Photoinduced charge separation and charge recombination in [60]fullerene-(benzothiadiazole-triphenylamine) based dyad in polar solvents

The molecular dyad C₆₀-(BTD-TPA) consisting of an electron donor triphenylamine-appended 2,1,3-benzothiadiazole chromophore (BTD-TPA) unit covalently linked to an electron acceptor [60]fullerene has been synthesized. The photoinduced electron transfer in C₆₀-(BTD-TPA) has been studied in polar and nonpolar solvents using time-resolved transient absorption and fluorescence measurements. By fluorescence lifetime measurements in picosecond time regions, the excitation of the C₆₀ moiety leads to the formation of C₆₀^•--(BTD-TPA)^•+ efficiently via the singlet excited state of the C₆₀ moiety. Excitation of the BTD-TPA moiety leads to initial energy transfer to ¹C* ₆₀- (BTD-TPA), from which electron transfer occurs to form C ₆₀^•--(BTD-TPA)^•+. In the nanosecond time region, C₆₀^•--(BTD-TPA)^•+ in which the radical cation (hole) delocalizes in the BTD-TPA moiety is persistent for 690 ns in DMF at room temperature. From the temperature dependence of the charge-recombination rate constants, which gave the Marcus parameters, we attempted to reveal the origins of long persistent C₆₀ ^•--(BTD-TPA)^•+ in DMF.