Photoinduced electron transfer in carotenoporphyrin-fullerene triads: Temperature and solvent effects

A carotenoid (C) porphyrin (P) fullerene (C-60) molecular triad (C-P-C-60) has been synthesized and found to undergo photoinduced electron transfer from the porphyrin first excited singlet state or to the fullerene first excited singlet state to yield C-P.+-C-60(.-) Electron transfer from the carotenoid then gives a C.+-PC-(.-)(60) final charge-separated state. This state is formed with quantum yields up to 0.88 and has a lifetime of up to 1 mu s, depending upon the conditions. The various electron transfer rate constants are relatively insensitive to solvent and temperature. The quantum yield of C.+-P-C-60(.-) is relatively constant under conditions ranging from fluid solutions at ambient temperatures to a rigid organic glass at 8 K. In most solvents, recombination of C.+-P-C-60(.-) yields the carotenoid triplet state, rather than the ground state. The results suggest that the energies of the charge-separated states of fullerene-based systems are only about half as sensitive to changes in solvent dielectric constant as are those for similar molecules with quinone electron accepters, and that total reorganization energies for electron transfer are also smaller.