梅山 有和

Photo- and thermal-responsive polymers containing azobenzene units in the main chain have been utilized as removable dispersants for single-walled carbon nanotubes (SWNTs) in organic solvents. Intermolecular interactions between SWNTs and the polymers are reversibly controllable by tuning the trans-cis composition.

This mini review presents our recent developments in porphyrin-modified electrodes for solar energy conversion. Various porphyrins have been assembled on nanostructured semiconducting electrodes to achieve efficient photocurrent generation. First, porphyrins have been organized with fullerenes onto semiconducting electrodes to elucidate the relationship between the molecular structures, film structures, and photoelectrochemical properties of the modified electrodes. Formation of hole and electron-transporting highways in the porphyrin/fullerene composite film led to the remarkable enhancement of photocurrent generation. Second, porphyrin-modified single-walled carbon nanotubes have also been assembled onto semiconducting electrodes. The degree of chemical functionalization by the bulky porphyrins was found to have a large impact on the photoelectrochemical properties. Third, asymmetrically pi-elongated porphyrins have been successfully employed in dye-sensitized solar cells to improve the cell performance as well as the light-harvesting properties. The power conversion efficiency of the fused porphyrin cell was improved by 50% compared to the reference cell using the corresponding unfused porphyrin. Finally, carboxyquinoxalino derivatives of zinc porphyrin have been further developed to extend the concept of asymmetrically pi-elongated porphyrins. The maximum power conversion efficiency of 5.2% was obtained by using 5,10,15,20-tetrakis(2,4,6-trimethylphenyl)-6'-carboxyquinoxalino[2,3-b]porphyrinatozinc(II).

A series of ineso-tetraphenylzineporphyrins have been prepared to examine the effects of the porphyrin substituents and adsorption conditions on photovoltaic properties of the porphyrin-sensitized TiO2 cells. The cell performance strongly depended on the linking bridge between the porphyrin core and the TiO2 surface, the bulkiness around the porphyrin core, and the immersing solvents and tirnes for the porphyrin adsorption, In particular, the high cell performance of the porphyrin-sensitized TiO2 cells was achieved when protic solvent (i.e., methanol) and short immersing time (0.5-1 h) were used for the conditions of the dye adsorption on TiO2, which is in sharp contrast with Ru dye-sensitized TiO2 cells. The highest cell performance was obtained with 5-(4-carboxyphenyl)-10,15,20-tris(2,4,6-trimethylphenyl)porphyrinatozine(II) as a sensitizer and methanol as an immersing solvent with an immersing time of 1 h: a maximal incident photon-to-current efficiency of 76%, a short circuit photocurrent density of 9.4 mA cm(-2), an open-circuit voltage of 0.76 V, a fill factor of 0.64, and a power conversion efficiency of 4.6% under standard AM 1.5 sunlight. These results will provide basic and valuable information on the development of dye-sensitized solar cells exhibiting a high performance.

Conjugated polymers with alternating main chain structures of zinc porphyrin-furan (PZnPF) and zinc porphyrin-thiophene (PZnPT) have been synthesized by palladium(0)-catalyzed Stille coupling reaction. The optical, electrochemical, photophysical, and photovoltaic properties of PZnPF and PZnPT were investigated to elucidate the effects of the heterole bridges (i.e., furan vs. thiophene) in the porphyrin polymers. The optical bandgap of PZnPF (1.75 eV) is smaller than that of PZnPT (1.90 eV), implying the high delocalization of the pi-electrons along the polymer main chain of PZnPF relative to PZnPT. The more extended pi-conjugation in PZnPF results from the smaller steric repulsion of the meso-furan moiety with the porphyrin rings than that of the meso-thiophene. The time-resolved fluorescence spectrum of PZnPF showed a gradual Stokes shift to the longer wavelength in the subnanosecond time domain due to the relaxation from a twisted conformation with the large dihedral angles between the porphyrins and the furan rings to a coplanar conformation with the small dihedral angles, whereas the fluorescence spectrum of PZnPT did not exhibit the dynamic Stokes shift. Both PZnPF and PZnPT are electrochemically active in the oxidation and reduction regions and have suitable HOMO/LUMO levels that enable photoinduced electron transfer from the polymer to [6,6]-phenyl-C-61-butyric acid methyl ester (PCBM) in the blend films. Indeed, the blend films displayed strong fluorescence quenching from the porphyrin moieties together with appearance of charge-transfer emission arising from the interaction between the porphyrin and the C-60 moieties. This is the first observation on charge-transfer emission between conjugated porphyrin polymers and fullerenes. Bulk heterojunction solar cells were fabricated by using the blend films of PZnPF:PCBM and PZnPT:PCBM as a photoactive layer. The PZnPF:PCBM and PZnPT:PCBM devices revealed power conversion efficiencies of 0.048% and 0.027% under standard AM1.5 sunlight (100 mW cm(-2)). These results obtained here will provide fundamental information on the design of large chromophore-embedded conjugated polymers for solar energy conversion.

