Thermal-induced dynamic self-assembly of adenine-grafted polyoxometalate complexes

Interfacial Preparation of Polyoxometalate-Based Hybrid Supramolecular Polymers by Orthogonal Self-Assembly

The construction of organic-inorganic hybrid supramolecular polymers using polyoxometalate (POM) as building block is expected to bring new opportunities to the functionalization of supramolecular polymers and the development of novel POM-based soft materials. Here, by using the orthogonal self-assembly based on host-guest interactions and metal-ligand interactions, we report the in situ construction of a novel POM-based hybrid supramolecular polymer (POM-SP) at the oil-water interface, while the redox and competitive responsiveness can be triggered independently. Moreover, the binding energy of POM-SP at the interface is sufficiently strong so that the assembly of POM-SP jams, allowing the stabilization of liquids in nonequilibrium shapes, offering the possibility of fabricating all-liquid constructs with reconfigurability.

The recent progress of synergistic supramolecular polymers: preparation, properties and applications

Interactions for forming supramolecular polymers were reviewed together with their unique properties and applications with detailed examples.

Synthesis of dynamic imine macrocyclic supramolecular polymers via synchronized self-assembly based on dynamic covalent bonds and noncovalent interactions

The preparation of dynamic imine macrocycles and supramolecular polymers is combined into a single step to form supramolecular polymers (SPs). 1,4-Diazabicyclo[2.2.2]octane (DABCO) derived quaternary ammonium salts induce aldehyde and amine building blocks to covalently form imine macrocycles. Multiple noncovalent interactions between hosts (i.e., imine macrocycle) and guests (i.e., DABCO) act as driving forces. Thus, for the first time, dynamic imine macrocyclic supramolecular polymers (DIMPs) have been achieved through the synchronized self-assembly of dynamic covalent bond formed imine macrocycles and noncovalent interactions of hosts-guests.

Detection of organophosphorus pesticides with liquid crystals supported on the surface deposited with polyoxometalate-based acetylcholinesterase-responsive supramolecular spheres

Here, we demonstrate use of acetylcholinesterase (AChE)-responsive polyoxometalate (POM)/surfactant supramolecular spheres to build a liquid crystal (LC)-based sensing platform for detection of organophosphorus pesticides. The self-assembled spheres are composed of hybrid materials of a POM, sodium dodecatungstophosphate (PW₁₂), and a surfactant, myristoylcholine (Myr). It displays dark appearance when the aqueous solution is in contact with LCs supported on the octadecyltrichlorosilane-treated glass deposited with the supramolecular spheres, suggesting perpendicular orientation of LCs at the aqueous/LC interface. In contrast, LCs show bright appearance when the surface-deposited supramolecular spheres are enzymatically hydrolyzed by AChE, corresponding to planar orientation of LCs at the aqueous/LC interface. Detection of organophosphates are successfully achieved as they are potent inhibitors of AChE. The detection limit of the sensing platform reached 0.9 ng/mL for dimethoate. This method can avoid disturbance of external interference with excellent specificity and sensitivity, which makes it very promise in detection of organophosphorus pesticides.

Hybrid polyoxometalates as post-functionalization platforms: from fundamentals to emerging applications

Polyoxometalates (POMs) represent an important group of metal-oxo nanoclusters, typically comprised of early transition metals in high oxidation states (mainly V, Mo and W). Many plenary POMs exhibit good pH, solvent, thermal and redox stability, which makes them attractive components for the design of covalently integrated hybrid organic-inorganic molecules, herein referred to as hybrid-POMs. Until now, thousands of organic hybrid-POMs have been reported; however, only a small fraction can be further functionalized using other organic molecules or metal cations. This emerging class of 'post-functionalizable' hybrid-POMs constitute a valuable modular platform that permits coupling of POM properties with different organic and metal cation functionalities, thereby expanding the key physicochemical properties that are relevant for application in (photo)catalysis, bioinorganic chemistry and materials science. The post-functionalizable hybrid-POM platforms offer an opportunity to covalently link multi-electron redox responsive POM cores with virtually any (bio)organic molecule or metal cation, generating a wide range of materials with tailored properties. Over the past few years, these materials have been showcased in the preparation of framework materials, functional surfaces, surfactants, homogeneous and heterogeneous catalysts and light harvesting materials, among others. This review article provides an overview on the state of the art in POM post-functionalization and highlights the key design and structural features that permit the discovery of new hybrid-POM platforms. In doing so, we aim to make the subject more comprehensible, both for chemists and for scientists with different materials science backgrounds interested in the applications of hybrid (POM) materials. The review article goes beyond the realms of polyoxometalate chemistry and encompasses emerging research domains such as reticular materials, surfactants, surface functionalization, light harvesting materials, non-linear optics, charge storing materials, and homogeneous acid-base catalysis among others.

