Chiral Twin Research Articles

Determination of crystallographic chirality of MnSi thin film grown on Si (111) substrate

MnSi is a prototypical chiral cubic magnet, in which swirling magnetic objects referred to as magnetic skyrmions appear. It is expected that the thin film of MnSi may expand the range of the magnetic skyrmion lattice phase in the temperature--magnetic field diagram. On the other hand, the MnSi film on an achiral Si (111) substrate may suffer from chiral twin formation. We succeeded in visualizing the domain structure of the chirality and axis orientation of a MnSi thin film by using a combination of transmission electron microscopy and x-ray reflectivity measurement. Only two types of domains were found on a Si (111) substrate. The information is essential for the challenge of homochiral film fabrication.

Dynamic chirality in mass regions A=105 and A=130

Chiral symmetry is dichotomic and its spontaneous breaking by the axial angular momentum vector leads to a pair of degenerate ΔI = 1 rotational bands, called chiral doublet bands. In many cases the energy degeneracy of the chiral candidate bands has almost been observed but the transition probabilities of the analogue states have been found to be different. In the angular momentum region where chirality sets in the B(E2) values of the electromagnetic transitions deexciting analogue states of the chiral twin bands should be almost equal. Correspondingly the B(M1) values should exhibit staggering. Experimental data for two nuclei from different mass regions are compared with theoretical predictions. Exited states in 134Pr were populated in the fusion-evaporation reaction 119Sn(19F, 4n)134Pr, while excited states in 102Rh were populated in the fusion-evaporation reaction 94Zr(11B, 3n)102Rh. Our lifetime measurements and level-scheme investigations show that there are similarities in both chiral candidates nuclei related to dynamic chirality.

Examples of dynamic chirality in nuclei

In many cases the chirality was almost observed but the transition probabilities are different. This fact is clearly seen in the cases of 134Pr and 102Rh. In the case of chirality, the yrast and the side bands should be nearly degenerate. In the angular momentum region where chirality sets, the B(E2) values of the electromagnetic transitions deexciting analog states of the chiral twin bands should be almost equal. Correspondingly the B(M1) values should exhibit staggering. Our lifetime measurements in the cases of 134Pr and 102Rh and the theoretical analysis do not support static chirality. Chirality has mainly a dynamical character in both nuclei. In the present paper, we compare our results with the results for other chiral candidate nuclei, which fulfill static chirality conditions.

Diabatic crossing of chiral “twins” in the odd-oddAg106nucleus: A theoretical perspective

A systematic study of both the observed positive-parity magnetic rotation band and the negative-parity $\mathrm{\ensuremath{\Delta}}I=1$ doublet bands in an odd-odd $^{106}\mathrm{Ag}$ nucleus is carried out. The negative-parity doublet bands depict some unusual features that have not been observed in any isotope in the mass $A=100$ region. For instance, (i) the moment of inertia of the partner band is quite different from that of the yrast band, and (ii) these bands cross each other at an angular momentum of $I=14\ensuremath{\hbar}$. Also, the observed significantly large but constant $B(M1)$ transitions confirm that the strong $M1$ transitions are being reinforced by the contributions from collective rotation. To explain these features, a collective model has been developed whose kinetic and potential energies are extracted from the tilted-axis cranking model. Instead of the triaxial parameter $\ensuremath{\gamma}$, a second-order phase transition is found to be responsible for the spontaneous breakdown of chiral symmetry. Analytical solution of the Schr\odinger equation has generated a doublet nondegenerate eigenvalue spectrum. The ensuing model results based on the two-quasiparticle configuration $\ensuremath{\pi}{g}_{\frac{9}{2}}\ensuremath{\bigotimes}\ensuremath{\nu}{h}_{\frac{11}{2}}$ exhibit similarities with many observed features of the negative-parity doublet bands and hence confirm their chiral character. The cranking mass parameter in kinetic energy plays an important role in diabatic crossing between these emerged chiral twin bands.

In a search for a chiral symmetry in 102Rh

Excited states in 102Rh were populated in the fusion-evaporation reaction 94Zr(nB, 3n)102Rh at a beam energy of 36 MeV, using the INGA spectrometer at IUAC, New Delhi. The angular correlations and the electromagnetic character of some of the γ-ray transitions observed in 102Rh were investigated in detail. A new candidate for a chiral twin band was identified in 102Rh for the first time.

