Photoinduced energy and electron transfer in fullerene-oligothiophene-fullerene triads

A series of fullerene-oligothiophene-fullerene (C-60-nT-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 fluorescenc e spectroscopy in solution and in the solid state as thin films. The result s are compared to those of mixtures of oligothiophenes (nT) with N-methylfu lleropyrrolidine (NLP-C-60). Photoexcitation of the triads in the film resu lts in an electron-transfer reaction for n = 6 and 9, but not for n = 3. Th e lifetime of the charge-separated state in the film is on the order of mil liseconds. Photoexcitation of the oligothiophene moiety of the C-60-nT-C-60 triads, dissolved in an apolar solvent, results in a singlet energy-transf er reaction to the fullerene moiety with rates varying between 10(12) and 1 0(13) 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 quenc hing of the MP-C-60(S-1) fluorescence provides a lower limit to the rate of the intramolecular photoinduced electron transfer of 10(11) s(-1) in the C -60-nT-C-60 triads with n = 6 or 9 in polar solvents, assuming that charge separation occurs after singlet energy transfer from nT(S-1) to MP-C-60(S-1 ). A direct mechanism, i.e., charge separation from nT(S-1), cannot be excl uded experimentally but must occur in the femtosecond time domain to compet e effectively with energy transfer. The lifetime of the intramolecularly ch arge-separated state in the C-60-nT-C-60 triads is significantly reduced co mpared to the lifetime of the radical ions in the films, and hence, the lat ter results from charge migration to different molecular sites. Similar ene rgy- and electron-transfer reactions occur intermolecularly in solution fro m the nT and MP-C-60 triplet states. The preferences for intra- and intermo lecular energy- and electron-transfer reactions, as a function of conjugati on length and solvent permittivity, are in full agreement with predictions that can be made using the Weller equation for the change in free energy up on charge separation.