Significant improvement of photocatalytic hydrogen evolution performance in covalent organic frameworks: substituent fine-tuning.

Imine-linked covalent organic frameworks (COFs) have been widely studied in the field of photocatalytic hydrogen production due to their easy synthesis, good crystallinity, tunable pore size and excellent thermal and chemical stability. However, the effect of substituent type on the photocatalytic properties of COFs has been rarely studied. Herein, six COFs with COF-TpPa as the basic skeleton were synthesized by introducing different types of substituents in the p-phenylenediamine benzene ring, among which TpPa-CN₂ was prepared for the first time. The COFs showed similar structures but great differences in their photoelectric properties. TpPa-Cl₂ and TpPa-CN₂ demonstrated a superior hydrogen evolution performance compared to the other four COFs when ascorbic acid was used as the electron sacrificial agent and Pt as the cocatalyst. The photocatalytic hydrogen evolution (PHE) rate of TpPa-Cl₂ and TpPa-CN₂ was as high as 99.23 and 76.93 mmol g⁻¹ h⁻¹, respectively. The introduction of chlorine and cyanide improved the visible-light response, hydrophilicity and photo-carrier separation ability of TpPa COF, which contributed to the enhanced PHE efficiency of TpPa-Cl₂ and TpPa-CN₂. This study provides a strong basis for the systematic fine-tuning of the structural and physico-chemical properties of COFs aiming to achieve COF materials with ultra-high photocatalytic activity. [ABSTRACT FROM AUTHOR]