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Kent State University

Achiral molecules

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Full-Text Articles in Physics

Nanostructures Of Liquid Crystal Phases In Mixtures Of Bent-Core And Rod-Shaped Molecules, Seung Ho Hong, Rafael Verduzco, Jim T. Gleeson, Samuel N. Sprunt, Antal Jakli Jun 2011

Nanostructures Of Liquid Crystal Phases In Mixtures Of Bent-Core And Rod-Shaped Molecules, Seung Ho Hong, Rafael Verduzco, Jim T. Gleeson, Samuel N. Sprunt, Antal Jakli

Chemical Physics Publications

We report small angle x-ray scattering (SAXS) studies of isotropic, nematic, and smectic mesophases formed by binary mixtures of bent-core (BC) and rod-shaped (RS) molecules. While optical studies indicate that the components are fully miscible, SAXS reveals fascinating structures that are consistent with segregation on a nanoscopic scale. We find that tilted smectic clusters, which have been previously reported in both the nematic and isotropic states of the pure BC materials, are also present in mixtures with up to 50 wt% of the RS compound; this is consistent with previous dielectric and flexoelectric studies on such mixtures. Unexpectedly in this ...


Large Flow Birefringence Of Nematogenic Bent-Core Liquid Crystals, C. Bailey, Katalin Fodor-Csorba, Rafael Verduzco, Jim T. Gleeson, Samuel N. Sprunt, Antal Jakli Dec 2009

Large Flow Birefringence Of Nematogenic Bent-Core Liquid Crystals, C. Bailey, Katalin Fodor-Csorba, Rafael Verduzco, Jim T. Gleeson, Samuel N. Sprunt, Antal Jakli

Chemical Physics Publications

We have found that bent-core liquid crystalline materials show exceptionally large intrinsic flow birefringence in their isotropic liquid phase. This effect is more than 100 times larger than typical values measured for low molecular weight liquid crystals. The specific flow birefringence (i.e., normalized by the flow viscosity) is an order of magnitude larger than in both side-chain polymeric as well as low molecular weight liquid crystals. We propose that this large enhancement for bent-core compounds may be attributed to nanoscale smecticlike clusters that persist above the nematic-isotropic transition temperature, and shear align under shear flow; however, this mechanism has ...


Optical Waveguiding In Bent-Core Liquid-Crystal Filaments, Jake Fontana, C. Bailey, Wolfgang Weissflog, I. Janossy, Antal Jakli Sep 2009

Optical Waveguiding In Bent-Core Liquid-Crystal Filaments, Jake Fontana, C. Bailey, Wolfgang Weissflog, I. Janossy, Antal Jakli

Chemical Physics Publications

We demonstrate optical waveguiding in recently discovered free-standing bent-core liquid-crystal filaments. The bent-core liquid-crystal molecules in air self-assemble into a cylindrical geometry that is "solidlike" along the radial direction of the filament and liquid in the axial direction of the filament. These filaments are unique not only because they are fluids, but also because they are anisotropic. For this reason, their waveguiding properties not predictable need to be characterized. The light power transmitted through the filament was found to be independent of temperature from 180 degrees C to near room temperature. Initial defects of newly pulled filaments were found to ...


Converse Flexoelectric Effect In A Bent-Core Nematic Liquid Crystal, John Harden, R. Teeling, Jim T. Gleeson, Samuel N. Sprunt, Antal Jakli Sep 2008

Converse Flexoelectric Effect In A Bent-Core Nematic Liquid Crystal, John Harden, R. Teeling, Jim T. Gleeson, Samuel N. Sprunt, Antal Jakli

Chemical Physics Publications

Flexoelectricity is a unique property of liquid crystals; it is a linear coupling between electric polarizations and bend and/or splay distortions of the direction of average molecular orientation. Recently it was shown [J. Harden et al., Phys. Rev. Lett. 97, 157802 (2006)] that the bend flexoelectric coefficient in bent-core nematic liquid crystals can be three orders of magnitude higher than the effect with calamitic (rod-shaped) molecular shape. Here we report the converse of the flexoelectric effect: An electric field applied across a bent-core liquid crystal sandwiched between thin flexible substrates produces a director distortion which is manifested as a ...


