A Molecular Meanfield Model for the Biaxial Rectangular Discotic Phase with Herring-Bone Packing of Tilted Molecules
01 May 1995-Molecular Crystals and Liquid Crystals (Taylor & Francis Group)-Vol. 264, Iss: 1, pp 165-179
TL;DR: In this article, a molecular mean field model for a rectangular columnar to hexagonal columnar phase transition is presented, assuming the molecules basically to be biaxial objects.
Abstract: A molecular meanfield model for a rectangular columnar to hexagonal columnar phase transition, as observed in case of the HAT series, is presented here assuming the molecules basically to be biaxial objects The theory is developed for a rectangular lattice having herring-bone packing of the tilted molecules Various types of phase diagrams for a homologous series are reproduced for different values of molecular biaxiality Some of them are very similar to the experimentally observed cases but others, particularly involving biaxial nematic phase, are not yet observed It is shown that a gradual lowering of biaxiality with the addition of flexible chain segments can lead to the full phase sequence observed in the HAT series
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TL;DR: In this paper, the structures of columnar mesophases that depend on the presence of either N-H⋯O, or H-O-Bonds (subsumed under the term ‘N|H|O-bonds') are reviewed.
Abstract: The structures of supramolecular columnar mesophases that depend on the presence of either N–H⋯O, or N⋯H–O– hydrogen bonds (subsumed under the term ‘N|H|O-bonds’) are reviewed. The thesis is supported that supramolecular mesogenes bridge different fields of liquid crystal research, offering the opportunity to create a unified picture of thermotropic, and lyotropic mesomorphism. With the examples of N|H|O hydrogen bond mesogenes the molecular structures of the respective mesogenes are presented, and general principles to rationally construct such molecules are discussed. A connection is drawn to systems from isolated columns as found in low molecular weight organogels. Perspectives, and outlooks for possible applications of N|H|O hydrogen bonded columnar phases are given.
117 citations
TL;DR: The results point to the formation of dynamical structural defects along the columns in stacks of liquid-crystalline tetraalkoxy-substituted, metal-free phthalocyanines.
Abstract: We present a joint molecular dynamics (MD)/kinetic Monte Carlo (KMC) study aimed at the atomistic description of charge transport in stacks of liquid-crystalline tetraalkoxy-substituted, metal-free phthalocyanines. The molecular dynamics simulations reproduce the major structural features of the mesophases, in particular, a phase transition around 340 K between the rectangular and hexagonal phases. Charge transport simulations based on a Monte Carlo algorithm show an increase by 2 orders of magnitude in the hole mobility when accounting for the rotational and translational dynamics. The results point to the formation of dynamical structural defects along the columns.
82 citations
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TL;DR: In this article, a self-consistent equation for the parameters characterizing orientational and two-dimensional hexagonal translational order is derived from anisotropic model interaction; numerical solution yields the order parameters and entropy as functions of temperature and interaction range.
Abstract: McMillan's model of smectic ordering is applied to systems of disc-like molecules. Self-consistent equations for the parameters characterizing orientational and two-dimensional hexagonal translational order are derived from an anisotropic model interaction; numerical solution yields the order parameters and entropy as functions of temperature and interaction range.
34 citations
TL;DR: In this article, a homologous series of hexa-nalkoxytruxenes has been prepared with values of n from 6 to 14, and the interphase transitions between solid, mesomorphic and isotropic phases were studied by hot stage microscopy and differential scanning calorimetry.
Abstract: An homologous series of hexa-n-alkoxytruxenes has been prepared with values of n from 6 to 14. Interphase transitions between solid, mesomorphic and isotropic phases were studied by hot stage microscopy and differential scanning calorimetry. X-ray diffraction measurements showed that all of the mesophases were of the same type; a hexagonal ordered columnar phase (Dho), with an unusually large temperature range. Systematic X-ray studies were performed with respect to temperature and chain lenght.
25 citations
TL;DR: Deuterium nuclear magnetic resonance studies in discotic mesophases are reviewed in this article, with particular emphasis on comparing axially symmetric and biaxial phases, and analyzing the various orientational order parameters required to characterize their NMR spectra.
Abstract: Deuterium Nuclear Magnetic Resonance studies in discotic mesophases are reviewed. The studies include NMR measurements on neat compounds as well as deuterated probe molecules dissolved in the mesophase. The discotic compounds discussed include derivatives of benzene, triphenylene and truxene which exhibit a variety of mesophases. Particular emphasis is placed on comparing axially symmetric and biaxial phases, and on analyzing the various orientational order parameters required to characterize their NMR spectra. The conformation of side chains as derived from the deuterium resonance of the methylene and methyl deuterons is also reviewed. Finally, the properties of discotic mesophases as ordering solvents for solute probe molecules are discussed.
25 citations
TL;DR: In this paper, the deuterium NMR spectra of deuterated probe molecules dissolved in several discotic liquid crystals possessing biaxial mesophases are reported.
Abstract: Deuterium NMR spectra of deuterated probe molecules dissolved in several discotic liquid crystals possessing biaxial mesophases are reported. The spectra exhibit typical features characteristic of biaxial phases. This is particularly manifested through the asymmetry parameter of the average quadrupole tensor. The effect is demonstrated on two types of biaxial discotic phases: (i) a rectangular Drd mesophase which appears in homologs of triphenylenehexa‐n‐alkanoates; and (ii) a tilted Dt mesophase which occurs in homologs of benzenehexa‐n‐alkanoates. The theory relating to the motional constants in biaxial phases and the asymmetry parameter of the quadrupole tensor (and other tensorial properties) is reviewed, and applied to some specific examples.
24 citations
TL;DR: The existence of a uniaxial-biaxial phase transition in a smectic A phase is shown theoretically, based on the more realistic anisotropic pair potential and within the mean field approximation as mentioned in this paper.
Abstract: The existence of a uniaxial-biaxial phase transition in a smectic A phase is shown theoretically, based on the more realistic anisotropic pair potential and within the mean field approximation. The resulting phase diagram exhibits successive phase transitions of isotropic-uniaxial nematic-uniaxial smectic A-biaxial smectic A and a uniaxial nematic-uniaxial smectic A-biaxial smectic A triple point. The possible assignment of the predicted biaxial smectic A to some phase classified recently as a smectic C is discussed.
12 citations