Organic photoredox catalysis is an important component of an energy-efficient, sustainable future as these catalysts can access highly reactive states upon excitation and quenching to carry out reactions that are otherwise thermally inaccessible . We aim to identify sustainable photoredox routes for CO2 utilization. Prior experiments show that a simple organic chromophore, p-terphenyl, can reduce and transform CO2 into useful molecules such as amino acids. The steps of the photoredox cycle and the reasons for low turnover numbers of these catalysts are poorly understood. We utilize quantum chemistry methods to delineate mechanisms of key steps in this cycle. In particular, we and our collaborators are combining experimental fluorescence spectra with computational methods (TDDFT optimization, excited-state energy decomposition analysis, natural transition orbitals) to identify and characterize excited-state donor-acceptor complexes or exciplexes. These complexes, formed between excited-state p-terphenyl and ground-state triethylamine (TEA) (electron donor), are hypothesized to be precursors for degradation via Birch reduction. The exciplex emission frequency depends strongly on solvent dielectric but the extent of charge separation in an exciplex does not. Our results also suggest that the formation of solvent-separated ionic, radical states upon complete electron transfer competes with exciplex formation in higher-dielectric solvents, thereby reducing exciplex emission intensities in fluorescence experiments.

Understanding photoredox catalytic cycles: Experimental and computational characterization of excited-state complexes

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