Unravelling the pH-dependence of a molecular photocatalytic system for hydrogen production.

Photocatalytic systems for the reduction of aqueous protons are strongly pH-dependent, but the origin of this dependency is still not fully understood. We have studied the effect of different degrees of acidity on the electron transfer dynamics and catalysis taking place in a homogeneous photocatalytic system composed of a phosphonated ruthenium tris(bipyridine) dye () and a nickel bis(diphosphine) electrocatalyst () in an aqueous ascorbic acid solution. Our approach is based on transient absorption spectroscopy studies of the efficiency of photo-reduction of and correlated with pH-dependent photocatalytic H production and the degree of catalyst protonation. The influence of these factors results in an observed optimum photoactivity at pH 4.5 for the - system. The electron transfer from photo-reduced to is efficient and independent of the pH value of the medium. At pH <4.5, the efficiency of the system is limited by the yield of photo-reduction by the sacrificial electron donor, ascorbic acid. At pH >4.5, the efficiency of the system is limited by the poor protonation of , which inhibits its ability to reduce protons to hydrogen. We have therefore developed a rational strategy utilising transient absorption spectroscopy combined with bulk pH titration, electrocatalytic and photocatalytic experiments to disentangle the complex pH-dependent activity of the homogenous - photocatalytic system, which can be widely applied to other photocatalytic systems.