Achiral Host Research Articles

Solid-state Transformation of Achiral Crystals to a Chiral Crystal by Guest Release from Host–Guest Inclusion Crystals of Achiral Host Compound

Abstract Solid-state transformation of achiral crystals into a chiral crystal was induced by thermal guest release from the host–guest inclusion complexes of 2,5-bis(hydroxydiphenylmethyl)thieno[3,2-b]thiophene as a host component. The chilarity of the resulting guest-free crystals, also obtained by recrystallization from dichloromethane, is attributed to conformational freezing of the achiral molecule into a chiral molecular structure in the solid state.

Stereochemical signal

The enantiomeric purity of a guest molecule can be measured through the splitting of a signal in the proton NMR spectrum of an achiral host

Enantiodiscrimination of methamphetamine by circular dichroism using a porphyrin tweezer

Using exciton-coupled circular dichroism (ECCD) spectroscopy, our lab was able to differentiate between the two enantiomers of methamphetamine using a commercially available porphyrin tweezer as an achiral host. The host-guest complex formed with (+)-(S)-methamphetamine produced a negative bisignate-shaped ECCD spectrum, whereas the complex formed with (-)-(R)-methamphetamine produced a positive one. This sensitive technique could serve as an alternative method for the enantiodiscrimination of chiral methamphetamine, a commonly abused drug in the United States.

Nuclear Magnetic Resonance Signaling of Molecular Chiral Information Using an Achiral Reagent

Until now NMR spectroscopic detection of guest chirality using an achiral host has not been possible in the absence of a chiral medium or auxiliary since chiral discrimination is principally based on chiral discrimination by host and/or diastereomeric host-guest complex formation. In this paper, we demonstrate that an achiral oxoporphyrinogen works as a host capable of signaling chiral information of alpha-hydroxycarboxylic acids in (1)H NMR spectroscopy. In particular, enantiomeric excess (ee) can be determined by observing the splitting of (1)H NMR resonances of the achiral host. This differs from the case of chiral hosts (shift reagents) where % ee is generally determined from the ratio of peak areas due to diastereomeric host-guest complexes. UV/vis, CD, FT-IR, and NMR spectroscopic investigations suggest that the unusual phenomenon reported here is based on formation of a complex with 1:2 stoichiometry in concert with a protonation-driven tautomerization of the host.

On the balance between syn- and anticlinicity in smectic phases formed by achiral hockey-stick mesogens with and without chiral dopants

A series of achiral hockey-stick-shaped mesogens forming tilted smectic liquid crystal phases of synclinic SmC- as well as anticlinic SmCa-type was prepared and characterized. While all homologues exhibit both phases, the balance shifts from anticlinic to synclinic order upon elongation of the terminal chain at the meta- position, defining the hockey-stick shape. The elongation also leads to an increased kinetic hindrance of the transition between syn- and anticlinic phases and a decreased transition enthalpy. These observations indicate that a well-defined kink (short meta-substituted chain) promotes the anticlinic structure while a higher flexibility between kinked and rod-shape (long meta-substituted chain) promotes synclinic order. An intermediate chain-length homologue was selected as host material for doping with syn- and anticlinic rod-shaped chiral dopants, respectively, at varying concentrations. Opposite of what might be expected the balance between syn- and anticlinic order was not simply dictated by the choice of dopant. Instead, both types of tilting order prevailed with roughly the same strength as in the achiral host regardless of which chiral material was added, up to concentrations well beyond normal doping conditions. Thus, at least with hockey-stick-shaped achiral hosts, syn- as well as anticlinic chiral compounds can be used effectively as chiral dopants without necessarily having an important impact on the clinicity of the resulting mixture. The hockey-stick design concept should be useful in producing achiral anticlinic-forming mesogens for low-polarization, long-pitch antiferroelectric liquid crystal mixtures. Finally, we point out that a mixture study like the one carried out here yields a conclusive means of establishing the clinicity of achiral tilted smectics, an endeavour that can sometimes be far from trivial.

Similarity and Chirality Measures for Liquid Crystal

The mechanism for the transfer of chirality between a chiral dopant and an achiral liquid crystal host phase remains the subject of conjecture. A review of the theoretical approaches undertaken to create a better understanding of the concept of chirality in liquid crystals proposed a Hausdorff measure of chirality between two enantiomers. We compare this measure with a scaled chiral index. The latter has been successful in predicting the helical twisting power of a range of dopants in nematic solvents but possesses no solvent interaction. We propose a combined measure as a predictive tool for helical twisting power and the for the mechanism for chirality transfer.

Negative refraction in chiral composite materials

Based on the Maxwell–Garnett approximation, we obtain the negative refraction in chiral composites in which electrically small isotropic chiral inclusions are randomly embedded in an isotropic achiral host medium. Numerical results show that there exists negative refraction in high (or low) frequency region for right- (or left-) circular polarized wave, and the region is widened for large volume fraction and small damping factor. In addition, the magnitude of negative refractive index increases with decreasing the dielectric loss. Moreover, it is numerically shown that the negative refraction results from relatively large chirality parameter in chiral composites compared to the corresponding parameters in the components.

