Orthoconic Antiferroelectric Liquid Crystals Research Articles

Synthesis and Characterization of New Chiral Smectic Four-Ring Esters.

Orthoconic antiferroelectric liquid crystals (OAFLCs) represent unique self-organized materials with significant potential for applications in photonic devices due to their sub-microsecond switching times and high optical contrast in electro-optical effects. However, almost all known OALFCs suffer from low chemical stability and short helical pitch values. This paper presents the synthesis and study results of two chiral AFLCs, featuring a four-ring structure in the rigid core and high chemical stability. The mesomorphic properties of these compounds were investigated using polarizing optical microscopy and differential scanning calorimetry. Spectrometry and electro-optical studies were employed to estimate the helical pitch, tilt angle, and spontaneous polarization of the synthesized compounds and the prepared mixtures. All studied compounds exhibit enantiotropic chiral smectic mesophases including the SmA*, the SmC*, and a very broad temperature range of the SmCA* phase. Doping top-modern antiferroelectric mixture with synthesized compounds offers benefits such as increased helical pitch and tilt angle values without significantly influencing spontaneous polarization. This allows the prepared mixture to be regarded as an OAFLC with high optical contrast, characterized by an almost perfect dark state. These valuable physicochemical and optical properties suggest significant potential of studied materials for practical applications.

High tilted antiferroelectric liquid crystals: Polymer-based approach for phase stabilisation and device development

On doping orthoconic antiferroelectric liquid crystal (OAFLC) mixtures with a small concentration of reactive mesogens, new liquid crystalline material was engineered with improved opto-electronic performance. In-situ photo polymerisation of the mixtures resulted in the creation of a polymer network that helped removal of the metastability bought on by F-states (Synclinic Ferroelectric State) in tristate switching devices. These metastable states causes the switching timings of the device to be asymmetrical, with the fall time being significantly longer than the rise time due to the difficulty in relaxing from the Ferroelectric-synclinic to the Antiferroelectric-anticlinic state. The opto-electronic parameters of the device cells were measured before and after the mixtures were stabilised by Ultraviolet light (UV) polymerisation. Polymer stabilisation of the orthoconic state yielded symmetric optical rise and fall times, accompanied by a reduction in the electrical response times. Spontaneous polarisation in the OAFLC mixtures were found to decrease after UV polymerisation; however, this is an important trade-off vis-à-vis symmetric optical response times, considering the overall performance of the material from an application standpoint.

The comparison of self-assembling behaviour of phenyl biphenylcarboxylate and biphenyl benzoate compounds with the different length and shape of chiral terminal chain

Our recent advances on polymer-stabilised orthoconic antiferroelectric liquid crystals (PSOAFLCs) eliminated the asymmetry of switching times in electro-optical effect based on surface stabilized OAFLCs and paved the way for their effective use in many photonic and optoelectronic devices requiring very fast switching. To fabricate new modifications of PSOAFLCs, besides bi- and tetrafunctional monomers, multicomponent AFLCs mixtures with a very low meting point, high stability of the antiferroelectric phase at lower temperatures and higher helical pitch length are desirable. Herein, a very convenient synthetic method of new AFLC compounds with a phenyl biphenylcarboxylate structure of the rigid core with different length and shape of terminal chains is reported. The resulting mesomorphic properties and helical parameters are compared with those of analogues with a biphenyl benzoate structure of the rigid core. The designed AFLC compounds clearly possess definite advantages. Specifically, they exhibit lower meting points, higher clearing points, broader temperature range of the antiferroelectric phase (SmCA*) and much longer helical pitch length, especially for analogues with the shortest achiral terminal chain, but unexpectedly lower tendency to give the direct transition between the antiferroelectric and isotropic phase. The introduction of the cycloalkyl ring at the end of the chiral terminal chain increases the melting points, flattens the temperature dependence of the helical pitch and changes the sense of the helical twist, but also enhances the ability of the direct SmCA*-Iso phase transition, regardless the type of the rigid core. The obtained results highlight a possibility of utilizing compounds with the phenyl biphenylcarboxylate structure of the rigid core, designed under this work, together with previously prepared compounds with the biphenyl benzoate rigid core to formulate multicomponent OALFC mixtures with a very low melting point and a very high helical pitch length.

