A generalized model for the molecular arrangement in the columnar mesophases of polycatenar mesogens. Crystal and molecular structure of two hexacatenar mesogens

The columnar mesophases of two series of hexacatenar palladium(II) mesogens have been studied in detail by a combination of X-ray diffraction on aligned and unaligned samples and dilatometry. The results of these studies, combined with the results of two single crystal structure determinations, have allowed a model of the molecular arrangement in the columnar phases to be proposed. This model differs in detail from that generally accepted for the arrangement of polycatenar mesogens in columnar phases, and a new model is proposed which accounts for both new and existing data.     

Thermotropic and lyotropic liquid crystalline phases of rigid aromatic amphiphiles

Rodlike amphiphilic molecules that contain exclusively aromatic building-blocks and no flexible alkyl chains have been synthesized and their mesomorphic properties investigated. These novel compounds bear diol head groups of different size (2,3-dihydroxypropyloxy or 5,6-dihydroxy-3-oxahexyloxy groups) at one end of a biphenyl unit, various aromatic segments (benzyloxy, 4-, 3-, or 2-methylbenzyloxy, phenoxy groups) at the other, and additional methyl substituents in different positions. They were synthesized by using Suzuki cross-coupling reactions as the key steps. Their thermotropic mesomorphism was investigated by means of polarized light optical microscopy, differential scanning calorimetry, and, for enantiotropic phases, by X-ray scattering. The liquid crystallinity of this class of compounds is influenced by protic solvents, such as water and glycerol. Dependent on the temperature and the solvent content, different SA phases were found. Several mesophases resulting from the frustration of these layer structures (e.g., different columnar phases, optical isotropic mesophases, and nematic phases) were also present. The smectic phases have different degrees of intercalation (SAd, SA2). The columnar phases are supposed to be ribbon structures that result from the collapse of the smectic layers. They occur in some pure compounds or they are induced upon the addition of protic solvents. The particular phase sequences of the different compounds depend mainly on the position of the methyl substituents at the biphenyl cores and are largely determined by the degree of intercalation of the aromatic cores.     

Calamitic bolaamphiphiles with (semi)perfluorinated lateral chains: polyphilic block molecules with new liquid crystalline phase structures

Novel bolaamphiphiles consisting of a rigid biphenyl unit, two terminal polar 1,2-diol units and laterally attached (semi)perfluorinated chains have been synthesized via palladium-catalyzed cross coupling reactions as the key step. The thermotropic liquid crystalline behavior of these compounds was investigated by polarized light optical microscopy, DSC, and X-ray scattering, and the influences of the length, number, structure, and position of the lateral chain on the mesomorphic properties were studied. A wide variety of unique liquid crystalline phases were found upon elongation of the lateral semiperfluorinated chains. For short- and medium-chain length a series of columnar phases were observed, and upon further elongation of the lateral chain a series of novel mesophases with layer structures were found. In the columnar phases, the nonpolar lateral chains segregate into columns, which are embedded in honeycomb-like networks of cylinders consisting of the biphenyl units. Strings of hydrogen-bonding networks of the diol groups provide cohesive forces, which maintain the overall structure. Changing the length of the lateral chains influences the diameter of the columns and thus determines the number of biphenyl units which are required to surround these columns. The number of these units [four (c2mm, p4mm), five (p2gg), six (p6mm), eight (c2mm) or 10 (p2gg)] defines the shape of the cylinders as well as the lattice type of the columnar phase. It is proposed that the columnar phases with a p2gg lattice result from the regular organization of pairs of cylinders which have a pentagonal cross sectional shape. In the mesophases with layer structure the aromatic rodlike cores are arranged parallel to the layer planes, and the onset of orientational and positional ordering of the biphenyl segments leads to a sequence of subtypes for these lamellar phases (Lam(Iso)-Lam(N)-Lam(X)).     

A silica-based monolithic column in capillary HPLC and CEC coupled with ESI-MS or electrospray-atmospheric-pressure laser ionization-MS

We describe the successful coupling of CEC and capillary HPLC with the recently developed atmospheric-pressure laser ionization (APLI) method. APLI is suitable for selectively and sensitively ionizing nonpolar aromatic compounds at ambient pressure for subsequent mass-selective detection. The polycyclic aromatic hydrocarbons used as analytes are first separated either by CEC on a silica-based monolithic column or by capillary HPLC. The eluent, along with a sheath flow, is volatilized by microelectrospray and then selectively ionized by excimer laser (KrF*) radiation via two-photon excitation. A QTOF-MS is used as mass-selective detector. This interface combination makes soft ionization of thermally labile nonpolar aromatic analytes possible.     

