Polar nematic methyl (E)-[trans-4-cyclohexyl-substituted]allyl ethers: Synthesis, liquid crystal transition temperatures and some physical properties

We have introduced an oxygen atom and a carbon-carbon double bond with a trans-configuration (E) into the terminal alkyl chain attached to the cyclohexyl ring of a variety of two- and three-ring nematic liquid crystals of positive dielectric anisotropy. The new polar two-ring methyl (E)-allyl ethers often possess low melting points, but are not mesomorphic in general. The related three-ring methyl (E)-allyl ethers exhibit high clearing points and wide nematic ranges. Comparisons with the corresponding derivatives incorporating either just an oxygen atom or just a carbon-carbon double bond in the same position indicate that synergetic effects lead to broader nematic phases than would otherwise have been expected. This is partially due to the low smectic transition temperatures observed for the methyl (E)-allyl ethers. Selected physical properties of three binary mixtures of a weakly polar standard nematic liquid crystal and three difluoro-substituted (polar) liquid crystals (including the new ethers) differing only in the nature of the terminal chain show that, although some of the methyl (E)-allyl ethers exhibit longer switch-off times in TN cells than those of analogous liquid crystals incorporating either a methyl propyl ether or a 1-(E)-propenyl chain instead of the methyl (E)-allyl ether chain, they are still useful components for nematic mixtures, especially where a wide temperature range is required.     

The synthesis and liquid crystal transition temperatures of some weakly polar nematic methyl (E)-[trans-4-substituted-cyclohexyl]-allyl ethers

Abstract

We have introduced an oxygen atom and a carbon-carbon double bond with a trans-configuration (E) into the terminal alkyl chain of a wide variety of liquid crystalline cyclohexane derivatives to produce a variety of new methyl (E)-allyl ethers. The melting points and tendency to form smectic mesophases are often low, while nearly all of the compounds prepared exhibit a nematic phase. Thus, even two-ring derivatives can exhibit nematic phases over a wide temperature range (≤80°C), sometimes starting below room temperature (T _m≈10°C). Comparisons with the corresponding derivatives incorporating either just an oxygen atom or just a carbon-carbon double bond in the same position indicate that synergetic effects lead to broader nematic phases than would otherwise have been expected. Thus many of the new methyl (E)-allyl ethers exhibit nematic phases over a wider temperature range than the corresponding materials with an unsubstituted alkyl chain attached to the cyclohexyl ring. The new compounds are easily prepared from known starting materials. Many intermediates are themselves liquid crystalline. This allows investigation of the relationship between liquid crystal transition temperatures and the nature of the terminally substituted alkyl chain (for example, incorporating C[dbnd]C, OH, CO₂C₂H₅ and OCH₃ groups).     

The synthesis and liquid crystal transition temperatures of some weakly polar nematic trans-4-substituted-cyclohexyl (E)-alk-2-enoates

The effect on the liquid crystal transition temperatures of introducing various groups (for example incorporating C=C, O, CO₂ and CO) into the terminal alkyl chain of a weakly polar model compound 1-[trans-4-(trans-4-propylcyclohexyl)cyclohexyl]pentane has been investigated systematically. Only the compound containing both an ester function and a trans-carbon-carbon double bond exhibited a wide-range nematic mesophase at elevated temperatures. Therefore, a wide variety of trans-4-substituted-cyclohexyl (E)-alk-2-enoates incorporating a carbon double bond with a trans-configuration (E) in the terminal alkyl chain has been synthesized. Nearly all the two-ring esters prepared exhibit a nematic phase over a wide temperature range (≤ 100°C) at elevated temperatures (≤ 200°C). The tendency to form smectic mesophases is often low. Comparisons with the corresponding derivatives incorporating either just a carboxy group (COO) or just a carbon-carbon double bond (C=C) in the same positions indicate that synergetic effects lead to broader nematic phases than would otherwise have been expected. The new compounds are easily prepared from known starting materials.     

