(509j) Modeling Organic Photoredox Catalyst Reduction Kinetics, Donor-Acceptor Interactions, and Degradation Pathways in CO2 Reduction Catalytic Cycle

Rising anthropogenic CO₂ levels motivate widespread efforts to convert CO₂ into value-added products that are less environmentally harmful. Metal-free photoredox catalysts are competitive both industrially and synthetically but are under-described with regards to the electronically challenging reduction of CO₂. Organic photoredox catalysts can perform high-energy reductions from an anionic state formed from UV/Visible light exciting the catalyst followed by reduction by a sacrificial electron donor. Terphenyl is one such organic photoredox catalyst that can reduce CO₂; CO₂’s anionic state can then be protonated to produce formic acid or be photo-fixed into other small molecules to form amino acids. This work calculates the electronic states controlling electron transfer kinetics of para-substituted terphenyl (oligo-(para-phenylene) or OPP) to CO₂ and from the sacrificial electron donor. By varying the substituents and isomeric forms of terphenyl catalysts studied, this work aims to provide foundational knowledge about which electronic or structural properties determine catalytic activity and which lead to faster degradation. We also utilize energy decomposition analysis to explore the substituent impact on OPP, the sacrificial electron donor, and CO₂ inter-fragment interactions and further rationalize rate trends.

Calculated adiabatic electron transfer rates for the reduction of CO₂ by anionic terphenyl exhibit a plateauing dependence on electrophilicity (represented by the Hammett parameter, σ_p), which stems from the combination of a parabolic dependence of ∆G on σ_p and high λ’s for certain electron donating groups (EDG’s). EDG’s enhance ∆G of this step by making the radical anion unstable, and therefore more reducing. To determine substituent effects on degradation pathways, this research investigates the hypothesis that excited state complexes, or exciplexes, formed between the excited chromophore and the sacrificial electron donor can accelerate degradation via Birch reduction. By describing these degradation routes, we can develop a complete description of the electronic properties of terphenyl catalysts as they reduce CO₂.