Surfactant-encapsulated Polyoxometalate Research Articles

Geometric Frustration and Long-Range Ordering Induced by Surface Pressure Oscillation in a Langmuir-Blodgett Monolayer of Magnetic Soft Spheres.

As a step toward the bottom-up construction of magnonic systems, this paper demonstrates the use of a large-amplitude surface-pressure annealing technique to generate 2-D order in a Langmuir-Blodgett monolayer of magnetic soft spheres comprising a surfactant-encapsulated polyoxometalate. The films show a distorted square lattice interpreted as due to geometric frustration caused by 2-D confinement between soft walls, one being the air interface and the other the aqueous subphase. Hysteresis and relaxation phenomena in the 2-D layers are suggested to be due to folding and time-dependent interpenetration of surfactant chains.

Surfactant-Encapsulated Polyoxometalate Complex as a Cathode Interlayer for Nonfullerene Polymer Solar Cells

Surfactant-Encapsulated Polyoxometalate Complex as a Cathode Interlayer for Nonfullerene Polymer Solar Cells

A polyoxometalate-based self-cleaning smart material with oxygenic activity for water remediation with membrane technology

An unprecedented solution for alleviating non-restorable fouling in water treatment membrane processes is herein proposed and described. A novel smart material with oxygenic activity was developed starting from a polymerizable bicontinuous microemulsion (PBM) containing a surfactant encapsulated polyoxometalate (SEP) as catalyst. This material (PBM-SEP) was used for the surface functionalization of commercial ultrafiltration (UF) membranes by allowing the polymerization of the SEP-containing PBM to take place after casting on membrane surface. This has allowed obtaining a composite membrane with self-cleaning properties toward non-restorable fouling, thanks to the oxygenic activity of the embedded catalyst, resulting from H2O2 dismutation. The newly developed smart material may find practical application to implement membrane technology for an efficient wastewater treatment.

Redox-Regulated Dynamic Self-Assembly of a Lindqvist-Type Polyoxometalate Complex

Dynamic regulation of self-assembly is of vital importance in chemistry, biology and material science thanks to its great potential for development of smart materials and devices. Polyoxometalates (POMs) are a class of functional inorganic nanoclusters, which has become one of the excellent building blocks for supramolecular self-assemblies, especially when covalently or non-covalently modified by organic species. As typical stimuli-responsive functional clusters, the POMs could be photochemically or electrochemically reduced to mixed-valence states, of which the structural integrity remains even after encountering stepwise multi-electron redox process. The intriguing photochromism of the POMs in different states exhibits distinct photophysical properties, which motivates us to exploit the dynamic self-assemblies of POM-based complexes. The divalent Lindqvist-type hexamolybdate cluster [Mo6O19]2- is one of the least negative-charged POMs, which is the ideal building blocks to construct novel assembly structures. Based on this motivation, herein, a single chain surfactant-encapsulated polyoxometalate (POM) complex (ODTA)2[Mo6O19] was prepared by simple counterion replacement of Lindqvist-type (TBA)2[Mo6O19] with octadecyltrimethylammonium (ODTA) in acetonitrile solution. The structure of the POM complex was confirmed by 1H nuclear magnetic resonance (NMR), Fourier transform infrared (FT-IR) spectroscopy, thermogravimetric analysis (TGA) and elemental analysis. The solution of complex (ODTA)2[Mo6O19] in the mixed solvents of acetonitrile and isopropanol with the volume ration of 4 to 1 exhibited reversible photochromism upon alternate UV light irradiation and air exposure. Upon UV light irradiation, the light yellow transparent solution of (ODTA)2[Mo6O19] turned into blue quickly. The new broad absorption band appearing at ca.751 nm assigned to the MoV → MoVI intervalence charge-transfer (IVCT) transition, indicated the formation of reduced POM, as revealed by UV-Vis absorption spectra. After exposed to air, the blue solution was bleached. The alternate photochromism could be conducted for multiple cycles. Helical self-assembled morphology of (ODTA)2[Mo6O19] was formed in acetonitrile/isopropanol, characterized by scanning electron microscope (SEM), transmission electron microscopy (TEM) and X-ray diffraction (XRD) methods. More interestingly, morphology transformation of the complex from helical strips to spherical assemblies occurred accompanied by photochromism occurrence. The morphology evolution during the photochromism process experienced from shortened helical strips through sea urchin-like aggregates to spherical assemblies. Most significantly, the helical assemblies could be recovered again after air oxidation, implying the reversible morphology transformation driven by redox stimulus. The redox-modulated reversible self-assembly is driven by the variation of electrostatic attraction between organic cations and inorganic anions as well as the electrostatic repulsion between inorganic ionic clusters, proved by X-ray photoelectron spectroscopy (XPS) and 1H NMR spectra. The results will contribute to better understanding the mechanism of dynamic assemblies and inspire the precise fabrication of advanced smart materials.

