Hiroshi Imahori

Nanostructured artificial photosynthesis

Abstract We have proposed a novel strategy for artificial photosynthesis where porphyrins and fullerenes are assembled as building blocks into nanostructured artificial photosynthetic systems by the help of self-assembled monolayers. Photodynamical studies on porphyrin–fullerene-linked systems revealed that fullerenes accelerate photo-induced electron transfer and charge-shift and slow down charge recombination, which is in sharp contrast with the modalities of conventional two-dimensional aromatic acceptors such as quinones and imides. We proposed and demonstrated, for the first time, that such electron transfer properties are attributable to the small reorganization energies of fullerenes, which make it possible to optimize artificial photosynthetic multistep charge separation. The multistep electron transfer strategy was combined with our finding that fullerenes have small reorganization energies, which are well-suited for the construction of light energy conversion systems as well as artificial photosynthetic models. Highly efficient photosynthetic energy and electron transfers were realized at gold and indium–tin oxide (ITO) electrodes modified with self-assembled monolayers of porphyrin–fullerene-linked systems. Porphyrin-modified gold nanoclusters were found to have potential as artificial photosynthetic materials and photonic molecular devices. These results provide basic principles and concepts for the development of nanostructured artificial photosynthetic materials as well as molecular devices.

Fullerenes as Novel Acceptors in Photosynthetic Electron Transfer

We propose a novel strategy using fullerenes for the construction of solar energy conversion systems that mimic the primary electron transfer events in photosynthesis. Redox-active fullerenes such as C60 and C70 were covalently bound to a porphyrin and the photophysical properties of the resulting compounds were investigated. Regardless of solvent and linkage, the charge-separated state is produced efficiently in zincporphyrin–fullerene dyads, showing that fullerenes are good electron acceptors. The most intriguing characteristic of fullerenes in electron transfer is that they accelerate photoinduced charge separation as well as charge shift and slow down charge recombination, properties that are in sharp contrast with those of conventional two-dimensional aromatic acceptors such as quinones and imides. The peculiar electron transfer properties of fullerenes can be explained by the small reorganization energies, which make it possible to optimize artificial photosynthetic multistep charge separation. A combination of the two strategies, multistep electron transfer and small reorganization energy of fullerenes, has been applied to light energy conversion systems as well as the more complex molecular systems such as triads. Highly efficient photosynthetic multistep electron transfer has been realized at gold electrodes modified with self-assembled monolayers of fullerene-containing molecules. These results will provide new principles and concepts to develop artificial photosynthetic materials as well as molecular devices.

Giant multiporphyrin arrays as artificial light-harvesting antennas.

Synthetic giant multiporphyrin arrays with well-defined architectures are reviewed in terms of artificial light-harvesting materials. Meso,meso-linked porphyrin arrays and multiporphyrin dendrimers have successfully mimicked the light-harvesting function of bacterial photosynthetic systems. We have also developed novel multiporphyrin-modified metal nanoclusters where porphyrins employed as a light-harvesting unit are well organized onto metal nanoclusters by self-assembly processes. Multiporphyrin-modified metal nanoclusters have been applied to photocatalyses and photovoltaic cells. In particular, they have been assembled with fullerenes step-by-step to make large, uniform clusters on nanostructured semiconductor electrodes, which exhibit a high power-conversion efficiency close to 1%. These systems provide valuable information on the design of porphyrin molecular assemblies that can be tailored to construct molecular photonic devices as well as artificial photosynthetic systems.

