Synthesis and studies of some 4-substituted phenyl-4'-azopyridine-containing hydrogen-bonded supramolecular mesogens

Novel azopyridine-containing supramolecular liquid crystalline (LC) materials built via 1 : 1-heterointermolecular hydrogen bonding between some 4-substituted phenyl-4'-azopyridines and 4- n -alkyloxybenzoic acids are reported. These hydrogen-bonded LC complexes exhibit well defined nematic, smectic A and smectic C phases over wide ranges of temperature depending upon the number of carbon atoms present in the alkyl chains. The formation of pure LC materials on 1 : 1-complexation could be confirmed from the phase diagrams of the binary mixtures, which clearly indicated a melting maximum for the 50 mol % mixture as well as the presence of two eutectic points on either side of this mixture.     

Mesomorphic Characteristics of Induced Chiral Nematic Phase of [Smectic LCP,PS(4BC/DM)/Nematic LC,E7/Chiral Dopant,CB-15]-Ternary Composite System

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Optical microscopy, DSC and X-ray diffraction studies in binary mixtures of 4-pentyloxy-4'-cyanobiphenyl with three 4,4'-di(alkoxy)azoxybenzenes

A large number of binary mixtures of 4-pentyloxy-4'-cyanobiphenyl (5OCB) and 4,4'-di(alkoxy)azoxybenzenes (nOAB) with n = 5, 6 and 7 have been studied by optical microscopy, DSC and X-ray diffraction methods. Over a wide concentration range the smectic A (SmA) phase is induced and the nematic (N) phase of the parent compounds is absent. Instead of the tilted smectic C phase of 7OAB an orthogonal SmA phase is observed when only 12% of the host molecules are replaced by 5OCB molecules. Moreover, in some mixtures the thermal stability of the induced SmA phase is found to be more than 1.6 times the stability of the mesophses in the pure compounds. The N-isotropic or Sm-I transition temperatures slowly increases with increasing concentrations, reaching a maximum at x _5OCB = 0.50 and then falling quite rapidly. In all the mixtures the enthalpy changes at the N-I transitions follow the simple additivity rule. In no mixture, except mixture C2, could the SmA-N transition be detected by DSC, although in all the mixtures the ratio T _NA/T _NI is found to be more than 0.95, which is in contradiction to McMillan's theory. Average intermolecular distances are found to have a minimum value near the equimolar concentration. From the concentration dependence of the smectic layer spacings it also appears that in all the mixtures the smectic A_d phase evolves from the smectic A₁ phase with increasing concentrations.     

Novel phase behaviour in two (smectic/cholesteric) liquid crystal mixtures

The phase behaviours of mixed liquid crystal systems having either Sm/N or Sm/Ch properties have been studied. The (smectic/nematic) binary system formed smectic phases over a wide and much enhanced range of temperature (42 C) and a broad concentration range (0-90 wt %). The ternary smectic/cholesteric system, in appropriate concentration ranges, exhibited the smectic A phase, a TGBA-like twist grain boundary A phase, the cholesteric phase and blue phases. The TGBA-like phase appeared in the cholesteric-smectic phase transition range. Three textures (chiral pitch, fan-shaped and scale-like) for the cholesteric phase of the ternary smectic/cholesteric mixtures were observed in the ranges 0-7, 7-43 and 43 wt % respectively, of cholesteric CB15, in a binary Sm/N mixture.     

Miscibility studies of the smectic phases of trans,trans-4'-alkylbicyclohexyl-4-carbonitrile (CCH) liquid crystals

Complete temperature-composition phase diagrams for binary mixtures of 4'-ethyl-, 4'-propyl-, and 4'-butylbicyclohexyl-4-carbonitrile (CCH-2, CCH-3, and CCH-4, respectively) in each of the three possible combinations have been constructed from differential scanning calorimetry and thermal microscopy data. The highest temperature smectic phases of each of the pure mesogens are immiscible with one another, even though CCH-3 and CCH-4 have both been previously assigned the bilayer crystal-B structure on the basis of X-ray diffraction studies. The present studies indicate that the enantiotropic smectic phase of CCH-4 is slightly higher-ordered than is the monotropic smectic phase of CCH-3. The smectic phase of CCH-2, which previously has been found to be characterized by rhombohedral packing on the basis of X-ray diffraction data, is miscible with the second, previously uncharacterized (monotropic) smectic phase (S₂) of CCH-3. Photographs illustrating the subtle differences in the microscopic textures of these smectic phases are also presented.     

