Synthesis and liquid crystalline properties of novel compounds containing a 3‐fluoro‐4‐cyanophenoxy group

A series of novel compounds containing a 3‐fluoro‐4‐cyanophenoxy group were synthesized and fully characterized by IR and ¹H NMR, and their mesomorphic properties were studied. Seven compounds exhibited enantiotropic nematic phases and three compounds exhibited monotropic nematic phases, as confirmed by differential scanning calorimetry and polarizing optical microscopy. Selected properties of the liquid crystalline compounds synthesized were calculated by ab initio methods at a HF/6‐31G level. The bond lengths, bond angles and dihedral angles of the fragments with the same structure change little between the compounds. All the compounds with a terminal alkoxy chain approached a planar structure.     

Synthesis of mesomorphic aryl esters bearing a pyridine ring

Aryl carboxylic esters bearing disubstituted pyridine rings were synthesized starting with various pyridine mono- or dicarboxylic acids by means of reactions, principally, with phenolic compounds. Some of the pyridine monocarboxylates thus synthesized exhibited clear nematic phases at relatively low temperatures, whereas most of the 2,5-pyridine dicarboxylates bearing two benzene rings provided nematic phases, the ranges of which were some times wider and/or lower than those of the corresponding aryl esters bearing benzene rings alone.     

Synthesis and phase behaviour of hydrogen-bonded liquid crystalline complexes of allyloxybenzoic acid compounds with 4,4'-bipyridine

Three kinds of benzoic acid compounds were synthesized and characterized; the corresponding symmetric hydrogen-bonded liquid crystalline complexes based on these benzoic acid compounds and 4,4'-bipyridine (BPy) were then prepared. The existence of the hydrogen bonds was verified by Fourier transform infrared spectroscopy. The liquid crystalline properties of the complexes were demonstrated by polarizing microscopy and differential scanning calorimetry. 4-Allyloxy-2-fluorobenzoic acid and two fluorinated hydrogen-bonded complexes exhibited only nematic phases, whereas the non-fluorinated complex showed both smectic and nematic phases. The clearing points and mesophase thermal range of the complexes decreased with increasing number of lateral fluorine atoms on the rigid core.     

Competition between micro-segregation and anti-parallel alignment of an amphiphilic rod-like liquid crystal

Recently, we reported that a rod-like molecule, 4-[4-(7-hydroxyheptyloxy)phenyl]-1-(4-hexylphenyl)-2,3-difluorobenzene, exhibited a nematic phase with a layered structure and smectic C phases consisting of three states. We prepared a homologous series of the rod-like molecules in which a 2,3-difluoro-1,4-diphenylbenzene unit and a hydroxyl unit are connected via a flexible methylene spacer. We investigated their physical properties using polarised optical microscopy (POM), differential scanning calorimetry (DSC) and X-ray diffraction (XRD). Although they showed nematic, smectic A and smectic C phases, the phase structures were found to depend on the flexible spacer length. Those compounds possessing a shorter spacer length than a heptyl unit exhibited bilayered smectic A and smectic C phases, whereas those possessing a longer spacer length than a heptyl unit showed conventional monolayered smectic A and smectic C phases. A nematic phase with a smectic-like layer ordering was observed for the compound possessing an octyl unit. The structure–property relations of the amphiphilic compounds are discussed.     

Synthesis and physical properties of laterally fluorinated liquid crystals containing 1,3,2-dioxaborinane and cyclohexyl units

Six series of laterally fluorinated liquid crystals containing 1,3,2-dioxaborinane and cyclohexyl units have been synthesized and characterized. Their mesomorphic properties were characterized by polarizing optical microscopy and differential scanning calorimetry. All these compounds are thermotropic liquid crystalline materials. All three-ring compounds (series A, B and C) and most four-ring compounds (series D, E and F) exhibit only a nematic phase, while some four-ring compounds with long terminal alkyl chains show nematic, smectic A and even smectic B phases. In addition, four compounds containing a 1,3-dioxane unit were prepared, and their mesomorphic properties compared with their 1,3,2-dioxaborinane analogues. The dielectric anisotropies of selected target compounds were determined; some showed a negative anisotropy. The relationships between the properties and chemical structures of these new compounds are discussed.     

