Photoinduced Electron Transfer in Naphthalene Diimide End-Capped Thiophene Oligomers

Abstract

A series of linear thiophene oligomers containing 4, 6, 8, 10, and 12 thienylene units were synthesized and end-capped with naphthalene diimide (NDI) acceptors with the objective to study the effect of oligomer length on the dynamics of photoinduced electron transfer and charge recombination. The synthetic work afforded a series of nonacceptor-substituted thiophene oligomers, Tn, and corresponding NDI end-capped series, TnNDI2 (where n is the number of thienylene repeat units). This paper reports a complete photophysical characterization study of the Tn and TnNDI2 series by using steady-state absorption, fluorescence, singlet oxygen sensitized emission, two-photon absorption, and nanosecond-microsecond transient absorption spectroscopy. The thermodynamics of photoinduced electron transfer and charge recombination in the TnNDI2 oligomers were determined by analysis of photophysical and electrochemical data. Excitation of the Tn oligomers gives rise to efficient fluorescence and intersystem crossing to a triplet excited state that is easily observed by nanosecond transient absorption spectroscopy. Bimolecular photoinduced electron transfer from the triplet states, 3Tn*, to N,N-dimethylviologen (MV2+) occurs, and by using microsecond transient absorption it is possible to assign the visible region absorption spectra for the one electron oxidized (polaron) states, Tn+•. The fluorescence of the TnNDI2 oligomers is quenched nearly quantitatively, and no long-lived transients are observed by nanosecond transient absorption. These findings suggest that rapid photoinduced electron transfer and charge recombination occurs, NDI-1(Tn)*-NDI → NDI-(Tn)+•-NDI-• → NDI-Tn-NDI. Preliminary femtosecond-picosecond transient absorption studies on T4NDI2 reveal that both forward electron transfer and charge recombination occur with k > 1011 s-1, consistent with both reactions being nearly activationless. Analysis with semiclassical electron transfer theory suggests that both reactions occur at near the optimum driving force where -ΔG ∼ λ.