Mesophase Stability Research Articles

Mesophase behavior and DFT conformational analysis of new symmetrical diester chalcone liquid crystals

Synthesis, geometrical structures, conformational analysis, mesomorphic and optical characterizations of new angular chalcone liquid crystals, 4-(1,5-(3-oxopenta-1,4-dienyl))diphenyl bis-4-alkoxybenzoates, were investigated. These compounds contain chalcone core and diester phenyl rings attached to symmetrical terminal alkoxy chains with different number (n) of carbons from 6 to 16. Mesomorphic and optical properties were studied for the prepared homologues by differential scanning calorimetry (DSC) and polarized optical microscopy (POM). Elemental analyses, FT-IR, 1H, and 13C NMR spectroscopy were used for molecular structure confirmation. Density functional theory (DFT) theoretical calculations were estimated to confirm the experimental data and to deduce the most stable confirmal of prepared compounds. The results indicate that the stability of the conformers and the type of the mesophase are dependent on the length of the terminal alkoxy chains. Thermal parameters, dipole moments and polarizability of all estimated conformers were discussed. The relationships between the values of these parameters and the conformer type as well as the mesophase stability were illustrated.

Impact of the proportionation of dialkoxy chain length on the mesophase behaviour of Schiff base/ester liquid crystals; experimental and theoretical study

ABSTRACTNew non-symmetrical homologues series of Schiff base/ester series, 4-alkoxyphenylimino-4ʹ-phenyl-4ʹ’-alkoxybenzoates, have two terminal alkoxy groups (Im/n), were synthesised and investigated for their mesophase behaviour. Where the number of carbon in the alkoxy chains attached to the phenylimino and benzoate moieties named m and n respectively. The newly Schiff base structures were confirmed via elemental analyses IR, and NMR spectroscopy. Mesomorphic properties were investigated by differential scanning calorimetry and polarised light microscopy. All investigated compounds proved to be dimorphic exhibits semictic A and nematic phases with high stability. The present Schiff base groups and the previously prepared 4-alkoxyphenylazo-4ʹ-phenyl-4ʹ’-alkoxybenzoates were compared and results revealed that the replacement the azo linkage by phenylimino group change the mesophase stability and the type of the phase depending up on the length of alkoxy – chain m that attached to the phenylimino mesogenic part. Furthermore, we have performed density functional theory theoretical calculations to confirm the experimental data and the results are in consistent with the experimental investigation.

Effect of lateral bromo substituent on the phase behavior of four-ring azo/ester/azo liquid crystalline materials

ABSTRACTIn order to study the influence of lateral Br substitution on mesophase behaviour, five homologous series of 4-substituted phenylazo phenyl 4ʹ-(3ʺ-bromo-4ʺ-alkoxyphenylazo) benzoates (Ina–e) have been synthesised. Within each homologous series, the alkoxy group varies from 6 to 16 carbons, while other terminal group substituents, X, are CH3O, CH3, H, Br and NO2 groups; the mesophase behaviour of these series is compared with previously prepared laterally neat analogues, 4-substituted phenylazo phenyl 4ʹ-(4ʺ-alkoxyphenylazo) benzoates (IIna–e) and laterally methyl analogues, 4-substituted phenylazo phenyl 4ʹ-(3ʺ-methyl-4ʺ-alkoxyphenylazo) benzoates (IIIna–e). Similar to lateral methyl analogues, the present series, lateral Br substitution showed that, independent of the polarity of the substituent X or the alkoxy-chain length, the nematic phase is predominant with relatively high stability and broad temperature ranges. The mesophase stability varies between 204.0°C and 335.0°C for the nematic phase and 169.6°C and 281.0°C for the SmA phase. Their total mesophase temperature ranges vary between 87.2°C and 201.4°C. All compounds were found to be thermally stable within the mesophase temperature range, except the lower homologue of the nitro and Br substituted derivatives. The obtained results are discussed in terms of molecular polarisability.

