Dielectric modes in antiferroelectric liquid crystal observed at low temperatures

A broad temperature antiferroelectric binary mixture has been investigated by means of dielectric spectroscopy. The sample was cooled down to –70°C. It was found that the sample was still in antiferroelectric phase. This is the widest antiferroelectric mixture ever seen (～170°) in which three well-separated modes have been detected at room temperature. In addition, the bias field influence on existing modes has been observed. All modes change their strengths with bias field. Results show that the fastest mode, called X mode previously, gradually disappears around –30°C. When the temperature decreases below –40°C, one can indisputably observe additional mode, faster than X mode. This mode (named as Y mode) observed for extra low temperatures is bias independent. It can be the molecular mode, connected with rotation around long molecular axis. The rotation around short molecular axis seems to be blocked in antiferroelectric packing. To calculate parameters of observed modes, Cole–Cole model was used. The parameters of Y mode are discussed in this article.     

Chirality-induced phase transitions in some liquid crystalline binary mixtures

Novel binary mixtures have been prepared between an optically active antiferroelectric liquid crystal, (S)-4-(1-methylheptyloxycarbonyl)phenyl 4'-octyloxy-biphenyl-4-carboxylate, and an optically active twin liquid crystal, (R)-3-methyladipic acid bis[4-(5-octyl-2-(pyrimidinyl)phenyl] ester, and the liquid crystalline properties investigated. The stability of each liquid crystal phase was found to decrease by mixing these two liquid crystalline materials. Furthermore, a phase diagram between these compounds showed a clear discontinuity in phase sequences. These results indicate that the liquid crystal phases are different in nature between these materials. The mixture consisting of the antiferroelectric material (40 per cent) and the twin material (60 per cent) shows an unusual liquid crystal phase, where the texture is similar to that reported for the twist grain boundary (TGB) phase. Related binary mixtures have been prepared between optically active or racemic materials, where the chirality of the system is expected to be altered systematically. The TGB phase was found to be induced only in the mixture between the optically active materials. Two kinds of effect on the appearance of the TGB phase, i.e. a strong helical structure induced by the optically active twin liquid crystal and a decrease of the smectic layer strength achieved by mixing between two types of liquid crystalline materials, are discussed.     

Synthesis and properties of ferro- and antiferroelectric esters with a chiral centre based on (S)-(+)-3-octanol

A homologous series of chiral three ring esters were synthesised and their properties were studied. The materials possess an antiferroelectric phase (SmC_A*) or/and a ferroelectric phase (SmC*), which was confirmed by observations of microscopic textures using a polarising optical microscope. The phase transition temperatures and enthalpies were checked by differential scanning calorimetry. The helical pitch was measured by a spectrophotometry method. For all cases the helix is right-handed, in the antiferroelectric phase the helical pitch increases with temperature, and in the ferroelectric phase the helical pitch is short.     

The influence of structure and concentration of cyano-terminated and terphenyl dopants on helical pitch and helical twist sense in orthoconic antiferroelectric mixtures

A bicomponent mixture in the orthoconic aniferroelectric phase, based on three-ring esters with fluorine atoms and ether group in a nonchiral chain, was doped with a variety of cyano-terminated compounds containing different numbers of phenyl rings in a rigid core and a terminal chain of different chirality. Compounds with two chiral chains were added to the basic mixture. The influence of the structure and concentration of dopant on the temperature-dependence of helical parameters, such as helical pitch and twist sense, were assessed by spectrophotometric and polarimetric methods. Long cyano-terminated compounds were found to be better than the other dopants tested for improving the usable properties of the antiferroelectric mixtures.     