Modification in different physical parameters of orthoconic antiferroelectric liquid crystal mixture via the dispersion of hexanethiol capped silver nanoparticles

Herein we investigate an orthoconic anti ferroelectric liquid crystal (OAFLC) material, W1000 and its mixtures with the dispersion of hexanethiol capped silver nanoparticles (Ag NPs) in three different concentrations of 0.1 wt%, 0.25 wt% and 0.50 wt%. The pristine AFLC and its mixtures are characterized by the dielectric, electro-optical and spectroscopic techniques. Various parameters viz. dielectric permittivity, electrical conductivity, spontaneous polarization, rotational viscosity, response time, anchoring energy coefficients, and photoluminescence of the pristine AFLC are altered due to the addition of Ag NPs. The polarizing optical micrographs show modification in the molecular distribution of the host AFLC after the addition of Ag NPs. The spontaneous polarization of the Ag NPs dispersed system was reduced due to the anti-parallel correlation amid dopant and AFLC dipoles as a consequence the dielectric permittivity of the Ag NPs-AFLC mixtures has also been decreased. The electrical conductivity of the pristine AFLC has increased after the dispersion of Ag NPs. Faster switching response of Ag NPs dispersed system comparing to that of the pristine AFLC is manifested by the decrement in the rotational viscosity. The anchoring energy coefficients were found to be reduced for the Ag NPs dispersed system. The photoluminescence (PL) intensity of the pristine AFLC material has been enhanced after the dispersion of Ag NPs which strongly depends upon the dopant concentration. Faster switching response, enhanced electrical conductivity and PL Intensity of the Ag NPs-AFLC mixtures are the remarkable findings of this study.

Computational, dielectric and electro-optical analysis of an orthoconic antiferroelectric mesogen having superstructure in its reduced symmetry phase

The study presents a comprehensive analysis of a novel octan-2-yl 4'-(4-{4-[3-(butanoyloxy)propoxy]benzoyloxy}benzoyloxy)-[1,1'-biphenyl]-4-carboxylate (abb. as OAFLC-108), an orthoconic antiferroelectric liquid crystal. Its paramorphic sequence is estimated from defect studies. Its optimised structure, interatomic charge distribution and dynamics, global parameters and nonlinear optical properties are calculated by density functional theory. Vibrational modes are determined experimentally and theoretically. Low-frequency dielectric spectra are studied in view of molecular fluctuations, activation energy and relaxation time. The moderate value (29.5 nC/cm2) of spontaneous polarisation and a response time of 2 ms support the candidature of OAFLC-108 as a competitive optoelectronic material.

Ultra-performance supercritical fluid chromatography: A powerful tool for the enantioseparation of thermotropic fluorinated liquid crystals

A fast and simple supercritical fluid chromatography method for the enantioseparation of twenty newly synthesized orthoconic antiferroelectric liquid crystals is reported for the first time. The effects of alkoxy spacer length and fluorine atom presence and position in the phenyl ring on chromatographic behavior were investigated. Baseline enantioseparation of all compounds was achieved using simple mobile phases consisting of carbon dioxide and alcohol as cosolvent on (3,5-dimethylphenylcarbamate) derivative of amylose as chiral stationary phase. The analysis times ranged from 2 to 4 and from 4 to 7 min for most samples when using methanol and propane-2-ol, respectively. The significant effect of cosolvent type on the enantioseparation of these compounds was assessed and partial complementarity of methanol and propane-2-ol was observed.

Physical properties of new binary antiferroelectric liquid crystal mixtures

ABSTRACTThree newly prepared binary mixtures exhibiting chiral tilted smectic phases have been studied using differential scanning calorimetry, dielectric spectroscopy and electro-optic method, as well as X-ray diffraction. Broad temperature range of ferroelectric and antiferroelectric phases was detected in these mixtures and temperature dependence of spontaneous polarization, tilt angle and switching time were measured for all of them. It's occurred that all of the studied mixtures are orthoconic antiferroelectric liquid crystals. Based on the X-ray diffraction results, the temperature dependence of layer thickness in the paraelectric, ferroelectric and antiferroelectric phases was found. By using dielectric spectroscopy, Goldstone mode was identified in the ferroelectric phase, while antiphase fluctuations of azimuthal angle have been found in the antiferroelectric phase. Based on the results of the complementary methods, the transition temperatures were found as well as the order of the para-ferroelectric phase transition was determined as non-continuous one with critical parameter β equal to ca. 0.25.

Highly tilted liquid crystalline materials possessing a direct phase transition from antiferroelectric to isotropic phase

Pure compounds and multicomponent mixtures with a broad temperature range of high tilted liquid crystalline antiferroelectric phase and a direct phase transition from antiferroelectric to isotropic phase, were obtained. X-ray diffraction analysis confirms these kinds of materials form a high tilted anticlinic phase, with a fixed layer spacing and very weak dependency upon temperature, after the transition from the isotropic phase. Due to this, not only pure orthoconic antiferroelectric liquid crystals but also those with a moderate tilt should generate a good dark state. Furthermore, due to the increased potential for forming anticlinic forces, such materials could minimize a commonly observed asymmetry of a rise and fall switching times at a surface stabilized geometry.

