Guest Exchange Research Articles

Guest exchange in a biomimetic ZnII cavity-complex: kinetic control by a catalytic water, through pore selection, 2nd sphere assistance, and induced-fit processes

The kinetics of ligand exchange at a ZnII center confined in a biomimetic environment is studied.

NMR exchange dynamics studies of metal-capped cyclodextrins reveal multiple populations of host-guest complexes in solution.

Metal-capped molecular hosts are unique in supramolecular chemistry, benefitting from the inner cavity's hydrophobic nature and the metal center's electrochemical properties. It is shown here that the paramagnetic properties of the metals in lanthanide-capped cyclodextrins (Ln-α-CDs and Ln-β-CDs) are a convenient NMR indicator for different populations of host-guest complexes in a given solution. The paramagnetic guest exchange saturation transfer (paraGEST) method was used to study the exchange dynamics in systems composed of Ln-α-CDs or Ln-β-CDs with fluorinated guests, revealing multiple co-existing populations of host-guest complexes exclusively in solutions containing Ln-β-CDs. The enhanced spectral resolution of paraGEST, achieved by a strong pseudo contact shift induction, revealed that different molecular guests can adopt multiple orientations within Ln-β-CDs' cavities and, in contrast, only a single orientation inside Ln-α-CDs. Thus, paraGEST, which can significantly improve NMR detectability and spectral resolution of host-guest systems that experience fast exchange dynamics, is a convenient tool for studying supramolecular systems of metal-capped molecular hosts.

Molecular guest exchange and subsequent structural transformation in CH4 – CO2 replacement occurring in sH hydrates as revealed by 13C NMR spectroscopy and molecular dynamic simulations

This study adopted experimental and computational approaches to investigate CH4 – CO2 replacement in the structure H (sH) CH4 + methylcyclopentane (MCP) hydrate for its dual functions of CH4 recovery and CO2 sequestration. Hydrate phase equilibria, 13C NMR spectra, and molecular dynamics (MD) simulations of CH4 + MCP – CO2 replacement were examined and compared with those of CH4 + neohexane (NH) – CO2 replacement to elucidate the molecular guest exchange behaviors in both systems. The structure I (sI) hydrates were thermodynamically favored in CO2-rich gas mixtures (CH4 + CO2 + MCP systems), and a structural transformation from sH to sI occurred when CO2 composition in the feed gas was higher than 20 %. The 13C NMR spectra indicated that the CO2 molecules preferred to occupy the large (51262) cages of sI and the medium (435663) cages of sH compared to the small (512) cages of both sI and sH during replacement. Following CO2 injection into the sH CH4 + MCP hydrate, the initial sH hydrate transformed to the sI hydrate with CH4 recovery of approximately 78 %. The MD simulations also demonstrated that structural transformation in the CH4 + MCP – CO2 replacement would occur at the lower CO2 enclathration ratio compared to the CH4 + NH – CO2 replacement, thereby leading to a lower CO2 concentration in the newly formed sI hydrate. This is the first study to provide both experimental and computational evidence of guest-dependent structural transformation in sH CH4 + liquid hydrocarbons – CO2 replacement.

Synthesis, Structural Characterization, and Guest Exchange Properties of Hydrogen-Bonded Organic Frameworks Based on Bis(benzimidazole)ZnCl2 Complexes.

Hydrogen-bonded organic frameworks (HOFs) based on coordination compounds constitute a developing class of interesting porous materials. Herein, we report on the synthesis, crystal structures, and guest exchange properties of four HOFs based on zinc dichlorido complexes that bear a bis(benzimidazolyl)methane ligand (bis(benzimidazole)ZnCl2). The porous structures of these bis(benzimidazole)ZnCl2-based HOFs are characterized predominantly by intermolecular N-H···Cl hydrogen bonds in conjunction with π-π interactions. One of these HOFs was found to exchange guest molecules via single-crystal-to-single-crystal transformations with or without structural change. A single-crystal X-ray diffraction study revealed that the guest exchange accompanied by a structural change is induced by the cleavage of the N-H···Cl hydrogen bonds between the bis(benzimidazole)ZnCl2 complexes, followed by the formation of alternate hydrogen bonds with guest molecules. This result suggests that the use of weaker N-H···Cl hydrogen bonds than those typically used for the construction of HOFs (e.g., carboxylic acid dimers, N-heterocycles, and charge-assisted moieties) may represent a convenient strategy to synthesize flexible HOFs.