Schiff base/ester liquid crystals with different lateral substituents: mesophase behaviour and DFT calculations

ABSTRACTFour new groups of 4-((2ʹ-substitutedphenylimino)methyl)phenyl-4”-alkoxy benzoates, Ina-d, of Schiff base ester liquid crystals, were prepared and investigated for their mesophase formation and stability. Each group constitutes four homologous series that differ from each other by the lateral attached polar group X in the ortho position for the imine mesogen at terminal benzene ring that alternatively changed from F, Br, NO2 and lateral benzene ring. Within each homologous series, the number (n) of carbons in the alkoxy chain varies between 6, 8, 10 and 12. Molecular structures of the prepared compounds were confirmed via elemental analysis, FT-IR, and 1H NMR spectroscopy. Mesomorphic properties were investigated by differential scanning calorimetry (DSC) and the phase identified by polarised light microscopy (PLM). A comparative study was made between the investigated compounds and their previously prepared laterally neat, 4-((4ʹ-phenylimino)methyl)phenyl-4”-alkoxy benzoates (IIn); the result revealed that all lateral substituents not only decrease the melting temperature but also the mesophase stability and shown only nematic phase. Density functional theory (DFT) calculations for new lateral derivatives were discussed.

Influence of lateral methyl and terminal substituents on the mesophase behaviour of four rings azo-ester liquid crystal compounds

ABSTRACTThe effect of introducing a lateral methyl substitution into the previously investigated laterally neat four-ring analogues, 4-substituted phenylazo phenyl 4ʹ-(4ʺ-alkoxyphenylazo) benzoates (Ina–e), on their mesophase behaviour was investigated for the newly prepared five homologous series of 4-substituted phenylazo phenyl 4ʹ-(3ʺ-methyl-4ʺ-alkoxyphenylazo) benzoates (IIna–e). Within each homologous series, the alkoxy group varies between 6, 8, 10, 12, 14, and 16 carbons, while the substituent, X, is a polar group that alternatively changes between the electron-donating (CH3O and CH3) groups, and the electron-withdrawing (Br and NO2) groups, including the unsubstituted homologues (IInc). Their mesophase stabilities were determined by DSC and phases identified by PLM. The results showed that independent of the alkoxy-chain length or the polarity of the substituent X, the nematic phase is predominant with relatively high stability and wide temperature ranges. All compounds show a good thermal stability in the mesophases domain, except the nitro and Br substituted derivatives bearing short alkoxy chain length. Comparison of the mesophase behaviour was also made between the present series and corresponding three-ring laterally CH3-substituted azo/ester analogues. UV-vis absorption spectra revealed that derivatives with electron donating or an electron withdrawing groups exhibited redshifts of the π→π* transition compared with unsubstituded derivative.

Rich liquid crystal phase behavior of novel alkyl-tri(ethylene glycol)-glucoside carbohydrate surfactants

Carbohydrates are appealing non-ionic surfactant head-groups as they are naturally abundant, generally biocompatible and biodegradable, and readily functionalized. Herein, we explore the phase behavior of seven novel carbohydrate-based surfactants (CBS) containing a tri-ethylene glycol (TEG) linker between a glucose head-group and alkyl tail-group, with linear saturated (C8–18) and cis-unsaturated (C18:1) alkyl chains. At high aqueous concentrations, these glycolipid-like surfactants transition into a variety of lyotropic liquid crystalline phases following an expected concentration phase sequence: hexagonal (H1) → bicontinuous cubic (V1) → lamellar (Lα). Using polarizing light microscopy (PLM), a binary (surfactant–water) phase diagram for each surfactant was constructed across a temperature range (25–80 °C) revealing thermotropic behavior and a broadening of liquid crystal phase regions with increasing alkyl chain length. There was also a significant difference between saturated and unsaturated alkyl chains, due to the cis-unsaturated ‘statistical bend’ lowering the melting point. Small-angle X-ray scattering (SAXS) measurements were performed to characterize the liquid crystal phases, identifying highly-ordered p6m,Ia3d, and Lα crystallographic space-groups with up to 7 resolved Bragg peaks, likely due to the highly anisometric nature of the TEG-linked surfactants. The phases were shown to be more numerous and exhibited greater thermal-stability compared to well-characterized alkyl glucoside surfactants lacking an oligoethylene spacer in the literature. Finally, the characteristic dimensions of each phase were determined to enable visualization of the internal microstructures, providing insight into the impact of molecular shape and the distribution of hydro-philicity/phobicity on the formation and stability of liquid crystalline mesophases.

