Polarity and steric effect of the lateral substituent on the mesophase behaviour of some newly prepared liquid crystals

Eight homologous series of 2- (or 3-) substituted phenyl 4?-(4?-alkoxy phenylazo) benzoates (In_a–h) were prepared in which, within each homologous series, the length of the terminal alkoxy group varies between 6, 8, 10 and 12 carbons, while the other substituent, X, is a laterally attached polar group that alternatively changed from CH₃, H, F, Br and CN. Compounds prepared were characterised by infrared and ¹H-NMR spectroscopy, and their mesophase behaviour investigated by differential scanning calorimetry and identified by polarised light microscopy. The results were discussed in terms of polarity and steric effects. The stability of the mesophase was correlated once with the dipolar anisotropy of the whole molecule and another with the dipolar anisotropy of the substituent, X. A comparative study was made between the investigated compounds and their previously prepared linear 4-substituted isomers, namely 4-substituted phenyl 4?-(4?-alkoxy phenylazo) benzoates (In_i–k).     

Hydrogen Bonded Supramolecular Liquid Crystalline Complex of 2,4,6-Triarylamino-1,3,5-triazines with Semiperfluorinated Benzoic Acids

Hydrogen bonded supramolecular complexes of 2,4,6‐triarylamino‐1,3,5‐triazines ( Tn ) with semiperfluorinated benzoic acids have been prepared. The mesophase behaviors of such complexes were investigated with POM (polarized optical microscopy), DSC (differential scanning calorimetry) and X‐ray diffraction. Columnar mesophase was found in the equimolar mixtures of the triazines with the semiperfluorinated benzoic acids. While the equimolar mixture of triazines with the nonfluorinated three fold alkoxy‐modified carboxylic acid was not mesogen. The mesophase formation is analyzed as the hydrogen bonding between the Tn and benzoic acid which leads to discrete dimeric supermolecule and enhances the polarity of the core region of the dimeric supermolecule; simultaneously, an increased intramolecular polarity contrast upon replacing alkyl chains by semiperfluorinated chains, which favors a microsegregation, and leads to the fromation of the columnar phase.     

Preparation and properties of laterally multifluorinated benzoxazole-based nematic mesogens

Series of laterally multifluorinated heterocyclic compounds, 2-(2?,3-difluoro-4?-alkoxy-1,1?-biphenyl-4-yl)-benzoxazole derivatives (nPF(2)PF(3)Bx), are prepared and characterised. They mainly display enantiotropic nematic mesophases with wider mesophase ranges of 12–107°C (heating process) and 22–134°C (cooling process) than the corresponding analogues. The enhanced nematic mesophase stability is achieved via slightly increasing inter-ring twist angle with inter-ring lateral fluorine substitute in biphenyl unit, as well as through improving the molecular polarity with multifluorine substitutes. Meanwhile, two inter-ring lateral fluorine atoms lead to a decrease in melting/clearing points and a wide nematic mesophase range, which makes it possible for heterocyclic mesogens nPF(2)PF(3)Bx to use in nematic liquid crystal display mixtures.     

Dielectric Dispersion and Electric Relaxation Processes Induced by Ionic Conduction in Formamide, 2-Aminoethanol and Their Binary Mixtures

The complex dielectric permittivity, ionic conductivity, electric modulus and impedance spectra of the dipolar molecules formamide (FA), 2-aminoethanol (AE) and their binary mixtures were investigated in the frequency range from 20 Hz to 1 MHz at 303.15 K. Debye-type distributions of the frequency dependent electric modulus and complex impedance were found, corresponding to an ionic conduction relaxation process in the upper frequency regime of the spectra, whereas a spike in the impedance spectra at low frequencies confirms the contribution of an electrode polarization (EP) relaxation process induced by ionic conduction. Due to the high static permittivity of FA, its ionic conductivity was found more than one order of magnitude higher than that of the AE, which is also shown by the comparative values of their EP and ionic conductivity relaxation times. The dependences of dc ionic conductivity values of the binary mixtures on their relaxation times and static permittivity were explored. The concentration dependent static permittivity and the relaxation times led us to infer the formation of a 1:1 H-bonded stable complex between FA and AE molecules with reduction in the number of effective parallel-aligned dipoles.     

