Maximizing Photon-to-Electron Conversion for Atom Efficient Photoredox Catalysis

Photoredox catalysis is a powerful tool to access challenging and diverse syntheses. Absorption of visible light forms the excited state catalyst (*PC) but photons may be wasted if one of several unproductive pathways occur. Facile dissociation of the charge-separated encounter complex [PC^•-:D^•+], also known as (solvent) cage escape, is required for productive chemistry and directly governs availability of the critical PC^•- intermediate. Competitive charge recombination, either inside or outside the solvent cage, may limit the overall efficiency of a photochemical reaction or internal quantum yield (defined as the moles of product formed per mole of photons absorbed by PC). Measuring the cage escape efficiency (ϕ_CE) typically requires time-resolved spectroscopy; however, we demonstrate how to estimate ϕ_CE using steady-state techniques that measure the efficiency of PC^•- formation (ϕ_PC). Our results show that choice of electron donor critically impacts ϕ_PC, which directly correlates to improved synthetic and internal quantum yields. Furthermore, we demonstrate how modest structural differences between photocatalysts may afford a sizable effect on reactivity due to changes in ϕ_PC, and by extension ϕ_CE. Optimizing experimental conditions for cage escape provides photochemical reactions with improved atom economy and energy input, paving the way for sustainable design of photocatalytic systems.