Phthalocyanine Nanostructures 1588830640

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Encyclopedia of Nanoscience and Nanotechnology

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Phthalocyanine Nanostructures Eunkyoung Kim Korea Research Institute of Chemical Technology, Yusung, Daejeon, Korea

CONTENTS 1. Introduction 2. Phthalocyanine Thin Films 3. Phthalocyanine Stacks, Molecular Arrays, and Self-Assemblies 4. Phthalocyanine LB Films 5. Phthalocyanine Nanocomposites and Hybrid Materials 6. Phthalocyanine Polymers 7. Polymeric Stacks 8. Conclusion Glossary References

1. INTRODUCTION Phthalocyanine (1) is a very important organic macrocycle, with many applications for its photochemical, photophysical, optical, and electrochemical properties, biological functions, and high stability [1].

N

N

N

M

N

N

N N N

1 (M = H2, or metal oxo, metal halide, etc.)

Having mainly two characteristic absorption bands, a Soret band (300–400 nm) and a Q-band (600–800 nm), phthalocyanine (Pc) and its derivatives show characteristic color as well as photo- and electrochemical properties. They are extensively used as pigments and dyes, and they are models for biologically important species such as porphyrins, hemoglobin, and chlorophyll. They can be applied in chemical sensors, especially for the detection of NO2 [2], in optoelectronic devices [3], solar cells [4], and well-behaved field ISBN: 1-58883-064-0/$35.00 Copyright © 2004 by American Scientific Publishers All rights of reproduction in any form reserved.

effect transistors [5], molecular metals, conducting polymers, sulfur effluent pollutant control, and in optical storage devices [1]. Their catalytic properties have been studied for some time [6], most recently for redox catalysis such as in fuel cell applications [7–9]. Recently the assemblies of phthalocyanines have attracted intense attention since the elucidation of the structure of the light-harvesting antenna LH2 in a natural purple photosynthetic bacterium [10]. Furthermore, a large number of phthalocyanine doped or bound polymers involving various metals have been prepared to enhance and tailor these properties and to facilitate the exploitation of Pcs in such applications [11]. For example phthalocyanine embedded in some organic polymers has been applied in optical recording materials, which records with the infrared (IR)-laser hole burning technique [12, 13]. An important step in the development of Pcs for these application is the precise manipulation of molecular arrangements. This precise manipulation allows directional transport of electrons, photons, and ions. In this context, a number of Pc molecules have been assembled into controlled nanoscaled structures through covalent and noncovalent bonds, by controlling the intermolecular interactions in all ranges (van der Waals, electrostatic, covalent) and molecular symmetry. For molecular self-assemblies, relatively weak and reversible noncovalent types of interactions are involved. Such interactions are electrostatic, van der Waals, hydrogen bonding, hydrophobic interaction, etc. Covalently bonded nanoscaled Pc macromers are constructed by a polymerization method. It is a subject of great importance because it controls the aggregation of Pcs and macroscopic properties of nanoscopic molecular devices [14]. Several strategies have been explored to construct Pc polymers using different monomers and polymerization methods. The control of spatial arrangement and orientation of these planar
-conjugated compounds promised the creation or modification of functions and properties of molecular organizations. This chapter provides an overview on nanoscaled Pc assemblies and Pc polymers.

2. PHTHALOCYANINE THIN FILMS Phthalocyanines having planar structures form thin films with a film thickness at the nanometer level. Thin films containing phthalocyanines have attracted a lot ofinterest

Encyclopedia of Nanoscience and Nanotechnology Edited by H. S. Nalwa Volume 8: Pages (629–689)

Phthalocyanine Nanostructures

630 in the past decade [15–32]. Several strategies were implemented to build up phthalocyanine thin films: spin coating [33–36], casting in matrixes such as bilayer membranes or thermotropic liquid crystals [30, 37], deposition by the Langmuir–Blodgett technique [20, 23, 31, 37b–43], or selfassembly [15, 16, 18, 25–29, 44]. The latter method appears to be very attractive since it provides a stable chemical linkage between the substrate and the macrocycles. Pcs were attached to mineral substrates via electrostatic interactions [16, 25, 26, 45], sulfur-to-gold bonds [18, 27, 46], or complexation between bound ligands and metal centers [15, 28, 47]. Thin films are more generally deposited to a thin film by the vacuum deposition method [48–59]. Vacuum deposition on a highly ordered support such as MoS [60], metal [56, 59], or alkali halide surfaces (100) [61–66] tends to produce welldefined film structures. Organic molecular beam deposition (OMBE) provides various film phases of tri- and tetravalent metallophthalocyanines such as AlPcCl, InPcCl, TiOPc, and VOPc; some of these phases exhibit a highly redshifted Q-band in its electronic spectrum [67, 68]. This redshift originates from lateral stacking of the molecules in the film structure, which has been reported for cyanine aggregates [69–71] and is usually described by exciton coupling [71–73]. On the other hand, bivalent metallophthalocyanines prefer a coplanar central stacking owing to missing axial ligands and provide a blueshifted Q-band [39, 72, 74]. Nonplanar PbPc molecules were found [75] to have three adsorption phases on MoS2 in ultrahigh vacuum (UHV), instead of the two phases observed for CuPc on the same surface. For CuPc, a close-packed phase with a square unit cell and a rowlike phase were observed. For PbPc, the additional adsorption phase comprised three adjacent close-packed rows separated by one or two isolated single rows. Molecularly resolved images of PbPc on MoS2 show two different adsorption geometries, with the Pb atom above or below the Pc plane (the molecule is nonplanar). Different phases and structures in the Pc films depending on the metal, substituent, and the film thickness are described.

