Development of the nano devices with photo-hydrogen-evolving abilities

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There is an increasing understanding that the reserves of fossil fuels, such as oil, coal, and natural gas, are limited and the transition to renewable energies is necessary and urgent. In view of the global and regional environmental issues such as global warming and air pollution, it is generally considered that hydrogen would become an important supplement to oil, even though basic practical problems still need a solution. In spite of the rapid technological advance in the development of fuel-cell cars based on H_2 and 1/2O_2, the means of generating and storing hydrogen gas has not been well established as yet. In this context, the electrochemical decomposition of water into H_2 and 1/2O_2 is regarded as one of the candidates as a method of generating hydrogen, where wind power stations or solar batteries are considered as the source of electricity. Nevertheless, the energy return on energy invested in such systems still does not meet the criteria for the practical use. On the othe … More r hand, hydrogen is also known to be generated upon the treatment of methane with steam, which also leads to the emission of greenhouse gas (CO_2) as a byproduct. Moreover, this process requires the energy to produce the steam. Consequently, great attention has been paid to the studies on the photocatalytic systems promoting the solar-light-induced splitting of water into H_2 and 1/2O_2 because water and sunlight are abundant and such systems are free of the CO_2, emission.In this regard, we have focused on a well-known photosystem consisting of EDTA, tris(2,2′-bipyridine)ruthenium(II), methylviologen, and hydrogen-evolving catalysts, where particular Pt(II) complexes serve as the catalysts. Further, continuous efforts have also been made to develop the photo-hydrogen-evolving molecular devices consisting of photosensitizing rnthenium(II) centers and platinum(II)-catalyst centers. Various heteronuclear Ru(II)Pt(II) or Ru(II)_2Pt(II)_2 complexes have been prepared and evaluated. However we have long been unable to demonstrate the usefulness of such molecular devices. We recently succeeded in the preparation of the first Ru(II)Pt(II)-based molecular device which effectively catalyzes visible-light-induced EDTA-reduction of water into molecular hydrogen. It has been confirmed that the observed H_2-evolving activity is not due to any photodecomposition products.In the present study, to clarify the important factors which govern the photocatalytic activity of the Ru(II)Pt(II)-based molecular device, several other structural analogs have been prepared and their photo-H_2-evolving activities have been evaluated. As a result, the devices with peptide-linkage are found to evolve H_2 upon Xe-lamp irradiation in the presence of EDTA under Ar atmosphere. On the other hand, the compounds having two or more photosensitizing ruthenium(II) centers are found to be ineffective towards the photochemical H_2 generation, presumably due to the enhancement in intramolecular energy-transfer quenching processes. Less