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 (C60) has been studied at 20 K by time-resolved EPR spectroscopy following laser excitation. Excitation of the porphyrin moiety yields C−1P−C60, which decays by photoinduced electron transfer to yield C−P•+−C60•-. This state rapidly evolves into a final charge-separated state C•+−P−C60•-, 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−C60•- state decays to give the carotenoid triplet in high yield with a time constant of 1.2 μs. The spin polarization of 3C−P−C60 is characteristic of a triplet formed by charge recombination of a singlet-derived radical pair. The kinetics of the decay of 3C−P−C60 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.