Enhancing charge transfer efficiency through carboxyl-modification to improve the photocatalytic activity of covalent organic frameworks for hydrogen evolution from water splitting

The design of photocatalysts based on covalent organic frameworks (COFs) has attracted considerable interest. Nevertheless, the low efficiency in separating photogenerated carriers remains a substantial challenge. Herein, carboxyl modification is employed to enhance the separation efficiency of photogenerated carriers within imine-based porphyrin COFs through the resultant built-in electric field, thereby improving the photocatalytic performance of COFs in hydrogen evolution from water splitting. Spectroscopic and electrochemical analyses reveal that, compared with pristine two-dimensional porphyrin-based COFs (Por-COF), an isostructural carboxyl-modified COF (Por-COOH-COF) exhibits a prolonged excited-state lifetime, reduced exciton binding energy and decreased deactivation probability via radiative processes, and an improved photocurrent response. These improvements result in a hydrogen evolution of 12773 ± 297 μ mol⋅g^-1⋅h^-1 for Por-COOH-COF, approximately four times higher than that of Por-COF (3351 ± 197 μmol⋅g^-1⋅h^-1), positioning it among the most efficient metal-free COF photocatalysts reported to date. Density functional theory calculations and in situ X-ray photoelectron spectroscopy analyses indicate that carboxyl modification facilitates charge transfer from the porphine core to the active site of the imine bond (-CN-) under the ultraviolet-visible light irradiation, thereby contributing to the remarkable photocatalytic activity of Por-COOH-COF. This study elucidates how carboxyl groups positively influence photogenerated carrier separation within COFs and provides valuable insights into the development of high-performance metal-free COFs photocatalysts.