Morphological phase behaviour under pressure of polycatenar mesogens with a perfluorinated moiety

Two polycatenar materials composed of a four-aromatic-ring core with a perfluorinated moiety attached in one terminal position through either butylene- or pentylene spacer groups, and three tetradecyloxy chains at the other end (abbreviated as 14PC₄F and 14PC₅F), were investigated to study the effect of pressure on the phase transition behaviour. A polarizing optical microscope equipped with a high pressure optical hot stage, was used for the purpose. The T vs. P phase diagrams of 14PC₄F and 14PC₅F were constructed in the pressure region up to 100 MPa. 14PC₄F showed the stable crystal (Cr₁)-columnar tetragonal (Col_tet)-smectic A (SmA)-columnar hexagonal (Col_h)-isoropic liquid (I) phase transition sequence under all pressures. 14PC₅F exhibited the phase sequence metastable crystal (Cr₂)-cubic (Cub)-Col_tet-SmA-I in a melt-cooled sample on heating under pressure. But when the melt-cooled Cr₂ sample was annealed at 52-54°C for 2-3 h, the stable crystal (Cr₁) was formed slowly, giving a stable Cr₁-Cub-Col_tet-SmA-I phase sequence. The temperature region of the stable cubic phase broadened with increasing pressure. Furthermore a new mesophase of 14PC₅F was pressure-induced between the I and SmA phases on cooling at pressures above about 16 MPa. Since the monotropic mesophase exhibited a texture very similar to that of the high temperature Col_h phase of 14PC₄F with planar orientation, the new phase was assigned at a high temperature columnar hexagonal phase of 14PC₅F.     

Synthesis and properties of new materials with banana-shaped achiral cores and chiral end groups

New chiral bent-core mesogens, derivatives of 1,3-phenylene bis[4-(alkanyloxyphenyliminomethyl)benzoate], were synthesized with variation of a substituent (X=F, Cl); their antiferroelectric properties are described. The mesomorphic and switching properties were characterized by differential scanning calorimetry, polarizing optical microscopy, triangular wave method, and X-ray diffractometry in the small and wide angle regions. The presence of chiral tails at the terminals of side wings in the bent-core molecules induced a decrease in transition temperature and formation of the switchable SmC* phase in the melt. In addition, the introduction of a lateral halogen substituent in the Schiff's base moiety prevented the regular stacking of the molecules, resulting in the formation of very complex optical textures. The smectic phase with F-substituted PBFDOB and Cl-substituted PBCDOB showed layer spacings of 39 and 38.5?Å, respectively, corresponding to the end-to-end distance of molecules with a bent conformation. Significantly, the smectic phases of PBFDOB and PBCDOB exhibited a period of 179.5 and 131?Å, respectively, compatible with a helical structure with periodicity about 4.6 and 3.4 times the layer spacings.     

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.     

Columnar mesophases and phase behaviors of novel polycatenar mesogens containing bi-1,3,4-oxadiazole

A new series of liquid-crystalline bi-1,3,4-oxadiazole derivatives (2,2′-bis(3,4,5-trialkoxyphenyl)-bi-1,3,4-oxadiazole, BOXD-Tn, n=3, 4, 5, 6, 7, 8, 10, 14) were designed and synthesized. They have been confirmed to give rise to columnar mesophases. The columnar mesophases for BOXD-Tn (n=5, 6, 7, 8, 10) could be supercooled to −20 °C on the cooling runs. A room temperature Col_ho phase was obtained for BOXD-T14. All BOXD-Tn exhibit good fluorescence properties either in cyclohexane or in solid state.     

Polycatenar mesogens with a perfluorinated moiety showing a variety of liquid crystalline polymorphism

Polycatenar materials composed of a four-aromatic-ring core, with either a bulky or end-branched perfluorinated moiety attached at one end through a methylene spacer group, and three peripheral alkoxy chains of varying length (the carbon number n = 4, 6, 8, 10, 12, 14, 16) on the other end-ring, were synthesized to investigate the roles of the chemical structure of the compositional segments in relation to liquid crystalline phase formation. It was found that a homologous series of polycatenar materials exhibited a variety of liquid crystalline phases strongly dependent on the length of the peripheral alkyl chains. Moreover it was shown that changes in the spacer group and in the structure of the perfluorinated segment had a significant influence on mesophase formation.     

