Strong Host-guest Interactions Research Articles

Gating the photochromism of an azobenzene by strong host-guest interactions in a divalent pseudo[2]rotaxane.

The ability of an E-configured azobenzene guest to undergo photoisomerisation is controlled by the presence of a complementary host. Addition of base/acid allowed for a weakening/strengthening of the interactions in the divalent pseudo[2]rotaxane complex and hence could switch on/off photochromic activity.

Interstitial modification of palladium nanoparticles with boron atoms as a green catalyst for selective hydrogenation.

Lindlar catalysts comprising of palladium/calcium carbonate modified with lead acetate and quinoline are widely employed industrially for the partial hydrogenation of alkynes. However, their use is restricted, particularly for food, cosmetic and drug manufacture, due to the extremely toxic nature of lead, and the risk of its leaching from catalyst surface. In addition, the catalysts also exhibit poor selectivities in a number of cases. Here we report that a non-surface modification of palladium gives rise to the formation of an ultra-selective nanocatalyst. Boron atoms are found to take residence in palladium interstitial lattice sites with good chemical and thermal stability. This is favoured due to a strong host-guest electronic interaction when supported palladium nanoparticles are treated with a borane tetrahydrofuran solution. The adsorptive properties of palladium are modified by the subsurface boron atoms and display ultra-selectivity in a number of challenging alkyne hydrogenation reactions, which outclass the performance of Lindlar catalysts.

Novel morphology-controlled hierarchical core@shell structural organo-layered double hydroxides magnetic nanovehicles for drug release.

Novel hierarchical core@shell structured salicylate (SA) intercalated ZnAl-LDH (layered double hydroxides) magnetic nanovehicles were obtained via a special double-drop coprecipitation strategy assembling organo-ZnAl-LDH nanocrystals onto the surface of Fe3O4 submicrospheres (∼480 nm) from cheap aspirin and Zn- and Al-nitrates in alkaline solutions. The obtained Fe3O4@SA-LDH-r nanovehicles exhibit varied morphologies with hexagonal LDH ab-face horizontal, vertical, and vertical/slant/horizontal to the surfaces of Fe3O4 upon proper mass ratio (r) of Zn-salt to Fe3O4 from 1.93 to 7.71 in a low supersaturation system and possess moderate drug loadings and strong superparamagnetism. An in vitro release study reveals that under "no MF" mode (without external magnetic field) the SA release exhibits the higher accumulated release amount and smaller half-life (t0.5) for Fe3O4@SA-LDH-3.85 (41.2%, 1.63 min) and Fe3O4@SA-LDH-7.71 (51.1%, 1.66 min) probably owing to their mainly vertical LDH orientations, while the dramatically reduced SA release (10.0%) and greatly elongated t0.5 (25.6 min) for Fe3O4@SA-LDH-1.93 may be due to its relatively stronger host-guest interaction and compact horizontally oriented LDH shell stack. Under "MF on" mode, all the magnetic samples show a detectable reduced SA release owing to the particle-particle interactions among the magnetic nanovehicles. The kinetic fittings show that the release processes of all the samples involve the bulk and surface diffusion. The SA release from Fe3O4@SA-LDH-1.93 is mainly determined by the interparticle diffusion among the horizontally oriented LDH shell nanocrystals while those of Fe3O4@SA-LDH-3.85 and Fe3O4@SA-LDH-7.71 mainly involve the interlayer intraparticle diffusion between LDHs layers due to their largely vertical LDH shell nanocrystals.

