Solvent-Free Mechanochemical Synthesis of High Transition Biphenyltetracarboxydiimide Liquid Crystals

A series of high temperature alkyl and alkoxy biphenyltetracarboxydiimide liquid crystals have been prepared under ball mill method using solvent-free mechanochemical approach. The thermal properties of the prepared compounds were investigated by deferential scanning calorimetry (DSC) measurements and the textures were identified by polarized optical microscope (POM). The compounds showed smectic mesomorphic behaviour. The results showed the increasing nature of transition temperature Cr-SmC with chain length with increments of the SmC mesophase range. However, the mesophase range of the SmA was decreased with the terminal chain length either for the alkyl or alkoxy terminal groups. Moreover, the DFT theoretical calculations have been conducted give a detailed projection of the structure of the prepared compounds. A conformational investigation of the biphenyl part has been studied. A deep illustration of the experimental mesomorphic behaviour has been discussed in terms of the calculated aspect ratio. A projection of the frontier molecular orbitals as well as molecular electrostatic potential has been studied to show the effect of the polarity of the terminal chains on the level and the gap of the FMOs and the distribution of electrostatic charges on the prepared molecules.     

New mesogenic compounds possessing a biphenyl ester and ether moiety comprising 1,3-dimethylbarbituric acid: synthesis,characterisation and mesomorphic studies

New substituted derivatives of 5-vinyl-1,3-dimethylbarbituric acid were synthesised and evaluated for liquid crystal properties. Two sets of molecules were prepared. One end of all the molecules possesses the 1,3-dimethylbarbituric core. The first set comprises biphenyl ethers, 4a–n and the second set biphenyl esters, 5a–g. Liquid crystalline properties were investigated by POM and DSC techniques. All the compounds exhibited enantiotropic smectic A and nematic mesophases. The LC properties were found to depend on the spacer and terminal alkoxy- chain and alkoxy- ester moiety of the molecules. Smaller alkyl chain members showed a smectic phase, while higher alkyl chain members showed a nematic phase.     

New 1,3-dioxane type ionic liquid crystal materials having a terminal double bond

New 1,3-dioxane type ionic liquid crystal materials having a terminal double bond were synthesized. The mesomorphic behaviour of these compounds was measured. The principal features of these compounds are a smectic A phase over a very wide range including room temperature, and a low isotropic to mesophase transition temperature (for example, compound 6-1: Cr-22 SmA 25 I).     

New 1,3-dioxane type ionic liquid crystal materials having a terminal double bond

New 1,3-dioxane type ionic liquid crystal materials having a terminal double bond were synthesized. The mesomorphic behaviour of these compounds was measured. The principal features of these compounds are a smectic A phase over a very wide range including room temperature, and a low isotropic to mesophase transition temperature (for example, compound 6-1: Cr-22 SmA 25 I).     

On the balance between nematic and smectic phases in 2′,3′-difluoro-4,4″-dialkyl-p-terphenyls

ABSTRACT

The full homologous series of 2?,3?-difluoro-4,4″-dialkyl-p-terphenyls, where alkyl chains are from methyl to nonyl length, have been synthesised and mesomorphic properties, phase transition temperatures and enthalpies were measured. The influence of the terminal chain length on thermal mesophase range, phase sequence has been analysed and discussed. The odd–even and symmetric–non-symmetric effects have also been studied and its influence on smectic to nematic balance was analysed.     

The influence of methoxy side groups and halogen nature on the mesomorphic and optical properties of symmetrical azomethines

The preparation, characterisation, thermotropic and optical properties of low-molecular azomethines with or without methoxy side group are described in this paper. The azomethine compounds were synthesised by condensation reaction of o-dianisidine/benzidine with para-halogen substituted benzaldehyde. Their properties were analysed by differential scanning calorimetry, thermogravimetry analysis, polarised optical microscopy, X-ray diffraction and optical spectroscopy. The azomethines present liquid crystalline behaviour with large mesophase range and high thermal stability. The compounds without lateral methoxy groups showed smectic A phase, while those with methoxy groups exhibited only nematic phase. The effect of methoxy group and different terminal substituents on the mesomorphic behaviour, molecular and optical properties was estimated in terms of parameters such as molecular polarisability, dipole moment, interdigitation parameter and axial ratio.     

Synthesis and X-ray investigation of liquid crystalline polymers containing laterally methyl-substituted tolane-based mesogenic side groups

A series of polymethacrylates containing laterally methyl-substituted 4-alkoxy-4-nitrotolane, 4-alkoxy-4'-cyanotolane, and 4-alkoxy-4'-trifluoromethyltolane as mesogenic side groups are presented. The thermal behaviours of the prepared compounds were characterized by differential scanning calorimetry, optical polarized microscopy and X-ray diffraction. All of the obtained methacrylate monomers exhibit no mesophase, while most of the synthesized polymers reveal enantiotropic mesomorphism. The polymer containing 4-propyloxy-2'-methyl-4'-nitrotolane side groups was the only one to show no mesomorphic behaviour. The spacer length and the nature of terminal group have a profound influence on the type of mesophase formed and the thermal stability. Incorporation of a lateral methyl-substituent into the mesogenic groups reduces the thermal stabilities of the mesophases. X-ray diffraction reveals that polymers with a smectic A or smectic E phase have the inclination to form a bilayer structure. Increasing the temperature within the smectic phase range, the intermolecular spacing increases while the layer spacing decreases. On the other hand, introducing a lateral substituent into the mesogenic side groups leads to increase in both layer and intermolecular spacing.     