ZnO nanorods and nanoparticle electrodes have been applied to solar cells possessing characteristics of bulk heterojunction and dye-sensitized devices. First, the ZnO electrodes were covered with fullerene acid molecules to yield the monolayer on the electrodes. Then, zinc porphyrin and fullerene acid molecules were spin coated onto the modified surfaces to give porphyrin-fullerene-modified ZnO electrodes. The porphyrin-fullerene-modified ZnO nanorod devices with the intervening fullerene monolayer exhibited efficient photocurrent generation compared to that of the reference systems without the fullerene monolayer. The significant improvement of the photocurrent generation efficiency by the fullerene monolayer may be associated with efficient charge separation in the porphyrin-fullerene composite layer, followed by electron injection into a conduction band of the ZnO nanorod electrode together with the suppression of charge recombination between the separated charges by the fullerene monolayer. Cell performance was optimized by altering the length and diameter of the ZnO nanorods and density of the ZnO nanorod array. The effects of the ZnO electrode structures (i.e., nanorod versus nanoparticle) on the photoelectrochemical properties were also compared under the same conditions. Despite a larger surface area of the ZnO nanoparticle electrode by a factor of 3 compared with that of the ZnO nanorod electrode, we noted similar photocurrent generation efficiencies. This can be rationalized by the facts that only the top surface of the porous ZnO nanoparticle electrode is covered by the porphyrin-fullerene composite layers, whereas the entire surface of the ZnO nanorod electrode is wrapped in the composite layers. The photocurrent generation mechanism was also corroborated by steady-state fluorescence and fluorescence lifetime measurements. The results obtained from this study will provide basic and valuable information on the design of electrode structures and the donor-acceptor combination toward the improvement of cell performance in dye-sensitized bulk heterojunction solar cells.

Donor-acceptor molecules have been fabricated on a nanostructured semiconducting electrode for solar energy conversion (i.e., dye-sensitized bulk heterojunction solar cell). The device structure is similar to that of dye-sensitized solar cells, but the top surface of the nanostructured semiconducting electrode is covered with donor-acceptor multilayers. Thus, initial charge separation takes place at the blend interface of the donor-acceptor, which is a typical characteristic of bulk heterojunction solar cells, whereas subsequent processes resemble those in dye-sensitized solar cells. In this novel solar cell, donor-nanocarbons (i.e., fullerenes and carbon nanotubes) have been successfully deposited electrophoretically or spin-coated onto nanostructured SnO2 and TiO2 electrodes that exhibit efficient photocurrent generation. The bottom-up self-organization of porphyrin and fullerene molecules onto the nanostructured electrodes has led to moderate cell performance with an incident photon-to-current efficiency of up to similar to 60% and a power conversion efficiency of 1-2%. Importance of donor-acceptor nanoarchitecture on the nanostructured semiconducting electrodes is highlighted in terms of self-assembly of donor-acceptor molecules.

A novel strategy for constructing a vertical arrangement of bicontinuous donor-acceptor arrays on a semiconducting electrode has been developed. The relationship between the film structure and the photoelectrochemical properties has been elucidated as a function of the number of donor Layers for the first time. The maximum incident photon-to-current efficiency value (21%) is comparable to the highest value (20%) reported for vertical arrangements of bicontinuous donor-acceptor arrays on electrodes.

Substituent effects of porphyrin on the structures and photophysical properties of the J-aggregates of protonated 5-(4-alkoxyphenyl)-10,15,20-tris(4-sulfonatophenyl)porphyrin have been examined for the first time. Selective formation of the porphyrin J-aggregate was attained when suitable length of the alkoxy group was employed for the amphiphilic porphyrin. Namely, a regular leaflike structure was observed for the J-aggregates of protonated 5-(4-octyloxyphenyl)-10,15,20-tris(4-sulfonatophenyl)porphyrin, which was consistent with the results obtained by using the UV-visible absorption and dynamic light-scattering measurements. A bilayer structure in which the hydrophobic alkoxyl groups facing inside the bilayer are interdigitated to each other, whereas the hydrophilic porphyrin moieties are exposed outside, was proposed to explain the unique porphyrin J-aggregate. Fast energy migration and efficient quenching by defect site in the J-aggregates were suggested to rationalize the short lifetimes of the excited J-aggregates.

Substituent effects of porphyrin on the structures and photophysical properties of the J-aggregates of protonated 5-(4-alkoxyphenyl)-10,15,20-tris(4-sulfonatophenyl)porphyrin have been examined for the first time. Selective formation of the porphyrin J-aggregate was attained when suitable length of the alkoxy group was employed for the amphiphilic porphyrin. Namely, a regular leaflike structure was observed for the J-aggregates of protonated 5-(4-octyloxyphenyl)-10,15,20-tris(4-sulfonatophenyl)porphyrin, which was consistent with the results obtained by using the UV-visible absorption and dynamic light-scattering measurements. A bilayer structure in which the hydrophobic alkoxyl groups facing inside the bilayer are interdigitated to each other, whereas the hydrophilic porphyrin moieties are exposed outside, was proposed to explain the unique porphyrin J-aggregate. Fast energy migration and efficient quenching by defect site in the J-aggregates were suggested to rationalize the short lifetimes of the excited J-aggregates.

