Ready-to-be-addressed oxo-clusters: individualized, periodically organized and separated from the substrate.

Clusters and oxo-clusters are drawing attention for their amazing physical properties, especially at the scale of the single molecule. However, chemical methods to organize them individually on a surface are still lacking. In this study we show that it is possible to periodically organize individual polyoxometalates thanks to their ordering by a new supramolecular assembly.

Multi-Electron Visible Light Photoaccumulation on a Dipyridylamine Copper(II)-Polyoxometalate Conjugate Applied to Photocatalytic Generation of CF3 Radicals.

This article describes the synthesis and characterization of an organic-inorganic hybrid polyoxometalate functionalized by a short link with a tripodal N-based ligand and its copper complex. Upon visible light irradiation, the latter is able to store up to three reducing equivalents. The locus of the reduction is discussed based on physicochemical measurements and DFT calculations. In the presence of Togni's reagent, this complex allows for the photocatalytic generation of CF3 radicals, opening the road to valuable synthetic applications.

H2 Evolution at a Reduced Hybrid Polyoxometalate and Its Vanadium-Oxo Derivative Used as Molecular Models for Reducible Metal Oxides.

We herein report our investigations on the use of a tris-silanol-decorated polyoxotungstate, [SbW9O33(tBuSiOH)3]3-, as a molecular support model to describe the coordination of an isolated vanadium atom at metal oxides, focusing on the reactivity of the reduced derivatives in the presence of protons. Accumulation of electrons and protons at an active site is a main feature associated with heterogeneous catalysts able to conduct the (oxy)dehydrogenation of alkanes or alcohols. Our results indicate that two-electron reduced derivatives release H2 upon protonation, a reaction that probably takes place at the polyoxotungstic framework rather than at the vanadium center.

Lennard-Jones interaction parameters of Mo and W in He and N2 from collision cross-sections of Lindqvist and Keggin polyoxometalate anions.

Drift tube ion mobility spectrometry (DTIMS) coupled with mass spectrometry was used to determine the collision cross-sections (DTCCS) of polyoxometalate anions in helium and nitrogen. As the geometry of the ion, more than its mass, determines the collision cross-section with a given drift gas molecule, we found that both Lindqvist ions Mo6O192- and W6O192- had a DTCCSHe value of 103 ± 2 Å2, and both Keggin ions PMo12O403- and PW12O403- had a DTCCSHe value of 170 ± 2 Å2. Similarly, ion mobility experiments in N2 led to DTCCSN2 values of 223 ± 2 Å2 and 339 ± 4 Å2 for Lindqvist and Keggin anions, respectively. Using optimized structures and partial charges determined from density functional theory calculations, followed by CCS calculations via the trajectory method, we determined Lennard-Jones 6-12 potential parameters ε, σ of 5.60 meV, 3.50 Å and 3.75 meV, 4.40 Å for both Mo and W atoms interacting with He and N2, respectively. These parameters reproduced the CCS of polyoxometalates within 2% accuracy.

When Identification of the Reduction Sites in Mixed Molybdenum/Tungsten Keggin-Type Polyoxometalate Hybrids Turns Out Tricky.

The mixed molybdenum/tungsten Keggin-type polyoxometalate (POM) hybrid (TBA)4[PW9Mo2O39{Sn(C6H4I)}] (TBA = tert-butylammonium) has been prepared by the reaction between [α-PW9Mo2O39]7- and [Cl3Sn(C6H4I)] in dried acetonitrile, in the presence of tetra-n-butylammonium bromide. A further coupling reaction affords the ferrocenyl derivative (TBA)4[PW9Mo2O39{Sn(C6H4)C≡C(C6H4)Fc}]. The POM hybrids have been thoroughly characterized by NMR and IR spectroscopies. Electrochemical analysis confirms their ease of reduction compared to the all-W analogue, albeit with a second reduction process occurring at a lower potential than in the all-Mo species. It is noteworthy that the second reduction is accompanied by an unusual red shift of the electronic absorption spectrum. Whereas there is no doubt that the first reduction deals with Mo, the location of the second electron in the bireduced species, on the second Mo or on W, has thus been the subject of a cross-investigation by spectroelectrochemistry, electron spin resonance, and theoretical calculations. Finally, it came out that the second reduction is also Mo-centered with two unpaired and antiferromagnetically coupled extra electrons.

