Porphyrin-based Supramolecular Architectures. From Hierarchy to Functions 9781839161803, 9781839164934, 9781839164941

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Porphyrin-based Supramolecular Architectures From Hierarchy to Functions

Monographs in Supramolecular Chemistry Series editors: Philip Gale, The University of Sydney, Australia Jonathan Steed, Durham University, UK

Titles in this series: 1: Cyclophanes 2: Calixarenes 3: Crown Ethers and Cryptands 4: Container Molecules and Their Guests 5: Membranes and Molecular Assemblies: The Synkinetic Approach 6: Calixarenes Revisited 7: Self-assembly in Supramolecular Systems 8: Anion Receptor Chemistry 9: Boronic Acids in Saccharide Recognition 10: Calixarenes: An Introduction, 2nd Edition 11: Polymeric and Self Assembled Hydrogels: From Fundamental Understanding to Applications 12: Molecular Logic-based Computation 13: Supramolecular Systems in Biomedical Fields 14: Synthetic Receptors for Biomolecules: Design Principles and Applications 15: Polyrotaxane and Slide-Ring Materials 16: Boron: Sensing, Synthesis and Supramolecular Self-Assembly 17: Porous Polymers: Design, Synthesis and Applications 18: Pillararenes 19: Supramolecular Chemistry at Surfaces 20: Aromatic Interactions: Frontiers in Knowledge and Application 21: Naphthalenediimide and its Congeners: From Molecules to Materials 22: Functional Supramolecular Materials: From Surfaces to MOFs 23: Supramolecular Amphiphiles 24: Co-crystals: Preparation, Characterization and Applications 25: Molecular Gels: Structure and Dynamics 26: Understanding Intermolecular Interactions in the Solid State: Approaches and Techniques 27: Metallomacrocycles: From Structures to Applications 28: Cucurbiturils and Related Macrocycles

29: Dendrimer Chemistry: Synthetic Approaches Towards Complex Architectures 30: Supramolecular Protein Chemistry: Assembly, Architecture and Application 31: Reactivity in Confined Spaces 32: Porphyrin-based Supramolecular Architectures: From Hierarchy to Functions

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Porphyrin-based Supramolecular Architectures From Hierarchy to Functions

Edited by

Shengqian Ma University of North Texas, USA Email: [email protected] and

Gaurav Verma University of North Texas, USA Email: [email protected]

Monographs in Supramolecular Chemistry No. 32 Print ISBN: 978-1-83916-180-3 PDF ISBN: 978-1-83916-493-4 EPUB ISBN: 978-1-83916-494-1 Print ISSN: 1368-8642 Electronic ISSN: 2041-7144 A catalogue record for this book is available from the British Library r The Royal Society of Chemistry 2022 All rights reserved Apart from fair dealing for the purposes of research for non-commercial purposes or for private study, criticism or review, as permitted under the Copyright, Designs and Patents Act 1988 and the Copyright and Related Rights Regulations 2003, this publication may not be reproduced, stored or transmitted, in any form or by any means, without the prior permission in writing of The Royal Society of Chemistry or the copyright owner, or in the case of reproduction in accordance with the terms of licences issued by the Copyright Licensing Agency in the UK, or in accordance with the terms of the licences issued by the appropriate Reproduction Rights Organization outside the UK. Enquiries concerning reproduction outside the terms stated here should be sent to The Royal Society of Chemistry at the address printed on this page. Whilst this material has been produced with all due care, The Royal Society of Chemistry cannot be held responsible or liable for its accuracy and completeness, nor for any consequences arising from any errors or the use of the information contained in this publication. The publication of advertisements does not constitute any endorsement by The Royal Society of Chemistry or Authors of any products advertised. The views and opinions advanced by contributors do not necessarily reflect those of The Royal Society of Chemistry which shall not be liable for any resulting loss or damage arising as a result of reliance upon this material. The Royal Society of Chemistry is a charity, registered in England and Wales, Number 207890, and a company incorporated in England by Royal Charter (Registered No. RC000524), registered office: Burlington House, Piccadilly, London W1J 0BA, UK, Telephone: þ44 (0) 20 7437 8656. Visit our website at www.rsc.org/books Printed in the United Kingdom by CPI Group (UK) Ltd, Croydon, CR0 4YY, UK

Preface Porphyrins can be regarded as nature’s toolbox for the sustenance of life in all living systems. Present as an essential component in natural systems, from heme in animals to chlorophyll in plants, these are vital molecules for us. Evidenced in nature for up to 1.1 billion years in the remains of extinct organisms, they have been studied extensively to understand their evolutionary processes. Porphyrins allow us to gain a control that only nature can achieve and thus they have been regularly explored by researchers to mimic natural systems. Besides being used as individual compounds, porphyrins and their derivatives can act as building blocks to generate frameworks or assemblies. A control over the structural rigidity, connectivity, and the functionality of porphyrin provides a route to designing porphyrinbased supramolecular frameworks for various applications such as catalysis and photodynamic therapy, among many others. In this book, we summarize the significant advances made in the field of supramolecular chemistry of the porphyrins. With our experience in the early development of metal–metalloporphyrin frameworks, it gives us an immense pleasure to provide commentary on the advances made in this field. The present book contains a collection of 9 chapters written by authors that have contributed to the development of their respective field. We have focused on two major porphyrin-based supramolecular architectures, metal–organic frameworks (MOFs) and covalent organic frameworks (COFs). In Chapter 1, Zhou’s group introduces us to the riveting architectures and topicity of the porphyrin and metalloporphyrin MOFs built from various types of linkers and metals via the secondary building unit (SBU) approach. The Reek group delves further into the supramolecular assemblies constructed from the porphyrin building blocks for specific catalytic applications in Chapter 2. The coordination-driven design of porphyrin MOFs and their rich topological diversity are explored by the Choe group in Monographs in Supramolecular Chemistry No. 32 Porphyrin-based Supramolecular Architectures: From Hierarchy to Functions Edited by Shengqian Ma and Gaurav Verma r The Royal Society of Chemistry 2022 Published by the Royal Society of Chemistry, www.rsc.org

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Chapter 3. The specific design and systematic tunability in porphyrin MOFs can help us target the specific function in order to relish the desired applications, some of which are discussed in the following chapters. In Chapter 4, Ma’s group examines porphyrin MOFs as heterogeneous catalysts for various organic transformation reactions. The Larsen group (Chapter 5) details the developments made in metalloporphyrin-containing MOFs referred to as MOFZymes, to mimic the function of heme enzymes. The light harvesting properties of the porphyrin MOFs and their relevant photo/electrochemical applications are discussed in Chapter 6 by the Deria group. The biomedical applications of porphyrin MOFs are elucidated by the Tian group in Chapter 7, whereby they summarize the use of nanoscale porphyrin MOFs in photodynamic therapy (PDT) and provide an outlook for overcoming the challenges in their clinical PDT use. The last two chapters bring about the developments made in porphyrinbased COFs. In Chapter 8, the Jiang group addresses the topology-directed design of COFs built from porphyrins, phthalocyanines, and their metallocompounds; along with the examination of their various properties. Finally, in Chapter 9, the Fang group elaborates the significant developments in the catalytic applications of COFs. This book aims to provide an overview of the progress made in porphyrinbased supramolecular architectures, and an insight into the significant challenges and future goals for further progress in this area of porphyrinic materials. Shengqian Ma and Gaurav Verma

Contents Chapter 1 Structural Design of Porphyrin-based MOFs Peiyu Cai, Yutao Huang, Mallory Smith and Hong-Cai Zhou 1.1 1.2

Introduction Carboxylate Linkers 1.2.1 Tetratopic Carboxylate Linkers 1.2.2 Ditopic Carboxylate Linkers 1.2.3 Hexatopic Carboxylate Porphyrin Linkers 1.2.4 Octatopic Carboxylate Porphyrin Linkers 1.3 Pyridinyl Porphyrin Linkers 1.3.1 Tetratopic Pyridinyl Linkers 1.3.2 Ditopic Pyridinyl Linkers 1.4 Azole Porphyrin Linkers 1.4.1 Tetratopic Pyrazolate Linkers 1.4.2 Tetratopic Tetrazole Linkers 1.4.3 Ditopic Imidazole Linkers 1.5 Other Tetratopic Linkers 1.5.1 Phosphonate Porphyrin Linkers 1.5.2 Cyanoporphyrins 1.5.3 Hydroxy Porphyrins 1.6 Porphyrin Linkers References Chapter 2 Metalloporphyrins as Building Blocks to Supramolecular Architectures with Catalytic Functions T. Keijer and J. N. H. Reek 2.1

Introduction

1

1 2 2 23 26 26 40 40 42 44 44 44 45 46 46 46 47 47 49 59

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Monographs in Supramolecular Chemistry No. 32 Porphyrin-based Supramolecular Architectures: From Hierarchy to Functions Edited by Shengqian Ma and Gaurav Verma r The Royal Society of Chemistry 2022 Published by the Royal Society of Chemistry, www.rsc.org

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2.2

Self-assembled Porphyrin Structures in Catalysis 2.2.1 Confined Space Hydroformylation Catalysis 2.2.2 Catalyst Stabilization and Size Selectivity in Confined Space 2.2.3 Catalyst Stabilization and Lowering Overpotential in Confined Space 2.3 Supramolecular Light Harvesting Porphyrin Assemblies – Chromophores 2.3.1 Chromophore Organization 2.3.2 Supramolecular Porphyrin Antennae 2.3.3 Light Harvesting Using Supramolecular Polymer Assemblies of Porphyrins 2.3.4 Supramolecular Scaffolds Pre-organizing Porphyrins 2.4 Supramolecular Charge-separation with Porphyrin Assemblies – Charge-separation 2.4.1 Discrete Supramolecular Charge-separation Assemblies 2.4.2 Integrated Antenna/Charge-separation Assemblies 2.5 Integrated Supramolecular Light-harvesting Porphyrin Catalyst Assemblies – Chromophore– Catalyst Assemblies 2.5.1 Supramolecular Photochemical Proton Reduction 2.5.2 Supramolecular Photochemical Water Oxidation 2.6 Conclusion References Chapter 3 Design of Porphyrinic Metal–Organic Frameworks Soochan Lee and Wonyoung Choe 3.1 3.2

Introduction Symmetry-guided Coordination Networks with Porphyrin Building Units 3.2.1 Pyridyl-based Porphyrin Coordination Networks 3.2.2 Self-assembly of Porphyrinic Metal–Organic Frameworks with Secondary Building Units 3.2.3 Rod-packing Secondary Building Unit-based Porphyrinic Metal–Organic Frameworks

60 61 65 67 68 69 70 83 84 84 85 90

94 94 98 99 100 106

106 107 107 108 129

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3.3

Mixed-linker Approach for New Porphyrin-based Metal–Organic Frameworks 3.4 Other Noteworthy Porphyrin-based Coordination Networks 3.4.1 Coordination Networks Based on Sulfonate and Phosphonate-containing Porphyrin 3.4.2 Self-assembly of Polyoxometalate-based Porphyrinic Coordination Networks 3.5 Conclusions and Future Directions Abbreviations Acknowledgements References Chapter 4 Heterogeneous Catalysis of Porphyrin-based MOFs Zachary Magnuson and Shengqian Ma 4.1 4.2 4.3

Introduction Catalysis of Cycloaddition Reactions Catalysis of Reactions Involving Alkanes, Alkenes, and Alkynes 4.4 Other Reactions Catalyzed by Por-MOFs 4.5 Conclusion References Chapter 5 Porphyrin-encapsulating Metal–Organic Materials as Solid-state Mimics of Heme Enzymes Randy W. Larsen 5.1

5.2 5.3

5.4

Introduction 5.1.1 Bioinspired Heme-based Materials 5.1.2 Metal–Organic Framework Materials (MOFs) 5.1.3 Metalloporphyrin-based Metal–Organic Materials Porphyrin-encapsulated rhoZMOF MOMZymes: Porphyrin Encapsulated HKUST-1 MOFs 5.3.1 MOMZymes II: Porphyrin-encapsulated MOM-XX MOFs Fe Protoporphyrin IX MOFs I: FePPIX@MIL-101(Al)–NH2 5.4.1 Fe Protoporphyrin IX MOFs II: FePPIX/Cu–MOF-74 5.4.2 Fe Protoporphyrin IX MOFs III: FePPIX-encapsulated Zn MOF

131 135 135 138 139 141 142 142 149

149 150 152 158 164 164

166

166 169 170 171 172 173 177 178 180 181

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5.5 Summary and Future Perspectives Abbreviations References

183 184 184

Chapter 6 Light Harvesting in Porphyrin-based Metal–Organic Frameworks Xinlin Li and Pravas Deria

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6.1 6.2 6.3

Introduction Background of MOFs Introduction to PorMOFs 6.3.1 PorMOFs with Pyridyl Binding Groups 6.3.2 PorMOFs with Carboxylate Ligands 6.4 Electronic and Photophysical Properties 6.4.1 Energy Transfer 6.4.2 Photo-induced Charge Transfer 6.4.3 Charge Transfer in the Ground State 6.5 Light-harvesting of PorMOFs 6.5.1 Photocatalysis by PorMOFs 6.5.2 Photoelectrochemical Processes in PorMOFs 6.5.3 Electrocatalysis 6.6 Conclusion and Outlook References Chapter 7 Nanoscale Porphyrinic Metal–Organic Frameworks for Photodynamic Therapy Xiang Lian, Chuxiao Xiong and Jian Tian 7.1 7.2

Introduction Applications of Porphyrinic nMOFs in PDT 7.2.1 The Advantage of nMOFs as Photosensitizer Carriers 7.2.2 Overcoming Tumor Hypoxia to Enhance Photodynamic Effect 7.2.3 Improving the Method of Excitation to Enhance Photodynamic Effect 7.2.4 PDT Combined with Immunotherapy for Metastatic Tumor Treatment 7.2.5 Improving the Biosafety of Porphyrinic nMOFs in PDT 7.3 Conclusion and Prospects References

188 191 196 197 200 207 207 216 226 229 230 236 240 243 243

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256 257 257 261 269 273 275 279 279

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Chapter 8 Porphyrin and Phthalocyanine Covalent Organic Frameworks: Pre-designable Structures and Tailor-made Materials Ruoyang Liu and Donglin Jiang 8.1

Introduction 8.1.1 Catalysis 8.1.2 Adsorption 8.1.3 Semiconductors 8.1.4 Energy Storage 8.1.5 Summary and Perspectives References Chapter 9 Catalysis via Porphyrin-based COFs Y. Yusran, X. Guan, S. Qiu and Q. Fang 9.1 9.2

Introduction Design and Description of Porphyrin COFs 9.2.1 Structural Topology and Linkage Types 9.3 Catalysis by Porphyrin-based COFs 9.3.1 Catalytic Sites of Porphyrin COFs 9.3.2 Catalysis Application of Porphyrin COFs 9.4 Summary Abbreviations References Subject Index

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284 285 307 314 321 326 327 331

331 333 333 337 337 338 356 356 357 361

CHAPTER 1

Structural Design of Porphyrin-based MOFs PEIYU CAI, YUTAO HUANG, MALLORY SMITH AND HONG-CAI ZHOU* Department of Chemistry, Texas A&M University, College Station, Texas 77842-3012, USA *Email: [email protected]

1.1 Introduction Metal–organic frameworks (MOFs) are crystalline, porous materials assembled from inorganic nodes linked by multitopic organic ligands. The organic ligands, also known as linkers, typically have a rigid skeleton to prevent the collapse of the framework upon desolvation of the internal pore structure. The inorganic nodes can be monoatomic metal ions or clusters, the latter of which is the basis of the secondary building unit (SBU). SBUs are inorganic polynuclear clusters that have advantages over the use of monoatomic metal nodes, including enhanced stability, modularity, and structural predictability.1,2 Many reviews have been written to cover the topic of SBUs and their use in constructing polymeric organic/inorganic materials.3–6 MOFs are attractive materials due to their modularity, tunable porosity, structural diversity, and high crystallinity and have been targeted for various applications including gas storage/separations, catalysis, sensing, biomedicine, and water purification. Furthermore, the incorporation of metalloporphyrins into solid-state materials has long been sought after due to their interesting photophysical properties, catalytic abilities, and biological Monographs in Supramolecular Chemistry No. 32 Porphyrin-based Supramolecular Architectures: From Hierarchy to Functions Edited by Shengqian Ma and Gaurav Verma r The Royal Society of Chemistry 2022 Published by the Royal Society of Chemistry, www.rsc.org

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Chapter 1

relevance. Additionally, a variety of metals can be incorporated into the porphyrin core, making metalloporphyrins functionally diverse. Robson et al. reported the earliest example of a porphyrinic coordination network constructed from Cd ions and tetrapyridylporphyrin linkers.7 However, as discussed, the use of monoatomic nodes tends to lead to framework collapse, and pyridyl-based linkers are somewhat limited in coordination mode. Encapsulation of porphyrins is another approach, but structural order is difficult to achieve and leeching is inevitable.8–10 Incorporation of multitopic porphyrin linkers, usually carboxylates, has led to a plethora of stable porphyrinic MOFs with permanent porosity and high crystallinity using a variety of metals. The following chapter discusses primarily MOFs that have their singlecrystal structure reported over the last two decades, most of which use the SBU approach in the construction of permanently porous frameworks. Some porphyrinic MOFs were not included in this chapter as their single crystal structures were not reported. The chapter is divided first into linker types (carboxylate, pyridinyl, etc.) and topicity followed by the type of metal used in the SBU. The chapter begins with tetratopic carboxylate linkers which is the most frequently encountered linker and thus occupies a large portion of the discussion here. The porphyrin linkers depicted in this chapter are also tabulated at the end of the chapter.

1.2 Carboxylate Linkers 1.2.1 Tetratopic Carboxylate Linkers 1.2.1.1 Zr and Hf Clusters Construction of MOFs based on high oxidation state metals such as Zr41 is attractive due to their high stability and rich structural diversity. An interesting feature of Zr6 clusters is their ability to vary in the number of linker connections while maintaining the robust [Zr6(m3-O)4(m3-OH)4] core. Tetratopic porphyrin linkers such as tetrakis(4-carboxyphenyl)porphyrin (TCPP, Porphyrin 1), which is the most commonly encountered porphyrinic linker, can also adopt different coordination geometries due to the free rotation of the benzene rings. As a result, TCPP can form various MOFs with different topologies with Zr clusters that can be synthetically controlled. One series of MOFs that demonstrates the structural variation is Zr–TCPP MOFs: PCN-221(MOF-525),11,12 PCN-222(MOF-545),12,13 PCN-223,14 PCN-224,15 PCN-225,16 and NU-902.17 It should be noted that PCN-221 has unique Zr8 clusters, while the remaining five MOFs contain Zr6 clusters (Table 1.1). PCN-22111,12 is a microporous MOF with a Brunauer–Emmett–Teller (BET) surface area of 1936 m2 g1 when the free-base (non-metalated) TCPP linker is used to construct the MOF. PCN-222(Fe) exhibits a steep increase in its nitrogen adoption isotherm at P/P0 ¼ 0.3, suggesting mesoporosity.12,13 The BET surface area was observed to be 2200 m2 g1 for PCN-222(Fe), while the structurally

Cluster connectivity Topology Porosity

PCN-221

PCN-222

PCN-223

PCN-224

PCN-225

NU-902

12 ftw Microporous

8 csq Mesoporous

12 shp-a Microporous

6 she Microporous

8 sqc Microporous

8 scu Microporous

Structural Design of Porphyrin-based MOFs

Table 1.1 Series of MOFs based on TCPP linkers and Zr clusters.

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Chapter 1

related PCN-223 is a microporous MOF of surface area B1600 m g and porosity of B0.6 cm3 g1.14 PCN-224 is a microporous MOF and appears to be the most porous of the six-MOF series and has a surface area of 2600 m2 g1, which is in agreement with reduced connectivity.15 The calculated total pore volume of PCN-224 is as high as 1.59 cm3 g1. In addition to PCN-222, two other 8-connected microporous MOFs have been reported. PCN-225 exhibits an sqc topology and BET surface areas of 1902 and 2080 m2 g1, respectively, for PCN-225 and PCN-225(Zn).16 NU-902, has a scu topology and a BET surface area of 1580 m2 g1. When HCl was used for sample activation, the BET surface area increased to 2150 m2 g1 likely due to the removal of a residual benzoate modulator. The pore size distribution of NU-902 features a pore diameter of 12 Å. Three distinct distances were found between porphyrins within the framework of 10.5 Å, 18.2 Å, and 20.9 Å.17 Due to the high charge density and bond polarization of the Zr41 cations, the interaction with the carboxylate oxygen atoms is very strong, leading to the high aqueous stability observed in the MOFs listed in Table 1.1. The acid/base stability varies depending on the structure. PCN-222 is highly acid-stable, with no change observed in the PXRD pattern or adsorption isotherm after prolonged treatment with concentrated HCl, but is less stable in basic solutions.12,13 At the same time, PCN-223, PCN-224, and PCN-225 have demonstrated aqueous stability in the pH range of 1 to 10, 1 to 11, and 1 to 12, respectively.13–15 The coordinatively unsaturated PCN-224 has the ability to accommodate additional linkers including isonicotinic acid (INA) or 4,4 0 -dicarboxydiphenyl sulfone (DCDPS).18 The ditopic auxiliary linkers allow the fabrication of multi-component MOFs as depicted in Figure 1.1. The DCDPS linkers installed in PCN-224 were used to further incorporate Zr6/Hf6 clusters inside the cavity, and the INA installed PCN-224 was used to accommodate metal cations such as Cu and Ni to further increase their structural diversity. This subsequent incorporation of organic linkers and inorganic clusters into the cavity of PCN-224 provides a methodology for effectively increasing the structural diversity of MOFs, and the resulting MOF was proven to be efficient as a cooperative bimetallic catalytic system. Tetratopic carboxylate porphyrin linkers are not limited to TCPP. A series of functionalized and elongated linkers have also been synthesized for the construction of porphyrin MOFs with larger cavities (Figure 1.2). These MOFs, namely PCN-228, PCN-229, and PCN-230, are isoreticular to the 12-connected MOF PCN-221 and were constructed from elongated linkers  Porphyrin 2 (TCBPP), Porphyrin 3 (TCTTPP), and Porphyrin 4 (TCBPP), respectively.19 The three MOFs gradually increase in cavity size from 2.5 nm to 3.8 nm, allowing for exceptionally large surface areas: 4510 m2 g1 (PCN-228), 4619 m2 g1 (PCN-229), and 4455 m2 g1 (PCN-230). Other sets of elongated MOFs isoreticular to PCN-221 were also explored using similar ligands and were named MOF-526, MOF-527, MOF-528, MOF-536, and NU-1104.20,21 There is also a case where a MOF isoreticular to PCN-225 was reported by using an amide elongated tetratopic carboxylate porphyrin linker.22 2 1

Structural Design of Porphyrin-based MOFs

Figure 1.1

5

(a) TCPP linker, (b) PCN-224, (c) PCN-224(Ni), (d) PCN-224(Ni)–INA, (e) PCN-224(Ni)–DCDPS, (f) PCN-201(Ni)–Cu, (g) PCN-201(Ni)–Ni, (h) PCN-202(Ni)–Hf and (i) PCN-202(Ni)–Zr. Reproduced from ref. 18, https://doi.org/10.1038/s41467-018-03102-5, under the terms of a CC BY 4.0 license, https://creativecommons.org/ licenses/by/4.0/.

Several examples have also been reported where a secondary linker is used to bind to the Zr6 SBUs along with TCPP linkers (Porphyrin 1). For example, PCN-138 uses 4,4 0 ,400 -(2,4,6-trimethylbenzene-1,3,5-triyl)tribenzoate (TBTB) to form rhombicuboctahedrons with TCPP linkers and Zr6 clusters (Figure 1.3). The framework structure is further extended through face

6

Chapter 1

Figure 1.2

Isoreticular Zr MOFs from linkers TCTTPP, TCTPP, TCBPP, and TCPP. Adapted from ref. 19, https://pubs.acs.org/doi/10.1021/ja5111317, with permission from American Chemical Society, Copyright 2015.

Figure 1.3

The structure of PCN-138 built with TCPP and TBTB. Reproduced from ref. 23 with permission from American Chemical Society, Copyright 2019.

sharing between multiple rhombicuboctahedrons.23 In another example, phase-pure PCN-222 and PCN-224 were synthesized by incorporating ditopic linkers including 4,4 0 -biphenyldicarboxylic acid (BPDC) and 2,6-naphthalene carboxylic acid (NDC), respectively. The introduced secondary linkers can link the coordinatively unsaturated sites on the Zr6 clusters, which facilitates

Structural Design of Porphyrin-based MOFs

7

the formation of phase pure products. Otherwise, the 12-connected MOF PCN-223 often contaminates the low-connectivity products.14,24

1.2.1.2

Ti Clusters

Due to the photocatalytic potential of ultrasmall titanium oxide particles, MOFs with Ti–O clusters have been highly sought after. However, like Zr MOFs, the strong binding between Ti and carboxylates complicates the synthesis of phase pure materials, making Ti-MOFs difficult to obtain in pure form. Porphyrins, on the other hand, are ideal photosensitizers for increasing catalytic efficiency, therefore, building titanium MOFs with porphyrinic ligands continues to be in demand despite synthetic challenges. One example of a Ti-based MOF with TCPP linkers is known as DGIST-1, which has a homogeneous pore size of 0.75 nm (Figure 1.4).25 DGIST-1 consists of 1D Ti-oxo chains, within which Ti41 centers were octahedrally coordinated by four oxygen atoms from four TCPP ligands and two m2-O atoms, while two carboxylate groups and one m2-O bridge the neighboring Ti41 centers. Due to the photosensitized Ti41 cluster, DGIST-1 can photocatalytically oxidize alcohols. Another MOF containing Ti41 1D chains is Ti– (TiTBP) (TBP ¼ TCPP) which has a BET surface area of 527.7 m2 g1.26 In the repeat unit of the Ti–(TiTBP), five different Ti atoms are present with slightly different coordination environments. Different from DGIST-1, the five Ti atoms are not bridged by m2-O atoms but instead are bridged by oxygen atoms from acetate groups, reducing the MOF symmetry to the P21/c space group. Besides the 1D chains, Ti41 can also form a Ti7O6 cluster with TCPP linkers (Figure 1.5). Each Ti7O6 cluster is comprised of two Ti3O3 subunits constructed from a m3-O2 ion and six carboxylates. A seventh Ti atom bridges the two Ti3O3 clusters to form the 12-connected Ti7O6 cluster. This

Figure 1.4

Crystal structures of (a) Ti6O6(OiPr)6(t-BA)6 clusters, (b) DGIST-1 along the b-axis, and (c) DGIST-1 along the c-axis. Green ¼ Ti, yellow ¼ O; blue ¼ N, gray ¼ C. H atoms omitted for clarity. Reproduced from ref. 25 with permission from John Wiley & Sons, Copyright r 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

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Figure 1.5

Chapter 1

(a) TCPP linkers combined with 12-connected Ti7O6 clusters; PCN-22 along (b) the a-axis and (c) the b-axis. (Red ¼ O, black ¼ C, blue ¼ N, cyan ¼ Ti). H-atoms omitted for clarity. Reproduced from ref. 27 with permission from the Royal Society of Chemistry.

structure, named PCN-22, has tetragonal channels B1.5 nm in diameter and can also function as a catalyst for the oxidation of alcohols.27

1.2.1.3

Paddlewheel Clusters

Many divalent metal cations including Zn, Co, and Cu can form binuclear paddlewheel-like clusters with four carboxylate groups.28,29 Like many other ligands, TCPP can also form a variety of stable MOFs comprised of paddlewheel clusters. Owing to the equatorial ligand positions, TCPP usually builds 2D sheet-like MOFs with paddlewheel clusters. Each TCPP linker in these 2D sheets coordinates with four paddlewheel clusters, and each carboxylate group is shared between two metal cations within the paddlewheel. Due to the unsaturated coordination in the z-direction of the paddlewheel clusters, the 2D sheets can be further linked by ditopic linkers to form pillaredlayer structures. Various pillars can be used for constructing pillar-layer structures with TCPP linkers and paddlewheel clusters, but two motifs are most common. The first type has pillars separating the layers by coordinating between the paddlewheels, while the second type further coordinates via metalloporphyrin centers. Depending on the length and flexibility of the pillar, interpenetration sometimes occurs, generating more complex structures. 1.2.1.3.1 Non-pillared Paddlewheel 2D Sheets. Zhao et al. reported a series of ultrathin 2D MOF nanosheets for the first time using a

Structural Design of Porphyrin-based MOFs

9 30

surfactant-assisted synthetic method. In their approach, a 2D layered sheet was formed by linking one TCPP ligand with four Cu paddlewheel metal nodes (Figure 1.6). During the reaction, the TCPP ligand is coordinated with Cu21 ions. The layered sheets are further stacked in an AB pattern, in which copper atoms in the porphyrin rings are aligned with copper atoms in the paddlewheel metal nodes, forming 2D frameworks with space group I4/mmm. AFM imaging revealed that the thickness of the Cu–TCPP nanosheet is 4.5  1.2 nm (Figure 1.7).30 Although not commonly observed due to its high reactivity with oxygen, FeII can form paddlewheel clusters when bipyridine or pyrazine is used as the pillar linker.31 The FeII porphyrin core adopts an octahedral N6 geometry, with four donors from the porphyrin and two from the pillars. This MOF exhibits a BET surface area of 900 m2 g1. TCPP can also form 2D sheets with Cd paddlewheels; one resolved single-crystal structure has been reported. This Cd–TCPP 2D MOF sheet was bridged by salen-based bipyridyl pillar linkers yielding a 2-fold interpenetrated 3D structure with a surface area of 1087.6 m2 g1 and a pore diameter of B10.4 Å.28 Similarly, 2D Zn–TCPP sheets can stack in an AB fashion to create a crystal with I4/mmm space group. The axial positions of the Zn paddlewheels are occupied by two solvent molecules when no pillar ligand is present to bridge adjacent 2D layers.32,33

Figure 1.6

Structures of the building units for 2D Cu–TCPP nanosheets and the crystal structure of 2D Cu–TCPP nanosheets. Reproduced from ref. 30 with permission from John Wiley & Sons, Copyright r 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

10

Figure 1.7

Chapter 1

STEM images of Cu–TCPP nanosheets. Reproduced from ref. 30 with permission from John Wiley & Sons, Copyright r 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

1.2.1.3.2 Pillared Paddlewheel 2D Sheets. There are two primary methods of obtaining 3D pillar-layered MOFs: (a) through a one-pot synthesis where an N-donor pillaring linker is mixed with TCPP prior to the synthesis of the MOF sheets, and (b) by post-synthetically adding pillaring linkers to the preformed Zn–TCPP sheets. The former method in some cases allows the formation of 3D structures with larger/bulkier pillars and often results in interpenetrated structures. On the other hand, the latter method can be used to produce pure non-interpenetrated phases but cannot achieve 3D structures when larger pillars are desired. Post-synthetic linker insertion for 2D TCPP-M2 MOF sheets was observed to proceed by a two-stage process. During the first stage, the pillars bridge two adjacent 2D sheets (Figure 1.8). The new 2D bilayer still applies an AB style stacking without lateral shear movement (PPF-27). In the first linker insertion stage, large/bulky bipyridinyl linkers can be successfully installed despite their high steric hindrance. When a high pillar ligand concentration is used, the bilayer formed will shear by 11.7 Å allowing for an ABBA stacked structure. The direct transformation from independent layers to fullybridged ABBA stacked structure is viable for small pillar linkers, however, the transformation from bilayer structure to ABBA 3D structure is challenging for larger pillar ligands, although the ABBA counterparts can be synthesized via one-pot reactions.32 In many cases, the metal embedded in the porphyrin center will bridge to the paddlewheel clusters in the adjacent MOF sheet via bipyridine-type

Structural Design of Porphyrin-based MOFs

Figure 1.8

11

Stepwise linker insertion from PPF-1 to PPF-27 and finally PPF-4. Reproduced from ref. 32 with permission from the Royal Society of Chemistry.

pillars, yielding an AB-type 3D structure in the I4/mmm space group. This usually occurs when metals such as Mn, Fe, Zn, and Co were located in the porphyrin center since they prefer further axial ligand coordination.34–36 Metalloporphyrins with metals including V, Pt, Ni, and Pd have also been reported and tend to form AA type stacking where the porphyrins do not bridge (Figure 1.9). A method to prevent the formation of the AB type 3D structures is to increase the steric hindrance of the pillaring linker. When switching from 4,4-bipyridine to 2,2-dimethyl-4,4-bipyridine pillars, the steric repulsion of the methyl groups drastically lowers the interaction between the pyridyl group and the metal center. Thus, the AB stacking is prevented and AA stacking with P4/mmm space group can be observed.37 An interesting effect was observed when DMF (N,N 0 -dimethylformamide) is used for the synthesis of Zn–TCPP 2D sheets. When heated for five days in the absence of the pillaring linker, formate anions generated from the slow thermal decomposition of DMF fill the role of the pillar to bridge the 2D layers.38 The formate anions are charge-balanced by [(CH3)2NH2]1 cations generated from the decomposition of DMF and appear to adapt a linear geometry similar to carbon dioxide. Depending on the metal cation embedded in the porphyrin center, different formate-bridged MOFs can be obtained. When long pillars like N,N 0 -di-(4-pyridyl)-1,4,5,8-naphthalenetetracarboxydiimide (DPNI) are used as pillars, a symmetry reduction to the C2/m space group concomitant with 2-fold interpenetration will be observed.39 However, under different conditions, the ABBA stacked structure can still be obtained. Another series of MOFs were reported in which 2D TCPP–Zn sheets were bridged by bidentate dipyridyl porphyrin pillars (Porphyrin 5).40 The ligand addition sequence was found to be important and the targeted materials can only be obtained when the pillar linker was added after TCPP (Figure 1.10).

12

Chapter 1

Figure 1.9

The 2D sheets built with TCPP linker and Zn or Co paddlewheels are pillared with 4,4-bipyridine to give different resulting stacking arrangements that are dependent on the preferred coordination geometry of the metal embedded in the TCPP linker. Reproduced from ref. 36 with permission from American Chemical Society, Copyright 2009.

1.2.1.4

Zn Clusters (Non-paddlewheel)

One example of a MOF constructed from TCPP linkers and Zn4O clusters is PIZA-4.41 In PIZA-4, TCPP linkers coordinate to the edges of the tetrahedral

Structural Design of Porphyrin-based MOFs

Figure 1.10

13

Zn–TCPP 2D sheets bridged by ditopic pyridinyl porphyrinic linkers. Reproduced from ref. 40 with permission from American Chemical Society, Copyright 2011.

¯ Zn4O61 clusters, leading to a highly symmetric crystal in the cubic Fd3 space group. Despite being 2-fold interpenetrated, the framework still exhibits a Langmuir surface area of 800 m2 g1 and 74% free volume.41 Another interesting example is the complex cluster Zn8(tz)6(TDC)3, where Htz is 1H-1,2,3-triazole and H2TDC is 2,5-thiophenedicarboxylic acid.42 It adopts a trigonal prism geometry and can be considered as a cluster originated from two triangular clusters. The two triangular Zn4 clusters are comprised of three triazole groups and are further connected by triply bound TDC linkers. The Zn8 prism cluster can coordinate with ditopic, tritopic, and tetratopic linkers to form periodic nets. When tetratopic linker Zn–TCPP was used to connect the prism clusters, a MOF formulated as [(CH3)2NH2]2[Zn8(tz)6(TDC)3](Zn–TCPP)1.5 can be harvested which crystallizes in the orthorhombic space group Cmca. This MOF framework was named TPMOF-7 and has an stp topology and appears to be 2-fold interpenetrated but exhibits a free volume of 79.7%. When interpenetration is not considered, a single framework in TPMOF-7 is comprised of a channel with an average diameter of approximately 3.4 nm along the a-axis.42

1.2.1.5

MIII Chains (M ¼ Al, Ga, In, Fe)

Some trivalent metal cations including AlIII, GaIII, and InIII can form 1D chains with carboxylate porphyrin linkers. When forming MOFs with TCPP, they appear to adopt the same frl topology and Cmmm space group (Figure 1.11).43 In these isoreticular structures, the side-to-side distance between two adjacent

14

Figure 1.11

Chapter 1

Crystal structures viewed down [001], [100], and [010] of M(III)–TCPP isoreticular MOFs. Reproduced from ref. 43 with permission from John Wiley & Sons, Copyright r 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

porphyrins is approximately 0.5 nm while the face-to-face distance can be as short as 0.34 nm in Ga-PMOF.44 Each porphyrin linker is coordinated to eight Al atoms where each carboxylate group bridges two Al atoms. The Al coordination sphere consists of four carboxylate-derived oxygen atoms in the equatorial plane and two axial OH groups bridging adjacent Al31 centers to form an infinite Al(OH)O4 chain. When pyrazine (pz) pillar linkers are combined with TCPP to form MOF structures with FeCl3, a MOF with FeIII chains can be obtained with the formula of [FeIIpzTCPP(FeIIIOH)2]nxDMF.31 The FeII metalloporphyrin center is generated in-situ and has a near-perfect octahedral environment consisting of four nitrogen atoms from the porphyrin and two nitrogen atoms from two axial pyrazine pillars. However, the inorganic SBU is different from the expected well-known Fe3O cluster, rather than an infinite [Fe(OH)O4]n chain that is very similar to the Al-oxo chain. The infinite chain consists of four carboxylate-derived oxygen atoms in the equatorial plane and two m2 axial bridging hydroxyl groups adjacent to FeIII centers. Interestingly, if FeCl2 was used instead of FeCl3 as the iron source, only amorphous materials were obtained. For this MOF, a reversible type I isotherm was observed with a BET surface area of 760 m2 g1.

Structural Design of Porphyrin-based MOFs

1.2.1.6

15

M3O Clusters (M ¼ Co, Fe, Mg, Mn, Ni)

A typical metal cluster observed in MOFs constructed from TCPP linkers is the M3O(COO)6 6-connected trinuclear cluster. The cluster can consist of both trivalent and divalent metals in different combinations including MIII3, MIII2MII, MIIIMII2, and MII3. In each case, depending on the total charge on the cluster, the terminal ligand can vary to balance the overall charge on the cluster. These MOFs all adopt the same stp topology with honeycomb-like structures. Combined with the various metal ions that can be incorporated into the porphyrin center, many multivariate MOFs can be built (Figure 1.12).45 These MOFs are typically not very stable and tend to collapse upon desolvation. Thus, supercritical CO2 activation for these MOFs is needed for collecting adsorption isotherm measurements.

Figure 1.12

Isoreticular MOF structures built with a metalated TCPP linker and variable trinuclear metal cluster SBUs. Reproduced from ref. 45 with permission from American Chemical Society, Copyright 2016.

16

1.2.1.7

Chapter 1

MII Chains (M ¼ Mn, Ni, Ru, Cd, Pb)

MnII chains are different from the previously discussed MnIII chains that are constructed by single-carboxylate bridged MnII2 clusters. When forming a MOF with TCPP linkers, a crystal with the P21/n space group can be obtained.46 Two distinct Mn atoms can be found in the asymmetric unit, where Mn1 is accommodated with a distorted octahedral MnO6 coordination environment and Mn2 is seven coordinated in a MnO7 coordination environment. This MOF has a relatively low BET surface area of 320.5 m2 g1. Ni-carboxylate-based infinite chains can be found in MMPF-20.47 MMPF-20 crystallizes in the C2/c space group and has a BET surface area of 517 m2 g1. The asymmetric unit consists of two distinct NiII ions, Ni1 is six-coordinated with distorted octahedral geometry while Ni2 is four-coordinated with tetrahedral geometry. The two Ni motifs are bridged by carboxylates to form an infinite 1D chain. This MOF has 1D channels with dimensions of 4.6 Å  12.6 Å along the a-axis. Due to their potential as photocatalysts, Ru-based TCPP MOFs have also been studied. Metalized TCPP ligands can link Ru2 paddlewheels to form a 3D framework of sql topology.48 The two RuIII centers in each Ru2 paddlewheel are bridged by three carboxylate groups from different TCPP linkers, and adjacent Ru2 SBUs are further linked by an additional carboxylate group to form a 1D chain. The distance between adjacent porphyrin centers and Ru2 paddlewheel chains is approximately 1.1 nm, whereas the distance between adjacent Ru chains is 1.6 nm. This MOF has a BET surface area of around 400 m2 g1. TCPP with Cd(NO3)2 can be used to form MOFs containing a pseudo-Cd chain with monoclinic space group C2/c.49 In each asymmetric unit, two crystallographically independent CdII atoms are present. The first Cd atom coordinates to eight carboxyl oxygen atoms from four TCPP linkers, while the second Cd atom is only surrounded by four carboxyl groups from four TCPP ligands and two water molecules. These MOFs contain two channels along the a-axis that are filled with solvent molecules. When Pb(NO3)2 was used to build MOFs with TCPP, a series of isoreticular MOFs with different metals embedded in the porphyrin centers were harvested.50 These MOFs crystallize in the monoclinic P21/c space group with the exception of the VQO embedded motif where the oxo vanadyl group reduces the symmetry to ¯. In these MOFs, the Pb21 ion is chelated by two carboxylate groups from P1 two TCPP linkers as well as three carboxyl oxygen atoms from three other TCPP linkers. These three carboxyl oxygen atoms triply bridge two heptacoordinated Pb21 ions to form the observed network.

1.2.1.8

Linear Trinuclear Clusters MII3 (M ¼ Co, Mn, Ni)

Different from the commonly observed triangular trinuclear clusters, the Co3/Mn3 cluster described here appears to be a linear trinuclear cluster. Unlike triangular trinuclear clusters, linear trinuclear clusters have various types of slightly different coordination environments around the metal

Structural Design of Porphyrin-based MOFs

17

cations. Two known MOFs built with TCPP and Co3/Mn3 clusters have been reported, known as PIZA-2 and PIZA-3.51,52 Taking PIZA-3 as an example, each trinuclear cluster is coordinated to eight carboxylate groups from eight TCPP linkers, and each TCPP linker is connected to four distinct MnII trinuclear clusters. Two different size 1D channels were observed along the crystallographic a-axis with dimensions of 5 Å 9 Å and 7 Å 8 Å. Besides these two channels, PIZA-3 also has another set of 1D channels with dimensions of 3 Å 5 Å along the c-axis. Overall, this framework has 56% void volume. Under different synthetic conditions, another structure can be formed, known as PIZA-1, which consists of linear trinuclear Co3 clusters and TCPP linkers.51 The trinuclear CoII-carboxylate cluster in this MOF appears to be different from that of PIZA-2. The crystal structure of this MOF indicates the existence of a 7 Å  14 Å channel along the a-axis and a 7 Å  9 Å channel down both b- and c- axes, creating an open framework with 74% void volume. Ni can also form linear trinuclear clusters yielding a framework isostructural to the Co3 cluster in PIZA-1.

1.2.1.9

Mononuclear Nodes (InIII, FeIII)

Mononuclear indium(III) ions usually yield [In(COO)4] tetrahedral nodes with carboxylic acids. To date, three InIII MOFs with porphyrin carboxylates have been reported (In–TBP, MMPF-7, and MMPF-8).53,54 Both TCPP and the elongated 5,10,15,20-tetrakis(4-carboxybiphenyl)porphyrin (Porphyrin 2, TCBPP) adopt similar pts networks with slightly different space groups and different degrees of interpenetration depending on the synthetic conditions. In-TBP, MMPF-7, and MMPF-8 are 2-fold, 3-fold and 4-fold interpenetrated, respectively (Figure 1.13). Mononuclear FeIII can also be used as the metal node to build a 3-fold interpenetrated 3D anionic network with TCPP linkers named MIL-141.55 In this structure, each Fe atom is tetragonally coordinated to four oxygen atoms from four TCPP linkers, and the negative charge is counterbalanced by the incorporation of alkali cations including Li1, Na1, K1, Rb1, and Cs1.

1.2.1.10

Lanthanide/Actinide Nodes

Due to the similarity in coordination modes of lanthanide metals, many different crystal structures built with lanthanide metals can be classified into one of several categories. Here, we discuss a few of the commonly observed structure types. 1.2.1.10.1 Lanthanide M6 Clusters. Many lanthanide metals can form similar SBUs to the Zr/Hf clusters. The resulting MOFs built with TCPP linkers are isoreticular to the Zr/Hf-based frameworks. Zhou’s group reported rare-earth (RE)-metal-based cluster synthesized porphyrin MOFs ¯m and was PCN-900(RE). PCN-900(Eu) crystallized in space group Im3 formulated as [(CH3)2NH2]2[Eu6(m3-OH)8(TCPP)1.5(DCDPS)3](solvent)x.56

18

Figure 1.13

Chapter 1

(a) The structure of a mononuclear indium node and Co(TBP) linkers. The structure of (b) 3D MOF network and (c) two-fold interpenetrated MOF structure of In–Co(TBP)-MOF. (d) The adjacent porphyrin linkers resulted from the two-fold interpenetration. Reproduced from ref. 53 with permission from John Wiley & Sons, Copyright r 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

This framework is made up of Eu6 clusters linked by square-planar TCPP linkers and bent, ditopic DCDPS linkers. A structure is formed by connecting each Eu6 cluster with six TCPP linkers which is isostructural to PCN-224 (Figure 1.14). The scaffold structure can be simplified into a 4,6-connected net with she topology. The DCDPS linker bridges a pair of adjacent Eu6 clusters to achieve the 12-connected motif. Topologically, the 12-connected metal clusters and tetratopic TCPP linkers can be regarded as cuboctahedrons and square nodes, respectively. The overall structure was a 4,12-connected net with tam topology and point symbol

Structural Design of Porphyrin-based MOFs

Figure 1.14

19

Crystal structure of PCN-900(RE). Reproduced from ref. 56 with permission from John Wiley & Sons, Copyright r 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

{312  418  524  612}2{34  42}3. PCN-900(Eu)–NiTCPP exhibited a saturated uptake of 645 cm3 g1 at 1 atm and BET surface area of 2523 m2 g1. 1.2.1.10.2 Lanthanide Tetranuclear Clusters. In the first structural group, the porphyrin units are held together by tetranuclear bridging synthons with a 3 : 4 ratio of TCPP:metal. Single-crystals were isolated when Sm, Dy, and Eu were used, and crystalline powders were isolated when La, Ce, Pr, or Nd ions were used (Figure 1.15).57 Each inner lanthanide ion is coordinated by six carboxylate groups from TCPP and one molecule of DMF solvent. These ions are bridged by four carboxylates to peripheral lanthanide ions on

20

Chapter 1

Figure 1.15

Scheme of the coordination mode in M4(TCPP)3(DMF)2(H2O)4, where M ¼ Sm, Dy, or Er and R ¼ porphyrin linkers. DMF is coordinated to the inner ions, while water molecules are coordinated to the outer ions. Dashed bonds at either end denote hydrogen bonds between carboxylic acids and one water ligand. Reproduced from ref. 57 with permission from the Royal Society of Chemistry.

Figure 1.16

M(TCPP)(H2O)2, where M ¼ Dy, Sm, Pr, Gd or Er, and R ¼ the porphyrin framework. Reproduced from ref. 57 with permission from the Royal Society of Chemistry.

either side of the cluster. The peripheral lanthanide ions are coordinated to four carboxylate/carboxylic acid groups and two water molecules. 1.2.1.10.3 Lanthanide 1D Chains. The second structural group has a 1 : 1 ratio of TCPP : Metal and is usually observed when Dy, Sm, Pr, Gd, and Er ions are utilized for the construction of the inorganic SBUs (Figure 1.16).57 The lanthanide cations form 1D chains bridged by carboxylate groups with a distance between neighboring metal ions of B0.48 nm. 1.2.1.10.4 Lanthanide Binuclear Clusters. The third structural group also ¯) or Er (space group has a 1 : 1 ratio of TCPP : Metal when Yb (space group P1 ¯ or P21/c) were used as metal sources. In each binuclear cluster, the two P1

Structural Design of Porphyrin-based MOFs

Figure 1.17

21

Coordination mode of M(TCPP)(H2O)2 or M(TCPP)(H2O)(DMF) where M ¼ Yb or Er, and R ¼ the porphyrin framework. (a) and (b) demonstrate two distinct patterns of dinuclear linkages. Reproduced from ref. 57 with permission from the Royal Society of Chemistry.

lanthanide cations are bridged by two or four TCPP linkers. The bridged binuclear clusters can then yield 2D or 3D respectively (Figure 1.17).57 1.2.1.10.5 YIII Chains. It is not common to see a linear metal-oxo chain constructed from trinuclear rare-earth clusters, where the trinuclear clusters are further bridged by a single carboxylate group to form the 1D chains. One MOF structure with this type of building unit constructed from YIII chains and TCPP linkers is known as NUS-40.58 The molecular formula of NUS-40 was determined to be Y3O6(TCPP)2(DMF)2(H2O)¯ space (COO)DMF and crystallizes in the triclinic crystal system with P1 group. In the crystal structure, the trinuclear YIII clusters were bridged by a single carboxylate group from a TCPP ligand to form 1D metal chains arranged into a 3D framework. This MOF has a BET surface area of 450 m2 g1 and a Langmuir surface area of 663 m2 g1. Porosity analysis of NUS-40 resulted in predominantly micropores of diameter 6 Å.

22

Chapter 1

1.2.1.10.6 Lanthanide Nodes with Auxiliary Linkers. When additional monotopic benzoic acids were added to the synthesis, several lanthanide– TCPP MOFs known as CAU-19-X were obtained.59 These MOFs were shown to differ in space group with CAU-19-4NO2 having the highest symmetry and crystalizes into the orthorhombic Cmmm space group. Although the Ce41 precursor was used for the synthesis, these MOFs contain exclusively Ce31 ions which were believed to be formed during the synthesis. When Pr2(ox)3 is used for MOF synthesis with TCPP, the oxalate anion will also participate in the formation of the framework, and a MOF with a 2 : 1 Pr/porphyrin ratio can be obtained.60 A pair of Pr cations were bound by a bridging oxalate anion and further connected by four different TCPP linkers. Each of the Pr31 cations was found to bind two additional water molecules to satisfy coordination requirements. 1.2.1.10.7 Mononuclear Uranium. One special metal node that forms a framework structure with TCPP is the 3-connected mononuclear uranium SBU [UO2(COO)3] which adopts a hexagonal bipyramid geometry with two oxo groups located on both sides of the plane (Figure 1.18).61 At present, two uranyl-TCPP MOFs have been reported: TCPP-U1 and TCPP-U2 (U-IHEP-4). TCPP-U1 and TCPP-U2 adopt pto and tbo topologies, respectively. However, these uranyl MOFs are unstable in aqueous solutions. 1.2.1.10.8 Hexanuclear Thorium. Hexanuclear thorium can form a 4,8connected MOF with scu topology known as NU-905, which crystallizes in the monoclinic space group C2/m with the formula [Th6(m3-O)2(HCOO)4(H2O)6 (TCPP)4].62 Four formic acid molecules generated from the decomposition of DMF were also observed in the structure while no counterions were observed, indicating NU-905 is a neutral framework. Supercritical CO2 activated NU-905 exhibits permanent porosity and has a surface area of 800 m2 g1. The calculated pore-size distribution shows a broad peak from 4 to 12 Å. Since there are coordinatively unsaturated sites on the Th6 cluster, linkers including 2,2 0 -bipyridyl-5,5 0 -dicarboxylic acid and 4,40 -biphenyldicarboxylic acid can be installed as bridging linkers between open sites of adjacent Th6 clusters. Two examples were reported as Th-IHEP-5 and Th-IHEP-6, both of them crystallize in the monoclinic space group C2/m.63

1.2.1.11

Other Tetratopic Porphyrinic Linkers

A meso-substituted regioisomer of TCPP, T3CPP (Porphyrin 6), has also been used in combination with several metal nodes. Since the carboxylate groups are shifted to the meso position of the aryl rings, the geometry of the resulting MOFs is perturbed.64 Two geometries were observed: (1) the ‘‘chair’’ type with two adjacent carboxylate groups facing the same direction, and (2) the ‘‘table’’ type where all four carboxylate groups face the same direction. T3CPP was used with several types of metal nodes including

Structural Design of Porphyrin-based MOFs

Figure 1.18

23

The crystal structure of (a) TCPP-U1 and (b) TCPP-U2. The dihedral angle between In TCPP-U1 (c) The binding geometry of TCPP in TCPPU1 and (d) TCPP-U2. Reproduced from ref. 61 with permission from the Royal Society of Chemistry.

lanthanide metals as well as Cd-based nodes. Due to their low symmetry and unexamined porosity, these MOFs will not be discussed further.

1.2.2 Ditopic Carboxylate Linkers 1.2.2.1 Cu2 Paddlewheels Mori et al. have successfully synthesized a series of porphyrin-based MOFs with the general composition of [Cu2(M–DDCPP)] [M ¼ Zn21, Ni21, Pd21, Mn31(NO3), Ru21(CO)] using M–DDCPP linkers (Porphyrin 8).65 The [Cu2(Zn–DDCPP)] framework has a (62  82  102)(62  8)2 topology and crystallizes in the tetragonal I4/mmm space group. Within the MOF structure, large cavities were observed with diameters of approximately 20 Å (Figure 1.19). Another Cu2 paddlewheel MOF which has an elongated ligand compared

24

Figure 1.19

Chapter 1

(a) ZnDDCPP moiety of [Cu2(ZnDDCPP)]; (b) cage consisting of eight ZnDDCPP ligands and eight paddlewheel Cu2 nodes. Reproduced from ref. 65 with permission from John Wiley & Sons, Copyright r 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

to M-DDCPP [Cu2(ZnBDCBPP)]5DMF5H2O (where BDCBPP is Porphyrin 9) has also been reported with enlarged window size and internal space.65 The single-crystal structure showed the [Cu2(ZnBDCBPP)] framework has each of its cavities surrounded by sixteen accessible metal units from both the porphyrin linkers and the Cu2 paddlewheels.

1.2.2.2

Co2 Paddlewheel with Additional Linkers

Choe’s group reported a novel MOF with the formula of [Co(cis-ZnDCPP)(bpy)] 4DMFH2O (DCPP ¼ Porphyrin 7) which exhibits a unique layered topology isostructural to CdI2 layers.66 Co paddlewheels were linked by four cis-ZnDCPP linkers which form ribbon-like structures that are further linked by bpy ligands, connecting the zinc atoms within the porphyrin ring and the Co paddlewheels.

1.2.2.3

Mg, Ca, Sr, and Ba Clusters

Four porphyrin-alkaline earth MOFs [Mg(HBCPP)2(DMF)2]n(H2O)7n, [Ca(HBCPP)2(H2O)2]n(DMF)1.5n, [Sr(BCPP)(H2O)(DMA)]n and [Ba(BCPP)(H2O)(DMA)]n were separated from the solvothermal reaction between the free-base linker 5,15-bis(4-carboxyphenyl)porphyrin (BCPP, Porphyrin 10) and alkaline earth metal ions.67 Both 2D and 3D supramolecular networks have been reported. Mg and Ca MOFs crystallize in triclinic system P1 space group. The measured Mg–O bond length is in the range of 2.02–2.06 Å. Mg21 ions show a slightly distorted octahedral coordination geometry and are coordinated in a monodentate form by two oxygen atoms of two DMF solvent molecules and four carboxyl oxygen atoms from four HDCPP ligands (Figure 1.20). One carboxyl oxygen atom remains uncoordinated while each of the two DCPP ligands bridge two Mg ions through two terminal carboxyl oxygen

Structural Design of Porphyrin-based MOFs

Figure 1.20

25

(a) The coordination environments of the Mg center in the MOF; (b) The porphyrin–porphyrin distance in the 1D channel and between the layers along the a-axis. Reproduced from ref. 67 with permission from John Wiley & Sons, Copyright r 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

atoms, forming a porphyrin polymer double chain along the b-axis. Different from the double-stranded structure of the Mg and Ca MOF, the crystals of the Sr and Ba MOF with P21/n space group in the monoclinic crystal system are 3D network structures formed by interlacing DCPP ligands. Each BaII ion is chelated by two oxygen atoms in the terminal carboxyl group of one DCPP ligand, and further chelated with the four carboxyl oxygen atoms of the four DCPP ligands, two water molecules, and one DMA solvent molecule. The oxygen atoms combine to form Ba–O bonds, and the distance ranges from 2.62 to 2.96 Å (Figure 1.21).67

1.2.2.4

Zn4O Clusters

Smithenry et al. proposed a structural model consisting of a 3D framework of zinc trans-biscarboxylate tetraarylporphyrins called PIZA-4, whose carboxylates are coordinated with six edges of the tetrahedral Zn4O6 cluster, ¯ space group maintaining charge neutrality.41 It crystallizes in the cubic Fd3 and the cubic framework has 74% free volume with a pore dimension of 4  7 Å. The surface area calculated from the type I N2 adsorption isotherm was shown to be approximately 800 m2 g1. Ma’s group reported a robust metal-porphyrin MOF (MMPF-18) with the pcu topology, which is composed of quasi-nuclear zinc clusters Zn4(m4O)(COO)6 (Figure 1.22) and a linear porphyrin linker BCPP (Porphyrin 10).68 The strong p–p stacking between the porphyrins and the extended BCPP linkers leads to a 4-fold interpenetrated network with reduced void space and narrow channels. The nitrogen adsorption isotherm shows a typical type I behavior MMPF-18 with an adsorption capacity of 220 cm3 g1 at 1 atm,

26

Chapter 1

Figure 1.21

(a) The coordination environments of the Ba center in a MOF structure; (b) the 3D supramolecular network structure in the bc plane. Reproduced from ref. 67 with permission from John Wiley & Sons, Copyright r 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

indicating its microporosity. The BET surface area of MMPF-18 is calculated to be 700 m2 g1 and the pore size distribution is centered between 7 and 12 Å.

1.2.3

Hexatopic Carboxylate Porphyrin Linkers

Only one hexatopic carboxylate porphyrin linker has been reported, namely H6HCPP (Porphyrin 11). It consists of two monotopic carboxylates and two dicarboxylates which can form a variety of different structure types with metals including Zn, Mn, Co, In, Pb, and Ln (Ln ¼ Pr, Gd, Yb).69 A schematic illustration is included in Figure 1.23.

1.2.4 1.2.4.1

Octatopic Carboxylate Porphyrin Linkers Cu2 Paddlewheel

Chuan-De Wu’s group synthesized a MOF, namely ZJU-22, with a chemical formula of [Cu16(MnIIIOCPP)3(OH)11(H2O)17]21DMF65H2O, where octacarboxylic acid MnIII–H8OCPP (Porphyrin 13) was used as the organic linker.70 ZJU-22 crystallizes in the hexagonal space group P63/mcm, in which a

Structural Design of Porphyrin-based MOFs

Figure 1.22

27

(a) The ubiquitous SBU and H2BCPP linear porphyrin linker of Zn4(m4-O) (-COO)6 used in the construction of MMPF-18. (b) The MMPF-18 structure containing in-situ metalized Zn porphyrin. (c) Simplified noninterpenetrating pcu topology network. (d) The 4-fold interpenetrating network of MMPF-18. Reproduced from ref. 68 with permission from American Chemical Society, Copyright 2016.

quarter of the MnIII-porphyrin linkers were found in the asymmetric unit. The structure of ZJU-22 is stabilized by triply-linked networks via coordination bonds between MnIII and organic linkers from adjacent networks. The MnIII position is fully occupied providing stabilization of the skeleton and permits fixation of highly active CuII ions of Cu(COO)2(substrate)2-3 units on the surface of the skeleton. As a result, each of the CuII sites can synergistically bind two or three substrate molecules (Figure 1.24). At 77 K and 1 bar, ZJU-22 can adsorb 202 cm3 g1 N2 with a BET surface area of 809 m2 g1. In another example, the octatopicporphyrin-based MMPF-9 has a structure that is generated using TDCBPP (Porphyrin 12) linkers. It has a rare (4, 12)¨fli symbol: (316  424  520  66) connected network with smy topology (Schla 4 2 71 (3  4 )). X-ray single-crystal diffraction analysis shows that MMPF-9 crystallizes in the hexagonal space group P63/mmc with a chemical formula of Cu6(CuC76H36N4O16)(HCO2)4(H2O)6. Two types of copper paddlewheel units were found in MMPF-9. The first type consists of four carboxylate groups from

28

Figure 1.23

Chapter 1

(a) A six-wheel paddlewheel centered on Mn2. (b) The connection of each Zn–H2HCPP to six Mn2 paddlewheels. (c) The fraction of the 3D frame projected parallel to the ac-plane, showing an open 1D channel propagating parallel to the b-axis of the crystal. Reproduced from ref. 69 with permission from American Chemical Society, Copyright 2018.

four different TDCBPP ligands with two water molecules bound to the axial position. The second type is composed of two coplanar carboxylate groups with two different TDCBPP ligands. The N2 adsorption isotherm measurement showed a typical type I adsorption isotherm and can reach 245 cm3 g1 under saturation pressure. The BET surface area of MMPF-9 was determined to be 850 m2 g1, while the Langmuir surface area was determined to be 1050 m2 g1. Xun Wang and co-workers reported a porous MOF namely MMPF-10 with the chemical formula Cu4(CuTBCPPP)(H2O)4 that was built with the octatopic linker tetrakis(3,5-bis[(4-carboxy)phenyl]phenylporphyrin and copper paddlewheels.72 MMPF-10 crystallizes in the orthorhombic Immm space group, with the unit cell dimensions of a ¼ b ¼ 26.3 Å and c ¼ 34.0 Å. Each copper paddlewheel is connected to four porphyrin linkers, and each porphyrin linker is connected to eight copper paddlewheels, resulting in a 3D non-interpenetrating structure (Figure 1.25). Assuming that the copper paddlewheels are 4-connected nodes, and the porphyrin ligand is a 4-connected node (porphyrin ring) and four 3-connected nodes (triphenyl arm), then MMPF-10 will display a

Structural Design of Porphyrin-based MOFs

Figure 1.24

29

(a) X-ray single-crystal structure of ZJU-22, (b) fragments of the structure (larger light gray and black balls are three-way branch points). Reproduced from ref. 70 with permission from John Wiley & Sons, Copyright r 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

(3,4,4) fmj topology. The type I adsorption isotherm was observed with a maximum uptake of 120 cm3 g1 for MMPF-10 under saturation pressure. The BET surface area of MMPF-10 was measured to be approximately 419 m2 g1 while the Langmuir surface area was measured to be approximately 478 m2 g1.

1.2.4.2

Zn2/Cd2 Paddlewheel

UNLPF-1 consists of a common square paddlewheel [Zn2(COO)4(H2O)2], each of which is connected to four TBCPPP ligands.73 Each TBCPPP ligand is connected to eight SBUs (four above the porphyrin plane and four below the porphyrin plane) yielding a 3D non-interpenetrated structure. UNLPF-1 crystallizes in the tetragonal space group I4/mcm with cell parameters a ¼ 30.70 and c ¼ 28.96 Å. If it is assumed that the paddlewheel SBU is a 4-connected node, and the TBCPPP ligand is a 4-connected node (porphyrin center) and four 3-connected nodes (terphenyl arms), then UNLPF-1 adopts a rare three node (3,4,4)-c net with fjh topology. Tubular supermolecular building blocks in UNLPF-1 are constructed from paddlewheel SBUs with one axial water molecule pointing outwards and one pointing toward the tube center for each SBU. The distance between two opposing water molecules in the 1D channels is measured to be 5.4 Å. After the removal of the water molecules, these square 1D channels can accommodate a column of 8.0 Å in diameter (Figure 1.26). UNLPF-1 does not adsorb nitrogen under low temperature conditions. In contrast, the low-pressure CO2 isotherm at 273 K shows a high adsorption capacity, with a maximum of 85 cm3 g1 at 1.0 bar.

30

Figure 1.25

Chapter 1

(a) The hexagonal channels in MMPF-10 viewed along the c-axis; (b) the pentagonal cavity in MMPF-10; and (c) the side view of the hexagonal channel. Reproduced from ref. 72 with permission from the Royal Society of Chemistry.

Ma’s group described an example of the octatopic linker OCPP (Porphyrin 13) in a MOF with divalent metals including Zn and Cd.74 The faces of the OCPP linkers connect triangular M2(CO2)3 units, generating small cubic octahedral SBUs, which in turn merge with adjacent SBUs of opposite faces of each porphyrin. The resulting highly symmetrical pcu topology network named MMPF-4 (M ¼ Zn) and MMPF-5 (M ¼ Cd) exhibit two distinct polyhedral cage structures and are permanently microporous. MMPF-4 crystallizes in the ¯ with a cell length of 43.0 Å, while MMPF-5 crystallizes cubic space group Ia3 ¯m with a cell length of 22.5 Å. The higher in the cubic space group Pm3 symmetry observed in MMPF-4 can be attributed to the mirror surface of the Zn2(CO2)3 paddlewheel. The crystal structure of MMPF-4 shows that its structure has a window size of 7.8 Å  8.0 Å, with an inner diameter of 21.5 Å (Figure 1.27). As expected for a microporous material, the Ar adsorption isotherm at 87 K shows that MMPF-4 has an adsorption capacity of

Structural Design of Porphyrin-based MOFs

Figure 1.26

31

(a) Side view and (b) top view of a single square tubular supramolecular member; (c) 3D network connectivity along the [001] direction between the paddlewheel and the V-shaped terphenyl arm connected to the porphyrin linker; (d) side view of the one-dimensional channels (the cage is represented by the red sphere). Reproduced from ref. 73 with permission from the Royal Society of Chemistry.

376 cm3 g1 at saturation pressure and demonstrates typical type I adsorption behavior. From the Ar adsorption data, the Langmuir surface area of MMPF-4 was calculated to be 1205 m2 g1 while the BET surface area was calculated to be 958 m2 g1. Yang et al. reported a porphyrin MOF (CZJ-4) also from the octatopic linker OCPP (Porphyrin 13) with the formula of [Zn16(H2O)8(MnIIICl–OCPP)4] 19DMF34CH3COOH45H2O composed of MnIIICl–OCPP and ZnII ions.75 CZJ-4 crystallizes in the tetragonal I4/mcm space group. In the crystal structure shown in Figure 1.28, there are three SBUs in CZJ-4: a dual-core Zn2(COO)4(H2O) paddlewheel and two distinct OCPP ligands. The first OCPP is a 16-dentate ligand that couples eight Zn2 paddlewheels, while each of the eight carboxylate groups on the second OCPP linker type is monodentate and occupies axial sites of eight Zn2 paddlewheels. The binuclear Zn2 paddlewheel SBU contains a pair of tetrahedrally coordinated zinc ions coupled through three carboxylate groups from three OCPP ligands, along with monodentate OCPP carboxylates and water molecules. Two cage types were found in CZJ-4 both with a window diameter of approximately 1.0 nm.

32

Figure 1.27

1.2.4.3

Chapter 1

(a) The Zn–TDCPP ligand is fused with the square face of a small cubic octahedron, thereby providing a pcu network with two types of cavities in MMPF-4. (b) The octahedron cage formed by linkers and metal SBUs. Reproduced from ref. 74 with permission from the Royal Society of Chemistry.

Mn-based Clusters

The self-assembled MOFs named ZJU-18 and ZJU-19 are isostructural MOFs built with M-OCPP linkers where M ¼ MnIIICl for ZJU-18, and M ¼ NiII for ZJU-19.76 These MOFs all crystallize in the orthorhombic Fmmm space group. The structure is made up of an octatopic MnIIICl–OCPP metalloligand joined to two metal-containing SBUs, binuclear Mn2(COO)4 and trinuclear Mn3(COO)4(m-H2O)2 (Figure 1.29). The MOF ZJU-18 was found to have superior catalytic activities over the molecular MnIIICl–Me8OCPP counterpart. A MOF with the formula of [Mn5L(H2O)6(DMA)2]5DMA4C2H5OH (H10L ¼ Porphyrin 14) was reported using an octacarboxylic acid porphyrin linker. The MOF structure exhibits rtl topology where three structurally distinct Mn atoms can be found (Figure 1.30).77 Mn1 is coordinated to four pyrrolic N atoms of the porphyrin core with bis-axially coordinated oxygen atoms derived from DMA molecules. Mn2 is coordinated to six distinct carboxylic acid oxygen atoms of three different porphyrin linkers, while the coordination geometry of Mn3 is composed of three oxygen atoms from water and three carboxylic acid oxygen atoms derived from two porphyrin linkers. The Mn2(COO)4(H2O)3 paddlewheel SBUs were then built by bridging Mn2 and Mn3 atoms with three carboxylates. They crystallize in the monoclinic space group P21/c. The CO2 uptake of this MOF can reach 25.51 cm3 g1 at 298 K under 1 bar, which can be converted to about 4.24 CO2 molecules in each MOF unit cell. This MOF also has good catalytic activity in the cycloaddition of CO2 to epoxides.

Structural Design of Porphyrin-based MOFs

Figure 1.28

1.2.4.4

33

(a) Three SBUs in CZJ-4. (b) The crystal structure of CZJ-4 viewed along the c-axis. (c) The fragmented structure of CZJ-4 along the c-axis, showing the MnIII sites within the cage that are accessible to substrates (different colored balls indicate different cavities). Reproduced from ref. 75 with permission from American Chemical Society, Copyright 2014.

Cd/Cd2 Clusters

Ma et al. constructed a porphyrinic framework named MMPF-5, which comprises cubic octahedral cages.74 Each cage consists of six CdII–OCPP ligands which are connected by eight triangular Cd(CO2)3 SBUs. MMPF-5 showed no significant adsorption of Ar, N2 nor O2 at 77 K nor 87 K, however, it exhibited a CO2 adsorption capacity of 67 cm3 g1 (or 3.0 mmol g1) at 273 K.

34

Chapter 1

Figure 1.29

(a) MnIIICl-H8OCPP ligand with MnN4Cl square pyramid at the center, and (b) the OCPP ligand connected to binuclear Mn2(COO)4 and trinuclear Mn3(COO)4(m-H2O)2 SBUs in the crystal structure of ZJU-18. Reproduced from ref. 76 with permission from American Chemical Society, Copyright 2012.

Figure 1.30

View of the 3D framework of the MOF. Reproduced from ref. 77 with permission from John Wiley & Sons, Copyright r 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

1.2.4.5

Co3/Co2 Clusters

The octatopic linker TBCPP (Porphyrin 15) can be used to assemble paddlewheel clusters with Co to form the [Co2(COO)4(H2O)2] SBU. This assembly yields a MOF named UNLPF-2 (Figure 1.31).78 UNLPF-2 crystallizes in a tetragonal

Structural Design of Porphyrin-based MOFs

Figure 1.31

35

Structure of UNLPF-2. Reproduced from ref. 78 with permission from John Wiley & Sons, Copyright r 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

crystal system with the P42/mmc space group. In the crystal structure, each ligand connects to eight 4-connected paddlewheel clusters, and each cluster is connected to four ligands. This provides an appropriate M–M distance of 6.1 Å, allowing for selective CO2 capture. The BET surface area calculated from the N2 adsorption isotherms gave a value of 623 m2 g1 for UNLPF-2. Wang and co-workers also used OCPP (Porphyrin 13) to connect twisted cobalt triangular prism SBUs prepared in-situ to yield the robust (6,8,8)connected MOF with msq topology, denoted as MMPF-2 (OCPP referred to as TDCPP in the source text). MMPF-2 uses a rare twisted cobalt triangular prism SBU which consists of three cobalt atoms bridged by m3-OH groups connected to six carboxylate groups of six OCPP ligands.79 The twisted cobalt triangular prism SBU of MMPF-2 is constructed from four bidentate carboxylates and two monodentate carboxylates. Two distinct coordination modes were observed – one hexacoordinate Co atom and two pentacoordinate Co atoms. Each SBU is connected to six OCPP ligands with carboxylates adopting either single or double chelation modes, and each OCPP ligand is connected to eight SBUs. MMPF-2 crystallizes in the tetragonal space group P4/mbm. If SBU is assumed to be six connected nodes and the TDCPP linkers are 8-connected vertices, then the topology of MMPF-2 is an unprecedented (6, 8, 8)-connected three-node network of a new topology, msq ((413  62)4(420  68)2(424  64)4) (Figure 1.32). The Ar adsorption isotherm at 87 K indicates that MMPF-2 exhibits an adsorption capacity of 545 cm3 g1 under saturation pressure with typical type I adsorption behavior. From the

36

Figure 1.32

Chapter 1

(a) Cobalt metalated porphyrins in MMPF-2 in a ‘‘face-to-face’’ configuration. (b) The space filling model of the three types of channels in MMPF-2 from the c direction. Reproduced from ref. 79 with permission from the Royal Society of Chemistry.

Ar adsorption data, the Langmuir surface area of MMPF-2 is 2037 m2 g1 while the BET surface area is calculated to be 1410 m2 g1. The pore size distribution of MMPF-2 is centered around 9.5 Å.

1.2.4.6

Fe3O Cluster

FeMPF is a 3D framework that crystallizes in the P2/m space group with a formula of [Fe3(H2O)(m3-O)(m2-HCOO)]8 [(Zn(H2O)–OCPP)6]n. This MOF consists of OCPP linkers and Fe2[Fe(H2O)](m3-O) SBUs formed in-situ.80 Within the FeMPF framework, the Fe2[Fe(H2O)](m3-O)(HCOO)(COO)6 SBU contains six carboxylates and one formate anion, and the Fe ions were found to be in the trivalent oxidation state. Each porphyrin entity in the framework is regarded as an 8-connected node, and the Fe-containing clusters are regarded as 6-connected nodes, thus forming a (6,8,8) three-node network with a new ¨fli symbol of (413  62)4(420  68)2(422  64)4 (Figure 1.33). topology with the Schla

1.2.4.7

Mononuclear Indium

Zhang et al. described a strategy for generating non-coordinating anions using zwitterionic MOFs assembled from anionic indium-based SBU [In(CO2)4] with the cationic metalloporphyrin linker MIII–TBCPPP (Figure 1.34).81 Several MOFs named UNLPF-13–MnIIICl, UNLPF-14–MnIII, UNLPF-15–FeIIICl, and UNLPF-16–FeIII were reported. Single crystal X-ray diffraction structure analysis showed that these MOFs are all crystallized in the orthogonal Pnnm space group and are isostructural. The two zwitterionic MOFs (UNLPF-16–FeIII and UNLPF-14–MnIII) exhibit complete internal charge separation which effectively prevents the counter cations from potentially binding to counter anions with contradictory charges. With the complete charge separation, the Lewis acidity of MnIII- and FeIII-porphyrins

Structural Design of Porphyrin-based MOFs

Figure 1.33

37

(a) SBU represents the tri-nuclear homometallic Fe3O(COO)7 cluster. (b) The projection of the framework along the b-axis, showing the channel gap. (c) The 3D porous frame and its topological representation viewed along the b-axis. Reproduced from ref. 80 with permission from American Chemical Society, Copyright 2016.

was found to be enhanced.81 In the case of the two chlorinated MOF frameworks (UNLPF-13–MnIIICl and UNLPF-15–FeIIICl), the chloride anion is coordinated to the metal centers in porphyrin linkers, and the bond distance was measured to be 2.16 and 2.20 Å, respectively. An anionic indium porphyrin MOF UNLPF-10 consisting of dodecahedral cages was constructed from an octatopic porphyrinic linker (Porphyrin 15) and tetratopic [In(COO)4] SBUs.82 Each 8-connected TBCPPP ligand was connected to eight 4-connected [In(COO)4] SBUs. Interestingly, six TBCPPP ligands connect eight [In(COO)4] SBUs and form a rare Williams b-tetraopened dodecahedral cage consisting of fourteen faces, twenty-four vertices, and thirty-six sides. In each cage, two planar porphyrin cores were used as two square faces with pentagonal and hexagonal faces formed by two TBCPPP ligands, which pass through two [In(COO)4] SBUs to link the two porphyrin cores in vertical and parallel directions. UNLPF-10 has a high degree of porosity, the internal dimensions of the tetrahedral dodecahedron cage were measured to be 33  23  23 Å, while the hexagonal windows and pentagonal windows were measured to be 19  16 Å and 17  13 Å.