Investigations on liquid crystalline partially fluorinated alkyl and succinimidyl benzoates

The synthesis and mesophase properties of partially fluorinated alkoxy‐substituted benzoic alkyl and succinimidyl (NHS) esters with one, two and three perfluoroalkyl alkoxy chains are reported. The mesophases were studied using differential scanning calorimetry (DSC), polarizing optical microscopy and X‐ray diffraction of non‐oriented samples. The SmA phases of the one‐chain methyl esters are monotropic, while those of the one‐chain NHS esters are enantiotropic. The more wedge‐shaped two‐ and three‐chain alkyl esters do not form mesophases, whereas the succinimidyl analogues exhibit hexagonal columnar phases. Their enhanced mesophase stability is caused by the higher polarity of the succinimidyl ring with respect to the alkyl ester groups. Aggregation of the dipolar succinimidyl groups, together with the microsegregation of the lipophilic and fluorophilic segments of the partially fluorinated alkoxy chains, is assumed to lead to a threefold structured morphology in both the SmA and the Col_h phases. This threefold structuring can be regarded as analogous to known morphologies of ABC triblock copolymers.     

Static permittivity and molecular interactions in binary mixtures of ethanolamine with alcohols and amides

The relative static permittivity at 1 MHz and high frequency limit permittivity at wavelength of sodium-D line of the binary mixtures of ethanolamine (2-aminoethanol) with alcohols (ethyl alcohol, ethylene glycol and glycerol) and amides (formamide, N,N-dimethylformamide and N,N-dimethylacetamide) have been investigated over the entire concentration range at 30 °C. The excess permittivity and Kirkwood correlation factor of the binary mixtures were determined to explore the hydrogen-bonded hetero-molecular interactions and their dependence on the number of hydroxyl groups of alcohols molecules and the extent of substitution in amides molecules. Results confirm that ethanolamine form weak H-bond interactions with alcohols, N,N-dimethylformamide and N,N-dimethylacetamide, but the dipolar alignments in these mixtures vary with number of hydroxyl group of alcohols and their molecular size. Comparatively strong H-bond interactions were found between ethanolamine and formamide molecules with reduce in number of parallel aligned effective dipoles.     

Investigations on liquid crystalline partially fluorinated alkyl and succinimidyl benzoates

The synthesis and mesophase properties of partially fluorinated alkoxy-substituted benzoic alkyl and succinimidyl (NHS) esters with one, two and three perfluoroalkyl alkoxy chains are reported. The mesophases were studied using differential scanning calorimetry (DSC), polarizing optical microscopy and X-ray diffraction of non-oriented samples. The SmA phases of the one-chain methyl esters are monotropic, while those of the one-chain NHS esters are enantiotropic. The more wedge-shaped two- and three-chain alkyl esters do not form mesophases, whereas the succinimidyl analogues exhibit hexagonal columnar phases. Their enhanced mesophase stability is caused by the higher polarity of the succinimidyl ring with respect to the alkyl ester groups. Aggregation of the dipolar succinimidyl groups, together with the microsegregation of the lipophilic and fluorophilic segments of the partially fluorinated alkoxy chains, is assumed to lead to a threefold structured morphology in both the SmA and the Col_h phases. This threefold structuring can be regarded as analogous to known morphologies of ABC triblock copolymers.     

Smectic, columnar and cubic mesophases in binary systems of hemiphasmidic and calamitic amphiphilic diols

A first example of an amphiphilic hemiphasmid consisting of a biphenyl rigid core connected to a hydrophilic 5,6-dihydroxy-3-oxahexyloxy group at one end and carrying two lipophilic dodecyloxy chains at the other end has been synthesized by a Pd-catalysed cross coupling reaction. The liquid crystalline properties of this compound have been investigated by polarized light microscopy, by differential scanning calorimetry and by X-ray diffraction. It exhibits a thermotropic hexagonal columnar mesophase which is stabilized on addition of formamide. On addition of ethylene glycol, a bicontinuous cubic mesophase is induced. Furthermore, binary mixtures of this compound with structurally related single chain amphiphiles have been investigated. Besides a smectic A phase, an induced columnar mesophase and a cubic phase were found in these mixtures.     

