Enantiotropic Smectic Phases Research Articles

Oxadiazolyl-Pyridinium as Cationic Scaffold for Fluorinated Ionic Liquid Crystals

The synthesis and characterization of a new class of 1,2,4-oxadiazolylpyridinium as a cationic scaffold for fluorinated ionic liquid crystals is herein described. A series of 12 fluorinated heterocyclic salts based on a 1,2,4-oxadiazole moiety, connected through its C(5) or C(3) to an N-alkylpyridinium unit and a perfluoroheptyl chain, differing in the length of the alkyl chain and counterions, has been synthesized. As counterions iodide, bromide and bis(trifluoromethane)sulfonimide have been considered. The synthesis, structure, and liquid crystalline properties of these compounds are discussed on the basis of the tuned structural variables. The thermotropic properties of this series of salts have been investigated by differential scanning calorimetry and polarized optical microscopy. The results showed the existence of an enantiotropic mesomorphic smectic liquid crystalline phase for six bis(trifluoromethane)sulfonimide salts.

Methylation driven molecular packing difference at the smectic phases of a series of pyridinium-based chiral ionic liquid crystals

A series of pyridinium-based chiral ionic liquid crystals were designed and synthesized. All of them exhibit enantiotropic smectic phases. The methylation on the pyridine ring not only decreases the melting point and clearing point, but also drives structural transitions from bilayer molecular packing structures to interdigitated monolayer ones at the smectic A and C phases. Due to entropy increase, the clearing point of the liquid crystal derived from 2-methylpyridine is higher than that derived from 3-methylpyridine, and then that derived from 4-methylpyridine.

Main-chain biodegradable liquid crystal materials base on diosgenin: synthesis and mesomorphism

ABSTRACT Reinforcing the stability of aliphatic carbonate is an intractable problem that the mesomorphism of the material still needs to solve today. Here, novel main-chain liquid crystal compounds with mesomorphism are designed via the bulk ring-opening polymerisation to improve the stability and mesomorphic properties. The compounds were synthesised by bulk copolymerisation reaction of the monomers and initiator. The monomers of the 1, 3-trimethylene carbonate and the d, l-lactide were initiated by the diosgenyl derivative, without adding any catalyst. The chemical structures of obtained compounds were characterised by FT-IR and 1H NMR. The mesomorphic behaviour of the materials was determined by polarising optical microscopy and X-ray diffraction measurement. The thermal performance was investigated by differential scanning calorimetry and thermo gravimetric analyser. The results indicated that the phase transition temperature of the obtained compounds can be adjusted by the variation content of the monomers, and the compounds displayed an enantiotropic smectic phase in the room-temperature.

Synthesis and mesomorphism of the liquid crystal based on diosgenyl end-capped polycarbonate

ABSTRACTTo enhance the activity of the hydroxyl group of diosgenin, the diosgenyl derivative Dios–(CH2)6–OH with an alkyl spacer and end primary hydroxyl group was synthesised via the chain extension reaction between diosgenin and 1,6-hexanediol. Then the liquid crystal (LC) compounds Dios–(CH2)6–(trimethylene carbonate [TMC])n with different TMC content were obtained via the ring-opening polymerisation of TMC with Dios–(CH2)6–OH as initiator and mesogenic unit, importantly, there was no catalyst added. The chemical structures of the Dios–(CH2)6–OH and Dios–(CH2)6–(TMC)n were characterised by Fourier transform infrared (FT-IR) spectra and 1H NMR. The mesomorphism was characterised by polarising optical microscopy and X-ray diffraction measurement, and the phase transition was investigated by differential scanning calorimetry. The results showed that Dios–(CH2)6–(TMC)n displayed an enantiotropic smectic phase, and TMC content played an important role in the LC properties of Dios–(CH2)6–(TMC)n. The higher the TMC content, the lower the phase transition temperature of Dios–(CH2)6–(TMC)n.

