Ronald Y. Dong

Advances in NMR Studies of Liquid Crystals

Abstract Recent solid state NMR studies of liquid crystalline materials are surveyed. The review deals first with some background information in order to facilitate discussions on various NMR ( 13 C, 1 H, 2 H, 19 F etc.) works to be followed. This includes the following: spin Hamiltonians, spin relaxation theory, and a survey of recent solid state NMR methods (mainly 13 C) for liquid crystals on the one hand, while on the other hand molecular ordering of mesogens and motional models for liquid crystals. NMR studies done since 1997 on both solutes and solvent molecules are discussed. For the latter, thermotropic and lyotropic liquid crystals are included with an emphasis on newly discovered liquid crystalline materials. For the solute studies, both small molecules and weakly ordered biomolecules are briefly surveyed.

A comparative study of dynamics in the nematic and reentrant-nematic phases of 60CB and 60CB/80CB mixture by deuteron nuclear magnetic resonance relaxation

Deuteron longitudinal (T1Z) and quadrupolar (T1Q) spin-lattice relaxation times and quadrupolar splittings were measured over all the stable mesophases in a mixture of perdeuterated 4-n-hexyloxy-4′-cyanobiphenyl (6OCB) and 4-n-octyloxy-4′-cyanobiphenyl (8OCB) at 15.1 and 46 MHz, and compared with those reported previously for a pure 6OCB sample. The 6OCB/8OCB mixture has 28 wt.u2009% of 6OCB and shows a nematic, smectic A and reentrant-nematic (RN) phases. We have carried out data analyses for both samples in order to achieve a consistent physical picture. The additive potential method is employed to construct the potential of mean torque using the quadrupolar splittings in these samples. A decouple model is used to describe correlated internal motions of the end chain, which are independent of the molecular reorientation. The latter motion is treated using the small-step rotational diffusion model of Tarroni and Zannoni, while the former motion is described using a master rate equation. In comparing the NMR ...

On the role of collective and local molecular fluctuations in the relaxation of proton intrapair dipolar order in nematic 5CB

We investigate the role that local motions and slow cooperative fluctuations have on the relaxation of the intrapair dipolar order in the nematic 5CB. With this purpose we present a theoretical and experimental systematic study which allow us to quantify the contribution from each type of molecular fluctuation to the intrapair dipolar order relaxation time, T(1D). The experimental work includes measurements of Zeeman and intrapair dipolar order relaxation times (T(1Z) and T(1D)) as a function of temperature at conventional NMR frequencies, in three complementary samples: normal and chain deuterated 4-n-pentyl-4()-cyanobiphenyl (5CB and 5CB(d11)) and a mixture of normal 5CB and fully deuterated 4-n-pentyl-4-cyanobiphenyl (5CB(d19)), 50% in weight. Additionally we perform T(1Z) field-cycling Larmor frequency-dependent measurements to obtain the spectral density of the cooperative fluctuations. The obtained results are as follows. (a) The cooperative molecular fluctuations have a strong relative weight in the relaxation of the intrapair dipolar order state, even at Larmor frequencies in the range of conventional NMR. (b) Alkyl chain rotations are an important relaxation mechanism of the intrapair dipolar order at megahertz frequencies. (c) Intermolecular fluctuations mediated by translational self-diffusion of the molecules is not an efficient mechanism of relaxation of the intrapair dipolar order.

Biased motion about the long molecular axis in ‘ordered’ smectic phases as studied by deuterium NMR relaxation

Spectral densities of motion were determined by deuteron NMR relaxation measurements in the ‘ordered’ smectic-B and -G phases of 4-n-pentyloxybenzylidene-d 1-4′-heptylaniline and 4-n-pentyloxybenzylidene-4′-heptylaniline-2,3,5,6-d 4 at two different Larmor frequencies. Different forms of motional behaviour are involved in these phases in comparison with those in the high temperature ‘disordered’ phases. Specifically, internal ring rotation and direction fluctuation are not effective in the ordered smectic phases. In addition, the fast rotation of the molecule about its long molecular axis is now strongly hindered to give a libration motion of angular amplitudes of about 100°. The third-rate model is again used to describe the molecular reorientation, taking the restricted γ motion into account. The effects of phase biaxiality on spin relaxation in the smectic-G phase are also discussed.

Relaxation properties of proton magnetic spin quasi-invariants in liquid crystals

Abstract We present an experimental study of the thermodynamic properties of proton pairs in two thermotropic nematic liquid crystals: chain deuterated 4-n-pentyl-4′-cyanobyphenyl, 5CBd11, and normal 5CB. In the first sample, we find the existence of pure intra-pair and inter-pair magnetic quasi-invariants. In the second, the dipolar signal is more complex, due to the nonequivalence of protons pairs in the molecule, but it is still possible to prepare states for which only one kind of order strongly dominates. In both compounds, the dipolar quasi-invariants relax independently to thermal equilibrium with the lattice. Finally, we discuss the temperature dependence of the characteristic relaxation times in terms of local and long-range cooperative molecular dynamics.

