Sarabjot Kaur

The Nematic Phases of Bent-Core Liquid Crystals

Over the last ten years, the nematic phases of liquid crystals formed from bent-core structures have provoked considerable research because of their remarkable properties. This Minireview summarises some recent measurements of the physical properties of these systems, as well as describing some new data. We concentrate on oxadiazole-based materials as exemplars of this class of nematogens, but also describe some other bent-core systems. The influence of molecular structure on the stability of the nematic phase is described, together with progress in reducing the nematic transition temperatures by modifications to the molecular structure. The physical properties of bent-core nematic materials have proven difficult to study, but patterns are emerging regarding their optical and dielectric properties. Recent breakthroughs in understanding the elastic and flexoelectric behaviour are summarised. Finally, some exemplars of unusual electric field behaviour are described.

The influence of structure on the elastic, optical and dielectric properties of nematic phases formed from bent-core molecules

The physical properties of the nematic phases formed by four bent-core oxadiazole based materials are reported. In particular, the splay (K11), twist (K22) and bend (K33) elastic constants, the birefringence and the dielectric anisotropy of the materials are described and the effect of chain length and the presence of fluoro-substituents at the outer phenylene group of the aromatic core structure on these parameters is determined. The birefringence and order parameter are found to be independent of the modification of molecular structure. The dielectric anisotropy is quite strongly dependent on molecular structure; the fluoro-substituted material has the largest magnitude of dielectric anisotropy while the alkyl-substituted compound has the smallest. Changes in the molecular length and fluoro-substitution in the bent-core materials are found to have little influence on the splay, twist and bend elastic constants at equivalent reduced temperatures. However, the material substituted with an alkyl terminal chain exhibits both smaller elastic constants and a less marked dependence on temperature than the alkoxy-substituted compounds. A possible insight into the behaviour of the elastic constants relevant to the formation of the dark conglomerate phase, which underlies the nematic phase in one of the compounds studied, is suggested by following the analysis proposed by Berreman and Meiboom. Importantly, using molecular field theory and atomistic modelling, we calculate elastic constants that are in excellent agreement with the experimental values. Our conclusion that the elasticity in the nematic phase formed from bent-core molecules is not strongly influenced by changes to the terminal chains or the presence of fluoro-substituents at the outer phenylene group of the aromatic core structure is in agreement with our previous work showing that the dominant parameter is the bend angle.

Novel switching mode in a vertically aligned liquid crystal contact lens

Liquid crystal (LC) contact lenses are emerging as an exciting technology for vision correction. A homeotropically (vertical) aligned LC lens is reported that offers improved optical quality and simplified construction techniques over previously reported LC contact lens designs. The lens has no polarization dependence in the off state and produces a continuous change in optical power of up to 2.00 ± 0.25 D with a voltage applied. The variation in optical power results from the voltage-induced change in refractive index of the nematic LC layer, from 1.52 to a maximum of 1.72. One device substrate is treated with an alignment layer that is a mixture of planar and homeotropic polyimides, rubbed to induce a preferred director orientation in the switched state. Defects that could occur during switching are thus avoided and the lens exhibits excellent optical quality with a continuous variation in focal power.

Graphene electrodes for adaptive liquid crystal contact lenses.

The superlatives of graphene cover a whole range of properties: electrical, chemical, mechanical, thermal and others. These special properties earn graphene a place in current or future applications. Here we demonstrate one such application - adaptive contact lenses based on liquid crystals, where simultaneously the high electrical conductivity, transparency, flexibility and elasticity of graphene are being utilised. In our devices graphene is used as a transparent conductive coating on curved PMMA substrates. The adaptive lenses provide a + 0.7 D change in optical power with an applied voltage of 7.1 Vrms - perfect to correct presbyopia, the age-related condition that limits the near focus ability of the eye.

Flexoelectricity in an oxadiazole bent-core nematic liquid crystal

We have determined experimentally the magnitude of the difference in the splay and bend flexoelectric coefficients, |e 1 − e 3|, of an oxadiazole bent-core liquid crystal by measuring the critical voltage for the formation of flexodomains together with their wave number. The coefficient |e 1 − e 3| is found to be a factor of 2–3 times higher than in most conventional calamitic nematic liquid crystals, varying from 8 pCm−1 to 20 pCm−1 across the ∼60 K—wide nematic regime. We have also calculated the individual flexoelectric coefficients e 1 and e 3, with the dipolar and quadrupolar contributions of the bent-core liquid crystal by combining density functional theory calculations with a molecular field approach and atomistic modelling. Interestingly, the magnitude of the bend flexoelectric coefficient is found to be rather small, in contrast to common expectations for bent-core molecules. The calculations are in excellent agreement with the experimental values, offering an insight into how molecular parameters contribute to the flexoelectric coefficients and illustrating a huge potential for the prediction of flexoelectric behaviour in bent-core liquid crystals.

Dielectric spectroscopy of polymer stabilised ferroelectric liquid crystals

Dielectric measurements were carried out in the frequency range from 20Hz to 500kHz on Polymer Stabilised Ferroelectric Liquid Crystals (PSFLCs). Polymerisation in the Smectic A* (Sm A*) and the Smectic C* (Sm C*) phase at equal polymer concentration results in a dielectric strength which is nearly twice the value in the latter case. An increase of the polymer concentration results in a decrease of the dielectric strength and an increase in relaxation frequency. The textural morphology and transmission intensity due to the residual birefringence of the polymer network in the isotropic phase, revealed a correlation between the interactions of the liquid crystal molecules with the polymer network. Results for polymerising in the tilted Sm C* phase with a large bias field are also reported which show that the structure of the phase in which the system was polymerised affects the dielectric properties. The observed differences in dielectric behaviour can be explained by the polymer network morphology formed due to the interplay of phase and temperature on the stabilised ferroelectric liquid crystal materials.

