Strategic design of triphenylamine- and triphenyltriazine-based two-dimensional covalent organic frameworks for CO₂ uptake and energy storage

Hexagonally ordered covalent organic frameworks (COFs) are interesting new crystalline porous materials that have massive potential for application in gas storage. Herein, we report the synthesis of two series of two-dimensional hexagonally ordered COFs - TPA-COFs and TPT-COFs - through one-pot polycondensations of tris(4-aminophenyl)amine (TPA-3NH₂) and 2,4,6-tris(4-aminophenyl)triazine (TPT-3NH₂), respectively, with triarylaldehydes featuring different degrees of planarity, symmetry, and nitrogen content. All the synthesized COFs exhibited high crystallinity, large BET surface areas (up to 1747 m² g^-1), excellent thermal stability, and pore size distributions from 1.80 to 2.55 nm. The symmetry and planarity of the monomers strongly affected the degrees of crystallinity and the BET surface areas of the resultant COFs. In addition, these COFs displayed excellent CO₂ uptake efficiencies of up to 65.65 and 92.38 mg g^-1 at 298 and 273 K, respectively. The incorporation of the more planar and higher-nitrogen-content triaryltriazine unit into the backbones of the TPA-COFs and TPT-COFs enhanced the interactions with CO₂, leading to higher CO₂ uptakes. Moreover, the synthesized COFs exhibited electrochemical properties because of their conjugated structures containing redox-active triphenylamine groups. This study exposes the importance of considering the symmetry and planarity of the monomers when designing highly crystalline COFs; indeed, the structures of COFs can be tailored to vary their functionalities for specific applications.