Synthesis and characterization of indigo photoswitches

Abstract

Controlling the function and properties of materials, pharmaceuticals and catalysts after manufacturing is essential for contemporary chemists. Such control can be achieved via use of molecular switches, which change their chemical and/or physical properties in response to external stimuli. Light is a particularly attractive stimulus because of its high degree of controllability without physical contact, and clean nature. Therefore, organic light-responsive molecules – photoswitches – are studied for such purposes. The synthesis of new photoswitches with different photochemical properties is needed for various applications. In the molecular design of such photoswitches, compatibility with different matrices such as polymers or elastomers, is necessary to be taken into account.

In this thesis, three different previously unreported indigo photoswitches were designed and synthesized and their photochemical properties characterized in different environments. Photoswitches with similar indigo-core had been previously syntheized and characterized in solution, so the focus was to enable implementation to liquid crystalline and polymer matrices and study the photochemical properties in these environments, with a particular aim of obtaining indigo photoswitching in the solid state.

The photochemical properties of the indigo photoswitches were first studied in solution, after which they were incorporated as dopants in liquid crystals and in polymer films. Photoisomerization efficiency, molar absorption coefficient, thermal stability of the metastable Z-isomer and fatigue resistance were determined. All studied molecules showed similar properties to reference molecules in solution: reversible and efficient photoswitching with moderate thermal life-times and high fatigue resistance. Photoswitching was less efficient in liquid crystals than in solutions, probably due to shorter Z-lifetimes dominating the photostationary states. All compounds showed fatigue resistance of over 50 cycles in liquid crystals. In a polymer film the photoswitching efficiency was higher than in liquid crystals but lower than in solution. Thermal lifetimes were multiple times longer than in a solution, and all photoswitches could be switched to both direction with 660 nm and 525 nm light.

Indigo photoswitches studied in this work were compatible with liquid crystals and polymer films. Photoswitching was observed in all studied environments with moderate E to Z conversion with low-energy irradiation. Regarding the synthetic routes, more studies for optimization of the reactions is required. The synthesized indigo photoswitches are ready for further studying as crosslinkers in liquid crystal elastomers or polymer networks