Optimum electromagnetic wave excitations of complex media characterized by positive or negative refractive indices and by chiral properties

The chiral properties of naturally occurring biological or chemical threat agents are usually weak, and the expressions in the constitutive relations associated with charility are relatively small compared with the familiar terms associated with the electric permittivity and magnetic permeability of the medium. Therefore it is necessary to optimize the excitation of the chiral materials in order to detect and identify the threat agents. Moreover chiral materials characterized by positive or negative refractive indices have several novel applications in nanophotonics. For these nanophotonic devices to perform efficiently, it is also necessary to optimize the excitation of these devices. Since the Mueller matrices that relate the scattered to the incident Stokes vectors completely characterize scattered polarized electromagnetic waves, much attention is given to the explicit expressions for the reflection and transmission matrices that also account for the cross-polarized coefficients. To first order, the cross-polarized coefficients are proportional to the chiral parameter. It is therefore important to determine the relationships between the cross-polarized coefficients and the incident angle of excitation, the medium parameters (such as the refractive index and the intrinsic impedance of the achiral host medium), and the wavelength. The optimum polarization and the mode of operation (reflection or transmission) are also important factors that need to be determined.

Chiral Recognition in Cucurbituril Cavities

For the first time, achiral cucurbiturils (CBs) were endowed with significant enantiomeric and distereomeric discrimination by incorporating a strong chiral binder. Calorimetric, nuclear magnetic, light-scattering, and mass spectral studies revealed that (S)-2-methylbutylamine (as a strong binder) can be discriminated by two enantiomeric supramolecular hosts, composed of CB[6] and (R)- or (S)-2-methylpiperazine, with an unprecedented 95% enantioselectivity in aqueous NaCl solution. This is the highest enantioselectivity ever reported for a supramolecular system derived from an achiral host. Similarly, CB[7], with a larger cavity, exhibited diastereoselectivities up to 8 times higher for diastereomeric dipeptides, as demonstrated for L-Phe-L-Leu-NH3+ versus L-Phe-D-Leu-NH3+.

Chiral Spaces in Supramolecular Assemblies

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF.

Supramolecular Chirogenesis with Bis-chlorin versus Bis-porphyrin Hosts: Peculiarities of Chirality Induction and Modulation of Optical Activity

[structure: see text] The complexation behavior, chirality induction and inversion in the achiral host, a racemic mixture of ethane-bridged bis(zinc octaethylchlorin) (1), and optical activity modulation in the chiral hosts, enantiopure 1(R) and 1(S), upon interaction with chiral and achiral amine guests have been investigated by means of the UV-vis, CD, and (1)H NMR techniques and compared with the corresponding spectral data of the bis-porphyrin analogue. It was found that the chirogenesis pathway is strongly dependent upon the structures of both major components (hosts and guests) of these supramolecular systems. Particularly, the distinct orientation of electronic transitions in the chlorin chromophores arisen from the reduced pyrrole ring, which makes it radically different from that of the porphyrin chromophores, and the size of the guest's substituents lead to the remarkable phenomenon of chirality induction-inversion in racemic 1 originating from the process of asymmetry transfer from enantiopure guests of the same homologous type and absolute configuration. This surprising chirogenic behavior is found to be in a sharp contrast to that observed in the analogous porphyrin-based systems. Furthermore, these structural and electronic phenomena also account for the effective optical activity quenching of enantiopure 1(R) and 1(S) upon interaction with chiral and with achiral amines, which results in formation of supramolecular complexes of opposite chirality.

Rationalization of supramolecular chirality in a bisporphyrin system.

A sign of the times: A 1:1 tweezer complex consisting of an achiral bis(zinc porphyrin) and (R,R)-1,2-diaminocyclohexane has been characterized by X-ray crystallographic analysis (see picture, Zn red, N blue). This has enabled a full and unambiguous rationalization of the highly efficient transfer of chirality information from an optically active guest to an achiral host in a supramolecular system based on a bisporphyrin to be carried out.

Supramolecular chirogenesis in zinc porphyrins: interaction with bidentate ligands, formation of tweezer structures, and the origin of enhanced optical activity.

The complexation behavior, binding properties, and spectral parameters of supramolecular chirality induction in the achiral host molecule, syn (face-to-face conformation) ethane-bridged bis(zinc porphyrin), upon interaction with chiral bidentate guests (diamines and amino alcohols) have been studied by means of UV-vis, CD, fluorescence, (1)H NMR, and ESI MS techniques. It was found that the guest structure plays a decisive role in the chirogenesis pathway. The majority of bidentate ligands (except those geometrically unsuitable) exhibit two major equilibria steps: the first guest ligation leading to formation of the 1:1 host-guest tweezer structure (K(1)) and the second guest molecule ligation (K(2)) forming the anti bis-ligated species (1:2). The second ligation is much weaker (K(1) >> K(2)) due to the optimal geometry and stability of the 1:1 tweezer complex. The enhanced conformational stability of the tweezer complex ensures an efficient chirality transfer from the chiral guest to the achiral host, consequently inducing a remarkably high optical activity in the bis-porphyrin.

