Discotic liquid crystalline hydrazone compounds: synthesis and mesomorphic properties

[reaction: see text] 1,3,5-Trisacetoacetamidobenzene with three 1,3-diketo groups was synthesized by the reaction of 1,3,5-triaminobenzene with diketene. Discotic hydrazone compounds were prepared by the diazo coupling reaction between 1,3,5-trisacetoacetamidobenzene and diazonium salts of 4-alkyloxyphenylamines. The compounds existed in hydrazone forms exclusively, being stabilized by the intramolecular hydrogen bonds, and showed discotic nematic or columnar hexagonal mesophases.     

Main-chain liquid crystalline copolymers: synthesis and comparative study of the mesomorphic properties and non-linear optical properties

Linear main-chain polymers (P1, P2 and P3) have been synthesised and characterised. The polymers were designed to understand the rigidity of the mesogen’s linking groups and its effect on transition temperature. The liquid crystalline behaviour of these polymers was investigated using polarised optical microscopy and differential scanning calorimetry. All polymers exhibited phase transition on heating and cooling runs. The mesophase stability decreases with the decrease of rigidity of the second mesogen (P2>P1>P3). Similarly, the enthalpy and entropy changes are dependent on the rigidity of the second mesogen (Mesogen B). Polymer P1 undergoes photoisomerisation under UV-visible light irradiation and it attains a photo-stationary state at 275 s. This provides an opportunity to investigate the third-order non-linear optical properties. The non-linear behaviour of polymer P1 in chloroform presents a distinct deviation in a different concentration, where polymer P1 also exhibits a negative non-linear refractive index. Thus, polymer P1 can be used as a potential candidate for optical device applications such as optical limiters.     

Heterocyclic benzothiazole-based liquid crystals: synthesis and mesomorphic properties

Two homologous series of 2-(4-alkanoyloxybenzylidenamino)benzothiazoles and 2-(2-hydroxy-4-alkanoyloxybenzylidenamino)benzothiazoles were synthesised and characterised. Their molecular structures differed wherein the latter comprised a lateral hydroxyl group, unlike the former. Spectroscopic techniques such as FT-IR, ¹H & ¹³C NMR and mass spectrometry together with elemental analysis were employed to elucidate the molecular structures. The transition temperatures and their mesophases were determined by differential scanning calorimetry, optical polarising microscopy and X-ray diffraction techniques. Members with decanoyloxy till hexadecanoloxy chain in the series without the lateral hydroxyl group each exhibited a smectic A phase, while those in the series with the lateral hydroxyl group were non-mesogenic. The mesomorphic properties of the present series were compared with other structurally related series to establish the chemical structure–mesomorphic properties relationship.     

Heterocyclic pyridine-based liquid crystals: synthesis and mesomorphic properties

A series of new calamitic liquid crystals, 4-{[(pyridin-4-yl)methylidene]amino}phenyl 4-alkoxybenzoates comprising a heterocyclic (pyridine) and two phenyl rings core system, terminal alkoxy chain, imine and ester linkers were synthesised and characterised. This series consists of nine members wherein the members differ by the length of alkoxy chain (C_nH_2n+1O–, where n = 2, 4, 6, 8, 10, 12, 14, 16, 18). Spectral analysis results were in accordance with the expected structure. Their thermotropic behaviours were studied by using differential scanning calorimetry, optical polarising microscopy and powder X-ray diffraction techniques. A single mesophase (nematic) was observed for the first three members of the series (n = 2, 4 and 6). As the alkoxy chain increased to n = 8 and n = 10, the nematic phase appeared together with an additional smectic A (SmA) phase. When moving from n = 12 until the highest members (n = 18), the nematic phase disappeared and these compounds only exhibited a single mesophase (SmA).     

Oxadiazole-based unsymmetrical chiral liquid crystal dimers: synthesis and mesomorphic properties

The synthesis and liquid crystalline properties of novel chiral Schiff's base dimers containing the 1,3,4-oxadiazole ring are reported. The length of the terminal S-alkyl chain has been varied. All the compounds synthesised were thermally stable and exhibited enantiotropic mesomorphism, showing either SmC*–SmA–TGB–N*–BP or SmC*–SmA phase sequence.     

