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 chiral dopants for ferroelectric liquid crystals

New chiral dopants for application in ferroelectric liquid crystal mixtures are presented. They derive from chirally modified biphenylcyclohexylcarbonitriles, lactates and optically active 2-cyano-2-methylalkanoates. Synthetic routes to the new compounds are shown and the physical properties of the new compounds in a non-chiral smectic C host are reported.     

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.     

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 series of ferroelectric liquid crystals with four ester groups

Three series of ferroelectric liquid crystals have been synthesized, having three ester groups in the mesogenic core and one ester group in the chiral chain. The ester groups introduced into the core decrease the temperature of crystallization, which results in a broad temperature range ferroelectric smectic C phase in homologues with a long non-chiral chain. These phases exhibit a high spontaneous polarization and relatively short pitch of the helical structure.     

New series of ferroelectric liquid crystals with two or three chiral centres exhibiting antiferroelectric and hexatic phases

New series of liquid crystals with a chiral chain containing two asymmetric carbon atoms in two lactate groups have been synthesized. Some compounds have a third asymmetric centre in an (S)-2-methylbutyl alkyl chain. All the new compounds have SmA, SmC* and hexatic phases. Two of the homologues with three chiral centres also exhibit very wide SmC_A phases and one of them a re-entrant SmC* phase. The phases are characterized by DSC, dielectric, and X-ray studies. Spontaneous polarization and tilt angle are determined for the polar phases.     

New chlorine-substituted ferroelectric liquid crystals with four aromatic rings in the mesogenic core

New series of liquid crystals with an alkyl-lactate chiral chain and four aromatic rings in the mesogenic core and chloro-substitution were synthesized. For all the compounds studied, SmA and SmC* phases, existing over very broad temperature ranges, were detected and characterized. Mesomorphic properties were analysed with respect to the lengths of the chiral and/or non-chiral chains.     

New chlorine-substituted ferroelectric liquid crystals with four aromatic rings in the mesogenic core

New series of liquid crystals with an alkyl-lactate chiral chain and four aromatic rings in the mesogenic core and chloro-substitution were synthesized. For all the compounds studied, SmA and SmC* phases, existing over very broad temperature ranges, were detected and characterized. Mesomorphic properties were analysed with respect to the lengths of the chiral and/or non-chiral chains.     

New antiferroelectric liquid crystals with a trifluoromethyl group at the chiral centre

New antiferroelectric liquid crystals (AFLCs) with a trifluoromethyl group at the chiral centre were synthesized using optically active 3-(4-methoxyphenyl)-4,4,4-trifluorobutanoic acid. The new AFLCs showed the phase sequence on cooling: isotropic liquid (I)-smectic A (S_a) phase-antiferroelectric (AF) phase-crystal (Cr). In the antiferroelectric phase, a d.c.-threshold, a double hysteresis loop, a small dielectric constant value and two sharp switching-current peaks were observed. These are the first examples that are AFLCs which do not have a carbonyl moiety in the molecule.     

New ferroelectric liquid crystals having a trifluoromethyl group at the asymmetric centre and a three-ring heterocyclic core

New ferroelectric liquid crystals with a trifluoromethyl group at the asymmetric centre and a three-ring heterocyclic core have been synthesized. These compounds possessed a large spontaneous polarization and exhibited a S*_c phase over a wide temperature range. Their properties were largely affected by the structure of the spacer group and the core part.     

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.     

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.     

Directed metalation route to ferroelectric liquid crystals with a chiral fluorenol core: the effect of restricted rotation on polar order

A new series of smectic C* (SmC*) mesogens containing a chiral (R)-2-octyloxy side chain and either a fluorenone (2a-e) or chiral fluorenol (3a-e) core were synthesized using a combined directed ortho metalation-directed remote metalation strategy. The SmC phase formed by the fluorenol mesogens is more stable and has a wider temperature range than that formed by the fluorenone mesogens, which may be ascribed to intermolecular hydrogen bonding according to variable-temperature FT-IR measurements. The C11 fluorenol mesogens (R,R)-3d and (S,R)-3d were obtained in diastereomerically pure form and gave reduced polarization (Po) values of +106 and +183 nC/cm2, respectively, at 10 K below the SmA*-SmC* phase transition temperature. The difference in Po values suggests that the chiral fluorenol core contributes to the spontaneous polarization of the SmC* phase. This is ascribed to the bent shape of the fluorenol core, which should restrict its rotation with respect to the side chains in the SmC* phase and favor one orientation of its transverse dipole moment along the polar axis, and to steric coupling of the core to the chiral 2-octyloxy side chain.     

Helix twist inversion in ferroelectric liquid crystals with one chiral centre

The helix twist inversion in the chiral smectic C phase was studied for two new homologous series of lactic acid derivatives. The inversion phenomenon was attributed to a competition between spatial conformers leading to opposite helix handedness. The temperature of inversion was found to increase with increasing length of the chiral molecular tail. With all substances studied, high values of the spontaneous polarization P_s were found which did not reach saturation on cooling, in spite of a saturated tilt angle θ_s.     

The first series of ferroelectric steroidal fluorinated liquid crystals

A series of cholesteryl p-perfluoroalkylphenyl carbonate (CPC) [C₂₇H₄₅OCO₂C₆H₄(CF₂)_nF, n=1,4,6,8] liquid crystals was synthesized and their phase transition behaviours were studied. The results show that the compounds containing a not too long perfluoroalkyl chain (n=6,8) have monotropic chiral smectic C phases.     

The first series of ferroelectric steroidal fluorinated liquid crystals

A series of cholesteryl p-perfluoroalkylphenyl carbonate (CPC) [C₂₇H₄₅OCO₂C₆H₄(CF₂)_nF, n=1,4,6,8] liquid crystals was synthesized and their phase transition behaviours were studied. The results show that the compounds containing a not too long perfluoroalkyl chain (n=6,8) have monotropic chiral smectic C phases.     

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.     

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.     

A convenient synthesis of ferroelectric liquid crystals bearing the trifluoromethyl group

A practical synthetic route to a series of (R)- or (S)-(1-trifluoromethyl)alkyl 4-hydroxybenzoateshas been developed. These materials were prepared by the reaction of p-hydroxybenzoic acid with (R)- or (S)-1-(trifluoromethyl)alkanols in the presence of p-toluenesulfonic acid under reflux conditions. Further reaction with the acid chloride of4′-alkoxybiphenyl-4-carboxylic acid gave the desired liquid crystals.     

Antiferroelectric liquid crystals with amide linking group positioned at chiral tail

A homologous series of chiral liquid crystal compounds, N‐methyl‐N‐pentyl‐(S)‐2‐(6‐(4‐(4‐alkyloxyphenyl)benzoyloxy)‐2‐naphthyl)propionamide, with an amide linkage in a chiral tail was synthesized and their mesomorphic properties studied. All the materials possessed an antiferroelectric smectic C (SmC_A*) phase, which was confirmed by observations of microscopic texture, switching current behaviour and electro‐optical responses. The spontaneous polarization, P _s, and apparent tilt angle, θ, were also measured. The maximum P _s values are in the range of 173–222 nC cm^?2, and the maximum θ values are in the range of 26–30°.