38

Figure 1.34

1.2.4.8

Chapter 1

The MOF structures with the anionic indium SBU. Reproduced from ref. 81 with permission from American Chemical Society, Copyright 2016.

Y1/Y3 Clusters

He et al. reported a 3D non-interpenetrating anionic porphyrin MOF named CZJ-22 which consists of 4-connected [Y(COO)4(H2O)2] SBUs and octatopic Cu–OCPP (OCPP ¼ Porphyrin 13) metalloporphyrin linkers with an overall formula of [(CH3)2NH2]2[Y2(Cu–OCPP)(H2O)4]8DMF22H2O.83 The [(CH3)2NH2]1 cations that balance the framework charge can be systematically exchanged with redox-active metal ions to adjust the catalytic performance. The CuII encapsulated material Cu[Y2(Cu–OCPP)(H2O)4]10DMF18H2O (CZJ-22–Cu) has the CuII ions residing within the anionic pores (Figure 1.35). The directional connection between these subunits produced nanocages with pore diameters of approximately 1.4 and 2.6 nm to accommodate counter cations and solvent molecules. These MOFs have high catalytic activity in the aerobic oxidation of ethers to esters at atmospheric pressure. CZJ-22 and CZJ-22-Cu adsorb 452 and 545 cm3 g1 N2 at 77 K and 1 bar, with their BET surface areas calculated to be 242.4 m2 g1 and 236.8 m2 g1, respectively. He et al. reported another kind of Y-based porphyrin MOF with a formula of [Y3(H2O)4(ML)(CH3COO)]Solvent, named CZJ-18(M), where M ¼ 2HI, MnIIICl, or CuII and L refers to TBCPP (Porphyrin 15).84 Crystals of CZJ-18(M) can be synthesized by heating M-H8L and Y(NO3)36H2O in a mixed solvent of DMF and acetic acid at 80 1C for 1 week. These isoreticular MOFs were made of triangular [Y3(H2O)4(benzoate)8(acetate)] SBUs (abbreviated as Y3) and eight TBCPP linkers. In the crystal structure, each deprotonated TBCPP linker coordinates to eight triangular Y3 SBUs, and each Y3 SBU coordinates eight M–L

Structural Design of Porphyrin-based MOFs

39

Figure 1.35

The ball and stick model and polyhedron representation of the CZJ-22 anion skeleton viewed downward in the b-axis. Reproduced from ref. 83 with permission from Elsevier, Copyright 2017.

Figure 1.36

The crystal structure of CZJ-18 (Cu) including accessible CuII sites of porphyrin. Reproduced from ref. 84 with permission from John Wiley & Sons, Copyright r 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

connectors to form a porous 3D non-interpenetrating network with apertures of about 1.8 nm (Figure 1.36). CZJ-18(Cu) adsorbs 434 cm3 g1 N2 at 77 K and 1 bar, and the BET surface area was calculated to be 490.5 m2 g1.

40

1.2.4.9

Chapter 1

M(III)Na Cluster (M ¼ Sm, Eu, Gd, Tb, and Dy)

Another example of a lanthanide-metal porphyrin porous framework named [(Zn(H2O)–OCPP)Ln2(H2O)4Na2]n (LnMPF-1, where Ln ¼ Sm, Eu, Gd, Tb, and Dy) can be synthesized using octatopic OCPP linkers (Porphyrin 13) in the presence of NaOH as a regulator.80,85 The prepared MOF adopts a (4,8) connectivity and has a 1D tubular zig-zag channels with a diameter of about 1.5 nm. The microporous nature of the framework is depicted by the N2 adsorption which has a maximum uptake of 124 cm3 g1 under saturation pressure. The Langmuir surface area for the Gd MOF is calculated to be 200 m2 g1.

1.3 Pyridinyl Porphyrin Linkers 1.3.1

Tetratopic Pyridinyl Linkers

As a soft ligand, pyridine-based tetratopic porphyrin linker 5,10,15,20tetra(4-pyridyl)porphyrin (TPyP, Porphyrin 16) typically forms MOFs when combined with soft metal cations, such as CuII, CdII, AgI, and ZnII. Some of the well-resolved structures are discussed here.

1.3.1.1

Mononuclear AgI and Equatorially Capped CuII

Silver can bind TPyP linkers via two different coordination modes since AgI can bind both two or four pyridine linkers to form 2D sheets. These MOFs are related to the square grid of (4,4) type as shown in the scheme in Figure 1.37.86 When half or one-quarter of the TPyP linkers from the original (4,4) square grid were removed, the two types of network can be observed. It should be noted that the AgTPyP MOF with the original (4,4) square grid is not experimentally observed and is only used as a structural model here. Since the pyridinyl linkers are neutral, the framework is cationic due to the AgI cationic nodes. A counteranion is needed to balance the framework charge. In some cases, the counter anion will bridge the adjacent 2D MOF sheets and form a 3D structure.86,87 A complex 3D structure was observed when the TPyP linker was metalated with Zn.87 ZnTPyP linkers first form 2D layers with AgI cations that are doubly perpendicularly interpenetrated. This 3D network is further connected by ZnTPyP linkers where four pyridyl groups form coordination bonds toward the metalloporphyrin Zn centers from the z-axial direction in four different surrounding layers and two-thirds of the Zn centers on each layer were involved. Although CuII does not prefer to adopt the tetracoordinate square planner geometry, when CuII is capped by 1,1,1,5,5,5-hexafluoroacetylacetonate (hfacac) anion, a MOF with the (4,4)-net topology can be obtained with TPyP linkers.88 For each mononuclear Cu node, two bidentate hfacac ligands occupy the equatorial positions while the axial position is utilized in

Structural Design of Porphyrin-based MOFs

Figure 1.37

41

(a) The structure of the Porphyrin 16 linker. (b) The structure of AgTPyP MOF with the original (4,4) square grid. (c) The structure of AgTPyP MOF with one-quarter and (d) half of the TPyP linkers removed from the original grid. Reproduced from ref. 86 with permission from John Wiley & Sons, Copyright r 2002 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

constructing the framework. This structure is isoreticular to the previously mentioned AgI–TPyP (4,4)-net. The distance between adjacent porphyrin centers was measured to be 1.94 nm.

1.3.1.2 89

Other MOFs with Tetratopic Pyridinyl Porphyrin Linkers 90

Pb, Cd, Mn,90 and Zn90 can all form similar (4,4) square grid structures but without the node removal as observed with AgI. Since the metal cations prefer higher coordination numbers to satisfy their coordination sphere and balance the charge of the 2D MOF, counter anions are usually found functioning as pillars coordinated to the mononuclear nodes in the axial position. These 2D (4,4) square grid sheets are thus bridged to form 3D

42

Chapter 1 II

networks. Cd can also form a MOF with a different topology named HMOF-1 with cds topology, but unlike many other MOFs with cds topology, HMOF-1 is not interpenetrated.91 This hinged framework can respond to thermal stimuli and expands upon heating. ZnII can also be found in a tetragonal coordination environment when forming MOFs with TPyP linkers.92 The ZnII–TPyP MOF crystallizes in the tetragonal P4/mbm space group. In the crystal structure, the Zn atom is coordinated to two pyridines from TPyP linkers and two water molecules to form a distorted tetrahedral coordination geometry, yielding a 2D framework structure in the ab-plane. The 2D planes were then bridged by formate anions to further extend into a 3D porous network. When secondary linkers like 1,4-benzene dicarboxylic acid (BDC) are present, Zn can form a binuclear cluster bridged by three nitrates with clusters connected by TPyP linkers to form several different 3D nets. TPyP can also form a 3D cubic framework with FeII cations, adopting the ¯ space group. Two crystallographically distinct Fe centers can be found Pn3 in the cationic framework: the first type is embedded in the porphyrin center and is axially coordinated to adjacent [Fe(TPyP)] units, while the second type is octahedrally coordinated to six pyridyl nitrogen donors. The counter anions reside within the framework to balance the cationic framework. Neutral CuTPyP linkers can link bimetallic Cu2Mo3O11 oxide chains to form a unique MOF structure that crystallizes in the monoclinic P21/c space group.93 The copper molybdate chains are perpendicular to the porphyrin cores, while the porphyrin linkers stack parallel to each other as well as to the oxide chains. Li’s group reported an iron MOF prepared by a one-pot solvothermal reaction of FeII with the TPyP linker, yielding two layered structures with unprecedented 2D paddlewheel SBUs.94 By reacting TPyP or T3PyP (Porphyrin 17) with various lanthanide metal nitrates several porphyrinic MOFs can be constructed. In these MOF structures, tetradentate porphyrin units with coordination bonding and hydrogen bonding functions were connected to each other through inorganic SBUs to form a self-assembled 3D structure. CuI can also form a MOF with TPyP. The structure reported crystallizes in ¯2c that is very similar to the structure formed with the the space group of P4 TCP ligand.95 The CuI nodes in this MOF have a tetrahedral geometry with Cu connected to four TPyP linkers, forming a net with pts topology. The CuI tetrahedral can be capped by a bidentate pyridine linker such as 1,10-phenanthroline. Under this circumstance, the 3D framework structure can be cut down to 2D sheets by replacing the two pyridyl units from two Cu-TPyP ligands to 1,10-phenanthroline.

1.3.2

Ditopic Pyridinyl Linkers

Wang’s group reported a cobalt MOF using a trans meso-bifunctional porphyrin ligand DPyP, (Porphyrin 18).96 The MOF framework,

Structural Design of Porphyrin-based MOFs

43

[Co3(DPyP)3]4DMF, has a ribbon-like coordination structure composed of tetranuclear metal porphyrin cages. Within the ribbon, two types of metalloporphyrin units can be found: one of which occupies the nodes while the other type occupies the corners. The nodal porphyrins in this MOF are flat and are parallel to each other while the porphyrins are folded, with pyrrole rings alternately arranged above and below the porphyrin plane. This combination of coordination geometry results in a unique one-dimensional extended structure of [Co3(DPyP)3]4DMF. A porphyrin–Co MOF [Co(DpyDtolP)]612H2O where DpyDtolP is a ditopic porphyrin linker Porphyrin 19 was reported by Lee et al.97 The MOF crys¯ space group and can adsorb 142.8 cm3 g1 (6.37 mmol g1) talizes in the R3 CO2 at 196 K under saturation pressure. The two axially coordinated pyridine groups tilt from the porphyrin ring at an inclination angle of 71.11. This MOF appears to be microporous and has a hexagonal channel with a radius of about 8.13 Å. Hosseini et al. reported another ditopic Zinc MOF.98 The coordination sphere around zinc consists of six nitrogen atoms, with two pyridine groups at the axial position (Zn–N distance about 2.35 Å). The geometry around the metal center appeared to be a deformed octahedron, with the two axial pyridines inclined by 69.91 rather than perpendicular to the porphyrin plane. Wilson et al. synthesized a MOF that has a band-like structure in which the six-coordinated Zn centers were connected to two five-coordinate Zn cations via the [Zn(TPyP)] linker, forming a 1D arrangement with the width of three porphyrin units.99 It should be noted that the TPyP linker in this MOF only utilizes 2 of its 4 pyridinyl groups thus it is concluded in the ditopic part. Combining Zn(NO3)26H2O and DPBPFP (Porphyrin 5) under solvothermal conditions, a purple pillared paddlewheel MOF can be obtained named as ZnPO.100 A reversible CO2 adsorption isotherm corroborated the permanent microporosity of the ZnPO-MOF and indicated a gas-accessible surface area of B500 m2 g1. The structure is shown in Figure 1.38. Two MOFs with the formula [Zn(C42H16F10N6)]2C2H7N and [Co(C42H16F10N6)]C2H7N were reported by Li et al.101 Both MOFs crystallize in the monoclinic crystal system with the C2/c space group and exhibit similar 2D layered structures. In both MOF frameworks, each porphyrin molecule connects four adjacent molecules through coordination bonds to self-assemble into a 2D layer. Each Zn and Co located in the center of the porphyrins coordinates with four pyrrole nitrogen atoms and forms an octahedral geometry by further coordinating with two trans-pyridyl ligands in the axial positions. Each porphyrin in the two isostructural MOFs connects two adjacent porphyrins through coordination bonds formed by the pyridyl substituents and the metal centers of adjacent porphyrins. Further connections are formed via the remaining two porphyrins through their own metal center and the pyridyl groups of the two porphyrins.

44

Chapter 1

Figure 1.38

The stick representation of the unit cell. Reproduced from ref. 100 with permission from American Chemical Society, Copyright 2009.

1.4 Azole Porphyrin Linkers 1.4.1

Tetratopic Pyrazolate Linkers

Due to strong coordination bonds that form between pyrazolates and soft acids, pyrazolate porphyrin MOFs have extraordinary stability in harsh chemical environments, especially in alkaline conditions. Several MOFs were reported based on the linker TPyzP (Porphyrin 20) and its derivative ligands including PCN-601,102 PCN-602,103 and PCN-624.104 All three MOFs were built with the [Ni8(OH)4 (H2O)2Pz12] SBU and adopt the same ftw-a topology. PCN-601 (Figure 1.39) is the first identified pyrazolate MOF that retains its crystallinity and porosity after saturated NaOH treatment at room temperature, with stability up to 100 1C. It has a surface area of 1309 m2 g1 and an N2 uptake of 505 cm3 g1.102 An elongated pyrazolate (Porphyrin 21) ligand was used to build PCN-602, which leads to a larger window size than PCN-601.103 The window size in PCN-602 is about 6.0 14.0 Å. As a comparison, the window size in PCN-601 is much smaller, at around 2.1 8.0 Å. PCN-624 has the same window size as PCN602, but the ligand is partially fluorinated. The pore size of PCN-624 was calculated to be 1.9 nm in diameter and exhibits a surface area of 2010 m2 g1.104

1.4.2

Tetratopic Tetrazole Linkers

An 8-connected Mn4Cl cluster can form MOFs with porphyrin tetrazole linkers. The MOF known as UTSA-57 was the first example of a

Structural Design of Porphyrin-based MOFs

Figure 1.39

45

Topological and geometrical comparison between PCN-221 and PCN601: (a) The structure of PCN-221; (b) [Zr8O6 (CO2)12]81 SBU; (c) TCPP linker; (d) ftw-a topology; (e) Oh symmetric 12-connected SBU; (f) D4h symmetric 4-connected SBU; (g) the structure of PCN-601; (h) [Ni8] cluster SBU; (i) TPP linker. Reproduced from ref. 102 with permission from American Chemical Society, Copyright 2016.

(tetrazolyl)porphyrin-based (Porphyrin 21, TPPyzP) MOF and has scu topology.105 It crystallized in the tetragonal crystal system with the P4/mmm space group. The four tetrazole groups in the TPPyzP linkers and their connecting phenyl rings are almost coplanar, whereas the porphyrin core and the phenyl rings are nearly perpendicular. This microporous MOF has a maximum N2 uptake of 137 cm3 g1 with Langmuir and BET surface areas of 330.5 m2 g1 and 206.5 m2 g1, respectively. The MOF UTSA-57 also contains 1D square channels with a dimension of 20.3 Å. Two FeIIFeIII mix-valence MOFs with the chemical formula [FeIIpzTTP (FeII1xDMF1xFeIIIxOHx)]n have been reported with the orthorhombic Cmmm space group and fry topology.106 A freshly prepared sample was determined to have approximately 75% of high-spin FeII and 25% low-spin FeIII. The 3D framework is created by the connection of four tetrazolates to 1D Fe chains formed by corner-sharing FeN4O2 octahedra. Each Fe porphyrin center is bridged by either pyrazine or 1,4-diazabicyclo[2.2.2]octane (DABCO) as the N-donor. These two MOFs also contain 1D channels and have BET surface areas of 430 m2 g1 and 370 m2 g1, respectively.

1.4.3

Ditopic Imidazole Linkers

Li’s group reported several ditopic imidazolyl porphyrins for MOF growth with both Co and Fe.107 Three MOFs were obtained using Porphyrin 22,

46

Chapter 1

Porphyrin 23, and Porphyrin 24. The first two were isostructural Co 2D grid layers with a rotaxane-like bilayer structure. The Fe analog appears to be a 4-connected 3D framework with lvt topology and slightly folded porphyrin cores. This MOF contains not only unique spiral channels but also tetrahedral cages.

1.5 Other Tetratopic Linkers 1.5.1

Phosphonate Porphyrin Linkers

Zirconium phosphates have attracted research attention but only a handful of porous ones have been successfully made with porphyrins. Two isoreticular MOFs named Zr-CAU-30 and Hf-CAU-30 were constructed from a tetratopic porphyrin phosphonate linker, tetra(4-phosphonophenyl)porphyrin (TPPP, Porphyrin 26).108 The inorganic SBU consists of chains of trans corner-sharing ZrO6/ZrO4F2 octahedra with bridging m2-OH or m2-F ions. Inside the framework, the porphyrin–porphyrin distance was measured to be as close as 4.48 Å. The Zr–CAU-30 and Hf–CAU-30 have BET surface areas of 970 and 910 m2 g1, respectively. The two MOF structures are both chemically stable in the pH range of 0 to 12 and are thermally stable up to 420 1C in air. Co-CAU-36 (chemical formula, [Co2(Ni–TPPP)]2DABCO6H2O) has a surface area of 700 m2 g1 with a pore volume of 0.31 cm3 g1.109 It crys¯c2 space group with a close porphyrin–porphyrin distance tallizes in the P4 again at 4.48 Å. This 3D framework contains 1D channels of approximately 9 Å in diameter and DABCO is present in the structure. Under different synthetic conditions, other divalent metal cations can be utilized to form similar MOF structures but with reduced symmetry. These divalent metals include Mn21, Co21, Ni21, and Cd21. For example, M-CAU-29 with the chemical formula of [M(Ni–H6TPPP)(H2O)] (M ¼ Mn, Co, Ni, Cd) crystal¯. Due to the reduced lizes in a triclinic crystal system with space group P1 symmetry, two 1D channels with the dimensions 2.8  5.0 Å and 3.8  3.8 Å were observed in Ni–CAU-29.

1.5.2

Cyanoporphyrins

Tetratopic cyano-porphyrin linker 5,10,15,20-tetrakis(4-cyanophenyl)porphyrin (TCP, Porphyrin 27) has been reported to form a MOF with mononuclear CuI nodes with the formula [CuII(TCP)CuI]n1.95 The Cu–TCP MOF exhibits 2-fold interpenetration and crystallizes in the Cccm space group. The counter anions (BF4) were observed along with solvent molecules to reside inside the cavity of this MOF. Each CuI ion adopts a tetrahedral coordination sphere bound to four nitrogen atoms from four different TCP linkers. This framework has a channel along the c-axis approximately 2 nm across, with the distance between the two nearest porphyrin centers across a channel measured to be 1.56 nm.

Structural Design of Porphyrin-based MOFs

Figure 1.40

1.5.3

47

Structure description of MIL-173(M) (M ¼ Zr, La, Ce, Y). (a) The coordination modes between Porphyrin 28 and MO8 polyhedra chain. (b) Perspective views of MIL-173(M) along with M chains. Reproduced from ref. 110 with permission from the Royal Society of Chemistry.

Hydroxy Porphyrins

A unique hydroxy porphyrin linker 5,10,15,20-tetrakis(3,4,5-trihydroxyphenyl)porphyrin (PorphGal, Porphyrin 28) has been used to grow MOFs with both Zr(IV) and several other trivalent rare earth metals (REIII) including La, Ce, and Y (Figure 1.40).110 These MOFs are named after MIL-173(M) where M ¼ Zr, La, Ce, Y. MIL-173(Zr) crystallizes in a tetragonal I41/amd space group built with edge-sharing ZrO8 polyhedra. Each Zr cation chelates four 1,2,3-trioxobenzene groups originating from four different PorphGal linkers. The 1,2,3-trioxobenzene groups in the PorphGal linker were found to be perpendicular to their central porphyrin rings. This MOF has disk-shaped pores with diameters of B16 Å and thickness of B4 Å. The pores are accessible through elliptic windows of dimensions approximately 8  4 Å2. Structure-wise, the REIII frameworks are isoreticular to ZrIV MOFs with slight variations of interatomic distances due to slightly different ionic sizes. The strong coordination interaction in the Zr-trioxobenzene cluster gave rise to remarkable chemical stability in MIL-173(Zr), which can maintain its crystallinity after being soaked in pure water as well as phosphate buffer saline solution (pH ¼ 7.4) for one week at 37 1C. In comparison, MIL-173(Y) loses all crystallinity after one week.

1.6 Porphyrin Linkers (Table 1.2)

48 Table 1.2

Chapter 1 Porphyrin linkers.

Porphyrin 1 TCPP

Porphyrin 2 TCBPP

Porphyrin 3 TCBPP

Porphyrin 4 TCTTPP

Porphyrin 5 DPBPFP

Porphyrin 6 T3 CPP

Porphyrin 7 DCPP

Porphyrin 8 DDCPP, BDCPP

Porphyrin 9 BDCBPP

Porphyrin 10 BCPP

Porphyrin 11 HCPP

Porphyrin 12 TDCBPP

Porphyrin 13 OCPP

Porphyrin 14 H10 L

Porphyrin 15 TBCPPP

Porphyrin 16 TPyP

Porphyrin 17 T3PyP

Porphyrin 18 DPyP

Porphyrin 19 DpyDtolP

Porphyrin 20 TPyzP

Porphyrin 21 TPPyzP

*Some labeled TDCPP

Structural Design of Porphyrin-based MOFs

49

Table 1.2 (Continued)

Porphyrin 22 TTP

Porphyrin 23 (tBu-Ph)2(Im-Ph)2Por

Porphyrin 24 (F3-Ph)2(Im-Ph)2Por

Porphyrin 25 (Me2N,F4-Ph)2 (Im-Ph)2Por

Porphyrin 26 TPPP

Porphyrin 27 TCP

Porphyrin 28 PorphGal

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70. X.-L. Yang, C. Zou, Y. He, M. Zhao, B. Chen, S. Xiang, M. O’Keeffe and C.-D. Wu, A Stable Microporous Mixed-Metal Metal–Organic Framework with Highly Active Cu2þ Sites for Efficient Cross-Dehydrogenative Coupling Reactions, Chem. – Eur. J., 2014, 20(5), 1447–1452. 71. W.-Y. Gao, L. Wojtas and S. Ma, A porous metal–metalloporphyrin framework featuring high-density active sites for chemical fixation of CO2 under ambient conditions, Chem. Commun., 2014, 50(40), 5316–5318. 72. X. Wang, W.-Y. Gao, Z. Niu, L. Wojtas, J. A. Perman, Y.-S. Chen, Z. Li, B. Aguila and S. Ma, A metal–metalloporphyrin framework based on an octatopic porphyrin ligand for chemical fixation of CO2 with aziridines, Chem. Commun., 2018, 54(10), 1170–1173. 73. J. A. Johnson, Q. Lin, L.-C. Wu, N. Obaidi, Z. L. Olson, T. C. Reeson, Y.-S. Chen and J. Zhang, A ‘‘pillar-free’’, highly porous metalloporphyrinic framework exhibiting eclipsed porphyrin arrays, Chem. Commun., 2013, 49(27), 2828–2830. 74. X.-S. Wang, M. Chrzanowski, W.-Y. Gao, L. Wojtas, Y.-S. Chen, M. J. Zaworotko and S. Ma, Vertex-directed self-assembly of a high symmetry supermolecular building block using a custom-designed porphyrin, Chem. Sci., 2012, 3(9), 2823–2827. 75. X.-L. Yang and C.-D. Wu, Metalloporphyrinic Framework Containing Multiple Pores for Highly Efficient and Selective Epoxidation, Inorg. Chem., 2014, 53(10), 4797–4799. 76. X.-L. Yang, M.-H. Xie, C. Zou, Y. He, B. Chen, M. O’Keeffe and C.-D. Wu, Porous Metalloporphyrinic Frameworks Constructed from Metal 5,10,15,20-Tetrakis(3,5-biscarboxylphenyl)porphyrin for Highly Efficient and Selective Catalytic Oxidation of Alkylbenzenes, J. Am. Chem. Soc., 2012, 134(25), 10638–10645. 77. W. Jiang, J. Yang, Y.-Y. Liu, S.-Y. Song and J.-F. Ma, A. Porphyrin-Based Porous rtl Metal–Organic Framework as an Efficient Catalyst for the Cycloaddition of CO2 to Epoxides, Chem. – Eur. J., 2016, 22(47), 16991– 16997. 78. J. A. Johnson, S. Chen, T. C. Reeson, Y.-S. Chen, X. C. Zeng and J. Zhang, Direct X-ray Observation of Trapped CO2 in a Predesigned Porphyrinic Metal–Organic Framework, Chem. – Eur. J., 2014, 20(25), 7632–7637. 79. X.-S. Wang, M. Chrzanowski, C. Kim, W.-Y. Gao, L. Wojtas, Y.-S. Chen, X. Peter Zhang and S. Ma, Quest for highly porous metal– metalloporphyrin framework based upon a custom-designed octatopic porphyrin ligand, Chem. Commun., 2012, 48(57), 7173–7175. 80. B. K. Tripuramallu and I. Goldberg, Assembling Coordination Frameworks of Tetrakis[meso-(3,5-biscarboxyphenyl)]-Metalloporphyrins with Polynuclear Metallic Nodes: Mechanistic Insights into the Synthesis and Crystallization Process, Cryst. Growth Des., 2016, 16(3), 1751–1764. 81. J. A. Johnson, B. M. Petersen, A. Kormos, E. Echeverrı´a, Y.-S. Chen and J. Zhang, A. New Approach to Non-Coordinating Anions: Lewis Acid

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CHAPTER 2

Metalloporphyrins as Building Blocks to Supramolecular Architectures with Catalytic Functions T. KEIJER AND J. N. H. REEK* Van’t Hoff Institute for Molecular Sciences, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands *Email: [email protected]

2.1 Introduction Porphyrins are an interesting category of macrocyclic ligands for numerous reasons. The synthetically accessible porphyrins have been studied as structural analogs of nature’s chlorophylls. Simultaneously, Fe–porphyrins are used as heme-model compounds that show oxygen transfer activity.1 Non-natural catalytic applications also abound, such as Co–porphyrin radical type chemistry for cyclopropanations or aziridinations.2,3 The large aromatic p-system makes porphyrins excellent light-absorbers with high extinction coefficients. Therefore, a lot of spectroscopic methods are available to study physical properties of porphyrin chromophores and catalysts. In short, (metallo)porphyrins are studied widely across the chemical disciplines.4 Unsurprisingly, porphyrins have not gone unnoticed in the field of supramolecular chemistry,5 where they fulfill roles as structural building blocks,6 light-harvesters and catalysts.7 The rigid planar structure of the porphyrin Monographs in Supramolecular Chemistry No. 32 Porphyrin-based Supramolecular Architectures: From Hierarchy to Functions Edited by Shengqian Ma and Gaurav Verma r The Royal Society of Chemistry 2022 Published by the Royal Society of Chemistry, www.rsc.org

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macrocycle makes them a valuable asset in the synthesis of polyhedral supramolecular assemblies.8 The macrocycle planes act as walls that generate a confined space where substrates bind using aromatic interactions. Porphyrins are successful as cage walls for several reasons, namely, their structural rigidity diminishes entropy penalties upon encapsulation of a guest and their flat planar surface allows for easy polarization, which facilitates van der Waals and p–p interactions with guests. Besides, metal–ligand interactions involving the metalloporphyrin are still possible at the interior of porphyrin supramolecular assemblies.9 Bound guests often show altered reactivity, changing selectivity and efficiency for organic and catalytic reactions. This chapter aims to provide a selected overview of the applications of porphyrins in supramolecular chemistry. Some of these applications utilize the porphyrin as a rigid building block in supramolecular assemblies. Others exemplify the effect of supramolecular preorganization on catalytic properties such as selectivity and stability. Most of these properties culminate in the field of supramolecular artificial photosynthesis.10 Porphyrins are preorganized to obtain light-harvesting assemblies that transfer energy and electrons over vast distances, where porphyrin catalysts may collect hot electrons obtained via these processes to perform energetically up-hill redox reactions for overall water-splitting. Such supramolecular assemblies often reveal novel properties, though designed at the molecular level, that emerge when simple building blocks become adequately preorganized.11 This is the strength and potential of supramolecular chemistry. Metalloporphyrins epitomize this potential because of their synthetic versatility, broad range of spectroscopic handles, and excellent light-harvesting and catalytic properties. We discuss several examples of catalysis using supramolecular porphyrin structures. The porphyrins either act as rigid walls of a cage that encapsulate the catalyst and the substrate or both catalyst and the supramolecular cage are metalloporphyrins. These structures find application in catalytic oxidations, hydroformylation and radical-type transformations. Attention is then shifted toward the photophysical properties of supramolecular porphyrin architectures. Namely, excited state energy transfer systems often described as antennae, followed by asymmetric donor–acceptor systems that lead to charge separation by photo-excited electron transfer (PET) processes. Attention is shifted back to the application of such systems for catalysis where porphyrin structures act as both light harvesters and structural backbones in supramolecular artificial photosynthesis.

2.2 Self-assembled Porphyrin Structures in Catalysis When porphyrins are metalated, coordination of axial ligands is possible. This axial binding often produces supramolecular assemblies.12 In particular, Zn– porphyrins are well explored, since axial binding occurs only once, leading to discrete soluble assemblies. Coordination to Zn–porphyrins is usually done with pyridine and imidazole functionalities. This section highlights two strategies for supramolecular catalysis using porphyrins to generate a confined

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space, that vary in rigidity of the cage formed. Whether substrates can enter the most rigid nanostructures depends on their size and shape leading to size selectivity effects. Moreover, bimolecular deactivation pathways can be avoided increasing catalyst stability expressed into increased turn-over numbers (TON). Additionally, bulky dormant intermediates in the catalytic cycle cannot form in the confined space leading to increased catalytic rates.

2.2.1 Confined Space Hydroformylation Catalysis 2.2.1.1 The Template Ligand Strategy In 2001 we developed a template–ligand strategy for catalyst encapsulation, which uses relatively simple bifunctional ligand building blocks that by a self-assembly to porphyrin building blocks lead to encapsulated transition metal catalysts (see Figure 2.1).13

Figure 2.1

The encapsulation of template-ligand 1 using Zn(II)TPP building blocks results in the formation of a phosphine in a dynamic self-assembled cage 2. The crystal structure of 2 shows that next to pyridyl–zinc coordination, CH–p interactions are keeping the cage together (highlighted in green). Catalyst 3 is the triscarbonyl hydride compound that forms under syngas pressure in the presence of a rhodium precursor. Reproduced from ref. 14, https://doi.org/10.1021/acs.accounts.8b00345, with permission from American Chemical Society, Copyright 2014.

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Figure 2.2

Chapter 2

The product selectivity of octene hydroformylation is shifted toward the minor product using porphyrin-encapsulated Rh-catalyst 3. Reproduced from ref. 14, https://doi.org/10.1021/acs.accounts.8b00345, with permission from American Chemical Society, Copyright 2014.

In the first example we used pyridyl-phosphine ligands that coordinate to zinc(II)porphyrin building blocks.14 From the X-ray crystal structure, it is immediately apparent that the phosphine ligand coordinates the metal porphyrin via pyridyl–zinc interactions. In addition, the structure is stabilized by interporphyrin CH–p interactions of neighboring Zn(II)TPP building blocks. These additional interactions provided an explanation for the cooperative binding that is observed, as the binding strength of the third porphyrin to a pyridine on the trispyridyl phosphine ligand is increased with a factor of 5 compared to the binding strength of the first porphyrin (KB103–104).19,23 The potential of this supramolecular template-ligand approach was initially shown by hydroformylation of 1-octene using capsule 2 (1(Zn(II)TPP)3) to encapsulate rhodium catalysts (Figure 2.2). Catalyst 3 leads to altered product selectivity toward the minor product in the hydroformylation of terminal alkenes, and the branched aldehyde product is now favored over the linear product. Catalyst encapsulation may decrease catalytic rates if diffusion of substrates into the confined space is inhibited. However, for the dynamic catalyst 3 the unprecedented selectivity occurs in conjunction with increased activity of the encapsulated catalyst. The rate is increased ten-fold compared to the rhodium catalysts obtained from ligand 1 without Zn(II)TPP to encapsulate the metal atom. Higher rates can be partly explained by the formation of activated rhodium complexes coordinating only one phosphine ligand, due to the encapsulation of the template ligand. The same encapsulated rhodium-based catalysts were used in the hydroformylation of internal alkenes, resulting in unprecedented regioselectivity. Tuning the porphyrin dynamicity is easily

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achieved by exchanging Zn(II)tetraphenylporphyrin for Ru(II)(CO)tetraphenylporphyrin.15 This leads to a ten-fold stronger binding to the template ligand and resulting Rh-catalysts perform hydroformylation of 1-octene with even higher selectivity for the branched product at room temperature. However, using building blocks based on abundant metals such as Zn is highly desirable for applications. After exploring modifications at the porphyrin ligand, it was found that using zinc(II)tetraphenylporpholactone instead of zinc(II)tetraphenylporphyrin increased the binding affinity to the template ligand.16 By porphyrin ligand modification the metal becomes more electron poor, leading to increased pyridyl–zinc bond strength to ligand 1. This enables the application of this strategy in more polar solvents and at elevated temperatures relevant for industry. Interestingly, the product selectivity is highly dependent on the size of the cage. Computer modeling of the bulkier Zn(II)phthalocyanine cage analog predicted a cavity volume increase of five times the size of the capsule formed with Zn(II)tetraphenylporphyrin.17 In line with the resulting hydroformylation of trans-2-octene, product selectivity is shifted toward the 2-aldehyde product (70%). As such, the template ligand strategy is analogous to strategies encountered in nature, since product selectivity is determined by the 2nd coordination sphere which is facilitated by supramolecular interactions. Moreover, the comparison to nature also holds for supramolecular chiral induction, as the template ligand approach was applied in the asymmetric hydroformylation of internal alkenes. Using a chiral template ligand and achiral porphyrins usually leads to enhanced enantiomeric excess. Using a chiral (R,S)-BINAPHOS ligand equipped with pyridine moieties in the hydroformylation of trans-2-octene to the 2-aldehyde only 25% ee could be achieved.18 Upon addition of Zn(II)tetraphenylporphyrin to the reaction mixture, the enantiomeric excess increased to 45% ee. When the template ligand approach was applied to bidentate phosphine–phosphoramidite ligand equipped with pyridine moieties high enantiomeric excess could be achieved with Ru(II)(CO)tetraphenylporphyrin.19 Styrene analogs were converted to the branched chiral aldehyde with 82% ee compared to 18% ee with just the ligand. It is clear that the supramolecular ligand template approach may be generally used in catalysis. For example, strongly binding Ru(II)(CO)tetraphenylporphyrin capsule building-blocks are used with the same bidentate ligand in Rh-catalyzed hydrogenation reactions.20 Dimethyl itaconate was converted to the racemic product by the catalyst alone. Upon addition of the porphyrin building blocks the dimethyl 2-methylsuccinate product could be obtained in enantiopure form (499% ee).

2.2.1.2

The Supramolecular Cage Strategy

Besides producing a supramolecular host around the catalyst template using metal–ligand interactions, another approach is the assembly of supramolecular metal–organic cages,8 which may profit from a technique denoted subcomponent self-assembly.21 The latter is a subclass of metal–organic

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self-assembly using both dative and covalent bonds. Cages of various shapes and sizes can be produced that are highly modular in character. Such hosts are tolerant of many synthetic modifications without hampering cage formation. These structures can be modified for selective binding of guests. Interestingly, the guests in these nanocontainers may experience entirely different chemistry, such as changes in selectivity and activity. One such example was reported in 2019 in a collaboration between the group of Nitschke and ours.22 The same ligand template approach discussed above was combined with the more rigid metal–organic cage strategy. A Zn–porphyrin was equipped with amines that condense with picolinaldehyde in the presence of Fe(II)triflate precursor. The product is a tetrahedral cage that can be obtained in high yields (see Scheme 2.1). With an internal cavity volume of 1750 Å3 bis-tripyridylphosphine rhodium and bis-tripyridylphosphine gold complexes could be encapsulated into this cavity, facilitated by the coordination to Zn–porphyrin on the inner side. Because of a limited cavity volume, encapsulated mono-tripyridylphosphine rhodium complexes showed a strong size selectivity in the hydroformylation of terminal alkenes. We showed that the substrate needs to be fully encapsulated inside the pore to be converted by the encapsulated catalyst. This selectivity inhibits aromatic substrate conversion (e.g. styrene) and higher alkenes over shorter chain substrates. Interestingly, an odd/even effect for conversion was observed, meaning that odd-numbered 1-alkene chains were more readily converted than evennumbered chains. Computational studies correlated this to the relative energies of alkene conformers inside the porphyrin confined space. A related approach squeezes a chiral BINOL-ligand equipped with pyridine moieties in-between two Zn–porphyrins.23 The porphyrins were held together by coordinating bis-palladium complexes to obtain the square cage (see Figure 2.3). When this supramolecular architecture was exposed to rhodium

Scheme 2.1

Sub-component self-assembly of an Fe4(Zn–L)6 tetrahedral porphyrin cage. Adapted from ref. 22 with permission from John Wiley & Sons, Copyright r 2018 The Authors.

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catalyzed hydroformylation conditions, the styrene starting material could be converted to a chiral product with 74% ee. A significant increase when compared to the enantioselectivity obtained with the free catalyst (16% ee).

2.2.2

Catalyst Stabilization and Size Selectivity in Confined Space

Nature’s enzymes such as cytochrome P450 have inspired chemists to look at metalloporphyrin catalysts for efficient oxidation reactions.24 Mn–porphyrins are well known for the catalytic epoxidation of alkenes. A selective oxidation catalyst was developed in the group of Nolte by closing one face of a porphyrin macrocycle with a glycoluril clip.25 By choosing appropriately bulky ligands, axial coordination of the Mn(III)Cl–porphyrin occurs on the exterior face of the porphyrin. As a result, alkene epoxidations occur at the interior of the clip cavity, preventing dimeric m-oxo-bridged Mn(IV) porphyrin structures. Therefore, catalyst deactivation is strongly inhibited when the substrate is encapsulated by the clip, while the activity is also greatly enhanced. In a subsequent work rotaxanes were formed, showing that the catalyst moves along a polybutadiene chain while epoxidizing the alkene moieties.26 Supramolecular porphyrin squares were developed in the group of Hupp that encapsulated Mn–porphyrin catalysts for the catalytic epoxidation of styrene.27 However, catalyst lifetimes are generally low, due to competing bimolecular deactivation pathways. The authors attempted to inhibit these pathways by singling out the molecular catalyst and protecting it in a confined space. Ditopic Zn(II)–trans-bispyridyl porphyrin coordinated Re-complexes form a square. Tetrapyridyl Mn–porphyrins nicely fit inside the square plane (see Figure 2.4). The lifetime of the catalyst was considerably increased under catalytic conditions. Though the rate of styrene epoxidation was somewhat lower, the desired effect was observed. The TON increased from 65 for the free catalyst to 1500 for the supramolecular architecture. Diluting the amount of catalyst by a factor of a thousand, while keeping the cage concentration constant, yielded a TON of 21 000. The cavity of the cage could be tuned further by encapsulating a Mn–transbipyridyl porphyrin guest. Only two out of four cage porphyrins were thus occupied leaving a coordination site for additional Zn–pyridine interactions. Pyridines functionalized with bulky groups could be co-encapsulated with the porphyrin catalyst, producing a fine-tuned environment right above the catalyst’s porphyrin plane. When applied in catalytic epoxidations substrate size selective catalysis could be performed. Therefore the authors conclude that these artificial supramolecular catalysts are functionally reminiscent of cytochrome P450 where stability and selectivity are induced by the protein environment around the active site. Co(II)porphyrins are interesting catalysts that form carbon and nitrogen centered radicals from suitable carbene and nitrene precursors respectively.2,28 These metalorganic radical intermediates are tamed by the Co–porphyrin and lead to selective and unique radical-type chemistry. Nevertheless, catalyst

66 (Top) Zn–porphyrin and bis-palladium complex self-assemble into a supramolecular cage. (Bottom) Asymmetric hydroformylation of styrene shows greatly enhanced chiral product selectivity when the catalyst is encapsulated into the supramolecular cage. Reproduced from ref. 14, https://doi.org/10.1021/acs.accounts.8b00345, with permission from American Chemical Society, Copyright 2014.

Chapter 2

Figure 2.3

Metalloporphyrins as Building Blocks to Supramolecular Catalysts

Figure 2.4

67

Supramolecular encapsulation complex of a Mn(III)(Cl)tetrapyridylporphyin catalyst into a supramolecular porphyrin square. As reported in ref. 27.

deactivation usually proceeds via a bimolecular radical–radical coupling reaction limiting the TON of catalysis.29 In an attempt to inhibit such deactivation pathways, our group again looked at porphyrins.30 By combining Zn–tetraaminoporphyrin with bipyridyl aldehyde and Fe(II)triflate or Fe(II)triflimide, a novel M8L6 cubic porphyrin cage could be obtained (see Figure 2.5). When Co(II)tetrapyridylporphyrin was added to these cages in solution, the Co-catalyst could be fully encapsulated into the cavity. Not only did the encapsulated catalyst remain active in cyclopropanation reactions of styrene using ethyl diazoacetate, but the overall catalyst stability was also enhanced. This was apparent due to increased TON compared to the free catalyst. Furthermore, the confined space catalysis again depicted size selective substrate conversion.31 Encapsulated cobalt–porphyrin as a catalyst for size-selective radical-type cyclopropanation reaction mixtures of styrene and bulkier styrene analogs led to favorable formation of the smaller product over the larger one.

2.2.3

Catalyst Stabilization and Lowering Overpotential in Confined Space

A recent study utilizes metal–ligand interactions to synthesize a supramolecular cage and subsequently encapsulates an [FeFe]-hydrogenase mimic

68

Figure 2.5

Chapter 2

A spartan model of Co(II)porphyrin catalyst encapsulated in an M8L6 supramolecular cage. Reproduced from ref. 30 with permission from John Wiley & Sons, Copyright r 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

with an appended pyridine containing a phosphine ligand (see Figure 2.6).32 The catalyst encapsulation is facilitated according to the ligand template strategy, by Zn–pyridine interactions of the ligand with the porphyrin cage wall. These phosphine-containing complexes are known to disproportionate upon electrochemical reduction in solution, which is easily observed by IR-spectroelectrochemistry. However, no disproportionation is observed when the catalyst is bound in a 1 : 1 ratio in a supramolecular cage. Most interestingly, the overpotential for proton reduction is lowered by 150 mV in the cage compared to the catalyst free in solution. The authors relate this to the highly cationic nature of the cage, lowering the barrier to deliver a negative charge. It is interesting to denote that, compared to the cyclodextrin embedded catalyst,33 now a second coordination sphere lowers the overpotential, rather than raising it. Thus, the catalyst is stabilized in the porphyrin cage just as in cyclodextrin capsules, but the catalytic activity is improved with the porphyrin cage.

2.3 Supramolecular Light Harvesting Porphyrin Assemblies – Chromophores The following sections will highlight how supramolecular chemists have attempted to create architectures with similar properties to the natural

Metalloporphyrins as Building Blocks to Supramolecular Catalysts

Figure 2.6

69

Encapsulation of the [FeFe]-hydrogenase mimic in a supramolecular porphyrin cage lowers the overpotential of catalysis and protects the catalyst from disproportionation reactions. Reproduced from ref. 32 with permission from the Royal Society of Chemistry.

system by assembling supramolecular porphyrin structures in homogeneous solution. First, we provide a general overview of the benefit of chromophore organization.

2.3.1

Chromophore Organization

The organization of chromophores leads to high absorption cross sections and localization of photon energy due to two reasons.34 (1) Close packing of light-absorbing molecules results in delocalization or rapid transfer of excitons. Distant chromophores can then deliver the harvested energy to a redox partner (e.g. a catalyst). Delivery of photon energy is therefore no longer limited by direct interaction of photons with the reactive center. The catalytic properties can be optimized without altering the lightharvesting capabilities of the surrounding antenna. (2) Energetically down-hill energy transfer results in an energy cascade toward a reactive center. Though some photon energy is lost, back-transfer is inhibited since this process has become energetically uphill. Similarly, the downward energy cascade for electron transfer (i.e. redox reactions) as observed in natural photosynthesis leads to long-lived charge-separated states. Long lifetimes are required to synchronize with the slower redox processes such as catalysis.

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2.3.2 Supramolecular Porphyrin Antennae 2.3.2.1 Metalated Porphyrin Axial Ligand Coordination Pyridine–Zn interactions show binding constants of 103–104 M in apolar solvents and to get more stable supramolecular assemblies several of these interactions are typically used. However, transiently bound Zn–porphyrin interactions have still led to interesting (photo)catalytic properties. Because of the nature of the porphyrin ligand, the porphyrins are often used with dual functionality. They act as the structural backbones and rigid walls and/or as the light-harvesting functionality. Zn–porphyrins are often combined with pyridine substituted H2–porphyrins for example. The fluorescence emission spectra of Zn–porphyrins overlap with the Q-band absorption of H2–porphyrins. Therefore, exchange of a virtual photon happens readily, such that exciting the zincated porphyrin transfers that energy to the free-base porphyrin. The first measurements performed on the (ZnP)2-H2P trimer indicated the energy transfer rate was below the detection limit of 10 ps (see Figure 2.7a).35 Using a similar strategy porphyrin pentads were prepared and the authors concluded that singlet–singlet ET occurred in these complexes (see Figure 2.7a).36 Other metalloporphyrins may be used, such as Mg, Co, or Ru(CO) porphyrins.37 For example, tetrakis(pyridyl)porphyrin axially coordinated four Ru21(CO)porphyrins.38 Selectively exciting the peripheral Ru21(CO)porphyrins transferred excitation energy to both the free-base and zincated central porphyrin. Zn–porphyrin dimers are useful architectures as well, as a rigid and bent backbone produces clamp-like hosts that allow for encapsulation of pyridine functionalized H2–porphyrins (see Figure 2.8).39 So-called gable type porphyrins showed efficient energy transfer up to 98% with a rate constant of 50 ps.40,41 Interestingly, supramolecular organization does not have to originate from a substituted porphyrin. One may use a pyridine-functionalized template to organize porphyrins in close proximity. Such a strategy was used to lock the planar geometry of butadiyne coupled porphyrin dimers.42 The rigidity

Figure 2.7

Porphyrin triad and pentad complexes that self-assembled by using Zn–pyridine axial binding interactions. These assemblies depict energy transfer from the Zn(II)porphyrin to the H2–porphyrin upon illumination.

Metalloporphyrins as Building Blocks to Supramolecular Catalysts

Figure 2.8

71

The Gable-type ZnP are pre-organized by hydrogen bonds to hold a clamp-like geometry. The pyridine-functionalized H2P is also rigidified by hydrogen bonds. When combining the components on the LHS, a self-assembled structure is obtained as depicted on the RHS. Adapted from ref. 39 with permission from John Wiley & Sons, Copyright r 2000 WILEY-VCH Verlag GmbH, Weinheim, Fed. Rep. of Germany.

72

Chapter 2

induced by the template allowed for optimal conjugation in the dimer molecule and revealed its panchromatic character. More importantly, these discrete nano-aggregates showed energy transfer among chromophores. The authors found an additional ultrafast excited-state decay pathway (3.1  0.1 ps) with an amplitude that was quadratically dependent on the laser fluence. Such a dependency is indicative of singlet–singlet annihilation, meaning that energy transfer occurred among the dimers (see Figure 2.9). More complex architectures, like supramolecular porphyrin cages,43 boxes,44 ladders,45–47 and rings48 (see Figure 2.10) could be formed using Zn-pyridine interactions.49 These different topologies all show fast (picosecond) energetically downhill energy transfer processes as a result of supramolecular preorganization. However, supramolecular aggregation is not necessarily innocent for the photophysical properties of the components. This was nicely illustrated for the porphyrin nanorings. Increasing the ring sizes (consisting of 6, 8, 10, 12, and 30 linked porphyrins) did not increase the energy transfer rate of (1.3 ps)1 to the coordinating porphyrin dimers.50 In fact, little change was ¨rster theory dictates that observed at all. This is counter-intuitive because Fo larger rings should transfer energy at a slower pace. The expected increased delocalization of the exciton (the electron–hole pair) across the ring would entail a larger distance between the donor’s and acceptor’s center-of-mass, resulting in a decreased rate of energy transfer. As the rate for ET did not decrease, the authors concluded that the energy transfer to the ring acceptor is localized below the donor due to the strain induced by the supramolecular complexation of the donor itself. Computer modeling elucidated that the coordination of the pyridine-functionalized dimer desymmetrized the ringacceptor leading to exciton delocalization across 6 porphyrins located beside the excited dimer donor. This desymmetrization occurred for all assemblies and was entirely independent of the ring size. Besides pyridyl Zn–porphyrin assemblies, imidazolyl-coordinating Zn– porphyrins are also interesting light-harvesting systems. Kobuke and Miyaji attempted to mimic the slipped co-facial orientation of the special pair in photosynthetic reaction centers and prepared the first imidazolyl Zn–porphyrin motif (see Figure 2.11).51 This was employed to prepare porphyrin polymers that could be end-capped with energy-accepting Mn(III)porphyrins.52,53 Inspired by gable-type porphyrin assemblies their motif led to the synthesis of a circular supramolecular antenna, that could be permanently locked by ring-closing metathesis reactions using the Grubbs catalyst (see Figure 2.11).54,55 After characterizing its photophysical properties,56 these supramolecular porphyrin rings were used to prepare a combined light-harvesting antenna/charge-separation assembly (vide infra).

2.3.2.2

Porphyrin Interactions at the Substituted Macrocycle

In addition to axial binding, derivation of the porphyrin macrocycle can result in the supramolecular organization of porphyrins using metal–ligand,

Metalloporphyrins as Building Blocks to Supramolecular Catalysts Porphyrin dimers are orientationally locked in place by the tritopic pyridine templates (SL and LL). The energy transfer rate (red arrows) depends on the size of the template. Adapted from ref. 42 with permission from American Chemical Society, Copyright 2008.

73

Figure 2.9

74

Chapter 2

Figure 2.10

The porphyrin dimer is excited, resulting in energy transfer to the ring. However, independent of ring size (N ¼ 6, 8, 10, 12, or 30) energy transfer happens with the same rate (1.3 ps). The authors used the line-dipole model (right) showing the effect of coordination and concurrent strain on the ring. As a result the excitation is delocalized over 6 porphyrins for every ring size. Reproduced from ref. 50, https://doi.org/10.1021/acsnano.6b01265, under the terms of the CC BY 4.0 license, https://creativecommons.org/ licenses/by/4.0/.

Figure 2.11

Supramolecular porphyrin rings developed by Kobuke use the slipped cofacial supramolecular assembly accessed by N-methyl imidazolyl substituents interacting with the Zn metal. Reproduced from ref. 54 with permission from American Chemical Society, Copyright 2003.

Metalloporphyrins as Building Blocks to Supramolecular Catalysts

75

hydrogen bonding, ion–dipole and p–p stacking interactions. Again, these interactions will not only preorganize chromophores in close proximity, but also alter the photophysical properties of dyes due to the supramolecular interaction itself. We will illustrate this by discussing supramolecular systems using metal–ligand and hydrogen bonding interactions. Supramolecular squares and cages can be formed from metal–ligand interactions as first reported by Fujita.57 Often geometrically rigid metals like square planar Pd21 and Pt21 are used to bind pyridine ligands.58 Other metals like Rh, Cu, Fe, or Re may also be used. When porphyrins act as the rigid ligand, discrete supramolecular assemblies form that act as lightharvesting assemblies through energy transfer.59 Metal–ligand assemblies should not competitively bind the porphyrin-bound metal. Therefore freebase porphyrins are often employed. One could wonder what the influence of metal–ligand interactions is on the photophysical properties of porphyrin arrays. Self-assembled multiporphyrin squares were first reported by Drain and Lehn.60 They prepared discrete trans and cis complexes that could be formed from mixed phenyl/ pyridyl porphyrins when combined with the corresponding PdCl2(NCPh)2 or PtCl2(NCPh)2 precursor (see Figure 2.12). All these complexes seemed to show decreased fluorescence intensity which was attributed to singlet to triplet intersystem crossing due to the heavy-atom effect. Similar conclusions were drawn when Drain et al. reported on the formation of nonameric porphyrin arrays.61 They observed a quadratic relationship in the fluorescence quenching upon successive binding of Pd complexes to a tetra pyridyl porphyrin. Moreover, Stang and Fleischer denoted that the fluorescence quenching became more pronounced when Pt was used rather than Pd to assemble supramolecular porphyrin squares.62 Showing that quenching is more efficient with heavier atoms. Scandola et al. attempted to shed light on these fluorescence deactivation pathways by studying the photophysics of pyridylporphyrin Ru(II) adducts.63 They proposed that S1-T3 intersystem crossing was facilitated by the Ru(II) atom both (i) at the excited chromophore subunit and (ii) in the process of energy transfer to the accepting chromophore via the bridging metal atom. Interestingly, the efficiency of the second pathway depended on the ligand field of the connecting Ru complex. A weak ligand field resulted in efficient singlet quenching. This work suggests that supramolecular light-harvesting assemblies using heavy metal ions may retain singlet energy by an increased crystal field splitting due to a strong ligand field at the connecting ion or by preventing orbital overlap. In short, metal–ligand interactions are a feasible strategy to produce light-harvesting antennae when taking the heavyatom effect into account. Formation of triplets complicates the photoexcited energy landscape and always leads to a loss of energy. In addition, the long lifetime of triplets may lead to bimolecular side reactions and decomposition pathways such as the generation of reactive oxygen species (e.g. singlet oxygen). Despite the heavy-atom effect, metal–ion assemblies can participate in downhill energy-transfer pathways. Zincated pyridyl porphyrins coordinating Re(CO)3Cl form fluorescent squares for example.64 These are able to transfer

76

Representative tetrameric supramolecular porphyrin assemblies with cis- (left) and trans-substitutions (right). Reproduced from ref. 60 with permission from the Royal Society of Chemistry.

Chapter 2

Figure 2.12

Metalloporphyrins as Building Blocks to Supramolecular Catalysts

77

energy to a H2–tetrapyridylporphyin guest quenching the fluorescence intensity of the host near completion (B90%). Although the free-base guest does not show any remaining emission, suggestive of non-emissive triplet formation. Clearly, these assemblies are very interesting when triplets are the desired photoproduct. Nevertheless, a properly chosen energy acceptor may still profit from the metal–ligand supramolecular assembly for light-harvesting as the rate for energy transfer competes with the rate for intersystem crossing. For example, Harvey et al. prepared several Pd and Pt coupled Zn–porphyrin and H2–porphyrin dyads (see Figure 2.13).65 They showed that an increasing size of the linker species (PdOoPtOoPtS) resulted in an increasing rate of energy transfer (kET) and an increasing rate of intersystem crossing (kisc). More importantly kisc (s1) was significantly lower than the kET (s1) in the case of the PdO linker (2.31010 s1 and 1.51012 s1 respectively). Therefore, the ultrafast energy transfer occurred after 660 fs and with an efficiency of 98%, where the remaining 2% led to triplet formation of the donor. The lifetime of the S1 energy transfer product was not reported. Another approach is to use lighter ions, such as Al31 and Ga31, which do not result in quenching of the S1 state, yet provide stable octahedral geometries like the heavier transition metals. Kobuke et al. reported three oxinyl-substituted H2–porphyrins coordinating a Ga31 ion (see Figure 2.14).66 Rapid energy transfer occurs among the porphyrins (B10 ps) and the singlet energy is retained at the ns timescale (8.6 ns) implying the singlet energy is transferred around 800 times across the assembly before decaying. A doubly oxinyl-substituted porphyrin resulted in the formation of an extended network with similar photophysical properties as the trimer.67 The light-harvesting systems described above consist of only a limited amount of porphyrin or require tedious organic synthesis (such as the porphyrin rings). However, the power of supramolecular chemistry lies in the

Figure 2.13

Supramolecular porphyrin dimers by Dekkiche et al. Reproduced from ref. 65 with permission from American Chemical Society, Copyright 2016.

78

Figure 2.14

Chapter 2

Oxinyl-substituted porphyrins assemble into light-harvesting trimers, that rapidly transfer singlet excitation energy. Reproduced from ref. 66 with permission from Elsevier, Copyright 1999.

synthesis of highly complex architectures with relatively simple building blocks. The potential of metal–ligand interactions for the organization of chromophores was recognized in the Fujita group, where spherical coordination cages consisting of 36 small components self-organized because of pyridine–Pd interactions (see Figure 2.15).68 The resulting nanoball was decorated with 24 porphyrin units. However, the added complexity is obvious as the photophysical properties and possible energy transfer pathways of these species were not characterized. Besides metal–ligand interactions, hydrogen bonds are commonly explored in the formation of supramolecular porphyrin assemblies. Like metal–ligand interactions, they influence the photophysical processes, not only by preorganization, but as a result of the hydrogen bond motif itself. Charge-separation assemblies using hydrogen bonds were explored first, but light-harvesting antennae have scarcely been reported.69 Sessler, Wang, and Harriman therefore decided to functionalize porphyrins with Watson–Crick-type nucleic acids and assemble porphyrin dimers and trimers. They found that the guanine (G)-substituted Zn–porphyrins bind singly and doubly functionalized cytosine (C) H2–porphyins selectively in a 1 : 1 and 2 : 1 ratio respectively. The authors were curious whether the hydrogen bonds facilitated the energy transfer process, or whether the energy transfer properties emerged as a result of spatial organiza¨rster thetion solely. Indeed, singlet–singlet ET could be wholly described to Fo ory, but triplet–triplet energy transfer was also observed. Since triplet–triplet energy transfer always occurs via the Dexter mechanism, orbital overlap is required. The observed T3-T3 ET decay rate (106 s1) was attributed to Dexter energy transfer because of orbital overlap facilitated by the hydrogen bonds.

Metalloporphyrins as Building Blocks to Supramolecular Catalysts

Figure 2.15

79

Self-assembled porphyrin nanoball developed in the Fujita group. The porphyrins (green) are functionalized with a ditopic pyridine ligand (blue) that coordinates Pd(II) ions (yellow). Reproduced from ref. 68 with permission from John Wiley & Sons, Copyright r 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

A decade later, Otsuki et al. prepared dimers and pentads (see Figure 2.16) of porphyrins through amidinium–carboxylate salt bridges.70 Similar experiments were used to characterize the supramolecular structures as described above. However, these assemblies showed S1 ET at a much higher rate than ¨rster theory. The dyad showed an energy transfer rate of predicted by Fo 4.0109 s1 observed versus 5.1108 s1 as predicted by theory and a quantum yield of 89%. Meanwhile, the pentad showed an energy transfer rate of ¨rster theory with a 1.21010 s1 observed versus 6.8108 s1 predicted by Fo quantum yield of 96%. Therefore, it was inferred that the hydrogen bonds, here augmented by electrostatic interactions, mediate singlet energy transfer. The question remained whether the observed S1 ET was merely determined by preorganization in the assembly or that orbital overlap was required as in the case of Sessler, Wang, and Harriman’s assemblies (vide supra). Therefore, Otsuki et al. prepared dyads of the amidinium–carboxylate bridged porphyrins with different electronic structures (see Figure 2.17).71 In the TPP analog the HOMO is located at the meso-carbon next to the hydrogen bonding bridge, whereas in the case of the b-octaalkyl substituted porphyrins a node is localized at this carbon atom. It was observed that the second assembly indeed showed a rate an order of magnitude faster than predicted ¨rster theory, but three times slower than the TPP analogues. Hence, by Fo orbital overlap facilitated by supramolecular interactions contributes considerably to efficient energy transfer across light-harvesting assemblies.

80

Figure 2.16

Chapter 2

Supramolecular light-harvesting assembly by Otsuki et al. Reproduced from ref. 70 with permission from John Wiley & Sons, Copyright r 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Metalloporphyrins as Building Blocks to Supramolecular Catalysts

Figure 2.17

2.3.2.3

81

Porphyrin assemblies used to study contribution of the supramolecular amidinium–carboxylate bond to the energy transfer rate. The TPP porphyrin (top) has considerable electron density at the meso positions, while the b-octaalkyl porphyrin (bottom) electronic structure has a node at the meso position. Reproduced from ref. 71 with permission from John Wiley & Sons, Copyright r 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Combined Axial Binding/macrocycle Substitution

Hydrogen-bonding interactions can be used in conjunction with metal– porphyrin coordination, resulting in dyads with high binding constants.72 Realizing the reported discrete assemblies contained relatively few components, the group of Kuroda attempted to increase the sheer size of a supramolecular antenna system to enhance light-harvesting efficiency.73 H2-tetraphenylporphyrin was substituted with pyrazine units on a covalent spacer. Pyrazines are ditopic nitrogen ligands that acted as a guest for coordination of Zn–porphyrin cages that themselves were formed via hydrogen bonds. A single H2–porphyrin was able to bind 8 light-harvesting Zn– porphyrins in this manner (see Scheme 2.2). The energy transfer efficiency from a single Zn–porphyrin unit to the H2–porphyrin was 82%. Though this efficiency is lower than previously reported for covalent systems or the hydrogen bonded assemblies discussed above, a very large antenna effect was observed. The fluorescence of the H2–porphyrin was increased by a factor of 18. This is a clear depiction that an increased donor/acceptor ratio

82

Scheme 2.2

Chapter 2

(Top) Structure of pyrazine-functionalized porphyrin that can coordinate (bottom) the donor Zn–porphyrin cage itself assembled with hydrogen bonds. When these components are combined in CH2Cl2, the antenna system on the RHS forms. Some atoms are omitted for clarity. Reproduced from ref. 10 with permission from the Royal Society of Chemistry.

Metalloporphyrins as Building Blocks to Supramolecular Catalysts

83

1

leads to enhanced development of the desired S photoproduct on the acceptor. Thus, the H2–porphyrin in the supramolecular complex can be excited by photons otherwise inaccessible to it, as excitation wavelengths were chosen where the absorption coefficient of the H2–porphyrin is low. More elaborate H2–porphyrins with increasing pyrazine functionalities linked in a linear or branching topology revealed that the enhanced lightabsorption could be further increased (77-fold) when the donor/acceptor ratio of 16 : 1 for the Zn–porphyrin/H2–porphyrin was obtained.74

2.3.3

Light Harvesting Using Supramolecular Polymer Assemblies of Porphyrins

Inspired by large arrays of chromophores called chlorosomes, supramolecular chemists have attempted to prepare stacked assemblies that absorb light and transfer excited-state energy over large distances. These assemblies can consist of several thousands of chromophores. Especially porphyrin assemblies have been used in this regard, because they tend to form so-called H- or J-aggregates when properly aligned. J-aggregates are easily prepared by supramolecular interactions upon modification of the macrocycle. For example, para tetrasulfonated porphyrin H2TPPS44 stacks in a slipped-co-facial alignment when acidified.75 The anionic sulfonates interact with the four pyrrole appended protons via ion–ion bonds. Formed rod-like structures can be huge (B0.5 mm long).76 A more recent investigation of these aggregates using transient absorption spectroscopy yielded a diffusion constant for energy migration across the rod of 3–6 cm2 s1, which would be too fast to be ascribed to FRET.77 Coherent energy transfer effects must play a role. The delocalized exciton changes the relaxation pathways in aggregated assemblies considerably when compared to the freely diffusing molecule. Theoretical models for dimers and aggregates have been available for some time,78,79 though supramolecular structures often deviate from ideal situations leading to complex deactivation pathways. Furthermore, these pathways depend strongly on the packing of aggregates, and they occur at the sub-ps timescale. Ultrafast spectroscopy is required to elucidate all pathways resulting from supramolecular organization.80 Furthermore, imperfections and impurities in aggregates often lead to additional deactivation pathways that may not be general for the studied system. In order to fully understand the emergent properties of supramolecular aggregates it is therefore crucial to be able to prepare and interrogate welldefined structures. Among others, J-type porphyrinoid aggregates resulting from a slipped co-facial alignment, have been formed selectively.81,82 Besides ionic interactions, substituting metalated porphyrin analogues can lead to J-aggregates.83 Hydroxyl substituents on Zn-chlorins coordinate the metal resulting in the desired alignment of the porphyrins. These aggregates can also be formed from porphyrin–catechol molecules via hydrogen bonds.84 Interestingly, the authors were able to tune the stacking process by changing the pH and via selecting different counterions (Cl, NO3). In this manner they were

84

Chapter 2

able to selectively prepare J-aggregates and the less-studied H-aggregates with the same porphyrin. Thus, using ultrafast transient spectroscopy the authors were able to map the different decay pathways for both types of aggregates. As a result, both types show very fast deactivation pathways, but the J-aggregates show small exciton–vibration coupling while the H-aggregates’ face-to-face arrangement leads to large exciton–vibration coupling.

2.3.4

Supramolecular Scaffolds Pre-organizing Porphyrins

Synthesis of light-harvesting assemblies is not limited to intercomponent interactions. Chromophores can be preorganized on a scaffold, harboring functionalities for supramolecular interactions. For this viable approach a multitude of natural or (partly) artificial constructs have been explored.85 Scaffolds include proteins, DNA, viral capsids, and lipid membranes, but also (non-natural) peptides and dendrimers. Most of these scaffolds use similar interactions as described previously, though the increased number of porphyrins in the system leads to complicated photophysical properties. Only dendrimers are highlighted here leading to unique topologies showing altered photophysical properties.

2.3.4.1

Dendrimer Scaffolds for Light-harvesting

Dendrimers are synthetically arduous branching covalent structures, though interesting for applications as light-harvesting antennae. One such example is the formation of a supramolecular coordination polymer from porphyrinbased light-harvesting dendrimer donors and pyridyl-functionalized porphyrin acceptors. When zincated porphyrin dendrimers were simply combined with pyridine-functionalized porphyrins in a 1 : 1 stoichiometry, a supramolecular polymer formed.86 Energy transfer that initially occurred intramolecularly across the dendrimer, happens intermolecularly from the zincated porphyrin wings to the coordinated free base pyridinyl porphyrin upon formation of the supramolecular polymer. Another example is the study by Sakamoto, Ueno, and Mihara which was based on dendrimers end-capped with artificial a-helix peptide chains containing histidine residues.87,88 They showed that Zn–mesoporphyrin IX was bound between two helices. As they prepared dendrimers containing 64 helices, up to 32 porphyrins could be incorporated in the assembly (see Figure 2.18).

2.4 Supramolecular Charge-separation with Porphyrin Assemblies – Charge-separation Comparable interactions are used for controlled electron transfer as we have seen in the previous section regarding strategies for efficient lightharvesting. Without going into the details of these strategies again, exemplary studies are highlighted, including examples that deliver new

Metalloporphyrins as Building Blocks to Supramolecular Catalysts

85

Figure 2.18

A 64-peptide dendrimer binds 32 Zn–porphyrins in supramolecular fashion by metal–ligand interactions of Zn with a histidine residue. Reproduced from ref. 87 with permission from the Royal Society of Chemistry.

Figure 2.19

Supramolecular assembly of a porphyrin donor and p-benzoquinone electron acceptor using H-bonds and p–p interactions.

insights. It is argued that supramolecular assemblies for charge-separation are not outcompeted by their covalent analogues and that these dynamic systems show a positive effect on decreasing recombination rate. Some examples, integrating light-harvesting and charge-separation functionalities in a supramolecular fashion are also discussed here.

2.4.1

Discrete Supramolecular Charge-separation Assemblies

Natural Photosystem II uses the collected photon energy to reduce quinones. These reduced molecules then diffuse through the membrane to deliver electrons to cytochrome b6f which eventually transfers electrons to Photosystem I. Many covalent porphyrin–quinone assemblies exist to model natural photo-induced electron transfer (PET) events.89 The group of Ogoshi et al. took the supramolecular route and prepared a 5,15-bis(hydroxynaphthyl)porphyrin that interacted efficiently with para-quinones (see Figure 2.19).90 The porphyrin–quinone adducts had a face-to-face separation as close as 3 Å and binding constants up to 4.2102 at room temperature in CHCl3 controlled by sterics, strength of H-bonding, and p–p interactions. The adducts were non-fluorescent due to this complexation, which was attributed to electron transfer. Higher binding constants (Ka ¼ 2.0104) were achieved in a follow-up study when four hydrogen bonds were used.91

86

Figure 2.20

Chapter 2

Cyclodextrin-sandwiched porphyrin donor binds electron-accepting quinones. Reproduced from ref. 92 with permission from American Chemical Society, Copyright 1993.

In a related approach, porphyrins were sandwiched between cyclodextrins via covalent bonds. Quinones, both free and adamantane functionalized, formed host–guest assemblies that preorganized the electron donor and acceptor (see Figure 2.20).92 Harriman, Kubo and Sessler used complementary base-pairing like that used for energy transfer (Section 2.3.2.2) to append a Zn–porphyrin to quinones.93 They describe a through bond-mechanism for electron transfer, though the functionalities are connected via hydrogen bonds only. Often, taking a known supramolecular light-harvesting antenna and changing the energy acceptor for an electron acceptor yields chargeseparation assemblies. For example, the zincated cis-pyridyl porphyrin dimers connected via RuCl2(CO)2 by Iengo et al. (Section 2.3.2.1) were

Metalloporphyrins as Building Blocks to Supramolecular Catalysts

Figure 2.21

87

Supramolecular squares of Zn–porphyrin donors and PBI struts acting as electron acceptors. Reproduced from ref. 95 with permission American Chemical Society, Copyright 2010.

assembled into a supramolecular square using trans-pyridyl free-base por¨rthner placed pyridine-functionalized phyrins.94 When the group of Wu perylene bisimides (PBI) in-between the Zn–porphyrins, a chargeseparating square95 (see Figure 2.21) or cage96 was obtained. This resulted in a short-lived (440 ps) PBI Zn1 charge-separated-state for the square, nearing quantitative yield (495% depending on the substituents at PBI). These PBIs are generally applicable in supramolecular electron transfer assemblies.97 PBIs coordinate Ru(CO)porphyrins98 (PET quantitative yield, 270 ps lifetime) and Ru(CO)phtalocyanines99 (PET quantitative yield, 115  5 ns lifetime) to form triads and pentads100 (PET 94% yield, 260  20 ps lifetime) that produce charge-separated states (see Figure 2.22). The lifetimes of the triads are higher than for the pentad, because of the orthogonal assembly in the triads. Moreover, the phthalocyanine triad profits from a large driving force for recombination, resulting in kinetics in the Marcus inverted region, slowing the ´nez et al. also prepared triads by using recombination rate considerably. Jime hydrogen-bonding melamine substituted phthalocyanines to PBIs that resulted in charge-separated states in a ns time regime (48 ns).101 Gunderson et al. assembled a cyclic tetramer of zincated chlorophyll a using axial pyridinyl interactions.102 The substituents were decorated with one or

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Figure 2.22

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Charge separating trimers of porphyrins (top), phthalocyanine (middle), and pentad phthalocyanines (bottom). Reproduced from ref. 97 with permission from the Royal Society of Chemistry.

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Figure 2.23

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Self-assembled tetramers have longer charge-separated states than the monomeric constituents. Reproduced from ref. 102 with permission American Chemical Society, Copyright 2012.

two naphthalene dicarboximide (NDI) acceptors. Interestingly, the assembly of this tetramer resulted in three times slower charge-recombination than observed in the monomer (see Figure 2.23). These examples for chargeseparation have focused on symmetric assemblies, partly due to the ease of synthesis and study. However, charge-separation is an intrinsically asymmetric process. This does not have to hamper the synthesis of supramolecular systems, when proper orthogonal interactions are chosen. The group of Scandola developed a metal-mediated strategy using porphyrins.103 They prepared monopyridyl Al–porphyrins that can bind carboxylic acids at the metal and another porphyrin (Ru(CO)porphyrin) at the pyridyl substituent. When a carboxylic acidfunctionalized NDI was combined with Al porphyrin and Ru(CO)porphyrin in a 1 : 1 : 1 ratio, a triad selectively and quantitatively formed (see Figure 2.24). Electron transfer occurs to NDI when the Al–porphyrin is excited in a mere 3 ps. The Ru–porphyrin provides an electron with a time constant of 30 ps to prepare a Ru1 –Al–NDI species that lives for 2–10 ns. Considerably longer (B20–100 fold) than the Al1 –NDI species also investigated. Because of its small reorganization energy, the Buckyball easily accepts electrons. It is therefore no surprise that porphyrins are thoroughly studied for PET using C60 fullerene acceptors.104 In a similar sense, carbon nanotubes and graphene materials are studied as electron acceptors. However, the reported assemblies rely strongly on similar interactions and strategies as discussed in this chapter. Mainly, porphyrinoids substituted with covalently linked lightharvesting functionalities such as BODIPYs, or covalently linked to a scaffold, are combined with fullerenes equipped with N-ligands. Upon axial ligand coordination a self-assembled charge-separation species is formed. We could have easily replaced all our discussions and examples for those reports using C60. Nevertheless, we shall highlight one interesting supramolecular triad implementing fullerenes as it uses orthogonal interactions as discussed above. Bis(amidopyridine) receptors bind barbiturate hosts with very high binding constants (105–108) in non-competitive solvents.105 Guldi et al. used this approach to assemble PBI–porphyrin dyads, but added a pyridinyl-functionalized C60 fullerene that selectively binds the Zn porphyrin (see Figure 2.25).106 Exciting the PBI unit resulted in energy transfer to the porphyrin (53  3 ps), followed by electron transfer to the fullerene (12  1 ps). The charge-separated

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Figure 2.24

An asymmetric supramolecular trimer selectively forms and performs charge-separation. Reproduced from ref. 103 with permission from the Royal Society of Chemistry.

state is longer lived in the triad (3.8  0.2 ns) compared to the porphyrin– fullerene dyad (1.0  0.1 ns).

