Twist Sense Research Articles

X-ray diffraction study of DL-1,4-dimethyl-2,5-dioxabicyclo[2.2.1]heptane-3,6-dione: the sense of twist and folding

The title dilactone exists in a crystalline state (110 K) as enantiomeric synchro-(+,+)-(R,R)- and synchro-(–,–)-(S,S)-twist forms (τ0 = |1.8°|) with the retention of C2 symmetry, linked by infinite zig-zag heterochiral chains (space group C2/c) by mutual H-bonds of the CH2–H‥O=C type.

Transmissive liquid crystal cell parameters measurement by spectroscopic ellipsometry

A powerful method for liquid crystal (LC) cell twist angle and retardation measurement is presented. This method is based on the spectroscopic ellipsometry. By varying the polarizer and analyzer angles only, a transmission spectrum containing null transmission at some particular wavelengths can be obtained. Analytical equations derived bases on a polarization analysis of twisted birefringent layers are then solved to give all the LC cell parameters. In general, the twist angle, twist sense, retardation, and the rubbing direction can all be determined. Measurement results on commercial liquid crystal display panels are presented.

The Counterkink Rotation of a Non‐Hale Active Region

We describe the long-term evolution of a bipolar non-Hale active region which was observed from October, 1995, to January, 1996. Along these four solar rotations the sunspots and subsequent flux concentrations, during the decay phase of the region, were observed to move in such a way that by December their orientation conformed to the Hale-Nicholson polarity law. The sigmoidal shape of the observed soft X-ray coronal loops allows us to determine the sense of the twist in the magnetic configuration. This sense is confirmed by extrapolating the observed photospheric magnetic field, using a linear force-free approach, and comparing the shape of computed field lines to the observed coronal loops. This sense of twist agrees with that of the dominant helicity in the solar hemisphere where the region lies, as well as with the evolution observed in the longitudinal magnetogram during the first rotation. At first sight the relative motions of the spots may be miss-interpreted as the rising of an $\Omega$-loop deformed by a kink-instability, but we deduce from the sense of their relative displacements a handedness for the flux-tube axis (writhe) which is opposite to that of the twist in the coronal loops and, therefore, to what is expected for a kink-unstable flux-tube. After excluding the kink instability, we interpret our observations in terms of a magnetic flux-tube deformed by external motions while rising through the convective zone. We compare our results with those of other related studies and we discuss, in particular, whether the kink instability is relevant to explain the peculiar evolution of some active regions.

Conformational Ordering of Apolar, Chiral m-Phenylene Ethynylene Oligomers

A series of m-phenylene ethynylene oligomers containing nonpolar, (S)-3,7-dimethyl-1-octanoxy side chains have been synthesized and studied. In apolar alkane solvents, oligomers of sufficient length (n > 10) were found to adopt a helical conformation with a large twist sense bias. In contrast, in chloroform the oligomers adopt a random coil conformation. Surprisingly, the strong twist sense bias was determined to be highly time dependent and is partially attributed to intermolecular aggregation.

8.2: A New Method for TN Cell Twist Angle and Retardation Measurement

AbstractWe propose a new measurement method to determine the twist angle and the retardation value in a twisted nematic(TN) or super twisted nematic (STN) cell. No prior knowledge of the cell is required. The surface director direction and the twist sense are also determined.

Twist Sense Bias Induced by Chiral Side Chains in Helically Folded Oligomers.

Cooperative interactions among the side chains of the helically folded phenylene-ethynylene oligomer shown (n=2, 4, 6, 8, 10, 12, 14, 16, 18) can induce a twist sense bias. Therefore, the side chains can play more than just an ancillary role in these conformationally ordered oligomers. The onset of the twist sense bias lags significantly behind the appearance of helical conformations, possibly because a large ensemble of "collapsed" conformations is initially formed.