Synthesis, characterization and liquid crystalline properties of novel benzimidazol-8-hydroxyquinoline complexes

The synthesis, characterization and liquid crystalline properties of N4,N4′-bis((1H-benzo[d]imidazol-2-yl)methyl)-3,3′-dimethyl-[1,1′-biphenyl]-4,4′-diamine and of their corresponding Mn(II), Fe(II), Ni (II), Cu(II), and Zn(II) complexes are described. The ligand and complexes have been characterized by elemental analysis, magnetic susceptibility measurements (µeff), conductometric measurements and Fourier Transform Infrared (FTIR), Nuclear Magnetic Resonance (1H NMR), (13C-NMR) and UV–Vis spectroscopy. Spectral investigations suggested octahedral coordination geometrical arrangement for M(II) complexes. The phase transition temperatures were detected by differential scanning calorimetry (DSC) analysis and the phases are confirmed by optical polarizing microscopy (POM). The DSC and POM supported the mesomorphic properties of the uncoordinated ligand in which the enantiotropic smectic phases were recorded. However, not all of their corresponding M(II) complexes are liquid crystal.

Design of advanced multicomponent ferroelectric liquid crystalline mixtures with submicrometre helical pitch

ABSTRACTNew functional multicomponent ferroelectric liquid crystalline mixtures have been designed while searching smart self-assembling materials with submicrometre periodicity of the helical structure responding definite demands for application in electro-optic devices and photonics that exploit the deformed helix ferroelectric effect. The resulting designed mixtures possess the paraelectric smectic A* and the tilted ferroelectric smectic C* phases over a very broad temperature range down below the room temperature. The mesomorphic, electro-optic and dielectric properties have been studied and discussed. The presence of a very stable enantiotropic ferroelectric smectic phase exhibiting almost temperature independent helical pitch within 150–250 nm range and reasonably high values of the tilt angle might allow these mixtures to be applicable for practical purposes.

Phase Diagram and Dielectric Studies in Hydrogen-Bonded Liquid Crystal System

ABSTRACTThe thermal and dielectric behaviors of mixtures were investigated for systems of p-n-hexyloxybenzoic acid (6OBA) and p-n-octyloxybenzoic acid (8OBA) by differential scanning calorimetry, polarized optical microscopy, and dielectric measurements. The T-X phase diagram was obtained for this system. Eutectic composition was calculated and received experimentally. Enthalpies of mixing were calculated to describe phase transitions physically. All mixtures show enantiotropic smectic and nematic phases. Dielectric permittivity has higher value for the mixture with fractional proportion of 30 to 70% mol for 8OBA than that for initial acid. It is assumed that co-crystallization of initial compound occurs when the ratio of components is close to equimolar ratio.

Study of phase behavior in a system of linear hydrogen-bonded carboxylic acid homologues

The system of hydrogen-bonded liquid crystals formed from binary mixtures p-n-heptyloxybenzoic (I) acid and p-n-undecloxybenzoic (II) acid has been investigated by polarizing optical microscopy, differential scanning calorimetry, X-ray diffraction, and scattering and dielectric measurements. The T–X phase diagram was obtained for this system. All mixtures show enantiotropic smectic and nematic phases. The crystalline phase represents a (α, β) solid solution. At the 2:1 ratio of I to II, an intermediate phase – co-crystal γ – is formed. The co-crystal γ possesses a much wider mesophase range than the corresponding initial components. In contrast to mixtures based on the solid solutions having a positive anisotropy, the co-crystal shows a negative dielectric anisotropy in the mesophase.

Molecular and crystal structures of 4-acylphenyl 4′-alkyloxybenzoates

The crystal structures of four liquid-crystal (LC) compounds belonging to 4-acylphenyl 4′-alkyloxybenzoates with the general formula C n H2n + 1-C(O)-C6H4-O-C(O)-C6H4-O-C m H2m + 1 numbered as 1/6, 1/7,2/7, and 3/7 (the numbers in the code indicate the ratios of the numbers of alkyl-chain units, n/m) are determined. Compounds 1/6 and 1/7 form smectic and nematic phases, but they are monotropic mesogens, whereas compounds 2/7 and 3/7 form only an enantiotropic smectic phase. Compound 3/7 crystallizes in two crystal modifications—triclinic with Z′ = 2 (3/7 tr ) and monoclinic with Z′ = 4 (sp. gr. P2, 3/7 mon ). All crystals consist of alternating aromatic and aliphatic regions and are characterized by the presence of weak directional interactions, such as C-H...O hydrogen bonds and π-stacking interactions. This provides the necessary conditions for the formation of a mesophase, and the existence of two types of structure-forming units is in line with the formation of a smectic phase. An explanation is proposed for the monotropic properties of 1/6 and 1/7.

