Bent-core V-shaped mesogens consisting of salicylaldimine mesogenic segments: synthesis and characterization of mesomorphic behaviour

A series of bent-core V-shaped mesogens consisting of salicylaldimine mesogenic segments have been synthesized and their mesomorphic behaviour characterized. In an attempt to understand structure–property relationships, the lengths of the terminal alkoxy chains have been varied from C₂ to C₁₂, C₁₆ and C₁₈, resulting in 13 new bent-core V-shaped molecules. The thermal behaviour of these new compounds has been investigated by optical microscopy, calorimetry and X-ray diffraction studies. In general the compounds show conventional mesophases similar to those shown by calamitic LCs. The materials exhibit good thermal stability, even though their melting and clearing transition temperatures are high as a result of the presence of intramolecular hydrogen bonding between the H-atom of the hydroxyl group and the N-atom of the imine functionality. The phase appearing in the first member of the series with ethoxy chains is a nematic, while the C₃ to C₆ derivatives exhibit a smectic A phase as well as a nematic phase. The higher homologues, C₇ to C₁₂, C₁₆ and C₁₈, show only the smectic A phase. X-ray studies reveal that the SmA phase has a partially bilayer (interdigitated) structure. Remarkably, in some cases, the smectic A phase supercools well below room temperature. It is apparent from our studies that increasing the length of the alkoxy chains promotes smectic behaviour, in agreement with the general observation made for such bent-core molecules.     

Heterocyclic reactive mesogens: synthesis, characterisation and mesomorphic behaviour

Novel heterocyclic and photopolymerizable liquid crystalline materials (reactive mesogens) with smectic phases have been synthesized and characterized. A selection of heterocyclic rings, such as benzothiazole, benzothiadiazole and pyrimidine, has been incorporated into the aromatic core to control the electrochemical/luminescence properties and the structural geometry. Particular emphasis is focused on structure-property relationships, in which the variation of molecular structure and its subsequent effect on the liquid crystalline transition temperatures have been investigated.     

Heterocyclic reactive mesogens: synthesis,characterisation and mesomorphic behaviour

Novel heterocyclic and photopolymerizable liquid crystalline materials (reactive mesogens) with smectic phases have been synthesized and characterized. A selection of heterocyclic rings, such as benzothiazole, benzothiadiazole and pyrimidine, has been incorporated into the aromatic core to control the electrochemical/luminescence properties and the structural geometry. Particular emphasis is focused on structure–property relationships, in which the variation of molecular structure and its subsequent effect on the liquid crystalline transition temperatures have been investigated.     

Novel mesogenic asymmetric diacylhydrazines: synthesis, mesomorphic behaviour and X-ray study

The synthesis and liquid crystalline properties of a new series of diacylhydrazine derivatives (2a-g) are reported. All compounds of series 2 exhibit a smectic C (SmC) phase. The first homologues (2a, 2b) display a monotropic SmC mesophase, whereas the highest homologues (2c-g) exhibit an enantiotropic SmC phase. The liquid crystalline properties were investigated by differential scanning calorimetry, polarizing optical microscopy and X-ray measurements.     

Unusual mesomorphic behaviour of new bent-core mesogens derived from 4-cyanoresorcinol

New five-ring bent-core mesogens derived from 4-cyanoresorcinol as the central core have been synthesized. The mesophases were assigned by polarizing microscopy, X-ray diffraction and electro-optical measurements. It was found that the mesophase behaviour clearly depends on the direction of the carboxyl linking groups between the aromatic rings. If the outer and inner ester linking groups of a wing are in the same direction then nematic phases and, for long chain members, SmC and SmCP_A phases are observed. If the outer and inner linking groups are in opposing directions a dimorphism, SmA-SmAP_A, is observed and the clearing temperatures are increased by about 55 K. It is remarkable that in the SmCP_A phases of the first series, polar switching preferably takes place through a collective rotation of the molecules around their long axes which is accompanied by the inversion of the chirality of the smectic layers.     

Design of calamitic self-assembling reactive mesogenic units: mesomorphic behaviour and rheological characterisation

Several calamitic reactive mesogens containing only two benzene rings in the molecular core and with or without lateral substitution by the methyl/methoxy groups have been designed and their mesomorphic behaviour was characterised. Depending on the molecular structure, some of the materials exhibit the nematic and the orthogonal smectic mesophases. The reactive mesogens are aimed for further design of the macromolecular materials like polysiloxane-based polymers and elastomers. Beyond the mesomorphic and structural properties, the electrorheological properties within the temperature range of the nematic and smectic A mesophases were studied with and without applied electric field for the selected reactive mesogen. The increase of viscosity was found not to be higher than three times under applied electric field strength of 2 kV/mm. The mesomorphic, structural and rheological properties of the newly designed reactive mesogens are discussed in order to contribute to better understanding of the molecular architecture–nano-organisation properties relationship of such mesogenic materials.     