Supramolecular assemblies of organo-functionalised hybrid polyoxometalates: from functional building blocks to hierarchical nanomaterials.

This review provides a comprehensive overview of recent advances in the supramolecular organisation and hierarchical self-assembly of organo-functionalised hybrid polyoxometalates (hereafter referred to as hybrid POMs), and their emerging role as multi-functional building blocks in the construction of new nanomaterials. Polyoxometalates have long been studied as a fascinating outgrowth of traditional metal-oxide chemistry, where the unusual position they occupy between individual metal oxoanions and solid-state bulk oxides imbues them with a range of attractive properties (e.g. solubility, high structural modularity and tuneable properties/reactivity). Specifically, the capacity for POMs to be covalently coupled to an effectively limitless range of organic moieties has opened exciting new avenues in their rational design, while the combination of distinct organic and inorganic components facilitates the formation of complex molecular architectures and the emergence of new, unique functionalities. Here, we present a detailed discussion of the design opportunities afforded by hybrid POMs, where fine control over their size, topology and their covalent and non-covalent interactions with a range of other species and/or substrates makes them ideal building blocks in the assembly of a broad range of supramolecular hybrid nanomaterials. We review both direct self-assembly approaches (encompassing both solution and solid-state approaches) and the non-covalent interactions of hybrid POMs with a range of suitable substrates (including cavitands, carbon nanotubes and biological systems), while giving key consideration to the underlying driving forces in each case. Ultimately, this review aims to demonstrate the enormous potential that the rational assembly of hybrid POM clusters shows for the development of next-generation nanomaterials with applications in areas as diverse as catalysis, energy-storage and molecular biology, while providing our perspective on where the next major developments in the field may emerge.

Polarizability is a key parameter for molecular electronics.

Identifying descriptors that govern charge transport in molecular electronics is of prime importance for the elaboration of devices. The effects of molecule characteristics, such as size, bulkiness or charge, have been widely reported. Herein, we show that the molecule polarizability can be a crucial parameter to consider. To this end, platinum nanoparticle self-assemblies (PtNP SAs) are synthesized in solution, including a series of polyoxometalates (POMs). The charge of the POM unit can be modified according to the nature of the central heteroatom while keeping its size constant. POM hybrids that display remote terminal thiol functions strongly anchor the PtNP surface to form robust SAs. IV curves, recorded by conductive AFM, show a decrease in Coulomb blockade as the dielectric constant of the POMs increases. In this system, charge transport across molecular junctions can be interpreted as variations in polarizability, which is directly related to the dielectric constant.

Photoactive Organic/Inorganic Hybrid Materials with Nanosegregated Donor-Acceptor Arrays.

The synthesis of the first mesogenic donor-acceptor polyoxometalate (POM)-based hybrid is herein described. The structural and electronic properties of the hybrid compound were evaluated through combination of small- and wide-angle X-ray scattering, optical microscopy, electrochemistry and photoluminescence. In the solid state, the compound behaves as a birefringent solid, displaying a lamellar organization in which double-layers of POMs and bis(thiophene)thienothiophene organic donors alternate regularly. Noticeably, the sub-unit organizations in the composite are similar to that observed for the individual POM and organic donor precursors. Photophysical studies show that in the hybrid, the fluorescence of the organic donor unit is considerably quenched both in solution and in the solid state, which is attributed to occurrence of intramolecular charge-separated state.

Tuning Photoinduced Electron Transfer in POM-Bodipy Hybrids by Controlling the Environment: Experiment and Theory.

The optical and electrochemical properties of a series of polyoxometalate (POM) oxoclusters decorated with two bodipy (boron-dipyrromethene) light-harvesting units were examined. Evaluated here in this polyanionic donor-acceptor system is the effect of the solvent and associated counterions on the intramolecular photoinduced electron transfer. The results show that both solvents and counterions have a major impact upon the energy of the charge-transfer state by modifying the solvation shell around the POMs. This modification leads to a significantly shorter charge separation time in the case of smaller counterion and slower charge recombination in a less polar solvent. These results were rationalized in terms of Marcus theory and show that solvent and counterion both affect the driving force for photoinduced electron transfer and the reorganization energy. This was corroborated with theoretical investigations combining DFT and molecular dynamics simulations.