2.4.2

Integrated Antenna/Charge-separation Assemblies

Artificial photosynthetic devices should integrate light-harvesting and charge-separation assemblies such as those described above. Similarly, supramolecular strategies can be developed to combine (individually optimized) antennae and charge-separation assemblies into a single species. Such architectures are rather limited and few examples are discussed here. Again, porphyrins play a major role. Cyclic tetramers of Zn–porphyrins such ´nez et al. in the group of Wasielewski can be as the one described by Jime considered as an integrated antenna/charge-separation (An/CS) assembly (vide supra). However, these architectures are fully symmetric. Cyclic tetramers of zinc chlorophylls were also explored in the Otsuki group (see Figure 2.26).107 Their strategy was somewhat different as Zn–chlorophylls were combined in a

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Figure 2.25

When the above PBI is excited, energy is transferred to the Zn–porphyrin. This chromophore reduces the coordinating C60 fullerene in turn. Reproduced from ref. 106 with permission from the National Academy of Sciences of the USA, Copyright 2012.

Figure 2.26

Synthesis of an integrated An/CS assembly yielding a statistical mixture in solution. Reproduced from ref. 107 with permission from John Wiley & Sons, Copyright r 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

3 : 1 ratio with fullerene acceptor-functionalized chlorophylls to prepare a statistical mixture of assemblies in solution. They mostly obtained the D3–A1 species (42%) but the D4 species was also predominant (32%). The rest was a mixture of multi-acceptor-containing species. Energy transfer was observed between the chlorophylls until electron transfer occurred (both 2.61011 s1) and with practically quantitative yield. The charge-separated state eventually recombines but with a time constant of 2 orders of magnitude slower (1.9109 s1) and a charge-separated state lifetime ofB0.53 ns. An important aspect of An/CS assemblies was highlighted by Terazono et al. when they prepared a supramolecular An/CS assembly heptad (see Figure 2.27).108 Namely, that natural light-harvesting systems use a plethora of

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Figure 2.27

Chapter 2

Multi-chromophoric antenna transfers excitation energy in a downward cascade resulting in charge-separation due to self-assembly with a C60 fullerene electron acceptor. Energy transfer pathways (solid arrows) and electron transfer pathways (dotted arrows) are shown. Reproduced from ref. 108 with permission from American Chemical Society, Copyright 2009.

chromophores to enable panchromatic light-absorption of the solar spectrum. As such, complementary chromophores such as anthracene, BODIPYs, and Zn– porphyrins were covalently attached to a hexaphenylbenzene core. The authors denoted that energy transfer among the chromophores occurred along a downward energy cascade until the exciton reached the Zn–porphyrins. By simple addition of a ditopic pyridyl-functionalized C60 fullerene to the mixture, this electron acceptor nested in-between the porphyrins. As a result, very rapid electron transfer occurred to the Buckyball with quantitative yield. The lifetime of the charge-separated state was 70 times larger than the rate of electron transfer, but relatively short compared to other systems (230 ps). Furthermore, the authors denote that higher binding constants may be required as a residual signal was observed for unbound fullerenes. A follow-up study appended four

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Coumarin dyes to the core in place of the anthracene and BODIPY functionalities to promote singlet–singlet energy transfer among the chromophores.109 However, no change in charge-separation properties was observed. The use of dendrimers as covalent light-harvesting assemblies was discussed above. However, a simple addition of a coordinating C60 acceptor turns the antenna in a An/CS assembly (see Figure 2.28).110 Firstly, several Zn porphyrin-containing dendrimers were prepared (4, 8, and 16). Then pyridinyl-functionalized fullerene was titrated into the solution. By comparing the determined binding constant via absorption and fluorescence, the authors noticed a discrepancy upon increasing dendrimer size. In short, the difference between binding constant determined by absorption and fluorescence became larger with dendrimers consisting of more porphyrins. The authors appointed this to excited-state energy migration. This resulted

Figure 2.28

Dendrimer with 16 porphyrin units acts as a light-harvesting antenna and achieves long-lasting charge separation when electron accepting fullerenes bind the porphyrin metal atom. Reproduced from ref. 110 with permission from the Royal Society of Chemistry.

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in an impressively long charge-separated lifetime (0.25 ms). Around the same time, these authors reported supramolecular complexes of multiporphyrinic oligopeptides.111 These porphyrin–peptide octamers bound up to eight equivalents of coordinating fullerene. Again, impressive lifetimes of the charge-separated state were reported (0.34–0.84 ms). The cyclic porphyrin macrocycles developed in Kobuke’s group, acting as light harvesting antennae and based on imidazolyl coordination to Zn–porphyrins, were discussed above. Notably, bent covalent Zn porphyrin trimers were also developed that form a supramolecular trimer of trimers via imidazolyl-Zn bonds.112 The central porphyrin unit retains the ability to bind a ligand. As such, tripodal pyridine ligands were developed that bind the available porphyrins in the light-harvesting macrocycle with very high binding constants (8108 in toluene). Next, a tripod was synthesized containing an energy accepting Zn–porphyrin with an attached C60 fullerene (see Figure 2.29).113 Energy transfer from the excited antenna occurred with high efficiency (91%). A lowest estimate efficiency for charge-separation by electron transfer to the fullerene was mentioned (94%). This led to a total energy conversion efficiency in the system of 85% where the charge-separated state had a half-life of 0.2 ms. The authors took their bioinspired approach even further when they reported incorporation of the supramolecular antenna into lipid bilayers (known as dynamic self-assembled interfaces) that could incorporate a tripod with appended fullerene into the membrane.114 Transient experiments were not performed, but the authors deduced interantenna energy transfer as they were assembled in the membrane. Namely, when one tripod acceptor was added to five equivalents of porphyrin antenna the fluorescence quenching efficiency was three times larger in the bilayer than in homogeneous solution (54% versus 19%).

2.5 Integrated Supramolecular Light-harvesting Porphyrin Catalyst Assemblies – Chromophore–Catalyst Assemblies Often light-absorption and efficient proton reduction catalysis are orthogonal molecular properties. Therefore, optimizing molecular properties separately is most interesting. Without much derivation, light-absorbing and catalytic components can be allowed to self-assemble into supramolecular architectures for light-driven proton reduction. This was primarily explored for metallosupramolecular dyads consisting of porphyrins and cobaloximes or [Fe2(CO)6(m-pdt)] (pdt ¼ propylene-1,3-dithiolate) complexes and their analogues as catalysts.

2.5.1

Supramolecular Photochemical Proton Reduction

If the diffusion of electron donors, the light-absorber and catalyst become rate limiting in photocatalysis, supramolecular interactions may be used to

Metalloporphyrins as Building Blocks to Supramolecular Catalysts

Figure 2.29

95

Model of the integrated An/CS assembly. Adapted from ref. 113 with permission from John Wiley & Sons, Copyright r 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

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Figure 2.30

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The supramolecular SED-Por-Cat assembly studied in the group of Sun consists of triethylamine SED coordinating a light-harvesting metalloporphyrin that reduces the coordinated cobaloxime catalyst upon excitation. Reproduced from ref. 115 with permission from the Royal Society of Chemistry.

pre-organize the electron donor and chromophore to prevent recombination. Therefore, the group of Sun prepared Mg and Zn porphyrins to bring the triethylamine sacrificial electron donor (SED) in the coordination sphere (see Figure 2.30).115 This was later expanded upon in the group of Scandola, who used Al(III)(OH)porphyrins and ascorbate as the SED to increase the TON.116 However, thorough ultra-fast spectroscopy studies revealed that increased efficiency was not derived from the assembled SED-Por-Cat assembly, but from its components in solution. The authors put the blame on an increased rate of charge recombination as the SED is coordinated to the chromophore. This denotes the careful balance between rigidity and dynamicity for supramolecular photocatalysis. For efficient artificial photosynthesis to occur, proper charge-separation is of paramount importance. As abundant metal catalysts [FeFe]-hydrogenase mimics are often considered for hydrogen production.117,118 Naturally, supramolecular dyecatalyst assemblies emerged as well. A Zn–porphyrin appended hydrogenase mimic was reported by the Sun group.119 They functionalized the nitrogen of [FeFe(CO)6(m-adt)] (adt ¼ azadithiolato) with a pyridine appended carbon chain. This strategy yielded H2 with a TON of 0.16 under illumination. Around the same time our group investigated pyridinyl phosphine-appended Fe2 clusters (see Figure 2.31).120 When Zn–porphyrins were added, a dyecatalyst assembly formed that was able to form hydrogen (TON ¼ 5) from NiPr2EtHOAc as the proton source and electron donor under illumination with a Xe-lamp. Interestingly, only experiments containing a mixture of porphyrins yielded hydrogen. The active species carried both porphyrins, highlighting that asymmetry in the structure may be required for catalysis, which coincides with the inherent asymmetry of the charge-separation process as discussed above. Interestingly, both groups highlight different advantages of supramolecular organization. Namely, the Sun group

Metalloporphyrins as Building Blocks to Supramolecular Catalysts

Figure 2.31

97

The supramolecular template ligand strategy was employed to prepare an asymmetric H2-producing [FeFe] hydrogenase mimic, coordinating two different porphyrin ligands. Reproduced from ref. 120 with permission from the National Academy of Sciences of the USA, Copyright 2012.

highlights the dynamicity of the pyridyl-Zn bond to inhibit recombination, while we denoted the potential of stabilization by encapsulating catalysts in the pocket formed by the bulky porphyrins. The CO ligands on these Fe2-clusters lend themselves for IR studies. Marked changes in the IR spectrum can be observed upon reduction. This was used by Li and Amirjalayer et al. to follow the PET when exciting a porphyrin appended to a [FeFe] hydrogenase mimic.121 Transient infrared spectroscopy revealed that the charge-separation was slowed considerably and more strongly than a covalent analogue. The authors denote that the supramolecular Zn–pyridine bond increased the driving force for chargeseparation, but decreased the driving force for charge recombination slowing the recombination rate as well. Tetrahedral porphyrin cages have been employed by our group to enhance the electrochemical stability and overall kinetic barrier of [FeFe]-hydrogenase mimics in electrocatalytic proton reduction (see Section 2.2.3). Acknowledging the potential dual-role of the porphyrin cage for site-isolating the catalyst and acting as a light-harvester, we have also investigated such assemblies for photocatalytic proton reduction using a sacrificial electron donor.122 This time a previously reported pyridine-functionalized, electron-accumulating phosphole ligand was encapsulated.123 The supramolecular cage strategy again allowed for quantitative encapsulation of the catalyst by interior coordination of pyridine ligands to the Zn–porphyrin walls. Using time-resolved IR spectroscopy the electron transfer process was monitored. This experiment pumps the sample with visible light (585 nm) to excite the porphyrin cage and probes the catalyst in the IR region with sub-picosecond

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resolution. These experiments elucidated very fast electron transfer with a time-constant of 0.5 ps. However, the lifetime of this charge separated state was equally fast, living only 37 ps.

2.5.2

Supramolecular Photochemical Water Oxidation

Light-driven photochemical water oxidation experiments using porphyrins in a supramolecular approach are scarcely reported in the literature. This is not only attributed to the limited solubility of porphyrins in water, the oxidized porphyrin radical cations usually do not have the oxidative power required for water oxidation.124 These limitations were also recognized by our group, as we explored porphyrins with increased oxidative power. We found that water soluble Pt(II)–tetracarboxylic acid porphyrins (TCPP) drove water oxidation catalysis with homogeneous Ir(II)N–heterocyclic carbene and Co4O4–cubane complexes.125 Halogenated tetrachloro-metalloporphyrin was also developed as a visible-light-driven sensitizer for water oxidation.126 These porphyrins were also very photostable and retained the ability to oxidize water, while using earth-abundant transition metals such as Ni and Cu. Interestingly, the conjugation of NaYF4:Yb31, Er31 upconversion nanoparticles with Pt(II)–TCPP delivered a highly stable sensitizer with the possibility to perform water oxidation using near-infrared light.127 Though interesting, these sensitizers have been preorganized with covalent chemistry. However, since the advent of poly-oxometallates (POMs) some assemblies using supramolecular interactions to preorganize porphyrin chromophores have been reported for oxidation chemistry. POMs based on Ru (Ru4POM) have potential as inorganic water oxidation catalysts, being able to accommodate several charges required for water oxidation (4 electrons) on multiple metal atoms. These POMs are also highly anionic, which facilitates ionic interactions with cationic porphyrin lightharvesters. Water-soluble hexacationic 5,10,15,20-tetrakis(N-methyl-pyridinium4-yl)porphyrins self-assemble with POM to function as a catalyst for light driven oxidation alcohols in water. Though the TON were in the order of 102 the reported overall yields were low, meaning a large excess of substrate was required.128 Actual supramolecular water oxidation catalysis using porphyrins was performed by Dolbecq, Mellot-Draznieks, and Fontecave. By using cobalt POMs as catalysts, they investigated the porphyrin-based metal organic framework (MOF), MOF-545, as a light absorbing supramolecular host for water oxidation catalysts.129 They chose a POM [(PW9O34)2Co4(H2O)2]10 as the WOC, which was immobilized in the MOF by mild aqueous impregnation (see Figure 2.32). This approach led to a TON of 70 per POM after one hour illumination. The catalyst retained most of its activity after the reaction when it was collected by filtration. So, the authors attribute a low rate to the sacrificial oxidant (and possibly, diffusion limitations). Therefore, when the supramolecular assembly was loaded on conductive thin films the TON was increased 23-fold after two hours of illumination.130

Metalloporphyrins as Building Blocks to Supramolecular Catalysts

Figure 2.32

99

The TCPP porphyrin building block has a dual role as a structural MOF ligand and light-harvesting functionality. The Co–POM catalyst is encapsulated in the large pores of the MOF, where it reduces excited porphyrin walls to oxidize water in turn. Reproduced from ref. 129 with permission from American Chemical Society, Copyright 2018.

2.6 Conclusion This chapter discussed the application of porphyrins in supramolecular systems to obtain catalytic function. Porphyrins are very versatile building blocks that facilitate cage formation around metal-based catalysts that can lead to stabilization and enhanced activity or selectivity. By the templateligand approach porphyrins encapsulate and highly enhance properties of transition metal catalysts. This template-ligand strategy was applied leading to improved catalysts for (asymmetric) hydroformylation, hydrogenation, epoxidation, radical reactions, and electrochemical hydrogen evolution. All these reactions depicted an increased catalyst stability when encapsulated by cages based on porphyrin building blocks. In addition, the supramolecular approach using porphyrins led to substrate size selectivity. All these properties are reminiscent of nature’s enzymes, where the active site is also protected by embedding into a supramolecular architecture. Besides substrate (size) selectivity is also controlled by the protein 2nd coordination sphere. Furthermore, the reaction barrier for proton reduction was significantly lowered for [FeFe]-hydrogenase mimics by encapsulation into a

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porphyrin cage, showing promising 2nd coordination sphere control in the field of artificial photosynthesis. Toward artificial photosynthesis supramolecular architectures have been studied thoroughly and porphyrins have proven to be indispensable as light-harvesting antenna systems, which has led to unprecedented artificial energy transfer systems. Porphyrins have also been used to form supramolecular structures in conjunction with electron acceptors. Such systems show PET with long charge-separation lifetimes. Furthermore, the combination of both antennae and charge-separation functionality has been facilitated by porphyrins using supramolecular interactions. These functionalities are essential for artificial photosynthesis and may lead to novel sophisticated photocatalytic systems. Initial results of supramolecular photocatalytic systems have shown promising results, but need to be optimized for light-harvesting and charge migration. A careful balance exists between supramolecular pre-organization and dynamicity of the photosynthetic system. Though a daunting challenge, developing supramolecular chemistry to integrate light-harvesting, charge separation, and catalysis in a single system may be unavoidable to achieve efficient artificial photosynthesis. Undoubtedly, metalloporphyrins will play a pivotal role in this development.

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CHAPTER 3

Design of Porphyrinic Metal–Organic Frameworks SOOCHAN LEE AND WONYOUNG CHOE* Department of Chemistry, Ulsan National Institute of Science and Technology, 50 UNIST-gil, Ulsan 44919, Republic of Korea *Email: [email protected]

3.1 Introduction Porphyrins are a series of heterocyclic macrocycles with the substituents on the parent backbone.1–4 Porphyrin derivatives are biologically important in nature, including in light-harvesting, electron and energy transfers, enzymatic catalysis, and oxygen transport.1,2 Especially heme, an important porphyrin compound in the human body, can transport oxygen in red blood cells. In green plants, another porphyrin derivative, chlorophyll, can act as a converter for the photosynthesis of sunlight to chemical energy.1,2 Due to their robustness and versatility, porphyrins are widely used as organic building blocks in coordination-driven self-assembly to form metal– organic materials, including metal–organic frameworks (MOFs).5,6 Porphyrinic MOFs have been developed by the rational design of porphyrin building blocks with varying sizes, substituents, and connectivity, thus forming numerous topological nets.7,8 The structural diversity of porphyrinic MOFs allows for the exotic catalytic properties within pores, as exemplified by applications such as Lewis acid catalysis, oxidation catalysis, photocatalysis, and electrocatalysis.9 New types of frameworks with

Monographs in Supramolecular Chemistry No. 32 Porphyrin-based Supramolecular Architectures: From Hierarchy to Functions Edited by Shengqian Ma and Gaurav Verma r The Royal Society of Chemistry 2022 Published by the Royal Society of Chemistry, www.rsc.org

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porphyrin building blocks provide not only catalytic performance, but also unveil new applications. In this chapter, we describe the coordination-driven design of MOFs with porphyrin building blocks as organic linkers and secondary building units (SBUs) as metal nodes.10,11 The main purposes of this chapter are (i) the presentation of porphyrin linkers in the design of MOFs with various types of functional groups, connectivity, and coordination environment, and (ii) the description of topological nets for porphyrinic MOFs based on various SBUs. The topologies of MOFs are referred to as three-letter codes (i.e. ‘xyz’) adopted from the Reticular Chemistry Structure Resource (RCSR) database.12 The porphyrin building blocks for MOFs are shown in Figure 3.1. For convenience in linkers and SBUs, k-coordinated nodes (nodes with k connectivity) are referred to as k-c nodes.13 We conclude this chapter with a future outlook on the coordination-driven self-assembly of porphyrinic MOFs.

3.2 Symmetry-guided Coordination Networks with Porphyrin Building Units 3.2.1

Pyridyl-based Porphyrin Coordination Networks

The porphyrin molecular backbone could be regarded as a 4-connected building block in the self-assembly of coordination networks (Figure 3.1). Porphyrin linkers widely used in the early stage of coordination networks were pyridine-based porphyrins (e.g. TPyP). Robson and co-workers, the pioneers of open coordination networks, firstly introduced the open channel coordination frameworks using the 4-c porphyrin building unit and Cu1 ion.14 These two structures were {[Cu(Cu–TPyP)]BF4} and {[Cu(Cu–TCyP)]BF4} with the same (4,4)-c pts topology (Figure 3.2). Cu1 ion and porphyrin linkers acted as tetrahedral and square 4-c units, respectively. Cu–TCyP-based MOFs show a two-fold interpenetrated pts net and short Cu1–Cu1 distance, while the TPyP-counterpart does not. In the pyridyl moiety, the blocked coordination geometry in Cu1 by four bulky pyridyl units keet the close Cu1 centers apart, preventing the interpenetration of a pts network. Robson et al. synthesized a 3D MOF {[Cd2(Pd–TPyP)(NO3)4(H2O)4]} assembled with a Cd21 ion and Pd–TPyP linker.15 TPyP linkers as 4-c square building units were linked with two types of single Cd ions. Two different types of Cd connectors are those with linear (N–Cd–N, 1801) and bent connections (N–Cd–N, 1041). Overall stacking of Pd–TPyP linkers is staggered along the [001] direction. The choice of linker found in most of the early studies of porphyrin-based MOFs is pyridyl-based porphyrins. Other 1D, 2D, and 3D MOFs from TPyP and its derivatives were summarized in the review by Choe et al.16 We will focus on the development of porphyrinic MOFs throughout this chapter.

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Figure 3.1

3.2.2

The building blocks used for the construction of porphyrinic MOFs.

Self-assembly of Porphyrinic Metal–Organic Frameworks with Secondary Building Units

To build robust frameworks, carboxylate-based linkers and metal-oxo SBUs are commonly employed. Suslick et al. reported PIZA-1, [Co3O2(Co–TCPP)2 (H2O)], assembled from a TCPP linker and Co3 cluster.17 Each Co3 cluster

Design of Porphyrinic Metal–Organic Frameworks

Figure 3.2

109

Structural illustration of [Cu(Cu–TPyP)]BF4 with pts topology.

was connected with eight carboxylates in PIZA-1. Among the eight carboxylates coordinated, four of them had syn–syn bidentate bridging coordination modes and the other ones showed syn monodentate terminal coordination to the Co cluster. Overall, the PIZA-1 network is (4,8)-c flu topology connected with 4-c Co–TCPP and 8-c Co3 cluster (Figure 3.3). PIZA-1 showed selective water adsorption acting as a superior desiccant. The synthesis of PIZA-1 paved a path to access porphyrinic MOFs. Suslick and co-workers synthesized another MOF, PIZA-4 ([Zn4O(Zn– Mes2BCPP)3] (Mes: mesityl)), based on the Zn4O cluster and a linear ditopic porphyrin linker (Figure 3.4).18 Since the birth of MOF-5 in 1999, building robust MOFs using a Zn4O cluster has been well established.19 PIZA-4 has two-fold interpenetrated pcu topology assembled from 6-c of the Zn4O cluster and the 2-c edge of Mes2BCPP. Even with an interpenetrated structure, PIZA-4 revealed a largely open void space (74% free volume) with a Langmuir surface area of B800 m2 g1. Ma et al. reported MMPF-18 ([Zn4O(Zn-BCPP)3]), a 4-fold interpenetrated pcu topology similar to PIZA-4, for selective CO2 adsorption and catalytic cycloaddition to epoxide of CO2.20 The difference between PIZA-4 and MMPF-18 is substituted functional groups in porphyrin linkers (PIZA-4: Mes, MMPF-18: H). The less bulky moiety in MMPF-18 exhibited a more interpenetrated pcu net (PIZA-4: 2-fold, MMPF-18: 4-fold) based on p–p interaction among porphyrins, despite the lengthy BCPP linker. A 2D square-grid-shaped (sql topology) porphyrin-based MOF was demonstrated by Ohmura et al.21 The assembly of a TPyP and Cu2(OAc)4 paddlewheel cluster (OAc ¼ acetate) afforded the 2-D array as [(Cu2(OAc)4)2 (Cu–TPyP)], with 2D sql lattice, packed as ABAB stacking (Figure 3.5). Pyridyl groups were coordinated to two axial Cu sites in the Cu paddlewheel. This compound showed microporosity with a Brunauer–Emmett–Teller (BET) surface area of 812 m2 g1, due to a large cavity (B2 nm).

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Figure 3.3

Structural illustration of PIZA-1 ([Co3O2(Co–TCPP)2(H2O)]).

Figure 3.4

(a) Crystal structure of PIZA-4 with Zn4O cluster and porphyrin linker. (b) Schematic illustration of 2-fold interpenetrated pcu net of PIZA-4. Reproduced from ref. 18 with permission from American Chemical Society, Copyright 2003.

Choe et al. reported another porphyrin-based MOF with 2D sql topology, PPF-1 ([Zn2(Zn–TCPP)]), assembled with a Zn paddlewheel and Zn–TCPP linker (Figure 3.6).22 Both the Zn paddlewheel and TCPP are 4-c square building units. Interestingly, 2D PPF-1 was decorated with tunable open

Design of Porphyrinic Metal–Organic Frameworks

Description of the assembly and packing of 2D [(Cu2(OAc)4)2(Cu–TPyP)]. Reproduced from ref. 21 with permission from American Chemical Society, Copyright 2006.

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Figure 3.5

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Figure 3.6

Chapter 3

Description of the assembly and packing of 2D PPF-1 ([Zn2(Zn–TCPP)]). Reproduced from ref. 22 with permission from the Royal Society of Chemistry.

mental centers. The lattice was stacked as an ABAB sequence. Modifying the metal content from Zn to Co afforded the isostructural MOF, PPF-1–Co ([Co2(Co–TCPP)]). This result is a good example of demonstrating that it is possible to alter the desired metal centers while keeping the same crystal structure. PPF-1 showed the permanent porosity with a Langmuir surface area of 622 m2 g1. Cryogenic H2 adsorption exhibited 9.8 adsorbed H2 molecules per formula unit (2.0 wt%) and the binding enthalpy of 7.5 kJ mol1 due to the abundant exposed open metal centers. PPF-1 proved to be an excellent platform for 3D MOFs using various pillar molecules. Synthesis of different metal-derived porphyrin-based MOFs with 2D sql topology followed. Kitagawa and co-workers successfully synthesized a Cu2(Co–TCPP) (denoted NAFS-1) lattice film onto the surface of a Si(100) substrate.23 NAFS-1 fabrication was conducted by layer-by-layer assembly with Co–TCPP, CuCl2, and pyridine. Rh and Mn paddlewheel-based 2D MOFs, [Rh2(Zn–TCPP)] and [Mn2(Mn–TCPP)], were also reported for various applications.24,25 A new series of Fe-based MOFs, MIL-141(A) ([Fe(Ni–TCPP)]A) (A ¼ Li, Na, K, Rb, Cs) was produced using a Ni–TCPP linker.26 These 3D MOFs had 3-fold interpenetrated pts topology. Fe31 and Ni–TCPP center act as 4-c tetrahedral and square nodes, respectively. Because four carboxylates are coordinating to Fe31, alkali metal cations were incorporated in MIL-141(A) to balance the charge. MIL-141(A) showed preferential O2 adsorption due to porphyrin open Ni-centers and A sites. The construction of (4,4)-c pts topology in porphyrin-based MOFs could be observed with 4-c tetrahedral building blocks such as In(COO)4. Ma et al. synthesized two types of pts MOFs, MMPF-7 [In(In–TCPP)] and MMPF-8 [In(In– TCBPP)] using In(COO)4 SBU (Figure 3.7).27 MMPF-7 with shorter TCPP exhibited 3-fold interpenetration, while MMPF-8 with longer TCBPP had 4-fold

Design of Porphyrinic Metal–Organic Frameworks

Figure 3.7

113

The non-interpenetrated and interpenetrated structures of (a) MMPF-7 and (b) MMPF-8 with pts topology. Reproduced from ref. 27 with permission from the Royal Society of Chemistry.

interpenetration, due to the strong p–p interactions among the porphyrin macrocycles. MMPF-8 demonstrated an enhanced CO2 uptake capacity. Du and co-workers reported the In(COO)4 cluster-based pts MOF, NUPF-3 [In(In–TPABP)] using an amido-bridged porphyrin linker.28 NUPF-3 showed 4-fold interpenetrated pts topology. The incorporated In31 in the porphyrin ring of NUPF-3 was used for catalysis of cycloaddition of CO2 and epoxides. Ma et al. also reported the (4,4)-c pts MOF, MMPF-14, based on bulky tert-butyl groups containing porphyrin (DCDTP).29 Zn2(COO)5 dimer SBUs as a 4-c tetrahedral node were linked with four DCDTP (4-c square node) and one HCO2 dangling. Due to a flexible tert-butyl moiety, MMPF-14 exhibited selective CO2 adsorption over CH4 and N2. A major issue of MOFs for practical applications has been water stability. Water molecules often attack metal–carboxylate bonds by binding to metal coordination, breaking the coordination linkage, resulting in the framework collapsing. In 2008, Lillerud et al. firstly synthesized the Zr-based MOF series including an iconic MOF, UiO-66.30 Following this work, the number of waterstable Zr-based MOFs has been rapidly growing. Since enzymatic catalysis is conducted in an aqueous medium, water stability is crucial for porphyrinic MOFs. Therefore the utilization of Zr nodes in porphyrinic MOFs, combined with 6, 8, 10, and 12-connectivity is required. The geometry and symmetry of

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the Zr-clusters could be tuned by their connectivity. This tunable coordination nature afforded the diverse topology of porphyrin MOFs. Yaghi and co-workers reported two types of Zr-based porphyrinic MOFs, MOF-525 ([Zr6O4(OH)4(TCPP)3]) and MOF-545 ([Zr6O8(H2O)8(TCPP)2]) with common Zr6 SBU (Figure 3.8).31 MOF-525 was assembled by a 12-c Zr6 cluster and 4-c TCPP linker. The ideal topology of MOF-525 is (4,12)-c ftw topology of face-centered cubic symmetry. 12-c Zr6 nodes and 4-c TCPP nodes were located at the corner and face of cubic cells, respectively (Oh symmetry of 12-c Zr node and D4h symmetry of 4-c TCPP). The combination of 8-c Zr6 nodes and 4-c TCPP afforded MOF-545 with (4,8)-c csq topology of the hexagonal space group. The four carboxylates on the equatorial position were vacant in the 8-c Zr6 cluster when compared to the 12-c Zr-cluster. The framework linkage could be represented by Oh symmetry of the 8-c Zr node and D4h symmetry of 4-c TCPP. MOF-545 had a triangular microporous channel (0.8 nm) and a hexagonal mesoporous channel (3.6 nm). The kagome lattice is shown in the view along the [001] direction (Figure 3.8). These two MOFs had good chemical and water stability. From the structural robustness, the permanent porosity was confirmed by gas sorption measurement. Especially, MOF-545 showed the mesoporous adsorption behavior due to a 3.6 nm-sized hexagonal mesoporous channel. Zhou’s group32 and Ma’s group33 also synthesized the same structure and named it PCN-222 and MMPF-6, respectively. They introduced Fe-TCPP to csq-type Zr MOFs. Fe-porphyrin centers in csq-type Zr MOFs exhibited heme-like bio-mimetic catalytic activities, especially oxidation reaction by hydrogen peroxide.32,33

Figure 3.8

Structural illustration and topology of six different Zr-based porphyrinic MOFs based on the TCPP linker.

Design of Porphyrinic Metal–Organic Frameworks

115

Zhou et al. reported another Zr-based porphyrinic MOF with 12-c node, PCN223 ([Zr6O4(OH)4(TCPP)3]) with high stability under aqueous and acidic conditions (Figure 3.8).34 PCN-223 was crystallized in the hexagonal space group and single-crystal X-ray diffraction revealed the disordered Zr18 cluster. To rationalize the structure and composition, the Zr18 cluster was separated into three Zr6 clusters oriented in different directions. The observed Zr6 cluster in PCN-223 was an unusual 12-connectivity compared to Oh cuboctahedral Zr6 node in UiO-66. The unusual 12-c Zr6 cluster showed a D6h hexagonal prism shape with four bidentate chelating coordination of carboxylates. The combination of 12-c D6h Zr node and 4-c D4h TCPP afforded (4,12)-c shp topology in PCN-223. PCN-223 (shp, 12-c) was a kinetic product obtained from a short reaction time compared to the thermodynamic counterpart, PCN-222 (csq, 8-c). PCN-223 and PCN-223(Fe) with Fe-TCPP showed excellent stability under harsh acidic conditions. PCN-223(Fe) was also a good recyclable heterogeneous catalyst for the hetero-Diels–Alder reaction.34 The lowest connectivity among Zr-based porphyrinic MOFs is 6-c of PCN224 ([Zr6O4(OH)10(H2O)6(TCPP)1.5]) (Figure 3.8).35 The Zr6 cluster of PCN-224 was connected with only six edges (equatorial position) of Zr6 octahedron. The vacancies from no carboxylates were capped by terminal OH and H2O. As a result the Zr to TCPP ratio was decreased when compared to PCN-222 (8-c) synthesis, and PCN-224 (6-c) could be obtained. The new 6-c Zr6 node had D3d point symmetry and was connected to the D4h TCPP linker. The overall topology of PCN-224 was (4,6)-c she topology. PCN-224 and its metalated PCN-224(M) (M ¼ Ni, Co, Fe) were highly porous (BET surface area of 2600 m2 g1) and exceptionally stable under acidic and basic conditions. Especially, the PCN-224(Co) sample exhibited reusability for the catalytic propylene oxide coupling reaction. Zhou et al. substituted the functional groups (F, Cl, Br, and Et (ethyl)) on the b-position of Fe-porphyrin without changing the topological structure of PCN-224.36 R–PCN-224(Fe) (R ¼ F, Cl, Br, Et) analogs were synthesized for enhancing the catalytic performance of PCN-224 structures. Although all the b-positions were replaced by different functional groups, R–PCN-224(Fe) exhibited chemical stability and porosity. Especially, Br–PCN-224(Fe) showed excellent activity and selectivity for the catalysis of 3-methylpentane oxidation. Zhou and co-workers synthesized the exceptionally stable Zr-based MOF, PCN-225 ([Zr6O4(OH)8(H2O)4(TCPP)2]), exhibiting pH-dependent fluorescence (Figure 3.8). PCN-225 was assembled with an 8-c Zr6 node and 4-c TCPP linker.37 The used 8-c Zr6 cluster in PCN-225 was similar to that of PCN-222, but the Zr location was distorted and not an exact octahedron. The lower symmetry of the 8-c Zr6 node as D2d symmetry was connected with D4h TCPP linkers. PCN-224 represented the (4,8)-c sqc network. The PXRD pattern showed the exceptional stability of PCN-225 in various pH ranges. The potential for fluorescent pH sensors of PCN-225 was confirmed as the fluorescent experiment under a wide pH condition range due to protonation and deprotonation processes of the porphyrin ring. Especially, PCN-225 could be utilized for pH sensing in the 7–10 pH range.

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The mainly synthesized cluster in the Zr-based MOF was the Zr6 cluster. Even in cluster chemistry, there are no Zr8 or Hf8 clusters. Zhou et al. reported the previously unknown Zr8/Hf8 cubic cluster-based porphyrin MOFs, PCN-221 ([Zr8O6(TCPP)3]) and [Hf8O6(TCPP)3]) (Figure 3.8).38 These new Zr8/Hf8 clusters (Zr8O6(COO)12/Hf8O6(COO)12) had a cubic shape with occupation by eight zirconium on the vertices and six oxygens on the faces. The twelve carboxylates could be bridged to Zr8/Hf8 clusters and the coordination geometry was ideally suitable for cubic symmetry MOFs. PCN-221 had (4,12)-c ftw topology, as did MOF-525. Accordingly, the pore and structure were very similar to MOF-525. Zr–PCN-221(Fe) exhibited the catalytic oxidation of cyclohexane from the high-density of active porphyrinic iron(III). Farha’s group introduced a new topology type Zr-based porphyrinic MOF, NU-902 ([Zr6O4(OH)8(H2O)4(TCPP)2]) (Figure 3.8).39 NU-902 also had an 8-c Zr6 cluster in the same manner as PCN-222 and PCN-225, however, the topology of NU-902 was (4,8)-c scu from different connectivity with PCN-222 (csq) and PCN-225 (sqc) of the Zr cluster and TCPP. The different PXRD patterns of PCN-223 (shp) with a similar structure, confirmed that NU-902 had no triangular channel. The authors demonstrated the correlation between the density of the catalytic centers on porphyrin and the specific spatial orientation of those sites. NU-902 (scu) was compared with PCN-222 (csq) and MOF-525 (ftw) to understand chemically identical catalytic sites, especially Lewis acidic Zn–porphyrin. Isoreticular pore expansion from lengthening of porphyrin linkers compared with that of TCPP was realized in ftw topology Zr/Hf-based porphyrinic MOFs (Figure 3.9). Su’s group synthesized the ftw Zr and Hf MOFs, FJI-H6 ([Zr6O4(OH)4(TCBPP)3]) and FJI-H7 ([Hf6O4(OH)4(TCBPP)3]), using the TCBPP linker which has one more phenyl ring than TCPP.40 The structures of FJI-H6 and FJI-H7 showed the expanded network of MOF-525. The adjacent phenyl ring with carboxylates could be located on the same plane with the porphyrin center in the TCBPP linker. Such geometry is suitable for direct coordination of the carboxylate to the Zr6 cluster of ftw topology. In the TCPP case, the ftw topology Zr MOF should have a large torsion angle between the phenyl and carboxylate, to make parallel geometry between the porphyrin and phenyl ring. The box-type pore in FJI-H6 and FJI-H7 had a 2.5 nm size. Due to the high porosity, FJI-H6 showed a high BET surface area of 5033 m2 g1. FJI-H6(Cu) and FJI-H7(Cu) were also synthesized from Cu-TCBPP. Similar work was conducted by Feng et al.41 They studied CPM-99(M) (M ¼ no metal, Zn, Co, Fe) to make heteroatom-doped carbon derivatives. CPM-99 was the isostructure of FJI-H6. Farha and co-workers reported the ultrahigh surface area of NU-1102 ([Zr6O4(OH)4(TCBPP)3]); isostructure with FJI-H6 and CPM-99 and NU-1104 ([Zr6O4(OH)4(Por-PTP)3]).42 These MOFs had ftw topology and NU-1104 contained Por–PTP which introduced a triple bond between two phenyl rings of TCBPP. The order of spacing between porphyrin and carboxylates for Por–PTP is P–T–P (P: phenyl; T: CRC). The calculated geometric surface areas of NU-1102 and NU-1104 were 4712 and 5290 m2 g1, respectively.

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Figure 3.9

117

Structural illustration of ftw net MOFs, (a) PCN-221 and (b) FJI-H6 (CPM-99, NU-1102, and PCN-238) with expanded pores.

Zhou et al. designed the isoreticular series ftw Zr MOFs using extended porphyrinic linkers.43 The used extended porphyrin linkers were TCP-1, TCP-2, and TCP-2 which have ethyl functional groups on the b-position of porphyrin. These linkers afforded the crystalline MOFs, PCN-228 ([Zr6O4(OH)4(TCP-1)3]), PCN-229 ([Zr6O4(OH)4(TCP-2)3]), and PCN-230 ([Zr6O4(OH)4(TCP-3)3]), respectively. Their pore sizes were increased in the order of PCN-228 (2.5 nm), PCN-229 (2.8 nm), and PCN-230 (3.8 nm). PCN230 had an extremely large pore (3.8 nm) and low density (0.189 g cm3). This result presented an example of the rational design of highly porous MOFs with a topology-guided approach toward the desired topology. Du et al. synthesized the scu topology Zr-based MOF, NUPF-1 ([Zr6O4(OH)8(H2O)4(TPABP)2]), from an amido-bridged porphyrin linker.44 NUPF-1 had 2-fold interpenetrated scu topology. The interpenetration of scu topology was not observed in the same topology type Zr–porphyrin MOF, NU-902. The extended linker (TPABP) caused the interpenetration of the scu net from more opened pores in NUPF-3. NUPF-3 had excellent stability in acidic conditions. NUPF-1– RuCO, a metalized RuCO species, nicely exhibited the size-selective catalytic performance for intermolecular C(sp3)-H amination with various azides. The hexanuclear [Zr6O4(OH)4]121 cluster could be replaced by various metal(IV) cation type [M6O4(OH)4]121 clusters, including Hf, Th, U, and Ce.

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Ce(IV)-based hexanuclear clusters were successfully introduced in UiO-type MOFs.45 The conventional synthetic procedure for Ce(IV) MOF synthesis was the mixing with (NH4)2Ce(NO3)6 source and linker. The synthesis with (NH4)2Ce(NO3)6 and porphyrin linker was unsuccessful to produce Ce6 cluster-based MOFs. De Vos et al. used a precursor method for the synthesis Ce(IV)-based porphyrinic MOFs.46 The pre-synthesized Ce6 cluster, [Ce6O4(OH)4(NH3CH2COO)8(NO3)4(H2O)6Cl8], was used for the synthesis of new Ce-PCN-224(Zn) (she) and Ce–MOF-545(Sn) (csq). However, the chemical and thermal stability of the synthesized Ce-based porphyrin MOFs were generally poor compared to their Zr analogs due to weaker Ce–carboxylate bonds. Rare-earth (RE) metals were commonly utilized in various MOF assemblies. Eddaoudi et al. used the 12-c rare-earth nonanuclear cluster, [RE9O2(OH)12(COO)12], to synthesize shp topology MOF with TCPP linker.47 The solvothermal reaction with RE31 (RE ¼ Y, Tb) and TCPP afforded the crystal of RE–shp–MOF-1, [RE9O2(OH)12(TCPP)3]. The overall structure of RE–shp–MOF-1 was very similar to PCN-223 with the same shp topology (Figure 3.10). The 12-c RE9 cluster had D6h symmetry for hexagonal prismatic coordination. The series of RE9 cluster-based porphyrinic MOFs with the same structure for RE–shp–MOF-1 using Gd, Dy, Er, and Yb were also synthesized by Du et al.48 Symmetry-guided synthesis using a pre-assembled D3h Fe3O(COO)6 cluster and D4h M–TCPP (M ¼ Mn, Fe, Co, Ni, Cu) linker produced the crystals of the PCN-600(M) series, ([(Fe3O)2(M–TCPP)3]).49 The topology of PCN-600 was (4,6)-c stp topology (Figure 3.11). PCN-600 preserved the permanent porosity after the treatment of acid and base. With the catalytic sites on the hexagonal mesopore wall (3.1 nm), PCN-600(Fe) showed good biomimetic catalytic activity for the co-oxidation of phenol and 4-aminoantipyrine.

Figure 3.10

Comparison of RE–shp–MOF-1 and PCN-223 with shp topology, but different SBU.

Design of Porphyrinic Metal–Organic Frameworks

Figure 3.11

119

The stp network of PCN-600 with D3h 6-c node and D4h 4-c node. Reproduced from ref. 49 with permission from American Chemical Society, Copyright 2014.

Lin et al. used the 2-c linear porphyrinic linker, BCPP, for the construction of UiO-type Hf porphyrinic MOFs.50 The synthesized UiO-type MOF had the formula of [Hf6O4(OH)4(BCPP)6] with fcu topology. The size and morphology of [Hf6O4(OH)4(BCPP)6] were nanosized and nano-plates which were suitable for photodynamic therapy (PDT), respectively. The isolated BCPP linkers in [Hf6O4(OH)4(BCPP)6] enhanced intersystem crossing by Hf clusters and the generated singlet-oxygen (1O2) diffusion for highly efficient PDT of resistant head and neck cancer. The Hf12 SBU instead of the common Hf6 cluster was also used to constitute [Hf12O8(OH)14(Co–BCPP)].51 [Hf12O8(OH)14(Co–BCPP)] supported on a carbon nanotube exhibited the efficient electrocatalytic proton reduction. A tetrazole-containing porphyrin linker was used for the discovery of new MOFs by Zhou et al.52 PCN-526, [Cd6Cl5(Cd–TTPP)2], was synthesized from the reaction of Cd chloride and TTPP linker. [Cd4Cl]71 cluster was considered as an 8-c node with connectivity from eight tetrazolates of the TTPP linker. For framework charge valence, extra Cd cations were coordinated to the [Cd4Cl(tetrazolate)8] cluster and bridged with nitrogen in tetrazole and chloride ions. The overall topology of PCN-226 was (4,8)-c scu topology. The reversible single-crystal-to-single-crystal (SC-SC) phase transition of PCN-226 was observed by a change in temperature. Mn-based PCN-528 ([Mn4Cl(Mn–TTPP)2]) with the same scu topology of PCN-226 afforded no

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temperature-dependent SC–SC transformation. The identical structure (UTSA-57) with PCN-228 was also reported by Chen et al.53 Zhou and co-workers synthesized pyrazolate-based porphyrinic MOFs with exceptional base-resistance.54,55 The self-assembly of Oh symmetric 12-c [Ni8(OH)4(H2O)2] cluster and D4h symmetric pyrazolate-based TPP linker produced PCN-601, [Ni8(OH)4(H2O)2(Ni–TPP)3], a structure with ftw topology (Figure 3.12).54 The extended TBPP linker was also used to construct PCN-602, [Ni8(OH)4(H2O)2(Ni–TBPP)3], from isoreticular design.55 PCN-601 and PCN-602 exhibited extraordinary stability in a basic environment. Pyrazolate (Pz) coordinated [Ni8(OH)4(H2O)2(Pz)12] cluster had a strong resistance to the attack of H2O and OH even under extremely basic conditions, compared to other MOFs. Mn-metalated PCN-602(Mn) could be an effective heterogeneous catalyst for C–H halogenation under basic conditions. MOFs constructed using titanium-oxo clusters might be promising porous materials for photoactive applications.56 The realization of Ti-porphyrinic MOFs could help to access the forthcoming photoactive catalytic system from photosensitizing porphyrin. Zhou et al. firstly synthesized a singlecrystalline Ti-porphyrinic MOF, PCN-22 ([Ti7O6(TCPP)3]), with an unprecedented Ti7O6 cluster which contains Ti41 (Figure 3.13).57 Ti7O6 cluster had the 12-c and PCN-22 showed the novel (4,12)-c topology. The band-gap of PCN-22 was 1.93 eV, which is smaller than those of other Ti-based MOFs.

Figure 3.12

(a) Reticular design and construction of ftw topology. (b and c) 12-c Ni8 cluster and 4-c pyrazolate porphyrin nodes for PCN-601 and PCN-602. Reproduced from ref. 55 with permission from American Chemical Society, Copyright 2017.

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Figure 3.13

121

(a) Optical microscope image of PCN-22 single crystals. (b) Structure of the Ti7O6 cluster. (c) Schematic illustration of the assembly and structure of PCN-22. Reproduced from ref. 57 with permission from the Royal Society of Chemistry.

PCN-22 exhibited a better photocatalytic activity for benzyl alcohol oxidation reaction under visible light compared to other MOF and TiO2 systems. Actinide-based porphyrinic MOFs were also discovered with uranyl and thorium SBUs.58,59 Shi et al. synthesized a uranyl-porphyrinic MOF, U-IHEP-4 ([(UO2)4(Co–TCPP)3]), with (3,4)-c tbo topology. UO2 SBU acted as the 3-c triangular node and Co–TCPP had a 4-c square node constituting U-IHEP-4.58 Farha and co-workers stabilized an unprecedented hexanuclear SBU in a Th-based porphyrinic MOF, NU-905 ([Th6O2(HCOO)4(H2O)6(TCPP)4]) (Figure 3.14).59 The [Th6O2(HCOO)4(H2O)6] SBU was never seen in any thorium-based compounds. This hexanuclear [Th6O2(HCOO)4(H2O)6] SBU could have the 8-connectivity and link the TCPP to form the (4,8)-c scu topology. These actinide-based porphyrinic MOFs might open the way to construct unexplored novel MOF structures with excellent catalytic efficiency. The custom-designed synthesis of porphyrin linkers has broadened the diversity of porphyrinic MOF structures. Ma’s group introduced a 3D metalloporphyrin MOF which is polyhedral cage contained, MMPF-1 ([Cu2(Cu–BDCPP)] (Figure 3.15).60 Both the Cu2 paddlewheel and BDCPP acted as 4-c square building units. The topology of MMPF-1 is (4,4)-c ssb topology and in the view of dividing BDCPP as two 3-c nodes, (3,4)-c stx topology (ssb-derived net) is also regarded as the topology of MMPF-1.

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Figure 3.14

Chapter 3

(a) Hexanuclear Th6 SBU and (b) packing of the TCPP linker in NU-905. (c) Pore windows and (d) simplified scu network in NU-905. Reproduced from ref. 59 with permission from American Chemical Society, Copyright 2019.

The framework contains the nanoscopic cage enclosed by eight Cu2 paddlewheel clusters, eight isophthalates, and eight porphyrins in BDCPP. This rhombicuboctahedral cage was packed as ABAB stacking from the view of polyhedral space-filling. Selective gas adsorption behaviors (H2 and O2 over N2, and CO2 over CH4) were observed in MMPF-1. Mori et al. reported the various metalloporphyrin-based MOFs, M–BDCPPs (M ¼ Zn, Ni, Pd, Mn(NO3), Ru(CO)), with the same structure as MMPF-1.61 Expansion of pore windows in MMPF-1 topology MOFs was obtained from the use of a linker, BDCBPP, longer than BDCPP.62 Isostructural [Cu2(Zn–BDCBPP)] was synthesized without alteration of the framework topology. Ma and co-workers synthesized MMPF-3, [Co2(m2-H2O)(H2O)4(Co–DCDBP)], using Co2 SBU and a custom-designed DCDBP porphyrin linker.63 The Co2(m2H2O)(H2O)4(COO)4 cluster and DCDBP linker acted as 4-c square nodes. The overall topology of MMPF-3 was (4,4)-c nbo topology, and (3,4)-c tfb topology was also possible after deconstruction of DCDBP into two 3-c nodes. MMPF-3

Design of Porphyrinic Metal–Organic Frameworks

Figure 3.15

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(a) Schematic illustration of the linking BDCPP and Cu2 paddlewheel to form a rhobicuboctahedral cage in MMPF-1. (b) ABAB packing model and molecular structure of MMPF-1. Reproduced from ref. 60 with permission from American Chemical Society, Copyright 2011.

was composed of three types of supramolecular building blocks, cubohemioctahedron, truncated tetrahedron, and truncated octahedron. Considering the 12 connectivity of cubohemioctahedron, MMPF-3 had fcu topology. MMPF3 exhibited an excellent catalytic property for trans-stilbene epoxidation over other Co-based MOFs due to the high density of catalytic active centers. Ma and co-workers synthesized the highly porous metal-metalloporphyrin framework, MMPF-2 ([{Co3(OH)(H2O)}4(Co–OCPP)3]), using a customdesigned octatopic porphyrin linker (OCPP) and rare Co3 SBU (Figure 3.16a).64 MMPF-2 was composed of a 6-c Co3 SBU and 8-c OCPP node. The distorted cobalt trigonal prismatic SBU was connected by six carboxylates on an OCPP linker. Topologically MMPF-2 possesses (6,8,8)-c msq topology. MMPF-2 showed a high Langmuir surface area of 2037 m2 g1 and structural robustness. The CO2 adsorption of MMPF-2 was the highest value among the metalloporphyrin-based MOFs (33.4 wt% at 273 K) due to the high-density of open metal centers on SBU and porphyrin. Supramolecular building block-based self-assembly using a customdesigned porphyrin and metal SBU was demonstrated by Ma et al.65 The coordination environment of triangular Zn2(COO)3 SBUs and OCPP linkers constructed MMPF-4, [(Zn2)8(Zn–OCPP)3] (Figure 3.16b). The overall topology of the topology of MMPF-4 is (3,8)-c the topology with 3-c Zn2(COO)3 and 8-c OCPP nodes. After deconstruction of the OCPP linker to 3-c isophthalate and 4-c porphyrin ring, topologically (3,3,4)-c tfe topology is for MMPF-4. The rhombicuboctahedron cage in MMPF-was formed by 6 Zn–OCPP squares

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Figure 3.16

Chapter 3

(a) Structural illustration of MMPF-2 assembled with Co3 cluster and OCPP linker. Reproduced from ref. 64 with permission from the Royal Society of Chemistry. (b) The packing of face-sharing rhombicuboctahedron in MMPF-4 and Zn2(COO)3 SBU connection. Reproduced from ref. 65 with permission from the Royal Society of Chemistry.

and 8 Zn2(COO)3 triangles. In view of supramolecular building blocks, MMPF-4 has pcu packing of the rhombicuboctahedron cages. In the same manner, MMPF-5 ([Cd8(Cd–OCPP)3]) which contained a 3-c Cd(COO)3 node instead of Zn2(COO)3 was synthesized with the same topology of MMPF-4. Co-incorporated MMPF-5(Co), [Cd8(Co–OCPP)3], was also reported for transstilbene catalysis.66 Wu et al. demonstrated the self-assembly of multi-cluster and multitopic porphyrin-based MOFs.67 Manganese-incorporated Mn–OCPP linkers and two Mn SBUs (Mn2 paddlewheel: Mn2Cl2(COO)4 and Mn3 cluster: Mn3(DMF)4(H2O)4) were used for the construction of ZJU-18, [Mn5Cl2(MnCl– OCPP)(DMF)4(H2O)4]. Two Mn SBUs have connected four carboxylates of OCPP linkers and could act as 4-c square nodes. When the benzene ring and porphyrin center in Mn–OCPP were regarded as the 3-c triangle node and 4-c square node, respectively, topologically ZJU-18 was represented as (3,4)-c tbo topology which is the same with HKUST-1 MOF (Figure 3.17). The isostructural frameworks, ZJU-19 ([Mn5Cl2(Ni–OCPP)(H2O)8]) and ZJU-20 ([Cd5Cl2(MnCl–OCPP)(H2O)6]) were also synthesized from synthetic modification. Especially, Mn-based ZJU-18 exhibited excellent catalytic performance for selective oxidation of alkylbenzene to form phenyl ketones.

Design of Porphyrinic Metal–Organic Frameworks

Figure 3.17

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The constituting 3-c and 4-c nodes to build tbo topology in ZJU-18. Reproduced from ref. 67 with permission from American Chemical Society, Copyright 2012.

Wu’s group synthesized metalloporphyrinic frameworks, CZJ-4 ([(Zn2)2(MnCl–OCPP)]), with multiple pores for efficient epoxidation catalysis.68 Binuclear Zn2 SBU was connected with three bidentate carboxylates on an equatorial position and one monodentate carboxylate on the axial position from OCPP linkers. The topology of CZJ-4 was (4,8)-c unprecedented topology. CZJ-4 had two types of cage as pores and the size of the cages was 1.0 nm and 2.3 nm, respectively. The Mn-site of Mn-OCPP in CZJ-4 could be utilized as a heterogeneous catalyst for efficient and selective epoxidation of olefins. Two types of Cu SBU-based metalloporphyrinic MOFs, ZJU-21 and ZJU-22, were synthesized with an OCPP porphyrin linker.69 The formula and topology of ZJU-21 are [(Cu2)2(Ni–OCPP)] and (3,4) tbo topology, respectively. Porphyrin center and Cu2 paddlewheel SBU acted as 4-c square nodes. Isophthalate of the OCPP linker was regarded as a 3-c triangular node. The coordination environment of ZJU-22, [Cu16(Mn–OCPP)3(OH)11(H2O)17], is quite complex. The Mn–OCPP center was connected with two Cu2O7 subunits. The CuO4 subunit connected two carboxylates on two adjacent OCPP linkers. The remaining one carboxylate on OCPP was coordinated to the Cu2O7 subunit. The simplified topology of ZJU-22 is (4,8)-c csq topology and the complicated version of the underlying net showed ZJU-22 with (3,6)-c xly topology. Cu2O7 and benzene acted as 3-c nodes. The Mn-OCPP center was a 6-c octahedral node. These MOFs were permanently porous and exhibited an

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efficient catalytic property for cross-dehydrogenative coupling reactions. ZJU-22 was a more effective catalyst because the accessible Cu21 sites on the CuO4 subunit were more active in catalysis. Longer octatopoic porphyrin linkers than OCPP could produce more diverse structure types from coordination-driven self-assembly. The assembly with Cu2 paddlewheel and TDCBPP afforded a porous framework, MMPF-9 ([(Cu2)3(Cu–TDCBPP)(HCOO)4]) (Figure 3.18).70 Two types of Cu2 paddlewheel existed in MMPF-9. The first type was bridged by four carboxylates of TDCBPP. The second type was linearly connected by two carboxylates of TDCBPP and the other two bidentate coordination sites were capped by HCOO ions produced from DMF decomposition. The truncated triangular and hexagonal channel type pores could be viewed along the [001] direction. The topology of MMPF-9 is rare, (4,12)-c smy net. MMPF-9 showed a high catalytic performance for chemical fixation of CO2 to cycloaddition of epoxide using Cu open metal centers. Zhang et al. used a TBCPPP linker to give the degree of rotational freedom between carboxylates and phenyl rings in isophthalates and perpendicular geometry of porphyrin macrocycles. Linker flexibility had the advantage to enhance the diversity of framework types. UNLPF-1, [(Zn2)2(Zn–TBCPPP)], was synthesized by the assembly of a Zn2 paddlewheel cluster and longer TBCPPP octatopic linker.71 The framework of UNLPF-1 could be represented as (4,8)-c scu topology with 4-c Zn paddlewheel and 8-c TBCPPP nodes. After deconstruction of TBCPPP to four 3-c nodes (isophthalates) and one 4-c square node (porphyrin), UNLPF-1 had (3,4,4)-c fjh topology (Figure 3.19a). The porous nature and high density of open metal centers led to it exhibiting an excellent CO2 adsorption capacity at 273 K and 1 bar (85 cm3 g1). The same group reported the paddlewheel SBU-based MOF, UNLPF-2 ([(Co2)2(Co–TBCPPP)]).72 The reduced formula is the same with UNLPF-1, but the connectivity of TBCPPP and Co2 paddlewheel is different. UNLPF-2 showed (3,4)-c pto topology. The origin of different topologies with UNLPF-1 (fjh) and UNLPF-2 (pto) could be attributed to the different conformation of TBCPPP

Figure 3.18

Structural illustration of MMPF-9 with two types of pores.

Design of Porphyrinic Metal–Organic Frameworks

Figure 3.19

127

Structural illustrations of (a) UNLPF-1 (fjh net) and (b) UNLPF-2 (pto net).

(Figure 3.19b). Zn-incorporated TBCPPP in UNLPF-1 showed ‘‘propeller’’-like conformation and D4 point symmetry with planar porphyrin conformation. The conformation of Co-TBCPPP in UNLPF-2 was ‘‘saddle’’-like and C2v symmetry from ruffled porphyrin conformation. In D4 symmetry, TBCPPP could not align to form closely spaced paddlewheel clusters. Interestingly, the CO2 molecule was captured in between two Co2 paddlewheel clusters of UNLPF- 2 in single-crystal X-ray analysis. The distance of M–M between the pair of paddlewheel clusters was 6.1 Å in UNLPF-2. This unsaturated metal site was a predesigned CO2 binding space as the single molecular trap.73 UNLPF-2 could not only release CO2 molecules but also recapture CO2. Ma et al. synthesized a new porous metal–metalloporphyrin framework, MMPF-10 ([(Cu2)2(Cu–TBCPPP)]), which had (3,4,4)-c fmj topology (Figure 3.20a).74 MMPF-10 showed the fmj net, which differs from the fjh topology of UNLPF-1 due to different torsion angles between porphyrin and meso-substituent phenyl ring (92.921 and 117.061, respectively). MMPF-10 exhibited excellent catalytic activity for the cycloaddition reaction of aziridines with CO2.

128

Figure 3.20

Chapter 3

Structural illustration of (a) MMPF-10 (fmj net) and (b) UNLPF-10 (jaj net).

Zhang et al. synthesized an anionic porphyrin MOF, UNLPF-10 ([In(In– TBCPPP)2]), for control of charge density and photocatalytic activity.75 UNLPF-10 was composed of tetrahedrally connected In(COO)4 SBU and octatopic TBCPPP linker. Topologically, (4,8)-c flu topology was underlying to UNLPF-10 as considering TBCPPP to 8-c node. After deconstruction of TBCPPP by 3-c and 4-c nodes, UNLPF-10 had the flu-derived (3,4,4)-c jaj topological net (Figure 3.20b). Williams b-tetrakaidecahedral cages were closely packed in UNLPF-10. They could tune varying the In to TBCPP linker ratio during MOF synthesis. With less indium introduction to the porphyrin of UNLPF-10, the framework charge could be more anionic. In the cationic dye adsorption experiment, UNLPF-10 (8% metalation to porphyrin) showed the highest methylene blue adsorption capacity due to enhancing the Coulombic interaction of the anionic framework and cationic dye. The flexible octacarboxylate-based TDCOPP was used to construct a porous [(Mn2)2(Mn–TDCOPP)] MOF.76 Paddlewheel-like Mn2 SBU was connected by three carboxylates with bidentate bridging and one carboxylate with bidentate chelating to one Mn atom. The authors simplified the topology of [(Mn2)2(Mn–TDCOPP)] to the (3,6)-c rtl (rutile TiO2) topology with 3-c Mn2 SBU and 6-c TDCOPP linker without bidentate chelating two of the carboxylates.

Design of Porphyrinic Metal–Organic Frameworks

Figure 3.21

3.2.3

129

Structural illustration of Al–PMOF analogs. Reproduced from ref. 85 with permission from American Chemical Society, Copyright 2019.

Rod-packing Secondary Building Unit-based Porphyrinic Metal–Organic Frameworks

Typically, the finite number of metal–oxygen clusters as SBUs was connected with organic linkers by coordination bonds. The second major structures in MOFs are rod-packing type MOFs which contain infinite metal-containing SBU in one dimension.77 In porphyrinic MOFs, many coordination networks are using rod-packing SBUs. Rosseinsky et al. synthesized a stable rod-packing porphyrin-based MOF, Al–PMOF ([(Al(OH))2(TCPP)]) for visible-light photocatalysis.78 The topology of Al–PMOF was fry net with zig-zag ladder SBU, Al(OH)O4 chain, and 4-c ideally planar TCPP linker (Figure 3.21). The porphyrin units were arranged as staggered in position along the [010] direction between adjacent pairs of the chain. This aluminum MOF showed waterstability and permanent porosity. Al–PMOF and Zn-metalated Al–PMOF(Zn) exhibited visible-light photocatalytic activity such as hydrogen evolution. Fateeva and co-workers reported the iron-based rod-packing MOF, Fe–PMOF ([(Fe(OH))2(Fe–TCPP)(Pyrazine)]), with a similar structure to Al-PMOF.79

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Chapter 3

The difference with these structures was contained-metal in SBU (Fe31 and Al31) and the existence of pyrazine between Fe–TCPP. Pyrazine molecules were coordinated between two porphyrin layers. The same authors also reported the extended MOF, [(Fe(OH))2(Fe–TTPP)(Pyrazine)], from Fe–PMOF using a tetrazolate-based TTPP linker.80 Two MOFs had the same topological networks, but coordinating linkers differed. Tetrazolates in TTPP were similarly coordinated to the Fe-oxo chain. In 2020, Su et al. synthesized the series of M–PMOF-3, [(Fe(OH))2(M–TCPP)] (M ¼ Fe, Co, Ni, Cu), which was the pyrazine absent version of fry-type MOFs to access porphyrin catalytic centers.81 Park and co-workers synthesized the Ti-carboxylate MOF, DGIST-1 ([(TiO)2(TCPP)] based on an unprecedented Ti-oxo chain cluster.82 The reaction with pre-synthesized Ti6O6(OiPr)6(t-BA)6 (OiPr ¼ isopropoxide, t-BA ¼ tert-butylacetate) and TCPP produced the fry topology MOF, DGIST-1. Topologically, DGIST-1 had the same structure as Al–PMOF. However, the oxidation state of Ti and Al was 4þ and 3þ in DGIST-1 and Al–PMOF, respectively. In DGIST-1 Ti41 ions were connected by m2-O, but m2-OH was introduced in Al–PMOF. The band-gap of DGIST-1 was 1.85 eV and the value was smaller than that of Ti7 cluster-based PCN-22 (1.93 eV). Interestingly, DGIST-1 exhibited the photocatalytic conversion of benzyl alcohol to benzaldehyde without scavengers. Similar MOF structures with fry topology mentioned above, particularly Al–PMOF analogs (Figure 3.21), were synthesized using different metal species such as Ga, In,83–85 Sc,86 Ce,87 V,88 Tb, Sm, Eu, and Yb89 for various applications. Lin et al. synthesized a similar metal–porphyrinic network series using Ru,90 W, Bi,91 and Ti.92 Ru2 SBU was connected by five TCPP linkers and afforded a 3D Ru–TBP MOF ([Ru2(TCPP)(H2O)2]) which exhibited visiblelight-driven hydrogen evolution.90 Three of the carboxylates were equatorially coordinated to Ru2 SBU by syn–syn bidentate bridging and two of the carboxylates were axially connected to two adjacent Ru2 SBUs by syn–anti bidentate bridging. Similar structures of W–TBP ([W2(TCPP)(H2O)2]) and Bi–TBP ([Bi2O(TCPP)]) were also synthesized for enhanced cancer immunotherapy.91 Nanoscale titanium-oxo chain-based MOF, Ti–TBP ([Ti5(OH)6(OAc)2(Ti–TCPP)2], was produced for type I PDT.92 Acetates were comprised of a Ti-oxo chain with carboxylates of the TCPP linker in the Ti–TBP nanocrystal. Goldberg et al. produced lanthanoid–metalloporphyrin frameworks, LnMPF-1 ([Na2Ln2(Zn–OCPP)] (Ln ¼ Sm, Gd, Eu, Tb, Dy)) using an octatopic Zn-OCPP linker.93 The observed rod-packing SBU in LnMPF-1 was [NaLn(COO)4], an infinite bimetallic chain. The [NaLn(COO)4] was composed of Na1 and Ln31 metals alternatively and exhibited the helical form. The overall topology of LnMPF-1 was (4,8)-c hqy net. Rod-packing SBUs from polyphenolate-based porphyrinic linker were observed in Zr41 and RE31 metal sources.94–96 Devic et al. synthesized the MIL-173(M) series (M ¼ Zr, La, Ce, Y) from the THPP linker, containing the 1,2,3-trihydroxy benzene moiety (Figure 3.22).94 The formula of MIL-173(M) was [M2(THPP)]. The chain scaffold of MO8 polyhedra was connected to

Design of Porphyrinic Metal–Organic Frameworks

Figure 3.22

131

(a) The MO8 chain of MIL-173(M) and connectivity between THPP and M-cation. (b) Schematic illustration of MIL-173(M). Reproduced from ref. 94 with permission from the Royal Society of Chemistry.

polyphenolates of the THPP linker. Each metal cation was chelated by four 1,2,3-trioxobenzene groups and two pairs were laid along 901 to the chain axis. The extended linker, THBPP, was used to form the isostructural network with MIL-173(Zr). Lin and co-workers synthesized ZrPP-1 ([Zr2(THPP)]; isostructure with MIL-173(Zr)) and ZrPP-2 ([Zr2(THBPP)]).95 The overall topology of two Zr-porphyrinic MOFs was rod-like 4-c nbo topology. 1D Zr-oxo chain acted as the infinite connection of the 4-c square node. The Cometalated ZrPP-1–Co showed the efficient CO2 photoreduction property. A linear connected DTPP linker was used to produce the Zr-oxo-chain MOF instead of the THPP linker.96 The tighter packing of the porphyrin linker exhibited J-aggregation and showed the enhanced photoinduced singlet oxygen generation than that of [Zr2(THPP)]. In 2020, a new porphyrinic zirconium MOF was synthesized with the infinite zig-zag Zr-chain SBU, [ZrO(COO)2].97 PCN-226, [Zr3O3(M–TCPP)(benzoate)2] (M ¼ Fe, Co, Ni, Cu, Zn), had a new topology of ztt topology with the assembly of Zr-oxo chain and TCPP linker. Each Zr atom showed heptacoordinated geometry, ZrO7. The extra benzoates were coordinated to the Zr-oxo chain. Similar to other Zr-based MOFs, PCN-226 exhibited excellent chemical stability and compatibility in aqueous solution as electrocatalysts. The chain-based PCN-226 showed an excellent oxygen reduction reaction activity compared to cluster-based MOFs.

3.3 Mixed-linker Approach for New Porphyrin-based Metal–Organic Frameworks Mixed-linker strategy in MOF synthesis could target novel type MOF structures with the multivariate system.98 In porphyrinic MOFs, the mixed-linker approach was useful to make new porphyrin-based MOFs. Choe et al. reported the pillared paddlewheel-based porphyrin framework series with structural variation depending on the coordination geometry of the porphyrin metal center.99 The synthesized 3D PPF-3 ([Co2(Co–TCPP)(BPY)2]),

132

Chapter 3

PPF-4 ([Zn2(Zn–TCPP)(BPY)1.5]), and PPF-5 ([Co2(Pd–TCPP)(BPY)]) had AB, ABBA, and AA stacking patterns of the PPF-1 sql layer with different location and amount of BPY pillars (Figure 3.23). The origin of the structural diversity of the PPF series was the preferred coordination geometry of the metalcenter for Co–TCPP (6-coordination; octahedral), Zn–TCPP (5-coordination; square-pyramidal), and Pd–TCPP (4-coordination; square-planar). Topologically, PPF-5 had (4,6)-c fsc topology with 4-c TCPP and 6-c paddlewheel. The heterometallic PPF-3 (Mn–TCPP and Fe–TCPP) and PPF-5 (Pt–TCPP, Ni–TCPP, and V–TCPP) series were also synthesized with various metalated TCPP linkers.100 The longer pillars than BPY, DPT, and DPNI were used to form the different types of PPF series MOFs (Figure 3.23).101 Notably, AB bilayer structures of PPF18 ([Zn2(Zn–TCPP)(DPNI)]) and PPF-21 ([Zn2(Zn–TCPP)(DPT)]) were formed which is not observed in BPY pillared PPFs. 3D PPF-19 (with DPNI; 2-fold interpenetrated fsc net; AA sequence), PPF-20, and PPF-22 (with DPNI and DPT, respectively; ABBA sequence) were also synthesized from different synthetic conditions. PPF-18 and PPF-20 were used as the structural backbone to realize ‘‘post-synthetic linker exchange’’ at first in MOF chemistry.102 These PPF platforms based on an sql layer by TCPP and paddlewheel were widely utilized with various organic pillar molecules for specific applications. Nguyen et al. synthesized the paddlewheel-based ZnPO-MOF, [Zn2(TCPB)(Zn–FDPyP)], having FDPyP as the porphyrinic pillars to construct fsc topology.103 The pillared Zn–FDPyP linker in ZnPO-MOF could be catalytically active and ZnPO-MOF showed the permanent microporosity with the accessible surface area of 500 m2 g1. The same group reported all porphyrin linkers (M1–TCPP for sql layer and M2–FDPyP for the pillar to connect sql layers) containing fsc topology MOF series.104 The synthesized RPM series, ZnMn-RPM, AlZn-RPM, and FeZn-RPM were named by M1M2 species in two types of porphyrin linkers. The porphyrin linkers could be utilized to not only form structural backbone but also catalytic active sites for further applications. Choe and co-workers demonstrated the mixed-linker paddlewheel MOF synthesis using the frustration of coordination of porphyrin linkers.105,106 PPF6, [Co(cis-Zn–DCPP)(BPY)], was synthesized with cis-Zn–DCPP which has two carboxylates along 901 and BPY linkers (Figure 3.24).105 BPY connected a Co2 paddlewheel and DCPP metal center based on a 1D tape motif. Interestingly, PPF-6 showed (3,6)-c CdI2-like topology, only a few examples in the MOF system. The assembly of trans-Zn–DCPP and BPY afforded PPF-25, [Zn(trans-Zn– DCPP)(BPY)], with the rare (3,6)-c ant topology (anatase TiO2) (Figure 3.25).106 The T-shaped 3-c nodes by BPY coordination to trans-Zn–DCPP were linked with the 6-c octahedral Zn2 paddlewheel node. The simple modification of the cis–trans DCPP form produced different types of topologies. Feng et al. synthesized the porphyrinic coordination lattices with fluoropillars, F and SiF62.107 Two crystals, CPM-131 ([Zn(Fe–TPyP)(SiF6)]) and CPM-132 ([Zn2(Zn–TPyP)2F2(SiF6)]) were synthesized with homo and heteropillared structures, respectively. CPM-131 had AA stacking of [Zn(Fe–TPyP)] layers by SiF6 pillar on the 6-c Zn node with (4,6)-c fsc topology. CPM-132 had ABBA stacking of [Zn(Zn–TPyP)] layers with (5,6)-c fsx topology. AA and BB

Design of Porphyrinic Metal–Organic Frameworks

Figure 3.23

(a) M-TCPP building unit and coordination geometry found in PPF-3 (Co), PPF-4 (Zn), and PPF-5 (Pd). (b) The construction of PPF-3, PPF-4, and PPF-5. Reproduced from ref. 99 with permission from American Chemical Society, Copyright 2009. (c) 2D bilayer structures of PPF-18 and PPF-21. (d) Perspective view of PPF-18 and PPF-21. Reproduced from ref. 101 with permission from American Chemical Society, Copyright 2009. 133

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Figure 3.24

The assembly of PPF-6 with CdI2 topology. Reproduced from ref. 105 with permission from the Royal Society of Chemistry.

Figure 3.25

Crystal structure of PPF-25 and its parent topology anatase (ant). Reproduced from ref. 106 with permission from American Chemical Society, Copyright 2009.

layers were connected by SiF6, but the AB layer was connected by F as a pillar with linking 5-c Zn–TPyP center and 6-c Zn node. Zhou et al. introduced thermodynamically guided synthesis of Zr-based porphyrin MOF, PCN-134 using BTB and TCPP.108 TCPP linker was inserted between 2D kgd Zr–BTB layers. The topology of PCN-134 was unprecedented (3,4,10)-c net with a 10-c Zr6 cluster. PCN-134 showed exceptional stability under acidic and basic conditions. The authors controlled the defect ratio of TCPP in PCN-134 to exhibit Cr2O72 adsorption and degradation.

Design of Porphyrinic Metal–Organic Frameworks

135

Mixed-linker synthesis using DCDPS and TCPP with RE metal (Eu, Y, Yb, Tb, Dy) afforded PCN-900, [RE6(OH)8(TCPP)1.5(DCDPS)3], with structural similarity to PCN-224.109 DCDPS linkers were bridged to the 6-c RE6 cluster (same manner with PCN-224) to form a 12-c RE6 cluster in PCN-900. PCN-900 had (4,12)-c tam topology from linear linker addition to the vacant position in she topology. This PCN-900 (RE) showed a high BET surface area of 2523 m2 g1. Trigonal-prismatic nodes were used to construct the porphyrin-based MOF, TPMOF-7 ([Zn8(triazolate)6(TDC)3(M–TCPP)1.5] (M ¼ Zn, Fe)).110 The assembled 6-c trigonal-prismatic [Zn8(triazolate)6(TDC)3] nodes were connected with 4-c TCPP linker and TPMOF-7 showed the 2-fold interpenetrated (4,6)-c stp topology. Deconstruction of the trigonal prism as two tetrahedral units of TPMOF-7 exhibited the stp-derived (4,4) xai topology. Zhou et al. designed the mixed-linker MOFs which have face-sharing Archimedean solids using 4-c TCPP and tritopic linkers.111 The synthesis with Zn21, BTB, and TCPP afforded the PCN-137 ([(Zn4O)4(Zn–TCPP)5(BTB)8/3]), packing of square face-sharing of rhombicuboctahedra (Figure 3.26a). PCN-137 had a novel (3,4,7)-c qyc topology from Zn4O SBU that acted as a 7-c node. Another square face-sharing of cuboctahedron MOF, PCN-138 ([Zr6O4(OH)4(TCPP)(TBTB)8/3]), was synthesized with the 12-c Zr6 cluster (Figure 3.26b). PCN-138 exhibited the (3,4,12)-c urr topology. Choe and co-workers reported the hinged cube tessellation of porphyrinic MOF, UPF-1, exhibiting negative Poisson’s ratio (NPR) (Figure 3.27).112 The [Zn2(COO)3(m2-O)Zn2(COO)3] SBU were linked with a bent 3-TCPP linker and these constructed rhombicuboctahedral nanocages (represented as a cube by the connection of Zn SBU). The cubes were axially connected with a BPY linker. The rotating mechanism based on the square tessellation of UPF-1 was confirmed as the NPR property toward the rare mechanical metamaterials.