Phthalocynanies with high periphearl substitutions and free from potential contamination by regioisomers have been synthesized and evaluated as photosensitizers for dye-sensitized solar cell applications. Each of the sterically hindered precursor compounds was accomplished by Suzuki-Miyaura cross-coupling reactions with the arylchloride and corresponding boronic acids. Metal free phthalocyanine-sensitized solar cells showed no photocurrent generation due to its low excited singlet state (LUMO) compared with the conduction band of TiO2. Upon zinc metalation, the LUMO level of the phthalocyanine was pushed up, and this variation afforded an exergonic free energy change for electron injection. the zinc phthalocyanine-sensitized solar cell displayed 0.57% power conversion efficiency (eta) and 4.9% maximal IPCE in the near infrared region. More importantly, the cell prepared with and without the presence of chenodeoxycholic acid revealed no difference in the power conversion efficiency. This implies that the well-known aggregation tendency of phthalocyanines that is considered to enhance the self-quenching of the phthalocyanine excited singlet state is effectively suppressed by the high degree of substitutions. The significance of the driving force for electron injection and the distance between the dye core and the TiO2 surface is also highlighted for devising high performance phthalocyanine photosensitizers.

Novel unsymmetrically pi-elongated porphyrins, in which the naphthyl moiety is fused to the porphyrin core at the naphthyl bridge with a carboxyl group (fused-Zn-1) or at the opposite side of the phenyl bridge with a carboxyl group (fused-Zn-2), have been synthesized to improve the light-harvesting abilities in porphyrin-sensitized solar cells. As the results of T-elongation with low symmetry, Soret and Q bands of fused-Zn-1 and fused-Zn-2 were red-shifted and broadened, and the intensity of Q-band relative to that of Soret band was enhanced. The fused-Zn-1 and fused-Zn-2-sensitized TiO2 solar cells showed the power conversion efficiencies (eta) of 4. 1 % and 1.1 %, respectively, under standard AM 1. 5 conditions. The eta value of the fused-Zn-1 cell was improved by 50% compared to the reference cell using unfused porphyrin (Zn-1). The fused-Zn-1-sensitized cell revealed high IPCE (incident photon-to-current efficiency) values of up to 55%, extending the response of photocurrent generation close to 800 nm. Thus, the improved photocurrent generation of the fused-Zn-1-sensitized cell relative to the Zn-1-sensitized reference cell is responsible for the remarkable difference in the eta values. The eta value of the fused-Zn-2 cell was much lower than that of the fused-Zn-1 cell. DFT calculations disclosed that there are significant electron densities on the carboxyl group in the LUMO of fused-Zn-1, whereas there are little electron densities on the carboxyl group in the LUMO of fused-Zn-2. Accordingly, the larger electronic coupling between the porphyrin and the TiO2 surface in the fused-Zn-1-sensitized cell may be responsible for the high cell performance, due to the efficient electron injection from the porphyrin excited singlet state to the conduction band of the TiO2 electrodes. To further improve the cell performance, 5-(4-carboxylphenyl)-10,15,20-tetrakis-(2,4,6-trimethylphenyl)porphyrinatozinc(II) (Zn-3), possessing different light-harvesting properties, was coadsorbed with fused-Zn-1 onto an TiO2 electrode. Under the optimized conditions, the cosensitized cell yielded maximal IPCE value of 86%, short circuit photocurrent density of 11.7 mA cm(-2), open-circuit voltage of 0.67 V, fill factor of 0.64, and eta of 5.0% under standard AM 1.5 conditions.

Fused five-membered zinc porphyrin carboxylic acid has been synthesized to improve light-harvesting, capability in the visible and near-infrared regions. The fused porphyrin-sensitized TiO2 cell exhibited the photocurrent generation extending over 800 nm, although the power conversion efficiency was found to be moderate (0.30%).

We have successfully developed a new methodology for the selforganization Of C(60) molecules on the sidewall of carbon nanotubes for use in photoelectrochemical devices. Novel nanocarbon composites of fullerene (e.g., C(60)) and highly soluble, chemically functionalized single-walled carbon nanotubes (f-SWNT) have been prepared by the rapid injection of a poor solvent (e.g., acetonitrile) into a mixed solution of C(60) and f-SWNT in o-dichlorobenzene. Measurements by using scanning electron microscopy of cast samples revealed that the composites are categorized into three groups; i) f-SWNT bundles covered with layers Of C(60) molecules, ii) round, large C(60) clusters (sizes of 500-1000 nm) containing f-SWNT, and iii) typical, round C(60) clusters (sizes of 150-250 nm). The electrophoretic deposition of the composites onto a nanostructured SnO(2) electrode yielded the hierarchical film with a gradient composition depending on the difference in the mobilities of C(60) and f-SWNT during the electrophoretic process. The composite film exhibited an incident photon-to-photocurrent efficiency as high as 18 % at lambda = 400 nm under an applied potential of 0.05 V vs. SCE. The photocurrent generation efficiency is the highest value among carbon nanotube-based photo-electrochemical devices in which carbon nanotubes are deposited electrophoretically, electrostatically or covalently onto semiconducting electrodes. The highly aligned structure of C(60) molecules on f-SWNT can rationalize the efficient photocurrent generation. The results obtained here will provide valuable information on the design of carbon nanotube-based molecular devices.

