Kojic acid—a new fragment for the preparation of bent‐core mesogens

Kojic acid was used to synthesize bent‐core mesogens. Antiferroelectric polar smectic mesophases, an oblique and a rectangular columnar mesophase were found. The mesophases were characterized by X‐ray scattering, electro‐optical measurements, polarizing microscopy and DSC. On the base of these measurements, models of the mesophases are proposed.     

Non‐symmetric bent‐core mesogens with one terminal vinyl group

Two series of non‐symmetric banana‐shaped compounds, both with one alkyl and one alkenyl terminal tail, have been synthesized and studied. Both series were compared with the corresponding series with two saturated terminal alkyl tails. All the compounds have a bent central 1,3‐phenylene bis(4‐benzoyloxy)benzoate core; their mesophases were characterized by polarizing optical microscopy, differential scanning calorimetry, X‐ray diffraction and switching current response experiments. In all four series one of the terminal tails is varied from OC₈H₁₇ to OC₁₆H₃₃. The other terminal tails are OC₁₁H₂₃, O(CH₂)₉CH?=?CH₂, OC₁₀H₂₁ and O(CH₂)₈CH?=?CH₂. The short‐tailed compounds show monotropic or enantiotropic B₁ phases and the long‐tailed compounds the B₂ phase. The introduction of one terminal vinyl group slightly lowers the transition temperatures. The introduction of a second terminal vinyl group further suppresses the liquid crystalline properties. All compounds with B₂ phases have layer spacings that suggest a tilt of ～45° of the bent molecules in the layers, and their switching behaviour is antiferroelectric.     

Field-induced uniaxial and biaxial nematic phases in the Maier–Saupe–Zwanzig (MSZ) lattice model

We report detailed statistical mechanics calculations to analyse the field-temperature phase diagram of a fully connected Maier–Saupe–Zwanzig (MSZ) lattice model for the nematic transitions in liquid-crystalline systems. If the field is applied along the uniaxial direction, with positive anisotropy, there is a first-order boundary between distinct uniaxial paranematic phases, which ends at a simple critical point. For negative values of the anisotropy, the first-order boundary becomes continuous at a tricritical point, and the transition is between field-induced uniaxial and biaxial structures. This elementary MSZ lattice model provides a way to systematically regain all of the main features of earlier phenomenological and model calculations. Due to the simplicity of the calculations, critical and tricritical points, as well as critical lines, can be located analytically, which leads to a possibility of comparisons with values for real experimental systems. Also, we formulate a generalised six-state MSZ model, which takes into account intrinsic molecular biaxiality, and perform calculations to determine the location of the critical point in the field-temperature phase diagram.     

Model‐independent structure and resonant X‐ray spectra of intermediate smectic phases

It is shown that the orientational structure of intermediate smectic phases can be determined using the symmetry properties of the general free energy with arbitrary orientational coupling between smectic layers, without addressing a particular model. The structure of three‐ and four‐layer intermediate phases, obtained in this way, corresponds to experimental data. The same method enables one to predict the structure of intermediate phases with periodicity of five and six layers, which have not been observed experimentally so far. The resonant X‐ray spectra of the five‐ and six‐layer intermediate phases with predicted structure have also been calculated. These spectra are characterized by a number of features which enable one to distinguish five‐layer and six‐layer intermediate phases from phases with smaller periods.     

Biaxial phase and coexistence of the two uniaxial nematic phases in the system sodium dodecyl sulphate–decanol–D2O

The system sodium dodecyl sulphate (SDS)/decanol (DeOH)/water presents, with temperature increase, an unusual phase transition between a stable biaxial phase and a coexistence region of the two uniaxial nematic phases (discotic N_D + cylindrical N_C). This has been detected previously by several methods for a sample with water/SDS molar ratio M_w = 36. Here, this system is investigated changing the ratio M_w to 32, where previously the coexistence region was reported after a discotic N_D phase, without the biaxial phase. We report now the existence of a biaxial N_B in the temperature range of ~2°C, defining for both values of M_w and the phase sequence as N_B – (N_D + N_C) – N_C. The change in temperature is followed by conoscopic and orthoscopic optical techniques and also analysed through the curve obtained by the technique of digital image processing of the textures, which reveals a continuous transition N_D – N_B – (N_D + N_C). While the biaxial phase is stable for at least 10 hours, in a reproducible way, the coexistence region evolves with time, and drops of the discotic phase grow immersed in a N_C matrix. Results are explained in terms of recent theories dealing with stabilisation of mixtures of cylinders and discs.     

