Polar Guest Research Articles

Cation-templated cyanometallate-based supramolecular rectangular cage compounds showing dielectric transitions

Relatively small displacements or thermal vibrations of the polar guests in Cd(ii)–Co(iii) cyanometallate frameworks A2{H(CdCl2)[Co(CN)6]} (A = monovalent cation) induce dielectric transitions and relaxations.

Dynamics of polar aromatic molecules confined in a nanocavity of δ-phase of syndiotactic polystyrene as studied by dielectric spectroscopy

Rotational dynamics was examined in detail for four polar aromatic molecules, benzonitrile (BN), p-methylbenzonitrile (p-MBN), o-methylbenzonitrile (o-MBN), and m-methylbenzonitrile (m-MBN), accommodated in a nano-cavity regularly arranged inside the crystalline region (δ-form) of syndiotactic polystyrene (sPS) by means of dielectric relaxation measurements. Except for m-MBN, for which the co-crystalline structure was monoclinic δ-intercalate and no dielectric relaxation process arising from the crystalline region appeared, other three polar guests in the monoclinic δ-clathrate form exhibited the specific dielectric relaxation process. The relaxation times of o-MBN, BN, and p-MBN at room temperature were ranged from 10−7s to 10−1s, and in the order of o-MBN<BN<p-MBN. From the analysis of dielectric intensity data as functions of temperature, new insights about the stable and quasi-stable states of the guest molecules inside the cavity were obtained based on the two site model.

Dielectric properties induced by the framework in the hybrid organic–inorganic compounds M(dca)2pyz M = Fe, Co and Zn

In this work we prepare and study the M(dca)2pyz compounds, where dca=polar dicyanamide N(CN)2− ligand, pyz=pyrazine (C4H4N2) ligand and M=Fe2+, Co2+ and Zn2+ that, according to DSC display a first order phase transition, at Tt∼270K (Fe), ∼250K (Co) and ∼240K (Zn). Very interestingly these compounds, which consist of a rather robust framework with incorporated polar dca ligands, and which does not host guest species in the cavities of the structure, display a dielectric anomaly associated to such phase transition. This latter in fact corresponds to the structural transition from the disordered HT-phase (space group Pnma) to the ordered LT-phase (space group P21/n). From a detailed analysis of the dipole distributions in each of these crystal structures we conclude that the polar dca ligands, which bridge two transition metal cations, display a cooperative electrical arrangement, in which the dipoles of each of the two interpenetrated networks show a ferroelectric (FE) arrangement; nevertheless, the relative antiparallel orientation of both networks finally results in net antiferroelectric (AFE) arrangements both in the LT- and HT-polymorphs. Very interestingly, the dynamic disorder of the dca ligands at Tt gives rise to the observed dielectric anomaly. These materials constitute, therefore, an example of hybrid organic–inorganic compounds where the dielectric response directly arises from the framework –and not from polar guest species– which is a very interesting and novel approach in the search for materials in which the dielectric and magnetic properties can couple more strongly.

A perovskite-type cage compound as a temperature-triggered dielectric switchable material

The perovskite-type cage compound [C3H6NH2]2[KCoIII(CN)6] shows a temperature-triggered dielectric switch. The switchable properties are derived from an order–disorder phase transition of a system, by changing the motions of the polar azetidine cation guests.

Citric acid encapsulation by a double helical foldamer in competitive solvents.

A new double helical aromatic oligoamide capsule able to bind to citric acid in polar and protic solvents was prepared. Aromatic amino acids in the sequence encode both structural (strand curvature and double helix formation) and functional features (recognition pattern) of the assembled capsule.

Effects of ligand and guest solvent molecules on the luminescence properties of Tb : Eu-codoped indium-based MOFs.

Luminescent lanthanide MOF materials are good sensors for analyzing some specific gas or volatile small molecules. However, some potential interference factors coming from the material itself, such as bridging ligands or guest solvent molecules entrapped in the channel of a MOF, were usually ignored during the sensing process. Here, two Tb : Eu-codoped indium-based MOFs with different bridging ligands were obtained for exploring the effects of ligand and guest solvent molecules on their luminescence properties. The current studies demonstrated that a ligand in the triplet state located between the excited states of Tb and Eu and polar guest solvent molecules encapsulated in the lanthanide MOF with such a type of ligand as the linker can interfere with the sensing process since they can substantially facilitate the energy transfer between Tb and Eu.

