Columnar Liquid Crystal Phase Research Articles

Features of the magnetic and dielectric behavior of mesophases of chromium (III) complexes with azacyclic ligands

Two paramagnetic (1, 2) chromium metallomesogens were studied by the EPR, magnetic-susceptibility, and dielectric-spectroscopy methods. These mesogens possess columnar liquid-crystal phase polymorphism. For mesogen 1 in the columnar Colxd phase, an irreversible structural phase transition was detected by the EPR method. An anomalous nonlinear temperature dependence of the resonance fields of the EPR line positions and of the fine-structure parameter D in the mesophases of compound 1 was observed. It was shown that this anomaly is associated with the soft mode of crystal lattice. The dielectric spectroscopy data provide support for the existence of the soft mode, which is probably due to the transition of the system from a paraelectric to a dipole-ordered state. The structure of the hexagonal columnar plastic phase of mesogen 2 is formed by a single type of paramagnetic monomer chromium center and does not exhibit any specific features.

CPI (complementary polytopic interaction) stabilised liquid crystal compounds formed by esters of 2­hydroxy­3,6,7,10,11­pentakis(hexyloxy)triphenylene

An improved synthesis of 2-hydroxy-3,6,7,10,11-pentakis(hexyloxy)triphenylene 6 is reported which is based on the oxidative coupling of 2-acetoxy-1-hexyloxybenzene 3 to 3,3′,4,4′-tetrakis(hexyloxy)biphenyl 4. Most of the esters of this phenol 7–15, give an enantiotopic Colh columnar liquid crystal phase, which is stable over a wide temperature interval. This can be further enhanced by formation of 1 ∶ 1 CPI compounds with either 2,3,6,7,10,11-hexakis(4-nonylphenyl)triphenylene {PTP9} 1 or hexakis(4-nonylphenyl)dipyrazino[2,3-f:2′,3′-h]quinoxaline {PDQ9} 2. As with other triphenylene derivatives, these additives can also be used to induce liquid crystal behaviour in otherwise non-mesogenic esters.

Thermotropic liquid crystal behaviour of alkaline-earth-metal dihexadecyl phosphate salts

Alkaline-earth-metal dihexadecyl phosphate salts were synthesized, and their thermal stability was evaluated by thermogravimetry. Their thermotropic liquid crystal behaviour was investigated by differential scanning calorimetry, polarizing optical microscopy, dilatometry, and X-ray diffraction. On heating, the calcium, strontium, and barium salts exhibit columnar liquid crystal phases, whereas the less ionic beryllium and magnesium salts melt directly into isotropic liquids.

An all-columnar bilayer light-emitting diode

The first organic light-emitting diode with both an electron-rich columnar liquid crystal as hole transport material and an electron-deficient fluorescent columnar liquid crystal as electron transport material is presented. Red fluorescence is observed above 10 V from the configuration indium–tin oxide (ITO)/hexabutoxy-triphenylene/tetraethyl perylenetetracarboxylate/Al. The excellent charge transport properties of columnar phases are thus exploited for light-emitting diodes. Both organic materials exhibit their columnar liquid crystal phase well above room temperature and the underlying ordered solid columnar phase is used in the diode. The I( V) and L( V) characteristics are very reproducible from device to device, with an emission peak at 620 nm and an FWHM of 80 nm, a current rectification ratio of about 30, I∼ V 2 at low voltages and I∼ L∼ V 7 at higher voltages. Initial lifetime measurements are encouraging.

Columnar phases and field induced biaxiality of a Gay–Berne discotic liquid crystal

We have studied a system of discotic particles with a central transverse dipole using Monte Carlo (MC) computer simulations at constant pressure. We have investigated several temperatures corresponding to nematic and columnar liquid crystal phases and determined the molecular and dipolar organizations. Low temperature columnar phases are characterized by local biaxial ordering of dipoles, even if the system is on the whole uniaxial. However, simulations in the presence of a transverse field for different field and dipole strengths show that the system has a large susceptibility and easily becomes biaxial, making it potentially interesting for switching applications. An explicit formulation for the susceptibility in terms of biaxial invariants is derived.

Eight-Vertex Metallomesogens: Zirconium Tetrakis-β-diketonate Liquid Crystals

Square antiprism zirconium tetrakis-β-diketonate complexes with 24 alkoxy chains organize in columnar liquid crystal phases. X-ray diffraction and polarized microscopy studies on complexes with n-alkoxy side chains revealed a columnar hexagonal phase. These sandwich-shaped compounds have much lower transition temperatures than their discotic analogues, which leads to the desirable attribute of room temperature liquid crystallinity. The addition of two branching methyl groups to the alkoxy chains was found to dramatically alter the properties of these materials. The branched side chain analogues exhibited a higher clearing point while the liquid crystallinity is maintained at room temperature. The branching methyl groups also induced a bulk reorganization of the material to a rare columnar oblique phase (Colob).

Self-Organization and Formation of Liquid Crystal Phases by Molecular Templates Related to Membrane Components of Archaebacteria.