3.4 Other Noteworthy Porphyrin-based Coordination Networks 3.4.1

Coordination Networks Based on Sulfonate and Phosphonate-containing Porphyrin

Examples of sulfonate or phosphonate-coordinated MOF structure are scarce. Especially, sulfonate or phosphonate-based porphyrinic MOF synthesis is exceptionally rare. The sulfonate-containing porphyrin linker, TPPS, afforded the rare (3,5)-c zbs topological framework assembled with [M(SOO2)4] rodSBU.113–115 Fukuzumi et al. synthesized the [HSm(VO–TPPS)] framework with vanadyl porphyrin linker, VO-TPPS.113 In the crystal structure, each Sm ion was coordinated by eight oxygen from different sulfonate groups in the TPPS linker. The [M(SOO2)4] rod SBU was linked to VO–TPPS linkers. Structurally same zbs topology MOFs, [Tb(Zn–TPPS)H3O] and [Eu(Zn–TPPS)H3O], were reported by Chen and co-workers.114,115 They also used a sulfonate-based Zn–TPPS linker for the construction of similar networks.

136

Figure 3.26

Chapter 3

Structural analysis of mixed-linker MOFs, (a) PCN-137 and (b) PCN-138. Reproduced from ref. 111 with permission from American Chemical Society, Copyright 2019.

Stock et al. firstly synthesized the phosphonate-based porphyrinic MOFs, M-CAU-29 ([M(Ni–TPPP)(H2O)]) (M ¼ Mn, Co, Ni, Cd).116 The M2O10 SBU was constructed by eight oxygens from the PO3 group and two oxygens from water. Two MO6 polyhedra were sharing the edge from two oxygens from phosphonate. One oxygen of each PO3 group in the TPPP linker was donated to M2O10 SBU. Various metals such as Mn, Co, Ni, and Cd exhibited an isostructural tendency. This M–CAU-29 series showed permanent porosity. Zr/Hf-based phosphonate porphyrin MOFs were synthesized using a Ni–TPPP linker.117 The produced Zr–CAU-30, [Zr2(Ni–TPPP)(OH/F)2], had the chain SBU of ZrO6/ZrO4F2 octahedra that were bridged by OH/F (Figure 3.28). Two oxygens of PO3 were coordinated to the Zr-oxo chain and Zr/Hf–CAU-30 exhibited the 1.3 nm-sized channel-type pores with the permanent porosity. The organized square lattice with SBU and Ni–TPPP linker was shown in view along the [001] direction. Zou et al. synthesized and determined the structure of Co–CAU-36 ([Co2(Ni– TPPP)]) based on the phosphonate group linker.118 CoO4 tetrahedra were linked by two oxygens of the PO3 moiety in Ni–TPPP and formed 1D infinite

Design of Porphyrinic Metal–Organic Frameworks

137

Figure 3.27

(a) A nanocage of UPF-1 is represented by rhombicuboctahedron and cube. (b) Schematic representation of the rotating mechanism based on square tessellation in UPF-1.

Figure 3.28

(a) The asymmetric unit, and (b) SBU chain with ZrO6/ZrO4F2 octahedra connected by PO3C tetrahedra in Zr–CAU-30. (c) Schematic illustration of the network of Zr–CAU-30. (d) Crystal structure and (e) pore of Zr–CAU-30. Reproduced from ref. 117 with permission from the Royal Society of Chemistry.

138

Chapter 3

chain SBU. The accurate structure of Co–CAU-36 was determined by the continuous-rotation electron diffraction (cRED) technique. The enhanced refinement solution by electron diffraction uncovered the precise location of metal, linker, and even guest molecule, DABCO (1,4-diazabicyclo[2.2.2]octane) in Co–CAU-36. Co–CAU-36 exhibited the BET surface area of 700 m2 g1. Yucesan and co-workers reported the alkali-phosphonate MOF, GTUB-1 ([Na2(Cu–TPPP)].119 This rare type of alkali-phosphonate porphyrin MOF had 1D chain SBU of Na and phosphonate coordination. The assembly of TPPP and 1D Na SBU afforded the channel-type porous nature with a specific surface area of 698 m2 g1. The Cu–TPPP linker showed the non-planar (saddle) deformations of the porphyrin macrocycle in GTUB-1.

3.4.2

Self-assembly of Polyoxometalate-based Porphyrinic Coordination Networks

Polyoxometalate is the representative multi-atom cluster to design unprecedented coordination networks. Lan et al. reported polyoxometalatemetalloporphyrin frameworks, M–PMOFs with Zn–e-Keggin (e-PMo8 V Mo4VIO40Zn4).120 M–PMOFs (M ¼ Co, Fe, Ni, Zn) were constructed by a 4-c M–TCPP linker and zig-zag Zn–e-Keggin chain (Figure 3.29). The connectivity of TCPP and Zn–e-Keggin could be represented by 2-fold interpenetrated mog topology. Especially, Co–PMOF exhibited superior catalytic performance for CO2 reduction.

Figure 3.29

Schematic illustration of the structures of M–PMOFs and M–TCPP connectivity with zig-zag Zn–e-Keggin chain. Reproduced from ref. 120, https://doi.org/10.1038/s41467-018-06938-z, under the terms of the CC BY 4.0 license, http://creativecommons.org/ licenses/by/4.0/.

Design of Porphyrinic Metal–Organic Frameworks

139

3.5 Conclusions and Future Directions Rational design of coordination-directed porphyrin-based MOFs has been developed by the efforts of synthetic and materials chemists over the last two decades. The many topologies for porphyrinic MOFs by the design of porphyrin building blocks were found in the crystalline form. The topologies from different porphyrin and SBU connectivity are summarized in Table 3.1. Accordingly, the tunable porphyrin building blocks have been utilized to explore the advanced porous platform exhibiting enhanced catalytic ability toward bio-mimic molecular design. Mixing of two or more organic linkers including porphyrins could be a beneficial approach to gain new types of porphyrinic MOFs for various applications. Novel topologies have been observed in multivariate porphyrinic MOFs from the mixed-linker approach. Moreover, light-harvesting energytransferring porous platforms containing porphyrins have been developed from the pillared-paddlewheel MOFs based on a multi-linker strategy.121–123 More porphyrinic MOFs exhibiting intriguing properties might be synthesized from the rationalized mixed-linker approach. Recently, some outstanding works showed the breaking bias against the typical self-assembly using porphyrins with common SBUs. Coordination-directed porphyrin box superstructures paved a new direction in the construction of design for functional hierarchical superstructures (Figure 3.30).124 Moreover, the Table 3.1

The topologies assembled from SBU and porphyrin connectivity.

Porphyrin SBU

Topologya

2-c 2-c 4-c 4-c 4-c 4-c 4-c 4-c 4-c 4-c 4-c 4-c 4-c 4-c 4-c 8-c 8-c 8-c 8-c 4-c 8-c 4-c 4-c

pcu18,20 fcu50 flu17 sql21–25 pts27 ftw31,38,40–43,54,55 shp34,47,48 csq31–33 sqc37 she35,36 scu39,44,52,53,59 stp49 tbo58 ssb(stx)60 nbo(tfb)63 msq64 the(tfe)65 scu(fjh),71 (pto),72 (fmj)74 flu(jaj)75 fry78–89 hqy93 ztt97 zbs113–115

a

6-c; [Zn4O] 12-c; [Hf6O4(OH)4] 8-c; [Co3O2] 4-c; [M2] 4-c; [In] 12-c; [Zr6O4(OH)4], [Zr8O6], [Ni8(OH)4(H2O)2] 12-c; [Zr6O4(OH)4], [RE9O2(OH)12] 8-c; [Zr6O4(OH)4] 8-c; [Zr6O4(OH)4] 6-c; [Zr6O4(OH)4] 8-c; [Zr6O4(OH)4], [M4Cl], Th6O2(HCOO)4(H2O)6 6-c; [M3O] 3-c; [UO2] 4-c; [M2] 4-c; [Co2O] 6-c; [Co3(OH)(H2O)] 3-c; [Zn2], [Cd] 4-c; [M2] 4-c; [In] [M(OH)]N [NaLn]N [ZrO]N [M]N

Topologies represented in the bracket (xyz) are derived nets by deconstruction of 8-c node to four 3-c nodes and one 4-c node.

140 (a) Scheme of the construction of the porphyrin box-based superstructure. (b) Crystal structure of porphyrin box-based superstructure. Reproduced from ref. 124 with permission from American Chemical Society, Copyright 2018.

Chapter 3

Figure 3.30

Design of Porphyrinic Metal–Organic Frameworks 125

141 126

exploration of corrole and phthalocyanine-based macrocyclic organic compounds similar to porphyrin, MOFs as porphyrinic MOF counterparts has been demonstrated for the new reticular design of functional coordination frameworks. The systematic tunability from porphyrin building blocks resulting in advanced porous materials allows us to explore the relationships between topological structures and desired functions. The tailorable platform to realize the intriguing property, including mechanical metamaterials with negative Poisson’s ratio, has been archived in porphyrinic framework materials (Figure 3.27).112 When the optimal correlation could be found in SBUs and metalloporphyrins in the MOF structure and its pore spaces, the tailorable MOFs will display the superior functions in the future for forthcoming advanced applications.

Abbreviations BCPP BDCPP BDCBPP BPY BTB DCDBP DCDTP DCDPS cis-DCPP trans-DCPP DPT DPNI DTPP FDPyP Mes2BCPP OCPP Por–PTP TCP-1 TCP-2 TCP-3 TDCBPP TPyP TCyP TCPP 3-TCPP

5,15-bis(4-carboxyphenyl)porphyrin 5,15-bis(3,5-dicarboxyphenyl)porphyrin 5,15-bis(dicarboxybiphenyl)porphyrin 4,4 0 -bipyridine 1,3,5-tris(4-carboxyphenyl)benzene 5,15-bis(3,5-dicarboxyphenyl)-10,20-bis(2,6-dibromophenyl)porphyrin 5,15-bis(3,5-dicarboxyphenyl)-10,20-bis(3,5-di-tertbutylphenyl)porphyrin 4,4 0 -dicarobxydiphenyl-sulfone 5,10-di(4-carboxyphenyl)-15,20-diphenylporphyrin 5,15-di(4-carboxyphenyl)-10,20-diphenylporphyrin 3,6-di-4-pyridyl-1,2,4,5-tetrazine N,N 0 -di-(4-pyridyl)-1,4,5,8-naphthalenetetracarboxydiimide 5,15-di(3,4,5-trihydroxyphenyl)porphyrin 5,15-dipyridyl-10,20-bis(pentafluorophenyl)porphyrin 5,15-di(4-carboxyphenyl)-10,20-di(2 0 ,4 0 ,6 0 trimethylphenyl)porphyrin 5,10,15,20-tetrakis(3,5-biscarboxylphenyl)porphyrin 5,10,15,20-tetrakis(4-((phenyl)ethynyl)benzoic acid)porphyrin b-octaethyl-meso-tetrakis(4-carboxybiphenyl)porphyrin b-octaethyl-meso-tetrakis(4-((phenyl)ethynyl)benzoic acid)porphyrin b-octaethyl-meso-tetrakis(4-(phenyl(2,5-dimethyl-1,4-diethynyl))benzoic acid)porphyrin 5,10,15,20-(3,5-dicarboxybiphenyl)porphyrin 5,10,15,20-tetra(4-pyridyl)porphyrin 5,10,15,20-tetrakis(4-cyanophenyl)porphyrin 5,10,15,20-tetrakis(4-carboxyphenyl)porphyrin 5,10,15,20-tetra(3-carboxyphenyl)porphyrin

142

TPP TBPP TTPP THPP THBPP TPPS TPPP TDCOPP TCBPP TBCPPP TPABP TCPB TDC TBTB

Chapter 3

5,10,15,20-tetra(1H-pyrazole-4-yl)porphyrin 5,10,15,20-tetrakis(4-(1H-pyrazol-4-yl)-phenyl)porphyrin 5,10,15,20-tetrakis(4-(2H-tetrazol-5-yl)phenyl)porphyrin 5,10,15,20-tetrakis(3,4,5-trihydroxyphenyl)porphyrin 5,10,15,20-tetrakis(3,4,5-trihydroxybiphenyl)porphyrin 5,10,15,20-tetra(4-sulfonatophenyl)porphyrin 5,10,15,20-tetra(4-phosphorylphenyl)porphyrin 5,10,15,20-tetra(4-(3,5-dicarboxylphenoxy)phenyl)porphyrin 5,10,15,20-tetrakis(4-carboxybiphenyl)porphyrin 5,10,15,20-tetrakis(3,5-bis((4-carboxy)phenyl)phenyl)porphyrin 5,10,15,20-tetrakis(4-(phenyl)amido-benzoic acid)porphyrin 1,2,4,5-tetrakis(4-carboxyphenyl)benzene 2,5-thiophenedicarboxylate 1,3,5-tris(4-carboxyphenyl)-2,4,6-trimethylbenzene

Acknowledgements This work was supported by the Korea Environment Industry & Technology Institute (KEITI) through Public Technology Program based on Environmental Policy Program, funded by the Korea Ministry of Environment (MOE) (2018000210002) and National Research Foundation of Korea (NRF2016R1A5A1009405 and 2020R1A2C3008226).

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CHAPTER 4

Heterogeneous Catalysis of Porphyrin-based MOFs ZACHARY MAGNUSONa AND SHENGQIAN MA*b a

Department of Chemistry, University of South Florida, 4202 E Fowler Ave, Tampa, FL 33620, USA; b Department of Chemistry, University of North Texas, 1508 W Mulberry St, Denton, Texas 76201, USA *Email: [email protected]

4.1 Introduction Porphyrins are ubiquitous in nature, often occurring as the cofactors (catalytic centers) of many enzymes in biological systems that are found in plants, animals, fungi, and insects among other creatures. They strongly interact with visible light, yielding various and intense coloration as well as interesting photophysical properties.1 Porphyrins as catalysts, however, offer a wide variety of chemical transformations and can be easily amended by metalation of or adding organic moieties to the macrocycle, allowing for fine adjustment that elicits a particular type of reaction to occur, selectivity of substrate, and/or specificity of product.2,3 MOFs provide an excellent foundation for the advancement of (metallo)porphyrin catalysis as a critical detriment is immediately addressed by inclusion of porphyrins into MOFs: prevention of deactivation via suicidal self-oxidation and dimer formation. These deactivating processes are common to homogeneous porphyrin catalysts and result in significant reduction in catalytic activity and sustainability, particularly when compared to natural counterparts which inspire the use of MOFs as scaffolds to isolate porphyrins. Indeed, Por-MOFs not only improve catalyst lifetimes of porphyrinic catalysts, Monographs in Supramolecular Chemistry No. 32 Porphyrin-based Supramolecular Architectures: From Hierarchy to Functions Edited by Shengqian Ma and Gaurav Verma r The Royal Society of Chemistry 2022 Published by the Royal Society of Chemistry, www.rsc.org

149

150

Chapter 4

they enhance sustainability through facile separations of the heterogeneous catalytic material which can then be reused, in some cases multiple times, without degradation of performance. The large surface area and open pores (particularly those of Por-MOFs) of MOFs prove beneficial to catalysis in that these accessible surfaces allow for a high density of catalytic sites without excluding the movement of substrate throughout the crystalline material, facilitating substrate–catalyst interaction. However, these large, open pores have also proven to be unstable at times, where degradation is a result of solvent evacuation or even solvent exchange from some Por-MOFs. Typically, Por-MOFs are prepared via modification of a tetraphenylporphyrin (TPP) model, functioning as ligands in MOF construction (though synthetic approaches do not necessarily include TPP or its modified forms in steps prior to the synthesis of the porphyrin macrocycle itself). Fitted with pyridines, phenyl rings, ethers, and/or carboxylates among other functionalizing groups these ligands provide the primary structural motif upon which the MOF is constructed, binding through Lewis acidic or basic sites to metal node secondary building blocks (SBBs).4 Metal salts commonly used in MOF synthesis to form SBBs can metalate free base porphyrin ligands, so pre- or post-synthetic metalation is common as opposed to in situ if a metalloporphyrin hosting a metal different from that of the SBB metal is required. The catalytic properties of the Por-MOFs can be affected by both the metalloporphyrin metal or the SBB metal.5 Common reactions catalyzed by Por-MOFs such as the oxidation of alkanes, alkenes, alcohols, and CO2 among others6 have been reported, as well as more complex ones like carbon–carbon bond formation, hydrogen evolution, and epoxidation. A selection of these, primarily more recent reports, will be covered in this chapter of the book in an effort to illustrate the diverse functionality and advantages of this unique class of materials. Catalysis results and details as to the role material properties play in said results will be the focus of discussion.

4.2 Catalysis of Cycloaddition Reactions Sequestration of carbon dioxide from the atmosphere has been investigated extensively as a means to affect climate change.7 Taking this in stride, the addition of CO2 to epoxides forming cyclic carbonates has been catalyzed by a flexible Mn por-MOF investigated by Wei Jiang and Jian-Fang Ma et al.8 In the presence of a quaternary ammonium cocatalyst, 1Mn ([Mn5L(H2O) 6(DMA)2]5DMA4C2H5OH where DMA ¼ N,N 0 -dimethylacetamide and L ¼ 5,10,15,20-tetra(4-(3,5-dicarboxylphenoxy)-phenyl)porphyrin) proves to be an effective heterogeneous catalyst, selectively adsorbing CO2 over N2 and transforming it in the presence of the model substrate epichlorohydrin under moderate conditions (1 atm CO2 at 80 1C for four hours) with a 499% yield. This is compared to a meager 31% yield in the presence of just the cocatalyst TBAB (tetrabutyl ammonium bromide). Yields were calculated by gas chromatography and 1H NMR. Epoxide substrates with ever larger

Heterogeneous Catalysis of Porphyrin-based MOFs Table 4.1

151

Cycloaddition of CO2 to epoxide substrates. Reaction conditions: epoxide (1 mmol), 1Mn (0.005 mmol) and nBu4NBr (1 mmol), CO2 (1 atm). Reproduced from ref. 8 with permission from John Wiley & Sons, Copyright r 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. Time [h]

Yieldsa [%]

1

4

499

2

4

499

3

4

30

4

4

41

5

4

43

Entry

a

Epoxides

Products

Yield of isolated product was calculated by GC and 1H NMR spectroscopy.

pendants were tested as well (see Table 4.1), illustrating a decrease in catalytic performance attributed to increasing steric hindrance, which would prevent substrate–catalyst interaction inside the crystalline structure of the por-MOF. The impact of the unique flexible structure of 1Mn on hosting substrate of increasing size is not characterized in this publication. Catalytic performance was shown to increase significantly with an increase in reaction time from four to twelve hours, more than doubling the conversion rate. The research also expounds catalyst recycling studies that show good reusability at 1 atm and 20 atm over four trials, as well as the TONs and TOFs of the catalyst on all five of the epoxide substrates tested. The transformation of aziridines to oxazolidinones is another interesting cycloaddition catalyzed by porphyrins that utilizes CO2. Oxazolidinones are useful in antimicrobial and antibiotic applications as protein synthesis inhibitors, preventing mRNA translation initiation.9,10 Xun Wang, Shengqian Ma et al.11 investigate the copper-metalated por-MOF [Cu4(CuTBCPPP)(H2O)4] (where H10TBCPPP ¼ tetrakis-3,5-bis[(4-carboxy)phenyl]phenyl porphine), deigned Metal Metalloporphyrin Framework 10 (MMPF-10), for catalysis of this reaction in the presence of cocatalyst TBAB, showing modest yields under mild conditions and a significant increase in yields with greatly increased pressure (see Table 4.2). MMPF-10 is compared to another coppercontaining MOF HKUST-1 in the transformation of model compound 1-methyl-2-phenylaziridine to 3-methyl-5-phenyl-2-oxazolidinone, where it outperforms under similar conditions. In both MOFs it is suggested that the copper paddlewheel SBB has catalytic functionality, so the enhanced catalysis observed in MMPF-10 is attributed to the addition of highly accessible

152 Table 4.2

Chapter 4 Cycloaddition reactions of different substituted aziridines with CO2 catalyzed by MOFs. Reproduced from ref. 11 with permission from the Royal Society of Chemistry.

+ Entry

Substrate

Catalyst

CO2 pressure (Mpa)

Yield (%)

a

1

0.1

63

2b

0.1

47

3c

2

499

4c

2

99

5c

2

96

6c

2

71

a

Reaction conditions: aziridine (1 mmol), MMPF-10 (0.625 mol% based on copper paddlewheel units), TBAB (0.05 mmol), 25 1C, 3 days. b Reaction conditions: Same reaction conditions catalyzed by HKUST-1 (0.625 mol%). c Reaction conditions: Same amounts of aziridines and catalysts, 100 1C, 10 h.

copper atoms in the metalloporphyrin ligand, as well as a reduction in substrate dimerization brought on by the increased availability of CO2 which is proposed to improve the rate of the reaction. However, the increased steric hindrance of larger substrate was shown to impact the percent yield of oxazolidinone, where 1-isopropyl-2-phenylaziridine (see entry 6 of Table 4.2) has a decreased yield under the more aggressive conditions which is attributed to the bulky isopropyl group on the aziridine nitrogen.

4.3 Catalysis of Reactions Involving Alkanes, Alkenes, and Alkynes C–H bond activation is highly sought after to access abundant, low cost, simple molecules such as methane, propane, cyclohexane, etc. for synthetic organic chemistry but has proved challenging due to the relative stability of saturated hydrocarbons.12 Ming-Hua Xie, Chuan-De Wu et al.13 have prepared and investigated CZJ-1 (Chemistry Dept. of Zhejiang University-1) with

Heterogeneous Catalysis of Porphyrin-based MOFs

153

unit formula [Zn2(MnOH–TCPP)–(DPNI)]0.5DMFEtOH5.5H2O – a mixed metal, mixed ligand MOF where TCPP (tetrakis(4-carboxyphenyl)porphyrin) is metalated with Mn and serves as the catalytic center (speculated to be Mn(V)QO) and DPNI (N,N 0 -di-(4-pyridyl)-1,4,5,8-naphthalenetetracarboxydiimide) functions as a pillar ligand with the SBBs being composed of zinc paddlewheels. CZJ-1 exhibits a high 94% yield for conversion of cyclohexane to a mixture of cyclohexanone and cyclohexanol at room temperature in the presence of iodosylbenzene (see Figure 4.1). A form of this mixture is commonly used in the production of Nylon-6.14 The research also reveals CZJ-1 is capable of epoxidation of olefins at room temperature with exceptional yields, also in the presence of iodosylbenzene as the oxidant (see Table 4.3). The TON of the epoxidation of model compound styrene was calculated to be 14 068 turnovers. Recycling of the heterogeneous catalyst was investigated, with simple filtration for separation from the reaction mixture revealing that the material maintains a moderate conversion rate (79%) after six trials. These reactions were performed against trials

Figure 4.1

Selective oxidation of cyclohexane to a mixture of cyclohexanol and cyclohexanone by CZJ-1. Reproduced from ref. 13 with permission from John Wiley & Sons, Copyright r 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Table 4.3

Epoxidation of olefins. Reaction conditions: 0.1 mmol olefin substrate, 0.15 mmol or 0.20 mmol iodosyl benzene from styrene and cyclohexane respectively, and 5 mol% catalyst in 1.5 mL acetonitrile at RT for 6 hours. Reproduced from ref. 13 with permission from John Wiley & Sons, Copyright r 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Entry

Substrate

Catalyst

1 2 3 4 5 6 7 8 9

Styrene Styrene Styrene Cyclohexene Styrene Styrene Styrene Styrene Styrene

CZJ-1 CZJ-1 CZJ-1 CZJ-1 MnCl2 DPNI MnOH–H4TCPP MnOH–H4TCPP and DPNI MnOH–H4TCPP, DPNI, and Zn(NO3)2

a

Conversion and selectivity were determined by GCMS. The second cycle of entry 1. c The sixth cycle of entry 1. b

Conversiona (%)

Selectivitya (%)

499 98 94 99 27 5 64 80 78

98 98b 95c 499 80 97 94 94 67

154

Chapter 4

containing components of the MOF (see Table 4.3) to illustrate how the MOF structure enhances the overall performance of the catalyst, the sum being greater than its parts. The paper details several additional experiments that show how the substrate enters the MOF pores, is transformed by the metalloporphyrin ligand, and later exits as the product. Furthermore, the researchers speculate that there may be a cooperative, catalysis-enhancing effect between neighboring Mn–porphyrin centers (with a distance of 13.9 Å). Enhancement of the system is also described with a comparison to homogeneous porphyrinic catalysts which can undergo u-oxo dimerization (bridging between manganese(v) oxide center through oxygen), deactivating the catalyst, which is prevented by fixation of the porphyrin ligand within the MOF. The hydration of terminal alkynes is a valuable tool in the production of intermediates for fine chemicals synthesis.15 This reaction is traditionally performed using hazardous mercury-based catalysts and as such research in finding alternatives is prevalent. Of interest are transition metals, some of which are quite abundant and safer to use. To this end, Zekai Lin, Wenbin Lin et al.16 have designed and tested an interpenetrated, mixed metal Co metalloporphyrin por-MOF, utilizing an indium SBB to produce In–TBP MOF, with the framework formula In(TBP)x[In(TBP)(H2O)(1x)[DMA]x (where H4TBP ¼ tetrakis(4-benzoic acid)porphyrine and DMA ¼ dimethylacetamide) for the cooperative catalysis of aromatic, terminal alkyne hydration reactions. It is important to note that during MOF synthesis of In–TBP MOF, Co–TBP is partially transmetalated with indium but this is shown to play no role in the catalysis. Indeed, the proposed mechanism relies on tandem action from neighboring Co–TBP that are rigidly positioned throughout the MOF at 8.8 Å apart (distance measured from the Co macrocycle core, see Figure 4.2).

Figure 4.2

Two CoIII(TBP) units brought near each other due to interpenetration. Yellow indicates the interpenetrated network. Reproduced from ref. 16 with permission from John Wiley & Sons, Copyright r 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Heterogeneous Catalysis of Porphyrin-based MOFs Table 4.4

155

Hydration of phenylacetylene in methanol at 80 1C (1 equiv. phenylacetylene, 4.4 equiv. H2O, 0.76 M MeOH). Reproduced from ref. 16 with permission from John Wiley & Sons, Copyright r 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

+

80 °C, MeOH

Entry

Catalysts (mol%)

Time (h)

Conversiona (%)

1 2 3 4 5 6 7 8

In–Co(TBP)–MOF (0.1) In–Co(TBP)–MOF (0.1) In–Co(TBP)–MOF (0.01) Na3[Co(TPPS)] (0.1) Co[III]–TBP (0.1) Co[III]–TBP (0.01) In–In(TBP)–MOF (0.2) Zr–Co(TBP)–MOF (0.1)

12 20 168 12 20 168 20 20

95 100 92 24 18 0 0 2

a

Conversion determined by GC with undecane as the internal standard.

Impressively, in the presence of methanol, the material transforms model compound phenylacetylene into acetophenone with 499% conversion at 0.1 mol% loading after twelve hours and maintains a good conversion (92%) at 0.01 mol% loading, a very small amount indeed, over the course of seven days. Conversion was determined by GC analysis. In comparison to the inspirational work of Naka et al.17 that describes a cobalt metalloporphyrin salt catalyst (see entry 4, Table 4.4), the yield is greatly enhanced – several other trials performed (see Table 4.4) attempt to illustrate the role the MOF has in increasing the yield as well as the cooperative effect proposed. With entry 7, an isostructural MOF wherein all of the porphyrin ligand is metalated with indium instead of the mixed metalation seen in In–TBP MOF, shows no activity while entry 8 demonstrates the cooperative mechanism clearly: Zr–Co(TBP) MOF, which has a greater metalloporphyrin Co–Co distance of 11 Å, has a significantly lower conversion rate. Indeed, the larger pores of this MOF should facilitate faster substrate diffusion and as such an enhanced conversion rate, but this is not observed, supposedly due to the increased Co–Co distance. The mechanism of this reaction is proposed as such: Co-coordinated MeOH attacks the likewise Co-coordinated (referred to as activated) alkyne across the 8.8 Å distance affording a methyl vinyl ether, which is then hydrolyzed by water finally yielding a ketone and the regeneration of methanol. Additional substrates were tested to characterize In–TBP MOF’s range of activity across compounds of different sizes. In most cases reported the conversion rate remains quite high, but it was observed to decrease, apparently due to steric crowding specifically at the meta position. Clearly the electron donating or withdrawing nature of the aromatic ring additions does not play a critical role in the yield, so the group-proposed alkyne activation occurs exclusively with a pi-stacking of substrate and porphyrin macrocycle that is prevented when the meta position on the substrate aromatic ring is

156

Chapter 4

occupied. Finally, recycling tests were conducted, showing no discernable change in performance of In–TBP MOF over six consecutive trials where the MOF was collected and reapplied after centrifugation. Carbon–carbon bond formation is critically important and widely used in organic synthesis. The transformation of small organic molecules into larger more complex structures is exacerbated by the methodology of prefunctionalizing these small molecules to access common carbon coupling reactions that yield products where subsequent removal of these excess functional groups is necessary.18 Extra steps such as these sap yields resulting in less efficient synthetic schemes and produce unnecessary waste. The ability to access carbon coupling without functionalization, i.e. through direct activation of simple molecules such as saturated hydrocarbons is profound. To this end, Xiu-Li Yang, Chuan-De Wu et al.5 synthesize two mixed metal por-MOFS, ZJU-21 and ZJU-22 (ZJU ¼ Zhejiang University) where ZJU-21 has the unit formula [Cu4(Ni–OCPP)(H2O)4] 10DMF11H2O and ZJU22 [Cu16(MnOCPP)3(OH)11(H2O)17]21DMF65H2O (H8OCPP ¼ 5,10,15,20-tetrakis(3,5-dicarboxyphenyl) porphyrin). Here, the porphyrin ligand in ZJU-21 is metalated with nickel while the porphyrin ligand of ZJU-22 is metalated with manganese. The structure of these MOFs is particularly interesting due to their construction of three independent nets comprised of three unique SBUs connected through M–OH–Cu (M is the metal within the porphyrin macrocycle) bridges which are described in greater detail in the publication and is proposed to explain the MOFs relatively stable structure despite large pores. It is important to note that ZJU-21 possesses a copper paddlewheel SBU while ZJU-22 instead exhibits a unique copper SBU that appears to contribute to the efficiency in catalysis observed – the copper sites are more free of ligation, potentially being capable of binding to two or three substrate molecules at a time. Indeed, carbon dehydrogenative coupling (CDC) catalyzed by ZJU-21 and 22 of 2-phenyl-1,2,3,4-tetrahydroisoquinoline (model substrate) and nitromethane in the presence of t-butyl hydroperoxide (TBHP) as oxidant shows good results, even with slight variations in substrate as illustrated (see Table 4.5). Clearly, ZJU-22 produces better conversion rates than that of ZJU-21 and even the well-known HKUST-1. The cause of this enhancement is elucidated by the comparison to HKUST-1, which contains hindered copper paddlewheel SBUs as catalytic sites that cannot perform as efficiently as those described in ZJU-21 or ZJU-22. It follows that the role of the porphyrin in this system is critical but indirect, only providing the topological structure and enhanced stability necessary for the open copper sites. Further research is done to show that catalysis takes place within the pores of these MOFs and not just on the surface. This is done through sampling and digestion of MOF crystals for 1H NMR, revealing amounts of substrate and product from within. It is also shown that leaching of the heterogeneous catalyst due to degradation does not occur over the course of the reactions performed. The introduction of halogens to simple molecules such as saturated hydrocarbons, which are otherwise relatively inert, activates them toward a

Heterogeneous Catalysis of Porphyrin-based MOFs Table 4.5

CDC reactions of 1,2,3,4-tetrahydroisoquinoline derivatives with nitroalkanes.a Reproduced from ref. 5 with permission from John Wiley & Sons, Copyright r 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

+ Entry

157

1

R

Catalyst, RT TBHP R

2

Yieldb (%) ZJU-21

ZJU-22

1

H

90

97

2

H

76c

—

3

H

66

93d

4

H

80

85e

5

H

88

94

6

H

81

84

7

H

70

77

8

CH3

79

78

9

CH3

83

90

10

H

11f

—

11

H

29

35g

12

H

84

94h

a

Tertiary amine (0.1 mmol), TBHP (0.15 mmol), catalyst (0.01 mmol), and nitroalkane (1.0 mL) were stirred at RT for 6 hours. b Yields of isolated products. c Catalyzed by HKUST-1. d In 1 mL of water containing 0.02 mL of nitroalkane for 12 h under otherwise identical conditions. e Tertiary amine (420 mg, 2 mmol), TBHP (3 mmol), catalyst (0.01 mmol), and nitroalkane (5.0 mL) were stirred at RT for 6 hours. f Without catalyst. g After 2 h of reaction, the solid catalyst was isolated by centrifugation. h Sixth cycle.

158

Chapter 4

wide array of transformations making it a very important process in organic synthesis.19 Xiu-Lang Lv, Hong-Cai Zhou et al.20 use a reticular-synthetic pathway to design and investigate two manganese-metalated, isoreticular por-MOFs utilizing nickel–pyrazolate cluster SBBs for the direct halogenation of saturated cyclic hydrocarbons in the presence of a hypohalite halogen source. PCN-601 and PCN-602 both possesses the [Ni8(OH)4(H2O)2Pz12]8 (Pz ¼ pyrazolate) cluster while PCN-601 is composed of a TPP4 (5,10,15,20-tetrakis(pyrazolate-4-yl)porphyrin linker and PCN-602 TPPP4 (5,10,15,20-tetrakis(4-(1H-pyrazol-4-yl)-phenyl)porphyrin). These linkers are essentially the same, but TPPP is elongated by the additional phenyl group, increasing pore volume within the MOF. Following the Hard-Soft Acid–Base theory (HSAB) the choice of linker and SBB that comprise these por-MOFs would indicate significant stability under basic conditions – this is particularly important as the halogen sources utilized are basic. Indeed, several studies are conducted showing no degradation after soaking crystal aqueous solutions of OH, F, CO32, and PO43 ions at 1 M for 24 hours. The difference in conversion rate due to the pore sizes of PCN-601 and PCN-602 are apparent in the catalytic halogenation of cyclohexane as shown (see Table 4.6). That these por-MOFs are isoreticular means in this case the only difference in their structures is pore size, and so the substantial decrease in yield (even with five-fold reaction time) can be attributed to reduced substrate access to catalytic Mn centers on the porphyrin ligand. The performance of PCN-602 however is quite good, even when compared to homogeneous Mn porphyrin catalyst equivalents21 which could be due to the isolation of porphyrin on the rigid MOF structure, preventing the formation of inactive dimers. The recyclability of PCN-602 was investigated; the heterogeneous catalyst was collected via centrifugation and after three consecutive cycles it was found to remain highly active, with a conversion rate of 92% for the final reaction.

4.4 Other Reactions Catalyzed by Por-MOFs A series of bifunctional lanthanide por-MOFs have been developed by Carla F. Pereira, Filipe A. Almeida Paz et al.22 capable of sensing nitroaromatics (not discussed herein) and catalyzing the sulfoxidation of thioanisole in the presence of H2O2 as the oxidant. The por-MOFs are prepared from H10TPPA (5,10,15,20-tetrakis(p-phenylphosphonic acid)porphyrin) and lanthanide SBBs, yielding [M(H9TPPA(H2O)x]Cl2yH2O (x þ y ¼ 7 and M31 ¼ La31 (1), Yb31 (2), and Y31 (3)). This oxidation process is of industrial and environmental importance due to the negative impact the use of sulfur-containing compounds found in petroleum products has on the environment as well as equipment or machinery that these materials are used in ref. 23. The model compound thioanisole was selected to illustrate this reaction because the electrophilic addition of oxygen benefits from the increased electron density on sulfur; the relative catalytic efficiencies of the three por-MOFs were found

Heterogeneous Catalysis of Porphyrin-based MOFs Table 4.6

Entry

159

Catalytic performance of PCN-602(Mn) and other catalysts in C–H bond halogenation of cycloalkanes. Reproduced from ref. 20 with permission from American Chemical Society, Copyright 2017. Substrate

Catalyst

Product

Yield (%)

1

PCN-602(Mn)

93a

2

Mn(TPP)Cl

8a

3

PCN-601(Mn)

15b

4

PCN-602(Mn)

95a

5

PCN-602(Mn)

89c

6

PCN-602(Mn)

85c

a

Standard conditions: substrate/NaClO/catalyst ¼ 2 : 0.66 : 0.013 mmol; in acetone–water (2 : 2 mL); and the suspension was stirred at RT for 2 h. b Standard conditions: The reaction time is extended to 10 h. c Standard conditions: NaBrO was used instead of NaClO.

to be 14243 with conversions of 89, 79, and 68%, respectively (see Table 4.7). In each case selectivity for the sulfoxide was 90% or greater with respect to the sulfone. Additional studies were performed on the three metal salts as well as the ligand, revealing enhanced conversion efficiency and selectivity when compared to the metal salts with inferior results for the por-MOFs when compared to the homogeneous ligand. However, recycling experiments reveal that the por-MOFs more or less maintain their conversion and selectivity results over the course of several reactions while the homogeneous H10TPPA is degraded and efficacy is significantly reduced (99% to 56% from first to third run) (see Figure 4.3). This clearly illustrates the benefits found in the por-MOFs and in addition their heterogeneous nature allows for facile separation of catalyst which the homogeneous catalyst is not availed of. Recycled catalysts were centrifugated, washed with acetonitrile and ethanol, and dried under atmospheric conditions prior to reapplication. Leaching tests were conducted on (1) by separation of the catalyst from the reaction mixture upon reaching 42% conversion after four hours. The filtrate was then treated to identical conditions as reported for twenty hours in the absence of light (the role light may play in these catalysis reactions is not otherwise discussed in the report). Increase in conversion after this time was negligible (42–50%), suggesting there were no soluble active species present in the mixture and that (1) is a completely heterogeneous catalyst. As mentioned earlier, creating value from the transformation of the greenhouse gas CO2 is an appealing route to sequester it from the

160 Table 4.7

Chapter 4 Catalytic activity of MOFs 1–3 and of the building blocks in the oxidation of thioanisole with H2O2. Reproduced from ref. 22 with permission from American Chemical Society, Copyright 2018.a Yield% (Selectivity%)

Catalyst

Conversion %

1 2 3 LaCl37H2O YbCl36H2O YCl36H2O H10TPPA

89 68 79 75 83 86 99

a

80 63 74 60 44 61 96

(90) (93) (94) (80) (53) (71) (97)

9 5 5 14 36 23 3

(10) (7) (6) (19) (43) (27) (3)

— — — 1 (1) 3 (4) 2 (2) —

Thioanisole (0.3 mmol) was dissolved in 2.0 mL of CH3CN and kept under magnetic stirring at 50 1C for 24 h, in the presence of 2 mol % of catalyst and 2 equiv. of H2O2.

environment, preventing its detrimental effects.24 As previously discussed, a por-MOF was shown to catalytically add CO2 to epoxides using thermal energy, producing valuable organic synthetic intermediates from relatively simple substrates. Similarly, the reduction of gaseous CO2 to formate, done without the need for reaction substrate, is induced with light energy absorbed by Rh–PMOF-1 with high selectivity for formate against other possible products such as carbon monoxide, methanol, or methane. This stable rhodium-metalated por-MOF (Rh–PMOF-1) synthesized and investigated by Jiewei Liu, Cheng-Yong Su et al.25 is constructed of RhTCCPCl (5,10,15,20tetrakis(4-carboxyphenyl)porphyrin) ligands with ZrCl4 generating the SBBs, yielding the formula unit [Zr6(u3-O)8(OH)2(H2O)10)2(Rh(TCPP)Cl)3]. Rh-PMOF-1 was found to remain stable up to 270 1C using thermogravimetric analysis and does not degrade in a host of common organic solvents, including water. The photocatalytic reduction of CO2 was performed in acetonitrile sparged with CO2, with triethanolamine (TEOA) as the sacrificial agent, and a 300 W Xe arc lamp (o400 filtered out) as the light source. Formate ion formation was determined using ion chromatography with H2 gas, CO, and CH4 determined by GC (see Table 4.8). The source of formate from the reaction was determined with an isotopic label tracing experiment using 13CO2 and carbon-13 NMR. It was shown that under reaction conditions with the labeled gas, products and intermediates (CO2, HCO3, HCOO) with the label were also observed, but this was not seen in parallel reactions under normal CO2, an oxygen and nitrogen atmosphere, in the dark, or without Rh–PMOF-1 – the latter with no formate ions present, suggesting it is produced by the proposed photocatalytic reaction. This rationale for selective formation of formate follows that a weak Co–Rh bond between substrate and catalyst has been shown26 to yield CO, while stronger bonding between the two can lead to favoring addition of

Heterogeneous Catalysis of Porphyrin-based MOFs

Figure 4.3

161

Catalytic recycling tests for the oxidation of thioanisole using MOFs 1 (a), 2 (b), and 3 (c). Reproduced from ref. 22 with permission from American Chemical Society, Copyright 2018.

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a hydrogen atom to the ligated substrate. Rh–PMOF-1 can be recycled up to three times without loss of activity. The catalyst was centrifugated and washed with acetone between each run and after the third run some degradation in crystallinity was apparent. Additionally, after the trial three analysis of filtrate revealed 0.9% of Rh content from Rh–PMOF-1 had leached into the reaction solution. However, the heterogeneous nature of the catalyst was confirmed after an incomplete reaction was separated from the catalyst and allowed to continue under identical conditions for additional time, yielding no appreciable increase in product. Investigating the mechanism, theoretical calculations were performed to characterize HOMO–LUMO energies and probability densities for CO2 adsorption in addition to electrochemical characterization that will not be discussed here. Suffice to say, it is suggested the rhodium-metalated porphyrin is bifunctional, both absorbing visible light as well as directly photocatalyzing the reaction; photon energy absorbed by the porphyrin can be transferred to the zirconium SBBs, reducing Zr(IV) to Zr(III) which can in turn reduce CO2 to formate. Direct photocatalysis can be confirmed by comparison to other rhodium-metalated por-MOFs. The role of TEOA is the same for both pathways. Development of alternative fuel sources has increased in importance with a growing understanding of the impact anthropogenic pollution has had on climate change and the health of people and their environment. Hydrogen is appealing for its efficiency, renewability, and that it produces zero harmful emissions (its byproduct being water).27 Facile production of hydrogen then is a key aspect of making it readily available for industrial and consumer use. To this end, Alexandra Fateeva, and Matthew J. Rosseinsky et al.28 developed a water-stable por-MOF from AlCl36H2O and H2TCPP (meso-tetra(4-carboxyphenyl)porphyrin), yielding H2TCPP[AlOH2](DMF3)(H2O)2, referred to as Al-PMOF1, as well as a post-synthetically metalated zinc H2TCPP (Al–PMOF2) derivative – both for the application of photocatalyzed hydrogen evolution from water in the presence of a sacrificial e donor EDTA and (or without) e transfer agent methyl viologen (MV), in the presence of colloidal platinum. It is noted that Al–PMOF1 is stable in water up to pH 5 and up to 350 1C after guest loss, while this increases with Al–PMOF2 up to 370 1C. First, the MOF/MV21/EDTA/Pt catalytic system was tested. The porphyrin is excited by visible light energy which then reduces MV to a radical cation by electron transfer. Following this, the porphyrin is oxidized by EDTA (which decomposes) and MV transfers an electron to the colloidal platinum, where hydrogen is evolved. For both Al–PMOF1 and Al–PMOF2, a small amount of it was observed by GC after 15 hours in an aqueous solution of all components with a small quantum yield estimated at less than 0.01%. This limited activity is attributed to the inability of MV to enter the MOF pores. Following this, MV was removed from the system (and Pt loading was increased) so that excited porphyrin would directly transfer e to the colloidal Pt, realizing a ten-fold increase in H2 generation where Al–PMOF2 had a rate of 100 mmol g1 h1

Entry

Solvent (4 mL)

1 2 3 4 5

CH3CN CH3CN CH3CN CH3CN CH3CN

Atmosphere

Irradiation time (h)

Sacrificial agent (TEOA)

Catalyst (mg)

H2 (mmol)

N2 O2 CO2 CO2 CO2

18 18 Dark 18 18

1 1 1 1 1

Rh–PMOF-1 Rh–PMOF-1 Rh–PMOF-1 Rh–PMOF-1 —

— — — — —

(10) (10) (10) (10)

CO (mmol)

CH4 (mmol)

HCOO1 (mmol)

— — — — —

— — — — —

— — — 14.03 —

Heterogeneous Catalysis of Porphyrin-based MOFs

Table 4.8 Controlled photoreduction of CO2 to elaborate on the role of Rh–PMOF-1 as the catalyst. Reproduced from ref. 25 with permission from Elsevier, Copyright 2018.

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1

and Al–PMOF2 had 200 mmol g h after an induction period of three hours. Repeated reactions with the same catalysts reproduced these results. Homogeneous catalytic activity was ruled out for both por-MOFs by filtering the reaction mixture of the catalyst after 24 hours and exposing the filtrate to direct light for 15 hours – no H2 was detected in the solution by GC. Overall, it is suggested that the low quantum yields achieved are due to selfquenching occurring inside the por-MOFs.

4.5 Conclusion The research covered in this chapter illustrates just a handful of reactions with simple molecules por-MOFs can facilitate, either directly or indirectly. It has been shown they possess not only direct catalytic activity, but other roles in catalytic systems as structural motifs or through their ability to access thermal and light energy, with readily adjusted functionality via metalation or as the freebase molecule. Truly, they are versatile implements in designing MOFs as heterogeneous catalysts. The benefit of heterogeneous catalysts in general is self-evident and with research of late addressing the structural stability of por-MOFs,29 this unique and interesting class of materials stands to grow greatly from further study.

References 1. J. W. Owens, R. Smith, R. Robinson and M. Robins, Inorg. Chim. Acta, 1998, 279, 226–231. ˜es, J. P. C. Tome ´ and F. A. Almeida Paz, 2. C. F. Pereira, M. M. Q. Simo Molecules, 2016, 21, 1348. ´ ska, W. Shan, K. Zawada, K. M. Kadish and D. Gryko, 3. K. Rybicka-Jasin J. Am. Chem. Soc., 2016, 138, 15451–15458. 4. O. M. Yaghi, M. O’Keeffe, N. W. Ockwig, H. K. Chae, M. Eddaoudi and J. Kim, Nature, 2003, 423, 705–714. 5. X. L. Yang, C. Zou, Y. B. He, M. Zhao, B. L. Chen, S. C. Xiang, M. O’Keeffe and C. D. Wu, Chem. – Eur. J., 2014, 20, 1447–1452. 6. L. Feng, K. Wang, E. Joseph and H.-C. Zhou, Trends Chem., 2020, 2, 555–568. 7. M. R. Allen, D. J. Frame, C. Huntingford, C. D. Jones, J. A. Lowe, M. Meinshausen and N. Meinshausen, Nature, 2009, 458, 1163–1166. 8. W. Jiang, J. Yang, Y. Y. Liu, S. Y. Song and J. F. Ma, Chem. – Eur. J., 2016, 22, 16991–16997. 9. G. G. Zhanel, R. Love, H. Adam, A. Golden, S. Zelenitsky, F. Schweizer, ´-Wiens, E. Rubinstein, A. Walkty, A. S. Gin, B. Gorityala, P. R. S. Lagace M. Gilmour, D. J. Hoban, J. P. Lynch and J. A. Karlowsky, Drugs, 2015, 75, 253–270. 10. J. B. Locke, G. E. Zurenko, K. J. Shaw and K. Bartizal, Clin. Infect. Dis., 2014, 58(Suppl 1), S35–42. 11. X. Wang, W.-Y. Gao, Z. Niu, L. Wojtas, J. A. Perman, Y.-S. Chen, Z. Li, B. Aguila and S. Ma, Chem. Commun., 2018, 54, 1170–1173.

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12. R. G. Bergman, Nature, 2007, 446, 391–393. 13. M. H. Xie, X. L. Yang, Y. B. He, J. Zhang, B. L. Chen and C. D. Wu, Chem. – Eur. J., 2013, 19, 14316–14321. 14. J. Ritz, Ullmann’s Encyclopedia of Industrial Chemistry, 7th edn, Wiley-VCH, Hoboken, 2011. 15. P. Hudrlik and A. Hudrlik, The Chemistry of the Carbon-Carbon Triple Bond, Part 1, ed. S. Patai, John Wiley and Sons, New York, 1978. 16. Z. K. Lin, Z. M. Zhang, Y. S. Chen and W. B. Lin, Angew. Chem., Int. Ed., 2016, 55, 13739–13743. 17. T. Tachinami, T. Nishimura, R. Ushimaru, R. Noyori and H. Naka, J. Am. Chem. Soc., 2013, 135, 50–53. 18. C. S. Yeung and V. M. Dong, Chem. Rev., 2011, 111, 1215–1292. 19. I. Saikia, A. J. Borah and P. Phukan, Chem. Rev., 2016, 116, 6837–7042. 20. X.-L. Lv, K. Wang, B. Wang, J. Su, X. Zou, Y. Xie, J.-R. Li and H.-C. Zhou, J. Am. Chem. Soc., 2017, 139, 211–217. 21. W. Liu and J. T. Groves, J. Am. Chem. Soc., 2010, 132, 12847–12849. 22. C. F. Pereira, F. Figueira, R. F. Mendes, J. Rocha, J. T. Hupp, O. K. Farha, ˜es, J. P. C. Tome ´ and F. A. A. Paz, Inorg. Chem., 2018, 57, M. M. Q. Simo 3855–3864. 23. V. Chandra Srivastava, RSC Adv., 2012, 2, 759–783. 24. R. K. Pachauri and A. Reisinger, Climate Change 2007. Synthesis Report. Contribution of Working Groups I, II and III to the Fourth Assessment Report, Intergovernmental Panel on Climate Change (IPCC), Geneva (Switzerland), 2008. 25. J. Liu, Y.-Z. Fan, X. Li, Z. Wei, Y.-W. Xu, L. Zhang and C.-Y. Su, Appl. Catal., B, 2018, 231, 173–181. 26. W. Tu, Y. Zhou and Z. Zou, Adv. Mater., 2014, 26, 4607–4626. 27. P. Nikolaidis and A. Poullikkas, Renew. Sust. Energy Rev., 2017, 67, 597–611. 28. A. Fateeva, P. A. Chater, C. P. Ireland, A. A. Tahir, Y. Z. Khimyak, P. V. Wiper, J. R. Darwent and M. J. Rosseinsky, Angew. Chem., Int. Ed., 2012, 51, 7440–7444. 29. Z. Y. Gu, J. Park, A. Raiff, Z. W. Wei and H. C. Zhou, ChemCatChem, 2014, 6, 67–75.

CHAPTER 5

Porphyrin-encapsulating Metal–Organic Materials as Solid-state Mimics of Heme Enzymes RANDY W. LARSEN Department of Chemistry, University of South Florida, Tampa, Florida 33620, USA Email: [email protected]

5.1 Introduction Heme proteins are one of the most widely distributed metalloproteins in nature and include cytochromes (electron transfer), monooxygenases (oxidation reactions), peroxidases, and catalases (hydrogen peroxide degradation), FixL, PAS, and HemeAT (small molecule sensing), CooA (transcription regulation), Heme-Copper Oxidases (energy transduction), hemoglobins and myoglobins (oxygen transport and storage), and lignan peroxidase (polymer synthesis/degradation).1–7 Heme proteins contain an iron protoporphyrin IX (PPIX) or derivatives of this macrocycle as active sites that are coordinated to the protein through a proximal amino acid ligand such as histidine, methionine, tyrosine, or cysteine (Figure 5.1). Although the heme group provides the catalytic/ligand binding center for heme proteins, the wide diversity of function is ultimately determined by the threedimensional structure of the protein matrix which defines ligand access Monographs in Supramolecular Chemistry No. 32 Porphyrin-based Supramolecular Architectures: From Hierarchy to Functions Edited by Shengqian Ma and Gaurav Verma r The Royal Society of Chemistry 2022 Published by the Royal Society of Chemistry, www.rsc.org

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Figure 5.1

167

Illustration highlighting the catalytic diversity of heme proteins.

channels between the heme iron and the bulk solvent, amino acid composition and orientation making up the distal ligand binding pocket, and the amino acid composition of the proximal ligand (Figure 5.2, top). Distal heme pocket residues are tuned to place the substrate in a specific orientation and at a specific distance from the heme unit to facilitate stereospecific chemistry as well as providing ligand-heme iron stability while the nature of the proximal heme ligand can influence the degree of electron back donation into p-accepting ligands. The protein tertiary structure also provides conduits for ligand access to and from the heme active site providing regulated (‘gated’) access from the solvent to the distal heme pocket that are modulated through conformational changes.8–14 The oxidative chemistry associated with heme proteins is of particular interest in biomimetics as these reactions are the primary avenue for the functionalization of molecules important to the development of pharmaceutically active compound.9 The oxidative chemistry associated with heme enzymes relies on the redox cycling of the iron porphyrin subsequent to oxygen or hydrogen peroxide binding (Figure 5.2, bottom). The peroxidase pathway begins with the Fe(III)PPIX binding of a hydrogen peroxide molecule (or other organic peroxides) to the Fe(III) ion, which is followed immediately by heterolytic cleavage of the O–O bond through a two-electron reduction. The electrons are derived from the Fe ion forming an Fe(IV)QO species and the second electron derived from the porphyrin ring forming a porphyrin p-cation radical with a second oxygen producing a water molecule.

168

Figure 5.2

Chapter 5

Top: General structural anatomy of heme proteins. Bottom: Diversity in oxygen utilization by heme proteins.

In the case of heme peroxidase enzymes, two electrons from any number of organic reductants result in the formation of a second water molecule and regeneration of the Fe(III)porphyrin completing the catalytic cycle.

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In contrast, oxygen can bind to Fe(II)porphyrin initiating an oxygen reduction catalytic cycle as exemplified by the cytochrome P450 (CYP) catalytic cycle (Figure 5.2, bottom). This oxidative chemistry is of great interest due to the ability of the oxygen reduction intermediates to perform alkane hydroxylation, alkene epoxidation, arene epoxidation, dealkylation, and oxygenation of hetero-atom substrates, NO synthase, and isomerizations.10–15 The catalytic cycle of the CYP class of enzyme begins with the reduction of the Fe(III)heme to Fe(II)heme by NADPH.15 Oxygen then binds forming an Fe(II)–O2 similar to that of oxyhemoglobin and oxymyoglobin. This species closely resembles an Fe(III)–O–O  intermediate due to the electron donating ability of the proximal cysteine.15 The addition of two protons results in heterolytic reduction of the O–O bond producing a water molecule and the highly valent Fe(IV)QO species. In the presence of a saturated hydrocarbon, such as an alkene, the oxygen atom is transferred leading, for example, to formation of the respective alcohol or epoxide, depending upon the substrate.15 The catalytic cycle of CYP can also be ‘shunted’ using H2O2 in the presence of the Fe(III) form of the enzyme.14,15 The catalytic cycle then proceeds through a mechanism similar to that of the peroxidases.

5.1.1

Bioinspired Heme-based Materials

The catalytic diversity of heme enzymes has inspired enormous efforts to mimic these complex structures in the form of both homogeneous and heterogeneous catalysts.16–22 The guiding structural paradigm associated with heme enzymes and, thus, the requirement for any effective biomimetic system, is a catalytic heme active site housed in an evolutionarily designed pocket that has been optimized for highly selective catalytic reactions (Figure 5.2, top).23,24 The heme-binding pocket is composed of amino acid residues on both the distal and proximal side of the heme plane that tune the heme iron catalytic environment coupled with protein-based access channels allowing for selective movement of substrates to and from the heme site. Effective biomimetic systems would then require a specific hemebinding cavity that can ultimately be functionalized for specific catalytic chemistry, as well as providing enough porosity for rapid substrate diffusion to and from the encapsulated metalloporphyrin. Early systems developed to mimic heme protein catalysis include both homogeneous and heterogeneous catalysts. With regard to homogenous catalysts, functionalized metalloporphyrins have been synthesized to enhance the catalytic lifetime of the catalyst as well as to add stereo control over the catalytic reaction. The ‘basket-handle’ porphyrins contain groups around the periphery of tetraphenyl porphyrins that form substrate binding pockets reminiscent of heme proteins.25,26 Depending upon the choice of tethering groups, significant improvements can be made in terms of enantio-selectivity. This is particularly evident for porphyrin peripheral groups containing chiral functional groups. In addition, as proximal bases provide electron density to the porphyrin’s central metal that delocalizes

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onto bound oxygen (either molecular oxygen or peroxides leading to the destabilization of bound O–O), functionalized porphyrins also include proximal histidine, thiolate, or pyridine covalently linked to the porphyrin periphery with a linker of sufficient length to allow for coordination to the central metal.22 The development of these homogeneous porphyrin-based catalysts has produced systems with high turnover rates, excellent stereospecificity, and extension of the catalyst lifetime in solution. However, remaining limitations include difficulties in the separation of the catalyst from the products, as well as catalyst degradation. The limitations associated with homogeneous porphyrin catalysts can be mitigated, to some extent, through the use of solid supports for the porphyrin catalyst. Solid supports include polymers,27,28 porous sol–gels,29,30 clay-like materials,31,32 and zeolites.33 Heterogeneous heme biomimetic materials have greatly improved properties over homogeneous functionalized porphyrins with regard to catalyst stability and reusability. However, these materials are lacking many of the critical features associated with heme enzymes including a functionalizable distal pocket, well defined ligand migration channels for kinetic control of ligand access, and flexible structures for dynamic modulation of catalysis.

5.1.2

Metal–Organic Framework Materials (MOFs)

The newest generation of biomimetic/bioinspired heme catalysts exploits the properties of metal–organic framework (MOF) materials that significantly expand the functional properties of the catalysts. The MOF class of materials features high porosity, wide ranging structural topologies, ease of synthesis, and tunable functionality.34–44 The versatility of MOFs is due to the fact that these materials contain molecular building blocks (MBBs) composed of metal clusters with multidentate organic ligands (Figure 5.3)

Figure 5.3

General concept of MOF design and assembly.

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which allows for (1) the geometry of the MBB to be ‘tuned’, (2) the organic ligand can be functionalized to provide functional diversity to the MOF, (3) both the MBB and ligand are modular, thus allowing for a wide range of structures, and (4) the scale of the structure can be tuned from nanoporous to mesoporous.35,36 In terms of structural diversity, these materials extend from discrete nanoscale-faceted polyhedra to large porous extended networks.34,35 Extended porous networks with nanoscale cavities are of particular importance to biomimetic heme catalyst development since these materials can accommodate a wide variety of guest molecules including various porphyrin macrocycles.

5.1.3

Metalloporphyrin-based Metal–Organic Materials

Two approaches have been explored to exploit the catalytic diversity of the heme active site in MOF materials (Figure 5.4). One approach has been to construct porphyrin framework solids in which the porphyrin macrocycle is utilized as the ligand that connects the MBBs and is thus a constituent of the material. Currently, a wide array of porphyrin materials has been developed using this methodology including discrete porphyrin ‘containers’,45,46 two-dimensional porphyrin sheets,47,48 and nanoporous MOFs.49–51 Although these frameworks demonstrate catalytic activity reminiscent of heme proteins, they lack the requisite feature of having diverse functionality associated with the proximal and distal sides of the porphyrin. An alternative approach to MOF heme biomimetics is to encapsulate catalytically active porphyrins within the nanoscale pores of existing frameworks. The pores must be of sufficient dimensions to

Figure 5.4

Top: Example of a MOF in which the catalytic porphyrin is a component of the MOF framework. Bottom: Example of a guest-based MOF containing catalytic porphyrins.

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Figure 5.5

Chapter 5

Assembly of porphyrin encapsulated of the rhoZMOF.

accommodate the porphyrin guest and allow easy access for molecules to diffuse from the bulk solvent to the porphyrin active site. The non-covalent encapsulation of porphyrin catalysts within MOFs allows for a more diverse set of porphyrin catalysts to be utilized as well as the exploitation of the vast library of existing MOFs.

5.2 Porphyrin-encapsulated rhoZMOF One of the first MOFs to contain a non-covalently encapsulated porphyrin was TMPyP@rhoZMOF (TMPyP ¼ free-base tetrakis-(N-methylpyridyl) porphyrin), which was synthesized using what has been described as a ‘ship-ina-bottle’ approach.52 The rhoZMOF is composed of rigid and directional tetrahedral InN4 building units, i.e. In(III) coordinated to four doubly deprotonated bis(bidentate) imidazoledicarboxylic acid ligands (HImDC) (Figure 5.5). Although the encapsulated porphyrin could not be crystallographically resolved within the framework, the most probable location for the porphyrin is the largest cavity with a diameter of B2.2 nm as the porphyrin ring diameter is B1 nm. The size of the cavity also precludes crystallographic identification of the porphyrin due to conformational disorder allowing for multiple orientations of the porphyrin. Encapsulation of free-base TMPyP was determined spectroscopically. In solution, TMPyP exhibits a Soret maximum centered at B416 nm with visible bands centered at 513 nm, 545 nm, 588 nm, and 643 nm in CH2Cl2 versus 421 nm, 518 nm, 554 nm, 583 nm, and 638 nm in water (Figure 5.6, left). The corresponding bands in the TMPyP@rhoZMOF appear at B434 nm, 522 nm, 556 nm, 593 nm, and 648 nm which are significantly bathochromically shifted, relative to the porphyrin in solution. The bathochromic shifts can arise from a distortion of the porphyrin ring as well as the dihedral angle of the pyridinium groups relative to the porphyrin plane. Rotation of the porphyrin pyridinium ring by up to 301 relative to the porphyrin plane can bathochromically shift the Soret band by as much as 35 nm.53,54

Metal–Organic Materials as Solid-state Mimics of Heme Enzymes

Figure 5.6

173

Left: Optical absorption spectra of various free bases and metalloporphyrins encapsulated within rhoMOF. Right: Catalytic monooxygenation activity of TMPyP@rhoZMOF. Reproduced from ref. 52 with permission from American Chemical Society, Copyright 2008.

The encapsulated free-base porphyrin could be readily metalated with Co(II), Cu(II), Mn(III), and Zn(II) (Figure 5.6, left). The Mn(III)TMPyP@rhoZMOF derivative exhibited limited activity toward the monooxygenation of organic molecules when using organic peroxidase as a substrate.54 Specifically, the crystalline solid was explored as a heterogeneous catalyst for cyclohexane oxidation.54 The initial oxidation reactions were performed at 65 1C (neat solvent) in the presence of tert-butyl hydroperoxide (TBHP) as the oxidant and chlorobenzene as an internal standard. After 24 h, the yield of cyclohexanol/hexanone was found to be 91.5% with a corresponding turnover number (TON) of 23.5 (catalyst loading of 3.8%). The reaction products were formed in near stoichiometric quantities. Overall, the rhoZMOF TMPyP system provided the first example of a noncovalent porphyrin catalyst guest within a MOF that displayed peroxidaselike biomimetic activity. Overall, the catalytic rates are complex and involve both surface reactions (guests near the surface) and slower processes involving diffusion through the interior pores. In addition, the lack of crystallographic resolution of the porphyrin limits our understanding as to the relationship between cavity–porphyrin interactions and catalytic properties.

5.3 MOMZymes: Porphyrin Encapsulated HKUST-1 MOFs A second MOF-based heme biomimetic system has been developed in which catalytically active porphyrins are encapsulated within specific cages associated with the HKUST-1 MOF framework (see Figure 5.7).55 The porphyrin@HKUST-1 results in functionalizable and orientationally-specific proximal and distal heme pockets, as well as substrate-selective access

174

Figure 5.7

Chapter 5

Diagram illustrating the formation of MOMZyme-1.

channels to and from the porphyrin active sites. As such, the new materials retain many of the critical catalytic features associated with heme enzymes that can serve as a platform for the development of bio-inspired materials spanning a wide range of catalytic chemistry. The HKUST-1(Cu) and HKUST-1(Zn) MOFS are formed through the assembly of benzene-1,3,5-tricarboxylate anions and copper(II)56 or zinc(II)57 cations and are well-suited to serve as a platform for heme biomimetic chemistry since its topology affords three distinctly different polyhedral cages capable of entrapping guest molecules. The first MOMzyme example, MOMzyme-1, contains a metalloporphyrin (either Fe(III) tetrakis-(4-sulfonatophenyl) porphyrin, Fe4SP, or Mn(III) tetrakis-(4-sulfonatophenyl) porphyrin, Mn4SP) encapsulated within the octahedral cage that is most suited to serve as a host for a metalloporphyrin based upon cage size and symmetry, while the remaining cavities allow small molecules to reach the active site for catalysis much like channels in heme proteins.55 The crystal structures of the MOMzyme-1 materials were determined through single-crystal X-ray diffraction and were found to be isostructural with HKUST-1 (Figure 5.7).55 These structures have the porphyrin benzenesulfonic acid peripheral groups oriented through four of the six square windows of the octahemioctahedral cages (Figure 5.7) that lock the porphyrin into a well-defined orientation within the cage. The axial sites on both planes of the porphyrin are therefore necessarily exposed to the access channels of the framework since they lie directly above and below the other two square windows of the cage. Although the porphyrin ring can occupy one of three equivalent orientations within the cavity due to cavity symmetry, the orientation between cavities varies leading to static disorder throughout the crystal. The porphyrin planes are clearly resolvable, as the D4h symmetry of the porphyrin core is a subgroup of the cage symmetry, and the core is located on the symmetry plane and

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55

axes. The porphyrin loading was estimated to be between 33% and 66% depending on the reaction conditions suggesting that 1/3 to 2/3 of the octahemioctahedral cages are occupied by porphyrin. The presence of porphyrin molecules within the HKUST-1 framework reduces the overall surface area from 1663 m2 g1 for HKUST-1(Cu) to 980 m2 g1 for the Mn(III)4SP HKUST-1(Cu).55 The single crystal optical absorption spectra of both Fe4SP@HKUST-1(Cu or Zn) or Mn4SP@HKUST-1(Cu or Zn) are displayed in Figure 5.8(top).55 The optical spectra of Fe(III)4SP depend upon the aggregation state of the porphyrin. In aqueous solution, at pH values above 7, the porphyrin exists predominantly as a m-oxodimer dimer, exhibiting a Soret maximum at B410 nm while in the presence of a cationic surfactant, the Soret band shifts to B414 nm indicative of a monomeric conformation.58 In mixed ethanolbuffer solutions the Soret band centered at B394 nm is also indicative of monomeric porphyrin with bis-H2O axial ligation. The bathochromic shift in the Soret observed for the Fe4SP@HKUST-1 is likely due to exclusion of water molecules from one side of the porphyrin surface. The single crystal optical spectra of Fe4SP@HKUST-1(Cu or Zn) exhibit a Soret maximum at

Figure 5.8

Top: Absorption spectra of M4SP@HKUST-1 and M4SP in solution where M ¼ Fe(3þ) (left) and Mn(3þ) (right). Bottom: Peroxidase activity of Fe4SP (left) and Fe4SP@HKUST-1 (right) vs. MP-11 and hhMb in solution. Reproduced from ref. 55 with permission from American Chemical Society, Copyright 2011.

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B419 nm consistent with complete exclusion of water from the porphyrin ring, indicating a relatively tight fit between the encapsulated porphyrin and the cavity interior.58 The data further indicate that the porphyrin remains in high spin with either no axial ligands present or only weak field ligands. Resonance Raman and FTIR results demonstrate that encapsulation of Fe4SP into the octahemioctahedral cages of the HKUST-1 MOF does not result in any significant perturbations to the porphyrin ring.59 The vibrational structure of the HKUST-1 framework is also unaffected by encapsulation. The encapsulated Fe4SP appears to retain the oxidation and spin state of the Fe4SP solubilized in ethanol. The most significant change to the heme is the cavity environment which participates in electron donation to the heme macrocycle p-system. The data also demonstrate that stabilization of the Fe4SP within the octahemioctahedral cages is a result, in part, from interactions between the porphyrin peripheral sulfonic acid groups and Cu paddle wheel secondary building units of the MOF. The corresponding optical spectra of Mn4SP@HKUST-1(Cu, Zn) also display slight bathochromic shifts of the Soret band relative to that of the porphyrin solution with a Soret maximum at B471 nm, also consistent with the hydrophobic nature of the HKUST-1 cavity. The fact that the single crystal optical spectra of the encapsulated porphyrins are nearly identical between M4SP@HKUST-1(Cu) and M4SP@HKUST-1(Zn) (M ¼ Fe(III) or Mn(III)) indicates that the electrostatic environment of the binding pockets is similar between the two frameworks. As a probe for heme protein biomimetic capacity of the MOMzyme-1, the peroxidase activity of the material was assayed using 2,2 0 azinodi(3-ethylbenzthiazoline)-6-sulfonate (ABTS) as an electron donor (indicator) and monitoring the rate of increase in absorbance at 660 nm subsequent to the addition of peroxide.60 The initial rate for ABTS1 formation by the Fe4SP@HKUST-1(Cu) material is lower than observed for MP-11, hhMb, or Fe4SP (all in solution), while the maximum yield of ABTS1 , relative to hhMb, is comparable to that of MP-11 and Fe4SP in solution (Figure 5.8, bottom). hhMb, MP-11, and Fe4SP were selected as preliminary bench-mark systems as each displays peroxidase activity with increasing levels of structural complexity. The lower initial rate for ABTS1 formation, relative to the three benchmark systems, is due to the fact that substrate molecules must diffuse into/ out of the channels of the HKUST-1(Cu) framework within the bulk material. The percentage of ABTS conversion, however, is comparable to both MP-11 and Fe4SP. The significant percent conversion demonstrates several important features of the new material: (1) the axial positions of the encapsulated porphyrins are accessible to small molecules diffusing from solution into the HKUST-1(Cu) framework, (2) the Fe4SP remain catalytically active within the framework, (3) the larger ABTS substrate still has access to the encapsulated active sites and (4) successive turnovers can take place without significant degradation of the porphyrin macrocycles (in contrast to free Fe4SP or hhMB in solution).

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One of the most significant limitations of homogeneous catalysts involving monooxygenation is the recycling of the catalyst. In the case of Fe porphyrins, both the ferryl and porphyrin p 1 are highly reactive with other porphyrin macrocycles in solution thereby deactivating the catalyst.61 Recovery and recycling of the Fe4SP@HKUST-1(Cu) and Mn4SP@HKUST-1(Cu) results in retention of B33% of the initial rate of ABTS 1 formation, while the maximal production of ABTS1 remains at B60% of the initial catalysis.55 The initial loss of activity is likely due to the presence of guest molecules within the framework that degrade the porphyrin catalyst but are consumed during the initial turnover cycle. No significant reduction in catalyst activity or percent ABTS conversion is observed after the initial catalytic cycle.

5.3.1

MOMZymes II: Porphyrin-encapsulated MOM-XX MOFs

The octahemioctahedral cages of HKUST-1-type nets are not unique to HKUST-1 type frameworks and there currently exists a plethora of polyhedral MOMs that exhibit high surface area and pore sizes suitable for metalloporphyrin encapsulation.62 Metalloporphyrins can also serve as templating agents for the synthesis of new MOF structures. The MOMZyme-2 class of porphyrin MOF catalysts was synthesized using a solvothermal process leading to three new crystalline porphine@HKUST-1 structures: Mn3(BTC)2(H2O)30.15MnTMPyP (MnTMPyP@HKUST-1–Mn), Fe3(BTC)2(H2O)30.15FeTMPyP (FeTMPyP@HKUST-1–Fe), and Co3(BTC)2(H2O)30.15CoTMPyP (CoTMPyP@HKUST-1–Co) (Figure 5.9).62 Like the M4SP@HKUST-1 (Cu, Zn) materials, the X-ray structures of these porphyrin–MOF materials demonstrate that the porphine molecules are also selectively trapped in the cuboctahedral cages. The new materials were synthesized by reacting a metal salt (M(II)Cl2, M ¼ Mn, Fe, Co) with BTC and TMPyP in DMF and H2O. Attempts to synthesize HKUST-1–M (M ¼ Mn, Fe, and Co) in the absence of TMPyP were unsuccessful and resulted in the formation of previously reported structures [M6(HCOO)(BTC)2(DMF)6]n (M ¼ Mn(III) and Co(II)) or inorganic polymer gels.62 Varying the proportion of TMPyP resulted in loadings of MTMPyP that vary from 14% to 88%, with the maximal loading much higher than the 66% achieved for the M4SP@HKUST-1(Cu, Zn) material. The templated MTMPyP@HKUST-1–M (M ¼ Mn, Fe and Co) materials were also examined for catalytic activity toward monooxygenation using organic peroxides.63 Of specific interest in heme biomimetic catalysis is the templated FeTMPyP@HKUST-1–Fe framework (50% loading), which has an experimentally measured surface area of 430 m2 g1. This material was examined to evaluate size-selective olefin oxidation with the conversion of styrene (4.2 Å  7.0 Å cross-section) reachingB85% (turnover ¼ 269 h1) after 10 h, relative to the conversion of B35% for an equivalent amount of commercial FeTMPyP in solution (Figure 5.10). Styrene oxide and benzaldehyde were identified as the major products (30% and 57%, respectively). In contrast, trans-stilbene (4.2 Å  11.4 Å cross-section) exhibited only B40%

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Figure 5.9

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Diagram illustrating the templating effect of TMPyP in the syntheses of MTMPyP@HKUST(M) where M ¼ Zn(2þ), Fe(3þ), Cu(2þ), etc. These templated MOFs are referred to as the Por@MOM-XX class of materials. Reproduced from ref. 63 with permission from American Chemical Society, Copyright 2012.

conversion under the same conditions (TON ¼ 126 h1) with trans-stilbene oxide being the major product (70% selectivity), relative to conversion ofB34% for FeTMPyP in solution. The conversion of triphenylethylene (9.0 Å  11.4 Å cross-section) by FeTMPyP@HKUST-1–Fe was o3% (TON ¼ 9 h1) under the same conditions, whereas FeTMPyP in solution exhibitedB14% conversion with diphenylmethanone being the major product (98% selectivity).62 These observations are consistent with the oxidation reaction occurring within the cages of FeTMPyP@HKUST-1–Fe since the pore (B9 Å  9 Å) in MTMPyP@HKUST-1–M is the window of the cuboctahedral cages. Molecules smaller than 9 Å  9 Å would therefore be expected to be able to readily access the interior of cages, whereas molecules larger than 9 Å  9 Å would be unable to access the cage.

