Discotic Nematic Research Articles

A new approach to the textural interpretation of magnetically aligned nematic and cholesteric lyotropic mesophases by using plate-λ accessory

ABSTRACT Lyotropic liquid crystals are substances formed by anisotropic micelles. Their unit structure change with the concentration and temperature and, as a consequence, the optical anisotropy changes. A measurable optical parameter, which reflects these anisotropies, is the refractive indices. Using these indices, it is possible to represent the anisotropic medium by indicatrix figures. In this sense, the indicatrix orientations will be interpreted in this work along the discotic nematic ( N D ) – biaxial nematic ( N B ) – calamitic nematic ( N C ) phases using a plate- λ accessory, from the polarized light microscopy, to modify the interference colors along the Michel-Lévy chart of the textures exhibited by the sample. Furthermore, the discotic cholesteric ( C h D ) – biaxial cholesteric ( C h B ) – N C ∗ [ unwounded calamitic cholesteric ( C h C ) ] phases are also investigated by the same methodology. The optic sign is also discussed here. In addition, we will also present a procedure that may help us to recognize the N B − N C phase transition from the microscopic textures.

First examples of room-temperature discotic nematic liquid crystals exhibiting ambipolar charge carrier mobilities.

Four new room-temperature nematic liquid-crystalline (LC) dimers consisting of a wedge-shaped 3,4,5-tridecyloxy gallic ester molecule linked to either cyanobiphenyl, cholesteryl, pentaalkynylbenzene or triphenylene based moieties are reported. Dimers with pentaalkynylbenzene and triphenylene moieties in their room-temperature discotic nematic (ND) mesophase show ambipolar charge carrier mobilities of the order of 10-5 cm2 V-1 s-1 and 10-3 cm2 V-1 s-1, respectively, as measured using a time-of-flight (ToF) technique.

Novel green synthetic approach for liquid crystalline materials using multi-component reactions

We report the synthesis of disk-shape liquid crystalline materials using multi-component reactions (MCR). Two new series, 3,9-di(3′,4′,5′-tris-alkyloxy)phenyl-2,3,4,8,9,10-hexahydronaphtho[1,2-e:5,6-e']bis([1,3]oxazine) and 3,10-di((3′,4′,5′-tris-alkyloxy)phenyl)-2,3,4,9,10,11-hexahydronaphtho[1,2-e:4,3-e']bis([1,3]oxazine) synthesized by coupling either 2,3-dihydroxy naphthalene or 2,6-dihydroxy naphthalene with 3,4,5-tris-O-alkylated anilines in a solvent free condition. The molecular structure of the new materials was confirmed using 1H NMR, 13C NMR and CHN elemental analysis. The phase behavior investigated using polarizing optical microscope (POM) and differential scanning calorimetry (DSC) experiments. All the materials exhibit room temperature discotic nematic (ND) during the cooling cycles as confirmed by X-ray diffraction studies. This synthetic approach provides multiple advantages such as high atom economy (AE), short reaction time, gentle reaction condition, simple workup, and do not require column chromatography purification, additionally this could pave a new way in the synthesis of liquid crystalline compounds.

Phase transition detection in liquid crystal analysis by mathematical morphology

Light polarizing-microscopy is a technique applied to identify different phases in liquid crystal analysis. With the linear increase of the temperature of the liquid crystal sample, in the image sequence acquisition stage, changes in the images, generated by the microscope, are observed. Such changes are related to the different phases and phase transitions. Based on Mathematical Morphology, this work aims to propose a new method for detection and quantification of phase and phase transitions. For this purpose lyotropic liquid crystal samples, exhibiting the classic sequence of phases: reentrant isotropic (Ir)→ discotic nematic (ND)→ biaxial nematic (NB)→ calamitic nematic (NC)→ isotropic (I) are studied. The connected morphological operators were shown to be very efficient to extract information from the complex structures. Once the image sequence is acquired, it may be processed by techniques designed to phase transition detection. From the image analysis point of view, texture changes are more evident when observed the all image sequence, and it allows to extract many features that may be applied to detect phase transitions and to characterize each phase as well. In this proposal, such features are extracted from the residue of a graylevel area opening operator.

Molecular Engineering for the Development of a Discotic Nematic Mesophase and Solid-State Emitter in Deep-Blue OLEDs.

