Oxidized covalent organic frameworks with enhanced local polarization for superior photocatalytic production of hydrogen peroxide†

Abstract

Polarization engineering is a promising approach to boost the photocatalytic performance of covalent organic frameworks (COFs). Nevertheless, the molecular structures of the building monomers employed in the construction of COFs with tailored polarization effects commonly exhibit a lack of central symmetry, posing formidable challenges in their synthesis. Herein, we report a facile post-oxidation strategy that efficiently converted a thiophene-based, non-substituted quinoline-linked COF (NQ-COF_S1) into its oxidized form NQ-COF_S1-O. This oxidation process introduced ionic N⁺–O⁻ and sulfone functionalities into the skeleton, which synergistically elicited an uneven distribution of electrons within NQ-COF_S1-O. Remarkably, the heightened polarization in NQ-COF_S1-O resulted in an extensive broadening of visible light absorption and enhanced carrier charge separation and migration efficiency. Additionally, it augmented hydrophilicity and activated the sites for the oxygen reduction reaction. As a result, NQ-COF_S1-O displayed much higher photocatalytic performance for hydrogen peroxide (H₂O₂) generation than the pristine NQ-COF_S1. This study presents a novel strategy for modulating the polarization effect in COFs, which might inspire the design of more active COF-based photocatalysts.