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.     

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.     

Synthesis and characterization of liquid crystalline elastomers bearing fluorinated mesogenic units and crosslinking mesogens

Several new side‐chain liquid crystalline (LC) polysiloxanes and elastomers ( IP ‐ VIP ) bearing fluorinated mesogenic units and crosslinking mesogens were synthesized by a one‐step hydrosilylation reaction with poly(methylhydrogeno)siloxane, a fluorine‐containing LC monomer 4′‐undec‐10‐enoyloxy‐biphenyl‐4‐yl 4‐fluoro‐benzoate and a crosslinking LC monomer 4′‐(4‐allyloxy‐benzoxy)‐biphenyl‐4‐yl 4‐allyloxy‐benzoate. The chemical structures and LC properties of the monomers and polymers were characterized by use of various experimental techniques such as FTIR, ¹H‐NMR, EA, TGA, DSC, POM and XRD. The effect of crosslinking mesogens on mesomorphic properties of the fluorinated LC polymers was studied as well. The obtained polymers and elastomers were soluble in many solvents such as toluene, tetrahydrofuran, chloroform, and so forth. The temperatures at which 5% weight loss occurred (T_d) were greater than 250°C for all the polymers, and the weight of residue near 600°C increased slightly with increase of the crosslinking mesogens in the fluorinated polymer systems. The samples IP , IIP , IIIP and IVP showed both smectic A and nematic phases when they were heated and cooled, but VP and VIP exhibited only a nematic mesophase. The glass transition temperature (T_g) of polymers increased slightly with increase of crosslinking mesogens in the polymer systems, but the mesophase–isotropic phase transition temperature (T_i) and smectic A–nematic mesophase transition temperature (T_S‐N) decreased slightly. It suggests that the temperature range of the mesophase became narrow with the increase of crosslinking mesogens for all the fluorinated polymers and elastomers. In XRD curves, the intensity of sharp reflections at low angle decreased with increase of crosslinking mesogens in the fluorinated polymers systems, indicating that the smectic order derived from fluorinated mesogenic units should be destroyed by introduction of more crosslinking mesogens. Copyright © 2008 John Wiley & Sons, Ltd.     

The synthesis and mesomorphic behaviour of tetracatenar di-hydrazine derivatives

Symmetrical four-chained (tetracatenar) di-hydrazine derivatives, namely oxalyl N',N'-bis(3,4-dialkoxybenzoyl)-hydrazide (BFH-n, n?=?4, 6, 8, 10), were synthesised. Investigations on the liquid crystalline properties by differential scanning calorimetry (DSC), wide-angle X-ray diffraction (WAXD) and polarising optical microscopy (POM) showed that the di-hydrazine derivatives exhibited columnar mesophases and the symmetry of the mesophase changes from rectangular to hexagonal on increasing the temperature. The rectangular columnar mesophases of BFH-n (n?=?6, 8, 10) remained stable down to 10°C during cooling and the subsequent recrystallisation from the Col_r phase of BFH-n (n?=?6, 8, 10) was observed on the second heating runs. Furthermore, the average number of molecules packing in a column slice was estimated to be three, based on their X-ray diffraction results. Intermolecular hydrogen bonding between –C=O and ?N?H groups in crystalline and liquid crystalline phases was confirmed.     

Non-symmetric chiral isoflavone dimers: synthesis,characterisation and mesomorphic behaviour

A new series of non-symmetric chiral isoflavone-based liquid crystalline dimers, α-(2-methylbutyl-4′-(4″-phenyloxy)benzoate)-ω-(3-(4′-decyloxyphenyl)-4H-1-benzopyran-4-one-7-oxy)alkanes, with 3–12 carbon atoms in the alkyloxy spacer, have been synthesised. A pronounced odd–even effect for the phase transition temperatures upon varying the spacer length was observed. The short dimers exhibited monolayer smectic A (SmA) and smectic C (SmC*) phases while for longer homologues a chiral nematic (N*) phase was found. The temperature range of the nematic phase was broadened with elongation of the alkyl spacer. Stabilisation of the nematic phase resulted from competition between the monolayer and intercalated smectic structures. The SmA–SmC* phase transition was second order for all studied compounds with a cross over to the de Vries type behaviour for the shortest homologue.     