Covalent Grafting of Polyoxometalate Hybrids onto Flat Silicon/Silicon Oxide: Insights from POMs Layers on Oxides.

Immobilization of polyoxometalates (POMs) onto oxides is relevant to many applications in the fields of catalysis, energy conversion/storage, or molecular electronics. Optimization and understanding the molecule/oxide interface is crucial to rationally improve the performance of the final molecular materials. We herein describe the synthesis and covalent grafting of POM hybrids with remote carboxylic acid functions onto flat Si/SiO2 substrates. Special attention has been paid to the characterization of the molecular layer and to the description of the POM anchoring mode at the oxide interface through the use of various characterization techniques, including ellipsometry, AFM, XPS, and FTIR. Finally, electron transport properties were probed in a vertical junction configuration and energy level diagrams have been drawn and discussed in relation with the POM molecular electronic features inferred from cyclic-voltammetry, UV-visible absorption spectra, and theoretical calculations. The electronic properties of these POM-based molecular junctions are driven by the POM LUMO (d-orbitals) whatever the nature of the tether or the anchoring group.

Hierarchical Self-Assembly of Polyoxometalate-Based Organo Palladium(II) Metallomacrocycles via Electrostatic Interactions.

The design and synthesis of a supramolecular square composed of polyoxometalate-based hybrid donors and ethylenediamine palladium(II) nodes are reported. The structure of the metallomacrocycle scaffold was inferred by diffusion NMR, small-angle X-ray scattering (SAXS), and molecular modeling. The metallomacrocycle scaffold that contains negatively and positively charged subunits can further self-assemble owing to a competition between the solvation energy of the discrete species and intermolecular electrostatic interactions. When the dissociating character of the solvent was lowered or when in the presence of a protic solvent, different types of multiscale organizations (vesicles and pseudo-1D structures) were selectively formed and were characterized by SAXS and transmission electron microscopy.

Exploring the self-assembly of dumbbell-shaped polyoxometalate hybrids, from molecular building units to nanostructured soft materials.

The formation of hierarchical nanostructures using preformed dumbbell-like species made of covalent organic-inorganic polyoxometalate (POM)-based hybrids is herein described. In this system, the presence of charged subunits (POM, metal linkers, and counter ions) in the complex molecular architecture can drive their aggregation, which results from a competition between the solvation energy of the discrete species and intermolecular electrostatic interactions. We show that the nature of the POM and the charge of the metal linker are key parameters for the hierarchical nanoorganization. The experimental findings were corroborated with a computational investigation combining DFT and molecular dynamics simulation methods, which outlines the importance of solvation of the counter ion and POM/counter ion association in the aggregation process. The dumbbell-like species can also form gels, in the presence of a poorer solvent, displaying similar nanoorganization of the aggregates. We show that starting from the designed molecular building units whose internal charges can be controlled by redox trigger we can achieve their implementation into soft nanostructured materials through the control of their supramolecular organization.

Molecular signature of polyoxometalates in electron transport of silicon-based molecular junctions.

Polyoxometalates (POMs) are unconventional electro-active molecules with a great potential for applications in molecular memories, providing efficient processing steps onto electrodes are available. The synthesis of the organic-inorganic polyoxometalate hybrids [PM11O39{Sn(C6H4)C[triple bond, length as m-dash]C(C6H4)N2}]3- (M = Mo, W) endowed with a remote diazonium function is reported together with their covalent immobilization onto hydrogenated n-Si(100) substrates. Electron transport measurements through the resulting densely-packed monolayers contacted with a mercury drop as a top electrode confirms their homogeneity. Adjustment of the current-voltage curves with the Simmon's equation gives a mean tunnel energy barrier ΦPOM of 1.8 eV and 1.6 eV, for the Silicon-Molecules-Metal (SMM) junctions based on the polyoxotungstates (M = W) and polyoxomolybdates (M = Mo), respectively. This follows the trend observed in the electrochemical properties of POMs in solution, the polyoxomolybdates being easier to reduce than the polyoxotungstates, in agreement with lowest unoccupied molecular orbitals (LUMOs) of lower energy. The molecular signature of the POMs is thus clearly identifiable in the solid-state electrical properties and the unmatched diversity of POM molecular and electronic structures should offer a great modularity.

Control of the hierarchical self-assembly of polyoxometalate-based metallomacrocycles by redox trigger and solvent composition.

