Formation of a homochiral antiferroelectric ground state in asymmetric bent-shaped molecules

New asymmetric bent-shaped molecules were synthesised and their mesomorphic properties and phase structures investigated by means of electro-optical measurements. All of the compounds formed fluid smectic phases and showed the homochiral antiferroelectric (SmC_AP_A) ground structure of the B2 phase. Comparing the new molecules with the previously reported classical symmetric bent-shaped molecules, we concluded that the modification of the chemical structure, especially the asymmetrical design of the bent-shaped molecules, is very significant for the interlayer interaction, influencing the formation of a specific polar order in banana phases.     

Polyimide-free homogeneous photoalignment induced by polymerisable liquid crystal containing cinnamate moiety

We designed and synthesised a reactive mesogen (CRM) containing cinnamate moiety in the core and two acrylate groups at both ends. The structure was characterised by ¹H NMR and FT-IR, and its liquid crystalline property was investigated by DSC and POM. The compound showed a nematic phase between 175.2 – 97.5 ^oC, followed by a smectic C phase between 97.5 and 57.9 ^oC during the cooling cycle. We proposed a fabrication method to achieve uniaxial homogeneous alignment of liquid crystals by irradiating the CRM-doped LC mixture in a sandwich cell of in-plane switching mode (CRM-IPS) with linearly polarised ultraviolet light. The results indicated that the CRM-IPS cell showed excellent initial dark state with a good alignment state, electro-optical performance and durability. This confirmed that the photo-induced alignment of LCs by CRM possesses outstanding alignment capability compared to the conventional rubbed polyimide alignment layer. We expect that this fabrication method is a promising candidate for cost-effective, green-manufacturing, and high-quality alignment method for the manufacturing of high-resolution liquid crystal displays (LCDs).     

Bulk-mediated in-situ homogeneous photoalignment induced by reactive mesogen containing diphenylacetylene moiety

ABSTRACT

We designed and synthesised a reactive mesogen containing diphenylacetylene moiety in the mesogenic core and two polymerisable acrylate groups at both ends. By irradiating linearly polarised UV light on the conventional host LC mixture containing a small amount of the synthesised reactive mesogen in a sandwiched cell without an alignment layer, we demonstrated an in-situ photo-induced homogeneous alignment of liquid crystals without a pre-treated alignment layer, which was achieved by an irreversible polarisation-selective [2 + 2] photodimerization of diphenylacetylene moiety with linearly polarised UV irradiation at above the isotropic temperature of LC mixture. The resulting homogeneous alignment showed a superior initial dark state, negligible pretilt angle and excellent stabilities. Furthermore, the in-plane switching (IPS) LC cell prepared by this method exhibited a better dark state and electro-optic performance compared to that with conventional-rubbed polyimide alignment layer. The single photoirradiation process automatically resulted in a perfect alignment matching of optical axes between the top and bottom substrates in the LC cell, giving rise to an excellent dark state overcoming an intrinsic misalignment issue and complex fabrication process. The proposed in-situ alignment method is a promising candidate for cost-effective, green-manufacturing, and high-quality alignment technique in the manufacturing of high-resolution liquid crystal displays.     

Symmetric bent-shaped liquid crystal dimers showing transitions between optically uniaxial and biaxial smectic phases

We report the synthesis of a new homologous series of symmetric bent-shaped liquid crystal dimers bearing oxybiphenylcarboxylate mesogens linked by a pentamethylene spacer. The dimers were characterised by optical, thermal, X-ray diffraction and computational techniques. Shorter dimers showed an I-N-SmA-SmC phase sequence with strong first-order phase transitions. For the longer dimers, the unconventional uniaxial SmA phase present in shorter members collapsed. A 9:1 binary mixture of a bent dimer with a non-mesogenic dimer exhibited upon the SmA-SmC phase transition a maximum layer contraction of 1%.     

