Influence of fluorination and chain length on the static dielectric properties of nematogenic bi-cyclohexyl phenyl derivatives

Effects of fluorination and chain length of six bi-cyclohexyl phenyl compounds (3ccp-f, 3ccp-ff, 3ccp-fff, 5ccp-f, 5ccp-ff, 5ccp-fff) on their physical properties have been investigated. All the compounds exhibit nematic phase over a wide range of temperatures. Molecular mechanics calculation reveals that dipole moments of the molecules increases (1.93D to 3.37D) with fluorination in both propyl- and pentyl-based systems; increment in f to ff derivatives is more than in ff to fff derivatives. However, no change in dipole moment is observed with increased chain length. In the nematic phase, average value of dielectric constant (?_avg) is less than the extrapolated values of ?_iso in isotropic phase. Increased fluorination also decreases splay elastic constant, and thus faster response is expected in triply fluorinated compounds.     

Study of optical parameters of two fluorinated isothiocyanato nematic liquid crystals exhibiting high birefringence

Optical studies have been carried out on two fluorinated isothiocyanato nematic liquid crystal (LC) compounds 4′-butylcyclohexyl-3, 5-difluoro-4-isothiocyanatobiphenyl and 4′-pentylcyclohexyl-3, 5-difluoro-4-isothiocynatobiphenyl. Transition temperatures of the two samples were confirmed using a polarizing microscope. The two LC compounds were found to exhibit fairly high clearing temperatures. Measurements of refractive indices of the two compounds were done by using thin prism method with He-Ne laser beam of wavelength 630 nm. Birefringence of the two LC compounds was calculated from the measured refractive indices. Both the compounds are found to display fairly high values of birefringence. Validation of a modified four-parameter model, based on Vuks equation describing the temperature dependence of refractive indices of the two liquid crystals, is also presented in this paper. The model is validated by fitting the experimentally measured values of refractive indices, birefringence and average refractive indices of the two nematic LCs with the theoretical values. In this paper, the calculation of order parameters of the LCs is presented by using two methods: direct extrapolation method based solely on the birefringence data and by using modified Vuks method based on Haller’s extrapolation. As observed from the obtained results, this procedure of calculating order parameter gives very reasonable results.     

Nematic liquid crystals exhibiting high birefringence

Two fluorinated isothiocyanato nematic liquid crystalline compounds, 4′-butylcyclohexyl-3, 5-difluoro-4-isothiocyanatobiphenyl and 4′-pentylcyclohexyl-3, 5-difluoro-4-isothiocynatobiphenyl are studied in detail to obtain their different physical parameters. Optical polarizing microscopy, differential scanning calorimetry, density and dielectric studies have been carried out for the two samples. Both the samples were found to have high clearing temperature (>100 °C) and exhibit small enthalpy of transition. The two samples exhibit high optical birefringence (Δn > 0.2). The values of order parameters for the two samples were obtained using different approaches, namely, Vuks’, Neugebauer’s, modified Vuks’ and direct extrapolation method from birefringence data. Experimentally obtained values of order parameters have also been compared with theoretical Maier–Saupe values. The parallel and perpendicular components of dielectric permittivity values of the two compounds were also calculated and their anisotropy values were found to be small. The effect of temperature on the molecular dipole moment μ and the angle of inclination β of the dipole axis with the director have also been investigated in this work.     

Dielectric properties of ten three-ring LC fluorosubstituted isothiocyanates with different mesogenic cores

Results of the static and dynamic dielectric investigations of 10 liquid crystalline compounds consisting of three rings (phenyl or cyclohexyl) with the n-propyl and isothiocyanato terminals, fluoro lateral substitutes, and different bridging groups (CH₂CH₂ and/or COO) are presented. These compounds form the nematic phase in a broad temperature range. The dielectric parameters are analyzed in relation to the chemical and dipole structures of the molecules. The influence of different structural elements such as rings, bridging groups and fluorosubstitution on the dielectric properties is discussed. The relaxation time and activation enthalpy characterizing the molecular rotations around the short axes were determined. In the nematic phase, the compounds having two fluorine atoms at the lateral positions exhibit a crossover of the principal permittivity components within the megahertz frequency range. This makes them interesting from the application point of view as the so-called dual frequency materials.     

