Effect of linkages on photocatalytic H2 evolution over covalent organic frameworks

• PMDA-COF features wide visible light absorption range up to 650 nm and hydrophilicity. • The D-A molecular heterojunction can promote separation of charges and holes. • PMDA-COF has the shorter band gap. Covalent organic frameworks (COFs) with two-dimensional (2D) structure are regarded as potential photocatalysts for light-induced hydrogen evolution due to their excellent visible light response performance, designable structure, and suitable energy bands for water reduction and oxidation. To investigate how linkages in the COF framework affect the holistic visible-light photocatalytic hydrogen production (PHP), herein, three tunable COF platforms with the same triazine benzene as the knot and phenyl as the linker, but with different linkages were constructed. The structure, photoelectrochemical properties and bandgap structure of these COFs were systematically discussed. The results showed that the dianhydride substituted imide-COF (PMDA-COF) possesses a sustainable and stable PHP rate of 435.6 μmol·g −1 ·h −1 under visible light irradiation, which is much higher than that hydrogen bond containing imine-COF (DHTA-COF, 56.2 μmol·g −1 ·h −1 ) and imine-COF (TPAL-COF, 6.8 μmol·g −1 ·h −1 ). In addition, PMDA-COF has excellent photo-generated charge separation capabilities, energy difference exists between PMDA and TAPT. Among them, D-A molecular heterojunction plays a crucial role., which helps not only to modify the electronic band structure but also to separate the reduction and oxidation sites. Our research is instructive for the further design of COF-based photocatalysts for H 2 evolution.