Trans−1,2-Difluorostilbenes: Promising Materials for High Dielectric Biaxiality Ferroelectric Mixtures

Abstract

A range of two- and three-ring liquid crystals have been synthesised that incorporate a trans?1,2-difluoroethene linking group. Some of the materials have surprisingly low melting points and high smectic C phase stability, and they exhibit the ideal phase sequence for ferroelectric hosts of smectic C to smectic A to nematic. The synthesis was accomplished in a convergent manner through the use of low-temperature lithiations to generate organometallics based on boron and tin. Palladium-catalysed cross-couplings and Michael-type reactions were used to generate the final mesogenic materials. The polar fluoro substituents enable the generation of ferroelectric mixtures with a high dielectric biaxiality which is very important in τV minimum driving schemes. The alkenic linking group in stilbenes normally confers instability, but the difluoro-substituted analogues are stable compounds.     

Dielectric Studies of Three Liquid Crystals HBT,E8 and PCH-1132

The dielectric behavior of a Schiff base N-(P-Hexyloxybenzylidene)-P-toluidine (HBT) and two technologically important mixtures, E₈ and PCH-1132, are reported in the radio frequency region. The value of the mean dielectric constant increases by ～5% on going from the nematic to the isotropic phase in the case of E₈ and PCH-1132. This may perhaps be due to antiferroelectric arrangement of the molecules in the nematic phase. These mesogens show unusual dielectric behavior and exhibit positive anisotropy at lower frequencies and negative dielectric anisotropy above a critical frequency termed as isotropic frequency. The isotropic frequency is found to be temperature dependent. All these nematogens exhibit single relaxation in the radio frequency region which is due to hindered rotation of the molecules under nematic potential. The relaxation frequencies are 3.98 MHz (at 61.4°C), 2.27 MHz (at 34.3°C), 1.87 MHz (at 33°C) in case of HBT, E₈ and PCH-1132 respectively. The activation energy is found to be 16.7 KCal/mole, 14.6 KCal/mole and 16.1 KCal/mole for HBT, Ea and PCH-1132 respectively. In smectic B phase of HBT, the dipolar orientations are found to be somewhat “locked” similar to that in solids. The inversion of the sign of dielectric anisotropy in S_B phase of HBT is also discussed.     

Effect of polarity of smectic Ad molecules on the induction of the nematic phase in binary mixtures of smectics A1 and Ad

The phase diagrams are determined of binary mixtures composed of 4-n-alkylbiphenylates 4-cyanobiphenyl (n-CBB)-smectics A₁, for n < 8 and esters of 4-(trans-4′-n-alkylcyclohexyl)benzoic acid (10CPCHB, 10NPCHB, 10FOPCHB)-smectics A_d. The effect is studied of the smectic layer spacing ratio and polarity of the components on the induction of the nematic phase in the tested series of compounds. The virtual N S_Ad phase transition points in series n-CBB are estimated and the effect is discussed of the stability of the smectic phase and polarity of the molecules of the mixture components on the width and position of the nematic gap.     

Liquid Crystals: A Bibliography for 1975

Abstract

The static dielectric constants of biphenyl-4-p-n-alkoxybenzoates have been measured as functions of temperature. The dielectric anisotropy (δ?) for the successive homologues (hexyl to undecyl) shows an alternation similar to that exhibited by the nematic-isotropic transition temperature (T _NI). The dispersion of ?_∥ in the frequency range of 0.1-15 MHz has been studied for four successive members of biphenyl-4-p-n-alkoxybenzoates (heptyl to decyl derivatives) as well as of trans-p-n-alkoxy-α-methyl-p'-cyanophenyl cinnamates (eighth to eleventh members). For both series the relaxation frequency (f _R) and the frequency of dielectric isotropy (f _o) evaluated at a common relative temperature show an alternation similar to T _NI. For any given compound the Arrhenius plots for f _R and f _o give the same value of the activation energy (W). In the case of the biphenyl esters the vaue of W is the same for the seventh and eighth members, but increases for the higher members which exhibit a smectic phase at lower temperatures. On the contrary for cinnamic acid esters W is found to be the same for all homologues irrespective of whether the nematic phase is preceded by a smectic phase or not.     

Anisotropy of Electric Properties in Binary Cholesteric Mixtures

Dielectric permittivity and electric conductivity anisotropies were determined for cholesterol laurate, cholesterol caprilate as well as for their mixtures 25:75%, 50:50% and 75:25% (by weight). Connection between anisotropies of pure substances and their mixtures were found. The mixtures 75:25% and 25:75% have positive conductivities anisotropies within the cholesteric range as for the initial esters; the sign of the dielectric anisotropy is the same as for the prevailing ester. Within the smectic range the dielectric anisotropy is negative for both mixtures and pure compounds, in accordance to Carr's hypothesis with reference to the conduction mechanism in smectics.