2.1. Metal-Free Phthalocyanine (H2 Pc) Two different polymorphic forms of free base phthalocyanine films have been grown on glass substrates by UHV organic molecular beam deposition. Postgrowth annealing of films grown at room temperature leads to transformation from the  to the 1 phase [76]. The effects of annealing lead to a number of transition states whose morphological, structural, and spectroscopic properties can be identified using atomic force and optical interference microscopy, X-ray diffraction, and Raman and electronic absorption spectroscopy. Detailed morphological studies indicate that the transition occurs via a discrete number of nucleations and is preceded by an elongation of the  crystallites. The film thickness plays a critical role and three regimes have been identified. The  → 1 transformation is only complete for films thicker than ∼940 Å, and thick films lead to a higher degree of orientation and larger domains. The morphology of the samples annealed for different times was assessed using atomic force microscopy (AFM) (Fig. 1). A film grown at room temperature with no annealing is shown as a reference in (a) and displays the high

(a)

(b)

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1 µm

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Figure 1. AFM images of 2330 Å thick H2 Pc films as a function of annealing time: (a) no anneal, (b) 1.0 h,  phase, (c) 1.0 h, 1 phase (minor component), (d) 1.25 h and (e) 2.0 h. Reprinted with permission from [76], S. Heutz et al., J. Phys. Chem. B 104, 7124 (2000). © 2000, American Chemical Society.

density of spherical islands that characterizes the  phase [77]. The first changes become apparent after annealing for 0.75 h. The underlying surface morphology is similar to that seen for growth at room temperature, but closer inspection reveals some elongation of the spheres and the presence of several larger islands on the surface, with a typical diameter of 0.2–0.3 m. Elongation of the  spheres becomes more pronounced after annealing for 1 h (b). This elongation prior to transformation has been reported in a previous electron microscopy study of CuPc films on muscovite [78]. Larger islands are also present on the surface, although these have not increased in size and number (number density of islands 4 m−1 . A small proportion of the sample displays regions covered in oriented slender crystallites (c). These areas are at least 49 m2 , the typical area of an AFM image, and are characteristic of a pure 1 phase film [77]. The organized growth of the 1 phase therefore starts after 1 h annealing, consistent with the Raman spectra shown in Figure 2. After 1.25 h annealing the surface is completely covered with the long thin crystals of the 1 phase. The crystallites are parallel to each other over large areas, and a boundary between two domains of different orientations can be seen in (d). Annealing for longer times leads to no appreciable change in the surface morphology until after 2 h (e). At this point, the appearance of smooth areas between remaining islands of 1 crystals indicates that the 1 phase crystals have grown together and there only remain a few well-defined thin 1 crystallites, mostly arranged in columns. However, the domain boundaries are still present and there are some features reminiscent of the oriented crystallites in the smooth areas, indicating that these have been formed by the merging of the slender 1 crystallites. Figure 3 shows the effect of film thickness on the larger scale morphology of the 1 phase. For the thinnest film for which it is possible to induce the  → 1 transformation, the result is isolated areas of fernlike morphology (a). There are large areas of nontransformed material between the 1 phase islands, and there is also space between the individual strands of the fernlike islands. When the starting film is slightly thicker, the 1 islands are larger, but they are still isolated from one another (b). This correlates well with the AFM results, which showed discrete domains of  and 1 phase H2 Pc. Figure 3c shows a sample that has the features

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631

h) g) f)

Intensity/a.u.

e) d) c)

b)

a)

500

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Wavelength/cm-1 Figure 2. Resonance Raman spectra of 2330 Å H2 Pc films acquired with a 632.8 nm laser source. (a) and (b) are reference spectra for 1 and  films, respectively. Different annealing times are shown: (c) 0.75 h, (d) 1.0 h,  phase, (e) 1.0 h, 1 phase, (f) 1.25 h, (g) 1.5 h, and (h) 2.0 h. The ratio of  (d) to 1 (e) for the film annealed for 1 h is approximately 5:2. Reprinted with permission from [76], S. Heutz et al., J. Phys. Chem. B 104, 7124 (2000). © 2000, American Chemical Society.

interlying space between the individual strands. It can be seen without any quantitative analysis that the size of the domains increases with film thickness for a given annealing time and temperature. This is a direct consequence of the higher degree of alignment observed of the crystallites for the thickest film. These observations suggest three regimes of behavior for films of different thickness. The extreme case is for thin films (i.e., less than 560 Å). In this case there is not enough material to facilitate any transformation from the  phase to the 1 . This is consistent with the suggestion that particles of the size found in these films are more stable in the  phase than in 1 . For film thicknesses between 560 and 940 Å, only partial transformation of the film occurs, indicating that the film is thick enough to allow transformation, but there is not enough material on the surface to ensure continuity of the film during transformation. Clearly, the 1 phase grows at the expense of the  phase directly in contact with it. The transformation occurs via discrete nucleations, since the remaining  areas coexist and do not intermix with completely transformed 1 phase regions. It is also likely that since the transformation is very close to the sublimation point, the regions that have not been transformed are thin enough to be in the regime where the  phase is more stable than the 1 . For film thicknesses above 940 Å, both the size of the  phase crystallites and the thickness of the film (and hence its continuity) allow complete transformation of the film.

2.2. Copper (II) Phthalocyanine typical of all films above a critical thickness where the transformation is complete (i.e., 940 Å). The surface is covered entirely by domains of aligned 1 phase crystals. On close inspection this thicker film can be seen to exhibit the same fernlike fine structure as the thinner films, but there is no a)

b)

c)

100 µm

Figure 3. Nomarski micrographs of H2 Pc films annealed for 2 h at 325  C, for different film thicknesses: (a) 560, (b) 990, and (c) 4880 Å. The thinnest film shows nucleation of the 1 phase and a fernlike morphology. Increasing thickness leads to an increase in the domain size. Reprinted with permission from [76], S. Heutz et al., J. Phys. Chem. B 104, 7124 (2000). © 2000, American Chemical Society.

