N-Rich 2D Heptazine Covalent Organic Frameworks as Efficient Metal-Free Photocatalysts

Heptazine serves as the solely repetitive unit of graphitic carbon nitride (g-C3N4), which represents a progressive metal-free photocatalyst. However, synthesizing high-quality 2D heptazine-based covalent organic frameworks (COFs) remains a formidable challenge, probably due to the scarcity of suitable and ample reactive heptazine-based building blocks. Herein, we designed and synthesized a 4-(methylene diacetate)phenyl-substituted heptazine monomer (HEP-OAc) by oxidation of tris(4-tolyl)heptazine. Accordingly, two highly ordered N-rich COFs (HEP-TAPT-COF and HEP-TAPB-COF) were readily prepared by the direct condensation of HEP-OAc with two C3-symmetric aromatic amines under solvothermal conditions. In comparison to the pristine g-C3N4, HEP-TAPT-COF exhibited greatly enhanced photocatalytic performance for benzylic C–H oxidation and selective sulfoxidation under mild conditions. This work provides meaningful insights into developing highly crystalline N-rich COFs with better photocatalytic capacities.