Synthesis and study of 4-(4'-n-alkoxybenzoyloxybenzoyl)-4'-n-butoxyanilides

Thirteen compounds with ester and amide linkages were synthesized and their mesogenic properties evaluated. Methyl to n-propyl derivatives exhibit nematic phases, n-butyl to n-decyl derivatives exhibit smectic and nematic mesophases, whereas n-dodecyl to n-octadecyl derivatives exhibit only smectic phases. All the smectic homologues exhibit smectic C phases. Middle members of the homologous series exhibit polymorphism of smectic mesophase. A plot of transition temperatures versus number of carbon atoms in the alkoxy chain reveals an odd-even effect for nematic-isotropic transition temperatures. Nematic-isotropic and smectic-cholesteric thermal stabilities of the prepared compounds (series I) are higher compared to those of previously reported compounds, series A, B and C. The results indicate that a simple reversal of a central linkage has a dramatic effect on the appearance of smectic mesophase in a homologous series. The structures of the synthesized compounds were characterized using elemental analysis, thin-layer chromatography and spectral data.     

Synthesis and mesomorphic properties of several series of fluorinated ester liquid crystals

Eight series of fluorosubstituted three-ring ester liquid crystals have been synthesized. Polarizing microscopic textural observations and DSC measurements of their phase transitions show that most are thermotropic liquid crystals with nematic and smectic A phases; furthermore, several show monotropic high order smectic phases. The results showed that the SmA phase is enhanced with the increasing number of fluoro-substituents at the para - and meta -positions of the terminal phenyl groups. The mesomorphic properties of these compounds are also affected by the direction of the ester bonds. The effect of the triple bond is also discussed.     

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.     

New side chain liquid crystalline polysiloxanes containing 1,3-dithiane or 1,3-dioxane rings as mesogenic side groups

New liquid crystal materials: 2-\[p-(9-decenyloxy)phenyl]-5-(p -alkoxyphenyl)-1,3-dioxane( 11 ) and 2-\[p-(9-decenyloxy)phenyl]-5-(p -alkoxyphenyl)-1,3-dithiane( 12 ) were synthesized. The mesomorphic behaviour of these compounds was determined. Though the 1,3-dioxane-type monomer exhibited both SmA and nematic phases, the 1,3-dithiane-type monomer only exhibited a nematic phase. By the hydrosilylation of poly(methylhydrosiloxane) and these monomers, side chain polysiloxanes were synthesized. For the monomers, the transition temperatures of the nematic to isotropic transitions of 1,3-dioxane-type compounds are higher than those of the corresponding 1,3-dithiane-type compounds. However, in the polysiloxane polymer this relation reversed.     

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.     

1,3-Dipolar cycloaddition reaction applied to synthesis of new unsymmetric liquid crystal compounds-based isoxazole

In this study, a regioselective, simple and versatile copper(I)-catalyzed procedure for preparation of a series of liquid crystals based on unsymmetrical 3,5-disubstituted isoxazole was developed. Using different substituted chloro oximes and phenyl acetylenes, 1,3-dipolar cycloaddition reaction was carried out. A second series containing the isoxazole ring and a triple bond in the rigid core was also synthesized. From the 3-(4-bromophenyl)-5-(4-(decyloxy)phenyl)isoxazole, new liquid-crystalline compounds were prepared by Sonogashira cross-coupling. All of the derivates of the isoxazole ring compounds exhibited mesomorphism. Smectic C, smectic A, and nematic phases were observed by optical microscopy and DSC analysis. The yield of these reactions varied from moderate to excellent (47-93%). The structure of the rigid core was investigated by single crystal X-ray diffraction, which confirmed the regioselectivity of the [3+2] dipolar cycloaddition reaction.     

