Rod-like Mesogens Research Articles

Phase behavior of some mono-substituted ferrocene- and [3]ferrocenophane-containing nematics with the cyclohexane ring in the rigid core

Generally, incorporation of the cyclohexane rings into the rigid core of rod-like mesogens leads to improved technological parameters, i.e. low viscosity, ambient transition temperatures and stability of the nematic state. Taking this into consideration, a series of novel cyclohexane-containing derivatives of ferrocene has been synthesized. The effect of various structural factors on liquid crystalline behavior of the synthesized ferrocene-containing nematics has been examined. Ferrocenophane compounds exhibited enhanced liquid crystalline properties in comparison with the derivatives of unbridged ferrocene. Depending on thermal prehistory of the samples, some of the synthesized ferrocenomesogens showed remarkable migration of the phase transition temperatures. In one case such behavior led to stabilization of the initially monotropic nematic mesophase in subsequent heating cycles. In another case, the phase transition shifts caused the lowering of the crystal-to-nematic transition temperature and the broadening of the mesophase range. There was also a case of alteration from the initially enantiotropic to monotropic behavior. The obtained novel metallomesogens are important footsteps toward the development of low-viscous and low-temperature materials for liquid crystal applications possessing a chromophoric, redox-switchable, polarizable and chemically stable superaromatic ferrocene unit.

Dynamics in the isotropic and nematic phases of bent-core liquid crystals: NMR perspectives

The dynamic properties of low-molecular-weight bent-core liquid crystals were revealed to be unusual with respect to the analogous rod-like mesogens. First pioneering investigations based on dynamic light scattering measurements were followed by intense and deep studies based on nuclear magnetic resonance (NMR) spectroscopy, mainly 2H NMR relaxation and line width analyses, 1H NMR relaxometry and diffusometry. These studies were of help in identifying the molecular motions responsible of the observed slow relaxation and the large 2H NMR spectral width. Viscosity and rheology measurements provided further elements confirming the slow dynamics of bent-core liquid crystals at a macroscopic level. All these experimental techniques indicate the presence of restricted dynamic motions as a distinctive feature of bent-core liquid crystals both in the isotropic and nematic phases. In this review, the case of several bent-core mesogens studied by different NMR methods is reported and the main results are discussed in view of the recent achievements concerning the tendency of bent-core molecules to form aggregates or temporary clusters both in the isotropic and nematic phases.

Film compensation of twisted nematic liquid crystal display using a rod-like reactive mesogen

A hybrid aligned nematic liquid crystal compensation film using UV curable rod-like mesogen has been made to compensate the residual birefringence of a twisted nematic (TN) device at oblique viewing directions in the dark state. Effective retardation value of the fabricated film was characterized by experiment and calculation using simulation. The results showed that the film has a hybrid alignment with a tilt angle variation from 3° to 19°. The optical compensation effects are evaluated in terms of light leakage in the dark state and contrast ratio (CR). The measured results showed that the light leakage is greatly suppressed with the compensation film and the viewing angle characteristics of TN device are greatly improved, most particularly in the up direction.

The role of NMR in the study of partially ordered materials: Perspectives and challenges

The development of NMR techniques applied in the last 10 years to partially oriented systems, and in particular to liquid crystals (LCs), is the object of this brief perspective. The evolution of NMR methods (i.e., new NMR pulse sequences) and the improvement of both theoretical models and mathematic tools for the analysis of NMR data (specifically, for partially ordered systems) allowed scientists to extend their research to increasingly complex materials, such as dendrimers, polymers, and membranes, and to investigate unique phenomena, such as field-induced alignment and confining effects. Furthermore, the fast development of nanoscience and biomedicine is offering a rich variety of new “physical chemical” problems related to partially ordered materials. Starting from a brief perspective of recent works on thermotropic and lyotropic LCs based on different NMR methods, new challenges in this field will be drawn. Moreover, recent selected research works will be discussed in detail with particular emphasis on: (i) the effect of high magnetic fields on the supramolecular structure of chiral liquid-crystalline phases, such as the SmC*, TGBA*, and “de Vries”-type SmA* phases, by means of solid-state 2H NMR; (ii) the slow dynamics in the isotropic phase of bent-core LCs (BLCs) and of liquid single-crystal elastomers evidenced by 2H NMR relaxation studies; and (iii) the influence of the LC environment on the conformational properties of rod-like mesogens studied by high-resolution solid-state 13C NMR methods. This work aims to offer an occasion of reflection on this field of physical chemistry with a glance at future trends and challenges in view of the celebration of the International Year of Chemistry, 2011.