Polar nematic trans-4-substituted-cyclohexyl (E)-alk-2-enoates The influence of dipoles and double bonds on the transition temperatures and other physical properties

As part of a systematic study of the factors affecting nematic phase formation, the influence of introducing dipoles (in the form of oxygen, carbonyl and carboxy groups) and steric restrictions (in the form of carbon-carbon double bonds) in various positions, configurations and combinations in a model system (4-[trans-4-pentylcyclohexyl]benzonitrile) has been investigated. On the basis of these results, we have introduced an ester group and a carbon-carbon double bond with a trans-configuration (E) into the terminal alkyl chain attached to the cyclohexyl ring of a variety of two- and three-ring nematic mesogens of positive dielectric anisotropy. This is a new combination of a polar ester group (dipole effect) and the added rigidity imposed by the double bond (steric effect). Most of the new (E) alk 2-enoates containing two rings in the molecular core possess high melting points. Only a few two ring esters exhibit nematic phase, although the clearing point of those esters exhibiting mesomorphic behaviour was high. The corresponding three-ring (E)-alk-2-enoates incorporating an additional phenyl or cyclohexane ring also possess high melting and clearing points, as well as wide nematic ranges. No smectic mesophases could be observed for any of the (E)-alk-2-enoates synthesized. Comparisons with the corresponnding derivatives incorporating either just an ester group, or just a carbon-carbon double bond in the same position indicate that synergetic effects lead to higher clearing points than would otherwise have been expected. The new (E)-alk-2-enoates possess a surprisingly moderate viscosity for esters. The high value of the elastic constant ratio k₃₃/k₁₁ is of advantage for mixtures designed for supertwisted nematic LCDs.     

The Effect Of The Position And Configuration Of Carbon-Carbon Double Bonds On The Mesomorphism Of Thermotropic, Non-Amphiphilic Liquid Crystals

The influence on their mesomorphic behaviour of introducing a carbon-carbon double bond into the chain, central linkage and alicyclic rings in the core of nematic and smectic liquid crystals (LCs) is discussed. Mesogens incorporating a trans-carbon-carbon double bond conjugated with an aromatic ring exhibit high mesophase-isotropic transition temperatures (T_c). However, they are photo-sensitive and can convert to the non-linear, non-mesogenic cis-isomers under the action of light. Non-conjugated double bonds in the terminal chain of mesogens can also lead to higher nematic and smectic C transition temperatures than those of the corresponding materials without a double bond, although the effect is not nearly as great. The position and trans-cis-configuration (E/Z) of the double bond are seen to be decisive. The combination of a hetero-atom (dipole effect) and the added rigidity imposed by the carbon-carbon double bond (steric effect) with a trans-configuration (E) in the terminal alkyl chain attached to the core of a liquid crystal molecule can give rise to a broad nematic phase. The double bond in the terminal chain of nematogens advantageously modifies the elastic constant ratios, as well as other properties of relevance to LCDs, especially for supertwisted TN-LCDs. The double bond in a central linkage gives rise to a broad spectrum of effects, sometimes suppressing undesired smectic phases and widening the nematic phase temperature range. Non-conjugated double bonds in the molecular core in the form of cyclohexene rings generally lead to lower transition temperatures, although smectic phases are sometimes suppressed and a nematic phase is observed. A conjugated double bond in a cyclohexene ring gives rise to a slightly higher T_NI. The effect on the transition temperatures of the double bond in steroid systems is complex.     

Four unit linking groups III. Liquid crystals of negative dielectric anisotropy

A series of 1-n-alkoxy-2,3-difluoro-4-[3-(trans-4-pentylcyclohexyl)propyloxy]-benzenes has been synthesized. Several homologues exhibit an enantiotropic nematic phase of moderately strong negative dielectric anisotropy (Δε ≈ 5) at or just above room temperature. An analogous series of three-ring diethers also including some (E)-3-allyloxy derivatives possesses enantiotropic, wide range nematic phases also of moderately strong negative dielectric anisotropy, but at elevated temperatures. The synthesis and liquid crystal transition temperatures of these 2,3-difluoroquinone derivatives are described and comparisons are made with the transition temperatures of the corresponding materials incorporating standard central linkages (-, C₂H₄, CH₂O and COO).     