A unique strategy for the attainment of a discotic nematic (ND) mesophase is reported consisting of a central benzene core to which are attached two 4-alkylphenyl and two 4-pentylbiphenyl moieties diagonally via alkynyl linkers. The rotational nature and incompatibility of unequal phenylethynyl units led to the disruption of π-π interactions within cores that aids to the realization of ND phase and favors high solid-state emission. When used in OLEDs, compounds act as an efficient solid-state pure deep-blue emitter with Commission Internationale de L'Eclairage (CIEx,y) coordinates of (0.16, 0.07).

From rod-like to disc-like Gay–Berne biaxial nematics and back

ABSTRACTBy means of Monte Carlo simulations at constant pressure and temperature, the phase behaviour of pure systems of soft biaxial Gay–Berne ellipsoids, prototypical models for thermotropic mesogens, was determined as a function of temperature and three different pressures. To reduce the number of free shape and energy parameters to only one, the medium ellipsoid axis , particles possessing identical total volume and length were studied, with interactions parameterised in terms of products of axes lengths. The resulting phase diagrams present a variety of crystalline and liquid crystalline phases characterised by different biaxiality and preferential axis alignment, with GB particles behaving like rods for more elongated shapes, and like discs for more squashed ones. In agreement with previous simulation and theoretical studies using simpler models, the transition between rod to disc behaviour (and back) occurs approximately when the so-called dual-shape condition is reached, i.e. when . This shape ideally identifies also the position of a Landau point, in which at a specific temperature, isotropic, calamitic nematic, discotic nematic, and biaxial nematic may coexist, and hence a direct isotropic to biaxial nematic transition could in principle occur. We did not observe such event in our simulations.

Temperature Behavior of Refractive Indices of Discotic Nematics with High Birefringence

Temperature Behavior of Refractive Indices of Discotic Nematics with High Birefringence

Effect of Dopant Structure on Birefringences and Symmetric Tensor Invariants of Lyotropic Nematic Phases

In this study, we investigated the effect of the dopant structure on the birefringences and the symmetric tensor invariants of lyotropic discotic nematic (ND), calamitic nematic (NC) and biaxial nematic (NB) phases. Lyotropic host mixture consisted of potassium laurate (KL)/decanol (DeOH)/water. Some dopant molecules were added to this host mixture to obtain quaternary mixtures and to investigate the nematic phase properties by polarizing optical microscopy and laser conoscopy. It was found that the dopant molecules with aromatic ring affect the birefringences and symmetric tensor invariants more efficiently than those with straight alkyl chain.

Frank elasticity of composite colloidal nematics with anti-nematic order.

Mixing colloid shapes with distinctly different anisotropy generates composite nematics in which the order of the individual components can be fundamentally different. In colloidal rod-disk mixtures or hybrid nematics composed of anisotropic colloids immersed in a thermotropic liquid crystal, one of the components may adopt so-called anti-nematic order while the other exhibits conventional nematic alignment. Focussing on simple models for hard rods and disks, we employ Onsager-Straley's second-virial theory to derive scaling expressions for the elastic moduli of rods and disks in both nematic and anti-nematic configurations and identify their explicit dependence on particle concentration and shape. We demonstrate that the splay, bend and twist elasticity of anti-nematically ordered particles scale logarithmically with the degree of anti-nematic order, with the bend-splay ratio for anti-nematic discotic nematics being far greater than for conventional nematic systems. The impact of surface anchoring on the elastic properties of hybrid nematics will also be discussed in detail. We further demonstrate that the elasticity of mixed uniaxial rod-disk nematics depends exquisitely on the shape of the components and we provide simple scaling expressions that could help engineer the elastic properties of composite nematic liquid crystals.

Phase Behavior of a New Class of Anthraquinone-Based Discotic Liquid Crystals.