Cyclodextrin-/photoisomerization-modulated assembly and disassembly of an azobenzene-grafted polyoxometalate cluster

Herein, a mono-lacunary Keggin-type polyoxometalate (POM), [SiW11O39]8-, grafted with an azobenzene group through Sn ion bridging was prepared, and the formed organic-inorganic hybrid cluster was characterized via elemental analysis, NMR, TGA, and IR techniques. A vesicular structure of the hybrid cluster assembly in aqueous media was observed in the TEM image, and it dissociated in the presence of α-/β-, γ-cyclodextrins (α-/β-, γ-CDs); this dissociation was driven by the host-guest interactions. The monodispersed inclusion complex further reassembled into smaller micelles under irradiation with 365 nm light, and this transformation was reversibly controlled by alternating the irradiation with 450 nm light. Moreover, in the case of the POM-Azo/β-CD system, reassembly from the monodispersed state to the vesicular state was achieved by the addition of a competitive guest molecule. Thus, the reversible host-guest interactions combining reversible photoisomerization of the azobenzene group provided multiple ways to modulate the assembly and disassembly of the POM hybrid as well as the changes between different assemblies. The present study inspires the potential use of these kind of hybrid POMs in enhanced catalytic reactions and recycling.

Photoluminescence Properties of Two Closely Related Isostructural Series Based on Anderson-Evans Cluster Coordinated With Lanthanides [Ln(H2O)7{X(OH)6Mo6O18}]•yH2O, X = Al, Cr

The paper describes synthesis and structural characterization of the whole series of two closely related lanthanide coordinated chromium or aluminum hexamolybdates (Anderson-Evans cluster) including twelve new members hitherto unreported: [Ln(H₂O)₇{X(OH)₆Mo₆O₁₈}]·4H₂O and [Ln(H₂O)₇{X(OH)₆Mo₆O₁₈}Ln(H₂O)₇]{X(OH)₆Mo₆O₁₈}·16H₂O where X = Al or Cr and Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, and Y. Crystal structures of all the solids were established by powder and single crystal X-ray diffraction techniques. The two series are dictated by a different aggregation of the same set of molecular species: Lighter lanthanides favor coordination interaction between lanthanide ions and molybdate cluster forming 1D chains (Series I) while the heavier lanthanides result in the stacking of a cation, a pair of lanthanide hydrates coordinating to the cluster, and an anion, the discrete cluster is further stabilized through a large number of water molecules (Series II). Crystallization with Er³⁺ and Tm³⁺ ions results in a concomitant mixture of Series I and II. Photoluminescence of single crystals of all the chromium molybdates was dominated by a ruby-like emission including those which contain optically active ions Pr, Sm, Eu, Tb, Dy, and Tm. In contrast, aluminum analogs showed photoluminescence corresponding to characteristic lanthanide emissions. Our results strongly suggest a possible energy transfer from f levels of lanthanide ions to d levels of chromium (III) causing the quenching of lanthanide emission when coordinated with chromium molybdates. Intensity measurements showed that the emission from chromium molybdates are almost two orders of magnitude lower than naturally occurring ruby with broader line widths at room temperature.

