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 linke d to a carotenoid polyene (C) and a fullerene (C-60) has been prepared and studied using time-resolved spectroscopic methods. In 2-methyltet rahydrafuran solution, the triad undergoes photoinduced electron trans fer to yield C-P-.+-C-60(.-), which evolves into C-.+-P-C-60(.-) With an overall quantum yield of 0.14. This state decays by charge recombin ation to yield the carotenoid triplet state with a time constant of 17 0 ns. Even in a glass at 77 K, C-.+-P-C-60(.-) is formed with a quantu m yield of similar to 0.10 and again decays mainly by charge recombina tion to give C-3-P-C-60. 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 form ation of a triplet state by charge recombination are phenomena previou sly observed mostly in photosynthetic reaction centers.