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-C-60-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 C-60-moiety, producing the radical ion-pair [TTF-spacer-C-60(center dot-)-spacer-TTFcenter dot+] 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-C-60(center dot-)-spacer-TTFcenter dot+] were observed, suggesting the charge-separation occurs via the excited triplet state of the C-60 moiety. The lifetime (tau(RIP)) of the radical ion- pair state depends on the solvent polarity; tRIP 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 C-60 moiety, when dissolved in non-polar solvents, appears to be a new criterion to maintain the charge-separated state for a relatively long time.