Order parameters of the mesogenic core in two nematic liquid crystals containing a four-unit link

Abstract

A series of compounds containing a four-unit link, 4′-n-alkylphenyl-4-n-hexyloxycinnamates, has been synthesized, and their mesomorphic behaviour has been determined. The orientational ordering of two compounds in this series has been studied in detail by 2D carbon-13 nuclear magnetic resonance with variable angle sample spinning. The carbon-proton dipolar coupling constants were obtained and the order parameters were calculated for different molecular segments (two phenyl rings, the central link containing an ethylenic motif, and the methylene carbons of the alkoxy chain). The temperature dependence of the local order parameters of each fragment were determined using these data and C-13 chemical shifts which were measured as a function of temperature. In these non-rigid molecules, the ratio between the order parameters of the two phenyl rings was found to be temperature dependent. The ring bearing the double bond is more ordered at low temperatures compared to the ring bearing the short alkyl chain, but this behaviour is reversed near the clearing point. The position of the major axis of the rigid core was calculated from the order parameter of one ring and the order parameter of the ethylenic link. It was found that this major axis forms a constant angle with respect to the para axis of the phenyl ring connected to the ethylenic group. On the other hand, the angle of this major axis with respect to the para axis of the phenyl ring connected to the carboxylic group is temperature dependent, varying from 11° at T/T_NI = 0·92 to 7° at T/T_NI = 0·98.