Structure and properties of the dendron-encapsulated polyoxometalate (C(52)H(60)NO(12))(12)[(Mn(H(2)O))(3)(SbW(9)O(33))(2)], a first generation dendrizyme

Accurately tuning the charge on giant polyoxometalate type Keplerates through stoichiometric interaction with cationic surfactants

We report an approach of exploring the interaction between cationic surfactants and a type of structurally well-defined, spherical "Keplerate" polyoxometalate (POM) macroanionic molecular clusters, {Mo72V30}, in aqueous solution. The effectiveness of the interaction can be determined by monitoring the size change of the "blackberry" supramolecular structures formed by the self-assembly of {Mo72V30} macroions, which is determined by the effective charge density on the macroions. Long-chain surfactants (CTAB and CTAT) can interact with {Mo72V30} macroions stoichiometrically and lower their charge density. Consequently, the blackberry size decreases continuously with increasing surfactant concentration in solution. On the other hand, for short-chain surfactants (e.g., OTAB), a larger fraction of surfactants exist as discrete chains in solution and do not strongly interact with the macroions. This approach shows that a controllable amount of suitable surfactants can accurately tune the charge on large molecular clusters.

Self-assembled monolayers of CH3COS- terminated surfactant-encapsulated polyoxometalate complexes

Self-assembled monolayers (SAMs) on gold surfaces based on three kinds of acetylthio-surfactant-encapsulated polyoxometalate clusters (thio-SECs) terminated with multiple CH(3)COS- groups, (NC(26)H(55)S(CO)CH(3))(6)H(2)[Co(H(2)O)CoW(11)O(39)], (NC(26)H(55)S(CO)CH(3))(13)H(3)[Co(4)(H(2)O)(2)(P(2)W(15)O(56))(2)], and (NC(26)H(55)S(CO)CH(3))(13)[Fe(4)(H(2)O)(2)(P(2)W(15)O(56))(2)]Br, have been prepared, which is representative of a general methodology to fabricate polyoxometalate-based SAMs. Thio-SECs self-assembled into monolayers on gold surfaces through the hydrolysis of CH(3)COS- groups and the subsequent formation of S-Au bonds, which was confirmed by grazing angle infrared spectroscopy, X-ray photoelectron spectroscopy, and ellipsometric and scanning tunneling microscopy (STM) measurements. Furthermore, the SAMs of the thio-SECs possess closely packed structures, and the local short-range order is clearly observed in the magnified STM image. We have also investigated the electrochemical behavior of SAMs of thio-SECs by cyclic voltammetry in detail, and the redox potential of the original polyoxometalates has been well retained. The electrochemical signals of the SAMs are very weak because of the small moiety of thio-SECs that are electrochemically accessible in the cyclic voltammetry experiments. The polyoxometalate-modified electrodes that we prepared are not only highly ordered in the local short range but also stable in electrochemical cycling because of the multiple S-Au bonds of thio-SECs on the gold substrates that aid in the construction of functional materials such as electrochemical and electrocatalytic devices.

Direct imaging of the structure, relaxation, and sterically constrained motion of encapsulated tungsten polyoxometalate lindqvist ions within carbon nanotubes

The imaging properties and observation of the sterically regulated translational motion of discrete tungsten polyoxometalate Linqvist ions (i.e., [W(6)O(19)](2-)) within carbon nanotubes of specific internal diameter are reported. The translational motion of the nonspheroidal anion within the nanotube capillary is found to be impeded by its near-perfect accommodation to the internal van der Waals surface of the nanotube wall. Rotational motion of the anion about one remaining degree of freedom permits translational motion of the anion along the nanotube followed by locking in at sterically favorable positions in a mechanism similar to a molecular ratchet. This steric locking permits the successful direct imaging of the constituent octahedral cation template of individual [W(6)O(19)](2-) anions by high resolution transmission electron microscopy thereby permitting meterological measurements to be performed directly on the anion. Direct imaging of pairs of equatorial W(2) atoms within the anion reveal steric relaxation of the anion contained within the nanotube capillary relative to the bulk anion structure.