Vacume deposited thin films of metal phthalocyanines such as copper (CuPc) [52, 58, 59, 79–82], lead (PbPc) [75] cobalt (CoPc), iron (FePc), and nickel (NiPc) [83] phthalocyanines have prepared and their film structures have been examined using scanning tunneling microscopy (STM). For example, a 0.3-nm layer of copper (II) phthalocyanine could be deposited on Au (111) to form a near monolayer [56, 59]. Tanaka and co-workers [81] observed that planar CuPc molecules are bonded in a stable manner on the SrTiO3 surface in UHV, whereas CuPc can diffuse on the much less chemically active Cu (111) surface to create low-dimensional structures. Maeda et al. [58] and Kanai et al. [52] obtained similar results in UHV, showing that CuPc molecules have three adsorption configurations on the dimer row of Si (100) with the molecular plane parallel to the surface, dominated by the interaction between the aromatic rings of CuPc molecules and Si dangling bonds. Kanai et al. [52] also observed a distinctive bias-voltage dependence of the STM images of CuPc molecules on Si (111) in UHV, suggesting that there is a strong interaction between the molecule and the surface. On the H-passivated Si (111) surface, the stacking and orientation of CuPc in UHV were found to be affected by the surface roughness [79]. On a rough surface, the CuPc molecules are stacked with the molecular columns parallel to the surface, whereas on an atomically flat surface, the CuPc molecules are stacked in columns perpendicular to the surface. Interestingly, CuPc was found [84] to induce faceting of misoriented Ag (110).

Phthalocyanine Nanostructures

632 In most cases, the predominant features of the molecular image show organic material alone with an apparent hole in the center of the molecule as shown in Figure 4 for a 0.3 nm layer of CuPc on Au (111) [56]. The four-leaf pattern of the phthalocyanine ring is clearly observed, with a hole in the center of the macrocycle. The Cu atom is not participating in the tunneling process. This result was also obtained with Pt/Ir tips and a variety of bias voltages and currents. The image for H2 Pc also shows an apparent hole in the center of the molecule [55]. The explanation for these “holes” was that both the occupied and unoccupied orbitals localized on Cu lay more than 1 eV from the Fermi energy, while the MPc ligand lowest unoccupied molecular orbital (LUMO) lay close to the Fermi energy [57]. As the metal in an MPc system is varied, the corresponding variation in metal d orbital participation near the Fermi surface should produce significant changes in the STM images. Alternatively, dramatic changes in the apparent molecular shape might also occur in systems where interactions between the metal d orbitals and a metallic substrate were significant. In the latter case, the metal surface density of states might “shine through,” giving enhanced height to the central metal. Calculations [85, 86] show that the CoPc dz2 orbital is roughly half-filled and lies very near the ligand highest occupied molecular orbital (HOMO). Calculations place the filled CuPc dz2 orbital about 2 eV below the ligand HOMO, and there is no significant d orbital contribution within a band 1 eV above and 2 eV below the ligand HOMO in CuPc [85]. STM images of CoPc and CuPc on Au (111) are fully consistent both with d orbital contribution and with these theoretical results [87]. STM images of CoPc and CuPc show that CuPc molecules have an apparent hole in the center of the ring, while CoPc units have what appears to be a very tall atom in the center. On the basis of crude orbital

energy arguments [88] (see Fig. 5) and the expectation that tunneling from the metal substrate to the STM tip will be most enhanced when the orbitals have a spatial distribution consistent with carrying charge from below to above the molecular plane, the apparent height of Co(II) relative to the Cu(II) in MPc complexes is easily rationalized as due to the half-filled dz2 orbital. The influence of steps on the orientation of copper phthalocyanine monolayers on Au (111) was investigated using scanning tunneling microscopy and low-energy electron diffraction [89]. CuPc molecules adsorb with their molecular plane parallel to the surface and form a highly ordered overlayer with a square unit cell. On terraces wider than ∼15 nm, the orientation of the monolayer is determined by the underlying substrate and the sides of CuPc square unit cells coincide very closely with the {112} and {110} directions of the Au (111) surface. On narrow terraces, the sides of the CuPc unit cells are aligned along the step edges. This effect is explained in terms of the maximization of coverage which favors the formation of the CuPc domains where the molecules are aligned parallel to the step edges. Figure 6a shows an STM image of a monolayer (ML) thick CuPc film on a Au (111) surface. A monolayer is defined as the amount of deposited CuPc that entirely covers the substrate surface. As evident from this image, CoPc

CuPc X

dx2-y2

X XX

XX

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XX

dz2 dxz,yz

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XX 0.0 0.0

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nm Figure 4. Top view STM image of a 0.3-nm layer of CuPc on Au (111) obtained with a W tip, +094 V sample bias, and 140 pA tunneling current. The image was Fourier filtered to reduce noise. Reprinted with permission from [56], X. Lu et al., J. Am. Chem. Soc. 118, 7197 (1996). © 1996, American Chemical Society.

dxy

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Figure 5. Schematic orbital energy diagrams for several transition metal phthalocyanine complexes. In the case of iron (II), a single orbital configuration cannot be used to describe the ground state, and at least two configurations are required. This situation is represented diagrammatically by use of a broken × to indicate partial occupation of an orbital by an electron, while a full × represents near complete occupation by a single electron. Reprinted with permission from [56], X. Lu et al., J. Am. Chem. Soc. 118, 7197 (1996). © 1996, American Chemical Society.

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633

(a) 2

Figure 6. (a) STM image (70 × 70 nm , sample bias voltage VS = −136 V, I = 014 nA) of the 1 ML CuPc film on Au(111). The dashed line marks the direction of the rows of CuPc square unit cells. The solid line follows the [112] direction of the Au(111) substrate which is defined by the line connecting the apexes of the bends of the Au(111) zigzag corrugation pattern. The CuPc molecular rows are aligned with the [112] direction within a ∼2 angle. (b) STM image (35 × 35 nm2 , VS = −150 V, I = 012 nA) of a 1 ML CuPc film. The square unit cell of the CuPc monolayer is outlined. Dashed lines follow the [112] direction of the Au(111) substrate. Arrows indicate the shifts of the CuPc molecular rows at the bends of the Au(111) zigzag corrugations. Crystalline directions of the Au(111) surface are shown on both images. Reprinted with permission from [89], I. Chizhov et al., Langmuir 16, 4358 (2000). © 2000, American Chemical Society.

