Tuning the linkage microenvironment in imine-based covalent organic frameworks for enhanced photocatalytic hydrogen production

Abstract

Rational design of covalent organic frameworks (COFs) at the molecular level is necessary to improve the photocatalytic hydrogen evolution (PHE) performance. Herein, a photosensitive two-dimensional (2D) COF, namely Im_TF–PD, is first constructed through the condensation of 1,3,6,8-tetra(4-formylphenyl)pyrene (TFPPy) and p-phenylenediamine (PD) under solvothermal conditions. The photocatalytic activity of Im_TF–PD is regulated by transforming imine linkages into quinoline through the Povarov reaction between the imine groups and phenylacetylene and 4-ethynylbenzonitrile, respectively. Such a post-synthetic modification (PSM) process enables the modulation of π-conjugation and simultaneously the introduction of the electron-deficient nitrile group improves the local electron delocalization of the skeleton of Im_TF–PD. As a result, the modified quinoline based COF (Qu-CN) exhibited outstanding and steady hydrogen production with a maximum hydrogen production rate of 245.17 mmol g⁻¹ h⁻¹ under visible-light irradiation (λ ≥ 420 nm). Experimental and computational results revealed that Qu-CN with better electron–hole separation and lower band gap significantly improved the catalytic activity regarding kinetics and thermodynamics. This study reveals that enhancing π-conjugation and tuning the local electronic environment of COF-based photocatalysts provide a convenient and effective approach for the development of new efficient photocatalysts.