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.
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