Enhancing Photocatalytic Hydrogen Evolution via Electron-Rich Aldehyde Incorporation in Covalent Organic Frameworks

Photocatalytic water splitting using visible light presents a promising route for producing sustainable hydrogen fuel from solar energy. Covalent organic frameworks (COFs) are an emerging class of semiconductors that show great potential for hydrogen production via photocatalysis. In this context, two hydroxyl-functionalized COFs, BTT-TaOH and Tp-TaOH, have been synthesized using a common diamine linker, 3,3'-dihydrooxybenzidine, with benzotrithiophene-1,3,5-tricarbaldehyde (BTT) and 1,3,5-triformylbenzene (Tp), respectively. The structural variation between BTT and Tp cores introduces significant differences in the optical and electronic behaviours of the resulting COFs. A highly conjugated and electron-rich benzotrithiophene unit is featured in BTT-TaOH, which enhances visible light absorption, facilitates more efficient charge carrier separation, and promotes faster electron transport. These distinctions are reflected in their photocatalytic performance: BTT-TaOH exhibits a significantly greater hydrogen evolution rate compared to the Tp-TaOH, attributed to its improved crystallinity, surface area, light absorption capability, and lower charge recombination. Therefore, the rational design of COFs incorporating extended π-conjugation and electron-donating moieties, as exemplified by BTT-TaOH, provides a viable strategy to enhance the photocatalytic hydrogen production efficiency.