The synthesis and liquid crystal properties of some 4-n-nonylphenyl esters of 3-(4'-n-alkoxybiphenylyl)- and 3-(4'-n-alkoxybiphenylyl)-3-methyl-propanoic acids and their laterally fluorinated analogues

A number of 4-n-nonylphenyl esters and mono- and di-fluorinated 4-n-nonylphenyl esters derived from 3-(4'-n-alkoxybiphenylyl)propanoic acids (II) have been synthesized and their thermotropic liquid crystal properties assessed with a view to obtaining tilted smectic phases for possible use in ferroelectric display devices. Many of these compounds exhibit wide temperature range S_C and S_I/F phases, but none gave the ideal phase sequence S_C-S_A-N-I mainly because the structure of these compounds was not conducive to the formation of the nematic phase. To try to alleviate this problem a series of 4-n-nonylphenyl esters based on 3-(4'-n-alkoxybiphenylyl)-3-methylpropanoic acid was prepared, where a lateral methyl group was incorporated in the β-position of the -CH₂CH₂CO₂- linkage. Incorporation of the lateral methyl group encouraged the formation of a nematic phase at the expense of both the smectic A phase and tilted smectic phases.     

Synthesis and characterization of thiophene-containing liquid crystals

Six series of liquid crystal materials containing a 2,5-disubstituted thiophene unit were synthesized. The liquid crystal compounds obtained were characterized by NMR, differential scanning calorimetry, polarizing optical microscopy and X-ray diffraction techniques. The properties of liquid crystalline phases were investigated as a function of spacer units, number of aromatic core rings and different terminal moieties. Cyano, methoxy and iodo groups were used as terminal groups. It is found that: (i) compounds having one thiophene ring and one phenyl ring connected by an ester group, with a length/breadth value of 2.1, exhibit no mesophase, while other compounds containing two biphenyl rings, with a length/breadth ratio of 2.7, show mesophases; (ii) the polarity of terminal groups and the flexible spacer length significantly affect the thermal behaviour of these compounds; (iii) the nematic transition range of cyano-containing compounds decreases with increasing length of the flexible spacer, and long alkenyloxy chains tend to facilitate the formation of the smectic phase and suppress the nematic phase in all the mesogenic compounds synthesized.     

Synthesis and characterization of thiophene-containing liquid crystals

Six series of liquid crystal materials containing a 2,5-disubstituted thiophene unit were synthesized. The liquid crystal compounds obtained were characterized by NMR, differential scanning calorimetry, polarizing optical microscopy and X-ray diffraction techniques. The properties of liquid crystalline phases were investigated as a function of spacer units, number of aromatic core rings and different terminal moieties. Cyano, methoxy and iodo groups were used as terminal groups. It is found that: (i) compounds having one thiophene ring and one phenyl ring connected by an ester group, with a length/breadth value of 2.1, exhibit no mesophase, while other compounds containing two biphenyl rings, with a length/breadth ratio of 2.7, show mesophases; (ii) the polarity of terminal groups and the flexible spacer length significantly affect the thermal behaviour of these compounds; (iii) the nematic transition range of cyano-containing compounds decreases with increasing length of the flexible spacer, and long alkenyloxy chains tend to facilitate the formation of the smectic phase and suppress the nematic phase in all the mesogenic compounds synthesized.     

First liquid crystalline cuneane‐caged derivatives: a structure–property relationship study

The synthesis and phase behaviour of the first cuneane derivatives having liquid crystalline properties are reported. This new class of liquid crystalline materials is the homologous series of bis[4‐(n‐alkoxyphenyl)]cuneane‐2,6‐dicarboxylates. They were synthesized by two methods: either by isomerization of the previously prepared bis[4‐(n‐alkoxyphenyl)]cubane‐1,4‐dicarboxylates or by direct esterification of the cuneane‐2,6‐dicarboxylic acid. Enantiotropic nematic phases were observed for the first four homologues. The higher homologues exhibited enantiotropic smectic A phases. The bis[4‐(n‐octylphenyl]cuneane‐2,6‐dicarboxylate exhibited the SmA phase at a lower temperature than its octyloxy analogue. The mesophases were investigated and established by polarizing optical microscopy, differential scanning calorimetry and X‐ray diffraction. Quantum chemical calculations suggest elongated structure for these new liquid crystalline compounds.     

