Series Of Ferroelectric Liquid Crystals Research Articles

Electro-optic and dynamic studies of biphenyl benzoate ferroelectric liquid crystals

Abstract We present electro-optic and dynamic properties on three homologous of biphenyl benzoate series of ferroelectric liquid crystals (FLCs) exhibiting the chiral smectic C phase (SmC*). The three compounds present high spontaneous polarization and show the SmC*–SmA*–N* phase sequence. Dielectric spectroscopy was used to provide the dynamics and dipolar ordering of the ferroelectric phase. The Goldstone relaxation mode was studied for sample cells on planar geometry without a DC bias voltage. The rotational viscosity corresponding to molecular motions in the SmC* phase was determined from dielectric measurements. The Arrhenius-type behavior of the rotational viscosity was found and the corresponding activation energies were evaluated.

Nature of smectic A*-C* phase transitions in a series of ferroelectric liquid crystals with little smectic layer shrinkage

The smectic layer spacing of two homologous series of ferroelectric liquid crystal compounds was characterized by small-angle x-ray diffraction and different degrees of smectic layer shrinkage on cooling from the SmA* into the SmC* phase were observed. The smectic A*-smectic C* phase transition was further studied by measuring the thermal and electric field effects on the optical tilt angle and the electric polarization. With decreasing length of the alkyl terminal chain the phase transition changes from tricritical exhibiting high layer shrinkage to a pure second-order transition with almost no layer shrinkage. This is explained by the increased one-dimensional translational order of the smectic layers, which seems to promote the "de Vries"-type [Mol. Cryst. Liq. Cryst. 41, 27 (1977)] smectic A*-C* phase transition with no or little layer shrinkage.

Thermal analysis and X-ray studies of chiral ferroelectric liquid crystalline materials and their binary mixtures

Thermal properties of a homologous series of ferroelectric liquid crystals S-(-)-[4-(2-n-alkoxy-propionyloxy)]biphenyl-4'-[n-alkoxy-(3,5-dimethyl)]benzoate have been investigated by polarizing optical microscopy and differential scanning calorimetry. The mesophases were identified and confirmed by X-ray too. Three binary mixtures were prepared from the individual homologues. In one of the mixtures (Mix1), the ferroelectric SmC* phase has broadened and became enantiotropic. This mesophase remained monotropic in the other two mixtures (Mix2, Mix3). The chiral nematic N* phase did not appear in Mix1, but remained monotropic for the other two mixtures. Two molecular parameters, the layer spacing and the average intermolecular distance have been calculated from the X-ray results for the homologues and their mixtures. An intercalated tail-to-tail packing of molecules was found both in the single compounds and their mixtures resulting in the layer spacing about half of the molecular length of the single compounds.

The coupling between polarization and tilt in homologous series of ferroelectric liquidcrystals

At temperatures well below the vicinity of the transition to the ferroelectric phase thecoupling between the spontaneous polarization of the SmC* phase with the director tiltangle exhibits nonlinear behaviour. In a generalized Landau model of ferroelectric liquidcrystals this is theoretically described by two terms in the free enthalpy density expansion:(i) the chiral bilinear or piezoelectric coupling term and (ii) the achiral biquadratic couplingterm, which describes the transversal quadrupolar ordering. We have investigated twohomologous series of ferroelectric liquid crystals of varying phase sequence with respect topiezoelectric and quadrupolar coupling. It is shown that the linear coupling increases withincreasing molecular length, while nonlinear contributions decrease. Effects ofvarying phase sequences N*–TGBA*–SmA*–SmC* versus N*–SmC* are 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.

SYNTHESIS AND PROPERTIES OF FLUORO- SUBSTITUTED FERROELECTRIC LIQUID CRYSTALS HAVING A 2-(BIPHENYL-4′-YL)-1,3-DIOXANE MESOGENIC CORE

Ferroelectric liquid crystals (FLCs) showing a short response time have been investigated with respect to their application for high resolution graphic displays addressed by thin film transistors. It is found that the introduction of fluoro substituents in the molecule at a suitable position such as the core and the asymmetric tail allowed the compounds to have a short response time. Here, in order to obtain further information about the fluorination effect onthestructure, a new series of ferroelectric liquid crystals having a 2-(biphenyl-4′-y)-1,3-dioxanyl group as the core has been synthesised. Their mesomorphic and physical properties have been measured, and the correlation between the structures and the properties is discussed.

Ferroelectric Liquid Crystals with Extremely Wide SmC* Phase Range

New series of ferroelectric liquid crystals containing four aromatic rings in the mesogenic core and the alkyllactate unit as a chiral centre were synthesized and investigated. All compounds have the SmA phase and the chiral SmC* phase in a very broad temperature range around 100 K. For the chlorine substituted materials the spontaneous polarization reaches the values of 110 - 150 nC/cm 2 at temperatures 50 K below the phase transition to the SmC* phase in contrast to the value of 60 nC/cm 2 for non-substituted compound. Similarly, increase due to the Cl substitution has been observed also for the values of spontaneous tilt angle.