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.     

Laterally fluorinated liquid crystals containing the 2,2'-bithiophene moiety

The synthesis and systematic evaluation of the influence of lateral mono-, di- and tetra-fluorination of the terminal phenyl ring on mesomorphic properties in seven novel series of suitably fluoro-substituted 5-n-alkyl-5'-(4-n-decyloxyphenyl)-2,2'-bithienyls is reported. Compared with their non-fluorinated parent counterparts, lateral fluorination eliminates high order smectic phases and reduces thermal stability, to reveal compounds exhibiting a selection of nematic, smectic A and smectic C phase types. As the number of fluoro-substituents increases from one to two, mesophase thermal stability drops drastically; the disposition of the second fluoro-substituent is important. Across-axis disposition is more detrimental than along-axis. However, complete fluorination does not destroy mesophase formation. Indeed, tetrafluorophenyl compounds are more stable than certain 3,5- and 2,6-difluoro compunds. The extrapolated birefringence for members of a series of 5-n-alkyl-5'-(2,3-difluoro-4-n-decyloxyphenyl)-2,2'-bithienyls is approximately 0.21.     

The synthesis and comparison of the liquid crystalline properties of certain 1-(4'-n-alkoxypolyfluorobiphenyl-4-yl)-2-(trans-4-n- pentylcyclohexyl)-ethenes and -ethanes carrying four, six or eight fluoro-substituents

The synthesis and mesomorphic properties of a variety of polyfluorinated alkylaryl-ethanes and-ethenes are reported. The number and position of the symmetrically disposed fluorosubstituents has been varied from six to four and the mesomorphic properties of the compounds are compared with those of the analogous octafluoro-substituted compounds previously reported. All the compounds are moderately low melting, mesomorphic and the type of mesophase formed is dependent upon the number and position of the fluoro- substituents. Increasing the number of fluoro-substituents from four, to six and to eight in a series of polyfluoro-alkylarylethanes decreases the mesophase thermal stability in a stepwise manner and eliminates smectic properties whilst retaining the nematic phase.     

Thiophene-based bent-shaped luminescent liquid crystals: synthesis and characterisation

Five new bent-shaped compounds based on the 2,5-disubstituted thiophene central core were successfully synthesised. The lengths of the rigid centres as well as the degree of conjugation were varied by coupling benzene, naphthalene and biphenyl systems to the thiophene through Sonogashira cross-coupling. The thermal, mesomorphic and optical properties of these variants were determined. Compounds 5a,b and 8a displayed liquid-crystalline smectic and nematic mesophases. Strong blue fluorescence was observed for the final compounds.     

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 mesomorphic properties of some platinum(II) and oxovanadium(IV) complexes

The synthesis and mesomorphic properties of a homologous series of N-(2-hydroxy-4-n-alkoxybenzylidene)-4'-n-decylphenylanilines and their platinum(II) and oxovanadium(IV) complexes are reported. All the ligands and their metal chelates exhibit enantiotropic mesophases, predominantly smectic A and smectic C phases. The transition temperatures and enthalpies have been determined for most of the compounds. The platinum(II) complexes have higher melting points and mesophase thermal stabilities. However, the oxovanadium(IV) complexes have a wider thermal range for the mesophase. Both platinum(II) and oxovanadium(IV) complexes containing only a chain on the biphenyl moiety exhibit a nematic phase.     

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,characterisation and liquid crystalline properties of some Schiff base and cinnamate central linkages involving 1,3,5-trisubstituted pyrazolone ring system and their Cu(II) complexes

