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.     

Synthesis and physical properties of tolane liquid crystals containing 2,3-difluorophenylene and terminated by a tetrahydropyran moiety

Four series of tolane liquid crystals containing 2,3-difluorophenylene and terminated by a tetrahydropyran (THP) moiety have been synthesised via multistep reactions based on 4-alkylcyclohexanecarboxylic acids and 4-bromo-2,3-difluorophenol. Their properties were measured by differential scanning calorimetry, polarising optical microscopy, Abbe refractometer and an electrical constants instrument. The results show that the THP-based liquid crystals nT reveal a relatively high value of the birefringence, negative dielectric anisotropy, good solubility and a broad nematic mesophase. The effects of the terminal alkyl chain, the lateral fluoro substituents and the THP terminal group on the mesomorphic and physical properties are discussed. Meanwhile, density functional theory calculations of molecular conformation and polarisability were conducted to correlate the experimental findings.     

Schiff base/ester liquid crystals with different lateral substituents: mesophase behaviour and DFT calculations

Four new groups of 4-((2?-substitutedphenylimino)methyl)phenyl-4”-alkoxy benzoates, In_a-d, of Schiff base ester liquid crystals, were prepared and investigated for their mesophase formation and stability. Each group constitutes four homologous series that differ from each other by the lateral attached polar group X in the ortho position for the imine mesogen at terminal benzene ring that alternatively changed from F, Br, NO2 and lateral benzene ring. Within each homologous series, the number (n) of carbons in the alkoxy chain varies between 6, 8, 10 and 12. Molecular structures of the prepared compounds were confirmed via elemental analysis, FT-IR, and ¹H NMR spectroscopy. Mesomorphic properties were investigated by differential scanning calorimetry (DSC) and the phase identified by polarised light microscopy (PLM). A comparative study was made between the investigated compounds and their previously prepared laterally neat, 4-((4?-phenylimino)methyl)phenyl-4”-alkoxy benzoates (IIn); the result revealed that all lateral substituents not only decrease the melting temperature but also the mesophase stability and shown only nematic phase. Density functional theory (DFT) calculations for new lateral derivatives were discussed.     

Substituted 1,4-dicyclohexyl-butan-1-ols A new class of liquid-crystalline compounds

Abstract

1,4-dicyclohexyl-butan-1-ols with a lateral hydroxyl group in the middle part of the molecule exhibit higher clearing temperatures than those of the respective compounds whithout this lateral group. A smectic B-like texture and a low temperature additional phase X were found by microscopic and calorimetric investigations. A further unexpected result was the strong increase of the dielectric constant at the S_B/X transition. These preliminary results are explained in terms of hydrogen bridges.     

Effect of lateral bromo substituent on the phase behavior of four-ring azo/ester/azo liquid crystalline materials

ABSTRACT

In order to study the influence of lateral Br substitution on mesophase behaviour, five homologous series of 4-substituted phenylazo phenyl 4?-(3?-bromo-4?-alkoxyphenylazo) benzoates (In_a–e) have been synthesised. Within each homologous series, the alkoxy group varies from 6 to 16 carbons, while other terminal group substituents, X, are CH₃O, CH₃, H, Br and NO₂ groups; the mesophase behaviour of these series is compared with previously prepared laterally neat analogues, 4-substituted phenylazo phenyl 4?-(4?-alkoxyphenylazo) benzoates (IIn_a–e) and laterally methyl analogues, 4-substituted phenylazo phenyl 4?-(3?-methyl-4?-alkoxyphenylazo) benzoates (IIIn_a–e). Similar to lateral methyl analogues, the present series, lateral Br substitution showed that, independent of the polarity of the substituent X or the alkoxy-chain length, the nematic phase is predominant with relatively high stability and broad temperature ranges. The mesophase stability varies between 204.0°C and 335.0°C for the nematic phase and 169.6°C and 281.0°C for the SmA phase. Their total mesophase temperature ranges vary between 87.2°C and 201.4°C. All compounds were found to be thermally stable within the mesophase temperature range, except the lower homologue of the nitro and Br substituted derivatives. The obtained results are discussed in terms of molecular polarisability.     