Unusual electro-optical response of an oblique columnar phase formed by a bent-core mesogen

Mesophases formed by bent-core mesogens have attracted special attention because they can organise into fluid phases with polar order and supramolecular chirality. In this paper, a new five-ring bent-core mesogen is presented which forms a columnar mesophase. The structure is built up by layer fragments and possesses an oblique two-dimensional lattice with a layer group of the type p112/a. The columnar phase can be transformed into a ferroelectric SmCP phase (SmCP_F) by application of a sufficiently high electric field (25 V µm^-1). This field-induced transition was found to be reversible. The mechanism of the polar switching depends on the frequency of the applied electric field. The switching takes place in the usual way by a collective rotation of the molecules around the tilt cone. At very low frequencies (0.1 Hz and lower), the polar switching is based on a collective rotation of the molecules around their long axes. In the latter case, the switching is accompanied by an inversion of the layer chirality.     

Corrosion inhibitor by a polymerizable columnar mesogen based on hexabenzocoronene derivative

A hexa-peri-hexabenzocoronene derivative (HBC-1,2,4-Ph-DA-C₁₂) with the central HBC core tethered with three diacetylene (DA)-containing chains and three saturated alkyl chains was applied for the anticorrosion of an iron surface. This compound, which exhibits a columnar liquid crystalline phase, could be polymerized by ultraviolet irradiation through reactions between the DA units. This polymer coating exhibited a good anticorrosion property with good hydrophobicity (contact angle [CA] ~ 84°–105°) and excellent mechanical property. It showed a high resistance of 149 kΩ/cm², much higher than that of known anticorrosion materials.     

The binary system composed of a bent-core compound forming a B7 phase and a nematogenic calamitic compound

The isobaric phase diagram for a binary system of liquid crystals is presented. One mixing component is a bent-core mesogen forming a B₇ phase and the other one is a nematogenic calamitic compound. The occurring mesophases have been identified by their optical textures as well as by X-ray investigations. The B₇ phase exists down to about 60 mol% of the bent-core compound A / 40 mol% of the calamitic compound B. Two additional mesophases are induced in the mixed phase region. At medium concentrations (around 50 mol%) an oblique columnar phase appears which shows a similar X-ray pattern to the B₇ phase but an utterly different nucleation on cooling the isotropic liquid and a completely dissimilar texture. At concentrations below 35 mol% A, a further phase is induced which possesses an irregularly modulated layer structure. This phase can be regarded as an intermediate state between the regularly modulated Col_r (B₁) phase and the intercalated B₆ phase. It transforms on cooling into the phase with a regularly modulated structure.     

The effects of azo-oxadiazole-based bent-shaped molecules on the temperature range and the light-responsive performance of blue phase liquid crystal

A series of light-responsive azo-oxadiazole-based bent-shaped molecules was synthesised and characterised. Their effects as dopants on the blue phases (BPs) range of the chiral nematic liquid crystal (N*LC) matrix and light-responsive properties including the photo-inducing change of UV absorption properties, photo-adjusting the BP structure as well as the temperature range were investigated. It was found that the azo-oxadiazole-based bent-shaped molecules with branched terminal had better miscibility in LC host than the molecules with straight terminal, and that increasing the length of rigid core of bent-shaped molecules will greatly improve the effect of widening the BP temperature range.     

Synthesis of steroid compounds containing a pyridazinone moiety

Russian Chemical Bulletin - The reaction of 17-ketosteroids and glyoxylic acid followed by treatment with hydrazine affords steroid compounds condensed in the 16 and 17 positions with the...     

Variety of mesophases in compounds with an increasing number of lactate units in the chiral chain

Liquid-crystalline compounds with different numbers of lactate units, n, in the chiral part were synthesised and mesomorphic properties studied. Physical properties were compared with respect to n. In the compound with one lactate unit in the chiral part the TGBA–TGBC–SmC* phase sequence was detected. For two lactate units the antiferroelectric SmC*_A phase occurs. Finally, three-lactate material exhibits the tilted hexatic SmI*(F*) phase below the ferroelectric SmC* phase. Dielectric spectroscopy and spontaneous tilt and polarisation were measured. For the three-lactate compound the temperature dependences were analysed in the vicinity of the SmC*–hexatic phase transition, and these properties compared with the theoretically predicted behaviour.     