CuPc molecules form an almost perfectly ordered layer. The molecular pattern corresponds to a square unit cell with dimensions of 15 Å × 15 Å as determined directly from the STM images (the absolute accuracy of the STM distance measurements is estimated to be ∼10%). The zigzag pattern that appears in the image is due to the corrugation of the underlying reconstructed Au (111) substrate and allows for a straightforward determination of the azimuthal orientation of the molecular layer with respect to the substrate lattice directions. The sides of the CuPc unit cell are very closely oriented to the {112} and {110} crystalline directions of the Au (111) lattice. This is illustrated in Figure 6a where the solid line indicates the direction of the CuPc molecular rows and the dashed line, which connects the apexes of the bends of the Au (111) zigzag pattern, indicates the {112} direction of Au (111). As determined from several STM images, the angle between the CuPc rows and the {112} direction ranges from ∼1 to ∼3 with the average value of ∼2 . A close-up STM image of the CuPc monolayer (Fig. 6b) shows the distinctive four-leaf shape of the CuPc molecules and their arrangement in a square unit cell (the unit cell is marked on the image). The molecular rows along the {110} direction undergo a lateral shift near the bend of the Au (111) zigzag corrugations, as shown by the solid lines and arrows in Figure 6b. This shift is accompanied by a change in rotational orientation of the individual CuPc molecules, as evident from Figure 6b, but without change in the unit cell dimensions. Figure 7a covers a wide terrace on the right and narrow terraces in the center and on the left. On narrow terraces, the molecular rows are aligned parallel to the step edges and actually follow the contour of these step edges. The influence of steps on the ordering of CuPc molecules is evident also on wide terraces near the step edges, where a region of disordered CuPc molecules is seen (Fig. 7a). The width

(b)

Figure 7. (a) STM image (50 × 50 nm2 , VS = −170 V, I = 012 nA) of a 1 ML CuPc film in a stepped region of the Au(111) surface. Solid lines show the alignment of the CuPc molecular rows along the step edges on narrow terraces. (b) Close-up STM image (30 × 30 nm2 , VS = +132 V, I = 013 nA) of the area with two Au steps covered with 1 ML of CuPc. Reprinted with permission from [89], L. Chizhov et al., Langmuir 16, 4358 (2000). © 2000, American Chemical Society.

of the terraces, on which the CuPc molecules are aligned parallel to the step edges, can be as large as 15 nm. The majority of steps on Au (111) run along the {110} direction. Thus, the orientation of CuPc molecules on narrow terraces formed by steps running along this direction coincides with the orientation of CuPc unit cells on wider terraces, where the orientation is defined by the {112} and {110} directions of the Au (111) substrate. An STM image in Figure 7b is an example of this situation. As on wider terraces, the CuPc molecules form a square grid pattern with a square unit cell and sides parallel/perpendicular to the step edges. However, on Au (111) steps that do not run along these directions, the CuPc molecules form rows which follow the contours of the steps (see Fig. 7a).

2.3. NiPc and FePc Iron(II) phthalocyanine (FePc) and nickel(II) phthalocyanine (NiPc) can be adsorbed on the Au(111) surface and provide STM images showing submolecular structure [56]. Interestingly, image contrast depended upon the identity of the central metal ion. Unlike NiPc, wherein the central metal appears as a hole in the molecular image, the iron ion in FePc is the highest point (about 0.25 nm) in the molecular image. These data are interpreted as indicating that the Fe(II) d 6 system has significant d orbital character near the Fermi energy while the Ni(II) d 8 system does not. This interpretation is consistent with theoretical calculations that predict a large contribution of iron d orbitals near the Fermi energy. Figure 8 presents good quality images of both NiPc and FePc that had once been exposed to air. The four-leaf pattern of the phthalocyanine ring is clearly observed, as is the hole (dark area) in the center of every NiPc molecule. On the other hand, the center of every FePc has a large hill (bright spot). The FePc images were always highest in the center, while the NiPc images always appear to have holes where the Ni(II) should be. A cross section image of a FePc sample (once exposed to air) shows the detailed variation with height observed as a function of position over the FePc as seen in Figure 9.

Phthalocyanine Nanostructures

634 nm

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Figure 8. Top view STM images of a 0.4 nm thick layer of NiPc and of 0.3 nm of FePc on Au(111) obtained with a W tip. Both samples had MPc layers that had once been exposed to air. The gray scale extends over 0.3 nm. For NiPc the sample bias was +050 V and the tunneling current was 300 pA. For FePc the sample bias was +010 V and the tunneling current was 1.6 nA. The image was Fourier filtered to reduce noise. Reprinted with permission from [83], X. Lu and K. W. Hipps, J. Phys. Chem. B 101, 5391 (1997). © 1997, American Chemical Society.

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Figure 9. Top view and sectional STM image of a 0.3 nm thick layer of FePc adsorbed on Au (111). The FePc was once exposed to air. The image was obtained with a PtIr tip at a sample bias of +015 V and a fixed tunneling current of 300 pA. Reprinted with permission from [83], X. Lu and K. W. Hipps, J. Phys. Chem. B 101, 5391 (1997). © 1997, American Chemical Society.

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The cross section was chosen to pass through the center of an FePc molecule (bottom center of image), across the benzene rings of two different arms of adjacent FePcs, and again through the center of another FePc, in a repeating pattern. In this image, the benzene rings all appear to be of roughly equal height (about 0.17 nm) and somewhat taller than the carbons of the five-member rings and the peripheral (noncoordinating) nitrogens. The Fe2+ ion region appears to be about 0.25 nm high, defining the total molecular height.