A new type of square columnar liquid crystalline phases formed by facial amphiphilic triblock molecules

A series of three novel liquid crystalline amphiphilic molecules is reported which are composed of three incompatible molecular parts, a rigid terphenyl core, two lipophilic decyloxy chains in the terminal 4- and 4' '-positions, and a polar group in the lateral 2'-position. The polar group comprises a polyether chain, an amide group, and a polyhydroxyalkyl end group (1-acylamino-1-deoxy-d-sorbitol derivatives). The self-organization of these compounds was studied by polarized light microscopy, differential scanning calorimetry, and different X-ray diffraction techniques. These investigations confirm a novel liquid crystalline phase with a square 2D-lattice (square columnar mesophase, plane group p4mm). This structure is built up by a set of three distinct columns, namely columns containing the polar lateral groups, columns incorporating the alkyl chains, and ribbons of the rodlike terphenyl units. The calamitic cores form walls bounding square-shaped channels occupied by the microsegregated polar lateral chains. The lipophilic columns containing alkyl chains are at the corners interconnecting the aromatic rods end-to-end.     

New liquid crystalline phases with layerlike organization

Novel lamellar mesophases which are quite distinct from conventional smectic mesophases were obtained with a bolaamphiphilic triblock molecule composed of a rigid biphenyl core, two polar 2,3-dihydroxypropoxy groups in the terminal 4- and 4'-positions, and a semiperfluorinated chain [O(CH2)6C10F21] in the lateral 3-position. The competitive combination of microsegregation and rigidity in this molecule leads to layer structures in which the bolaamphiphilic cores segregate from the lateral chains into distinct sublayers. In these sublayers the biphenyl cores are aligned parallel to the layer planes. Decreasing the temperature leads to a subsequent inset of orientational and positional order of the biphenyl unit, which leads to a transition from an uniaxial SmA phase to a biaxial SmAb phase and finally to a mesophase with an additional periodicity within the aromatic sublayers. Here, microsegregation occurs on two distinct levels: The segregation of the nonpolar chains from the aromatic cores leads to the "bulk" layer structure and segregation of polar and aromatic subunits within the aromatic sublayers gives rise to an additional periodicity within the aromatic sublayers. These phases can be regarded as smectic phases built up by quasi-2D layers with nematic, respectively SmA-like order, separated by isotropic layers of the lateral chains.     

High levels of electrochemical doping of carbon nanotubes: evidence for a transition from double-layer charging to intercalation and functionalization

We studied the transition from the electrochemical double-layer charging regime to intercalative doping of bundled single-walled carbon nanotubes (SWNT) in KCl and HCl aqueous solution. For this purpose we used high doping levels by applying constant potentials above 1000 mV approaching and slightly exceeding the oxidation potential for Cl(-) ions. At each potential in situ Raman measurements of the radial breathing mode (RBM), the high-energy tangential mode (HEM), and the disorder-induced (D) mode were performed. Furthermore, the conductivity and reflectivity of a set of SWNT samples were measured as a function of doping and subsequently the samples were examined by X-ray photoelectron spectroscopy (XPS). From a comparative analysis of the results we conclude that above 1000 mV a significant penetration of chlorine species into the interstitial channels of the SWNT bundles and possible covalent functionalization take place.     

Smectic A phases with strings of interdigitated molecules in swallow tailed compounds

X-ray diffraction studies are reported of smectic A phases exhibited by swallow-tailed compounds. The special molecular shape consisting of regions having differing space filling requirements results in antiparallel packing of the molecules; this influences the conformations of the alkyl chains. The layer spacings show unusually good agreement with the molecular lengths and this is the case for molecules with long and short aliphatic chains. We have observed also an additional incommensurable structural element which we believe to be unique for non-polar, low molar mass compounds. The X-ray diffraction patterns provide evidence for the existence of strings of interdigitated molecules.