Pressure effects in a plastic columnar discotic triphenylene

The effect of hydrostatic pressure on the structure of a plastic columnar discotic triphenylene has been investigated. The goal was to determine whether pressure can be used to modify electronic properties via changes in structural properties of columnar discotics to any significant extent. The findings are that (i) the intra‐ and inter‐columnar distances are reduced in a nearly isotropic fashion, (ii) that the crystal sizes are reduced and (iii) that a transition takes place from a more highly ordered plastic columnar to a less ordered hexagonal columnar state with increasing pressure. The induced decrease of the molecular distances, amounting to 6% for pressures up to 17?kbar, are clearly too small to induce an appreciable modification of the electronic structure and thus opto‐electronic properties.     

A polycatenar mesogen with a perfluorinated moiety showing continuous phase transformation between a smectic A phase and a structured,fluid, optically isotropic phase

A polycatenar mesogen consisting of a four aromatic ring system with three long alkoxy chains and a bulky perfluorinated substituent has been synthesized and found to show the polymorphism of a cubic phase, a columnar phase and a smectic A phase, including a structured, fluid, optically isotropic phase with the phase transformation between the smectic A phase and the isotropic phase occurring continuously.     

High-frame-rate liquid crystal phase modulator for augmented reality displays

We optimise a new liquid crystal mixture with wide nematic range, high birefringence (Δn), high resistivity, moderate dielectric anisotropy (?ε) and relatively low rotational viscosity for augmented reality display applications. High Δn and large Δε allow a thin cell gap (d ≈ 1.5 μm) to be employed in a reflective liquid-crystal-on-silicon (LCoS) device to achieve 2π phase change at 5 V and 2.87 ms average phase-to-phase response time at 40°C without complicated overdrive circuitry. Such a fast response time enables an LCoS panel to achieve 240 Hz frame rate for field sequential colour operation to avoid image flickering.     

The NTB phase in an achiral asymmetrical bent-core liquid crystal terminated with symmetric alkyl chains

ABSTRACT

The characteristics of the twist-bend nematic (N_TB) phase of an achiral asymmetrical rigid bent-core liquid crystal (LC), the ends of which are terminated by symmetric alkyl chains, are reported. The nematic–nematic phase transition and its properties are studied by differential scanning calorimetry (DSC), polarising microscopy and the electro-optic techniques. Large domains of opposite handedness are observed in the absence of the external field in the N_TB phase. Another set of periodic striped pattern consisting of domains with sharp boundaries is formed when a high-frequency electric field with a magnitude above its threshold is applied across a planarly aligned cell. The neighbouring domains are of opposite chirality. The temperature dependence of the heliconical angle θ₀ is determined from the birefringence measurements using Haller’s extrapolation technique. This material shows lower values of the heliconical angle (~9.3° at a temperature of 155°C within the N_TB phase) when compared with the previously reported dimer-based twist-bend nematic LCs (31°±3°).     

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.     

Mesophase behaviour and thermal stability of octa-alkoxy substituted phthalocyaninatocobalt (II) complexes

Cobalt (II) phthalocyanines substituted with eight alkoxy chains in the peripheral (2, 3, 9, 10, 16, 17, 23, 24) positions were prepared. The alkoxy chain length was varied between n-butyloxy (C₄H₉O) and n-octadecyloxy (C₁₈H₃₇O). Studies by polarizing optical microscopy and high temperature X-ray diffraction revealed that all the complexes are liquid crystalline and that they exhibit a hexagonal columnar mesophase (Col_h). Transition enthalpies were determined by differential scanning calorimetry. The clearing point could only be observed for compounds with a chain length longer than C₁₃H₂₇O. Both the melting and clearing points decrease with increasing chain length. The transition temperatures of these discotic metallomesogens are higher than those of the corresponding metal-free phthalocyanines, but are comparable with those of the corresponding copper (II) compounds. The thermal decomposition of the compounds was studied by thermogravimetry.     

