Tetrabenzocyclododecatetraene A new core for mesogens exhibiting columnar mesophases

Octasubstituted derivatives of tetrabenzocyclododecatetraene are found to exhibit thermotropic mesophases. The molecules of these mesogens are highly flexible with an average four-fold symmetry. Optical and differential scanning calorimetry measurements are presented on five ester and ether derivatives. All mesophases appear to be columnar with a negative optical anisotropy. From the five compounds studied the four which are derived from aliphatic ester and ether are optically uniaxial, while the mesophase of the decylbenzoyloxy derivative could be biaxial. Miscibility measurements performed with some discotic and pyramidic mesogens of similar molecular dimensions suggest that the mesophases of the tetrabenzocyclododecatetraene core form new families of columnar mesophases.     

Tetrabenzocyclododecatetraene A new core for mesogens exhibiting columnar mesophases

Abstract

Octasubstituted derivatives of tetrabenzocyclododecatetraene are found to exhibit thermotropic mesophases. The molecules of these mesogens are highly flexible with an average four-fold symmetry. Optical and differential scanning calorimetry measurements are presented on five ester and ether derivatives. All mesophases appear to be columnar with a negative optical anisotropy. From the five compounds studied the four which are derived from aliphatic ester and ether are optically uniaxial, while the mesophase of the decylbenzoyloxy derivative could be biaxial. Miscibility measurements performed with some discotic and pyramidic mesogens of similar molecular dimensions suggest that the mesophases of the tetrabenzocyclododecatetraene core form new families of columnar mesophases.     

Substituted tetrabenzocyclophanes as mesogenic units of new polycondensates exhibiting columnar mesophases

Recently a new family of flexible mesogens has been described namely hexa-alkylated derivatives of cyclotetraveratrylene. We have functionalized these compounds, obtaining monofunctionalized and bifunctionalized derivatives with heptyloxy side groups. A model dimer and polycondensates were synthesized. Calorimetric data as well as optical data prove that they exhibit columnar meso-phases. At this point it is not clear whether the conformation of the mesogenic units in the mesophase is tetragonal pyramidic with all of the mesogens being extended in one direction or discotic because of the high internal flexibility of the mesogenic core. The synthesis of the model dimer and the polycondensates as well as the calorimetric and optical data are the subject of this article.     

Novel zigzag-shaped compounds exhibiting apolar columnar mesophases with oblique and rectangular lattices

The design, synthesis and characterisation of some compounds whose constituent molecules have a completely new shape have been carried out. A total of eight different series/part of series of these zigzag-shaped compounds that are either symmetrical or unsymmetrical were synthesised. Only two mesophases were observed, and on the basis of polarising optical microscopy, differential scanning calorimetry, X-ray diffraction and electro-optical studies, these have been characterised as columnar phases with a rectangular or an oblique lattice. Perhaps, these represent the first examples of a rigid zigzag-shaped compound exhibiting a mesophase.     

New inductions of columnar mesophases in nematic discotic multiyne mesogens [1]

In examples of two large naphthalene- and triphenylene-centred hexaynes exhibiting only the nematic discotic (N_D) type of mesophase, a new kind of phase induction was observed. These disc-shaped nematogens were doped with numerous dipolar carbocyclic compounds leading to the induction of two types of columnar mesophase (i.e. D_ro and D_ho). The phase behaviour of these binary systems, depending on the structure of the dopant applied, was studied by polarizing microscopy.     

Discotic salicylaldimato metal complexes exhibiting columnar mesophases and their mixtures with rod-like salicylaldimato complexes

The synthesis and mesomorphism of a series of discotic metal complexes of salicylaldimine ligands is reported. None of the ligands was mesomorphic, but the longer chain length complexes showed a Col_h phase as evidenced by optical microscopy. Phase diagrams were constructed using rod- and disk-like salicylaldimato complexes, where one was designed to have Lewis acid characteristics and the other Lewis base properties. It was hoped that an associative Lewis acid/base interaction would prevent phase separation of the two components, which may point a way to the realization of a mixture with a biaxial nematic phase. The form of the phase diagrams is discussed.     