Physical characterisation of 4′-butyl-4-heptyl- bicyclohexyl-4-carbonitrile

We report measurements of birefringence, bend elastic constant, principal dielectric constants and conductivities as functions of temperature in the nematic phase of 4′-butyl-4-heptyl-bicyclohexyl-4-carbonitrile (CCN-47). The compound exhibits a small birefringence and a large negative dielectric anisotropy. Surprisingly, it shows a negative conductivity anisotropy, which probably owes its origin to the structural details of the molecules. The orientational order parameters (S) are estimated from both birefringence and dielectric anisotropy, and they agree well in the entire nematic range. The experimental results are compared with those on CCH7, which has a similar core structure but with a longitudinal dipole moment, in place of the transverse one of CCN-47.     

Corresponding states of nematic birefringence: an order parameter universality

In this work, experimental birefringence data of several nematic liquid crystals, which share the presence of a nematic–isotropic and a nematic–crystalline phase transition, are used to show that the birefringence presents a universal behaviour which encompasses the entire range of the nematic phase. The meaning and consequences of such behaviour are discussed; as the order parameter is proportional to the birefringence, this result implies that the order parameter has an extended universal profile, similar to a corresponding state law, which is independent of the liquid crystal compound and becomes determined once the temperature interval of the existence of the nematic compounds is obtained.     

Synthesis of novel fluorinated bis(biphenyl)diacetylene derivatives as highly birefringent nematic liquid crystalline materials

Three novel fluorinated bis(biphenyl)diacetylene (1,4-bis(biphenyl-4-yl)buta-1,3-diyne) derivatives, with alkoxy tails at the terminal positions, were prepared for use as high-birefringent nematic materials. To lower the thermal transition temperatures and determine the origin of the nematic phase of these compounds, fluorine atoms were introduced at lateral positions. Thermal transition temperatures and mesomorphic properties were investigated by differential scanning calorimetry and polarized optical microscopy. In addition, birefringence was measured using a microscopic method based on the interference method. Both the nematogenic properties and optical performances depended not only on the number of fluorine substituents, but also on the position of the substituents. The compound having the highest fluorine content exhibited the lowest birefringence, and the compound bearing fluorine atoms only on the inner benzene rings of the biphenyl cores exhibited an enantiotropic nematic phase with a maximum birefringence value of 0.48 at 550 nm and 140 °C.     

Mesomorphic, dielectric, and optical properties of fluorosubstituted biphenyls, terphenyls, and quaterphenyls

Compounds with moderate and large negative dielectric anisotropy (Δɛ) are very attractive liquid crystal (LC) for vertical alignment mode (VA). Materials with such properties can be achieved by lateral substitution of a polar group into a mesogenic molecule. We synthesized some new LC materials with a negative value of Δɛ, a moderately high birefringence (Δn), and a low viscosity. The mesomorphic and physical behaviour of the novel biphenyls, terphenyls and quaterphenyls fluorosubstituted in the rigid core and also with fluorinated alkyl and alkoxy chains are investigated. The prepared series of four LC compounds are promising for new LC mixtures for various applications. Examples of nematic mixtures with Δɛ∼−3.25 will be presented.     

Effect of lateral fluorination in antiferroelectric and ferroelectric mesophases: synchrotron X-ray diffraction,dielectric spectroscopy and electro-optic study

Effect of lateral fluorination in the rigid core on several macroscopic and microscopic properties of a terphenyl based mesogenic chiral ester has been studied by synchrotron X-ray, dielectric and electro-optic techniques. Correlation lengths across the smectic planes, in para-, ferro- and antiferroelectric phases, are found to be significantly less in the fluorinated compound. Para to ferroelectric transition is found to be tricritical in nature in both the compounds. Fluorination resulted in slower response under a square pulse. Collective mode relaxation behaviour, with and without bias field, in all the phases are also found to be different in the fluorinated compound.     

