Orientational Ordering of a Dimer Liquid Crystal by High-Resolution Solid-State 13C NMR Spectroscopy

The ordering of a dimer liquid crystal, α,ω-bis[(4,4′-cyanobiphenylyl)oxy]decane (CBA-10), was studied by variable-temperature high-resolution solid-state 13C NMR. A large transitional change of the 13C 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 13C 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 (Szzring) and Cγ−Cε axis (SzzCγ-Cε) was attempted here, using the obtained isotropic 13C chemical shift and principal values of the 13C 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 Szz increased from 0.79 to 0.89 with decrease of 17.5°C in temperature from nearby the nematic-isotropic transition point. Szzring increased from 0.74 to 0.80 with decrease of temperature whereas SzzCγ-Cε and the conformation of the spacer was essentially unaltered in the same temperature range.