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

Abstract

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.

Phase Transitions of Dimeric Liquid Crystals Containing Long Odd-Numbered Methylene Spacers

“…20 Shimizu et al also reported that the ordering of the mesogens and of the molecule itself to increase with a decrease in temperature in the nematic phase, whereas that of spacer remains almost constant. 21 It is thought that the difference in the n dependency of TN 1 and ASN1 results from an alteration of the ordering of mesogens in the nematic phase. This point will be clarified based on additional study on n dependence of TN 1 and ASN 1 for many dimeric liquid crystals which have various mesogens.…”

Spatial Configuration and Thermodynamic Characteristics of Main‐Chain Liquid Crystals

“…A homologous series of dimer, trimer, and polymer LCs having PM-type spacers have been studied carefully using various methods, such as 2 H-NMR (quadrupolar splittings) (Section 7.3.1), Faraday balance (magnetic susceptibility) [Sigaud et al, 1983], SQUID (magnetic susceptibility) [Esnault et al, 1988;Furuya et al, 1990Furuya et al, , 1991, and 13 C-NMR ( 13 C chemical shift tensor, 13 C-13 C dipolar couplings) [Shimizu et al, 1997[Shimizu et al, , 1998]. Studies on dimers were extended further to include the effect of the size and orientation of mesogenic units [Furuya et al, 1986], the effect due to the tilting angle at the junction joining the mesogenic unit with the spacer [Abe et al, 1995b], and the optical anisotropy of the partially ordered spacer and its contribution to the orientation-dependent attractive interactions [Furuya et al, 1995].…”

“…The oscillation representing the internal trans-gauche correlation of the bond rotation was found to be more pronounced for the spacer with n = even (Section 7.3.3). The molecular scheme described above has also been adopted successfully in an analysis of the observed 13 C chemical shift and the 13 C-13 C dipolar coupling data of the dimer and trimer LC compounds [Shimizu et al, 1997[Shimizu et al, , 1998]. …”

Thermodynamic Characteristics of Chain Molecules in the Nematic State: PVT Studies of Main-Chain Liquid Crystal Comprising a Long Oxyethylene-Type Spacer

“…Since the degree of polymerization (DP) dependence of physical properties is expected to be most distinct in the oligomer region, a homologous series of dimer (also called a twin compound), trimer, and polymer LCs having α,ω-dialkoxyalkane spacers were carefully examined in our previous work. Efforts were made to elucidate the conformational ordering of the spacer 4 by using various methods such as 2 H NMR (quadrupolar splitting), − SQUID (anisotropy of the magnetic susceptibility), , and 13 C NMR ( 13 C chemical shift tensor, 13 C− 13 C dipolar coupling). , Studies were further extended to include the effect of the size and orientation of mesogenic units, the effect due to the disorientation (tilting angle) at the junction jointing the mesogenic unit with the spacer, and the optical anisotropy of the partially ordered spacer and its contribution to the orientation-dependent attractive interactions . The main emphasis was placed on the thermodynamics involved in the formation of the LC state of the main-chain LC compounds .…”