Bent-core Mesogens Research Articles

Self-assembled hydrophobic surface generated from a helical nanofilament (B4) liquid crystal phase

Hydrophobic air/liquid crystal (LC) surfaces exhibiting self-assembled dual scale roughness have been made by simple cooling of a bent-core mesogen from its high temperature isotropic melt through two liquid crystal phases. The transition to the fluid smectic B2 phase generates micron-scale toric focal conic domains (TFCDs) at the surface. Upon further cooling into the hexatic smectic B4 phase these TFCD structures are preserved and become textured by the nanometer-sized helical nanofilaments (HNFs) of the B4. The resulting TFCD/HNF surface is hydrophobic and shows clear evidence for surface tension reduction characteristic of dual-scale roughness, suggesting a simple self-assembly-based approach to low surface tension surfaces using LC morphology.

Electric field effect on the phase diagram of a bent-core liquid crystal

We have investigated the effects of a low frequency electric field on the thermotropic behavior of a nematogenic bent-core liquid crystal by means of X-ray diffraction. We have determined for the first time the 2D phase diagram of the bent-core mesogen over an extended temperature-field (T, E) space. The results show that the electric field strongly affects both the mesophase sequence and the transition temperatures of the sample. An extraordinary field-induced shift of the nematic–isotropic phase transition is found, which has neither any experimental counterpart in conventional calamitic LCs, nor can be explained by the presently available theories. The phase diagram points out the existence of exotic phase transitions driven by either electric field or temperature along specific paths in the (T, E) space. In particular, the isothermal field-induced transition from the NCybC to the NCybA phase represents a breakthrough in the interpretation of previous XRD experimental results. These experimental findings provide further strong support of the now widely accepted cybotactic cluster picture of the nematic phase of bent-core mesogens.

Nanoconfinement of guest materials by helical nanofilament networks of bent-core mesogens

The helical nanofilament (HNF) liquid crystal phase is a complex, hierarchical self-assembly in which bent-core molecules cooled from the isotropic melt form dense periodic arrays of nanoscale filaments of twisted smectic layers, a structure driven by an inherent tendency of the layers to be locally saddle-splayed. Here we report the results of a study of mixtures of an HNF-forming bent-core liquid crystal with a variety of organic guest molecules, showing that the high temperature, isotropic phase is completely mixed but that upon cooling the filaments form and grow, expelling the guest material from their internal volume. The nanofilaments form a network which acts as a porous nanoconfinement medium of large internal area (∼100 m2 cm−3), with the guest material confined to nanoscale interstitial volumes between the filaments. Such helical nanofilament networks represent a robust platform for potential applications in chiral separations, asymmetric synthesis, and photonic metamaterials.

Bent-core compounds with two branched chains: evidence of a new dark conglomerate mesophase

Biphenyl-3,4′-diol derived bent-core compounds with branched side chains have been synthesized and characterized to study the influence of terminal branched groups on mesophase properties and transition temperatures of bent-core mesogens. The liquid crystalline properties of the compounds were investigated by differential scanning calorimetry, optical polarizing microscopy and X-ray scattering. Depending on the length of the chain, the size of molecular branches and the position of branched chains, either enantiotropic or monotropic rectangular columnar phases were observed. For one of the compounds combining two different branched side chains, a new type of optically isotropic dark conglomerate phase (MIso[*]) was found. It is proposed to be a distorted version of a non-tilted or only slightly tilted B6-like intercalated smectic phase with additional in plane order.

Tension of freely suspended fluid filaments

Stable fluid filaments with diameters of several micrometers and slenderness ratios well above 1000 are unique objects formed by some liquid crystalline phases of bent-core mesogens. We present a technique to determine filament tensions from their deflection under defined loads. A strong temperature dependence is observed, with a minimum near the clearing temperature. Both the nonlinear relation between filament tension and diameter and the substantial increase of the tension with lower temperatures indicate contributions of volume terms, in addition to surface capillary forces. We discuss a model that relates these bulk terms to elastic forces, originating from the undulated smectic layer structure. This model can explain the origin of the filament stability.

Liquid-crystalline heterodimesogens and ABA-heterotrimesogens comprising a bent 3,5-diphenyl-1,2,4-oxadiazole central unit.

