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London Dispersion and Solvent Effects on Azobenzene Switches

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2021

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Molecular photoswitches based on azobenzenes represent a widespread tool to manipulate properties of systems in the solid or liquid state as well as in solution by using light. In order to optimize the design of these azoswitches, comprehensive knowledge of all internal and external factors altering their properties is necessary. This includes photochromic but also thermodynamic and kinetic influences on their switching behavior. As part of this thesis, fundamental studies concerning the effect of London dispersion interactions on the stability of the cis-isomer of azobenzenes were conducted. In this context, multiple alkyl substituted azobenzenes were prepared and used as probes to determine the stabilizing contributions on the cis-state in dependence of the substituent length and position. It was possible to identify dispersion interactions originating from alkyl-aryl interactions as the main contributor to the stabilization of the cis-isomer. Surprisingly, even flexible alkyl chains like n-butyl can lead to an about five times higher half-life of the cis-isomer compared to the methyl-substituted derivative. Additionally, changes induced by different solvent environments were investigated. Replacing n-octane as the solvent with iso-octane, increased the activation barrier for the thermal cis-trans-isomerization by about 0.1 kcal mol-1, resulting in 20% higher half-lives. Cyclooctane had a destabilizing effect of the same magnitude except for a bulky tert-butyl substituted derivative, which was similarly stabilized as in iso-octane. It was further demonstrated that intramolecular dispersion interactions are not canceled in solution, since the overall trend of half-lives in dependence of the substituent chain length is constant in all investigated solvents. Moreover, several molecular systems for the investigation of London dispersion interactions were reviewed. Advantages and disadvantages of the different concepts were addressed, revealing the necessity to gain a deeper insight into London dispersion interactions between different molecular structures and especially concerning the solvent environment. Further studies were conducted in order to analyze the influence of an electronically decoupled ammonium tag attached to the azobenzene scaffold in different solvents. Such an ammonium functionality represents a common motif for biological active compounds, enhancing their solubility in aqueous environments even allowing binding with biological receptors. Photochromic as well as kinetic properties of the cis-trans isomerization were determined and compared to unsubstituted azobenzene. In this way, background data for the use of ammonium tagged azobenzenes for versatile applications in different systems could be provided.

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