Synthesis and mesomorphic properties of fishbonelike,liquid crystalline polysilsesquioxanes: 4. Pd-coordinating,fishbone-like imine-based liquid crystalline polysilsesquioxane

A new type of Pd-coordinating, imine-based, fishbone-like liquid crystalline polysilsesquioxanes (Pd-FBI) including μ-acetato-bridged Pd-FBI [Pd(μ-oAc)-FBI] and μ-chloro-bridged Pd-FBI [Pd(μ-Cl)-FBI] has been synthesized first by a hydrosilylation reaction to form fishbone-like, imine-based liquid crystalline polysilsesquioxane (H-FBI and MH-FBI respectively,) and then by an ortho-palladation reaction with palladium acetates to obtain Pd(μ-oAc)-FBI. The latter was further subjected to ligand substitution reaction with NaCl to form Pd(μ-Cl)-FBI. Their mesomorphic properties investigated by differential scanning calorimetry (DSC) and optical polarization microscopy (OPM). show that the Pd-FBI is a thermotropic liquid crystalline polymer and its clearing temperatures and mesophase range are much higher than those of the corresponding comb-like imine liquid crystalline polysiloxanes (IMLCP) by about 100 °C. In this paper the influence of the palladium content on mesogenic behavior is discussed.     

Synthesis and mesomorphic properties of novel fishbone-like liquid crystalline polysilsesquioxanes VI. Fishbone-like,ester-based liquid crystalline polysilsesquioxanes

Two novel kinds of fishbone-like, ester-based, liquid crystalline, random copolysilsesquioxanes, LC₁-CH₃-P and LC₂-CH₃-P, have been synthesized for the first time by stepwise coupling polymerization (SCP) from trichlorosilane monomers (RSiCl₃) containing mesogenic ester groups, using three main steps including preaminolysis, hydrolysis and polycondensation. Two new monomers 4-(11-trichlorosilylundecloyloxy)phenyl 4-methoxybenzoate (M₁) and 4-(3-trichlorosilylpropoxy)phenyl 4-methoxybenzoate (M₂) have been synthesized by hydrosilylation reaction of trichlorosilane and the mesogens 4-n-undecenoyloxyphenyl 4-methoxybenzoate and 4-allyloxyphenyl 4-methoxy-benzoate, respectively, in the presence of dicyclopentadienylplatinum(II) chloride (Cp₂Ptcl₂) catalyst. The structures and mesomorphic properties of LC₁-CH₃-P and LC₂-CH₃-P were investigated by FTIR, ¹H NMR, ¹³C NMR, ²⁹Si NMR, gel permeation chromatography, polarizing optical microscopy, differential scanning calorimetry and X-ray diffraction. It is proposed from the characterization data that these two polymers not only have an ordered ladder-like, double main chain, but also show thermotropic smectic behaviour with high clearing points and wide mesophase ranges (ΔT), which may hold promise for potential applications as highly stable gas chromatographic stationary phases.     

Synthesis and mesomorphic properties of fishbone-like liquid crystalline polysilsesquioxanes V. Pd-coordinating, fishbone-like azo-based liquid crystalline polysilsesquioxanes

New kinds of Pd-coordinating azo-based liquid crystalline polysilsesquioxanes, Pd-H-FBA and Pd-C-FBA, have been synthesized first by hydrosilylation reactions of a vinyl-terminated mesomorphic azo compound with ladder-like homopolyhydrosilsesquioxane (H-T) and copolymethylhydrosilsesquioxane (MH-T) to produce fishbone-like, azo-based liquid crystalline polysilsesquioxanes H-FBA and C-FBA, respectively, followed by a further ortho -palladation reaction with palladium chloride. The mesomorphic properties have been investigated by DSC, optical polarizing microscopy and temperature-variable X-ray diffraction. It is found that their clearing temperatures, Tcl, are much higher and the mesophase ranges, Delta T,are much wider than those of the corresponding comb-like polymers with a single main chain by more than 100 C. This indicates that the double main chain structures of the title polymers have a positive effect on mesomorphic properties. The influence of the contents of Pd2 ions on the liquid crystallinity is also discussed.     

Synthesis and mesomorphic properties of new liquid crystalline compounds VIII. Indolinobenzospiropyranyl aryliminobenzoates

A series of new liquid crystalline compounds, indolinobenzospiropyranyl aryliminobenzoates, 1a-d, have been synthesized and their mesomorphic properties characterized. Analysis by differential scanning calorimetry, polarizing optical microscopy, and X-ray diffraction showed that the benzoates 1a and 1d form a monotropic mesophase, whereas 1b and 1c show an enantiotropic mesophase. Enantiomorphic properties are uncommon to spiropyran-containing LC molecules. Among the compounds investigated, 1a was found to exhibit a monotropic SmC phase between 90 and 79°C.     

