A possible structural model of the SmC*beta phase

The so-called chiral smectic C-beta (SmC*beta) phase has been reported as distinct from the SmC* phase in several materials that exhibit antiferroelectric liquid crystal mesophases and subphases. The SmC*beta phase is known to be chiral, tilted and to exhibit ferrielectric switching, but no structure had been suggested which explains these effects. This paper presents a possible structure for the SmC*beta phase which can explain the ferrielectric properties. The model is proposed on the basis of complementary optical, electric and X-ray diffraction studies of a chiral liquid crystal. The layer spacing, optical and steric tilt and spontaneous polarization over the temperature range of the SmC*beta phase are described. The complementary experimental techniques used reveal the occurrence of inversion phenomena, on which this model is based.     

New ferroelectric liquid crystalline materials containing one and two lactate groups attached to the molecular core

Several new liquid crystalline materials containing one, two or three chiral centres and having one or two lactate groups in the molecular core have been synthesized. Most of the materials show the blue phase, chiral nematic phase, paraelectric smectic A phase and orthogonal hexatic smectic B phase; some possess the ferroelectric SmC* phase. A study of the mesomorphic properties has been performed using differential scanning calorimetry, optical microscopy and X-ray diffraction. The thickness of the smectic layers and the value of the average distance between the long axes of neighbouring molecules were determined. In the SmC* phase, the temperature dependence of spontaneous polarization, spontaneous tilt angle and helical pitch was measured. The influence of the number of lactate groups on mesogenic behaviour has been established.     

Influence of hydrogen bonding on phase abundance in ferroelectric liquid crystals

The synthesis and characterization of five hydrogen-bonded ferroelectric liquid crystal complexes (HBFLCs) prepared from mesogenic p-n-alkoxy benzoic acids and non-mesogenic propionic/butyric acids with different chiral centres are reported. Complementary intermolecular hydrogen bonding is confirmed through IR study. HBFLCs are found to exhibit chiral nematic (N*), smectic C* (SmC*) and smectic G* (monotropic) phases in their cooling profiles during polarizing thermal microscopy and differential scanning calorimetry. Phase coexistence regions are observed above the IN* transition. The chiral nematic to smectic C* transition is found to be of first order. The temperature variation of spontaneous polarization exhibited by these HBFLC complexes in their SmC* phase is presented. The effect of non-covalent interaction imparted by the soft hydrogen bonding in these LC complexes on enhanced or induced thermal stability of tilted LC phases is discussed.     

Influence of hydrogen bonding on phase abundance in ferroelectric liquid crystals

The synthesis and characterization of five hydrogen-bonded ferroelectric liquid crystal complexes (HBFLCs) prepared from mesogenic p-n-alkoxy benzoic acids and non-mesogenic propionic/butyric acids with different chiral centres are reported. Complementary intermolecular hydrogen bonding is confirmed through IR study. HBFLCs are found to exhibit chiral nematic (N*), smectic C* (SmC*) and smectic G* (monotropic) phases in their cooling profiles during polarizing thermal microscopy and differential scanning calorimetry. Phase coexistence regions are observed above the IN* transition. The chiral nematic to smectic C* transition is found to be of first order. The temperature variation of spontaneous polarization exhibited by these HBFLC complexes in their SmC* phase is presented. The effect of non-covalent interaction imparted by the soft hydrogen bonding in these LC complexes on enhanced or induced thermal stability of tilted LC phases is discussed.     

Evidence for a tilted smectic phase in Anopore membranes by dielectric spectroscopy

Collective relaxation processes are completely undetectable in a ferroelectric liquid crystal confined in porous Anopore membranes, as a result of perfect orientation of the smectic layers perpendicular both to the long axis of the pores and the direction of the measuring electric field. In the ferroelectric liquid crystal – Anopore composite only one relaxation process, assigned to rotation of the molecule around the molecular short axis, appears throughout all smectic phases. The temperature dependence of the relaxation frequency and of the dielectric strength of this process also shows no irregularity at the point of polarization sign reversal. The temperature dependence of the relaxation frequency follows the Arrhenius law with an activation energy slightly higher in the ferroelectric SmC* phase. Analysis of the non‐linear changes of temperature dependence of the dielectric strength at the SmA–SmC* phase transition enables one to obtain the temperature dependence of the tilt angle of the molecules in the SmC* phase in the Anopore membrane. Dielectric measurements confirm the existence of the tilted smectic phase in Anopore cylindrical channels with no tilt anomaly at the point of polarization sign reversal.     

Current topics in smectic liquid crystal research.

Interest in the smectic liquid-crystalline state of matter received a substantial boost with the discovery by Meyer in the mid-1970s that a chiral smectic C (SmC*) phase exhibits a spontaneous electric polarization, and with the subsequent demonstration by Clark and Lagerwall of the surface-stabilized SmC* ferroelectric liquid crystal at the beginning of the 1980s. Since then, chiral smectic phases and their plethora of polar effects have dominated the research in this field, which today has reached a mature state where the first commercial microdisplay applications are now shipping in millions-per-year quantities. In this Review we discuss some of the topics of highest interest in current smectic liquid crystal research, and address application-relevant research (de Vries-type tilting transitions without defect generation and high-tilt antiferroelectric liquid crystals with perfect dark state) as well as more curiosity-driven research (the nature and origin of the chiral smectic C subphases and their intermediate frustrated states between ferro- and antiferroelectricity).     

