A carotene-porphyrin-fullerene triad: Photoinduced charge separation and charge recombination to a triplet state

A molecular triad consisting of a porphyrin (P) covalently bonded to a fullerene (C-60) and a carotenoid polyene (C) has been synthesized and studied using time-resolved spectroscopic methods. In polar solvents, excitation of the porphyrin yields its first excited singlet state, C-P-1-C-60, which decays mainly by photoinduced electron transfer to the fullerene, giving C-P-.-C-60(.-). A second electron transfer, from the carotenoid, yields C.+-P-C-60(.-). This state decays by charge recombination to yield the carotenoid triplet state. Charge separation and recombination to the triplet is also observed in an organic glass at 77 K. Similar phenomena are observed in photosynthetic reaction centers, but are uncommon in synthetic systems.