Dielectric and electro-optical response of a room temperature tri-component antiferroelectric mixture

Frequency- and temperature-dependent dielectric and switching parameters of a room temperature tri-component antiferroelectric liquid crystal mixture W-287 have been determined. Dielectric, optical texture and thermodynamic studies show wide room temperature range antiferroelectric SmC*_a (?91.1°C to <–25°C) phase in addition to high temperature paraelectric SmA* (?2.6°C) and ferroelectric SmC* (?4.4°C) phases. The dielectric studies carried out in the frequency range of 1–35 MHz under planar anchoring condition of the molecules show five different relaxation modes appearing in the SmA*, SmC* and SmC*_a phases. Using Curie–Weiss law fit, ferroelectric SmC* to paraelectric SmA* transition temperature has been found to be 91.8°C. The dielectric response of SmC*_a phase exhibits unusually three relaxation modes due to collective as well as individual molecular processes in addition to phason mode in the SmC* phase and amplitudon mode in the SmA* phase. Spontaneous polarisation, switching time and rotational viscosity have also been determined. The maximum value of P_S is ?300 nC/cm², whereas viscosity is moderate. Switching time is of the order of few milli seconds.     

Optical switching and alignment of antiferroelectric liquid crystals containing an azo group

The optical properties of two kinds of photochromic antiferroelectric liquid crystal (AFLC) containing an azo group have been examined. Depending on the substituting group at the chiral centre, these AFLCs showed different photoinduced phase transitions by Ar⁺ laser light irradiation. This phase transition has led to a new form of optical switching controlled by a bias voltage. In addition, it was found that these AFLCs caused an azimuthal photoalignment effect regulated by linearly polarized Ar⁺ laser light irradiation. These phenomena were applied to image storage.     

Effect of the variation of swallow-tailed groups on the mesomorphic and electro-optical properties of chiral compounds derived from (S)-2-(6-hydroxy-2-naphthyl)propionic acid

Three homologous series of chiral swallow-tailed compounds, alkyl (S)-2-{6-[4-(4′-alkoxyphenyl)benzoyloxy]-2-naphthyl}propionates, (S)HNP(p,n,q) derived from (S)-2-(6-hydroxy-2-naphthyl)propionic acid in conjugation with a variety of swallow-tailed groups, attached to the external side of the chiral centre, have been synthesized and their mesomorphic and electro-optical properties studied. Both (S)HNP(p,1,2) and (S)HNP(p,1,3) exhibited an enantiotropic antiferroelectric SmC*_A phase. This implys that the swallow-tailed groups in the molecules favour zigzag pairing of the molecules in the smectic phase. The maximum P _S values of compounds (S)HNP(p,1,2) in the antiferroelectric phase were measured in the range 21–30 nC cm^-2; those of compounds (S)HNP(p,1,3) were in the range 15–23 nC cm^-2, indicating that these chiral compounds possess low polarity. The electro-optical response of the compounds in the antiferroelectric SmC*_A phase displayed thresholdless V-shaped switching.     

Highly tilted antiferroelectric (R) enantiomers useful for the formulation of eutectic mixtures

ABSTRACT

Novel chiral three-ring (R) enantiomers were synthesised using optically active (R)-(?)-2-octanol. Properties, such as the sequence of phases, the transition temperatures and enthalpies, were tested by a polarising optical microscope and differential scanning calorimeter. An antiferroelectric smectic phase (SmC_A*) with a direct transition from the antiferroelectric to the isotropic phase (SmC_A*-Iso) was observed for three esters with an achiral C₃F₇CH₂O(CH₂)₃O– terminal chain. Bi- and multicomponent mixtures with a broad temperature range of the antiferroelectric phase and good electro-optical properties were formulated. Helical pitch of pure esters and mixtures was measured by spectrophotometry method.     

High tilt antiferroelectric esters bearing a perfluorobutanoyloxy terminal chain: the influence of lateral fluoro substituents on mesomorphic behaviour,tilt angle,and spontaneous polarization

Novel liquid crystalline esters have been synthesized with a perfluorocarbonyloxy terminal chain and lateral fluoro substituents in order to determine their influence on mesomorphic behaviour, tilt angle and spontaneous polarization. Certain locations of lateral fluoro substituents provide for very low melting points and reasonably wide temperature ranges of the antiferroelectric phase, and can generate enhanced tilt angles of up to 45°, although spontaneous polarization is always lower than for the parent system without lateral fluoro substituents. Thus, these novel materials, particularly as components in mixtures, should be useful in high tilt antiferroelectric (orthoconic) applications.     

