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，向可再生能源过渡是必要和紧迫的。鉴于全球变暖和空气污染等全球性和区域性的环境问题，人们普遍认为氢将成为石油的重要补充，尽管基本的实际问题仍有待解决。尽管以H_2和1/2O_2为基础的燃料电池汽车的技术进步很快，但是产生和储存氢气的方法还没有完全建立起来。在这种情况下，将水电化学分解成H_2和1/2O_2被认为是产生氢的备选方法之一，其中风力发电站或太阳能电池被认为是电力来源。然而，在这种系统中投入的能源的能源回报仍然不符合实际使用的标准。另一方面，众所周知，用蒸汽处理甲烷也会产生氢气，这也会导致温室气体（CO_2）作为副产品的排放。而且，这个过程需要能量来产生蒸汽。因此，由于水和太阳光丰富，且无CO_2排放，促进水分解为H_2和1/2O_2的光催化体系的研究受到了广泛的关注。自首次发现具有较强金属-金属相互作用的中间体桥接Pt(II)_2二聚体的析氢活性以来，人们一直在努力开发由Ru(bpy)_3^2+光敏衍生物和析氢Pt（II）催化剂组成的“光-析氢（PHE）”分子器件。这种PHEMDs有望成为EDTA在可见光诱导下将H_2O还原为H_2的有效光催化剂。结果发现，在可见光照射下，由[Ru(bpy)_2（5-氨基-苯基）]^2+单元和PtCl_2（bpy）单元组成的特殊分子器件在EDTA存在下演变成H_2。为了更好地了解影响这类phd的PHE活性的因素，一些其他的相关化合物也被制备和评价。这些研究提示了以下关键因素：(1)与Pt(II) d_Z^2轨道相对应的HOMO的失稳增强了PHE活性；（ii）方形平面Pt（ii）配位球轴位周围的位阻降低了PHE活性；（iii）需要适当的芳香性来增强分子内电子转移过程；（iv） EDTA在生成H_2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。除了这些结果，电化学研究，停止流动实验，观察Pt（II）配合物与甲基紫素产生的自由基阳离子之间的反应，以及DFT MO研究也得到了检验。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