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 SmCA 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.

Synthesis and physical properties of laterally fluoro substituted ferroelectric liquid crystals with a fluoro substituted chiral terminal chain

A new series of ferroelectric liquid crystals (FLCs) has been designed for active matrix displays based upon the chiral smectic C phase. The FLCs have been derived from optically active fluorinated alkanols and a laterally fluoro substituted biphenylyl-1,3-dioxan core. Their physical properties such as spontaneous polarization, current response time, and tilt angle have been determined. The FLC derived from 2-fluoro-octanol showed a very short current response time of 10μs at T Sm*-N* - T= 10°C, while another FLC with the same core derived from 5-fluoro-octanol gave a value of 150μs.

Synthesis and characterization of halogen-containing ferroelectric liquid crystals and side chain liquid crystalline polymers

A new series of ferroelectric liquid crystals and side chain liquid crystalline polymers based on halogen-containing chiral centres has been synthesized. Chemical structures were analysed by NMR. Liquid crystal phases were characterized by differential scanning calorimetry, optical polarizing microscopy, and X-ray diffractometry. The behaviour of the liquid crystalline phases was investigated as a function of spacer units and differing terminal asymmetric moieties. It was found that phase transition temperatures decreased with increasing length of the oligooxyethylene spacer unit. Differing terminal asymmetric moieties led to differing mesophase phenomena. Furthermore, a wide temperature range (including room temperature) of a chiral smectic C phase was achieved.

Liquid Crystalline Properties of S-(-)-4-(2-n-Alkoxy-Propionyloxy) Biphenyl-4′-n-Alkoxy-3,5-Dimethylbenzoate

Four new chiral homologous series of ferroelectric liquid crystals S-(-)-4-(2-n-alkoxy-propionyloxy)biphenyl-4′-n-alkoxy-3,5-dimethylbenzoate (DMn/m series) having O-substituted S-(-)-lactic acid as terminal group were synthesized, and characterized. They showed monotropic chiral smectic C and chiral nematic mesophases, and two representatives exhibited blue phase as well. Binary mixtures of these compounds were investigated. The effect of the lateral methyl substitutents and the length of the terminal groups on the chiral and non-chiral parts were investigated.

New Series of Ferroelectric Liquid Crystals Incorporating Stilbene Unit in the Core

New liquid crystalline materials of the general formula have been synthesized (R1, R2 are alkyl chains). Mesomorphic properties were determined using polarizing microscopy. All compounds showed very broad smectic C* phase (about 70–90 K) and the cholesteric phase. No smectic A phase was observed. In the smectic C* phase the spontaneous polarization is very high, reaching values higher than 300 nC/cm2 at the low temperature limit of this phase for compounds with two chiral centres. The permitivity was measured showing a contribution of the Goldstone mode in the smectic C* phase.

Synthesis and characterization of new ferroelectric liquid crystals containing oligomethylene spacers

Two series of ferroelectric liquid crystals containing a phenyl biphenyl carboxylate mesogenic group and oligomethylene spacers (n 3, 4, 6, 8) were synthesized and characterized. The chiral moieties are the (S)-2-methylbutoxycarbonyl and (S)-1-methylheptoxycarbonyl groups. The mesomorphic behaviours of the compounds were characterized using differential scanning calorimetry and optical polarized microscopy. Spontaneous polarization (P) of the liquid crystals was measured by an automated polarization tester over a wide temperature range. The mesomorphic properties are discussed as a function of spacer length and position of the chiral centre. It is found that the phase transition temperature decreases with increasing number of oligomethylene units. The phase behaviours of the LC with (S)-1-methylheptoxycarbonyl are more complex than those with (S)-2-methylbutoxycarbonyl, due to the flexible tail in the former. Moreover, the spontaneous polarization depends mainly on the position of the chiral centre, with Ps 15 nC cm 2 and Ps 60 123 nC cm 2 for the series s with (S)-2-methylbutoxycarbonyl and (S)-1-methylheptoxycarbonyl groups, respectively.

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.

Ferroelectric liquid crystals derived from isoleucine I. Synthesis and characterization

A series of ferroelectric liquid crystals (FLCs), 4 (3-methyl-2-halopentanoyloxy)- 4-hexyloxybiphenyls (3M2XPHOB, X=F for fluorine, C for chlorine, B for bromine) and their racemates (3M2XPHOB-R), were synthesized and characterized. The FLCs contain a chiral tail comprised of alpha -halo acids which are derived from L-isoleucine (DL-isoleucine for the racemates). The mesogens were characterized by high-resolution 1H and 13C NMR and their phase behaviour was studied by optical microscopy and differential scanning calorimetry. The chloro and bromo derivatives show both chiral smectic C (SmC*) phases and smectic A (SmA) phases, while the fluoro derivatives exhibit only a SmA phase. The spontaneous polarization of 3M2CPHOB and 3M2BPHOB were measured in the respective SmC* phases; that of the fluoro derivative was inferred by extrapolating its concentration dependent polarization in an achiral SmC solvent, a racemic mixture of 3M2BPHOB.