Multiporphyrin-modified CdSe nanoparticles (CdSe-H2P) were prepared to elucidate the interaction between chromophores and luminescent semiconducting nonoparticles in the excited and ground states. The CdSe-H2P nanoparticles were obtained by place-exchange reactions of hexodecylamine-thiophenol-modified CdSe nanoparticles with porphyrin alkanethiols in toluene. The number of porphyrin molecules on the surface of a single CdSe nanoparticle increased with increasing reaction time to reach a saturated maximum of 21. The porphyrins as well as the core in CdSe-H2P can absorb UV/Vis radiation. Steady-state emission and emission-lifetime measurements reveal efficient energy transfer from the CdSe excited state to the porphyrins in the CdSe-H2P nanoparticles. The resulting porphyrin excited singlet state is not quenched by the CdSe core. These unique properties are in sharp contrast with those of multiporphyrin-modified metal and silica nanoparticles. Thus, semiconducting nanoparticle-multiporphyrin composites ore highly promising as novel artificial photosynthetic materials.

Trifluoroacetic acid-catalyzed condensation of pyrrole with electron-deficient and sterically hindered 3,5-bis(trifluoromethyl)benzaldehyde results in the unexpected production of a series of meso-3,5-bis(trifluoromethyl)phenyl- substituted expanded porphyrins including [22]sapphyrin 2, N-fused [22]pentaphyrin 3, [26]hexaphyrin 4, and intact [32]heptaphyrin 5 together with the conventional 5,10,15,20-tetrakis(3,5-bis(trifluoromethyl)phenyl)porphyrin 1. These expanded porphyrins are characterized by mass spectrometry, (1)H NMR spectroscopy, UV/Vis/NIR absorption spectroscopy, and fluorescence spectroscopy. The optical and electrochemical measurements reveal a decrease in the HOMO-LUMO gap with increasing size of the conjugated macrocycles, and in accordance with the trend, the deactivation of the excited singlet state to the ground state is enhanced.

This review article deals with recent advancements on the application of carbon nanotubes, especially single-walled carbon nanotubes (SWNTs), to a photoactive layer on electrodes of photoelectrochemical devices and cells using liquid electrolytes. Versatile methodologies such as electrophoretic deposition, layer-by-layer deposition, self-assembled monolayer technique, and spray-coating have been adopted to form films of SWNTs on semiconducting or metallic electrodes. In the photoelectrochemical devices and cells, SWNTs can act as an efficient charge-transport pathway suppressing the back electron transfer or as a one-dimensional nano- and meso-scaffold for assembling photoactive molecules to facilitate the charge transport through the molecule arrays. These results demonstrate that the unique optical, electrical, and structural properties of SWNTs are beneficial for developing highly efficient carbon nanotube-based photoelectrochemical devices and cells.

Trifluoroacetic acid-catalyzed condensation of pyrrole with electron-deficient and sterically hindered 3,5-bis(trifluoromethyl)benzaldehyde results in the unexpected production of a series of meso-3,5-bis(trifluoromethyl)phenyl- substituted expanded porphyrins including [22]sapphyrin 2, N-fused [22]pentaphyrin 3, [26]hexaphyrin 4, and intact [32]heptaphyrin 5 together with the conventional 5,10,15,20-tetrakis(3,5-bis(trifluoromethyl)phenyl)porphyrin 1. These expanded porphyrins are characterized by mass spectrometry, (1)H NMR spectroscopy, UV/Vis/NIR absorption spectroscopy, and fluorescence spectroscopy. The optical and electrochemical measurements reveal a decrease in the HOMO-LUMO gap with increasing size of the conjugated macrocycles, and in accordance with the trend, the deactivation of the excited singlet state to the ground state is enhanced.

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Hydrogen-bonding effects on film structures and photophysical, photoelectrochemical, and photovoltaic properties have been examined in mixed films of porphyrin and fullerene composites with and without hydrogen bonding on nanostructured TiO2 electrodes. The nanostructured TiO2 electrodes modified with the mixed films of porphyrin and fullerene composites with hydrogen bonding exhibited efficient photocurrent generation compared to the reference systems without hydrogen bonding. Atomic force microscopy, infrared reflection absorption and ultraviolet-visible absorption spectroscopies, and time-resolved fluorescence lifetime and transient absorption spectroscopic measurements disclosed the relationship between the film structures and optical and photoelectrochemical properties relating to the formation of hydrogen bonding between the porphyrins and/or the C-60 moieties in the films on the electrode surface. These results show that hydrogen bonding is a potential methodology for the fabrication of donor and acceptor composites on a nanostructured TiO2 electrode, which exhibits high open circuit potential relative to that of the corresponding SnO2 electrode.

Single-walled carbon nanotubes (SWNTs) covalently modified with large porphyrin molecules have been prepared to construct photoelectrochemical devices with nanostructured SnO2 electrodes on which the multiporphyrin-linked SWNTs are deposited electrophoretically. The film of the porphyrin-linked SWNTs on the nanostructured SnO2 electrode exhibited an incident photon-to-photocurrent efficiency as high as 4.9% under an applied potential of 0.08 V vs SCE. The more uniform film and moderate photocurrent generation in the porphyrin-linked SWNT devices can be rationalized by the exfoliation abilities of the bulky porphyrins that yield large steric hindrance around the SWNTs. Direct electron injection from the excited states of the SWNTs to the conduction band of the SnO2 electrode is responsible for the photocurrent generation. Despite the efficient quenching of the porphyrin-excited singlet state by the SWNTs in the porphyrin-linked SWNTs, the photocurrent action spectra revealed that the excitation of the porphyrin moieties makes no contribution to the photocurrent generation. The evolution of an exciplex between the porphyrin-excited singlet state and the SWNTs and the subsequent rapid decay to the ground state without generating the charge-separated state is proposed to explain the unusual photoelectrochemical behavior. The results obtained here will provide valuable information on the design of SWNT-based photoelectrochemical devices.

