Linker Nitrogen Tunes Charge Polarity to Strengthen Built-In Electric Fields in Covalent Organic Frameworks for Photocatalytic Oxidation.

Covalent organic frameworks (COFs) are attractive platforms for heterogeneous photocatalysis, yet efficient exciton dissociation and charge separation remain intrinsically challenging in ordered organic frameworks. Here, we show that these limitations can be addressed by regulating the local electrostatic environment via linker nitrogen engineering. Within a common imine-linked framework, systematic modulation of nitrogen content in the bridging linkers further tunes the overall electrostatic environment and strengthens the framework-scale built-in electric fields, as revealed by spatially resolved spectroscopic analyses and theoretical calculations. The strengthened built-in electric fields lower the effective exciton binding energy, suppress recombination, promote directional charge separation, and improve charge utilization under illumination. As a result, the polarity-engineered COFs exhibit excellent photocatalytic performance in two representative aerobic oxidation reactions under visible light and mild conditions, with TAPP-Bpy-COF affording >99% conversion and >99% selectivity within 1 h in both reactions. This work establishes linker nitrogen engineering as a chemically countable and general strategy for regulating exciton dynamics and charge utilization in COFs and provides a rational design principle for efficient organic photocatalysts.