Synthesis and mesomorphic properties of 4-(4-bromopropyloxy)-4′-(4-alkyloxybenzylidene)anilines

Abstract This article describes the preparation and liquid crystalline properties of a new homologous series of 4-(4-bromopropyloxy)-4′-(4-alkyloxybenzylidene)anilines in which the phenyl ring is armed by a bromopropyloxy chain. The thermal behavior and mesomorphic properties of the synthesized compounds were studied with particular attention given to the correlation between their phase transition temperatures and anisotropic change influenced by molecular structure. All the members displayed enantiotropic smectic phase except for the homologue with the longest alkyloxy chain (R = C 18 H 37 ), exhibiting only monotropic characteristic. The presence of bromine atom from the propyloxy side chain is found to be capable of altering and influencing the mesomorphic properties.

Studies of Calamitic Liquid Crystalline Compounds Involving Ester-Azo Central Linkages with a Biphenyl Moiety

Two mesogenic homologous series involving ester-azo central linkages with a biphenyl moiety have been synthesized, such as 4′-[(4-n-alkoxyphenyl)diazenyl]biphenyl-4-ol (series I) and 4′-[(4-n-alkoxyphenyl) diazenyl]-4-butoxy phenyl biphenyl-4-carboxylate (series II). Azobiphenyl of series I having a free hydroxyl group with strong hydrogen bonding exhibits a high-temperature enantiotropic smectic phase. Whereas in series II, compounds containing C1–C8 carbon atoms exhibit only a monotropic smectic phase and compounds with C10, C12, C14, and C16 atoms show an enantiotropic smectic phase. These compounds were characterized by elemental analysis, FT-IR, 1H-NMR, and mass spectral studies. The phase transition and mesogenicity of these substances were studied by polarizing optical microscopic and differential scanning calorimetric techniques. Their thermal stabilities and other characteristics are discussed.

Remarkable effect of a lateral substituent on the molecular ordering of chiral liquid crystal phases: A novel bromo-containing dichiralcompound showing SmC* variants

A bromo-substituted chiral smectic liquid crystal was synthesized for the purpose of the study on the sub-phases of the chiral smectic C* (SmC* variants) using resonant X-ray scattering. The introduction of a bromo atom was found to have a significant effect on the phase sequence. The bromo-substituted compound exhibited enantiotropic smectic phases in a wide temperature range of more than 130 K, in which at least five SmC* variants appeared below the smectic A phase. On the basis of the electro-optic and dielectric measurements, each SmC* variant was identified and assigned, and the structures of which were investigated by resonant X-ray scattering.

Thermal and current–voltage behaviour of liquid crystal compounds with rod and bent shapes comprising alkoxysemifluorinated and imine segments

The synthesis and mesomorphic properties of a series of thermotropic liquid crystalline symmetrical rod and bent-shaped imines containing two long alkoxysemifluorinated chains were reported. Three different diamines varying in the number of phenylene rings from 1 to 3 were used to understand structure–mesomorphic property correlation. Differential scanning calorimetry, X-ray diffraction and polarised optical microscopy were used to determine their phase transition behaviour. An enantiotropic smectic phase was observed for all the systems studied. The thermal properties of the azomethines were discussed in relation to the core shape and length of the core. Current–voltage measurements were performed on ITO/compound/Alq3/Al and ITO/TiO2/compound/Alq3/Al devices before and after irradiation with light (about 1000 W m−2). To prepare the TiO2 layer, the sol-gel technique was applied.

Novel Hydroxy- and Methyl-Terminated Triaromatic Schiff Base Compounds: Synthesis and Mesogenic Properties

A series of Schiff base hydroxy-mesogens and their model compounds have been synthesized. The mesogen is constructed from three-ring-containing mesogens linked through ester and azomethine groups with a terminal hydroxy group. The related model compounds were also synthesized with varying chain length with a terminal methyl group and their properties were compared with that of hydroxy mesogens. Extensive characterization of all mesogenic compounds was carried out by Fourier-transform IR, 1H and 13C NMR and mass spectroscopy. The phase characteristics such as nature of phase, melting and clearing temperatures and phase range were evaluated using a hot-stage optical polarizing microscope and differential scanning calorimetry. The appearance of enantiotropic smectic phases is due to the high molecular polarizability of the triaromatic core with azomethine and ester linkages.

