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.     

New ferroelectric liquid crystalline materials with an azo group in the molecular core

Six series of new liquid crystalline materials with an azo group (N=N) located in different parts of the mesogenic core of the molecule have been synthesized and their physical properties studied. The chiral segments of these materials are based on alkoxypropionate or alkyllactate units. It has been found that lateral methyl substitution on a phenyl ring in the molecular core disturbs the packing of the molecules. As a result smectic phases disappear and the phase transition temperatures decrease. In addition, shifting the N=N group closer to the chiral centre of the molecule leads to the disappearance of the ferroelectric SmC* phase. For the compounds which show the ferroelectric SmC* phase, the temperature dependence of the spontaneous polarization, the spontaneous tilt angle, the helix pitch length and the complex permittivity has been studied. The effect of shifting the azo group in the molecular core on the physical properties is discussed.     

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.     

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.     

Synthesis and SmC* properties of chiral liquid crystalline terephthalates

Chiral ester derivatives of terephthalic acid containing one, three, or five phenyl rings were prepared by using (S)-2-methylbutanol or (R)-2-chloropropanol as starting materials. The one-ring terephthalates did not exhibit any liquid crystalline phases, but one of them was used as a chiral dopant in ferroelectric mixtures. Elongation of the aromatic core structure to three and five phenyl rings stabilized the SmA phase, whereas a (monotropic) SmC* phase was detected in some three-ring and five-ring derivatives. In ferroelectric mixtures based on the terephthalates, spontaneous polarizations up to 340 nC cm2 were measured.     

Effect of fluorination on the phase sequence,dielectric and electro-optical properties of ferroelectric and antiferroelectric mixtures

Four multicomponent room temperature mixtures were formulated using a pyrimidine-based achiral matrix as host and four terphenyl-based ester chiral compounds as dopants. Among the four mixtures, two exhibit antiferroelectric and two other exhibit ferroelectric phases at room temperature. Dopant molecules differ from each other by position and number of fluorine atom substitution at the benzoate group of the molecular rigid core. Number and location of fluorine atoms in the dopant structure show significant effect on the phase sequences as well as on different physical properties. All the mixtures found to have moderate spontaneous polarisation, high tilt angle, very fast switching time and low viscosity which are important for liquid crystal-based display applications.     

Magnitude and direction of the spontaneous polarization of ferroelectric liquid crystals with several bond moments

The magnitude and direction of the spontaneous polarization in most ferroelectric liquid crystals have been confirmed to be determined by the location and magnitude of the bond moment around the chiral carbon and the core. In compounds with several bond moments their relative orientation is very important for obtaining a large spontaneous polarization. Compounds with benzene rings in the core substituted with OH also have a large spontaneous polarization, perhaps due to the formation of hydrogen bonds. Reversal of the direction of the spontaneous polarization with temperature has been found for EFPPOPB. This anomalous behaviour has been explained tentatively in terms of a conformation change due to the existence of a flexible -CH₂- unit between the chiral carbon and the dipole moment.     

Novel liquid crystals and dopants (for induced ferroelectricity) possessing a chiral 2-oxetanone unit. A comparison with corresponding dioxolanones

The mesomorphic properties, induced spontaneous polarizations, and response times of a new type of liquid crystal and ferroelectric liquid crystal mixtures containing di- or trisubstituted 2-oxetanones as a chiral unit are described. By heating above 140°C oxetanones, bearing a three core mesogenic part, suffer cycloelimination of carbon dioxide yielding liquid-crystalline olefines with nematic, smectic C and higher ordered smectic phases, which are best suited as host materials for induced S_c* phases using the corresponding oxetanones as dopants. Compared with 1,3-dioxolan-4-ones recently synthesized and investigated, the oxetanones show minor mesomorphism.     

Magnitude and direction of the spontaneous polarization of ferroelectric liquid crystals with several bond moments

The magnitude and direction of the spontaneous polarization in most ferroelectric liquid crystals have been confirmed to be determined by the location and magnitude of the bond moment around the chiral carbon and the core. In compounds with several bond moments their relative orientation is very important for obtaining a large spontaneous polarization. Compounds with benzene rings in the core substituted with OH also have a large spontaneous polarization, perhaps due to the formation of hydrogen bonds. Reversal of the direction of the spontaneous polarization with temperature has been found for EFPPOPB. This anomalous behaviour has been explained tentatively in terms of a conformation change due to the existence of a flexible -CH₂- unit between the chiral carbon and the dipole moment.     