Phthalocyanine-modified surfactant-encapsulated polyoxometalate and its self-assembly in solution

Firstly, a novel functional double-chain cationic surfactant was designed and prepared in this study. Zinc phthalocyanine (ZnPc) was bonded onto the dodecyl (11-hydroxy-undecyl) dimethylammonium bromide (DHDA), obtaining the novel surfactant ZnPc-O-DHDA. Then, the surfactant-encapsulated polyoxometalate (POM) complex (ZnPc-O-DHDA/PW12) was prepared by using the double-phase co-precipitation method, through ion replacement with the Keggin-type polyoxometalate H3PW12O40 (PW12). The composition and structure of the complex were characterized using FT-IR, 1HNMR and elemental analysis (EA). ZnPc-O-DHDA has a good absorption and emission spectral performance in the visible region. When ZnPc-O-DHDA combined with PW12 to form a complex, and its Q-band fluorescence emission was enhanced because of the increased singlet–triplet energy gap of ZnPc. Thermogravimetric analysis (TGA) and EA data indicated the ratio of ZnPc-O-DHDA and PW12 in the complex was 2.8:1. ZnPc-O-DHDA/PW12 can self-assemble in DMSO/H2O (v:v = 4:1) solvent to form a spherical structure with a size of about 500 nm. The resulting aggregates could be successfully transferred into water to produce pellets of about 50 nm, while transferred into ethanol to produce small cubes of about 150 nm. The successful synthesis of this functional surfactant ZnPc-O-DHDA and its POM complex provide a new approach for the further design and preparation of functional POM aggregates.

Honeycomb-patterned hybrid films of surfactant-encapsulated polyoxometalates by a breath figure method and its electrocatalysis for BrO3.

In this paper, a sandwich type of polyoxophosphotungstate K7Na3[Cu4(H2O)2(PW9O34)2]·20H2O (abbr. Cu4(PW9)2) was selected as an active unit to construct inorganic/organic hybrid materials. The ordered honeycomb structure of the surfactant-encapsulated polyoxometalate (SEP) complex was successfully obtained by using a famous breath figure method. Cu4(PW9)2 maintains typical features as confirmed by IR spectra, magnetization hysteresis studies and UV-vis spectra. An investigation of the effects of the relative humidity on the morphology of the honeycomb structures demonstrated a wide generality and high reproducibility of the formation of self-organized honeycomb-patterned films. In the present experiment, this magnetically ordered honeycomb was successfully deposited on indium tin oxide (ITO) substrates by directly casting the SEP chloroform solution. The cyclic voltammetric responses showed the redox couples of Cu4(PW9)2. The electrochemical behavior of the electrode was studied. In addition, the modified electrode exhibited high electrocatalytic activity for the reduction of BrO3-, which offered a new POM-based film material for the electrochemical fields.

Adsorption, Ordering, and Local Environments of Surfactant-Encapsulated Polyoxometalate Ions Probed at the Air-Water Interface.

