Beyond Water: Deep Eutectic Solvents Enable Dye‐Sensitised Photocatalytic Hydrogen Production

nanoparticles were sensitised with two carbazole-based organic dyes featuring hydrophobic or hydrophilic peripheral substituents, enabling a systematic investigation of solvent-catalyst interfacial effects. Photocatalytic experiments were conducted in both hydrophobic and hydrophilic DESs, while water was retained exclusively as a proton source, thereby drastically reducing its overall consumption. Comprehensive physicochemical characterisation, including UV-visible spectroscopy, cyclic voltammetry, transmission electron microscopy, infrared spectroscopy, and nitrogen physisorption, was employed to elucidate structure-property relationships. Under visible-light irradiation, DES-based systems exhibited significantly enhanced hydrogen production rates, turnover numbers, and light-to-fuel efficiencies compared to aqueous benchmarks using the same sacrificial electron donor. Remarkably, the highest photocatalytic activity was observed for heterogeneous dye-DES combinations, revealing a counterintuitive trend with respect to dye-solvent affinity. These findings demonstrate that photocatalytic performance is critically governed by the interplay between dye functionalisation, DES polarity, and interfacial solvation, which collectively modulate charge transfer processes, surface wettability, and hydrogen evolution kinetics. Overall, this study establishes DESs as low-cost, versatile, reusable, and environmentally benign reaction media, opening new avenues for solvent engineering in next-generation photocatalytic hydrogen production.