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Zhang S, Chen S, Ma T, Zou H, Li B, Miao X, Cheng X, Deng W. Terminal Group Effect on Two-Dimensional Self-Assembly of Fluorenone-Based Liquid Crystals at the Solid/Liquid Interface. LANGMUIR : THE ACS JOURNAL OF SURFACES AND COLLOIDS 2024; 40:1902-1908. [PMID: 38194665 DOI: 10.1021/acs.langmuir.3c03318] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/11/2024]
Abstract
Self-assemblies of two fluorenone-based derivatives (FE and FEC) consisting of a central 2,7-diphenyl-9-fluorenone polar moiety but differing in the flexible terminal groups were investigated by scanning tunneling microscopy (STM) at the 1-octanoic acid/HOPG interface under different concentrations and density functional theory calculation (DFT). STM results reveal a concentration-dependent polymorphic self-assembly behavior for FE, but without the presence of co-adsorbed solvents. As the concentration decreases, the dimer, bracket-like, and ribbon-like self-assembled structures were observed. On the contrary, FEC molecules assemble into only a type of oval-shaped morphology by the intermolecular N···H-O hydrogen bonds with the solvent molecules. Combined with DFT calculations, it can be deduced that the intermolecular van der Waals forces, dipole-dipole interactions, and hydrogen bonding are the main driving forces to stabilize the molecular packing of fluorenone-based polycatenars with strong polarity. Our work is of significance at the molecular level to further clarify the intermolecular interactions and conformational effects on the formation of molecular packing structures with liquid crystal property.
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Affiliation(s)
- Songyao Zhang
- College of Materials Science and Engineering, South China University of Technology, Guangzhou 510640, People's Republic of China
| | - Shibo Chen
- Key Laboratory of Medicinal Chemistry for Natural Resources, Chemistry School of Chemical Science and Technology, Yunnan University, Kunming 650091, People's Republic of China
| | - Tao Ma
- Key Laboratory of Medicinal Chemistry for Natural Resources, Chemistry School of Chemical Science and Technology, Yunnan University, Kunming 650091, People's Republic of China
| | - Hengqi Zou
- College of Materials Science and Engineering, South China University of Technology, Guangzhou 510640, People's Republic of China
| | - Bang Li
- College of Materials Science and Engineering, South China University of Technology, Guangzhou 510640, People's Republic of China
| | - Xinrui Miao
- College of Materials Science and Engineering, South China University of Technology, Guangzhou 510640, People's Republic of China
| | - Xiaohong Cheng
- Key Laboratory of Medicinal Chemistry for Natural Resources, Chemistry School of Chemical Science and Technology, Yunnan University, Kunming 650091, People's Republic of China
| | - Wenli Deng
- College of Materials Science and Engineering, South China University of Technology, Guangzhou 510640, People's Republic of China
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2
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Nematic ordering of model racemic mixture of stiff trimer chains. J Mol Liq 2021. [DOI: 10.1016/j.molliq.2021.116806] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
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3
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Kaur S, Mohiuddin G, Punjani V, Khan RK, Ghosh S, Pal SK. Structural organization and molecular self-assembly of a new class of polar and non-polar four-ring based bent-core molecules. J Mol Liq 2019. [DOI: 10.1016/j.molliq.2019.111687] [Citation(s) in RCA: 10] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/01/2022]
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Chrzanowska A. Mean-field model of boomerang nematic liquid crystals with diminished coupling of molecular uniaxial and biaxial susceptibilities. Phys Rev E 2019; 99:062703. [PMID: 31330725 DOI: 10.1103/physreve.99.062703] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/19/2018] [Indexed: 06/10/2023]
Abstract
The mean-field theory approach has been applied to the boomerang type particles from P. I. C. Teixeira, A. Masters, and B. Mulder [Mol. Cryst. Liq. Cryst. 323, 167 (1998)MCLCE91058-725X10.1080/10587259808048440] but with diminished strength of the interaction coefficient responsible for the coupling between molecular uniaxial and biaxial susceptibilities. For the rodlike particles, when the apex boomerang angle is larger than 107.35^{∘}, the stable uniaxial rodlike phase occurs. For smaller angles, beyond the point where the transition is of the second order (the Landau point) and for diminished parameter of molecular biaxial-uniaxial coupling, a biaxial phase is observed with the transition undergoing directly from the isotropic phase. According to the order parameters the character of this transition is of the first order. Such behavior is in accordance with the Sonnet-Durand-Virga model of the biaxial phases. The change in the type of the phase transition order is also illustrated by the changes in the equations of state and the changes in second and third derivatives of the free energy. The possibilities to tailor interaction coefficients of real molecules to obtain such a phase transition scenario are discussed.
