Microenvironment Engineering Vinylene-Linked Covalent Organic Frameworks for Highly Efficient CO 2 Photoreduction

Efficient and selective reduction of CO2 via photocatalysis is significantly challenging under demanding conditions, specifically in heterogeneous gas-solid systems that function without solvents, cocatalysts, or sacrificial agents. Here, we design and construct a robust vinylene-linked pyridazine covalent organic framework (COF) platform to programmably tune the microenvironment by varying the nitrogen content in aldehyde linkers. This molecular-level modulation governs charge distribution and photophysical behavior, establishing a clear structure-reactivity relationship in CO2-to-CO photoreduction. The pyridine-containing COF exhibits the most efficient charge separation and the highest CO production rate, surpassing all metal-free photocatalysts under gas-solid conditions. Building on this optimized microenvironment, site-specific coordination of Re(CO)5Cl at adjacent pyridazine nitrogen sites defines molecular catalytic centers and reinforces the acceptor-donor-acceptor (A-D-A) charge-transfer pathway. The resulting hybrid material delivers a record gas-solid CO2-to-CO activity among metal-loaded COF photocatalysts operating without a solvent, cocatalyst, or sacrificial agent and outperforms all presentative heterogeneous CO2 reduction systems under comparable conditions. In situ infrared spectroscopy, isotopic labeling, and density functional theory calculations provide insights into the impact of the N-site configuration on intermediate binding, transition-state energetics, and the overall reaction mechanism. These findings highlight N-microenvironment engineering as a versatile approach for advancing COF-based photocatalysts in high-performance CO2 conversion.