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.     

Driving Forces of the Self-Assembly of Supramolecular Systems: Partially Ordered Mesophases

The main aspects are considered of the self-organization of a new class of liquid crystalline compounds, rigid sector-shaped and cone-shaped dendrons. Theoretical approaches to the self-assembly of different amphiphilic compounds (lipids, bolaamphiphiles, block copolymers, and polyelectrolytes) are described. Particular attention is given to the mesophase structures that emerge during the self-organization of mesophases characterized by intermediate degrees of ordering, e.g., plastic crystals, the rotation-crystalline phase in polymers, ordered and disordered two-dimensional columnar phases, and bicontinuous cubic phases of different symmetry.     

Liquid crystalline bolaamphiphiles with semiperfluorinated lateral chains: competition between layerlike and honeycomb-like organization

Novel bolaamphiphilic triblockmolecules consisting of a rigid biphenyl unit, with a polar 2,3-dihydroxypropyloxy group and a phenolic OH group at opposite ends, as well as a semiperfluorinated chain in a lateral position have been synthesized via palladium catalyzed cross coupling reactions as the key steps. The thermotropic liquid crystalline behavior of these compounds was investigated by polarized light microscopy, DSC and X-ray scattering, and the influence of the length of the lateral chain on the mesomorphic properties was studied. The compound with the shortest chain as well as the long chain derivatives form lamellar mesophases composed of segregated layers of the bolaamphiphilic moieties and sublayers comprising the fluid lateral chains. The layers within the lamellar phases of the short chain compound adopt a positional correlation, leading to a 2D lattice (Col(r)/p2mm), whereas the layers of the lamellar phases of the long chain derivatives are noncorrelated (Lam). Compounds with a medium chain length organize into columnar phases, where the nonpolar lateral chains segregate into columns, which are embedded in networks of regular (Col(h)) or stretched (Col(r)/c2mm) hexagonal cylinder shells consisting of the bolaamphiphilic units. In total, an unusual phase sequence was found, where, with respect to the chain length, columnar mesophases occur between two mesophases with layer organization.     

Influence of flexible spacers on liquid-crystalline self-assembly of T-shaped bolaamphiphiles

T-shaped bolaamphiphiles composed of a biphenyl rigid core, a semiperfluorinated lateral chain, two polar 1,2-diol groups in the terminal positions and flexible alkyl spacers connecting the polar groups with the biphenyl core have been synthesized and investigated by polarizing microscopy, DSC and X-ray scattering. The influence of spacer length and position of the spacer on the self-assembly in liquid-crystalline phases was studied. A series of four different columnar phases (Col(hex)/p6mm, Col(rec)/p2gg, Col(squ)/p4gm and Col(squ)/p4mm), representing liquid-crystalline honeycomb structures composed of cylinders having hexagonal, pentagonal, and square cross section, were found on increasing the spacer length. It is also shown that introduction of aliphatic spacers in the backbone of the T-shaped bolaamphiphiles replaces the Col(rec)/c2mm phase made up of rhombic cylinders with the Col(squ)/p4mm phase composed of square cylinders. It also causes the 2d lattice of pentagonal cylinders to increase the symmetry from Col(rec)/p2gg to Col(squ)/p4gm. A temperature-dependent second-order phase transition between these two pentagonal cylinder structures was observed for the first time. Beside these effects on cylinder shape and phase symmetry the flexible spacer units also lead to reduced phase transition temperatures and allow adjustment of cylinder side length to envelop a wider range of side-chain sizes. Electron density maps suggest that this may involve sacrificing some of the hydrogen bonds.     

Liquid crystallinity of newly synthesized glucose derivatives with mesogenic side chains

Synthesis of glucose derivatives by direct esterification of the five available sites on D-(+)-glucose with side chains containing a biphenyl mesogenic moiety, a pentyl spacer and an alkyl tail is reported for the first time. Liquid crystalline phase behaviour of these glucose derivatives was studied by optical microscopy, thermal and X-ray diffraction methods. A layered arrangement of the smectic A (SmA) type was commonly observed in the above materials. An increase in the length of the alkyl tail results in a change of the phase structure from smectic Ai(SmAi), intercalated layer phase, to smectic As(SmAs), segregated bilayer phase. From the transition temperature and enthalpy observations, the SmAs phase has higher order than the SmAi phase. Cholesteric and chiral smectic C phases were also observed in addition to the SmA for some of the glucose derivatives, demonstrating a potential for preparing chiral liquid crystals.     