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Affiliation(s)
- Agnieszka Chrzanowska
- Institute of Physics, Kraków University of Technology, ul. Podchorążych 1, 30-084 Kraków, Poland
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Vaghela A, Teixeira PIC, Terentjev EM. Emergence of biaxial nematic phases in solutions of semiflexible dimers. Phys Rev E 2018; 96:042703. [PMID: 29347574 DOI: 10.1103/physreve.96.042703] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/30/2017] [Indexed: 11/07/2022]
Abstract
We investigate the isotropic, uniaxial nematic and biaxial nematic phases, and the transitions between them, for a model lyotropic mixture of flexible molecules consisting of two rigid rods connected by a spacer with variable bending stiffness. We apply density-functional theory within the Onsager approximation to describe strictly excluded-volume interactions in this athermal model and to self-consistently find the orientational order parameters dictated by its complex symmetry, as functions of the density. Earlier work on lyotropic ordering of rigid bent-rod molecules is reproduced and extended to show explicitly the continuous phase transition at the Landau point, at a critical bend angle of 36^{∘}. For flexible dimers with no intrinsic biaxiality, we find that a biaxial nematic phase can nevertheless form at a sufficiently high density and low bending stiffness. For bending stiffness κ>0.86k_{B}T, this biaxial phase manifests as dimer bending fluctuations occurring preferentially in one plane. When the dimers are more flexible, κ<0.86k_{B}T, the modal shape of the fluctuating dimer is a V with an acute opening angle, and one of the biaxial order parameters changes sign, indicating a rotation of the directors. These two regions are separated by a narrow strip of uniaxial nematic in the phase diagram, which we generate in terms of the spacer stiffness and particle density.
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Affiliation(s)
- Arvin Vaghela
- Cavendish Laboratory, University of Cambridge, J. J. Thomson Avenue, Cambridge CB3 0HE, United Kingdom
| | - Paulo I C Teixeira
- ISEL-Instituto Superior de Engenharia de Lisboa, Instituto Politécnico de Lisboa Rua Conselheiro Emídio Navarro 1, 1959-007 Lisbon, Portugal.,Centro de Física Teórica e Computacional Faculdade de Ciências da Universidade de Lisboa Campo Grande, Edifício C8, 1749-016 Lisbon, Portugal
| | - Eugene M Terentjev
- Cavendish Laboratory, University of Cambridge, J. J. Thomson Avenue, Cambridge CB3 0HE, United Kingdom
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Aliev MA, Kuzminyh NY, Ugolkova EA. Phase behavior of the thermotropic melt of asymmetric V-shaped molecules. Phys Rev E 2017; 95:042703. [PMID: 28505871 DOI: 10.1103/physreve.95.042703] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/24/2016] [Indexed: 11/07/2022]
Abstract
The phase behavior of the monodisperse melt of V-shaped molecules composed of two rigid segments of different lengths joined at their ends at an external angle α has been examined within the Landau-de Gennes approach. Each rigid segment consists of a sequence of monomer units; the anisotropic interactions in the system are assumed to be of the Maier-Saupe form. The coefficients of the Landau-de Gennes free-energy expansion have been found from a microscopic model of V-shaped molecule. A single Landau point at which the system undergoes direct continuous transition from isotropic to biaxial nematic phase is found for asymmetry parameter ϕ=1/3 or ϕ=2/3, where ϕ is the number fraction of monomer units in one of the segments. Two Landau points are found in a range 1/3<ϕ<2/3. Only isotropic and nematic states are found to be stable for 0≤ϕ<1/3 and 2/3<ϕ≤1. Regions of stability of isotropic, prolate uniaxial, and biaxial nematic phases are found for ϕ=1/3 and ϕ=2/3. In addition, a stable oblate uniaxial phase is revealed if asymmetry parameter falls in the range 1/3<ϕ<2/3. The region of stability of the biaxial nematic phase becomes smaller as parameter ϕ increases from 1/3 to 1/2 (or decreases from 2/3 to 1/2). Coefficients of the gradient terms have been found; for certain values of asymmetry parameter these coefficients can become negative in some ranges of angles.
