Effect of Absolute Configuration on the Formation of TGBA* Phase in Liquid Crystallines Containing Two Stereogenic Centres

Abstract

Our previous work has shown that a homologous series of chiral materials containing two sereogenic centres, (R)-2-pentyl (S)-2-(6-(4-(4-alkoxyphenyl)benzoyloxy-2-naphthyl)propionates (R,S)PmPBNP (m=7−14). exhibited TGBA* phase. To understand the effect of relative configuration of two stereogenic centres at chiral tail on the appearance of TGBA* phase, two stereoisomers of (R,S)P11PBNP, (S,S)P11PBNP and (±,S)P11PBNP were synthesized and their mesophases investigated. The results showed that both isomers also possessed TGBA* phase with the phase sequence of I-N*-TGBA*-SA*-Sc*-C, The temperature range of TGBA* phase in (R,S)-isomer was significantly larger than that in (S,S)-isomer, indicating the stability of TGBA* phase was affected by the relative spatial configuration of the second chiral centre at chiral tail. In addition, the temperature range of TGBA* phase in (±,S)-isomer was larger than (S,S)-isomer, suggesting that an introducing methyl branched group at the external side of mono chiral centre could cause a weakening of smectic layer order and result in an occurrence of TGBA* phase. The measured Ps values in the Sc* phase from the isomers, also indicated the absolute configuration of second chiral centre plays an important role in determining the magnitude of spontaneous polarization.