The first mesomorphic benzimidazolium-based silver(I)-N-heterocyclic dicarbene complexes: Synthesis, characterization and phase properties

The first mesomorphic benzimidazolium based mononuclear silver (I) N-heterocyclic dicarbene complexes, Ag(I)-NHC were prepared via in situ deprotonation of 1-octadecyl-3-alkyl/arylbenzimidazolium salts with silver oxide (Ag2O). Both series of Ag (I)-NHC complexes contained a C18 alkyl chain on one N atom and differed in term of the substituent (alkyl and substituted benzyl moiety) on the other N atom in the benzimidazolium ring. All the synthesized compounds were characterized by elemental analysis, Fourier-transform infrared, 1H and 13C NMR spectroscopy. Cation geometry, anion and alkyl chain length were found to be essential parameters for mesophase formation in the salts and Ag(I)-NHC complexes in this study. All dialkylated salts except the one with the shortest chain, n = 10 were smectogenic. On the other hand, a large width-to-length ratio of the cationic core in the benzyl analogues had a negative effect on the formation and stability of mesophase. No mesophases were observed for these compounds. Wider mesophase range was seen with decrease in anion size and elongation of alkyl chains. A metastable SmA phase was observed in two Ag(I)-NHC complexes. The minimum chain length required for compounds with different anions to exhibit liquid crystal (LC) properties was addressed. Mesophase was observed for bromide salts with C12 onwards while C14 is the minimum chain length required for the hexaflurophosphate containing Ag(I)-NHC complexes to display LC properties. All Ag(I)-NHC complexes except the one bearing a cyano group could be considered ionic liquids with melting/clearing temperatures below 100 °C.

Synthesis and mesophase behaviour of Schiff base/ester 4-(arylideneamino)phenyl-4″-alkoxy benzoates and their binary mixtures

Four new groups of Schiff base ester liquid crystal compounds, 4-(arylideneamino)phenyl-4″-alkoxy benzoates, Ina–d, were prepared and investigated for their mesophase formation and stability. An alkoxy group of variable chain length (n = 6, and 16 carbons) is attached to the terminal phenylazo benzoate moiety. Each group constitutes two homologous series that differ from each other by the polar substituent X (CH3O, H, NO2, and F). The molecular structure of the prepared compounds was confirmed via FT-IR, 1H NMR, mass spectroscopy and elemental analysis. The mesomorphic properties were investigated by differential scanning calorimetry (DSC) and polarized light microscopy (PLM). For n = 6, all compounds exhibit nematic phase. Amongst them methoxy and nitro derivatives have widest mesophase range. For n = 16, nitro and flouro derivatives exhibit SmA phase, while methoxy and unsubstituted derivative are nematogenic. The change of the mesophase stability is well correlated with van der Waals volume and polarizability of the substituent X and not satisfactory with the dipole moment of the whole molecule. The mesophase behavior of binary mixtures made from each individual derivatives bearing the same substituent X but varies with alkoxy chains length is presented. Results provide insights into the role of substituent X and alkoxy chain length on the mesophase behavior within the whole composition range of the mixtures.

