside the cells and thereby improving intracellular accumulation (Fig. 30). However, while this approach may be important to improve uptake from the medium, it lacks the biospecificity presented by targeting proteins or antibodies. White and coworkers244 have reported two very interesting dinuclear complexes [(bpy)2Ru(BL)Mn(CO)3Br](PF6)2, where BL is the bridging ligand 2,3-bis(2-pyridyl)pyrazine (dpp) or 2,20 -bipyrimidine (bpm). These hybrid systems serve both as photoCORMs and as PDT sensitizers. The tris(diimine)Ru(II) unit is a visible range antenna with broad, strongly absorbing MLCT bands (lmax

Fig. 28 Illustration of the photoCORM-ferritin conjugate undergoing CO uncaging in HEK293 cells and targeting the protein NF-kB. Figure reprinted with permission from Ref. Fujita et al. Angew. Chem. Int. Ed. 2016, 55, 1056. Published by John Wiley and Sons.

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Fig. 29 A Mn photoCORM conjugated to the peptide LPLGNSH-OH. Reprinted with permission from Ref. Pai et al. Eur. J. Inorg. Chem. 2014, 2886– 2895. Published by John Wiley and Sons.

510 nm) extending further into the red. Excitation of these complexes in aerobic solution leads to the release of all 3 CO’s with good quantum yields (Eq. 12). The residual (bpy)2Ru(BL)2 þ unit is capable of sensitizing singlet oxygen formation, thereby providing the potential for delivering both CO and 1O2 at the same physiological site.

2+ N N

Br N

N

Ru N

N N

CO

hn

CO

O2

Mn N CO

(bpy)2Ru(bmp)2+ + Mn2+ + 3 CO

(12)

Fig. 30 Schematic of the intracellular conversion of a lipophilic photoCORM to the hydrophilic form thereby making it unsuitable for back diffusion through the cellular membrane. Figure reprinted with permission from Ref. Mede et al. Chem. Eur. J. 2018, 24, 3321. Published by John Wiley and Sons.

Fig. 31

Some iron photoCORMs.

Photoactivated metal complexes for drug delivery 8.06.5.2.3

287

Group 8 photoCORMs Fe and Ru

Several iron and ruthenium carbonyl complexes have been developed as compounds that release CO spontaneously under physiological conditions and can be designated as CORMs. While CO release is often accelerated from such molecules by light, we will focus on those Fe and Ru compounds that are generally stable in the dark so that CO uncaging is specifically a photochemical event. Relative to the extensive studies of Mn(I) carbonyls, the activity regarding photoCORMs based on iron is considerably less. As noted above, Motterlini and coworkers qualitatively studied CO uncaging photoactivation from Fe(CO)5, but its aqueous insolubility and volatility are serious downsides.217 Ferroporphyrin carbonyl complexes undergo reversible photo-induced CO release,260 but there has been little interest in exploiting these as photoCORMs. In early studies, Westerhausen and co-workers261 described the Fe(II) complex Fe(cysteamine)2(CO)2 (Fig. 31a), which is soluble in water and photoactive toward CO by visible light (> 400 nm). Further studies with the same photoCORM262 showed that the Fe2þ is released simultaneously and has distinct physiological properties. In another study, the non-heme [(N4Py)FeII(CO)] complex (Fig. 31b) was prepared and studied by Kodanko and coworkers263 who reported that the photo-induced CO release demonstrated toxicity of prostate cancer cells. That the N4Py polypyridyl ligand can be modified by attaching a peptide offers the opportunity to enhance targeting for therapeutic applications. There has also been significant attention paid to dinuclear iron(I) carbonyl complexes with bridging thiolate ligands,264–267 some of which were first studied owing to an interest in the reaction mechanisms of iron-iron hydrogenase.264 For example, photolysis of the hydrogenase model m-pdt-[Fe(CO)3]2 (Fig. 31c) (pdt ¼ 1,3-propane dithiolate) in non-aqueous media leads to CO release with a quantum yield of 0.15 when irradiated at 365 nm.222b Photoreactions of similar dinuclear iron carbonyls [Fe(CO)2(m-SR)]2 (Fig. 31d; R ¼ an alkyl or aryl group) have been reported in the context of possible roles as photoCORMs.265,266 An example is the salt Na2[Fe(CO)2(m-SCH2CH2CO2)]2 which is water-soluble and non-toxic to normal epithelial cells. However, like m-pdt-[Fe(CO)3]2, CO photo-uncaging from these complexes requires irradiation at near-UV and short visible wavelengths. Not surprisingly, the diimine ruthenium(II) platform described for other photo-uncaging applications has also drawn attention for CO release.268–273 PhotoCORMs described in the literature include both mono- and bis-diimine complexes of the general formulas RuII(LL)(CO)2  2, RuII(LLL)(CO)2 , and RuII(LL)2(CO)Y (LL ¼ a bidentate aromatic heterocycle; LLL ¼ tridentate aromatic heterocycle.; X ¼ halide, Y ¼ X or some other ligand). Several examples are shown in Fig. 32. One feature of most complexes of this type is that coordination of the strongly p-accepting CO directly to the metal center shifts the MLCT band(s) characteristic of RuII(diimine) complexes significantly to the blue. As a consequence, the spectra of the majority of such complexes show strong absorptions in the near UV that do not extend very far into the visible. An exception is the compound reported by Geri et al.272 and depicted in Fig. 32a, for which a BODIPY (boron-dipyrromethene) dye is pendant on the coordinated bpy. The BODIPY chromophore provides a very strong visible absorbance (lmax 509 nm, 4 1 3max ¼ 3.68  10 M cm 1 in aqueous medium) and is also photoluminescent, allowing one to image the uptake and location of the complex in biological structures. Given that BODIPY dyes have been shown to be two-photon absorbing chromophores274 this or related systems may show promise under TPE. Interestingly, it appears that the CO photo-uncaging studies in this case were carried out in the near-UV (lirr 350 or 390 nm), so it is unclear whether energy transfer from the BODIPY chromophore to the metal center along the lines illustrated in Scheme 1 is effective in this case. Both CO’s were labilized, but the quantum yield for release of the second CO was about an order of magnitude less than for the first. In vitro studies with the cell line A431 (human squamous carcinoma) using this photoCORM and lirr 350 nm showed some cytotoxicity from the UV light itself, but only modestly enhanced cytotoxicity attributed to the CO uncaging. In an earlier study, Bischof et al.268 described a series of Ru(II) complexes of the type Ru(LL)(CO)2Cl2 (LL ¼ 4,40 -Me2bpy, 40 CH3-bpy-4-carboxyaldehyde, 40 -CH3-bpy-4-carboxylic acid, 2-(pyridin-2-yl)pyrimidine-4-carboxylic acid, or dipyrido[3,2-a:20 ,30 c]phenazine-11-carboxylic acid), as well as [Ru(LLL)(CO)2Cl] þ (LLL ¼ 6-(2,20 :60 ,200 -terpyridine-40 -yloxy)-hexanoic acid); the latter

Fig. 32

Several ruthenium photoCORMs. Structure (a) was adapted from Ref. Geri et al. Chem. Eur. J. 2020, 26, 10992–110060.

288

Photoactivated metal complexes for drug delivery

is depicted in Fig. 32b. The functional groups on the bidentate and tridentate ligands were then used to couple the complex to a peptide nucleic acid monomer backbone for targeting purposes. These complexes are stable in solution but photo-uncage CO when irradiated in the near-UV (365 nm), the MLCT absorption bands all being shifted to the blue by coordination to CO as discussed above. Even at this lirr, the quantum yields for CO release as measured using the myoglobin assay were quite small (< 10 3). Several Ru(II) carbonyl complexes not involving diimine ligands have also been examined as potential photoCORMs.275–277 Lorett Valasquez et al.275 have prepared complexes of the type M(SCH2CH2NH2)2(CO)2 and M(SC6H4-2-NH2)2(CO)2 where M ¼ Fe(II) or Ru(II). The iron complexes proved to be photosensitive to CO release but CO was not uncaged from the ruthenium analogs with visible light. In another example, Yang et al.277 prepared several dinuclear Ru(I) compounds such as that shown in Fig. 32, which they refer to as “sawhorse” complexes. These proved to be non-cytotoxic toward cell cultures, but again required near-UV light (365 nm) to trigger CO release.

8.06.5.3

Photorelease of H2S

Hydrogen sulfide (H2S) is another gasotransmitter with therapeutic potential. In a similar fashion to CO and NO, the photorelease of H2S is of interest because of its purported roles as a biological signaling molecule.138,139 H2S is implicated in the nervous system, neurotransmission, and reducing oxidative stress and apoptosis. Many organic photocages for H2S have been developed and described in other works.141b There are unique difficulties to photo-uncaging H2S from metals, relative to N- or even O- donors because of their tendency to form unreactive metal sulfides. In 2018, Woods et al.278 described a complex capable of photoreleasing GYY4137, an agent that forms H2S in aqueous solution (GYY4137 ¼ morpholin-4-ium 4-methoxyphenyl(morpholino)phosphinodithiolate). To our knowledge, this is the first and only example of H2S release from an inorganic system. GYY4137 is photoreleased from a familiar RuII polypyridyl platform, specifically [Ru(tpy)(biq)]2 þ; the authors cite its distorted octahedral center as lowering the energy of its 3LF state to allow for its easy thermal population from the 3MLCT. This photocage is activated by red light. While the literature is relatively scant, these are promising findings for future studies on inorganic H2S photorelease. The Pluth group has also demonstrated that photorelease of COS, which quickly hydrolyzes to form H2S in biological systems, is an interesting frontier for therapeutic H2S delivery.279,280

8.06.5.4

Nanocarriers and other delivery mechanisms

When designing metal complexes for the photo-uncaging of bioactive small molecules, one has to consider the methodology for delivering these photochemical precursors to the desired physiological site. Mentioned already are targeting polypeptides and antibodies that, when attached to the complex, provide cellular specificity. Other examples include building conjugates with triphenylphosphonium or folic acid groups to address receptors overexpressed by certain cells.281 These modifications entail engineering into a single complex the compound to be uncaged, an antenna to absorb the desired long wavelength visible or NIR wavelength, and the targeting molecule for certain cellular receptors, while ensuring that this conjugate in not toxic and is soluble in biological media. Targeting can also be accomplished by direct injection of photocages into the desired site or by implanting a device formed from a solid material incorporating the caged SMB. Indeed, if such an implant were attached to an optical fiber,282 there would be less need to make the specific photoNORM or photoCORM responsive to tissue-transmitting wavelengths. With this methodology, hydrophilicity of the caged compound would not be important, and might be undesirable, but it does require that the bioactive compound once uncaged diffuses from the polymer implant to the desired target. This should not be a serious problem for NO or CO, but might not be effective with larger molecules. A less invasive approach than injection or implantation would be to design nanocarriers that could be introduced to sites of therapeutic interest via the circulatory system. Such nanoparticles (NPs) are sufficiently large that multiple functions can easily be incorporated. With quantum dots (QDs),283 upconverting nanoparticles (UCNPs)1 and metallic NPs153,284 (for examples), the nanoparticle can also be an antenna that greatly enhances light absorption over the desired wavelengths. The applications of such NPs are separate topics in their own right. We will illustrate each of these with examples involving the uncaging of NO or of CO, but such uncaging applications has much broader implications to other bioactive compounds. Alternatively, NPs that are

Fig. 33 Silicon dioxide nanoparticles with tripyrazolyl methane modified through an azide alkyne linkage reaction. Reprinted with permission from Ref. Dördelmann et al. Inorg. Chem. 2011, 50, 10, 4362–4367, Copyright 2011 American Chemical Society.

Photoactivated metal complexes for drug delivery

289

benign with regard to photochemical behavior, such as those composed of silica, can transport caged complexes to targeted sites. The numerous surface sites on these nanoparticles can be decorated with multiple caged compounds and targeting molecules to provide tissue specificity; while the response to excitation remains the property of the specific photoNORM, photoCORM or other complexes on the NP surface. Several recent studies285–287 have demonstrated the use of silica-based NPs for photoNORM and photoCORM delivery agents. Schatzschneider et al.252a and Mascharak et al.239 have synthesized silicon dioxide NPs surface decorated with photoCORMs based on the fac-Mn(CO)3 or fac-Re(CO)3 platforms. An example is illustrated in Fig. 33. In both cases, near UV excitation led to CO release. Together, these studies show that silicon dioxide nanocarriers display low cytotoxicity in biological systems and are potential delivery methods for photoCORMs to tissues. In the latter case, mesoporous silica nanoparticles (100 nm diameter) were packed with a photoCORM fac-[Re(CO)3(pbt)(PPh3)](CF3SO3) that is strongly luminescent. In vitro experiments showed that these constructs were endocytosed into human breast cancer cells (MDA-MB-231) where they were visualized by their luminescence. UV excitation led to the death of theses MDA-MB-231 cells in a mechanism attributed to CO-induced toxicity. Liposomes were also examined as possible vehicles for delivering lipophilic photoNORMs to specific targets owing to the enhanced permeation and retention (EPR) effect which accumulates particles in tumors.288 For example, Ostrowski and coworkers185 encapsulated the BF4 salts of the anthracene tethered complex Anth-CrONO (Fig. 21) in phosphatidylcholine liposomes (Fig. 34). The Anth-CrONO fluorescence allows one to track these vessels simultaneous with therapeutic NO release, thus liposomes loaded with this luminescent photoNORM have theranostic potential. Nakanishi et al.289 have described another example of a photoNORM loaded liposome, in this case, a lipophilic Ru salen nitrosyl complex with pendant cholesterol groups. Semiconductor QDs display enormous absorption cross sections for both single and two photon absorption. Furthermore, one can tune the QD spectral properties by varying the size,290 and QDs show strong photoluminescence (PL), thus providing imaging opportunities in biological media. Neuman, Ostrowski and others showed that the photoNORM CrONO quenches the PL from water soluble CdSe:ZnS and CdSeS:ZnS core:shell QDs concomitant with enhanced NO release.186 A following study showed that Förster resonance energy transfer (FRET) from the QD was responsible for the PL quenching.187 Similar photo-sensitization of NO release from the photoNORM cis-[Ru-(NO)(4-ampy)(bpy)2]3 þ (4-ampy ¼ 4-aminopyridine) by water soluble CdTe QDs was proposed by Franco et al.205 to occur via a charge transfer, rather than a FRET, mechanism. An analogous photo-induced surface electron-transfer has been argued to be responsible for the photosensitized uncaging of CS2 from the 1,1-dithiooxalate anion by CdSe QDs.291 A problem with the CdSe and CdTe QDs is the toxicity of cadmium, thus providing an incentive to develop conjugates with less toxic nanomaterials.292,293 As a consequence, there is growing interest in nanomaterial graphitic photosensitizers.294,295 For example, Lui et al. described several luminescent nanoplatforms consisting of N-doped graphene QDs (GQDs, diameters 7– 10 nm) functionalized with a ruthenium photoNORM (Fig. 35).281,295 These NPs were also decorated with a triphenylphosphonium moiety to direct this platform to mitochondria. Photolysis at 808 nm leads to in vitro generation of NO in HeLa cancer cells leading to decreased viability by triggering apoptosis. Another platform for photo-induced drug delivery involves metallic nanoparticles. The affinity of metals such as gold or platinum for thiols facilitates the assembly of different functionalities in a single NP. For example, Sortino and coworkers296 reported NO release upon 400 nm excitation of systems constructed from platinum NPs on which they assembled a nitroaniline based organic photoNORM surface attached through a thiol. In vitro experiments with cervical cancer (HeLa) cells demonstrated the biocompatibility of these nanoparticles in the dark and the capability to induce cellular death upon visible light irradiation. In another example, Levy et al.153 utilized the near-infrared plasmonic absorption properties of hollow gold nanoshells (HGNs,  65 nm diameters) to photo-uncage NO. The ensemble was built from a HGN core, a thiol functionalized organic NO precursor (ammonium N-nitroso(4-mercaptomethylphenyl)-hydroxylamine, TCF), and a thiolated polyethylene glycol (PEG) with terminated with a targeting peptide TPEGRP (Fig. 36). The HGN core displayed a broad, very strong absorption band centered at  750 nm while the peptide mediates cellular endocytosis only for cells that overexpress the Neuropilin-1 receptor.259 Thus, these conjugates were readily internalized by PPC-1 or 22Rv1 prostate cancer cells, but not by HeLa cells that lack this receptor. NIR photolysis at 800 nm of nanocarriers internalized in 22Rv1 clearly demonstrated intracellular NO-release.

Fig. 34 Illustration of a liposome encapsulating the luminescent salt [Anth-CrONO]BF4. Photolysis leads both to NO release and detectable emission. Reprinted with permission from Ref. Ostrowski et al. Mol. Pharmaceutics 2012, 9, 2950–2955. Copyright 2012 American Chemical Society.

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Fig. 35 Cartoon illustrating the covalent attachment of both a ruthenium photoNORM and a triphenylphosphonium moiety on a N-doped graphene QD. Adapted with permission from Ref. Guo et al. Chem. Commun. 2017, 53, 3253–3256. Published by The Royal Society of Chemistry.

Lanthanide-doped UCNPs have changed the landscape of biological imaging and photosensitization owing to their unique optical properties and low toxicities.297,298 These can display NIR-to-visible (or UV) up-conversion by sequential multi-photon excitation, which is much more efficient than simultaneous TPE allowing one to use NIR diode lasers for such excitation. The upconversion efficiency remains a function of In with n > 1, so spatial resolution is a potential feature. Most importantly, the ability to effect uncaging with NIR light from a simple CW diode laser is a potential game changer in photo-activated drug delivery. This was demonstrated by Garcia et al.299 who prepared UCNPs with a NaYF4:Yb/Er core ( 10 nm) coated with NaYF4 and mesoporous silica layer deposited on the outside. Roussin’s black salt was then infused into the mesoporous silica and the pores sealed by coating the device with an amphiphilic polymer to give a water-soluble nanocarrier. Upon irradiation at 980 nm with a CW diode laser, this UCNP displayed bright visible luminescence that overlapped the RBS absorption spectrum (Fig. 37). The result was a water-soluble nanocarrier that released biologically relevant amounts of NO upon excitation in the NIR (Eq. 13). hv

Fe4 S3 ðNOÞ7  / 3:9Fe2þ þ 5:9 NO þ 3S2 þ other products O2

(13)

For such devices, energy transfer is the result from UCNP emission being reabsorbed by the photoNORM (the “trivial” mechanism), requiring that the precursor has strong absorption bands that overlap with the UCNP emissions. In this context, Burks et al.300,301 prepared small biocompatible polymers disks in which they suspended the UCNP and infused the photoNORM (RBS). Excitation with a diode laser operating at 980 nm through a tissue filter led to NO release. Independent studies by Ostrowski and coworkers302 with related silicone polymer disks containing UCNPs and the hydrophobic CrONO salt trans-[Cr(cyclam)(ONO)2]BPh4 gave similar results. However, the latter researchers showed that NO escape from the device depended on how the photoNORM was incorporated into the disk Huang et al.301 extended these studies by examining the rates of NO release from disks prepared with a variety of silicon polymers. These disks incorporated Nd3þ sensitized UCNPs that can be activated at 800 nm, a wavelength that avoids the absorbance from water found at 980 nm, were shown to be effective. Similarly, Pierri et al.303 designed nanocarriers consisting of UCNP cores terminated with hydrophobic oleate surface ligands made water-soluble by coating with an amphiphilic phospholipid-functionalized PEG (Fig. 38). The hydrophobic photoCORM trans-Mn(CO)2(PPh3)2(bpy)þ, which has an absorption spectrum well-matched to the emission from the NaGdF4(Yb20Tm0.2) @NaGdF4 core:shell UCNPs used, was then infused into the lipid-like interior of these NCs to give a stable, water-soluble ensemble. Excitation (980 nm) of the resulting NC led to CO release. A similar approach to activating a photoCORM by NIR-to-visible upconversion has since been reported by X. Liu and coworkers.304

Fig. 36 HGN surfaces are coated by co-absorption of TCF and TPEG or TPEGRP including a cell-targeting peptide. Laser excitation (800 nm) of these conjugates results in NO release with spatial-temporal control. Adapted with permission from Ref. Levy et al. Chem. Commun. 2015, 51, 17692– 17695. Published by The Royal Society of Chemistry.

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291

Fig. 37 The overlap of the upconversion emission spectrum water soluble NaYF4:Yb/Er@SiO2@mpSiO2 UCNP with the absorption spectrum of RBS. Insert: Illustration of a nanocarrier with a UCNP core and a mesoporous silica shell loaded with the photoNORM RBS and capped with an amphiphilic polymer. NIR photolysis results in NO uncaging. Adapted with permission from Ref. Garcia et al. Small 2012, 8, 3800–3805, published by John Wiley and Sons.

Another biological approach to targeting is to recruit immune cells to serve as “Trojan Horses” to carry desired payloads to the targeted tissue (Fig. 39).305 Unlike other circulatory cells, macrophages and monocytes can cross the blood brain barrier306 and can accumulate in tumors and other sites of inflammation.307 Tumor hypoxia generates inflammatory signals that recruit monocytes and macrophages from the blood via chemotaxis. Once the precursors are deposited in the targeted tissue, the relevant bioactive compound can be photo-uncaged by NIR excitation. As a proof of principle, Evans, Huang et al.178 demonstrated the ability of murine monocytes to carry micron-sized biodegradable polymer carriers loaded with the photoNORM [Mn(NO)dpaqNO2]BPh4 and an UCNP antenna to target sites. These microcarriers were essentially miniaturized versions of the biocompatible polymer disks described above, and confocal microscopy showed that macrophages ingest such micron-sized particles. Ingestion efficiency improved when IgG antibodies were incorporated in the microcarrier preparation. The microcarrier loaded macrophages were then shown to invade tumor spheroids, where 794 nm irradiation led to quantifiable NO release and UCNP emission indicated microparticle location. With low NIR light doses, the result was a reduction in hypoxia inducible factor 1 alpha (HIF-1a) levels in the tumor spheroids. In contrast, high doses promoted cell death, presumably by triggering apoptosis. Thus, macrophage Trojan Horses carried microparticles with a NIR-activated theranostic payload into a tumor model and were able to control biological response by varying the amount of irradiation. In principle, this study178 provides a model for the transport to and uncaging of NO and other therapeutic drugs at physiological targets.

Fig. 38 Water-soluble photoCORM nano-carriers with a UCNP core. Adapted with permission from Ref. Pierri et al. Chem. Commun. 2015, 51, 2072–2074, published by The Royal Society of Chemistry.

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Fig. 39 Trojan horse mechanism using macrophages to transport PLGA microcarriers (PLGA ¼ poly(lactic-co-glycolic acid)) loaded with UCNPs and a photoNORM to tumors or other sites of inflammation. Adapted with permission from Ref. Evans et al. Chem. Sci. 2018, 9, 3729–3741, published by The Royal Society of Chemistry.

8.06.6

Summary

This chapter has described the applications of transition metal complex photochemistry for the uncaging of a number of different species for potential therapeutic applications. These include neurotransmitters, anti-cancer chemotherapeutics, and the small molecule bioregulators NO and CO, among others. In some cases, it is noted that the complexes of interest may have dual functionalities combining photo-uncaging with PDT properties. Although the applications may differ, the fundamental principles remain the same. (a) The caged therapeutic should be biocompatible and relatively benign before activation with light. (b) In general, the caged compound needs to be responsive to the photolysis wavelengths that can be delivered to the desired target? For deep tissue such as a tumor, these wavelengths will be in the deep red or near infrared which are the most penetrating wavelengths. (c) Can the photochemical precursor be delivered to the target site with some selectivity? This could be accomplished by injection or implantation, but biological targeting mechanisms have a great deal of appeal. There has been a great deal of exciting research in this field with increasing attention from a growing number of individuals and research institutions, so the future of this biomedical application would appear to be very bright.

Acknowledgment Studies in our laboratories on photo-uncaging have long been supported by the US National Science Foundation.

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8.07

Photochemical CO2 reduction

Yusuke Tamaki and Osamu Ishitani, Tokyo Institute of Technology, Meguro, Tokyo, Japan © 2023 Elsevier Ltd. All rights reserved.

8.07.1 8.07.2 8.07.2.1 8.07.2.2 8.07.3 8.07.3.1 8.07.3.2 8.07.4 Acknowledgments References

Background Homogeneous photocatalytic systems Mixed system of photosensitizer and catalyst Supramolecular photocatalysts Hybrid systems comprising metal complexes and solid materials Hybrids with light-harvesting materials Hybrids with semiconductors Conclusion and future perspective

299 299 300 304 310 310 310 315 315 315

Nomenclature BIH 1,3-Dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole BNAH 1-Benzyl-1,4-dihydronicotinamide bpy 2,2’-Bipyridine Cat Catalyst dac 4,40 -Bis(methylacetamidomethyl)-2,20 -bipyridine dmb 4,4’-Dimethyl-2,20 -bipyridine DMA N,N-Dimethylacetamide DMF N,N-Dimethylformamide F Quantum yield MLCT Metal-to-ligand charge-transfer MOF Metal-organic framework NADP Nicotinamide adenine dinucleotide phosphate NADPH Dihydronicotinamide adenine dinucleotide phosphate OERS One-electron reduced species phen 1,10-Phenanthroline ppyH 2-Phenylpyridine PS Photosensitizer SP Supramolecular photocatalyst TADF Thermally activated delayed fluorescence TEA Triethylamine TEOA Triethanolamine TOF Turnover frequency TON Turnover number tpy 2,20 :60 ,200 -Terpyridine PMO Periodic mesoporous organosilica dcbpy 4,4’-Dicarboxy-2,20 -bipyridine dpbpy 4,40 -(CH2PO3H2)2–2,20 -bipyridine

Abstract In this chapter, we focused on CO2 reduction photocatalytic systems using metal complexes as “main players”. Two components are required for constructing efficient photocatalytic systems for CO2 reduction, i.e., redox photosensitizer and catalyst. The transition-metal complexes can be used for both components. In addition, hybrid systems of the metal-complex photocatalyst or catalyst with other (photo)functional solid materials (semiconductors and light harvesting materials) are also discussed.

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Comprehensive Inorganic Chemistry III, Volume 8

https://doi.org/10.1016/B978-0-12-823144-9.00123-0

Photochemical CO2 reduction

8.07.1

299

Background

Owing to global warming and shortage of fossil resources, which are or will be serious concerns for humanity, photocatalytic CO2 reduction has been receiving increasing attention. It is one of the fields of research related to artificial photosynthesis, which was initiated immediately after the oil crisis in the 1970s. From the beginning, semiconductor materials, metal complexes, metal particles, and enzymes have been investigated as photocatalysts and/or catalysts that are used in the presence of redox photosensitizers. Although semiconductor photocatalysts took precedence in the initial stage, the following problems were reported: (1) hydrogen evolution was superior to CO2 reduction, consequently the selectivity of CO2 reduction was low in most of the systems, (2) because the carbon sources of the products were not verified, the decomposition products of the organic contaminants may have been misidentified as the CO2 reduction products in many cases, and (3) the efficiency of CO2 reduction was considerably low. Recently, Kudo and co-workers reported on UV-light-driven Ag-metal-loaded metal oxide semiconductor photocatalysts such as Ag/NaTaO3:Ba, in which the loaded Ag particles functioned as co-catalysts for CO2 reduction using water as a reductant with a relatively high selectivity for CO2 reduction, even in an aqueous solution. In these, the carbon source was confirmed to be CO2 using isotope experiments.1 While visible-light-driven semiconductor photocatalysts for CO2 reduction using water as the reductant have been developed, the selectivity for CO2 reduction still remains low.2 In systems consisting of enzyme catalysts and redox photosensitizers such as [Ru(bpy)3]2þ (bpy ¼ 2,20 -bipyridine), Willner and co-workers reported pioneering studies using methylviologen as an electron relay, malic enzyme or isocitrate dehydrogenase as a catalyst for CO2 reductive fixation (carboxylation) into organic acids, the coenzyme nicotinamide adenine dinucleotide phosphate (NADP)/dihydronicotinamide adenine dinucleotide phosphate (NADPH) as a hydride mediator, ferredoxin-NADP-reductase as another catalyst for the reduction of NADP, and [Ru(bpy)3]2þ as a photosensitizer.3 Although the products were unique, including malic acid and isocitric acid, the systems were complicated and their stability was not high. Recently, systems consisting of enzymes and various photosensitizer/photocatalysts have been reported.4 As described in details in 1983, Lehn and co-workers reported that Re(I) bipyridine tricarbonyl complexes work as photocatalysts for CO2 reduction.5 This is the first photocatalytic system that can selectively reduce CO2 with a high quantum yield (FCO ¼ 14%). In this study, the carbon source was confirmed to be CO2 by isotope experiments using 13CO2. This research has led to realization of tremendous efforts in the development of photocatalytic systems using metal complexes as photocatalysts, catalysts, and/or photosensitizers. The advantages of metal complexes as photocatalysts or catalysts include selectivity for CO2 reduction as opposed to proton reduction. As photosensitizers, metal complexes with a variety of different colors, that is, strong visible-light absorption properties, are useful for using solar light. In this chapter, we focused on CO2 reduction photocatalytic systems using moleculesdprimarily metal complexes as “main players”, which include hybrid systems of the metal-complex photocatalyst or catalyst with other (photo)functional solid materials.

8.07.2

Homogeneous photocatalytic systems

Because one-photon excitation typically initiates only a one-electron transfer reaction, homogeneous photocatalytic systems for multi-electron CO2 reduction should require two functions as follows: one is a redox photosensitizer, which absorbs visible light and triggers the electron transfer process, and the other is a catalyst, which accepts electrons from the photosensitizer and activates CO2 for reduction. The reaction processes via reductive quenching of the excited photosensitizer can be summarized as follows: (1) the photosensitizer absorbs visible light to convert to its excited state (*PS), (2) *PS is reduced by a sacrificial electron donor (this is so-called “reductive quenching”) to give one-electron reduced species (PS), (3) an electron is transferred from PS to the catalyst, and (4) CO2 is reduced on the reduced catalyst. Based on these processes, we suggested that redox photosensitizers should present the following properties: (1) strong absorption of visible light toward efficient solar light utilization and suppression of the innerfilter effect by catalysts and/or substrates, (2) long lifetime in their excited state to initiate electron-transfer processes efficiently, and (3) strong reducing and oxidizing power to promote electron transfer processes. Catalysts should present low or no absorption in the visible region because visible-light absorption by catalysts suppresses visible-light absorption by photosensitizers. Excess amounts of photosensitizers are used in many mixed systems of a photosensitizer and a catalyst with strong visible region absorption, which are frequently converted from electrocatalytic system, to reduce inner filter effects. However, in the presence of an excess amount of photosensitizer, in other words, in the presence of an extremely low concentration of catalyst, the amount of products was frequently smaller than the amount of the photosensitizer. Therefore, the use of the too excess amount of photosensitizer compared to that of the catalyst should be avoided, and simple diversion from the electrochemical catalysts for CO2 reduction into photocatalytic systems is sometimes inappropriate. Ingenious devices are necessary; for example, tailoring the photophysical properties of catalysts to prevent them from absorbing visible light and using photosensitizers that can absorb longer wavelength light. The photon utilization efficiency, photocatalyst durability, reaction rate, and product selectivity are typically used to evaluate photocatalytic CO2 reduction reactions, as follows. (1) Quantum yield (Fproduct)

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Photochemical CO2 reduction

Fproduct ¼

amount of product ðmolÞ number of absorbed photons ðeinsteinÞ

(1)

Fproduct represents the efficiency of using absorbed photons for product formation, which is one of the most important indices in photocatalytic reactions. In some studies analyzing product formation via multi-electron processes, the values of quantum yield multiplied by number of used electrons (n) (n  Fproduct) have been used as quantum yields instead of Fproduct themselves. However, the value of “n  Fproduct” is scientifically meaningless and therefore should not be used because the electrons used for product formation are not always generated via photon absorption, especially in systems using sacrificial electron donors. In most studies, the electrons used to generate multi-electron reduced products have not necessarily been confirmed to be obtained via photoexcitation. Therefore, in this review, the Fproduct values are used as quantum yields even for the systems originally described using n  Fproduct as the quantum yield. (2) Turnover number (TONproduct) TONproduct ¼

amount of product ðmolÞ amount of added photocatalyst ðmolÞ

(2)

TONproduct represents the amount of product obtained per a molecule of photocatalyst. TONproduct after photocatalysis termination is a measure of photocatalyst durability. Many papers describing mixed photosensitizer-catalyst systems with excess photosensitizer amount express only TONproduct in terms of the amount of catalyst. However, photosensitizers absorb photons during photocatalytic reactions; therefore, TONproduct values based on the amount of photosensitizer are critical and should be described in the literature, which indicate the durability of “photo”catalytic reactions. (3) Turnover frequency (TOFproduct) TOFproduct ¼


TONproduct h1 ; min1 ; or s1 irradiation time ðh; min; or sÞ

(3)

TOFproduct, which represents the turnover number per unit of time, indicates the photocatalysis rate. Because photon absorption is the rate limiting step for many photocatalytic reactions, TOFproduct depends on the intensity of irradiated light. Therefore, when comparing the TOFproduct values for different reaction conditions, light intensity should be taken into consideration. (4) Product selectivity Product selectivity is calculated as the ratio of the amount of target product to the sum of amounts of all the products. The photocatalytic CO2 reduction reaction frequently competes with the H2 evolution reaction.

8.07.2.1

Mixed system of photosensitizer and catalyst

The structures and symbolisms of photosensitizers and catalysts are presented in Charts 1 and 2, respectively. Ishida, Tanaka and coworkers used a mixed photocatalytic system employing the ruthenium(II) tris-bipyridine complex (PS1) and ruthenium carbonyl 3+

Cl

R R

R

N N Ru N N N N

CO N Re N CO CO

2+

R R

MeO MeO

PS1: R = H PS11: R = CH3

R

Ph R R Ph

PS5: R = H PS8: R = SO3– Chart 1

P(OEt)3

+

CO N Re N CO CO

PS3 Ph

Ph Ph P N O Cu N P Ph Ph

P CO OC N Re CO OC Re N N N P P

PS2

R

R

P

+

Ph

P P=

Ph P Ph N Cu Ph N P Ph Ph P Ph Ph P Ph

Structures and symbolisms of photosensitizers.

NN Re P P OC CO

N Cu N

PS6

Ph

P Ph

N

Ph P Ph

Ir N

N

PS9

PS4

Ph Ph

2+

Ph Ph P N O Cu N P Ph Ph

PS7

Ph

+

NC

N

CN

N

Ph

N

PS10

N

Photochemical CO2 reduction

N N N Ru N CO CO

O

Cat4: H

Cat3: R =

Cat1

L

R

n

N

–O

O NCMe

N

SiMe3 Fe CO CO CO

R

N N Fe N NCS NCS

Cat9

R

Cat10

Ar

N Ni N MeCN NCMe

Cat17

Ar

N N M N N

Cat21: Co

S

Cat22: Co Cat23: Fe-Cl

N

SO3– N+

2+

2+

N N M N N OH2

N N Cu N N

Cat14: M = Co Cat15: M = Fe

Cat16

N+Me3

Cat24

2+

N

N

N Fe N N N

N

N

Cat25

Cat18 Chart 2

Cat8

N Cl N Fe Cl N Cl

Ar Cat20: Fe-Cl

N Ni N H2O OH2 N

Ar

Cl

OH2

Cat13: R =

Cat19: Co

2+

S

M

Ar

2+

Cat7

S

N

Cy2P Ir PCy2

N Ru N N NCMe

R Br CO N Mn N CO R CO

Cat11: R = OMe Cat12: R = H

2+

S

1 0

Br CO N Mn N CO CO

N

Ph Ph N P

2+

O

Me3Si

H

Cat6

OH 2 0

Cat5: H Cat26: CH3 Cl–

+

N N N Ir N Cl

CO N Re N CO CO

R

Cat2: R = H

+

n+

L

R

R Cl CO N Ru N CO R Cl

2+

301

Structures and symbolisms of catalysts.

complex (Cat1) as the photosensitizer and catalyst, respectively, to photocatalytically reduce CO2 under visible-light irradiation in the presence of a sacrificial electron donor, which is a typical example of the mixed photocatalytic system.6 Ruthenium(II) trisdiimine complexes are the most frequently used redox photosensitizers because of their visible-light absorption (lmax z 460 nm, 4 1 3 z 1.6  10 M cm1), which is attributed to the metal-to-ligand charge-transfer (MLCT) transitions, their relatively long lifetimes in their excited states ranging from several hundred nanoseconds to several microseconds, and their relatively strong oxidizing/reducing power.7,8 Cat1, which does not absorb visible light owing to the strong ligand field of its carbonyl ligands and has been used as a catalyst for electrochemical CO2 reduction,9,10 is a suitable catalyst used in the photocatalytic CO2 reduction. Visible-light irradiation to the mixed system of PS1 and Cat1 under CO2 selectively produced HCOOH using triethanolamine (TEOA) as the sacrificial electron donor. When 1-benzyl-1,4-dihydronicotinamide (BNAH), a model compound of coenzyme NAD(P)H, was used as the sacrificial electron donor, the primary product was CO. Similar ruthenium(II) complex of Cat2 also functioned as a catalyst for photocatalytic CO2 reduction in combination with a photosensitizer of PS1.11 One of the disadvantages of ruthenium(II) carbonyl catalysts is the formation of the polymer with RueRu bonds via reduction (Eq. 4).12–14 The polymer deteriorates the photocatalysis of the system owing to its high reduction potential and strong absorption in the visible region (the inner filter effect). Ishida and co-workers reported that Cat3, a ruthenium(II) complex with mesityl groups in the 6,60 positions of 2,20 -bipyridine, suppressed polymerization and served as a catalyst.15 Tanaka and co-workers16 and Meyer and co-workers17,18 used ruthenium(II) complexes with similar coordination structure, [Ru(tpy)(bpy)(L)]2þ (tpy ¼ 2,20 :60 ,200 -terpyridine, L ¼ CO, acetonitrile), as electrochemical catalysts for the reduction of CO2.

n

Cl CO N Ru N CO Cl

2n e– –2n Cl–

CO N Ru N CO

(4)

n

The rhenium(I) complex with a 2,20 -bipyridine and three carbonyl ligands (PS2) was first reported as a CO2 reduction “photocatalyst” in 1983. Lehn and co-workers demonstrated that photoirradiation of PS2 in the presence of TEOA and the absence of other components, such as photosensitizers or catalysts, generated CO with a high FCO of 0.14.5,19 Recently, the reason why the single

302

Photochemical CO2 reduction

component of PS2 functions as the “photocatalyst” was clarified.20 During the initial photocatalysis step, PS2 was partially converted into Cat4 via a photochemical ligand substitution reaction (Eq. 5). The Cl dissociated from PS2, followed by coordination of deprotonated TEOA to the vacant coordination site and insertion of CO2 between the ReeO bond to form the Re–CO2–TEOA adduct, Cat4. Residual PS2 and the obtained Cat4 served as the photosensitizer and catalyst, respectively, and photocatalytic CO2 reduction proceeded over the mixed system of PS2 and Cat4.

O

Cl CO N Re N CO CO

CO2

hv TEOA

–Cl–,

–H+

O O NR2 CO N Re N CO CO R = C2H4OH

(5)

This example of “photocatalyst consisting of a single mononuclear complex” highlights the importance of analyzing the mechanisms of photocatalytic reactions. Even though single metal complex is used for photocatalytic CO2 reduction, we cannot declare that it functions as photocatalyst. Metal complexes can change their structure via ligand substitution during multi-step reactions, such as CO2 reduction, and the obtained complexes present various functions. For example, light irradiation of PS1, a typical photosensitizer, in the presence of a sacrificial electron donor and the absence of other metal complexes promotes CO2 reduction.21 The photoinduced electron transfer from the sacrificial electron donor to the excited PS1 generates one-electron reduced species (OERS) of PS1, and photon-absorption by the OERS of PS1 produces [Ru(bpy)2(solvent)2]2þ via ligand dissociation (Eq. 6). Subsequently, [Ru(bpy)2(solvent)2]2þ serves as the CO2 reduction catalyst, and the residual PS1 serves as the photosensitizer.11 Because [Ru(bpy)2(solvent)2]2þ accepts an electron from the OERS of PS1, the durability of PS1 is improved after the formation of [Ru(bpy)2(solvent)2]2þ, and photocatalytic CO2 reduction proceeds over the mixture of PS1 and [Ru(bpy)2(solvent)2]2þ.

2+

N N Ru N N N N

2+

hv sacrificial electron donor

N N Ru N N N N

hv –bpy

N N Ru N N

solvent

(6)

solvent

Therefore, we should investigate the structural changes of metal complex(es) for discussing the photocatalytic reaction mechanisms. Based on the mechanistic insight gathered by analyzing CO2 photocatalysis over PS2, two different rhenium(I) complexes were designed and optimized as photosensitizer and catalyst, respectively. Because the OERS of a rhenium(I) complex with triethylphosphite as the ligand, PS3, was relatively stable and the quantum yield for the formation of the OERS of PS3 was significantly high (FOERS ¼ 1.6), PS3 was used as a photosensitizer. When a rhenium(I) complex with labile acetonitrile as the ligand, Cat5, was used as the catalyst, it was quantitatively converted into a Re–CO2–TEOA adduct, Cat4, in the presence of TEOA and CO2. The mixed system of PS3 and Cat5 with a PS3:Cat5 ratio of 24:1 photocatalyzed CO2 reduction to CO with a high FCO of 0.59.22 A ring-shaped trinuclear rhenium(I) complex, PS4, is another superior photosensitizer. The three rhenium(I) units with two carbonyl ligands and one 5,50 -dimethyl-2,20 -bipyridine ligand are connected to each other using bidentate phosphine ligands. PS4 exhibited a longer wavelength absorption (lmax ¼ 409 nm) than rhenium(I) tri-carbonyl complexes (lmax z 360 nm) and a remarkably long lifetime in the excited state (sem ¼ 5.4 ms). The strong p-p interactions between the phenyl groups of the phosphine ligands and diimine ligands were observed because of the limited mobility of the ring-shaped structure. The mixed system of PS4 photosensitizer and Cat5 with a PS4:Cat5 ratio of 1:1 was used to photocatalyze the selective reduction of CO2 to CO in the presence of 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole (BIH) as the sacrificial electron donor with high photocatalytic ability (FCO ¼ 0.82, TONCO ¼ 526). The FCO ¼ 0.82 was the highest value reported to date.23–25 Sato et al. reported an iridium(III) complex, Cat6, as a photocatalyst for CO2 reduction.26 A CO2-saturated MeCN-TEOA solution containing Cat6 was irradiated at lex > 410 nm to selectively produce CO. TONCO reached 38 after 6 h of irradiation and FCO was determined to be 0.13. Mechanistic analysis revealed that the hydrido complex, [Ir(tpy)(ppy)H]þ (ppyH ¼ 2-phenylpyridine), was an intermediate for photocatalytic CO2 reduction. Masaoka and co-workers reported that a ruthenium(II) polypyridyl complex ^ with N P-type bidentate ligand, Cat7, photocatalyzed CO2 reduction by itself.27 Visible-light irradiation of Cat7 in the presence of BIH as the strong sacrificial electron donor produced CO catalytically and selectively (TONCO ¼ 353, TOFCO ¼ 14.7 h1). The structural change of Cat7 during photocatalysis has not been described in the literature. Jung, Saito and co-workers used an iridium(III) complex with a PNNP-type tetradentate ligand, Cat8, as a CO2 reduction photocatalyst.28 Visible-light irradiation (lex > 400 nm) of Cat8 in the presence of BIH photocatalyzed CO2 reduction to HCOOH (TONHCOOH ¼ 2080 after 144 h of irradiation and FHCOOH ¼ 0.21). The authors proposed that CO2 insertion into the IrIIIeH bond of one-electron reduced Cat8 triggered CO2 reduction to HCOOH. Metal complexes of rare elements, such as ruthenium(II), rhenium(I), iridium(III), and osmium(II) have been used as photosensitizers and catalysts for many photocatalytic CO2 reduction systems. From the strategic point to utilize abundant elements, recently, photocatalytic systems using first-row transition metal complexes, such as manganese(I), iron(II), cobalt(II), nickel(II),

Photochemical CO2 reduction

303

and copper(I) complexes have been reported. For example, Beller and co-workers described the photocatalytic reduction of CO2 to CO using a Cu(I) complex, PS5, as the photosensitizer, an iron(II) complex, Cat9, as the catalyst, and BIH as the sacrificial electron PS5 donor.29 In the mixed system of PS5:Cat9 ¼ 5:1, the quantum yield was FCO ¼ 0.13 and turnover numbers were TONCO ¼ 97 and ^ ^ Cat9 ¼ 487. Heteroleptic copper(I) complexes with a diimine lingand (N N) and bidentate phosphine ligand (P P) exhibit TONCO photophysical properties suitable as a redox photosensitizer such as visible-light absorption (e.g. PS7: lmax ¼ 384 nm) and long lifetimes in their excited states (e.g. PS7: sem ¼ 0.99 ms).30 However, several shortcomings still remain as follows. [1] The coordi^ nation structures of these copper(I) complexes are relatively unstable, and homoleptic complexes, [Cu(N N)2]þ, which should 31 not serve as photosensitizers, are formed via ligand exchange reactions (Eq. 7).

2

P

N Cu N P

+

N

N Cu N N

+

+

P

P Cu P P

+ (7)

[2] Copper(I) complexes present d10 electron configurations with tetrahedral coordination, whereas their MLCT excited states exhibit pseudo-d9 configurations and tend to change their structure to square-planar coordination. These structural changes induce solvent coordination on the vacant axial positions, resulting in excited state quenching (exciplex quenching).32 The first shortcoming indicates a decrease in the amount of photosensitizer and the second indicates that the lifetimes of the excited states of the photosensitizers are shortened. To suppress exciplex quenching, steric hindrance was increased by introducing substituent groups such as methyl groups at the 2,9 positions of 1,10-phenanthroline. Beller and co-workers reported a method for adding excess phosphine ligands to inhibit the disproportionation reaction (Eq. 7).29 To develop more stable copper(I) photosensitizers, a tetradentate ligand, in which the diimine ligand was connected to phosphine ligands via eC4H8e chains, was synthesized. Complexing this ligand with copper(I) ion produced a binuclear copper(I) complex, PS6, in which copper(I) was coordinated by a diimine moiety, one of the phosphine moieties of the single tetradentate ligand, and a phosphine moiety of the other tetradentate ^ ligand. PS6 was highly stable, and no [Cu(N N)2]þ species formed via disproportionation even in acetonitrile solution. The excited state lifetime of PS6 was longer as sem ¼ 3.19 ms than that of the corresponding mononuclear model complex, PS7 (sem ¼ 0.99 ms). A mixed system of PS6 and Cat10 with a PS6:Cat10 ratio of 5:1, which was irradiated in the presence of BIH, yielded CO as the primary CO2 reduction product.30 The photocatalytic activity of this mixed system (FCO ¼ 0.067, TONCOPS6 ¼ 55, TONCOCat10 ¼ 273) was superior to that using PS7 instead of PS6 (FCO ¼ 0.026). The photocatalytic systems comprising PS6 and manganese(I) catalysts of Cat11-Cat13 exhibited excellent photocatalytic activity. Visible-light irradiation of the mixed systems of PS6 and Cat11-Cat13 with PS6:Cat11-Cat13 ratios of 5:1 in the presence of BIH produced CO and/or HCOOH.33 Both CO and HCOOH were produced when Cat11 with electron-donating 4,40 -dimethoxy-2,20 -bipyridine moieties was used as the catalyst, and PS6 PS6 the highest quantum yields for CO2 reduction was obtained (FCO ¼ 0.33, FHCOOH ¼ 0.24, TONCO ¼ 201, TONHCOOH ¼ 62, Cat11 Cat11 TONCO ¼ 1004, TONHCOOH ¼ 310) among the mixed systems of PS6 and Cat11-Cat13. HCOOH was the primary product (FHCOOH ¼ 0.30) when Cat12 without any substituents on the 2,20 -bipyridine moieties was used as the catalyst. When bulky mesityl groups were introduced in the 6,60 positions of 2,20 -bipyridine (Cat13), CO was produced selectively (FCO ¼ 0.41). Although the reasons for different product distributions depend on the substituents on 2,20 -bipyridine have not been elucidated yet, two intermediates of catalytic reaction have been observed (Scheme 1). One-electron reduction of fac-Mn(N^N)(CO)3Br dissociated Br ions and formed a 17-electron species, [Mn(N^N)(CO)3]. For Cat11 and Cat12, dimerization of [Mn(N^N)(CO)3] into the MneMn dimer was observed using spectroscopy during photocatalysis and electrochemical measurements. The subsequent reduction of the MneMn dimer produced an 18-electron species, [Mn(N^N)(CO)3]. Conversely, Cat13 did not form the MneMn dimer via one-electron reduction because of the steric hindrance of the mesityl groups, and subsequent reduction produced the 18electron species, [Mn(N^N)(CO)3], directly. CO2 reduction via the 18-electron species produced CO selectively, whereas CO2 reduction via the MneMn dimer produced both CO and HCOOH. It is noteworthy that the OERS of PS6 presented strong reducing ^ power to form [Mn(N N)(CO)3]. Robert and co-workers used first-row transition metal complexes with a tetradentate ligand, 2,20 :60 ,200 :600 ,2000 -quaterpyridine, as catalysts for CO2 reduction. The cobalt(II) complex, Cat14, and PS1 mixed system photocatalyzed CO2 reduction to CO in the presPS1 Cat14 ¼ 83 and TONCO ¼ 497. Under similar reaction conditions, CO2 was conence of BIH.34 At a PS1:Cat14 ratio of 6:1, TONCO PS1 verted into CO using the iron(II) complex, Cat15, and PS1 mixed system with a PS1:Cat15 ratio of 4:1 (TONCO ¼ 470, Cat15 34 TONCO ¼ 1879). In addition, CO was the primary reaction product of the photocatalytic CO2 reduction over the copper(II) PS1 Cat16 complex, Cat16, and PS1 mixed system with TONCO ¼ 6.2 and TONCO ¼ 12400 (PS1:Cat16 ratio of 2000:1).35 Kojima and co-workers developed a nickel(II) complex with a S2N2-type tetradentate ligand (Cat17) and used it as a CO2 reduction catalyst. The mixed system of Cat17 and PS1 with a PS1:Cat17 ratio of 50:3 was irradiated with visible light at lex ¼ 450 nm in PS1 Cat17 ¼ 43, TONCO ¼ 713).36 Cat18, which was designed and the presence of BIH to selectively form CO (FCO ¼ 0.014, TONCO synthesized by the introduction of “pyridine arms,” exhibited substantially improved photocatalytic ability in the presence of Mg2þ cations (FCO ¼ 0.11).37 The authors claimed that the Lewis acidity of the Mg2þ cations coordinated by pyridine moieties promoted CO2 coordination to the nickel center of the intermediate, which served as a Lewis acid cocatalyst in the second coordination sphere. Iron(III) and cobalt(II) porphyrins have been used as CO2 reduction catalysts. In 1999, Neta, Fujita and co-workers reported photocatalytic systems comprising an organic photosensitizer, p-terphenyl, which requires UV light (lex > 300 nm) irradiation,

304

Photochemical CO2 reduction R

1/2 CO2 1/2 HCOOH 1/2 CO

+ R R

Br CO N Mn N CO CO

N N OC CO Mn CO OC Mn OC CO N N

1/2

or

e–

(R = OMe, H)

PS6 R R

PS6,

Br–

R

R

R

+ e–

CO N Mn N CO CO

+ e–

R R

R=

17-electron species

CO N Mn N CO CO

–

18-electron species

+ e– CO Scheme 1

CO2

Proposed mechanisms of CO2 reduction over Mn(N^N)(CO)3Br catalysts.

a cobalt(II) porphyrin, Cat19, and triethylamine (TEA) as a sacrificial electron donor for CO2 reduction to CO and HCOOH Cat19 Cat19 (TONCO ¼ 31, TONHCOOH ¼ 27; p-terphenyl:Cat19 ratio of 30:1).38,39 An iron(III) porphyrin, Cat20, can be used as a catalyst Cat20 Cat20 instead of Cat19 (TONCO ¼ 20, TONHCOOH ¼ 31; p-terpheyl:Cat20 ratio of 30:1).39 Call, Sakai and co-workers developed a water-soluble catalyst, Cat21, by introducing sulfonate groups at the meso positions. CO was the primary product of the CO2 reduction reaction that occurred upon irradiating an aqueous solution containing a mixed system of PS1 and Cat21 with PS1 Cat21 PS1 a PS1:Cat21 ratio of 50:1 and sodium ascorbate as a sacrificial electron donor (TONCO ¼ 19, TONCO ¼ 926, TOFCO ¼ 9 h1, Cat21 1 40 TOFCO ¼ 456 h ). A copper(I)-based photosensitizer, PS8, is soluble in water owing to the sulfonate groups. In the combination of PS8 and another water-soluble catalyst Cat22 with pyridinium groups, the photocatalytic CO2 reduction also proceeded PS8 Cat22 PS8 Cat22 in aqueous solutions (TONCO ¼ 27, TONCO ¼ 2680, TOFCO ¼ 26 h1, TOFCO ¼ 2600 h1 at PS8:Cat22 ratio of 100:1).41 Robert and co-workers reported that the photocatalytic CO2 reduction reaction using the mixed system of an iron porphyrin with ammonium groups at the meso positions, Cat23, and an iridium(III) photosensitizer, PS9, with the PS9:Cat23 ratio of 100:1 and PS9 Cat23 TEA generated an eight-electron reduced product, CH4, along with CO as a main product (TONCO ¼ 3.7, TONCO ¼ 367, PS9 Cat23 42 TONCH4 ¼ 0.8, TONCH4 ¼ 79). A larger amount of CH4 was produced under similar reaction condition when CO was used PS9 Cat23 as the reagent instead of CO2 (FCH4 ¼ 0.0003, TONCH4 ¼ 1.6, TONCH4 ¼ 159). For most photocatalytic systems employing metal complex catalysts, two-electron reduced species of CO2, that is, CO or HCOOH, were the main products, and four-, six-, and eight-electron reduced products were scarcely detected. This should be attributed to the weak MeCO and/or MeO(O)CH bonds of the intermediates, especially after subsequent reduction, which could easily dissociate, resulting in the formation of CO and/or HCOOH before additional electrons were injected into the catalyst. For the system developed by Robert and coworkers, the strong FeeCO bond and strong reducing power of PS9 (E0(IrIV/IrIII*) ¼ 1.73 V vs. SCE) probably contributed to the formation of CH4. Recently, metal-free organic compounds have been used as redox photosensitizers. The lifetimes of most organic compounds in their singlet excited states are short (several nanoseconds or shorter), which rapidly deactivate via fluorescence and non-radiative decay; therefore, their photophysical properties render them unsuitable as photosensitizers. Adachi and co-workers reported organic compounds that exhibit thermally activated delayed fluorescence (TADF) and long lifetimes in their excited states ranging from several microseconds to several tens of microseconds as emitters for organic EL devices.43,44 These compounds presented small energy gaps (DE < 0.1 eV) between the lowest singlet (S1) and triplet (T1) excited states, resulting in accelerated intersystem crossing and reverse intersystem crossing processes. These compounds have been used in redox photosensitizing reactions, primarily for organic synthesis. Recently, Chao and co-workers used a TADF-type compounds, PS10, to the photocatalytic CO2 reduction in combination with an iron(III) catalyst (Cat24). Visible-light (lex > 420 nm) irradiation in the presence of TEA produced CO as PS10 Cat24 PS10 Cat24 the main product (TONCO ¼ 450, TONCO ¼ 2250, TOFCO ¼ 12 min1, TOFCO ¼ 60 min1, FCO ¼ 0.02 at PS10:Cat24 45 ratio of 5:1). Better photocatalytic abilities were achieved using another iron(II) catalyst (Cat25). The 10:1 mixed system of PS10 Cat25 PS10 Cat25 ¼ 632, TONCO ¼ 6320, TOFCO ¼ 12.7 min1, TOFCO ¼ 127 min1, FCO ¼ 0.095.46 PS10 and Cat25 exhibited TONCO

8.07.2.2

Supramolecular photocatalysts

Balzani et al. proposed that multinuclear complexes comprising multiple units with different functions, such as photosensitizer and catalyst units, were termed as supramolecular photocatalysts.47 In this section, we summarize the supramolecular photocatalysts for CO2 reduction and extract the architecture of molecular design to construct effective supramolecular photocatalysts. The electron

Photochemical CO2 reduction

305

transfer between the photosensitizer and catalyst in simple mixed systems of photosensitizer and catalyst occurs only via collision between the one-electron reduced or excited photosensitizer and the catalyst, which is limited by diffusion. The covalent bond between the photosensitizer and catalyst should accelerate electron transfer and is not limited by diffusion collision. The accelerated electron transfer also shortens the lifetime of the OERS or excited state of the photosensitizer, suppressing photosensitizer decomposition. Consequently, the photocatalytic abilities can be improved.48 Chart 3 displays the structures of Ru(II)eNi(II) and Ru(II)eCo(III) binuclear complexes. The first trial to develop supramolecular photocatalysts for CO2 reduction was reported by Kimura et al. in 1992. Although the binuclear complex (SP1) comprising nickel(II) cyclam and [Ru(phen)3]2þ-type (phen ¼ 1,10-phenanthroline) complex produced mainly CO using ascorbic acid as SP1 a sacrificial electron donor under light irradiation at lex > 350 nm, the TONCO was lower than 1, indicating that SP1 did not serve 49,50 Komatsuzaki et al. developed a binuclear complex comprising [Ru(bpy)2(phen)]2þ-type photosensitizer and as a photocatalyst. [Co(phen)(bpy)2]3 þ-type catalyst units, SP2. Visible-light irradiation of SP2 in the presence of TEOA under a CO2 atmosphere SP2 SP2 ¼ 5, TONH2 ¼ 1), whereas a mixture of the corresponding mononuclear model complexes, produced mainly CO (TONCO 3þ namely PS1 and [Co(bpy)3] , produced a larger amount of CO (TONCO ¼ 9, TONH2 ¼ 16).51 The structures and symbolisms of the Ru(II)eRe(I) binuclear complexes are presented in Chart 4, and the photocatalytic activities of the Ru(II)eRe(I) binuclear complexes are summarized in Table 1. The first successful supramolecular photocatalyst for CO2 reduction was reported in 2005. The photocatalytic properties of this supramolecular photocatalyst were superior to those of a mixed system of corresponding mononuclear complexes. ^ SP3, which comprised a [Ru(dmb)2(N N)]2þ (dmb ¼ 4,40 -dimethyl-2,20 -bipyridine) photosensitizer unit and a fac^ Re(N N)(CO)3Cl catalyst unit connected via an eCH2CH(OH)CH2e chain between two 4-methyl-2,20 -bipyridine moieties, efficiently photocatalyzed CO2 reduction in the presence of a sacrificial electron donor, BNAH, under visible-light irradiation (lex > 500 nm).52 CO was produced with high durability (Table 1, Entry 1: FCO ¼ 0.12, TONCO ¼ 170). The photocatalytic activity of SP3 was superior to that of the mixed system of its corresponding mononuclear complexes, namely PS11 and Cat26 (Table 1, Entry 2: FCO ¼ 0.062, TONCO ¼ 101). The reaction mechanism of SP3 was as follows. [Step 1] Visible light was selectively absorbed by the [Ru(dmb)2(N^N)]2þ photosensitizer unit to form the triplet MCLT (3MLCT) excited state via intersystem crossing from the corresponding singlet (1MLCT) excited state. [Step 2] The excited photosensitizer unit was reduced by BNAH to the OERS of the photosensitizer unit. [Step 3] The unpaired electron of the photosensitizer unit was intramolecularly transferred to the catalyst unit. [Step 4] CO2 was reduced to CO over the catalyst unit. The photocatalytic activities of SP4 (Table 1, Entry 3: TONCO ¼ 50) and SP5 (Table 1, Entry 4: TONCO ¼ 3) with 2,20 -bipyridine or 4,40 -bis(trifluoromethyl)-2,20 -bipyridine as the peripheral ligand of the ruthenium(II) unit instead of the dmb ligand were significantly lower than those of SP3 and even the mixed system of PS11 and Cat26. The unpaired electron of the OERS of SP4 and SP5 was primarily located in the peripheral ligands of the ruthenium(II) unit 5+

N N Ru N N N N

4+

N N H

SP1 Chart 3

Ni

H N

N N Ru N N N N

N H

N N Co N N N N

SP2

Structures and symbolisms of Ru(II)eNi(II) and Ru(II)eCo(III) binuclear complexes.

R R

R

N N Ru N N N N R

OH

CO N Re N CO CO

N N Ru N N N N

SP3: R = CH3 SP4: R = H SP5: R = CF3

N N Ru N N N N

P

P

N N H

2+

Cl

CO N Re N CO CO

SP6 O

F

3+ 3

CO N Re N CO

SP7 Chart 4

2+

Cl

N N Ru N N N N

O

SP8 F

3

Structures and symbolisms of Ru(II)eRe(I) binuclear complexes.

O O CO N Re N CO CO

2+

N

OH

2

306 Table 1

Photochemical CO2 reduction Photocatalytic properties of Ru(II)eRe(I) binuclear complexes.a

Entry

Photocatalyst

FCO

TONCO

TOFCO / min1

Sacrificial electron donor

Ref.

1 2 3 4 5 6 7

SP3 PS11 þ Cat26 SP4 SP5 SP6 SP7

0.12 0.062 – – – 0.15 0.45

170 101 50 3 28 207 3029

– – – – – 4.7 36

BNAH BNAH BNAH BNAH BNAH BNAH BIH

52 52 52 52 52 53 54

a CO2-saturated N,N-dimethylformamide (DMF)–TEOA (5:1 v/v) solutions containing the complex and a sacrificial electron donor (0.1 M) were irradiated using visible light (lex > 500 nm).

rather than the bridging ligand because the p* level of the peripheral ligands was more positive than that of the bridging ligand. Therefore, the intramolecular electron transfer from the reduced ruthenium(II) unit to the rhenium(I) unit became endergonic red red (RuI/II) ¼  1.23 V, E1/2 (Re0/I) ¼  1.76 V vs. Ag/AgNO3). Conversely, both the ruthenium(II) ([Step 3]; for example, for SP5, E1/2 red and rhenium(I) units of SP3 were reduced at approximately the same potential (E1/2 ¼  1.77 V). Moreover, the bridging ligand that connected the photosensitizer and catalyst units significantly affected photocatalytic activity. The photocatalytic ability of SP6 was low (Table 1, Entry 5: TONCO ¼ 28) although the intramolecular electron transfer from the OERS of the ruthenium(II) unit to the rhenium(I) unit should proceed rapidly. This was attributed to the conjugated bridging ligand lowering the reduction power of red the OERS of the rhenium(I) catalyst unit (E1/2 ¼  1.10 V), which inhibited the catalytic activity for CO2 reduction on the rhenium(I) unit. Note that the relationship between the photocatalyses using the mononuclear rhenium(I) complexes, i.e., fac[Re(4,40 -X2-bpy)(CO)3(PR3)]þ, and their first reduction potentials exists (Table 2).55 The data indicate that a mononuclear red rhenium(I) complex can serve as a catalyst if its first reduction potential (E1/2 ) is more negative than  1.4 V. Based on these results, the requirements for constructing effective supramolecular photocatalysts for CO2 reduction can be summarized as follows. (1) Not to inhibit light absorption by the photosensitizer unit ([Step 1]), the catalyst unit without absorptivity or with weak absorptivity in the same wavelength range as the photosensitizer unit should be used. The use of a large excess of the photosensitizer units per catalyst unit is difficult. (2) Because the efficient intramolecular electron transfer should proceed from the OERS of the photosensitizer unit to the catalyst unit ([Step 3]), the unpaired electron of the OERS of the photosensitizer unit should be localized on the bridging ligand. In addition, the first reduction potential of the photosensitizer unit must be equal to or more negative than that of the catalyst unit. (3) To maintain the reducing power and catalytic activity of the catalyst unit, the introduction of a conjugated bridging ligand should be avoided because this often lowers the reduction power of the catalyst unit ([Step 4]). The supramolecular photocatalyst (SP7) comprising a rhenium(I) biscarbonyl complex, cis,trans-[Re(N^N)(CO)2(PR3)2]þ, instead of fac-Re(N^N)(CO)3Cl photocatalytically produced CO (Table 1, Entry 6: FCO ¼ 0.15, TONCO ¼ 207, TOFCO ¼ 4.7 min1).53 The quantum yield, turnover number, and turnover frequency for the photocatalytic CO2 reduction over SP7 were significantly improved by using BIH as a sacrificial electron donor instead of BNAH (Table 1 Entry 7: FCO ¼ 0.45, TONCO ¼ 3029, TOFCO ¼ 36 min1).54 These results can be explained from the following reasons. (1) The reductive quenching of the excited photosensitizer unit by a sacrificial electron donor ([Step 2]) compete against phosphorescence and non-radiative ox ¼ 0.33 V vs. SCE)56 is stronger than that of BNAH (E ox ¼ 0.57 V)57, BIH quenched decay. Because the reducing power of BIH (E1/2 the excited photosensitizer unit almost quantitatively, whereas BNAH quenched only 62% of the excited photosensitizer unit. (2) The effects of the oxidized compounds of sacrificial electron donors, which accumulated during photocatalysis, on the photocatalytic activities were clarified. As shown in Eq. (8), after the reductive quenching by BNAH, the dimers (BNA2s) formed via oneelectron oxidation, deprotonation, and rapid dimerization. By contrast, upon using BIH, BI$ was produced via one-electron oxidation and deprotonation, and dimerization did not occur. Because the reducing power of BI$ was quite strong (Epox ¼  2.06 V vs. Fcþ/Fc)56, BI$ can donate one more electron to SP7, converting to BIþ (Eq. 9). During photocatalytic reactions using BNAH and BIH, BNA2s and BIþ quantitatively accumulated in solution, respectively. Although BIþ did not affect the photocatalytic reaction, BNA2s inhibited the reductive quenching of the excited photosensitizer unit by BNAH ([Step 2]) owing to their high quenching abilities. Therefore, BIH is a more suitable sacrificial electron donor with superior photocatalytic abilities than BNAH. Bn H H

CONH2

*SP7

SP7

H H

N

+ N

Bn

Bn

BNAH

CONH2

H+

H N Bn

CONH2

N

CONH2

(×2)

H2NOC

CONH2 N Bn

4,4'-BNA2



Bn

N CONH2 N Bn

4,6'-BNA2

(8)

Photochemical CO2 reduction

307

Relationship between photocatalyses of fac-[Re(4,40 -X2bpy)(CO)3(PR3)]þ and their first reduction potentials.a

Table 2

fac-[Re(4,40 -X2bpy)(CO)3(PR3)]þ X

R

FCO

TONCO

red E1/2

Me H H H H H CF3

OEt OiPr OEt OMe Et n Bu OEt

0.18 0.20 0.16 0.17 0.024 0.013 0.005

4.1 6.2 5.9 5.5 0.83 0.65 0.10

1.55 1.44 1.43 1.41 1.39 1.39 1.03

a A 4 mL solution in DMF containing the complex (2.6 mM) and TEOA (1.26 M) as a sacrificial electron donor was irradiated at lex ¼ 365 nm under a CO2 atmosphere. The light intensity was 1.27  10 8 einstein$s1.

*SP7

H+

SP7

N

SP7

N

+ N H

N H

BIH

SP7

N

N

N

N

BI•

BI+

+

(9)

The aforementioned ReeCO2eTEOA adduct can be synthesized using a rhenium(I) complex with a labile ligand such as DMF, ^ i.e. fac-[Re(N N)(CO)3(DMF)]þ, as a starting complex (Eq. 10).20 The dissolution of fac-[Re(N^N)(CO)3(DMF)]þ in a DMF–TEOA mixed solvent produced a fac-Re(N^N)(CO)3{OC2H4N(C2H4OH)2} and fac-[Re(N^N)(CO)3(DMF)]þ equilibrium mixture. Purging the mixture with CO2 induced CO2 insertion between the ReeO bond of fac-Re(N^N)(CO)3{OC2H4N(C2H4OH)2} forming a ReeCO2eTEOA adduct, fac-Re(N^N)(CO)3{OC(O)OC2H4N(C2H4OH)2}. The CO2 insertion process was reversible, and its equilibrium constant was extremely large as KCO2 ¼ 1.5  103 M1. Therefore, the CO2-containing supramolecular photocatalyst, SP8, formed easily via CO2 capture from mixed gases with low CO2 concentrations (Eq. 10).58

DMF [Ru-Re]

+

TEOA

O

N

O OH

2

[Ru-Re]

CO2 KCO2

O O

N

OH

2

[Ru-Re] (10)

[Ru-Re] =

N N Ru N N N N

2+

O

CO N Re N CO CO

Hence, SP8 can be used to capture CO2 into its rhenium(I) catalyst unit. Upon irradiating a DMF–TEOA (5:1v/v) solution containing SP8 and BIH with visible light under a pure CO2 atmosphere, CO produced with high efficiency and selectivity (FCO ¼ 0.50, TONCO > 1000). The results of photocatalyses under a mixed gas of Ar and CO2 are presented in Fig. 1. The CO production rate under a 10% CO2 (CO2:Ar ¼ 1:9) atmosphere was almost equal to that under a pure CO2. Moreover, the CO production rates under 1% or 0.5% CO2 were 0.74-times and 0.53-times lower, respectively, than those under a pure CO2.58 The CO formation rate was proportional to the concentration of the Re–CO2–TEOA adduct, i.e. SP8. By contrast, visible-light irradiation in the absence of TEOA under a 1% CO2 atmosphere did not produce CO at all. That is, the rhenium(I) catalyst unit captured and reduced CO2, and then, a low concentration of CO2 was reduced to CO photocatalytically and directly. Another method for reducing low concentration of CO2 relies on the adsorption capability of porous materials, such as metalorganic frameworks (MOFs) for CO2. Tanaka, Kitagawa and co-workers introduced a [Ru(tpy)(bpy)(CO)]2þ-type catalyst into a CO2- adsorptive zirconium(IV)-based MOF. The primary product of CO2 photocatalytic reduction over a mixture of Zr-MOF/ Zr-MOF/Ru(CO) PS1 ¼ 31, TONHCOOH ¼ 3). Ru(CO) composite and PS1 under 100% CO2 in the presence of TEOA was HCOOH (TONHCOOH Zr-MOF/Ru(CO) ¼ 33, Furthermore, the photocatalytic ability of the mixture was maintained even under 5% CO2 (TONHCOOH

308

Photochemical CO2 reduction

Fig. 1 (A) P Photocatalytic reactions under 100, 10, 1, and 0.5% CO2 atmosphere. (B) Linear relationship between the initial rate of CO formation and the ratio of the CO2-capturing complex SP8. PS1 TONHCOOH ¼ 3).59 Conversely, the photocatalytic ability of a [Ru(tpy)(bpy)(CO)]2þ and PS1 mixture in the absence of MOF under 5% CO2 was 50% lower than that under 100% CO2. These results indicated a synergistic effect between the CO2 adsorption ability and catalytically active sites within Zr-MOF/Ru(CO) composite. MOFs have also been used as platforms to fabricate systems featuring photosensitizer and neighboring catalyst units. Zhang and co-workers fabricated Eu-MOF/Ru/Cu composite by introducing a [Ru(bpy)3]2þ-type photosensitizing complex and a Cu(bpy)Cl2type catalytic complex into an europium-based MOF. HCOOH (3040 mmol g1) was produced by photo-irradiating Eu-MOF/Ru/ Cu composite at lex > 400 nm for 10 h in the presence of triisopropanolamine as a sacrificial electron donor.60 Kim, Son, Kang and co-workers synthesized MOF using Zr6O8 cluster and zinc porphyrin with carboxylate anchoring groups and Re(bpy)(CO)3Cl-type complex was doped on the MOF giving Zr-MOF/Zn(por)/Re composite. Visible-light (lex > 500 nm) irradiation of Zr-MOF/ Re Zn(por) Zn(por)/Re composite for 59 h in the presence of BIH produced primarily CO (TONCO ¼ 1893 and TONCO ¼ 25).61 Based on the results and investigations as described above, the photosensitizer and/or catalyst unit(s) can be exchanged from ruthenium(II) and rhenium(I) complexes to construct various efficient supramolecular photocatalysts and develop supramolecular photocatalysts with additional functions (Chart 5). In the case using an osmium(II) tris-diimine complex as a photosensitizer unit instead of the ruthenium(II) analogue, the osmium(II)-rhenium(I) binuclear complex (SP9) photocatalyzed CO2 reduction to CO ^ even under red-light irradiation (lex > 620 nm) in the presence of BIH (FCO ¼ 0.12, TONCO ¼ 1138).62 [Os(N N)3]2þ-type complexes absorb longer-wavelength light (labs < 730 nm) owing to the direct transitions from the ground state to the 3MLCT excited states (S–T transitions) induced by the heavy-atom effect of osmium(II), whereas [Ru(N^N)3]2þ complexes cannot absorb light at labs > 550 nm. In addition, the lifetime of [Os(N^N)3]2þ in its excited state was several tens of nanoseconds, and its first reduction potential was similar to that of the ruthenium(II) analogs. Therefore, [Os(N^N)3]2þ can serve as a photosensitizer at longer wavelengths than [Ru(N^N)3]2þ.

N N Os N N N N

Cl

P CO N Re N CO P

SP9

Cl

N Ir N N N

3+ 3

SP10

3

+

Br CO N Re N CO CO

6+

N N N Ru N N N

+

Ph N N

O

N Zn

Ph

N Ph

N H

SP11

N CO N Re N CO CO

Br(CO)3Re

SP12 Chart 5

N N Ru N N N N

N N Zn N N

Structures and symbolisms of supramolecular photocatalysts.

N N

N N N Ru N CO CO

SP13

Photochemical CO2 reduction

309

[Ir(C^N)2(N^N)]þ-type complexes (C^N ¼ cyclometalated ligand) can absorb visible light, their lifetimes in their excited states are long (several microseconds), and their reduction potentials are similar to that of [Ru(N^N)3]2þ; however, their oxidation potentials are more positive than that of [Ru(N^N)3]2þ. Therefore, [Ir(C^N)2(N^N)]þ-type complexes can also serve as photosensitizers. The Ir(III)-Re(I) binuclear complex, SP10, selectively photocatalyzed CO2 reduction to CO in the presence of BNAH (FCO ¼ 0.21, TONCO ¼ 130) or BIH (FCO ¼ 0.41, TONCO ¼ 1700).63 When [Ru(N^N)3]2þ was used as a photosensitizer, as described at Eq. (6), long irradiation often generates a ligand-substituted product, [Ru(N^N)2(solvent)2]2þ, which can serve as a CO2 reduction catalyst. In such cases, it should be difficult to evaluate the real catalysis of the added catalyst in the photocatalytic system because of the additional catalytic activity of [Ru(N^N)2(solvent)2]2þ. Conversely, [Ir(C^N)2(N^N)]þ photosensitizers do not form other iridium(III) complexes that can serve as catalysts even after long-time irradiation. Therefore, from the viewpoint of evaluating product distribution in photocatalytic systems, [Ir(C^N)2(N^N)]þ are superior photosensitizers to [Ru(N^N)3]2þ. Several researchers have used metalloporphyrins and chlorophylls with strong absorption capacity in the visible region (Soret and Q bands) as photosensitizer units in combination with rhenium(I) catalyst units. Perutz and co-workers reported that SP11, in which the zinc(II) porphyrin and rhenium(I) complex are connected via an eNHC(O)CH2e chain, photocatalyzed CO2 reduction to CO using TEOA as a sacrificial electron donor (TONCO ¼ 332).64 The photocatalytic activity of SP11 was superior to that of a mixed system of zinc(II) tetraphenylporphyrin and fac-[Re(bpy)(CO)3(3-picoline)]þ (TONCO ¼ 103). Recently, Kuramochi, Satake and co-workers reported a unique supramolecular photocatalyst featuring a zinc(II) porphyrin photosensitizer unit (SP12).65 Visible-light irradiation of SP12 in the presence of BIH produced CO selectively with a high durability (TONCO ¼ 1300). Because fac-Re(N^N)(CO)3Br was directly connected at the meso-position of zinc(II) porphyrin, the heavy-atom effect of rhenium(I) induced strong spin-orbit coupling to accelerate intersystem crossing from the singlet to the triplet excited states of the zinc(II) porphyrin photosensitizer unit. In this case, the electron transfer between the photosensitizer and the catalyst and the photophysical properties of the photosensitizer unit were improved by the covalent bond between the photosensitizer and catalyst units. Ruthenium(II) bis-carbonyl complexes have also been used as catalyst units of supramolecular photocatalysts instead of rhenium(I) catalyst units. The multinuclear complexes comprising [Ru(N^N)3]2þ photosensitizer and [Ru(N^N)2(CO)2]2þ catalyst units, with [Ru(N^N)3]2þ:[Ru(N^N)2(CO)2]2þ ratios of 2:1 or 1:1 served as effective supramolecular photocatalysts; moreover, the complex with a [Ru(N^N)3]2þ:[Ru(N^N)2(CO)2]2þ ratio of 2:1 (SP13) exhibited superior photocatalytic activity. HCOOH was produced with high selectivity by irradiating a DMF–TEOA (4:1 v/v) solution containing SP13 and BNAH under a CO2 atmosphere (FHCOOH ¼ 0.041, TONHCOOH ¼ 562, TOFHCOOH ¼ 7.8 min1).66 Conversely, the mixed systems of PS11 and [Ru(dmb)2(CO)2]2þ with PS11:[Ru(dmb)2(CO)2]2þ ratios of 1:2 or 1:3 were deactivated rapidly during photocatalytic reactions, especially at high [Ru(N^N)2(CO)2]2þ concentrations (typically > 0.3 mM). The deactivation of photocatalyses was caused by the changes in catalyst structure, namely dissociation of one dmb ligand and subsequent formation of polymer with RueRu bonds, [Ru(N^N)(CO)2]n. Polymer formation was completely suppressed for SP13. The photocatalytic activity of SP13 was significantly improved by using 1,3-dimethyl-2-(o-hydroxyphenyl)-2,3-dihydro-1H-benzo[d]imidazole as a sacrificial electron donor instead of BNAH (FHCOOH ¼ 0.46, TONHCOOH ¼ 2766, TOFHCOOH ¼ 45 min1).67 Although the photocatalytic properties of supramolecular photocatalysts are superior to those of the mixtures of corresponding mononuclear model complexes, significant synthetic efforts should be dedicated to preparing such multinuclear complexes. Recently, systems featuring non-covalent linkages, such as hydrogen bonds or coordinative interactions, have been used for the construction of supramolecular photocatalysts (Chart 6). For example, Kubiak and co-workers introduced amide substituents into the diimine ligand of [Ru(N^N)3]2þ and fac-Re(N^N)(CO)3Cl. The photocatalytic properties of the mixed systems comprising these complexes, which photocatalyzed CO2 reduction to CO in the presence of BIH (FCO ¼ 0.23 and TONCO ¼ 100) were superior to those of the mixed system comprising PS1 and fac-Re(dac)(CO)3Cl (dac ¼ 4,40 -bis(methylacetamidomethyl)-2,20 -bipyridine) (FCO ¼ 0.07, TONCO ¼ 28).68 The hydrogen bonds between the amide groups formed a heterodimer (SP14) to facilitate photocatalysis. The association constant for the formation of SP14 in acetonitrile was estimated to be K ¼ 300 M1. For this system, the in situ assembly method did not work uniquely and the possibility of forming [(bpy)2Ru(dac)$$$(dac)Ru(bpy)2]4 þ and

+

2+

N N Ru N N N N

O

N Ir N N N

N H N NH O O HN H N O

Cl CO N Re N CO CO

N N N Co N N N N

SP14 Chart 6

N N

Structures and symbolisms of supramolecular photocatalysts without covalent bonding.

SP15

310

Photochemical CO2 reduction

[fac-Re(dac)(CO)3Cl$$$fac-Re(dac)(CO)3Cl] homodimers cannot be excluded. Ouyang and co-workers used coordination interactions between an iridium(III) photosensitizer with pyridine pendants as free ligands and a cobalt(II) phthalocyanine catalyst with vacant sites in the axial positions. The binding constant for the formation of the SP15 heterodimer in DMF-d7 was estimated to be K ¼ 2362 M1. Visible-light irradiation of an acetonitrile-TEA-phenol solution containing SP15 and BIH produced CO with high selectivity and efficiency (FCO ¼ 0.10, TONCO ¼ 391).69 The photocatalytic activity of SP15 was superior to that of the mixed system of [Ir(ppy)2(bpy)]þ and cobalt(II) phthalocyanine without coordinating interactions (FCO ¼ 0.024, TONCO ¼ 152).

8.07.3

Hybrid systems comprising metal complexes and solid materials

As described in the previous section, some photocatalytic systems constructed solely from metal complexes exhibit fascinating selectivity, durability, and CO2 reduction efficiency. However, from a practical viewpoint, some drawbacks must be overcome. They are as follows. 1) Weak absorption ability of the molecules, even metal complexes: light-harvesting systems such as green plants are required. 2) Weak oxidation power: water, as an abundant resource, should be used as an electron donor for CO2 reduction in the future. This requires the complete four-electron oxidation of two molecules of water, which is a challenging task for metal-complex photocatalysts because one-photon excitation of molecules induces only one-electron transfer. 3) Separation of the reduction and oxidation products: reoxidation of reduction products and reduction of oxidation products should be suppressed. Recently, hybrid systems of meta-complex photocatalyst/catalysts with solid materials have been developed to address these deficiencies.

8.07.3.1

Hybrids with light-harvesting materials

Hybrids comprising metal-complex photocatalytic systems for CO2 reduction and periodic mesoporous organosilica (PMO) as light-harvesting materials were reported by Inagaki, Ishitani, and co-workers70 The Ru(II)eRe(I) supramolecular photocatalyst (SP16 in Fig. 2) was fixed in the mesopore of PMO, and numerous acridone groups were embedded in the silica framework as visible-light absorbers. The embedded acridone groups absorbed visible light, and the excitation energy was funneled to the Ru photosensitizer unit. The energy accumulation led to electron transfer and catalytic reduction of CO2 to CO on the Re catalyst unit. The light-harvesting ability of this hybrid enhanced the photocatalytic CO evolution rate by up to a factor of ten compared with that of SP16 adsorbed on mesoporous silica without a light harvester.

8.07.3.2

Hybrids with semiconductors

Hybrid systems consisting of semiconductor particles/electrodes and metal-complex photocatalysts/catalysts can be categorized into three types: (Type 1) [semiconductor photocatalyst þ metal-complex catalyst], in which only the semiconductor initiates photochemical electron transfer. (Type 2) [organic-dye photosensitizer þ semiconductor þ metal-complex catalyst], in which only the organic-dye photosensitizer absorbs light, and photochemical electron transfer from the excited photosensitizer to the catalyst proceeds through the conduction band of the semiconductor.

Fig. 2

Light-harvesting photocatalyst comprising PMO and SP16.

Photochemical CO2 reduction

311

(Type 3) [supramolecular photocatalyst þ semiconductor photocatalyst], in which step-by-step excitation of both the semiconductor and the supramolecular photocatalyst, that is, Z-scheme electron transfer, occurs. Sato and Morikawa et al. first reported a Type 1 photocatalytic system: N-doped Ta2O5 particles (N-Ta2O5) linked with [Ru(dcbpy)2(CO)2]2þ complexes (4,40 -dicarboxy-2,20 -bipyridine) as a catalyst (Fig. 3).71 The excitation of N-Ta2O5 caused electron injection from its conduction band to the Ru catalyst to proceed with CO2 reduction to HCOOH with H2 and CO as minor products. The hole produced in the valence band of N-Ta2O5 oxidized a sacrificial electron donor (TEOA). TONHCOOH ¼ 89 and FHCOOH ¼ 1.9% were reported. Maeda and co-workers reported similar systems, including carbon nitride polymer (C3N4, semiconductor) with Ru(dpbpy)(CO)2Cl2 (dpbpy ¼ 4,40 -(CH2PO3H2)2–2,20 -bipyridine), wherein the main product was HCOOH (selectivity ¼ 80%) with a higher TON (> 1000) and F (5.7%).72 The deposition of metallic Ag nanoclusters on C3N4 nanosheets with Ru(dpbpy)(CO)2Cl2 increased the durability of the Ru catalyst (TONHCOOH ¼ 5775, F ¼ 4.2%).73 Recently, Suzuki, Morikawa, Kudo, and co-workers successfully constructed a photocatalytic system for CO2 reduction with water oxidation using the Type 1 photocatalyst [Ru(dcbpy)2(CO)2]2þ/(CuGa)1-xZn2zS2 coupled with another semiconductor, BiVO4, which photocatalytically oxidizes water to O2.74 They explained that the excitation of both semiconductors causes Z-scheme electron transfer from BiVO4 to an electron mediator [Co(tpy)2]3 þ, and then the produced [Co(tpy)2]2þ injects the electron into the valence band of the excited (CuGa)1-xZn2zS2. The electron transfer from the conduction band of (CuGa)1-xZn2zS2 to the Ru catalyst causes CO2 reduction to HCOOH (Fig. 4). Arai, Sekizawa, Morikawa and co-workers applied the Type 1 system as a photocathode to a photoelectrochemical cell for CO2 reduction, which can be coupled with an n-type semiconductor photoanode for water oxidation. The polypyrrole polymer (polyPyr) incorporated with the catalyst Ru(polyPyr-bpy)(CO)2Cl2 was attached to the surface of a p-type semiconductor, InP75 or N,Zncodoped Fe2O3 with a multiheterojunction structure (TiO2/N,Zn-Fe2O3/Cr2O3)76 was used as the photocathode, and reduced SrTiO3 (SrTiO3  x) was used as the photoanode (Fig. 5). The semiconductor moieties in both electrodes were irradiated in an aqueous solution under a CO2 atmosphere to give reduction products of both CO2 (mainly HCOOH) and O2; the conversion efficiency of light to chemical energy was 0.14% (InP) and 0.15% (TiO2/N,Zn-Fe2O3/Cr2O3). Son, Pac, Kang, and co-workers reported Type 2 systems in which the excited organic dye injects an electron into the conduction band of the TiO2 particle, and subsequently, the electron transfer proceeds to the catalyst attached to the same TiO2, that is, Re(dpbpy)(CO)3Cl (TONCO  435)77 or Mn(dpbpy)(CO)3Br (TONHCOOH  250: Fig. 6)78. In Type 3 systems, both the photosensitizer and catalyst units of the metal complexes should be fixed on the surface of the semiconductor materials. Controlling the distances between these two components is difficult in cases using two types of photosensitizer and catalyst molecules. In contrast, the photosensitizer and catalyst units in the supramolecular photocatalysts are connected to each other. To clarify the differences in photocatalysis systems, the mixed photocatalytic system and supramolecular photocatalyst with methyl phosphonic acid groups as anchors were fixed on the insulator Al2O3 particles, and the suspension of these solid materials was irradiated in N,N-dimethylacetamide (DMA)-TEOA mixed solutions containing BNAH as a reductant, which is the

Fig. 3 Type 1 photocatalytic system comprising N-Ta2O5 and Ru(dcbpy)(CO)2Cl2. Reprinted from reference Sato, S.; Morikawa, T.; Saeki, S.; Kajino, T.; Motohiro, T. Angew. Chem. Int. Ed. 2010, 49, 5101–5105 with permission. Copyright 2010, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

312

Photochemical CO2 reduction

Fig. 4 Type 1 Z-scheme electron transfer in [Ru-complex/(CuGa)1-xZn2xS2þ [Co(tpy)2]2þ þ BiVO4]. Reproduced from reference Suzuki, T. M.; Yoshino, S.; Takayama, T.; Iwase, A.; Kudo, A.; Morikawa, T. Chem. Commun. 2018, 54, 10199–10202 with permission. Copyright 2018, Royal Society of Chemistry.

Fig. 5 Type 1 photoelectrochemical cell. Reprinted from reference Sekizawa, K.; Sato, S.; Arai, T.; Morikawa, T. ACS Catal. 2018, 8, 1405–1416 with permission. Copyright 2018, American Chemical Society.

Fig. 6 Type 2 system: Organic dye þ TiO2 þ Mn(I) catalyst. Reprinted from reference Woo, S.-J.; Choi, S.; Kim, S.-Y.; Kim, P. S.; Jo, J. H.; Kim, C. H.; Son, H.-J.; Pac, C.; Kang, S. O. ACS Catal. 2019, 9, 2580–2593 with permission. Copyright 2019, American Chemical Society.

one-electron donor, as described in Section 8.07.2.2, under CO2 atmosphere.79 In systems using the Ru(II)eRe(I) supramolecular photocatalyst (SP16 in Fig. 7), photocatalytic CO formation proceeded even when the average distance between supramolecular photocatalysts (6.3 nm, Fig. 7A) was considerably longer than the size of the molecule (1.2  2.4 nm, Fig. 7B). In the case of the system using the Ru(II) and Re(I) mononuclear complexes, the produced CO was much less, and photocatalytic reduction of CO2 did not proceed when the average distance between the photosensitizer and catalyst was 6.3 nm. Therefore, the supramolecular photocatalyst works significantly better than the mixed system, particularly on the surface of solid materials. Domen, Ishitani, and co-workers first reported a hybrid system comprising Ag-loaded TaON particles (Ag/TaON) and a Ru(II) eRu(II) supramolecular photocatalyst with methyl phosphonic acid groups (SP17 in Fig. 8). Step-by-step excitation of both the Ag/ TaON and Ru(II) photosensitizer unit of the supramolecular photocatalyst induces a Z-scheme type electron transfer that induces

Photochemical CO2 reduction (A)

313

(B)

Fig. 7 Average distance between SP16 molecules on the surface of Al2O3 particles and the size of the SP16 molecule. Reproduced from reference Saito, D.; Yamazaki, Y.; Tamaki, Y.; Ishitani, O. J. Am. Chem. Soc. 2020, 142, 19249–19258 with permission. Copyright 2020, American Chemical Society.

selective CO2 reduction to HCOOH (Fig. 8).80 Maeda, Ishitani, and co-workers developed hybrid systems of various semiconductor particles with the same supramolecular photocatalyst,81 and reported high selectivity for HCOOH formation and significantly high durability of photocatalysis in a system using a hybrid consisting of mesoporous graphitic carbon nitride loaded with both Ag nanoparticles (Ag/C3N4) as the semiconductor and SP17: TONHCOOH > 33000.82 Abe, Ishitani, and co-workers synthesized a dye-sensitized molecular photocathode in which SP16 was adsorbed on a p-type semiconductor NiO electrode (NiO-SP16).83 Visible-light irradiation to this photoelectrode with external bias at  0.7 V vs. Ag/ AgNO3 induced selective CO2 reduction to CO. This dye-sensitized molecular photocathode was coupled with a photoanode constructed with the n-type semiconductor TaON incorporating cobalt oxide as a co-catalyst for water oxidation (CoOx/TaON). Both electrodes were irradiated with visible light in an aqueous solution under a CO2 atmosphere to give CO and O2 simultaneously in the photocathode and photoanode compartments, respectively (Fig. 9).84 Excitation of TaON causes electron transfer into NiO, and excitation of the Ru(II) photosensitizer unit causes electron transfer from the valence band of NiO to the excited photosensitizer unit (Z-scheme electron transfer). Water oxidation and CO2 reduction proceed on CoOx and at the Re(I) catalyst unit, respectively. This is the first example of a molecular photocatalyst that reduces CO2 with water as a reductant and visible light as the energy source. However, external bias to the photocathode was required and detachment of SP16 from the NiO electrode was an additional problem; the photocatalysis of this photoelectrode could only be maintained for several hours, indicating low TONCO (Fig. 9). The low light absorption ability due to the presence of only a single molecular layer should also be solved to improve the light-energyconversion efficiency (h). A no-external-bias photocatalytic system was developed using another p-type semiconductor electrode, CuGaO2, which has a lower valence band potential than NiO.85 The introduction of a polymer layer containing Ru(II) photosensitizer units improves the performance with respect to the other two issues. The dye-sensitized molecular photocathode consisted of a polyethylene layer containing both a Ru(II) photosensitizer and Ru(II) catalyst units on the NiO electrode, which was produced by reductive polymerization of the vinyl groups in the bipyridine ligand of the photosensitizer unit, photocatalyzed CO2 reduction at a more positive applied potential with significantly higher efficiency and durability. Its photocatalysis also continued for longer than 1 day.86 This dye-sensitized molecular photocathode was combined with a CoOx/BiVO4 photoanode to photocatalyze CO2 reduction with water oxidation using only visible light as energy without any external bias; the conversion efficiency of light to chemical energy was 0.017% (Fig. 10). Oxidative polymerization of pyrrole groups in the bipyridine ligand, instead of the reductive polymerization of the vinyl groups, was also successfully applied to construct another dye-sensitized molecular photocathode. In the full-cell system constructed with this photocathode and the CoOx/BiVO4 photoanode, a higher conversion efficiency of light to chemical energy was obtained (0.083%).87

Ag

eCH3OH HCHO Fig. 8

e-

e-

Type 3 hybrid photocatalyst comprising SP17 and Ag/TaON.

e-

HCOOH CO2

314

Photochemical CO2 reduction

Fig. 9 Type-3 photoelectrochemical cell: SP17 on NiO þ CoOx/TaON. Reproduced from reference Sahara, G.; Kumagai, H.; Maeda, K.; Kaeffer, N.; Artero, V.; Higashi, M.; Abe, R.; Ishitani, O. J. Am. Chem. Soc. 2016, 138, 14152–14158 with permission. Copyright 2016, American Chemical Society.

Fig. 10 Type-3 photoelectrochemical cell: Ru(II) complexes in a polyethylene layer on NiO þ CoOx/BiVO4. Reproduced from reference Kamata, R.; Kumagai, H.; Yamazaki, Y.; Higashi, M.; Abe, R.; Ishitani, O. J. Mater. Chem. A 2021, 9, 1517–1529 with permission. Copyright 2021, Royal Society of Chemistry.

Photochemical CO2 reduction

8.07.4

315

Conclusion and future perspective

Various photocatalytic systems for CO2 reduction have been developed over the last 10 years. In particular, some photocatalysts consisting of metal complexes show extremely high efficiency (up to 82% quantum yield), high durability (higher than 3000 turnover number), and high selectivity for CO2 reduction. However, there are many features that metal-complex photocatalysts should have for real applications, such as: light-harvesting ability, use of water as the reductant, reconciliation of oxidation and reduction powers, and direct use of exhaust gases containing low concentrations of CO2 and contaminated molecules such as NOx and SOx. As described in Sections 8.07.3.1 and 8.07.3.2, some new systems and hybrids of metal-complex(es) with solid materials have been developed to add one or two of these necessary features. However, for the practical applications of photocatalytic systems for CO2 reduction, all these features should be combined in one system. New chemical engineering is also necessary for practical artificial photosynthesis to mitigate the serious and difficult problems faced by humans.

Acknowledgments The authors would like to thank Professors Akihiko Kudo (Tokyo University of Science) and Yutaka Amao (Osaka City University) for their useful suggestions. JSPS are acknowledged for financial support (KAKENHI Grant Numbers JP20K20367, JP17H06440, and JP20H00396).

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8.08

Water oxidation catalysis in natural and artificial photosynthesis

Yu Guo , Alexander Kravbergc,*, and Licheng Suna,b,c, a Institute of Natural Sciences, Westlake Institute for Advanced Study, Hangzhou, China; b Center of Artificial Photosynthesis for Solar Fuels, School of Science, Westlake University, Hangzhou, China; and c Department of Chemistry, School of Engineering Sciences in Chemistry, Biotechnology and Health, KTH Royal Institute of Technology, Stockholm, Sweden a,b,*

© 2023 Elsevier Ltd. All rights reserved.

8.08.1 8.08.2 8.08.2.1 8.08.2.1.1 8.08.2.1.2 8.08.2.1.3 8.08.2.1.4 8.08.2.2 8.08.2.2.1 8.08.2.2.2 8.08.2.2.3 8.08.2.3 8.08.2.3.1 8.08.2.3.2 8.08.2.3.3 8.08.2.4 8.08.2.4.1 8.08.2.4.2 8.08.2.4.3 8.08.2.4.4 8.08.2.4.5 8.08.2.4.6 8.08.3 8.08.3.1 8.08.3.1.1 8.08.3.1.2 8.08.3.2 8.08.3.3 8.08.3.3.1 8.08.3.3.2 8.08.3.3.3 8.08.3.3.4 8.08.3.4 References

Introduction Natural water oxidation Photosynthesis, photosystem II, and oxygen-evolving complex Photosynthesis Photosystem II (PSII) Oxygen-evolving complex (OEC) Crystallographic structures The Kok cycle, oxidation state schemes and the structural flexibility The Kok cycle Oxidation state schemes Structural flexibility Substrate-water exchange, substrate identifications, and water delivery channels Substrate-water exchange Substrate identifications Water delivery channels Mechanisms of the S-state transitions and OeO bond formation S0 / S1 S1 / S2 S2 / S3 S3 / S4 S4 / S0 Mechanism of OeO bond formation Artificial water oxidation Development of artificial water oxidation catalysts Molecular catalysts Material catalysts Water oxidation mechanisms Artificial water oxidation catalyst design Role of metal First coordination sphere Second coordination sphere Microscopic environment Artificial analogs of OEC

318 318 318 318 318 320 320 321 321 323 324 326 326 326 329 330 330 331 331 332 333 333 337 337 337 339 341 343 344 345 346 346 347 348

Abstract Energy shortage and environmental pollution limit the sustainable development of human beings. Water, as a clean and renewable resource, provides a solution for sustainable energy conversion from water oxidation catalysis. Lessons should be learned from nature to explore efficient artificial catalysts. In this chapter, we will review recent major progress in natural photosynthesis, from the structure and functions to the mechanisms for the water-oxidizing center of the biological enzyme. Later, the development of molecular and material water oxidation catalysts is discussed, with a focus on the mechanisms and rational catalyst design.

*

These authors are contributed equally.

Comprehensive Inorganic Chemistry III, Volume 8

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Water oxidation catalysis in natural and artificial photosynthesis

8.08.1

Introduction

With the rapid consumption of fossil energy, energy shortage has been the primary problem that restricts human survival and development. Solar energy is the only clean energy that can be used sustainably in the future and how to take its advantage in industrial catalysis has become a hot topic in the current international academia. If we use solar energy to split water into hydrogen and oxygen under the action of catalysts, a large amount of clean hydrogen energy can be provided for human beings, so as to get rid of our dependence on fossil energy, and reach an environmentally friendly way for production and life. Consequently, the research and development of water catalysts play an important role in the conversion and utilization of solar energy.1,2 Enzyme is the most efficient catalyst in nature, which plays an irreplaceable role in human metabolism, plant photosynthesis, and biological nitrogen fixation, etc. Compared with the traditional homogeneous and heterogeneous catalysis, the enzyme reactions, especially metalloenzymes, show the advantages of high activity, specific substrate selectivity, and mild reaction conditions. Over the past decades, the importance of bio-inspired catalysis has been greatly concerned, as well as the related syntheses and applications.3,4 Artificial photosynthesis, as the typical representative in the field of water splitting, simulates the plant photosynthesis to produce clean fuels using sunlight instead of electricity. People are committed to solving the process of transforming carbon dioxide and water into organic matter and releasing hydrogen and oxygen, and building a kind of artificial leaf, or even a more optimized device, aiming for a more efficient solar energy conversion equipment.5–7 It is supposed to be an attractive solution to the world’s resource consumption and environmental pollution. However, improvement of the photocatalytic efficiency is still challenging, which, to a great extent, depends on the properties of catalysts. Boosting the catalytic performance and the energy conversion efficiency require ingenious preparations of the catalysts. One effective rule for artificial synthesis is to maximize the similarity to nature’s water oxidation enzyme, not only for the structure and composition of the active site, but also its operating mechanism.8 It can be seen that learning from nature reflects the basic idea of bio-inspired catalysis. In this chapter, we will review the recent progress of water oxidation in natural and artificial photosynthesis with an emphasis on the mechanisms. For the natural water oxidation, we will focus on the core issues and present five main sections, including a brief introduction of photosynthesis, photosystem II, oxygen-evolving complex, and crystallographic structures; the Kok cycle, oxidation state schemes and the structural flexibility; substrate identifications, water delivery, and substrate-water exchange; mechanisms of the S-state transitions and OeO bond formation. For the artificial photosynthesis, we will first briefly outline the history of artificial water oxidation catalysts, marking the milestones that defined the course of their development, then present the known mechanisms of water oxidation with artificial catalysts, and finally discuss the key features of catalyst design. The chapter will be concluded with a section, describing artificial models of natural oxygen-evolving cluster.

8.08.2

Natural water oxidation

8.08.2.1

Photosynthesis, photosystem II, and oxygen-evolving complex

8.08.2.1.1

Photosynthesis

Photosynthesis usually refers to the process that green plants, algae or bacteria absorb light energy, synthesize carbon dioxide and water into energy-rich organic substances and release molecular oxygen at the same time: light

CO2 þ H2 O / ½CH2 O þ O2

(1)

This is known as ‘the most important chemical reaction on the earth’, which is the basis of the survival and development of all lives. In history, eight Nobel prizes have been awarded to scientists engaged in photosynthesis research. The isotope labeling method showed that O2 fully comes from H2O, while CO2 is converted into [CH2O]. In terms of energy conversion, light energy is converted into chemical energy and stored in saccharides. Photosynthesis can be divided into two stages: the light and dark reactions. The light reaction takes place in the thylakoid membranes of chloroplasts. Herein, water is split for the subsequent production of nicotinamide adenine dinucleotide phosphate (NADPH) and adenosine triphosphate (ATP), in addition to O2 release; The dark reaction, also known as the Calvin cycle, continuously consumes NADPH and ATP for carbon fixation and provides raw materials for the light reaction in turn (Fig. 1). Both stages, driven by various enzymes, constitute the whole photosynthetic electron transfer chain.9–11 Photosynthesis, as a common and fundamental botanic physiological phenomenon, constitutes the material basis for all the metabolic processes on earth, maintaining biological subsistence and normal development of human life activities.

8.08.2.1.2

Photosystem II (PSII)

PSII is a homodimer transmembrane protein with more than 40 subunits locating in the thylakoid membranes, the major place for the light reaction. It uses sunlight to split water into protons, electrons and oxygen (2H2O / 4Hþ þ 4e þ O2). The specific process is as follows: the antenna molecules absorbing photons transfer energy to chlorophyll P680 in the light reaction center, which will be electronically excited to become P680$þ due to charge separation. Then its electrons will be gradually delivered to pheophytin, plastoquinone QA and then QB. After twice charge separations, QB is reduced and hydrogenated to QBH2, which $þ is a strong biological oxidant, whose electron hole is will leave PSII and enter cytochrome b6f for subsequent reactions. P680 $ $ compensated by D1-Tyr161 (YZ), oxidizing YZ to YZ radical. Next, YZ will abstract electrons from the oxygen evolving complex and increase its oxidation states before water is oxidized and decomposed. The charge separation events caused by four consecutive

Water oxidation catalysis in natural and artificial photosynthesis

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Fig. 1 A diagram of material transfer in photosynthesis. Reproduced with permission from Ref. Li, D.; Li, W.; Zhang, H.; Zhang, X.; Zhuang, J.; Liu, Y.; Hu, C.; Lei, B. Far-red Carbon Dots as Efficient Light-Harvesting Agents for Enhanced Photosynthesis. ACS Appl. Energy Mater. 2020, 12 (18), 21009–21019. Copyright 2020 American Chemical Society.

illuminations make the oxygen-evolving complex accumulate enough oxidation equivalent to split water.12,13 Therefore, PSII is also called ‘photo-driven water plastid quinone oxidoreductase’, as shown in Fig. 2.

Fig. 2 (A) A sectional view of PSII and (B) electron transport route in the light reaction. Reproduced with permission from ref. Cox, N.; Pantazis, D. A.; Neese, F.; Lubitz, W. Biological Water Oxidation. Acc. Chem. Res. 2013, 46, 1588–1596. Copyright 2013 American Chemical Society.

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8.08.2.1.3

Oxygen-evolving complex (OEC)

The OEC is the catalytic active center embedded in PSII. It is composed of a manganese-calcium cluster (Mn4CaO5) and its surrounding water and protein environment, responsible for water splitting and oxygen evolution. X-ray crystal structure shows the dark stable S1 state cluster is made up of five bridging groups O2 connected with four Mn ions and one Ca ion, and W1/ W2 and W3/W4 are the ligands of Mn4 and Ca, respectively. The magnetic Mn is the direct oxidant of water and responsible for electron transfer; Ca has little contribution to the electronic and geometric structure of Mn clusters, but it could regulate the hydrogen bond network and proton-coupled electron transfer (PCET) between the Mn cluster and YZ$, and may act as the binding site of substrate water. The metals are connected by oxygen bridges and the strong covalent interactions maintain the spatial structure of Mn cluster, in which O5 may be one of the substrates. The Mn4CaO5 cluster is coordinated by seven first-layer amino acid ligands, namely, six carboxyl ligands D1-Asp170 (Mn4/Ca), D1-Glu189 (Mn1), D1-Glu333 (Mn3/Mn4), D1-Asp342 (Mn1/Mn2), D1-Ala344 (Mn2/Ca), CP43-Glu354 (Mn2/Mn3) and an imidazole ligand D1-His332 (Mn1). In this way, all Mn ions are six coordinated and the Ca ion are seven coordinated, and they together constitute the basis of the special electronic and geometric structure of the cluster.14,15 There are three essential amino acid ligands in the second layer, D1-Asp61, D1-His337 and CP43-Arg357, forming hydrogen bonds with the bridging oxygens or ligand water of the cluster and playing an important role in stabilizing the geometry, balancing charge or transporting protons and waters during the water oxidation catalysis (Fig. 3). Besides, the hydrogen-bonded pair D1-Tyr161/D1-His190 near Ca participates in PCET during the redox processes.15–17 Site-directed mutagenesis studies showed that replacement or elimination of some amino acids would seriously affect the O2 release activity.12 The two distal chloride ions could affect the pKa of the surrounding groups through hydrogen bonds, maintaining the architecture of the proton transfer channel.18 In addition, the crystal water molecules in the environment and other peripheral proteins together preserve the activity of oxygen evolution.

8.08.2.1.4

Crystallographic structures

Only with a good understanding of the structure of the macromolecular complexes at atomic resolution level, can its function be better understood. The fundamental step to make acquaintance of PSII/OEC should be getting its clear three-dimensional structural image from the biological organism. Almost two decades have been spent to refine the crystal structure (mostly from Thermosynechococcus elongatus) with increasing resolution.14,19–29 The traditional method of crystallography is X-ray diffraction (XRD), for which in 2004 Barber and coworkers were the first to discover the cluster as ‘oxo-bridged Mn3CaO4 unit linked to a distant dangling Mn’, though with 3.5 Å resolution.23 In 2011, Shen and coworkers promoted the resolution to 1.9 Å and found an additional oxygen bridge (named ‘O4’) linking Mn3 and Mn4 in the dark-stable S1 state (For the concept of ‘S state’, see Section 8.08.2.2.1).14 However, the obtained data suffers from radiation damage which unavoidably causes reduction of Mn and elongation of Mn-ligand distances. This undoubtfully leads to some proportion of unexpected mixture of lower S states. To ameliorate this disadvantage, X-ray free-electron laser (XFEL) technique has been adopted to get ‘radiation-damage-free’ PSII from 2011 by Shen and coworkers, who have made landmark contributions. Interestingly in 2015, an extra oxygen ligand (named ‘O6’) bound to Mn1 was observed in the S3 state (compared with the S1 state) but with a bonded distance of 1.46 Å to the bridging O5,21 which implies peroxide formation before the S4 arrival and gave rise to disputes about the nature of the S3 state. The quality is improving year by year (Fig. 4) although it is argued that perfect purification radiation-damage-free state is still not reached.26 In 2018, Yano and coworkers determined structures of all the (meta)stable intermediates (S0, S1, S2, S3) of the Kok cycle by timeresolved XFEL experiments at room temperature.24 The key O5-Ox (‘Ox’ equivalent to ‘O6’) distance was announced to 2.09 Å in the S3 state, which excludes their bonding feature and confines Ox as hydroxo or oxo (Fig. 5). One year later, Shen and coworkers

Fig. 3 A diagram of the oxygen-evolving complex and its surroundings in the dark-stable S1 state (left: a close-up of the OEC in PSII; right: the Mn cluster and its ligands, adapted with permission from Ref. Umena, Y.; Kawakami, K.; Shen, J.-R.; Kamiya, N. Crystal Structure of Oxygen-Evolving Photosystem II at a Resolution of 1.9 Å. Nature 2011, 473, 55–60. Copyright 2011 Springer Nature Limited).

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Fig. 4 Crystallographic structures of PSII with promoted qualities year by year. Reproduced with permission from Ref. Cox, N.; Pantazis, D. A.; Lubitz, W. Current Understanding of the Mechanism of Water Oxidation in Photosystem II and Its Relation to XFEL Data. Annu. Rev. Biochem. 2020, 89 (1), 795–820. Copyright 2020 Annual Reviews Inc.

published their refined XFEL structures from the S1 to S3 states, which showed 1.90 Å O5–O6 distance in S3 and suggested an oxyl nature for O6, as well as an oxyl/oxo coupling mechanism for OeO bond formation in the S4 state (Fig. 6).19 To sum up, the usage of XFEL technique has made huge progress on the structural characterization on PSII/OEC, whose geometric parameters are greatly refined although probable mixture of lower S states cannot be avoided. For the nature of O6/ Ox in the S3 state, we favor hydroxo or oxo over oxyl, because the latter would cause Mn4 reduction to valence (III), which seems in conflict with electron paramagnetic resonance (EPR) evidence that all Mn are in valence (IV) in the S3 state.30 More experiments and theoretical studies should be carried out to reach a convergence.

8.08.2.2 8.08.2.2.1

The Kok cycle, oxidation state schemes and the structural flexibility The Kok cycle

In 1970, Kok et al. established a classical four-step ‘Si state cycle’ (i ¼ 0–4, five intermediates) on the basis of the flash-induced polarography experiment and outlined the rudiment of photosynthetic oxygen release, laying a foundation for future research in this field.31 Here ‘S’ stands for ‘storage’, which means the gradual accumulation of oxidation equivalent in the OEC. ‘S1’ is the darkstable state in the cycle, and O2 is finally produced during the ‘S4–S0’ transition under continuous illuminations. The whole Sstate cycle is accompanied by the transfer of protons and electrons, and two water molecules are absorbed during the ‘S2–S3’ and ‘S4–S0’ transitions, respectively. Spectroscopic studies show that the S-state transitions cause change of the geometry and

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Fig. 5 XFEL structures of the OEC cluster in the S1, S2, S3 and S0 states at the full 2.07 Å resolution by Yano and coworkers. Reproduced with permission from Ref. Kern, J.; Chatterjee, R.; Young, I. D.; Fuller, F. D.; Lassalle, L.; Ibrahim, M.; Gul, S.; Fransson, T.; Brewster, A. S.; Alonso-Mori, R.; Hussein, R.; Zhang, M.; Douthit, L.; de Lichtenberg, C.; Cheah, M. H.; Shevela, D.; Wersig, J.; Seuffert, I.; Sokaras, D.; Pastor, E.; Weninger, C.; Kroll, T.; Sierra, R. G.; Aller, P.; Butryn, A.; Orville, A. M.; Liang, M.; Batyuk, A.; Koglin, J. E.; Carbajo, S.; Boutet, S.; Moriarty, N. W.; Holton, J. M.; Dobbek, H.; Adams, P. D.; Bergmann, U.; Sauter, N. K.; Zouni, A.; Messinger, J.; Yano, J.; Yachandra, V. K. Structures of the Intermediates of Kok’s Photosynthetic Water Oxidation Clock. Nature 2018, 563, 421–425. Copyright 2018 Springer Nature Limited.

Fig. 6 XFEL structures of the OEC cluster in the S3 state by Shen and coworkers in 2015 (A) and 2019 (B). Reproduced with permission from Ref. Cox, N.; Pantazis, D. A.; Lubitz, W. Current Understanding of the Mechanism of Water Oxidation in Photosystem II and Its Relation to XFEL Data. Annu. Rev. Biochem. 2020, 89 (1), 795–820. Copyright 2020 Annual Reviews Inc.

electronic structure of the OEC, which enables efficient water oxidation in the catalytic cycle.17,32 Dau and coworkers determined the kinetic data of each step and the detailed sequences of proton and electron transfer using photothermal beam deflection (PBD) experiments, and extended the primitive S-state cycle to a process involving nine intermediates.33–37 The updated S-state cycle

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includes not only the four electron transfer steps from the Mn cluster to YZ$, but also transport of the four protons released from the Mn cluster to the protein environment, as shown in Fig. 7. These long-range (above 30 Å) proton transfers are multiple steps participated by water molecular chains and acid-base groups of amino acids, so it cannot occur within the same time scale as electron transfer. The sequence of proton and electron transfer events has been basically formed, while the assignments of the donors and acceptors are still ambiguous, although some assumptions have been made.38 The sequence was formulated based on a large number of experimental facts, yet some details remained to be confirmed. Proton transfer in advance can stabilize the oxidized YZ$ formed later, which may be resulted from decreasing the oxidation potential of the local site and more beneficial with respect to energetics. Still, it is a great challenge to monitor proton transfer and identify the key groups in the process of proton movement using time-resolved X-ray experiments.

8.08.2.2.2

Oxidation state schemes

Although the above-mentioned S-state cycle outlined the basic material change and the sequence of events, it does not directly correlate to the absolute (formal) oxidation states of the four manganese ions. The description of this step-by-step single electron change does not even include any information regarding the specific sites that lead to the increase of oxidation equivalent, and whether ligand oxidation is involved. Different interpretations of the observed data according to various experimental methods, as well as different assumptions of the water oxidation conditions, lead to the contradictory judgment of Mn oxidation states. In general, there are two paradigms in this field, called ‘high-valent’ and ‘low-valent’ schemes,39–42 as shown in Fig. 8. The difference between the two schemes is ‘two net charges’, which will affect analysis of experimental phenomena, electronic structures of the OEC during the S-state cycle and the exploration of water oxidation mechanism. Time dependent density functional theory (TD-DFT) calculations combined with X-ray absorption near edge structure (XANES) spectral analysis support the low-valent scheme.43–45 However, the model used in their study is not completely consistent with the latest crystal structure at atomic resolution level, and the low-valent scheme cannot reproduce the experimentally observed spectral properties for Mn K edge region.42 Recent theoretical calculations revealed the disadvantages of the low-valent scheme: (i) too long interatomic distances between MneMn regarding the S0 and S1 states, deviating too much compared with those measured by the extended X-ray absorption fine structure (EXAFS) and XFEL; (ii) because of the insufficient intensity of antiferromagnetic exchange, the corresponding ground states of S0 and S1 are much higher than experimental values; (iii) poor consistence between the theoretically predicted hyperfine coupling constant (HPC) with experiments, especially for the S3 state; (iv) the predicted XANES spectra of S0–S3 cannot reproduce the trend of Mn K-edge region observed experimentally. On the contrary, the calculated results by high-valent scheme can meet the requirements of structural, magnetic and spectral observations of all S states, and conform to the experimentally measured geometries; it can reflect the consistency between optimal geometries and minimum energies; it obeys the experimentally determined ground spin state; it can explain the bistability of the S2 isomers; the 55Mn HFCs in S0/S2/S3 state were correctly produced; the X-ray absorption spectroscopy (XAS) curve of Kedge region fits experiment.42,46 Cox et al. analyzed the EPR lineshape and 55Mn electron-electron double resonance-detected nuclear magnetic resonance (EDNMR) data and support assignment of the S3 state to an all-octahedral (MnIV)4 complex,30 which corresponds to the high-valent scheme. Recently, Messinger and coworkers using a simple counting experiment showed compelling evidence that high-valent scheme should be dominant in the catalytic cycle.47

Fig. 7 A diagram of the S-state cycle (left: the classical cycle outlined by Kok et al.; right: the extended cycle by Dau et al. with the detailed sequence of proton and electron transfer, adapted with permission from Ref. Mäusle, S. M.; Abzaliyeva, A.; Greife, P.; Simon, P. S.; Perez, R.; Zilliges, Y.; Dau, H. Activation Energies for Two Steps in the S2 / S3 Transition of Photosynthetic Water Oxidation From Time-Resolved SingleFrequency Infrared Spectroscopy. J. Chem. Phys. 2020, 153 (21): 215101. Copyright 2020 American Institute of Physics).

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Fig. 8 The ‘high-valent’ and ‘low-valent’ schemes of Mn oxidation states from S1 to S3 states. Adapted with permission from Ref. Krewald, V.; Retegan, M.; Cox, N.; Messinger, J.; Lubitz, W.; DeBeer, S.; Neese, F.; Pantazis, D. A. Metal Oxidation States in Biological Water Splitting. Chem. Sci. 2015, 6, 1676–1695. Copyright 2015 Royal Society of Chemistry. (Reproduced under Creative Commons Attribution 3.0 Unported Licence).

8.08.2.2.3

Structural flexibility

At room temperature, S1, S2 and S3 may exist in multiple states, whose protonation forms, oxidation state distributions and geometric conformations may be different. Kusunoki proposed a variety of isomers in the S1 state,48 some of which are similar to the high-resolution 1.9 Å crystal structure.14 Pace and Stranger put forward a similar viewpoint in order to explain structural differences between crystal structures with different resolutions.43,45 Renger first proposed the idea of polymorphism for the S3 state, that is, there can be redox equilibrium between Mn ions and substrate water, which leads to formation of peroxide intermediates with tautomeric species.49 Isobe et al. also proposed spin, valence, and structural isomerism in chemical equilibrium models of the S3 state by hybrid DFT calculations,50,51 which has different protonation, charge, spin and conformations being responsible for reversible activation/deactivation processes of substrate oxygen species, and is related to formation of OeO bond in the S4 state (Fig. 9). In recent years, the most significant progress on the structural heterogeneity of the OEC belongs to the theoretical study of the EPRactive S2 state by Pantazis et al., who revealed that generation of the different EPR signals in S2 should be due to the flexible movement of O5, which leads to existence of two isomers: the open-cubane S2-A and closed-cubane S2-B.52 Their electronic and geometric structures are completely different, but their energies are very close (S2-A is more stable by about 1 kcal/mol), and they can convert to each other at room temperature (the barrier is less than 10 kcal/mol). The isomerization of valence state and spin distribution exists because of Mn(III) displacement between Mn1 and the dangling Mn4. Their ground state spin multiplicities and EPR signals are different, i.e., SG ¼ 1/2, g z 2 for S2-A and SG ¼ 5/2, g  4.1 for S2-B, as well as magnetic coupling constants (Fig. 10).53–55 The dynamic equilibrium between the two conformations of the S2 state may be the reason why the exchange rate of the slowexchanging water Ws (supposed to be O5) is much faster than that of any bridging group in common Mn compounds. It also provides a structural basis to explain the abnormal phenomenon that the slow exchange rate of S2 state is about 100 times faster than that of S1 state. Using ab initio molecular dynamics (AIMD), Bovi et al. studied the tautomerism in depth, and explained the dependence of isomer distribution and reaction rate on temperature, light conditions and treatment procedures in low-temperature EPR

Fig. 9 Chemical equilibrium models for the S3 state proposed by Isobe et al. Reproduced with permission from ref. Isobe, H.; Shoji, M.; Suzuki, T.; Shen, J.-R.; Yamaguchi, K. Spin, Valence, and Structural Isomerism in the S3 State of the Oxygen-Evolving Complex of Photosystem II as a Manifestation of Multimetallic Cooperativity. J. Chem. Theory Comput. 2019, 15, 2375–2391. Copyright 2019 American Chemical Society.

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Fig. 10 (A) The two S2 valence isomers found by theoretical calculations; (B) the formal oxidation states, HPC and spin states; (C) the two EPR signals rationalized by the isomers. Adapted with permission from Ref. Orio, M.; Pantazis, D. A., Successes, Challenges, and Opportunities for Quantum Chemistry in Understanding Metalloenzymes for Solar Fuels Research. Chem. Commun. 2021, 57, 3952–3974. Copyright 2021 Royal Society of Chemistry (reproduced from an Open Access Article under Creative Commons Attribution 3.0 Unported Licence.).

experiments.56 Later, Narzi et al. and Retegan et al. found another significance of the S2 tautomerism, that is, the OEC cluster should enter the S3 state in the form of closed-cubane instead of open-cubane, and the incoming water residing in the O4 channel would bind to the five-coordinated Mn4(IV) by the pivot mechanism during the S2–S3 transformation.57,58 It is noted that besides the Pantazis et al.’s valence isomerism, there are two more schemes accounting for the high-spin form of the S2 state: (i) protonation of oxo bridge proposed by Corry et al.59 and (ii) water coordination proposed by Pushkar et al.60 For the former, the high-spin form is created by protonating the O4 oxo bridge of the S ¼ 1/2 conformation, which weakens the J34 antiferromagnetic exchange coupling (while Mn oxidation states remain III–IV–IV–IV) and make a higher spin state become the ground state. For the latter, a high-spin form is attributed to OH binding to Mn1 of the normal S ¼ 1/2 form. Although the three proposals are not mutually exclusive, we prefer the valence isomerism option because only this scenario can accommodate the EPR observation that the high-spin state of S2 places the z-axis of the zero field splitting (ZFS) tensor of Mn(III) explicitly on the Mn4 ion and along Mn4-W1 vector, and produces two almost isoenergetic isomers which can coexist at very low temperatures required by experiments; while the O4 protonation model is strongly disfavored from energetic consideration and water binding model is hardly rationalized in an interconvertible way for isomerization. Very recently, combining X- and Q-band EPR experiments on native and methanol-treated PSII, Pantazis and coworkers resolved a previously uncharacterized high-spin (S ¼ 6) species of the S3 state, which coexists with the previously known intermediate-spin conformation (S ¼ 3) and corresponds to a closed-cubane water-unbound form with a coordinatively unsaturated Mn4(IV),61 as shown in Fig. 11. Shortly later, Pantazis and coworkers proposed orientational Jahn-Teller isomerism in the S1 state, which is consistent with available structural data and can explain the unresolved EPR observations in S1 and is shown to be the electronic form of valence isomerism in the S2 state (Fig. 12).62 It should be pointed out that the structural flexibility of the OEC cluster proposed theoretically (combined with EPR spectroscopic simulations) has not been so far observed by any XFEL crystallographic study (only open-cubane observed), even so in a recent dynamically refined S2–S3 transition conducted by Yano and coworkers,63 who had also declared that the closedcubane structural isomer of the S2 state may not be necessarily existent at room temperature and play a functional role during the catalytic cycle.64 However, in our opinion, the current inconsistency between DFT/EPR and XRD/XFEL should be taken cautiously and a premature negation of the flexibility should not be advisable. The reasons could be as follows: (i) the closedcubane might form but decay quickly before it can be detected; (ii) the fraction is too small to be detected. Consequently, improvement of the experimental technique should be necessary for a final conclusion.

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Fig. 11 (A) The ‘water-unbound’ closed-cubane structure of the S3 state with a 5-coordinated Mn4(IV); (B) Simulation (orange curve) of the Q-band EPR spectrum (black curve) of the S ¼ 6 state in methanol-treated PSII. Adapted with permission from Ref. Zahariou, G.; Ioannidis, N.; Sanakis, Y.; Pantazis, D. A. Arrested Substrate Binding Resolves Catalytic Intermediates in Higher-Plant Water Oxidation. Angew. Chem. Int. Ed. 2021, 60 (6), 3156–3162. Copyright 2021 Wiley-VCH GmbH. (Reproduced from an Open access Article published under Creative Commons CC BY license).

8.08.2.3 8.08.2.3.1

Substrate-water exchange, substrate identifications, and water delivery channels Substrate-water exchange

Time-resolved membrane inlet mass spectrometry (TR-MIMS) is an important experimental method to study the substrate water binding sites. It can monitor the rate change of the isotope-labelled H218O water exchanging to the substrate binding sites of the Mn4CaO5 cluster. After a certain S state (S0, S1, S2, or S3) with new water H218O injected, the timing of occurrence of water exchange can be determined by detecting the radioactivity of O2 molecule by mass spectrometry. Initially the produced O2 is totally made up of 16O16O (32O2), and then 16O18O (34O2) appears as substrate-water is exchanging. At this stage, the oxygen in the fast-exchanging water Wf is converted into one of the oxygens in 34O2. By detecting the fast exchange rate kf of 34O2 formation during this period, it is shown that the two substrate water molecules are bound to the OEC in different ways. And then formation of 18O18O (36O2) indicates that the oxygens in H218O have fully participated in substrate-water exchange, and the slow exchange rate ks can be obtained in the same way as kf (Fig. 13).65 By probing into the sensitivity of ks and kf under Ca/Sr substitution, the information of binding sites of Wf and Ws can be obtained.66 The experiment reveals that the chemical environment of the binding sites of the two substrate waters are quite different. Ws can exchange with the bulk water slowly in all S states, indicating that it is already bound to the OEC in the early stage of S-state cycle (S0). Wf can exchange rapidly in all S states, but only the exchange rates in S2 and S3 states can be measured (Table 1).65,66,68–75 Fourier transform infrared (FTIR) spectra shows that there is one new water molecule binding during the S2–S3 conversion, but this water may not be the substrate of the current cycle, but a structural water molecule instead.76

8.08.2.3.2

Substrate identifications

The mechanism of photosynthetic water splitting is closely associated with identifications of the binding sites of substrate waters. It has been proved that Ws is tightly bound to Mn as revealed by 17O hyperfine spectroscopy experiment on the substrate water

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Fig. 12 (A) The Jahn-Teller isomers in the S1 state proposed by Pantazis and coworkers; (B) simulations of the two types of S1 EPR signal. Adapted with permission from ref. Drosou, M.; Zahariou, G.; Pantazis, D. A., Orientational Jahn–Teller Isomerism in the Dark-Stable State of Nature’s Water Oxidase. Angew. Chem. Int. Ed. 2021, 60 (24), 13493–13499. Copyright 2021 Wiley-VCH GmbH (Reproduced from an Open access Article published under Creative Commons CC BY license).

binding mode of the S2 state.77 The substitution of Ca with Sr can obviously accelerate the exchange rate of Ws in all S states, strongly demonstrating that Ws also binds to Ca in addition to Mn.66,68 In this way, three candidates of Ws can be identified: O1, O2 and O5, which are bridging groups of both Ca and Mn. EDNMR at W-band frequency found that only the oxygen bridge O4 or O5 can exchange with 17O in the buffer solution of H217O within 1 h incubation time.78 Consequently, O5 is the most suitable candidate for Ws, combined with the kinetics from Ca/Sr substitution experiments.79 O5 as Ws is strongly supported by experimental evidence, whereas the attribution of Wf is still unclear. According to Siegbahn, Wf is a water molecule (termed as ‘Wx’) which is weakly bound to Mn1 in the S2 state and is converted into OH ligand of Mn1 in the S3 state.80,81 Although this viewpoint can satisfy a low energy pathway of OeO bond formation in the S4 state, it has some intrinsic drawbacks: (i) the position of Wx cannot be reflected in any high-resolution PSII crystal structure in the S2 state; (ii) the fact that kf slows down about three times from S2 to S3 does not support the hypothesis that Wf changes from a weakly bound water to Mn1coordinated OH ligand. If there is no significant structural change from S2 to S4, only the Ca-bound W3is suitable for coupling with O5. However, Ca/Sr substitution has little effect on kf, which does not support Wf is a Ca-bound water. The magnitude of kf is quite dependent on the S state, that is, the kf of S0 and S1 are too fast to be resolved, while the kf of S3 is only 20 times faster than that of ks. This observation is also contradictory with Wf as a Ca-bound water.65Therefore, Wf should be W1 or W2, of which W2 is more favorable for its more advantageous position to form OeO bond with O5. This is consistent with the experimental phenomenon that the water exchange rate of Wf can be detected for the first time in the S2 state, because Mn4 is supposed to be oxidized from S1 to S2, which leads to significant decrease of kf. In this way, the slight decrease of kf from S2 to S3 can also be rationalized, i.e., the last Mn(III) locating at distal end of W2 will be oxidized when the S3 state arrives. In brief, although not as compelling as ascription of Ws to O5, the suggestion of W2 as Wf can satisfy the qualitative analysis of the kinetics of substrate water exchange. However, the Mn4CaO5 cluster in the catalytic cycle may show great structural flexibility, i.e., the existing forms and binding sites of O5 and W2 may change, and the substrates corresponding to the fast and slow water exchange rates may be exchanged. These reflect the complexity of structural changes in the cycle. As recently proposed by Messinger and coworkers, substrate water exchange in the S2 state of photosystem II is dependent on the conformation of the Mn4Ca cluster, which diversifies the possible substrate water exchange pathways and Wf identification, as well as water oxidation mechanism.79

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Fig. 13 Substrate-water exchange kinetics measured by TR-MIMS. Adapted with permission from ref. Cox, N.; Messinger, J., Reflections on Substrate Water and Dioxygen Formation. Biochim. Biophys. Acta, Bioenerg. 2013, 1827 (8–9), 1020–1030. Copyright 2013 Elsevier (Reproduced from an Open access Article published under Creative Commons CC-BY-NC-ND license). Table 1

Substrate water exchange rates of Ca/Sr-OEC in various S states measured by TR-MIMS.

Si state

Ca

Sr

S0 S1 S2 S3

ks, s 1 10 0.02 2.0 2.0

kf, s 1 >120 >120 120 40

ks, s 1 y 0.08 9.0 6.0

kf, s 1 y >120 >120 23

Using DFT calculations, Siegbahn constructed plausible mechanisms for substrate water exchange in the S1, S2, and S3 states,67 which can reproduce and explain the experimental kinetics that are uncommon to Mn-containing model systems.

Water oxidation catalysis in natural and artificial photosynthesis 8.08.2.3.3

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Water delivery channels

Studies on the interactions of water analogs (such as methanol and ammonia) with the OEC can also provide useful information on the substrate binding sites.82–102 Although they do not dramatically alter the exchange rates of the two substrate waters, the additions will affect the oxygen release efficiency to a certain extent, which indicates that the entry of substrate waters into the catalytic active site is interfered. The crystal structure shows that there are at least three water transport channels to the Mn4CaO5 cluster as shown in Fig. 14 (a): (i) the channel through Ca2þ and the redox-active tyrosine Yz; (ii) the Mn4/W1/Asp61 channel; (iii) Mn4/ W1Cl channel.53,103,104 Recently, Shen and coworkers found the importance of O1-channel as the water-inlet, serving as a conduit for substrate water entry into the OEC (Fig. 15).19 The site for ammonia binding was initially identified as an amino/imino/nitrogen bridge substituting the bridging group O5,100,105 but later more experimental and theoretical evidences support that ammonia replaces the terminal ligand W1 of Mn4 instead of O5.91,94,95,106 The pulse EPR experiment combined with site mutations binding provides compelling evidence that W1 is substituted by NH3.93 The energy cost of the W1 substitution mode is lower, which can well explain the observed hyperfine and quadrupole couplings, while the asymmetric quadrupole coupling is the prominent feature of the W1/NH3-Asp61 hydrogen bonding network. When NH3 is added to the bulk, the hyperfine coupling constant of the solvent-exchangeable oxygen bridge is strongly affected by 30%. This result can be understood from the trans position of W1 respect to O5, that is, the substitution of W1 by NH3 lengthens the distance of O5-Mn4 and interferes with the observed hyperfine coupling (Fig. 14B). As for the binding site of methanol, one opinion is that it is near the ammonia binding site,87 while another viewpoint is that it is bound to Ca.84 However, a recent theoretical study suggests that methanol is not directly coordinated to the OEC cluster, but locating at the end of O4water channel.83 The competitive binding of small molecules near O4 suggests the importance of O4 channel for water delivery. In general, these studies prefer a terminal ligand of Mn4 as the fast exchange substrate water. Water binding to Mn4 via the Asp61 channel involves rotation of water ligands around Mn4, which is benefit from the flexible shift of O5 from Mn4 to Mn1 in the S2 state (Fig. 14C).52,58 Only in this way can the Mn4CaO5 cluster transform to a closed-cubane structure with the unsaturated five-coordinated Mn4 in trigonal bipyramid geometry, which is suitable for the ‘pivot/carousel’ binding of a new water during the S2–S3 transition.107,108 In this model, W2 is Wf in this cycle, and the incoming water should be the substrate of the next cycle. In the open-cubane structure of S2 state, the unsaturated coordination of Mn1 is close to hydrophobic group, which keeps water molecules away from Mn1. It is the movement of O5 that makes the Mn4 accommodate new water molecules from the O4 channel. However, possibility of new water from other channels cannot be excluded. Regarding how the closed-cubane transforms to the open-cubane structure after water binding during the S2–S3 transition for the formation of S3 state, Capone et al. suggested W2 and O5 exchange on Mn3 in the S3 state, accompanied by protonation exchange (Fig. 16).109 This proposal makes it compatible

Fig. 14 (A) Three water channels around Mn cluster; (B) NH3 replaces W1 in the S2 state; (C) Mn4 ligand rearrangement derived from O5 movement. Reproduced with permission from ref. Navarro, M. P.; Neese, F.; Lubitz, W.; Pantazis, D. A.; Cox, N., Recent Developments in Biological Water Oxidation. Curr. Opin. Chem. Biol. 2016, 31, 113–119. Copyright 2016 Elsevier Inc.

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Fig. 15 O1 and O4 channels in the S1–S3 states proposed by Shen and coworkers. Adapted with permission from Ref. Suga, M.; Akita, F.; Yamashita, K.; Nakajima, Y.; Ueno, G.; Li, H.; Yamane, T.; Hirata, K.; Umena, Y.; Yonekura, S.; Yu, L.-J.; Murakami, H.; Nomura, T.; Kimura, T.; Kubo, M.; Baba, S.; Kumasaka, T.; Tono, K.; Yabashi, M.; Isobe, H.; Yamaguchi, K.; Yamamoto, M.; Ago, H.; Shen, J.-R., An oxyl/oxo Mechanism for Oxygen-Oxygen Coupling in PSII Revealed by An X-Ray Free-Electron Laser. Science 2019, 366, 334–338. Copyright 2019 American Association for the Advancement of Science (AAAS).

for the substrate identifications from substrate water exchange, structural analysis from EPR spectroscopies, the closed-cubane structure as the intermediate of the S2–S3 transition, and the open-cubane oxo-oxyl coupling mechanism in the S4 state.

8.08.2.4 8.08.2.4.1

Mechanisms of the S-state transitions and OeO bond formation S0 / S1

EPR and 55Mn Electron nuclear double resonance (ENDOR) experiments strongly support that the total spin quantum number of S0 state is S ¼ 1/2, without Mn(II). From spin states of other S states, it can be inferred that the S0 state contains (III)3(IV).110 Quantum mechanics/molecular mechanics (QM/MM) calculations showed Mn oxidation state distribution is Mn1(III) Mn2(IV)Mn3(III)Mn4(III), and the protonated oxygen bridge is O5H.111 This is consistent with Cox and coworkers’ assignment from EPR spectroscopies that the exchangeable hydroxo bridge is O5H (Fig. 17),112 and the subsequent Krewald et al.’s QM calculations on the geometric, electronic, electronic and spectroscopic properties compared to experimental data.42 In this way, the observed evolution of water exchange kinetics can be readily explained. Thus, we favor O5H in S0 and its deprotonation coupled to Mn3(III) oxidation during S0–S1. However, there are some different opinions. Amin et al.’s continuum electrostatic model suggests Mn1(III)Mn2(III) Mn3(IV)Mn4(III) and O1 should be protonated instead of O5.113 Ishikita and coworkers proposed O4 as a m-hydroxo bridge which undergoes deprotonation during the S0–S1 transition.114 As for the prevalent S0 state model, the initial charge separation of P680 leads to the oxidation of YZ into YZ$ radical, which further oxidizes S0–S1 and Mn3 from (III) to (IV), resulting in sharp decrease of pKa of O5 and proton release. Compared with artificial Mn compounds, pKa decreases by about 10 pH units. Continuous oxidation and deprotonation make the same net charge in S0 and S1 states, resulting in the diamagnetic S1 state Mn1(III)Mn2(IV)Mn3(IV)Mn4(III).115 All the oxygen bridges in the S1 state are connected to at least one Mn (IV) and all of them are deprotonated O2. However, Shen and coworkers presume O5 should be hydroxide in the S1 state based on their structural analysis on the 1.95 Å-resolution XFEL PSII.21

Fig. 16 W2/O5 exchange in the S3 state that leads to transformation from the closed-cubane to open-cubane structure. Reproduced with permission from ref. Capone, M.; Bovi, D.; Narzi, D.; Guidoni, L., Reorganization of Substrate Waters Between the Closed and Open Cubane Conformers During the S2 to S3 Transition in the Oxygen Evolving Complex. Biochemistry 2015, 54, 6439–6442. Copyright 2015 American Chemical Society.

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Fig. 17 O5H assignment in the S0 state and its correlation to EPR spectroscopy by Cox and coworkers. Reproduced with permission from ref. Lohmiller, T.; Krewald, V.; Sedoud, A.; Rutherford, A. W.; Neese, F.; Lubitz, W.; Pantazis, D. A.; Cox, N. The First State in the Catalytic Cycle of the Water-Oxidizing Enzyme: Identification of a Water-Derived m-Hydroxo Bridge. J. Am. Chem. Soc. 2017, 139, 14412–14424. Copyright 2017 American Chemical Society.

8.08.2.4.2

S1 / S2

8.08.2.4.3

S2 / S3

Only Mn oxidation is involved during the S1 / S2 transition, without proton release, resulting in the accumulation of positive charge in the system.35 The S2 state can be realized by a flash illumination on the S1 state PSII at room temperature or continuous illumination at cryogenic temperature (130–220 K). It is paramagnetic and has been widely studied by EPR spectroscopy.116–118 For the S2 state obtained by continuous illumination at cryogenic temperature (< 140 K), only an EPR signal of approximate g ¼ 4.1 is produced; when the sample is heated to > 160 K in the dark, an obvious g ¼ 2 EPR multiline spectrum is observed. Both signals can be generated from the S1 state under 200 K illumination, but their relative intensities are obviously different depending on the sample preparations.119 The g ¼ 2 and g ¼ 4.1 EPR signals represent spin isomers of the S2 state, and the interconversion necessitates spin crossover.52,120 According to Pantazis et al., the ground spin state of the open-cubane g ¼ 2 isomer is S ¼ 1/2, and the oxidation state distribution is Mn1(III)Mn2(IV)Mn3(IV)Mn4(IV); the five-coordinated Mn1(III) is ferromagnetically coupled to Mn3(IV) and antiferromagnetically coupled to Mn2(IV) and Mn4(IV). The ground spin state of the closed-cubane g ¼ 4.1 isomer is S ¼ 5/2, and the oxidation state distribution is Mn1(IV)Mn2(IV)Mn3(IV)Mn4(III); the five-coordinated dangling Mn4(III) is antiferromagnetically coupled to the other three Mn(IV).52

The S2 / S3 transition involves ligand deprotonation, Mn oxidation and water binding, resulting in the S3 state containing four six-coordinated Mn(IV).30,35 Although the structure of the S3 state has been recognized, how it forms from the S2 state is still controversial. Specifically, what is the origin of ‘O6/Ox’ and how it binds to Mn1 remain unclear and these are central issues of this transition. Siegbahn according to his DFT calculations first proposed that a water molecule that weakly bound to Mn1(III) (about 2.4 Å) in the S2 state of the open-cubane structure, undergoes deprotonation and then directly coordinates to Mn1(IV) (once it is oxidized) in the form of OH when the S3 state forms.80,81 Debus and Yano and their coworkers proposed the Ca-bound W3 inserts to Mn1 in the open-cubane structure during the S2–S3 transition by Fourier transform infrared (FTIR) spectroscopy and XFEL crystallography, respectively.63,121 Yamaguchi, Guidoni, Kaila and their coworkers, using DFT calculations and ab initio molecular dynamics (AIMD) simulations, suggested that the Ca-bound W3 in the closed-cubane structure would transfer to the oxidized Mn4(IV) during S2–S3, and in the meanwhile, the water molecule originally interacted with W3 through hydrogen bond near the Mn cluster would move to the original position of W3.56,122,123 The last hypothesis is the so-called ‘pivot/carousel’ mechanism proposed by Pantazis, Cox and Batista, Brudvig and their coworkers, who suggested waters in the O4 hydrogen bond network would bind to Mn4, along with the rotation/rearrangement of the terminal ligands of the five-coordinated Mn4(IV) in the closed-cubane structure.26,53-55,58,103,104,107,108,124 The core for this mechanism is the ‘5-coordinate Mn4(IV)’ of the closed-cubane structure, which was first proposed by Retegan et al.’s DFT calculations,58 showing that it is the entry structure from S2 to S3, while the open-cubane structure cannot play such a role. This conclusion is subsequently supported by AIMD simulations by Guidoni and coworkers.56 Recently, Cox and coworkers performed EPR experiments which provides strong evidence for the concept of ‘5-coordinate Mn(IV)’.124 The Ca-H2O insertion mechanism (for both open and closed cubane structures) and pivot/carousel mechanism (closed cubane structure) are depicted in Fig. 18. At current stage, we would not prematurely show an arbitrary preference because each mechanism seems plausible under the methodology used but some questionable points have been raised in consideration of inconsistencies with other experimental data or theoretical calculations.

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Fig. 18 Proposed structures for the S2 state and possible pathways for water insertion during the S3 state formation. Adapted with permission from ref. Ibrahim, M.; Fransson, T.; Chatterjee, R.; Cheah, M. H.; Hussein, R.; Lassalle, L.; Sutherlin, K. D.; Young, I. D.; Fuller, F. D.; Gul, S.; Kim, I.-S.; Simon, P. S.; de Lichtenberg, C.; Chernev, P.; Bogacz, I.; Pham, C. C.; Orville, A. M.; Saichek, N.; Northen, T.; Batyuk, A.; Carbajo, S.; Alonso-Mori, R.; Tono, K.; Owada, S.; Bhowmick, A.; Bolotovsky, R.; Mendez, D.; Moriarty, N. W.; Holton, J. M.; Dobbek, H.; Brewster, A. S.; Adams, P. D.; Sauter, N. K.; Bergmann, U.; Zouni, A.; Messinger, J.; Kern, J.; Yachandra, V. K.; Yano, J. Untangling the Sequence of Events During the S2 / S3 Transition in Photosystem II and Implications for the Water Oxidation Mechanism. Proc. Natl. Acad. Sci. U.S.A. 2020, 117 (23), 12624–12635. Copyright 2020 National Academy of Sciences (reproduced from an Open Access Article published under Creative Commons Attribution-NonCommercialNoDerivatives License 4.0).

8.08.2.4.4

S3 / S4

So far, information of the structural change of the OEC lacks after the S3 state formation, until the S0 arrival in the next cycle. The current XFEL technique is unable to resolve any intermediate between S3 and S0, probably because of the extreme short-lived S4 state. For the S3 / S4 transition, the only available information comes from the kinetics from time-resolved X-ray, photothermal beam deflection (PBD) and polarography experiments.33,35-38,49,125–127 It is summarized as the following: a lag phase (about 250 ms) followed by a slow phase (1–2 ms) exist are detected after the formation of the S3YZ$ state, prior to the S4 state formation

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where YZ• reduction and O2 formation occurs (Fig. 19). The lag phase has been mostly referred to as a proton release because of the large H/D kinetic isotope effect (KIE), while the slow phase is known as the slowest process throughout the whole cycle, which may be attributed to some kind of structural rearrangement of the Mn cluster. However, the specific changes during S3–S4 remain elusive and hypothetical. More experimental and theoretical work should be involved to determine the exact events.

8.08.2.4.5

S4 / S0

8.08.2.4.6

Mechanism of OeO bond formation

The key step during the S4 / S0 transition is OeO bond formation, which is followed by O2 release, water binding and proton release for recovery of the S0 state of the next cycle. The sequence and kinetics of these events are basically solved, whereas the specific reaction styles are unclear.33 The challenge of directly studying OeO bond formation by experiments is to design new technology to deal with the S3–S0 stage from the catalytic cycle separately. It is very difficult to understand the essence of the deprotonation step of S3YZ$ and how this transient state will evolve. The equilibrium constant K of the S4 / S0 transition determined by Messinger and coworkers using oxygen water isotope exchange (OWIE) is more than 1.0  107, indicating that it is very exothermic.128 The S4 / S0 transition contains multiple reactions, and the information for the single OeO bond step is still limited. A promising method is to use systems with structural mutations which lead to decrease of the catalytic activity, including site-directed mutations (such as Asp61, Val185, etc.) or replacing non-redox ions (Ca2þ/Sr2þ, Cl/Br, etc.), which will reduce the rate of OeO bond formation by more than 100 times, and the latter has been used to study the kinetics of water exchange at a certain stage of the catalytic cycle.65,66,129 The specific reason why these structural modifications lead to the rate decrease of oxygen release is still unclear. It may be that the OeO bond formation step itself is slowed down, or these changes block the S-state progression by controlling the timing of proton transfer or the addition of the second substrate water. Further spectroscopic characterizations of these perturbed systems are needed for more evidences. Messinger and coworkers arrested the substrate water exchange in the S3Yz$ state, showing that the substrate should be tightly bound to Mn before OeO bond formation, and Wf is likely W2.65,130,131 Mn K-edge X-ray spectra show that Mn is not oxidized after the S3YZ$ formation, which means that the final electron hole should not localized on Mn, or Mn(V) would be immediately reduced once it is formed.36 Siegbahn suggested the S4 state involves Mn(IV)-oxyl,132 while Pecoraro and Brudvig and their coworkers favor the isoelectronic form of Mn(V)-oxo.133,134 Mn(IV)-oxyl and its adjacent m-oxo bridge can form OeO bond through low-barrier oxo-oxyl radical coupling under certain spin alignment. Mn(V)-oxo has great electrophilicity and may attract water for nucleophilic attack to form OeO bond. The former has been approved by theoretical calculations, and the latter was proposed from inorganic Mn catalysts for water oxidation. However, it may be an over simplification to describe S4 state as Mn(IV)-oxyl or Mn(V)-oxo, because the electron hole should be delocalized in the domain between Mn and O atoms. The spin density of O of Mn(IV)-oxyl in the OEC calculated by Li and Siegbahn is about 0.7,135 while it is only 0.45 in the synthesized Mn(V)-oxo compound by Borovik and coworkers136 It can be seen that the subtle changes or artificial modifications of the OEC cluster or the external environment may affect the spin density distribution of Mn and O atoms, as well as the electrostatic OeO bond formation, and even determine the reaction style. Fig. 20 illustrates the proposed S4–S0 transition centered on the ‘internal’ and ‘external’ mechanisms of OeO bond formation by the oxo/oxyl coupling and nucleophile-electrophile attack, respectively.26 So far, a final conclusion is still too early to support or abandon the radical coupling or nucleophilic attack mechanism, before irrefutable experimental evidences come out.

The mechanism of OeO bond formation is the central and most concerned subject in natural water oxidation. For a long time, there are mainly two prevalent theories: (i) the nucleophilic attack (NA) mechanism involving acid-base reactions proposed by Barber, and Brudvig, Batista and their coworkers23,137–142; (ii) the oxo-oxyl radical coupling (RC) mechanism proposed by Siegbahn.80,132,135,143–147 Besides, recent studies diversify the mechanistic possibilities, which will be also introduced below. 8.08.2.4.6.1 Nucleophilic attack (NA) mechanism Specifically, the NA mechanism refers to nucleophilic attack of the Ca-bound W3, either in the form of H2O or OH, onto the electron-deficient terminal ligand W2 that is bound to the dangling Mn4, which could be high-spin Mn4(V) ¼ O or Mn4(IV)oxyl. Barber and coworkers first proposed the NA mechanism occurring on the surface of the OEC cluster by analyzing their resolved PSII crystal structure.23 Sproviero et al.’s QM/MM calculations support the nucleophilic attack of the Ca-bound water onto Mn4(IV)oxyl141,142,148–151; Brudvig and coworkers disfavor the m-O5 bridge as a substrate according to substrate-water exchanging kinetics in Mn model systems, and prefer the NA mechanism of W3 (H2O) onto the Mn4-bound W2 in the form of high-spin Mn4(V)oxo.140,152–156 Gupta et al.’s spectroscopic evidences support the high-spin form of Mn(V)-oxo (rather than Mn(IV)-oxyl) from their synthesized OEC analogs, suggesting the possibility of the NA mechanism in natural water oxidation.136 However, the recent XFEL study excluded this mechanism by comparing the structural intermediates of the cycle.24 Figs. 21 and 22 show the NA mechanism of OeO bond formation embedded in the S-state cycle proposed by Barber, and Brudvig and their coworkers, respectively.

Fig. 19

A diagram for the lag phase and slow phase during the S3 / S4 transition.

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Fig. 20 Proposed scenarios of S4–S0 transition by an oxo/oxyl coupling (top) or nucleophile-electrophile attack mechanism (bottom). Reproduced with permission from Ref. Cox, N.; Pantazis, D. A.; Lubitz, W., Current Understanding of the Mechanism of Water Oxidation in Photosystem II and Its Relation to XFEL Data. Annu. Rev. Biochem. 2020, 89 (1), 795–820. Copyright 2020 Annual Reviews Inc.

8.08.2.4.6.2 Oxo-oxyl radical coupling (RC) mechanism The RC mechanism means the Mn1(IV)-bound oxyl radical (O6/Ox) couples with the m-O5 bridge,132,145 or its variant with a rotation of the ligand coordination by about 30 degrees.135 Mn(V) intermediate was not observed in Haumann et al.’s experiments, which supports the RC mechanism after oxidation of a substrate oxygen36; Cox et al.’s analysis of the EPR spectroscopy and electronic structure of the S3 state,30 and Suga et al.’s XFEL crystal structures of the S1–S3 states both favor the RC mechanism.19 Siegbahn’s recent DFT calculations proved the infeasibility of the NA mechanism, because the protonated peroxide product is always very high in energy.157 Fig. 23 illustrates Siegbahn’s S-state cycle with the RC mechanism of OeO bond formation. A problem within this mechanism is that Siegbahn presumed a loosely Mn1-bound water in the S2 state would bind to Mn1 as a hydroxide after its deprotonation,81 but there is not such a water in the XFEL-S2 structure.19,24 Then he modified the proposal and suggested the Ca-bound W3 as the origin of Ox.158 In order to satisfy W2 and O5 as substrates required by substrate water exchange, Li and Siegbahn presented a variant of this mechanism, using O5-W2 coupling using ‘inner’ oxo arrangement and spatially rotating Asp170.135 In parallel to this, Cox, Pantazis, and Lubitz and coworkers suggested OeO bond formation in an open-cubane structure using W2 and O5 as substrates, but structural rearrangements during the S2–S3 transition and O4-channel for water delivery are needed (Fig. 24).26,55,103,104

Fig. 21 The mechanism of S-state cycle using the NA mechanism of OeO bond formation proposed by Barber and coworkers. Reproduced with permission from ref. Barber, J., Photosynthetic Water Splitting Provides a Blueprint for Artificial Leaf Technology. Joule 2017, 1, 5–9. Under Computer Physics Communications (CPC) User License. Copyright 2017 Elsevier Inc.

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Fig. 22 The mechanism of S-state cycle using the NA mechanism of OeO bond formation proposed by Brudvig and coworkers. Adapted with permission from Ref. Vinyard, D. J.; Khan, S.; Brudvig, G. W., Photosynthetic Water Oxidation: Binding and Activation of Substrate Waters for O-O Bond Formation. Faraday Discuss. 2015, 185, 37–50. Copyright 2015 Royal Society of Chemistry (reproduced from an Open Access Article under Creative Commons Attribution 3.0 Unported Licence).

8.08.2.4.6.3 O4-W1 radical coupling mechanism Using QM/MM approach and molecular dynamics simulations, Ishikita and coworkers proposed oxo-oxyl radical coupling between OW1•- and corner m-oxo O4, and deprotonation via D1-Asp61 leads to OeO bond formation for OW1]O4. Then, O2 release is followed by W539 incorporation into the vacant O4 site, to form a new m-oxo bridge and recovery of the Mn4CaO5 cluster (Fig. 25).159 Although this mechanism seems plausible according to their calculations, the assignment of O4 and W1 as both substrates is not consistent with kinetics of substrate water exchange, which strongly supports O5 as one of the substrates.65,79,130,131 Also, the ammonia perturbation experiment excluded W1 as the second substrate water,95 which disfavors the mechanism in discussion. The ‘O4-lacking’ structure in Barber and coworkers’ 3.5 Å resolution PSII23 and Zhang et al’s synthetic OEC model160 should not be persuasive enough to support the mechanism here, because of the low-resolution quality and artificial character, respectively. 8.08.2.4.6.4 Nucleophilic OeO coupling mechanism In line with the theoretical studies and EPR findings by Cox and coworkers,124 Pantazis and coworkers proposed that the closedcubane structure with five-coordinate Mn4(IV) should be the entry point to the S3 state, and water binding may not be a necessary event during the S2–S3 transition.58 Then, the water-unbound form of the S3 state would progress to the S4 state after proton release and Mn4 oxidation from (IV) to (V).161,162 The electron-deficient high-spin Mn(V)-oxo would couple with the m-O5 bridge for OeO bond formation adopting an ‘intramolecular nucleophilic coupling mechanism’, followed by water binding to Mn4 from the O4 channel for the S0 state formation (Fig. 26). This hypothesis needs in depth investigation from viewpoint of energetics. 8.08.2.4.6.5 High-valent Mn(VII)-dioxo mechanism Inspired by the active Mn(VII)-oxo species identified during electrochemical water oxidation by a synthetic Mn oxide,163 Zhang and Sun proposed a novel mechanism for OeO bond formation in PSII (Fig. 27).164 There are two unique points: charge rearrangement involving disproportionation of 4 Mn(IV) to 3 Mn(III) and 1 Mn(VII); and OeO bond formation at the Mn(VII)-dioxo site. Substantial experimental evidences from Mn model systems were provided to support the possibility of this proposal. For the charge rearrangement, the examples of Mn(IV) sulphate, [(bpy)2Mn(mO)2Mn(bpy)2](Mn-bpy, bpy ¼ 2,20 -bipyridine), [(H2O)(tpy)Mn(m-O)2Mn(tpy)(H2O)] (Mn-tpy, tpy ¼ 2,20 :60 ,200 -terpyridine)

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Fig. 23 The mechanism of S-state cycle using the RC mechanism of OeO bond formation proposed by Siegbahn, including the structural (left) and energetic changes (right). Reproduced with permission from ref. Siegbahn, P. E. M., Structures and Energetics for O2 Formation in Photosystem II. Acc. Chem. Res. 2009, 42, 1871–1880. Copyright 2009 American Chemical Society.

dimer and Mn-tpy dimer catalysts were taken to demonstrate the feasibility of Mn(IV) / Mn(VII). Furthermore, the theoretical $þ and 1.21 V for Yz/YZ$, which potential 1.05 V at pH 7 for the formation of MnO4 is lower than those of 1.26 V for P680/P680 are thermodynamically adequate for initiating the formation of Mn(VII) species. It is supposed that the changedisproportionation induced significant structural rearrangement may be responsible for the kinetic slow phase during the S3–S4 transition. For OeO bond formation on Mn(VII), they presented some reported cases where O2 is evolved from MnO4 by various promoters, as shown in Fig. 28. Later, the proposed mechanism was updated considering proton transfer accompanied by electron transfer during the S3–S4 transition, which leads to change of the Mn4 coordination sphere to reach the high-valent Mn(VII), i.e., de- and re-coordination of the carboxylates Glu333 and Asp170, prior to OeO bond formation between W2 and O5 in the S4 state (Fig. 29).165 The newly inserted oxygen on Mn1 during the S2–S3 transition would serve as a new O5 for the next cycle. In this way, the dangling Mn4 is the open reactive site and the remaining Mn3CaO5 part functions as a battery to facilitate charge accumulation, and also a Lewis acid resembling ‘BF3’ to lower the barrier of OeO bond formation. This new scenario matches the XFEL data with O6/Ox relocating to the original O5 position, and the assignment of O5 and W2 as substrates is consistent with kinetics from substrate water exchange. Our recent DFT calculations showed O5-W2 coupling cannot happen in a non-rearranged OEC cluster.166 In other words, significant structural rearrangement is necessary for OeO bond formation if O5 and W2 are substrates. In addition to the structural changes during the S2–S3 transition proposed by Cox, Pantazis and Lubitz,26,55,58,103,104,124 the scenario for S3–S4 shown above by Zhang and Sun may be an alternative, and may account for the 1 2 ms ‘slow phase’ prior to the S4 state. For OeO bond formation, despite the vast reported oxygen-evolving reactions on Mn(VII)-chemistry, direct experimental and computational studies on the OEC are crucial to check the possibility. In brief, important aspects of photosynthetic water splitting are basically summarized above with emphasis on the mechanisms, from universal knowledge to current controversies pending a convergence. Uncovering the secretes in biological water oxidation would help development of artificial photosynthesis for energy conversion and utilization. Artificial water oxidation will be introduced in the next section.

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Fig. 24 The mechanism of S-state cycle using the RC mechanism of OeO bond formation proposed by Cox, Pantazis and Lubitz and coworkers. Reproduced with permission from ref. Krewald, V.; Retegan, M.; Neese, F.; Lubitz, W.; Pantazis, D. A.; Cox, N. Spin State as a Marker for the Structural Evolution of Nature’s Water-Splitting Catalyst. Inorg. Chem. 2016, 55, 488–501. Copyright 2016 American Chemical Society.

8.08.3

Artificial water oxidation

The ingenuity of natural selection, resulting in the abundant presence of photosystems in living organisms, has been always challenging scientists for the development of artificial systems, mimicking natural water splitting. It was early realized that the energetic bottleneck of the overall water splitting reaction lies in water oxidation, prompting researchers to focus on water oxidation catalysis. In the course of last 50 years, the goals of artificial water oxidation research evolved from simple mechanistic studies, uncovering the mechanisms of natural water oxidation, to the development of sophisticated electro- and photoelectrocatalysts, expected to not only outperform natural systems, but also be affordable and durable on an industrial scale. This section gives a brief overview of the artificial water oxidation catalysts, with the focus on important underlying mechanisms and design principles that take inspiration from the natural water oxidation.

8.08.3.1 8.08.3.1.1

Development of artificial water oxidation catalysts Molecular catalysts

Early studies on the water oxidation catalysis relied on a limited information about Photosystem II, such as: presence of a visible light source, presence of a metal (in particular, manganese), and organic environment. Consequently, most of the models for water oxidation were simply based on a photocatalytic oxidation of water by transition metal ions.167,168 Such models were reinforced by experimental observations of oxygen evolution in systems, containing only metal ions.169 In certain cases, the experiments were compromised by electrolytic generation of high-valent metals, which could potentially result in the formation of active heterogeneous species, undetected at the time.170 One of the key issues, raised in the metal ion-catalyzed water oxidation studies, was the mismatch between the high oxidation potential of water and low oxidation potentials of metal ions. This problem gave rise to a metal cooperation mechanism, where a cluster of oxygen-bridged oxidized metal ions utilizes the combined oxidizing power of several metal centers to drive OeO bond formation.31,168 Such idea, albeit not detailed correctly at the time, was an important step towards the elucidation of the real structure of OEC and the discovery of first successful artificial water oxidation catalysts. This, on the other hand, made the mechanistic studies more challenging due to complex synthesis and structure of metal clustersduntil the first mononuclear molecular catalyst has been developed. Development of photovoltaic materials increased the focus on the electrocatalysts, which also started to leverage proton-coupled electron transfer as a way of decreasing oxidation potentials. The latter was found its application in Ru complexes, where high-valent

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Fig. 25 The S4 and pre-S0 states of the O4-W1 radical coupling mechanism proposed by Ishikita and coworkers. Reproduced with permission from an Open Access article ref. Kawashima, K.; Takaoka, T.; Kimura, H.; Saito, K.; Ishikita, H., O2 Evolution and Recovery of the Water-Oxidizing Enzyme. Nat. Commun. 2018, 9, 1247, under a Creative Commons Attribution 4.0 International License. Copyright 2018 Springer Nature Limited.

Fig. 26 Proposed OeO intramolecular nucleophilic coupling mechanism based on the Mn4(V)-oxo species S4 proposed by Pantazis and coworkers. Reproduced with permission from Ref. Krewald, V.; Neese, F.; Pantazis, D. A., Implications of Structural Heterogeneity for the Electronic Structure of the Final Oxygen-Evolving Intermediate in Photosystem II. J. Inorg. Biochem. 2019, 199, 110797. Copyright 2019 Elsevier Inc.

Ru states could oxidize proton-rich ligands (Fig. 30).171 As aqua ligand also contains removable protons, this approach was successfully implemented in 1978 by Meyer and coworkers in a Ru complex Ru(bpy)2(py)(H2O), which could be easily oxidized to a Ru(IV) ¼ O complex (Fig. 31).172 This complex became a precursor for the first well-defined molecular water oxidation catalyst [(bpy)2(H2O)RuIIIORuIII(H2O)(bpy)2]4 þ, known as a “blue dimer”.173 The presence of multiple metal centers was long considered a requirement for reaching the necessary oxidizing powerda hypothesis, which was in part reinforced by a successful intramolecular OeO bond formation between two independent Ru]O units within complex Ru-Hbpp-trpy.174 Detailed mechanistic studies on

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Fig. 27 The proposed S-state cycle by Zhang and Sun, involving charge rearrangement to reach Mn(VII) during S3–S4 and OeO bond formation at Mn(VII)-dioxo site in the S4 state. Adapted with permission from Ref. Zhang, B.; Sun, L., Why Nature Chose the Mn4CaO5 Cluster as Water-Splitting Catalyst in Photosystem II: A New Hypothesis for the Mechanism of O-O Bond Formation. Dalton Trans. 2018, 47, 14381–14387. Copyright 2018 Royal Society of Chemistry (reproduced from an Open Access Article under Creative Commons Attribution 3.0 Unported Licence).

a “blue dimer” revealed mostly supporting roles of the second metal center in it, such as stabilization of one (bridged) oxo ligand and provision of a proton acceptor.175 Further investigation of the electronic structure of the intermediates showed that there is a significant electronic coupling between oxygen-bridged Ru centers, which defines key properties of the “blue dimer”.176 These findings are primarily important for the studies of oxygen-bridged clusters, including natural OEC. In an attempt to prove the need of minimum two metal centers, Thummel and coworkers developed a series of mononuclear complexes, which were discovered to be active in water oxidation.177 Numerous Ru-based mononuclear water oxidation catalysts have been developed since, with excellent performance in photochemical,178 chemical,179,180 and electrochemical water oxidation, both in basic181 and acidic182 conditions (Fig. 32). Well-defined monomolecular complexes became an excellent platform for the mechanistic studies, allowing to elucidate the roles of ligands and determine structure-activity relationships for water oxidation catalysts.184,185

8.08.3.1.2

Material catalysts

Since the solar energy was identified as a viable alternative to fossil fuels, artificial photosynthesis became one of the research areas with potentially significant economic and ecological value.186–188 Ubiquitous use of catalysts implies availability of raw materials, low price, and reliable production. As most artificial catalysts for water oxidation were based on ruthenium or other noble metals, in contrast to abundant manganese in natural OEC, many efforts have been devoted to the development of molecular catalysts, based on group IV transition metals. Early studies did not yield well-defined catalysts with particularly high activity, as molecular complexes were rarely outperforming metal ions in basic solutions.189 With further development of microscopic characterization techniques in 2000s, it was realized that earth-abundant transition metals tend to form nanostructured metal oxides, which are extremely active in water oxidation.190,191 Although the potential of metal oxides as electrocatalysts was recognized earlier,192 the efficiency of molecular catalysts in water oxidation was often confused with the formation of metal oxides via unnoticeable levels of decompositionda problem, appearing even in the modern studies.193–196

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Fig. 28 Highly reactive Mn(VII) species and related O2 evolution reactions. Adapted with permission from Ref. Zhang, B.; Sun, L., Why Nature Chose the Mn4CaO5 Cluster as Water-Splitting Catalyst in Photosystem II: A New Hypothesis for the Mechanism of O-O Bond Formation. Dalton Trans. 2018, 47, 14381–14387. Copyright 2018 Royal Society of Chemistry (reproduced from an Open Access Article under Creative Commons Attribution 3.0 Unported Licence).

Active inorganic oxide catalysts were further developed to improve the efficiency, stability and overpotential in water oxidation reaction, resulting in MnOx,197,198 CoOx,199 Fe2O3,200 and other metal oxide materials.201 An important supportive role of iron was observed in mixed metal oxides, such as cobalt iron oxide202 and nickel iron oxide,203 leading to the abundant use of related structuresdlayered double hydroxides (LDHs). LDH catalysts offer better control of morphology, hydrogen bond network and efficient diffusion of water to the active sites.204 Similarly to the molecular photocatalytic systems, where the active catalyst and light harvester can be connected in a one molecule or supramolecular ensemble, inorganic catalysts can combine both parts of photocatalytic systems in one material. Since many oxide materials are semiconductors, and doping with various elements allows to tune the band gap, a number of photoanode materials with water oxidation properties have been developed, including BiVO4,205 WO3,206 a-Fe2O3,207,208 and ternary chalcogenides.209,210 Another class of well-defined inorganic materials, active in water oxidation, is polyoxometalates (POMs), such as polytungstate Rb8K2[[Ru4O4(OH)2(H2O)4](g-SiW10O36)2]$25H2O.211,212 POMs represent a convenient intermediate between molecular and material catalysts, combining their advantages, as they have an oxide-like structure and small size, suitable for solution studies. Similar to molecular catalysts, their homogeneous applications can be limited due to instability,213 but overall oxidative stability makes them promising candidates for both electrode materials and cocatalysts.214,215 A number of strategies have been used in the development of better material-based water oxidation catalysts

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Fig. 29 The updated proposal of the catalytic mechanism for water oxidation in PSII involving Mn(VII)-dioxo species, with details of structural rearrangement. Reproduced with permission from Ref. Zhang, B.; Sun, L., Across the Board: Licheng Sun on the Mechanism of O-O Bond Formation in Photosystem II. ChemSusChem 2019, 12, 3401–3404. Copyright 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Fig. 30

Key steps towards the discovery of molecular catalysts, operating via metal oxo.

(Fig. 33),[216–218] including but not limited to: exfoliation of multi-layered bulk materials and intercalation with co-catalysts,219 increasing active sites exposure via surface engineering,220 and tailoring elemental surface composition.221 Numerous properties of both organic and inorganic materials are responsible for the stability and water oxidation activity: catalyst morphology, electronic structure, ligand stabilization of high-valence states etc. Nevertheless, the main principles are common for all water oxidation catalysts and can even be easily transferred between the organic and inorganic systems. These principles, which are based on known water oxidation mechanisms, will be discussed in the following sections.

8.08.3.2

Water oxidation mechanisms

While the detailed mechanisms of key water oxidation steps in the natural OEC are still under debate, the choice for primary mechanism of OeO bond formation is quite limited. Overall, reaction between two oxygen atoms can occur in a symmetric or asymmetric manner. Symmetric reaction implies that both oxygen atoms have similar electronic characteristics, and asymmetric reaction requires one electron-rich and one electron-deficient oxygen. Many early suggestions for the water oxidation mechanism

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Fig. 31

First molecular ruthenium-based water oxidation catalysts.

Fig. 32 Modern molecular ruthenium-based water oxidation catalysts Ru-bda (bda ¼ 2,20 -bipyridine-6,60 -dicarboxylate) and Ru-tda (tda ¼ 2,20 :60 ,200 -terpyridine-6,600 -dicarboxylate).183

proposed a symmetric pathway without detailed insights into the driving force of the reaction.168,173 A symmetric coupling cannot be charge-controlled, which means that the orbital interaction should govern the bond formation. Such interaction, strong enough to form a bond, is very likely to involve singly-occupied molecular orbitals (SOMOs)doxyl radicals in this case, that react without a noticeable energetic barrier.222 This mechanism, also known as “radical coupling” (RC, see Section 8.08.2.4.6.2), is often abbreviated I2M (interaction of two metal oxo units, Fig. 34). After the formation of an OeO bond, the peroxide is oxidized one again, and limited electron density on it becomes not sufficient to ligate a highly oxidized metal ion, which results in decoordination, formation of a side-on peroxo complex, which eventually releases a dioxygen molecule. Depending on whether two radicals are located on separate molecules or not, I2M mechanism can be either intramolecular or intermolecular. Intermolecular case is quite appealing in homogeneous catalysis, as its turnover frequency (TOF) grows linearly with the catalyst concentration, allowing to reach very high activity per catalytic unit.179 A representative example of a catalyst operating via intermolecular I2M mechanism is Ru-bda complex (Figs. 32 and 34).185 Intramolecular I2M can operate in both binuclear and mononuclear catalysts: between two separate metal oxo units within the same molecule174,223,224 or between two oxyl radicals sharing the same metal ion.225 Despite mechanistic studies on materials are often complicated by inhomogeneity and a different set of suitable operando techniques, I2M mechanism has also been observed in inorganic catalysts, including manganate ions.164,226 As metal oxo species do not always possess a radical character, and certain metals are much less likely to form those,227,228 most water oxidation reactions operate via an asymmetric OeO bond formation, which includes an electrophile and a nucleophile. Both water and a hydroxide ion feature a (partially) negatively charged oxygen atom, therefore constituting a nucleophile. On the other hand, an electrophilic or positively charged oxygen is a rare occurrence, and it would take a much higher oxidizing power to create one, compared to a theoretical oxidation to a neutrally charged oxygen in O2. Instead, oxygen undergoing a nucleophilic attack can be only a buffer to transfer an electron density from an incoming nucleophile to another electrophile. This setting is perfectly realized in a reaction between a high-valent metal oxo unit and a water molecule, called water nucleophilic attack (WNA or NA, see Section 8.08.2.4.6.1, Fig. 34). The oxidized metal ion is a primary recipient of electron density in this reaction, and if it is coordinatively saturated, a water molecule cannot attack it directly, and instead forms an electron-rich OeO bond in a metal hydroperoxide. This step is associated with a concomitant proton loss even in acidic conditions, making bases extremely important in this reaction.229–232 Further oxidation and proton transfer result in the release of oxygen. Various Ru complexes, such as Ru-bda and Rutda, are state-of-the-art water oxidation catalysts, operating via WNA mechanism.175,181,233 To differentiate between two mechanisms, WNA and I2M, several techniques can be used. An intermolecular homogeneous I2M can be identified by following reaction progress with varying catalyst concentrations, as I2M involves a reaction with second order in

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Fig. 33 Representative structures of material water oxidation catalysts and some of the design strategies for improving the catalytic performance. (A) left: crystal structure of Mn3O4, right: crystal structure of a layered Mn oxidedbirnessitedwith significant exposure of active sites. Adapted with permission from Ref. Gorlin, Y.; Lassalle-Kaiser, B.; Benck, J. D.; Gul, S.; Webb, S. M.; Yachandra, V. K.; Yano, J.; Jaramillo, T. F., In Situ X-ray Absorption Spectroscopy Investigation of a Bifunctional Manganese Oxide Catalyst with High Activity for Electrochemical Water Oxidation and Oxygen Reduction. J. Am. Chem. Soc. 2013, 135 (23), 8525–8534. Copyright 2013 American Chemical Society. (B) CoOx structural diversity in electrocatalysis. Reprinted with permission from Ref. Jung, H.; Ma, A.; Abbas, S. A.; Kim, H. Y.; Choe, H. R.; Jo, S. Y.; Nam, K. M. A New Synthetic Approach to Cobalt Oxides: Designed Phase Transformation for Electrochemical Water Splitting. Chem. Eng. J. 2021, 415, 127958. Copyright 2021 Elsevier. (C) FeOx structure in bulk and nanoparticles, doped with Cr. Reprinted with permission from ref. Fan, L.; Zhang, B.; Timmer, B. J. J.; Dharanipragada, N. V. R. A.; Sheng, X.; Tai, C.-W.; Zhang, F.; Liu, T.; Meng, Q.; Inge, A. K.; Sun, L. Promoting the Fe(VI) Active Species Generation by Structural and Electronic Modulation of Efficient Iron Oxide Based Water Oxidation Catalyst Without Ni or Co. Nano Energy 2020, 72, 104656 according to the Creative Commons Attribution License CC BY 4.0. (D) NiFe-LDH structure, doped with carbon quantum dots. Reprinted with permission from Ref. Tang, D.; Liu, J.; Wu, X.; Liu, R.; Han, X.; Han, Y.; Huang, H.; Liu, Y.; Kang, Z., Carbon Quantum Dot/NiFe Layered DoubleHydroxide Composite as a Highly Efficient Electrocatalyst for Water Oxidation. ACS Appl. Mat. Interf. 2014, 6 (10), 7918–7925. Copyright 2014 American Chemical Society.

respect to the catalyst.234 2H and 18O isotope labeling are commonly used to identify the atoms participating in a rate-determining step via observations of a kinetic isotope effect (KIE). Since OeO bond formation is often a rate-determining step, 2H labeling experiments provide information on whether hydrogen atom is being abstracted from one of the participating oxygens, which is typically associated with WNA-type mechanisms. In cases when metal oxo can be prepared separately, and water acts as a sole nucleophile, 18O labeling in combination with mass spectrometry of the formed oxygen species clearly shows the sources of both oxygen atomsdwhich is particularly useful for chemical water oxidation catalysis, where oxygen-rich oxidants can directly participate in oxygen formation.235 In order to keep track of oxidation processes, an oxidation state of the metal is often monitored by EPR236–238 and XANES.214,239–241 EXAFS is often used along other X-ray techniques to evaluate the first coordination sphere of metal ions.240,242 Complicated mechanisms, where the nucleophile in WNA mechanism is quickly exchanged with m-oxo bridges or even a metal oxo,243 or where nucleophile simply triggers a concerted rearrangement involving an OeO bond formation,244 require a combination of techniques and often are rationalized through computational studies (Fig. 35).245 There exist certain intermediate mechanisms, such as hypothetical HO-OH coupling accelerated by a base, that do not have substantial experimental evidence so far and therefore not discussed here.246

8.08.3.3

Artificial water oxidation catalyst design

The structure of many water oxidation catalysts is inspired by natural systems.247 As there is only one type of natural water oxidation complex, the information about structure-activity relationship in natural catalysis is insufficient for the development of new artificial catalysts. Therefore, artificial catalyst evolution took an extensive approach, collecting a large amount of information about

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Fig. 34 Overview of water oxidation mechanisms. Cobalt cubane structures reproduced with permission from ref. Nguyen, A. I.; Wang, J.; Levine, D. S.; Ziegler, M. S.; Tilley, T. D., Synthetic Control and Empirical Prediction of Redox Potentials for Co4O4 Cubanes Over a 1.4 V Range: Implications for Catalyst Design and Evaluation of High-Valent Intermediates in Water Oxidation. Chem. Sci. 2017, 8 (6), 4274–4284. (from an Open Access Article under Creative Commons Attribution 3.0 Unported License). Copyright 2017 Royal Society of Chemistry.

