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Szydłowska J, Krówczyński A, Górecka E, Pociecha D. Calamitic and discotic liquid crystalline phases for mesogens with triangular cores. SOFT MATTER 2019; 15:7195-7202. [PMID: 31465073 DOI: 10.1039/c9sm01080g] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/10/2023]
Abstract
Mesogenic compounds having new triangular mesogenic cores containing salicylaldimine and enaminocarbonyl or barbituric moieties have been synthesized and their liquid crystalline, dielectric and optical properties have been studied. Appropriate substitution of such cores with various numbers of lateral chains (alkoxy, perfluorinated or polyether) leads to mesogens forming either lamellar (SmC), nematic or columnar (Colhd and Colho) phases. Some of the materials show an unusual phase sequence with a re-entrant isotropic phase appearing below the columnar (Colhd) phase; others form two columnar hexagonal phases with a weakly first order transition between them. Also a new columnar superstructure with p2gg symmetry (Colros) was found, in which the crystallographic lattice is decorated with a two-molecule motive.
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Affiliation(s)
- Jadwiga Szydłowska
- University of Warsaw, Faculty of Chemistry, ul. Żwirki I Wigury 101, 02-089 Warsaw, Poland.
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Polineni S, Figueirinhas JL, Cruz C, Wilson DA, Mehl GH. Capacitance and optical studies of elastic and dielectric properties in an organosiloxane tetrapode exhibiting a N(B) phase. J Chem Phys 2013; 138:124904. [PMID: 23556748 DOI: 10.1063/1.4795582] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/15/2022] Open
Abstract
Biaxial (N(B)) and uniaxial nematic (N(U)) phase behavior was detected and confirmed for an organosiloxane tetrapode material using capacitance and birefringence measurements. Elastic constants, permittivities at two distinct low frequencies, and birefringencies were determined as a function of temperature over both the N(U) and the N(B) phase ranges. The N(U)-N(B) transition is clearly observed in the birefringencies and conoscopy data. A temperature dependent cross-over frequency is also detected in this material for the permittivities, allowing the electrical switching of both planar and homeotropic aligned samples.
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Affiliation(s)
- S Polineni
- IST-Technical University of Lisbon, Dept. de Física, Av. Rovisco Pais, P-1049-001 Lisboa, Portugal
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Kishikawa K. Utilization of the Perfluoroarene-Arene Interaction for Stabilization of Liquid Crystal Phases. Isr J Chem 2012. [DOI: 10.1002/ijch.201200028] [Citation(s) in RCA: 22] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/09/2022]
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Grolik J, Dudek Ł, Eilmes J. Tuning the mesomorphic properties of liquid-crystalline dibenzotetraaza[14]annulenes — discotic nematic phases of tetraalkoxy-substituted derivatives. Tetrahedron Lett 2012. [DOI: 10.1016/j.tetlet.2012.07.053] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/28/2022]
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Ostapenko T, Zhang C, Sprunt SN, Jákli A, Gleeson JT. Magneto-optical technique for detecting the biaxial nematic phase. PHYSICAL REVIEW. E, STATISTICAL, NONLINEAR, AND SOFT MATTER PHYSICS 2011; 84:021705. [PMID: 21929003 DOI: 10.1103/physreve.84.021705] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 04/15/2011] [Revised: 05/26/2011] [Indexed: 05/31/2023]
Abstract
The existence of the elusive biaxial phase has been the subject of much discussion since it was predicted by Freiser in 1970. More recently, there have been numerous attempts to find a thermotropic liquid crystal that exhibits a biaxial phase and with this, conflicting reports about whether such a phase has been positively identified in bent-core liquid crystals. One reason for the discrepancy is that there is currently no way to rule out surface effects or anchoring transitions, both of which may give a false positive identification of a uniaxial-biaxial nematic transition. We have developed a technique that uses a magnetic field to align the uniaxial director, thus widening its application to any bent-core nematic material.
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Affiliation(s)
- T Ostapenko
- Department of Physics, Kent State University, Kent, Ohio 44242, USA.