Ferroelectric properties of liquid-crystalline SC* phases induced by optically active cyanocyclohexylcyclohexanone derivatives

Derivatives of optically active cyanocyclohexylcyclohexanone have been synthesized and used as dipolar chiral dopants to induce ferroelectric S_C* phases in an achiral host phase. The dopant molecules are the first examples in which the chiral centres are incorporated into a rigid core with transverse dipoles directly attached. The spontaneous polarization P_s and the tilt angle θ of the induced S_C* phases have been measured. P_s is strongly influenced by the relatively small changes of the molecular structure of the cyclohexanones, for example a change of the sign of P_s or a vanishing value of P_s. These effects are discussed in terms of a sterically hindered rotation of the dopant molecules around their long axes and explained by the assumption that the transverse dipole must not be necessarily parallel to P_s in the equilibrium state of rotation.     

New ferroelectric liquid crystals with a trifluoromethyl group

New chiral dopants, possessing a trifluoromethyl group, have been synthesized. These dopants have different polar groups, located between the chiral centre and the diphenylacetylene core. The polar part is chosen from ester, methylene ether and ether linkages. Each new chiral dopant was mixed with a non-chiral liquid crystal mixture or a ferroelectric liquid crystal mixture, and their transition temperatures and electrooptic properties measured. The largest spontaneous polarization, 4·1 nC cm^-2, was obtained at 25°C for the ferroelectric liquid crystal mixture in which the dopant, with a methylene ether linkage, was incorporated. Molecular orbital calculations (MNDO method) for the dopants show that the calculated dipole moments are correlated with the measured spontaneous polarization, and the calculated results explain the experimental results quite well. In addition, it has been found that the conformation of the chiral part is an important factor which affects the magnitude of spontaneous polarization.     

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.     

Ferroelectricity of induced S*C phases with novel chiral dopants

Induced S*_C phases can be obtained by dissolving chiral dopants in achiral S_C host phases. If the chiral guest molecules bear a transverse dipole, ferroelectricity will occur. The novel dopants under discussion are characterized by chiral centres and the transverse dipole situated not in the alkyl end groups of the mesogenic molecules, but directly in their rigid cores. As a rigid core, analogues of decalin were used. In those dopants, rotation around the molecular long axis is sterically restricted. According to the microscopic model of Zeks, this leads to enhanced values of the spontaneous polarization P_s. The magnitude as well as the sign of the spontaneous polarization P_s of the S*_C phases induced by the novel dopants in different host phases has been investigated. It has been found for the first time that for a given dopant, the polarization as well as the sign of P_s depends on the structure of the host phase. The results are discussed in terms of two microscopic models. They can be understood taking into account the situation that the potential of the restricted long axial rotation is determined by the hard core interactions of the molecules involved or that an orientation of the host dipoles by a guest/host interaction takes place.     

New ferroelectric liquid crystals with a trifluoromethyl group

Abstract

New chiral dopants, possessing a trifluoromethyl group, have been synthesized. These dopants have different polar groups, located between the chiral centre and the diphenylacetylene core. The polar part is chosen from ester, methylene ether and ether linkages. Each new chiral dopant was mixed with a non-chiral liquid crystal mixture or a ferroelectric liquid crystal mixture, and their transition temperatures and electrooptic properties measured. The largest spontaneous polarization, 4·1 nC cm^?2, was obtained at 25°C for the ferroelectric liquid crystal mixture in which the dopant, with a methylene ether linkage, was incorporated. Molecular orbital calculations (MNDO method) for the dopants show that the calculated dipole moments are correlated with the measured spontaneous polarization, and the calculated results explain the experimental results quite well. In addition, it has been found that the conformation of the chiral part is an important factor which affects the magnitude of spontaneous polarization.     