Oxygen contamination during sample transfer between UHV systems also affects the images as presented in Figure 10, wherein both a top view and sectional view are shown. While the maximum height of the FePc now appears to be about 0.20 nm, the benzene rings now appear only about 0.11 nm tall, making the relative height of the central metal even greater than in the once air exposed sample. Moreover, the relative resolution is clearly better than in Figure 9, since the drop in apparent height over the inner carbon atoms and the noncoordinating nitrogens is more clearly observed. It is well known that both the electron density and the local density of surface states near the Fermi energy play a critical role in determining the STM image as observed for other examples [90, 91]. STM imaging of electronegative or electropositive elements, for example, can result in the observation of anomalous heights [92]. Another example is the observation of the dangling bonds on the silicon surface [93]. In the present case of Pc, the occupancy of the d orbitals is playing a significant role in the STM image. In these examples, Fe(II) may be acting as a conductor while Ni(II) may not be. Since the STM images reflect contours of constant current, the tip must dip toward the Au surface above the Ni(II) center and pull back from the Fe(II) in order to maintain constant current flow. There are, however, at least three separate mechanisms that could lead to the observed differences in height [83]. A different metalloporphyrin, Ni(II) octaethylporphyrin (NiOEP), was investigated by STM imaging [94]. Figure 11 is a typical constant current image of NiOEP on Au (111) observed at low resolution. In this image the individual NiOEP molecules appear as dots making up well-defined single molecule thick islands (e.g., region A). The regions

0

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Figure 10. Top view and cross-sectional plot of a 0.2 nm thick FePc film on Au(111) prepared and measured in the STM UHV chamber. A PtIr tip was used at a sample bias of −050 V and a current of 400 pA. Reprinted with permission from [83], X. Lu and K. W. Hipps, J. Phys. Chem. B 101, 5391 (1997). © 1997, American Chemical Society.

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635 200

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Figure 11. A typical constant current image of NiOEP on Au(111) observed at low resolution. The image was obtained with a W tip at a sample bias voltage of −12 V and a set point of 300 pA. The image has been flattened. The region marked B is uncovered Au(111) and the region marked A is a single molecule thick layer of NiOEP. The area marked C is a single gold atomic step. Reprinted with permission from [94], L. Scudiero et al., J. Phys. Chem. B 106, 996 (2002). © 2002, American Chemical Society.

between the islands (e.g., region B) appear to be uncovered Au (111) surface and the I–V curves obtained in these regions are similar to those from porphyrin-free substrates. The large height step (feature C) along the lower right side of the figure is due to a monatomic step of the Au (111) substrate. One can also observe striations running through both the NiOEP islands and portions of the uncovered surface. These are due to reconstruction of the Au (111) surface and appear as pairs of lines. The small grain gold films used in this study (about 0.3 m in diameter) generally have a larger reconstruction line spacing than is seen on large single-crystal Au (111) surfaces. Unlike true single-crystal gold [95], these small crystal grains show reconstruction line spacing ranging from 6.3 to about 9.0 nm. Figure 12 is a high-resolution image of NiOEP on Au (111) that clearly shows the fourfold symmetry expected for the molecule and the somewhat dark center associated with the central Ni(II) ion. The metal ion contrast has been explained in terms of occupation of the dz2 orbital. Also shown as an inset in Figure 12 are space filling (CPK) models of NiOEP placed to form a single primitive surface unit cell with basis vectors of length 1.65 and 276 ± 020 nm. Note that there are two molecules per unit cell because of the small angular offset (ca. 15) of alternating rows. Another interesting aspect of the NiOEP image is the prominence of the ethyl groups. While the individual hydrogens cannot be resolved, the terminal methyl groups are clearly seen with contrast similar to, but slightly greater than, the OEP ring. Moreover all the ethyl groups are turned up so that there is maximal contact between the OEP ring and the gold surface (as shown in Fig. 13). The mechanism by which the ethyl groups attain their prominence in the STM image is of interest. The porphyrin ring shows good contrast because of both HOMO- and LUMO-mediated tunneling in the energy region close to EF (vide infra).

Figure 12. High-resolution constant current STM image of NiOEP at near-monolayer coverage on Au(111). The image was acquired with a W tip at −06 V bias, with a setpoint of 0.3 nA. The image was flattened and low-pass filtered. Inset is a group of CPK models arranged as a single unit cell. Reprinted with permission from [94], L. Scudiero et al., J. Phys. Chem. B 106, 996 (2002). © 2002, American Chemical Society.

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Figure 13. CPK model of a typical metal(II) OEP. The four nitrogens that bind the central metal ion are shown in light gray. The molecule is shown both in top and side view. The configuration of the ethyl groups is chosen to be consistent with the structure of NiOEP adsorbed on Au(111). Reprinted with permission from [94], L. Scudiero et al., J. Phys. Chem. B 106, 996 (2002). © 2002, American Chemical Society.

Phthalocyanine Nanostructures

636 2.4. TiOPc TiOPc is a nonplanar polar molecule [96] with the titanyl group located perpendicular to the macrocycle and the outer phenyl rings making an angle of 7 with respect to the C–N inner ring [97]. Both the nonplanarity and the dipolar character of the molecule result in a specific polymorphism that differs significantly from that of planar phthalocyanines. Crystal structures have been reported so far [1, 97, 98], a monoclinic phase I, a triclinic phase II (hereafter -TiOPc), and a triclinic phase Y (Fig. 14) [96, 98]. Moreover, in -TiOPc, the geometry of the molecule was found to be strongly distorted with respect to molecules in solution: the molecular symmetry is reduced from C4v to C1 . This molecular distortion was attributed to strong
–
interactions [96]. As for planar phthalocyanines, polymorphism governs the crystal packing in thin films and is found to depend strongly on the growth conditions (substrate temperature, deposition rate, and nature of the substrate) [99–102]. The optical properties (absorption, photoconductivity, and second harmonic generation) turn out to be correlated with the crystal structure of the films [100, 101, 103]. Thin films of -TiOPc (phase II) show the largest third-order nonlinear susceptibility (X 3 = 17 × 10−10 esu) [104]. Highly oriented films of titanyl phthalocyanine (TiOPc) were obtained by high-vacuum sublimation onto an oriented poly(tetrafluoroethylene) (PTFE) substrate [99b]. In Figure 15, the evolution of the thin film morphology is deficted as probed by AFM as a function of increasing Ts in the range of 75–200  C for films with thicknesses of 40–50 nm. For Ts = 75  C, the films consist of a dense packing of small grains (mean radius around 35 nm) completely covering the PTFE substrate. The PTFE macrosteps are decorated by larger grains whose shape reflects that of the crystalline TiOPc phase. This observation suggests that TiOPc films grown at Ts < 75  C onto oriented PTFE consist of an amorphouslike material (hereafter a-TiOPc) with a few microcrystallites localized at the PTFE macrosteps.