Binding Geometry of Hydrogen-Bonded Chain Motif in Self-Assembled Gratings and Layers on Ag(111)

Upon adsorption on the (111) facet of Ag, 4-[trans-2-(pyrid-4-yl-vinyl)] benzoic acid (PVBA) self-assembles into a highly ordered, chiral twin chain structure at submonolayer coverages with domains that can extend for micrometers in one dimension. Using polarization-dependent measurements of C and N K-shell excitations in near-edge X-ray absorption fine structure (NEXAFS) spectra, we determine the binding geometry of single PVBA molecules within this unique ensemble for both low and high coverage regimes. At submonolayer coverage, the molecule is twisted to facilitate the formation of hydrogen bonds. The gas-phase planarity is gradually recovered as the coverage is increased, with complete planarity coinciding with loss of order in the overlayer. Thermal treatment of the PVBA film results in deprotonation of the carboxyl tail of the molecule, but despite the suppression of the stabilizing hydrogen-bonds, the overlayer remains ordered.

Shell chirality in Cambrian gastropods and sinistral members of the genus Aldanella Vostokova, 1962

Shell chirality among Cambrian gastropods is discussed. It is demonstrated that the earliest members of the class include chiral aberrations with abnormal opposite coiling of the shell. It is assumed that, in Cambrian gastropods, speciation could have occurred by mutation in the locus determining the chirality, as is proposed for extant gastropods. In contrast to modern gastropods, the existence of chiral morphs within single species has not been recorded in Cambrian mollusks, whereas the presence of chiral twin species is possible. The systematic position of sinistral representatives of the genus Aldanella Vostokova, 1962 is considered. Aldanella golubevi sp. nov. with sinistral shell is described from the base of the Tommotian Stage of the Anabar Region. Aberrant sinistral specimens of the normally dextral species Aldanella utchurica Missarzhevsky in Rozanov et al., 1969 and Pelagiella adunca Missarzhevsky in Rozanov et al., 1969 are figured.

Effect of introducing thioether linkages on the molecular organization of chiral twin liquid crystals

Novel symmetric and non-symmetric chiral twin compounds possessing one or two thioether groups in a central spacer were prepared, and the effect of substituting oxygen for sulphur on the liquid crystalline properties investigated. Chiral twin compounds possessing an alkylsulphanyl spacer showed an antiferroelectric phase exclusively. However, replacing the alkyloxy chain of the analogous monomer by the alkylsulphanyl chain has no significant effect on the phase transition behaviour, i.e. both of the monomers showed the ferro- and ferri-electric phases as well as the antiferroelectric phase. Thus, different effects of introducing the thioether linkage were for the first time observed between twin and monomeric systems. The introduction of oxygen or sulphur atoms into the central alkyl spacer of the chiral twin was also investigated, and these modifications were found to stabilize the SmA phase. Furthermore, the twin compound possessing a thiaalkyl spacer showed two different molecular assemblies in the smectic A phase.

A Chameleon Chiral Polar Liquid Crystal: Rod-Shaped When Nematic, Bent-Shaped When Smectic

The first antiferroelectric liquid crystal (AFLC) exhibiting a (chiral) nematic phase, a combination which has long been the goal of synthetic chemists working with polar liquid crystals but which at the same time represents a fundamental contradiction in terms of translational order, was recently reported by Nishiyama and co-workers. We have investigated this chiral twin dimer by optic, electrooptic, and dielectric methods and conclude that it is not an ordinary AFLC material, but one where the peculiar properties of bent-core smectics are combined with those of ordinary rod-shaped liquid crystals. The compound exhibits a new type of nematic−smectic phase transition, connected with a change of molecule conformation from rod- to bent-shaped. This also has an important impact on the chiral interactions in the system. Toward the high-temperature end of the smectic phase, the energy balance between bent conformation smectic and straight conformation nematic can be shifted by an electric field such that the t...

