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.