Orientational order parameter studies in two symmetric dimeric liquid crystals – an optical study

The optical technique developed by [W. Kuczynski, B. Zywucki, and J. Malecki, Determination of orientational order parameter in various liquid-crystalline phases, Mol. Cryst. Liq. Cryst. 381 (2002), pp. 1–19; B.J. Zywucki and W. Kuczynski, IEEE transactions on optical phenomena – The orientational order in nematic liquid crystals from birefringence measurements, Dielectr. Electr. Insul. 8 (2001), pp. 512–515] is fabricated and used to determine the orientational order parameter in two dimeric liquid crystalline compounds nematic and SmA phases of α,ω-bis(4-alkylanilinebenzylidene-4′-oxy)alkane (m.OnO.m) homologous series. The compounds studied are 5.O8O.5 and 5.O10O.5 which exhibit nematic and SmA, and nematic phases, respectively. The orientational order parameter in both the phases of nematic and SmA phases of the compound one and the nematic phase of the compound two are obtained using the principle of Newton's rings which gives directly the birefringence, δn of the liquid crystal dimer. The merits of the technique used are presented over the conventional techniques for the determination of orientational order parameter. The results for the two compounds are compared with those values estimated from n _e, n _o and density using the two internal field models due to Vuks and Neugebauer applicable to nematic phase.     

Dielectric and electrooptical properties of hockey-stick-shaped liquid crystal with a negative dielectric anisotropy

We investigated dielectric properties of a hockey-stick-shaped liquid crystal (HLC). Two dielectric relaxation modes were observed at 0.91 kHz and 4.51 MHz. The low frequency relaxation modes in isotropic, smectic, and nematic phases are related to the motion of ions, collective tilt fluctuation, and rotation around the long molecular axis, respectively. Meanwhile, the high frequency relaxation modes in nematic and smectic phases was due to the rotation around the short axis of the molecules and hindered by the resistance of the ITO layers. We also examined the electrooptical response of the 5.0 wt% HLC-doped commercial nematic liquid crystal (LC) mixtures. The birefringence of the LC mixture was slightly increased, while the falling time and the rotational viscosity was decreased in the nematic phase.     

Correlation of magnetic anisotropy with dielectric anisotropy in fluorinated phenyl bicyclohexane liquid crystal

Temperature dependence of magnetic anisotropy of homologous nematic fluorinated phenyl bicyclohexane liquid crystals is measured by a magneto-electric method.The result shows that the diamagnetic property is slightly influenced by the positions and the numbers of fluorine atoms substituted at the phenyl ring.By investigating the correlation of the dielectric anisotropy with the magnetic anisotropy,a novel explanation is proposed for the behaviour of the molecular dipole-dipole dimerization in the polar liquid crystal compounds.     

Deuterium NMR investigation of the biaxial nematic phase in an organosiloxane tetrapode

Deuterium NMR is used to examine the molecular order exhibited by an organosiloxane tetrapode giving the first experimental evidence, using a bulk sample, for the existence of a biaxial nematic phase in this type of compounds. The temperature dependence of the averaged quadrupolar coupling constant and asymmetry parameter was determined in the compound's nematic phase. Two distinct regimes could be identified, one with a vanishing asymmetry parameter corresponding to a uniaxial nematic phase and another with a significant temperature dependent asymmetry parameter, corresponding to a biaxial nematic phase. The high values obtained for the asymmetry parameter at the lower end of the nematic range are well above experimental error and constitute a definite proof of the biaxial nature of the nematic phase exhibited by the studied compound for those temperatures.     

Dielectric investigation on some binary mixtures of hockey-stick-shaped liquid crystal and octyloxy-cyanobiphenyl

We report the measurement of temperature-dependent dielectric parameters in some binary liquid crystal mixtures comprising of a hockey-stick-shaped mesogen 4-(3-decyloxyphenyliminomethyl) phenyl-4-decyloxycinnamate (SF7) and calamitic compound 4′-octyloxy-4-cyanobiphenyl (8OCB). All the investigated mixtures possess a large positive dielectric anisotropy (Δε), although a noticeable reduction has been found by increasing the diverse-shaped dopant concentration. Investigation on the pretransitional behavior in the vicinity of isotropic to nematic (I–N) phase transition suggesting a tricritical character for all the studied mixtures. Parameterization of dielectric permittivity close to the nematic to smectic-A (N–Sm-A) phase transition exhibits non-universal values of the critical exponents describing a second-order nature of the transition. Systematic variation of critical exponents against dopant concentration and McMillan ratio reveals a well consistency with those obtained from the high-resolution optical birefringence measurements.     

