Study of Nematic Liquid Crystals by Means of the Guest Dye Dichroism

By measuring the dichroism of rod-like probe molecules dissolved in a nematic matrix the value of the liquid crystal orientational order parameter (SLC) can be obtained without any prior knowledge of the properties of this material in a solid state. In this case it is important that the guest and host molecules be isomorphic, and the anisotropy of the local light field be taken into account. To this effect the refractive indices of some liquid crystals have been measured spectrophotometrically and the dichroism of spherical ions has been investigated. The order parameters of liquid crystals were deduced from values of the dichroic ratio measured with the specially choosen probe dye and the principal molecular polarizabilities were calculated for the same substances, using our SLC values and refractive indices. The results reported have been obtained for p-azoxyanisole, MBBA, Merck (Phase IV) and some liquid crystals with a strong positive dielectric anisotropy.