Synthesis of Vinylene-Linked Covalent Organic Frameworks by Monomer-Self-Catalyzed Activation of Knoevenagel Condensation

Reticular chemistry on the basis of thermodynamically controlled linking modes and numerous organic building blocks has constituted versatile crystalline frameworks in molecular-level precision. However, vinylene-linked organic frameworks (COFs) are still quite far from flexible tailoring either in their structures or topologies, due to the lack of monomers with sufficient activities. Herein, we established a strategy to synthesize vinylene-linked COFs via Knoevenagel condensation of a tetratopic monomer 2,2’,6,6’-tetramethyl-4,4’-bipyridine (TMBP) with linear aromatic dialdehydes in a mixed solvent of benzoic anhydride and benzoic acid. Mechanism investigation suggested that the condensation was promoted by a pyridine-self-catalyzed benzoylation upon the cleavage of benzoic anhydride solvent molecules. The layered structures of the resultant COFs were highly crystallized into orthorhombic lattice with vertically aligned AA stacking mode, delivering high surface areas up to 1560 m2 g-1. The pi-extended conjugated skeletons comprising para-bipyridyl units and vinylene linkages endow these COFs with substantial semiconducting properties, releasing visible light-stimulated catalytic activity in water-splitting hydrogen evolution with a rate as high as 3300 μmol g-1 h-1.