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

Abstract 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(CF 3 )ppy) 2 (dtbbpy)]PF 6 ( 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.