Ferroelectric Liquid Crystalline Compounds Research Articles

Self-Assembling Behavior of Smart Nanocomposite System: Ferroelectric Liquid Crystal Confined by Stretched Porous Polyethylene Film.

The control and prediction of soft systems exhibiting self-organization behavior can be realized by different means but still remains a highlighted task. Novel advanced nanocomposite system has been designed by filling of a stretched porous polyethylene (PE) film with pore dimensions of hundreds of nanometers by chiral ferroelectric liquid crystalline (LC) compound possessing polar self-assembling behavior. Lactic acid derivative exhibiting the paraelectric orthogonal smectic A* and the ferroelectric tilted smectic C* phases over a broad temperature range is used as a self-assembling compound. The morphology of nanocomposite film has been checked by Atomic Force Microscopy (AFM). The designed nanocomposite has been studied by polarizing optical microscopy (POM), differential scanning calorimetry (DSC), small and wide-angle X-ray scattering and broadband dielectric spectroscopy. The effect of a porous PE confinement on self-assembling, structural, and dielectric behavior of the chiral LC compound has been established and discussed. While the mesomorphic and structural properties of the nanocomposite are found not to be much influenced in comparison to that of a pure LC compound, the polar properties have been toughly suppressed by the specific confinement. Nevertheless, the electro-optic switching was clearly observed under applied electric field of low frequency (210 V, 19 Hz). The dielectric spectroscopy and X-ray results reveal that the helical structure of the ferroelectric liquid crystal inside the PE matrix is completely unwound, and the molecules are aligned along stretching direction. Obtained results demonstrate possibilities of using stretched porous polyolefins as promising matrices for the design of new nanocomposites.

Design and electro-optic investigations of de Vries chiral smectic liquid crystals for exhibiting broad temperature ranges of SmA* and SmC* phases and fast electro-optic switching

Two new ferroelectric liquid crystalline compounds using 5-phenyl-pyrimidine core terminated by achiral trisiloxane chain, and chiral (S) alkyl chain on the opposite ends of their respective mesogens are designed with better characteristics of switching speed and broader temperature range.

Dielectric Properties of Chiral Ferroelectric Liquid Crystalline Compounds with Three Aromatic Rings Connected by Ester Groups

The tilted ferroelectric SmC* phase of three structurally different series having three aromatic rings in the core structure connected by ester groups with different end alkyl chain lengths, all of which are derived from lactic acid, have been observed by broadband dielectric spectroscopy. Introduction of structural variations within the liquid crystalline compounds has led to the formation of chiral nematic N*, or the paraelectric orthogonal SmA* phase at higher temperatures. The dielectric spectra strongly depend both on the temperature as well as the specific molecular structure of the self-assembling compounds possessing the ferroelectric polar order. The results reveal a strong Goldstone mode in the ferroelectric SmC* phase with ~kHz relaxation frequency. In the SmC* phase, the real and imaginary parts of the complex permittivity increase up to certain temperature near the SmC*-N*/SmA* transition and then decrease with increasing temperature, perhaps due to the disruption of the molecular domains at the onset of the SmA*/N* phase transition. The dielectric strength attains a maximum value in the SmC* phase and then decreases near the SmA*/N* phase transition. The dielectric strength is also influenced by the lengths of the alkyl chain and the nature of the connecting unit of the constituent molecules. The relaxation time and the relaxation frequency are found to vary with the molecular structure of the studied ferroelectric compounds.

Effect of molecular structure on dielectric and electro-optic properties of chiral liquid crystals based on lactic acid derivatives

Self-assembling behaviour of a few lactic acid derivatives with several ester linkage groups in the molecular core has been studied. The molecular structure–physical property correlations has been discussed in the light of the temperature dependences of the static permittivity, dielectric anisotropy, spontaneous polarization, response time and torsional bulk viscosity for eight chiral ferroelectric liquid crystalline compounds. Depending on the molecular structure these compounds exhibit the chiral nematic (N*) phase, the paraelectric orthogonal smectic A* (SmA*) phase and the ferroelectric tilted smectic C*(SmC*) phase being thermally stable over relatively broad temperature range. The temperature behaviour of the spontaneous polarization clearly confirms the second order nature of the SmA*-SmC* phase transition and the first order nature of the N*-SmC* phase transition. The effect of temperature and molecular configuration on the polarization and on the dispersion anchoring energy coefficients has been established. The obtained results are discussed in order to contribute to the structure – property relationship for the specific class of soft organic materials which can be used for design of smart multifunctional liquid crystalline mixtures aimed for optoelectronic and photonic applications.

