Light-Driven Hydrogen Generation from Microemulsions Using Metallosurfactant Catalysts and Oxalic Acid.

A unique microemulsion-based photocatalytic water reduction system is demonstrated. Iridium- and rhodium-based metallosurfactants, namely, [Ir(ppy)2(dhpdbpy)]Cl and [Rh(dhpdbpy)2Cl2]Cl (where ppy = 2-phenylpyridine and dhpdbpy = 4,4'-diheptadecyl-2,2'-bipyridine), were employed as photosensitizer and proton reducing catalyst, respectively, along with oxalic acid as a sacrificial reductant in a toluene/water biphasic mixture. The addition of 1-octylamine is proposed to initiate the reaction, by coupling with oxalic acid to form an ion pair, which acts as an additional surfactant. Concentration optimizations yielded high activity for both the photosensitizer (240 turnovers, turnover frequency (TOF) = 200 h-1) and catalyst (400 turnovers, TOF = 230 h-1), with the system generating hydrogen even after 95 h. Mechanistic insights were provided by gas-phase Raman, electrochemical, and luminescence quenching analysis, suggesting oxidative quenching to be the principle reaction pathway.