Hydrogen peroxide (H₂O₂) is a green oxidant with applications in various industrial processes including wastewater treatment, paper bleaching and chemical synthesis. In recent years, the photo-driven two-electron oxygen reduction reaction (ORR) has emerged as a promising way for H₂O₂ synthesis due to advantages such as environmental friendliness and flexibility.

Among various photocatalysts, covalent organic frameworks (COFs) have garnered attention for exceptional light-harvesting properties and programmable structures. However, the photocatalytic efficiency of COFs in H₂O₂ production is limited by rapid charge recombination and undefined electron transfer pathways caused by redundant inactive sites around catalytic centers.

In a study published in Journal of the American Chemical Society, a research team led by Prof. CAO Rong from Fujian Institute of Research on the Structure of Matter of the Chinese Academy of Sciences developed a strategy that transforms the imine units in COFs from electronic recombination centers into active catalytic sites for the ORR.

Researchers synthesized three types of COFs with systematically tuned imine polarization: azine-linked COF (A-COF) with low polarization, imine-linked COF (I-COF) with medium polarization, and hydrazone-linked COF (H-COF) with high polarization. 

Through ultrafast spectroscopy and theoretical calculations, they demonstrated that reducing the polarization of imine units in A-COF could simultaneously suppress excited-state deactivation, enhance O₂ adsorption and activation, and enable direct electron transfer from the triphenylbenzene photosensitizer units to the imine catalytic center via the proximity effect. This unique configuration minimized charge recombination and maximized electron utilization efficiency.

Moreover, researchers found that A-COF demonstrated exceptional performance in photocatalytic H₂O₂ production. It achieved a yield of 2311 μmol g^-1 in 2 hours under visible light, which is 3.8 and 2.9 times higher than those of I-COF and H-COF, respectively. This superiority was attributed to A-COF's low imine polarization, which enhanced O₂ adsorption via Pauling-type end-on bonding and stabilized the excited state for prolonged electron availability. 

Furthermore, researchers revealed the critical role of spatial organization within COFs in optimizing electron transfer pathways and catalyst efficiency.

This study provides a molecular-level strategy to engineer catalytic sites in COFs through polarization control. It offers insights into the design of high-performance photocatalytic systems with optimized spatial and electronic structures and the development of efficient catalysts for other energy-related reactions.