不同拓扑构型嵌段液晶分子的合成与相变行为

本文对近来报道的三类嵌段液晶分子：多羟基1－苯甲酰胺－2，3－丙二醇两亲性分子I,星状季戊四醇苯甲酸酯II及带侧链的波拉化合物III的合成及自组装行为作了综述。这些分子由各向异性嵌段、两亲性嵌段和烷基链（或全氟链）以不同的拓扑形式组合而成，其合成的关键步骤是Pd(0)催化下碘代全氟烷烃对双键的自由基加成反应以及Pd(0)催化的偶合反应。通过调节亲水部分和亲脂部分的比例, 在I和II-F中观察到了从近晶层相（Sm）到柱相（Col）再到三维立方相（Cub）的液晶相序列；在波拉分子III-F中通过改变侧链的长度, 获得了一系列柱相及新型层相。这些研究表明, 利用多元化竞争组合理念，将分子中不相容的部分以不同的拓扑形式结合在一起，通过它们的微观相分离, 形成不同的微观区域, 获得不同的界面, 产生不同的界面曲率, 可设计合成具有不同微观结构的超分子体系， 从而在分子水平上控制分子的自组装行为； 同时全氟链的引入所带来的氟效应能促进微观分离，稳定液晶相，并有利于复杂超分子体系的产生。     

Synthesis and mesomorphic properties of rigid-core ionic liquid crystals

Ionic liquid crystals combine the unique solvent properties of ionic liquids with self-organization found for liquid crystals. We report a detailed analysis of the structure-property relationship of a series of new imidazolium-based liquid crystals with an extended aromatic core. Investigated parameters include length and nature of the tails, the length of the rigid core, the lateral substitution pattern, and the nature of the counterion. Depending on the molecular structure, two mesophases were observed: a bilayered SmA2 phase and the more common monolayered SmA phase, both strongly interdigitated. Most materials show mesophases stable to high temperatures. For some cases, crystallization could be suppressed, and room-temperature liquid crystalline phases were obtained. The mesomorphic properties of several mixtures of ionic liquid crystals were investigated. Many mixtures showed full miscibility and ideal mixing behavior; however, in some instances we observed, surprisingly, complete demixing of the component SmA phases. The ionic liquid crystals and mixtures presented have potential applications, due to their low melting temperatures, wide temperature ranges, and stability with extra ion-doping.     

Synthesis and Mesophase Behavior of Phenylthiophene Based Amphiphilic Molecules

Novel amphiphilic molecules consisting of a rigid 2‐phenylthiophene core, with a polar flexible tri(oxylethylene) moiety attached to the phenyl ring and one or two alkyl chains attached to the thiophene ring at the other side have been synthesized by using Ni(II) and Pd(0) catalyzed coupling reaction as key steps. The tri(oxylethylene) moieties were terminated with hydroxyl group, sodium carboxylate group and lithium carboxylate group respectively. The thermotropic and solvent induced liquid crystalline behavior of these substances was investigated by polarized optical microscopy, differential scanning calorimetry and X‐ray diffraction. Thereby the influence of the terminal groups attached to the tri(oxylethylene) moities as well as the influence of the length and the number of the alkyl chains on the mesophase behavior were investigated. The single alkyl chain Na‐carboxylate termianted derivatives show smectic A phases, double alkyl chain Na‐carboxylate terminated derivatives show a thermo tropic hexagonal columnar mesophase, while columnar mesophases are found in both single and double alkyl chain Li‐carbonate terminated derivatives. The model for molecular organization in the hexagonal columnar mesophase is established.     

