Terminal Dipole Research Articles

1PA04 末端双極子と末端側鎖をもつGay-Berneモデルを用いた5CBのMD計算

1PA04 末端双極子と末端側鎖をもつGay-Berneモデルを用いた5CBのMD計算

Separate-Path Electron and Hole Transport Across π-Stacked Ferroelectrics for Photovoltaic Applications

Electron and hole separate-path transport is theoretically found in the π-stacked organic layers and columns. This effect might be a solution for the charge recombination problem. The building molecules, named 1,3,5-tricyano-2,4,6-tricarboxy-benzene, contain the mesogenic flat aromatic part and the terminal dipole groups which make the system ferroelectric. The diffusion path of the electrons cuts through the aromatic rings, while holes hop between the dipole groups. The transmission function and the charge mobilities, especially for the holes, are very sensitive to the distance between the molecular rings, due to the overlap of the π-type orbitals. We verified that the separation of the diffusion paths is not destroyed by the application of the graphene leads. These features make the system suitable for the efficient solar cells, with the carrier mobilities higher than those in the organometal halide perovskites.

Role of terminal dipole charges in aggregation of α-helix pair in the voltage gated K+ channel

The voltage sensor domain (VSD) of the potassium ion channel KvAP is comprised of four (S1–S4) α-helix proteins, which are encompassed by several charged residues. Apart from these charges, each peptide α-helix having two inherent equal and opposite terminal dipolar charges behave like a macrodipole. The activity of voltage gated ion channel is electrostatic, where all the charges (charged residues and dipolar terminal charges) interact with each other and with the transmembrane potential. There are evidences that the role of the charged residues dominate the stabilization of the conformation and the gating process of the ion channel, but the role of the terminal dipolar charges are never considered in such analysis. Here, using electrostatic theory, we have studied the role of the dipolar terminal charges in aggregation of the S3b–S4 helix pair of KvAP in the absence of any external field (V=0). A system attains stability, when its potential energy reaches minimum values. We have shown that the presence of terminal dipole charges (1) change the total potential energy of the charges on S3b–S4, affecting the stabilization of the α-helix pair within the bilayer lipid membrane and (2) the C- and the N-termini of the α-helices favor a different dielectric medium for enhanced stability. Thus, the dipolar terminal charges play a significant role in the aggregation of the two neighboring α-helices.

Surface and disorder effects in aerosil dispersed liquid crystals

Aerosil nanoparticles are dispersed in liquid crystals to introduce disorder. The particles form a hydrogen bonded network, which interferes with the normal liquid crystalline order. Among other techniques, broadband dielectric spectroscopy has been applied to these systems, and three main differences have been observed. First, a new slow relaxation process has been found, about two decades slower than the bulk rotation around the short axis. Its strength increases with aerosil concentration and it is interpreted as a retarded relaxation due to surface interaction with the aerosil particles. In the particular case of cyanobiphenyl liquid crystals, a hydrogen bond is involved. Second, an acceleration of the rotation around the short molecular axis is observed in the ordered nematic phase. The liquid crystalline phase becomes subdivided into domains by the aerosil network. The presence of such domains disturbs the long-range orientational order, and the sample appears more isotropic-like. Therefore the relaxation shifts towards the values in the isotropic phase, and for the rotation around the short axis this means an acceleration. Third, in the isotropic phase a similar acceleration is observed, but only for the associating cyanobiphenyl molecules. These molecules have a strong terminal dipole moment, and form dynamically antiparallel correlated pairs. The aerosil particles disturb the normal pair formation.

Bilayered smectic phase polymorphism in the dipolar Gay–Berne liquid crystal model

We present computer simulations of the Gay-Berne model with a strong terminal dipole. We report the existence of different stable antiferroelectric interdigitated bilayered phases in this model with diverse in-plane organization. The occurrence of these phases depends crucially on the value of the molecular elongation kappa. For kappa=3 we find an interdigitated bilayered smectic-A phase (absent when there is no dipole) and a bilayered smectic-T (or crystal) with positional in-plane tetragonal ordering, different from the hexatic observed in the absence of the molecular dipole. For kappa=4, bilayered smectic-A and in-plane hexatic-ordered smectic-B (or crystal) phases are observed.

