Photoinduced Electron Transfer in Naphthalene Diimide End-Capped Thiophene Oligomers

• Published in: The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory Vol. 121; no. 50; pp. 9579 - 9588
• Main Authors: Jones, Austin L, Gish, Melissa K, Papanikolas, John M, Schanze, Kirk S
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 21-12-2017
• ISSN: 1089-5639; 1520-5215
• DOI: 10.1021/acs.jpca.7b09095

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 n NDI 2 (where n is the number of thienylene repeat units). This paper reports a complete photophysical characterization study of the T n and T n NDI 2 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 n NDI 2 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, 3 T 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 n NDI 2 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 T 4 NDI 2 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.