Assemblies of Fluorine Containing Bent-Shaped Liquid Crystal Molecules Studied by Using Scanning Tunneling Microscopy

In this work we prepared a fluorine containing bent-shaped liquid crystal from biphenyl as the central core and rod-like azobenzene mesogens as the side arms, namely 4',3-biphenyl bis[4-(4'-hexadecanloxy-3-fluorophenylazo)benzoate] (L104). The self-assembly behaviors of L104 molecules on graphite surface were investigated by using scanning tunneling microscopy (STM) under ambient conditions. The high-resolution STM images of L104 assemblies revealed three kinds of structures showing the joint effects of dipole-dipole interactions originated from the fluorine and the bent-core alignments for maximizing pi-pi interactions. These observations may be beneficial for understanding the assembly mechanism and designs for novel banana-shaped liquid crystal molecules.

Ferrocene-Based Rod-Like Mesogens Terminated with Oxadiazole Unit

A series of ferrocene-based rod-like mesogens has been synthesized in which the oxadiazole unit is in a terminal position. The length of the terminal S-alkyl chain, alkyl spacer length, and substitution on lateral position have been varied. All the synthesized compounds are thermally stable and exhibit mesomorphism. Most of them display two enantiotropic mesophases nematic-smectic C. Only one compound exhibits enantiotropic smectic C phase.

The phase behavior, structure, and dynamics of rodlike mesogens with various flexibility using dissipative particle dynamics simulation

We present a systematic dissipative particle dynamics (DPD) study on the phase behavior, structure, and dynamics of rodlike mesogens. In addition to a rigid fused-bead-chain model with RATTLE constraint method, we also construct a semirigid model in which the flexibility is controlled by the bending constant of k(φ). Using this notation, the rigid model has an infinite bending constant of k(φ)=∞. Within the parameter space studied, both two kinds of models exhibit the nematic and smectic-A phases in addition to the isotropic and solid phases. All of the phase transitions are accompanied by the discontinuities in the thermodynamical, structural, and dynamical quantities and the hysteresis around the transition points, and are therefore first order. Note that the obtained solid state exhibits an in-layer tetragonal packing due to the high density. For the rigid model, the simulations show that the liquid crystal phases can be observed for mesogens with at least five beads and the nematic phase is the first one to appear. More importantly, the phase diagram of seven-bead-chain models is obtained as a function of k(φ) and temperature. It is found that decreasing the value of k(φ) reduces the anisotropy of molecular shape and the orientational ordering, and thereby shifts the liquid crystal phases to the lower temperature end of the phase diagram. Due to the different k(φ) dependence of phase transition temperatures, the nematic phase range exhibits a more marked narrowing than the smectic-A phase as k(φ) is reduced, implying that the flexibility has a destabilizing effect on the nematic and smectic-A phases. We also have investigated the anisotropic translational diffusion in liquid crystal phases and its temperature and flexibility dependence. In our study, we find that the phases formed, their statical and dynamic properties, as well as the transition properties are in close accord with those observations in real thermotropic liquid crystals. It is clear that both the rigid and semirigid models we used are valuable models with which to study the behavior of thermotropic liquid crystals using DPD algorithm.