Convexe pairs Concave pair Convexe pairs

c b a

Figure 14. Molecular model of -TiOPc (phase II) showing the concave and convex pairs. Reprinted with permission from [99b], M. Brinkmann et al., Chem. Mater. 14, 904 (2002). © 2002, American Chemical Society.

75°C

85°C

100°C

200°C

1 µm Figure 15. Evolution of the film morphology probed by AFM in TiOPc thin films of thickness in the range of 40–50 nm grown onto an oriented PTFE substrate. Films were obtained at different substrate temperatures in the range of 75–200  C at a constant deposition rate of 0.8–1.0 nm/min. Reprinted with permission from [99b], M. Brinkmann et al., Chem. Mater. 14, 904 (2002). © 2002, American Chemical Society.

The grain size distribution is rather broad and suggests that coarsening has already occurred during growth. For Ts = 85  C (Fig. 15), two type of grains are observed: (i) small spherically shaped aggregates of radius extravirgin > ordinary > refined. In contrast, the sensors based on the tertbutyl substituted PrPct2 show a different pattern of response. In this case, the change in conductivity provoked by the headspace of the ordinary olive oil is bigger than the variation produced by the extra virgin olive oil. This behavior can be related to the electron donor character of the tertbutyl groups, which can affect the redox process between the volatile components and the sensitive material. The difference can also been related to the different structure of the monolayer due to the presence of the tertbutyl groups that can affect the mobility of the charge carriers.

40

20

0

2.0

2.5

3.0

3.5

Particle Size (nm) Figure 81. TEM micrographs of colloids: (a) TiO2 , (b) H2 Pc, (c) VOPc. Reprinted with permission from [298], W. Liu et al., Langmuir 15, 2130 (1999). © 1999, American Chemical Society.

particles in the TiO2 colloid and the VOPc colloid are well separated from each other without heavy aggregation. The first particles of H2 Pc can be clearly observed although there are some aggregates that may be formed during the desiccation process of TEM samples. This suggests that the dispersability of the H2 Pc particles is poorer than that of metal phthalocyanine (such as VOPc) particles. The obtained transparent blue complex films are bleached when illuminated with UV light due to the UV-light-induced catalysis of the titanium dioxide particles for the oxidizing decomposition of phthalocyanine. These nanoscopic materials are important for UV-light-induced decoloration of a new kind of UV-light recording material that is a complex film composed of nanoscopic particles of titanium dioxide and phthalocyanine.

Phthalocyanine Nanostructures

667

Figure 82a–c shows the UV-vis spectra of the VOPc/TiO2 , H2 Pc/TiO2 , and H2 Pc/PVA complex films, respectively, which are illuminated and measured under the same conditions. The complex films of VOPc/TiO2 and H2 Pc/TiO2 are obviously bleached after being illuminated by the UV light; however, the spectra of the H2 Pc/PVA film did not change significantly. This clearly shows the light-induced catalytic activity of the TiO2 particles in the complex films. Further evidence for the light-induced catalytic decomposition process is that the complex film decolored very slowly when covered by a piece of quartz glass having a coating film of TiO2 on it, while the complex film covered by quartz glass

(a)

0.08

Absorbance

0.06

a b c d e

0.04

f g h

0.02

0.00 300

400

500

600

700

800

Wavelength (nm)

itself decolored just as the quartz glass did not exist. These results prove that the light in the UV region (about 280 nm) absorbed by TiO2 leads to the light-induced decoloration of the film. Since the dispersion is in nanoscale, the titanium dioxide/ phthalocyanine film has high transprancy, which is requisite for optical device application. Similarly, a phthalocyaninesensitized nanostructure was reported [300]. In this example, phthalocyanine substituted with ester groups was anchored onto nanostructured TiO2 films. Thus TiO2 film is pretreated with (CH3 3 COLi to change the surface hydroxyl groups (-OH) into oxygen anions (-O− , to allow the surface to be more reactive toward the ester functionalities of the dye. The dye can then be anchored onto the semiconductor surface through the produced carboxylate group(s). The amount of anchored dye on the semiconductor shows a dependence on both the time of base treatment and the time of dye treatment. Electrodes treated with the free base phthalocyanine and zinc Pc were characterized by absorption spectroscopy, photocurrent action spectroscopy, and photocurrent–photovoltage measurements. The homogeneous blue–green color and the absorption bands in the far-red region are indicative of an attachment of the dye on TiO2 film. A monochromatic incident photo-to-current conversion efficiency of 4.3% was achieved at 690 nm for a cell where the base-treated electrode was treated with ZnPcBu.

0.20

6. PHTHALOCYANINE POLYMERS

Absorbance

(b) 0.15

a

0.10

b c d e f

0.5 g 0.00 300

400

500

600

700

800

900

Wavelength (nm) 0.10

Covalently bound phthalocyanine nanostructures are constructed by polymerization. The type of Pc polymers are (1) linear polymer (14–15) including block or random copolymer and comblike side chain, (2) ladder (network) type (16), and (3) cofacial one-dimensional type (17) (Figs. 83 and 84). As described in the previous section, many examples of ordered stacks, produced through only noncovalent bonds, are known. These stack structures have low kinetic stability. Therefore, the covalent linkage of side chains needs to be enhanced to increase the stability of ordered stacks. Only a few examples of such polymeric Pc materials are known to date [302, 303].

(c)

0.08

Absorbance

N N

0.06

N

N

N

N

M N

n

m

N

14

0.04

R2 R1

0.02

R2 n

m

R1

R2

p

o

R1

15a

R1 = n

m

15b

N N N

0.00 300

400

500

600

700

800

Wavelength (nm) Figure 82. UV-vis spectra of TiO2 /phthalocyanine complex films. (a) TiO2 /H2 Pc: a, before illumination; b–e, each after additional 10 min of illumination; f–h, each after additional 20 min of illumination. (b) TiO2 /VOPc: a, before illumination; b–f, each after additional 10 min of illumination; g, after additional 120 min of illumination. (c) PVA/H2 Pc, measured under the same conditions of (a) VOPc. Reprinted with permission from [298], W. Liu et al., Langmuir 15, 2130 (1999). © 1999, American Chemical Society.

