Electronic effect, photothermal effect, and bandgap simultaneously engineering pyridinium covalent organic framework for promoting photocatalytic organic transformation

Covalent organic frameworks (COFs) have emerged as promising photocatalysts for organic transformations due to their tunable structures and porosity. However, their efficiency is often limited by the rapid recombination of photogenerated charge carriers, restricted light absorption primarily in the UV region, and the lack of localized heat to enhance reaction kinetics at ambient conditions. To address these intertwined challenges, we propose a multi-faceted engineering strategy that simultaneously integrates a narrow bandgap, a strong internal electric field, and an efficient photothermal effect into a single COF platform. Herein, we demonstrate the deliberate design and synthesis of two pyridinium-based COFs (P-HCl-COF and Q-HCl-COF) via Schiff-base condensation followed by hydrochlorination. The incorporation of cationic pyridinium moieties serves as powerful electron-withdrawing centers, creating a robust internal electric field that drastically promotes charge separation and migration. Furthermore, the constructed donor-acceptor (D-A) architecture ensures a narrow bandgap for broad visible-light harvesting, while also enabling efficient conversion of absorbed photons into thermal energy. This synergistic combination of enhanced charge separation, extended light absorption, and accelerated local reaction kinetics via the photothermal effect results in exceptional photocatalytic performance. This is exemplified in the regioselective thiocyanation of indoles, where Q-HCl-COF achieves a near-quantitative yield (99 %) under mild conditions, significantly outperforming its precursors. This work provides a novel and effective strategy for the multi-dimensional design of high-performance COF photocatalysts by synergistically engineering electronic, optical, and thermal properties.