Bipyrimidine-Boosted Preparation and Post-Functionalization of Vinylene-Linked Covalent Organic Frameworks for...

In nature, photocatalytic reduction of carbon dioxide is a critical process for maintaining the global carbon cycle, which is initiated by the enzymes consisting of chromophoric organic macrocycles in the presence of water. It is extremely desirable to develop an artificial organic photosynthetic system for the efficient reduction of CO₂ to syngas for addressing the energy crisis and achieving the strategic environmental goal of carbon neutrality. In this work, we synthesized a series of vinylene-linked covalent organic frameworks (COFs) by Knoevenagel condensation of tetra-methylbipyrimidine as a tetratopic monomer with ditopic linear aromatic dialdehydes. The resultant COFs were crystallized in orthorhombic lattices with eclipsed AA stacking modes. Such tetravinylbipyrimidine-containing frameworks possess π-extended conjugation and highly dense accessible coordination sites, leading to substantially tunable semiconducting properties. Through simple post-functionalization, these COFs were coordinated with rhenium mono-cation with high utilization rates (up to ∼97%) of bipyrimidine units. The resultant rhenium-modified COFs enable efficient photocatalytic CO₂ reduction to CO in water without any additional sacrificial-agent. The CO generation reached a rate of up to 234.4 µmol·g^{- 1}·h^{- 1} with high selectivity of 100%, which is among the highest values so far for organic porous materials. Such results seem align more closely with environmental sustainability principles.