Photoinduced Microsecond-Charge-Separation in Retinyl-C60 Dyad

Abstract

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*-C60 by 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.