Charge Modulation of the Cobalt Center in An Azo-Linked Porphyrin Covalent Organic Framework: A Strategy for Boosting CO2 Photoreduction

Photocatalytic CO2 reduction driven by solar energy offers a sustainable solution to mitigate environmental issues. Covalent organic frameworks (COFs), characterized by their crystallinity, tunable porosity, and exceptional visible-light absorption, have recently gained prominence as promising catalysts for CO2 photoreduction. Herein, we report a novel azo-linked porphyrin COF (Azo-COF-366) via in-situ linker exchange from an imine-linked COF (Im-COF-366). The integration of azo (-N=N-) linkages effectively lowers the conduction band energy level and amplifies the photocurrent response of Azo-COF-366. This synergistic effect promotes charge carrier separation efficiency and optimizes transport dynamics. Furthermore, Azo-COF-366 exhibits exceptional resistance to degradation under harsh acidic and alkaline conditions. Attributed to its superior photoelectronic properties, Azo-COF-366 exhibits superior catalytic performance, achieving an increased CO production rate (13.4 mmol·g‒1·h‒1) compared to Im-COF-366, along with a 30% improvement in CO and H2 selectivity. The crucial role played by azo linkages is revealed by density functional theory (DFT) calculations. The results indicate that the incorporation of azo linkages modulates the charge density at the cobalt center and enhances its 3d-orbital splitting, which promotes more efficient electron transfer from the photosensitizer to Azo-COF-366 and reduces the energy barrier for the formation of the *COOH intermediate. These findings provide fundamental design principles for the development of high-performance COF-based photocatalysts, with significant implications for sustainable energy technologies and environmental remediation strategies.