Supramolecular structures and columnar mesophase induction in nondiscoid pyrazoles by complexation to rhodium(I)

Several new cis-[RhCl(CO)2(Ln)] complexes have been prepared using different polycatenar pyrazole ligands (Ln) in order to obtain columnar liquid crystalline arrangements. The topology of the ligand plays an essential role, and a mesophase is induced at room temperature from a nonmesogenic pyrazole only when it is symmetrically substituted with six decyloxy chains. The single-crystal structure of a methoxy-substituted analogue, 3,5-bis(3,4,5-trimethoxyphenyl)pyrazole, is formed by globular tetrameric structures held together by H-bonding. However, parallel dimers are present in the corresponding cis-chlorodicarbonylrhodium(I) complex, a situation that explains the induction of a columnar mesophase in the decyloxy-substituted complex. The XRD pattern of the mesophase is consistent with a hexagonal symmetry in which the columns are formed by molecules assembled in an antiparallel mode. The crystal-to-mesophase transition was also detected by spectroscopic techniques as a shift in the IR carbonyl stretching bands and the appearance of a charge-transfer band in the absorption spectrum with thermochromic behavior.     

Magnetic properties of rare-earth β-enaminoketone metallomesogens

Reaction of β-enaminoketones with lanthanide salts leads to mesomorphic complexes displaying an S_A mesophase. The mesophase was characterized using X-ray scattering, EPR and magnetic susceptibility. A very high magnetic anisotropy was deduced for a complex containing dysprosium.     

A monotropic liquid crystal polyester with a biphasic nematic-smectic phase

The polyesters made from the condensation of isophthalic acid (IA) or terephthalic acid (TA) with the mesogenic diol 4,4′-bis (6-hydroxy hexoxy)biphenyl (BHHBP) are both reported to show a smectic mesophase. However, in our previous work, we found evidence for a mesophase in the polyester containing BHHBP and IA, but not in the one with BHHBP and TA. For the BHHBP-IA polyester, we thought the phase sequence on cooling was I → S_A → K, while on heating the situation was unresolved; there was some indication that on heating, a K → I transition occurs without any intervening mesophase. In other words, it was suspected that the mesophase in the IA polyester was monotropic. In this work, we have now obtained firm evidence that the mesophase behavior of the BHHBP-IA polyester is indeed monotropic. Further, an additional complexity has been found: on cooling, there is in fact a nematic phase, besides a smectic A, and a crystal phase. However, the mesophase appears to be complex. As the nematic and smectic textures occurred simultaneously, it seems that the mesophase is a nematic-smectic biphase. © 1996 John Wiley & Sons, Inc.     

Relaxations in liquid crystalline poly(tetraethylene oxide terephthaloyl-bis-4-oxybenzoate)

The relaxation behavior of poly(tetra ethylene oxide terephthaloyl-bis-4-oxybenzoate), PTETOB, was analyzed by thermally stimulated depolarization currents, TSDC, and dynamic mechanical techniques, DMTA, and the results compared with those obtained by differential scanning calcrimetry, thermo-optical analysis, and x-ray diffraction. In the low temperature region, ?173–30°C, three main transitions were observed and assigned to the γ relaxation, the glass transition of the mesophase and the glass transition temperature of the amorphous material. The complex behavior observed in the range 110–160°C was as signed to a crystal-crystal transition which competed with the formation of a mesophase and afterward the formation of a smectic A mesophase. At higher temperatures, was observed the transition from the smectic A mesophase to a nematic one, prior to the isotropization temperature. In the TSDC experiments the formation of a permanent electret was detected and the charges trapped in the mesophase were canceled only at the isotropization temperature. © 1995 John Wiley & Sons, Inc.     

Twofold light and magnetic responsive behavior in nanoparticle-lyotropic liquid crystal systems

We demonstrate the dual magnetic and light responsive nature of hybrid mesophases constituted by Fe(3)O(4) nanoparticles dispersed in lipid-based lyotropic liquid crystals (LC). When subjected to an external magnetic field in the mesophase isotropic state, the nanoparticles aggregate and orient along the magnetic field direction, and upon cooling the system through the disorder-order transition the aggregates drive the orientation of the mesophase via heterogeneous nucleation; furthermore, order-disorder transitions in the lipidic mesophase can be triggered by Fe(3)O(4)-induced photothermal effect under visible light exposure. Both the orientational order and the photothermal effect of the hybrid mesophase can be tuned by the nanoparticle content, offering a general route for controlled assembly of complex fluids with combined magnetic and light responsiveness.     