5.4 Fe Protoporphyrin IX MOFs I: FePPIX@MIL101(Al)–NH2 Initial MOMZyme development utilized symmetrical metalloporphyrins based upon a tetraphenyl porphyrin (TPP) framework including the tetrasulfonatophenyl and tetra-N-methyl pyridyl porphyrins encapsulated within polyhedral MOF cavities. However, the biological porphyrin active site in heme proteins, as discussed earlier, is based upon the asymmetrical

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Figure 5.10

179

Catalytic Por@MOM-4 toward substrates of differing diameters using t-butylperoxide as a co-substrate. Reproduced from ref. 63 with permission from American Chemical Society, Copyright 2012.

protoporphyrin IX (PPIX) core. Initial attempts to encapsulate Fe(III)PPIX into the HKUST-1(Cu) or rhoZMOF were unsuccessful and it was thought that this was due to the bulky porphyrin substituents as well as the overall asymmetry of the ring. The first MOF-PPIX composite material (H@M) was prepared using a MIL-101(Al)–NH2 aluminum-based MOF and FePPIX (Figure 5.11).64 Rather than encapsulating the FePPIX during the synthesis, the composite was prepared by soaking crystalline powder of the MOF with FePPIX in N,N 0 -dimethyl formamide (DMF) solution for 12 h. The resulting X-ray powder diffraction matched closely to that of MIL-53(Al)–NH2 which is formed from MIL-101(Al)–NH2 under reflux with DMF. The corresponding H@M composite exhibits vibrational spectra consistent with the formation of MIL-53(Al)–NH2 with FePPIX and a BET surface area reduced byB13% and pore volume reduced by B56%.

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Figure 5.11

Diagram illustrating the diffusional encapsulation of FePPIX into the MIL-101(Al)–NH2 MOF. After porphyrin entry, conformational rearrangement of the MOF produces FePPIX@MOF-53(Al)–NH2.

The H@M materials displayed catalytic activity toward the oxidation of 3,3,5,5-tetramethylbenzidine (TMB) using hydrogen peroxide (Figure 5.12). The effective Michaelis–Menten constant (KM) and maximum reaction rate (Vmax) were determined to be 0.07 mM and 6.1  108 M s1, respectively toward TMB and 11 mM and 9  108 M s1 toward H2O2. The activity of H@M toward H2O2 and TMB provided the basis for a colorimetric glucose detection system with a linear range of 10–300 mM in glucose, which was selective for glucose over fructose, maltose, and lactose.64 Although the H@M materials contain both the MIL-53(Al)–NH2 framework and FePPIX there is very little evidence of the encapsulation of the FePPIX within the framework pores. This is due to the fact that the FePPIX is added post-synthetically and the aperture diameter (B14 Å) of the MOF is slightly smaller than the diameter of the porphyrin ring (B15 Å with the propionic acid and methyl group substituents). Thus, it is unlikely that the FePPIX can diffuse into the interior cavities. Alternatively, the NH2 groups associated with the MOF can provide coordination sites for the FePPIX on the exterior of the material and would lead to the peroxidase activity being a surface phenomenon.

5.4.1

Fe Protoporphyrin IX MOFs II: FePPIX/Cu–MOF-74

A second bioinspired system involves FePPIX associated with a Cu–MOF-74 system (Figure 5.13).65 The Cu–MOF-74 is composed of Cu-carboxylate nodes linked through 2,2-dihydroxyterepthalic acid. Similar to the MIL-101(Al) Fe PPIX system, preparation of the FePPIX/Cu–MOF-74 materials was accomplished by soaking crystalline Cu–MOF-75 with an aqueous-methanol solution of hematin (FePPIX) for six hours followed by centrifugation to collect the new material. The optical spectra of the FePPIX/Cu–MOF-74 displayed a Soret band at 396 nm with broad visible bands centered at B590 nm consistent with monomeric Fe(III)PPIX. A colorimetric assay using H2O2 and 3,3 0 ,5,5 0 -tetramethylbenene (TMB) demonstrated catalytic activity of the

Metal–Organic Materials as Solid-state Mimics of Heme Enzymes

Figure 5.12

181

Top: Glucose oxidase biosensor prepared from the FePPIX@MIL-53(Al)– MH2 MOF. In this coupled biosensor, the glucose oxidase enzyme produces gluconic acid and H2O2. The corresponding H2O2 reacts with the FePPIX@MIL-53(Al)–NH2 which then oxidizes 3,3,5,5-tetramethylbenzidine (TMB) producing a colorimetric signal. Bottom: Colorimetric detection of various sugars using the coupled sensor system. Reproduced from ref. 64 with permission from the Royal Society of Chemistry.

FePPIX/Cu–MOF-74 material. The materials were applied to a glassy carbon electrode through solvent evaporation followed by Nafion immobilization and then used to electrochemically detect 2,4,6-trichlorophenol (TCP).

5.4.2

Fe Protoporphyrin IX MOFs III: FePPIX-encapsulated Zn MOF

One of the first successful ‘ship-in-a-bottle’ type encapsulations of FePPIX into a MOF was performed within a Zn-based framework formed from the

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Figure 5.13

Illustration of the diffusional encapsulation of FePPIX into the Cu–MOF-74 material.

Figure 5.14

Illustration of the ‘ship-in-a-bottle’ encapsulation of FePPIX within the Zn–FePPIX-1 MOF.

reaction of Zn21 ions, 4,4 0 ,400 -s-triazine-2,4,6-triyl-tribenzoic acid (H3TATB), and FePPIX in DMF containing a small amount of nitric acid (Figure 5.14).66 The resulting framework contains two tetrahedral terminal zinc ions coordinated to six carboxylate oxygens as nodes. The framework (referred to as FePPIX-1) is a non-interpenetrated (10,3)-a network with chiral helical channels that most likely contain the FePPIX active sites. Like many porphyrin frameworks, the FePPIX could not be crystallographically resolved within the framework. The data indicated that the FePPIX occupied B40% of the channel volume. The optical spectrum of FePPIX-1 is entirely consistent

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with monomeric heme encapsulation with the heme iron either 5- or 6- coordinated and with high spin. The FePPIX-1 MOF displayed peroxidase activity with a turnover number of 3.4 with 1.4 mole percent catalyst loading. The percent conversion of ABTS (redox indicator) was found to beB11%. The ability of substrates to enter the MOF was examined using methyl orange as a probe. The FePPIX displayed biphasic uptake consistent with fast surface adsorption of the dye followed by diffusion through the interior channels. The slower phase of methyl orange uptake demonstrates the ability of catalytic substrates to approach the encapsulated FePPIX sites. Examination of various substrates with respect to molecular volume, however, indicated the materials were active only for relatively small molecules with molecular volumes o100 Å3. Interestingly, Dare et al.65 also attempted to encapsulate FePPIX within the cavities of HKUST-1(Zn) using the ‘ship-in-a-bottle’ method. Although the synthesis produced crystals (referred to as FePPIX-2), the MOF appeared to be unstable and exhibited poor sorption of methyl orange. The poor stability is consistent with previous observations that asymmetric or neutral porphyrins do not effectively encapsulate within the polyhedral cages of the HKUST-1 class of MOFs.

5.5 Summary and Future Perspectives The desire to produce materials that mimic heme protein catalytic efficiency and versatility continues to be an area of significant importance in green chemistry for industrial process. Although the concept of ‘biomimetic’ MOFs has emerged as a potential pathway for the green chemistry aspect of heme protein catalysis, the term ‘biomimetic’ must be used with some caution when referring to these materials. A better term is ‘bioinspired’ as the inspiration for both encapsulation of porphyrin guests and incorporation of porphyrins into MOF frameworks is inspired by heme protein structure and function. The representative examples of heme-encapsulated MOFs described here highlight both the advances made toward biomimetic materials but also clearly spotlight the limitations. In all examples described above, the metalloporphyrin active sites are encapsulated within rigid frameworks and in many are not crystallographically resolved. The lack of structural detail precludes the determination of solvent environment, central axial ligation, and heme proximal and distal environments, all of which are critical to heme protein function. Perhaps one of the most challenging aspects of true MOF heme protein biomimetics is illustrated in Figure 5.15. Heme proteins, by their polypeptide nature, are flexible and undergo complex dynamics during even the simplest processes of ligand binding. Conformational dynamics have evolved to provide both regulated reaction dynamics and substrate selectivity. These dynamics also regulate, quite extensively, electron transfer dynamics during catalytic processes. The more rigid MOF frameworks lack such dynamics, and the engineering of such dynamics into MOF materials is

184

Figure 5.15

Chapter 5

Illustration of the differences between MOFs and proteins with regard to simple reactions in proteins such as ligand binding. Myoglobin (right) exhibits multi-step dynamically regulated ligand release from the heme active site while rigid heme-guest-based MOFs undergo unregulated ligand release.

extraordinary complex and thermodynamically work against the crystallization process. New generations of covalent organic frameworks afford better opportunities to integrate regulatory dynamics as these systems are composed of more flexible linkages.

Abbreviations Fe(III)PPIX CYP NADPH MOF MBB T4MPyP HimDC 4SP ABTS MP-11 hhMb BTC TPP

Fe protoporphyrin IX Cytochrome P450 Nicotinamide adenine dinucleotide phosphate Metal–organic Framework Molecular building blocks Tetrakis-(N-methylpyridyl) porphyrin imidazoledicarboxylic acid ligands tetrakis-(4-sulfonatophenyl) porphyrin 2,2 0 azinodi(3-ethylbenzthiazoline)-6-sulfonate Microperoxidase-11 Horse heart myoglobin Benzene tricarboxylate Tetraphenyl porphyrin

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CHAPTER 6

Light Harvesting in Porphyrinbased Metal–Organic Frameworks XINLIN LI AND PRAVAS DERIA* Department of Chemistry and Biochemistry, Southern Illinois University, 1245 Lincoln Drive, Carbondale, Illinois 62901, USA *Email: [email protected]

6.1 Introduction Over the past two decades, the development of porous materials has attracted great interest due to their remarkable applications in laboratories and industries. Zeolites,1,2 which are essentially porous aluminosilicates, porous carbon materials3 with graphitic structures, and porous polymers4 capable of incorporating multiple organic guest components are prime examples. Their high surface area and large pore volume, highly accessible channels with diverse pore geometries, and superior mass transfer performance enable them to be ideal platforms for separation, gas storage, energy conversion, and catalysis.5 Given the considerable benefits of porous materials in multiple fields, further exploration has surged. Since the early 1990s, attempts to bring coordination chemistry into porous materials by linking together organic and inorganic building blocks have arisen. The pioneering works by Robson,6,7 Yaghi,8,9 and Moore10 revealed the potentially extensive class of coordination-chemistry-driven ordered materials. This general class of materials was initially termed as ‘‘coordination polymers’’ (CPs), which signifies the extended connection of metal and organic Monographs in Supramolecular Chemistry No. 32 Porphyrin-based Supramolecular Architectures: From Hierarchy to Functions Edited by Shengqian Ma and Gaurav Verma r The Royal Society of Chemistry 2022 Published by the Royal Society of Chemistry, www.rsc.org

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linkers through coordination bonds along 1D, 2D, and 3D. A distinctive subclass caught the most attention; as coined by Yaghi, the 2D and 3D crystalline solid materials were termed as ‘metal–organic frameworks (MOFs)’ (Scheme 6.1).11 Since then, this subclass of CPs has been described in different ways; however, the most important distinction for a CP to be called a MOF is its porosity. Thus, MOFs are synonymous with porous coordination polymers (PCPs) or porous coordination networks (PCNs) formed via moderate to strong coordination bonds between multitopic organic linkers or struts and metal-ion-based nodes or secondary building units (SBUs). Contrary to the early developments, the IUPAC recommendation does not include crystallinity as a required condition for MOFs (PCP/PCN), neither does it recognize being ‘hybrid’ as widely described in the earlier literature. Thus, MOFs broadly encompass both crystalline and amorphous porous compositions. However, traditionally, the crystalline compositions gained significant lead over the non-crystalline variants mainly due to the fact that the crystalline structures can be mathematically formulated, described with topological identities and divided into various reticular categories.12 It is not surprising that in just over two decades, more than 20 000 MOFs have been reported. A corollary is that the 3D extended PCNs/MOFs yield unique pore geometries, where the linkers/struts are arranged in precise positions around the pores, often in hierarchical fashion. These well-defined chemical environments became a major development to tune the ratio of surface area and void volumes, and gravimetric to volumetric surface area, mostly critical for tuning gas storage and separation capacity.13–17 The surface area of MOFs typically ranges from a few hundreds to several thousand m2 g1 and can exceed various traditional porous materials such as zeolites and carbons.18 With the variable topicity (number of connecting points) and symmetry of the wide range of available metal SBUs and organic struts, virtually endless possibilities of topological variety can be envisioned. Altering the metal identity of the SBUs and/or chemical functionality of the struts, an enormous number of frameworks can be generated with the same (or related subset of) topology – the chemistry more widely described as reticular diversity.

Scheme 6.1

Schematic representation of MOF construction and functionalization.

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Reticular chemistry widens the accessible functionalities of the frameworks. Additionally, MOFs can be post-synthetically functionalized19 via, for example, linker modification,20 metal or linker exchange,21,22 or solventassisted ligand incorporation (SALI)23 to integrate complementary functionalities for a plethora of desired chemistry. Owing to these fascinating features, researchers have been assembling the desired chemical entities (e.g. photo, redox, and/or catalytically active components) into MOFs to fulfill the targeted chemistry and functionality, which expands the horizon of MOF applications far beyond its original depiction as a molecular sponge. The precise control of concentration and the relative positioning of the complementary functionalities into a porous polymeric solid can be critical in the development of next-generation low-density practical compositions for photocatalysis,24–28 luminescent sensing,29–31 and photovoltaics.32–34 For these applications, light-harvesting (LH) is a key process, and developing MOF-based artificial compositions has attracted considerable attention, especially to meet the demand for clean, reproducible energy, and fuel. Inspired by the light-harvesting complexes of LH2 and LH1 in the natural photosynthetic systems (i.e. PS I/II) that are defined by the delicate assembly of chlorophyll pigments in circular antenna, macromolecular and polymeric artificial light-harvesting systems have been developed with porphyrin derivatives.24,35–37 The key function in these bio-inspired assemblies is (directional) energy transfer (EnT) involving molecular exciton migration. Consider a multielectron transformation of a redox-catalyst assembly from its resting state to an active state via a photo-induced charge transfer (PCT) process: the intermediates, after each such electron transfer process can be vulnerable to various deactivation processes or mass diffusion related challenges (like poor solubility). The biggest challenge, perhaps, is the mismatch in the respective lifetimes of the excited-state species and redox intermediates compared to the reactivation of the sensitizer, thus, the desired antenna system can not only be used to capture photons in a low-light scenario but also to improve the probability of multielectron injection to a single ‘reaction center’ (RC) or redox catalysts. Consider now, the entire antenna system is electronically coupled with the redox-active catalyst and fixed periodically in porous solid-phase assembly as the working composition. Likewise, if we view this design in a reverse fashion, i.e. one such ‘reaction center’ or redox quencher can quench many excited chromophores (or molecular excitons) – it can be used to sense an analyte (that can get involved in an EnT or PCT) at very low concentrations via an amplified emission quenching process. In this regard, MOFs can serve as an ideal platform to precisely arrange porphyrin-based light-harvesting arrays, where a complementary component can be fixed within the required relative positions and this entire arrangement can be periodically replicated in a dense form with wide accessible design flexibility. Motivated by this pursuit, various porphyrin-based MOFs, hereon referred to as PorMOF, have been developed with varying topology: a few classic examples are PCN-222/ MOF-545 (csq),38,39 PCN-225 (sqc),40 MOF-525 ( ftw),39 and NU-902 (scu).41

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Scheme 6.2

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Schematic representation of exciton hopping/delocalization along chromophore assemblies in MOFs.

While these MOFs are constructed by interconnecting tetratopic carboxy-functionalized porphyrins, e.g. tetrakis(4-carboxyphenyl)porphyrin (TCPP(M); M ¼ H2, Zn, Co, Fe, Mn, etc.) with a Zr6O4(OH)4 SBU, compositions with other porphyrin derivatives and metal nodes adopting a wide range of structures were also developed. With their transition dipole aligned by the adopted topology, these multi-chromophoric assemblies can potentially circumvent the extant challenges of unproductive exciton recombination processes existing in common molecular aggregates to ensure high photon absorptivity for driving long-range EnT (Scheme 6.2). Much like the PS I/II, the initially prepared excited states in PorMOFs can directly participate in PCT processes in the presence of appropriate complementary units (Scheme 6.3), where the resulting charges with appropriate potential can be used to drive redox reactions or initiate a migration chain to be carried away for further utilization.37 Depending on the concentration of the complementary units, an EnT may come into play before the excitedstate charge transfer. Such PCT can technically proceed between two different types of linkers (e.g. pillar-paddlewheel MOF); alternatively, excited porphyrin linkers can drive PCT with appropriate metal SBUs,42,43 or post-synthetically incorporated complexes,44 metallic clusters,45 or semiconducting nanostructures46 to fulfill the targeted chemistry such as photocatalysis, electrocatalysis and luminescent sensing. In this chapter, we aim to highlight the intrinsic electronic and photophysical properties of PorMOFs and their relevant photo/electrochemical applications, which are primarily organized in Sections 6.4 and 6.5, respectively. Considering ample concurrent developments, we will stress on various classical works involving rigorous spectroscopic investigations of photophysical events. A short introductory MOF chemistry relevant to the structure and synthesis is included in Sections 6.2 and 6.3.

6.2 Background of MOFs Over the last two decades, the number of reported MOF structures has increased tremendously, which has helped establish the design principle of

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Scheme 6.3

Figure 6.1

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Schematic representation of PCT including metal-to-ligand charge transfer (MLCT), ligand-to-metal charge transfer (LMCT) within MOFs, and PCT between guest species and MOF components. Reproduced from ref. 37 with permission from the Royal Society of Chemistry.

The topicity of organic linkers used in MOF synthesis ranging from 2 to 12 points of extension. Only high symmetry forms are shown.

synthetically accessible MOF structures. The widely regarded one, coined by Yaghi and co-workers, is known as reticular chemistry. It is the study of linking discrete chemical entities, typically organic linkers, and metal SBUs by strong bonds to make extended structures including MOFs. In the context of linker design, three fragments are generally considered: (i) the ‘‘core unit’’ defining the geometric extension motif of the backbone; (ii) the ‘‘binding groups’’ that connect the linkers to SBUs; and (iii) the ‘‘extending units’’ that are usually inserted between the core unit and binding groups defining the size of the linker and thereby the metrics of MOFs (e.g. pore dimension). The number of extension points of the ‘‘core unit’’, is called topicity which commonly ranges from 2 to 12 (Figure 6.1), providing enormous possibilities to construct MOFs with distinct topologies. The core unit can be a chromophore such as a porphyrin or a pyrene derivative, which primarily defines the photophysical features of the linker as well as the MOF. Since the

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electronic structure and symmetry defines the photophysics of a chromophore, particularly the energy and probability for the lowest energy transition; electronic modulation of the core via an appropriate bridging group should define the optical properties of the linker and MOF as well as the topology of the framework itself. This is a powerful strategy and synthetically accessible tool as the MOF topology, in turn, would define the interchromophoric interaction to further modulate the optoelectronic properties and photophysics of the assembly.47 To ensure structural directionality and robustness, linkers are usually designed bearing chelating groups such as carboxylates, pyrazolates,48,49 imidazolates,50,51 and pyridines.52 Being the most commonly used functional group in unit linkage so far, carboxylates exhibit advantages in moderate-to-strong bonding and immutable coordination motifs with metal clusters as they form robust structures without the need of counter ions (by balancing the positive charges of metal SBUs). Since the crystal growth requires ‘corrections’, strong metal–carboxylate bonds may not be the best choice to establish a self-correcting ‘reversible’ solvothermal synthesis condition, which may lead to amorphous structures via rapid precipitation. To address such issues, monocarboxylic acids (e.g. benzoic acid53 and acetic acid40) are used as modulators (sometimes amines like triethylamine as well54) to prime the kinetics of metal–carboxylate bond formation and thereby steer the synthesis toward (thermodynamically controlled) crystalline porous solids. Given that the N-base coordinations are more thermally labile, it is relatively easy to establish such self-correcting solvothermal conditions with pyridine-type likers (e.g. bipyridines, imidazoles, dinitriles, 1,4-diazabicyclo[2.2.2]octane (DABCO), etc.). SBUs are typically metal clusters with diverse geometries and a broad range of symmetry adopted directional connectivity. The structural diversity of SBUs makes them an ideal building block to offer a large number of distinct frameworks despite various stringent coordination geometryrelated constraints for interconnecting the linkers. SBUs are often formed in situ during MOF synthesis dictated by the given reaction conditions including metal sources, modulators, pH, etc. The most critical feature of SBUs is the connecting number which can vary from 4 to 12. The paddlewheel M2(–COO)4 (M ¼ Cu21, Zn21, Co21) represents one of the most common 4-connected (4-c) SBUs, featured with two metal centers in a square-pyramidal motif (with solvent taking the apex position). One of the benchmark MOFs, HKUST-1,55 is reticulated by the 4-c dinuclear copper paddlewheel and tritopic linker benzenetricarboxylic acid (BTC) (Figure 6.2b) that forms a tbo topology (Figure 6.2c). The trinuclear cluster M3OL3(–COO)6 (M ¼ Al31, In31, Cr31, V31) stands for the typical 6-connected (6-c) SBU, based on which two extensively studied MOFs MIL-10056 and MIL-10157 (Figure 6.2f) are constructed from its linkage with a ditopic (benzenedicarboxylic acid, BDC) and tritopic (BTC) linker, respectively. Interestingly, both MOFs show the same mtn topology and contain two types of cages likewise. High valent zirconium ions (Zr41)

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Figure 6.2

The chemical structure of (a) the dinuclear copper paddlewheel SBU Cu2(–COO)4; (b) benzenetricarboxylic acid, and (c) the crystal structure of HKUST-1. (d) Trinuclear chromium SBU; (e) BDC linker and (f) the crystal structure of MIL-101(Cr).

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form a well-defined octahedral cluster with the chemical formula of [Zr6(m3-O)4(m3-OH)4(–OH)12n(–OH2)12n]n1 that can bind a maximum of twelve carboxylates. The valence n varies between 6 and 12 depending on the number of bonded carboxylates, which defines SBUs with different connectivity (i.e. 6-c to 12-c) and the charge can be balanced by 12-n number of terminal hydroxy ligands.58 Thus, for a 6-c or 8-c Zr-SBU, the ‘empty’ sites may be occupied by 6 or 4 pairs of hydroxy-aqua or monotopic carboxyl ligands used as a modulator. This variation in the number of connectivity may also stem from the linker topicity: for example, a tetratopic linker that cannot form ftw topological net may not form a 12-c node. The first Zr-based MOF, UiO-66 (Figure 6.4) (from University of Oslo) was constructed with a 12-c Zr cluster: Zr6(m3-O)4(m3-OH)4(COO)12.59 This MOF forms a face-centered cubic ( fcu) network and exhibits excellent thermal, mechanical and hydrolytic stability (including in aqueous acidic medium) owing to its saturated 12-c structure involving a strong ZrIV-carboxylate bond. Figure 6.3 summarizes Zr-based SBUs having 6, 8, 10, and 12 connectivity. A series of Zr-PorMOFs exhibiting interesting electronic properties is introduced in the next subsection. We have discussed the importance of the symmetry of linkers and SBUs in defining the desired MOFs topologies; looking beyond the symmetry, the sizes of the building units need to be considered. Both size and shape, along with the connecting network, define the void volume. These parameters eventually dictate if two or more frameworks can interpenetrate

Figure 6.3

The structure of (a) 6-c, (b) 8-c, (c) 10-c and (d) 12-c Zr–oxo SBUs.

Figure 6.4

The chemical structure of (a) 12-c Zr–oxo SBU, (b) BDC linker, and (c) the crystal structure of UiO-66.

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together when long linkers (particularly linkers with longer bridging arms relative to the smaller core) are selected for MOF synthesis. The interpenetration usually results in the blockage of pores. In other words, if the diameter of the coordination polymer chain is less than that of the cavity, interpenetration may occur. Therefore, it is desired to design frameworks where interpenetration is forbidden either by the inherent dimensions of the building units or the topology.

6.3 Introduction to PorMOFs Given the excellent light absorptivity in the UV–Vis range and excellent redox activity, porphyrins have been incorporated in MOFs for light-harvesting and photocatalytic transformations. Thus, PorMOFs can be exploited as an ideal platform to develop entirely artificial energy conversion systems. Interestingly, PorMOFs were among the early developed frameworks, even before the term MOF was coined. Such developments, dating back to the early 1990s, were intimately tied with the rich chemistry of porphyrin; while various ligating functionalities can be incorporated at the meso-positions (either all four or only at the 5,15-positions), the coordination chemistry of the central metal also adds to generating the coordination cages and/or polymers. Furthermore, critical chemical functional groups (offering, for example, modulation of the ring electronic structure) are also synthetically accessible at the b-positions. Nevertheless, porphyrin-linkers can be categorized into two groups based on their ligating functionalities forming pyridyl-based

Figure 6.5

Typical pyridyl-functionalized porphyrin linkers.

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porphyrinic MOFs (PyPorMOF) and carboxy-based porphyrinic MOFs (CyPorMOF). Early developments mainly include one (1D),60 two (2D),61 and three-dimensional (3D)62 PyPorMOFs owing to the easier metal–pyridyl crystal engineering. On the contrary, the development of CyPorMOFs was deferred by the difficulty to establish a self-correcting solvothermal condition (vide supra). In this section, the lens is primarily focused on 3D PorMOFs which is of significant interest with LH utilities.

6.3.1

PorMOFs with Pyridyl Binding Groups

Figure 6.5 represents a few pyridyl-appended porphyrinic linkers. Among them, the tetratopic linker TPyP(H2) and its metalated-derivatives (TPyP(M)) have been extensively studied in PorMOF synthesis. Robson and co-workers reported the first porphyrin-based crystalline framework (Cd–TPyP(Pd) MOF) in 1991, which is assembled by a tetratopic pyridyl-functionalized metalloporphyrin, 5,10,15,20-tetrapyridylporphyrin palladium (TPyP(Pd)) and 2-c cadmium ion nodes in octahedral shape (Figure 6.6a).63 The cadmium nodes are coordinated by pyridine in two modes: linear (N–Cd–N, 1801) and bent-shaped connection (N–Cd–N, 1031), and the palladium porphyrins are stacked along the [001] direction. This pioneering example showed the great potential of pyridyl-based linkers in MOF construction with various metal SBUs. Thereon, a plethora of PorMOFs was made with pyridine-based porphyrin linkers over the next two decades. In 1994, the same group published another PorMOF (Cu–TPyP(Cu)) of the platinum–sulfide-like (PtS) structure (Figure 6.6b), comprised of two distinct building blocks: squareplanar TPyP(Cu) and tetrahedral CuI-SBUs, which are connected alternatively in a one-to-one ratio to form an open channel.7 However, the framework collapses upon the thermal solvent removal from the channels. 5,15-Dipyridyl-porphyrins, typically 5,15-dipyridyl-10,20-diarylporphyrin ¨hn et al. (DPyDAP(H2)) represents another commonly used linker. Ku synthesized a series of PorMOFs (ZnDPyDAP(Zn)) by the self-assembly of DPyDAP(Zn)64 (Figure 6.7a) in which the porphyrin-chelated Zn21 serves as a metal building unit that adopts a distorted octahedral geometry with six nitrogen atoms in the coordination sphere, four from the porphyrin ring (Zn–N distance in the range of 2.057(7) and 2.072(2) Å) and two coming from the pyridyl ligands of the adjacent porphyrinic linkers (Zn–N distance in the range of 2.317(11) to 2.357(3) Å). The 3D hexagonal-shaped PorMOF possessing channels filled with solvent molecules are obtained. Furthermore, ZnDPyDAP(Zn) can be functionalized with variable substituent groups at the para-position of phenyl rings to modulate the size of the hexagonal channels and thereby the volume of cavities. Zubieta and co-workers reported two 3D PorMOFs with the chemical formula [Cu2Mo3O11–TPyP(Cu)] (MoP(Cu)) and [Fe(Mo6O19)2–TPyP(Fe)] (MoP(Fe)).65 The structure of MoP(Cu) revealed by single-crystal X-ray diffraction (SCXRD) is shown in Figure 6.7b, which is constructed by the linkage between TPyP(Cu) linker and bimetallic Cu2Mo3O11 clusters.

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Figure 6.6

The crystal structure of (a) Cd–TPyP(Pd) MOF and (b) Cu–TPyP(Cu) MOF.

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Figure 6.7

The crystal structure of (a) ZnDPyDAP MOF and (b) MoP(Cu) MOF.

Figure 6.8

The crystal structure of (a) F-MOF and (b) DA-MOF.

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The porphyrin units stack in a face-parallel fashion, with a porphyrin-toporphyrin distance of 4.815 Å. In contrast, FeP(Mo) exhibits a different linking motif, in which the SBU Fe(Mo6O19)2 joins six porphyrin linkers around it, yielding a primitive cubic (pcu) structure. The 3D structure can be essentially resolved into two 2D layers, which are chemically the same but orient differently, stacking in an AB fashion. By adopting a mixed-ligand strategy, Hupp, Nguyen, and co-workers prepared a pillared paddlewheel MOF known as ZnPO-MOF (which is also named as F-MOF) by pillaring the Zn21-paddlewheel layer of 1,2,4,5tetrakis(4-carboxyphenyl)benzene (H4TCPB), with metallo 5,15-dipyridyl10,20-bis(pentafluorophenyl)porphyrin (DPyPFP(Zn)) under solvothermal conditions (Figure 6.8a).66 The pillared TCPB linker constructed a robust non-interpenetrated framework enabling it to display permanent porosity with a high surface area of B500 m2 g1. In another study, Hupp and co-workers studied a new PorMOF namely DA-MOF (Figure 6.8a and b) constructed from 5,15-di(4-pyridylacetyl)-10,20-diphenylporphyrin (DPADPP(Zn)) pillaring the Zn21-paddlewheel layer of TCPB.67

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6.3.2 PorMOFs with Carboxylate Ligands 6.3.2.1 PorMOFs Using TCPP as Linkers The development of PorMOF with carboxy-appended linkers was started with H4TCPP(M) and it is still the most commonly used porphyrinic linker, owing to its symmetric structure accessible through relatively easy condensation (Figure 6.9). One of the early examples is the microporous PIZA-1 (porphyrinic Illinois zeolite analog) developed by Suslick and co-workers (Figure 6.10).68 PIZA-1 was prepared solvothermally by heating an aqueous solution of H4TCPP(H2) together with cobalt (II) chloride in the presence of

Figure 6.9

Figure 6.10

Typical carboxy-functionalized porphyrin linkers.

Crystal structure of PIZA-1. Reproduced from ref. 116 with permission from American Chemical Society, Copyright 2014.

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pyridine/KOH at 150 1C. The crystallographic structure (determined via SCXRD analysis) suggests that TCPP(CoIII) interconnected through trinuclear Co(II)-carboxylate SBUs and possesses 7 Å14 Å channels down the crystallographic a-axis, and 7 Å9 Å channels down both the b- and c-axes, leading to a framework with an impressive void volume of 74%. The robust metal–carboxy bond ensured structural stability over the pore filling and evacuation cycles with the solvent. Interestingly, PIZA-1 shows preferable sorption toward hydrophilic guests including water, amines, and alcohols. In an attempt to utilize manganese(III) instead of cobalt, PIZA-3 was obtained where TCPP(Mn) is interconnected through bridging bent trinuclear manganese cluster. PIZA-3 possess three types of pores: 5 Å9 Å and 7 Å8 Å pores down the crystallographic a-axis, and 3 Å5 Å pores down the c-axis, different from those of PIZA-1. In an alternative approach, carboxy–porphyrins were used to construct the 2D layers that were pillared using secondary linkers. Adopting this common strategy, the Choe group prepared a series of pillared paddlewheel PorMOFs by the directional assembly of mixed linkers, TCPP(M) (M ¼ Co, Zn, and Pd), and bipyridine (bpy) pillared via cobalt/zinc SBUs. The resulting structures PPF-3 ([Co2(TCPP(Co))(bpy)2]), PPF-4 ([Zn2(TCPP(Zn))(bpy)1.5]) and PPF-5 ([Co2(TCPP(Pd))(bpy)]) were crystallographically established (Figure 6.11),69 adopting three distinct stacking patterns: AB, ABBA and AA for the 2D TCPP(M)-layers in PPF-3, PPF-4, and PPF-5, respectively. In PPF-3 and PPF-4, the bpy links either two SBUs or one SBU and the metal at the porphyrin core. In contrast, in PPF-5, the bpy linkage only exists between two SBUs. The variable stacking modes possibly stem from the geometry and coordination preference of the central metal ion of the metalloporphyrin. MOFs constructed by hexanuclear Zr–oxo SBUs and a wide range of carboxybased linkers stand out as an extremely important development – relevant to the current chapter – due to the versatile connecting motifs of Zr–oxo SBUs and the robustness of Zr–carboxy bonds. A family of Zr-based PorMOFs using Zr–oxo clusters, [Zr6(m3-O)4(m3-OH)4]n1 and TCPP(H2)/TCPP(M) as building blocks were reported by Zhou, Yaghi, Hupp, and several other groups. These Zr–oxo–TCPP(M) can offer chemically similar but topologically different PorMOFs, which stems from the tunable connecting patterns of Zr–oxo clusters dictated by different reaction conditions. Yaghi reported a

Figure 6.11

Crystal structure of (a) PPF-3; (b) PPF-4; (c) PPF-5. Reproduced from ref. 69 with permission from American Chemical Society, Copyright 2009.

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framework with the formula Zr6(m3-O)4(OH)4(TCPP(H2))3 denoted as MOF-525 with ftw topology (Figure 6.12a),39 note that mathematically ftw topology cannot produce interpenetrated frameworks, and thus has been used to extend developing large-pore MOFs (Section 6.3.2.2; Figure 6.15).70 While during this early development, MOF-525(FeIII) was only achieved via a post-synthesis metalation, later developments suggested that fine-tuning synthetic conditions may lead to the formation of phase-pure MOF-525(M) starting from H4TCPP(M) (M includes FeIII).71,72 MOF-525 features a 12-c cuboctahedronshaped Zr-SBU, where acetic acid is employed as the modulator establishing the self-correcting solvothermal conditions during the MOF synthesis (vide supra) and ensuring the porous crystalline structure. The crystal structure revealed that MOF-525 possesses a cubic cage with a pore diameter of 20 Å and each cage is constituted of eight corner-sharing Zr6O4(OH)4 clusters bridged with six face-sharing porphyrins. A chemically identical MOF PCN-223 (Figure 6.12b), reported by the Zhou group, has an unprecedented 12-c Zr-SBU with a unique hexagonal prismatic geometry different from MOF-525.73 PCN-223 features a shp topology and has uniform 1D triangular channels with a porphyrin-surface. PCN-223(M) was prepared from both H4TCPP(Fe) and H4TCPP(H2) in the crystalline phase by altering the reaction

Figure 6.12

Crystal structure of (a) MOF-525; (b) PCN-223; (c) PCN-222/MOF-545; (d) NU-902; (e) PCN-225; (f) PCN-224.

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conditions using either benzoic or acetic acid as modulators. Compared to MOF-525 synthesis, less modulator to linker ratio and shorter reaction time are adopted to prepare PCN-223, suggesting that PCN-223 could be a kinetically favored product. The highly tunable reticular chemistry of Zr-based SBUs is well demonstrated in the synthesis of three chemically identical PorMOFs, PCN-222/MOF-545 (csq), PCN-225 (sqc), and NU-902 (scu), (Figure 6.12c–e) having the chemical formula [Zr6O4(OH)8 (H2O)4(TCPP)2] with an 8-c Zr–oxo node. Unlike the 12-c Zr6–oxo clusters, each Zr6O4(OH)8 core herein is connected to eight TCPP(M) linkers while the remaining positions are capped by terminal hydroxy/aqua ligands; these sites may also be occupied with capping monocarboxylic ligands (i.e. in pristine form these are modulators). The Zr–oxo clusters in the three MOFs share the same 8-c geometry yet result in different topologies when reticulating with TCPP. Notably, subtle changes in the reaction conditions, such as temperature, solvent, and monocarboxylic modulator (often correlated to their pKa or acidity) will significantly affect the structure of the resulting MOFs. For example, PCN-222/MOF-545 can be afforded in both dimethylformamide (DMF) and diethylformamide (DEF) as a solvent and either benzoic acid (reported by Zhou38) or formic acid (by Yaghi39) as modulators; where DEF and/or a stronger acid as a modulator may provide larger crystals. The beauty of the csq topological net is two types of 1D pores running parallel to the c-axis: triangular microporous and hexagonal mesoporous channels with a pore size of 13 and 32 Å, respectively. In contrast, the preparation of PCN-225 (sqc) uses mixed modulators involving benzoic acid and acetic acid in a DEF solvent. The sqc network consists of two types of pores with a dimension of 158 Å and 229 Å.40 Both the csq and sqc networks were realized at relatively higher temperatures of 120–130 1C compared to NU-902,41 which is another chemically identical 8-c Zr–oxo PorMOF with a scu topology and formed at a lower temperature of 90 1C. The NU-902 consists of a single 1D pore channel (d ¼ 12 Å) along the c-axis. While a preformation of the Zr–oxo cluster may assist such MOF synthesis, this step often becomes essential for those MOFs which are neither a thermodynamic product nor that can be made under a kinetic condition: for NU-902, incubation of the zirconiummodulator solution (DMF) is carried out at 80 1C for 45 min before the introduction of the linker to form the MOF. Besides the 12-c and 8-c Zr–oxo nodes, Zr–oxo PorMOFs with sixconnected (6-c) nodes were also developed. The Zhou group synthesized PCN-224 (Figure 6.12f) which consists of 6-c [(Zr6O4 (OH)4(H2O)6(OH)6)]61 interconnecting with a TCPP(H2) linker.74 In PCN-224, only six edges of the 6-c Zr–oxo SBUs are bridged by carboxylates from TCPP(M) linkers. A higher ZrIV to TCPP(M) ratio (i.e. Zr/TCPP ratio in PCN-224 is B3 times compared to that in PCN-222) was used to implement steep competitive coordination for the TCPP in limiting connection numbers. Interestingly, the robust ZrIV-carboxy connectivity is sufficient to provide PCN-224 hydrolytic stability over a large pH range, even though it bears less connectivity (more open and accessible sites at the node). Given the maximum 12-point connectivity for

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Figure 6.13

Crystal structure of (a) MMPF-1 and (b) MMPF-4.

Figure 6.14

Crystal structure of ZJU-18.

the Zr6–oxo SBUs, the unoccupied coordinating sites on an 8-c or 6-c Zr–oxo SBU can be exploited for post-synthetic functionalization via a widely used solvent-assisted ligand incorporation approach (SALI) (Scheme 6.4).23,75 A vast array of carboxy-terminated compounds can be incorporated for complementary functionality: among these, chromophores with complementary

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Scheme 6.4

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Schematic representation of the SALI process to functionalize the Zr–oxo node.

optical absorption76 and complexes77 with reciprocal redox properties can be anchored periodically at the Zr–oxo nodes within the frameworks for optoelectronic and/or catalytic developments. SALI relies on acid–base chemistry and thus the incoming carboxy-appended species can either replace a pair of hydroxyl and aqua ligand (releasing two molecules of H2O) or a labile carboxylate (releasing R–COOH): note that the outgoing moieties must be weaker acid (higher pKa value – i.e. a weaker conjugate base replaces a stronger conjugate base) or can undergo thermal decomposition. As displayed in Scheme 6.4, both monodentate and bidentate products can potentially be generated depending on the geometry, electronic properties of the R group and the pKa of the carboxylic acid. The binding motif can be determined by single crystal analysis detecting distance between terminal oxygens on Zr node, along with diffuse reflectance infrared Fourier transform spectroscopy that probes signal from terminal aqua or hydroxy groups at the Zr-SBU. This thermodynamically controlled reaction condition can thus enforce the homogeneous distribution of the anchored functionalities across the MOF crystal which is useful for various photophysical processes (see Sections 6.4 and 6.5).

6.3.2.2

PorMOFs Using Other Carboxy Porphyrins as Linkers

Although TCPP(H2)/TCPP(M) derived PorMOFs are well established, the extended selection of carboxy-elaborated porphyrin linkers is necessary to meet the demand for multi-functional PorMOFs depending on applications. Ma’s group adopted a custom-designed linker 5,15-bis(3,5-dicarboxyphenyl)porphine (H4BDCPP(H2)) when solvothermally assembled with copper salt, forming a 3D framework MMPF-1 where a dinuclear copperpaddlewheel interconnects a BDCPP(Cu) linker (metalated in situ) forming nanoscopic polyhedral cages (with dimension ca 18.62 Å16.17 and a cage volumeB2340 Å3; Figure 6.13a).78 The exposed small cage windows show the molecular-sieve-like property for the selective separation of a gas mixture, which can exclude larger molecules like N2 and CH4, but allow the entry of smaller molecules such as O2 and CO2. Ma and co-workers have extended the connectivity beyond tetratopic porphyrin linkers with an octatopic tetrakis(3,5-dicarboxyphenyl)porphine

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(H8TDCPP). Triangular Zn2(CO2)3 or Cd(CO2)3 metal building blocks were used to link TDCPP, forming MMPF-4 and MMPF-5, respectively, with the same topology (pcu) (Figure 6.13b).79 The crystal structures of the two PorMOFs display cubicuboctahedron cages stacked within the framework with a window dimension of 7.83 Å8.05 Å. By choosing different SBUs; Wu, Chen, and their co-workers synthesized a series of PorMOFs with tbo topology via the coordination between TDCPP(M) (M ¼ Mn or Ni) and MnCl2/CdCl2, which are called ZJU-18 [Mn5Cl2(TDCPP(Mn))(DMF)4(H2O)4], ZJU-19 [Mn5Cl2(Ni–TDCPP)(DMF)4(H2O)8], and ZJU-20 [Cd5Cl2(TDCPP(Mn))(H2O)6] (Figure 6.14).80 The representative crystal structure of ZJU-18 is made up of an octatopic Mn–TDCPP metalloligand joined to two SBUs, binuclear Mn2(COO)4 and trinuclear Mn3(COO)4(m-H2O)2. This tbo network, which was first found in HKUST-1, exhibits a large cavity of around 21.3 Å and a window size of about 11.5 Å. If we recall the linker design strategy introduced in Section 6.2, the insertion of extended ‘arm’ units between the ‘‘core’’ and binding moieties could alter the pore size and structure; and possibly endow new physical and chemical properties onto MOFs. This strategy has been applied in the design of PorMOFs. The Zhou group synthesized a series of isoreticular Zr6–oxo PorMOFs by using three tetracarboxy porphyrins, H4TCP-1, H4TCP-2, and H4TCP-3, (H4TCP ¼ tetrakis(4-carboxyphenyl)-porphyrin) whose extending units are elongated systematically by introducing ethynylene and phenyleneethynylenes.81 The resulting PCN-228, PCN-229, and PCN-230 (Figure 6.15) can be considered as the extended variants of MOF-525 with ftw construction, albeit displaying impressive and increasing pore apertures ranging from 25 to 38 Å as a function of the length of extending arms. Interestingly, all three PorMOFs, especially PCN-230 (with the longest arm) show excellent hydrolytic stability in aqueous solutions with a wide range of pH from 0 to 12. Besides some of the exemplary carboxy-based linkers and their PorMOFs discussed above, other well-known frameworks generated from TCBPP,82 TCPEP83 and TBCPPP84 linkers can be found in the literature.

Figure 6.15

Crystal structure of (a) PCN-228, (b) PCN-229, and (c) PCN-230. Reproduced from ref. 81, https://doi.org/10.1021/ja5111317, with permission from American Chemical Society, Copyright 2015.

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6.4 Electronic and Photophysical Properties The coordination chemistry-derived assembly of porphyrin linkers with chemically and electronically diverse SBUs has surged significant photophysical and photochemical developments. Furthermore, the incorporation of a wide range of redox/optical active species as guests widens the possibility of PorMOF compositions for LH-functionality including EnT, PCT, and/or their combined tandem processes displaying antenna phenomena. In these systems, porphyrins are commonly used as light sensitizers that absorb photons preparing excited states (excitons). Given the significant intersystem crossing (ISC) efficiency for common metalloporphyrins, the excitons can be manipulated both in their singlet or triplet manifold. Like various well-studied photophysical properties in molecular and macromolecular (dendritic and oligomeric) systems in homogeneous solutions, the underlying fundamental photophysical features need to be established in these solid porous MOFs. The key factor here is the pore geometry, its size and shape dictate the inter-porphyrin distance and orientation, which in turn determines the transition–dipole interaction defining the excitonic properties. Understanding excitonic properties such as exciton size, the dimension of its delocalization and diffusion length (or volume) will define the dynamics and utilities of these photo-generated excitons. While a wide range of experimental and theoretical studies are underway and many more to evolve mapping the excited-state properties, one factor is clear: the photophysics of MOF-assembled porphyrins are different than any common solid aggregate or homogeneous solution of porphyrin derivatives, and can be modulated by their topological net. This section will describe and discuss various photophysical processes including EnT, ISC, and PCT with appropriate complementary moieties. The understanding of these fundamental processes will help us design efficient PorMOF-based artificial photo-, electro-, and photoelectro-catalysts, as discussed in Section 6.5.

6.4.1 Energy Transfer 6.4.1.1 Singlet-to-singlet Energy Transfer Porphyrins assembled in MOFs can deliver excited-state energy to neighboring chromophores and thus undertake long-range EnT via site-to-site exciton hopping: this process can occur both in singlet or triplet manifold (Scheme 6.5) depending on the packing density. These self-exchange processes have been analyzed, mostly as a chromophore-to-chromophore process. However, since the chromophore-assembly works in the cohort, the EnT process in the singlet manifold can be partially and relatively ana¨rster resonance energy transfer (FRET) rate constant: lyzed via the Fo kFRET ¼

2p 2 J Y h 

(6:1)

208

Scheme 6.5

Chapter 6

Energy diagram displaying EnT processes from photoexcited chromophore assemblies (denoted as D) to acceptor species (denoted as A). 1D* and 3D* are the singlet and triplet excited state of the donor respectively; 1A* and 3A* are the singlet and triplet excited-state of the donor respectively; 1A* and 3A* are the singlet and triplet excited state of the acceptor respectively. SSET, TTET, and ISC are the abbreviation of singlet-to-singlet energy transfer, triplet-to-triplet energy transfer, and intersystem crossing, respectively.

where J (eV1) is the interchromophoric electronic coupling and Y is the spectral overlap integral (eV1) between the area-normalized donor fluorescence and acceptor absorption bands. J depends upon the magnitudes of the transition dipole, their orientation and distance. Thus, J can be modulated by the MOF topology. Y (eV1) can be tuned by the spectroscopic properties of the chromophore, especially for the lowest energy transition. This is because radiative decays of the excited-state population occur through the lowest-energy state (S1-S0), and increasing the lowest energy oscillator strength should improve the Y. For MOFs with a single chromophore, the targeted metrical parameter would be to long-range energy transport. The radiative lifetime of common chromophores falls within the range of few tens of nanoseconds; only an efficient kFRET in the order of a few picoseconds or so would be useful to drive long-distant excited-state EnT (B100 nm). The challenge here is the random-walk type hopping path (involving the equal possibility of an exciton hopping that takes it one step forward and brings it one step backward), where an asymmetric design can lead to directional exciton delivery, much like the one seen in the natural LH-systems. In contrast, triplet excitons are long-lived due to a forbidden T1-S0 relaxation and offer the opportunity to drive a long-range exciton transport if two criteria are met: (a) complete O2-free environment, and (b) short interchromophoric distance as it is driven by a Dexter-type double exchange process. Hence, to realize efficient EnT in MOFs, the key factors include the selection of appropriate D–A (or D–D) pairs with favorable

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energetic alignment and geometrical positioning within the framework. The determination of EnT efficiency and related metrical parameters involves amplified emission quenching experiments. Commonly, a redox quencher is used to determine the effective radius of its quenching ability, for no or ineffective EnT, the quencher can only quench its neighboring chromophore via a PCT. Alternatively, (non-emissive) acceptor species can also be used. In a typical experiment, samples with varying amounts of quencher (e.g. ferrocene derivatives attached either at the linker or at the node via post-synthesis functionalization) are prepared and the Stern– Volmer analysis of the respective quantum yields can provide the KSV: Q0 I 0 ¼ ¼ 1 þ KSV ½R QS I S

(6:2)

Q0 and I0 are the intrinsic fluorescence quantum yield and intensity of the chromophore without quencher, whereas QS and IS are for the saturated sample. The [R] is the concentration or loading ratio for a quencher. For MOFs where the quenchers are fixed, i.e. a modified diffusion-controlled scenario, KSV, would reflect the number of linkers/sites the exciton visits within its lifetime. Hupp and co-workers investigated the efficiency and maximum length of singlet exciton migration within F-MOF and DA-MOF by quenching experiments. As described earlier (Section 6.3.1), these MOFs consist of pyridineappended porphyrins pillaring the TCPB(Zn) layers.85 The porphyrin linkers are designed with varying conjugation lengths by which the pyridyl moiety is appended. For the F-MOF, the pyridine is directly attached to the porphyrin at its (5,15) meso-positions. In DA-MOF the attachment is elaborated via ethyne bonds to increase the conjugation. This kind of functionalization strategy breaks the electronic symmetry with the extended conjugation along the x-axis (Figure 6.16a).67 For the emission quenching experiment, pyridylferrocene (Py-Fc) was used as a quencher, decorated post-synthetically by coordinating the zinc center at the porphyrin (Figure 6.16c). The amplified fluorescence quenching experiment (Figure 6.16b) and the corresponding KSV indicate that a small amount of ferrocene is sufficient to drastically reduce the emission due to an efficient exciton migration. For DA-MOF, the exciton was estimated to visit B90 chromophore sites within its lifetime by undergoing 2000 hops. In contrast, for the F-MOF that consists of symmetric porphyrin, the exciton can only visit 6 porphyrin sites by making only 8 hops. This difference stems from the improved lowest-energy transition oscillator strength for the porphyrin in DA-MOF. A higher oscillator-strength means an enhanced Y and larger electronic coupling (due to enlarged transition dipole) for the ethyne-conjugated DA-porphyrin. Using various crystallographic positions and inter-porphyrin distances, the energy migration anisotropy can be determined based on the strongest J. In DA-MOF, exciton hopping is most probable along the AB direction (Figure 6.16a; 55% of total hops) to migrateB38 nm; whereas the exciton can travel the longest distance

210

Figure 6.16

Chapter 6

(a) Synthetic routes of the isostructural DA-MOF and F-MOF. The labeling A to E on linkers marks out the four nearest neighbors B, C, D, and E around A and indicate the anisotropic exciton-hopping direction. (b) Schematic representation of the exciton migration and quenching processes. (c) The scheme of quenching binding with the porphyrin linker. Reproduced from ref. 67 with permission from American Chemical Society, Copyright 2013.

(B58 nm) along the AE direction, but with B21% probability. In contrast, the F-MOF shows only ca. 3.5 nm of exciton displacement. The substantial distances and sizable anisotropy of singlet exciton migration revealed by this work show the importance of MOF composites as an antenna-type lightharvesting system. Two chemically identical but topologically distinct Zr-based PorMOFs, NU-902 and MOF-525 in both freebase and Zinc metalated versions, were studied by Deria and co-workers to investigate their topology-dependent photophysical properties.44 The co-facial alignment of the linkers defines the porphyrin–porphyrin torsional angle of 901 and 601, with the center-tocenter interchromophoric distance of 13.5 Å and 10.5 Å for MOF-525 and NU-902, respectively (Figure 6.17a). The closer packing of porphyrins is anticipated to boost J, which is verified by the steady-state emission spectra showing a greater extent of red-shifted Q band emission from NU-902 compared to that of MOF-525. Furthermore, the time-resolved fluorescence decay profiles indicate a significantly shorter lifetime for the MOF samples (i.e. 5.6 ns for MOF-525, 4.6 ns for NU-902) than H4TCPP(H2) solution (9 ns; Figure 6.17c and d). Such a difference is derived from the diminished electronic transition energy gaps caused by the interchromophoric interaction. In contrast, NU-902(Zn) and MOF-525(Zn) show a greater extent of shortened emission lifetime than their freebase variants (Figure 6.17c and d) due to a better inter-porphyrin interaction among the planar TCPP(Zn)

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Figure 6.17

(a) DFT optimized structures of closely positioned porphyrin dimers in MOF-525 (left) and NU-902 (right). (b) Schematic presentation of the installation of Fc-COO onto the Zr–oxo node via SALI; Transient emission decay profile for (c) free-base and (d) zinc(II)- metalated NU-902 and MOF-525. Reproduced from ref. 44 with permission from the Royal Society of Chemistry.

Figure 6.18

Stern–Volmer plot of (a) TCPP and (b) PCN-223 in the pH range of 5.5 to 8.5. Reproduced from ref. 86 with permission from the Royal Society of Chemistry.

compared to the TCPP(H2). Amplified emission quenching experiments with ferrocene installed samples (Fc-COOH at 8-c Zr–oxo node via SALI; Figure 6.17b) indicate that the exciton travels about 170 and 114 chromophores in the free base NU-902(H2) and NU-902 (Zn), respectively, and the estimated hopping rate (khop) is 1.541013 s1 and 7.691013 s1 accordingly. This example shows that by tuning the topology, interchromophoric interaction and the EnT efficiency can be modulated.

212

Chapter 6

Morris and co-workers studied the EnT in another Zr-based PorMOF, PCN223 by systematically protonating a portion of pyrrole nitrogen on TCPP(H2) as an internal quencher.86 The electronic structure of the porphyrin gets significantly perturbed upon protonating the N-bases, which changes the redox potential and promotes the non-radiative decay pathway. Here, the loading of the quencher can be tuned by varying the pH of the solution. The overall khop in PCN-223 is calculated from the SV analysis, (Figure 6.18a and b) being 1.91012 s1. In the above three cases, we discussed the dynamics of exciton migration among highly symmetric TCPP-chromophores assembled in different PorMOFs and found that depending on the interchromophoric distance, a low-to-moderate J (B5104  4103 eV1) can be expected due to the low oscillator strength of Q-band derived transition of TCPP ( f ¼ 0.0287). A strategy that adopts electronically less symmetric porphyrin linkers like the one in DA-MOF ( f ¼ 0.21),85 can provide a stronger J. Likewise, the Y depends on the transition–dipole moment of the lowest energy excitation. Therefore, both the porphyrin electronic symmetry as well as MOF structural symmetry (i.e. topology) play a pivotal role in achieving the desired exciton migration efficiency. Apart from the EnT among single-type linkers, PorMOFs with complementary linkers possessing appropriate electronic properties have been studied for strut-to-strut EnT. In 2010, Hupp’s group, for the first time, reported FRET-type EnT in ZnII–TCPP(Zn) paddlewheel BOP MOF consisting of a dipyridine–bodipy pillar (Figure 6.19a).88 Given the significant Y between the porphyrin absorption and bodipy emission, and a good electronic coupling expected for their proximal arrangement within the framework, efficient FRET is observed from excited bodipy to porphyrin linkers as BOP shows porphyrin-based emission at 650–710 nm upon bodipy excitation at

Figure 6.19

(a) Crystal structure of BOP MOF and scheme of EnT and emission. (b) Emission spectra of BOB and BOP MOFs. Spectra were obtained by excitation at 520 nm. Reproduced from ref. 88 with permission from American Chemical Society, Copyright 2011.

Light Harvesting in Porphyrin-based Metal–Organic Frameworks

Figure 6.20

213

(a) Scheme showing EnT between two coupled chromophores in a green fluorescent protein variant (EGFP) and Cytochrome b562. (b) Spectral overlap of donor emission (solid green) and acceptor absorption (dashed red), which is the prerequisite for FRET. (c) Coordinative and (d) non-coordinative immobilization of HBI into PorMOF. Reproduced from ref. 89 with permission from American Chemical Society, Copyright 2016.

520 nm, without any bodipy emission detected at 560–615 nm (as seen in control non-porphyrinic BOB MOF with tetratopic TCPB (Figure 6.19b). Shustova and co-workers successfully mimicked the functionality of an EGFP-cyt b562-based efficient EnT process by incorporating 4-hydroxybenzylidene imidazolinone (HBI) based chromophores into PorMOF through a non-covalent and covalent approach.89 HBI is known to be emissive in site-isolated encapsulation within the b-barrel protein, and non-emissive outside (in an aggregated form). The HBI chromophores were highly emissive within the rigid MOF scaffold that provides the required siteisolated environment for the incorporated HBI. Furthermore, highly efficient FRET was achieved between the HBI moiety, both in non-covalent or covalent variants, and the porphyrin linkers (Figure 6.20c and d). Synthetic ease of the HBI derivatives needed for non-covalent incorporation was exploited to modulate the electronic structure of the HBI core to achieve a high Y with the porphyrin linker (serving as acceptor), resulting in an impressive 72% EnT from the HBI to the porphyrin.

6.4.1.2

Triplet-to-triplet Energy Transfer

The triplet-to-triplet energy transfer (TTET) occurs in a ‘‘spin allowed’’ Dexter electron exchange mechanism, which involves two electrons transfer commencing simultaneously with a different spin (a-a, b-b), which arises from exchanging electrons of the same spin but different energies.90 For this, TTET requires proximity between D and A species (i.e. RDA of the order of 10–15 Å is expected), and the rate constant should be within the range of

214

Chapter 6

PCT in MOF (or slower). Palladium-metalated porphyrins (Por(Pd)) are common choices for excited-state studies in triplet manifold as it provides high triplet population through an efficient ISC owing to a strong spin–orbit coupling induced by Pd. Howard and co-workers fabricated a SURMOF (surface-anchored MOF) film of ditopic 5,15-bis(4-carboxyphenyl)-10,20-diphenylporphyrinato palladium(II) (BCDP(Pd)) linker with 4-c Zn-paddlewheel SBU.91 Structural characterizations reveal that the planar Zn–BCDP(Pd) sheets are stacked with an interlayer distance of 6 Å, and the length of a unit cell within a sheet is 25 Å (Figure 6.21a). Triplet excited state dynamics of this MOF were probed by nanosecond transient absorption spectroscopy (ns-TA; Figure 6.21b), which shows faster decay kinetics with the increase of the excitation fluence. This observation indicates that a rapid migration of triplet excitons and triplet– triplet annihilation (TTA) are dominating processes than a slower monomolecular (T1-S0) decay. The TTA rate can be mathematically expressed as below:   dT ðtÞ gðtÞ ¼ T ðtÞ2 (6:3) dt where g(t) is the TTA rate constant; and T is the concentration of triplet exciton that can be determined from the ns-TA kinetics with the knowledge of T0. Figure 6.21c displays the plot of g(t) relative to the delay time (in log scale); the plot indicates a similar value of TTA rate under high but different

Figure 6.21

(a) Schematic drawing of the Pd–porphyrin-based linker, Zn paddlewheel, and the assembled MOF sheets on the surface of glass substrate. (b) Normalized TA kinetic traces for different excitation fluences; the monomolecular lifetime is shown for reference. In panel (c), the scattering dots are the TTA rate coefficient g(t) for the highest two excitations plotted against time. The solid line represents the 1D diffusion model. Reproduced from ref. 91 with permission from American Chemical Society, Copyright 2019.

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excitation fluence. This corroborates the assumption that TTA is a faster process than monomolecular decay. The monotonic decay of g(t) over time is consistent with the character for a 1D exciton diffusion model. In contrast, the 3D exciton diffusion model does not fit into the collected kinetics result. Therefore, it is speculated that triplet excitons primarily transport through different porphyrin layers rather than diffusing within the same porphyrin layer. The diffusion coefficient D of triplet excitons is calculated by eqn (6.4): 1 gð t Þ ¼ an

rffiffiffiffiffiffi 8D 1 ¼ g0 pffiffi pt t

(6:4)

where a is the exciton transport distance between chromophores (6 Å), and n is the concentration of chromophores within the MOF, which can be calculated from the unit cell dimensions. The extracted diffusion coefficient, D ¼ 2.7104 cm2 s1, and the khop (¼D/a2) was found to be 81010 s1. Note that the TTET rate in Zn–BCDP(Pd) SURMOF is more than an order of ¨rster SSET rate in DA-MOF (1012 s1) and magnitude slower than the Fo 13 1 TCPP-based MOFs (10 s ). Regarded as an electron exchange process, the rate of TTET can be generally expressed by the semiclassical Marcus theory:92 2p j VTTET j2 kTTET ¼ h

rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi  l   1 e 4kB T 4pkB Tl

(6:5)

where VTTET is the electronic coupling matrix element between D and A (here BCDP(Pd)), and l is the reorganization energy associated with the geometric change of D and A during TTET. VTTET is essentially governed by the electronic wavefunction of D and A and can be expected to exponentially decay with distance. However, the contribution of the electron density overlap and virtual CT intermediates which are not considered in the Dexter mechanism should not be neglected.90,93,94 The estimated l of the Zn-BCDP(Pd) is 0.256 eV, and when plugged into eqn (6.5) gives a kTTET value of 1.011010 s1, comparable to the experimental result. TTET coupled with TTA was exploited for the upconversion (UC) process within the MOFs. H.-C. Zhou and co-workers reported a Zr–oxo-based mixed ligand MOF, comprising of TCPP(Pd), a triplet sensitizer and 4,4 0 -(9,10-anthracenediyl)dibenzoic acid (DCDPA) serving as an annihilator (Figure 6.22a and b).95 The excited T1 state of TCPP(Pd) delivers its energy to populate the T1 state of DCDPA (Figure 6.22c). By virtue of the framework positioning, two adjacent 3DCDPA undergo a TTA-UC process generating a singlet 1DCDPA, which then relaxes to the ground state by emitting a relatively higher energy photon (UC). This MOF-based TTA-UC system shows a triplet exciton diffusion constant of 7.7106 cm2 s1 and a diffusion length of 1.6 mm.

216

Chapter 6

Figure 6.22

6.4.2

(a) Chemical structure of MOF linkers and metalloporphyrin dopant. (b) Schematic illustration of the distribution of the sensitizer and annihilator in a pillar-layered TTA-UC MOF system. (c) Proposed energy scheme for the TTA-UC process. Reproduced from ref. 95 with permission American Chemical Society, Copyright 2018.

Photo-induced Charge Transfer

Since the chromophoric struts are fixed in a periodic arrangement in high concentration, their excited states can be spatially dispersed, and shared by multiple chromophores. This delocalized excited state defines molecular excitons. Subsequently, when the exciton is delivered to a site that contains a complementary redox species, a photo-induced charge (i.e. electrons or holes) transfer can be initiated. These complementary species can be chromophores,76 or redox species like metal SBUs,96–98 or complexes25,77 with appropriate energetics to form a charge separation (CS) state upon PCT. The PCT processes in MOFs are shown in Scheme 6.6 as a function of the relative energy of the components. The relative energetic alignment of the respective ground and excited-state redox potentials determine the driving force for the PCT process. In scenario (a), the excited-state energy level of D D* (i.e. Eox ) lies higher than the LUMO (or LUCO for MOF), which provides the driving force for PCT from D* to A (i.e. when the ionization potential (ID*) of the excited D* is lower than the sum up of electron affinity of A and the Coulomb energy of the separated radicals). In scenario (b), A is excited and the

Light Harvesting in Porphyrin-based Metal–Organic Frameworks

Scheme 6.6

217

Energy diagram showing PCT processes between D and A depending on the relative energetics between D and A. For simplicity, the optical and electrochemical bandgaps are assumed to be the same (which may be different in a given system). ET is the abbreviation of electron transfer.

LUMO (LUCO for MOF) of D is still higher than that of A. This energy gap alignment restricts the excited electron of A* being transferred to D, and instead, the electrons located in the HOMO/HOCO (highest occupied crystal orbital) of D will be delivered to that of A* (reducing the HOMO/HOCO). As a consequence, the hole-polaron in the excited acceptor is transferred to the donor at the ground state. Both scenarios electron transfer involving LUMO/ LUCO (lowest unoccupied crystal orbital) and hole transfer involving HOMO/ HOCO – can be generally described as PCT processes. PCT among various entities within MOFs can be analyzed using Marcus theory – much like what we commonly see for various homogeneous systems99 or electrochemical reactions on solid electrodes.100–103 The kCT is expressed as the function of the free energy DG0 and reorganization energy: " # 2 2p 2 1 ðlt þ DG0 Þ H pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi exp  kCT ¼ (6:6) h DA 4plt kb T  4plt kb T where HDA represents the electronic coupling between the reactants and products, DG0 is the thermodynamic driving force, and lt is the total reorganization energy. The lt term is the sum of internal reorganization energy (li) and solvent oscillators (ls). The li stems from the oscillators within the redox species and can be determined by the vibrational energy of all the molecules in reactants (R) and products (P), whereas the ls is calculated by the dielectric continuum model of the solvent. Despite the fact that the major factor defining the kCT is usually either the HAD or DG* at a given temperature, the rigid and ordered structure defines the unique feature of PCT in MOFs. The relative positioning of the D–A species contributes to the HAD (pe(a(r–r0)); where r0 is the D and A contact distance, and a is the distance and dielectric-dependent decay factor of the tunneling electron wave-functions. In MOFs, especially those with microporous structures, various factors such as the structural rigidity, the unique solvent

218

Figure 6.23

Chapter 6

(a) Potential energy diagrams of reactant (R) and product (P) for endergonic reaction. Potential energy surface for the R and P in (b) nonadiabatic ET with insignificant and (c) adiabatic with strong electronic coupling (HDA). Reproduced from ref. 37 with permission from the Royal Society of Chemistry.

orientation (relative to the bulk) and the micro/local environment can play a critical role in defining the lt that may be unique and difficult to achieve otherwise, especially in homogeneous systems, and only found in elegant biological design. Figure 6.23 depicts the potential energy diagram as a function of nuclear coordinate in an endergonic reaction. A CT process can occur at the intersection of the potential energy surface where the overlap of wavefunctions of R and P reaches maximum. The activation energy DG* required to pass this intersection is mathematically related to DG0: DG* ¼ (lt þ DG0)2/4lt

(6.7)

Eqn (6.7) explains a quadratic relation of the rate with the driving force. The highest rate is obtained when the driving force is the same as the reorganization energy. DG1 can be calculated by using the Rehm–Weller equation;104,105 and these values are often used to guide the design for the feasibility of PCT between the D and A species. A 0 DG0CT ¼ e(ED ox  Ered)  DE0,0  DG (E)

(6.8)

A Here ED ox and Ered represent the ground state oxidation and reduction potential of D and A respectively; and E0,0 is the first excited-state energy of D. The DG0(E) is the dielectric correction term based on the Born eqn (6.4):105    e2 e2 1 1 1 1 0 DG ðEÞ ¼ þ   (6:9) Eref ES 4pE0 ES RDA 8pE0 rD rA

where e0 is the permittivity of free space, es is the static dielectric constant of the solvent, and eref is the dielectric constant of the solvent where the redox potentials were measured. The first term of eqn (6.4) reflects the interaction energy between the D–A radical ions with the radius rD and rA at a center-tocenter distance of RDA, where the second term corrects the difference of ion solvation between a solvent of interest and the solvent where the redox potential was measured.

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6.4.2.1

219

Classic Porphyrin–Fullerene Dyad in PorMOFs

PCT in PorMOFs can be initiated from the excited porphyrin-based linkers (or the linker assembly) within an intrinsic structural component (e.g. secondary linker, metallic building blocks), or a guest component that was postsynthetically installed or infiltrated. The fullerene–porphyrin dyad has been a well-established PCT system in homogeneous macromolecular constructs, in PorMOF this can be developed as an LH mimic owing to the more precise periodic organization, either by using a fullerene-derived strut or incorporating within the pore channels (dispersive force).106,107 Based on their relative redox potentials, fullerenes within the PorMOF pores can serve both purposes, PCT component (electron acceptor) and electron-transport channel (especially highly loaded 1D channels). Heinke and co-workers found that fullerene infiltrated PorMOF can display two orders of magnitude improvement in charge conductivity upon 455 nm light irradiation: here the MOF was constructed by interconnecting 5,15-bis-(3,4,5-trimethoxyphenyl)10,20-bis-(4-carboxyphenyl) porphyrinato zinc(II) (TMPCP(Zn)) with zinc based node.108 A thin film (50 nm) of a fullerene embedded Zn–TMPCP(Zn) MOF sample (Figure 6.24a) shows a conductivity of 1.3107 S m1 under 455 nm irradiation – a huge improvement compared to its dark conductivity (1.51011 S m1). The linear correlation between photocurrent and the intensity of incident light indicated a single photon process (Figure 6.24b). According to the computational results, the authors find that the electronic coupling between neighboring porphyrin HOMO is one order of magnitude larger (6.20103 eV) than for the other intermolecular pairs in this MOF. This suggests that the porphyrin frameworks may serve as a suitable holetransport medium along the z-axis, where the closest distance between the linkers molecules is 2.27 Å. Therefore, this work shows that CT between porphyrins and fullerenes provides a high concentration of charge carriers.

Figure 6.24

(a) The structure of fullerene@PorMOF-1. (b) The photocurrent of fullerene@PorMOF-1 at different intensities of the 455 nm light irradiation. Reproduced from ref. 108 with permission from John Wiley & Sons, Copyright r 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

220

Figure 6.25

Chapter 6

(a) Relative position between BPCF and TCPP(Zn) organized in the framework. (b) Difference density for the charge-transfer excitation in the BPCF–TCPP(Zn) molecular dyad. (c) Band structure of BPCF– TCPP(Zn)-based MOF. (d) Normalized diffuse reflectance of BPCF (dashed line) and emission of the BPCF–TCPP(Zn)-based MOF (solid line). Reproduced from ref. 109 with permission from John Wiley & Sons, Copyright r 2016 The Authors.

A different approach was adopted by the Shustova group, where they assembled a fullerene-based strut, bis(pyridin-4-ylmethyl)-3 0 H-cyclopropa[1,2](C60-Ih)[5,6]fullerene-3 0 ,3 0 -dicarboxylate (BPCF), as a pillar to the zincpaddlewheel layers of TCPP(Zn) (Figure 6.25a).109 The MOF can be defined as slipped-layers of TCPP(Zn), where BPFC links one zinc(II) from TCPP(Zn) and the other from a zinc-paddlewheel node (from the neighboring layer). Band-structure calculations revealed flat bands near the Fermi level with moderate dispersion (Figure 6.25c), which demonstrates the localized character of the frontier orbitals. Considering a significant spectral overlap between TCPP(Zn) and fullerene, a porphyrin-fullerene EnT scheme was ¨rster critical radius (R0, the distance at which 50% of exconsidered: the Fo cited donors are deactivated by FRET) between TCPPZn and BPCF was estimated to be 18.8 Å, which is longer than the D–A distance determined from the structure. Based on the fluorescence lifetime quenching of TCPP(Zn) (i.e. from 1.07 to 0.54 ns), the energy-transfer efficiency and rate constants were estimated to be 49.5% and 9.18108 s1, respectively. However, based on the relative ground- and excited-state redox potentials, a PCT scheme was also considered to be a possibility. Theoretical work by the Cramer group studied PCT between fullerene and two porphyrin-based pillar-paddlewheel MOFs: DA-MOF and F-MOF.110 Here, the fullerenes were infiltrated within the MOF pores via a non-covalent fashion. The calculated band gap of fullerene@DA-MOF (Figure 6.26c, lower panels) and fullerene@F-MOF (Figure 6.26d, lower panels)

Light Harvesting in Porphyrin-based Metal–Organic Frameworks

Figure 6.26

221

Optimized crystal structures of (a) fullerene@DA-MOF and (b) fullerene@F-MOF; Calculated density of states for (c) DA-MOF and (d) F-MOF before (upper panel) and after (lower panel) fullerene incorporation. The energy is relative to the Fermi level, and the y axis for the s- and p-orbitals is scaled by a factor of 1.75. Reproduced from ref. 110 with permission from American Chemical Society, Copyright 2020.

lie 0.16 and 0.36 eV lower than their respective pristine DA-MOF (1.66 eV) and F-MOF (2.04 eV). Partial density of states (PDOS) of the fullerene@PorMOF-2 composite reveals that the HOCO is mainly comprised of p-orbitals from the porphyrins and the LUCO predominantly consists of p-orbitals from fullerene. The significant orbital overlap between fullerene and porphyrins indicates that PCT can be facilitated at the porphyrin– fullerene interfacial region. This study theoretically demonstrated that the strategy of incorporation of electronic complementary moieties into MOF channels, even without covalent bonding to the framework, can alter the electronic properties of MOFs leading to PCT. These experimental and theoretical studies highlight that the incorporation of fullerene into PorMOFs defines a synthetically accessible dyad that can be extended along the MOF channels or backbone. The long-range organization of such dyads may facilitate charge transport media for the PCT generated charge carriers and electronic interactions.

6.4.2.2

Photoinduced Ligand-to-metal Charge Transfer (LMCT) in PorMOFs

Upon light irradiation, electronic transition corresponding to LMCT from populated excited states of porphyrins to adjacent metal SBU clusters can be

222

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observed, given the prerequisites of energy-levels alignment and geometries between D and A species are met. Titanium dioxide has been explored as a promising photocatalyst due to a low-lying TiIV–TiIII valence exchange band featuring a UV responsive bandgap. Researchers have pursued to integrate this unique semiconducting species into MOFs together with porphyrin linkers. The anticipated LMCT can realize the TiIV–TiIII valence exchange that can participate in photocatalysis.96–98,111 A typical example of Ti-based PorMOF named PCN-22 was reported by the Zhou group in 2015.112 The MOF was prepared by the reaction of a pre-synthesized Ti6O6(OiPr)6(abz)6 (abz ¼ 4-aminobenzoate) cluster, H4TCPP(H2), and benzoic acid under solvothermal conditions (Figure 6.27a). PCN-22 shows a catalytic activity in alcohol oxidation. The proposed mechanism speculated that the PCT generates TCPP(H2)1 after photo-reducing the TiIV SBU (Figure 6.27b). However, no spectral evidence was available to follow the dynamics of events presumed in this mechanism. Park et al. have synthesized a Ti-based PorMOF (DGIST-1) by using a different Ti6O6(OiPr)6(t-BA)6 cluster; and upon reacting it with H4TCPP(H2) they obtained a different crystal (Figure 6.28a and b).113 Electron paramagnetic resonance (EPR) spectroscopic measurement captured a signal with a g ¼ 1.971 when DGIST-1 is irradiated by visible light (400–800 nm; Figure 6.28c). This paramagnetic signal indicates the TiIII species in a distorted octahedral oxygen ligand field, stemming from the LMCT from TCPP(H2)* to TiIV SBU. The photo-reduced Ti-SBU can produce O2 species, which can be utilized for chemical transformations (e.g. alcohol oxidation). There are a few examples reporting PCT from porphyrin* to metal SBUs other than Ti in PorMOFs, which lead to CS applicable to drive redox reactions. A few examples include the Ru-based SBU reported by Lin et al.43 and Ni SBU by Zhou and co-workers.42 These are further discussed in Section 6.5.1.

Figure 6.27

(a) The structure of PCN-22 along the a-axis. (b) Energy diagram of PCT from TCPP(H2)* to Ti7O6 clusters. Reproduced from ref. 112 with permission from the Royal Chemical Society.

