Symmetry Basis Engineered Covalent Organic Frameworks for Water Purification Under Ultralow Light Intensity

Achieving efficient solar-to-chemical energy conversion of low-intensity and natural sunlight is a promising but challenged for sustainable water purification. Herein, an electron-deficient pyridine units into functional basis with symmetric and asymmetric is newly pre-designed to form covalent organic frameworks (COFs). It is found that the bidirectional push-pull effect of the bipyridine units in the symmetric Bby-COF induces an increase in charge density and enhances the electron sink effect. This transformation optimizes the activation pathway of dissolved oxygen, establishing a pathway of micropollutants decomposition mediated by superoxide free radicals and photoexcited holes oxidation. Specifically, the first-order rate constant of ofloxacin (OFL) removal for Bby-COF for is 28.14 × 10^-2 min^-1, surpassing that of asymmetric Bpy-COF by 6.3-times (4.45 × 10^-2 min^-1). Remarkably, Bby-COF can achieve complete OFL removal within 30-40 min under winter sunlight conditions, demonstrating unprecedented ultra-low-light-intensity (36 mW cm^-2) catalytic performance. In this mode, an array-type plate-and-frame flow-through reactor can be consecutively operated for treating a total volume of 58.8 L wastewater using outdoor sunlight, meeting the potentiality of large-scale applications. This study pioneers a symmetry-engineered molecular strategy for developing high-performance organocatalysts, bridging the critical gap between laboratory photocatalysis and real-world solar wastewater treatment applications.