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Lee JH, Lim TK, Yoon TH. Biaxial-uniaxial transition in a self-assembled nematic liquid crystal by field-induced molecular conformation. PHYSICAL REVIEW. E, STATISTICAL, NONLINEAR, AND SOFT MATTER PHYSICS 2011; 83:051705. [PMID: 21728554 DOI: 10.1103/physreve.83.051705] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 12/30/2010] [Revised: 03/02/2011] [Indexed: 05/31/2023]
Abstract
A cylindrical uniaxial liquid crystal was hydrogen-bonded with a tripod-shaped molecule. As the molecular number ratio of cylindrical liquid crystal to tripod molecule converges to 2:1, |s| = 1/2 disclination defects, which are a necessary condition of biaxial nematic phase, appeared predominantly. After the application of an electric field, the |s| = 1/2 disclinations converted to |s| = 1 disclinations which imply a transition from a biaxial to uniaxial state. IR dichroism measurements support the hypothesis that the liquid crystal-tripod molecule complex undergoes a molecular conformation change from bent shape to parallel shape, and this conformation change is thought to be related to a field-induced biaxial-uniaxial transition.
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Affiliation(s)
- Ji-Hoon Lee
- School of Electrical Engineering, Pusan National University, Pusan, Korea.
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Senyuk B, Wonderly H, Mathews M, Li Q, Shiyanovskii SV, Lavrentovich OD. Surface alignment, anchoring transitions, optical properties, and topological defects in the nematic phase of thermotropic bent-core liquid crystal A131. PHYSICAL REVIEW. E, STATISTICAL, NONLINEAR, AND SOFT MATTER PHYSICS 2010; 82:041711. [PMID: 21230299 DOI: 10.1103/physreve.82.041711] [Citation(s) in RCA: 15] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 04/03/2010] [Revised: 09/29/2010] [Indexed: 05/30/2023]
Abstract
We study optical, structural, and surface anchoring properties of thermotropic nematic bent-core material A131. The focus is on the features associated with orientational order as the material has been reported to exhibit not only the usual uniaxial nematic but also the biaxial nematic phase. We demonstrate that A131 experiences a surface anchoring transition from a perpendicular to tilted alignment when the temperature decreases. The features of the tilted state are consistent with surface-induced birefringence associated with smectic layering near the surface and a molecular tilt that changes along the normal to the substrates. The surface-induced birefringence is reduced to zero by a modest electric field that establishes a uniform uniaxial nematic state. Both refractive and absorptive optical properties of A131 are consistent with the uniaxial order. We found no evidence of the "polycrystalline" biaxial behavior in the cells placed in crossed electric and magnetic fields. We observe stable topological point defects (boojums and hedgehogs) and nonsingular "escaped" disclinations pertinent only to the uniaxial order. Finally, freely suspended films of A131 show uniaxial nematic and smectic textures; a decrease in the film thickness expands the temperature range of stability of smectic textures, supporting the idea of surface-induced smectic layering. Our conclusion is that A131 features only a uniaxial nematic phase and that the apparent biaxiality is caused by subtle surface effects rather than by the bulk biaxial phase.
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Affiliation(s)
- B Senyuk
- Liquid Crystal Institute and Chemical Physics Interdisciplinary Program, Kent State University, Kent, Ohio 44242, USA
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Acharya BR, Kang SW, Prasad V, Kumar S. Role of Molecular Structure on X-ray Diffraction in Uniaxial and Biaxial Phases of Thermotropic Liquid Crystals. J Phys Chem B 2009; 113:3845-52. [DOI: 10.1021/jp810333r] [Citation(s) in RCA: 27] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Affiliation(s)
- Bharat R. Acharya
- Platypus Technologies, 5520 Nobel Drive, Madison, Wisconsin 53705, Department of Physics, Kent State University, Kent, Ohio 44240, and Center for Liquid Crystal Research, Jalahalii, Bangalore-560013, India
| | - Shin-Woong Kang
- Platypus Technologies, 5520 Nobel Drive, Madison, Wisconsin 53705, Department of Physics, Kent State University, Kent, Ohio 44240, and Center for Liquid Crystal Research, Jalahalii, Bangalore-560013, India
| | - Veena Prasad
