Release Of Guest Molecules Research Articles

A Reversibly Photoswitchable Zinc Metallocycle Based on Azobenzene

Photoswitchable metallocycles and cages are of special interest for a photocontrolled uptake and/or release of guest molecules. While most compounds undergo a disassembly‐re‐assembly mechanism among photoisomerization, examples for reversibly switchable compounds without decomposition are extremely rare. Furthermore, most metallocycles and cages are based on 4d and 5d metal ions that form kinetically inert bonds. We here present a dinuclear Zn(II) metallocycle that despite the usually high ligand exchange rate of Zn(II) reversibly undergoes photoisomerization without disassembly. Therefore, we prepared the novel ligand 3,3’‐Azobenz(BPA)2 (1) that is based on an azobenzene scaffold equipped with two benzylpicolylamine (BPA) moieties for metal complexation. With this ligand, the Zn(II) complexes [{ZnCl2}2(1)] (2) and the cyclic 2:2 complex [Zn2(1)2](BF4)4 (3) were prepared and characterized. The photophysical switching properties of all compounds have been assessed using NMR and UV/Vis spectroscopy as well as quantum chemical calculations by density functional theory (DFT) at the CAM‐B3LYP/D4/def2‐TZVPP level. These studies show the highly quantitative and reversible photoinduced cyclic E/Z isomerization of 1 – 3. To the best of our knowledge, 3 thus represents the only second metallocycle based on azobenzene and first one using Zn(II), which reversibly undergoes photoisomerization.

Visible Light-Responsive Crystalline B←N Host Adducts with Solvent-Induced Allosteric Effect for Guest Release.

Photo-responsive organic crystals, capable of converting light energy into chemical energy to initiate conformational transitions, present an emerging strategy for developing lightweight and versatile smart materials. However, visible light-triggered tailored guests capture and release behaviors in all-organic solids are rarely reported. Here, we introduce a photoreactive crystalline boron-nitrogen (B←N) host adduct with the ability to undergo [2+2] photocycloaddition upon 447 nm light exposure. This process facilitates single-crystal-to-single-crystal (SCSC) photodimerization in the mother liquor, maintaining the original B←N host structure. Weakened intermolecular interactions within the photodimer host contribute to fast guest release in air under irradiation. Furthermore, the dynamic B←N bonds enable reversible transformations between organic host adducts and adduct cocrystals under the solvent-induced allosteric effect. As a result, four B←N host adduct crystals containing individual alkane guest are easily obtained and exhibited the ability of photo-controlled alkane release. Therefore, the integration of photo reactivity and structural transformation within B←N host adduct enables customized capture and release of guest molecules.

Visible Light‐Responsive Crystalline B←N Host Adducts with Solvent‐Induced Allosteric Effect for Guest Release

AbstractPhoto‐responsive organic crystals, capable of converting light energy into chemical energy to initiate conformational transitions, present an emerging strategy for developing lightweight and versatile smart materials. However, visible light‐triggered tailored guests capture and release behaviors in all‐organic solids are rarely reported. Here, we introduce a photoreactive crystalline boron‐nitrogen (B←N) host adduct with the ability to undergo [2+2] photocycloaddition upon 447 nm light exposure. This process facilitates single‐crystal‐to‐single‐crystal (SCSC) photodimerization in the mother liquor, maintaining the original B←N host structure. Weakened intermolecular interactions within the photodimer host contribute to fast guest release in air under irradiation. Furthermore, the dynamic B←N bonds enable reversible transformations between organic host adducts and adduct cocrystals under the solvent‐induced allosteric effect. As a result, four B←N host adduct crystals containing individual alkane guest are easily obtained and exhibited the ability of photo‐controlled alkane release. Therefore, the integration of photo reactivity and structural transformation within B←N host adduct enables customized capture and release of guest molecules.

The Application of Bilayer Heterogeneous MOFs in pH and Heat-Triggered Systems for Controllable Fragrance Release.

