Dual non-noble-metal-immobilized covalent organic frameworks for visible-light-driven photocatalytic hydrogen evolution

Abstract

Covalent organic frameworks (COFs) have emerged as highly versatile platforms for the spatially controlled immobilization of metals in heterogeneous photocatalytic hydrogen evolution reactions (HERs). Herein, we have integrated both a Cu(I) diimine-based photosensitizer unit and a Co(II) bipyridine-based catalyst unit into a two-dimensional COF, TpBpy, which serves as a macro-ligand scaffold. The resulting hybrid material, TpBpy–Cu/Co, achieves an optimized photocatalytic H₂ evolution rate of 12.16 mmol g⁻¹ h⁻¹, representing a 25-fold enhancement relative to the pristine TpBpy framework. To elucidate the origin of this enhancement, ultrafast transient optical spectroscopy, electrochemical measurements, and photoluminescence studies were employed. These studies reveal that the incorporation of the Cu(I) diimine unit enhances the photosensitizing capability of the framework, while the excited-state lifetime of the photosensitizer is prolonged due to the confinement effects within COF structure. Furthermore, comparative invetigations demonstrate that the TpBpy–Cu/Co outperformes the system consisting of separate TpBpy–Cu and Co(bpy)Cl₂, highlighting the critical role of metal coordination in facilitating intra-framework charge transfer from photosensitizers to catalysts. This study provides strategic insights into the design of COF–metal coordination systems, emphasizing the importance of integrating multifunctional units to tailor and enhance photocatalytic performance.