Brownian reorientation in smectic phases

The rotation of molecules around their long axes is influenced by the in-plane order in smectic phases. Assuming a Brownian reorientation process, the effect of polar and quadrupolar order on the dielectric spectrum is investigated. In the case of quadrupolar order two modes should be visible, while only one mode arises if the order is polar.     

Brownian reorientation in smectic phases

The rotation of molecules around their long axes is influenced by the in-plane order in smectic phases. Assuming a Brownian reorientation process, the effect of polar and quadrupolar order on the dielectric spectrum is investigated. In the case of quadrupolar order two modes should be visible, while only one mode arises if the order is polar.     

Layer frustration, polar order and chirality in liquid crystalline phases of silyl-terminated achiral bent-core molecules

A novel class of bent-core molecules with oligo(siloxane) or carbosilane units at both ends was synthesized and the self-organization of these molecules was investigated by polarizing microscopy, DSC, X-ray scattering, dielectric and electrooptical methods. Depending on the size of the silicon-containing segments, smectic and columnar liquid crystalline phases are formed. Most smectic phases are low birefringent and composed of macroscopic domains of opposite handedness (dark conglomerate phases). The switching process in these smectic phases is surface stabilized ferroelectric and, depending on the conditions, two distinct slow relaxation processes to nonpolar structures were observed. It is proposed that the smectic phases are built up by chiral and polar SmCsPF layer stacks which are separated by anticlinic interfaces. If the size of these layer stacks is sufficiently large a coupling to the substrate surfaces takes place and ferroelectric switching is observed. It is also suggested that the sponge-like layer distortion, occurring in the low birefringent mesophases, is due to an escape from the local polar order within these SmCsPF layer stacks. For compounds with larger silylated units a steric frustration arises, which leads to layer modulation (columnar ribbon phases) and this is associated with a transition from ferroelectric to antiferroelectric switching. All compounds show a switching of the molecules around the long axis which reverses the layer chirality.     

X-ray diffraction, DFT theoretical, and IR polarized spectroscopic studies of the crystal-like E-mesophase of 4′-hexyloxy-isothiocyanatotolane (6OTOLT)

The dynamics of 6OTOLT molecules was analyzed based on IR absorption spectra and DFT calculations. Of particular interest was the mode ascribed to the ν_as(NCS) vibrations, the transition dipole moment of which is directed along the long axes of the molecules. Polarized IR spectra in the region of this mode allowed a characterization of the ordering of molecules in the analyzed phases. In a case of the smectic E phase, a random distribution of molecules aligned laterally in two-dimensional space between KRS-5 windows was found. The crystals of 6OTOLT were successfully grown and are characterized by an interesting Pc space group. The molecules are parallely arranged in layers and cross the adjoining ones underneath at an angle of 86.04°. The distance between the ring planes is 3.48 Å. The packing of molecules indicates a tendency to maintain short contacts between NCS groups and alkyl chains. The crystals grown do not correspond to the ordering of the smectic E phase and thus to the solid state obtained after cooling the smectic phase. Calculations of the interaction energy for three possible arrangements of dimeric species show a predominance of the core-to-core units. It was also shown that in this case an increase in ν_as(NCS) frequency should be expected, in good agreement with experimental data.     

Induced smectic phases in phase diagrams of binary nematic liquid crystal mixtures

To elucidate induced smectic A and smectic B phases in binary nematic liquid crystal mixtures, a generalized thermodynamic model has been developed in the framework of a combined Flory-Huggins free energy for isotropic mixing, Maier-Saupe free energy for orientational ordering, McMillan free energy for smectic ordering, Chandrasekhar-Clark free energy for hexagonal ordering, and phase field free energy for crystal solidification. Although nematic constituents have no smectic phase, the complexation between these constituent liquid crystal molecules in their mixture resulted in a more stable ordered phase such as smectic A or B phases. Various phase transitions of crystal-smectic, smectic-nematic, and nematic-isotropic phases have been determined by minimizing the above combined free energies with respect to each order parameter of these mesophases. By changing the strengths of anisotropic interaction and hexagonal interaction parameters, the present model captures the induced smectic A or smectic B phases of the binary nematic mixtures. Of particular importance is the fact that the calculated phase diagrams show remarkable agreement with the experimental phase diagrams of binary nematic liquid crystal mixtures involving induced smectic A or induced smectic B phase.     

