Regulating the Tautomerization in Covalent Organic Frameworks for Efficient Sacrificial Agent-Free Photocatalytic H₂O₂ Production

The efficiency of photocatalytic production of H₂O₂ is constrained by the low selectivity toward oxygen reduction, and the active sites are still under debate. Herein, analogous covalent organic framework photocatalysts were synthesized from triformylphloroglucinol (Tp) and predesigned diamines, in which a molecular engineering strategy was employed to manipulate the energy barrier for the targeted proton transfers. The tautomerization of enol-imine to keto-enamine introduced abundant alkene bonds (CC), which serve as the primary adsorption sites and have a lower energy barrier for the reduction of the O₂ reduction. DHAA-Tp COF displayed a remarkable photocatalytic H₂O₂ production rate of 219.5 μmol h^–1 g^–1 without any sacrificial reagent, which stands out among the structure-related materials. A switch from a concerted one-step 2e^– to a two-step single e^– process in O₂ reduction was observed in TCNAQ-Tp COF, which is presumably ascribed to the suppressed tautomerization mediated by the strong electron-withdrawing cyano groups. The results demonstrate a novel concept for the photocatalytic production of H₂O₂ using an efficient, stable, and recyclable metal-free photocatalytic system.