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Affiliation(s)
- M A Aliev
- Institute of Biochemical Physics, 4, Kosygina Str., 119334, Moscow, Russia
| | - N Yu Kuzminyh
- Institute of Biochemical Physics, 4, Kosygina Str., 119334, Moscow, Russia
| | - E A Ugolkova
- Institute of General and Inorganic Chemistry, 31 Leninsky pr., 119991, Moscow, Russia
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Mohiuddin G, Punjani V, Pal SK. Three-Ring-Based Room-Temperature Bent-Core Nematic Compounds: Synthesis and Characterization. Chemphyschem 2015. [DOI: 10.1002/cphc.201500435] [Citation(s) in RCA: 20] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022]
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8
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Figueirinhas JL, Dong RY. NMR of bent-core nematogens: a mini-review. MAGNETIC RESONANCE IN CHEMISTRY : MRC 2014; 52:614-624. [PMID: 24975632 DOI: 10.1002/mrc.4097] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 04/09/2014] [Accepted: 05/26/2014] [Indexed: 06/03/2023]
Abstract
This review focuses on two of our studied bent-core liquid crystals 2-methyl-3-[4-(4-octylbenzoyloxy)-benzylidene]-amino-benzoic acid 4-(4-dodecylphenylazo)-phenyl ester A131, and a shape-persistent fluorenone mesogen, F1a by means of temperature-dependent (13)C NMR spectra and deuterium NMR spectra, respectively. Orientational order parameters, molecular packing, and/or conformations in the nematic phases or nematic glass can be extracted from the observed NMR spectra. These will be discussed in terms of certain motional models. In particular, F1a has been found to form a biaxial nematic glass. The derived orientational order parameters would put the biaxial system of phase symmetry lower than orthorhombic.
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Affiliation(s)
- João L Figueirinhas
- Departamento de Física, Instituto Superior Técnico, Universidade de Lisboa, Lisboa, Portugal; CFMC, Universidade de Lisboa, Lisboa, Portugal
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Gleeson HF, Kaur S, Görtz V, Belaissaoui A, Cowling S, Goodby JW. The nematic phases of bent-core liquid crystals. Chemphyschem 2014; 15:1251-60. [PMID: 24700653 PMCID: PMC4257080 DOI: 10.1002/cphc.201400014] [Citation(s) in RCA: 72] [Impact Index Per Article: 7.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/07/2014] [Indexed: 11/07/2022]
Abstract
Over the last ten years, the nematic phases of liquid crystals formed from bent-core structures have provoked considerable research because of their remarkable properties. This Minireview summarises some recent measurements of the physical properties of these systems, as well as describing some new data. We concentrate on oxadiazole-based materials as exemplars of this class of nematogens, but also describe some other bent-core systems. The influence of molecular structure on the stability of the nematic phase is described, together with progress in reducing the nematic transition temperatures by modifications to the molecular structure. The physical properties of bent-core nematic materials have proven difficult to study, but patterns are emerging regarding their optical and dielectric properties. Recent breakthroughs in understanding the elastic and flexoelectric behaviour are summarised. Finally, some exemplars of unusual electric field behaviour are described.