A conjugated polymer, poly[(p-phenylene-1,2-vinylene)-co-(p-phenylene-1,1-vinylidene)] (coPPV), with the structural defect caused by 1,1-vinylidene moieties in the main chain of all-trans phenylene-1,2-vinylenes, has been prepared to examine interactions with single-walled carbon nanotubes (SWNTs) in organic solvents. UV-vis-NIR absorption and AFM measurements revealed that SWNTs are dispersed well in organic solvents by pi-pi interaction with coPPV. The composite solution of coPPV SWNTs exhibited a remarkable enhancement of the NIR emission of SWNT with the excitation of coPPV at 400-500 nm, demonstrating the occurrence of energy transfer from the excited coPPV to SWNTs in the composite. (C) 2007 Elsevier B.V. All rights reserved.

The reaction of phenyl propynyl ether and diphenyl disulfide in the presence of 1 mol % tetrakis(triphenylphosphine)palladium as a model reaction of the polymerization of bis(4-prop-2-ynyloxyphenyl) disulfide (1a) gave a Z-substituted dithioalkene. No E-substituted dithioalkene was formed in this reaction. The palladium-catalyzed bisthiolation polymerization of a diethynyl disulfide derivative, 1a, in benzene, was carried out to give a hyperbranched polymer (5a) containing a Z-substituted dithioalkene unit after reaction for 4 h at 70 degrees C. From the gel permeation chromatography analysis (chloroform, PSt standards), the number-average and weight-average molecular weights of 5a were found to be 8,100 and 57,000, respectively. The structure of 5a was confirmed by H-1 and C-13 NMR spectra. The obtained polymer was soluble in common organic solvents such as benzene, acetone, and CHCl3. Polymerization for more than 5 h gave insoluble products. (C) 2007 Wiley Periodicals, Inc.

A conjugated polymer containing a phosphorus atom in the main chain, poly(vinylene-phosphine), was synthesized by ring-collapsed radical alternating copolymerization (RCRAC) of a cyclooligophosphine and phenylacetylene (1a). Under a nitrogen atmosphere, pentamethylcyclopentaphosphine (cyclo-(MeP)(5)) and 1a were reacted in the presence of a catalytic amount of 2,2'-azobis(isobutyronitrile) (AIBN) at 78 degrees C in deaerated benzene for 8 h. Poly(vinylene-phosphine) (2) was obtained as a yellow powder after purification by reprecipitation into n-hexane three times and freeze-dried for 10 h. The number-average molecular weight of polymer 2 was 2500, which is estimated by gel permeation chromatographic (GPC) analysis (chloroform, PSt standards). The structure of 2 was supported by H-1, C-13, and P-31 NMR spectroscopies, suggesting that trans isomers of diphosphaalkene units were mainly formed in the main chain. The UV-vis absorption spectrum of polymer 2 in chloroform showed small absorption at a visible region derived from the n-pi* transition in the main chain as well as strong absorption at a UV region derived from aromatic groups. The fluorescence spectrum of a dilute chloroform solution of 2 showed an emission peak at 470 nm, which derived from the n-pi* transition along the polymer backbone.

Sidewalls of acid-treated, shortened single-walled carbon nanotubes (SWNTs) with long alkyl chains at the open ends and defect sites have been functionalized by Bingel reaction to examine the structures and spectroscopic properties in detail for the first time. The microwave-assisted Bingel reaction has been successfully applied to the sidewall functionalization of which the reaction rate is ca. 50 times faster than that under the conventional conditions. The degree of the sidewall functionalization (one diester unit per 75-300 carbon atoms of SWNTs) was found to be controllable by changing the output power of the microwave under the same temperature. Atomic force microscopy and transmission electron microscopy showed the progressive exfoliation of the SWNT bundles by the double chemical modification. Resonant Raman and UV-vis-NIR absorption spectroscopies revealed that the electronic properties of SWNT are largely retained after a significant degree of sidewall modification by the Bingel reaction without apparent selective reactivity for metallic and semiconducting SWNTs. This is in remarkable contrast with the conventional sidewall functionalization of SWNTs leading to the loss of their electronic properties (one functional group per 10-100 carbon atoms on the sidewall). Thus, our covalent functionalization methodology can provide SWNT materials with both excellent solubility and inherent electronic properties which are highly desirable in solution-phase processing for the fabrication of SWNT-based molecular devices.

Novel perylene imide derivatives with both electron-donating and bulky substituents have been synthesized for dye-sensitized solar cells. The power conversion efficiency reached 2.6%, which is the highest value among perylene-sensitized TiO2 solar cells.