Physical properties of new binary ferroelectric mixture

Physical properties of new binary mixture of two ferroelectric liquid crystalline compounds, regarding the properties of the components, were studied. It turned out that both components and the mixture exhibit the ferroelectric smectic C* phase in a wide temperature range. Moreover, the calorimetric, texture observations as well as dielectric studies revealed an additional monotropic liquid crystalline phase for one of the mixture component, which was identified, based on X–ray diffraction results, as tilted one with hexagonal order within the layers. Dielectric spectroscopy has shown the strong low frequency relaxation process in the SmC* identified as related to Goldstone mode based on the mean–field model. Additionally, the molecular reorientation around the short axis was recognized in the monotropic hexatic phase. The temperature dependencies of the spontaneous polarization and switching time for new binary mixture and its components were compared and analyzed in terms for possible applications.

Dielectric response of ferroelectric liquid crystals in samples of finite thickness

The structure in real planar samples of ferroelectric liquid crystals as well as the collective modes characterizing these structures, are strongly affected by surface anchoring of molecules. This is reflected in dielectric properties, which become significantly dependent on the sample thickness. The dielectric strength and the relaxation frequency of the characteristic modes (pinned Goldstone mode for the thick samples and twisted mode for the thin ones) are determined and found that for both types of structures the dielectric strength increases and the relaxation frequency decreases with increasing the sample thickness. We proved theoretically predicted thickness dependences of various physical parameters for new ferroelectric liquid crystalline compound. We concluded the experimental results are in accordance with the theory taking into account the real structure of the samples.

Anomalous photovoltaic effect based on molecular chirality: influence of enantiomeric purity on the photocurrent response in π-conjugated ferroelectric liquid crystals.

(S)- and (R)-forms of chiral π-conjugated ferroelectric liquid crystals were synthesized. The dielectric properties in the ferroelectric liquid-crystalline (FLC) phases were evaluated using the Sawyer-Tower method. Spontaneous polarization of (S)-1 reached 68 nC cm-2 at 127 °C. Hole mobilities in the FLC phases estimated using the time-of-flight (TOF) method were on the order of 10-4 cm2 V-1 s-1. Each chiral π-conjugated compound exhibited a photovoltaic effect based on spontaneous polarization without p-n or Schottky junctions. This phenomenon could be attributed to an anomalous photovoltaic (APV) effect that has been observed in ferroelectric ceramics. In addition, liquid-crystalline enantiomeric mixtures of (S)-1 and (R)-1 were prepared and the APV response under UV illumination was studied. The APV response was enhanced with an increase in enantiomeric purity and was minimized in the racemic mixture. From this result, it was concluded that the APV effect in this FLC compound originated from the molecular chirality.

ChemInform Abstract: Syntheses and Properties of Ferroelectric Liquid Crystalline Compounds Having a Chiral Center Directly Connected to the Core Aromatic Ring

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Antiferroelectric dielectric relaxation processes and the interlayer interaction in antiferroelectric liquid crystals

Antiphase relaxation process of antiferroelectric phase is investigated in binary mixtures of antiferroelectric and ferroelectric liquid crystalline compounds. It is found that the frequency of the antiphase mode ranges from 2to4MHz, being one of the fastest collective processes reported so far. This can be exploited in a photonic switch with a submicrosecond response time. However, the frequency of the antiphase mode decreases by the mixing ratio of the ferroelectric compound. The decrease in the frequency is shown to be due to a reduction in the elastic constant in between the adjacent layers.