Synthesis and mesomorphic properties of Cu-coordinating liquid crystalline polysilsesquioxanes with heptyl side groups

In order to study the influence of the concentration of a flexible group on the liquid crystal behaviour of copper-containing complexes, series of Cu-coordinated, beta-diketone-based liquid crystalline polysilsesquioxanes (abbr. H-DK-Cu) and random copolymethylsilsesquioxanes (abbr. Me-DK-Cu), have been synthesized by the hydrosilylation reaction of a vinyl-terminated beta-diketone and 1-heptene with the polyhydrosilsesquioxane (H-T) and with the random copolymethylhydrosilsesquioxane (Me-T), followed by reaction with Cu(NH₃)₄Cl₂. Their mesomorphic properties were investigated by differential scanning calorimetry and optical polarizing microscopy. The H-DK-Cu and Me-DK-Cu complexes with M_w of approximately 1 x 10⁴ are thermotropic liquid crystalline polymers and their clearing temperature and mesophase ranges are lower than those of the corresponding Cu-coordinated, beta-diketone-based liquid crystalline polymethylsilsesquioxanes (Cu-FBDKLCP) without the n-heptyl flexible group. However, their liquidity within the temperature range of the liquid crystal state is better. The results are mainly attributed to the heptyl, flexible side group.     

Synthesis and mesomorphic properties of Cu-coordinating liquid crystalline polysilsesquioxanes with heptyl side groups

In order to study the influence of the concentration of a flexible group on the liquid crystal behaviour of copper-containing complexes, series of Cu-coordinated, beta-diketone-based liquid crystalline polysilsesquioxanes (abbr. H-DK-Cu) and random copolymethylsilsesquioxanes (abbr. Me-DK-Cu), have been synthesized by the hydrosilylation reaction of a vinyl-terminated beta-diketone and 1-heptene with the polyhydrosilsesquioxane (H-T) and with the random copolymethylhydrosilsesquioxane (Me-T), followed by reaction with Cu(NH₃)₄Cl₂. Their mesomorphic properties were investigated by differential scanning calorimetry and optical polarizing microscopy. The H-DK-Cu and Me-DK-Cu complexes with M _w of approximately 1 x 10⁴ are thermotropic liquid crystalline polymers and their clearing temperature and mesophase ranges are lower than those of the corresponding Cu-coordinated, beta-diketone-based liquid crystalline polymethylsilsesquioxanes (Cu-FBDKLCP) without the n-heptyl flexible group. However, their liquidity within the temperature range of the liquid crystal state is better. The results are mainly attributed to the heptyl, flexible side group.     

Synthesis and mesomorphic properties of new liquid crystalline stilbene derivatives containing vinyloxyalkoxy chains

New liquid crystalline stilbene derivatives containing vinyloxyalkoxy chains 1 (n=5, 7, 9, 11) have been synthesised. Their mesomorphic properties have been studied by polarising optical microscopy and differential scanning calorimetry. It is interesting to note that even their alcohol precursors showed liquid crystalline behaviour. The effect of terminal alkoxy chain length and UV irradiation on the mesomorphic behaviour is discussed.     

Synthesis,mesomorphic behaviour and optoelectronic properties of phosphorus-based thermotropic liquid crystalline dendrimers

