Computational Analysis of Electron Transfer Kinetics for CO2 Reduction with Organic Photoredox Catalysts

We present a fundamental description of the electron transfer (ET) step from substituted oligo(p-phenylene) (OPP) radical anions to CO2, with the larger goal of assessing the viability of underexplored, organic photoredox routes for utilization of anthropogenic CO2. This work varies the electrophilicity of para-substituents to OPP and probes the dependence of rate coefficients and interfragment interactions on the substituent Hammett parameter, σp, using constrained density functional theory (CDFT) and energy decomposition analysis (EDA). Large electronic coupling elements across substituents indicate an adiabatic electron transfer process for reactants at contact. As one might intuitively expect, free energy changes dominate trends in ET rate coefficients in most cases, and rates increase with substituent electron-donating ability. However, we observe an unexpected dip in rate coefficients for the most electron- donating groups, due to the combined impact of flattening free energies and a steep increase in reorganization energies. Our analysis shows that flattening OPP LUMO levels lower the marginal increase in free energy with decreasing σp. Reorganization energies do not exhibit a direct dependence on σp. They are higher for substituents containing lone pairs of electrons since substituent orientation varies with OPP charge. EDA reveals that interfragment orbital relaxation, or charge transfer interaction, plays a critical role in stabilizing the vertically excited charge transfer state. Subsequent relaxation to the final state geometry lowers charge transfer stabilization. A concurrent increase in long-range electrostatic interactions is observed, which are more favorable for electron-withdrawing substituents. Our study therefore suggests that while a wide range of ET rates are observed, there is an upper limit to rate enhancements achievable by tuning substituent electrophilicity.