V-shaped electro-optic response observed in a chiral ferroelectric smectic liquid crystal

We report on the observation of V-shaped switching in a ferroelectric liquid crystal cell over a wide range of temperatures. Results of the optical transmittance in the visible region give us the helical pitch for various temperatures of the ferroelectric liquid crystalline compound used. We show that the helical pitch, in addition to the spontaneous polarization (PS) and thickness of the alignment layer of the cell, is an important factor in giving V-shaped switching. A longer or helical compensated mixture gives a better V-shaped switching.

Effect of lateral substitution by fluorine and bromine atoms in ferroelectric liquid crystalline materials containing a 2‐alkoxypropanoate unit

Two homologous series of ferroelectric liquid crystalline compounds with 2‐alkoxypropanoate chiral unit containing biphenyl benzoate core laterally substituted by fluorine and bromine have been synthesized and studied. All compounds possess the ferroelectric smectic C* phase over a broad temperature range. For bromine‐substituted compounds values of spontaneous polarization reach high values up to 250 nC cm−2. The effects of the lateral substitution on the phenyl ring far from the chiral centre by methyl and methoxy groups, fluorine, chlorine and bromine atoms on mesomorphic properties and on values of the spontaneous polarization are discussed.

Optical microscopy, DSC and dielectric relaxation spectroscopy studies on a partially fluorinated ferroelectric liquid crystalline compound MHPO(13F)BC

Phase transitions in a partially fluorinated compound, MHPO(13F)BC, have been studied by optical microscopy, calorimetry and miscibility methods. Spontaneous polarization (PS), helical pitch measurements, and frequency-domain dielectric study, have also been carried out. These studies indicate the presence of an intermediate alpha sub-phase between a ferroelectric and a paraelectric phase. Both the Goldstone-mode (GM) and the soft-mode (SM) relaxation processes have been observed, and by fitting the spectra to the Cole–Cole function, values of dielectric strengths, critical frequencies and dispersion parameters have been determined. Anomalies have been observed in the temperature dependence of PS and inverse dielectric strength, which are interpreted as a signature of a phase between the SmC* and SmA* phases. Microscopic patterns, helical pitch measurements, and miscibility studies give further evidence in support of this inference.

Dielectric spectroscopy of de Vries-type smectic-A*–smectic-C*transitions

We report results of dielectric investigations on a number of ferroelectric liquid crystalline (FLC) compounds with different degrees of layer shrinkage in the smectic-A(*) -smectic-C(*) phase transitions. With a decreasing extent of layer shrinkage the investigated FLCs exhibit a significantly increasing soft-mode absorption, decreasing leading Landau coefficient alpha, and a considerably broader mean-field regime. We explain these tendencies by the fact that the low layer shrinkage materials come closer to the diffuse cone model of de Vries than to the common model of rigid rods which maintain their orientational order during tilting. In the case of the diffuse cone model the tilt-angle fluctuations are decoupled from variations of the smectic layer spacing, which explains the observed behavior.

Ellipsometric monitoring of molecular evolution in freely suspended films of M12/10 ferroelectric liquid crystal

This work is devoted to the ellipsometric investigation of the evolution of the molecular order in free standing films of the ferroelectric liquid crystalline compound M12/10 after its alignment by electric field. Creation of pronounced oscillations of anisotropic properties of films was found and monitored over a scale of hours. These oscillations are explained as the mutual precession of molecular directors.

New SmC* V-shaped switching FLC compounds

In a previous publication [1] we showed that chiral aryl esters of 4-alkyl-3-chlorobiphenyl-4-carboxylic acids form smectic C phases at low temperatures and over wide temperature ranges. Continuing our interest in ferroelectric liquid crystalline compounds we have synthesized new aryl esters of 4-(6-alkylcyclohex-2-enon-3-yl)benzoic acids and investigated their properties.

