Bakir A. Timimi

A deuterium nuclear magnetic resonance investigation of the symmetry and orientational order of the nematic phase of 4-[3,4,5-tris(4-dodecyloxybenzyloxy)benzoyloxy]-4′- (4-dodecyloxybenzoyloxy)-1,1′-biphenyl. A biaxial nematic?

The first compound for which the previously predicted biaxial nematic phase was claimed is 4-[3,4,5-tris(4-dodecyloxybenzyloxy)benzoyloxy]-4′-(4-dodecyloxybenzoyloxy)-1, 1′-biphenyl (I). This assignment was based on the observation of the optical texture and the x-ray diffraction pattern. To confirm this identification of the biaxial nematic phase, we have studied the deuterium nuclear magnetic resonance (NMR) spectra of nonuniformly aligned samples with the deuterons located specifically in the mesogen itself or in the disklike solute, hexamethylbenzene-d18, dissolved in the mesogen. These experiments allow us to determine the biaxiality in the partially averaged quadrupolar tensor. For both systems, the biaxiality parameter is found to be zero within the experimental error which is estimated to be less than ±0.08 for the pure mesogen and ±0.06 for the solute. The orientational order parameters determined from the quadrupolar splitting change discontinuously at the nematic-isotropic transition thus confi...

On the twist-bend nematic phase formed directly from the isotropic phase

Abstract The intriguing twist-bend nematic (NTB) phase is formed, primarily, by liquid crystal dimers having odd spacers. Typically, the phase is preceded by a nematic (N) phase via a weak first-order transition. Our aim is to obtain dimers where the NTB phase is formed directly from the isotropic (I) phase via a strong first-order phase transition. The analogy between such behaviour and that of the smectic A (SmA)–N–I sequence suggests that this new dimer will require a short spacer. This expectation is consistent with the prediction of a molecular field theory, since the decrease in the spacer length results in an increase in the molecular curvature. A vector of odd dimers based on benzoyloxybenzylidene mesogenic groups with terminal ethoxy groups has been synthesised with spacers composed of odd numbers of methylene groups. Spacers having 5, 7, 9 and 11 methylene groups are found to possess the conventional phase sequence NTB–N–I; surprisingly, for the propane spacer, the NTB phase is formed directly from the I phase. The properties of these dimers have been studied with care to ensure that the identification of the NTB phase is reliable. Graphical Abstract

Twist-bend nematic phase of the liquid crystal dimer CB7CB: orientational order and conical angle determined by 129Xe and 2H NMR spectroscopy

The liquid crystal dimer 1ʺ,7ʺ-bis(4-cyanobiphenyl-4ʹ-yl) heptane (CB7CB) has been shown to possess a nematic–nematic phase transition at 376 K. The phase below this temperature has been identified as a globally uniaxial twist-bend nematic phase, NTB. Within the temperature range 376–388 K, a classic uniaxial nematic phase, N, appears. The NTB phase has a helical structure and the liquid crystal director, n, is tilted with respect to the helix axis; these are formed into a conglomerate of degenerate domains having opposite handedness. In a magnetic field, the helix axis orients along the field. In the present studies, the properties of CB7CB are investigated using 129Xe NMR spectroscopy of dissolved xenon and 2H NMR of CB7CB-d4 and the probe 4-octyl-4ʹ-cyanobiphenyl-d2 also dissolved in CB7CB. In a uniaxial liquid crystalline environment, the 129Xe shielding tensor is cylindrically symmetric due to the deformation of the electron cloud by anisotropic forces associated with the director. The anisotropic part of the shielding tensor depends upon the orientational order parameter of the liquid crystal with respect to the applied magnetic field and the conical (aka tilt) angle of the director. The temperature dependence of the orientational order parameter and of the conical angle has been determined independently from 129Xe and previous 2H NMR experiments. In the NTB phase, the averaged Saupe ordering matrix contains three off-diagonal elements. The temperature dependence of one of these, resulting from the phase chirality, has been determined from the 2H quadrupolar splittings.

The dynamics of field-induced director reorientation for a nematic phase comprised of flexible molecules

The effect of rotating the nematic phase of 4-n-pentyl-4′-cyanobiphenyl (5CB) at a frequency ωr about an axis perpendicular to an applied magnetic field has been studied using deuterium NMR spectroscopy. The alignment of the directors produced by the competing magnetic and rotational torques was monitored by the effect produced on Δνi, the deuterium quadrupolar splittings at all five alkyl chain and one biphenyl core sites in 5CB. The director orientation was found to be independent of the site monitored. The change in the director orientation produced by stopping the rotation of a sample was followed by recording deuterium spectra during the reorientation process. The ratios of the quadrupolar splittings, Δνi(t)/Δνi(t=0), are found to be site independent, which shows that measuring the director reorientation rates by this method will produce the same result independently of which molecular site is monitored. This contrasts with recent reports of experiments which found that monitoring director reorientat...