Coordination of alkaline-earth metal cations to a symmetrical octamethyl-substituted cucurbituril in the presence of polychlorido cadmium(ii) anions

The interaction of the alkaline-earth metal ions Mg2+, Ca2+, Sr2+ and Ba2+ with the symmetrical octamethyl-substituted cucurbituril, OMeQ[6], in the presence of [CdCl4]2−/[Cd2Cl8]4− anions as structure-directing agent(s) in 3 M HCl has been investigated. Ca2+ and Sr2+ yielded closely related metallosupramolecular assemblies whose X-ray structures show that in the former, [Cd2Cl8]4− anions are arranged to form parallelogram-like frameworks that surround linear OMeQ[6]/Ca2+ polymeric units. The structure of the OMeQ[6]/Sr2+ assembly is similar except that the [Cd2Cl8]4− anions in the OMeQ[6]/Ca2+ assembly are replaced by [Cd2(H2O)2Cl8]4− anions. In contrast to the above structures, interaction of the larger Ba2+ ion with OMeQ[6] gives rise to a different assembly type displaying a porous linear polymeric arrangement incorporating two different Ba2+ coordination modes; [Cd2Cl4]4− anions are associated with the Ba2+ bridges linking adjacent OMeQ[6] units in the polymeric chain. In contrast to Ca2+ and Sr2+, the smaller Mg2+ ion under similar synthetic conditions failed to yield a solid product. Isothermal titration calorimetry (ITC) studies revealed that OMeQ[6] interacts with Ca2+, Sr2+ and Ba2+, but not Mg2+, in neutral aqueous solution. The respective X-ray structures reveal the presence of both channels and other voids in the three assemblies. Following application of a vacuum until constant weight was obtained, samples of the Sr2+ and Ba2+ assemblies were subjected to the saturated vapour of each of the following volatile ligands: acetonitrile, methanol, ethanol, acetone, diethylether, dichloromethane, cyclohexane and n-hexane. The respective absorption profiles show maximum absorption for methanol, with reduced uptake for the remaining larger and/or less polar guest molecules.

Switchable dielectric constant in an inclusion compound bis(thiourea) imidazolium chloride

Inclusion compounds that contain dynamic polar guests possess rich electrical properties such as ferroelectricity and switchable dielectric constant upon undergoing order–disorder-type phase transitions. We here report a thiourea-based inclusion compound (Him)[(TU)2Cl] (Him = imidazolium, TU = thiourea) that undergoes two reversible structural phase transitions at 139 K and 145 K, which are induced by the order–disorder transformations of the Him cation guests in the channels formed by the anionic [(TU)2Cl] components. Switching of the dielectric constant is thus triggered between the low- and high-dielectric states.

Photoswitchable Adsorption in Metal-Organic Frameworks Based on Polar Guest-Host Interactions.

Reversible remote-controlled switching of the properties of nanoporous metal-organic frameworks (MOFs) is enabled by incorporating photoswitchable azobenzene. The interaction of the host material with different guest molecules, which is crucial for all applications, is precisely studied using thin MOF films of the type Cu2 (BDC)2 (AzoBipyB). A molecule-specific effect of the photoswitching, based on dipole-dipole interactions, is found.

Syntheses, structures and properties of four second-sphere coordination complexes via metal halide anion and naphthalene-based ligand

