Synergistic Enhancement of Photocatalytic Hydrogen Evolution in Covalent Organic Frameworks via Isoreticular Design, Isomerism, and Protonation

Rational structural engineering of covalent organic frameworks (COFs) has significantly advanced their application in photocatalytic hydrogen evolution (PHE) from water splitting. However, the practical application of COFs for scalable solar-to-hydrogen conversion remains limited due to their insufficient photocatalytic efficiencies. Herein, we achieved an exceptionally high hydrogen evolution rate of 364 mmol g^-1 h^-1 using HCl-protonated COF-920 under ultraviolet-visible light, as confirmed by water-drainage and gas-gathering experiments, exceeding most previously reported COF-based photocatalysts. Three series of isoreticular COFs were synthesized, incorporating benzene, biphenyl, and triphenyl linkers, along with their structural isomers featuring reversed imine bond orientations, to systematically investigate the collaborative influences of linker length, structural isomerism, and protonation on the photocatalytic performance. Experimental results revealed that COFs employing biphenyl-based linkers exhibited superior photocatalytic activity compared to those with benzene or triphenyl linkers. Moreover, the performance difference resulting from variations in imine orientation decreases with increasing linker length, supported by density functional theory (DFT) calculations revealing narrower energy differences between units connected by imine bonds (─C═N─). Our findings not only provide new insights into structure-activity relationships in COF photocatalysts but also highlight the potential of COF materials for sustainable solar-to-fuel conversion.