Non-symmetrical bent-shaped liquid crystals based on a laterally substituted naphthalene central core with four ester groups

In the continuation of our study of the role of direction of the linking ester group and lateral substitution, we present three series of bent-core liquid crystals based on 7-hydroxynaphthalene-2-carboxylic acid. The mesomorphic properties were investigated by polarised optical microscopy, differential scanning calorimetry, X-ray diffraction and electro-optical methods. For non-substituted and laterally methyl-substituted compounds, columnar phases of broken-layer type and a smectic A phase were found. With the increasing length of the terminal alkyl chain, an increasing stability of the lamellar smectic A phase was observed. Substitution with a methyl group led to substantial narrowing of the mesophase range and the chloro-substituted materials exhibited no mesophases. The results are also discussed in context with the earlier studied naphthalene based mesogens.     

Synthesis and liquic crystalline properties of copper tetra-4-(n-alkoxycarbonyl)phthalocyanines

Copper tetra-4-(n-alkoxycarbonyl)phthalocyanines were synthesized, and their thermotropic and lyotropic mesomorphism at different lengths of lateral substituents was studied. Their possibility of forming both thermotropic and lyophilic mesophases (amphotropy) was found. The length of lateral substituents affects the type of supramolecular packing in the mesophase rather than the temperature of the crystal→mesophase phase transition. A rarecase of the mesogeneity of disk-like compounds of the same chemical nature possessing only four substituents is discussed. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 129–136, January, 2000.     

Synthesis and liquic crystalline properties of copper tetra-4-(n-alkoxycarbonyl)phthalocyanines

Copper tetra-4-(n-alkoxycarbonyl)phthalocyanines were synthesized, and their thermotropic and lyotropic mesomorphism at different lengths of lateral substituents was studied. Their possibility of forming both thermotropic and lyophilic mesophases (amphotropy) was found. The length of lateral substituents affects the type of supramolecular packing in the mesophase rather than the temperature of the crystal→mesophase phase transition. A rarecase of the mesogeneity of disk-like compounds of the same chemical nature possessing only four substituents is discussed. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 129–136, January, 2000.     

1,2-Propanediol-linked chiral symmetric and non-symmetric liquid crystal dimers containing trifluoromethyl

Four symmetric and non-symmetric chiral liquid crystal dimers containing trifluoromethyl groups, termed as TFBA-PD-TFBA, UEBBA-PD-TFBA, UEBA-PD-TFBA and UEA-PD-TFBA, respectively, have been synthesised and characterised. UEA-PD-TFBA exhibited chiral nematic phase, whereas the other three dimers displayed chiral smectic A phase. X-ray diffraction (XRD) has revealed the structure of the smectic A phase for TFBA-PD-TFBA to be intercalated, whereas that for UEBBA-PD-TFBA and UEBA-PD-TFBA to be monolayer and interdigitated, respectively. In addition, the weaker peak corresponding to a shorter layer spacing was observed for UEBBA-PD-TFBA and UEBA-PD-TFBA. Considering the results of XRD measurements and computer simulations, the structural model corresponding to the shorter layer spacing is assigned as horseshoe-like shape. The absence of smectic behaviour for UEA-PD-TFBA reveals that the weaker aromatic–aromatic interactions cannot stabilise the smectic A phase.     

From smectic to columnar phase of polypedal liquid crystals based on tetrathiafulvalene/1,3-dithiol-2-thione and cholesterol

Bipedal 1,3-dithiole-2-thiones attached two cholesteryl through a ω-thioalkanoyloxy spacer of varying length were synthesized. The bipedals were easily transformed to the appropriate tetrapedal tetrathiafulvalene derivatives by self-coupling reaction in net triethyl phosphite. All the synthesized compounds exhibit mesogenic phases in a wide temperature region, no crystallization but vitrifying to form glassy mesogens during cooling from the isotropic melt. The liquid crystals with shorter spacer (n=2-6) exhibited only a smectic A phase and those with the longest spacer (n=7) exhibited only a hexagonal columnar. In these series, the molecular packing depended on the length of spacers.     

