Photoinduced Microsecond-Charge-Separation in Retinyl-C₆₀ Dyad

Intramolecular photoinduced charge separation and recombination processes in a retinyl-C 60 dyad molecule (Ret-C 60 ) have been investigated in various solvents by time-resolved absorption and fluorescence techniques. Upon laser excitation of the C 60 -moiety in nonpolar toluene, the intersystem crossing proceeded from the excited singlet state of the C 60 -moiety (Ret- 1 C 60 *) to the excited triplet state (Ret- 3 C 60 *), followed by energy transfer yielding the excited triplet state of the retinyl-moiety ( 3 Ret*-C 60 ) without charge separation. On the other hand, in polar solvents such as N, N -dimethylformamide and benzonitrile, the charge separation occurred from Ret- 1 C 60 * at rate on the order of 10 10 s -1 . The quantum yield was close to unity in these polar solvents. Most parts of the ion pair (Ret •+ -C 60 •- ) changed to Ret- 3 C 60 * and 3 Ret*-C 60 by the charge recombination which took place at rate on the order of 10 9 s -1 . However, some parts of the charge-separated state were kept in microsecond time-region: The lifetimes of Ret •+ -C 60 •- were 16 μs and 19 μs in DMF and benzonitrile, respectively, which were as long as those of Ret- 3 C 60 * and 3 Ret*-C 60, suggesting an equilibrium between the charge-separated state and the excited triplet states.