Ferroelectric liquid crystals derived from isoleucine II. Orientational ordering by carbon-13 separated local field spectroscopy

The orientational ordering of a series of ferroelectric liquid crystals 4'-[(2S, 3S)-3-methyl- 2-halopentanoyloxy]-4-hexyloxybiphenyls (3M2XPHOB, X=F for fluorine, C for chlorine, B for bromine) and their racemates have been studied by two-dimensional carbon-13 separated local field spectroscopy combined with off-magic-angle spinning. All carbons are well-resolved in the carbon-13 NMR spectrum so that carbon-proton dipolar coupling constants for all carbons in the liquid crystal molecule can be determined, giving detailed segmental orienta- tional ordering information. (The order parameters of the biphenyl core segments and each carbon-proton bond in the aliphatic chains were measured as a function of temperature.) The results show that the substitution of different halogens on the chiral chain affects the carbon- proton bond orientational ordering of the entire chiral chain, while the orientational ordering of the rest of the liquid crystal molecule (core and achiral chain) remains unchanged. For the fluoro-substituted liquid crystals, carbon-fluorine dipolar couplings are also observed. At the SmA-to-SmC* transition, there is a discontinuous change in the magnitudes of all C-H bond order parameters. However, the aliphatic ordering relative to the core ordering is unchanged, suggesting that there is a negligible change in the mesogen conformation and the environment of the aliphatic chain at this transition, i.e. there is no evidence for substantial 'chiral interactions' in the SmC* phase.

Tilt-polarization coupling in a homologous series of ferroelectric liquid crystals

The synthesis of the homologous series 4-[(S)-2-chloro-4-methylpentanoyloxy]-4′-[4-alkylbenzoyloxy]biphenyl and their chiral and mesomorphic properties are reported. The enantiomeric excess is deduced from 1H NMR spectroscopy. For the hexyl derivative, a twist grain boundary state (TGBA*) mediates the cholesteric to SmC* phase transition and its stability is discussed in terms of chirality and surface interactions. In the SmC* phase, measurements of the spontaneous polarization are reported and the temperature dependence of the tilt angle is determined from the electro-optical switching process or deduced from X-ray measurements of the smectic layer thickness, respectively. The tilt-polarization coupling is discussed within the framework of the ‘generalized Landau model’ proposed by Žekš and Blinc.

The synthesis and mesomorphic properties of ferroelectric liquid crystals with a fluorinated asymmetric frame

A novel series of ferroelectric liquid crystals with a fluorinated asymmetric frame were synthesized by utilizing optically active (S)-2-, (S)-3-, (S)-4- and (S)-5-fluoroalkanols prepared from corresponding (R)-1,2-epoxyalkanes. Their mesomorphic and physical properties, such as spontaneous polarization, optic tilt angle and response time, were investigated systematically in a series of homologous compounds having the chiral centre at different positions on tails of various lengths. All the compounds exhibited the chiral smectic C phase in a wide range of temperatures and were found to possess a fast response time in spite of the small magnitude of the spontaneous polarization.

The effect of a lateral substituent on the mesomorphic properties in a series of ferroelectric liquid crystals with a 2-alkoxypropionate unit

A new series of ferroelectric liquid crystal materials containing a lateral methyl group on the aromatic ring of the alkoxybenzoate unit has been synthesized and investigated. These materials exhibit a wide temperature range of ferroelectric SmC* phase on cooling (including supercooling) with a very high spontaneous polarization. All phase transition temperatures are lower when compared with those of similar non-substituted materials. In comparison with the compounds containing the lateral methoxy group, only the SmC*-Cr transition changed significantly. The values of spontaneous polarization are mostly increased compared with both non-substituted and methoxy substituted compounds.

Synthesis and characterization of new ferroelectric liquid crystals containing a phenyl biphenyl carboxylate mesogenic group and oligooxyethylene spacers

Abstract Two series of ferroelectric liquid crystals containing a phenyl biphenyl carboxylate mesogenic group and oligooxyethylene spacers were synthesized. The mesomorphic behaviours of the compounds were characterized using differential scanning calorimetry (DSC), optical polarizing microscopy (POM) and powder X-ray diffraction measurements. These series all contain a smectic A (SA), a chiral smectic C (SC) and a crystal E (Cr E) phase sequence. The ordered smectic Cr E phase was observed for the short spacer chain (n = 0, 1, 2) homologues of the two series. Moreover, the mesomorphic properties are discussed as a function of the oligooxyethylene spacer length, and the position of the aromatic core rings. These results indicate that the structures with a rigid part (biphenyl group) closer to the flexible spacer chain (oxyethylene unit) tend to form the symmetrical smectic SA and Cr E phases.