Relative Local Electron Density Tuning in Metal-Covalent Organic Frameworks for Boosting CO₂ Photoreduction

The high local electron density and efficient charge carrier separation are two important factors to affect photocatalytic activity, especially for the CO₂ photoreduction reaction. However, the systematic studies on the structure-functional relationship regarding the above two factors based on precisely structure model are rarely reported. Herein, as a proof-of-concept, we developed a new strategy on the evaluation of local electron density by controlling the relative electron-deficient (ED) and electron-rich (ER) intensity of monomer at a molecular level based on three rational-designed vinylene-linked sp² carbon-covalent organic frameworks (COFs). As expected, the as-prepared vinylene-linked sp² carbon-conjugated metal-covalent organic framework (MCOFs) (VL-MCOF-1) with molecular junction exhibited excellent activities for CO₂ -to-HCOOH conversion (283.41 μmol g^-1  h^-1 ) and high selectivity of 97.1 %, much higher than the VL-MCOF-2 and g-C₃₄ N₆ -COF, which is due to the synergistic effect of the multi-electronic metal clusters (Cu₃ (PyCA)₃ ) (PyCA=pyrazolate-4-carboxaldehyde) as strong ER roles and cyanopyridine units as ED roles and active sites, as well as the boosted photo-induced charge separation efficiency of vinyl connection and increased light utilization ability. These results not only provide a strategy for regulating the electron-density distribution of photocatalysts at the molecular level but also offers profound insights for metal clusters-based COFs to effective CO₂ conversion.