Structural and dielectric behaviour of non-aqueous lyotropic mixtures: influence of amphiphile chain lengths and counter ions

This study highlights the effects of amphiphile chain length and counter ions on the self-assembly and dielectric behaviour of non-aqueous lyotropic liquid crystals. Two-dimensional hexagonal mesophase is seen for short-chain length sodium dodecyl sulphate, while lamellar and multiwall lamellar mesophases are noticed for long-chain length cetyltrimethylammonium bromide and polyoxyethylene (20) sorbitan monolaurate amphiphiles in the non-aqueous domains of ethylene glycol. A strong influence of amphiphile counter ions is seen on static dielectric constant, loss factor, relaxation frequency and relaxation time of these lyotropic mixtures. Refractive indices of these lyotropic phases are also highlighted.     

Enhancement of the stability and temperature range of the mesophases in the binary mixtures of cholesteryl myristate and 4-n-decyloxy benzoic acid

Thermodynamical, optical, and electrical properties of the binary mixtures of cholesteryl myristate (ChM) and 4-n-decyloxybenzoic acid (DOBA) have been carried out by the differential scanning calorimeter, polarized optical microscopy (POM), and impedance spectroscopy. Through thermodynamic study, various phase transition temperatures, transition enthalpies, and transition entropies have been determined to investigate temperature range and stability of the mesophases of the mixtures. Phase diagrams in the heating and cooling cycles have been drawn for the ChM–DOBA binary system. Optical textures of different mesophases have identified with the help of POM. Dielectric permittivity has been determined in planar and homeotropic alignments of the mixtures having DOBA concentrations 30.0 and 92.3 mol %. The experimental dielectric data in the frequency range of 1 Hz to 35 MHz do not show any relaxation mode in both the alignments of these mixtures. Dielectric anisotropy has been determined for the various observed phases of the mixtures.     

Preferential solvation of Brooker's merocyanine in binary solvent mixtures composed of formamides and hydroxylic solvents

The ET polarity values of 4-[(1-methyl-4(1H)-pyridinylidene)-ethylidene]-2,5-cyclohexadien-1-one (Brooker's merocyanine) were collected in mixed-solvent systems comprising a formamide [N,N-dimethylformamide (DMF), N-methylformamide (NMF) or formamide (FA)] and a hydroxylic (water, methanol, ethanol, propan-2-ol or butan-1-ol) solvent. Binary mixtures involving DMF and the other formamides (NMF and FA) as well as NMF and FA were also studied. These data were employed in the investigation of the preferential solvation (PS) of the probe. Each solvent system was analyzed in terms of both solute-solvent and solvent-solvent interactions. These latter interactions were responsible for the synergism observed in many binary mixtures. This synergistic behaviour was observed for DMF-propan-2-ol, DMF-butan-1-ol, FA-methanol, FA-ethanol and for the mixtures of the alcohols with NMF. All data were successfully fitted to a model based on solvent-exchange equilibria, which allowed the separation of the different contributions of the solvent species in the solvation shell of the dye. The results suggest that both hydrogen bonding and solvophobic interactions contribute to the formation of the solvent complexes responsible for the observed synergistic effects in the PS of the dye.     

H-bonded molecular interaction study on binary mixtures of mono alkyl ethers of ethylene glycol with different polar solvents by concentration dependent dielectric analysis

Static dielectric constant measurements on binary mixtures of the homologous series of mono alkyl ethers of ethylene glycol with six different characteristic polar solvents i.e. ethyl alcohol, ethylene glycol, glycerol, water, dimethyl formamide and dimethyl sulphoxide over the entire concentration range were carried out using precision LCR meter and a four terminal liquid dielectric test fixture at 1?MHz and 25°C. The concentration dependent excess dielectric constant and Kirkwood correlation factor were determined for the confirmation of solvent–cosolvent heterogeneous molecular interactions. The values of stoichiometric ratio corresponding to maximum interactions between the mixtures constituents were also estimated from the concentration dependent values of ?^E. It is observed that the behaviour of heterogeneous interactions significantly varies with the increase in molecular size of the homologous series and also with the change in the mixture constituents. Comparative dielectric parameters values of the studied binary mixtures were applied to recognize the dipolar orientation due to heterogeneous interactions of mono alkyl ethers with hydroxyl group(s) containing solvents and non-hydroxyl group containing solvents.     

