Photoinduced Energy and Electron Transfer in Fullerene−Oligothiophene−Fullerene Triads

A series of fullerene−oligothiophene−fullerene (C 60 − n T−C 60 ) triads with n = 3, 6, or 9 thiophene units has been synthesized, and their photophysical properties have been studied using photoinduced absorption and fluorescence spectroscopy in solution and in the solid state as thin films. The results are compared to those of mixtures of oligothiophenes ( n T) with N -methylfulleropyrrolidine (MP−C 60 ). Photoexcitation of the triads in the film results in an electron-transfer reaction for n = 6 and 9, but not for n = 3. The lifetime of the charge-separated state in the film is on the order of milliseconds. Photoexcitation of the oligothiophene moiety of the C 60 − n T−C 60 triads, dissolved in an apolar solvent, results in a singlet energy-transfer reaction to the fullerene moiety with rates varying between 10 12 and 10 13 s -1 . In more polar solvents, an intra molecular photoinduced charge separation occurs for n = 6 and 9 and, to some extent, for n = 3. The quenching of the MP−C 60 (S 1 ) fluorescence provides a lower limit to the rate of the intra molecular photoinduced electron transfer of 10 11 s -1 in the C 60 − n T−C 60 triads with n = 6 or 9 in polar solvents, assuming that charge separation occurs after singlet energy transfer from n T(S 1 ) to MP−C 60 (S 1 ). A direct mechanism, i.e., charge separation from n T(S 1 ), cannot be excluded experimentally but must occur in the femtosecond time domain to compete effectively with energy transfer. The lifetime of the intra molecularly charge-separated state in the C 60 − n T−C 60 triads is significantly reduced compared to the lifetime of the radical ions in the films, and hence, the latter results from charge migration to different molecular sites. Similar energy- and electron-transfer reactions occur inter molecularly in solution from the n T and MP−C 60 triplet states. The preferences for intra - and inter molecular energy- and electron-transfer reactions, as a function of conjugation length and solvent permittivity, are in full agreement with predictions that can be made using the Weller equation for the change in free energy upon charge separation.