Evidence for a tilted smectic phase in Anopore membranes by dielectric spectroscopy

Collective relaxation processes are completely undetectable in a ferroelectric liquid crystal confined in porous Anopore membranes, as a result of perfect orientation of the smectic layers perpendicular both to the long axis of the pores and the direction of the measuring electric field. In the ferroelectric liquid crystal - Anopore composite only one relaxation process, assigned to rotation of the molecule around the molecular short axis, appears throughout all smectic phases. The temperature dependence of the relaxation frequency and of the dielectric strength of this process also shows no irregularity at the point of polarization sign reversal. The temperature dependence of the relaxation frequency follows the Arrhenius law with an activation energy slightly higher in the ferroelectric SmC* phase. Analysis of the non-linear changes of temperature dependence of the dielectric strength at the SmA-SmC* phase transition enables one to obtain the temperature dependence of the tilt angle of the molecules in the SmC* phase in the Anopore membrane. Dielectric measurements confirm the existence of the tilted smectic phase in Anopore cylindrical channels with no tilt anomaly at the point of polarization sign reversal.     

Dielectric, viscosity, and pitch measurements of a ferroelectric liquid crystal exhibiting the phase sequence smectic M-smectic C

A chiral liquid crystal compound exhibiting the ferroelectric smectic C phase and the recently discovered ferroelectric smectic M phase has been studied by measurements of the Goldstone-mode relaxation frequency and dielectric strength, the spontaneous polarization, the tilt angle and the helical pitch. The data allow the determination of the Goldstone-mode rotational viscosity and the pitch controlling elastic constant. The results indicate that the smectic M phase is characterized by a larger molecular order within the smectic layers compared to the smectic C phase confirming the assumption of a tilted hexatic structure for the smectic M phase.     

CdSe quantum dots in chiral smectic C matrix: experimental evidence of smectic layer distortion by small and wide angle X-ray scattering and subsequent effect on electro-optical parameters

CdSe quantum dots (QDs) dispersed ferroelectric liquid crystal (FLC) has been subjected to small and wide-angle X-ray scattering and atomic force microscopy to understand the molecular organization in chiral smectic C (SmC*) phase. SAXS indicates that the presence of QDs causes enhancement in the smectic layer separation. The smectic order parameter for neat FLC and FLC–QDs mixtures is obtained in the range of 0.6 to 0.85. Both smectic order parameter and structural tilt are found to be lesser for FLC–QDs mixtures as compared to neat FLC. The insertion of QDs in SmC* matrix causes localized smectic layer distortion in such a way that spontaneous polarization remains almost the same but the electro-optic switching of molecules becomes faster. We have outlined the superiority of FLC–QDs mixtures for electrical energy storage and their suitability in electronic devices.     

Direct transition from the SmA phase to the tilted hexatic phase in liquid crystals with several lactate units

Compounds with differing numbers of lactate units in the chiral part were synthesized. For all materials, at least two smectic phases were found. In addition to the SmA, the SmC* and/or the tilted hexatic SmI*(F*) phase appear according to the length of the non-chiral alkyl chain. For the shortest non-chiral chain, a direct transition from the SmA phase to the SmI*(F*) phase has been discovered and studied. For compounds with the 2-(S)-methylbutyl alkyl chain and two lactate units in the chiral part the antiferroelectric SmC*_A phase occurs. The ferroelectric character of the hexatic phase has been confirmed even just below the SmC*_A phase.     

Dielectric,viscosity, and pitch measurements of a ferroelectric liquid crystal exhibiting the phase sequence smectic M–smectic C

Abstract

A chiral liquid crystal compound exhibiting the ferroelectric smectic C phase and the recently discovered ferroelectric smectic M phase has been studied by measurements of the Goldstone-mode relaxation frequency and dielectric strength, the spontaneous polarization, the tilt angle and the helical pitch. The data allow the determination of the Goldstone-mode rotational viscosity and the pitch controlling elastic constant. The results indicate that the smectic M phase is characterized by a larger molecular order within the smectic layers compared to the smectic C phase confirming the assumption of a tilted hexatic structure for the smectic M phase.     

Microscopic origin of spontaneous polarization in ferroelectric SC* liquid crystals

The origin of spontaneous polarization in the ferroelectric smectic C* phase is investigated within a mean-field microscopic model which describes the coupling between the tilt of molecules from the normal to the smectic layers and the rotation of a molecule around its long axis. The mean-field potential is studied which takes into account a chiral polar and a non-chiral quadrupolar biasing of the rotation of molecules around the molecular long axes. Each molecule is characterized by three transverse molecular axes: the chiral axis which turns parallel to the macroscopic C₂ axis at small tilts, the polar axis in the direction of the transverse dipole moment and the quadrupolar axis which tends to be parallel to the C₂ axis at very large tilts. A numerical analysis of the model shows that there are four different types of spontaneous polarization dependent on the temperature, including the sign-reversal type. The influence of three microscopic parameters, i.e. two angles between the three characteristic axes and the relative strength of the chiral versus the non-chiral biasing, on the type of spontaneous polarization is investigated. The relationship between the microscopic and the equivalent Landau model is established and discussed.     