To facilitate the integration of a fragrance encapsulation system into different products to achieve effective releases, a dual-responsive release system with pH and thermal trigger control is designed in this work. A series of ZIF-8 (M) and bilayer ZIF-8-on-ZIF-8 (MM) materials are synthesized by a solvent method at room temperature. The fragrance is encapsulated into the ZIFs by dynamic adsorption or in situ encapsulation combined dynamic adsorption. The fragrance loading contributed by dynamic adsorption was as high as 80%. The fragrance loaded in the double-layer MM host was almost twice that of the monolayer host M due to the stronger electrostatic interaction between MM and vanillin. In the pH and thermal trigger response release experiments, the second MOF layer in the MM host, as a controlled entity, greatly improved the load and kinetic equilibrium time of vanillin, and realized the controlled release of guest molecules. The developed dual-responsive release system in this work exhibits great potential in daily chemical products.

Monte Carlo simulation of the ionization and uptake behavior of cationic oligomers into pH-responsive polyelectrolyte microgels of opposite charge - a model for oligopeptide uptake and release.

External stimuli can tune the uptake and release of guest molecules in microgels. Especially their pH responsiveness makes microgels exciting candidates for drug delivery systems. When both microgel and guest molecules are pH-responsive, predicting the electrostatically driven uptake can be complex since the ionization depends on many parameters. In this work, we performed Metropolis Monte Carlo simulations while systematically varying the pK of the monomers, the concentrations of microgel and guest molecules to obtain a better understanding of the uptake of weak cationic oligomers as a model for oligopeptides into a weak anionic polyelectrolyte microgel. Further, we varied the chain length of the oligomers. The polyelectrolyte networks can take up oligomers when both the network and the oligomers are charged. The presence of both species in the system leads to a mutual enhancement of their ionization. The uptake induces a release of counterions and results in complex formation between the oligomers and the network, leading to the collapse of the networks. Longer oligomers enhance the ionization of the network and, therefore, the complexation. A higher microgel concentration increases the uptake only around the isoelectric point but prevents the uptake due to lower entropy gain at counterion release at higher pH. The results give an insight into the uptake of cationic oligomers into oppositely charged polyelectrolyte microgels and provide hints for the design of anionic microgels as carriers for guest molecules e.g. antimicrobial peptides.

Computational Analysis of Vibrational Spectra of Hydrogen Bonds in sII and sH Gas Hydrates.

The amount of energy in natural gas hydrates is thought to be equivalent to twice that of all other fossil fuels combined. However, economic and safe energy recovery has remained a challenge till now. To develop a novel method of breaking the hydrogen bonds (HBs) surrounding the trapped gas molecules, we investigated the vibrational spectra of the HBs of gas hydrates with structure types II and H. Two models of 576-atom propane-methane sII hydrate and 294-atom neohexane-methane sH hydrate were built. A first-principles density functional theory (DFT) method was employed using the CASTEP package. The simulated spectra were in good agreement with the experimental data. Compared with the partial phonon density of states of guest molecules, we confirmed that the experimental infrared absorption peak in the terahertz region mainly arose from HB vibrations. By removing the components of guest molecules, we found that the theory of two kinds of hydrogen bond vibrational modes applies. The use of a terahertz laser to enable resonance absorption of HBs (at about 6 THz, to be tested) may therefore lead to the rapid melting of clathrate ice and release of guest molecules.

Electric field induced release of guest molecules from clathrate hydrates and its consequences for electrochemical CO2 conversion

When CO2 is encapsulated in clathrate hydrate, where the concentration is nearly-two orders of magnitude greater than in saturated aqueous solution, the rate of the CO2 reduction reaction (CO2RR) increases, and the corresponding Faradaic efficiency becomes up to three times higher than that of the competing hydrogen evolution reaction (HER) at applied potential of −0.5 to −0.7 vs RHE. The energy efficiency of the CO2RR is correspondingly increased. The enhanced CO2RR in clathrate is ascribed to non-equilibrium release of the CO2 due to the electric field near the electrode, analogous to what has been observed recently for tetrahydrofuran [Li et al. J. Phys. Chem. Letters 125, 13802 (2021)]. This raises the chemical potential of CO2 beyond that of saturated aqueous solution, thereby reversing the relative rates of the CO2RR and the HER. The experimental demonstration of this release mechanism for the application of CO2 up-conversion illustrates how the electric field can not only influence electrochemical reactions at the surface but also the reactant supply from clathrate hydrates to the surface. Experimental focus rests on the low applied-potential regime, where the consequences of the reactant release mechanism are well-observable unobscured by mass-transport effects.

