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

A molecular triad consisting of a diarylporphyrin (P) covalently linked to a carotenoid polyene (C) and a fullerene (C₆₀) has been prepared and studied using time-resolved spectroscopic methods. In 2-methyltetrahydrofuran solution, the triad undergoes photoinduced electron transfer to yield C-P·⁺-C₆₀·^-, which evolves into C·⁺-P-C₆₀·^- with an overall quantum yield of 0.14. This state decays by charge recombination to yield the carotenoid triplet state with a time constant of 170 ns. Even in a glass at 77 K, C·⁺-P-C₆₀·^- is formed with a quantum yield of ~0.10 and again decays mainly by charge recombination to give ³C-P-C₆₀. The fullerene triplet, formed through normal intersystem crossing, is also observed at 77 K. The generation in the triad of a long-lived charge separated state by photoinduced electron transfer, the low-temperature electron transfer behavior, and the formation of a triplet state by charge recombination are phenomena previously observed mostly in photosynthetic reaction centers.