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Affiliation(s)
- Helen F Gleeson
- [a] Prof. H. F. Gleeson, Dr. S. Kaur School of Physics and Astronomy, The University of Manchester, Oxford Road, Manchester, M13 9PL (UK) E-mail:
| | - Sarabjot Kaur
- [a] Prof. H. F. Gleeson, Dr. S. Kaur School of Physics and Astronomy, The University of Manchester, Oxford Road, Manchester, M13 9PL (UK) E-mail:
| | - Verena Görtz
- Department of ChemistryUniversity of York, Heslington, York YO10 5DD (UK)
- Department of ChemistryLancaster University, Lancaster LA1 4YB (UK)
| | - Abdel Belaissaoui
- Department of ChemistryUniversity of York, Heslington, York YO10 5DD (UK)
| | - Stephen Cowling
- Department of ChemistryUniversity of York, Heslington, York YO10 5DD (UK)
| | - John W Goodby
- Department of ChemistryUniversity of York, Heslington, York YO10 5DD (UK)
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Bisi F, De Matteis G, Romano S. Calamitic and antinematic orientational order produced by the generalized Straley lattice model. PHYSICAL REVIEW. E, STATISTICAL, NONLINEAR, AND SOFT MATTER PHYSICS 2013; 88:032502. [PMID: 24125280 DOI: 10.1103/physreve.88.032502] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 01/04/2013] [Indexed: 06/02/2023]
Abstract
We consider here a classical model, consisting of D_{2h}-symmetric particles in a three-dimensional simple-cubic lattice; the pair potential is isotropic in orientation space, and restricted to nearest neighbors. The simplest potential model is written in terms of the squares of the scalar products between unit vectors describing the three interacting arms of the molecules, as proposed in previous literature. Two predominant antinematic couplings of equal strength (+1) are perturbed by a comparatively weaker calamitic one, parameterized by a coupling constant -z ranging in [-1,0]. This choice rules out thermodynamically stable phases endowed with macroscopic biaxiality. The antinematic terms favor states with the corresponding molecular axes mutually orthogonal. Although the low-temperature phase of the special case with null calamitic term (PP0) is uniaxial and antinematically ordered, in the general case presented here both Monte Carlo and molecular-field approaches show that, for z close to zero, the models exhibit a low-temperature uniaxial nematic phase, followed by an antinematic one, and finally by the orientationally disordered one. On the other hand, for sufficiently large values of z, we only find evidence of uniaxial calamitic behavior, as expected by following the limiting cases.
