New chiral liquid crystal materials based on menthol: Synthesis and phase behavior

To study structure-mesomorphism relationships of chiral materials based on menthol, a series of chiral compounds containing menthyl group were synthesized. Their chemical structures, formula, and the purity were characterized by Fourier transform infrared (FTIR), proton nuclear magnetic resonance (¹H NMR), and elemental analyses. The corresponding phase behavior was investigated with differential scanning calorimetry (DSC) and polarizing optical microscopy (POM). The effect of the mesogenic core, flexible spacer, and terminal groups on the mesophase formation and type was discussed. The compound 4-menthyloxyacetoxybenzoyloxy-4′- (6-bromohexyloxy)biphenyl showed the smectic A (S_A) and cholesteric phases on heating cycle, and a cubic Blue Phase, chiral smectic C (S_C^*), S_A and cholesteric phases on cooling cycle. However, the compound 4-menthyloxyacetoxybenzoyloxy-4′-(4-bromobutoxy)biphenyl showed enantiotropic S_C^*, S_A and cholesteric phases, moreover, the platelet texture of a cubic Blue Phase was also observed on cooling cycle. The methacrylate monomer with six flexible methylene units and three phenyl rings as mesogenic core only showed cholesteric phase, while the methacrylate monomer with four methylene unit and three phenyl rings showed the S_C^* and cholesteric phases. However, when the terminal methacryloyloxy groups in the monomers were substituted by thiophenylacryloyloxy groups, the corresponding monomers all show no mesophase. With increasing the rigidity of mesogenic core or decreasing the flexible methylene spacer length, the corresponding melting temperature (T_m) or isotropic temperature (T_i) increased.     

A dinuclear cyano-bridged complex based on hexacyanoferrate(III) and samarium(III) nitrate: synthesis and crystal structure

A new complex, [Sm(DMF)₄(H₂O)₄Fe(CN)₆]·H₂O (DMF = N, N-dimethylformamide), has been synthesized and characterized by X-ray single crystal structure and thermogravimetric analyses. The complex crystallizes in the P2₁/n space group, with lattice parameters a = 17.583(4) Å, b = 8.870(2) Å, c = 19.845(6) Å, = 95.98(3)°, V = 3078(1) ų, D _x = 1.679 Mg m^–3, D _m = 1.65(1) Mg m^–3, Z = 4. The molecular structure shows that a cyano-bridged bimetallic structure is obtained. The Sm atom is coordinated by eight oxygen atoms of four water molecules and four DMF molecules and one nitrogen atom of the bridging cyanide ligand. The iron atom assumes approximately an octahedral environment surrounded by six CN ligands. The hydrate water molecule is hydrogen-bonded to one of the O atoms bound to Sm. Each terminal CN ligand of the Fe(CN) ₆ ^3– entity is hydrogen-bonded to some O atoms of water molecules. An infrared spectrum is also reported.     

Hydrothermal synthesis and ionic conductivity of CdF2 and low-temperature modifications of PbF2 and PbSnF4

Single crystals of CdF₂ and low-temperature modifications of PbF₂ and PbSbF₄ are synthesized from high-temperature hydrothermal solutions in the MF₂-M′F₂-HF (KF, NH₄F)-(Pb)-H₂O systems, where M = Cd and Pb and M′ = Sn. Ionic conductivity of the synthesized phases showed pronounced anisotropy of fluoride-ion transport. Conductivity of α-PbF₂ measured along two directions equals σ_∥a = 1.4 × 10^?3 Sm/cm and σ_∥c = 2.3 × 10^?4 Sm/cm.     