Twist Sense Bias Induced by Chiral Side Chains in Helically Folded Oligomers

Twist Sense Bias Induced by Chiral Side Chains in Helically Folded Oligomers

Inherent chirality of the retinal chromophore in rhodopsin-A nonempirical theoretical analysis of chiroptical data.

CASSCF and GAUSSIAN CIS calculations were performed on ground and excited states of different conformations of 11-cis-retinal protonated Schiff bases, the chromophore of rhodopsin, in order to study their chiroptical properties and attempt a correlation between absolute conformation and CD-spectral data. Geometries were taken from molecular models, from published rhodopsin models, and from retinal conformations obtained from molecular dynamics with geometry restraints. In all the cases studied we find that a positive sense of twist about the C12-C13 bond correlates with a calculated positive CD of the long wavelength absorption band; the twist of the C6-C7 bond modulates this primary contribution of the C12-C13 bond. The correlation of the beta-band with structural features is not straightforward. Calculations on bathorhodopsin lend support to the idea that this intermediate is a highly twisted all-trans-conformation.

Helical twist sense bias in oligo(phenylene ethynylene)s induced by an optically active flexible tether

[graph and structure: see text] A series of tethered phenylene ethynylene oligomers, which undergo a solvent-dependent conformational transition from a random coil to a helix, has been synthesized. The use of trimethylsilyl ether protecting groups on the (+)-tartaric acid-derived tether results in the formation of helices with a large twist sense bias. In contrast, an isopropylidene ketal protecting group or no protecting group is not only ineffective at helical discrimination but may even inhibit helix formation.

Mesogenic derivatives of 2S,3S-2-halogeno-3-methylpentanoic acid with helix twist inversion in the smectic C* phase

Two homologous series of chloro and bromo derivatives of S,S-3-methylpentanoic acid were synthesized and studied. For some of these compounds an inversion of helix twist sense in the smectic C* phase was observed. Dielectric dispersion measurements showed a pronounced anomaly of the Goldstone mode relaxation frequency and strength close to the helix inversion point. Moreover, a qualitative difference in behaviour of the mode in samples with planar and with homeotropic alignment was detected; this is due to surface interactions.

Gaussian Schell-model beams and general shape invariance

We present two six-parameter families of anisotropic Gaussian Schell-model beams that propagate in a shape-invariant manner, with the intensity distribution continuously twisting about the beam axis. The two families differ in the sense or helicity of this beam twist. The propagation characteristics of these shape-invariant beams are studied, and the restrictions on the beam parameters that arise from the optical uncertainty principle are brought out. Shape invariance is traced to a fundamental dynamical symmetry that underlies these beams. This symmetry is the product of spatial rotation and fractional Fourier transformation.

Light Induced Colour Change of Cholesteric Copolymers**

We demonstrate an easy method for the colour patterning of cholesteric liquid crystalline copolymers based on photoisomerization of a chiral comonomer. The UV dose determines the reflection colour. If two chiral compounds with opposite twist sense are copolymerized, an inversion of helical twisting sense can be observed during photoisomerization.

Enantioselective binding of an 11-cis-locked cyclopropyl retinal. The conformation of retinal in bovine rhodopsin.

[formula: see text] The conformation of the retinal chromophore in rhodopsin is central for understanding the visual transduction process. The absolute twist around the 12-s bond of the chromophore in rhodopsin has been determined by studies with 11-cis-locked 11,12-cyclopropylretinal analogues (11S,12R)-2 and (11R,12S)-3, enantioselectively synthesized with the aid of an enzyme. The finding that enantiomer 2 binds to opsin while the other 3 does not defines the absolute sense of twist around the 12-s bond.