Cholesterol-based dimeric liquid crystals: synthesis and mesomorphic behaviour

Chiral non-symmetric dimeric liquid crystals consisting of a cholesteryl ester moiety as chiral entity and a biphenyl aromatic core, interconnected through n-butyl (C₄) or n-pentyl (C₅) parity alkylene spacers, have been synthesized and investigated for their liquid crystalline properties. All the dimers exhibit enantiotropic mesophases. The first member of the dimers having the C₄ central spacer exhibit only the chiral nematic (N*) mesophase, while the higher homologues also show smectic A (SmA) and twist grain boundary (TGB) mesophases. The dimers of the other series containing the C₅ central spacer also have stable SmA, TGB and N* mesophases, except for the first which does not show the TGB phase. Both series of compounds show a weak odd-even effect with terminal alkyl chain substitution, while the spacer length has a marked influence on the phase transition temperatures.     

Correlated layer structures: a novel type of liquid crystalline phase with 2D-lattice

A novel liquid crystalline quaternary five-block molecule is reported which is composed of four incompatible molecular parts, a rigid biphenyl core, two polar 2,3-dihydroxypropoxy groups in the terminal 4- and 4'-positions, and a branched semiperfluorinated chain in the lateral 3-position, consisting of a perfluorinated and a lipophilic hydrocarbon wing. The self-organization of this compound was studied by polarized light optical microscopy, differential scanning calorimetry, and X-ray diffraction of aligned samples. These investigations confirm a novel liquid crystalline phase with two-dimensional (2D) lattice (columnar mesophase), which results from the positional correlation of smectic layers. The layer structure results from the segregation of the bolaamphiphilic parts from the side chains. Within the aromatic sublayers the biphenyl cores are arranged parallel to the layer planes, and the hydrogen-bonding networks of the terminal diol groups are segregated from the biphenyl cores, forming separate columns. The correlation between adjacent layers is due to the (partial) segregation of the fluorinated and hydrogenated parts of the lateral chains in the nonpolar sublayers.     

New side chain cholesterol-functionalised aliphatic polycarbonate copolymer: synthesis and phase behaviour

A new side cholesterol-functionalised liquid crystal (LC) copolymer based on aliphatic polycarbonate backbone was synthesised. The chemical structures of the block copolymers obtained in this study were characterised with Fourier Transform Infrared Spectroscopy (FT-IR) and Proton Nuclear Magnetic Resonance (¹H NMR) spectra. Their thermal stability and phase behaviours were investigated with thermogravimetric analysis (TGA) measurements, differential scanning calorimetry, and polarising optical microscopy. The molecular organisation in the mesophase was studied by temperature-dependent X-ray diffraction (XRD). As a result, the block copolymer bearing side cholesteryl groups showed a glass transition at 15.8°C and a smectic A (SmA) to isotropic phase transition at 151.3°C on heating cycle. XRD indicated that the block LC copolymer showed an interdigitated molecular arrangement of the mesogenic units within the smectic layers. This partial bilayer structure was similar to the SmA phase formed by polar mesogens.     

Liquid crystal dimers possessing chiral rod-like anisometric segments: synthesis, characterization and electro-optic behaviour

Several new optically active liquid crystal dimers comprising pro-mesogenic cholesterol and a chiral diphenylacetylene (tolane) segment, covalently linked in an end-to-end fashion through a flexible spacer, have been synthesized and investigated for their mesomorphic behaviour with the aid of optical, calorimetric and X-ray diffraction studies. Five unsymmetrical dimers, designed on the basis of recent work, involve molecular structural variations of the tolane mesogenic entity with a view to stabilizing a wide thermal range smectic A (SmA) phase featuring the electroclinic effect. Three different chiral chains, namely, (S)-1-methylheptyloxy, (S)-2-methylbutyloxy, (3S)-3,7-dimethyloctyloxy, with or without polar (nitro or fluoro) lateral substituents, were incorporated, while keeping the length (C₆) of the spacer constant. As expected, all the dimers exhibited a SmA phase. A few also showed chiral nematic (N*) and/or twist grain boundary and/or chiral smectic C (SmC*) phases. Remarkably, some of these oligomesogens, upon melting, had a stable SmA phase over a wide thermal interval (100-150°C); this state seems to be stable for a long period of time. Electro-optic studies, including optical tilt angle as well as temporal response as a function of temperature, were carried out in the SmA phase. The SmC* phase was also investigated for its electrical switching and optical tilt angle, as well as spontaneous polarization as a function of temperature. These studies showed that the mesophase response to an applied field is weak and is independent of variations in the dimer investigated.     