New polymer liquid-crystalline CdS nanocomposites forming a chiral nematic phase

A new approach has been developed for the design of liquid-crystalline polymer nanocomposites combining the unique properties of polymer cholesterics and quantum dots of CdS.     

Dibenz[a,c]anthracene derivatives exhibiting columnar mesophases over broad temperature ranges

We report the synthesis and characterization of a series of novel hexaalkoxydibenz[a,c]anthracenes. While the parent compound is not mesomorphic, the introduction of substituents in the 10- and 13-positions yields compounds with columnar liquid crystalline phases over very broad temperature ranges.     

Liquid crystalline dendrimer of the fifth generation: From lamellar to columnar structure in thermotropic mesophases

The structure of a liquid crystalline (LC) carbosilane dendrimer of the fifth generation bearing 128 terminal cyanobiphenyl mesogenic groups has been studied. This dendrimer was synthesized by a hydrosilylation reaction and then the cyanobiphenyl mesogenic groups were chemically linked to the dendritic matrix via a-OOC-(CH2)10-Si(CH3)2OSi(CH3)2-spacer. Structural studies carried out by polarizing optical microscopy, differential scanning calorimetry and X-ray diffraction methods revealed unusual phase behaviour. At room temperature the dendrimer forms a lamellar (smectic A) phase which develops in-plane ordering above 40C.This is due toa tendency to form columns ofmolecules which areprobably perpendicular to the layers. Above 121C the material transforms into another more disordered mesophase which is probably a disordered hexagonal columnar phase. The proposed structures and molecular packing in these different types of mesophase are discussed.     

An X-ray diffraction study of the different nematic mesophases of liquid-crystalline poly(urethane-ester)s

An x-ray investigation on powder specimens and stretched oriented fibers of poly(urethaneester)s TDI-CmCn, derived from various mesogenic alkylene di[4-(ω-hydroxyalkyloxy-4-oxybenzoyl)oxybenzoate]s (CmCn; m = 2, 4, or 6, and n = 4, 6, 8, or 10) and 2, 4-toluenediisocyanate (TDI), is reported. Evidence is provided for the formation of two different nematic mesophases in the polymers, namely a cybotactic nematic and a conventional nematic mesophase. Whereas samples TDI-C2C6, TDI-C6C4, and TDI-C6C10 formed one cybotactic nematic mesophase, samples TDI-C4C6, TDI-C6C6, and TDI-C6C8 exhibited both cybotactic nematic and conventional nematic mesophases in a sequence with increasing temperature, which were connected by a first-order transition. The analysis of the various features of the small-angle x-ray diffraction patterns indicates that two structural arrangements, namely smectic C-like and conventional nematic structures, coexist inside the cybotactic nematic mesophase of these poly(urethane-ester)s. © 1995 John Wiley & Sons, Inc.     

Induction of cholesteric mesophases in nematic liquid crystals by some chiral aryl alkyl carbinols : A quantitative investigation

A series of chiral aryl alkyl carbinols, dissolved in MBBA and other nematic solvents, induce cholesteric structures. The handedness of the induced mesophases and the twisting power of the dopant alcohols are studied by means of CD and Grandjean-Cano microscopic techniques. The investigation points out the possibility of obtaining information on the stereochemistry of the dopant by studying the characteristics of the induced helices.Two different types of H- bonds between alcohols and MBBA are discussed in connection with a possible model of induction.     

Re-entrant isotropic phase between lamellar and columnar mesophases

Tetradental cis-enaminoketone Ni (II) complexes with different molecular shape have been synthesized. Intramolecular hydrogen bonds, which stiffen the mesogenic core and restrict rotation of some molecular parts, have been introduced in these compounds. In the case of molecules with two hydrogen bonds and alkoxy terminal chains filling the inner molecular space, the uncommon phase sequence Iso-D(h)-Iso(re)-SmA (series III-3) was detected. For the first time, it was observed that the isotropic re-entrant (Iso(re)) phase (short-range order) is separating the columnar (D) (high-temperature) and the lamellar (SmA) (low-temperature) phases, both revealing long-range ordered structures.     

