Structural design of small-molecule carbon-nitride dyes for photocatalytic hydrogen evolution

Carbon nitrides are an emerging class of metal-free polymeric photocatalysts but limited by the structural modifications. With the same central skeleton, small-molecule carbon-nitride dyes (also called heptazine dyes) have attracted widespread attention in the past decades. Herein, by introducing the electron-rich aryl substituents at three peripheral positions, four donor-acceptor (D-A) heptazine derivatives have been synthesized via Friedel-Crafts reactions, and their photocatalytic hydrogen evolution properties have been carefully investigated. Presumably, due to the most effective conjugation and reasonable HOMO and LUMO positions, the optimized heptazine 4 with three 4-methylbiphenyl substituents exhibited an impressive thermally activated delayed fluorescence (TADF), which indicated it possessed the maximized photoinduced charge separations, and thus presented the highest photocatalytic activity. This work not only contributes to the small-molecule carbonnitride dyes, but also has an instructive significance on understanding the electron-transfer mechanism of carbon nitride photocatalysts at the molecular level.