A liquid crystal based method for detection of urease activity and heavy metal ions by using stimulus-responsive surfactant-encapsulated phosphotungstate clusters

A liquid crystal (LC) based method is described for the sensitive determination of the activity of urease and of heavy metal ions which acts as inhibitors. Stimulus-responsive surfactant-encapsulated phosphotungstate clusters (SECs) were fabricated and deposited onto octadecyltrichlorosilane-coated glass. A copper TEM grid filled with LCs was placed on the substrate to construct the LC optical cell. Upon addition of water to the LC interface, the optical appearance of LCs on the glass undergoes a bright-to-dark shift due to an orientational transition of the LCs from a planar to a homeotropic state. However, the LCs display a bright appearance if they are pretreated with an aqueous solution containing urea and urease. This is caused by the disassemby of the SECs from the glass surface due to an increase of the pH value that is induced by the enzymatic hydrolysis of urea by urease. The method is highly sensitive and can detect urease activities as low as 0.03 mU/mL. It can also be applied to the determination of heavy metal ions which exert an inhibitory effect on the activity of urease. For example, Cu(II) can be quantified via urease inhibition in 1 nM concentration. Graphical abstract Schematic presentation of a liquid crystal-based sensor for detection of urease and heavy metal ions by using stimulus-responsive surfactant-encapsulated phosphotungstate clusters.

Solvent Dielectricity-Modulated Helical Assembly and Morphologic Transformation of Achiral Surfactant-Inorganic Cluster Ionic Complexes

Ionic complexes comprising single/double chain cationic surfactant and Lindqvist-type polyoxomolybdate anionic cluster were used for controlled self-assembly in organic solutions. In the solvent with low dielectric constant the complexes self-assembled into flat ribbon like lamellar aggregations with an inverse bilayer substructure where the cluster located at the middle. Under the condition of increased dielectric constant, the solvent triggered the formation of helical self-assemblies, which finally transformed from helical ribbons to the flower-like assemblies due to the bilayer becoming excessively twisted. The self-assembled morphology and the substructure were characterized by SEM, TEM, and XRD. The solvent dielectricity-controlled morphologic transformations modulated by the variation of electrostatic interactions between organic cations and inorganic polyanions were demonstrated by ¹H NMR and IR spectra. The strategy in this work represents an effective route in targeting the chirality-directed functionalization of inorganic clusters by combining controllable and helical assemblies of achiral polyoxometalate complexes in one system.

Thermal stability of mesoscopic compounds of cetyltrimethylammonium and Keggin metatungstates

A hybrid surfactant/polyoxometalate compound was synthesized by combining isopolytungstate anions with the cationic surfactant cetyltrimethylammonium bromide (CTA-Br) to produce a hierarchical compound that we identify as (CTA)₇[H₂W₁₂O₄₀]Cl·2H₂O. At room temperature the compound consisted of hexagonally ordered sheets of Keggin ions, with an intervening gallery containing alkyl-chains of the organic cations. The synthesis was highly dependent on solution pH, reaction time and the order in which the reactants were added. We examined the effect of temperature on the stability of (CTA)₇[H₂W₁₂O₄₀]Cl·2H₂O using thermal gravimetric analysis, differential scanning calorimetry, FT-IR spectroscopy and in situ synchrotron X-ray diffraction, and found a step-wise conversion to monoclinic WO_xvia a series of intermediates. Heating under nitrogen atmospheres accelerated transition events by ∼100 °C when compared to heating in air. During heating, the interplanar gallery at first expanded in a series of steps starting at 90 °C as the CTA⁺ amphiphiles changed orientation, before collapsing rapidly at 240 °C, a temperature coinciding with the removal of about 40% of the organic material. Between 240 and 320 °C, the material consisted of fragments of the Keggin ion cores, arranged in 2D hexagonally-packed sheets. At ∼330 °C, the Keggin ions were completely destroyed and replaced by bulk W₁₇O₄₇ which, upon further heating, transformed to bulk WO₂ or WO₃ depending on the environment.

Heterogeneous oxidative desulfurization of diesel fuel catalyzed by mesoporous polyoxometallate-based polymeric hybrid

In this work, the simple preparation of novel polymer supported polyoxometallates (POMs) catalysts has been reported. Soluble task-specific cross-linked poly (ionic liquid) (PIL) was prepared with N,​N-​dimethyl-​dodecyl-​(4-​vinylbenzyl) ammonium chloride and divinylbenzene as co-monomers. The as-prepared cationic PILs were assembled with different commercial POMs to form the interlinked mesoporous catalysts, and the formation mechanism was provided. The catalytic oxidation activities of the catalysts were closely related to the formation pathway of their corresponding peroxide active species. The catalyst with H₂W₁₂O₄₂^10- as counterion, which exhibited the best activity in the oxidation of benzothiophene (BT) and dibenzothiophene (DBT) to sulfones in model oil with hydrogen peroxide (H₂O₂, 30wt%) as oxidant, was characterized by different techniques and systematically studied for its sulfur removal performance. As for the oxidative desulfurization of a real diesel, it was observed that almost all of the original sulfur compounds could be completely converted, and the catalyst could be reused for at least eight cycles without noticeable changes in both catalytic activity and chemical structure. In the end, a catalytic mechanism was put forward with the assistant of Raman analysis.

