Solvent effects and electron transfer dynamics in a porphyrin-fullerene dyad and a carotenoporphyrin-fullerene triad

Electron transfer dynamics were studied in a molecular triad consisting of covalently linked carotenoid (C), porphyrin (P), and fullerene (C-60) chromophores (C-P-C-60) and in a related porphyrin - fullerene dyad (P-C-60) using optical spectroscopy. After porphyrin photoexcitation, electron transfer to the fullerene was observed, forming the charge-separated state P.+-C-60(.-). In toluene solution, this state recombines to the C-60 first excited singlet state, leading to fullerene singlet sensitization by an unusual two-step electron transfer mechanism. In polar solvents, the carotenoporphyrin-fullerene triad was shown to undergo a different two-step electron transfer process, forming the long-lived C.+-P-C-60(.-) charge-separated state. The solvation dynamics of this state were investigated by monitoring carotenoid radical cation absorption band hypsochromic spectral shifts. Picosecond solvation processes were observed in six solvents, with solvation lifetimes ranging from 0.4ps to 35ps.