Local high-density distributions of phospholipids induced by the nucleation and growth of smectic liquid crystals at the interface

Amphiphilic molecules adsorbed at the interface could control the orientation of liquid crystals (LCs) while LCs in turn could influence the distributions of amphiphilic molecules. The studies on the interactions between liquid crystals and amphiphilic molecules at the interface are important for the development of molecular sensors. In this paper, we demonstrate that the development of smectic LC ordering from isotropic at the LC/water interface could induce local high-density distributions of amphiphilic phospholipids. Mixtures of liquid crystals and phospholipids in chloroform are first emulsified in water. By fluorescently labeling the phospholipids adsorbed at the interface, their distributions are visualized under fluorescent confocal microscope. Interestingly, local high-density distributions of phospholipids showing a high fluorescent intensity are observed on the surface of LC droplets. Investigations on the correlation between phospholipid density, surface tension and smectic LC ordering suggest that when domains of smectic LC layers nucleate and grow from isotropic at the LC/water interface as chloroform slowly evaporates at room temperature, phospholipids transition from liquid-expanded to liquid-condensed phases in response to the smectic ordering, which induces a higher surface tension at the interface. The results will provide an important insight into the interactions between liquid crystals and amphiphilic molecules at the interface.     

IR dichroism study of orientational ordering of liquid-crystalline acrylates

Orientational ordering of four liquid-crystalline acrylates induced by the surface of KBr plates over a wide temperature range was studied by the IR dichroism technique. IR spectra of homogeneously aligned samples were used to calculate the angles between the direction of the transition moment for a series of vibrations of the mesogenic fragment and its long axis. A method for calculation of the homeotropic orientation parameter of molecules was proposed. The orientation parameters of homogeneously and homeotropically aligned samples in the nematic, smectic (A), and chiral smectic (I andH) phases were calculated. Thein situ photopolymerization of acrylates in the smectic phases occurs with retention of the orientational ordering in the polymer films formed. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 319–323, February, 1999.     

Competition between micro-segregation and anti-parallel alignment of an amphiphilic rod-like liquid crystal

Recently, we reported that a rod-like molecule, 4-[4-(7-hydroxyheptyloxy)phenyl]-1-(4-hexylphenyl)-2,3-difluorobenzene, exhibited a nematic phase with a layered structure and smectic C phases consisting of three states. We prepared a homologous series of the rod-like molecules in which a 2,3-difluoro-1,4-diphenylbenzene unit and a hydroxyl unit are connected via a flexible methylene spacer. We investigated their physical properties using polarised optical microscopy (POM), differential scanning calorimetry (DSC) and X-ray diffraction (XRD). Although they showed nematic, smectic A and smectic C phases, the phase structures were found to depend on the flexible spacer length. Those compounds possessing a shorter spacer length than a heptyl unit exhibited bilayered smectic A and smectic C phases, whereas those possessing a longer spacer length than a heptyl unit showed conventional monolayered smectic A and smectic C phases. A nematic phase with a smectic-like layer ordering was observed for the compound possessing an octyl unit. The structure–property relations of the amphiphilic compounds are discussed.     

Thermotropic liquid crystals from a monosubstituted 2,2'-bipyridine

Unsymmetric functionalization of 5-bromo-2,2'-bipyridine and 5,5'-dibromo-2,2'-bipyridine with 4-n-alkoxyphenylethynyl leads to two new families of mesomorphic compounds. The smectic B, smectic A and nematic phases observed were studied by polarizing optical microscopy, differential scanning calorimetry and X-ray diffraction. The structural parameters of the smectic phases were determined. The internal structure of the smectic phases, particularly the range of the in-layer ordering of the molecules, was investigated as a function of temperature.     

A simple molecular model for the interpretation of some properties of smectic C and A phases

We present a simple geometrical model in which the molecular shape is approximated by a small number of parameters for the dumbbell-like middle group and cylinder-like alkyl end chains. The pair potentials of nearest neighbours are approximated by the sum of anisotropic repulsive terms due to the contact of the different parts of the molecules and attraction due to dispersion forces between different parts of the molecules and attraction due to dispersion forces between different parts of the molecules. Since the number of nearest neighbours at the smectic C/A phase transition is unchanged, the resulting pair potentials are able to describe well the cooperative behaviour of the molecules in the non-ordered layers of the smectic C and A phases. The dependence of the tilt angle on the alkyl chain length and on the temperature and other thermodynamic and structural properties can be interpreted qualitatively very well.     

