Effect of the chiral chain length on structural and phase properties of ferroelectric liquid crystals

Studies of structural and phase properties obtained on several ferroelectric liquid crystalline materials with 2-alkoxypropionate group used as a chiral centre and without any lateral substitution are presented. In dependence on the chiral chain length these compounds exhibit the cholesteric N* phase, the ferroelectric smectic C* and a low-temperature SmX phase. Values of the spontaneous polarization and spontaneous tilt angle have been determined within the whole range of the SmC* phase. A low-temperature SmX phase has been identified as the orthogonal hexatic SmB* phase. The molecular parameters, namely the layer spacing in the SmC* and SmB* phases and the average intermolecular distances (D) between neighbouring parallel molecules in all investigated phases have been determined using the results of the X-ray diffraction obtained on non-oriented samples. The effect of the chiral chain length on mesomorphic, structural and physical properties of the studied ferroelectric liquid crystalline materials is discussed.     

Chain dynamics in a chiral C phase by deuteron spin relaxation study

Molecular reorientations and internal conformational transitions of an aligned chiral liquid crystal (LC) 10B1M7 are studied by means of deuterium spin-lattice relaxation in its smectic A (SmA) and smectic C* (SmC*) phase. The motional model which is applicable to uniaxial phases of many LCs is found to be adequate even when the phase is a tilted SmC* phase. The deuterium NMR spectrum in this phase cannot discern rotations of the molecular director about the pitch axis. The basic assumption is that the phase biaxiality is practically unobservable. However, the relaxation rates can be accounted for by the tilt angle between the molecular director and the layer normal in the SmC* phase. The tumbling motion appears to show a higher activation energy upon entering from the uniaxial SmA into the SmC* phase.     

Symmetries and induced effects in bilayer and multilayer antiferroelectric and ferrielectric liquid crystal phases

The stable antiferroelectric and ferrielectric smectic phases which may arise below a chiral SmA* phase are investigated theoretically. The symmetry and physical properties of the bilayer and multilayer configurations are worked out. Antiferroelectric and ferrielectric bilayer and multilayer configurations, possessing an induced spontaneous ferroelectric polarization component perpendicular to the smectic layers, are shown to take place, as the result of a nonlinear piezoelectric effect. These states of low polar symmetries occur when the angle between the inlayer projections of the dipoles and the director of the molecules is different from 90 degrees.     

A self-consistent approach for the SmA–SmC* phase transition

By using an analogy with the Maier–Saupe theory of the nematic phase, a mean field theory of the chiral smectic-C (SmC*) phase is constructed. As an order parameter of the SmC* phase, the tilt angle θ is selected, and the feedback effect of θ is introduced into the hindered rotational potential about the molecular long axis. By solving the self-consistent equation for the order parameter θ, the second-order phase transition appears. Also, to describe the SmA–SmC* phase transition under an electric field, a coupling term between a molecular dipole moment and an external electric field is introduced to the potential function. The electroclinic effect in the SmA phase and the hysteresis effect in the SmC* phase, which were described phenomenologically in the past, are found directly from the microscopic theory.     

Thermodynamic,optical and switching parameters of a ferroelectric liquid crystalline material having SmA*-SmC*-SmBh* phase sequence

Thermodynamic and electro-optical characterization of a ferroelectric liquid crystalline material, namely ((S) (+) 4-(1-methylheptyloxy) phenyl 4′-octyloxybiphenyl-4-carboxylate) possessing paraelectric SmA*, ferroelectric SmC*, hexatic SmB_h* and SmI* phases has been carried out. Phase identification has been done by optical and thermodynamic studies. Switching parameters viz. spontaneous polarization, switching time and rotational viscosity have been determined. The spontaneous polarization has been found to increase with decreasing temperature in SmC* phase. The switching time is found of the order of few milliseconds.     

Optical second-harmonic generation in ferroelectric chiral smectic polysiloxane

The second-order nonlinear optical properties of ferroelectric liquid crystalline polymer have been studied. Angular phase-matched Second-Harmonic Generation (SHG) is observed in the chiral smectic C (SmC*) phase of siloxane copolymer with chiral mesogen in the side chain. The effective nonlinear optical coefficient d_eff under the phase-matching condition is determined. The intense SHG is observed even in the crystalline phase by cooling down from the SmC* phase under the electric field. The effective coefficient of SHG in the crystalline phase is more than ten times larger than that of the phase-matched SHG in the SmC* phase. The enhancement of SHG in the crystalline phase is observed only in a homeotropically aligned cell and maintained even in the non-biased state for at least several days. The angular dependence of the SHG in the crystalline phase is confirmed.     