Three new types of terminally connected ABA-heterotrimesogens and heterodimesogens, composed of a bent 3,5-diphenyl-1,2,4-oxadiazole central unit and one or two rod-shaped 4-cyanobiphenyl cores or one 2-phenyl-1,3,4-thiadiazole core, connected by flexible spacers, have been synthesized, and their mesomorphic behavior was studied by optical polarizing microscopy (PM), differential scanning calorimetry (DSC) and X-ray diffraction (XRD). All dimesogens exhibit broad ranges of cybotactic nematic phases (NcybA and NcybC), in some cases accompanied by additional mesophases (CybA or SmC) at lower temperature. The combination of the 3,5-diphenyl-1,2,4-oxadiazole unit with one cyanobiphenyl core leads to the removal of tilted smectic and cybotactic nematic phases (SmC, NcybC), which are replaced by the nontilted CybA phases and nematic phases composed of SmA-type clusters (NcybA). The orthogonal cybotactic nematic phases of bent-core mesogens are of special interest for achieving biaxial nematic phases of the orthorhombic type. The orthogonal (NcybA) and skewed (NcybC) cybotactic nematic phases were distinguished by XRD and optical observations.

Theoretical Design of the Molecular Structure of Bent-Core Mesogens with Large Second-Order Nonlinear Optical Properties.

The first hyperpolarizability of two series of molecules with bent-shaped structures has been calculated at the ab initio level. The two series consist of carboxyl derivatives for which some molecules are known to exhibit banana phases and of their sulfinate homologues that have not been synthesized yet. The final purpose is to reveal the relevance or not in synthesizing these latter molecules. The strategy is based on reporting the effect of the positions of donor and acceptor groups on the central benzene ring on the structural parameters and on the first hyperpolarizabilities. It is shown that in the sulfinate series the larger first hyperpolarizabilities are incompatible with efficient geometrical character while for the carboxyl series, the opening angle between the molecular arms is typical of banana-shaped molecules but the first hyperpolarizability is only slightly enhanced. Calculations thus reveal that the sulfinate series does not need to be synthesized, whereas they show that our approach can ...

Laterally substituted symmetric and nonsymmetric salicylideneimine-based bent-core mesogens

Bent-core mesogens have gained considerable importance due to their ability to form new mesophases with unusual properties. Relationships between the chemical structure of bent-core molecules and the type and physical properties of the formed mesophases are relatively unknown in detail and differ strongly from those known for calamitic liquid crystals. In this paper symmetric and nonsymmetric five-ring salicylideneaniline-based bent-core mesogens are presented, and the effect of lateral substituents attached at the outer phenyl rings (F, Cl, Br) or the central phenyl ring (CH3) on the liquid-crystalline behaviour and on the physical properties is studied. Corresponding benzylideneaniline-based compounds were additionally prepared in order to study the influence of the intramolecular hydrogen bond. The occurring mesophases were investigated by differential scanning calorimetry, polarising microscopy, X-ray diffraction and dielectric and electro-optical measurements. The paper reports on new findings with respect to the structure–property relationships of bent-core mesogens. On one hand, the disruptive effect of laterally substituted halogen atoms, F, Cl and Br, on the mesophase behaviour of three isomeric series was much lower than expected. On the other hand, an increase of the clearing temperature by 34 K was observed, caused by small lateral substituents. The electro-optical behaviour, especially the type of polar switching and corresponding molecular movements, is sensitive to variations in the molecular structure.

Progress in liquid crystal chemistry II

It is a great pleasure to introduce this second Thematic Series on Progress in liquid crystal within the Beilstein Journal of Organic Chemistry. Why liquid crystals? Liquid crystals have a major impact on our modern society, reaching from simple digital calculators to high-resolution flat-panel displays, and their market volume is expected to further increase. For example, in 2010 revenues of €1,013 million from sales of liquid crystals were reported by Merck, the world’s largest supplier of liquid crystal materials for electro-optical applications. This increase of 38% on 2009 was attributed to the demand for liquid crystal displays (LCDs). Forecasts predict annual global shipments of over 268.8 million LCD television sets by 2014 [1]. However, the chemistry behind liquid crystals, and particularly the synthesis, is the platform for such tremendous technological achievements, and this is true for both calamitic (rod-like) as well as discotic (disk-shaped) or other types of liquid crystals [2–4]. Since our initial Thematic Series Progress in liquid crystal two years ago [5], the research field has further broadened and stretches currently from materials science, through energy conversion and storage, to life sciences. The interested reader might be directed to a very recent special issue of Liquid Crystals edited by John W. Goodby to find more detailed information [6]. The review articles and original research papers of this second Thematic Series in the Beilstein Journal of Organic Chemistry, written by known experts of their field, cover diverse topics such as novel discotic and calamitic compounds, bent-core mesogens, amphiphiles, and liquid-crystalline nanoparticles. It is our goal to give the reader insight into both synthetic aspects of liquid crystal chemistry as well as application-oriented properties, such as photoconductivity or chirality transfer, to name just a few selected examples. Finally, we would particularly like to inspire people with complementary expertise, working in different fields of chemistry, to join forces with the liquid crystal community and to introduce their specific knowledge and objectives in order to aid the further advancement of this topic. Sabine Laschat Stuttgart, January 2012

