Photoinduced charge-separation in a [TTF-spacer-C₆₀-spacer-TTF] triad involving two long flexible spacers

Photoinduced charge-separation and charge-recombination processes in a [TTF-spacer-Ceospacer-TTF] triad, in which both flexible spacers are identical, have been investigated by timeresolved absorption and fluorescence techniques. The observed short fluorescence lifetimes of this triad in polar solvents indicate that the charge-separation takes place via the singlet excited state of the C₆₀-moiety, producing the radical ion-pair [TTF-spacer-C ₆₀^.-spacer-TTF^.++] as shown from the transient absorption spectra in the 800-1100 nm region. In the case of non-polar solvent, although shortening of the fluorescence decay was not observed, transient absorption bands indicating the generation of [TTF-spacer-C6o ^.--spacer-TTF^.+] were observed, suggesting the charge-separation occurs via the excited triplet state of the C₆₀ moiety. The lifetime (τ_RIP) of the radical ion-pair state depends on the solvent polarity; τ_RIP values in non-polar solvents are larger than those in polar solvents. This is reasonably explained by the spin character of the radical ionpair, which is strongly influenced by the precursor of the charge-separation, and by Marcus theory. The triad possessing two TTFs connected by a flexible spacer to the C₆₀ moiety, when dissolved in non-polar solvents, appears to be a new criterion to maintain the charge-separated state for a relatively long time.