Induction of smectic A phases with calamitic molecules and 2,4,7-trinitrofluorenone: the influence of branching and chain length

Binary systems consisting of 2,5-diphenyl-1,3,4-thiadiazole derivatives incorporating an allene unit in one of the terminal chains and the electron acceptor 2,4,7-trinitrofluorenone (TNF) have been investigated. Though the diphenylthiadiazole cores do not represent typical electron donor units, the nematic and smectic C phases observed for the pure compounds were suppressed and replaced by smectic A phases which in most cases have a higher stability than the nematic phases of the pure compounds. The substitution pattern around the allene moiety allowed a systematic study of the influence of steric effects on the mesophase induction by TNF. Compounds with long and especially those with branched terminal chains can take up a larger number of TNF molecules and can reach a higher stability of the induced SmA phase than those with shorter and unbranched chains. The induction of SmA phases is explained as the result of attractive intermolecular interaction between the diphenylthiadiazole rigid cores and TNF molecules provided by donor-acceptor interactions and quadrupole interactions, as well as a consequence of microsegregation and space filling effects.     

Thermotropic liquid crystals from a monosubstituted 2,2'-bipyridine

Unsymmetric functionalization of 5-bromo-2,2'-bipyridine and 5,5'-dibromo-2,2'-bipyridine with 4-n-alkoxyphenylethynyl leads to two new families of mesomorphic compounds. The smectic B, smectic A and nematic phases observed were studied by polarizing optical microscopy, differential scanning calorimetry and X-ray diffraction. The structural parameters of the smectic phases were determined. The internal structure of the smectic phases, particularly the range of the in-layer ordering of the molecules, was investigated as a function of temperature.     

Molecular dynamics simulations of a liquid crystalline molecule in the smectic A and E phases and in vacuum

Molecular dynamics simulations are performed in this work at 393 and 323 K for a mesogenic molecule ( R )-1-methylheptyl 4\[4-(2-allyloxyethoxy)biphenyl-4-carbonyloxy]benzoate in the simulated smectics A and E, respectively, and in a vacuum at 300 K, for a period of 1.0ns. The trajectories obtained from molecular dynamics simulations allow us to investigate the dynamical behaviour of this mesogenic molecule in the simulated smectic phases. This dynamical behaviour of a single molecule is presented using the distributions of dihedral angles and rotational diffusion around the C-axis defined by the simulated cells. Simulation results indicate that, except for the bonds near the end of the spacer segment, the dihedral angles all exhibit a single Gaussian-like distribution in the smectic A and E phases. Fluctuations of a dihedral angle about its mean value are more restricted in the smectics A and E than in those simulated in a vacuum. The average value of the fluctuations of the dihedral angles at the bonds in the spacer is found to be about 2 fold larger than that of fluctuations in the tail of the same molecule in the smectic A and E phases. In the smectic A phase, the distribution of orientations of a molecule about its long axis in a 36 molecule cell in which the outer molecules are fixed is found to have three distinct peaks. This result shows that the orientational fluctuations of single molecules are limited by confinement due to neighbouring molecules, i.e. that the layers have short-range structural correlations. The orientational distributions show larger fluctuations at the ends of the molecules.     

Polar and quadrupolar order in smectic liquid crystals

In several smectic phases the long molecules are tilted towards the layer planes. The molecules in the layers of smectic C phases have a preferred tilt direction with a C2 rotation axis perpendicular to the tilt plane. If the molecules have a brick-like shape, a configuration possessing the D2h symmetry is also possible. For molecules shaped like chevrons or bananas, the existence of a smectic phase with the symmetry Cv was recently reported. We consider different in-plane configurations of smectics using a geometrical approach based on the 'scaled particle theory'. Varying the geometrical parameters of hard rod particles, a phase diagram for several smectic configurations is predicted. Depending on the particle shape, phases with dipolar order (C2,Cv) and quadrupolar order (D2h) can be stable.     

Polar and quadrupolar order in smectic liquid crystals

In several smectic phases the long molecules are tilted towards the layer planes. The molecules in the layers of smectic C phases have a preferred tilt direction with a C2 rotation axis perpendicular to the tilt plane. If the molecules have a brick-like shape, a configuration possessing the D2h symmetry is also possible. For molecules shaped like chevrons or bananas, the existence of a smectic phase with the symmetry Cv was recently reported. We consider different in-plane configurations of smectics using a geometrical approach based on the 'scaled particle theory'. Varying the geometrical parameters of hard rod particles, a phase diagram for several smectic configurations is predicted. Depending on the particle shape, phases with dipolar order (C2,Cv) and quadrupolar order (D2h) can be stable.     

