Host-guest Structures Research Articles

A Versatile Fréchet‐Dendron Compound Unifies Host‐Guest and Templated Heterogeneous Self‐Assembly

To date, heterogeneous patterns on surfaces could assemble either in host guest or templated structures only. A Frechet-type dendron now shows both classes of ordering and thus connects both assembly types. Depending on the type of interaction with the guest molecule, saturated adamantane is included in an existing host structure, while aromatic coronene templates a new molecular ordering.

Isolation of Chlorogenic Acid from Flaveria bidentis (L.) Kuntze by CCC and Synthesis of Chlorogenic Acid-Intercalated Layered Double Hydroxide

Preparative counter-current chromatography (CCC) was successfully used for isolation and purification of chlorogenic acid from Flaveria bidentis (L.) Kuntze with a solvent system composed of ethyl acetate–methanol–water at a volume ratio of 50:1:50, v/v. Using a preparative unit of the CCC centrifuge, about 800 mg of the crude extract was separated, yielding 3.2 mg of chlorogenic acid at a purity of 92.0%. The blood pressure lowering and antivirus chlorogenic acid (C16H18O9) was intercalated into magnesium–aluminum–layered double hydroxides, which was used as host materials for drug-LDH host-guest supermolecular structures by anion exchange under a nitrogen atmosphere. Chlorogenic acid–LDH is a functional and effective drug. The product chlorogenic acid–LDH has been characterized by powder X-ray diffraction (XRD), Fourier transform infrared (FT-IR), thermogravimetric analysis (TGA) and scanning electron micrographs (SEM). The X-ray diffraction patterns of NO3 − form of LDH and chlorogenic acid–LDH were compared, and the basal d spacing value of NO3 −-LDH layer was 8.75 A (2θ = 10.100°); however, the basal reflection (003) of chlorogenic acid–LDH shifts to lower 2θ (for 003 reflection: 2θ = 5.119°) that is expanded to 17.25 A, indicating the intercalation of chlorogenic acid into the interlayer of Mg–Al-LDH. Thermogravimetric analysis showed that chlorogenic acid stability had improved, and scanning electron micrographs showed that the morphology of the chlorogenic acid–LDH was irregular masses of distinctly thicker flakes, which was similar to the morphology of NO3 − form of LDH.

Structural study of liposomes loaded with a GM3 lactone analogue for the targeting of tumor epitopes

Therapeutic vaccination with tumor antigens is a new approach in cancer treatment, which aims at inducing immune response while avoiding the side effects generally associated to many conventional therapies. To improve the efficacy of vaccines, suitable carriers may be used. Herein the insertion of a thioether analogue of GM3 lactone (SNeuAC-C14) into liposomes is reported. SNeuAC-C14 is a potential vaccine for the targeting of saccharide-based tumor epitopes. Different liposome formulations were designed to act as carriers and to generate recognition by tumor epitopes. The structural study of pure and loaded liposomes was carried out by synchrotron Small Angle X-ray Scattering and was complemented by Dynamic Light Scattering and Zeta potential measurements. This provided detailed information on relevant properties of the investigated host-guest structures and showed that the active unit of SNeuAC-C14, i.e. its spiro tricyclic moiety, was located in the polar head region of the liposome bilayer, which is an important requirement for recognition phenomena. Moreover, it was found that most of the SNeuAC-C14/liposome complexes were positively charged. The obtained results allow these systems to be considered as candidates to promote immunoresponse in tumor cells.

Molecular Absorptive Behavior of Liquid Crystal Molecular Templates

A new discotic liquid crystal template has been found and the host-guest architecture structures on highly oriented pyrolytic graphite (HOPG) surface have been identified with scanning tunneling microscope (STM). This discotic liquid crystal molecule can function as a new molecular template for distribution and dispersion of organic molecules. The present molecular template may provide a facile way to fabricate new functional materials.

Mimicking the antenna system of green plants

We report the preparation and investigation of hierarchically organized host-guest structures, presenting successive ordering from the molecular up to macroscopic scale, thus supporting the relationship between the molecular arrangements and the macroscopic properties. Size, shape and surface composition of the host which is zeolite L play a decisive role. Its base and coat have distinctively different chemical properties. The guests, organic dye molecules or complexes, are well oriented inside the channels and can be organized into distinctive patterns. Zeolite L crystals containing oriented fluorophores in their parallel nanochannels possess remarkable fluorescent properties and they can be arranged in nearly any desired manner by means of self-organization methods. This makes them ideal host-guest structures for the exploitation of energy transfer and energy funneling processes. Size, shape and surface composition of the objects but also the properties of the surface on which they should be organized play a decisive role. We present a simple model of an artificial antenna based on supramolecular organization of dyes in nanochannels of the host, and we explain why zeolite L can be considered as an ideal host for this purpose. The preparation of different dye-zeolite L materials is described, and Förster energy transfer experiments carried out with them. Further, increasing supramolecular organization is discussed: the first unidirectional antenna system on a macroscopic level, organization of crystals and communication of the crystals interior with the environment. Additionally, we explain spectroscopy on monolayers of dye-zeolite L micro-crystals. The materials are shown to be new building blocks for optical, electro-optical and sensing devices.