NMR studies of the ferroelectric SmC* phase

In this work an attempt is made to compare and rationalise the structural and dynamic behaviour of some ferroelectric rod-like mesogens studied by our group in recent years, mainly by means of 2H- and 13C-nuclear magnetic resonance (NMR) spectroscopy. This comparison concerning the local orientational order, the average molecular conformation and the chiral smectic C (SmC*) phase structure, with a brief overview on the reorientational dynamic properties, allowed us to identify some common behaviour of these ferroelectric rod-like mesogens when decreasing the temperature from lower ordered phases to and within the SmC* phase. The main results obtained by means of NMR studies on these mesogens are rationalised and discussed in comparison with detailed discrete Fourier transform computations of their molecular conformations (in the limit of isolated molecules) with the purpose of enlightening the role of intermolecular and packing interactions in the ferroelectric phase with respect to the smectic A and nematic phases.

Molecular complexity and the control of self-organising processes

In this article we investigate the complexity of the molecular architectures of liquid crystals based on rod-like mesogens. Starting from simple monomeric systems founded on fluoroterphenyls, we first examine the effects of aromatic core structure on mesophase formation from the viewpoint of allowable polar interactions, and then we model these interactions as a function of terminal aliphatic chain length. By incorporating a functional group at the end of one, or both, of the aliphatic chains we study the effects caused by intermolecular interfacial interactions in lamellar phases, and in particular the formation of synclinic or anticlinic modifications. We then develop these ideas with respect to dimers, trimers, tetramers, etc. We show, for dendritic systems, that at a certain level of molecular complexity the local mesogenic interactions become irrelevant, and it is gross molecular shape that determines mesophase stability. The outcome of these studies is to link the complexity of the molecular interactions at the nanoscale level, which lead to the creation of the various liquid-crystalline polymorphs, with the formation of mesophases that are dependent on complex shape dependencies for mesoscopic supermolecular architectures.

Ordered Gold Nanoarrays: 3D Ordered Gold Strings by Coating Nanoparticles with Mesogens (Adv. Mater. 17/2009)

Liquid-crystal-coated gold nanoparticles exhibit 3D long-range ordering in strings jacketed by rod-like mesogens (shown in green) with controllable interparticle spacing. The work reported on p. 1746 by Goran Ungar and co-workers demonstrates a new method of assembling ordered metal nanoparticle superlattices, in a step towards developing self-assembled metamaterials. The cover shows red, yellow, and blue nanoparticles lying at different heights and forming a rhombohedral lattice.

Supra-Molecular Structure of TGBC* Phases Studied by Means of Deuterium NMR Line-Shape Analysis

In this work the structure of the tilted TGBC* phase is investigated by means of the line-shape analysis of Deuterium NMR spectra. The case of a rod-like mesogen containing a (S)-2-methylbutyl-(S)-lactate unit in the chiral chain, whose acronym is HZL 7/* , is taken as a representative example for the large temperature range of stability of its tilted TGB* phases. By comparing the experimental and computed Deuterium NMR spectra several structural features can be extracted. For instance, the analysis of Deuterium NMR line-shapes put in evidence the occurrence of helical deformations due to the effect of the external magnetic field.

Stability of Nematic and Smectic Phases in Rod-Like Mesogens with Orientation−Dependent Attractive Interactions

The stability of isotropic (I), nematic (N), smectic A (Sm A), and hexatic (Hex) liquid crystalline phases is studied for a fluid of molecules with a rod-like shape and dispersive interactions dependent on orientation. The fluid is modeled with the spherocylindrical Gay-Berne-Kihara interaction potential proposed in a recent work, with parameters favoring parallel pair orientations. The liquid crystal phase diagram is characterized for different molecular aspect ratios by means of Monte Carlo simulations in the isobaric-isothermal ensemble. Three types of triple points are observed, namely, I-Sm A-Hex, I-N-Sm A, and N-Sm A-Hex, leading to island-shape domains for the smectic A phase. The resulting phase diagrams are compared with those derived previously for prolate fluids of ellipsoidal and spherocylindrical symmetry. It is concluded that the stability of the layered phases with respect to the nematic phase is enhanced in the spherocylindrical fluids due to geometrical constraints. Furthermore, the anisotropy of the dispersive interactions induces a stronger dependence of the overall phase diagram on temperature and aids in the energetic stabilization of the hexatic crystalline phase with respect to the fluid smectic A phase.

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.