Fig. 35 Proposed intramolecular nucleophilic attack (NA), triggered by water nucleophilic attack in molecular245 and material244 catalysts. (A) Reproduced with permission from ref. Ding, Q.; Liu, Y.; Chen, T.; Wang, X.; Feng, Z.; Wang, X.; Dupuis, M.; Li, C., Unravelling the Water Oxidation Mechanism on NaTaO3-Based Photocatalysts. J. Mat. Chem. A 2020, 8 (14), 6812–6821. Copyright 2020 Royal Society of Chemistry. (B) Reprinted with permission from ref. Zhan, S.; De Gracia Triviño, J. A.; Ahlquist, M. S. G., The Carboxylate Ligand as an Oxide Relay in Catalytic Water Oxidation. J. Am. Chem. Soc. 2019, 141 (26), 10247–10252. Copyright 2019 American Chemical Society.

catalytic performance of various molecular and material structures.248 Obtained structure-activity relationships took shape of various design rules,183,249 which will be discussed in the present section.

8.08.3.3.1

Role of metal

One of the key variables of a water oxidation catalyst is the metal center, as it is the active site of a catalyst, and the choice of metal often governs the overall properties of a catalytic system: overpotential, operating pH window, decomposition routes. As shown in section 8.08.3.2, metal oxo is a crucial element of the catalytic cycle, as it usually participates in the rate-determining step and is typically the most oxidized species in the cycle. These characteristics imply that metal oxo stability is both essential and challenging. While it is usually the primary role of the ligands to stabilize the high oxidation states of a metal, the choice of a metal itself is determining. Group 4 metals are the first d-elements, which makes their high-valent states less stable than those of heavier d-elements, such as noble metals. Similarly, late transition metals typically reach high oxidation states more easily. Within the same period, molecular complexes with group 4 metals tend to be less stable, in part due to higher susceptibility to nucleophilic attacks by the solvent molecules and ions.184

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Fig. 36 (A) Diversity of the formation energy of a metal oxo (DEoxo) and b-HOMO level energy of a non-oxo metal complex, which is often used to estimate the stability of metal oxo. Absence of correlation indicates that more advanced methods must be used to assess the probability of a stable metal oxo formation. Reprinted with permission from ref. Nandy, A.; Zhu, J.; Janet, J. P.; Duan, C.; Getman, R. B.; Kulik, H. J., Machine Learning Accelerates the Discovery of Design Rules and Exceptions in Stable Metal–Oxo Intermediate Formation. ACS Catal. 2019, 9 (9), 8243–8255. Copyright 2019 American Chemical Society. (B) HOMO-LUMO mixing in broken-symmetry methods allows to clearly differentiate the complexes with O-radical character. Reproduced with permission from ref. Yamaguchi, K.; Isobe, H.; Shoji, M.; Miyagawa, K.; Yamanaka, S.; Kawakami, T.; Nakajima, T., Development of Broken-Symmetry (BS) Methods in Chemical Reactions. A Theoretical View of Water Oxidation in Photosystem II and Related Systems. J. Photochem. Photobiol. A 2020, 402, 112791. Copyright 2020 Elsevier.

Metal oxo species can be hard to isolate or even observe, as they are the most reactive species.241,250–252 Computational methods are commonly used to assess the stability, spin state and geometry of a certain metal oxo complex, as all these parameters can be severely influenced by the ligands (Fig. 36A).228,253 The spin state of the complex, in particular, the spin density on the terminal oxygen atom, defines the possibility of radical mechanisms, such as I2M, and the likelihood of the ligand oxidation (Fig. 36B).227,253

8.08.3.3.2

First coordination sphere

In the majority of inorganic materials with high-valent metals the first coordination sphere consists of oxygen atoms, less oftendchalcogenides or nitrides. While both chalcogenides254,255 and nitrides256,257 were successfully used in water oxidation catalysis, especially photoelectrocatalysis,210 they are prone to oxidation and therefore decomposition. Oxygen-rich materials, in turn, possess an advantage of sharing atoms in their structure with the substrate (water). This enables self-reconstruction properties,258,259 inner-sphere nucleophilic attack,244 and structure flexibility.243 Most importantly, electronegative atoms and generally electronwithdrawing groups in the first coordination sphere can act as an internal base for the incoming water or hydroxide ion, facilitating nucleophilic attack by abstracting a proton. In the inorganic material catalysts, this effect can be assessed via computational studies or isotope labeling. Molecular catalysts, on the other hand, can be tailored precisely for a more direct comparison. The influence of the introduction of negatively charged ligands in the first coordination sphere is best illustrated by typical mononuclear Ru complexes that operate via a WNA mechanism (Fig. 37). Progressive replacement of pyridine groups with monodentate

Fig. 37 Influence of negatively charged ligands on the water coordination and proton abstraction in: (A) Ru-hqc catalyst (Ru(hqc)(pic)2, hqc ¼ 8hydroxyquinoline-2-carboxylate). Reprinted with permission from ref. Tong, L.; Wang, Y.; Duan, L.; Xu, Y.; Cheng, X.; Fischer, A.; Ahlquist, M. S. G.; Sun, L., Water Oxidation Catalysis: Influence of Anionic Ligands upon the Redox Properties and Catalytic Performance of Mononuclear Ruthenium Complexes. Inorg. Chem. 2012, 51 (6), 3388–3398. Copyright 2012 American Chemical Society. (B) Ru-tda catalyst. Reprinted with permission from ref. Matheu, R.; Ertem, M. Z.; Benet-Buchholz, J.; Coronado, E.; Batista, V. S.; Sala, X.; Llobet, A., Intramolecular Proton Transfer Boosts Water Oxidation Catalyzed by a Ru Complex. J. Am. Chem. Soc. 2015, 137 (33), 10786–10795. Copyright 2015 American Chemical Society.

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carboxylates both facilitates the coordination of water substrates via hydrogen bonding and the abstraction of the proton during water nucleophilic attack.232,260–263 Apart from the composition of the first coordination sphere, its dynamics is vitally important for the reaction to occur. Coordinative saturation stabilizes oxidized metal but can be detrimental, if no available site for water is present. Inner sphere ligand exchange243 indicates that the structure is flexible, and that temporary decoordination might occur, opening a free site for water nucleophilic attack. Both decoordination and flexibility of the first coordination sphere are beautifully illustrated in a 7coordination phenomenon in Ru-bda and Ru-tda catalysts.184,232,264

8.08.3.3.3

Second coordination sphere

The second coordination sphere in molecular and material catalysts (including POMs) are typically quite different. Molecular catalysts usually bear organic ligands, therefore, second coordination sphere controls the flexibility of the ligand framework and electronic effects. The so-called FAME (flexible, adaptive, multidentate, and equatorial) ligands in one of the most successful catalytic structures Ru-bda and Ru-tda are the best examples of the second coordination sphere design (Fig. 38).183,184 Flexibility and adaptability ensure that both coordination of water and accommodation of three different oxidation steps (RuIII to RuV) are possible. To a certain degree, flexibility needs to be negotiated with the stability of a complex, to avoid excessive entropically-favored decoordination of the ligands. Multi-denticity allows controlled decoordination232,264 and minimizes associated entropic losses. Finally, a separation between apical and equatorial positions in molecular catalysts makes a significant difference, as one type of the ligands located in the trans-position to the oxo ligand controls the overall reactivity of the oxygen, and another type of ligands can be used for adjusting the net electron density on the metal center or engaging in other interactions.224,265 In material catalysts, second coordination sphere typically consists of another metal unit center, surrounded by m-oxo bridges. In cases when one of the metal centers in the second coordination sphere is also undergoing a catalytic transformation into a metal oxo, it can either participate in an I2M mechanism266 or serve as a hydrogen acceptor in the two successive deprotonations of a water molecule.208 Sometimes even a metal vacancy can create a significant electronic effect.267 For catalysts, functionalized with organic ligands, second coordination sphere has a similar role to the one in molecular catalysts: electronic modulation268–270 and assistance in deprotonations.159

8.08.3.3.4

Microscopic environment

Definition of the environment beyond the second coordination sphere is quite vague, but it can include the fragments in proximity to the metal center, which do not belong to the ligands or connected to the metal via a short covalent bond network. Natural OEC is surrounded by a sophisticated protein environment, that plays several crucial roles in both proton/electron transfers and other processes, directly related to the OeO bond formation.271 One of such roles is a formation of a stable hydrogen-bond network that allows fast delivery of water substrate and removal of protons.272 While such level of protein engineering is currently not reachable in artificial water oxidation catalysis, certain efforts in mimicking this architecture has been made. For example, a stable hydrogen-bonded network of water molecules, detected by X-ray diffraction, has been observed in a confined cavity of a macromolecular catalyst m-F-MC3.178 Placing a water oxidation catalyst into an existing metalloenzyme is another strategy.273 For molecular catalysts, operating via intermolecular I2M mechanism, a number of strategies has been used to promote intermolecular coupling, including embracing non-covalent interactions274,275 and integrating the catalysts in a polymer network276 (Fig. 39).

Fig. 38 First and second coordination sphere design, exemplified by a Ru-bda catalyst. Reprinted with permission from ref. Zhang, B.; Sun, L., Rubda: Unique Molecular Water-Oxidation Catalysts with Distortion Induced Open Site and Negatively Charged Ligands. J. Am. Chem. Soc. 2019, 141 (14), 5565–5580. Copyright 2019 American Chemical Society.

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Fig. 39 Beyond-second coordination sphere engineering in Ru-bda water oxidation catalysts: (A) Network of water molecules near the active site. Reproduced from ref. Meza-Chincha, A.-L.; Lindner, J. O.; Schindler, D.; Schmidt, D.; Krause, A.-M.; Röhr, M. I. S.; Mitric, R.; Würthner, F. Impact of Substituents on Molecular Properties and Catalytic Activities of Trinuclear Ru Macrocycles in Water Oxidation. Chem. Sci. 2020, 11 (29), 7654–7664 with permission from the Royal Society of Chemistry (from an Open Access Article under Creative Commons Attribution 3.0 Unported License). (B) Set of weak interactions, making I2M pathway preferred over WNA mechanism. (C) Pre-organization of catalyst molecules between two monomers in a polymer network. Adapted from ref. Li, Y.; Zhan, S.; Tong, L.; Li, W.; Zhao, Y.; Zhao, Z.; Liu, C.; Ahlquist, M. S. G.; Li, F.; Sun, L., Switching the OO Bond Formation Pathways of Ru-pda Water Oxidation Catalyst by Third Coordination Sphere Engineering. Research 2021, 2021, 9851231 according to the Creative Commons Attribution License CC BY 4.0.

8.08.3.4

Artificial analogs of OEC

Since the structure of natural OEC has been elucidated, it has served as an inspiration to develop artificial catalysts of similar architecture.23,277 One of the objectives of this research was not only to create an active artificial analog of OEC, but primarily to develop a simpler homogeneous system, serving as a model for natural OEC, which could be studied by a wider range of techniques. Even before the actual structure of OEC was determined, a cubane-like structure with manganese ions bridge by oxygen atoms has been proposed.278 A phosphinate-bridged cluster Mn4O4(O2PPh2)6 was developed by Dismukes and coworkers,279 becoming one of the earliest models to study the dynamics of the cubane core under oxidation processes.280 The success of this model was enhanced by its catalytic performance: over 1000 turnovers were reached during electrolysis in neutral conditions.281 Further development of Mn4O4(O2PPh2)6 is hampered by significant differences in the geometry from the actual OEC, realization of the importance of calcium ion in the cubane structure,282,283 and potential instability leading to the formation of self-healing oxide-like catalyst.258 Some of these issues have been mitigated by Agapie and coworkers through the synthesis of more flexible and stable clusters LMn4O4(OAc)3 (L ¼ 1,3,5tris(2-di(20 -pyridyl)hydroxymethylphenyl)benzene or its derivatives),284 capable of exchanging metal ions (Fig. 40).283,288 In contrast to Mn4O4 clusters, cobalt cubanes are more common due to the simpler synthesis,289 and are often used as cocatalysts for (photo)electrocatalysts.290 Unlike many manganese cubanes, which are primarily used to model S0–S3 states,291 Co4O4-based catalysts are active in water oxidation and therefore are extremely useful in modelling a crucial OeO bond formation step in the OEC. Both WNA and I2M mechanisms were proposed for cobalt cubanes. Based on the observations of a reaction between oxidized CoIII3CoIVO4 cluster and hydroxide ion, resulting in the oxygen evolution, Tilley et al. proposed a WNA mechanism.292 Dismukes and coworkers proposed a different mechanism, in which a geminal coupling of two hydroxyls bounded to a single cobalt ion in a CoIII2CoIV2O4 cluster yields an OeO bond, however, it is not detailed whether one of the hydroxyls gets oxidized, leading to an intramolecular WNA, or two hydroxyls participate in a radical-type coupling.293 A strikingly different I2M mechanism was proposed by Nocera and coworkers, resembling an intramolecular radical coupling between two ligandbridged metal oxos with radical character in molecular catalyst Ru-bda.266 Various attempts to alter the electronic properties of cobalt cubanes have been made, thanks to the robust structure of the cluster, allowing to vary both peripheral organic ligands and oxygen bridging ligands.269,294 Increased electron donation stabilizes high-valent cobalt sites, effectively resulting in higher stability and activity of the catalysts.269 Such modifications push the cobalt cluster performance to its probable limit, highlighting the superiority of manganese in natural OEC, which features simple oxygen bridges and weakly bound amino acid ligands.164 A long-standing ultimate goal in artificial water oxidation is an exact synthetic recreation of a natural OEC. Aforementioned Mn4O4 clusters developed by Agapie and coworkers were capable of exchanging one or more manganese ions in the structure with other cations, including redox-inactive, such as Ca2 þ.283,284 This allowed to rationalize the value of calcium ion and its role in the OEC as an auxiliary ion, facilitating Mn oxidation.283 The presence of a single big ligand and the absence of a dangling manganese ion were the key differences between clusters LMn4O4(OAc)3 and OEC. Both of these issues were solved in a breakthrough work of Christou and coworkers, presenting cluster Mn3Ca2O4(O2CtBu)8(tBuCO2H)4 with only carboxylate groups as peripheral ligands and a dangling calcium ion.285 While the calcium ion was simply coordinated to one of the bridging oxygen atoms, Agapie et al. further developed a CaMn3O4 cluster with a silver ion attached through a oxo bridge to two of the manganese ions, effectively mimicking the dangling manganese fragment in the OEC.286 The final effort by Zhang et al. afforded an almost completely identical core structure of the OEC (missing only an oxygen bridge between the dangling manganese atom and the cluster core) with an impressively similar peripheral ligand framework.160 An access to the complete synthetic OEC model now allows researchers to modify it and compare directly with natural OEC in order to establish the exact structure of S1–S3 states in the OEC.295

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Fig. 40 Evolution of artificial models of natural OEC: (A) Mn4O4(O2PPh2)6 by Dismukes and coworkers.279 Phenyl groups of phosphinate ligands are omitted for clarity. (B) LMn4O4(OAc)3 by Agapie and coworkers.284 (C) Mn3Ca2O4(O2CtBu)8(tBuCO2H)4 by Christou and coworkers.285 Tert-butyl groups are displayed as single carbons for clarity. (D) LMn3CaO4(ON4O)(OAc)$AgOTf by Agapie and coworkers.286 (E) CaMn4O4 cluster by Zhang et al. and its structural and electronic similarity to a natural OEC.160,287 Reproduced from ref. Zhang, C., The First Artificial Mn4Ca-Cluster Mimicking the Oxygen-Evolving Center in Photosystem II. Sci. China Life Sci. 2015, 58 (8), 816–817 according to the Creative Commons Attribution License CC BY 4.0.

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Photochromic materials

H. Maeda , M. Nishikawaa, R. Sakamotob, and H. Nishiharaa, a Tokyo University of Science, Noda, Chiba, Japan; and b Tohoku University, Sendai, Miyagi, Japan a

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8.09.1 8.09.2 8.09.2.1 8.09.2.2 8.09.2.2.1 8.09.2.2.2 8.09.2.2.3 8.09.2.2.4 8.09.2.2.5 8.09.2.2.6 8.09.2.3 8.09.2.3.1 8.09.2.3.2 8.09.2.4 8.09.2.4.1 8.09.2.4.2 8.09.2.5 8.09.2.5.1 8.09.2.5.2 8.09.2.5.3 8.09.3 8.09.3.1 8.09.3.2 8.09.3.3 8.09.3.3.1 8.09.3.3.2 8.09.3.3.3 8.09.3.3.4 8.09.3.3.5 8.09.3.4 8.09.3.4.1 8.09.3.4.2 8.09.3.4.3 8.09.3.4.4 8.09.3.5 8.09.3.6 8.09.3.7 8.09.4 References

General introduction Photochromic transition metal complexes and organometallics Introduction Photochromism with linkage isomerization Nitrito complexes and organometallics Nitrosyl complexes and organometallics Ruthenium sulfoxide complexes Rhodium dithionite cluster complexes Linkage isomerization with organic ambidentate ligands Other linkage isomerization complexes Photochromism with intramolecular ligand exchange reactions Rotaxanic and catenanic copper complexes Catenanic ruthenium complexes Photochromism with bond reorganization Fulvalene-containing organometallics Haptotropic rearrangement Photochromism without large structural changes Light-induced excited spin state trapping (LIESST) Light-induced charge transfer-induced spin transition Light-induced electron transfer-induced second-order nonlinear optical switching Interplay among transition metal complexes, organometallics, and organic photochromics Introduction Organic photochromics Control over the photo-, electro-, and magneto-properties of transition metal complexes and organometallics via photoisomerization of organic photochromics ON/OFF switching of luminescence Modulation of electronic communication in MV states Control over the magnetism Regulation of the coordination environment around a metal center NLO switching Control over the isomerization behavior of organic photochromics by transition metal complexes and organometallics Electrochromism triggered by redox switching of metal moieties ON/OFF switching of photochromism via redox switching of the metal centers Modulation of the photoresponsive wavelength NIR photochromism Mutual controls Multiphotochromic systems Other types of conjugates Conclusion

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Abstract This chapter considers transition metal complexes and organometallics that display photochromic properties. First, coordination compounds that possess photochromic characteristics inherently are described. Especially, physical properties of coordination compounds controlled by photoisomerization are highlighted. Second, chromic transition metal complexes and organometallics that are themselves nonchromic but are linked to photoisomerizable organic molecules are treated. Organic photochromics can modulate the photo-, electro-, and magneto-properties of transition metal complexes and organometallics. Contrarily, the photochromic properties of organic photochromics are controllable by metal-containing molecules. In addition, sophisticated molecular systems where coordination compounds and organic photochromic

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https://doi.org/10.1016/B978-0-12-823144-9.00089-3

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molecules control their properties mutually are known. Several multiphotochromic systems and uncategorizable conjugates are also enumerated.

8.09.1

General introduction

Photochromism is a reversible transformation of a single chemical species being induced in one or both directions by electromagnetic radiation between two states having different distinguishable absorption spectra. The radiation changes may be induced by UV visible and IR radiation. Reversibility is the important criterion. All irreversible reactions are normal photochemistry.1,2 Photochromic molecules have attracted a lot of attention due to their potential applications in the field of photon-mode high-density information storage and photo-switching devices. Many photochromic molecules have been discovered so far. The study of such compounds considers adjusting photochromics and other properties by introducing appropriate substituents, as well as the development of hybrid materials with polymers and liquid crystals to achieve conversion of photo-signals into other signals. On the other hand, in the field of molecular-scale electronics, molecules that can function as devices by themselves are attracting much recent attention. Such molecules must exhibit bistable or multi-stable state that can be reversibly interconverted by applying external stimuli such as photons, electrons, protons, and magnetic field. Metal complexes are excellent candidates known to exhibit unique physical properties that can respond to a variety of external stimuli. Therefore, the combination of metal complex units with photochromic molecular units within a single molecule is an effective approach to creating a device-performing single molecule. This chapter considers transition metal complexes and organometallics that display photochromic properties. Section 8.09.2 describes photochromic coordination compounds that possess such characteristics inherently. These sections especially emphasize control over the physical properties of coordination compounds. Section 8.09.3 treats chromic transition metal complexes and organometallics that are themselves nonchromic but are linked to photoisomerizable organic molecules. In addition to modulations of the photo-, electro-, and magneto-properties of transition metal complexes and organometallics by organic photochromics, this chapter focuses on control over the photochromic properties of organic photochromics by metal-containing molecules. Also discussed are sophisticated molecular systems where coordination compounds and organic photochromic molecules control their properties mutually. Several multiphotochromic systems and uncategorizable conjugates are also enumerated in this section. Several reviews related to photochromic metal complexes have been published.3–19

8.09.2

Photochromic transition metal complexes and organometallics

8.09.2.1

Introduction

Some photochromic properties based on linkage isomerization and ligand substitution can be associated with the corresponding electrochemical linkage isomerization and ligand substitution. Photoexcitation and subsequent intermolecular CT are practically indistinguishable from electrochemical redox reactions. Some photoexcited states can also induce linkage isomerization and ligand substitution reactions. MLCT and LMCT transitions can change the oxidation number of a metal center in the photoexcited states virtually, thereby promoting linkage isomerization in a manner similar to the corresponding electrochemical linkage isomerization. The eg ) t2g photoexcited state in octahedral metal complexes promotes ligand dissociation because the t2g orbitals are nonbonding and the eg orbitals are antibonding.20 Transition metal complexes and organometallics tend to have long-lived triplet photoexcited states due to intersystem crossing induced by the heavy atom effect. This feature gives metal-ligand bonds a greater chance to reconstitute. Other types of photochromic behavior are available in transition metal complexes and organometallics. Metal-metal, intraligand bond rearrangements, haptotropic rearrangements, as well as intramolecular electron transfer have been observed upon photoexcitation. At low temperatures, metastable photoexcited states, such as d-d and IVCT excited states, can be trapped. These metastable states are not accompanied by significant structural changes; however, changes in the electronic structures greatly impact the observed colors and physical properties. This series of phenomena is chiefly appreciated in photomagnetism.

8.09.2.2 8.09.2.2.1

Photochromism with linkage isomerization Nitrito complexes and organometallics

Nitrito complexes, which were the first examples of linkage isomerizable coordination compounds studied, later were found to display photochromism.21–30 For example, the nitrito-N,N form of trans-RuII(py)4(NO2)2 could be transformed into the metastable nitritoO,O form below 250 K with conversion ratios of either 50% or 80% in either the solid state or in a frozen methanol-ethanol solution, respectively, upon irradiation with 325 nm (Scheme 1).22 Heating above 250 K afforded full conversion to the stable nitrito-N,N isomer. On the other hand, the illumination of light over the range 405–442 nm resulted in an imperfect back reaction, presumably giving rise to the nitrito-N,O isomer as well as the nitrito-N,N isomer. Both forward and backward photoreactions were initiated by excitation of Ru(d) / p*(NO2) and Ru(d) / p *(py) transitions, which was supported by time-dependent density functional theory (TDDFT) calculations. Photoisomerization between the nitrito-N and nitrito-O isomers was found in CoIII,24–29 PtIV,30 and IrIII 21

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Scheme 1

Photochromism of trans-RuII(py)4(NO2)2.

complexes. Organometallic CpRu(CO)2NO2 was reported to undergo a photoresponse that involved formation of a h2-coodination complex with respect to the nitrito ligand.31

8.09.2.2.2

Nitrosyl complexes and organometallics

8.09.2.2.3

Ruthenium sulfoxide complexes

Scheme 2

Photochromism of SNP.

Since the first discovery of the titled compounds,32 a wide variety of transition metal complexes and organometallics that feature nitrosyl linkage isomerization have been synthesized.33,34 Linkage isomerization of one of the most frequently studied molecules, sodium nitroprusside (Na2[Fe(CN)5NO]$ 2H2O, SNP), is described in Scheme 2.35–42 In the ground state, SNP adopted the Nbound configuration. Upon irradiation with 350–580 nm light, which was ascribable to the 3dxz, 3dyz / p*(NO), and 3dxy / p*(NO) transitions,35,36 crystals of SNP underwent a transformation to form two metastable states with decay temperatures of 195 K and 150 K, respectively.37 Carducci, Coppens, and coworkers performed XRD analysis at low temperatures to identify the two metastable states as O-bound (isonitrosyl) and h2-NO (side-on) species (Scheme 2).38–40 Photoisomerization of SNP in solution was also investigated. Schaniel and coworkers performed transient absorption experiments in aqueous solutions and detected the formation of the h2-NO species with a lifetime of 110 ns.41 Khalil and coworkers investigated this photoisomerization series in methanol at room temperature using transient IR spectroscopy techniques.42 Linkage photoisomerization of nitrosyl ligands has also been observed in Ru and Os complexes, porphyrinic Fe and Ru complexes, and organometallic Ni, Mn, and V compounds.33,34

Yeh et al. identified reversible electrochemical isomerization of S-bound [Ru(NH3)5(dmso)]2 þ (dmso dimethylsulfoxide, Scheme 3), where dmso acted as an ambidentate ligand.43 Oxidation of this complex from RuII to RuIII results in linkage isomerization of the coordinating dmso ligand to the O-bound [Ru (NH3)5(dmso)]3 þ. On the other hand, the reduction of RuIII to RuII results in conversation to the original S-bound [Ru(NH3)5(dmso)]2 þ. This behavior can be intuitively rationalized based on the hard and soft (Lewis) acids and bases (HSAB) theory. The RuII center has a preference for ligands that can accept p-back-bonding (soft ligands), whereas RuIII prefers ligands with strong s-donating properties (hard ligands). Because dmso presents both soft (S) and hard (O) donor atoms, it coordinates through the atom better suited to the nature of the metal center. The dramatic difference between the redox potentials is noteworthy: the E1/2 values for the RuIII/RuII redox couple are 1.0 V (vs. standard hydrogen electrode, SHE) and 0.01 V in the S- and O-isomers, respectively. Taking advantage of this result, Sano and coworkers set up a ‘molecular hysteresis’ system using (1,5-dithiacyclooctane 1-oxide)bis (pentaammineruthenium).44,45

Photochromic materials

Scheme 3

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Electrochemical linkage isomerization in [Ru(NH3)5(dmso)]2 þ.

Similar electrochromic properties were reported by Johansson and Lomoth in ruthenium-polypyridine complexes that contained 1-[6-(2,20 -bipyridyl)]-1-(2-pyridyl)-ethanol as an ambidentate ligand and featured N- and O-bound isomers in the RuII and RuIII states, respectively.46 This molecule was also incorporated into an electrochemical hysteretic system.47 This series of studies on redox-induced linkage isomerization in Ru-dmso complexes encouraged researchers to construct a photochemically driven isomerization system comprising RuII complexes with polypyridine ligands because this class of complexes possesses a relatively long-lived (ms) photoexcited state assignable to a triplet MLCT (3MLCT). In this photoexcited state, the Ru center has a RuIII-like character,48 which should prefer O-bonding to S-bonding, as in the case of an electrochemically oxidized RuIII center. Smith et al. disclosed that [Ru(bpy)2(dmso)2]2 þ undergoes photoisomerization from the stable S-bound species to the metastable O-bound species, with a subsequent thermal back reaction in DMSO.49 This intermolecular reaction occurred between the complex and the surrounding solvent. The S- to O-photoisomerization occurred only in DMSO or noncoordinating solvents containing DMSO. The authors asserted that the back O- to S-reaction proceeds by an intermolecular mechanism. Rack, Winkler, and Gray clarified that [Ru(tpy)(bpy)(dmso)]2 þ showed photochromism in solution (in DMSO, acetone, and tetrahydrofuran (THF)), in a polymer matrix (poly(methyl methacrylate), PMMA), and, surprisingly, in the crystalline state.50 Based on these experimental data, the authors concluded that this series of isomerization reactions proceeded exclusively via intramolecular processes. The photochromic behavior of this series of compounds was extensively investigated by McClure et al.51; the authors substituted the bpy ligand of [Ru(tpy)(bpy)(dmso)]2 þ with other bidentate ligands to tune the electronic state of the complex.52 With the aid of transient absorption spectroscopy53 and computational chemistry,54 the authors concluded that the isomerization proceeded not via an 3LF (3d-d) state, but through a 3MLCT state via an adiabatic process on a timescale of subnanoseconds to nanoseconds (Fig. 1A). The authors also reported that the trans effect of the coordinating O atom of picolinate significantly increased the quantum yield of isomerization, resulting in fairly different isomerization efficiencies for the two dmso ligands coordinating to the same ruthenium center (Scheme 4).55 This series of photoisomerization reactions was accompanied by a large change in the redox potential for the RuIII/RuII couple.52 An osmium analog was also investigated by the same authors.56 The monodentate dmso ligand is labile in the O-form, resulting in substitution by, for example, solvent molecules. In an attempt to overcome this shortcoming, several types of chelating sulfoxide were designed and installed.57–60 Among them, [Ru(bpy)2(OSO)]þ (OSO ¼ 2-methylsulfinylbenzoate) followed a different S- to O-photoisomerization mechanism, or nonadiabatic pathway, from the 3MLCT excited state (Fig. 1B).59 Another defect in the ruthenium photochromic system was a lack of photochemical channels by which the O- to S-reaction could proceed: Only the thermal channel was available. A two-way photochromic molecule with a chelating sulfoxide was described by McClure et al. (Scheme 5).61 Light of wavelength 355 or 470 nm induced excitation of the 1MLCT band in the S- or O-isomers, respectively. Suzuki et al. synthesized [Ru(tpy)(picSO)]BF4 (picSO: 6-[(methylsulfinyl)methyl]picolinate), which undergoes linkage isomerization behavior between stable S-bound and metastable O-bound isomers upon irradiation with 436 nm and 546 nm light, respectively (Scheme 6).62 This complex shows also electrochemical linkage isomerization. Chelating double sulfoxide complexes which cause two photoisomerization by a single photon was found by Garg et al. (Scheme 7).63 Femtosecond transient absorption spectroscopy indicates that this reaction is complete within a few hundred picoseconds and suggest that isomerization occurs along a conical intersection seam formed by the ground-state and excited-state potential energy surfaces.

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Fig. 1 State diagrams for: (A) [Ru(tpy)(bpy)(dmso)]2 þ and (B) [Ru(bpy)2(OSO)]þ. The superscripted asterisk and ‘3’ denote a photoexcited state and a triplet state, respectively. Reproduced from McClure, B. A.; Rack, J. J. Eur. J. Inorg. Chem. 2010, 3895–3904.

Scheme 4 Photochromism of [Ru(pic)2(dmso)2] upon irradiation with 344 nm UV light. F and k indicate the photoisomerization quantum yield and the first-order rate constant for thermal isomerization, respectively.

Photochromic materials

Scheme 5

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Photochromism of [Ru(bpy)2(pySO)]2þ. bpy ¼ 2,20 -bipyridine, pySO ¼ 2- ((isopropylsulfinyl)methyl)pyridine.

Scheme 6 Two-way linkage photoisomerization of [Ru(tpy)(pic-SO)]þ. Reproduced from Suzuki, S.; Sakamoto, R.; Nishihara, H., Chem. Lett. 2013, 42, 17–18.

Scheme 7 Two photochromic ruthenium sulfoxide complexes that feature two isomerization reactions following absorption of a single photon. Reproduced from Garg, K.; King, A. W.; Rack, J. J. J. Am. Chem. Soc. 2014, 136, 1856–1863.

Instead of diimine ligands, triphenylphosphine based chelating sulfoxide ligands were employed for the photochromism by Ziegler, Webster, and Rack (Scheme 8). 64 Introduction of different substituents on the para position of the phenyl group on phosphine causes a dramatic range in photoisomerization reactivity. This is ascribed to differences in the electron density of the phosphine ligand donated to the ruthenium and the nature of the excited state. Computational approaches have been applied to this series of ruthenium complexes to gain insight into the photoisomerization process.54,59,65

8.09.2.2.4

Rhodium dithionite cluster complexes

Nakai and Isobe have described the photochromic behavior of rhodium dinuclear cluster complexes (Scheme 9).66–70 These photochromic reactions involve the isomerization of a dithionite ligand that bridges two rhodium centers: The m-O2SSO2 bridge is

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Photochromic materials

Scheme 8 Molecular structures determined from the X-ray analysis of the S-bonded isomers of (bpy)2Ru(P,S-SOMeC6H4P(C6H5)2) (left), (bpy)2Ru(P,S-SOMeC6H4P(C6H4OCH3)2) (center), and (bpy)2Ru(P,S-SOMeC6H4P(C6H4CF3)2) (right). Reproduced from Kosgei, G. K.; Breen, D. J.; Lamb, R. W.; Livshits, M. Y.; Crandall, L. A.; Ziegler, C. J.; Webster, C. E.; Rack, J. J., J. Am. Chem. Soc. 2018, 140, 9819–9822.

Scheme 9 Photochromism of rhodium dithionite dinuclear cluster complexes based on linkage isomerization of the dithionite ligand. The asterisk indicates an asymmetric sulfur atom.

converted to the m-O2SOSO bridge. The reaction proceeded without destruction of the crystallinity, thereby attaining complete conversion between the isomers in the crystal phase. The photoproduct features an asymmetric sulfur atom. When R ¼ n-propyl, chiral crystals are generated, which undergo enantioselective photoisomerization with an enantiomeric excess (ee) of > 90%.68,69 This photoisomerization involves the conformation change of the ethyl group of the n-propyl group. According to time-dependent X-ray diffraction analyses, the n-propyl group flips by the photoisomerization (circled by blue and red lines in Scheme 10), and the degree of the flipping changes non-linearly with the degree of the photochromic reaction.71

8.09.2.2.5

Linkage isomerization with organic ambidentate ligands

Burkey and coworkers found that cyclopentadienylmanganese(I) dicarbonyl displays photochromism in the presence of an additional ligand, 3-(cyanomethyl)pyridine, which is nonchelatable but is ambidentate (Scheme 11A).72 Illumination with visible light of the pyridine-coordinating form, which is associated with excitation of an MLCT transition from Mn(d) to pyridine(p*), affords linkage isomerization, giving rise to the nitrile-appended isomer. On the other hand, the nitrile-appended form undergoes reverse linkage isomerization upon excitation with an MLCT transition from Mn(d) to nitrile (p*) with UV light. This series of photoreactions lacks reversibility with respect to irradiation with UV light and produces unidentified byproducts; however, the addition of free 3-(cyanomethyl)pyridine allows this system to acquire sufficient reversibility. These experimental data indicate that some portion of the photoisomerization reaction proceeded via a bimolecular process. The authors also performed linkage one-way photoisomerization and thermal reversion in cyclopentadienylmanganese(I) dicarbonyl tethered with an ambidentate ligand, but again, this photoisomerization system suffered from slight photodegradation.73 The authors also reported a two-way linkage photoisomerization in arenechromium(0) dicarbonyl with an ambidentate ligand containing olefin and pyridine moieties (Scheme 11B).74 This series of isomerization

Scheme 10 ORTEP drawings before photoirradiation (left) and after 90 min photoirradiation (right). Reprodueced from Nakai, H.; Miyata, S.; Kajiwara, Y.; Ozawa, Y.; Abe, M., Dalton Trans. 2020, 49, 1721–1725.

Photochromic materials

Scheme 11

363

Linkage photoisomerizations: (A) cyclopentadienylmanganese(I) dicarbonyl and (B) arenechromium(0) dicarbonyl.

reactions is based on the excitation of an MLCT transition from Cr(d) to the olefin or pyridine(p*). Taking advantage of the IR-active carbonyl groups, this photochromic system was subjected to time-resolved infrared (TRIR) studies to identify an ultrafast photoisomerization process ( 200 ns). Unfortunately, this photoisomerization was again accompanied by the generation of byproducts. Another type of linkage isomerization of organometallic compound synthesized by Kelbysheva et al. is a manganese complex bearing cyclopentadienyl ligand which connects to thioureido cymantrene moiety (Scheme 12).75 The manganese has bond with carbon-carbon double bond in the ligand in one isomer, and the manganese binds sulfur atom in the ligand in another isomer. The two isomers are repeatedly changed upon light and heat stimuli.

8.09.2.2.6

Other linkage isomerization complexes

The ambidentate SO2 ligand can undergo linkage photoisomerization at low temperature in the solid state.76 N2 usually adopt an h1 coordination mode, whereas irradiation with light (330 < l < 460 nm) at 100 K in the solid state results in h2-(side-on) coordination.77 Linkage isomerization in ruthenium complexes containing 3-(pyridine-2-yl)-1,2,4-triazole derivatives was intensively studied, and the process proceeded through ligand dissociative 3d-d excited states.78–81

8.09.2.3 8.09.2.3.1

Photochromism with intramolecular ligand exchange reactions Rotaxanic and catenanic copper complexes

Sauvage and coworkers developed a catenanic CuI complex exhibiting electrochromic behaviors.82,83 One of the two rings that formed the catenane structure includes only one coordination site: bidentate 1,10-phenanthroline (phen), whereas two coordination sites, phen and the tridentate 2,20 :60 ,200 -terpyridine (tpy), are available on the other ring (Scheme 13).82 In the CuI state, this complex exhibits a tetrahedral configuration with respect to the metal center, which is formed by the two phen ligands. Oxidation of the metal center to CuII yielded a slow color change (completion within a few days). A significant shift in the redox potential of the CuII/CuI couple occurs upon the electrochemical linkage isomerization. The authors customized this supramolecular electrochromic system to undergo more rapid redox reaction by employing rotaxane chemistry.84–86

Scheme 12 Combined scheme of transformations of dicarbonyl complexes of cyclopentadienylmanganese with thioureido cymantrene derivatives. Reproduced from Kelbysheva, E. S.; Telegina, L. N.; Strelkova, T. V.; Ezernitskaya, M. G.; Smol’yakov, A. F.; Borisov, Y. A.; Lokshin, B. V.; Konstantinova, E. A.; Gromov, O. I.; Kokorin, A. I.; Loim, N. M., Organometallics 2019, 38, 2288–2297.

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Scheme 13 Electrochemically driven linkage isomerization in a catenanic copper complex. Reproduced from Livoreil, A.; Dietrich-Buchecker, C. O.; Sauvage, J.-P. J. Am. Chem. Soc. 1994, 116, 9399–9400.

Based on the systems above, Armaroli, Balzani, Sauvage, and coworkers set up a photochromic rotaxanic system in combination with the electrochromic rotaxane and intermolecular CT reactions.87 A copper-containing rotaxane (Fig. 2) showed electrochromism upon the CuII/CuI redox switching (Scheme 14A). The principal mechanism of this electrochromism resembles that shown in Scheme 15. The CuI center prefers tetracoordination afforded by the two phen ligands, whereas the CuII nucleus favor pentacoordination with one phen and one tpy ligands. This series of intramolecular ligand exchange reactions was successfully driven through photochemical means (Scheme 14B). In the presence of an electron acceptor (p-NO2C6H4CH2Br) and tetrabutylammonium tetrafluoridoborate (TBABF4) in acetonitrile, the tetracoordinated CuI center underwent photochemical oxidation and subsequent isomerization to the pentacoordinated CuII center upon irradiation with 464 nm light. However, an addition of ascorbic acid to the resultant pentacoordinated CuII species induced reduction to CuI, which was accompanied by isomerization to the tetracoordinated CuI form. The author noted that TBABF4 was essential for the reversible isomerization, which may facilitate the change in the coordination number at the copper center. Sauvage, Balzani, and coworkers also set up a catenanic photochromic system driven by the identical mechanism.88

8.09.2.3.2

Catenanic ruthenium complexes

Photodissociation of ligands from metal complexes has been extensively studied in a variety of derivatives over the past several decades.89–92 Photoreactions are characterized by loss of a ligand and coordination of a solvent molecule. In nonpolar solvents, the coordinating solvent molecule can be replaced by counterions, added ions, or residual water. Thus, in most cases, ligand photodissociation is an irreversible process. Sauvage and coworkers sophisticatedly circumvented the drawback of ligand photodissociation to fabricate a reversible system.93–95 They constructed a unique supramolecular photoisomerizable architecture (Scheme 16). A catenanic tris(diimine)RuII complex is characterized by a visible band (lmax ¼ 458 nm) that is assignable to an 1MLCT transition from Ru(d) to the diimine (p*), as in the other members of this series of complexes. Upon excitation of this band in the presence of excess Et4NCl salt, this supramolecular complex underwent a color change from red to purple (lmax ¼ 561 nm). The authors identified this photoreaction as dissociation of the bipyridine ligand and subsequent coordination of Cl. They proposed a mechanistic pathway via the 3LF state, which is thermally accessible from the 3MLCT state. The quantum yield for the photochemical reaction at 25  C and lmax ¼ 470 nm was estimated to be 0.014. The catenanic structure tactically prevented the labile bipyridine ligand from dissipating into the solution. The recomplexation of the bipyridine unit and resultant displacement of chloride ions were provided by heating the solution. The photochemical decoordination and the thermal recomplexation processes were almost quantitative (> 95%).

Fig. 2 Photochromic rotaxane with a CuI center. Reproduced from Armaroli, N.; Balzani, V.; Collin, J.-P.; Gaviña, P.; Sauvage, J.-P.; Ventura, B. J. Am. Chem. Soc. 1999, 121, 4397–4408.

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365

Scheme 14 (A) Square scheme for the electrochromism in the rotaxane. (B) Principle of the photochemically and chemically triggered rearrangement of the rotaxane. Reproduced from Armaroli, N.; Balzani, V.; Collin, J.-P.; Gaviña, P.; Sauvage, J.-P.; Ventura, B. J. Am. Chem. Soc. 1999, 121, 4397–4408.

Scheme 15 Electrochemically driven linkage isomerization in rotaxanic copper complexes. Reproduced from Poleschak, I.; Kern, J. M.; Sauvage, J.-P. Chem. Commun. 2004, 474–476.

Scheme 16 Photochemical and thermal ligand exchange in a supramolecular catenanic Ru-diimine complex. Reproduced from Bonnet, S.; Collin, J.-P.; Koizumi, M.; Mobian, P. Sauvage, J.-P. Adv. Mater. 2006, 18, 1239–1250.

8.09.2.4 8.09.2.4.1

Photochromism with bond reorganization Fulvalene-containing organometallics

Vollhardt and coworkers found that the titled dinuclear compounds show photoisomerization to (m2-h1: h5-cyclopentadienyl)2Ru2(CO)4 upon irradiation with light at the absorption maximum, 350 nm, or upon exposure to sunlight (Scheme 17).96 This photoexcitation is associated with Ru-Ru, s-s*, and dp-s* transitions. The backward reaction can be affected by the application of heat (> 65  C). Surprisingly, this thermal reaction proceeds even in the crystalline state at 208  C without loss of crystallinity.

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Photochromic materials

Scheme 17

Photochromism of (fulvalene)tetracarbonyldiruthenium.

This series of reaction involves a formal dinuclear oxidative addition to the CeC bond of the fulvalene moiety and vice versa, which contrasts with the former thesis that the CpeCp linkage in fulvalene is robust and inert in its organometallics forms.97–99 After a decade, the authors sought insight into the mechanism underlying this series of isomerization reactions, and DFT calculations suggested that at least the thermal isomerization takes place via a Ru biradical species as an intermediate.100 Prior to this work, a molybdenum analog of this dinuclear organometallic had shown a series of complicated photolysis, which had included a reaction similar to that shown in Scheme 17.101,102

8.09.2.4.2

Haptotropic rearrangement

Various organometallics have been reported to undergo photochemical haptotropic rearrangement.103–112 A good example of such compounds is (h6-indolyl)Mo(PMe3)3H, which experiences h5-photoconversion and a thermal back transformation (Scheme 18).111 (m2-h3:h5-Acenaphthylene)Fe2(CO)5 yields solid-state photochromism in KBr pellets.109

8.09.2.5

Photochromism without large structural changes

8.09.2.5.1

Light-induced excited spin state trapping (LIESST)

Scheme 18

Photochromism between (h6-indolyl)Mo(PMe3)3H and (h5-indolyl)Mo(PMe3)3H.

Spin crossover (SCO)113–116 is a phenomenon in which the spin state of a metal center is switched between low-spin (LS) and highspin (HS) states under external stimuli, such as temperature, pressure, and light. Octahedral, first-row transition metal complexes with electronic configurations of d4–d7 are known to exhibit SCO. Light-induced excited spin state trapping (LIESST),113–116 in which light-induced transitions occur from a stable LS state to a metastable HS state, has been extensively investigated. This photomagnetic effect was first reported by Decurtins and coworkers in 1984 in a study of [FeII(ptz)6](BF4)2 (ptz ¼ 1-propyltetrazole) in the solid state.117 Since then, a large number of FeII complexes with LS (S ¼ 0) and HS (S ¼ 2) states have been investigated. In the case of [FeII(ptz)6](BF4)2, the stable LS state 1A1 may be irradiated with 514.5 nm light to give rise to the photoexcited state 1T1, which relaxes to the metastable HS state 5T2 via the intermediate state 3T1 (Fig. 3). The relaxation 5T2 / 1A1 is spin-forbidden; hence, at low temperatures the 5T2 state has relatively a long lifetime: The thermal transition 5T2 / 1A1 is active at temperatures of > 50 K. Furthermore, the photoinduced 5T2 / 1A1 transition is also observed upon irradiation with 820 nm light (reverse LIESST, Fig. 3). LIESST can be regarded as a type of photochromism because the transition between the LS and HS states gives rise to a color change (i.e., MLCT and LF transitions are modulated). This transition is also accompanied by slight structural changes with respect to the coordination environment of the FeII center: The bond lengths between the metal center and the ligated atoms are extended by  0.2 Å upon the LS to HS transition due to the occupation of the antibonding eg orbitals in the latter. The thermal stability of the metastable HS state is central for the applicability of LIESST materials. A convenient parameter describing the thermal stability, T(LIESST), was proposed by Létard and coworkers.116,118–120 Fig. 4 shows a typical cMT–T plot for a LIESST compound, [FeII(PMBiA)2(NCS)2] (cMT: molecular magnetic susceptibility; PMBiA: N-20 -pyridylmethylene-4aminobiphenyl). Upon illumination with light, the cMT value increases dramatically due to the LS / HS transition. In the early stage of heating, a gradual increase in cMT, ascribable to the zero-field splitting of the HS state is observed, but cMT eventually undergoes an abrupt decrease. T(LIESST) corresponds to T and gives a minimum value of dcMT/dT when warming is conducted at a rate of 0.3 K min 1. Many studies have exploited cooperative interactions among complex molecules to express LIESST phenomena and to raise T(LIESST). For example, intramolecular interactions, such as p-p interactions,121–124 and hydrogen bonding125–128 in the crystalline state have been utilized for this purpose. With the rapid progress in the field of metal-organic framework (MOF) chemistry, SCO frameworks (SCOFs) have been developed.129–134 Kepert and coworkers reported the observation of LIESST in a SCOF134 that would lead to guest-responsive LIESST and profound cooperative effects. The champion value of T(LIESST), 132 K, was recorded by Sato and coworkers for [FeIIL(CN)2]$H2O (L ¼ [2,13-dimethyl-6,9-dioxa-3,12,18-triazabicyclo [12.3.1]octadeca-1(18), 2,12,14,16-pentaene]).135–137 Guionneau and co-workers found that a significant change accompanied

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Fig. 3 Photoexcitation and subsequent relaxation pathway of LIESST in an iron(II) complex. Reproduced from Real, J. A.; Gaspara, A. B.; Muñoz, M. C. Dalton Trans. 2005, 2062–2079.

this phase transition; the HS state featured hepta-coordination, whereas the LS state adopted hexacoordination.137 Létard and coworkers empirically described the relationships among the coordination environment at the FeII center, T(LIESST), and the thermal spin transition temperature (T1/2),116,118–120 TðLIESSTÞ ¼ T0  0:3T1=2

Fig. 4 cMT-T plot for the determination of T(LIESST) in [Fe(PMBiA)2(NCS)2]. The temperature dependence of cMT recorded in the cooling mode without irradiation is indicated by the (A) data points; the changes recorded during 1 h of irradiation at 10 K correspond to the (B) data points, and the behavior recorded during the warming mode (0.3 K min 1) after the light irradiation had been switched off is indicated by the (,) data points. Reproduced from Létard, J.-F. J. Mater. Chem. 2006, 16, 2550–2559.

(1)

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where T0 is a constant governed by the coordination environment and spans the range from 100 K to 200 K. According to the authors, multidentate ligands yield higher T0 values than monodentate ligands, and employment of the former is expected to lead to high T(LIESST). As shown above, since the first finding of LIESST, most studies have concentrated on FeII-based systems. Some LS FeIII (S ¼ 1/2) complexes were reported to exhibit transient generation of the HS FeIII (S ¼ 5/2) upon excitation of their LMCT bands.138,139 The quite short lifetimes of the metastable HS state in the FeIII complexes were believed to arise from the small changes in the bond lengths between the FeIII center and the ligands during the spin transition, as well as the resultant fast relaxation to the ground LS state.113 Sato, Hayami, and coworkers developed the first FeIII-based LIESST system using [FeIII(pap)2]ClO4$H2O (pap ¼ 2(2-pyridylmethyleneamino)phenolate) and [FeIII(pap)2]PF6$MeOH by exploiting proper intermolecular cooperative interactions, whereby the lifetime of the HS FeIII phase was extended.140–144 In the crystal structure of [FeIII(pap)2]ClO4$H2O, intermolecular p-p interactions between the pap ligands were confirmed (Fig. 5). The p-p interactions produced a two-dimensional (2D) network structure. As the authors expected, upon irradiation with 400–600 nm light corresponding to excitation of the LMCT band, these complexes showed LIEEST behavior. Another big challenge in LIESST chemistry lies in the realization of a ferromagnetic phase transition. To bring such systems into reality, proper interactions among the LIESST complexes are needed. Ohkoshi and coworkers synthesized air-stable FeII2[NbIV(CN)8](4-pyridinealdoxime)8$2H2O by mixing two aqueous solutions, a mixture of FeCl2$4H2O and 4-pyridinealdoxime, and K4[Nb(CN)8]$2H2O, and by stirring for 1 h under an argon atmosphere at room temperature.145 Fig. 6 shows the crystal structure of FeII2[NbIV(CN)8](4-pyridinealdoxime)8$2H2O. Each NbIV center is coordinated by eight cyanides, and four out of the eight cyanides participate in ligation with the FeII centers. The FeII centers feature pseudo-octahedral coordination. Two out of the six coordination sites are occupied by the N atoms of the cyanide ligands on the Nb centers in an axial fashion, and 4pyridinealdoxime occupies the other four coordination sites. In addition to the NbeCNeFe linkage, three types of hydrogen bonds, involving the hydroxy groups of 4-pyridinealdoxime, are responsible for the 3D bimetallic network: (1) with the nitrogen atoms of the nonbridged cyanide, (2) with noncoordinating water molecules, and (3) with the nitrogen atoms of the other 4pyridinealdoximes. FeII2[NbIV(CN)8](4-pyridinealdoxime)8$2H2O featured SCO behavior. At 290 K, the cMT value was 7.15 K cm3 mol 1, whereas at 50 K it was 1.72 K cm3 mol 1. The value for T1/2 was 130 K, and no hysteresis was observed. UV-vis-NIR spectra (sensitive to the d-d transitions) and XRD studies (sensitive to the decrease in the CeN bond lengths upon cooling) identified this change in the magnetic property as a phase transition between FeII(LS) and FeII(HS). 57Fe Mössbauer spectroscopy determined the relative populations of FeII(LS) and FeII(HS) to be FeII(HS)2 and FeII(HS)0.44FeII(LS)1.56 for the high- and low-temperature phases, respectively. Irradiation with a 473-nm diode laser light at 2 K, which excited the 1d-d (1A1 / 1T2) transition of the FeII(LS) center, induced spontaneous magnetization with a Curie temperature (TC) of 20 K (Fig. 7A). In addition, a magnetic hysteresis loop with Hc ¼ 240 Oe was observed at 2 K (Fig. 7B). The saturation magnetization, Ms, was 7.4mB at 7 T. The theoretical Ms for ferromagnetic coupling between NbIV(S ¼ 1/2) and FeII(HS) (S ¼ 2) is predicted to be 7.7mB. After photoirradiation, the magnetization progressively decreased by 30% then flattened out, preserving 70% of the original magnetization.

Fig. 5 (A) Crystal structure of [FeIII(pap)2]ClO4 $ H2O and (B) schematic illustration of the molecular arrangement of [FeIII(pap)2]ClO4 $H2O. The Fe complexes form intermolecular p-p stacking. Reproduced from Sato, O. Acc. Chem. Res. 2003, 36, 692–700.

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Fig. 6 Crystal structure of FeII2[NbIV(CN)8](4-pyridinealdoxime)8 $ 2H2O. (A) Coordination environments around the Fe and Nb centers. The Fe atom is coordinated by two cyanide nitrogen atoms of [NbIV(CN)8] and four pyridyl nitrogen atoms of 4-pyridinealdoxime. Four CN groups of [NbIV(CN)8] bridged to four of the Fe centers, and the other four remain free. Red and green balls represent the [FeN6] and [NbC8] moieties, respectively. Light blue, blue, and pink balls represent C, N, and O atoms in 4-pyridinealdoxime. (B) The cyano-bridged FeeNb three-dimensional framework viewed along the c-axis. (C) View from the diagonal direction. 4-Pyridinealdoxime molecules are drawn as light blue wire frames or orange sticks with planes. Zeolitic water molecules are omitted for clarity. (D) The organic 4-pyridinealdoxime ligands are drawn as spheres, with consideration for their van der Waals radii. Light gray, gray, blue, and red spheres denote C, H, N, and O atoms, respectively. Small red and green balls and gray sticks represent Fe and Nb atoms and the cyano-bridged FeeNb framework, respectively. Reproduced from Ohkoshi, S.; Imoto, K.; Tsunobuchi, Y.; Takano, S.; Tokoro, H. Nat. Chem. 2011, 3, 564–569.

A proposed mechanism for the observed ferromagnetism is shown in Scheme 19. Prior to photoirradiation, the diamagnetic 1A1 Fe and paramagnetic NbIV do not interact, yielding paramagnetism. Upon visible light irradiation, the photoexcited FeII(LS) relaxes to a metastable 5T2 FeII(HS) state, which interacts with the NbIV center in an antiferromagnetic fashion through the bridging cyanide ligands. As a result, ferrimagnetism is expressed. II(LS)

8.09.2.5.2

Light-induced charge transfer-induced spin transition

Charge transfer-induced spin transitions (CTISTs) are phase transitions that feature valence changes upon intramolecular CT induced by external stimuli, with the resultant modulation of the magnetic properties.146 Several transition metal complexes and Prussian blue (PB) analogs undergo light-induced CTISTs. A CTIST phenomenon of [CoII(HS)(3,5-dbsq)2bpy] (3,5-dbsq ¼ 3,5-di-tert-butyl-1,2-semiquinonate) was described by Buchanan and coworkers in 1980.147 At high temperatures, the form of [CoII(HS)(3,5-dbsq)2(bpy)] was stable; upon cooling, the structure underwent CTIST, giving rise to [CoIII(LS)(3,5-dbcat)(3,5-dbsq)(bpy)] (3,5-dbcat ¼ 3,5-di-tert-butyl-1,2-catecholate). This transformation was triggered by intramolecular CT from CoII(HS) to 3,5-dbsq, and was accompanied by a change in the

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Photochromic materials

Fig. 7 Light-induced ferromagnetism in FeII2[NbIV(CN)8](4-pyridinealdoxime)8 $2H2O. (A) Magnetization vs. temperature curves at 100 Oe. Light irradiation induces spontaneous magnetization with a Curie temperature of 20 K. (B) Magnetic hysteresis curves at 2 K. After irradiation, a magnetic hysteresis loop with a coercive field of 240 Oe appeared. Blue and red circles denote measurements before and after irradiation with 473 nm light. The measurement error is included within the marks. Reproduced from Ohkoshi, S.; Imoto, K.; Tsunobuchi, Y.; Takano, S.; Tokoro, H. Nat. Chem. 2011, 3, 564–569.

magnetization and a significant color change. This work was further advanced by synthesizing a number of analogs.148–153 Adams et al. demonstrated that this series of Co complexes could experience a ‘transient’ photoinduced CTIST.154,155

Scheme 19 Mechanism of light-induced ferromagnetism in FeII2[NbIV(CN)8](4-pyridinealdoxime)8 $ 2H2O. Reproduced from Ohkoshi, S.; Imoto, K.; Tsunobuchi, Y.; Takano, S.; Tokoro, H. Nat. Chem. 2011, 3, 564–569.

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Fig. 8 Magnetic properties of [CoII(HS)(3,5-dbsq)2(tmeda)] before and after irradiation. The inset shows changes in the magnetization at 5 K. Reproduced from Sato, O.; Cui, A.; Matsuda, R.; Tao, J.; Hayami, S. Acc. Chem. Res. 2007, 40, 361–369.

Sato and co-workers challenged a more stable photoinduced CTIST phenomenon, in which the metastable state generated by photoexcitation possessed a certain lifetime.156–162 In the low temperature phase, the series of Co complexes exhibited an LMCT transition from 3,5-dbcat to CoIII(LS) in the visible region,154,155 which was directly associated with the generation of CoII(HS) and 3,5-dbsq states. In fact, [CoIII(LS)(3,5-dbcat)(3,5-dbsq)(tmeda)] exhibited a meff of 1.7mB at 5 K, whereas after irradiation at 532 nm, the light increased meff to 2.3mB (Fig. 8, tmeda ¼ N,N,N0 ,N0 -tetramethylethylenediamine).158 This change was ascribable to the formation of [CoII(HS)(3,5-dbsq)2(tmeda)], which is a metastable state at low temperatures. This assignment was also supported by the observation that the MLCT transition from CoII(HS) to 3,5-dbsq around 800 nm, found in this series of Co complexes in the high-temperature phase,155 increased in intensity, whereas the LLCT transition between the 3,5-dbcat and 3,5-dbsq ligands around 2500 nm lost intensity. The disappearance of the CeO stretching vibrations in the 3,5-dbcat ligand, observed in IR spectroscopy, was also consistent with this assignment. The lifetime of the metastable state was 1.05  104 s at 5 K, and at 60 K it immediately reverted to the stable LS state. The relatively long lifetime of the metastable state was associated with the extension of ligand-metal bond lengths ( 0.18 Å) upon CoIII(LS) / CoII(HS) tautomerism, which prevented this system from fast thermal relaxation to the ground state. The application of heat was not the only method for inducing a backward reaction. Illumination of metastable [CoII(HS)(3,5dbsq)2(tmeda)] with NIR light at 830 nm, which corresponded to the excitation of the MLCT transition, shifted the magnetization value slightly from 2.3mB to 2.2mB. The authors identified this subtle magnetic change as a transition to the stable [CoIII(LS)(3,5dbcat)(3,5-dbsq)(tmeda)] state using UV-vis-NIR and IR spectroscopies. The authors also characterized the incomplete recovery of the CoIII(LS) state in terms of concurrent excitation of the LMCT band of [CoIII(LS)(3,5-dbcat)(3,5-dbsq)(tmeda)], yielding the corresponding photostationary state. The authors claimed that the [CoII(HS)(3,5-dbsq)2(phen)] (C6H5Cl)/[CoIII(LS)(3,5dbcat)(3,5-dbsq)(phen)]$(C6H5Cl) couple provided a more distinct photoinduced magnetic switch.161 meff increased from 1.7mB to 2.7mB upon irradiation with 532 nm light, whereas a decrease from 2.7mB to 2.3mB was observed upon irradiation with 832 nm light. It should be noted that Carbonera and Sato independently reported dinuclear cobalt complexes that showed photoinduced CTISTs similar to that observed for the mononuclear species.163,164 PB analogs with the general formula of A2xM0 II(1.5  x)[MIII(CN)6]$zH2O (x ¼ 0–1; A ¼ alkali metal ion; M0 II, MIII ¼ transition metal ions) display photochromic effects that are accompanied by modulations of the magnetic properties in the crystalline solid state.142,165–167 These phenomena were first reported by Hashimoto, Fujishima, and coworkers in a study of FeCo PB (M0 ¼ Co, M ¼ Fe), with a composition of K0.2Co1.4[Fe(CN)6]$6.9H2O.168 After this finding, intensive efforts have been devoted to this research area. Fig. 9 shows typical crystal structures of PB-type compounds.165,169–176 The structure shown in Fig. 9B constitutes an fcc lattice without lattice defects, and the alkali cations are placed at the interstitial sites of the lattice. In the structure shown in Fig. 9A, in which no alkali metal ions are present, one-third of the [FeIII(CN)6] units are removed to compensate for the charges. Instead, water molecules are stored at the defect sites, some of which coordinate to the neighboring M0 II sites. These coordinating water molecules play an important role in the expression of photomagnetization.166 Additional water molecules can be included in the interstitial sites of the lattice in a noncoordinating fashion. At 340 K, Na0.4Co1.3Fe(CN)6$5H2O, which is a member of the FeCo PB family,168,175–185 displayed a cmT value of 3.3 cm3 K mol 1. This was ascribable to the presence of LS FeIII and HS CoII centers. Cooling of this material abruptly decreased cmT to 0.8 cm3 K mol 1 at 20 K. This behavior was characterized as thermal CT from CoII(HS) to FeIII(LS), giving rise to CoIII(LS) and FeII(LS).175 Thus, Na0.4Co1.3Fe(CN)6$5H2O underwent a phase transition driven by entropy, as shown in Eq. (2):

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Fig. 9 Schematic illustrations of the crystal structures of the PB analogs A2xM0 II(1.5  x)[MIII(CN)6]$ zH2O: (a) M0 II1.5[MIII(CN)6]$ zH2O (x ¼ 0) and (b) AM0 II[MIII(CN)6] (x ¼ 0.5, z ¼ 0). Reproduced from Tokoro, H.; Ohkoshi, S. Dalton Trans. 2011, 40, 6825–6833.

    Na0:4 CoIIðHSÞ 0:3 CoIIIðLSÞ FeðCNÞ6 4Na0:4 CoIIðHSÞ 1:3 FeðCNÞ6

(2)

The left and right sides of Eq. (2) indicate the LS and HS phases, respectively. It should be noted that the phase transition was accompanied by a hysteresis loop,178–180 and the largest hysteresis width was 40 K.179 Such hysteresis is expected to be derived from cooperative effects afforded by the 3D network. The LS phase of K0.4Co1.3Fe(CN)6$5H2O displayed a visible band at 550 nm that was assignable to a MMCT band from FeII(LS) to CoIII(LS).181 Significant photomagnetism was induced upon excitation of this band with 500–750 nm visible light at 5 K (Fig. 10). Prior to illumination, the magnetization was negligible, whereas after illumination the magnetization increased. The induced magnetization was canceled at 26 K. The lifetime of the photoinduced metastable state reached several days, as long as the sample was stored at 5 K. A plot of the magnetization as a function of the external magnetic field, after illumination, displayed hysteresis loops (Fig. 11). Mössbauer,176 IR,173,177 X-ray absorption fine structure (XAFS),178,179 and UV-vis-NIR173,177 spectroscopies identified the photoinduced phase transition in FeCo PB species as the FeII(LS)-CN-CoIII(LS) 4 FeIII(LS)-CN-CoII(HS) transition. In the FeIII(LS)-CN-CoII(HS) state, the two metal ions, FeIII(LS) (S ¼ 1/2) and FeII(HS) (S ¼ 3/2), are antiferromagnetically coupled, which results in ferrimagnetism throughout the 3D crystalline network. In addition, the metastable HS state displayed absorption peaks for FeIII(LS) and CoII(HS) around 400 nm and 1300 nm, respectively.181 Irradiation with NIR light with a maximum at 1319 nm

Fig. 10 Field-cooled magnetization vs. temperature curves for K0.4Co1.3Fe(CN)6 $ 5H2O at H ¼ 5 G (n) before light illumination, (C) after visible light (hn1) illumination, and (:) after near-IR light (hn2) illumination. Reproduced from Sato, O. Acc. Chem. Res. 2003, 36, 692–700.

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Fig. 11 Hysteresis loops for K0.4Co1.3Fe(CN)6 $ 5H2O at 2 K (n) before illumination, (C) after visible light (hn1) illumination, and (D) after near-IR light (hn2) illumination. Reproduced from Sato, O. Acc. Chem. Res. 2003, 36, 692–700.

depleted the stimulated magnetization (Fig. 10).181 The photochromism and the accompanying magnetism of FeCo PB, therefore, are reversible. The photomagnetism of PB analogs can be dramatically tuned by changing the ratios among A, M0 II, MIII, and water molecules, and by substituting each component with another element. The most profound effects were observed by changing M0 II or MIII. Ohkoshi, Hashimoto, and coworkers investigated the Rb2xMnII(1.5  x)[FeIII(CN)6]$zH2O series.165,186–197 As in the case of FeCo PB, RbMn[Fe(CN)6] underwent a thermal phase transition. The cmT value at 320 K was 4.65 cm3 K mol 1 (high-temperature (HT) phase), whereas at 200 K, it declined to 3.16 cm3 K mol 1 (low-temperature (LT) phase). The transition temperatures HT / LT (T1/2 Y) and LT / HT (T1/2 [) were 231 and 304 K, respectively. The width of the thermal hysteresis loop (DT ¼ T1/2 [ T1/2 Y) was 73 K.186 This phase transition was accompanied by a color change from light brown (HT phase) to dark brown (LT phase).186 The crystal lattice transformation also followed this transition, from cubic (F  43 m, a ¼ 10.533 Å) to tetragonal (I  4 m2, a ¼ b ¼ 7.090 Å, c ¼ 10.520 Å).187 X-ray photoelectron spectroscopy (XPS) and IR spectra showed that this HT / LT phase transition was associated with a CT from MnII to FeIII.187 In addition, the cMT values, XPS, synchrotron radiation X-ray powder structural analysis (SR-XRD), and X-ray absorption near-edge structure (XANES) studies led to assignment of the spin state of the HT and LT phases as FeIII(S ¼ 1/2)-CN-MnII(S ¼ 5/2) and FeII(S ¼ 0)-CN-MnIII (S ¼ 2), respectively.187–189 It should be noted that Jahn-Teller distortion is responsible for the transformation of the crystal lattice in the LT phase.187 Rb0.88Mn[Fe(CN)6]0.96 $ 0.5H2O,190 which also exhibited a phase transition similar to that of RbMn[Fe(CN)6], showed ferromagnetism in the LT phase (Fig. 12A). Irradiation of the LT phase with 532 nm light, which excited the MMCT transition from FeII to MnIII, produced a photoinduced HT phase. This state presented much weaker magnetization than the LT phase (Fig. 12A). On the other hand, illumination of the photoinduced HT phase with 410 nm light recovered the magnetization (Fig. 12A). This backward reaction was induced by excitation of the LMCT transition from CN to FeIII. This series of photomagnetism reactions proved to be reversible (Fig. 12B). Neutron powder diffraction measurements190 showed that the LT phase is a ferromagnet with ferromagnetic coupling among the MnIII (S ¼ 2) sites, whereas the photoinduced HT phase is an antiferromagnet, as shown in Fig. 13. In total, Rb0.88Mn[Fe(CN)6]0.96 $ 0.5H2O displayed photoinduced magnetism exactly opposite to that of the FeCo systems. Other analogs are discussed below. Dunbar and coworkers synthesized another member of the PB family, CoII3[OsIII(CN)6]2 $ 6H2O, and disclosed its photomagnetic properties.198 Cobalt octacyanotungstate199,200 and copper octacyanomolybdate201–204 were also reported to show photomagnetic effects: Strictly speaking, these two species were not the PB type of complexes; however, similarities were observed in the fact that they are bimetallic and comprise polycyanometallates.

8.09.2.5.3

Light-induced electron transfer-induced second-order nonlinear optical switching

Wang and Guo synthesized the electron transfer (ET) photochromic crystalline compound, [ZnBr2(CEbpy)]$3H2O (CEbpy ¼ Ncarboxyethyl-4,40 -bipyridinium), with photoswitchable second-order NLO properties.205 They also synthesized a new photochromic and thermochromic bifunctional compound, b-[(MQ)ZnCl3] (MQþ ¼ N-methyl-4,40 -bipyridinium), with an acentric crystal structure.206

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Fig. 12 Reversible photomagnetism in Rb0.88Mn[Fe(CN)6]0.96 $ 0.5H2O: (A) M-T plots under 200 Oe before (,) and after (C) hn1 (l ¼ 532 nm) irradiation, after hn2 (l ¼ 410 nm) irradiation (B), and after thermal annealing at 180 K (n); (B) M vs. irradiation-time plot at 3 K upon alternating irradiation with light at hn1 (red arrows) and hn2 (black arrows). Reproduced from Tokoro, H.; Ohkoshi, S. Dalton Trans. 2011, 40, 6825–6833.

Fig. 13 Schematic illustration of the reversible photomagnetism in Rb0.88Mn[Fe(CN)6]0.96 $ 0.5H2O: (A) mechanism of the reversible photoinduced CT between the LT and photoinduced (PI) HT phases and (B) spin ordering in the LT (left) and PI HT (right) phases. Reproduced from Tokoro, H.; Ohkoshi, S. Dalton Trans. 2011, 40, 6825–6833.

8.09.3

Interplay among transition metal complexes, organometallics, and organic photochromics

8.09.3.1

Introduction

Transition metal complexes and organometallics possess several useful functionalities, such as phosphorescence from triplet excited states, reversible redox switches, and peculiar magnetism, which are rarely accessible in ordinary organic compounds and bulk metals. These characteristics stem from the d-orbitals at the metal centers, p-orbitals on the ligands that are electronically perturbed and coupled with the metal atoms or ions, and the heavy atom effects of the metal centers. The properties of transition metal

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complexes and organometallics are surely important components of molecule-based devices, and their utility could be enhanced by introducing photochemical switchability. The structural transformations associated with photochromism are always more or less accompanied by changes in the electronic structures; hence, photochromism can potentially switch the photo-, electro-, and magnetic properties of transition metal complexes and organometallics. The number of coordination compounds that possess alluring functionalities as well as photochromism, however, is highly limited. An alternative approach to preparing compounds with these properties involves the incorporation of organic photochromics into transition metal complexes and organometallics. Recent progress in both organic and inorganic syntheses makes it possible to fabricate this type of conjugates. The photochromic properties of an organic photochromic moiety in organic photochrome-transition metal complexes or organometallic conjugates can be regulated by the metal centers. In fact, the trans-cis isomerization reactions of thioindigo,207 all-trans-bcarotene,208 and stilbene209 can be induced by the photoexcitation of additives, metallated porphyrinoids, and subsequent triplet energy transfer to the organic photochromes. In these cases, photoisomerization is attained at wavelengths longer than the absorption limit of the organic photochromes. This behavior can be regarded as an extension of the photoresponse or a modification of the photochromic properties. Such systems become more reproducible, controllable, quantitative, and useful if the photosensitizer is tethered to the organic photochrome. In the context described above, several conjugates between organic photochromics and transition metal complexes or organometallics have been prepared. Among the methods used to conjugate organic photochromics with coordination compounds, pconjugation through the p-ligands of transition metal complexes and organometallics is frequently adopted. The greatest advantage of this method is that interactions in the ground state are available, as well as with the photoexcited state. Direct coordination to the metal center by coordinating groups on the organic photochromics, such as nitrogen or oxygen, is also widely employed as a convenient method of forming conjugates with strong interactions. Some organic photochromics can inherently form coordination bonds. If this is not the case, elaboration is needed to introduce a coordination ligand. 4-Stylylpyridine210 and dithienylethene with 4-pyridyl groups211 are good examples. To achieve direct coordination, a large degree of orbital mixing between the metal center and the organic photochrome is expected. Undoubtedly, a maximum effect is achieved if coordination and dissociation can be switched.212 Non-p-conjugated linkages can also be used, such as alkyl, ester, ether, and amide bonds. In these cases, structural and mechanical factors are more significant than the interactions in both the ground and the photoexcited states. The following section starts with a brief introduction on representative organic photochromes. Examples of conjugates comprising organic photochromics and transition metal complexes or organometallics are then described. Control over the photo-, electro-, and magneto-properties of the transition metal complexes or organometallics via photoisomerization of the

Scheme 20

The organic photochromics discussed in this chapter.

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organic photochromics is described first. Then, control over the isomerization behavior of the former by the latter is enumerated. Next, conjugates that realize mutual regulation of the functionalities are introduced. Molecules in which two or more organic photochromics are attached onto one metal center are then described. Finally, other uncategorizable examples are discussed.

8.09.3.2

Organic photochromics

Scheme 20 shows representative organic photochromics. Organic photochromics are roughly divided into two categories, based on whether bond formation and dissociation occur during photoisomerization. Among the photochromics introduced in this section, dithienylethene (diarylethene),213–215 spiropyran,216,217 flugide,218 and dimethyldihydropyrene (DHP)219–222 are good examples of compounds in which bonds form during photoisomerization. In these systems, the transformation is accompanied by dramatic changes in the electronic structure and the absorption property. These features are appreciated by, for example, quantitative conversion and significant color changes. Azobenzene,223–225 stilbene,226 and ethynylethene227,228 are examples of compounds in which bonds dissociate during photoisomerization. This class of compounds includes E (trans) and Z (cis) isomers, which are mutually transformed via rotation or inversion along the CeC or NeN double bond. Changes in the electronic structure and the absorption spectrum of this type are less prominent than those of the former group. Organic photochromics may be classified another way. Compounds in which the thermal isomerization channel from the metastable isomer to the stable isomer is available, for example, in spiropyran, DHP, and azobenzene, are called T-type. P-type compounds do not undergo thermal isomerization at ambient temperatures, for example, diarylethene, stilbene, and ethynylethene. Several organic photochromics inherently can form coordination bonds, which can be generated or dissociated via photochromism. For example, spiropyran does not bind strongly to metal ions, whereas the merocyanine form has an affinity to metals due to its zwitterionic structure.229,230 Finally, several exceptional organic photochromics have been described. Salicylideneaniline231,232 shows T-type photochromism that accompanies hydrogen transfer. The T-type photochrome hexaarylbiimidazole (HABI) features a radical metastable state.233,234

8.09.3.3 Control over the photo-, electro-, and magneto-properties of transition metal complexes and organometallics via photoisomerization of organic photochromics 8.09.3.3.1

ON/OFF switching of luminescence

One of the unique and important properties of luminescence from transition metal complexes and organometallics is its triplet nature rendered by the heavy atom effect.235,236 Heavy metal-induced triplet states display properties that differ from those of the excited states of organic luminophores, which are ordinarily singlet. The triplet nature of luminescence from transition metal complexes and organometallics is exploited in organic light emitting diodes (OLEDs): The theoretical efficiency of forming emitting excited states is unity in triplet luminophores, whereas in singlet ones it is 25%.237,238 The long lifetimes and large Stokes shifts of luminescence from transition metal complexes and organometallics are highly appreciated in bioimaging because these features

Scheme 21

Reversible photochromism of pyridyldithienylethene and Ru-porphyrin conjugates.

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facilitate the separation of excitation and emission spectral bands.239,240 Switching of the phosphorescence properties by the application of external stimuli would enlarge the scope of phosphorescent material applications. Norsten and Branda fabricated a conjugate of pyridyldithienylethene and phosphorescent ruthenium-based metalloporphyrin (Scheme 21).241 The open form undergoes photoisomerization upon irradiation with 365 nm UV light to afford the closed form. The open form is regenerated with 470–680 nm light. The authors also reported that the absorption spectrum of the conjugate was merely the sum of each component, which indicates that the system is free from ground state interactions among the components. The authors then measured the phosphorescence intensity modulation from the porphyrinic moiety upon photoisomerization. They selected the region of 400–480 nm for excitation because both the open and the closed forms of the pyridine-appended dithienylethene moiety display negligible absorption in this region. The fluorescence intensity changes reversibly, yielding stronger phosphorescence in the open form and weaker one in the closed form. The authors confirmed that the probe light used for stimulating the phosphorescence produced negligible photoisomerization of the pyridyldithienylethene moiety. In total, this system permitted nondestructive photonic readout of the status of the molecule, in open and closed forms. Fernández-Acebes and Lehn reported observation of the reverse emission behavior, that is, an emissive closed form and nonemissive open form, in W(CO)5pyconjugated dithienylethene.211 In this case, the probe light (240 nm) used to induce emission does not afford significant photoisomerization behavior. Zou et al. synthesized unsymmetrical diarylethene derivatives with one coordination site of CuII ion. These compounds become photoluminescent in the presence of both CuII ion and cyanide ion, and the luminescence is quenched by UV irradiation to cause isomerization from the open form to the closed form (Scheme 22).242 An iridium(III) cyclometalated complex bearing photochromic and acid-sensitive dithienylethene ligands was synthesized by Tian, Ceroni, Credi and coworkers (Scheme 23).243 Optical or electrochemical excitation of the complex generates phosphorescence emission that can be switched on/off by light and chemical stimulation. Wong et al. prepared a new class of dithienylethene alkyne-ligated platinum(II) phosphine complexes showing red phosphorescence and reversible photochromism.244 They also synthesized a cyclometalated platinum(II) 1,3-bis(N-alkylbenzimidazol-20 yl)benzene (bzimb) system with the dithienylethene unit (Scheme 24).245 Upon photoexcitation, the yellow solutions of the complexes in benzene display green phosphorescence originating from the triplet intraligand (3IL) excited state. This

Scheme 22 Proposed sensing processes of unsymmetrical diarylethene ligands with Cu2þ, and the Cu2þ complexes with CN and the photochromic processes responding to light stimuli. Reproduced from Zou, Q.; Li, X.; Zhang, J.; Zhou, J.; Sun, B.; Tian, H., Chem. Commun. 2012, 48, 2095–2097.

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opcal or electrochemical excitaon

phosphorescence O

+\– H+

O

IrIII N

N S

S

2

+\– e–

hQ UV-VIS Scheme 23 ON/OFF switching of phosphorescence emission of Ir(III) cyclometalated complex bearing photochromic and acid-sensitive dithienylethene ligands. Reproduced from Monaco, S.; Semeraro, M.; Tan, W.; Tian, H.; Ceroni, P.; Credi, A., Chem. Commun. 2012, 48, 8652–8654.

phosphorescence intensity is decreased by photoisomerization from the open form to the closed form. Recently, the authors reported photochromic thienylethene-containing copper(I) diimine complexes. The open form of the complexes is emissive with lem ¼ 532–543 nm and fem ¼ 0.014–0.026, whereas the closed form is non-emissive.246 These complexes were employed for fabrication of photomodulated resistive memory by sandwiching it between Al and ITO by Wong et al. (Scheme 25). They also developed photoresponsive tris(8-hydroxyquinolinato)aluminum(III) (Alq3) complexes bearing photochromic dithienylethene units. Photocontrollable memory performance was achieved with a binary memory behavior, with high ON/OFF ratio and long retention time (Scheme 26).247 Winkler, Gray, and coworkers synthesized [Re(diimine)(CO)3(E-dpe)](PF6) (E-dpe ¼ E-1,2-di(4-pyridyl)ethylene), which displayed a significant increase in the phosphorescence from the Re complex moiety in dichloromethane at ambient temperatures upon conversion of the E-dpe ligand to the Z form with 350 nm light.248,249 Using TRIR measurements, the authors concluded that the emissive 3MLCT state was efficiently quenched by the triplet excited state localized on the dpe ligand (3IL) in the E form, whereas the Z form circumvented quenching due to destabilization of the 3IL state stemming from the peculiar distortion of stilbenoids in the Z form. Other systems, in which the phosphorescence intensity is switched upon photoisomerization of the organic photochromics, were reported.250–254 Li et al. synthesized zinc complex based on Rhodamine B salicylaldehyde hydrazone (Scheme 27).255 The solution color of the metal complex gradually changes from yellow to purple by the increase of the absorbance at 554 nm upon irradiation at 365 nm accompanying with the decrease of the fluorescence emission at 527 nm and the transformation into two weak fluorescence at 513 and 582 nm. The photochromic property of the Zn complex is also maintained in solid matrix, poloxamer 407. The degree of the color change depends on the UV radiant intensities, suggesting the applicability of the complex to a UV sensor. Bhattacharyya et al. synthesized a series of self-assembled functional PtII molecular hexagons containing spiropyran in the molecular backbone. The complexes inherit the enhanced emission with aggregate formation.256 Switching of luminescence properties of lanthanide complexes by photoirradiation have been a target of research. Norel, Rigaut and coworkers showed that a dithienylethene modified dipicolinic amide ligand can modulate EuIII and YbIII luminescence using light as an external stimulus (Scheme 28).257 The nature of the modulation depends on the lanthanide emitter: with the EuIII ion, the dithienylethene ligand quenches the red luminescence upon ring closure, whereas with the YbIII ion, ring closure can be used to turn on the luminescence in the NIR range. They further reported a hybrid photochromic molecule with a ruthenium complex, ytterbium ion, and dithienylethene, of which NIR luminescence can be repeatedly switched by photoirradiation with 350 nm and 650 nm lights (Scheme 29).258 This luminescence switching is also controlled by redox reactions. Zhang et al. performed theoretical

Scheme 24 Photochromic cyclometalated alkynylplatinum(II) complexes with tridentate 1,3-bis(N-alkylbenzimidazol-20 -yl)benzene (bzimb) ligands exhibiting reversible photochromism. Reproduced from Chan, M. H.-Y.; Wong, H.-L.; Yam, V. W.-W., Inorg. Chem. 2016, 55, 5570–5577.

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Scheme 25 Photoinduced color changes of dithienylethene-containing copper(I) diimine complexes, which can be employed as an active component in the fabrication of solution-processed resistive memory devices. Reproduced from Wong, C.-L.; Cheng, Y.-H.; Poon, C.-T.; Yam, V. W.-W., Inorg. Chem. 2020, 59, 14785–14795.

analyses about the geometry structures, photophysical properties, and energy-transfer channels of different isomers of a series of Eu(acac)3-dethylethene complexes for the purpose of understanding the mechanism of regulating the luminescence efficiency of the EuIII complex by isomerization of the ligand.259

Scheme 26 Photo-controllable electron-transporting and resistive memory behaviors of dithienylene-containing tris(8-hydroxyquinolinato) aluminum(III) complexes. Reproduced from Wong, C.-L.; Ng, M.; Hong, E. Y.-H.; Wong, Y.-C.; Chan, M.-Y.; Yam, V. W.-W., J. Am. Chem. Soc. 2020, 142, 12193–12206.

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Scheme 27 Proposed mechanism and the complex in poloxamer 407 exposed under 2-min UV irradiation with the intensity of 0, 25, 50, 75 and 100 mW/cm2 from left to right. Reproduced from Li, K.; Xiang, Y.; Wang, X.; Li, J.; Hu, R.; Tong, A.; Tang, B. Z., J. Am. Chem. Soc. 2014, 136, 1643– 1649.

Scheme 28 Eu(III) and Yb(III) complexes with a dithienylethene (DTE) modified dipicolinic amide ligand, which can be a versatile tool to modulate luminescence using light as an external stimulus. Reproduced from He, X.; Norel, L.; Hervault, Y.-M.; Métivier, R.; D’Aléo, A.; Maury, O.; Rigaut, S., Inorg. Chem. 2016 55, 12635–12643.

The material consists of MOF based on EuIII ion and 4,40 -bipyridinium salt derivatives reported by Sun et al. undergoes reversible switching of luminescence of accompanied with photochromism (Scheme 30).260 At the initial state, the compound is colorless and emissive. After light irradiation the emission of the compound is negligible compared to that at the initial state. Wang et al. reported the synthesis of an EuIII complex bearing a photochromic N^C-chelate organoboron functionalized dipicolinic acid and the photomodulation of the emission from EuIII ion.261 The organoboron group in the ligand undergoes photoisomerization upon irradiation at 365 nm and produces the dark isomer along with the quenching of the luminescence at 530 nm. The photoirradiation at 365 nm to the EuIII complex causes the photoisomerization of the organoboron group and quenches the emission of the EuIII complex at 615 nm almost completely (ff ¼ 12% to 0.1% within 120 s). Heating the solution at 80  C recovers the initial absorption and emission spectra.

8.09.3.3.2

Modulation of electronic communication in MV states

Electronic communication in the mixed valence (MV) compounds can be induced if redox sites are connected via organic p-conjugation systems, as demonstrated in the Creutz-Taube salt, [(NH3)5RuIIpzRuIII(NH3)5]5þ.262,263 Electronic communication affords the delocalization of valences between redox sites. Robin and Day categorized MV compounds into three groups:264 (1) Class I, in which the valence is firmly trapped in either redox site; (2) Class II, in which valence is distinguishable but slightly delocalized over the redox sites; and (3) Class III, in which valence is completely delocalized. Control over the electronic communication via external

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Scheme 29 Redox/optical commutation among three forms of a Yb complex. Reproduced from Al Sabea, H.; Norel, L.; Galangau, O.; Hijazi, H.; Métivier, R.; Roisnel, T.; Maury, O.; Bucher, C.; Riobé, F.; Rigaut, S., J. Am. Chem. Soc. 2019, 141, 20026–20030.

stimuli (photons,265–271 protons,272–275 ions,276 etc.) has attracted much attention because these types of systems can constitute molecular devices.277,278 The compounds also provide models for switchable molecule-based electronic circuits,279–283 in which the p-bridges may be regarded as conductive nanowires. The electronic coupling matrix Hab, which represents the intensity of the electronic communication in MV compounds, can be quantified using parameters that are extracted from the IVCT bands.284 On the other hand, MV compounds with electronic communication are oxidized or reduced in a stepwise rather than a simultaneous manner. A split in the formal potential, DE00 , and the comproportionation constant Kc associated with DE00 , are also used as indicators for electronic communication.285 IVCT band analysis286,287 and quantification of DE00 and Kc 285,288–290 have several drawbacks; hence, they are utilized in combination. The first system in which the extent of electronic communication was modulated by photon stimuli was disclosed by Fraysse et al.265 They appended two cyclometallated RuII complexes to dithienylethene. Although no significant changes in DE00 and Kc (11 and 12 for the open and closed isomers, respectively) were observed with respect to the one-electron oxidized MV state, only the closed isomer exhibited an IVCT transition, with Hab of 200 cm 1. The extended Hückel calculation gave Hab of 0.021 eV and 0.003 eV to the closed and open isomers, respectively, in good agreement with the experimental values. The lack of changes in DE00 and Kc may be interpreted as a cancelation between the resonance factor, the genuine component of electronic communication, and other factors.286,287 The ON/OFF switching of the electronic communication in this system is induced by reorganization of the p-conjugated system in the dithienylethene moiety, from the open form with unfavorable p-orbitals to the closed form with favorable ones.

Scheme 30 An illustration of photoswitching of a compound in which Eu(III) ions have been introduced into a photoactive viologen system to yield a polyrotaxane-like metal-organic framework. Reproduced from Sun, J.-K.; Cai, L.-X.; Chen, Y.-J.; Li, Z.-H.; Zhang, J. Chem. Commun. 2011, 47, 6870–6872.

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Scheme 31 Photochromism and accompanying switching of electronic communication in diethynylated dithienylethene terminated by two Fe(h5C5Me5)(dppe).

Tanaka et al. fabricated a more profound switching system using dithienylethene (Scheme 31).266,267 They attached a pianostool-type FeII acetylide, which displays stronger electronic communication than other redox-active compounds, such as ferrocene.291 In the open form, Kc ¼ 16 (DE00 ¼ 65 mV). However, the closed form yielded Kc ¼ 510 (DE00 ¼ 160 mV), which was much greater. The authors also conduced IVCT band analysis, which proved to be consistent with DE00 (Hab ¼ 0 cm 1 for the open form and Hab ¼ 381 cm 1 for the closed form). Xu et al. synthesized heteronuclear complexes in which redox-active ferrocenyl and ruthenium centers are separated by a photochromic dithienylethene moiety to achieve photoswitchable charge delocalization and FeeRu electronic communication (Scheme 32).292 The NIR absorptions in mixed-valence species are gradually intensified following the conversion of the open form to the closed form of dithienylethene moiety (moieties), which demonstrates that the extent of charge delocalization shows progressive enhancement with stepwise photocyclization.

8.09.3.3.3

Control over the magnetism

A new strategy for photomagnetic switching, ligand-driven light-induced spin change (LD-LISC), was firstly proposed by Roux et al.293 The prerequisite of LD-LISC is that ligands bound to the metal center are based on organic photochromics, and their photoisomerization affects the spin equilibrium state that arises from a fast exchange between LS and HS states. The advantage of LD-LISC over LIESST is that it is potentially available even at ambient temperatures and proceeds in the solid state, in solution, or in a polymer matrix. The authors investigated the photochromic and the magnetic properties of [FeII(E-stpy)4(NCBPh3)2] (E-stpy ¼ E-4styrylpyridine, NCBPh3 ¼ Cyanotriphenylborate) in cellulose acetate films to demonstrate LD-LISC.294 The polymer-dispersed system reveals photochromism upon irradiation with light at 322 and 260 nm at 140 K, which affords conversion from E-stpy to Z-stpy, and vice versa, respectively. The E form exclusively possesses the LS state (S ¼ 0), whereas the Z form mostly occupies the HS state (S ¼ 2). The authors ascribed this spin state difference to a weaker LF in the Z form because the p* orbital energy increased due to distortion of the planarity and the resultant disturbance of the p-conjugation. Zarembowitch and coworkers also characterized the first example of LD-LISC in the solution state at room temperature using [Fe(E-msbpy)2(NCS)2] (Emsbpy ¼ E-4-methyl-40 -styryl-2,20 -bipyridine).295 Nishihara and coworkers demonstrated the reversible photomagnetic effects of [FeL3](BF4)2 $ 3H2O (L ¼ phenyl(2-pyridin-2-yl-3H-benzoimidazol-5-yl)diazene) at room temperature based on LD-LISC (Scheme 33). This complex showed thermal spin crossover in both the solid and solution states, with a transition temperature of T1/2 ¼ 279(16) K in acetone. The trans-cis photoisomerization of the azobenzene moieties upon photoexcitation afforded reversible photomagnetic effects.296 The examples of LD-LISC described above utilize modulation of the LF strength of the metal center induced by the electronic perturbation of the organic photochromics upon photoisomerization. Jana and coworkers adopted another method for manipulating the LF split that involved changing the coordination mode and number.212 For example, NiII forms complexes in two different spin states. In square planar complexes, the NiII center tends to render the LS (S ¼ 0) state. On the other hand, octahedral complexes are HS (S ¼ 1). Square pyramidal complexes are either HS or LS, depending on the nature of the ligands. Thus, changes in the coordination mode and the number can have profound influences in the magnetism in NiII complexes. The authors synthesized [tetrakis(pentafluoridophenyl)porphyrinato]nickel(II), in which 4-pyridylazobenzene is appended to one of the aryl groups (Scheme 34).212 This system shows trans-to-cis photoisomerization upon irradiation with 500 nm light in DMSO (trans:cis ¼ 30:70). On the other hand, 435-nm photoirradiation produces cis-to-trans isomerization (trans:cis ¼ 97:3). The cis form is stabilized by the coordination of the 4-pyridylazobenzene group to the NiII. Light-induced changes in the paramagnetic molar susceptibility were detected using Evans measurements. The pure cis form exhibits a meff of 2.989  0.084 Bohr magneton (BM), which predicts the HS state (2.8–3.4 BM). However, the meff of the trans form is zero (LS state). The authors described the durability of this system over 10,000 cycles, and proposed its utility in dynamic magnetic resonance imaging (MRI) applications. Single-molecule magnets (SMMs) feature slow relaxation of the magnetization, thereby behaving as nanometer-sized magnets below the blocking temperatures (TB).297–304 Their relaxation dynamics are attributed to the intrinsic properties of the molecule, such as the presence of large easy axis-type magnetoanisotropy (negative zero-field splitting parameter: D < 0), and a large spin ground state (ST). These parameters result in a large energy barrier (D) of |D |S2T and |D |(S2T–1/4) for integer and half-integer spins,

Photochromic materials

Scheme 32 The nine states of a FeeRu complex through photo- and electroswitchable processes. Reproduced from Xu, G.-T.; Li, B.; Wang, J.-Y.; Zhang, D.-B.; Chen, Z.-N., Chem. Eur. J. 2015, 21, 3318–3326.

383

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Photochromic materials

Scheme 33

LD-LISC phenomenon in [FeL3](BF4)2$3H2O (L ¼ phenyl(2-pyridin-2-yl-3H-benzoimidazol-5-yl)diazene).

Scheme 34 Reversible light-induced magnetic switching of azopyridine functionalized Ni-porphyrin. Reproduced from Venkataramani, S.; Jana, U.; Dommaschk, M.; Sönnichsen, F. D.; Tuczek, F.; Herges, R. Science, 2011, 331, 445–448.

respectively, between spin-up and spindown configurations, ms ¼  ST.300,301 Chemists and physicists have devoted significant effort to this field of research in an attempt to realize SMM applications, for example, memory devices that exploit SMMs as bits305 or quantum computers based on quantum tunneling of the magnetization (QTM).306–310 To achieve photoregulation of the SMM behavior, Miyasaka and coworkers designed a one-dimensional (1D)-chain-type SMM based on the photochromic diarylethene ligand, dae2  (Scheme 35A).311,312 An example of SMMs is depicted in Scheme 35C. MV tetranuclear [MnII2MnIII2] units

Scheme 35 (A) Photochromic diarylethene ligand dae2 ; (B) Mn tetranuclear SMM unit; and (C) photochromic 1D SMM chain. Reproduced from Morimoto, M.; Miyasaka, H.; Yamashita, M.; Irie, M. J. Am. Chem. Soc. 2009, 131, 9823–9835.

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Scheme 36 Proposed mechanism for the occurrence of the antiferromagnetic ordering in the 1D SMM based on the closed-dae2  and [MnII2MnIII2] unit after irradiation with visible light. The blue arrows indicate the magnetization of SMM aligned on an easy axis. Reproduced from Morimoto, M.; Miyasaka, H.; Yamashita, M.; Irie, M. J. Am. Chem. Soc. 2009, 131, 9823–9835.

(Scheme 35B), which are responsible for the expression of magnetism, are bridged by the dae2  ligands to form a 1D-coordination chain. The authors synthesized 1D SMM chains separately with open- and closed-dae2  ligands. These two 1D SMMs had different unit cells and different ligands (ClO4 or H2O) in the first coordination sphere. In both isomers, the [MnII2MnIII2] unit exhibited SMM behavior with a ground state of ST ¼ 9, which was ascribable to two types of intracluster ferromagnetic exchange interactions. D/kB was estimated to be 25.1 and 26.7 K for the open and closed isomers, respectively. Both open and closed isomers showed reversible photochromism in the solid state, which was characteristic of the dae2  ligand; however, variations in the magnetic properties upon induction of the photochromism were observed only in the 1D SMM that was originally in the closed form. The SMM in the closed form yielded quite weak antiferromagnetic interactions of zJ/kB z  1  10 2 K among the [MnII2MnIII2] units, whereas a remarkable enhancement in the antiferromagnetism (zJ/kB z  0.19 K) was induced upon visible light irradiation, which

Scheme 37 Two complexes [Mn2(saltmen)2(daeopen)] (upper one) and [Mn(saltmen)(daeclose)]$H2O$Et3N (bottom one), where H2dae ¼ 1,2-bis((5carboxyl-2-methyl-3-thienyl)perfluorocyclopentene) and H2saltmen ¼ 2,20 -((1E,10 E)-((2,3-dimethylbutane-2,3-diyl)bis(azaneylylidene)) bis(methaneylylidene))diphenol, which show reversible photochromic responses to UV/vis light and single-molecule magnet-like behavior. Reproduced from Fetoh, A.; Cosquer, G.; Morimoto, M.; Irie, M.; El-Gammal, O.; El-Reash, G. M. A.; Breedlove, B. K.; Yamashita, M., Inorg. Chem. 2019, 58, 2307–2314.

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Scheme 38 [EuIII(18C6)(H2O)3]FeIII(CN)6$2H2O (18C6 ¼ 18-crown-6) exhibiting the photochromism and photomagnetism of 3d  4f hexacyanoferrates at room temperature. Reproduced from Cai, L.-Z.; Chen, Q.-S.; Zhang, C.-J.; Li, P.-X.; Wang, M.-S.; Guo, G.-C., J. Am. Chem. Soc. 2015, 137, 10882–10885.

prepared the open form. Two plausible explanations were proposed for this switching behavior upon photoisomerization: switching of the through-bond superexchange interactions among the [MnII2MnIII2] clusters in the same chain through the p-system of the dae2  ligands, or switching of through-space interactions among the SMM units in different chains due to the structural transformation. The former was ruled out because the 1D SMM synthesized as the open form did not display significant changes in its magnetism. The authors, therefore, concluded that the magnetic switching behavior in the closed form was derived from intrachain interactions triggered by structural changes upon photoisomerization (Scheme 36). Fetoh et al. have synthesized two coordination assemblies by combining the open and close forms of 1,2-bis(5-carboxyl-2-methyl-3-thienyl)perfluorocyclopentene (H2dae) with [Mn2(saltmen)2(H2O)2](PF6)2, where H2saltmen ¼ 2,20 -((1E,10 E)-((2,3-dimethylbutane-2,3-diyl)bis(azaneylylidene)) bis(methaneylylidene))diphenol. Dithienylethene as bridging ligand between two MnIII complexes have been shown to exhibit photochromism. The complexes showed reversible photochromic responses to UV/vis light and showed SMM-like behavior (Scheme 37).313 Photochromism and photomagnetism of 3d-4f hexacyanoferrates [EuIII(18C6)(H2O)3]FeIII(CN)6$2H2O (18C6 ¼ 18-crown-6) have been demonstrated by Cai et al.314 Photoinduced electron transfer (PET) from crown to FeIII yields long-lived charge-separated species at RT in air in the solid state and also weakens the magnetic susceptibility significantly (Scheme 38). Ma et al. have synthesized a series of chain complexes [Ln3(H-HEDP)3(H2-HEDP)3]$2H3-TPT$H4-HEDP$10H2O (QDU-1; Ln ¼ Dy (QDU-1(Dy)), Gd (QDU1(Gd)), and Y (QDU-1(Y)); HEDP ¼ hydroxyethylidene diphosphonate; TPT ¼ 2,4,6-tri(4-pyridyl)-1,3,5-triazine). All the compounds exhibited reversible photochromic and photomagnetic behaviors via UV light irradiation at RT, induced by the photogenerated radicals via a photoinduced electron transfer (PET) mechanism. Strong ferromagnetic coupling with remarkably slow magnetic relaxation without applied dc fields was observed between Dy3þ ions and photogenerated O radicals, showing SMM behavior after RT

Scheme 39 A series of chain complexes [Ln3(H  HEDP)3(H2  HEDP)3]$2H3  TPT$H4  HEDP$10H2O (QDU-1; Ln ¼ Dy (QDU-1(Dy)), Gd (QDU1(Gd)), and Y (QDU-1(Y)); HEDP ¼ hydroxyethylidene diphosphonate; TPT ¼ 2,4,6-tri(4-pyridyl)-1,3,5-triazine) exhibiting reversible photochromic and photomagnetic behaviors via UV light irradiation at RT, induced by the photogenerated radicals via a photoinduced electron transfer (PET) mechanism, to show SMM behavior. Reproduced from Ma, Y.-J.; Hu, J.-X.; Han, S.-D.; Pan, J.; Li, J.-H.; Wang, G.-M., J. Am. Chem. Soc. 2020, 142, 2682–2689.

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Scheme 40 A one-dimensional coordination solid synthesized by reaction of a bispyridyl dithienylethene (DTE) photochromic unit with the highly anisotropic dysprosium-based single-molecule magnet [Dy(Tppy)F (pyridine)2]PF6, exhibiting a single-crystal-to-single-crystal transformation through photoisomerization of the bridging DTE ligand. Reproduced from Hojorat, M.; Al Sabea, H.; Norel, L.; Bernot, K.; Roisnel, T.; Gendron, F.; Le Guennic, B.; Trzop, E.; Collet, E.; Long, J. R.; Rigaut, S., J. Am. Chem. Soc. 2020, 142, 931–936.

illumination (Scheme 39).315 Hojorat et al. has prepared a one-dimensional coordination solid by reaction of a bispyridyl dithienylethene (DTE) photochromic unit with the highly anisotropic dysprosium-based single-molecule magnet [Dy(Tppy)F(pyridine)2]PF6 (Tppy ¼ tris(3-(2-pyridyl)pyrazolyl)hydroborate). Slow magnetic relaxation characteristics are retained in the closed form of the chain compound, and photoisomerization of the bridging DTE ligand induces a single-crystal-to-single-crystal transformation. The resulting open form exhibits faster low-temperature relaxation than that of the closed form (Scheme 40).316

8.09.3.3.4

Regulation of the coordination environment around a metal center

The large structural changes associated with photochromism tend to be regarded as a disadvantage for operation in a crystalline solid state, rigid polymer medium, or self-assembled monolayers (SAMs); however, several systems exploit the structural changes in a surprising way. Shinkai et al. took advantage of the unique large conformational changes in the azobenzene structure for the capture of alkali metal cations.317,318 They designed azobenzene derivatives in which crown ether analogs are incorporated onto the two phenyl rings (Scheme 41A).317 This modification results in the intriguing property that the cis form can extract alkali metal cations much more efficiently than the trans form. This difference is ascribed to intramolecular complexation in the cis form and intermolecular complexation in the trans form. In addition, a decrease in the thermal cis-to-trans isomerization rate was observed, which should correlate with the intramolecular complexation affair in the cis form. The author also prepared azobenzenophane-type crown ethers (Scheme 41B).318 The trans forms, in which the polyoxyethylene chains are linearly extended, completely lacked the ability to form complexes with alkali metal cations, whereas the cis isomers with crown-like loops exhibited excellent affinities with them. By selecting the length of the polyoxyethylene chain, Kþ, Caþ, or Rbþ can be selectively extracted. In this system, the reversible photoconversion of the azo moiety is preserved, and the cis forms are thermally stabilized by the coordination to alkali metal ions. Photonic systems capable of reversibly capturing and releasing transition metal ions and complexes upon photoisomerization have been immobilized onto substrates and electrode surfaces. Russell and coworkers prepared SAMs containing a 4pyridylazophenoxy chromophore on a gold-covered glass substrate.319 In this system, laser-induced evanescent light, which can excite molecules only in the vicinity of the surface,320 was utilized to stimulate the photochromic azo group, taking advantage of the immobility of molecules in SAMs and the transmittance of the glass substrate. The authors realized a photocontrollable association-dissociation of Zn-porphyrin onto the 4-pyridyl group (Scheme 42). The evanescent excitation is greatly appreciated in this system because direct illumination can result in intense absorption by Zn-porphyrin, leading to its photobleaching. Tian and Willner grafted dithienylethene derivatives modified with two piperazine units onto gold electrodes (Scheme 43).321 The open form of the complex shows a moderate affinity with Cu2þ ions with an association constant (Ka) of 4.6  105 M 1, whereas photoconversion to the closed form affords a substantially lower binding affinity, Ka ¼ 4.1  104 M 1. More profound selectivity was observed to Agþ ions, with a Ka ¼ 3.0  105 M 1 for the open form, and no detectable binding by the closed form. The piperazine moieties act as binding sites for the metal ions, and a higher affinity in the open form is expected based on structural flexibility, which facilitates accommodation of the guest ions. The benefit of immobilization on electrodes arises from the fact that the authors were able to quantify the uptake of ions using electrochemical techniques, such as cyclic voltammetry and chronoamperometry. This result suggests applications, such as the uptake of metal ions from contaminated solutions or the controlled release of contaminants.

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Scheme 41 (A) Photoresponsive azobis(benzocrown ether)s, and proposed structure of its Rbþ complex in the cis form. (B) Photoresponsive azobenzenophane-type crown ethers.

Scheme 42 Schematic representation of an evanescent wave-driven release/recoordination cycle of self-assembled monolayer films containing the photoswitchable 4-pyridylazophenoxy structure. Reproduced from Wang, Z.; Nygård, A.-M.; Cook, M. J.; Russell, D. A. Langmuir, 2004, 20, 5850– 5857.

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Scheme 43 Binding and release of ions to and from the photoisomer states of bis-piperazine-functionalized dithienylethene. Reproduced from Zhang, J.; Riskin, M.; Tel-Vered, R.; Tian, H.; Willner, I. Langmuir, 2011, 27, 1380–1386.

Riskin et al. exploited the switchable coordination affinity to metal ions of the spiropyran/merocyanine system for the photocontrollable and electrochemically reversible deposition of magnetic Co nanoparticles (Scheme 44).322 SAMs of a merocyanine ligand were fabricated on Au electrodes. This merocyanine ligand undergoes reversible photoisomerization to the spiropyran form. Fine patterning of merocyanine and spiropyran domains is available using photomask and illumination techniques. The modified Au electrode was immersed in an aqueous solution of Co2þ ions to measure the selective binding of Co2þ to the merocyanine region. The bound Co2þ is electrochemically active. At an electrode potential of  0.88 V versus Ag/AgCl, the bound Co2þ ions are reduced to ferromagnetic Co nanoclusters, whereas at  0.2 V, the Co nanoclusters are reversibly oxidized to the divalent cationic form. It is significant for this system that the re-oxidized Co2þ ions neither dissociate into the electrolyte solution nor migrate onto the spiropyran region. This feature assures the reversibility of the system. Furthermore, in the presence of the cobalt species, the photochromic property of the merocyanine domain is lost. The authors ascribed this change to irreversible conversion of the nitro group to a hydroxyamino group. In total, this system displayed write-read-erase properties, where ‘write’ is manipulated with photoirradiation and immobilization of Co2þ ions, ‘read’ with electrochemical reduction of Co2þ ions to Co nanoparticles and detection of a magnetic signal, and ‘erase’ with the electrochemical re-oxidation of the Co nanoparticles to Co2þ ions. Fine reversibility was present in the ‘read’ and ‘erase’ functions. Nishihara and co-workers succeeded in converting photon energy into electronic potential energy by exploiting a photo-driven ligand exchange reaction in a copper complex (Scheme 45).323–325 Fig. 14A and B illustrates [CuI(trans2-oAB)2]þ (oAB ¼ 6,60 bis(400 -tolylazo)-4,40 -bis(4-tert-butylphenyl)-2,20 -bipyridine), which is one of the important metal complex components in this square scheme. The trans2-oAB ligand comprises 4,40 -bis(4-tert-butylphenyl)bipyridine and two trans-azobenzene units. Remarkably, in [Cu(trans2-oAB)2]þ, the trans-azobenzene units form p-stacking interactions with the 4,40 -bis(4-tert-butylphenyl)bipyridine units of the counterpart ligands (Fig. 14B). [Cu(trans2-oAB)2]þ is stabilized by this interligand p-stacking, which prevents ligand exchange with the free bpy ligands present in solution. Upon irradiation with UV light, [Cu(trans2-oAB)2]þ is converted to [Cu(cis2-oAB)2]þ. The bent structure of the cis-azobenzene units forces the Cu complex to lose the p-stacking interactions and subsequent thermodynamic stabilization, which leads to a ligand exchange reaction with the free bpy ligands to form [Cu(bpy)2]þ. On the other hand, irradiation of the solution with visible light induces a cis-to-trans conversion of the free cis2-oAB ligands, which results in the backward ligand exchange to regenerate [Cu(trans2-oAB)2]þ. The E00 of [CuII/CuI] is higher in [Cu(trans2-oAB)2]þ than in [Cu(bpy)2]þ by ca. 0.5 V. This shift arises from the steric effect of the substituent adjacent to the coordinating nitrogen atoms.326 Thanks to the difference in E00 [CuII/CuI], this reversible photoresponse can be extracted as a shift in the open circuit potential and a polarity reversal of the current. This system is the first example of a photoelectric conversion system based on a photochromic reaction. Xu et al. have reported a dinuclear copper complexes with dithienylethene based diphosphine ligands acting as a photoswitchable catalyst for a hydroboration reaction.327 Li et al. have created photochromic [Pd2L4] coordination cages based on DTE ligands allow triggering guest uptake and release by irradiation with light of different wavelengths. The process involves four consecutive electrocyclic reactions to convert all chromophores between their open and closed photoisomeric forms. The intrinsic chirality of the DTE backbones was used as reporter for monitoring the fate of a chiral guest (Scheme 46).328 Stang, Mukherjee and coworkers

Scheme 44 Photochemical encoding and readout of magnetic patterns on a photoisomerizable monolayer associated with a gold electrode, and the subsequent erasure of the patterns while retaining the encoded information. Reproduced from Riskin, M.; Gutkin, V.; Felner, I.; Willner, I. Angew. Chem. Int. Ed. 2008, 47, 4416–4420.

Scheme 45 Photoelectric conversion scheme based on a photoisomerization of a 6,60 -bis(400 -tolylazo)-4,40 -bis(4-tertbutylphenyl)-2,20 -bipyridine ligand which induces the ligand exchange reaction accompanying redox potential change. Reproduced from Kume, S.; Murata, M.; Ozeki, T.; Nishihara, H., J. Am. Chem. Soc. 2005, 127, 490–491.

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Fig. 14 (A) Structure of [Cu(trans2-oAB)2]BF4. (B) Crystal structure of [Cu(trans2-oAB)2]BF4. The BF4 anion is omitted for clarity. Aromatics in red and blue are p-stacked with one another.

have synthesized spiropyran-based multi-stimuli-responsive self-assembled PtII macrocycles, rendering coordination-assisted enhanced photochromism relative to the corresponding ligands. The switching behavior of the macrocycles can also be precisely controlled by tuning the pH of the medium.329 Frank and coworkers evaluated the changes in ligand field accompanied with the photoisomerization of spirooxazine by introducing spirooxazine-conjugated phenanthroline ligand to molybdenum-tetracarbonyl complex.330

8.09.3.3.5

NLO switching

For the construction of practical molecule-based SHG materials, it is essential to assemble them into bulk materials in a noncentrosymmetric fashion. Even if the b value of a certain molecule is enormous, centrosymmetric single crystals based on this molecule do not exhibit SHG. In the case of flexible polymers, the electrical poling method331–335 is available, which takes advantage of the electric dipole moments of dipolar NLO-phores. Quadrupolar and octupolar NLO-phores tend to possess NLO properties superior to the corresponding dipolar NLO-phores; however, electrical poling methods cannot be applied to polymer materials containing these molecules due to the absence of electric dipole moments. Le Bozec and coworkers demonstrated another method, the ‘all-optical poling’ technique, in polymers that include an octupolar NLO-phore.336 The octupolar NLO-phores used feature a [Zn(bpy)3]2þ core that serves as the origin of the D3-symmetry 3D

Scheme 46 Schematic illustration of photochromic behavior of [Pd2L4] coordination cages for the excretion of the encapsulated guest molecule and the CD spectra of [Pd2Cl4] coordination cages with and without a chiral guest, 1R() or 1S(þ) camphor sulfonate. Reproduced from Li, R.J.; Holstein, J. J.; Hiller, W. G.; Andréasson, J.; Clever, G. H. J. Am. Chem. Soc. 2019, 141, 2097–2103.

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Fig. 15

Octupolar NLO-phores based on Zn(bpy)2þ 3 .

framework and the acceptor site (Fig. 15). On the other hand, azobenzene-inserted dialkylaminostyryl units function as both donor sites and photoresponsive moieties. This complex exhibits exceptionally fine NLO activity, with b and b0 values of 863  10 30 esu and 590  10 30 esu, respectively. The authors embedded this octupolar NLO-phore into polycarbonate by means of atom transfer radical polymerization (ATRP). They also prepared a polycarbonate sample doped with the monomer complex as a reference. Excitation of the ligand moieties’ p-p* transition with 488 nm light induced trans-to-cis photoisomerization in the complexes and polymer films, followed by a decrease in the p-p* band and an increase in the absorptivity around 370 nm. The cis form underwent a thermal cis-to-trans isomerization. The authors conducted optical poling of the thin polymer films. After reaching the photostationary state, the grafted polymer possessed an SHG intensity that was 40% higher than that of the doped polymer. In addition, a 10% loss in the SHG intensity associated with thermal relaxation was observed in the grafted polymer sample; this loss was far smaller than that measured in the doped polymer (25%). This experiment demonstrated that grafting of the octupolar chromophores allowed molecular reorientation processes to more efficiently dominate the molecular Brownian motion, thereby inducing a higher and more stable photoinduced noncentrosymmetry. Aubert et al. also set up a molecular system in which the intrinsic NLO properties of NLO-phores themselves can be tuned by photochromism (Scheme 47).337 They incorporated dithienylethene between donor and acceptor sites in a p-conjugated fashion. The open and closed forms are switched with UV and visible light. This photochromism is accompanied by the modulation of the mb0 value, which is a significant parameter for determining the quadratic NLO properties. The closed form shows far greater mb0 (1020–1800  10 48 esu) than the open form (75–160  10 48 esu) with a nonresonant incidence wavelength of 1.91 mm. The authors concluded that the more effective p-conjugation in the closed form enhanced the quadratic NLO properties. Jacquemin, Le Bozec and coworkers synthesized an octupolar Cu-based chromophore featuring four photochromic dithienylethene units showing the modulation of the quadratic NLO response. Quantum mechanical simulations are consistent with a full switching of the DTE units and reproduce the strong enhancement of the NLO response (Scheme 48).338 Samoc, Humphrey, and coworkers also fabricated a dithienylethene-containing NLO-phore and described the photoswitchability of its cubic NLO property (Scheme 49). The closed form shows higher NLO capabilities.339 In combination with the acid–base responsivity and reversible redox behavior of the Ru-acetylide units, this system possesses six states with different s2 values (0–500 GM), which may lead to its potential use in logic gate applications.

Photochromic materials

Scheme 47

393

Photochromism of noncentrosymmetric dipolar Zn(II) complex.

Scheme 48 Chemical structure of Cu-based chromophore with four photochromic dithienylethene units showing the modulation of NLO response. Reproduced from Nitadori, H.; Ordronneau, L.; Boixel, J.; Jacquemin, D.; Boucekkine, A.; Singh, A.; Akita, M.; Ledoux, I.; Guerchais, V.; Le Bozec, H. Chem. Commun. 2012, 48, 10395–10397.

Boixel et al. have synthesized photochromic DTE-based PtII complexes (C^N^N)Pt(C^CeDTEeC6H4eD) ((C^N^N) ¼ 4,40 -di(nhexyl)-6-phenyl-2,20 -bipyridine; D]H, NMe2). Photoinduced switching of their second-order NLO properties appears in solution and in polymer films with an excellent NLO contrast (Scheme 50).340 The authors have also synthesized another type of DTE-based PtII complex, which acts as a sequential double NLO switch induced first by protonation and next upon irradiation with UV light (Scheme 51).341

8.09.3.4 8.09.3.4.1

Control over the isomerization behavior of organic photochromics by transition metal complexes and organometallics Electrochromism triggered by redox switching of metal moieties

Some organic photochromics display electrochromism. For example, Coudret, Launay, and coworkers reported the cyclization and cycloreversion in the cation radical state of dithienylethene derivatives;342 however, the redox products of organic compounds tend to be labile, and, thus, they are difficult to use in practical applications. This hurdle can be circumvented by conjugating redox-active

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Scheme 49 Interconversions of six states by external stimuli (photon, redox, acid-base) in a ruthenium acetylide-dithienylethene conjugate. Reproduced from Green, K. A.; Cifuentes, M. P.; Corkery, T. C.; Samoc, M.; Humphrey, M. G., Angew. Chem. Int. Ed. 2009, 48, 7867–7870.

Scheme 50 Photocyclization of (C^N^N)Pt(C^CeDTEeC6H4eD) complex with the modulation of NLO response. Reproduced from Boixel, J.; Guerchais, V.; Le Bozec, H.; Jacquemin, D.; Amar, A.; Boucekkine, A.; Colombo, A.; Dragonetti, C.; Marinotto, D.; Roberto, D.; Righetto, S.; De Angelis, R., J. Am. Chem. Soc. 2014, 136, 5367–5375.

transition metal complexes and organometallics to organic photochromics. Reduction or oxidation of the metal center can stimulate the isomerization of the organic photochromic moiety. Tanaka et al. fabricated a conjugate of the dithienylethene and piano-stool Ru-acetylide (Scheme 52).267 This type of organometallics shows reversible redox behavior at the RuIII/RuII couple. In the RuII state, photochromic behavior is similar to that of purely organic dithienylethene, with ring closure upon irradiation with UV light and ring opening upon illumination with visible light. Oxidation of the two RuII sites to RuIII induces fast thermal isomerization of the open form, giving rise to the corresponding closed form. The authors ascribed this redox-assisted ring closing to a possible cumulenic resonance structure. The closed form with the RuIII center is not photoresponsive and can be reductively converted to the open form with the RuII center. In total, this system experiences dual chromism. Similar electrochromism properties are observed in dithienylethene derivatives modified with bis(tpy)MII complexes (M ]Ru, Fe), yielding electrocyclization upon oxidation of the metal centers to MIII.343 Zhong et al. reported FeII, CoII and RuII trisbipyridines containing three dithienylcyclopentenes (Scheme 53).344 They undergo reversible photochromic behavior when irradiated with UV or visible light, and the open form can be cyclized electrochemically.

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395

Scheme 51 Three-state system of DTE-based Pt(II) complex; (i) HBF4,OEt2, CH2Cl2; (ii) CH2Cl2, hn (350 nm); (iii) CH2Cl2, hn (580 nm). Reproduced from Boixel, J.; Guerchais, V.; Le Bozec, H.; Chantzis, A.; Jacquemin, D.; Colombo, A.; Dragonetti, C.; Marinotto, D.; Roberto, D., Chem. Commun. 2015, 51, 7805–7808.

Scheme 52

Switching of photochromic and electrochromic behavior of a dithienylethene conjugate decorated with a ruthenium acetylide.

8.09.3.4.2

ON/OFF switching of photochromism via redox switching of the metal centers

Transition metal complexes and organometallics can quench photoexcited molecules via both electron transfer and energy transfer mechanisms. For example, ferrocene acts as an electron donor, displays fast electron transfer, and possesses a low-lying 3LF excited state.345–348 These characteristics trigger reductive electron transfer quenching and triplet-triplet energy transfer quenching via a Dexter mechanism. Thus, metal complexes and organometallics with quenching properties can potentially stop photochromic reactions. Such quenching abilities are quite sensitive to the oxidation state of the metal center; hence, the ON/OFF switching of photochromism can be realized by taking advantage of reversible redox switching. Nishihara and coworkers found that ferrocenylspiropyran displays redox-regulated photochromism (Scheme 54).349 The neutral species, with an iron oxidation number of þ 2, shows general photoisomerization behavior. Upon irradiation with UV light,

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Scheme 53 Photochromic metal complexes with dithienylethene-based ligands. Reproduced from Zhong, Y.-W.; Vilà, N.; Henderson, J. C.; Abruña, H. D., Inorg. Chem. 2009, 48, 7080–7085.

Scheme 54

Redox-regulated photoisomerization of ferrocenylspiropyran.

it converts to the merocyanine form accompanied by a blue color change in dichloromethane (the proportion of the merocyanine form is 56%). However, visible light irradiation or incubation in the dark facilitates a back reaction, yielding the colorless spiropyran form (the proportion of the merocyanine form is 0% with visible light). The ferrocene moiety can be easily and reversibly oxidized (E00 ¼ 0.03 V vs. ferrocenium/ferrocene) using mild oxidants, such as 1,10-dichloroferrocenium hexafluoridophosphate, or electrochemical oxidation. The oxidized spiropyran species undergo almost full conversion to the merocyanine form ( 100%) upon irradiation with UV light. In contrast, the back reaction to the spiropyran form does not occur under irradiation with visible light or in the dark. This result indicates that both the photo- and thermal isomerization reactions are locked in the oxidized form. The authors regarded this redox-regulated photochromism series as ‘shallow memory’ versus ‘deep memory.’ Photochromism and thermochromism in the reduced state correspond to ‘shallow memory,’ in which the merocyanine form (memorized state) can be converted to the spiropyran form (unmemorized state) either by a photonic stimulus or by a thermal stimulus. In the oxidized state, however, the ‘memory’ cannot be erased by these external stimuli unless the ferrocenium moiety is reduced. This ferrocenylspiropyran redox-conjugated photochromism series was also examined in a polyvinyl chloride (PVC) matrix. Ferrocenylspiropyran-containing PVC films were fabricated as follows. A ferrocenylspiropyran solution was swollen in PVC films,

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397

Fig. 16 Photochromism of ferrocenylspiropyran in a PVC film. Reproduced from Nagashima, S.; Murata, M.; Nishihara, H., Angew. Chem. Int. Ed. 2006, 45, 4298–4301.

and the solvent was then evaporated in vacuo. When the film was irradiated with UV light through a T-shaped photomask, a letter ‘T’ was printed on the film (Fig. 16). Photoisomerization was therefore available even in the PVC matrix. Doping of a supporting electrolyte (tetra-n-butylammonium tetrafluoridoborate) in the PVC matrix yielded redox-coupled photochromism, similar to that seen in solution (Fig. 17).

Fig. 17 Photographs of the redox-conjugated photoisomerization of ferrocenylspiropyran in PVC films doped with tetra-n-butylammonium tetrafluoridoborate as a supporting electrolyte. A voltage was applied to the left-hand polymer films in each picture, and no voltage was applied to the right-hand polymer films: (A) As-prepared films; (B) irradiation with UV light; (C) application of a voltage of 2 V; (D) irradiation with visible light; (E) application of a voltage of 0 V; and (F) irradiation with visible light. Reproduced from Nagashima, S.; Murata, M.; Nishihara, H., Angew. Chem. Int. Ed. 2006, 45, 4298–4301.

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8.09.3.4.3

Modulation of the photoresponsive wavelength

Organic photochromic molecules have inherent photoresponsive wavelengths; for example, azobenzene undergoes trans-to-cis photoisomerization upon irradiation with 300–350 nm UV light, whereas its cis-to-trans photoconversion takes place with 400–500 nm visible light. Tuning of the photoresponsive wavelength, particularly extension in the long wavelength direction, is desired in several applications, such as a development of full-color optical materials.350,351 Extension of the photoresponse in long wavelengths would be also appreciated by applications in the body, which require the use of wavelengths that are harmless to tissue352,353 and have appropriate transparency properties.354 The photoresponsive wavelength of organic photochromics may be tuned by transition metal complexes and organometallics via two approaches: (1) photosensitization of organic photochromics using photoexcited transition metal complexes and organometallics and (2) expression of new electronic transitions. In both cases, p-conjugation between the organic photochromics and the transition metal complexes or organometallics is of great importance. Taking the first approach, (1), Yam, Phillips, and coworkers fabricated phen-conjugated dithienylethene and the corresponding ReI tricarbonyl complex (Scheme 55).251,252 The dithienylethene-appended ligand alone shows ordinary photocyclization (l < 340 nm), which was produced by the 1IL (intra-ligand p-p* excited state of the dithienylethene moiety). However, the corresponding Re complex displays a red-shifted photoresponse with respect to the cyclization (l < 480 nm). The shift in the photoresponse of the Re complex by 140 nm is due to the 1MLCT transition from the Re (d) to the phen (p*). Transient absorption and time-resolved emission spectroscopies clarified that the photocyclization induced by the excitation of the 1MLCT band in the Re complex proceeds from the 3IL state, which is formed via intersystem crossing from the 1MLCT to the 3MLCT, accompanied by intramolecular energy transfer or internal conversion (Fig. 18). On the other hand, the ligand and Re complex show cycloreversion upon illumination at comparable wavelengths (540 and 580 nm, respectively). Instead of Re ion, Zn ion also can form photochromic

Scheme 55

Photochromic reactions of phen-conjugated dithienylethene, and the corresponding Re(I) tricarbonyl complex.

Fig. 18 Proposed qualitative energetic scheme for photosensitized photochromism of the Re(I) complex by MLCT excitation. Reproduced from Ko, C.-C.; Kwok, W.-M.; Yam, V. W.-W.; Phillips, D. L. Chem. Eur. J. 2006, 12, 5840–5848.

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Scheme 56 Proposed qualitative energy state diagram of the ZnII complexes. Reproduced from Ngan, T.-W.; Ko C.-C.; Zhu, N.; Yam V. W.-W., Inorg. Chem. 2007, 46, 1144–1152.

complexes with the phen-conjugated dithienylethene ligand which show photocyclization at 313 nm by the 1IL state and photocycloreversion at 515 nm (Scheme 56).355 Jukes et al. fabricated dithienylethene derivatives decorated with p-conjugated [MII(bpy)3]2 þ (M ¼ Ru and Os, Scheme 57), which is an excellent triplet photosensitizer.356 In the open form of the Ru complex, excitation of the 1MLCT band (lmax ¼ 458 nm) at wavelengths much longer than that used for the excitation of the 1IL state of the dithienylethene moiety (lmax ¼ 345 nm) results in photocyclization, whereas the corresponding Os complex shows no photochromic reaction upon excitation with the 1MLCT band. The contrasting behavior between the Ru and the Os complexes arises from the different fates of the 1MLCT excited state. In the Ru complex, the 1MLCT excited state undergoes intersystem crossing to the 3MLCT state, followed by internal conversion to the 3IL state of the dithienylethene moiety, which is responsible for the ring closure (Fig. 19A). In contrast, the 1MLCT of the Os complex merely experiences intersystem crossing to the 3MLCT state because its excitation energy is lower than that of the 3 IL state (Fig. 19B). Taking the second approach, (2), Nishihara and coworkers reported several systems that incorporated transition metal complexes and organometallics that could act as donors to organic photochromics via p-conjugation. One good example is 3ferrrocenylazobenzene (3-FcAB), which includes ferrocene as an organometallics donor site (Scheme 58).357–360 Its molar absorptivities in the range 400–550 nm (lmax ¼ 444 nm, 3 max ¼ 1.86  103 M 1 cm 1) exceed those of the trans-azobenzene

Fig. 19 Qualitative energetic scheme for the efficient and insufficient sensitized photocyclization of the Ru complex and Os complex, respectively. Reproduced from Jukes, R. T. F.; Adamo, V.; Hartl, F.; Belser, P.; De Cola, L. Inorg. Chem. 2004, 43, 2779–2792.

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Scheme 57

Photoresponse upon excitation with the 1MLCT band of dithienylethene derivatives decorated with M(bpy)32 þ (M]RuII and OsII).

(lmax ¼ 444 nm, 3 max ¼ 5.15  102 M 1 cm 1). Upon excitation of the visible band with 546 nm green light, 3-FcAB undergoes trans-to-cis photoisomerization, opposite to the behavior of normal azobenzene. The authors investigated the nature of the visible band of 3-FcAB with the help of time-dependent DFT (TDDFT). They found that the enlargement of the visible absorption stems from the emergence of a new 1MLCT transition from the ferrocene (d) to the azo (p*), the intensity of which far exceeds that of the original n-p* band of the azo group. In the cis form, the intensity of the 1MLCT band significantly decreases, which is the driving force for shifting the photostationary state toward the cis-rich direction. Chemical oxidation of the ferrocene moiety of 3-FcAB removes the 1MLCT band in both isomers. Instead, an LMCT band assignable to a transition from azo (p*) to ferrocene (d) emerges at a maximum of 730 nm. In this case, the 546 nm irradiation exclusively excites the n-p* band of the azo moiety, which affords the cis-to-trans isomerization, as is observed in ordinary azobenzene. The LMCT band is not associated with the photoisomerization reaction. In total, 3-FcAB is the first system that realizes trans-cis conversion using a single-light source (546 nm) in combination with the redox switch of the ferrocenyl group. Toward use of 3-FcAB in a novel high-density optical data storage device, the authors fabricated SAMs of 3-FcAB on transparent ITO electrodes (Scheme 59).359 The response of ferrocenylazobenzene to an induced electrical potential, without permitting diffusion, can allow the construction of molecular devices. The photoswitching behavior was monitored by careful measurement of the absorbance changes caused by molecular switching. The direction of photoswitching induced by green light with a positive electrode voltage was opposite that induced without an electrical stimulus. This behavior indicates that the system can be switched by a single-light source, just like 3-FcAB in solution. This property is relevant to molecular-sized switching devices because it

Scheme 58

Schematic illustration of the redox-coupled single-light trans-cis photoisomerization of 3-ferrocenylazobenzene (3-FcAB).

Photochromic materials

Scheme 59

401

Schematic illustration of the redox-coupled single-light trans-cis photoisomerization of 3-FcAB on ITO electrode.

eliminates the need for focusing dual light sources onto a single small region to enable switching of the molecules. The authors also embedded 3-FcAB into polymer particles and demonstrated redox-coupled photoisomerization in this medium.360 Dithiolene-dppe-MII (M ]Ni, Pd, Pt) has quasi-aromaticity with respect to the M-S-C-C-S five-membered ring and good donor properties that stem from the metalladithiolene (p) orbitals.361 Nishihara and coworkers synthesized azobenzene-conjugated dithiolene-dppe-MII complexes, in which the azo group is linked to the benzenedithiolato ligand via p-conjugation.362 In this series of complexes, the authors identified new electronic bands around 400–500 nm that could not be observed in either the dithiolenedppe-MII or azobenzene moieties alone. These bands were assigned to intra-ligand charge transfer (CT) transitions from the metalladithiolene (p) to the azo (p*). Excitation of this intra-ligand CT band, for example, at 405 nm, afforded trans-to-cis photoisomerization of the azo moiety. In this system, the trans-to-cis conversion is realized with visible light. Moreno et al. have synthesized IrIII complex with a covalently tethered azobenzene fragment and found that selective irradiation of the 1MLCT band of the IrIII complex with 380 nm-light induced an efficiently sensitized Z-E photoswitching of the dyad over a wide concentration range and even at high dilution (Scheme 60).363 Li et al. synthesized a dinuclear gold(I) complex with three dithienylethene units acting as a four-state and four-color photochromic switch upon irradiation with appropriate wavelengths of light (Scheme 61).364

8.09.3.4.4

NIR photochromism

The extreme extension of the photoresponsive wavelengths in photochromics reaches NIR photochromism, which enhances the semiconductor diode laser susceptibility for applications in optical memory storage. Several pure organic photochromics have realized NIR photochromism by exploiting p-extension reactions,365–372 though, their synthetic costs are not trivial, and the wavelength shifts would eventually be hampered by the p-conjugation convergence limit. Yam and coworkers synthesized a photochromic ligand, dithienyl-substituted 2-(2-pyridyl)imidazole and its ReI complex (Scheme 62).373 The ligand displayed photochromism at wavelengths characteristic of dithienylethene derivatives. Incorporation of the ReI unit shifted the absorption of the cyclized form toward much longer wavelengths (lmax ¼ 710 nm). Upon excitation with this band with NIR light, the Re complex underwent a ringopening reaction. The authors ascribed this NIR absorption to the 1IL transition of the ligand, which was perturbed by the 1MLCT transitions, and the redshift to the coplanarization of the pyridyl and imidazolyl rings induced by the Re complexation: This coplanarization afforded more intense p-conjugation. In total, this work suggested a new methodology for generating NIR photochromic systems: metal coordination-assisted planarization and the subsequent extension of p-conjugation. Yam and coworkers finally fabricated a conjugate system involving a dithienylethene and a cyclometallated PtII complex, which also displayed NIR photochromism.374 Tan et al. described a pyridine-appended dithienylethene and its corresponding cyclometalated IrIII complex,375 which underwent NIR photochromism with cycloreversion of the dithienylethene moiety. The authors also fabricated thienyl-substituted tetraazaporphyrins and phthalocyanines exhibiting NIR photochromism (Scheme 63).376–379 The highly developed p-conjugation of the metal-containing macrocycles gave rise to a redshift toward the NIR region in the absorption bands of the closed form to produce NIR-active photochromic compounds. Also noteworthy is that the NIR luminescence properties from tetraazaporphyrins and phthalocyanines could also be regulated via photochromic reactions.

Scheme 60 Ir(ppy)2acac-AB complex where the 1MLCT band of the IrIII complex induced an efficiently sensitized photoswitching of azobenzene. Reproduced from Moreno, J.; Grubert, L.; Schwarz, J.; Bléger, D.; Hecht, S., Chem. Eur. J. 2017, 23, 14090–14095.

Scheme 61 A dinuclear gold(I) complex with both DTE-acetylide and DTE-diphosphine exhibiting multistep and multiple photocyclization/ cycloreversion reactions. Reproduced from Li, B.; Wu, Y.-H.; Wen, H.-M.; Shi, L.-X.; Chen, Z.-N., Inorg. Chem. 2012, 51, 1933–1942.

Photochromic materials

Scheme 62

Photoisomerization of dithienyl-substituted 2-(2-pyridyl)imidazole and its Re(I) complex.

Scheme 63

Photochromism of unsymmetrical phthalocyanine hybrids based on bisthienylethene (M ]MgII, ZnII).

8.09.3.5

403

Mutual controls

The semiconductor industry has long relied on top-down methods for the fabrication of materials and devices; however, in the development of molecule-based devices, bottom-up methods may be more advantageous and powerful. The ultimate goal is to establish unimolecular devices. It is very important for the fabrication of high-end materials and devices to integrate a variety of functionalities (e.g., electro-, photo-, and magneto-properties) within a single molecule. Several conjugates of organic photochromics and transition metal complexes or organometallics permit mutual control over the physical properties of each component. With the goal of integrating multiple functionalities into a single molecule, mutual interactions present an excellent advantage. Several groups have developed systems with such properties. Yutaka et al. studied a series of azobenzene-bridged dinuclear terpyridine complexes where the metal is Fe,380 Co,380 Ru,381,382 and Rh,382,383 showing the alteration of photoisomerization behavior and redox potential shift. Katan, Malval, Fillaut and coworkers reported the influence of the conjugation pathway on the photoisomerization of aminoazobenzene-substituted RuII tris(bipyridine) complexes.384 Nishihara and coworkers synthesized bis(ferrocenylethynyl)fumarate (E form) and maleate (Z form) (Scheme 64), comprising ferrocene, an organometallics donor,345,346 and dimethyl 2,3-bis(ethynyl)fumarate/maleate, a new class of diethynylethene frameworks with photochromic and acceptor propreties.268,269 As mentioned in Section 8.09.3.2, ethynylethene derivatives undergo E-Z conversion upon excitation of the p-p* bands derived from their highly developed p-systems. This class of compounds yields a limited photoresponse (at 500 nm); hence, its extension toward longer wavelengths would be of great value. In this system, strong donor-acceptor interactions yield a CT band at wavelengths longer than that corresponding to the original p-p* band. As a result, red-shifted photochromism is observed upon excitation of this CT band. This corresponds to a control of the photoisomerization behavior of dimethyl 2,3-bis(ethynyl)fumarate/maleate by the two ferrocene moieties (see Section 8.09.3.4.3).

404

Photochromic materials

Scheme 64 Visible right photochromism of bis(ferrocenylethynyl)fumarate (E form) and maleate (Z form), in which electronic communication switches between the two ferrocene moieties in the mixed-valent monocationic state.

Diethynylethene can be regarded as a minimal unit of the polydiacetylene polymer, which displays a number of intriguing physical properties derived from the extended p-orbitals, including electroconductivity.385 The exploration of electronic communication via organic p-bridges in MV compounds is related to electron transfer and transport. Therefore, a survey of electronic communication in diethynylethene-based compounds would potentially advance this field toward the creation of a molecule-based photoswitchable electronic circuit. The authors quantified the electronic communication between the ferrocene moieties in the MV cationic state, focusing on the difference between the E and Z forms. This can be regarded as control over the electrochemical property of the ferrocene via dimethyl 2,3-bis(ethynyl)fumarate/maleate (see Section 8.09.3.3.2). Both bis(ferrocenylethynyl)fumarate (E form) and maleate (Z form) display peculiar electronic absorption bands in the visible region, together with an intense p-p* bands intrinsic to the diethynylethene derivatives in the UV region (Fig. 20A). TDDFT calculations suggest that these visible bands may be assigned to CT transitions from the two ferrocenes (d) to the diethynylethene (p*) (Fig. 20B). The maxima corresponding to the p-p* (E: lmax ¼ 365 nm; Z: lmax ¼ 362 nm) and the CT (E: lmax ¼ 524 nm; Z: lmax ¼ 517 nm) bands are slightly blue-shifted in the Z form, although ethynylethene derivatives without steric hindrance do not display such blue shifts.386 This change stems from the steric repulsion between the methyl ester moieties, and indicates that the p-system of the diethynylethene moiety in the Z form is disturbed. Single crystal X-ray structure analysis and DFT calculations also suggest the existence of this steric hindrance. Irradiation to the E form in dichloromethane with a mixture of green (546 nm) and yellow (578 nm) light, which leads to excitation of the CT band, produces an E-to-Z transformation with stepwise decreases in the p-p* band (characteristic of the E-to-Z

Fig. 20 (A) Electronic spectra of the E isomer (solid line) and the Z isomer (dotted line) in dichloromethane. (B) Main transition in the intramolecular CT band of the E form calculated by DFT calculation.

Photochromic materials

405

transformation of ethynylethene) and the CT band. The Z form is the dominant species in the photostationary state (89%). In addition, one isosbestic point is observed, confirming the absence of side reactions. The authors evaluated electronic communication in the monocationic MV state by studying the splitting between the formal potentials of the two equivalent ferrocene sites, DE00 . The results revealed a greater DE00 in the E form (70 mV) than in the Z form (48 mV), which corresponds to Kc of 15 and 6.5, respectively. Because the through-space distance between the two redox sites is shorter in the Z forms (the FeeFe distances in the neutral species, determined by DFT calculations, are 11.9 Å in the E form and 7.5 Å in the Z form), the splitting pattern in E00 is mainly derived from through-bond electronic communication between the redox sites. The weaker through-bond interactions in the Z forms are consistent with the conclusion that the p-system of the diethynylethene is disturbed by the steric hindrance between the two methyl ester moieties, as indicated in the structural analyses, electronic spectra, and DFT calculations. Nishihara and coworkers synthesized another mutually interacting system based on DHP and ferrocene (Scheme 65).270,271 The ethynylferrocene-appended DHP shows photochromism typical of ordinary DHP, with retrocyclization to cyclophanediene (CPD) upon visible light (578 nm) illumination and cyclization to DHP upon UV light (303 nm) illumination. The cyclic voltammogram of the DHP form shows a split in E00 corresponding to the oxidation of two equivalent ferrocene moieties (DE00 ¼ 63 mV, Kc ¼ 28). However, the CPD form does not exhibit a clear split (DE00 ¼ 16 mV, Kc ¼ 2.3). These results indicate that the DHP form displays stronger electronic communication than the CPD form. This is consistent with the fact that the potential of the p-orbital in the DHP form is closer to that of the highest occupied d-orbital of ferrocene than in the CPD form. In addition, a redox-assisted ring closing reaction occurs after oxidation of the ferrocene moieties. This behavior was quantitatively investigated by fitting the cyclic voltammograms in 0.1 M Bu4NClO4-1,3-dichloropropane at 218 K with simulations. The first-order kinetic parameters for the redoxassisted isomerization were found to be k1 ¼ 3.7 s 1 and k2 ¼ 0.50 s 1 for the cationic and dicationic species, respectively. These values are much greater than those obtained for the thermal ring closure of the neutral CPD form. In summary, the photochromic DHP/CPD moiety provides two ferrocene moieties with ON/OFF switching properties with respect to electronic communication in the MV state. On the other hand, the ferrocenemoieties provide the DHP/CPD moiety with electrochromic properties. The operation of LD-LISC in the crystalline solid state was first described by Boillot and coworkers using FeII(E-stpy)4(NCSe)2 (Scheme 66).387 In the all-trans form, FeII(E-stpy)4(NCSe)2 undergoes SCO from 293 K (HS) to 104 K (LS) with a T1/2 of 163 K, whereas the all-cis form FeII(Z-stpy)4(NCSe)2 is HS at all temperatures tested.388 At room temperature in the solid state, both the all-trans and the all-cis complexes show an absorption band characteristic of the p-p* transition of the stpy moieties. In addition, a weaker visible absorption with maxima at 455 nm (all-trans) and 435 nm (all-cis), respectively, is observed. The authors assigned this band to an MLCT transition from FeII (d) to the stpy (p*), typical of this type of HS FeII complexes.389,390 In a PMMA matrix at

Scheme 65 Photochromism, redox-assisted ring closure, and switch in electronic communication in ethynylferrocene-conjugated dimethyldihydropyrene/cyclophanediene.

406

Photochromic materials

Scheme 66

One-way photochromism in FeII(stpy)4(NCSe)2 upon excitation with the MLCT band in the solid state.

130 K, irradiation with UV light (355 nm) affords trans-cis photoisomerization, and the LD-LISC phenomenon is observed.388 However, in the crystalline solid state at room temperature, this complex does not undergo photochromism upon irradiation with UV light. Instead, excitation of the MLCT band with 532 nm visible light promotes a one-way cis-to-trans photoisomerization. With the aid of DFT calculations on the free stpy ligand, the authors tentatively assigned this peculiar photoisomerization to the 3 p-p* excited state, which is generated from the 5MLCT photoexcited state and subsequent relaxations. Stilbenoids, including styrylferrocene, undergo one-way cis-to-trans isomerization from the triplet excited state.391–393 The visible-light photoisomerization of FeII(stpy)4(NCSe)2 in the crystalline solid state can be potentially utilized LD-LISC phenomena at low temperatures. In this system, the stpy moieties trigger the spin state change in the FeII complex, whereas the d-orbitals of FeII afford the MLCT transition and yield peculiar visible-light photochromism in the stpy ligands. Nishihara and coworkers independently investigated L2FeII (L ¼ 2,6-di(1Hpyrazol-1-yl)-4-styrylpyridine), in which an LD-LISC phenomenon is observed at ambient temperatures.394 Also in this molecular system, one-way Z to E photochromism of the stilbene moiety stimulated by MLCT excitation plays a crucial role.

8.09.3.6

Multiphotochromic systems

Several transition metal complexes and organometallics have been synthesized to include two or more organic photochromics in the pursuit of synergistic effects among the organic photochromics; however, most of these efforts led to naught. For example, [CoII(dmAB)3](BF4)2 and [CoIII(dmAB)3](BF4)3 (dmAB ¼ 4,40 -bis[300 -(400 -tolylazo)phenyl]-2,20 -bipyridine), which feature six azobenzene units bound to one tris(bipyridine)CoII or CoIII unit, did not display synergetic phenomena: The six azobenzene units photoisomerized independently.395 Nishihara and coworkers prepared three types of azobenzene-conjugated dithiolato-bipyridine-PtII complexes,396,397 which include an azo group on either the bipyridine or the dithiolato ligand (Scheme 67A and B). Dithiolato-bipyridine-PtII complexes feature an interligand CT band at around 500–700 nm, which results in a long-lived triplet photoexcited state.398–401 The authors expected that this photoexcited state could sensitize photoisomerization of the azobenzene moiety to extend the photoresponse to longer wavelengths. The authors were also interested in the effects of the position of the azo groups on their photoisomerization behavior. A complex that included two azo groups on both ligands was also synthesized (Scheme 67C). The PtII complex with one azo group on the bipyridine ligand displays regular photoisomerization behavior: trans-to-cis and cisto-trans photoisomerization upon 334 or 405 nm illumination, respectively (Scheme 67A). However, a complex with an azo group on the dithiolato ligand shows irregular photoisomerization behavior. Illumination at 405 nm induces the trans-to-cis conversion, whereas 312 nm light makes the complex adopt the trans phase (Scheme 67B). In addition, the extension of the photoresponse to longer wavelengths is noteworthy in the two PtII complexes shown above. Irradiation at 578 nm, which corresponds to the excitation of the interligand CT band, results in cis-to-trans photoisomerization, irrespective of the position of the azo group (Scheme 67A and B). The interligand CT excited state of the PtII complex moiety is consumed in the photoisomerization of the azobenzene moieties. In fact, phosphorescence from the triplet interligand CT excited state was significantly quenched compared to that of a reference compound398–401 without azobenzene. Finally, the authors investigated photochromism in the Pt complexes with two azo groups on both ligands and found that the photoresponse nearly coincides with those of the two complexes described above. Illumination with 334 and 405 nm light permits the selective trans-to-cis photoisomerization of the azo groups on the bipyridine and dithiolato ligands, respectively. However, the all-trans state is regenerated upon irradiation at 578 nm. In summary, the Pt complex with two azo groups displays photocontrollable tristability (Scheme 67C). Yam and coworkers synthesized a tetranuclear metallomacrocyclic molecule composed of four AuI acetylide and two azobenzene units (Scheme 68),402 where the two azobenzenes are parallel. This molecule undergoes trans-to-cis isomerization, although it is thought that only one of the two azobenzene units is transformed into the cis form. When both azobenzenes are in the trans state, the two acetylide moieties lie in such close proximity that they encapsulate Agþ ions in a sandwich fashion. The intercalated Agþ ions suppress photoisomerization of the azobenzene units. The addition of nBu4NCl removes the encapsulated Agþ ions from the metallomacrocycle to form insoluble AgCl, thereby recovering the original photoisomerization behavior.

Photochromic materials

407

Scheme 67 (A and B) Photocontrollable bistability in dithiolato-bipyridine-Pt(II) complexes, in which the azo groups are conjugated on the bipyridine and dithiolato ligands, respectively. (C) Photocontrollable tristability in a dithiolato-bipyridine-Pt(II) complex, in which the azo groups are conjugated on both the dithiolato and bipyridine ligands.

Scheme 68 Entanglement of AgI coordination and photoisomerization in an AuI metallomacrocyclic molecule. Reproduced from Tang, H.-S.; Zhu, N.; Yam, V. W.-W. Organometallics, 2007, 26, 22–25.

408 8.09.3.7

Photochromic materials Other types of conjugates

Muraoka, Kinbara, and Aida fabricated a molecular machine system that realized, for the first time, the mechanical manipulation of a guest molecule in a controlled and reversible fashion (Scheme 69).403–405 In this system, the contraction and the elongation of an azobenzene moiety upon trans-cis photoisomerization is translated into rotations of the ferrocene moiety, which is further transmitted to the tip zinc porphyrins as a pedal-like motion. This series of movements is exploited to twist a guest molecule trapped by the host zinc porphyrins. The unique shape and coordination mode of the Fe2þ ion and cyclopentadienyl anions in ferrocene play a crucial role in this system. Shionoya and coworkers controlled the crystallinity of a supramolecular system via photoisomerization of a guest molecule (Scheme 70).406 The authors had previously found that a molecular cage comprising a pyridine-appended banana-shaped ligand and a square planar PdII center could hold bis(sulfonate) molecules in its cavity through electrostatic attraction between the cationic tetrakispyridine-PdII complex unit and the anionic bis(sulfonate).407 Taking advantage of this knowledge, they introduced photoswitchable 4,40 -azobenzene bis(sulfonate) as a new guest molecule. First, they investigated the photoregulation of the host-guest interactions in a polyethyleneglycolated (PEGylated) cage with good solubility in organic solvents. When the guest molecule assumed the cis configuration, quantitative inclusion was observed; in the transform, a loosely associated complex was generated. They also tested the fine reversibility of the two states upon illumination at two wavelengths, UV and visible light. The authors ascribed this switching behavior to the dramatic change in the distance between the two sulfonates upon photoisomerization. The length of the cis form is more suitable in the guest cage, in this case. A cage without PEG substituents yielded an intriguing photoinduced crystallization property. When the cage holding cis-4,40 azobenzene bis(sulfonate) in solution was irradiated with visible light, immediate generation of crystals was observed. Single crystal X-ray structure analysis showed that this crystal contains two trans-4,40 -azobenzene bis(sulfonate) guests per host cage, the former of which connect the latter via crosslinking. On the other hand, when two equivalents of cis-4,40 -azobenzene bis(sulfonate) are added to the host cage, the mixture undergoes spontaneous crystallization, even upon exclusion of visible light, giving rise to crystals with a different morphology. Although the structure was not identified, the composition likely features a 1:2 ratio of the host cage and the guest molecule in the cis configuration. Yam and coworkers synthesized a series of novel benzo[b]phosphole alkynylgold(I) complexes which display photochromic and mechanochromic properties upon applying the respective stimuli of light and mechanical force (Scheme 71).408 These multistimuli responsive properties were achieved by incorporation of the photochromic dithienylethene-containing benzo[b]phosphole into the triphenylamine containing arylethynyl ligand that is susceptible to mechanical force-induced color changes via AuI complexation. The complexes undergo repeatable photochromic and mechanochromic cycles without apparent loss of reactivity. [BIIIWVI11O39CoIII]6 borotungstates which is incorporated by CoIII have been functionalized by diarylethene which contains pyridyl unit.409 Irradiation at 365 or 400 nm of a cyclohexane solution of the complex progresses the cyclization of the diarylethene unit. The photochromic performance of a spiropyran derivative grafted on polyoxotungstates is reported in solution and in the solid state.410 Mirkin and coworkers synthesized a series of PtII and PdII complexes incorporating phosphino-aminoazobenzene ligand.411 Either addition or extraction of chloride ion induces the reversible change of AB-type complex and aAB- or pAB-type complex. This conformation switching modulates their half-lives of their cis conformers over a 3 order of magnitude range. Walkey et al. reported an ethynylspiropyran ligand coordinating to a ruthenium complex showed the twentyfold longer lifetime of the ring-opened form than that of the non-coordinating state.412 According to the DFT calculation, the presence of ruthenium complex enhances the charge delocalization and makes the higher barrier to thermal ring-closing process from the ring-opened form.

Scheme 69 Schematic representation of photoisomerization of a 1:1 complex of a molecular pedal and a rotary guest. Reproduced from Muraoka, T.; Kinbara, K.; Aida, T. Nature, 2006, 440, 512–515.

Photochromic materials

409

Scheme 70 Schematic illustration of the photoregulation of supramolecular interactions and crystallization. Reproduced from Clever, G. H.; Tashiro, S.; Shionoya, M. J. Am. Chem. Soc. 2010, 132, 9973–9975.

Park et al. reported the control of singlet oxygen (1O2) generation in living cells by introducing a porphyrine-based photosensing ligand and a diarylethene-based photochromic ligand into a zirconium metal-organic framework.413 When the diarylethene unit takes a closed form, the excited electron at the photosensing ligand is quenched by the energy transfer to the closed diarylethene. On the other hand, when the diarylethene unit is in the open form, the energy transfer from the excited state of photosensing ligand to 3O2, progressing 1O2 generation reaction. A Zn complex, [ZnLBr2] (L ¼ 4,40 -bipyridinium-N-propionate), exhibits the reversible photochromism induced by soft and hard X-ray irradiation at room temperature.414 According to in situ X-ray photoelectron spectroscopy determination, the electron transfer form Br to L is an important factor for this X-ray induced photochromism. Other metal complexes ligated a viologen derivative have been reported as photochromic materials based on the electron transfer mechanism.415–424 Cindric et al. reported the series of MoIV oxalate, M2[Mo2O5(C2O4)2(H2O)2] (M ]Na, K, Rb, Cs) exhibit a photochromic behavior under UV irradiation.425 ESR spectra investigation suggests that the partial reduction of MoVI to MoV only at crystal surfaces is a key role of this photochromism.

410

Photochromic materials

Scheme 71 Photochromic benzo[b]phosphole alkynylgold(I) complexes with mechanochromic property to serve as multistimuli-responsive materials. Reproduced from Wu, N. M.-W.; Ng, M.; Yam, V. W.-W., Angew. Chem. Int. Ed. 2019, 58, 3027–3031.

8.09.4

Conclusion

This chapter introduced transition metal complexes and organometallics that inherently display photochromic behavior. Section 8.09.1 presented a general introduction of the topic. Section 8.09.2 addressed coordination compounds and organometallics that exhibit photochromism. These compounds are divided into three categories: linkage isomerization, intramolecular ligand exchange, and metastable state trapping at low temperatures upon photoinduced electron transfers or photoexcitation. A convenient methodology for introducing photochromic properties into transition metal complexes and organometallicsdconjugation with organic photochromicsdwas explained in Section 8.09.3. These molecules display synergy between metal-containing fragments and organic photochromics, including regulation of the physical properties of the former and control over the isomerization behavior of the latter. Also introduced were tactical molecules that simultaneously realize mutual interactions between the two components, multiphotochromic systems in which two or more organic photochromics are conjugated to one metal center, and other uncategorizable systems. Beyond the fundamental interests of chemists, photochromic molecular systems can be exploited in a wide range of applications, the ultimate goal of which is the fabrication of molecule-based devices.

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Chem. 2012, 51, 1933–1942. Gilat, S. L.; Kawai, S. H.; Lehn, J.-M. Chem. A Eur. J. 1995, 1, 275–284. Gilat, S. L.; Kawai, S. H.; Lehn, J.-M. J. Chem. Soc. Chem. Commun. 1993, 1439–1442. Peters, A.; McDonald, R.; Branda, N. R. Chem. Commun. 2002, 2274–2275. Ahmed, S. A.-M. J. Phys. Org. Chem. 2002, 15, 392–402. Liang, Y.; Dvornikov, A. S.; Rentzepis, P. M. J. Mater. Chem. 2003, 13, 286–290. Ahmed, S. A. J. Phys. Org. Chem. 2006, 19, 402–414. Odo, Y.; Matsuda, K.; Irie, M. Chem. A Eur. J. 2006, 12, 4283–4288. Heller, H. G.; Hughes, D. S.; Hursthouse, M. B.; Rowles, N. G. Chem. Commun. 2000, 1397–1398. Lee, P. H.-M.; Ko, C.-C.; Zhu, N.; Yam, V. W.-W. J. Am. Chem. Soc. 2007, 129, 6058–6059. Chan, J. C.-H.; Lam, W. H.; Wong, H.-L.; Zhu, N.; Wong, W.-T.; Yam, V. W.-W. J. Am. Chem. Soc. 2011, 133, 12690–12705. Tan, W.; Zhang, Q.; Zhang, J.; Tian, H. Org. Lett. 2009, 11, 161–164. Chen, B.; Wang, M.; Wu, Y.; Tian, H. Chem. Commun. 2002, 1060–1061. Tian, H.; Chen, B.; Tu, H.-Y.; Müllen, K. Adv. 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Photochromic materials Luo, Q.; Cheng, S.; Tian, H. Tetrahedron Lett. 2004, 45, 7737–7740. Yutaka, T.; Mori, I.; Kurihara, M.; Tamai, N.; Nishihara, H. Inorg. Chem. 2003, 42, 6306–6313. Yutaka, T.; Kurihara, M.; Nishihara, H. Mol. Cryst. Liq. Cryst. 2000, 343, 193–198. Yutaka, T.; Mori, I.; Kurihara, M.; Mizutani, J.; Kubo, K.; Furusho, S.; Matsumura, K.; Tamai, N.; Nishihara, H. Inorg. Chem. 2001, 40, 4986–4995. Yutaka, T.; Kurihara, M.; Kubo, K.; Nishihara, H. Inorg. Chem. 2000, 39, 3438–3439. Amar, A.; Savel, P.; Akdas-Kilig, H.; Katan, C.; Meghezzi, H.; Boucekkine, A.; Malval, J.-P.; Fillaut, J.-L. Chem. A Eur. J. 2015, 21, 8262–8270. Takami, K.; Mizuno, J.; Akai-kasaya, M.; Saito, A.; Aono, M.; Kuwahara, Y. J. Phys. Chem. B 2004, 108, 16353–16356. Gobbi, L.; Seiler, P.; Diederich, F.; Gramlich, V. Helv. Chim. Acta 2000, 83, 1711–1723. Tissot, A.; Boillot, M.-L.; Pillet, S.; Codjovi, E.; Boukheddaden, K.; Daku, L. M. L. J. Phys. Chem. C 2010, 114, 21715–21722. Boillot, M.-L.; Pillet, S.; Tissot, A.; Rivière, E.; Claiser, N.; Lecomte, C. Inorg. Chem. 2009, 48, 4729–4736. Toftlund, H. Coord. Chem. Rev. 1989, 94, 67–108. Sénéchal-David, K.; Zaman, N.; Walko, M.; Halza, E.; Rivière, E.; Guillot, R.; Feringa, B. L.; Boillot, M.-L. Dalton Trans. 2008, 14, 1932–1936. Arai, T.; Karatsu, T.; Sakuragi, H.; Tokumaru, K. Tetrahedron Lett. 1983, 24, 2873–2876. Hamaguchi, H.; Tasumi, M.; Karatsu, T.; Arai, T.; Tokumaru, K. J. Am. Chem. Soc. 1986, 108, 1698–1699. Arai, T.; Ogawa, Y.; Sakuragi, H.; Tokumaru, K. Chem. Phys. Lett. 1992, 196, 145–149. Hasegawa, Y.; Takahashi, K.; Kume, S.; Nishihara, H. Chem. Commun. 2011, 47, 6846–6848. Yamaguchi, K.; Kume, S.; Namiki, K.; Murata, M.; Tamai, N.; Nishihara, H. Inorg. Chem. 2005, 44, 9056–9067. Sakamoto, R.; Murata, M.; Kume, S.; Sampei, H.; Sugimoto, M.; Nishihara, H. Chem. Commun. 2005, 1215–1217. Sakamoto, R.; Kume, S.; Sugimoto, M.; Nishihara, H. Chem. A Eur. J. 2009, 15, 1429–1439. Zuleta, J. A.; Bevilacqua, J. M.; Eisenberg, R. Coord. Chem. Rev. 1991, 111, 237–248. Cummings, S. D.; Eisenberg, R. J. Am. Chem. Soc. 1996, 118, 1949–1960. Paw, W.; Cummings, S. D.; Mansour, M. A.; Connick, W. B.; Geiger, D. K.; Eisenberg, R. Coord. Chem. Rev. 1998, 171, 125–150. Hissler, M.; McGarrah, J. E.; Connick, W. B.; Geiger, D. K.; Cummings, S. D.; Eisenberg, R. Coord. Chem. Rev. 2000, 208, 115–137. Tang, H.-S.; Zhu, N.; Yam, V. W.-W. Organometallics 2007, 26, 22–25. Muraoka, T.; Kinbara, K.; Aida, T. Nature 2006, 440, 512–515. Muraoka, T.; Kinbara, K.; Kobayashi, Y.; Aida, T. J. Am. Chem. Soc. 2003, 125, 5612–5613. Muraoka, T.; Kinbara, K.; Aida, T. J. Am. Chem. Soc. 2006, 128, 11600–11605. Clever, G. H.; Tashiro, S.; Shionoya, M. J. Am. Chem. Soc. 2010, 132, 9973–9975. Clever, G. H.; Tashiro, S.; Shionoya, M. Angew. Chem. Int. Ed. 2009, 48, 7010–7012. Wu, N. M.-W.; Ng, M.; Yam, V. W.-W. Angew. Chem. Int. Ed. 2019, 58, 3027–3031. Xu, J.; Volfova, H.; Mulder, R. J.; Goerigk, L.; Bryant, G.; Riedle, E.; Ritchie, C. J. Am. Chem. Soc. 2018, 140, 10482–10487. Parrot, A.; Izzet, G.; Chamoreau, L.-M.; Proust, A.; Oms, O.; Dolbecq, A.; Hakouk, K.; El Bekkachi, H.; Deniard, P.; Dessapt, R.; Mialane, P. Inorg. Chem. 2013, 52, 11156– 11163. Park, J. S.; Lifschitz, A. M.; Young, R. M.; Mendez-Arroyo, J.; Wasielewski, M. R.; Stern, C. L.; Mirkin, C. A. J. Am. Chem. Soc. 2013, 135, 16988–16996. Walkey, M. C.; Byrne, L. T.; Piggott, M. J.; Low, P. J.; Koutsantonis, G. A. Dalton Trans. 2015, 44, 8812–8815. Park, J.; Jiang, Q.; Feng, D.; Zhou, H.-C. Angew. Chem. Int. Ed. 2016, 55, 7188–7193. Wang, M.-S.; Yang, C.; Wang, G.-E.; Xu, G.; Lv, X.-Y.; Xu, Z.-N.; Lin, R.-G.; Cai, L.-Z.; Guo, G.-C. Angew. Chem. Int. Ed. 2012, 51, 3432–3435. Liu, J.; Lu, Y.; Lu, W. Dalton Trans. 2020, 49, 4044–4049. Kan, W.-Q.; He, Y.-C.; Wen, S.-Z.; Zhao, P.-S. Dalton Trans. 2019, 48, 17770–17779. Kan, W.-Q.; Wen, S.-Z.; He, Y.-C.; Xu, C.-Y. Inorg. Chem. 2017, 56, 14926–14935. Lin, R.-G.; Xu, G.; Wang, M.-S.; Lu, G.; Li, P.-X.; Guo, G.-C. Inorg. Chem. 2013, 52, 1199–1205. Wu, J.; Tao, C.; Li, Y.; Yan, Y.; Li, J.; Yu, J. Chem. Sci. 2014, 5, 4237–4241. Chen, C.; Sun, J.-K.; Zhang, Y.-J.; Yang, X.-D.; Zhang, J. Angew. Chem. Int. Ed. 2017, 56, 14458–14462. Shen, J.-J.; Li, X.-X.; Yu, T.-L.; Wang, F.; Hao, P.-F.; Fu, Y.-L. Inorg. Chem. 2016, 55, 8271–8273. Hao, P.; Guo, C.; Shen, J.; Fu, Y. Dalton Trans. 2019, 48, 16497–16501. Yu, T.-L.; Hao, P.-F.; Shen, J.-J.; Li, H.-H.; Fu, Y.-L. Dalton Trans. 2016, 45, 16505–16510. Mallick, A.; Garai, B.; Addicoat, M. A.; St. Petkov, P.; Heine, T.; Banerjee, R. Chem. Sci. 2015, 6, 1420–1425. Cindric, M.; Strukan, N.; Vrdoljak, V.; Devcic, M.; Veksli, Z.; Kamenar, B. Inorg. Chim. Acta 2000, 304, 260–267.

8.10 Photochemically driven molecular machines based on coordination compounds Alberto Credia,b, Serena Silvic,b, Massimo Baroncinid,b, Leonardo Andreonic,b, and Chiara Taticchia,b, a Dipartimento di Chimica Industriale “Toso Montanari”, Università di Bologna, Bologna, Italy; b CLAN-Center for Light Activated Nanostructures, Bologna, Italy; c Dipartimento di Chimica “G. Ciamician”, Università di Bologna, Bologna, Italy; and d Dipartimento di Scienze e Tecnologie Agroalimentari, Università di Bologna, Bologna, Italy © 2023 Elsevier Ltd. All rights reserved.

8.10.1 8.10.1.1 8.10.1.2 8.10.2 8.10.3 8.10.4 8.10.4.1 8.10.4.2 8.10.4.3 8.10.5 References

Introduction Molecular machines Coordination compounds: the role of inorganic chemistry Coordination compounds as structural elements Coordination compounds as photoactive triggers Coordination compounds as structural and photoactive components Intramolecular photosensitizer Photoactive structural components Phototrigger of supramolecular transformations Conclusion

417 417 419 421 422 428 428 432 434 435 436

Abstract The marriage of molecular machines and coordination compounds has been fruitful since the early stages of the research on molecular machines. First only structural elements, but soon also functional components, metal complexes played a crucial part in the development of sophisticated systems, thanks to their excellent and versatile geometrical, photophysical and electrochemical properties. In this Chapter photoactive molecular machines will be divided in three main classes, depending on the role of the coordination compounds: structural elements, triggers of the photochemical processes, or photoactive structural components.

8.10.1

Introduction

8.10.1.1

Molecular machines

Since the famous talk by Richard Feynman in 1959,1 the design and construction of molecular machines have been an ambition for visionary scientists.2–8 The conviction of the feasibility and utility of molecular machines came from the disclosure of Nature’s sophisticated devices at the basis of the functioning of living systems.9,10 Natural molecular machines are ubiquitous in our body, and they are responsible of most of the operations at the basis of our lives.9 Like their macroscopic counterparts, molecular machines are assemblies of components that individually perform simple functions, like the catalysis of a defined reaction, the absorption of light, a spatial rearrangement of the functional groups. When the molecular components are properly assembled, these simple operations collectively give rise to more complex functions. The peculiarity of molecular machines resides on the non-trivial long-range motions of their components associated with their operations. The most studied and famous example is probably the enzyme responsible for the synthesis of ATP.11–13 This wonderful nanoscale device is one of the most abundant and ubiquitous proteins in the biological world, the synthesis of ATP being the most prevalent chemical reaction. ATP-synthase is made of two rotary motors, an enzyme and an ion pump, which cooperate to synthesize ATP by exploiting a proton gradient across the cell membrane, or, by working in a reverse manner, to create a proton gradient by consuming ATP. As a matter of fact, one of the reasons that raise such an admiration for this molecular machine resides in the use of rotation of its subunit as a catalytic step: indeed, rotation is not a favorite motion in living organisms.13 It is obviously impossible to date to reproduce in a laboratory the functional and structural complexity of natural molecular machines.14 Indeed, all the artificial molecular machines realized so far attains to the following general and accepted definition: “A molecular machine is a particular type of device in which the (molecular) component parts can display changes in their relative positions as a result of some external stimulus”.3 The majority of the artificial molecular machines realized so far are based on interlocked components, the so called Mechanically Interlocked Molecules (MIMs).8 The advantage of such systems is the presence of the mechanical bond,15 which prevents the molecules from falling apart, but still preserves the capability to perform relative long-range motion of the molecular components. The most representative prototypes of such architectures are (pseudo)rotaxanes and catenanes. Briefly, a pseudorotaxane is composed of

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an axle-like molecular component surrounded by at least one macrocycle (Fig. 1): these supramolecular complexes are not mechanically interlocked, as they can dissociate and exist in equilibrium between the associated and dissociated forms. Nevertheless, they are precursors and prototypes for the construction of rotaxanes: these MIMs have a similar structure to pseudorotaxanes, but the dissociation of the components is prevented by the presence of an energetic barrier for the dethreading process (Fig. 1). This barrier can be due to steric or electronic effects exerted by bulky groups at the extremities of the axle. Indeed, the interaction that holds together the components of a rotaxane is the mechanical bond, i.e. “an entanglement in space between two or more component parts, such that they cannot be separated without breaking or distorting chemical bonds between atoms”.15 The mechanical bond is also at the basis of the existence of catenanes, which are made of two (or more) interlocked molecular rings (Fig. 1): the system can dissociate only by breaking one of the rings. Pseudorotaxanes, upon association and dissociation, can perform threading and dethreading motions; rotaxanes can perform linear motions, by shuttling of the ring along the axle molecule, or rotary motion, by pirouetting of the macrocycle around the axle; catenanes can perform rotary motions, by mutual rotation of the two rings one with respect to the other (Fig. 1). Both for synthetic and functional purposes, the molecular components of these systems are equipped with complementary interaction sites. Indeed, the synthesis of these interlocked systems most commonly relies on the initial formation of a threaded structure, which is templated by a non-covalent interaction between the axle and the ring components (like, e.g., charge-transfer, hydrogen bonding, or coordination interactions). By tuning these weak bonds with external stimuli (e.g., redox or acid/base reactions), it is possible to control the interaction between the molecular components and trigger their relative motions. For example, rotaxanes can be equipped on the axle molecule with two different sites (“stations”), which give stronger and weaker interactions with the ring (called primary and secondary stations, respectively). If an external stimulus can modify the primary station, for instance deactivating it, also its interaction with the ring will be affected; therefore, the ring will move (shuttle) toward the secondary station, which is now the preferred one. In principle, these processes should be reversible, to be able to perform a cyclic and repetitive sequence of reactions. Other classes of molecular machines are based on rotation around single16 or double17,18 bonds, the most famous examples being Feringa’s overcrowded alkenes.19 In these systems, a double bond connects two bulky moieties: because of the hindrance exerted by these units, the double bond is forced out of plane and the molecule has an intrinsic helicity. Therefore, these systems can undergo light-triggered trans/cis isomerization around the double bond and thermally activated helical inversion reactions. Indeed, by following a sequence of photochemical and thermal reactions, a unidirectional rotation of one half of the molecule (the rotor) with respect to the other (the stator) is accomplished (Fig. 2). Before dealing with some illustrative examples, it is worth defining the difference between machine and motor, a concept that is of the utmost importance in the field of molecular machinery.2,20 A motor is a particular kind of machine, wherein the molecular components move unidirectionally. In this context pseudorotaxanes become particularly relevant, as they can perform unidirectional threading and dethreading motions, in resemblance to the motion of a pump.21–23 Rotatory motors based on catenanes were realized, wherein one macrocycle moves clockwise or counterclockwise with respect to the other, in response to a specific sequence of stimuli,24 or to the continuous supply of an input.25 Feringa’s overcrowded alkenes were the first19 (and to date most developed) examples of rotary motors: indeed, the four-step sequence of reactions (Fig. 2) corresponds to a 360
rotation of the rotor upper half with respect to the stator. Without going into much detail, it is nevertheless worth mentioning that the control on the direction is accomplished by a fine tuning of the kinetic barriers, together with the relative stabilities at each step of the operation. Indeed, movements at the molecular level are biased by ratcheting Brownian thermal molecular motions.26,27

(A)

primary sta on

(B) +X

secondary sta on

-X

deac vated primary sta on (C)

(D) +X -X

Fig. 1 Schematic representation of (A) threading/dethreading motion in a pseudorotaxane; (B) linear motion (shuttling) of the ring component between the two interaction sites (stations) of a [2]rotaxane; (C) pirouetting of the macrocycle around the molecular axle of a [2]rotaxane; (D) ring rotation in a [2]catenane. Deactivation of the primary station by an external stimulus (X), causes the shuttling of the ring in the [2]rotaxane (B) or the circumrotation in the [2]catenane (D).

