Development of molecular photoswitches for the effective control of the pH value of aqueous solutions by visible light

该项目的目的是开发某些化合物，以下称为pH光开关，其水溶液在可见光照射下会经历pH值的变化，从而可以通过改变光的颜色来逆转最初的变化。通过光照射控制pH值，可以在高空间和时间分辨率下进行，这在研究和技术中提供了几个潜在的应用，因为许多生物和技术过程依赖于pH，因此，使用这种pH光开关，即使主要不是光敏的，也可以通过辐射间接控制。然而，到目前为止，所有用于可逆调节pH的物质都有很强的局限性，没有一种得到广泛的应用。在这个项目过程中需要克服的典型限制是pH值的微小变化或这种变化的短期持续时间，以及使用紫外光的必要性。在众所周知的光致变色剂中，吲哚类染料，特别是硫代蓝色衍生物，似乎最适合于开发新型的pH光开关。硫代吲哚衍生物以两种几何构型(反式和顺式)存在，它们可以在可见光照射下相互转化，同时自发转化为反式异构体，半衰期为几个小时。构型的改变可用于控制分子内酸性或碱性基团之间分子内氢键(H键)的形成。这种氢键作为一个整体调节分子的酸度，并可能最终改变周围溶液的pH值几个单位。实现这一概念的一个主要挑战是设计允许形成最大强度的氢的分子结构，从而最大化酸度的变化。另一个挑战是确保染料在水中有良好的溶解性。在利用成熟的技术进行新化合物的化学合成之后，在UV/Vis光谱确定光化学和酸碱化学参数之后，氢键的表征，主要是通过核磁共振光谱，将在pH光开关的结构优化中起到重要的作用。

The aim of this project is the development of certain chemical compounds, hereafter called pH photoswitches, whose aqueous solutions experience a variation in pH values upon irradiation by visible light in such a way that an initial variation can be reversed by a change in the color of the light. Controlling the pH through irradiation by light, which can be performed in high spatial as well as time resolution, offers several potential applications in research and technology, as many biological and technical processes are pH dependant and may thus, using such pH photoswitches, be controlled indirectly by irradiation even if primarily not light sensitive. All the substances for the purpose of reversible pH modulation so far available, however, have strong limitations and none of them have found broad application. Typical limitations, to be overcome in the course of this project, are meager changes in pH or short time durations of such changes and the necessity to use UV light. Among the well known photochromes, indigoid dyes, especially thioindigo derivatives, seem to be best suited for the development of novel pH photoswitches. Thioindigo derivatives exist in two geometrical configurations (trans und cis isomers) which may be interconverted by irradiation by visible light while the concurrent spontaneous conversion to the trans isomer takes place with a half life of a few hours. The change in configuration may be exploited in the control over the formation of intramolecular hydrogen bonds (H bonds) between acidic or basic groups within the molecule. Such H bonds modulate the acidity of the molecule as a whole and may eventually change the pH of the surrounding solution by a few units. One major challenge in the realization of this concept lies in the design of molecular structures that allow for the formation of hydrogen of maximum strength and to thus maximize the variation in acidity. Another challenge is to ensure good solubility of the dyes in water. Following the chemical synthesis of the novel compounds which is expected to be feasible using well established techniques, and after the UV/Vis spectroscopic determination of parameters of photo chemistry and acid base chemistry, characterization of the hydrogen bonds, primarily by NMR spectroscopy, will be of major importance in the structure optimization of pH photoswitches.