Induced smectic C* phases

Induced ferroelectric S*_C phases are formed by non-chiral S_C host phases doped with chiral dipolar guest molecules. In those mixtures the spontaneous polarization P_s and the tilt angle Θ has been investigated as a function of the mole fraction x_G of the chiral dopant. In most cases the reduced polarization P₀ = P_S/ sin Θ has been found to depend linearly on x_G. The polarization power which is defined by δ_P=(∂P₀/∂x_G)ΔT is discussed in terms of the molecular structure of the chiral dopants. There are systems in which P₀(x_G) deviates positively from linearity. This behaviour can be understood by considering a local field correction to P₀. By assuming a local field of Lorentz type a theoretical relation for P₀(x_G) has been derived which explains the experimental results. The effect of a local field is considerable if the transverse dipole moment and the polarizability of the chiral dopant are large.     

New low molar mass organosiloxanes with unusual ferroelectric properties

A new low molar mass chiral organosiloxane mesogen and its racemic analogue have been synthesized and their mesomorphic and ferroelectric properties investigated. The chiral derivative, denoted A*B, exhibits one tilted enantiotropic ferroelectric smectic C mesophase over a broad temperature range, with very high tilt angles and moderate spontaneous polarization (36° and 19 nC cm^-2 at 20°C). The achiral siloxane derivative, denoted A*B, exhibits one broad enantiotropic smectic C phase. Preliminary electro-optic measurements indicate that the spontaneous polarization is weakly dependent on temperature between 10°C and 50°C, the latter being the S*_c to isotropic phase transition. The tilt angle and layer spacing are temperature independent, and current response times of less than 200 μs were measured at 25°C for fields of 10 V μ^-1. These results are discussed in comparison with those for side chain polymer liquid crystal structures and other low molar mass ferroelectric materials.     

Synthesis and ferroelectric properties of chiral swallow-tailed liquid crystals derived from (L)-lactic acid

Three homologous series of chiral swallow-tailed materials derived from (L)-lactic acid were prepared. Structural effects on the mesomorphic and physical properties were investigated in terms of (i) the variation of non-chiral peripheral length chain, (ii) the variation of swallow-tailed groups and straight alkyl chain at the chiral tails, and (iii) lateral halogen substituents in the core of the molecules. The mesophases and their corresponding transition temperatures were identified by optical polarized microscopy and differential scanning calorimetry. The physical properties of the ferroelectric SmC * phases such as switching current, spontaneous polarization and electro-optical response were also measured and compared.     

Synthesis and ferroelectric properties of chiral swallow-tailed liquid crystals derived from (L)-lactic acid

Three homologous series of chiral swallow-tailed materials derived from (L)-lactic acid were prepared. Structural effects on the mesomorphic and physical properties were investigated in terms of (i) the variation of non-chiral peripheral length chain, (ii) the variation of swallow-tailed groups and straight alkyl chain at the chiral tails, and (iii) lateral halogen substituents in the core of the molecules. The mesophases and their corresponding transition temperatures were identified by optical polarized microscopy and differential scanning calorimetry. The physical properties of the ferroelectric SmC* phases such as switching current, spontaneous polarization and electro-optical response were also measured and compared.     

New dimeric liquid crystals with chiral flexible spacers

In order to obtain chiral compounds of low molecular weight expected to exhibit mesogenic and ferroelectric properties, the asymmetric carbon was introduced into the central part of the molecule. The series consists of nine chiral dimers in which the chiral flexible spacer derives from S-2-chlorosuccinic acid. The mesomorphic properties of the compounds were studied and all of them were found to be mesogenic. From n = 6 onwards the mesophases were chiral smectic C. Some analogous compounds derived from S-2-chloroglutaric acid were also prepared; these were not found to be mesogenic. The thermal instability of the 2-chlorosuccinic acid derivatives prevented the study of their ferroelectric properties. We attempted to perform this study for several mixtures involving 3-methyladipic acid derivatives and both 2-chlorosuccinic and 2-chloroglutaric acids derivatives. All the mixtures studied show chiral smectic C phases. The mixtures containing 2-chlorosuccinoyl derivatives show thermal instability; however those containing 2-chloroglutaroyl derivatives are stable, but do not show significant values of spontaneous polarization.