Directed synthesis of stable large polyoxomolybdate spheres

Polyoxometalates or POMs, a class of inorganic transition metal-oxide based clusters, have gained significant interest owing to their catalytic, magnetic, and material science applications. All such applications require high surface area POM based materials. However, chemically synthesized POMs are still at most in the range of a few nanometers, with their size and morphology being difficult to control. Hence, there is an immediate need to develop design principles that allow easy control of POM morphology and size on mesoscopic (50-500 nm) length scales. Here, we report a design strategy to meet this need. Our method reported here avoids a complex chemical labyrinth by using a prefabricated cationic 1,2-dioleol-3-trimethylammonium-propane (DOTAP) vesicle as a scaffold/structure directing agent and gluing simple anionic heptamolybdates by electrostatic interaction and hydrogen bonds to form large POM spheres. By this method, complexity in the resulting structure can be deliberately induced either via the scaffold or via the oxometalate. The high degree of control in the matter of the size and morphology of the resulting POM superstructures renders this method attractive from a synthetic standpoint.

Metallo-supramolecular modules as a paradigm for materials science

Metal ion coordination in discrete or extended metallo-supramolecular assemblies offers ample opportunity to fabricate and study devices and materials that are equally important for fundamental research and new technologies. Metal ions embedded in a specific ligand field offer diverse thermodynamic, kinetic, chemical, physical and structural properties that make these systems promising candidates for active components in functional materials. A key challenge is to improve and develop methodologies for placing these active modules in suitable device architectures, such as thin films or mesophases. This review highlights recent developments in extended, polymeric metallo-supramolecular systems and discrete polyoxometalates with an emphasis on materials science.

Direct Evidence on the Existence of [Mo132] Keplerate-Type Species in Aqueous Solution

We demonstrate the existence of discrete single molecular [Mo(132)] Keplerate-type clusters in aqueous solution. Starting from a discrete spherical [Mo(132)] cluster, the formation of an open-basket-type [Mo(116)] defect structure is shown for the first time in solution using analytical ultracentrifugation sedimentation velocity experiments.

A General Strategy for Ligation of Organic and Biological Molecules to Dawson and Keggin Polyoxotungstates

The copper-catalyzed azide/alkyne cycloaddition (click chemistry) is used for the first time in polyoxometalate chemistry to graft any kind of organics (lipophilic, water-soluble, biologically relevant) to polyoxotungstates to generate hybrids. The method is not limited by solvent matching between the polyoxometallic platforms and the organic substrates.

Polyoxometalate clusters, nanostructures and materials: From self assembly to designer materials and devices

Polyoxometalates represent a diverse range of molecular clusters with an almost unmatched range of physical properties and the ability to form structures that can bridge several length scales. The new building block principles that have been discovered are beginning to allow the design of complex clusters with desired properties and structures and several structural types and novel physical properties are examined. In this critical review the synthetic and design approaches to the many polyoxometalate cluster types are presented encompassing all the sub-types of polyoxometalates including, isopolyoxometalates, heteropolyoxometalates, and reduced molybdenum blue systems. As well as the fundamental structure and bonding aspects, the final section is devoted to discussing these clusters in the context of contemporary and emerging interdisciplinary interests from areas as diverse as anti-viral agents, biological ion transport models, and materials science.

Progress in the characterization of synthetic (supramolecular) polymers by analytical ultracentrifugation

Analytical ultracentrifugation (AUC) is the leading technique for determining the molar mass, state of association in solution, and association constants of biological macromolecules. Even though it is little used for supramolecular polymers, this technique has already been shown to represent one of the best options in the characterization of these systems. The use of supramolecular assemblies in the field of nanotechnology requires the arrangement of such systems on surfaces, from solution. Therefore, the control and investigation of solution properties is of major importance. In this contribution we highlight the applicability and advantages of using AUC by presenting a summary of the results of the past few years on the characterization of various types of synthetic polymers and supramolecular polymer systems.

Fabrication and photoluminescence of hybrid organized molecular films of a series of gemini amphiphiles and europium(III)-containing polyoxometalate

