What are the Quenching Processes in Photocatalyzed Reductive N-Heterocyclic Carbene Organocatalysis?

The mechanistic ambiguity surrounding ketyl radical generation via single-electron reduction of acyl azoliums in dual N-heterocyclic carbene (NHC)/photoredox catalytic systems has been systematically investigated. In this notation, a series of Stern-Volmer fluorescence quenching experiments employing two benchmark photocatalysts 4CzIPN (PC-1) and [Ir(dF(CF₃)ppy)₂(dtbbpy)]PF₆ (PC-2) with substrates S1-S11 demonstrates that acyl azoliums could be either quenched via an oxidative quenching process or a reductive quenching process. Noteworthy, we also highlight that the reduction potential of acyl azoliums derived from 1,2,4-triazolylidene was -0.8 V vs. SCE, resolving prior discrepancies attributed to hydrolytic degradation. This work underscores the necessity of using native intermediates for mechanistic accuracy and establishes oxidative quenching as an alternative pathway in NHC/photoredox systems.