Effective CH4 production and novel CO2 storage through depressurization-assisted replacement in natural gas hydrate-bearing sediment

In this study, the guest exchange behaviors in the hydrate-bearing sediment of a one-dimensional reactor specially designed for depressurization-assisted replacement were experimentally investigated for CH4 production and CO2 storage. The longitudinal distributions of vapor compositions, replacement efficiency, and hydrate weight fractions, as well as the average efficiency for depressurization-assisted replacement, were examined using gas chromatography and powder X-ray diffraction (PXRD). The immediate re-formation of gas hydrates after CO2 injection implied a rapid recovery of the geo-mechanical strength of the sediment. The unique changes in the gas compositions in the pore space caused by the re-formation of gas hydrates resulted in a distinctive longitudinal distribution of the replacement efficiency in the hydrate-bearing sediment. PXRD analysis of the hydrate samples revealed that the gas hydrate saturation nearly recovered after depressurization-assisted replacement. Despite CH4 re-enclathration, the replacement efficiency was remarkably enhanced through depressurization-assisted replacement, and a larger amount of CO2 was stored in the hydrate-bearing sediment than the amount of CH4 produced. Furthermore, the production rate of CH4 through depressurization-assisted replacement was significantly higher than that through replacement only, and the guest exchange rate increased with an increase in the initial hydrate dissociation ratio. The experimental results demonstrated that depressurization-assisted replacement could solve the weakening of the geo-mechanical strength of the sediment for depressurization only and the slow production rate for replacement only; thus, it would be useful for low-carbon energy production from natural gas hydrate-bearing sediments.

A Versatile 3D-Confined Self-Assembly Strategy for Anisotropic and Ordered Mesoporous Carbon Microparticles.

Mesoporous carbon microparticles (MCMPs) with anisotropic shapes and ordered structures are attractive materials that remain challenging to access. In this study, a facile yet versatile route is developed to prepare anisotropic MCMPs by combining neutral interface‐guided 3D confined self‐assembly (3D‐CSA) of block copolymer (BCP) with a self‐templated direct carbonization strategy. This route enables pre‐engineering BCP into microparticles with oblate shape and hexagonal packing cylindrical mesostructures, followed by selective crosslinking and decorating of their continuous phase with functional species (such as platinum nanoparticles, Pt NPs) via in situ growth. To realize uniform in situ growth, a “guest exchange” strategy is proposed to make room for functional species and a pre‐crosslinking strategy is developed to preserve the structural stability of preformed BCP microparticles during infiltration. Finally, Pt NP‐loaded MCMPs are derived from the continuous phase of BCP microparticles through selective self‐templated direct carbonization without using any external carbon source. This study introduces an effective concept to obtain functional species‐loaded and N‐doped MCMPs with oblate shape and almost hexagonal structure (p6mm), which would find important applications in fuel cells, separation, and heterogeneous catalysis.

Preparation of γ-Cyclodextrin Inclusion Compounds Using Solid-Phase Guest Exchange Method

Preparation of γ-Cyclodextrin Inclusion Compounds Using Solid-Phase Guest Exchange Method

Ternary host-guest complexes with rapid exchange kinetics and photoswitchable fluorescence

SummaryConfinement within molecular cages can dramatically modify the physicochemical properties of the encapsulated guest molecules, but such host-guest complexes have mainly been studied in a static context. Combining confinement effects with fast guest exchange kinetics could pave the way toward stimuli-responsive supramolecular systems—and ultimately materials—whose desired properties could be tailored “on demand” rapidly and reversibly. Here, we demonstrate rapid guest exchange between inclusion complexes of an open-window coordination cage that can simultaneously accommodate two guest molecules. Working with two types of guests, anthracene derivatives and BODIPY dyes, we show that the former can substantially modify the optical properties of the latter upon noncovalent heterodimer formation. We also studied the light-induced covalent dimerization of encapsulated anthracenes and found large effects of confinement on reaction rates. By coupling the photodimerization with the rapid guest exchange, we developed a new way to modulate fluorescence using external irradiation.