The continued development and application of surfactant-encapsulated polyoxometalates (SEPs) relies on understanding the ordering and organization of species at their interface and how these are impacted by the various local environments to which they are exposed. Here, we report on the equilibrium properties of two common SEPs adsorbed to the air-water interface and probed with surface-specific vibrational sum-frequency generation (SFG) spectroscopy. These results reveal clear shifts in vibrational band positions, the magnitude of which scales with the charge of the SEP core, which is indicative of a static field effect on the surfactant coating and the associated local chemical environment. This static field also induces ordering in surrounding water molecules that is mediated by charge screening via the surface-bound surfactants. From these SFG measurements, we are able to show that Mo132-based SEPs are more polar than Mo72V30 SEPs. Disorder in the surfactant chain packing at the highly curved SEP surfaces is attributed to large conic volumes that can be sampled without interactions with neighboring chains. Measurements of adsorption isotherms yield free energies of adsorption to the air-water interface of -46.8 ± 0.4 and -44.8 ± 1.2 kJ/mol for the Mo132 and Mo72V30 SEPs, respectively, indicating a strong propensity for the fluid surface. The influence of intermolecular interactions on the surface adsorption energies is discussed.

Rational design of photo-responsive supramolecular nanostructures based on an azobenzene-derived surfactant-encapsulated polyoxometalate complex

Using an ionic self-assembly (ISA) approach, photoresponsive surfactant-encapsulated polyoxometalate complexes (SECs) were fabricated using 4-ethyl-4′-(trimethylaminohexyloxy) azobenzene bromide (ETAB) and H3[PW12O40] in water.

Self-Assembling a Polyoxometalate–PEG Hybrid into a Nanoenhancer To Tailor PEG Properties

The unique performance of natural materials stems from their hierarchical hybrid structures formed through self-assembly. The self-assembly principles of natural materials have been exploited to create artificial materials. Herein, we demonstrate a bottom-up approach that produces polymer nanocomposites as well as a self-assembled nanoenhancer for tailoring the polymer properties. The polymer is a poly(ethylene glycol) (PEG), and the nanoenhancer is aggregates formed by self-assembly of a hybrid. The hybrid is prepared through covalent bonding of a surfactant-encapsulated polyoxometalate (S-POM) complex with a PEG chain and can form aggregates composed of an S-POM complex bilayer sandwiched by two PEG layers. The lateral size of aggregates changes, depending on the conditions used in the sample preparation. Hence, we examined four nanostructures in the solid samples of nanocomposites: hybrid self-assembled nanosheets, PEG crystallized lamellae, PEG/hybrid cocrystallized lamellae, and hybrid crystallized l...

Hybrid Liquid Crystals from the Self‐Assembly of Surfactant‐Encapsulated Polyoxometalate Complexes

AbstractThe construction of nano‐hybrid liquid crystals (LCs) has been intensively pursued because the unique traits of nano‐objects together with the self‐assembly features of LCs allow the development of novel anisotropic materials. Self‐assembly of surfactant‐encapsulated polyoxometalate (SEP) complexes, consisting of nano‐sized cores and organic shells, provides particularly important contribution in this area. The main motivation for developing those nano‐hybrid liquid crystals originates from the added‐value combination between anionic polyoxometalates and cationic surfactants. This account describes recent work in our group to develop thermotropic SEP complexes that result from the sophisticated molecular design. Since the polyoxometalates possess well‐defined topology, strictly mono‐dispersed size, and precise surface charges, it is possible to get more insight into understanding of the influence of nature of components on the thermal property of nano‐hybrid LCs. We briefly describe the general importance and advantage of nano‐hybrid LCs. We then highlight the synthesis and characterization of thermotropic liquid crystals based on polyoxometalate nano‐clusters. The driving forces behind the molecular self‐assembly are discussed in depth. The various factors, including chain length, surfactant density and the size‐matching effect, affecting the interfacial curvature and the LC properties are summarized. We expect that the structure‐property relationships are virtually helpful for the design of new nano‐hybrid LC materials.

