Modulating Benzothiadiazole-Based Covalent Organic Frameworks via Halogenation for Enhanced Photocatalytic Water Splitting: Small Changes Make Big Differences.

Two-dimensional covalent organic frameworks (2D COFs), which is an emerging class of crystalline porous polymers, have been recognized as a new platform for efficient solar-to-hydrogen energy conversion owing to their pre-designable structures and tailor-made functions. Herein, we demonstrate that slight modulation of the chemical structure of a typical photoactive 2D COF (Py-HTP-BT-COF) via chlorination (Py-ClTP-BT-COF) and fluorination (Py-FTP-BT-COF) can lead to dramatically enhanced photocatalytic H 2 evolution rates (HER = 177.50 μmol h -1 with a high apparent quantum efficiency (AQE) of 8.45% for Py-ClTP-BT-COF). Both experimental and computational studies reveal that halogen modulation at the photoactive benzothiadiazole moiety can efficiently suppress charge recombination and significantly reduce the energy barrier associated with the formation of H intermediate species (H*) on polymer surface . Our findings provide new prospects toward design and synthesis of highly active organic photocatalysts toward solar-to-chemical energy conversion.