Five novel columnar liquid crystalline compounds (4.1-4.5) consisting of a central anthraquinone core carrying four alkoxy chains (R = n-C6H13, n-C8H17, n-C10H21, n-C12H25, and 3,7-dimethyl octyl) with two diagonally opposite 1-ethynyl-4-pentylbenzene units were synthesized, and their phase transitions were investigated between changes in the molecular structure and their self-assembly into the columnar mesophases. Small and wide-angle X-ray scattering (SAXS/WAXS) studies were performed to deduce the exact nature of the mesophases, and their corresponding electron density maps were derived from the intensities of the peaks observed in the diffraction patterns. A comparison of compounds with different alkoxy chains indicated that the soft crystal columnar rectangular (Crcolrec) phase was stable at lower temperature for the shortest peripheral alkoxy chain (4.1; R = n-C6H13) and was found to exhibit the columnar hexagonal (Colh) phase and then the discotic nematic (ND) phase with increasing temperature. In contrast, increasing the peripheral chain length to n-C8H17 or the branched one (4.2 and 4.5) stabilized the Colh phase at lower temperature and showed the ND phase at higher temperature. Further increase in chain length (4.3 and 4.4; n-C10H21, n-C12H25) demonstrated the formation of the ND phase. Conductivity measurement in the Colh mesophase was found to be almost 10 times higher in magnitude than the corresponding Crcolrec phase. The HOMO-LUMO band gap of all the compounds was found to be in the range from 2.79 to 2.82 eV, which is quite less and comparable with the optical energy band gap.

Room-Temperature Oligomeric Discotic Nematic Liquid Crystals over a Wide Temperature Range: Structure-Property Relationships.

The design and synthesis of three room-temperature discotic nematic (ND ) liquid crystals (LCs) is presented. The LC consists of an azobenzene core attached to which are four pentaalkynylbenzene (PA) units through flexible alkyl spacers. The presence of a short azo linking group provides more disorder in the system, thereby reducing the packing efficiency among the PA units and resulting into the formation of a room-temperature ND phase over a wide temperature range. Dielectric constant and birefringence studies were performed to gain further insights into the physical properties of the mesophase.

Stereodynamic control of star-epoxy/anhydride crosslinking actuated by liquid-crystalline phase transitions.

The epoxy/anhydride copolymerization kinetics of an original star-epoxy monomer (TriaEP) was explored in dynamic heating mode using a series of isoconversional methods. Negative values of the apparent activation energy (Eα) related to an anti-Arrhenius behavior were observed. The transition from Arrhenius to anti-Arrhenius behavior and vice versa depending on the Eα of polymerization was correlated with the dynamics of mesophasic fall-in/fall-out events, physically induced transition (PIT) and chemically induced transition (CIT). This self-assembly phenomenon induces the generation of an anisotropic crosslinked architecture exhibiting both nematic discotic (ND) and nematic columnar (NC) organization. Particular emphasis was placed on evaluating the juxtaposition/contribution of the liquid-crystalline transitions to crosslinking, considering both the reaction dynamics and the macromolecular vision.

A new strategy towards the synthesis of a room-temperature discotic nematic liquid crystal employing triphenylene and pentaalkynylbenzene units.

A new approach is reported for the design of a room-temperature discotic nematic (ND) liquid crystal (LC) dimer consisting of a triphenylene and a pentaalkynylbenzene unit linked via flexible alkyl spacers. The formation of the ND phase is realized most likely through folding of the dimeric molecule that prevent stacking between the triphenylene units, as suggested by modelling in the mesophase derived from X-ray scattering results and high-level DFT calculations.

Spinodal instabilities in polydisperse lyotropic nematics.

Many lyotropic liquid crystals are composed of mesogens that display a considerable spread in size or shape affecting their material properties and thermodynamics via various demixing and multi-phase coexistence scenarios. Starting from a generalized Onsager theory, we formulate a generic framework that enables locating spinodal polydispersities as well as identifying the nature of incipient size fractionation for arbitrary model potentials and size distributions. We apply our theory to nematic phases of both hard rods and disks whose main particle dimension is described by a unimodal log-normal distribution. We find that both rod-based and discotic nematics become unstable at a critical polydispersity of about 20%. We also investigate the effect of doping nematic assemblies with a small fraction of large species and highlight their effect on the stability of the uniform nematic fluid. Our main finding is that while rod-based are only weakly affected by the presence of large species, doping discotic nematics with very large platelets leads to a remarkable suppression of the spinodal instabilities. This could open up routes towards controlling the mechanical properties of nematic materials by manipulating the local stability of nematic fluid and its tendency to undergo fractionation-driven microphase separation.

Lattice model for biaxial and uniaxial nematic liquid crystals.

We use a lattice gas model to describe the phase transitions in nematic liquid crystals. The phase diagram displays, in addition to the isotropic phase, the two uniaxial nematics, the rod-like and discotic nematics, and the biaxial nematic. Each site of the lattice has a constituent unit that takes only six orientations and is understood as being a parallelepiped brick with the three axes distinct. The possible orientations of a brick are those in which its axes are parallel to the axes of a Cartesian reference frame. The analysis of the model is performed by the use of a mean-field approximation and a Landau expansion of the free energy.