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Affiliation(s)
- Fulvio Bisi
- Dipartimento di Matematica "F. Casorati", Università di Pavia, via Ferrata 1, I-27100 Pavia, Italy
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11
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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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12
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Chen HH, Lin HA, Lai YH, Lin SY, Chiang CH, Hsu HF, Shih TL, Lee JJ, Lai CC, Kuo TS. Enantiotropic Nematics From Cross-Like 1,2,4,5-Tetrakis(4′-alkyl-4-ethynylbiphenyl)benzenes and Their Biaxiality Studies. Chemistry 2012; 18:9543-51. [DOI: 10.1002/chem.201103453] [Citation(s) in RCA: 14] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/19/2011] [Revised: 04/14/2012] [Indexed: 11/07/2022]
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13
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Zhao C, Wang H, Ran X, Bai B, Zhang Y, Li M. Liquid Crystalline Behaviours of the Complexes Based on 1,3,4-Oxadiazole Derivative and Benzoic Acid. CHINESE J CHEM 2012. [DOI: 10.1002/cjoc.201100276] [Citation(s) in RCA: 3] [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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14
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Speetjens F, Lindborg J, Tauscher T, LaFemina N, Nguyen J, Samulski ET, Vita F, Francescangeli O, Scharrer E. Low nematic onset temperatures and room temperature cybotactic behavior in 1,3,4-oxadiazole-based bent-core mesogens possessing lateral methyl groups. ACTA ACUST UNITED AC 2012. [DOI: 10.1039/c2jm33705c] [Citation(s) in RCA: 46] [Impact Index Per Article: 3.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
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Grzybowski P, Longa L. Biaxial nematic phase in model bent-core systems. PHYSICAL REVIEW LETTERS 2011; 107:027802. [PMID: 21797641 DOI: 10.1103/physrevlett.107.027802] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 09/28/2010] [Revised: 05/05/2011] [Indexed: 05/31/2023]
Abstract
We determine the bifurcation phase diagrams with isotropic (I), uniaxial (N(U)) and biaxial (N(B)) nematic phases for model bent-core mesogens using Onsager-type theory. The molecules comprise two or three Gay-Berne interacting ellipsoids of uniaxial and biaxial shape and a transverse central dipole. The Landau point is found to turn into an I-N(B) line for the three-center model with a large dipole moment. For the biaxial ellipsoids, a line of Landau points is observed even in the absence of the dipoles.
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Affiliation(s)
- Piotr Grzybowski
- Marian Smoluchowski Institute of Physics, Department of Statistical Physics and Mark Kac Center for Complex Systems Research, Jagellonian University, Reymonta 4, Kraków, Poland
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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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Figueirinhas JL, Feio G, Cruz C, Lehmann M, Köhn C, Dong RY. Nuclear magnetic resonance spectroscopic investigations of phase biaxiality in the nematic glass of a shape-persistent V-shaped mesogen. J Chem Phys 2010; 133:174509. [DOI: 10.1063/1.3496491] [Citation(s) in RCA: 22] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
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Lehmann M, Köhn C, Figueirinhas J, Feio G, Cruz C, Dong R. Biaxial Nematic Mesophases from Shape-Persistent Mesogens with a Fluorenone Bending Unit. Chemistry 2010; 16:8275-9. [DOI: 10.1002/chem.201001214] [Citation(s) in RCA: 27] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022]
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Yoon HG, Kang SW, Dong RY, Marini A, Suresh KA, Srinivasarao M, Kumar S. Nematic biaxiality in a bent-core material. PHYSICAL REVIEW. E, STATISTICAL, NONLINEAR, AND SOFT MATTER PHYSICS 2010; 81:051706. [PMID: 20866246 DOI: 10.1103/physreve.81.051706] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 06/18/2009] [Revised: 02/21/2010] [Indexed: 05/29/2023]
Abstract
The results of a recent investigation of the nematic biaxiality in a bent-core mesogen (A131) are in apparent disagreement with earlier claims. Samples of mesogen A131 used in the two studies were investigated with polarized optical microscopy, conoscopy, carbon-13 NMR, and crossover frequency measurements. The results demonstrate that textural changes associated with the growth of biaxial nematic order appear at ∼149 °C. The Maltese cross observed in the conoscopic figure gradually splits into two isogyres at lower temperatures indicating phase biaxiality. Presence of the uniaxial to biaxial nematic phase transition is further confirmed by temperature trends of local order parameters based on 13C chemical shifts in NMR experiments. Frequency switching measurements also clearly reveal a transition at 149 °C. Differences between the two reports appear to be related to the presence of solvent, impurities, and/or adsorbed gases in samples of A131 used in the study of Van Le [Phys. Rev. E 79, 030701 (2009)].
