Microenvironment Engineering Vinylene-Linked Covalent Organic Frameworks for Highly Efficient CO₂ 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.