Second-sphere coordination refers to any intermolecular interactions with the ligands directly bound to the primary coordination sphere of a metal ion. Four supramolecular complexes, 0.5[L·2H]2+·0.5[MCl4]2−·[CH3OH]·0.5[CH2Cl2] (M = Co, crystal 1; M = Mn, crystal 2), 0.5[L·2H]2+·0.5[ZnBr4]2−·[CH3OH]·0.5[CH2Cl2] (crystal 3), and 0.5[L·2H]2+·0.5[Cu2Br4]2−·H2O (crystal 4), based on naphthalene-based ligand N,N,N′,N′-tetra-p-methylnaphthyl-ethanediamine (L), have been synthesized. X-ray analysis reveals that 1–3 are isostructural, in which the methanol molecules are bridges, connecting the protonated L and metal chloride anions via N–H⋯O and O–H⋯Cl (Br) interactions to construct the host framework, and forming X-shaped cavity accessible for the inclusion of weakly polar guest molecules of dichloromethane. Dichloromethane is connected with the host framework through van der Waals forces. In 4, a dinuclear anion [Cu2Br4]2− is connected with the ligand through N–H⋯Br interactions, in which the water molecules are accommodated between chains formed by the ligand and [Cu2Br4]2−. Structure stability, thermal analysis, and photoluminescent properties were studied for 1–4.

Solvent effects and driving forces in pillararene inclusion complexes.

Pillararenes, a recently discovered class of aromatic macrocycles, form inclusion complexes with a large number of guest molecules, but not much is known about the driving forces of complexation, including the role of the solvent. We have measured the binding thermodynamics for a small number of model complexes in several solvents and used computational chemistry to rationalize the obtained results and identify the driving forces of complexation. Favorable electrostatic interactions between the host and guest are obtained when the charge distribution in the guest matches the negative electrostatic potential in the cavity of the pillararene. Polar guests, however, also interact strongly with polar solvents, thereby shifting the complexation equilibrium away from the complex. The shape of the solvent molecules is another important factor as some solvents are sterically hindered from entering the pillararene cavity. By changing solvent from acetonitrile to o-xylene the binding constant in one case increased more than 4 orders of magnitude. Even electrostatically similar solvents such as o-xylene and p-xylene have very different impacts on the binding constants due to their different abilities to fit into the cavity. The study illustrates the importance of taking into account the interactions between the solvent and the complexing species in the investigation and design of molecular host:guest systems.

Ordered co-encapsulation of chloride with polar neutral guests in a tetraurea calix[4]pyrrole dimeric capsule.

In this work, we describe the stoichiometrically controlled self-assembly process of tetraurea calix[4]pyrrole 1 with a polar neutral guest, trimethylamine N-oxide or beta-alanine betaine, and methyltrioctylammonium chloride (MTOACl) salt into two supramolecular architectures which differ in morphology and stoichiometry. Whereas an equimolar solution of tetraurea calixpyrrole 1, polar guest and MTOACl produces a four-particle inclusion assembly, a mixture of the same components in a 2 : 1 : 1 molar ratio induces the formation of a dimeric capsular assembly displaying multiple guests which are orderly co-encapsulated. The influence of other polar guests and ammonium salts on the self-assembly process is also described.

Switchable guest molecular dynamics in a perovskite-like coordination polymer toward sensitive thermoresponsive dielectric materials.

A new perovskite-like coordination polymer [(CH3)2NH2][Cd(N3)3] is reported which undergoes a reversible ferroelastic phase transition. This transition is due to varied modes of motion of the [(CH3)2NH2](+) guest accompanied by a synergistic deformation of the [Cd(N3)3](-) framework. The unusual two-staged switchable dielectric relaxation reveals the molecular dynamics of the polar cation guest, which are well controlled by the variable confined space of the host framework. As the material switches from the ferroelastic phase to the paraelastic phase, a remarkable increase of the rotational energy barrier is detected. As a result, upon heating at low temperature, this compound shows a notable change from a low to a high dielectric state in the ferroelastic phase. This thermoresponsive host-guest system may serve as a model compound for the development of sensitive thermoresponsive dielectric materials and may be key to understanding and modulating molecular/ionic dynamics of guest molecules in confined space.