- Platypus Technologies, 5520 Nobel Drive, Madison, Wisconsin 53705, Department of Physics, Kent State University, Kent, Ohio 44240, and Center for Liquid Crystal Research, Jalahalii, Bangalore-560013, India
| | - Satyendra Kumar
- Platypus Technologies, 5520 Nobel Drive, Madison, Wisconsin 53705, Department of Physics, Kent State University, Kent, Ohio 44240, and Center for Liquid Crystal Research, Jalahalii, Bangalore-560013, India
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Van Le K, Mathews M, Chambers M, Harden J, Li Q, Takezoe H, Jákli A. Electro-optic technique to study biaxiality of liquid crystals with positive dielectric anisotropy: the case of a bent-core material. PHYSICAL REVIEW. E, STATISTICAL, NONLINEAR, AND SOFT MATTER PHYSICS 2009; 79:030701. [PMID: 19391886 DOI: 10.1103/physreve.79.030701] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 11/05/2008] [Indexed: 05/27/2023]
Abstract
We propose that, for materials having positive dielectric anisotropy, the biaxiality can be clearly verified or excluded by measuring the transmitted light intensity as a function of electric field. If the material is biaxial and is observed in homeotropically aligned cells, the schlieren texture should not disappear (transmitted intensity is not zero) even at very high fields, since the field does not affect the distribution of the second director normal to the main director. On the other hand, if the material is uniaxial the transmitted intensity should decrease with increasing field and a perfect homeotropic texture can be achieved at high fields. We have studied a bent-core compound in which a uniaxial-biaxial nematic (N_{u}-N_{b}) transition has been reported. This material has a positive dielectric anisotropy at low frequencies, so we could apply the technique described above. Our studies indicate that the material is uniaxial in the entire nematic range.
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Affiliation(s)
- Khoa Van Le
- Department of Organic and Polymeric Materials, Tokyo Institute of Technology, O-okayama, Meguro-ku, Tokyo 152-8552, Japan
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Cordoyiannis G, Apreutesei D, Mehl GH, Glorieux C, Thoen J. High-resolution calorimetric study of a liquid crystalline organo-siloxane tetrapode with a biaxial nematic phase. PHYSICAL REVIEW. E, STATISTICAL, NONLINEAR, AND SOFT MATTER PHYSICS 2008; 78:011708. [PMID: 18763974 DOI: 10.1103/physreve.78.011708] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 05/14/2008] [Indexed: 05/26/2023]
Abstract
High-resolution adiabatic scanning calorimetry and differential scanning calorimetry have been employed to study the thermal behavior of an organo-siloxane tetrapode reported to exhibit a biaxial nematic phase. No signature of the uniaxial to biaxial nematic phase transition could be retraced in sequential heating and cooling runs under different scanning rates, within the experimental resolution. The results obtained reveal that an extremely small heat should be involved in the uniaxial to biaxial nematic phase transition. The isotropic to uniaxial nematic transition at 318+/-0.01 K is very stable, and it is weakly first order with a rather small latent heat of 0.20+/-0.02 J/g .
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Affiliation(s)
- George Cordoyiannis
- Laboratorium voor Akoestiek en Thermische Fysica, Departement Natuurkunde en Sterrenkunde, Katholieke Universiteit Leuven, Celestijnenlaan 200D, Leuven, Belgium
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Neupane K, Kang SW, Sharma S, Carney D, Meyer T, Mehl GH, Allender DW, Kumar S, Sprunt S. Dynamic light scattering study of biaxial ordering in a thermotropic liquid crystal. PHYSICAL REVIEW LETTERS 2006; 97:207802. [PMID: 17155716 DOI: 10.1103/physrevlett.97.207802] [Citation(s) in RCA: 14] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 06/29/2006] [Indexed: 05/12/2023]
Abstract
Dynamic light scattering from orientational order fluctuations in a liquid crystalline tetrapode reveals successive, weakly first-order isotropic to uniaxial and uniaxial to biaxial nematic phase transitions. The order parameter relaxation rates exhibit temperature dependences consistent with Landau-de Gennes mean field theory. Combined with previous evidence of a second-order uniaxial-biaxial transition in a closely related tetrapode, the present study supports the existence of a nematic-nematic tricritical point in thermotropic liquid crystals.
