Engineering Imine Carbon Catalytic Sites in Covalent Organic Frameworks for Enhanced Overall H₂O₂ Photosynthesis

Covalent organic frameworks (COFs) have emerged as ideal photocatalysts for hydrogen peroxide (H₂O₂) photosynthesis from oxygen and water due to their broad light harvesting and programmable structures. However, their catalytic efficiency remains constrained by undefined electron transfer pathways caused by redundant inactive sites around the catalytic sites. Herein, we report a polarization engineering strategy that converts the imine unit from an electronic recombination-prone center into a catalytic center for the oxygen reduction reaction. Ultrafast spectroscopy and theoretical calculations reveal that reducing imine polarization could simultaneously inhibit excited-state deactivation, promote O₂ adsorption and activation, and achieve direct electron transfer from the triphenylbenzene photosensitizer units to the imine catalytic center via the proximity effect. Consequently, azine-linked COF with low polarization of imine units delivers high H₂O₂ production yields of 2311 μmol g^-1 from O₂ and H₂O for 2 h, which is 3.8 and 2.9 times higher than that of the imine-linked COF with medium polarization and hydrazone-linked COF with high polarization, respectively. This work pioneers a paradigm for engineering imine catalytic sites in COFs, providing a molecular blueprint for designing high-efficiency photocatalytic systems with spatially optimized photosensitizer centers and active sites.