Induction of smectic mesomorphism by mixing a non-liquid-crystalline methacrylate with a nematic liquid-crystalline compound having a cyano-containing mesogen

The structure and phase diagram of a binary mixture of a non-liquid-crystalline, mono-functional (meth)acrylate monomer and a liquid-crystalline compound having a cyano group-containing mesogen were examined. The monomers had mesogenic units such as biphenylene and phenyl benzoate, but did not show liquid-crystallinity. The liquid-crystalline compounds possessed a cyanobiphenyl or a cyanophenyl benzoate mesogenic unit. The liquid-crystalline 4-cyanophenyl 4'-n-octyloxybenzoate did not show a smectic A phase, while the binary mixture of the 4-butoxy-4'-(ω-methacryloyloxyhexyloxy)biphenyl monomer with 4-cyanophenyl 4'-n-octyloxybenzoate showed the induction of a smectic A phase in a nearly equimolar composition range. However, the binary mixture of the monomer, containing a phenyl benzoate group, and the liquid-crystalline compound did not induce a smectic phase.     

Density studies on various smectic liquid crystals

Density measurements as a function of temperature for four homologues of the 5-n-alkyl-2-(4-n-alkyloxy-phenyl)-pyrimidines (PYP nOm) which exhibit nematic, smectic A and smectic C phases are reported. Furthermore 1-butyl-c-4-(4'-octyl-biphenyl-4-yl)-r-cyclo-hexan- carbonitrile (NCB84) is studied; this has additionally a smectic G phase. From these data the thermal expansion coefficients are calculated. Comparing PYP 907 and PYP 709, differing in their exchanged alkyl chains, we observe in the smectic A and the smectic C phase a distinctly lower density for PYP 709 whereas their densities nearly agree in the isotropic phase. The pyrimidines PYP 709 and PYP 808 exhibit a continuous volume change on crossing the smectic A-smectic C transition which differs dramatically from PYP 909 which shows a small volume jump. Furthermore a binary mixture of PYP 708 and PYP 706 is analysed which shows only a nematic and a smectic C phase. The associated phase transition is probably first order revealing nearly no pretransitional behaviour. The smectic A-smectic C transition of NCB84 seems to be second order exhibiting a continuous change of volume across the transition whereas the smectic C-smectic G transition shows a volume discontinuity and is first order. In order to induce ferroelectric smectic C* phases all smectic C materials were doped with a chiral pyrimidine dopant. Astonishingly the thermal expansion coefficient across the smectic A-smectic C* transition is influenced by the dopant in a very different way.     

Atomistic simulations of liquid crystal mixtures of alkoxy substituted phenylpyrimidines 2PhP and PhP14

We study liquid crystal mixtures of alkoxy substituted phenylpyrimidines 2-[4-(butyloxy)phenyl]-5-(octyloxy)pyrimidine (2PhP) and 2-[4-(tetradecyloxy)phenyl]-5-(tetradecyloxy)pyrimidine (PhP14) using molecular dynamics simulations at the all atom level. The molecular length of PhP14 is 1.8 times that of 2PhP, resulting in an interesting binary mixture phase diagram. Our simulations are composed of 1000-1600 molecules for a total of 80,000-130,000 atomic sites, with total simulation times of 60-100 ns. We first show that a pure 2PhP system self-assembles into isotropic, nematic, smectic A and smectic C phases, and a pure PhP14 system self-assembles into isotropic and smectic C phases. Binary mixtures of PhP14 and 2PhP display a stabilization of the smectic A phase at the expense of the smectic C and nematic phases. We determine that the concentration-induced phase transition from the smectic C to the smectic A phase in the mixture is driven by an out-of-layer fluctuation arrangement of the molecules. We also observe that the tilt angle in the smectic C phases formed in the mixtures is concentration dependent. The results of our simulations are in good agreement with the experimental findings of Kapernaum et al. [J. Org. Chem. 5, 65 (2009)], thus showing that atomistic simulations are capable of reproducing the phase behavior of liquid crystal mixtures and can also provide microscopic details regarding the mechanisms that govern phase stability.     