Effect of water on the formamide-intercalation of kaolinite

The molecular structures of low defect kaolinite completely intercalated with formamide and formamide-water mixtures have been determined using a combination of X-ray diffraction, thermoanalytical techniques, DRIFT and Raman spectroscopy. Expansion of the kaolinite to 10.09 A was observed with subtle differences whether the kaolinite was expanded with formamide or formamide-water mixtures. Thermal analysis showed that greater amounts of formamide could be intercalated into the kaolinite in the presence of water. New infrared bands were observed for the formamide intercalated kaolinites at 3648, 3630 and 3606 cm(-1). These bands are attributed to the hydroxyl stretching frequencies of the inner surface hydroxyls hydrogen bonded to formamide with water, formamide and interlamellar water. Bands were observed at similar positions in the Raman spectrum. At liquid nitrogen temperature, the 3630 cm(-1) Raman band separated into two bands at 3633 and 3625 cm(-1). DRIFT spectra showed the hydroxyl deformation mode at 905 cm(-1). Changes in the molecular structure of the formamide are observed through both the NH stretching vibrations and the amide 1 and 2 bands. Upon intercalation of kaolinite with formamide, bands are observed at 3460, 3344, 3248 and 3167 cm(-1) attributed to the NH stretching vibration of the NH involved with hydrogen bonded to the oxygens of the kaolinite siloxane surface. In the DRIFT spectra of the formamide intercalated kaolinites bands are observed at 1700 and 1671 cm(-1) and are attributed to the amide 1 and amide 2 vibrations.     

Synthesis of HCN Polymer from Thermal Decomposition of Formamide

Prolonged heating of formamide (HCONH₂) at 185°C or 220°C produces a black insoluble product. The FT-IR spectroscopy and the X-ray photoelectron spectroscopy (XPS) suggest that the product has the chemical structure of a polymer of hydrocyanic acid: (HCN)_x. The pyrolysis of (HCN)_x prepared from formamide produces a large amount of gaseous HCN in a wide range of temperatures together with ammonia (NH₃) and isocyanic acid (H─N─C═O).

During the thermal decomposition of formamide to produce (HCN)_x, the volatile products evolved were monitored with gas phase infrared spectroscopy. At 185°C, the gaseous product released were CO₂, CO and NH₃ while at 220°C, also HCN was detected. In both cases, a white sublimate was collected in the upper part of the reaction vessel. It consists of ammonium carbamate and its hydrolysis products ammonium carbonate and hydrogen carbonate. It is therefore possible to synthesize the polymer of hydrocyanic acid (HCN)_x starting from formamide avoiding to handle the dangerous hydrocyanic acid.     

Synthesis and characterization of thermotropic liquid crystalline polyurethanes from 4,4′‐bis(6‐hydroxyhexoxy) biphenyl and aliphatic diols

A series of thermotropic liquid crystalline polyurethanes (LCPUs) were synthesized by the polyaddition reactions of 2,4‐toluene diisocyanate (2,4‐TDI) with 4,4′‐bis(6‐hydroxyhexoxy)biphenyl (BHHBP) and aliphatic diol. The intrinsic viscosities of the polymers were measured by Ubbelohde viscometer, and the chemical structure was confirmed by Fourier transform infrared spectroscopy (FT‐IR). The LCPUs were examined by differential scanning calorimetry (DSC), polarized optical microscopy (POM), wide angle X‐ray diffraction (WAXD), and thermogravimetric analysis (TGA). The intrinsic viscosities were 0.56–0.83 dl/g. According to the melting point (T_m) and the isotropic temperature (T_i) of the LCPUs, the temperature range of the liquid crystalline phase became wider with increased number of methylene spacers in the polyurethane. The LCPUs exhibited a nematic phase with a threaded texture and had a wide mesophase temperature range. The decomposition temperature of the LCPUs was >300°C. On WAXD, the LCPUs give a dispersing peak at 2θ ≈ 20°, and a strong diffraction peak at 2θ ≈ 25°. Copyright © 2008 John Wiley & Sons, Ltd.     