Two new mesogenic homologous series, each containing 1,3,5-trisubstituted pyrazolone derivatives, 4-n-alkoxyphenyl and Schiff base–cinnamate central linkages, have been synthesised to give 4-[(5-hydroxy-3-methyl-1-phenyl-4,5-dihydro-1H-pyrazol-4-yl) methyleneamino] phenyl 3-(4-n-alkoxyphenyl)acrylate [Series-A] and 4-[(5-hydroxy-3-methyl-1-p-tolyl-4,5-dihydro-1H-pyrazol-4-yl)methyleneamino] phenyl 3-(4-n-alkoxyphenyl)acrylate [Series-B] and their Cu(II) complexes have also been synthesised. These compounds were characterised by elemental analysis, Fourier transform infrared (FT-IR), proton nuclear magnetic resonance (¹H NMR), carbon-13 NMR (¹³C NMR) and ultraviolet (UV)-visible and mass spectral studies. Their mesomorphic behaviour was studied by polarising optical microscope (POM) with a heating stage. POM data were compared with differential scanning calorimetry thermograms. In Series-A and -B, all compounds exhibit mesomorphism. Series-A compounds exhibit an enantiotropic nematic mesophase except propyl derivative, while a smectic A (SmA) mesophase is observed from the heptyl derivative and persists up to the last member of the homologous series. n-Heptyloxy derivative is monotropic for SmA phase. Series-B compounds also exhibit the enantiotropic nematic mesophase, while the SmA mesophase is observed from the heptyl derivative and persists up to the last member of the homologous series. n-Dodecyloxy derivative exhibits monotropic SmA and nematic mesophases. The mesomorphic properties of both series are compared with each other and the other structurally related compounds. The study reveals that cinnamate linkage containing liquid crystals have higher thermal stability compared to structurally related series containing chalcone linkage. In case of complex series, only one compound from each series gives nematic mesophase.     

Synthesis and characterization of side-chain liquid crystalline copolymers for non-linear optics

A number of acrylate copolymers having nematogenic groups with ring-X (X = CN, CF₃) terminal substituent and highly polarizable groups with ring-NO₂ terminal substituent linked to the backbone have been synthesized. The liquid crystalline nature of these copolymers has been established by calorimetric measurements, optical texture observations and X-ray investigations. Ring-CN substituents enhance nematic thermal stability and tend to repress smectic properties while terminal CF₃ groups favor smectic A mesophase formation. These copolymers should possess interesting non-linear optical properties.     

New isothiocyanatotolane liquid crystals with terminal but-3-enyl substitute

Four kinds of new but-3-enyl-based isothiocyanate liquid crystals composed of tolane core and but-3-enyl terminal group (A1–A4) were synthesised via seven step reactions based on 2-(4-bromophenethyl)-1,3-dioxolane, and four n-butyl analogues B1–B4 as comparison structures were also prepared. The mesomorphic properties and physical properties of the compounds were investigated. Single fluorinated compounds A2 and A3 exhibit monotropic nematic phase, and they have lower melting enthalpy and higher clearing points than those of the comparison compounds B2 and B3. The non-fluoro-substituted compound A1 and difluorinated compound A4 exhibit no nematic phase. Replacement of n-butyl chain by but-3-enyl as terminal group is enabled to increase birefringence (Δn ~ 0.394–0.430) and reduce rotational viscosity. These isothiocyanatotolanes with terminal but-3-enyl substitution have potential application for high birefringence mixtures.     

Mesomorphic properties of achiral halogen-substituted banana-shaped compounds

Three banana-shaped achiral compounds, derivatives of 4,6-dichloro-1,3-phenylene bis-[4-(4-n-octyloxyphenyliminomethyl) benzoate], were synthesized by varying the substituent (X = H, F and Cl). Their mesomorphic properties were investigated by differential scanning calorimetry, polarizing optical microscopy, and X-ray analysis. The compound with X = H exhibited an enantiomeric nematic phase. The compounds with X = F and Cl formed a nematic phase on heating, while on cooling they formed a nematic phase at high temperature and a smectic A phase at a lower temperature (monotropic). A schematic representation of the mesophase structures for the compound with X = F is also illustrated.     

Non-symmetric liquid crystal dimers based on 1,3,4-oxadiazole derivatives: synthesis,photoluminescence and liquid crystal behaviour

A series of non-symmetric liquid crystal compounds consisting of two different semi-rigid anisometric cores, namely 1,3,4-oxidiazole and biphenyl units, and two short terminal groups, have been synthesised in good yield. It has been shown by polarising optical microscopy and differential scanning calorimetry that all these compounds display liquid crystalline behaviour, with nematic and/or smectic A mesophases. The nature of the mesophases is dependent on the electronic properties of the terminal groups. In methylene chloride solution all the compounds displayed a room temperature emission with λ_max at 358–396 nm and quantum yields of 0.29–0.56. The effect of the terminal groups on the mesomorphic and photoluminescent properties is briefly discussed in the context of their electronic characteristics.     

Synthesis,characterization and thermal properties of 4,4'-bis(4-n-alkoxybenzoyloxy)benzylideneanilines and bis(4-benzylidene-4'-n-alkoxyaniline) terephthalates

Series of novel thermotropic liquid crystalline compounds which serve as models for mesogenic diols have been synthesized. The mesogen molecules are constructed from 1,4-disubstituted benzene rings linked through ester and azomethine units. Based on the number of phenyl rings, a tetrad or pentad classification has been assigned to the core molecule. The structures of these compounds were established by FTIR and NMR spectroscopy, while the mesomorphic behaviour has been confirmed using polarizing optical microscopy and DSC. All the compounds of the homologues series are mesomorphic in nature. The lower members of the series are nematogenic, while the higher members exhibit both nematic and smectic character, with high melting behaviour and wide mesophase ranges.