Comparison of the Mesomorphic Behaviour of 1:1 and 1:2 Mixtures of Charged Gay-Berne GB(4.4,20.0,1,1) and Lennard-Jones Particles

We present a Molecular Dynamics study of mixtures of charged Gay-Berne (GB) ellipsoids and spherical Lennard-Jones (LJ) particles as models of ionic liquids and ionic liquid crystals. The GB system is highly anisotropic (GB(4.4,20.0,1,1)) and we observe a rich mesomorphism, with ionic nematic and smectic phases in addition to the isotropic mixed phase and crystalline phases with honeycomb structure. The systems have been investigated by analyzing the orientational and translational order parameters, as well as radial distribution functions. We have directly compared 1:1 mixtures, where the GB and LJ particles have a charge equal in magnitude and opposite in sign, and 1:2 mixtures where the number of LJ particles is twice as large compared to the GB and their charge half in magnitude. The results highlight the role of the long-range isotropic electrostatic interaction compared to the short-range van der Waals anisotropic contribution, and the effect of the stoichiometry on the stability of ionic mesophases.

DFT Calculations and Mesophase Study of Coumarin Esters and Its Azoesters

Two groups of coumarin derivatives, 4-methyl-2-oxo-2H-chromen-7-yl 4-alkoxybenzoates (coumarin esters), In, and 4-methyl-2-oxo-2H-chromen-7-yl 4-(2-(4-alkoxyphenyl)diazenyl) benzoates (coumarin azoesters), IIn, were synthesized and investigated for their mesophase behavior and stability. Each group constitutes five series that differ from each other by length of the mesogenic part. Within each homologous series, the length of the terminal alkoxy group varies between 6, 8, 10, 12 and 16 carbons. Mesophase behavior was investigated by differential scanning calorimetry (DSC) and identified by polarized light microscopy (PLM). Density functional theory (DFT) calculations for coumarin derivatives were discussed. The results revealed that the incorporation of azo group incorporated in the mesogenic core decreases the energy differences, increases the dipole moments and stabilities of coumarin azoesters series more than coumarin esters.

Lord of The Crowns: A New Precious in the Kingdom of Clustomesogens

AbstractReplacing pure inorganic materials by functional organic–inorganic hybrid ones to lower production costs has become a major challenge, in particular for the optoelectronic industry. Adding nanostructuration abilities meanwhile preserving homogeneity is even more challenging for this class of new materials. Here we show that red‐NIR emissive ternary molybdenum cluster salts can be assembled to liquid crystalline 15C5 crown ethers. The resulting hybrids are homogeneous and stable up to high temperature despite the weakness of the supramolecular interactions binding both components. These are illustrated by 133Cs MAS NMR. All hybrids show hexagonal columnar arrangements and strong red‐NIR emission. Surprisingly, when chlorinated clusters are used instead of brominated ones, the mesophase stability is largely enhanced.

Lord of The Crowns: A New Precious in the Kingdom of Clustomesogens.

Replacing pure inorganic materials by functional organic-inorganic hybrid ones to lower production costs has become a major challenge, in particular for the optoelectronic industry. Adding nanostructuration abilities meanwhile preserving homogeneity is even more challenging for this class of new materials. Here we show that red-NIR emissive ternary molybdenum cluster salts can be assembled to liquid crystalline 15C5 crown ethers. The resulting hybrids are homogeneous and stable up to high temperature despite the weakness of the supramolecular interactions binding both components. These are illustrated by 133 Cs MAS NMR. All hybrids show hexagonal columnar arrangements and strong red-NIR emission. Surprisingly, when chlorinated clusters are used instead of brominated ones, the mesophase stability is largely enhanced.