A new series of thermotropic phosphorus-based liquid crystalline (LC) dendrimers based on a thiophosphoryl-phenoxymethyl(methylhydrazono) core (thiophosphoryl-PMMH) up to the fifth generation has been synthesised by solution condensation of aldehyde groups, surface-functionalised thiophosphoryl-PMMH dendritic substrates of generation numbers G_0.5 to G_5.5, with the appropriate molar equivalents of the pro-mesogenic n-hexadecylaniline mono-functional building block. Their chemical composition has been confirmed by ¹H/¹³C/³¹P nuclear magnetic resonance spectroscopy, Fourier transform infrared spectroscopy, elemental analysis. Optical properties have been studied by ultraviolet-visible absorption, photoluminescence spectroscopy and polarised optical microscopy, and thermal characteristics by differential scanning calorimetry. Electrical studies have been made using the current-voltage characteristics of organic light-emitting diodes consisting of multi-layered indium tin oxide/dendrimer/aluminium tris(8-hydroxyquinoline)Al architecture. It has been demonstrated that the molecular engineering approach adopted can successfully lead to phosphorus-containing dendritic organic semiconductors (OSCs) which show tunable mesomorphic behaviour (extension of the observed smectic mesophase) and (opto) electronic properties, owing to their peripheral decoration with a tunable number of azomethine-based optically active chromophoric units. This rare combination of ‘tunable by design’ properties makes this series of thermostable thiophosphoryl-PMMH-based LC dendrimers a particularly appealing class of OSCs for use in optically and/or electronically active layers of (opto)electronic devices such as light-emitting diodes, field-effect transistors, solar cells and lasers.     

Ionic liquid crystals: synthesis and mesomorphic properties of dimeric 2,4,6-triarylpyrylium tetrafluoroborates

The synthesis of some dimeric pyrylium salts is described. The link between the two cationic parts of the molecule can be flexible or rigid. Surprisingly, the starting dimeric chalcones with a rigid link exhibit monotropic columnar hexagonal phases. Results concerning the spectroscopic and mesomorphic properties of the pyrylium salts and the chalcones are given.     

Synthesis and mesomorphic properties of new compounds exhibiting TGBA and TGBC liquid crystalline phases

We synthesised a series of rod-like mesogens with a (S)-2-methylbutyl-(S)-lactate unit in the chiral chain that exhibited extremely wide temperature ranges in the TGBA and TGBC* phases. TGB phases were identified, based on typical textures in confined samples and in free-standing films, by Grandjean-Cano texture and by NMR studies on a deuterium-labelled isotopomer. A sufficiently high electric or magnetic field transformed the TGBA and TGBC* phases into their respective SmA and SmC* phases, the TGB structures being restored within some 20-30 minutes. Therefore values of the spontaneous polarisation and spontaneous tilt angle, when measured under a sufficiently high field, gave evidence of the properties of the SmC* phase. Temperature dependencies of relaxation frequency, dielectric strength, selective reflection and layer spacing showed anomalies at a certain temperature within the TGBC* phase range. Also, changes in textures, as well as in ²H-NMR spectra, occurred at this same temperature. These results suggest the existence of two TGBC* phases.     

Ionic liquid crystals: synthesis and mesomorphic properties of dimeric 2,4,6-triarylpyrylium tetrafluoroborates

The synthesis of some dimeric pyrylium salts is described. The link between the two cationic parts of the molecule can be flexible or rigid. Surprisingly, the starting dimeric chalcones with a rigid link exhibit monotropic columnar hexagonal phases. Results concerning the spectroscopic and mesomorphic properties of the pyrylium salts and the chalcones are given.     

Polymerizable liquid crystalline twin molecules: synthesis and thermotropic properties

The synthesis of 14 novel low molar mass liquid crystalline twin molecules is described and experimental details are given. The twin monomers contain two mesogenic units which are connected by a flexible spacer. Two terminal acrylate groups make these twins suitable for photopolymerization. The insertion of lateral groups into the mesogen leads to glassforming properties. We tested several substituents (-OCH₃, -CH₃) in different positions of the mesogenic unit and investigated their thermotropic properties as well as their crystallization behaviour by polarizing microscopy and DSC experiments. Some of the novel twin molecules with lateral substituents in the mesogenic core have unusually broad mesophases of about 150^oC. Below T _g stable LC glasses are formed. At room temperature a slow, kinetically hindered crystallization starts after about three hours. The broad mesophases of the twin molecules allow investigations of the photopolymerization kinetics over a wide temperature range. The addition of chiral non-liquid crystalline comonomers and subsequent photopolymerization leads to cholesteric networks with interesting optical properties. Last but not least, the twins are suitable mixing agents which suppress the crystallization of classical mono-rods.     

A new liquid crystalline derivative of dibenzotetraaza[14]annulene: synthesis, characterization and the preliminary evaluation of mesomorphic properties

A new dibenzotetraaza[14]annulene ligand has been synthesized that contains two 2-hydroxybenzoyl and four 3,7-dimethyloctyloxy peripheral substituents. Its mesomorphic textures were observed by means of a polarizing optical microscopy.     

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.