Switchable Guest Molecular Dynamics in a Perovskite‐Like Coordination Polymer toward Sensitive Thermoresponsive Dielectric Materials

AbstractA new perovskite‐like coordination polymer [(CH3)2NH2][Cd(N3)3] is reported which undergoes a reversible ferroelastic phase transition. This transition is due to varied modes of motion of the [(CH3)2NH2]+ guest accompanied by a synergistic deformation of the [Cd(N3)3]− framework. The unusual two‐staged switchable dielectric relaxation reveals the molecular dynamics of the polar cation guest, which are well controlled by the variable confined space of the host framework. As the material switches from the ferroelastic phase to the paraelastic phase, a remarkable increase of the rotational energy barrier is detected. As a result, upon heating at low temperature, this compound shows a notable change from a low to a high dielectric state in the ferroelastic phase. This thermoresponsive host–guest system may serve as a model compound for the development of sensitive thermoresponsive dielectric materials and may be key to understanding and modulating molecular/ionic dynamics of guest molecules in confined space.

The multi-domain nanoparticle structure of a universal core-multi-shell nanocarrier

Nanoparticles with a multi-domain structure have been identified as a new class of highly potent nanocarriers with a dual transport mechanism for polar and non-polar guests. In this paper we present a detailed analysis of their spatial structure, which has been analyzed by small-angle neutron scattering (SANS) including contrast variation. The nanocarriers consist of a hyperbranched polyglycerol (hPG) core onto which di-block segments consisting of non-polar alkyl chains and hydrophilic poly(ethylene glycol) methyl ether (mPEG) has been grafted. The hPG cores have been analyzed separately for comparison. SANS demonstrates that the hPG-cores can be described as fractal structures. The nanocarriers form rather compact structures in the non-polar solvent toluene. A fractal structure with a correspondingly rough surface is found here as well, which is explained by the backfolding of the hydrophilic mPEG-chains in the non-polar environment. In water the nanocarriers associate to form well-defined nanoparticle clusters with an ellipsoidal shape. Contrast variation demonstrates that these particles have no distinct core–shell structure but the separation of the incompatible parts of the structure takes place on a much smaller length scale.

Encapsulation of diatomic molecules in fullerene C60: implications for their main properties.

The endohedral complexes of diatomic guest molecules H2, N2, O2, F2, HF, CO, LiH, LiF, BN, and BeO with C60 have been characterized computationally by employing second-order Møller-Plesset (MP2) theory and its density-fitting local (DF-LMP2) variant. The interaction energies, equilibrium geometries, dipole moments and harmonic vibrational frequencies of these complexes have been systematically calculated. It was found that all guest molecules are stabilized inside the C60 cage, with the most pronounced stabilization effect (of about 50 kcal mol(-1)) observed for the polar covalent BeO and BN molecules. It is noteworthy that the normally short-lived BN molecule is the only guest molecule that was found to chemisorb on the inner surface of C60. When encapsulated, all guest molecules (except for BN) exhibit bond elongation (up to 0.07 Å) and, consequently, a red shift in vibrational stretching frequencies. In fact, the calculated vibrational properties of the H2@C60 complex agree well with those derived from experiment. The C60 geometry is not perturbed significantly upon encapsulation, but a subtle tendency to decrease the carbon-carbon bond alternation is observed. Polar guest molecules inside C60 are located at an off-center position and a significant decrease in their dipole moments upon encapsulation is observed. The importance of explicitly taking into account electron correlation effects, as well as full geometry relaxation, to yield a correct description of the complexes investigated is clearly demonstrated. The present results may serve as a guide for future attempts to synthesize such complexes employing the "molecular surgery" approach.

Supramolecular disassembly of facially amphiphilic dendrimer assemblies in response to physical, chemical, and biological stimuli.

