Tweaking the electron push-pull effect of vinylene-linked thienothiophene covalent organic frameworks for enhanced selective photocatalysis

Abstract

Covalent organic frameworks (COFs) unveil exceptional potential for selective photocatalysis, owing to their molecularly tunable structures. Herein, four COFs are designed with vinylene-linked isomeric thienothiophenes. Thereby, the condensation of 4,7-bis(2,6-dimethylpyridin-4-yl)benzo[c] [1, 2, 5]thiadiazole (MPBTD) and 4,7-bis(2,6-dimethylpyridin-4-yl)benzo[c][1, 2, 5]oxadiazole (MPBO) with thieno[3,2-b]thiophene-2,5-dicarbaldehyde (T32T) and thieno[2,3-b]thiophene- 2,5-dicarbaldehyde (T23T) yields MPBTD-T32T-COF, MPBTD-T23T-COF, MPBO-T32T-COF, and MPBO-T23T-COF, respectively. Comprehensive characterizations and theoretical calculations confirm the well-defined crystalline porous structures and distinct optoelectronic properties of these four COFs. Notably, the electron-withdrawing units, benzo[c][1, 2, 5]thiadiazole and benzo[c][1, 2, 5]oxadiazole, tweak the electron push-pull effect of COFs, leading to distinct photocatalytic performances. In the selective photocatalytic oxidation of three sulfide substrates, the observed performance exhibits the following order: MPBTD-T32T-COF > MPBTD-T23T-COF > MPBO-T32T-COF > MPBO-T23T-COF. Mechanistic studies reveal that superoxide is the predominant reactive oxygen species powering the selective photocatalytic sulfoxidation over MPBTD-T32T-COF. Furthermore, MPBTD-T32T-COF demonstrates recyclability and broad substrate applicability for the selective photocatalytic sulfoxidation.