Second-harmonic generation and the influence of flexoelectricity in the nematic phases of bent-core oxadiazoles

ABSTRACT Second-harmonic generation (SHG) in the nematic phase of bent-core oxadiazole-based liquid crystals (LCs) was studied and compared to that for the rod-like compound 4-cyano-4ʹ-n-octylbiphenyl (8CB). Weak, isotropically scattered second-harmonic (SH) light was observed for all materials, consistent with SHG by nematic director fluctuations. The SH intensity produced by the bent-core materials was found to be up to ~ 3.4 times that of 8CB. We discuss this result in terms of the dependence of SH intensity on temperature, elastic constants and flexoelectric coefficients. We have calculated the latter by using a molecular field approach with atomistic modelling, thus demonstrating how molecular parameters contribute to the flexoelectric coefficients and illustrating the potential of this method for predicting the flexoelectric behaviour of bent-core LCs. We show that the increased SH signal in the bent-core compounds is partly due to their nematic phases being at a much higher temperature, and also potentially due to them having greater flexoelectric coefficients, up to ~1.5 times those of 8CB. These estimates are consistent with reports of increased flexoelectric coefficients in bent-core compounds in comparison to rod-like compounds. GRAPHICAL ABSTRACT

Observing the emergence of phase biaxiality in a polar smectic A system via polarised Raman spectroscopy

We report polarised Raman spectroscopy, and optical and dielectric properties of an asymmetric bent-core compound derived from 3-hydroxybenzoic acid with a long terminal chain at one end and a nitro group at the other. Earlier X-ray scattering experiments on the compound suggested a partial bilayer smectic A phase (SmAd) and a partial bilayer biaxial antiferroelectric smectic A phase (SmAdPA) in the material. The dielectric behaviour, the microscopic textures and conoscopy experiments all explicitly show that the compound exhibits two different phases, with the lower temperature phase biaxial in nature. Raman spectroscopy was used to determine the temperature evolution of the uniaxial order parameters 〈P2〉 and 〈P4〉, deduced from the analysis of the depolarisation ratio, informed by modelling the bent-core structure. Anomalously low values were measured (less than 0.5 and 0.15 respectively) which could suggest that the smectic A phase may be de Vries like in nature, rather than a partial bilayer structure. Raman spectroscopy was also used to investigate the biaxial nature of the SmAdPA phase. The effect that the biaxial order parameters 〈P220〉, 〈P420〉 and 〈P440〉 have on the depolarisation ratio is calculated. By making the assumption of an approximately continuous increase in the 〈P2〉 and 〈P4〉 order parameters, it was possible to deduce the behaviour of the biaxial order parameters in the biaxial SmAdPA phase; the emergence of biaxial order in the system is clearly demonstrated as all of the biaxial order parameters increase in magnitude as the temperature decreases in the (SmAdPA) phase. The dielectric studies show that the perpendicular component of the dielectric permittivity increases from 10 to 70 in the SmAd phase and decreases from 70 to 45 in the SmAdPA phase. A strongly temperature dependent relaxation frequency with a large value of ∼400 kHz is observed in the SmAd phase. On the other hand, the SmAdPA phase exhibits a weakly temperature dependent relaxation frequency at ∼100 kHz.

Electronic contact lenses may replace reading glasses

A leading problem facing modern optometry is correcting presbyopia, the reduction of near-visual acuity arising from the natural deterioration of the eyes’ accommodation mechanism. It affects everyone over age 45. This deterioration is thought to arise from a reduction in the crystalline lens’ flexibility that renders the aging eye unable to provide the additional focal power for near-vision tasks. The most common form of correction for presbyopia is wearing a pair of reading glasses for near-vision tasks. Contact lens correction of presbyopia is more problematic due to the fundamental restrictions caused by various forms of optical compromise associated with simultaneous vision devices. This results in reduced vision quality and user satisfaction.1 Currently, correcting presbyopia with contact lenses relies on a concept known as simultaneous vision, with a single contact lens possessing multiple regions of different corrective power. Simultaneous vision contact lenses provide both focused and unfocused images on the retina. The brain’s visual system is required to select the correct image and ignore out-of-focus information. This process can result in reduced visual acuity and contrast sensitivity and is a major area of contact lens research and development. We are researching a new correction method that uses electro-active liquid-crystal lenses and avoids the visual compromises associated with current technology.2, 3 Liquid-crystal lenses can be turned on and off electronically and show great promise for recreating the accommodation mechanism of the crystalline lens. Liquid crystals are a state of matter between a solid and a liquid, and are best known for their use in flat-screen displays. Liquid crystals generally form from long, rodlike molecules that exhibit some degree of orientational order, but with no or limited positional order. The long molecules tend to point in the same direction, be it dictated by surfaces or applied Figure 1. A prototype liquid-crystal contact lens. Voltage is applied via connections wired to a flat ring around the optical region in the center.