Studies on the reaction mechanism and activity control of single-molecule photo-hydrogen-evolving molecular devices.

人们越来越认识到，石油、煤炭和天然气等化石燃料的储量是有限的，向可再生能源的过渡是必要和紧迫的。鉴于全球变暖、大气污染等全球性和地区性环境问题，人们普遍认为氢气将成为石油的重要补充，尽管基本的实际问题仍然需要解决。尽管以氢和1/2O_2为基础的燃料电池汽车的技术发展很快，但氢气的产生和储存手段尚未完全建立起来。在这种背景下，将水电化学分解为H_2和1/2O_2被认为是一种候选的制氢方法，其中风力发电站或太阳能电池被认为是电能的来源。然而，投入这类系统的能量回报仍然不符合实际使用的标准。在另一个…上另一方面，用蒸汽处理甲烷也会产生氢气，这也会导致温室气体(CO_2)的排放。此外，这一过程需要产生蒸汽的能量。因此，由于水和阳光充足，且无CO_2排放，促进太阳光诱导水分解为H_2和1/2O_2的光催化体系的研究受到了极大的关注。自从首次发现具有较强金属-金属相互作用的酰胺桥联的铂(II)_2二聚体的析氢活性以来，人们花了大量的努力来开发由光敏化Ru(BPY)_3^2+衍生物和析氢铂(II)催化剂组成的光放氢(Phe)分子器件(MD)。这些PHEMDs有望作为一种有效的光催化剂，用于EDTA在可见光下将H_2O还原为H_2。结果，我们发现了一个由[Ru(Bpy)_2(5-氨基邻菲)]^2+单元和PtCl2(Bpy)单元组成的特殊分子装置在可见光照射下放出H_2。还制备和评价了其他几种相关化合物，以更好地了解影响这类PHEMD的Phe活性的因素。这些研究表明以下关键因素：(I)填充的铂(Ii)d_Z^2轨道对应的HOMO的失稳增强了Phe的活性；(Ii)正方形平面铂配位球轴位置周围的空间位阻降低了Phe的活性；(Iii)需要适当的芳香性来增强分子内的电子转移过程；(Iv)EDTA在发生H_2生成之前与PHEMD形成加合物；(V)H_2的形成是通过PHEMD的双分子过程进行的。除这些结果外，还对铂(II)络合物与甲基紫精生成的自由基阳离子之间的反应进行了电化学研究、停流实验和密度泛函理论研究。较少

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 other … More 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.Since the original discovery in the H_2-evolving activities of amidate-bridged Pt(II)_2 dimers having a relatively strong metal-metal interaction, considerable efforts have been made to develop the 'photo-hydrogen-evolving (PHE)' molecular devices (MDs) that are made up of photosensitizing Ru(bpy)_3^2+ derivatives and H_2 evolving Pt(II) catalysts. Such PHEMDs are expected to serve as an efficient photocatalyst' towards the visible-light-induced reduction of H_2O by EDTA into H_2. As a result, we discovered that one particular molecular device consisting of a [Ru(bpy)_2(5-amino-phen)]^2+ unit and a PtCl_2(bpy) unit evolves H_2 in the presence of EDTA under the visible light illumination. Several other related compounds have also been prepared and evaluated to better understand the factors affecting the PHE activities of such PHEMDs. These studies suggested the following key factors: (i) destabilization of the HOMO corresponding to the filled Pt(II) d_Z^2 orbital enhances the PHE activity; (ii) steric hindrances around the axial site of the square-planar Pt(II) coordination sphere lower the PHE activity; (iii) an appropriate aromaticity is needed to enhance the intramolecular electron transfer processes; (iv) EDTA forms an adduct with a PHEMD before undertaking the H_2 generation; (v) the H_2 formation undergoes via a bimolecular process of the PHEMD. In addition to these results, the electrochemical studies, stopped flow experiments looking at the reactions between the Pt(II) complexes and a radical cation generated from methylviologen, and DFT MO investigations have been examined. Less