Transient states in photoinduced electron transfer reactions of porphyrin- and phthalocyanine-fullerene dyads

This paper combines the most important results on studies performed by the authors during the last decade on photoinduced electron transfer reactions of pheophytin-, phthalocyanine-, and porphyrin-fullerene dyads, in which donor and acceptor moieties are covalently linked to each other. Practically all studied molecules form an intramolecular exciplex as a transient state before the formation of the charge separation state or tight ion pair. When the center-to-center distance of the donor and acceptor pair is short (7-10 angstrom) both the exciplex formation and primary electron transfer are extremely fast with rate constants of 7-23 x 10(12) s(-1) and 40-1400 x 10(9) s(-1), respectively. Rates become slower when the distance and orientational fluctuation increases. No systematic correlation between free energies and the rates of the formation and recombination of the exciplex and the charge separation state, respectively, were observed. The mechanism is discussed in frames of the Marcus electron transfer and the radiationless quantum transition theories.