On the structure of binary mixtures of non-symmetric liquid crystal dimers: the alpha-(4-cyanobiphenyl-4-yloxy)-omega(4-alkylanilinebenzylidene-4-oxy) alkanes

Liquid crystal dimers, in which two mesogenic groups are linked by a flexible spacer, exhibit a rich smectic polymorphism for both symmetric and non-symmetric dimers which differ in the nature of the mesogenic groups. For example, smectic phases having monolayer, interdigitated and intercalated structures have been discovered. We have extended our studies of such systems to binary mixtures in an attempt to understand the origin of the different phase structures at the molecular level. The dimers studied include non-symmetric systems differing in the parity of the spacer and in the length of the terminal chains; for comparison we have also studied a mixture of symmetric dimers differing solely in the parity of the spacer. We have constructed the phase diagrams for the various mixtures and found that for certain systems the smectic phases exhibited by either one or both components can be destroyed. To investigate the local structure of the nematic phase for mixtures in which a smectic A phase is eliminated from the phase diagram we have determined their orientational order using NMR and ESR spectroscopy. To provide more direct information on the local structure an X-ray diffraction study was undertaken on certain of the mixtures.     

Creation of a nematic phase in mixtures of smectic A1 phases

Phase diagrams were determined for binary mixtures consisting of two 5-n-alkyl-2-(4'-isothiocyanatophenyl)-1,3-dioxane compounds (n-DBT) or 4'-isothio-cyanatophenyl 4-(trans-4'-n-decylcyclohexyl)benzoate and n-DBT. All compounds investigated have monolayer smectic A phases. A nematic phase in the upper temperature range and a nematic gap between two smectic regions also were observed, with the smectic layer spacing ratio, d/d', of 1.23 and 1.87 respectively. The variation of the enthalpy of transition with mixture composition in relation to changes of layer spacing ratio are also discussed for these systems.     

Synthesis and studies of some 4-substituted phenyl-4′-azopyridine-containing hydrogen-bonded supramolecular mesogens

Novel azopyridine-containing supramolecular liquid crystalline (LC) materials built via 1 : 1-heterointermolecular hydrogen bonding between some 4-substituted phenyl-4′-azopyridines and 4-n-alkyloxybenzoic acids are reported. These hydrogen-bonded LC complexes exhibit well defined nematic, smectic A and smectic C phases over wide ranges of temperature depending upon the number of carbon atoms present in the alkyl chains. The formation of pure LC materials on 1 : 1-complexation could be confirmed from the phase diagrams of the binary mixtures, which clearly indicated a melting maximum for the 50 mol% mixture as well as the presence of two eutectic points on either side of this mixture.     

Effect of properties of the smectic Ad phase on the induction of a nematic phase in binary mixtures of smectics A1 and Ad

The phase diagrams of two series of binary mixtures composed of 5-trans-n-butyl-2-(4-isothiocyanatophenyl)-1,3-dioxane (smectic A₁) and 4-(trans-4'-n-alkyl-cyclohexyl)-1-(2-cyano-ethenyl)benzene or 4-cyanophenyl-4'-n-alkoxybenzoates (both smectic A_d) are determined. It is shown that the smectic layer spacing ratio and smectic phase transition enthalpy are the factors influencing the width and the position of the nematic gap created between the two smectic regions. It is found that their influence on this ability is in the opposite direction.     

