(4e) Catalyst Discovery for Metal-Free, Photoredox CO2 Reduction

Our goal is to identify sustainable, light-driven routes for CO₂ utilization. Prior experiments show that upon activation by light and subsequent reduction, a simple organic chromophore, p-terphenyl, can reduce and transform CO₂ into valuable molecules such as amino acids. These photoredox reactions are attractive because organic chromophores are metal-free and can access highly reactive states upon excitation and quenching that are otherwise energy-intensive. However, the steps of the photoredox cycle and the reasons for the low turnover numbers of these catalysts are poorly understood. My group uses state-of-the-art quantum chemistry methods and automated workflows to delineate mechanisms of steps constituting the catalytic cycle and leverage these insights to drive the discovery of viable chromophores.

We demonstrated that the electron transfer (ET) step from the p-terphenyl radical anion to CO₂ is adiabatic and that ET barriers are lower when electron-donating groups are substituted at the p-terminal positions of the catalyst. While ET rates are higher for o- and m- isomers, they also exhibit faster degradation via carboxylation. To probe degradation pathways from the excited state, we established a protocol for calculating and characterizing excited-state donor-acceptor charge transfer complexes, or exciplexes. Furthermore, we constructed a first-of-its-kind benchmark database to identify the theoretical methods that provide physically meaningful descriptions of these excited-state quenching processes. Ongoing and future studies include probing other degradation pathways (e.g., Birch reduction), understanding the role played by the solvent in the catalytic cycle, and incorporating solvent contribution to our discovery protocol.