When less is more: harnessing optimal quencher concentrations to maximize quantum yields in TADF photoredox reactions

Here, we report a counterintuitive "less is more" effect, in which lower quencher concentrations improve overall quantum yields in photoinduced electron transfer - key reaction steps in organic photoredox catalysis. This behavior arises because lower quencher concentrations favor quenching of the triplet rather than the singlet excited state. In the latter undesired case, lower efficiencies for free radical or radical ion formation following electron transfer are generally expected, due to reduced cage escape (i.e., separation of geminate radical pairs before recombination). This less productive pathway kinetically competes with the more productive triplet channel. Using nanosecond transient absorption spectroscopy, we directly quantified state-specific cage escape efficiencies and incorporated them into a model to predict radical production quantum yields across different quencher concentration regimes. Additionally, we assessed how the TADF-specific photophysical properties contribute to the observed behavior and examined how these processes translate into catalytic efficiency under continuous irradiation. In contrast to expectations derived from conventional emission quenching experiments, our results clearly demonstrate - across four photocatalyst-quencher systems and three solvents spanning a wide polarity range - that dilution of a reaction system or avoiding a quencher excess can greatly improve the achievable reaction quantum yields in photoredox catalysis. This behavior is not only demonstrated by quantitative transient absorption spectroscopy but also by an initial lab-scale photocatalysis experiment. These findings have important implications in the context of maximizing quantum yields of photoreactions, which is essential for enabling competitive applications on a larger scale.