Paramagnetic relaxation enhancement NMR as a tool to probe guest binding and exchange in metallohosts

Paramagnetic metallohost systems can bind guest molecules and find application as biomimetic catalysts. Due to the presence of the paramagnetic metal center, rigorous characterization of these systems by NMR spectroscopy can be very difficult. We report here that metallohost-guest systems can be studied by using the paramagnetic relaxation enhancement (PRE) effect. Manganese(III) porphyrin cage compounds are shown through their PRE to thread and bind viologen guests, including a polymeric one. The binding constants and dethreading activation parameters are lower than those of the metal-free porphyrin cage compounds, which is proposed to be a result of charge repulsion of the trivalent metal center and dicationic viologen guest. The threading rate of the manganese(III) porphyrin cage onto the polymer is more than 10 times faster than that of the non-metallated one, which is ascribed to initial binding of the cage to the polymer chain prior to threading, and to an entron effect.

A Rare Flexible Metal-Organic Framework Based on a Tailorable Mn8 -Cluster Showing Smart Responsiveness to Aromatic Guests and Capacity for Gas Separation.

The design and creation of soft porous crystals combining regularity and flexibility may promote potential applications for gas storage and separation due to their deformable framework's responsiveness to external stimuli. The flexibility of metal-organic frameworks (MOFs) relies on alterable degrees of freedom that are mainly provided by organic linkers or the junctions linking organic and inorganic building units. Herein, we report a new dynamic MOF whose flexibility originates from an unprecedented tailorable Mn8 O38 -cluster and shows simultaneous coordination geometry changes and ligand migration that are reversibly driven by guest exchange. This provides an extra degree of freedom to the framework's deformation, resulting in three-dimensional variations in the framework that subtly respond to varied aromatic molecules. The gas adsorption behavior of this flexible MOF was evaluated, and the selective separation of light hydrocarbons and Freon gases is achieved.

A Rare Flexible Metal–Organic Framework Based on a Tailorable Mn8‐Cluster Showing Smart Responsiveness to Aromatic Guests and Capacity for Gas Separation

AbstractThe design and creation of soft porous crystals combining regularity and flexibility may promote potential applications for gas storage and separation due to their deformable framework's responsiveness to external stimuli. The flexibility of metal–organic frameworks (MOFs) relies on alterable degrees of freedom that are mainly provided by organic linkers or the junctions linking organic and inorganic building units. Herein, we report a new dynamic MOF whose flexibility originates from an unprecedented tailorable Mn8O38‐cluster and shows simultaneous coordination geometry changes and ligand migration that are reversibly driven by guest exchange. This provides an extra degree of freedom to the framework's deformation, resulting in three‐dimensional variations in the framework that subtly respond to varied aromatic molecules. The gas adsorption behavior of this flexible MOF was evaluated, and the selective separation of light hydrocarbons and Freon gases is achieved.

Hyper-CEST NMR of metal organic polyhedral cages reveals hidden diastereomers with diverse guest exchange kinetics

Guest capture and release are important properties of self-assembling nanostructures. Over time, a significant fraction of guests might engage in short-lived states with different symmetry and stereoselectivity and transit frequently between multiple environments, thereby escaping common spectroscopy techniques. Here, we investigate the cavity of an iron-based metal organic polyhedron (Fe-MOP) using spin-hyperpolarized 129Xe Chemical Exchange Saturation Transfer (hyper-CEST) NMR. We report strong signals unknown from previous studies that persist under different perturbations. On-the-fly delivery of hyperpolarized gas yields CEST signatures that reflect different Xe exchange kinetics from multiple environments. Dilute pools with ~ 104-fold lower spin numbers than reported for directly detected hyperpolarized nuclei are readily detected due to efficient guest turnover. The system is further probed by instantaneous and medium timescale perturbations. Computational modeling indicates that these signals originate likely from Xe bound to three Fe-MOP diastereomers (T, C3, S4). The symmetry thus induces steric effects with aperture size changes that tunes selective spin manipulation as it is employed in CEST MRI agents and, potentially, impacts other processes occurring on the millisecond time scale.