Photochemically driven molecular machines based on coordination compounds

419

photoisomeriza on hν

Δ

helix inversion

Δ

helix inversion

photoisomeriza on hν Fig. 2 Schematic representation of the operation mechanism of Feringa’s rotary motors based on overcrowded alkenes: the rotor part of the molecule (upper half) rotates unidirectionally with respect to the stator part (lower half). The operation mechanism relies on a sequence of photochemical trans to cis and cis to trans isomerization reactions (horizontal processes), followed by thermal helix inversion reactions (vertical processes), governed by the hindrance exerted by the bulky groups.

The added value of a molecular motor with respect to a simple molecular machine is that the former, in principle, can perform useful work: indeed, the majority of the natural molecular machines are motors. We will not distinguish between machines and motors in the following discussion, but we will point out the (few) examples of molecular motors.

8.10.1.2

Coordination compounds: the role of inorganic chemistry

Among the plethora of examples of molecular machines realized to date, the ones based on coordination compounds deserve a special mention,28–35 as the first example36 ever of an interlocked structure in the shape of a catenane was designed and realized by exploiting the peculiar properties of coordination compounds. First of all, metal-ligand complexes are characterized by a defined geometry: ligands are arranged around the metal center in a precise geometry, which can be predicted on the basis of the nature and oxidation state of the metal, and on the type of ligand(s). This feature of coordination compounds was at the basis of the design and construction of the first [2]catenane, reported by Sauvage and Dietrich-Buchecker.36 The successful intuition of these two researchers was the exploitation of the tetrahedral [Cu(phen)2]þ complex (Fig. 3) (phen ¼ 1,10-phenanthroline), where the two phenanthroline ligands are inherently “criss-crossed.” The metal-ligand complex was the point of intersection of the two macrocycles of the catenane formed by intra-ligand cyclisation, and it was used to template the synthesis of the interlocked architecture. With the same approach a rotaxane structure was synthesized,37 and from then on, other metal-ligand complexes, with different coordination geometries, were used as templating agents.33 Indeed, besides their use in the design and operation of molecular machines, coordination compounds are extremely powerful tools for the realization of challenging structures, with complicated geometries and topologies.33 The high degree of predictivity and geometrical precision of the coordination chemistry enables to selectively obtain defined architectures also from complex mixtures. This predictability translates into the possibility to exploit the self-recognition and organization of the molecular components, to get the desired structures. A well-known but still amazing example are Lehn’s helicates,38,39 in which a mixture of different ligands and metal ions resulted in self-assembly with selfrecognition.40 By mixing Cu(I) ions with [oligo(2,20 )-bipyridine] strands of different length, only discrete helicates with matching ligands were formed, and no mixed species were detected.39 Moreover, different coordination modes of Cu(I) and Ni(II) ions were exploited to drive the formation of double and triple helicates from a mixture of oligobipyridine strands: indeed, tetrahedral and

Fig. 3 Synthesis of the first catenane as reported in the original paper36: the two phenanthroline ligands around the Cu(I) ion template the synthesis of the interlocked structure.

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octahedral requirements of Cu(I) and Ni(II) ions are fulfilled by 6,60 - and 5,50 -connected tris-bipyridine strands, respectively.40 Hence, the mixture of 11 “instructed” pieces of four different types leads to a clean self-assembly into two supramolecular species. More recently, the synthesis of molecular Borromean rings from the self-assembly of 18 components41 and of a [2]catenane that consists of two triply entwined rings (a Star of David catenane)42 were reported: here too, the crossing points of the final structure are characterized by the presence of coordination bonds. These examples, as many others in the literature,33 suggest that coordination chemistry is a robust and effective instrument for the design and construction of complex architectures. Another fundamental characteristic of coordination compounds that makes them relevant for the design of molecular machines is the possibility to control the nature, number and geometrical arrangement of ligands around the metal center. As already mentioned, these features are obviously very important when metal complexes are used as templating units, as they can be exploited to finely position the components of the interlocked structure. But the geometrical arrangement can be tuned also after the formation of the desired structure, and it can be used to change the relative arrangement of the molecular components, i.e., to cause mechanical motion. Therefore, coordination compounds can be included in mechanically interlocked structures as “switchable stations,” i.e. interaction sites, whose binding ability can be modulated by means of external inputs. In the present context, two significant features of coordination compounds appear relevant for the design and operation of molecular machines. First of all, transition metal ions often show good redox properties: they exhibit several oxidation states, which can be easily populated by reversible oxidation or reduction reactions. The oxidation state of the metal ion affects the coordination geometry of the complex: for example Cu(I) ions prefer a tetrahedral coordination, whereas oxidation to Cu(II) favors higher coordination numbers. As mentioned above, the rearrangement of the ligands can thus be exploited to induce mechanical motion of the components of a molecular machine. A second important quality of most coordination compounds is their rich photochemistry and photophysics: in many instances they are colored and/or have long-lived emissive excited states. It is very common to report as an example the properties of [Ru(bpy)3]2 þ (bpy ¼ 2,20 -bipyridine) (Fig. 4), a unique compound that gathers all the main features of coordination complexes.43–45 In the ground state, [Ru(bpy)3]2 þ can be reversibly oxidized (on the metal center) and reduced (on the bipyridine ligands). The complex is colored, and it absorbs both in the visible and in the UV. Upon absorption of light, the triplet metal-toligand charge transfer state (3MLCT) is populated via a 100% efficient intersystem crossing from the singlet 1MLCT by excitation in the visible. The 3MLCT is emissive and long-lived. Moreover, the excited state of [Ru(bpy)3]2 þ is both a stronger oxidant and a stronger reductant than its ground state. This feature deserves a particular note, as it implies that redox reactions on coordination compounds can be triggered also by means of photoinduced electron transfer processes, that is, by using light. Eventually, the ligands coordinated to the metal center can be addressed not only by redox reactions on the metal, but also by exploiting photodissociation processes.36,46 The metal-centered (MC) state of coordination compounds may have a dissociative nature, therefore when it is populated, release of the ligands follows. Most ruthenium polypyridyl complexes possess a 1MLCT state, which is populated by absorption in the visible region, and from which an efficient intersystem crossing to the 3MLCT occurs (Fig. 5). This excited state can deactivate thermally or via emission of a photon. Nevertheless, when sterically hindering ligands are used and the geometry of the coordination compound is distorted, the MC state can be sufficiently lowered in energy to be thermally populated by deactivation from the 3MLCT excited state (Fig. 5).47

Absorption λ max = 450 nm εmax = 15000 M-1 cm-1

E0-0 = 2.1 eV τ ≈ 800 ns

-0.77 V

+0.81 V Luminescence λ max = 610 nm Φlum = 0.045

+1.33 V

-1.29 V

Fig. 4 Summary of the main electrochemical and photophysical properties of [Ru(bpy)3]2 þ: in the ground state [Ru(bpy)3]2 þ can be reversibly oxidized and reduced; the complex is colored and it can be excited with visible light; the excited state of [Ru(bpy)3]2 þ is luminescent and long lived, and it is both a better oxidant and reductant than the ground state.

Photochemically driven molecular machines based on coordination compounds

(A)

421

(B)

Fig. 5 Schematic representation of the potential energy surfaces for a ruthenium polypyridyl complex: absorption of a photon in the visible region is followed by efficient energy transfer to the 3MLCT via intersystem crossing: if the energy gap between the 3MLCT and the 3MC states is large, the excited state may deactivate via emission of a photon (A); if the 3MLCT-3MC energy gap is small, thermal population of the dissociative 3MC state may occur (B).

Depending on the role of the coordination compounds in the molecular machine (structural and/or functional), we will classify the systems in three categories. 1 Coordination compounds as structural elements: the metal ligand complex in these systems plays only a structural role, but it is not involved in the photoactivated process that actuates the machine. 2 Coordination compounds as photoactive triggers: this category comprises all those molecular machines that are intermolecularly photosensitized by a coordination compound, which is not part of the molecular machine. 3 Coordination compounds as structural and photoactive components: the metal complex is the photoactive component integrated in the molecular machine and responsible for its operation.

8.10.2

Coordination compounds as structural elements

A few examples of light driven molecular machines comprising coordination compounds as structural elements have been reported. These prototypes are minimally composed of a photoswitchable unit and a metal complex, with a structural or functional role. In these systems, therefore, the metal complex is not photoactive, but it can be involved in the operation of the machine anyhow. Recently a light-driven molecular assembler was reported,48 based on an azobenzene photoswitch functionalized at the two extremities with two Zn-cyclene moieties, which are ligands for vanadates (Fig. 6). The design of this system couples the photochemical reaction with the condensation reaction of monovanadate to cyclic tetravanadate. The azobenzene central unit is isomerized from trans to cis with UV light and from cis to trans with visible light, moving the two Zn-cyclene moieties closer or further, respectively. When the azobenzene is in the cis configuration the receptor cannot bind any vanadate species, as the two sites are too close to each other; but when the azobenzene is photoisomerized to its trans configuration two molecules wrap around a tetravanadate, which is then coordinated by four Zn-cyclen moieties. The operation cycle is summarized as follows (Fig. 6): two trans molecules bind to the monovanadate present in solution, promoting its condensation to the cyclic tetravanadate; upon trans to cis photoisomerization the tetravanadate is released in solution, where it is unstable and it hydrolyses back to monovanadate; back cis to trans isomerization, triggered by irradiation with visible light, restarts the cycle. Overall, a photoactivated molecular machine-type ligand mediates an endoergonic reaction, thus converting light energy into chemical energy. Schmittel and coworkers developed a supramolecular rotor,49 whose speed can be controlled by light.50 The nanorotor is based on a stator molecule, comprising a Zn-porphyrin core and four phenanthroline ligands, and a rotator unit, made of a Zn-porphyrin functionalized with a pyridine and a pyrimidine unit. The rotator and the stator are held together by a 1,4-diazabicyclo[2.2.2]octane (DABCO) axle, which interacts with the two Zn-porphyrin cores, and two Cu(I) ions, that complex one phenanthroline ligand on the stator and one pyridine or pyrimidine ligand on the rotator, in anti-positions (Fig. 7). Two more Cu(I) ions are positioned on the free phenanthroline moieties of the stator, and form a binding site for a fifth component, a 2,20 -diazastilbene photochromic compound which can play the role of a photoactive brake. When in the trans configuration, the stilbene moieties do not interact with the supramolecular rotator, but when photoisomerized to the cis configuration they form a complex with the Cu(I) bound to the free phenanthroline ligands of the stator. Overall, when all the components are present in solution, the rotator oscillates

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I

III

II Fig. 6 Operation mechanism of a molecular assembler based on an azobenzene photoswitch functionalized with two Zn-cyclene moieties: the trans isomer promotes the condensation of four monovanadate to the tetravanadate, forming a 2:1 complex (I➔II); upon trans to cis photoisomerizaton, the Zn-cyclene moieties are moved closer, thus releasing the tetravanadate, which, on its turn, hydrolyses back to monovanadate (II➔III); irradiation with visible light restores the trans isomer and restarts the cycle (III➔I). Adapted with permission from Sell, H.; Gehl, A.; Plaul, D. et al Towards a Light Driven Molecular Assembler. Commun. Chem. 2019, 2, 62. Copyright © The Author(s) 2019. Published by Springer Nature.

between the two pairs of Cu(I)-phenanthroline stations on the stator, with a speed of rotation of 86 kHz (Fig. 7). Upon irradiation with UV light, trans to cis isomerization of the molecular brake occurs, followed by complexation, and the speed of rotation slows down to 38 kHz, on account of the hindrance exerted by the coordination complex between the brake and the stator (Fig. 7). Back isomerization is accomplished by heating the system, which then recovers its speed of rotation. Haberhauer reported two examples of rotary motors,51,52 based on a chiral peptidic molecular clamp motif,53,54 which imparts directionality to the rotary motions. The first prototype is based on the following design: the chiral imidazole-containing peptide macrocyclic scaffold is connected to an azobenzene photoswitch, which, on its turn, is functionalized with two arms containing 2,20 -bipyridine units.52 The operation mechanism of the system is described by a four-state cycle, in which the two arms rotate and fold (Fig. 8). The rotary motion is photoactivated, as it is related to the trans to cis isomerization of the azobenzene switch; after rotation, the folding of the arms is controlled by the formation of copper-bipyridine complexes. Overall, the authors highlight the resemblance of the light and chemically driven motion of the molecular machine with the arm movements of a human breaststroke swimmer. The second example reported by Haberhauer is based on similar molecular components, but the structure and movement of the machine are different. The same chiral clamp is the scaffold for the construction of the final structure, which contains two bipyridine moieties and one azobenzene (Fig. 8). One bipyridine unit controls the direction of movement, the other one, connected to the azobenzene switch, is the “pushing blade.” A four-state cycle describes the operation of the machine: the formation of a Zn-bipyridine complex drives the rotation of the blade, whereas the photoisomerization of the azobenzene controls the folding. Overall, the alternate supply of the chemical and photochemical stimuli drives the molecule through the four-states cycle, after which a 360
rotation of the phenyl group of the azobenzene unit around a virtual axis is accomplished. It must be recalled here that these two molecular machines perform unidirectional movements, thanks to the control exerted by the chiral clamp, and are therefore classified as molecular motors. Moreover, it is worth noting that the cases reported in this section are among the few examples of molecular machines comprising a coordination compound, which are not based on interlocked architectures.

8.10.3

Coordination compounds as photoactive triggers

As already mentioned, coordination compounds exhibit outstanding photophysical properties, including long-lived excited states. Such a feature enables the use of metal complexes as “external triggers,” as they can initiate photoinduced intermolecular electron or

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Fig. 7 A supramolecular rotor. (A) Structural formulas and schematic representation of the molecular components: the stator, comprising a Znporphyrin core and four phenanthroline ligands (top), the rotator, a Zn-porphyrin functionalized with a pyridine and a pyrimidine unit (bottom left) and the brake, a 2,20 -diazastilbene (bottom right). (B) Spinning motion of the nanorotor: one DABCO molecule interacts with the two Zn-porphyrin cores and four Cu(I) ions are bound to the four phenanthroline ligands on the stator and to a pyridine or pyrimidine ligand on the rotator. (C) Operation mechanism of the photoactivated brake: the trans configuration of the stilbene derivative does not interact with the nanorotor, whereas the cis isomer binds to the [Cu(phen)]þ complexes, thus slowing down the rotation of the supramolecular rotor.

energy transfer processes. This strategy is extremely convenient and successful, as, in principle, it would enable to operate any molecular machine driven by electrochemical stimuli,55 provided that the redox potentials and rates of mechanical movements of the machine are compatible with the energy and lifetime of the excited state of the coordination compound. Indeed, charge transfer interactions are commonly employed to template the synthesis and/or stabilize the interaction of the molecular components in mechanically interlocked molecules. In these systems, the reciprocal motion of the components can be triggered by destabilizing the charge transfer interaction via oxidation or reduction of the electron donor or acceptor unit, respectively (Fig. 9). On their turn, these reactions can be initiated either electrochemically or photochemically, by means of photoinduced electron transfer processes. The working principle for the intermolecular photosensitization of mechanical motions was first described by Balzani and coworkers in 199356 (Fig. 10): in this seminal paper the authors demonstrate the light-controlled dethreading of a pseudorotaxane stabilized by charge transfer interactions, via photoinduced electron transfer from a photosensitizer.

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Fig. 8 (A) Left: molecular structure of the molecular “swimmer” and motion principles of the bipyridine arms folding. Right: schematized sequence of motions: irradiation with UV light isomerizes azobenzene from trans to cis (I➔II, rotation of stretched arms), addition of Cu(II) ions causes the folding of the arms (II➔III), irradiation with visible light isomerizes cis-azobenzene to the trans configuration (III➔IV, rotation of stretched arms), addition of cyclam unfolds the arms (IV➔I). (B) Molecular structure (left) and schematic representation (right) of the four-stroke molecular motor: irradiation with UV light causes trans to cis isomerization of the azobenzene (I➔II, folding), Zn(II) ions form a complex with the bipyridine unit (II➔III, rotation of the “closed blade”), irradiation with visible light isomerizes the azobenzene unit from cis to trans (III➔IV, opening of the “blade”), addition of cyclam removes the Zn(II) ions (IV➔I, rotation of the “open blade”).

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+ e-

- e-

electron donor

electron acceptor

Fig. 9 Schematic representation of a pseudorotaxane stabilized by charge transfer interactions: upon reduction of the electron accepting unit, the supramolecular complex is destabilized and becomes dissociated.

(A)

(B)

e-

e*P

P+

S

*P hν

P

Prod

P+

R

hν P

R+ e-

Fig. 10 Schematic representation of the working mechanism for the operation of a molecular machine based on a pseudorotaxane by intermolecular photoinduced electron transfer processes: the excited state of the photosensitizer *P transfers an electron to the electron-accepting unit of the pseudorotaxane, forming the oxidized Pþ and the reduced pseudorotaxane, which then disassembles. To prevent back-electron transfer from the reduced pseudorotaxane to Pþ, two strategies are proposed: (A) a sacrificial reductant S reduces Pþ to P, generating waste products; (B) an electron relay R reduces Pþ to P, generating Rþ, which, on its turn, oxidizes the reduced axle, generating R and the pseudorotaxane.

The excited state of the photosensitizer *P can reduce (or oxidize) the electron acceptor (or donor) unit of the molecular machine, thus causing its destabilization. The mechanical motion competes with the back electron transfer between the reduced (or oxidized) molecular machine and the oxidized (or reduced) photosensitizer. As the electron transfer processes are typically much faster with respect to mechanical motions, two strategies can be employed to scavenge the oxidized (or reduced) photosensitizer (Fig. 10). In a first instance, a sacrificial reductant (or oxidant) S is added to the solution, which scavenges the oxidized (or reduced) photosensitizer Pþ (or P) by an irreversible redox reaction, which restores P and transforms the scavenger into waste products (Prod). Another strategy is based on the reaction of an electron relay R with the oxidized (or reduced) photosensitizer by a reversible redox reaction, which restores the reduced (or oxidized) form of the photosensitizer P and transforms the electron relay into its oxidized (or reduced) form Rþ (or R) that, on its turn, reacts with the reduced (or oxidized) molecular machine, thus generating the initial species and closing the cycle. In the first attempts,56,57 the external photosensitizers were either ([Ru(bpy)3]2 þ or 9-anthracenecarboxylic acid, and the molecular machine was based on the cyclobis(paraquat-p-phenylene) (CBPQT) electronacceptor macrocycle and a 1,5-dioxynaphthalene (DNP) electron-donor group in the axle component. It is worth mentioning that the word “molecular machine” was reported in the title of a paper for the first time in 1993,56 in a paper that is considered one of the milestones of the research on molecular machines. Moreover, in this pioneering work the authors already mention the possibility to integrate the photosensitizer in the components of the molecular machine and they put on paper the designing principle of a linear molecular motor (vide infra). More recently, this strategy was used to photoactivate a molecular pump based on a pseudorotaxane architecture.22 A molecular pump is a linear motor whose components are directionally translocated one with respect to the other. In a pseudorotaxane-based molecular machine the linear directional motion is achieved by unidirectional threading and dethreading of the axle and ring components. The photoactive pump reported by Stoddart and coworkers is based on a CBPQT electron-acceptor macrocycle and a non-symmetric axle containing a DNP electron-donor site. The pseudorotaxane is stabilized by charge transfer interactions and dissociation of the components is achieved by reduction of the bipyridinium units on the CBPQT ring. The axle component is

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Photochemically driven molecular machines based on coordination compounds

functionalized at the extremities with two different units: a neutral and bulky 2-isopropylphenyl (IPP) group and a positively charged 3,5-dimethylpyridinium (PY) unit. The directionality of the threading and dethreading motions is imparted by the different kinetic barriers experienced by the CBPQT macrocycle in its tetracationic or reduced form. PY exerts a strong Coulombic repulsion on the CBPQT ring, which slides preferentially over the IPP side upon threading. On reduction of the ring, though, the two kinetic barriers invert, as the Coulombic repulsion between the reduced CBPQT and PY decreases, and IPP is bulkier than PY. Therefore, the reduced macrocycle slides over the PY side upon dethreading. It was proposed that the complete redox cycle of reactions can be accomplished by using light, in the presence of [Ru(bpy)3]2 þ as photosensitizer and of phenothiazine (ptz) as electron relay (Fig. 11): upon irradiation with visible light, an electron is transferred from the 3MLCT state of [Ru(bpy)3]2 þ to the CBPQT station. While the reduced macrocycle dethreads (sliding over the PY unit), the oxidized photosensitizer [Ru(bpy)3]3 þ is reduced by ptz to [Ru(bpy)3]2 þ again. On its turn, the oxidized ptz reacts with the free reduced ring, restoring the neutral ptz and the tetracationic CBPQT, which now threads on the axle through the IPP extremity. By using an electron relay, this cycle of reactions can be repeated in principle as long as the energy source (i.e., light) is kept constant: indeed, systems of this kind could operate autonomously. It must be noted that the designing principle at the basis of the operation of the machine is the same as the one reported in 1993 by Balzani and coworkers,56 thus confirming the robustness and versatility of this approach: the same strategy can be applied to any system that is governed by redox processes. Recently, the interaction between radical pairs of bipyridinium radical cations was exploited as a template for the synthesis58 and as switchable station59 of mechanically interlocked molecules in aqueous environment. Indeed, the propensity to dimerization of bipyridinium radical cations is harnessed to design and synthesize interlocked architectures stabilized by the strong interaction between a diradical dicationic CBPQT ring and a monoreduced bipyridinium cation. The synthesis of a highly energetic rotaxane containing one bipyridinium ion in the axle and two bipyridinium ions in the ring (CBPQT) was prepared by a threading followed by stoppering reaction using [Ru(bpy)3]2 þ as a photocatalyst: indeed, upon irradiation of a mixture of the two molecular

Fig. 11 (A) Molecular structures and (B) operation mechanism of a light-driven molecular pump: after irradiation with visible light (1) one electron is transferred from the excited photosensitizer to the CBPQT ring (2); the oxidized Ru complex is reduced by ptz (3), while the pseudorotaxane dethreads unidirectionally (4); the reduced macrocycle is oxidized by ptzþ (5) and the pseudorotaxane assembles via unidirectional threading (6). Adapted with permission from Li, H.; Cheng, C.; McGonigal, P. R. et al. Relative Unidirectional Translation in an Artificial Molecular Assembly Fueled by Light. J. Am. Chem. Soc. 2013, 135(49), 18609–18620. Copyright © 2013 American Chemical Society.

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components and of the photosensitizer, in the presence of a sacrificial electron donor, all the bipyridinium units are monoreduced. The interaction between the radical cations drives the formation of a pseudorotaxane complex, which is then converted into a rotaxane by functionalization of the extremities of the axle. After reoxidation of the radical ions, a “unlikely [2]rotaxane” is achieved, wherein three bipyridinium units are held together by the mechanical bond. Recently, an original and clever strategy to enhance the efficiency of intermolecular electron transfer processes has been reported.60 One of the parameters that influence the feasibility of photoinduced processes is the lifetime of the excited state of the photosensitizer, which affects the probability of the electron transfer. One of the expedients available to photochemists to influence the excited state lifetime is reversible electronic energy transfer (Fig. 12).61–63 When the excited states of two chomophores lie close in energy, excitation of one molecule can be followed by energy transfer to the second chromophore, and then back to the first one. Typically, transition metal complexes are coupled with organic molecules, having their lowest triplet excited state close in energy to the triplet metal-to-ligand charge transfer state of the metal-based compounds. By exploiting the possibility to populate the triplet state of the metal complex by efficient energy transfer from the lower energy singlet excited state, the long-lived triplet state of the organic chromophore can be then populated by energy transfer. If proper energetic and kinetic parameters are satisfied, the energy can be reversibly transferred back to the metal complex: overall, the organic chromophore acts as a “storage element,”63 leading to a longer lifetime of the metal complex excited state. Bichromophoric systems can be designed for this purpose, wherein one or more “storing” units are appended to a metal complex, for instance via proper functionalization of the surrounding ligands.62,63 By combining a ruthenium complex functionalized with one or more pyrene units with a pseudorotaxane prototype stabilized by charge-transfer interactions (Fig. 13),60 McClenaghan and coworkers developed a photoactivated molecular piston. Once more, the molecular components of the supramolecular complex are a CBPQT macrocycle and a DNP based axle, which can be disassembled by reduction of the bipyridinium units of the CBPQT ring. Irradiation of the photosensitizer on the 3MLCT state of the ruthenium complex is followed by electronic energy transfer to the triplet excited state of the pyrene unit(s), which then redirects the energy to the 3MLCT state. The excited state lifetime of the bichromophoric photosensitizers is longer with respect to the parent ruthenium model complex lacking the pyrene unit, and it increases on increasing the number of pyrene chromophores. As a consequence, the yield of the molecular machine dethreading also increases depending on the number of pyrene units: the yield in presence of the three-pyrenes photosensitizer is circa doubled and tripled with respect to the yield in presence of the monofunctionalized and model non-functionalized ruthenium complexes, respectively. This smart strategy gives new life to and expands the limits of one of the most common methodologies used to operate molecular machines with light. Indeed, it confirms the strength and versatility of coordination compounds. Intermolecular energy transfer processes were exploited to trigger photoisomerization reactions with longer wavelength, by using a triplet sensitizer. Feringa’s rotary motors19 are based on overcrowded alkenes, which, upon constant irradiation with directly absorbed UV-light, perform a unidirectional rotation around the double bond, following a sequence of photoisomerization and thermal helix inversion reactions (Fig. 2). A prototype of such molecular rotary motors was coupled with a Pd-porphyrin complex (Fig. 14), which has an absorption band in the visible region, where no absorption from the motor molecules is present.64 The photoisomerization reaction of the motor molecules is accomplished via the triplet state by sensitization from the Pd-complex. Upon irradiation with visible light, energy transfer takes place from the Pd-complex to the alkene, and unidirectional rotation was demonstrated.

1(π-π*)

1MLCT

2 3 3MLCT 3(π-π*)

1

ΔE 20% in molar ratio) can promote NIR harvesting. Although Yb3þ-sensitized upconversion provides handy design and high efficiency, it only responds to single-wavelength excitation at 980 nm. To overcome this deficiency, Nd3þ can be codoped as a sensitizer with Yb3þ for cascade energy transfer. Unlike Yb3þ, Nd3þ features multiple absorption bands in the optical window (730 nm, 800 nm, 860 nm), largely mitigating solution overheating, typically associated with continuous 980-nm excitation.59–61 As shown in Fig. 3B, Nd3þ-sensitized excitation energy can be transferred to Yb3þ and activators in succession. Notably, the absorption cross-section of Nd3þ is one order-of-magnitude larger than that of Yb3þ. Energy transfer efficiency from Nd3þ to Yb3þ is exceptionally high (> 50%); therefore, one can expect much

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Lanthanide-doped upconversion nanomaterials

Fig. 3 Three major schemes of NIR-harvesting using lanthanide-doped upconversion nanocrystals. (A) Yb3þ sensitization, (B) Nd3þ/Yb3þ cosensitization, (C) organic dye-mediated upconversion sensitization. Printed with permission from Ref. Liang, L.; Qin, X.; Zheng, K.; Liu, X. Acc. Chem. Res. 2019, 52, 228–236. Copyright © 2018 American Chemical Society.

brighter upconversion nanocrystals co-sensitized with Nd3þ and Yb3þ than those sensitized with solely Yb3þ. However, direct contact between Nd3þ sensitizers and activators may severely quench luminescence. In this regard, core-shell structures are usually designed to ensure spatial isolation for bright emission.62 The absorption cross-section of organic molecules is around 1  10 16 cm2, over three orders of magnitude larger than those of lanthanides. To boost light-harvesting of lanthanide-doped nanoparticles, organic dyes have been introduced.63,64 Organic dyes can be tethered to nanoparticle surfaces for enhanced NIR absorption (Fig. 3C). Nd3þ and Yb3þ sensitizers are activated by energy transfer from dye molecules to Nd3þ and Yb3þ. Subsequent energy transfer from Yb3þ to emitters gives rise to luminescence. Although dye sensitization can enhance upconversion luminescence, especially under low-power excitation, this design only works well in low-oxygen environments where limited triplet states are quenched.65,66 Moreover, dye sensitizers generally have low photostability, making their long-term application impractical.

8.11.3.2

Optimization of energy transfer pathways

Energy cross-talk between lanthanide activators is usually the major factor leading to severe luminescence quenching (Fig. 4A).67,68 Excited-state depopulation of upconversion nanocrystals with low activator doping through cross-relaxation is insignificant. However, with an increase in activator concentration, upconversion luminescence may increase initially but then declines rapidly. Although more activators facilitate energy transfer from sensitizers, energy cross-talk between activators occurs, depopulating excited states and ultimately luminescence intensity. Furthermore, energy back transfer from activators to sensitizers depopulates the excited states. Quite unlike Yb3þ, the energy levels of Nd3þ are highly complex. Significant energy cross-talk between common activators (Er3þ, 3þ Ho , and Tm3þ) and Nd3þ sensitizer can occur.60 For Nd3þ-sensitized upconversion nanocrystals, core-shell structures can be employed to tune luminescence (Fig. 4B). Confinement of Nd3þ and activators in different spatial layers substantially mitigates energy cross-talk. It should be noted that Yb3þ sensitizers are doped in both core and shell layers as linkers to bridge energy transfer from Nd3þ to activators. An additional layer of doped Yb3þ is usually inserted between layers of Nd3þ and activators to eliminate cross-relaxation through the core-shell interface. An interesting observation for activators is that the optimal doping concentration can be substantially increased. For example, for the commonly used NaYF4:Yb/Er host, the optimum doping concentration of Er3þ is around 2%. However, this value can be increased to 20% under 106 W cm2 excitation.69 Under low-power irradiance, a high doping concentration leads to severe crossrelaxation between activators, and high-lying excited energy levels are likely to undergo significant depopulation (Fig. 4C). However, when irradiance is markedly increased, intermediate excited states are immediately populated to higher excited states through ESA, albeit energy cross-relaxation continues to occur. In other words, a high photon flux could combat cross-relaxation and alleviate the overall depopulation of excited states, making high-concentration doping possible. For applications involving high excitation irradiance, such as super-resolution imaging, highly doped upconversion nanocrystals are usually employed. For NaGdF4 nanocrystals codoped with Yb/Er ions, the luminescence intensity of 2% Er-activated samples is higher than those doped with 20% Er3þ when the excitation power is below 3  105 W cm2 (Fig. 4D). However, with increased power density, the luminescence intensity of 2% Er-activated samples saturates while their counterparts show drastically increased intensities. Microscopic imaging of single nanoparticles indicates that the emitted photons from low doping samples are suppressed by highly doped ones at high excitation irradiance (Fig. 4E). Similar phenomena have also been verified using Tm3þ- or Nd3þ-activated upconversion nanocrystals (Fig. 4F and G).70,71 As described above, energy mitigation represents an effective way of ensuring bright upconversion luminescence. Notably, this method enhances all emission bands without selectivity. However, multiple emission bands may interfere with luminescence detection and post-analysis. Introducing energy reservoir centers can improve the luminescence intensity of a specified emission band. Upon NIR excitation, activators in high-lying excited states depopulate by transferring their energy to adjacent reservoir centers.

Fig. 4 (A) Energy cross-talk between activators and sensitizers. EBT: energy back transfer; CR: cross-relaxation. (B) Design of core-shell upconversion nanocrystals with minimized energy cross-talk. (C) Dependence of excited-state population in upconversion nanocrystals on dopant concentration and power density. (D) Luminescence intensity of single 8-nm upconversion nanocrystals with 20% (blue circles) and 2% (red circles) Er3þ, each codoped with 20% Yb3þ. (E) Confocal luminescence images of single UCNPs containing a mixture of 2% and 20% Er3þ. (F) Emission spectra of NaYF4:Yb/Tm (20/0.2–8 mol%) UCNPs as a function of Tm3þ doping concentration upon 980 nm excitation ( 2.5  106 W cm2) through a microstructure optical fiber (top panel). (G) Schematic of ICG-sensitized upconversion in NaYF4:Nd nanoparticles (top panel) and emission intensity of NaYF4:Nd (1–50 mol%) nanocrystals versus Nd3þ doping concentration with and without Indocyanine green (ICG) sensitization (bottom panel). (A, B) Printed with permission from Ref. Liang, L.; Qin, X.; Zheng, K.; Liu, X Acc. Chem. Res. 2019, 52, 228–236. Copyright © 2018 American Chemical Society. (D, E) Printed with permission from Ref. Gargas, D. J. et al. Nat. Nanotechnol. 2014, 9, 300–305. Copyright © 2014, Nature Publishing Group. (F) Printed with permission from Ref. Zhao, J. et al. Nat. Nanotechnol. 2013, 8, 729–734. Copyright © 2013, Nature Publishing Group. (G) Printed with permission from Ref. Wei, W. et al. J. Am. Chem. Soc. 2016, 138, 15130–15133. Copyright © 2016 American Chemical Society.

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Fig. 5 (A) Working mechanism of upconversion nanocrystals with Mn2þ ions as reservoir centers and single-band emission spectra of KMnF3:Yb/ Er (top panel) and KMnF3:Yb/Tm (bottom panel) nanocrystals. (B) Illustration of topological energy clustering within crystal sublattice and population processes of upconversion nanocrystals with Yb3þ energy clusters (top panel). Emission spectra of KYb2F7:Er/Lu (2/0–80 mol%) nanocrystals upon 980 nm excitation (bottom panel). (A) Printed with permission from Ref. Wang, J.; Wang, F.; Wang, C.; Liu, Z.; Liu, X. Angew. Chem. Int. Ed. 2011, 123, 10553–10556. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. (B) Printed with permission from Ref. Wang, J. et al. Nat. Mater. 2014, 13, 157–162. Copyright © 2013, Nature Publishing Group.

Subsequently, energy back transfer from reservoir centers to activators populates low-lying excited states, enhancing photon emission at long wavelengths. Energy reservoir centers can redistribute activator populations by promoting low-lying excited states. Mn2þ is frequently used as the energy reservoir in lanthanide-doped upconversion nanocrystals to achieve quasi-single-band luminescence.72,73 Upon Mn2þ codoping, the green emission band of Yb3þ/Er3þ-codoped KMnF3 nanocrystals disappears, leaving only a red emission band (Fig. 5A). Similarly, for Yb3þ/Tm3þ-codoped samples, only 800-nm NIR emission is detectable. After population with Er3þ/Tm3þ ions in high-lying excited states, Mn2þ reservoir centers transfer energy back to activators for population redistribution in low-lying excited states. In addition to Mn2þ, Tm3þ has been used as the energy reservoir to enhance upconversion emission at long wavelengths.74 Quasi-single-band red emission can be further enhanced by codoping Tm3þ with Er3þ in NaErF4 nanocrystals. As an energy reservoir, Mn2þ performs much better than Tm3þ, but the latter is much easier to integrate into NaLnF4 lattices. Yb3þ can also work as an energy reservoir center in special lattice environments.75 In KYb2F7:Er3þ nanocrystals, Yb3þ and Er3þ ions distribute in arrays of tetrad clusters (Fig. 5B). Yb-Yb distances between inter clusters are much larger than those between intraclusters. Excitation energy can be trapped in every cluster, increasing Yb3þ-Er3þ interaction. The much enhanced short-wavelength emission provides direct evidence of excitation energy confinement. However, the existing energy reservoir strategy is limited to a small set of lanthanides. For efficient energy transfer, the spectral overlap between adjacent donors/acceptors must be considered. Unlike common upconversion activators (Er3þ, Ho3þ, and Tm3þ), other lanthanides cannot be directly sensitized using Yb3þ. As in Tb3þ and Eu3þ activators without long-lived intermediate energy levels, energy transfer from Yb3þ to Tb3þ and Eu3þ is inefficient because of large energy gaps. Upconversion from these activators is possible through cooperative sensitization, but it requires high input power (Fig. 6A). This problem has been addressed by introducing energy migration in core-shell nanocrystals with significantly augmented emission intensity.53,54 To achieve energy migration upconversion, sensitizers and activators must be spatially separated in the core and shell, respectively (Fig. 6B). In the core, Yb3þ/Tm3þ are codoped to populate Tm3þ activators to high-lying excited states upon NIR excitation. The critical factor triggering energy migration through the core-shell interface is selecting gadolinium (Gd) ions as the host lattice for the core and shell. After a five-photon pumping process, the energy of Tm3þ activators can be efficiently extracted by the Gd sublattice. Subsequently, the energy diffuses and migrates from the core to the shell layer and eventually to activators (e.g., Tb3þ, Eu3þ, Dy3þ, and Sm3þ) doped in the shell(Fig. 6C). As such, activators without intermediate states can be efficiently sensitized by Yb3þ. Energy migration-mediated upconversion luminescence of Tb3þ and Eu3þ is much brighter than that of Dy3þ and Sm3þ. For this reason, Tb3þ and Eu3þ activators have been more extensively studied than Dy3þ and Sm3þ activators.

8.11.3.3

Blocking energy leakage

Surface quenching occurs when excited-state emitters are deactivated by contact with quenchers, such as defects, solvent molecules or surface ligands.76,77 Nanocrystals have a high probability of surface luminescence quenching because of their high surface-tovolume ratios. Yb3þ/Tm3þ-codoped nanocrystals show significant enhancement in upconversion emission when particle sizes

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Fig. 6 Proposed energy transfer pathway and schematic design for (A) energy transfer upconversion with a large gap and (B) energy migrationmediated upconversion. (C) Energy migration upconversion emission spectra of nanocrystals with different activators (Tb3þ, Eu3þ, Dy3þ, Sm3þ). (B and C) Printed with permission from Ref. Wang, F. et al. Nat. Mater. 2011, 10, 968–973. Copyright © 2011, Nature Publishing Group.

are increased (Fig. 7A). After coating an inert protection layer, smaller particles exhibit more prominent luminescence enhancement (Fig. 7B). Energy migration upconversion can be further optimized by adding an inert layer to prevent energy loss through the Gd sublattice. In the absence of inert-layer protection, suppression of energy migration by surface defects and ligands is highly possible, hindering energy transfer to lanthanide activators (Fig. 8A). To enhance emission, the activator doping concentration needs to be significantly increased. However, with a NaYF4 inert layer, only 1 mol% of activators is required to achieve comparable luminescence intensity (Fig. 8B). Further evidence that is underpinned by inert shell-dependent luminescence enhancement is cooperative sensitization upconversion. In the vein of multiphoton excited luminescence, cooperative sensitization upconversion requires an intense energy flux for excitation. Without inert shell protection, the excitation energy of Yb3þ sensitizers would severely dissipate through surface quenching.52 With NaYF4 layer protection, excitation energy can be confined in the core region, permitting intense emission through cooperative sensitization (Fig. 9A and B). As with energy migration upconversion, cooperative sensitization upconversion can also be linked to other activators doped in an adjacent layer (Fig. 9C and D). For instance, the emitting energy of Tb3þ activators can be transferred to Eu3þ and Nd3þ activators for additional emission spectral modification. Inert shell protection prevents surface quenching of luminescence. A particular concentration of sensitizers in the inert layer may harvest more light and further enhance luminescence.15,41 Upon activation with sensitizers, the inert layer works as an active layer for incident excitation harvesting (Fig. 10A). Although more sensitizer doping improves light-harvesting, an excessive concentration could exacerbate the situation of surface quenching. Thus, doping of sensitizers in the active shell must be carefully optimized. It is important to note that the optimum doping concentration of sensitizers in the active shell can be quite different, especially when the activator/sensitizer in the core is slightly or highly doped (Fig. 10B). For slightly sensitized/activated core nanoparticles, once an active layer is coated, the protective effect from the coating remains dominant. Thus a certain concentration of activator ( 20 mol%) can still be doped in the active shell for luminescence enhancement. On the contrary, the excitation energy in the highly doped core will be linked to surface quenchers by sensitizers doped in the active shell, resulting in luminescence attenuation.

Fig. 7 (A) Upconversion emission spectra of NaGdF4:Yb/Tm nanocrystals with different sizes upon 980 nm excitation. (B) Comparison of upconversion luminescence enhancement factors for nanocrystals with varied sizes after coating protecting layer. Printed with permission from Ref. Wang, F.; Wang, J.; Liu, X. Angew. Chem. Int. Ed. 2010, 49, 7456–7460. Copyright © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

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Fig. 8 (A) Schematic illustration of the protective effect of inert NaYF4 layer for energy migration upconversion nanocrystals. (B) Spectra comparison between energy migration upconversion crystals with and without NaYF4 inert layer protection. Printed with permission from xRef. Su, Q. et al. J. Am. Chem. Soc. 2012, 134, 20849–20857. Copyright © 2012 American Chemical Society.

Fig. 9 (A) Schematic illustration of cooperative sensitization with inert shell protection. (B) Upconversion emission spectra of Yb/Tb-codoped upconversion nanocrystals with and without inert shell coating. (C) Cooperative sensitization upconversion-induced interfacial energy transfer mechanism. (D) Upconversion emission spectra of core-shell nanocrystals with different acceptors in the shell layer. Printed with permission from Ref. Zhou, B.; Yang, W.; Han, S.; Sun, Q.; Liu, X. Adv. Mater. 2015, 27, 6208–6212. Copyright © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

As proof in NaErF4@NaLuF4:Yb core-shell nanoparticles reported in the literature, the core section was activated with 100 mol% Er3þ ions that functioned as both sensitizers and activators (Fig. 10C and D). When Yb3þ sensitizers were added to the active shell, upconversion luminescence rapidly declined, although Yb3þ ions could harvest incident photons.78

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Fig. 10 (A) Balance between NIR-harvesting ability and surface quenching for sensitizers doped at different concentrations in the active shell. Inset shows the energy flow pathways in a core-active shell upconversion nanocrystal. (B) The optimum doping concentration of sensitizers in the active shell can be quite different for nanocrystals with low and high activator doping in the core. (C) Energy transfer mechanism of core-active shell nanocrystals sensitized by Yb3þ. (D) Upconversion luminescence spectra of NaErF4@NaLuF4:Yb nanocrystals with a different dopant concentration of Yb3þ in the active layer. (C, D) Printed with permission from Ref. Johnson, N. J. J. et al. J. Am. Chem. Soc. 2017, 139, 3275–3282.

8.11.4

Recent strategies for enhancing upconversion luminescence

Although lanthanide-doped upconversion nanocrystals have found broad applications in bioimaging, multiplexing sensing, anticounterfeiting, lasing, and optogenetics, their low conversion efficiency has remained a formidable challenge, especially at the single-particle level. In recent decades, innovative solutions have been developed to synthesize upconversion nanocrystals with much-improved upconversion brightness. By way of illustration, by exploiting the energy flux behavior in upconversion nanocrystals, factors such as light-harvesting capability, energy cross-talking, and surface quenching can be rationally balanced for developing efficient nanocrystals with targeted optical properties. Surface passivation is the most frequently used approach for upconversion luminescence enhancement and can be applied to all types of lanthanide-doped phosphors at nanoscale.79–81 As a result of high surface-to-volume ratios of luminescent nanocrystals, many dopant ions are trapped on the outermost layer of the nanocrystals, and their luminescence can be readily quenched by surface impurities, ligands, and solvent molecules through multiphonon relaxation. Since this topic has been exhaustively reviewed previously,40,82 we only summarize two key points of this approach. i) Smaller upconversion nanocrystals benefit more from this approach. In particular, with a 2.5-nm NaGdF4 shell, an emission enhancement factor of 450-fold has been reported for 10 nm NaGdF4:YbTm nanocrystals. However, this value declines to 70fold when the core size is around 25 nm. Therefore, for micrometer-sized upconversion crystals, the surface passivation approach will not work.

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ii) Surface passivation is a prerequisite for bright upconversion emission with Yb3þ sensitization. This is because energy dissipation through the Yb3þ sensitizer sublattice to surface quenchers is responsible for surface quenching; For Nd3þ-sensitized upconversion nanocrystals, surface passivation would not lead to significant luminescence enhancement. This is because Nd3þ sensitizers do not form energy dissipation sublattices, which is quite different from Yb3þ sensitizers. However, it is important to note that surface passivation should still be applied when Yb3þ sensitizers coexist with Nd3þ sensitizers in the same spatial layer. Besides optimization of upconversion nanocrystals, approaches involving external stimuli have been applied for luminescence enhancement. On account of intense light absorption and scattering of noble-metal nanoparticles, surface plasmon coupling has been widely used for optoelectronic controlling.83–85 Upon illumination, free electrons of metal nanoparticles oscillate at frequencies similar to those of passing photons and subsequently enter localized surface plasmon resonance. Resonance peaks can be finely tuned from the visible to the NIR region by controlling morphology, chemical composition, and spatial design.86,87 For upconversion luminescence enhancement, the surface plasmon can contribute in two ways (Fig. 11A): (i) Enhancing light absorption of lanthanide ions. In view of the nonlinear emission nature of lanthanide upconversion, a slight enhancement in excitation intensity can result in significant luminescence enhancement, especially for those emission transitions featuring 4- or even 5-photon processes. (ii) Improving radiative decay rates of activators. The small gap between adjacent energy levels of lanthanide activators induces nonradiative decay, which could significantly reduce the quantum yield of lanthanide luminescence. Improvement of radiative decay rates of activators can be a direct and effective way for lanthanide luminescence enhancement. One important issue is that the distance between metal nanoparticles and upconversion nanocrystals should be carefully controlled for upconversion luminescence amplification.88 For example, to study upconversion luminescence enhancement with surface plasmon coupling, an Al2O3 layer was used to block direct contact between the Au nanoparticle film and the upconversion nanocrystal monolayer (Fig. 11B). A threshold distance existed for luminescence enhancement. When the separation was inadequate, upconversion luminescence was quenched, and significant upconversion enhancement occurred once the separation distance increased to 5 nm. The enhancement factor declined when reducing interactions between the Au nanoparticles and upconversion nanocrystals by increasing the separation distance (Fig. 11C). Lifetime variation was used to probe the occurrence of improved radiative decay rates. Results showed that the lifetime of upconversion luminescence from Tm3þ activators was shortened when coupled with plasmonic Au nanoparticles (Fig. 11D). Single-particle level plasmon coupling-induced luminescence enhancement has also been achieved by researchers.89 The distance between the gold nanotip and single upconversion nanoparticles can be finely controlled while collecting luminescence simultaneously (Fig. 11E). When the tip was proximal to the particle, emission was enhanced with a concomitant lifetime decline from 105 to 36 ms (Fig. 11F and G). Surface plasmon resonance of Au nanorods can be tuned to the NIR region by carefully adjusting their aspect ratios. With a considerable spectral overlap between Yb3þ sensitizer absorption and the resonance band of Au nanorods, excitation intensity can be indirectly enhanced (Fig. 12A). The distance between Au nanorods and lanthanide-doped nanocrystals can be controlled by the size of the silica sphere (Fig. 12B).90 With spectral overlap, the pumped power density of 980-nm excitation near Au nanorod antennas was increased by local field enhancement (Fig. 12C), leading to a greater number of excited Yb3þ ions and thus enhanced upconversion luminescence (Fig. 12D). Although surface plasmon coupling has long been applied to upconversion enhancement from less than one to over three orders of magnitude, practices such as nanoscale pattern/distance controlling are quite challenging. Photonic-crystal engineering has also been employed for upconversion luminescence modulation.91–96 Made from artificial periodic patterns of materials with different permittivities, light can be confined in photonic crystals with an enhanced density of states. The stop-band position of the photonic crystal structure can be rationally designed by tuning the size, structure, and refractive index of periodic units. Luminescence enhancement of lanthanide upconversion through photonic crystal engineering can be attributed to two pathways: enhancing the electric field through resonance of excitation photons with photonic crystal modes and enhancing light collection by matching emission bands with stopbands. As shown in Fig. 13, PMMA polymer beads can be used to fabricate 3D photonic crystals through self-assembly.97 By depositing upconversion nanocrystals on the surface of PMMA photonic crystal structures, luminescence can be effectively augmented ( 30fold) once the stopband is tuned to overlap with the excitation (Fig. 13C). It should be noted that about 15-fold enhancement in blue emission can be achieved when the stopband is designed to 450 nm (Fig. 13D). This enhancement is attributed to photonic crystal reflection since blue emission largely overlaps with the stop band where photons are strongly reflected. For example, by coupling with photonic crystals fabricated using polystyrene (PS) beads,98 upconversion emission bands (green and red) of NaYF4:Yb/Er nanocrystals were selectively enhanced by changing the size of PS beads (Fig. 13E and F). Besides the common enhancement strategies discussed above, several new techniques have emerged in the past 5 years. Here, we highlight recently developed strategies for boosting luminescence, including organic dye sensitization, plasmon nanocavity coupling, and dielectric superlens modulation.

8.11.4.1

Organic dye sensitization

Despite achievements in controlling the composition, size, shape and optical properties of upconversion nanocrystals, their weak and narrow-band NIR absorption remain a major challenge. This is because low-rate 4f-4f electronic transitions fundamentally limited the absorption of doped lanthanide sensitizers. Recently, organic dyes have been utilized as antennas to enhance

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Fig. 11 (A) Schematic illustration of surface plasmon mediated upconversion luminescence enhancement. (B) Experimental design of Au nanoparticle surface plasmon assisted photon upconversion luminescence modulation. (C) Upconversion luminescence variation as a function of the thickness of the Al2O3 blocking layer. (D) Luminescence lifetime of NaYF4: Yb/Tm nanocrystals with and without surface plasmon coupling. (E) Experimental design of noble metal tip-enhanced upconversion luminescence. (F) Upconversion luminescence of single Er3þ activated nanoparticle while tipping up and down. (G) Luminescence lifetime of Er3þ activators with and without tip attachment. (A) Printed with permission from Ref. Han, S.; Deng, R.; Xie, X.; Liu, X. Angew. Chem. Int. Ed. 2014, 53, 11702–11715. Copyright © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. (B, C, D) Printed with permission from Ref. Saboktakin, M. et al. ACS Nano 2010, 6, 8758–8766. Copyright © 2012 American Chemical Society. (E, F, G) Printed with permission from Ref. Mauser, N. et al. ACS Nano 2015, 9, 3617–3626. Copyright © 2015 American Chemical Society.

upconversion emission brightness. Because of the large absorption cross-sections ( 10 16 cm2) of organic dyes, which are several orders of magnitude larger than that of lanthanide ions (10 20–10 19 cm2), dye sensitization can significantly enhance the lightharvesting capacity of upconversion nanocrystals. Through efficient energy transfer from surface attached dye to lanthanide ions, upconversion luminescence can be remarkably promoted. The energy transfer mechanism from dye antenna to lanthanide ions is illustrated in Fig. 14A. Upon NIR excitation, the dye molecule can be excited from its ground singlet state (S0) to excited singlet state (S1). The excited dye can depopulate through two pathways to its ground state: (i) relaxation from S1 to S0 and (ii) S1 relaxes to a triplet state (T1) through fast inter-system crossing (ISC) and then depopulates to S0. Both the S1 and T1 states can be depopulated through energy transfer to adjacent lanthanide ions.

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Lanthanide-doped upconversion nanomaterials

Fig. 12 (A) Schematic illustration gold nanorod induced lanthanide luminescence enhancement. (B) TEM image for Au nanorod@silica (top) and Au nanorod@silica@lanthanide emitter (bottom) nanostructures. (C) Surface plasmon resonance spectra of pure gold nanorods and nanorod@SiO2 composite. (D) Upconversion emission spectra of Silica@LaF3:Yb, Er nanocrystals with and without Au nanorod coupling. Printed with permission from Ref. Zhang, C.; Lee, J. Y. J. Phys. Chem. C 2013, 117, 15253–15259. Copyright © 2013 American Chemical Society.

Dye-sensitized upconversion is typically realized with two antenna-nanocrystal configurations (Fig. 14B): core-only or core-shell nanocrystals. For the first configuration, organic dye antennas work with Yb3þ sensitizers through direct energy transfer from the dye to Yb3þ ions. Then the excitation energy is delivered to upconversion activators (such as Er3þ, Tm3þ, and Ho3þ). Although this design is rather simple, both sensitizers and activators are exposed to surrounding luminescence quenchers. Although significant upconversion luminescence can be achieved, overall brightness remains low due to persistent surface quenching. Dye-sensitized core-shell structures are much more rational for upconversion enhancement. In this design, Nd3þ sensitizers are usually selected for co-sensitization with Yb3þ sensitizers.59,99 Under NIR excitation, excited dye antennas transfer energy to Nd3þ sensitizers in the shell layer. Then the energy can be passed to Yb3þ sensitizers, and subsequently, energy transfer upconversion occurs. Unlike the core-only design, the core-shell design enables much brighter emission due to surface passivation of the Nd3þ sensitization layer. In 2012, pioneering work in enhancing upconversion luminescence was published by coupling nanocrystals with organic dye molecules (Fig. 15A).64 Through nucleophilic substitution, a carboxylic acid-functionalized derivative IR-806 dye was synthesized based on a commercial cyanine dye, IR-780. With the carboxylic group, the IR-806 dye can be attached to the surface of NaYF4:Yb/Er nanocrystals. Due to the considerable overlap between the emission spectrum of IR-806 dye and the absorption spectrum of Yb3þ sensitizers, efficient energy transfer from IR-806 to Yb3þ ions occurs once dye antennas are stimulated (Fig. 15B). Experimental results showed that the extinction coefficient of IR-806 was about six orders of magnitude higher than that of prepared nanocrystals. As a result, upon 800-nm excitation, NIR-806 dye-sensitized nanocrystals are more than 1100 times brighter than nanocrystals without sensitization upon 980-nm excitation (Fig. 15C). More importantly, as shown in Fig. 15D, by virtue of the broadband

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Fig. 13 (A) Experimental design for upconversion luminescence enhancement through photonic crystal coupling. (B) SEM image of the prepared sample with NaYF4:Yb/Tm upconversion nanocrystals dispersed on the surface of PMMA structure. (C) Upconversion emission spectra with and without PMMA photonic crystal coupling. (D) Demonstration of the relationship between luminescence enhancement factors and the position of stopband of the prepared photonic crystal structure. (E, F) Upconversion luminescence of NaYF4:Yb/Er with and without 275 nm (E) and 335 nm (F) polystyrene photonic crystal coupling. (B-D) Printed with permission from Ref. Yin, Z. et al. Chem. Commun. 2013, 49, 3781–3783. Copyright © 2013 Royal Society of Chemistry. (E, F) Printed with permission from Ref. Liao, J. et al. J. Mater. Chem. C 2013, 1, 6541–6546. Copyright © 2013 Royal Society of Chemistry.

absorption of organic dyes, IR-806-sensitized nanocrystals can be enlightened using excitation photons with wavelengths from 720 to 830 nm. To eliminate surface quenching of dye-sensitized core-only nanocrystals, core-shell structures can be introduced for energy cascade upconversion.100 To illustrate, the core particle codoped with Yb3þ/Tm3þ ions is protected by a shell with Nd3þ sensitization, and the core-shell structure is further sensitized by NIR dye antennas attached to nanoparticle surfaces (Fig. 16A). Under NIR excitation, energy from excited dye molecules is transferred to adjacent Nd3þ sensitizers in the shell layer. Afterward, efficient energy transfer from Nd3þ to Yb3þ sensitizers occurs, followed by energy transfer to Tm3þ activators. Unlike lanthanide ions, organic dyes feature broad absorption and emission bands, ensuring broadband excitation and sufficient spectral overlap. For Nd3þ/Yb3þ sensitized upconversion nanocrystals, multiple narrow excitation bands can be detected (740 nm, 800 nm, and 980 nm). However, once sensitized by the organic dye, the excitation band can be significantly broadened and enhanced (Fig. 16B). The strong enhancement in blue emission is clearly shown from inserted images. The excitation band can be further broadened since Nd3þ sensitization with multiple IR dyes is plausible. As shown in Fig. 16C, intense excitation bands were detected for core-shell nanocrystals sensitized with IR-806 and IR-808. However, for IR 820, sensitization efficiency is relatively low, which can be ascribed to the smaller spectral overlap between the emission band of the IR dye and the Nd3þ absorption spectrum. However, compared with dye-sensitized coreonly nanocrystals, Nd3þ-sensitized core-shell nanocrystals allow more freedom in IR dye selection. As demonstrated in Fig. 16D, strong blue emission from Tm3þ-activated core/shell nanocrystals can be triggered by multiple excitation bands centered at

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Lanthanide-doped upconversion nanomaterials

Fig. 14 (A) Schematic illustration of nonradiative energy transfer processes from organic dye to lanthanide ions. R, IC, and ISC represent the radiative emission, internal conversion, and intersystem crossing, respectively. (B) Commonly designed organic dye-sensitized upconversion. Printed with permission from Ref. Wang, X. et al. Chem. Soc. Rev. 2017, 46, 4150–4167. Copyright © 2017 Royal Society of Chemistry.

Fig. 15 (A) Principal concept of dye-sensitized upconversion. (B) The absorbance of Yb3þ sensitized upconversion nanocrystals and photoluminescence of IR-806 dye. (C) Upconversion luminescence spectra of NaYF4:Yb/Er nanocrystals with different IR dye sensitization excited by a 2 mW continuous-wave laser. (D) Experimental upconversion excitation spectra of b-NaYF4:Yb/Er nanoparticles/IR-806 (blue circles) and b-NaYF4:Yb, Er nanoparticles (green triangles), both dissolved in CHCl3. Printed with permission from Ref. Zou, W.; Visser, C.; Maduro, J. A.; Pshenichnikov, M. S.; Hummelen, J. C. Nat. Photon. 2012, 6, 560–564. Copyright © 2012, Nature Publishing Group.

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Fig. 16 (A) Scheme of the proposed nanostructure and energy transfer pathway for energy-cascaded upconversion in dye-sensitized fluoride core/ shell nanocrystals. (B) Excitation spectra (for: IR-808 dye-sensitized (NaYbF4:Tm3þ 0.5%)@NaYF4:Nd3þ 30% (blue), (NaYbF4:Tm3þ 0.5%) @NaYF4:Nd3þ 30% (red), and canonical (NaYF4:Yb3þ 30%, Tm3þ 0.5%)/NaYF4 core/shell UCNPs (black). (C) Excitation spectra of upconversion nanocrystals sensitized with different NIR dyes. (D) Photographic image for dye-sensitized upconversion nanocrystals upon different wavelength excitation. Printed with permission from Ref. Chen, G. et al. Nano Lett. 2015, 15, 7400–7407. Copyright © 2015 American Chemical Society.

740 nm, 780 nm, 800 nm, 810 nm, 830 nm, and 850 nm. Thus, in contrast to pure nanocrystals that usually require laser excitation, dye-sensitization endows upconversion nanocrystals with the capacity to emit intense luminescence with incoherent broadband excitation. As demonstrated in the bottom panel of Fig. 16D, the pattern with blue upconversion luminescence can be distinguished upon incoherent light excitation. The dye-sensitized core-shell configuration can be further optimized. Although exposure of Yb3þ sensitizers can lead to severe surface quenching, low-content co-doping of Yb3þ ions in the active shell eventually enhances the overall upconversion output. Despite efficient interfacial energy transfer from Nd3þ to Yb3þ, their spatial isolation in core and shell layers leads to inefficient excitation energy extraction. Furthermore, unlike Yb3þ sensitizers in which sublattice energy can migrate long distances, inter-sensitizer energy transfer for Nd3þ ions is extremely inefficient. Thus, introducing Yb3þ sensitizers with Nd3þ sensitizers in the same spatial layer facilitates energy extraction efficiency from the shell to the core, where activators exist.101 As can be seen in Fig. 17A, multiple energy transfer pathways are simultaneously involved in relaying energy from IR dye antennas to activators in the core. Apart from direct energy transfer from IR dyes to Nd3þ activators, Yb3þ sensitizers can be activated by accepting energy from dye molecules without the help of Nd3þ sensitizers (Fig. 17B). As shown in Fig. 17C, compared to configurations based on dye-Nd and dyeYb, the dye-Nd/Yb design shows much-improved excitation efficiency from 700 to 850 nm. Importantly, these IR dye-sensitized upconversion nanocrystals can be well dispersed in a polymer matrix for potential applications in display. As shown in Fig. 17D, dye-sensitized core/shell upconversion nanocrystals activated with different activators (Er3þ, Ho3þ, Tm3þ) can be integrated into PDMS film for multicolor emission demonstration. In principle, upconversion nanocrystals are sensitized with one type of IR dye antenna to achieve broadband sensitization. In 2016, a UV emitting dye-sensitized upconversion nanostructure harvesting photons of a wide wavelength range from 450 to 975 nm was reported. To achieve this, multiple dye molecules were modified on the surface of Yb3þ/Tm3þ codoped upconversion nanocrystals.102 As illustrated in Fig. 18A, three types of Vis/IR dye antennas were selected for sensitization. Sensitizer III (IR antenna) can directly transfer its energy to Yb3þ sensitizers for energy transfer upconversion. More importantly, the absorption spectra of sensitizers I and II covered the range from 450 to 650 nm, and their energy was eventually transferred to Yb3þ sensitizers because of a suitable energy gradient (Fig. 18B). Therefore, intense UV emission (340–370 nm) was obtained upon broadband excitation from 450 to 1000 nm (Fig. 18C). Multi-dye-sensitized UV emitting nanocrystals were integrated with resistive switching

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Fig. 17 (A) Schematic illustration of multidimensional energy transfer pathways from the dye molecules on the surface of the core/active shell nanocrystal of (NaYF4:Yb/X)@NaYF4:Nd/Yb to the lanthanide ions in the core. (B) Absorption and emission spectra of ICG dye versus the absorption spectra of Nd3þ ions and Yb3þ ions. (C) Excitation spectra of upconversion nanocrystals with different energy transfer pathways. (D) Photographic images of a transparent polydimethylsiloxane (PDMS) cylinder doped with ICG-sensitized core/active shell nanocrystals. Printed with permission from Ref. Chen, G. et al. Adv. Opt. Mater. 2016, 4, 1760–1766. Copyright © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

random access memory (RRAM) to develop a memory system featuring the novel function of unrecoverable data erasure. UV emission from upconversion nanocrystals triggered chemical destruction of the ultrathin RRAM. As demonstrated in Fig. 18D, for pure nanocrystals coated RRAM, 980 nm laser exposure induced chemical destruction patterns, however, the film showed no response to 800-nm excitation. In contrast, both 980 and 800 nm illumination resulted in RRAM destruction when sensitizer III-sensitized upconversion nanocrystals were used. Moreover, 800-nm exposure can store data in the memory array with a power density of 25 mW mm 2 (Fig. 18E). We should note that all of the above-mentioned dye-sensitized nanocrystals are designed based on the spectral overlap between the dye singlet luminescence (S1 to S0 transition) and the sensitizer absorption. In 2018, it was shown that an increase in lanthanide doping content shifts the primary energy donor from the dye singlet to its triplet, and resultant triplet states then mediate energy transfer to nanocrystals, resulting in significant luminescence enhancement.103 Fig. 19A shows an IR-806 dye antenna bound to a NaYGdF4:Yb/Er nanocrystal. Photon upconversion occurs inside the nanoparticle where two Yb sensitizers excite an Er activator into a higher energetic state. Upon 808-nm excitation, the IR-806 antenna is excited to its first singlet state. By increasing the doping concentration of Gd3þ in the host lattice, singlet-to-triplet intersystem crossing can be significantly enhanced because of heavy atom effects induced by Gd3þ. Compared to luminescence from pure nanoparticles (lex ¼ 980 nm), IR-806-sensitized NaYF4:Yb/Er nanocrystals showed approximately 500-fold enhancement in upconversion emission under 808-nm excitation. This enhancement factor was further increased to 15,000-fold with 30% Gd3þ dopants. The red-shift of phosphorescence relative to fluorescence from the IR-806 antenna was considered the main reason for enhanced emission. With the Gd3þ co-doping, the excited IR-806 antenna quickly populates to the triplet state to generate phosphorescence, which has a much larger spectral overlap compared to that between the singlet stemmed fluorescence and the Yb3þ absorption spectrum, thus significantly enhancing energy transfer from Yb3þ sensitizers to dye antennas (Fig. 19B and C). The efficient energy transfer from the dye to lanthanide sensitizers is signified by the fluorescence lifetime variation of IR-806 antennas. As can be seen in Fig. 19D, attaching a dye antenna to nanocrystal surfaces shortens their fluorescence lifetimes to some extent; however, adding Gd3þ ions to the host lattice can amplify this phenomenon, indicating that more efficient energy transfer can be triggered once Gd3þ ions are introduced. Although heavyatom-induced triplet enhancement improves emission brightness, this strategy is only appliable under inert gas conditions. Because dissolved singlet oxygen molecules severely depopulate triplet states of dye antennas and block energy transfer to lanthanide sensitizers, elimination of moisture and oxygen is a prerequisite for applying this approach for upconversion enhancement. As shown in Fig. 19E, upconversion luminescence of dye-sensitized nanocrystals decays rapidly in air, but their stability can be significantly improved with nitrogen protection. A recent study published in Nature suggested that the above-mentioned heavy-atom effect is not the exact reason for enhanced upconversion luminescence (Fig. 20).66 To prove the heavy atom effect for triplet-state accumulation, other lanthanide ions with comparable atomic numbers, such as Lu3þ and La3þ, should be employed to investigate luminescence enhancement. However, previous research concluded, using only Gd3þ as an example, that the heavy atom effect is the main factor governing luminescence

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Fig. 18 (A) Normalized absorption/emission spectra of dye molecules used for upconversion sensitization. (B) Cascaded energy transfer between surface-modified dye molecules for upconversion sensitization. (C) Absorption spectra of upconversion nanoparticles with dye sensitization and emission spectra of nanocrystals upon different excitation sources. (D) Images of the etched Mg layer coated with upconversion nanocrystals (left) and nanoparticle/sensitizer III (right) by exposure to 980 and 800 nm laser with the same power density of 25 mW mm 2. (E) Magnified view of the integrated device without (left) and with light exposure (right) for photo-induced chemical destruction. Printed with permission from Ref. Lee, J. et al. Adv. Mater. 2017, 29, 1603169. Copyright © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

enhancement. Systematically, in the Nature study, researchers found that only lanthanide ions with unpaired electrons can facilitate intersystem crossing from the excited singlet state to a triplet state. For example, compared to pristine 9-[3-carboxyl-4-(diphenylphosphinoyl)phenyl]-9H-carbazole (CPPOA) molecules, the triplet state rise rate of those molecules attached to NaYF4@NaGdF4 nanocrystal surfaces is almost three orders of magnitude faster (Fig. 20B and C). As with Gd3þ-doped nanocrystals, those doped with lanthanide ions containing unpaired electrons (e.g., Eu3þ, Tb3þ, and Yb3þ) could remarkably shorten the triplet state generation of attached dye antennae (Fig. 20D). In contrast, for Lu3þ doping, the triplet state generation rate could not be facilitated, although the atomic number of Lu3þ is much larger than that of Gd3þ. Thus, with fast intersystem crossing, dye sensitizers could be quickly populated to their triplet states, which may promote efficient energy transfer from dyes to lanthanide sensitizers, which should explain the significant upconversion enhancement, instead of the heavy-atom effect. Dye-sensitization has proven effective for luminescence enhancement; however, this strategy has not found many practical applications. One reason is that even though upconversion nanocrystals have high photostability, the stability of attached organic dyes is extremely low. Moreover, with dye sensitization, further surface modifications for specific bio-applications are rather challenging. In addition, for triplet exciton-mediated sensitization, inert gas protection is required, which makes practical applications difficult.

8.11.4.2

Nanocavity-assisted surface plasmon coupling

There have been many reports on plasmon-enhanced upconversion luminescence in solution and on lithographically prepared substrates. However, precise control of orientation and distance of the plasmonic nanoparticles with respect to the upconversion nanocrystals is quite difficult. Lithographic fabrication of nanostructures allows better control of the morphology of plasmonic structures and the distance between the plasmonic structures and upconversion nanocrystals. In 2018, a paper reported over a 1000-fold emission enhancement of NaYF4:Yb/Er nanocrystals by incorporating them in a metal-insulator-metal (MIM) structure

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(B)

Yb3+ Effective absorption cross-section

Er3+

1.0

Dye phosphorescence Smoothed Yb3+ absorption

Normalized intensity

(A)

NIR

Yb3+

Dye singlet 0

Oleic acid Triplet

ET

Energy (eV)

off 4

ISC a Z

ET

O –

Visible

1.5

S

4

F9/2

ET

ET

C

4

2

F5/2

l11/2

0.5

0 S 0

30% Gd3+ UCNP 0% Gd3+ UCNP Free dye

10–1

10–2

2

Dye / molecular concentrator

F7/2

4

Yb3+

l15/2

Er3+

(E)

upconverted emission

Normalized intensity

IS

S1

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Fig. 19 (A) Cartoon schematic of the dye-sensitized upconversion nanocrystal system, showing IR-806 bound to the crystal surface, and an upconversion event inside the nanoparticle. (B) Phosphorescence of NIR-806 dye and absorption spectrum of Yb3þ sensitizers. (C) Energy transfer scheme for dye-sensitized photon upconversion. (D) Luminescence lifetime variation for NIR-dye before and after attached to nanocrystal surfaces with and without Gd3þ doping. (E) Upconversion luminescence intensity versus exposure time under different atmospheric conditions. Printed with permission from Ref. Garfield, D. J. et al. Nat. Photon. 2018, 12, 402–407. Copyright © 2018, Nature publication group.

(Fig. 21).104 The fabrication procedure of the MIM structure is illustrated in Fig. 21A. Through a few relatively simple steps without complicated wet-chemistry techniques, the lithographic fabrication method is readily scalable to large areas for higher yield while maintaining excellent uniformity. For the fabricated MIM structure, gold is used as top and bottom metal layers and a monolayer of NaYF4:Yb/Er nanocrystals works as the insulator layer (Fig. 21B–D). Electric field simulation of the MIM structure upon incident plane wave excitation shows that the plasmon mode exhibits strong field enhancement in the insulator layer, significantly enhancing the excitation intensity (Fig. 21B). With the finite-difference time-domain (FDTD) method, the resonance peak of prepared MIM can be tuned to around 980 nm, corresponding to the absorption transition energy of Yb3þ sensitizers. Fig. 21E shows the simulated diameter (D) dependence of plasmonic resonance. As the diameter is increased from 160 to 280 nm, the resonance peak shifts from 800 to 1200 nm. Importantly, for plasmonic structures with 220-nm D, the insulator layer shows the strongest absorption enhancement at 980 nm (Fig. 21F). For practical testing, MIM structures with diameters from 220 to 300 nm were fabricated and their luminescence spectra were measured (Fig. 21G). The green upconversion luminescence of all MIM samples showed significant enhancement compared to the control sample. Besides, the maximum enhancement occurs at a diameter of

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Fig. 20 (A) Schematic illustration of a NaYF4@NaLnF4 core-shell nanoparticle modified with CPPOA. (B) Extracted kinetics showing the singlet (S1) decay and triplet (T1) growth of a solution containing pristine CPPOA molecules and of a solution of CPPOA-modified NaYF4@NaGdF4 nanoparticles. (C) The interaction between the lanthanides and the molecules accelerates the ISC from the singlet to triplet exciton states of the molecule. (D) Triplet state generation speed of CPPOA molecules attached on nanocrystals with different lanthanide ion doping. Printed with permission from Ref. Han, S. et al. Nature 2020, 587, 594–599. Copyright © 2020, Nature publication group.

around 240 nm, which matches the simulation results well. More importantly, the MIM structure performs quite differently upon varied excitation power density (Fig. 21H). Upon low-power excitation (< 0.02 kW cm 2), an enhancement factor of  1200-fold can be achieved, which declines with increased excitation power. Only a 200-fold enhancement was obtained at 100 kW cm 2 power density. This is because upconversion luminescence saturates under high power excitation. Thus, with increased external excitation power, upconversion luminescence from a MIM structure and the reference finally approach each other. For this work, the MIM-induced enhancement in upconversion luminescence can be attributed to the spectral overlap between the MIM plasmonic resonance and the Yb3þ absorption. In other words, the excitation power density surrounding upconversion nanocrystals is significantly enhanced. Besides increasing the excitation power density by tuning the resonance peak to match sensitizer absorption, the spectral overlap between upconversion emission and resonance bands can be another practical approach for enhancing luminescence. For upconversion luminescence, quantum yield is usually ultra-low because it is limited by intense nonradiative relaxation of excited lanthanide activators. Thus, strategies that reduce nonradiative or increase radiative processes can directly enhance the quantum yield and ultimately the upconversion output. By harnessing the Purcell effect, the radiative decay of lanthanide activators in the resonance cavity can be remarkably accelerated, resulting in a large increase in luminescence. This approach has been successfully applied to augment the fluorescence of organic dyes, quantum dots, and 2D materials. In 2019, with plasmonic nanocavities constructed with gold film and silver nanocubes, significant luminescence enhancement for upconversion nanocrystals was observed.105

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Fig. 21 (A) Metal-insulator-metal (MIM) fabrication process. (B) Simulated field profile under 980 nm normal incidence plane wave excitation. E field is normalized by incident plane wave amplitude E0. (C) SEM image and (D) cartoon illustration of fabricated MIM nanocavity structure. (E) Simulated diameter dependence of plasmonic resonance and (F) absorption enhancement factor at 980 nm for various diameters. (G) Green upconversion emission spectra of the reference sample and metal-insulator-metal (MIM) structure of varying diameter under 980 nm excitation. (H) Upconversion luminescence enhancement factors for 250-nm-diameter MIM structure as a function of excitation power density. Printed with permission from Ref. Das, A.; Mao, C.; Cho, S.; Kim, K.; Park, W. Nat. Commun. 2018, 9, 4828. Copyright © 2018, the authors.

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According to the SEM image and schematic illustration in Fig. 22A, the fabricated nanocavity consists of a large-area gold film, silver cubes, and a sandwiched monolayer of upconversion nanocrystals. The size of the silver cube can directly determine the resonance band of the nanocavity structure. As simulated in Fig. 22B, a substantial electric field enhancement between the substrate and the silver cube can be observed. Substantial resonance band overlap with red upconversion emission can be achieved by selecting silver cubes of  90 nm in edge length (Fig. 22C). Based on simulations through coupling in the gold film-silver cube nanocavity, upconversion luminescence can be guided to a small corner and almost totally collected by an objective lens (Fig. 22D). The significant enhancement of upconversion luminescence induced by plasmonic nanocavity is demonstrated in Fig. 22E. Upon 980-nm excitation, some random red-emitting regions can be observed. Referenced by the SEM image (Fig. 22F and G), only fluorescence from areas with silver cubes on top of the upconversion nanocrystal monolayer can be distinguished. Fig. 22H shows green and red upconversion luminescence enhancement factors as a function of excitation power density. It is clear that upconversion emission can be significantly enhanced by the plasmonic resonance nanocavity, but the red emission (660 nm) features much higher luminescence enhancement. This can be ascribed to the reduced spectral overlap between the green emission (554 nm) and the plasmonic resonance. In addition, power-dependent investigations of the emission enhancement factor revealed that the effect of pump fluence on emission intensity is more predominant under low-power than under high-power excitation. To confirm the radiative rate acceleration, the luminescence lifetimes of samples with and without nanocavity coupling were compared (Fig. 22I and J). Interestingly, through coupling in the nanocavity, the lifetime of 660 nm luminescence can be shortened from 232 to 1.4 ms (Fig. 22J). For the lifetime of green emission, a change from 143 to 6 ms can also be achieved (Fig. 22I). Since the nonradiative decay cannot be significantly changed under current conditions, the drastic decline in lifetime can be ascribed to accelerated radiative decay, which eventually contributes to the upconversion output. Therefore, we predict that further enhancement of upconversion luminescence will be achieved by coupling upconversion nanocrystals in plasmonic resonance nanocavities with large, simultaneous resonance spectral overlap with the absorption and emission band of nanocrystals. Besides silver and gold, aluminum has been used to construct plasmonic resonance cavities for luminescence enhancement. By coupling NaGdF4:Yb/Er nanocrystals in the Al disk-Al film nanocavity, the position of the cavity resonance can be tuned to selectively enhance red or green emission.106 As illustrated in Fig. 23A and B, upconversion nanocrystal-coupled Al nanocavities were fabricated by placing NaGdF4:Yb/Er nanocrystals between a substrate and an Al disk. By increasing the diameter of the Al disk from 90 to 220 nm (Fig. 23C–E), reflection spectra and resonance bands were shifted from 400 to 1100 nm. Importantly, the red-togreen ratio of overall color output can be customized by changing the disk size that affects resonance band position. For instance, with a disk diameter of 160 nm, the constructed nanocavity shows a strong resonance band around 660 nm, which overlaps considerably with red emission band. Correspondingly, the resulting upconversion luminescence exhibited the highest red-to-green ratio and an orange color disk was observed in Fig. 23C. Since the size of each cavity can be controlled minutely, large-area color images exhibiting different luminescent color schemes of the same image can be printed (Fig. 23F). The left panel of Fig. 23F is a brightfield optical micrograph of “The Starry Night” (Vincent van Gogh, 1889), observed under white light illumination. However, upon 980 nm laser excitation, the same image shows a totally different color scheme consisting of green, yellow, and orange. This technique is up-and-coming for data encryption applications.

8.11.4.3

Electric hotspot generation through dielectric superlensing modulation

Unlike conventional luminescent probes, such as organic dyes and quantum dots, lanthanide upconversion materials feature distinct nonlinear optical properties. By absorbing multiple NIR photons sequentially, one photon with an appreciable antiStokes shift is emitted. Thus, upconversion luminescence power dependence follows an interesting rule of I ¼ P^n, where I, P, and n represent upconversion luminescence intensity, excitation power density, and the number of NIR photons absorbed in order to generate a single short-wavelength photon in a given upconversion process, respectively.45 For instance, for a 4-photon upconversion process, its luminescence intensity can be amplified up to 16-fold, once the excitation power density is doubled, and this enhancement factor can be further amplified to a value of 81 with a corresponding power density increased three-fold. Therefore, increasing the excitation power density can be an effective approach for significant upconversion luminescence enhancement, especially for those cases governed by a larger number of photons. With strong light confinement capability, dielectric microlenses can converge incident NIR light into a sub-micron-sized hotspot with ultra-high-power density.107–109 Thus, by coupling dielectric microlenses with upconversion nanocrystals, remarkable enhancement in upconversion luminescence can be anticipated. Liang et al. have demonstrated five orders of magnitude enhancement in upconversion luminescence using polymeric microbeads for efficient excitation confinement (Fig. 24).45 Polymeric microlenses were fabricated using the microfluidic method. By controlling the injection rate of the oil phase, poly (ethylene glycol) diacrylate (PEGDA) microbeads were customized from 5 to 100 mm in size. Fig. 24A shows the microscopy image of prepared PEGDA microbeads with a diameter of 20 mm. These polymeric microbeads are quite uniform and transparent. The incident light beam can be effectively focused on the rear side of the microbeads (inset image). To study the enhancement effect of prepared dielectric microlenses, PEGDA microbeads were self-assembled to form a large-area monolayer on a PDMS layer consisting of upconversion nanocrystals (Fig. 24B). Here, NaYF4:Yb/Er@NaYF4 and NaGdF4:Yb/Tm@NaGdF4:Eu nanocrystals are prepared and embedded in the PDMS layer for luminescence enhancement investigation. As simulated in Fig. 24C, prepared PEGDA microbeads can confine the incident NIR light (980 nm) into an ultra-small hotspot with full-width at the half-maximum (FWHM) of about 680 nm, which is over two orders of magnitude smaller than the size of microlens selected. Importantly, according to the electric field profile, the incident light intensity can be amplified over 300-fold. With the assistance of microbeads,

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Fig. 22 (A) Schematic illustration and SEM image of plasmon nanocavity constructed by Au film-Upconversion nanocrystal-Ag nanocube. (B) Filed profile simulation for fabricated nanocavity. (C) Upconversion luminescence of Yb/Er codoped nanocrystals upon 980 nm excitation and simulated scattering spectrum of a single nanocavity. (D) Schematic of the experimental set-up designed for upconversion luminescence investigation (left). The simulated far-field radiation pattern of upconversion emission in nanocavity. (E) Upconversion photoluminescence imaging of the enhancement effect induced by plasmonic nanocavity coupling. (F) Enlarged SEM image of the circled region marked in (E). (G) High-resolution SEM imaging of the nanocavity. (H) Upconversion luminescence enhancement factor for nanoparticles located in the plasmonic nanocavity as a function of excitation power. (I, J) Luminescence lifetime for green (I) and red (J) upconversion emission of samples on the glass substrate and in nanocavity. Printed with permission from Ref. Wu, Y. et al. Nat. Nanotechnol. 2019, 14, 1110–1115. Copyright © 2019, Nature publication group.

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Fig. 23 (A) Schematic showing tunable resonator-upconverted emission (TRUE) color pixel arrays. (B) SEM image of the assembled upconversion nanocrystals in PMMA nanoholes (left) and image of fabricated pixel arrays. (C) SEM i), reflected bright field ii), and upconversion luminescence iii) color patches of pixel arrays. (D) Measured reflectance spectra of TRUE color pixels. (E) Upconversion luminescence of pixel arrays with different diameters. (F) Optical micrograph and its corresponding luminescence image of “The Starry Night” using NaYF4:Yb/Er as upconversion nanocrystals. Scale bar is 10 mm. Printed with permission from Ref. Liu, H. et al. Adv. Mater. 2019, 31, 1807900. Copyright © 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

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Fig. 24 (A) Photographic image of the as-prepared PEGDA polymeric microbeads. (Inset) Photographic image showing the convergence of two intersected orange and green LED beams into two small focal spots after passing through the microbeads. (B) Schematic illustration of the experimental setup designed for luminescence amplification investigation. (C) FDTD simulation of the electric field distribution and phase variation of NIR excitation light after passing through the dielectric microbead. (D) Comparative simulations of the far-field emission collection efficiency for upconversion enhancement, obtained in the presence or absence of a dielectric microbead. (E) Upconversion luminescence images of the upconversion nanocrystal-embedded PDMS upconverting films recorded with and without the microbead coverage upon 980-nm excitation at different intensities. (F) Power-dependent investigations of microbead monolayer induced-luminescence enhancement for PDMS films comprising NaYF4:Yb/Er@NaYF4 and NaGdF4:Yb/Tm@NaGdF4:Eu nanoparticles. (G) Photographic image of a PDMS composite sheet comprising 50 mm BaTiO3 microbeads. Inset shows the ensemble of the microbeads used for making the planar sheet. (H) Demonstration of document security application using the composite sheet. Printed with permission from Ref. Liang, L. et al. Nat. Commun. 2019, 10, 1391. Copyright © 2019, the authors.

upconversion luminescence from nanocrystals embedded in the PDMS film can be efficiently collected in the far-field (Fig. 24D). For the upconverting PDMS film as the control, only 0.77% of the emitted photons can be collected by a 10X objective lens with a NA of 0.3. However, once the microbead is placed on the film, the collection efficiency was increased to about 6.14%, an 8-fold enhancement in luminescence collection efficiency. Therefore, the dielectric microlens monolayer can contribute to the enhancement in both excitation power density and luminescence collection efficiency. Fig. 24E shows the microscopic fluorescence imaging of PDMS films doped with NaYF4:Yb/Er@NaYF4 and NaGdF4:Yb/Tm@NaGdF4:Eu upconversion nanocrystals with and without

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the coverage of a microlens monolayer. Upon 980 nm excitation, Er3þ-activated nanocrystals showed yellow-green emission with two characteristic bands centered around 540 and 654 nm. For crystals co-activated with Tm3þ/Eu3þ dopants, characteristic bands centered around 450 and 614 nm were observed, arising from 1D2 -> 3F4 transition of Tm3þ and 5D0-> 7F2 transition of Eu3þ. Without coupling of the PEGDA microbead monolayer, no upconversion emission from PDMS films can be observed. However, bright luminescence from both Er3þ and Tm3þ/Eu3þ coactivated PDMS films can be clearly seen, signifying that significant upconversion luminescence enhancement can be achieved through microlens monolayer coupling. For systematic investigation of the enhancement contributed by the dielectric microlens monolayer, upconversion luminescence spectra of both NaYF4:Yb/Er@NaYF4 and NaGdF4:Yb/Tm@NaGdF4:Eu embedded PDMS films are recorded at varied excitation power densities. As shown in Fig. 24F, utilization of a dielectric microbead monolayer can induce an immense luminescence enhancement in all-optical transitions of activators under investigation. It is important to note that in all cases, luminescence enhancement is more significant under low-power excitation. Luminescence enhancement with dielectric superlenses depends strongly on the upconversion population of different activators. For example, under 1.5 W cm 2 power excitation, the upconversion luminescence of Eu3þ was enhanced by over five orders of magnitude. However, only  100-fold enhancement for emission at 540 nm (Er3þ) was recorded. The diminished enhancement factor at higher excitation power can be attributed to luminescence saturation at high power. Importantly, the huge difference in the enhancement factor for emission originating from different optical transitions can be ascribed to the difference in the number of photons involved in each upconversion process. For example, by accepting migrated energy from Gd3þ sublattices, Eu3þ emission features a five-photon upconversion process. In contrast, the green emission of Er3þ comprises only two-photon energy transfer. As mentioned above, luminescence governed by more steps of NIR pumping shows much larger luminescence enhancement under increased excitation power. For convenience and improved mechanical strength, a transparent, planar composite PDMS sheet was produced by incorporating a close-packed BaTiO3 microbead monolayer into a PDMS precursor (Fig. 24G). Due to the high relative refractive index between BaTiO3 (2.1) and PDMS (1.4), the composite film worked well for light confinement. The composite film was applied to improve document security applications involving upconversion-based encoding techniques. As shown in Fig. 24H, with the help of the as-fabricated PDMS sheet, an encrypted quick-response code, printed on a piece of paper using green-emitting NaYF4:Yb/Er@NaYF4 nanoparticle inks, was readily decoded with a tungsten lamp at subsolar irradiance ( 12 mW cm 2). In addition to PEGDA and BaTiO3 materials, other dielectric microlenses (e.g., SiO2 and PMMA microbeads) have proven efficacious for upconversion luminescence enhancement. Interestingly, yeast cells or human cells with spherical or disk-like shapes can also act as natural bio-microlenses for luminescence enhancement (Fig. 25).110 Due to the photonic nano-jet effect of the bio-microlens, NIR excitation can be confined in a subwavelength region, enhancing upconversion luminescence by two orders of magnitude. As a result of enhanced luminescence, single-cell imaging and real-time detection of pathogenic bacteria can be achieved. Fig. 25A illustrates the schematic of luminescence enhancement and signal collection of an E. coli chain with a bio-microlens at the end of an optical fiber. With an average diameter of 2 mm, yeast cells have smooth surfaces and can work as dielectric micro-lenses for light confinement. To conduct measurements, a single yeast cell was first trapped at the end of a fiber probe by the incident NIR laser beam (Fig. 25B). Then the probe was inserted into a solution containing blue-emitting NaYF4:Yb/Tm nanocrystals for luminescence collection. Upon launching a 980-nm laser with an optical power of 3 mW, strong blue-emitting luminescence can be imaged with a biomicrolens trapped on the fiber tip. In contrast, the emission is difficult to detect when only the fiber was used (Fig. 25C), indicating bio-microlens-mediated luminescence amplification. Moreover, using the bio-microlens, optical trapping of single or even chained nanocrystals can be achieved with significantly boosted luminescence. As demonstrated in Fig. 25D, upconversion nanocrystalmodified E. coli and S. aureus cells can be trapped on the rear side of the bio-microlens for luminescence investigation. With bio-microlens coupling, upconversion luminescence from trapped single or chained cells can be amplified over 110 times (Fig. 25E). More interestingly, using this bio-microlens optical trapping system, trapping and releasing of fluorescent E. coli cells can be distinguished through tracing of collected luminescence intensity with high signal-to-noise ratios (Fig. 25F). This realtime detection strategy using a bio-microlens can be used for monitoring the trapping process of the bacteria, especially in very limited spaces where direct observation by an optical microscope cannot be performed. Apart from the above-mentioned single bio-microlens and dielectric microlens monolayer obtained through self-assembly, polymeric microlens arrays (MLAs) consisting of abundant hemisphere-like microlens units are commercially available and have been applied to improve photodetector sensitivity in the NIR region (Fig. 26).36 Fig. 26A illustrates a configuration employing MLAs for upconversion luminescence enhancement. By attaching the MLA layer (see the SEM image in Fig. 26B) on top of a film consisting of gold nanorods and upconverting nanocrystals, localized excitation power density in the film can be remarkably enhanced. Moreover, gold nanorods mixed with core-shell-shell (CSS) upconversion nanocrystals can also enhance luminescence intensity due to the large overlap between the plasmonic resonance band with the absorption spectra of lanthanide sensitizers and activators (Nd3þ, Yb3þ, and Er3þ) (Fig. 26C and D). Thus, by simultaneous coupling with dielectric superlensing and surface plasmons, the excitation field surrounding upconversion nanocrystals can be remarkably improved, drastically enhancing luminescence. Selection of MLA of suitable height for nanocrystal coupling can enhance emission up to 6000-fold (Fig. 26E). However, MLAs with longer focusing distances perform worse since the optical hot spot is beyond the upconverting film (see Fig. 26F). For upconverting film co-coupled with the MLA film and Au nanorods, the enhancement factor can reach over 22,000-fold. Considering the large upconversion luminescence enhancement, a multiband responsive NIR photodetector (MLA/Au NR/CSS/MAPbI3) was fabricated using MAPbI3 film as the photon-to-current converting material. By virtue of the spectral overlap between the blue/ green upconversion emission and the absorption spectrum of MAPbI3 film, efficient upconverted photon-to-current conversion can be achieved using the MLA/Au NR/CSS/MAPbI3 photodetector (Fig. 26G). Fig. 26H presents the performance of the prepared

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Fig. 25 (A) Schematic showing the upconversion luminescence enhancement and optical trapping features of the microlens. (B) SEM image of the yeast cell bio-microlenses with an average diameter of 2.0 mm (left). An optical image showing a bio-microlens was trapped at the tip of an optical fiber probe (right). (C) Fluorescent images of outputted light from a fiber probe without and with a bio-microlens. (D) Optical microscope images showing a single E. coli (a2)/a single S. aureus (b2)/and an S. aureus and E. coli (c2) were optically trapped by a fiber probe with a 2 mm yeast cell bio-microlens. Insets of (a2)  (c2) show the corresponding fluorescent images of the trapped E. coli or S. aureus. (E) The upconversion fluorescent intensity of single S. aureus and E. coli was detected by a fiber probe with or without a bio-microlens. (F) Real-time detection of Bs in the trapping and releasing process of the E. coli chain by the fiber probe with and without a bio-microlens. Printed with permission from Ref. Li, Y. et al. ACS Nano 2017, 11, 10672–10680. Copyright © 2017 American Chemical Society.

photodetector. Since the upconversion nanocrystal can be stimulated by multiple excitation bands around 808, 980 and 1540 nm, the fabricated photodetector responds strongly to these excitation bands. Additionally, the device with simultaneous coupling of superlensing and plasmonic resonance features the best NIR photon-to-current performance, especially upon 808-nm excitation. Taken together, dielectric superlensing coupling provides an efficient approach for upconversion luminescence enhancement, holding promise for many emerging applications.

8.11.5

Emerging applications

The study of upconversion nanomaterials has made considerable progress in terms of optical control and luminous efficiency enhancement, especially at single-particle levels. Thus, upconversion nanocrystals have enabled applications in various fields, including super-resolution imaging, lasing, and optogenetics.

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Fig. 26 (A) Experimental design for upconversion luminescence enhancement using dielectric superlensing and surface plasmon coupling. (B) SEM image of the microlens. (C) SEM image for prepared Au nanorods. (D) Absorption spectra of gold nanorods with different aspect ratios. (E) Upconversion luminescence enhancement factors based on different microlenses upon 808 nm, 980 nm, and 1540 nm excitation. (F) Simulation results for light confinement effect of different microlenses. (G) Spectra overlap between upconversion luminescence and perovskite film absorption. (H) Photoresponse of different photodetector samples upon 808 nm, 980 nm, and 1540 nm excitation. Printed with permission from Ref. Ji, Y. et al. Light: Sci. Appl. 9, 184 (2020). Copyright © 2020, Nature publication group.

8.11.5.1

Super-resolution imaging

Compared to commonly used luminescent probes, such as organic dyes and quantum dots, lanthanide-doped upconversion nanocrystals offer many distinct features, including ultrahigh photostability, a nonlinear optical emitting process, a tunable emission band from the deep-UV to the NIR region and ultralong luminescent lifetime, as well as low background noise. Recently, upconversion nanocrystals have been developed as super-resolution imaging nanoprobes based on their unique optical properties.28,29,111–116

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In 2017, upconversion nanocrystals highly doped with Tm3þ activators were applied to stimulated emission depletion (STED) microscopy.29 For conventional STED microscopy employing organic dye molecules, a gaussian excitation beam and a donutshaped depletion beam were applied simultaneously to a sample through the objective lens (Fig. 27A).117–120 The luminescent sample was pumped to an excited state and then generated spontaneous emission at nanosecond scale. Upon addition of the depletion beam, stimulated emission was triggered, and the fluorescence was suppressed in which the excitation and depletion beams overlapped. In such a way, the effective point spread function can be effectively reduced compared to the original gaussian excitation beam (Fig. 27B). Upon further increasing the depletion power, the emitting area was remarkably reduced to sub-20-nm scale. Although organic dye-mediated STED microscopy has been successfully applied to studies of subcellular structures and bioevents, this technique suffers many limitations. To achieve effective fluorescence suppression, an ultra-high power pulsed depletion beam is usually required to compete with the fast spontaneous decay process. Thus, the whole system can be very complicated due to the synchronization of excitation and detection. More importantly, because the organic dye molecules suffer severe photobleaching, especially upon high-power depletion, image brightness quickly declines after several frames. Thus, super-resolution imaging of long-term bio-events is not plausible by traditional STED microscopy. However, these challenges can be overcome using upconversion nanocrystal-based STED microscopy. As illustrated in Fig. 28A, high (8%) and low (1%) Tm-activated NaYF4:Yb/Tm upconversion nanocrystals with intense blue emission were selected for a STED imaging demonstration. Upon 980-nm excitation, both high and low Tm-doped nanocrystals showed strong blue emission. However, when an 808-nm depletion beam was applied, emission from highly Tm-doped nanocrystals was turned off, whereas nanocrystals with low Tm doping remained unaltered. Therefore, blue-emitting upconversion nanocrystals with highly doped Tm activators can be engineered for STED imaging. The mechanism for upconversion luminescence depletion is illustrated in Fig. 28B. Upon 980-nm pumping, multiple band emission from Yb/Tm-codoped NaYF4 nanocrystals was produced through conventional energy transfer upconversion. Importantly, with increased Tm3þ dopants in the host lattice, the distance among adjacent activators can be greatly reduced; strong cross-relaxation would dominate the Tm3þ activator sublattice. As such, activated Tm3þ emitters would be redistributed to an energy level close to 3H4, from which effective stimulated emission could be triggered by 808-nm depletion. Once the 3H4 level is depopulated by the depletion beam, energy transfer upconversion for 455-nm emission could be suppressed. In contrast, for a sample with low Tm3þ activation, cross-relaxation is subsidiary and the depletion beam contributes to upconversion to a less extent. Upconversion nanocrystals with different Tm3þ dopant contents showed large differences in luminescence quenching upon 808-nm depletion (Fig. 28C). Nanocrystals with high-Tm doping exhibited more efficient luminescence suppression, and their saturation power density (0.19 MW cm 2) was significantly reduced to a value two orders of magnitude lower than that of organic dye-mediated STED microscopy. The high performance was attributed to high-quality population inversion induced by cross-relaxation and long luminescence lifetime. STED imaging with 13-nm and 40-nm upconversion nanocrystals was also conducted (Fig. 28D). Resolution was considerably enhanced by increasing the power of the 808-nm depletion beam. Furthermore, selection of smaller upconversion nanocrystals improves imaging resolution slightly. Single-particle imaging results are profiled in Fig. 28E–G. Upon 980-nm excitation, the measured FWHM for a single particle is higher than 400 nm. However, this value can be reduced to around 30 nm once a depletion beam is applied. Similar work was published by Zhan’s group with 10% Tm3þ-activated NaYF4:Yb/Tm upconversion nanocrystals.28 Their investigations show that the optimum doping with Tm3þ activators is 10 mol%, which results in the highest depletion efficiency of 96% (Fig. 29A). For nanocrystals with Tm3þ doping lower than 3 mol%, luminescence enhancement instead of suppression occurs.

Fig. 27 (A) Experimental setup for conventional STED microscopy. (B) Illustration of point-spread function for the excitation, depletion beam, and the effective point-spread function (PSF) upon excitation and depletion beam overlapping. Printed with permission from Ref. Qin, X.; Xu, J.; Wu, Y.; Liu, X. ACS Cent. Sci. 2019, 5, 29–42. Copyright © 2019 American Chemical Society.

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Fig. 28 (A) Upconversion luminescence imaging of 8% and 1% Tm activated-NaYF4:Yb/Tm nanocrystals upon 980-nm excitation and 808-nm depletion. (B) Schematic showing the blue upconversion depletion mechanism of highly Tm doped upconversion nanocrystals. (C) Depletion efficiency versus depletion power of nanocrystals doped with different Tm concentrations. (D) Super-resolution curve of highly doped upconversion nanocrystals at different depletion powers. (E) Comparison of confocal and STED imaging of highly doped upconversion nanocrystals with diameters of 40 nm (top panel) and 13 nm (bottom panel), respectively. (F, G) Corresponding line profiles of upconversion imaging with and without depletion beam. Printed with permission from Ref. Liu, Y. et al. Nature 2017, 543, 229–233. Copyright © 2017, Macmillan Publishers Limited, part of Springer Nature.

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Further increasing the doping amount of Tm3þ ions reduces depletion efficiency slightly, but no less than 80%. Therefore, the high doping of Tm3þ activators is a prerequisite for efficient luminescence suppression. Importantly, Zhan’s group also found that the most efficient wavelength for the depletion beam is 810 nm (Fig. 29B). Importantly, this study proposed a different mechanism for efficient luminescence depletion of highly Tm3þ-activated upconversion nanocrystals. As illustrated in Fig. 29C, the depletion transition is from 1D2 to 1G4, instead of the 3H4-to-3H6 transition proposed by Jin’s group.29 By coupling Tb3þ emitters in core-shell upconversion nanocrystals (NaGdF4:Yb/Tm@NaGdF4:Tb), dual-color STED imaging was achieved. As demonstrated in Fig. 29D, upon 975-nm excitation, multiple nanoparticles can be imaged. By adding suitable filters, blue and green upconversion luminescence from Tm3þ and Eu3þ activated nanocrystals could be directly extracted or achieved through image substruction. After adding the depletion beam (Fig. 29E), dual-color STED imaging can be achieved. For practical application, desmin on the cytoskeletons of fixed HeLa cancer cells was labeled with antibody-conjugated upconversion nanocrystals (Fig. 29F and G). With only 975nm excitation, subcellular structures labeled with NaYF4:Yb/Tm nanocrystals can be clearly imaged. When the 810-nm depletion beam was added, resolution can be effectively enhanced to about 80 nm. This is the first demonstration of successful immunolabeling of fine subcellular structures, as well as the first demonstration of super-resolution cell imaging using upconversion nanocrystals. However, labeling quality using upconversion nanocrystals larger than 10 nm still has ample room for improvement. Not all upconversion nanocrystals meet the critical criteria for STED imaging application. However, all of them can be used for sub-diffraction imaging due to their intrinsic nonlinear optical nature. At the end of the last century, a conceptually different super-resolution approach was developed to conveniently achieve 3D sub-diffraction imaging on a standard confocal microscope without the need for setup modifications and image post-processing. By using super-linear emitters, imaging resolution can be effectively improved since the central part of the scanning beam yields the most significant emission, which results in a region smaller than the size of the beam. Thus, the stronger the super-linearity, the smaller the region of significant emission, and the better the resolution. Since lanthanide photon upconversion is usually governed by two /three/four- and even five-photon processes, its superlinearity is much higher than that of organic dyes that are conventionally used for two-photon imaging. More importantly, the multiphoton absorption process of lanthanide upconversion offers an excitation threshold orders of magnitude lower.121 Thus, lanthanide upconversion nanocrystals present a highly promising class of luminescent materials for sub-diffraction imaging applications. The design of upconversion super-linear excitation-emission (uSEE) microscopy is illustrated in Fig. 30A.116 Upon NIR excitation, the blue emission of NaYF4:Yb/Tm nanoparticles can be collected through the same objective lens and the sub-diffraction image can be directly obtained after one scanning. The point spread function of the excitation beam (976 nm) was obtained by scanning around a gold nanoparticle. The measured FWHM for the axial and lateral directions is 977 nm and 408 nm, respectively (Fig. 30B). For linear emitter imaging using a 977-nm beam, resolution would be extremely low in both x-y and z directions. This situation is quite different when upconversion nanocrystals are selected as emitters. For NaYF4:Yb/Tm nanocrystals chosen in this work, their super-linear optical properties were characterized by measuring their luminescence intensity at 455 nm as a function of the excitation power density (Fig. 30C). In a log-log plot, the value of s represents the slope of the curve at a certain excitation power. At low excitation intensity (< 1 mW mm 2), the nanocrystals are barely luminescent. Further increasing the excitation power results in steep luminescence intensity increase where the particle is in the super-linear regime with s larger than 1. In a power region from around 1 to 2.5 mW mm 2, the slope achieves a constant of  4.1. However, the slope drops quickly to  1 upon further increases in excitation power density. These data suggest that upconversion nanocrystals can work as superior super-linear emitters for uSEE microscopy imaging only in a certain power range. As evidenced in Fig. 30D, the axial and lateral FWHM of excitation PSFs can be reduced to 542 and 216 nm when the excitation power is tuned into the super-linear region. However, when working far away from this region, corresponded values can be extended to 1050 and 576 nm, even larger than those of the original PSF. Considering the large improvement in resolution at 3D, sub-diffraction uSEE 3D microscopy imaging of cells labeled with upconversion nanocrystals is possible (Fig. 30E–I). For a sample labeled with the commonly used Alexa 647 dye, the contrast of the obtained 3D image is poor due to strong autofluorescence (Fig. 30E). The noisy background can be totally eliminated when upconversion nanocrystals are used for labeling (Fig. 30F). Furthermore, when operating in the super-linear region, the imaging resolution of Z-stack imaging is clearly improved (Fig. 30G). Figs. 30H and I show the measured PSFs in 3D upon excitation at different power regimes. Working in the super-linear regime, a 450-nm resolution along the z-axis and a 210-nm resolution in the x-y plane can be achieved. Although the current work succeeds in achieving sub-diffraction imaging, resolution is still quite poor and sub-100 nm lateral resolution imaging is anticipated. Besides designing upconversion nanocrystals that can be excited by short-wavelength lasers, nanocrystals featuring much higher super-linear emission slopes should be developed to further improve uSEE microscopy performance. Similarly, donut beam scanning of upconversion nanocrystals can be applied for deep-tissue imaging with much-improved resolution (Fig. 31).113,122 To achieve this, Yb3þ/Tm3þ-doped NaYF4 nanocrystals were selected. As illustrated in Fig. 31A, upon 980nm excitation efficient energy transfer upconversion was obtained from Tm3þ activators. Beyond blue emissions at 455 and 470 nm, intense NIR emission at 800 nm through two-photon upconversion was detected. These nanocrystals are highly promising for deep-tissue imaging. The emission saturation curve shows an early onset of 800-nm upconversion emissions at low power density

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Fig. 29 (A) Depletion efficiency of NaYF4:Yb/Tm upconversion nanocrystals with different Tm dopant amounts. (B) Depletion efficiency of upconversion luminescence at 455 nm versus depletion beam wavelength. (C) Proposed cross-relaxation-mediated luminescence depletion upon 975nm excitation and 810-nm depletion. (D, E) Dual-color upconversion confocal and STED imaging using NaYF4:Yb/Tm and NaGdF4:Yb/Tm@NaGdF4:Tb nanoparticles upon 975-nm excitation and 810-nm depletion. (F, G) Immunofluorescence labeling and super-resolution imaging of cellular cytoskeleton protein desmin with antibody-conjugated upconversion nanocrystals upon 975-nm excitation and 810-nm depletion. Printed with permission from Ref. Zhan, Q. et al. Nat. Commun. 2017, 8, 1058. Copyright © 2017, the authors.

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Fig. 30 (A) Setup for achieving sub-diffraction resolution imaging using a conventional confocal microscope. (B) Experimentally obtained point spread function of the 977-nm excitation beam by scanning a gold particle. (C) Upconversion luminescence (455 nm) power dependence of NaYF4:Yb/Tm nanocrystals. (D) Measured point spread function of the 977-nm beam by scanning a single upconversion nanocrystal under low and high power excitation. (E) Confocally imaged 3D z-stack of a cell incubated with Alexa 647 dye. (F, G) Confocal images of upconversion nanocrystals inside the cell were taken at power densities of 11.8 (F) and 1.7 mW mm 2 (G). (H, I) Measured axial (H) and lateral (I) resolution of upconversion nanocrystals inside the cell. Printed with permission from Ref. Denkova, D. et al. Nat. Commun. 2019, 10, 3695. Copyright © 2019, the authors.

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Fig. 31 (A) Energy transfer upconversion scheme of Yb/Tm-doped upconversion nanocrystals. (B) Power-dependent feature of prepared upconversion nanocrystals. (C) Point spread function of upconversion nanocrystals upon scanning with a donut beam at different power densities. (D) Corresponding PSF images from (C). (E) Illustration of the mouse liver tissue with 93-mm thickness. (F) Single-particle imaging at different depths in liver tissue. Confocal images from 455-nm emission (left); Confocal images from 800-nm emission (middle); Corresponding NIRES images (right). (G) Normalized attenuation for 455 and 800 nm emissions at different tissue depths. (H) Corresponding FWHM in (F). Printed with permission from Ref. Chen, C. et al. Nat. Commun. 2018, 9, 3290. Copyright © 2018, the authors.

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and a sharply increasing slope, reflecting its non-linear energy transfer upconversion process (Fig. 31B). The decrease in the saturation curve under high-power excitation is due to energy being further pumped from 3H4 to higher energy states. To achieve super-resolution imaging of a single nanocrystal by NIR-emission saturation (NIRES) microscopy, a donut excitation beam was selected to scan the nanocrystal sample (Fig. 31C). Upon increasing the pumping power from 0.1 to 3 MW cm 2, the single-particle PSF (negative) was efficiently reduced (Fig. 31D). The penetration depth and resolution of NIRES imaging through deep tissues were further examined in a mouse liver slice (Fig. 31E). Due to the strong attenuation of visible emission, only 11% of the blue emission remains after passing through a 93-mm slice. In contrast, still nearly 40% of the 800-nm emission can be detected in confocal and NIRES microscopy imaging (Fig. 31F and G). Encouragingly, unlike STED imaging employing two laser beams, aberration distortion can be eliminated with singlebeam excitation. Through 980-nm donut beam excitation, a consistent resolution of  50 nm can be maintained throughout the 93-mm liver slice (Fig. 31F and H). In addition to the abovementioned lanthanide upconversion techniques for super-resolution imaging, upconversion nanocrystals have found applications in structured illumination microscopy with a resolution below 131 nm in the optical window.112 However, the current challenge of upconversion nanocrystal-mediated super-resolution imaging in biological studies lies in the poor capacity for cell structure labeling using nanocrystals over 10-nm. Thus, developing bright upconversion nanocrystals with much smaller sizes (e.g., < 4 nm) and a new protocol for uniform cell labeling are two urgent tasks.

8.11.5.2

Lanthanide upconversion lasing

Compared to organic dyes, quantum dots and transition metals, lanthanide ions feature complex excited states and luminescence lifetimes orders of magnitude longer. Population inversion, which is the usual prerequisite for lasing, can be easily achieved from lanthanide-activated gain materials with a much lower pumping threshold, and by tuning the doped activators, lasing output can be easily programmed from the deep UV to the NIR region. More importantly, multicolor lasing can also be realized by placing multiple or even single types of lanthanide activators in the gain medium. Miniaturized lasers are an emerging platform for generating coherent light for quantum photonics, in vivo cellular imaging, solid-state lighting, and fast 3D sensing in smartphones. Recently, lanthanide-doped upconversion nanocrystals have been utilized to achieve UV-to-NIR lasing action in stand-alone microcavities.33,34,123 Remarkably, the pumping threshold can be reduced to the sub-100 W cm 2 level using a continuous wave (CW) pumping laser source at room temperature. Lanthanide-doped upconversion materials can be easily prepared using several synthesis approaches, such as co-precipitation, thermal decomposition and hydrothermal/solvothermal methods.124–128 Meanwhile, their size can be tuned from sub-5 nm to several micrometers. Microrods with smooth surfaces can usually be obtained using the hydrothermal method. With a refractive index of  1.45, these micro rods are suitable for working as a whispering gallery mode (WGM) lasing cavity for upconversion lasing generation in a stand-alone microcavity. As shown in Fig. 32, by codoping Yb3þ/Er3þ/Tm3þ in a NaYF4 microrod that supports WGM resonance, NIR-pumped white lasing (RGB) output was realized at room temperature.123 The SEM image of prepared NaYF4 micro rods are shown in Fig. 32A. With a radius of  3 mm, these micro rods have six flat surfaces and the ends have a hexagonal pyramid structure. Meanwhile, homogeneous doping of lanthanide sensitizers (Yb3þ) and activators (Er3þ, Tm3þ) can be confirmed by elemental mapping. Fig. 32B shows the numerical investigation of the optical field profile (cross-section) of a NaYF4:Yb/Er/Tm microrod at 654 nm (Er3þ emission). A whispering gallery mode via total internal reflection can be supported by the six flat surfaces of the microrod; thus, single microrod upconversion lasing can be expected. For NaYF4 microrods codoped with Yb3þ/Er3þ(100%/1%), Yb3þ/Er3þ(20%/1%) and Yb3þ/Tm3þ(40%/2%), red (R), green (G) and blue (B) lasing can be detected under 980-nm ns-pulsed excitation, respectively (Fig. 32C). This verifies that the microrod cavity supports lasing at all these wavelengths. By codoping Yb3þ/Er3þ/Tm3þ (40%/0.5%/ 2%) in a single 4-mm microrod, white lasing was achieved upon 980-nm ns-pulsed excitation at room temperature (Fig. 32D inset). The threshold of RGB microrod lasing was effectively reduced by increasing the radius of microrods. However, to maintain stable single-mode lasing, 4 mm was chosen in this work. For lanthanide upconversion, visible light output is the most frequently studied and it is relatively easy to achieve upconversion lasing in this spectral region. As deep ultraviolet lasers are useful for environmental, health and industrial applications, fabrication of cost-effective, compact diode-pumped, solid-state deep-UV lasers using upconversion nanomaterials may revolutionize this field. In 2016, deep-UV upconversion lasing at 311 nm upon 980-nm pumping at room temperature was reported by Chen et al. (Fig. 33).129 The energy migration upconversion mechanism is illustrated in Fig. 33A. To achieve intense deep-UV emission, Gd3þ ions are selected as the activators for 311-nm emission through energy migration upconversion. Upon 980-nm pumping, the incident energy is accepted by Yb3þ sensitizers and then transferred to Tm3þ activators. Subsequently, the accumulated energy on Tm3þ activators can be accepted by the Gd3þ sublattice. With a NaYF4 inert shell, excitation energy can be confined in the Gd3þ sublattice to support intense deep-UV emission at 311 nm. Although deep-UV upconversion emission around 290 nm can be obtained from Tm3þ-activated NaYF4 nanocrystals, its intensity is much lower than that through Gd3þ migration. Moreover, the luminescence lifetime of the excited energy levels for Tm3þ (290 nm, 0.3 ms) is much shorter than that from Gd3þ (310 nm, 4.07 ms). Considering the requirement of high-quality population inversion for lasing, Gd3þ instead of Tm3þ was selected for deep-UV upconversion lasing. NaYF4@NaYbF4:Tm/Gd(1/30%)@NaYF4 nanocrystals were prepared for lasing (Fig. 33B). With a thinner inner shell, energy loss to the host lattice can be effectively suppressed, as evidenced by the significantly prolonged luminescence lifetime of Yb3þ sensitizers and the gradually increased ratio of five-photon upconversion emission to whole emission spectra. The probability distribution function of the excitation energy as a function of space within the Yb shell has also been simulated,

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Fig. 32 (A) SEM imaging and corresponding elemental mapping of a Yb/Er/Tm-coactivated upconversion microrod. (B) Numerical simulation of the optical field distribution inside the NaYF4:Yb/Er microrod. Here, the emitting wavelength of 654 nm is selected for simulation. (C) Room-temperature emission spectra of three individual 3-mm NaYF4 microrods with (left) 100%Yb-1%Er, (middle) 20%Yb-1%Er, and (right) 40%–2%Tm doping concentrations upon 980-nm pulsed pumping. (D) White upconversion lasing from a single microrod codoped with 40%Yb-2%Tm-0.5%Er. Printed with permission from Ref. Wang, T. et al. ACS Photon. 2017, 4, 1539–1543. Copyright © 2017 American Chemical Society.

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Fig. 33 (A) Energy transfer deep-UV upconversion process from Gd3þ and corresponded emission spectra. (B) Upconversion emission intensity versus inner shell thickness (1–17 nm). (C) The probability of finding the excitation energy on the equatorial section of core-shell-shell nanoparticles of varying inner shell thickness. With increasing inner shell thickness (from left to right panels), energy migrates to a larger area, and the probability of finding the excitation energy in the vicinity of the starting point drops significantly. (D) Logarithmic plot of output intensity versus excitation power of a microresonator with Dm ¼ 75 mm. Inset shows the photographic image of the microresonator. (E) The corresponding lasing spectra at different excitation power. (F) Single-mode lasing spectra measured from a microresonator with Dm ¼ 20 mm. Printed with permission from Ref. Chen, X. et al. Nat. Commun. 2016, 7, 10304. Copyright © 2016, the authors.

assuming that the excitation energy hops randomly in the inner shell layer (Fig. 33C). The simulation results show that the probability of finding the excitation energy in a thin Yb shell can be much higher than that in a thicker one, validating favorable energy transfer from Yb3þ to adjacent Tm3þ activators. A drop of silica resin containing upconversion nanocrystals was coated onto a standard optical fiber to fabricate a bottle-like micro resonator laser cavity (Fig. 33D inset). Upon pulsed 980-nm excitation, upconversion emission exhibits a well-defined, Slike nonlinear power dependence, suggesting the occurrence of amplified spontaneous emission and lasing (Fig. 33D). Fig. 33E shows the corresponding emission spectra upon different pumping powers. At low pump power, a broad spontaneous emission band is observed. As the excitation power increases slightly above Pa, a sharp peak can be detected. By further increasing the excitation power above Pth, well-defined sharp peaks with a linewidth < 0.11 nm emerge. Using a smaller resonator (D ¼ 20 mm), single-mode lasing can be realized, but with a higher pumping threshold (Fig. 33F). Although an upconversion micro resonator laser can be achieved at room temperature, the above-mentioned approaches are based on a pulsed pumping laser. To realize lasing upon continuous-wave excitation, working at low-temperature is usually required to combat the heat-induced increases in lasing thresholds. Moreover, CW laser-pumped upconversion lasing can be obtained by selecting a microresonator that weakly absorbs the pumping laser and has a high gain-to-loss ratio. More importantly, upconversion nanocrystals with long excited-state lifetimes and high-quality population inversion can also contribute to CW lasing.

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In 2018, a room-temperature CW microlaser, based on a combination of polystyrene microspheres as the WGM microresonators and Tm3þ-doped energy-looping nanoparticles (ELNPs) as the gain media, was reported (Fig. 34).33 For preparation of WGM microresonators, ELNPs were coated to the surfaces of polystyrene microspheres with a 5-mm diameter. SEM and cross-sectional TEM images of prepared microresonators reveal that a 100-nm thick shell of ELNPs is uniformly coated to resonator surfaces (Fig. 34A). Fig. 34B illustrates energy transfer and emission mechanisms of ELNPs used for upconversion lasing. Upon 1064nm laser pumping (step 1), Tm3þ activators can be populated to the 3F4 state and subsequently the 3F2,3 state. Energy looping could be triggered by cross-relaxation, enabling a fraction of excitation energy from a highly excited Tm3þ ion to be transferred to a neighboring Tm3þ ion in its ground state. This process results in two Tm3þ ions in the same intermediate excited state (step 3). In such a way, the looping process doubles the population of an excited state, thereby populating excited states nonlinearly. Ultimately, a population inversion in the 3H4 state triggers the 800-nm emission (step 4). With a strong population inversion, lowthreshold CW lasing at room temperature can be expected. Fig. 34C and D show the experimentally obtained and simulated WGM spatial distribution in a 5 mm polystyrene bead, respectively. The good match between the experimental lasing spectra and simulated optical modes are shown in Fig. 34E, and high Q-factors about 103–104 for these ELNP-coated microresonators can be calculated.

Fig. 34 (A) SEM and cross-section TEM images of a polystyrene bead coated with upconversion nanocrystals. (B) Upconversion looping mechanism in Tm3þ-doped NaYF4. (C) Wide-field image of a lasing microsphere. (D) Numerical simulation of electric field intensity for the transverse magnetic WGM at 807 nm around a dielectric sphere. (E) Experimental emission spectra of upconversion nanocrystals and simulated NIR spectra of WGM supported by a 5-mm polystyrene microsphere. (F) NIR emission spectra of Tm3þ-doped NaYF4 nanocrystals with pump intensity below, near, and above the lasing threshold. (G) Schematic and wide-field image of the microlaser operation in blood serum. Printed with permission from Ref. Fernandez-Bravo, A. et al. Nat. Nanotechnol. 2018, 13, 572–577. Copyright © 2018. Nature publication group.

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Upon CW 1064-nm laser pumping, lasing was observed from these ELNP microresonators (Fig. 34F). When the pumping power was below the lasing threshold (17 kW cm 2), a broad upconversion emission band was detected. With increased pumping above the threshold, a primary lasing mode at around 807 nm emerged with narrowed bandwidth and increased intensity. The low lasing threshold obtained can be attributed to the high Q-value of the microresonator and the strong population inversion of the NaYF4:Tm3þ gain medium. Interestingly, an application for these stand-alone microlasers in biological media has been demonstrated. After immersing these microresonators in fetal bovine serum (FBS), NIR microcavity emission spectra exhibit an in situ CW lasing threshold of about 250 kW cm 2 (Fig. 34G). The lasing threshold can be further reduced by selecting microresonators with a higher refractive index. The spectral shift of lasing modes can be an effective signature for sensing the changes, such as local index, temperature, and pressure in the environment. More importantly, these microlasers are stable over several hours of continuous excitation at high power (300 kW cm 2) and show promising applications for subcellular imaging, tracing, and probing. In 2019, room-temperature CW upconversion microlasers with a greatly reduced threshold (70 W cm 2) were achieved using plasmon nanocavities (Fig. 35).34 A plasmonic array consisting of Ag nanopillars (80-nm diameter, 50-nm height) arranged in a square lattice with a 450-nm periodicity was fabricated as the resonator (Fig. 35A). A solution of NaYF4:Yb/Er nanoparticles was drop-cast onto the Ag array to form a 150-nm thick film. After nanoparticle coating, a resonance band of the fabricated nanocavity was measured around 660 nm, which overlapped the upconversion emission band of Er3þ activators. Fig. 35B shows the simulated electric field intensity profile at 664 nm. The simulation result indicates that the near-field enhancement (| E |2/|E0 |2) around the nanopillars can be higher than 1000-fold. With strong plasmonic coupling, upconversion lasing around 660 nm can be successfully observed with a record low threshold of 70 W cm 2. Fig. 35C shows the upconversion emission spectra as a function of pumping power at 980 nm. Upon increasing the pumping power above the threshold, a significant increase in the rising slope of the input-output curve and simultaneous linewidth narrowing of the lasing mode to < 1 nm can be measured. Apart from low thresholds, Ag plasmonic array nanocavities offer directional coupling, in contrast to the aforementioned spherical WGMs, in which input coupling is nontrivial and emission is less directional. According to Fig. 35D, upon 980 nm laser excitation, beam-like upconversion lasing with a low divergence angle of 0.5 was obtained. Moreover, the output lasing beam was polarized in the same direction as the incident pump and the all-solid-state system can work at room temperature for more than 6 h under continuous pumping. Therefore, this plasmonic upconversion nanolaser platform offers prospects to realize lab-on-a-chip quantum-optical technologies.

Fig. 35 (A) Schematic showing upconversion nanocrystal coating onto an array of Ag nanopillars. (B) FDTD simulation of the | E|2 plot for the Ag nanopillar array at resonance. (C) Power-dependent lasing spectra of Yb/Er-doped NaYF4 nanocrystals placed on top of Ag nanopillars that resonate at 664 nm. (D) Schematic of the experimental setup used to measure the far-field beam profile of the upconversion laser. Printed with permission from Ref. Fernandez-Bravo, A. et al. Nat. Mater. 2019, 18, 1172–1176. Copyright © 2019, Nature publication group.

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To date, lanthanide upconversion micro-nanolasers working at room temperature with an ultra-low threshold have been realized. This technology will find applications in nanophotonics, biophotonics, and even quantum optics.

8.11.5.3

Upconversion optogenetics

Membrane ion channels are responsible for propagation and integration of electrical signals in the muscular, nervous, and many other systems. Recently, one technology termed “optogenetics,” has been utilized for manipulation of neural activities optically with high specificity and millisecond-timescale resolution. Despite remarkable achievements, currently established optogenetic tools are mainly confined to the visible window (Fig. 36A), which significantly limits in vivo applications due to the high attenuation of visible light in biological tissues.40,130–134 Upconversion nanocrystals have long been applied to deep-tissue imaging due to their high brightness in the UV-Vis-NIR region upon excitation with NIR light, which features reduced tissue attenuation (Fig. 36B).135–140 Furthermore, upconversion emission bands can be designed to overlap with targeted channelrhodopsins (ChRs) or other light-sensitive proteins (Fig. 36C).45 Therefore, in recent years, upconversion nanocrystals have also been developed as emerging nano-illuminators to selectively control the activity of specific membrane ion channels in order to study physiological and pathophysiological processes39,41,44,141–143 (Fig. 36D). Upconversion nanocrystal-mediated optogenetics at the single-cell level has been demonstrated by many research groups. For example, by embedding upconversion nanocrystals with strong blue emission into polymeric scaffolds, optogenetic control of activities of neuron cells located on the scaffold surface has been successfully realized upon 980-nm NIR excitation (Fig. 37).43 First, a polymeric upconversion nanocrystal hybrid scaffold was fabricated by spin-coating a solution containing upconversion nanocrystals onto a substrate (Fig. 37A). Channelrhodopsin-2 (CHR2) is one microbial ion channel that conducts cation influx upon blue light illumination (450–475 nm). Thus, NaYF4:Yb/Tm@NaYF4 core-shell nanocrystals were selected to trigger the ChR2. As shown in Fig. 37B, upon 980-nm excitation, strong blue upconversion luminescence around 475 nm was observed from the hybrid scaffold. To study NIR-triggered optogenetics, hippocampal neurons were cultured on the surface of the prepared hybrid scaffold and their activities were investigated upon 980-nm photoexcitation (Fig. 37C). The SEM image in Fig. 37D clearly shows the success of culturing neurons on the polymer scaffold and the high loading of upconversion nanocrystals in the scaffold. In current-clamp recording mode, nerve impulse generation was checked by applying brief pulses of light to neurons (Fig. 37E). It can be seen

Fig. 36 (A) Optogenetic tools for membrane ion channel manipulation and working spectra of commonly selected light-sensitive channelrhodopsins and proteins. (B) Schematic illustration of the structure and working mechanism of lanthanide-doped upconversion nanocrystals. (C) Photographic images and upconversion luminescence from different lanthanide-activated upconversion nanocrystals dispersed in cyclohexane upon 980-nm NIR excitation. (D) Illustration of selective activation of channelrhodopsins and light-sensitive proteins by designing upconversion nanocrystals with strong emission in a certain spectral range. (A, B, D) Printed with permission from Ref. Wang, Z.; Hu, M.; Ai, X.; Zhang, Z.; Xing, B. Adv. Biosyst. 2019, 3, 1800233. Copyright © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. (C) Printed with permission from Ref. Liang, L. et al. Nat. Commun. 2019, 10, 1391. Copyright © 2019, the authors.

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Fig. 37 (A) Experimental processes of embedding blue-emitting upconversion nanocrystals into a cell-supporting polymeric scaffold. (B) Photoluminescence spectrum of Yb3þ/Tm3þ-doped NaYF4 nanocrystals upon 980-nm excitation. Inset is a photographic image of the hybrid scaffold upon 980-nm excitation. (C) Schematic application of the upconversion nanocrystal-embedded polymeric scaffold for optogenetic neuronal activation. (D) SEM imaging of the distribution of a hippocampal neuron cell cultured on the surface of the hybrid scaffold. (E) Membrane action potentials induced by blue (470 nm, top panel) and NIR light (980 nm, bottom panel), respectively. (F) Traces showing repetitive action potentials in a currentclamped hippocampal neuron evoked by 1-, 5-, and 10-Hz train of light pulses from blue (top row) and NIR light (bottom row). Printed with permission from Ref. Shah, S. et al. Nanoscale 2015, 7, 16571–16577. Copyright © 2015 Royal Society of Chemistry.

that both blue and NIR excitation can elicit nerve impulses. Importantly, impulses elicited by NIR light are comparable to those elicited with 470-nm blue light, implying that blue upconversion emission is strong enough to activate ChR2 and drive neuronal depolarization beyond the nerve impulse firing threshold. Moreover, time-locked and sustained trains of impulses can also be generated upon 980-nm illumination of ChR2-expressing neurons on the polymeric scaffold (Fig. 37F). Therefore, this work provides an excellent demonstration of rationally designed upconversion-mediated optogenetics for advanced neural applications. Beyond working at the single-cell level, in 2016, in vivo control of the movement of ChR2- expressing nematodes with NIR optogenetics was also demonstrated (Fig. 38).44 In that study, to activate ChR2 expression in worms, silica-coated NaYF4:Yb/Tm

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Fig. 38 Schematic showing differences in movement behavior in worms when exposed to NIR light with (A) and without (B) the presence of blueemitting upconversion nanocrystals. (C) Effect of the upconversion nanocrystal concentration on the percentage of worms showing reversal phenotype upon NIR illumination. (D) Percentage of worms showing reversal phenotype upon different incubation and excitation situations. (E) Images showing NIR -induced reversal phenotype of worms with upconversion nanocrystals. Printed with permission from Ref. Bansal, A.; Liu, H.; Jayakumar, M. K. G.; Andersson-Engels, S.; Zhang, Y. Small 2016, 12, 1732–1743. Copyright © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

nanocrystals with strong blue emission were prepared and consumed by worms. With ChR2 expression in neurons, illumination with blue light to activate ChR2 was interpreted as “touch,” resulting in an aversive response. Thus, a forward worm movement should be observed upon NIR excitation, once upconversion optogenetics is at work. As shown in Fig. 38A, without nanocrystal uptake, the movement of worms is not disturbed by 980-nm stimulation. But the opposite behavior could be observed when UCNPs are introduced (Fig. 38B). With a greater consumption of upconversion nanocrystals, a higher probability of triggering opposite behavior of worms expressing ChR2 can be observed (Fig. 38C). Control experiments confirm that blue upconversion emission induced by NIR excitation is required for ChR2 activation (Fig. 38D). The opposite response of a worm that consumed upconversion nanocrystals was immediately observed once NIR stimulation was applied (Fig. 38E), indicating the effectiveness of upconversion-mediated in vivo optogenetics. Similarly, in vivo upconversion optogenetics in zebrafish have also been demonstrated upon 808-nm NIR stimulation (Fig. 39).46 Upconversion nanocrystals sensitized with Yb3þ ions are usually stimulated using a 980-nm laser. However, the 980-nm excitation beam can be severely attenuated by biological tissues through scattering and absorption. To achieve deeper penetration and less overheating, Nd3þ-sensitized upconversion nanocrystals can be selected with greatly mitigated attenuation at 800 nm. In that work, NaYF4:Yb/Tm/Nd@NaYF4:Nd nanocrystals were prepared to generate blue (Tm3þ) upconversion luminescence upon 808-nm excitation. Fig. 39A shows the protocol of metabolic labeling for site-specific covalent localization of NIRresponsive nanocrystals on cell membranes to achieve precise regulation of ion channel function. By simply feeding the Ac4ManNaz precursor, N3-tagged glycans were introduced to human embryonic kidney 293 (HEK293) cells. A DBCO-conjugated fluorophore, working as a biorthogonal linkage, triggered a copper-free click reaction in Ac4ManNAz-treated HEK293 cells (Fig. 39B). After surface modification of Nd-sensitized nanocrystals with polyacrylic acid (PAA), these nanocrystals were covalently anchored on the membrane through DBCO coupling. To accept blue upconversion emission, ChR2 was engineered on cell surfaces to mediate the influx of essential signaling ions in the cytoplasm (Fig. 39B). To regulate membrane channel activities in vivo in zebrafish using 808-nm light, a Ca2þ indicator (Rhod-3 AM) was used to monitor the cation influx in zebrafish. As shown in Fig. 39C, upon 808-nm illumination for 2 h, strong fluorescence can be recorded from zebrafish treated with DBCO/Cy5.5-upconversion nanocrystals. However, no distinguishable change in fluorescence can be found from zebrafish without NIR illumination. Notably, this long-term experiment cannot be performed under 980-nm irradiation due to the limitation of severe overheating.

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Fig. 39 (A) Labeling strategy for site-specific localization of upconversion nanocrystals on cell membranes for precise optogenetic modulation. (B) Imaging of membrane localization of upconversion nanocrystals through glycan metabolic strategy. (C) In vivo optogenetic activation of ChR2 in zebrafish. Printed with permission from Ref. Ai, X. et al. Angew. Chem. Int. Ed. 2017, 56, 3031–3035. Copyright © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

In another step forward in 2018, deep-brain neuron stimulation using upconversion nanocrystal-mediated optogenetics in mouse brain was reported (Fig. 40).39 Upconversion nanocrystal-mediated NIR optogenetics evoked dopamine release from genetically tagged neurons and induced brain oscillations through activation of inhibitory neurons. Such upconversion optogenetics also silenced seizure by inhibition of hippocampal excitatory cells and triggered memory recall. In a typical experiment, an adeno-associated virus (AAV) encoding ChR2-EYFP (enhanced yellow fluorescent protein) was injected into ventral tegmental area (VTA) of tyrosine hydroxylase-driven Cre recombinase transgenic mice to achieve Credependent expression of ChR2 in dopamine neurons Subsequently, blue-emitting NaYF4:Yb/Tm@NaYF4@SiO2 nanocrystals were injected into the VTA (Fig. 40A and B). Upon 980-nm excitation, photocurrents and evoked spikes in VTA DA neurons were detected, indicating successful triggering of membrane depolarization (Fig. 40C and D). Notably, multiple spikes were elicited by trains of NIR illumination at 10 to 50 Hz, showing no significant dependence on excitation frequency (Fig. 40E). Furthermore, in vivo utility of NIR optogenetics was tested through viral delivery of ChR2, followed by upconversion nanocrystal injection in TH-Cre mice with TA DA neurons sensitized to transcranial NIR stimulation (Fig. 40F). Phasic spike activity of VTA DA neurons was reflected by striatal DA transients. As shown in Fig. 40G, a 2-s NIR stimulation can trigger striatal DA release lasting for more than 15 s while no significant DA release can be detected in control mice without NIR stimulation and nanoparticle injection, or ChR2 expression. Upconversion nanocrystals embedded in implantable microdevices (e.g., glass microneedles and flexible PEGDA waveguides) for remote in vivo optogenetic control of neuronal functions have also been reported. Although implantation of millimeter-sized devices in the mouse brain is relatively invasive, stimulation efficiency can be remarkably improved, benefiting from high nanoparticle loading in these microdevices and the waveguiding of the stimulating light. An exciting demonstration of upconversion nanocrystals for night vision in an animal model was reported in 2019 (Fig. 41).144 Upon surface conjugation of concanavalin A protein (ConA), green-emitting NaYF4:Yb/Er@NaYF4 nanocrystals bind to glycoproteins on the outer photoreceptor segment (Fig. 41A). After injection into the subretinal space, surface-modified nanocrystals remain tightly bound to the inner and outer segments of both rods and cones (Fig. 41B–D). Therefore, a layer of nanoantennae capable of converting NIR excitation to green emission can be constructed. As evidenced in Fig. 41E, strong pupil constriction of nanocrystalinjected mice can be triggered upon 980-nm illumination. However, no response was detected in non-injected control mice,

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Fig. 40 (A) AAV-DIO-ChR2-EYFP was injected into the VTA of TH-Cre transgenic mice for Cre-dependent expression of ChR2 in DA neurons. (B) Upconversion emission spectra of NaYF4:Yb/Tm@SiO2 nanocrystals upon 980-nm excitation. Inset shows the upconversion emission intensity of upconversion nanocrystals as a function of excitation power density at 980 nm. (C) Voltage-clamp traces of neurons from VTA slice in response to NIR stimulation at various intensities. (D) Photocurrent amplitude as a function of NIR stimulation intensity. (E) Current-clamp traces of ChR2expressing DA neurons in response to NIR pulses at different frequencies. (F) In vivo transcranial NIR stimulation of the VTA in anesthetized mice. (G) Transient DA concentrations in the ventral striatum in response to transcranial VTA stimulation under different conditions and corresponding cumulative DA release after the start of transcranial stimulation. Printed with permission from Ref. Chen, S. et al. Science 2018, 359, 679–684. Copyright © 2018 American Association for the Advancement of Science.

indicating that injection of nanocrystals enhances NIR vision. Furthermore, the light-dark box experiment (Fig. 41F and G) shows that nanocrystal-injected mice exhibit a significant preference for a dark box, whereas non-injected control mice cannot distinguish the NIR-illuminated dark box. By introducing Nd3þ or Er3þ sensitization at 730, 800, 860, and 1532 nm, current NIR vision can be further broadened. Beyond vision enhancement, this approach may serve as an integrated, light-controlled nanosystem for visual function correction.

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Fig. 41 (A) Surface modification of ConA-functionalized, photoreceptor-binding green-emitting upconversion nanoparticles (pbUCNPs). (B) Illustration of sub-retinal injection of modified upconversion nanocrystals in mice. (C–D) Overlaid green (pbUCNPs)/violet (rods) and green (pbUCNPs)/red (cones) channel fluorescence images of retina from PBS-injected mice (C) and pbUCNP-injected mice (D). Scale bars, 10 mm. (E) Images of pupil constriction from non-injected controls and pbUCNP-injected mice under 980-nm illumination. (F) Light-dark box experiment diagram. (G) Preference index for dark box under three different lightbox conditions (light off, 980 nm, and 535 nm). Printed with permission from Ref. Ma, Y. et al. Cell 2019, 177, 243–255. Copyright © 2019 Elsevier Inc.

8.11.6

Conclusion

Fundamental study of photon upconversion in nanomaterials has been explored, and customization of upconversion phosphors with a targeted physical dimension, morphology, and specific optical properties is no longer a challenging task. In the next decade, one crucial mission will be to optimize available materials further and promote their practical applications. New mechanisms based on lanthanide-doped upconversion may be developed for super-resolution imaging that offers low power and fast imaging in deep tissues. Moreover, small (< 5 nm), bright upconversion nanocrystals are anticipated to improve cell labeling. For nano/microcavity

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lasing, directional, single-mode laser output with programmable emission wavelengths upon low-power CW beam pumping at room temperature is still challenging. Upconversion nanocrystals with orthogonal excitation/emission features may be an alternative to achieve this goal. Furthermore, improving the NIR-to-Vis efficiency of upconversion nanocrystals will facilitate upconversionmediated optogenetics in deep tissues. In addition to consolidating the above applications, we should further explore applications of lanthanide-doped upconversion nanomaterials in nanophotonics and biophotonics. With a better understanding of the stimulus-response model of emission behavior, disruptive technical innovations may enable significant advances in conversion efficiency and sensitivity.

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Lanthanides as luminescence imaging reagents

Laura France´s-Sorianoa, Niko Hildebrandta,b,c, and Loı¨c J. Charbonnie`red, a nanoFRET.com, Laboratoire COBRA (Chimie Organique, Bioorganique, Réactivité et Analyse - UMR6014 & FR3038), Université de Rouen Normandie, CNRS, INSA, Normandie Université, Rouen, France.; b Université Paris-Saclay, Saint-Aubin, France; c Department of Chemistry, Seoul National University, Seoul, South Korea; and d Équipe de synthèse pour l’analyse (SynPA), Institut Pluridisciplinaire Hubert Curien (IPHC), UMR 7178, CNRS/Université de Strasbourg, ECPM, Strasbourg, France © 2023 Elsevier Ltd. All rights reserved.

8.12.1 8.12.2 8.12.2.1 8.12.2.2 8.12.2.3 8.12.3 8.12.3.1 8.12.3.2 8.12.3.3 8.12.3.4 8.12.4 8.12.4.1 8.12.4.2 8.12.4.3 8.12.4.4 8.12.4.5 8.12.4.6 8.12.5 References

Introduction Spectroscopic properties of luminescent lanthanide complexes Basic spectroscopic properties of lanthanide complexes Förster resonance energy transfer (FRET) with luminescent lanthanide compounds Anti-Stokes properties of lanthanide compounds Lanthanide compounds as luminescent probes, the choice of the physical state Lanthanide complexes as luminescent probes Lanthanide nanoparticles (NPs) as luminescent probes Lanthanide metal-organic frameworks (MOFs) as luminescent probes Other materials for lanthanide-based luminescence imaging Lanthanide-based luminescence imaging Introduction Continuous wave (CW) or steady-state microscopy Time-resolved (TR) or time-gated (TG) microscopy Multiplexed microscopy Near infrared (NIR) microscopy Upconversion (UC) microscopy Conclusion

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Abstract Since their very first discovery, the luminescence properties of lanthanide (Ln) compounds have attracted the interest of researchers worldwide and have found numerous applications, particularly as very effective tools for luminescence imaging. Covering the whole spectral domain from the UV to the NIR, depending on the utilized Ln, this series of elements offers a unique palette of colors to play with. This article aims at summarizing the properties that make Ln compounds so effective for luminescence imaging, what kind of imaging applications can be developed based on these properties and how the chemists can play with the chemical composition of lanthanide compounds to optimize the imaging capabilities.

8.12.1

Introduction

Lanthanides (Ln) are the series of 15 elements ranging from Lanthanum (Z ¼ 57) to Lutetium (Z ¼ 71), obtained by the successive filling of the 4f shell in the periodic table. Although the IUPAC recommended group name is “lanthanoids,” to avoid confusion with other groups where the “ide” ending indicates a negative charge, in this chapter we have followed the still common practice in using the old group name, “lanthanides.”. When associated to Yttrium and Scandium, the family is also referred to as the rare earths. Although the beginning of the discovery of Ln dates back to the end of the 18th century,1 observation of luminescence phenomena attributable to Ln are as old as the history of fluorescence, because the fluorescence observed in fluorite (calcium fluoride), from which the term fluorescence was derived from George Stokes, appeared to be due to samarium and europium impurities.2,3 In contrast to organic compounds or coordination compounds from the lower series, the very specific and exceptional luminescence properties of Ln cations arose from their 4f shell and the first section of this review will be devoted to the description of these fundamental properties. In the next section, the different physical states of the Ln based luminescent probes will be examined in parallel to their potentials as luminescence probes for imaging. Finally, the use of Ln probes in luminescence imaging will be exemplified by discussing various recent applications to help the reader to direct their choice of Ln probes as a function of the targeted imaging exercise.

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Spectroscopic properties of luminescent lanthanide complexes

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Basic spectroscopic properties of lanthanide complexes

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The luminescence properties of Ln compounds are essentially arising from electronic transitions within the 4f shell, as for Ln3 þ cations, the general electronic configuration is [Xe]4fn (n ¼ 0–14). As can be observed in Fig. 1A,4 the radial expansion of these orbitals is weak and 4f orbitals are shielded from the chemical environment by filled 5s and 5p orbitals, resulting in very weak perturbations due to the ligand or solvent molecules around the ion. As a consequence, the transitions, which can be observed in absorption or emission spectroscopies, are energetically narrow and one deals with “atom like” bands. A typical example is shown in Fig. 1B for the emission bands of Tb3þ and Eu3þ that are observed in the visible domain. For the most intense Tb emission band at 545 nm (5D4 / 7F5 transition), the full width at half maximum is only circa 440 cm 1, while the corresponding value is circa 1450 cm 1 for Rhodamine 6G, a typical organic fluorescent compound, illustrating that the emission bands of organic compounds are three to four times broader than those of Ln compounds. In the typical case of Eu3þ, the highest energy emission band (the 5D0 / 7F0 transition) is observed at circa 17250 cm 1 (around 580 nm), and perturbations associated to the ligand field effect, also called nephelauxetic effect,5 are generally of the order of tens to thousands of cm 1 units.6 As for all chemical entities, the intensities of these transitions are governed by different selection rules.7 These transitions from an initial state to an excited state (denoted with a star hereafter) are due to interactions between the electronic cloud of the ion and electric or magnetic fields, resulting in electric dipole transitions or in magnetic dipole transitions having different selection rules. For both kinds of transitions, the rule concerning the change of spin quantum number S are the same, i.e., D S ¼ 0. Concerning Laporte’s rule, the change in the orbital angular momentum quantum number L, transitions with D L  6 are allowed (except if L or L* ¼ 0) for electric dipole transitions and D L ¼ 0 for magnetic dipole transitions. Finally, concerning the total angular momentum quantum number J, the rule is D J  6 (D J ¼ 2, 4 or 6 if J or J* ¼ 0) for electric dipole transitions and D J ¼ 0,  1 for magnetic dipole transitions (but transitions between level with J ¼ J* ¼ 0 are forbidden).8 Magnetic and electric dipole transitions generally have the same order of magnitude, but for both of them the numerous forbidden characters lead to very weak molar absorption coefficients rarely exceeding few units of M 1 cm 1, with a noticeable exception for the 2F5/2 / 2F7/2 transition of Yb.9 Considering the weakness of the absorption coefficients of f-f transitions, the luminescence of Ln cations might have encountered far less interest if two important points were not existing. The first one is that in some instances, charge transfer transitions may be observed. These include the possible transfer of electrons from the 4f orbitals to 5d ones, as can be observed for example with Ce3þ compounds, for which the charge may be exchanged between the 4f orbitals and those of a neighboring ligand, resulting in metal-to-ligand or ligand-to-metal charge transfer transitions (respectively MLCT and LMCT transitions). In both cases, the selection rules lead to far larger absorption probabilities, but the electronic mixing between the 4f and 5d or ligand orbitals made these transitions highly dependent on the chemical environment, resulting in broad absorption or emission bands. The second important point that boosted the field of Ln luminescence was the discovery of Weissman in 1942 that the coordination of some aromatic ligands with Eu3þ cations in solution allows an indirect excitation of the Eu cation by excitation through the ligand absorption bands.10 This phenomenon was later called the antenna effect,11 by which photons are collected by the organic antenna and their energy is transferred to the excited states of Ln cations. The basic principle of the antenna effect is represented by the Perrin-Jablonski diagram of Fig. 2 and illustrated for the well-known [Eu(dipic)3] complex (dipic ¼ 2,6dipicolinate).12 The first step of the antenna effect is the excitation of the ligand. This is generally achieved with UV light for Ln emitting in the visible region (Eu, Tb, Dy, and Sm) or in the near-infrared (NIR) region,13,14 but the light can be extended up to the visible region for

Fig. 1 (A) Representation of the radial charge densities of the 4f, 5s, 5p, and 6s electrons calculated for the Gdþ cations. (B) Typical emission spectra of Tb3þ (green) showing the narrow 5D4 / 7FJ (J ¼ 6 to 3) emission bands, of Eu3þ (red) with the 5D0 / 7FJ (J ¼ 0 to 4) transitions, and of Rhodamine 6G (yellow), a fluorescent compound. (A) Reproduced with permission from Freeman, A. J.; Watson, R. E. Theoretical Investigation of Some Magnetic and Spectroscopic Properties of Rare-Earth Ions. Phys. Rev. 1962, 127 (6), 2058–2075. doi:10.1103/PhysRev.127.2058. Copyright 1962 American Physical Society.

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Lanthanides as luminescence imaging reagents

Fig. 2 (A) Principle of the antenna effect in the [Eu(dipic)3] complex. (B) Perrin-Jablonski diagram representing the different steps leading to emission from the Ln ion within the [Eu(dipic)3] complex.

Eu15 or for Ln emitting in the near-infrared (NIR) region,16 such as in the case of Yb porphyrins.17 Absorption of light by the ligand leads to the population of the ligand centered excited singlet state, which can relax to the ground state by emission of light, affording fluorescence of the complex, or by non-radiative deactivation (knr). Alternatively, the excited singlet state can transfer its energy to the ligand centered excited triplet state by intersystem crossing (ISC). This phenomenon is amplified by the presence of heavy atoms on the ligand, which is typically the case for Ln complexes, as the presence of these heavy atoms favors spin-orbit coupling and the mixing between singlet and triplet states. At that stage, the de-excitation of the triplet state can be obtained by radiative relaxation to the ground state (i.e., phosphorescence), non-radiative loss of the excess energy (k0 nr), or by energy transfer (ET) to the Ln centered excited states to form Ln*L. Once in its excited state, the Ln cation can at its turn relax to the ground state by emission of photons at a rate kLn or lose its excess energy by non-radiative de-excitation (knrLn). Possibly, if the energy difference between the ligand centered triplet state and the Ln excited state is too small, an energy back-transfer can be observed from Ln* to the triplet state. This energy difference was assessed by Latva and co-workers to be of 1850 cm 1 in the case of Tb,18 and was proposed to be larger than 3500 cm 1 for fast and irreversible triplet-to-Ln* energy transfer.19 A deep insight into the energy transfer mechanisms in Ln complexes, including ligand to metal energy transfer (antenna effect) and the other energy pathways was recently proposed by Tanner and co-workers.20 If this global scheme is generally admitted, it is certainly not the only one and the sensitization pathways can also occur directly from the singlet state,21,22 or even from charge transfer states, generally arising from MLCT when d block metal ions are present in the complexes such as Ru2þ,23 Pt2þ,24 Os2þ,25 or Cr3þ.26 Whatever the pathway, the excitation of the compound is generally occurring at a large energy difference from the Ln emitting states, which can be referred to as large pseudo (absorbing and emitting entities are different) Stokes shift. Because emission and absorption are not originating from the same excited state and if one considers only f-f transitions, the actual Stokes shift becomes very small as a result of the weak perturbations of 4f electrons from the ligand field. Nevertheless, this large energy difference between excitation and emission can be used as an asset in spectroscopy and for imaging with Ln, in contrast to most organic fluorescent compounds, which only display small Stokes shifts and for which the absorption and emission signals are significantly overlapping, thereby decreasing their effective potentials as luminescent probes. Basically, the luminescence efficiency of a compound can be defined by the luminescence quantum yield F: f¼

number of emitted photons number of absorbed photons

(1)

There exist two main techniques to determine the luminescence quantum yield of a compound. When the luminescence spectrometer is equipped with an integration sphere able to collect the light emitted in all directions, F can be determined directly by comparing the number of photons emitted by the sample to the number of absorbed photons obtained by comparing the intensity of the signal of the excitation source in the absence and in the presence of the luminescent compound.27 In absence of an integration sphere, F can be determined by comparison with a reference sample with a known Fref. For a compound x, the luminescence quantum yield Fx is obtained with Eq. (2):   R 1  103 ref lcref Ix ðlÞdl I0;ref n2x R   fx ¼ fref  2    (2) Iref ðlÞdl I0;x nref 1  103 x lcx in which n is the refractive index of the solvent used, I(l) represents the fluorescence intensity spectra corrected for the wavelength dependence of the detector, I0 is the intensity of the lamp at the excitation wavelength and 3 lc corresponds to the absorbance at the excitation wavelength corresponding to the product of the absorption coefficient 3 at the excitation wavelength multiplied by the length of the cell, l, and the concentration of the sample, c. For most recent spectrometers, the intensity of the lamp is

Lanthanides as luminescence imaging reagents

489

automatically corrected to have a constant value over the spectral domain of the lamp (I0ref/I0x ¼ 1). In case of optically diluted samples (3 lc  0.05),3 Eq. (2) is simplified to: R Ix ðlÞdl 3 ref lcref n2  fx ¼ fref  2x  R (3) 3 x lcx Iref ðlÞdl nref By recording the absorption and luminescence spectra of the compounds and of the reference sample, one can thus have access to the luminescence quantum yield. Considering the entire antenna effect, the overall luminescence quantum yield of a Ln compound, Fov, is the product of the sensitization efficiency, hsens, which represents the capacity of the antenna to transfer its energy to the Ln excited state, multiplied by the Ln centered quantum yield, FLn. fov ¼ hsens  fLn (4) Emission of photons, will then be governed by an exponential decay law: Iðt Þ ¼ Ið0Þ  e

t= sobs

where the observed rate of de-excitation (1/sobs) is the sum of the rates of radiative and non-radiative (nr) processes: X 1 1 ¼ þ knr sobs srad

(5)

(6)

sobs is the observed luminescence lifetime, which can be determined experimentally using time-resolved (TR) spectrometers. From Eq. (1), the luminescence quantum yield can be represented as the ratio of the rates of the radiative processes, krad, over the sum of all de-excitation processes, should they be radiative or non-radiative (nr). The luminescence lifetime and the metal centered quantum yield are then related to each other by equation krad s P fLn ¼ ¼ obs (7) krad þ knr srad In the case of Ln compounds, the forbidden character of most of the selection rules seen above results in mostly forbidden electronic transitions, and as a result, generally very long radiative lifetimes of the order of micro to milliseconds (up to 9.7 and 9.0 ms for the visible emission bands of Eu and Tb respectively for their aqua ions).28 For compounds having reasonable quantum yields, this represents luminescence lifetimes of the same order of magnitude, which affords another asset for Ln compounds compared to conventional organic or inorganic dyes, which possess luminescence lifetimes in the nano to microsecond range. This is the basis of one of the largest applications of luminescent Ln compounds, namely TR spectroscopy and microscopy. It is important to notice here that if the radiative lifetime can be long for Ln, the observed lifetime, which is the useful parameter for experimental purposes, can be substantially shortened in the presence of very efficient non-radiative processes. This is typically the case for NIR emitting Ln cations as a result of efficient energy transfer to the vibrational overtones of neighboring molecules (vide infra). At that point, the chemical composition of the compound should play a significant role for a targeted imaging application and one should prefer a nanoparticle (NP) system, in which the ion is protected from the environment. For instance, the luminescence lifetime of Yb complexes in water rarely exceeds tens of ms,29 whereas it easily reaches the ms timescale in solids.30 Considering Eq. (7), some authors tried to decrease the radiative lifetimes of Ln, in particular for Yb, to increase the Ln centered luminescence quantum yield, by playing with the ligand field.31 However, the real effect of the ligand field on srad is still scarcely understood and a systematic influence is up to now not predictable. Unfortunately, srad can be determined experimentally only for few Ln ions.32 This is typically the case for Eu and Yb ions. When the absorption spectrum of the electronic transition can be determined, as for the 2F5/2 ) 2F7/2 transition of some Yb compounds,33 the use of Einstein’s coefficient for spontaneous emission leads to: Z 8pcn2 n2ul gl 1 ¼ 2303   3 ðnÞdn (8) NA gu srad In Eq. (8), c is the speed of light in vacuum (3.108 m s 1), n is the refractive index of the medium, NA is Avogadro’s constant (6022  1023 mol 1), nul is the barycentre of the transition (in cm 1), gl and gu are related to the degeneracies of the lower (l, Jl ¼ 7/ 2 for Yb) and upper (u, Ju ¼ 5/2 for Yb) excited states (gi ¼ 2Ji þ 1), and 3 (n) is the molar extinction coefficient (in M 1 cm 1) at the wavenumber n (in cm 1). The calculation of srad and the measurement of sobs then allow for the determination of the Yb centered quantum yields.9 This approach has also been used for Nd3þ and Er3þ.32 In the case of Eu, the measurement of the emission spectrum corresponding to the 5D0 / 7FJ transitions (J ¼ 0–4) spanning from circa 580 to 750 nm (see Fig. 1) allows for the determination of srad.32 This calculation is based on the assumption that the magnetic dipole allowed 5D0 / 7F1 transition of Eu is independent of the environment and that the 5D0 / 7F5,6 transitions are weak and negligible. Then, the shape of the emission spectrum allows to calculate the branching ratios, i.e., the relative intensity of each emission band and to determine the ratio of the total intensity Itot over that of the 5D0 / 7F1 transition, I1, from which one can deduce srad with:   1 Itot (9) ¼ AMD n3 srad I1

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Lanthanides as luminescence imaging reagents

in which AMD is the spontaneous emission probability of the 5D0 / 7F1 transition in vacuum, calculated to be 14.65 s 1 and n is the refractive index of the medium. Once srad has been determined, it allows access to the metal centered quantum yield, and for coordination complexes displaying an antenna effect, to the sensitization efficiency of the ligand using Eq. (4). When considering Eq. (7), one can realize that non-radiative processes play a crucial role in the luminescence efficiency. Among those, one of the most important non-radiative de-excitation pathways is energy transfer from the Ln excited state to vibrational overtones of OH, NH, and CH bonds of the surrounding ligand and solvent molecules. A representative example is given by the presence of water molecules in the first and second coordination sphere of Ln3 þ cations. It was early demonstrated that the luminescence lifetime of Ln ions was noticeably increased by changing the solvent from water to deuterated water.34 The phenomenon was then studied by Horrocks and co-workers,35,36 who first proposed a correlation between q, the hydration number or number of water molecules in the first coordination sphere, and the luminescence lifetimes of the Ln cation of interest (in their study Eu and Tb) in H2O and D2O, under the form:   1 1 q¼A  B C (10) sH2 O sD2 O in which A, B and C are constants depending of the Ln ion. In these first reports from Horrocks, B and C values were not taken into account, but this relationship was further extended by different research groups, noticeably by the one of Parker or Kimura, to refine the constants,37 to take into account other vibrations in the molecule and second sphere interactions,38 the effect of other solvent such as MeOH,38 or to extend it to other Ln ions such as NIR emitting Ln as Yb38 and Nd,39 or dual visible-NIR emitting ions, such as Sm and Dy.40 Table 1 summarizes the main values for A, B, and C of the different Ln studied. Finally, the effectiveness of a Ln complex to be used in luminescence applications will be determined by the brightness of the probe. The brightness, B(l) represents the ability of the complex to absorb photons at a wavelength l, represented by its molar extinction coefficient 3 (l), multiplied by its ability to restore those absorbed photons into emitted photons represented by the overall luminescence quantum yield Fov. Intrinsic brightness, corresponding to direct excitation into the f-f absorption bands, is rarely discussed as a result of the weak absorption coefficients of the Ln centered absorption bands. BðlÞ ¼ 3 ðlÞ  fov

(11)

With regards to the interest of long luminescence lifetimes of Ln probes, some authors have also proposed a definition of brightness taking into account the time delay td and the temporal acquisition window tacq, considerations applied in TR luminescence experiments using the relationship:41 Z td þtacq BðlÞ ¼ 3 ðlÞ  fov eðt=slum Þ dt (12) td

Similarly, considering the spectral narrowness of Ln emission bands compared to those of organic dyes (Fig. 1), other authors defined an effective brightness, which takes into account the full width at half maximum for an emission peak, u1/2, the brightness being then defined by:42 BðlÞ ¼ 3 ðlÞ  f=u1=2

(13)

Considering the brightness as a standard to compare luminescent probes, lanthanide complexes have long suffered from the comparison with conventional organic dyes, being far less bright.43 However, the most recent examples of bright Ln based

Table 1

Phenomenological constants A, B and C determined for different Ln cations for the determination of the hydration number using Eq. (10).

Ln

A

B

C

References

Nda,b Smb,c Euc Euc Tbc Dyb,c Yba Yba (in CH3OH)

0.358 0.026 1.2 1.11 5.0 0.024 1.0 2.0

0 0 0.25 0.31 0.06 0 0.2 0.1

1.97 1.6 0.075nd 0 0 1.3 0 0

39 40 38 37 38 40 38 38

s determined in ms. In the cases of Nd, Sm and Dy, the calculation in Eq. (10) did not take 1/sD2O into account. c s determined in ms. d n refers to the number of NH oscillators in the second sphere of Eu atoms. a

b

Lanthanides as luminescence imaging reagents

491

luminescent probes display values of the brightness exceeding 104 M 1 cm 1 for coordination complexes,44,45 and up to 106 M 1 cm 1 for Ln-based NPs.46,47

8.12.2.2

Förster resonance energy transfer (FRET) with luminescent lanthanide compounds

The acronym FRET stands for Förster resonance energy transfer, derived from the name of Theodor Förster, who first theorized the phenomenon allowing for a quantification with observables.48 The “F” is often also used for fluorescence, although in the case of lanthanide compounds, LRET (Luminescence RET) would be more appropriate. FRET is a non-radiative energy transfer between an energy donor and an energy acceptor in energetic resonance.49 The observation of FRET requires two important pre-requisites. The emission spectrum of the donor must overlap the absorption spectrum of the acceptor (energy resonance condition) and the distance between the donor and the acceptor must be short enough. In FRET theory, an important parameter is the Förster distance or critical radius, R0, which is the distance R between the donor and the acceptor at which the donor transfers half of its energy to the acceptor. R0 can be calculated by:

with

9ðln 10Þk2 fD J 128p5 NA n4

Z J¼ Z

1 6 =

 R0 ¼

(14)

ID ðlÞ3 A ðlÞl4 dl

(15)

ID ðlÞdl ¼ 1

(16)

k ¼ cosqDA  3cosqD cosqA

(17)

n is the refractive index of the medium, k is the orientation factor between the transition dipole moments of the donor emission and the acceptor absorption, FD is the luminescence quantum yield of the donor in absence of the acceptor, NA the Avogadro’s number, J is the overlap integral obtained with 3 A the molar absorption coefficient of the acceptor, l the wavelength and ID is the donor emission spectrum normalized to unity (Eq. 16). In the definition of k, the angles qDA, qD and qA respectively refer to the angles between the transition dipole moments of the donor and the acceptor, and between the donor (or the acceptor) and the vector joining the donor and the acceptor. When the donor and acceptor are simultaneously present in solution, R0 can be linked to the FRET efficiency EFRET with the different following relationships: EFRET ¼

1 6

1 þ ðR=R0 Þ

¼

R60

R60 f sDA IDA ¼ 1  DA ¼ 1  ¼ 1 fD sD ID þ R6

(18)

Therefore, by measuring the luminescence quantum yield or the luminescence intensity or the luminescence lifetime of the donor in the absence (subscript D) and in the presence (subscript DA) of the acceptor, one can determine EFRET and the distance between the donor and the acceptor. Considering the possibility of determining the donor-acceptor distance at the subnanometer range via spectroscopy, the FRET phenomenon is also called a spectroscopic ruler.50,51 The interest of luminescent Ln compounds in FRET experiments mainly resides in two important aspects of their properties. As a first point, the excited state lifetimes of Ln compounds are generally of the order of micro- to milliseconds, which is three to six orders of magnitude longer than conventional fluorescent dyes, typically emitting in the one to ten nanosecond regime. As FRET between Ln and an acceptor arise from a long-lived excited state of Ln, the process is rather long and the FRET sensitized emission of the acceptor is a slow process. This is translated into the acceptor emission containing a long-lived emission component, in addition to the short-lived normal fluorescence. By applying time-gated (TG) detection, with imposition of a delay between the pulsed excitation of the sample and the record of the emitted signal, the fluorescence of the acceptor arising from its direct excitation vanished and only the FRET-sensitized emission of the acceptor is obtained. A second important point of a long lived Ln donor is an important simplification in the calculation of the term k2. For short lived donor atoms, the excited-state lifetime (ns) is of the same order of magnitude than the rotational correlation time of the donor or acceptor molecules and it is important to have a good estimation of the qDA, qD and qA angles as the value of k2 can vary between 0 and 4, depending on these angles. For a long-lived donor, the tumbling of the donor and of the acceptor occurs at a faster rate than the emission. The orientation of the dipoles then becomes dynamically randomized and an average value of their orientation can be taken into account, resulting in a fixed value of k2 of 2/3. Additionally, in the case of long-lived donor atoms with short lived acceptors, the theory of FRET shows that the luminescence decay time of the donor in the presence of the acceptor, sDA, is equal to the FRET-sensitized luminescence decay time of the acceptor sAD.52 Measuring both sDA and sAD ensures that the values are accurate and improves the accuracy of the determination of the other parameters such as the FRET efficiency.

492

Lanthanides as luminescence imaging reagents

The second important aspect of the use of Ln in FRET arises from the line-like emission spectra of Ln compounds, which allow for multiplexed detection of different acceptors within a same sample. As a proof of concept, Hildebrandt and co-workers reported the possibility to observe FRET between a single type of Tb donor and up to five different luminescent semiconductor quantum dots (QDs) as acceptors (Fig. 3).53 Considering the typical emission spectrum of Tb (see Fig. 3B), there are several spectral gaps between the different peaks, in which there is almost no Tb emission. Providing the energy acceptor can reemit in these spectral gaps, the emission induced by FRET from the Tb donor to the acceptor can be measured with low Tb background emission in a multiplexed manner. In their case, Hildebrandt et al.53 used five different QD acceptors, the absorption and emission properties of which can be modulated by their size.54 The spectral overlap was evidenced by absorption and emission spectroscopy (Fig. 3A) and the close proximity of the Tb donor and QDs acceptors was fulfilled using the strong interaction between streptavidin, labeled with Tb complexes, and biotin molecules linked to the surfaces of QDs.

8.12.2.3

Anti-Stokes properties of lanthanide compounds

Photoluminescence is the result of the emission of photons after absorption of light (or photons). Generally, the absorption of the exciting photon is followed by different processes such as vibrational relaxation or energy transfer to lower excited states (Fig. 2). These intermediate processes result in a partial loss of energy and in an emission at a longer wavelength than the excitation source. Considering the pioneering work of George Stokes in the field, the phenomenon was called a Stokes shift.55 However, there exist few instances for which the emission of the photon occurs at a higher energy than the excitation. On a general ground, and although the denomination is sometimes misused, these Anti-Stokes phenomena are referred to upconversion (UC).56 The general classification will separate two kinds of UC processes (Fig. 4). The first one is related to nonlinear optical processes, implying the presence of a virtual excited state. These entail two photon absorption, second harmonic generation, and cooperative luminescence. In practice, only some few complexes based on the two photon absorption process have been applied to luminescence imaging, particularly by the group of Maury and coworkers (see Section 8.12.4).58 The other processes are combining the sum of linear optical processes and are therefore far more efficient. They are related to excited state absorption, energy transfer UC, and cooperative photosensitization. Here also, even if cooperative sensitization or excited state absorption have been reported for some Yb/Tb59 or Yb/Eu60 compounds in the solid state, there exist no reported work using these processes for Ln luminescence applications, even if a recent example demonstrated molecular Yb/Tb upconverting devices in water that might find applications to biolabelling.61 In contrast, examples with NPs that are based on the principle of energy transfer UC are flourishing in the literature for their theranostic (therapeutic þ diagnostic) applications62,63 and for the study of their potential in vivo and in cellulo toxicities.64 The applications of UC probes to luminescence imaging application will be discussed in Section 8.12.4.

8.12.3

Lanthanide compounds as luminescent probes, the choice of the physical state

When using Ln compounds as luminescent probes, one has to critically examine the expected spectroscopic properties of the device to ensure an optimal observation as a function of the physical state of the probe. As a basic example, NIR emitting Ln complexes are

(A)

(B)

Fig. 3 (A) Representation of the overlap between the emission spectrum of a Tb complex as donor (black curve) with the absorption spectra of five different QDs as energy acceptors. (B) Representation of the emission spectrum of the Tb donors and the emission spectra of the five QDs, showing the spectral gaps between the Tb emission peaks at which the FRET induced emission of the QDs can be measured. Adapted with permission from Geißler, D.; Charbonnière, L. J.; Ziessel, R. F.; Butlin, N. G.; Löhmannsröben, H.-G.; Hildebrandt, N. Quantum Dot Biosensors for Ultrasensitive Multiplexed Diagnostics. Angew. Chem. Int. Ed. 2010, 49 (8), 1396–1401. doi:10.1002/anie.200906399. Copyright 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Lanthanides as luminescence imaging reagents

493

Nonlinear op cal UC processes Two photon absorp on (K = 10-13)

Second order harmonic genera on (K = 10-11)

Coopera ve luminescence (K = 10-8)

Excited state absorp on (K = 10-5)

Energy transfer UC (K = 10-3)

Coopera ve sensi sa on (K = 10-6)

Successive op cally linear UC processes

Fig. 4

Schematic representation of the different UC processes with their relative efficiencies.57

known to be prone to luminescence quenching, especially in water, rendering their use hazardous in this spectral window. The following sections aim at giving some insights for the right choice of the physical state of the probe depending on the spectral window of interest.

8.12.3.1

Lanthanide complexes as luminescent probes

Ln complexes are obtained by the coordination of a ligand with a Ln cation. One of the basic requirements for the use of Ln complexes as luminescent probe is the stability of the complexes formed, which is related to the stability constant K by the relationship: Lx þ Ln3þ 4½LnLð3xÞþ K ¼

½ML ½L  ½M

(19)

In theory, K is related to the association of the Ln cation with the fully deprotonated ligand. In practice, the association constants are determined at a fixed pH and one refers to the measurement of a conditional stability constant, which can be calculated from K, knowing the dissociation constants of the ionisable functions of the ligands, the pK values of the ligand.65,66 In order to better compare the relative stability of the complexes formed with different ligands, the pLn value was introduced by Raymond,67 which is a measure of the concentration of free metal (Ln here) in the presence of an excess (generally a 10-fold) of ligand with defined pH, Ln concentration and buffer conditions, pLn ¼  Log[Ln3 þfree]. In addition to a strong complexation related to a large association constant, the kinetic inertness is also often determined by competition experiments in the presence of excess of competitors, which can be anions or cations. This is particularly true if the biological conditions envisaged for the imaging application are related to the presence of endogenous cation such as Zn2þ or Cu2þ, or anions, such as citrates or phosphates. As previously mentioned, the use of Ln complexes also requires the synthetic developments of ligands containing antennae to boost the photosensitization of the complexes.68,69 This can be a large part of the preparation of luminescent Ln probes requiring large synthetic efforts, but has led to the development of actually very bright Ln complexes in the case of the red emitting Eu45 or the green Tb ones.44,70 As an important part of the synthesis, it is to be kept in mind that the presence of water molecules in the first coordination sphere of Ln ions is very detrimental to the luminescence and that the ligand should be at least octadentate. Finally, the labeling of biological material requires the introduction of a chemically active function in the design of the ligand. This function is generally targeted toward the amine residues present in the lysine amino acids of proteins, or towards cysteine residues, either natives or obtained after reduction of cystine bridges.71 Additionally, some other linkers may be specifically developed as for example for the labeling of specific amino acids.72

8.12.3.2

Lanthanide nanoparticles (NPs) as luminescent probes

In contrast to Ln complexes, the Ln atoms embedded in NPs are fairly well protected from the environment and from the deactivation from solvent molecules, except for those at the surface.73 Under these conditions, only the vibrations of the lattice, the phonons, are susceptible to quench the luminescence. By choosing a matrix with low phonon energy, such as in fluoride

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Lanthanides as luminescence imaging reagents

structures,74 the luminescence lifetimes of Ln atoms can be very long with concomitant very large quantum yields (see Eq. 7). This is particularly true for NIR emitting Ln atoms and it opens the way to imaging applications in the NIR domain.75 In addition, the long-lived excited states allow for the process of upconversion (see Section 8.12.2.3) to be easily obtained with doped NPs.76 By playing with the matrix and the doping with different NIR and visibly emitting Ln atoms, emission can then be observed in almost all the UV to NIR domain.62 Finally, it may happen that energy migration phenomena within Ln atoms in the NPs are funneling the energy to surface Ln atoms, which are quenched by the interaction with molecules present at the surface. In that case, core/shell structures can be designed to add one protecting shell.77 A design with multiple layers can also be used to finely tune the spectroscopic properties of the NPs.78 In contrast, Ln emitting in the visible domain such as Tb and Eu are less prone to quenching by solvent molecules47 and do not really benefit from the protection afforded by the matrix. In contrast to QDs, which benefit from the confinement effect,79 the absorption properties of the Ln in the NPs are basically the sum of the absorption of each atom, which are themselves weak, as previously mentioned. Such nanoprobes are however widely used for bio-analytical applications and imaging,80 although they suffer from poor absorption, and there are increasing examples in the literature, in which the antenna effect is used to boost the absorption capacities, by developing surface capping ligands providing sensitization of the core Ln cations.47 Ln NPs have the advantage of being rather easy to synthesize and to functionalize,81 however, the reproducibility of the synthesis is often questionable and they can suffer from leaching when dissolved in water, thereby losing some of their optical properties.82 Ligand exchange at their surface can drastically improve their long term stability.83 The question of the cytotoxicity of such NPs is still debated,64 and their relatively large sizes question their potential for a correct bio-distribution. In that respect, the size can also be an advantage in some instances, as large NPs have been shown to display an enhanced permeability and retention (EPR) effect in tumor tissues, leading to their increased retention in the tumor neighborhood.84 Finally, Ln NPs are of special interest regarding the possibility to combine multimodal imaging applications.85,86

8.12.3.3

Lanthanide metal-organic frameworks (MOFs) as luminescent probes

MOFs are a class of materials obtained by a crystalline arrangement of metals linked by organic ligands.87 They are generally easily obtained by hydrothermal methods and display submicrometer scales, a large porosity and some rich inclusion properties. The advantage of Ln based MOFs may be found in the possibility to dope them at will, as for Ln NPs, and the possible use of the organic linkers as antenna to boost the luminescence of the Ln. Despite these advantages and probably because of their large size (larger than 100 nm in general) and large polydispersity, Ln MOFs are far less used than their NPs counterparts in imaging applications.88

8.12.3.4

Other materials for lanthanide-based luminescence imaging

If Ln complexes and NPs are the most prominent forms used for Ln imaging, one also has to mention that other vehicles of Ln materials can be used. Those generally take advantage of hybrid composition such as the decoration of Ln-free inorganic NPs by surface anchored Ln complexes. Inorganic NPs might be composed of semiconducting NPs,89 silica NPs,90 or superparamagnetic iron oxides (SPIO).91 A second largely employed architecture is the embedding of Ln complexes into organic NPs formed of polymers,46,92 dendrimers,93 or inorganic silica NPs.94

8.12.4

Lanthanide-based luminescence imaging

8.12.4.1

Introduction

Although much less frequently used compared to lanthanide-based spectroscopy, luminescence microscopy with lanthanides can look back on a long history. Early developments of measuring delayed luminescence (or phosphorescence) date back to 1867, when Becquerel presented the phosphoroscope,95 which was also used to determine fluorescence lifetimes of uranyl salts in 1929 by Delorme and Perrin,96 or in 1942 by Weissman when he discovered the antenna effect.10 Based on the microscope-centrifuge presented in 1930 by Harvey and Loomis,97 Harvey and Chase developed a phosphorescence microscope to observe phosphorescence of different biological materials (including cells) as early as 1942.98 In the late 1980s and early 1990s, new time-delayed luminescence microscopy concepts were developed by Jovin and Arndt-Jovin,99 Tanke,100 and Soini,101 which were well adapted for TR lanthanide imaging. Although Soini and Hemmilä already reported about TR microscopy in relation to lanthanide chelates in the 1970s (with contributions to symposia in Finland),102 arguably the first lanthanide-based luminescence imaging paper in an international scientific journal was published in 1989 by Elster et al.,103 in which Eu-DTPA (diethylenetriamine penta-acetic acid) was imaged in various rat organs with a standard epifluorescence microscope setup. In 1990, the first TR lanthanide microscopy paper by Beverloo et al. reported about the use of europium-activated yttrium oxysulfide phosphors (Y2O2S:Eu3þ) for immunocytochemistry.104 In 1992, Seveus et al. demonstrated TR luminescence imaging of a Eu chelate for in situ hybridization and immunohistochemistry,101 which was further confirmed in 1994 by Marriott et al. using a different Eu chelate labeled to streptavidin.105 During the last 10 years, advancements in lanthanide-based luminescence microscopy have been largely dominated by NPs.106–108 In particular, the more classical approach of packing many lanthanide ions into one NP for improved brightness,109 has moved to the exploitation of new photophysical properties, such as upconversion,110–112 persistent luminescence,113–115 or NIR emission.116,117

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Another recent trend is the use of lanthanide-based metal-organic-frameworks MOFs.118,119 Although sometimes more difficult to find among the many recent developments that imply lanthanide nanomaterials, luminescence imaging with molecular lanthanide complexes and their related imaging technologies (e.g., TR, time-gated, or confocal imaging) have strongly advanced over the last 20 years.7,68,120–128 Depending on the point of view, luminescence imaging of biological systems can be classified (i) biologically, i.e., by the disease, organism, organ, tissue, cell, organelle, or subcellular compartment to be investigated; (ii) technically, i.e., by the method, setup, wavelength range, resolution, luminescent material to be applied; or (iii) biotechnically, i.e., via in vitro, in vivo, or in situ analysis of the biological system. The focus of this review being lanthanides, we will discuss their specific technical benefits, advances, and limitations sub-divided in continuous-wave (CW) or steady-state luminescence (wide-field, confocal, two-photon excitation), TR (time-gating, lifetime imaging, afterglow imaging), multiplexed (CW and TR), NIR (and short wavelength infrared, SWIR), and upconversion microscopies. Each section will discuss representative examples from the most recent literature. Many of those studies have been discussed in the recent review articles cited above.13–37 While the properties of lanthanide-based luminescent probes, such as brightness, cell penetration, or selective and quantitative sensing, have also significantly advanced both for lanthanide complexes and NPs,68,71,106,128 we focus on the actual imaging applications to illustrate the state-of-the-art concerning usability of lanthanide-based imaging agents for biological in vitro, in vivo, and in situ microscopy analysis.