Light Harvesting in Porphyrin-based Metal–Organic Frameworks

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Figure 6.28

Crystal structure viewing of DGIST-1 along the (a) b-axis and (b) c-axis; (c) EPR spectra of DGIST-1 in the dark (black) and after visible light (400–800 nm) irradiation (blue). Reproduced from ref. 113 with permission from John Wiley & Sons, Copyright r 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Figure 6.29

(a) Scheme of photoinduced electron/energy transfer in a typical TPP(Zn)@NU-1000 structure, depicting the energy and charge transfer between TPPZn and pyrene under the excitation of 403 and 507 nm. (b) Transient emission decay profiles of NU-1000, TPP(Zn), and TPP(Zn)@NU-1000 in two solvents (lex ¼ 403 nm, lprobe ¼ 660 nm). (c) Representative fs-TA spectra of TPP(Zn)@NU-1000 under the excitation of 400 nm. Reproduced from ref. 76 with permission from American Chemical Society, Copyright 2019.

6.4.2.3

PCT Involving Non-framework Porphyrin as Complementary D–A Pair

The great flexibility of post-synthetic functionalization enables the installation of species other than fullerene; such as complexes and chromophores into MOFs for PCT study. In an attempt to emulate the functionality of the antenna behavior in the natural LHs, Deria and co-workers post-synthetically

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anchored tetraphenylporphyrinatozinc(II) (TPP(Zn)) derived complementary pigment within the pores of a pyrene-based NU-1000 MOF. The TPP(Zn) moiety was attached systematically at the ZrIV–oxo nodes via SALI to define a densely arranged D–A system in a periodic fashion (Figure 6.29a).76 This design was based on the appropriate alignment of the ground and excitedstate redox potentials of donor and acceptor such that it facilitates an EnT from the excited MOF (i.e. NU-1000*) to TPP(Zn) forming [TPP(Zn)]*. The excited TBAPy linker-assembly in the framework serves as the antenna to harvest photonic energy and efficiently deliver it to the adjacent TPPZn anchored on the Zr–oxo node via FRET; kEnTE4.71011 s1 (tEnT ¼ 2 ps; Figure 6.29b).76 Given that the excited-state oxidation potential of [TPP(Zn)]* lies 0.54 eV higher than the ground-state reduction potential of NU-1000, a spontaneous CT was observed from [TPP(Zn)]* to NU-1000 with a CT rate kCT ¼ 1.21010 s1. Emission quenching experiments collected for the TPPZn@NU-1000 relative to the respective pristine components (Figure 6.29c) revealed a solvent polarity dependent rate: kCT ¼ 6.2108 s1 and 1.21010 s1 (i.e. tCTB1.6 ns and 80 ps) in MeTHF and CF3Tol solvents, respectively. The femtosecond transient absorption (fs-TA) spectroscopic data collected for TPP(Zn)@NU-1000 (in CF3Tol solvent) clearly highlights the broad NIR transient at 976 nm evinced for [TPP(Zn)] 1 species as the PCT intermediate. The global fitting of the transient dynamical data probed at 541, 693, and 976 nm revealed a 283 ps charge recombination time constant with a 80 ps charge separation and a 1.47 ns S1-S0 emission lifetime. The photocurrent observed in the photoelectrochemical measurement demonstrated that the PCT-generated charge-carriers in TPP(Zn)@NU-1000 can migrate within the framework to be collected at the electrode, and therefore such a system holds promise for further photoelectrochemical developments. Alternatively, a redox quencher can be installed in a PorMOF. Deria et al. immobilized carboxy-appended Fc, Fc-COO, onto the ZrIV–oxo node of NU-902 and studied the EnT process via the amplified fluorescence quenching involving a PCT described in Scheme 6.6b. Here, a hole transfer process can be postulated from the excited TCPP(M) to Fc (HOMO), entailing an amplified quenching that enabled determination of the kEnT (see Section 6.4.1); with the relevant PCT timescales estimated to be faster than 1 ns. It is important to note that unlike the TiIV–oxo based PorMOFs, the ZrIV–oxo clusters are not involved in LMCT from porphyrin*. The optoelectronic ‘‘inertness’’ of the ZrIV–oxo nodes, in the UV–Vis region, is stemming from a much wider bandgap relative to most p-conjugated organic linkers (Figure 6.31).36 Gascon and co-workers showed that in NH2-UiO-66(Zr), ZrIV ions do not promote overlap with the organic linker, resulting in a linker-centered, short-lived excited-state.114 The EPR spectra showed the absence of any paramagnetic signals from a possible PCT product involving a ZrIII species, generated via LMCT from an excited linker (Figure 6.30). Furthermore, the electronwithdrawing ability of the ZrIV was reported by Deria and co-workers via probing the potential of the atom-centered hole using a core-level X-ray photoelectron spectroscopy (XPS) technique. The XPS data collected for various pyrene-based MOFs revealed unchanged Zr 3d5/2 and 3d3/2, and C1s core potentials of the

Light Harvesting in Porphyrin-based Metal–Organic Frameworks

225

Figure 6.30

EPR spectra of dark (black) and UV-illuminated (red) MOFs: (a) NH2–MIL-125(Ti); (b) NH2–UiO-66(Zr). Reproduced from ref. 114, https://doi.org/10.1038/srep23676, under the terms of the CC BY 4.0 license, http://creativecommons.org/licenses/ by/4.0/.

Figure 6.31

Total (black) and projected (red and blue) density of states of UiO-66 with (a) Zr and (b) Ti as SBUs. The unoccupied Zr 4d and, Ti 3d orbitals are highlighted with a yellow background. Reproduced from ref. 36 with permission from American Chemical Society, Copyright 2018. Reproduced from ref. 150 with permission from American Chemical Society, Copyright 2018.

MOF-incorporated units relative to the Zr6IV–oxo–benzoate clusters (ZrO-BA) and protonated free linker (solid form), respectively (Figure 6.32c, d).36 The XPS data highlight that the assembly of the aromatic linkers through a ZrIV–carboxylate bond will not alter the electronic energy of the Zr ions, nor the linker carbon atoms. Therefore, the Zr6IV–oxo node has no influence whatsoever, on the electronic structure of the linker (and their assembly). Besides chromophores and complexes, external redox species like metal nanoparticles or polyoxometalates can be immobilized into MOFs as an electron acceptor from porphyrin*. The photo-generated electrons subsequently can take part in a chemical transformation like water splitting reactions which are discussed in Section 6.5.1.

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Figure 6.32

6.4.3

(a) Structure of ZrO–BA; note the coordination of one propanol (or its deprotonated anion) in lieu of a m2Z2 benzoate binding. (b) PXRD of the synthesized ZrO–BA cluster (red); simulated PXRD pattern for the panel a cluster (blue). XPS spectra for (c) zirconium 3d5/2 and 3d3/2 and (d) carbon 1s in NU-901 (blue), NU-1000 (red) MOFs compared to the ZrO–BA and H4TBAPy benchmark samples (black). Reproduced from ref. 36 with permission from American Chemical Society, Copyright 2018.

Charge Transfer in the Ground State

In a natural LH system, the harvested photonic energy delivered to RC drives PCT, the generated charge-carriers then enter sequential charge-carrier chains driven by primed downhill potential force. With the efficient PCT in MOF (tCT ranging from a few tens to a few hundreds of ps), it is important to study how efficiently these charges (or at least the residual charges after utilizing one in a photo-redox process) can be transported to the distal catalytic site for chemical transformation (or to the external electrical contact for collection). From a minimalist perspective, understanding dark-state charge-carrier migration within MOFs can provide a lot of key information. This is because the criteria for PCT and photo-induced charge-carrier migration (aka photoconductivity) are somewhat orthogonal, e.g. improving the driving force for a PCT may mean acceptor localized charge carriers. Thus, sophisticated design parameters need to be considered for such developments. In contrast, studying the electrodeinjected charge carrier migration can be relatively straightforward.

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227

As we have seen in Section 6.3, PorMOFs are exclusively constructed by metal–carboxy or metal–pyridyl based nodes. Electronically, such connectivity is defined by mismatched energy levels and poor orbital overlap amongst the relevant frontier orbitals of the porphyrin linkers and SBUs.115 Thus, the charge carriers can be expected to be localized either in porphyrin or the SBUs, depending on their relative potentials, which makes PorMOFs drive electron transport through a charge hopping mechanism. Morris et al. studied the redox hopping process in PIZA-1 (or CoPIZA), a PorMOF constructed with TCPP(Co) and CoIII SBUs (see Figure 6.11).116 The measured conductivity, determined by the reciprocal of the area, and thickness corrected low-frequency intercept of the Nyquist plot, was 3.62108 S cm1 (possibly at a biased potential corresponding to CoIII/II event). The cyclic voltammetric (CV) profile of this MOF displays two reversible cathodic peaks at 1.1 and 1.45 V, corresponding to TCPP(CoIII/II) and TCPP(CoII/I) in PIZA-1 (Figure 6.33c). Since the electrochemical reductions of the CoIII–SBU occur at a comparable potential (0.87 V for CoIII/II and 1.39 V for CoII/I) as that of the TCPP(Co) linkers, it would be difficult to rule out if the Co–SBUs participate to augment the redox hopping/tunneling. The non-zero, scan-rate-dependent redox pair separation (DEp) at the scan rate of 10–1000 mV s1 indicates a diffusionlimited (the charge transfer rate is in the order of the scan rate) charge transport kinetics. Thus, the charge hopping rate can be represented by the apparent diffusion coefficient (Dhop) calculated to be 7.551014 cm2 s1, which is the diffusion of charge coupled with the counterions (from electrolyte). Nevertheless, it must be at the highest limiting range where slow charge hopping kinetics leads to semi-conductivity. To investigate the correlation between MOF topology and CT rate, meanwhile eliminating the influence from LMCT, Deria et al. examined the

Figure 6.33

(a) Co clusters (up) and TCPP(Co) linker (down); (b) CV of CoPIZA in 0.1 M LiClO4/DMF at 100 mV s1. Inset shows the peak current of the reduction at 1.1 V vs ferrocyanide to be linear with the square root of the scan rate (u)1/2. Reproduced from ref. 116 with permission from American Chemical Society, Copyright 2014.

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Figure 6.34

Chapter 6

(a–d) Crystal structures and respective short hopping pathways within (a) PCN-222(Fe); (b) NU-902(Fe); (c) MOF-525(Fe); (d) PCN-225(Fe); (e–f) Cottrell plots of transient currents measured in the (e) absence and the (f) presence of MIM for TCPP(Fe)-MOF films of (A) NU-902(Fe); (B) PCN225(Fe); (C) MOF-525(Fe); (D) PCN-222(Fe); (E) MOF-525(Fe)/MIM; (F) NU-902(Fe)/MIM; (G) PCN-225(Fe)/MIM; and (H) PCN-222(Fe)/MIM. Reproduced from ref. 72 with permission from American Chemical Society, Copyright 2019.

redox hopping rate (khop) within four chemically similar but topologically different ZrIV–oxo TCPP(Fe)-MOFs, namely PCN-222(Fe), MOF-525(Fe), NU-902(Fe), and PCN-225(Fe).72 The selection of these Zr-based MOFs is based on the fact that the nodes offer a large electrochemical window due to their large bandgap (Figure 6.34), leaving the TCPP(Fe) to manifest their electronic and electrochemical behaviors based on their assembly unperturbed by the nodes. Thus, a redox hopping involving a TCPP(FeIII/II) event would depend on their distance. From the transient step currents (slope of I vs. t0.5; Figure 6.34e) measured for the MOF-films on FTO electrodes, the diffusion coefficients for the FeIII/II redox pair at ca. 0.5 V (vs. Ag/AgCl) was determined using the Cottrell equation: rffiffiffiffiffiffiffiffiffi Dhop I ðtÞ ¼ nFAC (6:10) pt where n is the number of electrons transferred (here, n ¼ 1), F is the Faraday constant (9.65104 C mol1), A is the geometric area of the electrode, and C is the concentration of the redox-active species in the film (i.e. concentration of TCPP(Fe) in a given MOF). These diffusion coefficients are for the charge carrier coupled with the counterions, where the khop can be extracted as: khop ¼

Dhop r2

(6:11)

Light Harvesting in Porphyrin-based Metal–Organic Frameworks Table 6.1

229

Summary of the hopping parameters for the TCPP(Fe)-MOFs in the absence and presence of MIM.

MOF

dFe–Fe (Å)

khop (s1)

khop (MIM; s1)

PCN-222(Fe) MOF-525(Fe) NU-902(Fe) PCN-225(Fe)

15.9 13.6 10.5 8.29

64 118 813 890

146 440 1480 1214

with r being the dFe–Fe between two adjacent TCPP(Fe) sites (Figure 6.34 a–d). The khop data (Table 6.1) suggest that center-to-center distances as a function of the framework topology can sufficiently trach the electronic coupling (HDA) to define the hopping rate: the hopping is facile for MOFs with shorter dFe–Fe and can vary ca one order of magnitude within this series of MOFs. For these TCPP(Fe) frameworks, altering the central metal spin-state was found to tune the reorganization energy of the self-exchange process in TCPP(FeIII/II). A significant increase in the hopping rate was observed upon the axial coordination of 1-methyl imidazole (MIM), which transforms a halide-bound high-spin (HS) TCPP(FeIII/II) to a six-coordinate low-spin (LS) complex.117 From the electrochemical and resonance Raman data, it was found that pore geometry dictates the steric demand to accommodate two MIM-molecules as axial ligands and hence, the relative population of LS and HS species. This indicates that for the smaller pore MOFs, there is a significant population of the TCPP(Fe) centers that will be pentacoordinated by the MeIm, where a small substrate (e.g. O2 or CO) can improve the khop which will positively impact the electrocatalytic performances.

6.5 Light-harvesting of PorMOFs Porphyrin and metalloporphyrin derivatives have been established as promising photo- or electro-catalysts for energy-related transformations like CO2 reduction,118,119 and water splitting.120 PorMOFs offer such catalytic sites in high density into a heterogeneous phase, and as we saw, their electrochemical behaviors depend on their structures. While the multitude of beneficial aspects that are offered by MOF assembled metalloporphyrins have been discussed in the introductory part, the densely arranged porphyrins can facilitate improved photon absorptivity where both EnT and PCT, as well as the charge carrier transport, can be facilitated to desired sites for a battery of applications. For example, in PorMOFs that have been studied for CO2 reduction, the porphyrin linkers can serve both as light sensitizers and catalytically active species in their photo-reduced form. However, some sophisticated PorMOFs or MOFs with porphyrins can be designed to facilitate the PCT involving a porphyrin* and a complimentary unit with appropriate energetics required to drive photoreactions.

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Furthermore, the porosity in the PorMOFs should endow a high substrate (CO2) capacity and/or facilitate its diffusion making highly accessible catalysts.

6.5.1 6.5.1.1

Photocatalysis by PorMOFs CO2 Reduction

Metalloporphyrin-based homogeneous photocatalysis has been known to drive CO2 reduction since the 1990s.121,122 The early attempts of photocatalytic CO2 reduction in PorMOFs were realized much later. In 2013, Huang and co-workers reported the photocatalytic conversion of CO2 to methanol by a TCPP(Cu)-based MOF with Al31 SBU.123 Compared to the freebase variant, the Cu-metalated MOF showed enhanced CO2 binding capacity, which is confirmed by in situ Fourier-transform infrared spectroscopy (FT-IR). The photocatalytic experiment was conducted in an aqueous solution containing 1% triethylamine as an irreversible sacrificial regenerator under CO2 flow, and the reaction system is irradiated by a Xe lamp. The gas chromatography results reveal the major products as methanol with an evolution rate of 262 ppm g1 h1 in Cu metalated MOF, in contrast to 37.5 ppm g1 h1 in the freebase one. In this regard, the FeIII and CoII-metalated porphyrins are also known photocatalysts for CO2 reduction in selective CO production.124 Wang, Guo, and co-workers arranged TCPP(FeIII) and TCPP(CoII) into 3D frameworks by reticulating with In31 building block-based SBU, forming a MOF interconnected by In-SBU and TCPP (TCPP linkers are partially metalated).45 The photocatalytic CO2 conversion was conducted in the presence of L-ascorbyl palmitate (L-AP) as a sacrificial reagent. The catalytic results show that In–TCPP(Fe)1.91 (i.e. the overall Fe loading in the MOF is 1.91 wt%) and In–TCPP(Co)1.71 yield 3469 mmol g1 and 929 mmol g1 of CO in 24 h, respectively. Moreover, the In–TCPP(Fe)1.91 MOF displayed more efficient CS than the cobalt-variant, which is seen from the significantly high photocurrent measured for the In–TCPP(Fe)1.91 deposited film on the FTO electrode compared to the cobalt and freebase variants. The authors treated the MOFs as a semiconductor to elucidate the photocatalytic mechanism that involved injection of electrons to the Fe-center, where the MOF-arranged TCPP(Fe) facilitates a multielectron PCT process. To computationally investigate the CO2 reduction activity of a Co-based PorMOF, Su and Li et al. established the monolayer model of three 2D TCPP(Co)-MOFs, namely Co–TCPP(Co), Zn–TCPP(Co), and Zr–TCPP(Co), which are constructed by the planar assembly of TCPP(Co) with Co–oxo, Zn–oxo, and Zr–oxo clusters, respectively.125 The calculated PDOS suggested that these MOFs feature a bandgap of 1.21, 1.72, and 1.68 eV, respectively (Figure 6.35) where the Co 3d states of TCPP(Co) dominate the valence band (VB) of Co–TCPP(Co) and the 3d states of Co–oxo clusters significantly contribute to conduction band (CB) as well as VB. Thus, the lower bandgap

Light Harvesting in Porphyrin-based Metal–Organic Frameworks

Figure 6.35

231

PDOS of (a) TCPP(Co); (b) Co–TCPP(Co) MOF; (c) Zn–TCPP(Co) MOF; (d) Zr–TCPP(Co)MOF. Reproduced from ref. 125 with permission from American Chemical Society, Copyright 2019.

of Co–TCPP(Co) and the electronic overlap between TCPP(Co) and the CoSBU is predicted to promote PCT from TCPP* to Co–oxo SBU. The band edge positions of these MOFs lie more negative than the CO2/CH4 potential, reflecting that all three MOFs may be able to reduce CO2 to CH4. Figure 6.36 illustrates the calculated CO2 reaction pathway taking place on the photogenerated CoI center of TCPP(Co) (via PCT involving TCPP(Co)*). The reduction of TCPP(Co)-bound *CHO intermediate to *CH2O (* represents bound species on catalytic sites) is regarded as the rate-determining step with a DG of 0.39, 0.64, and 0.82 eV, corresponding to Co–TCPP(Co), Zn– TCPP(Co), and Zr–TCPP(Co), respectively. The reduction cascade from *CH2O to *1CH4 is all thermodynamically favorable, and thus, CH4 is predicted as the final reduction product. Experimentally, however, Yang and Yaghi et al. reported that the major product in TCPP(Co)-based 2D MOF is CO for an electrochemical CO2 reduction. This is probably due to the relatively weak bonding between CO and the CoI center which leads to easy desorption. This case will be further discussed in detail in Section 6.5.3. As we have learned that Fe and Co-metalated porphyrins selectively produce CO, rhodium–porphyrins, however, show the capability of driving photocatalytic CO2 reduction beyond CO due to its sizable CO binding affinity.126,127 Su et al. synthesized RhIII-metalated PCN-224 acting as a photocatalyst for the transformation of CO2 mainly to formic acid using

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Figure 6.36

Chapter 6

Gibbs free energy diagrams of CO2 reduction for (a) TCPP(Co), (b) Co–TCPP(Co)MOF, (c) Zn–TCPP(Co)MOF, and (d) Zr–TCPP(Co)MOF monolayers. Note that optimal pathways to CH4 and the ratedetermining DG are shown in red and competing pathways are shown in cyan. The asterisk (*) represents the surface-active sites for adsorption and reaction. Reproduced from ref. 125 with permission from American Chemical Society, Copyright 2019.

triethanolamine (TEOA) as a sacrificial reagent with a turnover frequency (TOF) of 0.34 h1. The strong CO binding affinity of TCPP(RhIII) led to a high product selectivity toward formic acid, possibly assisted by the proton coupling steps. The frontier orbital analysis of Zr6O8–TCPP(H2) and Zr6O8– TCPP(Rh) show characteristic localized electronic distribution on porphyrin rings which is not related to Zr–oxo clusters. And, the LUCO energy of both Zr6O8–TCPP(H2) and Zr6O8–TCPP(Rh) lies above the energy of CO2–CHxOy conversion, which indicates that the PCT-driven CO2 reduction is thermodynamically favorable. In this PCN-224(Rh) system, the authors also proposed that PCT could be conducted from porphyrin* to ZrIV clusters to generate ZrIII as a catalytic center. Another example of such LMCT can be found in the literature, for example, the Jiang group reported CO2 reaction by PCN-222(H2) as the photocatalyst, in which the ZrIII generated via PCT from TCPP* to ZrIV clusters was speculated as a catalytic center to bind and reduce CO2 molecules.128 These speculative claims bring up new debates on whether an energetically unfavorable LMCT (i.e. from porphyrin* to ZrIV SBU) is possible from a low bandgap, large aromatic chromophore core and then how exactly ZrIII–CO2 binding occurs at the ZrIV-oxo/carboxy node. Recall our discussion regarding the experimental and theoretical evidence

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233

that shows negligible electronic interaction between a wide bandgap BDC linker and Zr SBU in UiO-66. While it will be beyond the scope of this chapter, a complete characterization including the possibility of the presence of trace TiIV (a possible contaminant) in the ZrIV-source needs to be excluded before such conclusions, even though an EPR signal of paramagnetic MIII is observed (see Figure 6.30 and related text). Furthermore, Morris et al. have shown that only weak interactions exist between the adsorbed CO2 molecules and the framework of UiO-66, meaning that CO2 does not interact strongly with the Zr clusters.129 Therefore, the porphyrin linkers are more reasonable catalytically active sites than Zr. As we have discussed so far, the metalloporphyrins can drive both the PCT and CO2 reduction within PorMOFs. Alternatively, the catalytic center can be separated, where the photoinduced electrons can be delivered. Cao and co-workers synthesized a PorMOF PCN-601 bearing Ni-oxo clusters as SBU interconnecting pyrazolyl-functionalized Ni-metalated porphyrin linkers.42 Different from the traditional carboxy linkers, the pyrazolyl binding group provides more pp–dp orbital overlaps, thereby giving rise to more electronic delocalization with the Ni–oxo nodes. PCT from porphyrin* to NiII SBUs generates NiI as binding sites for CO2 reduction. EPR results collected under N2 showed that visible-light irradiation remarkably enhanced NiI signals at g>¼ 2.05 and g>¼ 2.11. With a substitution of N2 for CO2, these EPR signals disappeared, which explains that the electron at NiI is utilized for CO2 activation and then NiI gets turned back into diamagnetic NiII. However, the origin of the NiI signal, either at the Ni–oxo clusters or porphyrinato nickel center cannot be distinguished. To address this issue, DFT computation was performed. PDOS results disclosed that both linkers and Ni–oxo SBUs contribute to the LUCO of PCN-601, and the ligand solely contributes to the HOCO (Figure 6.37d). The distribution of states suggests the possibility of an LMCT from porphyrins* to the Ni–oxo clusters; such CS-state may be achieved via a PCT from locally excited porphyrin* to Ni SBUs rather than a direct excitation. Therefore, the mechanism involves an efficient PCT to NiII nodes to generate NiI (Figure 6.37a and b). This example validates the feasibility to involve a complementary redox species other than porphyrins into MOFs as both electron-acceptors and reactive sites for CO2 reduction.

6.5.1.2

Photocatalytic Water Splitting

Metal porphyrins have long been examined as light sensitizers to convert solar energy into clean fuel including H2 as well as O2 via photo-driven watersplitting reactions.130 Early attempts for such energy conversions employed colloidal Pt as catalysts for photocatalytic hydrogen evolution reactions (abbreviated as PHER to be distinguished from electrochemical HER); with methyl viologen (MV21) as an electron shuttle to deliver the PCT-generated charge to the Pt, and ethylenediaminetetraacetic acid (EDTA) as sacrificial agents to regenerate the photosensitizer.131 Development of a PorMOF-based

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Chapter 6

Figure 6.37

(a and b) Schematic illustration of PCT through a coordination sphere. (c) Reaction pathways for the products. (d) Calculated PDOS for PCN-601. Reproduced from ref. 42 with permission from American Chemical Society, Copyright 2020.

Figure 6.38

CT scheme of the reaction in the presence of PorMOF, MV21, and EDTA (left); in the presence of Pt-2DPorMOF and EDTA (right).

PHER system involved porphyrin-assemblies as a photosensitizer, where the pores were exploited to host Pt nanoparticle-based catalysts as well as to facilitate molecular diffusion. Rosseinsky et al. examined this conception in two PorMOFs prepared by interconnecting TCPP(H2) or TCPP(Zn) with Al31 SBU. Both MOFs, along with a Pt catalyst, were tested for PHER in two different photocatalytic systems (Figure 6.38).132 In one, MV21 was used as the electron shuttle that delivers the highly reducing PCT-generated electron to the Pt-NPs for driving the HER, and in the other EDTA was used as the sacrificial electron donor that photo-reduces the porphyrin (quenches the photogenerated hole), which possibly in turn transfers the charge via hopping to a nearby Pt-nanoparticle driving HER.

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Instead of employing Pt nanoparticles as the external catalytic surface, Mai and Zhou et al. exploited TCPP(Pt) based Cu2-(COO)4 paddlewheel 2D MOF (TCPP(Pt)  2DMOF) nanosheets as a catalyst for PHER.133 XPS data showed two major peaks (4f5/2 and 4f7/2) at 76.3 and 72.9 eV, respectively, indicating a mixed oxidation state of the Pt (PtII and Pt0). PHER performance of the TCPP(Pt)  2DMOF is evaluated in an aqueous solution with ascorbic acid as a sacrificial agent under visible light irradiation (l4420 nm). The TCPP(Pt)  2DMOF exhibits better PHER activity (H2 generation rate 11320 mmol g1 h1) than the freebase variant TCPP(H2)  2DMOF (2.0 mmol g1 h1), and a composite of Pt nanoparticles with freebase TCPP(H2)  2DMOF (317 mmol g1 h1). This significant efficiency difference indicates that the single platinum site at the TCPP(Pt) may be an efficient catalyst here. While various computational methods support this assumption, it remains unclear if the TCPP(Pt)-derived longlived triplet states improve the probability of PHER. The DFT-computed DOS illustrates that the incorporation of Pt contributes to the states of both VB and CB, which narrows down the bandgap compared with the freebase sample. Furthermore, the free energy of H atom adsorption (DGH*) at the TCPP(H2)  2DMOF surfaces was found to be more stable (0.47 eV) compared to that for the TCPP(Pt)  2DMOF (þ0.02 eV; Figure 6.39). Thus, strong adsorption of H* (atom) on the TCPP(H2)  2DMOF surface limits H2 release. Lin’s group reported two MOFs, Ru–TBP(H2) and Ru–TBP(Zn); built with TCPP(H2) or TCPP(Zn) and Ru2 paddlewheel SBUs. They used these for PHER via a PCT mechanism.43 The PHER performance is examined in an acetonitrile solution with water as a proton source and TEOA as a sacrificial agent, where H2 production increased linearly with time at a rate of 0.13 mmol h1 for Ru–TBP(H2) and 0.24 mmol h1 for the Ru–TBP(Zn). The higher PHER activity of Ru–TBP(Zn) is likely due to a better photosensitizing ability of the TCPP(Zn) with the Ru2-node than TCPP(H2). Based on separate homogeneous emission quenching experiments (using a soluble Ru2-tetraacetate paddlewheel cluster compound), it was ascribed that PCT involving TCPP(Zn)*-Ru2-node is a more likely pathway rather than the photoreduction of the TCPP(Zn)* by TEOA, which eventually regenerates a TCPP(Zn)1 formed upon the PCT (Figure 6.40c and d). In a separate electrochemical experiment, it was found that RuIIIRuII to RuIIRuII (at 0.08 V vs. NHE in CH3CN) precedes (Figure 6.40b) a catalytic HER peak with an onset potential of 0.47 V for the Ru-PD. A similar voltammetric response observed for Ru-TBP(Zn) suggests that the Ru2 SBUs were first photo-reduced to RuIIRuII as the active catalyst for PHER. In a followup computational work, Tavernelli, Smit, and co-workers studied this Ru-TBP(Zn)–Cl system to investigate the PCT process by TDDFT.134 The PDOS plot displays that the porphyrin species mainly contribute to the VB, and two main bands fully localized in Ru-SBUs in the CB (Figure 6.41a). The partial separation of VB and CB on porphyrin and RuIII hints a lowlying LMCT excitation from TCPP(Zn)* to Ru. The experimental UV–vis

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Figure 6.39

The top and side view for the models of H* adsorbed on (a) Freebase2DPorMOF and (b) Pt-2DPorMOF. (c) DG* on Freebase-2DPorMOF and Pt-2DPorMOF respectively. Reproduced from ref. 133 with permission from John Wiley & Sons, Copyright r 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. Reproduced permission needed from ref. 133. Copyright 2019, Wiley-VCH. 345 USD.

spectrum of Ru–TBP(Zn) reported by Lin (Figure 6.41c), along with the computed UV–vis spectrum shows a LMCT band corresponding to PCT from TCPP(Zn)* to Ru SBU.

6.5.2

Photoelectrochemical Processes in PorMOFs

The development of photocatalytic PorMOFs has been seen to rely on a highconcentration of MOF-assembled photocatalysts, where the activity was facilitated by an equivalent amount of irreversible regenerator consumption. The essentially short-lived excited states and the rapid recombination of the charge carriers seem to be major challenging factors for utilizing the photogenerated redox equivalents (i.e. charges with appropriate potential based on the ground and excited-state redox potential of the respective PCT components). To overcome this issue, the charge-carriers need to be transported either to catalytic sites or collected at the external electrical contact. Within a solid assembly, the separation of an electron–hole pair can be promoted by the electrical field that is either spontaneously formed at the interface of the photoelectrode or by an external electrical field, which directs the electrons

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Figure 6.40

237

(a) Schematic representation showing the PCT pathway in PHER; (b) CV curve of Ru-PD and Ru–TBP(Zn) in 20 mL 0.1 M tetrabutylammonium hydroxide/CH3CN solution with 500 mL H2O and 500 mL trifluoroacetic acid; Emission evolving of Me4TBP–Zn (0.1 mg mL1) after the addition of different amounts of (c) Ru-PD and (d) TEOA in 2 mL of CH3CN with 410 nm excitation. Reproduced from ref. 43 with permission from American Chemical Society, Copyright 2018.

and holes to participate in reduction and oxidation reactions before they recombine (Figure 6.42).135–138 In the classic semiconducting scenario, when the n-type semiconductor is employed as the photoelectrode, electrons and holes are formed at the surface by splitting the photo-generated excitons; the electrons are locally driven to the interior bulk and holes to the surface, which spontaneously forms a local electrical field, a phenomenon represented by the band bending (Figure 6.42a).139 Likewise, the p-type semiconductor (Figure 6.42b) forms a regional electrical field pointing toward the opposite direction to the n-semiconductor case. This principle may not be fully implanted on MOF cases, because charge transport in most of the metal–carboxylate MOFs is governed by hopping due to discrete, localized electronic states coupled with the diffusion limitation of counter ions (vide supra).140–144 For the photoelectrochemical study, MOFs can be deposited on a transparent conducting electrode such as fluorine-doped tin oxide (FTO) or indium tin oxide (ITO) to ensure electrical conductivity between the MOF–electrode interface and sufficient light transmissivity. To make the MOF-modified electrode serve as photocathode or photoanode, commonly p-type (e.g. NiO) or n-type (e.g. TiO2) large bandgap semiconducting layers are deposited as a barrier (i.e. in between the MOF layer and FTO). These semiconducting layers not only facilitate exciton splitting at their interfaces with the dye (here MOF), but also provide an augmented connection that provides charge-carrier doping based on their potential and therefore collects the electron or hole. In contrast, by switching the redox

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Figure 6.41

Chapter 6

(a) Projected spin density of states of RuTBPZn; (b) Isosurface representation of the densities of the electron (green) and hole (orange) in RuTBPZn–Cl, displaying an interacting charge-hole separation; (c) Experimental UV–vis spectra of Ru–TBP(Zn), (taken from ref. 43) compared with the calculated UV–Vis spectra of CAM-B3LYP TDDFT calculations on the cluster models for Ru-TBP(Zn) variants. The inset shows the enlarged spectra of the Q and LMCT bands. Reproduced from ref. 134 with permission from American Chemical Society, Copyright 2020.

mediator (D or A species) in the electrolyte, the MOF-based photoelectrode can be made to serve as a photoanode or photocathode accordingly. Such fabrication is different from the standard semiconducting layered electrode, because for the MOF-only system, the kinetics of PCT at the MOF–electrolyte interface dictate the polarity of the MOF-modified electrode. Hod and co-workers demonstrated that the interfacial electron transfer kinetics at the surface of porphyrin-based MOF-525(H2)/FTO photoelectrode could be tuned by introducing electron or hole-accepting species, such as water or TEOA into the electrolyte solution (Figure 6.43a and b).145 The introduction of a hole-acceptor TEOA displayed a photo-anodic response: i.e. under light illumination, a positive increment of current density was observed with a shift of the onset oxidation potential from 0.85 V to 0 V (vs. NHE; Figure 6.43c). In the case of a proton (from water) as an electron acceptor, photo-cathodic behavior was observed upon light illumination, where electrons from the MOF-525(H2)* are used to generate H2. In the latter

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Figure 6.42

Scheme of (a) n-Type and (b) p-Type semiconductor cell, depicting the formation of a space chare region, locally bending of bands under irradiation and the direction of electron flow. The interfacial electron transfer between the photoelectrode and D/Dþ or A/A species in solution are illustrated too. Reproduced from ref. 37 with permission from the Royal Society of Chemistry.

Figure 6.43

(a) The components for MOF-525 assembly and the fabrication of photoelectrode; (b) scheme illustration of the PCT in photoanode and photocathode fabricated by MOF-525, in the presence of TEOA as D or H2O as A respectively; LSV curves for MOF-525 (c) photoanode (i.e. TEOA as the donor species) and (d) photocathode (i.e. H2O as the electron acceptor) in the dark (black) and under light (red). Reproduced from ref. 145 with permission from the Royal Society of Chemistry.

case, dark and light LSV measurements exhibit an increasing current density in a negative direction (Figure 6.43d), attributed to the PCT from MOF-525(H2)* to water. Photoinduced oxidation of TEOA to TEOA 1 is a fast

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Figure 6.44

(a) Schematic structure of Cu–PPF-1. (b) Photocurrent responses from bare FTO, Cu–PPF-1, and Cu–PPF-2 films deposited on FTO with onand-off light illumination (100 mW cm2) at a bias voltage of 0 V applied to a three-electrode system. Expansion of the photo-response shown in panel (b) from (c) Cu–PPF-1 and (d) Cu–PPF-2. Reproduced from ref. 146 with permission from American Chemical Society, Copyright 2018.

one-electron process, whereas photo-induced reduction of water to H2 at the photocathode is a relatively kinetically sluggish two-electron process. Hence, the photogenerated charges spend a longer time in the photocathode by simply increasing its probability to recombine. Tian et al. studied the photoelectrochemical properties of 2D PorMOF sheets Cu–PPF-1 and Cu–PPF-2, which are built by interconnecting TCPP(H2) and TCPP(Cu) with Cu2–(COO)4 paddlewheel SBUs, respectively.146 The CV measurement conducted on a Cu–PPF-2 deposited glassy carbon electrode revealed the electrochemical bandgap (the gap between HOCO and LUCO) being 1.25 and 1.31 eV for Cu–PPF-1 and Cu–PPF-2, respectively. Upon light irradiation, similar photocurrent density (140 nA cm2) was observed in Cu–PPF-1 and Cu–PPF-2 deposited on FTO at a bias voltage of 0 V in a threeelectrode setup (Figure 6.44b), possibly due to a comparable bandgap of the two MOFs. It is noteworthy that the two MOFs exhibit different photocurrent profiles: the photocurrent produced by Cu–PPF-1 increased steadily and reached a stable stage, which has been fitted to a second-order polynomial plot (Figure 6.44c). The Cu–PPF-2 generated a transient photocurrent spike (Figure 6.44d) then underwent an exponential decay. The transient photocurrent response of Cu–PPF-2 is possibly due to the accumulation of photoinduced carriers at the interface.

6.5.3

Electrocatalysis

In PorMOF-based photocatalysis and photoelectrochemical systems, we have noticed that the overall activity and the behaviors of the key components

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during various critical steps may be unique and beneficial in such solidporous assembly while keeping some of the molecular porphyrin behaviors more-or-less intact. For example, the inherent photophysical properties like high photon-absorptivity and catalytic mechanism regarding the substrate (e.g. CO2) binding mode at the central metal site seem to be translated well in the solid phase. However, the photoelectrochemical developments hit serious roadblocks due to the mismatch between the excited states, precisely the charge-separated state, and the charge-hopping timescale to drive chemical transformations. This mismatch, particularly a sluggish charge hopping, makes PorMOF-based working composition vulnerable for energy wasting charge-recombination (i.e. PCT-derived charges do not migrate sufficiently long enough to improve their chances to drive a chemical transformation). However, the charge hopping rates (a few thousand per second) can be sufficient to drive chemical transformations (occurring in ms timescale). Thus, electrode-injected charges within the PorMOFs can be exploited as the working electrode for critical electrocatalysis. Note that the basic idea here is to separate the PCT derived CS-state; and instead, solar photovoltaic generated charges can be plugged in to drive these darkelectrochemical transformations. Yaghi, Yang, and co-workers demonstrated the capability to reduce CO2 to CO by the PorMOF comprised of TCPP(Co) and Al31-based SBUs.147 The thin film of Al–TCPP(Co) MOF was fabricated by an atomic layer deposition of metal oxide thin layer as metal precursors onto the electrode, followed by the introduction of linker solution onto the coated electrode in a microwave reactor to form MOF films. The thickness was controlled by altering the depositing cycles. The electrochemical measurements suggested that the current density as a function of the square root of the scan rate indicates a diffusion limited CoII/I redox event involving the counterions (Figure 6.45b and c). Spectro-electrochemical measurements under the operating conditions, in the 0.2 to 0.7 V range (Figure 6.45a) suggested that the

Figure 6.45

(a) The first derivative of in situ spectro-electrochemical results, revealing the oxidation state of the cobalt catalytic unit of the MOF under reaction conditions. (b) CV of Al–TCPP(Co) MOF in Ar and CO2. (c) The plot of electrochemical peak current density against the square root of the scan rate. Reproduced from ref. 147 with permission from American Chemical Society, Copyright 2015.

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TCPP(Co ) is the active catalyst, as known for the homogeneous porphyrinato-cobalt system.148 In an interesting study, the Hupp group highlighted the impact of diffusion-limited charge transport on electrocatalysis. The MOF-525(Fe) deposited FTO working electrode displayed CO2 reduction in the presence of trifluoroethanol (TFE) producing CO with a TON of 272 and an average TOF of 64 h1.149 Dihydrogen presumably derives from electrochemical reduction of residual water in the organic solvent (acetonitrile), or by abstracting a proton from the TBAPF6 electrolyte via Hofmann-type degradation. The comparison between the CV profiles of the working electrode recorded in argon and CO2 environments unambiguously showed that the catalytic current for CO2 reduction appears at the TCPP(FeI/0) redox event (Figure 6.46c), which indicates that MOF-525(Fe) exploits TCPP(Fe0) as the active catalyst. More importantly, when comparing with a homogeneous Feporphyrin catalyst, two major distinct features can be seen for MOF-525(Fe): the catalytic current is less potential-dependent (Figure 6.46b and c) and a significantly small (16 times) TOF than the homogeneous TPP(Fe). This difference is due to a rate-limiting charge diffusion within the heterogenized TCPP(Fe) in MOF. A small charge-diffusion coefficient (D) of 51013 cm2 s1 was determined from chronoamperometry, which suggests that the timescale for near-complete charge diffusion through a hundred nanometer MOF film would be in the order of a few seconds, which is much longer than the turnover of one single TPP(Fe) molecule. Therefore, the sluggish charge diffusion limits the catalytic efficiency of PorMOF.

Figure 6.46

(a) Schematic representation of MOF-525(Fe) deposited on FTO as the working electrode and the proposed electron hopping pathway. (b) CV of homogeneous TPP(Fe) in DMF, showing the comparison between N2 and CO2 atmosphere. (c) CV of MOF-525(Fe) in the presence of 1 M trifluoroethanol. Reproduced from ref. 149, https://doi.org/10.1021/acscatal.5b01767, with permission from American Chemical Society, Copyright 2015.

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6.6 Conclusion and Outlook In this chapter, we have discussed the fundamental photophysical processes, mainly in terms of energy transfer (EnT) and photoinduced charge transfer (PCT) within PorMOFs. Since porphyrin-assemblies within MOFs are reminiscent of pigment assembly seen in natural light-harvesting (LH) complexes in photosystems, we highlighted that the facile EnT and PCT within PorMOFs can be utilized for developing an artificial antenna system. Fixed within crystalline frameworks with tunable inter-porphyrin distance and orientation, MOFs offer excellent platforms to develop various photophysical and photochemical architectures. Based on amplified quenching experiments, we saw extremely facile exciton hopping (khop ¼ 1010–1013 s1) within these densely organized chromophores ensuring a long-range migration – the direction of which can be modulated and biased by tuning interchromophoric coupling. We also saw that the linkers need to be carefully crafted as its electronic symmetry becomes a key player to manifest a large transition dipole for the lowest energy transition, which impacts both J and Y for EnT in singlet manifold. This is quite important for highly symmetric porphyrin cores, and an extended conjugation at the mesoposition can be a way out. Nevertheless, it is the topology of the framework that dictates the inter-porphyrin distance and orientation – this is important for EnT in both the singlet and triplet manifold. Likewise, PCT can be realized with complementary species incorporated within the frameworks either as an inherent structural component (i.e. second linkers or SBUs) or post-synthetically installed non-essential structural entity. A PCT involving porphyrin* can be implemented by carefully choosing secondary components based on their relative ground and excitedstate redox potentials. The PCT-derived charges can be utilized in photocatalytic or photoelectrocatalytic conversions such as CO2 reduction. In such processes, metalloporphyrin linkers can serve as both light sensitizers and catalytic centers due to their substrate-binding ability. However, the catalytic activity of PorMOFs is still in its infancy, only a few of the unique features have been exploited and relevant mechanistic pathways need to be fully established. For example, the polar ZrIV–oxo node can play a role in protoncoupled transformations, which we have not covered. Overall, charge migration within the common metal-carboxy-based PorMOFs remains a major roadblock for both photo and electrochemical transformations. Developing strategies that exploit various unique features in MOFs with improved charge conductivity will define future directions in PorMOF-based LH and photocatalytic developments.

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140. I. Hod, O. K. Farha and J. T. Hupp, Modulating the rate of charge transport in a metal-organic framework thin film using host:guest chemistry, Chem. Commun., 2016, 52(8), 1705–1708. 141. K. Maindan, X. Li, J. Yu and P. Deria, Controlling Charge-Transport in Metal-Organic Frameworks: Contribution of Topological and Spin-State Variation on the Fe-Porphyrin Centered Redox Hopping Rate, J. Phys. Chem. B, 2019, 123(41), 8814–8822. ˘, Electrically Conductive Porous 142. L. Sun, M. G. Campbell and M. Dinca Metal–Organic Frameworks, Angew. Chem., Int. Ed., 2016, 55(11), 3566– 3579. 143. C.-W. Kung, S. Goswami, I. Hod, T. C. Wang, J. Duan, O. K. Farha and J. T. Hupp, Charge Transport in Zirconium-Based Metal–Organic Frameworks, Acc. Chem. Res., 2020, 53(6), 1187–1195. 144. S. Lin, P. M. Usov and A. J. Morris, The role of redox hopping in metal– organic framework electrocatalysis, Chem. Commun., 2018, 54(51), 6965–6974. 145. R. Ifraemov, R. Shimoni, W. He, G. Peng and I. Hod, A metal–organic framework film with a switchable anodic and cathodic behaviour in a photo-electrochemical cell, J. Mater. Chem. A, 2019, 7(7), 3046–3053. 146. K. M. Ishihara and F. Tian, Semiconducting Langmuir–Blodgett Films of Porphyrin Paddle-Wheel Frameworks for Photoelectric Conversion, Langmuir, 2018, 34(51), 15689–15699. 147. N. Kornienko, Y. Zhao, C. S. Kley, C. Zhu, D. Kim, S. Lin, C. J. Chang, O. M. Yaghi and P. Yang, Metal-Organic Frameworks for Electrocatalytic Reduction of Carbon Dioxide, J. Am. Chem. Soc., 2015, 137(44), 14129–14135. 148. D. Behar, T. Dhanasekaran, P. Neta, C. M. Hosten, D. Ejeh, P. Hambright and E. Fujita, Cobalt Porphyrin Catalyzed Reduction of CO2. Radiation Chemical, Photochemical, and Electrochemical Studies, J. Phys. Chem. A, 1998, 102(17), 2870–2877. 149. I. Hod, M. D. Sampson, P. Deria, C. P. Kubiak, O. K. Farha and J. T. Hupp, Fe-Porphyrin-Based Metal-Organic Framework Films as High-Surface Concentration, Heterogeneous Catalysts for Electrochemical Reduction of CO2, ACS Catal., 2015, 5(11), 6302–6309. 150. X.-P. Wu, L. Gagliardo and D. G. Truhlar, Cerium Metal–Organic Framework for Photocatalysis, J. Am. Chem. Soc., 2018, 140(25), 7904– 7912.

CHAPTER 7

Nanoscale Porphyrinic Metal–Organic Frameworks for Photodynamic Therapy XIANG LIAN, CHUXIAO XIONG AND JIAN TIAN* School of Pharmaceutical Sciences, Wuhan University, Wuhan 430071, P. R. China *Email: [email protected]

7.1 Introduction Photodynamic therapy (PDT), as a relatively new clinical approach for the treatment of numerous medical conditions including malignant cancers, has attracted significant research interest in the past decade. It usually consists of three essential components: Photosensitizers (PSs), light, and molecular oxygen. Upon light irradiation, the PSs are activated and subsequently transfer their excited-state energy to the surrounding oxygen for generating reactive oxygen species (ROS), which are toxic and elicit the death of cancer cells.1,2 Compared with conventional cancer treatments, such as surgical resection, chemotherapy, and radiotherapy, PDT has the advantages of minimal invasiveness, high selectivity, minimal side effects, and wide anticancer spectrum without drug resistance, making it an emerging solution for cancer therapy.3–5 Despite the substantial advancements, the therapeutic efficacy of PDT in cancer treatment is still compromised by several unfavorable factors, such as nonspecific accumulation of PSs in tumor sites,6,7 hypoxic tumor microenvironments (TME),8,9 short diffusion distance of ROS,10,11 and limited tissue penetration depth of light. Monographs in Supramolecular Chemistry No. 32 Porphyrin-based Supramolecular Architectures: From Hierarchy to Functions Edited by Shengqian Ma and Gaurav Verma r The Royal Society of Chemistry 2022 Published by the Royal Society of Chemistry, www.rsc.org

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To circumvent these limitations, numerous nanomedicine strategies have been developed and explored with a focus on developing porphyrin-based nanomedicines for improved clinical applications of PDT. Porphyrins and their analogs have been successively employed for PDT as the first generation of PSs, with the approval of Photofrin for clinical use.12 However, these porphyrin-based organic PSs exhibit poor selectivity toward tumors, and high accumulation in the skin. The second-generation PSs, such as phthalocyanines and chlorines, demonstrate enhanced selectivity toward tumors and less accumulation in the skin. Unfortunately, their hydrophobic nature results in poor circulation half-life and serious selfaggregation, compromising the therapeutic effect severely. Nanophotosensitizers, which use inorganic/organic nanocarriers to deliver PSs, were developed as third-generation PSs with greatly enhanced accumulation within tumors. The dispersion of molecular PSs in nanocarriers not only prohibits their self-aggregation, but also greatly improves their loading capacity. A wide range of inorganic/organic nanocarriers, including mesoporous silica, liposomes, micelles, and dendrimers have been investigated for PSs delivery. Significantly, nanoscale metal–organic frameworks (nMOFs), a new class of organic–inorganic hybrid porous nanomaterials have emerged as a versatile platform in the past decade for high photosensitizer loading and delivery, as well as numerous biomedical applications. nMOFs are constructed from metal ions/ion clusters and organic linkers. Owing to their three-dimensional ordered structure, high porosity, adjustable composition, tunable functions, and intrinsic biodegradability, nMOFs significantly increase the capabilities of existing nanomedicines.13–17 In this regard, nanoscale porphyrinic metal–organic frameworks (nPMOFs) constructed from porphyrin-based organic linkers and various metal ions/ion clusters can have a very high loading of PSs, and they demonstrate great potential as next-generation PDT systems. The constituent porphyrins in MOF structures are well dispersed to prevent their self-aggregation and facilitate interactions with molecular oxygen, enabling highly effective PDT. Besides, nPMOFs can also be employed as nanocarriers to load various therapeutic agents or combined with other functional nanoparticles (NPs) for synergistic effects.18–22 This chapter aims to briefly review the recent advances in the development of nPMOFs and porphyrinic MOF-based multifunctional nanoplatforms for PDT applications, with a particular focus on the strategies for overcoming the bottlenecks of current clinical PDT technologies.

7.2 Applications of Porphyrinic nMOFs in PDT 7.2.1

The Advantage of nMOFs as Photosensitizer Carriers

Porphyrin and metalloporphyrin derivatives have been extensively investigated as molecular PSs for the PDT of cancer. Although some of them have

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already been approved for clinical use, several drawbacks such as poor aqueous solubility, aggregation-induced quenching, and non-specific targeting hamper their practical therapeutic efficacy. The introduction of nMOFs provides a promising platform to overcome these shortcomings. The nMOFs can deliver porphyrin-based PSs in two effective ways. Firstly, porphyrins can be loaded in the ordered pores of nMOFs by physical adsorption, decorating them on the surface through ligand exchange23 or chemical reactions.24–26 Secondly, porphyrin-derived bridging ligands can be used to directly synthesize nPMOFs22 in which porphyrinic molecules are wellisolated in the framework to avoid aggregation and self-quenching of the excited states. Besides, the porous network structure allows the facile diffusion of ROS to exert their cytotoxic effects on cancer cells. Moreover, nPMOFs can combine with other treatment strategies to further enhance the efficacy of PDT and realize synergistic therapy. Herein, we summarize the development of several classic nPMOFs to illustrate their applications in PDT. In 2014, Lin et al. reported the design and synthesis of a Hf–porphyrin nMOF, DBP–UiO, via the coordination-driven self-assembly of Hf41 and organic linker 5,15-di(p-benzoato)porphyrin (H2DBP), in which the loading content of photosensitizer (DBP) could reach as high as 77 wt% (Figure 7.1a). DBP–UiO displayed a nanoplate morphology with a dimension of B100 nm in diameter and B10 nm in thickness, particularly beneficial for the generation and diffusion of ROS (Figure 7.1b and c). The singlet oxygen generation efficiency of DBP–UiO, as determined using Singlet Oxygen Sensor Green was at least twice that of free H2DBP (Figure 7.1d). This potent PDT effect was presumably due to the site isolation of porphyrin ligands, enhanced intersystem crossing with heavy Hf centers, and facile 1O2 diffusion through porous frameworks. Furthermore, both in vitro and in vivo PDT efficacy studies proved that DBP–UiO exhibited great antitumor effect against human head and neck tumor cells (SQ20B) and cancer (Figure 7.1e and f).22 This study revealed that nPMOFs represent a new class of highly potent PDT agents. Since the report of DBP–UiO as the first nPMOFs used in PDT, significant research efforts have been devoted to the development of new nPMOFs for enhanced PDT effects. Representative nPMOFs such as PCN-224,27 PCN-222,28 TBC–Hf,29 Hf–TCPP,30 and Fe–TBP31 have been reported successively in the past five years. Effective photodynamic cancer therapy requires significant accumulation of nanophotosensitizers within the tumor tissues. Most nPMOFs are enriched in the tumor through passive targeting via the enhanced permeability and retention (EPR) effect. To improve the accumulation of NPs at the tumor site, Zhou et al. successfully achieved the construction of porphyrinic Zr-MOF NPs (PCN-224) with controllable sizes using a solvothermal method (Figure 7.2a). The size of PCN-224 NPs could be precisely tuned from 30 nm to 190 nm with incremental amounts of benzoic acid as the modulator (Figure 7.2b). Moreover, further modification of PCN-224 with folic acid (FA) through the interaction between FA and unsaturated Zr6 SBUs afforded PCN-224–FA NPs with active-targeting

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(a) Synthesis of Hf–DBP nMOFs and a schematic description of the singlet oxygen generation process. (b) TEM image of DBP–UiO. (c) High-resolution TEM images of DBP–UiO. (d) Singlet oxygen generation by DBP–UiO, H2DBP, and H2DBPþHfCl4. The dots are experimental data and the solid lines are fitted curves. (e) In vitro PDT cytotoxicity of H2DBP, DBP–UiO, and PpIX at different PS concentrations and irradiation times. (f) Tumor growth inhibition curve after PDT treatment. Black and red arrows refer to injection and irradiation time points, respectively. Reproduced from ref. 22, https://pubs.acs.org/doi/10.1021/ja508679h, with permission from American Chemical Society.

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Figure 7.1

260 (a) 6-connected Zr6 cluster (Zr6O4(OH)4(H2O)6(OH)6(COO)6), tetratopic linker (tetrakis(4-carboxyphenyl) porphyrin (H2TCPP)), and 3D nanoporous framework of PCN-224. (b) TEM images of PCN-224 with different sizes. (c) In vitro PDT cytotoxicity efficacy of different sized PCN-224. (d) In vitro PDT cytotoxicity efficacy of pristine PCN-224 and 1/4FA–PCN-224 in HeLa cells. Reproduced from ref. 27 with permission from American Chemical Society, Copyright 2016.

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Figure 7.2

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27

abilities. Under 420 nm light irradiation, PCN-224 NPs were capable of producing 1O2 efficiently. Besides, in vitro studies further revealed a sizedependent cellular uptake and promising PDT effect, in which 90 nm PCN224 NPs had the best killing effect to HeLa cells (up to 81%) while 190 nm PCN-224 had the least killing effect (49%) (Figure 7.2c). Notably, the FAmodified PCN-224 showed stronger cytotoxicity as a result of active targeting (Figure 7.2d).

7.2.2

Overcoming Tumor Hypoxia to Enhance Photodynamic Effect

O2 is one of the three essential elements of PDT. The growth and metabolism of tumor cells are unusually vigorous and thus their oxygen consumption is much higher than normal cells. Meanwhile, the oxygen consumption in PDT further aggravates hypoxia in tumor tissues, resulting in a decrease in PDT efficacy.32–34 Therefore, overcoming tumor hypoxia is the key to enhance the efficacy of PDT. nPMOFs coupled with self-supplying oxygen systems would be an ideal solution for the PDT treatment of hypoxic tumors. Alternatively, nPMOFs that have PDT mechanisms independent of O2 would also be greatly desired.

7.2.2.1

Oxygen Production by a Self-supplying System

It is well-known that the concentration of H2O2 in TME is much higher than that in normal tissues. Hydrogen peroxide enzyme (catalase, CAT)20,21 and numerous artificial hydrogen peroxide enzymes,35–38 including MnO2, Fe3O4, and Pt NPs can catalyze the conversion of H2O2 to generate O2. Besides, C3N4 and other photodegradable materials can decompose water to produce O2 under light excitation.33 These substances thus offer opportunities to produce O2 in situ, once delivered to the tumor sites. Compared with biological enzymes that are susceptible to the external environment, catalase-like artificial enzymes have higher stability and a lower cost, thus attracting great research interest.35–39 In 2020, Gao et al. designed and synthesized a novel type of twodimensional nanosheet Sm–TCPP through the coordination of metal ion Sm31 and photosensitizer tetrakis(4-carboxyphenyl) porphyrin (TCPP). Then, catalase-like platinum nanozymes were grown in situ on the nanosheet and modified with mitochondrial-targeted molecule triphenylphosphine (TPP) to obtain the Sm–TCPP–Pt/TPP nanocomposite (Figure 7.3a). Due to the heavy atomic effect of Sm, the energy conversion efficiency of TCPP could be enhanced. With an ultra-thin and unique 2D nanoplate structure (Figure 7.3b and c), Sm–TCPP was more conducive to generate and diffuse 1 O2. More importantly, with the integration of catalase-like Pt nanozyme the nanocomposite system could effectively catalyze the over-expressed H2O2 in the TME to produce O2 in situ, alleviating tumor hypoxia and improving PDT

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Figure 7.3

(a) Schematic illustration of the preparation of Sm–TCPP–Pt. (b) TEM image of Sm–TCPP nanosheets. (c) AFM image of Sm–TCPP. (d) Cell viability of Sm–TCPP, Sm–TCPP–Pt, and Sm–TCPP–Pt/TPP to MCF-7 cells under hypoxia environments after 15 min of light irradiation (660 nm). (e) Tumor volume. Reproduced from ref. 40 with permission from American Chemical Society, Copyright 2020. Chapter 7

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effect when exposed to laser irradiation. The mitochondrion is the most important organelle that produces H2O2 and is highly sensitive to ROS. By introducing TPP molecules the nanocomposite could target mitochondria, and thus the reactive oxygen species produced nearby were able to induce tumor cell apoptosis to a greater extent, making full use of its PDT effect (Figure 7.3d). In vitro and in vivo experiments showed that Sm–TCPP–Pt/TPP could effectively relieve tumor hypoxia, overcome the shortcomings of traditional PDT, and significantly enhance the anti-cancer effect (Figure 7.3e).40 The great diversity and tunable chemical compositions of nMOFs allow them to be developed as catalytic nanoreactors themselves. Several nMOFs, including Fe–TBP,31 MIL-100(Fe),18 Mn-MOF,8 and PCN-222(Mn)41 were reported to function as catalase nanoreactors which could significantly alleviate tumor hypoxia and improve the efficacy of PDT. In 2019, He et al. used Mn(III)-chelated TCPP (Mn–TCPP) as the organic ligand and Zr41 as metal ions to prepare PCN-222(Mn) (Figure 7.4a). The scanning electron microscope (SEM) analysis showed that PCN-222(Mn) NPs had a uniform olivary morphology with an average dimension of 300130 nm (lengthwidth)

Figure 7.4

(a) Illustration showing the construction of a PCN-222(Mn) framework by Zr6 clusters and Mn–porphyrin ligands. (b) SEM image (inset, TEM image) of PCN-222(Mn) NPs. (c) Evaluation of O2 generation by PCN-222 and PCN-222(Mn) in 10 mM H2O2 at pH 7.4 and 6.5. Reproduced from ref. 41 with permission from American Chemical Society, Copyright 2020.

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(Figure 7.4b). The organic ligand Mn–TCPP could combine the photodynamic property of TCPP with T1-weighted magnetic resonance imaging (MRI) and the catalase-like performance of Mn(III). Unfortunately, in the physiological environment Mn–TCPP was prone to aggregation easily, which seriously hindered its PDT effect. When Mn–TCPP was constrained in the network structure of PCN-222 (Mn), the self-aggregation was averted and the highly ordered porous structure provided abundant accessible catalytic sites to decompose H2O2 into O2 (Figure 7.4c). Thus PCN-222 (Mn) could realize O2 self-sufficient PDT guided by MRI and achieved effective tumor inhibition.41

7.2.2.2

Type I PDT Independent of O2

Although PDT is an efficient anticancer treatment, it largely relies upon an oxygen-dependent type II mechanism in which excited PSs transfer energy to the molecular oxygen generating singlet oxygen. The therapeutic efficacy of type II PDT is diminished in the hypoxic environments found in many solid tumors. In contrast, type I PDT is more hypoxia tolerant by generating cytotoxic radicals via electron transfer (ET) from excited PSs to O2 and organic molecules. Over recent years, the research on nMOFs with PDT applications has mainly focused on the oxygen-dependent type II mechanism. The development of nMOFs with type I PDT remains scarce but proves to be an effective strategy to combat tumor hypoxia. In 2019, Lin et al. reported the fabrication of a new type of nPMOF (Ti–TBP) and its use in the first type I PDT mediated by nMOFs. Ti–TBP was composed of tetravalent titanium and the photosensitive 5,10,15,20-tetra(p-benzamino) porphyrin (TBP) ligand. Under light irradiation, since the Ti-oxo SBU is close to the TBP ligand (about 1.1 nm) (Figure 7.5a), the excited TBP (TBP*) could also transfer electrons to Ti41 in addition to producing 1O2 (Figure 7.5b). As a result, TBP 1 ligands and Ti31 centers were generated, which ultimately promoted the generation of three distinct ROS: O2 , H2O2 and  OH (Figure 7.5c–e). In addition, the Ti31 centers could reduce the excessive H2O2 in the tumor sites, thereby generating a large amount of highly cytotoxic  OH (Figure 7.5f). Through the above mechanisms, Ti–TBP-mediated PDT exhibited an excellent anti-cancer effect and the tumor regression rate was greater than 98% (Figure 7.5g).42

Figure 7.5

(a) Perspective view of the Ti–(TiTBP) structure along the (010) direction. (b) Schematic showing both type I and type II PDT enabled by Ti–TBP. Time-dependent 1O2 generation (c), and time-dependent H2O2 generation (d) and  OH generation (e) upon light irradiation under oxygenated conditions. (f) Time-dependent enhanced  OH generation from H2O2 upon light irradiation under oxygen-free conditions. (g) In vivo anticancer effect on CT26 tumor-bearing BALB/c mice. N ¼ 5. Black and red arrows refer to intratumoral injection and light irradiation, respectively. Reproduced from ref. 42 with permission from American Chemical Society, Copyright 2020.

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ROS Production Independent of O2 Used in PDT

With the ongoing research on PDT mechanism, some new ROS production mechanisms independent of O2 such as heat-induced ROS production have been proposed. High heat will increase the thermo-electron emission of electrons and further react with the surrounding media to produce ROS. Besides, some organic molecules with large-conjugated structures can store 1O2 through the formation of endoperoxides at low temperatures and release 1O2 at high temperatures to achieve the cell-killing effect. These new mechanisms circumvent the traditional O2 dependent PDT and provide a new perspective to improve the PDT.2 Inspired by Russell’s mechanism, Wang et al. developed an ultra-thin two-dimensional nPMOF Cu–TCPP nanosheet having a rectangular shape with an average length of B106 nm, an average width of B37 nm and a thickness ofB1 nm (Figure 7.6b–d). In the TME featuring a low pH and a high concentration of H2O2, the acidic H2O2 could peroxidize TCPP ligands followed by the reduction of H2O2 to peroxy radicals (ROOC) by peroxidase-like nanosheets and Cu21. Finally, through the spontaneous recombination reaction of the Russell mechanism, the ultra-thin two-dimensional Cu–TCPP nanosheets could selectively produce 1O2 (Figure 7.6a). In addition, the nanosheets could deplete glutathione in cells through a cyclic oxidation mechanism, thereby inhibiting glutathione from clearing 1O2. Based on these processes, the Cu–TCPP nanosheets efficiently and selectively eradicated tumors (Figure 7.6e–g). This Russell mechanism gets rid of the traditional PDT’s dependence on oxygen. Meanwhile, the reactions require a slightly acidic and high H2O2 environment so they can act selectively in tumor tissues thereby reducing damage to the normal tissues.43

7.2.2.4

PDT Combined with Hypoxia-activated Prodrugs

Hypoxia-based agents (HBAs), such as anaerobic bacteria and bioreductive prodrugs exert hypoxia-based cytotoxic effects against solid tumors. HBAs can take advantage of the exaggerated hypoxia during PDT. Therefore, PDT combined with HBAs can be an effective strategy to enhance the overall therapeutic effects.44,45 Liu et al. synthesized a spherical nPMOF platform [Hf/tetra(4-carboxyphenyl) porphyrin (TCPP)] loaded with hypoxia-activated prodrug Terazamine (TPZ), which was further modified with dopaminederived polyethylene glycol (DOPA–PIMA–mPEG) (Figure 7.7a). Due to the high loading of PSs in the nPMOF and heavy atom effect of Hf, the Hf–TCPP exhibited an excellent photodynamic performance. The consumption of O2 to generate 1O2 aggravated the hypoxia in the TME which further accelerated the activation of TPZ, thereby generating more cytotoxic free radicals (oxidized hydroxyl radicals and benzotriazinyl radicals) to achieve enhanced synergistic and efficient anti-tumor effects through the combination of photodynamic and hypoxia-responsive therapy (Figure 7.7b). Meanwhile, the Hf centers could be used for CT imaging with strong X-ray attenuation (Figure 7.7c). TPZ/Hf–TCPP/PEG showed significant cytotoxicity to both HeLa cells and 4T1 cell lines (Figure 7.7d and e), higher than that of

(a) Synthesis and therapeutic mechanism of the Cu–TCPP nanosheets. (b) TEM, (c) HRTEM, and (d) AFM images of the nanosheets with their height (inset). MTT assay (e) to test the cytotoxicity of the nanosheets. (f) Tumor growth curves of each group (n ¼ 5/group, **P o0.01). (g) Mice weight growth curves of each group. Reproduced from ref. 43 with permission from John Wiley & Sons, Copyright r 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

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Figure 7.6

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268 (a) Schematic of the synthesis route of TPZ/Hf/TCPP/PEG. (b) In vivo synergetic photodynamic and hypoxia-activated therapy of TPZ/Hf/TCPP/PEG. (c) In vivo computed tomography (CT) imaging of TPZ/Hf/TCPP/PEG. In vitro cytotoxicity of TPZ/Hf–TCPP/ PEG against (d) HeLa and (e) 4T1 cells. (f) Time-dependent tumor growth curves via tail vein administration at a concentration of 5.0 mg mL1 (200 mL). Reproduced from ref. 46 with permission from American Chemical Society, Copyright 2018.

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Figure 7.7

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Hf–TCPP/PEG under both normal oxygen and hypoxia conditions. These results further demonstrated the synergistic effect of PDT and hypoxiaactivated prodrug TPZ (Figure 7.7f), which proved the advantages of the combined effect of PDT and HBAs by applying drug-loaded nPMOFs.46

7.2.3

Improving the Method of Excitation to Enhance Photodynamic Effect

Besides oxygen, the tissue penetration depth of the excitation light is another critical parameter for PDT. For most PSs, the wavelength of the excitation light is in the visible region, where biological tissues have strong absorption and scattering. As a result, the light reaching the deep lesion region is not enough to effectively activate PSs and trigger photodynamic effects, seriously limiting the efficacy of PDT for deep tumors. Therefore, current clinical applications of PDT are still limited to the treatment of superficial diseases. It is highly desirable to improve the tissue penetration depth of the excitation sources. NIR light displays a good tissue penetration ability and can be used as the excitation source for deep-tissue fluorescence imaging and PDT. Apart from NIR light, X-ray radiation has no limit on tissue penetration depth making it suitable for activating PDT in the treatment of deep-seated tumors. Moreover, PSs can also be activated by internal self-luminescence that is not limited by the tissue penetration depth. In general, excitation modes with deep tissue penetration depth such as NIR light, X-ray, and internal self-luminescence may provide a novel technique of PDT for the efficient treatment of deep-seated tumors.

7.2.3.1

NIR-light Stimulated PDT

At present, the excitation light of most PSs approved clinically is visible light which possesses high energy but low penetration depth. One way to increase the penetration depth is to irradiate these PSs indirectly with NIR light. The light in the NIR region (700–1300 nm) lies in the ideal phototherapy window, which promises deeper penetration and less attenuation during its tissue propagation. Upconversion nanoparticles (UCNPs) doped with rare-earth elements can convert low energy but high penetration NIR light (usually 980 nm) into visible ultraviolet light, and then deliver the energy to photosensitive agents through the effect of fluorescence resonance energy transfer (FRET), thus achieving indirectly stimulated PDT by NIR light. In this regard, UCNP–MOF heterodimers,47 MOF–UCNP core–satellite nanostructures,48 and UCNP@MOF core–shell structures20,45 have been reported for the treatment of deep tumors successively. Liu et al. obtained UCNP–MOF heterodimers (UCM) by growing nPMOF PCN-224 on the surface of Nd31-sensitized UCNPs and further fabricated nanophotosensitizer UCMTs with mitochondrial targeting capability via coordination of the TPP moiety on the surface of UCM (Figure 7.8a–d).

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Figure 7.8

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(a) Schematic of the UCMT Janus nanostructure. TEM images of (b) NaGdF4:Yb,Er@NaGdF4:Yb,Nd@NaGdF4 UCNPs and (c and d) UCMTs. (e) The tumor growth curves after intra-tumoral injection of different samples followed with or without NIR irradiation. Data are means  SD; N ¼ 5. ***P o0.001. Reproduced from ref. 49 with permission from John Wiley & Sons, Copyright r 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

In these Janus nanostructures, the fluorescence emitted by UCNPs in UCMTs after absorbing 808 nm photons is transmitted to the adjacent PCN-224 through the FRET effect which activates the production of 1O2. Notably, UCMTs showed a more obvious growth inhibition effect on tumors compared with UCMs owing to the mitochondrial-targeting effect (Figure 7.8e), which caused the depolarization of the mitochondrial membrane and the initiation of the intrinsic apoptotic pathway. It is worth mentioning that the introduction of Nd31 successfully reduced the excitation wavelength of UCMTs from 980 nm to 808 nm, thus avoiding the thermal damage to normal tissues due to the overheating effect of traditional 980 nm laser radiation and improving the biosafety of PDT.49 Another strategy to improve the penetration depth of PDT is to develop nano-photosensitizers that can be directly excited by NIR light. Lin et al. reported a bacteriochlorin-based nMOF (Zr–TBB) for both type I and type II photodynamic therapy (Figure 7.9a). Bacteriochlorin, a highly reduced derivative of porphyrins and chlorins, possesses strong absorption in the NIR region (700–850 nm). Meanwhile, bacteriochlorin can realize type I PDT independent of molecular oxygen, further amplifying the efficiency of PDT. Through experimental and computational studies, it was proved that

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Figure 7.9

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(a) Stabilization of Bacteriochlorin ligands in Zr–TBB for Type I and Type II PDT. (b) Energy profiles of TBB photofragmentation in H4TBB and Zr–TBB calculated by DFT. (c) CLSM images of various ROS species generated in 4T1 cells after light irradiation. Total ROS was detected by H2DCFDA assay. Scale bar is 20 mm. (d) Antitumor efficacy on MC38-bearing C57Bl/6 mice. Reproduced from ref. 50 with permission from American Chemical Society, Copyright 2020.

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bacteriochlorin could be stabilized toward oxygen and light in the Zr–TBB framework, and could generate various types of cytotoxic ROS under 740 nm irradiation, including singlet oxygen (1O2), hydroxyl radicals ( OH), superoxide anions (O2), and hydrogen peroxide (H2O2) (Figure 7.9b and c). An in vivo study demonstrated a superb antitumor effect on breast and colon cancers (Figure 7.9d).50

7.2.3.2

X-ray Stimulated PDT

The photon energy of an X-ray (keVBMeV) does not match the energy required to activate the photosensitizer (eV). Thus, an energy converter which generally contains high Z elements (Hf, Au, Bi, etc.) is required. These substances first absorb X-rays and then transfer the energy to PSs through the FRET effect. Therefore, X-ray excited PDT is also referred to as radiodynamic therapy (RDT).52 In 2016, Liu et al. prepared Hf–TCPP nMOFs with a strong X-ray attenuation ability by a solvothermal method using hafnium (Hf41) and tetrakis (4-carboxyphenyl) porphyrin (TCPP) (Figure 7.10a). Most of the Hf–TCPP NPs were between 80 and 150 nm in diameter (Figure 7.10b). After PEG modification, the afforded NPs could be dispersed and stabilized in physiological solutions, and used for combined PDT and radiotherapy in vivo. Under X-ray irradiation, Hf could effectively absorb X-ray photons and transfer energy to TCPP, thereby being proved beneficial for the production of 1O2. Meanwhile, Hf as a high-Z element could interact with ionizing radiation to produce toxic free radicals. As a result, Hf also acted as a radio-sensitizer to enhance the responsiveness to X-rays (to produce  OH), increasing the damage to tumor cells. Moreover, the ultrathin monolayer structure of Hf–TCPP nMOF greatly promoted the diffusion of ROS and free radicals thereby further improving the photodynamic efficacy. Notably, the PEG coating on the Hf–TCPP nMOF significantly prolonged its half-life in blood circulation (B3.27 h) (Figure 7.10c) and improved its tumor homing upon intravenous injection, which resulted in a remarkable anti-tumor effect with an efficient combination of PDT and radiotherapy (Figure 7.10d).30 In 2018, Lin et al. constructed 5,15-di(p-benzoato)porphyrin–Hf (DBP–Hf) for both radiotherapy (via the production of  OH radicals) and radiodynamic therapy (by exciting the PSs to generate 1O2).51 X-ray induced RT–RDT exhibits much deeper penetration than PDT. Under low-dose X-rays, the high-Z Hf cluster could absorb the energy and directly transfer it to the organic ligands (Figure 7.11a), resulting in efficient radiotherapy and radiodynamic therapy with minimal side effects. In this study, both in vitro and in vivo results demonstrated that DBP–Hf could eradicate several types of tumors under low doses of X-rays (Figure 7.11b and c). Furthermore, when loaded with an inhibitor of the immune checkpoint molecule indoleamine 2,3-dioxygenase, the nPMOF displayed strong systemic antitumor results in both irradiated and distant tumors, significantly increasing the response rates of cancer immunotherapy (Figure 7.11d–f).

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Figure 7.10

7.2.4

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(a) The schematic illustration of the synthesis of Hf–TCPP nMOFs and the processes of light-triggered ROS generation by TCPP for PDT, as well as X-ray absorbance by Hf for enhanced RT. (b) TEM images of Hf–TCPP nMOFs. The inset is a magnified TEM image. (c) Blood circulation of nMOFs–PEG in mice by measuring the fluorescence of TCPP in blood at different time points post-injection (Ex: 414 nm, Em: 651 nm). (d) Tumor growth curves of different groups after various treatments indicated: (i) untreated control, (ii) i.v. injection with nMOFs–PEG, (iii) X-ray irradiation, (iv) i.v. injection with nMOFs– PEG þ Laser, (v) i.v. injection with nMOFs–PEG þ X-ray, and (vi) i.v. injection with nMOFs–PEG þ X-ray þ Laser. Error bars were based on SD of five mice per group. P values were calculated by Tukey’s post-test (***p o0.001, **p o0.01, or *p o0.05). Reproduced from ref. 30 with permission from Elsevier, Copyright 2016.