Characterization, optical and thermal properties of new azomethines based on heptadecafluoroundecyloxy benzaldehyde

New thermotropic liquid crystals containing a long alkoxysemiperfluorinated chain (-O-(CH2)3-(CF2)7-CF3), one linking unit in mesogenic cores (HC=N-) and different functional end-groups such as 4-hexadecyl-, 4-n-hexadecyloxy- chain, or biphenyl-4-carbonitrile, 4-diazenyl-N,N-dimethylbenzene or pyren were synthesized via a one-step route. The methods of nuclear magnetic resonance (NMR), Fourier transform infrared (FTIR), ultraviolet–visual (UV-vis) and photoluminescence (PL) spectroscopy as well as differential scanning calorimetry (DSC) and polarizing optical microscopy (POM) were used. The absorption (UV-vis) and PL features of all compounds are documented. The amine had an effect on the mesomorphic properties of the azomethines. An enantiotropic smectic phase was observed for all of the systems studied. As a result of DSC and POM investigations, it is shown that liquid crystalline properties of the azomethines exhibit a strong dependence of the end-groups. The mesomorphic behaviour of the compounds was investigated also by FTIR(T) and UV-vis(T) spectroscopy. Current–voltage measurements were performed on an ITO/compound/Alq3/Al device.

Synthesis and properties of liquid crystalline conjugated disubstituted polyacetylene containing cyanoterphenyl mesogenic pendant

Novel acetylene monomers containing cyanoterphenyl groups, namely, 4-[(4′-cyano-4-terphenylyl)oxy]-1-butyl-1-butyne M(1) and 3-[(4′-cyano-4-terphen-ylyl)oxy]-1-phenyl-1-proyne M(2) are synthesized. M(1) was polymerized with WCl 6–PhSn 4 catalyst successfully to give the liquid crystalline conjugated disubstituted polyacetylene containing cyanoterphenyl mesogenic pendant P(1). Polymerization of monomer M(2) was carried out in a series of different solution, but did not obtain any product. The results indicate that the stereoeffect of the bulky cyanoterphenyl group and phenyl seems to inhibit the reaction. The structures and properties of the disubstituted polyacetylene P(1) and monomers were characterized and evaluated with nuclear magnetic resonance, infrared spectroscopy, thermogravimetry, differential scanning calorimetry, polarized optical microscopy, ultraviolet spectroscopy, and photoluminescence. The monomers show enantiotropic smectic phases in the heating and cooling processes, while the polymer P(1) exhibits a nematic phase when observed with a polarizing optical microscope. The existence of the chromophoric cyanoterphenyl core endows the monomers with high photoluminescence, and the polymer P(1) prepared from M(1) can emit a strong UV light of 411 nm.

Symmetric liquid crystal dimers containing hydrazide groups: parity‐dependent smectic structure, hydrogen bonding and substitution effect

Dimeric hydrazide derivatives with long terminal alkoxy chains, i.e. α,ω‐bis[N‐(4‐hexadecyloxybenzoyl)‐N′‐(benzoyl‐4′‐oxy)hydrazine]alkanes (C16. n. C16, C16 indicates the terminal hexadecyloxy chain; n = 3, 5, 6, 10, indicates the number of the carbon atoms in the spacer) were synthesised. The liquid crystalline properties were investigated by differential scanning calorimetry, polarising optical microscopy and X‐ray diffraction. The dimers with shorter spacer (C16. n. C16, n = 3, 5, 6) exhibit enantiotropic smectic phases, whereas C16.10.C16 with the longest spacer is non‐mesomorphic. Furthermore, the smectic structure is parity dependent, i.e. C16.3.C16 and C16.5.C16 with odd spacer exhibit a monolayer smectic A phase, whereas C16.6.C16 with even spacer exhibits a monolayer smectic C phase, in which the incompatibility between the terminal chains, mesogenic groups and the spacers is considered to be the driving force. The results are compared with those for the terminally nitro‐substituted series, α,ω‐bis[N‐(4‐nitrobenzoyl)‐N′‐(benzoyl‐4′‐oxy)hydrazine]alkanes (N n ), which exhibit an intercalated smectic A phase. Temperature‐dependent IR spectroscopic analysis on these two kind of dimers suggests that intermolecular hydrogen bonding, as well as the dipole–dipole interaction arising from the strong polar substituents (–NO2) synergistically drives the intercalated structure of N n . Although hydrogen bonding exists in the monolayer smectic phase of C16. n. C16, microphase segregation is in favour of the monolayer smectic structure, preventing the formation of an intercalated structure.