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.     

Polarity and steric effect of di-lateral substitution on the mesophase behaviour of some azo/ester compounds

Eight homologous series of 2-(or 3-)substituted phenyl 4?-(4″-alkoxy (2?-, or 3″-substituted phenylazo) benzoates (In_XY) were prepared in which the suffix ‘X’ refers to the lateral substituent X attached to the terminal benzene ring that carries the alkoxy group, and the suffix ‘Y’ refers to the substituent attached to the other terminal phenyl group. Within each homologous series, the length of the terminal alkoxy group varies from 8 to 16 carbons, while the lateral polar substituents, X and Y, alternatively varies between CH₃ and F. The mesophase behaviour was investigated by differential scanning calorimetry and identified by polarised optical microscopy. The results were discussed in terms of the polarity and steric effects of the two lateral substituents. Comparative correlations were made to investigate the effect of the second lateral substituent on the mesophase behaviour of the previously investigated mono-laterally substituted analogues. UV–vis spectroscopic study revealed that the compounds I8_XY exhibited two absorption bands: low intense bands at 254–263 and a broad band at 364–376 nm. These bands are attributed to the π–π? transition of the phenyl rings and the whole mesogenic portion.     

Mesophase behaviour of laterally di-fluoro-substituted four-ring compounds

Four new groups of the di-fluoro-substituted 4-(2′-(or 3′)-fluoro phenylazo)-2-(or 3-) fluoro phenyl-4″-alkoxyphenylazo benzoates (In–IVn) were prepared and investigated for their mesophase behaviour. An alkoxy group of variable chain length (n = 6, 10 and 14 carbons) is attached to the terminal phenylazo benzoate moiety, and two lateral fluoro substituents are attached individually with different orientations to the other two adjacent rings. The molecular structures of the prepared compounds were confirmed by Fourier transform infrared spectroscopy and ¹H NMR spectroscopy. The study aims to investigate the steric effect of the spatial orientation and relative positions of the two lateral fluorine atoms on the mesomorphic properties in their pure states. The mesophase behaviour was investigated via differential scanning calorimetry and mesophases were identified by polarised light microscopy. The investigation shows that these compounds exhibit high enantiotropic mesophases (SmC and N) and broad mesophase temperature range. The type and stability of the mesophase depends on the length of the terminal alkoxy chain and the position the two fluoro substituents. A comparison between these investigated compounds with their corresponding three-ring analogues was discussed.     

The effect of locations of triple bond at terphenyl skeleton on the properties of isothiocyanate liquid crystals

Three series of new tolanyl benzene isothiocyanate liquid crystals A, B and C were synthesised with lateral fluorine substituent at different positions and 3,5-difluoro terminal groups. Series D and compound E4 with carbon–carbon triple bond connected with the isothiocyanated benzene were also prepared as reference. Their structures were confirmed using infrared spectrometry, nuclear magnetic resonance and mass spectrometry. The mesomorphic properties, birefringence, dielectric anisotropy and miscibility of these compounds were investigated. Series A exhibits higher clearing points than that of the corresponding compounds D. Series B displays the lowest melting points, the broadest nematic phase temperature range and no smectic phase with increase of the carbon number of alkyl chain. Lateral fluorine in the position of Y (series C) results in an increase of smectic phase. The birefringence (~0.454–0.490) of series A is almost the same as that of the corresponding reference compounds D, while series B shows a decrease of Δn about 0.05 compared with compounds D. For comparison, mixtures formulated by A and B exhibit higher clearing points and better low temperature stability than those of the mixtures based compounds D in the same fraction.     