Ferrielectric and antiferroelectric chiral twin liquid crystals showing a stable chiral nematic phase

A homologous series of chiral twin liquid crystals possessing identical chiral moieties at both peripheral ends, i.e. optically active α,ω -bis{4-[(4′-(1-methylheptyloxycarbonyl)-4-biphenylyl)oxycarbony]phenoxy}alkanes, has been prepared and their liquid crystalline properties investigated. The homologues preferred to show the ferrielectric and/or antiferroelectric phase rather than the ferroelectric phase. With ascending central spacer length, the temperature range of the ferrielectric phase became narrow and eventually disappeared for the dodecyl homologue, suggesting that the coupling in motion and/or direction between two mesogenic parts of each twin molecule has an important effect on the stabilization of the ferrielectric phase. The octyl and dodecyl homologues showed a wide temperature range chiral nematic phase (ca. 10°C or more), so that these compounds were found to be the first examples showing antiferroelectric and/or ferrielectric phases with a broad temperature range of the chiral nematic phase. X-ray diffraction studies showed that the homologues tend to form a mono-layered structure. The formation of a relatively well defined layered structure was also indicated, which is considered to be important for generating anticlinic ordering in the antiferroelectric and ferrielectric phases. An isotropic-isotropic transition characterized by the emergence of a broad diffuse DSC peak was observed for the even-membered homologues.     

Achiral swallow-tailed materials with 'antiferroelectric-like' structure and their potential use in antiferroelectric mixtures

Achiral 'swallow-tailed' liquid crystalline materials are known to give alternating-tilt smectic C phases (S_Calt) which have structural similarities to the chiral antiferroelectric phases denoted as S*_CA. This paper describes the synthesis and characterization of three achiral branchedalkyl 4-(4'-dodecyloxybiphenyl-4-carbonyloxy)-3-fluorobenzoates. Optical microscopy and differential scanning calorimetry confirm that these materials show S_Calt and overlying S_A phases. The compounds were investigated as potential hosts which could be doped with a chiral ferroelectric liquid crystal so as to provide a viable- antiferroelectric mixture. These studies (microscopy and differential scanning calorimetry), to characterize the properties of the mixtures, show that antiferroelectric phases are induced. However, switching studies show that the antiferroelectric phases are extremely stable, a property which is almost certainly a consequence of the length of the lateral branching groups (ethyl, propyl and butyl).     

X-ray diffraction and dielectric spectroscopy studies on a partially fluorinated ferroelectric liquid crystal from the family of terphenyl esters

The fluorinated compound, (S)-4′′-(6-perfluoropentanoyoxyhexyl-1-oxy)-2′,3′-difluoro-4-(1-methylheptyloxycarbonyl)-[1,1′:4′,1′′]-terphenyl, which exhibits antiferroelectric SmC_A*, ferroelectric SmC* and paraelectric SmA* phases, has been investigated by polarising optical microscopy, differential scanning calorimetry, X-ray diffraction and frequency-dependent dielectric spectroscopy methods. X-ray studies have revealed that the layer thickness remains almost constant in the SmA* phase but within the SmC* and SmC_A* phases it decreases with decreasing temperature, a step jump being observed only at the SmA*–SmC* transition. The tilt angle in the SmC_A* phase decreases from 22.2° to 19.5°, and in the SmC* phase it decreases from 18.8° to 5.5°. Spontaneous polarisation is found to be quite high and varies between 74.1 and 118.7 nC cm^?2. The variation in ε′ and ε′′ with temperature shows a discontinuous change at the transition temperatures. Goldstone mode relaxation is only observed in the ferroelectric and antiferroelectric phases and is found to be of the Cole–Cole type. The soft mode is observed on application of a bias field near the SmC*–SmA* transition. Neither the soft mode nor the anti-phase azimuthal angle fluctuation mode is observed in SmC_A*. Rotational viscosity decreases quite rapidly with temperature but in a different manner in the ferroelectric and antiferroelectric phases. Activation energy for this process is found to be 48.14 kJ mol^?1 in the SmC* phase.     

