Synthesis and properties of allyloxy-based tolane liquid crystals with high negative dielectric anisotropy

A series of tolane liquid crystals (LCs) containing 2,3-difluorophenyl and allyloxy terminal groups have been synthesised via multi-step reactions. Their thermotropic mesophases, birefringences and dielectric anisotropy properties are discussed by comparison with the non-fluorinated analog. The results show that the allyloxy-based tolane LC 3TOV reveals a high value of the birefringence (0.29), negative dielectric anisotropy (?4.44) and a broad nematic mesophase with a low melting point and high clearing point. The effects of the terminal alkyl chains, the lateral fluoro substituents, the allyloxy terminal group and alkyne bridge on the mesomorphic and physical properties were also discussed. Meanwhile, Density Functional Theory (DFT) calculations of molecular conformation and polarisability were used to correlate the experimental findings.     

Dielectric behaviour of the B1 phase

Dielectric measurements on two different samples formed by banana-shaped molecules and showing a B₁ phase were carried out. Three relaxation ranges were detected: a very fast intramolecular motion, the fast reorientation about the long axes and a slow collective process. The data are discussed in relation to known X-ray investigations and dielectric results for homologous samples showing the B2 state. The main difference between B₁ and B₂ is that in B₁ no evidence for ferroelectric order in the short range was detected.     

Electrical orientation behaviour of a nematic liquid crystal with negative dielectric anisotropy above the microelectrodes

Factors for the electrical orientation on interdigitated microelectrodes deposited on a substrate, a microdielectrometric sensor (or a sensor), were discussed experimentally using a model nematic liquid crystal with negative dielectric anisotropy. The electrical orientation behaviour of the nematic liquid crystal was investigated using microdielectrometry and polarised optical microscopy. The experimental results were discussed in relation to electric-field line, surface topology, anchoring of the liquid crystalline molecules to the microelectrodes, and transversal rotation of a rode-like molecule of the liquid crystal with negative dielectric anisotropy.     

Synthesis and mesomorphic properties of chiral fluorinated liquid crystals

Some fluorinated chiral liquid crystals were synthesized and the compounds characterized by IR, ¹H NMR, ¹⁹F NMR and mass spectroscopy and elemental analysis. Their phase transition behaviour was investigated by differential scanning calorimetry and polarizing optical microscopy. Nearly all of the compounds synthesized are liquid crystals with an enantiotropic cholesteric phase. Some of them exhibit a blue phase. Lateral tetrafluoro substitution decreases the clearing point and molecular polarity affects the formation of liquid crystalline phases.     

The birefringence and extinction coefficient of positive and negative liquid crystals in the terahertz range

Birefringence and extinction coefficients of positive nematic liquid crystal (NLC) MLC-2142 and negative NLC BHR28000-400 are measured by terahertz time-domain spectroscopy system (THz-TDS). Frequency ranging from 0.1 to 0.6 THz, the birefringence of positive NLC MLC-2142 increases with the increase of frequency, and keeps larger than 0.23, which exhibits application potential in tunable broadband LC THz device where a large birefringence is required. In contrast, the birefringence of BHR28000-400 decreases with the increased frequency, which shows a completely different optical property from the positive NLC. The extinction coefficient of the above two kinds of liquid crystals are also investigated.     

New terphenyl liquid crystals terminated by 2-chloro-3,3,3-trifluoropropenyl group

New liquid crystal (LC) compounds composed of terphenyl as a core and 2-chloro-3,3,3-trifluoropropenyl terminal group were synthesised. Their structures were confirmed by nuclear magnetic resonance, mass spectra and infrared spectrometry. The mesomorphic and physical properties were evaluated. The results show that these compounds have a smectic phase, large dielectric anisotropy, high birefringence and suitable rotational viscosity. The lateral fluorine substitutions lead to a large perpendicular component of the dielectric constant, and a small value of Δε/ε_⊥. These characteristics are significant for the fringe-field switching LC display mode.     

Design and synthesis of nematic liquid crystals with negative dielectric anisotropy

Axially fluorinated cyclohexane derivatives are a promising new type of liquid crystals with negative dielectric anisotropy. Up to now, the technical application of these materials in LCDs was impeded by their strong tendency to form smectic B mesophases. We present a convenient synthetic pathway leading to new homologues carrying alkenyl side chains which induce nematic phases.     

Fast switching of vertically aligned negative liquid crystals by optically hidden relaxation

We propose a method for fast switching of vertically aligned (VA) negative liquid crystals (LCs) by hiding the relaxation process of LCs. During the turn-off process, a strong in-plane electric field is applied for a short duration of time instead of relying solely on the slow relaxation of LCs. The LC molecules are rotated to the transmission axis of one of the polarisers by the applied in-plane electric field, resulting in turn-off switching that is 5.8 times faster than that of a conventional VA cell. By applying an overdriving scheme, we experimentally obtained a total response time of 3.3 ms.     