Theory of the intermediate tilted smectic phases and their helical rotation

A molecular-statistical theory for the entire sequence of the chiral tilted smectic phases is derived. Uniaxial and biaxial subphases were found to be stable in different temperature ranges depending on the molecular parameters. The model of a chiral molecule possessing a strong transverse terminal dipole moment and a quadrupole moment located in the molecular core was used. Direct dispersion and electrostatic interactions (modulated by shape) between molecules located in the same or in the neighboring smectic layers are taken into account. An effective long-range interaction arises after the minimization of the free energy with respect to polarization vectors. If the molecular quadrupole moment is small, only uniaxial phases with different periodicities arise. Their periodicity may be tens and hundreds of layers (Sm-C*), or approximately two layers (Sm-C*)(Sm-C*A), or several layers (Sm-C*alpha). In the presence of the nonpolar biaxial ordering (in addition to polarization) there is a cap-shaped border in the phase diagram that separates Sm-C*A, Sm-C*, and Sm-C*alpha. If the molecules are nonchiral, Sm-CA, Sm-C, and the de Vries's phases arise instead of the three phases mentioned above. If the molecular quadrupole moment is large, the left "arm" of the border breaks into two lines, and a sequence of biaxial subphases arises in the area between these two lines. Among these biaxial subphases, the one with periodicity of three smectic layers appears to be the broadest in the temperature range. In addition, the subphases with different periodicities were found to be stable in narrow temperature ranges. The long helical rotation in every biaxial subphase is calculated. It is found to change sign between the three-layer subphase and Sm-C*, and may diverge in the four-layer subphase if it arises. All calculations are done with help of (A)FLC Phase Diagram Plotter software developed by the first author and available at his web-page.

Formation of tilted smectic-C liquid crystal phase in polar Gay–Berne molecules

We perform molecular dynamics simulation for a system of Gay–Berne molecules having two terminal dipole moments to generate tilted smectic-C liquid crystal phase. We investigate the effect of dipolar orientation with respect to the long molecular axis on phase behaviour. The study indicates that larger dipolar angle can give rise to greater tilt in molecular organization within a layer.

Molecular dynamics simulation study on the phase behavior of the Gay-Berne model with a terminal dipole and a flexible tail.

To study the effect of the alkyl tail and the terminal dipole on the stability of the liquid crystalline phase of mesogens, we have carried out molecular dynamics simulations for 1CB(4-methyl-4'-cyanobiphenyl) and 5CB(4-n-pentyl-4'-cyanobiphenyl) by using a coarse-grained model. In the coarse-grained model, a 5CB molecule is divided into the rigid part of 1CB moiety, which is represented by an ellipsoid, and the remaining flexible part which is represented by a chain of united atoms. The nonbonded potential between coarse-grained segments is represented by the generalized Gay-Berne (GB) potential and the potential parameters are determined by directly comparing the GB potential with the atomistic potentials averaged over the rotation of the mesogen around its axis. In addition, a dipole moment is placed at one end of the ellipsoid opposite to the flexible tail. The ordered state obtained in the polar 5CB model was assigned as the nematic phase, and the experimental static and dynamical properties were reproduced well by using this coarse-grained model. Both the dipole-dipole interactions and the thermal fluctuation of the flexible tail increase the positional disorder in the director direction, and stabilize the nematic phase. Thus, the nematic phase in the polar 5CB is induced by a cooperative effect of the flexible tail and the terminal dipole. It is noted that a local bilayer structure with head-to-head association is formed in the nematic phase, as experimentally observed by x-ray diffraction measurements.

Influence of Dipole-Dipole Interactions on Structural Properties of Gay-Berne Particles

Molecular dynamics simulations were performed for a liquid crystal system composed of the Gay—Berne particles with terminal transverse dipole moments. When the dipole—dipole interactions are weak, the isotropic-smectic transition is observed and the transition point shifts towards lower densities as the dipole moment increases. For higher dipole moments the system shows isotropic structure in the whole range of density, and strong dipole—dipole interactions produce two types of dimers that cannot be accommodated into smectic layers.

Simulation study of the phase behavior of a primitive model for thermotropic liquid crystals: Rodlike molecules with terminal dipoles and flexible tails

A primitive model for small mesogenic molecules is proposed, consisting of three elements: (i) a rigid rodlike core, modeled as a hard spherocylinder of length/diameter ratio L/D=5; (ii) a flexible end group, consisting of five segments of length D, which is “ideal” in the sense that it has no volume; (iii) a terminal dipole, located in the end cap opposite the flexible tail. This model is studied using Monte Carlo computer simulation, and the dipolar interactions are evaluated using the reaction field method. The hard spherocylinder model displays four phases: isotropic, nematic, smectic-A and crystal. Previously, it was found that the addition of the terminal dipole to hard spherocylinders without tails greatly enhances the range of stability of the nematic phase, at the expense of the smectic-A phase [McGrother et al., J. Phys.: Condens. Matter 8, 9649 (1996)]. Conversely, adding the flexible tail to hard spherocylinders without dipoles is found to suppress the nematic phase, whereas the smectic-A and crystal phase are little affected. Combining the effects of the terminal dipole and the flexible tail, all four phases survive. Because of the dipoles, the particles prefer to adopt a staggered antiparallel arrangement. In the smectic-A and crystal phases, this gives rise to interdigitation of the smectic layers. In the crystal phase a tendency towards columnar ordering is observed. The results are compared with experimental observations.

