Unveiling the latent reactivity of imines on pyridine-functionalized covalent organic frameworks for H2O2 photosynthesis

Unveiling the active site for the oxygen reduction reaction (ORR) holds the key to understanding and improving the photocatalytic activity of covalent organic frameworks (COFs) for H2O2 evolution. However, for imine-linked COFs, the role of the imine group is often overlooked in photosynthesis compared with other groups with electrophilicity or light-harvesting capabilities. Herein, a strategy is presented for eliciting the latent photoreactivity of imines by introducing an electron-acceptor structure (pyridine unit) near the imine bonds to enhance the kinetic and thermodynamic advantages of imines for the photocatalytic ORR to H2O2. Experiments and theoretical simulations indicate that the hindered visible light absorption and charge carrier separation caused by the weak electron delocalization can be substantially improved by introducing pyridinic N, leading to full solar spectrum absorption. Meanwhile, the pyridinic N can act with the N atom of the imine as an enhanced site for O2 adsorption and activation, and the strong electron transfers from COFs to O2 and ORR intermediates enable a two-step single-electron reduction route of O2 in pyridine-functionalized COFs for a more feasible H2O2 generation (706.2 µmol g−1 h−1) than original imine-linked COF (372.7 µmol g−1 h−1) under visible light irradiation. This work provides a new idea for designing and modifying imine-linked COFs in advanced photocatalytic applications.