Excited-state dynamics of an electron-donating, pyrrolidine-substituted perylenediimide-C-60 linked dyad have been investigated by means of time-resolved transient absorption spectroscopy and fluorescence lifetime measurements. By the picosecond transient absorption measurements at a selective excitation of the perylenediimide moiety, a charge-separated state has been successfully detected in polar solvents (i.e., benzonitrile, pyridine, and o-dichlorobenzene), demonstrating the occurrence of photoinduced electron transfer from the perylenediimide to the C-60 moiety. In contrast, in nonpolar solvents (i.e., toluene), singlet-singlet energy transfer takes place from the perylenediimide to the C-60, followed by intersystem crossing to the C-60 excited triplet state and subsequent triplet-triplet energy transfer to yield the perylenediimide excited triplet state. Rate constants of the charge recombination in the polar solvents are found to be comparable to or even larger than those of the charge separation, which is in sharp contrast with electron transfer behavior in typical donor-C-60 linked systems. A reorganization energy (0.86 eV) of the perylenediimide-C-60 linked dyad obtained in the polar solvents is significantly larger than those of similar porphyrin-C-60 linked dyads (0.51-0.66 eV) in which both have comparable edge-to-edge distances between donor and acceptor. The large reorganization energy of the perylenediimide-C-60 linked dyad relative to the porphyrin-C-60 linked dyads results from a relatively large conformational change in the pyrrolidine groups at the perylenediimide moiety accompanied by one-electron oxidation. This agrees with the fact that charge recombination to the ground state rather than the excited triplet state of the perylenediimide moiety is predominant in benzonitrile, irrespective of the lower energy level of the excited triplet state than that of the charge-separated state.

Novel 5-(5-carboxy-2-thienyl)- 10, 15,20-tris(2,4,6-trimethylphenyl)-porphyrinatozinc(II) (Zn5S), 5-(5-carboxy2-furyl)- 10, 15,20-tris(2,4,6-trimethylphenyl)porphyrinatozinc(II) (Zn5O), and 5-(4-carboxy-2-thienyl)- 10, 15,20-tris(2,4,6-trimethylphenyl)porphyrinatozinc(II) (Zn4S) were synthesized to evaluate the spacer effects on the structures of the porphyrin films and the photovoltaic properties of the porphyrin-sensitized TiO2 solar cells. Each of the porphyrins showed different adsorption behavior and saturated coverage on the TiO2 surface and photovoltaic properties depending on the identity of heteroatoms in the bridge and the position of carboxylic acid. Specifically, Zn5S-sensitized TiO2 cell displayed larger, maximum incident photon-to-current efficiency of 65% and maximum power conversion efficiency of 3.1% than Zn5O-sensitized TiO2 cell by similar to 20% and similar to 40%, respectively. We interpret that these results are stemmed from ancillary electron-transfer pathway through specific interaction between a sulfur atom in the bridge of Zn5S and the TiO2 surface. Optical spectroscopy, attenuated total reflectance-Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, density functional theory calculations, and photovoltaic measurements under standard AM 1.5 conditions were employed to support our proposal.

We have systematically examined the substituent effects of meso-tetraphenylporphyrins on film structures and the photoetectrochemical properties of the composite clusters of free-base porphyrin and C-60 electrophoretically deposited on nanostructured SnO2 electrodes. The photocurrent generation efficiency was found to correlate with the complexation ability of the porphyrin for C-60. Basically, the incident photon-to-current efficiency (IPCE) value was increased with increasing relative amounts of the porphyrin versus C-60 in the films. The unique molecular arrangement of the porphyrin with the simple, specific substituents (i.e., methoxy groups at the ineta-positions of the meso-phenyl rings of tetraphenylporphyrins (3,5-OMeTPP; TPP=tetraphenylporphyrin)) and C-60 on SnO2 electrodes resulted in the largest IPCE value (ca. 60%) among this type of photoelectrochemical device. The rapid formation of the composite clusters and microcrystals from the combination of 3,5-OMeTPP and C60 in a mixed solvent is unique as the association is accelerated by intermolecular interactions (i.e., hydrogenbonding and CH-pi interactions) between the methoxy groups of the porphyrins and the porphyrin/C-60, in addition to the pi-pi interactions between the porphyrins/C-60 and C-60 molecules. Both the films and single crystals composed of the porphyrin and C60 exhibited remarkably high electron mobility (7 x 10(-2) and 0.4 cm(2)V(-1) s(-1)), which is comparable to the value for highly efficient bulk heterojunction solar cells. Our experimental results have successfully demonstrated the importance of nanostructured electron- and hole-transporting pathways in bulk heterojunction solar cells. Such a finding will provide basic and valuable information on the design of molecular photovoltaics at the molecular level.

We have systematically examined the substituent effects of meso-tetraphenylporphyrins on film structures and the photoetectrochemical properties of the composite clusters of free-base porphyrin and C-60 electrophoretically deposited on nanostructured SnO2 electrodes. The photocurrent generation efficiency was found to correlate with the complexation ability of the porphyrin for C-60. Basically, the incident photon-to-current efficiency (IPCE) value was increased with increasing relative amounts of the porphyrin versus C-60 in the films. The unique molecular arrangement of the porphyrin with the simple, specific substituents (i.e., methoxy groups at the ineta-positions of the meso-phenyl rings of tetraphenylporphyrins (3,5-OMeTPP; TPP=tetraphenylporphyrin)) and C-60 on SnO2 electrodes resulted in the largest IPCE value (ca. 60%) among this type of photoelectrochemical device. The rapid formation of the composite clusters and microcrystals from the combination of 3,5-OMeTPP and C60 in a mixed solvent is unique as the association is accelerated by intermolecular interactions (i.e., hydrogenbonding and CH-pi interactions) between the methoxy groups of the porphyrins and the porphyrin/C-60, in addition to the pi-pi interactions between the porphyrins/C-60 and C-60 molecules. Both the films and single crystals composed of the porphyrin and C60 exhibited remarkably high electron mobility (7 x 10(-2) and 0.4 cm(2)V(-1) s(-1)), which is comparable to the value for highly efficient bulk heterojunction solar cells. Our experimental results have successfully demonstrated the importance of nanostructured electron- and hole-transporting pathways in bulk heterojunction solar cells. Such a finding will provide basic and valuable information on the design of molecular photovoltaics at the molecular level.