Determination of reorganization energy of fluorenone and 4-hydroxyfluorenone in neat and binary solvent mixtures

Steady-state absorption and fluorescence measurements of fluorenone and 4-hydroxyfluorenone in neat and binary solvent mixtures were used to explore the reorganization energy in liquid system. The results of spectroscopic measurements were used to calculate, according to Marcus theory, the outer-sphere solvent reorganization energy, lambda(0), and the internal molecular reorganization energy, lambda(in). Preferential solvation of fluorenone and 4-hydroxyfluorenone in binary solvent mixtures has been studied by monitoring the outer-sphere solvent reorganization energy. In cyclohexane-tetrahydrofuran mixtures, the deviation from linearity in the lambda(0) versus the solution polarity is due to non-specific dipolar solvent-solute interactions. For cyclohexane-ethanol binary mixtures, both non-specific and specific (hydrogen bond) interactions contribute to the observed changes.     

Laser photolysis study of the hematoporphyrin IX—ℓ-tryptophan system in solvent mixtures at different polarity

The oxygen and ?-tryptophan (TRP) effect on the triplet state of hematoporphyrin IX (HP) has been investigated by means of the laser photolysis technique at room temperature, in various pH 7.4 buffered aqueous/formamide or methanol mixtures with different polarity. The quenching of the HP triplet state by oxysgen is not affected by the medium polarity (k_q O₂ = 1.5 × 10⁹ M^?1 s^?1), whereas the quenching by TRP appears dependent on the composition. The mechanism of the HP-sensitized photooxidation of TRP is discussed.     

Gas hydrate formation in the system C2H6–H2–H2O at pressures up to 250 MPa

Decomposition curves of gas hydrates formed in the ethane–hydrogen–water system were studied in the pressure interval 2–250 MPa. Gas hydrates synthesized at low (up to 5 MPa) pressures were also studied with use of X-ray powder diffraction and Raman spectroscopy. It was shown that ethane–hydrogen mixtures with hydrogen contents 0–30 mol.% form cubic structure I gas hydrates. Higher hydrogen concentration most probably results in appearance of another hydrate phase. We speculate that the gas mixtures with the hydrogen content above 60 mol.% form cubic structure II double hydrate of hydrogen and ethane at temperatures below ≈280 K and pressures above 25 MPa.     

Effect of quenching on the structural and dynamic properties of non-aqueous lyotropic mesophases

We focused to highlight the effect of quenching on the development and ordering of non-aqueous lyotropic liquid crystalline phases. Lyotropic mesophases are prepared from binary mixtures of sodium dodecyl sulphate and ethylene glycol at varying concentrations 30:70 and 50:50 wt%. The obtained self-assembled phases are characterised by X-ray diffraction, polarisation optical microscopy, differential scanning calorimetry and dielectric spectroscopy to evaluate the structural, optical, thermal and dielectric behaviours. Structural and textural measurements confirmed mesomorphic and crystalline phases for both mixtures. Calorimetric study gives insight about the growth of new phases at ≈335 K and isotropic temperatures of these mixtures. Both the mixtures are quenched from 335 K to the 303 K to analyse the effect of quenching on the structure and ordering of mesophases. We noticed well-defined hexagonal liquid crystalline mesophases for both concentrations after quenching at 303 K. Dielectric and relaxation behaviours of quenched mesophases were also examined. Higher capacitance and dielectric strength are noticed for quenched mixtures. The application prospective of such phases is also discussed.     

On the nature of solubilized water clusters in aerosol OT/alkane solutions. A study of the formation of hydrated electrons and 1,8-anilinonaphthalene sulphonate fluorescence

The pulse radiolysis of dioctyl sulphosuccinate (aerosol-OT) H₂O/heptane solutions leads to formation of hydrated electrons in the aqueous core of the inverted aerosol-OT micelles. The hydrated electrons are produced via direct interaction of the radiation with the aqueous regions and scavenging of electrons formed initially in the hydrocarbon phase by the water bubbles. The scavenging efficiency decreases with decreasing radius of the water cluster. Hydrated electrons are not formed below a critical size of the solubilized water particles.The quantum yield and wavelength of the maximum of the aniline-naphthalene sulphonate (ANS) fluorescence are strongly dependent on the water content of aerosol-OT inverted micelles. The fluorescence behavior indicates an increase of polarity with increasing cluster radius. The polarity of very large water clusters (r ≈ 73 Å) is still lower than that of bulk water. Water which is bound to Na⁺ counterions cannot effectively participate in the solvation of the dipolar ANS excited states.