Photoinduced charge-separation and charge-recombination processes of fullerene[60] dyads covalently connected with phenothiazine and its trimer

Photoinduced charge-separation and charge-recombination processes of fullerene[60] dyads covalently connected with phenothiazine and its trimer (PTZ(n)-C-60, n = 1 and 3) with a short amide linkage were investigated. A time-resolved fluorescence study provided evidence of charge separation via the excited singlet state of a C-60 moiety (C-1(60)*), which displayed high efficiencies in various solvents; Phi D-CS(s) (quantum yield of charge separation via (C-1(60)*) = 0.59 (toluene) to 0.87(DMF) for PTZ(1)-C-60 and 0.78 (toluene) to 0.91 (DMF) for PTZ(3)-C-60. The transient absorption measurement with a 6 ns time resolution in the visible and near-IR regions showed evidence of the generation of radical ion pairs in relatively polar solvents for both dyads. In nonpolar toluene, only PTZ(1)-C-3(60)* was observed for PTZ(1)-C-60, whereas PTZ(3)-C-3(60)* as well as the radical ion pair state in equilibrium were observed for PTZ(3)-C-60. The radical ion pairs had relatively long lifetimes: 60 (DMF) to 910 ns (o-dichlorobenzene) for (PTZ)(1)(center dot+)-C-60(center dot-) and 230 (PhCN) to 380 ns (o-dichlorobenzene) for (PTZ)(3)(center dot+)-C-60(center dot-). The small reorganization energy (lambda) and the electronic coupling element (vertical bar V vertical bar) were estimated by the temperature dependence of the charge-recombination rates, i.e., lambda = 0.53 eV and vertical bar V vertical bar = 1.6 cm(-1) for (PTZ)(3)(center dot+)-C-60(center dot-).