ABSTRACT:The ordering of a dimer liquid crystal, IX,w-bis [(4,4'-cyanobiphenylyl)oxy]decane (CBA-10), was studied by variable-temperature high-resolution solid-state 13 C NMR. A large transitional change of the 13 C chemical shift was observed on going from the isotropic to the nematic phase due to uniaxial alignment of the molecule. The principal values of the 13 C chemical shift tensor have been determined by the Herzfeld-Berger method for CBA-10 itself. The molecular axis is assumed to lie parallel to the line connecting the centers of both mesogenic cores, and the evaluation of the order parameters along the molecular axis, (Szz), mesogene axis (S~~ng) and C,-C, axis (S~,;'-c") was attempted here, using the obtained isotropic 13 C chemical shift and principal values of the 13 C chemical shift tensor, and taking into account the conformational behavior of the flexible spacer in the frame of rotational isomeric state (RIS) approach. The order parameter of the molecule S., increased from 0.79 to 0.89 with decrease of I 7.5°C in temperature from nearby the nematic-isotropic transition point. S~~"• increased from 0.74 to 0.80 with decrease of temperature whereas S~,:--c· and the conformation of the spacer was essentially unaltered in the same temperature range.KEY WORDS High-Resolution Solid-State 13 C NMR / 13 C Chemical Shift Tensor/ Conformation/ Rotational Isomeric State / Order Parameter / Dimer Liquid Crystal / Low molecular weight monomer liquid crystals are composed of rod-like molecules which form an uniaxially oriented mesophase. The chemical shift anisotropy of the 13 C nucleus has proved to be a powerful tool for probing the orientational ordering at the molecular level of such compounds 1 · 2 and for a side-chain liquid-crystal polymer. 3 Though the principal values of the chemical shift tensor needed for the analysis are in some cases assumed to be the same for analogous compounds, this may not be true 2 .4 and may lead to error in constructing models of microscopic configurations from the observed data.In the case of dimer liquid crystals consisting of two mesogenic moieties linked by a flexible alkyl chain, as the model of polymer liquid crystals, the molecules align as a whole in the nematic phase, the conformation of the spacer being quite stable in a large temperature range. 5 Our goal in this work is to prepare a 13 C-labeled dimer liquid crystal, ll (,w-bis[( 4,4' -cyanobiphenylyl)oxy ]decane (CBA-10) and to analyze its ordering behavior through the observation of the 13 C chemical shift anisotropy, determining the principal values of the 13 C chemical shift tensor for CBA-10 itself. This is the first attempt with a dimer liquid crystal in which the conformation of the spacer must be considered in the analysis of ordering being different from simple monomer liquid crystals. Spacer conformation is considered in the frame of the rotational isomeric state (RIS) approach.
