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.