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Affiliation(s)
- Hyung Guen Yoon
- Department of Physics, Kent State University, Kent, Ohio 44242, USA
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Porzio W, Destri S, Pasini M, Giovanella U, Ragazzi M, Scavia G, Kotowski D, Zotti G, Vercelli B. Synthesis and characterisation of fluorenone–thiophene-based donor–acceptor oligomers: role of moiety sequence upon packing and electronic properties. NEW J CHEM 2010. [DOI: 10.1039/c0nj00045k] [Citation(s) in RCA: 29] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
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Lehmann M, Seltmann J. Low temperature enantiotropic nematic phases from V-shaped, shape-persistent molecules. Beilstein J Org Chem 2009; 5:73. [PMID: 20300458 PMCID: PMC2839554 DOI: 10.3762/bjoc.5.73] [Citation(s) in RCA: 26] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/22/2009] [Accepted: 11/17/2009] [Indexed: 11/30/2022] Open
Abstract
A series of V-shaped, shape-persistent thiadiazole nematogens, based on an oligo(phenylene ethynylene) scaffold with ester groups connected via alkyloxy spacers, was efficiently prepared by a two-step procedure. Phase engineering results in an optimum of the mesophase range and low melting temperature when the nematogens are desymmetrised with a butoxy and a heptyloxy spacer. The mesophases are enantiotropic and over the whole temperature range nematic. For the optimised mesogen structure, optical investigations by conoscopy monitored a uniaxial nematic phase upon cooling from the isotropic phase to room temperature (ΔT = 150 °C). X-ray studies on magnetic-field-aligned samples of this mesogen family revealed a general pattern, indicating the alignment of two molecular axes along individual directors in the magnetic field. These observations may be rationalised with larger assemblies of V-shaped molecules isotropically distributed around the direction of the magnetic field.
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Affiliation(s)
- Matthias Lehmann
- Institute of Chemistry, Chemnitz University of Technology, Strasse der Nationen 62, 09111 Chemnitz, Germany.
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Vaupotic N, Szydlowska J, Salamonczyk M, Kovarova A, Svoboda J, Osipov M, Pociecha D, Gorecka E. Structure studies of the nematic phase formed by bent-core molecules. PHYSICAL REVIEW. E, STATISTICAL, NONLINEAR, AND SOFT MATTER PHYSICS 2009; 80:030701. [PMID: 19905050 DOI: 10.1103/physreve.80.030701] [Citation(s) in RCA: 24] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 06/10/2009] [Indexed: 05/28/2023]
Abstract
In recent years there are several reports showing that bent-core mesogenic molecules are able to form biaxial nematic phase in which molecular rotation around the long molecular axis is strongly hindered. The x-ray pattern with azimuthally split signals at low angle region of diffraction is usually given as evidence for the biaxial nematic phase. We show experimentally and theoretically that such x-ray pattern is due to the local smectic- C fluctuations ("cybotactic" groups) in the uniaxial nematic phase.
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Affiliation(s)
- N Vaupotic
- Department of Physics, Faculty of Natural Sciences and Mathematics, University of Maribor, Koroska 160, 2000 Maribor, Slovenia
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Lehmann M. The dawn in the area of thermotropic biaxial nematics Symposium on Biaxial Liquid Crystals, Kent, 27–28 October 2008. LIQUID CRYSTALS TODAY 2009. [DOI: 10.1080/13583140902853961] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/20/2022]
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26
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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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Lehmann M, Seltmann J, Auer AA, Prochnow E, Benedikt U. Synthesis and mesomorphic properties of new V-shaped shape-persistent nematogens containing a thiazole or a thiadiazole bending unit. ACTA ACUST UNITED AC 2009. [DOI: 10.1039/b818240j] [Citation(s) in RCA: 57] [Impact Index Per Article: 3.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
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Berardi R, Muccioli L, Orlandi S, Ricci M, Zannoni C. Computer simulations of biaxial nematics. JOURNAL OF PHYSICS. CONDENSED MATTER : AN INSTITUTE OF PHYSICS JOURNAL 2008; 20:463101. [PMID: 21693834 DOI: 10.1088/0953-8984/20/46/463101] [Citation(s) in RCA: 46] [Impact Index Per Article: 2.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/30/2023]
Abstract
Biaxial nematic (N(b)) liquid crystals are a fascinating condensed matter phase that has baffled, for more than thirty years, scientists engaged in the challenge of demonstrating its actual existence, and which has only recently been experimentally found. During this period computer simulations of model N(b) have played an important role, both in providing the basic physical properties to be expected from these systems, and in giving clues about the molecular features essential for the thermodynamic stability of N(b) phases. However, simulation studies are expected to be even more crucial in the future for unravelling the structural features of biaxial mesogens at the molecular level, and for helping in the design and optimization of devices towards the technological deployment of N(b) materials. This review article gives an overview of the simulation work performed so far, and relying on the recent experimental findings, focuses on the still unanswered questions which will determine the future challenges in the field.
