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

The molecular dyad C-60-(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 C-60-(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 C-60 moiety leads to the formation Of C-60(.-)-(BTD-TPA)(.+) efficiently via the singlet excited state of the C-60 moiety. Excitation of the BTD-TPA moiety leads to initial energy transfer to C-1(60)*-(BTD-TPA), from which electron transfer occurs to form C-60(.-)-(BTD-TPA)(.+). In the nanosecond time region, C-60(.-)-(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 C-60(.-)-(BTD-TPA)(.+) in DMF.