N N

N

N

M

N

N

N

N

N

N

N

M

N

N

N

N

N

m

m

n

n

N

N

N

N

M

N

N

N

N

N

N

N

N

M

N

N

N

N

m

m

N M N

N N N

n

n

16

Figure 83. Linear and network type of polymeric phthalocyanines.

Phthalocyanine Nanostructures

668

L

L

M

M

L

17

M = Fe2+, Fe3+, Co2+, Co3+, Ru2+, Mn2+, Mn3+, Cr3+ L=

C

C , CN : , :N

N: , NG

CN : ,:N

N:

= Phthalocyanine ring

Figure 84. Cofacial 1-D type of polymeric phthalocyanines.

6.1. Synthesis of Soluble Phthalocyanine Polymers Network type phthalocyanine polymers are synthesized from tetramerization of dianhydride and urea as has been well documented [1, 11, 304]. These polymers are usually insoluble and thus structures of them are not well characterized. In order to increase solubility a solubilizing group was introduced into the network polymer as a spacer, or a linear type of phthalocyanie polymers has been synthesized by using a mixture of dianhydride (for the construction of Pc ring) and monoanydride (for linearity). Soluble phthalocyanine polymers were prepared from tetramerization of hexafluoroisopropylidene-benzenetetracarboxylicacid dianhydride and urea in the presence of Ti(OBu)4 and (NH4 2 MoO4 [305]. Modification of end groups and bridging groups between the phthalocyanine rings of the polymer has allowed development of soluble phthalocynine polymers. A general scheme for the synthesis of poly(titanyloxo-phthalocyanines), PTiOPc, is summarized in Figure 85. The resultant polymer was deep blue–green in color with a metallic glint and soluble in DMF. It was noteworthy that PTiOPc with carboxylic acid end groups (19) showed higher solubility in DMF than the imide type. Polymers with imide end groups (18) were stable up to ∼350  C

(a) 2.0

18a

1.5

ABS (a.u.)

M

and temperatures for 10% weight loss (T10  were 386, 454, 386, and 458  C for 18a, 18b, 18c, and 18d, respectively. Figure 86 shows UV-vis spectra of poly(titanyloxophthalocyanines) in DMF solution (∼10 × 10−6 M). In dilute chloroform solution, poly(titanyloxo-phthalocyanines) are present mainly as monomers, characterized by the sharp absorption bands in the Soret (350–400 nm) and in the Q-band region (around 680 nm) [306, 307]. For the sample with carbonyl bridged (18b), the Q-band is broadening and the maximum shifts slightly to around 670 nm, which is ascribed to the Q-band(s) of the polymer aggregates [304]. The intensity of the Q-band(s) of the polymer was significantly reduced when the bridging group was sulfone (18d), possibly due to the insolubility of the polymer 18d in DMF. However, the absorbance of the sulfone-bridged polymer was much increased after end group modification to carboxylic acid (19d) as shown in Figure 86b. It was noteworthy that PTiOPc with carboxylie acid end groups showed higher solubility in DNF than the imide type.

18b

1.0

18c

0.5

0 18d 200

400

600

800

1000

Wavelength (nm) (b)

1.5

KOH, aq-NaCl, 90°C/5hrs

19a, X = C(CF3) 19b, X = CO 19c, X = 19d, X = SO2

19d

ABS (a.u.)

18a, X = C(CF3)2 18b, X = CO 18c, X = 18d, X = SO2

1.0

19a

0.5

0.0

200

400

600

800

1000

Wavelength (nm)

Figure 85. General scheme for the synthesis of poly(titanyloxophthalocyanines). Reprinted with permission from [305c], D. S. Han et al., Synthetic Metals 101, 62 (1999). © 1999, Elsevier Science.

Figure 86. UV-vis spectra of poly(titanyloxo-phthalocyanines) in DMF solution. (a) Imide end group. (b) Carboxylic acid end group. Reprinted with permission from [305c], D. S. Han et al., Synthetic Metals 101, 62 (1999). © 1999, Elsevier Science.

Phthalocyanine Nanostructures

669

The surface of the film prepared by the spin coating method showed a homogeneous surface with an average pore size of less than 0.2 m. The soluble phthalocyanine polymers could be processed to a thin film without binder by the spin coating method. The resultant film was transparent, indicating that the aggregation of Pc polymer is much reduced and the dispersion of Pc is in nanoscale. Irradiation of the films of poly(titanyloxo-phthalocyanines) under the potential of 0.6 V (vs Ag/Ag/Cl) resulted in photocurrent generation. Figure 87 shows photoamperommetric measurement of an ITO glass coated with 18b prepared by the solution coating method. As potential was stepped from −02 to 0.6 V, current pulse was decayed under dark. Photocurrent was generated when the electrode was irradiated and the anodic current was decayed in the dark. The photocurrent generation was repetitive by a light and dark switching under a potential of 0.6 V. Such current generation can be ascribed to a redox process of hydroquinone and quinone [304], which can quench the photogenerated hole carrier from PTiOPc film. Relative efficiency (&rel  was defined as the ratio of photocurrent of the polymer film (ip  and that of TiOPc (ir  ( &rel = ip /ir ip and iT were detemined as ip (or iT  = i/Al, where I A, and l are observed photocurrent, area, and thickness of the film (cm), respectively. Thus &*e, reflects relative charge carrier generation efficiency of PTiOPc. The current intensity was linearly dependent on the film thickness up to 1.5 m. Table 1 summarizes the photocurrent generation for different poly(titanyloxo-phthalocyanines). Films of PTiOPc showed higher photoconductivity than that of the monomeric -type titanium oxophthalocyanine (-TiOPc) dispersed in polyvinyl butyral (PVB). Photocurrent could be affected by the charge generation efficiency of the phthalocaynine chromophores in the polymers and by the charge mobility between the polymer chains. Particularly important are the structure and arrangement of the polymer in film. In the film of 18b, smaller bridging of C O induces ordering of polymer chains, resulting in high photocurrent generation. On the other hand, large hexafluoroisopropylidene groups in 18a favor amorphous irregular structures, resulting in low photocurrent for 18a. Indeed

Table 1. Photocurrent generation of the films of PTiOPc. Polymer

Thickness (m)

ihv A/cm2 )

&rel

105 023 054 051 064 129 047 054 058

267 107 1027 200 533 207 467 200 267

1 18 75 15 33 06 39 15 18

TiOPc 18a 18b 18c 18d 19a 19b 19c 19d

Note: Film prepared by a milling method using a mixture of -TiOPc (40 wt%) and PVB (60 wt%). Source: Reprinted with permission from [305c], D. S. Han et al., Synthetic Metals 117, 203 (2001). © 2001, Elsevier Science.