New mesogenic materials bearing 1,2,4-oxadiazole moiety: Synthesis,characterization and investigation their mesomorphic behaviors

In this work, two series containing a 1,2,4-oxadiazole ring as a central core were synthesized and characterized by common spectroscopic techniques, including FT-IR, ¹H-NMR, and elemental analysis. The first series was 3-(4-alkoxyphenyl)-5-(p-methylphenyl)-1,2,4-oxadiazole (C_n), which consisted of alkoxy group in the terminal arm and a methyl group in the other, while, the oxidation reaction of methyl group in a series (C_n) to the carboxy group was the method used to synthesize the second series in this work, 4-(3-(4-alkoxyphenyl)-1,2,4-oxadiazol-5-yl)benzoic acid (D_n). The mesophase behaviors of these two series were studied by optical polarized microscopy (OPM) and differential scanning calorimetry (DSC). The liquid crystalline investigations of the compounds (C_n and D_n) show that the last six homologeus of the series (C_n), (C₆–C₁₁), have a monotropic nematic phase, while only the intermediate compounds in the series (D_n), (D₃–D₉), displayed monotropic nematic phase, also, the liquid crystalline properties in the first and last two compounds in this series (D₁, D₂, D₁₀, and D₁₁) had disappeared. The differences in liquid crystalline properties between the two series, (C_n and D_n), were discussed through the influences of the different terminal groups (–CH₃ and –COOH) in addition to the effect of the 1,2,4-oxadiazole ring and the length of the terminal alkyl chain.     

Heterocyclic columnar pyrimidines: synthesis, characterization and mesomorphic properties

The synthesis, characterization, and mesomorphic properties of two series of heterocyclic compounds derived from a pyrimidine core are reported. These series, 1a and 1b, are substituted with a variety of functional groups (X=NHSO₂CF₃, F, Cl, Br, I, OCH₃, CH₃, C₂H₅) at the C^3' (meta)- or C^4' (para)-position of the terminal phenyl ring, and the substituent effect on mesophase formation was studied. The compounds were characterized by ¹H and ¹³C NMR spectroscopy and elemental analysis, and the mesomorphic behavior of the compounds was characterized and studied by differential scanning calorimetry, polarizing optical microscopy and powder X-ray diffraction. Most of the compounds were mesogenic at room temperature, and the mesophases were assigned as lamellar columnar (Col_L) phases, as expected for disk-like molecules. The results also revealed that compounds with a para-substituent (1a; except for -OCH₃) at the C^4'-position, exhibited higher clearing temperatures and wider temperature mesophase ranges than those of compounds with a meta-substituent (1b) at the C^3'-position. The higher clearing temperatures may be attributed to stronger dipolar interactions resulting from a greater resonance effect with the central core for para-substitution than for meta-substitution. The results also indicated that the columnar mesophases observed show a correlation with the electronic properties of the substituents; compounds containing electron-withdrawing substituents (X=F, Cl, Br, I, NHSO₂CF₃) also have higher clearing temperatures than compounds containing electron-donating substituents (X=Me, Et, OMe).     

Hydrogen-bonded complexes between mesogenic heterocyclic Schiff's bases and mesogenic 4- n -nonyloxybenzoic acid: mesomorphic behaviour,FTIR study and PM3 semi-empirical calculations

A new series of intermolecular hydrogen-bonded complexes has been obtained using mesogenic 4-n-nonyloxybenzoic acid and mesogenic 5-(4-pyridyl)-2-(4-n-alkoxy) benzylideneamino-1,3,4-thiadiazole moieties. The thermal and phase behaviour of these complexes were studied by thermal microscopy and differential scanning calorimetry. Intermolecular hydrogen bonding was studied by FTIR spectroscopy, from crystalline to the isotropic state. A study by PM3 semi-empirical calculations is also described.     

Hydrogen-bonded complexes between mesogenic heterocyclic Schiff's bases and mesogenic 4- n -nonyloxybenzoic acid: mesomorphic behaviour, FTIR study and PM3 semi-empirical calculations

A new series of intermolecular hydrogen-bonded complexes has been obtained using mesogenic 4- n -nonyloxybenzoic acid and mesogenic 5-(4-pyridyl)-2-(4- n -alkoxy) benzylideneamino-1,3,4-thiadiazole moieties. The thermal and phase behaviour of these complexes were studied by thermal microscopy and differential scanning calorimetry. Intermolecular hydrogen bonding was studied by FTIR spectroscopy, from crystalline to the isotropic state. A study by PM3 semi-empirical calculations is also described.     

