Helical Pitch Values Research Articles

Synthesis and Characterization of New Chiral Smectic Four-Ring Esters.

Orthoconic antiferroelectric liquid crystals (OAFLCs) represent unique self-organized materials with significant potential for applications in photonic devices due to their sub-microsecond switching times and high optical contrast in electro-optical effects. However, almost all known OALFCs suffer from low chemical stability and short helical pitch values. This paper presents the synthesis and study results of two chiral AFLCs, featuring a four-ring structure in the rigid core and high chemical stability. The mesomorphic properties of these compounds were investigated using polarizing optical microscopy and differential scanning calorimetry. Spectrometry and electro-optical studies were employed to estimate the helical pitch, tilt angle, and spontaneous polarization of the synthesized compounds and the prepared mixtures. All studied compounds exhibit enantiotropic chiral smectic mesophases including the SmA*, the SmC*, and a very broad temperature range of the SmCA* phase. Doping top-modern antiferroelectric mixture with synthesized compounds offers benefits such as increased helical pitch and tilt angle values without significantly influencing spontaneous polarization. This allows the prepared mixture to be regarded as an OAFLC with high optical contrast, characterized by an almost perfect dark state. These valuable physicochemical and optical properties suggest significant potential of studied materials for practical applications.

Alignment layer and helix controlled unconventional operational switching in ferroelectric liquid crystal

Substrate interface driven helical unwinding led dynamical switching is observed and analysed for the low voltage operation switching of ferroelectric liquid crystal (FLC). The surface driven FLC molecules force the bulk molecules to switch the sample cell device at very low voltage as compared to conventional cells. It has been shown in the present investigation using three FLC materials that the surface anchoring of the FLC molecules and the thickness of the cell play a primary role for such devices because these conditions contrive the surface interface particle helical unwinding which is very essential for the low power switching. However, P s, helical pitch value, and the dielectric strength parameters of the FLC materials are also taken into account for the analysis of bulk switching assisted by interfacial switching. Dielectric and electro-optics investigation has proven that once the relaxation process associated with the interface is confirmed by dielectric spectra then only, the low power operation is observed in the sample which is interesting for fundamental and potential electro optical devices application.

Experimental investigation on performance of downhole electric heaters with continuous helical baffles used in oil shale in-situ pyrolysis

To rapidly produce oil, downhole heating technology requires an electric heater, which is limited by the diameter of the heat injection well, with long-term working stability and high-efficiency heat transfer. Simulations indicated that heater with continuous helical baffles is more suitable for downhole heating technology than heater with segmental baffles in terms of long-term working stability. Then, experiments were conducted on four continuous helical schemes with pitches of 50, 110, 160, and 210 mm. The test results showed that, in the test scope, the heater with the helical pitch of 110 mm (H110) had the best comprehensive performance, which was 2.95–3.75 times better than that of the heater with the helical pitch of 210 mm (H210). Under the same working conditions, heaters with smaller helical pitch values produce less irreversibility in the heat transfer process. In addition, the heaters with helical pitch values of 110, 160, and 210 mm were more effective at a lower Reynolds number, whereas the heater with the helical pitch of 50 mm could be used under a wide effective range of Reynolds numbers. The conclusions are benefit to setting the heating parameters of electric heater during the process of oil shale in-situ pyrolysis.

Determination of the temperature dependence of the helical pitch in 1.m.7(HH) liquid crystalline compounds

Homologous series 1.m.7(HH) of liquid crystalline compounds, varying in the chiral chain length m (m=4-9), were studied. These compounds are characterized by a long helical pitch in an antiferroelectric phase, which is higher than the measuring range of the available instruments. The temperature dependence of the helical pitch of these compounds was determined by the extrapolation of the results obtained for the mixtures of 1.m.7(HH) compounds with 1.4.7(HF) compound of a known helical pitch value. The helical pitch measurements were made by spectrophotometry and phase transition temperatures were measured by polarizing thermomicroscopy. Extrapolation of the results let established that the investigated liquid crystals are characterized by the helical pitch increasing and then decreasing upon heating, which is accompanied by the helical twist sense inversion from the right-handed structure to the left-handed one. Keywords: liquid crystals, antiferroelectric phase, helical pitch, miscibility study

Significant Enhancement of the Chiral Correlation Length in Nematic Liquid Crystals by Gold Nanoparticle Surfaces Featuring Axially Chiral Binaphthyl Ligands.

