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-C₆₀, 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 C₆₀ moiety ( ¹C₆₀*), which displayed high efficiencies in various solvents; Φ^s_cs (quantum yield of charge separation via ¹C₆₀*) = 0.59 (toluene) to 0.87 (DMF) for PTZ₁-C₆₀ and 0.78 (toluene) to 0.91 (DMF) for PTZ ₃-C₆₀. The transient absorption measurement with a 6 ns time resolution in me 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 PTZ₁-³C₆₀* was observed for PTZ₁-C₆₀, whereas PTZ₃- ³C₆₀* as well as the radical ion pair state in equilibrium were observed for PTZ₃-C₆₀. The radical ion pairs had relatively long lifetimes: 60 (DMF) to 910 ns (o-dichlorobenzene) for (PTZ)₁^.+-C₆₀^.- and 230 (PhCN) to 380 ns (o-dichlorobenzene) for (PTZ)₃^.+-C^{60 .-}. The small reorganization energy (IVI) and the electronic coupling element (IVI) were estimated by the temperature dependence of the charge-recombination rates, i.e., λ -0.53 eV and IVI = 1.6 cm ^-1 for (PTZ)₃^.+-C60^.-.