Organic Photoredox Catalysts for CO2 Reduction: Understanding the Mechanisms of Catalyst Deactivation.

Organic photoredox catalysts, such as terphenyls, offer sustainable alternatives to precious-metal-based catalysts for carbon dioxide utilization. However, their practical application is hindered by low turnover numbers, believed to be caused by rapid catalyst deactivation via reactions, such as Birch reduction. This computational study examines two possible deactivation mechanisms of the three terphenyl isomers: (1) hydrogen atom transfer from an exciplex ([OPP-3δ--TEAδ+]*) formed as a result of incomplete excited-state quenching of the catalyst and (2) protonation of the radical anion terphenyl post-quenching by the triethylamine (TEA) radical cation. Calculations reveal that deactivation from the exciplex state is less likely owing to large intrinsic barriers. The TEA radical cation is the most likely proton source for the ground state reaction, and the intrinsic barriers to protonation are lower for the meta- and ortho- isomers compared to the para- isomer. The solvent dielectric plays an important role; exciplex formation is less likely and protonation barriers increase with increasing dielectric. We also identify a computationally accessible descriptor for the terphenyl position(s) that are most susceptible to protonation.