EPR Investigation of Photoinduced Radical Pair Formation and Decay to a Triplet State in a Carotene−Porphyrin−Fullerene Triad

Abstract

The photochemistry of a molecular triad consisting of a porphyrin (P) covalently linked to a carotenoid polyene (C) and a fullerene derivative (C 60 ) has been studied at 20 K by time-resolved EPR spectroscopy following laser excitation. Excitation of the porphyrin moiety yields C− 1 P−C 60, which decays by photoinduced electron transfer to yield C−P • + −C 60 • - . This state rapidly evolves into a final charge-separated state C • + −P−C 60 • -, whose spin-polarized EPR signal was observed and simulated. There is a weak exchange interaction between the electrons in the radical pair ( J = 1.2 G). The C • + −P−C 60 • - state decays to give the carotenoid triplet in high yield with a time constant of 1.2 μs. The spin polarization of 3 C−P−C 60 is characteristic of a triplet formed by charge recombination of a singlet-derived radical pair. The kinetics of the decay of 3 C−P−C 60 to the ground state were also determined. The photoinduced electron transfer from an excited singlet state at low temperature and the high yield of charge recombination to a spin-polarized triplet mimic similar processes observed in photosynthetic reaction centers.