Liquid crystal dimers possessing chiral rod-like anisometric segments: synthesis, characterization and electro-optic behaviour

Several new optically active liquid crystal dimers comprising pro-mesogenic cholesterol and a chiral diphenylacetylene (tolane) segment, covalently linked in an end-to-end fashion through a flexible spacer, have been synthesized and investigated for their mesomorphic behaviour with the aid of optical, calorimetric and X-ray diffraction studies. Five unsymmetrical dimers, designed on the basis of recent work, involve molecular structural variations of the tolane mesogenic entity with a view to stabilizing a wide thermal range smectic A (SmA) phase featuring the electroclinic effect. Three different chiral chains, namely, (S)-1-methylheptyloxy, (S)-2-methylbutyloxy, (3S)-3,7-dimethyloctyloxy, with or without polar (nitro or fluoro) lateral substituents, were incorporated, while keeping the length (C₆) of the spacer constant. As expected, all the dimers exhibited a SmA phase. A few also showed chiral nematic (N*) and/or twist grain boundary and/or chiral smectic C (SmC*) phases. Remarkably, some of these oligomesogens, upon melting, had a stable SmA phase over a wide thermal interval (100-150°C); this state seems to be stable for a long period of time. Electro-optic studies, including optical tilt angle as well as temporal response as a function of temperature, were carried out in the SmA phase. The SmC* phase was also investigated for its electrical switching and optical tilt angle, as well as spontaneous polarization as a function of temperature. These studies showed that the mesophase response to an applied field is weak and is independent of variations in the dimer investigated.     

Bent-core 6-thioxoverdazyl: a comparison of mesogenic properties with the 6-oxo analogue

Two bent-core verdazyl radicals, 2[16] and 3[16], containing 6-thioxo and 6-oxo groups, respectively, were prepared and their physical properties were compared. Analysis of binary mixtures with bent-core mesogen 1[12] gave virtual N-I ([T_NI]) and N-I_re ([T_NIre]) transition temperatures. Results indicate that the thioxo derivative 2[16] has greater stabilising effect on the nematic phase than the 6-oxo analogue 3[16], presumably due to the higher dipole moment of the core (1.90 D vrs 0.83 D) despite the less favourable conformational properties.     

Symmetrical and asymmetrical liquid crystal dimers: synthesis,characterisation and mesomorphic behaviour

ABSTRACT

A series of new asymmetrical liquid crystal dimers, R_nO–S2O–OR_n (n = 2–10), linked by an ethylene spacer having carbothiol – COS – and carboxyl – COO – linkages to the core centre of the molecule and bearing different lengths of terminal alkoxy chains were synthesised in order to study the effects of the length of the terminal alkoxy chains on mesomorphic properties. As well as, five symmetrical and asymmetrical dimers linked by butylene spacers bearing different linkages to the core and various terminal chains were also synthesized in order to study the effect of the nature of the spacer and terminal groups. The structures of the synthesised dimers were confirmed by physico-chemical techniques, i.e. FTIR, NMR and mass spectra. Differential scanning calorimetry and polarising optical microscopy verified the liquid crystal behaviour transition temperatures. The isotropic transition temperatures of the dimers R_nO–S2O–OR_n (n = 2–10) decreased with increasing length of alkoxy chain. Structural effects on the mesomorphic and physical properties were investigated in terms of alteration of carboxylate and thioester groups linking the spacer. The mesomorphic investigation reveals that all the dimers formed an enantiotropic Nematic phase except for dimer HO2SH which is not a liquid crystal.     

Synthesis and mesomorphic properties of mesogens containing 1,1,2,2-tetrahydroperfluorodecaoxy terminal chains

One series of two-ring and two series of three-ring mesogens with 3,3,4,4,5,5,6,6,7,7,8,8,9,9,10,10-heptadecafluorodecaoxy terminal chains containing tolane or biphenyl units have been synthesized. Their mesomorphic properties were observed and measured by optical polarizing microscopy and differential scanning calorimetry. The three-ring mesogens only exhibit a wide smectic C phase when the length of their hydrocarbon chains is intermediate.     

Synthesis and mesomorphic behaviour of new mesogenic compounds possessing a cholesteryl ester moiety connected to a pyrimidine core

New mesogenic compounds containing a cholesteryl ester and a pyrimidine moiety connected through a polymethylene spacer have been prepared. The mode of linkage has been made via -CC- and -CN- to understand the structure-property relationship. Only two compounds with a pentamethylene spacer show mesomorphic behaviour. The mesomorphic behaviour has been investigated by polarizing optical microscopy, differential scanning calorimetry and HRXRD studies. Enantiotropic smectic A, twist grain boundary (TGB) and chiral nematic mesophases are exhibited by the newly synthesized compounds.