A comparative study of dynamics in the nematic and reentrant-nematic phases of 60CB and 60CB/80CB mixture by deuteron nuclear magnetic resonance relaxation

Deuteron longitudinal (T1Z) and quadrupolar (T1Q) spin-lattice relaxation times and quadrupolar splittings were measured over all the stable mesophases in a mixture of perdeuterated 4-n-hexyloxy-4′-cyanobiphenyl (6OCB) and 4-n-octyloxy-4′-cyanobiphenyl (8OCB) at 15.1 and 46 MHz, and compared with those reported previously for a pure 6OCB sample. The 6OCB/8OCB mixture has 28 wt. % of 6OCB and shows a nematic, smectic A and reentrant-nematic (RN) phases. We have carried out data analyses for both samples in order to achieve a consistent physical picture. The additive potential method is employed to construct the potential of mean torque using the quadrupolar splittings in these samples. A decouple model is used to describe correlated internal motions of the end chain, which are independent of the molecular reorientation. The latter motion is treated using the small-step rotational diffusion model of Tarroni and Zannoni, while the former motion is described using a master rate equation. In comparing the NMR results of the pure 6OCB sample and of the 6OCB/8OCB mixture, both the dynamic and static behaviors appear to be similar, and there are no dramatic changes upon entering the RN phase of 6OCB/8OCB, supporting the belief that the effects driving the reentrancy in this mixture are very subtle. The tumbling motion of 6OCB molecules, however, shows quite different behaviors in the two studied samples. Both 6OCB and 8OCB possess a strong terminal dipole and tend to form “loose” dimers. The degree of dimerization can be inferred from the tumbling motion of 6OCB molecules and their internal chain dynamics.

Computer simulation of dipolar liquid crystals

Abstract The aim of the work presented here is to use computer simulation techniques to examine the effect of dipolar interactions on the structures formed by model mesogenic molecules. The most wide-ranging and substantial part of the work involves an in-depth Monte Carlo (MC) study of the effect of dipolar interactions on the phase behaviour of hard spherocylinders with dipoles in a number of positions and orientations. A reaction-field (RF) method, with a simple self-consistent iterative method to accurately describe the dielectric constant of the surrounding medium, is used to treat of the long-range dipolar interactions. For the system with a central longitudinal dipole the density range of the nematic phase is found to be considerably reduced. The nematic phase is destabilised relative to both the isotropic and the smectic-A phases when compared with the non-polar system, and disappears altogether below the I-N-SmA triple point. No evidence of ferroelectricity is found although some short-range anti-ferroelectric ordering is seen. As for the molecules with central longitudinal dipoles, the smectic-A phase is stabilised for a system with central transverse dipoles with strong nose-to-tail interactions. The densities of the isotropic-nematic transitions are essentially unchanged from that of the nonpolar system. The most interesting features of this system are the domain structures formed by the dipoles within the smectic-A layers. At high temperature the dipoles are orientationally disordered, but as the temperature is lowered the stable structures are rings and chains of dipoles. As for the system with central longitudinal dipoles, the terminal longitudinal dipole is seen to slightly destabilise the nematic phase relative to the isotropic phase. More interestingly, the smectic phase is destabilised, and is only seen at the very highest densities. This is in stark contrast to what is seen for the systems with longitudinal and transverse central dipoles, and appears to be a consequence of the anti- parallel geometry of the dipoles in their minimum- energy conformation.

The effect of hydrogen bonding, molecular shape, dipole moments, and chain length on the mesomorphism of some D-glucose and D-glucosamine derivatives

Diverse 4-substituted-benzyl 1-O-alpha- and beta-D-glucopyranosides, 4-substituted-benzyl 2-deoxy2-trifluoroacetamido-alpha- and beta-D-glucopyranosides and 4-substituted-benzyl 2-acetamido2-deoxy-alpha- and beta-D-glucopyranosides have been synthesized. They were prepared directly by a one-step alkylation of commercially available carbohydrate starting materials. The 4-substituted-benzyl 2-deoxy-2-trifluoroacetamido-alpha- and beta-D-glucopyranosides were cleaved to yield the corresponding free amines. The influence of hydrogen bonding, terminal dipole moments, molecular shape and chain length on the thermotropic mesomorphic behaviour of these benzyl-substituted carbohydrate amphiphiles is discussed. Comparisons between the mesomorphic behaviour of the new benzyl-substituted carbohydrates and that of the related phenyl-substituted carbohydrates, as well as the non-aromatic n-alkyl D-glucosides, are made. Although the new carbohydrates do not exhibit lyotropic phase behaviour, the 4-substitutedbenzyl 2-deoxy-2-trifluoroacetamido-alpha- and beta-D-glucopyranosides have been found to gel water at very low sugar concentrations.