Induction of smectic A phases with calamitic molecules and 2,4,7-trinitrofluorenone: the influence of branching and chain length

Binary systems consisting of 2,5-diphenyl-1,3,4-thiadiazole derivatives incorporating an allene unit in one of the terminal chains and the electron acceptor 2,4,7-trinitrofluorenone (TNF) have been investigated. Though the diphenylthiadiazole cores do not represent typical electron donor units, the nematic and smectic C phases observed for the pure compounds were suppressed and replaced by smectic A phases which in most cases have a higher stability than the nematic phases of the pure compounds. The substitution pattern around the allene moiety allowed a systematic study of the influence of steric effects on the mesophase induction by TNF. Compounds with long and especially those with branched terminal chains can take up a larger number of TNF molecules and can reach a higher stability of the induced SmA phase than those with shorter and unbranched chains. The induction of SmA phases is explained as the result of attractive intermolecular interaction between the diphenylthiadiazole rigid cores and TNF molecules provided by donor-acceptor interactions and quadrupole interactions, as well as a consequence of microsegregation and space filling effects.     

Liquid crystals derived from 2-phenylisoindoles: synthesis and characterization

2-Phenylisoindole was investigated as the rigid core unit in a series of asymmetric mesogenic molecules. When the 2-phenylisoindole core was terminated with a hexyl tail, no mesophase formation could be observed. When 4- n -(tridecafluorohexyl) was used, however, both monotropic and enantiotropic phase behaviour were observed. It was found that most functionalities at the anhydride 5-position results in the formation of smectic A (SmA) phases in the temperature range 70-180°C. Functionalities at the anhydride 4-position suppress mesophase formation. Large substituents (-Br, -NO2) and symmetric substitution patterns (5,6-dichloro, 4,7-dichloro and 4,5,6,7-tetrachloro) on the anhydride moiety increase the melting point and destabilize the mesophase. Temperature dependent X-ray diffraction experiments suggest an interdigitated SmA packing for this family of compounds.     

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)).     

Azomesogens with polar chloro,nitro and phenolic –OH substituents

Two new mesogenic homologous series containing chloro, nitro and phenolic hydroxy groups were synthesised and their molecular structures characterised by a combination of element analysis and standard spectroscopic methods. The mesomorphic behaviour of the compounds of both series was investigated by polarising optical microscopy and, in some cases, differential scanning calorimetry. All the compounds of both the series exhibit an enantiotropic nematic mesophase. In series I higher members also exhibit an enantiotropic smectic A (SmA) mesophase, whereas in series II the enantiotropic SmA mesophase commences somewhat earlier for the middle members. The mesomorphic properties of both series are compared with each other and also with the properties of other structurally related series to evaluate the effects of different polar substituents on mesomorphism.     

Self-assembly of amphiphilic liquid crystal polymers obtained from a cyclopropane-1,1-dicarboxylate bearing a cholesteryl mesogen

We study the self-assembly of a new family of amphiphilic liquid crystal (LC) copolymers synthesized by the anionic ring-opening polymerization of a new cholesterol-based LC monomer, 4-(cholesteryl)butyl ethyl cyclopropane-1,1-dicarboxylate. Using the t-BuP(4) phosphazene base and thiophenol or a poly(ethylene glycol) (PEG) functionalized with thiol group to generate in situ the initiator during the polymerization, LC homopolymer and amphiphilic copolymers with narrow molecular weight distributions were obtained. The self-assemblies of the LC monomer, homopolymer, and block copolymers in bulk and in solution were studied by small-angle X-ray scattering (SAXS), differential scanning calorimetry (DSC), polarizing optical microscopy (POM), and transmission electron microscopy (TEM). All polymers exhibit in bulk an interdigitated smectic A (SmA(d)) phase with a lamellar period of 4.6 nm. The amphiphilic copolymers self-organize in solution into vesicles with wavy membrane and nanoribbons with twisted and folded structures, depending on concentration and size of LC hydrophobic block. These new morphologies will help the comprehension of the fascinating organization of thermotropic mesophase in lyotropic structures.     