Columnar versus smectic order in systems of charged colloidal rods

We study the stability of inhomogeneous liquid crystalline states in systems of monodisperse, stiff, charged rods. By means of a bifurcation analysis applied to the Onsager free energy for charged rods in strongly nematic states, we investigate nematic-smectic and nematic-columnar instabilities as a function of the Debye screening length kappa(-1). While the nematic-smectic transition clearly preempts the nematic-columnar one in the regime of strong screening (i.e., small kappa(-1)) a marked stability of hexagonal columnar order is observed at larger screening lengths. The theoretical results are substantiated by Brownian dynamics computer simulation results based on the Yukawa site model. Our findings connect to experiments on tobacco mosaic virus rods, in particular, but might be relevant for soft rodlike mesogens in strong external directional fields in general.

Frustrated smectic layer structures in bent-shaped dimer liquid crystals studied by x-ray microbeam diffraction

The layer structures in bent-shaped liquid crystal dimers mOAM5AMOm (m=6-16) have been investigated by x-ray microbeam diffraction. These liquid crystal molecules have two rodlike mesogens connected with an odd-numbered alkylene spacer and form a bent shape. In these compounds it is found that the structure changes from the single (m=6) to frustrated-layer structures (m=8, 10, and 12) and switchable frustrated-layer structures (m=14 and 16) with increasing terminal chain length. An anticlinic antiferroelectric structure is suggested in the compound with m=16, based on the different electric-field-induced reorientation behavior from those in the other dimers.

Dimeric Salicylaldimine-Based Mesogens with Flexible Spacers: Parity-Dependent Mesomorphism

New series of dimeric molecules with flexible spacers are described in which the mesogenic salicylaldimine group is linked to the alkylene spacer via an imino group. To study the structure−property relations, the lengths of the alkylene spacers (m = 5−8) as well as those of the terminal alkoxy chains (n = 4−14) have been varied. The compounds with even-numbered spacers behave as classic rodlike mesogens displaying nematic and/or smectic C phases, while those with odd-numbered spacers show nematic, columnar nematic, and columnar rectangular phases, behaving as bent-core mesogens. The effect on the liquid-crystalline behavior of the effective geometric change arising from the spacer parity and the imino linkage is discussed.

Mesogenic dimers composed of a calamitic and a bent-core mesogenic unit.

Three mesogenic dimers have been synthesized in which a five-ring bent-core moiety is connected with different calamitic units flexible spacers. The mesophase behavior of the dimers have been investigated by polarizing microscopy, differential scanning calorimetry, X-ray diffraction on oriented samples and by dielectric and electro-optical measurements. We found that two dimers exhibit a dimorphism columnar-nematic whereas the third one forms a columnar phase only. On the basis of the X-ray data a possible structure model of one of the columnar phases is proposed. The nematic phase exhibits unusual properties. A smectic-like texture can be induced by applying an electric field, which is unknown for nematic phases formed by rod-like mesogens.

A Low‐Molar‐Mass, Monodispersive, Bent‐Rod Dimer Exhibiting Biaxial Nematic and Smectic A Phases

A peelable banana is formed when a bent-core molecule is linked to a rodlike mesogen through a flexible aliphatic spacer. This is an appropriate description of this novel low-molar-mass organic system, which displays a transition from a biaxial nematic (N b ) phase to a biaxial smectic A phase. The illustration gives a schematic representation of the dimeric molecules in the N b phase as well as the corresponding textural and conoscopic patterns obtained.

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).

Ferroelectric smectic phase formed by achiral straight core mesogens.

We report electro-optic experiments in liquid crystalline freestanding films of achiral hockey stick shaped mesogens with a straight aromatic core. The material forms two smectic mesophases. In the higher temperature phase, a spontaneous polarization exists in the smectic layer plane and the films show polar switching in electric fields. It is the first example of a ferroelectric phase formed by nearly rodlike achiral mesogens. Mirror symmetry of the phase is spontaneously broken. We propose a molecular configuration similar to a synclinic ferroelectric (C(S)P(F)) high temperature phase and an anticlinic, probably antiferroelectric (C(A)P(A)) low temperature phase.