Photoinduced Electron Transfer in Naphthalene Diimide End-Capped Thiophene Oligomers

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, T_n, and corresponding NDI end-capped series, T_nNDI₂ (where n is the number of thienylene repeat units). This paper reports a complete photophysical characterization study of the T_n and T_nNDI₂ 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 T_nNDI₂ oligomers were determined by analysis of photophysical and electrochemical data. Excitation of the T_n 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, ³T_n*, 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, T_n^+•. The fluorescence of the T_nNDI₂ 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(T_n)*-NDI → NDI-(T_n)^+•-NDI^-• → NDI-T_n-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.