High Surface Area Nanoporous-Crystalline Polymer Films

Based on a commercially available thermoplastic polymer (poly(2,6-dimethyl-1,4-phenylene) oxide, PPO), nanoporous-crystalline (NC) polymer films with a surface area (SA) higher than 600 m2 g–1 and with exceptionally fast organic pollutant uptakes are presented. Preparation procedures require fast crystallizations of amorphous films, as induced by high uptakes of nonvolatile guest molecules (typically in the range 70–90 wt %), followed by guest exchange with a volatile guest. The high SA of these NC PPO films is associated with a nanofibrillar morphology, analogous to those observed in physically cross-linked PPO aerogels. Organic pollutant diffusivities for high-SA NC PPO films are much higher than for other high-diffusivity PPO films, like for example those exhibiting NC phases with chain axes (and hence crystalline channels) being preferentially perpendicular to the film plane. Much higher diffusivities are observed both for guest sorption from vapor phase and from diluted PPO aqueous solutions and for guest desorption. Surprisingly, kinetics of guest uptakes from high-SA NC PPO films are even faster than for NC PPO aerogels and then much faster when referred to volume of the sorbent material. Because of their extremely fast kinetics of pollutant uptakes, associated with high thermal and mechanical stability, these new high-SA NC PPO films are expected to find many applications, mainly in pollutant remediation and molecular sensors.

Preparation of polyfluorinated azobenzene/niobate composite as electrochemical sensor for detection of ascorbic acid and dopamine

The polyfluorinated cationic azobenzene derivative, trans-[2-(2,2,3,3,4,4,4-heptafluorobutylamino)ethyl]-{2-[4-(4-hexyphenylazo)-phenoxy]ethyl}-dimethylammonium (abbreviated as C3F7-azo+Br-) was intercalated into layer semiconductor KNb3O8 by guest–guest exchange method to synthesize a sandwich-structured C3F7-azo+-Nb3O8 composite, which can be utilized as an electrochemical sensor for simultaneous detection of ascorbic acid (AA) and dopamine (DA). X-ray diffraction (XRD), scanning electron microscope (SEM), high-resolution transmission electron microscopy (HRTEM), energy dispersive spectrometer analysis (EDS) and Fourier transform infrared spectrometer (FTIR) were used to characterize the structure, morphology and composition of C3F7-azo+-Nb3O8. It is proposed that C3F7-azo+ ions were intercalated into [Nb3O8]- lamellar forming a bi-molecular layer at angle of 47.85°. The results of cyclic voltammetry (CV) and differential pulse voltammetry (DPV) show that the as-prepared C3F7-azo+-Nb3O8/GCE can detect AA and DA simultaneously in N2-saturated PBS (0.2 M, pH = 5.0), and the detection limits of AA and DA are 4.71 μM and 1.81 μM severally. Moreover, C3F7-azo+-Nb3O8/GCE has a long-term stability and good anti-interference capability.

Dynamic nanosurface reconfiguration by host-guest supramolecular interactions.

The dynamic functionalization of the nanoparticle surface with biocompatible coatings is a critical step towards the development of functional nano-sized systems. While covalent approaches have been broadly exploited in the stabilization of nanoparticle colloidal systems, these strategies hinder the dynamic nanosurface chemical reconfiguration. Supramolecular strategies based on specific host-guest interactions hold promise due to their intrinsic reversibility, self-healing capabilities and modularity. Host/guest couples have recently been implemented in nanoparticle platforms for the exchange and release of effector molecules. However, the direct exchange of biocompatible hydrophilic oligomers (e.g. peptides) for the modulation of the surface charge and chemical properties of nanoparticles still remains a challenge. Here, we show the intracellular reconfiguration of nanoparticles by a host/guest mechanism with biocompatible oligomeric competitors. The surface of gold nanoparticles was functionalized with cyclodextrin hosts and the guest exchange was studied with biocompatible mono and divalent adamantyl competitors. The systematic characterization of the size and surface potential of the host/guest nanoparticles allowed the optimization of the binding and the stabilization properties of these supramolecular systems. The in cellulo host/guest-mediated direct reconfiguration of the peptide layer at the surface of nanoparticles is achieved by controlling the valence of adamantane-equipped peptides. This work demonstrates that host/guest supramolecular systems can be exploited for the direct exchange of pendants at the surface of nanoparticles and the intracellular dynamic chemical reconfiguration of biocompatible colloidal systems.