Conformational analysis of 1,2-dimethoxyethane and 1,2-diphenyloxyethane incorporated in nematic liquid crystals

The conformations of flexible chain molecules incorporated in a nematic environment have been investigated. The phase behaviours and orientational characteristics of 1,2-dimethoxyethane (DME) dissolved in 4'-methoxybenzylidene-4-n-butylaniline at low solute mol fraction have been reported in our previous paper. In this work, proton-proton and carbon-carbon dipolar coupling constant measurements were attempted in addition to ²H NMR observations of quadrupolar splittings. These conformation-dependent properties were analysed according to the rotational isometric state (RIS) simulation scheme previously proposed. Our treatment rests on the assumption that the molecular axis of the chain should tend to align along the nematic field. Studies were further extended to a mixture of 1,2-diphenyloxyethane (DPE) with a nematic liquid crystal, 4,4'-azoxyanisole. Replacement of the terminal methyl groups in DME by phenyl groups leads to DPE. The results of the analysis indicate that the fraction of elongated conformers such as ttt tends to increase significantly on going from the isotropic to the liquid crystalline solution: DME; 9·4 to 15·6 per cent and DPE; 12·3 to 20·0 per cent. The tgt form (tg⁺ t or tg^-t) is the lowest-energy arrangement of these molecules. The fraction of this conformer also increases in DPE (26·8 to 40·0 per cent), while it decreases in DME (25·7 to 18·8 per cent). The conformational distributions of chain molecules were found to remain invariant over the range of concentration (0·5-6·0 mol per cent) and temperature (22·0-44·5°C) studied. It has been concluded that the observed variation of proton dipolar and deuterium quadrupolar couplings with concentration and temperature mainly arises from the orientational order of the molecular axis, which varies sensitively with the alignment of the surrounding solvent molecules. These results suggest that flexible chains are also participating in the nematic interaction by adjusting their configurations so as to enhance favourable interactions and suppress unfavourable steric repulsions when accommodated in an anisotropic potential field.     

Synthesis of novel nematic liquid crystals containing 3,3'-dimethyl-2,2'-bipyridyl

Liquid-crystalline 3,3'-dimethyl-2,2'-bipyridyl derivatives with long 4-(alkoxyphenyl)ethynyl (-C=C-C₆H₄-OR) groups in the 5,5'-positions were synthesized by palladium-catalysed crosscoupling reactions. The compounds exhibit exclusively nematic behaviour; for example, the hexyl derivative showed a nematic phase over the temperature range 145.2-205.0°C. On increasing the length of the terminal chain, the transition temperatures were lowered; for example, the hexadecyl derivative was nematic in the range 117.0-126.8°C.     

The synthesis and liquid crystalline behaviour of alkoxy-substituted derivatives of 1,4-bis(phenylethynyl)benzene

Despite the prevalence of organised 1,4-bis(phenylethynyl)benzene derivatives in molecular electronics, the interest in the photophysics of these systems and the common occurrence of phenylethynyl moeties in molecules that exhibit liquid crystalline phases, the phase behaviour of simple alkoxy-substituted 1,4-bis(phenylethynyl)benzene derivatives has not yet been described. Two series of 1,4-bis(phenylethynyl)benzene derivatives, i.e. 1-[(4'-alkoxy)phenylethynyl]-4-(phenylethynyl)benzenes (5a-5f) and methyl 4-[(4'-alkoxy)phenylethynyl-4'-(phenylethynyl)] benzoates (18a-18f) [alkoxy = n-C₄H₉ (a), n-C₆H₁₃ (b), n-C₉H₁₉ (c), n-C₁₂H₂₅ (d), n-C₁₄H₂₉ (e), n-C₁₆H₃₃ (f)] have been prepared and characterised. Both series have good chemical stability at temperatures up to 210°C, the derivatives featuring the methyl ester head-group (18a-18f) offering rather higher melting points and generally stabilising a more diverse range of mesophases at higher temperatures than those found for the simpler compounds (5a-5f). Smectic phases are stabilised by the longer alkoxy substituents, whereas for short and intermediate chain lengths of the simpler system (5a-5c) nematic phases dominate. Diffraction analysis was used to identify the SmB_hex phase in (5d-5f) that is stable within a temperature range of approximately 120-140°C. The relationships between the organisation of molecules within these moderate temperature liquid crystalline phases and other self-organised states (e.g. Langmuir-Blodgett films) remain to be explored.     

