Photoinduced electron transfer processes of a fused C₆₀-TTF- C₆₀ dumbbell triad

Intramolecular photoinduced charge-separation and charge-recombination processes in a fused C₆₀-TTF-C₆₀ dumbbell triad molecule, which has been designed to be soluble in organic solvents thanks to the substitution of C₆₀ with polyalcoxy ester groups, have been investigated in various solvents by time-resolved absorption and fluorescence techniques. Appreciable interaction between the C₆₀-moiety and TTF-moiety in the ground state has been suggested by steady-state absorption spectra. The fluorescence spectra suggest also considerable interaction in the singlet excited state. The observed short fluorescence lifetimes of C ₆₀-TTF-C₆₀ compared with methanofullerene in toluene. THF, and benzonitrile indicate that the charge-separation takes place via the singlet excited state of the C₆₀-moiety, producing the ion-pair (C₆₀-TTF^.--C₆₀^.-), which was confirmed with picosecond transient absorption spectra. The nanosecond transient absorption spectra at 1000 nm indicate the formation of C ₆₀-TTF^.--C₆₀^.-, which decays with a lifetime of 20 ns as an upper limit in benzonitrile. In THF and toluene, clear transient absorption bands of C₆₀-TTF^.+-C ₆₀^.- were not observed within nanosecond laser pulse of ca. 6 ns duration, suggesting that C₆₀-TTF^.+-C ₆₀^.- state presents a lifetime within 10 ns. These observations were compared with the reported data for C⁶⁰-TTF dyads.