A novel naphthyl-fused zinc porphyrin carboxylic acid has been synthesized and employed successfully in a dye-sensitized TiO2 solar cell, with a power conversion efficiency of 4.1%, which is improved by 50% relative to the unfused porphyrin reference cell.

Electrode structures and photovoltaic properties of porphyrin-sensitized solar cells with TiO(2) and Nb-, Ge-, and Zr- added TiO(2) composite electrodes were examined to disclose the effects of partial substitution of Ti atom by the other metals in the composite electrodes. The TiO(2) and Nb-, Ge-, and Zr- added TiO(2) composite electrodes were prepared by sol-gel process using laurylamine hydrochloride as a template for the formation of micellar precursors yielding well-defined mesoporous nanocrystalline structures, as in the cases of the formation of silica and titania tubules and nanoparticles by the templating mechanism. The TiO(2) and Nb-, Ge-, and Zr- added TiO(2) composite electrodes were characterized by transmission electron microscopy, BET surface area analysis, X-ray diffraction analysis, Raman spectroscopy, and impedance measurements. The TiO(2) anatase nanocrystalline structure is retained after doping a small amount (5 mol %) of Nb, Ge, or Zr into the TiO(2) structure, suggesting the homogeneous distribution of the doped metals with replacing Ti atom by the doped metal. The power conversion efficiency of the porphyrin-sensitized solar cells increases in the order Zr- added TiO(2) (0.8%) Nb-added TiO(2) (1.2%) < TiO(2) (2.0%) < Ge- added TiO(2) cells (2.4%) under the same conditions. The improvement of cell performance of the Ge-added TiO(2) cell results from the negative shift of the conduction band of the Ge-added TiO(2) electrode. The Ge-added TiO(2) cell exhibited a maximum power conversion efficiency of 3.5% when the porphyrin was adsorbed onto the surface of the Ge- added TiO2 electrode with a thickness of 4 mu m in MeOH for 1 h.

Very high photocurrent generation is seen for an ordered supramolecular composite structure Of C-60 and porphyrin assembled on Sno(2) electrodes. The simple substitution of methoxy groups at the meta positions of the meso-phenyl groups on the porphyrin ring leads to the ordered stacking arrangement shown in the figure, with separate electron and hole transport paths along the 1D porphyrin chains and 2D C-60 sheets.

An electron-rich perylenediimide-C-60 dyad has been prepared to explore a new type of donor-acceptor system. Time-resolved absorption measurements in benzonitrile revealed unambiguous evidence for the formation of a charge-separated state consisting of perylene diimide radical cation and C-60 radical anion via photoinduced electron transfer, showing a new class of artificial photosynthetic models in terms of charge separation.

A silica nanoparticle has been successfully employed as a nanoscaffold to self-organize porphyrin and C-60 molecules on a nanostructured SnO2 electrode. The quenching of the porphyrin excited singlet state on the silica nanoparticle is suppressed significantly, showing that silica nanoparticles are promising scaffolds for organizing photoactive molecules three-dimensionally in nanometer scale. Marked enhancement of the photocurrent generation was achieved in the present system compared with the reference system, where a gold core was employed as a scaffold of porphyrins instead of a silica nanoparticle. The rather small incident photon-to-current efficiency relative to a similar photoelectrochemical device using a silica microparticle may result from poor electron and hole mobility in the composite film due to poor connection between the composite clusters of a porphyrin-modified silica nanoparticle and C-60 in micrometer scale.

Clusters of phthalocyanine and phthalocyanine-perylene diimide have been prepared and electrophoretically deposited on nanostructured SnO(2) electrodes. The structure and photoelectrochemical properties of the clusters have been investigated by using UV-visible absorption, dynamic light scattering (DLS), atomic force microscopy (AFM), transmission electron microscopy (TEM), and photoelectrochemical and photodynamical measurements. Enhancement of the photocurrent generation efficiency in the composite system has been achieved relative to that in the phthalocyanine reference system without the perylene diimide. Such information will be valuable for the design of molecular photoelectrochemical devices that exhibit efficient photocurrent generation.

Structure and photoelectrochemical properties of nanostructured SnO(2) electrodes deposited electrophoretically with the composite clusters of porphyrin-modified gold nanoparticle with a long, flexible spacer and C(60) molecules have been examined to obtain basic information on the development of organic solar cells with a high performance. The photoelectrochemical system with the long, flexible spacer between the porphyrin and the gold nanoparticle in the porphyrin-modified gold nanoparticle exhibited comparable external quantum yield in the UV-vis regions relative to porphyrin-modified gold nanoparticle with a relatively short spacer-C(60) Composite reference system. These results demonstrate that a suitable spacer to incorporate C(60) molecules efficiently between the porphyrins in porphyrin-modified gold nanoparticles is a prerequisite for improving the performance of porphyrin and fullerene-based organic solar cells. (c) 2005 Elsevier Ltd. All rights reserved.