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Affiliation(s)
- Roberto Berardi
- Dipartimento di Chimica Fisica e Inorganica, and INSTM-CRIMSON, Università di Bologna, Viale Risorgimento 4, 40136 Bologna, Italy
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Cross TA, Davis MC. Synthesis of Hydroxyalkyl‐Substituted, Push–Pull Chromophores based on Diphenylacetylenes and 1,4‐Bis(phenylethynyl)benzenes. SYNTHETIC COMMUN 2008. [DOI: 10.1080/00397910701796584] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/22/2022]
Affiliation(s)
- Tara A. Cross
- a Chemistry and Materials Division, Michelson Laboratory , Naval Air Warfare Center , China Lake, California, USA
| | - Matthew C. Davis
- a Chemistry and Materials Division, Michelson Laboratory , Naval Air Warfare Center , China Lake, California, USA
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Lehmann M, Köhn C, Kresse H, Vakhovskaya Z. Synthesis and properties of oxadiazole based V-shaped, shape persistent nematogens. Chem Commun (Camb) 2008:1768-70. [DOI: 10.1039/b718348h] [Citation(s) in RCA: 35] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
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High nematic ordering in a bent-core smectogen showing a biaxial nematic phase: A 13C NMR study. Chem Phys Lett 2007. [DOI: 10.1016/j.cplett.2007.09.070] [Citation(s) in RCA: 38] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/19/2022]
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Kaznacheev K, Hegmann T. Molecular ordering in a biaxial smectic-A phase studied by scanning transmission X-ray microscopy (STXM). Phys Chem Chem Phys 2007; 9:1705-12. [PMID: 17396182 DOI: 10.1039/b616010g] [Citation(s) in RCA: 17] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Abstract
Results of STXM investigations of a binary mixture (-TNF = 2 : 1; SmA(b) 140 M 180 Iso) known to form a SmA(b) phase [T. Hegmann, J. Kain, S. Diele, G. Pelzl and C. Tschierske, Angew. Chem. Int. Ed., 2001, 40, 887] are presented. Near edge X-ray absorption fine spectra (NEXAFS) of the -TNF board-like aggregates, in particular the intensity of the low energy peaks associated with aromatic ring pi* orbitals (284.5-286.5 eV), show that the molecular plane of these aggregates is very sensitive to the relative orientation of electric field vector E of linearly polarized light, which is used to determine the molecular orientation in the LC phase. The observed strong in-plane dichroic signal suggests the predominant orientation of the -TNF aggregates to be along the smectic layer normal as well as long-range ordering of the in-plane molecular orientation (biaxiality). Orientational maps derived from series of measurements at different sample rotation angles around the specimen normal clearly show a Schlieren-type texture, and permit a detailed examination of exclusive +/-(1/2) disclination theoretically predicted for the SmA(b) phase.
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Affiliation(s)
- Konstantin Kaznacheev
- Department of Physics & Engineering Physics and Canadian Light Source Inc., University of Saskatchewan, Saskatoon (SK), CanadaS7N 0X4.
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