The phase transitions are revealed in most cases by maxima or minima of the dielectric anisotropies.     

Dielectric studies on binary mixtures containing 4-trans-4′ -n-hexyl-cyclohexyl-isothiocyanato-benzene

We have studied binary mixtures with an induced smectic phase of 4-trans-4′ -n-hexylcycohexyl isothiocyanatobenzene (6CHBT) with 4-n-ethyl-4′-pentylazoxybenzene (EPAB) by means of dielectric, microscopic and refractometric measurements. The effective values of components p ⟂ and p∥ of the dipole moment per one molecule of the mixture have been calculated from values of ϵ ⟂ and ϵ ∥. As a result of our studies, we have established that the heterocomplexes formed in the mixture are so weak that they do not cause any essential changes in the distribution of molecule charges. The observed deviations from additivity of the dielectric constant of the mixture are caused solely by a non-additive behavior of the mixture density.     

Electrical Conductivity of Liquid Crystal Mixtures with Induced Smectic Phases

The electrical conductivity of the two liquid crystal mixtures N-(4-ethoxybenzylidene)-4′-amino benzonitrile (PEBAB)/4,4′-di-n-hexyloxy-azoxybenze (HEXOAB) and 4-n-heptyloxy-4′-cyanobiphenyl (7 CBP)/HEXOAB, which exhibit induced smectic A phases is investigated. In the smectic phases, the conductivity anisotropy of the PEBAB/HEXOAB mixtures is negative at the lowest PEBAB concentrations; this behaviour is usually expected for a smectic layer structure. With increasing PEBAB concentration the anisotropy increases and becomes positive. Possibly, this is an indication for a growing double layer structure, which was observed in polar smectic phases. In the nematic phases the conductivity anisotropy of the pure HEXOAB is considerably reduced by adding a relatively small amount of the polar component. An addition of 10 mole% 7 CBP reduces the anisotropy ratio of the electrical conductivity to V = K_∥/K ₁ ≈ 0.2, which probably is the lowest value observed in a nematic phase so far. Besides the negative conductivity anisotropy, these mixtures also exhibit a positive anisotropy of the dielectric constant. They thus fulfil the conditions for inverse dynamic scattering.     

Induction of Mesomorphic Properties in Non-Mesogenic Octa(decyloxy) phthalocyanines

Abstract

As it is well known [1, 2], non-peripheral substituted octa (alkoxy)-phthalocyanines (Pc) do not exhibit neither thermotropic nor lyotropic mesomorphism (in binary mixtures with organic solvents). In this work we induced an enantiotropic mesophase due to charge-transfer (CT) interactions in mixtures of PC derivatives I-III (viscous or oil-like liquids) with a strong electron acceptor 2,4,7-trinitro-9-fluorenone (TNF). Decreasing of number and length of substituents and modification of chemical structure of phthalocyanine macrocycle by introduction of four sulfur atoms (compound IV) resulted in crystallinity of this compound. In contact preparation of IV with TNF a wide area of CT-complex showing the monotropic mesomorphic phase is found out. All above mentioned CT-complexes do not form any lyomesophases in binary mixtures with number of organic solvents.     

Twist Grain Boundary Phases in the Binary Mixtures of Smectic and Cholesteryl Compounds

Abstract

The binary mixture of Terephthal-bis-p-n-hexadecylaniline (TB16A) and cholesteryl nonanoate (CN) exhibits I - N* - TGB _A - TGB _c*- Sm A - Sm I - Sm F - Sm G - K phases in the concentration region between 95 to 98% of TB16A at higher temperature range. The 90% of TB16A exhibits I - N* - TGB _A - TGB _c - Sm A - Sm I - Sm F - K phases. The mixtures of concentration between 50 to 80% of TB16A exhibits I - Sm A - Sm I - K phases. The frustrated blue phase is also observed in the mixtures of lower concentration of TB16A in CN at higher temperature. Optical, DSC and X-ray studies were carried out for the identification of the different phases. The phase diagram of the Chen-Lubensky model in this context is discussed.     

Influence of a.c. Electric Field on Infrared Absorption Spectra of Liquid Crystals and Determination of Orientational Order Parameter by Infrared Dichroism. II. Molecules with Weak Negative Dielectric Anisotropy

The influence of a.c. electric field (10 kHz) on the IR spectra of the nematic phase of six p-alkoxybenzylidene-p,n-butylanilines with weak negative dielectric anisotropy was studied and the temperature dependence of the orientational order parameter and threshold voltage have been investigated. The values of band elastic constants K₃₃ were calculated for MBBA and EBBA.     