Preparation and mesomorphic properties of 1-methyl-1H-benzimidazole-based compounds

ABSTRACTA series of 1-methyl-1H-benzimidazole-based compounds, 2-(4ʹ-alkoxy-1,1ʹ-biphenyl-4-yl)-1-methyl- 1H-1,3-benzimidazole derivatives (nPPMx-M) with terminal hydrogen, methyl and nitro moieties (coded as nPPMH-M, nPPMM-M and nPPMN-M, respectively), were prepared and their structures were characterised. The compounds display enantiotropic smectic mesophases for hydrogen and methyl terminated compounds (nPPMH-M and nPPMM-M), and enantiotropic nematic mesophases for nitro terminated compounds (nPPMN-M) with short alkoxy chain below than 10 carbon atoms, where the mesophase ranges are 24–72°C and 74–104°C on heating and cooling processes for nPPMH-M, 90–119°C and 110–135°C for nPPMM-M, and 102–129°C and 113–207°C for nPPMN-M, respectively. It is noted that the compounds nPPMx-M exhibit much lower melting points and much wider mesophase range both in heating and cooling than non-1-methyl substituted analogs, which are ascribed to the disruption of hydrogen bonding among the molecules caused by methyl substitution at 1-position of benzimidazole. Meanwhile, among the compounds nPPMx-M, much wider mesophase ranges are obtained for nPPMM-M and nPPMN-M, indicating a much high mesophase stability for the compounds bearing terminal moiety (CH3 and NO2).

Synthesis and Mesogenic Properties of Liquid Crystals Based on Cholesterol and Cinnamic Acid Derivatives

A group of new mesomorphic compounds with cholesterol-based, which have derivative of the cinnamic acid, with different lengths of terminal alkoxy chains, are synthesized and characterized. The mesomorphic properties were investigated by DSC and POM. All target compounds showed a cholesteric phase. For 12-TC and 14-TC besides a cholesteric phase, also revealed a SmA* phase. The experimental results demonstrated that with the increase in the terminal chain lengths, the thermal stability of the smectic mesophases of these liquid crystals was increased, and these compounds shows brilliant color changes in the temperature ranges of cholesteric phase.

Effect of position of the lateral fluoro substituent on the mesophase behaviour of aryl 4-alkoxyphenylazo benzoates in pure and binary mixtures

ABSTRACTFive groups of 4-substituted phenyl 4ʹ-(2ʺ- (or 3″-) substituted-4ʺ-alkoxyphenylazo) benzoates (Ina-c to Vna-c) were investigated in which, within each group, the length of the terminal alkoxy group varies between 8 and 16 carbons, while the other terminal substituent, X, is a polar group that alternatively changed between the electron-donating CH3O and the electron-withdrawing Br group, in addition to the un-substituted analogue (X = H). The lateral substituent (Y) in the five groups I–V varies, respectively, between H, 3-CH3, 2-CH3, 3-F and 2-F. Their mesophase stabilities were determined by differential scanning calorimetry and phases identified by polarised light microscopy. The two newly prepared groups of compounds (IVna-c and Vna-c) are structurally characterised by infrared, 1H-NMR, mass spectroscopy, thermogravimetric and elemental analyses. Binary phase diagrams were constructed for each pair of isomers from groups IV and V bearing the same wing substituents but the lateral F is attached to different positions (2ʺ or 3″).

Liquid-Crystalline Tris[60]fullerodendrimers.

Liquid-crystalline tris[60]fullerodendrimers based on first- and second-generation poly(arylester)dendrons carrying cyanobiphenyl mesogens were synthesized for the first time by the olefin cross-metathesis reaction between type I (terminal) and type II (α,β-unsaturated carbonyl) olefinic precursors, using a second-generation Grubbs or Hoveyda-Grubbs catalyst. The modular synthetic approach developed here also allowed the selective preparation of the [60]fullerene-free, mono[60]fullerodendrimer, and bis[60]fullerodendrimer derivatives from the appropriate precursors. As revealed by polarized optical microscopy, differential scanning calorimetry, and small-angle X-ray scattering, all of the materials displayed liquid-crystalline properties. In agreement with the nature of the dendritic building blocks, the emergence of lamellar mesophases (smectic C and/or smectic A phases), with the segregation of the various constitutive parts, was systematically observed. The small variation of the mesomorphic temperature range and of the mesophase stability suggested that the mesomorphism is essentially dominated by the dendrimer itself and is regulated by a subtle adaptive mechanism, in which the proportion of monolayering and bilayering arrangements of the multisegregated lamellar mesophases is modified in order to compensate the space requirements of each of the elementary building blocks, namely, the [60]fullerene units, the cyanobiphenyl mesogens, and the dendritic matrix.

