Three-dimensional covalent organic framework photocatalyst with asymmetrically coordinated single-atom cobalt for highly efficient CO2 reduction reactions

Three-dimensional (3D) covalent organic frameworks (COFs) have attracted extensive attention as photocatalysts for CO₂ reduction reactions. Introducing metal atoms is essential for enhancing activity, but previous metal sites in 3D COFs predominantly exhibit symmetrical coordination, making them unsuitable for CO₂ activation. Here, we design a 3D COF with 2,2′-pyridine linked around tetra-(4-anilyl)methane (TCM-Bpy-COF), where Co²⁺ is asymmetrically coordinated by bipyridine and acetates (TCM-Bpy-COF-CoAc). The TCM-Bpy-COF-CoAc exhibits outstanding photocatalytic CO₂ reduction performance under weak visible light, achieving a CO evolution rate of 26,650 μmol g⁻¹ h⁻¹ under 5 W of light-emitting-diode (LED) lamp and high apparent quantum efficiency. The performance far exceeds that of symmetrically coordinated bipyridine-Co-bipyridine TCM-Bpy-COF and surpasses most reported COF-based photocatalysts. In-situ spectral characterizations and theoretical calculations show that asymmetric N, O-coordination around the Co²⁺ center polarizes electron density and lowers reaction energy barriers of *COOH intermediates, enhancing the conversion of CO₂ to CO. This work inspires the design of 3D COF-based photocatalysts with highly catalytic efficiency.