Orientational ordering of a ferroelectric liquid crystal with high spontaneous polarization

The orientational ordering of the liquid crystal, 3M2CPHOB ((2S, 3S)-3-methyl-2-chloropentanoic acid-4',4'-hexyloxybiphenyl ester) in its smectic A and chiral smectic C phases has been studied by carbon-13 2D N.M.R. The technique used is separated local-field spectroscopy, which produces a first-order splitting for each carbon signal, from which carbon-proton dipolar constants can be determined. The order parameters for different molecular segments of the liquid crystal can then be calculated. The C-H bond order parameter at the chiral centre with a chlorine atom is unusually large, which is a strong indication of hindered rotation of the molecular segment at the chiral centre. The carbon-13 chemical shifts of 3M2CPHOB were also measured as a function of temperature, and the relation between the chemical shifts and the order parameters is discussed.     

Orientational ordering of a ferroelectric liquid crystal with high spontaneous polarization

The orientational ordering of the liquid crystal, 3M2CPHOB ((2S, 3S)-3-methyl-2-chloropentanoic acid-4′,4″-hexyloxybiphenyl ester) in its smectic A and chiral smectic C phases has been studied by carbon-13 2D N.M.R. The technique used is separated local-field spectroscopy, which produces a first-order splitting for each carbon signal, from which carbon-proton dipolar constants can be determined. The order parameters for different molecular segments of the liquid crystal can then be calculated. The C-H bond order parameter at the chiral centre with a chlorine atom is unusually large, which is a strong indication of hindered rotation of the molecular segment at the chiral centre. The carbon-13 chemical shifts of 3M2CPHOB were also measured as a function of temperature, and the relation between the chemical shifts and the order parameters is discussed.     

Liquid crystalline properties of asymmetric molecules: II. Smectic properties of 4-alkoxyphenyl 4-(4-trifluoromethylbenzoyloxy)benzoates

The thermal properties of a homologous series of 4-alkoxy-phenyl 4-(4-trifluoromethylbenzoyloxy)benzoates were examined. The homologues show remarkable smectic properties involving a smectic A and a higher order smectic phase characterized by microscopic and X-ray experiments, indicating that the molecules have a partially bilayer arrangement in both smectic phases. Based on these results, molecular arrangements of the two smectic phases are proposed. The effects of the trifluoromethyl group on the thermal properties of the smectic and crystalline phases are discussed in terms of the fluorophilic interaction around the boundary of the smectic layer.     

Dielectric studies of a 5-n-alkyl-2-(4'-isothiocyanatophenyl)- 1,3-dioxane (nDBT) homologous series (n=4-10)

The results of dielectric studies of seven members of a 5-n-alkyl-2-(4'-isothiocyanatophenyl)- 1,3-dioxane (nDBT) homologous series (n=4-10) in the isotropic and smectic A phases are presented. The complex dielectric permittivity, epsilon* (nu) = epsilon'(nu) - iepsilon' (nu), was measured with the aid of two experimental set-ups: an impedance analyser (10kHz-13 MHz) and a dielectric time domain spectrometer (TDS, 10 MHz-4 GHz). This allowed two main relaxation processes in both the phases studied to be separated: the low frequency (l.f.) process connected with molecular reorientations around the short axes, and the high frequency (h.f.) process connected with the rotations around the long axes. The measured dielectric increments enabled us to estimate the value and direction of the dipole moment of the nDBT molecules. The l.f. relaxation process in the isotropic and smectic A phases of the nDBT compounds exhibits some peculiar features which distinguish the materials from other similar substances. The observed decrease of the relaxation times and activation enthalpy with increasing n is discussed in relation to the molecular arrangements in the smectic layers.     

A novel biaxial smectic liquid crystalline phase formed by rodlike molecules with a 1,3-diazaazulene skeleton

Novel rodlike liquid crystalline (LC) materials incorporating the nonbenzenoid aromatic 1,3-diazaazulene unit have been synthesized and investigated by polarizing microscopy, differential scanning calorimetry, and X-ray diffraction of oriented samples. Some representatives form, in addition to conventional SmC phases, a novel biaxial smectic LC phase, in which the rotation of the molecules around their long axes is greatly restricted. Attractive face-to-face interactions of the flat aromatic units lead to columnar aggregates which are organized in layers. As there is no positional correlation between adjacent layers, these mesophases are related to lamellar columnar phases formed by some disklike and boardlike molecules. Such LC materials may be promising candidates in the field of organic semiconductors, photoconduction, and other applications.