Rapid Electrochemical Assessment of Excited-State Quenching Dynamics.

Recent advancements in electro-photoredox catalysis (e-PRC) and consecutive photoinduced electron transfer (conPET) have pushed the energy limits of conventional photocatalysis. Both methods produce open-shell intermediate catalysts that, upon light absorption, become highly reducing or oxidizing, enabling challenging reactions. Despite their widespread use, the mechanisms of e-PRC and conPET reactions remain debated, in part due to a lack of quantitative data in most studiesparticularly single-electron transfer rate constants (k SET) between excited-state catalysts and substrates. We present a straightforward electrochemical method for determining k SET using cyclic voltammetry (CV) under light irradiation, paired with electrochemical simulation. Using inexpensive LEDs and standard potentiostats, we investigated the reactivity of excited-state anions of a perylene diimide dye (PDI), the seminal catalyst of conPET reactions. CV was used to study the photochemical reactivity of both reduced species of PDI, *PDI•- and *PDI2-, in the reductive cleavage of carbon-halogen bonds in alkyl and aryl halides. The extreme reactivity of these excited-state anions is confirmed, with quenching rate constants of 107 and 1010 M-1 s-1 for *PDI•- and *PDI2-, respectively, consistent with theoretical and experimental data. The voltammetric approach presented here provides a rapid and reliable tool for studying the excited-state reactivity of labile intermediates utilized in e-PRC and conPET systems, including both radical anions and dianions.