A silica microparticle has been successfully employed as a nanoscaffold to self-organize porphyrin and C-60 molecules on a nanostructured SnO2 electrode which exhibits efficient photocurrent generation.

We have constructed a variety of molecular assemblies of porphyrin as a donor and fullerene as an acceptor on electrodes for molecular photoelectrochemical devices. Highly efficient energy- and electron-transfer processes have been realized at gold electrodes modified with self-assembled monolayers of porphyrin- or fullerene linked systems mimicking light-harvesting and charge separation in bacterial photosynthesis. Highly ordered organization of porphyrins and fullerenes has also been achieved using step-by-step self-assembly of porphyrin and fullerene units by association with gold nanoparticles or dendrimers on tin oxide electrodes, which exhibit high power-conversion efficiency of up to 1.5%. These results will provide valuable information on the design of donor-acceptor type molecular assemblies that can be tailored to construct highly efficient organic solar cells.

Novel gold nanoparticles modified with a mixed self-assembled monolayer of porphyrin alkanethiol and short-chain alkanethiol were prepared (first step) to examine the size and shape effects of surface holes (host) on porphyrin-modified gold nanoparticles. ne porphyrin-modified gold nanoparticles with a size of about 10 nm incorporated C-60 molecules (guest) into the large, bucket-shaped holes, leading to the formation of a supramolecular complex of porphyrin-C-60 composites (second step). Large composite clusters with a size of 200400 nm were grown from the supramolecular complex of porphyrin-C-60 composites in mixed solvents (third step) and deposited electrophoretically onto nanostructured SnO2 electrodes (fourth step). Differences in the porphyrin:C-60, ratio were found to affect the structures and photoelectrochemical properties of the composite clusters in mixed solvents as well as on the SnO2, electrodes. The photoelectrochemical performance of a photoelectrochemical device consisting of SnO2 electrodes modified with the porphyrin-C-60 composites was enhanced relative to a reference system with small, wedged-shaped surface holes on the gold nanoparticle, Time-resolved transient absorption spectroscopy with fluorescence lifetime measurements suggest the occurrence of ultrafast electron transfer from the porphyrin excited singlet states to C-60 or the formation of a partial charge-transfer state in the composite clusters of supramolecular complexes formed between porphyrin and C-60 leading to efficient photocurrent generation in the system. Elucidation of the relationship between host-guest interactions and photoelectrochemical function in the present system will provide valuable information on the design of molecular devices and machines including molecular photo-voltaics.

Hydrogen bonding effects on surface structure, photophysical properties, and photoelectrochernistry have been examined in a mixed film of porphyrin and fullerene composites with and without hydrogen bonding on indium tin oxide and nanostructured SnO2 electrodes. The nanostructured SnO2 electrodes modified with the mixed films of porphyrin and fullerene composites with hydrogen bonding exhibited efficient photocurrent generation compared to the reference systems without hydrogen bonding. Atomic force microscopy, infrared reflection absorption, and ultraviolet-visible absorption spectroscopies and time-resolved fluorescence lifetime and transient absorption spectroscopic measurements disclosed the relationship between the surface structure and photophysical and photoelectrochernical properties relating to the formation of hydrogen bonding between the porphyrins and/or the C-60 moieties in the films on the electrode surface. These results show that hydrogen bonding is a highly promising methodology for the fabrication of donor and acceptor composites on nanostructured semiconducting electrodes, which exhibit high photoelectrochernical properties.

Organic photovoltaic devices based on supramolecular complexes of porphyrin-modified gold nanoclusters with fullerene molecules are described and their promising photoelectrochemical performance reported. The incorporation of C-60 in surface holes on the porphyrin-modified gold nanoclusters (see Figure)-caused by the addition of a short-chain alkanethiol to the porphyrin modification mixture-plays a key role.

Two kinds of fullerene derivatives have been designed to examine the effect of the fullerene substituents on the structure and photoelectrochemical properties of fullerene clusters electrophoretically deposited on nanostructured SnO2 electrodes. The cluster sizes increase and the incident photon-to-current efficiencies decrease with introduction of large substituents into C-60. The trend for photocurrent generation efficiency as well as surface morphology on the electrode can be explained by the steric bulkiness around the C-60 molecules. A C-60 molecule with two alkoxy chains is suggested to give a bilayer vesicle structure, irrespective of the hydrophobic nature of both the C-60 and alkoxy chain moieties. Such information will be valuable for the design of photoactive molecules, which are fabricated onto electrode surfaces to exhibit high energy conversion efficiency.

Cross-linked poly(vinylene-arsine)s (4) were synthesized by a free-radical terpolymerization of phenylacetylene (2), hexaphenylcyclohexaarsine (1), and p- or m-diethynylbenzene (p- or m-3) at different compositions in the presence of a catalytic amount of AIBN, The number-average molecular weights of the resulting polymers were a few thousands estimated by gel permeation chromatography (GPC) analysis. By gas chromatography (GC) analysis p-3 was found to be more reactive during the polymerization than m-3 and the resulting cross-linked polymer (p-4) showed lower solubility than that of m-4. The cross-linked poly(vinylene-arsine) s showed higher glass transition temperature in the measurement of differential scanning calorimetry (DSC) and more red-shifted emission in chloroform solution than that of the linear poly(vinylene-arsine). These properties could be tuned by varying the monomer feed ratio.