Photoinduced charge separation and charge recombination in the [60]fullerene-diphenylbenzothiadiazole-triphenylamine triad: Role of diphenylbenzothiadiazole as bridge

Photoinduced electron-transfer processes of the newly synthesized [60]fullerene-diphenylbenzothiadiazoletriphenylamine (C₆₀-PBTDP-TPA) triad in polar and nonpolar solvents have been studied by using time-resolved transient absorption and fluorescence measurements from picosecond to microsecond regions. By fluorescence lifetime measurements in picosecond time regions, excitation of the charge-transfer transition of the PBTDP-TPA moiety in C₆₀-PBTDP-TPA induces energy transfer to the C₆₀ moiety generating ¹C₆₀*-PBTDP-TPA, competitively with charge separation generating C₆₀^̇--PBTDP-TPA ^̇+. From ¹C₆₀*-PBTDP-TPA, which is generated directly and indirectly, charge separation occurs generating C ₆₀^̇--PBTDP-TPA^̇+ in polar solvents. The C^60̇--PBTDP-TPA^̇+ formed via the singlet excited states decayed within a few nanoseconds as revealed by the picosecond transient absorption spectra. In the nanosecond time region, C ₆₀^̇- PBTDP-TPA^̇+ is produced slowly, probably via ³C₆₀*-PBTDP-TPA. Lifetimes of such slowly generated C₆₀^̇--PBTDP-TPA^̇+ were longer than 1 μs, which are the longest values among the C ₆₀-bridge-TPA triad systems reported hitherto at room temperature. Roles of the PBTDP-TPA moiety with twisted intermolecular charge-transfer character playing as energy donor and electron donor in addition to the bridge have been disclosed.