Kossel diagram of an aligned cholesteric phase

The selective reflection of light by the cholesteric phase has been examined using the Kossel technique. The Kossel diagram observed for an aligned monodomain sample, viewed down the helicoidal axis, using crossed polarizers, consists of a single bright annulus. Within this annulus two features are visible---a bright central ring and a pattern of four spiral arcs. When the analyzer is rotated from the crossed to the uncrossed position, one diagonally opposite pair of arcs remains stationary and the other pair rotates around the ring until it becomes superimposed on the stationary pair. The handedness of the spiral pattern is related to the chirality of the phase and offers a direct way of experimentally determining the twist sense---a cholesteric phase with a right-handed twist giving a clockwise spiral pattern. The presence of the more intense central ring and the spiral ``extinction'' pattern are in accordance with the analytical modeling of the selective reflection of cholesteric phases by Oldano et al.

Synthesis and Conformational Characteristics of Poly(phenyl isocyanate)s Bearing an Optically Active Ester Group

Optically active phenyl isocyanates (1−9) bearing an optically active ester group were synthesized and polymerized with an achiral anionic initiator. The resulting polymers from 1 and 4−9 showed a much greater specific rotation than that of the corresponding monomer and an intense circular dichroism (CD) band caused by the main-chain absorption, indicating that these polymers have a predominantly one-handed helical conformation in solution. On the other hand, the polymers obtained from 2 and 3 showed a much smaller specific rotation and a weaker CD band than poly-1, which suggests that, as the distance between the polymer main chain and the asymmetric center on the side chain increases, the asymmetric center cannot effectively induce an excess of one helical twist sense of the polymer main chain at room temperature. However, the specific rotation and CD intensity of poly-2 greatly increased because of a predominantly one-handed helical conformation at low temperatures. The polymers obtained from 3−9 did not show such a temperature effect on conformational change. Specific rotation of the copolymers of chiral 8 and achiral 10 changed from a positive to a negative value with an increase in the content of the optically active unit. The helicity of the copolymer may depend on the composition.

Conformers of n-Si5Me12: A Comparison of ab Initio and Molecular Mechanics Methods

Optimized geometries of the conformers of permethylated linear pentasilane, n-Si5Me12, were calculated by the HF/3-21G*, MM3, MM2, and MM+ methods, which predict eight, nine, six, and six energetically distinct enantiomeric conformer pairs, respectively, at geometries representing various combinations of the anti (similar to 165 degrees), ortho (similar to 90 degrees), and gauche (similar to 55 degrees) SiSiSiSi dihedral angles in the backbone. The results of the MM2 and MM+ methods, based on the same force field, differ insignificantly. The barriers between conformers appear to be exaggerated by the molecular mechanics methods, particularly MM2. Contour maps showing the groundstate energy as a function of the full range of two backbone SiSiSiSi dihedral angles, with all other geometrical variables optimized, computed by each of the methods (only a limited range of angles near the anti,anti geometry in the case of HF/3-21G*) are compared with each other and with analogous results for a model compound, Si4Me10. Conformer interconversion paths are discussed, and two meso transition states for enantiomer interconversion have been located at the HF/3-21G* level of calculation. At the eight HF/3-21G* optimized geometries, single-point energies (HF/6-31G* and MP2/6-31G*) and vibrational frequencies (HF/3-21G*) were computed. The predicted IR and Raman spectra suggest that about half of the expected conformers will be identifiable by vibrational spectroscopy under conditions of matrix isolation. Relative conformer energies calculated by the MM2 and HF methods are similar and favor the anti dihedral angles over gauche and ortho, in agreement with results of solution experiments. Those calculated by the hIM3 and HF methods are similar to each other and favor both anti and gauche dihedral angles nearly equally over ortho, in agreement with indications provided by gas-phase experiments. A rationalization of these solvent effects is proposed. The energies of the conformers of Si4Me10 and Si3Me12 were used to set up a system of additive increments at the MM2, MM3, HF/3-21G*, HF/6-31G*, and MP2/6-31G* levels of calculation, which can be used to predict conformational energies of longer permethylated oligosilanes. An intrinsic energy value is assigned to each of the a, o, and g dihedral angles, and interaction energy values are assigned to each combination of two dihedral angles. The interaction values follow the expected rules in that equal twist sense is favored for adjacent aa, ag, oo, and gg pairs, whereas opposite twist sense is generally favored for adjacent so and go pairs. The MM3-derived set of increments has been tested against results computed for Si6Me14 and found to perform well.

