Photoinduced microsecond-charge-separation in retinyl-C₆₀ dyad

Intramolecular photoinduced charge separation and recombination processes in a retinyl-C₆₀ dyad molecule (Ret-C₆₀) have been investigated in various solvents by time-resolved absorption and fluorescence techniques. Upon laser excitation of the C₆₀-moiety in nonpolar toluene, the intersystem crossing proceeded from the excited singlet state of the C₆₀-moiety (Ret-¹C₆₀*) to the excited triplet state (Ret-³C₆₀*), followed by energy transfer yielding the excited triplet state of the retinyl-moiety (³Ret*-C₆₀) without charge separation. On the other hand, in polar solvents such as N,N-dimethylformamide and benzonitrile, the charge separation occurred from Ret-¹C₆₀* at rate on the order of 10¹⁰ s^-1. The quantum yield was close to unity in these polar solvents. Most parts of the ion pair (Ret^.+-C₆₀^.-) changed to Ret-³C₆₀* and ³Ret*-C₆₀by the charge recombination which took place at rate on the order of 10⁹ s^-1. However, some parts of the charge-separated state were kept in microsecond time-region: The lifetimes of Ret^.+-C₆₀^.- were 16 μs and 19 μs in DMF and benzonitrile, respectively, which were as long as those of Ret-³C₆₀* and ³Ret*-C₆₀, suggesting an equilibrium between the charge-separated state and the excited triplet states.