Porphyrin–fullerene linked systems as artificial photosynthetic mimics

We have prepared a variety of porphyrin-fullerene linked systems to mimic photoinduced energy and electron transfer (ET) processes in photosynthesis. Photodynamical studies on porphyrin and analogs-fullerene linked systems have revealed the acceleration of photoinduced electron transfer and charge-shift and the deceleration of charge recombination, which is reasonably explained by the small reorganization energies of electron transfer in fullerenes. In this context, we have proposed two strategies, photoinduced single-step and multi-step electron transfers, for prolonging the lifetime of a charge-separated state in donor-acceptor linked systems. The single-step ET strategy allowed a zinc chlorin-fullerene linked dyad to extend the lifetime up to 120 seconds in frozen PhCN at 123 K, which is the longest value of charge separation ever reported for donor-acceptor linked systems. Unfortunately, however, the quantum yield of formation of the charge-separated state was as low as 12%, probably due to the decay of the precursor exciplex state to the ground state rather than to the favorable complete charge-separated state. In contrast, the multi-step ET strategy has been successfully applied to porphyrin-fullerene linked triads, tetrads, and a pentad. In particular, a ferrocene-porphyrin trimer-fullerene pentad revealed formation of a long-lived charge-separated state (0.53 s in frozen DMF at 163 K) with an extremely high quantum yield (83%), which is comparable to natural bacterial reaction centers. These results not only provide valuable information for a better understanding of photoinduced energy and electron transfer processes in photosynthesis, but also open the door for the development of photoinitiated molecular devices and machines.

Photoinduced Charge Carrier Dynamics of Zn−Porphyrin−TiO2 Electrodes: The Key Role of Charge Recombination for Solar Cell Performance

Time resolved absorption spectroscopy has been used to study photoinduced electron injection and charge recombination in Zn-porphyrin sensitized nanostructured TiO(2) electrodes. The electron transfer dynamics is correlated to the performance of dye sensitized solar cells based on the same electrodes. We find that the dye/semiconductor binding can be described with a heterogeneous geometry where the Zn-porphyrin molecules are attached to the TiO(2) surface with a distribution of tilt angles. The binding angle determines the porphyrin-semiconductor electron transfer distance and charge transfer occurs through space, rather than through the bridge connecting the porphyrin to the surface. For short sensitization times (1 h), there is a direct correlation between solar cell efficiency and amplitude of the kinetic component due to long-lived conduction band electrons, once variations in light harvesting (surface coverage) have been taken into account. Long sensitization time (12 h) results in decreased solar cell efficiency because of decreased efficiency of electron injection.

Supramolecular donor-acceptor heterojunctions by vectorial stepwise assembly of porphyrins and coordination-bonded fullerene arrays for photocurrent generation.

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.

Carbon nanotube-modified electrodes for solar energy conversion

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.

Electron Transfer Cascade by Organic/Inorganic Ternary Composites of Porphyrin, Zinc Oxide Nanoparticles, and Reduced Graphene Oxide on a Tin Oxide Electrode that Exhibits Efficient Photocurrent Generation

A bottom-up strategy has been developed to construct a multiple electron transfer system composed of organic/inorganic ternary composites (porphyrin, zinc oxide nanoparticles, reduced graphene oxide) on a semiconducting electrode without impairing the respective donor-acceptor components. The hierarchical electron transfer cascade system exhibited remarkably high photocurrent generation with an incident-photon-to-current efficiency of up to ca. 70%.

Novel unsymmetrically π-elongated porphyrin for dye-sensitized TiO2 cells

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.

Highly Asymmetrical Porphyrins with Enhanced Push–Pull Character for Dye‐Sensitized Solar Cells

A porphyrin π-system has been modulated by enhancing the push-pull character with highly asymmetrical substitution for dye-sensitized solar cells for the first time. Namely, both two diarylamino moieties as a strong electron-donating group and one carboxyphenylethynyl moiety as a strong electron-withdrawing, anchoring group were introduced into the meso-positions of the porphyrin core in a lower symmetrical manner. As a result of the improved light-harvesting property as well as high electron distribution in the anchoring group of LUMO, a push-pull-enhanced, porphyrin-sensitized solar cell exhibited more than 10% power conversion efficiency, which exceeded that of a representative highly efficient porphyrin (i.e., YD2)-sensitized solar cell under optimized conditions. The rational molecular design concept based on highly asymmetric, push-pull substitution will open the possibilities of further improving cell performance in organic solar cells.