Synthesis and Mesomorphic Properties of Naphthyl Propionate Ferroelectric Liquid Crystals

Ferroelectric liquid crystalline compounds containing (2 S )-2-(6-hydroxy-2-naphthyl)propionic acid moiety were synthesized from ( S )-naproxen ( 1 ). Reactions of ( S )-naproxen chloride with alkanols (C 4 -C 6 ) in the presence of propylene oxide gave chiral esters 4 . Demethylation of 4 with dimethylsulfide-AlCl 3 produced naphthols 5 . Esterfication of naphthols 5 with 4-(4-alkoxyphenyl)benzoic acid 6 using 1,3-dicyclohexylcarbodiimide and 4-dimethylaminopyridine afforded alkyl ( S )-2-{6-[4-(4'-alkoxyphenyl)benzoyloxy]-2-naphthyl} propionates 7 in good yields. Naphthyl propionates 9 having a fluorine substituent on the biphenyl core were similarily prepared. HPLC analysis using chiral column confirmed that racemizations did not occur in all processes. All of the optically pure naphthyl propionate compounds 7 and 9 showed an enantiotropic mesomorphic phase. Some derivatives of 7 and 9 with a straight terminal chain attached to the external side of the chiral center exhibited blue phase (BP) and/or twist grain boundary (TGB A ) phase.

Optically Active Mesogenic Dispiro[2.0.2.1]Heptane Derivatives and their Unusual Physical Properties

Physical properties, i.e. phase transition temperatures, response times, Ps values, tilt angles, helical twisting powers and helical twisting senses of a series of novel ferroelectric liquid crystalline compounds with non-substituted and halogen-substituted dispiro[2.0.2.1]heptyl moieties in their side chains have been measured. When mixed with an achiral SmC base mixture, the resulting SmC* mixtures containing the novel compounds showed ferroelectricity, and the novel compounds were estimated as useful dopants for ferroelectric liquid crystal mixtures. Further unusual properties were observed with the combinations of the two diastereomeric novel compounds, for example (1S,3R,4S)-4a and (1S,3R,4R)-5a. They showed opposite signs for the Ps and the same sense for the N* helical twisting, which clearly suggests that the direction of the Ps and helical sense of N* must be derived from different origins.

Synthesis and Liquid Crystalline Properties of (S)-[4-n-Alkyloxy-benzoyloxyphenyl]-4′-[(2-n-alkyloxy)propionyloxy]benzoate

New series of chiral liquid crystalline compounds, (S)-[4-n-alkyloxy-benzoyloxyphenyl]-4′-[(2-n-alkyloxy)propionyloxy]benzoate (QMn/m), were synthesised. Polymorphism of the new compounds was investigated by polarising microscopy and differential scanning calorimetry. All members of the homologous series showed enantiotropic SmC* and N* mesophases except one compound having the longest alkyl chain in both terminal groups. The spontaneous polarisation of these ferroelectric liquid crystalline compounds is about 110–130 nC/cm2. The temperature dependence of the dielectric permittivity was measured for all the compounds studied. Several ferroelectric binary mixtures were prepared in order to get a wide enantiotropic chiral SmC* phase at ambient temperature.

Synthesis and Properties of Optically Active Dispiro[2.0.2.1]heptane Derivatives as Novel Ferroelectric Liquid Crystalline Compounds

Enantioselective enzymatic acylation, using Lipase PS® (Pseudomonas sp., immobilized on Celite, Amano Pharmaceutical Co., Ltd.), of endo-dihalosubstituted and nonsubstituted dispiro[2.0.2.1]heptylmethanol and endo-4-methylenespiropentylmethanol provided the corresponding optically active compounds in high enantiomeric excesses (> 95% ee). Difluorocarbene addition onto the optically active endo-4-methylenespiropentylmethanol yielded the first enantiomerically pure difluorodispiro[2.0.2.1]heptylmethanols. The obtained optically active dispiro[2.0.2.1]heptane derivatives were used in syntheses of phenylpyrimidine derivatives, providing novel ferroelectric liquid crystalline compounds with unique physical properties.

Spontaneous polarization, tilt angle and dielectric measurements on a ferroelectric liquid crystal

A new ferroelectric liquid crystalline compound 4-(p-butyloxy-2-chloro-tyrosinate)-2-hydroxy-benzylidene-4'-nonylaniline, abbreviated as BCTHBN, was characterized by various techniques. Phase sequence and transition temperature data were obtained by thermal microscopy and DSC. The values of spontaneous polarization, tilt angle, response times and viscosity, along with the wide thermal range of the SmC* phase, make this compound a candidate for applicational studies. Dielectric studies included variation of capacitance with temperature and calculation of activation energies in the soft and Goldstone modes.