Effect of position of the lateral fluoro substituent on the mesophase behaviour of aryl 4-alkoxyphenylazo benzoates in pure and binary mixtures

Five groups of 4-substituted phenyl 4?-(2?- (or 3″-) substituted-4?-alkoxyphenylazo) benzoates (In_a-c to Vn_a-c) were investigated in which, within each group, the length of the terminal alkoxy group varies between 8 and 16 carbons, while the other terminal substituent, X, is a polar group that alternatively changed between the electron-donating CH₃O and the electron-withdrawing Br group, in addition to the un-substituted analogue (X = H). The lateral substituent (Y) in the five groups I–V varies, respectively, between H, 3-CH₃, 2-CH₃, 3-F and 2-F. Their mesophase stabilities were determined by differential scanning calorimetry and phases identified by polarised light microscopy. The two newly prepared groups of compounds (IVn_a-c and Vn_a-c) are structurally characterised by infrared, ¹H-NMR, mass spectroscopy, thermogravimetric and elemental analyses. Binary phase diagrams were constructed for each pair of isomers from groups IV and V bearing the same wing substituents but the lateral F is attached to different positions (2? or 3″).     

Structure–property investigation of 2‐ and 3‐thienylacrylates bearing laterally fluorinated azobenzene moieties

The synthesis, transition temperatures and structure–property relationships of a variety of thiophene‐containing azobenzene esters derived from either 3‐(2‐thienyl)acrylic acid (series I, IV, VI, VIII, X and XII) or 3‐(3‐thienyl)acrylic acid (series II, V, VII, IX, XI and XIII) and appropriate fluoro‐ and non‐fluoro‐substituted ‘azophenols’ are reported. For comparative purposes, the non‐heterocyclic counterparts, i.e. cinnamates (series III), were also prepared and are reported. All 70 final esters are mesomorphic, exhibiting the nematic phase alone. Their mesomorphic properties are dependent on the disposition of the terminal thiophene moiety. In general, 3‐thienyl‐substitution gives thermally more stable compounds than 2‐thienyl‐substitution. The influence of mono‐ (series IV, V, VI and VII) and di‐lateral (series VIII, IX, X, XI, XII and XIII) fluoro‐substitution on mesomorphic properties is investigated in detail. Lateral fluorination lowers mesophase thermal stability and its extent is dependent on the number and disposition of the lateral fluoro‐substituents. Di‐lateral fluorination across the long molecular axis is more detrimental to mesophase thermal stability than along the long molecular axis.     

New laterally aromatic branched liquid crystal materials with large nematic ranges

Abstract

Three new homologous series of mesogens containing three rings in the main structure and a lateral aromatic branch have been synthesized. When the lateral aromatic branch is attached to the middle ring, monotropic phases are observed, and there is a strong tendency to form the smectic C phase. When the lateral aromatic branch is attached to an outer ring, the compounds possess purely enantiotropic nematic phases. The terminal alkoxy chain in the outer ring which has a lateral aromatic substituent can be replaced by a rigid group without disturbing the large nematic range. Selective introduction of a particular substitutent may be useful for the construction of devices possessing desirable characteristics such as non-linear optical properties.     

Lateral substituted phenyl biphenylcarboxylates ‒ non-chiral analogues of ferroelectric liquid crystals

ABSTRACT

We report mesomorphic properties of non-chiral liquid crystalline molecules and study the effect of lateral halogen (fluoro, chloro) substitution in the molecular core and the length of terminal chains. We have prepared nine homologue series with the molecular core substituted at different positions with respect to the ester linkage group. Additionally, we have modified the length of both terminal alkyl chains (hexyl, octyl, decyl and dodecyl) symmetrically for both terminal chains. The effects of the lateral substitution and the chain length are analysed with respect of the possibility to reduce the transition temperatures and tune the properties of presented liquid crystalline molecules.     