Dye removal by surfactant encapsulated polyoxometalates

A novel surfactant encapsulated polyoxometalate (SEP) has been synthesized by using a simple ion-exchange reaction. The prepared SEP complex was found to self-assemble into nanospherical particles whose morphology and component were characterized by TEM and XPS. The SEP was further incorporated into polyvinylidene fluoride (PVDF) to fabricate SEP incorporated composite membrane (SEP-M). Both the SEP and SEP-M exhibited excellent dye removal activities, which is for the first time reported as an intriguing property of the SEP. A regeneration scheme for SEP-M was successfully proposed without any loss of dye removal efficiency. Detailed mechanism studies were carried out to elucidate the nature of dye decolorization. Ion exchange was revealed to play a dominant role in the dye removal process. The current research not only renders a new example for the simple and direct synthesis of SEP but more importantly provides an efficient dye removal methodology.

Multicomponent hybrids with surfactant-encapsulated europium polyoxometalate covalently bonded ZnO and tunable luminescence

In this paper, europium polyoxometalate (Na9EuW10O36·32H2O, abbreviated as POM) is encapsulated into the surfactant dodecyl(11-(methacryloyloxy)undecyl) dimethylammonium bromide (DMDA) to form surfactant-encapsulated polyoxometalate (SEP). Methacrylic group-modified ZnO nanoparticles (designated ZnO-MAA) are also prepared through a sol–gel reaction between zinc methacrylate and LiOH. Finally, the synthesized SEP, ZnO-MAA and three different ester units (ethyl methacrylate (EMA), 2-hydroxyethyl methacrylate (HEMA) and 2,2,3,4,4,4-hexafluorobutyl methacrylate (HFMA)) are covalently bonded through a copolymerization reaction in the presence of benzoyl peroxide (BPO). A detailed physical characterization and especially the photoluminescence of these hybrid materials are studied in detail. The results provide a method to assemble multicomponent hybrid materials with europium polyoxometalate, a semiconductor compound and polymer unit, whose luminescence can be tuned and integrated.

A Photo-driven Polyoxometalate Complex Shuttle and Its Homogeneous Catalysis and Heterogeneous Separation

A "smart" core-shell complex is designed to combine a catalytic reaction and automatic separation through remote light control. Here, we present the induced amphiphilic behavior of a surfactant-encapsulated polyoxometalate complex with photoresponsive azobenzene units on the periphery. The reversible phase transfer of the complex shuttle between two incompatible phase termini, driven by a photoisomerization-induced polarity change, further facilitates the separation and recycle of the catalyst.

The oxidation of pyridines catalyzed by surfactant-encapsulated polyoxometalate [(C18H37)2(CH3)2N]8[HBW11O39] with the temperature-responsive property of solubility

Temperature-responsive characterization of solubility based on a surfactant-encapsulated polyoxometalate ([(C18H37)2(CH3)2N]8[HBW11O39]) in tert-butyl alcohol was described and used in catalytic oxidation of pyridines. The catalyst could be recovered and reused several times by controlling the temperature.

Nematic Ion‐Clustomesogens from Surfactant‐Encapsulated Polyoxometalate Assemblies

AbstractThree surfactant‐encapsulated polyoxometalate (SEP) complexes were synthesized by grafting T‐shaped quaternary ammonium units onto the surface of K5BW12O40, K6SiW11O39Co, and Na10[Mn4(H2O)2(VW9O34)2]·34H2O. All the complexes possess shell–core structures, in which the peripheral mesogenic units adopt a lateral attachment. The bulk organization of the complexes with thermal stimulus was studied by polarized optical microscopy, differential scanning calorimetry, and X‐ray diffraction. The quaternary ammonium surfactant alone was found to form smectic structures, whereas the SEPs exhibited nematic characteristics. The detailed structural characterizations suggested the presence of a delicate synergy between the surfactants and polyoxometalates (POMs). The orientation of the mesogenic units was demonstrated to have a pivotal influence on the formation of liquid crystalline structures of the SEPs by comparison with the reported SEPs with head‐to‐tail attached mesogenic units. The clearing temperatures of the nematic complexes were found to be dependent on the number of peripheral surfactants. Finally, the magnetism of the polyoxometalates was proven to be well maintained in the liquid crystalline matrices, which allows the potential development of magnetically tunable liquid crystals bearing nematic structures.