PDT Combined with Immunotherapy for Metastatic Tumor Treatment

As a local therapy, PDT cannot be directly used to treat metastatic tumors. Due to the limited tissue penetration depth of excitation light, it is difficult to completely eradicate deeper tumors and the residual cancer cells will lead to recurrence. Although the immune system can recognize and eliminate cancer cells, the tumor immunosuppressive microenvironment greatly restricts its effect and can achieve immunologic escape. Immunotherapy can restore or strengthen the body’s immune response to the tumor. However, its activation is insufficient and the anticancer effect is limited. Studies have shown that PDT can not only induce apoptosis, but also cause immunogenic

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Figure 7.11

Chapter 7

(a) Schematic illustration of the mechanisms of X-ray-induced RT–RDT by nMOFs. Tumor growth curves of SQ20B (b), and U87MG (c) tumor-bearing mice treated with PBS, DBP–Hf (0.11 mg), or TBP–Hf (0.60 mg), with (þ) or without () X-ray irradiation (n ¼ 6 mice per group). (d) nMOFs enable synergistic RT–RDT and immunotherapy using extremely low doses of X-rays. Growth curves for treated (e) and untreated (f) tumors of TUBO tumor-bearing mice intratumorally injected with PBS, DBP–Hf, IDOi@DBP–Hf, or DBP–Hfþintravenously administrated IDOi. P values for comparisons with controls are indicated by asterisks: *P o0.01; **P o0.001. Reproduced from ref. 51 with permission from Springer Nature, Copyright 2018.

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cell death (ICD) to release antigens and then stimulate the immune response. In view of this, PDT combined with immunotherapy can enhance the anti-tumor immune response, which is expected to eradicate the primary tumor and eliminate the metastatic tumor at the same time.29 The overexpressed programmed death molecule ligand 1 (PD-L1) on the surface of tumor cells will bind to the anti-programmed death protein 1 (PD-1) on the surface of activated T cells and send a ‘‘don’t eat me’’ signal, which can prevent the attack on tumor cells. One way to achieve immunotherapy is to apply an immune checkpoint blockade (ICB), which uses antibodies to block negative immune regulatory pathways. Lin et al. have developed a series of nPMOFs for PDT combined with immune checkpoint blockade therapy, revealing excellent outcomes for the treatment of primary and metastatic tumors.29,31,51–53 For instance, they reported a novel nPMOF Fe–TBP for synergistic PDT and ICB treatment (Figure 7.12a). Fe–TBP is a nanophotosensitizer build from iron-oxo clusters and 5,10,15,20-tetrad (p-benzoic acid)-porphyrin (H4TBP). It can decompose H2O2 in tumor tissues into O2 through the Fenton reaction to alleviate the degree of tumor hypoxia and provide O2 for the photodynamic reaction (Figure 7.12b). Thus even under hypoxic conditions, Fe–TBPmediated PDT exhibited great anticancer effect in CT-26 tumor models (Figure 7.12c and d), alleviated the immunosuppression, and increased the infiltration of effector T cells. By intraperitoneal injection of PD-L1 antibodies (a-PD-L1), the PD-1/PD-L1 pathway would be inhibited and a systematic immune response would then be activated. Both primary and distant tumors were completely inhibited by PDT plus a-PD-L1 treatment (Figure 7.12e and f). More importantly, the combined photodynamic therapy and immunotherapy could inhibit tumor growth after re-challenge with CT-26 tumors, suggesting great potential to treat metastatic tumors (Figure 7.12g).31 In 2018, Zhang et al. synthesized a benzoporphyrin-based nMOF (TBP-MOF) with a superior PDT effect (Figure 7.13a). Compared with traditional porphyrin, benzoporphyrin exhibits an extensive p-conjugation which leads to a red-shift of absorption bands (at l4650 nm) and improved 1O2 generation (Figure 7.13b and c). After poly(ethylene glycol) modification, the nanoscale TBP-MOF could act as an efficient PDT agent under hypoxia conditions to suppress the growth of primary tumors. Furthermore, when combined with aPD-1 checkpoint blockade therapy, adaptive immune responses could be stimulated to achieve metastatic tumor inhibition (Figure 7.13d).54 These results showed that PDT combined with immunotherapy could achieve a complementary anticancer effect, providing a new strategy for the treatment of metastatic tumors.

7.2.5

Improving the Biosafety of Porphyrinic nMOFs in PDT

The long-term toxicity caused by the retention of PSs in the body is a safety issue that cannot be ignored. In particular, most MOF-based

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Figure 7.12

(a) Schematic illustration of using Fe–TBP to overcome hypoxia for PDT primed cancer immunotherapy. (b) Time-dependent O2 generation detected by an oxygen sensor. (c) Cytotoxicity tests under hypoxic conditions. (d) Tumor growth curves of CT26 tumor-bearing mice treated with PBS, H4TBP, Hf–TBP, or Fe–TBP with light irradiation or Fe–TBP without light irradiation. N ¼ 6. Growth curves of primary tumors (e) and distant tumors (f) of bilateral CT26 tumor-bearing mice. (g) Tumor growth curves after challenge with CT26 cells. N ¼ 6. *P o0.05, **P o0.01, and ***P o0.001. N ¼ 6. Reproduced from ref. 31 with permission from American Chemical Society, Copyright 2018. Chapter 7

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Figure 7.13

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(a) Crystal structure and underlying network topology of TBP-MOF (Cu). (b) The 10-connected Zr6 cluster in TBP-MOF (the carbon atoms from benzoic acid, single-coordinated carboxylate oxygen of TBP, and double-coordinated carboxylate oxygen of TBP are shown in purple, yellow, and light black, respectively). (c) UV–vis spectra of TBP and TBPnMOF. Inset shows expanded views of the Q-band regions. (d) The relative tumor volume changes in 22 days after various treatments; P values were calculated by a Student’s t test (**P o0.01, ***P o0.001). Reproduced from ref. 54 with permission from American Chemical Society, Copyright 2018.

nano-photosensitizers have a large particle size (4100 nm) and are not efficiently eliminated by the body’s metabolism. Besides, the ROS generated inside a large NP may become inactive toward killing tumor cells because of their short half-life and difficulty in diffusing out of the nMOFs interior. To solve the above challenges, Wang et al. utilized the liquid phase stripping method to break PCN-224 NPs into ultra-small nanodots (QDs) with an average size of B4 nm, that could be quickly cleared by kidneys (Figure 7.14a). The materials, PCN-224 QDs, were modified with PEG on the surface to obtain MOF QDs which exhibited good biological stability and could be used for efficient PDT. More importantly, due to their ultra-small size these MOF QDs could promote the diffusion of internally generated ROS by shortening the distance within the framework, which greatly avoided the annihilation of singlet oxygen inside the material and significantly improved the PDT efficiency. Compared with bulk nMOF, ROS production of MOF QDs was twice that of PCN-224 NPs upon the same light irradiation (Figure 7.14b and c). By using MOF QDs with efficient tumor accumulation and rapid renal clearance in vivo, tumors could be effectively eradicated by PDT, and the long-term biological toxicity of MOF materials was greatly reduced (Figure 7.14d and e).55 This work provides a new perspective to develop effective nPMOFs with great biosafety.

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Figure 7.14

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(a) Schematic illustration of the rational design of MOF QDs and their usage as renal-clearable nanoagents for the enhanced PDT of cancer. Concentration-dependent (b) and time-dependent (c) PDT efficacy of PCN-224 nMOFs, PEG-nMOFs, PCN-224 QDs, and MOF QDs under 650 nm laser irradiation. (d) Tumor growth curves of the tumor-bearing mice after various treatments. (e) Tumor masses collected from the tumor-bearing mice at 3 weeks post-initiation of various treatments. Reproduced from ref. 55 with permission from American Chemical Society, Copyright 2018.

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7.3 Conclusion and Prospects Porphyrins and their derivatives are promising PSs for PDT, while nMOFs are emerging as advanced functional materials with great potential in biomedical applications owing to their versatile components, tunable functions, and inherent biodegradability. The construction of nPMOFs from porphyrin-based organic linkers could combine the distinct advantages of porphyrinic PSs and nMOFs, affording a superior nanoplatform for advancing the current PDT technology. Since the first report in 2014, nPMOFs have been widely studied for enhanced PDT to overcome the current limitations of PDT such as light penetration depths, hypoxic TME, and physiological safety. Although significant progress has been made, the study of nPMOFs in clinical settings is still in the preliminary stage. Several challenges need to be overcome in order to realize nPMOFs for clinical PDT. Firstly, the in vivo stability of most nPMOFs is not ideal, and a balance between their stability and metabolic clearance is needed. Secondly, there are few studies on the metabolic kinetics and toxicology of nPMOFs in the body, thus a comprehensive and in-depth assessment of its long-term biological safety is greatly needed. Thirdly, the selective targeting ability of nPMOFs toward tumor tissues still needs to be improved, and the relationship between the properties of nPMOFs (particle size, charge, surface chemistry, etc.) and tumor targeting ability need to be established. Finally, the complexity of tumors makes the response and efficacy of the pre-designed nPMOFs-PDT system significantly different as a result of different types of tumors and the different stages of the diseases. Nevertheless, these challenges will guide the future development of stable and functional nPMOFs for practical applications. The excellent properties of nPMOFs endow them with great clinical development potential. In summary, with the joint efforts of researchers in the fields of chemistry, materials science, and medicine, the clinical applications of PDT based on nPMOF platforms could be expected.

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CHAPTER 8

Porphyrin and Phthalocyanine Covalent Organic Frameworks: Pre-designable Structures and Tailor-made Materials RUOYANG LIUa AND DONGLIN JIANG*a,b a

Department of Chemistry, Faculty of Science, 3 Science Drive 3, Singapore 117543, Singapore; b Joint School of National University of Singapore and Tianjin University, International Campus of Tianjin University, Fuzhou 350207, China *Email: [email protected]

8.1 Introduction Nature evolves 18-p macrocycles to realize unique biological functions, including chlorophylls for light harvesting in plants and bacteria; and heme for oxygen-carrying proteins of hemoglobin, myoglobin, and coenzyme cytochrome C oxidase.1–3 These functions are specific to these 18-p macrocycles and their assembled structures, inspiring chemists to develop artificial functional materials using the 18-p macrocycles. In synthetic systems, 18-p macrocycles are established with two major classes i.e. porphyrin, phthalocyanine, and their metallo-compounds,4–6 which resemble the structures of chlorophyll, heme, and cytochrome C. Over past decades, a broad diversity of materials has been explored based on porphyrin, phthalocyanine, and their metallo-compounds, showing different properties and Monographs in Supramolecular Chemistry No. 32 Porphyrin-based Supramolecular Architectures: From Hierarchy to Functions Edited by Shengqian Ma and Gaurav Verma r The Royal Society of Chemistry 2022 Published by the Royal Society of Chemistry, www.rsc.org

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functions. Despite the great progress that has been made, construction of well-defined architectures of porphyrin, phthalocyanine, and their metallocompounds remains a key fundamental issue from the perspective of their synthetic chemistry. Covalent organic frameworks (COFs) are a class of crystalline porous polymers which enable the organization of organic building units into ordered structures via topology-guided polymerization.11–19 This becomes possible as the reaction merges covalent bonds for connecting organic units to form specific polygonal skeletons and non-covalent interactions for assembling these skeletons to form layer or folded frameworks. This feature endows COFs with structural pre-designability and long-range structural orderings, thus offering a unique platform for exploring organic materials with tailor-made structures. Porphyrin, phthalocyanine, and their metallocompounds have been developed as building blocks to design and synthesize a diversity of porphyrin and phthalocyanine COFs, which greatly enhance our capability of designing ordered structures and materials. In this chapter, we summarize the design and synthesis of crystalline porous COFs to construct tailor-made structures of porphyrins, phthalocyanines, and their metallo-compounds, with an aim to show the chemistry of precise structural design and the underlying diversity of structural scope. We scrutinize different properties of these frameworks ranging from photocatalysis to electrocatalysis, organocatalysis, metal nanoparticle catalysis, adsorption, semiconductors, and energy storage, by emphasizing their interactions with photons, electrons, phonons, holes, ions, and molecules. We disclose their structure–function relationships and elucidate the way to develop unique properties and functions that are specific to the framework structures. We predict key issues to be addressed and show future directions from the viewpoints of design, synthesis, and properties.

8.1.1

Catalysis

Owing to the conjugated 18-p electron aromatic core, porphyrin and its derivatives are commonly recognized for their remarkable photophysical properties, where they often exhibit intense absorption in the visible light region, and even extend into the near-infrared region when chemically modified with suitable groups or coordinated with metal ions. One unique feature of COFs is that they integrate porphyrins, phthalocyanines, and their metallo-compounds into periodically ordered p arrays and constitute stacked p columns across the material. This highly ordered structure is hardly achieved with the traditional self-assemblies of their polymers and is inaccessible even with the single crystals of their organic compounds. The well-defined p architectures offer the possibility of triggering specific interactions between p units and interplays with photons and excitons, electrons, holes, ions, and molecules. This unprecedented possibility greatly attracts researchers to explore the catalytic functions for a wide range of different reactions.

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Photocatalysis

Light-harvesting antennae are essential for photocatalysis, as elaborated in nature’s photosynthetic systems. In synthetic systems, how to organize porphyrin, phthalocyanine, and their metallo-compounds into ordered structures is key to light harvesting and photocatalysis. Over the past 15 years, we have shown the design principles and synthetic strategies to develop porphyrin, phthalocyanine, and their metallo-compounds for the construction of photocatalytic COFs. In order to leverage on the excellent light absorption properties of porphyrin as well as the outstanding structural orderings, we designed and synthesized for the first time, the squaraine linkage for the construction of copper(II) porphyrin COFs, i.e. CuP-SQ COFs (Figure 8.1). The CuP-SQ COF features extended intralayer p conjugation over the two-dimensional (2D) layer and interlayer p electronic coupling, demonstrating three distinct

Figure 8.1

CuP-SQ COF for photocatalytic singlet oxygen generation.

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differences in electronic properties compared to its porphyrin monomer, TAP-CuP. Firstly, the absorption edge of CuP-SQ is extended from 650 to 750 nm, forming a smaller band gap (1.7 eV vs. 1.9 eV) and broader light absorbance. Secondly, the intensity of the Q-bands of CuP-SQ is greatly enhanced, reflecting a greater absorption capability in the long wavelength visible region. Lastly, the HOMO level of CuP-SQ (5.7 eV vs. vacuum) is raised compared to TAP-CuP (6.0 eV), suggestive of better delocalization of p electrons across the framework. These combined advantages allow CuP-SQ COFs to be an efficient photosensitizer for singlet oxygen generation. Using 1,3-diphenylisobenzofuran (DPBF) as a label, an oxygen-saturated DMF solution (2.3 mL) of DPBF (50 mM) experienced 80% decrease in absorbance after 600 min of irradiation at 500 nm in the presence of 0.1 mg of CuP-SQ COF, while the monomeric TAP-CuP hardly enables the generation of singlet oxygen under otherwise the same conditions.20 This work thus not only highlights the superior structural features of Cu-SQ-COF, but also offers a new strategy for designing efficient photosensitizers to enable photocatalytic applications. Besides having desirable properties, it is equally important for COFs to have robust stabilities that are essential for prolonged and repeated uses in real-world applications. One intuitive approach is the integration of hydroxy groups at specific positions on the linker unit to serve as hydrogen-bond donors for the formation of intramolecular hydrogen bonds with the imine nitrogen. As demonstrated by DhaTph (Figure 8.2; ¼ H2P-DHPh), this strategy not only proves to rigidify the connection between the knot and linker units, leading to high crystallinity and porosity, but also deactivates the reactivity of the imine group so that a high stability against hydrolysis is achieved, compared to the non-hydrogen bonded analog DmaTph (Figure 8.2; ¼ H2P-DMPh).21 In addition to the enhanced stability, we recognized that hydrogenbonding interaction drives the planarization and thus induces a greater extension of p conjugation over the 2D polymer, which in turn leads to a reduction in the band gap and improves the light-harvesting capability. As such, we prepared a series of porphyrin and metallo-porphyrin-based COFs with varying degrees of intramolecular hydrogen bonds and investigated their abilities in photocatalytic singlet oxygen generation (Figure 8.2). We found that as a greater proportion of terephthalaldehyde (TA) was replaced with dihydroxyterephthalaldehyde (DHTA), the COFs displayed increased crystallinities and porosities. Generally, MP-DHPh COFs show higher absorption edge than their porphyrin and metalloporphyrin monomers, as well as amorphous analogs. Together with the increased absorbance intensity at longer wavelengths, these signify a greater extension of p conjugation and boosted light-harvesting capability. Besides, the increase in intramolecular hydrogen bonding is accompanied by an increase in photocatalytic activity in singlet oxygen generation. Notably, H2P-DHPh exhibited the highest efficiency, achieving almost full decomposition of DPBF in 50 mins. Hence, through this extensive study we were able to establish the structure–property

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Schematic of the synthesis of 2D porphyrin COFs with tunable content of hydrogen-bonding structures.

Chapter 8

Figure 8.2

Porphyrin and Phthalocyanine Covalent Organic Frameworks

289

relationship between intramolecular hydrogen bonding and photocatalytic activity.22 Despite displaying promising photocatalytic activities, porphyrincontaining COFs mainly absorb light in the visible region, limiting their full potential in photocatalysis. Therefore, it would be of immense interest if their absorption range can be extended into the near-infrared region. In an attempt to achieve this goal, we incorporated both porphyrin and phthalocyanine units for the designed synthesis of tetragonal microporous M1TPP-M2Pc-COF and mesoporous M1DPP-M2Pc-COF (Figure 8.3). We rationalize that the integration of complementary visible-light absorbing porphyrin and near-infrared absorbing phthalocyanine moieties can give rise to a synergistic effect, which when combined with the ordered and periodically aligned p-columns produces COFs with exceptional optoelectronic properties. Undoubtedly, all the porphyrin-co-phthalocyanine COFs synthesized showed larger absorption onsets than the corresponding monomer mixtures. Furthermore, most COFs have stronger absorption intensity, especially in the near-infrared region. In particular, ZnTPP-CuPcCOF has an absorption onset of 1350 nm and a small band gap of 0.92 eV. When utilized as a photosensitizer for singlet oxygen generation, ZnTPPCuPc-COF fully quenched the absorbance of DPBF in about 20 mins, which is unachievable using previously reported COF systems. Likewise, ZnDPPCuPc-COF showed similar properties, albeit with a slightly slower rate of reaction (30 min). Interestingly, triplet oxygen excitation can also be carried out with near-infrared light (750 nm), where a competitive performance is still observed. Through the rational design and selection of monomers, we precisely modulated the optoelectronic properties of COFs, providing further insights for the designed synthesis of highly efficient photocatalytic systems.23 Condensation of C4-symmetric freebase porphyrin or Cu(II)-porphyrin with Td-symmetric tetra(p-aminophenyl)methane affords the 2-fold interpenetrated 3D-Por-COF and 3D-CuPor-COF with pts topology, respectively (Figure 8.4). Specifically, both COFs are able to generate singlet oxygen in oxygensaturated acetonitrile solution (40 mL). Using 9,10-dimethylanthracene (DMA) as a label, 8 mg of 3D-Por-COF almost fully degraded 10 mM of DMA in 90 mins, under 500 nm light irradiation, while retaining 94% catalytic activity even after three cycles. In contrast, owing to the paramagnetic Cu(II) species, 3D-CuPor-COF only degrades 55% of DMA after 720 mins. The good photocatalytic activity of 3D-Por-COF is mainly attributed to its moderately high surface area of 1398 m2 g1, which exposes the porphyrin active sites for catalysis.24 Thus far, it has been shown that porphyrin-containing COFs are good photosensitizers for singlet oxygen generation. Nevertheless, most of these COFs were constructed from at least two monomers and involved extensive screening of reaction conditions. To circumvent this problem, a bifunctionalized A2B2 type porphyrin monomer was designed. The utilization of

290

M1TPP-M2Pc-COF and M1DPP-M2PC-COF for singlet oxygen generation.

Chapter 8

Figure 8.3

Porphyrin and Phthalocyanine Covalent Organic Frameworks

Figure 8.4

291

3D-Por-COF and 3D-CuPor-COF for singlet oxygen generation.

only one monomer greatly enhanced efficiencies of screening reaction conditions, where crystalline A2B2-Por-COF (Figure 8.5) can be obtained in various solvents such as dichloromethane, chloroform, and 1,2dichloroethane. Notably, in the presence of visible light and oxygen, A2B2-Por-COF can generate singlet oxygen that oxidizes aryl sulfides to sulfoxides, tolerating both electron-donating (4-methyl and 4-methoxy) and withdrawing substituents (4-fluoro, 4-chloro, and 4-bromo). The involvement of singlet oxygen species is further confirmed by the addition of sodium azide (singlet oxygen scavenger), which almost completely quenches the reaction.

292

Figure 8.5

Chapter 8

A2B2-Por-COF for sulfide oxidation.

Additionally, the basic pyrrolic nitrogen atoms render A2B2-Por-COF being able to catalyze the Knoevenagel condensation reaction between various aryl aldehydes and malononitrile, with up to 99% yield.25 Other than the energy transfer from the photoexcited triplet state of photosensitizers to triplet oxygen, it is also known that electron transfer can occur under certain circumstances to generate the superoxide radical anion, which can also oxidize sulfides to sulfoxides.26,27 In a bid to investigate the effects of dimensionality on photocatalytic activity, 3D-PdPor-COF and 2D-PdPor-COF (Figure 8.6) were prepared. Compared to 2D-PdPor-COF, 3D-PdPor-COF has a larger BET surface area (1406 m2 g1 vs. 1120 m2 g1) and smaller pore size (0.58 nm vs. 1.9 nm). Moreover, in 3D-PdPor-COF, the porphyrin units are less aligned with one another. As a result, the lifetime of the triplet state of 3D-PdPor-COF (26.34 ms) is significantly longer than 2D-PdPor-COF (0.41 ms), indicating better photosensitizing abilities. Indeed, 3D-PdPor-COF generally performs better

Porphyrin and Phthalocyanine Covalent Organic Frameworks

3D-PdPor-COF and 2D-PdPor-COF for sulfide oxidation.

293

Figure 8.6

294

Chapter 8

in the oxidation of aryl sulfides to sulfoxides, achieving almost quantitative yields for simple aryl sulfides in only 0.4 h, as well as showing a size-selective effect. The involvement of superoxide radical anions was confirmed by both a spin-trapping experiment using electron spin resonance (ESR) spectroscopy and the use of p-benzoquinone as a superoxide scavenger in control experiments.28 The imine condensation reaction has been widely utilized for the preparation of a great variety of COFs. However, unless other stabilization effects are present, they are prone to hydrolysis under acidic conditions. Moreover, being partially conjugated limits the degree of p-conjugation across the 2D layers. In lieu of these issues, we developed the fully p-conjugated CQC linked sp2c-COF via the Knoevenagel condensation reaction for the first time.29 Not only do such COFs exhibit unprecedented spin alignment characteristics, their excellent chemical stabilities and optoelectronic properties also make them an attractive material for function exploration.30 Inspired by the unique properties of CQC linked COFs, porphyrin-containing Por-sp2c-COF (Figure 8.7) was designed for use as a photosensitizer. Under white light at ambient conditions, Por-sp2c-COF can efficiently generate superoxide radical anions that oxidize various secondary amines to imines in more than 86% yield. Furthermore, Por-sp2c-COF retains almost the same efficiency after five cycles, which is much better than that of iminelinked Por-COFs (Figure 8.4), as it decomposes after one cycle. The high chemical stability is further evident from its high resistance toward strongly acidic (9 M HCl) and basic (9 M NaOH) solutions, whereby no significant change in crystallinity was observed.31 To further illustrate the superiority of Por-sp2c-COF, it was used as a photosensitizer for reactions under red light.32 This was made possible by the extensive p conjugation present in Por-sp2c-COF. Compared with white light, photosensitizers that are responsive to red light harvest a greater portion of energy from the sun, constituting more efficient catalytic systems. When combined with (2,2,6,6-tetramethylpiperidine-1-yl)oxyl (TEMPO), the system can promote the aerobic oxidation of primary amines into imines under red light with excellent yields (Z90%) in a short reaction time (30 min). Due to the instability of imine linkages in the presence of amines, such transformations cannot be achieved by Por-COF. The same catalytic system can promote the oxidation of secondary amines into imines with almost equal efficiencies (Z88% yield in 21 min). Mechanistic studies reveal that the reaction initiates by the photoexcitation of Por-sp2c-COF, followed by an electron transfer to generate the superoxide radical anion. The superoxide then participates in the catalytic cycle involving TEMPO, which acts as a cooperative catalyst. As can be seen, these works highlight the exceptional properties of CQC linked COFs, inspiring future investigations of related systems. Being the most abundant greenhouse gas, carbon dioxide capture and conversion has garnered increasing attention over the past few decades. This has inevitably led to an increase in the research for heterogeneous catalysts

Porphyrin and Phthalocyanine Covalent Organic Frameworks

Figure 8.7

295

Por-sp2c-COF for oxidation of amine to imine.

capable of converting carbon dioxide into value-added chemicals. Nonetheless, due to the inertness of carbon dioxide, such transformations are energy intensive. Hence, it would be imperative to be able to use renewable resources such as the sun to promote these reactions. Although there have been a few COFs reported for CO2 photoreduction, they either involve rare rhenium metal complexes or additional sacrificial donors such as triethanolamine (TEOA), which are hardly sustainable in the long run.33–36 On the other hand, TEOA-free systems were also reported but with unsatisfactory conversion rates.37 Nevertheless, such systems are often challenging to construct because it requires the precise tuning of HOMO and LUMO levels to allow both redox reactions to be thermodynamically feasible. As strong absorbers of visible light, porphyrin and its derivatives have been attractive scaffolds in the fabrication of many materials capable of performing artificial photosynthesis.38–41 Furthermore, their electronic

296

Chapter 8

properties can be fine-tuned by additional substituents or metal species. Despite these advantages, porphyrin-based COFs for photocatalytic reduction of CO2 have remained largely unexplored. Recently, a series of TTCOF-M (Figure 8.8) were designed by combining strongly electron-donating tetrathiafulvalene (TTF) with porphyrin and metalloporphyrin units. Out of the four COFs, TTCOF-Zn exhibited the highest CO2 adsorption capacity of 52 cm3 g1. It also has the most ideal HOMO level for water oxidation and LUMO level for CO2 reduction, and a band gap of 1.49 eV. As a result, under visible light irradiation (420–800 nm) TTCOF-Zn could catalyze the CO2 reduction in water more efficiently than other COFs, achieving nearly a constant CO production rate of 2.06 mmol g1 h1 for 60 h and is stable for up to five cycles. Density functional theory calculations suggest that upon photoexcitation, electrons are excited from the HOMO on TTF units to the LUMO on porphyrin units, thereby achieving charge separation. The electrons are then transferred to CO2, while water donates electrons to recombine with the holes on TTF, completing the reaction cycle. Therefore, the design of COFs by rational integration of appropriate donor and acceptor moieties represents a novel strategy to achieve CO2 photoreduction.42

Figure 8.8

TTCOF-M for photoreduction of carbon dioxide.

Porphyrin and Phthalocyanine Covalent Organic Frameworks

Figure 8.9

297

TAPBB for photoreduction of carbon dioxide.

By condensation of 2,5-dibromo-1,4-benzenedialdehyde with [5,10,15,20tetrakis(4-aminophenyl)-porphyrin], TAPBB-COF was prepared (Figure 8.9). Compared to COF-366 (Figure 8.2; ¼ H2P-Ph) which contains terephthalaldehyde linker units, the introduction of electron-withdrawing bromine substituents heavily modulates the electronic properties of TAPBB-COF. Under simulated sunlight (200–1000 nm) in a gas–solid phase photocatalytic device, TAPBB-COF exhibited three times the yield of CO as compared to COF-366 (24.6 mmol g1 h1 vs. 8.5 mmol g1 h1); while under visible light irradiation (Z430 nm) TAPBB-COF achieves a CO production rate of 12.4 mmol g1 h1, three times higher than the COF-366 (3.9 mmol g1 h1) and g-C3N4 (3.2 mmol g1 h1), with 95.6% selectivity.43 The effectiveness of the donor–acceptor strategy in designing COFs for photocatalytic CO2 reduction is also demonstrated with NiP-TPE-COF (Figure 8.10), where electron-donating tetraphenylethene is paired with

298

Figure 8.10

Chapter 8

MP-TPE-COF for photoreduction of carbon dioxide.

electron-accepting Ni-porphyrin. In the presence of [Ru(bpy)3]Cl2 photosensitizer and TEOA donor, NiP-TPE-COF could achieve a CO evolution rate of 525 mmol g1 h1, with 93% selectivity. Even when the pressure of CO2 is reduced from 1 to 0.1 atm, the CO evolution rate was maintained at 386 mmol g1 h1 with 77% selectivity. NiP-TPE-COF shows negligible changes in activity after four cycles. When Co-porphyrin-based CoP-TPE-COF (Figure 8.10) is subjected to the same conditions, a higher CO evolution rate of 2414 mmol g1 h1 was achieved. However, a lower selectivity of 61% over H2 evolution was recorded. This was attributed to the stronger binding of CO to Co, and a smaller energy barrier for proton reduction compared to Ni.44 Besides electronic properties, the morphology of COFs exerts a great influence on its photocatalytic activities. Notably, COF films are attractive targets for their large surface area and exposed active sites, which maximize its light absorption capabilities and promote facile photoredox transformations. By adding a large excess of bulky 2,4,6-trimethylbenzaldehyde to the COF synthesis, the interlayer p–p interactions can be effectively disrupted, such that ultrathin nanosheets with a thickness of 1.1  0.1 nm can be produced. Amongst the series of imine-based COFs prepared, COF-367-Co (Figure 8.11) exhibited an outstanding photocatalytic CO2 to CO reduction rate, up to 10162 mmol g1 h1, with 78% selectivity. However, in this case,

Porphyrin and Phthalocyanine Covalent Organic Frameworks

Figure 8.11

299

COF-367-Co for photoreduction of carbon dioxide.

additional [Ru(bpy)3]Cl2 photosensitizer and ascorbic acid as the sacrificial donor are required. Nevertheless, COF-367-Co nanosheets outperform bulk COF-367-Co (CO production rate of 124 mmol g1 h1, selectivity of 13%) and most other COFbased materials. Even under 0.1 atm of CO2, a CO production rate of 2587 mmol g1 h1 and 72% selectivity can be achieved. Most importantly, COF-367-Co nanosheets can be reused for at least six cycles, without significant decrease in efficiency and selectivity.45

8.1.1.2

Electrocatalysis

Similar to the photocatalytic reduction of CO2, the electrocatalytic reduction of CO2 faces key issues such as sluggish reaction kinetics, multiple possible reaction pathways, and competitive hydrogen evolution reaction. However, unlike photocatalytic COFs which could achieve concurrent light absorption

300

Chapter 8

and redox reactions, electrocatalytic reduction of CO2 requires external electrochemical stimuli, which could be derived from renewable resources such as the sun, using solar cell semiconductors. Nevertheless, it is highly desirable to design electrocatalysts with low overpotential and high Faradaic efficiency, yet retaining their pristine structures after prolonged usage. Inspired by the capabilities of cobalt porphyrin in CO2 reduction, a series of Co–porphyrin-based COFs were prepared. Amongst them, COF-366-Co (Figure 8.2; ¼ CoP-Ph) exhibited an overpotential of 0.55 V, Faradaic efficiency for CO (FECO) of 90%, and turnover number per electroactive cobalt (TONEA) of 34 000; which is four times higher than molecular Co(TAP) (TONEA ¼ 8300). Replacement of the linker with biphenyl-4,4 0 -dicarboxaldehyde produces COF-367-Co (Figure 8.11). As a result of the larger pore size, a greater exposure of active sites is possible, which translates to a higher FECO of 91% and TONEA of 48 000. However, it was rationalized that the density of cobalt active sites overwhelms the degree of interaction between COF-367-Co and dissolved CO2. Hence, a lower amount of cobalt loading would be sufficient. Indeed, dilution via the multi-component strategy with copper porphyrin yields COF-367-Co(10%) and COF-367Co(1%), with improved turnover frequency per electroactive cobalt (TOFEA) of 4400 h1 and 9400 h1, respectively; compared to COF-367-Co (1900 h1). In particular, COF-367-Co(1%) could reach a high TONEA of 296 000 in the long-term electrolysis experiment.46 In a follow-up study, the linker units of COF-366-Co were decorated with electron-withdrawing methoxy and fluoro groups to give COF-366-(OMe)2Co, COF-366-F-Co, and COF-366-(F)4-Co (Figure 8.12). It was hypothesized that such modification could promote the reduction of cobalt(II) to cobalt(I), an important intermediate in CO2 electroreduction. To further enhance the contact of the catalyst with the electrode surface, these COFs were grown onto highly oriented pyrolytic graphite (HOPG) electrodes as oriented thin films. Taking COF-366-Co as the reference, the oriented films on HOPG exhibited higher TONEA and improved stability in the long-term compared to bulk COF samples and those grown on glassy carbon. As expected, when electron-withdrawing groups are introduced to the linker units, the current density for CO formation increases from 45 mA mg1 for COF-366-Co to 46 mA mg1 for COF-366-(OMe)2, and 65 mA mg1 for COF-366-F-Co. Despite having the highest number of electronwithdrawing groups, COF-366-(F)4-Co exhibits a current density of about 38 mA mg1, which is the lowest amongst the series. This is likely due to the hydrophobicity of the COF material, which reduces the contact of the electrolyte and solute with the cobalt active sites. Thus, through this work, it becomes clear that morphology as well as electronics tuning are effective strategies for designing more efficient electrocatalysts. Furthermore, other factors such as hydrophobicity should also be taken into consideration when designing COF catalysts for electroreduction.47 Besides introducing electron withdrawing moieties to enhance the efficiency of cobalt(II) to cobalt(I) reduction, electron donating groups can be

Porphyrin and Phthalocyanine Covalent Organic Frameworks

Figure 8.12

301

COF-366-Co, COF-366-(OMe)2-Co, COF-366-F-Co, and COF-366-(F)4-Co for electroreduction of carbon dioxide.

integrated to increase the rate of electron transfer to cobalt(II) for carbon dioxide reduction. With this in mind, tetrathiafulvalene-containing TTF-Por(Co)-COFs (Figure 8.8; ¼ TTCOF-M, M ¼ Co) were designed. Being a strong electron donor, tetrathiafulvalene endows TTF-Por(Co)-COF with superior electron conduction of 1.32107 S m1, more than twenty-fold higher than COF-366-Co (6.5109 S m1). Correspondingly, it also boasts of higher carrier mobility (0.26 cm2 V1 s1 vs. 0.06 cm2 V1 s1), as well as smaller impedance compared to COF-366-Co. TTF-Por(Co)-COF also showed FECO of up to 95% at 0.7V vs. RHE and CO partial current density of 6.88 mA cm2 at 0.9V vs RHE, higher than COF-366-Co (FECO ¼ 70.8%, current density ¼ 2.17 mA cm2). As a comparison, TTF-Por(2H)-COF

302

Chapter 8

(Figure 8.8; ¼ TTCOF-M, M ¼ 2H) was found to be active for the hydrogen evolution reaction (HER), with a FECO of up to 93.5%, while TTF-Por(Co)-COF (Figure 8.8; ¼ TTCOF-M, M ¼ Co) has a FECO of only 67.2%.48 Conductivity and durability of the catalyst are important aspects to consider in electrocatalysis. For this reason, a fully p-conjugated phenazinelinked CoPc-PDQ-COF (Figure 8.13) was designed to facilitate electron conduction along its horizontal planar 2D layers, as well as vertically-aligned p columns. In addition, the framework gained stability advantage from the strong phenazine linkages. As a result, CoPc-PDQ-COF exhibited a bulk conductivity of 3.68103 S m1 at 298 K, which is higher than the Ni analog COF-DC-8, reported for the sensing of various gases.49 CoPc-PDQ-COF exhibits almost unchanged crystallinities and is free of weight loss after soaking in various solvents such as dimethylformamide, dimethyl sulfoxide, tetrahydrofuran, methanol, cyclohexane, hot water, HCl (12 M), and NaOH (14 M). When coated on carbon fiber paper, CoPc-PDQ-COF exhibits a cathodic current density of up to 49.4 mA cm2 (equivalent to 762 mA mg1)

Figure 8.13

CoPc-PDQ-COF for electroreduction of carbon dioxide.

Porphyrin and Phthalocyanine Covalent Organic Frameworks

303

at 0.66 V (vs. RHE), with FECO of 96%, while the aforementioned COF-367-Co only achieves FECO of 53% at 0.67 V. Besides, CoPc-PDQ-COF also has a higher accessibility of electrocatalytically active CoPc units than COF-367-Co (4.72% vs 4%). This change results in a higher TOFEA of 11 412 h1 and TONEA of 320 000, compared to the best imine-linked COF-367-Co(1%) (TOFEA ¼ 9400 h1, TONEA ¼ 290 000).46 Most importantly, the high stability of CoPc-PDQ-COF is reflected in its long-term current density and Faradaic efficiency, whereby no significant change was observed even after 24 hours.50

8.1.1.3

Organocatalysis

Owing to the pre-designability of COFs, reactive sites can be deliberately installed onto the monomers and subsequently fabricated onto the pore walls. These walls can then be post-synthetically modified to integrate different types of organocatalytic sites, which transform the COF material into a heterogeneous catalyst. Compared to the homogeneous counterparts, COF catalysts benefit from higher stability, reusability, and ease of separation. In this context, our group developed a series of porphyrin-based COF catalysts via a multiple-component polymerization approach; employing varying ratios of 2,5-bis(2-propynyloxy)terephthalaldehyde (BPTA) and 2,5-dihydroxyterephthalaldehyde (DHTA) as linkers (Figure 8.14). Subsequently, the ethynyl-containing COFs undergo a click reaction with chiral azidefunctionalized pyrrolidine units to provide a series of [Pyr]X-H2P-COFs with varying densities of chiral catalytic sites (Figure 8.14). Compared to the homogeneous analog ((S)-4-(phenoxymethyl)-1-(pyrrolidin-2-ylmethyl)-1H-1H-1,2,3-triazole), although similar enantioselectivity was observed (49%), a shorter reaction time (1 h vs. 3.3 h) and slightly higher diastereoselectivity (70/30 vs. 60/40) was achieved when [Pyr]25-H2P-COF was utilized in an asymmetric Michael addition reaction. Notably, the COF catalyst can be isolated after the reaction by centrifugation and reused for at least four times. The excellent performance is largely attributed to its good crystallinity and porosity, which not only expose the catalytic sites, but also aid in the delivery of reactants to and the release of products from the catalytic sites. Subsequently, a [Pyr]25-H2P-COF-containing column was prepared and incorporated into a continuous flow system, where 100% conversion together with good stereoselectivities (44% ee, 65/35 dr) were maintained for more than 48 h. Thus, based on the aforementioned results, anchoring catalytic sites to COFs via post-synthetic modification represents an effective strategy to improve both efficiency and recyclability.51 In addition to post-synthetic modification with chiral molecules, direct usage of chiral monomers represents another common strategy to prepare chiral COFs. Recently, Cu-porphyrin-based chiral COF containing the BINOL-type, 6,6 0 -dichloro-2,2 0 -diethoxy-1,1 0 -binaphthyl-4,4 0 -dialdehyde linker was prepared. In consideration of the Lewis acidity of Cu(II), the photothermal conversion ability of Cu–porphyrin, as well as the chiral

304

Figure 8.14

The general strategy for the pore surface engineering of imine-linked COFs via a condensation reaction and click reaction (the case for X ¼ 50 was exemplified). Chapter 8

Porphyrin and Phthalocyanine Covalent Organic Frameworks

Figure 8.15

305

(R)-CuTAPBN-COF for asymmetric Strecker reaction.

environment provided by the BINOL-type linker, the resulting (R)CuTAPBN-COF (Figure 8.15) was anticipated to catalyze visible light induced asymmetric reactions. As expected, (R)-CuTAPBN-COF catalyzes the three-component asymmetric Strecker reaction between 2-chlorobenzaldehyde, 4,5,6,7-tetrahydrothieno[3,2c]pyridine, and trimethylsilyl cyanide. In the presence of only 2.1 mol Cu% equivalent of (R)-CuTAPBN-COF at room temperature and under visible light irradiation, the resulting (S)-2-(2-chlorophenyl)-2-(6,7-dihydrothieno[3,2c]pyridin-5(4H)-yl)acetonitrile ((S)-CIK) was produced in 98% yield and 94% enantioselectivity, in merely 3 h. (R)-CuTAPBN-COF can be incorporated into a continuous flow system, which under visible light irradiation produces over 1.29 g of (S)-CIK (90% yield) with 93% enantioselectivity in 8 h. Being an important intermediate in the production of (S)-clopidogrel, which is an antiplatelet and antithrombotic drug; (R)-CuTAPBN-COF has exhibited

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Chapter 8

remarkable performance in the large-scale synthesis of (S)-CIK. The catalyst is not only energy-efficient, but also opens new possibilities of using COFs as heterogeneous catalysts for pharmaceutical applications.52 Other than providing organocatalytic sites, incorporating catechol units into imine-linked COFs can afford bifunctional catalysts, which possess both acidic (catechol units) and basic sites (imine nitrogen). One such example is 2,3-DhaTph COF (Figure 8.16), which could catalyze the cascade acetal deprotection-Knoevenagel reaction between benzaldehydedimethylacetal and malononitrile to give 2-benzylidenemalononitrile in 96% yield, within 90 min. In addition, several other substituted dimethyl acetal reactants containing electron donating (methyl and methoxy) and withdrawing groups (nitro) were also tolerated, with more than 80% conversion rate, despite having different molecular dimensions. Compared to 2,3-DhaTph,

Figure 8.16

2,3-DhaTph COF and 2,3-DmaTph COF for cascade acetal-deprotectionKnoevenagel reaction.

Porphyrin and Phthalocyanine Covalent Organic Frameworks

307

the 2,3-DmaTph COF (Figure 8.16) which does not contain acidic sites, gave 2-benzylidenemalononitrile in only 52% yield in 90 min. Thus, this result shows that acidic catechol sites are crucial for the reaction. 2,3-DhaTph COF can be reused for at least five cycles with a slight decrease in activity.53 In lieu of the acidic sites in 2,3-DhaTph COF, the possibilities have been extended to using it to catalyze the reaction between epoxides, aziridines, and carbon dioxide to form cyclic carbonates and oxazolidinones, respectively. Interestingly, in the presence of catalytic amounts of tetrabutylammonium iodide (TBAI) and atmospheric pressure of CO2, 2,3DhaTph COF allows the formation of various cyclic carbonates with up to 94% yield, without additional solvents. The high efficiency arises from the hydrogen bond interactions between the catechol units and incoming epoxide, which promotes the ring opening as well as stabilizing the negatively-charged anion intermediate. Under similar conditions, a variety of oxazolidinones were produced in more than 86% yield with high regioselectivity (497 : 3) in a short time (o15 h).54

8.1.1.4

Metal Nanoparticle Catalysis

Being stable and porous materials with multiple anchoring sites, COFs have become attractive heterogeneous scaffolds to support the growth of metal nanoparticles. Inspired by the above, as well as the photothermal conversion properties of Cu–porphyrin, chiral CCOF-CuTPP (Figure 8.17) was prepared by Buchwald–Hartwig coupling between Cu–porphyrin and S-(þ)  2-methylpiperazine. Subsequently, Au nanorods (mean width ¼ 3 nm, mean length ¼ 30 nm) and Pd nanoparticles (2–5 nm) were loaded to give Au@CCOF-CuTPP and Pd@CCOF-CuTPP, respectively. Au@CCOF-CuTPP was found to catalyze the asymmetric Henry reaction between a range of benzyl alcohol and nitromethane. In the presence of only 1 mol Au% equivalent of Au@CCOF-CuTPP, the resulting b-nitro alcohol was produced in almost quantitative yield with at least 94% enantioselectivity, under visible light irradiation. Notably, Pd@CCOF-CuTPP can catalyze the one-pot A3-coupling (aldehyde–alkyne–amine) reaction to generate a variety of propargylamines in up to 98% yield and 98% enantioselectivity under visible light irradiation. In fact, both catalytic systems can proceed even under natural sunlight irradiation, maintaining high enantioselectivities (96% ee and 92% ee, respectively) albeit with lower yields (49% and 45%, respectively). Nevertheless, the capability of a heterogeneous catalyst to perform asymmetric catalysis for many cycles using renewable energy from the sun makes CCOF-CuTPP an attractive alternative to traditional homogeneous and heterogeneous catalysts.55

8.1.2

Adsorption

Although COFs possess aligned one-dimensional channels, dense functional group distribution, large surface areas, and permanent porosities, selective

308

Figure 8.17

Chapter 8

CCOF-CuTPP for asymmetric Henry reaction and A3-coupling reaction.

adsorption of gases often requires modification of the pore walls. Conventionally, this was achieved by the integration of specific functional groups that allow for specific interactions with the gaseous molecules.56,57 Besides introducing these functional groups in the form of monomers or generating them in situ during COF synthesis, our group has developed a facile post-synthetic pore surface engineering strategy to increase the CO2 uptake capacity of COFs while retaining their crystallinity. Specifically, a series of [HO]X%-H2P-COFs with different densities of phenolic groups were prepared via the multiple-component approach. These COFs were then treated with succinic anhydride, which allows the attachment of carboxylic acid groups via a ring opening reaction, yielding the corresponding [HO2C]X%-H2P-COFs (Figure 8.18). Amongst the series, [HO2C]100%-H2P-COF demonstrated a CO2 uptake capacity of 174 mg g1 at 1 bar and 273 K, comparable to the state-of-the-art organic and inorganic adsorbents. Moreover, [HO2C]100%-H2P-COF displayed a higher selectivity for CO2 uptake over N2 from flue gas (15% CO2, 85% N2)

Porphyrin and Phthalocyanine Covalent Organic Frameworks

Figure 8.18

The general strategy for the pore surface engineering of imine-linked COFs via a condensation reaction and ring opening reaction (the case for X ¼ 50 was exemplified).

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compared to [HO]100%-H2P-COF. Even at higher pressures of 100 kPa, [HO2C]100%-H2P-COF maintains an adsorption selectivity of 77. The performance of [HO2C]100%-H2P-COF under kinetic flowing gas conditions was also examined. In a packed-bed adsorber, [HO2C]100%-H2P-COF displayed a longer breakthrough time than [HO]100%-H2P-COF, signifying a greater tendency for CO2 uptake upon functionalization with carboxylic acids. Most importantly, after CO2 adsorption [HO2C]100%-H2P-COF can be regenerated in pristine conditions under vacuum and heating at 353 K. In fact, it has been demonstrated that [HO2C]100%-H2P-COF can tolerate more than ten adsorption–desorption cycles without sacrificing its uptake capacity or structural integrity.58 Following our pioneering work, the effect of introducing azo groups to [HO]X%-TAPH-COFs was also examined. Starting from the same series of hydroxyl functionalized [HO]X%-H2P-COFs (Figure 8.19), azobenzene and stilbene units were attached to the COF channels via nucleophilic acyl substitution, producing [NQN]X%-TAPH-COFs and [CQC]X%-TAPH-COFs, respectively (Figure 8.19). In lieu of the CO2-philic and N2-phobic azo units, [NQN]25%-TAPH-COF exhibited high CO2 uptake of 207 mg g1 at 273 K as well as a high selectivity of 111 at 298 K for CO2 from a flue gas mixture.59 To further diversify the possible functional groups that can be anchored to the pore walls, our group prepared a series of COFs with different densities of ethynyl groups, [HCRC]X-H2P-COFs. Subsequently, these were reacted with different azide-functionalized units, such as ethyl, acetate, hydroxyl, carboxylic acid, and amino groups (Figure 8.20), via click reactions. In lieu of the different properties that these units possess, the resultant [R]X-H2P-COFs have drastically different pore environments ranging from hydrophobic to hydrophilic, and acidic to basic. In this study, it was observed that the integration of acetate, hydroxyl, carboxylic, and amino groups could enhance CO2 uptake compared to [HCRC]X-H2P-COFs and [Et]X-H2P-COFs. Notably, as a result of the acid–base interactions with CO2, [EtNH2]50-H2P-COF displayed CO2 uptake of 157 mg g1 at 273 K, which is the highest amongst the series. The performance of [EtNH2]50-H2P-COF and [HCRC]50-H2P-COF under kinetic flowing gas conditions were also examined in a packed-bed absorber. [EtNH2]50-H2P-COF displayed a longer breakthrough time than [HCRC]50-H2P-COF, signifying a greater tendency for CO2 uptake upon functionalization with amino groups.60 Hence, from these examples, it can be concluded that the pore surface engineering strategy has proven to be effective in designing COFs for adsorption of gases. Other than CO2 adsorption, bromate (BrO3) adsorption from drinking water is another important area of research, as the latter is known to be a carcinogen. It would be highly desirable to design a cationic adsorbent that can promote electrostatic attraction with BrO3 to facilitate its removal. Based on this principle, cationic viologen-linked PV-COF (Figure 8.21) was prepared via the Zincke reaction and it was found to exhibit remarkable BrO3 removal efficiency.

Porphyrin and Phthalocyanine Covalent Organic Frameworks

Figure 8.19

The general strategy for the pore surface engineering of imine-linked COFs via a condensation reaction and nucleophilic acyl substitution reaction (the case for X ¼ 50 was exemplified).

311

312

Figure 8.20

The general strategy for the pore surface engineering of imine-linked COFs via a condensation reaction and click reaction (the case for X ¼ 100 was exemplified).

Chapter 8

Porphyrin and Phthalocyanine Covalent Organic Frameworks

Figure 8.21

PV-COF, Zn-PV-COF, and Red-PV-COF for bromate removal.

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Specifically, 5 mg of PV-COF is sufficient to decrease BrO3 concentration from 50 mg L1 to less than 3 mg L1 in 20 min. This translates to an adsorption rate of 191.4 g mg1 min1, which is the highest value compared to other common adsorbents. Furthermore, it exhibited the highest BrO3 adsorption capacity of 203.8 mg g1. Despite the presence of other anionic species (HCO3, SO42, Cl, NO3, and F) at a thousand times the concentration of BrO3, PV-COF is still able to remove up to 93% of BrO3 within 20 min from commercial water samples. Most importantly, PV-COF can be regenerated and recycled for at least three cycles without an obvious change in adsorption efficiency. By metalation and reduction of PV-COF, Zn-PV-COF and Red-PV-COF can be prepared, respectively (Figure 8.21). However, both COFs displayed lower BrO3 adsorption efficiencies than PV-COF. This was attributed to the loss in hydrogen bonding interactions between pyrrole hydrogen and BrO3, as well as the absence of electrostatic interactions between the COF skeleton and BrO3.61

8.1.3

Semiconductors

COFs are characterized by strong interlayer p–p interactions, which drive the formation of highly ordered and crystalline architectures. The aligned p-columns, together with the shortened intermolecular distances, create highways for electron and hole transport. These unique features open a new platform for designing semiconductors, with the potential to be fabricated into electronic and optoelectronic devices. The imine-linked COF-366 (Figure 8.2; ¼ H2P-Ph) and boronate ester-linked COF-66 (Figure 8.22) were prepared and examined for semiconducting properties. Notably, both COFs experienced a surge in current when 0.2 V bias voltage was applied across them. Moreover, laser flash photolysis time-resolved microwave conductivity (FR-TRMC) measurements registered currents with a maximum fSm of 4.1105 cm2 V1 s1 and 1.7105 cm2 V1 s1, respectively. Moreover, the number of charge carriers at 0 V bias was estimated to be 3.2109 and 4.5109, respectively. Accordingly, both COFs are p-type semiconductors with hole mobilities of 8.1 cm2 V1 s1 and 3.0 cm2 V1 s1, respectively. Remarkably, such mobilities are even higher than inorganic amorphous silicon (1 cm2 V1 s1) and most other organic semiconductors.62 Our group has pioneered the synthesis of porphyrin COFs as semiconductors. We prepared a series of boronate ester-linked MP-COFs (Figure 8.23, M ¼ H2, Zn, and Cu) with free base, zinc, and copper porphyrin knot units. The FR-TRMC analysis shows that H2P-COF is a p-type semiconductor, with fmh of 1.8104 cm2 V1 s1, and CuP-COF is a n-type semiconductor with fme of 1.16104 cm2 V1 s1, while ZnP-COF is ambipolar with fmh of 3.36105 cm2 V1 s1 and fme of 5.4105 cm2 V1 s1. Comparatively, H2P-COF exhibits a hole mobility of 3.5 cm2 V1 s1, while ZnP-COF has hole and electron mobilities of 0.032 and 0.016 cm2 V1 s1, respectively. Lastly, CuP-COF has an electron mobility of 0.19 cm2 V1 s1. Henceforth, these

Porphyrin and Phthalocyanine Covalent Organic Frameworks

Figure 8.22

315

COF-66 for hole transport.

observations suggest that the central metal ion plays an indispensable role in modulating the charge carrier properties of COFs. Specifically, without metal species, the porphyrin columns with a high electron density facilitate hole transport. In the presence of Cu(II), ligand-to-metal charge transfer occurs, thereby lowering the electron density on porphyrin macrocycles. Thus, electron transport is more favorable through metal-on-metal columns. On the other hand, ligand-to-metal charge transfer does not occur in the case of Zn(II), resulting in balanced charge carrier transport. Hence, ZnP-COF is ambipolar. As a result of its ambipolar characteristics, ZnP-COF shows the highest photocurrent of 26.8 nA when irradiated with visible light (4400 nm), with a high on–off ratio of 5104. Additionally, ZnP-COF achieves higher photocurrent when irradiated with long-wavelength visible and near infrared light (4600 nm).63 Although porphyrin units are often used as knots in the construction of tetragonal COFs, desymmetrization can result in C2-symmetric porphyrin

316

Figure 8.23

Chapter 8

H2P-COF, Zn-COF, and CuP-COF for hole, ambipolar, and electron transport, respectively.

units that can be incorporated as struts. One example is the hexagonal TPPor COF (Figure 8.24), prepared via the condensation between triphenylene (Tp) and porphyrin (Por) subunits. Owing to the aligned donor and acceptor p-columns, a bicontinuous heterojunction is created, endowing TP-Por COF with unique electronic properties. Specifically, upon photoexcitation, electrons are transferred from triphenylene to porphyrin units, forming Tp1 and Por as evident from the photoinduced absorption spectroscopy (PIA) spectrum. Tp-Por COF sandwiched between ITO/MoOx and ZnO/Al electrodes, produced an open-circuit voltage of 312 mV and a short-circuit current density of 44.6 mA cm2 upon irradiation with simulated sunlight.64 Other than porphyrin, electron-rich planar phthalocyanine derivatives are also attractive units for the construction of COFs. Owing to the large macrocyclic p-system, these COFs have strong interlayer p-interactions and

Porphyrin and Phthalocyanine Covalent Organic Frameworks

Figure 8.24

317

TP-Por COF with semiconducting properties.

well-aligned p columns for exploring semiconducting properties. Fully p-conjugated phenazine-linked metallophthalocyanine-containing MPc-pzCOF (Figure 8.25) was prepared and characterized in terms of its electronic properties. Unlike the situation in MP-COF, it was found that the central metal species (Zn and Cu) had negligible effects on the resulting conductivity (B5107 S cm1), charge density (B1012 cm3), charge carrier scattering rate (B31013 s1), and effective mass (B2.3 m0) of hole carriers. Hole mobilities of up to 5 cm2 V1 s1 were also registered.65 Our group reported the first metallophthalocyanine NiPc COF (Figure 8.26), which features high carrier mobility and photoconductivity. Specifically, FR-TRMC measurements revealed that NiPc COF has a transient conductivity (fSm) of 3.9105 cm2 V1 s1, with a hole mobility of 1.3 cm2 V1 s1. Additionally, the photocurrent of up to 3 mA can be detected when irradiated (4400 nm), with high sensitivities for deep-red and near-infrared photons.66 As well as than NiPc COF, we investigated CuPc COF, CoPc COF, and ZnPc COF (Figure 8.26); whereby they yielded a fSm of 1.4104 cm2 V1 s1, 2.6104 cm2 V1 s1, and 2.2104 cm2 V1 s1, respectively. The charge carriers mainly originate from holes and the trend is dependent on

318

Figure 8.25

Chapter 8

MPc-pz COF for hole transport.

the electron density of the phthalocyanine macrocycle (CuPc COFoZnPc COFoCoPc COF). The three COFs are photoresponsive, and currents of 110 nA, 0.14 nA, and 0.6 nA are registered respectively, upon irradiation with visible light (4400 nm).67 Subsequently, our group replaced the linker with the electron-deficient benzothiadiazole unit for the construction of donor–acceptor 2D-NiPc-BTDA COFs (Figure 8.27). Interestingly, the 2D-NiPc-BTDA COF exhibited distinct differences as compared to the NiPc COF. Notably, unlike the NiPc COF, the 2D-NiPc-BTDA COF is an n-type semiconductor with high electron mobility of 0.6 cm2 V1 s1, a fSm value of 5.8104 cm2 V1 s1, and it can generate up to 15 mA when irradiated (4400 nm). Similar to the NiPc COF, the 2D-NiPc-BTDA COF is sensitive to near-infrared photons.68 The donor–acceptor strategy was further extended to the construction of DZnPc-ANDI-COF (Figure 8.28). The DZnPc-ANDI-COF consists of periodically ordered alternating p-columns of electron-rich zinc phthalocyanine and electron-deficient naphthalenediimide units, forming bicontinuous yet segregated super heterojunctions that promote photoinduced electron transfer and charge separation. Furthermore, the aligned p-columns offer

Porphyrin and Phthalocyanine Covalent Organic Frameworks

Figure 8.26

319

MPc COFs for hole transport.

independent pathways for hole and electron transport to electrodes. Specifically, when dispersed in benzonitrile solution, charge separation completes within 1.4 ps upon photoexcitation at 430 nm to achieve a long lifetime of 10 ms, indicating a good suppression of charge recombination. In DMF solution, delamination results in the formation of COF layers which further decreases the lifetime to 217 ps. On the other hand, in the solid state, the DZnPc-ANDI-COF exhibited lifetimes of 1.8 and 1500 ms at 280 and 80 K, respectively.69 In order to demonstrate the generality of this strategy, our group designed and prepared a series of metallophthalocyanine-diimide COFs (Figure 8.29). These COFs contain different types of electron-rich metallophthalocyanine (CuPc, NiPc, and ZnPc), as well as different types of electron-deficient diimide derivatives (pyromellitic diimide, naphthalene diimide, and perylene diimide), allowing for the systematic study of donor–acceptor systems. We found that DCuPc-APyrDI-COF, DCuPc-ANDI-COF, DNiPc-APyrDI-COF, and DNiPc-ANDI-COF exhibited two-component decays with CuPc-containing COFs having longer final lifetime values (33 ms) than NiPc-containing COFs (26 and 29 ms). These lifetimes are longer than the one reported for

320

Figure 8.27

Chapter 8

2D-NiPc-BTDA COF for electron transport.

DZnPc-ANDI-COF (11 ms). Through detailed investigations, we found that charge separation and lifetime are not affected significantly by the lattice size of COFs.70 In consideration of the large and ordered one-dimensional pore channels of the COFs, bulky electron-accepting fullerene units can be integrated to convert electron-rich frameworks into donor–acceptor heterojunctions. In particular, via a three-component reaction, our group installed azide units onto the pore walls to form X%N3-ZnPc-COFs, which can subsequently undergo facile click reactions to anchor different ratios of fullerene in the pore spaces (Figure 8.30). We found that without fullerene, charge-separated states were not formed upon photoexcitation, while in the presence of fullerene acceptors charge transfer occurs to form ZnPc1 and C60 , with a long lifetime of 2.37 ms ([C60]0.4-ZnPc-COF). Likewise, long lifetimes of 2.66 and 2.49 ms were observed for [C60]0.3-ZnPc-COF and [C60]0.5-ZnPc-COF, respectively.71

Porphyrin and Phthalocyanine Covalent Organic Frameworks

Figure 8.28

8.1.4

321

DZnPc-ANDI-COF for charge separation and transport.

Energy Storage

Energy storage devices such as supercapacitors, pseudocapacitors, and batteries have garnered increasing attention over the past decades. However, combining high power and energy density with fast charge–discharge rates into one material has always remained a challenge. Fortunately, the predesignable skeletons of COFs allow facile integration of redox-active units, while its porous and ordered network allows for quick ion and electrolyte diffusion without sacrificing its stability. In lieu of these merits, our group developed the first radical COFs, Ni-porphyrin-based COFs, [TEMPO]X%-NiPCOFs (Figure 8.31) for capacitive energy storage. Using a multi-component approach we first prepared a series of ethynyl-containing [HCRC]X%-NiP-COFs, before undergoing a click reaction to convert electrochemically inactive COFs to redox-active [TEMPO]X%-NiP-COFs (Figure 8.31).

322 DCuPc-APyrDI-COF, DCuPc-ANDI-COF, DCuPc-APDI-COF, DNiPc-APyrDI-COF, DNiPc-ANDI-COF, and DZnPc-APDI-COF for charge separation and transport.

Chapter 8

Figure 8.29

Porphyrin and Phthalocyanine Covalent Organic Frameworks

Figure 8.30

Schematic representation of converting open lattice structures into segregated donor–acceptor arrays.

323

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Figure 8.31

The general strategy for the pore surface engineering of imine-linked COFs via a condensation reaction and click reaction (the case for X ¼ 50 was exemplified). Chapter 8

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325

Owing to the ability of TEMPO to reversibly switch between the neutral radical and oxoammonium cation, [TEMPO]X%-NiP-COFs exhibited remarkably high reversibility in charge–discharge experiments across a range of current densities. [TEMPO]100%-NiP-COF and [TEMPO]50%-NiP-COF exhibited capacitances of 167 and 124 F g1 at a current density of 100 mA g1, respectively. Even when the current density is increased to 2 A g1, capacitances of 113 F g1 and 101 F g1 can be maintained, respectively. Most importantly, [TEMPO]50%-NiP-COF retains its redox capabilities even after 100 charge–discharge cycles at a current density of 500 mA g1.72 As can be seen, this work not only expanded the possibility of developing radical COFs, but also demonstrated the advantageous structural features of COFs for energy storage applications. Following our findings, another Ni-porphyrin-based COF, JUC-511 (Figure 8.32) was prepared to exhibit promising properties as an electrochemical double-layer capacitor (EDLC). A series of COFs was prepared via solvothermal synthesis, before chemical exfoliation to produce e-COF nanosheets of thickness 19–22 nm. Compared to the parent COFs, e-COFs are expected to have higher accessibilities of surface area for the formation of EDL, as well as improved ion and electrolyte diffusion kinetics. Indeed, this was reflected in the cyclic voltammetry (CV) curves of e-COFs, which exhibited near rectangular shapes even at a high scan rate of 10 000 mV s1. Specifically, e-JUC-511 and e-JUC-512 achieved areal capacitance of 5.46 and 5.85 mF cm2 at a current density of 1000 mV s1, while power densities of 5.32 and 4.08 W cm3 were attained, respectively. Both e-COFs maintained almost 100% capacitance even after 10 000 cycles showing potential for long-term repeated usage. Finally, capacitor cells consisting of the respective e-COFs were prepared to achieve power densities of 4.1–5.4 W cm3 with moderate energy densities of about 0.27 mW h cm3. Certainly, these outstanding results make e-COF capacitor cells highly competitive amongst other commercially available materials.73 With a high theoretical capacity (1675 mA h g1) and energy density (2600 Wh kg1), lithium–sulfur batteries represent an attractive candidate for energy storage. However, the shuttle effect caused by the dissolution of polysulfide has been a major challenge in the development of electrode materials. For this reason, electrode materials capable of confining polysulfide within the porous framework are highly desirable. Intrigued by the designable pore environment and the high porosity of COFs, Por-COF (Figure 8.6) was prepared and impregnated with sulfur for lithium–sulfur batteries. Specifically, Por-COF achieves a 55wt% sulfur loading content in the framework. In a coin cell set-up employing a Por-COF/S cathode, a discharge capacity of 929 mAh g1 was achieved at the 2nd cycle, which decreases to 633 mAh g1 after 200 cycles at a current density of 840 mA g1, whilst retaining a coulombic efficiency of 96%. This not only reflects a reduced likelihood of the shuttle effect, but also the structural integrity of Por-COF. Furthermore, Por-COF tolerates a higher current density of 1680 mA g1, while retaining a capacity of 670 mAh g1.74

326

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Figure 8.32

8.1.5

JUC-511 and JUC-512 for capacitive energy storage.

Summary and Perspectives

Porphyrin, phthalocyanine, and their metallo-compounds are typical 18-p macrocycles with unique structures and various functions. Topologydirected polycondensation of porphyrin, phthalocyanine, and their metallo-compounds with other building units enables the construction of various COFs to achieve different topologies. The resulting frameworks constitute ordered p columns of porphyrin, phthalocyanine, and their metallo-compounds, and well-defined pores with discrete pore size and shape. Pore surface engineering offers a general approach to design pore environments by integrating a diversity of functional groups onto the pore walls with the desired density and components to generate tailor-made interfaces. This designability and synthetic controllability of the structures opens up a new area of chemistry to produce a broad diversity of organic materials whose structures are unique and inaccessible with other

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327

supramolecular and macromolecular approaches. Porphyrin and phthalocyanine COFs exhibit a diversity of functions which are highly correlated with their ordered p columns and pores, showing great potential for producing advanced materials. From a chemistry perspective, the designed synthesis of these frameworks in the form of single crystals other than the current crystallite solids remains a substantial challenge and deserves further efforts. The deployment of single crystals helps in the further elucidation of the nature of phenomena. From the viewpoint of physics, investigating the elementary processes involved in the photocatalysis, electrocatalysis, and energy storage at different spatiotemporal scales to elucidate the effects of structures and the mechanisms is necessary, which will disclose the uniqueness of COFs. From the perspective of materials science, the development of methods to prepare large-scale homogenous thin films is still a difficult issue to be addressed, which will extend these well-defined p framework materials to important applications and practical industrial use.

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CHAPTER 9

Catalysis via Porphyrin-based COFs Y. YUSRAN, X. GUAN, S. QIU AND Q. FANG* State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, Jilin University, Changchun 130012, P. R. China *Email: [email protected]

9.1 Introduction Porphyrins can be easily found in nature and generally act as catalysts for certain reactions.1 The molecular derivatives such as heme and chlorophyll are typical supramolecular-based porphyrin compounds that function as essential catalysts in the red blood cells and green plants.2,3 Porphyrin is a heterocyclic 18p macrocycle compound with four pyrrole units and four bridging carbon atoms within its planar conformation.4 The strong aromaticity and the richness of the metal coordination chemistry promise huge utilization of porphyrin across a wide range of research disciplines. Particularly, porphyrin can be functionalized with modifiable active sites that are necessary properties for catalysis application. In addition, porphyrin and its derivatives are optically active and photo-active compounds.4,5 Hence, they have great potential for photocatalysis. As well as being an individual and small molecular compound, porphyrin is also widely polymerized into molecular frameworks (polymers), acting as a building block. The control over the structural rigidity and connectivity of porphyrin, provide an access to design porphyrin building blocks for constructing organic polymers (see Scheme 9.1). Various types of

Monographs in Supramolecular Chemistry No. 32 Porphyrin-based Supramolecular Architectures: From Hierarchy to Functions Edited by Shengqian Ma and Gaurav Verma r The Royal Society of Chemistry 2022 Published by the Royal Society of Chemistry, www.rsc.org

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332

Scheme 9.1

Chapter 9

Typical porphyrin building blocks.

functional groups (e.g., –OH, NH2, COOH, –CN) can be settled at the mesoor b-position of the porphyrin. Meanwhile, numerous reactive metal ions can be incorporated onto the free-base sites to generate metalloporphyrin. Hence, these functionalized building blocks are readily employed for designing functional porous frameworks. Indeed, porphyrin-based building blocks have been widely employed to construct porous organic polymers (POPs) and metal–organic frameworks (MOFs).6,7 In the rapid development of porous materials research, the new discoveries of highly crystalline porous organic polymers which were termed covalent organic frameworks (COFs) have attracted huge attention in the recent years.8 COFs demonstrate low density, functional tunability, and high structural precision.9,10 With the well-positioning of the organic building blocks into 2D and 3D structures, COFs provide accessible surfaces and active sites within the pore.11 Such properties are beneficial for applications in catalysis, whereby the porosity and exposed catalytic active sites are paramount. Indeed, like MOFs, COFs have also been widely researched in the field of catalysis.12,13 Intensive research and development of functional COFs has been clearly observed in the last decade.10 This encompasses the precise integration of catalytically active building blocks, such as porphyrin, into the COF structures. Various types of porphyrin-COFs have been synthesized and explored for catalysis applications. The controlled synthesis of COFs with high chemical and thermal stabilities, along with their high porosity, procures porphyrin COFs as excellent candidates for various types of catalysis applications.

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9.2 Design and Description of Porphyrin COFs 9.2.1

Structural Topology and Linkage Types

The topology describes the structures and pore dimensions of porous materials which greatly affects the active surface and the exposed catalytic sites. The topology of COFs relies on the specific shapes, geometries, and functionalities of the building blocks. Porphyrin COFs have been prepared as 2D or 3D networks with distinct pore shapes and functional sites. On the other hand, the functional groups of the porphyrin building blocks determine the linkage types of the resultant porphyrin COFs. The modular nature of COFs serves the designable structural topologies, adjustable porosities, and tunable functionalities within the frameworks by selecting suitable building blocks.9,14,15 Hence, porphyrin moieties can judiciously be aligned and assigned on the framework of COFs at the accurate positions. Such flexible structural design strategy provides an excellent platform to design efficient catalysts. Most of the porphyrin COFs are based upon the tetratopic building blocks (see Scheme 9.1) with D4h symmetry or ditopic linear building blocks. Depending on the linkers chosen, porphyrin-COFs possess structural topologies belonging to sql, pts, and hcb nets.16 One of the wellknown examples of porphyrin COFs was reported by Yaghi and co-workers in 2011 (COF-366 and COF-66).17 COF-366 was a crystalline 2D COF, prepared by condensing the square-shape 5,10,15,20-tetrakis( p-aminophenyl)porphyrin (TAPP, M ¼ H2) with a linear terephthalaldehyde (TPA) linker (see Figure 9.1a). This [4 þ 2] symmetry combination generates a porphyrin COF with 2D sql topology and square-shaped channels. Upon condensing the longer linear linker (e.g., 2,3,4,5-tetrahydroxy anthracene, THAn) with 5,10,15,20-tetrakis( p-boronic acid-phenyl)porphyrin (TBPP, M ¼ H2), COF-66 was obtained with a similar topology (sql) and a larger pore size. The combination of [4 þ 4] building blocks also produces porphyrin COFs with sql topology. For instance, the condensation of the square TAPP (M ¼ H2, Zn) with 5,10,15,20-tetrakis(4-formylphenyl)porphyrin (TFPP, M ¼ H2, Ni, Cu) yields crystalline 2D porphyrin COFs (Por-COF-HH, Por-COFZnNi, Por-COF-ZnCu) with sql topology and typical square-shape pores.18 Such COF structures are composed of two porphyrin cores at the adjacent vertices; hence they possess dense catalytically active porphyrin sites. On the other hand, stitching rectangular (D2h) building blocks with square shape porphyrin (D4h) could also produce porphyrin COFs with sql topology (see Figure 9.1b). For example, a rectangular shaped 1,3,6,8-tetrakis(4formylphenyl) pyrene (TFFPy) was condensed with square-shaped TAPP (M ¼ H2) to produce SB-PORPy COF with permanent microporosity.19 This 2D COF showed a unique kind of distorted layers and formed a crystalline quasi-2D structure due to the combination of slightly distinct building block shapes.

334 Typical design structure of porphyrin-COFs. (a) Design synthesis of porphyrin-COFs (COF-366) via [4 þ 2] symmetry combination. Reproduced from ref. 17 with permission from American Chemical Society, Copyright 2011. (b) Design synthesis of porphyrin-COFs (COF-366) from square-shaped porphyrin and a rectangular-shaped linker. Reproduced from ref. 19 with permission from American Chemical Society, Copyright 2017.

Chapter 9

Figure 9.1

Catalysis via Porphyrin-based COFs

335

Stitching porphyrin and organic linkers with varied geometries would generate porphyrin COFs with diverse topologies (e.g., hcb and pts). The linear linker 5,15-bis(4-boronophenyl)porphyrin (BBP, M ¼ H2) was reacted with the trigonal 2,3,6,7,10,11-hexahydroxytriphenylene (HHTP) to produce 2D Tp-Por COF with hcb net (see Figure 9.2a).20 Since Tp-Por COF was obtained via [2 þ 3] symmetry combination, it exhibited hexagonal-shaped pores with a pore size of 4.6 nm. Structurally, this COF is composed of porphyrin subunits located at the edges and triphenylene subunits at the vertex. In a recent report,21 a 3D porphyrin COF was successfully constructed. The crystalline 3D-Por-COF was obtained by [4 þ 4] symmetry combination of a square unit TFPP (M ¼ H2) with tetrakis( p-aminophenyl)methane (TAPM) (see Figure 9.2b). This 3D COF possesses pts topology featuring microporosity and a 2-fold interpenetrated structure. Indeed, this successful design of 3D porphyrin COFs will inspire the wider design of porphyrin COFs with higher surface areas and accessible catalytic sites via 3D frameworks. Another important aspect of porphyrin COFs is the linkages that stitch the building blocks. The linkage greatly affects the chemical and thermal stabilities of the framework and may also function as active sites for the catalytic reaction.14,22 A majority of the porphyrin COFs are governed by imine linkages, which are based on the Schiff-base condensation reaction of aldehyde and amine functional groups.23 Hence, imine-based porphyrin COFs may contain abundant imine (–CQN–) bonds within their structures. Another linkage that is widely adopted to construct porphyrin COFs is the boronate ester linkage. Porphyrin COFs based on this linkage are obtained by stitching building blocks with boronic acid and catechol functional groups. Hence, the resultant frameworks are connected via boron–oxygen bonds. For instance, the condensation reaction between TBPP (M ¼ H2, Zn, Cu) and 1,2,4,5-tetrahydroxybenzene (THB) produces a series of 2D MP-COFs with boronate ester linkages.24 The polymerization reaction of arylhydroxylamine groups (e.g., 5, 10, 15, 20-tetrakis (arylhydroxylamine)porphyrin, TAHAP) has also been used to produce porphyrin COFs with electron-rich azodioxy (–ONQNO–) linkages.25 The imide bonds with high thermal stability have also been employed to connect the porphyrin building blocks to produce polyimide-based porphyrin COFs. For instance, the condensation between TAPP (M ¼ H2) and perylenetetracarboxylic dianhydride (PTCA) produced 2D PI-COF with very high thermal stability (up to 500 1C).26 The rigidity and high chemical stability of the CQC bond have also been adopted to design porphyrin COFs. In a recent report, a sp2-carbon-conjugated porphyrin COF (Por-sp2c-COF) was prepared by reacting TFPP (M ¼ H2) with 1,4phenylenediacetonitrile (PDAN) via Knoevenagel condensation.27 Eventually, the presence of CQC bonds enhanced the chemical stability of the COF even under harsh conditions. An intriguing advantage of the pre-designed porphyrin COFs is the flexibility and tunability for the incorporation of additional functions into the framework that may contribute to the catalytic performance. For example, hydrogen bonding that can interlock the layer

336

Figure 9.2

Chapter 9

Design structures of porphyrin-COFs obtained from varied building block geometries. (a) Synthesis of 2D Tp-Por-COF with hcb topology from the combination of a linear porphyrin and a trigonal linker. Reproduced from ref. 20, https://pubs.acs.org/doi/10.1021/ja509551m, with permission from American Chemical Society, Copyright 2014. (b) Synthesis of 3D-Por-COF and 3D-CuPor-COF with pts topology from the combination of two square-shaped building blocks. Reproduced from ref. 21 with permission from American Chemical Society, Copyright 2017.

structure can be judiciously installed in the framework by condensing a dihydroxyterephthalaldehyde (DHTA) linker with porphyrin (e.g., TAPP).28 Similarly, enforcing porphyrin J-aggregate in the layer structure of porphyrin

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COF has also been demonstrated to enhance the planarity and crystalline packing of the COF.29 Nevertheless, such tailorable structural composition will contribute to the great potential of porphyrin COFs in the field of catalysis.