Unexpected Factors Affecting the Kinetics of Guest Molecule Release from Investigation of Binary Chemical Systems Trapped in a Single Void of Mesoporous Silica Particles.

In this work, we present results for loading of well-defined binary systems (cocrystal, solid solution) and untreated materials (physical mixtures) into the voids of MCM-41 mesoporous silica particles employing three different filling methods. The applied techniques belong to the group of "wet methods" (diffusion supported loading - DiSupLo) and "solvent-free methods" (mechanical ball-mill loading - MeLo, thermal solvent free - TSF). As probes for testing the guest1-guest2 interactions inside the MCM-41 pores we employed the benzoic acid (BA), perfluorobenzoic acid (PFBA), and 4-fluorobenzoic acid (4-FBA). The guests intermolecular contacts and phase changes were monitored employing magic angle spinning (MAS) NMR Spectroscopy techniques and powder X-ray diffraction (PXRD). Since mesoporous silica materials are commonly used in drug delivery system research, special attention has been paid to factors affecting guest release kinetics. It has been proven that not only the content and composition of binary systems, but also the loading technique have a strong impact on the rate of guests release. Innovative methods of visualizing differences in release kinetics are presented.

PH-Sensitive Polyacrylic Acid-Gated Mesoporous Silica Nanocarrier Incorporated with Calcium Ions for Controlled Drug Release

In this work, polyacrylic acid-functionalized MCM-41 was synthesized, which was made to interact with calcium ions, in order to realize enhanced pH-responsive nanocarriers for sustained drug release. First, mesoporous silica nanoparticles (MSNs) were prepared by the sol-gel method. Afterward, a (3-trimethoxysilyl)propyl methacrylate (TMSPM) modified surface was prepared by using the post-grafting method, and then the polymerization of the acrylic acid was performed. After adding a calcium chloride solution, polyacrylic acid-functionalized MSNs with calcium-carboxyl ionic bonds in the polymeric layer, which can prevent the cargo from leaking out of the mesopore, were prepared. The structure and morphology of the modified nanoparticles (PAA-MSNs) were characterized by X-ray diffraction (XRD), Fourier-transform infrared (FT-IR) spectroscopy, transmission electron microscopy (TEM), and N2 adsorption–desorption analysis, etc. The controlled release of guest molecules was studied by using 5-fluorouracil (5-FU). The drug molecule-incorporated nanoparticles showed different releasing rates under different pH conditions. It is considered that our current materials have the potential as pH-responsive nanocarriers in the field of medical treatment.

Fluorescence Resonance Energy Transfer Measurements in Polymer Science: A Review.

Fluorescence resonance energy transfer (FRET) is a non-invasive characterization method for studying molecular structures and dynamics, providing high spatial resolution at nanometer scale. Over the past decades, FRET-based measurements are developed and widely implemented in synthetic polymer systems for understanding and detecting a variety of nanoscale phenomena, enabling significant advances in polymer science. In this review, the basic principles of fluorescence and FRET are briefly discussed. Several representative research areas are highlighted, where FRET spectroscopy and imaging can be employed to reveal polymer morphology and kinetics. These examples include understanding polymer micelle formation and stability, detecting guest molecule release from polymer host, characterizing supramolecular assembly, imaging composite interfaces, and determining polymer chain conformations and their diffusion kinetics. Finally, a perspective on the opportunities of FRET-based measurements is provided for further allowing their greater contributions in this exciting area.

Supramolecularly cross-linked nanoassemblies of self-immolative polyurethane from recycled plastic waste: high encapsulation stability and the triggered release of guest molecules

Recycled plastic waste based self-immolative polyurethane has been synthesized and nanoformulated for tumor relevant pH induced charge modulation, efficient guest encapsulation and triggered release in a controlled fashion.

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.

Coordination Cages Selectively Transport Molecular Cargoes Across Liquid Membranes.

