Synthesis and photoswitching properties of some cholesterol based liquid crystals

The synthesis and photoswitching properties of some azobenzene linked cholesterol derivatives (3a-d and 5a,b) have been described. Thermal properties of these compounds were studied by differential scanning calorimetry (DSC) and polarizing optical microscopy (POM) techniques. Compounds 3a and 3b exhibit a Smectic A phase, whereas 3d and 5b exhibit Smectic A, TGB and cholesteric phases. Compounds 3c and 5a show a cholesteric mesophase. The presence of azobenzene moiety imparts photochromic properties to these compounds. Trans-cis photoisomerization of me azo-moiety in these compounds in eutectic mixtures consisting of cholesteryl oleyl carbonate, cholesteryl chloride and cholesteryl nonanoate was studied by irradiating at 355 nm. Irradiation of the thin films of these mixtures lead to trans-cis isomerization of the azobenzene chromophore present in the mixture which is accompanied by a large shift in the reflectance band resulting in visual changes in the colour of these films.