Heterocyclic benzoxazole-based liquid crystals

The synthesis, characterization, and mesomorphic properties of a new type of heteronuclear compounds 1a-c and a Pd complex 1d derived from benzoxazole as the core group are reported. These compounds were prepared by the ring closure reaction of 4-alkoxybenzoic acid 4-[(4-alkoxy-2-hydroxyphenylimino)methyl]phenyl esters 6 in the presence of lead(IV) acetate. All the compounds were characterized by ¹H and ¹³C NMR spectroscopies and elemental analysis. The phase behaviour of these mesogenic compounds was characterized and studied by differential scanning calorimetry and polarizing optical microscopy. All the compounds 1a exhibited nematic (N) and/or smectic C (SmC) phases, as expected for rod-like molecules; however, the compounds 1b and 1c exhibited crystal phases. For those compounds 1a having shorter carbon chains (n = 1, 3, 4) nematic phases were observed, whereas for compounds having longer carbon chains (n = 6, 7, 8, 10, 12, 14) smectic C behaviour was also observed at lower temperatures. The greater aspect ratio (l/d) of compounds 1a compared with 1b and 1c was found to be required for the observation of liquid crystallinity. The fluorescent properties of these compounds were also examined. All λ_max peaks of the absorption and photoluminescence spectra of compounds 1a-1c occurred at c. 316-322 nm and 371-382 nm, respectively. The quantum yields of some compounds were relatively low, and also slightly solvent-dependent.     

Synthesis and physical properties of laterally fluorinated liquid crystals containing 1,3,2-dioxaborinane and cyclohexyl units

Six series of laterally fluorinated liquid crystals containing 1,3,2-dioxaborinane and cyclohexyl units have been synthesized and characterized. Their mesomorphic properties were characterized by polarizing optical microscopy and differential scanning calorimetry. All these compounds are thermotropic liquid crystalline materials. All three-ring compounds (series A, B and C) and most four-ring compounds (series D, E and F) exhibit only a nematic phase, while some four-ring compounds with long terminal alkyl chains show nematic, smectic A and even smectic B phases. In addition, four compounds containing a 1,3-dioxane unit were prepared, and their mesomorphic properties compared with their 1,3,2-dioxaborinane analogues. The dielectric anisotropies of selected target compounds were determined; some showed a negative anisotropy. The relationships between the properties and chemical structures of these new compounds are discussed.     

Tailoring the physical properties of some high birefringence isothiocyanato-based liquid crystals

This paper describes the synthesis and characterization of several liquid crystal compounds having a tolane or terphenyl core structure and a high polarizability, isothiocyanato (NCS), terminal group. The synthesized compounds have high optical birefringerce (0.35 and 0.52), the highest being exhibited by the olefin-tolanes. Several eutectic mixtures are formulated and show improved properties over the single compounds, such as a broad nematic range, low melting temperature and relatively high clearing point. The birefringence of these mixtures is in the range 0.35-0.37. UV, viscosity and electro-optical measurements of the synthesized compounds are also reported. Dipole moment and polarizability calculations were made using CS-MOPAC and Hyperchem software computational programmes.     

Tailoring the physical properties of some high birefringence isothiocyanato-based liquid crystals

This paper describes the synthesis and characterization of several liquid crystal compounds having a tolane or terphenyl core structure and a high polarizability, isothiocyanato (NCS), terminal group. The synthesized compounds have high optical birefringerce (0.35 and 0.52), the highest being exhibited by the olefin-tolanes. Several eutectic mixtures are formulated and show improved properties over the single compounds, such as a broad nematic range, low melting temperature and relatively high clearing point. The birefringence of these mixtures is in the range 0.35–0.37. UV, viscosity and electro-optical measurements of the synthesized compounds are also reported. Dipole moment and polarizability calculations were made using CS-MOPAC and Hyperchem software computational programmes.     

Three‐Ring‐Based Room‐Temperature Bent‐Core Nematic Compounds: Synthesis and Characterization

We report the synthesis and characterization of a new class of achiral three‐ring bent‐core compounds with an amide and ester linkage at the molecular bend, which are shown to exhibit nematic/phases in wide temperature ranges around room temperature (RT) and undulated SmC phases below RT. In contrast to previous studies, the compounds reported in this Communication show a true RT nematic phase with fluid physical appearance. They show strong photoluminescence in the mesophase and are found to display a one‐dimensional array of intermolecular hydrogen bonding. Furthermore, the nematic phases exhibited by these compounds show a good homeotropic alignment that can be exploited in applications such as optics and sensing. Considering the scarcity of bent‐core materials exhibiting an RT nematic mesophase, this new class of materials is promising.