The SmCPA phase in five‐ring bent‐core compounds derived from 5‐methoxyisophthalic acid

The synthesis and mesophase characterization of a homologous series of five‐ring bent‐core compounds derived from 5‐methoxyisophthalic acid are described. Most of the compounds exhibit a polar antiferroelectric smectic C phase. Replacement of the terminal n‐alkoxy chains by n‐alkyl carboxylate groups, not only destabilizes the formation of mesophases but induces a calamitic mesophase. However, extension of the arms of the bent‐core molecule by a phenyl moiety stabilizes the switchable phase. The mesophases were investigated using a combination of polarizing optical microscopy, differential scanning calorimetry, X‐ray diffraction and electro‐optical methods.     

X‐ray diffraction,differential scanning calorimetry and evolved gas analysis of aged plutonium tetrafluoride (PuF4)

Journal of Radioanalytical and Nuclear Chemistry - A 30 year-old PuF4 sample consisting of brown powder (PuF4-b) and pink granules (PuF4-p) was analyzed. X-ray diffraction shows the bulk...     

X-ray investigation of the nematic and reentrant nematic phases of N-[(4-n-octyloxybenzoyloxy)-salicilidene]-4′-cyanoaniline

We have performed an X-ray scattering study of the nematic-smectic A and reentrant nematic-smectic A phase transitions in N-[(4-n-octyloxybenzoyloxy)-salicilidene]-4′-cyanoaniline (OOBOSCA). A diffractometer with a linear position sensitive detector was used. The results show that the smectic phase in OOBOSCA is of the A_d type with an interlayer spacing incommensurate with the molecular length L; d ∽ 1·2L. In the reentrant nematic phase two types of fluctuation modes were found. One of them corresponds to the monolayer wavevector q ₁ ∽ 2π/L, and the other is due to the partial bilayer fluctuations with the wavevector q ₂ ∽ 0·8q ₁. The temperature dependences of the interlayer spacing, X-ray scattering intensity and longitudinal correlation length for both types of layering in the reentrant nematic phase are presented. The change of the fluctuation regime from S_Ad, to S_Cd type with decreasing temperature in the reentrant nematic phase of OOBOSCA was found. The results are discussed on the basis of models with competing order parameters. The influence of alkyl chain flexibility on the stability of a partial bilayer smectic phase is also considered.     

Banana‐shaped mesogens: mesomorphic properties of seven‐ring esters derived from 5‐chlororesorcinol

Two new homologous series of seven‐ring bent‐core compounds derived from 5‐chlororesorcinol have been synthesized. Many of the lower homologues in both series exhibit the rectangular columnar B₁ phase. However, the three analogous compounds having long terminal alkyl chains exhibit a switchable lamellar phase, which is different from the usual B₂ phase. Although the ground state structure of the mesophase is antiferroelectric, it shows chiral conglomerates with opposite tilt and polarity. Simultaneously, racemic structures are also seen. The mesophases have been characterized using a combination of polarized light microscopy, X‐ray diffraction and electro‐optical studies.     

Sulfamic Acid as an Effective Catalyst in Solvent‐Free Synthesis of β‐Enaminoketone Derivatives and X‐ray Crystallography of Their Representatives

Two types of β‐enaminoketone derivatives of 3‐(2‐oxo‐2‐arylethylidene)‐3,4‐dihydro‐1H‐quinoxalin‐2‐ones and 3‐(2‐oxo‐2‐arylethylidene)‐3,4‐dihydro‐benzo[1,4]oxazin‐2‐ ones were effectively and conveniently prepared in good to excellent yields under solvent‐free conditions via the catalysis of sulfamic acid in the corresponding condensations of o‐phenylenediamine and o‐aminophenol with ethyl 2,4‐dioxo‐4‐arylbutyrate respectively. The compounds were confirmed by IR, ¹H NMR, and ¹³C NMR, and a representative was further determined by X‐ray crystallography.     

