EPR investigation of photoinduced radical pair formation and decay to a triplet state in a carotene-porphyrin-fullerene triad

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-P-1-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 mu s. The spin polarization of C-3-P-C-60 is characteristic of a triplet formed by charge recombination of a singlet-derived radical pair. The kinetics of the decay of C-3-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.