Eu(TTA)_3/MCM-41介孔复合体的溶胶凝胶法组装

利用溶胶 凝胶法将稀土配合物Eu(TTA) 3 组装到MCM 41介孔分子筛的孔道中 ,并初步认定客体分子Eu(TTA) 3 是以加合物形式包裹于表面活性剂胶束中。该法制得的介孔复合体Eu(TTA) 3/MCM 41,用XRD、HRTEM技术证实具有短程有序的、规整的六方介孔结构和大小分布均匀的纳米晶粒。对其光致发光和荧光寿命的研究发现 :与乙醇溶液中相比 ,Eu3 的荧光寿命没有发生改变 ,但Stokes位移却明显增大 ;复合体中 ,能量是从主体MCM 41传递到客体Eu(TTA) 3 上。     

基于多金属氧簇的手性超分子液晶

以手性诱导为切入点,采用含胆固醇基团的季铵盐表面活性剂静电包覆缺位的Keggin结构多金属氧簇K7PW11O39·12H2O,得到了手性介晶阳离子修饰的多金属氧簇杂化超分子复合物。圆二色谱对该复合物光学活性的表征说明外围的手性表面活性剂可以通过静电相互作用诱导复合物显示出手性。利用差示扫描量热曲线法、偏光显微镜观察和变温X射线衍射详细研究了该复合物的热性质和相行为,结果表明该复合物在较宽的温度范围内具有热致液晶性质,是一种典型的手性近晶A相离子液晶材料。     

A highly ordered hydrogen-bonded network

Self-assembled, hydrogen bonded, supramolecular complexes based on the interaction of pyridine with carboxylic acids have been synthesized. These contain different proportions of a compound with two terminal benzoic acid residues and a pentaerythritol-based tetra-pyridyl. They have been investigated for possible mesomorphic properties. Surprisingly for a tetrahedral pentaerythritol-based complex, a mesophase was observed with a grainy microscopic texture, which was fluid and could be sheared. The mesogenic unit is formed on hydrogen bonding complexation between the pyridyl residues and the carboxylic acid residues. Infrared spectra, as a function of the temperature, were recorded for the 1:1 complex; these show pyridyl complexation and changes on isotropisation. These changes are discussed in terms of the hydrogen bonding. Fibres could be drawn from the (mesophase) melt, supporting a polymeric, extended-chain structure for the complexes.     

Dynamics and structure of a flexible columnar liquid crystal based on tetrabenzocyclododecatetraene

The dynamical and conformational behaviour of a flexible tetrabenzocyclododecatetraene derivative exhibiting a columnar mesophase has been studied by a combination of deuteron solid state NMR spectroscopy and molecular dynamics (MD) simulations. As shown by two-dimensional (2D) exchange NMR, the mesophase is characterized by slow axial reorientations (∼10^-3s) of single molecular units where the phenylene rings exhibit a well-defined quasi-fourfold potential, while the 2D spectra of the core methylene sites are sensitive to the molecular conformation and reorientation mechanism. Motional narrowing of one-dimensional (1D) spectra reveals additional fast librations due to the internal flexibility of the mesogenic moiety. The various reorientation pathways comprising interconversions and pseudo-rotations between different energetically stable conformations are elucidated on a microscopic level by molecular dynamics simulations. The mesophase dynamics is ascribed to a complex axial motion involving rotational jumps combined with a pseudo-rotation between two symmetry related sofa forms. This is confirmed quantitatively by comparing the experimental 2D NMR spectra of the core methylene sites and the simulations which are based on the molecular geometries obtained by MD simulations. The lineshapes of one- and two-dimensional spectra of magnetically aligned samples specific to the orientation behaviour of the sofa conformer are discussed.     

Discotic liquid-crystalline materials based on porphycenes: a mesogenic metalloporphycene-tetracyanoquinodimethane (TCNQ) adduct

A number of substituted zinc(II) porphycenes and porphyrins have been synthesized as potentially mesogenic materials. One of the resulting porphycenes, bearing eight decyloxy chains, exhibits two mesophases, a transient lamellar phase (Lam) and a highly ordered lamello-columnar phase (L(Col)), with remarkably different structural characteristics. The same zinc(II) porphycene also forms an electron donor-acceptor (EDA) complex with tetracyanoquinodimethane (TCNQ), generating a hexagonal columnar mesophase (Col(h)) that is thermally stable up to ca. 200 degrees C. The EDA interaction between porphycene and TCNQ has been probed using electronic and vibrational spectroscopy. A mixture of zinc(II) porphyrins, isomeric with the above porphycene complex, forms a rectangular columnar mesophase (Col(r)).     

