Porphyrin-Triazine Covalent Organic Framework with Periodic Z -Scheme Molecular Junctions for Visible-Light-Driven CO 2 Reduction

Covalent organic frameworks (COFs) have attracted growing attention as tunable, stable, and highly designable photocatalysts for CO2 reduction reactions (CO2RR). Metalloporphyrins, recognized for their superior photophysical properties and well-defined M-N4 sites, are widely employed in photocatalysis systems. In this work, two imine-linked porphyrin-based COFs, denoted as CoPor-Tz and CoPor-Bz, were constructed by integrating cobalt(II) 5,15-bis(4-aminophenyl)-10,20-diphenylporphyrin (CoPor) with either 2,4,6-tris(4-formylphenyl)-1,3,5-triazine (Tz) or 1,3,5-tris(4-formylphenyl)benzene (Bz). The electron-deficient Tz linker creates a microenvironment in CoPor-Tz COF that promotes more efficient oxidative half-reactions and charge separation compared with CoPor-Bz COF. Periodic Z-scheme molecular junctions are established between the CoPor and Tz units, enabling directional electron transfer from Tz to the Co centers, where CO2 activation and reduction occur. Under visible-light (λ ≥ 420 nm) irradiation, CoPor-Tz COF exhibits a remarkable CO yield of 12,909 μmol g-1 h-1, which is 2.2 times higher than that of CoPor-Bz COF (5638 μmol g-1 h-1). Experimental and theoretical studies reveal that the enhanced CO2RR activity of the CoPor-Tz COF originates from its superior electron transfer efficiency, stronger CO2 adsorption/activation capability, and accelerated interfacial redox kinetics compared to the CoPor-Bz COF. This study provides mechanistic insight into periodic molecular junction engineering within COFs, highlighting an effective strategy for constructing highly efficient photocatalytic CO2RR systems.