Guest-host interactions in dichroic dye-liquid crystal mixtures in smectic A and nematic phases

The polarized absorption spectra of six azo dyes dissolved in the liquid crystals 8CB and 8OCB have been measured as a function of temperature in the smectic A and nematic phases. On the basis of these spectra the order parameter of the dyes oriented in the liquid crystalline host has been determined. The guest order parameter has been compared with that of the host, obtained from the optical birefringence measurements. Moreover, the influence of the dichroic dye on the phase transition temperatures of the pure liquid crystal has been studied. A correlation between the orientation of the guest molecules in the uniaxial liquid crystalline phases, the guest molecular geometry and the nematic-isotropic transition temperature of the dye-liquid crystal mixture has been discussed. (C) 1997 Elsevier Science B.V.