Induced smectic phases in phase diagrams of binary nematic liquid crystal mixtures

To elucidate induced smectic A and smectic B phases in binary nematic liquid crystal mixtures, a generalized thermodynamic model has been developed in the framework of a combined Flory-Huggins free energy for isotropic mixing, Maier-Saupe free energy for orientational ordering, McMillan free energy for smectic ordering, Chandrasekhar-Clark free energy for hexagonal ordering, and phase field free energy for crystal solidification. Although nematic constituents have no smectic phase, the complexation between these constituent liquid crystal molecules in their mixture resulted in a more stable ordered phase such as smectic A or B phases. Various phase transitions of crystal-smectic, smectic-nematic, and nematic-isotropic phases have been determined by minimizing the above combined free energies with respect to each order parameter of these mesophases. By changing the strengths of anisotropic interaction and hexagonal interaction parameters, the present model captures the induced smectic A or smectic B phases of the binary nematic mixtures. Of particular importance is the fact that the calculated phase diagrams show remarkable agreement with the experimental phase diagrams of binary nematic liquid crystal mixtures involving induced smectic A or induced smectic B phase.     

Asymmetric dimeric liquid crystals The preparation and properties of the α-(4-cyanobiphenyl-4'-oxy)-ω-(4-n-alkylanilinebenzylidene-4'-oxy)hexanes

Abstract

The first eleven members of the homologous series of α-(4-cyanobiphenyl-4'-oxy)-ω-(4-n-alkylanilinebenzylidene-4'-oxy)hexanes have been synthesized. The compounds are all enantiotropic nematogens and, with the exception of the heptyl, octyl and nonyl homologues, exhibit smectic phases. The thermal stability of the smectic A phase initially increases with the length of the terminal alkyl chain, passes through a maximum and then falls dramatically before disappearing. The smectic A phase subsequently reappears with the decyl homologue which has the highest smectic A-nematic transition temperature of the series. In order to understand this unusual behavior we have determined the entropies of transition for the compounds and we have measured the layer spacing of the smectic A phase for three of them.     

LC polyimides. XXXVI. Poly(ester imide)s derived from N,N′‐Bis(4‐hydroxyphenyl)biphenyl‐3,3′,4,4′‐tetracarboxylic diimide and aliphatic dicarboxylic acids

A new mesogenic monomer was prepared from biphenyl‐3,3′,4,4′‐tetracarboxylic dianhydride and 4‐aminophenol followed by the acylation of OH groups with propionic anhydride. This diphenol propionate was polycondensed by transesterification with decane‐1,10‐dicarboxylic acid, dodecane‐1,12‐dicarboxylic acid, and eicosane‐1,20‐dicarboxylic acid or with equimolar mixtures of two dicarboxylic acids. The resulting poly(ester imide)s were characterized by elemental analyses, ¹H NMR spectra, inherent viscosities, DSC measurements, optical microscopy, and X‐ray measurements with synchrotron radiation at variable temperatures. An enantiotropic smectic A phase in the molten state and a crystalline smectic E (or H) phase in the solid state were found in all cases. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 3019–3027, 2000     

Chirality-induced phase transitions in some liquid crystalline binary mixtures

Novel binary mixtures have been prepared between an optically active antiferroelectric liquid crystal, (S)-4-(1-methylheptyloxycarbonyl)phenyl 4'-octyloxy-biphenyl-4-carboxylate, and an optically active twin liquid crystal, (R)-3-methyladipic acid bis[4-(5-octyl-2-(pyrimidinyl)phenyl] ester, and the liquid crystalline properties investigated. The stability of each liquid crystal phase was found to decrease by mixing these two liquid crystalline materials. Furthermore, a phase diagram between these compounds showed a clear discontinuity in phase sequences. These results indicate that the liquid crystal phases are different in nature between these materials. The mixture consisting of the antiferroelectric material (40 per cent) and the twin material (60 per cent) shows an unusual liquid crystal phase, where the texture is similar to that reported for the twist grain boundary (TGB) phase. Related binary mixtures have been prepared between optically active or racemic materials, where the chirality of the system is expected to be altered systematically. The TGB phase was found to be induced only in the mixture between the optically active materials. Two kinds of effect on the appearance of the TGB phase, i.e. a strong helical structure induced by the optically active twin liquid crystal and a decrease of the smectic layer strength achieved by mixing between two types of liquid crystalline materials, are discussed.     

