Order Parameters in Liquid Crystals.

Available from UMI in association with The British Library. The extent to which an anisotropic dye dissolved in a liquid crystal host is ordered by the host and the effect of the dye upon the host, the Guest-Host interaction, is a matter of current interest. The Guest-Host interaction between two groups, one simple and one complex, of anthraquinone dyes with hydrogen- and non-hydrogen-bonding substituents dissolved in cyanobiphenyl or phenylcyclohexane/bicyclohexane hosts has been investigated by refractive index and absorption polarization techniques. A fluorescent dye in the same hosts has also been studied using these techniques and also using the method of fluorescence polarization. Results from refractive index measurements show that the inclusion of small amounts of dye in the liquid crystal host can significantly change the refractive indices and birefringence of the host material. It is found that some dyes in the cyanobiphenyl hosts changed the ordinary refractive index of the host such that the dyed host refractive index curves crossed the pure host curve and the mixtures which exhibited this crossing behaviour always have a higher order parameter than that of the pure host. This crossing behaviour is exhibited by both hydrogen-bonding and non -hydrogen-bonding dyes, but only occurs in the cyanobiphenyl hosts. Absorption polarization results reveal that dye order parameter is increased by increasing the length of the alkyl chain attached to the anthraquinone skeleton and also by the addition of thiophenyl groups to the anthraquinone molecule, whereas the methylation of amino groups attached to the anthraquinone skeleton significantly decreases the dye order parameter. Hydroxy groups attached to the dye molecule lower the dye order parameter in PCH/BCH hosts and also influence the shape of the order parameter curve. Results using a dye with two absorption maxima in the visible suggest that the molecular orientational distribution function of the dye is asymmetrical. It is noted that the dye order parameter is very sensitive to the chemical nature of the host. Order parameters determined from fluorescence polarization measurements are found to be lower than the corresponding values measured by absorption. The fourth rank order parameters measured by this method are found to be mainly negative and are thought to be due to neglecting the effect of rotational depolarization of the fluorescent dye molecules.