8.12.4.2

Continuous wave (CW) or steady-state microscopy

Although lanthanide complexes based on Dy3þ, Sm3þ, and Yb3þ lanthanide ions have also been used for luminescence imaging (see Section 8.12.4.4), the major players remain Eu3þ and Tb3þ, mainly due to the higher brightnesses.128 Recent interesting developments for luminescence imaging include (i) pixel-by-pixel spectrally resolved imaging of the pericellular matrix by visible light excitation of Eu3þ and Tb3þ ions,129 (ii) visualization of changes in peroxynitrite concentration within the mitochondria of live cells via Eu3þ complexes,130 and (iii) Eu3þ complex-based imaging of time-dependent lysosomal acidification of aging cells.131 In the first study, Arppe-Tabbara et al. used simple Eu3þ and Tb3þ acetate to stain fixed Chinese Hamster Ovary (CHO) cells for demonstrating that these very inexpensive lanthanide materials can be used for specific lanthanide ion binding to the glycocalyx (intercellular space) and that direct excitation of the lanthanide ions (without the antenna effect via coordinated ligands) in the visible wavelength range (465 nm for Eu and 488 nm for Tb) can result in sufficiently bright signals for confocal imaging.129 The cells were imaged in 60  60 pixel arrays (with approximately 1 mm2 per pixel) and each individual pixel could be spectroscopically resolved with an impressively high resolution (Fig. 5). Using a previously developed spectral analysis “infinite contrast” algorithm that exploits the specific narrow-band spectra of the lanthanide emission to distinguish it from all other components,132 such as auto-fluorescence or other fluorescent dyes (e.g., F18 membrane and Syto-9 nucleus stains), the authors were able to locate the lanthanide ions in the glycocalyx of the cells. The high spectral resolution of their setup was further used to analyze the fine structure of the Eu3þ emission in the 5D0 / 7F4 band at 690 nm, which showed that all Eu ions were bound in identical, symmetric binding pockets, i.e., a single structural motif of the glycocalyx. The results showed that a combination of highly sensitive photon detectors, high spectral resolution of lanthanide luminescence, and a smart spectral separation algorithm can lead to high-contrast imaging of specific cell structures by direct excitation of lanthanide ions with visible light. Because the same technology can be used for other lanthanide-based probes (e.g., lanthanide complexes or NPs) with different excitation wavelengths and/or brightnesses (that may reduce the relatively long acquisition time), it has the potential to significantly advance lanthanide-based luminescence imaging. In the second study, Breen et al. used Eu3þ complexes to image peroxynitrite (OONO) levels in mitochondria of live HeLa cells.130 The emissive complex Eu.1 (maximum absorption of the antenna ligand at 321 nm) was designed to form the nonemissive complex Eu8.HQ via OONO-mediated oxidative cleavage of its phenyl boronic acid group (Fig. 6A). After 4 h of incubating HeLa cells with Eu.1, the typical Eu emission could be visualized (by laser scanning confocal microscopydLSCM) primarily in the mitochondria, as confirmed by co-incubation with MitoTracker Green (Fig. 6B top). When the cells were subsequently incubated with 3-morpholinosydnonimine (SIN-1), which releases the reactive oxygen and nitrogen species (RONS) nitric oxide (NO) and superoxide (O2) leading to spontaneous and sustained production of ONOO, the Eu luminescence was strongly quenched (by  90%). Considering that the ONOO produced in the cell culture medium was unlikely to penetrate the cells, the authors concluded that NO would penetrate the cell membranes and form ONOO with superoxide in the mitochondria, thus, reducing the Eu luminescence intensity. Co-incubation of the Eu.1-stained cell with SIN-1 and the superoxide scavenger TEMPO, resulted in significantly lower Eu luminescence intensity decrease (by  35%), because of reduced intracellular formation of ONOO compared with the SIN-1 treatment alone (Fig. 6B bottom). The importance of this work relied on the demonstration of sensitive and specific lanthanide-based intracellular (cell penetration of the Eu3þ complexes) RONS biosensing with confocal imaging resolution. Further improvements of this approach may become possible by also exploiting the spectral (narrow-band emission) and temporal (long luminescence lifetime) luminescence components for imaging the Eu3þ luminescent probes. The third study by Starck et al. applied Eu3þ complexes for sensing lysosomal pH in living mouse skin fibroblasts (NIH-3T3) via LSCM and 355 nm excitation.131 The pH-sensitive (increasing luminescence intensity with decreasing pH) complexes [EuL4] (Fig. 7A) were uptaken by NIH-3T3 cells and primarily localized in the lysosomes (as confirmed by LysoTracker co-incubation). The Eu luminescence intensity steadily increased with incubation time until approximately 16–18 h, then decreased until  24 h, and increased again until 40 h (Fig. 7B and C). In contrast, the structurally analogous but pH-insensitive complex D[EuL5] (Fig. 7D) resulted in much faster Eu luminescence intensity increase and reached its maximum after only  4 h of incubation with NIH-3T3 cells with a minor inflection at around 16–18 h (Fig. 7E and F). The intensity decrease (and subsequent increase) for

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Fig. 5 30  60 mm images of single regions of CHO cells stained with Eu3þ acetate, F18, and Syto-9 (A–D) or Tb3þ acetate (E–H) imaged in a confocal scanning microscope equipped with a CCD spectrometer and following 465 nm (Eu3þ) or 488 nm (Tb3þ) excitation. (A and E) Pseudocolored images. (B and F) Emission spectra from all pixels in A and E. (C and G) Image of photons arising from lanthanide centered emission. (D and H) Emission spectra from all pixels in C and G. The green and red dashed lines in the spectra illustrate the spectral regions used for pseudocoloring. Reproduced with permission from Arppe-Tabbara, R.; Carro-Temboury, M. R.; Hempel, C.; Vosch, T.; Sørensen, T. J. Luminescence from Lanthanide(III) Ions Bound to the Glycocalyx of Chinese Hamster Ovary Cells. Chem. A Eur. J. 2018, 24 (46), 11885–11889. doi: 10.1002/ chem.201802799. Copyright 2018 Wiley-VCH Verlag GmbH & Co. KGaA.

both Eu3þ complexes could be explained by the NIH-3T3 cell division after approximately 16 h followed by continued uptake of Eu complexes from the cell growth medium. The longer and stronger luminescence increase of the [EuL4]-stained cells could be explained by a combination of time-dependent uptake (as also found for D-[EuL5]) and the strong luminescence intensity increase with decreasing pH in the aging endosomes when they transform into lysosomes over time. The results demonstrated that

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Fig. 6 (A) ONOO-mediated transformation of Eu.1 into Eu.8HQ, resulting in a luminescence on-off switch sensing mechanism. (B) After 4 h of incubation of HeLa cells with Eu.1 (250 mM), mainly the mitochondria were stained (top images). Incubation of the Eu.1 stained cells for 30 min with SIN-1 (500 mM) resulted in intracellular ONOO production, which switched off the Eu luminescence (bottom center). Coincubation with TEMPO resulted in less ONOO production and less decrease of the Eu luminescence intensity (bottom right). Reproduced with permission from Breen, C.; Pal, R.; Elsegood, M. R. J.; Teat, S. J.; Iza, F.; Wende, K.; Buckley, B. R.; Butler, S. J. Time-Resolved Luminescence Detection of Peroxynitrite Using a Reactivity-Based Lanthanide Probe. Chem. Sci. 2020, 11 (12), 3164–3170. doi:10.1039/C9SC06053G. Copyright 2020 The Royal Society of Chemistry.

lanthanide complexes are well suited to follow cellular uptake and sub-cellular processes over time in live cells with confocal resolution. Again, exploitation of the unique spectral and temporal luminescence properties of lanthanides may lead to further improvements of such lanthanide-based live cell luminescence imaging approaches.

8.12.4.3

Time-resolved (TR) or time-gated (TG) microscopy

In principle, fluorescence or phosphorescence lifetime imaging microscopy (FLIM or PLIM) can also be performed with luminescent lanthanide probes. However, as sample volumes and/or concentrations are usually small in bioimaging (e.g., on and in cells), it can take a long time to acquire a luminescence decay curve of several hundreds of microseconds or even up to several milliseconds with a sufficient amount of photons, such that lifetimes (single or multiexponential) can be reliably determined. Different microscopy technologies, such as pinhole shifting in scanning confocal microscopy or TG orthogonal scanning automated microscopy, have

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Fig. 7 Upon incubation of NIH-3T3 cells with pH-sensitive (A) and pH-insensitive (D) Eu3þ complexes, LSCM revealed that the Eu luminescence increases stronger and longer for the pH sensitive complexes (B and Cdimages in B correspond to 30 mM Eu complex concentration) compared to the pH-insensitive ones because of endosomal/lysosomal acidification over time within the aging cells. Reproduced with permission from Starck, M.; Fradgley, J. D.; Pal, R.; Zwier, J. M.; Lamarque, L.; Parker, D. Synthesis and Evaluation of Europium Complexes That Switch on Luminescence in Lysosomes of Living Cells. Chem. A Eur. J. 2021, 27 (2), 766–777. doi:10.1002/chem.202003992. Copyright 2020 The Authors.

been developed to adapt imaging to lanthanide probes.122 However, similar to biosensing and screening approaches using lanthanide probes on plate reader systems, TR microscopy mainly relies on time-gating approaches, for which the short-lived autofluorescence can be suppressed by taking luminescence images after a certain delay (usually some microseconds) following pulsed excitation. Such TG microscopy can be realized by mechanical optical choppers or gated camera systems, such as ICCD cameras.122 Various luminescent probes (including lanthanide complexes and NPs) with different luminescence lifetimes (from tens of microseconds to several milliseconds) or persistent luminescence (up to hundreds of seconds) have been used for both in vitro (including live-cell) and in vivo imaging with very high signal-to-background ratios and TG detection limits down to single lanthanide NPs.46,47,133–136 Recently, Chen et al. could demonstrate live-cell intracellular TG imaging of protein-protein interactions using a single-chain Tbto-GFP (green fluorescent protein) FRET biosensor.135 The biosensor contained GFP as FRET acceptor and the FK-binding protein 12 (FKBP12) on the C terminus and an Escherichia coli dihydrofolate reductase (eDHFR), which can bind heterodimers of trimethoprim (TMP) linked to a luminescent Tb3þ complex (TMP-Tb) as FRET donor, and the rapamycin-binding domain of m-Tor (FRB) on the N terminus (Fig. 8A). To permit efficient cell penetration of TMP-Tb, such that it can bind to the eDHFR domain of the intracellularly expressed biosensor, it was linked to nine arginines (TMP-Tb-R9). The rapamycin-induced activation of FKBP12/FRB interaction resulted in closer proximity between Tb and GFP leading to more efficient FRET (“high FRET”), which could be imaged inside live NIH3T3 fibroblasts with TG microscopy (Fig. 8B). Depending on the length of the linker between GFP and eDHFR (10, 20, or 30 nm), the dynamic range (between “low FRET” and “high FRET”) of the TG FRET ratio (GFP-to-Tb TG intensity ratio) without and with rapamycin could reach up to 520% (Fig. 8C). This very high FRET ratio change showed that TG lanthanide imaging can be very useful for highly sensitive intracellular imaging of protein-protein interactions. The recent development of similar Tb complexes for cell penetration without the addition of cell-penetrating peptides may further facilitate such intracellular TG microscopy.137 Cho et al. used FRET between Eu3þ complexes and fluorescent dyes for TG imaging of molecular interactions in vivo.133 They first characterized technical drawbacks of lanthanide-based TG imaging, and found that efficient FRET to a dye acceptor can strongly reduce the FRET-sensitized luminescence decay time of the dye (down to few tens of ms), which allowed for excitation with much higher repetition rates (18 kHz) and a concomitant increase of the photon flux (more photons per time). Excitation with short pulses of low energy and in-efficient epi-illumination were identified as possible limitations to accomplish high signal-to-background ratios, which showed that careful optimization of a TG imaging setup can be highly beneficial for efficient lanthanidebased luminescence microscopy. Using their optimized setup with Q-switched laser excitation and transreflected illumination,

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Fig. 8 (A) Single-chain biosensor containing an eDHRF (D) domain for binding a Tb FRET donor, a GFP FRET acceptor, two domains (FRB and FKBP) for rapamycin(rap)-induced interaction, and a flexible ER/K helix. (B) TG luminescence (excitation pulse width: 1.5 ms; pulse period: 3 ms; delay time: 0.001 ms; camera intensifier-on time: 1.48 ms) images of NIH3T3 fibroblasts expressing the FRB-eDHFR-(ER/K)20-GFP-FKBP12 biosensor circa 20 min after stimulation with 1 mM rapamycin in Tb (lex ¼ 365 nm; lem ¼ 620  10 nm) and GFP (lex ¼ 365 nm; lem ¼ 520  10 nm) detection channels. Scale bars: 20 mm. (C) The dynamic range of the TG-FRET biosensor increases with the length of ER/K linker due to reduction of “low FRET” signals. FRET ratios refer to pixel-wise GFP/Tb TG intensity ratios (averaged from 10 or more cells) acquired before and 25 min after addition of rapamycin. Percentage values present the relative FRET ratio increase from “low FRET” to “high FRET”. (D) TG luminescence (excitation time: 1 ms; delay time: 1 ms; collection time: 60 ms; repetition rate: 15 kHz; total imaging time: 3.3 s) micrographs of zebrafish embryos (24 hpf) injected at one-cell stage with a Eu3þ-complex-labeled MO and either a complementary (yes) or noncomplementary (no) MO labeled with Atto Rho14 dye. TG Euþdye: lex ¼ 355 nm; lem > 575 nm; TG dye: lex ¼ 355 nm; lem ¼ 655  20 nm; ratio: Ratio of TG dye/TG Euþdye normalized to unity for the highest value. (E) Eu3þ-complexes encapsulated into poly(methyl methacrylate)-based NPs (I) can be used for single NP TG luminescence (lex ¼ 349 nm; lem ¼ 605  8 nm; excitation pulse width: 12 ns; delay time: 10 ms; gate width: 0.6 ms; repetition rate: 300 Hz; total imaging time: 0.3–0.7 s) imaging (II) and the analysis of single-NP live cell uptake (III, HeLa cells with membranes shown in turquoise and NPs in magenta). Scale bars: 10 mm. (F) Coordination of antenna-ligand-functionalized antibodies to Tb-doped LaF3 NPs results in simultaneous efficient photosensitization and antibody conjugation (I). The Tb-NP-antibody conjugates were used for TG luminescence (lex ¼ 349 nm; lem ¼ 542  10 nm; excitation pulse width:  12 ns; delay time: 11 ms; gate width: 2 ms; repetition rate: 200 Hz; total imaging time: 4–15 s) imaging of transmembrane EGFR on A431 cells after 30 min incubation (II) and inside A431 cells after 5 h of incubation (III). (A–C) Reproduced with permission from Chen, T.; Pham, H.; Mohamadi, A.; Miller, L. W. Single-Chain Lanthanide Luminescence Biosensors for Cell-Based Imaging and Screening of Protein-Protein Interactions. iScience 2020, 23 (9), 101533. doi:10.1016/j.isci.2020.101533. Copyright 2020 The Authors. (D) Reproduced with permission from Cho, U.; Riordan, D. P.; Ciepla, P.; Kocherlakota, K. S.; Chen, J. K.; Harbury, P. B. Ultrasensitive Optical Imaging With Lanthanide Lumiphores. Nat. Chem. Biol. 2018, 14 (1), 15–21. doi:10.1038/nchembio.2513. Copyright 2017 Nature Publishing Group. (E) Reproduced with permission from reference Cardoso Dos Santos, M.; Runser, A.; Bartenlian, H.; Nonat, A. M.; Charbonnière, L. J.; Klymchenko, A. S.; Hildebrandt, N.; Reisch, A. Lanthanide-Complex-Loaded Polymer Nanoparticles for Background-Free Single-Particle and Live-Cell Imaging. Chem. Mater. 2019, 31 (11), 4034–4041. doi:10.1021/ acs.chemmater.9b00576. Copyright 2019 American Chemical Society. (F) Reproduced with permission from Charpentier, C.; Cifliku, V.; Goetz, J.; Nonat, A.; Cheignon, C.; Cardoso Dos Santos, M.; Francés-Soriano, L.; Wong, K.-L.; Charbonnière, L. J.; Hildebrandt, N. Ultrabright Terbium Nanoparticles for FRET Biosensing and in Situ Imaging of Epidermal Growth Factor Receptors. Chem. A Eur. J. 2020, 26 (64), 14602–14611. doi:10.1002/chem.202002007. Copyright 2020 Wiley-VCH GmbH.

the authors were able to acquire bright TG luminescence images of zebrafish embryos (injected with Eu3þ complexes at the zygote stage) at 22 h postfertilization (hpf). They could also demonstrate TG FRET imaging of molecular interactions in zebrafish embryos (24 hpf) that were injected at the one-cell stage with a Eu3þ-complex-labeled morpholino oligonucleotide (MO) and a dye-labeled MO. TG-FRET-sensitized dye luminescence was much stronger when the dye-MO was complementary to the Eu3þ-MO (Fig. 8D), which demonstrated the MO hybridization in vivo. Recently, Cardoso dos Santos et al. demonstrated that TG-FRET nanoprobes, consisting of Tb3þ-complexes attached to QDs and injected at the one-cell stage, could be imaged in developing zebrafish embryos over seven days with toxicity similar to injected RNA and strongly improved signal-to-background ratios compared to non-TG imaging.134 These studies showed the enormous potential of lanthanide-based TG-FRET for bright in vivo imaging with strongly reduced autofluorescence background. Lanthanide-based TG imaging has also been extended to NPs. Such NPs usually exploit the large amount of lanthanides (brightness of one NP scales with the amount of lanthanides) and their better protection from the environment (less luminescence quenching). Cardoso dos Santos et al. encapsulated Eu3þ-complexes into polymer NPs of 10–30 nm by simple nanoprecipitation, which resulted in up to 5000 complexes per NP and a brightness of up to 4  107 M 1 cm 1.46 Autofluorescence-free TG luminescence microscopy provided high sensitivity down to single NP imaging and single-NP tracking during cellular uptake (Fig. 8E). In another

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example, Charpentier et al. used Tb3þ-doped LaF3 NPs with diameters around 20 nm and attached antenna ligands to the NPs, such that only the Tb ions close to the surface of the NPs would efficiently emit luminescence.47 Interestingly, those ligands could be conjugated to proteins (e.g., antibodies), which resulted in a dual ligand functionality of protein-conjugation and luminescence sensitization of the NPs. The surface emitting Tb-NPs could reach a brightness of up to 1.8  106 M 1 cm 1 and antibodycoated Tb-NPs could be used for live-cell TG imaging of epidermal growth factor receptors (EGFR) on cell membranes and their subsequent cellular uptake (Fig. 8F). These studies demonstrated the potential of bright and stable lanthanide-based NPs for extracellular and intracellular live-cell autofluorescence-free TG imaging with high signal-to-background ratios.

8.12.4.4

Multiplexed microscopy

The extremely long luminescence lifetimes of lanthanides and their multi and narrow band luminescence spectra from the UV to the IR make them ideally suited for multiplexing, i.e., the detection of various different luminescence signals for analyzing different biological or chemical targets or interactions or for optical encryption, authentication, and barcoding. Lanthanide-based multiplexed imaging has been demonstrated for excitation, emission, and energy transfer selected lanthanide luminescence wavelengths (spectral or color multiplexing) and decay times (temporal or lifetime multiplexing).134,136,138–145 Recently, Hamon et al. developed a series of pyclen-based Ln3 þ complexes that contained trivalent Eu, Sm, Yb, Tb, or Dy ions with luminescence in the approximately 400–1200 nm wavelength range upon excitation of the antenna ligands between circa 300 and 400 nm.145 All Ln complexes could internalize into permeabilized T24 human epithelial bladder cancer cells and were suitable for two-photon-excitation (2PE), such that they could be used for intracellular 2PE confocal luminescence microscopy with excitation between 720 and 800 nm and emission between 470 and 610 nm, which corresponded to Eu, Sm, Tb, and Dy as well as antenna ligand (for the Yb complex) luminescence (Fig. 9A). For the Yb3þ complex, the authors could also show dual (ligand and Yb3þ) emission detection, i.e., VIS (< 740 nm) and NIR (> 850 nm) intracellular luminescence imaging, upon NIR 2PE of the ligand (765 nm). In particular, the NIR-NIR imaging could become useful for deeper tissue penetration. Potential suitability for in vivo imaging was demonstrated with Eu3þ complexes injected into zebrafish embryos. Applying both conventional onephoton excitation (365 nm) and 2PE (720 nm) microscopies revealed the advantages of 2PE concerning reduced autofluorescence and higher spatial resolution. Although actual multiplexing was not demonstrated, the study provided an important proof-ofconcept for multicolour confocal in vitro and in vivo luminescence imaging with up to five different Ln complexes using both one and two photon excitation. Cardoso dos Santos et al. demonstrated live cell intracellular multiplexed TG imaging using Tb3þ complexes as FRET donors and QDs and FPs as FRET acceptors.134 The Tb complexes were self-assembled via hexahistidine-appended peptides to three different QDs (emitting around 503, 600, and 635 nm) and those Ln-nanohybrids were microinjected into live African green monkey kidney fibroblast-like cells (COS-7). Intracellular FRET from Tb to all three QDs resulted in long luminescence decays from the Tb donor and all three QD acceptors, which could be imaged in four different TG detection channels without the contribution of short-lived autofluorescence or direct (non-FRET) fluorescence from QDs (Fig. 9B top). To also demonstrate that intracellular interactions can be monitored by TG FRET microscopy, the Tb-QD nanohybrids were injected into COS-7 cells that expressed the FP mCherry. The cytosolic assembly of mCherry to the Tb-QD nanohybrids (via the hexahistidine-tag of mCherry) could be imaged by multistep TGFRET from Tb-to-QD-to-mCherry (and also minor contribution of Tb-to-mCherry FRET on the QD surface) only in mCherry expressing cells, whereas there was no TG-FRET sensitized mCherry luminescence in cells without mCherry expression (Fig. 9B bottom). Such bright and stable Ln based nanohybrid TG-FRET probes may become very useful for autofluorescence-free multiplexed imaging of intracellular biomolecular interactions. In a third example, Li et al. developed persistent (or afterglow) NPs136 in which SiO2/CdSiO3 NPs were doped with different ions, including Tb3þ and Dy3þ, to perform multicolor in vitro and in vivo imaging several seconds after UV excitation. All NPs showed spectrally broad emission but with higher intensities at specific colors, i.e., In3þ doped NPs emitted stronger in the blue, In3þ/Tb3þ doped NPs in the green, and In3þ/Mn2þ doped NPs in the orange spectral region, which could be used for true-color imaging or filter based encoding in a conventional bioluminescence imaging system (Fig. 9C). Chen et al. proposed a luminescence lifetime-based multiplexed encoding strategy, in which they coupled Eu3þ and Tb3þ complexes to QDs and used distance-tunable FRET to adjust the signal intensities in different TG luminescence detection windows.139 Two different luminescence lifetimes of the Eu3þ and Tb3þ complexes (1.1 and 2.7 ms) and two different thicknesses of the QD coatings (6 and 12 nm) resulted in four distinct FRET-sensitized QD luminescence decays (all excited at the same wavelength of 349 nm and detected at the same wavelength of 640 nm), which could be distinguished by defining three distinct TG detection windows, which were identified as red, green, and blue for intuitive RGB imaging (R: 0.05–0.5 ms; G: 0.5–1 ms; and B: 1–3 ms after the excitation pulse). With this temporal multiplexing approach, the authors were able to specifically distinguish HeLa cells that were labeled with the different Ln-QD nanohybrids using a single excitation and a single emission wavelength within a single measurement on a TG luminescence microscope (Fig. 9D). In a follow-up study, the approach was extended to Tb-to-QD-to-dye FRET, which resulted in single-wavelength brightness-equalized temporally multiplexed optical barcodes, which did not require the adjustment of imaging contrast or excitation intensities.140 These different NP-based multiplexed in vitro and in vivo encoding strategies highlight the potential of Ln-based luminescence microscopy to provide unique benefits to biological imaging, which cannot be accomplished with conventional fluorescent probes.

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Fig. 9 (A) 2PE confocal images (fluorescence, transmission, and overlay from left to right) of fixed T24 cells stained with Eu (lex ¼ 750 nm; lem: 557–646 nm), Sm (lex ¼ 750 nm; lem: 560610 nm), Yb (lex ¼ 800 nm; lem: 470 610 nm), Tb (lex ¼ 720 nm; lem: 470 610 nm), and Dy (lex ¼ 720 nm; lem: 470610 nm) complexes. Scale bars: 10 mm. (B) Transmission (left) and TG luminescence (excitation pulse width: 12 ns; delay time: 10 ms; gate width: 2.3 ms; repetition rate: 200 Hz; total imaging time: 2 s for top images, 4 s for Tb and 6 s for QD and mCherry bottom images; lex ¼ 349 nm; lem(Tb) ¼ 542  10 nm; lem(QD1) ¼ 510  10 nm; lem(QD2 & mCherry) ¼ 605  8 nm; lem(QD3) ¼ 640  7 nm) images of COS-7 cells injected with Tb-QD nanohybrids. The bottom images show COS-7 cells with (purple stars in DIC image) and without (black star in DIC image) transiently expressed cytosolic mCherry. Scale bars: 10 mm. (C) Intensity scaled (blue for low and red for high intensities) images of In3þ (top row), In3þ/Tb3þ (middle row), and In3þ/Mn2þ (bottom row) doped SiO2/CdSiO3 NPs in multiwell plates (left) and subcutaneously injected in a mouse (right). The images were acquired directly after switching off the UV radiation (254 nm, 6 W, 5 min) in a standard animal imaging system coupled with an EMCCD camera (1 s exposure). When using different standard filters (GFP: 515–575 nm; DsRed: 575–650 nm), the luminescence intensities were NP specific (yes ¼ Y or no ¼ N) and could be encoded as YNN for In3þ, YYN for In3þ/Tb3þ, and YYY for In3þ/Mn2þ doped NPs. (D) TG luminescence (excitation pulse width: 12 ns; delay times and gate widths shown above the images and encoded in different RGB colors; lex ¼ 349 nm; lem ¼ 640  7 nm) images, their overlay (mixed RGB image), and transmission bright field (BF) images (top rows show several cells and bottom rows show only few cellsdscale bars on the top right: 20 mm) of HeLa cells labeled with individual Ln-QD nanohybrids (I: Tb-QD with 6 nm SiO2 coating; II: Tb-QD with 12 nm SiO2 coating; III: Eu-QD with 6 nm SiO2 coating; IV: Eu-QD with 12 nm SiO2 coating). (A) Reproduced with permission from Hamon, N.; Roux, A.; Beyler, M.; Mulatier, J.-C.; Andraud, C.; Nguyen, C.; Maynadier, M.; Bettache, N.; Duperray, A.; Grichine, A.; Brasselet, S.; Gary-Bobo, M.; Maury, O.; Tripier, R. Pyclen-Based Ln(III) Complexes as Highly Luminescent Bioprobes for In Vitro and In Vivo Oneand Two-Photon Bioimaging Applications. J. Am. Chem. Soc. 2020, 142 (22), 10184–10197. doi:10.1021/jacs.0c03496. Copyright 2020 American Chemical Society. (B) Reproduced with permission from Cardoso Dos Santos, M.; Colin, I.; Dos Santos, G.R.; Susumu, K.; Demarque, M.; Medintz, I. L.; Hildebrandt, N. Time-Gated FRET Nanoprobes for Autofluorescence-Free Long-Term In Vivo Imaging of Developing Zebrafish. Adv. Mater. 2020, 32 (39), 2003912. doi:10.1002/adma.202003912. Copyright 2020 Wiley-VCH GmbH. (C) Reproduced with permission from Li, Z.; Yu, N.; Zhou, J.; Li, Y.; Zhang, Y.; Huang, L.; Huang, K.; Zhao, Y.; Kelmar, S.; Yang, J.; Han, G. Coloring Afterglow Nanoparticles for High-Contrast Time-Gating-Free Multiplex Luminescence Imaging. Adv. Mater. 2020, 32 (49), 2003881. doi:10.1002/adma.202003881. Copyright 2020 Wiley-VCH GmbH. (D) Reproduced with permission from Chen, C.; Ao, L.; Wu, Y.-T.; Cifliku, V.; Cardoso Dos Santos, M.; Bourrier, E.; Delbianco, M.; Parker, D.; Zwier, J. M.; Huang, L.; Hildebrandt, N. Single-Nanoparticle Cell Barcoding by Tunable FRET From Lanthanides to Quantum Dots. Angew. Chem. Int. Ed. 2018, 57 (41), 13686–13690. doi:10.1002/anie.201807585. Copyright 2018 The Authors.

8.12.4.5

Near infrared (NIR) microscopy

Despite the many advantages of luminescence imaging, the use of more conventional visible light (400–700 nm) lacks penetration depth due to the many interactions of light and biological tissues, i.e., high photon absorption, scattering, or autofluorescence.116,146–148 Therefore, increasing attention has been paid to the use of NIR (700–1700 nm) excitation and/or emission to improve luminescence imaging performance.116,117,146–149 Imaging in the first NIR window (NIR-I, 700–900 nm) reduces significantly the absorption by biological samples and scattering caused by the heterogeneity of tissues, thus increasing the penetration depth into tissues up to several millimeters.150,151 However, the remaining autofluorescence and background interference are still a drawback for obtaining high-quality images. Subsequently, the second NIR or short-wave IR window (NIR-II or SWIR, 1000– 1700 nm) offers some advantages to improve the image fidelity.152 The significantly lower NIR-II light scattering and decreased tissue autofluorescence (almost zero beyond 1500 nm), can increase the penetration depth up to centimeters (see Fig. 10), the signal-to-background ratio, and the temporal and spatial resolution, facilitating real-time visualization and high-contrast imaging.146,153 Over the last decades, many luminescent probes have been designed for NIR microscopy, such as organic chromophores, QDs, or carbon-nanomaterials, demonstrating the benefits of NIR microscopy compared to conventional microscopy.153–156 Most luminescent NIR probes have lower quantum yields compared to their visible counterparts. The main drawbacks of organic dyes are their photobleaching and small Stokes shift, whereas the application of nanomaterials can be limited by their cytotoxicity and their

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Fig. 10 NIR-II imaging (right) provides deeper tissue penetration than NIR-I imaging (left). Reproduced with permission from Chen, C.; Tian, R.; Zeng, Y.; Chu, C.; Liu, G. Activatable Fluorescence Probes for “Turn-On” and Ratiometric Biosensing and Bioimaging: From NIR-I to NIR-II. Bioconjug. Chem. 2020, 31 (2), 276–292. doi:10.1021/acs.bioconjchem.9b00734. Copyright 2020 American Chemical Society.

bigger size. Nowadays, the challenge focuses on developing biocompatible NIR-II probes with enhanced luminescence properties, i.e., high quantum yields and photostability.157 In this section, we discuss some recent examples concerning Ln3 þ-based complexes and NPs developed over the last few years for NIR-II bioimaging. Motivated by the successful application of Ln coordination complexes in visible luminescence microscopy (vide supra), researchers have intended to develop NIR emissive Ln3 þ-based compounds for NIR microscopy.158 Ln3 þ coordination compounds not only possess the advantages of organic dyes (i.e., small size, tunable excitation, and relatively large extinction coefficient), but also show resistance to photobleaching, large Stokes shift, and long luminescence lifetimes.8 Several groups have contributed to the development of NIR Ln3 þ probes, especially, those containing Yb3þ (band centered around  1000 nm), Nd3þ ( 900 nm,  1060 nm, and  1340 nm), or Er3þ ( 1550 nm). For example, a series of biocompatible b-fluorinated Yb porphyrinate complexes was successfully applied for NIR confocal intracellular visualization by recording the luminescence signals of Yb in the 900–1100 nm biological window upon 405 nm excitation.29 Different synthetic strategies were carried out to render the porphyrinate complexes water-soluble, such as meso-modification, b-cycloaddition, or b-lactonization (Fig. 11A). The described probes possessed strong luminescence in water (QYs of 5–13%), stability, and long decay times (> 100 ms), allowing effective discrimination from cell autofluorescence, and a superior signal-to-noise ratio (Fig. 11B–F). Because of the long luminescence lifetimes of these complexes, they have also been used for TR FLIM in the NIR window, which also offers quantitative imaging at molecular scale but is independent from the luminescence intensity and emitter concentration.159 In addition, the porphyrinates complexes are easily modifiable, which can be harnessed to make them sensitive to biomolecules or environmental biological changes. The reported NIR pH-responsive Yb porphyrinate probe was able to quantitatively and dynamically monitor real-time gastrointestinal intracellular pH changes in situ, which evidenced the potential application of these Ln3 þ porphyrinate complexes for in vivo visualization. Concerning the use of Ln3 þ-based NPs for NIR bioimaging, Jia et al. developed a novel bioimaging method for tissue multichannel NIR-II imaging, i.e., simultaneous imaging in different wavelength ranges, which can provide a better understanding of complex biological processes.160 They synthesized two Ln-doped nanoprobes, specifically, NaYF4:Gd@NaYF4:Nd@NaYF4 (cssNd) and NaYF4:Gd@NaYF4:Er@NaYF4 (cssEr), showing orthogonal NIR-II emissions with different excitation wavelengths: (i) 1064 and 1330 nm for cssNd upon their excitation at 730 nm, and (ii) 1550 nm for cssEr after 980 nm irradiation (Fig. 12). Both

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Fig. 11 (A) Synthesic routes of biocompatible b-fluorinated Yb3þ complexes. (B) Comparison of the emission spectra of b-fluorinated Yb3þ complex (Yb-4) and the analogous non-fluorinated Yb3þ complex (Yb-4c) in water (0.1% DMSO, lex ¼ 410 nm). Inset: their decay curves monitored at 980 nm. (C–F) NIR confocal images of living HeLa cells incubated with 10 mM Yb-4 for 12 h followed by 30 min incubation with 75 nM LysoTracker Green. (C) Bright field; (D) Yb3þ NIR signal (lex ¼ 620 nm; lem ¼ 935/170 nm); (E) LysoTracker Green visible signal (lex ¼ 470 nm; lem ¼ 530/43 nm); (F) merged image of D and E showing colocalization. Scale bars: 10 mm. (A) Reproduced with permission from Jin, G.-Q.; Ning, Y.; Geng, J.-X.; Jiang, Z.-F.; Wang, Y.; Zhang, J.-L. Joining the Journey to Near Infrared (NIR) Imaging: The Emerging Role of Lanthanides in the Designing of Molecular Probes. Inorg. Chem. Front. 2020, 7 (2), 289–299. doi:10.1039/C9QI01132C. Copyright 2020 Royal Society of Chemistry. (B) Reproduced with permission from Ning, Y.; Tang, J.; Liu, Y.-W.; Jing, J.; Sun, Y.; Zhang, J.-L. Highly Luminescent, Biocompatible Ytterbium(III) Complexes as Near-Infrared Fluorophores for Living Cell Imaging. Chem. Sci. 2018, 9 (15), 3742–3753. doi:10.1039/C8SC00259B. Copyright 2018 Royal Society of Chemistry.

NPs were further modified with PEG polymer to render them water-soluble and biocompatible. They carried out tissue mimic experiments by using a glass capillary tube containing the NPs and covered by various thicknesses of beef tissue. The tube was then irradiated with a 730 or 980 nm laser (for cssNd and cssEr, respectively) with a fixed power density of 66 mW cm 2 (730 nm) or 69 mW cm 2 (980 nm), and the emission signal was collected by using a corresponding NIR CDD detector. Their findings suggested that the tissue penetration depth of the NIR-II emission for both NPs was up to 4.5 mm. Additionally, further experiments were also realized in vivo. The results demonstrated the feasibility of multichannel NIR imaging with separable signals in a mouse, enabling to distinguish spatially different tissues with a high resolution, i.e., liver and stomach, under two different excitation wavelengths. An outstanding feature of lanthanide-doped NPs is the possibility of doping them with different lanthanide cations with diverse optical properties, allowing the simultaneous performance of multiple imaging techniques to overcome the individual limitations of each.161–163 As an illustrating example, NaGdF4: Nd (5%)@NaGdF4 NPs have been described as a powerful NIR-II nanoprobe for visualization of small hepatocellular carcinoma lesions.164 The NP surface was functionalized with liposomes to render them hydrophilic. Interestingly, the quantum yield of the Gd3þ emission upon 808 nm excitation was calculated to be 0.36% at 1057 nm and 0.17% at 1335 nm. The NPs displayed good photostability in PBS and low cytotoxicity. Because of the magnetic properties of the Gd3þ cation, the described NPs were able to perform simultaneously both magnetic resonance imaging and NIR-II imaging with high sensitivity and contrast allowing the differentiation between the liver cancer and surrounding normal liver, opening new opportunities for its application in intra-operative guidance.

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Fig. 12 Schematic diagram of the orthogonal NIR imaging system with cssNd and cssEr NPs to separate the different tissues by tuning the excitation wavelength. Reproduced with permission from Jia, Q.; Ma, L.; Zhai, X.; Fu, W.; Liu, Y.; Liao, X.; Zhou, J. Orthogonal Near-Infrared-II Imaging Enables Spatially Distinguishing Tissues Based on Lanthanide-Doped Nanoprobes. Anal. Chem. 2020, 92 (21), 14762–14768. doi:10.1021/ acs.analchem.0c03383. Copyright 2020 American Chemical Society.

Persistent luminescence has also been used as a promising tool for NIR imaging. After turning off the excitation light, the luminescence remains for a long time, which can range from some seconds to several days. Therefore, the autofluorescence is efficiently suppressed since the excitation has ceased.165 Wu et al. synthesized a nanophosphor doped with Nd3þ, Ce3þ, and Cr3þ with persistent luminescence properties.166 Upon 10 min of excitation with a 410-nm LED, the nanophosphor showed the Nd3þ emission lines in the NIR-I (at 890 nm) and NIR-II window (1063 nm) during 60 min. While the NIR-I emission allowed a penetration depth in a chicken breast tissue of 2.5 mm, the NIR-II penetration was up to 3.9 mm. Pei et al. prepared persistent luminescence NPs doped with Nd3þ, Ho3þ, Tm3þ, or Er3þ, which could be activated by X-ray radiation and emit NIR-II luminescence over several hours (still detectable after more than 72 h).167 The NPs could be used for NIR-II color encoding with Nd, Ho, and Er persistent luminescence and for in vivo imaging via Er and Nd persistent luminescence. Unfortunately, the brightness of the NP persistent luminescence was still too low to permit the use of lower X-ray doses (below the safety threshold) with a possibly deeper tissue penetration depth, which may become very useful to recharge the luminescent probes directly in vivo for long-term nanoprobe tracking and monitoring.

8.12.4.6

Upconversion (UC) microscopy

UC refers to a non-linear optical process that involves the emission of one single photon of higher energy by the absorption of two or more lower-energy photons, i.e., UC materials can emit UV, Vis, or even NIR light after absorption of NIR light.168 With the advances of nanoscience over the last decades, UCNPs have gained much popularity concerning imaging applications. UCNPs are lanthanide-based NPs that display UC luminescence.169 Traditionally, the energy transfer upconversion (ETU) mechanism is the predominant one, for which one Ln cation absorbs the NIR light (sensitizer) and transfers the energy to another Ln cation which emits the light (activator). UCNPs have narrow emission bands, large anti-Stokes shift, low cytotoxicity, good (photo)chemical stability, and are neither photoblinking nor photobleaching.170,171 Importantly, their surfaces can be modified with a broad range of organic molecules, biomolecules, or dyes, which makes them applicable as a new generation of luminescent biolabels.172,173 Owing to their unique NIR irradiation followed by UV/Vis emission, UCNPs have been extensively studied in microscopy for a broad range of applications,174 such as intracellular pH imaging,175 temporally multiplexed imaging,144 drug tracking,176 or anticounterfeiting.177 In a recent study, Tang et al. built an orthogonal emissive core-shell-shell UCNPs architecture composed of NaErF4:Yb/Tm@NaYF4:Yb@NaNdF4:Yb for imaging-guided on-demand photodynamic therapy (PDT).178 The NP matrix was specially designed to precisely control the energy migration processes between the Ln3 þ doping cations by using different excitation wavelengths to get green or red emission from a single activator, i.e., the Er3þ. Thus, when the nanostructure was excited under 808 nm, i.e., the Nd3þ acts as sensitizer, the green emission from Er3þ was mainly observed while when irradiated at 980 nm, i.e., Yb3þ excitation, the Er3þ red emission was predominating. That was possible because Tm3þ cations enhanced the red UCL of Er3þ through energy trapping, and the Yb3þ sensitizers were confined in the core, inner shell, and outer shell to sensitize 980 nm light while Nd3þ

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were just confined in the outer shell to harvest the 808 nm light. Later, a well-known photosensitizer with a green absorption, zinc phthalocyanine, was embedded into the mesoporous silica layer on the NP surface to carry out PDT. The authors demonstrated that the coupling between therapy and imaging was feasible by selecting the excitation source, 808 and 980 nm, respectively. These results are promising for the application of UCNPs for imaging-guided therapy. Single-particle and super-resolution imaging are among the most emergent and relevant optical imaging technologies with UCNPs.179,180 They demonstrated both high sensitivity and high resolution, in particular, when NIR-to-NIR UC was accomplished.117 Single-nanoparticle tracking consists of the measurement of isolated particles, which is essential to better characterize the NPs and determine the statistical brightness behavior of many NPs that may discover additional chemical or physical properties.117 Liu et al. presented a systematic study of the UCL brightness by combination of wide-field and confocal microscopy to evaluate different sizes and compositions of UCNPs at the single NP level with a laser power density between 8 W cm 2 and 6 MW cm 2 (Fig. 13A and B).179 Contradictorily to the previous studies where it had been assumed that the most luminescent UCNPs were those with a NaYF4:Yb(20%);Er(2%) composition, they found that NaYF4:Yb(20%);Er(8%) showed a 150-fold enhancement of the luminescence intensity. These discrepancies were explained by the observation of an enhancement by the inert shell. Super-resolution imaging can afford images with a resolution beyond the diffraction limit. Recently, small core-shell NaYF4,Tm (8%)@NaYF4,Gd (20%) UCNPs were demonstrated as exceptional probes for super-resolution NIR-to-NIR imaging.181 The UC emission generated at 800 nm displayed the unconventional photon avalanching (PA) UC mechanism, which allowed an extremely large UCL response upon their excitation at 980 nm. The authors observed the PA mechanism in UCNPs and exploited this unconventional feature for their use in photon-avalanche single-beam super-resolution imaging (PASSI). Moreover, the PA phenomenon was dependent on the pumping intensity, with a 500–10000-fold enhancement of the UCNP brightness beyond the PA threshold (Fig. 13C). The PASSI images were obtained by using scanning confocal microscopy and without any further computational analysis, achieving a spatial resolution of sub-70-nm with relatively low excitation powers. These recent findings will be of importance for further super-resolution imaging applications of the UCNPs.

8.12.5

Conclusion

Thanks to their exceptional luminescence properties, Ln probes allow to play with photons from the UV to the NIR and vice versa. Using time-gating, their large pseudo Stokes shift, multiplexing, or upconversion, Ln probes open numerous applications to

Fig. 13 (A) SEM images and three-dimensional representation of the wide-field UCL images at 8 W cm 2 and 625 W cm 2 of NaYF4@NaYbF4:Er(8%)@NaYF4 (top) and NaYF4:Yb(20%),Er(2%) (bottom). Scale bars: 500 nm. (B) Wide-field images (625 W cm 2) for NaYF4:Yb(20%),Er(2%) (left) and NaYF4@NaYbF4:Er(8%)@NaYF4 (right). Scale bars: 2 mm. (C) Experimental PASSI images of 8% Tm3þ UCNPs, separated by 300 nm, excited at different intensities, from near saturation regime (left) to near PA regime (right). (A and B) Adapted with permission from Liu, Q.; Zhang, Y.; Peng, C. S.; Yang, T.; Joubert, L.-M.; Chu, S. Single Upconversion Nanoparticle Imaging at Sub-10 W cm 2 Irradiance. Nat. Photonics 2018, 12(9), 548–553. doi:10.1038/s41566-018-0217-1. Copyright 2018 The Authors. (C) Reproduced with permission from Lee, C.; Xu, E. Z.; Liu, Y.; Teitelboim, A.; Yao, K.; Fernandez-Bravo, A.; Kotulska, A. M.; Nam, S. H.; Suh, Y. D.; Bednarkiewicz, A.; Cohen, B. E.; Chan, E. M.; Schuck, P. J. Giant Nonlinear Optical Responses From Photon-Avalanching Nanoparticles. Nature 2021, 589 (7841), 230–235. doi:10.1038/s41586020-03092-9.Copyright 2021 The Authors.

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imaging at the cellular or at the whole animal scale with low background fluorescence. The perspectives in the field are widely opened with large potentials remaining such as for improvements in persistent luminescence NPs,165 for multimodal imaging or for the development of molecular upconverting devices luminescent in biological media.61 To meet these future challenges, it is expected that the creativity of chemists, biologists, and instrumentalists will again be requested to repel the frontiers of the known world.

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8.13 Ultrafast dynamics of photoinduced processes in coordination compounds Fernandez-Teran Ricardo J. Ferna´ndez-Tera´n and Julia A. Weinstein, Department of Chemistry, University of Sheffield, Sheffield, United Kingdom © 2023 Elsevier Ltd. All rights reserved.

8.13.1 8.13.2 8.13.2.1 8.13.2.2 8.13.2.3 8.13.2.4 8.13.3 8.13.4 8.13.5 8.13.5.1 8.13.5.2 8.13.5.3 8.13.5.4 8.13.6 8.13.6.1 8.13.6.2 8.13.7 References

Introduction Pump–probe spectroscopy Brief principles Transient electronic absorption spectroscopy (TAS) Transient infrared spectroscopy (TRIR) Time-resolved Raman spectroscopy Time-resolved emission spectroscopy Time-resolved structural methods: X-ray spectroscopy and molecular movies Ultrafast multidimensional spectroscopy General concepts Two-dimensional infrared spectroscopy (2D-IR) Two-dimensional electronic spectroscopy (2D-ES) Mixed spectroscopies: Two-dimensional electronic-vibrational and vibrational-electronic spectroscopies (2D-EV and 2D-VE) Multi-pulse experiments Transient 2D-IR spectroscopy UV pump–IR pump–IR probe spectroscopy: IR control of electron transfer Concluding remarks

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Abstract This Chapter briefly introduces modern and emerging methods to study ultrafast processes in coordination compounds, which are key to any photon-driven application from artificial photosynthesis to information storage. Ultrafast optical methods [namely electronic transient absorption, fluorescence upconversion, time-resolved infrared and resonance Raman], structural methods [X-ray spectroscopies and scattering], and ultrafast multidimensional methods are discussed. The relevance of each method to photochemistry of coordination compounds is illustrated on specific examples. The importance of using multiple methodsde.g., combining electronic, vibrational, and X-ray spectroscopiesdto resolve the complex and entangled electronic, structural and spin dynamics happening on the short, fs–ps timescales, is emphasized. Finally, the approaches to direct and control photochemical dynamics are discussed.

8.13.1

Introduction

The tremendous advances of laser technologies in the last decades have prompted a rapid development of multiple experimental techniques that allow one to capture the ever-faster events in a molecular system. The revolutionary work of Porter and Norrish in 1950s enabled first modern-age investigation of light-induced molecular reactions in a time-resolved manner.1 In their apparatus, the reaction was triggered by a short and intense flash from gas-filled flash discharge tubesdfrom which the name “flash photolysis” originated. The Nobel Prize in Chemistry 1967, to Manfred Eigen, and to Ronald George Wreyford Norrish and George Porter, was awarded “for their studies of extremely fast chemical reactions, effected by disturbing the equilibrium by means of very short pulses of energy.” The article “A Century of Chemical Dynamics Traced through the Nobel Prizes. 1967: Eigen, Norrish, and Porter” gives a detailed account of the field at the time.2 Fast-forward through invention and development of lasers, to femto-chemistry (Nobel Prize in Chemistry 1999, awarded to Ahmed Zewail) to chirped-pulse amplification (for which Donna Strickland and Gérard Mourou shared half of the Nobel Prize in Physics in 2018), and X-ray free-electron lasers. We now live in a world where the increased brightness, stability and tuneability of photon sources, along with tremendous advances in sensitivity of the detectors, brings the “real-time molecular movies” within reach of researchers.3,4 This chapter will review, briefly, recent advances in applications of these technologies to light-induced processes in transition metal complexes, which underpin their applications in artificial photosynthesis and photocatalysis, light-emitting devices, information storage, development of photoactive materials, applications in imaging and photomedicine, and so forth.

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Upon absorption of light, a molecule can undergo a series of transformations in their structure and composition (e.g., photoreactions), engage in intermolecular processes such as electron or proton transfer, energy transfer, and transit deactivation pathways unavailable from their electronic ground state. While the majority of organic molecules react mostly within a singlet spin manifold, transition metal complexes have a more varied and complex spin landscape, where the electron configuration of the metal and the interaction with the ligands play a significant role. Typically, transition metal complexes strongly favor intersystem crossing from a manifold of singlet excited states to a triplet manifold,5,6 and/or, especially in first-row complexes further higher spin multiplicity states can play a fundamental role in their deactivation, as we shall discuss in the following sections. A graphical representation of the relative energy levels of these electronically excited states, and the transitions that they are engaged in, is exemplified on a so-called Jablonski diagram (Fig. 1), which summarizes the photophysical properties of a molecular system. An example of the complex involvement of higher multiplicity states in the relaxation pathways of metal complexes is given in Fig. 2, which illustrates the complex excited state landscape of a series of [Fe(bpy)3]2 þ complexes. Excitation to a singlet metal-toligand excited state (MLCT) leads to very fast intersystem crossing into the triplet manifold, followed by a quick decay into the metal-centered states of higher spin multiplicity. These states finally deactivate via radiationless transitions back to the ground state.7 To elucidate the dynamics of excited states of transition metal complexes, all modern techniques and their combinations have been applied. We will begin this chapter by discussing pump–probe spectroscopy in both the electronic and vibrational domains (Section 8.13.2), followed by time-resolved emission spectroscopy (Section 8.13.3). Direct structural information and elementspecific information can nowadays be extracted using time-resolved X-ray based methods, introduced in Section 8.13.4. Pump–probe experiments have been traditionally performed with a single excitation wavelength. To obtain further informationdtypically inaccessible for these “1D” methodsdmultidimensional spectroscopic methods using broadband excitation (or by scanning the excitation wavelength), both in the vibrational and electronic domains (i.e., two-dimensional infrared and twodimensional electronic spectroscopies), are needed. We briefly introduce such methods and their applications to ultrafast photoinduced processes in transition metal complexes in Section 8.13.5, concluding the chapter with mixed two-dimensional vibrational-electronic spectroscopies (2D-EV and 2D-VE). Finally, multiple-pulse experiments that can reveal information about, for example, conical intersections, vibronic couplings and which could control the outcome of certain excited-state reactions are discussed in Section 8.13.6.

8.13.2

Pump–probe spectroscopy

8.13.2.1

Brief principles

We aim here to give an overview of the applications of pump–probe methods to the study of transition metal complexes and their exciting photochemical dynamics. The majority of the illustrative examples chosen are from publications that appeared in the past 5

Fig. 1 A schematic Jablonski diagram. The solid arrows indicate radiative transitions, while the wavy arrows indicate non-radiative processes. Absorption of a photon (1) leads to population of an electronically and vibrationally excited state. Vibrational cooling and internal conversion (2) lead to fast relaxation on the excited state manifold to the lowest singlet state. In systems where the spin–orbit coupling is large, efficient intersystem crossing (3) leads to the triplet manifold, where internal conversion can also take place between triplet states. Finally, having reached the lowest level of the excited singlet or triplet manifolds, the molecule can either relax back to the ground state by internal conversion and vibrational cooling (4), or emit light by fluorescence (5) or phosphorescence (6).

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Fig. 2 Representative potential energy surface diagram of Fe(II)-based spin crossover complexes, as discussed in Ref.7. Reproduced with permission from Bressler, C.; Milne, C.; Pham, V.-T.; ElNahhas, A.; van der Veen, R. M.; Gawelda, W.; Johnson, S.; Beaud, P.; Grolimund, D.; Kaiser, M.; Borca, C. N.; Ingold, G.; Abela, R.; Chergui, M. Femtosecond XANES Study of the Light-Induced Spin Crossover Dynamics in an Iron(II) Complex. Science 2009, 323 (5913), 489–492. doi:10.1126/science.1165733. Copyright 2009, American Association for the Advancement of Science.

years or so, and are by no means comprehensivedthe reader is referred to multiple comprehensive reviews on the subject,8–12 a recent special collection (“Developments in Ultrafast Spectroscopy”dRef.13 and other articles in the special collection), and to the recent reviews on theoretical methods in application to metal complexes.14–16 In an ultrafast time-resolved pump–probe experimentdable to resolve dynamics from fs to a few nsda sample is excited by a first pulse (the pump), and the spectroscopic changes associated with this excitation are monitored in a time-dependent manner with a second, probe pulse, which interacts with the sample after a pre-set time delay (the pump–probe delay). This delay is varied in discrete steps, and the experiment is repeated until the desired time window has been mapped (with appropriate signal-to-noise ratio). The time resolution attainable with pump–probe spectroscopy is limited by the duration of the pump and probe pulses, as the experimentally detected response results from the convolution of the temporal profiles of the pump and probe pulses. The maximum achievable time delay is limited by the repetition rate of the laser. A typical experimental implementation starts with a common laser pulse, which is split into the pump and probe branches using a beam splitter (Fig. 3). The pump–probe delay (population delay) is adjusted by moving a translation stage, increasing (decreasing) the path length of the probe (pump) branchdin such a way that the probe pulse arrives at the sample after the pump (defined as a positive population delay). The specific way the pump and probe pulses are generated will of course depend on the region of the electromagnetic spectrum that is being explored. For example, in a UV pump–IR probe experiment, the UV pump pulses are generated typically by frequency doubling or tripling of a fundamental laser wavelength (e.g.,  800 nm in Ti:Sapphire, 1064 nm in Nd:YAG, or 1035 nm in Yb-based lasers), or by using an optical parametric amplifier (OPA), while the mid-IR probe pulses are generated by differential frequency mixing or in an OPA.17 In contrast, an IR pump–IR probe experiment where the spectra of the two pulses are identical can be performed by splitting off the probe pulses from the output of the OPA with the help of a wedged window (ca. 2% intensity is reflected for the very weak probe beams).18 Two-color IR pump–IR probe experiments can be implemented by using two OPAs tuned to different frequencies/ranges of frequencies. Finally, in a UV pump/UV–Vis probe experiment, the Vis probe can be generated either by the use of a broadband

Fig. 3

A simplified pump–probe experiment.

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non-collinear OPA (NOPA),19–22 or by tightly focusing a weak beam into a non-linear medium which generates a supercontinuum (white light).23–25 Probe pulses extending into the UV can be generated by achromatic doubling of the NOPA output,26 or by focusing the frequency-doubled output of a Ti:Sapphire laser into a translating CaF2 plate.23 As mentioned before, the general principle of a transient absorption spectrometer relies on using the probe pulse to monitor small changes induced in the absorption of the sample. For this, typically two consecutive shots are collected: one in absence and one in presence of the pump pulse. This ON/OFF switching of the pump beam is achieved through a mechanical chopper (shown in Fig. 3), which blocks every second pump pulse, and is synchronized to the laser repetition rate. The femtosecond TAS experiments usually acquire a full spectrum at a specific time delay, with the kinetics then reconstructed from the spectra. Time-resolved spectroscopy in the nanosecond and slower time regimes can be directly performed by monitoring the changes in the probe intensity as a function of time after photoexcitation. This technique is typically performed using a fast photomultiplier tube (PMT) detector and fast acquisition electronics (e.g., a fast digitizing oscilloscope), and is often called flash photolysis or nanosecond transient absorption. In this case, the kinetic information is collected in one shot, but at a specific detection wavelength, and the experiment is repeated at multiple wavelengths. The spectrum at a given time-delay is then reconstructed from the kinetic data. Regardless of the method of data acquisition, a differential absorption spectrum contains three contributions: (i) the groundstate bleach (GSB), corresponding to the decreased number of molecules in the ground state; (ii) excited-state absorption (ESA), due to absorption of the probed state into higher excited states or due to the absorption of newly generated species (photoproducts); and (iii) stimulated emission (SE), due to stimulated fluorescence of the sample induced by the probe pulse (assuming a singlet ground state). These features are exemplified in Fig. 4. The total signal corresponds to the sum of these responses, which may have different amplitudes and may overlap, resulting in partial apparent cancelation of spectral features.

8.13.2.2

Transient electronic absorption spectroscopy (TAS)

The most common time-resolved experiment involves electronic excitation of a molecule with a pulse in the UV/Visible region (300–800 nm), and detection of the corresponding changes in the absorption spectrum in the same region. We will refer to this method as Transient Electronic Absorption Spectroscopy (TAS). The pump pulse is typically “narrowband” (a few nm FWHM depending on the pulse duration, typically in the order of 40–120 fs), which allows for a somewhat selective excitation into an absorption band in a metal complex. Perhaps the most studied and well-known complex from the point of view of inorganic photochemistry is [Ru(bpy)3]2 þ and its derivatives,12,27–31 which continue to be so to the present day. An example of an important fundamental question which caused considerable debate is whether the excitation into an MLCT transition results in the localized or delocalized excess electron density on the one or several bpy-ligands, at a specific point in time after the excitation. An extension of TASdfemtosecond transient absorption anisotropy,32 has shown that ultrafast interligand randomization of the MLCT state takes place within 1 ps, and hence the complex lost its “memory” of which bipyridine ligand was initially photoselected. Of direct relevance to development of artificial photosynthetic systems are pH-sensitive photosensitizers and catalysts. A recent study33 showed that protonation isomers of the linearly p-extended [(tbbpy)2Ru(L)]2 þ-type complexes bearing a dppz (dipyridophenazine) ligand with directly fused imidazole (im) and methyl-imidazole units (mim) as L, have strong absorbance in the visible region, and two long-lived excited states, 3ILCT and 3MLCT, at pH values between 3 and 12 (Fig. 5). It was founddby combining transient absorption and emission spectroscopydthat protonation of the (methyl-)imidazole unit at pH  2 causes decreased excited-state lifetimes and causes an emission switch-off. This is evidenced in Fig. 6, where the differences in the transient spectral signatures between the anionic, neutral and protonated forms of the complex serve to explain the differing relaxation pathways in the excited states at different pH.

Fig. 4 Different responses observed in a typical pump–probe spectrum: the positive ESA, and the negative GSB and SE contributions. Their position is arbitrary and merely for illustrative purposes.

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Fig. 5 Structural formula (a) and absorption and emission spectra of Ru-im, Ru-imL, and Ru-imHD (b) as well as Ru-mim and Ru-mimHD (c). The spectra are recorded in acetonitrile (emission upon excitation at 400 nm). Reproduced from Müller, C.; Isakov, D.; Rau, S.; Dietzek, B. Influence of the Protonation State on the Excited-State Dynamics of Ruthenium(II) Complexes with Imidazole p-Extended Dipyridophenazine Ligands. J. Phys. Chem. A 2021, 125 (27), 5911–5921. doi:10.1021/acs.jpca.1c03856 with permission. Copyright 2021, American Chemical Society.

Fig. 6 Transient absorption (top row) and decay-associated spectra (bottom row) of Ru-imHD (a and b, violet), Ru-imL (c and d, orange), and Ruim (e and f, green) in acetonitrile upon excitation at 400 nm (0.4 mW, excitation densities of 10%). Reproduced from Müller, C.; Isakov, D.; Rau, S.; Dietzek, B. Influence of the Protonation State on the Excited-State Dynamics of Ruthenium(II) Complexes with Imidazole p-Extended Dipyridophenazine Ligands. J. Phys. Chem. A 2021, 125 (27), 5911–5921. doi:10.1021/acs.jpca.1c03856 with permission. Copyright 2021, American Chemical Society.

Multielectron accumulation in transition metal complexes is also of central importance toward the development of functional artificial photosynthetic systems.34 A recent example35 reports the photophysical properties of a charge-photoaccumulating Ru(II) complex with an extended dppz ligand (Fig. 7). The oxime-containing complex was able to store two electrons, coupled to two protons on the p-extended ligand framework, as illustrated by TAS (Fig. 8) and suitable energy level diagrams (derived from experimental quantities). Transient absorption spectroscopy is also well suited to provide mechanistic details about electron and energy transfer processes in multichromophoric and multicomponent systems, such as Ru-Pt dyads. For instance, a Ru-Pt dyad has been studied by means of

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Fig. 7 Structure of [(bpy)2Ru(L)]2 þ bearing an oxim-dppqp (R ¼ N–OH, Ru) or oxo-dppqp (R ¼ O) ligand as L. The dipyridophenazine (dppz) and pyridoquinolinone (pq) moieties are highlighted in yellow and green, respectively. Violet arrows indicate considered protonation sites (gray circles) and their thermodynamical preference for Ru as predicted at the B3LYP/def2SVP level of theory. Blue arrows indicate positions of the second protonation site (first protonation occurs at the pq-moiety) upon two-electron reduction, relative energies are given accordingly. Reproduced with permission from Müller, C.; Schwab, A.; Randell, N. M.; Kupfer, S.; Dietzek-Ivansic, B.; Chavarot-Kerlidou, M. A Combined Spectroscopic and Theoretical Study on a Ruthenium Complex Featuring a p-Extended Dppz Ligand for Light-Driven Accumulation of Multiple Reducing Equivalents. Chem. A Eur. J. 2022, 28 (18), e202103882. doi:10.1002/chem.202103882. Copyright 2022, The Authors. Chemistry–A European Journal published by Wiley-VCH GmbH.

TAS, ns time-resolved emission and photocatalytic studies of H2 evolution in aqueous solution (Fig. 9).36 The authors found that a charge-separated (CS) state was formed in the sub-picosecond time regime and recombined in the picosecond time regime. The CS-state formation was found to compete with reductive quenching of the triplet excited state by EDTA-dianion that forms ion-pairs with one of the complexes. It was also shown that some of the conformers in solution possess a CS lifetime sufficiently long to drive hydrogen evolution from water. Another example of the use of time-resolved spectroscopy to study multichromophoric systems comes from a Ru-Pt dyad which shows a self-repairing reoxidation by in-situ generated singlet oxygen (Fig. 10).37 While a lot of work has been focused on the complexes of precious metalsdsuch as Ru, Os, Pt, Ir or Reddue to their kinetic stability, photostability, strong absorbance in the visible region and often long excited-state lifetimes, there is a strong need to move away from such systems toward cheaper and more scalable alternatives. Fe(II) complexes have emerged as valuable contenders,38–42 together with Cr(III),43,44 Mn(I),45 and Cu(I).46–48 We believe that further investigation of complexes with firstrow transition metals using ultrafast spectroscopic methods will lead to a richer understanding of their deactivation mechanisms, which could hopefully be disabled by a judicious ligand choice. The examples discussed so far were mainly concerned with the dynamics of complexes in solution. The operation of both the dye-sensitized solar cells, and heterogeneous catalysts rely on the efficient electron transfer between a semiconductor and a transition metal complex immobilized on its surface. One such example,49 focused on the study of the ultrafast hole and electron transfer processes that take place after photoexcitation of a covalently-linked organic dye–cobaloxime catalyst system (Fig. 11) immobilized on mesoporous NiO. In this case, TAS confirmed the presence of the oxidized state of the dye (when absorbed on TiO2), the reduced state of the dye (when absorbed on NiO) or the excited state (when absorbed into ZrO2), illustrating how these three semiconductor surfaces can be used as templates for electron or hole injection, or as a neutral scaffold to derive mechanistic studies on light-induced electron and charge injection processes. A brief summary of the observed processes for this particular system on NiO is shown in Fig. 12, illustrating the complex nature of the photophysical events occurring in these artificial systems leading to hydrogen production. Another example of the use of TAS to follow electron transfer processes between a dye and a catalyst50,51 reports on a dye and proton reduction catalyst co-immobilized on a NiO semiconducting substrate (Fig. 13). The authors found ultrafast sub-ps injection of holes into NiO from the excited organic dye, followed by a rapid reduction of the catalyst on the surface (in ca. 10 ps). This reduced catalyst was long-lived (2 ms to 20 ms), allowing for potential protonation and a second reduction to take place. The photoelectrochemical device made using this dye–catalyst combination showed excellent yields but a notable deterioration due to catalyst degradation and desorption form the semiconductor surface. TAS studies on sensitized TiO2 photoelectrodes, decorated with Ru(II) and Os(II) polypyridyl photosensitizers,52 probed the ultrafast electron injection dynamics between the components, and the effect of the excited-state energetics and the nature of the metal center on such processes. The results indicated that electron injection from nonthermalized excited states competes more effectively with ultrafast intersystem crossing for the Ru sensitizers than for the Os sensitizers. The breadth of the use of TAS is illustrated further by its application to study the photoinduced dynamics of electron transfer in, for example, coordination cages (host–guest interactions),53 BODIPY–ZnPorphyrin–C60 assemblies,54 and Prussian blue analogues.55 In application to understanding catalysis, a TAS study of the Ni(II) aryl halide complexes common to crosscoupling and Ni/photoredox reactions, was illustrated by a recent report (Fig. 14).56 Computational and ultrafast spectroscopic studies revealed the presence of long-lived 3MLCT excited states (Fig. 15), highlighting Ni complexes as an underexplored alternative to precious metal photocatalysts. It was also shown that 3MLCT in Ni(II) complexes engages in bimolecular electron transfer

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Fig. 8 Species associated spectra (SAS, a–c) and their corresponding reconstructed decay associated spectra (DAS, d–f) of the fs-TA data of Ru (left), RuHD (middle), and RuH2 upon 400 nm excitation (0.4 mW) in acetonitrile. The employed target models are shown on top. States (solid lines) and processes (dotted lines) that are not included in the target model are shown in gray in the Jablonski diagrams: Since the pp* excited states dominate the TA signals the proximal MLCT states (MLCTprox: MLCTbpy and MLCTphen), which typically show strong GSB between 400 and 450 nm and weak ESA ranging from 500 to 750 nm, were not included in the target models (cf. gray states and dashed lines in the Jablonski schemes). The DAS were qualitatively constructed from the SAS. Reproduced with permission from Müller, C.; Schwab, A.; Randell, N. M.; Kupfer, S.; DietzekIvansic, B.; Chavarot-Kerlidou, M. A Combined Spectroscopic and Theoretical Study on a Ruthenium Complex Featuring a p-Extended Dppz Ligand for Light-Driven Accumulation of Multiple Reducing Equivalents. Chem. A Eur. J. 2022, 28 (18), e202103882. doi:10.1002/chem.202103882. Copyright 2022, The Authors. Chemistry–A European Journal published by Wiley-VCH GmbH.

with ground-state Ni(II), enabling access to Ni(III) in the absence of external oxidants or photoredox catalysts. As such, it is possible to facilitate Ni-catalyzed CeO bond formation solely by visible light irradiation, thus representing an alternative strategy for catalyst activation in Ni cross-coupling reactions. A series of RCpTi[-C^C-Fe(Cp)2]2 complexes with different substituents were recently studied with transient absorption, TDDFT, and in the presence of Cu(I) (Fig. 16).57 The metal-to-metal charge transfer (MMCT) excited state of the complexes with Fe(II) donors and Ti(IV) acceptors undergoes back-electron transfer (BET) and has a lifetime of 20–40 ps, while coordination of Cu(I) increases the MMCT lifetime by 3 orders of magnitude. Their investigations suggest that the BET lifetime of the Cu-free parent complex is dominated by mixing of the MMCT state with a 1Fc excited state, while relaxation of the complex containing coordinated Cu(I) proceeds through a long-lived 3Fc state. Performing TAS at variable temperatures,58 and in the presence of magnetic fields59,60 allows one to resolve in even more detail a complex interplay of excited state processes in transition metal complexes. While transient electronic absorption spectroscopy is undoubtedly an extremely useful tool, the electronic absorption spectra of excited states are generally very broad and not always distinct. Electronic spectroscopy is best combined with other methods, such as time-resolved vibrational spectroscopy. Time-resolved vibrational spectroscopy (TRVS) provides a powerful and complementary method to resolve a manifold of close-lying excited states because it probes molecular structure. TRVS is also invaluable for

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Fig. 9 Energy level diagram showing the photoinduced processes in the complexes studied in Ref.36. Reproduced with permission from Suneesh, C. V.; Balan, B.; Ozawa, H.; Nakamura, Y.; Katayama, T.; Muramatsu, M.; Nagasawa, Y.; Miyasaka, H.; Sakai, K. Mechanistic Studies of Photoinduced Intramolecular and Intermolecular Electron Transfer Processes in RuPt-Centred Photo-Hydrogen-Evolving Molecular Devices. Phys. Chem. Chem. Phys. 2014, 16 (4), 1607–1616. doi:10.1039/c3cp54630f. Copyright 2013, Royal Society of Chemistry.

elucidating the dynamics of vibrationally hot electronic states which are frequently formed upon initial photo-excitation and play a key role in the ultrafast intramolecular energy redistribution. The frequently used types of TRVS are time-resolved infrared (TRIR), and time-resolved Raman methods, which are discussed in the following section.

8.13.2.3

Transient infrared spectroscopy (TRIR)

After excitation of a molecule using UV/Visible pulsesdleading to an electronically excited statedthe vibrational spectrum will reflect changes in the electron density distribution at each chemical bond, as vibrational frequencies are highly sensitive to subtle changes in electron-density. This bond-specific information allows one to assign the processes occurring in the excited state, and evolution of the states of different origin in a much clearer way than the usually congested electronic transient absorption spectra. TRIR uses UV/Vis excitation and probe/detection in the mid-IR range of the spectrum, typically from 1200 up to 3500 cm-1 (depending on the sample and the solvent). It was initially applicable mainly to molecules bearing strong IR reporter groups, such as C]O, C^C, or C^N.61 Major developments in detector sensitivity and improvements in the stability of the ultrafast mid-IR sources17 have made it possible to investigate much weaker IR bands, essentially covering the entire molecular framework. Changes in the energy of the infrared bands of a metal complex in the excited statedwhen compared to the ground statedindicate changes of electron density distribution, and assist in assignment of the nature of the excited states involved. Changes in the relative intensity of the IR bands between ground and excited electronic states can yield further structural information. Transition metal carbonyls are arguably the class of compounds most studied by TRIR, owing to the very high IR absorption cross-section of the carbonyl group vibrations, which also typically occur in a relatively background- and overlap-free region of the spectrum (1800–2100 cm-1). The C^O ligands are sensitive and selective reporters of the electronic density around the metal center, since a competition between s donation and p backbonding influences the absorption frequency of the C^O stretching modes. The combined use of spectroelectrochemical and time-resolved IR methods, which illustrates the value of complementary methods used in parallel, is illustrated for instance by a report on TRIR and IR-spectroelectrochemical experiments of a Pt(II) diimine complex Pt(bpyam)Cl2 (bpyam ¼ 4,40 -{C(O)NEt2}2-2,20 -bipyridine).62 The bottom panel in Fig. 17 demonstrates the 19 cm-1 shift to lower energy of the n(CO) of the neutral molecule upon electrochemical reduction to [Pt(bpyam)Cl2]•. This species, according to density functional theory (DFT) calculations, should be represented as [Pt(bpyam•)Cl2] following a ligand-centered reduction, with an excess of electron density on the p* orbital which is

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Fig. 10 Molecular structure of the Ru(tpphz)PtI2 photocatalyst, consisting of a bis(di-tert-butylbipyridine)ruthenium(II) photocenter, a tpphz bridging ligand and a diiodoplatinum(II) catalytic center. b, Photocatalytic hydrogen production with Ru(tpphz)PtI2 (TON, turnover number; n, amount of substance). Through the short exposure of the hydrogenated PMD to in situ generated singlet oxygen (1O2), the photocatalyst is reoxidized to its active form and can again be used for light-driven hydrogen production. Through the repetitive active repair of the damaged photocatalyst, the longterm activity of the photochemical molecular device (PMD) is prolonged to weeks of active hydrogen formation. The solid lines of the TON curves are not fits, but included to guide the eye. Reproduced from Pfeffer, M. G.; Müller, C.; Kastl, E. T. E.; Mengele, A. K.; Bagemihl, B.; Fauth, S. S.; Habermehl, J.; Petermann, L.; Wächtler, M.; Schulz, M.; Chartrand, D.; Laverdière, F.; Seeber, P.; Kupfer, S.; Gräfe, S.; Hanan, G. S.; Vos, J. G.; Dietzek-Ivansic, B.; Rau, S. Active Repair of a Dinuclear Photocatalyst for Visible-Light-Driven Hydrogen Production. Nat. Chem. 2022, 14 (5), 500– 506. doi:10.1038/s41557-021-00860-6 with permission. Copyright 2022, Springer Nature.

largely eC]O localized. On the other hand, upon promotion to the excited state, the ground-state n(CO) is transiently shifted to lower energy by about the same amount such that it almost coincides with n(CO) of [Pt(bpyam)Cl2]•. Thus, the TRIR behavior is consistent with the Pt / bpyam metal-to-ligand charge transfer (MLCT) nature of the lowest excited state, in which the same n(CO) p* antibonding orbital is populated. Re(I) carbonyl diimine complexes represent a large family of molecular photosensitizers, which have been extensively studied, in particular using TRIR spectroscopy, due to the strong n(CO) stretching vibrations of the {Re(CO)x}þ (x ¼ 2, 3) moiety.

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Fig. 11 Molecular structures of the dye (PB-1), catalyst (Co-N3), and dye–catalyst (PB-2) compounds. Reproduced from Pati, P. B.; Zhang, L.; Philippe, B.; Fernández-Terán, R.; Ahmadi, S.; Tian, L.; Rensmo, H.; Hammarström, L.; Tian, H. Insights into the Mechanism of a Covalently Linked Organic Dye–Cobaloxime Catalyst System for Dye-Sensitized Solar Fuel Devices. ChemSusChem 2017, 10 (11), 2480–2495. doi:10.1002/ cssc.201700285 with permission. Copyright 2017, The Authors. ChemSusChem published by Wiley-VCH GmbH.

Fig. 12 The charge separation and recombination processes and the corresponding rate constants of PB-2/NiO upon light illumination. Reproduced from Pati, P. B.; Zhang, L.; Philippe, B.; Fernández-Terán, R.; Ahmadi, S.; Tian, L.; Rensmo, H.; Hammarström, L.; Tian, H. Insights into the Mechanism of a Covalently Linked Organic Dye–Cobaloxime Catalyst System for Dye-Sensitized Solar Fuel Devices. ChemSusChem 2017, 10 (11), 2480–2495. doi:10.1002/cssc.201700285 with permission. Copyright 2017, The Authors. ChemSusChem published by Wiley-VCH GmbH.

The early studies illustrated by means of TRIR and TAS the diverse photochemistry and photophysical landscape of Re(I) tricarbonyl photosensitizers with different diimine ligands.63–71 The benefits of combining experimental and computational methods are also illustrated in two recent reports,72,73 where a series of Re(I) carbonyl complexes with terpyridine ligands bearing electron donating and withdrawing groups were studied using a combination of experimental methods (including TRIR and spectroelectrochemistry) as well as TD-DFT calculations (Fig. 18). In the tricarbonyl series, upon substitution with the strongly-donating NMe2 group, the character of the lowest excited state switched from MLCT to intra-ligand charge transfer (ILCT), as evidenced by the change in the TRIR spectroscopic signatures (Fig. 19). These also resemble closely the differential spectrum obtained from IR spectroelectrochemical studies, showing that indeed excitation corresponds to a HOMO–LUMO transition, whichdfrom the perspective of the metal centerdinvolves an increased electronic density. The change in character was also accompanied by a red shift and a significant increase in the absorption cross sections across the visible, together with an increase in the lifetime from ca. 2 ns to ca. 380 ns. Despite the slightly lower excited-state redox potential of this complex, significant and prolonged H2 evolution was observed in solution when combining the [Re(k2N-tpy-NMe2)(CO)3Cl] complex and a standard Co-dimethylglyoxime catalyst upon photoirradiation with 450 nm light. This showcases the potential for long-lived ILCT excited states as an alternative to enhance the photosensitizer performance in artificial photosynthetic devices. In contrast, the dicarbonyl series did not show a switching in the character of the lowest excited state with the NMe2 substituent. This was also evidenced by TRIR spectroscopy and time-dependent density functional theory (TD-DFT) calculations. In the TRIR

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Fig. 13 (a) Sequence of Photoinduced Electron-Transfer Reactions Observed and the Subsequent Protonation Suggested; (b) Suggested Mechanism of Proton Reduction by the [FeFe] catalyst under Photochemical Conditions. Reproduced from Antila, L. J.; Ghamgosar, P.; Maji, S.; Tian, H.; Ott, S.; Hammarström, L. Dynamics and Photochemical H2 Evolution of Dye-NiO Photocathodes With a Biomimetic FeFe-Catalyst. ACS Energy Lett. 2016, 1 (6), 1106–1111. doi:10.1021/acsenergylett.6b00506 with permission. Copyright 2016, American Chemical Society.

spectra of [Re(k2N-tpy-H)(CO)2Cl] complex (Fig. 20), the ESA bands corresponding to the C^O stretches both shift to higher wavenumbers (a blue shift), which is consistent with a diminished electron density around the metal center,74 hence supporting the assignment of the excited state to an MLCT character. This picture is unchanged regardless of the electron donating/withdrawing character of the substituent, showing that the coordination environment indeed prevents access to the ILCT states in these complexes. A report on similar Re(I) dicarbonyl complexes illustrated how the replacement of the axial halide ligand for a phosphine ligand leads to a recovery of the emission,75–77 while an increase in the energy of the HOMO offers another way to tune the electronic properties of these complexes.78 Application of TRIR resolved dynamics of immobilized complexes,79,80 where different semiconductor surfaces (ZrO2 and TiO2) were sensitized with Re(I) carbonyl bipyridine derivatives, and in some cases even co-sensitized with an electron donor (Fig. 21) or a molecular proton reduction catalyst (Fig. 22). The complex electron transfer pathways were elucidated with the help of a kinetic model and a series of control experiments, aiming to answer whether diffusion is affected upon immobilization on the surface and its role in altering the cage escape and recombination yields. When the quencher (electron donor) was co-adsorbed alongside the Re complex, the reaction cycle changed completely. Electron transfer occurred only from quencher molecules that sit next to an excited Re complex, with no possibility of cage escape. Varying the ratio of quencher molecules and Re complexes, it was concluded that molecules do not cluster on the surface and that excitation energy migration is not a very efficient process. In contrast, upon co-adsorption of a catalyst, significant quenching of the excited state of the Re(I) complex by the Co-based catalyst was observed. Furthermore, TRIR was used to reveal the non-innocent role of TiO2 in the photoreduction of CO2 (Fig. 23).81 It was found that attaching the phosphonate-modified molecular catalyst [ReBr(phosphonate-bpy)(CO)3] to the wide-bandgap semiconductor TiO2 strongly enhances the rate of visible-light-driven reduction of CO2 to CO in DMF with triethanolamine (TEOA) as sacrificial electron donor. The mechanism of catalyst photoreduction is initiated by ultrafast electron injection into TiO2, followed by rapid (ps-ns) and sequential two-electron oxidation of TEOA that is coordinated to the Re center. The injected electrons can be stored in the conduction band of TiO2 on a ms-s time scale, and they propose that these stored electrons lead to further reduction of the Re catalyst and completion of the catalytic cycle. Thus, the excited Re catalyst gives away one electron and would eventually receive three electrons back. Of particular note is the use of TRIR spectroscopy to elucidate the charge dynamics, since TRIR provides a clear spectroscopic signature for the free electrons on the conduction band of the semiconductor (Fig. 23, B). A combination of TRIR and TAS was also used to study a series of Re(CO)3 complexes with naphthalimide (NI)-appended phen ligands (Fig. 24).82 It was found that with the more electron-rich phen-NI ligands, the equilibrium shifts toward the 3NI states, since the gap between the 3NI and 3MLCT states becomes larger (Fig. 25). Another application of TRIR spectroscopy has also been (in combination with TAS) to study photophysical and photochemical processes of DNA-intercalating metal complexes (Fig. 26).83 In this regard, our understanding of photoreactions of metal complexes bound to DNA and the structural changes that take place upon binding have been greatly advanced by these studies. For instance, the direct observation of the so-called bright and dark excited states of [Ru(phen)2(dppz)]2 þ both in solution and when bound to DNA (Fig. 27) has been reported.84 This so-called “light-switch” complex shows emission in organic solvents or

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Fig. 14 (A) Electronic structure of d6 photocatalysts compared to Ni(II) aryl halide complexes. (B) Structure of the Ni(II) aryl halide complexes and X-ray crystal structure of a representative complex, Ni(dtbbpy)(Ar)Cl. (C) Comparison of component absorption spectra. (D) Absorption spectra for aryl halide complexes. Reproduced from Shields, B. J.; Kudisch, B.; Scholes, G. D.; Doyle, A. G. Long-Lived Charge-Transfer States of Nickel(II) Aryl Halide Complexes Facilitate Bimolecular Photoinduced Electron Transfer. J. Am. Chem. Soc. 2018, 140 (8), 3035–3039. doi:10.1021/jacs.7b13281 with permission. Copyright 2018, American Chemical Society.

when bound to DNA, but is non-emissive in water. The authors could characterize both bright and dark states using TRIR spectroscopy and understand the identity and nature of the binding site of this complex. In a similar study, the binding of the same Ru(II) complex to guanine-quadruplex structures in both Kþ and Naþ containing solutions (Fig. 28), which offer different structural arrangements of the DNA quadruplex, was reported.85 TRIR permitted once more the simultaneous monitoring of both the “dark” and “bright” states of the complex and of the quadruplex nucleobase bases, the latter via a Stark effect induced by the excited state of the complex. The authors use the TRIR data to establish the contribution of guanine base stacking and loop interactions to the binding site of this biologically relevant DNA structure in solution. A particularly striking observation was the strong thymine signal observed for the Naþ form of the human telomere sequence, which is expected to be in the anti-parallel conformation. Further studies with DNA-intercalated metal complexes involved the photooxidation of adenine by a Cr(III) complex (Fig. 29).86 A combination of TRIR and TAS reveal the presence of relatively long-lived dppz-centered states, which eventually yield the emissive metal-centered state. The dppz-localized states are fully quenched when bound by GC base pairs and partially so in the presence of an AT base-pair system to generate purine radical cations. The sensitized formation of the adenine radical cation species (A•þ T) was identified by assigning the TRIR spectra with help of DFT calculations. In natural DNA and oligodeoxynucleotides containing a mixture of AT and GC of base pairs, the observed time-resolved spectra are consistent with eventual photo-oxidation occurring predominantly at guanine through hole migration between base pairs. Apart from metal carbonyls, other strong IR absorbersdwhich receive somewhat lesser attentiondare azide groups coordinated to the metal center (e.g., Ir and Rh azides), where application of TRIR allowed to investigate potential ligand dissociation.87 Application of TRIR to platinum diimine acetylide complexes allowed to resolve the dynamics of photoinduced charge-transfer processes.62,88–92 Other examples of the power of TRIR to investigate photochemical reactions include understanding the factors

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Fig. 15 Initial transient absorption experiments on Ni(II) potential catalysts. (A) Full contour plot for TA spectrum of 1-Cl (pump 295 nm). (B) Comparison of TA spectra of free dtbbpy (1–1000 ps, pump 305 nm), Ni(dtbbpy)Cl2 (1–60 ps, pump 295 nm), and Ni(dtbbpy)(Ar)Cl (1–1000 ps, pump 295 nm). (C) Single-wavelength kinetics for Ni(dtbbpy)Cl2 (367 nm, pump 295 nm) and Ni(dtbbpy)(Ar)Cl (476 nm, pump 400 nm). (D) Experimental spectrum vs. simulated 3MLCT1 spectrum of Ni(dtbbpy)(Ar)Cl. Reproduced from Shields, B. J.; Kudisch, B.; Scholes, G. D.; Doyle, A. G. Long-Lived Charge-Transfer States of Nickel(II) Aryl Halide Complexes Facilitate Bimolecular Photoinduced Electron Transfer. J. Am. Chem. Soc. 2018, 140 (8), 3035–3039. doi:10.1021/jacs.7b13281 with permission. Copyright 2018, American Chemical Society.

Fig. 16 Changes in the decay pathways upon complexation of a copper halide to a Ti(IV) bis-ferrocenylidene complex. Reproduced with permission from Livshits, M. Y.; Turlington, M. D.; Trindle, C. O.; Wang, L.; Altun, Z.; Wagenknecht, P. S.; Rack, J. J. Picosecond to Nanosecond Manipulation of Excited-State Lifetimes in Complexes with an FeII to TiIV Metal-to-Metal Charge Transfer: The Role of Ferrocene Centered Excited States. Inorg. Chem. 2019. doi:10.1021/acs.inorgchem.9b02316 with permission. Copyright 2019, American Chemical Society.

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Fig. 17 Bottom panel: Ground state FTIR spectrum (black) and a series of spectra obtained in the course of electrochemical reduction. Top panel: a series of TRIR spectra of Pt(bpyam)Cl2 in CH2Cl2, recorded at 1, 2, 3, 5, 7, 10,15, 20, and 25 ps time delays after initial excitation with a 400 nm, 150 fs laser pulse. Adapted from Best, J.; Sazanovich, I. V.; Adams, H.; Bennett, R. D.; Davies, E. S.; Meijer, A. J. H. M.; Towrie, M.; Tikhomirov, S. A.; Bouganov, O. v.; Ward, M. D.; Weinstein, J. A. Structure and Ultrafast Dynamics of the Charge-Transfer Excited State and Redox Activity of the Ground State of Mono- and Binuclear Platinum(II) Diimine Catecholate and Bis-Catecholate Complexes: A Transient Absorption, TRIR, DFT, and Electrochemical Stud. Inorg. Chem. 2010, 49 (21), 10041–10056. doi:10.1021/ic101344t with permission. Copyright 2010, American Chemical Society.

Fig. 18 Synthetic route and molecular structures of the [Re(k2N-tpy-R1)(CO)3Cl] and [Re(k3N-tpy-R1)(CO)2Cl] complexes studied by FernándezTerán and Sévery.72,73 Adapted from Fernández-Terán, R. J.; Sévery, L. Coordination Environment Prevents Access to Intraligand Charge-Transfer States through Remote Substitution in Rhenium(I) Terpyridinedicarbonyl Complexes. Inorg. Chem. 2021, 60 (3), 1325–1333. doi:10.1021/ acs.inorgchem.0c02914 with permission. Copyright 2020, American Chemical Society.

influencing C–H activation by Rh complexes,93 or the formation and reactivity of organometallic methane and ethane complexes in room-temperature solutions.94 A combination of TAS and TRIR established stabilization of the lowest energy charge-separated state in a {metal chromophore–fullerene} assembly.95 A recent development in time-resolved infrared spectroscopy is that of pulse radiolysis–TRIR.96 There, an electron beam is used as an excitation source, generating excited states, anions or cations, which are then interrogated by an infrared pulse (Fig. 30). The development of this method, already being applied to understanding of CO2 reduction using Mn(I) diimine catalysts, is likely to open up a new chapter in our understanding of photo- and electrocatalysis, as well as multitude of other processes involving metal complexes, in photochemistry and radiation chemistry.

8.13.2.4

Time-resolved Raman spectroscopy

Another type of time-resolved vibrational spectroscopic method, time-resolved resonance Raman spectroscopy (TR3), highlights vibrations coupled to a particular electronic transition in the excited state, assisting in assignment of the nature of the frontier

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Fig. 19 FT-IR spectrum (top) and UV pump–IR probe transient spectra (bottom) of the [Re(k2N-tpy-NMe2)(CO)3Cl] complex (5 mM in nitrogenpurged DMF) at different delays (picosecond to microsecond time scales). The IR-SEC difference spectrum at  1.8 V vs. Fcþ/Fc (first reduction) is overlaid with the pump–probe spectra (purple dotted line, DAbs  0.4). Reproduced from Fernández-Terán, R.; Sévery, L. Living Long and Prosperous: Productive Intraligand Charge-Transfer States from a Rhenium(I) Terpyridine Photosensitizer with Enhanced Light Absorption. Inorg. Chem. 2021, 60 (3), 1334–1343. doi:10.1021/acs.inorgchem.0c01939 with permission. Copyright 2020, American Chemical Society.

Fig. 20 FT-IR spectrum (top) and magic-angle TRIR spectra (bottom) of complex 3d (5 mM in DMF) at different pump–probe delays. Reproduced from Fernández-Terán, R. J.; Sévery, L. Coordination Environment Prevents Access to Intraligand Charge-Transfer States through Remote Substitution in Rhenium(I) Terpyridinedicarbonyl Complexes. Inorg. Chem. 2021, 60 (3), 1325–1333. doi:10.1021/acs.inorgchem.0c02914 with permission. Copyright 2020, American Chemical Society.

orbitals. Both steady-state and time-resolved resonance Raman methods have been applied extensively to polypyridyl complexes of Ru(II) and Re(I),97 among other metal complexes (e.g., Fig. 31).98–105 The development and implementation of Kerr-gated Raman spectroscopy enabled interrogation of even strongly emissive species by time-resolved resonance Raman.106 The TR3 method is limited by picosecond (or longer) time-resolution. In order to access Raman spectra at shorter time scales, femtosecond stimulated Raman spectroscopy (FSRS) has been developed as a powerful tool for investigating ultrafast structural and vibrational dynamics in light absorbing systems.107,108 It is however power-hungry, and requires exceptional photostability of the samples. Recent advances including high repetition rate systems109 and impulsive Raman spectroscopy may help broadening applications of this method to photochemistry of coordination compounds.110,111 For instance, FSRS has been applied to study the ultrafast dynamics of a photoexcited Re(I) tricarbonyl complex (Fig. 32).112 This study revealed that the shifts of Raman features upon excitation agree with the predominant ReCl(CO)3 / bpy CT character of the lowest excited state, demonstrating the diagnostic value of low-frequency Raman modes in excited-state characterization and the need for reliable assignment provided by anharmonic calculations. Furthermore, the temporal evolution of FSRS features is mode-specific and provides information on the dynamics and mechanism of the population of the lowest excited state and its relaxation, namely, the development of the charge separation in the relaxed MLCT state.

8.13.3

Time-resolved emission spectroscopy

Time-resolved emission spectroscopy traditionally relied on fast detectors, being limited by the detector’s time response. The two most used methods would be Time-Correlated Single Photon Counting (TCSPC), in which the time resolution would be

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Fig. 21 Scheme of the investigated system of the rhenium carbonyl complex on the surface with the quencher in solution (RePOH@ZrO2) and with the quencher coadsorbed (RePOH D Phtz@ZrO2). For comparison, a system with all compounds in solution has been considered as well with RePOH and RePO as the electron acceptor. Reproduced from Oppelt, K.; Fernández-Terán, R.; Pfister, R.; Hamm, P. Geminate Recombination Versus Cage Escape in the Reductive Quenching of a Re(I) Carbonyl Complex on Mesoporous ZrO2. J. Phys. Chem. C 2019, 123 (32), 19952–19961. doi:10.1021/ acs.jpcc.9b04950 with permission. Copyright 2019, American Chemical Society.

Fig. 22 Re-based photosensitizer PS (yellow) and cobalt-based water reduction catalyst WRC (green) co-adsorbed on ZrO2 with a reversible electron donor ED (methylphenothiazine, red) in solution. Possible processes in the reaction cycle and their rates are kr (PS* excited state relaxation), ket (reduction of PS* by the ED), kWRC (non-productive quenching of PS* by the WRC), kred (electron transfer to the WRC), and ket-1 (charge recombination with the oxidized EDþ). Reproduced from Oppelt, K.; Mosberger, M.; Ruf, J.; Fernández-Terán, R.; Probst, B.; Alberto, R.; Hamm, P. Shedding Light on the Molecular Surface Assembly at the Nanoscale Level: Dynamics of a Re(I) Carbonyl Photosensitizer with a Coadsorbed Cobalt Tetrapyridyl Water Reduction Catalyst on ZrO2. J. Phys. Chem. C 2020, 124 (23), 12502–12511. doi:10.1021/acs.jpcc.0c02556 with permission. Copyright 2020, American Chemical Society.

determined by the instrument response function of the PMT detector. Another way is to use a streak camera, which would allow the resolution of up to several picoseconds. These methods, being detector-limited, do not match the resolution of the pump–probe methods discussed so far. To achieve higher time resolution, one needs to implement an optical delay between the event of a photon being emitted, and the time it arrives at the detector. Fluorescence upconversion is the method which transforms emission spectroscopy into an ultrafast method with the time resolution determined by the time delay between a pair of laser pulses (Fig. 33).113

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Fig. 23 (A) fs-TRIR for the ZrO2–[Re(bpy)(CO)3]þ system in DMF, (B) fs-TRIR for the TiO2–[Re(bpy)(CO)3]þ system without TEOA, and (C) fs-TRIR for the TiO2–[Re(bpy)(CO)3-OC(O)O-(CH2)2Nþ R2]þ system in DMF/TEOA solution (5:1) and CO2 bubbling (the absorption at 2100 cm–1 was subtracted from the spectra to emphasize molecular signals). Reproduced from Abdellah, M.; El-Zohry, A. M.; Antila, L. J.; Windle, C. D.; Reisner, E.; Hammarström, L. Time-Resolved IR Spectroscopy Reveals a Mechanism with TiO2 as a Reversible Electron Acceptor in a TiO2-Re Catalyst System for CO2 Photoreduction. J. Am. Chem. Soc. 2017, 139 (3), 1226–1232. doi:10.1021/jacs.6b11308 with permission. Copyright 2016, American Chemical Society.

In traditional emission spectroscopy, emitted photons would travel to the detector. An alternative method is to combine the emitted photon (of frequency u1) with another photon (frequency u2) of different energy, in a non-linear optical medium, and deliver either u1 þ u2 (an “upconverted emission”), or u1  u2 (the “downconverted emission”) to the detector. The advantage of this method is that the arrival time of the signal at the detector is determined by the second pulse, often called the “gate.” Thus, by varying the delay between the excitation pulse and the gate pulse, one can achieve temporal resolution limited only by the duration of the laser pulses used. For technical reasonsdmainly the higher sensitivity of the detectors in the UV/visible range in comparison to the near-IR rangedthe preferred configuration for the studies relevant to coordination compounds are conducted using the upconverted emission signal. Time-resolved fluorescence upconversion was first reported in 1975, and has evolved since to become a very sensitive and broadly applicable technique.114

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Fig. 24 Shifting of the excited-state triplet equilibria toward the ligand-centered 3NI states with more electron-rich NI substituents on the phen ligand. Reproduced from Yarnell, J. E.; Wells, K. A.; Palmer, J. R.; Breaux, J. M.; Castellano, F. N. Excited-State Triplet Equilibria in a Series of Re(I)Naphthalimide Bichromophores. J. Phys. Chem. B 2019, 123 (35), 7611–7627. doi:10.1021/acs.jpcb.9b05688 with permission. Copyright 2019, American Chemical Society.

One way of achieving an upconverted emission spectrum is to record the emission kinetics at a single wavelength, and reconstruct the spectrum by scanning the detected wavelength. The pioneering work on [Ru(bpy)3]2 þ demonstrated how this method can be used to resolve the rate of intersystem crossing in a metal complex (Fig. 34).28,115 This method has also been used to study the vibrational relaxation and intersystem crossing of binuclear metal complexes in solution, where a Pt(II) dimer, [Pt2(P2O5H2)4]4 (PtPOP) was studied using TAS and fluorescence upconversion spectroscopy FLUPS (Fig. 35).116 Both datasets exhibit clear signatures of vibrational relaxation and wave packet oscillations of the Pt–Pt stretch vibration in the 1A2u state with a period of 224 fs, that decay on a 1–2 ps time scale, and of intersystem crossing (ISC) into the 3A2u state. The vibrational relaxation and ISC times exhibit a pronounced solvent dependence. The intersystem crossing of Re(I) carbonyl-bipyridine complexes has also been elucidated using fluorescence upconversion (Fig. 36).117 The luminescence is characterized by a broad fluorescence band at very short times, and evolves to the steady-state phosphorescence spectrum from the a3A00 state at longer times. The analysis of the data allowed for the identification of three spectral components. The first two are characterized by decay times s1 ¼ 85  150 fs and s2 ¼ 340  1200 fs, depending on the axial ligand, and were identified as fluorescence from the initially excited singlet state and phosphorescence from a higher triplet state (b3A00 ), respectively. The electronic relaxation processes of metallo-porphyrins studied by fluorescence upconversion spectroscopy118 revealed that in all cases, the relaxation from the Soret (B)-state to the Q-states (S1) occurs on ultrafast time scale of < 50–100 fs, regardless of the metal, its oxidation state, or the peripheral groups of the macrocycle (Fig. 37). The upconversion method discussed in the previous section provides a detailed kinetic information, but at the expense of a long acquisition time, potential sample degradation, and limited spectral resolution. An alternative method is that of broadband fluorescence upconversion, where an optical set-up is designed to deliver a certain portion of the spectrum at a particular time delay. The advantages of this method include high spectral resolution, a simultaneous capture of the entire emission spectrumdor of a large portion thereof, and a much faster acquisition time. The detailed description of this recent development is given in Ref.119. This method is particularly useful if several short-lived emissive states are formed, whether in parallel or consecutively, as part of the photophysics/photochemical processes of the system of interest. An application to resolving multiple timescales of intersystem crossing in a Pt(II) donor-acceptor assembly is illustrated in Fig. 38. Using a combination of transient absorption, FLUPS and transient stimulated Raman spectroscopy, the authors can extract the multiple timescales (1.6 ps to  20 ps) associated with the rise of triplet species, which have been assigned to multiple ISC pathways from a manifold of hot charge-transfer singlet states.

8.13.4

Time-resolved structural methods: X-ray spectroscopy and molecular movies

The technological breakthroughs of the last decades enabled interrogation of light-induced phenomena in molecules and materials on ever shorter time-scales, coming close to real-time monitoring of breaking and formation of a chemical bond. The majority of photophysical and photochemical processes usually evoke simultaneous electronic, structural, and spin changes. The optical spectroscopies discussed above give one tool to follow electronic changes, and indirectly (through vibrational spectroscopies) structural

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Fig. 25 Top: Molecular structures of the complexes studied in Ref.82. Bottom: Qualitative energy diagrams for the several Re(I) complexes with a phen ligand substituted with a naphthalene-imide electron acceptor, in acetonitrile at room temperature. Adapted from Yarnell, J. E.; Wells, K. A.; Palmer, J. R.; Breaux, J. M.; Castellano, F. N. Excited-State Triplet Equilibria in a Series of Re(I)-Naphthalimide Bichromophores. J. Phys. Chem. B 2019, 123 (35), 7611–7627. doi:10.1021/acs.jpcb.9b05688 with permission. Copyright 2019, American Chemical Society.

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changes. Yet methods based upon the optical region of the electromagnetic spectrum cannot measure sub-Ångström displacements and femtosecond motions. In order to “watch chemistry happen,” with femtosecond temporal and sub-Å spatial resolution, timeresolved X-ray methods are required. Direct structural interrogation of transient species can be achieved in a pump-probe experiment whereby an optical (usually in the UV/Vis range) excitation laser pulse is paired with a probe pulse in the X-ray region,121 or with an electron pulse.3,4 The pioneering work of Phillip Coppens using synchrotron radiation opened up this field of researchddirect structural interrogation of excited states, photocrystallography.122,123 Much effort has been dedicated to investigating light-induced structural changes in crystals of metal complexes, a notable example being a 100% solid-state reversible transformations between the ground and metastable states in single-crystals of a series of nickel(II) nitro complexes.124 The initial difficulties in such experiments were “blowing up” of crystals due to heating with the excitation laser pulse. The new methods of serial X-ray crystallography, sample delivery of a “stream” of microcrystals in a cooled jet, and others, are making such experiments easier. The focus of this sub-section is on application of time-resolved X-ray methods to excited state dynamics of molecules in solution. The general philosophy of such experiments is similar to that of optical methods: an (electronic) excited state is populated with an ultrafast UV/Vis pulse, and the subsequent evolution of the excited state(s) is interrogated by an X-ray pulse (Fig. 39).125 X-ray absorption spectroscopy would investigate absorption of X-ray photons by inner electrons; X-ray emission spectroscopy would detect emission in the X-ray range following X-ray excitation in the valence shell. X-ray diffraction methodsdperhaps the most familiar to chemists due to the ubiquitous presence of steady-state X-ray diffraction for determining structures of stable speciesdcan be brought into time-resolved domain, by preceding the X-ray diffraction experiment with an excitation pulse. Finally, X-ray scattering methods such as resonance inelastic X-ray scattering (RIXS) can be visualized as an X-ray equivalent of Raman scattering in optical methods. Time-resolved X-ray diffraction and X-ray absorption spectroscopy [X-ray absorption near edge structure (XANES), and the extended X-ray absorption fine structure (EXAFS), Fig. 40] allow unprecedented insight into both dynamics and structure of short-lived excited states, intermediates, and approach atomic resolution for interrogating processes on surfacesdof direct importance to DSSC and photocatalysis, including plasmonics. The key advantage of XAS is that it probes both the electronic structure of an absorber and the nuclear structure around it in the same measurement and is element-specific. Of particular importance for applications in chemistry is that XAS investigates the valence orbitals, i.e., those which are engaged in bond formation and breaking. XES is used to provide information about the occupied density of states projected on the absorbing atom. Many investigations have been performed on the study of the Ka and Kb emission lines of 3d transition metals in molecules, taking advantage of the sensitivity of these emission lines to the oxidation state and the number of unpaired electrons of the transition. Time-resolved X-ray studies, using a variety of available methods, have been conducted, for example, on light-absorbing Cucomplexes which undergo ultrafast structural change in the excited state (Fig. 41),48 aiming at establishing both the degree and the timescale of the change, and how the structural changes is linked to the rate of intersystem crossing.126,127 Dinuclear complexes often used as a photosensitizer-catalyst combination in photocatalytic applications. Recent advances in Xray detection methods enabled simultaneous detection of X-ray data from two metal centers simultaneously: the ultrafast dynamics in bridged bimetallic complexes have been recently studied by using optical and X-ray transient absorption spectroscopies where a Cu(I) center is linked to a Os(II) center.128 In this study, the in-phase vibrational coherence was evidenced in the time evolution of the X-ray transient absorption (XTA) signals at both the Cu K- and Os LIII-edges, revealing that metal–metal interactions can be modulated via the bridging ligand over a relatively long distance, and suggests an opportunity to control electronic coupling between metal centers. Multinuclear metal complexes often undergo a change in metal-metal distance in their excited states. Application of timeresolved EXAFS allowed one to elucidate transient changes in the Pt–Pt distance in the excited state of the famously emissive dinuclear Pt(II) photocatalyst (Fig. 42),129 while vibrational coherence transfer in the ultrafast intersystem crossing of this complex has been investigated in Ref.130. Various pyrazolate-bridged Pt(II) dimers have been investigated by both optical,131 and X-ray methods.132 For example, a combination of ultrafast X-ray transient absorption and TAS elucidated excited-state bond contraction and electronic and nuclear structural dynamics of two photoexcited pyrazolate-bridged [Pt(ppy)(m-R2pz)]2-type Pt(II) dimers (ppy ¼ 2-phenylpyridine, mR2pz ¼ 3,5-substituted pyrazolate): [Pt(ppy)(m-H2pz)]2 and [Pt(NDI-ppy)(m-Ph2pz)]2, both of which have distinct ground-state Pt–Pt distances.133 X-ray transient absorption at the Pt LIII-edge revealed a new d-orbital vacancy due to the one-electron oxidation of the Pt-centers in both cases. However, while a transient Pt–Pt contraction was observed in the NDI-ppy complex, this effect was completely absent in the ppy complex, demonstrating how the excited states of these complexes are determined by the overlap of the Pt d(z2) orbitals, which is tuned by the steric bulk of the pyrazolate R-groups (Fig. 43). A butterfly deformation mode in a photoexcited pyrazolate-bridged Pt(II) complex was investigated by time-resolved X-ray scattering in solution.134 A combination of ultrafast transient absorption and an ultrafast X-ray based methods are often used to elucidate the excited state dynamics. Additional insights can be obtained by combining the X-ray methods with time-resolved infrared spectroscopy, which is possible if the complex in question has IR-active groups, whether bridging, or peripheral. For example, d8-d8 isocyanide complexes

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Fig. 26 Example transient IR (left) and TAS (right) studies of DNA-intercalating metal complexes. Reproduced from Keane, P. M.; Kelly, J. M. Transient Absorption and Time-Resolved Vibrational Studies of Photophysical and Photochemical Processes in DNA-Intercalating Polypyridyl Metal Complexes or Cationic Porphyrins. Coord. Chem. Rev. 2018, 364, 137–154. doi:10.1016/j.ccr.2018.02.018 with permission. Copyright 2018, Elsevier.

Fig. 27 TRIR spectroscopic signatures of the dark (blue) and bright (yellow) states of the Ru(II) photosensitizer used to study binding between metal complexes and DNA. Reproduced from Devereux, S. J.; Poynton, F. E.; Baptista, F. R.; Gunnlaugsson, T.; Cardin, C. J.; Sazanovich, I. V.; Towrie, M.; Kelly, J. M.; Quinn, S. J. Caught in the Loop: Binding of the [Ru(Phen)2(Dppz)]2þ Light-Switch Compound to Quadruplex DNA in Solution Informed by Time-Resolved Infrared Spectroscopy. Chem. A Eur. J. 2020, 26 (71), 17103–17109. doi:10.1002/chem.202002165 with permission. Copyright 2020, Wiley-VCH GmbH.

of Ir(I) and Rh(I) are amenable to both TRIR and X-ray studies. There, following an excitation into either ds*ps or dpps singlet states, structural changes and intersystem crossing across various pathways has been resolved (Fig. 44).135 Moving to even larger systems, a trinuclear Mn-cluster offers an interesting insight into correlated spin-vibronic dynamics in single molecule magnets (Fig. 45),136 where potential X-ray studies can offer insight into the magnetic behavior of such systems which are promising candidates for future information storage. Another fascinating process of considerable practical importance is excited state spin-crossover. Here, transient X-ray methods brought us a step closer to the understanding of the mechanism of this process in iron complexes.7,137,138 A model spin-

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Fig. 28 TRIR spectroscopy revealsdvia the sensitive DNA base vibrationsdthe presence of the locally excited “dark” and “bright” states of the intercalated [Ru(phen)2(dppz)]2 þ complex. Reproduced from Devereux, S. J.; Poynton, F. E.; Baptista, F. R.; Gunnlaugsson, T.; Cardin, C. J.; Sazanovich, I. V.; Towrie, M.; Kelly, J. M.; Quinn, S. J. Caught in the Loop: Binding of the [Ru(Phen)2(Dppz)]2þ Light-Switch Compound to Quadruplex DNA in Solution Informed by Time-Resolved Infrared Spectroscopy. Chem. A Eur. J. 2020, 26 (71), 17103–17109. doi:10.1002/ chem.202002165 with permission. Copyright 2020, Wiley-VCH GmbH.

Fig. 29 Transient IR spectroscopy reveals the transient signatures of the adenine radical cation, produced by photooxidation of DNA from an intercalated Cr(III) complex. Reproduced from Baptista, F. A.; Krizsan, D.; Stitch, M.; Sazanovich, I. V.; Clark, I. P.; Towrie, M.; Long, C.; MartinezFernandez, L.; Improta, R.; Kane-Maguire, N. A. P.; Kelly, J. M.; Quinn, S. J. Adenine Radical Cation Formation by a Ligand-Centered Excited State of an Intercalated Chromium Polypyridyl Complex Leads to Enhanced DNA Photo-Oxidation. J. Am. Chem. Soc. 2021, 143 (36), 14766–14779. doi:10.1021/jacs.1c06658 with permission. Copyright 2021, American Chemical Society.

crossover system is [Fe(bpy)3]2 þ. The studies of its dynamics in solution using synchrotrons as an X-ray source revealed structural dynamics of the SCO process. Improved time resolution achieved with an X-ray free electron laser (XFEL) source TR-XANES allowed one to investigate nuclear wavepacket oscillations in solution and extract the information of the Fe–N breathing vibrational mode (Fig. 46).7,137,138 The valence electronic structure of transition metal complexes in solution can also been studied with X-ray spectroscopy in the soft X-ray range with femtosecond time resolution at XFEL sources. Time-resolved RIXS (TR-RIXS) at the Fe L3 edge has been performed to study the photoinduced ligand exchange dynamics of Fe(CO)5, concluding that the photoinduced removal of CO generates a 16-electron Fe(CO)4 species in an excited singlet state, that either converts to the triplet ground state or combines with a solvent molecule to regenerate a penta-coordinated low spin singlet state Fe species (Fig. 47).139 Other applications include studies of catalytic mechanisms, such as atomistic characterization of the active-site solvation dynamics of a model photocatalyst. [Ir2(dimen)4]2 þ, where dimen is para-diisocyanomenthane.140 The time-dependent structural changes in this model photocatalyst, as well as the changes in the solvation shell structure, have been measured with ultrafast diffuse X-ray scattering (XDS) and simulated with Born–Oppenheimer molecular dynamics (BOMD). Both methods provide direct access to the solute-solvent pair distribution function, enabling the solvation dynamics around the catalytically active iridium sites to be robustly characterized. The results provide evidence for the coordination of the iridium atoms by the acetonitrile solvent (Fig. 48). The development of X-ray free electron lasers is pushing this field of research toward direct structural imaging on the ultrafast time-scale, of bonds breaking and reforming. Please see recent reviews on the technique developments and their potential.141–145

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Fig. 30 Pulse radiolysis–TRIR investigation of a Mn(I) diimine CO2 reduction catalyst. Reproduced from Grills, D. C.; Farrington, J. A.; Layne, B. H.; Lymar, S. V.; Mello, B. A.; Preses, J. M.; Wishart, J. F. Mechanism of the Formation of a Mn-Based CO2 Reduction Catalyst Revealed by Pulse Radiolysis with Time-Resolved Infrared Detection. J. Am. Chem. Soc. 2014, 136 (15), 5563–5566. doi:10.1021/ja501051s with permission. Copyright 2014, American Chemical Society.

Fig. 31 Molecular structures of the complexes studied (left) and resonance Raman spectra (right) of RuL4 in CH2Cl2 (10 3 M). Vibrations associated with the phen0 ligand and/or the R substituent are indicated in bold. Solvent bands: *. Reproduced from Shillito, G. E.; Bodman, S. E.; Mapley, J. I.; Fitchett, C. M.; Gordon, K. C. Accessing a Long-Lived 3LC State in a Ruthenium(II) Phenanthroline Complex with Appended Aromatic Groups. Inorg. Chem. 2020, 59 (23), 16967–16975. doi:10.1021/acs.inorgchem.0c02102 with permission. Copyright 2020, American Chemical Society.

The theoretical approachesddeveloping alongside the experimental advancesdwill be extremely important in interpretation and understanding of the data.146 Overall, time-resolved X-ray methods, with simultaneous detection of different elements, and using multiple detection methods in the same experiment, from the same sample, are likely to unlock the next level of understanding of excited state reactions involving metal complexes (Fig. 49).147 Description of the capabilities and further details can be found in, for example, Ref.148.

8.13.5

Ultrafast multidimensional spectroscopy

8.13.5.1

General concepts

The methods examined until now cover a “single wavelength” excitation and focus on resolving the absorption or emission frequency axis in time. In a multidimensional spectroscopic experiment, both the pump and probe axes of a broadband pump–broadband probe

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Fig. 32 FSRS spectra of the [Re(bpy)(CO)3Cl] complex upon photoexcitation at 400 nm. Reproduced from Pizl, M.; Picchiotti, A.; Rebarz, M.; Lenngren, N.; Yingliang, L.; Zális, S.; Kloz, M.; Vlcek, A. Time-Resolved Femtosecond Stimulated Raman Spectra and DFT Anharmonic Vibrational Analysis of an Electronically Excited Rhenium Photosensitizer. J. Phys. Chem. A 2020, 124 (7), 1253–1265. doi:10.1021/acs.jpca.9b10840 with permission. Copyright 2020, American Chemical Society.

experiment are frequency resolved, which gives rise to the two dimensions. The pump–probe delay is called population delay or t2 delay, and the pump (excitation) and probe (detection) axes are called u1 and u3, respectively. The excitation axis can be resolved by either (i) scanning the frequency of the excitation pulse and collecting a series of pump– probe spectra using a narrowband excitation source; or (ii) exciting the sample using two copies of a broadband pump pulse and detecting the pump–probe spectra as a function of the coherence delay, which is then Fourier transformed to reveal the excitation axis. The first approach is often called frequency-domain while the latter is called time-domain, and offers the ultimate time resolution. The probe is often detected in the frequency domain using a multichannel detector array (i.e., a CMOS array for UV/Vis spectra or a HgCdTe array in the mid-IR). These concepts are illustrated in Fig. 50:[149]

Fig. 33 Schematics of (A) time-correlated single photon counting, and (B) fluorescence upconversion spectroscopy (FLUPS) experiments. Reproduced from Nibbering, E. T. J. Ultrafast Technology: Femtosecond Condensed Phase Spectroscopy: Structural Dynamics. In Encyclopedia of Modern Optics, Five-Volume Set; Elsevier, 2004; pp 253–263. doi:10.1016/B0-12-369395-0/00944-1 with permission. Copyright 2005, Elsevier.

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Fig. 34 (a) 2D time-resolved luminescence spectrum of aqueous [Ru(bpy)3]2 þ under excitation at 25000 cm 1 (intensities (0–300) in arbitrary units). The signal at z21,600 cm 1 is the Raman line of water. (b) Luminescence spectra recorded at different time delays. Reproduced from Cannizzo, A.; van Mourik, F.; Gawelda, W.; Zgrablic, G.; Bressler, C.; Chergui, M. Broadband Femtosecond Fluorescence Spectroscopy of [Ru(Bpy)3]2 þ. Angew. Chem. Int. Ed. 2006, 45 (19), 3174–3176. doi:10.1002/anie.200600125 with permission. Copyright 2006, John Wiley & Sons.

In the following sections, we will discuss the fundamentals and applications of two-dimensional spectroscopies in the vibrational and electronic domains, and mixed spectroscopies involving electronic (UV/Vis) excitation and vibrational (mid-IR) detection (2D-EV), or vibrational excitation with UV/Vis detection (2D-VE). These multidimensional spectroscopic techniques are illustrated in Fig. 51:150

8.13.5.2

Two-dimensional infrared spectroscopy (2D-IR)

We begin this section on multidimensional spectroscopies with two-dimensional infrared (2D-IR) spectroscopy. The first implementation of 2D-IR spectroscopy was reported in 1997 by Hamm, Lim and Hochstrasser.151 Since then, numerous technological advances including pulse shaping in the mid-IR,152–154 and fast repetition rate sources (reaching up to 100 kHz),155,156 have allowed 2D-IR spectroscopy to gain an ubiquitous place in the ultrafast toolbox of a modern ultrafast spectroscopy laboratory. The first “unusual” feature of a 2D-IR spectrum is that two transitions are typically observed for each vibrational band (instead of the single feature that is observed for each uncoupled spin in, e.g., 2D-NMR spectroscopy). The two features appearing in the 2D IR spectrum of an isolated oscillator originate from the different frequencies of the 0 / 1 and 1 / 2 transitions, originated by the anharmonicity of the vibrational potential (Fig. 52). An excellent and complete introduction to 2D-IR spectroscopy is given by

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Fig. 35 Time-resolved fluorescence data for PtPOP in water excited with a  120 fs laser pulse at 380 nm. (a) 2D time  wavelength plot of timeresolved fluorescence spectra. The inset shows a zoom into the initial 1.7 ps time window. (b) Fluorescence time traces (open circles) at various wavelengths (same data as in a), together with their fit functions using a global fit model (solid lines, see text). The inset shows a zoom into the initial 3.7 ps time window. The weak modulation during the first 3 ps is due to a slow fluctuation in the laser power. Reproduced from van der Veen, R. M.; Cannizzo, A.; van Mourik, F.; Vlcek, A.; Chergui, M. Vibrational Relaxation and Intersystem Crossing of Binuclear Metal Complexes in Solution. J. Am. Chem. Soc. 2011, 133 (2), 305–315. doi:10.1021/ja106769w with permission. Copyright 2010, American Chemical Society.

Hamm and Zanni in their book.157 Kiefer and Kubarych have also nicely summarized recent studies on 2D-IR spectroscopy of coordination complexes.158 For strictly harmonic oscillators, both the diagonal (Dii) and off-diagonal (Dij) anharmonicities are equal to zero. Hence, strictly harmonic oscillators cannot be coupled, and will also show no 2D-IR signalda zero anharmonicity would cause the negative (GSB and SE) and positive (ESA) contributions to perfectly overlap and cancel out. A 2D-IR spectrum contains a vast amount of information, which is often unavailable from pump–probe (“1D”) methods. On the first place, it can reveal the underlying inhomogeneous and homogeneous frequency distributions of a given mode (through the line shapes of the diagonal peaks). This aspect is crucial to understanding solvation, through its manifestation by spectral diffusion in the timescale of the experiment. A second piece of information one can obtain from a 2D-IR spectrum are cross peaks, indicative of vibrational coupling or population transfer. Finally, through the polarization dependence of the intensities of the cross peaks, the relative orientations of the transition dipoles from the coupled modes can be obtained.159 The applications of 2D-IR to investigate photoinduced processes in coordination complexes are diverse, and span a range of fundamental questions, which we aim to summarize in the following section. A well-known family of metal carbonyls, derivatives of Vaska’s complex, trans-[IrCl(CO)(PPh3)2] (VC), and complexes derived from oxidative addition of oxygen, iodine and other substrates, have been studied using 2D-IR spectroscopy.160–164 Upon studying

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Fig. 36 2D maps of time-resolved luminescence spectra of [Re(L)(CO)3(bpy)]n in CH3CN, measured after 400 nm excitation. (a): L ¼ Etpy; (b): L ¼ Cl; (c): L ¼ Br, (d): L ¼ I. Intensities are color-coded. The peak at 457 nm is a CH3CN Raman line. Reproduced from Cannizzo, A.; Blanco Rodríguez, A. M.; El Nahhas, A.; Sebera, J.; Zális, S.; Vlcek, A.; Chergui, M. Femtosecond Fluorescence and Intersystem Crossing in Rhenium(I) Carbonyl-Bipyridine Complexes. J. Am. Chem. Soc. 2008, 130 (28), 8967–8974. doi:10.1021/ja710763w with permission. Copyright 2008, American Chemical Society.

these adducts in mixtures of benzene-d6 with benzyl alcohol or chloroform, the carbonyl frequency and width were found to vary nonlinearly across the two binary solvent series, indicating preferential solvation. In contrast, the vibrational lifetime changes linearly with solvent composition, and is correlated with the mole fraction of chloroform but anticorrelated with the mole fraction of benzyl alcohol. The results are explained by differences in the densities of solvent modes that affect intermolecular relaxation of the carbonyl mode.161 Similarly, Vaska’s complex iodine adduct, cis,trans-[Ir(CO)(Cl)I2(PPh3)2] (VC-I2), has been studied in different solvent mixtures (Fig. 53). Similarly, the selective vibrational coupling between metal carbonyl and metal hydride vibrations in hydridocarbonyl complexesdan industrially relevant family of complexesdwhen the two ligands are oriented in a trans configuration has been recently reported.165 This is exemplified in Fig. 54, where the 2D-IR spectrum of the complex cis,trans-[Ir(CO)(Cl)(H)2(PPh3)2] (VC-H2)danother derivative of Vaska’s complexdshows a set of cross peaks corresponding to the coupling of the trans Ir–H and Ir(C^O) vibrations, while the isolated and decoupled Ir–Hcis vibration does not show any cross peaks. The coupling of these two modes enhances the often-weak M–H stretching mode, allowing for direct determination by infrared spectroscopy of both the oxidation (by frequency shifts) and the protonation state (via vibrational coupling) of a metal hydridocarbonyl complex. Vibrational couplings and energy transfer in metal carbonyl complexes, between different ligands and across the metal center have also been studied using 2D-IR spectroscopy. These processes play a fundamental role on the ultrafast energy redistribution, in particular after photoexcitation of a metal complex. For instance, the dynamics of ground and excited-state vibrational relaxation and energy transfer in the Re(Cl)(CO)3(4,40 -diethylester-2,20 -bipyridine) complex (ReCOe) have been studied.166 It was shown

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Fig. 37 Fluorescence upconversion spectra of 5,10,15,20-tetraphenylporphin (TPP) and 2,3,7,8,12,13,17,18-octaethylporphin (OEP) with open 3dshell metals in cyclohexane. Reproduced from Bräm, O.; Cannizzo, A.; Chergui, M. Ultrafast Broadband Fluorescence Up-Conversion Study of the Electronic Relaxation of Metalloporphyrins. J. Phys. Chem. A 2019, 123 (7), 1461–1468. doi:10.1021/acs.jpca.9b00007 with permission. Copyright 2019, American Chemical Society.

Fig. 38 Left: Broadband FLUPs data for PTZ-CH2-Ph-CC-Pt(PBu3)2-CC-NAP, the Pt(II) donor-bridge-acceptor complexes studied in Ref.120, in DCM. The Y-axis is expressed as pump-induced optical gain (rel.). Excitation 400 nm, gate 1340 nm. Right: Energy level diagram constructed using FLUPS, TAS, TRIR, and FSRS data. Symbols: FCdFranck–Condon excited state, Zdrelaxed “thermalized” lowest 1CT state. Multiexponential descriptions of the observed evolutions call for states X and Y (either virtual or true intermediates). Time constants extracted from FSRS and FLUPS results are indicated; percentages refer to relative amplitudes of decay or rise terms. Reproduced from Farrow, G. A.; Quick, M.; Kovalenko, S. A.; Wu, G.; Sadler, A.; Chekulaev, D.; Chauvet, A. A. P.; Weinstein, J. A.; Ernsting, N. P. On the Intersystem Crossing Rate in a Platinum(Ii) Donor-BridgeAcceptor Triad. Phys. Chem. Chem. Phys. 2021, 23 (38), 21652–21663. doi:10.1039/d1cp03471e with permission. Copyright 2021, Royal Society of Chemistry.

that in the ground state, intramolecular anharmonic coupling may be solvent-assisted through solvent-induced frequency and charge fluctuations, and as such the vibrational relaxation rates are solvent-dependent (Fig. 55). Further results from this work concerning the excited state vibrational relaxation pathways are discussed later in this chapter, as they correspond to transient 2D-IR spectroscopy. Understanding the rules of energy relaxation and energy transfer across the metal center in such compounds can help optimize their electron transfer switching properties. A technique called relaxation-assisted 2D-IR has been developed to understand longdistance vibrational energy transfer dynamics.167,168 The transfer of vibrational energy has been tracked across a Pt(II) metal center in a recent report that studied a series of complexes containing polyyne ligands (Fig. 56).169 In this study, the energy transport

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Fig. 39 A summary of X-ray methods. Processes that are triggered when an X-ray beam intensity (I0) is incident on a sample. Different types of core-level spectroscopies are possible: X-ray absorption spectroscopy (XAS), and photon-in/photon-out spectroscopies, which are broadly classified as X-ray emission (XES) and photoelectron spectroscopy (photon-in/electron-out). X-ray scattering is used to probe structures of molecules in solution and X-ray diffraction for crystalline systems. Reproduced from Chergui, M.; Collet, E. Photoinduced Structural Dynamics of Molecular Systems Mapped by Time-Resolved X-Ray Methods. Chem. Rev. 2017, 117 (16), 11025–11065. doi:10.1021/acs.chemrev.6b00831 with permission. Copyright 2017, American Chemical Society.

pathways between the ligands in the high-frequency regime were identified. The complementary regime involves redistribution via low-frequency delocalized modes, which does not lead to interligand energy transport. The energy transfer pathways in various Pt(II) alkyne complexes have been studied, whereby the alkyne vibrations have been used to control the excited state deactivation pathways. These works are discussed in more detail in Section 8.13.6 of this chapter, in the context of IR-control of electron transfer.170–176 From the early studies on systems such as W(CO)6 and Rh(CO)2(acac), a myriad of 2D-IR studies have been performed on transition metal carbonylsdthe brightest ground-state IR chromophores known. W(CO)6 has been used as a vibrational probe to explore the ultrafast dynamics in different environments. From the interior of aerosol-OT/water mixtures (Fig. 57),177 to planar phospholipid multibilayer model cell membranes and vesicles (Fig. 58);178,179 and more recently in ionic liquids (Fig. 59).180 The observable that gives fundamental information about frequency fluctuationsdand hence about the “excitation memory” of the systemdis the degree of elongation along the diagonal (often quantified through the so-called centerline slope), which is intimately related to the frequency fluctuation correlation functions (FFCF).157,181–183 As the different inhomogeneous environments which are initially excited randomize and exchange, the initially high degree of correlation between excited and probed frequencies is lost, yielding a “round” line shape and evidencing that spectral diffusion has taken place.157,158 The timescale of this process and the relative amplitude of the homogeneous and inhomogeneous contributions to the 2D-IR line shapes are of central importance and reveal crucial information about solvation dynamics. In another study, the equilibrium reaction dynamics of a nearly barrierless molecular rotordthe Cr(h6-Ph)(CO)3 complex (BCT)dwere explored with 2D-IR spectroscopy.184 The FFCFs obtained for this complex in apolar solvents of increasing viscosity show no change in either the relaxation time constants or the relative homogeneous/inhomogeneous amplitudes (Fig. 60). The spectral dynamics is attributed to low-barrier internal torsional motion. This tripod complex has two stable minima corresponding to staggered and eclipsed conformations, which differ in energy by roughly half of kBT. The solvent independence is due to the

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Fig. 40 Top: X-ray absorption spectroscopy. Excitation of a core orbital to the orbitals below the ionization potential (E0) gives rise to pre-edge transitions, which probe the unoccupied density of states. Above E0, there is a jump in the absorption cross-section giving rise to an edge and a photoelectron is generated with an excess energy (E  E0). Bottom: Pattern of an outgoing and backscattered photoelectron wave in the case of EXAFS (single scattering events) and XANES (multiple scattering events). Low energy electrons have high scattering cross sections, so if the absorbing atom is embedded in an assembly of atoms, multiple scattering events occur (right panel), giving rise to modulations of the spectrum in the above ionization region, called the X-ray absorption near edge structure (XANES). As the kinetic energy of the electrons increases, the cross section decreases and weaker modulations appear, which form the extended X-ray absorption fine structure (EXAFS). Adapted from Chergui, M.; Collet, E. Photoinduced Structural Dynamics of Molecular Systems Mapped by Time-Resolved X-Ray Methods. Chem. Rev. 2017, 117 (16), 11025– 11065. doi:10.1021/acs.chemrev.6b00831 with permission. Copyright 2017, American Chemical Society. Adapted from Bressler, C.; Chergui, M. Ultrafast X-Ray Absorption Spectroscopy. Chem. Rev. 2004, 104 (4), 1781–1812. doi:10.1021/cr0206667 with permission. Copyright 2004, American Chemical Society.

relative size of the rotor compared with the solvent molecules, which create a solvent cage in which torsional motion occurs largely free from solvent damping. Since the one-dimensional transition state is computed to be only 0.03 kBT above the higher energy eclipsed conformation, this model system offers an unusual, nearly barrierless reaction, which nevertheless is characterized by torsional coordinate dependent vibrational frequencies. Following this line of thought, the FFCF dynamics of a series of Cu(II)-azido complexes have been explored as a function of the steric bulk around the N3 ligand.185 A series of Cu(II) complexes [(R3P3tren)Cu(N3)]BAr4F (R3P3tren ¼ tris[2-(phosphiniminato) ethyl]amine, BAr4F ¼ tetrakis(pentafluorophenyl)borate) were studied, where the number of methyl and phenyl groups in the PR3 ligand were systematically varied across the series (PR3 ¼ PMe3, PMe2Ph, PMePh2), as shown in Fig. 61. The results of the pump–probe measurements indicate that the vibrational energy of azide dissipates through intramolecular pathways, and that the bulkier phenyl groups lead to an increase in the spatial restriction of the diffusive reorientation of bound azide. From 2D-IR experiments, the authors characterize the spectral diffusion of the azide group and find that an increase in the number of phenyl groups maps to a broader inhomogeneous frequency distribution. This indicates that an increase in the steric bulk of the secondary coordination sphere acts to create more distinct configurations in the local environment that are accessible to the azide group. Their work demonstrates how ligand structural variation affects the ultrafast dynamics of a small molecular group bound to the metal center, which could provide insight into the structure–function relationship of the copper coordination complexes and transition-metal coordination complexes in general.

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Fig. 41 Excited state distortion and intersystem crossing in a Cu(I) complex. Reproduced from Iwamura, M.; Takeuchi, S.; Tahara, T. Ultrafast Excited-State Dynamics of Copper(I) Complexes. Acc. Chem. Res. 2015, 48 (3), 782–791. doi:10.1021/ar500353h with permission. Copyright 2015, American Chemical Society.

2D-IR spectroscopy has also been used to study biomimetic catalysts like [FeFe]Hydrogenase active site model complexes. In particular, both the photolysis products,186 and the effect of the solvent environment on the vibrational dynamics of these model systems have been studied (Fig. 62).187–189 The exchanging conformations of a [CpRu(CO)2]2 and [CpFe(CO)2]2 have also been tracked in real time with 2D-IR spectroscopy (Fig. 63).190 By treating the energy transfer as an equilibrium process, the rate constants associated with both the uphill and the downhill transfer of vibrational energy were extracted, finding that the difference in the rate constants of the two metal complexes maps to the difference in the energy gap between the two modes involved. Furthermore, the direct exchange between distinct chemical species was not observed in either case, probably due to the very low population of the complementary isomers. A similar study used 2D-IR spectroscopy to monitor the chemical exchange between the different isomers of Co2(CO)8.191 In this case, exchange of population between bridged and unbridged isomers was found to take place on the time scale of a few picoseconds, corresponding to activation barriers of several kcal mol 1. Despite overlapping spectral features in the 2D-IR spectrum, the exchange component of the waiting time dependence was isolated by exploiting the well-characterized coherent modulation of nonexchange cross peaks. The temperature dependence of the forward and reverse rate constants enabled extraction of isomerization energy barriers, where analysis using the Eyring equation indicated a substantial entropic contribution to the free energy barrier (Fig. 64). 2D-IR spectroscopy has also been used to monitor transition metal complexes immobilized on surfaces. For instance, it has been shown192 that atomic layer deposition (ALD) induces a rigidization of the molecular environment around an immobilized dye, effectively covering it from the environment. This strategy has been shown to benefit the long-term stability of catalysts and solar cell devices anchored on semiconductor supports.193 While most of the work discussed so far concerns intramolecular processes, it is worthwhile mentioning that intermolecular processes may also be observed with 2D-IR spectroscopy. For instance, the process of vibrational energy transfer (VET) on surfaces has been studied in detail.194–197 The transfer of vibrational excitation between the 12CO and 13CO isotopologues of a Re(CO)3(dcb)X complex (X ¼ Cl, Br; dcb ¼ 2,20 -bipyridine-4,40 -dicarboxylic acid) was used as a “molecular ruler” to monitor surface morphology, the impact of plasmonic enhancements on the VET rates, and to gain further detail on the intricacies of vibrational energy transport on immobilized molecular surfaces (Fig. 65). It was found that: (i) the molecules attach on the surface forming a close-to-closest packing arrangement, without evidence for clustering even in the presence of other co-adsorbed species;194,197 (ii) the VET rates are not significantly changed by plasmonic enhancements;195 (iii) a reversible morphology change was observed when the solvent was changed from MeOH to MeCN;196 (iv) vibrational energy transfer becomes a collective phenomenon on the surface, where a delocalized network of chromophores operates, and it can no longer be thought as a collection of pairwise energy transfer processes.197 The aggregation of Re(I) carbonyl dyes on a surface has been investigated using 2D-IR spectroscopy. In particular, the use of one vs. two anchoring groups (Fig. 66) and impact of the different surface binding motifs has been examined.198,199 It was found that aggregation of dyes on the surface is energetically favorable. Adsorbate–adsorbate interactions may play an important role in defining surface structure and electronic properties of dye-sensitized solar cells and related organic/inorganic interfaces, something which has also been discussed previously in this context.200

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Fig. 42 Static Pt L3 XAS spectrum of [Pt2(P2O5H2)4]4 in solution (black line, left axis) and the transient (excited–unexcited) XAS spectrum (red circles, right axis, same units as left) integrated up to 150 ns after excitation. The inset zooms into the XANES region. (b) Transient EXAFS data (circles; binned from the data in (a)) and best fit (solid line) with the following results: a Pt–Pt contraction of 0.31(5) Å, a platinum–ligand elongation of 0.010(6) Å, zero energy shift, and 7% excitation yield. The error bars represent the standard error of the measurement. The best-fit structural distortions are indicated in the upper right corner. Reproduced from van der Veen, R. M.; Milne, C. J.; El Nahhas, A.; Lima, F. A.; Pham, V. T.; Best, J.; Weinstein, J. A.; Borca, C. N.; Abela, R.; Bressler, C.; Chergui, M. Structural Determination of a Photoehemieally Active Diplatinum Molecule by Time-Resolved EXAFS Spectroscopy. Angew. Chem. Int. Ed. 2009, 48 (15), 2711–2714. doi:10.1002/anie.200805946 with permission. Copyright 2009, John Wiley and Sons.

In their report, the authors have identified characteristic frequencies of monomers, dimers, and trimers. A comparison of 2D-IR spectra in solution vs. those deposited on TiO2 shows that the propensity to dimerize in solution leads to higher dimer formation on TiO2, but that dimers are formed even if there are only monomers in solution. Aggregates cannot be washed off with standard protocols and are present even at sub-monolayer coverages. We observe cross peaks between aggregates of different sizes, primarily dimers and trimers, indicating that clusters consist of microdomains in close proximity. This is also supported by evaluating the 2DIR spectra of amorphous deposits (Fig. 67) and comparing them to the 2D-IR spectra of the sensitized TiO2 films (Fig. 68). Other intermolecular cross peaks have been observed in, e.g., a mixture of a Re(I) tricarbonyl complex and NaSCN in THF.201 The preferential association between the metal carbonyl catalyst and the NaSCN ion pairs was found to facilitate intermolecular energy transfer on a few picoseconds time scale. Monitoring the cross peak between the highest frequency metal carbonyl band and the CN bands of NaSCN contact ion pairs, the authors found a striking time evolution of the cross-peak position on the detection axis. It is argued that the energy transfer, a second-order Förster process, effectively increases the dimensionality of the 2D-IR spectroscopy and thus enables sensitivity to non-Gaussian dynamics (Fig. 69). Further recent technical developments in 2D-IR spectroscopy have enabled a selective enhancement of surface-bound molecules,202–205 and polarization schemes to suppress diagonal peaks.206 The fundamental insights discussed in this section are only attainable using ultrafast two-dimensional infrared spectroscopy, showcasing its utility for the study of coordination complexes. We firmly believe that many more questionsdespecially those

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Fig. 43 Left: Structures of photoexcited pyrazolate-bridged [Pt(ppy)(m-R2pz)]2-type Pt(II) dimers. Right: Ground-state XANES with excited-state difference spectra (excited minus ground) of 1 (a) and 2 (c) and XTA kinetic traces at 11.561 keV of 1 (b) and 2 (d). Adapted from Weingartz, N. P.; Mara, M. W.; Roy, S.; Hong, J.; Chakraborty, A.; Brown-Xu, S. E.; Phelan, B. T.; Castellano, F. N.; Chen, L. X. Excited-State Bond Contraction and Charge Migration in a Platinum Dimer Complex Characterized by X-Ray and Optical Transient Absorption Spectroscopy. J. Phys. Chem. A 2021, 125 (40), 8891–8898. doi:10.1021/acs.jpca.1c07201 with permission. Copyright 2021, American Chemical Society.

Fig. 44 Time-resolved infrared spectroscopic studies of excitation-wavelength dependent photophysics of Ir(I) and Rh(I) d8-d8 dimeric complexes. Reproduced from Pizl, M.; Hunter, B. M.; Sazanovich, I. V.; Towrie, M.; Gray, H. B.; Zális, S.; Vlcek, A. Excitation-Wavelength-Dependent Photophysics of D8d8Di-Isocyanide Complexes. Inorg. Chem. 2022, 61 (6), 2745–2759. doi:10.1021/acs.inorgchem.1c02645 with permission. Copyright 2022, American Chemical Society.

involving photoactive and reactive transition metal complexesdwill continue to illustrate the beauty and power of 2D-IR spectroscopy.

8.13.5.3

Two-dimensional electronic spectroscopy (2D-ES)

It is now our turn to get (electronically) excited and discuss the complementary technique of two-dimensional electronic spectroscopy (2D-ES). Among the questions addressed with 2D-ES, the study of light-harvesting complexes found in green plants and photosynthetic bacteria constitutes a significant body of literature.207–217 The ability of 2D-ES to reveal excitonic couplings, coherent vs incoherent energy transfer, conformational changes and other ultrafast processes in these systems makes it an especially attractive investigation method.218,219 Furthermore, since the light-harvesting complexes often have relatively narrow absorption bands (since the actual chromophores are porphyrinoid derivatives, e.g. Fig. 70), this makes them especially amenable to 2D spectroscopy. In other words, both the system and the method are a perfect match.

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Fig. 45 A trinuclear m3-oxo-bridged Mn(III)-based SMM, whose magnetic anisotropy is closely related to the Jahn–Teller distortion. Reproduced from Liedy, F.; Eng, J.; McNab, R.; Inglis, R.; Penfold, T. J.; Brechin, E. K.; Johansson, J. O. Vibrational Coherences in Manganese Single-Molecule Magnets after Ultrafast Photoexcitation. Nat. Chem. 2020, 12 (5), 452–458. doi:10.1038/s41557-020-0431-6 with permission. Copyright 2020, Springer Nature AG.

This is illustrated by the schematic linear and 2D-ES spectra shown in Fig. 71, together with the corresponding energy level diagram and model system.220 For instance, using 2D-ES, the downhill energy equilibration on a 50 fs time scale was observed in the photosystem I complex of a cyanobacterium (Fig. 72).221

8.13.5.4 2D-VE)

Mixed spectroscopies: Two-dimensional electronic-vibrational and vibrational-electronic spectroscopies (2D-EV and

Apart from two-dimensional electronic and vibrational spectroscopies, other mixed spectroscopies have been developed and applied to the study of transition metal complexes. In first place we will discuss two-dimensional electronic–vibrational spectroscopy (2D-EV). 2D-EV results from frequency resolving the excitation axis of a broadband UV/Vis pulsedwhich electronically excites the complex, and detecting the corresponding absorption changes in the mid-IR. In other words, it can be seen as a collection of TRIR experiments with varying excitation wavelengths. For the same reasons discussed above regarding 2D-ES, 2D-EV spectroscopy has also been largely applied to light-harvesting complexes from photosynthetic organisms.222–231 2D-EV allows one to directly explore the interplay between nuclear and electronic motion in the ultrafast events that take place after photoexcitation, and also allows for the characterization of the cross correlation between electronic solvation in the excited state and vibrational spectral diffusion.232,233 In this manner, 2D-EV provides a unique picture and access to these cross-correlations, since both 2D-IR and 2D-ES spectroscopies measure purely vibrational or electronic correlations, respectively. Example 2D-EV spectra are shown in Fig. 73,228 where the transfer of electronic excitation energy between specific pigments within the light-harvesting complex II (LHCII) from spinach are illustrated. In a complementary manner, two-dimensional vibrational–electronic spectroscopy (2D-EV) has been recently introduced.234 In this technique, the roles of the electronic and vibrational pulses are reversed: the changes in the electronic (UV–Vis–NIR) absorption of a system after broadband mid-IR excitation are interrogated, with the IR excitation axis being frequency resolved. This technique has been applied to study the vibrational-electronic couplings between high frequency cyanide stretching vibrations, n(CN) and either a ligand-to-metal charge transfer transition [FeIII(CN)6]3 (Fig. 74); or a metal-to-metal charge transfer (MMCT) transition in the mixed-valence complex [(CN)5FeIICNRuIII(NH3)5] (Fig. 75).234 The 2D-VE spectra of both molecules reveal peaks resulting from coupled high- and low-frequency vibrational modes to the charge transfer transition. The time-evolving amplitudes and positions of the peaks in the 2D-VE spectra report on coherent and incoherent vibrational energy transfer dynamics among the coupled vibrational modes and the charge transfer transition. The selectivity of 2D-VE spectroscopy to vibronic processes is evidenced from the selective coupling of specific n(CN) modes to the MMCT transition in the mixed valence complex. The line shapes in 2D-VE spectra report on the correlation of the frequency fluctuations between the coupled vibrational and electronic frequencies in the mixed valence complex, which has a time scale of 1 ps. Both 2D-EV and 2D-VE techniques, as well as similar approaches based on mixed vibrational–electronic spectroscopies provide a window to unique spectroscopic information, not readily available to “diagonal” spectroscopies that operate in the same regime.235–240 While these techniques are extremely challenging and more demanding, we believe that their application to coordination compounds may help answer crucial questions regarding the vibrational modulation of charge and energy transfer in a wide

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Fig. 46 Fit to the femtosecond transient XANES of [Fe(bpy)3]2 þ measured at 7121.5 eV, showing the contribution of the MLCT and of the oscillating high spin state. Reproduced from Lemke, H. T.; Kjær, K. S.; Hartsock, R.; van Driel, T. B.; Chollet, M.; Glownia, J. M.; Song, S.; Zhu, D.; Pace, E.; Matar, S. F.; Nielsen, M. M.; Benfatto, M.; Gaffney, K. J.; Collet, E.; Cammarata, M. Coherent Structural Trapping through Wave Packet Dispersion during Photoinduced Spin State Switching. Nat. Commun. 2017, 8 (1), 1–8. doi:10.1038/ncomms15342 with permission. Copyright 2017, Springer Nature AG.

range of complex molecular, material, and biological systems, as well as the complex interplay between nuclear and electronic degrees of freedom in photoexcited transition metal complexes.

8.13.6

Multi-pulse experiments

8.13.6.1

Transient 2D-IR spectroscopy

Transient 2D-IR spectroscopy (T2D-IR) is a complementary technique that combines the principles of both TRIR and 2D-IR spectroscopy, yielding information about correlations between vibrations in the excited state, as well as correlations between groundand excited-state vibrationsddepending on the time ordering of the pulses.241–244

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Fig. 47 (a) Scheme with optical-laser pump and soft X-ray probe after the pump–probe time delay Dt. The intensity of RIXS is measured at the Fe L3-absorption edge with a dispersive grating spectrometer. (b) Electron configuration of ground-state Fe(CO)5 with single-electron transitions of X-ray probe and laser-pump processes (orbital assignments according to Fe 2p and 3d or ligand 2p character and according to symmetry along the FeeCO bonds; the asterisk marks antibonding orbitals). RIXS at the Fe L3-absorption edge with 2p / d(s*) excitation involves scattering to final dp7ds* ligand-field excited states. Optical dp/2p* excitation triggers dissociation. (c) Measured Fe L3-RIXS intensities (encoded in color) vs. energy transfer and incident photon energy. Top: ground-state Fe(CO)5 (negative delays, probe before pump). Middle and bottom: difference intensities for delay intervals of 0–700 fs and 0.7–3.5 ps, respectively, isolating transients by subtracting scaled intensities of unpumped Fe(CO)5 from the measured intensities (scaling factor 0.9). Reproduced from Wernet, P.; Kunnus, K.; Josefsson, I.; Rajkovic, I.; Quevedo, W.; Beye, M.; Schreck, S.; Grübel, S.; Scholz, M.; Nordlund, D.; Zhang, W.; Hartsock, R. W.; Schlotter, W. F.; Turner, J. J.; Kennedy, B.; Hennies, F.; de Groot, F. M. F.; Gaffney, K. J.; Techert, S.; Odelius, M.; Föhlisch, A. Orbital-Specific Mapping of the Ligand Exchange Dynamics of Fe(CO)5 in Solution. Nature 2015, 520 (7545), 78–81. doi:10.1038/nature14296 with permission. Copyright 2015, Springer Nature AG.

This is illustrated in Fig. 76, which shows two possible time orderings for a frequency-domain T2D-IR experiment: (i) UVpump  IRpump  IRprobe (the “normal” sequence); and (ii) IRpump  UVpump  IRprobe (the exchange, labeling or EXSY sequence). The “normal” T2D-IR pulse sequence simply reveals a 2D-IR spectrum of the photogenerated species. The only important difference is that the signs of the ESA and GSB/SE contributions are now reversed. This has been used to track, for instance, the photolysis products of [FeFe]Hydrogenase active site model systems (the TRIR aspects of this work were discussed in a previous section),186 and to investigate the vibrational dynamics of a 17 e metallocarbonyl intermediate formed during the photolysis of [(nPr-Cp) W(CO)3]2.245 T2D-IR has also been used to study vibrational energy migration in the excited state, which allows for a direct comparison with the vibrational energy transfer processes that take place in the ground state. This has been studied for a Re(I) tricarbonyl complex with an ester-substituted bipyridine ligand (ReCOe).166 The increased vibrational relaxation rates in the excited state are attributed to increased intramolecular electrostatic interactions helping overcome structural and thermodynamic barriers for this process in the vicinity of the central heavy atom, a feature which may be of significance to nonequilibrium photoinduced processes observed in transition metal complexes in general (Fig. 77). A similar increase in the vibrational relaxation dynamics in the excited state was observed using T2D-IR in [Ru(4,40 -(COOEt)22,2-bpy)2(NCS)2], a close relative of the efficient N3 dye used in dye-sensitized solar cells (Fig. 78).246

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Fig. 48 Scheme and results of the XDS experiments on [Ir2(dimen)4]2 þ. (a) Shows a snapshot of [Ir2(dimen)4]2 þ in acetonitrile solution from BOMD simulations. (b) Shows the experimental set-up. (c) The recorded difference scattering data and fit, each consecutive curve has been offset by 150 e.u. for visibility. (d) Shows examples of the four components used to fit the data; The contraction signal is simulated for a 4.2–2.9 Å contraction of the Ir-Ir distance with no change in the ligand twist. The ligand twist component is simulated for a 0–15 degree increase in the N-Ir-IrN ligand dihedral twist at an Ir-Ir distance of 2.9 Å. The two solute components are extracted directly from the analysis. (e) Shows a sketch of four dynamics giving rise to the signals presented in (d). Reproduced from van Driel, T. B.; Kjaer, K. S.; Hartsock, R. W.; Dohn, A. O.; Harlang, T.; Chollet, M.; Christensen, M.; Gawelda, W.; Henriksen, N. E.; Kim, J. G.; Haldrup, K.; Kim, K. H.; Ihee, H.; Kim, J.; Lemke, H.; Sun, Z.; Sundström, V.; Zhang, W.; Zhu, D.; Møller, K. B.; Nielsen, M. M.; Gaffney, K. J. Atomistic Characterization of the Active-Site Solvation Dynamics of a Model Photocatalyst. Nat. Commun. 2016, 7 (1), 1–7. doi:10.1038/ncomms13678 with permission. Copyright 2016, Springer Nature AG.

Fig. 49 Time-resolved X-ray spectroscopy with simultaneous multi-element detection and multiple detection methods at femtosecond X-ray experiment (FXE) at EU XFEL. Retrieved from Capabilities of the EU XFEL. https://www.xfel.eu/facility/instruments/fxe/current_capabilities_of_fxe/ index_eng.html (accessed 2022-08-02).

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Fig. 50 Schemes of multidimensional spectroscopies that can provide time- and excitation/detection frequency-resolved information. (a) Pump– probe spectroscopy. (b) Homodyne transient grating spectroscopy. (c) Colinear self-heterodyne 2D spectroscopy. (d) Non-colinear heterodyne 2D spectroscopy. Top left (all parts), the spatial arrangement of the pulses, sample and detector; bottom left (all parts), temporal arrangement of the pulses and signal; right, the procedure to obtain 2D maps and characteristic features of the technique. Reproduced from Gelzinis, A.; Augulis, R.; Butkus, V.; Robert, B.; Valkunas, L. Two-Dimensional Spectroscopy for Non-Specialists. Biochim. Biophys. Acta, Bioenerg. 2019, 1860 (4), 271–285. doi:10.1016/j.bbabio.2018.12.006 with permission. Copyright 2019, Elsevier.

The transient 2D-IR studies discussed so far were performed in the frequency domain, by scanning the central frequency of a narrowband IR pump pulse. An alternative implementation, time-domain transient 2D-IR spectroscopy, offers the ultimate time resolution attainable in 2D-IR experiments, albeit at the cost of increased acquisition times.242 A summary of all potential pulse sequences for both ground- and excited-state 2D-IR spectroscopy is given in Fig. 79. The orientational dynamics of transient molecules have also been recently studied using T2D-IR,247 in particular during the photodissociation of Mn2(CO)10 to Mn(CO)5. The orientational relaxation was shown to slow down as the photoproduct cools, providing a measure of the transient temperature decay time of 70  16 ps.

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Fig. 51 Schematic representation of the spectral ranges covered by ultrafast multidimensional optical techniques, together with sketches of typical molecules investigated with these methods. Reproduced from Buckup, T.; Léonard, J. Multidimensional Time-Resolved Spectroscopy; Buckup, T., Léonard, J., Eds.; Topics in Current Chemistry Collections; Springer International Publishing, 2018 with permission. Copyright 2019, Springer Nature Switzerland AG.

Fig. 52 Anatomy of a 2D-IR spectrum of an isolated oscillator. On the left, the 2D-IR spectrum, with the different contributions (GSB, SE, ESA) being detailed in the vibrational potential energy surface on the right. The difference between u01 and u12 is called the vibrational anharmonicity, D.

Transient 2D-IR has also been applied to study ultrafast charge injection from a Re(I) carbonyl complex into a TiO2 semiconductor surface.248 By initiating electron transfer in the middle of the infrared pulse train (i.e., using the “EXSY” or labeling pulse sequence), the authors were able to assign the excited state features by correlating them to the ground state vibrational modes and determine that the three conformations of this molecule on the surface have different time scales and cross sections for electron injection (Fig. 80). Transient 2D-IR combines the advantages of 2D-IR spectroscopy (i.e., structural sensitivity, elucidation of broadening mechanisms) and those of TRIR spectroscopy (i.e., formation and detection of excited states and photoproducts), yet is important to highlight that T2D-IR is a fifth order methoddhence the excitation densities of both the UV–IR and IR–IR experiments need to be very high in order to obtain a significant response. While typical TRIR or IR–IR pump–probe signals are in the order of 1–50 mOD, the typical signal sizes for T2D-IR are in the order of 0.05–0.5 mOD, making the acquisition of T2D-IR spectra a significant challenge.

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Fig. 53 2D-IR spectra of VC-I2 collected at Tw ¼ 0.3 (top row) and 30 ps (bottom row) for (a,b) neat CHCl3, (c,d) 0.5 mol fraction CHCl3 in d6benzene, (e,f) neat d6-benzene, (g,h) 0.5 mol fraction benzyl alcohol (BA) in d6-benzene, and (i,j) neat BA. Reproduced from Jones, B. H.; Massari, A. M. Origins of Spectral Broadening in Iodated Vaska’s Complex in Binary Solvent Mixtures. J. Phys. Chem. B 2013, 117 (49), 15741–15749. doi:10.1021/jp4064627 with permission. Copyright 2013, American Chemical Society.

Fig. 54 (A) Normal modes of VC-H2 with their corresponding scaled displacement vectors (phosphine ligands simplified for clarity). (B) Normalized absorption spectra of VC (black), VC-H2 (blue), and VC-D2 (red) in CHCl3. The symbols * and ⋄ in the spectra refer to traces of VC and VC-O2, respectively. In the spectrum of VC-D2, the additional, very weak bands due to partially deuterated products or traces of VC-H2 are indicated with y. (C) Absorption and (D) 2D-IR spectrum of VC-H2 in CHCl3 in the 1960–2280 cm 1 spectral region (for D, t2 ¼ 3 ps). The dashed circles emphasize the missing cross-peaks between n3 and the other modes. Reproduced from Fernández-Terán, R.; Ruf, J.; Hamm, P. Vibrational Couplings in Hydridocarbonyl Complexes: A 2D-IR Perspective. Inorg. Chem. 2020, 59 (11), 7721–7726. doi:10.1021/acs.inorgchem.0c00750 with permission. Copyright 2020, American Chemical Society.

These technical challenges can be mitigated with the use of higher repetition rate sources,155,156,249 allowing future explorations of more molecular systems, in particular of photoactive transition metal complexes.

8.13.6.2

UV pump–IR pump–IR probe spectroscopy: IR control of electron transfer

Charge transfer modulation by vibrational excitation was originally reported for an organic donor–bridge–acceptor system.250 The charge-separated (CS) state yield was lowered by high-frequency bridge mode excitation. The effect was linked to a dynamic modulation of the donor–acceptor coupling interaction by weakening of H-bonding and/or by disruption of the bridging base-pair planarity.

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Fig. 55 Ground state 2DIR spectra of ReCOe in CD2Cl2. (A, B) 2D maps at 1 and 10 ps IR pump–IR probe delay, respectively. Colors correspond to 25 mOD (blue) and þ25 mOD (red). (C) Pump–probe 1D spectrum at representative delay times after exciting a0 (1) (ca. 2025 cm 1). (D) Closeup of the a(es) probe region at early time delays following a0 (1) excitation. (E) Pump–probe 1D spectrum at representative delay times after exciting a(es) (ca. 1730 cm 1). (F) Normalized spectra of the a(es) response (probe) to the excitation (pump) of several different vibrations. The a(es) diagonal peak pair clearly changes shape between 1 and 5 ps, indicating a shift from diagonal to off-diagonal character. Reproduced from Delor, M.; Sazanovich, I. V.; Towrie, M.; Spall, S. J.; Keane, T.; Blake, A. J.; Wilson, C.; Meijer, A. J. H. M.; Weinstein, J. A. Dynamics of Ground and Excited State Vibrational Relaxation and Energy Transfer in Transition Metal Carbonyls. J. Phys. Chem. B 2014, 118 (40), 11781–11791. doi:10.1021/ jp506326u with permission. Copyright 2014, American Chemical Society.

Fig. 56 2D-IR spectroscopy reveals different energy transfer processes in Pt(II) polyyne complexes. Reproduced from Leong, T. X.; Collins, B. K.; Dey Baksi, S.; Mackin, R. T.; Sribnyi, A.; Burin, A. L.; Gladysz, J. A.; Rubtsov, I. V. Tracking Energy Transfer across a Platinum Center. Chem. A Eur. J. 2022. doi:10.1021/acs.jpca.2c02017 with permission. Copyright 2022, American Chemical Society.

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Fig. 57 2D-IR vibrational echo spectra of the CO stretch of W(CO)6 in l ¼ 24 lamellae. The top panel is at short time (t2 ¼ 4 ps). It is substantially elongated along the diagonal (black dashed line). The bottom panel is at long time (t2 ¼ 95 ps). The shape of the spectrum has changed substantially, and it has little elongation along the diagonal showing spectral diffusion is almost complete. Reproduced from Kumar, S. K. K.; Tamimi, A.; Fayer, M. D. Dynamics in the Interior of AOT Lamellae Investigated with Two-Dimensional Infrared Spectroscopy. J. Am. Chem. Soc. 2013, 135 (13), 5118–5126. doi:10.1021/ja312676e with permission. Copyright 2013, American Chemical Society.

Fig. 58 Ultrafast dynamics in the interior of planar aligned multibilayers of 1,2-dilauroyl-sn-glycero-3-phosphocholine (dilauroylphosphatidylcholine, DLPC), investigated using 2D-IR vibrational echo spectroscopy. Reproduced from Kel, O.; Tamimi, A.; Thielges, M. C.; Fayer, M. D. Ultrafast Structural Dynamics inside Planar Phospholipid Multibilayer Model Cell Membranes Measured with 2D IR Spectroscopy. J. Am. Chem. Soc. 2013, 135 (30), 11063–11074. doi:10.1021/ja403675x with permission. Copyright 2014, American Chemical Society.

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Fig. 59 2D-IR spectrum for W(CO)6 in [EMIM][FAP] at different waiting times: 0.5 ps, 3 ps, 10 ps. 20 ps, 40 ps, and 100 ps. Reproduced from Mora, A. K.; Singh, P. K.; Nath, S. Dynamics in Tris(Pentafluoroethyl)Trifluorophosphate (FAP) Anion Based Ionic Liquids: A 2D-IR Study with Tungsten Hexacarbonyl. J. Mol. Liq. 2022, 358, 119189. doi:10.1016/j.molliq.2022.119189 with permission. Copyright 2022, Elsevier.

Fig. 60 The normalized FFCF for each solvent is shown, offset by 0.4 and showing every fourth point for clarity. BCT spectral diffusion is independent of solvent viscosity among the tested set of alkanes. Reproduced from Nilsen, I. A.; Osborne, D. G.; White, A. M.; Anna, J. M.; Kubarych, K. J. Monitoring Equilibrium Reaction Dynamics of a Nearly Barrierless Molecular Rotor Using Ultrafast Vibrational Echoes. J. Chem. Phys. 2014, 141 (13), 134313. doi:10.1063/1.4896536 with permission. Copyright 2014, American Institute of Physics.

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Fig. 61 The motion of the azido ligand in a series of Cu(II) complexes with differing steric bulk was studied using 2D-IR spectroscopy and IR pump–probe anisotropy. Reproduced from Weng, W.; Weberg, A. B.; Gera, R.; Tomson, N. C.; Anna, J. M. Probing Ligand Effects on the Ultrafast Dynamics of Copper Complexes via Midinfrared Pump-Probe and 2DIR Spectroscopies. J. Phys. Chem. B 2021, 125 (44), 12228–12241. doi:10.1021/ acs.jpcb.1c06370 with permission. Copyright 2021, American Chemical Society.

Fig. 62 FTIR and 2D-IR spectra of an [FeFe]hydrogenase active site model complex in heptane (a), acetonitrile (b) and 1,7-heptanediol (c). 2D-IR spectra were recorded with a pump–probe delay time of 2 ps. All spectra are plotted using a color scheme ranging from red (negative values) to blue (positive values). Reproduced from Bonner, G. M.; Ridley, A. R.; Ibrahim, S. K.; Pickett, C. J.; Hunt, N. T. Probing the Effect of the Solution Environment on the Vibrational Dynamics of an Enzyme Model System with Ultrafast 2D-IR Spectroscopy. Faraday Discuss. 2010, 145, 429–442. doi:10.1039/b906163k with permission. Copyright 2009, Royal Society of Chemistry.

An “IR control” experiment consists of a pulse sequence very similar to that of frequency-domain transient 2D-IR spectroscopy. Rather than measuring the excited-state 2D-IR contribution, however, the aim is to change the dynamics of the excited statedpotentially altering the deactivation pathways of the complex and/or directing electron transfer along one branch. The concept of vibrational control of electron transfer with mode-specific excitation was realized in transition metal complexes, in particular, to Pt(II) donor–bridge–acceptor complexes with alkyne bridges.170–176 Using an ultrafast electronic-vibrationalvibrational pulse-sequence, it was demonstrate how the outcome of light-induced ET can be radically altered by modespecific infrared (IR) excitation of vibrations that are coupled to the electron transfer pathway. Picosecond narrow-band IR excitation of high-frequency bridge vibrations in an electronically excited covalent trans-acetylide platinum(II) donor-bridgeacceptor system in solution alters both the dynamics and the yields of competing ET pathways, completely, 100%, switching a charge separation pathway off (Fig. 81).

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Fig. 63 Absolute-value non-rephasing spectra of [CpRu(CO)2]2 in n-hexane at t2 ¼ 150 fs and 10 ps. Spectra are normalized to the maximum peak at the given t2 value, and 30 contours are plotted for each spectrum. Reproduced from Anna, J. M.; King, J. T.; Kubarych, K. J. Multiple Structures and Dynamics of [CpRu(CO)2]2 and [CpFe(CO)2]2 in Solution Revealed with Two-Dimensional Infrared Spectroscopy. Inorg. Chem. 2011, 50 (19), 9273–9283. doi:10.1021/ic200466b with permission. Copyright 2011, American Chemical Society.

Fig. 64 2D-IR spectroscopy reveals the timescales for the forward and backward isomerization processes, illustrated by the growth of cross peaks between different vibrational modes. Reproduced from Anna, J. M.; Ross, M. R.; Kubarych, K. J. Dissecting Enthalpic and Entropic Barriers to Ultrafast Equilibrium Isomerization of a Flexible Molecule Using 2DIR Chemical Exchange Spectroscopy. J. Phys. Chem. A 2009, 113 (24), 6544–6547. doi:10.1021/jp903112c with permission. Copyright 2009, American Chemical Society.

Fig. 65 2D-IR experiment in a pseudo-ATR geometry, showing the vibrational energy transfer between two isotopologues of a Re(I) tricarbonyl CO2 reduction catalyst and photosensitizer. Reproduced from Fernández-Terán, R.; Hamm, P. A Closer Look Into the Distance Dependence of Vibrational Energy Transfer on Surfaces Using 2D IR Spectroscopy. J. Chem. Phys. 2020, 153 (15), 154706. doi:10.1063/5.0025787 with permission. Copyright 2020, American Institute of Physics.

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Fig. 66 Dyes used to study aggregation (upper panel) and the different possible binding motifs (lower panel) upon immobilization on TiO2. Reproduced from Laaser, J. E.; Christianson, J. R.; Oudenhoven, T. A.; Joo, Y.; Gopalan, P.; Schmidt, J. R.; Zanni, M. T. Dye Self-Association Identified by Intermolecular Couplings between Vibrational Modes as Revealed by Infrared Spectroscopy, and Implications for Electron Injection. J. Phys. Chem. C 2014, 118 (11), 5854–5861. doi:10.1021/jp412402v with permission. Copyright 2014, American Chemical Society.

Fig. 67 Waiting time 2D-IR spectra for ReC and ReCC in an amorphous deposit on sapphire substrate. (a) and (d) at waiting time ¼ 0 ps, (b) and (e) at waiting time ¼ 1 ps, and (c) and (f) at waiting time ¼ 3.5 ps for ReC and ReCC. A dashed circle is given in each spectrum to help identify where cross peaks should appear if the different vibrational modes are coupled. Reproduced from Oudenhoven, T. A.; Joo, Y.; Laaser, J. E.; Gopalan, P.; Zanni, M. T. Dye Aggregation Identified by Vibrational Coupling Using 2D IR Spectroscopy. J. Chem. Phys. 2015, 142 (21), 212449. doi:10.1063/ 1.4921649 with permission. Copyright 2015, American Institute of Physics.

The “IR-control” strategy has also been applied to Re(I) donor–acceptor complexes.251 While no significant rate modulation was found when the cyano-group stretching mode of the 3-dimethylaminobenzonitrile (3DMABN) donor was excited, a sizable effect was found when the ring stretching mode of the 4,40 -dicarboxyethyl-2,20 -bipyridine (DCEB) acceptor was excited. The accumulation of the charge separated state (LLCT state) in the 3-pulse experiment was observed as a sharp excited-state vibrational peak of the symmetric stretch of the three facial carbonyl groups, nSS(CO). Modeling indicated that the rate of charge separation was increased

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Fig. 68 Waiting time 2D-IR spectra for ReC and ReCC adsorbed on TiO2 nanoparticle thin films for both full coverage and submonolayer coverage samples. (a), (d), (g), and (j) at waiting time ¼ 0 ps; (b), (e), (h), and (k) at waiting time ¼ 1 ps; and (c), (f), (i), and (l) at waiting time ¼ 3.5 ps for ReC and ReCC. Intensity changes within the dashed circles show cross peak growth. Reproduced from Oudenhoven, T. A.; Joo, Y.; Laaser, J. E.; Gopalan, P.; Zanni, M. T. Dye Aggregation Identified by Vibrational Coupling Using 2D IR Spectroscopy. J. Chem. Phys. 2015, 142 (21), 212449. doi:10.1063/1.4921649 with permission. Copyright 2015, American Institute of Physics.

by ca. 28% when vibrational excitation was present. The vibronic coupling signal of the bpy ring mode and nSS(CO) as well as the energy transport dynamics from bpy to carbonyl contributed to the 3-pulse signal and were studied as well using the 3-pulse method. Energy transport between the same modes in the ground electronic state was measured by relaxation-assisted twodimensional infrared (RA 2D-IR) spectroscopy. The energy transport times of 4  0.7 and 5  1.5 ps were found for the ground and excited electronic states (Fig. 82). In order to direct electron transfer along a preselected pathwaydthe goal inspired by multiple pathways in photosynthesisda model system has to be designed, whereby two electron transfer pathways are electronically identical, but vibrationally distinct. The first example of such explicit vibrational control was achieved through judicious design of a Pt(II)-acetylide charge-transfer donor– bridge–acceptor–bridge–donor “fork” system: asymmetric isotopic labeling of one of the two eC^Ce bridges with 13C makes the two parallel and otherwise identical donor / acceptor electron-transfer pathways vibrationally distinct (Fig. 83),252 enabling independent vibrational perturbation of either (Fig. 84).173 This work demonstrated that applying an ultrafast UVpump(excitation)  IRpump(perturbation)  IRprobe(monitoring) pulse sequence, we show that the pathway that is vibrationally perturbed during UV-induced electron transfer is dramatically slowed down compared to its unperturbed counterpart. One can thus choose the dominant electron transfer pathway. Further developments of the IR-control control in solution are currently focusing on both the investigations of the mechanism of the IR-control effect,176 and its application to the Pt(II) donor-acceptor molecules with various coupling modes and energetics of

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Fig. 69 Intermolecular vibrational energy transfer effectively increases the dimensionality of 2D-IR spectroscopy, revealing non-Gaussian dynamics in the solvation of a Re(I) tricarbonyl complex and NaSCN. Reproduced from Kiefer, L. M.; Kubarych, K. J. NOESY-Like 2D-IR Spectroscopy Reveals Non-Gaussian Dynamics. J. Phys. Chem. Lett. 2016, 7 (19), 3819–3824. doi:10.1021/acs.jpclett.6b01803 with permission. Copyright 2016, American Chemical Society.

Fig. 70 Structural models of the light-harvesting pigment protein complexes from green plants where (a) shows a model of the Photosystem II supercomplex, (b) a structural model from X-ray crystallography of LHCII, and (c) of the minor complex, CP29. (d) The linear absorption spectra of LHCII and CP29 at 77 K. Reproduced from Schlau-Cohen, G. S.; Ishizaki, A.; Fleming, G. R. Two-Dimensional Electronic Spectroscopy and Photosynthesis: Fundamentals and Applications to Photosynthetic Light-Harvesting. Chem. Phys. 2011, 386 (1–3), 1–22. doi:10.1016/ j.chemphys.2011.04.025 with permission. Copyright 2011, Elsevier.

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Fig. 71 A model system consisting of three identical chromophores in a protein scaffold (a) along with the corresponding linear spectrum (b) and energy level diagram (c). Schematic two-dimensional spectra at t2 ¼ 0 fs (d) and a later waiting time (e). At early waiting times, cross-peaks indicate that the corresponding diagonal peaks are electronic transitions involving common electronic orbitals. At later waiting times, the appearance of crosspeaks indicates energy transfer between the different electronic transitions. Reproduced from Anna, J. M.; Scholes, G. D.; van Grondelle, R. A Little Coherence in Photosynthetic Light Harvesting. Bioscience 2014, 64 (1), 14–25. doi:10.1093/biosci/bit002 with permission. Copyright 2013, Oxford University Press.