Novel high birefringence bistolane liquid crystals with lateral fluorosubstituent

Twelve compounds based on lateral fluorinated bistolane moieties with cyano or trifluoromethyl terminal groups were synthesised with Hagihara–Sonogashira cross-coupling reaction. The molecular structures of the compounds were confirmed by Fourier transform infrared, ¹H-nuclear magnetic resonance and mass spectrometry. The transition temperatures, optical textures and optical anisotropies were determined by differential scanning calorimetry, polarizing optical microscopy and ultraviolet microscopy. The effects of the terminal substituents and lateral fluorosubstituent group on their physical properties were investigated and discussed. The compounds with lateral double fluorosubstituent and cyano terminal group exhibited nematogenic texture, while other compounds showed smectic phase. All the compounds exhibited wide liquid crystal temperature ranges and high clearing points. Interestingly, the compounds have the high birefringence in the range of between 0.480 and 0.633.     

Photo-induced alignment behaviour of polymerisable liquid crystals on bisazide in a polymer matrix

Photo-reactive bisazide in a polymer matrix containing acryloyl groups on the side chain was investigated as a photoalignment layer for polymerizable liquid crystals (PLC). We found the thin film of bisazide (2,6-bis(4-azidobenzylidene)-4-methyl-1-cyclohexanone) in a polymer matrix, irradiated by linearly polarised ultraviolet light (LPUVL), was able to homogeneously align PLC. The LPUVL irradiation dose changed the orientation direction of the PLC on the thin film of bisazide in the polymer matrix. In addition, the direction of the slow axis for the retardation of the photoalignment layer changed from parallel to perpendicular to the LPUVL electric field with the irradiation dose. From these results, it was suggested that the PLC was likely to be aligned along the slow axis of the retardation of the photoalignment layer. We concluded that the key mechanism that changed the direction of the slow axis in a plane was the photoreaction of azide–acrylate at low irradiation dose and that of bis(benzylidene)cyclohexanone at high irradiation dose. Although the photoalignment as a result of a simple photo cross-linking was previously little known except for photo-dimerisation, we revealed that the photoaddition of azide–acrylate is able to achieve the photoalignment.     

Lyotropic liquid crystalline phase behaviour of a monomeric and a polymeric monosaccharide amphiphile in aqueous solution

Monomeric and polymeric amphiphiles were synthesized which exhibit lyotropic liquid crystalline phases in aqueous solution. The hydrophobic group of the monomeric surfactant is a dodecane-group esterified with acrylic acid. The hydrophilic unit is a monosaccharide derivative. By radical polymerization the monomer (N-D(-)-gluco-N-methyl-(12-acryloyloxy)-dodecane-l-amide) is converted into the corresponding polymeric surfactant. The monomer as well as the polymer exhibit a lyotropic 1.c. phase of lamellar structure. Owing to the polymerization the regime of the lamellar phase is greatly enlarged for the polymer, compared to the monomeric sufactant. These results confirm earlier investigations on non-ionic ethylene-glycol surfactants.     

Electrooptical properties of new type fluorinated phenyl-tolane isothiocyanate liquid crystal compounds

A series of 4?-(4-alkyl-phenyl)-2?,6?-difluorotolane isothiocyanate liquid crystal (LC) compounds were synthesised, and their phase transitions and electrooptical properties were investigated. These compounds exhibit high birefringence, about 0.47–0.52. As the number of carbon atoms in the alkyl chains increases, these LC compounds can exhibit smectic phases. When these LCs were mixed into the commercial LCs, the birefringence values of LC mixtures become higher than pure commercial LCs, and the visco-elastic coefficients of five LC mixtures are very close to each other at every test temperature. The results of response properties indicate that the compounds with a tri-fluorine substitution and n-propyl end group possess better response performance than the others. These LC compounds are particularly attractive for achieving fast response times in LC optic devices.     