Synthesis of N‐Benzylated Anilines from the Reaction of Anilines and Benzyl Chloroformate

Abstract

Reactions of benzyl chloroformate with a series of substituted anilines produced N‐carbobenzyloxy “CBZ” products along with the unexpected N‐benzylated “Bn” compounds. Reaction of aniline, 1a, gave the CBZ, or 2a, and Bn, or 3a, products in 29% and 14% yield, respectively. For 2‐nitro‐, 2‐bromo‐, and 2‐bromo‐5‐nitroanilines, the N‐benzylated compounds were produced exclusively. However, 2‐methoxy‐, 4‐bromo, 4‐iodo, and 4‐ethylanilines gave mainly CBZ products. Other compounds reported in this study gave mixtures of the two products. For 4‐chloro‐3‐nitroaniline, in addition to the Bn and CBZ products (53% and 14% yield, respectively), a N,N‐dibenzylated product was isolated in 27% yield. Collectively, the results indicated that electron‐withdrawing groups, particularly at the ortho position, directed the formation of Bn compounds, whilst electron‐donating groups, especially at the ortho and para positions, favored the synthesis of CBZ products.     

Detection of Intermediate during Electrochemical Polymerization of Azure A and X‐ray Photoelectron Spectroscopy of Poly(azure A)

The electrochemical polymerization of azure A has been investigated by the rotating ring‐disk electrode (RRDE) and the X‐ray photoelectron spectroscopy (XPS). The electrolysis of azure A was studied in the pH region of 1.0 and 12.0. The results from the RRDE experiments show that the electrochemical polymerization of azure A was performed by two steps, i.e., oxidation of azure A and followed by polymerization. The intermediates generated at the disk were detected at the ring electrode in the ring potential region of ?0.2–0.20 V (vs. Ag/AgCl reference electrode). The collection efficiencies of the intermediates increase with decreasing the ring potential as well as increasing the rotation rate of the RRDE. Thus, the intermediate is to carry a positive charge species and not too stable. The results from the XPS experiments show that the anions can be doped into poly(azure A) film and be dedoped from it during oxidation and reduction processes.     

Novel mesogenic asymmetric diacylhydrazines: synthesis,mesomorphic behaviour and X‐ray study

The synthesis and liquid crystalline properties of a new series of diacylhydrazine derivatives (2a–g) are reported. All compounds of series 2 exhibit a smectic C (SmC) phase. The first homologues (2a, 2b) display a monotropic SmC mesophase, whereas the highest homologues (2c–g) exhibit an enantiotropic SmC phase. The liquid crystalline properties were investigated by differential scanning calorimetry, polarizing optical microscopy and X‐ray measurements.     

The Determination of Trace Elements Release from Dental Cements in Artificial Saliva by Energy Dispersive X‐ray Fluorescence Spectrometry

Abstract

The application of energy dispersive X‐ray fluorescent analysis (EDXRF) for the determination of elements released from five different dental luting cements such as Zinc Polycarboxylate (Carbchem), Zinc carboxylate (Adhesor Carbofine), Glass ionomer (Meron), Resin cement (Duo‐cement kit), and Carboxylate (Durelon) in artificial saliva is described. The equipment used for this study is a Si(Li) detector, a multichanel analyser, an amplifier and ⁵⁵Fe and ²⁴¹Am radioisotope sources. The physical basis of the analytical method used the procedure of sample preparation and results are presented. The detected elements were Cl, P, K, Ca. The results show high concentrations of Ca being released from dental cements in artificial saliva. Chemical disintegration of dental cements can adversely affect their long‐term performance. Fixed prosthodontic restorations cemented with carboxylate cement (Durelon) may be capable of withstanding long‐term clinical use compared to other agents. This material showed the highest resistance to dissolution or disintegration.     

