Disc-like Mesogens Research Articles

Low–temperature biaxial nematic from rod and disc mesogen mixture

Molecular dynamics computer simulations have been employed to demonstrate that the addition of a suitable uniaxial discotic nematic to a biaxial nematic (Nb) of elongated mesogens can give rise to a low temperature Nb liquid crystal (where the rod-like particles would form a biaxial smectic). The two species are made to be fully miscible over the entire isotropic and nematic temperature range by a suitable parameterisation of shapes and pair interactions. The disc-like mesogen, even though not forming a biaxial nematic by itself gives a stable low-temperature discotic nematic which strongly disfavours the formation of columnar and smectic phases in the two-component system.

Synthesis and Luminescence Properties of a Lollipop-Shaped Molecule Combined with Rod and Disc-Like Mesogens

A novel lollipop-shaped molecule containing 1,3,4-oxadiazole moieties was designed and synthesized. The molecular structure of this 1,3,4-oxadiazole derivative was confirmed by FT-IR, 1H-NMR spectroscopy and elemental analyzer. Its phase behavior and mesomorphic property were investigated by DSC, POM and X-ray diffraction measurements. We found that this compound showed a typical mosaic texture of columnar phase during cooling run. The electron excitation and luminescence properties of this compound were investigated by UV-vis absorption spectroscopy and photo-luminescence spectroscopy both in CHCl3 solution and in solid state.

Columnar mesophases constructed by hierarchical self-organization of rod-like diacetylene molecules

We report a new class of liquid crystalline system exhibiting columnar mesophases, which are constructed by the hierarchical supramolecular self-organization of a rod-like diacetylene derivative as the primary building block. A series of 1,3-alkadiynyl-4-benzoic acids (mmDA) with different alkyl chain lengths (m = 8–16) were synthesized and their mesophase properties were investigated by differential scanning calorimetry, polarized optical microscopy, and X-ray diffraction under temperature control. The mmDA exhibited three types of mesophases, i.e., a discotic lamellar phase, DL, and two columnar rectangular phases, Colrd and Colro, with disordered and ordered stacking structures of the mesogens in a column, respectively, depending on the length of the alkyl substituent and temperature. We have revealed that the columnar mesophase structures are constructed with the rod-like mmDA molecules as the starting building blocks by hierarchical self-organization. The hydrogen-bonded dimers of the carboxylic acid with a rigid framework are assembled to provide a disc-like mesogen unit, which consists of a π-stacked core-part and peripheral alkyl-chains. The discotic supramolecular mesogens further stack in ordered and disordered columnar structures, producing the Colro and Colrd mesophases, or in the layer DL mesophase. This is a rare example for the discotic mesophases using the rod-like diacetylenes with a non-branched molecular structure.

Synthesis and Luminescence Properties of Dumbbell-Shaped Molecules Combined with Rod and Disc-Like Mesogens

A novel series of dumbbell-shaped molecules containing 1,3,4-oxadiazole moieties (OXDn: n = 1, 2, and 3) was designed and synthesized. The molecular structures of these 1,3,4-oxadiazole derivatives were confirmed by FT-IR, 1H-NMR spectroscopy and elemental analyzer. The electron excitation and luminescence properties of these compounds were investigated by UV-vis absorption spectroscopy and photoluminescence spectroscopy both in CHCl3 solution and in solid state. The absorption peaks were found at 336 nm-351 nm, and the photoluminescent peaks at 410 nm-450 nm. The thermal properties were investigated by DSC method. Their melting temperatures were in the range of 81-95°C depending on the structure. Especially, OXD1 could show a discotic columnar mesophase at 75°C monotropically.