9.3 Catalysis by Porphyrin-based COFs 9.3.1

Catalytic Sites of Porphyrin COFs

COFs are highly porous and stable materials, exhibiting necessary properties for catalysis applications.12,14 To enable porphyrin COFs catalyzing a certain reaction, specific design of the framework with accessible catalytic centers is crucial. The –NH groups present on the pyrrole rings of the free base porphyrin possess basic properties that have the potential to act as catalytic sites for the organocatalytic reactions.30 Hence, metal-free porphyrin COFs can be promising as heterogeneous catalysts. Furthermore, the imine bonds of the iminelinked porphyrin COFs can act as additional basic sites. Thus, imine-linked porphyrin COFs are attractive heterogeneous organocatalysts. Besides the free base sites, introduction of reactive metals at the porphyrin or at the linkage could transform porphyrin COFs into catalytic materials. Indeed, metalloporphyrin compounds have been explored as prime examples of electrocatalytic materials.31,32 Hence, metalloporphyrin COFs are great candidates for electrocatalytic applications. Figure 9.3a depicts a typical metalloporphyrin COF structure that is able to electro-catalyze the reduction of CO2.33 As another strategy, appending reactive functional groups on the pore wall could also mediate porphyrin COFs to possess catalytic sites. Specifically, such porphyrin COFs may contain more than one active site (the porphyrin cores and the functional groups). Interestingly, the incorporation of these reactive functional groups can be performed in both bottom-up (building block self-assembly) and post-synthetic approaches.34–36 Figure 9.3b illustrates a typical example of a porphyrin COF (2,3-DhaTph) bearing two catalytic sites (in the free-base porphyrin core and in the –OH functional groups).37 Hence, 2,3-DhaTph was able to catalyze a cascade reaction. Another common strategy to transform porphyrin COFs into an ideal catalyst material is compositing with other conductive or catalytic materials (e.g. conductive carbon, carbon nanotubes (CNTs), graphene, polymer, and metals). In this kind of catalyst design, the resultant composites work synergistically to catalyze the desired reaction. For instance, a 2D porphyrin COF (CuPor-Ph-COF) was in-situ grown onto the surface of 2D graphitic carbon nitride (g-C3N4) to produce a 2D/2D heterojunction photocatalyst (CuPor-Ph-COF/g-C3N4) which could perform excellent electrocatalytic degradation of organic pollutants.38 As clearly elaborated, catalytic sites in porphyrin COFs are adjustable and can be settled in various strategies. Thus, porphyrin COFs are attractive catalytic materials for a wide range of reactions.

338

Chapter 9

Figure 9.3

9.3.2

Typical design of catalyst-based porphyrin COFs. (a) The structure of metalloporphyrin COF electrocatalyst with reactive metal on the porphyrin for reducing CO2. Reproduced from ref. 33 with permission from American Chemical Society, Copyright 2018. (b) The structure of a porphyrin COF heterogeneous catalyst with bifunctional catalytic sites for catalyzing a tandem reaction. Reproduced from ref. 37 with permission from the Royal Society of Chemistry.

Catalysis Application of Porphyrin COFs

The unique structures, high porosity, and abundant catalytic sites in porphyrin COFs make them essential as catalytic materials. Nevertheless, the

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339

high chemical and thermal stabilities as well as adjustable functionalities on the framework enable porphyrin COFs to execute a wide range of catalytic reactions with excellent performance. Table 9.1 summarizes the application of porphyrin COFs in the field of catalysis, including for organocatalysis, biocatalysis, photocatalysis, and electrocatalysis. As clearly tabulated, a number of porphyrin COFs have been designed and studied as promising catalysts with notable activities.

9.3.2.1

Organocatalysis

The presence of organocatalytic sites in porphyrin COFs promise great potential for catalyzing various types of organic reactions. Organocatalysis is one of the important catalysis reactions having large reaction scope in pharmaceutical and food industries.39,40 Recent reports indicate that a number of porphyrin COFs have been examined as promising heterogeneous catalysts for a variety of organic reactions, including organic C–C coupling reactions, organic oxidation–reduction reaction, deacetylation reaction, and addition reaction. A metal-free mesoporous porphyrin COF (A2B2-Por-COF) was designed as an excellent heterogeneous organocatalyst.41 The A2B2-Por-COF was prepared by self-polycondensation of a pre-designed A2B2 type porphyrin (4 0 ,4 0 0 0 -(10,20-bis(4-aminophenyl)porphyrin-5,15-diyl)bis-(([1,1 0 -biphenyl]-4carbaldehyde)), BAPPBPC) precursor (see Figure 9.4). This porphyrin COF exhibited mesoporous pores (up to 2.31 nm) and a high Brunauer–Emmett– Teller (BET) surface area (up to 785 m2 g1) which could allow for efficient mass transport and provide accessible catalytic sites. Structurally, A2B2-PorCOF is rich with –NH free-base (on the pyrrole of the porphyrin core) possessing high basicity. Hence, it displayed potential as a heterogeneous organocatalyst for typical CQC bond forming reactions such as Knoevenagel condensation.42 Accordingly, A2B2-Por-COF was employed to catalyze the coupling reaction between benzaldehyde compounds and malononitrile. Remarkably, the catalyst demonstrated high reactant conversion (499%) even using water as a co-solvent. More intriguingly, this catalyst could show excellent activity for a wide range of reactant scopes. Benefiting from its heterogeneous nature and structural stability, the catalytic activity of A2B2-Por-COF can be preserved even after eight consecutive cycles. Besides the free-base on the porphyrin, additional catalytic sites such as reactive functional groups could result in unique catalytic activity. For example, Banerjee and co-workers designed an imine-linked porphyrin COF (2,3DhaTph) that contains acid–base dual catalytic sites.37 The 2,3-DhaTph was synthesized by condensing metal-free porphyrin (TAPP, M ¼ H2) with a hydroxyl-containing linear linker. Hence, this porphyrin COF possesses hydroxyl (–OH) functional groups tethered on the pore wall. Interestingly, the free-base of the pyrrole ring on the porphyrin moieties and N of imine bonds in the 2,3-DhaTph could act as basic sites, while the hydroxyl (catechol) groups functioned as weak acidic sites.43 The 2,3-DhaTph exhibited high crystallinity,

340 Table 9.1

Chapter 9 Tabulated porphyrin COFs and their application in catalysis.

Porphyrin COFs

Catalysis Types

A2B2-Por-COF

Organocatalysis

2,3-DhaTph

Organocatalysis

[Pyr]X-H2P-COFs Pd/H2P-Bph-COF PCOFs-Fe

Organocatalyst Organocatalyst Biocatalyst

CuP-SQ COF

Photocatalyst

CuP-DHPh COF

Photocatalyst

2D/3D-PdPorCOFs

Photocatalyst

Por-sp2c-COF

Photocatalyst

Por-sp2c-COF þ TEMPO

Photocatalyst

CuPor-Ph-COF/ g-C3N4 COF-366-Co COF-367-Co COF-366-R-Co Co-TTFCOF Co-TTFCOF NSs Py-POR-COF SB-POR-COF

Photocatalyst Electrocatalyst Electrocatalyst Electrocatalyst Electrocatalyst Electrocatalyst Electrocatalyst Electrocatalyst

PCOFs-Co

Electrocatalyst

Reactions Knoevenagel condensation Cascade deacetalizationKnoevenagel condensation Michael addition Suzuki-coupling Organic oxidation Activation of molecular oxygen Activation of molecular oxygen Oxidation of sulfides to sulfoxides Oxidation of amines to imines Oxidation of amines to imines Degradation of RhB Reduction of CO2 Reduction of CO2 Reduction of CO2 Reduction of CO2 Reduction of CO2 Reduction of O2 Oxygen evolution reaction Oxygen evolution reaction

Activity Conv.a

Sel.b/Eff.c

499%

—

41

96%

—

37

100% 98.5% —

70/30 — 1.5104

45 47 49

80%

—

55

B100%

—

28

99%

—

56

99%

—

58

94%

99%

60

—

86%

38

36 mL 100 mL — — — — —

90% 91% 87% 91.3% 99.7% — 90%

67 67 33 68 68 70 19

—

—

49

References

a

Conversion. Selectivity. Efficiency.

b c

large pore size (2.2 nm) and high BET surface area (up to 1019 m2 g1). In addition, the imine bonds (–CQN–) in the 2,3-DhaTph structure were in trans conformation with suitable positions between nitrogen atoms and hydroxyl groups. Hence, intramolecular hydrogen bonds were generated to enhance the stability of the framework. Considering all these properties, 2,3-DhaTph was an attractive bifunctional catalyst. The one-pot tandem conversion of benzaldehydedimethylacetal into 2-benzylidenemalononitrile was chosen to examine the catalytic ability of 2,3-DhaTph. Furthermore, another COF analog (2,3-DmaTph, without catechol group) was also prepared for comparison.

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For the catalysis reaction, a certain amount of benzaldehydedimethylacetal was reacted with malononitrile in the presence of toluene and water (co-solvent). Interestingly, once 2,3-DhaTph was introduced into the reaction, a high reactant conversion into the final product was observed (up to 96%) and the reaction was completed within 90 min. It is worth noting that this organic coupling reaction occurs through sequential steps. Specifically, the acidic sites of 2,3-DhaTph catalyze the deacetalization of benzaldehydedimethylacetal into benzaldehyde, while the basic sites continue to boost the Knoevenagel condensation reaction to produce the final product (2-benzylidenemalononitrile). In addition, excellent conversions (480%) were also noticed for various substituted dimethyl acetal reactants. On the other hand, 2,3-DmaTph without acidic catecholic sites demonstrated lower conversion (52%) for a similar time duration. Hence, the presence of catecholic acidic site is paramount to efficiently catalyze this tandem reaction. Furthermore, the catalytic performance of 2,3-DhaTpH can be maintained for at least 5 consecutive cycles with a convincing conversion (B81%) over a similar time duration. This report clearly defines the controllable structure– activity relationship in designing porphyrin COFs as catalysts. The judicious design of porphyrin COFs with high chemical stabilities and reactive functional groups provides a perfect platform for grafting catalytic sites under a controlled post-synthetic approach.44 Jiang and team postsynthetically decorated the pore wall of pre-designed ethynyl functionalizedporphyrin COFs ([HCRC]X-H2P-COFs) with pyrrolidine groups to produce [Pyr]X-H2P-COFs as heterogeneous chiral organocatalysts.45 The pristine COFs were prepared by quantitatively appending the ethynyl groups (X ¼ 0, 25, 50, 75, 100) which can readily be bonded with pyrrolidine groups via common CuI-assisted click chemistry.46 Surface area and pore size reduction were clearly seen on the resultant [Pyr]X-H2P-COFs, especially for X ¼ 100 (63 m2 g1 and 1.4 nm, respectively). In contrast, with relatively lower functional constituent loading, [Pyr]25-H2P-COF successfully preserved a high surface area of 960 m2 g1 and a pore size of 1.9 nm relative to its pristine COF ([HCQC]25-H2P-COFs, 1092 m2 g1 and 2.0 nm, respectively). Indeed, this porosity analysis confirmed the well-organized chiral pyrrolidine groups on the framework which was attractive for catalyzing the Michael addition reaction in aqueous solution. In the catalytic experiment, the [Pyr]X-H2P-COFs (X ¼ 25, 50, 75, 100) were all tested, while the amorphous polymers and molecular catalyst analogs were also included for comparison. All administered catalysts successfully converted the reactant into the desired product with varied time durations. In particular, [Pyr]25-H2P-COF needed only 1h to catalyze the reaction with ee and dr values of 49% and 70/30, respectively. Meanwhile, with higher pyrrolidine group loading, [Pyr]100-H2P-COF needed a much longer time (9 h) to do so with a lower ee value (44%) and unfavorable dr value (65/35). These results exemplify that the porosity influenced the catalytic activities of the resultant catalysts. On the other hand, the molecular catalyst (control sample) and the two amorphous polymers (1,2) exhibited even longer times to catalyze the

342 Design synthesis of mesoporous A2B2-Por-COF as a heterogeneous catalyst and its catalytic performances toward the Knoevenagel condensation reaction of benzaldehyde with malononitrile. Reproduced from ref. 41 with permission from American Chemical Society, Copyright 2019.

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Figure 9.4

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similar reaction (3.3, 43, and 65 h, respectively). Again, these results signify the advantage of the crystallinity and regular pores of the COFs. Notably, [Pyr]25-H2P-COF was also applicable for a continuous flow catalytic system in which 100% conversion with ee and dr values of 44% and 65/35 was achieved for 448 h. Besides having functional groups on the pore wall, another research group attempted to intercalate reactive Pd in between the layered structure of imine-linked 2D porphyrin COF (H2P-Bph-COF) to yield the porphyrin COF as the catalyst for a typical Suzuki-coupling reaction.47 The H2P-BphCOF was prepared by condensing TAPP (M ¼ H2) with a linear 4,4-biphenyldialdehyde (BPDA) linker and displayed abundant imine bonds and a large pore size (up to 2.92 nm). Hence, this COF proved to be a perfect host for the intercalation of metal salt (e.g., Pd(OAc)2). Practically, the Pd(OAc)2 was intercalated into H2P-Bph-COF via wet-chemistry affording Pd/H2PBph-COF. The surface area was well-preserved being as high as 147 m2 g1 with almost retained pore size. Reductions in the powder X-ray diffraction (PXRD) peaks were also observed indicating the lattice disturbance due to the metal intercalation. Having highly loaded (12.87%, based on ICP-AES analysis) reactive Pd on the pore wall, the resultant Pd/H2P-Bph-COF is a promising catalyst for the Suzuki-coupling reaction between bromoarenes and arylboronic acids under mild conditions. Thus, Pd/H2P-Bph-COF could convincingly produce coupling products in up to 97.8% yield for the Suzuki-coupling reaction of 1-bromo-4-methoxybenzene with benzeneboronic acid, while another tested catalyst (Pd/C) could only yield 65% product conversion. Furthermore, the catalytic activity of Pd/H2P-Bph-COF can be maintained for four consecutive cycles with negligible loss in activity. On the other hand, Pd/C could retain only 5.4% yield after four consecutive cycles. These results exemplify the advantage in using highly porous COFs as catalysis hosts that can strongly hold the reactive metal species compared to amorphous porous carbon. In addition, Pd/H2P-Bph-COF also exhibited very good yield (up to 98.5%) for wide reactant scopes. Further investigation clarified that negligible Pd leaching was observed (o0.02 ppm). The reports mentioned in this sub-section clearly highlight the potential of porphyrin COFs as heterogeneous organocatalysts for catalyzing a wide range of organic reactions. Furthermore, the catalytic sites could not only originate from the porphyrin core, but can also be settled on the linkage or the pore wall in a controlled manner.

9.3.2.2

Bio-catalysis

Similarly as found in heme, a metalloporphyrin such as Fe–porphyrin is one of the most attractive bio-catalysts. Specifically, Fe–porphyrin can stimulate the cytochrome P450 monooxygenase and activate molecular oxygen under mild conditions.48 Hence, metalated porphyrin COFs can be designed for bio-catalysis applications.

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In a recent report, 3D porphyrin COFs were designed as a perfect host to accommodate Fe to produce Fe–porphyrin COFs as a bio-catalyst.49 The so-called PCOF-1 and PCOF-2 were synthesized by designing a rigid tetrahedral aldehyde 3,3 0 ,5,5 0 -tetrakis(4-formylphenyl)bimesityl (TFBM) as a knot. The presence of six methyl groups on TFBM limited the rotation on the biphenyl plane due to the strong steric hindrance of the methyl groups. Hence, the TFBM gives rise to geometrically tetrahedral building blocks once connected with square-shaped linkers.50 The porphyrin-based TAPP (M ¼ H2) and 5,10,15,20-tetrakis(4-aminobiphenyl)porphyrin (TABPP, M ¼ H2) were condensed with TFBM to produce two 3D porphyrin COFs

Figure 9.5

The synthesis of 3D PCOF-1 and PCOF-2 and the biocatalytic activity of PCOFs-Fe towards the oxidation reaction of ABTS and TMB. Reproduced from ref. 49 with permission from the Royal Society of Chemistry.

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(PCOF-1 and PCOF-2, respectively) with pts topology (see Figure 9.5). The resultant PCOF-1 and PCOF-2 were highly crystalline with BET surface areas of 316 and 234 m2 g1, respectively. Two pore sizes (2.3 and 4.3 nm) were observed for PCOF-1, while a single pore size (1.9 nm) was found in PCOF-2. Having large pore sizes and the presence of free-base porphyrin rings in both COFs, the ferric iron (Fe) can judiciously be introduced via wet-chemistry to generate PCOF-1-Fe and PCOF-2-Fe. As high as 0.663103 mol g1 (91%) of Fe were deposited in PCOF-1-Fe, while 0.554103 mol g1 of Fe were determined in PCOF-2-Fe. According to the X-ray photoelectron spectroscopy (XPS) and energy dispersive X-ray (EDX) spectroscopy analyses, the Fe metals were coordinatively bonded on the N species (probably at the imine linkage and at the free-base porphyrin ring). According to the further porosity analysis, the surface area and the porous structure of the COFs were well-maintained. Having these results, both catalysts were studied toward biomimetic catalytic oxidation of 2,2 0 -azinodi(3-ethylbenzothiazoline)-6sulfonate (ABTS) and 3,3 0 ,5,5 0 -tetramethylbenzidine (TMB). With only 40 mg of the catalysts, a rapid color change in the reaction system was observed signaling the rapid oxidation of ABTS. The initial reaction rate (kcat) was calculated to be as high as 23.4 min1 for PCOF-1-Fe and 3.96 min1 for PCOF-2-Fe. Furthermore, PCOF-1-Fe exhibited a catalytic efficiency of 1.5104. More importantly, both the catalysts were recoverable and could be re-used for three consecutive reactions. For the oxidation of TMB, as low as 10 mg of the catalysts were used and the kcat values of 3.8 and 2.44 min1 were obtained for PCOF-1-Fe and PCOF-2-Fe, respectively. The molecular Fe–porphyrin (TAPP, M ¼ Fe) was also tested as a catalyst and showed relatively lower catalytic efficiency. In addition, compared with the other porous catalysts tested in the paper, the bio-catalytic activities of PCOFs-Fe were far more superior. It was assumed that this excellent biocatalytic activity may be associated with the high density of metalloporphyrin active sites within the COF frameworks and their highly stable structure. Although the studies in this field are still in their infancy, the results presented in this report could inspire the design of other porphyrin catalysts for various kinds of bio-catalytic reactions.

9.3.2.3

Photocatalysis

2D COFs such as porphyrin COFs are generally composed of an extended p-conjugated structure which possesses low band-gap energy and aligned pore channels.51–53 Such materials are effective for utilization of light energy for photocatalysis applications. Porphyrin is a typical kind of highly conjugated p-electron macrocycle with unique photophysical and redox properties.4,54 In addition, porphyrin compounds have been widely explored for their photoactivity. Hence, porphyrin COFs are a promising photocatalysis material. Table 9.1 compiles a number of porphyrin COFs that have been explored for photocatalysis applications, including molecular activation, oxidation reaction, and degradation of organic pollutants.

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The Jiang team in 2013 designed a highly conjugated metalloporphyrin COF (CuP-SQ COF) by condensing squaric acid (SA) with TAPP (M ¼ Cu).55 The reaction of SA with TAPP produced squaraine-linked porphyrin COF with planar zig-zag and zwitterionic resonance structures. The CuP-SQ COF was a crystalline 2D layer COF with a highly p-conjugated structure and excellent chemical stability. This COF managed a high BET surface area of 539 m2 g1 and a pore size of 2.1 nm. Such high porosity signals the potential for catalysis applications. Furthermore, the UV–Vis absorption spectroscopy analysis confirmed the photoactivity of CuP-SQ COF, whereby it absorbed light energy in the visible and near-infrared regions. In addition, it revealed that the COF managed a band gap energy as low as 1.7 eV which again signaled an extended p-conjugation over the 2D skeletons of the COF. Indeed, the low band gap energy promises huge potential for photocatalysis applications. Accordingly, CuP-SQ COF was employed as a heterogeneous photocatalyst for the activation of molecular oxygen (O2). A steady conversion of molecular O2 into singlet oxygen (up to 80%) was clearly observed once CuP-SQ COF (0.1 mg) was mixed with oxygen saturated-DMF in the presence of 1,3-diphenylisobenzofuran (DPBF) as a label under visible-light irradiation. On the other hand, molecular CuP (TAPP, M ¼ Cu) exhibited relatively lower conversion. These results exemplify the advantage of the well-ordered and defined catalytic sites (TAPP, M ¼ Cu) and the high porosity of CuP-SQ COF for catalysis applications. Furthermore, the use of non-noble metals in this work inspires a new platform to design efficient and economic photocatalysts. Again by the Jiang team in 2015, intralayer H-bonding was introduced on the layered structure of 2D porphyrin COF for minimizing the torsion of the edge units.28 Hence, the tetragonal sheets are locked into a planar conformation. Such structural fashion will amplify the crystallinity, porosity, and light-harvesting capability. The porphyrin COFs (MP-Ph polymers, MP-DHPhx COFs and MP-DHPh COFs) were prepared by systematically controlling the quantity of hydroxyl (–OH) functional groups through threecomponent condensation reactions (see Figure 9.6). These groups could govern H-bonding with the adjacent electron-rich imine linkages. Both fully (MP-DHPh COFs) and partially H-bonded COFs (MP-DHPhx COFs) exhibited improved crystallinity and surface areas, depending on the OH content in the framework. COFs with a fully locked framework (MP-DHPh COFs, M ¼ H2, Cu, Ni) managed the highest BET surface areas (916–1094 m2 g1) and high intensity PXRD peaks relative to other samples with partial and without H-bonding (MP-Ph polymers). These results clarify that the presence of H-bonding enhances the planarity of the structure, which leads to the improved structural rigidity, chemical stability, and porosity for the frameworks. More importantly, this H-bonding increased the photoactivity of the COFs. For instance, the NiP-DHPh COF is absorbed in the wavelength region from 250 to 600 nm without an obvious boundary between the Soret and Q-bands. This phenomenon may be associated with an increase of extended p conjugation over the 2D sheets. Meanwhile, the Cu-DHPh COF

Catalysis via Porphyrin-based COFs

Figure 9.6

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The design and photocatalytic activity of H-bonding porphyrin COFs. (a) The synthesis of MP-Ph, MP-DHPhx COFs, and MP-DHPh COFs. (b) Absorption spectral changes of DPBF in the presence of CuP-DHPh COFs in an oxygen-saturated DMF solution. (c) Absorption spectral changes of DPBF in the presence of an amorphous CuP-Ph polymer in an oxygen-saturated DMF solution. Reproduced from ref. 28 with permission from American Chemical Society, Copyright 2015.

exhibited a broad Soret band centered at 494 nm, a red-shift relative to the amorphous CuP-Ph polymer. Furthermore, the fully H-bonded COFs exhibited smaller band gap energies (1.36, 1.31, and 1.54 eV for CuP-DHPh COF, H2P-DHPh COF, and NiP-DHPh COF, respectively) compared to the amorphous MP-Ph polymers (1.40, 1.36, and 1.58 eV). Indeed, this photoactivity signals the great potential for photocatalysis. Similarly in CuP-SQ COFs, the singlet oxygen generation from molecular O2 was chosen to examine the catalytic activity of the resultant H-bonded COFs by using DPBF as a label under visible-light irradiation. By using

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0.5 mg of CuP-DHPh COF, a gradual molecular O2 conversion (up to 100% for 800 min, based on the absorption spectral changes of DPBF) was clearly observed. In contrast, the amorphous CuP-Ph polymer (without H-bonding), monomeric CuTPP (copper tetrakisphenyl porphyrin), and linear CuP polymer could only show a sluggish O2 conversion. For instance, the monomeric CuTPP converted only 5% molecular O2. Similarly, the other two materials also demonstrated lower photocatalytic activity towards this reaction. These results further emphasize the great effect of H-bonding in enhancing the photo activity of the predesigned COFs which lead to the enhanced photocatalytic performances. Recently, 2D and 3D metalloporphyrin COFs were designed and their photocatalytic activity was studied.56 The 2D COF (2D-PdPor-COF) was prepared by reacting the TFPP (M ¼ Pd) with a p-phenylenediamine (PPDA) linker, while the 3D one (3D-PdPor-COF) was obtained by using tetra(paminophenyl)methane (TAPM) instead of PPDA. Both 2D and 3D porphyrin COFs were crystalline materials with different topologies. The 2D-PdPor-COF was a typical square-shape pore 2D COF with sql net, while 3D-PdPor-COF was a five-fold interpenetrated pts net. Accordingly, both porphyrin COFs exhibited distinct porosity and metalloporphyrin arrangement within the frameworks. The 2D-PdPor-COF managed a BET surface area of 1120 m2 g1 and a large pore size of 1.9 nm. In contrast, 3D-PdPor-COF possessed a narrower pore size (0.58 nm) and a higher BET surface area of 1406 m2 g1. The metalloporphyrin (Pd-porphyrin; PdPor) moieties in the 2D-PdPor-COF were arranged in a layer-stacked fashion with a 4.02 Å distance to each other. Meanwhile, in the 3D-PdPor-COF, the PdPor moieties were separated 4.76 Å to each other. Since PdPor is a good photosensitizer and because of the distinct PdPor alignment in both COFs, their photocatalytic activities will be unique and interesting to explore. The visible-light-induced oxidation of sulfides to sulfoxides was chosen to examine the photocatalytic activity of both porphyrin COFs. The 3D-PdPorCOF oxidized 98% of thioanisole into methyl phenyl sulfoxide within 24 min. Notably, this photocatalytic activity can be retained even after three consecutive runs. In contrast, the 2D-PdPor-COF could only oxidize 48% of thioanisole within a similar time duration. Further analysis revealed that the PdPor arrangement in the 3D-PdPor-COF allowed for more efficient oxidation of the reactant relative to 2D-PdPor-COF. More interestingly, 3DPdPor-COF could convincingly oxidize other sulfide reactants with higher activity (up to 99%) compared to 2D-PdPor-COF (only up to 60%). However, with a relatively larger reactant size (e.g., 4-tert-butylphenyl methyl sulfide), 3D-PdPor-COF showed lower conversion (48%) compared to 2D-PdPor-COF (up to 59%) as a consequence of the narrower pore size. Indeed, this result indicates the potential of 3D-PdPor-COF for size selective photocatalysis. The stability of the materials is a paramount criterion for photocatalysis. Hence, chemically stable porphyrin COFs are attractive for photocatalysis. Recently, Jiang and team developed an extremely stable COF (sp2c-COF) with CQC (or olefin) linkages that showed stability in the open air for over

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one year. This type of COF is interesting to develop for photocatalysis. Accordingly, Wang and co-workers prepared a new carbon-conjugated porphyrin COF with olefin linkages as a metal-free heterogeneous photocatalyst.58 The so-called Por-sp2c-COF was prepared through the Knoevenagel condensation reaction of TFPP (M ¼ H2) with PDAN. The Por-sp2c-COF was a crystalline AA stacking COF with a BET surface area of 689 m2 g1 and a pore size of 1.85 nm. Benefiting from the olefin linkage, this porphyrin COF was highly chemically stable. For example, it displayed good stability in various organic solvents and even in high concentration acid (9 M HCl) and base (9 M NaOH) solutions. As predicted, Por-sp2c-COF was an optically active material as revealed from the solid-state UV–Vis spectroscopy measurements. It absorbed a wide range of visible light with a red-shift of 32 nm relative to the reported imine-linked porphyrin COF analog.59 This result signals the higher efficiency of olefin linkage in transmitting p conjugation over the 2D skeleton compared to the imine linkage. More interestingly, Por-sp2c-COF exhibited very low band gap energy, going as low as 1.75 eV, indicating a semiconductor property. Nevertheless, the highly conjugated structure, high porosity and chemical stability, and notable optical properties call for its use in photocatalytic applications. The visible-light induced aerobic oxidation of secondary amines to imines was chosen to access the photocatalytic activity of Por-sp2c-COF. Notably, Por-sp2c-COF demonstrated superior oxidation of benzylamine to N-benzylidenebenzylamine relative to several reported metal-free heterogeneous photocatalysts and even comparable to other metal-based heterogeneous photocatalysts compared in the work. Specifically, Por-sp2c-COF could produce 99% N-benzylidenebenzylamine within 30 min under visible-light irradiation. This activity was higher than other tested catalysts. Furthermore, Por-sp2c-COF could also exhibit notable activity for a wider scope of reactants (reactants with electron-withdrawing or electron-donating substituents). This excellent photocatalytic activity may be associated with the well-adjusted porphyrin active sites and the high chemical stability of the COF. This report inspires the huge potential of adopting olefin linkage in designing COFs as photocatalysts. Compositing porphyrin COFs with other molecular catalysts or optically active materials shall give unique photoactivity due to the synergetic effects from both the components. Lang and team re-synthesized the previously discussed highly stable Por-sp2c-COF as a perfect host to accommodate the (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) molecular catalyst.60 TEMPO is a stable radical molecule that has been widely employed as a molecular catalyst for various kinds of chemical transformations.61 Hence, one can conclude that the incorporation of TEMPO (as co-catalyst) into the photoactive porous materials (e.g., Por-sp2c-COF) will be interesting to explore. The resultant highly crystalline Por-sp2c-COF managed a high BET surface area of 714 m2 g1 and a pore size of 1.91 nm. This porosity character is sufficient enough to allow a TEMPO molecule with molecular dimensions of 0.9 nm0.5 nm0.5 nm62 penetrating into the pores of Por-sp2c-COF.

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Furthermore, Por-sp c-COF absorbed a wide range of visible-light including the spectrum of violet, blue, green, yellow, and red LEDs. This optoelectronic property signals the potential of Por-sp2c-COF as a photocatalyst. Indeed, TEMPO was employed as a co-catalyst (Por-sp2c-COFþTEMPO) in the photocatalytic experiment. The oxidation of amines into imines was employed to study the photocatalytic activity of Por-sp2c-COFþTEMPO under 623 nm red-light LED irradiation in acetonitrile. For comparison, both 5,10,15,20-tetraphenylporphyrin (Por) and Por-sp2c-COF were also tested under similar conditions. With only 2 mol% of TEMPO, Por-sp2c-COFþTEMPO produced doubled imine compared to Por-sp2c-COF, while negligible conversion was observed for the Por sample. These results revealed the synergetic performance of both TEMPO and Por-sp2c-COF for photocatalysis. For instance, high conversions of 73–84% were achieved for various amine reactants (electron-deficient benzyl-amine, benzylamine, and electron-rich benzylamine) within minutes using Por-sp2c-COF (0.005 mol)þTEMPO (2 mol %). Meanwhile, Por-sp2c-COF could show only 32–43% conversion for similar reactants. It was assumed that the high chemical stability of Por-sp2c-COF and the presence of TEMPO as a co-catalyst led to the excellent photocatalytic activities even with high concentrations of benzylamine (0.5 mol L1). Furthermore, a high conversion (up to 94%) and selectivity (99%) were achieved for benzylamine as a reactant in 30 min. More importantly, the photocatalytic activity of Por-sp2c-COF þ TEMPO can be maintained for three consecutive cycles. In another report, a porphyrin COF (CuPor-Ph-COF) was hybridized with g-C3N4 to produce a CuPor-Ph-COF/g-C3N4 photocatalyst hybrid.38 The g-C3N4 is an important visible-light-responsive semiconductor photocatalyst that has been widely used for environmental pollutant degradation.63 Accordingly, a CuPor-Ph-COF/g-C3N4 hybrid was employed as the photocatalyst for degradation of rhodamine B (RhB). Interestingly, the hybrid with 8% CuPor-Ph-COF could show degradation efficiency up to 86%, which was far superior than those of g-C3N4 (23%) and CuPor-Ph-COF (36%). This activity corresponded to a rate constant (k) of 0.021 min1, which was 7.78 times higher than that of pure g-C3N4 (0.0027 min1). This report demonstrates the tunability of porphyrin COFs in terms of structural design with robust composition and enhanced photocatalytic activity. The representative studies discussed in this sub-section describe the rich design strategies of porphyrin COFs with unique structural properties which are necessary for photocatalytic applications. The photoactive sites can be settled controllably in the framework, thus the photocatalytic activity is well-tuned.

9.3.2.4

Electrocatalysis

Electrocatalysis is a renewable energy conversion technology which is efficient, low-cost, and environmentally benign.64 Porphyrin and metalloporphyrin derivatives (M-Por) are electroactive building blocks that hold huge

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potential for applications in the field of electrochemical conversion and water splitting technology.32,65,66 Hence, porphyrin or metalloporphyrin COFs are attractive candidates for electrocatalysis applications. Table 9.1 lists porphyrin COFs that have been reported in electrocatalysis applications. The typical electrocatalytic CO2 reduction reaction (CO2RR), oxygen reduction reaction (ORR), hydrogen evolution reaction (HER), and oxygen evolution reaction (OER) have been studied using porphyrin COFs as an electrocatalyst. Yaghi and team explored the potential of 2D porphyrin COFs as electrocatalyst materials for the first time in 2015.67 The team designed 2D COFs that were composed of electroactive cobalt porphyrin (TAPP, M ¼ Co) and linear linkers linked via covalent imine linkage. The so-called COF-366-Co was prepared by linking the TAPP (M ¼ Co) with 1,4-benzenedicarboxaldehyde (BDA) and managed a high BET surface area of 1360 m2 g1 with a pore size of up to 1.8 nm. Using the longer linker BPDA instead of BDA, COF367-Co was obtained with a BET surface area of 1470 m2 g1 and pore size up to 2.3 nm. Indeed, cobalt porphyrin has been widely researched as a molecular electrocatalyst. Hence, both COF-366-Co and COF-367-Co are attractive materials for electrocatalysis applications. Furthermore, the high porosities of these COFs are another beneficial aspect as electrocatalysts since they can allow a high amount of reactant loaded (e.g., CO2 absorbed) on the pores to react with the electroactive sites. The electrocatalytic CO2RR was performed to study the electroactivity of both porphyrin COFs. The experiments were conducted in CO2-saturated aqueous bicarbonate buffer solution (pH ¼ 7.3) under applied potentials between 0.57 and 0.97 V (vs reversible hydrogen electrode (RHE)). By using COF-366-Co, CO was observed as the major reduction product. Interestingly, as high as 36 ml of CO (equivalent to (1.6 mmol at STP)/mg of catalyst) was produced in 24 h at 0.67 V with a 5 mA mg1 catalyst with a Faradaic efficiency (FECO) of 90%. Indeed, this activity accounted for a turnover number (TON) of 1352 (or equivalent to TONEAE34 000 per electroactive cobalt) with an initial turnover frequency (TOF) of 98 per hour (or equivalent to TOFEAE2500 per electroactive cobalt). Interestingly, the PXRD pattern of COF-366-Co was retained signaling the high chemical stability of the porphyrin COF. Furthermore, no evidence of Co nanoparticle formation was observed, proving the structural integrity of the COF. Meanwhile, by using COF-367-Co with an expanded lattice and higher BET surface area, more than 100 mL of CO (or 4.5 mmol at STP)/mg of catalyst was produced at a similar voltage (–0.67 V) and this activity could also be extended for 24 h. Indeed, it was equal to a TON of 3901 (equivalent to TOFEAE48 000) with FECO of 91%. This result clearly describes how the structural modulation affected the electroactivity of COFs. The authors also prepared bimetallic (Co/Cu) porphyrin COF analogs (COF-367-Co (10%) and COF-367-Co (1%)) by co-condensing TAPP (M ¼ Co) and TAPP (M ¼ Cu). The new COF analogs exhibited a substantially higher performance for similar electrocatalytic CO2RR. Indeed, the X-ray absorption data revealed that these remarkable

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electrocatalytic activities arose from the COF environment and the electronic structure of the catalytic cobalt centers (Co-TAPP). The team further engineered the structure of COF-366-Co by appending various functional groups (R ¼ –H2, –(OMe)2, –F or –(F)4) on the pore wall (see Figure 9.3a).33 The resultant COF-366-R-Co series managed crystallinity and topology preservation. These functional groups provide an efficient charge transport along the backbone and stimulate electronic connectivity between the remote functional groups and the catalytic site (Co-TAPP). Hence, the COF-366-R-Co series could exhibit distinct catalytic activities toward electrocatalytic CO2RR. The COFs were prepared into thin-film electrodes for electrocatalytically reducing CO2. Under an applied potential of 0.67 V (vs RHE), a COF-366-R-Co (R ¼ H2) thin-film electrode showed an improved current density up to 45 mA mg1 of cobalt for the formation of CO. In addition, it exhibited a 9-fold CO evolution improvement relative to the previous report (COF-366-Co microcrystalline powder) with a FECO of 87%. Meanwhile, with electron withdrawing groups, the other COF-366-R-Co series demonstrated distinct activities. For instance, COF-366-R-Co (R ¼ (OMe)2) exhibited a current density of 46 mA mg1, while as high as 65 mA mg1 current density was achieved by COF-366-R-Co (R ¼ F) for producing CO. In contrast, COF-366-R-Co (R ¼ F4) did not follow the trend which might be associated with its high hydrophobicity. Nevertheless, this report clearly demonstrated how the electroactivity of COFs can be tuned at the molecular level. Another 2D porphyrin COF series was reported for a similar electrocatalytic CO2RR application. The so-called M-TTCOFs were prepared by condensing metalloporphyrin (TAPP, M ¼ H2, Co or Ni) with rectangular shaped 2,3,6,7-tetrakis(4-formylphenyl)-tetrathiafulvalene (4-formyl-TTF).68 Structurally, M-TTCOFs composed of electroactive metalloporphyrin and the TTF units were arranged in a layer-eclipsed structure. The TTF is a typical electron donor species69 that is able to promote charge-transfer within the crystal once connected with electron acceptor species such as metalloporphyrin17 (see Figure 9.7). Thus, the TTF units in M-TTCOFs could donate electrons to build an oriented electron transmission channel with the metalloporphyrin. This type of structural electronic property is favorable for electrocatalysis applications. The M-TTCOFs are highly crystalline materials with high BET surface areas of 675, 481, and 531 m2 g1 and pore volumes of 0.612, 0.633, and 0.483 cm3 g1 for H2-TTCOF, Co-TTCOF, and Ni-TTCOF, respectively. These high porosities are attractive for CO2 uptake. Indeed, CoTTCOF was able to show a CO2 uptake of 20 cm3 g1 at 293 K which was higher than that of H2-TTCOF (11 cm3 g1) and comparable with Ni-TTCOF (21 cm3 g1). Indeed, these results showed that the presence of the metal enhanced the CO2 uptake of the COFs. The ability to engage CO2 would be beneficial for the electrocatalytic CO2RR activity. More intriguingly, M-TTCOFs were chemically stable even in boiling water, 0.5 M KHCO3 (pH 7.2), 0.1 M HCl (pH 1), and in 0.1 M KOH (pH 13) for more than 5 days. Overall, M-TTFCOFs possess the necessary properties to be promising electrocatalysts for CO2RR.

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Figure 9.7

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The design and electrocatalytic activity of M-TTCOFs towards the reduction of CO2RR. (a) Design synthesis of M-TTCOFs (M ¼ Co or Ni). (b) Tafel plots of M-TTCOFs. (c) The FECO curve of M-TTCOFs calculated over the potential range 0.5 to 0.9 V. (d) Cycling stability test of Co-TTCOF at the potential of 0.7 V vs RHE. Reproduced from ref. 68, https://doi.org/10.1038/s41467-019-14237-4, under the terms of the CC BY 4.0 license, http://creativecommons.org/ licenses/by/4.0/.

In the electrocatalysis experiment, the packed cells composed of M-TTCOFs were prepared and tested under a three-electrode H-type cell with CO2 or Arsaturated 0.5 M KHCO3 solution. Comparatively, Co-TTCOF showed a

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higher current density (at 0.3 to 1.0 V vs RHE) in the CO2-saturated KHCO3 solution than in Ar-saturated KHCO3 solution. Thus, this preliminary result indicates the higher possibility of electrocatalytic CO2RR rather than HER. Furthermore, both CO and H2 were detected as the primary products without any liquid product detected. Subsequent experiments revealed that Co-TTCOF managed a Tafel slope of 237 mV dec1, much lower than the Tafel slope of Ni-TTCOF (629 mV dec1) and H2-TTCOF (433 mV dec1). Indeed, these results signal that Co-TTCOF performed more effectively in CO2RR than the other two samples. Furthermore, the CO can be evolved at an initial CO partial current density of 0.10 mA cm1 (at 0.45 V) using Co-TTCOF. A maximum FECO of 91.3% was determined when the potential was increased to 0.7 V. Comparatively, Co-TTCOF demonstrated superior selectivity (FECO) than that of Ni-TTCOF (20.9%) and H2-TTCOF (4.22%). In addition, Co-TTCOF could manage a TOF of 1.28 s1 at 0.7 V. Intriguingly, this activity can be maintained at more than 90% even after 40 h testing, signaling the highly stable electrocatalytic activity of Co-TTCOF. From this result, the TON (for CO) was found to be as high as 40 142 for 10 h and increased up to 141 479 after 40 h. Meanwhile, as for H2, the TON rose from 4014 to 14 148 over a similar time duration. To enhance the accessibility of the electroactive sites, Co-TTCOF in particular was exfoliated to produce nanosheets (Co-TTCOF NSs with 5B6 nm thick) and these were further tested for a similar electrocatalytic application. Notably, enhanced electrocatalytic activities were clearly seen for the Co-TTCOF NSs. For instance, a high FECO of 99.7% was achieved at 0.8 V, higher than that of Co-TTCOF (91.3%). Indeed, this result exemplifies that the electroactivity of the COFs can be enhanced by exposing more active sites by processes such as exfoliating the COFs. Besides CO2RR, the electrocatalytic ORR has also been explored using porphyrin COFs. For instance, TAPP (M ¼ H2) was electrochemically deposited on a substrate to produce a dendrite-like crystalline porphyrin COF(POR-COF) film that was able to reduce oxygen under electrical stimulus.70 This rapid and brand-new electrochemical deposition method produced thin film COFs with phenazine linkages. Pyridine was also included during the preparation to aid the structural regulation of the film (Py-POR-COF film). The presence of pyridine promoted the selfpolymerization of the sole building block (TAPP) and stabilized the formation of the crystalline lattice. The presence of delocalized p-electron porphyrin moieties in the framework of the Py-POR-COF film calls for electrocatalysis applications.71 Accordingly, the film was deposited on the glassy-carbon (GC) electrode and employed for electrocatalytic ORR. The electrocatalytic testing was conducted using a three-electrode system with an aqueous phosphate buffer solution as the electrolyte. In the cyclic voltammetry (CV) experiments, a clear cathodic peak (at B0.54 V vs. RHE) emerged signaling that the ORR happened on the deposited film. Meanwhile, under rotating-disk experiments, the electron transference number (n) was determined to be as high as 3.97  0.44 using the Koutecky–Levich

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355

(K–L) equation. Indeed, this high value indicated that the ORR occurred via a mixed four/two-electron pathway. Comparatively, the wire-like pTAPP could only achieve an n value of 3.21  0.51. The higher n value signals more efficient and superior electrocatalytic activity. Electrochemical water splitting is a green energy conversion technology that involves electrocatalyst materials.72 The electrocatalytic HER and OER are the main reactions involved in water splitting technology, whereby noble metal catalysts (Pd, RuO2) are the benchmark materials.73 The electroactivity of porphyrin or metalloporphyrin promises applications in electrochemical water splitting as HER or OER electrocatalysts. For example, a predesigned 2D COF composed of porphyrin and pyrene units was employed as a metal-free HER electrocatalyst.19 The so-called SB-PORPy-COF was prepared by reacting TAPP (M ¼ H2) with TFFPy. The resultant porphyrin COF was crystalline and highly porous. The SB-PORPyCOF managed a BET surface area of B869 m2 g1 and permanent micropores of 1.7 nm. Intriguingly, this porphyrin COF was chemically stable even in strong acid (e.g., 0.5 M H2SO4) and base (e.g., 0.5 M NaOH) solutions, a necessary property for electrocatalytic HER. The electrocatalytic HER experiments were conducted in acidic (0.5 M H2SO4) solution. Interestingly, a notably low onset overpotential of B50 mV was shown by SB-PORPy. Indeed, this value was the lowest reported overpotential for all COF-based electrocatalysts in acidic solution. Furthermore, a relatively small overpotential (only 380 mV) was needed to reach 5 mA cm2 exchange current density. These activities signify the superior catalytic performances of SB-PORPy toward the HER. In addition, SB-PORPy reported a Tafel slope of B116 mV dec1. More intriguingly, this electrocatalyst was stable even after 500 cycles at a scan rate of 100 mV s1 and a potential range of 0.2 V to 0.5 V (vs. RHE). Nevertheless, it achieved a high Faradaic efficiency of 90%, signaling the high selectivity of the catalyst towards the HER. The 3D porphyrin COFs have also been employed as promising OER electrocatalysts. For example, the previously discussed 3D PCOFs were also explored as OER electrocatalysts.49 Accordingly, the PCOFs were loaded with electroactive Co to produce two OER electrocatalysts (PCOF-1-Co and PCOF-2-Co). Both the COFs were then transferred for electrocatalyst testing using 1 M KOH in a three-electrode system. The PCOF-1-Co managed a lower overpotential of 473 mV relative to PCOF-2-Co (487 mV) to reach a current density of 10 mA cm2. Meanwhile, to reach a 1 mA cm2 anodic current density, PCOF-1-Co showed even lower overpotential of 386 mV while an overpotential as high as 396 mV was observed for PCOF-2-Co. This distinct activity may be associated with the higher active site density in PCOF-1-Co compared to PCOF-2-Co. Furthermore, a lower Tafel slope of 89 mV dec1 was documented by PCOF-1-Co relative to PCOF-2-Co (95 mV dec1). This work shows that COFs are highly promising potential electrocatalysts with a controllable structure–activity relationship.

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9.4 Summary In this chapter, the significance of porphyrin in the catalysis applications was elaborated. The integration of porphyrin catalytic units into the structures of highly porous and crystalline COFs was further discussed. The COFs with the porphyrin demonstrate specific catalytic activities since the porphyrin units enable them to catalyze various types of reactions. In addition, new catalytic sites in porphyrin COFs can be further affixed to give unique catalytic activities. These include appending reactive functional groups, intercalation of reactive metals, as well as hybridization with other conductive or highly porous constituents. It was clearly observed that the resultant porphyrin COFs showed excellent catalytic activities toward organocatalysis, bio-catalysis, photocatalysis, and electrocatalysis. The high porosity, exceptional chemical and thermal stabilities, as well as the precise integration of functional catalytic sites in the COFs led to the robust and controllable structure–activity relationship even at a molecular level. Despite their remarkable catalytic performances, a lot of work is still needed to improve the activity of the porphyrin COFs as catalyst materials. For instance, installation of various kinds of reactive pendant groups would promise wider reaction scopes that can be catalyzed. Meanwhile, the exploration of porphyrin COFs in the field of bio-catalysis is still in its nascency. Hence, intensive research is required to design porphyrin COFs for bio-catalysis and discover more interesting aspects. Similar is the case for the field of electrocatalysis, in which the utilization of porphyrin COFs for the HER and OER is still relatively less explored. Hence, the design of porphyrin COFs with HER or OER catalysts (e.g., reactive noble metals) can be one of the alternative solutions. In all, we propose that the development of novel porphyrin COFs as catalysts is paramount and their catalytic exploration should be oriented towards overcoming certain bottlenecks in heterogeneous catalysis where the conventional catalysts fail.

Abbreviations ABTS AFM BAPPBPC BBP BDA BET BPDA CNTs CO2RR COFs CuTPP CV

2,2 0 -azinodi(3-ethylbenzothiazoline)-6-sulfonate atomic force microscopy 4 0 ,400 -(10,20-bis(4-aminophenyl)porphyrin-5,15-diyl)bis(([1,1 0 -biphenyl]-4-carbaldehyde)) 5,15-bis(4-boronophenyl)porphyrin 1,4-benzenedicarboxaldehyde Brunauer–Emmett–Teller 4,4-biphenyldialdehyde carbon nanotubes CO2 reduction reaction covalent organic frameworks copper tetrakisphenyl porphyrin cyclic voltammetry

Catalysis via Porphyrin-based COFs

DHTA DPBF EDX FE g-C3N4 HER K–L MOFs OER ORR PDAN PPDA PTCA PXRD RhB RHE SA TABPP TAHAP TAPM TAPP TBPP TEM TEMPO TFBM TFFPy TFPP THAn TMB TOF TON TPA TTF XPS

357

dihydroxyterephthalaldehyde 1,3-diphenylisobenzofuran energy dispersive X-ray Faradaic efficiency graphitic carbon nitride hydrogen evolution reaction Koutecky–Levich metal organic frameworks oxygen evolution reaction oxygen reduction reaction 1,4-phenylenediacetonitrile p-phenylenediamine perylenetracarboxylic dianhydride powder X-ray diffraction rhodamine B reversible hydrogen electrode squaric acid 5,10,15,20-tetrakis(4-aminobiphenyl)porphyrin 5,10,15,20-tetrakis(arylhydroxylamine)porphyrin tetrakis(p-aminophenyl)methane 5,10,15,20-tetrakis(p-aminophenyl)porphyrin 5,10,15,20-tetrakis(p-boronic acid-phenyl)porphyrin transmission electron microscopy (2,2,6,6-tetramethylpiperidin-1-yl)oxyl 3,3 0 ,5,5 0 -tetrakis(4-formylphenyl)bimesityl 1,3,6,8-tetrakis(4-formylphenyl) pyrene 5, 10, 15, 20-tetrakis(4-formylphenyl)porphyrin 2,3,4,5-tetrahydroxy anthracene tetramethylbenzidine turnover frequency turnover number terephthalaldehyde tetrathiafulvalene X-ray photoelectron spectroscopy

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Subject Index ABTS. See 2,2 0 -azinodi(3-ethylbenzothiazoline)-6-sulfonate (ABTS) adsorption, 307–314 anti-programmed death protein 1 (PD-1), 275 2,2 0 -azinodi(3-ethylbenzothiazoline)6-sulfonate (ABTS), 176, 345 aziridines, 151 azole porphyrin linkers ditopic imidazole linkers, 45–46 tetratopic pyrazolate linkers, 44 tetratopic tetrazole linkers, 44–45 bacteriochlorin, 270 ‘basket-handle’ porphyrins, 169 BDC. See benzenedicarboxylic acid (BDC) benzaldehydedimethylacetal, 340 benzenedicarboxylic acid (BDC), 193 benzenetricarboxylic acid (BTC), 193 benzoporphyrin, 275 2-benzylidenemalononitrile, 306, 340 bio-catalysis, 343–345 4,4-biphenyldialdehyde (BPDA) linker, 343 2,5-bis(2-propynyloxy)terephthalaldehyde (BPTA), 303 BPDA linker. See 4,4-biphenyldialdehyde (BPDA) linker BPTA. See 2,5-bis(2-propynyloxy)terephthalaldehyde (BPTA) BTC. See benzenetricarboxylic acid (BTC)

carbon–carbon bond formation, 156 carbon dehydrogenative coupling (CDC), 156 carboxy-based porphyrinic MOFs (CyPorMOF), 197 carboxylate ligands, PorMOFs with PorMOFs using other carboxy porphyrins as linkers, 205–206 PorMOFs using TCPP as linkers, 200–205 carboxylate linkers ditopic carboxylate linkers Co2 paddlewheel with additional linkers, 24 Cu2 paddlewheels, 23–24 Mg, Ca, Sr, and Ba clusters, 24–25 Zn4O clusters, 25–26 hexatopic carboxylate porphyrin linkers, 26 octatopic carboxylate porphyrin linkers Cd/Cd2 clusters, 33 Co3/Co2 clusters, 34–36 Cu2 paddlewheel, 26–29 Fe3O cluster, 36 M(III)Na cluster, 39 Mn-based clusters, 32 mononuclear indium, 36–37 Y1/Y3 clusters, 38–39 Zn2/Cd2 paddlewheel, 29–31 tetratopic carboxylate linkers

362

carboxylate linkers (continued) lanthanide/actinide nodes, 17–22 linear trinuclear clusters, 16–17 M3O clusters, 15 MII chains, 16 MIII chains, 13–14 mononuclear nodes, 17 paddlewheel clusters, 8–12 Ti clusters, 7–8 Zn clusters (non-paddlewheel), 12–13 Zr and Hf clusters, 2–7 cascade acetal deprotectionKnoevenagel reaction, 306 catalysis, 285 electrocatalysis, 299–303 metal nanoparticle catalysis, 307 organocatalysis, 303–307 photocatalysis, 286–299 catalysis application of porphyrin COFs, 338 bio-catalysis, 343–345 electrocatalysis, 350–355 organocatalysis, 339–343 photocatalysis, 345–350 catalyst stabilization and lowering overpotential in confined space, 67–68 and size selectivity in confined space, 65–67 catalytic sites of porphyrin COFs, 337 CDC. See carbon dehydrogenative coupling (CDC) charge transfer in the ground state, 226–229 chromophore–catalyst assemblies, 94 supramolecular photochemical proton reduction, 94–98 supramolecular photochemical water oxidation, 98

Subject Index

chromophores, 68 chromophore organization, 69 light harvesting using supramolecular polymer assemblies of porphyrins, 83–84 supramolecular porphyrin antennae combined axial binding/ macrocycle substitution, 81 metalated porphyrin axial ligand coordination, 70–72 porphyrin interactions at substituted macrocycles, 72–79 supramolecular scaffolds pre-organizing porphyrins, 84 dendrimer scaffolds for light-harvesting, 84 CO2 reduction, 230–233 coenzyme cytochrome C oxidase, 284 COFs. See covalent organic frameworks (COFs) confined space hydroformylation catalysis supramolecular cage strategy, 63–65 template ligand strategy, 61–63 continuous-rotation electron diffraction (cRED) technique, 138 coordination polymers (CPs), 188 Cottrell equation, 228 Coulomb energy, 216 covalent organic frameworks (COFs), 285, 314, 332 CPs. See coordination polymers (CPs) cRED technique. See continuousrotation electron diffraction (cRED) technique

Subject Index

CV profile. See cyclic voltammetric (CV) profile cyanoporphyrins, 46 cyclic voltammetric (CV) profile, 227 CyPorMOF. See carboxy-based porphyrinic MOFs (CyPorMOF) DABCO. See 1,4-diazabicyclo[2.2.2]octane (DABCO) DBP–Hf. See 5,15-di(p-benzoato)porphyrin–Hf (DBP–Hf) DCDPS. See 4,4 0 -dicarboxydiphenyl sulfone (DCDPS) death protein 1 (PD-1), 275 DEF. See diethylformamide (DEF) dendrimer scaffolds for light-harvesting, 84 2,3-DhaTph, 340–341 DHTA. See dihydroxyterephthalaldehyde (DHTA) 1,4-diazabicyclo[2.2.2]octane (DABCO), 45, 46 4,4 0 -dicarboxydiphenyl sulfone (DCDPS), 4, 18 diethylformamide (DEF), 203 dihydroxyterephthalaldehyde (DHTA), 287, 303, 336 dimethylformamide (DMF), 203 5,15-di( p-benzoato)porphyrin–Hf (DBP–Hf ), 272 1,3-diphenylisobenzofuran (DPBF), 287, 346 5,15-dipyridyl-porphyrins, 197 ditopic carboxylate linkers Co2 paddlewheel with additional linkers, 24 Cu2 paddlewheels, 23–24 Mg, Ca, Sr, and Ba clusters, 24–25 Zn4O clusters, 25–26 ditopic imidazole linkers, 45–46 ditopic pyridinyl linkers, 42–43 DMF. See dimethylformamide (DMF) DPBF. See 1,3-diphenylisobenzofuran (DPBF)

363

EDLC. See electrochemical double-layer capacitor (EDLC) EDTA. See ethylenediaminetetraacetic acid (EDTA) 18-p macrocycles, 284 electrocatalysis, 299–303, 350–355 electrochemical double-layer capacitor (EDLC), 325 electrochemical water splitting, 355 electron spin resonance (ESR) spectroscopy, 294 energy storage, 321–325 energy transfer (EnT), 190 singlet-to-singlet energy transfer (SSET), 207–213 triplet-to-triplet energy transfer (TTET), 213–215 enhanced permeability and retention (EPR) effect, 258 EnT. See energy transfer (EnT) ESR spectroscopy. See electron spin resonance (ESR) spectroscopy ethylenediaminetetraacetic acid (EDTA), 233 Fe protoporphyrin IX MOFs I, 180–181 Fe protoporphyrin IX MOFs III, 181–183 fluorescence resonance energy transfer (FRET), 269 fluorine-doped tin oxide (FTO), 237 4-formyl-TTF. See 2,3,6,7-tetrakis(4-formylphenyl)-tetrathiafulvalene (4-formyl-TTF) ¨rster resonance energy transfer Fo (FRET) rate constant, 207 Fourier-transform infrared spectroscopy (FT-IR), 230 FRET. See fluorescence resonance energy transfer (FRET) ¨rster FRET rate constant. See Fo resonance energy transfer (FRET) rate constant

364

FT-IR. See Fourier-transform infrared spectroscopy (FT-IR) FTO. See fluorine-doped tin oxide (FTO) GC electrode. See glassy-carbon (GC) electrode glassy-carbon (GC) electrode, 354 hafnium, 270 Hard-Soft Acid–Base theory (HSAB), 158 HBAs. See hypoxia-based agents (HBAs) HBI. See 4-hydroxybenzylidene imidazolinone (HBI) heme enzymes, 166 bioinspired heme-based materials, 169–170 Fe protoporphyrin IX MOFs I, 180–181 Fe protoporphyrin IX MOFs III, 181–183 future perspectives, 183–184 metalloporphyrin-based metal–organic materials, 171–172 metal–organic framework (MOF) materials, 170–171 MOMZymes, 173–178 porphyrin-encapsulated rhoZMOF, 172–173 hemoglobin, 284 heterogeneous catalysis of porphyrin-based MOFs, 149 catalysis of cycloaddition reactions, 150–152 catalysis of reactions involving alkanes, alkenes, and alkynes, 152–158 reactions catalyzed by Por-MOFs, 158–164 hexatopic carboxylate porphyrin linkers, 26 highly oriented pyrolytic graphite (HOPG) electrodes, 300

Subject Index

HOPG electrodes. See highly oriented pyrolytic graphite (HOPG) electrodes HSAB. See Hard-Soft Acid–Base theory (HSAB) 4-hydroxybenzylidene imidazolinone (HBI), 213 hydroxy porphyrins, 46 hypoxia-activated prodrugs, 266–269 hypoxia-based agents (HBAs), 266 ICB. See immune checkpoint blockade (ICB) ICD. See immunogenic cell death (ICD) imine condensation reaction, 294 immune checkpoint blockade (ICB), 275 immunogenic cell death (ICD), 273–275 INA. See isonicotinic acid (INA) indium tin oxide (ITO), 237 integrated supramolecular lightharvesting porphyrin catalyst assemblies, 94 supramolecular photochemical proton reduction, 94–98 supramolecular photochemical water oxidation, 98 intersystem crossing (ISC) efficiency, 207 ISC efficiency. See intersystem crossing (ISC) efficiency isonicotinic acid (INA), 4 ITO. See indium tin oxide (ITO) K–L equation. See Koutecky–Levich (K–L) equation Knoevenagel condensation, 335, 339, 341 Koutecky–Levich (K–L) equation, 354–355 lanthanide/actinide nodes, 17 hexanuclear thorium, 22 lanthanide 1D chains, 20

Subject Index

lanthanide binuclear clusters, 20–21 lanthanide M6 clusters, 17–19 lanthanide nodes with auxiliary linkers, 22 lanthanide tetranuclear clusters, 19–20 mononuclear uranium, 22 YIII chains, 21 L-AP. See L-ascorbyl palmitate (L-AP) L-ascorbyl palmitate (L-AP), 230 LH. See light harvesting (LH) ligand-to-metal charge transfer (LMCT), 221–222 light harvesting (LH), 188, 190 electronic and photophysical properties, 207 charge transfer in the ground state, 226–229 energy transfer, 207–215 photo-induced charge transfer, 216–225 metal–organic frameworks (MOFs), background of, 191–196 of PorMOFs, 229 with carboxylate ligands, 200–206 electrocatalysis, 240–242 photocatalysis by PorMOFs, 230–236 photoelectrochemical processes in PorMOFs, 236–240 with pyridyl binding groups, 197–199 linear trinuclear clusters, 16–17 LMCT. See ligand-to-metal charge transfer (LMCT) Marcus theory, 217 MBBs. See molecular building blocks (MBBs) metalloporphyrin-based metal– organic materials, 171–172

365

metalloporphyrin derivatives (M-Por), 350–351 metalloporphyrins, 59 integrated supramolecular light-harvesting porphyrin catalyst assemblies, 94 supramolecular photochemical proton reduction, 94–98 supramolecular photochemical water oxidation, 98 self-assembled porphyrin structures in catalysis, 60 catalyst stabilization and lowering overpotential in confined space, 67–68 catalyst stabilization and size selectivity in confined space, 65–67 confined space hydroformylation catalysis, 61–65 supramolecular chargeseparation with porphyrin assemblies, 84 discrete supramolecular charge-separation assemblies, 85–90 integrated antenna/chargeseparation assemblies, 90–94 supramolecular light harvesting porphyrin assemblies, 68 chromophore organization, 69 light harvesting using supramolecular polymer assemblies of porphyrins, 83–84 supramolecular porphyrin antennae, 70–81 supramolecular scaffolds pre-organizing porphyrins, 84 metal nanoparticle catalysis, 307

366

metal–organic frameworks (MOFs), 1–2, 149, 170–171, 189, 201, 332 background of, 191–196 Michael addition reaction, 303 MOFs. See metal–organic frameworks (MOFs) molecular building blocks (MBBs), 170 MOMZymes, 173–178 mononuclear nodes, 17 M-Por. See metalloporphyrin derivatives (M-Por) myoglobin, 284 nanodots, 277 nanophotosensitizers, 257, 270 nanoscale metal–organic frameworks (nMOFs), 257 advantage of nMOFs as photosensitizer carriers, 257–261 improving the biosafety of porphyrinic nMOFs in PDT, 275–277 photodynamic effect, overcoming tumor hypoxia to enhance, 261 oxygen production by a self-supplying system, 261–265 type I PDT independent of O2, 265 photodynamic therapy (PDT) combined with hypoxiaactivated prodrugs, 266–269 combined with immunotherapy for metastatic tumor treatment, 273–275 NIR-light stimulated, 269–272 X-ray stimulated, 272 reactive oxygen species (ROS) production independent of O2 used in PDT, 266

Subject Index

nanoscale porphyrinic metal– organic frameworks (nPMOFs), 257 naphthalene dicarboximide (NDI) acceptors, 89 NDI acceptors. See naphthalene dicarboximide (NDI) acceptors negative Poisson’s ratio (NPR), 135 NIR-light stimulated PDT, 269–272 nMOFs. See nanoscale metal– organic frameworks (nMOFs) non-pillared paddlewheel 2D sheets, 8–10 nPMOFs. See nanoscale porphyrinic metal–organic frameworks (nPMOFs) NPR. See negative Poisson’s ratio (NPR) octatopic carboxylate porphyrin linkers Cd/Cd2 clusters, 33 Co3/Co2 clusters, 34–36 Cu2 paddlewheel, 26–29 Fe3O cluster, 36 M(III)Na cluster, 39 Mn-based clusters, 32 mononuclear indium, 36–37 Y1/Y3 clusters, 38–39 Zn2/Cd2 paddlewheel, 29–31 organocatalysis, 303–307, 339–343 oxazolidinones, 151 paddlewheel clusters, 8 non-pillared paddlewheel 2D sheets, 8–10 pillared paddlewheel 2D sheets, 10–12 palladium-metalated porphyrins (Por(Pd)), 214 partial density of states (PDOS), 221 PCNs. See porous coordination networks (PCNs) PCPs. See porous coordination polymers (PCPs)

Subject Index

PCT. See photo-induced charge transfer (PCT) PD-1. See death protein 1 (PD-1) PDAN. See 1,4-phenylenediacetonitrile (PDAN) PDOS. See partial density of states (PDOS) PDT. See photodynamic therapy (PDT) perylenetetracarboxylic dianhydride (PTCA), 335 PET processes. See photo-excited electron transfer (PET) processes 1,4-phenylenediacetonitrile (PDAN), 335 PHER. See photocatalytic hydrogen evolution reactions (PHER) phosphonate porphyrin linkers, 46 photocatalysis, 286–299, 345–350 photocatalytic hydrogen evolution reactions (PHER), 233 photocatalytic water splitting, 233–236 photodynamic effect, overcoming tumor hypoxia to enhance, 261 oxygen production by a selfsupplying system, 261–265 type I PDT independent of O2, 265 photodynamic therapy (PDT), 256 combined with hypoxiaactivated prodrugs, 266–269 combined with immunotherapy for metastatic tumor treatment, 273–275 NIR-light stimulated, 269–272 X-ray stimulated, 272 photo-excited electron transfer (PET) processes, 60, 85 photoinduced absorption spectroscopy (PIA) spectrum, 316 photo-induced charge transfer (PCT), 216, 223–225 ligand-to-metal charge transfer (LMCT) in PorMOFs, 221–222

367

photo-induced charge transfer (PCT), 223–225 in PorMOFs, 219 porphyrin–fullerene dyad in PorMOFs, 219–221 photosensitizers (PSs), 256, 257 PIA spectrum. See photoinduced absorption spectroscopy (PIA) spectrum pillared paddlewheel 2D sheets, 10–12 polyoxometalate-based porphyrinic coordination networks. self-assembly of, 138 poly-oxometallates (POMs), 98 POMs. See poly-oxometallates (POMs) POPs. See porous organic polymers (POPs) PorMOFs, 196, 227, 229 with carboxylate ligands, 200–206 electrocatalysis, 240–242 photocatalysis by CO2 reduction, 230–233 photocatalytic water splitting, 233–236 photoelectrochemical processes in, 236–240 with pyridyl binding groups, 197–199 using other carboxy porphyrins as linkers, 205–206 using TCPP as linkers, 200–205 porous coordination networks (PCNs), 189 porous coordination polymers (PCPs), 189 porous organic polymers (POPs), 332 porphyrin, 345, 350–351 and metalloporphyrin derivatives, 229

368

porphyrin and phthalocyanine covalent organic frameworks, 284 adsorption, 307–314 catalysis, 285 electrocatalysis, 299–303 metal nanoparticle catalysis, 307 organocatalysis, 303–307 photocatalysis, 286–299 energy storage, 321–325 semiconductors, 314–320 porphyrin-based coordination networks polyoxometalate-based porphyrinic coordination networks. self-assembly of, 138 sulfonate and phosphonatecontaining porphyrin coordination networks, 135–138 porphyrin-based covalent organic frameworks (COFs) catalysis application of, 338 bio-catalysis, 343–345 electrocatalysis, 350–355 organocatalysis, 339–343 photocatalysis, 345–350 catalytic sites of, 337 porphyrin COFs, 335 porphyrin covalent organic frameworks, design and description of structural topology and linkage types, 333–337 porphyrin encapsulated HKUST-1 MOFs, 173 porphyrin-encapsulated MOM-XX MOFs, 177–178 porphyrin-encapsulated rhoZMOF, 172–173 porphyrin–fullerene dyad in PorMOFs, 219–221 porphyrinic metal–organic frameworks, design of, 106 future directions, 139–141

Subject Index

mixed-linker approach for new porphyrin-based metal– organic frameworks, 131–135 symmetry-guided coordination networks with porphyrin building units, 107–131 porphyrin linkers, 47–49 porphyrins, 59, 257 light harvesting using supramolecular polymer assemblies of, 83–84 porphyrin structures in catalysis, 60 catalyst stabilization and lowering overpotential in confined space, 67–68 and size selectivity in confined space, 65–67 confined space hydroformylation catalysis supramolecular cage strategy, 63–65 template ligand strategy, 61–63 Por-sp2c-COF, 294 powder X-ray diffraction (PXRD), 343 PSs. See photosensitizers (PSs) PTCA. See perylenetetracarboxylic dianhydride (PTCA) PXRD. See powder X-ray diffraction (PXRD) Py-Fc. See pyridyl-ferrocene (Py-Fc) PyPorMOF. See pyridyl-based porphyrinic MOFs (PyPorMOF) pyrazine, 45 pyridinyl porphyrin linkers ditopic pyridinyl linkers, 42–43 tetratopic pyridinyl linkers, 40–42 pyridyl-based porphyrin coordination networks, 107 pyridyl-based porphyrinic MOFs (PyPorMOF), 196–197 pyridyl binding groups, PorMOFs with, 197–199 pyridyl-ferrocene (Py-Fc), 209

Subject Index

rare-earth (RE) metals, 118 RC. See reaction center (RC) reaction center (RC), 190 reactive oxygen species (ROS), 256 production, independent of O2 used in PDT, 266 redox catalysts, 190 Rehm–Weller equation, 218 RE metals. See rare-earth (RE) metals rhombicuboctahedrons, 5 rhoZMOF, porphyrin-encapsulated, 172–173 ROS. See reactive oxygen species (ROS) sacrificial electron donor (SED), 96 SALI. See solvent-assisted ligand incorporation (SALI) SBBs. See secondary building blocks (SBBs) SBUs. See secondary building units (SBUs) secondary building blocks (SBBs), 150 secondary building units (SBUs), 1, 189, 192, 193 SED. See sacrificial electron donor (SED) semiconductors, 314–320 singlet-to-singlet energy transfer (SSET), 207–213 solvent-assisted ligand incorporation (SALI), 190, 204–205 Soret band shifts, 175 sqc network, 203 sulfonate and phosphonatecontaining porphyrin coordination networks, 135–138 supramolecular charge-separation with porphyrin assemblies, 84 discrete supramolecular charge-separation assemblies, 85–90 integrated antenna/chargeseparation assemblies, 90–94 supramolecular light harvesting porphyrin assemblies

369

charge-separation, 84–94 chromophores, 68–84 supramolecular photochemical proton reduction, 94–98 supramolecular photochemical water oxidation, 98 Suzuki-coupling reaction, 343 symmetry-guided coordination networks with porphyrin building units pyridyl-based porphyrin coordination networks, 107 rod-packing secondary building unit-based porphyrinic metal–organic frameworks, 129–131 self-assembly of porphyrinic metal–organic frameworks with secondary building units, 108–128 TAPM. See tetrakis( p-aminophenyl)methane (TAPM) TBAB (tetrabutyl ammonium bromide), 150 TBAI. See tetrabutylammonium iodide (TBAI) TBCPPP ligand, 29, 37, 38 TBHP. See tert-butyl hydroperoxide (TBHP) TCPP. See tetrakis(4-carboxyphenyl)porphyrin (TCPP) TEMPO. See 2,2,6,6-tetramethylpiperidine-1-yl)oxyl (TEMPO) TEOA. See triethanolamine (TEOA) terephthalaldehyde (TA), 287 terminal alkynes, hydration of, 154 tert-butyl hydroperoxide (TBHP), 156, 173 tetrabutylammonium iodide (TBAI), 307 tetrahedral porphyrin cages, 97 1,2,4,5-tetrahydroxybenzene (THB), 335

370

tetrakis(4-carboxyphenyl)porphyrin (TCPP), 2, 8–9, 18, 261, 272 3,3 0 ,5,5 0 -tetrakis(4-formylphenyl)bimesityl (TFBM), 344 5,10,15,20-tetrakis(4-formylphenyl)porphyrin (TFPP), 333 2,3,6,7-tetrakis(4-formylphenyl)-tetrathiafulvalene (4-formyl-TTF), 252 tetrakis( p-aminophenyl)-methane (TAPM), 335 3,3 0 ,5,5 0 -tetramethylbenzidine (TMB), 180, 345 2,2,6,6-tetramethylpiperidine-1-yl)oxyl (TEMPO), 294, 325, 349–350 tetraphenylporphyrin (TPP) model, 150 tetrathiafulvalene (TTF), 296 tetratopic carboxylate linkers lanthanide/actinide nodes, 17–22 linear trinuclear clusters, 16–17 MII chains, 16 MIII chains, 13–14 M3O clusters, 15 mononuclear nodes, 17 paddlewheel clusters, 8–12 tetratopic porphyrinic linkers, 22–23 Ti clusters, 7–8 Zn clusters (non-paddlewheel), 12–13 Zr and Hf clusters, 2–7 tetratopic pyrazolate linkers, 44 tetratopic pyridinyl linkers, 40 mononuclear AgI and equatorially capped CuII, 40–41 other MOFs with tetratopic pyridinyl porphyrin linkers, 41–42 tetratopic tetrazole linkers, 44–45 tetrazole-containing porphyrin linker, 119

Subject Index

TFBM. See 3,3 0 ,5,5 0 -tetrakis(4-formylphenyl)bimesityl (TFBM) TFE. See trifluoroethanol (TFE) TFPP. See 5,10,15,20-tetrakis(4-formylphenyl)porphyrin (TFPP) THB. See 1,2,4,5-tetrahydroxybenzene (THB); 3,3 0 ,5,5 0 tetramethylbenzidine (TMB) TME. See tumor microenvironments (TME) TPP. See triphenylphosphine (TPP); tetraphenylporphyrin (TPP) model triethanolamine (TEOA), 160, 232, 295 trifluoroethanol (TFE), 242 triphenylphosphine (TPP), 261 triplet-to-triplet energy transfer (TTET), 213–215 triplet–triplet annihilation (TTA), 214 TTA. See triplet–triplet annihilation (TTA) TTET. See triplet-to-triplet energy transfer (TTET) TTF. See tetrathiafulvalene (TTF) tumor microenvironments (TME), 261 UCNPs. See upconversion nanoparticles (UCNPs) upconversion nanoparticles (UCNPs), 269 XPS technique. See X-ray photoelectron spectroscopy (XPS) technique X-ray photoelectron spectroscopy (XPS) technique, 224–225 X-ray stimulated PDT, 272 zirconium phosphates, 46 ZnPO-MOF, 199