Structural, electro-optical and dielectric characterizations of ferroelectric liquid crystals showing the SmC*–SmA*–N* phase sequence

Structural (helical pitch), electro-optical (tilt angle, spontaneous polarization and response time) and dielectric (Goldstone mode) characterizations have been performed on two pure ferroelectric liquid crystals of a biphenyl alkyloxy benzoate series and they show the N*–SmA*–SmC* phase sequence. The different results are discussed: the helical pitch, the spontaneous polarization and the rotational viscosity which is determined as a function of temperature by two methods using electro-optical or dielectric measurements. An Arrhenius behaviour of the rotational viscosity is found for the two compounds. The corresponding activation energies are determined.     

Critical behaviour of the smectic A*–C* phase transition and electroclinic effect of ferroelectric liquid crystals with terphenyl rigid core

The SmA*–SmC* phase transition was studied by measuring the temperature and electric field dependences of the optical tilt angle, the electric polarisation and the dielectric spectra collected in a wide frequency range. Critical behaviour of the phase transition was analysed by varying the length of the fluorinated part of the alkyl terminal chain and by differing fluorine substitution in the terphenyl core. Both tilt and polarisation show tricritical mean-field behaviour for all homologues with n?>?2. Almost all coefficients that describe the SmA*–SmC* transition in the frame of the Landau theory were derived for homologue series. Double fluorine substitution in the central ring of the core seems to promote the ‘de Vries'-type smectic A*–C* phase transition with a little layer shrinkage. These well correspond with the lower tilt angle and smaller changes of the birefringence at the phase transition compared to the other homologues.     

Characteristic dielectric parameters of para-, ferro- and antiferro-electric phases of (S)-4-(1-methylheptyloxycarbonyl)phenyl-4′-(6-pentanoyloxyhex-1-oxy)biphenyl-4-carboxylate

Frequency and temperature dependence of dielectric parameters of a liquid crystalline compound (S)-4-(1-methylheptyloxycarbonyl)phenyl-4′-(6-pentanoyloxyhex-1-oxy)biphenyl-4-carboxylate under planar orientation of the molecules have been investigated in the frequency range 1 Hz-10 MHz. This compound possesses smectic paraelectric (SmA*), ferroelectric (SmC*) and antiferroelectric (SmC_A*) phases. Dielectric spectroscopy suggests the existence of a relaxation mechanism in the SmA* phase, which behaves as a soft mode. In the SmC* phase two relaxation modes are observed. One mode continues from the SmA* phase with decreasing dielectric strength and the other has characteristics of the Goldstone mode. Two dielectric relaxation modes have been observed for the SmC_A* phase. These two modes are related to the antiferroelectric ordering and the helical structure of the SmC_A* phase.     

‘Guest–host’ effect in liquid crystal mixtures

The most important goal of our research is to show the influence of the ‘guest’ (bent-core mesogen, 1,3-phenyldicarboxylatebis{4-[(4-octylbenzoyl)sulphanyl]phenyl} [IFOS8], banana-shaped liquid crystal [BLC]) on the ‘host’ (calamitic liquid crystal [CLC], (S)-(+)-1-methylheptyloxybiphenyl-(4-n-octylphenyl)thiobenzoate [MHOBS8]), on the stability and the destabilization of the antiferroelectric B2 and the ferroelectric smectic C* (SmC*) phases, and change of the temperature ranges of other phases in the binary liquid crystal mixtures. This work is focused on polymorphism of three new binary liquid crystal mixtures, exhibiting a ‘guest–host’ (guest liquid crystal–host liquid crystal [GH-LC]) effect. MHOBS8 has, among others, a ferroelectric SmC* phase, and IFOS8 assumes the B2 phase with antiferroelectric properties. The observed properties of the mixtures, such as variation of the phase transition temperatures, spontaneous polarization, tilt angle and switching time, are characteristic of a ‘guest–host’ mixture. The influence of BLC on the character of the interactions within the CLC host is discussed, with particular attention paid to electro-optical properties of the GH-LC mixtures.     

Field-induced transition from homeotropic to planar geometry in the SmC* phase of an electroclinic liquid crystal

The optical and electrical behavior was investigated of a symmetric liquid crystal (LC) cell: ITO–silane–LC–silane–ITO. The silane layer induces a perfect homeotropic alignment of the molecules of the studied electroclinic liquid crystal (ELC) material, BDH 764E. A field-induced transition from the perfect homeotropic to planar orientation in the chiral smectic C (SmC*) and smectic A (SmA) phases of the ELC was observed. Optical and dielectric studies were performed for both alignment (geometry) modes. The field-induced transition from the homeotropic to planar orientation was studied vis-à-vis the high negative dielectric anisotropy obtained in the studied material. Such an ELC with large negative dielectric anisotropy and perfect homeotropic alignment may have important implications for modern LC display technology.     

