Liquid crystals with negative dielectric anisotropy: the effect of unsaturation in the terminal chain on thermal and electro-optical properties

Abstract

(E)-4-Decenyl 2 and a series of pentenyloxy 3 derived from 2,3-difluoro-4-(2-(trans-4-pentylcyclohexyl)ethyl)biphenyl and an analogues series of pentenyloxy derivatives of 2,2′,3-trifluoro-4-(2-(trans-4-pentylcyclohexyl)ethyl)biphenyl (4), in which the position and configuration of the C=C bond was varied, were investigated. All materials exhibit a nematic phase, while compounds 2 and 4 also display a SmA phase. The dependence of T NI on the position and configuration of the C=C bond follows the order 2-(E) > 4 > none > 3-(Z) > 3-(E) for series 3 and is similar for series 4. Dielectric parameters, elastic constants and rotational viscosity were obtained as function of temperature and compared to those of their saturated analogues: decyl 1 and pentyloxy 3a and 4a. Dielectric anisotropy is lower for difluorobiphenyl mesogens 3 (Δϵ = −1.4 to −1.8 at ΔT = –40 K) than for 4b (Δϵ = –2.8 at ΔT = –40 K). The unsaturation in series 3 generally lowers the elastic constants K11 and K33 and their temperature dependence, and also increases rotational viscosity activation energy E a, which follows the order: none < 3-(Z) < 4 < 2-(E) < 3-(E).