Microwave‐assisted Synthesis, Structure, and Tunable Liquid‐Crystal Properties of 2,5‐Diaryl‐1,3,4‐Thiadiazole Derivatives through Peripheral n‐Alkoxy Chains

A series of substituted 2,5-diaryl-1,3,4-thiadiazole derivatives are prepared by microwave-assisted synthesis in the absence of an organic solvent. All derivatives are well characterized by (1)H and (13)C NMR, MS, and elemental analyses. The X-ray crystal structure of 2,5-di-(4-decyloxyphenyl)-1,3,4-thiadiazole reveals the tilt lamellar arrangement of rod-shaped molecules, which are stabilized by a variety of weak non-covalent interactions. The liquid crystalline properties are studied by polarized-light optical microscopy (POM), differential scanning calorimetry (DSC), and in situ variable temperature X-ray diffraction (VTXRD). By variations in the peripheral n-alkoxy chains, the calamitic mesogens exhibit enantiotropic smectic (SmC and/or SmA) mesophases with wide mesomorphic temperature ranges, whilst the disc-like mesogens form hexagonal columnar mesophase (Col(h)) at room temperature. The bulk electrical conductivity values of the smectic mesophases of 1-3 are in the range of 10(-3)-10(-4) S cm(-1), which are slightly higher than that of their solid films. In contrast, the solid film made from 2,5-di-(3,4,5-trioctyloxyphenyl)-1,3,4-thiadiazole shows poor conductivity (2x10(-7) S cm(-1)).

Biaxial Molecular Arrangement of Rod−Disc Molecule under an Electric Field

We developed rod−disc liquid crystalline molecules consisting of six calamitic mesogens and one discotic mesogen, which exhibit a biaxial nematic phase. In this ordered nematic phase, rodlike mesogens and disclike mesogens are completely mixed and intercalated.

Molecular Parameters of Glass Forming Disc-Like Mesogens

With the help of the HyperChem Pro 6.0 program we have constructed and optimized the models for a new series of molecular structures, that comprise 43 non-mesogenic and mesogenic disc-like, glass-forming and non-glass-forming compounds of known structure with the domination of glass-forming, disc-like mesogens. We have calculated the molecular parameters for all the compounds of the series. Later, with the help of these parameters, the possibility of selection of the next two series was analyzed: 1) the class of disk-like mesogens 2) the sub-class of glass-forming, disc-like mesogens in the disc-like mesogen class. It has been defined that in this series of compounds the disc-like mesogen class can be selected the probability within a 90% probability range, while the sub-class of glass-forming disc-like mesogens was selected within the 60% probability range.

The phase behavior of a binary mixture of rodlike and disclike mesogens: Monte Carlo simulation, theory, and experiment

The phase behavior of a binary mixture of rodlike and disclike hard molecules is studied using Monte Carlo NVT (constant number of particles N, volume V, and temperature T) computer simulation. The rods are modeled as hard spherocylinders of aspect ratio LHSC/DHSC=5, and the discs as hard cut spheres of aspect ratio LCS/DCS=0.12. The diameter ratio DCS/DHSC=3.62 is chosen such that the molecular volumes of the two particles are equal. The starting configuration in the simulations is a mixed isotropic state. The phase diagram is mapped by changing the overall density of the system. At low densities stabilization of the isotropic phase relative to the ordered states is seen on mixing, and at high densities nematic–columnar and smectic A–columnar phase coexistence is observed. Biaxiality in the nematic phase is not seen. The phase diagram of the mixture is also calculated using the second virial theory of Onsager for nematic ordering, together with the scaling of Parsons and Lee to take into account the higher virial coefficients. The disc–disc and rod–disc excluded volumes are evaluated numerically using the exact overlap expressions, and the lower-order end-effects are incorporated. The exact rod–rod excluded volume is known analytically. In the case of the theoretical calculations, which are limited to translationally disordered phases, coexistence between two uniaxial nematic phases is predicted, as well as the stabilization of the disc-rich isotropic phases. As found in the simulation, biaxial nematic phases are not predicted to be stable. The phase equilibria of an experimental system is also reported which exhibits a behavior close to the system studied by computer simulation. As in the model mixtures, this system exhibits a marked destabilization of the ordered phases on mixing, while nematic–columnar demixing is observed at lower temperatures (the higher-density states).

