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

Bifunctional photocatalysts capable of medi-ating both single-electron transfer (SET) and energytransfer (EnT) processes are rare and typically metalbased. Here, we present 3-thioaryl-4-hydroxycoumarins,a new family of cost-effective organic photocatalysts thatleverage a stabilized charge-transfer (CT) excited stateto achieve both strong reducing power and efficientenergy transfer. The spatial separation of the HOMO andLUMO stabilizes the CT state, enhancing SET reactivity(E*red = −3.08 V vs. SCE) while maintaining a sufficientlyhigh triplet energy (ET = 67 kcal mol−1 ) for EnT-driven transformations. This dual reactivity enables theactivation of redox-inert substrates (Ered < −2.8 V vs.SCE) via SET reduction, generating radicals suitablefor diverse C─S, C-─P, C─B, and C─C bond-formingtransformations, alongside EnT-based processes such asE/Z olefin isomerization and [2 + 2] photocycloadditions.Mechanistic studies, supported by photophysical and the-oretical analyses, confirmed the catalyst’s bifunctionality.