Synthesis and characterization of main chain liquid crystalline ionomers containing sulphonate groups

A series of main chain liquid crystalline ionomers containing sulphonate groups pendent to the polymer backbone were synthesized by an interfacial condensation reaction of 4,4'-bis(1,10-sebacyloxo)benzoic acid, brilliant yellow (BY), and 4,4'-biphenyldiol. 4,4'-Bis(1,10-sebacyloxo)benzoic acid exhibited nematic schlieren texture during heating and cooling. The ionomers are thermotropic liquid crystalline polymers and thermally stable to about 270°C. They exhibit broad mesophase regions over a range of 220°C and the same nematic mesomogen with a colourful thread texture as B₀-LCP, which implies that the introduction of an ionic group did not change the texture of the B₀-LCP. However, the thermotropic liquid crystalline properties were somewhat weakened when the concentration of BY was more than 5%. The inherent viscosity in N,N-dimethylformamide solution suggested that intermolecular associations of sulphonate groups occurred at low concentration, and intermolecular associations predominated at higher concentration.     

Mixed-valent diruthenium (II,III) aliphatic carboxylates: columnar mesophases in dodecylsulfate and octylsulfonate derivatives

Two series of polymeric mixed-valent diruthenium (II,III) aliphatic carboxylates of formula Ru₂[O₂C(CH₂)_n-2CH₃]₄X (where X=dodecylsulfate (DOS) anion and n=8, 9, 16 and 18, or X=octylsulfonate (OS) anion and n=8, 10, 12, 14 and 18) were synthesized and characterized. Their liquid crystalline properties and crystalline (Cr) structures were analysed; the Cr phases are lamellar in all cases. For long chain DOS derivatives (n=16 and 18) hexagonal and rectangular columnar mesophases (Col_h and Col_r) with melting temperatures close to 140°C were observed. For long chain OS derivatives (n≥10) a Col_h mesophase was observed, with melting temperatures between 140 and 190°C.     

Novel bent-shaped liquid crystalline compounds II. Synthesis and properties of the 1,3-bis4-[4-(4-alkyloxybenzoyloxy)benzylidene]aminophenoxypropan-2-ols

A series of new symmetric dimer compounds was synthesized, constaining 2-hydroxy-1,3-dioxypropylene as the central linkage and terminal alkyl chains with different lengths. The chemical structures of the liquid crystal dimers (2ES-n) were examined by FTIR and ¹H NMR spectroscopy. Their mesomorphism, thermodynamic properties and optical textures were investigated by differential scanning calorimetry, polarizing optical microscope and X-ray diffraction. For homologues with terminal propyloxy and butyloxy chains, no liquid crystalline phase was observed. Homologues with pentyloxy and hexyloxy terminal chains showed nematic phases, while those with heptyloxy, octyloxy, nonyloxy and decyloxy terminal chains displayed nematic phases and smectic phases. The results confirmed that the liquid crystalline phase changes from nematic to smectic as the terminal chain length increases.     

A novel series of styrene-based liquid crystal monomers displaying either nematic or chiral nematic phases

A new series of liquid crystalline styrene-based monomers is described. These monomers are prepared by the DCC-mediated esterification reaction between 4-[11-(4-vinylphenoxy)undecyloxy]benzoic acid and a range of phenols chosen due to their proven utility in the synthesis of liquid crystals. Most members of the series display thermally stable (enantiotropic) nematic phases, although a few give only monotropic nematic phases. By incorporating the (S)-2-methylbutyl side chain, monomers that exhibit the chiral nematic phase can be obtained. Predictably, monomers derived from phenols containing an additional ring as substituent (e.g. 4-cyano-4'-hydroxybiphenyl) display relatively high transition temperatures. In contrast, monomers derived from simple 4-n-alkylphenols possess a nematic phase, which is accessible at moderate temperatures. In addition, a eutectic mixture derived from these monomers has a melting point only just above room temperature, which is an advantage for the fabrication of robust films via the in situ photopolymerization process. Standard free radical polymerization of a number of these monomers provides side chain liquid crystal polymers, SCLCPs, with mesophases that are stable over a wide temperature range. For a homologous series of SCLCPs containing a terminal n-alkyl chain on the mesogenic group, an unexpected but distinct odd-even effect is observed.     