Non‐monotonic temperature dependence of the alignment of a nematic mixture on a ferroelectric substrate

Polarization optic techniques have been applied to study specific features of the anisotropic interaction between a dye‐doped eutectic mixture of nematic liquid crystals p‐methoxybenzylidene‐p‐n‐butylaniline and p‐ethoxybenzylidene‐p‐n‐butylaniline and a polar surface of a ferroelectric triglycine sulphate crystal over the temperature range including the substrate Curie point T _c. It has been found that the temperature‐induced structural changes in the nematic layer in the vicinity of T _c are related to the changes in the orientational part of the tensor order parameter Q_ik . The temperature dependence of the director angle θ¯, averaged over the nematic layer, has been obtained from the effective dichroism values of solute absorption. The experimental data were interpreted using the model, in which the anisotropic part of the surface energy has two terms with orthogonal easy axes.     

Synthesis and mesomorphic behaviour of wedge‐shaped nematic liquid crystals with flexoelectric properties

The synthesis and mesomorphic behaviour of a series of wedge‐shaped liquid crystals and some reference compounds are reported. These unusual liquid crystals possess smectic C, smectic A and nematic phases. These new wedge‐shaped materials with a high degree of shape anisotropy and a large dipole moment can be used to induce an increase in the flexoelectric effects of nematic guest–host mixtures as dopants at low concentrations.     

On the balance between nematic and smectic phases in 2′,3′-difluoro-4,4″-dialkyl-p-terphenyls

ABSTRACT

The full homologous series of 2?,3?-difluoro-4,4″-dialkyl-p-terphenyls, where alkyl chains are from methyl to nonyl length, have been synthesised and mesomorphic properties, phase transition temperatures and enthalpies were measured. The influence of the terminal chain length on thermal mesophase range, phase sequence has been analysed and discussed. The odd–even and symmetric–non-symmetric effects have also been studied and its influence on smectic to nematic balance was analysed.     

Superoxide‐Mediated Synthesis of N‐Aminoaziridines from N‐Aminoheterocycles and Olefins

Oxidation of 3‐amino‐2‐methyl‐4(3H)‐quinazolone and N‐aminophthalimide by superoxide ion generated in situ in the presence of olefinic substrates gives rise to the formation of N‐aminoaziridine derivatives.     

MM3 Conformational Analysis and X‐ray Crystal Structure of 2,3,4,5‐Tetra‐O‐Acetyl‐N,N′‐Dimethyl‐D‐Glucaramide as a Conformational Model for the d‐Glucaryl Unit of Poly(Alkylene 2,3,4,5‐Tetra‐O‐Acetyl‐d‐Glucaramides)

This report describes the MM3 conformational analysis and X‐ray crystal structure of tetra‐O‐acetyl‐N,N′‐dimethyl‐d‐glucaramide as a conformational model for the D‐glucaryl monomer unit of poly(alkylene tetra‐O‐acyl‐d‐glucaramides). The driving force for this study was to determine the conformational preferences for the diacid unit as a function of the increasing steric bulk of pendant O‐acyl groups: acetyl, propanoyl, 2‐methylpropanoyl, and 2,2‐dimethylpropanoyl. The model dialkyl d‐glucaramides all displayed a large vicinal proton coupling between the central backbone glucaryl hydrogens, indicating an essentially fixed anti conformational arrangement of these protons. The MM3 molecular mechanics program was then applied to calculate the corresponding low‐energy conformations of the structurally simplest of these molecules, tetra‐O‐acetyl‐N,N′‐dimethyl‐d‐glucaramide (4). Given the large number of dihedral angles to be considered and the apparent rigidity of these molecules around the central carbons of the glucaryl backbone, a number of conformational approximations based upon model compounds were applied regarding the rotameric disposition of the pendant O‐acetyl and terminal N‐methyl groups. The calculated, and dominant, lowest energy conformer has a sickle structure very similar to the global minimum conformation previously calculated for unprotected d‐glucaramide. The x‐ray crystal structure data from 4 indicated an extended conformation in the solid state and gave solid‐state torsion angle information that was comparable to that obtained computationally.