New ferroelectric and antiferroelectric liquid crystals studied by complementary methods

Four new synthesised liquid crystalline compounds belonging to the homologous series of fluorinated biphenyl benzoate esters have been studied to compare their dielectric and electrooptic properties. Three of the studied compounds exhibited ferro- and antiferroelectric phases while one of them exhibited only one liquid crystalline phase – ferroelectric SmC*. No paraelectric phase was detected and straight transition between isotropic liquid and ferroelectric phases was observed for all studied compounds. Tilt angle for all of the studied compounds was equal to ca. 45? in the liquid crystalline phases, except temperature range close to the isotropic liquid–ferroelectric smectic phase transition. Temperature dependences of helical pitch, spontaneous polarisation and switching time have been determined. Based on XRD results, temperature dependence of the layer thickness has also been found. Only one relaxation process has been revealed in the ferroelectric as well as antiferroelectric phases, even the bias field up to 8 V/µm was applied. The dielectric and electrooptic data are discussed based on the mean-field theory predictions.     

V-shaped switching in binary mixtures of an achiral swallow-tailed material with the antiferroelectric liquid crystal (S)-MHPOBC

An achiral swallow-tailed material, 2-propylpentyl 4-(4'-decyloxybiphenyl-4-carbonyloxy)benzoate, p, showing SmA and SmC_alt phases was prepared for mixing (by weight percentage) with an antiferroelectric liquid crystal, (S)-MHPOBC, m, for the study. The binary mixture p15/m85 using (S)-MHPOBC (85%) as a host doped with achiral material (15%) resulted in a phase sequence SmA-SmC*-SmC*_A. The electro-optic response of this mixture in the ferroelectric SmC* phase displayed V-shaped switching, while that in the antiferroelectric SmC*_A phase displayed a double hysteresis switching. The mixture p85/m15 possessed SmA* and SmC*_A phases; V-shaped switching was found in the antiferroelectric SmC*_A phase of this mixture. These optical phenomena implied that a binary mixture containing a larger amount of achiral swallow-tailed material and/or possessing relatively lower polarization favours the occurrence of V-shaped switching in the antiferroelectric phase. The results of this work also suggested that thresholdless V-shaped switching in chiral smectic liquid crystals can be achieved by mixing an achiral swallow-tailed material with an antiferroelectric liquid crystal.     

V-shaped switching in binary mixtures of an achiral swallow-tailed material with the antiferroelectric liquid crystal (S)-MHPOBC

An achiral swallow-tailed material, 2-propylpentyl 4-(4′-decyloxybiphenyl-4-carbonyloxy)benzoate, p, showing SmA and SmC_alt phases was prepared for mixing (by weight percentage) with an antiferroelectric liquid crystal, (S)-MHPOBC, m, for the study. The binary mixture p/15/m85 using (S)-MHPOBC (85%) as a host doped with achiral material (15%) resulted in a phase sequence SmA-SmC*-SmC*_A. The electro-optic response of this mixture in the ferroelectric SmC* phase displayed V-shaped switching, while that in the antiferroelectric SmC*_A phase displayed a double hysteresis switching. The mixture p85/m15 possessed SmA* and SmC*_A phases; V-shaped switching was found in the antiferroelectric SmC*_A phase of this mixture. These optical phenomena implied that a binary mixture containing a larger amount of achiral swallow-tailed material and/or possessing relatively lower polarization favours the occurrence of V-shaped switching in the antiferroelectric phase. The results of this work also suggested that thresholdless V-shaped switching in chiral smectic liquid crystals can be achieved by mixing an achiral swallow-tailed material with an antiferroelectric liquid crystal.     