Tubular supramolecular aggregates are formed from glycolipids 1, which are closely related to natural components of membranes of archaebacteria. The glycolipids exhibit disordered columnar thermotropic and hexagonal lyotropic liquid crystal phases. Whereas formation of the mesophases is insensitive to stereochemical factors, formation of the tubules is dependent on the configuration of the stereocenters that are shown in the picture but not specified.

A rational synthesis of polyacrylates with discogenic side groups

The ferric chloride mediated oxidative coupling of 3,3,4,4-tetrahexyloxybiphenyl with 2-hexyloxyanisole followed by demethylation gave 3,6,7,10,11-pentahexyloxy-2-hydroxyor triphenylene. Reaction with tert -butyldimethylsilyl-protected 6-bromohexanol, removal of the protecting group, and condensation with acryloyl chloride methacryloyl chloride gave acrylate and methacrylate monomers with hexamethylene 'spacers'. Alternatively, reaction of 3,6,7,10,11-pentahexyloxy-2-hydroxytriphenylene with 2-(2-chloroethoxy)ethanol followed by acryloyl chloride or methacryloyl chloride gave acrylate and methacrylate monomers with diethylenoxy 'spacers'. Both of the poly(acrylate) homopolymers and the poly(methacrylate) homopolymer with the diethylenoxy 'spacer' gave columnar liquid crystal phases. This is contrary to the current perception that the methacrylate polymer backbone is too inflexible to be incorporated in columnar phases.

Electron transport across metal/discotic liquid crystal interfaces

Electron transport across micron thick films of columnar hexagonal discotic liquid crystal phases homeotropically aligned between metal electrode surfaces has been studied both experimentally and theoretically. These molecules are unique in their combination of charge transport along individual molecular columns with liquidlike self-organization. Typical of organic insulators, a high resistance Ohmic regime is evident at fields of less than 0.05 MV cm−1, due to a low concentration of chemical impurities (n<109 cm−3), and a space-charge injection regime at higher fields. Breakdown fields are reasonably high: in hexakis(hexyloxy)triphenylene they reach ∼5 MV cm−1 at room temperature. Our results show that triphenylene-based discotics form an excellent class of highly ordered optically transparent insulators. At high temperatures and high fields the current is injection controlled and exhibits typical tunneling and space charge limited, nonlinear I–V characteristics. Dramatic jumps in injection currents are observed at phase transitions. The change at the crystalline to liquid crystalline phase transition is mainly due to more efficient “wetting” of the electrode surface in the liquid crystalline phase, whilst at the liquid crystalline to isotropic phase transition it arises from the enhancement in the molecular mobility. The concepts of semiconducting gaps, band mobilities, and carrier injection rates are extended to these new materials. The experimental observations are interpreted in a framework which takes into account the important role played by liquidlike dynamics in establishing the microscopic structural order in, what is, otherwise a highly anisotropic and weakly bonded “molecular crystal.”

Columnar Mesophases from Nondiscoid Metallomesogens: Rhodium and Iridium Dicarbonyl Diketonates

A series of nondiscoid rhodium and iridium dicarbonyl β-diketonate complexes have been shown to organize in columnar liquid crystal phases with hexagonal disordered structures (Colhd). The shape and dipolar attributes of these materials produce highly correlated antiparallel pairwise arrangements. This organization was investigated by X-ray diffraction and miscibility studies. The liquid crystallinity was found to be stabilized by decreasing the number of carbon atoms in the side chains, and single component room-temperature liquid crystals were developed. The nature of the metal is also important, and iridium mesogens exhibited higher clearing points than the analogous rhodium materials.

Monte Carlo simulation of discotic Gay–Berne mesogens with axial dipole

R. Berardi, S. Orlandi and C. Zannoni, J. Chem. Soc., Faraday Trans., 1997, 93, 1493 DOI: 10.1039/A607571A

Stabilization of Nondiscoid Columnar Liquid Crystals: Studies of Unsymmetrical Copper Bis-β-diketonates

A series of copper(bis-β-diketonate) complexes are reported which display columnar liquid-crystal phases with a hexagonal disordered structure (Dhd). The complexes do not have the disc-shape characteristic of most Dhd materials, but produce a disc shape by forming dimers with 90o rotations between nearest neighbors. In the liquid-crystalline state this dimerized nature produces short-range rotational correlations. Three side-chain copper bis-β-diketonates with a single phenyl substituent, 2, are not liquid crystalline, and it was found that an extension of the mesogenic core is necessary to introduce liquid crystallinity. The simple phenyl analogues, 3a, are monotropic, and the addition of electron-withdrawing substituents to the phenyl moiety, 3b−d, results in a stabilization of the mesophase. These substituents produce favorable dipolar interactions which stabilize the mesophase. Consistent with this explanation, electron-donating substituents are not effective at stabilizing the mesophase. Substitution of the complex with thiophene groups rather than phenyls, 4, produces stable mesophases with greatly lowered melting and clearing points. This latter result indicates that thiophene substitution provides dispersive forces which destabilize the crystal phase. Thiophene substitution may provide a general method for reducing transition temperatures in metallomesogens.