Direct decoration of disclinations by solidification-induced band texture for a nematic side chain liquid crystalline polymer

For a nematic polymethacrylate side chain liquid crystalline polymer, g 154 N 298 I (°C), the solidification-induced band texture has been observed aligned along the disclination under a polarizing optical microscope, when the specimen was quenched from 280°C to room temperature. The decoration technique of solidification-induced band texture, which is usually reported for main chain liquid crystalline polymers, was then introduced to reveal the director field pattern along a disclination for this side chain liquid crystalline polymer. It was found by infra-red dichroism measurements that the director orientation is parallel with the direction of the band. On this basis, disclinations with strength s=±1/2 and s=±1 were mapped according to the corresponding pattern of solidification-induced band texture. In addition, two types of inversion wall, loop-like and splay-type walls, were also found to be decorated by the solidification-induced band texture.     

Four unit linking groups IV. Liquid crystals of positive dielectric anisotropy

Three separate series of new materials of weak to strongly positive dielectric anisotropy have been prepared. Each series contains four sub-sets of materials each incorporating a different four unit linking group (i.e., C₄H₈, C₄H₆, C₃H₆O and C₃H₄O) and the same series of end groups (i.e. F, CN and OCF₃) in various substitution patterns. The synthesis and liquid crystal transition temperatures of these novel substances are described and compared with those of the corresponding materials incorporating standard central linkages (i.e.-, C₂H₄, CH₂O, COO). The effect of an additional trans carbon-carbon double bond in the terminal alkyl chain and in the central linking unit has also been studied.     

Electron transfer of quinone self-assembled monolayers on a gold electrode

Dialkyl disulfide-linked naphthoquinone, (NQ-C_n-S)₂, and anthraquinone, (AQ-C_n-S)₂, derivatives with different spacer alkyl chains (C_n: n = 2, 6, 12) were synthesized and these quinone derivatives were self-assembled on a gold electrode. The formation of self-assembled monolayers (SAMs) of these derivatives on a gold electrode was confirmed by infrared reflection-absorption spectroscopy (IR-RAS). Electron transfer between the derivatives and the gold electrode was studied by cyclic voltammetry. On the cyclic voltammogram a reversible redox reaction between quinone (Q) and hydroquinone (QH₂) was clearly observed under an aqueous condition. The formal potentials for NQ and AQ derivatives were −0.48 and −0.58 V, respectively, that did not depend on the spacer length. The oxidation and reduction peak currents were strongly dependent on the spacer alkyl chain length. The redox behavior of quinone derivatives depended on the pH condition of the buffer solution. The pH dependence was in agreement with a theoretical value of E_1/2 (mV) = E′ − 59pH for 2H⁺/2e⁻ process in the pH range 3–11. In the range higher than pH 11, the value was estimated with E_1/2 (mV) = E′ − 30pH , which may correspond to H⁺/2e⁻ process. The tunneling barrier coefficients (β) for NQ and AQ SAMs were determined to be 0.12 and 0.73 per methylene group (CH₂), respectively. Comparison of the structures and the alkyl chain length of quinones derivatives on these electron transfers on the electrode is made.     

Synthesis, micro structure, and thermal stability of side chain liquid crystalline polysiloxane polymers with an oligo (ethylene oxide) unit in the side chain

A series of new alkene monomers [MS3BDBEn, n=1-3] containing 4-oligo (ethylene oxide) monomethyl ether 4-biphenyl ether carboxyl benzoate as terminal groups were synthesized. These polymers were prepared by grafting these monomers onto the poly (methylhydrosilox-ane) (PMHS) backbone. The transition temperatures, liquid crystalline textures, and thermal stability of the polysiloxane polymers have been determined by thermal data, by optical texture, and by X-ray diffraction patterns. Polymers PS3BDBEn showed smectic or smectic and nematic phases which were not analogous to their precursor nematic monomers. The terminal length of the polymers affects not only the mesophase transition temperatures but also the layer-spacing length (d₁) and the side-chain distance (d₂). The long- and short-range orders can remain to some extent above the isotropization temperature and below the melting point. The polymer PS3BDBE3 decomposed in air 20°C above the isotropization temperature and lost its short range orders as detected by the X-ray diffraction analysis.     