Design and Constitutional Features of Mesophase-Forming Uni- or Supramolecular Discotic Materials [1,2

After introductory comments on the situation of liquid crystal work and its importance in scientific, technological, and social respect before the discovery and start of systematic exploration of non-linear, flat, i.e., disc-shaped mesogens twenty-five years ago, illustrative thermotropic liquid crystalline uni- or supramolecular discotics are discussed in relation to geometric features of their constituent molecules or aggregates. These materials are divided into two main groups: radial-symmetric disc-like and unconventional non-linear mesogens. The first main group deals, first of all, with representative standard and thereafter with organometallic mesogens, whereas the second one is sub-divided into four parts: bulky unimolecular mesogens; hydrogen bonding and molecular recognition; polycatenar types of mesogens; inter- and intramolecular EDA/CT complexes. Clearly, the significance of micro-segregation for the mesophase formation in cases of unconventional molecular conditions is emphasized as well.

Columnar mesophase from a new hybrid siloxane-triphenylene

The design, the synthesis and the mesomorphic properties of a new disc-like mesogen based on a hexasubstituted triphenylene core terminated with pentamethyldisiloxane pendant groups are reported. The siloxane fragments were grafted to the six terminal olefinic branches of a triphenylene precursor by a Pt-catalysed hydrosilylation reaction. Both the hexaolefinic and the hexasiloxane derivatives are mesomorphic, showing a rectangular columnar (ColR—p2gg) mesophase between 55 and 88 °C for the former, and a broad hexagonal (ColH—p6mm) mesophase between ambient temperature and 125 °C for the latter. The liquid crystalline behaviour was investigated by means of differential scanning calorimetry, polarised-light optical microscopy, and X-ray diffractometry. The mesophase temperature range and stability were considerably extended for the hybrid compound with respect to the siloxane-free compound. This behaviour is explained on the basis of enhanced microsegregation between the chemically distinct constituent components of the molecule.

Columnar Mesophase from a New Disclike Mesogen Based on a 3,5-Dicyano-2,4,6-tristyrylpyridine Core

We report on the design and synthesis of a new disclike mesogen based on a 3,5-dicyano-2,4,6-tristyrylpyridine core. This compound was characterized by spectroscopic methods (NMR, UV−vis, and photoluminescence) and cyclic voltammetry. The mesogenic behavior was investigated by differential scanning calorimetry, polarized-light optical microscopy, and X-ray diffraction. The disclike structure self-assembles into a hexagonal columnar mesophase and, because of the permanent dipole exhibited by the isolated molecule, the stacking along the columnar axis is antiparallel. Besides two-dimensional charge transfer and fluorescence, the disclike molecule exhibits high electron affinity as a result of the strong acceptor character of the 3,5-dicyanopyridine core.

Structural Effects on the Mesophase Formation of Certain Thermotropic Liquid Crystalline Random Copolyesters

Thermotropic random copolyesters containing 2,2′-biphenylene rings, disclike mesogens in the polymer chain, were synthesised by the polycondensation method and characterised by viscosity measuremen...

Acoustical Properties in the Region of Phase Transitions in the Aqueous Systems of Lyotropic Mesogens

Abstract The results of the acoustic investigation are reported for phase transitions in aqueous systems of rod-like and disc-like mesogens, forming the lyotropic mesophases of different types.

Self-order in flexible non-mesogenic macromolecules

Some very flexible linear poly(di-n-alkylsiloxane)s and cyclolinear oligo- and polyorganosiloxanes, which do not contain classical rigid rodlike or disclike mesogens, similar to other semi-organic polymers like polyphosphazenes and polysilanes, can form thermodynamically stable two-dimensionally ordered thermotropic mesophases. In many respects the mesophases in polysiloxanes resemble columnar liquid crystals which seems to be the result of the amphiphilic constitution of the polyorganosiloxanes. The origin of the mesomorphic state seems to be closely related to the competition between the interaction of the polar inorganic backbones and those of the organic side chains facilitated by the high flexibility of the inorganic main chain. Similarily to discotic liquid crystals, mesophases in polysiloxanes consist mostly of columns packed hexagonally (or closely so) with lateral long-range positional order, but with only short-range order along the columns. The unique feature of the columnar mesophases is that the axes of macromolecules lie along the column axes. The influence of molecular structure and molecular mass of linear macromolecules is discussed. It has been demonstrated that the temperature interval of mesophase stability is critically dependent on the length of the alkyl side chains. Both the process of formation of the mesophase from the isotropic melt and the crystallization of the mesophase are nucleation controlled processes, although the crystallization of the mesomorphic state is characterized by some specific features. The phase behaviour of cyclolinear polyorganosiloxanes is described with particular emphasis on the temperature stability as a function of molecular structure, tacticity, substituents and flexible spacers.

