Synergistic Enhancement of Photoredox Catalysis Beyond the Singlet Bottleneck Through Triplet Charge Recombination and Magnetic Field Effects

Organic photosensitizers with redox-active excited states have revitalized photoredox catalysis, but their efficiency is often hindered by spontaneous back electron transfer (BET), particularly in singlet-driven pathways. Enhancing triplet utilization is therefore critical for improving catalytic performance but remains challenging, especially without structural modifications to the catalyst. Here, using phenothiazine-type photosensitizers as a model system, we demonstrate a synergistic strategy that enhances reaction efficiency by almost 300%. Triplet charge recombination (TCR) is shown to be significantly promoted within the nonpolar micellar core, enhancing triplet generation and facilitating efficient triplet-substrate interactions. Furthermore, an external magnetic field is introduced along the triplet pathway to depress BET by inhibiting spin conversion of radical-ion-pair intermediates from triplet to singlet. The developed kinetic modeling provides quantitative insights and mechanistic validation of the magnetic field effect. This work establishes a powerful synergistic strategy to control photoredox mechanisms, offering a new framework for advancing catalytic performance.