Molecular models for ferroelectric liquid crystals with conventional and anomalously weak layer contraction

A molecular theory of the ferroelectric smectic C* phase has been developed using the simple model of a chiral molecule composed of a uniaxial core and a pair of off-center nonparallel dipoles which determine molecular chirality and polarity. The interaction between uniaxial cores is modeled by a rather general effective potential which can be used to describe smectic materials with both conventional and anomalously weak layer contraction in the smectic C* phase. Spontaneous polarization, tilt, and layer spacing are calculated numerically as functions of temperature, and it is shown that the variation of the polarization generally deviates from that of the tilt angle. It is shown that this deviation is more pronounced in smectic materials tilting with low layer contraction which corresponds to existing experimental data. The model has been used to reproduce qualitatively the experimental data for polarization, tilt and layer spacing for two similar mixtures exhibiting conventional and anomalously weak layer contraction. The polarization and the tilt are also calculated in the case when the smectic A-smectic C* transition is characterized by the biaxial primary order parameter.     

Biaxial modeling of the structure of the chevron interface in smectic liquid crystals

We have included the inherent molecular biaxiality of the smectic C phase in a model of the chevron structure. This molecular biaxiality is related to a hindered rotation about the molecular long axis which for chiral, polar molecules induces a spontaneous polarization. Through the coupling between biaxiality and the smectic cone angle, continuity of the molecular distribution at the chevron interface leads to changes in the cone angle. Under certain approximations we are able to find analytic expressions for the chevron structure and consequently estimate the width of the chevron interface. There are in fact two correlation lengths which govern variations in the cone angle and the biaxiality.     

X-ray studies of the smectic B phase of the 4-bromobenzylidene-4′-alkoxyanilines

The results of X-ray studies of the nine members (4?÷?12) of the 4-bromobenzylidene-4′-alkyloxyaniline homologous series (nBBAA) in smectic B (SmB) and smectic A (SmA) phases are presented. Interestingly, no SmC phase was identified. The thickness of smectic layers in the SmA and SmB phases was estimated showing unexpectedly that in SmA it is bigger than in the SmB. For the SmB phase, orthorhombic unit cell parameters were determined as a function of temperature. The obtained data are discussed in relation to rotational dynamics.     

Optical activity produced by layer chirality in bent-core liquid crystals

Large optical activity has recently been observed in chiral smectic liquid crystalline phases formed from achiral bent-core molecules. The origin of the optical activity remains unclear and has been attributed to both a helical superstructure and the layer chirality produced by simultaneous tilt and polar ordering of bent-core mesogens in the B2 phase. Here, we directly demonstrate that layer chirality produces optical activity in the well ordered SmC(A)P(A) subphase.     

Banana-shaped 1,2,4-oxadiazoles

3,5-Disubstituted 1,2,4-oxadiazoles are a new type of liquid crystalline (LC) compounds with asymmetrical five-membered heterocycle as a central unit. They have a bent shape and are very convenient model-compounds for studying the dependence of the LC properties on the molecular design. We have also synthesized and investigated ‘banana-shaped’ 1,2,4-oxadiazoles using the ester groups as the linkage units. The new compounds exhibit spontaneous polarization in the smectic phase, even if there is no chiral group in the molecules. Preliminary experimental data suggest the presence of spontaneous polarization in the nematic phase as well. In order to study the structural properties of the LC phases, X-ray diffraction (XRD) measurements on powder samples have been carried out. Based on the XRD data, a model of the structural arrangement of the bent molecules in the smectic phase is provided, which accounts for the macroscopic spontaneous polarization as well as the ferroelectric switching behavior.     

Highly ordered hexatic phases with large spontaneous polarization

In this article the physical properties of hexatic phases of three substances MHPNBC, FOOPP and FNHPP have been studied by differential scanning calorimetry, texture observation and dielectric spectroscopy. Experimental results are discussed from the point of view of existing theories. It is interesting that two of the substances studied, the FOOPP and FNHPP, exhibit enhanced spontaneous polarization in the highly ordered SmI* phase and show a jump of the spontaneous polarization in the vicinity of the SmC*–SmI* transition. In the SmI* phase of FOOPP a very high value of spontaneous polarization of the order of 530?nC?cm^?2 was found. Based on the results obtained the macroscopic and microscopic properties of the hexatic phases are discussed.     

Dielectric relaxation spectroscopy of guest-host ferroelectric liquid crystals: influence of anthraquinone dye

The dielectric spectroscopy of a short pitch and high spontaneous polarization ferroelectric liquid crystal mixture and its guest-host derivatives with different wt/wt ratio of anthraquinone blue dichroic dye has been studied over a wide frequency range of 50 Hz-1 MHz. The increase in dye concentration results in the decrease of the permittivity of the material in the SmC* phase, however, an opposite effect is observed in the SmA phase. The influence of bias voltage on the dielectric parameters has also been investigated. A new relaxation mode has been observed with a relaxation frequency of ∼300 kHz and dielectric strength of ∼5 at room temperature.