Regulation on photocatalytic debromination by the reconfiguration of carbonyl groups on the covalent organic frameworks

Abstract

Covalent organic frameworks (COFs) as promising organic semiconductor photocatalysts have become a research hotspot in the field of photocatalytic organic synthesis. Herein, a series of imine-linked EN-COF-x were synthesized by condensation reaction of amine monomer with triformylbenzene monomer. Then, β-ketoamine-linked KE-COF-y were obtained by the reconstruction of EN-COF-x in alkaline solution. The photocatalytic performance of COFs before and after the isomerization from enols to ketones were investigated. The characterization results and density functional theory calculations clearly demonstrated that the carbonyl groups not only make the conduction band energy level more negative, but also lead to a more inhomogeneous charge distribution in the molecule, which improved the separation efficiency of the photogenerated electrons. In addition, the increase in the number of hydroxyl groups on triformylbenzene monomer contributed to the formation of more carbonyl groups in the COF frameworks, which is beneficial to improving its photocatalytic performance. KE-COF-3 synthesized from 2,4,6-Trihydroxy-1,3,5-benzenetricarbaldehyde has more carbonyl group units and more suitable redox potential, which effectively promoted the debromination reaction with 99% yield under light irradiation. Meanwhile, it can be reused at least five times without loss of catalytic activity. This work provides insights for the rational design of functionalized β-ketoamine COFs for photocatalytic organic transformations.