Freeze-fracture electron microscopy of thermotropic cubic and columnar mesophases

Freeze-fracture electron microscopy has been applied to study thermotropic columnar and cubic phases of a polycatenar liquid crystal of silver(I). The columnar phases displayed hexagonal type II characteristic fracture planes. The cubic phases showed a fracture behaviour highly characteristic of the Ia 3 d type II (Q230II) cubic space group, namely the high frequency of well ordered domains perpendicular to the \[2 1 1] crystallographic axis and, much less frequently, the fragmented aspect of domains perpendicular to the \[1 1 0] crystallographic axis. For both types of domain, the observed 2-D lattice parameters were in excellent agreement with the ideal values expected for the Q230 cubic space group.     

Highly birefringent nematic and chiral nematic liquid crystals

We report a simple interference method to determine the dispersion of the extraordinary refractive index and birefringence of highly conjugated and coloured nematic liquid crystals used as light-emitting materials in organic electroluminescent devices. The measurements are made in the nematic glass phase at room temperature. The birefringence is highly dispersive and values up to 1.1 are obtained. Chiral groups are incorporated into the end chains giving a chiral nematic liquid crystal with a very wide stopband in the visible region. The Berreman matrix method is used to simulate transmission through the chiral nematic liquid crystal cell using the refractive index parameters obtained experimentally. Excellent agreement between theory and experiment is found.     

Highly birefringent nematic and chiral nematic liquid crystals

We report a simple interference method to determine the dispersion of the extraordinary refractive index and birefringence of highly conjugated and coloured nematic liquid crystals used as light‐emitting materials in organic electroluminescent devices. The measurements are made in the nematic glass phase at room temperature. The birefringence is highly dispersive and values up to 1.1 are obtained. Chiral groups are incorporated into the end chains giving a chiral nematic liquid crystal with a very wide stopband in the visible region. The Berreman matrix method is used to simulate transmission through the chiral nematic liquid crystal cell using the refractive index parameters obtained experimentally. Excellent agreement between theory and experiment is found.     

Low-frequency dielectric relaxation in columnar hexagonal and micellar inverse cubic mesophases

Dielectric measurements on four diols showing columnar hexagonal and micellar cubic phases were carried out. Besides the known fast local reorientation in theGHz-range, a new absorption in theMHz region was detected. This mechanism was interpreted as a collective motion of the molecules inside the micelles or cylindrical aggregates. The absorption was also found in the isotropic phase. Based upon the interpretation given, the molecular aggregates should also exist in the isotropic state. The electrical conductivity strongly supports the classification as inverse phases.     

Equilibria of extended-chain polymers exhibiting crystalline and liquid-crystalline phases

The phase diagram of poly(p-benzamide) (PBA) in N,N-dimethylacetamide/LiCl solutions was determined for two PBA samples having weight-average molecular weights of about 10,000. The various equilibria were studied using analytical, viscometric, and optical microscope measurements. The phase diagram at 25°C, taking as variables the concentration of polymer (C_p) and LiCl (C_s), involves several equilibria which can be summarized as follows: solid ? isotropic solution when 0.75 ≤ C_s ≤ 2 g/dl, solid ? anisotropic solution when 2 ≤ C_s ≤ 4 g/dl, isotropic solution ? anisotropic solution when 2 ≤ C_s ≤ 4 g/dl, and C_p > 6 g/dl, and dilute isotropic solution ? gel when C_s > 4 g/dl. In the C_p range in which the isotropic and anisotropic phases coexist, enrichment of the high-molecular-weight component of the polymer in the anisotropic phase becomes more marked as the volume fraction of the latter phase is decreased. The two PBA samples exhibit noticeable differences in solubility, absolute viscosity, and in their viscosity-concentration behavior. The location of the maximum in the latter dependence does not necessarily coincide with the first appearance of the anisotropic phase. In the absence of a flow field, anisotropic solutions exhibit an irreversible increase in viscosity. Inclusion of the equilibria involving the crystalline state furnishes insight into some of the common observations for extended-chain polymers. A diagram illustrates the superposition of the solubility curves for a crystalline polymer and the liquid-crystal regions. This indicates that, for the high melting crystalline polymers, the crystalline phase should be stable relative to the concentrated anisotropic phase of the wide biphasic region.     