Study of phase situation in a banana-shaped 7OSOR compound from thiobenzoate series

This article reports new results on mesomorphic properties of 1,3-phenylene bis{4-[(4-heptyloxybenzoyl)sulfanyl]benzoate}, from the homologous series of banana-shaped resorcinol-based thioesters, referred to as 7OSOR, synthesized by us. The 7OSOR compound has got three enantiotropic smectic phases. At higher temperatures, 7OSOR exhibits the B6 phase and the B1 and SX phases at lower temperatures. The mesomorphic properties were studied by three complementary methods: Differential Scanning Calorimetry (DSC), Polarized Light Optical Microscopy (PLOM) and Transmitted Light Intensity (TLI). X-ray diffraction, static dielectric permittivity and electro-optic studies (EO) were also carried out.

Bridged 3,5-disubstituted pyrazolate ligands as support of metallomesogens containing [Pd(η 3-C 3H 5)] + fragments : X-ray crystal structure of [Pd(η 3-C 3H 5)(μ-pz R2)] 2·CH 2Cl 2 (R=C 6H 4OC 12H 25). Part III

The mesomorphic properties of two new long-chain 3,5-(4- n-alkoxyphenyl)pyrazoles Hpz R2 (R=C 6H 4OC n H 2 n+1 , n=16, 18) have been studied and compared with those of related compounds containing shorter chains ( n=4, 6, 8, 10, 12, 14). For liquid crystal purposes, the derivative with 14 carbon atoms in the chain behaves as the best compound on the basis of the lowest melting point and the widest stability range of the mesophases. New Pd(II) complexes of formula [Pd(η 3-C 3H 5)(μ-pz R2)] 2 (R=C 6H 4OC n H 2 n+1 , n=8, 10, 12, 14, 16, 18) ( 1– 6) containing the mesomorphic ligands as pyrazolate-bridging groups have been prepared and characterised. In solution, three conformational isomers (two symmetric and one asymmetric corresponding to different orientations of the allyl groups) have been found. In the solid state, the X-ray crystal structure of 3 ( n=12) evidenced the asymmetric isomer. The structure shows a bowlic core drawn on the basis of the boat-like conformation of the six-membered ring Pd(NN) 2Pd. However, a rod-like shape could also be considered by taking the molecular dimensionality into account (47.80 Å in length×4.75 Å in wide). The Pd-complexes 2– 6 were found to have liquid crystal properties exhibiting enantiotropic smectic phases consistent with the molecular characteristics. Moreover, the molecular packing of 3 is described as the layer-like type in which the molecules are parallel to the c-axis but tilted off the layer, this fact appearing similar to the molecular ordering in the smectic fluid phases.

Liquid crystalline compounds having a tribranched structure: substituted malonic esters consisting of two Schiff's base units and a cholesteryloxyalkyl substituent

New tribranched thermotropic liquid crystal compounds were synthesized and their liquid crystalline properties studied by differential scanning calorimetry, X-ray diffraction and polarizing optical microscopy. The compounds are the bis-{10-[4-(4-alkylphenyliminomethynyl)phenoxy]decyl} 2-[6-(cholesteryloxy)hexyl]malonates and the corresponding alkoxy derivatives. These compounds contain three mesogenic units, two identical Schiff's base type mesogens and one cholesteryl either moiety, interconnected in a tribranched structure via spacers. The cholesterly moiety is attached to the malonic acid core through an oxyhexamethylene spacer while the two Schiff's-base moieties are attached through oxydecamethylene spacers. The terminal alkyl group of the Schiff's base unit is either a butyl or decyl group, and the alkoxy terminal group is either a butoxy or decyloxy chain. All the compounds form only an enantiotropic smectic phase, most probably of the smectic C type. The larger spacings determined by small angle X-ray diffraction range from 3.3 to 4.1 nm, which are much shorter than the end-to-end distance (5.9-7.4 nm) of the molecules estimated using molecular models assuming an all trans extended conformation for all the alkyl spacers.