The photophysical properties of a homologous series of oligothienylenevinylenes (nTVs) and the inter- and intramolecular photoinduced energy- and electron-transfer processes between an nTV as a donor and N-methylfulleropyrrolidine (MP−C60) as an acceptor are described as a function of oligomer length (n = 2, 3, 4, 6, 8, and 12) in apolar and polar solvents. Whereas the shorter oligomers fluoresce and have singlet excited-state [nTV(S1)] lifetimes of 280 ps (2TV) and 1360 ps (3TV), the S1 lifetimes of the longer oligomers (n > 3) are extremely short because of a fast thermal decay, resulting in negligible quantum yields for fluorescence and intersystem crossing to the triplet state [nTV(T1)]. Using photoinduced absorption (PIA) spectroscopy, we demonstrate, however, that the nTV(T1) state can be populated indirectly via intermolecular triplet energy transfer using photoexcited MP−C60 as a triplet sensitizer for n > 2. This enabled the spectral determination of the nTV (T n ← T1) transition as a function of chain length (n = 3−12) in toluene (ε = 2.38). In a more polar solvent, o-dichlorobenzene (ODCB, ε = 9.93), the MP−C60(T1) state acts both as an oxidizing agent toward the nTVs, resulting in the formation of a metastable radical ion pair (nTV+• + MP−C60 -•) for n > 2, and as a triplet sensitizer, to produce the nTV(T1) state. In addition to the intermolecular transfer reactions, we investigated the corresponding intramolecular photoinduced energy- and electron-transfer reactions in systems in which MP−C60 and nTV (n = 2−4) are covalently linked. The results indicate that, after photoexcitation of the nTV moiety, an ultrafast singlet energy transfer to MP−C60 occurs, followed by an intramolecular electron transfer. The intramolecularly charge-separated state is the lowest-energy excited state in polar media, e.g., ODCB. In apolar media, e.g., toluene, the formation of the intramolecularly charge-separated state occurs only for n > 2 and concurrent with fluorescence and intersystem crossing to the MP−C60(T1) state. The discrimination between energy and electron transfer is rationalized using a continuum model.

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 1012 and 1013 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 1011 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.