Synthesis and Binding Properties of Chiral Macrocyclic Barbiturate Receptors: Application to Nitrile Oxide Cyclizations.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Synthesis, Structure and Chiral Inclusion Crystallisation of Tetrakis(4-Ethynylphenyl)Ethylene Derivatives

Achiral host molecules, tetrakis(4-ethynylphenyl)ethylene 3 and tetrakis(4-bromoethynylphenyl)ethylene 4, formed chiral host–guest inclusion crystals with achiral guest molecules via weak intermolecular interactions.

Phase-Sensitive Supramolecular Chirogenesis in Bisporphyrin Systems

To date, the field of supramolecular chirality has attracted much attention in the scientific community, in the context of its direct connection with various natural systems and many practical applications in the areas of catalysis,[1] nonlinear optics,[2] polymer and materials science,[3] molecular recognition,[4] molecular devices,[5] and absolute configuration determination.[6] As noncovalent interactions are the key elements of supramolecular chemistry, there are various external and internal factors controlling chirality induction in these systems. Among these influences, the phase transition, or aggregation state, takes on particular significance in the light of the wide prospective applications of optically active systems in the solid state.[7] However, this important factor has not yet been well investigated and rationalized. Here, by applying supramolecular chemistry, solid-state optical spectroscopy, and exciton chirality principles, we describe a novel and intriguing phenomenon of phasetransition controlled chirality induction in a supramolecular system, which consists of an achiral host and a chiral guest.[8] Ethane-bridged bis(zinc octaethylporphyrin),[9] which is in a syn conformation in nonpolar solvents (see syn ZnD in Figure 1), was chosen as an achiral host molecule. This

Chiral inclusion crystals constructed with achiral host and achiral guest molecules.

Achiral molecules of tetrakis(p-bromophenyl)ethylene and 2,3,6,7,10,11-hexahydroxy triphenylene were found to arrange in a chiral form in their inclusion crystal with an achiral guest molecule. The chiral arrangement of these achiral molecules was studied by X-ray analysis and CD spectral measurement in the solid state.

Supramolecular chirogenesis in zinc porphyrins: equilibria, binding properties, and thermodynamics.

Complexation mechanism, binding properties and thermodynamic parameters of supramolecular chirality induction in the achiral host molecule, syn (face-to-face conformation) ethane-bridged bis(zinc porphyrin), upon interaction with chiral monoamine and monoalcohol guests have been studied by means of the UV-vis, CD, (1)H NMR, and ESI MS techniques. It was found that the chirogenesis process includes three major equilibria steps: the first guest ligation to a zinc porphyrin subunit of the host (K(1)), syn to anti conformational switching (K(S)), and further ligation by a second guest molecule to the remaining ligand-free zinc porphyrin subunit (K(2)), thus forming the final bis-ligated species possessing supramolecular chirality. The validity of this equilibria model is confirmed by the excellent match between the calculated and experimentally observed spectral parameters of the bis-ligated species. The second ligation proceeds in a cooperative manner as K(2) > K(1) for all supramolecular systems studied, regardless of the structure of the chiral ligand used. The binding properties are highly dependent on the nature of the functional group (amines are stronger binders than alcohols) and on the structure of the chiral guests (primary and aliphatic amines have overall binding constant values greater than those of secondary and aromatic amines, respectively).

Structures and Properties of Induced Lyotropic Cholesteric Phases

Phase chirality in disk-like lyotropic cholesteric phases (ChD) was investigated, which were induced by center and axial chiral dopants to achiral lyotropic nematic host phases (ND). In a lyotropic nematic matrix of the ND phase in the ternary system hexadecyldimethylethylammoniumbromid (C16Me2EABr)/water/n-decanol, a disk-like lyotropic cholesteric phase ChD was induced by addition of the axial optically active compound R(−)-1,1′-binaphthalene-2,2′-diyl-hydrogenphosphate (BDP). The HTP value of the BDP is generally lower than the HTP value of inducing substances with center chirality as cholesterol, prednisolon etc. At constant composition of the ND phase, the helix lengths were determined in dependence on the BDP concentration by means of evaluation of the “spaghetti-like texture” using polarizing microscopy. The reciprocal helix-lengths are changing linearly with rising BDP concentration. Furthermore selected results of X-ray scattering and diffraction on the structure formation in the ChD phase were discussed.

The Development of Materials, Mixtures and Gels for Ferroelectric Displays

Ferroelectric liquid crystals have developed from the early single component chiral systems, that brought a wealth of fundamental knowledge, to multi-component achiral host systems with optimised physical properties that have a small quantity of chiral dopant added to provide the required ferroelectricity. The research into ferroelectric liquid crystals has been intense, but the technology has not yet proved commercially viable. This paper discusses the developments in ferroelectric materials and focuses on the ferroelectric mixtures generated from ortho-difluoroterphenyl host materials and the chiral cyanohydrin dopants which have a high dielectric biaxiality and are suitable for use in τVmin devices. Such ferroelectric mixtures have been incorporated into structurally matched diacrylates and polymerised to form a gel network, and the interesting results of these systems are discussed in comparison with the conventional ferroelectric mixtures.