Chirality is a fundamental scientific concept best described by the absence of mirror symmetry and the inability to superimpose an object onto its mirror image by translation and rotation. Chirality is expressed at almost all molecular levels, from single molecules to supramolecular systems, and present virtually everywhere in nature. Here, to explore how chirality propagates from a chiral nanoscale surface, we study gold nanoparticles functionalized with axially chiral binaphthyl molecules. In particular, we synthesized three enantiomeric pairs of chiral ligand-capped gold nanoparticles differing in size, curvature, and ligand density to tune the chirality transfer from nanoscale solid surfaces to a bulk anisotropic liquid crystal medium. Ultimately, we are examining how far the chirality from a nanoparticle surface reaches into a bulk material. Circular dichroism spectra of the gold nanoparticles decorated with binaphthyl thiols confirmed that the binaphthyl moieties form a cisoid conformation in isotropic organic solvents. In the chiral nematic liquid crystal phase, induced by dispersing the gold nanoparticles into an achiral anisotropic nematic liquid crystal solvent, the binaphthyl moieties on the nanoparticle surface form a transoid conformation as determined by imaging the helical twist direction of the induced cholesteric phase. This suggests that the ligand density on the nanoscale metal surfaces provides a dynamic space to alter and adjust the helicity of binaphthyl derivatives in response to the ordering of the surrounding medium. The helical pitch values of the induced chiral nematic phase were determined, and the helical twisting power (HTP) of the chiral gold nanoparticles calculated to elucidate the chirality transfer efficiency of the binaphthyl ligand capped gold nanoparticles. Remarkably, the HTP increases with increasing diameter of the particles, that is, the efficiency of the chirality transfer of the binaphthyl units bound to the nanoparticle surface is diminished as the size of the particle is reduced. However, in comparison to the free ligands, per chiral molecule all tested gold nanoparticles induce helical distortions in a 10- to 50-fold larger number of liquid crystal host molecules surrounding each particle, indicating a significantly enhanced chiral correlation length. We propose that both the helicity and the chirality transfer efficiency of axially chiral binaphthyl derivatives can be controlled at metal nanoparticle surfaces by adjusting the particle size and curvature as well as the number and density of the chiral ligands to ultimately measure and tune the chiral correlation length.

An Influence of the Fluorosubstitution and the Spacer Length on the Phase Transitions and Helical Pitch Value in High Tilted Antiferroelectric Esters

Phase transitions temperatures and temperature dependence of helical pitch p for S-(+)-4′-(1-methylheptyloxycarbonyl)biphenyl-4-yl 4-[(4,4,4,3,3,2,2-heptafluorobutoxy)alkoxy] benzoates, and 3-fluoro-, and 2-fluoro-, 2,3-difluoro-benzoates upon length of alkoxy spacer were tested. For etoxy and propoxy spacer the pitch p in antiferroelectric phase is increasing with temperature to maximal value of about 1000 nm in high temperatures region. For pentoxy, hexoxy, and heptoxy spacer the pitch p is longer and reverse temperature dependence is observed. The compounds convenient for formulation of antiferroelectric orthoconic mixture with pitch length of about 2000 nm were selected.

New Helical Zinc Complexes with Schiff Base Derivatives of β‐Diketonates or β‐Keto Esters and Ethylenediamine

AbstractThe tetradentate Schiff bases H2L = H2AC [N,N′‐ethylene‐bis(4‐iminopentan‐2‐one)], H2MAL1 [N,N′‐ethylene‐bis(methyl‐3‐iminobutanoate)], and H2MAL2 [N,N′‐ethylene‐bis(tert‐butyl‐3‐iminobutanoate)] were found to possess sufficient flexibility and were tailored to self assemble with zinc ions to form neutral bimetallic helicates with the overall composition Zn2L2. In solution for all ligands, an equilibrium between the tautomeric forms exists, which was confirmed by IR and NMR spectroscopy. X‐ray single‐crystal analysis performed for H2MAL1 (1) and H2MAL2 (2) reveals that they have similar geometrical parameters and crystallize in a trans conformation. In the crystal structures of the zinc complexes [Zn2(AC)2]·C6H6 (3), [Zn2(MAL1)2]·0.5C7H8 (4), and [Zn2(MAL2)2]·C7H8 (5), the Schiff base ligands are helically wrapped around the two four‐coordinate metal ions, which leads to an idealized D2 symmetry. The coordination polyhedron around the Zn ions in 3–5 can be described as a tetrahedral geometry comprising two O‐ and two N‐donor atoms of the Schiff base ligands. The intramolecular separation Zn···Zn varies from 3.465 to 3.628 Å. The total length of the helix is 5.85, 5.84, and 6.01 Å for structures 3, 4, and 5, respectively, while the helical pitch values are in the range 7.8–9.2 Å. Both the ligands H2L and the zinc helicates are thermally stable up to ≈150–160 °C, as shown by thermogravimetric analysis under a nitrogen atmosphere. The formation of the zinc complexes in solution is confirmed by spectrophotometric titrations, along with ESI‐MS, and has almost no effect on the absorption spectra of H2L. The Schiff bases and the Zn2L2 complexes display broad‐band, blue emission in the range 350–600 nm under a 337‐nm excitation at 77 K. Finally, the impact of electronic effects of the Schiff base substituents on the structure of the Zn complexes, especially on bond lengths within the chelate rings, and the absorption and photoluminescence spectra, are discussed. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