On the Two Closely Related Phases of [Ru(C5Me5)(η6-1,3-(Me2NCH2)2C6H4)](BF4) and the Reversible Solid–Solid Order–Disorder Phase Transition

Abstract  

Crystals of the complex [Ru(C₅Me₅)(η⁶-1,3-(Me₂NCH₂)₂C₆H₄)](BF₄) have been examined over the temperature range 150–300 K via X-ray diffraction measurements. This study shows that the Ru complex is a two-phase system in this T-range and the solid–solid transition is reversible. At 150 K, phase II (P2₁/c, Z′ = 4) is ordered and non-merohedrally twinned, a = 16.4396 (9) ?, b = 17.3226 (4) ?, c = 32.1874 (11) ?, β = 91.375 (2)°. At 295 K, phase I (Pbca, Z′ = 1) is disordered, a = 8.5071 (3) ?, b = 17.1567 (3) ?, c = 32.8250 (8) ?. The relationship between the two phases is obvious because the packing remains similar in the two phases. The greatest structural changes between the two phases are found in the rows of adjacent cations [Ru(C₅Me₅)(η⁶-1,3-(Me₂NCH₂)₂C₆H₄)]⁺ packed along the a direction. These rows are ordered in phase II but are disordered in phase I. The phase transition is first order. Significant changes in thermal motion for the cations are considered as being the driving force for the occurrence of this phase transition.     

Dielectric and pyroelectric studies of an antiferroelectric liquid crystal

Polymorphism and polar properties of an antiferroelectric (R)-2-methylheptyloxycarbonylphenyl-4-[(4-decyloxy-3-fluoro)benzoyloxy]benzoate liquid crystal are studied. The phases are identified, and the phase transition points are determined. Dielectric constant, dielectric losses, and pyroelectric properties are studied for the orthogonal smectic SmA and tilted smectic, SmC _α ^* , SmC ^*, SmC _γ ^* , and SmC _A ^* phases. The temperature dependence of spontaneous polarization is measured by the repolarization current technique and integration of the pyroelectric constant.     

Formation Entropy of Schottky Defects and Free Formation Enthalpy of Vacancy Pairs in Alkali Halide Crystals

The melting of alkali halides occurs probably at a critical mole fraction x_c = 3.2 · 10⁻⁴ of thermally created vacancies (Schottky defects). Using this value the formation entropy of Schottky defects is found to be s_S = (962.1 K mol/kJ L/T_m – 16.1) k (L, T_m heat and temperature of fusion, respectively). As to the free association enthalpy g_A = h_A – T_sA of (impurity-vacancy) complexes it is s_A = (10.8 h_A/eV – 2.56) k. Using this relation the concentration vacancy pairs is found to be smaller than that of isolated vacancies up to the melting point.     

Growth kinetics of ammonium oxalate monohydrate single crystals from aqueous solutions containing Co(II) and Ni(II) impurities

The growth kinetics of different faces of ammonium oxalate monohydrate (AO) single crystals from aqueous solutions containing different concentrations of Co(II) and Ni(II) ions at a constant temperature are described and discussed. It was found that: (1) at a given supersaturation σ, both Co(II) and Ni(II) ions lead to a decrease in the growth rates R of different faces of AO crystals, (2) the growth of a particular face of the crystals occurs above a critical supersaturation σ_d, but there is also another supersaturation barrier σ* when the rate abruptly increases with σ, and (3) the values of σ_d and σ* increase with increasing concentration c_i of the impurity. The experimental R(σ) data for different concentrations c_i of the impurities were analysed according to the model involving complex source of cooperating screw dislocations and concepts of instantaneous and time‐dependent impurity adsorption. Analysis of the data showed that: (1) adsorption of Co(II) and Ni(II) impurities occurs on the surface terrace of AO crystals, (2) there is a simple relationship between Langmuir constant K and the impurity concentration c_i* corresponding to maximum surface coverage, and (3) the ratio σ_d/σ* of the supersaturation barriers observed in the presence of both impurities increases with an increase in impurity concentration c_i, and may be explained from the standpoint of the mechanism of adsorption of impurity particles at kinks and ledges. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Electrical Conductivity of Liquid Crystal Mixtures with Induced Smectic Phases

