Photosynthetic electron transfer using fullerenes as novel acceptors

A variety of porphyrin-linked C-60 dyads and triads have been designed and synthesized to elucidate the special properties of fullerenes in electron transfer (ET). C-60 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 (A) in C-60 compared with that in conventional planar aromatic accepters, as we have already proposed. Porphyrin-pyromellitimide-C-60 triads have been prepared to mimic photosynthetic ET. Based on the fluorescence quenching experiments, it was concluded that the C-60 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-C-60 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. (C) 2000 Elsevier Science Ltd. All rights reserved.