Induced Smectic-G Phase through Intermolecular Hydrogen Bonding. Part VI: Thermal and phase Behavior of 2-(4-Decyloxybenzylidene Imino)-5-Chloropyridine (Dacp : Aba) Complexes

A novel series of liquid-crystalline complexes involving intermolecular hydrogen bonding between p - n -alkoxybenzoic acids (ABA) (where alkoxy = propoxy- to decyloxy- and dodecyloxy-) and 2-(4-decyloxybenzylidene imino)-5-chloropyridine (DACP) have been synthesized and their thermal and phase behaviour are studied by Thermal Microscopy (TM) and Differential Scanning Calorimetry (DSC). A detailed IR spectral investigation in solid and solution states confirms the formation of H-bonding between pyridine ring nitrogen and -COOH group of DACP and ABA respectively. Comparative thermal analyses of both free p - n -alkoxybenzoic acids and H-bonded complexes suggest the inducement of smectic-G phase in all the complexes, with a simultaneous quenching of smectic-C phase. The comparative thermal and phase behaviour with analogous series 2-(4-nonyloxybenzylidene imino)-5-chloropyridine : p - n -alkoxybenzoic acids (NACP : ABA) and 2-amino-5-chloropyridine : p - n -alkoxybenzoic acids (ACP : ABA) also imply the stabilization of the induced phase with a wide thermal span.     

Dynamics and phase behaviour of materials in confined geometry

Phase transitions and dynamics of the liquid crystal MBBA [N-( p-methoxybenzylidene)-p-n-butylaniline] microconfined within porous glasses of average pore diameters: 82 and 337 Å were investigated by ¹H Nuclear Magnetic Resonance (NMR). Confinement is found to favour the depression of phase transition temperatures below the corresponding transitions of bulk MBBA. Confinement induces further effects for the molecular dynamics related to the phase investigated. The correlation times of the molecular motions were obtained and the results are discussed by making comparison with the bulk analysis.     

A study on polymorphism of hydrogen-bonded thermotropic liquid crystals

A novel mesogenic homologous series comprising of eight hydrogen-bonded liquid crystalline complexes are isolated and analyzed by forming a hydrogen bond between p-n alkyloxy benzoic acids (where n represent alkyloxy carbon number which varies from 5 to 12) and mesaconic acid, respectively. Eight synthesized complexes are subjected to Fourier transform infra-red spectroscopy, polarizing optical microscopy and differential scanning calorimetry studies to meet the basic characterization. The variation of optical tilt angle with respect to temperature in various conventional and smectic X phases are investigated and analyzed. A phase diagram is constructed to elucidate the mesogenic behavior of novel liquid crystalline series. The Cox ratio that reveals the order of various mesophases and the stability factor, to invoke the thermal stability of mesophases, is studied under the results of DSC thermogram.     

Bent-shaped mesogenic oxadiazoles and thiadiazoles with terminal methyl group

Rod-shaped 4-methylbenzoic acid-N′-(4′-n-alkoxybenzoyl) hydrazide (series I) have been synthesized by the Schotten–Baumann reaction of 4-methylbenzhydrazide with 4-n-alkoxy benzoyl chloride using dry pyridine, as a solvent. The series I compounds have been cyclized to bent-shaped mesogenic 2-(4′-methylphenyl)-5-(4″-n-alkoxyphenyl)-1,3,4-oxadiazoles (series II) and 2-(4′-methylphenyl)-5-(4″-n-alkoxyphenyl)-1,3,4-thiadiazole (series III) using POCl₃ and Lawesson's reagent, respectively. The synthesized compounds are characterized by the combination of elemental analysis and standard spectroscopic methods. In series II, lower and middle members are non-mesogenic. n-Dodecyloxy to n-hexadecyloxy derivatives exhibit enantiotropic nematic mesophase. In series III, all the compounds synthesized exhibit enantiotropic nematic mesophase. n-Tetradecyloxy and n-hexadecyloxy derivatives also exhibit enantiotropic SmA mesophase. The mesomorphic properties of the series II and III used in this study are compared with each other and with other structurally related compound to evaluate the effect of different heterocyclic moieties as well as terminal substituents on mesomorphism.     

Statistical parameters-based image enhancement techniques in pure and nanodispersed 6O.O8 liquid crystalline compounds

Synthesis, characterization and phase-transition studies in pure and 20?µl citrate-capped Au nanodispersed liquid crystalline (LC) N-(p-n-hexyloxybenzylidene)-p-n-octyloxy aniline, 6O.O8, compounds are carried out using a polarizing microscope attached with hot stage and camera. The polarizing microscopy technique is used to measure the phase-transition temperatures. Differential scanning calorimetry is used to determine the transition temperatures and enthalpy values. By the dispersion of citrate-capped Au nanoparticles into the LC compounds, the nematic transition temperature is decreased. Furthermore, statistical parameters-based image enhancement algorithm is proposed to identify the crystalline phases in the pure and nano-doped LC compound. Empirical result analysis clearly reveals the robustness of this method over existing image enhancement methods.     