Novel Chiral Effects on the Molecular Organization in the Liquid-Crystalline Phases

An optically active chiral twin compound SS4 showed two novel liquid-crystalline phases. One is optically isotropic M1 phase, and the other exhibits parquet-like texture (M2 phase). Both phases changed into the antiferroelectric phase with reduction of the optical purity, showing a new type of chiral effect on the mesogenic behavior.

HELICAL STRUCTURES PRODUCED IN SOME BINARY MIXTURES BETWEEN TWIN AND MONOMERIC LIQUID CRYSTALS

Binary mixtures between a chiral twin possessing two identical chiral moieties and the analogous parent monomeric liquid crystal were prepared, and liquid-crystalline properties investigated. TGB phase was found to be induced in some mixtures between (R, R)-twin and (R)-monomer. A mixture between (S, S)-twin and (R)-monomer showed an induced nematic phase due to the pitch compensation, however, also showed an induced TGB phase. XRD measurements showed that the intra-layer molecular distance increases by mixing the twin and monomeric compounds, which may produce a weaker layered structure, consequently, the TGB structure was stabilized.

Ferrielectric and antiferroelectric chiral twin liquid crystals showing a stable chiral nematic phase

A homologous series of chiral twin liquid crystals possessing identical chiral moieties at both peripheral ends, i.e. optically active α,ω -bis{4-[(4′-(1-methylheptyloxycarbonyl)-4-biphenylyl)oxycarbony]phenoxy}alkanes, has been prepared and their liquid crystalline properties investigated. The homologues preferred to show the ferrielectric and/or antiferroelectric phase rather than the ferroelectric phase. With ascending central spacer length, the temperature range of the ferrielectric phase became narrow and eventually disappeared for the dodecyl homologue, suggesting that the coupling in motion and/or direction between two mesogenic parts of each twin molecule has an important effect on the stabilization of the ferrielectric phase. The octyl and dodecyl homologues showed a wide temperature range chiral nematic phase (ca. 10°C or more), so that these compounds were found to be the first examples showing antiferroelectric and/or ferrielectric phases with a broad temperature range of the chiral nematic phase. X-ray diffraction studies showed that the homologues tend to form a mono-layered structure. The formation of a relatively well defined layered structure was also indicated, which is considered to be important for generating anticlinic ordering in the antiferroelectric and ferrielectric phases. An isotropic-isotropic transition characterized by the emergence of a broad diffuse DSC peak was observed for the even-membered homologues.

PB15 非対称ツイン液晶が示すカイラリティーに敏感な特異な液晶相(化学・材料)

PB15 非対称ツイン液晶が示すカイラリティーに敏感な特異な液晶相(化学・材料)

Novel non-symmetric chiral twin liquid crystals possessing two identical chiral moieties at both peripheral ends: highly chirality-sensitive phases appeared between smectic and isotropic liquid phases

A novel strategy for preparing non-symmetric chiral twin liquid crystals was proposed, where two identical chiral moieties were designed to be located at both peripheral ends and the non-symmetric character was introduced by the non-symmetric central connecting structure. Thus, optically active chiral non-symmetric twin homologues, [4′-(1-methylheptyloxycarbonyl)-1,1′-biphenyl-4-yl] 4″″-[ω-[{4″-(1-methylheptyloxycarbonyl)-1″,1″′-biphenyl-4″′-yl}oxycarbonyl]alkyloxy]benzoate, were prepared, which were found to exhibit the antiferro-, ferri-, and ferroelectric phases. Furthermore, two more phases were observed between the ferroelectric and isotropic liquid phases. The lower-temperature phase was optically isotropic under the polarized light microscopy for both of the homogeneously and pseudo-homeotropically aligned thin samples, whereas the high-temperature phase showed a mosaic texture. Both of these anomalous phases were found to have a layered structure with a smaller layer spacing than the smectic phase. The stability of these phases was quite sensitive to the optical activity. With reduction of the optical purity, these phases disappeared but the Twist Grain Boundary phase was observed. A 1 ∶ 1 mixture of (S,S)- and (R,R)-isomers just showed the smectic A phase instead of these anomalous phases.

Ferro-, Ferri-, Antiferroelectric and TGB Phases Created by Chiral Twin Liquid Crystals

Chiral twin compounds, possessing two chiral moieties in such a way that the chiral centres are located at the both peripheral sides of a molecular twin structure, have been prepared and the liquid-crystalline properties investigated. The chiral twins showed stable ferrielectric and antiferroelectric phases. A twist grain boundary phase was found to appear by mixing the twin and the analogous monomer.

