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-4-hydroxycoumarins, 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 (E_T = 67 kcal mol^-1) for EnT-driven transformations. This dual reactivity enables the activation of redox-inert substrates (E_red < -2.8 V vs. SCE) via SET reduction, generating radicals suitable for diverse C─S, C-─P, C─B, and C─C bond-forming 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.