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) typically requires time-resolved spectroscopy; however, we demonstrate how to estimate ϕ using steady-state techniques that measure the efficiency of PC formation (ϕ). Our results show that choice of electron donor critically impacts ϕ, 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, and by extension ϕ. 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.