Cationization-Engineered Modulation of Photocatalytic Pathways for Enhanced H2O2 Production in COFs

Abstract

Covalent organic frameworks (COFs) have attracted considerable attention as promising photocatalysts for hydrogen peroxide (H2O2) production. To further improve their catalytic performance, a quaternary ammonium functionality was introduced into the bipyridine units of the COF via a post-synthetic modification strategy. Under identical irradiation conditions, the cationized Pry-COF-QA achieved a H2O2 production rate of 7.31 mmol·h–1·g–1, which is markedly higher than that of the non-protonated Pry-COF (5.50 mmol·h–1·g–1). To gain mechanistic insight into the enhanced activity, density functional theory (DFT) calculations were performed. The results indicate that the incorporation of the quaternary ammonium group fundamentally reconfigures the photophysical behavior. In addition to inducing a red shift in the absorption spectrum by narrowing the HOMO–LUMO energy gap, it more importantly converts the excitation character from a localized excitation (LE) state, which is unfavorable for charge separation, to an intramolecular charge transfer (ICT) state with well-separated spatial distribution. This efficient ICT pathway effectively suppresses electron–hole recombination, thereby significantly prolonging the lifetime of photogenerated charge carriers and ultimately facilitating efficient photocatalytic H2O2 production, while also providing a valuable guideline for the rational design of COF-based systems toward efficient photocatalytic H2O2 generation.