Molecular Clips with Extended Aromatic Sidewalls as Receptors for Electron-Acceptor Molecules. Synthesis and NMR, Photophysical, and Electrochemical Properties

We have synthesized molecular clips 1 comprising (i) two benzo[k]fluoranthene sidewalls and (ii) a dimethylene-connected benzene bridge that carries two acetoxy (1a), hydroxy (1b), or methoxy (1c) substituents in the para position. Their NMR spectra, single-crystal structures, and photophysical (fluorescence intensity, lifetime, depolarization) and electrochemical properties are discussed. For the purpose of comparison, similar compounds (2 and 3) containing only one benzo[k]fluoranthene unit have been prepared and studied. The strongly fluorescent clips 1 form stable complexes with electron-acceptor guests because of a highly negative electrostatic potential on the inner van der Waals surface of their cavity. The complexation constants in chloroform solution for a variety of guests, determined by NMR and fluorescence titration, are much larger than those of the corresponding anthracene and naphthalene clips (4 and 5), particularly in the case of extended aromatic guests. The effect of the substituents in the para position of the benzene spacer unit of clips 1 is discussed on the basis of the host-guest complex structures obtained by X-ray analysis and molecular mechanics simulations. In the case of 9-dicyanomethylene-2,4,7-trinitrofluorene (TNF) guest, complex formation with clip 1a causes dramatic changes in the photophysical and electrochemical properties: (i) a new charge-transfer band at 600 nm arises, (ii) a very efficient quenching of the strong benzo[k]fluoranthene fluorescence takes place, (iii) shifts of both the first oxidation (clip-centered) and reduction (TNF-centered) potentials are observed, and (iv) reversible disassembling of the complex can be obtained by electrochemical stimulation.

Displacive mechanisms and order-parameter symmetries for the A7-incommensurate-bcc sequences of high-pressure reconstructive phase transitions in Group Va elements

The high-pressure structural transitions observed in the Group Va elements P, As, Sb, and Bi are described by displacive atomic mechanisms. A theoretical analysis of the proposed mechanisms shows that two different structural paths are followed relating the low-pressure A7 structure to the highest-pressure bcc structure. One structural path gives rise to the host-guest structures found in As, Sb, and Bi, whereas another path yields the incommensurate P IV structure recently disclosed in P. The incommensurate character of the host-guest and P IV structures is shown to be imposed by the energetically most favorable path relating the A7 and bcc structures.

Competition between Burgers mechanism and Bain deformation in alkaline-earth metals: Host-guest structures of barium and strontium

The structural mechanisms underlying the high-pressure phase transitions in alkaline-earth metals are described and analyzed theoretically in the framework of an extension of the Landau approach to reconstructive phase transitions. All the high-pressure structures are interpreted as resulting from the full or partial realization of the Burgers and Bain deformation mechanisms. The host-guest structures found in strontium and barium are shown to reflect the competition between the two preceding mechanisms, which expresses at the structural level the competition occurring at the electronic level between the bonding changes induced on pressurization. Further experimental measurements and calculations are suggested for testing our theoretical analysis of the transition mechanisms.

High-pressure Raman spectroscopy of antimony: As-type, incommensurate host-guest, and bcc phases

The vibrational dynamics of elemental solids that form incommensurate host–guest structures are of fundamental interest. High-pressure Raman scattering has been used to examine the vibrational spectrum of the group-V element Sb up to 33 GPa. A 1 g and E g phonons of the ambient pressure rhombohedral A7 phase display a marked decrease with pressure, i.e., prior to the transition to the tetragonal host–guest Sb-II phase at 8.6 GPa, via the monoclinic host-guest Sb-IV phase. The Raman spectrum of the incommensurate host-guest Sb-II phase, has five bands between 80 cm −1 and 200 cm −1 that increase with pressure. For the bcc structure stable above 28 GPa, we observe one weak disorder-induced band that increases with pressure.

