beta-Diketone, pyrazole and isoxazole derivatives with polar groups: Liquid crystalline and non-linear optical properties

The liquid crystalline behaviour of 1-(4-n-decyloxyphenyl)-3-(4-X-phenyl)propan-1,3-diones and their pyrazole and isoxazole derivatives has been studied by optical microscopy, DSC and X-ray techniques. The di-substituents (X) were chosen to include a range of different polar and non-polar substituents: H, OCH3, Cl, Br and CN. A monotropic S-A phase is observed for the beta-diketone derivative in which X = CN and this is the first example of this phase found in a 1,3-diphenylpropan-1,3-dione derivative. The majority of the pyrazole and isoxazole compounds show S-A phases. As regards the cyano-substituted compounds, X-ray diffraction studies on the mesophase show that the layer spacing is consistent with a partial bilayer S-A mesophase. The first hyperpolarizabilities of the cyano-derivatives due to their push-pull structure have been measured by the EFISH method. Values for these compounds were found to be comparable to those for other conjugated CH3O-pi-CN systems.