Investigation of thermal behavior of Heliotropium indicum L. lyophilized extract by TG and DSC

Thermogravimetric (TG) techniques and differential scanning calorimetry (DSC) used for the study of pre-formulation or drug–adjuvant compatibility have been gaining importance in Brazil. These techniques are being used for the verification of possible interactions between drugs and adjuvants. Aiming at studying the behavior of a plant extract and its mixture with adjuvants, using these thermoanalytical techniques the plant species Heliotropium indicum L. was used. This plant which is originally from India and has been well acclimatized in Brazil has healing and anti-inflammatory properties. The methodology for obtaining the extract followed the Brazilian Pharmacopoeia methodology. And the incorporation of the extract with adjuvants was through binary mixtures (1:1 w/w). The TG and DSC curves were obtained under nitrogen atmosphere (25 mL min^?1) at a heating rate of 5 °C min^?1; TG tests were analyzed within a temperature range from 25 to 600 °C and DSC from 25 to 300 °C. The TG curves show good thermal stability of the extract and its mixtures with adjuvants up to 150 °C, except the propylene glycol (PLG). The DSC curves revealed an incompatibility of the extract with methylparaben and PLG mixture.     

Novel sulphur-containing banana-shaped liquid crystal molecules

In this preliminary report we present the mesomorphic properties and electro-optic investigations of three new 'banana-shaped' mesogens. The materials are structurally similar to Niori's the original bent-core materials but possess alkylthio substituents in place of alkoxy substituents. Microscopic investigations revealed an unusual 'spiral-domain texture', similar to that observed in the 'B₇' phase of related materials. Further investigations support our earlier observations that the materials display antiferroelectric switching in this phase, in two of the new materials studied. The nature of this phase, in comparison with the 'switchable' (B₂) and 'chiral-domain' (B₄) phases of the original materials, is discussed.     

Development and validation of TLC–densitometric method for simultaneous determination of two binary antihypertensive mixtures containing felodipine in fixed dose combinations

A new densitometric thin‐layer chromatographic method has been developed for simultaneous determination of two binary mixtures containing felodipine in combination with either metoprolol (mixture I) or ramipril (mixture II). The two mixtures were quantitatively separated on 60 F254 silica gel plates using toluene–ethyl acetate–methanol–ammonia as mobile phase with UV detection at 233 and 229 nm for mixtures I and II, respectively. The studied drugs were satisfactorily resolved with retention factor (R_f) values of 0.34 ± 0.03 and 0.65 ± 0.03 for metoprolol and felodipine, respectively, in mixture I and 0.35 ± 0.03 and 0.74 ± 0.03 for ramipril and felodipine, respectively, in mixture II. Linearity ranges were 2000–7000 and 200–700 ng/band for metoprolol and felodipine, respectively, in mixture I and 1500–4000 ng/band for both ramipril and felodipine in mixture II. Correlation coefficient (r) values were 0.9968 for both metoprolol and felodipine in mixture I and 0.9993 for ramipril and 0.9989 for felodipine in mixture II. The method has been validated according to International Conference on Harmonization guidelines and has been successfully applied for determination of the studied drugs in their dosage forms without interference from commonly encountered excipients. Copyright © 2015 John Wiley & Sons, Ltd.     

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.     

Synthesis and liquid-crystalline properties of a homologous series of antiferroelectric liquid crystals and their respective racemates

New chiral, as well as racemic 1-methylalkyl terephthalidene-bis-aminocinnamates have been synthesized and their liquid-crystalline properties investigated. Generally, the chiral compounds possess the antiferroelectric S*_o phase, whereas in the homologous series of racemates, the appearance of the S_o phase depends on the length of the alkyl chain. For racemic compounds derived from 2-alkanols with an odd number of carbon atoms, a S_o phase is generally observed. On the other hand when the number of carbon atoms is even, a S_c phase occurs. Moreover, in racemic 1-methylbutyl terephthalidene-bis-aminocinnamate, a S_c-S_o-S_c reentrant phase sequence has been established.