The effect of terminal epoxy modification on the mesomorphic and thermal stability of biphenyl ester liquid crystals

ABSTRACT

A series of biphenyl ester compounds with alkenyl, alkyl, oxirane, and 1,3-dioxolane as the terminal groups have been synthesised for investigating the mesomorphic and thermal stability. Based on the same carbon skeleton, the allyl-terminated compound V1OLC has no mesophase, while the oxirane-terminated compound POLC not only has wider multiple smectic mesophase compared with the propyl-terminated compound 3OLC but also exhibits the best thermal stability. However, when the terminal groups change from oxirane to 1,3-dioxolane, EOLC shows neither mesophase nor the good thermal stability. In addition, with the extension of the terminal chains, the butyl-terminated compound 4OLC gives the widest smectic mesophase. These imply that the mesomorphic and thermal stability of these biphenyl ester compounds are dependent on the suitable terminal chain lengths and epoxy sizes. Meanwhile, Density Functional Theory (DFT) calculations were conducted to assist in analysing the experimental results.     

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.     

Investigation of terminal olefine in the isothiocyanatotolane liquid crystals with alkoxy end group

Three series of liquid crystals based on isothiocyanatotolane core with terminal alkenoxy groups (4A-1?4C-2) have been synthesised via multi-step reactions. Their thermotropic mesophases and physical properties are evaluated by comparison with the alkoxyl analogues (4a-1?4c-2). The results show that the isothiocyanatotolane LCs with alkenoxyl terminals deliver higher birefringence (0.410~0.436), lower melting points, lower melting enthalpies and viscosity. The effects of the alkenoxy terminal groups and the lateral fluoro substituents on the mesomorphic behaviour and physical properties are discussed. The formulation of the mixture containing isothiocyanatotolane liquid crystals with alkenoxyl terminals provides evidence of an useful candidate for liquid crystal photonics.     

Polarography in Propylene Carbonate: Preliminary Considerations

Abstract

A preliminary investigation is reported of a relatively new solvent for polarography. Its aprotic nature, stability and high dielectric constant should prove useful for studies of organic and inorganic compounds. Preparation and stability of salt bridges and reference electrodes are described. Behavior of dropping mercury, pyrolytic graphite and platinum indicator electrodes is described.     

A two dimensional liquid crystal simulation for thin film transistor liquid crystal displays

Abstract

A two-dimensional liquid crystal simulation, whose electrode configuration corresponds to that in a thin film transistor liquid crystal display (TFT-LCD), was carried out. Simulation results show that the lateral field between buslines and pixel electrode forms a reverse tilt domain. This reverse tilt domain leads to the disclination on the pixel electrode. The distance from the pixel electrode edge to this disclination location depends on the dielectric anisotropy and elastic constant for the liquid crystal. A small dielectric anisotropy or large elastic constant makes this distance small.     

Experimental and theoretical approaches of molecular geometry and mesophase behaviour relationship of laterally substituted azopyridines

New 10 laterally chloro azopyridine-based derivatives, with terminal alkoxy chain length varies between 6 and 16 carbons, were prepared. The mesophase behaviour was investigated for prepared homologues by differential scanning calorimetry and polarised light microscopy. Each homologous series differ from each other by the lateral attached Cl group in the ortho- or meta-position with respect to the ester carbonyl core in the central benzene ring. Elemental analyses, FT-IR, ¹H NMR and C¹³ NMR spectroscopy were used for molecular structure confirmation of the prepared compounds. All prepared homologues were found to be nematogenic. Density functional theory theoretical calculations used to confirm the experimental data and the results are consistent with the experimental investigations. It was found that the nematic range and stability is influenced by the length of the alkoxy chain and the orientation of the lateral chloro group. Moreover, an experimental and theoretical comparative study between the Cl-lateral isomers with respect to the previously reported methyl lateral derivatives as well as the laterally neat one in determination of the type, the stability of the mesophase and the molecular geometry.     

Molecular relaxation of components of LC mixture 2f- 3333 (ROLIC) for dual- frequency electrooptic shutters

ABSTRACT

Dual-frequency liquid crystal materials are used in fast response electrooptic modulators in different devices for digital information displaying and processing. In an NLC mixture 2f-3333 (ROLIC, Switzerland) there are components with lateral dipole moment. Their molecular relaxation is investigated by a method of dielectric spectroscopy. Values of relaxation time and its activation energy, molecular friction coefficient, rotational diffusion are obtained. They are compared with data obtained by other methods like LC viscosimetry.