Texture transitions in binary mixtures of 6OBAC with compounds of its homologous series

Recently the authors have observed in compounds of the 4,n-alkyloxybenzoic acid series, with the homologous index n ranging from 6 to 9, a texture transition in the nematic range which subdivides the nematic phase in two sub-phases displaying different textures in polarised light analysis. To investigate a persistence of texture transitions in nematic phases, we prepared binary mixtures of 4,6-alkyloxybenzoic acid (6OBAC) with other members (7-, 8-, 9-, 12-, 16OBAC) of its homologous series. Binary mixtures exhibit a broadening in the temperature ranges of both smectic and nematic phases. A nematic temperature range of 75°C is observed. In the nematic phase, in spite of the microscopic disorder introduced by mixing two components, the polarised light optics analysis of the liquid crystal cells reveals a texture transition. In the case of the binary mixture of 6OBAC with 12OBAC and with 16OBAC, that is of compounds with monomers of rather different lengths, the texture transition temperature is not homogeneous in the cell, probably due to a local variation in the relative concentrations of compounds.     

Optical properties of a liquid crystal with small ordinary and extraordinary refractive indices and small optical anisotropy

Optical properties of a nematic liquid crystal with small refractive index and small birefringence were studied. The ordinary and extraordinary refractive indices and birefringence were measured as functions of temperature by using an Abbe refractometer and wedge nematic cells. From values of these indices the nematic orientational order parameter was calculated by using several methods and corresponding mathematical models. Kuczyński et al. method was found to be suitable for determining the order parameter also for materials featuring small ordinary refractive index, with unknown density.     

Conductivity relaxation in NiTiO3 at high temperatures

NiTiO₃ ceramics were prepared via the traditional solid-state reaction route. The dielectric properties of NiTiO₃ ceramics have been systematically investigated in the temperature range from room temperature to 1073 K NiTiO₃ ceramics exhibit intrinsic dielectric response in the temperature range below 400 K. Two relaxations were observed in the temperature range higher than 400 K. The relaxation activation energy is 0.95 eV and 1.17 eV for the low- and high-temperature relaxations, respectively. Our results strongly indicate that the two relaxations are related to conductivity relaxation associated with the singly and doubly ionized oxygen vacancies.     

Syntheses and mesomorphic properties of laterally fluorinatedphenyl isothiocyanatotolanes and their high birefringent mixtures

Syntheses and mesomorphic properties of di-, tri- and tetrafluorosubstituted 4′-(4-alkylphenyl)-4-isothiocyanatotolanes are described. Four fluorine substituted compounds in the position 2′, 3′, 3, and 5 show stronger nematic character than difluorosubstituted ones in the position 3, 5 or 2′, 3′. Example of high birefringence nematic mixture and its properties are given.     

Mesomorphic and dielectric properties of esters useful for formulation of nematic mixtures for dual frequency addressing system

Mesomorphic and dielectric properties of three homologous series of two and three ring fluorosubstituted esters are described. They are 4-cyano-3-fluorophenyl 4-alkyloxy-2-fluorobenzoates, 4-cyano-3-fluorophenyl 4-(4-alkylbenzoyloxy)-2-fluorobenzoates and 3-fluoro-4-cyanophenyl, or 3-fluoro-4-isothiocyanatophenyl or 3,4-difluorophenyl 4′-alkyl-2,3-difluoro-biphenyl-4-carboxylates. The compounds exhibit the nematic mesophase accompanied by the smectic A or smectic C mesophase in some cases. Most of them show strong dependence of the dielectric anisotropy Δε upon frequency, at low frequencies Δε reaches a value ∼200, while at high frequencies a small negative dielectric anisotropy appears.     

Study of dielectric anisotropy and other related parametersof a liquid crystal compound 7OAOB

Temperature-dependent dielectric anisotropy has been measured for the compound 4,4′-di-n-heptyloxy-azoxy benzene (7OAOB) by using a digital LCR bridge (HIOKI 3532-50 LCR HiTESTER). The compound 7OAOB is found to exhibit negative dielectric anisotropy in the nematic and smectic-C phase. Frequency-dependent electrical conductivity has been measured for this compound. The dc conductivity has also been obtained from frequency-dependent total conductivity studies by using universal power law equation. The activation energy of the dc conduction process and relaxation time has been determined in different mesophases. The behavior of splay elastic constant (K₁₁) with temperature has been examined for this compound. From the studies of UV-visible spectra, the values of energy band gap have been evaluated.