基于多金属氧簇的手性超分子液晶

以手性诱导为切入点,采用含胆固醇基团的季铵盐表面活性剂静电包覆缺位的Keggin结构多金属氧簇K₇PW₁₁O₃₉·12H₂O,得到了手性介晶阳离子修饰的多金属氧簇杂化超分子复合物。圆二色谱对该复合物光学活性的表征说明外围的手性表面活性剂可以通过静电相互作用诱导复合物显示出手性。利用差示扫描量热曲线法、偏光显微镜观察和变温X射线衍射详细研究了该复合物的热性质和相行为,结果表明该复合物在较宽的温度范围内具有热致液晶性质,是一种典型的手性近晶A相离子液晶材料。     

枝杈状Bola型两亲分子包覆的层状近晶相多金属氧簇液晶

我们利用离子自组装的方法以含苯甲酸基团的两亲分子A6包覆了含铕的无机多金属氧簇Na9[EuW10O36]。利用红外光谱、热重分析、元素分析等方法对所得有机无机杂化复合物A6Eu进行了详细的结构表征。苯甲酸基团通过分子间氢键相互作用发生的二聚使有机组分形成了枝杈状的Bola型结构,这种结构导致外围的介晶基元以侧向方式连接在无机簇上。我们通过差示扫描量热法、偏光显微镜和变温X射线衍射对A6Eu的热致液晶性质进行了表征。虽然单独的两亲分子A6组装成了简单的近晶结构,但是复合物A6Eu表现出新颖的层状近晶相液晶结构。层状近晶相液晶结构中介晶基元是平行于层面的。由此可见,介晶基元的侧向连接方式对复合物的热致组装结构具有重要影响。多金属氧簇的荧光性质在液晶材料中得到了很好的保持。     

Fluoroalkylated discotic liquid crystals

The concept of mesophase stabilization by partial fluorination of side chains has been extended to discotic systems. The mesophase structure is essentially unchanged, whereas the temperature range of the mesophase is strongly affected by the fluorinated side chains. Triphenylene substituted with only one partially fluorinated side chain exhibits a decrease of the clearing temperature, whereas for symmetrically substituted systems a broad range mesophase has been observed. This behaviour can be attributed to a segregation of the incompatible parts of the molecule giving rise to a stabilized columnar structure for the symmetrical substitution pattern.     

Liquid crystalline materials with complex mesophase morphologies

Liquid crystals, which combine order and mobility on a molecular and supramolecular level are increasingly accepted as a fourth state of matter. Besides the well-established nematic, smectic and columnar mesophases, more complex mesophase morphologies attracted increasing interest during the recent years. These are bicontinuous and discontinuous cubic mesophases and other two- and three-dimensionally ordered intermediate phases, superstructures induced by molecular chirality or by polar order of bent core molecules, novel biaxial smectic phases, and novel mesophase morphologies of polyphilic block molecules and dendrimers.     

Investigation of the Structures of the Crystalline and Columnar Phases of Linear Chain Copper(II) Alkanoates

Binuclear copper(11) complexes of fatty acids crystallize at room temperature in a lamellar lattice that has been characterized by X-ray diffraction. A transition to a thermotropic columnar mesophase is observed at about 110-120°C for each compound of the series n = 12 to n = 22, n being even and equal to the number of carbon atoms in the corresponding fatty acid. This columnar mesophase has been investigated by polarizing optical microscopy, differential scanning calorimetry and X-ray diffraction. Columns of polar copper carboxylate groups are surrounded by disordered aliphatic chains, and form a two-dimensional hexagonal lattice. The repeating unit in a column is a binuclear dicopper tetracarboxylate complex.     

Probing the electric field alignment of a thermotropic liquid crystalline polymer by synchrotron radiation

The orientation of a cyclic side-chain thermotropic liquid crystalline material in an AC field was monitored in real-time using synchrotron radiation. Monitoring the realignment processes in the millisecond-to-minute time-scale was made possible by the high X-ray flux. Orientation parameters and response times were calculated as a function of temperature and frequency. Response times decreased exponentially with temperature due to a decrease in the viscosity. Very little dependence of the response time on frequency was observed, except at low temperatures, where a switch from homeotropic to planar alignment of the molecules was detected. This reorientation of the director was studied in real-time and the resulting complex diffraction patterns were due to equal but opposite director rotations from an alignment parallel to the applied electric field to an alignment perpendicular to the applied electric field. The orientation parameters were highest in the central portion of the mesophase temperature range. At temperatures near clearing, the net degree of orientation diminished. Cooling through the mesophase with an applied electric field resulted in much larger orientation parameters than could be obtained by aligning at a fixed temperature in the mesophase.     