Investigation of the structure and mechanisms of thermal motion in nanostructured undecylenic acid

Abstract

We have compared the structure, phase transitions and the thermal motion mechanisms of the undecylenic acid in bulk and inside a porous matrix using powder X-ray diffraction, calorimetry and IR-spectroscopy methods. The matrix used was a porous silicon with cylindrical pores (D?=?20?nm). The interaction between the walls of the porous silicon and the nanocrystals of the undecylenic acid results in a crystalline structure, which is significantly different from the structure of bulk undecylenic acid. A phase transition in the wall-adjacent layer of the undecylenic acid nanocrystals was shown to take place. The solid-state phase transition temperatures, as well as the melting point, were shown to be lower for nanocrystals. Topological soliton motion mechanism was suggested to explain the thermal motion of the undecylenic acid molecules in the rotator phase.     

Raman Spectroscopy Studies of Reorientations in Isotropic Phase of Alkoxyazoxybenzenes

Abstract

Raman spectra for isotropic phase of PAA, PAP and HOAB were recorded. Three benzene rings vibrations were chosen for quantitative analysis and the choice is discussed for different PAA deuteration. The separation of vibrational and rotational relaxation was performed. Vibrational relaxation is a dominating process, but ca. 10% of total bandshape can be attributed to rotational relaxation. Rotational correlation times τ2 were calculated for all studied substances to be of the order 10^?11 s. τ2 is connected with the rotation around the long molecular axis and supports the correlation time τ₁ received from the dielectric relaxation measurements.     

Amphotropic Properties of Cyclic Scyllitol Hexaesters [1]

Abstract

The new Scyllitol hexaesters with cyclic fragments in lateral substituents of series 1 were synthesized and their thermo- and lyotropic properties (with various organic linear or cyclic solvents) have been studied. It was established that the presence of benzene fragments in the substituents of the scyllitol hexaesters (1a-e) reduces the ability of the mesophase formation. The cyclohexane unit instead (sf. 1f) increases the thermostability of mesophase. While using above mentioned solvents a lot of cases of the induction of mesomorphic properties for non-mesogenic compounds of series 1 have been described. With cyclic solvents lyotropic mesophases were induced in binary mixtures of the hexaester 1a which showed mesomorphic properties neither under thermotropic conditions nor in binary mixtures with linear alkanes.     

Phase Transitions in Chlorobenzene-Cis-Decalin Mixtures: A Nuclear Magnetic Resonance,Thermal Analysis and Phase Diagram Study

Using nuclear magnetic resonance and differential scanning calorimetry we have been able to observe the molecular rotation properties of chlorobenzene-cis-decalin mixtures in their glassy, amorphous and crystalline phases. The results indicate: that in “rapidly” cooled samples the behavior of the host dominates the properties of the mixtures; that reorientation of the guest molecule is less restricted in the amorphous phase than in the glassy phase; that when the material crystallizes from the amorphous phase on warming, reorientation again becomes severely restricted. The temperatures at which these phenomena occur agree with the phase diagram that has been determined for these materials. Similar experiments on t butyl chloride-cis-decalin mixtures support the above conclusions. These conclusions are in agreement with the previous dielectric studies.

Our magnetic resonance and thermal analysis experiments support the argument that the behavior of the CD host dominates the behavior of guest polar molecules in rapidly cooled mixtures. At the lowest temperature (110 K) CD forms a glassy phase unaccompanied by any heat of crystallization. On warming this glassy phase transforms near 150 K to an amorphous phase where considerable motion of both host and guest molecules occurs. Above 160 K the material crystallizes and reorientation is once again severely restricted up to the eutectic melting of 210 K. There appears to be no time dependence to these transformations and, as long as one stays below the crystallization temperature, the glassy-amorphous transition is reversible.     

Crystal structure and phase transitions in the new crystals of [(CH3)2NH2]2CuCl4[(CH3)2NH2]Cl

Crystals grown from a solution of dimethylammonium and copper chlorides are studied using electron paramagnetic resonance (EPR) and X-ray diffraction. The dielectric properties of the crystals grown are measured. It is established that the crystals have the composition [(CH₃)₂NH₂]₂CuCl₄[(CH₃)₂NH₂]Cl and, in phase I at room temperature, are described by the orthorhombic space group Pna2₁ with the unit cell parameters a = 11.338 Å, b = 9.981 Å, and c = 15.675 Å. At temperatures of 279 K and 253 K, the crystals undergo jumpwise phase transitions into the incommensurate modulated ferroelectric phase II and commensurate modulated phase III, respectively. __________ Translated from Kristallografiya, Vol. 49, No. 1, 2004, pp. 92–100. Original Russian Text Copyright ? 2004 by Kirpichnikova, Pietraszko, Bednarski, Waplak, Sheleg. Dedicated to the 80th Birthday of L.A. Shuvalov     

