Angewandte Chemie, Accepted Article.

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.