Intra- and Intermolecular Photoinduced Energy and Electron Transfer between Oligothienylenevinylenes and <i>N</i>-Methylfulleropyrrolidine

Abstract

The photophysical properties of a homologous series of oligothienylenevinylenes ( n TVs) and the inter- and intramolecular photoinduced energy- and electron-transfer processes between an n TV as a donor and N -methylfulleropyrrolidine (MP−C 60 ) 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 [ n TV(S 1 )] lifetimes of 280 ps (2TV) and 1360 ps (3TV), the S 1 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 [ n TV(T 1 )]. Using photoinduced absorption (PIA) spectroscopy, we demonstrate, however, that the n TV(T 1 ) state can be populated indirectly via intermolecular triplet energy transfer using photoexcited MP−C 60 as a triplet sensitizer for n > 2. This enabled the spectral determination of the n TV (T n ← T 1 ) 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−C 60 (T 1 ) state acts both as an oxidizing agent toward the n TVs, resulting in the formation of a metastable radical ion pair ( n TV +• + MP−C 60 -• ) for n > 2, and as a triplet sensitizer, to produce the n TV(T 1 ) state. In addition to the intermolecular transfer reactions, we investigated the corresponding intra molecular photoinduced energy- and electron-transfer reactions in systems in which MP−C 60 and n TV ( n = 2−4) are covalently linked. The results indicate that, after photoexcitation of the n TV moiety, an ultrafast singlet energy transfer to MP−C 60 occurs, followed by an intra molecular electron transfer. The intra molecularly 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 intra molecularly charge-separated state occurs only for n > 2 and concurrent with fluorescence and intersystem crossing to the MP−C 60 (T 1 ) state. The discrimination between energy and electron transfer is rationalized using a continuum model.