Photosynthetic electron transfer using fullerenes as novel acceptors Original Research Article

A variety of porphyrin-linked C60 dyads and triads have been designed and synthesized to elucidate the special properties of fullerenes in electron transfer (ET). C60 or naphthalenediimide with comparable reduction potentials was linked to a porphyrin with similar spacers. Accelerated photoinduced charge separation (CS) was observed in the former compared with the latter by picosecond fluorescence lifetime measurements. It may be explained by the small reorganization energy (λ) in C60 compared with that in conventional planar aromatic acceptors, as we have already proposed. Porphyrin–pyromellitimide–C60 triads have been prepared to mimic photosynthetic ET. Based on the fluorescence quenching experiments, it was concluded that the C60 moiety accelerates the initial ET process via through-bond or enhances the direct through-space ET from the excited singlet state of the porphyrin. To optimize CS in triad models, a ferrocene–porphyrin–C60 triad has been synthesized. The triad produced a long-lived, charge-separated state with a high quantum yield, compared with the previously reported similar conventional triads. These results clearly show that a combination of multistep ET strategy with small reorganization energies of fullerenes is a promising methodology toward the construction of a solar energy conversion system.