When Better Quenching Means Lower Yields: Electrostatic Control of Cage Escape

Abstract

Photoredox catalysis often relies on excited-state quenching data to rationalize performance, yet such metrics can obscure the impact of solvent cage escape on overall efficiency. We report a systematic study of the effect of electrostatic interactions on the excited-state quenching, cage escape, and back-electron transfer processes in a benchmark system comprising methyl viologen (MV2+) and differently carboxylated ruthenium polypyridyl complexes with net charges from 2+ to 4–. Increasing electrostatic attraction between photosensitizer and MV2+ enhances the quenching rate constant (kq) up to the diffusion limit but simultaneously suppresses cage escape quantum yields, resulting in an inverse correlation between kq and photochemical MV•+ production. Transient absorption spectroscopy confirms that cage escape, rather than quenching or back-electron transfer, governs the quantum yield of product formation. Protonation of carboxylate groups to yield uniformly 2+ complexes equalizes quenching rates and substantially increases cage escape efficiency for the originally anionic species. These results establish electrostatic control of charge separation as a decisive factor in photoredox catalysis and challenge the practice of predicting yields solely from quenching experiments. Consideration of both the initial and post-electron-transfer charges of the photocatalyst/quencher pair emerges as a general design principle for maximizing cage escape and, consequently, photoredox reaction efficiency.