XRD of the polymer films indicated higher ordering in 18b than in 18a as shown in Figure 88.

6.2. Polymeric Pc Nanocomposite Although the previous soluble phthalocyanine polymers could be used in the preparation of photoconductive film with high optical clarity, the hardness of the film (pencil strength 1. Bleaching and absorption bands are recognized as increases and decreases with respect to the baseline, respectively. Reprinted with permission from [355], J. Ern et al., J. Phys. Chem. A 103, 2446 (1999). © 1999, American Chemical Society.

Phthalocyanine Nanostructures

682

8. CONCLUSION The nanostructure of phthalocyanines and their polymers has been controlled through covalent and noncovalent bonds, by controlling the intermolecular interactions in all ranges. Thus phthalocyanine thin films, nanorods and stacks, molecular arrays of Pc with templates, self-assembly, layer-by-layer assembly of oppositely charged materials, Langmuir–Blodgett films, and other methods have been developed for construction of nanostructured phthalocyanines, as a noncovalent method. Complex film composed of nanoscopic particles of Pcs with nanostructured materials is also an example of a noncovalent method of nanostructured Pcs. Since the dispersion is in nanoscale, the titanium dioxide/phthalocyanine film has high transprancy, which is requisite for optical device application. Having planar structures in the central ring, phthalocyanines easily form thin films with a film thickness on the nanometer level, by deposition methods. STM images of the thin films were highly dependent on the electron density and the local density of surface states near the Fermi energy. Disk-shaped rigid phthalocyanines and metallophthalocyanines can easily stack through strong
–
interaction and form one-dimensional rodlike assemblies with interesting electronic and optic properties. Such cofacially stacked Pc aggregates are present even in dilute solution for some examples. In the Langmuir and Langmuir–Blodgett films containing phthalocyanine derivatives, it is shown that the molecular parameters of the amphiphilic Pc derivative (i.e., the nature, the number, and the location of the polar substituents, the number and the location of the aliphatic chains) strongly affect the orientation of the molecules at the air–water interface and in the LB multilayers. Two extreme examples are given: LB films containing Pc molecules lying parallel to the substrate and LB films containing Pc molecules standing edge-on. In both cases, good-quality and well-ordered LB films are only obtained when the packing parameter  is sufficiently close to unity. It is shown that “sacrificial” single-chain amphiphilic molecules, mixed with the Pc derivatives, can significantly improve the quality of the mono- and multilayers and in some cases modify the inner structure of the film. Methods for the identification of the nanostructures are developed using various techniques such as STM, AFM, surface-enhanced Raman scattering spectrum, and UV-vis spectral methods. Such methods enable the precise manipulation of molecular arrangements and photoelectronic properties of Pcs in nanoscale. In films of metallophthalocyanine stacks, the Pc rings are spaced by van der Waals distances and exciton coupling of neutral–electronic transitions between neighboring macrocycles becomes very efficient. On the other hand, linear polymers of (phthalocyaninato)siloxane in solution strictly limit the exciton transport to one dimension and heating effects can be avoided in flowing solutions. Chemical modification of the core of phthalocyanines by electron withdrawing groups such as fluorine provides a high versatility in molecule structure and hence intermolecular interactions. The film structure as well as electrical and optical characteristics can be adjusted in a rather wide range. Several polymeric nanostructures are achieved by cofacial arrangement of Pc stacks by bridging with an organic

spacer, linear, or network type by, for example, using olefin metathesis and condensation polymerization methods. The stacking of phthalocyanines can be controlled by putting certain metals in the middle. The bridging ligands can be organic ligands with unsaturated or heteroatoms containing aromatic molecules for high charge transport. A better conductor has been made by substituting biphenyl to separate the porphyrin rings. Among those, phthalocyaninto-polysiloxanes are one example showing edge-on phthalocyanines structures, which are linked through a siloxane chain by their centers, thus forming a rigid rod nanostructures. These are promising for designing molecular devices because of their one-dimensionality and high stability. Although polymeric Pc nanostructures are rare, compared to the monomeric stacks, the polymer stacks ensure increased stack stability, processibility, and wider applications. The control of spatial arrangement and orientation of these nanostructured Pc materials allowed directional transport of electrons, photons, and ions, to improve photoelectronic properties of phthalocyanines. Judging from the unique properties of Pc and availability of nanoscaled manipulations, there will be increasing applications of nanoconstruction based on Pc in the future.

GLOSSARY Fermi energy (EF ) When a number of electrons are put into an energy level, electrons will occupy higher energy levels when the lower ones are filled up. EF is the energy level of the highest occupied state at zero temperature. Langmuir–Blodgett film A set of monolayers, or layers of organic material one molecule thick, deposited on a solid substrate. An LB film can consist of a single layer or many, up to a depth of several visible-light wavelengths. Layer-by-layer (LBL) assembly Assembly of oppositely charged materials. Molecular self-assemblies Spontaneous formation of molecules into covalently bonded, well-defined, stable structures via electrostatic, van der Waals, hydrogen bonding, hydrophobic interaction, etc. Molecular wire A common name to indicate a long, highly conjugated molecule. Monolayer A one-molecule thick insoluble layer of an organic material spread onto an aqueous subphase.

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