Fig. 72 Monitoring the waiting-time-dependent changes of the line shape of the inhomogeneously broadened Qy(0–0) transition in the trimeric photosystem I of the cyanobacterium Thermosynechococcus elongatus, downhill energy equilibration on the 50 fs time scale was observed. Reproduced from Anna, J. M.; Ostroumov, E. E.; Maghlaoui, K.; Barber, J.; Scholes, G. D. Two-Dimensional Electronic Spectroscopy Reveals Ultrafast Downhill Energy Transfer in Photosystem i Trimers of the Cyanobacterium Thermosynechococcus Elongatus. J. Phys. Chem. Lett. 2012, 3 (24), 3677– 3684. doi:10.1021/jz3018013 with permission. Copyright 2012, American Chemical Society.

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Fig. 73 2DEV spectra of LHCII at t2 ¼ 0 ps (a), 1 ps (b), 5 ps (c), and 15 ps (d). Slices through these spectra at u1 ¼ 14,800 and 15,400 cm 1, the positions marked by vertical dashed lines on (a–d), are shown in (e) and (f) normalized to their absolute maxima at t2 ¼ 0, where the dynamics have been normalized by the fit to the first right singular vector V1 to facilitate visualization of the excited state dynamics, with the overall relaxation removed. Reproduced from Lewis, N. H. C.; Gruenke, N. L.; Oliver, T. A. A.; Ballottari, M.; Bassi, R.; Fleming, G. R. Observation of Electronic Excitation Transfer Through Light Harvesting Complex II Using Two-Dimensional Electronic-Vibrational Spectroscopy. J. Phys. Chem. Lett. 2016, 7 (20), 4197–4206. doi:10.1021/acs.jpclett.6b02280 with permission. Copyright 2016, American Chemical Society.

individual steps,170–173 in order to formulate the design criteria for IR-control of electron transfer in donor–bridge–acceptor systems. IR control in other systems and of complementary reactionsdsuch as proton-coupled electron transferdis an ongoing field of research, and we believe that both experimental and synthetic advances will allow for this to become a reality.

8.13.7

Concluding remarks

The ever faster, reaching into attosecond regime, and ever more broadband and intense laser sources, covering the ranges from THz to hard X-rays, are now used to interrogate excited state dynamics and reaction pathways in molecules and materials, and transition metal complexes are no exception.

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Fig. 74 Vibrational-electronic (VE) spectra of [FeIII(CN)6]3 in formamide (FA) ((a)-(e)) and neat FA ((f)-(j)). Top panels: IR pump pulse spectra (solid gray lines) and FTIR spectra of both FA (dashed black lines) and solvent-subtracted [FeIII(CN)6]3 (solid black lines). Left panels: UV/visible probe pulse spectra (solid gray lines) and linear electronic spectrum of [FeIII(CN)6]3 in FA (solid black line). All linear (pulse) spectra are plotted in black (gray) to correspond with absorbance (intensity, Int.) axes. ((a)–(d)) 2D VE spectra of [FeIII(CN)6]3 for a series of waiting times show bleaches (red) at the cyanide stretching mode frequency. ((f)–(i)) Electronically non-resonant solvent signals in 2D VE spectra of neat FA underlie [FeIII(CN)6]3 2D VE signals, most notably in (f). Panels ((a)–(d) and (f)–(i)) are normalized to panel (a). 1D vibrational-electronic (VE) spectra of [FeIII(CN)6]3 in FA (e) and neat FA (j). Panel (j) spectrum is normalized to (e); 2D VE spectra are plotted using contour lines at the following positions: 0.05, 0.1, 0.2, 0.3, 0.4,  0.5, 0.6, 0.7, 0.8, 0.9, and 1.0; 1D VE spectra have an additional 0.025 contour line. Reproduced from Zhao, W.; Wright, J. C. Spectral Simplification in Vibrational Spectroscopy Using Doubly Vibrationally Enhanced Infrared Four Wave Mixing. J. Am. Chem. Soc. 1999, 121 (47), 10994–10998. doi:10.1021/ja9926414 with permission. Copyright 2015, American Institute of Physics.

Fig. 75 2D vibrational-electronic (2D VE) spectra of [(CN)5FeIICNRuIII(NH3)5] (FeRu) dissolved in formamide (FA). Top panels: IR pump pulse spectra (solid gray lines) and FTIR spectra of both FA (dashed black lines) and solvent-subtracted FeRu (solid black lines). Cyanide stretching modes are indicated by dashed vertical lines as follows: ntrans (2002.0 cm 1, blue), nradial (2050.6 cm 1, red), naxial (2065 cm 1, green), and nbridge (2089.4 cm 1, purple). Left panel: near-IR probe pulse spectrum (solid gray line) and linear electronic spectrum of FeRu in FA (solid black line). All linear (pulse) spectra are plotted in black (gray) to correspond with absorbance (intensity, Int.) axes. ((a) and (b)) Early waiting time 2D VE spectra of FeRu are dominated by vibrational mode-specific absorptions (blue) at ntrans and nbridge as well as naxial modes, which lie on the charge-transfer axis of the molecule. ((c)–(e)) The sign of the nbridge mode signal oscillates, negative signal emerges at the perpendicular nradial mode, and a broad positive signal develops at the ntrans mode. Panels ((a)–(e)) are normalized to (a). Each spectrum is plotted using contour lines at the following positions: 0.025, 0.05, 0.1,  0.2, 0.35, 0.5, 0.65, 0.8,  0.95, and 1.0. Reproduced from Zhao, W.; Wright, J. C. Spectral Simplification in Vibrational Spectroscopy Using Doubly Vibrationally Enhanced Infrared Four Wave Mixing. J. Am. Chem. Soc. 1999, 121 (47), 10994–10998. doi:10.1021/ja9926414 with permission. Copyright 2015, American Institute of Physics.

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Fig. 76 (a) Time-resolved IR spectrum after 20 ps. (b) T2D-IR spectrum. (c) Pulse sequence for normal T2D-IR. First, the UV-pump pulse arrives, then a 2D-IR measurement is carried out. (d) Time-resolved IR spectrum after 2 ps. (e) Labeling T2D-IR spectrum. (f) Pulse sequence for labeling T2D-IR. The narrowband IR-pump pulse arrives first now and labels the electronic ground state. The UV-pump pulse transfers the molecule into the excited state. The IR-probe pulse arrives last. Reproduced from Bredenbeck, J.; Helbing, J.; Hamm, P. Labeling Vibrations by Light: Ultrafast Transient 2D-IR Spectroscopy Tracks Vibrational Modes during Photoinduced Charge Transfer. J. Am. Chem. Soc. 2004, 126 (4), 990–991. doi:10.1021/ja0380190 with permission. Copyright 2004, American Chemical Society.

Fig. 77 Normalized kinetic traces extracted from bleach kinetics of 2DIR and T2D-IR data for ReCOe in CD2Cl2. (A) Ground state a0 (1) diagonal peak and a(es) cross peak. (B) Ground state a(es) diagonal peak and a0 (1) cross peak. Comparison of a0 (1) (C) and a(es) (D) diagonal peak lifetimes in the ground (blue) and 3MLCT (black) states. Solid lines represent best fits deconvoluted from a Gaussian instrument response function. (44) Extracted lifetimes are represented schematically (E) for the ground state and (F) for the 3MLCT state. Note that VET from a0 (1) to a(es) in 3MLCT was not satisfactorily resolved due to the low signal-to-noise of the a(es) cross-peak in T2D-IR experiments. Reproduced from Delor, M.; Sazanovich, I. V.; Towrie, M.; Weinstein, J. A. Probing and Exploiting the Interplay between Nuclear and Electronic Motion in Charge Transfer Processes. Acc. Chem. Res. 2015, 48 (4), 1131–1139. doi:10.1021/ar500420c with permission. Copyright 2015, American Chemical Society.

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Fig. 78 Time profiles of 1 plotted as integrated peak intensity of n(CN) diagonal peak in the ground state and in the excited state in DCM. The profiles were fitted with the convolution of Gaussian (IRF  1.5 ps) and biexponential function. Reproduced from Fedoseeva, M.; Delor, M.; Parker, S. C.; Sazanovich, I. V.; Towrie, M.; Parker, A. W.; Weinstein, J. A. Vibrational Energy Transfer Dynamics in Ruthenium Polypyridine Transition Metal Complexes. Phys. Chem. Chem. Phys. 2015, 17 (3), 1688–1696. doi:10.1039/c4cp04166f with permission. Copyright 2014, Royal Society of Chemistry.

Fig. 79 Schematic diagram of the pulse sequences employed in 2D-IR and T2D-IR spectroscopy. Red pulses numbered 1–3 indicate the three time-ordered interactions between IR laser pulse and sample necessary to generate the 2D-IR signal (Sig). The blue pulse marked “UV” indicates the UV or visible excitation pulse. Asterisks indicate the use of tunable narrow bandwidth pulses. In the pseudo-collinear pump–probe geometry, t ¼ 0 since the probe acts as a third pulse and local oscillator. Reproduced from Hunt, N. T. Transient 2D-IR Spectroscopy of Inorganic Excited States. Dalton Trans. 2014, 43 (47), 17578–17589. doi:10.1039/c4dt01410c with permission. Copyright 2014, Royal Society of Chemistry.

Fig. 80 Transient 2D-IR spectroscopy was used to study ultrafast charge injection onto TiO2. Reproduced from Xiong, W.; Laaser, J. E.; Paoprasert, P.; Franking, R. A.; Hamers, R. J.; Gopalan, P.; Zanni, M. T. Transient 2D IR Spectroscopy of Charge Injection at Organic-Inorganic Interfaces; Vol. 131 Optics InfoBase Conference Papers, 2010, (50), 18040–18041. doi:10.1364/up.2010.fa4 with permission. Copyright 2010, Optical Society of America.

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Fig. 81 Results of the IR-modulation experiments. The top panel shows the TRIR at representative delay times. The bottom panel shows DAbs(IRpumpON  IRpumpOFF) 200 ps after UV excitation (198 ps after IR excitation) that demonstrate 100% suppression of the CSS and an increase of the 3NAP pathway by IR excitation. Right panelda simplified Jablonski diagram of the DBA system shown. Reproduced from Delor, M.; Scattergood, P. A.; Sazanovich, I. V.; Parker, A. W.; Greetham, G. M.; Meijer, A. J. H. M.; Towrie, M.; Weinstein, J. A. Toward Control of Electron Transfer in Donor-Acceptor Molecules by Bond-Specific Infrared Excitation. Science (1979) 2014, 346 (6216), 1492–1495. doi:10.1126/ science.1259995 with permission. Copyright 2014, American Association for the Advancement of Science.

Fig. 82 (A) The 3-pulse t2 kinetics measured at 2023 cm 1 at two t1 delays: 2 ps (blue) and 40 ps (red). A normalization factor of 1.2 was applied to the kinetics curve at t1 ¼ 40 ps to match the values at t2 close to zero with those of the kinetics at t1 ¼ 2 ps. (B) The difference (green) between the two kinetics shown in panel A. The red curve represents the modeling, with the equation S(t2) ¼ A exp.(k1t1)  QY[exp(k1t2)  exp.(k2t2)] where QY denotes the vibrational excitation probability (MLCT*/MLCT), evaluated to be 3% and A is the maximum of the LLCT peak at 2023 cm 1 formed under the actual conditions of both 2-pulse and 3-pulse experiments. The k1 rate constant of (10 ps) 1 was used in the modeling. The k2 value of (7.8 ps) 1 was obtained from the fit. Reproduced from Yue, Y.; Grusenmeyer, T.; Ma, Z.; Zhang, P.; Schmehl, R. H.; Beratan, D. N.; Rubtsov, I. V. Electron Transfer Rate Modulation in a Compact Re(i) Donor-Acceptor Complex. Dalton Trans. 2015, 44 (18), 8609–8616. doi:10.1039/ c4dt02145b with permission. Copyright 2015, Royal Society of Chemistry.

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Fig. 83 Structure of the “molecular fork” which has two electronically identical, but vibrationally distinct Donor-Bridge-Acceptor pathways. Isotopic substitution changes the frequency of n(CC) from 2104 cm-1 in n(12C12C) to 2016 cm-1 for n(13C13C). This difference allows the two vibrations to be excited independently by a ca. 12–20 cm 1 FWHM picosecond IR-pump pulse.173,252 Reproduced from Delor, M.; Archer, S. A.; Keane, T.; Meijer, A. J. H. M.; Sazanovich, I. V.; Greetham, G. M.; Towrie, M.; Weinstein, J. A. Directing the Path of Light-Induced Electron Transfer at a Molecular Fork Using Vibrational Excitation. Nat. Chem. 2017, 9 (11), 1099–1104. doi:10.1038/NCHEM.2793 with permission. Copyright 2017, Springer Nature AG.

Fig. 84 a, FTIR of the molecular fork complex. b, TRIR spectra at 2 and 50 ps time delay following 400 nm excitation, displaying the spectral profiles of n(12C) and n(13C) in 3MLCT* and CSS, respectively. The apparent difference in shape of the 12CSS vs. 13CSS TRIR signals is due to a larger bleach/transient overlap for the latter. c, IR-control experiments: the IRpump is introduced 2 ps after the UVpump, and the spectra shown are recorded with the IRprobe at 1 ps (black), 3 ps (green) and 50 ps (blue) after the IRpump. The bleach–transient peak pairs are signatures of depopulation of the v ¼ 0 and population of the v ¼ 1 state of the excited mode. The off-diagonal signals arising at frequencies not excited by the IRpump are due to a change in electronic dynamics, specifically the rate of formation of the 12CSS and 13CSS (see text for details). The presence and sign of the off-diagonal signals suggests that vibrational excitation of one pathway accelerates electron transfer through the opposite pathway. Reproduced from Delor, M.; Archer, S. A.; Keane, T.; Meijer, A. J. H. M.; Sazanovich, I. V.; Greetham, G. M.; Towrie, M.; Weinstein, J. A. Directing the Path of Light-Induced Electron Transfer at a Molecular Fork Using Vibrational Excitation. Nat. Chem. 2017, 9 (11), 1099–1104. doi:10.1038/ NCHEM.2793 with permission. Copyright 2017, Springer Nature AG.

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This Chapter touches upon the diverse transition metal complexes that are being developed for applications in all lightassociated processes, but also to gain more fundamental understandingdat ever faster time scales, and with atomic resolutiondon how chemistry happens.

We believe the future is bright for transition metal complexes. The future directions in method development are likely to be in combining multiple detection techniques for simultaneous interrogation of the sampledexamples include optical and X-ray methods, as well as multi-pulse methods, where pulse sequences are used to perturb excited state reaction or transiently change a property of the material studies, while several methods are probing the changes induced. The philosophy of multiple detection is being realized at this very moment at major facilities around the world, e.g., Diamond Plc, CLF, EU XFEL, LCLS, SACLA, Swiss FEL, and many others, and are driving new developments in light sources, detection methods and theoretical and computational advances. All such developments bring us one step close to the real-time molecular movie, where the researchers can observe and direct chemical change.

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Luminescent supramolecular assemblies

Vonika Ka-Man Aua, Michael Ho-Yeung Chanb, and Vivian Wing-Wah Yamb, a Department of Science and Environmental Studies, The Education University of Hong Kong, Hong Kong, P R China; and b Institute of Molecular Functional Materials and Department of Chemistry, The University of Hong Kong, Hong Kong, P R China © 2023 Elsevier Ltd. All rights reserved.

8.14.1 8.14.2 8.14.2.1 8.14.2.1.1 8.14.2.1.2 8.14.2.1.3 8.14.2.1.4 8.14.2.2 8.14.2.3 8.14.2.4 8.14.3 8.14.3.1 8.14.3.1.1 8.14.3.1.2 8.14.3.2 8.14.3.2.1 8.14.3.2.2 8.14.3.3 8.14.3.3.1 8.14.3.3.2 8.14.4 Acknowledgments References

Introduction Luminescent supramolecular assemblies of d8 metal complexes Platinum(II) Platinum(II) complexes with cyanide and/or isocyanide ligands and platinum(II) double salts Platinum(II) complexes with chelating N-donor ligands Platinum(II) complexes with chelating cyclometalating ligands Discrete multinuclear platinum(II) complexes Palladium(II) Rhodium(I) Gold(III) Luminescent supramolecular assemblies of d10 metal complexes Copper(I) Copper(I) clusters Copper(I) metallacycles Silver(I) Silver(I) clusters Silver(I) metallacycles Gold(I) Low-nuclearity gold(I) complexes Gold(I) clusters Conclusion

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Abstract There has been a continuous interest in the study of transition metal complexes. In particular, the variation in coordination modes and geometries of d8 and d10 metal centers have allowed the formation of supramolecular assemblies with different structures and topologies through the interplay of coordination bonds, non-covalent metal
metal interactions and other intra-/intermolecular interactions, which is in sharp contrast to octahedral d6 metal complex systems. Not only are metal complexes of d8 and d10 transition metal centers luminescent, many of their supramolecular assemblies are luminescent in nature due to the availability of different excited states; some of which could be modified by metal–metal interactions. The present chapter will give an overview on the luminescent supramolecular assemblies based on d8 metal centers including platinum(II), palladium(II), rhodium(I) and gold(III), and d10 metal centers including gold(I), silver(I) and copper(I), that were reported after 2013. The discussion will begin with luminescent supramolecular assemblies based on platinum group metals with d8 electronic configuration, followed by an overview of the luminescent assemblies based on d10 coinage metal centers.

8.14.1

Introduction

There has been a continuous interest in the study of transition metal complexes for over a century after Werner’s work on the linkage of atoms in molecules. In particular, the variation in coordination modes and geometries of d8 and d10 metal centers have allowed the formation of supramolecular assemblies with different structures and topologies through the interplay of coordination bonds, non-covalent metal
metal interactions and other intra-/intermolecular interactions, which is in sharp contrast to octahedral d6 metal complex systems. Not only are metal complexes of d8 and d10 transition metal centers luminescent, many of their supramolecular assemblies are luminescent in nature due to the availability of different excited states; some of which could be modified by metal–metal interactions. As an extension of our chapter on non-covalent metal–metal interactions in Comprehensive Inorganic Chemistry II,1 the present chapter will give an overview on the luminescent supramolecular assemblies based on d8 and d10 metal centers that were reported after 2013. The discussion will begin with luminescent supramolecular assemblies based on platinum

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group metals with d8 electronic configuration, followed by an overview of the luminescent assemblies based on d10 coinage metal centers. Due to the limitation of the scope, only homometallic systems will be discussed here and readers are encouraged to refer to other review articles2,3 for related works on heterometallic systems.

8.14.2

Luminescent supramolecular assemblies of d8 metal complexes

8.14.2.1

Platinum(II)

Square-planar platinum(II) complexes have been known to show a strong propensity in exhibiting non-covalent metal–metal interactions for decades. Discrete mononuclear, dinuclear and metallacyclic complexes have been found to show intriguing physical, optical, photophysical, conductivity and assembly properties, attributed to the unique non-covalent Pt(II)/Pt(II) interactions. They represent a unique and important class of transition-metal coordination compounds.

8.14.2.1.1

Platinum(II) complexes with cyanide and/or isocyanide ligands and platinum(II) double salts

The [Pt(CN)4]2
system represents an important class of platinum(II) complexes that is capable of the formation of onedimensional linear stacks in the solid state.4–6 The photophysical properties of this system were found to be attributed to the alteration of the Pt(II)/Pt(II) separations by counter-cations and the number of hydration.4–6 Early demonstration showed that partial oxidation could be brought about by various cations, which can alter the Pt(II)/Pt(II) separation within the linear chains, leading to a reduction of the Pt(II)/Pt(II) separation from 3.1 to 2.8 Å.7 Recently, Iwamura and Tahara reported the excited-state dynamics of K2[Pt(CN)4] in aqueous solution associated with the non-covalent metal–metal interactions by femtosecond time-resolved absorption and picosecond time-resolved studies (Fig. 1).8 The time-resolved emission studies of a 0.6 M K2[Pt(CN)4] solution showed that a singlet emission of a trimer (S1 trimer) at 407 nm was found to decay with a time constant of 0.1 ns, while a lower-energy singlet emission (S1 tetramer) of a tetramer at 460 nm was found to decay with a time constant of 0.44 ns.8 Upon vanishing of these two emission bands, a weak and broad emission band at 500 nm was observed at  4 ns, where this band was further red-shifted to 530 nm at 8.2 ns. The emission band at 530 nm was found to remain at up to 50 ns, which was attributed to the phosphorescence of the oligomers. The 500-nm emission band was assigned as the phosphorescence from the triplet states of the trimer and tetramer respectively, which were generated from the intersystem crossing (ISC) of S1 trimer and S1 tetramer as these excited states were indistinguishable from the measurement. The 8.2-ns dynamics of the red shift was found to be diffusion-controlled, where oligomerization of the trimers and tetramers upon the collision with the monomers was believed to occur to afford high-order oligomers responsible for the phosphorescence of K2[Pt(CN)4] with lifetime of 0.66 ms.8 In addition, the femtosecond time-resolved absorption studies of 0.6 M K2[Pt(CN)4] solution showed that 88-ps and 0.4-ns dynamics were observed, assignable to the ISC of S1 trimer and S1 tetramer. More importantly, oscillatory components in the first few picoseconds were observed. Such oscillatory components have been analyzed by Fourier transform analysis, from which 135-cm
1oscillation was recognized. Time-dependent density functional theory (TD-DFT) calculation of the S1 trimer showed that a symmetric vibrational mode of a Pt
Pt stretch appeared at 137 cm
1. Thus, the 135-cm
1 oscillation was assigned to the Pt
Pt stretching frequency. It was believed that such a stretching mode was attributed to the bent-to-linear structural change of the S1 trimer upon photoexcitation associated with the strengthening of the Pt(II)/Pt(II) interaction in the excited state.8 Upon the successful preparation of the Magnus’ green salt (MGS), [Pt(NH3)4][PtCl4],9 which was found to exhibit onedimensional infinite chains of Pt(II)/Pt(II) contacts of 3.23–3.25 Å from the alternating stacks of [Pt(NH3)4]2þ cations and [PtCl4]2
anions,9 there has been increasing attention in the investigation of non-covalent Pt(II)/Pt(II) interactions.6,10–13

Fig. 1 Relaxation pathways of the photoexcited oligomers of [Pt(CN)4]2
and the time constants of each process. Reproduced from Iwamura, M.; Fukui, A.; Nozaki, K.; Kuramochi, H.; Takeuchi, S.; Tahara, T. Angew. Chem. Int. Ed. 2020, 59, 23154–23161 © Wiley, 2020.

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However, the exploration of the metal–metal interaction in MGS has been limited to the solid state due to its poor solubility in water and common organic solvents. The group of Caseri prepared a series of soluble derivatives of MGS, [Pt(NH2R)4][PtCl4], and demonstrated their semiconducting behaviors with measurable charge carrier mobilities due to the presence of short Pt(II)/Pt(II) contacts.14–17 Recently, Perevedentsev, Bargardi and Caseri reported the synthesis, processing and characterization of another series of soluble derivatives of MGS, [Pt(NH2R)4][PtCl4] (R ¼ branched alkyl or u-phenylalkyl group).18 The complex salts were found to dissolve in dimethylformamide (DMF) solutions with individual ion pairs as revealed by the small-angle X-ray scattering (SAXS) analysis due to the presence of a zero-slope SAXS pattern, implying the absence of extensive Pt(II) stacks.18 In order to assist the formation of one-dimensional linear stacks of Pt(II) arrays in the thin film, matrix-assisted assembly of the salts has been employed by co-dissolving the salt with poly(ethylene oxide) (PEO) with subsequent film casting, thermal activated assembly and removal of PEO.18 The resulting assembled inorganic polymer in the thin film was found to exhibit bright bluegreen photoluminescence with photoluminescence quantum yield (PLQY) reaching 13%,18 which is significantly higher than that reported for another light-emitting Magnus’ salt-like derivative (anhydrous [Pt(bpy)2][Pt(CN)4]; PLQY ¼ 0.2%).19 In 2014, Autschbach and Schurko unraveled the structure of the previously unknown crystal of Magnus’ pink salt, which is an isomer of the MGS and an elusive product of MGS synthesis.20 Since the isolation of Magnus’ pink salt is challenging, a certain reaction condition is needed in the synthesis without the presence of MGS as a co-product. It was believed that the pink color of the salt was attributed to a longer Pt
Pt distance than that of MGS, and corresponded to a weaker Pt/Pt interaction. Despite the optimized isolated yield of 70% of the Magnus’ pink salt, the salt was found to readily convert to MGS in solution, further hindering the growth of single crystals for X-ray diffraction analysis.20 Thus, the elucidation of the structure of Magnus’ pink salt has been limited to the solid state. They utilized solid-state nuclear magnetic resonance (NMR), X-ray absorption near edge spectroscopy (XANES), extended X-ray absorption fine structure (EXAFS) measurements, synchrontron-based powder X-ray diffraction (PXRD), together with theoretical calculation to illustrate the structure of the Magnus’ pink salt.20 Upon Rietveld refinements of the d

PXRD data, together with theoretical calculation, a crystal structure with P1 symmetry, residing in a pseudotetragonal unit cell was resulted. A Pt
Pt distance of greater than 5.5 Å was observed for adjacent square-planar platinum(II) units. The long Pt
Pt distances and the non-parallel orientation of the complexes hindered the formation of intermolecular Pt/Pt interactions within the Magnus’ pink salt (Fig. 2).20 To further investigate the conductivity of the Magnus-type double salts, Walsh and Sakai reported the synthesis of a series of double salts containing alternating cationic and anionic Pt(II) building blocks of [Pt2(bpy)2(m-pivalamidate)2]2þ and [Pt(ox)2]2
, respectively (Fig. 3).21 The color of the resultant double salt was found to be converted between yellow, orange and blue, attributed

Fig. 2 Structural model of the Magnus’ pink salt, generated from a Rietveld refinement of the PXRD data together with theoretical calculation. Reproduced from Lucier, B. E. G.; Johnston, K. E.; Xu, W.; Hanson, J. C.; Senanayake, S. D.; Yao, S.; Bourassa, M. W.; Srebro, M.; Autschbach, J.; Schurko, R. W. J. Am. Chem. Soc. 2014, 136, 1333–1351 © American Chemical Society, 2013.

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Fig. 3 Preparation of double salts containing alternating cationic and anionic Pt(II) building blocks of [Pt2(bpy)2(m-pivalamidate)2]2 þ and [Pt(ox)2]2
by slow diffusion. Reproduced from Hendon, C. H.; Walsh, A.; Akiyama, N.; Konno, Y.; Kajiwara, T.; Ito, T.; Kitagawa, H.; Sakai, K. Nat. Commun. 2016, 7, 11950 © The Author(s), 2016.

to the change in the extent of intermolecular Pt/Pt interactions between adjacent units. However, the air-oxidized Pt2.33þ needles (1e
partial oxidation per double salt) of these colorful crystals were of particular interest due to their high conductivity and strong absorption. The electronic structure was elucidated by quantum chemical calculations, revealing a metallic state with delocalized electrons at the Fermi level. The needles were found to display a conductivity of 11 S cm
1 at room temperature.21 In 2018, Yam and co-workers reported the synthesis of the first water-soluble double complex salt (DCS), [Pt{bzimpy(TEG)2} Cl][Pt{bzimpy(PrSO3)2}Cl] (bzimpy ¼ 2,6-bis(benzimidazol-20 -yl)pyridine) (Fig. 4).22 The DCS was synthesized by salt metathesis of the respective cationic and anionic precursors. The DCS was found to be soluble in water attributed to the incorporation of the ethylene glycol and sulfonate groups. Upon increasing the water content in the yellow dimethylsulfoxide (DMSO) solution of the DCS, a growth of a low-energy absorption band associated with the formation of Pt/Pt and p–p stacking interactions was observed. Further nuclear overhauser effect spectroscopy (NOESY) experiments revealed a twisted head-to-tail configuration between the cationic and anionic units. The transmission electron microscopy (TEM) images revealed the formation of nanofibers from the DCS in water, in which these fibers were drop-casted onto a gold electrode-containing silicon wafer for the investigation of its charge-transport properties. A current of 1 nA was obtained upon a bias voltage of þ 8 and
8 V with a nonlinear I-V profile, suggesting the semiconducting properties of the nanofibers.22 This water-soluble DCS opens up a new avenue in the investigation of the non-covalent Pt/Pt interactions and the assembly properties of these DCSs in the solution state, which can hardly be achieved by those insoluble Magnus-type double salts. Recently, following a similar strategy reported by Yam and co-workers, Ma, Zhao and Wong reported the reversible luminescence switching properties of a DMSO–water-soluble DCS, [Pt(tpp)(ed)][Pt(ftpp)(CN)2] (tpp ¼ 2-(4-(trifluoromethyl)phenyl)pyridine, ed ¼ ethane-1,2-diamine, ftpp ¼ 2-(4-fluoro-3-(trifluoromethyl)phenyl)pyridine) (Fig. 5).23 This DCS was found to dissolve in DMSO–water mixture and showed drastic color and luminescence changes upon increasing water content in the DMSO solution of the DCS, attributed to the change in the extent of non-covalent Pt/Pt interactions. Nanofibers with lengths of 0.2–2 mm and widths of 20–50 nm have been revealed from the TEM images prepared from a DMSO–water (1:3, v/v) solution of the DCS. More importantly, the solid-state phosphorescence of the DCS was found to be red-shifted from 595 to 644 nm upon excitation at various wavelengths (360–520 nm). The reversible switching of the emission of this DCS has been demonstrated through the

Fig. 4

Structure of [Pt{bzimpy(TEG)2}Cl][Pt{bzimpy(PrSO3)2}Cl] (bzimpy ¼ 2,6-bis(benzimidazol-20 -yl)pyridine).

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Fig. 5 Structure of [Pt(tpp)(ed)][Pt(ftpp)(CN)2] (tpp ¼ 2-(4-(trifluoromethyl)phenyl)pyridine, ed ¼ ethane-1,2-diamine, ftpp ¼ 2-(4-fluoro-3(trifluoromethyl)phenyl)pyridine).

alteration in the extent of Pt/Pt distances in the DCS by mechanical grinding and vapor fuming. Together with the excitationwavelength-dependent emission properties, which may likely be a result of sample heterogeneity, this DCS is thought to be able to serve as a potential candidate in anticounterfeiting applications.23

8.14.2.1.2

Platinum(II) complexes with chelating N-donor ligands

Various classes of platinum(II) polypyridine complexes have aroused tremendous interest in the past few decades attributed to their rich luminescence and solid-state polymorphism. Some of their spectroscopic features have been attributed to the formation of non-covalent Pt/Pt interactions. Incorporation of various classes of ancillary ligands including halide,24–26 cyanide,27–31 dithiolate,32–36 and alkynyl37–41 into platinum(II) compounds with N-donor bidentate ligands was found to alter the nature of the emissive excited state, conferring them with intriguing luminescence properties. One of the most representative examples of platinum(II) complexes with bidentate N-donor ligands is the chloridoplatinum(II) bipyridine complex, [Pt(bpy)Cl2], which has been wellknown to exhibit solid-state polymorphism with red and yellow crystal forms. These two crystal forms were found to have identical chemical composition, while the difference in their optical properties was attributed to the Pt/Pt distance of 3.45 and 4.44 Å for the red and yellow forms respectively as revealed by their X-ray crystal structures.42–44 Recently, Fernández and co-workers designed a V-shaped bipyridine-containing dichloridoplatinum(II) complex that showed a two-step cooperative aggregation pathway in the self-assembly (Fig. 6). This complex was found to undergo the formation of an on-pathway kinetic intermediate upon the head-tohead stacking of the V-shaped platinum(II) complex assisted by non-covalent Pt/Pt interactions.45 These intermediates were found to rapidly convert into a thermodynamically stable aggregate, which afforded the formation of fibrous structures with red luminescence as revealed by the scanning electron microscopy (SEM) and photoluminescence microscopy. The energy landscape of such supramolecular polymerization has been established.45 Recently, Yoshida and Kato reported a series of polyhalogenated bipyridine platinum(II) complexes, [Pt(N^N)X2] (X ¼ Cl, Br) that was found to show elastic flexibility dependent on the solvent of crystallization. In particular, the methanol-solvated crystals were found to exhibit luminescence arising from a triplet metal–metal-to-ligand charge transfer (3MMLCT) excited state upon assembly assisted by non-covalent Pt/Pt interactions. The factors governing the elastic deformability were explored, providing this class of materials with interesting functions for soft materials.46 Adopting the strategy of incorporating alkynyl in the design of luminescent metal complexes,6,13,47–53 luminescent bis-alkynyl platinum(II) diimine system was first reported by Che and co-workers.37 Since then, a number of derivatives of the complex with

Fig. 6

Structure of a V-shaped bipyridine-containing dichloridoplatinum(II) complex reported by Fernández and co-workers.

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different substituent groups on the alkynyl and diimine ligands were developed, in which their electrochemical, electronic absorption and emission properties were studied, showing correlation to the electron-richness of the alkynyl and diimine ligands.38,39,41,54–56 In 2014, Yam and co-workers synthesized a series of unsymmetrical bipyridine-platinum(II)-alkynyl complexes by a post-click reaction (Fig. 7).57 These complexes were found to exhibit significant luminescence enhancement from solution (PLQY: 0.03) to solid-state thin films (PLQY: up to 0.72), which was found to be attributed to the aggregation-enhanced emission properties of the complexes.57 Solution-processable phosphorescent organic light-emitting diodes (PHOLEDs) were fabricated with current efficiency of 18.4 cd A
1 and an external quantum efficiency (EQE) of 5.8% reported. This represented the first demonstration of efficient PHOLEDs, achievable through the aggregation-enhanced emission characters of transition metal complexes.57 At the same time, De Cola and Bäuerle reported the synthesis of a series of bis-alkynyl bipyridine platinum(II) complexes with flexible bridging units by a modular click reaction. The resulting dinuclear platinum(II) complexes were found to show different extents of intramolecular Pt/Pt interactions and excimer formation upon the change in solvent, conferring them with rich photophysical and electrochemical properties.58 In 2018, Yam and co-workers reported a series of triangular metallacycle-containing bipyridine bis-alkynyl platinum(II) complexes.59 The X-ray crystal structures were found to contain dimeric units of the complexes packed in a zig-zag fashion (Fig. 8). The emission properties could be tuned upon the variation of the size of the triangular metallacyclic alkynyl ligands. Near-infrared (NIR) emission was observed upon the aggregation of these complexes, which was found to be associated with the formation of non-covalent Pt/Pt and p
p stacking interactions.59 Later, the same group reported the synthesis of L-glutamine containing bis-alkynyl platinum(II) bipyridine complexes (Fig. 9). These complexes showed tunable emission color and aggregation characteristics upon the interplay of various non-covalent interactions including hydrogen bonding, p
p stacking and Pt/Pt interactions.60 Various nanostructures such as honeycomb and nanospheres were formed upon the variation of solvent composition. In addition, metallogels with yellow and red emission colors, which were tunable by controlling the temperature, were formed at room temperature in chloroform
tetrahydrofuran (THF) and acetone. The formation of these metallogels was found to be cooperatively altered by hydrogen bonding, p
p stacking and Pt/Pt interactions, demonstrating the importance in the balance of these non-covalent interactions in a supramolecular system.60 Platinum(II) polypyridine complexes with cyano ligands represented another important class of complexes, in which the photophysical properties of [Pt(N^N)(CN)2] (N^N ¼ bpy and phen), [Pt(en)(CN)2] and the corresponding double salts related to Pt/Pt interactions were studied by Miskowski and Houlding.24,31 In 1989, Che and co-workers first reported the solid-state emission properties of [Pt(bpy)(CN)2] and showed the presence of Pt/Pt interactions in the solid state with Pt/Pt distance of 3.33 Å by

Fig. 7

Synthetic route of unsymmetrical bipyridine-platinum(II)-alkynyl complexes.

Fig. 8 Crystal packing diagram of a triangular metallacycle-containing bipyridine bis-alkynyl platinum(II) complex. Reproduced from Ai, Y.; Ng, M.; Hong, E. Y.-H.; Chan, A. K.-W.; Wei, Z.-W.; Li, Y.; Yam, V. W.-W. Chem.-Eur. J. 2018, 24, 11611–11618 © Wiley, 2018.

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Fig. 9

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Structure of L-glutamine containing bis-alkynyl platinum(II) bipyridine complex.

X-ray crystallography.28 Che and Yam first reported the solution-state luminescence of [Pt(5,50 -Me2bpy)(CN)2], showing a strong luminescence at 502 nm which was found to be attributed to the strong s-donating effect of the cyano ligands.27 In 2015, Kobayashi and Kato reported the synthesis and vapochromic shape-memory behavior of a dicyanoplatinum(II) diimine complex, [Pt(CN)2(H2dcphen)] (H2dcphen ¼ 4,7-dicarboxy-1,10-phenanthroline).61 The amorphous purple solid of the complex was found to transform to a red crystalline, porous, vapor-adsorbed form upon exposure to alcoholic vapors. The release of the adsorbed vapors in the vapor-adsorbed form upon heating was found to retain the porous structure, in which guest vapors such as water or n-hexane could be further detected. Mechanical grinding was found to readily transform the vapor-free crystalline form to the amorphous purple solid. Such a formation/collapse of the porous framework associated with the emission changes upon vapor adsorption/grinding endowed this complex with ON-OFF switching functions as a vapor-history sensor.61 In 2016, Yam and co-workers reported the synthesis of a series of dicyanoplatinum(II) bipyridine complexes, where the bipyridine ligands were incorporated with L-valine amino units of various hydrophobic motifs.62 These complexes were found to exhibit self-assembly behaviors in the solution state, controlled by the involvement of hydrogen bonding, p
p stacking and Pt/Pt interactions from the L-valine-containing substituent groups.62 The dichloromethane solution of one of the representative complexes showed interesting circularly polarized luminescence (CPL) at
88  C with the highest calculated emission dissymmetry factor (glum) of
0.04, attributed to the formation of chiral spherical aggregates. Moreover, systematic morphological transformation from less uniform aggregates to fibers and rods was observed upon changing the solvent compositions of hexane and dichloromethane at room temperature. Such a transformation was attributed to the perturbation of the extent of Pt/Pt interactions by solvent polarity.62 Recently, as an update to the previously reported vapochromic studies of [Pt(CN)2(H2dcphen)], Ishii and Kato further reported the vapochromic studies of a platinum(II) complex, [Pt(CN)2(H2dcbpy)] (H2dcbpy ¼ 2,20 -bipyridine-4,40 -dicarboxylic acid), by super-resolution microscopy techniques such as structured illumination microscopy and confocal laser microscopy.63 The amorphous-to-crystal transition and the meso-/microscopic solid-state crystallization processes were directly probed under methanol vapor by the changes in phosphorescence. It was found that the vapochromic behaviors were revealed along the direction of the methanol vapor applied when the size of the single particles exceeds 10 mm (Fig. 10). The vapor-induced crystallization was found to be initiated at the surface, where the voids generated assisted the propagation of the crystallization processes inside the particles. Such a finding provided insights into the solid-state crystallization processes and can be readily investigated by phosphorescence changes.63 Tridentate N-donor ligand-containing platinum(II) complex systems represent another important and distinct class of complexes attributed to their rich photophysical properties associated upon the possible formation of Pt/Pt interactions both in the solid and solution states. Compared to the bidentate N-donor counterparts, the tridentate ligands possessed strong preference of coordination to metal centers in a planar fashion with enhanced rigidity, and thus reduced the possible energy dissipation through non-radiative decay. One of the classical examples of such platinum(II) systems is the chloridoplatinum(II) terpyridine complex, [Pt(tpy)Cl]þ, in which the synthesis was first reported by Morgan and Burstall in 1934.64 Subsequent work of related thiolato analogs, [Pt(tpy)(SCH2CH2OH)]þ, focused on the dimerization of the complexes in the solution state, and the observation of Pt/Pt contact of 3.572 Å was reported by Lippard and co-workers.65 Despite these earlier studies of this class of complexes on the extent of Pt/Pt contact, extensive studies on their intriguing photophysical properties were reported by the groups of Che66 and Gray67 in the 1990s. The solid-state colors and emission energies were found to be dependent on the nature of the ancillary ligands and counter-anions with Pt/Pt separations of 3.269–3.329 Å.66,67 The chloridoplatinum(II) terpyridine system was found to exhibit strong luminescence in both solid state and in butyronitrile glass. However, it was found to be non-emissive in fluid solution, possibly due to the thermally accessible ligand field excited state resulting from a weak field chlorido ligand that provided facile non-radiative decay pathway. It was only until the first report of the preparation and solution-state assembly alkynylplatinum(II) terpyridine system by Yam and co-workers that the luminescence and self-assembly properties of this class of complexes can be readily investigated since the incorporation of alkynyl ligands was found to efficiently reduce the non-radiative decay through the ligand field excited state and to provide enhanced solubility in the solution state.68,69

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Fig. 10 Time-dependent confocal laser phosphorescence microscopy images overlapped with transmission light microscopy images of single particles of [Pt(CN)2(H2dcbpy)] upon exposure to MeOH vapor from 1 to 8 min. Reproduced from Ishii, K.; Takanohashi, S.; Karasawa, M.; Enomoto, K.; Shigeta, Y.; Kato, M. J. Phys. Chem. C 2021, 125, 21055–21061 © American Chemical Society, 2021.

Water-soluble alkynylplatinum(II) terpyridine complexes were prepared by Yam and co-workers for real-time monitoring of enzymatic activities through the induced assembly of the complexes by intermolecular Pt/Pt interactions (Fig. 11a).70 The formation of Pt/Pt and p
p interactions from the cationic complexes was found to be induced by polyanionic phosphate derivatives like adenosine triphosphate (ATP) and phosphopeptide. The negative charge densities of the phosphate derivatives were found to distinguish the target substrates from metabolic products by the cationic complexes through structural differences. Upon the monitoring of the NIR emission changes at 805 nm, the enzymatic activities of ATPase, v-Src kinase and alkaline phosphatease (ALP) were readily probed on a real-time basis, providing insightful kinetic parameters from Michaelis–Menten analyses for the catalytic conversions of the respective target substrates (Fig. 11b).70

Fig. 11 (a) Structures of the water-soluble alkynylplatinum(II) terpyridine complexes. (b) Cartoon showing the aggregation characteristics of the complexes upon ATPase-catalyzed reaction of ATP, v-Src kinase- and alkaline phosphatease-catalyzed reactions on the sequences of P1 and pP1. Reproduced from Yeung, M. C.-L.; Yam, V. W.-W. Chem. Sci. 2013, 4, 2928–2935 © Royal Society of Chemistry, 2013.

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By incorporating motifs that are capable of hydrogen bonding, Yam and co-workers reported the synthesis and assembly studies of a series of platinum(II) terpyridine complexes with L-valine-containing alkynyl ligands (Fig. 12a).71 The alkynylplatinum(II) complex with one L-valine unit in the alkynyl ligand was found to show interesting gelation properties in acetonitrile, distinctive from its organic counterpart. The critical gelation concentration and the sol-gel transition temperature were found to be 2.5 mg mL
1 and 68 C respectively. A color change from yellow to red was observed upon the sol-to-gel transition (Fig. 12b), suggestive of the involvement of the intermolecular Pt/Pt interactions. This red gel was found to exhibit a structureless NIR emission at 755 nm, which was attributed to a 3MMLCT excited state by intermolecular Pt/Pt and p–p interactions upon gelation. Further investigation into the temperature-dependent NMR spectra of the gel revealed the involvement of p–p stacking and hydrogen-bonding interactions. Fibrous networks were revealed from the TEM and SEM images of the xerogel of the complex, showing a thickness of about 30–100 nm (Fig. 12c). A delicate balance of various non-covalent interactions was found to be crucial to the gelation of this class of complexes since precipitation of the complexes would occur, instead of gelation, when the number of 71 L-valine units in the alkynyl ligand increases to two or three. Yam and co-workers further reported a series of sulfonate-containing amphiphilic alkynylplatinum(II) 2,6-bis(N-alkylbenzimidazol-20 -yl)pyridine (bzimpy) complexes to investigate the relationship between molecular structure, packing and morphologies of the nanostructures by variation of alkyl chain length in the alkynyl ligands.72 The incorporation of long alkyl chains into the alkynyl ligands was found to readily lead to the formation of cylindrical micelles with diameters that matched well to twice the molecular length scales. In sharp contrast, the curvature required for the formation micelles was not fulfilled due to the absence of long alkyl chains as suggested by the packing parameter, and ultimately lead to planar bilayer nanostructures. Thus, the morphologies of this class of complexes can be readily tuned through the variation in alkyl chain lengths and can be rationalized by the changes in the packing parameters.72 The same group further reported another class of triethylene glycol (TEG)-containing alkynylplatinum(II) complexes that show unusual thermoresponsive properties due to distinct morphological transformation associated with the alteration of Pt/Pt and p–p stacking interactions.73 The growth of the MMLCT absorption at 500 nm was observed upon heating an aqueous solution of the complex. The solution color was found to change from yellow to orange-red with a concomitant growth of a low-energy 3MMLCT emission at 710 nm associated with the formation of intermolecular Pt/Pt and p–p stacking interactions. Spherical aggregates with diameters of about 8–10 nm were observed in the electron microscopy images prepared from the orangered solution at high temperature, while sheet-like aggregates were observed from that of the yellow solution at 0  C. More importantly, a large hysteresis in the heating and cooling cycles was revealed in the temperature-dependent absorption studies, indicative of the transformation of two distinct aggregated species. Such a unique thermoresponsive behavior was found to be governed by the solubility of the alkyl chains on the alkynyl ligands.73 Mauro, De Cola and co-workers reported the synthesis of a neutral platinum(II) complex with 2,6-bis(3-(trifluoromethyl)-1H1,2,4-triazol-5-yl)pyridine as the pincer ligand and its solvent-induced aggregation study (Fig. 13a).74 This complex was found to

Fig. 12 (a) Structure of L-valine-containing alkynylplatinum(II) terpyridine complex. (b) Photographs of sol–gel transition between the yellow solution and the red gel of the complex in acetonitrile. (c) SEM image of the xerogel of the complex. Scale bar ¼ 1 mm. Reproduced from Po, C.; Ke, Z.; Tam, A. Y.-Y.; Chow, H.-F.; Yam, V. W.-W. Chem.-Eur. J. 2013, 19, 15735–15744 © Wiley, 2013.

Fig. 13 (a) Structure of a neutral platinum(II) complex with 2,6-bis(3-(trifluoromethyl)-1H-1,2,4-triazol-5-yl)pyridine as the pincer ligand. (b) Fluorescence microscopy image of the luminescent microcrystalline fibers. Scale bar ¼ 100 mm. Reproduced from Mauro, M.; Aliprandi, A.; Cebrián, C.; Wang, D.; Kübel, C.; De Cola, L. Chem. Commun. 2014, 50, 7269–7272 © Royal Society of Chemistry, 2014.

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show yellow luminescent microcrystalline fibers with the length in the micrometer scale as revealed from the fluorescence microscopy image (Fig. 13b). The PLQY of the microcrystalline fibers was found to be 74% with emission origin assignable to the 3MMLCT emission associated with the formation of intermolecular Pt/Pt interactions.74 The same group further reported a similar series of complexes by replacing the pyridine ancillary ligand with a hydrophilic 4-hydroxypyridine.75 This class of complexes was found to exhibit uptake by HeLa cells in which the luminescence in the cell was attributed to the self-assembly of the complexes by Pt/Pt and p–p stacking interactions, rendering them a luminescent probe for bioimaging.75 At the same time, Du, Cao, Eisenberg and co-workers reported the reversible mechanochromic luminescence of cationic 40 -(pnicotinamide-N-methylphenyl)-containing platinum(II) terpyridine complexes with various counter-anion including PF6
, SbF6
, Cl
, triflate (OTf
) and BF4
(Fig. 14a).76 The yellow solid of the complex with triflate anion was found to turn red upon grinding, reversible through the addition of drops of acetonitrile (Fig. 14b). The X-ray crystals of the complex with triflate anion of the yellow form revealed long Pt/Pt distances (> 3.9 Å), while that of the red form showed short Pt/Pt contacts of 3.289 Å, indicating the involvement of Pt/Pt interactions upon grinding.76 Apart from incorporating hydrophilic moieties into the platinum(II) terpyridine system, Yam and co-workers recently incorporated a hydrophobic polyhedral oligomeric silsesquioxane (POSS) moiety into the alkynylplatinum(II) system, showing intriguing morphological transformation of distinguishable nanostructures from ring to rods upon varying the solvent polarity.77 These changes were found to be governed by the delicate balance and interplay of different non-covalent interactions including hydrophobic, Pt/Pt and p–p stacking interactions. The well-dispersed POSS-containing alkynylplatinum(II) complex gave a yellow solution in THF. Upon reduced solvation of the complex in non-polar solvent like hexane, a drastic color change from yellow to red has been observed, associated with the enhancement in the low-energy MMLCT absorption and 3MMLCT emission bands, leading to the formation of a rod-like aggregate as revealed from the electron microscopy. The stabilization of the nanorods in hexane
THF mixture was attributed to the formation of directional Pt/Pt interactions as revealed by the spectroscopic responses. The well-dispersed THF solution was also found to turn red upon reduced solvation by increasing solvent polarity by adding water. This was associated with an abrupt increase in the low-energy MMLCT absorption and 3MMLCT emission bands at 60% water
THF mixture. Ring-like nanostructures was observed upon in 30% water
THF, which was believed to be resulted from the stabilization of the aggregates through the less directional hydrophobic interactions. Further increase in the water content to 70% resulted in the formation of rod-like aggregates, which was believed to be associated from the switching-on of the directional Pt/Pt interactions at high water content.77 Yam and co-workers further developed and designed a POSS-containing alkynylplatinum(II) terpyridine complex with amphiphilic character by incorporating a hydrophilic, zwitterionic sulfobetaine alkynyl ligand, which provided solubility in polar medium.78 The complex was found to dissolve in THF to give a red solution, which showed drastic color changes from red to yellow and back to red upon addition of water. This observation was attributed to the aggregation
partial deaggregation
aggregation via the formation or weakening of Pt/Pt and/or p
p interactions. It was found that the rather poor solvation of the zwitterionic sulfobetaine moiety in THF would facilitate assembly by Pt/Pt and/or p
p interactions, resulting in a red solution with NIR emission arising from the MMLCT absorption and 3MMLCT emission. Thus, rod-like nanostructures were observed in the TEM images due to the directional Pt/Pt interactions. A yellow solution was formed upon further increase in the water content in the THF
water mixture, probably resulting from the improved solvation of the zwitterionic moiety that led to disassembly, where ill-defined nanostructures were observed in the TEM images. Further increase in the water content to 50% were found to facilitate the formation of ring-like aggregates while the solution remained yellow in color, suggestive of the switching-on of the less directional hydrophobic interactions resulting from the reduced solvation of the POSS moiety. Further reduction in solvation of the hydrophobic POSS moiety in 70% water
THF mixture allowed the adjacent complexes to come into close proximity, leading to the formation of Pt/Pt and/or p
p interactions to give the red color solution with the formation of plate-like aggregates upon the formation of directional Pt/Pt interactions. Such a drastic color change was found to accompany with the morphological transformation from short fibrils to spherical aggregates through circular/ring-like intermediates to plate-like aggregates.78 Apart from controlling the formation of stable aggregates thermodynamically, Manners and co-workers reported an investigation into another synthetic approach in accessing the kinetically-trapped assemblies as an off-pathway product by using a single-component building block.79 They reported the synthesis and control of length in supramolecular nanofibers by using a platinum(II) complex with (2,6-di(1H-tetrazol-5-yl)pyridine) as the pincer ligand and a hydrophilic TEG-containing pyridine

Fig. 14 (a) Structure of a cationic 40 -(p-nicotinamide-N-methylphenyl)-containing platinum(II) terpyridine complex with triflate as the counter-anion. (b) Reversible mechanochromic responses of the complex. Reproduced from Han, A.; Du, P.; Sun, Z.; Wu, H.; Jia, H.; Zhang, R.; Liang, Z.; Cao, R.; Eisenberg, R. Inorg. Chem. 2014, 53, 3338–3344 © American Chemical Society, 2014.

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Luminescent supramolecular assemblies

as the ancillary ligand. Polydisperse fibers were observed upon the self-assembly of the complexes through Pt/Pt and p
p stacking interactions. These fibers were found to be kinetically trapped since there is no observable difference in the average contour length and the length distribution over 3 days.79 The control in the length of this supramolecular polymer was found to be achieved by the addition of various amounts of unimer into the seeds, in which the seeds were prepared by sonication of the polydisperse fibers. A narrower length distribution of the resultant fibers was achieved when compared to the polydisperse fibers.79 Mauro and De Cola reported a luminescent charge-neutral platinum(II) complex with a hydrophobic tridentate N-donor ligand and a hydrophilic triethylene glycol-containing ancillary ligand and investigated their self-assembly properties including the utilization of an off-pathway aggregation process.80 In sharp contrast to the work of Manners that only showed one kinetically trapped state, the self-assemblies of this platinum(II) complex were found to display distinctive kinetically trapped metastable aggregates (A and B), and a thermodynamically stable aggregate (C) (Fig. 15). The kinetically metastable state A was found to interconvert into the thermodynamically stable aggregate C in 3 weeks at room temperature.80 Surprisingly, a transient species B was found to form during the conversion from A to C. The aggregate B exhibited green luminescence which was found to be attributed to the axial Pt/Pt interactions in a face-to-face manner but of a longer Pt/Pt distance when compared to that of aggregate A, which exhibited orange luminescence. The interconversion of C to B was found to be achieved by irradiating C with a laser.80 The control and understanding of the on-pathway and off-pathway aggregates in their studies demonstrated real-time visualization of these aggregates attributed to the alteration of the extent of intermolecular non-covalent Pt/Pt and p
p stacking interactions. Distinctive from the demonstration of kinetically trapped off-pathway aggregation by the utilization of the single-component metal complex by Manners79 and De Cola,80 Yam and co-workers adopted another strategy to design a two-component ensemble system to realize the kinetics of the supramolecular assemblies. They designed and synthesized a two-component co-assembly system that involved both alkynylplatinum(II) terpyridine complexes and block copolymers, poly(ethylene glycol)-b-poly(acrylic acid) (PEG-b-PAA), for the formation and manipulation of supramolecular assembly.81 The self-assembly of the platinum(II) complexes through Pt/Pt and p–p stacking interactions was observed, which was believed to be induced by the electrostatic attractions toward the block copolymer PEG-b-PAA in an aqueous medium since the PAA units were deprotonated. Interestingly, highly crystalline nanofibers were observed in the TEM images.81 Further characterization of these nanofibers revealed the core-shell structures of the fibers in which the core was constructed by the hexagonally packed columns of platinum(II) complexes on the PAA blocks, while the shell was constructed from the solvated PEG blocks. Short patchy nanofibers were formed upon subsequent sonication of these nanofibers, which were believed to be the kinetically trapped aggregates since the controlled formation of long nanofibers as the kinetic product and wider nanobelts as the thermodynamic product was further achieved through these short patchy nanofibers, demonstrating the control and manipulation through longitudinal and transverse growth of one-dimensional selfassembly of supramolecular structures.81 Later, the same group further demonstrated the living supramolecular polymerization of this class of two-component systems. A segmented growth of the supramolecular polymer through Pt/Pt and p–p stacking interactions by using two different platinum(II) complexes was achieved, demonstrating the controlled formation of a heterojunction that allowed a large structural difference and lattice mismatch between the two different platinum(II) complexes.82 Further establishment of the energy landscapes of this

Fig. 15 A schematic diagram showing the dynamic equilibrium of assemblies A, B and C with the monomeric charge-neutral platinum(II) complex. Reproduced from Aliprandi, A.; Mauro, M.; De Cola, L. Nat. Chem. 2016, 8, 10–15 © Springer Nature, 2015.

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585

two-component ensemble system was reported by the same group.83 The nanobelts were found to be the thermodynamically stable aggregates, while the core-shell nanofibers were the kinetically trapped assemblies. The longitudinal and transverse growth of the platinum(II) complexes were achieved by room-temperature incubation and thermal annealing of the co-assembly mixture respectively.83 Based on the energy landscape, the systematic fabrication of nanosphere-block-nanobelt nanostructures with distinct segmented architectures was further achieved through thermal annealing of a ternary mixture of platinum(II) complexes, PEG-bPAA and PAA brushes.83 Later, Yam and co-workers utilized the unique features of this class of two-component ensemble system to fabricate a supramolecular DNA hydrogel using kinetic control with tunable gel-to-sol transition.84 Upon mixing the platinum(II) complex with double-stranded DNA in aqueous medium, the complexes were found to stack into columnar phases via intermolecular Pt/Pt and p–p stacking interactions, leading to the formation of luminescent supramolecular hydrogels.84 This was the first example reported for the competition between the on- and off-pathway aggregates in platinum(II)-DNA metallosupramolecular system. Recently, Yam and co-workers reported the design and synthesis of a series of anthracene-containing alkynylplatinum(II) terpyridine complexes with photo-modulated self-assembly behaviors in solution.85 Facilitated by various kinds of intermolecular noncovalent interactions including p–p stacking and hydrophobic interactions and directional Pt/Pt interactions, supramolecular assemblies, with well-defined sheet-like nanostructures, of these dinuclear platinum(II) complexes were found to occur in concentrated DMSO solutions, demonstrating intriguing temperature-responsive spectroscopic features with drastic color changes associated with the change in the extent of Pt/Pt interactions. More interestingly, this series of complexes can undergo photooxygenation, as revealed by UV–vis absorption spectroscopy and mass spectrometry.85 Such photooxygenation reaction was found to perturb the self-assembly behaviors of the complexes as revealed by TEM and SEM images as well as dynamic light scattering experiments before and after photoirradiation.85 Further extension of the use of platinum(II) complexes to direct phototriggered self-assembly processes was demonstrated by the same group.86 A new class of photoactivatable ortho-nitrobenzyl ester-based alkynylplatinum(II) 2,6-bis(N-decylbenzimidazol-20 yl)pyridine (bzimpy) complexes was strategically designed and synthesized. Given that the ortho-nitrobenzyl moieties can be readily cleaved by UV irradiation to afford the corresponding benzoic acids, an interesting morphological transformation from ill-defined architecture to sheet-like nano-layers was observed for these platinum(II) complexes upon UV irradiation.86 It was believed that the removal of the ortho-nitrobenzyl ester group attributed to an enhancement of the amphiphilic character and a better molecular alignment of the amphiphilic complexes upon UV irradiation, leading to the formation of well-defined nanostructures directed by intermolecular Pt/Pt and p
p interactions.86 Apart from the preparation of mononuclear alkynylplatinum(II) terpyridine complexes, the dinuclear analogs were also found to exhibit interesting luminescence and self-assembly properties due to the formation of intra- and/or intermolecular Pt/Pt and p
p interactions. In 2012, Yam and co-workers first reported the design, synthesis and assembly studies of a series of meta-phenylene ethynylene (mPE)-containing dinuclear platinum(II) terpyridine complexes backbone capable of folding back to form a single helical turn stabilized by intramolecular Pt/Pt and p
p stacking interactions.87 Such a folding process was found to be mediated by solvent polarity and temperature, in which it was probed by the emergence of lower-energy MMLCT absorption and 3MMLCT emission bands due to the formation of intramolecular Pt/Pt interactions.87 Later, the same group incorporated an inherently chiral binaphthol moiety into the mPE backbone of the alkynyl ligand to prepare chiral alkynylplatinum(II) metallofoldamers.88 Various spectroscopic studies including absorption and emission spectroscopy revealed the stabilization of the folded state by the intramolecular Pt/Pt interactions, in which helix stabilization energies from
0.84 to
1.80 kcal mol
1 were determined. In particular, the circular dichroism (CD) spectra were found to show a bisignate Cotton effect and the sigmoidal increase in the ellipticity at 307 nm, indicative of a cooperative formation of a single helical turn. Interestingly, preferred handedness of M- or P-helices could be constructed through the use of the pure enantiomers of the chiral binaphthol backbone.88 Modification by introduction of amphiphilic character into the mPE-containing binaphthol alkynyl backbone by incorporation of triethylene glycol moieties was made by the same group (Fig. 16).89 The amphiphilic alkynylplatinum(II) terpyridine metallofoldamer was found to feature a drastic increase in the Cotton effect and a dramatic luminescence enhancement upon progressive increase in water content in the acetonitrile solution of the complex. Such changes were found to be attributed to the significant change in the secondary structures, attributed to the formation of a columnar stack from the foldamers assisted by the formation of intermolecular Pt/Pt and p–p stacking interactions. Further evidence showed a significant decrease in Cotton effect with two well-defined isodichroic points upon heating the 70% water–acetonitrile solution of the complex, indicative of a deaggregation process attributed to the dissociation of the individual folded helical turn from the helices of helices. These intriguing findings revealed the unique role of Pt/Pt interactions in assisting the construction of high-order hierarchical architectures of this class of metallofoldamers.89 In 2013, Wolf and co-workers prepared a dinuclear terthiophene-containing alkynylplatinum(II) terpyridine complex that incorporated flexible terthiophene linkers.90 The folding of the complexes in CHCl3
CH3CN solution was investigated by electronic absorption spectroscopy, 1H and NOESY NMR spectroscopy. The folded and unfolded solid-state structures were successfully determined from their single crystals by X-ray crystallography, in which the folded structure was found to be stabilized by revealing weak intermolecular interactions C
H/O and C
H/Cl interactions, Cl
p and p
p interactions (Fig. 17).90 Recently, Yam and co-workers reported the preparation of a luminescent supramolecular metallogel based on metallofoldamers by the incorporation of mPE-containing alkynyl ligand into the platinum(II) 2,6-bis(N-alkylbenzimidazol-20 -yl)pyridine complex.91 The complex containing five mPE repeating units was found to form a single-turn helix in acetonitrile stabilized by

586

Luminescent supramolecular assemblies

Fig. 16

Structures of mPE-containing binaphthol alkynylplatinum(II) metallofoldamers.

Fig. 17 X-Ray crystal structures of the folded and unfolded state of a dinuclear terthiophene-containing alkynylplatinum(II) terpyridine complex and its electronic absorption spectra in CHCl3
CH3CN solution. Reproduced from Cao, Y.; Wolf, M. O.; Patrick, B. O. Inorg. Chem. 2013, 52, 5636–5638 © American Chemical Society, 2013.

intramolecular Pt/Pt interactions. Interestingly, this single-helical turn was found to self-associate to form a metallogel at high concentrations, resulting from the formation of intermolecular Pt/Pt interactions. In sharp contrast to the platinum(II) complex with five mPE repeating units, there was a lack of gelation behaviors in the platinum(II) complexes with four or six mPE units, attributed to the differences in their conformations and the extent of Pt/Pt interactions. This synthetic strategy in the preparation of luminescent supramolecular metallogel by the intermolecular stacking of the helical complex was the first of its kind, which has provided insights into the construction of high-order hierarchical architectures by metallofoldamers.91 As an extension of the mPE-containing dinuclear platinum(II) terpyridine metallofoldamers reported in 2012, recently, Yam and co-workers further incorporated oligo(ethynylpyridine)-containing foldamer backbone into the alkynylplatinum(II) terpyridine system.92 This series of metallofoldamers not only was able to exhibit rich spectroscopic responses, showing red and green luminescence in the folded and unfolded forms respectively, due to the modulation of the intramolecular Pt/Pt interactions mediated by solvent composition and temperatures, the addition of acids/bases was also found to perturb the conformation of the folded state, attributed to charge repulsion between the positively charged pyridinium moiety and the Pt(II) pincer that led to the unfolding of the metallofoldamer.92 The studies provided insights into the rational molecular design and multidimensional control of metallofoldamers for the preparation of multi-stimuli-responsive luminescent supramolecular materials. Phosphorescent molecular tweezers based on di- or trinuclear alkynylplatinum(II) terpyridine complexes were reported by Yam and co-workers, demonstrating the host
guest interactions of these tweezers and various guest molecules including cationic, charge-neutral and anionic platinum(II), palladium(II), gold(I), and gold(III) complexes93 as well as polyaromatic hydrocarbons.94 The guests were found to be encapsulated into the binding cleft of the tweezers which were often accompanied by absorption and emission changes in the low-energy region as a result of the formation of metal
metal, p
p and electrostatic interactions. The differences in these non-covalent interactions upon guest binding revealed the binding affinity of the guests to the tweezers.93,94 Further extension of the molecular tweezers was made to the design of a double-decker tweezers structure, ultimately leading to the binding of two equivalents of guests that induce a drastic color change due to the formation of extensive metal
metal

Luminescent supramolecular assemblies

587

interactions.95 Recently, Yam and co-workers reported the design and synthesis of a dinuclear platinum(II) guest that was found to complementarily fit into the two binding sites of a double-decker tweezer.96 More importantly, this work represented the first report of the single-crystal structure of the discrete multinuclear stack constructed by platinum(II) tweezers attributed to the enhanced stability of the host
guest system by extended Pt/Pt interaction with a linear array of five Pt(II) centers and a p-stack of pincer ligands.96 The formation of such a discrete metal array and p-stack was also found to be accompanied by drastic color changes with the switching on of a NIR emission associated with the formation of Pt/Pt and p
p interactions (Fig. 18). Following along the same line of research initiated by Yam,93–96 Wang and co-workers reported the preparation of an alkynylplatinum(II) terpyridine molecular tweezers with pyrene as the pendant.97 The cavity of the tweezers was found to encapsulate the pyrene pendant of an adjacent tweezers, leading to the formation of an extended head-to-tail supramolecular polymer. The reversible disassembly and reassembly processes were found to be triggered by the addition of anthracene derivatives and bis(2methoxyethyl) dicyanofumarate to the heteroditopic monomers.97 Later, Wang, Yao and co-workers reported the host–guest interactions between a bis-alkynylplatinum(II) terpyridine tweezers and an alkynylgold(III) diphenylpyridine guest, in which the specific complexation was found to be maintained in the presence of a benzo-21-crown-7 (B21C7)-secondary ammonium salt recognition motif, leading to the formation of supramolecular hyper-branched polymers.98 Further extension of the host architecture from tweezers to molecular rectangles was achieved to restrict the cavity for selective capture of certain guest molecules. Yam and co-workers reported the design and synthesis of a series of alkynylplatinum(II) terpyridine molecular rectangles with different geometries, topologies and electronic properties.99 The rectangles were found to selectively encapsulate transition metal complexes based on the size and steric microenvironment of the cavity. More importantly, the reversible guest capture and release were found to be manipulated upon the changes in pH environment, demonstrating a possible approach for smart delivery of guest molecules such as therapeutic reagents to the slightly acidic microenvironment in cancer cells.99 By utilization of a similar approach in pH modulation by Yam and co-workers, the pH-responsive transformation of an alkynylplatinum(II)-based molecular tweezers was recently reported by Wang and co-workers.100 The transformation from a “U”- to “W”-shaped conformation was found to be triggered by the addition of trifluoroacetic acid, leading to the release of the guest molecules as a result of the disruption of the Pt/Pt and p
p interactions in the host–guest adduct.100

8.14.2.1.3

Platinum(II) complexes with chelating cyclometalating ligands

von Zelewsky and co-workers reported the first synthesis of a class of bis-cyclometalated platinum(II) complexes, [Pt(C^N^C)X] (HC^N^CH ¼ 2,6-diphenylpyridine; X ¼ pyridine, pyrazine, Et2S) in 1988,101,102 while Rourke and co-workers further optimized the synthetic route to obtain this class of complexes in high yield in 1999.103,104 The photophysical properties of this class of complexes were independently reported by the groups of Yam105 and Che.106 Yam and co-workers reported the photophysical properties and ion-binding properties of a series of cyclometalated C^N^C platinum(II) complexes with crown ether pendants.105 The complexes exhibited structureless emission bands in dichloromethane at high concentration, which were suggested to be originated from the self-assembly of the complexes. Further cation-binding studies of this class of neutral C^N^C platinum(II) complexes were found to give rise to a larger binding constant when compared to the positively charged terpyridine platinum(II) counterparts.105 Che and co-workers reported the photophysical properties of a class of cyclometalated C^N^C platinum(II) complexes with 2,6-diphenylpyridine as the pincer ligand.106 The solid-state emission bands of this class of complexes were found to be broad and structureless, which were assignable to the excimeric emission since there were no observable Pt/Pt interactions as revealed

Fig. 18 X-Ray crystal structure of the discrete multinuclear stack constructed by platinum(II) tweezers with a linear array of five Pt(II) centers and a p-stack of pincer ligands showing a switching on of a NIR emission. Reproduced from Kong, F. K.-W.; Chan, A. K.-W.; Ng, M.; Low, K.-H.; Yam, V. W.-W. Angew. Chem. Int. Ed. 2017, 56, 15103–15107 © Wiley, 2017.

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Luminescent supramolecular assemblies

from the X-ray crystal structure. With the incorporation of a bis(diphenylphosphano)methane as the bridging ligand, the complex was found to exhibit vapochromism, where the orange crystals were found to turn bright yellow upon the exposure to organic solvents including benzene, pentane, dichloromethane, acetone and methanol.106 In 2013, Che and co-workers prepared a series of luminescent cyclometalated N^C^N platinum(II) complexes with auxiliary isocyanide ligand, [4-R-(N^C^N)-PtC^N(C6H3-2,6-Me2)]PF6 (4-R-(N^CH^N) ¼ substituted 1,3-bis(20 -pyridyl)benzene) (Fig. 19).107 The complex without any substituent groups on the pincer ligand (R ¼ H) was found to exhibit two solvatomorphic crystal structures, the red and yellow forms. The red form was found to show a Pt/Pt contact of 3.43 Å, which was slightly shorter than the sum of van der Waals radii of 3.44 Å, suggesting the presence of metal–metal interactions. On the other hand, the yellow form exhibited long and short Pt/Pt separations of 3.74 and 5.76 Å, indicating the absence of Pt/Pt interactions. The drop-casted sample of the red form on a thin film was found to exhibit vapoluminescence upon the exposure to acetonitrile vapor, leading to a change in emission color from red to orange. It was believed to be attributed to the structural rearrangements by the sorption or desorption of the solvated molecules as revealed from the X-ray crystal structure.107 Later, the same group reported the synthesis and photophysical studies of a class of cyclometalated C^N^N platinum(II) allenylidene complexes.108 This class of complexes was found to exhibit prominent luminescence and self-assembly properties attributed to the formation of intermolecular Pt/Pt and/or p
p interactions. It was noteworthy that cellular imaging of the HeLa cells was demonstrated where the complexes were found to be predominantly localized in the nucleus or cytosolic region.108 In 2014, they further reported another class of luminescent cyclometalated C^N^N platinum(II) complex, [Pt(C^N^N)(C^NtBu)]ClO4 (HC^N^N ¼ 6phenyl-2,20 -bipyridine), that exhibit luminescence enhancement upon the intercalation of the complexes into the nucleobases of nucleic acids.109 More importantly, one of the complexes was found to show potent anticancer activity toward cancer cell in vitro and the inhibition of a tumor growth in a mouse model was demonstrated. The anticancer activity was believed to be due to the stabilization of the topoisomerase I–DNA complex with resulting DNA damage by the complex.109 Apart from mononuclear cyclometalated platinum(II) complexes, the group of Che also reported a series of dinuclear cyclometalated platinum(II) complexes in 2014, which was synthesized by covalently connecting [Pt(C^N^N)(CNR)]þ (HC^N^N ¼ 6-phenyl2,20 -bipyridyl, CNR ¼ 2,6-dimethylphenyl isocyanide) motifs by oligo(oxyethylene) chains (Fig. 20).110 The complexes were found to form luminescent lyotropic chromonic liquid crystals/hydrogels in aqueous dispersions, assisted by the formation of intra- and intermolecular Pt/Pt and p
p interactions. Interestingly, the complexes containing short oxyethylene chains were found to act as gelating agents for the mononuclear cyclometalated complex, [Pt(C^N^N)(CNR)]þ, which was found to exhibit insignificant gelation properties in water.110 The spontaneous gelation of the mononuclear complexes was achieved by the addition of a small amount of dinuclear complexes. It was suggested that the dinuclear complexes provided cross-linkage mediated by Pt/Pt and p
p

Fig. 19

Structures of a series of luminescent cyclometalated N^C^N platinum(II) complexes with auxiliary isocyanide ligand.

Fig. 20

Structures of a series of dinuclear cyclometalated platinum(II) complexes.

Luminescent supramolecular assemblies

589

Fig. 21 Structure of [(C^N)Pt(m-pz)2Pt(C^N)] (C^N ¼ 2-(2,4-difluorophenyl)pyridine). Reproduced from Han, M.; Tian, Y.; Yuan, Z.; Zhu, L.; Ma, B. Angew. Chem. Int. Ed. 2014, 53, 10908–10912 © Wiley, 2014.

interactions between the one-dimensional chains formed from the mononuclear complexes, and thus facilitated the gelation processes in aqueous medium.110 At the same time, Ma and co-workers reported a pyrazolate (pz) bridged dinuclear cyclometalated platinum(II) complex, [(C^N) Pt(m-pz)2Pt(C^N)] (C^N ¼ 2-(2,4-difluorophenyl)pyridine) that was found to be butterfly-like in shape (Fig. 21).111 The complex was found to show no significant involvement of Pt/Pt interactions in the ground state as revealed by the density functional theory (DFT) calculations. Interestingly, this complex was found to undergo a molecular structural change upon photoexcitation, in which the Pt/Pt distance was shortened, leading to the formation of two distinct excited states, the triplet ligand-centered/metal-to-ligand charge transfer (3LC/MLCT) and the 3MMLCT excited states, that are responsible for the dual emission.111 As a result, a broad white emission was achieved upon the control and manipulation of the surrounding of this butterfly-like platinum(II) complex due to the co-existence of the two excited states.111 In 2014, Swager and co-workers reported a series of cationic cyclometalated platinum(II) complexes with 2-phenylpyridine and pyridyl triazole as the ligands (Fig. 22).112 These complexes were found to display thermotropic columnar liquid crystalline behavior despite having only a single side chain. It was believed that the strong intermolecular association of the complexes promoted liquid crystalline and mechanochromic behavior associated with the formation of significant intermolecular Pt/Pt interactions and enhanced dispersive forces due to distortions from square planarity. Two distinct polymorphs were identified, which were found to be interconvertible by mechanical force, melting or solvent exposure.112 Later, Li, You and co-workers reported a series of chiral cyclometalated platinum(II) complexes, [Pt(L)(Dmpi)]Cl (L ¼ (þ)- or (
)-4,5-pinene-60 -phenyl-2,20 -bipyridine; Dmpi ¼ 2,6-dimethylphenylisocyanide) in 2015.113 Two polymorphs, the yellow and red forms, of the (
)-complex were obtained which were found to be associated with different extent of Pt/Pt interactions in the crystal packing of the complex. The yellow forms of the (þ)- and (
)-complexes were found to undergo crystal-to-amorphous transformation upon mechanical grinding, leading to the switching characteristics of the luminescence and chiroptical behaviors as revealed by emission and solid-state electronic CD spectroscopy. Fine-tuning of such switching behaviors was achieved by replacing the counter-anion from chloride to a bulkier trifluoromethanesulfonate anion (OTf
).113 Kato and co-workers reported the dual-emissive properties of an ionic liquid based on an anionic cyclometalated platinum(II) complex with an imidazolium cation.114 The dual-emissive properties were found to be attributed to the monomeric and aggregated forms of the platinum(II) complex anions, giving rise to distinct thermochromic luminescence. It was believed that the energy transfer from the monomeric excited states to the aggregated state led to the dual emission due to the co-existence of monomeric and aggregated forms of the complex in disordered liquid and glass phases.114

Fig. 22

Structures of a series of cationic cyclometalated platinum(II) complexes with 2-phenylpyridine and pyridyl triazole as the ligands.

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Luminescent supramolecular assemblies

In 2016, Li, Yam and co-workers reported the synthesis of a series of cyclometalated platinum(II) complexes, [Pt(bpzb)Cl] (bpzb ¼ 1,3-bis(1-n-alkylpyrazol-3-yl)benzene) and [Pt(bpzb)(C^C-R)] (R ¼ C6H5, C6H4–OCH3-p, C6H4–NO2-p, C6H4–NH2-p, 4-cholesteryl phenyl carbamate, and cholesteryl methylcarbamate).115 Two of the complexes were found to show short intermolecular C–H/Pt contacts in their X-ray crystal structures. Vibronic-structured emission bands, assignable to triplet intraligand (3IL) [p / p*(bpzb)] excited state mixed with some triplet 3MLCT [dp(Pt) / p*(bpzb)] character, were realized in the solution state. Interestingly, the complex incorporated with a hydrophobic cholesteryl 4-ethynylphenyl carbamate ligand was found to form a stable metallogel in various organic solvent such as n-butanol, DMSO, and cyclohexane with the critical gelation concentration determined to be 7.5, 3.9, and 2.6 mg mL
1, respectively. Such a stable gel was found to be responsive to mechanical sonication, thermal stimuli and to show CD activity.115 Li, Yam and co-workers further developed a platinum(II) molecular hinge in which the motions of the hinge were readily visualized by phosphorescence changes (Fig. 23).116 The molecular hinge was synthesized by the incorporation of a rigid aromatic alkynyl ligand into a cyclometalated platinum(II) moieties to yield a dinuclear cyclometalated platinum(II) complex. The reversible molecular motions of the hinge were found to be modulated by solvent and temperature, showing green and red phosphorescence for the open and closed form of the hinge respectively.116 Upon addition of a poor solvent to the closed form of the hinge, it was found that the reduced solvation provided the driving force for the molecular rotation to the open form via the formation of intermolecular aggregates of the open form by p
p stacking interactions. Interestingly, such a rather loosely packed p-stacks of the open form was found to be disrupted upon heating, leading to a visual phosphorescence color change from green to red, which was attributed to the rotation of the wings or flaps of the hinge back to the closed form, where intramolecular p
p stacking interactions become dominant.116 In 2019, You and co-workers reported the amplification of circularly polarized phosphorescence by the formation of helical coassemblies between an achiral cyclometalated platinum(II) complex and a small fraction of homochiral cyclometalated platinum(II) complex using sergeants-and-soldiers principle.117 Large dissymmetry factors in absorption (gabs) and phosphorescence (glum) of 0.020 and 0.064 respectively were observed from the co-assemblies, in which these values were found to be two orders of magnitude improved when compared to those of individual molecules. The PLQYs of the co-assemblies were also found to be enhanced by a factor of 10. The chiroptical amplification was found to be ascribed to both the magnetically allowed MMLCT state of the phosphorescence and enantiomeric enrichment of the circular polarization via absorption through the asymmetrically coupled MLCT states prior to populating the emitting MMLCT state (Fig. 24).117 Kato and co-workers reported a series of platinum(II) complexes containing N-heterocyclic carbene (NHC) and cyanide ligands and investigated their self-assembly properties.118 All the complexes showed photoluminescence with emission colors of blue, green, yellow and red, in which the emission origin was believed to be originating from triplet 3MMLCT excited state. The solidstate PLQYs of the complexes were determined to be of 0.51 to 0.81. The fine-tuning of the emission color was found to be achieved by the modification of the bulkiness of the NHC ligands, ultimately perturbing the extent of Pt/Pt interactions in the solid state.118 Recently, Yam and co-workers reported the modulation of the apparent color, solubility, luminescence properties, and selfassembly behaviors of a series of dinuclear cyclometalated platinum(II) complexes through the variation of the alkoxy chain lengths in the alkynyl ligand.119 The solid-state luminescence properties were found to be perturbed upon a change in the alkoxy chain length, giving rise to a red triplet 3MMLCT emission and a yellow 3IL emission for the series of complexes.119 Interestingly, the red solid was found to adopt an overlapped packing of the cyclometalated platinum(II) moieties, showing bathochromic shifts upon increasing pressure due to the isotropic compression-induced shortening of the Pt/Pt and p
p distances. On the other

Fig. 23

Structures of the molecular hinge.

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Fig. 24 A schematic representation of the amplification of circularly polarized phosphorescence by the formation of helical co-assemblies using sergeants-and-soldiers principle. Reproduced from Park, G.; Kim, H.; Yang, H.; Park, K. R.; Song, I.; Oh, J. H.; Kim, C.; You, Y. Chem. Sci. 2019, 10, 1294–1301 © Royal Society of Chemistry, 2019.

hand, it was found that there was insignificant shift of the emission band of the yellow solid under high pressure, attributed to the laterally shifted stacks between the cyclometalated platinum(II) planes.119 The same group further developed a series of cyclometalated platinum(II) complexes with benzaldehyde and its derived iminecontaining alkynyl ligands that was capable of showing phosphorescent, chiroptical, and self-assembly behaviors.120 The chiral sense of the enantiomers was found to be transferred from the chiral alkynyl ligands to the cyclometalated platinum(II) dipyridylbenzene chromophore, which was further amplified through supramolecular assembly via intermolecular interactions. The R- and S-enantiomers were found to self-assemble into nanosphere and leaf-like nanostructures respectively.120 Such a formation of distinctive nanostructures was found to be attributed to the tightness in the packing of the aggregates, in which the R-enantiomer was found to assemble loosely, probably due to a larger molecular curvature that led to the formation of spherical aggregates.120

8.14.2.1.4

Discrete multinuclear platinum(II) complexes

Huang, Stang and co-workers reported the synthesis of highly emissive discrete platinum(II) metallacages in 2015 by coordinationdriven self-assembly, in which these discrete supramolecular coordination complexes (SCCs) were found to preserve their emissive behavior at both low and high concentrations.121 The tetragonal prismatic SCCs were self-assembled on mixing a platinum(II) acceptor, [Pt(PEt3)2(OSO2CF3)2], a pyridyl-containing tetraphenylethylene (TPE) as the organic donors, and one of two benzene dicarboxylate species (Fig. 25). The monomeric blue emission was found to be imparted by the rigid SCC ligand frameworks in dilute solutions while the yellow emission was found to be originated from the aggregated species of the discrete SCCs. Interestingly, a white-light emission was observed in THF solution of a SCC which was due to the gradual aggregation of the SCCs and the co-existence of the blue and yellow emission originating from the monomeric and aggregated species respectively.121 In 2017, Zhang, Saha, Yin, Stang and co-workers reported another series of tetragonal prismatic platinum(II) metallacages with tunable emission which was capable of amino acid sensing.122 These metallacages were constructed by eight 90 platinum(II) acceptors as the corners, four linear dipyridyl ligands as the pillars and two TPE-based sodium benzoate ligands as the faces. These cages were found to be luminescent in dilute solutions due to the metal-coordination-induced partial restriction of intramolecular rotation of the TPE units.122 The emission properties were found to be readily tuned by different dipyridyl ligands and variation in solvent composition. One of the metallacages has been employed as a turn-on fluorescent sensor for thiol-containing amino acids through a self-destructive reaction. Such cages were found to be regenerated upon the addition of platinum(II) acceptors.122

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Fig. 25 Synthetic route of the highly emissive discrete platinum(II) metallacages. Reproduced from Yan, X.; Cook, T. R.; Wang, P.; Huang, F.; Stang, P. J. Nat. Chem. 2015, 7, 342–348 © Springer Nature, 2015.

Recently, Wang and Zhang reported the preparation of two tetragonal prismatic platinum(II) metallacages via metal coordination in which the ligands in the faces and pillars were rationally designed with a reverse fluorescence resonance energy transfer (FRET) process.123 The fine-tuning of these luminescent ligands was found to tune the FRET process and thus emission of the cages by altering the efficiency of FRET, rendering the cages responsive to solvents, pressure and temperature. The exploration on the ability to distinguish structurally similar alcohols such as n-butanol, t-butanol, and i-butanol was made.123 The cages were also found to show decrease in emission intensity and bathochromic shifts under high pressure. More importantly, the thermochromic properties of the cages rendered them as potential candidates as fluorescent ratiometric thermometers.123 Huang, Stang and co-workers reported the design and preparation of optically pure pillar[5]arene-based platinum chiral metallacycles as planar chiral platinum triangles by metal coordination using 60 and 90 platinum(II) acceptors.124 The two enantiomeric metallacycles (pS and pR) were found to show planar chirality, showing a negative and a positive Cotton effect by the pS and pR enantiomers respectively as revealed by the mirror image of the CD signals. Interestingly, these metallacycles exhibited CPL at  400 nm, attributed to the emission by the conjugated aromatic ligands. This system represented a new strategy in planar chiralitybased coordination-driven metal-organic complexes for constructing CPL systems.124

8.14.2.2

Palladium(II)

In 2013, Lützen and co-workers reported the preparation of a palladium(II) coordination complex, [Pd2(L)4], where L is a 1,10 binaphthyl-based bis(pyridine) ligand in racemic and enantiomerically pure forms.125 The degree of chiral self-sorting of the complexes was investigated, in which the self-assembly proceeded with a highly selective narcissistic self-recognition manner to give only homochiral metallosupramolecular Pd2L4 cages. The degree of self-sorting has been determined to be 3 using the algorithm of Schmittel and Mahata.125 In 2014, Vilar and co-workers reported the assembly of a palladium(II) metallo-rectangle with a guanosine-substituted terpyridine ligand and the study of the interactions with quadruplex DNA (Fig. 26).126 The palladium(II) complex was successfully characterized by single crystal X-ray diffraction analysis, revealing the [2 þ 2] metallo-rectangle in the solid state. The complex was found to adopt a self-filling rectangular conformation with the pyridine ring of the terpyridine unit involved in a back-to-back intramolecular p
p stacking interaction, with the mean interplanar separation of about 3.80 Å. Further investigation into the ability of this complex to interact with quadruplex and duplex DNA was made by fluorescent intercalator displacement (FID) assays, FRET melting studies, and electrospray ionization (ESI) mass spectrometry. From these studies, this complex was found to interact selectively with human telomeric DNA and G-rich promoter region of c-myc oncogene over duplex DNA.126 In 2016, Fujita and co-workers reported the construction of a spherical structure by self-assembly of 30 palladium ions and 60 bent ligands, in which the structure was found to belong to a shape family that has not been previously observed experimentally at that time (Fig. 27).127 The structure consisted of 8 triangles and 24 squares, and possessed the symmetry of a tetravalent Goldberg polyhedron. They further utilized graph theory to predict the formation of a larger and more stable tetravalent Goldberg polyhedron involving the self-assembly from 144 components (48 palladium ions and 96 bent ligands). This M48L96 polyhedron was found to be the first report of self-assembly using the largest number of components at a molecular level.127 The tetravalent Goldberg

Luminescent supramolecular assemblies

Fig. 26

593

Structure of the palladium(II) metallo-rectangle with a guanosine-substituted terpyridine ligand.

Fig. 27 X-Ray crystal structure of M30L60. Reproduced from Fujita, D.; Ueda, Y.; Sato, S.; Mizuno, N.; Kumasaka, T.; Fujita, M. Nature 2016, 540, 563–566 © Springer Nature, 2016.

polyhedra were rarely observed in nature, contrary to the hexagonal packing in graphite and the trivalent Goldberg polyhedra in fullerenes. The square-planar geometry of the palladium(II) ions was found to be attributed to the formation of a square motif that can induce an unusual packing that enabled the construction of a tetravalent Goldberg polyhedron.127 Almost at the same time, Crowley, Gordon and co-workers reported the synthesis of palladium(II) luminescent cages, [Pd2L4]4þ, from the self-assembly of a tripyridyl-1,2,3-triazole backbone and palladium(II) ions.128 The photophysical studies of these metallosupramolecular cages were reported, in which the emission bands were observed at emission maxima of 525–638 nm. The insignificant changes in the emission maxima and nanosecond-emission lifetime upon metal coordination of the ligand backbone were observed, revealing the contribution of the emission from the ligand scaffolds. Such an insignificant change enabled the tunability of the emission color by the rational modification of the ligand backbone.128 In 2017, Che and co-workers reported the self-assembly studies of a series of luminescent pincer-type palladium(II) isocyanide complexes (Fig. 28).129 The complexes were found to exhibit intermolecular Pd/Pd and ligand–ligand interactions in the aggregated state at room temperature where the emission band at 540 nm was attributed to the 3MMLCT excited state.129 Such a supramolecular polymerization was found to be living and was driven by these non-covalent interactions, as revealed by the time-dependent growth of the low-energy MMLCT absorption and emission bands. The X-ray crystal structure of the palladium(II) complexes with PF6
counter-anion exhibited an infinite PdII–PdII chain with Pd
Pd distances of 3.3–3.4 Å, in which the complexes were stacked in a head-to-tail fashion. Further spectroscopic studies revealed an off-pathway kinetic trap that can be modulated by the change in counter-anion and metal atom. DFT and TD-DFT calculations were performed, supporting the assignment of the MMLCT absorption and emission attributed to the metal–metal interactions.129

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Fig. 28

Structures of palladium(II) isocyanide complexes.

Later, Lu and co-workers reported the synthesis of a series of N-heterocyclic allenylidene-containing cyclometalated palladium(II) complexes in 2018.130 This series of complexes was found to exhibit extended Pd/Pd contacts of 3.3 Å as revealed by the X-ray crystal structure, and featured a low-energy absorption band at 528 nm and emission bands beyond 600 nm in the solution state. These absorption and emission bands of the complexes in toluene solutions were found to be temperature-responsive, showing a red shift upon decrease in temperature, in which the absorption and emission bands were assignable to be originating from MMLCT transition and 3MMLCT excited state, respectively.130 Interestingly, one of the complexes exhibited luminescence with mechanochromic and vapochromic properties at room temperature. Upon grinding or fuming with toluene vapor of the yellow solid, a red color solid was obtained. The yellow solid was found to be restored upon grinding the red solid in methanol.130 Yang and co-workers explored the hierarchical coordination-driven self-assembly of alkynylplatinum(II) complexes upon the coordination of palladium(II) ions to form a metallacage (Fig. 29).131 Such a discrete supramolecular metallacage exhibited solvent-induced emission enhancement with a drastic color change from pale yellow to orange upon addition of water into a DMF solution of the metallacage. The luminescence enhancement was believed to be attributed to the increase in the extent of intermolecular Pt/Pt and p
p stacking interactions.131 Interestingly, the metallacage was found to spontaneously selfassemble into a stable transparent metallogel in DMF–water (1:1, v/v) mixture at room temperature with a heating–cooling cycle. The critical gelation concentration was determined to be 7.5 mg mL
1. Further SEM studies revealed the fibrous structures of the gel.131 Preston, Gordon and Crowley further reported the synthesis of two nonanuclear Pd3Pt6 and Pd9 “Donut”-shaped metallacages by the coordination-driven self-assembly of a low-symmetry 2-(1-(pyridine-4-methyl)-1H-1,2,3-triazol-4-yl)pyridine ligand that possessed both monodentate and bidentate binding sites.132 The heterometallic Pd3Pt6 metallacage was assembled quantitatively in the solution state, while the homometallic Pd9 cage was able to form quantitative assembly only in the solid state upon crystallization. These cages were characterized by NMR spectroscopy, ESI mass spectrometry and X-ray crystallography. The Pd3Pt6 cage was found to serve as a molecular container and a photosensitizer for anthracenyl substrates, featuring the photocatalytic conversion of anthracenyl substrates to form aromatic endoperoxides with a turnover number of  120.132 However, the cage was found to be photolabile and eventually decomposed that led to the photodegradation over time with loss of catalytic activities. Nevertheless, the Pd3Pt6 cage showed catalytic activities in the conversion of anthracenyl substrates to aromatic endoperoxides.132 At the same time, Clever, Lützen and co-workers independently reported a metallosupramolecular Pd6L12 aggregate, constructed from the coordination-driven assembly between a BODIPY-based bis(3-pyridyl) ligand and tetravalent palladium(II) cations.133 The Pd6L12 aggregate was found to form an unprecedented structural motif that resembled a rotaxane-like cage-in-ring arrangement with two distinct conformations adopted by the cages. From the X-ray crystal structure of Pd6L12, it was found that a C-shape Pd2L4 cage was located in the center of a Pd4L8 ring, in which the ligands were found to adopt a W-shape conformation (Fig. 30).133 It was suggested that the Pd6L12 ({[Pd2L4]@Pd4L8}) assembly was not mechanically interlocked and it was stabilized only by the p
p stacking interactions between the peripheral BODIPY moieties and the ligand scaffold, and the van der Waals interactions between the long alkoxy chains.133 The observation of ions corresponding to [Pd2L4] and {[Pd2L4]@Pd4L8} fragments/aggregates, with none of the [Pd4L8] ions, might indicate the requirement of a [Pd2L4] template for the formation of the Pd6L12 aggregates.133 In addition, the absorption properties of the BODIPY chromophores were found to be not significantly altered upon the formation of aggregates by the coordination-driven assembly. On the other hand, the emission characteristics were found to be significantly, but not completely, quenched in the presence of the palladium ions.133 Later in 2019, Clever and co-workers further reported the coordination sphere engineering of pyridine cages, quinoline bowl and heteroleptic pills by coordination-driven assembly of various ligands and palladium(II) ions for the binding of one or two fullerenes.134 The pyridine-containing cage, [Pd2L4]4þ, was found to be highly selective toward the binding of C60, serving as an induced-fit receptor.134 The quinoline-containing assembly, [Pd2L3(MeCN)2]4þ, was found to exhibit an unprecedented bowlshape geometry, in which this bowl not only was found to show a wider guest encapsulation ability including C70 and fullerene derivatives, but also the ability to serve as a supramolecular protecting group for selective monofunctionalization of the fullerene guest, demonstrated through the monoaddition of anthracene.134 Interestingly, dicarboxylates were found to act as the bridging ligands of the bowls for the formation of a pill-shaped dimer upon hierarchical assembly of the bowls, in which the dimer was

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Fig. 29 Self-assembly of alkynylplatinum(II) complexes upon the coordination of palladium(II) ions to form a metallacage. Reproduced from Zhang, Y.; Zhou, Q.-F.; Huo, G.-F.; Yin, G.-Q.; Zhao, X.-L.; Jiang, B.; Tan, H.; Li, X.; Yang, H.-B. Inorg. Chem. 2018, 57, 3516–3520 © American Chemical Society, 2017.

Fig. 30 X-Ray crystal structure of the cationic {[Pd2L4]@Pd4L8} and its schematic representation of the rotaxane-like cage-in-ring structural motif (color code: C: gray (red in [Pd2L4]), N: blue, O: red, B: light-blue, Pd: orange). Reproduced from Käseborn, M.; Holstein, J. J.; Clever, G. H.; Lützen, A. Angew. Chem. Int. Ed. 2018, 57, 12171–12175 © Wiley, 2018.

capable of binding two fullerene guests, which was anticipated to transfer bound fullerene guests and their derivatives into organic solvents, allowing the selective uptake, regioselective modification, derivatization and processing of fullerenes.134 Sun, Bonnet and co-workers reported the self-assembly of a series of cyclometalated palladium(II) photosensitizers which were capable of triggering drug uptake in vitro and in vivo.135 The X-ray crystal structure of the complexes revealed the presence of Pd/Pd interactions with Pd/Pd distances of 3.275–3.353 Å. The complexes were found to be weakly luminescent in water with emission maxima at 509–593 nm, and to self-assemble in aqueous medium, possibly through the formation of Pd/Pd and p
p stacking interactions to form soluble molecular nanorods.135 When compared to a control palladium(II) complex that show no aggregation

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Luminescent supramolecular assemblies

properties, the aggregates of these cyclometalated complexes were found to demonstrate colloidal stability in the presence of serum and enhanced cellular uptake in vitro. In cell culture, such self-assembled nanorods were found to be responsible for the uptake of the complexes through endocytosis, which led to a drastic enhancement of the photodynamic properties under blue light irradiation, demonstrated by the efficient cyclometalated drug uptake by multicellular tumor spheroids and a mice tumor xenograft.135 It was suggested that serum proteins was found to be a major element in the aspect of drug design since they significantly affect the size and bioavailability of supramolecular drug aggregates as well as their efficacy.135

8.14.2.3

Rhodium(I)

Similar to the synthetic strategy in the formation of Magnus green salt, Haukka and co-workers reported the formation of onedimensional metal chains by alternate assembly of anionic, [RhCl2(CO2)]
, and cationic, [Rh(CO)2L]þ (L ¼ 2,20 -bipyridine or 1,10-phenanthroline), rhodium(I) carbonyl complexes in 2013.136 Short Rh/Rh contacts of 3.32 Å and 3.33 Å were observed in the X-ray crystal structure of the two complex salts, suggesting the formation of Rh/Rh interactions in the solid state.136 The existence of such metal
metal interactions was further elucidated through TD-DFT and quantum theory of atoms in molecules (QTAIM) analysis. These computational results revealed the intermolecular charge transfer from the anionic complex to the cationic complex, which contributed to the absorption properties of the complex salts significantly.136 Later in 2014, Mitsumi, Kitagawa, Miyazaki and co-workers reported a comprehensive study of a one-dimensional rhodium(I)semiquinonato carbonyl complex, [Rh(3,6-DBSQ-4,5-(MeO)2)(CO)2]N (3,6-DBSQ-4,5-(MeO)2
¼ 3,6-di-tert-butyl-4,5dimethoxy-1,2-benzosemiquinonato radical anion), demonstrating magnetic and electrical properties.137 The complex was found to stack in a staggered arrangement, resulting in the formation of one-dimensional chains with short Rh/Rh distances of 3.08 and 3.10 Å at 226 K. It was found to exhibit bistable multifunctionality from its magnetic and conductive properties, in which such bistability was believed to be originating from the thermal hysteresis across a first-order phase transition that accompanies an exchange of the interchain C
H/O hydrogen-bond partners between the semiquinonato ligands.137 Moreover, the magnetic properties of the chains were found to be highly dependent on the temperature, in which strongly ferromagnetic interactions at low temperature were found to drastically change to antiferromagnetic with hysteresis upon increasing temperature to room temperature. Interestingly, the electrical conductivity of the complex was determined to be 4.8  10
4 S cm
1, which is relatively high although the rhodium centers were not found to exhibit mixed-valence characters.137 Such a high electrical conductivity was believed to be owing to the reduced energy gap between the filled d orbitals of the rhodium centers and the vacant semiquinonato p* orbitals associated with the formation of Rh/Rh interactions in the one-dimensional chains.137 Besides the formation of one-dimensional chains by the assembly of rhodium(I) carbonyl complexes, Yam and co-workers reported the supramolecular assembly of a series of tetrakis(isocyano)rhodium(I) complexes consisting of various chain lengths in the alkoxy substituents of the isocyanide ligands in 2015.138 The complexes were found to show a strong tendency in exhibiting solution-state assembly associated with the alteration in the extent of intermolecular Rh/Rh interactions upon the variation in concentration, temperature and solvent composition.138 A rainbow array of solution aggregate color was observed upon variation in the solvent composition of the complexes, in which the aggregation behavior was found to be attributed to the formation of Rh/Rh interactions that were believed to be synergistically assisted by van der Waals interactions between the alkoxy chains.138 It was worth noting that a cooperative growth of the assembly was resulted, in which the cooperativity was found to be dependent on the long alkoxy chains in the isocyanide ligands. The synergy in the non-covalent interactions including Rh/Rh, p
p stacking interactions and van der Waals forces was found to be crucial to the cooperative growth.138 Nanoplates and nanovesicles were observed in the aggregates prepared form hexane–dichloromethane mixtures of the complexes, which was believed to be arising from different association mechanisms based on the alkoxyl chain lengths.138 The group of Yam further reported the synthesis and self-assembly properties of a series of tridentate N-donor rhodium(I) complexes in 2016 (Fig. 31).139 These complexes were found to exhibit interesting induced assembly upon application of external stimuli such as temperature, associated with the formation of extensive Rh/Rh and p
p stacking interactions. An isodesmic growth mechanism was determined for the assembly of the complexes as revealed by a sigmoidal curve in the temperaturedependent absorption studies.139 Interesting wire-like nanostructures, in which their shape was found to be dependent on the extent of p-conjugation of the tridentate ligands, were observed from the assembly prepared from acetone solutions of the complexes. More importantly, a crystalline needle that was obtained from recrystallization of the complexes, was found to be

Fig. 31

Structures of tridentate N-donor rhodium(I) complexes.

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electrically conducting with the conductivity determined to be on the order of 10
3 S cm
1.139 Such conducting properties were believed to be attributed to the extensive Rh/Rh interactions with staggered molecular conformations together with the crystalline ordered structure, leading to maximized electronic delocalization.139 Later, the same group reported the versatile control of directed assembly of pincer-type rhodium(I) complexes by subtle modification on the tridentate N-donor ligands in 2018.140 Self-assembly properties with the formation of extensive Rh/Rh interactions were observed. Interestingly, the incorporation of an electron-deficient
CF3 group onto the pincer ligand was found to alter the stacking mode of the complex significantly from head-to-tail to head-to-head stacking.140 It was believed that the p
p stacking of the complex was facilitated, attributed to the reduced electronic repulsion between the complex molecules and the introduction of F/F interactions upon the incorporation of the electron-deficient
CF3 group, ultimately leading to the directed formation of the head-to-head stacked structure.140 An isodesmic growth mechanism was determined for the solvent-induced assembly of the complexes. Low-energy absorption bands corresponding to the formation of dimers, trimers and high-order oligomers upon aggregation was identified, in which the absorption energies were related to the electronic properties of the tridentate N-donor ligands.140 Mochida and co-worker reported the synthesis of multifunctional ionic liquids based on tetrakis(isocyano)rhodium(I) complexes.141 The complexes were found to exhibit thermochromism in the liquid state, in which the color of the complexes changes from orange to blue-purple upon cooling associated with the formation of Rh/Rh interactions (Fig. 32).141 The color of the complexes can also be tuned by oxidative addition reaction with methyl iodide vapor. The complexes were found to exhibit luminescence properties from the dimer in the liquid and glassy states. Emission bands at 717 nm were observed when the temperature was below the glass transition temperature (
85  C) while a slight redshift of the emission band to 724 nm was observed upon increasing temperature to 300 K.141 This example represents the demonstration of multifunctional organometallic ionic liquids with thermochromic, luminescent, chemochromic and reactive properties. Recently, based on the exploration of the one-dimensional chain formed from rhodium(I)-semiquinonato carbonyl complex, Mitsumi, Kitagawa, Miyazaki and co-workers further reported a one-dimensional conductive rhodium(I)-dioxolene complex that exhibited large-amplitude thermal vibration-coupled valence tautomeric transition.142 The complex was found to exhibit drastic changes in properties during a first-order phase transition, originating from the mixed-valence character of the complex at room temperature. In particular, the conductivity was found to be semiconducting in low-temperature phase, while it drastically changed to a relatively high electrical conductivity in the room-temperature phase.142 Interestingly, the temperature-dependent X-ray crystal structures revealed the drastic increase in the mean square displacements of the rhodium atoms along the one-dimensional chain in the room-temperature phase, suggestive of the large-amplitude thermal vibration of the Rh–Rh bonds.142 A possible mechanism based on the thermal electron transfer from the one-dimensional d-band to the semiquinonato p* orbitals, which was coupled with the large-amplitude thermal vibration, was proposed for the appearance of the mixed-valence state at room-temperature.142

8.14.2.4

Gold(III)

Since the first report of Au(III)/Au(III) interactions between discrete gold(III) complexes in the solid state with Au(III)/Au(III) distances of 3.507 and 3.584 Å in the polymeric stacking of tetraazidoaurate(III), [Au(N3)4]
, by the group of Klapötke,143 the groups of Yam and Che independently reported a series of cationic cyclometalated alkynylgold(III) complexes that was found to exhibit Au(III)/Au(III) interactions with the shortest Au(III)/Au(III) separation of 3.495 Å in 2012.144,145a Later, a shorter Au(III)/Au(III) distance of 3.4435 Å was observed in a related neutral alkynylgold(III) complex by Yam and co-workers.145b To date, examples of gold(III) complexes demonstrating Au(III)/Au(III) interactions in the solid state and/or in solution remain scarce, possibly attributed to the contraction of orbital in Au(III) center with relatively high electrophilicity, let alone the utilization of such aurophilic interactions for supramolecular assembly.

Fig. 32 Photographs of the liquid sandwiched between quartz plates showing the thermochromism of a tetrakis(isocyano)rhodium(I) complex. Reproduced from Tominaga, T.; Mochida, T. Chem.-Eur. J. 2018, 24, 6239–6247 © Wiley, 2018.

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In 2019, Che and co-workers reported a series of cyclometalated cationic Au(III) complexes with different auxiliary ligands, of which the Au(III)-allenylidene complex was found to exhibit close intermolecular Au(III)/Au(III) contact of 3.367 Å in the X-ray crystal structure.146 Weak attractive Au(III)/Au(III) interactions were revealed in the Au(III)-isocyanide and Au(III)-allenylidene complexes by DFT/TD-DFT studies.146 Moreover, the Au(III)-isocyanide and the Au(III)-acetylene complexes were found to undergo kinetically controlled self-assembly, in which the emissive excited states of the Au(III) aggregates were believed to be originated from a mixture of major triplet intraligand 3[p
p*] and minor triplet ligand-to-metal–metal charge transfer (3LMMCT) excited states as revealed by a combination of spectroscopic and computational studies.146 At the same time, Yam and co-workers independently reported the supramolecular assembly of a series of luminescent cationic cyclometalated gold(III) amphiphiles.147 Positively charged trimethylammonium (
CH2NMe3þ) containing alkynyl ligands have been incorporated to introduce the electrostatic interactions. The X-ray crystal structure of two of the complexes revealed the contribution of p
p stacking interactions with an interplanar distance of 3.31 Å.147 Interestingly, this class of amphiphiles was found to exhibit self-assembly behavior in solution, which was assisted by the formation of p
p stacking interactions, van der Waals forces between the alkoxy chains, and possibly weak Au(III)/Au(III) interactions as revealed by the emergence of low-energy absorption and emission bands upon cooling.147 The presence of weak Au(III)/Au(III) interactions in solution was also corroborated with the computational studies of the dimer of the model complexes, in which the noncovalent interactions (NCI) analysis revealed an isosurface of the non-covalent interactions between the two coordination planes and between the phenyl rings on the pincer ligand, suggestive of the presence of p
p stacking and weak Au(III)/Au(III) interactions.147 Two distinctive nanostructures were prepared from the assembly of this class of gold(III) complexes where nanofibers were observed in polar solvent, while nanorods were formed in nonpolar solvent medium.147 It was believed that the incorporation of long, hydrophobic alkoxy chains in the pincer ligand was important to the formation of aggregates in polar solvent where van der Waals forces play a crucial role in governing the intermolecular interactions.147

8.14.3

Luminescent supramolecular assemblies of d10 metal complexes

Apart from the platinum group metals, luminescent supramolecular assemblies of coinage metals with d10 electronic configurations, namely those of copper(I), silver(I) and gold(I), have attracted much attention over the years. One major reason is that these metal centers tend to exhibit metallophilic interactions1,52,148–151 which can drive and facilitate the formation of self-assemblies.

8.14.3.1 8.14.3.1.1

Copper(I) Copper(I) clusters

Half a century after the pioneering studies of the emissive copper(I) phosphanes and arsanes by Dori and co-workers,152 luminescent copper(I) supramolecular assemblies have continued to attract research interests over time. One major class of luminescent copper(I) clusters is the tetranuclear clusters that feature a Cu4I4 cubane core. The first example of copper(I) cubanes, namely [Cu4I4(py)4] (py ¼ pyridine), was reported in 1972153 and the corresponding photophysical studies were reported since the 1980s.154–156 Similar to the work of Vogler in using saturated amines like in [Au4Cl4(piperidine)4] to elucidate the spectroscopic properties of polynuclear Au(I) complexes,157 Maini, Fattori and co-workers prepared derivatives of [Cu4I4(py)4] by replacing pyridine with saturated cyclic amines including piperidine, N-methyl-piperidine, quinuclidine and 3quinuclidinol (Fig. 33) so as to exclude the effect of the ligand’s p system on the photophysical properties on the clusters.158 The powders of the clusters were found to emit at 540–570 nm from a triplet cluster centered (3CC) excited state at room temperature. The clusters exhibited PLQYs of 0.30–0.51, with the exception of [Cu4I4(piperidine)4] which showed a higher PLQY value of 0.76. The crystal structure of [Cu4I4(piperidine)4] showed a columnar structure in which adjacent cubanes were held together by N
H/I interactions with a short H
I distance of 2.935(3) Å.158 The extensive hydrogen bonding interactions would increase the rigidity of the cluster and render non-radiative decays less favorable, thus enhancing the PLQY of [Cu4I4(piperidine)4]. Instead of nitrogen-donor ligands, phosphane ligands have also been used for the synthesis of copper(I) clusters, as demonstrated by the seminal works of White and co-workers on [Cu4X4(PPh3)4] (X ¼ Cl, Br or I)159 and related [Cu2X2(PPh3)3] clusters by Hardt and co-workers.160 More recently, Perruchas and co-workers introduced various P(C6H4-R)3 phosphane ligands to prepare a series of luminescent copper(I) cubanes (Fig. 34).161 Single crystal X-ray diffraction studies revealed that all of the clusters exhibited the typical cubane structure with the copper and iodine atoms occupying the corners of a cube, and the phosphorus atoms of the phosphanes coordinating to copper. The mean Cu(I)/Cu(I) distance was found to be the longest (3.126(1) Å) in [Cu4I4(P(C6H4-OCH3)3)4] in the presence of electron-donating
OCH3 substituents. On the other hand, the presence of electron-withdrawing
CF3 substituents would give rise to a shorter mean Cu(I)/Cu(I) distance of 2.9530(9) Å.161 When the crystalline powders of the clusters were illuminated under UV light, the clusters containing electron-donating
CH3 and
OCH3 groups were found to emit in the green (515 nm) and yellow (558 nm) regions, respectively, which are typical emission colors of Cu4I4 cubanes. On the other hand, the introduction of electron-withdrawing
CF3 groups would lead to a cyan-blue emission, which was derived from an admixture of a more intense band at 440 nm and a weaker band at 525 nm. When the temperature was gradually lowered to 8 K, a higher-energy band would appear at 415–420 nm for the clusters with
CH3 and
OCH3. On the other hand, the lower-energy band at 525 nm of the CF3-containing cluster would completely disappear, leaving only the higher-energy

Luminescent supramolecular assemblies

Fig. 33

599

Structures of the [Cu4I4L4] clusters.

emission at low temperatures. On the basis of DFT calculations, such a thermochromic phenomenon was assigned to a thermal equilibrium between 3(X,M)LCT and 3CC excited states.161 In a related study, the cluster [Cu4I4(P(m-Tol)3)4] was allowed to aggregate in an oil-in-water microemulsion and aggregationinduced emission (AIE) was observed.162 Because of the high solubility of [Cu4I4(P(m-Tol)3)4] in organic solvents such as chloroform, the organic phase could be readily mixed with an aqueous phase that contained sodium dodecyl benzene sulfonate (SDBS) to prepare microemulsion droplets with clusters confined and aggregated inside.162 Subsequent vacuum treatment led to demulsification and the isolation of [Cu4I4(P(m-Tol)3)4] nanoparticles which exhibited AIE properties. Upon complete vacuum evaporation, the PLQY of the nanoparticles was found to be 19% and the intensity of the emission continued to increase with time. Because of the small particle size, the nanoparticles could be employed as inks for painting on parchment papers. Later on, luminescent ink based on another copper(I) iodide complex was reported by the same group.163 In this case, polyvinylpyrrolidone was used as a high-affinity polymer to stabilize the copper(I) iodide cluster units and give rise to luminescent inks with improved dispersibility. The reaction of CuI with alkyl-tris(2-pyridyl)phosphonium halides, [R-Ppy3]HaI (R ¼ Me, Pr, Bu, Bn), has led to the preparation of a series of luminescent copper(I) compounds.164 In the presence of different substituents on the R-Ppy3 ligand, the structures of the copper(I) complexes were found to vary from an ionic compound (R ¼ Me) to clusters with a stepwise (R ¼ Pr) or linear

Fig. 34

Structures of the copper(I) cubanes with various phosphane ligands.

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Luminescent supramolecular assemblies

(R ¼ Bu or Bn) Cu4I6 core (Fig. 35). Despite the different structures, all of the copper(I) complexes were found to exhibit red phosphorescence at 623–660 nm in the solid state at ambient conditions. The red emission of the complexes was assigned as a thermally activated delayed fluorescence (TADF) emission originating from the 1(M þ X)LCT excited state mixed with 3CC character.164 Using bis-phosphane complexes which contained pyridylethyl groups at phosphorus atoms and phenyl or tolyl substituents at nitrogen atoms, white-light-emitting hexanuclear copper(I) clusters were prepared by Strelnik and co-workers in 2019.165 The single crystal structure of the hexanuclear cluster with tolyl groups, which was recrystallized from acetonitrile, revealed that the complex was composed of two Cu3I3 clusters linked together by two bis-phosphane ligands (Fig. 36), and the shortest Cu(I)/Cu(I) distances observed were approximately 2.5 Å. According to PXRD and thermogravimetric analysis-differential scanning calorimetry (TGA-DSC) data, the loss of acetonitrile solvent molecules would result in the transformation of the crystal to another phase with some modification of crystal packing.165 On the other hand, crystals of the hexanuclear Cu(I) clusters were observed to show blue luminescence when immersed in acetonitrile, which gradually turned white upon exposure to air along with the crystal phase transformation. Based on quantum chemical computations, it was suggested that the white emission was a combination of a higherenergy band at 465–466 nm, which originated from a 3MLCT state mixed with 3XLCT character, and a lower-energy 3CC emission at 610–615 nm.165 While tetranuclear Cu4I4-containing compounds constitute a large class of luminescent copper(I) iodide clusters, highernuclearity molecular copper(I) iodide compounds are relatively rare. A heptanuclear cluster, [Cu7I7(P(C6H4CF3)3)6(CH3CN)], was prepared as a white crystalline powder by Perruchas and co-workers in 2018.166 As shown in Fig. 37, the Cu7I7 core structure was surrounded by six phosphane ligands and an acetonitrile solvent molecule. Depending on the coordination environment, the Cu/Cu distances in the cluster ranged from 2.8684(18) to 5.750(1) Å. Upon photoexcitation, the cluster was found to phosphoresce at 470 and 477 nm respectively at 298 and 77 K. Based on DFT calculations, the phosphorescence was assigned as a 3XLCT/ MLCT origin.166 In addition to photoluminescence, the same group has extended their work to the study of the mechanoluminescence of copper(I) clusters.167 In a recent example, the cluster [Cu4I4(PPh2-iPr)4] with isopropyl-substituted diphenylphosphane ligands was reported to show a contrasted off-on effect upon mechanical stimulation.168 Although [Cu4I4(PPh2-iPr)4] exhibited a typical cubane geometry, the Cu(I)/Cu(I) distances of 3.411(2)
3.438(2) Å were among the longest reported in the literature and hence cuprophilic interaction was negligible in the ground state.168 When the white crystalline powder of [Cu4I4(PPh2-iPr)4] was ground mechanically, the weak green emission at 540 nm of the copper(I) cluster was observed to turn bright yellow (590 nm). This red shift in emission was accompanied by an increase in the PLQY from < 1% to 4%. If the powder was exposed to acetonitrile, the mechanochromism could be reversed and the emission would diminish. From PXRD analysis and Rietveld pattern matching, it was confirmed that there was no long-range structural change nor crystal-to-amorphous phase transition during the

Fig. 35

Structures of the copper(I) clusters.

Luminescent supramolecular assemblies

Fig. 36

Structures of the copper(I) clusters.

Fig. 37

Structure of [Cu7I7(P(C6H4CF3)3)6(CH3CN)].

601

mechanochromic process. Instead, mechanical grinding was suggested to cause a local deformation of the molecular structure of the copper(I) cluster, which in turn led to a change in the luminescence properties.168 The unusual mechanochromic behavior could be attributed to the presence of long Cu(I)/Cu(I) distances in the cluster, which would destabilize the typical emissive triplet excited states. Upon grinding, the mechanical forces would result in the shortening of the Cu(I)/Cu(I) distances and thus gave rise to the recovery of photoluminescence properties.168 Despite the rich luminescence properties of copper(I) halide clusters, their practical applications as emitters in organic lightemitting diodes (OLEDs) have been limited by their low tendency to sublime at high temperatures and relatively low stability in solutions.169 To tackle this issue, Thompson and co-workers developed a co-deposition technique for the in situ generation of copper(I) clusters from copper(I) iodide and various N-donor ligands during the preparation of thin films.170 In one example, the group utilized 3-(carbazol-9-yl)-5-((3-carbazol-9-yl)phenyl)pyridine (CPPyC) as both the ligand and host matrix to prepare co-deposited thin films that were composed of copper(I) clusters.171 A phosphorescent copper(I) cluster with a Cu2I2 core as shown in Fig. 38 was suggested to be formed in situ during the co-deposition process. When the molar ratio of CuI:CPPyC in the thin films was 1:10, an emission band at 532 nm with quantitative PLQY was observed, and such a high PLQY was associated to the efficient energy transfer between CPPyC and the copper(I) cluster.171 Using these co-deposited films, green-emitting OLED devices were fabricated and a maximum EQE of 15.7% has been obtained.171 In another study, Xie, Xu and co-workers prepared a phosphane-chelated Cu4I4 cluster, [DBFDP]2Cu4I4 (Fig. 39; DBFDP ¼ 2,9di(diphenylphosphane)-dibenzofuran) and investigated its use in the fabrication of OLEDs by spin-coating.172 While the assynthesized copper(I) cluster was non-emissive in nature, it became emissive at 492 nm upon mechanical grinding. The cluster was suggested to be dual emissive in nature and the predominant radiative pathway for its room-temperature luminescence was TADF.172 Using [DBFDP]2Cu4I4 as a dopant, white-emitting OLEDs with a maximum EQE of 0.73% were fabricated. The white emission was derived from the combination of the blue TADF emission and the red luminescence of the cluster from the 3CC excited state.172 Subsequently work of the group involved the introduction of additional carbazole and 3,6-di-tert-butylcarbazole moieties as substituents of the DBFDP ligand to suppress the 3CC excited state, giving rise to sky-blue-emitting OLEDs with improved EQE values of up to 7.9%.173 Copper(I) self-assemblies other than those of iodides are equally attractive in terms of their photoluminescence properties. Bräse and co-workers developed a strategy to prepare luminescent copper(I) complexes based on a series of bidentate P^N ligands derived from 2-di-phenylphosphanopyridine and related compounds.174–177 Using the bidentate P^N ligands, a series of homoleptic and

602

Luminescent supramolecular assemblies

Fig. 38

Proposed structure of the copper(I) cluster formed in situ from CuI and CPPyC.

Fig. 39

Structures of the copper(I) clusters used for the fabrication of OLEDs.

heteroleptic dinuclear copper(I) halides has been prepared.178 These dinuclear copper(I) complexes were found to be luminescent in the solid state at ambient conditions, exhibiting an emission band of XLCT/MLCT origin. The emission maxima of the complexes could be readily tuned across the visible region by changing the N-heterocycle or the phosphane moiety of the P^N ligand.179–181 These complexes were demonstrated to be useful as light-emitting materials in OLEDs. In a representative example of solutionprocessed OLEDs fabricated with the heteroleptic complex as shown in Fig. 40, a maximum current efficiency of 9 cd A
1 was obtained at a turn-on voltage of 4.1 V.174 In 2016, Kato and co-workers reported a series of luminescent thiolato-bridged copper(I) clusters, namely [Cu2(P^S)2(PPh3)2], [Cu4(P^S)4(CH3CN)2] and [Cu6(P^S)6] (Fig. 41).182 The emission colors showed a red shift with an increase in the number of copper centers in the cluster, with the emission maxima ranging from 482 nm (blue-green) for the dinuclear complex to 526 (green) and 553 nm (yellow) in the tetranuclear and hexanuclear complexes, respectively, in the solid state at room temperature.182 While the emission maxima of [Cu2(P^S)2(PPh3)2] and [Cu4(P^S)4(CH3CN)2] were almost independent of temperature, the luminescence lifetimes were found to increase with decreasing temperature. Using the two-state model, the room-temperature emission of the dinuclear and tetranuclear complexes were suggested to originate from the TADF of thermally equilibrated 1CT and 3CT excited states. On the other hand, [Cu6(P^S)6] showed a red shift in the emission to the orange region at 78 K, and this thermochromic luminescence was assigned to a 3CC state.182 Following a strategy for the synthesis of copper(I) nanoclusters first reported by Mak and co-workers,183 the groups of Feng, Wang and Sun prepared a tetradecanuclear copper(I) alkynyl cluster, [Cu14(tBuC^C)10(CH3COO)4], via the comproportionation reaction between copper(II) acetate and copper powder.184 Extensive cuprophilic interactions were observed within the tetradecanuclear cluster, as shown by short Cu(I)/Cu(I) distances of 2.439(9)–2.800(9) Å in the crystal structure.184 Upon photoexcitation, the cluster was found to emit at 580 nm in the solid state. The origin of this yellow emission was assigned as a mixture of triplet

Luminescent supramolecular assemblies

Fig. 40

Structures of the copper(I) cluster with a bidentate P^N ligand.

Fig. 41

Structures of the copper(I) clusters.

603

ligand-to-metal charge transfer (3LMCT) and metal-centered d-s excited states with perturbation by cuprophilic interactions,184 similar to those of copper(I) alkynyls studied earlier by Yam and co-workers.49,185–190 In 2019, van Zyl and co-workers reported the synthesis of a series of copper(I) dithiophosphonate clusters, [Cu4L4] (L ¼ S2PR(OR0 )
).191 The clusters featured a Cu4 core at the center with relatively short Cu(I)/Cu(I) distances of 2.7545(10)– 2.7600(10) Å between the copper(I) centers that were bridged by sulfur, whereas the Cu(I)/Cu(I) distances of non-bridged copper(I) centers were of 2.9042(18)–2.9353(14) Å.191 The presence of different substituents on the dithiophosphonate ligands showed negligible influence on the photophysical properties of the tetranuclear copper(I) clusters. All of the clusters were observed to exhibit yellow-green emission at 519–534 nm, which was assigned to a MLCT origin with perturbation by Cu(I)/Cu(I) interactions.191 Thiolate ligands have also been used to prepare luminescent copper(I) clusters with reversible chiral and achiral self-assemblies. Dong, Zang and co-workers reported the preparation of the luminescent anionic cluster, [Cu5(StBu)6]
, and used it to prepare a racemic mixture of chiral crystals with achiral [K(CH3OH)2(18-crown-6)]þ cations.192 The chiral complex was found to emit at about 680 nm in the solid state at room temperature, originating from a mixture of 3LMCT (S / Cu) and 3CC (3d / 4p) excited

604

Luminescent supramolecular assemblies

states.192 Although Cu(I)/Cu(I) interactions were absent, extensive O
H/S hydrogen bonds were observed between the H of the methanol units in the cations and the S atoms of the anionic clusters to stabilize the chiral self-assembled structure. When the crystals were slowly heated to release the methanol molecules, disordered stacking of the anionic units and relative displacement of [K(18-crown-6)]þ would be resulted and a mirror plane would be created, destroying the chirality of the system.192 The structural rearrangement was accompanied by a reduction in the intensity of the red emission at about 680 nm, and the changes were reversible if the achiral sample was treated with methanol. Using the chiral amino alcohols D/L-valinol, homochiral crystals of [D/L-valinol(18-crown-6)]þ[Cu5(StBu)6]
(Fig. 42) could be obtained. In addition to red luminescence at about 660 nm, the enantiomers were observed to show both CD and CPL activities. The same group later reported the use of a pair of chiral alkynyl ligands, (R/S)-2diphenyl-2-hydroxylmethylpyrrolidine-1-propyne (R/S-DPM), for the synthesis of copper(I) alkynyl clusters, namely [Cu14(R/SDPM)8](PF6)6.193 Dichloromethane solutions of the copper(I) alkynyl clusters were found to be non-emissive but showed CD signals at 300–460 nm. When increasing fraction of n-hexane was used in the solvent, the solution was found to show photoluminescence with stronger intensity, reaching a peak quantum yield of 1.6% in the presence of 90% n-hexane. The change in emission intensity was attributed to the solvent-induced aggregation in higher n-hexane content.193 On the other hand, the crystals of the (R)enantiomer were found to emit at about 726 nm at ambient temperature and showed CPL response. The red emission was assigned to originate from an LMCT excited state mixed with a metal-centered nd9 (n þ 1)s1 state.193 Related work on luminescent copper(I) clusters bearing phosphane ligands has also been reported by the same group.194 Vogt and co-workers reported a tetranuclear cationic cluster, [Cu4(PCP)3]þ (Fig. 43; PCP ¼ 2,6-(PPh2)2C6H3), which exhibited intense narrow-band green emission with a high PLQY.195 The cationic cluster featured a small, rigid Cu4 core coordinated by three bulky tridentate PCP ligands. It was remarkable that the shortest Cu(I)/Cu(I) distance in the Cu4 core was 2.316(1) Å, which was among the shortest ones reported in the literature.195 In the presence of [BArF4]
(ArF ¼ 3,5-(CF3)2C6H3) counteranions, crystals of the copper(I) complex exhibited green emission at 518 nm at room temperature with a PLQY of 0.50. Temperature-resolved emission studies revealed that the intensity of the photoluminescence decreased gradually when the temperature was increased from 11 K to 440 K, along with a concomitant blue-shift of the peak maxima.195 Such emission was assigned to be a mixture of prompt photoluminescence and TADF. The complex was used as a dopant in the emissive layer to fabricate green-emitting OLEDs by solution processing, and a maximum EQE of 11.2% was obtained at 42 cd m
2.195 Recently, a series of hexanuclear copper(I) clusters were found to exhibit luminescence properties that were regulated by cuprophilic interactions.196 Using different N,S-donor ligands, discrete hexanuclear copper(I) clusters with the formula [Cu6L6] (L ¼ BTT (benzo[d]thiazole-2-thiolate), BPT (5-bromopyridine-2-thiolate) or TPT (5-(trifluoromethyl)pyridine-2-thiolate)) were prepared. While [Cu6(BTT)6] was reported to exhibit NIR phosphorescence in an earlier report,197 the crystals of [Cu6(BPT)6] and [Cu6(TPT)6] were found to exhibit bright rosy red and red emission, respectively at room temperature.196 The emission bands of [Cu6(BPT)6] and [Cu6(TPT)6] were both assigned to a 3CC origin which involved a combination of LMCT (S / Cu) and metal-centered (3d / 4s) characters. Remarkably, the emission was found to be independent of the excitation wavelength and the excitation spectra of both complexes spanned from 250 nm to 600 nm for [Cu6(BPT)6] and 500 nm for [Cu6(TPT)6]. Hence, a blue LED could be used to excite the complexes to generate white emission.196

Fig. 42 Crystal structures of [D/L-valinol(18-crown-6)]þ[Cu5(StBu)6]
. Insets show the luminescence of the single crystals upon photoexcitation at 365 nm. Color code: brown, Cu; yellow, S; green, H; gray, C; red, O; blue, N. Reproduced from Jin, Y.; Li, S.; Han, Z.; Yan, B.-J.; Li, H.-Y.; Dong, X.Y.; Zang, S.-Q. Angew. Chem. Int. Ed. 2019, 58, 12143–12148 © Wiley, 2019.

Luminescent supramolecular assemblies

Fig. 43

605

Structure of [Cu4(PCP)3]þ.

8.14.3.1.2

Copper(I) metallacycles

By using the pre-assembled bimetallic complex, [Cu2(m2-dppm)2(CH3CN)4](PF6)2, as a molecular clip to react with CuCN, Costuas, Yam, Lescop and co-workers prepared a highly luminescent tetranuclear metallacycle, [Cu4(m2-dppm)4(CN)2](PF6)2, with an emission quantum yield of 72% in the solid state at room temperature.198,199 Temperature-dependent emission measurements revealed that the tetranuclear complex exhibited TADF with red shift in its emission maximum from 457 nm at 298 K to 486 nm at 80 K. Remarkably, the tetranuclear metallacycle was structurally rigid and displayed no observable change in its shape, dimension and intermetallic distances upon a change in temperature.198 The same group later used this tetranuclear metallacycle as a preorganized adaptive supramolecular precursor and prepared a series of one-dimensional helical coordination polymer and discrete Cu8M (M ¼ Ni, Pd, Pt) circular heterobimetallic assemblies by reacting the metallacycle with KCN or K2[M(CN)4] (Fig. 44).200 The structures of the one-dimensional helical coordination polymer and Cu8Pd assembly are depicted in Fig. 45. These supramolecular assemblies were found to exhibit solid-state emission properties that are different from that of tetranuclear [Cu4(m2dppm)4(CN)2](PF6)2. The one-dimensional helix was found to exhibit a metal-perturbed dppm intraligand phosphorescence at 538 nm at room temperature.200 Upon a decrease in temperature to 77 K, the emission maximum was blue-shifted to 468 nm, which could be assigned to originate from a MLCT [ds*(Cu) / p*(CN)/p*(dppm)] state with mixing of a copper-centered d-s/ d-p excited state modified by weak Cu/Cu interactions.200 On the other hand, the heterobimetallic Cu8Ni assembly only exhibited very weak emission whereas the Cu8Pd and Cu8Pt assemblies emitted in the turquoise region at 500–510 nm at 298 K and showed red-shifted emission at low temperature. While the emission of the Cu8Pd assembly could be assigned to TADF, it was difficult to confirm the same assignment for the Cu8Pt derivative as the possibility of ISC to give phosphorescence could not be excluded.200 Subsequently, bis(diphenylphosphanomethyl)phenylphosphane (dpmp) was used as a bridging ligand to obtain pre-assembled

Fig. 44 (a) Molecular structure of the dicationic copper(I) precursor A. (b) Syntheses of the one-dimensional helical coordination polymer C and discrete Cu8M (M ¼ Ni, Pd, Pt) circular heterobimetallic assemblies DM. Reproduced from Evariste, S.; Khalil, A. M.; Moussa, M. E.; Chan, A. K.-W.; Hong, E. Y.-H.; Wong, H.-L.; Le Guennic, B.; Calvez, G.; Costuas, K.; Yam, V. W.-W.; Lescop, C. J. Am. Chem. Soc. 2018, 140, 12521–12526 © American Chemical Society, 2018.

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Luminescent supramolecular assemblies

Fig. 45 (a, b) Different views of the molecular X-ray structure of the bimetallic repetition unit of the one-dimensional helical coordination polymer C. (c) Molecular X-ray structure of the Cu8Pd circular heterobimetallic assembly DPd. Reproduced from Evariste, S.; Khalil, A. M.; Moussa, M. E.; Chan, A. K. -W.; Hong, E. Y. -H.; Wong, H. -L.; Le Guennic, B.; Calvez, G.; Costuas, K.; Yam, V. W. -W.; Lescop, C. J. Am. Chem. Soc. 2018, 140, 12521–12526 © American Chemical Society, 2018.

[Cu3(m3-dpmp)2] fragments and a related Cu11 cluster with TADF properties was successfully prepared.201 Recent works by the same group have also been extended to luminescent heteronuclear Au2Cu10 and Pt4Cu11 clusters.202 The luminescence properties of trinuclear copper(I) pyrazolate metallacycles have been well documented in the literature.6 Very recently, Giménez, Elduque and co-workers introduced long alkoxy chains into the system to prepare a series of trinuclear copper(I) complexes with liquid crystalline properties.203 The complexes were found to exhibit hexagonal columnar mesophases with a stacking distance of 3.6 Å along the column, involving weak Cu(I)/Cu(I) interactions. The liquid crystalline complexes were found to emit at 661–664 nm in neat thin films at room temperature, which was assigned as excimeric metal-centered phosphorescence. As an extension of their earlier work in 2003 on phosphorescent trinuclear copper(I) pyrazolato complexes,204 Dias and coworkers reported a series of trinuclear coinage metal pyrazolates with fluoro alkyl substituents, namely {[3,5-(3,5-(CF3)2Ph)2Pz] M}3.205 The presence of bulky aryl groups in the metallacycles gave rise to the formation of twisted M3N6 cores which hindered inter-trimer metallophilic interactions in the complexes. The copper metallacycle, {[3,5-(3,5-(CF3)2Ph)2Pz]Cu}3 (Fig. 46), was only found to be emissive at low temperature. Upon photoexcitation at 315 nm, {[3,5-(3,5-(CF3)2Ph)2Pz]Cu}3 was found to exhibit light blue ligand-centered emission at 460 nm with a large Stokes shift of about 8200 cm
1.205 Similar light blue emission was shown by the silver(I) derivative at 77 K whereas the gold(I) derivative was also emissive at room temperature.

8.14.3.2

Silver(I)

Compared to copper(I), the luminescence properties of silver(I) are relatively less investigated mainly because of the light sensitivity and thermal instability of silver. In the following, the recent advances in the photophysical studies of silver(I) complexes will be discussed. Luminescent coordination polymers and metal-organic frameworks (MOFs) of silver will not be discussed in view of the scope of this chapter.

Fig. 46

Structure of {[3,5-(3,5-(CF3)2Ph)2Pz]Cu}.

Luminescent supramolecular assemblies 8.14.3.2.1

607

Silver(I) clusters

In 2014, Liu and co-workers reported a luminescent dodecanuclear silver(I) cluster, [Ag12(m12-I)(m3-I)4{S2P(CH2CH2Ph)2}6](I), which featured a rare example for a halide ion to coordinate to 12 metal centers at the same time.206 The crystal structure of the cluster revealed a Ag12 cuboctahedron encapsulating the m12-I at the center (Fig. 47). The shortest Ag(I)/Ag(I) distances were 3.111(2) Å, which could be found at the edges of the trigonal faces of the cuboctahedron. Upon photoirradiation by UV light, the dodecanuclear cluster showed a strong yellow emission with an emission maximum at 533 nm, both in dichloromethane solution and in the solid state. A small red shift was observed when the temperature was reduced to 77 K. The emission was suggested to be of charge-transfer character on the basis of TD-DFT calculations. Steffen, Finze and co-workers obtained a series of hepta- and octanuclear silver(I) clusters by reacting the precursor silver(I) ethynylcarba-closo-dodecaborate complex, {Ag2(12-C^C-closo-1-CB11H11)}n with pyridine, 4-methylpyridine, 4-tert-butylpyridine and 3,5-lutidine. A similar reaction using 4-trifluoromethylpyridine was found to yield either a hexadecanuclear cluster at room temperature, or a tetradecanuclear derivative at
30  C instead.207 All of the newly-formed clusters were found to be photoluminescent under ambient conditions. In the presence of different pyridine ligands, the silver(I) clusters were found to exhibit identical emission in crystals and in the powder form, with the emission peaks ranging from 511 nm to 640 nm across the visible spectrum.207 The emission of most of the complexes were in the microsecond timescale, and could be assigned to originate from a 3CC excited state modified by argentophilic interactions, although the possibility of 3LMMCT and 3MMLCT origins could not be eliminated. On the other hand, the octanuclear cluster with 4-tert-butylpyridine was found to show fluorescence at 521 nm and this emission was dependent on the excitation wavelength. Such exceptional emission behavior was associated to the disorder of the silver core in the structure.207 To investigate the effect of ligands on the luminescence properties, Torrens and co-workers used a combination of diphosphane and fluorophenylthiolate ligands to prepare a series of dinuclear silver(I) complexes (Fig. 48).208 All of the complexes exhibited dimeric structures with bridging sulfur atoms from the thiolate ligands. The dinuclear complexes were found to show either an anti or syn arrangement of the flurophenyl moieties in the crystal structures, accompanied by different extents of argentophilic interactions with Ag(I)/Ag(I) distances ranging from 3.0709(5) to 3.4356(6) Å. However, the photoluminescence of the complexes were mainly affected by the nature of the diphosphane ligands. In the presence of 1,2-bis(diphenylphosphane)

Fig. 47 (a) Structure of the m12-I-centered cuboctahedral Ag12 skeleton in [Ag12(m12-I)(m3-I)4{S2P(CH2CH2Ph)2}6](I). (b) Molecular structure of [Ag12(m12-I)(m3-I)4{S2P(CH2CH2Ph)2}6]þ with alkyl groups omitted for clarity. Reproduced from Liao, J.-H.; Latouche, C.; Li, B.; Kahlal, S.; Saillard, J.Y.; Liu, C. W. Inorg. Chem. 2014, 53, 2260–2267 © American Chemical Society, 2014.

Fig. 48

Structure of the dinuclear silver(I) complexes.

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benzene (dppBz), the complexes were found to emit at 481–491 nm in the solid state at ambient conditions.208 When 1,2-cis-bis(diphenylphosphane)ethylene (dppE) was used as the ligand, the emission was red-shifted to 514–523 nm. The origin of the emission bands were suggested to be either LMCT or MLCT states modified by the thiolate ligands, and there was no involvement of argentophilic interactions in the emission. In another study, Sun and co-workers reported a series of C3-symmetric nonanuclear silver(I) clusters with thiolate and phosphane ligands.209 One of the clusters, [Ag9(tBuC6H4S)6(dpph)3(CF3SO3)] (dpph ¼ 1,6bis(diphenylphosphano)hexane), was found to exhibit yellow emission at 584 nm at ambient conditions. The yellow luminescence was assigned to be originated from a S2
(3p) / Ag(5s) LMCT excited state, or mixed with metal-centered character. Upon cooling to 80 K, the emission peak was observed to shift to 564 nm, probably due to the reduced non-radiative decays caused by the restricted rotation of the peripheral phenyl and the tert-butyl moieties at low temperatures.209 Apart from alkyl- and arylthiolates, metal complexes with silanethiolato ligands are relatively rare due to their low stability in air caused by the susceptibility of silicon-sulfur bonds to hydrolysis. Dołe˛ ga and co-workers overcame the issue and successfully prepared a series of silver(I) silanethiolates by attaching aryloxy substituents to silicon.210 One of the silver(I) complexes (Fig. 49) was found to feature a trinuclear silver core with intramolecular Ag(I)/Ag(I) interactions, accompanied by short intermetallic distances of 2.9006(3) to 3.1336(3) Å. This complex was shown to be emissive at 513.8 nm upon photoexcitation at 305 nm in the solid state under ambient conditions. The emission of the trinuclear silver(I) complex showed a red shift when compared to that of the uncoordinated silanethiolato ligand, and was assigned to be originated from LMCT/MLCT transitions modified by argentophilic interactions.210 Hosseini and co-worker prepared a series of multivalent ligands based on calix[4]arene and thiacalix[4]arene backbones in 1,3alternate conformation, and utilized these ligands to prepare a series of dinuclear silver(I) complexes.211 Remarkably, the calix[4] arene ligand with ortho-amino-methylpyridine units was found to yield a cationic tetranuclear silver(I) complex in the presence of excess Ag(I) ions (Fig. 50). The tetranuclear complex was found to exhibit argentophilic interactions with short Ag(I)/Ag(I) contacts of 2.8928(7) Å in the presence of BF4
anions and 2.8775(5) Å in the presence of NO3
anions, respectively. Upon photoexcitation at 295 nm, the tetranuclear complex showed an emission peak at 425 nm, which was suggested to be a result of the argentophilic interactions. On the other hand, the related dinuclear complexes were non-emissive in nature. Besides, Chujo and co-workers utilized 4,7,12,15-tetrasubstituted [2.2]paracyclophane as a chiral template to prepare enantiopure dimeric silver(I) complexes.212 In dilute dichloromethane solutions at room temperature, the silver(I) complexes were found to show bathochromic shifts in both the UV–vis absorption and emission spectra when compared to the ligands, mainly because of the increase in planarity of the compounds upon metal coordination.212 CD and CPL spectroscopies were employed to study the chiroptical properties of the complexes in dichloromethane solutions. Both the silver(I) complexes and the uncoordinated ligands were observed to show mirror-image Cotton effects in the CD and CPL spectra. When an extended phenylene-ethylene ligand was used, the CD signal of the silver(I) complex was greatly enhanced and the CPL signal disappeared. These observations were suggested to be associated to the predicted formation of higher-ordered structures of either a zigzag or a double-helical shape upon coordination of the extended ligand to silver.212 In 2019, Shamsieva and co-workers utilized pyridine-substituted phospholanes as bidentate P^N ligands to prepare a series of luminescent dinuclear silver(I) complexes (Fig. 51).213 According to the X-ray crystal structures, the complexes exhibit short Ag(I)/Ag(I) distances of 2.7854(8)–2.9917(7) Å. In acetonitrile solutions, the complexes were found to exhibit weak blue

Fig. 49

Structure of the trinuclear silver(I) complex.

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Fig. 50 Structure of the tetranuclear silver(I) complexes formed by combination of the ligand with AgBF4 (top) and AgNO3 (bottom), respectively. Reproduced from Noamane, M. H.; Ferlay, S.; Abidi, R.; Kyritsakas, N.; Hosseini, M. W. Eur. J. Inorg. Chem. 2017, 3327–3336 © Wiley, 2017.

luminescence at 378–416 nm at room temperature upon photoexcitation. This emission was assigned to originate from an excited state of mixed 3MLCT and 3ILCT character.213 Remarkably, the complexes were found to show an unusual triplet NIR emission at 765–902 nm in the solid state at room temperature, which is in sharp contrast to the blue to green luminescence of many dinuclear Ag(I) complexes with other ligands. Although the origin of the NIR emission was not completely clear, it was suggested that redshifted solid-state emission was mainly caused by the dense crystal packing of the Ag(I) complex with a tight placement of the cationic dinuclear Ag(I) part and the tetrafluoroborate anions.213

Fig. 51

Structure of the dinuclear silver(I) complexes.

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Luminescent supramolecular assemblies

In another study, silver(I) p-toluenesulfonate was allowed to react with 2,20 -(1,4-butanediyl)bis-1,3-benzoxazole (BBO) to generate a dinuclear complex, [Ag2(BBO)2(p-toluenesulfonate)2] (Fig. 52).214 The silver(I) complex exhibited a distorted tetrahedral structure with each of the silver(I) center respectively coordinated to two N atoms of the BBO ligands and two O atoms of the p-toluenesulfonate ligands. There was no considerable Ag(I)/Ag(I) interactions in the complex due to the relatively long separation between the two metal centers (3.808 Å). The dinuclear complex was found to emit at 428 nm upon photoexcitation at 350 nm at room temperature, and this emission was assigned to a LMCT origin.214 On the other hand, compounds with different chemical structures and photophysical properties were obtained when other silver salts were used as the starting materials. The use of silver picrate and silver o-coumarate would give rise to one-dimensional coordination polymers, whereas the use of silver hexafluorophosphate would give a two-dimensional coordination polymer. In addition, it was found that the presence of these anions would lead to partial quenching of the intraligand emission of the coordination polymers due to the electron-withdrawing effect of the anions.214 In 2020, Zhou, Bi and co-workers reported a large, discrete silver(I) thiolate cluster that contained 31 Ag(I) centers.215 This Ag31 cluster was reported to exhibit a thermally and chemically stable turtle-like structure, in which the Ag9S9 turtle belly and the Ag20S11 turtle back were connected by two capped Ag(I) centers (Fig. 53), with Ag(I)/Ag(I) contacts ranging from 2.855(3) to 3.3427(9)

Fig. 52 (a) Molecular structure of [Ag2(BBO)2(p-toluenesulfonate)2]. (b) Coordination structure of the central silver ion in the complex. (c) Argentophilic interaction and double-ring structure of the complex. Reproduced from Wu, H.; Xia, L.; Qu, Y.; Zhao, K.; Wang, C.; Wu, Y. Appl. Organomet. Chem. 2020, 34, e5297 © Wiley, 2019.

Fig. 53 Molecular structure of the turtle-like silver(I) cluster viewed perpendicular to the (a) “turtle back” and (b) “turtle belly”. Reproduced from Feng, Y.-H.; Lin, Z.-S.; Liu, S.-Q.; Shi, J.-F.; Zhou, K.; Ji, J.-Y.; Bi, Y.-F. New J. Chem. 2020, 44, 663–667 © Royal Society of Chemistry, 2019.

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Å.215 Upon excitation at 365 nm, the cluster was found to emit at 631 and 648 nm, respectively, at room temperature and 77 K. In addition, the maximum emission intensity of the cluster was observed to increase linearity with decreasing temperature, suggesting that the cluster could potentially be used as a temperature probe. Dong, Zang and co-workers prepared a hexanuclear silver(I) cluster, [Ag6(dpppy)2(CF3COO)6] (Fig. 54; dpppy ¼ 2,6-bisdiphenylphosphanyl-pyridine).216 In this cluster, the dpppy ligands contained both electron-donating phosphorus atoms and electron-accepting aromatic moieties. At room temperature, the hexanuclear cluster was found to show bright blue luminescence at room temperature with a PLQY of up to 22%.216 Based on DFT and TD-DFT calculations, the emission was speculated to be originated from an interligand transmetallic charge-transfer (ITCT, O/Ag / dpppy) excited state.216 Upon grinding, the emission of the cluster in the solid state would change from blue (453 nm) to green (493 nm) with a negligible decrease in the PLQY, as the cluster changed from the crystal phase to the less-ordered amorphous phase with a higher extent of p–p stacking.216 Such changes were reversible when a small amount of dichloromethane was added to the ground sample. Another example of blue-emitting silver(I) clusters was reported by Chen and co-workers. The group used 2diphenylphosphanepyridine (dppy) and dicyanamide (N(CN)
2 ) as bridging ligands to synthesize the silver(I) complex, {Ag2(dppy)2[N(CN)2]}2[m-N(CN)2]2.217 From the single crystal structure, it was observed that the Ag(I)/Ag(I) distance across the bridging dppy ligand was 3.1088(3) Å, whereas the Ag(I)/Ag(I) distance across the N(CN)2 bridge was 8.957 Å.217 Upon photoirradiation by UV light, the silver(I) cluster was found to emit at 454 nm with a PLQY of 13.4% in the solid state at room temperature, and the emission was tentatively assigned as originated from a 3MLCT (Ag2 / dppy) state with perturbation by Ag(I)/Ag(I) interactions.217 A related coordination polymer, {Ag2(dppy)2[m-C(CN)3]2}n, was synthesized when tricyanomethanide (C(CN)
3 ) was 217 used in place of N(CN)
2 as the bridging ligand, and it exhibited similar blue emission at 441 nm. Very recently, a luminescent silver(I) cluster, namely [Ag9(PBZ)8(indole-2,3-diide)2](CF3SO3)5 (PBZ ¼ 2-(pyridine-2-yl)-1Hbenzo[d]imidazole), was synthesized by Zhao and co-workers via the reaction of o-ethynylaniline and PBZ with an excess of silver triflate.218 The crystal structure of the cluster revealed two twisted Ag5 rings that shared the same vertex with Ag(I)/Ag(I) distances of 2.729(1)–2.906(1) Å. Upon photoexcitation, the dilute methanol solution of the cluster emitted at 366 nm, which could be assigned as the intraligand p–p* fluorescence of the PBZ ligand.218 When increasing amount of water was added to a concentrated methanol solution of the complex, a new phosphorescence band was observed at 569 nm, which was assigned to originate from a Ag-to-PBZ 3MLCT state. Such a change from fluorescence to phosphorescence with increasing water content was caused by the aggregation of the cluster. At the same time, the heavy atom effect of the silver centers would promote the ISC between singlet and triplet states.218 Interestingly, when the o-ethynylaniline was replaced by a chiral diamino derivative, an infinite chiral chain based on the Ag9 cluster cores could be formed via the cross-linking the pendent amino groups of the diide ligands through an additional bridging silver atom. This one-dimensional coordination polymer was also found to emit at 558 nm from 3MLCT states.218

8.14.3.2.2

Silver(I) metallacycles

Trinuclear silver(I) metallacycles, in particular those of silver(I) pyrazolates, have been extensively reported in the literature as a class of luminescent materials.1,52,219 In 2013, Dias and co-workers introduced a heavier halo substituent at the 4-position of the pyrazole moiety and prepared the trinuclear silver(I) complex, {[4-X-3,5-(CF3)2Pz]Ag}3 (X ¼ Cl or Br), and its copper(I) analog.220 Unlike the unsubstituted {[3,5(CF3)2Pz]Ag}3 which exhibited short inter-trimer metal
metal distances, {[4-X-3,5-(CF3)2Pz]Ag}3 complexes do not form extended structures with short Ag/Ag contacts. However, relatively short Ag/Cl and Ag/Br distances were observed in the complexes. As illustrated in the molecular packing diagram of {[4-Cl-3,5-(CF3)2Pz]Ag}3 in Fig. 55, each Ag3 core was in close

Fig. 54 Structure of [Ag6(dpppy)2(CF3COO)6] (color legend: green, Ag; cyan, F; gray, C; blue, N; purple, P; red, O). Reproduced from Yang, J. -S.; Zhang, M.-M.; Han, Z.; Li, H.-Y.; Li, L.-K.; Dong, X.-Y.; Zang, S.-Q.; Mak, T. C. W. Chem. Commun. 2020, 56, 2451–2454 © Royal Society of Chemistry, 2020.

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Fig. 55 Molecular structure (top) of {[4-Cl-3,5-(CF3)2Pz]Ag}3 and a view showing the intertrimer Ag.Cl contacts in its molecular packing (bottom). Reproduced from Hettiarachchi, C. V.; Rawashdeh-Omary, M. A.; Korir, D.; Kohistani, J.; Yousufuddin, M.; Dias, H. V. R. Inorg. Chem. 2013, 52, 13576–13583 © American Chemical Society, 2013.

proximity to two Cl atoms in the neighboring molecules to form a distorted trigonal bipyramidal arrangement of Cl2Ag3 units.220 The bromo derivative was observed to show a similar packing arrangement. Also unlike the unsubstituted analog which only emit at low temperatures, the crystalline solid samples of both {[4-X-3,5-(CF3)2Pz]Ag} complexes exhibited weak blue luminescence at 450–480 nm under both ambient and low-temperature conditions.220 The blue luminescence could be assigned to an intraligand origin based on the emission measurements of the respective metal-free pyrazole ligands. Related copper(I) analogs that showed visible luminescence color change in the presence of vapors of volatile organic compounds have also been reported.220 On the basis of trinuclear silver(I) pyrazolates, a hexanuclear silver(I) trigonal prismatic cage was synthesized by Yang, Raptis and co-workers using a bis-pyrazole ligand that contained a 2,7-naphthalene spacer.221 As revealed from the crystal structure in Fig. 56, the hexanuclear molecular cage featured two trinuclear silver(I) pyrazolate metallacycles as the faces of the trigonal prismatic structure, and three naphthalene spacers at the periphery. The inter-trimer Ag(I)/Ag(I) distance was 7.651 Å, whereas the intra-trimer distances were in the range of 3.387(2)–3.528(2) Å, indicative of the absence of argentophilic interactions.221 The silver(I) cage could be used as a host to bind cyclohexane to give an adduct that emitted at about 390 nm at room temperature in the solid state. On the other hand, the molecular cage demonstrated stronger binding affinity to cyclooctasulfur (S8) over cyclohexane and other hydrocarbons. A 1:1 binding ratio toward S8 was confirmed by elemental analysis and X-ray diffraction studies. The host
guest interaction with S8 was suggested to occur via weak Ag(I)/S interactions, since the Ag(I)/S distances (3.226(4)– 3.501(5) Å) in the adduct were observed to be shorter than the sum of the van der Waals’ radii of silver and sulfur atoms (3.52 Å).221 A copper(I) molecular cage with similar dimensions and structure was prepared, but it showed less well-defined binding with S8.221

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Fig. 56 Molecular structure of the silver(I) molecular cage binding a S8 molecule. Reproduced from Duan, P.-C.; Wang, Z.-Y.; Chen, J.-H.; Yang, G.; Raptis, R. G. Dalton Trans. 2013, 42, 14951–14954 © Royal Society of Chemistry, 2013.

Although phosphane-containing silver complexes have been commonly studied,222 relevant work on phosphane-containing silver(I) pyrazolate metallacyles are relatively rare. Tunik, Shubina and co-workers prepared a trinuclear silver(I) complex and its copper(I) derivative by reacting the cyclic precursors, {[3,5-(CF3)2Pz]M}3 (M ¼ Ag, Cu), with bis(diphenylphosphano)methane (dppm), in benzene or toluene.223 As shown in the molecular structure in Fig. 57, each of the P atoms in dppm was coordinated to a silver(I) center while the third silver(I) center did not coordinate to the dppm ligand, leading to a large distortion of the planarity of the trinuclear macrocycle. Upon photoexcitation, the 1,2-dichloroethane solution of the trinuclear silver(I) complex exhibited a weakly-structured emission band at 362 nm from a singlet ligand-centered (1LC) origin.223 At low temperature, dual emission was observed at 341 nm and 505 nm. The higher-energy emission was assigned to a 1LC origin, whereas the lower energy emission band displayed a large Stokes shift and was assigned to be originated from a triplet MLCT state.223 In the solid state at room temperature, the complex showed a broad emission band at 430 nm with a shoulder at 530 nm, which could be similarly assigned to the 1LC and 3MLCT states.223 Such emission properties were found to be very different from the parent {[3,5(CF3)2Pz]Ag}3 complex, in which the planar structure promoted the formation of exciplex and in turn affected the emission behavior. Excimeric emissions could not be observed in the {[3,5-(CF3)2Pz]Ag}3(dppm) complex since the Ag3 core became non-planar and the dppm ligands also prevented the stacking of the molecules in the solid state. On the other hand, the copper(I) derivative, {[3,5-(CF3)2Pz]Cu}3(dppm), was observed to show TADF behavior attributed to the relatively small energy gap (1080 cm
1) of the singlet and triplet MLCT states.223 Using the bis(dicyclohexylphosphano)methane (dcmp) ligand which is spectroscopically silent in UV–vis absorption spectroscopy, the team of Riehn and Thiel prepared a trinuclear silver(I) hydride complex, [Ag3(m3-H)(m2-dcpm)3](PF6)2, in which the almost equilateral triangular Ag3H core was stabilized by three m2-bridged phosphane ligands at the periphery (Fig. 58).224 The average Ag(I)/Ag(I) distance was 3.082 Å, showing that argentophilic interactions exist within the trinuclar complex. Upon

Fig. 57 Molecular structures of the silver(I) complex. Reproduced from Titov, A. A.; Filippov, O. A.; Smol’yakov, A. F.; Godovikov, I. A.; Shakirova, J. R.; Tunik, S. P.; Podkorytov, I. S.; Shubina, E. S. Inorg. Chem. 2019, 58, 8645–8656 © American Chemical Society, 2019.

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Fig. 58

Structure of [Ag3(m3-H)(m2-dcpm)3](PF6)2.

photoexcitation, the trinuclear complex showed an emission peak at about 490 nm in acetonitrile solution, whereas two emission peaks were observed at around 480 nm and 347 nm in methanol solution. Together with the large Stokes shifts in each solvent, the dual emission was associated to the complex and solvent-dependent Frank-Condon state deactivation processes of the molecule, possibly involving the formation of exciplexes.224 By inserting 1,10-phenanthroline and its derivatives into trinuclear silver(I) {[3,5-(CF3)2Pz]Ag}3 metallacycles, Shubina and coworkers reported a series of luminescent adducts.225 When the bulkiness of the diimine ligand increased from 1,10-phenanthroline to 2,9-dimethyl-1,10-phenanthroline followed by 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline, the initial Ag3Pz3 core was found to be disrupted to different extents and gave rise to the formation to different structures. All of the adducts were emissive at 450–550 nm region in the solid state at room temperature, showing phosphorescence derived from the ligand centered excited states of the respective diimine ligands.

8.14.3.3 8.14.3.3.1

Gold(I) Low-nuclearity gold(I) complexes

There has been extensive research interest in the study of closed-shell aurophilic interactions over the past decades since the pioneering reports by the group of Schmidbaur.148,226–228 Aurophilic interactions have widely been reported to be associated with the selfassembly of gold(I) complexes, despite recent debate on the exact nature of these metallophilic interactions.229 In 2013, Ito, Yagai and co-workers reported a dinuclear gold(I) isocyanido complex that was functionalized with cholesterol groups (Fig. 59).230 Upon the irradiation of UV light, the powder of the complex exhibited vibronically structured emission at 427 and 454 nm, which was assigned as p–p* phosphorescence. When this blue-emitting powder was ground, the emission color would change to green, corresponding to a broad emission at 515 nm. Aurophilic interactions were suggested to be involved in this red-shifted emission. Based on PXRD studies, it was suggested that the blue-emitting powder showed considerable molecular tilt and interdigitation of the cholesterol groups between adjacent layers.230 On the other hand, the ground powder was amorphous in nature and did not show any defined peaks in its PXRD pattern. The change in emission color from blue to green via mechanical grinding could hence be associated with a semicrystalline to amorphous transition. It was further observed that the semicrystalline phase with belt-like morphologies could be prepared by vapor diffusion of acetone, a solvent that could solvate both the aromatic and cholesterol units, into a dichloromethane solution of the complex. When a polar solvent such as methanol was used instead of acetone, strong aggregation of the cholesterol moieties would be induced to give the amorphous phase as globular agglomerates. Because of the aggregation, the organometallic core of the complex could no longer be very orderly packed and this would favor intermolecular aurophilic interactions. Thus, the green emission of the amorphous phase could be assigned as originated from an excited state associated with aurophilic interaction of the complex.230 As an extension of their work on mechanochromic gold(I) isocyanides,231,232 Ito and co-workers introduced a biphenyl moiety to synthesize a dinuclear gold(I) bis-isocyanido complex.233 The dihedral angles between the phenyl rings in the biphenyl moiety would lead to restricted freedom in rotation and hence would allow the structural variation of the gold(I) complex in the crystalline phases. By introducing various solvents into the as-prepared non-solvated gold(I) bis-isocyanido complex, a series of 11 solvated crystal structures with distinct crystalline phases and emission colors has been obtained.233 In the molecular packing, the gold(I) bis-isocyanide molecules arranged in dimers. These dimers further interacted with each other through Au(I)/Au(I) (3.480(2)– 3.649(3) Å) and p–p interactions (3.248–3.421 Å) to form an infinite columnar motif, with the exception of the cholorformsolvated complex which showed isolated dimers.233 While the solid-state emission color of the complex varied from 490 to 580 nm in the presence of different solvents, there was no clear correlation between the Au(I)/Au(I) or p–p stacking distances with the emission maxima, probably because the photophysical properties were governed by multiple structural factors.

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(a)

(b)

Fig. 59 (a) Structure of the gold(I) isocyanido complex with cholesterol groups; (b) Schematic representation of solvent-assisted self-organization of the complex into distinct microstructures. Reproduced from Kawaguchi, K.; Seki, T.; Karatsu, T.; Kitamura, A.; Ito, H.; Yagai, S. Chem. Commun. 2013, 49, 11391–11393 © Royal Society of Chemistry, 2013.

On the other hand, Naota and co-workers prepared an ionic gold(I) bis-N-heterocyclic carbene (bis-NHC) complex with bis(trifluoromethanesulfonyl)imide (NTf2
) as the anion (Fig. 60).234 The as-synthesized complex was a solid with weak orange emission, which was derived from a mixture of monomeric and excimeric emission of the complex at 380 and 600 nm, respectively. When the complex was subjected to a pinpoint stimulus by a needle, intense violet-blue emission would appear at 420 nm. It was suggested that the as-synthesized complex, which was prepared by cooling the molten liquid of the complex to 90  C, was a metastable solid. Such a change in the emission properties was specific to the presence of NTf2
anions, as the flexibility of these anions would give rise to a much lower melting point of the complex when compared to the use of other counter-anions. Based on the crystal structure of the same cationic gold(I) complex with BPh
4 as the anion, the metastable solid was proposed to align as loosely-packed dimers with weak p–p stacking interactions.234 Upon stimulation by a needle, the metastable solid would spontaneously convert to a thermodynamically stable state that would emit in the violet-blue region. The crystal structure of the stimulated solid revealed that the gold(I) cationic moieties changed to align as

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Fig. 60

Structure of the gold(I) bis-NHC complex.

trimers with an (ABA)-type staggered conformation with short intra-trimer aurophilic interactions of 3.29 Å.234 The trimers would in turn align to form infinite arrays without any significant aurophilic interactions. Hence, the intense violet-blue luminescence was associated with the emission from trimers with modifications by aurophilic interactions. Trinuclear metallacycles represent another class of gold(I) complexes with interesting photophysical properties. Apart from pyrazolate ligands that have been well-documented in the literature,235 recent studies of trinuclear gold(I) metallacycles have been extended to the use of other classes of ligands. In 2016, Fackler and co-workers reported the luminescent trinuclear gold(I) clusters, [Au3f2y] and [Au3fy2] based on formamidinate (f) and ylide (y) ligands (Fig. 61).236 The intramolecular Au(I)/Au(I) distances ranged from 3.0543(4) to 3.900(2) Å in the complexes. Upon photoexcitation by UV light, the complexes were observed to show bright green phosphorescence in the solid state at room temperature and in 2-methyltetrahydrofuran at 77 K. This emission was suggested to be LMCT emission influenced by the heavy metal center, probably with some association with the formamidinate ligands.236 In spite of the luminescence properties of [Au3f2y] and [Au3fy2], the related cyclic trinuclear complex [Au3y3] with three ylides was non-emissive in nature. Muñoz-Castro and co-workers prepared a ligand-supported hexanuclear gold(I) complex using a bulky N-phosphanylfunctionalized NHC as a tridentate ligand (Fig. 62).237 The hexanuclear complex featured a trinuclear Au(I) metallacycle that was surrounded by three peripheral Au(I) centers, with Au(I)/Au(I) distances of 3.444–3.545 Å in the central Au3 core and 3.043–3.130 Å in the peripheral Au2 units. The hexanuclear complex showed an absorption peak at 283 nm which was associated with the singlet metal-centered [5ds* / 6ps] transition. Upon photoexcitation at 300 nm, the complex showed dual emission with a green fluorescence band at 512 nm and a red phosphorescence band at 694 nm, both attributed to an excited state of [ps / ds*] character.237 Such dual emission was suggested to be caused by the slow ISC in the complex, which in turn resulted in the incomplete equilibrium of the singlet and triplet excited states at ambient conditions. In a related study, Wang and co-workers reported a nonanuclear gold(I) cluster, [Au9(PNC)6](BF4)3 (PNC ¼ 2diphenylphosphanopyridyl monoanion), in which six of the nine gold(I) centers arranged like a trigonal prism with two trinuclear Au3(PNC)3 metallacycles on the top and bottom faces as shown in Fig. 63a and b.238 The two Au3(PNC)3 units were planar and parallel to each other, with short Au(I)/Au(I) distances of 3.022(2)–3.329(2) Å within each trinuclear unit.238 The nonanuclear cluster exhibited intense green emission at 497–499 nm, both in dichloromethane solution and in the solid state at ambient temperature. When [Au9(PNC)6](BF4)3 was allowed to react with excess [(Ph3P)Au]BF4, a larger cluster, namely [Au11(PNC)6(PPh3)2](BF4)5 (Fig. 63c), would be obtained. Unsupported aurophilic interactions were observed between the Au3(PNC)3 and Ph3PAuþ units in the Au11 cluster, as revealed by a short Au(I)/Au(I) distance of 2.738(2) Å.238 Upon photoexcitation, the Au11 cluster was found to emit strongly at 607 nm in the solid state, and at 578 nm in dichloromethane solution at ambient temperature. Based on DFT calculations, the emission origin of [Au9(PNC)6](BF4)3 was proposed. The emission of [Au11(PNC)6(PPh3)2](BF4)5 was suggested to be originated from a PPh3-to-gold LMCT excited state.238 When these gold(I) clusters were introduced into living cells, selective imaging of mitochondria could be achieved.

Fig. 61 Crystal structures of (a) [Au3f2y] and (b) [Au3fy2] with displacement ellipsoids at 30% probability and hydrogen atoms omitted. Reproduced from Melgarejo, D. Y.; Chiarella, G. M.; Fackler, J. P., Jr. Inorg. Chem. 2016, 55, 11883–11889 © American Chemical Society, 2016.

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Fig. 62

617

Structure of the hexanuclear gold(I) complex.

8.14.3.3.2

Gold(I) clusters

Other than the self-assembly of discrete low-nuclearity gold(I) complexes, the availability of aurophilic interactions has facilitated the self-assembly of gold(I) centers to yield polynuclear clusters, many of which have been demonstrated to show remarkable photoluminescence properties. It should be noted that the following discussion will be focused on luminescent molecular clusters of gold(I), whereas atomically precise nanoclusters will be out of the scope of this chapter. Yam and co-workers reported a series of gold(I) sulfido complexes with alkyl chains of different lengths on the aminophosphane ligands, [Au10{Ph2PN(CnH2n þ 1)PPh2}4(m3-S)4](ClO4)2 (n ¼ 8, 12, 14, 18).239 At the center of the structure as shown in Fig. 64, there were two interstitial gold(I) atoms with the S4 symmetry axis passing through them, and the Au(I)/Au(I) distances ranged from 2.956(11) to 3.329(12) Å.239 In dichloromethane solutions, the complexes showed dual luminescence in the green and orange-red regions. The higher-energy emission was assigned as the metal-perturbed intraligand phosphorescence. On the other hand, the lower-energy emission was assigned to originate either from a triplet LMCT and/or metal-centered excited state modified by aurophilic interaction, or from a LMMCT excited state.239 Remarkably, when the acetone stock solutions of the gold(I) complexes with long alkyl chains (n ¼ 12, 14, 18) were injected into pure water and methanol-water mixtures with increasing methanol content, the emission colors were observed to change from green to yellow to red. The changes in emission was associated with the self-assembly of the gold(I) complexes to form nanoaggregates in non-solvents. When the methanol content was increased, a morphological transformation took place in which the shape of the nanoaggregates changed from spheres to cubes as observed by SEM and TEM.239 The group subsequently extended this work to the preparation of Au10 clusters with alkylpyridine pendant groups (Fig. 64; R ¼ CH2Py, CH2CH2Py).240 Interestingly, the cluster with CH2Py pendants was observed to show three pseudopolymorphs that respectively showed blue, green and red luminescence, even though the different forms were revealed to be identical to each other by 1H NMR spectroscopy and high-resolution electrospray ionization (HR-ESI) mass spectrometry. The single crystal structures of the blue and red forms have been determined, with the blue form showing a more orderly packed structure. Similar to the aforementioned Au10 clusters with alkyl pendants, these pseudopolymorphs exhibited dual emission. The difference in the relative ratios of the high- and low-energy emission bands hence contributed to the variation in luminescence of the three forms of the clusters.240 On the other hand, when the longer CH2CH2Py pendants were present, only a single green-emitting form could be observed. This was probably because of the involvement of the additional CH2 unit in the molecular packing, although the possibility of other mechanisms could not be completely excluded.240 In a related work, the same group systematically reduced the steric effect of the same type of bis(diphenylphosphano)amine ligands by changing the substituents (R ¼ CH3CH2, CH3, H).241 In this case, only the ligand with ethyl substituents could yield decanuclear gold(I) clusters. On the contrary, a mixture of decanuclear and dodecanuclear gold(I) clusters was obtained in the presence of methyl substituents, whereas only a dodecanuclear gold(I) cluster was obtained when the substituent was hydrogen.241 At 77 K, these clusters exhibited dual emission in the solid state, with a green emission originated from metal-perturbed intraligand phosphorescence and an orange-red emission attributed to a triplet LMCT excited state modified by aurophilic interactions.241 In 2014, Koshevoy, Tunik, Chou and co-workers reported a series of hexanuclear gold(I) alkynyl clusters, [Au6(C^CR)4(PR0 2-XPR0 2)2]2þ (Fig. 65 top).242 These clusters consisted of two dialkynylgold [Au(C^CR)2]
rods that were connected by [Au2(diphosphane)]2 þ moieties via aurophilic interactions and Au-C^C p-coordination. These gold(I) clusters were found to emit intensely in the solid state at 482–572 nm with PLQYs of 0.61 to about 1.0. In particular, the cluster with the ligand PCy2(CH2)2-PCy2 (R ¼ diphenylmethanolyl) was found to give two polymorphs with contrasting yellow (572 nm) and sky blue (482 nm) emission signals. In addition, a related octanuclear cluster, [Au8(C^CC13H11O)6(PPh2C4H8PPh2)2]2þ, was reported to exhibit intense emission at 586 nm.242 According to DFT calculations, the T1 excited state of this class of gold(I) clusters was mainly dominated by Au and p*(C^C) characters. ISC from the S1 state to the T1 state also occurred readily due to heavy atom

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Luminescent supramolecular assemblies

Fig. 63 (a) Top view and (b) side view of [Au9(PNC)6]3þ in [Au9(PNC)6](BF4)3. (c) Side view of [Au11(PNC)6(PPh3)2]5þ in [Au11(PNC)6(PPh3)2](BF4)5 (color legend: orange, Au; purple, P; blue, N; gray, C). Hydrogen atoms are omitted for clarity. Reproduced from Lei, Z.; Zhang, J.-Y.; Guan, Z.-J.; Wang, Q.-M. Chem. Commun. 2017, 53, 10902–10905 © Royal Society of Chemistry, 2017.

effect. As a result, the radioactive decay from T1 to S0 was enabled and contributed to the intense luminescence of the system.242 The same group later collaborated and reported a series of tetranuclear gold(I) complexes with various alkynyl and thiolate ligands, [Au4(P^P^P)2(C^CR)2]2þ and [Au4(P^P^P)2(SPh)2]2þ (P^P^P ¼ PPh2CH2PPhCH2PPh2) (Fig. 65 bottom).243 In these tetranuclear gold(I) complexes, the alkynyl ligands were only bonded to the gold(I) centers by s bonds in h1-mode. Relatively short intramolecular Au(I)/Au(I) distances of 3.04034(17)
3.2325(3) Å were observed.243 The tetranuclear gold(I) clusters emitted at 443– 615 nm in the solid state at room temperature. In particular, the cluster [Au4(P^P^P)2(C^CC6H4NMe2)2]2þ showed a relatively high PLQY of 0.51 and was hence used as a dopant in the emissive layer for the fabrication of OLEDs. A device with CIE coordinates of (0.52, 0.46) and an EQE of 3.1% has been obtained.243 On the other hand, the cis-trans isomerization of 1,2-bis(diphenylphosphano)ethene (dppee) in the presence of photoirradiation was demonstrated to convert a decanuclear gold(I) sulfido cluster, [Au10(m-cis-dppee)4(m3-S)4]2þ, to an octadecanuclear cluster, [Au18(m-trans-dppee)6(m3-S)8]2þ.244 This photoisomerization process could be completed in a few minutes and was accompanied by a change in the color of the solution from bright yellow to orange-red. In the electronic absorption spectra, the Au10 cluster

Luminescent supramolecular assemblies

Fig. 64

Structure of [Au10{Ph2PN(R)PPh2}4(m3-S)4]2þ [R¼CnH2n þ 1, CH2Py, CH2CH2Py].

Fig. 65

Structure of the gold(I) alkynyl clusters.

