A Bifunctional Organic Photocatalyst for Efficient Single‐Electron and Energy Transfer Activation

Bifunctional photocatalysts capable of mediating both single‐electron transfer (SET) and energy transfer (EnT) processes are rare and typically metal‐based. Here, we present 3‐thioaryl‐4hydroxycoumarins, a new family of cost‐effective organic photocatalysts that leverage a stabilized charge‐transfer (CT) excited state to achieve both strong reducing power and efficient energy transfer. The spatial separation of the HOMO and LUMO stabilizes the CT state, enhancing SET reactivity (E*red = −3.08 V vs SCE) while maintaining a sufficiently high triplet energy (ET = 67 kcal/mol) for EnT‐driven transformations. This dual reactivity enables the activation of redox‐inert substrates (Ered < −2.8 V vs SCE) via SET reduction, generating radicals suitable for diverse C–S, C–P, C–B, and C–C bondforming transformations, alongside EnT‐based processes such as E/Z olefin isomerization and [2+2] photocycloadditions. Mechanistic studies, supported by photophysical and theoretical analyses, confirmed the catalyst’s bifunctionality.