Modeling Internal Chain Dynamics in Nematogens of Different Chain Lengths: Deuteron NMR Study

Abstract Aligned liquid crystals with deuterated chains show well-resolved deuterium NMR spectra. The site specificity of these quadrupolar doublet signals allows measurements of both static and dynamic observables, which can provide information of the rotation dynamics within the flexible end chain. We use a realistic geometry and the rotational isomeric model of Flory to generate all possible configurations and a set of C-D bond orientations in the chain. A master rate equation is used to describe transitions among all allowed configurations of the chain. In this paper, we report on motional parameters derived from quantitative fittings of quadrupolar splitting and spectral density data in three different nematogens of various chain lengths. The deuterated samples are p-methoxy-d3-benzylidene-d1-p-n-butyl-d9-aniline (10.4-d13), 4-n-pentyl-dn-4‘-cyanobiphenyl-d4 (5CB-d 15) and 4-n-hexyloxy-d13-4‘-cyanobiphenyl-d8 (6OCB-d21).

Correlated chain dynamics in the nematic phase of 4-cyano-4′-hexyloxybiphenyl

Abstract We report in this paper measurements of the Zeeman and Quadrupolar spin-lattice relaxation times at two different deuteron Larmor frequencies for the nematic phase of the perdeuteriated nematogen 6OCB. A model of correlated internal motion is used to account for both the quadrupolar splittings and the spectral densities of motion. The nematic mean field is constructed using the additive potential method, while the conformational transitions among the allowed configurations are described by a master equation. For modelling the quadrupolar splittings, we used 729 conformations, although this seemed impossible when the spectral densities were fitted by minimizing the sum of squares of per cent errors. The pentane effect has, therefore, been used to limit the size of the transition rate matrix in the master equation. We found that the dynamic model for liquid crystals proposed by one of us (1991, Phys. Rev. A, 43, 4310) is essentially correct for 6OCB.

SPIN RELAXATION IN ORIENTATIONALLY ORDERED MOLECULES

Nuclear Magnetic Resonance (NMR) has been shown to be an extremely powerful technique for investigating molecular orientational order and dynamics in partially ordered systems such as thermotropic and lyotropic liquid crystals (LC) [1]. In this Chapter, nuclear spin relaxation of orientationally ordered molecules is described. In addition, theoretical models explaining NMR observables are outlined for various dynamical processes in LCs. It is known that nuclear spin-lattice relaxation rates contain information on how a nuclear spin system exchanges energy with its surrounding “lattice”, i.e., all degrees of freedom in the physical system of interest except those of the nuclear spins. Pulsed NMR provides a highly versatile tool for measuring various spin relaxation rates which can probe the entire spectrum of molecular motions in LCs. As in ordinary liquids, mesogenic molecules can reorient and translate, as well as execute internal motions if they are non-rigid. Furthermore, these molecules align preferentially in a certain direction labeled by the director n0 and possibly also arrange spatially to form various layered structures. When these molecules move collectively, the local director fluctuates both spatially and temporally. These unique motions are known as order director fluctuations (ODF). All the dynamical processes mentioned can contribute to the spin relaxation in LCs. In addition, cross relaxation due to possible couplings between different motions may also exist.

On a motional model for spin relaxation in liquid crystals

Abstract An alternative decoupled model is proposed to account for the correlated internal rotations in flexible alkyl chains of liquid crystals. Both gauche migration and gauche pair production in the chain are now taken into account. Spectral densities of motion for deuterons on various atomic sites in the chain are predicted and compared to the experimental values in the liquid crystal 4-n-hexyloxy-4′-cyanobiphenyl. Transition rates for the chain dynamics are discussed in conjunction with overall rotational diffusion constants.

Intramolecular character of the intrapair dipolar order relaxation in the methyl deuterated nematic para-azoxyanisole

Abstract The proton intra-pair dipolar order relaxation time ( T 1D ) was measured for nematic methyl-deuterated para -azoxyanisole (PAA d6 ) as a function of both the temperature and concentration in perdeuterated PAA (PAA d14 ), at 27 MHz. Since the results coincide for all measured concentrations in all the nematic temperature range, we conclude that the intermolecular contribution to the relaxation of the dipolar energy in this compound is negligible, at the studied frequency. The observed temperature dependence of T 1D is typical of the order fluctuations of the director (ODF), which clearly indicates that the ODF is the relevant mechanism producing dipolar order relaxation, in accordance with previous field cycling experiments.