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

A series of fullerene-oligothiophene-fullerene (C60-nT-C60) 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 (nT) with N-methylfulleropyrrolidine (MP-C60). 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 C60-nT-C60 triads, dissolved in an apolar solvent, results in a singlet energy-transfer reaction to the fullerene moiety with rates varying between 10e12 and 10e13 s-1. In more polar solvents, an intramolecular photoinduced charge separation occurs for n = 6 and 9 and, to some extent, for n = 3. The quenching of the MP-C60(S1) fluorescence provides a lower limit to the rate of the intramolecular photoinduced electron transfer of 10e11 s-1 in the C60-nT-C60 triads with n = 6 or 9 in polar solvents, assuming that charge separation occurs after singlet energy transfer from nT(S1) to MP-C60(S1). A direct mechanism, i.e., charge separation from nT(S1), cannot be excluded experimentally but must occur in the femtosecond time domain to compete effectively with energy transfer. The lifetime of the intramolecularly charge-separated state in the C60-nT-C60 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 intermolecularly in solution from the nT and MP-C60 triplet states. The preferences for intra- and intermolecular 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.