Magnetic anisotropy of liquid crystals based on lanthanide mesogenic complexes

The values of magnetic anisotropy of smectic A-phases for a number of lanthanide complexes (LH)₂LM(NO₃)₂, where M=Nd, Eu, Gd, Tb, Dy, Ho, and Er, and LH is a Schiff's base), were measured. These values are two orders of magnitude larger than those normally found for diamagnetic liquid crystals and are well correlated with magnetic birefringence constants and molecular mangetic anisotropy of nomesogenic lanthanide diketonates. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 694–697, April, 1999.     

Kerr effect of low melting nematic liquid crystals with negative dielectric anisotropy

The two low melting nematic liquid crystals, 2-chloro-4-heptylphenyl 4-pentylbicyclo[2,2,2]octane-1-carboxylate (7CP5BOC) and 2-chloro-4-heptylphenyl 4-heptylbicyclo[2,2,2]octane-1-carboxylate (7CP7BOC) have been investigated to determine their electro-optical behaviour and third order non-linearity by the static Kerr effect method. Both liquid crystals are laterally substituted by a single chlorine atom located close to the ester linking group. The temperature dependence of the electric Kerr constant in the isotropic phase and the pretransitional behaviour have been investigated for these low birefringence nematic liquid crystals in the isotropic phase. Both the compounds, with negative dielectric anisotropy, have a positive Kerr constant. The Landau-de Gennes model was obeyed for these compounds.     

Nematic compounds and mixtures with high negative dielectric anisotropy

Syntheses, mesogenic and dielectric properties of three-ring alkylcyclohexyl, alkoxy and alkylphenyl benzoates substituted with four fluorine atoms or with two fluorine atoms and simultaneously a cyano group or two cyano groups are described. Their properties are compared with analogous three-ring phenyl tolanes. Conformation analysis and the calculation of dipole moments with the quantum chemistry method are performed to explain the experimental results. Several four fluorine-substituted two-ring tolanes and biphenyls are also prepared, which are used as a solvent decreasing viscosity, melting point and clearing point of the formulated mixtures. Examples of mixtures prepared from the esters and tolanes with high dielectric anisotropy are given. Curiosity is observed that 4?-(4-ethoxyphenyl)-4-pentyloxy-2,2?,3,3?-tetrafluorotolane is not the mesogenic compound, while analogous ester 4?-ethoxy-2,3-difluorobiphenyl-4-yl 2,3-difluoro-4-pentyloxybenzoate exhibits a broad temperature range of the nematic phase.     

Polarisation-dependent dielectric processes in ferroelectric liquid crystals

The behaviour of dielectric relaxation process has been investigated in four ferroelectric liquid crystal (FLC) materials having different spontaneous polarisation (Ps) values. Ps effect on the permittivity in four different FLCs has been carried out in highly anchored sample cells around ~8 μm thick. It has been found that the main contribution to the dielectric permittivity in chiral Smectic C (SmC*) phase is due to Goldstone mode (GM) and partially unwound helical mode (p-UHM). In higher PS value FLC materials, the p-UHM process is found to dominate the dielectric properties. It has also been observed that p-UHM process is highly dependent on the probing ac voltage and temperature, whereas GM is found to be weakly dependent of probing voltage and temperature in SmC* phase of all the studied FLC materials. The influential contribution of p-UHM has exhibited the dielectric properties in its intrinsic frequency range making the materials suitable for futuristic display and photonics devices.     

Transverse dipole association and negative dielectric anisotropy of nematic liquid crystals

Measurements are reported of the dielectric properties of three laterally substituted dicyano compounds with large molecular dipole moments perpendicular to the long molecular axis. Results are given for the materials in solution, in the isotropic liquid state, and for one compound in the nematic state. There is evidence of a high degree of local ferroelectric order in all phases, and dipole correlation factors in excess of 3 were obtained. The significance of these observations for macroscopic ferroelectrically ordered and biaxial liquid-crystalline phases is considered.     

A simple transflective liquid crystal display with composite dielectric layer

ABSTRACT

A simple transflective liquid crystal display with a vertically aligned cell using a composite dielectric layer is demonstrated. The top substrate has a top planar common electrode, two transparent dielectric layers with different dielectric constants are coated on the bottom planar pixel electrode to generate linearly varying electric potential from the transmissive region (T region) to the reflective region (R region), while two bumpy reflectors are coated under the bottom substrate. In this device, with the composite dielectric layer, the common and pixel electrodes generate a strong electric potential in the T region and a relatively weak electric potential in the R region. Consequently, the T and R regions accumulate the same electro-optical characteristics. The simulation results show that the display exhibits reasonably low operating voltage, high optical efficiency and well-matched voltage-dependent transmittance (VT) and reflectance (VR) curves. Besides, the driving mode and the fabrication process of the transflective liquid crystal display are fairly simple and it is suitable for mobile applications.     

