Highly hydrophilic covalent organic frameworks as efficient and reusable photocatalysts for oxidative coupling of amines in aqueous solution

Abstract

As a class of new emerging crystalline porous materials, photoactive covalent organic frameworks (COFs) have exhibited great potential for photocatalytic applications for metal-free organic transformations. In this study, we report three hydrophilic two-dimensional (2D) COFs achieved by a polycondensation reaction of 2,5-dimethoxyterephthalohydrazide (DMTH), 1,3,5-triformylbenzene (TFB) and 2-hydroxy-1,3,5-benzenetricarbaldehyde (SOH) under solvothermal conditions. The resulted COF materials, TFB-XX-DMTH (XX = 33, 50 and 66, corresponding to a molar ratio of TFB/SOH = 1 : 2, 1 : 1, and 2 : 1), are successfully constructed by a three-component in situ assembly strategy and thus exhibit high crystallinity, large surface areas (up to 1808 m² g⁻¹) and good thermal and chemical stability. Moreover, benefiting from the enhanced charge separation efficiency and high hydrophilicity, the TFB-XX-DMTH materials could be employed as photocatalysts for visible-light-driven oxidative coupling reactions of benzylamines in aqueous solution. The photocatalytic performance with high conversion, selectivity and recyclability could be attributed to a combined result of efficient generation, migration and separation of photogenerated carriers in such a platform. Our results provide insights into the structure–function correlation of COF photocatalysts and also highlight their potential in developing photofunctional COFs for sustainable organic transformations.