Preparation and Study of the Electro-optical Properties of Binary Mixtures of Orthoconic Anti-ferroelectric Esters and Achiral Phenyl Pyrimidine Liquid Crystal

We have prepared three different binary systems by mixing an achiral low viscous phenyl pyrimidine liquid crystalline SmC compound with three different orthoconic antiferroelectric analogous of (S)MHPOBC exhibiting SmA*-SmC*-SmCA* phase sequences. In these binary mixtures, ferroelectric and antiferroelectric phases were induced and their mesomorphic parameters (spontaneous polarization, response time, effective torsional viscosity, and anchoring energy coefficient) were determined by thermo-microscopic and electro-optical study. All the physical parameters show a nonadditive behavior with respect to the concentration of chiral mesogens due to strong dipole-dipole and/or intermolecular interactions between polar fluorinated chiral esters with heterocyclic phenyl pyrimidine compound. Interestingly the achiral mesogen reduces the response time of these binary mixtures at the cost of lower Ps. Therefore, these mixtures can be useful for high speed technological applications where low polarization, long pitch, and very fast switching ferro or anti-ferroelectric liquid crystal mixtures are desirable.

Electro-optical properties of a new series of fluorinated antiferroelectric orthoconic liquid crystalline esters

The electro-optical properties of orthoconic antiferroelectric liquid crystals of some analogues of (S)‑MHPOBC ((S)‑4‑(1‑methylheptyloxy)carbonylphenyl 4'-octyloxy-4-biphenylcarboxylate) have been investigated and discussed in terms of the structure property correlations of liquid crystalline molecules. Polarisation field reversal technique has been used for their characterisation. In this study the occurrence of such a high value of spontaneous polarisation (Ps) for the investigated compound is especially noteworthy as they comprise a bulky chiral unit, zigzag ester linkage and polar fluorinated terminal. Lengthening of oligomethylene spacer in the terminal chain and placement of fluorine atom in the molecular core structure leads to reduction in the value of Ps. The Ps data suggest that the nature of the chiral smectic C (SmC*) to smectic A (SmA) phase transition is of second order and the critical exponent β is in the range 0.19–0.34. The ‘V-shaped’ mode or thresholdless switching in such high-polarisation ferroelectric liquid crystals has also been discussed.

Helix parameters in bi- and multicomponent mixtures composed of orthoconic antiferroelectric liquid crystals with three ring molecular core

Eight types of bicomponent systems composed of antiferroelectric compounds with different polarity of achiral chain and different temperature dependence of helical pitch as well as three multicomponent mixtures composed of antiferroelectric compounds with opposite helical twist sense were studied. The phase miscibility was tested by polarising optical microscopy. The results of the helical pitch and helical twist sense measurements obtained by a spectrophotometric method of selectively reflected light and by polarimetric method, respectively, are presented. It was found that it is possible to control helical pitch length and temperature of helix twist inversion in antiferroelectric mixtures by controlling the ratio of compounds with left- and right-handed helix in such mixtures, although all of them have the same chiral terminal chain.

Orthoconic antiferroelectric liquid crystals

The ubiquitous liquid crystal display (LCD) is based on nematic liquid crystals (LCs) and has during 40 years developed from simple few digit displays into high-resolution flat-panel displays. A last and very important step towards the present TVs was the combination with thin film transistors that also led to the introduction of several new switching modes. Despite the enormous success of this technology, there is presently a renewed interest in LCs with faster electrooptic response, especially for future 3D vision display systems and possibly for field-sequential-colour (FSC) generation displays. Here, I focus on the so-called orthoconic antiferroelectric LCs (OAFLCs), which can provide the fast switching of conventional antiferroelectric LCs, but combined with a potentially much higher optical contrast. The reason is that the dark state of orthconic materials has a homeotropic optic axis, which makes the extinction ideally complete between crossed polarisers and independent of in-plane alignment fluctuations characteristic of all antiferroelectric LCs. The basic features and device physics issues of orthoconic LCs are reviewed and the most important remaining challenges to be met in terms of materials development are discussed in this article. Furthermore, a few examples of new applications made possible with the use of present and future orthoconic materials are given.

Efficient one-step novel synthesis of ZnO nanospikes to nanoflakes doped OAFLCs (W-182) host: Optical and dielectric response

We are reporting a one-step solvo-chemical to hydrothermal technique under high temperature and pressures with varying concentration. Without exchange of any chemical compounds compared to the previous treatment, we obtained a significant novel uniform nanoflakes synthetic approach. The proportionality ratio of solvent, reaction time and temperature, leads to new prominent growth of ZnO nanoflakes structure having diameter of the order of 180–185nm through hydrothermal approach. A comparative study with pure and doped orthoconic antiferroelectric liquid crystals (W-182) revealed that, the optical and electrical behaviors drastically differed. This fabrication strategy mainly nano doped technique are phenomenologically unique and has promising applications to dc switching devices. Additionally, the uniform growth mechanism of these nano objects doped OAFLCs have been clarified.