619

exhibited low-energy absorption shoulders at 230, 365 and 425 nm whereas the Au18 showed red-shifted absorption shoulders at about 380 and 465 nm. These absorption shoulders were tentatively assigned to the LMCT (S / Au) transitions modified by aurophilic interactions.244 Upon photoisomerization, an increase in the absorption shoulder at 450–550 nm region was observed and the overall red shift was probably associated with the narrowing of the HOMO-LUMO energy gap caused by the increase in nuclearity from Au10 to Au18.244 The emission maximum of the cluster also showed a similar red shift from about 630 nm to about 645 nm upon the transformation from Au10 to Au18.244 In 2015, a tetranuclear gold(I) cluster was synthesized by Veige and co-workers from [(R3P)Au(C^CC6H5)] and [(R3P)Au(N3)] (R ¼ Et, PhMe2) (Fig. 66 left) via azide-acetylide cycloaddition and subsequent dimerization through Au(I)/Au(I) interactions.245 Both clusters were found to form crystals with similar metric parameters, in which the Au4 core appeared as a distorted tetrahedron with C2 symmetry. Short Au(I)/Au(I) distances of 3.0334(4)–3.1318(4) Å were observed.245 Although the complex could be dissociated in DMSO to give an equilibrium of tetranuclear and dinuclear units, the tetranuclear structure remained intact in chloroform solution and this suggested that the aurophilic interactions were adequately strong to hold the structure in place. A similar synthesis was performed with a gold(I) diacetylide precursor with non-bulky PEt3 groups, such as [(Et3P)Au]2-(C^CC6H4C^C), such that the cycloaddition could take place at both ends to form polymeric and oligomeric derivatives (Fig. 66 right).245 Both the tetranuclear cluster and the oligomeric complex with PEt3 groups were found to be emissive in dichloromethane solution at room temperature. The tetranuclear cluster exhibited an emission maximum at 324 nm from an excited state associated with aurophilic interactions. The oligomer also emitted at 324 nm in dichloromethane solution, which was indicative of the presence of similar Au4 units in the oligomeric structure.245

620

Luminescent supramolecular assemblies

Fig. 66

Structure of the gold(I) clusters.

Using both bridging diethyldithiocarbamate (Et2dtc) and diphosphane/triphosphane ligands, Chen and co-workers prepared a series of polynuclear gold(I) complexes with different extents of Au(I)/Au(I) interactions.246 The gold(I) complexes with 1,4(bisdiphenylphosphano)naphthalene and 9,10-(bisdiphenylphosphano)anthracene ligands, in particular, formed tetranuclear structures with distorted linear structures (Fig. 67 left and middle), in which the gold(I) centers were connected through aurophilic interactions supported by Et2dtc ligands. All of the gold(I) complexes were emissive in the solid state, but only the complex with 9,10-(bisdiphenylphosphano)anthracene could emit in the solution state at ambient temperature. The gold(I) complexes were found to emit at 528–543 nm in the solid state at room temperature, and this emission could either be assigned to a metalcentered (5d/6 s / 6p) or a S / Au LMCT excited state.246 In another study by Deák and co-workers, a trinuclear gold(I) complex with two bridging xanthene-containing phosphane ligands (Fig. 67 right) was prepared via CeH bond activation of its dinuclear gold(I) xantphos precursor complex.247 In this case, the trinuclear gold(I) complex was almost linear with an angle of 166.9(1) Å for Au
Au
Au and short Au(I)/Au(I) distances of 2.783(2) and 2.850(1) Å between adjacent gold(I) centers. Upon photoirradiation at 365 nm under ambient temperature, the trinuclear gold(I) complex showed intense bright blue emission at 480 nm, which could be assigned to a mixture of triplet metal-centered and LMMCT excited states.247 Besides, the introduction of NHCs has been reported to enhance the stability and photophysical properties of gold(I) chalcogenido clusters. Corrigan and co-workers prepared [Au4M4(m3-E)4(IPr)4] (M ¼ Au, Ag; E ¼ S, Se, Te; IPr ¼ 1,3-bis(2,6diisopropylphenyl)imidazole-2-ylidene) (Fig. 68) from various [(IPr)AuESiMe3] precursors.248 In the crystal structures, metallophilic interactions were observed and the four “M” metal centers were nearly coplanar to each other. As the size of the chalcogenide increased from S to Te, the structure would become more distorted and the metal
metal distances would lengthen. In general, the clusters were found to show yellow-green to red emission in glassy 2-methyltetrahydrofuran, and the emission was red-shifted in the solid state at low temperature due to the strengthening of metallophilic interactions when solvents are absent. The emission origin was assigned as a mixture of 3LMMCT (IPr / AuM2) and 3IL (IPr / E) excited states.248 Due to the involvement of the IPr ligand, it was expected that the luminescence properties of the system could be ready tuned upon variation of the NHC moieties. A rare example of gold(I) carbide clusters was reported by Roesky and co-workers.249 They prepared a dinuclear gold(I) complex, [{Me3SiC^C(NDipp)2}2Au2] (Dipp ¼ 2,6-diisopropylphenyl), with an alkynyl-functionalized bisamidinate, and this dinuclear complex was further used to prepare an octanuclear cluster (Fig. 69). Intramolecular aurophilic interactions were observed in both complexes, as revealed by relatively short Au(I)/Au(I) distances of 2.7009(11) Å in the dinuclear complex and 2.9824(4)–2.9928(3) Å in the octanuclear complex, respectively.249 Both complexes were luminescent in the solid state and in THF solution at room temperature. Upon photoexcitation, the dinuclear complex showed a vibronically structured emission band at 518 nm in the solid state and a similar emission band at 492 nm in solution, which could be assigned as originated from either the 1[p
p*(C^C)] or the 3[p
p* (C^CR)] excited state. On the other hand, the octanuclear complex emitted

Fig. 67

Structure of the gold(I) clusters.

Luminescent supramolecular assemblies

Fig. 68

Structure of the gold(I) clusters.

Fig. 69

Structures of the gold(I) carbide clusters.

621

similarly at 483 nm in the solid state but exhibited a red-shifted emission band at 690 nm when dissolved in THF. The red-shifted emission was suggested to be of a p
p* or s
p* origin.249 The groups of Olmstead and Balch constructed a series of luminescent gold(I) complexes, [Au6(Triphos)4Cl](PF6)5$2(CH3C6H5), [Au6(Triphos)4Cl](AsF6)5$8(CH3C6H5) and [Au6(Triphos)4Cl](SbF6)5$7(CH3C6H5) (Triphos ¼ bis(2-diphenylphosphanoethyl)phenylphosphane), which exhibited a box-like structure.250 As shown in the representative crystal structure of the [Au6(Triphos)4Cl]5þ cation in the SbF6
salt, two Triphos ligands were connected together by two gold(I) centers to form a face. Another two gold(I) centers functioned to link two Au2(Triphos)2 faces to give the overall box structure, and a chloride ion was found at the center of the molecular box (Fig. 70).250 The molecular boxes were found to show blue phosphorescence although the Triphos ligand was non-emissive. As the Au(I)/Au(I) distances were long (> 6.34 Å) and aurophilic interactions were absent, it was proposed that the emission might originate from a charge transfer excited state, or from an exciplex involving the central chloride ion.250 When the complexes were ground, the emission color would change from blue to green due to the transformation of the molecular box to a bridged helicate dimer, [m-Cl-{Au3(Triphos)2}2](PF6)5 $ 3CH3OH, which exhibited short Au(I)/Au(I) distances of 2.9486(4)–2.9915(4) Å.250 Yam and co-workers discovered that the cluster-to-cluster transformation (CCT) of gold(I) complexes could be induced by a click reaction.251,252 In 2019, a chiral gold quartet framework was reported to be generated from four decanuclear [Au10(dpptrz)4S4]2
(dpptrz ¼ 4,5-bis-(diphenylphosphanyl)-1,2,3-triazol-2-ide) clusters, which were in turn prepared from [(dppa)6Au18S8]2þ (dppa ¼ 1,2-bis(diphenylphosphano)acetylene) via a click-reaction-induced CCT process (Fig. 71).251 The gold quartet framework was found to exhibit dual emission in the solid state at room temperature, namely a mixture of a green emission at about 500 nm from a triplet intraligand state with mixing of LMCT character, and a red emission at about 650 nm from a 3LMMCT sulfide-to-goldcluster (S / Aun) excited state, possibly with the involvement of solvent exciplex formation.251 In a related work, the same group

622

Luminescent supramolecular assemblies

Fig. 70 Transformation of the gold(I) clusters. Reproduced from Walters, D. T.; Aghakhanpour, R. B.; Powers, X. B.; Ghiassi, K. B.; Olmstead, M. M.; Balch, A. L. J. Am. Chem. Soc. 2018, 140, 7533–7542 © American Chemical Society, 2018.

Fig. 71 Schematic representation of the click-reaction-induced cluster-to-cluster transformation from [(dppa)6Au18S8]2þ to [Au10(dpptrz)4S4]2
in the presence of NaN3 under heat at 110  C. The Au18 cluster is represented by the single crystal structure (color legend: light-gray, C; orange, P; red, S; yellow, Au). Hydrogen atoms, counter-anions and solvent molecules are omitted for clarity. Reproduced from Yao, L.-Y.; Low, K.-H.; Yam, V. W.W. Chem 2019, 5, 2418–2428 © Elsevier, 2019.

has demonstrated the self-assembly of luminescent chiral hexanuclear and decanuclear gold(I) clusters from pure enantiomers of precursor compounds.253 By reacting the dinuclear gold(I) precursor, [(AuCl)2(bdppmapy)] (bdppmapy ¼ N,N-bis-(diphenylphosphanylmethyl)-2aminopyridine) with Na2S in a molar ratio of 1:1 or 1:2, either the tetradecanuclear Au(I) sulfido cluster [Au14S6(bdppmapy)5] Cl2 or the octadecanuclear cluster [Au18S8(bdppmapy)6]Cl2 could be obtained respectively.254 Aurophilic interactions were present in both clusters, with Au(I)/Au(I) distances of 2.937(8)–3.361(8) Å in [Au14S6(bdppmapy)5]Cl2 and 2.985(7)– 3.367(7) Å in [Au18S8(bdppmapy)6]Cl2.254 Both clusters showed bright yellow-green emission in the solid state and in DMSO/PBS buffer solution (1:99 v/v; pH 7) at room temperature. Based on DFT calculations, the emission was assigned to a MLCT origin.254 In addition, the clusters were found to be useful in bioimaging and could be used to track lysosomes in living cells. In a recent study, a series of chloridogold(I) precursors with diphosphane ligands of different substituents has been used to react with H2S to synthesize a series of pentanuclear gold(I) sulfido clusters.255 Remarkably, when electron-withdrawing fluoro or cyano substituents were present, the pentanuclear gold(I) clusters could further react with H2S and transform into octadecanuclear gold(I) clusters. Both pentanuclear and octadecanuclear gold(I) clusters were observed to exhibit aurophilic interactions. In the UV–vis absorption spectra, the pentanuclear clusters were found to absorb at 320 nm whereas the octadecanuclear clusters showed a red-shifted absorption peak at 400 nm. The absorption was assigned tentatively as LMMCT (S / Aun) or LMCT (S / Au) transitions modified by aurophilic interactions, and the red shift was probably due to the increase in nuclearity as well as the shorter Au(I)/Au(I) distances in the octadecanuclear clusters.255 The pentanuclear clusters were non-emissive in solution but showed red emission at 633–668 nm in the solid state at room temperature. At low temperature, the pentanuclear clusters exhibited dual emission at 440–550 and 629–695 nm. On the other hand, the octadenuclear clusters emitted at 580–637 nm in dichloromethane solution and at 600–705 nm in the solid state at room and low temperatures. The low energy emission bands near 580–705 nm were assigned to be originated from a 3LMMCT or 3LMCT excited state modified by aurophilic interactions, while the higher energy band was assigned as ligand-centered phosphorescence with metal perturbation.255

Luminescent supramolecular assemblies

623

Furthermore, Tang and co-workers investigated the chiral optical properties of a pair of chiral gold(I) alkynyl-phosphane clusters, (R)- and (S)-[Au6(C2H10H17O)4(PPh2C3H6PPh2)2](PF6)2.256 Relatively short intramolecular Au(I)/Au(I) distances of 3.190– 3.244 Å were observed in the single crystal structure of the (S)-enantiomer.256 When 30% (v/v) of water was added to a DMF solution of these gold(I) clusters, the resulting mixture was found to emit at about 490 nm and show strong CPL signals.256

8.14.4

Conclusion

The supramolecular assemblies and luminescence of d8 and d10 metal centers have continued to attract considerable research interest in recent years. As illustrated in this chapter, the photophysical properties of supramolecular assemblies based on platinum group metals and coinage metals are readily tunable via the judicious combination of ligands and metal centers and the delicate control of the metal–metal interactions in the systems. The rich photoluminescence properties have rendered these metal-based assemblies versatile candidates for applications in fields ranging from sensing, catalysis, optoelectronics to biomedicine and others. With the development of synthetic protocols, improvement in characterization techniques and advancement in computational studies, the structure-property relationship of the luminescent supramolecular assemblies can be better understood to provide guiding principles for the exploration of next-generation luminescent metal-based materials.

Acknowledgments V.W.-W.Y. acknowledges support from the Key Program of the Major Research Plan on “Architectures, Functionalities and Evolution of Hierarchical Clusters” of the National Natural Science Foundation of China (NSFC 91961202), the Collaborative Research Fund (CRF) (C7075-21G) and the General Research Fund (GRF) (HKU17303421) from the Research Grants Council of Hong Kong, and the CAS-Croucher Funding Scheme for Joint Laboratory on Molecular Functional Materials for Electronics, Switching and Sensing. V.K.-M.A. acknowledges the support by the Early Career Scheme (ECS) from the Research Grants Council of Hong Kong (EdUHK 28300220) and the Dean’s Research Fund (Research Output Prize Project no. FLASS/ DRF 04715) of the Faculty of Liberal Arts and Social Sciences, The Education University of Hong Kong. M.H.-Y.C. acknowledges the receipt of a University Postdoctoral Fellowship from The University of Hong Kong.

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Lo, H.-S.; Zhu, N.; Au, V. K.-M.; Yam, V. W.-W. Polyhedron 2014, 83, 178–184. Siu, S. K.-L.; Ko, C.-C.; Au, V. K.-M.; Yam, V. W.-W. J. Clust. Sci. 2014, 25, 287–300. Pillay, M. N.; Liao, J.-H.; Liu, C. W.; van Zyl, W. E. Inorg. Chem. 2019, 58, 7099–7106. Jin, Y.; Li, S.; Han, Z.; Yan, B.-J.; Li, H.-Y.; Dong, X.-Y.; Zang, S.-Q. Angew. Chem. Int. Ed. 2019, 58, 12143–12148. Zhang, M.-M.; Dong, X.-Y.; Wang, Z.-Y.; Li, H.-Y.; Li, S.-J.; Zhao, X.; Zang, S.-Q. Angew. Chem. Int. Ed. 2020, 59, 10052–10058. Kong, Y.-J.; Yan, Z.-P.; Li, S.; Su, H.-F.; Li, K.; Zheng, Y.-X.; Zang, S.-Q. Angew. Chem. Int. Ed. 2020, 59, 5336–5340. Olaru, M.; Rychagova, E.; Ketkov, S.; Shynkarenko, Y.; Yakunin, S.; Kovalenko, M. V.; Yablonskiy, A.; Andreev, B.; Kleemiss, F.; Beckmann, J.; Vogt, M. J. Am. Chem. Soc. 2020, 142, 373–381. Liu, G.-N.; Xu, R.-D.; Guo, J.-S.; Miao, J.-L.; Zhang, M.-J.; Li, C. J. Mater. Chem. C 2021, 9, 8589–8595. Yue, C.; Yan, C.; Feng, R.; Wu, M.; Chen, L.; Jiang, F.; Hong, M. Inorg. 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Rev. 2009, 253, 325–342. Titov, A. A.; Filippov, O. A.; Smol’yakov, A. F.; Godovikov, I. A.; Shakirova, J. R.; Tunik, S. P.; Podkorytov, I. S.; Shubina, E. S. Inorg. Chem. 2019, 58, 8645–8656. Kruppa, S. V.; Groß, C.; Gui, X.; Bäppler, F.; Kwasigroch, B.; Sun, Y.; Diller, R.; Klopper, W.; Niedner-Schatteburg, G.; Riehn, C.; Thiel, W. R. Chem. Eur. J. 2019, 25, 11269– 11284. Titov, A. A.; Filippov, O. A.; Smol’yakov, A. F.; Averin, A. A.; Shubina, E. S. Dalton Trans. 2019, 48, 8410–8417. Schmidbaur, H. Gold Bull. 1990, 23, 11–21. Schmidbaur, H.; Cronje, S.; Djordjevic, B.; Schuster, O. Chem. Phys. 2005, 311, 151–161. Schmidbaur, H.; Schier, A. Chem. Soc. Rev. 2012, 41, 370–412. Wan, Q.; Yang, J.; To, W.-P.; Che, C.-M. Proc. Natl. Acad. Sci. U. S. A. 2020, 118, e2019265118. Kawaguchi, K.; Seki, T.; Karatsu, T.; Kitamura, A.; Ito, H.; Yagai, S. Chem. Commun. 2013, 49, 11391–11393. 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8.15

Multicomponent supramolecular photochemistry

Fausto Puntoriero, Francesco Nastasi, Giuseppina La Ganga, Ambra M. Cancelliere, Giuliana Lazzaro, and Sebastiano Campagna, Dipartimento di Scienze Chimiche, Biologiche, Farmaceutiche ed Ambientali, University of Messina, Messina, Italy © 2023 Elsevier Ltd. All rights reserved.

8.15.1 8.15.2 8.15.2.1 8.15.2.2 8.15.2.3 8.15.3 8.15.4 8.15.5 References

Introduction Polynuclear metal complexes Light harvesting antennas based on metal complexes subunits Molecular multi-chromophoric systems for photoinduced charge separation Coordination cages Metal-containing supramolecular compounds based on non-covalent linkages Supramolecular systems based on host-guest interactions Conclusions

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Abstract Metal complexes have strongly contributed to the development of supramolecular photochemistry. Indeed, photoinduced energy (EnT) and electron transfer (ET) processes involving different components of (multicomponent) supramolecular species, as well as changes in the luminescence output of supramolecular subunits, as a consequence of interaction of other subunits of the assembly with environment components, are key features of photo-active supramolecular systems. Here we review recent progresses in the field, ranging from (i) multicomponent supramolecular species made of photo-active metalbased building blocks, often explored as synthetic light-harvesting and charge separation molecular devices for artificial photosynthesis, to (ii) coordination cages and host-guest species, also investigated as biological sensors, to (iii) metalcontaining supramolecular compounds based on non-covalently linkages, including systems exhibiting aggregation induced emission, often investigated as environmental probes.

8.15.1

Introduction

Supramolecular photochemistry, since the introduction of the field at the end of the nineth decade of the last century by the pioneering work of Balzani, Scandola and coworkers,1–3 has permeated a large part of photochemistry as well as of coordination chemistry. Indeed, supramolecular photochemistry has significantly contributed to many topics, both of fundamental and applicative aspects, such asdfor exampledphotoinduced electron (ET) and energy transfer (EnT)4 luminescence sensing including biological issues,5 solar energy conversion.6 A supramolecular system is a multicomponent species in which each individual component keeps in the supramolecular array most of its own properties. Within such a definition, which is based on a functional viewpoint, the nature of the bonds bringing the components together is less important: they can be hydrogen bonding, stacking interactions or coordination or covalent bonds, until the relevant individual properties are maintained. Therefore, multicomponent, supramolecular photochemistry deals with multicomponent systems in which at least one component is a photochemically-active species whose basic properties (that is, electronic absorption properties, redox behavior, intrinsic radiative and radiationless decays) are not significantly affected in the assembly, but can interact with other component(s) to (i) give arise to new properties, such as photoinduced charge separation or energy migration, or (ii) can substantially modify their luminescence output by shifting the emission energy, for example (iia) by simple perturbation of the (emissive) excited state or (ii-b) by the emergence of new emissive excited states as a consequence of the supramolecular structure. Within case (i) are most of the covalently-linked systems involved in photoinduced electron and energy transfer, whereas in case (ii) are involved host-guest systems, luminescence sensors (whose emission output can be quenched, enhanced or shifted upon interaction with specific substrates), as well as aggregated systems capable to exhibit new photo-active properties. To review the multicomponent supramolecular photo-active species investigated in the last decades is a without hope mission. We temporarily limit our chapter to the last 10 years and to some specific classes of species belonging to the supramolecular photochemistry realm: (a) polynuclear metal complexes. Within this class of supramolecular species, we will report on species involved in intercomponent energy and electron transfer processes, including multicomponent systems relevant for solar energy conversion. Photo-active species containing excited-state reservoir components (giving arise to excited-state equilibration and most

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frequently prolonged emission lifetimes) as well as molecular square, racks and polyhedrons in general also belong to this class of supramolecular compounds; (b) metal-containing supramolecular compounds based on non-covalently linkages. This class of compounds also includes covalently-linked species in which non covalent bonds are formed between individual components: for example, most of the species exhibiting aggregation-induced light emission belong to this general area; (c) supramolecular systems based on host-guest interaction. Most of the luminescent sensors, including sensors for biologicallyrelevant species, are part of this class of compounds. For former extensive reviews on supramolecular photochemistry the reader is invited to refer to fundamental papers published before 2010.7–20

8.15.2

Polynuclear metal complexes

In the big picture provided by the study of multinuclear supramolecular complexes, the study of information transfer at the molecular level has over time become an increasingly fascinating and explored area. Over the years, much attention has been devoted to the design of molecular structures in which the individual components are optimally and functionally organized both energetically and spatially. In particular, ruthenium(II) and osmium(II) complexes with bidentate and tridentate ligands and in general systems based on different metal centers have been extensively studied,21–32 thus providing a library of systems in which intra- and intermolecular energy and/or electron transfer processes have been optimized for use in various fields of application.33

8.15.2.1

Light harvesting antennas based on metal complexes subunits

Among the many examples reported in the literature, photo- and redox-active dendrimeric structures have received much attention as they are capable of exhibiting a unique combination of relatively easy synthetic strategy for their preparation and ability to absorb light, intercomponent energy and or electron transfer, and luminescence in the lowest-energy excited state, which makes them capable of behaving as light-harvesting antennae and as (supra)molecular multicomponent systems for light-induced charge separation.34,35 Among the systems made up of metal centers, those consisting of Ru(II) and Os(II) polypyridine building blocks are the most studied, due to the extraordinary photophysical and redox properties of Ru(II) and Os(II) polypyridine compounds.8,36,37 In this type of system, nuclearity plays an important role in the efficiency of inter-component photoinduced processes. Indeed, in dendrimers with more than four metal centers, the presence of metal subunits that have a higher excited-state energy than the donor (usually peripheral) and acceptor (usually core of the system) subunits plays a limiting factor for the efficiency of the energy transfer from periphery to core (EnT), thus playing the role of an energy barrier in down-hill transfer processes.34a,38,39 It has recently been reported that an energy-transfer process that is not active in dendrimers with high nuclearity becomes efficient at relatively high concentrations, i.e. under aggregation conditions; this information is highlighted by investigating the energytransfer process in a second-generation, decanuclear dendrimers composed of an Os(II) core and branches based on Ru(II) subunits, see Fig. 1.40 In such a species, the energy transfer from the metal-ligand charge-transfer (MLCT) triplet state that localized on the peripheral chromophoric {(m-2,3-dpp)Ru(bpy)}2 þ to the less energetic MLCT triplet state, localized on the core {Os(m-2, 3dpp)3}2 þ is known to be inefficient in dilute acetonitrile solution, due to the MLCT excited states involving the intermediate subunits, which reside at higher energy and act as barriers to the core-directed transfer process. It has been shown that with increasing concentration, irrespective of the excitation wavelength, energy is quantitatively transferred to the MLCT state located on the central building block, as a consequence of aggregation. This inter-dendrimeric process occurs with a time constantdas determined by pump-probe transient absorption spectroscopydof 18ps, and mimics what occurs in the energy migration processes between the various light-harvesting complex (LH2 and LH1) subunits in the natural photosynthetic system. The aggregation process was later corroborated by molecular dynamics and microscopy experiments.41 In 2016, the synthesis and study of a mixed-metal luminescent heptanuclear dendrimer was reported in the literature, in which all the light energy absorbed by the six Ru(II) chromophores is funneled to the central Os(II) core, within 11ps.42 Compound 2, shown in Fig. 2, is the first mixed OseRu dendrimers obtained using a tris-chelating ligand bridge. The electronic characteristics of the ligand mean that the electronic coupling between the nearby metal centers is practically negligible from a redox point of view. This species is also one of the very few examples of metal dendrimers whose energy transfer (EnT) has been studied by transient absorption pump spectroscopy (TAS). From a structural point of view, the multinuclear systems mentioned so far are based on octahedral metal subunits coordinated by three bidentate ligands, which leads to the formation of numerous stereoisomers during synthesis.43 This can be remedied by the use of tridentate ligands, which enable well-defined geometrical patterns to be obtained.21,44,45 However, Ru(II) bis-terpyridine complexes are poorly emissive and the properties of the lowest-energy excited state are poor as it is, in general, very close in energy to the 3MC (metal-centered) state. This drawback can be partially overcome by appropriate design of ligands and complexes aimed at stabilizing the 3MLCT state or destabilizing the 3MC state. In most cases, however, significant intervention in the synthesis and design of the terpyridine ligand is required to ensure that the lifetime and quantum yield of the excited state is consistent.

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Multicomponent supramolecular photochemistry

Fig. 1

Schematic representation of the decanuclear mixed metal dendrimer 1.

Fig. 2

Schematic representation of the heptanuclear mixed metal dendrimer 2.

Multicomponent supramolecular photochemistry

631

It has recently been reported that the simple direct connection of terpyridine subunits on top of each other leads to a significant increase in both the luminescence quantum yield and the duration of the excited state of the corresponding ruthenium complexes. Furthermore, due to the robustness of the bis-tridentate ruthenium(II) complexes, it is possible to put several subunits together in order to obtain multinuclear Ru(II) complexes as promising candidates for the formation of dendrimers and light-harvesting systems, some examples of which are shown in Fig. 3.46 The complexes 3–5, shown in Fig. 3, manage to combine delocalization of the excited state (which influences the photophysical properties of the lower-energy excited state localized on the central ruthenium center) with an efficient antenna effect. In fact, in these systems, all the energy absorbed by the ‘peripheral’ chromophores is transferred, with a driving force of  0.12 eV, to the central Ru(II) subunit, which, thanks to the delocalization of its excited state 3MLCT emits with a lifetime and quantum yield of 2 orders of magnitude larger than the reference complex [Ru(tpy)2]2 þ. A further modification in the structure of this type consists in introducing an [Os(tpy)2]2 þ-type subunit as an energy trap, since it is known that the electronic properties of osmium give this type of luminophore more important photophysical properties than the ruthenium analogue and localized 3MLCT excited states at lower energy. Efforts in this direction led to the synthesis of the heptanuclear complex 6 shown in Fig. 4.

Fig. 3

Chemical structure of complexes 3–5.

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Fig. 4

Multicomponent supramolecular photochemistry

Chemical structure of complex 6.

The energy stored via absorption of visible light by the four peripheral Ru(II) units is efficiently transferred to the intermediate subunits and finally-with a unitary energy transfer efficiencydto the Os(II) center that emits with s ¼ 161 ns.47 A further interesting approach is to mix the two types of ligand, so as to exploit the interesting photophysical properties of the [Ru(bpy)3]2 þ-like subunit with those of the [Os(tpy)2]2 þ, see Fig. 5.48 The luminescence of the multinuclear 7–8 complexes is weak and originates from the deactivation of the excited state localized on the Ru-tpy or Os-based centers. In 7 and 8, the energy absorbed by the ruthenium chromophore is efficiently transferred to the osmium subunit, which emits in the near-infrared (approx. 833 nm and 816 nm respectively). The proper choice of suitable ligands can significantly change the energy of excited metal-to-ligand charge transfer type states, as demonstrated by the molecular thread containing subunits of Ru(II) and Os(II) in which photoinduced energy transfer from osmium to ruthenium in a polypyridyl complex was demonstrated for the first time, see complex 10 in Fig. 6.49 This effect is due to the strong donation of the nonsymmetrical ligand phen-hpp (phen ¼ phenanthroline and hpp ¼ hexahydro-pyrimidopyrimidine), which heavily affects the optical and electrochemical properties of the ruthenium fragment, leading to

Fig. 5

Chemical structures of complexes 7–9.

Multicomponent supramolecular photochemistry

Fig. 6

633

Chemical structure of complex 10.

a red shift in its emission and its excited state at lower energy than that of the osmium fragment, such that it can act as an energy acceptor. In the case of complex 6 the final acceptor possesses an excited state that exhibits a sufficiently long lifetime for possible use in diffusion-limited processes such as electron transfer reactions in homogeneous-phase photocatalysis processes. Indeed, the peculiar light absorption properties, in particular the large molar absorption and the extension towards the red, have made metal dendrimers particularly interesting in applications such as photoinduced water oxidation processes,50 relevant for photocatalytic water splitting.51 In the context of studying the processes involved in artificial photosynthesis, water oxidation poses a formidable challenge being the energy-demanding bottleneck of overall process.52–56 A modular approach to photosynthesis finds the oxidation of H2O to O2 the key process for the accumulation of electrons and protons needed to drive the reductive half-reaction for hydrogen production. The urgency of finding renewable and environmentally friendly energy sources therefore makes the sunlight-driven water oxidation process the focus of numerous scientific research. In 2010, it was shown that the use of a dendrimeric Ru4 system in combination with a suitably designed catalyst (Ru4POM) can utilize around 60% of the absorbed photons to produce oxygen,51 optimizing a result previously achieved using IrO2 nanoparticles as a catalyst.57 This result adds up to the possibility of accessing a large fraction of the sun’s energy, including photons in the red region of the solar spectrum down to 700 nm, and is of considerable importance as it paves the way towards the goal of a sustainable hydrogen economy from light-activated water splitting. A further approach to the use of multinuclear systems is to incorporate specific subunits with special properties into their structure. An example are the multi-cromophoric structures of Ru(II) and Re(I) used as multi-component systems for the photocatalytic reduction of CO2, see compounds 11 and 12 in Fig. 7.

Fig. 7

Chemical structures of complexes 11 and 12.

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Multicomponent supramolecular photochemistry

A series of supramolecular photocatalysts for the photo-reduction of CO2 consisting of trinuclear metal complexes containing Ru(dmb)32 þ-type chromophores (dmb ¼ 4,4’dimethyl-2,2’bipyridine) and Re(dmb)(CO)3Cl-type catalysts for the reduction of CO2 in different ratios by means of made-to-order synthesis has been reported in the literature.58 The properties of the bridging ligand used mean that the different metal subunits of the multinuclear systems retain their light absorption and oxidation-reduction properties even in supramolecular assemblies, thus demonstrating that the electronic interactions between the various metal canters are negligible. Although the inter-subunit distance imposed by the ligand is substantial, during photocatalytic cycling in the presence of a sacrificial agent, rapid electron transfer occurs between the reduced chromophores and the catalytic units. In addition, a very high CO catalytic formation efficiencydunder visible light irradiationdwith exceptional turnover number (over 6000 under the correct conditions), high selectivity and durability is demonstrated. Overall, the results show that the structural arrangement imposed by the ligand and the multinuclearity of the system make it possible to overcome the size limitation of bridging ligands represented by ethylene bridges, which are normally used as a connection between photosensitizer and catalyst subunits, thus opening up new avenues in the field of designing more efficient and stable multifunctional molecular systems for photocatalysis.

8.15.2.2

Molecular multi-chromophoric systems for photoinduced charge separation

As previously described, the conversion of solar energy into chemical energy is based on elementary photoinduced chemical reactions of electron transfer and accumulation of multiple redox equivalents. Therefore, in addition to the processes of absorption of sunlight and its convection, of particular interest is the study of organized systems capable of generating long-lived photoinduced charge separation and subsequent unidirectional electron transfer processes.59–62 An interesting example of systems based on metal complexes are dyads of the D-B-A type (D ¼ electron donor; B ¼ bridging ligand; A ¼ electron acceptor), see species 13–15 shown in Fig. 8.63 These systems differ from each other in the presence of differently substituted biphenylene bridges at the connection of a Ru(II)bis-terpy (terpy ¼ 2,20 :60 ,200 -terpyridine) type metal chromophore and a bipyridinium type fragment expanded and fused across differently substituted biphenylene bridges. The chromophores were selected to be selectively activated-through different spectral absorption regions-and it was shown that selective excitation of the two different subunits leads to different excited-state dynamics. Specifically, by transient pump-probe spectroscopy the excitation of the Ru subunit leads to photoinduced oxidative electron transfer (OPET) from the 3MLCT state involving ruthenium and bridged ligand. Under these conditions, it is not possible to accumulate the charged-separated species, which recombines faster at the fundamental state. Excitation, on the other hand, at 400 nm, leads to the population of the 1p-p* state localized on the organic fragment, which undergoes a series of decay processes a series of intercomponent decay events: (i) an energy transfer to the MLCT state (SS-EnT), which decays in the same manner as observed for direct excitation at 570 nm, and (ii) photoinduced reductive electron transfer (RPET), with formation of the charged-separated state (CS). The formation of the latter state is much faster than the oxidative process, which allows for the accumulation of the charge-separated state. Photoinduced electron transfer has been also studied, in two molecular triads composed of a triarylamine donor, a d6 metal diimine photosensitizer, and a covalently linked 9,10-anthraquinone-type acceptor (compound 16 in Fig. 9).64 In these systems, it has been shown that the excitation of ruthenium(II) and osmium(II) leads to the formation of the chargedseparated states with oxidized triarylamine and reduced anthraquinone. The kinetics of formation of these states is strongly influenced by hydrogen bond donors such as trifluoroethanol, which is able to thermodynamically and kinetically stabilize the

Fig. 8

Chemical structures of complexes 13–15.

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Fig. 9

635

Chemical structure of complex 16.

charged-separated states due to the interaction between the alcoholic solvent and the reduced anthraquinone. In the ruthenium triad, the addition of alcohol leads to the elongation of the lifetime of the charged-separated state from  750 ns in dichloromethane to  3000 ns in hexafluoro-isopropanol, while in the osmium triad the respective lifetime increases from  50 to  2000 ns respectively. An evolution of this system is the pentad 17 shown in Fig. 10 in which, after excitation with visible light, a long-lived state of charge separation can be observed (s z 870 ns) with two electrons accumulated on the acceptor unit by exploiting a redox cascade method that does not require the use of sacrificial reagents.65,66 Similarly to the previous triad example, a dinuclear species of Ru(II) based on terpyridine ligands as light-absorption subunits, decorated with an electron donor group on one side and an electron acceptor group on the other side so as to obtain a linearly arranged D-(PeP)-A molecular triad, see compound 18 in Fig. 11, has been reported in the literature.67 In the presence of trace amounts of methanol, which is necessary to stabilize the reduced form of anthraquinone, photoinduced oxidative electron transfer occurs in D-(PeP)-A in about 380 ps, with formation of the charged-separated state of the type: D(PeP)þ-A. However, although the formation of the Dþ-(PeP)-A state is thermodynamically allowed, this species is not formed because, charge recombination within the D-(RueRu)þ-A state turns out to be faster.

8.15.2.3

Coordination cages

Another class of multinuclear compounds are molecular cages, which have become a class of compounds of great interest in supramolecular chemistry in recent decades. The structure of these compounds arises from the appropriate combination of bridging ligands and metal centers, which under appropriate conditions, can generate so-called coordination cages i.e., a closed pseudospheric structure with a central cavity.68 The progenitor of these species is the supramolecular system with stoichiometry M4L6 consisting of a metal ion at each vertex of a tetrahedron and a bridging ligand defining each of the six edges of the structure.69,70 Through the very principles of chemical symmetry71,72 able to dictate the guidelines for the realization of self-assembly processes.73,74 In just over a decade, complex systems such as Fujita’s Pd ion-based “nanospheres” have been achieved.75,76 Moving on from what were pure stylistic exercises in research in this field, attention was turned to the useful functional properties of these cages, such as those associated with the ability to bind small molecules as hosts in the central cavities and to achieve output through the use of at least one of the photophysically active components. The attraction in this context is that cage structures allow the assembly of a large number of photoactive units surrounding a host molecule in the cavity. Such a high local concentration of chromophores near a host that can interact with the excited states of chromophores promises exciting developments in the field of supramolecular photochemistry.

Fig. 10

Chemical structure of complex 17.

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Multicomponent supramolecular photochemistry

Fig. 11

Chemical structure of complex 18.

As already highlighted for dendrimers, the photophysical properties for coordination cages can also be modulated depending on the metal ion used and/or the ligands that connect them. Therefore, the use of luminescent complexes of Ru(II), Os(II), Re(I), Ir(III), Pt(II), and in general ions with low-spin d6/d8 electronic configurations is particularly interesting. Although most of these metal centers are kinetically inert, so not ideal species to allow self-assembly, interesting compounds have been obtained. For example, Lusby and colleagues reported the preparation and study of a system of an octahedral coordination cage in which 6 units of {Ir(ppy)2}-type units at the top of the cage are connected by four ligands of 1,3,5-tricyanobenzene (see compound 19 in Fig. 12)77. The luminescence of this system results from the deactivation of the 3LC excited state typical of the {Ir(ppy)2  2} subunits (lem ¼ 570 nm; f ¼ 0.04). The luminescence of this system is affected by the presence of anions within the cavity. Other examples using unreactive luminescent complexes have been obtained, using Ru(II) and Os(II) with particular ligand systems and the principle behind the “complexes-as-ligands/complexes-as-metals” (cal/cam) strategy. Within this synthetic strategy, metal complexes in which coordination sphere there is a ligand containing a free coordination site - the “complex as ligand”dcan coordinate a second metal center belonging to a metal complex containing a labile ligand (the “complex as metal”), which is removed by the “complex as ligand” species. By taking advantage of this strategy it is possible to achieve the formation of a molecular cage such as those (20) shown in Fig. 13.78–80 Combining complexes of the type [Ru(terpy)2]2 þ containing four accessory pyridine units with Pd(II) ions resulted in cages in which three or four of the Ru(II) complex units combine with six or eight Pd ions through the formation of PdeN(pyridyl) bonds (Fig. 13). The resulting species retain the typical photophysical properties of the ruthenium subunit. Using the same strategy, a C3-symmetric Ru(II) complex decorated with three ancillary pyridyl groups was used as a complexligand with Pd(II) ions to obtain the Pd6(RuL3)8 cage (L ¼ L1 in Fig. 14).81 The cavity of this system has a volume of > 5000 Å and encapsulates a wide range of aromatic and hydrophobic hosts, which inside the cavity are protected from irradiation-induced degradation by becoming protected from UV light. Similar systems have been obtained using as photoactive subunits, Os(II) complexes as ligand in combination with cobalt or cadmium ions, by using ligand L2 in Fig. 14.82 Os(II) tris-diimine subunits, in the mentioned systems, exhibit luminescence in the red portion of the spectrum with a relatively long lifetime.

Fig. 12

Chemical structure of species 19.

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Fig. 13

637

Chemical structure of species 20.

The most common metal ions used in cage assembly are lanthanoids. Systems that can be obtained with these metals have tetrahedral geometries of the Ln4L4 type in which a metal ion is located at each vertex and a tris chelating bridge ligand between three metals on each triangular face. Hamacek and coworkers prepared a series of cages Ln4L4 based on tripodal ligands such as L3-L5 (see Fig. 14) containing three tridentate pyridin-dicarbonyl O,N,O chelating arms, so make up for the large coordination number of Ln(III) ions, since the two O and one N atoms of L3-L5 binds to the metals.83,84By playing with the lanthanide/ligand

Fig. 14

Chemical structures of ligands L1–L8.

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Multicomponent supramolecular photochemistry

combination, cages with cavities of different sizes can be obtained in order to encapsulate molecules of discrete sizes85,86 or simply anions.87 The choice of ligand allows one to drive the property of the system. In fact, the use of ligands in which the O,N,O chelating units are chiral (type) allows one to induce chirality on the metal coordination and thus obtain a single diastereoisomer of the supramolecular array. The use of racemic L6 allows for a mixture of the two enantiomers of the tetrahedral Ln4L4 cages as if the ligands had undergone homochiral self-mixing during the assembly process.88 He, Duan and coworkers have developed a series of cages based on Ce(III) or Ce(IV) ions and O,N,O-type chelating bridge ligands, used as luminescent sensors for a wide range of hosts.89–97 The 4f-5d transition that generates Ce(III) luminescence is much more sensitive to the environment than the 4f-4f transitions associated with Eu(III) and Tb(III) luminescence and is therefore sensitive to perturbations resulting from host interactions. For example, tetrahedra type Ce4L4 and Ce4L6 obtained using tritopic ligands type (L8)98 and ditopic ligands, type (L7)99 are able to exhibit increased luminescence in the presence of saccharides due to the hydrogen bonding interaction between the bound substrates and the inward-directed amide groups. The ability to modulate the size of the cage with higher nuclearities and different ligands,86,100 show a similar luminescence response only when the host is complementary to the cavity dimensions thus becoming size-selective luminescent sensors. The nature of the ligand and the introduction of specific fragments into it, such as, for example, triamine-triazine, produce cerium ion-based cages that can selectively luminesce in the presence of guanosine compared with other nucleosides, even in organic solvents.101 If, on the other hand, a dihydropyridine amide group is introduced into the ligand skeleton, it is possible to selectively bind the RDX explosive, with a much more intense luminescence enhancement for this species than for other types of explosives.102 The use of the triphenylamine subunit in the ligand, see for example L7, makes the Ce4L4 cage an intense blue emitter.103 This species, is able to bind with a particularly high constant with the organic radical 4,4,5,5,5-tetra-methyl-imidazolinoxyl-3-oxide and as a whole the host-guest system thus obtained is able to spin-trap NO with resultant quenching of fluorescence even at concentrations of 5 nM, much lower than the most commonly used EPR spectroscopic assay.97 The same cage showed selectivity in binding tryptophan relative to other amino acids as also demonstrated by tests performed for the identification of tryptophan in human serum.104 The high selectivity and sensitivity of this type of sensor also makes them applicable in the study of the reactivity of complex systems to signal the progress of a reaction when a component is consumed. Some Ce4L4 tetrahedra of this family based on a tritopic ligand containing amide groups have been used to monitor cyanosilylation of aromatic aldehydes by restoring the cage luminescence signal.105

8.15.3

Metal-containing supramolecular compounds based on non-covalent linkages

In the last 10 years, in the field of supramolecular photochemistry many scientists focused on the development of self-assembled aggregate systems, whose photophysical properties strongly depend on the nature of the non-covalent linkage between complexes. A fundamental role in this story is certainly played by the Pt (II) complexes whose electronic and therefore structural properties allow the formation of aggregates under suitable conditions.106 In recent years, many efforts were made to study aggregation behavior of Pt(II) complexes in solution.104,107–109 In particular, a series of square-planar platinum(II) terpyridyl complexes with enhanced solubility due to the presence of the alkynyl group exhibited intense emission in solution and were reported by Yam and coworkers more than 10 years ago. For these compounds, both the lower-energy absorption and emission bands involve MLCT and LLCT (ligand-to-ligand chargetransfer) transitions. An interesting feature lies in the pronounced color change and increased luminescence quantum yield as a function of the amount of diethyl ether used in solvent mixture. The drastic “solvatochromism” and “solvatoluminescence” effects are induced by the solvent on metal-metal-to-ligand charge transfer transitions (MMLCT). In fact, the non-solubilizing solvent (diethyl ether) effect results in reduced solvation that increases Pt$$$Pt and p-p stacking interactions resulting in self-assembly or aggregate formation. Both the spectral range in absorption and the emission energy of these self-assemblies are also influenced by the nature of the anions, which, to some extent, drive both the degree of self-assembly and the extent of intermolecular interactions in the aggregates. The process of aggregation and self-assembly of alkynyl-platinum(II) terpyridine species with positive charge and solubility in water has been studied in the presence of various negatively charged polymers (under basic conditions) and oligonucleotides (whose surface charge is known to be negative). In these cases, the presence of electrostatic interaction between the molecules of the complex and the polymer forces the cationic complexes into a position that induces Pt$$$Pt and p-p interactions. This type of interaction again results in pronounced color changes and an increase in luminescence depending on the nature of the polymer and obviousness of the complex used. In addition, the magnitude of the interaction drives the conformation of the polymer by stabilizing it in helicoidal form.110 Self-assembly and formation of aggregate involving Pt(II) complexes find application in biological field and, in particular, by monitoring the spectral changes that arise from the modulation of such self-assembly behavior new sensing strategies and protocols can be developed for the label-free detection of various biological macromolecules.111,112 In this context, in 2013, Yam and coworkers113 reported the study of the self-assembly properties of water soluble alkynylplatinum(II) terpyridine complexes,114,115 21

Multicomponent supramolecular photochemistry

639

and 22 (Fig. 15), and their NIR-emissive behavior, governed by the polyanionic nature of ATP and phosphopeptide, have been demonstrated in aqueous media. By monitoring the extent of such self-assembly behavior based on the spectral changes from UV-vis and emission studies, these cationic platinum(II) complexes could be potentially utilized as detection probes for the quantification of phosphate derivatives. Based on their relatively high sensitivity towards microenvironmental changes, such behavior could be exploited for the differentiation of the substrates from other possible interfering candidates in a selective manner and for the monitoring of the activities of biologically important enzymes in a real-time manner, providing important information on the enzymatic kinetic parameters. By introducing guanidinium moieties, in 2014 Yam and co-workers synthesized and studied new water-soluble alkynylplatinum(II) terpyridine complexes and their self-assembled luminescence properties were also employed in biological application.116 Thanks to the strong and specific interactions between the cationic complex and citrate, an induced self-assembly of the complex molecules was demonstrated, with remarkable UV/Vis absorption and NIR emission spectral changes. The potential of complex, here mentioned as 23, Fig. 16, as an NIR luminescent probe for tracking changes in citrate concentration in aqueous media was proved because the unique changes in the NIR emission intensity were employed for the detection of citrate in aqueous media and Stain Buffer (FBS), with good sensitivity and selectivity over mono- and dicarboxylate substrates in the tricarboxylic acid cycle (TCA) cycle, as well as phosphate and lactate anions. In addition, the NIR emission of 23 was further exploited for real-time monitoring of the enzymatic activity of citrate lyase. During that year, De Cola and co-workers, in an interesting review,117 reported phosphorescent platinum(II) complexes that have been successfully employed as luminescent labels for bio-imaging application. In particular, they discussed how metallophilic interactions can be advantageously used for bio-imaging purposes leading to a new class of labels with potential in diagnostics and therapy.118,119 They have investigated on the self-assembly as a tool to obtain luminescent nanostructures based on neutral tridentate platinum(II) complexes also in cellular compartments. They have preliminarily demonstrated that the platinum complexes 24 and 25, showed in Fig. 17, are internalized in living cells and they self-assemble into nanosized aggregates.120

Fig. 15

Chemical structure of compounds 21 and 22.

Fig. 16

Chemical structure of 23.

Fig. 17

Chemical structures of 24–25.

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Multicomponent supramolecular photochemistry

The compounds have remarkable photophysical properties in the solid state and, in solution, their properties depend dramatically on the concentration and on the employed solvent. In particular, in water: DMSO mixture, both complexes are present in an aggregate form displaying an intense yellow emission arising from 3MMLCT transitions. An important feature of this assemblies is that, under excitation they are bright emitters, with long-lived triplet excited-state lifetimes, even in the presence of oxygen, and enhanced photostability. Moreover, complexes here reported are both internalized into HeLa cells within a few minutes and demonstrate how the choice of the coordinating ligands can determine the aggregate formation preferentially into the cytoplasmatic region. Interestingly, both species show luminescence that does not depend on the chemical environment; in fact, the energy of emission does not vary between aqueous solvent and the complex cellular environment, suggesting that even within HeLa cells they remain in aggregated form. Therefore, it’s important to underline how the photophysical properties of the aggregate are exploited not only in bulk solution but in particular within cells: thanks to the self-assembly and the photophysical properties that derive from it that this species can be used as probe for biological applications. Another important application for similar Pt(II) complex as luminescence probe and tags for drugs and toxins in water was reported by De Cola and coworkers.121 In an interesting work they reported a highly reactive platinum precursor that enables facile labelling of aza-heterocyclic compounds (e.g. pyridines, imidazoles) in aqueous media through the formation of highly emissive Pt(II) complex aggregates.122 An assay was established that allows analyte identification by means of the photophysical fingerprints of their labelled complexes. The emission-based assay is selective for those Pt(II)-analyte complexes that self-aggregate in aqueous media, because only those show a marked (mostly red-shifted) emission, on account of the rigidochromic effect. The tendency for self-assembly depends on the steric properties of the analytes and the interplay with the surfactant. Moreover, different Pt-analyte complexes that aggregate show readily distinguishable spectroscopic features (e.g. emission color) because the photophysics of the assemblies is highly dependent on the PtePt distance between neighboring complexes and the stereoelectronics of the coordinated analyte. This study provides a showcase for the superior analyte discrimination capabilities of a combined reactive-probe and supramolecular assay. Interesting supramolecular architectures made by novel Pt(II) complexes with tridentate N-donor ligands synthesized and reported by Yam in 2014,123 were also involved in the field of chemo-sensing. A similar complex was previously studied124 and it was demonstrated, by spectroscopic and luminescence changes, an interesting morphological transformation from vesicles to nanofibers of a new class of amphiphilic anionic platinum(II) bzimpy complexes via aggregation of Pt$$$Pt and p-p stacking interactions that can be systematically controlled by the variation of solvent composition. It is envisaged that with its high water solubility and strongly emissive nature in water, together with its drastic emission and absorption changes on aggregation and deaggregation, this class of platinum(II) complexes was an ideal candidate for potential sensing or imaging purposes. In fact, as it was reported by Yam in 2017,125 the bis(triphenylphosphine)iminium (PPN) salt of complex 26 (the chloroplatinum(II) complex in Fig. 18) is found to exhibit vapochromic and vapoluminescent properties upon the adsorption of alcohol or water vapor while the alkynylplatinum(II) complex (26a in Fig. 18) shows an aggregation-deaggregation-aggregation self-assembly behavior, leading to interesting spectroscopic and morphological changes. The colorful polymorphs and intriguing phosphorescence changes upon exposure to different vapors would have great potential applications in the fields of chemo-sensing and logic gates. The PPN salt of compound 26 was also used as active material to fabricate a resistive memory devices and it also demonstrate the potential of this class of platinum(II) complexes as a promising candidate for memory applications. To stress the importance of directional Pt$$$Pt and/or p-p interactions in the self-assembly process, we reported an example published by Yam in which different distinctive structures have been obtained by controlling the various intermolecular interactions between Pt complexes. This work has provided further insights into the development of metal complexes with an AIE (Aggregation Induced Emission) property as new functional materials. In particular, a series of alkynylplatinum(II) terpyridine complexes with TPE (tetraphenylethylene) moieties (27 and 28) has been reported (Fig. 19)126 The unique color and luminescence changes resulting from the interactions of the two different chromophores may lead to the development of functional materials for detecting microenvironment changes. With the aid of molecular engineering, the AIE effect that originated from the TPE moiety can be hindered through the

Fig. 18

Chemical structures of 26 and 26a.

Multicomponent supramolecular photochemistry

Fig. 19

641

Chemical structures of complexes 27 and 28.

introduction of the bulky tert-butyl groups on the terpyridine ligand. On the other hand, via molecular modification of the ligand, different superstructures have been obtained from the self-assembly processes by tuning the Pt$$$Pt and/or p-p stacking interactions, as well as the hydrophilicity of the complexes. So it was possible to form nanowires in the case of a stronger metal interaction or nanorods/nanoleaves when the interaction between Pt center was weaker. As we reported until now, multicomponent self-assembled aggregates of platinum (II) find application in many fields from biological/biomedical application to the functional materials. A very interesting work was reported by De Cola and coworkers in 2016; they synthetized a Pt(II) complex bearing an oligoethyleneoxide pendant, that is able to self-assembly in ultralong ribbons that display mechano-chromism upon nanoscale mechanical stimuli.127 Investigation with confocal microscopy has revealed that the mechanochromism process is activated within small distances of the location of mechanical stimulation. This results in such control that high-density information can be written using AFM nanolithography techniques, in fact each ribbon, can function as a microsystem for data storage.128 The difference in emission color (selectively activated by visible light) after mechanical stimulus (orange) compared to the native cyan luminescence makes for much contrast. An additional feature of these systems is photochromism and therefore sensitivity to light exposure that allows a variation in emission color. These types of systems, therefore, allow information to be stored using lithographic techniques and zeroed out using appropriate light input. Aggregation and tunable color emission behaviors of a glutamine-derived platinum(II) bipyridine complexes (29) was recently reported by Yam (Fig. 20).129 This work has provided insights and guiding principles into the design and construction of functional supramolecular soft materials, especially highlighting the importance of a balance of different non-covalent interactions that can give rise to sophisticated systems with unique optical and responsive properties. The introduction of L-glutamine-derived phenylacetylene to bis(arylalkynyl)platinum(II) bipyridine complexes led to distinct spectroscopic properties and aggregation behaviors. Given that the solvent or the temperature changed, the emission could be switched between the 3MLCT excited state and the triplet excimer state, which is attributed to the formation and disruption of hydrogen-bonding, p-p stacking, and metal-metal interactions. Different architectures with various morphologies, such as honeycomb nanostructures and nanospheres, were formed when the polarity of the solvents changed. Furthermore, the molecular chiral information derived from the L-glutamine moiety was transferred to the supramolecular architectures. The emission color of the metallogels was found to be tunable by controlling the temperatures and this found potential application in thermally emissive sensing materials. More interestingly, the assembly process was found to be cooperatively altered by the hydrogen- bonding interactions governed by the glutamate functional group, in addition to the p-p and metal-metal interactions to form the luminescent metallogel.

Fig. 20

Chemical structure of complex 29.

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Multicomponent supramolecular photochemistry

In recent years, a variety of complexes possessing AIE effect with bright luminescence by the aggregate formation were reported. Apart from platinum complexes here mentioned, also gold(I) species appears interesting, in particular in the field of stimuliresponsive materials. As we have seen so far, luminescent materials capable of responding to external stimuli are very interesting because of the many applications to which they are aimed. Further examples are vapochromic materials, which are molecular systems capable of changing their luminescence as a function of exposure to volatile organic vapors. High aggregative-state emission and obvious color contrast before and after stimulating are two extremely important factors for the highly efficient application of stimuli-responsive smart materials. So, it is very significant to synthesize multistimuli-responsive AIE materials with vapochromism characteristics. Over the past two decades, gold(I) chemistry has aroused the interest of many researchers owing to the occurrence of fascinating aurophilic.130,131 For these reasons, in 2014, Liu and coworkers reported the synthesis and study of a fluorene-based gold(I) complex.132 Its aggregation-induced emission behavior was investigated as well as its solid-state mechanochromic and thin-film vapochromic luminescence behaviors. The results demonstrated that complex showed obvious aggregation-induced emission property. In particular, it is important to highlight, that species 30, see Fig. 21, exhibits mechanochromism and vapochromism effects in high-contrast luminescence and, in addition, can emit phosphorescence at room temperature in the solid state with a half-life of up to 86 ms.133 During the same year, the authors synthesized a new fluorene-based dinuclear gold(I) complex. Also this novel complex, like the mononuclear one previous reported, is AIE-active and exhibits reversible mechanochromic fluorescence. Furthermore, they also found a noteworthy CIEE (Crystallization induced emission enhancement) effect in the complex. This dinuclear gold(I) complex (31) is the first example of an AIE-active metal-bearing luminogen with CIEE and switchable mechanochromism characteristics.134 Mechanochromism and aggregation-induced phosphorescence emission (AIPE) behavior was also showed by a novel bipyridine-based gold(I) complex (32). Although pyridine ligands are among the most versatile both synthetically and structurally, this is one of the few examples of gold(I) complexes in which emission is induced by aggregation and capable of exhibiting reversible mechanochromism. 32 is ungrateful to respond, in fact, to mechanical action by switching from weak luminescence in green to intense luminescence in yellow, representing the first example of an AIPE-type luminophore with mechanochromic activity.135 The development of gold-based smart and multiresponsive materials continued with the synthesis of a new class of amphiphilic tridentate cyclometalated gold(III) complexes.136 These species follow a self-assembly pathway driven by p-p stacking interactions and their hydrophobic nature with a likely weak Au$$$Au interaction, as evidenced by the presence of energy-driven absorption bands and changes in luminescence compared to the monomeric forms. The use of polar solvents leads, in most cases, to the formation of nanofibers, and it is generally confirmed that small structural changes or counterion exchange results in modifications of the structural organization of the aggregate. Among the square planar complexes studied for their interesting emission properties related to the non-covalent linkage, a new series of cyclometalating tridentate N^C^N and tetradentate N^C^N^O ligand-containing nickel(II) complexes has been designed and synthesized. The nickel complex, reported in Fig. 22 as 33, is the first example of nickel(II) complexes, featuring not only long-lived luminescence at low temperatures but also room temperature luminescence, with the improved molecular design of the incorporation of a strong s-donating carbazolyl ligand into the nickel(II) center. Meanwhile, the self-assembly property of the tetradentate ligandcontaining nickel- (II) complex in solution was investigated, and the work represents the first example of the self-assembly of tetradentate ligand-containing cyclometalated nickel(II) complex in the solution state. Interestingly, due to the nearly perfect square planar geometry of such a complex, weak Ni$$$Ni interactions have been shown to emerge in the aggregation process.

Fig. 21

Chemical structures of complexes 30–32.

Multicomponent supramolecular photochemistry

Fig. 22

643

Chemical structure of 33.

Again, studies on aggregation have shown that the use of variable portions of a non-solubilizing solvent leads to self-assembly into highly ordered supramolecular architectures. The reported approach can open up a new opportunity for the future design of nickel(II)-based phosphorescent emitters, as well as other possible applications ranging from catalysis to OLEDs and bioimaging.137 Until now we showed most of the examples for aggregation induced emission phenomenon by flat metal complexes such as Pt(II) or Au(I) or Ni(II) species. In recent years, it was possible to demonstrate that, by design, even iridium compounds can display this process without shifting the emission energy. Moreover, a new cationic iridium(III) complex exhibiting AIE activity, [Ir(PPh3)2(bpy)(H)2]A (bpy ¼ 2,20 -bipyridine; A ¼ counterions), has been synthesized and its behavior in solution and solid state was investigated.138 Furthermore, for these species, the color of the solid-state emission is a function of the counterion, and in particular for species 34 (shown in Fig. 23), crystallization-induced emission is observed. The emission properties for the compounds are discussed not only by analyzing the crystal packing but also by studying the frontier molecular orbitals, and by the calculation of the relevant low-lying excited states using time-dependent density functional theory. They understood that the restriction of internal rotation of the phenyl rings in the phosphine ligands due to intermolecular interactions is suggested as the most plausible origin of the observed AIE effect in these crystals. So, since the changes in coordination geometry and electronic delocalization seem to be out of the question, the most plausible origin of the AIE effect in the studied compounds appears to be the restriction of internal motion (RIM mechanism). The larger phosphorescence quantum yield observed for the crystals with PF6 seems to hint at a larger RIM in this case, perhaps due to the additional CeH$$$p interactions found in the crystal. The observation of some vibrational structure in the spectra for the PF6 case seems also to hint to a stronger restriction of the low frequency modes that would result in a larger spacing of their associated vibrational states and a more structured spectrum in which the vibrational progressions of the modes affected by the electron transfer are not blurred by the quasi continuum of low frequency modes active in solution or in loosely packed crystals.

Fig. 23

Chemical structure of 34.

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Multicomponent supramolecular photochemistry

Another interesting work about AIE active iridium(III) complexes was reported by Laskar and coworker in 2017.139 The complexes 35 and 36 are represented in Fig. 24. The two iridium(III) complexes, reported by the authors, are based on a simple Schiff base ligand. The study provided evidence that the AIE effect is induced by restriction of internal movement (RIM), consistent with previous studies.140 The properties of these systems, and in particular those exhibited by species 36, made it possible to use them as a selective probe for mitochondria in bioimaging. Progress in this field was provided by the results obtained by De Cola and collaborators, in 2019, focusing their attention on amphiphilic complexes. The study focused on the bola amphiphilic zwitterionic Ir(III) species (37 in Fig. 25), capable of emitting in the blue.141 Species 37 exhibits, in fact, centered emission at 450 nm in dilute solution with a quantum yield of 22% in deaerated solution, similar to that of the linked cationic amphiphilic complex. It also displayed significant emission enhancement in the solid state, with an emission quantum yield that reaches 52%. Interestingly, the emission of the cationic analogue suffers from aggregation quenching in the solid state (in fact, PLQY is reduced to 3%), as common for these types of complexes. The unique packing motif is responsible for the enhancement of the luminescence as indicated by the comparison with the analogous complex lacking the negatively charged sulfate group. Indeed, in dilute solution the two complexes possess identical photophysical properties. The rigid molecular arrangement of 37, is due to the electrostatic attraction of the positively-iridium moiety and the sulfate group that rigidify the long flexible alkyl chains. An interesting AIE behavior was also discovered for a red emissive ruthenium(II) complex, 38 (see Fig. 26) in hexafluorophosphate salt and its perchlorate analogues of the 4,7-dichloro phenanthroline ligand was reported in 2018.142 Compound 38 showed AIE enhancement in water, highly dense polyethylene glycol media, and also in the solid state. The possible reason behind the AIE property may be the weak supramolecular p$$$p, CeH$$$p, and CeCl$$$H interactions between neighboring phenyl ligands as well as CeCl$$$O halogen bonding (XB). The cell-imaging experiments revealed rapid staining of the nucleolus in HeLa cells via the interaction with nucleolar ribosomal ribonucleic acid (rRNA). It is expected that the supramolecular interactions as well as CeCl$$$O XB interaction with rRNA is the origin of aggregation and possible photoluminescence enhancement. This was actually the first example of emissive probes in the red based on AIE processes involving ruthenium(II) complexes that could selectively detect rRNA and enable nucleolar imaging.

Fig. 24

Chemical structures of complexes 35 and 36.

Fig. 25

Chemical structure of 37.

Multicomponent supramolecular photochemistry

Fig. 26

8.15.4

645

Chemical structure of 38.

Supramolecular systems based on host-guest interactions

Much of the progress in the field of biomedical research is closely related to the development of bioanalytical methodologies capable of precise and accurate detection of particular biomolecular substrates in complex systems such as biologics. Among the most widely used methods, biological analysis using luminescent probes has proved to be of great interest as it allows, thanks to a clean signal such as light, to understand the dynamics of biomolecules in living systems even at the subcellular level. In most cases, organic fluorophores are used, which usually have the implicit limitation of relatively low luminescence and Stokes shift lifetimes. Alternative luminophores to them are luminescent metal complexes, in particular those of certain lanthanides143–145 (species 39–42, in Fig. 27) and transition metal complexes146–149(e.g. Ru(II) and Ir(III)) which, due to their unique photophysical/chemical properties, allow for increasingly interesting applications in bio-imaging and biosensing. In particular, the design of these metal probes implies the use of peculiar structural functions capable of responding to a particular analyte by means of a specific response (usually variation in luminescence intensity or variation in luminescence color). A characteristic implication of these complexes is the excited-state lifetime, which, being usually of longer duration than the fluorescence of organic luminophores and the matrix under investigation, makes it possible to eliminate the interference of background autofluorescence and light scattering.150 Biological assays based on time gate luminescence detection (TGL), for example, use long-lasting luminescence probes and have been widely used in clinical diagnostics and biomedical investigations. Following pulsed excitation, the scattered light quickly disappears and the autofluorescence has the same fate, so only the luminescence due to the deactivation of a longer-lasting excited state of the metal complex-based molecular probe can be observed, collected after the decay of the previous two. In fact, by using a delayed readout time after excitation, the TGL technique allows only the luminescence from complex metalreactive probes to be collected for background-free detection of analytes in situ in complicated living systems, which cannot be achieved using short-lived fluorescence probes. Due to the effective elimination of background signals, the TGL technique significantly increases sensitivity and signal-to-noise (S/N) ratio and thus minimizes false positive/negative signals for biological assays. Some examples of lanthanide-based complexes are given in Fig. 28, and it has been shown, for example, that the species 43 is able to selectively recognize HOCl in aqueous solutions by time-gated luminescence. These sensors are based on the principle that specific recognition moiety, 4-aminophenyloxymethylene incorporated on the complex quenches the luminescence of the LnTerpy portion by electron transfer. The almost non-luminescent probes can selectivity distinguish HOCl from other reactive oxygen/ nitrogen species and react to yield highly luminescent products 44, with a very high increase in quantum yield and luminescence lifetime.151 On the other hand, the use of luminescent transition metal complexes as biological probes raises some concerns regarding (i) weaker emission intensities than organic fluorescent dyes; (ii) low solubility in water; and (iii) in some cases high cytotoxicity. However, many luminescent complexes such as the previously illustrated lanthanides and [Re(Ph2phen)(CO)3(pyridine)]þ and [Ir(ppy)3] exhibited extremely high emission quantum yields. Furthermore, it has been widely established from studies that it is possible to modulate the water solubility of these species and make them non-cytotoxic by using biocompatible substituents such as poly(ethylene glycol) (PEG).152 Lo and co-workers have, for example, decorated the luminescent polypyridine complexes of rhenium (I), iridium (III) and rhodium (III) with appropriate reactive functional groups such as isothiocyanate, aldehyde, maleimide and iodoacetamide in order to achieve bioconjugation. Some examples are represented by [Re(N^N)(CO)3(py-NCS)]þ,153 [Re(N^N)(CO)3(py-maleimide)]þ,154 [Ir(tpy-R)(tpy-C6H4-NCS)]3 þ,155 [Ir(N^C)2(phen-NCS)]þ, [Ir(N^C)2(phen-NHCOCH2I)]þ,156 [Ir(N^C)2(bpy-CHO)]þ,157 and

646

Multicomponent supramolecular photochemistry

Fig. 27

Chemical structures of complexes 39–42.

Fig. 28

Chemical structures of complexes 43–44.

[Rh(ppy-CHO)2(N^N)]þ.158 where N^N and N^C are diimine and cyclometalating ligands, respectively. The presence of the isothiocyanate and aldehyde groups allows further reaction with the primary amines while the maleimide and iodoacetamide groups with the sulphydryl ones, which allowed the conjugation of these complexes with amine- and sulphydryl-modified oligonucleotides, amino acids, peptides and proteins including serum albumins and avidin. In contrast to similar approaches on fluorescent organic systems, bioconjugation does not alter the photophysical properties of the complexes and the biological behavior of the biomolecules. Lo and co-workers, used a heterogeneous assay for the drug digoxin consisting of avidin that is labelled with the complex [Ir(pq)2(phen-NCS)]þ-species 45 in Fig. 29 (pq ¼ 2-phenylquinoline)da biotinylated antibody, and microsphere particles.158

Multicomponent supramolecular photochemistry

Fig. 29

647

Chemical structure of 45.

The system works on the competition between the immobilized digoxin and free digoxin analyte towards the binding to the biotinylated anti-digoxin in a real test the biotinylated anti-digoxin captured on the solid phase is recognized by the avidin conjugate and an increase of phosphorescence intensity. Lo and co-workers also reported a bimetallic Ir(III)-rhenium(I) complex containing a bridged disulfide bond that acts as a biological sensor for Cys, GSH and H2S.159 In particular, they demonstrated that a photo-induced energy transfer process from the rhenium(I) center to the Iridium center is active in this complex. After incubation with GSH at increasing concentrations, this complex shows a clear increase in emission at 517 nm (localized on the rhenium subunit) and a reduction in intensity of the band centered at 655 nm (centered on iridium), with good linearity in the ratio of emission intensity (I517nm/I655nm). This behavior is attributed to the cleavage of the disulfide bond by GSH and thus the consequent reduction in the photoinduced energy transfer efficiency from rhenium(I) to iridium(III). In addition to the identification of thiols, many metal complexes are suitably decorated to be able to provide a signal that allows discernment between different thiols. Zhang and co-workers, for instance, designed a ruthenium(II) complex that can detect and discriminate different thiols.160 A 7nitro-1,2,3-benzoxadiazole (NBD) group is present on the complex, which participates in a photoinduced electron transfer process responsible for the low luminescence of this species. The complex is linked to the NBD via a reactive bond against nucleophilic substitution by the biothiols to release. This reaction leads to the formation of a ruthenium-OH complex that exhibits longlived luminescence in the red, the NBD-NR that emits green for Cys/Hcy (with a much shorter lifetime) and the non-emitting NBD-SR in the case of GSH. The difference in the color response of the luminescence and the different life time of the excited state make it possible to differentiate between GSH and Cys/Hcy by steady-state luminescence analysis and to detect total biothiol by time-gated luminescence analysis capable of discerning between autofluorescence and NBD-NR fluorescence. Yuan and co-workers developed a ruthenium(II) complex with a 2,4-dinitrobenzene (DNB) unit (compound 46 in Fig. 30) as a luminescent probe for the detection and quantification of lysosomal formaldehyde.161 Indeed, high levels of formaldehyde have been shown to be an indicator of tissue cancer. The presence of the nitro group allows an electron transfer process that again results in low luminescence intensity. In the presence of formaldehyde, bond cleavage between the metal subunit and the -nitro fragment occurs, which leads to an intense increase in luminescence around 644 nm, observing a good linearity between the emission intensity at 644 nm and the formaldehyde concentration (0–650 mM). In particular, lysosomal formaldehyde luminescence imaging of the HeLa cell line demonstrates that this complex allows formaldehyde to be detected both in vitro and in vivo. A further example in this direction is shown by a specific probe for detecting the ClO anion in live cells, which was synthesized and studied by Lv and co-workers. The new multifunctional Ir(III) complex has a diaminomaleonitrile group on the pyridine ligand that serves as a luminescent probe for multi-signal detection and multimodal imaging of ClO with multiphoton activation in the IR region.162 Again, the luminescence is quenched by the presence of electron-attracting groups, which can be oxidized by the ClO anion, resulting in a significant increase in emission and a prolongation of the lifetime. The complex and dynamic nature of the cellular microenvironment is governed by a myriad of factors such as oxygen level, pH, viscosity, and temperature. Abnormal levels of these factors can be a cause of, or an indication of, disease.

648

Multicomponent supramolecular photochemistry

Fig. 30

Chemical structure of 46.

Huang and co-workers studied iridium(III) complexes with or without methyl amino groups on their cyclometallating ligands.163 In particular, the specie 47 shown in Fig. 31 is able to distinguish between hypoxia, normoxia and hyperoxia in live cells and in vivo. The operating principle is based on the fact that the complex exhibits dual phosphorescence with comparable intensities in phosphate buffer. This behavior is due to the presence on the cyclometallating ligands of unconjugated amine groups. The lone pare on these amino group allow the population of a 3NLCT (n to ligand charge transfer) excited state via transition to the p* localized on the polypyridine ligand (n / p*). This state inhibits the internal conversion between the 3IL and 3CT states, which emit separately with a total emission ranging from green to orange to red depending on the oxygen concentration. A direct visualization of viscosity change is instead evidenced by exploiting the photophysical properties of a complex such as the one shown in Fig. 32.164 Species 48 exhibits an intensity dependence of luminescence and viscosity lifetimes due to the mobility of aromatic rings on the diphosphine ligand and aldehyde groups on cyclometallated ligands. This type of behavior made it possible to monitor intramitochondrial viscosity, between about 35 cP and 100 cP, using FLIM microscopy with biphotonic resolution, and an increase in luminescence lifetime.

Fig. 31

Chemical structure of 47.

Multicomponent supramolecular photochemistry

Fig. 32

649

Chemical structure of 48.

In the field of multi-modal systems, two series of dinuclear heterobimetallic compounds based on the luminescent subunit Ru(bpy)2(N^N) and on a metal-based anticancer fragment (e.g. AuCl, (p-cymene)RuCl2, (p-cymene)OsCl2,) with anti-cancer potential have recently been reported in the literature (Fig. 33).165 The two subunits are linked by a bridging ligand with two different chelating sites, the first involved in the coordination of the Ru(II)-type bipyridine (49) or dipyridylamine (50), and the second a diphenyl-phosphine fragment capable of coordinating the pharmacologically active metal subunit. Photophysical studies have shown that while all dinuclear complexes based on the Ru(bpy)3-like fragment exhibit luminescence from a state of 3MLCT with a band at about 595 nm and relatively high quantum yields, in the case of the series based on the chelating fragment dipyridylamine the luminescence is dramatically quenched, most probably due to the distortion induced by the chelating amino ligand. Another particularly interesting field of application is closely related to the ability of metal complexes to possess a long-lived excited triplet state, and concerns the efficient ability to generate 1O2, due to good photostability and controllable cytotoxicity. For these reasons ruthenium(II) and iridium(III) polypyridine complexes have recently been explored as potential photosensitizers for PDT.166 In particular, among others, TLD1433, a ruthenium(II) polypyridine complex prepared and studied by McFarland, has even reached phase II clinical trials in humans.167 But most of these examples concern mononuclear species, while recently in the literature cases of multi-metallic systems capable of offering a response in photodynamic therapy via two-photon excitation has appeared. One example is a ruthenium(II)platinum(II) heterometallic metallocycle.168 The specific metallacyclic structure and the electronic communication between the metal centers offered by the bridge conjugation gives the complex a red-shifted emission and a high cross-sectional area for biphotonic absorption that reaches 1371 GM. The co-presence of positive charges and increased structure-induced lipophilicity means that the macrocyclic species can be localized in both the nuclei and mitochondria of A549 cells. Irradiation with lasers at 820 nm leads to cell death by photoinduced evolution of 1O2. In this direction, the result shown by Gasser and co-workers is particularly interesting. Who demonstrated that the efficiency of biphotonic uptake and ROS generation in PDT can be precisely regulated by modulating the conjugation of the coordination ligand

Fig. 33

Chemical structures of complexes 49 and 50.

650

Multicomponent supramolecular photochemistry

Fig. 34

Chemical structure of 51.

or by modifying it with functional groups such as trans-stilbene, a species capable of inducing biphotonic uptake efficiently. By incorporating ruthenium(II) polypyridine complexes (see complex 51 in Fig. 34), the conjugation of the ligand169 is extended, and thanks to the strong TPA property of the trans-stilbene unit,170 Ru(II) complexes are able to absorb with an efficiency of 6800 GM. In addition, this uptake is capable of stimulating singlet oxygen formation with great efficiency and induces photoinduced cytotoxicity against various tumor cells. A final example, far from being exhaustive, concerns the use of appropriate metal complexes in what is called photo-activated chemotherapy (PACT): an oxygen-independent phototherapy considered to be a very promising anti-cancer approach. An example has been reported by Lo and co-workers, who reported in the literature a series of iridium(III) complexes (e.g. compound 52 in Fig. 35) decorated with a nitrobenzyl-modified PEG pendant used as mitochondria-targeted anticancer drugs for PACT.171

8.15.5

Conclusions

In this article we have attempted to summarize few examples of multicomponent supramolecular systems based on photoactive metal complexes. We selected some key examples, which have emerged in the literature in the last decade, concerning specific classes of compounds with related applications in many research fields, including solar energy conversion, luminescence sensors, bioimaging, and phototherapy. We hope that this article will be useful as a source of information and as a possible starting point for future developments.

Fig. 35

Chemical structure of 52.

Multicomponent supramolecular photochemistry

651

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8.16 Recent progress and application of computational chemistry to understand inorganic photochemistry Thomas Penfold, Conor Rankine, and Julien Eng, Chemistry, School of Natural and Environmental Sciences, Newcastle University, Newcastle upon Tyne, United Kingdom © 2023 Elsevier Ltd. All rights reserved.

8.16.1 8.16.2 8.16.2.1 8.16.2.2 8.16.2.3 8.16.2.4 8.16.3 8.16.3.1 8.16.3.2 8.16.4 8.16.4.1 8.16.4.2 8.16.4.3 8.16.5 References

Introduction Computational considerations for understanding inorganic photochemistry Development in electronic structure theory for inorganic complexes The impact of machine learning in inorganic photochemistry Excited-state rate theory Excited-state nuclear dynamics Understanding the photophysical properties of OLED emitters Computational insights into phosphorescent molecules used for OLEDs Triplet harvesting by thermally-activated delayed fluorescence using carbene metal amides Probing inorganic photochemistry using ultrafast pulses of X-rays Electronic structure and dynamics for time-resolved X-ray spectra Ultrafast dynamics of Fe(II) carbenes photosensitisers Revealing the structure-function relationships of metalloproteins with experiment and theory Summary

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Abstract The photochemistry and photophysics of inorganic complexes is incredibly rich owing to a large number of competing outcomes including both radiative (fluorescence and phosphorescence) and non-radiative (internal conversion, intersystem crossing, and energy transfer) pathways. These are often highly coupled to, and strongly dependent on, structural changes which occur on photoexcitation. Theoretical studies of excited-state processes have tended to focused on organic photoactive molecules, largely due to the complexity of inorganic systems - in particular, the commonality with which multiple electronic configurations and energetically-close-lying electronic states are encountered. However, recent developments in both quantum chemical and quantum dynamics methodologies have made it increasingly possible to study challenging inorganic excited-state mechanisms from first principles. In this Chapter, we will review recent developments in electronic structure theory and excited-state dynamics methodologies that can be used to understand and describe the photochemistry of inorganic systems. Subsequently, to illustrate this progress, we will explore two case studies. The first of these looks at the effect of excited-state dynamics on triplet harvesting using phosphorescent and thermallyactivated-delayed-fluorescence (TADF) materials to achieve high performance in organic light-emitting diodes (OLEDs). The second of these highlights the synergy between experiment and theory while working with ultrafast pulses of X-rays to probe inorganic photochemistry, focusing expressly on the excited-state dynamics of Fe(II) carbene complexes and heme proteins.

8.16.1

Introduction

Molecules and materials that absorb and/or emit light form a central part of our daily lives. Indeed, lights at night are an effective measure of the economic development of a country,1 and the development of materials that are able to capture efficiently energy by harvesting sunlight is at the heart of our attempts to reduce our reliance on unsustainable fuel sources.2 Owing to the flexibility of their applications and their high degree of tunability, the photochemistry and photophysics of transition metal compounds has evolved into a major research theme over the past few decades, with the focus falling on challenges including elucidating the fundamental principles of their excited-state mechanisms and investigating these processes in action in photoactivated metalloproteins and molecular machines. Nonetheless, and despite great successes in the development of excited-state energy materials, there are fundamental aspects relating to the formation and harvesting of excited states that we simply do not understand at present, and which are now starting to impede progress. The challenges arise principally from the breakdown of the Born-Oppenheimer approximation and - in the particular context of inorganic photochemistry - are compounded by the commonality with which multiple electronic configurations, spin multiplicities, and energetically-close-lying electronic states are encountered, presenting a multitude of possible (and often competitive) excited-state pathways.

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The rate of population transfer, k, between two electronic states can be described using perturbation theory which - to first order is equivalent to Fermi’s “Golden Rule” approximation, and can be written as:  2p XD c  E2  k¼ (1)  jf H if ji  d Ei  Ef ; Z f c if is the where ji and jf are the total (electronic and vibrational) wavefunctions of the initial and final states, respectively, and H Hamiltonian that describes the coupling between them. Under the Condon approximation, i.e. the assumption that the coupling between the two electronic states is constant for all geometries, the electronic and vibrational wavefunctions can be separated out as: 2   2p XD c  E2 X nfk jnij  d Eij  Efk ; k¼ (2)  jf H if ji  Z f q0 jk   c if between the initial and Eq. (2) reveals the factors which control the rate of excited-state processes: of these, the coupling H final states is a crucial one. In inorganic photochemistry, it is especially important to appreciate that the presence of heavier elements means that spin-orbit couplings (SOCs) play a significant role alongside non-adiabatic couplings (NACs) between excited states and, therefore, that excited-state dynamics in inorganic systems are comparatively more likely to lead to changes in spin multiplicity - often a characteristic feature. This leads to a significant increase in the density of coupled electronically-excited states (indicated by the sum over f states in Eq. 2) that has the potential to exert an influence on the rate of population transfer. The overlap between the vibrational wavefunctions is also crucial; this can be tuned, in practice, by moderating the energy gap between the states. Many theoretical studies of excited-state processes have focused on organic photoactive molecules; this is largely because the size of these systems (i.e. number of electrons) tends to be smaller and, consequently, these systems are more amenable to, and affordable for, theoretical calculation. Predictably, high-level theoretical studies on the photochemistry of inorganic systems, e.g. transition metal complexes, are comparatively scarce. The primary reasons for this are found in the theoretical challenges associated with (i) accurately describing the electronically-excited states in transition metal coordination compounds, (ii) identifying and disentangling the excited states that are involved in the photochemical process of interest (due to the high density of electronic states present), (iii) determining the photochemical pathways evolving on the excited-state potential energy surface(s), and (iv) actually simulating the photodynamics of these systems in practice. It is acknowledged that these challenges are also present in many organic systems, but they tend to be most pronounced – and, consequently, most problematic – in inorganic systems. In this Chapter, we will review recent progress in electronic structure theory and excited-state dynamics methodologies that can be used to investigate the photochemistry of inorganic systems (Section 8.16.2). Following an outline of the requisite theory, we will introduce and explore two case studies. Firstly (in Section 8.16.3), we will explore the effect of excited-state dynamics on triplet harvesting in carbene-metal-amides which exploit thermally-activated-delayed-fluorescence (TADF)3,4 to achieve high performance in organic light-emitting diodes (OLEDs). Secondly (in Section 8.16.4), we will examine the synergy between experiment and theory while using ultrafast pulses of X-rays to probe inorganic photochemistry; specifically, we will review recent work that combines experiment and theory to deliver insight into the excited-state dynamics of Fe(II) carbene complexes.

8.16.2

Computational considerations for understanding inorganic photochemistry

The non-equilibrium nature of electronically-excited-state processes means that standard reaction rate theories used to describe thermal (i.e. electronic-ground-state) chemistry are inadequate. Instead, ultrafast dynamics that start immediately post-photoexcitation play a decisive role in subsequent processes that may deviate significantly from the limits of the Born-Oppenheimer approximation5,6 due to the strong coupling of nuclear and electronic motion. In these far-from-equilibrium situations, dynamical processes (e.g. spin, electronic, and vibrational dynamics) arising from non-Born-Oppenheimer behavior are crucial for understanding the functional properties, and unlocking the potential, of molecules and materials operating in electronically-excited states. Their description ideally calls for solutions to the time-dependent form of Schrödinger equation, which describes the wavefunction, j (R, r, t), of a quantum-mechanical system: b ðr; R; t Þ ¼ iZ vjðr; R; t Þ Hj vt

(3)

Defining an energy operator, b E, as in Eq. (4) allows Eq. (3) to be rewritten in the familiar form given in Eq. (5). v b E ¼ iZ vt

(4)

b ðr; R; t Þ ¼ b Hj Ejðr; R; t Þ

(5)

In order to obtain the time-independent form of Schrödinger’s equation, j (r, R, t) is separated out into spatial, j (r, R), and temporal, e–iEt/Z, components, as in Eq. (6), where E is the energy. The probability density, r, is then made time-independent, as in Eq. (7).

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Recent progress and application of computational chemistry to understand inorganic photochemistry jðr; t Þ ¼ jðr; RÞjðt Þ ¼ jðr ÞeiEt=Z

(6)

r ¼ jðr; R; t Þj ðr; R; t Þ ¼ jðr; RÞj ðr; RÞ

(7)

The time-independent form of Schrödinger’s equation, given in its familiar form in Eq. (8), is obtained via substitution of the expression for j (r,R, t) given in Eq. (6) into Eq. (5). b ðr; RÞ ¼ Ejðr; RÞ Hj

(8)

In the Born-Huang representation, the total wavefunction can be separated into nuclear (U) and electronic (F) terms: jðr Þ ¼

N X

Fðr; RÞUi ðR; t Þ

(9)

i

By substituting the expression in Eq. (9) into Eq. (3) and multiplying through by F*, one can obtain an expression for the timedependent nuclear wavefunction in an arbitrary electronic state, j: " # N X Z2 X vUj ðR; t Þ el ¼  iZ þ Ej Uj ðR; t Þ þ F ij ðRÞUi ðR; t Þ (10) vt g 2Mg i F ij allows excited-state population to transfer between different excited states, and is written in terms of first- and second-order NACs: " #  Z X Z2 X 1 Z  Fi ðr; RÞ þ (11) F ij ðRÞ ¼ drFj ðr; RÞ  drFj ðr; RÞ½  iZVFi ðr; RÞ ½  iZV g 2Mg g Mg Discarding F ij is equivalent to the Born-Oppenheimer approximation. While the Born-Oppenheimer approximation is a cornerstone of quantum theory and is used throughout quantum chemistry and condensed-matter physics, the rapid evolution of the nuclear wavepacket on the excited-state potential energy surface typically leads to the breakdown of this approximation.7 This makes it impossible to neglect the coupling between the electronic and nuclear degrees of freedom present in the timedependent Schrödinger equation. Importantly, the Born-Huang ansatz creates a picture of a time dependent nuclear wavepacket evolving over an excited-state potential energy surface generated by the electrons. To improve meaningfully our understanding of inorganic photochemistry, one consequently has to improve both the description of the electronic structure and the nuclear dynamics.

8.16.2.1

Development in electronic structure theory for inorganic complexes

A defining attribute of transition metal photochemistry is the commonality with which multiple electronic configurations, spin multiplicities, and energetically-close-lying electronic states are encountered. This gives rise to rich photochemistry, with a multitude of possible relaxation pathways including fluorescence, phosphorescence, internal conversion (IC), intersystem crossing (ISC), and intramolecular vibrational redistribution (IVR) routinely observed. In most cases these are coupled to changes in nuclear geometry. Additional complexity arises when multiple magnetically-interacting open-shell ions are present.8 While it is this level of complexity that makes transition metal photochemistry such a rich field, it also poses a challenge for electronic structure theory: this being able to calculate accurately and efficiently their properties ab initio, i.e. from first principles.9 Owing to the favorable balance between computational cost and accuracy, density-functional theory (DFT) and its excited-state extension, time-dependent DFT (TDDFT), are extensively employed for these kind of calculations. However, one has to exercise caution as transition metal complexes, especially in their electronically-excited states, often highlight some of the challenges and limitations of density-functional-based approaches. One of the challenges is associated with the assignment of spin states10; hybrid density functionals tend to favor high-spin-state solutions, while the earlier generalized gradient approximation (GGA) density functionals favor low-spin-state solutions. This is because Hartree-Fock-type theories, incorporated into hybrid density functionals, are able to describe satisfactorily the Fermi correlation but not the dynamic correlation.11 This has driven empirical fine-tuning of established density functionals to improve their performance - B3LYP*,11 for example, was developed in this way - and has, in some cases, led to density functionals that are capable of providing a balanced and accurate description of both the electronic ground and excited states.12–14 Beyond the presence of multiple spin states, care also has to be taken where states exhibit multireference and/or charge transfer (CT) character. The latter is a well-documented weakness of TDDFT15 and is particularly problematic where CT states, e.g. metal-to-ligand/ligand-to-metal CT (MLCT/LMCT) states, are in close proximity to locally-excited (LE) states, e.g. metalcentered (MC) states. From the perspective of post-Hartree-Fock theory, the challenge presented by transition metal complexes - particularly in exchange-coupled systems - is their multireference character; this demands a balanced and accurate description of both static (e.g. multiple electronic configurations) and dynamic electron correlation. Many post-Hartree-Fock electronic structure methods, e.g. algebraic diagrammatic construction (ADC)16 and coupled cluster (CC)17–19 theory, while providing a better description of the latter kind of correlation, can only provide a satisfactory description of transition metal complexes overall in the absence of strong multireference character. This said, there are exciting developments underway towards addressing this limitation; Sokolov

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and co-workers20–22 have recently introduced a multireference ADC (MR-ADC) formalism to the first-22 and second-order degrees of expansion20,21 that equips ADC theory to take on practical problems with multiconfigurational character, and progress is being made on multireference CC (MR-CC) theory, too, by Neese and co-workers.23 Although MR-ADC and MR-CC arguably lose some of the ‘black box’ character of their parent post- Hartree-Fock methods, and are certainly associated with greater computational expense, they nonetheless represent promising developments; Neese and co-workers,23 for example, benchmarked their MR-CC implementation against multireference CISD (MR-CISD; a ‘gold standard’ approach for multireference problems) on an inorganic problem - specifically, in calculations of the Fe L-edge XAS spectra of [Fe(II)Cl4]2 and [Fe(III)Cl4] - and reported that MR-CC outperformed MR-CISD on accuracy with respect to experiment and on scaling up to larger active space sizes.23 The authors emphasized the importance of active space selection - in particular, the inclusion of the orbitals associated with the Cl ligands - but found that MR-CC was less sensitive to the exact construction of the active space than MR-CISD and that it allowed, in general, for it to be kept comparatively compact.23 Nonetheless, to treat effectively the most difficult multireference cases, multiconfigurational selfconsistent field (MCSCF) methods are usually adopted.24 MCSCF methods use a full configuration interaction (FCI) ansatz, but do so cost-effectively by limiting it to a defined subspace of orbitals - the active space.24 The active space has to be chosen judiciously to span all of the important configurations and resolve any quasi-degeneracies.24 Configuring the active space is not always straightforward (indeed, it has been described as an art)25 as there are no transparent rules governing the choice and it is often made on intuition and/or experience alone, although quasi-systematic strategies do exist.25 The complete active space self-consistent field (CASSCF)26 method is one example of an MCSCF method; in the CASSCF method, FCI is carried out in an N-electron, n-orbital CAS space. The number of configurations spanned by a CASSCF expansion increases factorially with N and n, however, and it can quickly become prohibitively large. It is possible to use larger active spaces with, for example, the restricted active space self-consistent field (RASSCF)27 method; in the RASSCF method, the active space is split into three subspaces, RAS1, RAS2, and RAS3, with FCI carried out in the RAS2 space and a specified maximum number of holes and electrons allowed in the RAS1 and RAS3 subspaces, respectively. This concept can be further extended, and leads naturally to the generalized active space self-consistent field (GASSCF)28 method in which an arbitrary number of subspaces, each with independent rules governing the allowed orbital occupations, can be configured. MCSCF methods like CASSCF, RASSCF, and GASSCF are able to capture the multiconfigurational character of transition metal systems and, in combination with subsequent perturbative dynamic correlation treatments, e.g. CASPT229,30 and RASPT2,31 can deliver very high quality results at an increased computational cost, provided that the active space spans all of the configurations necessary to describe the excited state(s) of interest. The large number of electrons in the confined region of space spanned by the metal-centered d orbitals means that an accurate treatment of dynamic correlation is essential to provide quantitatively correct (and, in some cases, even qualitatively correct) results for transition metal complexes.32 An alternative approach for describing multiconfigurational character and incorporating an accurate account of dynamic electron correlation is to combine DFT with a multireference configuration interaction (MRCI) ansatz (DFT/MRCI).33,34 Here, the information contained within the DFT Kohn-Sham (KS) orbitals, coupled with the MRCI ansatz, delivers an accurate and cost-effective description for large systems and limits the number of configuration state functions (CSFs) via configuration selection.33,34 While care has to be taken to avoid the double counting of dynamic electron correlation contributions, the DFT/MRCI approach has been successfully applied to a number of transition metal systems.35 Beyond the calculation of electronically-excited states, determining their character unambiguously is one of the most important tasks for analyzing and understanding mechanistic details as it underlies any meaningful discussion of the photochemistry/photophysics of transition metal complexes. Indeed, there are a great number of possible state characters including MC, MLCT, LMCT, intra-ligand (IL), and ligand to- ligand charge transfer (LLCT), among others; these different state characters are crucial to many of the functional properties of transition metal complexes. Recently, Mai et al.36 have introduced an objective and automatable strategy for assigning state character based on the transition density matrix.37,38 Their approach can be used with a number of electronic structure methods and, indeed, holds value even in particularly challenging cases, e.g. transition metal complexes with spinorbit-mixed states. Advances in electronic structure methods are reported continually and, recently, there have been a number of exciting developments reported in the literature that promise to extend the most accurate electronic structure treatments to larger systems and to accelerate computations. Implementation of approaches based on the density matrix renormalization group (DMRG)39 strategy, for example, have opened up the possibility of using much larger active spaces in MCSCF methods.40 In addition, Thomas et al.41 have shown that CI quantum Monte Carlo (CIQMC) is able to deliver accurate transition metal ionization potentials; CIQMC has also been used in a practical setting to investigate the electronic structure of Fe(II) porphyrins.42 The adaptation of existing electronic structure theory codes to new architectures is also driving efficiency boosts for a number of popular algorithms.43,44 Graphical Processing Unit (GPU) acceleration is a particularly hot topic (with some new electronic structure theory codes being designed from the ground up with the intention of baking GPU support into the program),43,44 and great strides have been made in this area over the last decade; Martínez and co-workers, in particular, have actively taken the lead in implementing GPU acceleration for multiconfigurational (e.g. CASSCF, CASCI) and CI schemes.45–52 These developments, in parallel with the increased power and accessibility of computational resources and the development of improved architectures, are opening up new opportunities to investigate the electronically-excited states of ever-larger transition metal complexes and take on ever-more ambitious challenges in inorganic photochemistry.

658 8.16.2.2

Recent progress and application of computational chemistry to understand inorganic photochemistry The impact of machine learning in inorganic photochemistry

Supervised machine-learning/“deep learning” algorithms53 can be highly effective at mapping complex, non-linear relationships, and offer the attractive possibility of being able to make accurate predictions of properties/observables from very little input information. Consequently, as these algorithms have seen adoption in almost all aspects of modern life, so too have they exerted an impact across the natural sciences.54–61 Unsurprisingly, there are a growing number of examples of machine learning (ML) in inorganic photochemistry, especially in light of the challenges discussed in Section 8.16.2; ML models have been deployed to accelerate electronic structure calculations of large systems,54 predict excited-state properties from ground-state wavefunctions,62 and estimate experimental observables such as X-ray spectra (XS)63–65 - in addition to decoding them.66–74 Indeed, ML models have even been incorporated directly into the heart of the electronic structure theory in recent years.75 However, while undeniably exciting developments, our expectations have to be tempered; many of these ML-assisted approaches are in the very early stages of development, and many more that have seen success elsewhere are yet to be applied to inorganic systems. Among others, Kulik and co-workers have worked intensively to design ML models to accelerate materials discovery in inorganic chemistry.76–78 Beyond their work towards the accurate prediction of material properties, e.g. HOMO-level energies, the HOMOLUMO gap, and spin-state splittings,76,78 the authors have outlined a number of key targets for ML-assisted inorganic chemistry, including the crucial tasks of (i) automating the simulation of new compounds (traversing quickly a very large chemical space), (ii) assessing adequately the accuracy of ML predictions, and (iii) developing models that enable ‘faster-than-fast’ property predictions to overcome bottlenecks in first-principles characterisation.77 Penfold and co-workers63–65 have pushed the envelope beyond the prediction of scalar properties and developed a multi-output deep neural network (DNN) based on the multilayer perceptron (MLP) model for the instantaneous, qualitative prediction of XS spectra. Rankine et al.65 introduced the DNN, which is able to predict Fe K-edge XS spectra using nothing more than the local geometry of an arbitrary Fe absorption site, i.e. bypassing time- and resource-intensive electronic structure theory calculations (Section 8.16.2.1), and demonstrated that it was able to reproduce peak positions to sub-eV accuracy with respect to reference XS data and peak intensities with errors an order of magnitude smaller than the spectral variation in the reference XS data set.65 Madkhali et al.63,64 have worked with the DNN on practical problems, demonstrating - most significantly - that, with an appropriate reference XS data set, it can be reoptimized to be equally performant at other XS absorption edges.64 Madkhali et al.64 used the DNN to predict Co K-edge XS spectra and analyze T-jump-pump/XS-probe experimental data acquired by Chergui and co-workers79 in their recent work. The authors took advantage of the instantaneous predictions provided by the DNN to be able to sample a greater number of possible geometric configurations than would otherwise have been possible (as computational cost would otherwise have precluded an exhaustive electronic structure theory analysis), better modelling the effects of thermal disorder on the measurements of Chergui and co-workers,79 and discovered, in the process, hidden details about the near-infrared-driven ligand exchange processes of Co2þ in chlorinated aqueous solution.64 In the context of the present Chapter, it is the electronically-excited states of inorganic systems which are of particular interest. There has been some significant progress towards describing the electronically-excited states of (organic) molecular systems using ML models (see, for example, Ref. 80), but such ML models have yet to be extended to inorganic chemistry. Part of the challenge here is likely to be curating the reference dataset with which the ML model is optimized, as calculating accurately the properties of transition metal complexes requires high-level electronic structure theory and is often not ‘black box’, i.e. there is no easilyautomatable, ‘one-size-fits-all’ strategy and, for the volume of data that ML models typically require, a bespoke approach is out of the question.

8.16.2.3

Excited-state rate theory

Within the limit of perturbation theory, i.e. the assumption that the coupling between the initial and final states of interest is small compared to their adiabatic energy difference, it is possible to calculate the rate of population transfer between excited states using Fermi’s “Golden Rule.” The usual starting point is to approximate the initial- and final-state potentials using the harmonic oscillator model with vibrational frequencies Ui and Uf, respectively. The mass-weighted normal coordinates of the final state, Qf, can be mapped onto the mass-weighted normal coordinates of the initial state, Qi, via a Duschinsky transformation.81 The rate of population transfer, kISC, is then either estimated from only the direct coupling under the Condon approximation:  2pD c  E2 X  2  nfk nia d Eia  Efk (12) kFC  jf H SO ji  ISC ¼ Q0 Z k .or by including the effects of the nuclear motion of the electronic coupling with Herzberg-Teller-like terms: 0 0 E  D  c   2p BXBXv jf  H SO ji   HT Au-Cz > Ag-Cz (Fig. 6).186,242 Recent experiments reported by Feng et al.158 have confirmed the importance of the LE triplet excited state and the role of the spin-vibronic pathway for ISC.158 This is most significant for Ag-Cz; this is the complex which has the weakest direct ISC pathway and is, therefore, the complex which exhibits the slowest ISC. The experiments of Feng et al.158 also confirmed the ordering of the ISC rates reported in Ref. 60, demonstrating how a close interplay between experiment and theory can provide a deep complementary understanding. The experiments of Feng et al.158 were also able to measure the activation energy, which was found to be correlated with the expected exchange energy of the charge-transfer state; it was also found to be closely related to the length of the bonds joining the carbene and amide ligands.158 The findings for the CMA complexes highlight the importance of understanding rigorously the interplay between multiple electronically-excited states, as this is a route towards fine-tuning the functional properties of these complexes. For example, another approach, exploiting only triplet excited states, was recently proposed by Leung et al. The authors reported a thermally-stimulated delayed phosphorescence (TSDP) process208; in TSDP, multiple low-lying triplet excited states are accessed and are able to emit light; the emission color can be tuned by the environment/host polarity,208 similar to mechanisms proposed for organic molecules.188,192,220 Combining electronic excited states of different natures and spin multiplicity may offer an interesting design option for efficient TADF emitters. This is typically employed in hot-excitons TADF emitters,198,218,245 where the triplet population is harvested in higher lying electronic states. Emission then occur from a lower excited state better suited for efficient and narrow fluorescence. However, this requires the ability to describe a large variety of electronic states of different nature on an equal footing. TDDFT is known to badly describe long range interaction15,182 and may makes its usage problematic when relative position of local exciton

Fig. 3

Structure of a donor-bridge-acceptor carbene-metal-amide. M ¼ Cu (Cu-Cz), Ag (Ag-Cz), Au (Au-Cz).

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Fig. 4 Difference in electronic density associated with the 3CT, 1CT, 3LE(CAAC), and 3LE(Cz) electronically-excited states for Ag-Cz {CAAC ¼ cyclic (alkyl)(amino) carbene; Cz ¼ carbazole}. While the ordering of the electronically-excited states changes for Cu-Cz and Au-Cz, the difference in electronic density remains, qualitatively, the same. For each state, a front (left) and side (right) view is shown. A reduction in electronic density is shown in red; a gain in electronic density is shown in blue. Figure reproduced from Eng, J.; Thompson, S.; Goodwin, H.; Credgington, D.; Penfold, T. J. Competition Between the Heavy Atom Effect and Vibronic Coupling in Donor–Bridge–Acceptor Organometallics. Phys. Chem. Chem. Phys. 2020, 22, 4659–4667.

and charge transfer states is important. More advanced methods aforementioned may help circumventing this issue at the expense of computational time. The use of optimally tuned range corrected functional185,239 gives a less time-consuming option. In this case, Hartree-Fock exchange is used at long range to improve the TDDFT bias on long range interactions.

8.16.4

Probing inorganic photochemistry using ultrafast pulses of X-rays

Ultrafast pump-probe studies, starting with the development of femtosecond/ps linear and nonlinear optical spectroscopies, have contributed greatly to our understanding of transition metal photochemistry. However, for molecular systems with more than two atoms, the link between the spectroscopic observables in the optical domain and the underlying nuclear structure and - in particular - dynamics is ambiguous. The ability to study directly these dynamics with structural probes, e.g. light in the X-ray region of the spectrum, is very appealing.216,225 X-ray spectroscopy (XS) is a particularly powerful tool when implemented in a pump-probe setup as it provides users with element- and site-specific insight into the electronic and geometric structure of matter and - crucially - how it evolves with time.216,225 The use of time-resolved XS techniques is becoming increasingly common, driven by the evolution of highbrilliance light sources such as 3rd-generation synchrotrons and 4th-generation X-ray free-electron lasers (XFELs).163,164,184,199,202 In addition, outside of these facilities, researchers have been able to bring XS experiments closer to home with powerful miniaturized tabletop X-ray spectrometers based on high-harmonic generation (HHG).165,170,171,226,231,246,247 In this Section, we will focus on highlighting recent work, especially at XFELs, and point out how, in combination with electronic structure theory and dynamics simulations, it has been used to untangle excited-state dynamics of transition metal complexes. We will summarize some of the requisite theory - specifically that which is relevant to XS - and then, as a case study, we will explore an Fe(II) carbene photosensitizer.

8.16.4.1

Electronic structure and dynamics for time-resolved X-ray spectra

On the surface level, there appear to be many similarities between calculating core- and valence-excited states and spectra and, in fact, almost all of the popular electronic structure methods for calculating valence-excited states have been adapted to allow for the calculation of core-excited states as well.227 However, there are a number of challenges that are unique to the calculation of core-

(B)

(C)

Fig. 5 Evolution of the diabatic population of the electronically-excited states as a function of time for (A) Au-Cz (B) Ag-Cz (C) Cu-Cz following excitation to the 1CT state. Figure reproduced from Eng, J.; Thompson, S.; Goodwin, H.; Credgington, D.; Penfold, T. J. Competition Between the Heavy Atom Effect and Vibronic Coupling in Donor–Bridge–Acceptor Organometallics. Phys. Chem. Chem. Phys. 2020, 22, 4659–4667.

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(A)

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Fig. 6 Transient absorption spectra of (A) 5 wt% Au-Cz, (B) 5 wt% Ag-Cz, and (C) 5 wt% Cu-Cz in PS thin film on the picosecond-to-nanosecond time scales. Recorded at 298 K. The intensity is shown in DT/T, i.e. the fractional change in transmission. (D) The normalized deconvoluted singlet kinetics of Au-Cz, Ag-Cz, and Cu-Cz; the ISC time of each sample is estimated by the crossover of the singlet and triplet kinetics, and is labelled with a spot. The ISC time is ca. 6.0 ps for Au-Cz, ca. 32.2 ps for Ag-Cz, ca. 3.7 ps for Cu-Cz. Figure reproduced from Feng, J.; Reponen, A.-P. M.; Romanov, A. S.; Linnolahti, M.; Bochmann, M.; Greenham, N. C.; Penfold, T. J.; Credgington D. Influence of the Heavy Atom Effect on the Photophysics of Coinage Metal Carbene-Metal-Amide Emitters. Adv. Func. Mater. 2021, 31, 2005438.

excited states. The clearest difference is the high energy at which core-excited states are typically located; this is often in the thousands of electron volts. The challenge here is that electronic structure approaches tend to solve for the n lowest-energy states, making the calculation of the high-energy core-excited states problematic as a huge number of lower-lying states would need to be calculated to obtain the requisite information.227 This is solved in practice with the core-valence separation (CVS) approximation; the CVS approximation involves the generation of an approximate Hamiltonian which decouples the core from the valence-excited states, making it possible to obtain the core-excited states as the lowest-energy roots of the resultant expansion.227 Other challenges abound227; it is also important to bear in mind (i) the breakdown of the electric dipole approximation, which arises from the short wavelength of the X-ray radiation,168,211 (ii) the practical necessity of including relativistic effects,179,180 and (iii) the difficulties associated with obtaining an accurate description of very-high-lying core-excited states which are embedded in the ionization continuum.228–230 In the context of the present Chapter, it is important to remember that the calculation of experimental observables required for an in-depth analysis not only requires the computation of valence-excited states or core-excited states but, additionally, the interaction between the valence- and core-excited states.227 This can be achieved, for example, under the framework of the RASSCF method (Section 8.16.2.1); while valence-excited states are calculated in the usual way, core-excited states can be calculated with appropriately-configured RAS1, RAS2, and RAS3 subspaces, e.g. the RAS1 subspace, allowing at most one hole, can be configured with the core-orbitals associated with the core-excited state of interest. Ultimately, the interaction between the valence- and core-excited states is obtained from two separate calculations, and the (spin-orbit) RAS state interaction {(SO)-RASSI}214,215 scheme is typically used subsequently to obtain the relevant information that couples the two sets of states.200,219,244 Practicably, one of the challenges is that the unfavorable computational scaling of these schemes is compounded by the fact that the dynamics may require a large number of calculations at each sampling interval to represent correctly the nuclear wavefunction.227 A simplification of the analysis can be achieved by restricting the sampling of geometries for spectral simulations to critical points on the electronically-excited-state potential energy surface {e.g. transition geometries, or minimum-energy crossing points

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(MECP)/conical intersections (CI)}, but the increased efficiency usually comes at the cost of more severe approximations. For accessing the interaction between valence- and core-excited states, Northey et al.219 recently proposed a mixed TDDFT/maximum overlap method (MOM)169,193 scheme. The scheme described by Northey et al.219 used the MOM to converge a reference valenceexcited state which was then acted on by TDDFT to calculate the manifold of relevant core-excited states.219 The MOM method forms part of a class of electronic structure theory methods referred to as DSCF. It allows excited states dominated by a specified character (e.g. a HOMO-LUMO excitation) to be generated and maintained during the process of wavefunction optimisation.169,193 The MOM prevents variational collapse during iterative optimization cycles by setting the occupancy of the orbitals such that the overlap between the occupied orbitals of the current (nth) and previous ((n  1)th) iteration is maximized, rather than setting the occupancies according to the aufbau principle, as is traditional.169,193 An orbital overlap matrix, O, is obtained at each iteration via Eq. (23). y

O ¼ Cn1 SCn

(23)

The projection, pj, of the ith orbital for the current iteration onto the jth orbital from the previous iteration is found via Eq. (24), and the occupied orbitals are then set to those with the largest projections.169,193 X pj ¼ Oij (24) i

By carrying out complementary RASSCF/PT2 calculations on top of their mixed TDDFT/MOM approach, Northey et al.219 were able to obtain a substantial boost in efficiency without sacrificing qualitative accuracy. However, limitations clearly arise from the single-reference description of the valence-excited state given by the mixed TDDFT/MOM approach, which will be inadequate for describing critical points on the electronically-excited-state potential energy surface, e.g. MECP/CI. Alternative approaches which could also be investigated include quadratic response TDDFT methods232 or sequential double excitations within the framework of linear-response TDDFT.217 In other work, Capano et al.175 derived ultrafast Cu K-edge XAS (XANES and EXAFS) and Ka1,2, Kb1,3, and Kb2,5XES signals for a prototypical Cu phenanthroline complex ([Cu(dmp)]þ) using earlier wavepacket dynamics simulations based on the MCTDH scheme89,90,224 and, thus, adopted a reduced-dimensionality model Hamiltonian approach (Section 8.16.2.4). The authors’ conclusions were recently confirmed by state-of-the-art experiments at the SPring-8 Angstrom Compact Free Electron Laser facility,201 highlighting how theory can be used to propose, plan, and predict experiment, i.e. it is not only valuable postexperiment, where it is routinely used to interpret data.

8.16.4.2

Ultrafast dynamics of Fe(II) carbenes photosensitisers

Fe(II) complexes provide a playground for theory and experiment - in particular, the opportunity to test our understanding of, and our ability to control, complex nuclear and electronic excited-state dynamics. For example, Fe(II) complexes with strong charge transfer absorption bands in the visible spectral region can be utilized as photosensitisers for solar energy conversion applications. However, in addition to their intense MLCT bands, the partially-filled d orbitals of Fe(II) complexes give rise to low-lying MC states as well. These states contribute collectively to the extremely rich photochemistry and ultrafast excited-state dynamics that culminate in a phenomenon known as light-induced excited-state spin trapping (LIESST).181,197 A prototypical example of LIESST is found in the [Fe(bipy)3]2 þ complex.174 After photoexcitation at the 1MLCT band, this complex relaxes into a non-emissive quintet MC state within ca. 100 fs,166,207,248 a decay route that makes the complex unsuitable for applications as a photosensitizer because the MC states transfer the photoexcited electrons away from the accessible outer (ligand) reaches of the complex (Fig. 7). These observations led to Fe(II) complexes being discarded as potential photosensitisers in favor of rare, and often environmentally-unfriendly, alternatives, e.g. Ru(II), the complexes of which have larger crystal field splitting. However, independently of the transition metal, the crystal field splitting and, therefore, the energy of the lowest-lying MC states can be controlled by developing ligands which destabilize the MC states such that they no longer contribute to the photophysics and photochemistry of the transition metal complex. Towards this end, a significant amount of work has focused on N-heterocyclic carbenes.176– 178,187,209,213,241 Liu et al.,213 for example, have carried out work to understand the prototypical example, [Fe(bmip)2]2 þ, (bmip ¼ 2,6-bis(3-methyl-imidazole-1-ylidine)-pyridine), which exploits strongly s-donating N-heterocyclic carbene ligands.213 Using these ligands, Harlang et al.194 have demonstrated that the picosecond lifetime of the 3MLCT state of [Fe(bmip)2]2 þ anchored to titanium dioxide (TiO2) facilitates the injection of photoelectrons in the conduction band of the latter with a quantum yield of 92%, illustrating that the complex could be highly effective as a photosensitiser.194 To explore the excited-state decay processes of [Fe(bmip)2]2 þ, Fredin et al.191 used TDDFT scans between key optimized geometries on the electronically-excited-state potential energy surface to show that the exceptionally long excited-state lifetime of the MLCT states in [Fe(bmip)2]2 þ is a consequence of the greater destabilization of the triplet and quintet MC states {cf. to other Fe(II) complexes}; a product of the N-heterocyclic carbene ligands.191 Fredin et al.191 also reported that the 3MLCT potential was very flat - a finding that contributed to the slow decay into the 3MC state.191 Subsequently, and using first-principles quantum nuclear wavepacket simulations, Pápai et al.14 added the all-important dynamical dimension; using a model Hamiltonian comprising four of the most important excited-state nuclear degrees of freedom and 25 separate electronically-excited states, the authors identified excited state dynamics and decay channels which were in excellent agreement with earlier experimental

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Fig. 7 Schematic of the electronically-excited-state potential energy surfaces, proposed decay pathways, and time scales involved in the excitedstate deactivation processes of (A) [Fe(bipy)3]2 þ and (B) [Fe(bmip)2]2 þ. Figure reproduced from Pápai, M.; Vankó, G.; Rozgonyi, T.; Penfold, T. J. High-Efficiency Iron Photosensitizer Explained with Quantum Wavepacket Dynamics. J. Phys. Chem. Lett. 2016, 7, 2009–2014.

observations.14,213 The excited-state dynamics simulations of Pápai et al.14 demonstrated that ultrafast ISC takes place from the 1 MLCT to the 3MLCT state within 100 fs post-photoexcitation; the authors found that this occurs because of the initial neardegeneracy of the 1MLCT and 3MLCT states involved in the transfer and the limited nuclear motion required to reach the point of degeneracy.14 Subsequently, decay into the 3MC states was recorded; this process was found to be slower because the wavepacket did not readily reach the crossing between the states and the population transfer was found to occur instead where the energy gap between the states was large (Fig. 8), making it much less efficient.14 Pápai et al.12 subsequently extended their work towards understanding the effect of tert-butyl functionalization on the photoexcited decay.12 The [Fe(btbip)2]2 þ (btbip ¼ 2,6-bis(3-tert-butyl-imidazole- 1-ylidene)pyridine) complex showed faster deactivation from the 1,3MLCT) states; this was found to occur within 350 fs post-photoexcitation,12 in good agreement with earlier experimental measurements of 300 fs.213 In contrast to the unfunctionalized parent compound, [Fe(bmip)2]2 þ, population transfer was found to occur into the 1MC state which intersects the MLCT states close to the Franck-Condon geometry. Consequently, the 1 MC, usually neglected in modelling the decay channels of similar complexes, was demonstrated to be crucial in [Fe(btbip)2]2 þ.12

Fig. 8 Snapshots of the wavepacket density in the (A) 3MLCT and (B) 3MC excited states projected along two of the most important vibrational degrees of freedom at time delays of 50, 200, 400, and 1000 fs post-photoexcitation, and a (C) one-dimensional projection of the wavepacket in the 3 MLCT excited state along n6 overlaid with the form of the electronically-excited state potential energy surfaces. Figure reproduced from Pápai, M.; Vankó, G.; Rozgonyi, T.; Penfold, T. J. High-Efficiency Iron Photosensitizer Explained with Quantum Wavepacket Dynamics. J. Phys. Chem. Lett. 2016, 7, 2009–2014.

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The authors also demonstrated how the excited-state relaxation of the Fe(II) carbene complexes could be controlled by relatively minor changes in the ligand sphere; a finding which presents both significant challenges and opportunities for the optimization of these materials.12 Related work has also highlighted the importance of the solvent for mediating excited-state relaxation.222 The previously-detailed works have all investigated the ultrafast excited-state dynamics using model Hamiltonians and, therefore, have restricted the nuclear degrees of freedom of these complexes. In contrast, Zobel et al.249 performed TSHD simulations within a linear vibronic coupling model which included 244 nuclear degrees of freedom and 20 singlet and triplet coupled excited states in a related Fe(II) complex.249 The authors reported ultrafast ISC, consistent with the previous results, while a smaller fraction of the population yet was found to decay directly to the ground state with a timescale of ca. 300 fs.249 A powerful way to access directly the correlated spin-vibronic dynamics is to carry out ultrafast XS experiments. Penfold et al.95 have previously used quantum dynamics to predict the ultrafast XS observables for [Fe(bmip)2]2 þ; the authors demonstrated that it would be possible to observe the coherent wavepacket oscillations.95 Recently, Kunnus et al.206 have performed the proposed experiment using Ka and Kb XS and X-ray scattering (Fig. 9).206 The authors were able to observe a coherent oscillation with a period of ca. 300 fs206 which corresponds to symmetric breathing mode of [Fe(bmip)2]2 þ in the 3MC state.14,95 The authors also reported bifurcation of the excited-state population, not reported in the simulations, and, while ca. 60% of the wavepacket was found to follow the decay from the 1MLCT state into the 3MLCT state and then onwards to the 3MC state with a ca. 9 ps timescale (in excellent agreement with the theory), 40% of the wavepacket was found to decay directly into the 3MC state.206 However, the simulations carried out by Penfold et al.95 assumed excitation of the lowest-energy excited states at ca. 485 nm, consistent with previous optical

Fig. 9 A (A) UV–visible absorption spectrum of [Fe(bmip)2]2 þ in acetonitrile and its chemical structure (inset), with (B) a schematic of the experimental setup, (C) time-resolved difference signals from X-ray solution-phase scattering, (D) time-resolved difference Fe Kb XES spectra, and (E) time-resolved Fe Ka/Kb XES and X-ray solution-phase scattering traces. The inset on (E) highlights the oscillatory component of the XES and XSS signals. Figure reproduced from Kunnus, K.; Vacher, M.; Harlang, T. C. B.; Kjær, K. S.; Haldrup, K.; Biasin, E.; van Driel, T. B.; Pápai, M.; Chabera, P.; Liu, Y.; Tatsuno, H.; Timm, C.; Källman, E.; Delcey, M.; Hartsock, R. W.; Reinhard, M. E.; Koroidov, S.; Laursen, M. G.; Hansen, F. B.; Vester, P.; Christensen, M.; Sandberg, L.; Németh, Z.; Sárosiné Szemes, D.; Bajnóczi, É.; Alonso-Mori, R.; Glownia, J. M.; Nelson, S.; Sikorski, M.; Sokaras, D.; Lemke, H. T.; Canton, S. E.; Møller, K. B.; Nielsen, M. M.; Vankó, G.; Wärnmark, K.; Sundström, V.; Persson, P.; Lundberg, M.; Uhlig, J.; Gaffney, K. J. Vibrational Wavepacket Dynamics in an Fe Carbene Photosensitizer Determined with Femtosecond X-ray Emission and Scattering. Nature Comm. 2020, 11, 634.

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time-resolved spectroscopy measurements,212 while Kunnus et al. excited the sample at 400 nm and it is quite possible that this additional energy could contribute to the observation of different behaviour.206 It is especially interesting that the Ka and Kb X-ray emission (XES) spectra showed a strong dependence on the structural (i.e. nuclear) dynamics, as these XES spectra arise from transitions between core orbitals and have therefore been thought to be less sensitive to vibrational motion. However, Vacher et al.243 recently used the RASSCF/PT2 approach to demonstrate that the sensitivity of core-to-core transitions to structural dynamics arises from the different shapes of the potential energy surfaces for the 1s and 2p corehole states. Vacher et al.243 have suggested that the different shapes arise from the non-bonding 3s and 3p orbitals in the 1s core-hole states, which serve to decrease electron-electron repulsion and increases overlap in the metal-ligand bonds.243 Finally, Paulus et al.223 have recently demonstrated that the MLCT lifetime of Fe(II) complexes can be manipulated using information from excited-state quantum coherences as a guide to implementing synthetic modifications that change the reaction coordinate associated with MLCT decay.223 This leads to the idea of molecular design beyond the Born-Oppenheimer approximation: an essential direction for the intelligent design of next-generation photoactive materials. These ideas will bring new challenges for theory acutely into focus before it can be used to predict accurately the optimal strategies for tailoring ultrafast excited-state dynamics through synthetic design.

8.16.4.3

Revealing the structure-function relationships of metalloproteins with experiment and theory

Diatomic molecules generated within cells play an important role in various signaling pathways when bound to heme-based sensor proteins such as myoglobin (Mb). The description of the biological functions of proteins such as Mb relies largely on the use of static protein structures from crystallography which provide the basic input data to develop a mechanistic understanding. However, to understand more completely protein function, a detailed understanding of the interplay between structural and dynamical properties is required. Certainly, in the context of ligand binding to the heme protein, the most commonly adopted and effective approach to access this information is pump-probe spectroscopy.204,205,237 In such an experiment, an initial pump pulse is typically delivered to dissociate a ligand from the Fe core of the porphyrin, and a second (delayed) probe pulse is used to monitor the evolution of the system with time.204,205,237 However, the complicated nature of the underlying nuclear and electronic dynamics, which include many excited states of many different spin multiplicities, means that theory and computation play an important and complementary role alongside experiment. For example, Harvey and co-workers195,196,221,240 have used ab initio electronic structure theory and DFT to map out the region of the potential energy surface associated with the dissociation and recombination of a ligand with the heme, illustrating the deep insight that can be obtained by considering exhaustively states of different spin multiplicity for a variety of ligands.195,196,221,240 Recently, Falahati et al.189 have extended this work into the temporal domain; the authors developed a model Hamiltonian (Section 8.16.2.4) containing the key vibrational coordinates and electronically-excited states to enable an accurate quantum dynamics simulation of the initial stages of diatomic ligand photolysis and the spin crossover transition that initiates a conformational change in the protein.189 Their model Hamiltonian was based on the linear vibronic coupling model (Section 8.16.2.4), but also included SOCs to allow for transitions between electronically-excited states of different spin multiplicity. The potential energy surface along the key degrees of freedom was precomputed at the CASSCF/PT2 level of theory (Section 8.16.2.1) and fitted to the model until the total error was < 0.10 eV.189 Using this model, Falahati et al.189 showed that, after photoexcitation, the initial electronically-excited states decay onto the triplet manifold and, subsequently, populate a quintet state within the first 500 fs (Fig. 10).189 It is the ultrafast spin dynamics which ultimately drive the structural change associated with the doming of the heme. As an additional detail, their results revealed wavepacket dynamics along the FeeCO bond with a period of 42 fs; these dynamics are thought to play a significant role in making the CO dissociation irreversible (Fig. 10C).189 Their conclusions were supported by a recent time-resolved XS study233 which built upon previous time-resolved XS studies of nitrosyl Mb (MbNO).203,238 In this work, Shelby et al.233 observed rapid formation of a triplet excited state within 80 fs as the CO ligand dissociated.233 An expansion and concomitant doming of the heme was observed within 900 fs, consistent with the predicted formation of the quintet excited state.233 Unfortunately, as the time-resolution of the experiment was limited to 80 fs, the authors were unable to confirm the ultrafast wavepacket dynamics along the FeeCO bond that were reported by Falahati et al.189 In the work by Shelby et al.,233 the spin-crossover process was observed during ligand expulsion from the heme, although similar spincascade dynamics have also been reported in ferric hemes which do not exhibit ligand release.167 Finally, it is important to touch on the challenge associated with resolving the electronic structure of these systems using modern electronic structure theory. CASSCF/PT2 calculations, for example, are typically highly accurate (Section 8.16.2.1), but their computational cost frequently limits their applicability to model systems, e.g. the imidazole-heme-CO model system. In the work by Falahati et al.,189 these calculations were supplemented with a quantum-mechanics/molecular-mechanics (QM/MM) strategy to incorporate the effect of the protein environment on the properties of the heme,189 but the embedded QM/MM strategy does not natively include effects such as polarisability, which can be important for electronically-excited states. Recently, Linscott et al.210 have gone one step further by combining dynamical mean-field theory (DMFT) and linear-scaling DFT to enable the accurate description of strongly correlated electronic behavior while simultaneously including - at the quantum level of theory - the effects of the environment.210 Such strategies are ideal for challenging problems such as metalloproteins and, indeed, Linscott et al.210 demonstrated this practically by computing the photodissociation properties of a CO-ligated Mb (MbCO).

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Fig. 10 Quantum photodynamics of heme-CO complex: (A) evolution of diabatic populations for the 1Q (magenta), 1MLCT (orange), triplet band (green), and quintet band (blue) excited states. The 1Q population decays rapidly, populating the 1MLCT excited state in as little as 75 fs, at which point the triplet manifold population increases. The quintet manifold population builds up more slowly and evolves into the dominant component by around 350 fs. (B) A schematic representation of the reaction mechanism and interpretation in terms of time constants; upon initial photoexcitation into the Q-band, the 1MLCT state is populated in ca. 25 fs. In a second step, the system relaxes onto the triplet manifold in ca. 75 fs, and then onto the lowest-energy quintet state in ca. 430 fs. The black arrows indicate the direction of the electron transfer and the main nuclear motions. (C) The evolution of the FeeC(O) distance (magenta; left axis) and the Fe out-of-plane distance (black; right axis); large-amplitude motions are observed with a 40 fs oscillatory period. The amplitude of the oscillation is initially ca. 0.9 Å and converges towards of 2.2 Å at later times. At this distance, the CO can be considered to have been ejected. The standard deviation of these values is shown as a shaded area. In the inset, the Fourier transform of the FeeC(O) oscillations is shown (in cm 1). Figure reproduced from Falahati, K.; Tamura, H.; Burghardt, I.; Huix-Rotllant, M. Ultrafast Carbon Monoxide Photolysis and Heme Spin Crossover in Myoglobin via Nonadiabatic Quantum Dynamics. Nature Comm. 2018, 9, 4502.

8.16.5

Summary

In this Chapter, we have reviewed recent progress in the theoretical description of inorganic photochemistry and given a brief summary of the state-of-the-art in electronic structure theory (Section 8.16.2.1) and nuclear dynamics (Section 8.16.2.4) methods. We have also discussed the emergence of coupled ML methodologies (Section 8.16.2.2) which, in the future, could be used for translating high-level understanding in intelligent design procedures to optimize the properties of inorganic materials for applications, and for accelerating theory. We have highlighted how significant developments in electronic structure theory and excited-state nuclear dynamics methods towards describing accurately and cost-effectively the properties of sizeable inorganic systems has dramatically increased the scope for exploring their - often highly complex - dynamics. From a fundamental perspective, it is clear that the correlated dynamics of the vibrational, spin, and electronic subsystems have to be considered rigorously, and often in a balanced fashion, as they play a crucial role in determining excited-state properties and behavior. The ability to investigate increasingly complicated systems using electronic structure theory and excited-state dynamics methods is already opening up new opportunities for using theory to support the design and development of new functional inorganic materials. Indeed, developing new ways of using solar irradiation to drive commercially-significant and/or challenging chemical processes is an ongoing research goal. Recently, polyoxometalates (POMs) have emerged as cheap, robust, and easilysynthesized materials, with an added convenience being that they can act as versatile molecular building blocks. In particular, their photoactivity and ability to store multiple electrons makes them attractive for use in a variety of photochemical applications.172,173,183,190 In a slightly different direction, new materials and technologies for data storage are urgently needed to keep up with projected data use in ‘big data’-driven applications like ML and artificial intelligence (AI). A promising technique for increasing operational

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speed of hard-drives 1000-fold is optical control of magnetization using femtosecond laser pulses; this has already been demonstrated in some solid-state materials. To reduce the size of information centers in hard drives, and therefore increase the data storage density, single-molecule magnets (SMMs) are promising candidates because of their nanometer size. In a recent proof-of-principle study, Liedy et al.8 explored the dynamics occurring after photoexcitation of [Mn(III)3O(Et-sao)3(b-pic)3(ClO4)] whose magnetic anisotropy is closely related to the Jahn-Teller (JT) distortion of the Mn(III) ions.8 The anisotropy in the Mn(III) ion arises because the electronic configuration leads to a JT distortion and an axial zero-field splitting.8 Together with the spin-orbit interaction, this anisotropy in Mn(III)-based SMMs leads to two degenerate magnetic ground states where the ground-state spin is saturated either parallel or anti-parallel to the magnetic axis.8 Using ultrafast transient absorption spectroscopy in solution, Liedy et al.8 reported oscillations with a period of 180 fs superimposed on the decay traces due to a vibrational wavepacket.8 Based on complementary measurements and quantum chemical calculations on the monomer Mn(acac)3, it was concluded that the excited state corresponds to a shift of electron density from the antibonding dz2 to dx2  y2 orbitals which, in turn, leads to a compression of the axial bonds and an elongation of equatorial bonds via the formation of a vibrational wavepacket.8 In Mn(acac)3, this leads to a shift from an axial to equatorial JT distortion which, in turn, changes the magnitude and orientation of the magnetic anisotropy.8 The challenge from the perspective of electronic structure theory in these examples is handling the presence of multiple magnetically-interacting open-shell metal ions, the interaction of multiple electronic configurations, and a high density of electronically-excited states of different spin multiplicity. This requires a high-level treatment under electronic structure theory - the kind that can be very challenging to apply to these types of systems. This can already be a great challenge, even for a single-point electronic structure theory calculation, and the problem is compounded when one considers dynamics simulations.

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Site-Selective Orbital Interactions in an Ultrathin Iron-Carbene Photosensitizer Film. J. Phys. Chem. A 2020, 124, 1603–1609. 242. Thompson, S.; Eng, J.; Penfold, T. J. The Intersystem Crossing of a Cyclic Alkylamino Carbene Gold(I) Complex. J. Chem. Phys. 2018, 149, 014304. 243. Vacher, M.; Kunnus, K.; Delcey, M. G.; Gaffney, K. J.; Lundberg, M. Origin of Core-to-Core X-ray Emission Spectroscopy Sensitivity to Structural Dynamics. Struct. Dyn. 2020, 7, 044102. 244. Wernet, P.; Kunnus, K.; Josefsson, I.; Rajkovic, I.; Quevedo, W.; Beye, M.; Schreck, S.; Grübel, S.; Scholz, M.; Nordlund, D.; Zhang, W.; Hartsock, R. W.; Schlotter, W. F.; Turner, J. J.; Kennedy, B.; Hennies, F.; De Groot, F. M. F.; Gaffney, K. J.; Techert, S.; Odelius, M.; Föhlisch, A. Orbital-Specific Mapping of the Ligand Exchange Dynamics of Fe(CO)5 in Solution. Nature 2015, 520, 78–81. 245. Xu, Y.; Xu, P.; Hu, D.; Ma, Y. Recent Progress in Hot Exciton Materials for Organic Light-Emitting Diodes. Chem. Soc. Rev. 2021, 50 (2), 1030–1069. 246. Zhang, K.; Ash, R.; Girolami, G. S.; Vura-Weis, J. Tracking the Metal-Centered Triplet in Photoinduced Spin Crossover of Fe(phen)3 2 þ With Tabletop Femtosecond M-Edge Xray Absorption Near-Edge Structure Spectroscopy. J. Am. Chem. Soc. 2019, 141, 17180–17188. 247. Zhang, K.; Lin, M. F.; Ryland, E. S.; Verkamp, M. A.; Benke, K.; De Groot, F. M. F.; Girolami, G. S.; Vura-Weis, J. Shrinking the Synchrotron: Tabletop Extreme Ultraviolet Absorption of Transition-Metal Complexes. J. Phys. Chem. Lett. 2016, 7, 3383–3387. 248. Zhang, W.; Alonso-Mori, R.; Bergmann, U.; Bressler, C.; Chollet, M.; Galler, A.; Gawelda, W.; Hadt, R. G.; Hartsock, R. W.; Kroll, T.; Kjaer, K. S.; Kubıcek, K.; Lemke, H. T.; Liang, H. W.; Meyer, D. A.; Nielsen, M. M.; Purser, C.; Robinson, J. S.; Solomon, E. I.; Sun, Z.; Sokaras, D.; van Driel, T. B.; Vanko, G.; Weng, T.-C.; Zhu, D.; Gaffney, K. J. Tracking Excited-State Charge and Spin Dynamics in Iron Coordination Complexes. Nature 2014, 509, 345–348. 249. Zobel, J. P.; Bokareva, O. S.; Zimmer, P.; Wölper, C.; Bauer, M.; González, L. Intersystem Crossing and Triplet Dynamics in an Iron(II) N-Heterocyclic Carbene Photosensitizer. Inorg. Chem. 2020, 59, 14666–14678.

8.17 Structural characterization of excited state transition metal complexes by x-ray transient absorption spectroscopies Lin X. Chen, Department of Chemistry, Northwestern University, Evanston, IL, United States; and Division of Chemical Science and Engineering, Argonne National Laboratory, Lemont, IL, United States © 2023 Elsevier Ltd. All rights reserved.

8.17.1 8.17.2 8.17.3 8.17.3.1 8.17.3.2 8.17.3.3 8.17.3.4 8.17.3.5 8.17.3.6 8.17.4 8.17.4.1 8.17.4.2 8.17.4.3 8.17.4.4 8.17.5 8.17.6 References

Introduction X-ray transient absorption spectroscopy Experimental methods X-ray sources with intense short pulses Laser pump pulses Detector systems Signal processing Sample considerations Data analyses TMC excited state structural characterization examples Photoinduced ligand dissociation Metal-to-ligand-charge-transfer (MLCT) excited states of TMCs Photoinduced isomerization in the excited state TMCs Interfacial charge transfer from TMCs to semiconductor nanoparticles Metal-metal interactions in bimetallic transition metal complexes TMCs studied by L-edge XTA spectroscopy and soft X-ray spectroscopy

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Abstract Excited state transition metal complexes (TMCs) undergo various steps of light-matter interactions to become highly energetic species capable of driving many photochemical reactions with implications on different applications. Characterizing transient electronic and nuclear structures and trajectories of TMC excited states are crucial for understanding and control of photochemical properties, such as excited state lifetime, reaction branching ratio, energy or electron transfer and catalytic kinetics. Over the past decades, methods in direct determination of excited state TMC structures have been developed using pulsed X-ray sources in X-ray spectroscopy, scattering and diffraction at synchrotrons, X-ray free electron lasers and table-top sources. The chapter focuses on X-ray absorption spectroscopy (XAS), a particularly suitable method for characterization of local electronic and nuclear structures of the metal centers as well as their interactions with the ligands with examples on those photoactive TMCs. The basic and fundamental aspects of X-ray transient absorption (XTA) spectroscopy will be described with the laser “pump,” X-ray “probe” scheme and consideration of sample environments. Soft X-ray spectroscopy will be described briefly at the end. Other variations of X-ray spectroscopies, such as X-ray emission spectroscopy (XES) and resonance inelastic X-ray scattering (RIXS), will not be covered in this chapter. Due many relevant existing literature over the past two decades, the examples given in this chapter are most familiar and understood by the author.

8.17.1

Introduction

Photochemistry of transition metal complexes (TMCs) has been an established and continuously evolving field for decades.1–23 Many of these complexes have strong absorption in the solar spectrum and hence can utilize photons from sunlight to elevate their energetics to the excited states for driving subsequent photophysical and photochemical processes, such as emission,11,24–30 electron/hole generation,5,10,31–39 and catalytic functions.19,40–51 Consequently, these complexes have versatile applications in optoelectronics, solar energy conversion, catalysis, and so on. Here we need to first clarify that the excited states described in this chapter are created via transitions among valence molecular orbitals, or frontier orbitals and can be created by inducing electronic transition via visible, UV and near IR photons. As commonly known, these valence excited states have finite lifetimes ranging from femtoseconds to milliseconds due to difference decay pathways as described by the Jablonski diagram.52 TMCs have their characteristic sets of electronic transitions determined by the interactions between the metal center and its ligands electronically.53 Most of photoactive TMCs have aromatic ligands that have their own set of molecular orbitals (MOs) and undergo electronic transitions involving p-orbitals. Once these ligands are coordinated with the transition metal center, a new set of electronic transitions involving both metal d-orbitals and ligand p-orbitals emerge. The excited states created by these new transitions have been named as metal-to-ligand-charge-transfer (MLCT), ligand-to-metal-charge-transfer (LMCT), ligand-to-

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ligand-charge-transfer (LLCT), metal-metal-to-ligand-charge-transfer (MMLCT) transitions and so forth which have been comprehensively documented in various textbooks and reviews.9–11,15,54–57 These characteristic excited states can be induced by visible and UV photons with energies in the range of up to several eVs, resulting in electron density shift between the metal center and the ligands, and corresponding nuclear rearrangements. These highly energetic excited states function as vehicles for converting light energy to other forms of energies, such as light,25,27 chemical,19,20,22,40– 43,45–48,50,51,58–65 and thermal energies,66 and as reactants for many photophysical and photochemical processes, such as 30,37,67–69 and charge transfer,4,32,38,70,71 ligand dissociation,17,72–74 substrate binding and transformation. Because these energy excited states change their electronic and nuclear structures, they are transiently new species with their own sets of electronic and vibrational signatures that can be utilized to follow reaction trajectories. The energetics of MOs in TMCs can be quite complicated due to couplings of the d-orbital manifolds with MOs of the ligands. The energies and spin states of the metal center are highly sensitive to the oxidation state, electron occupancy and coordination geometry of the metal center as well as the ligand electronic structures.75–83 Therefore, photochemical pathways can be vary due to relatively minor chemical modifications that change relative energies of different coexisting excited states. It is in fact an ongoing extremely active research area to engineer molecular species for desirable final products.32,45,49,84–87 Thus, it is crucial to understand the structural control of photochemical reactions of TMCs. While interplays of different structural factors complicate the task of mapping out TMC excited state trajectories challenging, the energetic sensitivity with the above structural factors also provides means of detecting transient species with detailed structural information during photochemical processes. The TMC excited state dynamics originate from different steps of light-matter interactions and can span multiple time scales from femtoseconds (fs, 10
15 s) to milliseconds (Fig. 1). The initial step is electron density shift because of the interactions with photons or electromagnetic waves in sub-fs to fs, followed by nuclear rearrangements in response to the electron density shift within the time scale of vibrational motions from fs to sub-picoseconds (ps, 10
12 s). Some spin state transitions in certain TMCs via intersystem crossing (ISC) could also take place on this time scale or longer. In general, the vibrational relaxation from higher energy vibrational manifold to lower energy to dissipate the extra energies gained by absorbing the lights will be on 1–20 ps time scale. Finally, many TMCs reach to the thermally equilibrated final excited state, frequently a triplet state, with nanosecond and longer lifetimes. The long-lived TMC triplet states have been investigated for several decades using nanosecond (ns) flash photolysis and timecorrelated single photon counting (10 ps and longer) instrumentation that are relatively simple and more commonly installed in many laboratories. As ultrafast laser spectroscopy and laser technology advance, they are commonly used to study excited state dynamics on the time scales from fs to ps and longer since the turn of the century.12,17,88–126 Typically used are transient absorption spectroscopy (TA),33,127,128 time-resolved vibrational spectroscopy, including femtosecond stimulated Raman spectroscopy (FSRS),119,129 and more recently multi-dimension spectroscopy, such as two-dimensional electronic spectroscopy (2DES),130–133 two-dimensional infrared spectroscopy (2DIR) and their hybrid versions.134–140 Most of these optical spectroscopic methods share a general “pump-probe” scheme where the first laser pulse, called “pump,” triggers photoexcitation that generate the excited state, and the second laser pulse (s), called “probe,” interrogates the sample at different time delays from the “pump” to detect optical signatures for the excited states. The time resolution from this “pump – probe” spectroscopic measurements is limited only by the pulse durations of the pump and probe rather than detector time responses. These optical spectroscopy methods are very powerful in deciphering excited state dynamics, especially the energy and charge flow between different species in the excited states and their couplings and coherence. However, there are challenges/limitations in using some of those afore described spectroscopic methods to study the TMC excited states, because (1) no direct transient nuclear structural information with atomic precision (e.g., sub-0.1 Å); (2) no

Fig. 1 Photochemical and photophysical processes in transition metal complexes and their approximate time scales as well as the light sources for studying them.

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confirmation on the electron occupancy of the d-orbitals and intermediate oxidation and spin states of the metal centers during photochemical reactions; (3) no covalency and coordination geometry (number of ligands, vacancies) information on the metal-ligand bonding; and (4) many metal centered valence transitions are overlapping and overwhelmed by strong p-p* transition spectral features, thus their dynamics can be difficult to reveal. Clearly, methods are needed to capture electronic and nuclear structures of excited state TMCs along their pathways in photochemical processes. Complementary to the transient optical spectroscopies, direct transient structural determining methods for excited state TMCs, such as X-ray diffraction,141–150 X-ray scattering117,151–160 and X-ray spectroscopies,66,92,103,107,152,154,158–199 have been developed in the past two decades as intense and short X-ray pulses become available in the emerging light sources, such as synchrotrons, X-ray free electron lasers and table-top X-ray sources.200 Analogous to the “pump-probe” scheme in optical spectroscopy as described above, the “pump” pulse remains to be the laser pulse that induces dipole mediated electronic transitions among the frontier MOs, whereas the “probe” pulses are X-ray pulses from those new light sources.200 The laser pulse pump, X-ray pulse probe methods aim at solving electronic and nuclear structures of the TMC excited states especially whose could be ambiguous from optical transient spectroscopy. In this chapter, these excited state structural determination methods will be described followed by some examples from our own work and the literature. The chapter will focus X-ray spectroscopy, mainly on X-ray absorption spectroscopy (XAS) along brief introduction of X-ray emission spectroscopy (XES) and resonant inelastic X-ray scattering (RIXS) related to TMC excited state structural characterizations. There are many very exciting studies in X-ray diffraction and scattering which are unfortunately not covered here due to the scope of the chapter. The applications of these methods to the excited state TMC structural characterization are covered with brief descriptions on the key aspects of theory and instrumentation. Since the X-ray transient spectroscopy started two decades ago, numerous studies have been carried out on the excited state TMCs.

8.17.2

X-ray transient absorption spectroscopy

XAS covers a wide photon energy range from  100 eV to tens of keVs determined by inner shell transition thresholds of ejecting an electron to the continuum, namely the energy required to induce electronic transitions from 1s (K-edge), 2s, 2p (L-edge), 3s, 3p, 3d (M-edge), etc. to a vacant orbital with higher energy (Fig. 2).201 These transitions are induced by X-ray photons with an energy range of 100–1000 times higher than those in UV/vis/NIR spectroscopies. XAS probes a new set of electronic transitions specifically

Fig. 2

Inner shall electronic transitions and their relative energies (Atenderholt’s contribution to X-ray absorption spectroscopy via Wikipedia).

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associated with electronic configurations of X-ray absorbing atoms and thus is an element specific resonant process as described by conventional XAS literature.202–215 These inner shell transitions create a core hole and an electron onto a vacant orbital of the X-ray absorbing atom. Based on Heisenberg’s uncertainty principle, 6E 6t  Z, we expect the higher energy edge to have shorter core hole lifetimes (6 t) and broader experimental linewidth (6E) (assuming the optics do not limit the energy resolution). This, the core hole will be refilled on the femtosecond time scale by outer shell electrons, much shorter than the valence excited state with much longer lifetime. The inner shell to higher orbital transition energy can be sensitive to chemical transformation when the valence MO is vacant to accept the core electron. Analogous to optical transient absorption (TA), the XAS with the laser pump, and X-ray probe scheme is also referred as X-ray transient absorption (XTA). XTA is particularly useful for probing transition metal structures because the metal center in many TMC excited states may undergo transient changes in metal oxidation state, spin state, coordination symmetry, electronic configurations of frontier orbitals and nuclear structures during photochemical reactions. Some of these changes frequently lack distinct features or have weak features in optical absorption region or can be overshadowed by changes in strong electronic transitions involving other parts of the molecules/materials, such as ligand centered p to p* transitions. Hence, optical TA measurements could not definitively identify some electronic structural changes at certain metal centers. Moreover, the electron paramagnetic resonance (EPR) that is sensitive to the spin states of the metal, does not have the time resolution to capture many singlet excited states of TMCs. Therefore, XTA plays an important role in revealing structural dynamics of excited state TMCs. There are key differences between optical TA and XTA spectroscopies in the following aspects. First, the laser pump and X-ray probe pulses are electronically decoupled, while both of the pump and probe pulses in optical TA induce valence electronic transitions with the same or similar photon energies and could coherently interact.216–218 In contrast, the laser pump and X-ray probe pulses in the current XTA experiments induce two very different absorption processes that are not coupled coherently due to their photon energy difference (by a factor of several hundreds to thousands, e.g., 2 eV for the laser pump vs. 7 keV, for the X-ray probe) (Fig. 3). Second, the core-shell excitation is extremely short-lived - the valence excited electronic state lifetime ranges from 10
13 to

Fig. 3 Energy levels in the valence (left) and inner shall (right) electronic transitions respectively probed by optical and X-ray transient absorption spectroscopies.

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10
6 s, whereas those for excited states induced by X-ray photons are on the femtosecond time scale or shorter. Therefore, in the view of each X-ray photon, the valence electron excited state is static. In the rapidly developing attosecond (as) science, the pump and probe pulses with many electron volts of energies can be coherently interacting with each other from which the electron motions can be mapped out.219,220 However, this chapter will only cover valence electron transitions on slower time scales (typically 10
14–10
6 s range) which are relevant to chemical processes. Third, inner shell transitions in XAS is element and orbital specific - XAS can probe the configuration of the electron occupation of certain orbitals, such as the electron occupation in the 3d MO manifold. In contrast, the photoexcitation with UV/vis/NIR photons creates an excited state with a combination of multiple electronic transitions from one set of orbitals to another set of orbitals, which makes detecting electron configuration of individual orbitals extremely difficult. XAS includes both XANES (X-ray absorption near edge structure) and XAFS (X-ray absorption fine structure) (Fig. 4).201–215 In roughly 1 keV energy range from right before the transition edge to higher, XANES covers roughly 10 eV before and 50 eV after the transition edge, and XAFS covers the energy above that. In general, XANES region includes the inner shell transitions from a particular core level to upper vacant orbitals in the pre-edge region, and the edge region when the X-ray photon energy is high enough to eject the core electron to the continuum. Because the electron binding energy of the core shell will increase if the X-ray absorbing atom bears more positive charges, high energy X-ray photons are needed to eject the inner shell electron, resulting in the transition edge energy upshift. Thus the edge energy in general is sensitive to the oxidation state of the transition metal center. The features in the pre-edge region originate from electronic transitions from one to the other orbital, while the features in the edge region correspond to the rise of the absorption as the X-ray photon energy reaches to the threshold, and then multiple scattering of the photoelectron wave within the molecule. Using the first row transition metal centers as examples, some of their pre-edge features at their

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Fig. 4 XAS spectra division for XANES and XAFS. The XANES example shows the edge shift with the iron oxidation state, and the pre-edge region features 1s to 3d transitions that are enhanced due to the 3d-4p mixing in the tetrahedral coordination but are weak in octahedral coordination in different compounds. The XAFS spectra, A. after removal of the absorption profile for continuum with only oscillatory parts due to the interference between the outgoing and back-scattered photoelectron waves; B. Fourier transformed A., radial distribution centered at the metal atom; and C. back FT spectrum from a particular neighboring shell showing nearly single mode of oscillation arisen from a particular scattering path, from which the metal to neighboring atom distances can be extracted from data analysis using Eq. (2).

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corresponding K-edge appear from quadrupole-allowed 1s to 3d transitions which are weak compared to dipole allowed transitions, but yet can be informative about the vacancies in the 3d orbitals (Fig. 4. XANES spectra). These 1s to 3d transition features are also sensitive to the coordination geometry of the metal center. When the 3d-4p molecular orbital (MO) mixing takes place in the metal electronic states which breaks the total symmetry of the metal center coordination, the originally dipole forbidden 1s / 3d transition becomes slightly dipole allowed due to the 4p-component, and their intensity can be related to the extent of the d-p mixing. The higher energy part of the XANES region contains the transition edge where the electronic transitions and multiple scattering signals entangle. Some distinct and intense spectral features can appear in the middle of the transition edge for dipole allowed transitions, such as the 1s / 4pz transitions of first row transition metal complexes. Therefore, XANES spectra provide information about empty orbitals which the core electron can fill when excited. The electronic transitions observed at the pre-edge and edge regions can be expressed by the Fermi Golden rule, m ¼ ðEÞ a

EX f >EF

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(1)

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where m (E) is the total X-ray absorption at photon energy E, e is the unit vector of the electric field, r is a transition dipole moment operator [note: the transition dipole moment is normally represented by m. Here r is used in order to avoid confusion with the X-ray absorption. The EXAFS region covers the energy range above the core to vacuum transition where the inner shell electron is completely ejected to the continuum. EXAFS signals arise from the coherent interference of the outgoing photoelectron wave originating at the central X-ray absorbing atom and the back scattered photoelectron wave from neighboring atoms, which can be described by the equation below that has been derived from the real-space multiple scattering theory developed by Rehr and coworkers201,221:   2rj X
2s2 k2
lj ðkÞ sin 2kr j þ dij ðkÞ cðkÞ a Nj Fj ðkÞe e (2) kr 2j j where j is the index for the neighboring atom shells around the X-ray absorbing atom, F(k) is back-scattering amplitude, N, coordination number, r, average distance, s, Debye-Waller factor, l, electron mean free path, and d is the phase shift of the photoelectron wave. k is photoelectron wavevector, k ¼ [2 m(E-E0)/Z2]1/2, where m is the electron mass and E0 the threshold energy for the transition edge. The discovery and formulation of XAFS took place in the early 1970s by Stern, Lytle and Sayers who laid foundation of the current extended X-ray absorption fine structure (EXAFS) or XAFS.222–224 The experimental signals described by Eq. (2) can be Fourier transformed (FT) into a radial distribution function centered at the X-ray absorbing atom from which the local structure of the X-ray absorbing atom (e.g., transition metal atoms) can be extracted in terms of coordination number N and bond length r shown in Eq. (2). Because the scattering paths are sensitive to both inter-atomic distances and the orientation of the scattering wavevectors, XAS spectra can be used to extract atom-to-atom distances with high precision (XANES and XAFS) as well as angles between different bonds for distant coordination shells (XANES).204,205,225–227 XAS signals are detected using transmission or fluorescence mode as well as total electron yield, with the first two are most common. The electron yield detection mode is used in soft X-ray regime and occasionally in the hard X-ray regime. In most experiments, the fluorescence mode detects the total X-ray fluorescence due to the outer shell electrons refilling the hole in the inner shell and hence the X-ray fluorescence intensity is proportional to the absorption coefficient of the sample. For dilute samples with the X-ray absorbing atom concentration less than a few hundred ppm or one milli-molar, the X-ray optical density change across the transition edge is very small on top of a huge background. For such dilute samples, X-ray fluorescence detection is a much more sensitive and alternative method from the transmission detection because the majority of elastic background signals can be removed by low pass filters while only the X-ray fluorescence signals are collected, nearly background free.

8.17.3

Experimental methods

8.17.3.1

X-ray sources with intense short pulses

Since the third-generation synchrotron sources were built at the end of last century to provide intense X-ray pulses with the pulse duration around 100 ps, most of XTA experiments have been carried out on TMCs in these facilities. After two decades, XTA has been transformed to be standard measurements at some of the synchrotron sources. Since about one decade ago, hard X-ray free electron laser (XFEL) sources with intense femtosecond X-ray pulses have been available for users. However, the beam time at XFEL sources is very limited because each XFEL is essentially for a single user at a time, compared to tens of simultaneous users at a synchrotron source. General information about the light sources around the world can be found in a website https://lightsources.org/. Here we use the Advanced Photon Source (APS) as a synchrotron X-ray source example, to describe key features of XTA and its distinctions from optical TA and conventional XAS for the users. X-ray probe pulses - Most of the XTA measurements currently are carried out at undulator beamlines with one or more undulators in-line with tapered gap or continuously variable energy gap to provide high pulse photon flux (e.g., 106–107 photons/pulse) with sufficiently broad spectral range (e.g., 5–30 keV) for XAFS measurement for a broad range of elements from 3d to 5d TMCs. Since the X-ray focusing optics are implemented at the XTA facilities, the X-ray beam size commonly use is around < 0.5 mm or even

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smaller to a few microns diameter. Apparently, a very high X-ray source stability is crucial for XTA at synchrotron source, which are assured by improvements of beamline optics, mechanical controls, and sample delivery system. The XTA time resolution at synchrotron sources is limited by the X-ray pulse duration of  100 ps and the spectral range can cover the K-edges for the first row and some of the second-row transition metals and L-edge of the third row transition metals. The X-ray pulse repetition rates at the synchrotron can be as low as 1 Hz in some facilities and as high as several MHz in others, depending on the fill pattern of charges in the storage ring. The pulse timing modes at different synchrotron facilities also vary. For example, the APS provides X-ray pulses with a regular time separation of 153 ns as a normal timing mode (Fig. 5) as well as irregularly spaced pulses in a hybrid mode where a single intense pulse is separated from other pulses by 1.8 ms while several cluster of pulses with much weaker intensities coexist in this hybrid mode. The operating timing mode is a dynamic character at the synchrotron sources which can change for different user needs. X-ray pulses at the XFELs have pulse durations of a few tens of femtoseconds and most commonly operating with self-amplified spontaneous emission (SASE) mode where the spectrum and pulse shape vary from pulse-to-pulse with average band width of 30– 50 eV depending on the energy. Also in the first decade of the operation, the repetition rate of X-ray pulses is around tens to a couple hundred Hz (Fig. 5), much lower than that at synchrotron sources. The XFEL sources continue to be upgraded, making advances on the daily basis. Here, the Linac Coherent Light Source (LCLS) is use as an example to describe current and future characteristics very briefly, and the details can be found from the website of the LCLS (https://lcls.slac.stanford.edu/). The LCLS has been providing Xray pulses over the energy range 280 eV to 11.2 keV in the fundamental, at 120 Hz. The current LCLS-II upgrade aims at broader energy range up to 25 keV, 0.5–2 mJ pulse energy. The SASE pulse length can be varied from 10 to 50 fs for hard X-rays, while for soft X-rays the range is extended to 250 fs. Shorter pulses, < 10 fs, with a reduced number of photons per pulse can also be achieved. In general, the SASE radiation is centered at a particular energy with about 30–50 eV band width, therefore only XTA in the XANES region could be collected with the scanning of the monochromater. However, a recent publication have shown successful XAFS measurements with more than 500 eV range by scanning the electron energy, which opens promising opportunities for the future XAFS measurements with femtosecond time resolution.228

8.17.3.2

Laser pump pulses

An ideal laser pump source should have intense pulses with a pulse duration shorter than that of the X-ray pulse, and tunable wavelengths for exciting samples with various absorption spectra. There had been challenges in balancing requirements for a single laser pump pulse to create a sufficient excited state population (i.e. > 10%) and the repetition rate of the pump laser, so that many XTA pump-probe cycles have been in 1 kHz regime. Two main advances have taken place in recent years, the high stability of the XTA signals due to the improvements of the light source, data acquisition software and hardware, and more importantly ultrafast laser with high repetition rate and high pulse energy. Consequently, one can obtain useful XTA data with only  1% excited state (in certain samples with large spectral changes) and the pump-probe duty cycle has increased by 10–100 times depending on the laser repetition rate. The uses of now available high repetition rate pump lasers (e.g., from 10 kHz and even higher up to hundreds of kHz175), focused X-ray beams, and new detection systems has enabled efficient data acquisition and much higher signal to noise ratios in XTA spectra. It is realistic now to take only a few hours to acquire an XTA spectrum (XANES and XAFS) of a dilute sample

Fig. 5 Timing structure schematic of XTA at an XFEL and a synchrotron source. The figure only shows as examples while the timing structures vary at different light sources.

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(i.e., 1 mM or lower) with high quality data comparable to those of single scan steady-state XAS spectra taken at bending magnet beamlines. Because of the short X-ray pulses at the XFEL facilities, the pump lasers there also have 50 fs or shorter pulses, which brings a concern regarding nonlinear absorption of the laser photons by the sample which could cause artifact due to multiple photon absorption rather than desire photochemical processes. Also, the short laser pulses may also cause stimulated emission because the excited state does not have time to relax to another vibrational or electronic state, so that the excited state population is “dumped” back to the ground state. It is our observation of changing a pump laser with 5 ps pulse duration vs. 150 fs duration, the former creates higher fraction of the excited state population than the latter. However, if we need the time resolution, short femtosecond lasers will be necessary. In practice, a “power titration” procedure should be carried out before the laser pulse energy can be selected, where the XTA signal intensity vs. laser pulse intensity will be measured to ensure the signal vs. laser pulse energy is in the linear regime. This is especially important consideration if the UV pump pulse is used because the molecules or solvents could be ionized at high pulse fluence.

8.17.3.3

Detector systems

Similar to the conventional steady-state XAS, XTA measurements can use either transmission or fluorescence detection for concentrated and dilute samples respectively. An avalanche photodiode (APDs) gated with electronic signal synchronized with the X-ray probe pulses was used for the transmission detection while caution must be made to make sure that the transmitted X-ray photons will not damage or saturate the detectos.229 As stated earlier, many chemical systems are at concentrations lower than the transmission detection requires (e.g., > 20 mM), so that the fluorescence detection has been used for many dilute solution TMC samples whose low concentrations are limited by the solubility or the availability of the samples from the research laboratories (i.e., a few tens of mg). The early fluorescence detection XTA experiments were carried out by using single photon counting detectors (e.g., a solid-state Ge detector array from Canberra). While the current detection systems for XTA are largely based on current mode, such as avalanche photodiodes (APDs), because there will be multiple photons to be received within a single X-ray pulse due to increased photon flux. The concerns is the detector response time (rise and fall time) to be sufficiently fast to enable digitization of the pulse shape for data acquisition (see below) and recovery before subsequent pulse in the X-ray pulse train appears (e.g., 153 ns at the APS normal operation mode). To prevent the detector saturation by the elastic scattering signals from the solvent and air for dilute samples, a soller slits/Z-1 filter assembly is placed between the liquid sample jet and the APD, inspired by the design principle for the Lytle detector assembly. An APD placed upstream from the sample collects air scattering signals as the reference for pulse-to-pulse photon flux normalization which is important because the photon flux varied from pulse to pulse due to the electron refill operation in a so called “top-up” mode in the storage ring. The bunch current for a particular bunch can vary as much as up to > 20% of its peak value, while the total X-ray photon flux from all the pulses is very close to a constant. If the sample concentration is on the order of 10 mM or higher, the ratio of the X-ray fluorescence signals from the atoms of interest to background scattering will be sufficiently high, so that the soller slits/filter combo in front of the detector becomes less crucial which is most of the cases at XTA measurements at XFEL sources with several mM concentration of the X-ray absorbing transition metal.

8.17.3.4

Signal processing

The XTA data acquisition proceeds with a desired integration time at each energy step after the X-ray monochromator until necessary signal to noise ratio is achieved for an XTA spectrum. The signals from the APD detectors can be digitized by fast analyzer cards (e.g., Agilent) for all of the X-ray pulses in the entire pulse train at an interval of 1 ns/point between two consecutive laser pulses. The use of real-time data processing software (by Guy Jennings, APS personal communication) in the data acquisition is crucial for much improved data quality, especially for dilute samples. The software controls in situ data processing including fast digitizing, signal averaging, background subtracting, and pulse shape fitting, which finally extract signal amplitudes from individual X-ray pulses. Compared to peak height analysis or peak area integration, this approach removes the pulse shape dependence of the signal, improper background subtraction and high frequency electronic noise. An additional APD detector in the upstream of the X-ray beam propagation direction from the sample jet is used to collect air scattering signals as references for the pulse-to-pulse intensity normalization for the X-ray fluorescence signals from the same X-ray pulse collected by the other two APD detectors. This scheme has been proven necessary because the top-up mode at the APS fills different electron bunches at different time, and the bunch-tobunch intensity fluctuation can be as much as > 20% while the total flux only varies by 0.2–0.4%. The other important advantage of the fast digitizing is the capability of collecting signals from all the X-ray pulses from the pulse train, which provides the capability of detecting structural changes at a sequential time delays defined by the timing mode of the X-rays pulses, e.g. pulse separation by 154 ns in the 24-bunch mode at the APS, so that the sequential X-ray pulses will provide signal timing steps, e.g. 154 ns/step, for some slow processes in which the structural changes can last for a period covered by multiple consecutive X-ray pulses in the pulse train.

8.17.3.5

Sample considerations

As XTA grows into a more commonly used method in synchrotron and XFEL facilities around the world and many technical improvements have been made through the past decades, sample choices and environments become crucial for the success of

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excited state TMC structural characterizations. The main issues to be considered are (a) light absorption cross section, (b) solubility, (c) excited state lifetime, (d) stability under the illumination of both laser and X-rays, and (e) available amounts for the XTA measurements. These requirements are continuously to be revised as the method advances. It should be noted that the absorption cross sections for the solution TMCs in the UV/visible region are about 100–1000 times higher than those in the X-ray region, resulting in huge difference in the light penetration lengths for the photons from the pump and the probe pulses. A majority of the optical TA experiments are carried out on the TMC solution samples with 0.01–0.1 mM concentration and 1–2 mm optical path, assuming the extinction coefficient in the Beer-Lambert law is 1000–10,000 M
1 cm
1, while the concentration for XTA needs to be at least 10–100 times higher in order to obtain acceptable signal to noise ratio within an achievable beamtime. This requirement then brings up the second issue regarding the solubility of TMCs which need to fulfill the concentration requirements. Because the X-ray pulse duration from a synchrotron source is about 100 ps, the lifetime of the excited state of interest must be equal or longer than this time scale. The illumination of the intense X-ray pulses and laser pulses could cause the damage of the TMCs, thus the sample sustainability to at least a few hours is desirable. If the sample is abundant, one can afford to refresh the solution continuously and the stability issue can be circumvented. However, that brings the last issue regarding the availability of the TMC samples, which is a key obstacle for extending the excited state TMC studies by XTA to more systems.

8.17.3.6

Data analyses

Data analyses in XTA are mostly the same as in the steady-state XAS analysis with some import exceptions: simultaneous determination of local structures of the starting and transient states in the sample. Conventional XAS analyses utilize the total spectra from which structural parameters are extracted through normalization, background removal, Fourier transform (FT)/back FT, and data fitting in either distance or wavevector space.208,210 In XTA experiments, the spectra of the sample with the laser pump excitation contain contributions from both the ground state (GS) and the excited or transient state (ES or TS). Therefore, the XTA signal at a particular pump-probe time delay time t and a particular photon energy E can be expressed as m(E,t) below, X mðE; t Þ ¼ fi ðt Þmi ðE; t Þ (3) i

where i is the index of the components, fi is the fraction of the component (Sfi ¼ 1), and mi(E,t) is the XAS spectrum of the ith state coexisting in the sample after the photoexcitation. Because the spectra for transient species as well as their respective fractions are both unknown, the fitting is under-determined with more variables than the number of equations. For XTA results, both the starting and transient state spectra are simultaneously acquired at different time delays from the laser pulse, and the maximum difference over the spectral range can be as small as  1% of the total signal. Such small differences can be due to the small structural changes between the transient species and the ground state, or simply due to the small fraction of the transient species produced by the pump laser. This is analogous to the situation in optical TA spectroscopy, where one uses the changes of the optical density (DOD) instead of total OD to extract the dynamic information of transient species. Similar approaches have been developed in XAS and are now applied to XTA,230–233 where fits of the structural parameters are carried out on the theoretically calculated and experimentally obtained difference spectra instead of total spectra as in conventional XAS data analyses. The main complication in this approach is the uncertainty in the population fractions of the transient states. A number of approaches have been developed for analyzing structural information from XANES region using difference spectra, such as multiple dimensional approaches (MIA) and MXAN, which provide refinements of 3D local structural parameters. The method is based on the quantitative analysis of XTA spectra in the XANES region using the multidimensional interpolation as a function of structural parameters. The corresponding software212 are compatible with several extensively used computational methods for XANES calculation, including both multiple scattering muffin-tin algorithms and non-muffin-tin schemes. The calculations can couple to advanced algorithms, such as full-potential schemes, to gain more precise structural information for molecular systems. These approaches start with a structure of the ground state based on the results of XRD/XAS or DFT/MD (density functional theory/ molecular dynamics) calculations as the initial structure to calculate the corresponding XANES spectrum. Then the structural parameters are rationally selected to vary within chemically reasonable limits to gain the best agreement between theoretically calculated and experimentally obtained difference XANES spectra (i.e. the transient state – the starting state). An energy-dependent interpolation polynomial is constructed, which reproduces a theoretical spectrum for the starting values of the structural parameter sets, and then XANES spectra for all possible sets of structural parameters are calculated. After the polynomial construction, the minimal discrepancy between the two difference XANES spectra obtained experimentally and interpolated theoretically is searched by varying the selected set of structural parameters. The final values of the structural parameters corresponding to the minimal discrepancy between the two difference spectra are considered the closest to the correct structural parameters of the transient molecular structure. This approach can be a reliable alternative to the conventional XAS data analysis using a full EXAFS spectrum and is particularly powerful at resolving the structures from the XANES region where the largest spectral differences are observed. Such an application is only possible now due to significant improvements in the spectral quality of current “pump-probe” experiments. Similar approaches like these are expected to be used more extensively in the future for analyzing XTA spectra. Nevertheless, the fractions of the transient species are still unknown and but can obtained by (1) mapping fi(t) from optical transient spectroscopic results via global fitting; (2) using a few distinct features that are uniquely present in certain species in the XANES region to fit the edge features

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Structural characterization of excited state transition metal complexes by x-ray transient absorption spectroscopies

to obtain the fractions of different species; and (3) using steering fitting to systematically vary one or more structural parameters in search of the global minimum of the fits.168,234 As the excited state TMC structural studies evolve, advanced quantum mechanical calculations emerge to enable the inner shell transitions to be calculated for comparing and predicting experimental results from the XTA measurements. The advancement of computational chemistry and the capabilities of large scale computations enable the detailed electronic structures of the excited state TMCs to be calculated and their XANES spectra simulated. Such significant advances totally changed our understanding of the excited state dynamics of TMCs. The calculated results on each characteristic transitions help to decipher the electronic interactions that cannot be obtained with phenomenological interpretations of the XTA results or with optical transient absorption. For example, 1s to 4p transition features have been commonly observed in the first-row transition metal complexes and this feature is sensitive to the ligation coordination symmetry as well as the oxidation states of the metal center. However, some of the 1s to 4p transition energy shifts and intensity change are not immediately clear in the excited state due to the unknown transient structures, even though extensive investigations have been carried out in the ground state. In this case, the quantum mechanical calculation on the excited state electronic structures and wavefunctions are extremely helpful to aim the interpretation of the experiments to pinpoint the cause of the spectral changes, from the nuclear rearrangements or electronic configurations. For the XAFS region, the data fitting programs are available, such as Athena - Artemis, WinXAS, etc. as well as XAFS simulation program, Feff. They are readily available with low or no cost to the users. Once the excited state structures of TMCs available from quantum mechanical calculation, they can be the starting point to be used in Feff and the resulting XAFS spectra can be used to compare with the experiments.

8.17.4

TMC excited state structural characterization examples

Applying the structural determination principle established in steady-state XAS, XTA spectroscopy detects TMC excited state structural changes in metal center coordination geometry (number of ligands, metal to ligand band distance, etc.) and metal-centered electronic configuration focused on those vacant orbitals.92,161–163,235–238 There are several photochemical processes taking place in the TMC excited states that could cause changes in their electronic and nuclear structures and thus be detected by XTA: (1) photoinduced ligand dissociation; (2) electron/energy transfer; (3) isomerization, and (4) interfacial charge injection to semiconductor nanoparticles, etc. These are common photochemical processes appearing in TMCs and the structural details could really determine, confirm, verify, and predict impact of light to the molecules. Such structural information could also be used to guide and compare with theoretical calculations.

8.17.4.1

Photoinduced ligand dissociation

Metal-ligand dissociation occurs in many reactions in catalytic reactions74,239–241 and enzymatic functions.242,243 In order to study the structural transformation of this process, the dissociation steps among the molecule in the ensemble need to be synchronized. Thus, the pump-probe scheme is adapted using a laser pulse as a trigger for the ligand dissociation. Here we use an example of the XANES spectral change at the Fe K-edge (7.112 keV), where 1s electron is promoted by X-ray photons) for a CO adduct of Fe(II) myoglobin [Fe(II)MbCO] (Fig. 6A) and its CO removed product Fe(II)Mb that can only exist transiently.244 Two spectral regions in XTA probe the Fe center electronic configuration information that cannot be obtained directly from optical TA measurements. In the pre-edge region, Fe(II)MbCO XANES spectrum displays two distinct quadrupole-allowed transitions (Fig. 6C and inset), assigned as two possible transitions of 1 s / 3dx2-y2, 3dz2, at 7.112 keV, and 1 s / 3dxz, 3dyz at 7.115 keV (from DFT calculations).245 These two features arise from two sets 3d orbitals have vacancies and distinct energy difference due to a low spin configuration under the high field ligand CO binding (Fig. 6B).244 The energy separation of the two transitions reflects, but not necessarily equals to, the energy difference between the MOs because the presence of the core hole in 1s. The energetically well-separated preedge features provide the electronic configuration information in this pseudo-octahedral coordinated Fe(II) (3d6) which suggests a low spin state with all six 3d electrons occupying the lower energy orbitals and leaving the two higher energy orbitals empty due to binding a strong field ligand CO (Fig. 6). In contrast, after the photodissociation of CO ligand, the two distinct pre-edge features collapse into a broader feature without distinction of clear peaks, which is due to the spin state and orbital occupation change as the Fe(II) center becomes a high spin electronic configuration, with the energetic separations of 3d orbital vacancies smaller than the energy resolution of the measurements could resolve (Fig. 6B). The kinetics collected at the X-ray photon energies of the two distinct features display very different time constants (Fig. 6D and E). The depletion of the sharp feature at 7112 eV (Fig. 6D) takes place within the time resolution limit of  50 fs due to the CO dissociation that changes Fe(II) center from low spin to high spin, implying that the CO departure occurs within a fraction of a single vibration cycle of FeeCO stretch (i.e.  300–500 cm
1).246 In contrast, the second pre-edge feature at 7115 eV has multiple depletion time constants (Fig. 6E), one is an instantaneous depletion as in the 7112 eV feature, but the other time constant is much longer up to 0.9 ps, which later have been revealed by quantum mechanical calculations due to convolution of a rising absorption feature at a higher energy as the heme nuclear structure relaxes, e.g. doming of the macrocycle.245,247,248 In the rising edge region, one can clearly see the edge position down shifts to a lower energy after the CO dissociation, suggesting effective gain of the electron density on Fe(II) even its apparent oxidation state remains unchanged. Such an electron density gain is from the removal of the p-back bonding from Fe(II) to CO which draws electron density away from Fe(II) to CO. The kinetics of this process have been investigated by X-ray

Structural characterization of excited state transition metal complexes by x-ray transient absorption spectroscopies

MbFe(II)CO

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Fig. 6 (A) The heme site structure with the red and yellow dots represent before and after the CO dissociation; (B) possible 1s to 3d transitions before (left Fe(II) low spin) and after (right, Fe(II) high spin) the CO dissociation (red arrows indicate possible path for 1s to 3d transitions; C. XANES and pre-edge (inset) at the Fe K-edge before (red) and after (blue) the CO photodissociation; D&E, kinetics collected at 7112 and 7115 eV respectively indicating that two pre-edge change subject to different kinetics with signals collected at parallel and perpendicular polarization directions between the pump laser and the probe X-ray pulses. (A B, C) Adapted from Ref. Stickrath, A. B.; Mara, M. W.; Lockard, J. V.; Harpham, M. R.; Huang, J.; Zhang, X.; Attenkofer, K.; Chen, L. X. Detailed Transient Heme Structures of Mb-CO in Solution after CO Dissociation: An X-Ray Transient Absorption Spectroscopic Study. J. Phys. Chem. B 2013, 117, 4705–4712 with permission from the American Chemical Society. (D and E) Adapted from Ref. Shelby, M. L.; Wildman, A.; Hayes, D.; Mara, M. W.; Lestrange, P. J.; Cammarata, M.; Balducci, L.; Artamonov, M.; Lemke, H.; Zhu, D.; Seideman, T.; Hoffman, B. M.; Li, X.; Chen, L. X. Interplays of Electron and Nuclear Motions along CO Dissociation Trajectory in Myoglobin Revealed by Ultrafast X-Rays and Quantum Dynamics Calculations. Proc. Natl Acad. Sci. USA 2021, 118, e2018966118 with permission.

free electron laser source showing similar multiphasic rise with instantaneous, sub-ps and 2–3 ps components corresponding well with the pre-edge results. This example, along with several recent relevant studies,249–257 demonstrates that the XTA in the XANES region can reveal detailed electronic configuration information for the excited state which optical TA cannot achieve. Analogous with optical transient anisotropy measurements, where the TA signals parallel and perpendicular to the pump pulse polarization direction are respectively detected to obtain dynamics and directionality of the excitation, emission and excited state absorption, molecular motions and energy transfer,258 some recent XTA studies employed this principle in the X-ray regime to detect the Xray probe signal in parallel and perpendicular to the optical laser polarization.245,252,254,255,259 Such an optical polarization selected XTA method provides additional dimension to detail the structural dynamics in molecules, such as the relative electronic transition dipole moments in the excited state to the pump polarization direction. Photodissociation of the CO from Mb also changes the coordination number of Fe(II) from six to five, which not only changes the symmetry of the coordination but also the bond length of Fe with the ligands as well as distant Fe to other atom distances and all of these ultrafast motions take place within a couple of picoseconds while the dissociated Fe(II)Mb species lasts for many microseconds before it could recombine with CO again. From the data analysis using Eq. (2), the reduction of the Fe(II) coordination number from 6 to 5, disappearance of Fe(II)eC(CO) and Fe(II)eO(CO) distance can be confirmed. Meanwhile, the Fe(II)eN distance, averaged among four heme N atoms and one N from ligating histidine, elongated from 1.99 to 2.05 Å. Such changes reflect heme nuclear motions from a near planar with CO ligated hexacoordinated geometry with a shorter average Fe(II)eN distance to a domed conformation where Fe(II) atom is out-of-plane by approximately 0.09 Å.244 This example demonstrates the capability of XTA in solving structural dynamic of ligand photodissociation and other similar examples can be found from the literature.260–268

8.17.4.2

Metal-to-ligand-charge-transfer (MLCT) excited states of TMCs

The most commonly studied excited states among the TMCs are metal-to-ligand-charge-transfer (MLCT) or ligand-to-metal-chargetransfer (LMCT) states in which shifts of electron density between the metal centers and ligating groups take place upon photoexcitation, accompanied by nuclear geometry changes in response to the electron density rearrangement.1,38,269–271 These characteristic excited states in TMCs function as both light harvesting sensitizers and energy/electron donors or acceptors for light-to-electricity

690 Structural characterization of excited state transition metal complexes by x-ray transient absorption spectroscopies

Fig. 7 (A) Structures of [CuI(dmp)2]þ and [CuI(dpp)2]þ, (B) XANES spectra for the ground state (black), MLCT state (red) and chemically generated Cu(II) complex for the two complexes as labeled. The insets highlight the pre-edge feature due to the vacancy in 3d9 which is absent in the ground state [CuII(dmp)2]þ 2 due to the ligation with the solvent but it still present in the MLCT state. This feature remain unchanged between chemically generated [CuII(dppS)2]þ 2 and the MLCT state, indicative that solvent does not ligate with the Cu(II) center in both cases; C) XAFS spectra and FT-XAFS spectra of [CuII(dmp)2]þ showing the CueN shortening.

