Roles of Molecular Wires between Fullerenes and Electron Donors in Photoinduced Electron Transfer

In the photoinduced intramolecular electron transfer events of donor-bridge-acceptor molecular systems, the bridges play important roles. In the first part of this account, we review charge separation and charge recombination of porphyrin-bridge-fullerene systems, in which the porphyrin acts as a photosensitizing electron donor, whereas the ground state of the C-60 moiety acts as an electron acceptor. In such systems, the charge separation usually takes place through the LUMO of the bridges via a super-exchange mechanism and/or a hopping mechanism, depending on their relative LUMO energy levels. On the other hand, the charge recombination of the radical ion pairs usually takes place through the HOMO of the bridges. In the second part, research of charge separation via the excited state of fullerene through the HOMO of the bridges is reviewed. So far, very small damping factors have been reported for pi-conjugated bridges such as oligophenyleneacetylenes, oligothiophenes, and oligothiophenevinylenes. On summarizing these results, it is revealed that switching from the super-exchange mechanism in short bridges to hopping mechanism in longer bridges is important to achieve long distance electron transfer through the bridges.