Amide-Anchored Pyrazine-Bridged Cu–Phenanthroline Molecular Complex on Carbon Nitride Nanostructures for Enhanced Photocatalytic Hydrogen Evolution

The rational integration of molecular catalysts with polymeric semiconductors offers a powerful route to engineer well-defined interfaces for solar-driven hydrogen production. Herein, a robust and electronically coupled nanohybrid photocatalyst is reported, having a pyrazine-bridged Cu–phenanthroline molecular complex covalently linked to graphitic carbon nitride (g-CN) through an amide anchor. The ligand framework is constructed via a modular, building-block-based condensation of phenanthroline-5, 6-dione with 3, 4-diaminobenzoic acid, introducing a conjugated pyrazine linkage that promotes efficient electronic coupling between the g-CN backbone and the Cu coordination center. Postfunctionalization of terminal amine sites on g-CN nanosheets with the ligand afforded the intermediate g-CNCONHL, followed by postmetalation to generate the final catalyst g-CNCONHLCu. Photocatalytic hydrogen-evolution studies conducted in an aqueous medium containing 10% triethanolamine as a sacrificial electron donor, and in the absence of any noble-metal cocatalyst, revealed a clear activity trend of g-CNCONHLCu > g-CNCONHL > g-CN, with the fully metalated hybrid achieving an average HER rate of ∼880 μmol g–1 h–1 under visible-light irradiation. Transient photocurrent and electrochemical impedance spectroscopy measurements substantiated the photocatalytic results, indicating enhanced charge extraction and reduced interfacial charge-transfer resistance upon ligand functionalization and Cu coordination. Band-alignment analysis based on ultraviolet photoelectron spectroscopy and optical bandgap determination indicated that the overall band edges shifted upwards, suggesting that performance enhancements originated from the enhancement in electronic coupling between the g-CN matrix and the rationally designed ligand and the introduction of a catalytically active Cu site along with stronger reduction potential of the conduction band. This work is expected to establish a modular and scalable ligand-engineering strategy for constructing covalently integrated molecular–carbon nitride hybrids, providing a general platform for designing earth-abundant, noble-metal-free photocatalysts for solar hydrogen generation.