Self-assembly of the polyoxometalate–cholesterol conjugate into microrods or nanoribbons regulated by thermodynamics

The self-assembly of the polyoxometalate–cholesterol conjugate can be well controlled and regulated by means of a temperature-mediated approach.

Structure- and solvent-triggered influences in the self-assembly of polyoxometalate–steroid conjugates

The supramolecular structures formed by polyoxometalate–steroid conjugates can be greatly influenced by molecular structures and solution components.

A high fatigue resistant, photoswitchable fluorescent spiropyran–polyoxometalate–BODIPY single-molecule

In the reported spiropyran/polyoxometalate/BODIPY triad, the fluorescence of the BODIPY can be highly reversibly quenched by the activation of the photochromic spiropyran.

Metallo/clusto hybridized supramolecular polymers

The introduction of metal centers to a supramolecular polymer system is an important approach to fabricate hybrid supramolecular polymers with synergistic properties between their inorganic and organic components, which is mainly realized through two strategies: one is the embedment of metal ions through metal-ligand coordination to form metallo-supramolecular polymers (MSPs); the other is using metal-containing clusters as hybrid building blocks to prepare clusto-supramolecular polymers (CSPs). The available paradigms of MSPs and CSPs not only exhibit the unique functions of metal centers but also hold the good processing ability and the stimuli-responsibility of dynamically bonded polymeric structures, thus representing a new class of hybrid soft materials. In this review, the development and recent progress of MSPs and CSPs are discussed in detail, including their structure design, synthetic procedures and related properties. Finally, challenges and potential areas in metal-containing supramolecular polymers are outlooked.

Hybrid surfactant systems with inorganic constituents

Surfactants are molecules of enormous scientific and technological importance, which are widely used as detergents, emulsifiers, and for the preparation of diverse nanostructures. Their fascinating ability to form self-organized structures, such as micelles or liquid crystals, originate from their amphiphilic architecture-a polar head group linked to a hydrophobic chain. While almost all known surfactants are organic, a new family of surfactants is now emerging, which combines amphiphilic properties with the advanced functionality of transition-metal building blocks, for example, redox or catalytic activity and magnetism. These hybrid surfactants exhibit novel self-organization features because of the unique size and electronic properties of the metal-containing entities.

Charge and pressure-tuned surface patterning of surfactant-encapsulated polyoxometalate complexes at the air-water interface

In this paper, four organic-inorganic hybrid complexes were prepared using a cationic surfactant dimethyldioctadecylammonium (DODA) to replace the counter cations of four Keggin-type polyoxometalate (POM) clusters with gradually increased negative charges, PW(12)O(40)(3-), SiW(12)O(40)(4-), BW(12)O(40)(5-), and CoW(12)O(40)(6-). The formed surfactant-encapsulated POM (SEP) complexes showed typical amphiphilic properties and can be spread onto the air-water interface to form Langmuir monolayers. The interfacial behavior of the SEP monolayer films was systemically studied by multiple in situ and ex situ characterization methods including Brewster angle microscopy (BAM), atomic force microscopy (AFM), reflection-absorption infrared (RAIR), and X-ray photoelectron spectroscopy (XPS). We found that the increasing alkyl chain density of SEPs leads to an enhanced stability and a higher collapse pressure of SEP Langmuir monolayers. Moreover, a second layer evolved as patterns from the initial monolayers of all the SEPs, when the surface pressures approached the collapse values. The rational combination of alkyl chain density and surface pressure can precisely control the size and the morphology of SEP patterns transforming from disk-like to leaf-like structures on a micrometer scale. The pattern formation was demonstrated to be driven by the self-optimized surface energy of SEP monolayers. This finding can direct a new strategy for the fabrication of POM-hybrid films with controllable patterns, which should be instructive for designing POM-based thin film devices.