ENHANCED FLEXOELECTRIC SWITCHING BEHAVIOUR IN THE CHIRAL NEMATIC PHASE

Abstract

In order to develop materials that exhibit enhanced flexoelectric switching in the chiral nematic phase we have identified mesogenic units that display inherently strong flexoelectric coupling capabilities. Here we examine the oxycyanobiphenyl (OCB) moiety: homologues from the nOCB series exhibit significant electro-optic switching effects when doped with a highly chiral additive. Here we have examined lower dielectric anisotropy materials, since they allow the flexoelectric response to be extended to high field amplitudes. We show that dielectric coupling strength can be low in symmetric bimesogenic molecules. The flexoelectric response of such a molecular structure is tested by doping a homologue from the series CBOnOCB with a chiral additive: very significantly we find that the optic axis is rotated through 2φ = 45° in < 50 μs on reversing the polarity of the field (amplitude E = ±6 V μm^?1). Subsequently we have synthesized room temperature chiral nematic materials that exhibit 2φ > 90° at E = 10 V μ^?1.     

Seven-coordinated chelate [Cd(SCZ)3·H2O](HTNR)2(H2O)2: Synthesis,crystal structure and energetic properties

To deepen the study on heptacoordinate transition metal coordination complexes, a novel seven-coordinated coordination compound [Cd(SCZ)₃·H₂O](HTNR)₂(H₂O)₂ (1, SCZ = semicarbazide, H₂TNR = styphnic acid) was synthesized and characterized by elemental analysis and FTIR spectroscopy. Single-crystal X-ray diffraction analysis revealed that 1 crystallizes in the triclinic space group P-1. The thermal decomposition mechanism of 1 was studied by differential scanning calorimetry (DSC). The non-isothermal kinetics parameters were calculated by the Kissinger's method and Ozawa-Doyle's method, respectively. Sensitivity tests revealed that 1 is insensitive to mechanical stimuli. The heat of combustion was measured by oxygen bomb calorimetry and the enthalpy of formation was calculated.     

Nematic Thiophenes for STN-LCDS and Flexoelectric LCDS

Abstract

We report anew class of liquid crystals incorporating a 2,5-disubstituted thiophene ring and one or two conjugated trans-carbon-carbon double bonds in the terminal chain for use as components of nematic mixtures for TN-LCDs and STN-LCDs. The combination of these molecular elements contribute towards generating a high birefringence, a high nematic clearing point as well as a low viscosity for these materials. Several of these compounds exhibit a melting point below room temperature. This is the first time a room temperature nematic phase has been reported for phenyl thiophene derivatives. The new thiophene compounds can be used to induce a high birefringence in nematic mixtures for LCDs with short response times.     

Refractive Indices and Dielectric Studies of Three Phenylcyclohexane Liquid Crystals in the Nematic Phase

The refractive indices n_e, n_o and densities and the dielectric primittivities ε_∥ and ε_⊥ of three P-cyano-P-alkylcyclohexanes (PHC-3, PCH-5, PCH-7) have been measured at different temperatures. The order parameters calculated by using Neugebauer and Vuks' methods were comparable. A comparison of order parameters of the PCH with the corresponding alkylcyanobiphenyls at reduced temperatures τ (= T-T_NI/T_NI) shows that the replacement of a phenyl ring with a cyclohexyl ring decreases the order parameter appreciably. The experimental values of dielectric permittivities ε_⊥ in each of the PCHs are found to be less than half of the ε_⊥-value calculated from Maier and Meier equations using the polarizability and the order parameter obtained from refactive index measurements. This was explained as due to short range antiparallel ordering of strongly polar molecules in the nematic phase, as proposed by Chandrasekhar and Madhusudan, which was not taken into consideration in Maier and Meiers' theory     

Enol tautomer of the acetate ester of 3-methoxy-4-hydroxyphenylpyruvic acid: Crystallographic and NMR spectroscopic evidence

The para acetate ester azlactone of vanillin 2 was synthesized from vanillin 1 and hydrolyzed with sodium hydroxide. The yielded product 3 was investigated with X-ray Crystallographic and nuclear magnetic resonance techniques. Compound 3 crystallized in the orthorhombic Pbca space group (Z = 8) and with cell dimensions a = 14.732(2), b = 12.756(3), c = 12.747(6)Å revealing the enolate tautomer and not the keto form of 3-methoxy-4-hydroxyphenylpyruvic acid as the acetate ester. The structure exhibited the pyruvic acid side chain in the trans extended conformation. A single proton on the benzylic carbon atom further suggested the existence of the enolate tautomer form of 3 in solution. The chemical shift values and peak integration in the NMR spectra add additional support to this finding.