Viologen-Modified Platinum Clusters Acting as an Efficient Catalyst in Photocatalytic Hydrogen Evolution

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Abstract
A highly efficient photocatalytic system for hydrogen evolution with dihydronicotinamide coenzyme (NADH) as a sacrificial agent in an aqueous solution has been constructed by using water-soluble platinum clusters functionalized with methyl viologen−alkanethiol (MVA2+) and a simple electron-donor dyad, 9-mesityl-10-methylacridinium ion (Acr+−Mes), which is capable of fast photoinduced electron transfer but extremely slow back electron transfer. The mean diameter of the platinum core was determined as RCORE = 1.9 nm with a standard deviation σ = 0.5 nm by transmission electron microscopy (TEM). As a result, the hydrogen-evolution rate of the photocatalytic system with MVA2+-modified platinum clusters (MVA2+−PtC) is 10 times faster than the photocatalytic system with the mixture of the same amount of MVA2+ and platinum clusters as that of MVA2+−PtC under otherwise the same experimental conditions. The radical cation of NADH has been successfully detected by laser flash photolysis experiments. The decay of the absorbance due to NAD•, produced by the deprotonation from NADH•+, coincides with the appearance of the absorption band due to Acr•−Mes. This indicates electron transfer from NAD• to Acr+−Mes to give Acr•−Mes, which undergoes the electron-transfer reduction of MVA2+−PtC, leading to the efficient hydrogen evolution.