Fast spatially localised electrooptical mode in vertically aligned nematic LCs with negative dielectric anisotropy

ABSTRACT

Electro-optical switching and the liquid crystal (LC) director distribution are studied in spatially periodic electric field for vertically aligned LC with negative dielectric anisotropy. Two electro-optical switching modes characterised by different switching times are observed. These modes are well distinguished optically by choosing proper geometry for the polarisers axes orientation. One of these modes is significantly faster as compared to the other. The fast switching is explained in terms of localised near-to-surface director reorientation. The 3D-numerical simulation shows very good agreement with the experiment: it points out the existence of the disclination lines and field-stabilised walls responsible for the localised director field switching and its relaxation. Possibilities of enhancing the fast mode for high-speed light modulators are discussed.     

Experimental investigation of liquid crystals in the millimetre frequency range

Dielectric properties of n·CB (n = 5, 6, 7, 8) liquid crystals (LCs) are investigated in the frequency intervals of 50–80, 120–160 GHz, and at 0.6 THz. The measurements are carried out in the millimetre wave range by the resonance method and with use of a reflectometer designed on the basis of a Michelson interferometer and quasi-optical metal-dielectric waveguides. Of most interest are the room temperature measurements of the birefringence of LCs. These measurements show that LCs can be used as polarisation transformers in the short wavelength region of the millimetre wave band and the long wavelength region of the sub-millimetre wave band.     

Synthesis and characterization of novel imidazolium-based ionic discotic liquid crystals with a triphenylene moiety

Two novel triphenylene-tethered imidazolium salts were synthesized either by the quaternization of 1-methylimidazole with an ω-bromo-substituted triphenylene or by the quaternization of a triphenylene-substituted imidazole with methyl iodide. Their chemical structures were determined by ¹H NMR, ¹³C NMR, IR, UV spectroscopy and elemental analysis. The thermotropic liquid crystalline properties of these salts were investigated by polarizing optical microscopy and differential scanning calorimetry. These triphenylene-based imidazolium salts with bromide or iodide as counterion show columnar mesophase properties over a wide temperature range.     

Computer simulation of the homeotropic to planar transition in cholesteric liquid crystals

The transition from the homeotropic to the planar state in cholesteric liquid crystal displays is investigated through computer simulation. The simulation reproduces the observed relaxation from the homeotropic state to the long pitch transient planar state. The simulation also agrees with the suggestion that the transition from the transient planar state to the planar state proceeds through a bulk modulation resulting in folding and buckling of cholesteric layers without introduction of defect cores. The model obtained agrees well with earlier experimental observations showing that the process includes a tilting of cholesteric helices, and that the surface plays only a minor role in the relaxation process.     

Dynamics in ferro- and antiferroelectric phases of a liquid crystal with fluorinated molecules as studied by dielectric spectroscopy

For 1-[3-fluoro-4-(1-methylheptyloxycarbonyl)phenyl]-2-[4-2,2,3,3,4,4,4-heptafluorobutoxybutoxy)biphenyl-4-yl]ethane (1F7), built of chiral molecules, results of dielectric measurements of liquid-crystalline and solid phases are presented. Rich polymorphism of liquid-crystalline (SmC*, SmC*_A and SmI*_A) phases as well as of solid (Cr1 and Cr2) phases were observed down to –130°C. At a frequency range from 0.1 Hz to 3 MHz, the relaxation processes were detected in ferroelectric SmC*, antiferroelectric SmC*_A and highly ordered SmI*_A smectic phases. The mechanism of complex dynamics (moleculear and collective) was identified with the help of the bias field. Vitrification of conformationally disordered crystal phase Cr2 was found in accordance with calorimetric observations.     

Synthesis and liquid crystalline behavior of azulene-based liquid crystals with 6-hexadecyl substituents on each azulene ring

Hexakis(6-hexadecyl-2-azulenyl)benzene (1b) has been synthesized by Co₂(CO)₈-catalyzed cyclotrimerization reaction of bis(6-hexadecyl-2-azulenyl)acetylene (2b). The mesomorphic behaviors of 1b, 2b, and 6-hexadecyl-2-phenylazulene (3b) were studied by differential scanning calorimetry (DSC), polarizing optical microscopy (POM), and X-ray diffraction (XRD) techniques and their mesomorphic properties were compared with those of their 6-octyl derivatives 1a, 2a, and 3a. Increase of the number of carbon atoms in the peripheral side chains drops the isotropization temperatures of 1b, 2b, and 3b by 56.9 °C, 33 °C, and 23.6 °C, respectively. Additionally, the phase-transition behavior varied with increase of the number of the peripheral chains, as well as decrease of the crystalline-mesophase transition temperatures, except for compound 3b. As the results, spontaneous monodomain homeotropic molecular alignment was revealed by compound 1b in its Col_hd mesophase on non-treated glass substrate, which would be attracted to the application for the device fabrication of molecular materials.