The electrical conductivity of the two liquid crystal mixtures N-(4-ethoxybenzylidene)-4′-amino benzonitrile (PEBAB)/4,4′-di-n-hexyloxy-azoxybenze (HEXOAB) and 4-n-heptyloxy-4′-cyanobiphenyl (7 CBP)/HEXOAB, which exhibit induced smectic A phases is investigated. In the smectic phases, the conductivity anisotropy of the PEBAB/HEXOAB mixtures is negative at the lowest PEBAB concentrations; this behaviour is usually expected for a smectic layer structure. With increasing PEBAB concentration the anisotropy increases and becomes positive. Possibly, this is an indication for a growing double layer structure, which was observed in polar smectic phases. In the nematic phases the conductivity anisotropy of the pure HEXOAB is considerably reduced by adding a relatively small amount of the polar component. An addition of 10 mole% 7 CBP reduces the anisotropy ratio of the electrical conductivity to V = K_∥/K ₁ ≈ 0.2, which probably is the lowest value observed in a nematic phase so far. Besides the negative conductivity anisotropy, these mixtures also exhibit a positive anisotropy of the dielectric constant. They thus fulfil the conditions for inverse dynamic scattering.     

On the strengthening of Magnesium Single Crystals by Cadmium

Solid solution hardening in single crystals of magnesium containing cadmium (up to 2.4 at.%) has been investigated in the temperature range 77 K to 295 K. A strong temperature dependence of the critical resolved shear stress, τ₀, is observed below 230 K, while above this temperature τ₀ is temperature independent. At all temperatures τ₀ is found to increase linearly with c^2/3, where c is the concentration (in atomic fractions) of cadmium as solute. The concentration dependence of τ₀ can be explained by the theory of LABUSCH .     

Structural Polymorphism in MnGa2Se4

The polymorphic behaviour of the adamantane-type compound MnGa₂Se₄ is reinvestigated by X-ray powder diffraction and electron microscopy techniques carried out on annealed and quenched samples. The low-temperature modification (γ-phase) is confirmed to crystallize in the defect tetragonal thiogallate structure (space group I4-) with lattice parameters a = 5.677(1) Å and c = 10.761(2) Å. Samples quenched from high temperature (750 °C) show the coexistence of two ternary phases both having the MnGa2Se₄ composition but different crystal structures: the main phase is still tetragonal with slightly different lattice parameters (γ′-phase); the secondary phase exhibits an orthorhombic cell with approximate dimensions a = 13.5 Å, b = 8.0 Å, c = 6.5 Å and possible space group Pna2₁ (ε-phase). This latter polymorph is identified as the β-ZnAl2S₄-type form of MnGa₂Se₄. TEM analyses performed on the ε-phase show the presence of ferroelastic twins, confirmed by electron diffraction. The connections among the three structures are shortly discussed.     

The Order-Disorder Phase Transition in Liquid Crystals as a Function of Molecular Structure. I. The Alkyl Cyanobiphenyls

The pretransitional behaviour of a homologous series of mesomorphic compounds, the alkyl cyanobiphenyls (CNρρC _n H _{2n + 1}) for n = 5–12, has been studied as a function of temperature by light scattering in the isotropic phase. The higher homologues, n = 8–12, exhibit a smectic A phase that becomes increasingly important at the expense of the nematic phase for increasing n and for n = 10 and 12 no nematic phase is observed. We have shown from light scattering and differential scanning calorimetry measurements that the presence of the smectic A phase changes markedly the pretransitional behaviour, and gives rise to two distinct pretransitional regions. The first region is for temperatures greater than ～3°C above the clearing temperature T_c where the systems exhibit a typical pretransitional behaviour adequately described by the phenomenological Landau-de Gennes model of a second order phase transition with an intervering first order transition at T_c . The constants of this model for this region are given and show a distinct odd-even effect. The second region is close to T_c where a strong divergence from the already critical behaviour is observed. This secondary divergence is observed to be a function of the alkyl chain length and its conformation, and is attributed to coupling between smectic A like layers and the orientational order parameter in the isotropic phase.     

A pure reentrant cholesteric phase

Abstract

An optically active derivative of p-cyanobiphenyl exhibits a stable reentrant cholesteric phase with the seuqnce: K N?S_A N?I.     