Thermally stable azo-substituted bent-core nematogens: observation of chiral domains in the nematic mesophases composed of smectic nano clusters

In an effort to synthesise thermally stable bent-core nematogens with fairly low transition temperatures and wide nematic mesophase range, we have synthesised five new series of azo-substituted bent-core compounds without Schiff’s base unit. Here, we studied the effect of different lateral substituents (–CH₃ and –Cl), at two different positions in one of the arms, on the mesogenic properties. We found that such variation in the molecular architecture has a clear-cut impact on the transition temperatures as well as the liquid crystalline properties. The mesogenic properties of these compounds were studied using polarising optical microscopy and differential scanning calorimetry. The exact nature of different mesophases is investigated using X-ray diffraction studies. The results obtained indicated the presence of smectic nano clusters in the nematic mesophases of these compounds. We observed domains of opposite handedness in these nematic phases. The unusual properties of these mesophases were investigated by electro-optical studies. The electro-convection pattern study showed that these nematic mesophases are of negative dielectric anisotropic in nature. The compounds synthesised here exhibit photo-switching both in solution and in their nematic mesophases, a property that can be exploited for practical applications.     

Synthesis and characterization of fluorinated liquid-crystalline elastomers containing chiral liquid-crystalline crosslinking units

Fluorinated chiral liquid-crystalline elastomers (LCEs) were graft copolymerized by a one-step hydrosilylation reaction with polymethylhydrogenosiloxane, a fluorinated LC monomer 4-(2,2,3,3,4,4,5,5,6,6,7,7,8,8,8-pentadecafluorooctanoyloxy)phenyl 4-(undec-10-enoyloxy)benzoate (PPUB) and a chiral crosslinking LC monomer (3R,3aR,6S,6aR)-6-(undec-10-enoyloxy)hexahydrofuro[3,2-b]furan-3-yl 4′-(4-(allyloxy)benzoyloxy)biphenyl-4-carboxylate (UHAB). The chemical structure, liquid-crystalline behavior and polarization property were characterized by use of various experimental techniques. The effective crosslink density of the LCEs was characterized by swelling experiments. The thermal analysis results showed that the temperatures at which 5% weight loss occurred were greater than 250 °C for all the LCEs, and the residue weight nearby 600 °C increase with increasing chiral crosslinking components in the polymer systems. All the samples showed chiral smectic C mesophase when they were heated. The glass transition temperature and mesophase-isotropic phase transition temperature of fluorinated elastomers increased slightly with increase of chiral crosslinking mesogens in the polymer systems, but the enthalpy changes of mesophase-isotropic phase transition decreased slightly. In XRD curves, all the samples exhibited strong sharp reflections at small angles suggesting smectic layered packing arrangement. These fluorinated chiral LCEs showed 0.1–0.2 μC/cm² of spontaneous polarization with increasing chiral crosslinking component.     

Amphiphilic taper-shaped oligomer exhibiting a monolayer smectic A to columnar phase transition

We prepared two series of liquid-crystalline oligomers composed of phenylpyrimidine based mesogenic cores, alkyl spacers and semiperfluorinated alkyl chains in one series and only alkyl chains in the second series. Their physical properties were investigated using optical microscopy, differential scanning calorimetry and X-ray diffraction measurements. The oligomers possessing a semiperfluorinated alkyl chain show a phase sequence of SmC–SmA–Col when the number of phenylpyrimidine moieties was increased. The compound possessing three phenylpyrimidine cores was found to exhibit monolayer SmA and Col phases. On the other hand, the oligomer composed of an alkyl chain and three phenylpyrimidine cores showed only a nematic phase. We discuss effects of the shape amphiphilicity and the hydrocarbon–fluorocarbon amphiphilicity on phase transition behaviour of the amphiphilic oligomers.