Azo-Linked Pyrazine-Bridged Cu-Phenanthroline Molecular Complex on Carbon Nitride for Boosted Photocatalytic Hydrogen Evolution

Abstract The rational incorporation of molecularly defined catalytic sites onto polymeric semiconductor systems furnishes a reliable route to obtain precise interfaces for solar-driven water splitting. Herein, a potent and electronically conjugated hybrid photocatalyst is reported, comprising a pyrazine-bridged Cu-phenanthroline molecular complex decorated onto graphitic carbon nitride (g-CN) via a covalent azo (–N=N–) linkage. A broadly adaptive building-block-based condensation approach was taken for constructing the ligand framework, involving the condensation of phenanthroline-5,6-dione and 4-nitro-1,2-phenylenediamine to introduce a conjugated pyrazine bridge capable of boosting electronic coupling between the g-CN framework and the active Cu-phenanthroline site. Post-synthetic ligand functionalization of the amine termini of g-CN via a base-mediated azo coupling yielded the intermediate g-CN_NN_L, which underwent post-synthetic metalation to form the active catalyst g-CN_NN_L_Cu. Photocatalytic hydrogen-evolution experiments under visible-light irradiation in 10% aqueous triethanolamine without any noble-metal co-catalyst exhibited an activity trend of g-CN_NN_L_Cu > g-CN_NN_L > g-CN, with g-CN_NN_L_Cu illustrating an average HER rate of ∼1108 μmol g–1 h–1. The observation was supported by electrochemical impedance spectroscopy and transient photocurrent measurements, which unveiled reduction in interfacial charge-transfer resistance and increment in charge extraction upon ligand attachment and Cu-metalation. Band-alignment analysis employing ultraviolet photoelectron spectroscopy and optical bandgap measurements revealed an overall upward shift of the band structures, implying that the performance boost of the hybrid catalysts could be attributed to a combination of stronger reduction potentials, stronger electronic coupling between the g-CN backbone and the electron-withdrawing ligand, and the presence of a dedicated Cu catalytic site. This study is anticipated to outline a modular and scalable ligand-engineering concept for the construction of covalently incorporated molecular-carbon nitride hybrid architectures, thereby establishing a versatile platform for the rational design of earth-abundant, noble-metal-free photocatalysts for solar water splitting.