On the origin of supersaturation barriers during the growth of single crystals from aqueous solutions containing impurities – a review

A generalised treatment of the appearance of supersaturation barriers σ_d, σ* and σ** during the growth of single crystals is outlined from the standpoint of well‐defined critical values of relative step velocities on a face. The final theoretical expressions are based on the premise that: (1) there are critical values of the relative step velocities associated with different average distances between adsorbed impurity particles during instantaneous, time‐dependent and time‐independent adsorption of the impurity on the growing surface, (2) the growth rate of a face is proporptional to velocity of steps on the growing face, and (3) Freundlich and Langmuir adsorption isotherms apply for different impurities. The theoretical expressions are then used to critically analyse the experimental data on supersaturation barriers observed during the growth of ammonium oxalate monohydrate and potassium dihydrogen phosphate single crystals from aqueous solutions containing different impurities. It was found that: (1) Langmuir adsorption isotherm is more practical for the analysis of the experimental data of the dependence of supersaturation barriers σ_d, σ* and σ** on the concentration c_i of an impurity, and (2) the ratios σ_d/σ* and σ*/σ** of successive supersaturation barriers for an impurity either increases or remains constant with an increase in impurity concentration c_i, and may be explained in terms of the mechanism of adsorption of impurity particles. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Structures of the Triclinic and the Monoclinic Modification of Divascan®

Trielinic modification: C₁₂N₄O₃H₁₆. Mr = 264.26, triclinic P 1 , a = 7.702(3), b = 9.420(11),c = 10.893(8) A, α = 73.37(8), β = 82.22(5), γ = 66.30(6)°, V = 638(1) A³, Z = 2, D_m = 1.360 Mg m⁻³, D_x = 1.362 Mg m⁻³, λ(MoK_α) = 0,71062 A, μ = 0.1084 mm⁻¹, F(000) = 280, T = 296 K, final R = 0.0742, wR = 0.0758 for 1842 reflections. Monoclinic modification: C₁₂N₄O₃H₁₆·, 3/2H₂O, M_r = 291.28, monoclinic P 2₁,/c, α = 14.075(5), b = 12.631(6), c = 16.234(5) A, β = 99.25(3)°, V = 2849(1) A³,Z = 8, D_m = 1.338 Mg m⁻³, D_m = 1.359 Mg m⁻³, λ(MoK_α) = 0.71062 A, μ = 0.1134 mm⁻¹, F(000) = 1240, T = 296 K, final R = 0.0557, wR = 0.0691 for 1266 reflections. Comparison of the two independent Divascan® molecules of the monoclinic modification with the one of the triclinic modification demonstrates good agreement of related bond distances and angles in the molecules. Disorder of the hydroxyl groups of one Divascan® molecule of the monoclinic modification is announced by high vibration parameters of the related oxygen atom and a very short C O distance, which is not adequate to a real C O bond length.     

A Study of Flow Alignment Instability During Rectilinear Oscillatory Shear of Nematics

Rectilinear oscillatory shearing of nematic liquid crystals in thin large-area (10 cm × 8 cm × 10 μm) cells has been studied in the range 20 to 100 Hz for two uniform initial alignments, one planar and parallel to the direction of oscillation and the other homeotropic. In the former case, for certain positive values of the viscosity coefficient α₃ a flow-alignment instability occurs above a critical amplitude A_c for the displacement of the movable upper plate, leading to formation of regular cm-wide bands of alternating 2 π and -2 π twist, running parallel to the alignment direction. The width of the bands decreases with decreasing temperature and increasing frequency, the total number present depending on the size of the cell. In homeotropic alignment, similar bands of lower twist form at an angle to the direction of oscillation, in this case both for certain positive values of α₃ and, in addition, for α₃ slightly negative. A theoretical expression for A _c in the cases with α₃ > O has been derived and substantially confirmed experimentally. The theory further predicts the existence of a remarkable “stability gap” where, although the expression for A_c is a minimum, there is no instability and formation of bands should not occur. By exploiting the divergence of α₃ at a smectic A phase, theory and experiment have been compared over a wide range of α₃ values, and the stability gap confirmed experimentally for several materials. In the case of “weak” smectic phases (i. e., showing actual or incipient re-entrant behavior) the expected divergence of A_c does not occur until significantly inside the smectic phase range. Extrapolation of 1/A² _c against temperature can be used to locate accurately in the temperature at which α₃ is zero. A variety of materials has been studied, revealing that α₃ may be positive in a rather wider range of materials than had previously been realized, and that when α₃ is positive it usually remains so over most or all of the nematic range.     