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Affiliation(s)
- K Neupane
- Department of Physics, Kent State University, Kent, Ohio 44242, USA
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Prasad V, Kang SW, Suresh KA, Joshi L, Wang Q, Kumar S. Thermotropic Uniaxial and Biaxial Nematic and Smectic Phases in Bent-Core Mesogens. J Am Chem Soc 2005; 127:17224-7. [PMID: 16332069 DOI: 10.1021/ja052769n] [Citation(s) in RCA: 146] [Impact Index Per Article: 7.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
Two azo substituted achiral bent-core mesogens have been synthesized. Optical polarizing microscopy and synchrotron X-ray scattering studies of both compounds reveal the existence of the thermotropic uniaxial and biaxial nematic and three smectic phases at different temperatures in these single component small molecule systems. The transition from the uniaxial to biaxial nematic phase is confirmed to be second order. The transitions from the biaxial nematic to the underlying smectic phase and between the smectic phases have barely discernible heat capacity signatures and thus are also second order.
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Affiliation(s)
- Veena Prasad
- Center for Liquid Crystal Research, P.O. Box 1329, Jalahalli, Bangalore 560 013, India
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Luckhurst GR. V-förmige Moleküle: neue Kandidaten für die zweiachsig nematische Phase. Angew Chem Int Ed Engl 2005. [DOI: 10.1002/ange.200500709] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/11/2022]
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Affiliation(s)
- Geoffrey R Luckhurst
- School of Chemistry and Southampton Liquid Crystal Institute, University of Southampton, Southampton SO17 1BJ, UK.
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Figueirinhas JL, Cruz C, Filip D, Feio G, Ribeiro AC, Frère Y, Meyer T, Mehl GH. Deuterium NMR investigation of the biaxial nematic phase in an organosiloxane tetrapode. PHYSICAL REVIEW LETTERS 2005; 94:107802. [PMID: 15783524 DOI: 10.1103/physrevlett.94.107802] [Citation(s) in RCA: 33] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 09/22/2004] [Indexed: 05/24/2023]
Abstract
Deuterium NMR is used to examine the molecular order exhibited by an organosiloxane tetrapode giving the first experimental evidence, using a bulk sample, for the existence of a biaxial nematic phase in this type of compounds. The temperature dependence of the averaged quadrupolar coupling constant and asymmetry parameter was determined in the compound's nematic phase. Two distinct regimes could be identified, one with a vanishing asymmetry parameter corresponding to a uniaxial nematic phase and another with a significant temperature dependent asymmetry parameter, corresponding to a biaxial nematic phase. The high values obtained for the asymmetry parameter at the lower end of the nematic range are well above experimental error and constitute a definite proof of the biaxial nature of the nematic phase exhibited by the studied compound for those temperatures.
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Affiliation(s)
- J L Figueirinhas
- CFMC-UL, Avenida Professor Gama Pinto 2, 1649-003 Lisboa, Portugal
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Abstract
In this article, the search for the elusive biaxial nematic phase (NB) in liquid crystals is considered. The structure of the phase is described along with theoretical and computational work which suggests how it might be realised. An overview of the work of the Exeter group in this area is then given showing the different approaches adopted and illustrating how one of these has led to a new type of amphiphilicity based on shape.
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Affiliation(s)
- Duncan W Bruce
- Department of Chemistry, University of Exeter, Stocker Road, Exeter, EX4 4QD, UK.
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Syed IM, Percec V, Petschek RG, Rosenblatt C. Apparent tricritical behavior at a nearly second-order nematic-isotropic phase transition of a cyclic liquid crystalline trimer. PHYSICAL REVIEW. E, STATISTICAL, NONLINEAR, AND SOFT MATTER PHYSICS 2003; 67:011704. [PMID: 12636514 DOI: 10.1103/physreve.67.011704] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 10/15/2002] [Indexed: 05/24/2023]
Abstract
The cyclic liquid crystalline trimer TPB-(c)9(3) was investigated by optical retardation and Fréedericksz techniques within a few tens of millikelvins of the superheating limit of the nearly second-order nematic-isotropic phase transition. Both the optical retardation and the Fréedericksz bend threshold voltage are in good agreement with tricritical behavior for the transition.
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Affiliation(s)
- Ishtiaque M Syed
- Department of Physics, Case Western Reserve University, Cleveland, Ohio 44106-7079, USA
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Polysubstituted triphenylenes with active groups. Molecular parameters, synthesis, structure, and mesomorphism. J STRUCT CHEM+ 1998. [DOI: 10.1007/bf02873645] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/22/2022]
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