Synthesis of Doubly Ethyl‐Bridged Bis(p‐sulfonatocalix[4]arene) and Its Supramolecular Polymerization with Viologen Dimer

A water-soluble supramolecular polymer with a high degree of polymerization and viscosity has been constructed based on the strong host-guest interaction between p-sulfonatocalix[4]arenes (SC4As) and viologen. A homoditopic doubly ethyl-bridged bis(p-sulfonatocalix[4]arene) (d-SC4A) was prepared and its binding behavior towards methyl viologen compared with the singly ethyl-bridged bis(p-sulfonatocalix[4]arene) (s-SC4A) by NMR spectroscopy and isothermal titration calorimetry. By employing a viologen dimer (bisMV(4+)) as the homoditopic guest, two linear AA/BB-type supramolecular polymers, d-SC4A⊃bisMV(4+) and s-SC4A⊃bisMV(4+), were successfully constructed. Compared with s-SC4A⊃bisMV(4+), d-SC4A⊃bisMV(4+) shows much higher solubility and viscosity, and has also been characterized by viscosity, diffusion-ordered NMR spectroscopy, dynamic light scattering, and atomic force microscopy measurements. Furthermore, the polymer is responsive to electrostimulus as viologen is electroactive, which was studied by cyclic voltammetry. This study represents a proof-of-principle as the polymer can potentially be applied as a self-healing and degradable polymeric material.

Amphiphilic Inclusion Spaces for Various Guests and Regulation of Fluorescence Intensity of 1,8‐Bis(4‐aminophenyl)anthracene Crystals

A host framework for inclusion of various guest molecules was investigated by preparation of inclusion crystals of 1,8-bis(4-aminophenyl)anthracene (1,8-BAPA) with organic solvents. X-ray crystallographic analysis revealed construction of the same inclusion space incorporating 1,8-BAPA and eight guest molecules including both non-polar (benzene) and polar guests (N,N-dimethylformamide, DMF). Fluorescence efficiencies varied depending on guest molecule polarity; DMF inclusion crystals exhibited the highest fluorescence intensity (ΦF=0.40), four times as high as that of a benzene inclusion crystal (ΦF=0.10). According to systematic investigations of inclusion phenomena, strong host–guest interactions and filling of the inclusion space led to a high fluorescence intensity. Temperature-dependent fluorescence spectral measurements revealed these factors effectively immobilised the host framework. Although hydrogen bonding commonly decreases fluorescence intensity, the present study demonstrated that such strong interactions provide excellent conditions for fluorescence enhancement. Thus, this remarkable behaviour has potential application toward sensing of highly polar molecules, such as biogenic compounds.

Computational study of diffusion of propane in small pore acidic zeotypes AFX and AEI

Small pore zeolites, containing 8-rings as the largest, are widely employed in commercial processes such as methanol-to-olefins or methanol-to-hydrocarbons. Diffusion in these materials has mainly been studied in pure silica materials. In this study acidic versions of two zeolites (AFX and AEI) are considered under different chemical compositions, loading, and temperature. Propane molecule, one of the products in these processes, has a size very similar to those of zeolitic 8-rings and hence little variations in their dynamic size can affect diffusion coefficients by several orders of magnitude. The dynamics of intercavity motions, involving an 8-ring crossing, are thoroughly investigated with the help of atomistic molecular dynamics. Detailed analysis of trajectories, motions, orientations, conformations, 8-ring sizes and shapes, and energetic profiles including activation energies. The results show a preferential diffusion in SAPOs over zeolites, an influence of the diffusivity with the propane loading depending on the structure, and the need of high temperature to overcome the activation barriers defined by 8-ring crossing events. SAPO-18 (AEI) shows a certainly improved propane diffusivity due to the slightly larger size of its 8-rings compared to those of SAPO-56 (AFX). Framework deformations have also been found as a result of strong host–guest interactions in the AEI zeotype, especially in the case of the SAPO-18. The implications on diffusivity are analysed in detail depending on the loading and temperature.

Supramolecular Photosensitizers with Enhanced Antibacterial Efficiency

Photodynamic therapy: A novel supramolecular photosensitizer is efficiently fabricated based on a strong host–guest interaction, which shows an enhanced singlet oxygen generation ability of the porphyrin units and has thus a greatly improved antibacterial efficiency (see picture). The supramolecular photosensitizer is an adaptive system with switchable photophysical properties.