Orientational order parameter studies on 3.Om and 3O.Om liquid crystals

Orientational order parameter S is evaluated in the nematic phase of six liquid crystal compounds, N-(p-n-propyl benzylidene)-p-n-alkoxy anilines, 3.Om and N-(p-n-propyloxy benzylidene)-p-n-alkoxy anilines, 3O.Om compounds with m = 6, 7 and 8, using different methods. The techniques employed are S from birefringence δn, Haller's approximation from (1?T/T_c) ^β, effective geometry parameter α_g and Vuks’ scaling factor S_C. The values of S obtained using the above methods are compared with one another and with the results on a number of liquid crystals; the liquid crystals favor isotropic Vuks’ method.     

Phase transitions in crystals of protein amino acids

The temperature dependence of the piezoelectric response of many protein amino acids and their compounds exhibits a sharp increase in the piezoelectric response signals at a certain temperature. It has been shown using the example of the known ferroelectrics without a piezoelectric effect in the paraphase (triglycine sulfate, glycine phosphite, trisarcosine calcium chloride) that such an increase in the piezoelectric response is associated with the structural phase transition to the low-symmetry piezoelectric phase with a large number of piezoelectric coefficients. The specific features of the phase transitions with the symmetry change D ₂-C ₂ have been discussed.     

Phase diagram of the Mott transition in a two-band Hubbard model in infinite dimensions

The Mott metal-insulator transition in the two-band Hubbard model in infinite dimensions is studied by using the linearized dynamical mean-field theory recently developed by Bulla and Potthoff. The phase boundary of the metal-insulator transition is obtained analytically as a function of the on-site Coulomb interaction at the d-orbital, the charge-transfer energy between the d- and p-orbitals and the hopping integrals between p-d, d-d and p-p orbitals. The result is in good agreement with the numerical results obtained from the exact diagonalization method. Received 5 October 2000 and Received in final form 8 December 2000     

Undulation textures at the phase transitions of some alkyloxybenzoic acids

We observed undulated smectic textures for some compounds of the 4,n-alkyloxybenzoic (nOBAC) acid series, at transitions between the smectic and the isotropic phase and between the smectic and nematic phase. Studied compounds were 12OBAC, 16OBAC and binary mixtures of 12- and 16OBAC. The undulations are dressing a usual Schlieren texture. An interesting fingerprint pattern is observed at the smectic-nematic in the case of the binary mixture, approximately 1 : 1 in weight.     

Terahertz dichroism of MBBA liquid crystal on rubbed substrate

Terahertz (THz) dichroism of a nematic liquid N-(p-methoxybenzylidene)-p-butylaniline (MBBA) was measured using a GaP Raman THz spectrometer. MBBA on a rubbed plastic substrate generates a band at around 4.0 THz: its liquid crystal phase shows strong dichroism, which well corresponds to that of the IR absorption caused by π(CH) of MBBA molecule reported in the literature. Based on inferences drawn from the present THz and the published IR dichroic results, the 4.0 THz band probably stems from lateral intermolecular or intramolecular interactions of MBBA molecules aligned to the rubbing direction. The results clearly demonstrate that THz spectroscopy is powerful for discussing of phase transition and dichroism of liquid crystals.     

Phase transitions of new antiferroelectric bent-core thiobenzoate

This article reports the results on phase transitions, mesomorphic and electro-optical (EO) properties of new bis [4-(4′-alkylphenylthiocarbonyl)phenyl] isophtalates from a homologous series of banana-shaped thioesters, referred to as IFOSn, when n?=?8, synthesized by us. The IFOS8 compound has two enantiotropic phases: B2 with antiferroelectric order and B6. The mesomorphic properties were investigated by means of three complementary methods: differential scanning calorimetry, polarized light optical microscopy and transmitted light intensity. EO measurements were also carried out.     

Synthesis and characterization of wide range mesogenic esters based on asymmetrical 2,5-disubstituted 1,3,4-thiadiazole

A homologous series of new 13 esters, 4-(5-(p-tolyl)-1,3,4-thiadiazol-2-yl)-phenyl-4-alkoxybenzoate, (IV_n), based on 1,3,4-thiadiazole core has been synthesized. The structures of these esters were confirmed by Fourier transform infrared spectroscopy, proton nuclear magnetic resonance and mass techniques. Their mesophases behavior was investigated with hot-stage polarizing optical microscope and differential scanning calorimetry. The thermal stability for most of these derivatives was measured by thermal gravimetric analysis. All the target esters showed enantiotropic mesomorphic behaviors with nematic and nematic/smectic C phases. The phase transition temperatures and liquid crystalline properties were affected by the nature of heterocyclic ring and the length of the alkoxy chain. Only the nematic phase was observed in the first 10 derivatives, (n = 1–10), while the last 3, (n = 12, 16 and 18) showed nematic and smectic C phases. These compounds demonstrated high liquid crystalline ranges, both in heating and cooling cycles. The mesomorphic results obtained were compared with the reported analogs of similar constituents.     

