Creation of fullerene-based artificial photosynthetic systems

Various porphyrin-fullerene-linked systems have been prepared to elucidate the intrinsic electron-transfer properties of fullerenes. Photodynamical studies on porphyrin-fullerene-linked systems showed that spherical fullerenes accelerated photoinduced electron transfer and charge-shift, but slowed down charge recombination, which is in sharp contrast with those of conventional planar acceptors such as quinones and imides. For the first time, it was shown that the unique electron-transfer properties result from the small reorganization energies of fullerenes arising from the delocalized pi system on the sphere together with the rigid structure. The small reorganization energies make it possible to produce a long-lived charge-separated state with a high quantum yield in donor-fullerene systems. The finding also will allow us to construct light energy conversion systems as well as artificial photosynthetic models. Highly efficient photoinduced energy and electron transfer were achieved on gold and ITO electrodes modified with self-assembled monolayers of the porphyrin-fullerene-linked systems. We also developed dye-sensitized bulk heterejunction solar cell possessing both characteristics of dye-sensitized and bulk heterojunction solar cells. These results showed the advantages of fullerenes as electron acceptors in artificial photosynthetic model, photonic molecular devices and machines, and organic molecular electronics including solar cells.