A Ni(ii) metal–organic framework with helical channels for the capture of iodine via guest exchange induced amorphization

A Ni(ii) MOF based on a imidazole–tetrazole heterotopic tripodal ligand was constructed. It exhibits abilities for the stable capture of iodine molecules present in cyclohexane, water, and vapor, via amorphization induced by guest exchange.

A combination of a graphene quantum dots-cationic red dye donor-acceptor pair and cucurbit[7]uril as a supramolecular sensor for ultrasensitive detection of cancer biomarkers spermine and spermidine.

In a unique approach, the combination of a donor-acceptor pair of hydroxy graphene quantum dots (GQDs-OH) and a red-emissive donor-two-acceptor (D-2-A) type dye with pyridinium units (BPBP) and the well-known host cucurbit[7]uril (CB[7]) has been exploited as a supramolecular sensing assembly for the detection of cancer biomarkers spermine and spermidine in aqueous media at the sub-ppb level based on the affinity-driven exchange of guests from the CB[7] portal. In the binary conjugate, green fluorescent GQDs-OH transfers energy to trigger the emission of the dye BPBP and itself remains in the turn-off state. CB[7] withdraws the dye from the surface of GQDs-OH by strong host-guest interactions with its portal, making GQDs-OH fluoresce again to produce a ratiometric response. In the presence of spermine (SP) or spermidine (SPD), their strong affinity with CB[7] forces the ejection of the fluorophore to settle on the GQDs-OH surface, and the strong green emission of GQDs-OH turns off to device a supramolecular sensor for the detection of SP/SPD. The DFT studies revealed interesting excited-state charge-transfer conjugate formation between BPBP and GQDs leading to turn-on emission of the dye, and further supported the stronger binding modes of BPBP-CB[7], indicating the retrieval of the emission of GQDs. The assembly-disassembly based sensing mechanism was also established by Job's plot analysis, particle size analysis, zeta potential, time-resolved spectroscopy, ITC studies, microscopic studies, etc. The supramolecular sensing assembly is highly selective to SP and SPD, and showed nominal interference from other biogenic amines, amino acids, various metal ions, and anions. The limits of detection (LODs) were 0.1 ppb and 0.9 ppb for spermine and spermidine, respectively. The potential for the real-world application of this sensing assembly was demonstrated by spiking SP and SPD in human urine and blood serum with a high %recovery.

Subtle metal(ii) effects of 2D coordination networks on SCSC guest exchange

The multi-channel crystals consisting of 2-D networks G@[M(NO3)2L] are an unusually efficient, tolerant, and reproducible matrix offering M-dependent adsorption/desorption of various guest molecules in the single-crystal-to-single-crystal mode.

Molecular Guest Exchange and Subsequent Structural Transformation in Ch4 – Co2 Replacement Occurring in Sh Hydrates as Revealed by 13c Nmr Spectroscopy and Molecular Dynamic Simulations

Molecular Guest Exchange and Subsequent Structural Transformation in Ch4 – Co2 Replacement Occurring in Sh Hydrates as Revealed by 13c Nmr Spectroscopy and Molecular Dynamic Simulations

An elastic metal-organic crystal with a densely catenated backbone.

What particular mechanical properties can be expected for materials composed of interlocked backbones has been a long-standing issue in materials science since the first reports on polycatenane and polyrotaxane in the 1970s1-3. Here we report a three-dimensional porous metal-organic crystal, which is exceptional in that its warps and wefts are connected only by catenation. This porous crystal is composed of a tetragonal lattice and dynamically changes its geometry upon guest molecule release, uptake and exchange, and also upon temperature variation even in a low temperature range. We indented4 the crystal along its a/b axes and obtained the Young's moduli of 1.77 ± 0.16 GPa in N,N-dimethylformamide and 1.63 ± 0.13 GPa in tetrahydrofuran, which are the lowest among those reported so far for porous metal-organic crystals5. To our surprise, hydrostatic compression showed that this elastic porous crystal was the most deformable along its c axis, where 5% contraction occurred without structural deterioration upon compression up to 0.88 GPa. The crystal structure obtained at 0.46 GPa showed that the catenated macrocycles move translationally upon contraction. We anticipate our mechanically interlocked molecule-based design to be a starting point for the development of porous materials with exotic mechanical properties. For example, squeezable porous crystals that may address an essential difficulty in realizing both high abilities of guest uptake and release are on the horizon.