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

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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.