Mesophase stability of new Schiff base ester liquid crystals with different polar substituents

ABSTRACTFour new groups of Schiff base ester liquid crystal compounds, 4-((4ʹ-substituted phenylimino) methyl)phenyl–4″-alkoxybenzoates, Ina–d, were prepared and investigated for their mesophase formation and stability. Each group constitutes four homologous series that differ from each other by the polar substituent X (CH3O, CH3, H, and Cl). Within each homologous series, the number (n) of carbons in the alkoxy chain varies between 6, 8, 10, and 12. Molecular structures of the prepared compounds were confirmed via FT-IR, 1H NMR, mass spectroscopy, and elemental analysis. The mesomorphic properties were investigated by differential scanning calorimetry and polarised light microscopy. Comparative studies between the present series and previously investigated 4-(4-substituted phenylazo)phenyl 4″-alkoxybenzoates revealed that the phenylimino mesogenic core increases the mesophase stability rather than the azo analogues.

Ortho-Fluorination of azophenols increases the mesophase stability of photoresponsive hydrogen-bonded liquid crystals

We present a photoresponsive supramolecular liquid-crystalline (LC) system with enhanced stability of the LC phase due to ortho-fluorination of the bond-donating hydroxyazobenzene derivative, an important characteristic for their future use in photonics.

Supramolecular liquid crystals displaying competitive hydrogen bonding: Small molecule mesogens assembled through different hydrogen bond assemblies

ABSTRACTA series of supramolecular assemblies created through competitive hydrogen bonding of a n-alkoxy benzoic acid hydrogen bond donor and a mixture of hydrogen bond acceptors: a liquid crystal-forming rigid bi-pyridyl and a non-mesogenic tetrafunctional pyridyl species. The associative chain structures displayed mesogenic characteristics at loadings dependent on the concentration of the disruptive tetrafunctional agent- systems eliminated liquid crystallinity at 85% inclusion in heating transitions and 90% in cooling. The assembled structures displayed considerable mesophase stability. This was observed through long liquid crystalline lifetimes even at high compositions of 4PD, which generally dampened due to a generalized eutectic effect. It is believed that the freedom of mobility inherent in the C8 Acid allows for the reorganization of the hydrogen bonds in the liquid crystalline state providing for a very high loading of disruptor and long mesophase lifetimes.

Thermotropic liquid crystalline polyesters derived from 2-chloro hydroquinone

Synthesis of thermotropic liquid crystalline polyesters derived from bis[4-hydroxy benzoyloxy]-2-chloro-1,4-benzene (BHBOCB) and aliphatic dicarboxylic acid chlorides by interfacial polycondensation methodology is presented. Synthesised polyesters consist of bis[4-hydroxy benzoyloxy]-2-chloro-1,4-benzene as a mesogen and aliphatic diacid chloride as flexible spacer. The length of oligomethylene units in the polymer was varied from the trimethylene to the dodecamethylene groups. Synthesized polyesters were characterized by differential scanning calorimetry and optical microscopy. The transition temperatures and thermodynamic properties were studied for all these polymers. These polyesters exhibited thermotropic liquid crystalline behavior and showed nematic texture except decamethylene spacer. Decamethylene spacer based polyester showed marble texture of smectic C. Mesophase stability of these polyesters was higher than $$123{^{\circ }}\hbox {C}$$ (except first heating cycle of PE-1). SYNOPSIS The present study deals with the synthesis of thermotropic liquid crystalline polyesters derived from bis[4-hydroxy benzoyloxy]-2-chloro-1,4-benzene (BHBOCB) and aliphatic dicarboxylic acid chlorides by interfacial polycondensation methodology.