Flux‐Tube Twist Resulting from Helical Turbulence: The Σ‐Effect

Recent observational studies suggest that active region magnetic flux emerges in a twisted state and that the sense of twist depends weakly on solar hemisphere. We propose that this twist is imparted to the flux through its interaction with turbulent velocities in the convection zone. This process, designated the Σ-effect, operates on isolated magnetic flux tubes subjected to buffeting by turbulence with a nonvanishing kinetic helicity ${b u}$ --> ∇ × ${b u}$ -->. The Σ-effect leads to twist of the same sense inferred from observation and opposite to that predicted by the α-effect. A series of numerical calculations are performed to estimate the magnitude of the Σ-effect in the solar convective zone. The results compare favorably with observations in both mean value and statistical dispersion. We find a further relationship with total magnetic flux that can be tested in future observations. The model also predicts that twist is uncorrelated with the tilt angle of the active region.

Absolute Sense of Twist of the C12-C13 Bond of the Retinal Chromophore in Rhodopsin—Semiempirical and Nonempirical Calculations of Chiroptical Data

The sign of the dihedral angle of the 12-s-trans bonds in the protonated Schiff base of 11-cis-retinal (see picture on the right) correlates with the sign of the long-wavelength absorption in the circular dichroism spectra. This is the result of the high-quality excited-state calculations on the chromophore of the visual pigment rhodopsin.

A new chiral polyisocyanate: an optical switch triggered by a small amount of photochromic side groups

A new isocyanate monomer was synthesized which contains an azobenzene dye connected to the isocyanate group via a chiral spacer. Copolymers with the achiral hexyl isocyanate were prepared by anionic polymerization with NaCN. CD measurements in solution show that the chiral center in the spacer induces a shift in the equilibrium between P- and M-helical main chain conformations toward the left-handed conformation. The magnitude of this chiral induction can be varied reversibly by photochemical isomerization of the azo chromophore (trans → cis). The preference for one twist sense of the polymer chain is much larger than in previously studied systems, if copolymers with the same amount of chiral monomer units are compared. It is thus possible to build chirooptical switches with only small amounts of chiral monomers.

Spontaneous chiral domains in the nematic phase of achiral trans-4-alkylcyclohexanecarboxylic acids

We have discovered a spontaneous chiral ordering in thermotropic nematic liquid crystals forming hydrogen bonds, namely in trans -4-alkylcyclohexanecarboxylic acids, which do not contain any chiral group in the molecular structure, and have suggested a possible explanation of this phenomenon. The investigation was performed by polarizing light microscopy, using cells with glass substrates treated to give a uniform planar alignment in the nematic phase. On cooling from the isotropic phase, two types of nematic domains with different alignment were found to grow: some possessing a unidirectional planar orientation as expected, and others with a spontaneous twisted molecular arrangement. The appearance of the twisted domains and the sense of their twist were random, i.e. they always emerged in various regions of the cell with different twist sense: thus, they cannot be regarded as induced by stable defects in the surface aligning film. With the coalescence of the nematic droplets, the number of the domains with the spontaneous twist very quickly characterized by different handedness transformed into a racemic domain with uniform planar alignment. Here the explanation of the macroscopic texture is given in terms of molecular structure, based upon the ability of the carboxylic acids to organize through hydrogen bonds into not only dimers, but also trimers and oligomers. The microscopic structure influences the macroscopic twisted organization of the director inside the domains, and the surface lies normally to the helix axis. A brief review is given of work done in the fields of hydrogen bonded liquid crystals and of order transitions, together with a Landau-de Gennes model describing the mechanism of the spontaneous chiral ordering. decreased, since two chiral domains