Regulation of the Collisional Self-Quenching of Fluorescence in Clay/Porphyrin Complex by Strong Host–Guest Interaction

This paper proposes a novel methodology to improve noncollisional photochemical reactions such as Förster resonance energy transfer on solid surfaces. Since an excited guest molecule densely adsorbed on the solid surfaces is quenched by an unfavorable interaction between guests in general, the photochemical reactions such as electron and energy transfers tend to be inefficient compared to those in homogeneous systems. In this work, the mechanism of unfavorable quenching process of dyes on the clay surface as a typical solid surface for the photochemical energy transfer was systematically investigated by using a series of porphyrin derivatives. As a result, it was found that the quenching rate constants of excited guest dye determined by the time-resolved fluorescence measurements correlated well with the strengths of coulombic interaction between host and guest. The strong coulombic interaction should suppress the mobility and collision frequency of guests on the clay surface; thus, the collision of guest molecules was revealed as the origin of unfavorable quenching for photochemical reactions on the clay surface. According to this principle, we will be able to construct efficient photochemical reaction systems without any quenching process, such as efficient energy transfers toward an artificial light-harvesting system. In fact, we have already realized almost 100% energy transfer by the suppression of quenching process on the clay surface (e.g., J. Am. Chem. Soc.2011, 133, 14280-14286).

The Mechanism of the Porphyrin Spectral Shift on Inorganic Nanosheets: The Molecular Flattening Induced by the Strong Host–Guest Interaction due to the “Size-Matching Rule”

The mechanism inducing the unique absorption spectral shifts of porphyrin molecules upon adsorption on the clay surface was experimentally confirmed to be the flattening of the meso substituent with respect to the plane of the porphyrin ring. We investigated the spectral shift systematically by using seven types of porphyrin derivatives, differing in their center metal, meso substituent, and number of cationic sites. The aggregation, which usually induces the spectral shift, is suppressed in our clay/porphyrin systems. The adsorption strengths of porphyrin molecules on the clay surface were estimated as the relative adsorption equilibrium constants Krel. The seven types of porphyrins studied had different values of Krel, which can be explained by the steric effect and the Coulomb interaction due to the differences in the molecular structure. The obtained Krel values have interesting information of the photochemical property of the clay/porphyrin complexes. The absorption spectral shifts of the porphyrin Soret bands between those in the bulk solution and those on the clay surface are well related to Krel values. Like this, by the systematic experiments using the series of porphyrin derivatives, it was directly confirmed that the flattening of the molecule is the dominant mechanism for the spectral shift on the surface of inorganic nanosheets.

Two ligand-functionalized Pb(ii) metal–organic frameworks: structures and catalytic performances

A microporous Pb(II) metal-organic framework (MOF) [PbL(2)]·2DMF·6H(2)O (1) has been assembled from a N-oxide and amide doubly functionalized ligand HL (= N-(4-carboxyphenyl)isonicotinamide 1-oxide). Complex 1 features a three-dimensional (3D) framework possessing one-dimensional (1D) rhombic channels with dimensions of 13 × 13 Å(2). The 3D framework is built up from 1D PbO(2) chains that link ligands in parallel fashion to construct single-wall channels. When recrystallizing 1 in a DMSO-DMF mixture (3 : 5 v/v), a new coordination polymer, [PbL(2)]·DMF·2H(2)O (2), was obtained. Complex 2 is also a 3D framework containing 1D rectangular channels, but the channel dimensions become reduced in size to 13 × 8 Å(2) due to reorganization of the Pb(ii) coordination environment. The PbO(2) chains in 2 are reformed to link ligands in a double-wall fashion, significantly reducing the channel size. Even though, the guest exchange study indicates that the DMF molecules in 2 could be replaced with benzene molecules when immersing in benzene solvent, showing single-crystal-to-single-crystal (SC-SC) guest exchange in the solid state and leading to a daughter crystal [PbL(2)]·0.5C(6)H(6)·2H(2)O (2'). Desolvated 1 and 2 display preferential adsorption behaviors of water vapour over CO(2) due to the hydrophilic nature of the channels and the strong host-guest interactions. Catalytic tests indicate that desolvated 1 and 2 have size-selective catalytic activity towards the Knoevenagel condensation reaction.