Chemical purifications are critical processes across many industries, requiring 10–15% of humanity’s global energy budget. Coordination cages are able to catch and release guest molecules based upon their size and shape, providing a new technological basis for achieving chemical separation. Here, we show that aqueous solutions of FeII4L6 and CoII4L4 cages can be used as liquid membranes. Selective transport of complex hydrocarbons across these membranes enabled the separation of target compounds from mixtures under ambient conditions. The kinetics of cage-mediated cargo transport are governed by guest binding affinity. Using sequential transport across two consecutive membranes, target compounds were isolated from a mixture in a size-selective fashion. The selectivities of both cages thus enabled a two-stage separation process to isolate a single compound from a mixture of physicochemically similar molecules.

Integrating redox-response in crown ethers by disulfide incorporation: a computational approach

Reducing properties are investigated for the minimum energy conformers of selected thiacrown systems with redox-responsive disulfide linkage. The adiabatic electron affinity values of the investigated host systems in the gas phase lie in a range that makes them capable to undergo reversible redox reactions. In water, these show a high propensity to get reduced. Their binding energy with different alkali metal ions (Li+, Na+, and K+) is also determined under gas and aqueous phase conditions. The computational analysis reveals the weak metal binding interactions of disulfide systems than the parent thiacrown ethers. Such weak interaction energies in host-guest complexes indicate the possibility of reversible complexation-decomplexation. This assumably assists the fast release of guest molecules from the disulfide systems in response to the redox stimuli. Possibilities of incorporating such disulfide incorporated moieties in thiacalixarenes are also discussed. The results discussed in the present work are valuable for further development in the practical applications of crown ether–based host materials.

Insights into the intraparticle morphology of dendritic mesoporous silica nanoparticles from electron tomographic reconstructions

HypothesisAlthough many synthetic pathways allow to fine-tune the morphology of dendritic mesoporous silica nanoparticles (DMSNs), the control of their particle size and mesopore diameter remains a challenge. Our study focuses on either increasing the mean particle size or adjusting the pore size distribution, changing only one parameter (particle or pore size) at a time. The dependence of key morphological features (porosity; pore shape and pore dimensions) on radial distance from the particle center has been investigated in detail. ExperimentsThree-dimensional reconstructions of the particles obtained by scanning transmission electron microscopy (STEM) tomography were adapted as geometrical models for the quantification of intraparticle morphologies by radial porosity and chord length distribution analyses. Structural properties of the different synthesized DMSNs have been complementary characterized using TEM, SEM, nitrogen physisorption, and dynamic light scattering. FindingsThe successful independent tuning of particle and pore sizes of the DMSNs could be confirmed by conventional analysis methods. Unique morphological features, which influence the uptake and release of guest molecules in biomedical applications, were uncovered from analyzing the STEM tomography-based reconstructions. It includes the quantification of structural hierarchy, identification of intrawall openings and pores, as well as the distinction of pore shapes (conical vs. cylindrical).

Host‐Guest Inclusion Complexes of Geraniol and Nerol with Acyclic Cucurbit[n]urils: Preparation, Characterization and Controlled Release

AbstractTwo main components of rose essential oil, geraniol and nerol are highly volatile, and hardly water‐soluble, which limit their further development and application in food and cosmetics industry. Host‐guest inclusion complexation could be a promising strategy to address these issues. Herein, new inclusion systems of geraniol and nerol with two acyclic cucurbit[n]urils (ACBs) were fabricated, and their binding behaviors both in solution and solid states were investigated by fluorescence spectroscopy, NMR, XRD, FT‐IR spectroscopy and TGA. The results confirmed the formation of host‐guest inclusion complexes. The stoichiometry of the complexes was proved to be 1 : 1 by Job's plot. The possible inclusion modes were proposed by 2D‐ROESY experiments. The pH‐dependent release of guest molecules from the inclusion complexes has been studied via UV‐Vis spectroscopy. The heat‐controlled release properties of guest molecules from the solid inclusion complexes were studied via 1H NMR spectra. The complexes featured less volatilization under low temperature, longer retention time, better water solubility and improved heat‐controlled release properties. In brief, this work uncloses a new strategy to enhance the performance of essential oils and flavors/fragrances and further their application in the field of cosmetic and food industry.

Control of Guest Thermal Release by Crystalline Host Orientation

Thermal release of guest molecules from poly(2,6-dimethyl-1,4-phenylene)oxide (PPO) films, exhibiting host–guest co-crystalline (CC) phases with orientation of their chain axes being preferentially...