The liquid-crystalline phase behaviour of hard spherocylinders with terminal point dipoles

We examine the effect of dipolar interactions on the liquid-crystalline phase behaviour of L/D = 5 hard spherocylinders with a terminal point dipole. The hard spherocylinder consists of a cylinder of length L and diameter D with hemispherical caps on each end; the point dipole is located at the centre of the hemispherical cap (2.5D from the centre of the spherocylinder) and is oriented along the principal molecular axis. The phase transitions exhibited by this system are studied using the isothermal - isobaric Monte Carlo (MC-NPT) technique. As for systems with central dipoles, the terminal dipole is seen to slightly destabilize the nematic (orientationally ordered) phase relative to the isotropic phase when compared with the non-polar hard spherocylinders. More interestingly, the smectic (layered) phase is destabilized in the terminal dipole case, and is only seen at the very highest densities. This is in stark contrast to what is seen for systems with central point dipoles in which the smectic phase is stabilized relative to the nematic phase due to the strong anti-parallel dipolar interactions. We do not find any evidence for ferroelectric or anti-ferroelectric ordering in these systems.

Structural and dielectric investigations of a binary system of a bi-swallow-tailed and a terminally-polar compound

Abstract Induced smectic A phases in binary mixtures of a bi-swallow-tailed component and a second component are described as ‘filled smectic A phases’, since the second rod-like component is considered to occupy spaces between the terminal branches of the bi-swallow-tailed molecules. A new system of this kind is now described where the second rod-like component has a terminal dipole, thereby enabling dielectric studies of the mixtures. These measurements coupled with calorimetric and X-ray measurements give information about the packing and dynamics of the mixed phases.

Dielectric relaxation in a new cyclic liquid‐crystalline oligosiloxane

Polysiloxanes with mesogenic side chains are suitable model compounds dor the study of molecular dynamics. The first dielectric relaxation data are presented here for a cyclic oligosiloxane having a degree of oligomerization of 5, a cyanophenylbenzoate group as the terminal dipole, and a spacer length of 3. The phase sequence was determined by means of X‐ray diffraction, differential scanning calorimetry, and polarizing microscopy. Reasons for the strong dependence of physical properties on the degree of oligomerization are discussed.

Liquid crystalline materials: physical properties and intermolecular interactions

The physical properties and the phase behaviour of some nematic liquid crystals are discussed, with emphasis on the influence of short-range antiparallel dipole correlation that occurs in mesogenic compounds with a strong terminal dipole moment. Evidence for this effect, which stems especially from dielectric studies, is summarized. Variations of the dipole correlation with molecular structure can explain the sometimes unexpected phase behaviour and physical properties of these substances. A qualitative model is given in terms of a monomer-dimer equilibrium.

Electrocardiographic QRS changes induced by acute coronary ligation in the isolated rabbit heart

The electrical fields associated with augmented R- and decreased S-wave amplitudes during acute, severe myocardial ischemia were studied in fifteen isolated rabbit heart preparations. Hearts were suspended in a spherical tank and perfused with oxygenated electrolyte solution. Electrocardiographic signals were recorded from electrodes on the tank's surface and processed by computerized methods. Fifteen minutes after suture ligation of the left anterior descending coronary artery, records from electrodes overlying the lesion demonstrated increased R-wave amplitude and ST-segment elevation. Fitting a single moving dipole to pre- and post-ligation potentials demonstrated that the ligation increased the dipolarity of the field and shifted the terminal QRS dipole to a position topographically related to the location of the ischemic lesion. The effects of injury were further assessed by study of fields computed by subtracting control from post-ligation data. This generated a dipolar field at the instant of maximal dipole moment whose strength directly correlated (r=0.74) with the area of non-perfused epicardium, as determined by post-ligation methylene blue infusion. These results suggest that the electrical fields generated during mid to late dipolarization by coronary ligation are qualitatively similar to those generated during repolarization, and may therefore be of similar clinical and/or experimental value.