Erstmalige Haltung eines Halsbandmakis (Eulemur collaris) in einem deutschen Zoo

The synthesis of MCM-41 analogue mesoporous metallosilicates with high specific surfaces, containing the redox active metals Ti, V, Cr, Mn, Fe, or Co, from an aqueous gel, under reflux is described. Their X-ray diffractograms reveal a highly ordered array of the mesopores. A strong host-guest interaction between the incorporated template molecules and the pore walls, observed by 13C n.m.r., indicates the incorporation of the metals, which is further evidenced by u.v.-vis and 29Si n.m.r. spectroscopy. These mesoporous metallosilicates are active as catalysts for the liquid phase oxidation of cyclohexane with aqueous H2O2 or tert-butyl hydroperoxide.

Fabrication and Photoluminescent Properties of ZnO/Mesoporous Silica Composites Templated by a Chelating Surfactant

A novel anionic surfactant-templated synthesis of ZnO/mesoporous silica nanocomposites has been carried out by using N-hexadecylethylenediamine triacetate (HED3A), a triprotic surfactant, as the structure-directing agent. The chelating template can capture zinc ions in solution and then direct the mesophase formation, enabling an amount of zinc oxide to be embedded in the porous silica matrix during calcination. With variation of the molar ratio of Zn(2+) to HED3A in the template, a series of composites with different doping amounts were obtained after the removal of organic components. The variation of the zinc ion concentration in the initial template solution induces an evolution of the silica mesophase, presumably due to the change in electronegativity of the HED3A headgroup caused by the chelating effect. Spectroscopic studies show a strong host-guest interaction between the silica pore walls and ultrafine ZnO nanoparticles. The photoluminescence properties of the resulting composites exhibit a size-dependent light emission and quantum-confinement effect of ZnO, accompanied by an infrequent violet emission originating from the ZnO-SiO(2) interface.

Macrocyclic Multicenter Complexes of Nickel and Copper of Increasing Complexity

Multicenter (bi-, tri-, and tetranuclear) tetraazamacrocyclic complexes were self-assembled from Ni and Cu tetraazamacrocyclic mononuclear units and α,ω-diamines as building blocks. The structures of all compounds studied were proved by spectroscopic methods (ESI MS and NMR spectroscopy). Electrochemical experiments revealed reversible one-electron electrode processes at each of the Ni(2+) and Cu(2+) centers with formation of metal cations in oxidation state +3. Long linkers allow bi- and trinuclear complexes with noninteracting metal centers to be obtained. In the case of the short linkers (e.g. ethylenediamine) higher, trinuclear species are formed as major product. The structures of the bis- and tris-macrocyclic systems were confirmed by single-crystal X-ray diffraction. The tris-macrocyclic systems form cations in the shape of triangles partially filled with counterions and solvent molecules. The cations form positively charged layers, which interact in the crystal lattice with the neighboring negatively charged layers of anions. In solution, the trinuclear complexes exhibit strong host-guest interactions with 9,10-dimethyltriptycene due to complementarity of shape and size of this guest molecule. The association constants were determined by NMR spectroscopy and voltammetry, and very good agreement was obtained. The structural flexibility of the tetranuclear complex with long linkers allows for attractive interactions between the metal-complexing macrocycles that result in folding of the molecule. On the contrary, no folding is possible in the case of short linkers consisting of two CH(2) groups.

Synthesis and characterization of polycrystalline ZnO/HZSM-5 nanocomposites

Polycrystalline ZnO/HZSM-5 nanocomposites were synthesized by the impregnation method using a home prepared HZSM-5 zeolite as porous support and Zn(C5H7O2)2 or Zn(NO3)2 as zinc precursors. As-prepared samples were characterized by ICP, XRD, SEM (EDS), TEM (SAED), BET and DRS techniques. A small amount of sub-nanometeric ZnO clusters were introduced into the channels of HZSM-5 zeolite. These ZnO clusters exhibited absorption band onset at about 280 nm, different from ZnO particles at about 370 nm. The significant blue shift possesses high quantum size effect in sub-nanometeric ZnO clusters. SAED and TEM images revealed that the ZnO nanoparticles, supported on the surface of HZSM-5 zeolite, were identified as polycrystalline structure with the particle size of about 20-25 nm. XRD results provided evidence of the strong host-guest interactions between HZSM-5 framework and ZnO structure. The samples prepared by Zn(C5H7O2)2 were more porous and smaller than those prepared from the Zn(NO3)2. This was confirmed by SEM and XRD results.

