Boosting CO2 Photoreduction via Ligand-Engineered Single-Atom Cobalt Sites in Covalent Organic Frameworks.

The microenvironment of single-atom active sites plays a pivotal role in determining the activity and selectivity of the CO2 reduction reaction (CO2RR). Herein, we report a highly efficient single-atom catalyst, Co-DABT-COF, featuring Co(II)-bithiazole N motifs that achieve exceptional CO2 photoreduction performance (16.4 mmol g-1 h-1, 96% selectivity) under visible light irradiation in the presence of a photosensitizer and a sacrificial agent. Through isoreticular design maintaining identical Co-N2O2 primary coordination, Co-DABT-COF exhibits 2.8-fold higher activity than its bipyridine analogue Co-Bpy-COF with Co(II)-bipyridine N motifs (5.8 mmol g-1 h-1). Systematic characterization reveals the 2,2'-bithiazole ligand significantly enhances charge transfer efficiency and reduces the activation energy barrier for *COOH intermediate formation (0.79 eV versus 1.14 eV). Bader charge analysis demonstrates stronger electron donation from Co sites to CO2 (0.63 e for Co-DABT-COF versus 0.58 e for Co-Bpy-COF), facilitating CO2 activation. This work demonstrates a ligand-engineering strategy for precisely tuning the electronic structure of single-atom active sites to achieve efficient CO2 conversion.