A tensor model of liquid crystalline polymers: application to basic disclination processes

One of the features of liquid crystalline polymers (LCPs) is their strong elastic anisotropy, which means they have unequal elastic constants. Elastic anisotropy plays a crucial role in the microstructure and macroscopic properties of LCPs. In this paper, the effect of unequal elastic constants on microstructure is investigated without an external field by using a deterministic tensorial approach. In this model, the evolution of the director field can be viewed as a process driven towards the state of zero elastic torque. A tensor expression of the elastic torque is used so that the nematic symmetry is automatically conserved. In simulations of bulk samples, disclination lines of strength half and escaped integer disclinations are observed. The distortion fields around the disclinations are found to depend on elastic anisotropy. If the twist constant is the lowest, as is the case for main chain liquid crystalline polymers, the disclination lines are predominantly of the twist type.     

Structure–property relationship of photoactive liquid crystalline polyethers containing benzylidene moiety

A series of main chain photoactive liquid crystalline polyethers, containing rigid bisbenzylidene photoactive mesogen and flexible methylene spacers, were synthesized by polycondensation of bisbenzylidene diols and dibromoalkanes. The polyethers were characterized with ¹H NMR, gel permeation chromatography (GPC), differential scanning calorimeter (DSC), thermo gravimetric analyzer (TGA), and polarized light optical microscopy. The individual and combined effects of spacer length and number of methoxy substituents on mesogenic and photoactive properties were investigated. Both first order and second order transition temperatures decreased with increased spacer length and the number of substituents. The combined effect of spacers and substituents drastically reduced the transition temperatures. All monomers and polymers showed mainly the smectic mesophase. In a few cases, nematic droplets along with the smectic phase were observed. The width of the liquid crystalline phase reduced with an increasing number of methoxy substituents on mesogenic unit. Variation of spacer length has a negligible effect on photocycloaddition. However, steric hinderance caused by the substituents decreased the photoactivity as the number of substituents increased. Total energies of crosslinked dimers calculated from modeling studies supported the above findings. Intermolecular photocycloaddition was also confirmed by photoviscosity measurement. The refractive index change was found to be in the range of 0.017–0.031. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 2143–2155, 2009     

Photoinduced liquid crystal blue phase by bent-shaped cis isomer of the azobenzene doped in chiral nematic liquid crystal

A photoresponsive azobenzene molecule DCAZO2 with two cholesteryl groups linked to both sides of the azobenzene group is doped in a mixture of nematic liquid crystal E7 and chiral dopant S811 (61.9 wt% E7, 36.1 wt% S811 and 2.0 wt% DCAZO2). Cooled from isotropic phase to 33.0°C, chiral nematic liquid crystal (N*LC) was formed in the sample and then the temperature was kept unchanged at 33.0°C. UV light irradiation induces the trans–cis photoisomerisation and thus an obvious phase transition. When the azobenzene groups isomerise to a cis-saturated state, the UV light was turned off and the white light was turned on at the same time. The bent-shaped cis isomer then turns back to the planar trans isomer gradually. A blue–green platelet texture representing cubic blue phase (BP) was observed and the size of the platelets was increased along with the cis–trans isomerisation. UV–vis absorption spectra indicate that the photoinduced BP exists when the isomerisation degree is between 79% and 18%, and further cis–trans isomerisation change BP back into N*LC. The large geometric structure of the cholesteryl groups and the large bent angle θ of the cis isomer are supposed to be responsible for the interesting result.     

Preparation and characterization of side chain cholesteric liquid crystalline polysiloxanes containing two chiral groups

A series of liquid crystalline (LC) polysiloxanes containing diosgeninyl and menthyl groups (from monomers M ₁ and M ₂, respectively) were synthesized. The chemical structures of the monomers and polymers obtained were confirmed by elemental analysis, Fourier transform infrared spectroscopy, proton NMR and carbon‐13 NMR. The LC properties were investigated by differential scanning calorimetry, thermogravimetric analysis, polarizing optical microscopy, and X‐ray diffraction. Monomer M ₁ showed cholesteric oily‐streak and spiral textures. Copolymers P ₂–P ₅ exhibited cholesteric phases. With increasing concentration of M ₂ units, the glass transition and clearing temperatures decreased. Experimental results demonstrated that a flexible polymer backbone and a long flexible spacer tended to favour a lower glass transition temperature, higher thermal stability, and wider mesophase temperature range.