Density functional theory study of the potassium complexation of an unsymmetrical 1,3-alternate calix[4]-crown-5-N-azacrown-5 bearing two different crown rings

Theoretical studies of an unsymmetrical calix[4]-crown-5-N-azacrown-5 (1) in a fixed 1,3-alternate conformation and the complexes 1·K(+)(a), 1·K(+)(b), 1·K(+)(c) and 1·K(+)K(+) were performed using density functional theory (DFT) at the B3LYP/6-31G level. The fully optimized geometric structures of the free macroligand and its 1:1 and 1:2 complexes, as obtained from DFT calculations, were used to perform natural bond orbital (NBO) analysis. The two main types of driving force metal-ligand and cation-π interactions were investigated. NBO analysis indicated that the stabilization interaction energies (E (2)) for O…K(+) and N…K(+) are larger than the other intermolecular interactions in each complex. The significant increase in electron density in the RY or LP orbitals of K(+) results in strong host-guest interactions. In addition, the intermolecular interaction thermal energies (ΔE, ΔH, ΔG) were calculated by frequency analysis at the B3LYP/6-31G level. For all structures, the most pronounced changes in the geometric parameters upon interaction are observed in the calix[4]arene molecule. The results indicate that both the intermolecular electrostatic interactions and the cation-π interactions between the metal ion and π orbitals of the two pairs that face the inverted benzene rings play a significant role.

Controlled spatial distribution of tris(2,2′-bipyridine)ruthenium cation ([Ru(bpy) 3] 2+) in aluminum containing mesoporous silicas

The adsorption of tris(2,2′-bipyridine)ruthenium(II) ([Ru(bpy) 3] 2+) onto aluminum containing mesoporous silicas (BJH pore size of 2.2 and 3.0 nm and Si/Al ratio of ca. 26) was conducted and the photoluminescence of the products was examined as a function of the loaded [Ru(bpy) 3] 2+ amounts. The loading amount of the complex was controlled below ca. 0.1 mol L −1 based on the pore volume by changing the initial concentration of the [Ru(bpy) 3] 2+ ethanol solutions used for the adsorption, resulted from the strong host–guest interactions as evidenced by the H type adsorption isotherm. The concentration dependence of the luminescence intensity was discussed to show the self-quenching process of the complex in the mesopore. The Quasi Stern–Volmer plots showed a linear relationship, suggesting that the spatial distribution of [Ru(bpy) 3] 2+ was successfully varied in the mesopore by the loading amount of [Ru(bpy) 3] 2+, even at the high concentration of the complex in the mesopore (ca. 0.1 mol L −1 based on the pore volume). The difference of the self-quenching efficiencies (coefficients of self-quenching, K sv values) of [Ru(bpy) 3] 2+ adsorbed on aluminum containing mesoporous silicas from that on the mesoporous silica modified with phenethylsulfonic group [M. Sohmiya, Y. Sugahara, M. Ogawa, J. Phys. Chem. B 111 (2007) 8836–8841] suggests that the pore size and the interactions between [Ru(bpy) 3] 2+ and the pore surface affects the efficiency; the larger pore size and the weaker interactions between [Ru(bpy) 3] 2+ and the pore surface results in the higher self-quenching efficiency.