Delineation of moving targets with slow MVCT scans: implications for adaptive non-gated lung tomotherapy

Accurate imaging is a prerequisite for adaptive radiation therapy of mobile tumours. We present an evaluation of the performance of slow computed tomography (CT) for mapping and delineating the excursion boundary of a moving object using a tumour phantom scanned with the helical MVCT scanner of a tomotherapy unit. A spherical test object driven by sinusoidal motion in both the lateral and cranial-caudal directions was used to determine how well MVCT images depict the true envelope of the motion. Such information is useful in interpreting the CT images relative to the static object case when radiotherapy gating is to be used or in determining the internal target volume (ITV) when beam gating is not possible. A computer simulation of the CT imaging process was developed which incorporates the third generation fan beam geometry and helical acquisition technique of the tomotherapy MVCT system. Motion artefacts are mainly characterized by the parameter α = Tgantry/Trespiration which is interpreted as the period of the gantry rotation (Tgantry) in units of the respiratory period (Trespiration). Experimental tests were performed using a fixed gantry period of 10 s per full rotation and respiratory period ranging from 4.0 (α = 2.5) to 1.0 (α = 10) s. These cases represent typical clinical imaging conditions on the tomotherapy unit, as well as an extreme test case where the gantry period is intentionally set to be much greater than the respiratory period (termed an ‘ultra-slow’ scan). The accuracy of target (ITV) delineation is evaluated by comparing volumes generated using iso-density contours on the MVCT images to the true motion envelope, known a priori in this phantom study. As expected, motion artefacts are present in clinical MVCT images and they are not averaged over the slow gantry period of rotation. Furthermore, artefacts are not significantly affected by scanning with different helical pitch values. Greater distortions from the true density distribution are observed for lateral motion compared to cranial-caudal motion. Volumes generated by iso-density contours yield better agreement with the motion envelope for scans performed under ultra-slow conditions (α = 10) compared to typical clinical imaging conditions (α = 2.5). If the MVCT gantry cannot be rotated very quickly due to engineering constraints in order to achieve ultra-fast CT, we suggest an opposite approach as an interim measure for mapping the ITV. Adjusting MVCT scan conditions to a very slow acquisition (α = 10) may be a good compromise for determining the ITV for non-gated adaptive tomotherapy of moving lung tumours.

平成15年度後期国際研究集会派遣会員報告書(学術交流委員会だより)

With multi-slice helical scanning, increasing helical pitch causes changes in the effective slice sensitivity profile and thus in a thickening of the effective slice thickness. However, the increase in helical pitch alters the time sensitivity profile, thereby decreasing the effective scan time. In this paper we report the results of our research concerning the setting of optimum conditions to resolve this issue. The experiment was conducted with a moving phantom. The movement showed a variety of changes. When the subject was not in motion, increases in helical pitch resulted in deterioration in the image quality of the CT image. However, when the subject was in motion increases in helical pitch improved image quality. The general belief until now has been that helical pitch should be decreased in order to improve CT image quality. However, in the case of a moving subject, reduced helical pitch leads to deterioration in the time resolution and thus to a deterioration in the image quality. Through this study we have acquired an understanding of which helical pitch value is optimal in terms of the subject's rate of movement.

Optical Properties of Polymer Stabilized Cholesteric Liquid Crystals

Polymer Stabilized Cholesteric Liquid Crystals (PSCLC) were prepared from homogeneous mixtures including various contents of a nematic liquid crystal (E7), a chiral dopant (R1011), a photoinitiator (Darocur 1173) and a difunctional acrylate monomer (HDDA). Helical pitch measurements were carried out on several chiral dopant/LC blends and compared to mixtures including monomer. Similar helical pitch values were found for samples with and without monomer. The electric field-induced diffraction gratings in the cholesteric liquid crystal phase were efficiently stabilized by polymerization of the monomer. Isothermal photocalorimetry measurements were performed to follow the photopolymerization process. The averaged acrylate double bond conversion is of the order of 72%. Preliminary results on the electro-optical properties of PSCLC samples are presented and briefly discussed.

