Viologen-modified platinum clusters acting as an efficient catalyst in photocatalytic hydrogen evolution

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 R_CORE = 1.9 nm with a standard deviation σ = 0.5 nm by transmission electron microscopy (TEM). As a result, the hydrogenevolution 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 NAḊ, produced by the deprotonation from NADḢ, coincides with the appearance of the absorption band due to Acṙ-Mes. This indicates electron transfer from NAḊ to Acr⁺-Mes to give Acr⁺-Mes, which undergoes the electron-transfer reduction of MVA²⁺-PtC, leading to the efficient hydrogen evolution.