Ordered inclusion of endohedral fullerenes N@C 60 and P@C 60 in a crystalline matrix

The organic inclusion compound 2,4,6-tris-(4-bromophenoxy)-1,3,5-triazine was used to create an ordered arrangement of the endohedral fullerenes N@C 60 and P@C 60 in a crystalline host matrix. Two different host–guest structures were investigated by X-ray diffraction and electron paramagnetic resonance. In one of these, an axial deformation of the fullerene guests leads to a sizeable zero-field splitting of the quartet ground state of the encapsulated atoms. The small size of the zero-field strain leads to well-separated lines for all transitions, opening the possibility of transition selective pulses for future quantum computing experiments in these multi-level systems.

Silicon Analogues of Triarylmethanol Hosts. Inclusion Properties and Host–guest Structures: A Comparative Study

The simple triarylmethanol hosts, 2 and 4, and their silicon analogues, 1 and 3, have been studied for comparison of the formation of crystalline inclusion compounds. Clathrate formation experiments showed that replacement of the carbinol C atoms in 2 and 4 by Si atoms to give 1 and 3 resulted in a distinct increase of the capability to form inclusion compounds with organic guests, in particular with alcohols. Moreover, the naphthyl derivatives are much more efficient than the phenyl species, irrespective of the carbinol and silanol features. In order to investigate and compare the guest recognition modes and packing relations of hosts 1–4 in their crystalline inclusion compounds, 11 selected co-crystals, namely 1·DMSO (2:1), 3·EtOH (1:1), 3·i-PrOH (1:1), 3·acetone (1:1), 3·DMSO (1:1), 3·THF (1:1), 3·piperidine (1:1), 4·acetone (1:1), 4·DMSO (1:1), 4·1,4-dioxane (1:1) and 4·benzene (1:1), were studied by X-ray diffraction from single crystals. The survey contains additional 11 crystal structures from the literature and provides a detailed discussion of isostructurality relationships.

Organic Host—Guest Structures in the Solid State

AbstractFor Abstract see ChemInform Abstract in Full Text.

Combined diffraction and Raman studies on incommensurately modulated host-guest structures of elements

Combined diffraction and Raman studies on incommensurately modulated host-guest structures of elements

Novel chain structures in group VI elements

Recent developments in high-pressure methods and advances in X-ray crystallography have led to a new level of understanding of phase diagrams and structures of materials under pressure. Recently discovered phenomena such as complex phases of alkali metals, incommensurate host-guest structures, and incommensurately modulated structures have rendered obsolete our conventional wisdom about the range of structures possible in the elements. Using new in situ diffraction techniques, we have resolved the long-standing problem of the phase-transition sequence of sulphur in its non-metallic state. We demonstrate that it is very different from that previously proposed, with only two phases stable between 1.5 GPa and 83 GPa (the pressure of metallization), and temperatures from 300 K to 1,100 K. The phases have a triangular chain and a squared chain structure. The same squared chain structure is found in the heavier group VI element selenium.

Fine-tuning the weak-link approach: effect of ligand electron density on the formation of rhodium(I) and iridium(I) metallomacrocycles.

A novel bis(phosphinoalkyl-thioether)arene ligand with a fluorinated aryl group (1,4-(Ph(2)PCH(2)CH(2)S)(2)C(6)F(4)) has been synthesized. This ligand has been used to prepare symmetric bimetallic structures with Rh(I) and Ir(I) metal centers in high yield. Unlike their nonfluorinated counterparts, these complexes can be opened into large macrocyclic structures through straightforward ligand (i.e., carbon monoxide, nitriles, and isocyanides) substitution reactions at the metal-thioether linkage. In addition, the symmetric bimetallic structures have been shown to react with appropriately sized bifunctional aromatic molecules to form three-tiered host-guest structures.

High-pressure structural trends of Group 15 elements: simple packed structures versus complex host-guest arrangements.

The Group 15 elements P, As, Sb, and Bi all have layered structures consisting of six-membered rings under ambient conditions and attain the body-centered cubic (bcc) structure at the highest pressures applied. In the intermediate pressure region, however, phosphorus and its heavier congeners behave profoundly differently. In this region P first attains the open packed simple cubic (sc) structure for a wide range of pressures and then transforms into the rarely observed simple hexagonal (sh) structure. For the heavier congeners complex, incommensurately modulated host-guest structures emerge as intermediate pressure structures. We investigated the high-pressure behavior of P and As by ab initio density functional calculations in which pseudopotentials and a plane wave basis set were employed. The incommensurately modulated high-pressure structure of As was approximated by a supercell. Our calculations reproduced the experimentally established pressure stability ranges of the sc and sh structures for P and the host-guest structure for As very well. We found that the sc and especially the sh structure are decisively stabilized by the admixture of d states in the occupied levels of the electronic structure. This admixture releases s-s antibonding states above the Fermi level (s-d mixing). With pressure, s-d mixing increases rapidly for P, whereas it remains at a low level for As. As a consequence, the band energy contribution to the total energy determines the structural stability for P in the intermediate pressure region, giving rise to simple packed structures. On the other hand, in the intermediate pressure region of the heavier Group 15 elements, a delicate interplay between the electrostatic Madelung energy and the band energy leads to the formation of complex structures.