Twist-bend nematics, liquid crystal dimers, structure–property relations

ABSTRACT One of the current challenges in liquid crystal science is to understand the molecular factors leading to the formation of the intriguing twist-bend nematic phase (NTB) and determine its properties. During our earlier hunt for the NTB phase created on cooling directly from the isotropic phase and not the nematic phase, we had prepared 30 symmetric liquid crystal dimers. These had odd spacers and methylene links to the two mesogenic groups; desirable but clearly not essential features for the formation of the NTB. Here, we report the phases that the dimers exhibit and their transition temperatures as functions of both the lengths of the spacer and the terminal chains. In addition we describe the transitional entropies, their optical textures, the X-ray scattering patterns and the 2H NMR spectra employed in characterising the phases. All of which may lead to important properties of the twist-bend nematic phase. Graphical Abstract

A deuterium nuclear magnetic resonance investigation of field induced director dynamics in a nematic slab subject to magnetic and pulsed electric fields

Abstract Deuterium nuclear magnetic resonance (NMR) spectroscopy has been used to investigate the field-induced director dynamics in a nematic liquid crystal, 4-pentyl-d2-4′-cyanobiphenyl (5CB-d2) deuteriated in the α-position of the pentyl chain, confined between two glass plates. The NMR spectra have been measured as a function of time after turning an electric field on and off. It is demonstrated that the field-induced director dynamics in the nematic liquid crystal cells can be successfully time-resolved. In addition, it is found that the doublet NMR spectra become powder-like during the turn-on and turn-off processes. It is shown that the rotational viscosity and the diamagnetic anisotropy of 5CB-d2 can be determined from the time-resolved NMR spectra by assuming uniform alignment of the director.

Director reorientation processes in a monodomain thin nematic liquid crystal film: A deuterium NMR spectroscopy study

Deuterium nuclear magnetic resonance (NMR) spectroscopy has been used to investigate the director dynamics in the nematic liquid crystal, 4-pentyl-4′-cyanobiphenyl (5CB), confined between two glass plates and subject to magnetic and electric fields. The nematic cell was held in the NMR probe head so that the electric field, whose direction is normal to the substrate surface, makes an angle of about 45° with the magnetic field. This experimental geometry avoids the degeneracy in the field-induced alignment pathway for the director found for larger angles. A series of deuterium NMR spectra, obtained using a quadrupolar echo sequence, was acquired as a function of time. When the electric field, whose intensity is controlled so that the director makes an angle with the magnetic field is applied to the nematic film, the director moves from being parallel to the magnetic field to being at an angle with respect to the magnetic field because Δe and Δχ are both positive for 5CB. After the electric field is switc...

The surface-induced static director distribution in thin nematic liquid crystal films: A deuterium nuclear magnetic resonance spectroscopy study

We have studied the static director distribution in thin nematic liquid crystal cells with different film thicknesses and different surface anchoring strengths using a combination of deuterium nuclear magnetic resonance (NMR) spectroscopy and continuum theory. A nematic liquid crystal, 4-pentyl-d2-4′-cyanobiphenyl (5CB-d2) deuteriated in the α position of the pentyl chain, was confined between two glass plates with both weak and strong anchoring conditions; the anchoring strengths were measured by using a saturation voltage method. A series of deuterium NMR spectra, obtained using a quadrupolar echo sequence, was acquired as a function of the applied electric field, which can be used to explore the director deformation. The deuterium NMR spectra predicted by continuum theory involving the magnetic, electric, elastic, and unified surface anchoring energies are found to be in good agreement with experiment. The investigation also reveals that profiles of the director distribution for the case with a magneti...

Electric Field-Induced Alignment of the Directors in the Smectic A Phase of 4-Octyl-4′-Cyanobiphenyl. A Deuterium NMR Study

Abstract Deuterium nuclear magnetic resonance (NMR) spectroscopy has been used to investigate the electric field-induced alignment of the director of the smectic A phase of the liquid crystal, 4-α,α-d2-octyl-4′-cyanobiphenyl (8CB-d2), at 303.3K. The electric field is arranged to be orthogonal to the magnetic field. The alignment process has been investigated at different electric field strengths and the rate of director alignment was monitored by recording the deuterium NMR spectra as a function of time after the electric field was switched on. The results reveal a complex pattern of electric field-induced director alignment. At high electric field strengths a rapid process is observed in which the director switches from an orientation parallel to the magnetic field to one in which it is parallel to the electric field. An induction period is also observed in which no apparent change in director orientation occurs. This induction period becomes longer (hours in magnitude) as the electric field strength is lowered. Other, intermediate, director orientations are observed as the electric field strength is lowered further. The role of defects is invoked in trying to interpret some of the observed processes underlying the mechanism of the director alignment in the smectic A phase.

Deuterium NMR investigation of field-induced director dynamics: the role of backflow

Abstract Deuterium NMR spectroscopy has been used to investigate the director dynamics in the nematic liquid crystal, 4-pentyl-4′-cyanobiphenyl, confined between two glass plates and subject to orthogonal magnetic and pulsed electric fields. When the pulsed electric field, whose intensity is strong enough to make the director align normal to the magnetic field, is applied to the nematic film, the director moves from being parallel to the magnetic field to being parallel to the electric field. After the pulsed electric field is switched off, the director relaxes back to being parallel to the magnetic field. Deuterium NMR spectra were recorded during the turn-off realignment process as a function of time. With this experimental geometry the director alignment is not unique as it can rotate equally probably clockwise or counterclockwise in this realignment process. That is, the realignment pathway for the director is degenerate, which establishes a director flow pattern. We have studied the time dependence of the director orientation and distribution for the turn-off process. The deuterium NMR spectra corresponding to the director dynamics in the realignment pathways were predicted by a continuum theory analysis including a time dependent viscous torque with an effective rotational viscosity.