The orthoconic antiferroelectric smectic liquid crystals and their engineering by doping with homo- and heterostructural compounds

The attempt of the engineering of modern liquid crystalline orhoconic antiferroelectric smectic material OAFLC with remarkable optical properties is reported. The high tilted anticlinic antiferroelectric parent mixtures were formulated and studied. Subsequently those parent mixtures were doped with homo- and heterostructural dopants with selected properties including a molecular shape, molecular dipole moment μ value and orientation and molecular conformation. The effects of the doping are described. The doping affecting a single phase parameter is reported. Examples of functional dopants are presented and their influence on the physical and structural properties of doped mixture is shown.

Electro-optic and dielectric investigations of a perfluorinated compound showing orthoconic antiferroelectric liquid crystal

We observed microsecond order response ranging from 100 to 600μs with respect to temperature for a single component fluorinated rigid core antiferroelectric liquid crystals having highly tilted phases. In the antiferroelectric region, we observed V-shaped electro-optic switching in a 4μm indium tin oxide coated homogeneously aligned cell having PS, ranging from 100 to 200nC/cm2. Dielectric studies have revealed an interesting phase that exists during heating in the temperature region between 74°C and 84°C. From dielectric results studied we calculated activation energies associated with different relaxation modes using Arrhenius law.

34.3: Fast Orthoconic Antiferroelectric Liquid Crystals for Field‐Sequential Color Applications

AbstractOrthoconic antiferroelectric liquid crystals (OAFLCs) allow fast (submillisecond) in‐plane switching with a wide‐angle dark state insensitive to misalignment. In this paper we demonstrate RGB field‐sequential‐color using OAFLCs at a pixel switching rate of 720Hz, i.e., a “frame rate” of 240Hz.

Precise Dielectric Spectroscopy of a Long Pitch Orthoconic Antiferroelectric Working Mixture

W-1000 orthoconic antiferroelectric liquid crystal eutectic mixture was prepared in Military University of Technology. This mixture is probably the broadest antiferroelectric mixture, ever prepared. Precise dielectric measurements were performed in a broad temperature range. Some parameters of detected modes and their temperature characteristics are found. It seems that in different temperature regions of orthoconic antiferroelectric liquid crystal different numbers of modes should be anticipated to fit experimental results. Results are discussed with respect to the theoretical model.

Influence of a helical pitch on orthoconic antiferroelectric liquid crystals performance

AbstractOrthoconic antiferroelectric liquid crystals, whose half cone angle is 45°, are of interest in display technology because they are capable of providing greyscale in passively addressing waveform while exhibiting very good contrast. However, orthoconic antiferroelectrics having short pitch are difficult to be aligned, this makes surface stabilized alignment unfeasible. In this case, dynamic behaviour is usually poor, avoiding electronic addressing with standard electronics. In this work, we report the helical pitch measurements of OAFLC mixtures and we compare electrooptical performance of high and low pitch OAFLCs.

Dielectric relaxation spectroscopy and electro-optical studies of a new, partially fluorinated orthoconic antiferroelectric liquid crystal material exhibiting V-shaped switching

In this paper we report the results from detailed electro-optical and dielectric studies in various antiferroelectric and ferroelectric phases of an orthoconic antiferroelectric liquid crystal (OAFLC) material . The material possesses high tilt and high spontaneous polarisation. Such an OAFLC, because of its high tilt, provides an excellent dark state. The material exhibits V-shaped switching in the SmC* phase. Dielectric studies reveal the existence of another phase during heating in the range between 78.6 and 92°C which did not appear in the DSC curve and in polarising microscopy. This phase has been identified as the SmCγ* phase and is extremely sensitive with respect to the cell conditions, aligning material, purity, etc. Three dielectric modes have been assigned in the above-mentioned temperature range and their origins are discussed.

Molecular Structures and the Induction of the Orthoconic Properties in Antiferroelectric Liquid Crystals

Smectic Antiferroelectric Liquid Crystals (AFLC) could offer simpler solutions for high demanding applications than commonly known Ferroelectric Liquid Crystals (FLCs). Among all known AFLCs those exhibiting extremely high molecular tilt, reaching 45°, called Orthoconic AFLCs (OAFLCs), seems to be most promising for display and photonic applications. OAFLCs are still rather rare kind of AFLC phase and are most frequently exhibited by compounds belonging to homologous series of smectogens incorporating terphenyl, biphenyl benzoate and phenyl biphenylate molecular rigid cores with a fluorinated aliphatic terminal chain. Such parameters like the molecular shape and the molecular dipole moment obtained by using of the computer aided molecular modelling promoting the OAFLC phase induction are presented and discussed.