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(C) (A)

(B)

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(D)

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9050 9100 Energy (eV)

Fig. 8 (A) Copper K-edge XTA kinetic trace (orange) of the depletion of the Cu(I) species taken at a probe energy of 8.984 keV following 400 nm excitation, and the fit (solid black) to a linear combination of an impulsive and a non-impulsive exponential decay (dashed black). The time constants for the non-impulsive rise (sIMCT) and excited state decay (sMLCT) are also given. (B) ruthenium K-edge XTA kinetic trace (orange) taken at a probe energy of 22.126 keV and the fit (black) to a single exponential decay, corresponding to inter-metal charge transfer. (C) Scheme depicting electron transfer pathways following optical excitation into the MLCT bands of the Ru(II) (top) or Cu(I) (bottom) center. Blue arrows represent the excitation pulse, yellow arrows show the movement of electrons through the molecule, and the numbers adjacent to arrowheads indicate the order in which these processes occur. D) XTA XANES spectra at the Cu K-edge from which the biphasic depletion kinetics taken at 8984 eV (dashed blue line) was extracted shown in A). Adapted from Ref. Hayes, D.; Kohler, L.; Hadt, R. G.; Zhang, X.; Liu, C.; Mulfort, K. L.; Chen, L. X. Excited State Electron and Energy Relays in Supramolecular Dinuclear Complexes Revealed by Ultrafast Optical and X-Ray Transient Absorption Spectroscopy. Chem. Sci. 2018, 9, 860–875 with permission from the Royal Society of Chemistry.

and light-to-fuel functions. To determine photocatalytic pathways, optical spectroscopy alone cannot extract metal center structural information, because (1) the optical signatures of the functional metal centers in condensed phases are often overlaid and weaker than the p / p* transitions of aromatic ligands; (2) metal-centered excited states may be optically dark or optical absorption indistinguishable; and (3) most time-resolved vibrational spectroscopy studies of transition metal complexes are focused on characteristic frequencies of certain functional groups (e.g. carboxyl groups, etc.). Thus, one needs to use structural determination methods that are targeted particularly to the metal centers in excited state TMCs. As potential low cost replacements of the quintessential ruthenium polypyridyl complexes, the MLCT states of Cu(I) diimine complexes have been increasingly studied via the XTA method focusing on structural factors including conformation and solvent-solution interactions which can strongly influence the MLCT state properties, such as lifetimes and fluorescence yields.12,178,184,272 The Cu(I) to Cu(II) conversion from the ground to the MLCT state generates distinct features at the copper Kedge XAS spectra which can be used to monitor the reaction trajectories in photochemical reactions as shown in a review as well as references therein.184 These Cu(I) diimine complexes undergo the MLCT transition when they absorb visible photons iþ  r þ 527nm h Cu ðdmpÞ2 ! CukðdmpÞ
ðdmpÞ where one electron on the Cu(I) center in the ground state with a 3d10 configuration is shifted to the ligand, resulting in a Cu(II) center in the MLCT state with a 3d9 occupation, which causes the Jahn-Teller distortion from pseudo-tetrahedral to flattened tetrahedral geometry, accompanied by CueN bond shortening.162 Here we use two different Cu(I) diimine complexes to demonstrate the excited state structural characterization with XTA, [CuI(dmp)2]þ and [CuI(dppS)2]þ(Fig. 7A). With methyl groups at the 2,9-positions of phenanthroline, the [CuI(dmp)2]þ MLCT state flattens and enables solvent access to the transient Cu(II) center, resulting in shortening the MLCT triplet lifetime from  100 ns in chloroform to  1 ns in “coordinating” solvents, such as acetonitrile. In comparison, [CuI(dppS)2]þ has bulky phenyl groups at the 2,9 positions

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Structural characterization of excited state transition metal complexes by x-ray transient absorption spectroscopies

of the phenanthroline to impose two structural effects to the MLCT state: (1) a flattened coordination geometry in both the ground and MLCT states with little room for the Jahn-Teller distortion, and (2) a well-shielded Cu(II) center in the MLCT state to minimize the quenching from solvent interactions. The Cu K-edge (8.979 keV) XANES spectra for the ground and the MLCT state of the two complexes are shown in Fig. 7B. In the pre-edge region of the XANES spectra, the MLCT states of both complexes show a distinct feature, although weak, assigned to the quadrupole-allowed 1s to 3d transition, only appeared when there is a vacancy in 3d9 configuration in the MLCT state with the transient Cu(II) (Fig. 7B). Thus, this feature reveals the 3d vacancy and confirms the Cu(II) nature in the MLCT state. Moreover, a distinct feature at the Cu K-edge for these complexes is the peak corresponding to the 1s / 4pz transition, which is more pronounced in a flattened tetrahedral geometry than the one with a pseudo-tetrahedral geometry, presumably due to the difference in the “emptiness” of the 4pz orbital of the copper. Such a feature can be used to identify the coordination geometry of the complexes. Because the Cu(I) center is effectively oxidized to Cu(II) in the MLCT state, the edge position is upshifted to higher energy reflecting more positive charge at the metal center which will increase the required energy for ejection of electrons to the continuum. One debatable structural aspect is whether the “exciplex” forms in the MLCT state in coordinating solvents. Extensive optical TA results focused on the correlation of the accessibility of the solvent to the Cu(II) center in the MLCT state strongly suggest that the excited state lifetime shortening requires direct contact of the solvent with the metal center. However, the “exciplex” may not have the formal ligation but loose and dynamic ligation which may be hard for XTA to identify. A recent study indicated that the pre-edge feature intensity is a function of the solvent to Cu(II) center distance. Thus, we observed static [Cu(II)(dmp)2]2 þ in acetonitrile has a smooth rising edge and no apparent pre-edge feature, while the photogenerated [Cu(II)(dmp)2]2 þ in the MLCT state has still a shoulder feature and visible pre-edge peak. The difference indicated that thermally equilibrated Cu(II) species is solvent ligated as penta-coordinated while the MLCT Cu(II) species with finite lifetime of  1 ns does not have a bonding between the metal and solvent even though the excited state is quenched. In comparison, thermally equilibrated and photogenerated [Cu(II)(dppS)2]2þ have the same XANES spectra which does not indicate the solvent ligation, agreeable with the optical TA results.272 The XAFS and FT-EXAFS spectra of [CuI(dmp)2]þ clearly show CueN distance shortening at the MLCT state. Thus, XTA in these studies have revealed detailed electronic and nuclear structural changes in the MLCT state which can confirm the electron transfer from the Cu center. In addition, the CueN bonds in the MLCT state were also shortened by  0.06 Å due to the electrostatic interactions as shown in Fig. 7C. The Cu(I) diimine motif has been implemented in more complex supramolecular systems, such as bimetallic bridged complex with Cu(I) and Ru(II) ligated with phenanthroline ligands and bipyridyl ligands respectively (Fig. 8C).195,273 Optical TA measurement is challenged to sort out the excited state dynamics since the MLCT transition bands for Ru(II) and Cu(I) moieties overlapping in the same region. Therefore, either Ru(II) moiety or Cu(I) moiety in each molecule can be excited and their respective MLCT states coexist in the ensemble, and the excited pathways are different (Fig. 8C). Therefore, it is very challenging in optical TA to extract the kinetics of the electron transfer between the two metal centers. Because XTA spectra are elemental-specific, Cu K-edge (8.979 keV) and Ru K-edge (22.126 keV) respectively can be probed at a synchrotron source. Therefore, electronic structural changes corresponding to the MLCT formation and decay can be obtained for each metal center unambiguously. Based on the XTA measurements at both edges combined with optical TA results (Fig. 8A and B), we concluded that (1) light induced MLCT transitions from Ru(II) or

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Fig. 9 (A) A general description of potential energy surfaces associated with the angle between the two phen ligands in Cu(I) diimine complexes; in flattened pseudotetrahedral conformation in the structure shown, the blue and green lights induce the S0 / S2 and S0 / S1 transitions respectively; and S1 and T1 correspond to 1MLCT and 3MLCT respectively. The energy levels and time constants for photochemical processes involved in interfacial electron transfer are also shown with the corresponding time constants labeled. (B) XANES spectra of the complex in solution and (C) on TiO2 nanoparticle surface. The formation and recovery kinetics of transient Cu(II) center via photoexcitation and/or electron transfer to TiO2 are taken at the energy at 8981 eV labeled by the dashed vertical line as shown in the insets. Notice that time axes in the inset for (B) and (C) have different ranges, with the recovery time in (B) around 150 ns and that in (C) is biphasic with 150 ns and up to a few ms, and the relative ratio of the two gives the quantum yield for interfacial electron transfer. Adapted with permission from Ref. Mara, M. W.; Bowman, D. N.; Buyukcakir, O.; Shelby, M. L.; Haldrup, K.; Huang, J.; Harpham, M. R.; Stickrath, A. B.; Zhang, X.; Stoddart, J. F.; Coskun, A.; Jakubikova, E.; Chen, L. X. Electron Injection from Copper Diimine Sensitizers into TiO2: Structural Effects and their Implications for Solar Energy Conversion Devices. J. Am. Chem. Soc. 2015, 137, 9670–9684. Copyright 2015 American Chemical Society.

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Cu(I) centers with  1.3:1 ratio undergo two pathways (Fig. 8C); (2) instantaneously formed Ru*(III) center at the MLCT state quickly recovers to Ru(II) in  60 ps due to the hole transfer to the Cu(I) center; (3) instantaneously formed Cu*(II) center due to the direct formation of the MLCT state. Cu(I) depletion fit a linear combination of impulsive and non-impulsive exponential decays due to the two routes for the formation and depletion of Cu(I) center; and (4) Cu(II) returns to Cu(I) in over 1 ns. This example demonstrated powerfulness of XTA in excited state electron transfer and relay in multiple metal center complexes which have potential to implement energetic gradient to form artificial photosynthetic system with a “z-scheme” where multi-step electron transfer is unidirectional and effective. The above examples are from the first-row transition metal complexes that have relatively narrow transition band widths compared to those of higher row transition metals with homogeneously broadened pre-edge and edge features due to the shorter core-hole state lifetimes. There are other first row transition metal complexes, such as iron, and a number of the second and third row transition metal complexes studied by the XTA method, such as ruthenium,66,165,180,235,274–277 iridium,185 rhenium,182 platinum168,170 and tungsten179 complexes.

8.17.4.3

Photoinduced isomerization in the excited state TMCs

XTA has also been used on multiple-metal-center complexes, such as Ru3(CO)12276 and Ru2(Cp)2(CO)466 (where CO is carbonmonooxide and Cp, cyclopentadiene), these complexes undergo photoinduced isomerization, sometimes irreversible, while converting light energy to heat or catalyzing chemical reactions. Some of these complexes were extensively studied by time-resolved vibrational spectroscopy focused on the C^O stretching signal.276 While the C^O stretching frequency can be sensitive signatures to the local structure, they provide indirect information on transient coordination geometry and oxidation state of the metal center. XTA can offer complementary information to the time-resolved vibrational spectroscopy to enable us to capture snapshots of these transient species in a novel way. An example for the TMC isomerization for (fulvalene)diruthenium complexes278–280 that under the sunlight capture undergo RueRu bond rupture on photoexcitation to furnish a long-lived triplet, which requires thermal activation to reach a surface crossing point to enter the singlet surface on route to its thermal storage isomer. Energy storage material when the reverse reaction have more heat output and input. However, it was unclear about the reaction path from cis- to trans-isomer. Using XTA and time-resolved IR spectroscopies, an intermediate structure generated via photoexcited triplet state after the intersystem crossing was captured which has the two fulvalene rings at roughly 90 instead of 180 as predicted by theory calculated for the possible dip in the potential energy surface without the photoexcitation.66 This example indicates that the reaction pathways via the photoexcitation could travel in a different energetic pathway because its starting point on the potential energy surface (PES) is different from that based on thermodynamics.

Fig. 10 Orbital diagrams for (A) Pt2(POP)2 and (B) structure and molecular orbital diagram for pyrazolate-bridged Pt(II) binuclear complexes. (A) Adapted from Durrell, A. C.; Keller, G. E.; Lam, Y. C.; Sýkora, J.; Vlcek Jr, A.; Gray, H. B. Structural control of 1A2u-to-3A2u intersystem crossing in diplatinum (II, II) complexes. J. Am. Chem. Soc. 2012, 134, 14201–14207 with permission. (B) Adapted with permission from Ref. Haldrup, K.; Dohn, A. O.; Shelby, M. L.; Mara, M. W.; Stickrath, A. B.; Harpham, M. R.; Huang, J.; Zhang, X. Y.; Moller, K. B.; Chakraborty, A.; Castellano, F. N.; Tiede, D. M.; Chen, L. X. Butterfly Deformation Modes in a Photoexcited Pyrazolate-Bridged Pt Complex Measured by Time-Resolved x-Ray Scattering in Solution. J. Phys. Chem. A 2016, 120, 7475–7483. Copyright 2016 American Chemical Society.

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(B) 1.6

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Energy (eV) Fig. 11 (A) Static XANES (black) and difference XTA spectra for (A) Pt(pop), for (B) [Ptppy(m-tBu2pz)]2. (A) Adapted from van der Veen, R. M.; Milne, C. J.; El Nahhas, A.; Lima, F. A.; Pham, V.-T.; Best, J.; Weinstein, J. A.; Borca, C. N.; Abela, R.; Bressler, C.; Chergui, M. Structural Determination of a Photochemically Active Diplatinum Molecule by Time-Resolved EXAFS Spectroscopy. Angew. Chem. Int. Ed. 2009, 48, 2711–2714 with permission. (B) Adapted from Lockard, J. V.; Rachford, A. A.; Smolentsev, G.; Stickrath, A. B.; Wang, X.; Zhang, X.; Atenkoffer, K.; Jennings, G.; Soldatov, A.; Rheingold, A. L.; Castellano, F. N.; Chen, L. X. Triplet Excited State Distortions in a Pyrazolate Bridged Platinum Dimer Measured by XRay Transient Absorption Spectroscopy. J. Phys. Chem. A 2010, 114, 12780–12787 with permission.

XTA measurements have a great potential application in catalysis, which has been highlighted in a recent publication by Lamberti and co-workers,281 where they carried out the XTA investigations on a ruthenium complex, cis-[Ru(bpy)2(py)2]2 þ, identified intermediates of photoexcited Ru(II) complex derived from the MLCT state and metal-centered (MC) state, which has been supported by combined X-ray solution scattering results. Other examples of catalytic functional relevant XTA studies are Ru3(CO)2 solution phase photodissociation of CO ligands.

8.17.4.4

Interfacial charge transfer from TMCs to semiconductor nanoparticles

Photoinduced interfacial electron/charge transfer from the excited state TMC to their covalently bound semiconductor nanocrystals have been studied via optical TA extensively due to their important applications in solar cells and photocatalysis. The structural evolution of the TMC and the rearrangement of the nanocrystal surface associated with the electron density shift during and after the interfacial charge injection are of great interest. XTA measurements can contribute complementarily to reveal transient electronic and geometric structures during this interfacial charge transfer processes. The first such system studied by XTA was RuN3 (RuII (dcbpy)2(NCS)2 in which dcbpy is 4,40 -dicarboxy-2,20 -bipyridine) with the carboxyl linker, -COOH, anchored the molecule onto the surface of TiO2 nanoparticles (NPs). The laser pump pulse triggered a MLCT transition in RuN3, shifting one electron to the dcbpy ligands and then injecting it into the NP. At a time delay of 50 ps, the transient structure of the dye was captured by the XTA measurements. The data analysis212,213 combines quantitative XANES fitting using the multidimensional interpolation approach implemented in the FitIt code and full multiple scattering (FMS) calculations of XAS using FEFF8.2 to obtain the average RueN(NCS) bond length shortens by about 0.06 Å, from 2.05 Å to 1.99 Å upon the conversion from the ground state RuN3/TiO2 to RuN3þ/TiO
2 , whereas the average RueN (dcbpy) bond length only changes within the experimental error from 2.04 Å to 2.05 Å. The different responses in the RueN bond lengths in dcbpy and NCS ligands have been directly characterized and rationalized by the interplay between two important factors governing the metal to ligand bonds, the bond order and the steric hindrance.

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Another study on such a hybrid system is on Cu(I) diimine derivatives bound onto the TiO2 nanoparticle surface where the photoinduced interfacial charge injection occurs (Fig. 9).107,186,190,282–298 The important new aspect from this study is that the Cu(II) in the MLCT state of this complex and Cu(II) after the electron injection can be distinguished by the kinetics measurements in which the former displays a similar kinetics as in solution measurements, whereas the latter has a prolonged ground state recovery time into the microseconds indicative the successful injection into TiO2 nanoparticles. The relative depletion and recovery ratio could also tell the real efficiency of the injection from the germinated recombination. For example, the electron injection time constants for two Cu(I) diimine sensitizers were about 0.3 ps while the ISC time constants were about 14 ps. Thus, the injection efficiency should be  97%. However, the recovery kinetics revealed that only about 30–40% of injected electrons has sustained without fast recombination. Such studies at the interfaces of hybrid systems open new possibilities to conceptualize and understand details of photoexcited dyes during exciton generation, charge separation and charge transport processes in real-world device configurations (Fig. 9).

8.17.5

Metal-metal interactions in bimetallic transition metal complexes

Bimetallic transition metal complexes and clusters of mono-metallic TMCs emerge as important systems for biomimetic catalytic centers299–301 and as photoluminescence materials.302–305 For example, hydrogenases have been investigated in the enzymes and in model compounds mimicking the functions. Metal oxide clusters, like what found in the oxygen evolving the manganese oxygen cluster in the Photosystem I for natural photosynthetic proteins, have attracted interests for their potential in electroand photo-catalysis to oxidize water.306,307 XTA spectroscopy using synchrotron radiation sources has been used to capture excited state structures of platinum dimer complexes173,174,308 with the molecular orbital diagrams shown in Fig. 10.109,305 Fig. 10A shows the prototypal photoactive d8–d8 complex, tetrakis(m-pyrophosphito)diplatinate(II), [Pt2(m-P2O5H2)4]4
, abbreviated Pt(pop), which has been extensively studied since decades ago.305,309 Fig. 10B shows another group of emerging photoactive d8–d8 Pt dimer complexes, pyrazolate-

Fig. 12 (A) Molecular structure of [Fe(tren(py)3)]2 þ; (B) schematic of relevant molecular orbitals and energetics involved in time-resolved soft x-ray absorption measurements; (C) L-edge spectra of low spin ground state and laser generated high spin state at 90 ps delay; (D) L-edge spectra of reference compounds for different spin states, [Fe(tacn)2]2 þ (where tacn is 1,3,7-triazacyclononane, a low-spin FeII complex) and [FeCl6]4
(a highspin FeII complex) confirming the photoexcitation generated the high spin state. Adapted with permission from Huse, N.; Kim, T. K.; Jamula, L.; McCusker, J. K.; de Groot, F. M. F.; Schoenlein, R. W. Photo-Induced Spin-State Conversion in Solvated Transition Metal Complexes Probed Via Time-Resolved Soft x-Ray Spectroscopy. J. Am. Chem. Soc. 2010, 132(19), 6809–6816. Copyright 2010 American Chemical Society.

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bridged Pt(II) binuclear complexes,310–316 which has controllable charge transfer transition and dual emission by changing the PtePt distance. The two types of Pt dimer complexes share a common feature due to the interacting 5dz2-5dz2 orbitals as the PtePt distance in the ground state allows, forming fully filled bonding ds and autibonding ds* orbitals, and thus the PtePt bond order is zero in the ground state. When the molecule absorbs a UV–vis photon, the lowest energy transition dominantly promotes one electron from ds* to higher energy empty orbitals, effectively increases the PtePt bond order and shorten the PtePt distance in the excited state. The ds*-ps* transitions are dominating the lowest energy transition in Pt(pop), and the ds*-p* transitions, as commonly assigned as metal-to-ligand-charge-transfer (MLCT) or metal-metal-to-ligand-charge-transfer (MMLCT) transitions are in the pyrazolate-bridged Pt dimers. Fig. 11 displays the XTA results for the triplet excited state of Pt(pop)168 and for the triplet one of the pyrazolate-bridged Pt dimers [Ptppy(m-tBu2pz)]2173 in their respective triple states with sub-microsecond lifetimes. The most distinct feature is a sharp feature in the difference XANES spectra (laser on – laser off) at the rising Pt LIII-edge, indicative that a new 2p / 5d transition arises from the vacant ds* created by the valence transitions.168,173,174 Indeed, a vacancy suggests an oxidized Pt(II)ePt(II) core, confirming the model that has been proposed in earlier studies. The PtePt distance in the triplet state shrank by 0.31(5) Å for Pt(pop)168,174 and 0.20 Å for [Ptppy(m-tBu2pz)]2173 in their respective triplet states. The PteP distance elongation of 0.01 Å in Pt(pop)168,174 and PteN(C) distance (bridging pyrazolate ligand) shortening by 0.09 Å and PteN(C) distance (cyclometallic phenylpyridine ligand) elongation of 0.03 Å.173 Such structural details combined with theoretical calculations has significantly deepened our understanding of key factors that could influence the excited state pathways and provided guidance for molecular design. During recent years, several studies on these Pt dimers combining OTA and X-ray measurements on the femtosecond time scale have been published with strong emphasis on interplays between electronic and nuclear structures as the excited state undergoing intersystem crossing (ISC). These studies have been carried out at XFEL sources with a few tens of femtosecond time resolution. The questions to answer through these studies are the timing of nuclear motions and electronic migrations from one part of molecule to another during intersystem crossing, and structure control of the excited state processes. Due to the current operation mode of the XFEL sources the monochromator scan range is limited which hinders the energy scan required for EAFS data. Thus, nuclear structures of excited state TMCs have been obtained via ultrafast X-ray solution scattering or diffraction,317–320 which is beyond the scope of the current chapter.

8.17.6

TMCs studied by L-edge XTA spectroscopy and soft X-ray spectroscopy

So far most of the XTA studies on TMCs are at metal K-edges in the hard X-ray region, broadly speaking from  5 keV and up in energy, within which the 3d and 4d element K-edges and 5d element L-edge XAS spectra can be obtained as shown in the examples in the earlier sections. For the first row TMCs, the K-edge XTA provides electronic configuration through weak quadrupole-allowed 1s to 3d transitions in the pre-edge region and local nuclear coordinating geometry from the XAFS signal in the excited state, but it has certain limitations. For example, if the complex has a nearly perfect central symmetric coordination geometry, there will be extremely weak or no pre-edge features for 3d and 4d elements to provide vacant orbital electronic configuration. Using the dipole-allowed 2p to 3d transitions at L-edge for the 3d elements, e.g., 2p (LII- and LIII-edges) to 3d transitions, the inner shell transition features can be intense and narrower in probing the metal valence charges and spin density in 3d orbitals.321 More importantly, the narrower spectral features compared to K-edge transitions due to much longer 2p core-hole lifetime (e.g., 0.2 eV lifetime broadening at the LIII- edge) enables fine differences of the metal covalency, spin density and valence change in the excited state to be detected. Because of the close energy levels from the tender and soft X-ray regions for the inner shell transitions (e.g., LII- and LIIIedges for Fe are 721, and 708 eV, and for Ru, 2967 and 2838 eV), the XTA measurements in this energy range is limited to electronic transitions, and XAFS measurements could not be carried out. One example of using L-edge XTA to study excited state TMCs is [Fe(tren(py)3)]2 þ complex undergoing photoinduced charge transfer99,322 (Fig. 12A). This molecule is one of the “spin-crossover” compounds to undergo interconversion between a lowspin and a high-spin configuration (S ¼ 0 and S ¼ 2 in the case of FeII) induced by heat or light (Fig. 12B).323 There have been several works in the hard X-ray regime to study the spin-crossover complexes, because they display significant change in the XANES and XAFS signals and have been studied extensively by other means for decades due to their promising applications in information storage and quantum computing. In this particular complex, Huse and coworkers used light to induce the spin transition from low to high spin state of this Fe(II) complex. Fig. 12C and D present L-edge spectra of the low spin ground state and the laser generated high spin state at 90 ps delay of the laser pulse. These have been compared with L-edge spectra of reference compounds with known spin states to confirm the photoexcitation generated the high spin states. The detailed comparison between the low-spin L-edges of [Fe(tren(py)3)]2 þ with those of [Fe(tacn)2]2 þ revealed the influence of p-backbonding that only present in the former. Because the electron delocalization through conjugated aromatic ligand via the p-backbonding leads to the Fe-3d electrons shared by the ligand resulting in L2,3-absorption edges at energies higher than those of [Fe(tacn)2]2 þ (Fig. 12D). On the other hand, the L-edges of the transiently formed 5T2 excited state of [Fe(tren(py)3)]2 þ at 90 ps time delay and the 5T2 ground state of [FeCl6]4
(Fig. 12C and D) show remarkable similarity. [FeCl6]4
has t2g-symmetry ligand-group valence orbitals, Cl
can only serve as a p-donor. Thus, the t2g orbitals of Fe are formally pantibonding, and localized on the iron center. The strong similarity of both of the high-spin L-edges in Fig. 12C and D led a conclusion of 3d electron in the photoexcited high-spin state of [Fe(tren(py)3)]2 þ is essentially localized with the increased metal
ligand

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bond distance in the high-spin state. In this study, the detailed comparison of chemical bonding situation with the peak positions for the 2p to 3d transition provides insightful information regarding not only the electron occupancy but also electronic communication between the metal center and the ligands Notably large shift of the L3-edge by 1.7 eV reflects the reduced influence of the ligand p-electron system on the metal 3d orbitals. XTA measurements in the soft X-ray energy regions also enable detecting electronic structures of light elements, such as carbon, nitrogen, and oxygen, commonly appearing in the ligands to the transition metal centers. Numerous studies in the literature have shown that synthetically tuning the TMC properties can be accomplished via changing the ligand structures. The soft X-ray absorption features are sensitive to the chemical environment, such as aromatic vs. aliphatic carbon, nitrogen directly ligated with metal vs. in other bonding situations. Thus, complementary to the hard X-ray studies, soft X-ray XTA starts to emerge for studying TMC excited state structures. In the soft X-ray spectroscopy (sometimes called NEXAFS, near edge X-ray absorption fine structure), different chemical bonding type could influence the edge features of C, O and N to probe the covalency between the metal center and the ligands complement to the L-edge and K-edge spectra. In spite of these capabilities, XTA in the soft X-ray region has not been as common as its counterpart in the hard X-ray region, because most of these measurements need to be carried out in vacuum due to the air absorption and scattering, which has been a challenge for liquid samples until very recently when a flat sub-micron liquid jet combined with a cold trap are used under the vacuum.324,325 The other challenge for the soft X-ray XTA application is the specificity of the atoms because ligands frequently have multiple nearly equivalent N, C or O atoms. However, if the same kind of atoms in different chemical bonding situation with different electron structures, their absorption features can be distinguished in the soft X-ray absorption spectroscopy. Advances in soft X-ray sample environment and delivery systems will make such approach more common in XTA applications in the future. During recent years, table-top soft X-ray transient absorption spectroscopy have been developed rapidly to probe excited state transition metal oxides and TMCs using the inner shell transition at M-edge, from 3s and 3p orbitals to higher energy orbitals.102,113,324,326 These facilities are based on high harmonic generation derived from an intense laser fundamental radiation, such as Ti:sapphire lasers, interacting with helium gas medium.325,327–332 Because of the energy range for the soft X-ray, VUV/XUV, i.e. tens eV – 100 eV, the experiments have to be carried out in vacuum and most of the experiments done so far are on solid films, as they have similar sample environment challenges as in the soft X-ray XTA. The main advantage of this approach is the time resolution in femtosecond regime and the convenience of access the instruments without traveling to user facilities. However, the photon flux will be much lower than the synchrotron or XFEL facilities. Thus, these table-top facilities can be emerging way of studying TMC excited state electronic structures. In summary, XTA in both hard and soft X-ray energy regimes has become an increasingly common and important method for studying electronic and nuclear structures of transition metal complexes, complementary to optical transient absorption and emission spectroscopies, vibrational spectroscopies for kinetics and energetic information. In addition to the XTA, a variety of X-ray emission spectroscopies, such as non-resonant X-ray emission spectroscopy (XES), resonant inelastic X-ray scattering (RIXS), etc. have been pursued to gain electronic structures of the metal center, especially the spin state. XES and RIXS probe the occupied orbitals from which the electron refill the core hole. Because the dispersion optics used to disperse the emitted photons with different energies, the signals have higher energy resolution compared to XAS, but weaker at each energy interval. Experimentally, they are more challenging that XTA, and thus have been used in neat or very concentrate solution samples. There have been many published work using these methods, which unfortunately are not covered here due to the scope of the chapter.

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d-d and charge transfer photochemistry of 3d metal complexes

Matthias Dorn, Nathan Roy East, Christoph Fo¨rster, Winald Robert Kitzmann, Johannes Moll, Florian Reichenauer, Thomas Reuter, Laura Stein, and Katja Heinze, Department of Chemistry, Johannes Gutenberg University Mainz, Mainz, Germany © 2023 Elsevier Ltd. All rights reserved.

8.18.1 8.18.1.1 8.18.1.2 8.18.2 8.18.2.1 8.18.2.2 8.18.3 8.18.3.1 8.18.3.2 8.18.4 8.18.4.1 8.18.4.2 8.18.4.3 8.18.4.4 8.18.4.5 8.18.4.6 8.18.4.7 8.18.5 8.18.6 Acknowledgments References

Introduction General context and scope of this review Photophysical background and specific considerations for 3d TMs Luminescent 3d TMCsdPreservation of the coordination sphere 3d TM spin-flip emitters 3d TM charge transfer emitters Electron and energy transfer with 3d TMCsdBimolecular reactivity Photoinduced electron transfer from and to 3d TMCs Photoinduced energy transfer from and to 3d TMCs Unimolecular reactivity of 3d TMCsdModification of the coordination sphere CO dissociation from 3d TMCs NO isomerization and dissociation in 3d TMCs CO2 dissociation from carboxylato 3d TMCs Nx dissociation from azido 3d TMCs N2 splitting with 3d TMCs M–C homolysis in 3d TMCs Miscellaneous photodissociations and rearrangements of 3d TMCs Conclusion Note added in proof

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Abbreviations 2-MeTHF 2-Methyltetrahydrofuran A Acceptor Ad Adamantyl AdoCbl 50 -Desoxyadenosylcobalamin BArF4L Tetrakis(3,5-bis(trifluoromethyl)phenyl)borate bISC Back-intersystem crossing BODIPY 4,4-Difluoro-4-bora-3a,4a-diaza-s-indacene CAAC Cyclic (alkyl)(amino)carbene CAArC Cyclic (amino)(aryl)carbene CASPT2 Complete active-space second-order perturbation theory CASSCF Complete active space self-consistent field CORM CO-releasing molecule CPL Circularly polarized luminescence CSU Cooperative sensitization upconversion CT Charge transfer CTTS Charge transfer to solvent D Donor DFT Density functional theory DIPEA Diisopropylethylamine Dipp 2,6-Diisopropylphenyl DMF N,N-Dimethylformamide DMSO Dimethylsulfoxide DNA Deoxyribonucleic acid DSSC Dye-sensitized solar cell EnT Energy transfer

Comprehensive Inorganic Chemistry III, Volume 8

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ES Excited state ESA Excited state absorption ET Electron transfer ETU Energy transfer upconversion FRET Förster resonance energy transfer FTIR Fourier-transform infrared FTO Fluorine doped tin oxide FWHM Full width at half maximum GS Ground state HAT Hydrogen atom transfer HbO2 Oxyhemoglobin HER Hydrogen evolution reaction HFEPR High-field electron paramagnetic resonance HOMO Highest occupied molecular orbital IC Internal conversion IL Intraligand ILCT Intraligand charge transfer IPCT Ion pair charge transfer IR Infrared ISC Intersystem crossing IUPAC International Union of Pure and Applied Chemistry IVCT Intervalence charge transfer KGW Potassium gadolinium tungstate LC Ligand-centered LEC Light-emitting electrochemical cell LED Light-emitting diode LF Ligand field LL0 CT Ligand-to-ligand charge transfer LMCT Ligand-to-metal charge transfer LT Low temperature LUMO Lowest unoccupied molecular orbital Mb Myoglobin MbO2 Oxymyoglobin MC Metal centered MeCbl Methylcobalamin Mes Mesityl mIR Mid-infrared MLCT Metal-to-ligand charge transfer MO Molecular orbital NEVPT2 N-Electron Valence State Perturbation Theory NHC N-Heterocyclic carbene NIR Near-infrared NMR Nuclear magnetic resonance NORM NO-releasing molecule OHCbl Hydroxidocobalamin OLED Organic light-emitting diode PDT Photodynamic therapy PES Potential energy surface PFMCH Perfluoromethylcyclohexane RT Room temperature RTP Room temperature phosphorescence sc Supercritical SET Single-electron transfer SNP Sodium nitroprusside

d-d and charge transfer photochemistry of 3d metal complexes

SOC Spin-orbit coupling SOI Spectral overlap integral SOMO Singly occupied molecular orbital Sub Substrate TA Transient absorption TADF Thermally activated delayed fluorescence TBDMS tert-Butyldimethylsilyl TDDFT Time-dependent density functional theory THF Tetrahydrofuran TM Transition metal TMC Transition metal complex TMS Trimethylsilyl TRIR Time-resolved infrared TTA-UC Triplet-triplet annihilation upconversion UV Ultraviolet VR Vibrational relaxation XAS X-Ray absorption spectroscopy XES X-Ray emission spectroscopy XLCT Halogenido-to-ligand charge transfer YAG Yttrium aluminum garnet ZFS Zero field splitting

Nomenclature acacL Acetylacetonato(1–) alaL Alaninato(1) b-alaL b-Aminopropanoato(1) [15]ane-N4 1,4,8,12-Tetra-azacyclopentadecane Arbip 2,6-Bis(10 -(2,6-diisopropy)-phenylimidazol-20 -yl)pyridine ATL 11,13-Dimethyl-1,4,7,10-tetraazacyclotetradeca-10,12-dienato(1–) bath 4,7-Diphenyl-1,10-phenanthroline bbt 4,4000 -Bis(2,20 :60 ,200 -terpyridine) bbip-COOH 2,6-Bis(10 -benzimidazol-20 -yl)-4-carboxyl-pyridine bcp 2,9-Dimethyl-4,7-diphenyl-1,10-phenathroline [BIMPNMes,Ad,Me]L Anion of bis[2-(3-mesityl-imidazol-2-ylidene)ethyl-(3-adamantyl-2-hydroxy-5-methylphenyl)methyl] amine binc Bis(2-isocyanophenyl) phenylphosphonate Bn-TPEN N-Benzyl-N,N0 ,N0 -tris(2-pyridylmethyl)-1,2-diamino-ethane bpgL N,N-Bis(pyridyl-methyl)glycinato(1) bpm 2,20 -Bipyrimidine bpy 2,20 -Bipyridine btz 3,30 -Dimethyl-1,10 -bis(p-tolyl)-4,40 -bis(1,2,3-triazol-5-ylidene) t Bubip 2,6-Bis(10 -tert-butylimidazol-20 -yl)pyridine g-butL g-Aminobutanoato(1) 1,4-C2-cyclam 1,4,8,11-Tetraazabicyclo[10.2.2]hexadecane [cat-COOH]2L Catecholato-4-carboxylic acid(2) (C6F5)3tren 2,20 ,200 -Tris[(pentafluorphenyl)amino]triethylamine CpL Cyclopentadienide Cp*L Pentamethylcyclopentadienide cyclam 1,4,8,11-Tetraazacyclotetradecane cyclam-acL 1,4,8,11-Tetraazacyclotetradecane-1-acetato(1) dap 2,9-Bis(p-anisyl)-1,10-phenanthroline dbap 2,9-Bis(p-carboxyphenyl)-1,10-phenanthroline

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dcbpy 4,40 -Dicarboxy-2,20 -bipyridine dcpp 2,6-Bis(2-carboxypyridyl)pyridine ddpd N,N0 -Dimethyl-N,N0 -dipyridine-2-yl-pyridine-2,6-diamine depe 1,2-Bis(diethylphosphanyl)ethane dgpy 2,6-Bis(2,3,4,6,7,8-hexahydropyrimido[1,2-a]pyrimidin-1-yl)pyridine dgpz 2,6-Bis(2,3,4,6,7,8-hexahydropyrimido[1,2-a]pyrimidin-1-yl)pyrazine diamsar 1,8-Diamino-3,6,10,13,16,19-hexaazabicyclo[6.6.6]icosane dmgHL Anion of dimethylglyoxime (dmgH2) dmp 2,9-Dimethyl-1,10-phenanthroline dmpe 1,2-Bis(dimethylphosphanyl)ethane dpa2L 2,6-Pyridinedicarboxylato(2) dpma N,N-Di(2-pyridyl)methylamine dpaqL 2-(N,N-Bis(pyridin-2-ylmethyl))-amino-N0 -quinolin-8-yl-acetamido(1) DPEPO Bis[2-(diphenylphosphanoxide)-phenyl]ether dpp 2,3-Bis(2-pyridyl)pyrazine dppe 1,2-Bis(diphenylphosphanyl)ethane dpya 2,20 -Dipyridylamine dqp 2,6-Di(quinolin-8-yl)pyridine dtbbpy 4,40 -Di-tert-butyl-2,20 -bipyridine dtmaL 1-Diethylene-triamine-monoacetato(1) ebbt 4,4000 -(Ethynyl)-bis(2,20 :60 ,200 -terpyridine) en 1,2-Ethylene diamine glyL Glycinato(1) H2tpda 2,6-Bis(2-pyridylamino)pyridine HBpz3L Trispyrazolylborato(1) H-DAB 1,4-Diazabutadiene IMes 1,3-Dimesityl-4-imidazoline-2-ylidene IPr 1,3-Bis(2,4,6-trimethylphenyl)imidazole-2-ylidene iPr-DAB 1,4-Diisopropyl-1,4-diazabutadiene Dipp-DAB N,N0 -Bis(2,6-diisopropylphenyl)-1,4-diaza-1,3-butadiene LN3 N1,N1,N3,N3-Tetrakis(2-(pyridin-2-yl)ethyl)propane-1,3-diamine) LN5 N1,N1,N3,N3-Tetrakis (2-(pyridin-2-yl)ethyl)pentane-1,5-diamine 2,6-lut 2,6-Lutidine MabiqL Anion of HMabiq ¼ 2-4:6-8-bis(3,3,4,4-tetramethyldihydropyrrolo)-10–15-(2,20 -biquinazolino)-[15]-1,3,5,8,10,14hexaene-1,3,7,9,11,14-N6 mal2L Malonato(2) mbip 2,6-Bis(10 -methylimidazol-20 -yl)-pyridine mbip-COOH 2,6-Bis(10 -methylimidazol-20 -yl)-4-carboxyl-pyridine Me4cyclam 1,4,8,11-Tetramethyl-1,4,8,11-tetraazacyclotetradecane Me3cyclam-acL 4,8,11-Trimethyl-1,4,8,11-tetraazacyclotetradecane-1-acetato(1) Me5-D3htricosaneN6 1,5,9,13,20-Pentamethyl-3,7,11,15,18,22-hexaazabicyclo[7.7.7]tricosane MePy2tacn N-Methyl-N,N-bis(2-picolyl)-1,4,7-triazacyclononane mi-5edt2L 1-(N-methylindol-5-yl)-ethene-1,2-dithiolato(2) mi-5hdt2L 1-(N-Methylindol-5-yl)-hex-1-ene-1,2-dithiolato(2) morph Morpholine obtL 4-Oxobenzo[d][1,2,3]triazin-3(4H)-olato(1) ox2L Oxalato(2) PaPy3L Anion of PaPy3H ¼ N,N-bis(2-pyridylmethyl)amine-N-ethyl-2-pyridine-2-carboxamide PaPy2QL Anion of PaPyQH ¼ N,N-bis(2-pyridylmethyl)amine-N-ethyl-2-quinoline-2-carboxamide Pc2L Phthalocyaninato(2) [PhB(MeIm)3]L Phenyl tris(3-methylimidazol-2-yl)borato(1), phtmeimb [PhB(tBuIm)3]L Phenyl tris(3-tert-butyl-imidazol-2-yl)borato(1) phen 1,10-Phenanthroline Ph2phen 2,9-Diphenyl-1,10-phenanthroline

d-d and charge transfer photochemistry of 3d metal complexes

a-pic a-Picoline POP Bis(2-(diphenylphosphanyl)phenyl)ether por2L Porphyrinato(2) ppyL Anion of ppyH ¼ 2-phenylpyridine P3B Tris(2-(diisopropylphosphanyl)phenyl)borane 0 P2P Ph ((Phenylphosphanediyl)bis(2,1-phenylene))bis(diisopropylphosphane) i Prbip 2,6-Bis(10 -isopropylimidazol-20 -yl)pyridine PV-TMPA Bis(pyrid-2-ylmethyl){[6-(pivalamido)pyrid-2-yl]methyl}amine py Pyridine pyr3 Tris(pyrid-2-yl)methane qdt2L 2,3-Quinoxaline-dithiolato(2) quinL 8-Hydroxyquinolinato(1) salen N,N0 -Ethylene-bis(salicylidenimine) salphen N,N0 -o-Phenylene-bis(salicylidenimine) salchd 1,2-Bis(salicylidenimino)cyclohexane SBPy3 N,N-Bis(2-pyridylmethyl)amine-N-ethyl-2-pyridine-2-aldimine SBPy2Q N,N-Bis(2-pyridylmethyl)amine-N-ethyl-2-quinoline-2-aldimine sen 4,40 ,400 -Ethylidenetris(3-azabutane-1-amine) H4TAML 3,4,8,9-Tetrahydro-3,3,6,6,9,9-hexamethyl-1H-1,4,8,11-benzotetraazo-cyclotridecane-2,5,7,10-(6H,11H)tetrone tbta Tris[(1-benzyl–1H–1,2,3-triazole-4-yl)methyl]amine tet-a Meso-5,7,7,12,14,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane ThiaSO2 p-tert-Butylsulphonylcalix[4]arene TIMENmes Tris[2-(3-mesityl-imidazol-2-ylidene)ethyl]amine TIMENxyl Tris[2-(3-xylyl-imidazol-2-ylidene)ethyl]amine 2-tip Tris(imidazol-2-yl)phosphane 4-tip Tris(2-isopropylimidazol-4-yl)phosphane TMG3tren 2,20 ,200 -(Nitrilotris(ethane-2,1-diyl))tris(1,1,3,3-tetramethylguanidine) thioPOP 10,100 -(2,8-Dimethylphenoxathiine-4,6-diyl)bis(2,8-dimethyl-10H-phenoxaphosphinine) tmpa Tris(2-pyridylmethyl)amine tpa Tris(2-pyridylmethyl)amine tpe 1,1,1-Tris(pyrid-2-yl)ethane tpena N,N,N0 -Tris(2-pyridylmethyl)-ethylenediamine-N0 -acetato(1) tpm Tris(pyrazolyl)methane TPP2L 5,10,15,20-Tetraphenylporphyrinato(2) tpy 2,20 :60 ,200 -Terpyridine tpy-COOH 4-Carboxyl-2,20 :60 ,200 -terpyridine tren Tris(2-aminoethyl)amine tren(py)3 Tris(2-pyridyl-methyliminoethyl)amine trypL Tryptophanato(1) ttpy 4-p-Tolyl-2,20 :60 ,200 -terpyridine Xant 4,5-Bis(diphenylphosphanyl)-9,9-dimethylxanthene

Abstract The photophysics and photochemistry of 3d transition metal complexes strongly differ from their heavier 4d and 5d homologs. The distinct excited state dynamics are a direct consequence of the weaker ligand field splitting and the smaller spin-orbit coupling in 3d transition metal complexes. The often very fast non-radiative relaxation of photoexcited 3d transition metal complexes to the ground state has prevented a widespread use of these much more abundant transition metals in photophysical and photochemical applications so far. Recent exciting advancements in ligand design, synthesis, ultrafast spectroscopy, computational chemistry and understanding of excited state dynamics as well as the awareness for a more sustainable photochemistry led to a paradigm change in the reputation and emerging importance of 3d transition metals. The introduction chapter contrasts the photophysical background of 3d transition metal complexes with that of their heavier homologs. This is followed by overviews of the excited state reactivity of 3d transition metal complexes, namely

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d-d and charge transfer photochemistry of 3d metal complexes luminescence, bi- and unimolecular reactivity. Luminescence from excited states is divided into spin-flip and charge transfer luminescence. The charge transfer excited states and the resulting luminescence can be of ligand-to-metal, metal-to-ligand, ligand-to-ligand charge transfer and charge transfer to solvent character. Bimolecular reactivity covers photoinduced electron and energy transfer reactions. In solution, the lifetimes of the reacting excited states play a particularly important and challenging role. Dissociative unimolecular reactivity comprises dissociation of ligands such as CO and NO, CO2 dissociation from carboxylato complexes, N2 dissociation from azido complexes, MeC bond homolysis and other MeX dissociations as well as photoisomerizations. These unimolecular reactions can occur on ultrafast timescales.

8.18.1

Introduction

8.18.1.1

General context and scope of this review

Photoactive transition metal (TM) complexes (TMCs) are a topic of intense research for decades both from a fundamental and an application view point.1–5 Today, photoactive TMCs play a vital role in numerous photophysical and photochemical processes. TMCs can act as sensitizers in dye-sensitized solar cells (DSSCs),6–10 as emitters in (phosphorescent) organic light emitting diodes ((PH)OLEDs)11–14 and light-emitting electrochemical cells (LECs),15–18 as photosensitizers or photocatalysts in photochemical and photocatalytic reactions,19–24 as bioimaging reagents25–28 and as pro-drugs29–33 in medicinal photochemistry. This wide range of applications is a result of their favorable excited state ordering and excited state dynamics. TMCs of the noble metals, e.g., Ru, Ir, Pt or Au, and complexes of the rare-earth elements particularly qualify for these applications. However, the low abundance of these elements with associated high costs is a severe limitation for a worldwide industrial use. Furthermore, widespread use of precious and rare-earth metals can pose geopolitical problems as only a few countries mine and produce appreciable quantities. In addition, mining and extraction of noble and rare-earth metals are energy and water demanding, and often employ very toxic agents, such as cyanide or mercury in gold mining.34–36 The widespread use of Earth-abundant metals,37,38 namely most first-row TMs (3d elements) and certain second-row TMs, would be significantly more sustainable. For intense luminescence and efficient interaction with a suitable substrate, the excited states of TMCs should be sufficiently (photo)stable, and the relevant electronically excited states should possess a sufficiently long lifetime. Both requirements are challenging to achieve with 3d TM ions, and this has hindered their widespread application to date. In the last decade,39,40 ultrafast time-resolved spectroscopy,41–46 high-level quantum chemical methods47–49 and the exploitation of structural characterization of excited states44 provided deep insight into the electronic state ordering, and the often ultrafast excited state processes of photoactive 3d TMCs. This has led to a paradigm change in the reputation and importance of photoactive 3d TMCs. Indeed, first exciting and promising applications of tailored 3d TMCs in the fields of OLEDs,50,51 DSSCs,51,52 sensing53–57 and photoredox catalysis58,59 have already been reported. This review summarizes the current understanding and the specific challenges of photoactive 3d TMCs, with respect to absorption, excited state dynamics, luminescence (Section 8.18.2), bimolecular photoreactivity (Section 8.18.3) and unimolecular photoreactivity (Section 8.18.4).

8.18.1.2

Photophysical background and specific considerations for 3d TMs

The electronic structure of TMCs determines their photophysical and photochemical properties.60–63 The nature and energetic ordering of electronically excited states is primarily influenced by the ordering of molecular orbitals and their ground state (GS) occupancy. A convenient simplification adapts point group notations of coordination compounds (idealized geometry Oh, Td, D3h, C4v; Fig. 1: Oh) for the involved orbitals (e.g., a1g, t1u, eg, t2g in Oh; Fig. 1) and for the symmetry of resulting excited states. By formally partitioning a TMC into ligand and metal moieties, the excited states are further categorized in charge transfer (CT) states (ligand-to-metal CT (LMCT), metal-to-ligand CT (MLCT), ligand-to-ligand CT (LL0 CT), intervalence CT (IVCT) in mixedvalent systems) and more localized transitions, namely ligand field (LF) or metal-centered (MC) and ligand-centered (LC) or intraligand charge transfer (ILCT) transitions (Fig. 1).60–62 Differing MC states can be further classified into transitions between different metal d orbitals (e.g., t2g/eg in Oh; interconfigurational states) or spin-flip transitions, within a configuration (e.g., within a partially filled t2g subshell; intraconfigurational states). In a given coordination geometry, the electronic structure of a TMC in its ground and excited states is sensitive to interrelated effects. These include the variation of the central ion, its oxidation and spin state (intrinsic ligand field strength), the type of coordinated ligands and their substituents, which shifts eg and t2g orbitals by s-donating and paccepting effects, respectively, and the energy of non-bonding ligand-derived orbitals (e.g., t1g, t1u, t2u, Fig. 1). The challenge of obtaining photoactive complexes of the first-row TMs39,40 is strongly linked to their weaker ligand field strength Do (in Oh, Fig. 1) as compared to their heavier homologs. The increasing ligand field strength in 3d/4d/5d TM triads is apparent for many TMCs such as pseudo-square-planar ML4 and pseudo-octahedral ML6 complexes of the NiII, PdII, PtII and the FeII, RuII, OsII triads, respectively. Comparing the photodynamics of the low-spin d6-iron(II) complex [Fe(bpy)3]2þ 12D (Fig. 2A and B)65–67 with that of its heavier homolog [Ru(bpy)3]2þ 22D67–71 is particularly revealing (bpy ¼ 2,20 -bipyridine). While the prototypical 22D complex is phosphorescent (lem z 620 nm) with a long excited state lifetime (s) in the nanosecond regime67,72 enabling numerous photophysical and photochemical applications,20–24 the iron(II) complex is non-luminescent and possesses a very short excited state lifetime of the corresponding excited state (femtosecond regime).65–67 Due to the electron-rich MII centers and the presence of low-lying p*-orbitals of the bpy ligands, the initially

d-d and charge transfer photochemistry of 3d metal complexes

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Fig. 1 (A) Molecular orbital diagram for octahedral ML6 complexes with p backbonding and (B) categorization of derived electronically excited states.61,63

Fig. 2 (A) Simplified Jablonski diagram of 12D with relevant photophysical processes, absorption, ISC, IC, fluorescence, phosphorescence and vibrational relaxation (VR) are indicated. 3T comprises 3T1g and 3T2g states. Electronic state ordering according to ref. 64. Molecular structures of (B) [Fe(bpy)3]2þ 12D and (C) [Fe(btz)3]2þ 32D.

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d-d and charge transfer photochemistry of 3d metal complexes

populated excited state is of 1MLCT nature in both cases (Fig. 1). While 22D undergoes rapid intersystem crossing (ISC, kISC) from the Franck-Condon 1MLCT state to a long-lived 3MLCT state as luminescent lowest excited state, the excited state decay of 12D is much more intricate. According to the smaller ligand field splitting of the iron(II) complex, the interconfigurational MC states (5T2g, 3T1g, 3T2g in the Oh notation) are much lower in energy than the 1/3MLCT states. In agreement with Kasha’s rule,73 ultrafast population transfer via internal conversion (IC) and ISC occurs finally populating the long-lived non-luminescent 5T2g high-spin state with unity quantum efficiency (Fig. 2A).74 As kISC and kIC to the distorted, non-emissive MC states outcompete radiative processes of the 1MLCT and 3MLCT states (fluorescence kf; phosphorescence kp), 12D is non-luminescent (Fig. 2A).65–67 This intrinsically weak ligand field splitting of 3d TM ions is caused by the so-called primogenic effect.65,75,76 The absence of nodes in the electronic radial distribution functions of the first p, d and f shells (i.e., 2p, 3d and 4f) leads to a higher effective nuclear charge acting on these wavefunctions, accompanied by a radial contraction. Compared to metal 4d and 5d orbitals, the poorer overlap of the contracted metal 3d orbitals with ligand orbitals causes the smaller ligand field splitting. Consequently, the nonluminescent MC states of 3d TMCs possess lower energies.39,40,65,66 To overcome the intrinsic weak ligand field of the FeII ion, which causes short lifetimes of the potentially photoactive 1/3MLCT manifold, different strategies based on ligand tuning have been devised.66 Most approaches aim to destabilize the low-lying MC (3T1g, 3T2g, 5T2g in Oh notation) states by increasing the ligand field strength with strongly s-donating (destabilization of eg* orbitals) and/or p-accepting (stabilization of t2g orbitals) ligands. Push-pull concepts aim at lowering the MLCT states.40,65,66 For example, strong s-donating chelating mesoionic carbene ligands77 boost the lifetime of the 3MLCT state by several orders of magnitude to s(3MLCT) z 528 ps in [Fe(btz)3]2þ 32D (btz ¼ 3,30 -dimethyl-1,10 -bis(p-tolyl)-4,40 -bis(1,2,3-triazol-5-ylidene), Fig. 2C).78 While the general mechanism of the excited state dynamics from the 1MLCT to the 5T2g state of the prototypical 12D is well accepted, the detailed excited state dynamics are still subject of intense investigation and different pathways are discussed (Fig. 2A). While direct 1MLCT / 5T2g intersystem crossing, bypassing the 3MLCT state has been proposed,79 other possible pathways populate the 3MLCT state by ultrafast ISC (sISC < 30 fs, kISC ¼ 1/sISC)45,80 from the 1MLCT state. The 3MLCT state is also very short-lived and rapidly evolves to MC states within 100 fs.81–83 The 5T2g MC state either forms directly from the 3MLCT state by ISC81,82,84,85 or via intermediate population of the 3T MC state manifold via IC, followed by 3T / 5T2g ISC (Fig. 2A; kISC00 ).86–88 The parallel decay channels 1MLCT / 3MLCT / 5T2g and 1MLCT / 3MLCT / 3T / 5T2g have been evidenced by molecular dynamics.89 For the latter path, a cascade via 3T2g / 3T1g in the 3MC manifold was suggested.64 Finally, the lowest excited MC state, 5 T2g, decays in a spin crossover back to the low-spin 1A1g state within a 650 to ca. 1000 ps timescale, depending on the solvent.82,85,90 Clearly, ultrafast ISC outcompetes fluorescence (kf) and high kIC and kISC0 constants outcompete phosphorescence (kp) in 12D. The excited state dynamics of 12D with the 1MLCT / 3MLCT ISC as a “spin-forbidden process” is similarly ultrafast like that of the heavier homolog 22D, although iron(II) possesses a significantly smaller spin-orbit coupling (SOC) constant than ruthenium(II) (z(Fe2þ)exp ¼ 436 cm1, z(Ru2þ)exp ¼ 1159 cm1).45,67,80,91 Consequently, a “simple” heavy-atom effect,48 which roughly scales with Zeff4 , cannot be solely responsible for these decisive ultrafast ISC processes in iron(II) complexesdand other 3d metal complexes. In fact, ISC in 3d TMCs can occur in a non-Born-Oppenheimer regime with strongly coupled electronic and nuclear vibrational motions, acting on similar timescales.80,82,86,92,93 The conventionally accepted relative rates of kVR > kIC > kISC do not necessarily apply for TMCs generally, and 3d TMCs in particular. In the Fermi’s Golden Rule approximation, ISC rates kISC are described with the assumption that the coupling of initial and final states is small compared to their adiabatic energy difference.48,94,95 After separation of electronic and vibrational contributions, the ISC rate depends on the direct SOC between initial and final electronic states of different spin multiplicity and the Franck-Condon weighted density of states (Born-Oppenheimer regime). A high density of (vibrational) states of the final state increases kISC. Therefore, ISC is often fast in molecules with many atoms providing a high density of vibrational levels.96 In the absence of an operative heavy atom effect, e.g., in organic molecules with only light atoms, the conservation of the total momentum forms the basis of El Sayed’s rule,97,98 which invokes SOC.96,99 In fact, a change in orbital magnetic moment can compensate a change in spin magnetic moment enabling fast ISC between close-lying states, e.g., in ketones.95,96,99 Beyond the Born-Oppenheimer approximation, high molecular flexibility and resulting low energy vibrations open a path to nonadiabatic couplings. Spin-vibronic coupling describes the coupling of electronic, spin and nuclear dynamics.48,94 The perturbational description adds vibrational spin-orbit contributions and spin-vibronic contributions to the direct SOC of two states, e.g., a singlet state S1 and a triplet state T1 (termed together spinvibronic coupling). The vibrational SOC includes the change of the SOC matrix elements of two states with molecular vibrational motions. Furthermore, spin-vibronic contributions can induce SOC between two states, e.g., S1 and T1. Here, states of the singlet manifold Sn (n > 1) with vibronic coupling to S1 and effective SOC to the T1 state can induce SOC between S1 and T1. Analogously, SOC between S1 and T1 is induced by states of the triplet manifold Tn with SOC to S1, occurring indirectly by vibronic coupling.48 Similarly, the phosphorescence rate kp as radiative spin-forbidden process relies on effective SOC and the energy difference of the initial and final electronic states (energy gap law).94,100 For a radiative T1 / S0 transition, intensity borrowing from spin-allowed Sn / S0 transitions induces purely electronic SOC by mixing the T1 state with close-lying singlet states Sn that have large electric transition dipole moments for the Sn / S0 transition. Conversely, SOC can mix S0 with low-lying triplet states.94,100 As discussed for ISC processes, spin-vibronic coupling can also be operative to increase kp.100 Clearly, all these effects beyond the classical heavy atom effect play important roles for ISC and spin-forbidden radiative processes of 3d TMCs and generally applicable rules for these effects are sought after. Time-dependent density functional theory

d-d and charge transfer photochemistry of 3d metal complexes

715

(TDDFT) calculations and the derived Kohn-Sham molecular orbitals (MOs) have proven useful to estimate the amount of SOCbased intensity borrowing in emissive copper(I) complexes, which show thermally activated delayed fluorescence (TADF) via backintersystem crossing (bISC).13,101,102 For example, a descriptive point of view reliably predicts relative rates kp of related TADF copper(I) complexes by simple inspection of the frontier orbitals. This is possible, since SOC matrix elements vanish for states originating from the same orbital configuration, high metal orbital contributions in the MOs induce high SOC and only MOs involving different metal d orbitals can couple.101 As exemplified above, the ultrafast excited-state kinetics of iron(II) complexes illustrates the breakdown of the BornOppenheimer approximation. Therefore, control over vibrational degrees of freedom and vibrational energies, would allow careful tuning of (electronic) excited-state kinetics. Furthermore, the information gained from vibronic coherences43 can be used to increase the 3MLCT lifetime of [Fe(bpy)3]2 þ-type complexes.103 The specific vibrational modes associated with the trajectory of the 3MLCT decay have been extracted from transient absorption (TA) spectroscopic data and assigned via DFT calculations. Rigidification of the ligand backbone attenuates these vibrational motions raising the 3MLCT lifetime by a factor of 20. Beyond emissive CT excited states, spin-flip states originating from intraconfigurational states can be luminescent, the prototypical examples being octahedral chromium(III) complexes and Al2O3:Cr3þ (ruby).55 The d3-electron configuration with a quartet GS (4A2g) gives rise to several intraconfigurational doublet excited states (2Eg, 2T1g, 2T2g). Analogously, the d2-electron configuration with a triplet GS (3T1g) leads to spin-flip excited states with singlet multiplicity (1Eg, 1T2g).104 To obtain the weakly distorted d3or d2-spin-flip states as lowest electronically excited MC states, the interconfigurational MC states must shift to higher energy. Again, this can be accomplished by increasing the ligand field splitting using strong-field ligands and optimized metal-ligand orbital overlap. The complex [Cr(ddpd)2]3þ 43D (ddpd ¼ N,N0 -dimethyl-N,N0 -dipyridine-2-yl-pyridine-2,6-diamine, Scheme 1A) achieves high 2Eg/2T1 / 4A2g luminescence quantum yields and lifetimes (F ¼ 14.2%, s ¼ 1164 ms in deaerated D2O).105 As these spinflip transitions are not only forbidden by the change of multiplicity (kp; see above) but also by the parity conservation of the initial and final wavefunctions (Laporte’s rule for d-d transitions with gerade symmetry106,107), these spin-flip states are very long-lived. The effect of Laporte’s rule106 has been exemplified in chromium(III) complexes coordinated by six pyridine donor ligands with and without a center of symmetry. Lifting the inversion symmetry lowers the excited state lifetime from 4500 to 1164 ms in deaerated D2O/DClO4 by increasing the radiative rate (kp).105,108 As the spin-flip states are typically very low in energy, the emission occurs in the near-infrared (NIR) spectral region. This poses additional challenges with respect to non-radiative decay. High-energy oscillators with frequencies above 2800 cm1 (C–H, N–H, O–H) open an efficient pathway for non-radiative deactivation by the inductiveresonant mechanism of non-radiative transitions.55,109 Overtones of the respective high-energy X–H oscillators serve as energy accepting states. Solvent and ligand deuteration lowers the energy of the respective modes and overtones. Consequently, higher overtones are required to match the emission energy which reduces the energy transfer (EnT) probability and diminishes this non-radiative decay. For example, ligand deuteration of 43D increases the excited state lifetime from 1164 to 1700 ms and the photoluminescence quantum yield F from 14.2% to 22.4% in deaerated D2O.110 Understanding the electronic structure of 3d TMCs together with the control of their associated low and high energy vibrational motions, which are responsible for non-radiative relaxation, delivers a comprehensive picture of their excited state potential energy landscapes and their excited state dynamics, and ultimately enables the design of useful long-lived excited states of 3d TMCs. Luminescent spin-flip and charge transfer states of 3d TMCs are further discussed in Section 8.18.2. A diffusion-controlled bimolecular reaction of an excited state without any preorganization or precomplexation with a substrate (dynamic quenching)22,60 requires an excited-state lifetime in the nanosecond range at least.22 Provided that the excited state of the TMC is sufficiently long-lived, bimolecular photo-induced energy (EnT; kEnT; Dexter or Förster type) or electron transfer (ET; kET; reductive or oxidative)60,62 reactions can occur with suitable substrates. Beyond the excited state lifetime (s), decisive parameters are the energies and redox potentials of the excited state for EnT and ET, respectively. These excited state properties offer useful applications of photoactive 3d TMCs with long-lived spin-flip and CT excited states in energy conversion,51,52 sensing,53 or photochemical/photocatalytic reactions58,59 as summarized in Section 8.18.3. Unimolecular photochemical reactions (kdiss) of TMCs have been known for decades.2,4,5 However, the detailed excited state dynamics including large structural reorganizations often remained unclear, due to similar (ultrafast) timescales of ISC, IC, VR and dissociation. Photodecarbonylation reactions of carbonyl TMCs are prominent examples of unimolecular photochemical reactions with several excited states involved.63 These reactions are extremely useful for delivering highly unsaturated TM species

Scheme 1

Molecular structures of (A) [Cr(ddpd)2]3þ 43D and (B) [FeIII(cyclam-ac)(N3)]þ 8D.

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d-d and charge transfer photochemistry of 3d metal complexes

opening the way to a huge follow-up chemistry and for advanced applications in medicinal chemistry, e.g., delivering CO triggered by light (Section 8.18.4.1). In the prototypical octahedral Cr(CO)6 complex 5, irrespective of the initially populated excited state (CT, MC), population transfer to dissociative MC states occurs within a few femtoseconds.111 Typically, one photon dissociates only a single CO molecule in solution. However, this general view has been challenged as a second CO can thermally dissociate from hot states on the picosecond timescale under high energy excitation.111 The mechanism of CO dissociation from 5 does not involve triplet states, so ISC appears to be slower than VR, IC and dissociation in this particular complex.112 However, ISC can occur after the ultrafast CO loss, e.g., after the CO dissociation from Fe(CO)5 6.63,113 The formation of carbonyl complexes with triplet ground states, e.g., Fe(CO)4, is confined to 3d TMs due to their lower ligand field splitting. Bimetallic carbonyl complexes, such as Mn2(CO)10 7, undergo CO dissociation and competing M–M homolysis under UV light excitation.63,114 Excitation energy dependent population of different states can even lead to different photodissociation reactions underlining the complexity of the branching dynamics and the breakdown of Kasha’s rule73 in some TMCs. For example, the azido iron(III) complex [FeIII(cyclam-ac)(N3)]þ 8D (cyclam-ac ¼ 1,4,8,11-tetraazacyclotetradecane-1-acetato(1), Scheme 1B) dissociates an azide ion N3 upon excitation at 450 nm retaining the iron(III) oxidation state, while with high energy excitation (266 nm) the complex expels N2 giving a nitrido iron(V) complex in parallel with azidyl radical N3 homolysis and iron(II) formation. The latter reductive reaction predominates under excitation with medium energy (300 nm) light. Time-resolved infrared (TRIR) spectroscopy revealed specific Fe–N(azide) and N3 vibrations being responsible for the reaction pathway. The unimolecular dissociative photoreactions of 3d TMCs with diverse ligands (CO, NO, carboxylate, azide, N2, alkyl/aryl and other ligands), their photodynamics and light-triggered applications are described in Section 8.18.4.

8.18.2

Luminescent 3d TMCsdPreservation of the coordination sphere

This section covers recent advances in the field of CT and spin-flip luminescence of 3d TMCs.39 TMCs including several 3d TMCs with CT emission have been reviewed with respect to application in OLEDs.50 Chromium(III) complexes showing spin-flip luminescence have been reviewed due to their distinct emission properties.39,55,107,115,116 The importance of SOC in ISC processes relevant for the excited state dynamics and phosphorescence of TMCs has been summarized.48,95 MC emissive 3d TMCs are covered in Section 8.18.2.1, while CT emissive 3d TMCs are reported in Section 8.18.2.2.

8.18.2.1

3d TM spin-flip emitters

MC states in 3d TMCs are typically located at lower energy than CT states due to the intrinsically small ligand field splitting of 3d TMs. The shapes and crossing points of the GS and MC potential energy surfaces (PESs) of the TMC determine whether the MC states rapidly decay non-radiatively to the GS or enable luminescence. The lowest MC states of pseudo-octahedral TMCs with d1-, d4–7and d9-electron configurations involve d orbitals of different character (t2g/eg*; interconfigurational states), while TMCs with d2-, d3- and d8-configurations additionally possess low-energy intraconfigurational states (spin-flip states). In a strong field environment, the spin-flip states become the lowest energy excited states (Fig. 3). The minimum ligand field splitting Do/B required for spin-flip states as lowest energy MC states, namely the crossing point of intra- and interconfigurational states is marked with a circle in the respective Tanabe-Sugano diagrams (Fig. 3).117,118 Pseudo-octahedral TMCs with d2- and d3-electron configurations (e.g., vanadium(III), vanadium(II), chromium(III); Fig. 3A and B) can undergo spin-flip transitions 3T1 / 1T2/1E and 4A2 / 2E/2T1, respectively, within the partially filled t2g subshell, while d8-TMCs (e.g., nickel(II), Fig. 3C) can undergo a spin-flip (3A2 / 1E) within the partially filled eg*subshell. A spin-flip state typically possesses a similar equilibrium nuclear configuration as the GS (nested states), while interconfigurational states are often strongly distorted with elongated MeL bonds due to the population of antibonding orbitals. Fig. 4 illustrates this difference for a low-spin d6- and a d3-TMC with their corresponding PESs. Population of s-antibonding d orbitals (eg*) in the 5MC state of a d6-TMC horizontally displaces the 5MC PES relative to the 1GS PES along a nuclear coordinate involving MeL bonds. The enhanced vibrational overlap between wave functions of the 1GS and 5MC state enables vibronic coupling and efficient population transfer to the 1GS PES (strong coupling limit, Fig. 4A).119 On the other hand, a TMC in an excited spin-flip state experiences only small distortions relative to the GS as these states share the same electron configuration. Hence, spin-flip states are nested with the GS (weak coupling limit, Fig. 4B). As non-radiative decay to the GS is slow, spin-flip states can possess long excited state lifetimes and can show phosphorescence, even though the spin-flip luminescence is spin- and Laporte-forbidden.39,106 A few TMCs display phosphorescent interconfigurational states, in particular some high-spin d5-manganese(II) complexes and a low-spin d6-cobalt(III) complex (Scheme 2). The tetranuclear manganese(II) complex [Mn4(ThiaSO2)2F] 9L with sevencoordinate MnII ions incorporating the calixarene ligand ThiaSO2 (p-tert-butylsulphonylcalix[4]arene; Scheme 2) shows a broad emission band peaking at 666 nm after excitation at 350 nm. The photoluminescence quantum yield and lifetime amount to F ¼ 0.4% and s ¼ 35 ms in DMF in the presence of O2 at 298 K. Quantum yield and lifetime increase to F ¼ 15% and s ¼ 1080 ms under inert conditions. The long lifetime and the oxygen sensitivity confirm the emission as phosphorescence.120 As the coordination geometry is distorted from octahedral towards a capped trigonal prism due to the calixarene ligand and the bridging fluorido ligand, the lowest MC state with t24e1 electron configuration (4T1 in O) is significantly split. The emission occurs from the lowest 4T1 component to recover the 6A1 ground state (t2g3eg2).120 The broad emission band suggests a significantly distorted excited state in agreement with an interconfigurational state. Analogous radiative 4T1(G) / 6A1 transitions occur in the

d-d and charge transfer photochemistry of 3d metal complexes

717

Fig. 3 Tanabe-Sugano diagrams for octahedral ML6 complexes with d2-, C/B ¼ 4.42 (A), d3-, C/B ¼ 4.50 (B) and d8-electron configurations, C/B ¼ 4.71 (C). Spin-flip states and other relevant MC excited states are plotted in red and blue, respectively.117,118

718

d-d and charge transfer photochemistry of 3d metal complexes

Fig. 4 Ground state PES (black curves) and lowest excited MC state PES (red curves) for a low-spin d6-TMC (A) and a d2-TMC (B) in an octahedral field representing a strong- and a weak-coupling limiting case, respectively. Respective microstates are illustrated in black and red boxes.

Scheme 2 Molecular structures of high-spin d5-manganese(II) complexes 9L–13 and low-spin d6-cobalt(III) complex 14D showing luminescence from interconfigurational MC states.

d-d and charge transfer photochemistry of 3d metal complexes

719

pseudo-tetrahedral MnII complexes [Mn(DPEPO)X2] 10–12, (DPEPO ¼ bis[2-(diphenylphosphanoxide)-phenyl]ether; X ¼ Cl, Br, I; Scheme 2) in the solid state. The bathochromic shift of the emission band of the iodido complex 12 may be ascribed to the lower ligand field strength of iodide. The quantum yields and lifetimes of 10–12 amount to 32%, 70%, 64% and 2200, 500, and 100 ms, respectively. The broad bands and long lifetimes confirm interconfigurational character and change in multiplicity.121 The solid state luminescence of the heteroleptic pseudo-octahedral manganese(II) complex 13 (Scheme 2) around 495 nm is assigned to a combination of MC and XLCT/MLCT emission with different lifetimes.122 The low-spin d6-cobalt(III) complex [CoIII{PhB(MeIm)3}2]þ 14D ([PhB(MeIm)3] ¼ phenyl tris(3-methylimidazol-2-yl)borato; Scheme 2) with a rigid strong-field hexacarbene donor set displays interconfigurational MC emission at 690 nm in CH3CN solution at room temperature. The broad emission band, excitation spectra, O2 sensitivity and long lifetime of s z 1 ms suggest an assignment as 3T1 / 1A1 emission. The luminescence quantum yield is reported as F ¼ 0.01% for excitation at 266 nm.123 A key prerequisite for spin-flip luminescence is the appropriate order of inter- and intraconfigurational excited states as illustrated in the Tanabe-Sugano diagrams for d2-, d3- and d8-ions in an octahedral field (Fig. 3). As the energies of spin-flip states are essentially independent of the ligand field splitting, raising the interconfigurational states (blue) above the intraconfigurational states (red) with increasing ligand field splitting can enable spin-flip luminescence (Fig. 3). The largest group of emissive TMCs displaying spin-flip luminescence encompasses pseudo-octahedral chromium(III) complexes. Phosphorescence energies, lifetimes and quantum yields of selected chromium(III) complexes are summarized in Table 1 (Scheme 3). In strong ligand fields with Do > 20 B employing s-donating and/or p-accepting ligands, preferably with trans LeMeL bond angles close to 180 for optimum metal-ligand orbital overlap, the interconfigurational states 4T2 and 4T1 are located above the intraconfigurational states 2E and 2T1 (Fig. 3B).55,105,108,131,133,135 To prevent thermally activated back-ISC from the doublets to the (dissociative) quartet states, the energy gap between the 2E/2T1 and 4T2 states should be sufficiently large. For high photoluminescence quantum yields and excited state lifetimes, the rates of non-radiative decay from the 2E/2T1 to the 4A2 ground state should be reduced. This can be accomplished using rigidified ligands to diminish excited state distortions and consequently obtain nested states and avoiding high-energy oscillators such as O–H, N–H and C–H close to the metal center to diminish multiphonon relaxation.55 Weak and strong monodentate ligands as in [Cr(NCS)6]3, [CrF6]3, [Cr(urea)6]3 þ, [Cr(NH3)6]3þ and [Cr(CN)6]3 153L–193L lead to fluorescence and phosphorescence, respectively.115,124,127,145,146 In thiocyanato and fluorido complexes 153L124 and 163L127 fluorescence and non-radiative relaxation outcompete the slower phosphorescence.147 The 2E state of [Cr(urea3)6] [ClO4]3 17[ClO4]3, however, is slightly lower in energy than the 4T2 state145,146 and TADF is enabled. Application of external hydrostatic pressure shortens the MeL bonds, increases the ligand field splitting and shifts the 4T2 state to higher energy. Consequently, the broad, weak fluorescence from a crystal of 17[ClO4]3 at 120 K turns into sharp phosphorescence bands at lower energy upon increasing the pressure from 12 to 23 kbar.145 With increasing ligand field splitting, phosphorescence prevails as found in the hexaammine and hexacyanido complexes 183D129 and 193L.124 In the perfectly octahedral complex 193L spin-flip luminescence is strongly Laporte-forbidden,106 explaining its low quantum yield (Table 1). Excited state distortions and ligand flexibility in the excited state play a role as well. The related tris-bidentate polypyridine complexes [Cr(bpy)3]3þ 203D and [Cr(phen)3]3þ 213D demonstrate that rigidifying the chelate ligand backbone increases phosphorescence quantum yields and lifetimes (phen ¼ 1,10phenanthroline; Scheme 3, Table 1).130 Substituent effects and heteroleptic tris-bidentate polypyridine complexes 213D–273D have been studied in addition (Scheme 3, Table 1).131 Heteroleptic chromium(III) complexes of the general formula [Cr(N^N)(phen)2]3þ 223D–273D feature ligand field splittings of approximately 22,000 cm1 and 4T2/2E energy gaps of DE ¼ 5510–8600 cm1, with the exception of [Cr(phen)2(phen-NH2)]3þ 253D. Efficient bISC from doublet to quartet states due to a small 4T2/2E energy gap of DE ¼ 2950 cm1 and dissipation of excitation energy to N–H oscillators of the NH2 substituent significantly reduce the doublet lifetime of 253D. The non-radiative deactivation pathway via electronic-to-vibronic energy transfer from the doublet states to X–H overtones148–150 was initially proposed as “inductive-resonant mechanism of non-radiative transitions” (Fig. 5A, X–H ¼ N–H).109,151 The efficiency of this EnT process depends on the spectral overlap integral (SOI) of absorption bands of high-energy X–H overtones with a matching luminescence band.149 Beyond the NH2 substituent attached to the phen ligand, electronic effects of other substituents are minor, with luminescence lifetimes of 213D–243D between 177 and 356 ms (Table 1).131 The more flexible ligands bpy and dpma (N,N-di(2-pyridyl)methylamine) in 263D and 273D lead to shorter luminescence lifetimes compared to 213D (Table 1). Lowering the symmetry of the parent phen complex 213D in the heteroleptic complexes 263D and 273D leads to broadening of the spin-flip luminescence.131 In the series of tris-bidentate chromium(III) complexes, no reported modification of the homoleptic phen complex 213D increases the room temperature lifetimes (Table 1). The 2E lifetime of the bis(terpyridine) chromium(III) complex 283D of s ¼ 0.14 ms increases to s ¼ 0.6 ms via incorporation of additional aryl substituents in 293D–333D (Scheme 3, Table 1).134 The substituents also serve as light harvesting antennae.134 However, quantum yields and lifetimes of these bis-tridentate polypyridine complexes are poor. One reason might be the lower ligand field splitting in bis(terpyridine) complexes related to 283D due to the small trans N–Cr–N angle of z158 .152 Six-membered chelate rings offer smaller cavities and shorter M–L distances compared to five-membered ones and therefore may be unsuitable for large metal ions.153 Yet for chromium(III), the expanded tpy-type (tpy ¼ 2,20 :60 ,200 -terpyridine) ligand ddpd allows for similar CreN bond lengths, but imposes terminal N–Cr–N angles of z177 in the expanded complex [Cr(ddpd)2]3þ 43D (Scheme 1A, Table 1).105,152 Thanks to the better Cr–N orbital overlap, the ligand field splitting increases from 18,750 cm1 (283D) to 22,990 cm1 (43D).105,133 As bISC to quartet states is effectively prohibited in 43D due to the large 4 T2/2E energy gap of DE ¼ 10,090 cm1, the excited state lifetime increases to s ¼ 899 ms and the photoluminescence quantum

720

d-d and charge transfer photochemistry of 3d metal complexes

Table 1

Luminescence properties of selected mononuclear chromium(III) complexes.

Chromium(III) complex

lem/nm

sRT (sRT,deox)/ms

sLT (sLT,deox)/ms

F (Fdeox)/%

153L124–126 163L127 173D128 183D126,129 193L124,129 203D129,130 213D130–132 223D131 233D131 243D131 253D131 263D130,131 273D131 283D133,134 293D134 303D134 313D134 323D134 333D134 343D133 353D133 43D110 363D135 373D136 383D129,137 393D138 403D139 413D140 433D141 443D108 45aD142 45bD143 46144

776 738 704 667 z820 727 689/726 689/726 690/726 689/726 726 688/726 700/742 770 785 785 788 796 776 711/771 711/736/774 738/775 738/782 727/747 670 675l 685 690 700/740 748 720 735 785

0.01

4349a

2  105

2.2 0.14 52 (74) 74 (356) (214g) (259g) (177g) (17g) 70 (208g) (23g) < 0.14h ( 4 ps. These components are assigned to two transfer mechanisms between 3MLCT and 3MC states, i.e., non-adiabatic and via SOC. With the majority of the wave packet in the highest 3MLCT state, contributions from both nonadiabatic and SOC mechanisms give rise to the (initial) fast component of the decay dynamics. After population redistribution among the triplet states, the non-adiabatic coupling contribution diminishes and SOC finally dominates.207 The majority of 3 MLCT state wave packets resides around the lowest energy geometry, and is therefore distant to the 3MLCT/3MC energy crossing point.207 802D–822D (Scheme 7) are related to 742D with varying numbers of carbene donors. In all cases, the 1MLCT / 3MLCT ISC is fast (< 100 fs), yet the 3MLCT lifetimes increase with the number of carbene donors from < 100 fs (802D and 812D), 3.6 ps (822D) to 8.1 ps (752D).208 Substituting the complex at the pyridine moiety with a carboxylic acid results in complex 772D. This backbone-functionalized complex shows two distinct MLCT absorptions at 394 nm (7000 M1 cm1) and 520 nm (16,200 M1 cm1), corresponding to CT transitions from the iron center to the carbene and pyridine moieties, respectively.209–213 TA spectroscopy delivers 1MLCT and 3MLCT lifetimes of 20 fs and 16.5  0.7 ps, respectively. The 3MLCT lifetime is higher than that of the unsubstituted complex 742D thanks to the MLCT states being lower in energy by the electron-accepting carboxylic acid substituents. These carboxylic acid functional groups at the pyridines can also anchor these complexes to semiconductors for the use as photosensitizers in DSSCs (Section 8.18.3.1).209–213 Bichromophoric iron(II) complexes, [Fe(C^N^C-ant)2]2þ 782D and [Fe(C^N^C-pyr)2]2þ 792D (Scheme 7; ant ¼ 9-anthracenyl, pyr ¼ 1-pyrenyl) exhibit room temperature fluorescence with lifetimes in the nanosecond range, which originates from singlet states dominated by the anthracenyl and pyrenyl chromophores. The 3MLCT states of 782D and 792D possess lifetimes of 13.4 ps and 12.8 ps, respectively.214 Ligands that form 6-membered chelates (832D–852D) constitute an alternative to the chelate ligands forming 5-membered chelate rings employed in complexes 672L–822D (Scheme 7). The larger rings provide a better overlap between metal and ligand donor orbitals that could lead to larger ligand-field splitting and higher-energy 3/5MC states. Furthermore, the electron-poor pyridines of dcpp (dcpp ¼ 2,6-bis(2-carboxypyridyl)pyridine; Scheme 7) with low-energy p* orbitals202 lower the energies of the 1/ 3 MLCT states. The two dcpp ligands in 832D form nearly 90 and 180 N–Fe–N angles close to an ideal octahedron.202 Absorption spectroscopy confirmed the low energy of the 1MLCT state (610 nm).202 The lifetime of the excited state was determined by TA spectroscopy as s ¼ 280  1 ps. This lifetime is much shorter than the long 5MC lifetimes of 12D or 692D requiring double spin change DS ¼ 2 for the relaxation to the GS. Consequently, this lifetime was originally assigned to the 3MC state requiring only a spin change of DS ¼ 1 for the relaxation to the GS and the 5MC state would not be reached. Yet, time-resolved X-ray spectroscopy (XAS, XES) confirmed this state as 5MC state. EXAFS data provided clear evidence that this state is highly distorted with FeeN bonds to the terminal and central pyridines elongated by 0.23 Å and 0.09 Å, respectively. The comparably fast decay to the GS results from barrierless relaxation to the GS in contrast to the small barrier of 300 cm1 found for 12D.215 Replacing an electron-poor dcpp ligand by an electron-donating ddpd ligand yields the heteroleptic push-pull substituted complex [Fe(dcpp)(ddpd)]2þ 842D (Scheme 7).216 Similar to 832D, the coordination geometry of 842D is nearly octahedral. 842D shows broad absorption bands peaking at 592 nm and tailing into the NIR spectral region, suggesting CT states at even lower energy than in 832D. TDDFT calculations assigned mixed Fe/ddpd / dcpp MLCT/LL0 CT character to these bands. TA spectroscopy of 842D in acetonitrile at room temperature gave a monoexponential ground state recovery with s ¼ 548 ps, similar to 832D. In light of the revised interpretation of the decay dynamics of 832D, this lifetime should be assigned to the 5MC state as well. Obviously, the barrier for 842D to the GS is slightly higher than that of 832D. Lacking long-lived 3MLCT states, 832D and 842D are not emissive.202,216 The NH analog 852D can be reversibly deprotonated at the electron-rich tridentate ligand to give strongly colored complexes with panchromatic absorption reaching up to 1000 nm. TDDFT calculations assign the long-wavelength absorptions to LL0 CT transitions from the anionic amide of the deprotonated ligand plus some small metal contribution to a single CO moiety of the dcpp ligand.217 Electron-rich amides directly coordinated to iron(II) are present in the complexes 86 and 87 coordinating N-(quinoline-8-yl) phenanthridine-4-amido ligands, with an electron-donating amido and electron-accepting quinoline and phenanthridine moieties (Scheme 7).218 Both complexes absorb strongly across the UV/Vis region with CT absorption maxima at 450, 600 and 730 nm. The lowest energy bands were assigned as mixed LL0 CT/MLCT from the occupied p*(pN,amido þ dFe) orbitals to p* orbitals of the electron-poor N-heterocycle.218 After excitation with 780 nm pulses in toluene, both amido complexes decay monoexponetially with s > 2 ns (86: s ¼ 2041  23 ps/2625  35 ps; 87: s ¼ 2454  13 ps/2669  41 ps from kinetic traces at 486 and 600 nm,

d-d and charge transfer photochemistry of 3d metal complexes

731

respectively). Based on the interpretation of a persistent excited state absorption (ESA) around 485 nm, the long-lived excited states have been assigned as MLCT states. In spite of this long lifetime, the complexes appear non-luminescent.218 To enhance the absorptivity and lower the energy of the MLCT states, a carbene pyridine iron(II) complex of type 742D was combined with a meso-zinc(II) porphyrin linked by an alkynyl bridge to give the bimetallic complex 882D (Scheme 7). The bichromophore 882D showed strong absorptions in acetonitrile at approximately 440 nm (mixed porphyrin 1p-p* Soret band and iron MLCT transitions), 516 nm (iron MLCT transitions) and 550–650 nm (1p-p* Q-band). The red-shifted MLCT absorptions of 882D compared to the parent complex 742D underscores the stabilization of the MLCT states by extending the p-system. Ultrafast TA spectroscopy showed that complex 882D evolves to an excited state with a lifetime of s ¼ 160 ps within 300 fs after excitation with 650 nm pulses. The fast component is attributed to ISC from the porphyrin’s S1 state to a triplet state assigned as the 3 MLCT state of the [Fe(C^N^C)2]2þ moiety. The 3,5MC cascade plays a negligible role in the dynamics. Weak luminescence is observed at lmax ¼ 880 nm after excitation at 532 nm of 882D in an 1:1 CH3CN:H2O mixture at room temperature. The luminescence lifetime of 175 ps matches that obtained by TA spectroscopy. Encouraged by the long lifetime and useful excited state redox potentials, 882D has been successfully employed as sensitizer in a DSSC (Section 8.18.3.1).219 Sterically demanding tpy ligands with fluorine, chlorine or bromine substitution at the 6- and 600 -positions yield the complexes 2D 70 –722D, respectively (Scheme 7). In contrast to the parent low-spin complex 692D, the fluorine derivative 702D is a spin crossover complex with T½ of 220 K, while the chlorine and bromine substituted complexes 712D and 722D possess high-spin ground states at all temperatures due to substituent-induced intramolecular strain. The complexes in their high-spin states feature 5/7MLCT excited states. Increasing the amount of steric strain extends the lifetime of these 5/7MLCT states from 14.0 ps, over 16.0 ps to 17.4 ps for 702D, 712D and 722D, respectively. While these lifetimes are still too short to enable luminescence or bimolecular reactivity, this concept provides an alternative to the 3MLCT excited states of low-spin d6-TMCs.220,221 Isoelectronic to low-spin iron(II) is chromium(0). However, classical six-coordinate chromium(0) complexes with monodentate strong-field ligands such as CO in Cr(CO)6 5 or aryl isocyanides in Cr(CN-C6H3R2)6 (R ¼ H, 89 and R ¼ iPr, 90; Scheme 7)222 are prone to photodissociation (Section 8.18.4.1). In a 2-methylpentane glass at 77 K, 89 and 90 emit weakly at 590 nm and 583 nm, respectively. The lifetimes are below 10 ns and substantial singlet character has been attributed to this emissive state.222 No emission could be detected at room temperature. With bidentate chelating isocyanides (91), the photostability improved considerably.223 91 absorbs between 400 and 600 nm (MLCT) and luminesces at 630 nm in deaerated THF solution at room temperature after excitation at 500 nm, with an excited-state lifetime s ¼ 2.2 ns and a photoluminescence quantum yield of F z 0.001%. The emission is slightly solvatochromic with a blue-shift from THF to toluene and n-hexane, compatible with an MLCT emission. In frozen matrices (2-MeTHF, toluene, 2-methylpentane), the lifetimes increase to the microsecond range suggesting that the emissive state has significant triplet character.223 Sections 8.18.3.1 and 8.18.3.2 discuss first applications of 91. Square-planar d8-NiL4 complexes with p-accepting ligands exhibit low-energy MLCT states, similar to octahedral d6-ML6 complexes. NiII(C^N^N)X complexes 92 (X ¼ Br) and 93 (X ¼ CF3) show absorption bands in THF at 505 nm and 468 nm, respectively (Scheme 8A). These absorptions correspond to mixed MLCT/LL0 CT transitions.224 Efficient non-radiative decay from the MLCT/LL0 CT to MC states associated with the anti-bonding dx2y2 orbital in the square-planar d8-configuration prevents luminescence even at 110 K in frozen solution.224 The chlorido nickel(II) complex NiIICl(N^C^N) 94 is non-emissive as well (Scheme 8A).225 In contrast, the carbazolyl complex 95 (Scheme 8A) showed a broad absorption from 370 nm to 440 nm in CH2Cl2 peaking at 385 nm (3935 M1 cm1) and 425 nm (2750 M1 cm1), which were assigned to MLCT and metal-perturbed intraligand (IL) transitions. In the solid state at room temperature, 95 yielded a weak, structureless emission band at 600 nm. At 77 K a structured emission band peaking at 468 nm with vibrational progression of 1300–1400 cm1 appeared characteristic for vibrational modes of the N^C^N ligand. This band was assigned to a metal-perturbed 3IL [p-p*(N^C^N)] excited state with a lifetime of 0.11 ms (solid, 77 K), rather than a pure MLCT state.225 As tetrahedral nickel(0) complexes possess a closed-shell d10-configuration, MC states are absent. A major problem in nickel(0) carbonyl complexes is the tendency of photodissociation of the ligands (Sections 8.18.4.1). Upon excitation into the MLCT band at room temperature, the homo- and heteroleptic carbonyl complexes 96, 97 and 98 undergo CO photodissociation (Scheme 8B). Ni(CO)4 96 showed a MLCT absorption at 240 nm but no luminescence.226–228 The heteroleptic carbonyl phosphane nickel(0) complex Ni(CO)2(PPh3)2 97 absorbed at 356 nm in acetonitrile, assigned as MLCT transition. Excitation of 97 at 380 nm in the solid state yielded a red emission at 650 nm both at room temperature and 77 K with a quantum yield of 0.1%.226 Similarly, the mixed carbene carbonyl complex 98 exhibited an MLCT absorption at 365 nm in CH3CN and a 3MLCT emission at 510 nm in CH3CN solution at room temperature.229 Chelating isocyanide ligands can prevent photodissociation of nickel(0) carbonyls similar to chromium(0) carbonyls (see above), as in 99 and 100 (Scheme 8B). Both complexes feature isocyanides coordinating to the metal center in a pseudo-tetrahedral manner. Both compounds showed strong MLCT absorptions in THF at 420–425 nm, yet emission upon excitation at room temperature was not detectable while the complexes 99 and 100 emitted at 511 and 554 nm, respectively, at 77 K in frozen toluene solution. The emission decay of 99 and 100 occurred biexponentially with s1 ¼ 200 ns (53%)/s2 ¼ 1100 ns (47%) and s1 ¼ 230 ns (57%)/s2 ¼ 1200 ns (43%), respectively.230,231 The lack of 3MLCT luminescence at room temperature but the presence in frozen matrices is likely due to distorted MLCT excited states (flattening distortion) with subsequent coordination of solvent molecules similar to isoelectronic copper(I) complexes in their excited MLCT excited states (Scheme 9).230 In fact, pseudo-tetrahedral bis(diimine) copper(I) complexes [Cu(N^N)2]þ, e.g., [Cu(R2phen)2]þ complexes 101D–105D (Scheme 9) with 2,9-disubstituted 1,10-phenanthroline ligands (R ¼ H, Me, sBu, Ph, tBu) and heteroleptic derivatives, play

732

d-d and charge transfer photochemistry of 3d metal complexes

Scheme 8 (A) Square planar d8-nickel(II) complexes 92–95 with cyclometallating p-accepting ligands and (B) pseudo-tetrahedral d10-nickel(0) complexes 96–100 exhibiting MLCT transitions.

a dominant role in copper(I) photochemistry.46,232–238 Although deactivation via MC states is absent in the closed-shell d10-copper(I) complexes, other non-radiative decay pathways come into play, especially the excited state flattening distortion and subsequent coordination of nucleophiles to the formed copper(II) ion (Scheme 9). Excitation of 101D–105D at 400–500 nm populated a relatively high-lying 1MLCT0 state, which evolved via IC to the lowest energy 1MLCT state and further via ISC to a 3MLCT state. This 3 MLCT state with a lifetime in the nano- to microsecond range was phosphorescent at low temperatures.237 Formally, the 3MLCT state of 101D–105D and analogs can be described as a reduced (poly)pyridyl radical anion and a d9-copper(II) ion (Scheme 9). In contrast to the pseudo-tetrahedral (D2d) GS with a d10-copper(I) central ion, d9-copper(II) ions prefer a planar coordination geometry (D2) as the non-symmetric population of the degenerate d orbitals in the 1/3MLCT states induces a (second order) Jahn-Teller distortion.46 The major distortion in the MLCT states corresponds to a flattening of the initial D2d symmetric complex to a D2 symmetric geometry (Scheme 9). As highly distorted excited states (strong coupling limit) enable efficient

Scheme 9

Flattening distortion and coordination of nucleophiles in the MLCT excited states of [Cu(R2phen)2]þ complexes 101D–105D.

d-d and charge transfer photochemistry of 3d metal complexes

733

non-radiative decay, the flattening competes with radiative processes and consequently reduces luminescence quantum yields.46,237,238 The electronically excited unsubstituted complex 101D evolved directly from the 1MLCT to the 1GS state bypassing the 3MLCT state as lowest excited state, i.e., fast non-radiative relaxation from the 1MLCT state outcompeted ISC and prevented phosphorescence.46 Steric congestion in the bulky [Cu(tBu2phen)2]þ complex 105D (Scheme 9) damped the amplitude of the flattening motion and consequently, the 1/3MLCT states were less distorted and relatively nested (weak coupling limit) which reduced non-radiative relaxation and enabled ISC and 3MLCT luminescence.232,238 In addition to the increased non-radiative decay, the flattened geometry of the MLCT states rendered the copper ion accessible for coordination of nucleophiles, e.g., solvent molecules or counter ions, to the positively charged copper(II) center. The thus formed 5-coordinate excited complexes (exciplexes) were non-luminescent as illustrated by the CH3CN adduct of 101D.237 In fact, the BPh4 salt of 101D was only luminescent in the solid state as flattening and exciplex formation was prevented.239 In solution, donor strength and steric properties of the solvent influenced the exciplex luminescence quenching which increased in the series CH2Cl2 < THF < CHCl3  EtOH < CH3OH < CH3CN < DMF.238,240 Similarly, anion coordination to the (formally) CuII ion in the MLCT states diminished the luminescence in the series BPh4  PF6 < BF4 < ClO4 < NO3.238,240 A 1MLCT 3MLCT energy gap DE(1MLCT 3MLCT) below 1000 cm1, a long lifetime of the 3MLCT state and sufficient available thermal energy can enable thermally activated bISC populating the emissive 1/3MLCT states according to a Boltzmann distribution.46,237 As spin-allowed fluorescence from the 1MLCT state is faster than the spin-forbidden phosphorescence from the 3 MLCT state, TADF (lem ¼ 600–800 nm) prevails at higher temperature.101,241 In fact, two methyl groups in 2,9-positions of the phen ligands in 102D sufficed to enable TADF in CH2Cl2 solution.46 The group of neutral heteroleptic bis(2-(diphenylphosphanyl)phenyl) ether (POP) copper(I) complexes 106–108 (Scheme 10) absorbs between 310 and 370 nm (1MLCT transitions) in dichloromethane at room temperature. These TMCs exhibit very strong blue/white luminescence (436–498 nm) with emission quantum yields up to 90%, depending on the borate coligand, the environment (powder, poly(methyl methacrylate), CH2Cl2) and the temperature.242,243 Below 100 K, the emission decay times were in the order of many hundreds of microseconds in accordance with 3MLCT emission involving 3dCu and p*(POP) orbitals. At higher temperature, the emission decay time in the solid state shortened to 20 ms, 22 ms and 13 ms for 106, 107 and 108, respectively. This behavior is consistent with TADF with the fluorescent 1MLCT state being only 800–1300 cm1 above the phosphorescent 3 MLCT state.242,243 Beyond the vast family of four-coordinate copper(I) emitters, three- and two-coordinate heteroleptic copper(I) complexes have received increasing interest (Scheme 10) as they experience no Jahn-Teller distortion in the excited CT states. As examples for threecoordinate complexes, the dipyridylamine NHC copper(I) complexes 109D and 110D showed MLCT absorptions at 315 and 310 nm and emitted at 488 nm and 436 nm, respectively (Scheme 10).244 The quantum yields and lifetimes increased from 22%/13 ms (109D) to 86%/44 ms (110D) by backbone methylation. This enhancement likely reflects a less distorted excited triplet state of MLCT/IL character. Replacing the dipyridylamine by bpy or phen inhibits emission, probably due to enhanced nonradiative decay of the MLCT states.244 A trend towards IL and LL0 CT, rather than MLCT excited states in copper(I) complexes also emerges in two-coordinate copper(I) complexes (Scheme 10). Many highly emissive two-coordinate copper(I) complexes have been incorporated already into OLEDs. A family of two-coordinate copper(I) complexes 111–116 with monoamido and diamido-aminocarbene ligands along with unsubstituted and substituted carbazolyl ligands possessed emissive 3LL0 CT states deriving from the electron-rich carbazole and electrondeficient carbene ligands as well as close-lying localized triplet states of the carbazolyl ligand (3IL).245 The emission bands were solvatochromic due to the high and small dipole moments in the ground and excited states, respectively. Tuning the energy of the HOMOs and LUMOs by substituent effects varied the emission wavelengths from 448 to 666 nm, from 432 to 704 nm and from 438 to 658 nm in 2-MeTHF solution, in a polystyrene film and as neat solids, respectively. Quantum yields up to 100% were achieved for 112 in solution and in the film.245 The luminescence was based on the TADF mechanism, which allowed for fast radiative transitions. The efficiency of non-radiative decay in this series increased with lower emission energies245 consistent with the energy gap law.94,100 Likewise, cyclic (alkyl)(amino)carbenes (CAACs) in the complex series 117 (Scheme 10) enabled luminescent LL0 CT excited states emerging from the electron-donating carbazolyl and the electron-deficient CAAC ligand. Again, small energy gaps between 1LL0 CT and 3LL0 CT states enabled TADF.246 With chloride as coligand, the quantum yields were lower, e.g., 1.8% for 118 in THF at room temperature (Scheme 10). However, employing an enantiopure CAAC ligand derived from Lmenthol in 118 induced CPL at 555 nm with glum ¼ þ0.0011, while the enantiomer achieved glum ¼ 0.0012.247 Lowering the p-accepting orbital of the carbene in the cyclic (amino)(aryl)carbenes (CAArCs) shifted the emission maximum of the corresponding CAArC carbazolyl copper(I) complex 119 into the deep red region (Scheme 10). Again, TADF was responsible for efficient radiative relaxation.248 On the other hand, employing only CAAC ligands yielded the strongly blue-emissive complex 120D (F ¼ 65%, lem ¼ 398 nm, s ¼ 6.9 ms). These favorable luminescence properties were ascribed to an unusually strong SOC in 120D instead of TADF mechanisms operating in the heteroleptic carbene complexes 117–119.249 The photophysics of carbazolyl copper(I) complexes with classical NHC ligands (121, 122; Scheme 10) is dominated by carbazolyl localized excited states (1/3IL) instead of MLCT or LL0 CT excited states. Interestingly, complexes 121 and 122 showed dual emission in the solid state. The structured emission bands from 400 to 550 nm were insensitive towards O2 with nanosecond lifetimes (fluorescence from the 1IL state), while emission bands around 550–750 nm were highly air-sensitive with millisecond decay lifetimes (phosphorescence from the 3IL state). Deuteration of the carbazolyl ligand suppressed non-radiative decay pathways109 and prolonged the phosphorescence lifetime to 140 ms at room temperature (ultralong room temperature phosphorescence, RTP).250

734

d-d and charge transfer photochemistry of 3d metal complexes

Scheme 10

Selection of luminescent four-, three- and two-coordinate copper(I) complexes 106–122.

To conclude this section, the multifaceted photodynamics of CT (and IL) excited states of 3d TMCs spans from ultrafast ISC processes in the femtosecond regime to very long spin-forbidden phosphorescence in the millisecond time range.

8.18.3

Electron and energy transfer with 3d TMCsdBimolecular reactivity

Beyond radiative decay (Section 8.18.2), an excited state can follow several non-radiative deactivation pathways. The most dominant non-radiative bimolecular processes, energy transfer (EnT) and electron transfer (ET), can compete, depending on the excited state energies, excited state redox potentials and reaction partners. While both are non-radiative quenching processes that reduce the luminescence quantum yield of an emissive species, EnT and ET follow different paths after local excitation (Fig. 6).251 The IUPAC defines EnT as a “photophysical process in which an excited state of one molecular entity (the donor) is deactivated to a lower-lying state by transferring energy to a second molecular entity (the acceptor), which is thereby raised to a higher energy

d-d and charge transfer photochemistry of 3d metal complexes

735

Fig. 6 Excitation of a TMC MLn by light, followed by (A) EnT and VR results in an excited, thermally equilibrated substrate Sub* and the TMC in its GS or by (B) ET resulting in an oxidized TMC MLn þ and reduced substrate Sub for oxidative ET or vice versa for reductive ET.251

state.”252 An EnT pathway from a photoexcited chromophore to a substrate can offer access to otherwise elusive excited states of the substrate moiety. For a high efficiency, spectral or molecular orbital overlap needs to be optimized (Section 8.18.3.2). In contrast, ET describes the transfer of an electron from one molecular entity to another, or between two localized sites in the same molecular entity. ET takes advantage of the enhanced photoredox activity of a photoexcited chromophore. ET requires an appropriate match of the excited state redox potentials of the TMC and the redox potentials of the substrate. As the ET reorganization energy is larger than for EnT processes, an additional overpotential must be considered.253 Furthermore, photoinduced EnT is more distance sensitive, especially Dexter-type EnT (Section 8.18.3.2), due to the exchange of two electrons, compared to the photoinduced ET mechanism where only a single electron is transferred (Section 8.18.3.1).254 Stern-Volmer analyses provide valuable insights into the kinetics of bimolecular EnT or ET reactions.255,256 The ratio of the luminescence rate constants with (kobs) and without (k0) substrate or the ratio of the respective quantum yields F0 and Fobs are plotted versus the substrate concentration [Sub] according to Eq. (1).255,256 k0 þ kq ½Sub kq ½Sub kobs F0 ¼ ¼ ¼ 1þ k0 Fobs k0 k0

(1)

Eq. (1) allows derivation of the quenching constants kq from concentration dependent quenching experiments. Comparing SternVolmer plots constructed from steady-state and time-resolved emission data distinguishes between dynamic (bimolecular, diffusion controlled) and static (pre-association) processes.22 As Stern-Volmer analyses cannot discriminate between thermodynamically feasible ET or EnT mechanisms, it is required to analyze the products of a photoinduced reaction.22 ET leads to either oxidized photosensitizer and reduced substrate, or vice versa. On the other hand, EnT regenerates the photosensitizer in its GS, while the substrate is in its excited state with characteristic spectral properties and reactivity patterns. In addition to product analysis after photolysis, TA spectroscopy in the presence of a substrate can provide very useful information regarding the mechanism, by spectroscopically identifying radical cation/radical anion intermediates or further excited states.22 Chromium(III) polypyridine photosensitizers constitute instructive, yet complicated examples of 3d TMCs displaying both ET and EnT processes after excitation in addition to possible photodissociation (Section 8.18.4). Most chromium(III) complexes used as photosensitizers operate via EnT,257,258 yet parallel EnT and ET processes occur in several instances (Sections 8.18.3.1 and 8.18.3.2). The following sections summarize important electron and energy transfer pathways of electronically excited 3d TMCs with a particular focus on lifetime and stability requirements.

8.18.3.1

Photoinduced electron transfer from and to 3d TMCs

Photosensitizers for photoinduced ET processes should be highly absorptive in the desired spectral region or possess even panchromatic absorptions. Their excited state lifetimes should be high enough for efficient bimolecular ET or for charge injection into semiconductors. As the excited state character should promote electron transfer, directional CT excited states are most suitable, although MC states can be redox-active as well. For catalytic applications of a photoredox sensitizer, redox stability and photostability play major roles.22,58 Excited state redox potentials control the thermodynamics of photoinduced ET reactions. They drastically differ from the respective GS redox potentials.71 The [Fe(bpy)3]3þ/2þ 13D/2D redox couple serves to illustrate this notion. The 3MLCT excited state of 12D is formally described as an oxidation of the iron(II) center to iron(III) along with a reduction of one bpy ligand to its radical anion. Compared to the GS, the iron center is easier to reduce in the MLCT excited state, while the ligand is easier to oxidize (Fig. 7).59 The excited state redox potentials E*ox(MLnþ/MLn*) and E*red(MLn*/MLn) resulting from the respective GS potentials Eox(MLnþ/MLn) and Ered(MLn/MLn) and the one-electron potential E00 corresponding to the energy difference between ground and excited state at their zero vibrational levels (0–0 transition) are given by the Rehm-Weller equations (Eq. 2) and are schematically depicted in Fig. 7.260 E ox ¼ Eox  E00 ; E red ¼ Ered þ E00

(2)

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d-d and charge transfer photochemistry of 3d metal complexes

Fig. 7 Conceivable oxidative and reductive quenching pathways of [Fe(bpy)3]2þ 12D. E00 from the low-energy onset of the absorption band, the GS potentials from ref. 259 vs. ferrocene. One-electron configurations are shown for the relevant species with d orbital characters for Oh point group notation.

The one-electron potential E00 is accessible by fitting the emission spectral profile in a Franck-Condon analysis as described by Claude and Meyer.22,261 Other approximations for E00 include the high-energy onset of the emission band at low temperature, the intersection between normalized absorption and emission bands, the low-energy onset of an appropriate absorption band and the HOMO-LUMO gap calculated from the difference between oxidation and reduction potentials for CT excited states. A higher excited state energy increases the power of the photocatalyst rendering it a stronger oxidant and a stronger reductant, but it also raises the energy required for photoexcitation.58 Marcus theory of outer sphere electron transfer describes the rate constant kET for bimolecular ET between electron donor D and electron acceptor A (Eqs. 3 and 4).262,263  !  0 DG þ l 2 2p 1 2 (3) kET ¼ jHDA ðrDA Þj pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi exp  4lk b T 4plk b T jHDA ðrDA Þj2 ¼ jHDA ð0Þj2 exp ð  brDA Þ

(4)

HDA(rDA) denotes the electronic coupling between D and A, l the reorganization energy resulting from structural changes of D and A and reorganization of solvent molecules due to redistribution of charge, kb ¼ Boltzmann constant and DG0 the thermodynamic driving force of the ET.262 The electronic coupling HDA(rDA) exponentially decreases from its maximum HDA(0) with distance rDA with a damping factor b specific for the intervening medium (Eq. 4).264 While thermodynamic and kinetic properties of the ET step itself (DG0 and kET) need to be taken into account, the lifetime of the excited state s and the lifetime of the encounter complex with the substrate are decisive for the overall reaction as well. In fact, the 3 MLCT excited state lifetime of 12D is too short to allow productive encounters with a substrate,79 so an important aspect of photoredox sensitizer development with 3d TMCs is lifetime engineering. Encounter complex formation and dissociation has been studied using positively and negatively charged copper(I) sensitizers with cationic and neutral methyl viologen electron acceptors. Indeed, attractive electrostatic interactions between the anionic sensitizer and the cationic electron acceptor raise the quantum yield for photoinduced ET.265 Furthermore, the actual type of bimolecular excited state quenching operating in a catalytic cycle can be ambiguous. Polypyridine chromium(III) photosensitizers (Scheme 3) can undergo photoinduced ET and EnT quenching (Section 8.18.2). Mechanistic studies on the regioselectivity of Diels-Alder reactions between electron-rich styrenes and isoprene or 2,3-dimethyl-1,3-butadiene using the tris(phenanthroline) chromium(III) derivative [Cr(bath)3]3þ 1233D (bath ¼ 4,7-diphenyl-1,10-phenanthroline) as photocatalyst distinguish between ET and EnT (Scheme 11).266,267 1233D prefers an EnT pathway via triplet substrates that orients the

d-d and charge transfer photochemistry of 3d metal complexes

Scheme 11

737

Molecular structures of photosensitizers 1233D and 1243D preferably engaging in EnT and ET processes, respectively.

electron-poor alkene for regioselective cycloaddition, while an unselective outer-sphere ET pathway via substrate radical cations is favored for chromium(III) photosensitizers with electron-poor bpy ligands as in 1243D (Scheme 11). Other related CrIII and classical RuII photosensitizers likely do not participate significantly in the EnT pathway due to the accessibility of lower energy ET pathways or inappropriate sensitizer-substrate preorganization for selective EnT.268 Finally, the actual photoactive state is sometimes ambiguous and hard to discern from product analysis alone. Nevertheless, ET processes involving 3d TMCs discussed below are categorized with respect to the proposed photoactive MC and CT (MLCT, LMCT, LC/ILCT/LL0 CT, CTTS; CTTS ¼ charge transfer to solvent) excited states. Due to a lack of charge separation in MC excited states and the typically low lifetimes, only a few photoredox-active TMCs engage in ET processes from such states. Instead most of the MC states decay non-radiatively or undergo EnT instead of ET with suitable substrates (Section 8.18.3.2). Chromium(III) complexes with strong-field ligands can possess long-lived 2E excited states with excited state energies of 1.6– 1.7 eV (Section 8.18.2.1, Table 1).105,269,270 The CrIII/II GS redox potentials range from Ered ¼ 0.68 to 1.13 V vs. ferrocene.58,271 Consequently, these complexes can operate as photooxidants. However, the resulting reduced complexes can be quite sensitive when chromium(II) is formed.272 Upon ligand-centered reduction of chromium(III) complexes with suitable electron-poor pyridine ligands, the reduced intermediates are sufficiently stable for several catalytic cycles highlighting the importance of redox stability.268,273 While [Cr(ddpd)2]3þ 43D (Scheme 1A) undergoes a metal-centered reduction to the labile CrII complex 42D (Ered ¼ 1.11, E*red ¼ þ0.49 V vs. ferrocene),105,272 [Cr(tpe)2]3þ 443 D (Scheme 3) shows a mainly ligand-centered reduction at Ered ¼ 0.88 V vs. ferrocene. Its excited state redox potential (E*red ¼ þ0.87 V vs. ferrocene) is sufficient to oxidize azulene to its radical cation.108 Similarly, complexes of the type [Cr(bpy)3]3þ 203D and [Cr(tpy)2]3þ 283D (Scheme 3) are reduced to their stable ligand-centered radicals.273,274 This enables photoredox catalysis using [Cr(bpy)3]3þ and [Cr(phen)3]3þ type complexes, e.g., 1233D, as photooxidants.268 The ttpy ligands in [Cr(ttpy)2]3þ 313D (ttpy ¼ 4-p-tolyl-2,20 :60 ,200 -terpyridine; Scheme 3) can even be photoreduced twice using triethanolamine as reductant giving the doubly reduced complex [Cr(ttpy)2]þ 31D.275 Consequently, the 313D/312D/31D redox couples represent a multi-electron reservoir for photoinduced charge accumulation. Manganese(IV) complexes with long excited state lifetimes applicable in photoredox chemistry are rare. The six-coordinate oxido manganese(IV) complex [Mn(Bn-TPEN)(O)]2þ 1252D (Bn-TPEN ¼ N-benzyl-N,N0 ,N0 -tris(2-pyridylmethyl)-1,2-diamino-ethane; Scheme 12) is photoinactive as well, as its excited state lifetime, determined by fs-pump-probe spectroscopy, is very short. However, coordination of two equivalents of the Lewis acid Sc(CF3SO3)3 to the oxido ligand increases the excited doublet state lifetime to 6.4 ms and positively shifts the reduction potential by 0.58 V.276–278 These excited state properties enable photooxidation of benzene to its radical cation, yet a catalytic cycle is not achieved due to the instability of the resulting manganese(III) and manganese(II) species.278 The five-coordinate 3d5-cobalt(IV) complex [CoO(TAML)]2 1262L (H4TAML ¼ 3,4,8,9-tetrahydro-3,3,6,6,9,9-hexamethyl-1H1,4,8,11-benzotetraazo-cyclotridecane-2,5,7,10-(6H,11H)tetrone, Scheme 12) is an oxido TMC with a photoredox-active MC state. After excitation at 393 nm, the initially formed LMCT state with a lifetime of 1.4 ps evolves to an MC state with 0.6 ns lifetime.

738

d-d and charge transfer photochemistry of 3d metal complexes

Scheme 12

Molecular structures of 3d TMCs with photoredox-active MC states 1252D–1272D and iron(II) reference complexes 1282D–1302D.

Nanosecond laser TA spectroscopy in the presence of potential substrates confirmed that this MC state oxidizes m-xylene and anisole, but not benzene and toluene.279,280 For low-spin 3d6-TMCs the low-energy MC states (5T2) are populated from initially excited MLCT states on an ultrafast time scale, but some notable exceptions have been realized (see below).79 Therefore, ET from the short-lived MLCT excited states is unlikely in most cases. Furthermore, the lack of charge separation hampers ET from MC states. However, bimolecular quenching studies between [Fe(tren(py)3)]2þ 1272D (tren(py)3 ¼ tris(2-pyridyl-methylimino-ethyl)amine; Scheme 12) and a series of benzoquinoid electron acceptors demonstrated that ET can also occur from the lowest MC state of 1272D following excitation into the MLCT state (580 nm) and relaxation to the 5T2 state. An excited state redox potential of the 5T2 state of E*ox z 0.35 V vs. ferrocene in acetone has been estimated using a series of quinone substrates with different redox potentials.281 With the FeIII/II GS redox potential Eox ¼ 0.51 V, the 5T2 state energy can be estimated as 0.86 eV (Eq. 2).281 The related capped iron(II) complex 1282D possesses a high-spin ground state, while the uncapped analog 1292D is a low-spin species and the diamsar (diamsar ¼ 1,8diamino-3,6,10,13,16,19-hexaazabicyclo[6.6.6]icosane) cage complex 1302D undergoes spin crossover (Scheme 12).282,283 Consequently, subtle changes in geometry and ligand donor properties affect the nature of the GS and the energy difference between low- and high-spin states. Beyond these few examples of photoactive MC states of 1252D–1272D, CT states are much more suitable for ET due to the favorable charge accessibility in the excited state. A negative charge localized on a ligand in an MLCT state makes a good candidate for oxidative ET. Comparably electron-rich TMs are required for low-energy MLCT states, such as d6-, d8-, and d10-electron configurations (chromium(0), iron(II), nickel(II), nickel(0), copper(I)). However, short MLCT lifetimes of 3d TMCs (Section 8.18.2.2), poor photostability of TMCs due to the presence of accessible MC states (Section 8.18.4) and irreversible redox chemistry represent severe challenges for photoredox applications. Carbonyl and isonitrile chromium(0) complexes with a 3d6-electron configuration and low-energy MLCT states are often prone to photodissociation (Section 8.18.4.1). An exceptional example is Cr(CNtBuAr3NC)3 91 (Scheme 7) with chelating isonitriles. Although the lifetime of its MLCT state of 2.2 ns is shorter than that of the homologous 4d6-molybdenum(0) complex, it is still sufficient for ET.223 With an excited state oxidation potential of E*ox ¼ 2.43 V vs. ferrocene, 91 (Scheme 7) is a very strong photoreductant. Unfortunately, the corresponding CrI complex is too unstable to enable photoredox catalysis. However, EnT to anthracene was successful (Section 8.18.3.2).223 As most 3d6-iron(II) complexes possess very short MLCT lifetimes,86,88 photoinduced ET from MLCT states should occur only in pre-associated molecular systems or with surface-bound iron(II) sensitizers. [Fe(bpy)3]2þ 12D as photosensitizer (Figs. 2B and 7) in combination with an organocatalytic system promotes the enantioselective alkylation of aldehydes with various a-bromo carbonyl

d-d and charge transfer photochemistry of 3d metal complexes

739

compounds.284 NHC ligands elongate excited state lifetimes (Section 8.18.2.2). Tridentate C^N^C biscarbene pyridine ligands in the iron(II) sensitizers [Fe(iPrbip)2]2þ 752D and [Fe(Arbip)(tpy)]2þ 802D (iPrbip ¼ 2,6-bis(10 -isopropylimidazol-20 -yl)pyridine, Arbip ¼ 2,6-bis(10 -(2,6-diisopropy)-phenylimidazol-20 -yl)pyridine; Scheme 7) enable photocatalytic proton reduction with K2[PtCl4] as pre-catalyst.285 The homoleptic C^N^C biscarbene pyridine iron(II) complexes with N–Me and N–tBu substituents [Fe(mbip)2]2þ 742D and [Fe(tBubip)2]2þ 762D (mbip ¼ 2,6-bis(10 -methylimidazol-20 -yl)-pyridine, tBubip ¼ 2,6-bis(10 -tert-butylimidazol-20 -yl)pyridine; Scheme 7) possess 3MLCT state lifetimes of 9.5 ps and ca. 0.3 ps, respectively.206,213 These lifetimes are higher than for the bis(terpyridine) iron(II) complex [Fe(tpy)2]2þ 692þ (Scheme 7). Due to the stronger s-donating nature of carbenes the FeIII/II redox potential is lowered.206 Yet, the spectral absorption range of these carbene complexes covers less of the low energy range due to the blue-shifted CT absorptions of [Fe(iPrbip)2]2þ 752D, [Fe(mbip)2]2þ 742D and [Fe(tBubip)2]2þ 762D compared to [Fe(tpy)2]2þ 692D.206,285 MLCT lifetimes below the 50 ps threshold, required for complete electron injection into semiconductors, lower the efficiency of DSSCs with iron(II) sensitizers.286 Intramolecular relaxation namely IC and ISC (and VR) into low-energy MC states is faster than the photochemical processes needed for DSSCs.52 The metastable 3MLCT states of analogous polypyridine ruthenium(II) complexes possess a relatively long lifetime, as competitive non-radiative relaxation pathways are energetically uphill or associated with an activation barrier.287 Progress in this field has accelerated in recent years since the first examples of iron-containing DSSCs in 1998.288 Bipyridine cyanido iron(II) complexes cis-[Fe(bpy)2(CN)2] 131 and cis-[Fe(CN)2(dcbpy)2] 132 (dcbpy ¼ 4,40 -dicarboxy2,20 -bipyridine) have MLCT lifetimes below 25 ps.288–290 Studies employing different electron withdrawing and anchoring groups for applications in DSSCs revealed a comparatively poor DSSC performance.291–297 For photochemical applications further enhancement of the lifetime of the 1/3MLCT excited states is required, the global ET process must be accounted for, and a p-accepting site for efficient electron injection to the conduction band of the semiconductor is important.52 Pyridyl-NHC ligands chelating iron(II) meet these requirements with a p-accepting pyridine and strongly s-donating NHCs enabling a favorable MLCT transition.52 A carboxylic acid group at the C^N^C ligand ([Fe(mbip-COOH)2]2þ 772D, mbip-COOH ¼ 2,6-bis(10 -methylimidazol-20 yl)-4-carboxyl-pyridine; Scheme 7) serves to attach the NHC iron(II) complex to a TiO2 surface for use in DSSCs. This leads to a 92% efficiency of charge injection into the semiconductor with 0.13% overall efficiency of the DSSC.211,212 The 3MLCT lifetime of 772D increased to 16.5 ps (Eox ¼ 0.45 V vs. ferrocene) compared to 9.5 ps for [Fe(mbip)2]2þ 742D and < 0.1 ps for [Fe(tpyCOOH)2]2þ 1332D (tpy-COOH ¼ 4-carboxyl-2,20 :60 ,200 -terpyridine; Scheme 13).212,213 Upon TiO2 surface attachment, the 3 MLCT state lifetime of 772D decreased to 3.1 ps and the oxidation potential decreased to Eox ¼ þ0.39 V vs. ferrocene demonstrating the influence of surface attachment.212 The 3MLCT lifetime of iron(II) complexes increased further by expansion of the p-system as shown for [Fe(bbip-COOH)2]2þ 1342D (bbip-COOH ¼ 2,6-bis(10 -benzimidazol-20 -yl)-4-carboxyl-pyridine; Scheme 13) with a 3MLCT lifetime of 26 ps compared to 16.5 ps for 772D.210 However, the expanded p-system reduced the efficiency of charge injection into TiO2 because of a weaker coupling between the LUMO of the sensitizer and the conduction band of TiO2.210 With a zinc porphyrin for expansion of the p-system in 882D (Scheme 7), the 3MLCT lifetime increased to 160 ps, even sufficient for phosphorescence (Section 8.18.2.2). The long lifetime, the wide absorption range from 400 to 650 nm and suitable FeIII/II ground and excited state redox potentials enabled a successful incorporation into a SnO2/FTO-DSSC (FTO ¼ fluorine doped tin oxide).219 Diamagnetic, square-planar 3d8-NiL4 complexes with appropriate p-accepting ligands possess 1/3MLCT excited states, yet 3MC states are typically lower in energy. Aryl bipyridine halido nickel(II) complexes such as [Ni(dtbbpy)(o-tolyl)(X)] 135–137 (dtbbpy ¼ 4,40 -di-tert-butyl-2,20 -bipyridine; X ¼ Cl, Br, I, Scheme 13) have been suggested to undergo ET from their excited states assigned as 3MLCT with nanosecond lifetimes. With a GS complex as redox partner the corresponding nickel(I) and nickel(III) complexes are formed (photoinduced disproportionation). This could initiate a NiI/III catalyzed C–O cross coupling reaction.298 However, this interpretation has been challenged and a photoinduced Ni–C(aryl) bond homolysis (Section 8.18.4.6) from a longlived 3MC state has been invoked as initial photo-induced step.299 The nickel(II) complex [Ni(Mabiq)]þ 138D (Mabiq ¼ anion of HMabiq ¼ 2-4:6-8-bis(3,3,4,4-tetramethyldihydropyrrolo)-10–15-(2,20 -biquinazolino)-[15]-1,3,5,8,10,14-hexaene-1,3,7,9,11,14N6; Scheme 13) with a macrocyclic p-accepting ligand possesses an excited state with an estimated lifetime of 10 ns. This excited state is quenched by sacrificial amines and the resulting reduced complex Ni(Mabiq) 138 can reduce organic substrates such as bromoalkylsubstituted indoles. The assignment of the long-lived photoactive state as 3MLCT or 3MC in character is open to interpretation.300 The meso-tetraphenyl porphyrinato nickel(II) Ni(TPP) 139 (TPP2 ¼ 5,10,15,20-tetraphenylporphyrinato(2); Scheme 13) exhibits an even longer excited triplet state lifetime of 12.9 ms. This state can be oxidatively and reductively quenched using diazonium salts and N,N-dimethylaniline, respectively (E*ox(DMF) ¼ 2.17 V, E*red(DMF) ¼ þ0.57 V vs. ferrocene). Photocatalyzed CeC and CeX bond formations have been reported using 139 and blue LEDs.301 The nature of the photoactive excited triplet state of 139 remains unclear to date. To prevent population of (mostly) redox-inactive MC states, pseudo-tetrahedral 3d10-ML4 complexes (M ¼ Ni0, CuI) are widely used since the fully occupied 3d-shell prevents MC states. The 3MLCT excited states possess comparably long lifetimes because competitive relaxation pathways via MC states are lacking. However, the formal oxidation of the metal center (Ni0, CuI) in the 1/3 MLCT states leads to the Jahn-Teller active 3d9-configuration (NiI, CuII) suffering from flattening distortions and attack of solvent molecules (Section 8.18.2.2, Scheme 9). Furthermore, the NiI and CuII species resulting after ET might be substitutionally labile. The limited number of photo and redox stable nickel(0) complexes with redox-active ligands prevented their wide application in ET so far. The isonitrile nickel(0) complexes 99 and 100 (Scheme 8B) are luminescent at low temperature but not at room temperature (3MLCT) suggesting a viable non-radiative excited state distortion pathway at room temperature. In principle, their estimated

740

d-d and charge transfer photochemistry of 3d metal complexes

Scheme 13 Molecular structures of iron(II) (1332D–1342D), nickel(II) (135–139) and copper(I) complexes (140D–150) with photoredox-active MLCT states.

excited state redox potentials recommend them as photoreductants (Eox ¼ 0.32 V and 0.30 V; E*ox ¼ 2.85 V and 2.83 V, respectively; vs. ferrocene). However, room temperature excited state lifetime and redox stability need to be improved for photoredox catalytic applications.58,230,231 Copper(I) complexes form the largest subset of photoredox-active 3d TMCs. Several reviews have comprehensively summarized photoredox-active copper(I) complexes.302–304 Photoredox-active copper(I) complexes can be grouped into four categories, namely

d-d and charge transfer photochemistry of 3d metal complexes

741

homoleptic bis(diimine) complexes [Cu(N^N)2]þ, heteroleptic complexes with diimine and mono- or diphosphanes [Cu(N^N)(P^P)]þ, heteroleptic complexes with diimine and isonitrile ligands [Cu(N^N)(CN^NC)]þ and complexes with diimine and NHC ligands [Cu(N^N)(NHC)]þ (Scheme 13). Homoleptic [Cu(N^N)2]þ complexes undergo oxidative and reductive quenching and show good performances in DSSCs after surface anchoring.304–306 They feature excited state energies between 1.8 and 2.1 eV, e.g., 1.87 eV and 2.05 eV for [Cu(dmp)2]þ 102D (dmp ¼ 2,9-dimethyl-1,10-phenanthroline; Scheme 9) and [Cu(dap)2]þ 140D (dap ¼ 2,9-bis(p-anisyl)-1,10-phenanthroline; Scheme 13), respectively.307,308 In solution, they are often poorly emissive due to the presence of non-radiative deactivation pathways (Section 8.18.2.2). The first photoredox catalysis for copper(I) complexes was reported using [Cu(dap)2]þ 140D in 1987.307 The highly reductive 3[Cu(dap)2]þ (E*ox ¼ 1.83 V vs. ferrocene)271,307 was applied to photoactivated C–C coupling of para-nitrobenzyl bromide to form dibenzyl derivatives,307 to atom transfer radical addition,309,310 to CeH bond functionalization,311 to functionalization of alkenes312 and many more photoredox catalytic processes.303,313,314 The less photoreducing [Cu(Ph2phen)2]þ 104D (Ph2phen ¼ 2,9-diphenyl-1,10-phenanthroline; Scheme 9) has been applied in phenyl radical formation for allylation with allylsulfones and DIPEA.315 DFT calculations and time-resolved spectroscopy suggest that the phenyl substituents in 2,9-position in [Cu(Ph2phen)2]þ 104D sufficiently protect the photogenerated copper(II) center from quenching by coordinating solvents. This results in a solvent independent 3MLCT lifetime for [Cu(Ph2phen)2]þ 104D, while the MLCT lifetime of [Cu(dmp)2]þ 102D strongly depends on the solvent (Scheme 9, Section 8.18.2.2).316 The electron density of 104D localizes on the phen moiety in the 3MLCT state without extending onto the phenyl substituents, due to the non-coplanar arrangement of the rings.316 While homoleptic phosphane copper(I) complexes [Cu(P^P)2]þ lacking energetically low-lying p-accepting ligands usually have large HOMO-LUMO gaps and only weak absorptions in the visible spectral region58 unsuitable for ET processes, heteroleptic [Cu(N^N)(P^P)]þ complexes can be strategically tuned towards suitable redox potentials, low-energy absorptions and excited state dynamics.317,318 Consequently, they have been widely employed in electroluminescent devices and photoredox reactions.50 Even the overall complex charge has been adapted, e.g., in [Cu(dmp)(PPh3)2]þ 142D and [Cu(dmp)(PPh2(m-C6H4SO3))2] 143L (Scheme 13).265 Diphosphane ligands with large bite angles are especially suitable for copper(I) ligation, e.g., [Cu(dmp)(Xant)]þ 144D, [Cu(bcp)(Xant)]þ 145D, [Cu(bcp)(thioPOP)]þ 146D and [Cu(bcp)(POP)]þ 147D (bcp ¼ 2,9-dimethyl-4,7-diphenyl-1,10phenathroline, Xant ¼ 4,5-bis(diphenylphosphanyl)-9,9-dimethylxanthene, thioPOP ¼ 10,100 -(2,8-dimethylphenoxathiine-4,6diyl)bis(2,8-dimethyl-10H-phenoxaphosphinine); Scheme 13). [Cu(dmp)(Xant)]þ 144D has been used as photosensitizer in polymerization319 and oxidative cyclization of triarylamines to generate N-substituted carbazoles.320 The influence of different phosphane ligands on the redox potentials and MLCT absorption is negligible as shown by the complexes [Cu(bcp)(Xant)]þ 145D and [Cu(bcp)(thioPOP)]þ 146D. Yet their excited state lifetimes differ (s ¼ 6.4 and 16.3 ms, respectively), demonstrating differing non-radiative decay kinetics.321 Both complexes and further derivatives were tested in photocatalytic proton reduction with Fe3(CO)12 as catalyst using triethylamine as sacrificial reductant. An oxidative quenching pathway has been proposed. Both complexes show a similar turnover number, yet [Cu(bcp)(thioPOP)]þ 146D shows faster electron transfer to the iron carbonyl catalyst.321 Interestingly, [Cu(bcp)(POP)]þ 147D can be reductively quenched by amines, such as DIPEA (E*ox ¼ 1.42 V vs. ferrocene). The resulting neutral complex 147 (Ered ¼ 2.04 V vs. ferrocene) reduces aryl iodides and aryl bromides, producing aryl radicals, which are finally intercepted by pyrroles.271,322 The key intermediate 147 has been denoted as “copper(0) complex,”322 although a coordinated bathocuproine-centered radical is probably a more appropriate electronic description. A further subgroup of four-coordinate heteroleptic copper(I) complexes features chelating diisonitrile ligands with large bite angles in place of the diphosphane ([Cu(CN^NC)(N^N)]þ). For example, [Cu(binc)(Ph2phen)]þ 148D (binc ¼ bis(2-isocyanophenyl) phenylphosphonate; Scheme 13) was employed as photosensitizer in the allylation of diethyl bromomalonate with allyltrimethylsilane under visible-light irradiation.323 With a diimine and a monodentate NHC ligand heteroleptic three-coordinate copper(I) complexes ([Cu(NHC)(N^N)]þ) form. These complexes possess high GS CuII/I redox potentials. Consequently, the copper(II) complexes resulting from photoinduced ET are strong oxidants.244,324–329 Varying the bridging groups and substituents on the diimine moiety optimized these systems for applications in LECs and photoredox catalysis. For example, [Cu(IPr)(phen)]þ 149D and [Cu(dpya)(IPr)]þ 109D (IPr ¼ 1,3bis(2,4,6-trimethylphenyl)imidazole-2-ylidene, dpya ¼ 2,20 -dipyridylamine; Schemes 10 and 13) have promising excited state oxidation potentials with E*ox ¼ 2.09 V and 1.10 V vs. ferrocene, respectively (Eox ¼ þ1.14 V and þ1.52 V vs. ferrocene, respectively).58,244,271 NHC copper(I) complexes are also strong photooxidants (E*red z þ1.0 V vs. ferrocene) due to their high excited state energies.244 Yet, low spectral coverage of the absorption profile in the visible region and moderate redox stability are drawbacks for efficient ET. A related group of three-coordinate copper(I) complexes feature carbazolide and phosphane ligands, e.g., Cu(carbazolide)(Pmtol3)2 150 (E*red z 3.0 V, Ered z þ0.1 V vs. ferrocene; Scheme 13).271,330 This complex is formed in situ from copper(I) halides by Ullmann coupling reactions and found to be a photoactive catalyst for C–N coupling reactions.330,331 With similar photocatalytically active copper(I) complexes formed in situ CeC,332 and CeS bond formation,333 phenol–aryl coupling,334 decarboxylative CeN coupling,335C–N coupling with CeCl bond activation,336 alkylation of aryl halides337 and Csp–Csp homocoupling of terminal alkynes were performed.338,339 Several other photocatalytically active copper(I) complexes formed in situ operate in a similar fashion.340–343 Three-coordinate copper(I) complexes heterogenized on silica via bpy ligands have been suggested as intermediates in photocatalyzed C–H arylation reactions.344 Surface anchoring is also important for application of copper(I) complexes in DSSCs.345,346 The first copper(I) based DSSC contained the carboxylate-functionalized sensitizer [Cu(dbap)2]þ 141D (dbap ¼ 2,9-bis(p-carboxyphenyl)-1,10-phenanthroline; Scheme 13).347 As the C6H4-COO-substituent is not involved in the 3MLCT state, electron injection in the seminconductor is

742

d-d and charge transfer photochemistry of 3d metal complexes

poor and the efficiency of the cell is low.316 For efficient copper(I)-based DSSCs, the copper(I) sensitizer should absorb strongly in the visible spectral region, should possess suitable ground and excited state redox potentials, high photo and redox stability, a sufficiently long 3MLCT lifetime (> 25 ps) and suitable anchoring groups at the electron accepting diimine ligand.345 Heteroleptic pushpull complexes perform better in DSSCs, yet they are difficult to isolate because of the labile CueL bonds. A synthetic protocol for on-surface synthesis of heteroleptic complexes enabled screening of several copper(I) sensitizers in DSSCs.348–350 Structural modifications of [Cu(bpy)2]þ type dyes can enhance the DSSC efficiency. 6,60 -Functionalization enables a reversible CuI/II redox process.351 Aryl substituents at the 6,60 -position cause a flattening of the tetrahedral coordination geometry due to pp interactions resulting in two MLCT bands and enhanced spectral response.352,353 Expansion of the p-system with arene spacers between bpy and the anchoring group improves efficiency.353 Phosphonic acid/phosphonate anchors are superior to carboxylic acid/carboxylate anchors.354 Installation of hole-transporting triphenylamino-dendrons onto the ancillary ligands can enhance efficiency, resulting from an extended hole-transporting domain.355,356 However, a large number of arene rings on the ancillary ligand can also enhance aggregation of the sensitizer due to p-stacking and reduce the efficiency.355,356 As LMCT excited states formally offer a positive charge on the ligand, a reductive quenching pathway is preferred with suitable substrates. Only a few 3d TMCs show ET from LMCT excited states. The key step is either the generation of a radical cation from the substrate via reductive quenching of the photosensitizer193 or the reduction of the substrate by a photoreduced photosensitizer, acting as strong GS reductant.357 Comparably electron-poor TMs, e.g., with d0- or d1-electron configurations, and electron-rich, p-donating ligands are required for low-energy LMCT states. As scandium is not as abundant as copper or iron, the photochemistry of 3d0-scandium(III) complexes has received comparably less attention. Decamethyl scandocene halides and amides ScCp2*X 151–154 (Cp* ¼ pentamethylcyclopentadienide; X ¼ Cl, Br, I, NHPh; Scheme 14) with low-lying CT states are emissive with microsecond luminescence lifetimes. The excited state of the chlorido complex is quenched by aromatic substrates such as toluene, xylenes, hexachlorobenzene or 1,2,4,5-tetrachlorobenzene.358 Stern-Volmer analyses indicate static quenching. Semiempirical molecular orbital calculations suggest LMCT character of the excited state with Cp* and X being the donors.358 The photochemistry and the products of the excited state reaction are not yet further elaborated. Titanium(IV) oxide has found widespread use in heterogeneous photoredox catalytic reactions,359 yet molecular photoactive titanium(IV) complexes showing photoredox catalysis are scarce. Titanocene dihalides TiCp2X2 155 and 156 (Cp ¼ cyclopentadienide; X ¼ Br, Cl; Scheme 14) possess useful LMCT states. TiCl2Cp2 155 absorbs around 520 nm and emits at around 630 nm at 78  C.360 At this temperature, DIPEA quenches the emission. A reductive quenching is proposed, giving TiIIIClCp2 after chloride dissociation. In a catalytic process, TiClCp2 opens an epoxide-ring forming an alkoxide b-radical kO-coordinated to titanium(IV). Hydrogen atom transfer (HAT) from the photochemically generated amine radical cation gives the iminium ion and the titanium(IV) alkoxide. Substitution of the alkoxide by chloride closes the photocatalytic cycle. This has been developed into a photocatalytic reductive epoxide ring opening using green LEDs. Titanocene dibromide 156 was somewhat superior possibly due to better absorption properties, while other X ligands, such as fluorido, mesylate and trifluoroacetato resulted in no conversion.360 As most titanium(IV) complexes are colorless, only titanium(IV) complexes that contain organic chromophores for strong visible light absorption can be efficiently used in DSSCs, e.g., with phthalocyanine as chromophore and carboxy-functionalized catecholate as

Scheme 14

Molecular structures of 3d TMCs 151167 with photoredox-active LMCT states.

d-d and charge transfer photochemistry of 3d metal complexes

743

axial ligand and anchor in the sensitizer Ti(cat-COOH)Pc 157 ([cat-COOH]2 ¼ catecholato-4-carboxylic acid(2), Pc2 ¼ phthalocyaninato(2); Scheme 14).361,362 High-valent oxido vanadium(V) complexes bearing donor ligands with electron-withdrawing substituents possess high E*red values. Consequently, they are potent photooxidants.363 The vanadium(V) complex 158 (Scheme 14) with electronwithdrawing fluorine substituents at the phenoxide donors is highly absorptive between 400 and 500 nm, has a high excited state reduction potential of þ0.9 V vs. ferrocene and a rather high LMCT lifetime of s ¼ 420 ps, which is promising for ET.364 Electronwithdrawing pentafluorophenyl and nitrophenyl groups in the Schiff base complexes 159 and 160 (Scheme 14) lead to powerful oxidation agents due to very high excited state reduction potentials of E*red ¼ þ2.81 and þ3.18 V vs. ferrocene, respectively.58 These sensitizers have been successfully employed in photocatalyzed oxidative CeC bond cleavage reactions of aliphatic alcohols in air, using white light LEDs.365 Low-spin 3d5-iron(III) complexes such as [Fe(btz)3]3þ 33D (Fig. 2C for 32D) have a great potential in ET applications (E*red ¼ þ1.56 V vs. ferrocene). However, they possess excited state lifetimes that are too short (s ¼ 107 ps for 33D) for efficient bimolecular ET.58,190 Anionic tripodal carbene ligands in low-spin [Fe{PhB(MeIm)3}2]þ 63D (Scheme 6) achieve a high 2LMCT lifetime of s ¼ 2 ns (Section 8.18.2.2).191,366,367 This luminescent and photoactive 2LMCT excited state is highly oxidizing and reducing with excited state redox potentials of E*(FeIII/II) ¼ þ1.0 V and E*(FeIV/III) ¼ 1.9 V vs. ferrocene, respectively (E(FeIII/II) ¼ 1.2 V, E(FeIV/III) ¼ þ0.3 V vs. ferrocene).191 Oxidative and reductive quenching was successful with the methyl viologen dication and with N,N-dimethylaniline, respectively. The charge separation rate constant up to 1.25  1012 s1 for the latter quencher exceeds those found for typical ruthenium(II) based ET systems and outcompetes solvent dynamics.191,366 However, the fast spin-allowed charge recombination with rate constants of z2  1011 s1 competes with the cage escape of the ET product. Consequently, the photoproduct yields are moderate.366 The luminescent low-spin 3d6-cobalt(III) complexes [Co(dgpy)2]3þ 653D and [Co(dgpz)2]3þ 663D with electron-rich sdonating guanidine ligands (Scheme 6) possess long mono- and biexponential 3LMCT lifetimes of s ¼ 5.07 ns and 8.69 ns (61%)/3.21 ns (39%), respectively. Both are highly oxidizing in their excited states (E*red ¼ þ1.35 V and þ1.58 V vs. ferrocene, respectively).193,271 After photoexcitation at 420 nm, the 3LMCT state oxidizes trifluoromethylarene radicals to the corresponding carbocations. The concomitantly formed cobalt(II) complex acts as reductant closing the catalytic cycle. This ET cycle is connected to an EnT cycle, hence two photons are required per turnover (Section 8.18.3.2). Overall, the trifluormethylation of pyrene, perylene or coronene has been achieved using CF3SO2Cl, light and the cobalt(III) sensitizer, while benzene, naphthalene and phenanthrene were unreactive, suggesting the participation of excited states of the polycyclic aromatic hydrocarbon.193 The four- or five-coordinate 3d8-morpholine nickel(II) complex NiBr2(morph)n 161 has a triplet GS (n ¼ 2, 3). Excitation at 365 nm (LMCT) gives formally a nickel(I) ion and a morpholine radical cation which dissociates.368 After deprotonation of the morpholine radical cation and bromide dissociation from nickel(I), the morpholine radical adds to the substrate p-F3C-C6H4-Br with Br release which in turn regenerates the nickel(II) complex. The trick to overcome the short excited state lifetime appears to be pre-coordination of the amine substrate to nickel(II).368 A few nickel sensitizers in DSSCs exploit the LMCT absorption. The (formally) bis(dithiolato) nickel complexes 162–165 (Scheme 14) exhibit reversible redox processes and are highly absorptive in the UV and NIR regions with maxima around 320 nm (LMCT) and 870 nm (ILCT/LMCT). They have been employed as sensitizers in DSSCs with rather poor efficiencies between 0.07% and 0.11%.369 NIR-absorbing bis(dithiolato) nickel complexes containing polymerizable indolyl substituents Ni(mi-5edt)2 166 and Ni(mi-5hdt)2 167 have been incorporated in bulk heterojunction photovoltaic devices after electropolymerization (mi5edt2 ¼ 1-(N-methylindol-5-yl)-ethene-1,2-dithiolato(2), mi-5hdt2, 1-(N-methylindol-5-yl)-hex-1-ene-1,2-dithiolato(2); Scheme 14). Yet, the efficiencies were below 0.1%.370,371 The low-energy absorptions of the bis(dithiolato) nickel complexes exhibit only small LMCT and strong intraligand character. This could be further elaborated towards excited states of TMCs with essentially LL0 CT character. ILCT and LL0 CT states are indeed frequently present in 3d8-nickel(II) and 3d10-zinc(II) complexes with suitable ligands featuring extended p-systems and electron-rich and electron-poor moieties. The heteroleptic square-planar complex Ni(dcbpy)(qdt) 168 (qdt2 ¼ 2,3-quinoxaline-dithiolato(2); Scheme 15) displays LL0 CT absorption bands from the electron-rich dithiolate qdt2 to the electron-deficient bipyridine dcbpy up to 560 nm together with intense UV absorption bands.372 The carboxylic acid substituents at the bpy ligand serve to anchor the complex onto TiO2. The detrimental p-stacking of the complexes was reduced by addition of chenodeoxycholic acid during immobilization. However, the efficiency of a DSSC using this photosensitizer and chenodeoxycholic acid is still low, likely due to a short excited state lifetime.372 Porphyrcene nickel(II) sensitizers equipped with carboxylic acid substituents 169–171 (Scheme 15) were immobilized on NiO semiconductors to yield p-type DSSCs. Electron injection occurs from the NiO semiconductor into the excited nickel(II) porphyrin with efficiencies between 0.019% and 0.028%.373 As a result of the high redox potential of zinc(II), CT states are ligand-centered ILCT or LL0 CT states. Zinc(II) merely acts as a Lewis acid and stabilizes ligand-centered orbitals.58 For example, Zn(TPP) 172 operates as photoreductant (Eox ¼ þ0.28 V, E*ox ¼ 1.88 V vs. ferrocene)58,374 in polymerization reactions (ET reversible addition fragmentation chain-transfer polymerization)375 and as photooxidant (Ered ¼ 1.85 V, E*red ¼ þ0.31 V vs. ferrocene)58,374 in a photocatalytic formation of a-alkylated derivatives from aldehydes and diazo compounds.376 Zinc(II) complexes have been applied in DSSCs, using zinc(II) porphyrins377 or terpyridine zinc(II) complexes, including functionalizations with triphenylamine for efficient hole transport and with anchor groups leading to efficiencies between 0.34% and 0.71%.378 Push-pull substituted zinc(II) porphyrins 173 and 174 (Scheme 15) possess pp* excited states with directional CT character, yet without significant metal contribution. Combined with a suitable

744

d-d and charge transfer photochemistry of 3d metal complexes

Scheme 15

Molecular structures of nickel(II) 168–171 and zinc(II) 173–174 complexes with photoredox-active ligand-centered excited states.

cobalt(II/III) redox mediator in the DSSC, the rate of interfacial back-ET from the conduction band of the nanocrystalline TiO2 film to the cobalt(III) mediator was retarded and high photovoltages approaching 1 V were obtained. Combining 174 with an organic cosensitizer achieved a very high power conversion efficiency of 12.3% in an optimized DSSC.379 Beyond intramolecular charge transfer excitations, photo-induced CTTS has been observed with electron-rich, oxidizable TMCs. A formal oxidation of the metal center or an electron-rich ligand occurs along with reduction of a solvent molecule. CTTS typically requires comparably high excitation energies.380 CTTS bands are often difficult to identify due to interference with other absorptions.381 The energy of the corresponding absorption band depends on the redox potentials of the complex and the solvent.382 For example, CTTS bands blue-shift in the weaker oxidizing solvent CH2Cl2 compared to CHCl3 and red-shift in the stronger oxidizing solvent CCl4.382 Apart from direct reduction of solvent molecules by CTTS, solvated electrons can form leading to a characteristic blue color due to their long-wavelength absorption. Solvated electrons are usually only stable in certain solvents (e.g., liquid ammonia) or in lowtemperature matrices. In most solvents solvated electrons appear as transients at room temperature before recombination with the oxidized complex or reduction of the solvent or other substrates occurs.381 Numerous photoreactions of TMCs are associated with CTTS and a selection of 3d TMCs showing CTTS is presented below.383–388 Hexacyanidoferrate(II) [Fe(CN)6]4 1754L generates solvated electrons upon CTTS excitation between 214 and 313 nm in water.389–392 Solvated electrons have been found to be accessible by excitation of tetracarbonylferrate(–II) [Fe(CO)4]2 1762L at 588 nm in 10 M NaOH at 77 K.393 The CTTS band of ferrocene FeCp2 177 appears between 300 and 400 nm in chlorinated solvents. Irradiation of ferrocene in CCl4 or CHCl3 gives a ferrocenium cation, a chloride anion and CCl3$ or CHCl2$ radicals, respectively.394 Solvated electrons were not observed, suggesting that the s* orbitals of the CeCl bond are directly populated. The 1,2-dithiolato nickel complexes [Ni(S2C2(CN)2)]2 1782L, [Ni(S2C2Ph2)] 162L and Ni(phen)(S2C2Ph2) 179 undergo photooxidation in chlorinated solvents as well, indicating CTTS (Scheme 16).382,395,396 Photolysis of 179 in a toluene/CHCl3 glass at 77 K yields 179D which has been characterized by EPR spectroscopy as a nickel(II) complex with a monoanionic coordinated dithiolato-derived radical.382,395,396 The macrocyclic nickel(II) complexes [Ni(tet-a)]2þ 1802D and [Ni(AT)]þ 181D show CTTS absorption bands in the UV spectral region (tet-a ¼ meso-5,7,7,12,14,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane, AT ¼ 11,13-dimethyl-1,4,7,10-tetraazacyclotetradeca-10,12-dienato(1); Scheme 16).397 In aqueous solution in the presence of Hþ and O2, CTTS excitation of [Ni(tet-a)]2þ 1802D with a low pressure mercury vapor lamp (254 nm) leads to formation of HO2 via oxidation of the complex.397

d-d and charge transfer photochemistry of 3d metal complexes

Scheme 16

745

Molecular structures of nickel(II) complexes 1782––181D that show CTTS.

A variety of copper(I) complexes with simple inorganic ligands such as halides, cyanide or ammonia which lack acceptor orbitals at low-energy, e.g., tricyanido cuprate(I) [Cu(CN)3]2 1822L or [Cu(NH3)3]þ 183D, are also photooxidized in aqueous solution by CTTS, giving hydrated electrons as the primary photochemical step.385,398 Continuous irradiation of chlorido and bromido cuprates [CuXnþ1]n 184nL and 185nL (X ¼ Cl, Br) with l > 313 nm in deaerated acidic solution produces hydrogen and copper(II) species. The electron formed in the primary photochemical step in acidic solution is scavenged by Hþ, giving a hydrogen radical. The generated H could then add to [CuXn](n1) forming a hydrido copper(II) species [CuHXn](n1). Protonation of the hydride leads to H2 evolution and copper(II) species [CuXn](n2).385,398 Cyanido cuprates(I) initially yield copper(II) species as well, yet these are rereduced to copper(I) by excess cyanide giving cyano radicals CN which recombine to dicyan N^CeC^N.385,398

8.18.3.2

Photoinduced energy transfer from and to 3d TMCs

EnT can formally be described as an acceptor molecule (A) absorbing a part of the emission energy of a donor molecule (D).62 The A D Gibbs free energy for a bimolecular EnT process is given by DG0 ¼ E A00  E D 00 with the transition energies E 00 and E 00 at 0– 0 vibronic levels of acceptor and donor, respectively. Solvent reorganization and entropic effects are assumed to be very minor and are neglected. This allows for a Marcus-type description of EnT with a similar “Golden Rule” expression as for ET with the two-electron matrix element HEnT describing the electronic coupling and the Franck-Condon weighted density of states FCEnT (Eq. 5).62 The two-electron matrix element HEnT describes the electronic coupling between the HOMOs and LUMOs of D and A. This electronic factor can be split in two additive terms, an electronic Coulomb and an exchange term. FCEnT can be identified with the spectral overlap integral between the emission spectrum of D and the absorption spectrum of A. Hence, two limiting cases of non-radiative intermolecular EnT are possible. Both yield the same products, yet they differ in the mechanism, kinetics and distance dependence. EnT occurring via electron exchange interaction (Dexter or exchange mechanism) necessitates overlap of wavefunctions and thus requires donor and acceptor to be in close proximity (r0 < 10 Å). In a simple one-electron picture, an electron of the excited donor D* transfers to the LUMO of A, while the other electron travels from the HOMO of A to the lower energy SOMO of D (Fig. 8A). Dexter EnT requires conservation of the total spin of the D/A pair, while the local spin states of D and A can

Fig. 8

(A) Triplet-triplet Dexter-type EnT and (B) singlet-singlet Förster-type EnT in a one-electron description.

746

d-d and charge transfer photochemistry of 3d metal complexes

change. The Dexter EnT rate constant kEnTD (Eq. 6)62 is proportional to the Dexter overlap integral JD between the donor’s luminescence and the acceptor’s absorption, given by the molar extinction coefficient, normalized by the spectral areas of emission (D) and absorption (A). Increasing the distance rDA between D and A leads to an exponential decrease of the rate constant kEnTD. Electronic coupling and distance between D and A are included in the term HEnT, where HEnT(0) is the interaction at contact distance.62 The attenuation parameter bEnT (Eq. 7)62 is specific to the interacting species D and A in a given medium. kEnT ¼

4p2  EnT 2 EnT FC H h

(5)

4p2  EnT 2 JD H h

(6)

kD EnT ¼

  bEnT HEnT ðrDA Þ ¼ HEnT ð0Þ exp  ðrDA  r0 Þ 2

(7)

In bimolecular EnT, a certain excited state lifetime (sD > 1 ns) of D is required so that a contact between D* and A for orbital overlap can occur. Coupling mediated by an intervening medium such as a connecting bridge is possible.399 From inter- to intramolecular EnT, the distance between D and A drastically decreases, so that the exchange mechanism dominates.400,401 Since intraD molecular Dexter-type EnT occurs at almost fixed geometries, further contributing factors to kEnT can be easily disentangled. As ISC is 62,80,253,399–404 Since the spin selection rule often efficient in TMCs, EnT from the lowest spin-forbidden excited state is expected. requires EnT processes to obey spin conservation within the D/A pair, the exchange mechanism operates when the multiplicities of both involved excited states differ from the respective GS multiplicity, e.g., triplet-triplet EnT (Eq. 8 and Fig. 8A). A prominent example for Dexter-type EnT is quenching of excited triplet states of TMCs by molecular oxygen (3O2), forming 1O2. To exclude this common quenching path of spin-forbidden excited states, photoreactions and spectroscopic measurements of TMCs are typically carried out under oxygen-free conditions.22,252 D ðT 1 Þ þ AðS0 Þ/DðS0 Þ þ A ðT 1 Þ

(8)

The second EnT type is Förster resonance EnT (FRET). FRET additionally requires the conservation of the local spins of D and A, hence Förster-type EnT is typically a singlet-singlet EnT (Eq. 9 and Fig. 8B). This EnT is induced by dipole-dipole interactions and does not require physical contact between D and A. The distance between D and A can exceed the sum of their van der Waals radii252 and ranges between 10 and 100 Å.405 D ðS1 Þ þ AðS0 Þ/DðS0 Þ þ A ðS1 Þ F

(9)

62

The Förster EnT rate constant kEnT (Eq. 10) is proportional to the inverse sixth power of the distance rDA between D and A, to the inverse fourth power of the refractive index of the medium n, to the inverse lifetime sD of the excited donor and to the quantum yield of the donor FD. Additionally, the Förster overlap integral JF between the donor’s luminescence and the acceptor’s absorption spectrum, given by the molar extinction coefficient, normalized by the spectral area of the donor A and the relative orientation of the F . K2 ranges between 0 and 4 and equals to 2/3 for freely transition dipoles of D and A, given by the orientation factor K, affects kEnT 406–408 diffusing D/A pairs (Eq. 10). Donors suitable for FRET are required to have high quantum yields (FD) and small fluorescence lifetimes in the absence of A (sD). kFEnT ¼ 8:8  1025

K 2 fD 6 s JF n4 rDA D

(10)

Considering all these aspects, EnT processes from a donor excited state with different multiplicity to its GS can only occur by the Dexter mechanism, as a Förster-type EnT with a typical TMC donor in its triplet state (e.g., 3MLCT) would violate the spin conservation rule.409,410 However, intramolecular EnT via the Förster mechanism with a rate constant kEnT z 108 s1 has been suggested to occur between the excited 3MLCT state of the [ReI(bpy)(CO)3]þ donor unit and the metal-centered quartet state of the central CrIII(acac)3 acceptor moiety (4T2) in the tetrametallic complex 1863D (Scheme 17).411 The total spin angular momenta spanned by the coupled reactants (3MLCT(Re) and 4A2(Cr)) and the coupled products (1A1(Re) and 4T2(Cr)) are 5/2, 3/2, 1/2 and 3/2, respectively. Consequently, the reactant and product manifolds share a common S ¼ 3/2 state which enables a spin-allowed EnT via FRET.411 Reviews on Dexter- and Förster-type EnT have been published and selected examples of EnT with 3d TMCs as acceptors and donors are highlighted in the following.405,409,412 Diffusion-controlled EnT between 3d3-chromium(III) complexes involving the Reineckate anion trans-[Cr(NCS)4(NH3)2] 187L as donor and hexacyanidochromate(III) [Cr(CN)6]3 193L as acceptor was reported in solution in the late 1960s, by observing the quenching of the Reineckate emission in the presence of 193L and the appearance of the low-energy luminescence of 193L upon selective excitation of the Reineckate 187L.413–416 The hexacyanidochromate(III) ion 193L serves as acceptor in the salt [Cr(en)3][Cr(CN)6] 188 with [Cr(en)3]3þ 383D as donor (Scheme 3). The EnT efficiency is close to unity demonstrating high 4 T2 / 2E ISC efficiency in [Cr(en)3]3þ 383D.417 The series of mixed crystals [Rh0.99Cr0.01(bpy)3][NaAl1–xCrx(ox)3]ClO4 189 exhibits two EnT mechanisms with [Cr(ox)3]3 1903L as donor and [Cr(bpy)3]3þ 203D as acceptor (ox2 ¼ oxalato(2)).418 The short-range EnT path (kEnT > 106 s1) is mediated by superexchange coupling between the chromium(III) ions and p-overlap of the oxalato and bpy ligands, while the slower process (kEnT  200 s1) occurs via a dipole-dipole mechanism at low

d-d and charge transfer photochemistry of 3d metal complexes

Scheme 17

747

Molecular structure of the tetrametallic [CrIIIRe3I ] complex 1863D showing Förster-type EnT from ReI to CrIII.411

concentrations of 1903L.418 [Cr(ox)3]3 to [Cr(ox)3]3 resonant energy migration (self-exchange) assists the EnT from [Cr(ox)3]3 to [Cr(bpy)3]3þ as the [Cr(ox)3]3 concentration increases.418,419 Excitation of the [Ru(bpy)3]2þ cation 22D in the complex salts [Ru(bpy)3]3[CrX6]2 191 and 192 (X ¼ CN, NCS) and [Ru(bpy)3]3[CrL3]2 193 and 194 (L ¼ ox2, mal2; mal2 ¼ malonato(2)) yields emission from Cr(2E) states instead of phosphorescence from Ru(3MLCT) states.420,421 EnT from the Ru(1MLCT) state to the Cr(4T2g) state occurs with very high efficiency, when the Cr(4T2) state is lower in energy than the Ru(1MLCT) state, i.e., EnT to chromium(III) should be even faster than ISC in [Ru(bpy)3]2þ. If the Cr(4T2) state is higher in energy than the Ru(1MLCT) state, EnT from the Ru(3MLCT) state to the Cr(2E) state occurs, but with low efficiency and preservation of residual 3MLCT emission of [Ru(bpy)3]2þ. On the other hand, Otsuka et al. suggested that EnT in Na[Ru(bpy)3][Cr(ox)3] 195 occurs from Ru(3MLCT) to populate the Cr(2E) state even though the Cr(4T2) state lies below the Ru(1MLCT) state.422 Studies on crystalline Na[Ru(bpy)3][Cr(ox)3] 195 increased the understanding of EnT between Cr(2E) states in solids.423,424 Crystal size influences energy migration between the Cr(2E) states.425,426 For 670 and 140 nm crystallites, emission from the surface region dominates, indicating an approximate 100 nm average distance to be traveled by EnT, implying approximately 30 EnT steps. Close to the surface of the crystals, the effective symmetry around the chromium(III) ions is lowered, providing the driving force for directional energy migration from the core to the surface of the crystals. Quenching of the [Ru(bpy)3]2þ 3MLCT emission also occurs with cyanido complexes as counter ions.427,428 In the solid state, the shortest distance between the bpy carbon atoms of the [Ru(bpy)3]2þ donor 22D and the chromium(III) acceptor of [Cr(CN)6]3 193L is below 5 Å, enabling orbital overlap of the donor and acceptor. Consequently, EnT operates via the Dexter mechanism. Changing the metal center of the donor from ruthenium(II) to osmium(II) in the double-complex salts [M(bpy)3]2[Cr(CN)6]Cl 196 and 197 (M ¼ Ru, Os) further illustrates the dependence of EnT rates on spectral overlap with an eightfold increase of kEnT for the osmium(II) salt over the ruthenium(II) salt at 77 K.428 As the high-energy spin-flip state Cr(2T2) is only 1500 cm1 higher in energy than the Ru(3MLCT) state in 196, increasing the temperature opens a thermally activated EnT path from the Ru(3MLCT) state to the Cr(2T2) state. Consequently, the RuII salt 196 exhibits a temperature dependent decay rate constant for the Ru(3MLCT) state. In the OsII homolog 197 this path is unavailable due to the large energy difference of 4700 cm1 between the Os(3MLCT) and Cr(2T2) states.428 Chromium(III) complexes with macrocyclic ligands have been investigated with respect to cross-exchange and self-exchange dynamics including equilibrium and rate constants of EnT, for example trans-[Cr(dx-cyclam)X2]nþ 198D (x ¼ 0, X ¼ CN, n ¼ 1), 1993D (x ¼ 0, X ¼ NH3, n ¼ 3), 200D (x ¼ 4, X ¼ CN, n ¼ 1), 2013D (x ¼ 4, X ¼ NH3, n ¼ 3) and trans-[Cr([15]aneN4)(CN)2]þ 202D ([15]ane-N4 ¼ 1,4,8,12-tetra-azacyclopentadecane).429,430 As the luminescence lifetimes of N-deuterated chromium(III) complexes are substantially longer than those of their non-deuterated counterparts (Sections 8.18.1 and 8.18.2.1), excited-state emission quenching of the longer lived species by shorter lived species was studied by analyzing the decay profile following pulsed excitation. Flash photolysis experiments for deuterated/non-deuterated pairs yielded self-exchange rate constants of kEnT > 7  106 M1 s1 and kEnT ¼ 2.4  106 M1 s1 for 198D/200D and 1993D/2013D, respectively.429 Following the studies of EnT between ionic entities in complex salts and between individual complex ions in solution, interest in this field substantially increased to investigate EnT in chromophore-appended TMCs and in oligometallic TMCs. Release of nitric oxide (NO) from nitrito complexes such as trans-[Cr(cyclam)(ONO)2]þ 203D can occur after light excitation giving a prototype of photoactivated NO-releasing molecules (PhotoNORMs; Section 8.18.4.2).431 Using organic antennae appended to the cyclam-type ligand, such as anthracenyl or pyrenyl substituents in the nitrito complexes trans-[Cr(cyclam-R)(ONO)2]þ 204D and 205D (R ¼ anthracenyl or pyrenyl derivative), EnT from the excited organic chromophore to the chromium(III) center takes place and increases the NO photorelease compared to 203D without antenna.432,433 Dinuclear [(tpy)CrIII(m-L)CrIII(tpy)]6þ complexes 2066D and 2076D (with L ¼ bbt (4,4000 -bis(2,20 :60 ,200 -terpyridine)) and ebbt (4,4000 -(ethynyl)-bis(2,20 :60 ,200 -terpyridine)); Scheme 18A) with non-equivalent chromium sites in the crystalline state have slightly different CrIII(2E) level energies (D and A). Therefore

748

d-d and charge transfer photochemistry of 3d metal complexes

Scheme 18 (A) Molecular structures of dinuclear chromium(III) complexes 2066D and 2076D and (B) [CrIIIN6] “complex-as-ligand” building block 2083D for polymetallic architectures.434,435

they exhibit intramolecular EnT between the Cr centers via the bridging ligand with rate constants of kEnT ¼ 4.1  105 s1 and 2.1  107 s1, respectively.434 The significantly higher kEnT for 2076D with the longer ebbt bridging ligand results from the excellent through-bond conducting character of the alkyne-containing bridge.434 A “complex-as-ligand” synthetic strategy incorporates substitutionally inert, photophysically active [Cr(L)(phen)2]3þ 2083D (Scheme 18B, L ¼ 2,6-bis(N-methyl-benzimidazol-2-yl)-4((1,10-phenanthrolin-5-yl)ethynyl)pyridine) building blocks into polymetallic architectures.435 The tridentate bisbenzimidazole(pyridine) metal binding unit connected to the phenanthroline chromium(III) complex via a “conducting” alkyne bridge enables electronic communication for intermetallic EnT such as sensitizing lanthanide activators in 3d-4f luminescent complexes (see below). In the heterobimetallic cyanido-bridged RuII-CrIII complex 2092L (Scheme 19A) with a defined Ru/ Cr distance, the ruthenium(II) moiety acts as donor and the coordinated [Cr(CN)6]3 193L as acceptor with a quenched Ru(3MLCT) emission and the appearance of the Cr(2E) emission at 800 nm.436 The EnT performance of bridged RuII-CrIII systems depends on the efficient mixing of the donor and acceptor electronic states. The short cyanido bridge promotes strong coupling between metal ions,439–443 as exemplified by numerous oligometallic cyanide-bridged mixed-valence complexes.443 A-D-A assemblies of the CrIII-RuII-CrIII type that incorporate cyanido bridges 210–2154L (Scheme 19B) have been extensively used to build molecular wires,443,444 to attach antennae436,441,445,446 and in various other applications.447,448 A superposition of the energy diagrams of the fragments to a full Jablonski diagram of the heterometallic complex describes their basic photophysical behavior. Since the extinction coefficients of the allowed ruthenium(II) 1MLCT absorption bands are significantly larger than those of the chromium(III) MC bands, the ruthenium(II) fragment preferentially absorbs compared to the chromium(III) moiety. Following population of the Ru(1MLCT) state, ISC to the Ru(3MLCT) state takes place on the picosecond timescale449 and is efficient with unity quantum yield.450 Consequently, EnT occurs from the Ru(3MLCT) state to the lower-lying doublet states of the chromium(III) ion. The Jahn-Teller distorted chromium-centered quartet excited states are thus bypassed when exciting the ruthenium(II) antenna. This protects the chromium moiety from photodissociation via the distorted Cr(4T2) state (Section 8.18.4). The intramolecular EnT is Dexter-type in nature as the local spins at the D and A sites are not conserved, as a short, conducting bridging ligand is present and as the quantum yield for EnT approaches unity.436,442 In the trinuclear RuII-RuII-CrIII complex trans-[Ru(bpy)(tpy)(m-CN)Ru(py)4(m-NC)Cr(CN)5]438 (216, py ¼ pyridine; Scheme 19C) long-range EnT between the terminal RuII and CrIII moieties was observed. Poor spectral overlap between donor emission and acceptor absorption suggests negligible Coulomb coupling between the transition dipole moments for a Förster-type EnT. Although the distance between the terminal centers exceeds 10 Å, Dexter-type EnT occurs. In fact, throughbridge mixing of the ruthenium orbitals results in an extended excited donor moiety {RuIIþdþ(bpy)(tpy–)(m-NC)RuIIþdþ(py)4} and consequently a short distance to the chromium(III) acceptor suitable for Dexter-type EnT.438 Beyond downconversion from high-energy donor states to low-energy acceptor states at the 3d TM, upconversion processes can yield excited states at higher energy by two-photon mechanisms, namely excited state absorption (ESA), energy transfer upconversion (ETU) and cooperative sensitization upconversion (CSU, Scheme 20B). With [Cr(ddpd)2]3þ 43D (Scheme 1A) as acceptor and ytterbium(III) complexes [Yb(dpa)3]3 as donors in the [Cr(ddpd)2][Yb(dpa)3] salt 217 (dpa2 ¼ 2,6-pyridinedicarboxylato(2); Scheme 20A), an NIR-to-NIR CSU process has been realized.451 Irradiation into the ytterbium 2F7/2 / 2F5/2 band at 980 nm leads

d-d and charge transfer photochemistry of 3d metal complexes

749

Scheme 19 (A) Molecular structure of the cyanido-bridged RuII-CrIII complex 2092L with excitation and emission data in DMF,436 (B) general molecular structure of cyanido-bridged CrIII-RuII-CrII complexes 210–2154L251,437 and (C) molecular structure of the cyanido-bridged RuII-RuII-CrII complex 216.438

to the appearance of the chromium-centered 2E/2T1 / 4A2 phosphorescence at 775 nm at ambient temperature in the solid state. In the absence of a bridge between the metal centers and the metal-localized excited states, EnT from the excited ytterbium centers (2F5/2) to the chromium center (4T2) results most likely from dipole-dipole interactions. The spin-allowed Förster-type EnT is followed by ISC from Cr(4T2) states to the emissive doublet states Cr(2E/2T1).451 The detrimental back-EnT from the Cr(2E/2T1) states to give an Yb(2F5/2) excited state (Scheme 20B) is comparably inefficient as shown by the similar Cr(2E/2T1) luminescence quantum yields of the isostructural ytterbium(III) and lutetium(III) salts of 5.8% and 6.8%, respectively, upon excitation of the chromium(III) cation at 435 nm.451 Photocatalysis for organic synthesis using sensitized 3d TMCs as catalysts is an emerging field in EnT applications. Typically, an Ir(ppy)3 derivative (218, ppy ¼ anion of 2-phenylpyridine) acts as photosensitizer for C–X cross-coupling reactions with nickel(II) catalysts. During the catalytic cycle, the Ir(3MLCT/3LC) excited state can excite thermally stable and inert nickel(II) species by EnT. The excited nickel(II) species then undergoes reductive elimination to give the cross-coupled product. A current working hypothesis suggests that the initially formed NiII(3MLCT) state evolves into a long-lived tetrahedral NiII(3MC) state. Metal-ligand homolysis (Section 8.18.4.6) then generates an aryl radical and nickel(I) as first step of the reductive elimination.299 Oxidative addition of an aryl halide to the nickel(0) species regenerates nickel(II) and is followed by addition of a nucleophile to complete the cycle.

750

d-d and charge transfer photochemistry of 3d metal complexes

Scheme 20

(A) Structure of the 1:1 chromium(III) ytterbium(III) salt 217 and (B) schematic illustration of the CSU process.