The substituent effect on mesomorphic properties of 4-octyloxyphenyl 4-(4-R-3-nitrobenzoyloxy)benzoates and 4-(4-octyloxybenzoyloxy)phenyl 4-R-3-nitrobenzoates

The thermal properties of 4-octyloxyphenyl 4-(4-R-3-nitrobenzoyloxy) benzo-ates (1) and 4-(4-octyloxybenzoyloxy)phenyl 4-R-3-nitrobenzoates (2) have been examined, where R = hydrogen, halogens, alkyl and alkoxy groups. The derivatives of compound 1 incorporating hydrogen, halogens, methoxy and nitro groups show a smectic A phase having a bilayer arrangement, and the others with a long alkoxy group show the S_A phase with the monolayer arrangement. The derivatives of compound 2 incorporating halogens, and the nitro group show the S_A phase with the monolayer arrangement. The alkoxy derivatives show a smectic C phase as well as the nematic phase. The nitro group at the lateral position tends to increase the ratio of the S_A-N transition temperature to the N-I. The effect of the nitro group on the smectic properties has been discussed in terms of the structural and electrostatic nature of the nitro group.     

Synthesis and mesomorphic properties of 4-(1,1,7-trihydroperfluoroheptyloxycarbonyl)phenyl 4'-n-alkoxybiphenyl-4-carboxylate

A family of 4-(1,1,7-trihydroperfluoroheptyloxycarbonyl)phenyl 4'-n-alkoxybiphenyl-4-car- boxylates has been synthesized. The compounds show the smectic A phase while the mesogens with intermediate terminal alkyl chain ( n 7-12) also exhibit a broad smectic C phase.     

The substituent effect on mesomorphic properties of 4-octyloxyphenyl 4-(4-R-3-nitrobenzoyloxy)benzoates and 4-(4-octyloxybenzoyloxy)phenyl 4-R-3-nitrobenzoates

Abstract

The thermal properties of 4-octyloxyphenyl 4-(4-R-3-nitrobenzoyloxy) benzo-ates (1) and 4-(4-octyloxybenzoyloxy)phenyl 4-R-3-nitrobenzoates (2) have been examined, where R = hydrogen, halogens, alkyl and alkoxy groups. The derivatives of compound 1 incorporating hydrogen, halogens, methoxy and nitro groups show a smectic A phase having a bilayer arrangement, and the others with a long alkoxy group show the S_A phase with the monolayer arrangement. The derivatives of compound 2 incorporating halogens, and the nitro group show the S_A phase with the monolayer arrangement. The alkoxy derivatives show a smectic C phase as well as the nematic phase. The nitro group at the lateral position tends to increase the ratio of the S_A-N transition temperature to the N-I. The effect of the nitro group on the smectic properties has been discussed in terms of the structural and electrostatic nature of the nitro group.     

Vapor-liquid equilibrium properties for confined binary mixtures involving CO2, CH4, and N2 from Gibbs ensemble Monte Carlo simulations

The effects of solid-fluid interactions on the vapor-liquid phase diagram,coexistence density,relative volatility and vaporization enthalpy have been investigated for confined binary systems of CO 2-CH 4,CO 2-N 2 and CH 4-N 2.The Gibbs ensemble Monte Carlo(GEMC) simulation results indicate that the confinement and the solid-fluid interaction have significant influences on the vapor-liquid equilibrium properties.The confinement and the strength of the solid-fluid interaction make the p-x i phase diagram move to higher pressure regions.They also make the two-phase region become narrower for each binary mixture.The strength of the solid-fluid interactions can cause increases in the coexistence liquid and vapor densities,and cause the decrease of the relative volatility and the vaporization enthalpy for the systems studied.As the pore width is decreased,the two-phase region of the binary mixture becomes narrower.