Amphiphile/Water/Decanol Lyotropic Liquid Crystalline System: Study of Thermal States of Anisometric Micelles in Nematic-Calamitic and Nematic-Discotic Mesophases

Abstract Structural peculiarities of the rod-like micelles in the nematic-calamitic (NC) and disc-like micelles in the nematic-discotic (ND) lyotropic mesophases have been investigated. Hexadecyltrimethyl ammonium bromide/water/1-decanol ternary lyotropic system was used. Temperature changes of the shapes and anisometricity of the rod-like and disc-like micelles have been estimated. The method of the electrical conductivity anisotropy in the orientational shear flow has been used for these estimations. Sign of the diamagnetic anisotropy of the NC and ND mesophases has been also determined. The rod-like micelles of the NC mesophase were considered as the prolate ellipsoids and the disc-like micelles of the ND mesophase were considered as the oblate ellipsoids.

Liquid‐Crystalline Mesogens Based on Cyclo[6]aramides: Distinctive Phase Transitions in Response to Macrocyclic Host–Guest Interactions

AbstractProducing macrocyclic mesogens that are responsive to guest encapsulation presents a significant challenge. Cyclo[6]aramides, a type of macrocycle with a hydrogen‐bond‐constrained backbone, exhibit thermotropic lamellar, discotic nematic, hexagonal, and rectangular columnar mesophases over a considerably wide temperature range, including at room temperature. Additionally, cyclo[6]aramides show unusual mesophase transitions from lamellar to hexagonal columnar phase mediated by macrocyclic host–guest (H–G) interactions between the macrocycles and alkylammonium salts. The phase transition, triggered by an organic guest engaging in H–G interactions with a macrocyclic cavity, provides a novel strategy for manipulating the properties of liquid‐crystalline materials. The crystal structure of a homologous cyclo[6]aramide reveals a disk‐shaped, near‐planar molecular backbone that facilitates intermolecular π–π stacking and leads to columnar assembly.

Liquid-crystalline mesogens based on cyclo[6]aramides: distinctive phase transitions in response to macrocyclic host-guest interactions.

Producing macrocyclic mesogens that are responsive to guest encapsulation presents a significant challenge. Cyclo[6]aramides, a type of macrocycle with a hydrogen-bond-constrained backbone, exhibit thermotropic lamellar, discotic nematic, hexagonal, and rectangular columnar mesophases over a considerably wide temperature range, including at room temperature. Additionally, cyclo[6]aramides show unusual mesophase transitions from lamellar to hexagonal columnar phase mediated by macrocyclic host-guest (H-G) interactions between the macrocycles and alkylammonium salts. The phase transition, triggered by an organic guest engaging in H-G interactions with a macrocyclic cavity, provides a novel strategy for manipulating the properties of liquid-crystalline materials. The crystal structure of a homologous cyclo[6]aramide reveals a disk-shaped, near-planar molecular backbone that facilitates intermolecular π-π stacking and leads to columnar assembly.

Study of optical conoscopy in uniaxial and biaxial nematic lyotropic phases

The calamitic nematic (NC)–biaxial nematic (NB)–discotic nematic (ND) phase transitions are studied in a potassium laurate-decanol-potassium sulfate and water mixture by optical conoscopy. The NC−(ND+) phase is uniaxially negative (positive) as expected. A biaxial negative (NB−) nematic phase is optically characterized and confirmed through conoscopic image, inside the biaxial range, beyond the known biaxial positive (NB+) nematic phase. Conoscopic image of a biaxial nematic induced by material flow from ND+ phase is investigated and discussed in this work.

Phase diagram of tetradecyltrimethylammonium bromide (TTAB) + water + octanol system with application of mechanical deformation

Morphological properties of tetradecyltrimethylammonium bromide (TTAB)+water+octanol system in different concentrations have been studied. In the process, isotropic phase (L1) and nematic calamitic (NC), nematic discotic (ND), hexagonal E and lamellar D anizotropic mesophases have been determined by polarizing microscopy method and partial ternary phase diagram of the system set up. Textural properties of the anisotropic mesophases of the system have been discussed and their birefringence values measured. Mechanical deformation has been applied to the mesophases. The textural properties and the birefringence values have been observed to be changed by the deformation, after and before which changes have been compared.