Polyoxometalate Assemblies: Photo-Responsive Self-Assembly of an Azobenzene-Ended Surfactant-Encapsulated Polyoxometalate Complex for Modulating Catalytic Reactions (Small 20/2012)

Polyoxometalate Assemblies: Photo-Responsive Self-Assembly of an Azobenzene-Ended Surfactant-Encapsulated Polyoxometalate Complex for Modulating Catalytic Reactions (Small 20/2012)

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.

Surfactant-encapsulated polyoxometalate building blocks: controlled assembly and their catalytic properties

Polyoxometalates (POMs) are discrete anionic metal-oxide nanoclusters which exhibit unrivalled structural diversity, exceptional physical properties, and have many potential applications. Nonetheless, possessing high crystalline energy and hydrophilic nature, the assembly of POM clusters into rationally design architectures has been a long-standing bottleneck for their ultimate use in advanced materials and devices. To confront this challenge, both covalent and non-covalent modifications of POM nanoclusters are increasingly considered. This perspective reviews recent progress in the assembly of non-covalently modified surfactant-encapsulated POM nanoclusters with particular emphasis on our research work. The described solution-based assembly approach provides an excellent control on size, shape, and stability of the assembly structures. By effective exploitation of non-covalent interactions between the POM hybrid nanobuilding blocks, several unprecedented assembly structures including disks, cones, tubes, fullerene-like spheres, multiple shape flowers, wires, and thin films can be achieved. The assembly structures are highly robust and tunable in terms of size and shape and can act as hosts for guest nanomaterials to develop composite materials of combinatorial properties. In the last section of this manuscript, we present the catalytic properties of the assembly structures and their remote controlled manipulation in the reaction system.

Self‐Assembly and Structural Evolvement of Polyoxometalate‐Anchored Dendron Complexes

A cationic dendritic molecule that has alkyl chains has been synthesized and employed to encapsulate anionic polyoxometalates through electrostatic interactions. The prepared surfactant-encapsulated polyoxometalate (SEP) complexes were used as building blocks to fabricate self-assemblies in solution and the solid state. Monodispersion, lamellar, and columnar assemblies of SEP complexes have been characterized in detail. With increasing the number of peripheral cationic dendrons on inorganic clusters, the SEPs undergo changes from globular assemblies to monodispersions in solution and from lamellar assemblies to hexagonal columnar structures in the solid state, depending on the amounts of cationic dendrons in the complexes. The structural evolvement was simulated through consideration of the size and shape of the cationic dendron and polyanionic clusters, and the experimental results are in good agreement with the interpretation of the simulations. The present research demonstrates a new kind of dendritic complex and provides a route for controlling their assembling states by simply alternating the number of cationic dendrons in the complexes.

Surfactant‐Encapsulated Polyoxometalates as Immobilized Supramolecular Catalysts for Highly Efficient and Selective Oxidation Reactions

A type of interesting immobilized supramolecular catalysts based on surfactant-encapsulated polyoxometalates has been developed for oxidation reactions. Through a sol-gel process with tetraethyl orthosilicate, hydroxyl-terminated surfactant-encapsulated polyoxometalate complexes have been covalently and uniformly bound to a silica matrix with unchanged complex structure. The formed hybrid catalysts possess a defined hydrophobic nano-environment surrounding the inorganic clusters, which is conducive to compatibility between the polyoxometalate catalytic centres and organic substrates. The supramolecular synergy between substrate adsorption, reaction, and product desorption during the oxidation process has been found to have an obvious influence on the reaction kinetics, with the activity of the catalyst being greatly improved. The supramolecular catalysts performed effectively in the selective oxidation of several different kinds of organic compounds, such as alkenes, alcohols, and sulfides, and the main products were the corresponding epoxides, ketones, sulfoxides, and sulfones. More significantly, the catalyst could be easily recovered by simple filtration, and the catalytic activity was well retained for at least five cycles. Finally, the present strategy has proved to be a general route for the fabrication of supramolecular hybrid catalysts containing common polyoxometalates suitable for various purposes.