Ewa Gorecka

Antiferroelectric Chiral Smectic Phases Responsible for the Trislable Switching in MHPOBC

At least two antiferroelectric liquid crystalline phases were discovered in MHPOBC. These phases appear below the usual ferroelectric Sm C* phase. Because of the alternation of the molecular tilt directions as well as the dipole orientations in successive layers, the optic axis is along the layer normal. This strong stabilization along the layer normal brings about the so-called third stable state responsible for the tristable switching. The antiferroelectric structure was strongly supported by selective reflections in oblique incidence; a full-pitch band does not appear in the antiferroelectric phases, while it does appear in the ferroelectric phase.

Molecular Orientational Structures in Ferroelectric, Ferrielectric and Antiferroelectric Smectic Liquid Crystal Phases as Studied by Conoscope Observation

Molecular orientational structures in MHPOBC were studied by means of conoscope observation. Contrary to a ferroelectric response of the conoscope to an electric field in the smectic C* phase, the conoscopic figure in the antiferroelectric smectic CA* phase does not shift its center and is biaxial with its optic plane perpendicular to the field direction. This conoscope change is only due to a dielectric contribution, indicating the existence of the inherent threshold in the electric field induced transition to the ferroelectric phase. In between the ferroelectric and antiferroelectric phases, a kind of ferrielectric phase was clearly distinguished in the field response of the conoscopic figure; the shift of the conoscope center perpendicular to the applied field and the biaxial optic plane parallel to the field suggest a novel molecular orientational structure.

Why do non-symmetric dimers intercalate? The synthesis and characterisation of the α-(4-benzylidene-substituted-aniline-4′-oxy)-ω-(2-methylbutyl-4′-(4″-phenyl)benzoateoxy)alkanes

Ten new non-symmetric liquid crystal dimers belonging to the family of compounds α-(4-benzylidene-substituted-aniline-4′-oxy)-ω-(2-methylbutyl-4′-(4″-phenyl)benzoateoxy)-alkanes have been synthesised and their transitional properties characterised. The dimers contain either a hexamethylene or octamethylene spacer, while the terminal substituents on the 4-benzylideneaniline fragment are H, CH3, F, Cl and Br. The unsubstituted dimers are not liquid crystalline, while the remaining compounds exhibit enantiotropic nematic behaviour. The trends in the clearing temperatures, according to the chemical nature of the terminal substituent, are largely consistent with those established for conventional low molar mass liquid crystals. Three of the dimers also exhibit an intercalated smectic A phase, specifically the two bromo-substituted dimers and the chloro-substituted dimer containing a hexamethylene spacer. The driving force for the formation of this phase is considered to be, at least in part, the specific anisotropic interaction between the unlike mesogenic units. The absence of smectic behaviour for the isosteric methyl-substituted dimers reveals that steric factors alone cannot stabilise the intercalated smectic A phase.

Liquid-crystalline phases made of gold nanoparticles.

Spontaneous formation of smectic and columnar structures was observed when spherical gold nanoparticles were functionalized with mesogenic thiols (see layered structure and X-ray pattern of a sample in smectic phase). The particle ordering is stimulated by softening of the interparticle potential and flexibility for deformation of the grafting layer.

Switchable columnar phases

Disclike molecules form various columnar phases. Extensive efforts have been made to make these phases polar. In the course of these attempts, two columnar phases, in which dipoles along the column ferroelectrically switch, were discovered. In this Highlight article, we summarize the attempts to realize polar columnar phases and then introduce the two molecular systems possessing switchable columnar phases.

Ideal Liquid Crystal Display Mode Using Achiral Banana-Shaped Liquid Crystals

We have demonstrated a totally new liquid crystal display (LCD) mode using a smectic A-like phase of banana-shaped molecules. An in-plane electric field was applied to homeotropically aligned cells, resulting in fast polarization reorientation and the associated birefringence. The reported LCD mode has all the advantages of the existing LCD modes, such as vertical alignment (VA), in-plane switching (IPS), ferroelectric LC (FLC) or antiferroelectric LC (AFLC) and V-shaped switching (VS) modes; namely, fast response of the order of 100 µs, high contrast ratio (3000:1), wide viewing angle, continuous gray level, and small threshold voltage. These performances originate from the cooperative motion of bent molecules with quasi-long-range order of dipoles based on a two-dimensional Langevin process.

Nematic phase formed by banana-shaped molecules

Most mesogens formed by banana-shaped molecules exhibit liquid-like smectic phases. Here we present the synthesis and properties of homologous series of derivatives of resorcinol and 2,6-disubstituted pyridine. One of these classes of bent-core molecules displays the nematic phase.

Do the short helices exist in the nematic TB phase

Dimeric compounds forming twist-bend nematic, Ntb, phase show unusual optical textures related to the formation of arrays of focal conic defects (FCDs). Some of the focal conics exhibit submicron internal structure with 8 nm periodicity, which is very close to that found in the crystalline phase of the material, that might suggest surface freezing.

Dynamically self-assembled silver nanoparticles as a thermally tunable metamaterial

The availability of metamaterials with properties that can be actively tuned is crucial for the future development of various metamaterial-based technologies. Here we show that by using silver nanoparticles equipped with a thermally responsive organic coating a metamaterial is obtained with reversibly switchable properties. The material investigated exhibits dynamic self-assembly resulting from temperature-dependent changes of organic coating shape, which translates to a switchable spatial distribution of the silver nanoparticles. This in turn strongly influences the optical properties of the entire material. The measured optical characteristics of the material are in excellent agreement with theoretical calculations, which allow us to use the latter to predict a dynamically tunable epsilon-near-zero behaviour of the metamaterial. The suggested methodology opens new routes for tunable metamaterials that operate in the visible region and will enable various applications for soft-matter-based optical devices.

Electric-Field-Induced Transitions among Antiferroelectric, Ferrielectric and Ferroelectric Phases in a Chiral Smectic MHPOBC

A T-E phase diagram of MHPOBC (C8H17O–--COO–-COO*CH(CH3)C6H13) has been obtained by means of the temperature (T) dependence of the dielectric constant under a dc biased electric (E) field. It is presumed from the T-E phase diagram that there is a temperature range where the field-induced antiferroelectric-ferrielectric-ferroelectric phase transitions take place. These transitions have been confirmed as five transmittance states between crossed Nicols under a triangular wave, i.e., ferro-ferri-antiferro-ferri-ferro. The field-induced antiferroelectric-ferrielectric phase transition has also been identified through conoscopic observation.