The synthesis and liquid crystal transition temperatures of some weakly polar nematic trans-4-substituted-cyclohexyl (E)-alk-2-enoates

Abstract

The effect on the liquid crystal transition temperatures of introducing various groups (for example incorporating C˭C, O, CO₂ and CO) into the terminal alkyl chain of a weakly polar model compound 1-[trans-4-(trans-4-propylcyclohexyl)cyclohexyl]pentane has been investigated systematically. Only the compound containing both an ester function and a trans-carbon-carbon double bond exhibited a wide-range nematic mesophase at elevated temperatures. Therefore, a wide variety of trans-4-substituted-cyclohexyl (E)-alk-2-enoates incorporating a carbon double bond with a trans-configuration (E) in the terminal alkyl chain has been synthesized. Nearly all the two-ring esters prepared exhibit a nematic phase over a wide temperature range (≤ 100°C) at elevated temperatures (≤ 200°C). The tendency to form smectic mesophases is often low. Comparisons with the corresponding derivatives incorporating either just a carboxy group (COO) or just a carbon-carbon double bond (C˭C) in the same positions indicate that synergetic effects lead to broader nematic phases than would otherwise have been expected. The new compounds are easily prepared from known starting materials.