Low nematic onset temperatures and room temperature cybotactic behavior in 1,3,4-oxadiazole-based bent-core mesogens possessing lateral methyl groups

As part of our efforts to access the biaxial nematic phase at low temperatures, we prepared a series of 1,3,4-oxadiazole-based bent-core mesogens that possess either one or three lateral methyl groups. The phase behavior of these derivatives was characterized using polarizing microscopy and differential scanning calorimetry. Target compounds containing one methyl group showed large nematic ranges but relatively high clearing points. The derivatives with three methyl substituents showed lower nematic onset temperatures and two of these compounds supercooled in the nematic phase to room temperature. X-ray diffraction experiments confirmed the presence of cybotactic clusters in the nematic phase, as observed in other bent-core mesogens. However, for two of these derivatives, cybotaxis persists at room temperature, due to the formation of a glassy nematic phase at low temperatures. These results suggest that these materials could be promising candidates in the search for low temperature biaxial thermotropic nematics.

Laterally connected bent-core dimers and bent-core-rod couples with nematic liquid crystalline phases

First examples of laterally connected bent-core dimesogens and liquid crystalline dimesogens involving bent-core units and rod-like units are reported. Four distinct types of dimesogens have been synthesized and investigated by polarizing microscopy (PM), differential scanning calorimetry (DSC) and X-ray diffraction (XRD) studies. Two of them combine two bent-core mesogens derived from 4-cyanoresorcinol either in a lateral–lateral or lateral–terminal topology, the other two types combine a 4-cyanoresorcinol based bent-core mesogen with a rod-like mesogen in a lateral–lateral or lateral–terminal manner. All compounds, except one, exhibit nematic phases composed of SmC-type cybotactic clusters (NcybC) with wide phase ranges and low crystallization temperatures. Two compounds show additional low temperature phases, an SmC phase and a rectangular columnar phase were observed.

Uniaxial to biaxial nematic phase transition in a bent-core thermotropic liquid crystal by polarising microscopy

Polarising optical microscopy (POM) shows evidence for a reversible uniaxial nematic (NU) to biaxial nematic (NB) phase transition in a quiescent melt of bent-core mesogens (BCM). A second superimposed Schlieren texture, attributed to a second director field, grows into the NU phase on lowering the temperature and disappears again on heating, indicating a fully reversible NB to NU phase transition. This POM result together with recent X-ray data provides a new signature for the NB phase in this class of BCMs, and offers a new method for assessment of new biaxial nematic phases.

Chirality of an achiral bent-core nematic mesogen observed in planar and homeotropic cells under certain boundary conditions

It is shown that planar and homeotropic cells under certain boundary conditions containing an achiral bent-core C5 liquid crystalline material show chiral domains of opposite handedness. However such domains are not seen in other members of the homologous series which show cybotactic clusters of larger sizes. The stability of such chiral domains in a planar cell depends not only on temperature but also on the device parameters such as the cell gap and the angle between the rubbing directions on the substrates. The energy difference between the two states with different twist angles measured with respect to the rubbing direction is expressed in terms of the device parameters. The chirality of the material with a pitch length of 9 μm is discussed in terms of the molecular symmetry created by the formation of a pair of the two bent-core molecules, which acts as a chiral dopant. The results of the study on C5 are compared with a prediction from a theoretical model with higher order molecular interactions in the bent-core system and it is suggested that the observed phase is a tetrahedratic chiral nematic phase of D2 symmetry as recently suggested by [Longa et al., Phys. Rev. E: Stat., Nonlinear, Soft Matter Phys., 2009, 79, 040701].