Goldstone mode behaviour in deformed helix ferroelectric liquid crystal materials

The dielectric processes of a deformed helix ferroelectric liquid crystal material have been carried out in a 2.5 μm thick cells, having a helical pitch value of 0.35 μm in the frequency range of 10 Hz to 10 MHz. The measurements have been done in a strongly anchored planarly aligned cell which was done by treating one of the electrode cell by polymer and rubbed and no any treatment was given to the other electrode of the cell. A perfect alignment was obtained by applying an electric field just below the transition temperature of Sm C* – Sm A phase. It has been observed that the charge accumulation field on the rubbed surface affects the Goldstone mode behaviour. The soft mode could not be detected in Sm C* phase by suppressing the Goldstone mode. The temperature and bias field dependences with respect to the applied frequency of the dielectric processes in Sm C* and Sm A phases have been reported and discussed in the deformed helix ferroelectric liquid crystal material.

A correlation between surface effect and pitch value of flc material by dielectric method

The studies of two ferroelectric liquid crystal materials (FLCs), having different helical pitch values, have been performed in thin (2.5 μm) cell by dielectric methods in the frequency range of 10 Hz to 10 MHz. The results have been recorded on a strongly anchored planarly aligned ferroelectric liquid crystal material which was done by coating one electrode of the cell with polymer and rubbed. The influence of surface effect on the behaviour of the Gold-stone mode has been investigated for positive and negative polarity bias field to the rubbed surface of the cells. It has been observed that in thin cells the dielectric permittivity due to the Goldstone mode in the smectic C* phase is completely different by applying bias field in ferroelectric liquid crystal materials, having different helical pitch value. The results have been discussed in view of how the surface effect influences the phase fluctuation (Goldstone mode) behaviour in smectic C* phase. A correlation between surface effect and helical pitch value of FLC material is drawn and discussed.

A boundary element analysis of helically symmetric potential fields

This paper presents a boundary element analysis of helically symmetric potential fields. A two dimensional boundary integral equation is introduced in general curvilinear coordinates. On the basis of this formulation, a boundary integral equation for helically symmetric potential problems is obtained using twisted coordinates. Moreover, the boundary element solution by the present scheme is compared with perturbation and finite element solutions for a simple potential problem and it is shown that the boundary element solution is in good agreement with the finite element solution for arbitrary helical pitch values while the perturbed solution becomes inaccurate for large pitch values.

Scanning tunneling microscopy of a liquid crystalline phase of poly((dA-dT) · (dA-dT)) induced by a histone H 1 peptide

In this report, we present the first observations of uncoated poly((dA-dT)·(dA-dT)) molecules organized in a liquid crystalline phase induced by the binding of a histone H 1 peptide. The effect of the peptide on the polymer condensation is clearly illustrated on the large-scale STM images which reveal a well defined spacing between parallel DNA helices. High resolution images of rare isolated molecules of poly((dA-dT)·(dA-dT)) exhibit two sets of helical pitch values, 6 and 7.5 nm. While the lower value can be correlated with the pitch of poly((dA-dT)·(dA-dT)), the larger one may arise from peptide binding in the polymer minor groove.

Dielectric and Optical Measurements for Some Compounds Exhibiting an Is-Ch-SmC* Phase Transition Sequence

Abstract The Isotropic—Cholesteric—Chiral Smectic C—Crystal phase transition sequence for some terminal alkoxy derivatives of 2,2'-dihydroxybenzalazine has been studied. Dielectric and optical measurements for the compounds with n = 7, 8, 9 and 10 are reported. The low-frequency dielectric anisotropy is determined for the chiral meso-phases and the results are analysed as a function of the terminal chain length. The helical pitch values and tilt angles in the chiral smectic C phases are found to exhibit some peculiar characteristics which are qualitatively different from those of the materials undergoing the more fully studied smectic A—chiral smectic C phase transition.

Experimental Evidence of Permeation Effects in Induced Cholesteric Systems

Abstract Viscous properties of induced cholesteric systems with different steroidal and nonsteroidal opically active dopants were investigated in flat capillaries. In all the systems studied, when helical pitch values were sufficiently low we observed a pronounced non-Newtonian behaviour interpreted in the terms of “permeation” model. With high pitches and high shear rates, however, properties were much more nematic-like. Comparisons are made with data obtained for mesogenic cholesterol esters and tube capillaries.