Molecular engineering of liquid-crystalline polymers: architecture and functionalization

In the past few years molecular engineering of liquid-crystalline (LC) polymers with respect to molecular architecture and functionalization has become increasingly important. Molecular architecture of LC polymers, i.e. the variation of the arrangement of mesogens, the variation of their shapes (rod, disc, board) and the variation of the polymer backbone, leads to polymers with new LC phases and new properties. Functionalized or dye-containing polymers can be used for a destruction or even the formation of the LC phase by photoreactions as well as for photochromic effects. The induction of discotic phases in amorphous polymers with disc-like mesogens is possible by charge-transfer interactions and opens the accessibility for a wide variety of highly oriented polymer systems.

On the Occurrence of Point and Ring Defects in the Nematic-Nematic Coexistence Range of a Binary Mixture of Rod-like and Disc-like Mesogens

We report some observations on the director configurations in the coexistence range of two nematic phases, NR and ND in a binary mixture of rod-like and disc- like nematogens. The director alignment is continuous across the boundary of the NR and ND phases. Point defects of strength + 1 in NR droplets in an otherwise continuous ND medium are found to be annulled by singular ring disclinations of strength - 1/2 Further, the NR drops which orient homeotropically at the glass plates have a point defect of strength + 1 only if their size is less than about 7 μm. Larger drops have ring disclinations of strength + 1/2 instead of + 1 point defects. This changeover can be understood on the basis of simple energy considerations.

Observation of a re-entrant isotropic phase in a pure disc-like liquid crystal

Abstract We report the observation of an optically isotropic phase in the new disc-like mesogen 2,3,7,8,12,13-hexa(octadecanoyloxy)truxene. On cooling, this optically isotropic phase goes over into a nematic phase, while on heating a columnar phase grows in. To our knowledge this is the first example of a pure discotic mesogen exhibiting an isotropic phase below the temperature existence of a columnar phase. Our findings are supported by refractive index measurements of this compound as a function of temperature.

Generalized Asymmetry of Thermotropic and Lyotropic Mesogens

Abstract The multipole model of molecular asymmetry of rod-like and disc-like mesogens was developed by us earlier (1981). It included the electrical, sterical and biphilic asymmetry. That model was generalized later on (1985) by including the flexibility asymmetry as well. Accordingly, mesogens were classified as generalized multipoles: monopoles, dipoles, quadrupoles etc. Furthermore, the generalized dipoles were considered as functions of the generalized fields by introducing generalized polarizabilities. These new lines of development of the model are reviewed here. They are especially important for lyotropics and membranes. Since 1981 a number of new mesogens corresponding to the theoretical predictions of the model were synthesized. We are applying our general classification to the following substances: cone-shaped discotics, flexible core discotics, discoidal amphiphilics, phasmids, biforked mesogens etc. Peculiarities in the structural organization of the tilted compounds are qualitatively discuss...

Phase behaviour under pressure of two hexasubstituted triphenylenes

The phase behaviour under pressure of two disc-like mesogens are studied with a metabolemeter. The investigated compounds are (-)-2,3,6,7,10,11-hexa-[S-(3-methyl)- n-nonanoyloxy] triphenylene and 2,3,6,7,10,11-hexa-( n- dodecanoyloxy) triphenylene. The weakly first order or practically second order mesomorphic transitions are detected. The pressure-temperature phase diagrams are established. The volume changes for the first order phase transitions are determined.

Disc-like Mesogens: Synthesis and Characterization of a Series of Rufigallol-hexa-n-alkanoates

Abstract Aliphatic esters of rufigallol with formula have been prepared and their phase transitions studied by differential scanning calorimetry, x-ray diffraction, and optical methods. Compounds with 6 < m < 13 give enantiotropic columnar mesophases; those with 7 < m < 9 give also a monotropic mesophase. Both melting temperature and entropy are sensitive to the parity of the number of atoms contained in the flexible part of the molecules. No similar effect is present at the isotropization transition. Calorimetric data relative to the phase transitions, x-ray and optical patterns of the liquid crystalline phases are shown.