Elucidating Electronic Coupling of Bimolecular Excited State Electron Transfer

Electronic coupling is one of three parameters needed to predict and model excited state electron transfer kinetics yet has never been measured for bimolecular reactions. This knowledge gap is surprising given the central role that this reaction plays in photoredox catalysis and solar energy conversion. Herein, we provide an experimental approach with an analysis based on Marcus theory that provides the electronic coupling, H_ab, for electron transfer within the encounter complex. To test this approach, two photosensitizers of the general form Ir(dF-(CF₃)-ppy)₂(LL)]⁺, where LL was bipyrazine (bpz) or 4,4'-(di-tert-butyl)-2,2'-bipyridine (dtb) were characterized and utilized to photo-oxidize iodide, bromide, and chloride over a 40 °C temperature range in acetonitrile. For iodide photo-oxidation, H_ab was found to be 130 cm^-1 for Ir-dtb* and was 3300 cm^-1 for Ir-bpz*, being sufficiently large that the nonadiabaticity of the excited state electron transfer is called into question. The stark difference in coupling is attributed, in part, to a larger average separation by the sterically bulky tert-butyl groups. Ir-bpz* was found to oxidize all three halides efficiently, and the coupling increased with the halide radius. These findings have significant implications for the design of photosensitizers with applications in photoredox catalysis and solar energy conversion.