Mixed copper-lanthanide metallomesogens

This paper describes the first examples of heteropolynuclear metallomesogens that contain both a transition metal ion and a trivalent lanthanide ion. Adducts were formed between a mesomorphic [Cu(salen)] complex (salen=2,2'-N,N'-bis(salicylidene)ethylenediamine) with six terminal tetradecyloxy chains and a lanthanide nitrate (Ln=La, Gd). Different stoichiometries were found, depending on the lanthanide ion: a trinuclear copper-lanthanum-copper complex [La(NO(3))(3)(Cu(salen))(2)] and a binuclear copper-gadolinium complex [Gd(NO(3))(3)Cu(salen)]. The compounds exhibit a hexagonal columnar mesophase (Col(H)) over a wide temperature-range with rather low melting temperatures. Although the clearing point could be observed for the parent [Cu(salen)] complex, the mixed f-d complexes decomposed in the high-temperature part of the mesomorphic domain before clearing. On the basis of X-ray diffraction measurements and molecular modelling, a structural model for the mesophase of the metal complexes is proposed.     

KINETICSOF TRANSITION FROM ISOTROPIC TO CHOLESTERIC PHASE FOR A THERMOTROPIC CELLULOSE DERIVATIVE

The kinetics of mesophase formation of a thermotropic hydroxyethyl cellulose acetatefrom isotropic phase to cholesteric mesophase has been studied by means of depolariz-ing transmittance method. Avrami type analysis of the data gives an exponent n close to1, which suggests the nucleation followed by rod-like growth. It means that the kineticbehavior of phase transition from isotropic to cholesteric mesophase is very similar to thatof the mesophase formation from isotropic to nematic mesophase.     

Photoluminescent nickel(II)-metallomesogens derived from salphen ligands: influence of halogens at the spacer on mesomorphism and emission properties

Three new series of photoluminescent nickel(II) metallomesogens, [NiL]; H₂L = N,N′-Bis(4-n-alkoxysalicylidene)-4-fluoro/bromo/chloro-1,2-diaminobenzene (n = 12, 14, 16) based on ‘salphen’ ligands have been synthesised and their mesomorphic and photophysical properties explored. The complexes, isolated as orange microcrystalline solids were characterised by elemental analyses, FT-IR, ¹H NMR and UV-visible spectroscopy. Thermal studies show all the compounds to be enantiotropic liquid crystals displaying columnar mesophase over a wide temperature range. Electronegativity and steric requirement of the halogen substituent at the ligand’s spacer remarkably influence the 2-D packing of the columns in the lattice in these complexes controlling the supramolecular mesomorphic order and photoluminescence. The mesophase behaviour of the fluoro-substituted complex is characterised by a transition from a columnar oblique (p1) to columnar rectangular (p2mm) phase, former stable till ambient temperature. The chloro and bromo analogues, on the other hand, displayed exclusively columnar rectangular (p2mm) mesophase with the former transforming into a glassy state and latter into a crystalline phase during cooling to ambient temperature. Molecular model based on interdigitated anti-parallel and back to back arrangements in the different columnar mesophase are proposed on the basis of X-ray diffraction (XRD) studies. The complexes emit in the blue region when excited with near UV wavelength.     

Pyridinium salt liquid crystals Effect of mesogen extension and alkyl chain length

The thermotropism of 1-n-alkyl-(4-methyl and 4-tolyl)pyridinium bromides were compared for alkyl chain lengths ranging from n = 12 to 22 carbons. A smectic A mesophase is present in both series for the longer chain compounds, n ≥ 16, with the clearing temperature being similar for both series but increasing rapidly with chain length. The series with the elongated mesogen also possesses an ordered mesophase identified as smectic G. The transition between this mesophase and the S_A or isotropic phase in the 4-tolyl series, and the transition to and from the crystalline phase in both series, are affected relatively little by the alkyl chain length. It seems that the S_A mesophase is governed primarily by the amphiphilic character of the substances, whereas elongation of the ionic head group is responsible for the appearance of a more ordered mesophase at intermediate temperatures.