Double enhancement of protonation and conjugation in donor–imine–donor covalent organic frameworks for photocatalytic hydrogen evolution

Covalent organic frameworks (COFs) have emerged as highly promising platforms for photocatalytic water splitting. However, exploring the structure-activity relationships in different COF systems remains challenging. In this study, three donor-imine-donor (D-I-D) COFs as relatively pure model materials were carefully selected to investigate the effect of protonation and conjugation on the mechanism of photocatalytic H2 evolution. Unlike widely reported donor-acceptor (D-A) COF systems, these three ideal COFs have short electronic channels and lack chemical bond isomerism and heteroatoms in building blocks. These aspects are beneficial for a comprehensive investigation of the underlying mechanisms at the active sites of the imine bond. Both the calculation and experimental results indicate that increasing the conjugation intensity can enhance the efficiency of exciton dissociation and charge transfer rates. Protonation can also dominantly enhance the light absorption capacity and electron transport efficiency of D-I-D COFs. After protonation, the Py-hCOF with optimal conjugation intensity exhibits a remarkable H2 evolution rate of 44.2 mmol g-1 h-1 under visible light, which is 88.4 times higher than that of Tpe-hCOF. This result highlights the crucial roles of simultaneous enhancement of the protonation and conjugation in improving photocatalytic hydrogen evolution of COFs, providing valuable insights for the design of COF materials to achieve the superior electronic functions in photocatalysis.