Observation Of A Possible Tetrahedratic Phase In A Bent-Core Liquid Crystal, D. B. Wiant, K. Neupane, S. Sharma, Jim T. Gleeson, Samuel N. Sprunt, Antal Jakli, N. Pradhan, G. Iannacchione Jun 2008

Observation Of A Possible Tetrahedratic Phase In A Bent-Core Liquid Crystal, D. B. Wiant, K. Neupane, S. Sharma, Jim T. Gleeson, Samuel N. Sprunt, Antal Jakli, N. Pradhan, G. Iannacchione

Chemical Physics Publications

An experimental study of the heat capacity, mass density, magnetic-field-induced optical birefringence, linewidth and intensity of scattered light, and the viscosities associated with nematic order parameter fluctuations and fluid flow has been performed on an achiral bent-core liquid crystal above its clearing point temperature. The measurements reveal a transition between two optically isotropic phases that is consistent with recent theoretical predictions of a "tetrahedratic" form of orientational order.


Role Of Molecular Shape On Bent-Core Liquid-Crystal Structures, C. Bailey, Antal Jakli Nov 2007

Role Of Molecular Shape On Bent-Core Liquid-Crystal Structures, C. Bailey, Antal Jakli

Chemical Physics Publications

We show how frustrations in molecular packing generated by molecular substitutions and electrostatic potentials necessitate a local C(1) symmetry and lead to spontaneous polarization splay and layer bending in bent-core liquid crystals. By estimation of the entropic changes due to curvature, we calculated the elastic constants that drive the spontaneous layer bend and polarization splay and obtained length scales of these deformations. Finally, we propose a structural phase diagram which describes how the experimentally observed helical superstructures, layer undulation, polarization splay, and spontaneous fiber formation depend on the direction and magnitude of the spontaneous polarization.


Optically Isotropic Liquid-Crystal Phase Of Bent-Core Molecules With Polar Nanostructure, Guangxun Liao, S. Stojadinovic, Gerhard Pelzl, Wolfgang Weissflog, Samuel N. Sprunt, Antal Jakli Aug 2005

Optically Isotropic Liquid-Crystal Phase Of Bent-Core Molecules With Polar Nanostructure, Guangxun Liao, S. Stojadinovic, Gerhard Pelzl, Wolfgang Weissflog, Samuel N. Sprunt, Antal Jakli

Chemical Physics Publications

We found that the optically isotropic (I-M) mesophase observed recently below the nematic phase of the bent-core liquid crystal 4-chlororesorcinol bis[4-(4-n-dodecyloxybenzoyloxy)benzoate] shows ferroelectric-type switching. Polarizing microscopic, electric current, dielectric, and dynamic light scattering studies lead us to propose that the I-M phase is composed of interconnected orthoconic racemic smectic (Sm-CaPF) nanodomains with random layer orientations. Near the nematic phase, where the polarization can be saturated by electric fields, the system responds in a fashion analogous to the granular structure of a magnetic spin glass-in particular, we observed that the relaxation back to the nonpoled structure follows a ...


Light-Induced Changes Of Optical And Electrical Properties In Bent-Core Azo Compounds, Antal Jakli, V. Prasad, D.S.S. Rao, Guangxun Liao, I. Janossy Feb 2005

Light-Induced Changes Of Optical And Electrical Properties In Bent-Core Azo Compounds, Antal Jakli, V. Prasad, D.S.S. Rao, Guangxun Liao, I. Janossy

Chemical Physics Publications

We have studied the optical and electrical properties of two bent-core substances with an azo linkage in their cores. Pump-probe laser studies, direct textural observations, and spectrophotometric recordings show an initial decrease of light transmission, which at larger light intensities (similar to 1 mW/mm(2)) is followed by a bleaching. Simultaneously the electrical properties (electric conductivity, antiferroelectric polarization, switching threshold, and switching time) decreased monotonically with increasing light intensities. The monotonic decrease of electrical properties indicates that the darkening and bleaching have the same origin, namely, the photochemical isomerization of the azo linkage from the trans to the cis ...