ConspectusSupramolecular assemblies formed from spontaneous self-assembly of amphiphilic macromolecules are explored as biomimetic architectures and for applications in areas such as sensing, drug delivery, and diagnostics. Macromolecular assemblies are usually preferred, compared with their simpler small molecule counterparts, due to their low critical aggregate concentrations (CAC) and high thermodynamic stability. This Account focuses on the structural and functional aspects of assemblies formed from dendrimers, specifically facially amphiphilic dendrons that form micelle or inverse micelle type supramolecular assemblies depending on the nature of the solvent medium.The micelle type assemblies formed from facially amphiphilic dendrons sequester hydrophobic guest molecules in their interiors. The stability of these assemblies is dependent on the relative compatibility of the hydrophilic and hydrophobic functionalities with water, often referred to as hydrophilic–lipophilic balance (HLB). Disruption of the HLB, using an external stimulus, could lead to disassembly of the aggregates, which can then be utilized to cause an actuation event, such as guest molecule release. Studying these possibilities has led to (i) a robust and general strategy for stimulus-induced disassembly and molecular release and (ii) the introduction of a new approach to protein-responsive supramolecular disassembly. The latter strategy provides a particularly novel avenue for impacting biomedical applications. Most of the stimuli-sensitive supramolecular assemblies have been designed to be responsive to factors such pH, temperature, and redox conditions. The reason for this interest stems from the fact that certain disease microenvironments have aberrations in these factors. However, these variations are the secondary imbalances in biology. Imbalances in protein activity are the primary reasons for most, if not all, human pathology. There have been no robust strategies in stimulus-responsive assemblies that respond to these variations. The facially amphiphilic dendrimers provide a unique opportunity to explore this possibility.Similarly, the propensity of these molecules to form inverse micelles in apolar solvents and thus bind polar guest molecules, combined with the fact that these assemblies do not thermodynamically equilibrate in biphasic mixtures, was used to predictably simplify peptide mixtures. The structure–property relationships developed from these studies have led to a selective and highly sensitive detection of peptides in complex mixtures. Selectivity in peptide extraction was achieved using charge complementarity between the peptides and the hydrophilic components present in inverse micellar interiors. These findings will have implications in areas such as proteomics and biomarker detection.

Stepwise Assembly of Pd6(RuL3)8 Nanoscale Rhombododecahedral Metal–Organic Cages via Metalloligand Strategy for Guest Trapping and Protection

Stepwise synthesis of nanosized Pd-Ru heteronuclear metal-organic cages from predesigned redox- and photo-active Ru(II)-metalloligand and naked Pd(II) ion is described. The resulting cage shows rhombododecahedral shape and contains a 5350 Å(3) cavity and 12 open windows, facilitating effective trapping of both polar and nonpolar guest molecules. Protection of photosensitive guests against UV radiation is studied.

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.

Preparation and characterization of multi stimuli-responsive photoluminescent nanocomposites of graphene quantum dots with hyperbranched polyethylenimine derivatives.

Oxidized graphene sheets (OGS) were treated with a hyperbranched polyethylenimine (PEI) under hydrothermal conditions to generate nanocomposites of graphene quantum dots (GQDs) functionalized with PEI (GQD-PEIs). The influence of the reaction temperature and the PEI/OGS feed ratio on the photoluminescence properties of the GQD-PEIs was studied. The obtained GQD-PEIs were characterized by TEM, dynamic light scattering, elemental analysis, FTIR, zeta potential measurements and (1)H NMR spectroscopy, from which their structural information was inferred. Subsequently, isobutyric amide (IBAm) groups were attached to the GQD-PEIs through the amidation reaction of isobutyric anhydride with the PEI moieties, which resulted in GQD-PEI-IBAm nanocomposites. GQD-PEI-IBAm was not only thermoresponsive, but also responded to other stimuli, including inorganic salts, pH, and loaded organic guests. The cloud point temperature (Tcp) of aqueous solutions of GQD-PEI-IBAm could be modulated through changing the number of IBAm units in GQD-PEI-IBAm, by varying the type and concentration of the inorganic salts and loaded organic guests, or by varying the pH. All the obtained GQD-PEI-IBAm nanocomposites were photoluminescent, and their maximum emission wavelengths were not influenced by outside stimuli. Their emission intensities were influenced a little or negligibly by pH, traditional salting-out anions (Cl(-) and SO4(2-)), and the relatively polar aspirin guest. However, the traditional salting-in I(-) anion and the more hydrophobic 1-pyrenebutyric acid (PBA) guest could effectively quench their fluorescence. 2D NOESY (1)H NMR spectra verified that GQD-PEI-IBAm accommodated the relatively polar aspirin guest using the PEI-IBAm shell, but adsorbed the relatively hydrophobic PBA guest through the nanographene core. The release rate of the guest encapsulated by the thermoresponsive GQD is different below and above Tcp.