Cage escape governs photoredox reaction rates and quantum yields

Abstract

Photoredox catalysis relies on light-induced electron transfer leading to a radical pair comprising an oxidized donor and a reduced acceptor in a solvent cage. For productive onward reaction to occur, the oxidized donor and the reduced acceptor must escape from that solvent cage before they undergo spontaneous reverse electron transfer. Here we show the decisive role that cage escape plays in three benchmark photocatalytic reactions, namely, an aerobic hydroxylation, a reductive debromination and an aza-Henry reaction. Using ruthenium(II)- and chromium(III)-based photocatalysts, which provide inherently different cage escape quantum yields, we determined quantitative correlations between the rates of photoredox product formation and the cage escape quantum yields. These findings can be largely rationalized within the framework of Marcus theory for electron transfer. (Figure presented.)