Cholesterol-based dimeric liquid crystals: synthesis and mesomorphic behaviour

Chiral non-symmetric dimeric liquid crystals consisting of a cholesteryl ester moiety as chiral entity and a biphenyl aromatic core, interconnected through n-butyl (C₄) or n-pentyl (C₅) parity alkylene spacers, have been synthesized and investigated for their liquid crystalline properties. All the dimers exhibit enantiotropic mesophases. The first member of the dimers having the C₄ central spacer exhibit only the chiral nematic (N*) mesophase, while the higher homologues also show smectic A (SmA) and twist grain boundary (TGB) mesophases. The dimers of the other series containing the C₅ central spacer also have stable SmA, TGB and N* mesophases, except for the first which does not show the TGB phase. Both series of compounds show a weak odd-even effect with terminal alkyl chain substitution, while the spacer length has a marked influence on the phase transition temperatures.     

A new series of nematic and smectic liquid crystals with negative dielectric anisotropy: the effect of terminal chain substitution on thermal and electro-optical properties

Two alkyl (1b and 1c) and four fluoroalkyl derivatives (1d-1f) of 4-arylbutyric acid (1c, 1d and 1e) and 4-arylbutanol (1b, 1f and 1g) [aryl = 2',3'-difluoro-4'-(2-(E-4-pentylcyclohexyl)ethyl)-biphenyl-1-yl] were prepared and investigated in the pure form as nematic materials (1b and 1c) and as additives to a ferroelectric liquid crystal (FLC) host (1d-1f). A comparison of 1b and 1c with the decyl analogue 1a demonstrates the effect of terminal chain modification on thermal and electro-optical properties. The substitution of the -CH₂O- (1b) or -COO- (1c) for -CH₂CH₂- in 1a destabilized the N and SmA phases or completely eliminated the smectic behaviour (1c). Dielectric analysis revealed that the chain modification increased the negative Δε, reduced elastic constant K ₁₁ and moderately decreased rotational viscosity γ₁. The temperature dependence of the key electro-optical parameters was analysed for ester 2, the methyl analogue of 1c, which exhibits a 45 K wide N phase. All four fluoroalkyl derivatives 1d-1f showed enantiotropic SmA phases and 1e also exhibited a monotropic SmC phase. Solutions of 1d-1f in a FLC host (0.2 mmol g^-1) increased the tilt angle Θ (up to 45° for 1g), reduced rotational viscosity γ₁ and the risetime τ. The most dramatic changes were observed for 1g, which contains 15 fluorine atoms.     

Synthesis and characterization of side chain liquid crystalline polymers exhibiting cholesteric and blue phases

A series of cyclosiloxane-based cholesteric liquid crystalline (LC) polymers were synthesized from a cholesteric LC monomer cholest-5-en-3-yl(3β) 4-(2-propenyloxy)benzoate and a nematic LC monomer butyl 4-[4-(2-propenyloxy)benzoxy]benzoate. All the polymers exhibit thermotropic LC properties and show cholesteric phases. Most of the polymers display four types of phase transition behaviour corresponding to glass transition, melting point, cholesteric phase-blue phase transition and clearing point. The mesophase temperature range of the blue phases are as broad as 20°C. The blue phase was confirmed by the apperance of planar textures and cubic packings. With an increase of non-chiral component in the polymers, the clearing point decreases slightly, while the glass transition and melting temperatures change little. In the reflection spectra of the polymer series the reflected wavelength broadens and shifts to longer wavelength with increase of the non-chiral component in the polymer systems, suggesting that the helical pitch P lengthens.