Unexpected liquid crystalline behaviour of three-ring bent-core mesogens: bis(4-subst.-phenyl) 2-methyl-iso-phthalates

Download options Please wait... Supplementary files Supplementary information PDF (133K) Crystal structure data TXT (14K) Article information DOI https://doi.org/10.1039/C2SM07064B Article type Paper Submitted 30 Oct 2011 Accepted 02 Dec 2011 First published 25 Jan 2012 Download Citation Soft Matter, 2012,8, 2671-2685 BibTex EndNote MEDLINE ProCite ReferenceManager RefWorks RIS Permissions Request permissions Unexpected liquid crystalline behaviour of three-ring bent-core mesogens: bis(4-subst.-phenyl) 2-methyl-iso-phthalates W. Weissflog, U. Baumeister, M. Tamba, G. Pelzl, H. Kresse, R. Friedemann, G. Hempel, R. Kurz, M. Roos, K. Merzweiler, A. Jákli, C. Zhang, N. Diorio, R. Stannarius, A. Eremin and U. Kornek, Soft Matter, 2012, 8, 2671 DOI: 10.1039/C2SM07064B To request permission to reproduce material from this article, please go to the Copyright Clearance Center request page. If you are an author contributing to an RSC publication, you do not need to request permission provided correct acknowledgement is given. If you are the author of this article, you do not need to request permission to reproduce figures and diagrams provided correct acknowledgement is given. If you want to reproduce the whole article in a third-party publication (excluding your thesis/dissertation for which permission is not required) please go to the Copyright Clearance Center request page. Read more about how to correctly acknowledge RSC content. Social activity Tweet Share Search articles by author Wolfgang Weissflog Ute Baumeister Maria-Gabriela Tamba Gerhard Pelzl Horst Kresse Rudolf Friedemann Günther Hempel Ricardo Kurz Matthias Roos Kurt Merzweiler Antal Jákli Cuiyu Zhang Nicholas Diorio Ralf Stannarius Alexey Eremin Ulrike Kornek

Measurement of the Tension of Freely Suspended Smectic Liquid Crystal Filaments

Fluid filaments with diameters of several micrometers and large aspect ratios can be prepared from some layered liquid crystalline phases of organic bent-core mesogens. An internal structure stabilizes them against the Rayleigh-Plateau instability, but the filament material can flow freely along their axis. In equilibrium, they adopt a straight minimal shape as a consequence of the filament tension. We discuss methods for the measurement of this tension, and apply one of these methods, the deflection of horizontally suspended filaments under defined loads, to obtain quantitative tension data.

Photoisomerization-induced stable liquid crystalline cubic blue phase

We have succeeded in obtaining a photoinduced liquid crystalline cubic BP in a mixture of photoinactive and photoresponsive bent-core mesogens. The UV stimulus can convert N* to cubic BP through photoisomerization of the azobenzene linkages included in the photoresponsive bent-core molecule.

Alignment by Langmuir/Schaefer Monolayers of Bent-Core Liquid Crystals

Bulk alignment of liquid crystalline phases is achieved using self-organized thin films of bent-core mesogens transferred to a glass substrate by Inverse-Langmuir-Schaefer (ILS). We discuss the importance of the architecture of the aligning molecules (hydrophilic/hydrophobic balance for film stability as well as other structural factors) and show the dependence of the density of the monolayer with the alignment induced by it. These results are compared with molecular simulations for further understanding of molecular packing and interfacial interactions.

RECENT DEVELOPMENTS IN NEMATOGENIC BENT-CORE MESOGENS: AN X-RAY DIFFRACTION PERSPECTIVE

Bent-core mesogens are attracting great attention because of their potentially biaxial and polar nematic phase. In this paper, we review the results of our recent X-ray diffraction studies on the nematic phase of oxadiazole-based mesogens. The evolution of the small-angle diffraction signal with temperature points to the existence of nano-sized cybotactic clusters, characterized by short-range smectic-like fluctuations. However, in contrast to classical cybotaxis of calamitic nematics, these fluctuations do not have a pretransitional nature. They explain most of the extraordinary properties of bent-core mesogens and, in particular, are responsible for the unique ferroelectric response exhibited by one of our compounds.

Extraordinary Magnetic Field Effect in Bent-Core Liquid Crystals

A bent-core mesogen that forms a cybotactic nematic phase exhibits a giant magnetic field-induced shift of its nematic-isotropic and smectic-C-nematic transition temperatures: ΔT(H) = 4 K for H = 10 kOe. In contrast with molecular nematics, in cybotactic nematics the field couples with the anisotropic susceptibility of clusters containing several hundred partially ordered molecules. X-ray diffraction data corroborate a quantitative estimate of inferred cluster size (∼300 molecules). The results represent an unequivocal demonstration of the cluster picture of the nematic phase of this class of nonlinear liquid crystals.

Preparation and Single Crystal Structure of a Dirhenium(I) Complex of a Bent-Core Mesogen

Dinuclear complexes of ReI are prepared by functionalising a symmetric, bent-core mesogen containing two imine groups. The resulting complex is characterised by single crystal X-ray diffraction and presents the expected geometry, but unfortunately the introduction of the two Re(CO)4 moieties suppresses the mesomorphism of the parent ligand.