Opto-dielectric effect on a nematic liquid crystal doped with a photoactive azo mesogen

We report the detailed temperature variation of the dielectric constant across the nematic-isotropic transition in both the normal and UV-irradiated states of a commercially available room temperature nematic mixture (E7 from E-Merck) doped with a small quantity (4.5 mol %) of a liquid crystalline azo compound. The photoirradiation was done using a very low power (0.1 mW/cm2) UV source. Data of time-resolved measurements of the dielectric constant through both the UV-activated “trans-cis” transformation and the thermal back relaxation process are also presented and compared with time-dependent absorbance data. Even for this low concentration of the photoactive material, UV irradiation of as small a magnitude as 0.1 mW/cm2 is observed to lower the nematic-isotropic transition temperature by 15 °C.