EPR investigation of photoinduced radical pair formation and decay to a triple 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 (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 to 1.2 μs. The spin polarization of ³C-P-C₆₀ is characteristic of a triple formed by charge recombination of a single- 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.