Antiferroelectric Ordering Of Amphiphilic Glycolipids In Bent-Core Liquid Crystals, Sugat Abeygunaratne, Antal Jakli, Gotz Milkereit, Hans Sawade, Volkmar Vill Feb 2004

Antiferroelectric Ordering Of Amphiphilic Glycolipids In Bent-Core Liquid Crystals, Sugat Abeygunaratne, Antal Jakli, Gotz Milkereit, Hans Sawade, Volkmar Vill

Chemical Physics Publications

Lipids are the main constituents of biological cell membranes, and their liquid crystalline properties play a crucial role in cell functions. We have discovered that dodecyl-beta-D-glucopyranoside glycolipid layers can be electrically polarized in supramolecular structures of glycolipid and bent-core ("banana-shape") molecules. Interestingly the glycolipid molecules, known to be excellent chiral dopants, are not able to transfer their chirality to the bent-core layers. Our observations indicate that glycolipid molecules self-assemble into pairs of tilted and antiferroelectric double layers, sandwiched between layers of bent-core molecules. These systems may provide a basis for understanding "bioferroelectricity," which is important in biological cell membrane functions.


Unusual Sequences Of Tilted Smectic Phases In Liquid Crystals Of Bent-Shape Molecules, Antal Jakli, P. Toledano Dec 2002

Unusual Sequences Of Tilted Smectic Phases In Liquid Crystals Of Bent-Shape Molecules, Antal Jakli, P. Toledano

Chemical Physics Publications

The first theoretical description of the experimentally often-observed isotropic-polar smectic phase transitions of symmetric and asymmetric bent-shape molecules is presented. The isotropic-tilted polar smectic phase transition requires a coupling among three types of ordering processes: polar packing, director tilt, and layer ordering. For explaining the unusual triclinic-monoclinic phase transition with the lower symmetry phase at higher temperature, observed in fluoro-containing compounds recently, a mechanism is proposed in which the layer ordering triggers the simultaneous onset of the tilt and polar order.


Dynamics Of The Nematic Phase Of A Bent-Core Liquid Crystal, S. Stojadinovic, A. Adorjan, Samuel N. Sprunt, Hans Sawade, Antal Jakli Dec 2002

Dynamics Of The Nematic Phase Of A Bent-Core Liquid Crystal, S. Stojadinovic, A. Adorjan, Samuel N. Sprunt, Hans Sawade, Antal Jakli

Chemical Physics Publications

The dielectric fluctuations in the uniaxial nematic phase of a bent-core liquid crystal have been studied by dynamic light scattering. Polarization selection of the scattering cross-section reveals one mode due to ordinary director fluctuations and, in the lower part of the nematic phase, a second mode attributable to fluctuations of the biaxial order parameter. The director fluctuations are similar to100 times slower than observed in typical nematics based on straight-core molecules, suggesting that the bent core nematogenic units may be microscopic smectic domains ("cybotactic" clusters).


Evidence For Triclinic Symmetry In Smectic Liquid Crystals Of Bent-Shape Molecules, Antal Jakli, D. Kruerke, Hans Sawade, G. Heppke Jun 2001

Evidence For Triclinic Symmetry In Smectic Liquid Crystals Of Bent-Shape Molecules, Antal Jakli, D. Kruerke, Hans Sawade, G. Heppke

Chemical Physics Publications

The first experimental evidence for triclinic symmetry of bulk smectic liquid-crystal samples of achiral banana-shaped molecules is presented. This phase corresponds to the so-called Sm-C-G phase consisting of biaxial molecules and characterized by two tilt directions with respect to the layer normal: tilt of the molecular plane (clinic) and tilt of the molecular kink direction (leaning). Each smectic layer has a polarization component normal to the smectic layers (C-1 symmetry). The observations suggest that the phase tentatively labeled as B-7 is identical with the Sm-C-G phase.


Optical Studies Of The Liquid Crystal B2 Phase Formed By Banana-Shaped Molecules, D. A. Olson, M. Veum, A. Cady, M. V. D'Agostino, P. M. Johnson, H. T. Nguyen, Liang-Chy Chien, C. C. Huang Apr 2001

Optical Studies Of The Liquid Crystal B2 Phase Formed By Banana-Shaped Molecules, D. A. Olson, M. Veum, A. Cady, M. V. D'Agostino, P. M. Johnson, H. T. Nguyen, Liang-Chy Chien, C. C. Huang

Chemical Physics Publications

Null-transmission ellipsometry and depolarized light microscopy have been performed on free-standing films of three achiral banana-shaped compounds in the B2 phase. Our results support a two-layer unit cell previously proposed to explain the observed antiferroelectricity in thin films and bulk samples. We have studied thicker films than previously reported and have found no deviations in the film structure from the earlier findings. Moreover, we can determine the layer spacing, the molecular tilt from the layer normal, and the three principal indices of refraction in the molecular reference frame.