Structure of 18-cyanoprogesterone,C22H29O2N

The crystal structure of 18-cyanoprogesterone was determined by X-ray diffraction methods:P2₁2₁2₁ a=7.436(2),b=11.322(2),c=22.642(2) Å. The structure was solved usingShelx-86. Final conventionalR=0.054.R _w =0.051 for 1841 reflections. TheA ring has an intermediate sofa-half-chair conformation with asymmetry parameters C _s ^/1 =11.0, C ₂ ^3,4 =14.9. The steroid skeleton exhibits a flattening of theA ring relative to the rest of the molecule. The progesterone side chain has a typical conformation, and the C16-C17-C20-O20 torsion angle is –19.0(6)°.     

Synthesis and Liquid Crystal Behaviour of Further 4,4″-Disubstituted 2'-Fluoro-1,1':4',1”-Terphenyls

Further to earlier work on the liquid crystal properties of fluoro-1,1':4',1”-terphenyls¹ we have now extended this series to include terminally fluoro-and cyano-substituted 2'-fluoro-1,1':4',1″-terphenyls, chiral 2'-fluoro-1,1':4',1″-terphenyls, and esters derived from 2'-fluoro-1,1':4',1”-terphenyls and incorporating the alkylcyclobutyl group. The preparations and transition temperatures for these series of compounds are presented and their transition temperatures and mesophase types are discussed. An interesting result from this work was the appearance of an S^* _C phase for one chiral homologue and of S _c phases for the esters incorporating the alkylcyclobutyl group. The S _c formation is attributed to the presence of the fluoro-substituent.

Keywords: terphenyls, fluoroterphenyls, smectic C phases, cyclobutyl esters     

Anomalous High Pressure Properties in Fullerene Superconductors

Abstract

Correlations among lattice parameter, external pressure, and critical temperature T_c has been surveyed on fullerene superconductors with a wide range of lattice parameters and various valence states. The observed value of dT_c/dP for Na₂Rb_0.5Cs_0.5C₆₀ and Li₃CsC₆₀, having small interfullerene separations, is about a few times lager than that of K₃C₆₀. In contrast, small dT_c/dP values are found in fullerides with expanded unit cells, such as (NH₃)_xNaAA^?C₆₀ (A, A^? = K, Rb and Cs) and A₃Ba₃C₆₀. Interestingly, the chemical and physical pressure effects on T_c are considerably different in these fullerides with large interfullerene spacings. Our results suggest that the pressure dependence is scaled by an interfullerene distance when the interfullerene distances is small, but that this scaling fails when the unit cell is expanded.     

Pressure Effects of Nematic Liquid Crystals

There exists a variety of important new phenomena and plenty of experimental data on the pressure effects of liquid crystals. Yet, no systematic phenomenological or microscopic theory is available. In this paper, the ordinary Landau-deGennes theory is generalized so that the free energy becomes G = G _o(P, T) + a/2 [T - T*(P)]S ² - B(P)/3 S ³ + C(P)/4 S ⁴, T*(P) = T _o + bP - eP ². All the known pressure experiments of nematic PAA (where B and C are independent of P) are explained in one stroke and all parameters are determined. New results including reentrant I phase (I represents isotropic phase), and the independence of T _c—T* and T*—T _c on P, etc., are predicted. Simple methods of experimental confirmation are proposed. Similar discussions on the cases of MBBA, EBBA, etc. are also given. Our theory differs from that of Lin-Keyes-Daniels but agrees better with experiments. Results related to G _O(P, T) will be reported in a separate paper.