Direct transition from the SmA phase to the tilted hexatic phase in liquid crystals with several lactate units

Compounds with differing numbers of lactate units in the chiral part were synthesized. For all materials, at least two smectic phases were found. In addition to the SmA, the SmC* and/or the tilted hexatic SmI*(F*) phase appear according to the length of the non-chiral alkyl chain. For the shortest non-chiral chain, a direct transition from the SmA phase to the SmI*(F*) phase has been discovered and studied. For compounds with the 2-(S)-methylbutyl alkyl chain and two lactate units in the chiral part the antiferroelectric SmC*_A phase occurs. The ferroelectric character of the hexatic phase has been confirmed even just below the SmC*_A phase.     

New series of chiral ferroelectric liquid crystals with the keto group attached to the molecule core

Two new series of ferroelectric liquid crystals with a keto group attached to the molecular core and a lactate unit as a chiral centre were synthesized and studied. All the compounds exhibit the paraelectric SmA phase and the ferroelectric SmC * phase over very wide temperature ranges and down to room temperature. The main characteristics of the chiral ferroelectric SmC * phase, namely spontaneous polarization, spontaneous tilt angle, and dielectric constant, have been measured to establish their relationship to the length of the terminal chains. The influence of the chiral and non-chiral chain lengths on the helix pitch length is also discussed.     

New series of chiral ferroelectric liquid crystals with the keto group attached to the molecule core

Two new series of ferroelectric liquid crystals with a keto group attached to the molecular core and a lactate unit as a chiral centre were synthesized and studied. All the compounds exhibit the paraelectric SmA phase and the ferroelectric SmC* phase over very wide temperature ranges and down to room temperature. The main characteristics of the chiral ferroelectric SmC* phase, namely spontaneous polarization, spontaneous tilt angle, and dielectric constant, have been measured to establish their relationship to the length of the terminal chains. The influence of the chiral and non-chiral chain lengths on the helix pitch length is also discussed.     

Generation of frustrated liquid crystal phases by mixing an achiral nematic-smectic-C mesogen with an antiferroelectric chiral smectic liquid crystal

By mixing the achiral liquid crystal HOAB, exhibiting a nematic (N)-smectic-C (SmC) mesophase sequence, with the chiral antiferroelectric liquid crystal (AFLC) (S,S)-M7BBM7, forming the antiferroelectric SmC(a)(*) phase, at least seven different mesophases have been induced which neither component forms on its own: a twist-grain-boundary (TGB(*)) phase, two or three blue phases, the untilted SmA(*) phase, as well as all three chiral smectic-C-type "subphases," SmC(alpha)(*), SmC(beta)(*), and SmC(gamma)(*). The nature of the induced phases and the transitions between them were determined by means of optical and electro-optical investigations, dielectric spectroscopy, and differential scanning calorimetry. The induced phases can to a large extent be understood as a result of frustration, TGB(*) at the border between nematic and smectic, the subphases between syn and anticlinic tilted smectic organization. X ray scattering experiments reveal that the smectic layer spacing as well as the degree of smectic order is relatively constant in the whole mixture composition range in which AFLC behavior prevails, whereas both these parameters rapidly decrease as the amount of HOAB is increased to such an extent that no other smectic-C-type phase than SmC/SmC(*) exists. By tailoring the composition we are able to produce liquid crystal mixtures exhibiting unusual phase sequences, e.g., with a direct isotropic-SmC(a)(*) transition or a temperature range of the SmC(beta)(*) subphase of about 50 K.     

Effect of spontaneous polarization on smectic C*-smectic A* phase transition temperature and the thickness dependence of the spontaneous polarization of ferroelectric liquid crystal

The influence of spontaneous polarization on the smectic C*-smectic A* phase transition was studied theoretically by means of Landau theory. calculations that the transition temperature from a non-chiral to chiral smectic C phase is shifted due to the effect of bilinear and biquadratic couplings. A qualitative relationship has also been derived between spontaneous polarization and cell thickness which supports the experimental observations of the increase of spontaneous polarization with the increase of cell thickness for surface-stabilized ferroelectric liquid crystals. It was observed from the theoretical     

Twin‐like domains in chiral smectic C liquid crystals

In the chiral smectic C phase of liquid crystals with the phase transition N*–SmC*, texture development depending on the sample thickness is reported. In very thin samples, domains of rectangular‐like shape are observed. As two possible tilts of smectic layers are possible for one anchoring direction, smectic layers inside a domain, called twin‐like domains, are tilted with respect to layers in outer regions, similarly to crystalline planes in solid crystalline twins. An elastic model of such a twin domain is proposed and its energy determined.