Intra- and intermolecular photoinduced energy and electron transfer between oligothienylenevinylenes and N-methylfulleropyrrolidine

Abstract

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 (Tn ← T1) transition as a function of chain length (n = 3-12) in toluene (∈ = 2.38). In a more polar solvent, o-dichlorobenzene (ODCB, ∈ = 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.