Dielectric relaxation in two liquid crystalline polyacrylates under high hydrostatic pressure

For two polyacrylates with p-cyano-phenyl benzoate side groups and different spacer lengths the dielectric relaxation was measured in the nematic and isotropic phase at pressures up to 5000 bar. The relaxation of the cyano groups was observed as a separate relaxation process; it seems to be closely correlated with the glass relaxation. The pressure dependence of the glass temperature and of the clearing temperature could be derived from the experiments.     

Mesogenic behaviour of p-nitrophenyl group linked with biphenyl mesogens via flexible spacers

Three series of nematogens with a terminal p-nitrophenyl group linked with biphenyl groups via flexible spacers were synthesised and their transitional properties were studied by means of differential scanning calorimetry (DSC) and polarising microscopy (POM). The nematic–isotropic transition temperature and the associated entropy change showed an odd–even effect as the length of the spacers was varied, in which the even members exhibited higher values. Such an odd–even effect was in consistency with the feature of liquid crystal dimers and trimers. On the basis of these data, it seemed reasonable to assume that the terminal p-nitrophenyl group played a role of mesogen in the nematic liquid crystal. The p-nitrophenyl group may work as a mesogen as a result of the interaction with the biphenyl mesogens. The mesogenic behaviour of the terminal p-nitrophenyl group is explained on the basis of neighbouring group effect.     

Experimental studies on the phase diagram and physical properties of mixture of calamitic and discotic nematic liquid crystals

Physical studies on mixture of calamitic and discotic nematic liquid crystals are meagre although they are potential for optimising physical properties. Here, we report experimental studies on the phase diagram and physical properties of mixtures of ambient temperature discotic and calamitic nematic liquid crystals. A substantial decrease in several physical properties such as birefringence, dielectric anisotropy and elastic constants are observed with increasing wt% of discotic compound. On the other hand a large increase in the rotational viscosity is observed. Based on the experimental results a simple model of mutual orientation of the rod-like and disc-like molecules is proposed.     

Synthesis and properties of new non-symmetric 2,5-bis(4-alkylphenylethynyl)thiophenes

The synthesis and characterisation of new non-symmetric 2,5-bis(4-alkylphenylethynyl)thiophenes are described. Phase transition properties of these non-symmetric homologues are compared to those of the corresponding symmetric analogues. Furthermore, optical, dielectric and elastic data of investigated compounds and nematic mixture are provided. Compounds are characterised by ¹H NMR spectroscopy and mass spectrometry (electron ionization) analysis. They show an enantiotropic nematic behaviour in broad temperature ranges, confirmed by a polarising thermomicroscopy and differential scanning calorimetry. Detailed synthetic procedures are attached, showing a great synthetic versatility of the thiophene ring substitution. Synthesised compounds stand as promising components of highly birefringent liquid crystalline mixtures.     

Synthesis of cyclopentyloxy terphenyl liquid crystals with negative dielectric anisotropy

Liquid crystalline materials possessing negative dielectric anisotropy (Δε) have attracted considerable attention because they can be used to formulate eutectic mixture for several display applications. The negative Δε can be achieved by introducing a lateral polar substituent onto the mesogen core of a liquid crystalline material, and fluorine atoms are usually used because of the small size and high electronegativity. 2,3-ortho-Difluoro substituent liquid crystals exhibit stable and profound physical properties such as the low viscosity, strong negative Δε, and high solubility. To avoid the decrease of the mesophase range, 2,3-ortho-difluorophenyl is often placed next to the terminal chain. In this paper, we have synthesised a new series of 2,3-ortho-difluoroterphenyl liquid crystals with a negative dielectric anisotropy. Ether oxygen is introduced to the structure because it can further increase the lateral dipole moment. 3-Alkylcyclopentane end group decreases both the melting point and the viscosity, which makes it an appropriate substituent for negative Δε 2,3-ortho-difluoroterphenyl liquid crystals. The mesomorphic properties as well as the optical anisotropies of the synthesised liquid crystals are discussed in this paper. All compounds in the series are found to possess highly negative dielectric anisotropy, wide mesophase ranges; therefore, they have a great potential to formulate eutectic mixture for liquid crystal displays.