Carbon-(hetero)atom cross-coupling reactions, such as C–C,452,453C–O,454,455C–N456–459 and C–S460 formation, via nickel/photo dual catalysis have been studied using time-resolved spectroscopy. However, the details of the EnT mechanism are not yet fully understood.461 The majority of the pathways appears to be of Dexter-type,452–454,456,460 but also the Förster-type EnT pathway is discussed.457 Metallaphotocatalysis/metal photo dual catalysis with sensitized nickel and copper complexes was reviewed in 2017.462 Beyond acting as energy acceptors, 3d TMCs can also function as energy donors towards inorganic and organic substrates. In solution, the donor excited state lifetime must be sufficiently long to enable bimolecular EnT. A prominent application of EnT using 3d TMCs as donors is singlet oxygen (1O2) formation for use in organic synthesis and photodynamic therapy (PDT).463–465 [Cr(ddpd)2]3þ 43D (Scheme 1A) has been employed as sensitizer in the photocyanation of amines via the 1O2 pathway.160 Due to the short 4T2 lifetime and the fast ISC to the 2E/2T1 states (s ¼ 3.5 ps in CH3CN) of the sensitizer, intermolecular ET pathways from the 4T2 state to the amine substrate have been excluded. In addition, back-ISC is unlikely, based on the large energy difference between the 2E/2T1 and 4T2 states. Hence, only EnT to oxygen remains as viable pathway.105,160 The quantum yield for 1O2 formation with 43D is high (F(1O2) ¼ 61% in DMF).160 The singlet oxygen then abstracts a hydride from the amine substrate and the resulting iminium ion is intercepted by the cyanide nucleophile.160 Inspired by the high F(1O2), the application of 43D in PDT has been attempted, yet internalization into cancer cells is insufficient probably due to the high positive charge of 43D.465 The centrosymmetric complex [Cr(tpe)2]3þ 443D (Scheme 3) is a promising candidate for application in EnT reactions due to its extremely high doublet state lifetime of s ¼ 4500 ms and its efficient luminescence quenching by dioxygen.108 Beyond the lifetime optimization, the extinction coefficients of the chromium(III) sensitizers need to be improved for better light harvesting, for example by EnT from antennae (see above). [Cr(bath)3]3þ 1233D (Scheme 11) is an example of a 3d TM photoredox catalyst used for radical cation based regioselective Diels-Alder cycloadditions. Mechanistically, oxygen and singlet oxygen formation appear to play a decisive role in the photoredox catalytic cycle and excited 1233D appears to follow a dual pathway of ET and EnT in this scenario.258,266–268 Besides chromium(III), cobalt(III) complexes can operate as sensitizers in photocatalytic EnT reactions.193 The low-spin guanidine cobalt(III) complexes [Co(dgpy)2]3þ 653D and [Co(dgpz)2]3þ 663D (Scheme 6) transfer their triplet excited state energy (3LMCT; Sections 8.18.2.2 and 8.18.3.1) to pyrene as triplet energy acceptor. Due to the involved local spin changes, a Dextertype mechanism is likely. The nanosecond lifetimes of the excited states support this assignment. The thus formed triplet pyrene then participates in the catalytic cycle via SET forming trifluoromethyl radicals from CF3SO2Cl.193 In the proposed catalytic cycle the excited cobalt complexes operate via dual EnT and ET pathways (Section 8.18.3.1), conceptually similar to 1233D. Organic acceptors also play decisive roles in triplet-triplet annihilation upconversion (TTA-UC, Fig. 9) and a few examples of TTA with 3d TMCs as donors (sensitizers) have been reported. Clearly, a sufficiently long excited state lifetime of the excited 3d TMC is required for efficient bimolecular reactions with the organic acceptor. In the isonitrile chromium(0) complex 91 (Scheme 7), the chromium center is effectively shielded from its chemical environment by three sterically demanding chelate isonitrile

d-d and charge transfer photochemistry of 3d metal complexes

Fig. 9

751

General Jablonski diagram illustrating TTA-UC and resulting upconverted delayed fluorescence (green arrow).

ligands, increasing its robustness and enabling a long 3MLCT lifetime of s ¼ 2.2 ns at room temperature in THF.223 Anthracene quenches the emission of 91 after excitation at 532 nm and upconverted delayed fluorescence of anthracene is observed at 405 nm with s ¼ 170  5 ms, indicating TTA-UC (Fig. 9 with D ¼ 91 and A ¼ anthracene).223,466 The non-linear dependence of the anthracene’s singlet excited state fluorescence intensity on excitation power further confirms the two-photon upconversion process.223,467,468 Orange-red absorbing copper(I) complexes have been investigated as sensitizers for TTA with perylene and the commercial BODIPY dye pyrromethane 546 as acceptors.469 Using the 3MLCT sensitizer [Cu(Ph2phen)2]þ 104D (Scheme 9; Sections 8.18.2.2 and 8.18.3.1), orange-to-blue and red-to-green photon upconversion was achieved. By optimizing the substitution pattern of the phen ligand, the lifetime of the 3MLCT state of the copper(I) complex could be dramatically increased and the TTA-UC process became highly efficient.466 Direct excitation of lanthanides is inefficient as f-f transitions are Laporte-forbidden. Consequently, antenna moieties are often used to harvest light and achieve sufficient population of luminescent f-f excited states.470,471 Commonly employed broadband light harvesting organic chromophores can be replaced with d-block sensitizers, that have narrower and tunable absorption bands, enabling the design of more specific excitation/emission wavelength pairs.472 Long excited state lifetimes, high molar extinction coefficients and sufficient overlap of excited-state energies with lanthanide-based f-f states suggest a few 3d TMCs as candidate sensitizers.473 Pseudo-octahedral trivalent chromium chromophores are particularly attractive as sensitizers because of accessible transitions in the UV/Vis (4A2 / 4T1, 4T2) and NIR spectral regions (4A2 / 2T1, 2E).107,464 In cyanido-bridged d-f assemblies of lanthanide cations and hexacyanidochromate(III) (polymeric {cis-[Cr(CN)4(mCN)2Ln(H2O)2(dmf)4]$nH2O}N, 219) or hexacyanidocobaltate(III) (dinuclear 220, Scheme 21A) the lanthanide ion NIR emission is observed after exciting the hexacyanidometallates(III) by UV/Vis photons. Overlap of donor emission spectra and acceptor f-f absorption bands in these complexes facilitate EnT via the Förster mechanism, the short metal-metal separation (Scheme 21A) and the bridging cyanido ligands enable Dexter-type EnT. The latter path is furthermore supported by the selection rules for EnT to most LnIII ions.473 Heterometallic complexes with a chromium(III) donor and a lanthanide(III) acceptor allow intramolecular intermetallic LnIII / CrIII and CrIII / LnIII EnT (Ln ¼ Nd, Er, Yb). The specific EnT direction CrIII / LnIII or LnIII / CrIII depends on the incorporated lanthanide and its energy levels. Apparent lanthanide excited state lifetimes after CrIII / LnIII EnT are within the millisecond range.474–481 The donor-acceptor distance of 9.3 Å in the heterobimetallic CrIII-LnIII helicate [CrIIILn(N^N  N^N)3](CF3SO3)6  4 CH3CN 221 (Scheme 21B for ligand N^N  N^N) is suitable for dipole-dipole EnT while a Dexter-type EnT is less likely due to the absence of a short bridging ligand. After CrIII excitation and CrIII / LnIII intramolecular energy migration, the CrIII-NdIII and CrIII-YbIII helicates show lanthanide-based NIR emission (> 850 nm) with millisecond lifetimes along with diminished residual Cr(2E) emission.475 The LnIII lifetimes mirror those of the Cr(2E) donor levels confirming the CrIII / LnIII EnT process. Non-radiative EnT from the Cr(2E) state to the excited 4f levels of lanthanide(III) ions was also revealed through variable temperature emission spectroscopy of oxalato-bridged complexes [(acac)2CrIII(m-ox)LnIII(HBpz3)2] 222 (HBpz3 ¼ trispyrazolylborato(1); Ln ¼ La, Nd, Eu, Gd, Tb, Ho, Er, Tm, Yb, and Lu, Scheme 21C). The 3d-4f complexes show emission around 800 nm resulting from the 2E level of the chromium(III) moiety between 10 and 150 K. For Ln ¼ Nd, Ho, Er, Tm and Yb, the Cr(2E) emission is quenched upon warming to room temperature enabled by an efficient EnT from the Cr(2E) state to the lower lying or nearly isoenergetic 4f excited states of Nd, Ho, Er, Tm and Yb.482 In contrast, the CrIII-EuIII and CrIII-TbIII complexes exhibit LnIII / CrIII EnT and the lanthanide centered emission is quenched completely.483 Depending on the incorporated lanthanide ion and the excitation energy, intramolecular EnT in both directions (CrIII / LnIII or III Ln / CrIII) is possible in the trinuclear triple-stranded helicates [CrLnCr(N^N  N^N^N  N^N)3]9þ 2239D (Fig. 10A for Ln ¼ Er; Scheme 21B for ligand N^N  N^N^N  N^N). After ligand and LnIII-centered excitation of 2239D with Ln ¼ Eu or Tb, LnIII / CrIII, divergent energy migration occurs followed by chromium(III)-centered and residual lanthanide-centered emission with more than 90% efficiency,481,484 while CrIII / LnIII convergent energy migration followed by lanthanide(III)-centered NIR emission occurs with Ln ¼ Nd, Er and Yb ions.474 Compared to the dimetallic helicates [CrLn(N^N  N^N^N)3]6þ 2246D (Scheme 21B for ligand N^N  N^N^N), incorporation of a second CrIII sensitizer in the corresponding triple-stranded Cr-Ln-Cr helicates

752

d-d and charge transfer photochemistry of 3d metal complexes

Scheme 21 (A) Photosensitized lanthanide-centered emission in 220 (LnIII ¼ YbIII, NdIII) after intramolecular EnT from coordinated hexacyanidocobaltate(III),473 (B) polytopic ligands N^N  N^N ¼ 2-{6-[N,N-diethylcarboxamido]pyridin-2-yl}-1,10 -dimethyl-5,50 -methylene-20 (5-methylpyridin-2-yl)bis[1H-benzimidazole]; N^N  N^N^N ¼ 2-{6-[1-(methyl)-1H-benzimidazol-2-yl]pyridin-2-yl}-1,10 -dimethyl-5,50 -methylene20 -(5-methylpyridin-2-yl)bis[1H-benzimidazole]; N^N  N^N^N  N^N ¼ 1,10 -dimethyl-2,20 -bis(5-methylpyridin-2-yl)-5,50 -{pyridine-2,6-diylbis [(1-methyl-1H-benzimidazole-2,5-diyl)methylene]}bis[1H-benzimidazole] employed for heterometallic 3d-4f helicates and (C) oxalato-bridged 3d-4f bimetallic complexes 222 with Ln ¼ La, Nd, Eu, Gd, Tb, Ho, Er, Tm, Yb, and Lu.

[CrLnCr(N^N  N^N^N  N^N)3]9þ 2239D significantly enhances the lanthanide-centered emission intensity following chromium(III) sensitization.474 The opposite situation as in the CrIII/YbIII complex salt [Cr(ddpd)2][Yb(dpa)3] 217,451 namely EnT from excited 3d TMCs to lanthanide ions, can lead to upconverted photons as well. Exciting the CrIII-ErIII-CrIII helicate 2239D with NIR laser light, selective for the Cr(2E) state (750 nm) at 77 K, green erbium(III)-centered emission occurs at 543 nm. This is enabled by an ETU process (Fig. 10B, blue pathway).476 Upon increasing the number of sensitizers from one to two, the theoretically expected enhancement of upconversion efficiency in an activator-centered ETU mechanism is four. The experimentally observed enhancement for the CrIII-

d-d and charge transfer photochemistry of 3d metal complexes

753

Fig. 10 (A) Trinuclear triple-stranded helicate 2239D with Ln ¼ Er showing upconversion with 50% efficiency and (B) the proposed pathways for the upconversion processes.

ErIII and CrIII-ErIII-CrIII helicates is even larger, indicating an additional upconversion pathway in the latter. In 2239D, the energy of two photons can be stored on the CrIII excited states before transferring to the ErIII (Fig. 10B, red pathway). This additional sensitizer-centered mechanism (Fig. 10B, red pathway) is much less sensitive to the excited-state lifetime of the ErIII center.477 Further examples of LnIII luminescence sensitized by 3d TMCs were summarized in a comprehensive review in 2013.485

8.18.4

Unimolecular reactivity of 3d TMCsdModification of the coordination sphere

Historically, product analysis was the major characterization technique of photodissociation reactions. Modern spectroscopic and computational methods like femtosecond resolution TA spectroscopy, and DFT now have allowed a detailed investigation of the involved excited states and the elementary steps leading to photodissociation. The multi-dimensionality along nuclear coordinates in a given many-atom 3d TMC and the large number of relevant excited states (e.g., MC and CT states of various multiplicities) in a given energy region lead to a multitude of conceivable dissociative relaxation pathways along several X–Y reaction coordinates following photoexcitation as schematically depicted in Fig. 11. The TMCs and their unimolecular photoreactions are classified in the following, according to the cleaved XeY bond (MeCO, MeNO, MeOOCR, MeN3, NeN, MeC and others).

Fig. 11 Selection of general one-dimensional X–Y dissociation pathways after excitation with light. The region of the PES where dissociation occurs is colored in orange.

754 8.18.4.1

d-d and charge transfer photochemistry of 3d metal complexes CO dissociation from 3d TMCs

In the early 2000s, theoretical studies on the excited state character of carbonyl complexes led to a reassessment of the role of MC states in M–CO photodissociation reactions. To induce photodissociation, excitation to MC states is not necessary.486–488 In many cases, the lowest excited state manifold consists of MLCT instead of MC states.489,490 This view of excited-state ordering is supported by TDDFT and CASSCF/CASPT2 calculations on Cr(CO)6 5 and its heavier analogs.491,492 The low-energy shoulder of relatively weak intensity in the absorption spectrum of 5 corresponds to a set of symmetry forbidden excitations in MLCT states (predominantly 1A1g / 1T2u, Fig. 1)493,494 whereas typically allowed MLCT and other MC transitions lie at higher energies. The low energy MLCT states have only small M–CO antibonding character and yet are photoactive with respect to M–CO dissociation. Furthermore, the calculations show that the allowed and forbidden MLCT and MC transitions arise from interconfigurational mixing of t2g5t1u1 and t2g5t2u1 excited configurations demonstrating a delocalized character.489,491,492 DFT studies on mixed ligand chromium carbonyl complexes revealed an interplay of MLCT(non-CO ligand), with weakened M–CO p-backbonding, and MLCT(CO) excited states, that are responsible for the low energy bands in the absorption spectra.63,495–497 According to the primogenic effect, the ligand field splitting is smaller in 3d TMCs compared to their heavier analogs resulting in MC states of lower energies while the MLCT states are less affected.65,75,76 For example, CASPT2 and TDDFT calculations indicate that the lowest excited state has mostly MC character in Mn(CO)5H, but MLCT(CO) character in Re(CO)5H.498 MLCT states are intrinsically non-dissociative, yet the repulsive MC PESs play a key role in CO photodissociation even without direct MC state population by low-energy excitation, since the MC energies decrease rapidly with increasing MeC bond length. Along the M–C stretching coordinate, the antibonding MC orbitals decrease in energy due to short-range interaction and strongly dissociative MC states drop lower in energy than the MLCT states. The dissociation is therefore not direct but it requires a preceding relaxation step followed by a crossing-over to an MC PES (Fig. 11B).111,489 The dynamics and role of MLCT states in CO photodissociation focusses on PES of individual electronic states, their couplings and conical intersections along the dissociative coordinate. Experimental evidence supports a “prompt photodissociation.”63,495,497 In Cr(CO)6 5 a steep initial down-slope of the PES of the lowest MC state (T1g) along the M–C stretching or Jahn-Teller-active coordinates enables direct paths from the low-energy MLCT states 1T2u and 1T1u to the repulsive MC surface (anti-Kasha behavior) (Fig. 12). Both MLCT states of T character in Oh symmetry split along the Cr–CO stretching coordinate in C4v symmetry according to T2u(Oh) / E(C4v) þ B1(C4v) and T1u(Oh) / E(C4v) þ A1(C4v). This enables mixing of the states with E representation with the

Fig. 12

Schematic PESs and dynamics of 5 after excitation in the MLCT states 1T2u and 1T1u (345 and 270 nm).111

d-d and charge transfer photochemistry of 3d metal complexes

755

energetically falling MC state (1E(C4v) derived from the 1T1g(Oh) state) with an avoided surface crossing already at rather small deviation from the equilibrium geometry.494,499 Excitation into MLCT states 1T2u and 1T1u of 5 is followed by direct ultrafast relaxation to the dissociative PES of the 1E state (s1 ¼ 21 fs, s10 ¼ 12.5 fs) via avoided surface crossings in the vicinity of the Franck-Condon region. An initial CreCO bond elongation by only 10% caused by the Jahn-Teller splitting of MLCT states of T symmetry is enough to mix MLCT with dissociative MC states and to induce CO cleavage. Fast CreCO bond breakage takes place on the 1E surface along separated trajectories for each excitation producing an excited trigonal bipyramidal Cr(CO)5 intermediate (s2 ¼ 22 fs, s20 ¼ 18 fs).111 IC to a vibrationally excited square-pyramidal species with a 1A1 GS (s3 ¼ 25 fs, s3‘¼ 40 fs) via a Jahn-Teller-induced conical intersection (Fig. 12) is accompanied by coherent vibrational wavepacket motions along a pseudorotation coordinate. Only a single CO ligand dissociates from the MLCT excited states upon one-photon excitation. Yet, an excess of vibrational energy can initiate a CO dissociation to Cr(CO)4 in its S0 state (s4 ¼ 1.5 ps, s40 ¼ 0.93 ps) (Fig. 12).63,111,112,500 Excitation of trigonal-bipyramidal Fe(CO)5 6 (1A10 GS) into its MLCT state (1E0 ) leads to ultrafast electronic relaxation in MLCT and MC manifolds onto the dissociative PES of the lower lying 1E0 MC state (s1 ¼ 15–21 fs) (Fig. 13). Fast CO dissociation on the 1 B2 surface after state splitting according to E0 (D3h) / B2(C2v) þ A1(C2v) produces Fe(CO)4 in an MC excited state (s2 ¼ 30 fs) which rapidly converts to the excited singlet state 1A1 (s3 ¼ 47 fs). Thermal CO loss to give Fe(CO)3 (s4 ¼ 3.3 ps) driven by an excess of vibrational energy, ISC to the relatively stable 3B2 state (s5 ¼ 300 fs) or ligation by a solvent molecule (e.g., ethanol) resulting in a 1A10 state (s6 ¼ 200–300 fs) can follow (Fig. 13).501–503 The diverse ultrafast IC processes after excitation of 6 which also include CO dissociation are facilitated by many energetically close lying states combined with surface crossing and the Jahn-Teller effect present in E0 states.504 According to two-photon excitation coupled-cluster calculations, double CO loss could also proceed by sequential one-photon absorption events due to overlapping absorptions bands of 6 and Fe(CO)4.505 In addition to CO dissociation on a singlet MC pathway, dissociation directly from an MLCT state or an alternative triplet pathway in which ISC to the 3E0 state occurs prior to dissociation to the triplet GS (3B2) of Fe(CO)4 cannot be excluded.113,504 Excitation of Ni(CO)4 96 by UV/Vis photons leads to relaxation and IC processes within the MLCT states (22–70 fs) followed by NieCO bond dissociation (600 fs). The CO dissociation is relatively slow because the avoided crossing between MLCT and MC states occurs rather late on the reaction coordinate, which is consequently accompanied by a high energy barrier. The primary

Fig. 13

Schematic PESs and dynamics of 6 after excitation in the MLCT state 1E0 (267 nm).501

756

d-d and charge transfer photochemistry of 3d metal complexes

photoproduct Ni(CO)3 displays long-lived emission, most likely after ISC to a triplet state. A second CO elimination from the singlet excited state requires a longer time (55 ps), due to the absence of an easily accessible conical intersection.63,506,507 In the bimetallic carbonyl complex Mn2(CO)10 7, CO dissociation yielding Mn2(CO)9 (42 fs) competes with MneMn bond homolysis forming two Mn(CO)5 radicals upon UV irradiation. The initial transition is assigned as ss* and the dynamics are controlled by a reduction in bond order, caused by repulsion in the Mn–Mn and the Mn–CO coordinates. Along with the effect of Mn–CO having a smaller reduced mass, MneCO bond heterolysis occurs faster than MneMn bond homolysis. The initially excited ss* state couples non-adiabatically to other electronic states, contributing to a relatively fast cleavage of the MneCO bond.63,114,507,508 In heteroleptic carbonyl complexes of the first TM series the loss of another type of ligand can compete with CO dissociation by photochemical excitation. In the carbonyl phosphane complex Cr(CO)5(PH3) 225, the avoided crossing between initially populated MLCT(CO) states and dissociative MC states occurs further away from the equilibrium geometry along the Cr–C than along the Cr–P coordinate. This gives rise to a smaller energy barrier on the CreP dissociative pathway favoring CrePH3 bond cleavage.63,509 In hydrido complexes Co(CO)4H 226 and cis-Fe(CO)4(H)2 227, M–H dissociation is faster than M–CO dissociation (Section 8.18.4.7) due to a smaller energy barrier with tens of femtoseconds for M–H, compared with hundreds of femtoseconds for M–CO dissociation.510 In contrast, wave packet motions result in CO dissociation after MLCT(CO) excitation of facMn(CO)3(H-DAB)H 228 (H-DAB ¼ 1,4-diazabutadiene; Scheme 22).510 In Cr(bpy)(CO)4 231 (Scheme 22), the PES of singlet MLCT(bpy) states are also dissociative along the Cr–C coordinate, but the avoided crossing with MC states occurs far beyond the equilibrium distance leading to high energy barriers.511 As a result of the separated MC absorption band, Mn(CO)3(Cp) 233 represents an example of CO dissociation through selective excitation in the MC states. The first CO dissociation (66 fs) occurs via the MC state, followed by the dissociation of the remaining CO ligands (100 fs).512 In the heme-CO complex (Section 8.18.4.7), excitation into the Q-band triggers CO loss and a spin crossover of iron(II) from low- to high-spin. The vibronic mechanism can be summarized as a sequential 1Q / 1MLCT/MC / 3MLCT / 5MLCT cascade with CO dissociation occurring in the 1 MLCT excited states prior to the spin crossover.513 Photochemically induced CO elimination delivers coordinatively unsaturated, reactive compounds enabling subsequent reactions and/or formation of species that would be inaccessible by thermal pathways. Addition of weakly coordinating ligands, addition of stronger ligands and photoinduced isomerization reactions in combination with re-coordination of CO are typical follow-up reactions after CO dissociation (Table 2). Matrix isolated Fe(CO)4 reversibly forms complexes with weak s-donating matrix molecules including N2, CH4 or Xe.514 A similar photochemistry exists for matrix isolated Cr(CO)5 forming complexes

Scheme 22 Molecular structures of carbonyl manganese (228, 229, 230, 232, 234), iron (235D) and chromium (231, 236–239) complexes. Leaving CO ligands are highlighted in blue or red.

d-d and charge transfer photochemistry of 3d metal complexes Table 2

757

Photolysis products of carbonyl complexes in certain environments, categorized as addition of weak and strong ligands as well as isomerization after CO re-coordination.

Carbonyl complex

Photolysis product

Fe(CO)5(N2)514 Fe(CO)5(CH4)514 Fe(CO)5(Xe)514 Cr(CO)6 (5) Cr(CO)5(Ne)515 Cr(CO)5(Xe)515 Cr(CO)5(Ar)515 Cr2(CO)11516 Cr(CO)5(N2)516 Cr(CO)5(c-C6H12)516 Cr(CO)5(Kr)517 Cr(CO)5(CO2)517 Mn(CO)3(Cp) (233) Mn(Cp)(CO)2(C3H8)518 Mn(Cp)(CO)2(C4H10)518 Mn(Cp)(CO)2(c-C5H10)518 Mn(Cp)(CO)2(C2H6)519 Mn(Cp)(CO)2(i-C5H12)519 Mn(Cp)(CO)2(PFMCH)520 Mn(Cp)(CO)2(Xe)520 Mn(Cp)(CO)2(CO2)520 Fe(CO)5 (6) Fe(CO)3(PEt3)2521 Fe(CO)3(PMe3)2521 Fe(CO)3(P(nBu)3)2521 Fe(CO)3(PPh3)2521 Fe(CO)3(py)2521 Fe(CO)3(tBuNC)2521 Fe(CO)3(CH3CN)2521 fac-Mn(CO)3(NCCH3)(tryp) Mn(CO)2(NCCH3)2(tryp)522 (234) cis-Cr(CO)4(NHC)2 (236– cis-Cr(CO)3(NHC)2(py)523 239) cisCr(CO)3(NHC)2(a-pic)523 cis-Cr(CO)3(NHC)2(2,6lut)523 fac-MnBr(CO)3(iPr-DAB) MnBr(CO)2(iPr(229) DAB)(py)524 cis-Cr(CO)4(NHC)2 (236– trans-Cr(CO)4(NHC)2523 239) fac-Mn(bpy)Br(CO)3 (232) mer-MnBr(bpy)(CO)3525 Fe(CO)5 (6)

fac-MnBr(CO)3(iPr-DAB) (229)

mer-MnBr(CO)3(iPrDAB)524

Excitation

Environment

MLCT (Nernst-glower, >320 nm) MLCT (Hg arc lamp)

N2, CH4 or Xe low temperature matrix

MLCT (Hg arc lamp)

Ne-2% Xe low temperature matrix

MLCT (laser flash photolysis, 353 nm)

Ar-2% Xe low temperature matrix Solution (PFMCH, PFMCH under air or PFMCH doped with c-C6H12)

MLCT (Nd:YAG laser, 355 nm)

supercritical fluid (scKr or scCO2)

MLCT (Nd:YAG laser, 355 nm)

Solution (propane, butane, cyclopentane, ethane or isopentane)

MLCT (TOPAS-C OPA, 300 nm)

Solution (PFMCH, PFMCH doped with Xe or CO2)

MLCT (Xenon arc lamp with filter, 337 nm)

Solution (cyclohexane in the presence of PEt3, PMe3, P(nBu)3, PPh3, pyridine, tBuNC or CH3CN

LMCT (Yb:KGW amplifier, 400 nm)

Solution (CH3CN)

MLCT (Hg high pressure lamp)

Solution (THF with pyridine, a-picoline or 2,6lutidine)

MLCT/XLCT (optical parametric amplifier, 500 nm) MLCT (Hg high pressure lamp)

Solution (pyridine) Solution (THF)

MLCT (Nd:YAG laser with frequency doubling, Solution (THF) 532 nm) MLCT/XLCT (OPA amplifier, 500 nm) Solution (CH2Cl2)

with noble gas elements (Ne, Xe, Ar). Cr(CO)5 only occurs in a ligated form in its singlet state while Cr(CO)4 in its triplet state is less reactive due to a large geometrical reorganization and the required spin change for complexation.515 Kinetic investigations of photochemically treated 5 in perfluoromethylcyclohexane (PFMCH)516 and in supercritical media sc-Kr or sc-CO2517 determined the lifetime of the formed Kr and CO2 complexes. Spectroscopic investigations of manganese alkane complexes produced by photochemical CO dissociation of 233 in the presence of alkanes revealed different isomers with the alkane coordinating via a CeH bond of either the methyl or the methylene groups.518,519 Triplet adducts of 3Mn(CO)2(Cp) with alkanes, Xe or CO2 play a role in complexation kinetics, and account for the different experimentally observed timescales.520 In the presence of phosphanes, pyridines or nitriles, 6 undergoes photosubstitution of two CO ligands. A labile monosubstituted triplet intermediate that competitively loses CO and relaxes via ISC to the singlet GS is responsible for the substitution of two CO ligands. At low ligand concentrations, the quantum yield for generation of mono- and disubstituted products exceeds one, because coordinatively unsaturated intermediates and 6 produce Fe2(CO)9 which thermally reacts with donor ligands to form two mononuclear complexes.521 Light-induced CO substitution by a solvent molecule is observed in fac-Mn(CO)3(NCCH3)(tryp) 234 (tryp ¼ tryptophanato(1–); Scheme 22) upon LMCT/LL0 CT excitation from the indolyl group to orbitals based on the metal and the carbonyl/NCCH3 ligands. Ultrafast CO dissociation and ISC produce a vibrationally hot triplet intermediate which subsequently forms a long-lived species, assigned to all-cis-Mn(CO)2(NCCH3)2(tryp) (s ¼ 20 ps).522 A solvent molecule or a counterion can fill the vacancy in the

758

d-d and charge transfer photochemistry of 3d metal complexes

coordination sphere of the metal. Upon excitation, the ferracyclobutadiene carbonyl complex [Fe(k2-C3(NEt2)3)(CO)3]þ 235D (Scheme 22) relaxes non-radiatively to a vibrationally hot triplet GS from which CO dissociation occurs (Fig. 11D). Ultrafast consecutive substitution of a CO ligand by a solvent molecule (e.g., CH2Cl2) is most likely followed by thermal substitution of the solvent by a counterion such as BF4.526,527 Pseudo-octahedral cis-dicarbene carbonyl complexes cis-Cr(CO)4(NHC)2 236–239 (NHC ¼ 1,3-dimethyl-4-imidazoline-2ylidene, 1,2-dimethyl-4-pyrazoline-3-ylidene, 2,4-dimethyl-1,2,4-triazoline-3-ylidene, 1,3-dimethyl-benzimidiazoline-2-ylidene; Scheme 22) convert into trans isomers by photoinduced isomerization. The mechanism can be summarized as photolytic CO dissociation, followed by a pseudo-rotation of the bulky carbene ligands into the kinetically favored trans configuration of the fivecoordinate complex and a final re-coordination of CO. The isomerization reactions are reversible via thermal rearrangement of the trans isomers into thermodynamically more stable cis isomers.523,528 Excitation of fac-Mn(bpy)Br(CO)3 232 (Scheme 22) leads to CO dissociation within the MC state allowing isomerization to mer-Mn(bpy)Br(CO)3. On the other hand, the mer isomer undergoes homolysis of the MneBr bond from a sbp* state after CT excitation with metal/halide-to-bpy character. The different behavior of the isomers originates from different CT energies with the fac isomer possessing a smaller energy difference to M– CO dissociative MC states.525 In the presence of coordinating solvent molecules, substitution of the CO ligand can compete with isomerization. For example, excitation of fac-MnBr(CO)3(iPr-DAB) 229 (iPr-DAB ¼ 1,4-diisopropyl-1,4-diazabutadiene; Scheme 22) in an MLCT/XLCT state (dp(Mn)/pp(Br) / p*(iPr-DAB; XLCT ¼ halogenido-to-ligand transfer) leads to CO dissociation and migration of the bromido ligand from an axial to an equatorial position depending on the donor strength of the solvent.524 Substitution of CO by pyridine has been observed for cis-dicarbene complexes of chromium.523,528 Beyond the application of CO photolysis in preparative organometallic chemistry, the catalytic activity of the resulting unsaturated species, e.g., the capability to activate covalent bonds, is of particular interest. Either a single photon initiates the catalytic cycle to form the catalytically active species from the precatalyst (photo-induced catalysis), or photons are required for every catalytic cycle to maintain turnover (photo-assisted catalysis). An example of photo-induced catalysis is the chemoselective reduction of carboxylic acid esters to aldehydes with secondary silanes as reductants. UV activation of the precatalyst Fe(CO)4(IMes) 240 (IMes ¼ 1,3-dimesityl-4-imidazoline-2-ylidene; Scheme 23) forms the 16-electron unsaturated NHC iron(0) complex Fe(CO)3(IMes). The four-coordinate active species oxidatively adds the silane (diethyl- or diphenylsilane) to yield a hydrido silyl iron complex capable of inserting the ester carbonyl group of the substrate in the FeeH or FeeSi bond. Reductive elimination affords the silylated acetal and regenerates the active iron species.529 The mixed carbonyl phosphane complex trans-Fe(CO)3(PPh3)2 241 enables photo-assisted catalytic dehydrogenation of formic acid under ambient light. Formation of the active species [Fe(CO)xH(PPh3)y] as well as dissociation of phosphane and H2 during turnover require irradiation. DFT calculations confirm that PPh3 elimination to give the active hydrido iron species is energetically favored compared to CO dissociation, although additional initiation steps cannot be fully excluded.530 The eliminated CO molecule is the product of interest in carbon monoxide releasing molecules (CORMs). Advantages of photoCORMs over conventional CORMs activated by hydrolysis include precise spatial and temporal control over CO release, and a higher persistency of the photo-CORM in the dark. A challenge for the design of CORMs remains to enable an appropriate absorption wavelength due to the inverse correlation between tissue penetration depth of light and the incident light energy.30,32,531,532 [Mn(CO)3(tpm)]þ 242D (tpm ¼ tris(pyrazolyl)methane; Scheme 23) releases two equivalents of CO upon MLCT excitation at 365 nm within a timescale compatible with biological systems (20 min, myoglobin assay) and causes significant light-induced cytotoxic effects on human colon cancer cells.531,533 Both CO ligands are not released simultaneously and excess excitation energy is distributed among vibrational modes of the initial dicarbonyl photoproduct.534 The peptide bioconjugate of the photo-CORM [Mn(CO)3(tpmR)]þ 243D (R ¼ propargyl ether, Scheme 23) features CO release properties similar to the parent TMC 242D.535 In the tricarbonyl complexes 244D–248D with tris(imidazol-2-yl)phosphane (2-tip) and tris(2-isopropylimidazol-4-yl)phosphane (4-tip) ligands (Scheme 23) the substitution pattern influences the primary CO release efficiency and the reactivity of the dicarbonyl intermediate towards further photolytic CO release.536 The tricarbonyl polypyridyl manganese(I) functionalized metallodendrimer 249 (Scheme 23) liberates at least two CO ligands per Mn(CO)3 moiety upon MLCT photoactivation (dp(Mn) / p*(bpy)), whereby the individual complex headgroups act as independent chromophores.537 An approach to achieve red/NIR excitation exploits chelators with extended aromatic p-systems. fac-MnBr(CO)3(Dipp-DAB) 230 (Dipp-DAB ¼ N,N0 -bis(2,6diisopropylphenyl)-1,4-diaza-1,3-butadiene; Scheme 22) shows an MLCT/XLCT absorption band (dp(Mn)/pp(Br) / p*(iPr2Ph-DAB)) at 582 nm close to the therapeutic window. The CO photodissociation rate is high due to the labilized MneCO bond caused by the steric bulk around the metal center.538 Visible light-activated CO release is further promoted in the bimetallic complexes [(bpy)2Ru(m-dpp)MnBr(CO)3]2þ 2502D and [(bpy)2Ru(m-bpm)MnBr(CO)3]2þ 2512D (dpp ¼ 2,3-bis(2-pyridyl)pyrazine, bpm ¼ 2,20 -bipyrimidine; Scheme 23) at 627 nm through MLCT excitation (dp(Ru) / p*(dpp/bpm) and dp(Mn) / p*(dpp/bpm)) followed by decomplexation of manganese(II) and further CO release upon continued irradiation.539

8.18.4.2

NO isomerization and dissociation in 3d TMCs

Beyond the diatomic molecule CO in biology, nitric oxide NO plays a major role in bioregulation of cardiovascular and nervous systems and participates in the body’s immune response to pathogen invasion.540–542 Furthermore, NO can be used in radiation treatment of tumors, and has shown promise in other cancer chemotherapeutics.543–546 Consequently, there is significant interest in photo-NORMs (NO releasing molecules) that can be used to photochemically deliver this bioregulatory diatomic molecule to

d-d and charge transfer photochemistry of 3d metal complexes

Scheme 23 Molecular structures of carbonyl iron (240) and manganese complexes (242D–2512D) employed in photo-induced catalysis and as photo-CORMs. Leaving CO ligands are highlighted in blue.

759

760

d-d and charge transfer photochemistry of 3d metal complexes

specific physiological targets.547 In addition to light-induced NO release from nitrosyl or nitrito TMCs, some NO complexes undergo light-activated NO linkage isomerization (kN, kO and side-on kN,kO).548 Nitroprusside [Fe(CN)5(NO)]2 2522L (commercialized as sodium salt: sodium nitroprusside SNP) shows NO linkage isomerism in crystals irradiated at 50 K. The GS and two metastable states correspond to minima in a non-dissociative end-over-end isomerization (Scheme 24A). Irradiation of the linear nitrosyl GS with visible light is followed by luminescence or relaxation to a side-on bound kN, kO-NO ligand.549 This structure differs from that of the traditional bent nitrosyl ligand as the metal oxygen distance approaches the range of a covalent bond.550 The kN, kO isomer reversibly converts to a linear isonitrosyl isomer (kO) through continued irradiation. MLCT excitation (dp(Fe) / p*(NO)) of the linear or side-on isomers produces an excited state with slightly distorted or bent equilibrium geometry. Specific excitation wavelengths (l1 ¼ 350–600 nm, l2 ¼ 600–1200 nm) populate the three isomeric states (Scheme 24A).549,551 A related side-on kN, kO isomer with a compressed NieNeO bond angle of 92 compared with 179 for the kN isomer forms by excitation of Ni(Cp*)(NO) 253 (Scheme 24B), whereas the linear isonitrosyl isomer was only theoretically predicted.550 Mechanistic studies of NO photodissociation using velocity-mapped ion imaging showed that the bending in the excited state arises from a Jahn-Teller distortion prior to dissociation.552 The iron nitrosyl porphyrine Fe(NO)(por) 254 (por2 ¼ porphyrinato(2); Scheme 24C) represents the first example of photoinduced NO linkage isomerism in five-coordinate {FeNO}7 complexes. Upon irradiation, the bent nitrosyl isomer transforms into a bent isonitrosyl isomer, while experimental or theoretical evidence for a side-on kN,kO-isomer is lacking in this particular complex.553 Several strategies can photochemically release nitric oxide from a TMC. Excitation of a nitrosyl complex leads to M–NO cleavage (direct release), while excitation of nitrito complexes releases NO by N–O cleavage within the coordinated kO-nitrite (indirect release). Both scenarios can also be initiated by appended photosensitizers with a chromophoric substituent (direct and indirect sensitized release, cf. Section 8.18.3.2) (Scheme 25).547

Scheme 24 (A) Stable and metastable isomers of 2522L by photoinduced reversible linkage isomerism549 and molecular structures of nitrosyl nickel (B) and iron (C) complexes (253 and 254) undergoing photoisomerization. Involved NO ligands are highlighted in blue.

Scheme 25

Direct and indirect strategies for photochemical NO release.547

d-d and charge transfer photochemistry of 3d metal complexes

761

Prominent examples of direct NO release under aerobic conditions are the red [Fe2(NO)4S2]2 2552L and black [Fe4(NO)7S3] 256L Roussinate anions (Scheme 26A) with relatively small quantum yields for NO generation. The O2 oxidant intercepts key intermediates after NO elimination and prevents NO re-addition to reform the original iron-sulfur cluster.554,555 TDDFT calculations show that LMCT transitions (NO(p*) / Fe(dp)) around 374 nm are responsible for the photochemical reactivity of 2552L.556 Calculations on 256L predict that excitation around 350–370 nm mostly leads to CT from the [Fe4(NO)6S3] core to the apical NO, whereas excitation around 290–320 nm results in CT towards basal and axial NO ligands. This explains the different photoproducts through NO loss from the apical or basal positions.557,558 In Roussin‘s red salt esters Fe2(NO)4(m-SR)2 257–260 (R ¼ Me, Et, CH2Ph, CH2CH2OH; Scheme 26A), LMCT excitation (p*(NO) / dp(Fe)) and initial NO dissociation lead to decomposition of the cluster yielding up to four equivalents of NO.556,559 Roussin‘s red-salt ester with a pendant porphyrin chromophore 262 (Scheme 26A) shows sensitized direct NO release. 262 efficiently generates NO by excitation at longer wavelength. The strong absorption of the pendent porphyrin Q bands at 546 nm is followed by EnT (Section 8.18.3.2) to the iron nitrosyl, leading to

Scheme 26 (A) Molecular structures of nitrosyl iron complexes 2552L–262 and (B) nitrito manganese complex 263, nitrosyl manganese complexes 264D, 266D–272D and nitrosyl iron complex 2652D. Leaving NO moieties are highlighted in blue or red.

762

d-d and charge transfer photochemistry of 3d metal complexes

emission quenching of the porphyrin and subsequent photodecomposition of the cluster.560 Fluorescein derivatized Roussin’s redsalt ester 261 (Scheme 26A) enables sensitized direct NO release with a high rate of photochemical NO generation at 436 nm irradiation.561 Photosensitized generation of NO using NIR light via two-photon excitation occurs at 800 and 810 nm for 261 and 262, respectively (Section 8.18.3.2).562,563 The thermally stable complex trans-[Cr(cyclam)(ONO)2]þ 203D shows indirect NO generation under aerobic conditions via photolytic cleavage of coordinated nitrite, following population of a 4MC state. In the presence of O2, re-coordination of NO is prevented by trapping the putative chromium(IV) intermediate to give a stable chromium(V) complex.564 Direct photosensitization experiments indicate that a substantial fraction of NO generation occurs along a doublet pathway after ISC from the 4MC state to doublet states. Thus, dissociation to diamagnetic CrIV(cyclam)(NO2)O and doublet NO as primary photoproducts occurs on the doublet PES with spin conservation.565 The quantum yield is independent of the excitation wavelength confirming that the reactive state is likely the lowest (thermalized) MC excited state.107 An advantage associated with nitrito chromium complexes compared to manganese complexes (see below) is the greater efficiency of NeO bond cleavage even under excitation with relatively low energy light due to the higher oxophilic character of chromium.547 In analogous dinitrito chromium(III) complexes trans-[Cr(cyclam-R)(ONO)2]D 204D and 205D with R ¼ anthracenyl or pyrenyl appended to the cyclam, the organic chromophores serve as light-harvesting antennae and sensitize the intramolecular photoreaction at the metal center through EnT from ligand-centered pp* states to MC states (Section 8.18.3.2).432 Enhanced indirect NO photogeneration from 203D is achieved via electrostatic assembly of the cationic chromium(III) complex on anionic surfaces of water-soluble CdSe/ZnS core/shell quantum dots as sensitizers.566,567 The nitrito porphyrin manganese complex Mn(NO2)(TPP) 263 (Scheme 26B) undergoes MneO bond cleavage and loss of the nitrito ligand instead of NO dissociation.568–570 Efficient NO delivery systems with large NO quantum yields under continuous NIR excitation are the direct photo-NORMs [Mn(NO)(PaPy3)]þ 264D and [Mn(NO)(PaPy2Q)]þ 266D (PaPy3H ¼ N,N-bis(2pyridylmethyl)amine-N-ethyl-2-pyridine-2-carboxamide, PaPy2QH ¼ N,N-bis(2-pyridylmethyl)-amine-N-ethyl-2-quinoline-2carboxamide; Scheme 26B).571 Binding of NO in the iron complex [Fe(NO)(PaPy3)]2þ 2652D, which is isoelectronic to 264D (Scheme 26B), is fully reversible and NO is photolabile through LMCT excitation (s(PaPy3) / ds*(Fe)) with a 25 W tungsten lamp.572,573 For 264D, direct excitation to dissociative MLCT states (dp(Mn) / p*(NO)) or indirect dissociation within low lying 3 MLCT states (dp(Mn) / p*(NO)) after initial population of MLCT states of the coligand (dp(Mn) / p*(PaPy3/PaPy2Q)) and ISC are conceivable mechanisms.574 Related [Mn(NO)(SBPy3)]2þ 2672D and [Mn(NO)(SBPy2Q)]2þ 2682D (SBPy3 ¼ N,N-bis(2pyridylmethyl)amine-N-ethyl-2-pyridine-2-aldimine, SBPy2Q ¼ N,N-bis(2-pyridylmethyl)amine-N-ethyl-2-quinoline-2-aldimine; Scheme 26B) with imine donors trans to NO are photoactive towards NO release at a long wavelength excitation (800– 950 nm).575 The substituted complexes [Mn(dpaqR)(NO)]þ 269D–272D (R ¼ H, Cl, OMe, NO2; Scheme 26B) show that the efficiency of NO release relates to the substituent-dependent extinction coefficient, rather than to p-backbonding effects of the substituent.576

8.18.4.3

CO2 dissociation from carboxylato 3d TMCs

Photodecarboxylation of coordinated ligands in carboxylato complexes of the type [L4Mm(X(CH2)xCO2)]nþ (with X ¼ NR2, CO2) can be understood in terms of the radical pair model.577,578 Irradiation with light in the region of the LMCT band of LnMm-carboxylato complexes results in homolysis of the MeO bond, reduction of Mm to Mm1 and a solvent surrounded radical pair. The unstable alkylcarboxyl radical loses carbon dioxide forming an alkyl radical, which reoxidizes Mm1 to an organo-Mm complex (Scheme 27A).579 Photodecarboxylation followed by ring contraction to an organocobalt complex with a three-membered chelate ring was first observed in the aminocarboxylato complexes [Co(bpy)2(gly)]2þ 2722D and [Co(gly)(phen)2]2þ 2742D (Scheme 27B).580–582 The photochemical ring contraction of aminocarboxylato complexes appears to be a general reaction, and metallacyclic complexes were prepared by photodecarboxylation of [Co(gly)(tpa)]2þ 2752D as well as N-substituted complexes [Co(dtma)(phen)]2þ 2762D and [Co(bpg)(phen)]2þ 2772D (gly ¼ glycinato(1), tpa ¼ tris(2-pyridylmethyl)amine dtma ¼ 1-diethylene-triaminemonoacetato(1), bpg ¼ N,N-bis(pyridyl-methyl)glycinato(1); Scheme 27B).583,584 The contraction of aminocarboxylate chelate rings of different sizes in [Co(en)2(gly)]2þ 2782D, [Co(b-ala)(en)2]2þ 2792D and [Co(g-but)(en)2]2þ 2802D (gbut ¼ g-aminobutanoato(1); Scheme 27B) results in metallacyclic photolysis products of different stability depending on the chelate ring strain (Scheme 27A). Finally, the labile cyclometalated product can decompose via hydrolysis or a spontaneous redox process to cobalt(II) and ligand radicals.579 The product distribution after secondary reactions between the by-products of the photolysis shows a solvent dependence based on different electrochemical properties of the involved complexes in different solvents as investigated for 2722D and [Co(ala)(bpy)2]2þ 2732D (ala ¼ alaninato(1); Scheme 27B).585 An advantage of the photochemical strategy is that the synthesis of cyclometalated complexes is not limited to ortho-metalated complexes but could be expanded to TMCs with simple aminoalkyl ligands in aqueous solution under aerobic conditions.579,586–588 Beyond aminocarboxylato cobalt(III) complexes, photodecarboxylation following LMCT excitation can also take place for complexes of other metal ions, for other heteroatoms in the chelate ring and for chelate rings with a second carboxylato donor (Scheme 27A, X ¼ CO2), for example malonato and oxalato complexes of copper(II) and iron(III).2,589,590 Spectroscopic measurements on copper(II) complexes with amino carboxylato ligands derived from glycine, glutamic acid and b-alanine show that high energy excited LMCT states of p(NH2)/p(CO2) / ds*(Cu) character undergo an efficient conversion to lower photoreactive LMCT states (p(CO2) / ds*(Cu)) followed by fast decarboxylation.591,592 During the photodecarboxylation of oxalato cobalt(III) complexes [Co(NH3)5(kO-ox)]þ 281D, [Co(NH3)4(k2O-ox)]þ 284D and [Co(en)2(k2O-ox)]þ 285D (Scheme 27C) participation

d-d and charge transfer photochemistry of 3d metal complexes

Scheme 27 (A) General photodecarboxylation reaction of aminocarboxylates and dicarboxylates with chelates of different ring sizes,579 (B) molecular structures of aminocarboxylato cobalt complexes 2722D–2802D and (C) molecular structures of carboxylato and oxalato cobalt(III) complexes 281D–285D, of carbonato and formato copper complexes 286–289L and of the carboxylato iron(III) complex 2902D. Leaving CO2 moieties are highlighted in blue, when unambiguously known.

763

764

d-d and charge transfer photochemistry of 3d metal complexes

of an excited MC state is assumed, which labilizes the coordinated oxalate ion.593 A unique system for reversible carbon dioxide binding is the carbonato-bridged copper dimer Cu2Cl2(m-CO3)(phen)2 286 (Scheme 27C). Decarboxylation through excitation in the spectral region of chlorido-to-copper and oxido-to-copper CT transitions delivers CO2 and a m-oxido copper(II) complex. Thermal re-association of CO2 regenerates 286.594 Complexes with monodentate carboxylato ligands can be photolyzed with LMCT excitation to corresponding transient s-organometallic TMCs.595–597 This photolysis can also be exploited as a source for alkyl radicals that form MeC s bonds as shown for the photodecarboxylation of [Co(NH3)5(OAc)]2þ 2822D and [Co(NH3)5(O2CC2H3)]2þ 2832D (Scheme 27C).598,599 In the formato copper complexes [Cu2(HCO2)3] 287L, [Cu(HCO2)3] 288L and [Cu2(HCO2)5] 289L (Scheme 27C) with copper in different oxidation states, MC/LMCT transitions with ds*(CuI) / s/p(CuI) or dp(CuII)/p(HCO2) / ds*(CuII) character are followed by fast IC through conical intersections back to the GS. The redistributed energy is available for statistical dissociation in the electronic GS (Fig. 11C) inducing different decomposition channels with several elimination products including carbon dioxide, neutral copper, formic acid or copper(II) formate Cu(HCO2)2.600 The rate of photodecarboxylation and therefore light sensitivity could be enhanced in situ through transient formation of a more photoreactive adduct. For the iron(III) complex fac-[Fe(tpena)]2þ 2902D (Scheme 27C) photodecarboxylation is accelerated by addition of thiocyanate forming a seven-coordinate heteroleptic adduct with a significantly increased LMCT absorption. The additional anionic charge of the thiocyanato coligand facilitates creation of a vacant site via decarboxylation and O2 insertion in the FeeC bond.601 Photochemical decarboxylation of oxalates is used in preparative photochemistry and photocatalysis for actinometric measurement of radiant fluxes in photoreactors (Hatchard-Parker actinometer).602 Optical excitation of the ferrioxalate anion [Fe(ox)3]3 2913L from the high-spin GS to LMCT states dissociates a vibrationally hot CO2 molecule leaving a pentacoordinate dioxalato ferrous complex with a bent carbon dioxide radical anion ligand (Scheme 28).604 The CO2-derived ligand coordinates end-on (kO) to the metal center in this intermediate and represents a photoinduced reductively activated form of CO2 that offers carbon-centered chemical reactivities.603 Since the initial FeeO and CeC bond breakages compete with IC processes, the quantum yield for the initial photoreduction is limited to approximately 69%. The ferrous intermediate exists in three different isomeric forms, namely trigonal-bipyramidal with axial OCO–, square-pyramidal with apical OCO– and trigonal-bipyramidal with equatorial OCO– which isomerize within 40 ps.603,605 Subsequent loss of CO2– from the coordination sphere yields the long-lived square-planar dioxalatoferrate(II) [Fe(ox)2]2 and a carbon dioxide radical anion (Scheme 28).604 The radical anion CO2– reduces a further [Fe(ox)3]3 2913L under release of oxalate. In a catalytic application, the alkyl chain of aliphatic carboxylic acids can be transformed to a nucleophilic radical through photodecarboxylation of carboxylato iron(III) complexes. Light-assisted ligand-accelerated iron photocatalysis enables a decarboxylative alkylation of heteroarenes and CeC/CeN bond formation with Michael acceptors and azodicarboxylates, respectively. The catalytic system consists of iron(III) ligated by 2-picolinate (heteroarene coupling) or di(2-picolyl)amine (CeC and CeN bond formation) which coordinate the carboxylate. Visible light induced LMCT excitation of the carboxylato iron(III) complex generates an iron(II) species and an unstable carboxyl radical which delivers the reactive alkyl radical through decarboxylation. The

Scheme 28 Mechanism of the photodecarboxylation of 2913L including changes of coordination number and geometry.603 Leaving CO2 and COC– 2 moieties are highlighted in blue and red, respectively.

d-d and charge transfer photochemistry of 3d metal complexes

765

intermediate iron(II) complexes are easily reoxidized to iron(III) to close the catalytic cycle. A diverse range of carboxylic acids readily undergoes radical decarboxylation and couples with a broad scope of heteroarenes and alkyl radical acceptors.606,607

8.18.4.4

Nx dissociation from azido 3d TMCs

Azido TMCs are prone to a variety of photodissociations (Fig. 11) such as M–N homolysis, M–N heterolysis and photooxidation via Na–Nb splitting (Scheme 29). The pathway depends on the nature of the electronic excitation (MC, MLCT, LMCT, etc.). MC excited states are prone to N3 dissociation, LMCT states should facilitate homolysis as N3 is preformed and ILCT states should lead to photooxidation with N–N splitting.608 However, in many cases the complexity of the PES prohibits a clear-cut assignment and several pathways may be available simultaneously for a given excitation energy. This section examines key primary pathways and associated competing reactions, while thermal follow-up reactions, e.g., dimerization or ligand association, will not be discussed in detail. In the last two decades, the photochemistry of azido complexes including follow-up chemistry has been reviewed.608,609 LMCT excitation of azido complexes is expected to cleave the MeNa-bond leaving the metal center formally reduced, and with an unsaturated coordination sphere (Scheme 29, homolysis).608 The released azidyl radical N3 was detected indirectly by spintrapping610–613 and spectroscopically by its asymmetric stretching vibration at 1660 cm1.614 Photogenerated N3 from ferric azides has been used as an initiator in radical polymerization reactions.615 The generation of coordinatively unsaturated species sparked interest in the use of azido complexes as catalyst precursors. Vitamin B12 model complexes CoIII(chelate)(py)(N3) 292–294 (with chelate ¼ salen (N,N0 -ethylene-bis(salicylidenimine)), salphen (N,N0 -o-phenylene-bis(salicylidenimine)), (dmgH)2, dmgH2 ¼ dimethylglyoxime; Scheme 30) photogenerate cobalt(II) complexes upon LMCT excitation which are capable of catalytic dihydrogen formation from thiols, while population of MC states is unproductive.610,616 CuIICl2(dap) 295 featuring 2,9-bis(p-anisyl)-1,10-phenanthroline (dap) operates as precatalyst in the oxoazidation of vinyl arenes with TMSN3 and O2. The active species CuI(dap)X and the azidyl radicals are generated in situ by irradiation of [CuII(dap)X(m-N3)]2 296 or 297 with X ¼ OH, Cl.617 In rare cases, the azido ligand merely acts as a spectator while a different photoreduction occurs. The ion pair {[CoIII(NH3)5(N3)]2þ [BPh4]}þ 298D undergoes ion pair charge transfer (IPCT) following irradiation of the corresponding band to yield {[CoII(NH3)5(N3)]þ [BPh4]}þ.618 The simplest case of photoreactivity in a TMC is ligand dissociation (heterolysis), which is usually induced by MC excitation (Scheme 29).608 Occupation of anti-bonding orbitals in most MC states weakens the MeL bond. LMCT excitation of azido complexes can lead to a stepwise redox-neutral mechanism composed of reductive homolysis and SET (Scheme 29).619 [CrIII(NH3)5(N3)]2þ 2992D undergoes photoaquation to [CrIII(H2O)(NH3)4(N3)]2þ and homolytic cleavage of the CreN3 bond after MC and LMCT excitation, respectively.620 However, the photoproducts are not always directly associated with the character of the initially excited state (MC, LMCT). Photodissociation is sometimes observed after LMCT excitation as an additional pathway to photoreduction or oxidation, for example in the azido complexes [FeIII(cyclam)(N3)2]þ 300D (Scheme 30)619 and [FeIII(cyclamac)(N3)]þ 8D (Scheme 1B).614,621 Similarly, ultrafast UV-pump/IR-to-Vis-probe spectroscopy revealed that LMCT excitation (355 nm) of the diazido cobalt(III) complex [CoIII(cyclam)(N3)2]þ 301D (Scheme 30) mainly leads to ground state recovery (95%) and heterolytic Co–N cleavage followed by coordination of DMSO (5%).622 Dinitrogen is a superior leaving group resulting from its extraordinary stability. Thus, irradiation of azido complexes can lead to cleavage of the NaeNb-bond (Scheme 29) with subsequent loss of the preformed dinitrogen. This N2 release from azido complexes relates to the expulsion of NO from nitrito complexes (Section 8.18.4.2). While upon NO release from Mn–ONO TMCs, the metal is singly oxidized forming the Mnþ1]O fragment (Section 8.18.4.2), N2 dissociation from Mn–N3 TMCs, formally oxidizes the metal center twice giving the nitrido complex Mnþ2^N. Nitride N3 is a very strong p-donor ligand and thus stabilizes high oxidation states of the metal. The photogenerated nitrido TMCs typically exhibit a five-coordinate square pyramidal geometry due to the high trans influence of N3, while only a few octahedral nitrido complexes have been observed.608 The double oxidation of the

Scheme 29

Photolytic pathways of azido TMCs.

766

d-d and charge transfer photochemistry of 3d metal complexes

Scheme 30 and 308D.

Molecular structures of azido cobalt, iron and manganese complexes 292, 293, 294, 300D, 301D, 302D, 3032D, 304, 305, 306D, 307D

metal upon N2 loss can be exploited for the synthesis of high-valent metal complexes under mild conditions. Much attention was devoted to the photochemical synthesis of nitrido chromium(V) and manganese(V) complexes with cyclam,623–625 porphyrinato,626–628 phthalocyaninato,629 Schiff base ligands630 and derivatives as equatorial co-ligands. Nitrido iron(V) complexes served as models for intermediates of enzymatic cycles.631,632 The photochemistry of azido iron complexes with cyclam, cyclam-ac and Me3cyclam-ac (Me3cyclam-ac ¼ 4,8,11-trimethyl1,4,8,11-tetraazacyclotetradecane-1-acetato(1)) ligands has been thoroughly investigated using ultrafast time-resolved spectroscopy.614,619,633–636 Irradiation of [Fe(cyclam)(N3)2]þ 300D at 266 nm at room temperature in acetonitrile leads to photoreduction as well as photooxidation. The photooxidation product [Fe(cyclam)(N3)N]þ has a lifetime of z350 ms and was detected by quenching experiments with PnBu3.619 Time-resolved FTIR spectroscopy using 266 nm excitation pulses revealed that 300D undergoes a non-adiabatic transition to the vibrationally excited GS, and vibrationally cools within 7.5  1.0 ps. A fraction of these molecules in vibrationally hot states evolves to the FeV complex via NaeNb bond-cleavage and subsequent photooxidation within 10.0  0.5 ps (Fig. 11D). The excess energy is funneled in kinetic energy of N2 thus leaving the FeV complex vibrationally cold.633 The quantum yield of the photooxidation is therefore governed by the competition between vibrational quenching and vibrational cooling633 and may be further enhanced by selective pre-excitation of the asymmetric azide stretching vibration.637 [FeIII(cyclam-ac)(N3)]þ 8D shows wavelength dependent photoreduction, photooxidation and redox neutral heterolytic FeeN bond cleavage in solution within 200 fs at room temperature (Fig. 14).635 The absorption band of 8D at 460 nm was assigned to

d-d and charge transfer photochemistry of 3d metal complexes

767

Fig. 14 Photophysical processes of 8D in acetonitrile at room temperature after irradiation at different wavelengths determined via fs-UV/Vis pump mIR probe spectroscopy. The colored numbers represent upper limits for the quantum yields. The energy barrier for N–N cleavage is 14,000 cm1 according to DFT calculations.635

LMCT transitions, whereas excitation below 350 nm populates states of currently unidentified nature.614 A detailed discussion of the experimental data on various temporal resolutions has been reviewed.632 Upon LMCT excitation at 450 nm only heterolytic bond cleavage was observed on a fs-time scale with 4% quantum yield (Figs. 11A, 14 and 15).635 Figs. 14 and 15 illustrate the connection of the two-dimensional PESs along the N–N and Fe–N reaction coordinates for all investigated excitations and for the LMCT excitation, respectively. Only high-energy photons with a wavelength of 266 nm induced photooxidation in solution at room temperature. A competing process is homolytic cleavage of the FeeN bond resulting in coordinatively unsaturated [FeII(cyclam-ac)]þ and azide radicals, which were identified by their asymmetric stretching vibration at 1660 cm1.635 Step-scan IR experiments at 266 nm irradiation showed the formation of azide anions after 500 ns with a relative quantum yield of 20%.614 Most fragments undergo geminate non-adiabatic transitions, recombination and vibrational relaxation within 13 ps.635 Another faster component of 2 ps is present for all three excitation wavelengths and was assigned to IC in an earlier report focusing on excitation at 266 nm.634

Fig. 15

Schematic representation of the processes after LMCT excitation of 8D at 450 nm.635

768

d-d and charge transfer photochemistry of 3d metal complexes

While LMCT excitation at 450 nm at room temperature only led to redox-neutral photodissociation, the photooxidation product [FeV(cyclam-ac)(N)]þ appeared in high yields after continuous irradiation of 8D at 419 nm in frozen acetonitrile solution.621 It has been suggested that a small fraction of the excited molecules is trapped in high-energy quartet or sextet states with long enough lifetimes to absorb a second photon enabling the photooxidation pathway, with accumulation of the nitrido iron(V) complex. This mechanism is not available in laser flash photolysis experiments in solution, due to needed renewal of the sample after each pulse.635 The methylated analog [FeIII(Me3cyclam-ac)(N3)]þ 302D (Scheme 30) shows spin crossover at around 100 K in condensed phases with a low-spin doublet ground state.638 In frozen solutions of 302D photoreduction to high-spin [FeII(Me3cyclamac)(NCCH3)]þ and N3 occurs independent of excitation wavelength.636,639 The azidyl radical can be quenched with azide ions to yield N6–.636 Furthermore, CO2 was detected in time-resolved FTIR measurements of 302D which suggests a homolytic cleavage of the FeeO and CeC bonds likely followed by the formation of [FeII(Me4cyclam)(NCCH3)(N3)]þ (Me4cyclam ¼ 1,4,8,11tetramethyl-1,4,8,11-tetraazacyclotetradecane; Section 8.18.4.3).636 Interestingly, 302D can also be reversibly oxidized to the azido iron(IV) complex 3022D. Irradiation of a frozen solution of electrochemically generated 3022D in acetonitrile and [nBu4N][PF6] at 650 nm led to photooxidation, yielding a rare molecular iron(VI) species [FeVI(Me3cyclam-ac)N]2þ with a strongly covalent Fe^N triple bond (1.57  0.02 Å).639 The branching ratio between photooxidation and photoreduction was investigated using temperature dependent ion spectroscopy. The pseudo-octahedral complexes 300D, 8D, [FeIII(MePy2tacn)(N3)]2þ 3032D and 302D (MePy2tacn ¼ N-methyl-N,N-bis(2picolyl)-1,4,7-triazacyclononane; Schemes 1B and 30) possess doublet and sextet states with similar energy which are thermally populated. The spin state influences the preferred relaxation pathway after excitation (Fig. 16). The doublet state favors the formation of the nitrido iron(V) complex whereas the almost isoenergetic sextet state leads to photoreduction. Thus, the complexes 300D, 8D and 3032D with an S ¼ ½ ground state mostly showed N2 loss at low temperatures. Conversely, a reverse temperature dependence was found for 302D. Its sextet state is stabilized in the gas phase to become the GS.640 The ferrous azide FeII{PhB(tBuIm)3}(N3) 304 can be photooxidized to the FeIV nitride and subsequently chemically oxidized to the iron(V) complex ([PhB(tBuIm)3] ¼ phenyl tris(3-tert-butyl-imidazol-2-yl)borato(1); Scheme 30).641,642 V t þ [Fe {PhB( BuIm)3}(N)] has a half-life of 4 h at room temperature in solution. Attempts to synthesize the analogous CoIV nitride from the CoII azide 305 failed and instead resulted in photoreduction (Scheme 30).643 The first crystallographically analyzed FeIV complexes [FeIV(TIMENR)N]þ (R ¼ mes, xyl: TIMENxyl ¼ tris[2-(3-mesityl-imidazol-2-ylidene)ethyl]amine, TIMENmes ¼ tris[2-(3xylyl-imidazol-2-ylidene)ethyl]amine) are air stable and were synthesized by photooxidation of the corresponding ferrous azides 306D and 307D (Scheme 30).644 Carbene ligands stabilize high-valent nitrido manganese complexes. For example, [MnIV(TIMENxyl)N]þ was prepared by irradiation of [MnII(TIMENxyl)(N3)]þ 308D in solution (Scheme 30).645 Electron-rich “late” TMs are not able to support strongly p-donating nitrido ligands, because of their inherent higher electron densities. Instead, their nitrido complexes are transient intermediates for N-migratory insertion reactions. CoII(BIMPNMes,Ad,Me )(N3) 309 ([BIMPNMes,Ad,Me] ¼ anion of bis[2-(3-mesityl-imidazol-2-ylidene)ethyl-(3-adamantyl-2-hydroxy-5-methylphenyl) methyl] amine) featuring a mixed tripodal carbene phenolato ligand yields a highly reactive CoIV nitride after irradiation and N2 loss. The nitrido complex undergoes N-migratory insertion into the CoeC bond (Scheme 31A).646 Irradiation of the azido nickel(II) complex 310 featuring a P^N^P pincer ligand leads to N2 expulsion and generates a transient nickel(IV) nitride with significant nitridyl character (Scheme 31B). This radical is trapped by insertion into the NieP bond to give an iminophosphorane donor.647 A similar insertion reaction followed by dimerization was observed for the analogous cobalt(II) complex 311 after UV photolysis (Scheme 31C).648 The complex [CoII(tbta)(N3)]þ 312D (tbta ¼ tris[(1-benzyl–1H–1,2,3-triazole-4-yl)methyl]amine; Scheme 31D) on the other hand is photostable because of a very fast non-radiative decay to the GS. The excess energy is repartitioned in a non-statistical fashion with the out-of-plane N3-bending vibration excited most (s z 10 ps) followed by in-plane bending and anti-symmetric stretching modes (s z 2 ps).649

Fig. 16

Spin-state dependence of the photoreactivity of pseudo-octahedral azido iron(III) complexes.640

d-d and charge transfer photochemistry of 3d metal complexes

769

Scheme 31 N-migratory insertion reactions of nitrido/nitridyl complexes photogenerated from (A) 309, (B) 310 and (C) 311 and (D) molecular structure of 312D.

Aside from monomeric nitrido TMCs, bridged binuclear650,651 or polymeric complexes652,653 were obtained after photolysis of azido metal complexes in some cases. For certain TMCs, the existence of singlet or triplet nitrene intermediates was rationalized. Indirect evidence was provided via quenching experiments, by exploiting the different reactivity patterns of these two spin states. The singlet nitrene features an empty p-orbital and is a strong Lewis acid, whereas triplet nitrenes are diradicals which often undergo abstraction and insertion reactions.608 (Singlet) nitrenes were postulated as initial photoproducts for several azido nickel(II) complexes like NiII(N3)2(PEt3)2 313 and NiIICl(N3)(PMe3)2 314.654–656 Photooxidation of CrIII(N3)(salchd) 315 gave species absorbing at 420 nm assigned as arising from triplet nitrene intermediates by laser flash photolysis and quenching experiments with oxygen and olefins (salchd ¼ 1,2-bis(salicylidenimino)cyclohexane; Scheme 32).657 The manganese(III) complex MnIII(N3)(salchd) 316 displayed similar behavior (Scheme 32).657 For the photolytic decomposition of Ni(N3)2(tet-a) 317

Scheme 32

Molecular structures of azido 3d TMCs 315, 316 and 317.

770

d-d and charge transfer photochemistry of 3d metal complexes

both singlet and triplet nitrene pathways were postulated (Scheme 32).658 Quenching experiments with HCl suggested that [Cr(NH3)5(N3)]2þ 2992D decomposes via a nitrene intermediate after irradiation with UV light.659

8.18.4.5

N2 splitting with 3d TMCs

Dinitrogen is readily available in the atmosphere and the key component in the synthesis of ammonia via the Haber-Bosch process.660 However, its non-basic, non-polar nature, as well as high dissociation energy (941 kJ mol1) and ionization potential (15.6 eV)661 pose a great challenge for its activation and N–N splitting. High pressures and temperatures are required to promote reactivity,662 while nitrogenase enzymes accomplish this feat at ambient conditions.663 Substantial effort has been dedicated to the synthesis of complexes capable of binding and activating N2.664 In order to weaken the NeN bond by coordination to a TM, electron density needs to be withdrawn from the NeN bonding orbitals via s-donation and increased in N–N anti-bonding orbitals (pacceptation).665 Several binding modes (end-on/side-on; terminal bridging) of N2 have been experimentally found to date,666 the formal charge being N2, N2, N22, N23 or N24 with varying spin states.667 Irradiation can increase a molecule’s internal energy without risking thermal decomposition. However, activation of bound dinitrogen strongly depends on the nature of the transition. An MC transition populating s*(M–N) orbitals is expected to lead to ligand dissociation while p*(N–N) orbitals are occupied following an MLCT, which weakens the NeN bond and increases the ligands basicity, facilitating protonation.665,668 Photolysis of N2 complexes can give complex reactivity patterns. In frozen matrices, a rearrangement without dissociation of the N2 ligand changes the binding mode following electronic excitation. In an argon matrix at 20 K, an MLCT excitation of the linear complex Ni0(h1-N2)2 318 can lead to population of singlet or triplet states (1Fu, 3Pu) which relax radiatively to the 1Sgþ ground state (592–633 nm) (Fig. 17).669 Alternatively, 318 can photoisomerize upon MLCT excitation to three- and four-coordinate Ni0(h1-N2)(h2-N2) and Ni0(h2-N2)2, respectively. A similar photoinduced change from h1-N2 to h2-N2 was found for TiO(N2) in matrix isolation IR studies after photolysis.670 Matrix isolation IR studies and DFT calculations revealed that NiIIX2 (X ¼ Cl, Br) forms [X2NiII $$$(N2)n] 319 featuring physisorbed N2 in a nitrogen matrix. This species could be converted to NiIIX2(h1-N2)2 with end-on coordinated N2 via photolysis at 8 K if the deposition (not the photolysis) was carried out below 10 K. CoBr2 behaved similarly under analogous conditions.671 These h1-N2/h2-N2 isomerization reactions of coordinated N2 are formally analogous to those of NO coordinated to TMCs (Section 8.18.4.2). The most common reaction of many systems designed for N2 photosplitting is expulsion of N2 following cleavage of an MeN bond upon irradiation as found in several Mo, W, and Os complexes.672–676 This is not limited to 4d/5d transition metal complexes and has been observed for dinitrogen chromium, iron and nickel complexes as well. Trans-Cr0(dmpe)2(N2)2 320 (dmpe ¼ 1,2-bis(dimethylphosphanyl)ethane; Scheme 33) undergoes ligand substitution with CO and H2 under irradiation with UV light to form cis-Cr0(CO)2(dmpe)2 and eight-coordinate CrIV(dmpe)2(H)4 with a proposed coordinatively unsaturated Cr0(dmpe)2 intermediate.679 Monometallic Cr–N2 species 321 obtained from metal atoms sprayed onto cold KBr windows in an argon/nitrogen matrix at 20 K form dinitrogen containing clusters upon extended irradiation. IR spectroscopy revealed that this process was accompanied by a weakening of the NeN bond for M ¼ Cr while the NeN bond is strengthened for M ¼ Ni (322) in the cluster.680 Trans-[FeII(depe)2(N2)X]þ 323D and 324D (depe ¼ 1,2-bis(diethylphosphanyl)ethane; X ¼ Cl, Br; Scheme 33) decomposed with laser irradiation as demonstrated by IR spectroscopy.681 Currently, seven 4d/5d TMCs (5x Mo, 1x W, 1x Os) show light-induced cleavage of N2, as summarized in a review,665 while N2 photosplitting with 3d TMCs has not yet been achieved. However, two iron complexes show increased turnover of N2 to NH3 when 0 irradiated. Fe2I (H)2(m-N2)(P2P Ph)2 325 and [Fe(N2)(P3B)] 326L catalyze ammonia formation from N2 under reducing acidic 0 conditions (P2P Ph ¼ ((phenylphosphanediyl)bis(2,1-phenylene))bis(diisopropylphosphane), P3B ¼ tris(2-(diisopropylphosphanyl)phenyl)borane; Scheme 33). Turnover increased significantly with irradiation from a mercury Hg lamp from 7.5  0.8 to 18.1  0.8 equiv. NH3 and from 60.0  3.7 to 88.1  8.0 equiv. NH3 for 325 and 326L, respectively. Yet this acceleration entails

Fig. 17 Jablonski diagram and the isomerization reaction coordinate Ni–N2 (abbreviated as rotation of N2 in the coordination sphere of Ni) of 318 in an argon matrix.669

d-d and charge transfer photochemistry of 3d metal complexes

771

Scheme 33 Molecular structures of TMCs with coordinated N2 320, 323D and 324D and dinitrogen iron complexes that show increased turnover of N2 to NH3 under irradiation. 327 and 328 form from 325 and 326L, respectively, under turnover conditions (HBArF4, KC8, N2, 78  C, Et2O).677,678

no direct photo-activation of N2.677 Instead, stoichiometric reactivity and freeze-quench Mößbauer experiments revealed that 325 0 forms off-path hydrido species Fe(H)y(N2)x(P2P Ph) such as 327 (Scheme 33). These are active in hydrogen evolution reactions (HER), which compete with the nitrogen splitting cycle. Irradiation leads to photoinduced H2 elimination (Section 8.18.4.7) 0 and formation of Fe(N2)2(P2P Ph) which catalyzes N2 splitting more efficiently when reduced to the mono- or dianion. Thus, the reaction pathway shifts from the HER to N2 splitting.682 Whether photo-excitation is involved in the N2 splitting reaction, remains unclear so far.677,682 326L forms the borohydride complex Fe(m-H)(H)(N2)(P3B) 328 during catalysis as off-path species, which limits catalytic efficiency (Scheme 33).678 Photolysis of this borohydride complex yielded Fe(N2)(P3B), presumably via reductive elimination, which might also be responsible for the enhanced turnover. Again involvement of excited states of N2 TMCs remains unclear to date.677,678

8.18.4.6

M–C homolysis in 3d TMCs

Organometallic TMCs with carbanionic alkyl or aryl ligands are of great interest in organometallic chemistry, homogenous catalysis and bioinorganic chemistry.384,683,684 The MeC bond can in many cases be broken via photoexcitation, either homo- or heterolytically.683 This subsequently generates highly reactive species that can be utilized in artificial or biological catalytic processes. In both areas, the coenzyme B12 330 and its analogs have received much attention (Scheme 34).684,685 Methylcobalamin (MeCbl, 329) and 50 -desoxyadenosylcobalamin (AdoCbl, coenzyme B12, 330) undergo CoIII–C homolysis under irradiation in aqueous solution, forming a coordinatively unsaturated CoII complex and an alkyl radical. The initial photoproducts recombine very quickly (329: 1.4  109 s1, 330: (4  1)  109 s1),686,687 while under aerobic conditions the Co–C cleavage is irreversible and reactions with O2 follow to give hydroxidocobalamin (OHCbl).688,689 The initial photoexcitation in 330 was assigned as a pp* transition centered on the corrin ligand690 which evolves to a 1LMCT state (p(corrin) / Co, S1)691 on a subpicosecond timescale.686 The LMCT state enables the dissociation of the lower axial benzimidazole ligand, which is then followed by homolysis of the upper axial adenosyl group and subsequent radical formation.687,692 CoeC bond cleavage occurs with near unity quantum yield within 50 ps,687 independent of excitation wavelength.686 Subsequent geminate recombination of the two radicals results in an overall photolysis quantum yield of 20–24%.687,693,694 The photolysis mechanism of 329 is wavelength dependent. Green light generates a Co / Me MLCT, which subsequently leads to homolysis (Fig. 11B), while UV light opens up an additional pathway featuring prompt homolysis bypassing the MLCT state (Fig. 11A).695 Excess energy is funneled in

772

d-d and charge transfer photochemistry of 3d metal complexes

Scheme 34

Molecular structures of B12 derivatives with selected axial ligands.

translational modes of the expelled methyl radical, facilitating cage escape and impeding geminate recombination.696 Another relaxation pathway of the MLCT state in 329 is IC back to the GS.686 For both 329 and 330, the initial radical pairs form as singlets, thus they originate from a singlet excited state.697,698 The spin dynamics of these two compounds are largely independent of excitation wavelength.698 For most of the organisms that use coenzyme B12 330, only thermally driven reactions are known to date.699 Notable exceptions are the bacteria Myxococcus xanthus and Thermus thermophilus that utilize 330 to mediate light-dependent gene regulation.700,701 Light triggers Co–C heterolysis following MLCT excitation702 and starts a cascade leading to carotenogenesis, which protects the organism from photooxidative damage.700,701 Furthermore, cobalamins were shown to regulate photosystem gene expression in Rhodobacter capsulatus.703 The photochemistry of cobalamins and B12 analogs has also been exploited in organic synthesis to catalyze a variety of reactions.704–708 Selected M–C photodissociations beyond that of B12 analogs are highlighted below. The chromium(III) complexes cis[CrIII(bpy)2(R)2]þ 331D–333D (R ¼ Ph, 4-tert-butyl-phenyl, 4-methoxyphenyl), cis-CrIII(bpy)Ph2(quin) 334 and cis-CrIII(bpy) Ph2(S2CNMe2) 335 (quin ¼ 8-hydroxyquinolinato(1); Scheme 35) undergo a concerted reductive elimination of biphenyl upon irradiation. The four-coordinate intermediate [Cr(bpy–)2]þ resulting from cis-[CrIII(bpy)2(R)2]þ was trapped by bpy as [Cr(bpy–)2(bpy)]þ.709 Photoinduced M–C homolysis was also exploited in homogeneous catalysis. The chiral bisoxazoline benzyl copper(II) complexes of type 336 (Scheme 35) release benzyl radicals by visible light induced homolysis. The responsible excited state was assigned as LMCT. The generated alkyl radical combines with an imine substrate in a separate catalytic cycle.710

8.18.4.7

Miscellaneous photodissociations and rearrangements of 3d TMCs

A special case of M–C dissociation is the release of p-coordinated arene ligands. A comprehensive review has appeared recently.711 Irradiation of CH2Cl2 and CH3CN solutions of [FeII(h6-arene)(h5-Cp)]þ 337D complexes with alkyl- and chlorido-substituted arene ligands (1E2 absorption band) in the presence of phen led to the formation of [FeII(phen)3]2þ with quantum yields between 0.44  0.02% and 90  1%. The dissociative excited state a3E1 features a weakened Fe–aryl bond with a distorted geometry exposing the central ion to nucleophilic attacks.712 [FeII(h6-arene)(h5-Cp)]X complexes 338 (arene ¼ toluene, naphthalene, pyrene;

d-d and charge transfer photochemistry of 3d metal complexes

Scheme 35

773

Molecular structures of cis-diaryl chromium(III) complexes 331D–335 and suggested benzyl bisoxazoline copper(II) intermediates 336.

X ¼ PF6, CF3SO3, SbF6) delivered [FeII(h5-Cp)(NCPh)3]X in PhCN following irradiation at 532 nm. Time-resolved spectroscopic experiments were used to investigate the underlying photolysis mechanism. Following excitation to a singlet state and ISC to a triplet state, the coordination mode of the arene ligand changed from h6 to h4. The free coordination site is then occupied by the counter ion X or PhCN.713 Complete arene photoremoval gives access to useful synthons such as [FeII(h5-Cp)Ln]þ, and has been exploited in the classical synthesis of triple-decker complexes,714,715 piano-stool complexes of the type [Fe(h5Cp)(L1)(L2)(L3)]nþ,716 dendritic iron nitriles717 or ferracarborane complexes among others.718 Photoexcitation of (poly)hydrido complexes can lead to reductive elimination of H2 and the formation of coordinatively unsaturated, highly reactive species. A prominent example is FeII(dmpe)2(H)2 339 which releases H2 upon irradiation at 100  C in n-pentane. The photoproduct Fe0(dmpe)2 oxidatively adds alkanes, and transforms them to terminal alkenes via b-hydride elimination.719,720 DFT calculations employing Fe0(PH3)4 as a model for the 16 valence electron intermediate Fe0(dmpe)2 revealed a minimal activation barrier for the highly exothermic addition of H2 to the complex in the singlet state.721 Similarly FeII(dppe)2(H)2 340 (dppe ¼ 1,2-bis(diphenylphosphanyl)ethane) shows H2 release upon irradiation and forms Fe0(dppe)2 which can be used as a catalyst in the hydrosilylation of ketones.722 The dihydrido iron(II) complexes 327 and 328 that are generated from 325 and 326L during catalytic formation of ammonia from N2 are prone to H2 photoelimination (Scheme 33). This results in increased turnover for N2 splitting, as the competing HER is hindered (Section 8.18.4.5).677,678 Superoxido and peroxido TMCs can photolytically expel dioxygen. Myoglobin (Mb) containing an iron(II) heme unit at its active site binds O2 to yield the six-coordinate superoxido complex oxymyoglobin 341. Irradiation into the Soret (400 nm) or Q bands (580 nm) of 341 gives O2 with a quantum yield of 28  6%, while the carbon monoxide complex MbCO 342 undergoes CO photolysis (Section 8.18.4.1) with near unity quantum yield.723 At pH 7.5 and temperatures below 10 K, 55% of 341 appear to be inert to photolysis.724 These observations for 341 have been traced back to an unphotolyzable myoglobin side-on O2 complex instead of the commonly encountered end-on coordination mode.723,725 Similar to 341, O2 dissociates very rapidly from O2-loaded hemoglobin 343 within 45  5 fs after excitation.726,727 Photodissociation quantum yields of synthetic superoxido FeIII heme and heme/M (M ¼ CuI, FeII) model complexes are analogous to those of 341.728 In recent years there has been increased interest in understanding the excited state behavior and applications of TMCs with coordinated O2-derived ligands.729 Peroxido copper(II) complexes [CuII(O2)(TMG3tren)]þ 344D and [CuII(O2)(PV-TMPA)]þ 345D showed Cu–O homolysis to yield [CuI(TMG3tren)]þ and [CuI(PV-TMPA)]þ, respectively, when irradiated with a pulsed laser at 436 nm (LMCT states) or 683 nm (MC states) with wavelength-dependent quantum yield (TMG3tren ¼ 2,20 ,200 -(nitrilotris(ethane-2,1-diyl))tris(1,1,3,3-tetramethylguanidine), PVTMPA ¼ bis(pyrid-2-ylmethyl){[6-(pivalamido)pyrid-2-yl]methyl}amine; Scheme 36A). Supported by TDDFT calculations, the dissociation after excitation to MC states was explained by occupation of the strongly s-antibonding 3dz2 orbital, and surface crossing with LMCT excited states.730 A rare two-electron process following single photon excitation was observed in the m-peroxido complexes [CuII2(O2)(tmpa)2]2þ 3462D, [CuII2(LN3)(O2)]2þ 3472D and [CuII2(LN5)(O2)]2þ 3482D (tmpa ¼ tris(2-pyridylmethyl)amine, LN3 ¼ N1,N1,N3,N3-tetrakis(2-(pyridin-2-yl)ethyl)propane-1,3-diamine), LN5 ¼ N1,N1,N3,N3-tetrakis (2-(pyridin-2-yl)ethyl)pentane-1,5-diamine; Scheme 36B) at 80  C in acetone, leading to a two-electron oxidation of the O22 ligand. The process was found to be initiated by

774

d-d and charge transfer photochemistry of 3d metal complexes

Scheme 36

(A) Mononuclear superoxido and (B) dinuclear peroxido-bridged copper(II) complexes 344D–3482D showing O2 photorelease.

peroxido-to-copper LMCT excitation which resulted in the corresponding mixed-valent superoxido intermediates CuI–O2––CuII followed by an intramolecular SET. The m-h1:h1-end-on peroxido complex 3462D evolved to a dinuclear CuI2O2 complex with m-h2:h2-side-on coordinated O2, whereas 3472D and 3482D released dioxygen (3472D: F ¼ 14  1%).731 LMCT excitation of halido TMCs can homolytically split the MeX bond. The primary photolytic step of CuIICl2 349 in various organic solvents was tentatively assigned to Cu–Cl homolysis, based on reactions of the photoproduct with olefins and alcohols.732 Similar behavior was proposed for CuIICl2(dap) 295, which generates a CuI species that can subsequently reduce sulfonyl chlorides.733 LMCT excitation at 427 or 470 nm of [CuIICl(dmp)2]þ 350D homolytically cleaves the CueCl bond with a quantum yield of 2.8%, whereas no net photoreaction was obtained after low-energy MC excitation (785 or 800 nm).734 Light-induced Ni–X homolysis is an emerging field for energy storage and catalysis as summarized in recent reviews.735,736 NiIIICl3(dppe) 351 and its derivatives show dissociation of the apical ligand as Cl upon LMCT excitation. The released chlorine atom is stabilized by an aromatic ring in the second coordination sphere, which hinders exothermic back-reactions.737,738 Similarly, excitation of [NiIIICl(dtbbpy)(R)]þ 352D (R ¼ aryl) generates chlorine radicals that mediate HAT with C(sp3)-H bonds. The organic radical is then trapped by the nickel complex followed by a cross-coupling reaction.736 Beyond affecting the first coordination sphere, i.e., the MeL bonds, photoexcitation of TMCs can lead to cleavage of intraligand bonds. The S-dealkylation of thioether ligands is a well-known reaction path for several iron and nickel complexes under reductive conditions.739,740 This process can also be induced by photoexcitation of low-valent TMCs with coordinated thioether ligands. In the nickel(0) complex 353 with (2-(methylthio)phenyl)diphenylphosphane ligands, homolytic S–C cleavage results in a stepwise oxidation of Ni0 to NiI and NiII with concomitant release of methyl radicals (Scheme 37A).741 Analogous results were obtained for Ni0 complexes containing similar kP, kS chelate ligands.744 This reaction pathway is associated with low metal oxidation states and strong back-donation into the p*(S–C) orbitals.741 Intraligand N2 release after LMCT excitation in FeIII(obt)3 354 (obt ¼ 4oxobenzo[d][1,2,3]triazin-3(4H)-olato(1); Scheme 37B) produces localized ligand radical intermediates, capable of cleaving DNA in potential biomedical applications.742 Prominent examples for 3d TMCs applicable in medicinal photochemistry are curcumin cobalt(III) complexes for tumor therapy, as free curcumin is phototoxic with a short serum half-life (< 1 h). Substitutionally inert complexes such as [CoIII(curcumin)(tpa)]2þ 3552D and their derivatives act as stable carriers releasing curcumin only after LMCT using green light irradiation (Scheme 37C). Curcumin decomposes to cell-toxic photoproducts after excitation with blue light. In contrast to the systems based on generation of singlet oxygen via energy transfer (Section 8.18.3.2) these cobalt(III) complexes can be applied in hypoxic tumor environments.743

d-d and charge transfer photochemistry of 3d metal complexes

775

Scheme 37 (A) Stepwise S-demethylation in 353 under irradiation,741 (B) molecular structure of FeIII(obt)3 354742 and (C) molecular structure of [CoIII(curcumin)(tpa)]2þ 3552D.743

Beyond NO isomerization in nitrosyl TMCs (Section 8.18.4.2), other ambidentate ligands can undergo photorearrangements as summarized in a review.745 Selected examples of 3d TMCs serve to illustrate this phenomenon. In piano-stool complexes 356, 357 and 358 the ambidentate chelate ligand rearranges to yield a metastable state with a modified coordination sphere (linkage isomerism), e.g., kN / kO, kCC/ kN and kN /kN0 , respectively (Scheme 38A). The reactions can be categorized as type-T or type-P by being either thermally as for 356746 or photochemically reversible as for 357 and 358, respectively.747,748 Thermally reversible haptotropic light-induced rearrangements occur in the h4-polyolefin and h6-naphthalene complexes 359,749 360750 and in the dinuclear Fe0 complex (m2,h3:h5-acenaphthylene)Fe2(CO)5 361 (Scheme 38B).751 Irradiation of the [4]-phenylene cyclopentadienyl cobalt(I) complex 362 at low-temperature causes a motion of the CoICp unit from central coordination sites to terminal rings (Scheme 38C) as observed by NMR spectroscopy. Heating to 50  C moves the CoICp fragment back to the central ring.752

8.18.5

Conclusion

With a deeper understanding of the photophysics and photochemistry of transition metal complexes in general, related to the advancements in ultrafast time-resolved spectroscopy and theoretical methods, the specific issues of 3d metal complexes have been tackled. The lower ligand field splitting and smaller spin-orbit coupling compared to the respective 4d or 5d complexes, a high density of states, (photo)lability and redox activity in particular have been addressed by significant improvements in ligand development. First promising applications of photoactive 3d transition metal complexes in catalysis, sensing, bioimaging, medicinal chemistry and energy conversion have already evolved. The enormous progress in the past years gives rise to a great future perspective for Earth-abundant 3d transition metal complexes in numerous photoapplications.

8.18.6

Note added in proof

Section 8.18.2.1: Chromium(III) complexes with spin-flip emission maxima at 1067 nm (77 K, f < 0.00089% at room temperature) and at 709 nm (f ¼ 20%, s ¼ 1800 ms in D2O/DClO4 at room temperature) were reported.753,754 The vanadium(III) complex VCl3(ddpd) shows spin-flip emission at room temperature in the solid state.755,756 Section 8.18.2.2: An isonitrile chromium(0) complex with appended pyrenes displays 3MLCT luminescence at 682 nm with 6.1 ns lifetime and f ¼ 0.09% at room temperature.757 Manganese(I) complexes with bi- and tridentate chelating isonitrile ligands were reported to emit at 584 nm and 525 nm with f ¼ 0.05% and f ¼ 0.03%, respectively, and to engage in bimolecular reactions

776

d-d and charge transfer photochemistry of 3d metal complexes

Scheme 38 (A) Photo-induced rearrangements of 356, 357 and 358,746–748 (B) photo-induced thermally reversible haptotropic rearrangements of the complexes 359, 360 and the dinuclear complex 361749–751 and (C) light-induced motion of a CoCp moiety on a phenylene ligand in 362.752

after excitation.758,759 An iron(II) complex with two tripodal ligands achieves a 3MLCT lifetime of 9.2 ps with only two NHC and four pyridine donors.760 An iron(II) complex supported by tridentate benzannulated diarylamido ligands displays panchromatic absorption and a 3 ns excited state lifetime but no luminescence.761 Titanocenes of the type TiCp2(CCR) with CCR ¼ ethynylphenyl, 4-ethynyldimethylaniline, or 4-ethynyltriphenylamine exhibit LMCT absorption bands at 417, 540, and 525 nm and emit at 77 K at 575, 672, and 642 nm, respectively, but photodecompose at room temperature in 2-MeTHF.762 Section 8.18.3.1: The emissive iron(III) complex 63D undergoes efficient excited state electron transfer with organic electron donors under green light irradiation.763 Section 8.18.3.2: An iron(II) complex with two pyridine and four triazole donors with an excited state lifetime of s >1.6 ns sensitizes singlet oxygen.764

d-d and charge transfer photochemistry of 3d metal complexes

777

Acknowledgments This work was supported by Deutsche Forschungsgemeinschaft [DFG, Priority Program SPP 2102 “Light-controlled reactivity of metal complexes” (HE 2778/13-1)] and a scholarship for N. R. E. (GCS 266, Materials Science in Mainz). W. R. K. is grateful for a Kekulé scholarship of the Fonds der Chemischen Industrie.

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Chem. Soc. 2002, 124, 434–441. 696. Sension, R. J.; Harris, D. A.; Cole, A. G. J. Phys. Chem. B 2005, 109, 21954–21962. 697. Bussandri, A. P.; Kiarie, C. W.; van Willigen, H. Res. Chem. Intermed. 2002, 28, 697–710. 698. Lukinovic, V.; Woodward, J. R.; Marrafa, T. C.; Shanmugam, M.; Heyes, D. J.; Hardman, S. J. O.; Scrutton, N. S.; Hay, S.; Fielding, A. J.; Jones, A. R. J. Phys. Chem. B 2019, 123, 4663–4672. 699. Banerjee, R. Chem. Rev. 2003, 103, 2083–2094. 700. Ortiz-Guerrero, J. M.; Polanco, M. C.; Murillo, F. J.; Padmanabhan, S.; Elías-Arnanz, M. Proc. Natl. Acad. Sci. U. S. A. 2011, 108, 7565–7570. 701. Jost, M.; Fernández-Zapata, J.; Polanco, M. C.; Ortiz-Guerrero, J. M.; Chen, P. Y.-T.; Kang, G.; Padmanabhan, S.; Elías-Arnanz, M.; Drennan, C. L. Nature 2015, 526, 536–541. 702. Kutta, R. J.; Hardman, S. J. O.; Johannissen, L. O.; Bellina, B.; Messiha, H. L.; Ortiz-Guerrero, J. M.; Elías-Arnanz, M.; Padmanabhan, S.; Barran, P.; Scrutton, N. S.; Jones, A. R. Nat. 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d-d and charge transfer photochemistry of 3d metal complexes Hossain, A.; Engl, S.; Lutsker, E.; Reiser, O. ACS Catal. 2019, 9, 1103–1109. Fayad, R.; Engl, S.; Danilov, E. O.; Hauke, C. E.; Reiser, O.; Castellano, F. N. J. Phys. Chem. Lett. 2020, 11, 5345–5349. Wenger, O. S. Chem. A Eur. J. 2021, 27, 2270–2278. Kariofillis, S. K.; Doyle, A. G. Acc. Chem. Res. 2021, 54, 988–1000. Hwang, S. J.; Powers, D. C.; Maher, A. G.; Anderson, B. L.; Hadt, R. G.; Zheng, S.-L.; Chen, Y.-S.; Nocera, D. G. J. Am. Chem. Soc. 2015, 137, 6472–6475. Hwang, S. J.; Anderson, B. L.; Powers, D. C.; Maher, A. G.; Hadt, R. G.; Nocera, D. G. Organometallics 2015, 34, 4766–4774. Sellmann, D.; Reisser, W. J. Organomet. Chem. 1985, 297, 319–329. Cha, M.; Shoner, S. C.; Kovacs, J. A. Inorg. Chem. 1993, 32, 1860–1863. Kim, J. S.; Reibenspies, J. H.; Darensbourg, M. Y. J. Am. Chem. Soc. 1996, 118, 4115–4123. Maurer, T. D.; Kraft, B. J.; Lato, S. M.; Ellington, A. D.; Zaleski, J. M. Chem. Commun. 2000, 69–70. Renfrew, A. K.; Bryce, N. S.; Hambley, T. Chem. A Eur. J. 2015, 21, 15224–15234. Kim, J. S.; Reibenspies, J. H.; Darensbourg, M. Y. Inorg. Chim. Acta 1996, 250, 283–294. Nakai, H.; Isobe, K. Coord. Chem. Rev. 2010, 254, 2652–2662. To, T. T.; Duke, C. B., III; Junker, C. S.; O’Brien, C. M.; Ross, C. R., II; Barnes, C. E.; Webster, C. E.; Burkey, T. J. Organometallics 2008, 27, 289–296. To, T. T.; Barnes, C. E.; Burkey, T. J. Organometallics 2004, 23, 2708–2714. To, T. T.; Heilweil, E. J.; Duke, C. B.; Ruddick, K. R.; Webster, C. E.; Burkey, T. J. J. Phys. Chem. A 2009, 113, 2666–2676. King, J. A.; Vollhardt, K. P. C. J. Organomet. Chem. 1989, 369, 245–251. Jahr, H. C.; Nieger, M.; Dötz, K. H. Chem. Commun. 2003, 2866–2867. Niibayashi, S.; Matsubara, K.; Haga, M.-A.; Nagashima, H. Organometallics 2004, 23, 635–646. Albright, T. A.; Drissi, R.; Gandon, V.; Oldenhof, S.; Oloba-Whenu, O. A.; Padilla, R.; Shen, H.; Vollhardt, K. P. C.; Vreeken, V. Chem. A Eur. J. 2015, 21, 4546–4550. Sinha, N.; Jiménez, J.-R.; Pfund, B.; Prescimone, A.; Piguet, C.; Wenger, O. S. Angew. Chem. Int. Ed. 2021, 60, 23772–23728. Reichenauer, F.; Wang, C.; Förster, C.; Boden, P.; Ugur, N.; Báez-Cruz, R.; Kalmbach, J.; Carrella, L. M.; Rentschler, E.; Ramanan, C.; Niedner-Schatteburg, G.; Gerhards, M.; Seitz, M.; Resch-Genger, U.; Heinze, K. J. Am. Chem. Soc. 2021, 143, 11843–11855. Gerhards, M.; Seitz, M.; Heinze, K. Chem. Sci. 2021, 12, 10780–10790. Zobel, P.; Knoll, T.; Gonzalez, L. Chem. Sci. 2021, 12, 10791–10801. Wegeberg, C.; Häussinger, D.; Wenger, O. S. J. Am. Chem. Soc. 2021, 143, 15800–15811. Herr, P.; Kerzig, C.; Larsen, C. B.; Häussinger, D.; Wenger, O. S. Nat. Chem. 2021, 13, 956–962. Heinze, K. Nat. Chem. 2021, 13, 926–928. Reuter, T.; Kruse, A.; Schoch, R.; Bauer, M.; Lochbrunner, S.; Heinze, K. Chem. Commun. 2021, 57, 7541–7544. Larsen, C.B.; Braun, J.D.; Lozada, I.B.; Kunnus, K.; Biasin, E.; Kolodziej, C.; Burda, C.; Cordones, A.A.; Gaffney K.J.; and Herbert, D.E.; J. Am. Chem. Soc. 2021, 143. https:// doi.org/10.1021/jacs.1c06429. London, H. C.; Whittemore, T. J.; Gale, A. G.; McMillen, C. D.; Pritchett, D. Y.; Myers, A. R.; Thomas, H. D.; Shields, G. C.; Wagenknecht, P. S. Inorg. Chem. 2021, 60, 14399–14409. Aydogan, A.; Bangle, R. E.; Cadranel, A.; Turlington, M. D.; Conroy, D. T.; Cauët, E.; Singleton, M. L.; Meyer, G. J.; Sampaio, R. N.; Elias, B.; Troian-Gautier, L. J. Am. Chem. Soc. 2021, 143, 15661–15673. Dierks, P.; Kruse, A.; Bokareva, O. S.; Al-Marri, M. J.; Kalmbach, J.; Baltrun, M.; Neuba, A.; Schoch, R.; Hohloch, S.; Heinze, K.; Seitz, M.; Kühn, O.; Lochbrunner, S.; Bauer, M. Chem. Commun. 2021, 57, 6640–6643.

8.19

Luminescence properties of the actinides and actinyls

Laura Lopez-Odriozolaa,b, Lauren Walkera, and Louise S. Natrajana,b, a Centre for Radiochemistry Research, Department of Chemistry, School of Natural Sciences, The University of Manchester, Manchester, United Kingdom; and b The Photon Science Institute, The University of Manchester, Manchester, United Kingdom © 2023 Elsevier Ltd. All rights reserved.

8.19.1 8.19.2 8.19.3 8.19.3.1 8.19.3.2 8.19.3.3 8.19.4 8.19.5 8.19.5.1 8.19.5.2 8.19.5.3 References

Background Optical transitions in actinide ions and compounds Uranium Uranyl(VI) Uranyl(V) Uranium(IV) The neptunyl and plutonyl ions Transuranic actinide ions Americium(III) Curium(III) The late actinides Bk-Es

789 790 791 792 796 797 799 801 801 802 804 806

Abstract The chemistry of the elements that make up the actinide series is generally less well understood than other elements of the Periodic Table, but is currently receiving renewed interest, particularly in light of recent landmark reports of the isolation of actinides in non-traditional oxidation states and compounds of the trans plutonium elements. The optical properties of actinide and actinyl ions are very distinctive and can provide important information on the electronic structure, bonding and chemical environment of an individual ion in compounds and in the natural and engineered environment. However, it is only relatively recently that the optical properties of these important elements have begun to be explored and exploited in nuclear fission remediation activities for example. Here, we discuss the fundamental principles of the different types of luminescence that the actinide and actinyl cations exhibit and provide key examples of how the optical properties can be exploited to provide important information on the electronic structure and coordination environment of these ions.

8.19.1

Background

The actinide series comprise the elements actinium (Ac) to Lawrencium (Lr), are all radioactive, and are created artificially from nuclear fission of U-235 or initially isolated from thermonuclear explosions. Exceptions to this are those elements that occur naturally; principally uranium (U) and thorium (Th) alongside those found in U ores (Ac and Pa, protactinium), with Pa occurring in trace amounts as part of the U decay chain. Historically, interest in the physics and chemistry of these elements stemmed from the prospect of nuclear physics during the Manhattan Project, with the majority of the chemical and physical properties known today having being developed after this period. It is only relatively recently that research into the actinides has developed further; particularly in the past few decades. Remarkable efforts in handling small quantities of these often very radioactive elements has accelerated our understanding of the chemistry and given way to a variety of useful applications. These include the use of americium in smoke detectors, radioisotope thermoelectric generators (RTGs) and historically, pacemakers.1 Nowadays, with the increased interest in civil nuclear power for electricity production, a new period of actinide chemistry is upon us. As many countries approach a new era of long term deep geological disposal of high-level legacy wastes, alongside subsurface disposal of lower activity wastes after ca. 70 years of civil nuclear power, research focus has shifted to the chemistry of the transuranics and safe treatment and disposal of nuclear wastes. Future options of a (partially) closed fuel cycle are being considered as commercial uses for fission products (such as medically useful radioisotopes geared as alpha therapeutics for advanced cancer treatments using Ac-225 for example)2 and advanced separation strategies are developed. In addition, global concerns regarding climate change and targets to reduce CO2 emissions (e.g., net zero by 2050), has resulted in nuclear fission being considered by many as an essential zero carbon energy source in the future electricity mix. In this regard, new build reactors are being considered and commissioned alongside research and development into new treatment strategies. The electronic properties of the actinides and actinyls in solution, solids and in coordination compounds continues to develop as an intriguing topic of interest spanning chemistry, physics and environmental sciences.3 Contemporary research from both theoretical and experimental viewpoints are increasing in activity as the drive for advanced knowledge in actinide chemistry and materials science underpins understanding of the nuclear fuel cycle and its environmental implications. Optical spectroscopy of the actinides is one of several useful techniques that are vital experimental probes of chemical bonding, oxidation state, speciation

Comprehensive Inorganic Chemistry III, Volume 8

https://doi.org/10.1016/B978-0-12-823144-9.00191-6

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790

Luminescence properties of the actinides and actinyls

and reactivities of elements that are prevalent in the fuel cycle. This Chapter discusses the fundamental optical (absorption and emission) properties of the actinides and the actinyl ions with key examples illustrating how the optical properties can be utilized to provide crucial insights into the chemical properties of these important ions in solution.

8.19.2

Optical transitions in actinide ions and compounds

Unlike the more commonly encountered 4f ions (the lanthanides), the energy of actinide absorption and emission spectra vary significantly depending on the chemical species present. This is due to the fact that the 5f valence orbitals are relativistically more radially expanded than their 4f counterparts. Both spin orbit coupling and crystal field effects for these heavy ions are increased by approximately a factor of two, which, in many cases gives rise to a rich electronic landscape, and renders the absorption and emission spectra more complicated to record and interpret. In general, there are three types of electronic transitions that can occur in actinide and actinyl compounds: 1. Intra-configurational f-f transitions. These involve electronic transitions in all open shell actinide ions from one f-orbital to another, are predominately electric dipole in nature and therefore in a free ion are forbidden by the parity selection rule. However, mixing of states with opposite parity in compounds partly relaxes the selection rule. In actinides, the oscillator strengths of these transitions are 10–100 times greater than the trivalent lanthanide ions. An individual transition is relatively narrow in energy range and they commonly span the visible to near infra-red regions of the electromagnetic spectrum (Figs. 1 and 2). Other f-f transitions are magnetic dipole allowed in the free ion and a fully compatible with no change in parity, so are fully allowed by this mechanism. 2. Inter-configurational f-d transitions. These transitions occur between ground state 5f orbitals and higher lying 6d orbitals, and are more common in the early actinide series (e.g., U3þ, U4þ) as the 5f and 6d orbitals lie closer in energy. As the actinide series is traversed, the 6d orbitals drop in energy below those of the 5f and therefore intra-configurational f-f transitions dominate after plutonium. Inter-configurational transitions conserve the parity/Laporte rule (DL ¼  1) are therefore much more intense than ff transitions and tend to occur in the UV and visible region of the electromagnetic spectrum. 3. Ligand to metal charge transfer (LMCT) transitions. These electronic transitions are a prevalent feature of actinyl ions (AnO2n þ, n ¼ 1, 2, An ¼ U, Np, Pu) and involve transfer of a bonding electron from the oxygen orbitals with 2p character to an empty 5f orbital. According to the Laporte rule, in a centrosymmetric isolated actinyl ion of DN h symmetry, these transitions are parity forbidden, but in reality, are partially allowed due to vibronic coupling and/or orbital mixing which reduces the centrosymmetric character of the orbitals/system. Molar extinction coefficients are generally recorded in the range (10–100 M 1 cm 1). LMCT transitions are typically more energetic than f-f and f-d transitions, and they mainly occur in the UV-violet part of the electromagnetic spectrum. Given intra-configurational transitions are unique to open shell actinides (and lanthanides), they are commonly used to identify (or fingerprint) the presence of an oxidation state of a given actinide. However, the electronic absorption and emission spectra of An(III) and An(IV) ions in particular tend to be broader than their lanthanide counterparts and assignment is more difficult. This is due to a breakdown in the Russell-Saunders (L-S) spin-orbit coupling scheme that renders the electronic transitions more susceptible to ligand and crystal field effects, which can be an indicator of actinide ligand covalency (through the nephelauxetic Most stable oxidation state Only found in solid complexes

NpO23+ PuO23+ AmO65-

7 UO22+

NpO22+ PuO22+ AmO22+

PaO2+

UO2+

NpO2+

PuO2+

AmO2+

Th4+

Pa4+

U4+

Np4+

Pu4+

Am4+

Cm4+

Bk4+

Cf4+

Th3+

Pa3+

U3+

Np3+

Pu3+

Am3+

Cm3+

Bk3+

Cf3+

Es3+

Fm3+

Md3+

No3+

Cf2+

Es2+

Fm2+

Mf2+

No2+

Cf

Es

Fm

Md

No

6 5 4 Ac3+

3

Am2+

2 Ac

Th

Pa

U

Np

Pu

Am

Cm

Bk

Lr3+

Lr

Fig. 1 Chemically accessible oxidation states of the actinides and colors of ions in solution where applicable. Singly underlined ions are the principal oxidation states, doubly underlined ions are found in solid compounds. Note that uranium and thorium in the formal þII oxidation states have been prepared in anhydrous reducing conditions.4,5

U3+

Am3+

NpO22+

Es3+

Cm3+

nIR

Bk3+

UO22+

Am3+

UO22+

UO2+

U4+

Visible Cf3+

Pa4+

NpO22+

U4+

UV NpO2+

Pa4+

Pa4+

Pa4+

Fig. 2 Graphical representation of the typical regions of luminescent emission of the actinide ions and their compounds in energy (drawn by the authors).3

Luminescence properties of the actinides and actinyls

791

effect). Despite this, j-j coupling schemes may also not be a particularly accurate descriptor of actinide electronic levels, and an intermediate coupling scheme is likely to be most appropriate and accounting for the increased mixing of the ground and excited states compared with the trivalent lanthanides. Nevertheless, Russell-Saunders coupling can serve as a good approximation for the relative ordering of the 5f-f electronic levels especially for the lighter elements in high symmetry environments (for example in U(IV) cubic systems).6 An important consequence of the increased oscillator strengths of f-f and LMCT transitions is the application of Time Resolved Laser Induced Fluorescence Spectroscopy (TRLIFS) to selectively (directly) excite into an individual 5f-f absorption band. This takes advantage of the increased sensitivity of the 5f-f optical transitions to local coordination environment and is generally in contrast to the sensitized emission necessary to overcome the weak 4f-f oscillator strengths in equivalent solutions of the trivalent lanthanides. Finally, as Fig. 3 illustrates, the density of excited states arising from a given term is high, they order differently to lanthanide(III) ions, and the J manifolds can overlap. This is effectively illustrated by considering the valence isoelectronic 4f-5f pair Eu3þ and Am3þ which both possess an f6, 7F0 ground state configuration; the emissive state is 5D0 for Eu3þ and 5D1 for Am3þ. These, combined with the challenges associated with the handling of the late actinides are some of the reasons actinide electronic spectroscopy is much less well developed than that of the corresponding lanthanide ions.

8.19.3

Uranium

Uranium is the heaviest naturally occurring element; it is present in small amounts in the environment, in soil, rocks and underground water, and occurs in trace amounts in food and drinking water. Its average abundance in the Earth’s surface is about 0.2 mg per kg and 0.1–0.5 mg L 1 in water, but has a significant dependence on the quantity of uranium in associated materials, such as naturally occurring rocks and the presence of industries which may contribute to the release of uranium into the environment.7 Natural uranium consists of three isotopes, U-234 (0.0055%), U-235 (0.720%) and U-238 (99.2745%). The half-life of these isotopes varies from 4.46  109 years for the heaviest isotope to 2.45  105 years for the lightest.8 Depleted uranium contains 99.8% U-238, while it is an alpha emitter, the long half-lives result in a negligible build-up of alpha activity. Nevertheless, chemotoxicity is still of concern, as uranium can be absorbed into the blood and accumulates in the kidneys.9 The World Health Organization limit for uranium in drinking water is 2 mg L 1, which protects against any possible negative renal effects. Failing to control uranium from nuclear waste leaching into the environment can lead to elevated levels in the ground and have a negative health impact on the population. Understanding the interactions of uranium with minerals in the environment is essential to

25

Energy /103 cm-1

20

5/2

6

4

5/2

1

4 5/2

7/2 4

6

5

15

6

5/2

5 6 11/2

2

7

10

5 6

3/2

9/2

3

3

11/2

9/2 5

0

5

9/2 2 4

5

2

11/2

4

I

5

U

Np

I

4

7/2

5/2 6

4

1

0

H

7

Pu

Am

F

7/2 8

S

Cm

6 7

F

Bk

15/2 6

8

15/2

H

5

I

4

Cf

Es

Fm

I

6 3

H

Mo Md

2

F

7/2

No

Fig. 3 Energy level diagram for the free trivalent actinide ions doped in LaCl3, showing the ground state and excited state energy levels. The principle emissive states are depicted in red. Diagram modified and re-drawn from Carnall, W. T.; Crosswhite, H. M. ANL-84 Argonne National Laboratory; Argonne: Illinois, 1985.

792

Luminescence properties of the actinides and actinyls

prevent contamination, and help to realize remediation strategies for existing contaminated land. To fully understand these interactions, the speciation of uranium must be considered in depth. Uranium can exist in a variety of oxidation states, from þ III to þ VI (although the þ II oxidation state has recently been isolated).4 Of these, the þ IV and þ VI oxidation states are the only two which are sufficiently stable to be found in significant quantities in natural conditions10,11 although it should be noted that studies have shown uranyl(V) can form as an intermediate during the bioreduction of uranyl(VI) which is stable for up to 5 days in biotic environmental conditions12 as well as in Fe(II) bearing mineral samples.13 Generally uranyl(V) is unstable in aqueous conditions, undergoing disproportionation to form U(VI) as UO22þ and U(IV), and is only stable in very high pH conditions.14 Furthermore, as U(V) is rapidly oxidized to U(VI) in the presence of oxygen experimental work must be carried out in inert atmospheres; the instability of compounds makes the literature available on this oxidation state more limited than the others. Nevertheless, a number of pentavalent uranyl cation (UO2þ) compounds have recently been isolated.15–17 Quite remarkably, pyridyl amino carboxylate ligands have been used to form UO2þ complexes which are stable in anoxic water at high pH.18,19

8.19.3.1

Uranyl(VI)

In aqueous solutions, uranium is most stable in its þ VI oxidation state and is most commonly found as the uranyl moiety, UO22þ. In general, the stability of the different oxidation states of uranium in solution is dependent on pH and the presence of complexing ligands.20 As observed by Zhang and Pitzer, in 1999 approximately half of the known uranium compounds contained the uranyl ion, making its chemistry very important for the understanding of the behavior of uranium compounds.21 The uranyl(VI) ion (UO22þ) is by far the most well studied and understood actinide ion due to its natural prevalence and from an electronic spectroscopic point of view is the simplest of actinide ions to study as it contains no unpaired 5f electrons. Indeed, the nature of the chemical bonding and electronic structure of uranyl(VI) was a topic of great contention prior to the pivotal work of Denning, who developed the now accepted molecular orbital diagram of uranyl (Fig. 4).22,23 The molecular orbitals, in Fig. 4 are formed from the interaction between the empty valence 5f and 6d orbitals in U, the “pseudo-core” 6p and 6s orbitals in U, and the filled 2s and 2p orbitals in O. This molecular orbital diagram is sometimes drawn without inclusion of the 6p, 6s and 2p electrons, however, as explained by Denning. As a result of its thermodynamic stability and kinetic inertness, the uranyl ion is very chemically stable and has a substantial range of coordination chemistry. Most notably, uranyl(VI) compounds are often luminescent and the difference between absorption and emission of uranium was responsible for the discovery of the Stokes shift.24 Long luminescent lifetimes and high quantum efficiencies, and the marked electronic structure differences between actinides and the corresponding lanthanide ions, renders the chemistry of uranyl(VI) a topic of significant interest. The uranyl ion, like other actinyl ions, consists of a linear UO22þ unit, with DN h symmetry. The six filled 2p orbitals on the two oxygen atoms have symmetry allowed combinations with the valence 5f and 6d orbitals on uranium, these can have four types of symmetry, pu, pg, sg and su. The UeO bond is very strong, with a bond enthalpy of 701 kJ mol 1 for the gaseous uranyl ion. This is comparable to bond enthalpies of gaseous transition-metal dioxides and closest to the CeO bond strength in carbon dioxide (802 kJ mol 1). Despite this similarity, it is interesting to note the unexpected propensity of actinyl ions to maintain a linear geometry in structures, as opposed to their transition metal counterparts (like MoO22þ) which often adopt cis configurations between the

Uranium

Oxygen σg πg

6d

δg σu πu

5f

δu φu

LUMO HOMOσ

2 x 2p

u

σg πg πu

σu

6p 6s

σg πu

2 x 2s

σu σg

Fig. 4 Molecular orbital diagram of uranyl(VI) as developed by Denning, showing the participation of the uranium 6p atomic orbital in the highest occupied molecular orbital (HOMO). Redrawn by the authors from Denning, R. G. J. Phys. Chem. A 2007, 111, 4125–4143.

Luminescence properties of the actinides and actinyls

793

metal and oxido-ligands in complexes and has been rationalized in terms of the inverse trans effect in which strongly donating ligands in a trans position to one another mutually strengthen the bonding.22,25 Establishing the order of the molecular orbitals in uranyl has been a complex task with a number of contradicting theories proposed.26,27 The empty valence shells of U mix with the filled 2p orbitals of oxygen to form the highest occupied molecular orbitals, pu, pg, sg and su, these can be considered bonding orbitals, localized on the oxygen atoms and thus suggest a formal UeO bonding order of three.23 However, experimental data from Denning’s work shows the involvement of the 6p orbital of uranium in the formation of the su HOMO, which results in much weaker oxygen contribution and greater instability of the molecular orbital. This observation is supported by density functional theory (DFT) calculations carried out by Kaltsoyannis.28 The lack of compatible symmetry with the available orbitals on oxygen renders the non-bonding 5fd and 5f4 the lowest unoccupied molecular orbitals. The remaining orbitals are antibonding with respect to oxygen and as stated by Denning, too high in energy to be relevant to electronic transitions in the uranyl ion. The distinctive bright green photoluminescence of the uranyl ion is a result of the transitions between its electronic states; assignment of these transitions has not been straightforward and some speculation still exists. The aqua uranyl ion has two principal absorption bands, centered at 413.8 nm and approximately 360 nm,29 due to transitions from the su HOMO to the 5fd and 5f4 LUMO (Fig. 4). While the assignment of these transitions was confirmed by Denning in 199222 by detailed analysis of crystal optical spectroscopy and polarized crystal spectroscopy, it was the extensive analysis of the spectra of uranyl complexes using group and molecular orbital theory carried out by Görller-Walrand and Vanquickenborne in 1971 which first rationalized this hypothesis.30 1 1 þ They identified the observed transitions as Sþ g / Dg and Sg / Fg using the molecular Russell-Saunders coupling scheme. The 1

1

oscillator strength, and consequently molar extinction coefficient, of these transitions is unusually weak when compared to a 5f-f transition,  10 4 and  10 M 1 cm 1 respectively, and corresponds to a Laporte forbidden and spin forbidden transition. As well as these primary bands corresponding to transitions within the uranyl unit, there is a featureless broad absorption that can be observed at higher energies (below 330 nm). This is described by Görller-Walrand and Vanquickenborne as an excitation into the continuum (i.e., high energy excitations to  S2, Fig. 5). Generally speaking, there are three energy regions that can be excited that often result in LMCT emission of the uranyl unit itself these are shown below in Fig. 5. Note that competitive emission from the different excited states can sometimes be observed as a higher energy and broad emission band between 360 and 430 nm.29 Until recently, some ambiguity still existed over the character of the lowest energy excited state. As discussed by Ghosh, Mondal and Palit,31 theoretical calculations agree that the lowest energy excited states are triplet states, either 3 Dg and 3 Fg but different methods of calculation yield different results.32 In 2018, Kumke and co-workers33 demonstrated that the 3 Dg state has the lowest energy by ultrafast transient absorption spectroscopy of UO22þ in a water coordinated matrix. As the uranyl ion is found between coupling regimes, this cannot fully be triplet state as that would imply a complete Russell Saunders coupling scheme,34 but as long as some electron repulsion exists, the state which is lowest in energy will have a greater triplet character. Following Kasha’s rule, emission occurs from this state centered at approximately 520 nm, and thus is formally considered phosphorescence. Lifetime measurements of uranyl(VI) in solution also suggest that the emission has phosphorescent character, since it is generally long lived (in the order of microseconds, compared with nanoseconds for typical fluorescence) and is temperature dependent.3 As seen in Fig. 5, the emission profile of uranyl contains a marked vibrational progression, owing to the coupling of the electronic transition with the total symmetric vibrational stretch of the O¼ U ¼ O molecule, this can also be observed in the absorption profile. The frequency of vibration impacts the energy of photons emitted and thus the shape of the spectra observed. The electronic energy levels in the uranyl ion are affected by complexation of the ion and changes in bonding will affect the vibrational frequency of the O¼ U ¼ O bond. As a consequence, photoluminescence spectra for absorption, emission and excitation can be of great utility in identifying the speciation of uranyl in different systems. Stronger sigma donors bonded to uranyl in the equatorial plane tend to red shift the emission (to lower energy) as competition for bonding overlap with the uranium 6p, 6d and 5f orbitals with the oxide groups occurs, thereby lowering the uranyl bond order and the HOMO-LUMO energy gap.3 Likewise, elongation of the uranyl bond for example in complexes where the uranyl oxygen atom is coordinated to an adjacent uranium ion as in the tetrameric alkoxide complex [UO2{OCH(iPr)2}2]435 and the trimetallic complex [UO2(TPIP)2]3 (TPIP ¼ tetraphenylimidodiphosphinate)29 (Fig. 6) affords a significant red shift of in the uranyl(VI) LMCT emission and in the case of [UO2(TPIP)2]3, a corresponding reduction in the luminescence lifetime by a factor of approximately two (from 2.00 ms to 1.04 ms). In both of these examples, the presence of strong sigma donors coordinated to uranyl in the equatorial plane enhances the Lewis basicity of the uranyl oxido groups resulting in uranyl-oxido to uranium dative bonds. Such electrostatic bridging interactions are more commonly seen in actinyl(V) complexes (U, Np and Pu).3 The spacing between the two highest energy peaks observed in vibrationally resolved emission spectra more often than not correspond to the vibrational frequency of the symmetric O ¼ U ¼ O stretch. This stretch is Raman active and has a frequency of 880 cm 1 for the free uranyl ion,36 with values between 790 and 900 cm 1 reported for other uranyl compounds.37 Considering the total symmetric stretch, as the uranyl ion is linear it can be treated as a diatomic anharmonic oscillator, from which the potential energy curves and vibrational levels of the ion can be calculated for the electronic states involved in the transition, as demonstrated by de Jaegere and by Görller-Walrand in 1969.38 Their results show the strongest absorption peak corresponds to the excitation into the second vibrational level of the second excited state, which following the Franck-Condon principle has the structure (in this case bond length) most similar to that of the ground state. The 0–0 phonon transition (E0–0), from and to the lowest energy vibrational states in the first and ground electronic energy levels respectively, is the only one common to both absorption and emission spectra. Despite this, the low Frank-Condon factor for the

794

Luminescence properties of the actinides and actinyls

Fig. 5 A Jablonksi energy diagram depicting the excitation and emission of uranyl(VI). Three possible excitation routes are depicted, note that each results in the same emission. The solid purple arrow depicts excitation into a continuum of excited states (left) and the corresponding structureless character in the excitation spectrum at 275 nm. The solid dark blue arrow depicts excitation into an excited singlet state of uncertain origin and the corresponding structured character in the excitation spectrum at 370 nm (right). The solid light blue arrow depicts excitation into the first excited singlet state (left) and the corresponding structured character in the excitation spectrum. Three green arrows represent the emission from the excited triplet to the ground state (left) and the corresponding emission (right). Dotted arrows represent a radiationless transition (Kasha’s rule), which include but are not limited to a combination of internal conversion and vibrational relaxation. Red arrow represents intersystem crossing. The excitation and emission spectrum were recorded at 77 K, thus contributions from vibrational excited states are negligible. Figure redrawn and expanded by the authors from Drobot, B.; Bauer, A.; Steudtner, R.; Tsushima, S.; Bok, F.; Patzschke, M.; Raff, J.; Brendler, V.; Anal. Chem. 2016, 88, 3548–3555.

transition means it is usually very low in intensity, or not visible at all. When it is not observable, it can be approximated from the crossing of the absorption and emission spectra, or from the average of the maxima of absorption and emission spectra (in wavenumbers).39 While this should be the highest energy peak in the emission spectrum, a hot band, higher in energy, can sometimes be observed. This occurs as a result of a non-negligible Boltzmann population of a higher energy vibrational state, for example in the emission spectrum of the free uranyl ion in perchloric acid, a hot band can be observed at 473 nm.27

RO O U RO

R O O

O

O O U

OR O

N

OR U

O OR

N

P P

O O R

U

O

O

P

P

O

O

U

O

O O

O

O

P OR

P

U

P O O O P

N

P O

O

O

P

O

P

N

U

N

O P

O

N

Fig. 6 Elongation of the uranyl bonds in oligomeric complexes of uranyl(VI) assembled by uranyl-oxido-uranium electrostatic interactions induces a red shift in the emission spectra (R ¼ OCH(iPr)2).29,35

Luminescence properties of the actinides and actinyls

795

As would be expected from the mirror image properties of absorption and emission spectra, the most intense emission transition corresponds to de-excitation from the lowest vibrational level in the first excited state to the second vibrational level in the ground state (v00 ¼ 2 / v0 ¼ 0). Both absorption and emission spectra can be indicative of different uranyl species. Table 1 below shows some data demonstrating how significant the variation between spectral features can be for different complexes. The luminescent lifetimes of uranyl complexes are another property affected by speciation of the photoluminescent center. Lifetimes are heavily dependent on quenching of photoluminescence and thus are very sensitive to the local environment of the uranyl moiety. As the emission of uranyl(VI) is formally spin forbidden and Laporte forbidden with respect to the linear O¼ U ¼ O unit, uranyl(VI) species can have relatively long lifetimes (ms) compared to those expected for organic molecules (ns).44 Quenching of luminescence results in a decrease in the observed lifetime (and consequently emission intensity). Quenching can occur via a number of electron transfer or vibrational processes. For example, in solution, vibrational quenching can occur via interactions, such as the formation of a non-emissive species (static quenching), or collisions with solvent molecules (dynamic quenching). In a solid, non-radiative decay can occur due to coupling with lattice vibrations; as rates of non-radiative decay increase, the observed lifetime decreases. Consequently, temperature can be a quencher of luminescence. As the kinetic energy of the system increases, both lattice vibrations in solids and the movement of molecules in solution also increase, causing a reduction in the luminescence lifetime.45 This temperature effect cannot be ignored when evaluating experimental data; lifetimes obtained at different temperatures are not comparable. Nevertheless, within the same experimental conditions, different uranyl species have different lifetimes and fitting decay curves with the correct number of components provides information on the number of species in a system. Furthermore, the lifetimes of the different components can give information regarding the relative environments of two species.26 For example, a uranyl(VI) complex adsorbed on the surface of a solid will have a longer luminescence lifetime than a complex in solution, which has a greater chance of colliding with solvent molecules. The practicality of lifetime analysis for assignation of uranyl species has been demonstrated in results obtained by Baumann et al.46 and by Tits et al.47 in 2005 and 2014 respectively. Despite this, there is generally a lack of consistency between results obtained by different groups, which is likely a result of differences in experimental conditions, temperatures and fitting procedures, which can make the assignment of species using lifetimes challenging. Nevertheless, fluorescence and phosphorescence lifetime image mapping techniques are very useful tools and have recently been introduced in this field in biological systems, to identify uranyl(VI) location in uranium reducing bacteria and thus to examine speciation and heterogeneity in biogeochemical conditions.48 In addition, the propensity of the uranyl aqua ion in particular to undergo photochemical reactions in the excited state (including H atom abstraction and single electron transfer reactions) can reduce observed lifetimes further. The photochemistry of uranyl(VI) has been reviewed in depth elsewhere.49,50 Table 1 highlights the complexities of fingerprinting uranyl(VI) species using spectral features (emission maxima) and lifetime data, particularly in environmentally relevant conditions, where the speciation of uranyl is non-trivial and several hydrolyzed and carbonato species can co-exist in solution at a given uranyl concentration, pH, ionic strength and temperature. For this reason, thermodynamic geochemical modeling software packages can be utilized to aid in spectral assignments (Fig. 7). Nevertheless, unless the uranyl compound in question exists solely as one species (as in the case of UO2Cl4252 and in some coordination compounds where solvent exchange is absent or faster than the experimental timescale)3 spectral profiles are often a convolution of two or more species whose emission bands overlap and the resultant lifetimes are multiexponential. In such cases, experimental techniques such as variable temperature measurements and time resolved emission spectroscopy alongside spectral slicing can be employed to separate the signals (Fig. 7).53 Mathematical deconvolution techniques including principal component analysis, factor analysis and parallel factor analysis (for larger datasets) can also be employed to separate out signals (Fig. 8).54 For example, the variation of excitation spectra recorded at different emission wavelengths can be very informative regarding speciation where different uranyl species possess slightly different absorption (and therefore excitation) profiles. The stacking of emission spectra recorded at different excitation wavelengths, to produce a three-dimensional map of excitation and emission (an excitation emission matrix or EEM) can be used to determine the number and spectra of species in a sample using deconvolution methods such as parallel factor analysis (PARAFAC).55 This is particularly useful for the elucidation of uranyl speciation in different samples, which is often a complex undertaking. The first deconvolutions of luminescence data from aqueous uranyl systems using PARAFAC were carried out by Drobot and Richter in 2015 Table 1

Table showing variations in luminescence properties of the uranyl(VI) LMCT emission in common aqueous uranyl(VI) species at a given temperature.

Aqueous species

Emission/nm

T/K

lex/nm

Lifetime (ms)

Reference

UO22þ UO22þ UO2CO3 UO2(CO3)34 (UO2)2(OH)3CO3 (UO2)2(OH)22þ UO2(HPO4)22 Ca2UO2(CO3)3

492, 514, 538, 564 470.4, 488.8, 510, 533.2, 558.9 479, 498, 519, 542, 567 468, 487.4, 506.5, 528.5, 550.6 532, 542.3, 561.3 480, 497, 519, 542,570 496.3, 516.8, 540.3566.8 465, 484, 504, 525,550

6 298 6 255 6 293 6 298

375 266 375 266 375 266 375 266

270 1.7 – 282 144 9 564 37–42

40 41 40 41 40 42 40 43

796

Luminescence properties of the actinides and actinyls

0.001 mM U in 0.1 M NaNO3

Intensity

Intensity

Fig. 7 Aqueous speciation of uranyl in a 0.1 M NaNO3 solution containing 1 mM uranyl nitrate, modeled using PHREEQC51 geochemical modeling from a pH of 4 to 11 by the author.

Wavelength / nm

Tim e/

μs

Wavelength / nm

Fig. 8 Cartoon depicting the use of using time resolved emission spectroscopy to separate overlapping signals in a mixture using lifetime data and spectral slicing, drawn by the author.

where the speciation and stability constants of uranyl hydroxide complexes was investigated.55–57 This demonstrates the importance of using multiple techniques for characterization of multiple species and highlight the benefits of luminescence and timeresolved luminescence spectroscopy over other spectroscopic techniques for the assignment of speciation.

8.19.3.2

Uranyl(V)

In 2005, the first report of the existence of uranyl(V) during the biotic reduction of uranyl(VI) by the sulfur reducing bacterium Geobacter sulfurreducens was reported by X-ray absorption spectroscopy.58 Although uranyl(V) is kinetically unstable with respect to disproportionation, it has recently been shown that it can exist in the environment for much longer than originally thought.12 Uranyl(V) plays a vital mechanistic role in the enzymatic reduction of uranyl(VI), readily disproportionating to uranium(IV) and uranyl(VI), eventually forming the insoluble mineral phase uraninite (UO2). Since uranyl(V) possesses one unpaired 5f electron, if it can be isolated, it represents an ideal opportunity to study the simplest electronic configuration of an open shell actinyl species. Since 2005, a number of researchers have isolated an impressive number of uranyl(V) compounds in anhydrous media, many of which are oligomeric and are held together by uranyl-oxido-uranium electrostatic interactions.15 The uranyl(V) ion itself is only stable in a narrow pH range ( 2–3 and  11) in aqueous conditions and has been prepared photochemically and electrochemically under these conditions. The reduction reaction is readily monitored at suitable concentrations by the disappearance of the uranyl(VI) LMCT absorption band at  420 nm and the concomitant appearance of a broad higher energy band and several lower energy, sharper f-f absorption transitions spanning the range 1180–760 nm.59–63 The first solution luminescence study of uranyl(V) was reported by Steudtner in 2006, via photo-reduction of uranyl(VI) in perchloric acid and 2-propanol at pH 2.4.64 After several minutes of photolysis, a broad emission band centered at 440 nm was observed following excitation at 255 or 408 nm attributed to LMCT luminescence from the UO2þ ion. The measured luminescence lifetime was determined to be 1.1 ms. Residual luminescence from small amounts of UO22þ in the sample solution could be

Luminescence properties of the actinides and actinyls

797

deconvoluted from the UO2þ emission by the employment of gaussian fitting. Bulk electrochemical generation of uranyl(V) carbonate, [UO2(CO3)3]5 was separately achieved in aqueous Na2CO3 solution at pH 11.8.65 However, this complex was non emissive at room temperature, but upon cooling to 153 K, a broad emission with a maximum at 405 nm was recorded upon excitation at 225 nm with a lifetime of 120 ms. Notably, peak deconvolution indicated the presence of five component bands at 386, 393, 405, 420 and 443 nm, possibly indicative of some vibrational coupling to the uranyl(V) LMCT excited state. Given that photochemical and electrochemical generation of uranyl(V) may well be incomplete, to avoid the presence of any remaining uranyl(VI), attention has very recently shifted to the chemical preparation of analytically pure kinetically stable uranyl(V) complexes with a view to fully interrogating the electronic structure of the uranyl(V) ion in detail. In the water soluble uranyl(V) dipicolinic acid (dpa) complex [K(2.2.2.crypt)]2n{[KUO2(dpa)2]}n (Fig. 9) by Mazzanti in 202166 excitation at 360 nm, affords a broad emission spectrum with two maxima at 404 and 459 nm in 5 mM pyridine solutions with excitation maxima at 335 and 360 nm respectively. Of note, following 459 nm excitation, the lower energy maximum becomes red shifted, which indicates the presence of electronically different excited states. In a frozen glass at liquid nitrogen temperature, the emission spectrum exhibits vibronic fine structure, with seven resolved lines that can be distinguished and four sharp transitions (in the 360 nm excitation band). In agreement with the above observations, the luminescence lifetime was best fitted to a bi or multi exponential decay with the longest component being 11 ms. The same uranyl(V) complex also exhibited luminescence in water at pH 10 at room temperature where three separate emission bands were identified at 408, 434 and 524 nm (following excitation at 320, 380 and 445 nm). However, in frozen solution, different emission and excitation profiles were observed with excitation at 271 and 291 nm resulting in emission centered at 403 nm. Our own contributions in this area have focused on the synthesis and electronic structural investigations of a family of discrete molecular uranyl(V) compounds beginning with the monomeric tris-amide complex shown in Fig. 9 prepared by chemical reduction of the uranyl(VI) analog.67 Here, the well-known bis(trimethylsilyl)amide ligand, (N(SiMe3)2) was chosen due to its steric bulk to discourage the formation of oligomers and because the ancillary ligands lack any aromatic chromophores, whose optical properties may interfere with those of the uranyl(V) cation. Indeed, reducing the steric bulk around the uranyl(V) unit in the related bis amide derivative UO2{N(SiMe3)2}2(thf)2 results in the formation of the dimeric complex [K(2.2.2.crypt)]2[UO2{N(SiMe3)2}2]2 which is held together by uranyl-oxo-uranium electrostatic interactions. The monomeric tris amide [K(2.2.2.crypt)]2[UO2{N(SiMe3)2}3] is an unusual emerald green color in solution, due to an optical transition centered at 641 (3 ¼ 145 M 1 cm 1) which is assigned to an amide ) 5f1 charge transfer transition by comparison with the calculated excitations using theory. In dimethoxyethane (dme), tetrahydrofuran (thf) and 2-methyl tetrahydrofuran solutions (2-Me-thf), the resultant room temperature emission spectra exhibit two broad bands at lmax ¼ 355 and 475 nm. At 77 K in a frozen 2-methyl thf solution, the emission feature at lower energy (475 nm) becomes vibrationally resolved with a total of seven vibronic transitions (as seen for [K(2.2.2.crypt)]2n{[KUO2(dpa)2]}n), (Fig. 10). The estimated E0–0 transition of 737 cm 1 agrees well with the total symmetric uranyl stretch measured experimentally by Raman spectroscopy and that predicted computationally, but also possesses some U-amide stretching character. Notably, the excitation spectrum of this blue 475 nm emission band (at 390 nm) is well resolved in stark contrast to the absorption spectrum in this region, with six distinguishable peak maxima. The 475 nm emission band exhibits biexponential decay kinetic with luminescence lifetimes of 1.02 ms and 8.22 ms; the latter contributing 75% to the overall luminescence intensity, in broad agreement with the study by Mazzanti, whereas the lifetimes in fluid solution at room temperature are < 10 ns. Further theoretical calculations including time dependent density functional theory of the uranyl(V) ion itself in the gaseous phase, indicate that the blue vibrationally resolved emission arises from deactivation of an excited state of mainly quartet character where both the degenerate 5f du orbitals are occupied. Further calculations support the origin of this vibrationally resolved 475 nm transition to be an admixture of amide to U(5f) and O(yl) to U(5f) excitations, i.e., the equivalent LMCT transition in the uranyl(V) complex [K(2.2.2.crypt)]2[UO2{N(SiMe3)2}3] originates from electronically impure excited state. By contrast, the excited state uranyl(VI) ion is essentially a pure triplet state with single occupation of the 5fdu orbital (Fig. 4).

8.19.3.3

Uranium(IV)

To date, only a handful of U(IV) compounds have been investigated for their luminescence properties. Using the Russel-Saunders LS coupling scheme, the electronic terms of the 5f2 ion can be well approximated and are analogous to Pr3þ (4f2).3 With this in mind,

2-

3O

O N

O

Fig. 9

O O U

O O O

O 3[K(2.2.2.crypt)]+

N

(Me3Si)2N

O U

O

n

(Me3Si)2N

2[K(2.2.2.crypt)]+ N(SiMe3)2

O

Recently reported uranyl(V) emissive complexes. Left, [K(2.2.2.crypt)]2n{[KUO2(dpa)2]}n66 and right, [K(2.2.2.crypt)]2[UO2{N(SiMe3)2}3].17

798

Luminescence properties of the actinides and actinyls Wavenumber (cm-1) 150

00

100

Wavenumber (cm-1)

00

30000

400

25000

20000

30

Emission Intensity (a.u.)

Extinction coefficient (M-1 cm-1)

450

00 00 250 200

350 300 250 200 150 100

25 20 15 10 5

50 0 400

600

800

1000

Wavelength (nm)

1200

1400

0 350

400

450

500

550

600

Wavelength (nm)

Fig. 10 Left, UV–vis-nIR absorption spectrum of the uranyl(V) complex [K(2.2.2.crypt)]2[UO2{N(SiMe3)2}3] in DME solution (298 K). Right, steady state emission spectrum of [K(2.2.2.crypt)]2[UO2{N(SiMe3)2}3] in 2-Me-thf solution following UV excitation (280–300 nm). Figure modified and redrawn by the authors from Ortu, F.; Randall, S.; Moulding, D. J.; Woodward, A.; Kerridge, A.; Meyer, K.; La Pierre, H. S.; Natrajan, L. S. J. Am. Chem. Soc. 2021, 143, 13184–13194.

it may be expected that the emission profile of U(IV) salts and compounds would be dominated by intra configurational f-f transitions. Early reports focused on comparisons with the valence isoelectronic lanthanide ions in doped crystalline solids including LiYF4,68 ThBr4, ThCl4, ThSiO4 and Cs2 ZrBr6, Cs2GeBr6 and Cs2UCl6.69 In all these systems, relatively broad charge transfer emission and relatively less intense bands in the UV–visible region of the electromagnetic spectrum were recorded. For example, in LiYF4:U4þ, more intense UV emission bands at 262, 282, 304, 328 and 334 nm alongside two visible weaker bands at 438 and 492 nm were observed. These have been assigned with support from experiment and computation to be inter-configurational transitions from the 5f2 electronic ground state (3H4) to the 3F2 excited state arising from the 5f16d1 excited state configuration (Fig. 11). Luminescence lifetimes were noted to be essentially independent of temperature and were approximately 17 ns for both the UV and the visible transitions. Conversely, the U(IV) aqua ion in 1.0 M perchloric acid solution gave rise to 10 sharp emission bands (reminiscent of those seen for lanthanide(III) ions) with maxima at 525, 409, 394, 345, 338, 335, 320, 318, 291 and 289 nm upon high energy excitation at 245 nm.70 This excitation wavelength corresponds to the energy of the intra-configurational 1S0 ) 3H4 absorption band, with all other bands corresponding to f-f excitations as recorded in the electronic absorption spectrum. Deconvolution techniques indicated that a total of 12 bands could be assigned to transitions originating from the 1S0 excited state to the lower lying 1 1 I6, G4,3P0,1D2,3F3,3F4and3H5 spin orbit coupled states by comparison with existing data. While the luminescence lifetimes were shorter than 20 ns and could not be resolved from the envelope of the laser pulse, at 77 K luminescence lifetimes of 149 ns and 198 ns were determined in H2O and D2O respectively facilitating optical detection at low concentration (down to 10 6 M).71 Notably, subsequent theoretical investigations of the U(IV) aqua ion illustrated that including a correction for a large Stoke’s shift of the highest energy term (rather than treating the U4þ aqua ion as the free ion in a solvent continuum) results in slightly different assignments of the emission bands.72 Paralleling this work, a combined experimental and computational study of the U(IV) chlorides UCl4(thf)3, UCl5 and UCl62 in thf solutions affords emission spectra that resemble those recorded in U(IV) doped solids (Fig. 11) upon UV excitation (230– 350 nm).73 Three groups of transitions are observed with approximate energy maxima of 360, 420 and 510 nm (the latter overlapping spectrally with uranyl(VI) LMCT emission), supporting the origin of the transitions involving 5f16d1 / 5f2 de-excitations. Given that the emission is independent of excitation wavelength used, this indicates that the luminescent transitions occur from the lower energy part of the 5f16d1 manifold (the 3F2 state) and that this state lies lower in energy than the 1S0 state. Indeed, theoretical state averaged CASSCF (complete-active-space self-consistent-field) calculations are in agreement, and further support the conclusion that the emissive state in the family of U(IV) chlorides is the 5f16d1 state with the 1S0 state lying slightly higher in energy (Fig. 11). Accordingly, the energy gap law dictates that the emissive state will be the 3F2 state.74 By contrast, in the aqua ion, equivalent calculations suggest that the emissive state is the 1S0 state with the 5f16d1 relatively raised in energy. Each envelope of the three emission bands in UCl4(thf)3, UCl5 and UCl62 comprises transitions to groups of lower lying L-S coupled states (including the ground state, Fig. 11). Accordingly, the luminescence lifetimes are multiexponential and lie in the nanosecond range ( 2–10 ns). Note here, that at 77 K, the large quantum yield of trace amounts of uranyl(VI) enables both the U(IV) and uranyl(VI) chloride emission to be observed simultaneously, which has important implications for following REDOX reactions in solution in real time. Other emissive U(IV) compounds include the macrocyclic complex [U(DO3A)(dmso)2]Br (DO3A ¼ [4,7,10-tris-carboxymethyl,1,4,7,10-tetraaza-cyclododec-1-yl]-acetic acid, dmso ¼ dimethylsulfoxide),75 [Et4N][U(NCS)5(bipy)2] (bipy ¼ 2,20 -bipydridyl)76 and the U(IV) halides UF4, UBr4 and UI4 exhibit similar spectral profiles.77

Luminescence properties of the actinides and actinyls

799

Fig. 11 Top left, steady state emission spectrum of UCl4(thf)3 at 295 K following 331 nm excitation, recorded by the author; bottom left, steady state emission spectrum of [Li(thf)4][UCl5(thf)] at 77 K following 331 nm excitation, showing the presence of both U(IV) and uranyl(VI) emission bands. Right, computed mean ground and excited state energy levels and assignments for U4þ in H2O, [UCl6]2, [UCl5(THF)] and [UCl4(THF)3]. Note a solvent continuum of H2O was used in the calculations of U4þ, but no explicit water molecules ligated to the U(IV) center were included, meaning the 1S0 state may be artificially high in energy. Reprinted with permission from Hashem, E.; Swinburne, A. N.; Schulzke, C.; Evans, R. C.; Platts, J. A.; Kerridge, A.; Natrajan, L. S.; Baker, R. J.; RSC Adv. 2013, 3, 4350–4361, Copyright (2012) Royal Society of Chemistry.

It is worth mentioning here that trivalent uranium compounds are generally non-emissive due to the high density of spin orbit coupled states and the presence of broad charge transfer inter-configurational absorptions which both favor energy loss by nonradiative decay. However, U(III) doped LiYF4 crystals have been investigated for their potential as diode pumped lasing materials. In this system, intense emission in the near infra-red region between 2.1 and 2.6 mm has been documented with a long luminescence lifetime of 170 ms.78

8.19.4

The neptunyl and plutonyl ions

After uranium, as the actinide series is traversed, the 5f orbitals of the trans uranyl ions are progressively filled, which imparts important consequences on their chemistry. Importantly, he 5f1 neptunyl(VI) ion is much less thermodynamically stable than the 5f2 neptunyl(V) ion and readily undergoes reduction to NpO2þ in protic media (water, alcohols). By contrast, since plutonium can exist four oxidation states simultaneously79 the REDOX chemistry of plutonium is diverse and heavily depended on the precise chemical conditions and disproportionation and redox interchange reactions occur easily. These facts are probably partly why there is a scarcity of reports of luminescence of these ions. The absorption spectra of both NpO22þ and NpO2þ cations possess characteristic intra f-f transitions in the near infra-red region of the electromagnetic spectrum at  1230 and 980 nm respectively for the aqua ions and are routinely used to easily identify the solution oxidation state.20

800

Luminescence properties of the actinides and actinyls

Wilkerson et al. first investigated the near infra-red emissive properties of the high symmetry (D4h) neptunyl(VI) ion (NpO22þ) doped into Cs2[UO2Cl4] in Cs2U(Np)O2Cl4 crystals using both pulsed visible excitation (628 nm) and continuous wave excitation (633 nm) at room temperature and liquid nitrogen temperature.80–82 Two near infra-red emission features were observed at 1473.8 and 1509.4 nm resulting from transitions from two different low lying 5f centered excited states to the 5f2 ground level. Notably, at both temperatures vibrational progression is evident which is particularly resolved at low temperature, enabling identification of the electronic origins of the transitions at 1453 and 1451 nm. These assignments were further supported by the presence of hot bands; the energy difference between these and the electronic origin corresponding to the infra-red active vibrational modes of Cs2[NpO2Cl4]. The luminescence decay times measured from the most intense peak are long-lived and determined as 20 ms at 295 K (1453) nm and 71 ms at 75 K (1451 nm). A subsequent report on pure Cs2[NpO2Cl4]82 revealed that the electronic origin of the emission at 1452 nm is located at higher energy at room temperature compared to liquid nitrogen temperature (as seen in the doped system) and also contained a number of vibrational hot bands. The luminescence lifetime of 2 ms however is an order of magnitude shorter than that measured in the doped system Cs2[U(Np)O2Cl4] at room temperature indicative of a higher degree of vibrational quenching in the single actinide system Cs2[NpO2Cl4]. In a comparative study, near infra-red emission at 1490 and 1580 nm of the neptunyl(V) aqua ion along with the polyoxometalate coordination complexes ([Na2(NpO2)2(PW9O34)2]12 and [Na2(NpO2)2(GeW9O34)2]14) was documented upon excitation at 337 nm in aqueous solutions.83 In the case of the aqua ion, 337 nm corresponds to the Np ) O LMCT absorption, whereas in the coordination complexes, 337 nm corresponds to the more intense W ) O LMCT absorption. For the aqua ion in D2O, the decay kinetics are dominated by non-radiative pathways such that the lifetimes of both emission bands were indistinguishable from the envelope of laser pulse (< 10 ns). This indicates that for the aqua ion, the emission is almost completely quenched by efficient overlap of the wavefunctions of the excited state with the second vibrational harmonic of the OeH bond. Indeed, in the polyoxometalate complexes, where the neptunyl(VI) ion is sandwiched between two ligands limits the number of bound inner sphere water molecules. The luminescence lifetimes of the sensitized emission at 1490 nm in the complexes increased to 62 ns in D2O and the intensity ratios of both emission bands changed from the aqua ion upon complexation indicative of a change in local symmetry of the neptunyl ion. Lifetime measurements upon incremental addition of H2O to the D2O solutions importantly enabled an estimation of the near infra-red luminescence lifetime of the aqua ion (7.6 ns) by factoring outer sphere quenching. In 2015, our group reported the first UV–visible emission of the neptunyl(VI) ion in the compound NpO2(TPIP)2(Ph3PO) (where TPIP ¼ tetraphenylimidodiphoshinate) which was prepared from the unusual spontaneous oxidation of NpO2þ and also from NpO22þ salts in chloroform and methanol.84 Our rationale for employing the TPIP ligand was that the ligand p-p* transitions are restricted to the UV (ca. 260 nm) and therefore should not overlap with any LMCT absorptions in the neptunyl unit. Upon excitation into the neptunyl charge transfer region at 320 nm, a broad emission band centered at 438 nm was recorded with superimposed vibrational progression. The measured peak to peak separations of the first two highest energy vibronic bands of 1349 cm 1 indicated that this was a result of coupling of the electronic excited state to a PeN bond stretch in the TPIP ligand (Fig. 12) and suggests the excited state has some ligand character as seen with the uranyl(V) complex [K(2.2.2.crypt)]2[UO2{N(SiMe3)2}3].17

Fig. 12 Steady state emission spectrum (red trace) and excitation spectrum (black trace) of Np(TPIP)2(Ph3PO) (solid state molecular structure also depicted) in CH2Cl2 solution recorded at room temperature by the author.84 Adapted with permission from Woodall, S. D.; Swinburne, A. N.; Banik, N. L.; Kerridge, A.; Di Pietro, P.; Adam, C.; Kaden, P.; Natrajan, L. S. Chem. Commun. 2015, 51, 5402–5405, Copyright (2015) Royal Society of Chemistry.

Luminescence properties of the actinides and actinyls

801

Time dependent density functional calculations revealed three excitations at 363, 390 and 397 nm with substantial NpO22þ LMCT character in addition to significant contributions from N-TPIP(2p)-Np(5f) excitations (55%, 22% and 20%) respectively. The luminescence lifetime recorded at the emission maximum (438 nm) is biexponential in agreement with excited state(s) of mixed origin (1.3 (95%) and 5.0 (5%) ns). The decay constants of the visible emission in this complex are significantly shorter than all near infra-red emission, which suggests fast non-radiative decay to the low lying 5f-f manifold. This is evidenced in part by the long lived LMCT emission (1.66 ms) of in the isostructural uranium complex UO2(TPIP)2(Ph3PO). Since the actinyl units in the TPIP complexes possess differing excitation and emission profiles as well as decay kinetics it is possible to discriminate between the two in solution by spectral editing using time resolved measurements. In an approximate equimolar solution of UO2(TPIP)2(Ph3PO) and NpO2(TPIP)2(Ph3PO) (and even with an excess of UO2(TPIP)2(Ph3PO)), emission from both uranyl(VI) (lmax ¼ 523 nm) and neptunyl(VI) (lmax ¼ 438 nm) was observed using 320 nm excitation. Changing the excitation wavelength to better match the absorption maxima of each complex resulted in exclusively either uranyl(VI) emission (lexc ¼ 290 nm) or neptunyl(VI) emission (lexc ¼ 380 nm), suggesting that in the correct system, individual actinyl ions in a mixture can be detected at the same time, At the time of writing, there remains only one account describing the emissive properties of the neptunyl(V) ion. Visible (greenyellow) LMCT emission from the NpO2þ aqua ion and in aqueous solutions of the complexes with acetylacetonate and 2hydroxypyridine-1-oxide have been reported.85 Pulsed laser excitation at 337 nm or 405 nm did not reveal any background distinguishable near infra-red emission (as seen in the case of NpO22þ), but a broad feature centered at 560 nm with biexponential decay kinetics of 0.6 and 3.4 ns in D2O solutions were measured for the aqua ion. Similarly, the temporal profiles of the complexes of acetylacetonate (acac) and 2-hydroxypyridine-1-oxide (HPO) in D2O were also biexponential with lifetimes determined as 1.9 and 9.2 ns for [NpO2(acac)2] and 1.9 and 9.8 ns for [NpO2(HPO)2] respectively. The biexponential kinetic behavior was attributed to slow exchange of coordinated water molecules with the bulk solvent with respect to the experimental timescale. Similarly to the above, there is only one report concerning the emission of the plutonyl(VI) ion, PuO22þ in the doped system Cs2[U(Pu)O2Cl4].86 At liquid nitrogen temperatures, in the solid phase, 628 nm excitation led to several near infra-red emission bands between 980 and 1665 nm with the most intense grouping of transitions at  1025 nm. These were assigned to intraconfigurational f-f transitions based on examination of the excitation and absorption spectra Notably, the emission spectrum is much more complex than in Cs2[U(Np)O2Cl4] consistent with the increased number of spin-orbit coupled excited levels for the 5f2 electronic configuration. Time resolved studies showed rise times in the temporal profiles of between > 10 ns to a maximum of 0.1 ms before the exponential decay indicating population from a different excited state. Fitting the grow in component and the decay gave luminescence lifetimes that span 1.2–2.3 ms for the 30 emission bands that were resolvable in the emission spectrum indicating multiple non-radiative relaxation pathways. However, the similar temporal profiles between the transitions is evidence that the emission arises from deactivation of the same excited state or that the origin is a common longer-lived excited state that feeds into lower energy excited states.

8.19.5

Transuranic actinide ions

Transuranic actinides that have displayed luminescence in one or more of their oxidation states are americium (Am), neptunium (Np), plutonium (Pu), curium (Cm), berkelium (Bk), californium (Cf) and einsteinium (Es).87,88 Luminescence studies of transuranic actinides are mainly limited to Am and Cm as Bk, Cf and Es suffer from limited supply, and their high radioactivity renders luminescence studies very difficult to perform. Luminescence of NpF6 and PuF6 in the vapor phase was reported by Beitz et al.89 Both complexes were excited at 1064 nm and luminescence observed at 1360 nm for the Np(IV) compound and 2300 nm for PuF6. Luminescence of trivalent curium is the most studied of all the transuranic actinides, owing to the high quantum yield which allows for studies at very low concentrations of curium in solution.90 The Cm(III) ion shows a wide range of luminescence decay times and emission maxima in varying coordination environments and is an essential indicator of solution speciation. Importantly, the luminescence properties of the minor trivalent actinides Am(III) and Cm(III) are of great utility in liquid-liquid extraction processes for the quantification of lanthanide(III)/actinide(III) separation processes under development to reduce the overall heat load and radioactivity of high level nuclear wastes.91

8.19.5.1

Americium(III)

Luminescence from the Am(III) ion is often difficult to study owing to the short luminescence decay time, high radioactivity, and low molar absorption coefficient in solution87 reflected by the small number of reports that have focused on the luminescence properties. The first comprehensive report of Am(III) emission at room temperature was described by Beitz et al. in 1994.92 Direct f-f excitation was carried out at 503 nm and a spectrum of three well resolved bands between 570 and 1100 nm were observed; these transitions were assigned to intra-configurational 5f-5f transitions originating from the 5D1 subshell. In subsequent studies Yusov et al.93 studied Am(III) species in a range of organic and aqueous solvents and showed that the emission was consistent in all cases and three main emission bands at approximately 700, 840 and 1030 nm assigned to the 5D1 / 7FJ (J ¼ 1,2,3) transitions were observable in addition to the 5D1 / 7F0 band at 570 nm, albeit much weaker in intensity. In an analogous manner to the trivalent lanthanides (Nd, Eu, Tb, and Yb)94 energy matched O-H vibrational harmonics of bound and closely diffusing water molecules can be used to determine the hydration number of Am(III) complexes in aqueous

802

Luminescence properties of the actinides and actinyls

solution based on vibrational quenching differences in H2O and D2O solutions using the equation developed by Kimura (Eq. 1), where kH2O,Am is the experimentally determined luminescence decay constant for Am(III) in s 1.95 The luminescence lifetimes of the Am(III) aqua ion were determined to be 24.6 ns in H2O and 162 ns in D2O. NH2O;Am ¼ 2:56  107 k H2O;Am  1:43

(1)

Photoexcitation of americium is very sensitive to the solution speciation of the complex in question due to differences in local crystal fields and has been noted to range from 503 to 510 nm. This means that when using narrow band tuneable lasers with spectral linewidths (down to 0.01 nm), discrete absorption bands belonging to a given species can be selectively excited and probed. Investigations on the complexation of ligands with Am(III) have focused on the 5D1 / 7F1 transition which emits in the 660– 740 nm range and is sensitive to ligand coordination. Changes in peak energy, lifetime and luminescence intensity have been observed upon ligand coordination. Time resolved laser induced fluorescence has also been used to study the coordination and interaction of americium with small organic molecules in order to model the potential attenuation of Am(III) through the environment. For example, proligands including pyromellitic acid,96 anthranilic acid, picolinic acid97 and phthalic acid were reacted with Am(III) and the luminescence spectra recorded to determine the stability constants and number of ligands that coordinate to the Am(III) ion in the resultant complexes. A further study into the complexation of Am(III) with the carboxylic acid group, widely found in the geosphere, was undertaken by Kim et al.98 The oxalate ligand (C2O42) finds important uses as a ligand for separation of decay products from fission, yet the compounds formed are not well understood. The complexation of Am(III) with increasing oxalate concentrations was investigated, where the absorption maxima was shifted gradually from 503.0 to 506.9 nm and an increase in the molar absorption coefficient was observed. The gradual changes in the absorption spectra suggest that several Am(III)-oxalate species exist with different distribution ratios. Luminescence spectroscopy as also been utilized to probe heterogeneous solid state interactions. Stumpf et al. studied the incorporation of Am(III) into calcite by TRLFS in order to characterize the adsorption and incorporation of americium in environmentally important minerals.99 Calcite was doped with Am(III) using a mixed-flow reactor, allowing for crystal growth under constant conditions. Using an excitation wavelength of 497 nm the 5D1 / 7F1 luminescent transition was observed. In order to distinguish between surface-sorbed and Am(III) incorporated into the crystal structure, the calcite doped with Am(III) was cooled to 18 K. At low temperatures, the structurally incorporated Am(III) resolves the crystal field splitting, and narrowing of emission bands results. By contrast, the temperature change should not affect the surface-sorbed Am(III) owing to the highly disordered orientation in relation to the calcite crystal structure. The emission spectra differ depending on the excitation wavelength, indicating that different Am(III)/calcite species exist. The peaks at 685.7, 688.0, 709.5 and 690.5 nm are sharp and can be attributed to Am(III) species which have a well-defined orientation in relation to the calcite crystal lattice. At 503 nm excitation, a broader peak appears at 697.7 nm assigned to Am(III) sorbed onto the surface of the calcite. This conclusion was supported by lifetime measurements of the 697.7 nm band, which fitted well to a biexponential decay 414  16 and 1875  45 ns. After analysis of the spectra at different delay times (TRLIFS), it was concluded that the shorter lifetime belongs to the sorbed Am(III) species and long lifetime was attributed to incorporated Am(III). Regarding synthetic coordination chemistry, the isolated Am(III) complexes containing the ligand bis[(phosphino)methyl]pyridine-1-oxide (NOPOPO) ligand, [Am(NOPOPO)2(NO3)](NO3)2 was shown to luminesce in the solid state.100 The NOPOPO ligand was investigated as a selective extraction agent for trivalent actinides. Excitation at 503 nm resulted in strong emission at 690.5 nm. Time resolved studies showed that the total emission spectrum remained unchanged with increasing delay time, indicating a single emissive species. The inner hydration sphere of the crystal was determined to be zero.

8.19.5.2

Curium(III)

Curium luminescence is the most widely studied of the trivalent actinides. The large energy gap between the excited state and ground state of Cm3þ facilitates detection of the visible emission, even at concentrations as low as 10 7 M.87 The most commonly used isotope, 248Cm, has a long half-life (340,000 years) and the lower radioactivity in comparison to most transuranium elements renders it easier to handle. In aqueous solutions containing the aqua ion, excitation at 395 nm produces a spectrum with a maximum at 593.8 nm and is assigned to the 6D7/2 / 8S7/2 transition (Fig. 13).87 This transition is sensitive to the local coordination environment around the Cm3þ ion and has a typical emission range of 593–615 nm dependent on the species in question. Upon complexation, the emission of the aqua ion typically shifts to lower energy and more strongly coordinating ligands give a more pronounced red shift of the 6D7/2 / 8S7/2 emission band. Curium in its uncommon þ IV oxidation state has also displayed luminescence properties; this was observed in Cm doped into a CeF4 matrix. The Cm(IV) ion was excited at 501.3 nm and emission was observed around 602 nm with a decay time in the 50 ms region.87 The speciation of Cm(III) has been shown to effect the peak maximum and the decay times in the emission spectrum. Additionally, upon ligand complexation, crystal field splitting of the 8S7/2 ground state (resolved into 4 levels) and the 6D7/2 and 6P5/2 excited states occurs giving rise to splittings (often seen as shoulders) in the spectrum.101 These changes upon coordination can therefore be used to investigate the coordination of curium in different environments and quantify its interaction with different ligands. TRLIFS studies have been carried out on many different curium systems including nitrate,102 glycolic acids103 hydroxyquinoline,104

Luminescence properties of the actinides and actinyls

803

1.2

Normalised Emission Intensity

1

0.8

0.6

0.4

0.2

0 574

580

586

592

598

604

610

616

622

628

634

Wavelength (nm) Fig. 13

Emission spectrum of a 1.2  10 7 M Cm(III) aqueous solution excited at 396.6 nm recorded by the author.

proteins105,106 and pyrazine based ligands being investigated for lanthanide/actinide separation chemistry by liquid-liquid extraction.107 The luminescence lifetimes can be utilized to calculate the number of inner sphere coordinated water molecules (as with Am(III)), Kimura et al.,108 determined the linear relationship between lifetime and number of inner sphere coordinated water molecules through measuring lifetimes in different ratios of deuterated solvent. NH2O ¼ 0:65  103 kobsðCmÞ  0:88

(2)

Since Cm(III) will be an isotope present in higher level nuclear wastes, researchers have interest in studying the interactions of curium with relevant molecules in order to determine its potential for migration within the engineered and natural environment and biologically through the body. Barkleit et al.109 investigated the speciation of Cm(III) with the protein alpha-amylase (Amy), a major enzyme in saliva and the pancreatic secretions of mammals, to give insight to the potential biological transport of Cm(III). Spectrophotometric titrations of Cm(III) at constant pH (5.5) with increasing concentration of Amy (0.01–0.85 g L 1) and at constant Amy concentration (1.0 g L 1) with increasing pH (3.1–8.0) were carried out in order to characterize the Cm-Amy complexes. Increasing the Amy concentration and pH resulted in red shift of the 6D7/2 / 8S7/2 peak from 593.8 to 603.0 nm, indicating a complexation reaction between Cm(III) and Amy. The recorded lifetime measurements showed biexponential decay kinetics that indicated that at least two independent species were present in solution. The two calculated lifetimes from the lifetime decay measurements were 120  10 and 240  40 ms, these lifetimes were attributed to two species: one with 7 water molecules displaced from the inner coordination sphere of Cm(III)and one with 5 displaced water molecules. Using the lifetime data, it was concluded that the three possible species in solution were Cm3þ, Cm(Amy-COO)2þ and Cm(Amy-COO)3. The Cm(III) luminescence spectra were normalized to the whole peak area and analyzed using SPECFIT software to isolate the spectrum corresponding to the individual species and calculate binding constant (log b) values. The log b values calculated were 4.76  0.11 and 12.13  0.12 for Cm(Amy-COO)2þ and Cm(Amy-COO)3 respectively. The log b values were then used to calculate the distribution of the Cm(III)-Amy species as a function of Amy concentration and pH. The binding constants are important in reliable modeling of Cm(III) speciation in the digestive system, which can aid in medical treatment following accidental oral incorporation of radioactive heavy metals. In another study, emission spectra were measured at [Cm] ¼ 1.15  10 7 M, at a pH of 2.52 and 3.44 as a function of phosphate concentration.110 Increasing phosphate concentration resulted in a shift of the emission spectrum to a longer wavelength, suggesting a progressing complexation reaction was occurring in solution. The multicomponent spectra were decomposed into individual components by stepwise extraction. In the decomposed spectrum at pH 2.52, two additional Cm-phosphate complexes were observed at 599.2 and 600.4 nm, along with the peak associated with the non-complexed Cm(III) ion. The lifetimes of the samples were collected in order to calculate the number of inner sphere water molecules calculated. The obtained lifetime curves only displayed mono-exponential decay curves despite having multiple species in solution indicating fast chemical exchange on the experimental timescale between coordinated water and phosphate ligands. In order to confirm

804

Luminescence properties of the actinides and actinyls

the denticity of the ligands theoretical lifetimes were calculated according to Eq. 3. Lifetimes were assumed according to the number of displaced water molecules: 68 ms for Cm3þ(aq), 73 ms for Cm(H2PO4)2þ, and 83 ms for Cm(H2PO4)2þ and Cm(HPO4)þ. The experimental and calculated theoretical values were in good agreement and the binding modes were further confirmed with computational modeling.       1 1 1 1 ¼ a þ b þ ðc þ dÞ  (3) sðcalc:Þ 68 ms 73 ms 83 ms A further study into curium decontamination strategies using the octadentate ligand, 3,4,3-LI(1,2-HOPO) (1-hydroxy-N-[3-[(1hydroxy-6-oxo-pyridine-2-carbonyl)-[4-[(1-hydroxy-6-oxo-pyridine-2-carbonyl)-[3-[(1-hydroxy-6-oxo-pyridine-2-carbonyl) amino]propyl]amino] butyl]amino]propyl]-6-oxo-pyridine-2-carboxamide) which comprises four 1-hydroxy-pyridin-2-one (1,2HOPO) units linked to a spermine scaffold via amide groups was carried out by Sturzbecher-Hoehne et al.111 Luminescence was utilized to calculate the stability constant of 3,4,3 with Cm(III) in order to model its interactions within biological systems and assess the binding of the ligand to Cm(III). The photophysical properties of the curium-3,4,3-LI(1,2-HOPO) hydroxy pyridinone complex were investigated in buffered solutions at pH 7.4. The luminescence investigation showed the ligand acted as an antenna chromophore for Cm(III), resulting in highly sensitized luminescence emission upon complexation of the ligand. The emission spectrum of the complex showed the characteristic 6D7/2 / 8S7/2 transition, with peaks at 610, 589 and 579 nm. The splitting in the emission was a result of crystal field splitting of the emitting state, the most intense peak at 610 was from the lowest Stark level of the J ¼ 7/2 excited state and the higher energy splitting (at 589 and 579 nm) were the remaining Stark components. The sensitized emission possessed a long luminescence lifetime of 383 ms and high quantum yield (45%). Lifetime measurements enabled the average number of inner sphere coordinated water molecules to be determined as 1. Spectrophotometric titrations were carried out at low (mM) concentrations to calculate the binding constant of the complex by addition of a solution of the ligand to a solute on Cm3þ(aq) at pH 1.1, and the solution titrated to pH 4.0 using KOH. An immediate increase in emission intensity at 610 nm was observed, corresponding to the formation of the complex. No spectral change was observed after pH 3, indicating a high proton-independent stability constant (log b) which was calculated to be 21.8. With a large log b value good biological stability was expected, as the ligand had a very strong affinity for curium, meaning it can outcompete complexation with intrinsic proteins. This hypothesis was confirmed with in vivo experiments where 98% of ingested curium was excreted upon treatment with 3,4,3-LI(1,2-HOPO). Interestingly, the same complex has more recently been investigated for its two-photon excited ligand sensitized Cm(III) emission at 684 nm. The two photon cross sections were found to be two- to threefold higher than those in the corresponding lanthanide derivatives, highlighting important differences between the 4f and 5f series.112 Recently, Raymond and co-workers explored the optical properties of Cm(III) complexes of two chiral octadentate ligands derived from orthoamide phenol by circularly polarized luminescence (CPL) spectroscopy which is able to distinguish between different forms of chirality and is particularly effective when probing f-f transitions.113 They found that the two complexes exhibited significant differences in the emission maxima (8–10 nm compared to 0–3 nm in lanthanide analogs), consistent with ligand field effects and differences in electronic structure between the Cm(III) ion and the lanthanides. Such findings indicate that CPL spectroscopy can provide more insight into the electronic structure of actinide ions while discriminating between different chiral molecules, for example right handed vs left handed helices in metalloproteins.

8.19.5.3

The late actinides Bk-Es

The latter actinide ions in the 5f series are only produced in very small quantities (typically micrograms) from nuclear reactions and are extremely radioactive and very difficult to manipulate. This has meant that progress has been relatively slow and reports on the optical properties of Bk-Es are rare. Nevertheless, there have been some quite remarkable advances in this area in recent years that have complimented earlier studies. Luminescence from trivalent berkelium was reported in 1983 by Carnall, who attributed the absorption band in the UV at 299 nm to a 6d ) 5f inter-configurational transition in the aqua ion with the 5f76d1 electronic configuration corresponding to an 7F6 term.114 Excitation of a 0.5 M DCl in D2O solution (to minimize O-H vibrational quenching), produced red emission at 647 nm following pulsed excitation at 391 nm with a luminescence lifetime of 0.1  0.02 ms. Using the calculated intrinsic radiative lifetime of 0.5 ms (i.e., in the absence of any quenching), the quantum yield of this emission was estimated to be 2.0  10 4. When immobilized in a silicate glass, the trivalent oxidation state is stabilized with respect to the solution state, and two emission bands were recorded at 651.5 and 742 nm.115 The relative intensities of these two transitions are laser power dependent, so that at lower powers, the lower energy emission band dominates the spectrum and at higher laser powers, the 651.5 nm band is the principle transition. The tetravalent oxidation state of Bk was initially investigated in doped CeF4 crystals, where two emission lines at 610.7 and 491.2 nm were recorded and assigned to depopulation of the lowest energy 6D7/2 state and the 6P5/2 state respectively.92 More recently, the þ IV oxidation state of Bk was stabilized using the siderophore inspired ligand 3,4,3-LI(1,2-HOPO) that selectively coordinates to actinide(IV) ions over actinide(III) ions.116 Prior to this, the þ IV oxidation state of Bk was only stabilized in a narrow pH range at high pH and exhibited complicated speciation. The resultant synthesized complex exhibits ligand sensitized emission via energy transfer from the ligand to the emissive f-f state at 320 nm excitation and four narrow emission peaks at 590, 612, 569 and 702 nm were observed, where the most intense transition is the one centered at 612 nm. The presence of four emissive transitions (rather than one) is attributed to ligand field splitting of the 6D7/2 emissive state, rather than the 8S7/2 ground state (f7 ion). The authors

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805

additionally noted that upon complexation, all the emission maxima undergo pronounced red shifts from  10–100 nm, which is considerably larger than the complexation induced red shifts from the aqua ions in the case of Am(III) and Cm(III). The optical properties of californium(III) doped into LaCl3 were initially investigated in 1962,117 where the ionizing radiation emitted induces self-luminescence. A number of emission lines in this compound containing some small crystal field splittings were noted 370.6 and 378.9 nm, 384.4 and 389.3 nm and 426.7 and 433.5 nm. When doped into a borosilicate glass, an emissive transition at 684.9 nm was observed due to a transition between the 6H7/2 and 6H15/2 LS states.118 In addition, californium borate has been shown to possess a very broad visible emission band with a peak maximum around 500 nm and notable vibrational progression across the entire spectrum The emission was assigned to a transition from the 4P5/2 first excited state (Fig. 3), with a measured decay constant of 1.2  0.3 ms. Early studies of einstenium luminescence in the þ III oxidation state focused on the Es3þ ion itself in doped gadolinium iodide crystals,119 and doped in LaF3 crystals.120 In the latter case, very long lived (2060  100 ms) visible and near infra-red emission between 658 and 1010 nm arising from transitions from the 5F5 excited state were observed at an excitation wavelength of 526 nm. The decay constant is unusually very close in value to the intrinsic radiative lifetime suggesting that the Es3þ ion in the low phonon LaF3 lattice is essentially vibrationally isolated. In the crystalline coordination complex CsGd(hfac)4 (hfac ¼ hexafluoroacetylacetonate) doped with Es(hfac)4, Nugent et al., were able to demonstrate near infra-red ligand sensitized emission for the first time (1023–1120 nm) and assigned the emission band to the J ¼ 7 / J ¼ 8 transition.121 In the solution state however, measurement of the emission spectrum was much more challenging due to efficient vibrational quenching by closely diffusing solvent molecules, yet in the system di(2-ethylhexyl)orthophosphoric acid and Es3þ in n-hexane the corresponding emission is detectable and the lifetime measurable (s ¼ 2.34 ms).122 In aqueous solution, the Es(III) aqua ion emits at 1080 nm with decay lifetimes of 1.05 ms in H2O and 2.78 ms in D2O.122 Comparison of lifetime data with those lanthanide(III) ions that possess similar energy gaps between the ground state and excited state (e.g., Yb3þ) allude to the fact that non-radiative quenching is considerably greater in the 5f series compared to the 4f series, presumably related to the increase in spin-orbit coupling and crystal field effects in the actinides. Intriguingly, the emission maximum in the near infra-red shifts to higher energy relative to the free ion upon ligand coordination in stark contrast to the trivalent Am(III) and Cm(III) ions and tetravalent Bk, where noticeable shifts to lower energy are commonly observed. This blue shift of the emission (75 nm, 690.7 cm 1) was observed to be even more pronounced in the Es(III) complex of 3,4,3-LI(1,2-HOPO) prepared on a 200 ng scale by Kozimor, Abergel and co-workers.123,124 Ligand excitation in the UV results in well-defined near infra-red emission at 1005 nm assigned to the 5I5 / 5I8 transition (Fig. 14). The authors suggest that a change in

Fig. 14 Energy-level diagram for complexed HOPO and Es3þ(aq) showing sensitized emission from the ligand to the metal and the emissive transition (blue arrow). Adapted with permission from Carter, K. P.; Shield, K. M.; Smith, K. F.; et al. Nature 2021, 590, 85–88, Copyright (2021) Springer Nature.

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electronic structure around einsteinium in the actinide series may account for the differences and increased spin orbit coupling for the heaviest actinides may change the electronic regime toward a j-j coupling scheme from the Russell-Saunders spin-orbit coupling scheme. A shift in electronic structure regime has important effects on the chemical bonding and hence the chemical and physical properties of these essentially unexplored ions.125

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