Binuclear liquid crystals incorporating dia- or para-magnetic transition metals

Abstract Discotic binuclear Ni(II) and Cu(II) complexes of 1,2,4,5-tetrakis-(2′-alkyl-2′-homoalkylcarbonylvinylamino)benzene ligands were synthesized. These compounds exhibit columnar liquid crystal phases.

Structure and conductivity of liquid crystal channel‐like linic complexes of taper‐shaped compounds

AbstractMolecular packing and electrical conductivity were studied in complexes of alkali trifluoromethanesulphonates with low‐molar or polymeric compounds containing both taper‐shaped mesogens were esters of either 3,4,5‐tris [p‐(n‐dodecan‐l‐yloxy) benzyloxy] benzoic acid (I) or 3, 4, 5‐tris (n‐dodecan‐l‐yloxy) benzoic acid (II). In the hexagonal columnar liquid crystal phase the tapered mesogens fan out from the centre of the column, with the ionic receptors forming the central channel and the aliphatic tails constituting the continuum matrix. In the case of side‐chain polymethacrylates the column core also contains the backbone chain. The DC conductivity σ of unoriented samples increases greatly at the crystal‐columnar transition, with only a minor further change upon columnar‐isotropic transition. σ was in the range 10−9 − 10−6 in the columnar phase 40–90 °C, whilst the activation energies for conduction were between 28 kcal mol−1 for the crown ether and only 2 kcal mo−1 for the complex of LiCf3SO3 with the non‐polymeric ester of tri(ethylene oxide) with I.

Incoherent quasi-elastic neutron scattering study of highly oriented fibres of copper laurate in the columnar mesophase

Oriented fibres of dicopper tetrakis(dodecanoate) Cu2(COOC11H23)4 were investigated using the neutron backscattering technique both in the columnar liquid crystal phase and in the crystalline phase. Differences in the spectra recorded at various glancing angles in the columnar mesophase, on the time-scale 5 × 10-10 s, were assigned to anisotropic motions of the binuclear units within the columns. In the crystalline phase, on the 10-10 time-scale, the fixed-window technique yielded a determination of the elastic incoherent structure factor.

Alkyl chain motions in columnar mesophases

The dynamical behaviour of the alkyl chains in dicopper tetrapalmitate is investigated in the columnar liquid crystal phase, using incoherent quasielastic neutron scattering by several selectively deuterated derivatives. A mathematical model is developed for the evaluation of the scattering function which yields the free volume accessible to each hydrogen atom along the chains, approximated by a sphere. It is shown that the hydrogen atoms linked to the carbons close to the core undergo very little motion. The radius of the spheres is a nearly linear function of the distance from the core between the first and the fifth carbons and then reaches a limiting value directly related to the spacing between the stacked molecules. These results were fully confirmed by the analysis of the overall quasielastic broadening of the spectra as a function of the momentum transfer.

Alkyl chain motions of dicopper-tetraalkanoates in the crystalline and columnar phases

The dynamical behaviour of alkyl chains in a series of copper(II) alkanoate complexes was studied by incoherent quasielastic neutron scattering, both in their crystalline and columnar liquid-crystal phases. This series included compounds with different lengths of the alkyl chains and with different numbers (4 or 8) of chains bonded to the central tetracarboxylic core. The spectra were analysed in terms of their purely elastic and quasielastic parts and experimental values of the elastic incoherent structure factor were obtained for the different temperatures. The latter is found to vary with both the temperature and the nature of the compound. Conclusions about the dynamics of the alkyl chains are drawn.

Disordered hexagonal-disordered rectangular phase transitions in HAT series

Group theoretical techniques are used to discuss the disordered hexagonal columnar-disordered rectangular columnar (D hd −D rd ) liquid crystal phase transition in HAT series. On the basis of Landau theory the transition is predicted to be second order. The Ginzburg-Landau-Wilson Hamiltonian corresponds to a universality class with an n=3 component order parameter and with cubic anisotropy. The critical behaviour near the D hd −D rd transition is discussed by referring to the renormalization group results of Aharony and other authors and we present evidence in favour of the critical value of n, n c , being greater than 3. It is also predicted that a second order transition from the D hd phase to an as yet experimentally undiscovered phase is possible Analyse de la transition en appliquant les techniques de la theorie des groupes

Freely Suspended Strands of Tilted Columnar Liquid-Crystal Phases: One-Dimensional Nematics with Orientational Jumps

Observations of stable, freely suspended liquid-crystal strands are reported. Liquid crystals of triphenylene hexa-$n$-dodecanoate can be stretched into strands of diameter \ensuremath{\gtrsim}1 \ensuremath{\mu}m, with the disk-shaped molecules arranged into columns along the strand axis. In the $D1$ phase ($Tl110\ifmmode^\circ\else\textdegree\fi{}$ C) the molecules are tilted with respect to the strand axis, the tilt orientation behaving as a one-dimensional nematiclike field, interrupted by jumps in orientation between degenerate stable positions relative to the column lattice.