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

Abstract

Photoinduced charge-separation and charge-recombination processes in a [TTF–spacer–C₆₀–spacer–TTF] triad, in which both flexible spacers are identical, have been investigated by time-resolved 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–C₆₀˙⁻–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–C₆₀˙⁻–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 ion-pair, 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.