Some errors from the crystallographic literature, some amplifications and a questionable result.

The space groups of [[Mo(2)(O(2)CCH(3))(4)('linker')](n)] are corrected from P1 to C2/m for 'linker' = pyrazine and 1,4-diazabicyclo[2.2.2]octane (dabco) and from P1 to C2/c for 'linker' = 4,4'-bipyridine. Also, [[tris-(2-pyridylmethyl)amine]BrV(mu-O)VBr[tris-(2-pyridylmethyl)amine]]Br.H(2)O is corrected from P1 to C2/c. These Laue class changes allow more reliable crystallochemical comparisons to be made among families of related structures. Space groups are corrected for 4-methyl-2,6-bis(4-methylbenzylidene)cyclohexanone, 2,6-bis(4-dimethylaminobenzylidene)cyclohexanone, K[Cr(tetramethylenediamine-N,N,N',N'-tetraacetate)].H(2)O and [bis(11,-11-dimethyl-3,4:8,9-dibenzobicyclo[4.4.1]undeca-3,8-diene)-(tetracyanoethylene)]. The conflicting reports for Cu(H(2)O)-(phenanthroline)(2)(X)(2), where X = ClO(4), NO(3) and BF(4), are resolved. Three related examples of open framework host-guest structures with space groups 'Cc or C2/c' are discussed. Adding centers to 2,2'-bi-1H-imidazolium dipicrate and [tris(2,2'-bi-1H-imidazole)bis[2-(2-1H-imidazolyl)-1H-imidazolium]bis(iodide)] corrects discrepancies of up to 0.38 A between chemically similar hydrogen-bond distances. Wrongly identified atoms are corrected in theta-[bis(ethylenedithio)tetrathiafulvalene](2) (CsCd)(SCN)(4) and (purported) diaquadihydroxotetrakis(m-nitrobenzenesulfonate)discandium(III). The reported difference between the crystal structure of (CH(3)NH(3))(4)YbCl(7) and those of the other members of this family of (CH(3)NH(3))(4)MX(7) (M = In, Fe, V; X = Cl, Br) structures is pointed out in the context of possibly different N-H...Cl hydrogen bonding in the Yb structure.

New artificial host compounds containing galactose end groups for binding chiral organic amine guests: chiral discrimination and their complex structures.

New linear host (1) and cyclic hosts (2 and 3), which have galactopyranose skeletons as chiral origins and oxyethylenes skeletons as binding sites, were designed based on the structural features extracted from the fructo-oligosaccharide derivatives, having a large chiral discrimination ability, and were then synthesized. These hosts showed chiral discrimination toward chiral organic ammonium salts. For example, the chiral discrimination ability (the ratio of association constants: K(R)/K(S)) of host 1, which has the highest value among them, was K(R)/K(S) = 3 for Trp-O-(i)Pr(+) and K(R)/K(S) = 0.7 for 1-(1-naphthyl)ethylammonium (NEA(+)) at 298 K in CHCl(3). It was clarified that host 1 changed the conformation from a linear structure to the pseudo-ring structure by complexation with cations such as alkali metallic ions and chiral organic ammonium ions. The (1)H NMR induced shifts of host 1 by adding the NEA(+) guests showed that the host-guest complex structures are clearly different, depending upon the chirality of the guest; in the complex with (R)-NEA(+), the naphthyl group of the guest is located above the oxyethylene skeleton of the host and in the complex with (S)-NEA(+), and the naphthyl group is located between the edges of the pseudo-ring of the host. The clearly different structure of the complex of host 1 with NEA(+) may be caused by the dynamic molecular recognition, thus the induced-fitting mechanism.

Intercalation of Bismuth Chalcogenides with Molecular Fluorine

The intercalation of molecular fluorine into Bi2X3(X = S, Se, Te) was studied by x-ray diffraction, mass spectrometry, and thermogravimetry. The results show that, depending on fluorination conditions, the process yields either chalcogenide intercalation compounds, or substitutional intercalation compounds, or complex host–guest structures. The diffusion coefficient of Li in the intercalation compounds and their physical properties are shown to be strongly correlated with fluorine content.

Useful Techniques in Host-Guest Chemistry

While crystal structure analysis remains the most powerful analytical tool for the elucidation of host-guest structures, a greater understanding of the behaviour of these compounds is achieved by studying their thermal stabilities and kinetics of formation and desolvation. Various analytical techniques for the study of inclusion compounds are outlined and evaluated.