Irreversible structural changes of recovered hydrogen hydrate transforming from C0 phase to ice XVII

• Neutron diffraction on the recovered high-pressure hydrate C 0 . • Crystalline structure transformation from the recovered C 0 hydrate to ice XVII. • Spurious guest contamination in recovered gas hydrates. • Thermal annealing treatment on the recovered filled ice. The present work, performed by means of ex-situ neutron diffraction measurements and combined with complementary Raman data, reports the first structural characterization of the recovered hydrogen hydrate during the transformation from the C 0 phase to ice XVII. The measurement of the position of the Bragg reflections in the neutron data allows to obtain the behaviour of the lattice parameters for the C 0 filled ice as a function of temperature during hydrogen release. From this analysis, we demonstrate the irreversibility of the structural changes passing from the pristine filled hydrate to the empty one, i.e. ice XVII. Meanwhile, we observe also the release of the spurious nitrogen molecules previously absorbed by the hydrate crystal when recovered at low temperature and ambient pressure. A significant release of guest molecules starts around 60 K, reaching the complete evacuation of the water structure above 100 K.

Catechin or quercetin guests in an intrinsically microporous polyamine (PIM-EA-TB) host: accumulation, reactivity, and release.

Microporous polymer materials based on molecularly “stiff” structures provide intrinsic microporosity, typical micropore sizes of 0.5 nm to 1.5 nm, and the ability to bind guest species. The polyamine PIM-EA-TB contains abundant tertiary amine sites to interact via hydrogen bonding to guest species in micropores. Here, quercetin and catechin are demonstrated to bind and accumulate into PIM-EA-TB. Voltammetric data suggest apparent Langmuirian binding constants for catechin of 550 (±50) × 103 M−1 in acidic solution at pH 2 (PIM-EA-TB is protonated) and 130 (±13) × 103 M−1 in neutral solution at pH 6 (PIM-EA-TB is not protonated). The binding capacity is typically 1 : 1 (guest : host polymer repeat unit), but higher loadings are readily achieved by host/guest co-deposition from tetrahydrofuran solution. In the rigid polymer environment, bound ortho-quinol guest species exhibit 2-electron 2-proton redox transformation to the corresponding quinones, but only in a thin mono-layer film close to the electrode surface. Release of guest molecules occurs depending on the level of loading and on the type of guest either spontaneously or with electrochemical stimuli.

In situ reversible tuning of chemical interface damping in single gold nanorod-based recyclable platforms through manipulation of supramolecular host-guest interactions.

Recently, chemical interface damping (CID) has been proposed as a new plasmon damping pathway based on interfacial hot-electron transfer from metal to adsorbate molecules. It has been considered essential, owing to its potential implications in efficient photochemical processes and sensing experiments. However, thus far, studies focusing on controlling CID in single gold nanoparticles have been very limited, and in situ reversible tuning has remained a considerable challenge. In these scanning electron microscopy-correlated dark-field spectroscopic measurements and density functional theory calculations, cucurbit[7]uril (CB[7])-based host–guest supramolecular interactions were employed to examine and control the CID process using monoamine-functionalized CB[7] (CB[7]-NH2) attached to single gold nanorods (AuNRs). In situ tuning of CID through the CB[7]–oxaliplatin complexation, which can result in the variation of the chemical nature and electronic properties of adsorbates, was presented. In addition, in situ tuning of CID was demonstrated through the competitive release of the oxaliplatin guest from the oxaliplatin@CB[7] complex, which was then replaced by a competitor guest of spermine in sufficient amounts. Furthermore, nuclear magnetic resonance experiments confirmed that the release of the guest is the consequence of adding salt (NaCl). Thus, in situ reversible tuning of CID in single AuNRs was achieved through successive steps of encapsulation and release of the guest on the same AuNR in a flow cell. Finally, single CB[7]-NH2@AuNRs were presented as a recyclable platform for CID investigations after the complete release of guest molecules from their host–guest inclusion complexes. Therefore, this study has paved a new route to achieve in situ reversible tuning of CID in the same AuNR and to investigate the CID process using CB-based host–guest chemistry with various guest molecules in single AuNRs for efficient hot-electron photochemistry and biosensing applications.