A symmetric chiral liquid-crystalline twin exhibiting stable ferrielectric and antiferroelectric phases and a chirality-induced isotropic–isotropic liquid transition

In this article we report the first example of a pair of symmetrically substituted chiral twin enantiomers that exhibit stable wide temperature range ferrielectric and antiferroelectric phases, and which have large tilt angles of over 45°. In addition, both of the enantiomers exhibited an isotropic–isotropic liquid transition just above the transition to the liquid crystalline state.

Evidence for chiral symmetry breaking in136Pmand138Eu

High-spin states in the doubly odd $N=75$ nuclei ${}^{136}\mathrm{Pm}$ and ${}^{138}\mathrm{Eu}$ were populated following the ${}^{116}\mathrm{Sn}{(}^{24}\mathrm{Mg},p3n)$ and ${}^{106}\mathrm{Cd}{(}^{35}\mathrm{Cl},2pn)$ reactions, respectively. A new $\ensuremath{\Delta}I=1$ band is reported in ${}^{138}\mathrm{Eu}$ and new data are presented for the recently reported band in ${}^{136}\mathrm{Pm}.$ Polarization and angular correlation measurements have been performed to establish the relative spin and parity assignments for these bands. Both bands have been assigned the same $\ensuremath{\pi}{h}_{11/2}\ensuremath{\bigotimes}\ensuremath{\nu}{h}_{11/2}$ structure as the yrast band and are suggested as candidates for chiral twin bands.

Stereoselective activity of local anesthetics

Most local anesthetic molecules contain an asymmetric carbon atom in their structure.Therefore, they provide excellent examples of chiral drugs that exist as two enantiomers, where each form may exhibit striking differences in its pharmacologic profile. Bupivacaine is currently marketed as a racemic mixture of both its S-(-)- and R-(+)-enantiomers. Fuelled by concerns about the cardiovascular toxicity of this preparation, research has focused on the pharmacology of the single enantiomers.The R-(+)-enantiomer has a higher potency as a sodium channel blocker than its “chiral twin” but correspondingly, also causes higher cardiovascular and central nervous system toxicity at lower doses.This work on bupivacaine has in turn driven the development and release of ropivacaine, an optically pure S-(-)-propyl homologue of bupivacaine.When compared with racemic bupivacaine, it demonstrates a significantly improved toxicity profile.This paper reviews some of the more recent literature on this subject in order to provide an account of current thinking on this important group of drugs.

Amplification of the twisting power by modification of the chiral spacer in a smectic liquid-crystalline phase

We investigated twisting power of chiral twin molecules, (S)-2-methylnonanedioic acid bis[4-(5-octyl-2-pyrimidinyl)phenyl]ester (MNB-8-PYP) and (S)-2-methyldecanedioic acid bis[4-(5-octyl-2-pyrimidinyl)phenyl] ester (MDB-8-PYP), in N* and SmC* phases. The inverse of the SmC* pitch induced by MDB-8-PYP was found to vary non-linearly as a function of changing concentration, indicating cooperative twisting mechanism for MDB-8-PYP which induces stronger helical structure than MNB-8-PYP.

Odd-even effects in the optical properties of chiral triplet liquid crystals based on estradiol

A series of bis ω-cyanobiphenylyloxyalkanoate esters of estradiol with variable spacers have been prepared as chiral triplet liquid crystals. The compounds show very broad cholesteric phases and are transformed into cholesteric glasses at room temperature. They exhibit a relatively weak odd-even effect with spacer length for their cholesteric-isotropic transition temperatures and the associated entropy changes. This weak odd-even effect probably results from the orientation of the ester linkages to the estradiol moiety. This causes the cyanobiphenyl groups to be poorly ordered with respect to one another, regardless of the odd or even number of methylene units in the alkanoate spacer. The selective reflection wavelength of the planar cholesteric phase of most of the compounds increases strongly with temperature and changes from blue at low temperature to red at high temperature. An odd-even effect of the selective reflection wavelength with spacer length as observed with other chiral twin and triplet liquid crystals is hardly observed.