Behaviour of photosensitive soft materials: Thermo-optical, dielectric and elastic constant studies on azo-dye doped nematic liquid crystals

Abstract

Optical absorption, thermo-optical and dielectric studies have been performed on three guest–host nematic mixtures featuring photo-sensitized flexoelectric polarization. The liquid crystal 4-butyl-cyclohexane carboxylic acid 4-pentyloxy-phenyl ester, a room temperature nematic with a negative dielectric anisotropy served as the host material. Three different azo-dye compounds, also exhibiting liquid crystallinity were employed as the low-concentration guest component in the mixtures. In each case illumination of the sample with actinic (UV) light leads to strong photoisomerization driven effects. Correlations among the shift of the isotropic–nematic phase transition temperature, population of the cis isomers, and the change in the length of the azo-dyes upon UV illumination, are discussed. The UV-induced changes in the dielectric constant values have been shown to be dependent on the nature of the photoactive dopant. The article also describes the first observation of the lowering of the bend elastic constant upon photoisomerization, a feature ascribed to the formation of the bent-shaped cis isomers.