Electric performance and photosensitivity of heterostructures prepared by thermal decomposition of a gallium nitrate aqueous solution on an indium selenide (0001) cleaved surface

It is shown that thermal decomposition of a gallium nitrate aqueous solution on the surface of a semiconductor in air can basically be used for preparing structures sensitive to radiation from in the near-UV spectral range. The electric and photovoltaic properties of Ga₂O₃-n-(p-)InSe heterostructures are studied. It is found that a high-ohmic layer arising at the InSe-Ga₂O₃ interface affects not only the electric performance but also the photoelectric spectra of the heterostructures. It is supposed that current instability with Z- and N-shaped reverse branches in the I-V characteristics of the Ga₂O₃-p-InSe structures stems from doping nonuniformity in the bulk of quasi-two-dimensional p-type indium selenide.     

Pressure–temperature phase diagrams for four higher members (nonyl–dodecyl) of the homologous series of 4′-alkyl-4-isothiocyanato-biphenyl ( n BT)

Differential thermal analysis (DTA) method was applied to study the pressure–temperature phase diagrams for four members of the 4′-alkyl-4-isothiocyanato-biphenyl (nBT) homologous series (n?=?9–12). These compounds exhibit a smectic E polymorphism enriched by a smectic A phase for two longest members. Measurements were performed in the temperature range 25–110°C and the pressure range up to 165?MPa. For all substances studied the pressure-induced phases were established with one or two triple points in the examined pressure range. In addition, for the 11BT and the 12BT the phases existing under atmospheric pressure disappeared under relatively low pressures. The results are compared with recently studied smectogenic n-alkyl-cyanobiphenyl (nCB) compounds.     

Local electronic structure of phosphorus-doped ZnO films investigated by X-ray absorption near-edge spectroscopy

Using X-ray absorption near-edge structure spectroscopy (XANES), we investigate the local electronic structure of phosphorus (P) and its chemical valence in laser-ablated n-type (as-grown), and p-type (annealed) P-doped ZnO thin films. Both the P L ₁- and P L _2,3-edge XANES spectra reveal that the valence state of P is 3− (P³⁻) in the p-type as well as in the n-type P-doped ZnO. However, the peak intensity is stronger in the former than that in the latter, suggesting that P replaces O (O²⁻ sites with the P³⁻) after rapid thermal annealing. The Zn and O K-edges XANES spectra consistently demonstrate that, in the p-type state, P ions substitutionally occupy O sites in the ZnO lattice.     

Synthesis and phase transition studies on non-symmetric liquid crystal dimers: N-(4-(n-(4-(benzothiazol-2-yl)phenoxy)alkyloxy)-benzylidene)-4-chloroanilines

A new series of non-symmetric liquid crystal dimers N-(4-(n-(4-(benzothiazol-2-yl)phenoxy)alkyloxy)benzylidene)-4-chloroaniline containing benzothiazole and benzylideneimine units connected by a flexible alkyl spacer, –(CH₂) _n –, with n ranging from 4 to 12 in even parity have been prepared. All five members of this homologous series exhibit an enantiotropic nematic phase. The compounds with greater n of 8–12 exhibit both nematic and smectic phases upon cooling. A notable feature among this series is that for the member with n?=?10, the smectic–nematic transition is also present. The nematic–isotropic transition temperatures and associated entropy changes with respect to all compounds in this series exhibit a dramatic dependence on the length of the flexible spacer. A comparison of the transitional properties of this series with those of α-(4-benzylidenechloroaniline-4′-oxy)-ω-[4-(thiophene-2-carboxyl)benzylideneaniline-4′-oxy]alkanes reveals that replacing benzothiazole moiety at one side of the flexible alkyl spacer reduces the nematic–isotropic transition temperature.     

On some liquid crystalline phases exhibited by compounds made of bent-core molecules and their mixtures with rod-like molecules

In most homologous series of compounds made of bent-core (BC) molecules, the B₂ B₁ and B₆ phases occur as the chain length decreases. We have studied binary mixtures of the compound 1,3-phenylene bis[4-(3-methylbenzoyloxy)]4-n-dodecylbiphenyl 4’-carboxylate (BC12) which exhibits the B₂ phase with the compound 4-biphenylyl 4’-n-undecyloxybenzoate (BO11) made of rod-like (R) molecules. We find the above sequence of occurrence of the B phases with increasing concentration of BOH. In this paper we describe the physical origin for the formation of these phases in both pure compounds and in the mixtures. We have also found the occurrence of the biaxial smectic A phase when the BO11 concentration is increased to 87–95.5 mol%. We also report on another binary system composed of BC12 and 4-n-octyloxy 4’- cyanobiphenyl (8OCB) made of R molecules. This system exhibits the biaxial smectic A phase down to 30°C. Using polarized infrared spectroscopy we find that the mutual orientation of the R and BC molecules in the SmA_db liquid crystal is such that the arrow axes of the BC molecules are along the layer normal of the partial bilayer smectic structure formed by the rods. We also describe unusual growth patterns obtained when the nematic phase transforms to the SmA_db phase in a mixture with 24 mol% of BC12.