Solvent-Free Mechanochemical Synthesis of High Transition Biphenyltetracarboxydiimide Liquid Crystals

A series of high temperature alkyl and alkoxy biphenyltetracarboxydiimide liquid crystals have been prepared under ball mill method using solvent-free mechanochemical approach. The thermal properties of the prepared compounds were investigated by deferential scanning calorimetry (DSC) measurements and the textures were identified by polarized optical microscope (POM). The compounds showed smectic mesomorphic behaviour. The results showed the increasing nature of transition temperature Cr-SmC with chain length with increments of the SmC mesophase range. However, the mesophase range of the SmA was decreased with the terminal chain length either for the alkyl or alkoxy terminal groups. Moreover, the DFT theoretical calculations have been conducted give a detailed projection of the structure of the prepared compounds. A conformational investigation of the biphenyl part has been studied. A deep illustration of the experimental mesomorphic behaviour has been discussed in terms of the calculated aspect ratio. A projection of the frontier molecular orbitals as well as molecular electrostatic potential has been studied to show the effect of the polarity of the terminal chains on the level and the gap of the FMOs and the distribution of electrostatic charges on the prepared molecules.     

Amphiphilic N-benzoyl-1-amino-1-deoxy-D-glucitol derivatives forming thermotropic lamellar, columnar and different types of cubic mesophases

Novel amphiphilic glucamine derivatives have been synthesized. These are N-benzoyl-1-deoxy1-methylamino-D-glucitols and N-benzoyl-1-amino-1-deoxy-D-glucitols carrying one, two or three aliphatic chains (CnH2n 1O- with n 3, 6 and 12) grafted to the benzamido group. The thermotropic mesophases of these compounds were studied by thermal polarizing optical microscopy and differential scanning calorimetry, and some also by X-ray scattering. Depending on the number and the length of the alkyl chains lamellar, bicontinuous cubic, hexagonal columnar or inverted micellar cubic mesophases were detected by analogy with lyotropic systems. In the contact region between lamellar phases of the single chain amphiphiles and micellar cubic phases of the mesomorphic triple chain compounds, hexagonal columnar phases can be induced. A hexagonal columnar phase was also induced in the contact region between a bicontinuous and a micellar cubic mesophase. The lyotropic liquid crystalline behaviour of the dodecyloxy substituted N-benzoyl-1-deoxy-1-methylamino-D-glucitols was investigated by the solvent penetration method using ethylene glycol as protic solvent. On increasing the solvent content, the double chain compound forms a cubic and a lamellar mesophase and the triple chain compound forms a hexagonal columnar lyomesophase. The dodecyloxy substituted compounds were also investigated with respect to their behaviour as thin films at the air-water interface using a Langmuir film-balance. Different types of pi/Aisotherms were observed whereby the molecular areas at collapse were determined either by the size of the carbohydrate head group (single chain compounds) or by the number of alkyl chains (double and triple chain compound).     

Hole mobility and lifetime in a smectic liquid crystalline photoconductor of a 2-phenylnaphthalene derivative

We have investigated hole transport properties in the smectic mesophases of a 2-phenylnaphthalene derivative 6-(4'-octylphenyl)-2-dodecyloxynaphthalene in detail by using time-of-flight technique. The transient photocurrents were measured in liquid-crystal cells with various thickness from 5 to 700 microm. They were well defined and nondispersive in the smectic A (SmA) phase up to 500 microm and in the smectic B (SmB) phase within the entire thickness employed, while they exhibited an exponential decay in the SmA phase at 700 microm. The mobilities in the SmA and SmB phases were constant in each mesophase irrespective of the cell thickness, and were 2.5 x 10(-4) and 1.7 x 10(-3) cm2V s, respectively. The hole lifetimes were determined to be 10 ms and longer than 5 ms for the SmA and SmB phases, respectively. We discuss the origin of these lifetimes from the two points of view, i.e., hole trapping by a trace amount of existing impurities and recombination with negative ionic charges. We conclude that impurities are mainly responsible for the present hole lifetime test.