Optical properties of (nanometer MCM-41)–(malachite green) composite materials

Nanosized materials loaded with organic dyes are of interest with respect to novel optical applications. The optical properties of malachite green (MG) in MCM-41 are considerably influenced by the limited nanoporous channels of nanometer MCM-41. Nanometer MCM-41 was synthesized by tetraethyl orthosilicate (TEOS) as the source of silica and cetyltrimethylammonium bromide (CTMAB) as the template. The liquid-phase grafting method has been employed for incorporation of the malachite green molecules into the channels of nanometer MCM-41. A comparative study has been carried out on the adsorption of the malachite green into modified MCM-41 and unmodified MCM-41. The modified MCM-41 was synthesized using a silylation reagent, trimethychlorosilane (TMSCl), which functionalized the surface of nanometer MCM-41 for proper host–guest interaction. The prepared (nanometer MCM-41)–MG samples have been studied by powder X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy, low-temperature nitrogen adsorption–desorption technique at 77 K, Raman spectra and luminescence studies. In the prepared (nanometer MCM-41)–MG composite materials, the frameworks of the host molecular sieve were kept intact and the MG located inside the pores of MCM-41. Compared with the MG, it is found that the prepared composite materials perform a considerable luminescence. The excitation and emission spectra of MG in both modified MCM-41 and unmodified MCM-41 were examined to explore the structural effects on the optical properties of MG. The results of luminescence spectra indicated that the MG molecules existed in monomer form within MCM-41. However, the luminescent intensity of MG incorporated in the modified MCM-41 are higher than that of MG encapsulated in unmodified MCM-41, which may be due to the anchored methyl groups on the channels of the nanometer MCM-41 and the strong host–guest interactions. The steric effect from the pore size of the host materials is significant. Raman spectra firmly demonstrated the stable form obtained after the MG incorporation into the nanometer MCM-41. Therefore, nanometer MCM-41 appears to have a good potential for its use as a support for dyes and the (nanometer MCM-41)–MG composite materials may give a wide optical application.

Effective Enlargement of Fluorescence Resonance Energy Transfer of Poly-Porphyrin Mediated by β-Cyclodextrin Dimers

4,4-Difluoro-4-bora-3a,4a-diaza-s-indacene bridged bis(permethyl-beta-cyclodextrins) 1 was synthesized successfully through click chemistry, and it forms a well-defined linear assembly with 5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin 2 via extremely strong host-guest interactions, which was identified by the methods of UV-vis, NMR, AFM, and TEM, respectively. Furthermore, the fluorescence resonance energy transfer from host spacer to porphyrin guest was investigated, showing that the energy transfer quantum yield is high to 94% in virtue of such a noncovalent path.

Pattern‐Based Recognition of Heparin Contaminants by an Array of Self‐Assembling Fluorescent Receptors

Tracking down potential killers: Strong host-guest interactions enable the facile combination of polycationic cyclodextrin binding motifs (blue) with fluorescent reporters (orange) tethered to a hydrophobic guest molecule (dark green). An array of supramolecular fluorescent receptors prepared by this modular approach was used for the pattern-based recognition of negatively charged contaminants in the anticoagulant drug heparin.

Molecular Recognition and Spectroscopic Properties of Cyclobis(paraquat-p-anthracene)

Cyclobis(paraquat-p-anthracene) is a novel electron-deficient macrocyclic host. One of the most important applications of the host is molecular recognition. The binding energies of the host to several guest molecules like water, ammonia, alcohol, and heterocycles were investigated. The structures of the host-guest complexes were optimized using DFT (density functional theory) at B3LYP/3-21G level. The energies of the complexes were calculated at B3LYP/6-31G(d) level. The binding energies of the complexes were obtained after the correction of basis set superposition error. The 13C and 3He chemical shifts of the complexes were calculated by the gauge-including atomic orbital (GIAO) method at B3LYP/3-21G level. It indicates that the host can bind the guest molecules via hydrogen bonds between the heteroatoms in the guest and hydrogen atoms in the host. The binding energies of the complexes were mainly affected by the number and distance of the hydrogen bonds. The energy gaps for some of the complexes were increased owing to the formation of the hydrogen bonds. At the same time, the chemical shifts of the carbon atoms on the C-H bonds connected with the hydrogen bonds in the complexes relative to those of the same carbon atoms in the host were transferred downfield. The aromaticities of the complexes were relevant to the binding energies and ways. The aromaticity of the host was increased by the weak interaction between the host and guest molecules. The cyclic current of the host was decreased, thus the aromaticity was lowered owing to too strong host-guest interaction and incorporation of the guest molecules inside the cavity of the host.