Organic/inorganic hybrid monolayers, consisting of an oval decatungstoeuropate (EuW10) with photoluminescence and a series of gemini-type amphiphiles with various lengths of the flexible hydrophobic spacers, were formed through electrostatic interaction at the air/water interface. The ultrathin hybrid multilayer films could be fabricated through the horizontal transfer of the monolayers onto the solid substrates. The characteristic absorption band of EuW10 in the UV spectra of the hybrid films showed a slight red shift in comparison with that in solution, indicating the ordered arrangement of EuW10 in the hybrid films. FT-IR spectra of the hybrid films presented characteristic bands of EuW10, indicating that the structure of the cluster was kept in the films. X-ray diffraction measurements of the films revealed that the well-defined layer structures were formed in these multilayer films and the EuW10 cluster ions were orientated with their short axis parallel to the film surface. Typical photoluminescence of the 5D0 --> 7F2 and 5D0 --> 7F1 transitions of EuW10 was observed in the hybrid films. Interestingly, the relative intensity of the two emission bands varied with the spacer length of the gemini amphiphiles in the films. A size matching between EuW10 and the gemini amphiphile with a hexamethylene spacer was observed, which showed the lowest ratio of the emission intensity of 5D0 --> 7F2 to that of 5D0 --> 7F1. Circular nanoparticles were observed in the AFM images for all these hybrid films. In addition, the size of the formed nanoparticles showed a dependence on the spacer length.

Langmuir and langmuir-blodgett films of metallosupramolecular polyelectrolyte-amphiphile complexes

A detailed analysis of a metallosupramolecular polyelectrolyte-amphiphile complex (PAC) at the air-water interface is presented. Langmuir isotherms, Brewster angle microscopy, and X-ray reflectance and diffraction methods are employed to investigate the structure of the Langmuir monolayers. The PAC is self-assembled from 1,3-bis[4'-oxa-(2,2':6',2' '-terpyridinyl)]propane, iron acetate, and dihexadecyl phosphate (DHP). Spreading the PAC at the air-water interface results in a monolayer that consists of two strata. DHP forms a monolayer at the top of the interface, while the metallosupramolecular polyelectrolyte is immersed in the aqueous subphase. Both strata are coupled to each other through electrostatic interactions. The monolayers can be transferred onto solid substrates, resulting in well-ordered multilayers. Such multilayers are model systems for well-ordered metal ions in two dimensions.

Polyoxometalates matrixed into surfactants: synthesis, characterizations, and conformations of surfactants

We synthesized two surfactant-encapsulated polyoxometalate (POM) clusters (SECs), (DODA)(15)[H(3)Mo(57)V(6)(NO)(6)O(189)(H(2)O)(12)(VO)(6)].13H(2)O (SEC-2, DODA: dimethyldioctadecyl ammonium) and (DODA)(12)[Eu(H(2)O)P(5)W(30)O(110)].28H(2)O (SEC-3). SECs were characterized in detail by elemental analysis, UV-Vis absorption spectroscopy, FTIR, 1H NMR, thermogravimetric analysis, Langmuir isotherms, and powdered X-ray diffractions. SEC-2 can form a stable monolayer at the air-water interface. SEC-2 and SEC-3 possess a layered structure with periodicities of 3.8 and 5.05 nm, respectively. Alkyl chains in SEC-2 are disordered, but SEC-3 is indicative of well-ordered alkyl chains. According to the simplified structural analysis for SEC-2 and SEC-3, DODAs occupy areas of 1.12 and 0.589 nm(2) on the surface of the POMs, which are characteristic of the liquid-like and crystalline-like phases for DODA, respectively. The different conformations of DODAs then lead to the different packing structures of SECs. This work is aiming to control the organizational structures of the POM-based materials by selecting the appropriate POMs.

Nanoscopic Structure of a Metallo-supramolecular Polyelectrolyte-Amphiphile Complex, Elucidated by X-ray Scattering and Molecular Modeling

A combination of molecular modeling and X-ray scattering was used to elucidate the structure of the metallosupramolecular polyelectrolyte--amphiphile complex (PAC) self-assembled from FeII, 1,4-bis(2,2':6,'2"-terpyridin-4'-yl)benzene, and dihexadecyl phosphate (DHP). An approximate structure of the semi-ordered material was derived from the analysis of the X-ray scattering data. The experimental data provided sufficient input for obtaining a useful starting configuration for molecular modeling. Various models of the supramolecular architecture are presented and discussed in terms of their total energies and scattering patterns. In an iterative approach each level of the structural hierarchy was refined until satisfactory agreement of calculated and experimental scattering patterns was reached. The remarkable sensitivity of the simulated scattering curves to even the smallest structural changes at all length scales restricts the arbitrariness of modeling. The final model of PAC consists of flat lamellae of alternating strata of interdigitated DHP monolayers and nematically ordered polyelectrolyte chains.