Discrete metallomacrocycles are attractive scaffolds for the formation of complex supramolecular architectures with emergent properties. We herein describe the formation of hierarchical nanostructures using preformed metallomacrocycles by coordination-driven self-assembly of a covalent organic-inorganic polyoxometalate (POM)-based hybrid. In this system, we take advantage of the presence of charged subunits (POM, metal linker, and counterions) within the metallomacrocycles, which drive their aggregation through intermolecular electrostatic interactions. We show that the solvent composition and the charge of the metal linker are key parameters that steer the supramolecular organization. Different types of hierarchical self-assemblies, zero-dimensional (0D) dense nanoparticles, and 1D worm-like nanoobjects, can be selectively formed owing to different aggregation modes of the metallomacrocycles. Finally, we report that the worm-like structures drastically enhance the solubility in water of a pyrene derivative and can act as molecular carriers.

Rapid photoinduced charge injection into covalent polyoxometalate-bodipy conjugates.

Controlled design of photoactive hybrids would provide highly active materials for solar energy conversion and photo(electro) catalysis. We describe the kinetics of photoinduced electron transfer in a series of photoactive hybrids based on Keggin-type polyoxometalates (POMs) covalently grafted to bodipy photosensitizers. We show how the electronic properties and corresponding dynamics of these hybrids can be readily tuned by varying the POM metal ion, the anchoring functionalization and the spacer length. Ultrafast visible and IR transient absorption spectroscopy, supported by spectroelectrochemical measurements, reveals that photoinduced electron transfer from the bodipy chromophore to the organosilyl POM derivative occurs as rapidly as τ = 54 ps to generate a long-lived (τ = 4.8 ns) charge-separated (CS) state, making this system appropriate for applications in photoelectrochemical devices.

A calibration framework for the determination of accurate collision cross sections of polyanions using polyoxometalate standards.

RATIONALE: Polyoxometalates (POMs) are remarkable oxo-clusters forming compact highly charged anions. We measured their collision cross sections (CCS) in N2 with drift tube ion mobility spectrometry (DTIMS). These values were then used to calibrate a traveling wave ion mobility spectrometry (TWIMS) device and the accuracy of the calibration was tested. METHODS: Six POM standards were analyzed by DTIM-MS (Tofwerk, Thun, Switzerland) at different voltages to determine absolute DT CCS (N2 ) values. Five POM compounds (Lindqvist TBA2 Mo6 O19; decatungstate TBA4 W10 O32; Keggin TBA3 PMo12 O40 ; TBA3 PW12 O40 and Dawson TBA6 P2 W18 O62 ) were used for the calibration of the TWIM-MS instrument (Synapt G2 HDMS, Waters, Manchester, UK) and a sixth Dawson POM, TBA9 P2 Nb3 W15 O62 , was used to compare the accuracy of the calibrations with POM or with polyalanine and dextran reference ions. RESULTS: We determined 45 DT CCS (N2 ) values at 30°C or 60°C. Fourteen DT CCS (N2 ) values at 30°C were used to perform calibration of the TWIMS instrument. Better correlations were observed than when DT CCS values in helium from the literature were used. The accuracy tests on six ions of Dawson POM TBA9 P2 Nb3 W15 O62 led to relative errors below 3.1% while relative errors of 3.6% to 10.1% were observed when calibration was performed with polyalanine and dextran reference ions. CONCLUSIONS: Our novel calibration strategy for determination of CCS values of multiply negatively charged ions on TWIM-MS devices based on DT CCS (N2 ) of standard POM structures covered a wider range of CCS and improved the accuracy to 2.1% relative error on average compared with 6.9% using polyalanine and dextran calibration.

Self-assembly study of nanometric spheres from polyoxometalate-phenylalanine hybrids, an experimental and theoretical approach.

Herein, we report on the study of supramolecular assemblies based on polyoxometalates (POMs) upon their modification with amino acids. Two POM-amino acid hybrids were synthesized by coupling a functionalized Keggin type polyoxoanion [PW11O39{Sn(C6H4)C[triple bond, length as m-dash]C(C6H4)COOH}]4- with carboxyl-protected (methyl-ester) phenylalanine or diphenylalanine peptides. Surprisingly, all compounds, including the initial POM, formed supramolecular nanospheres in different solvent mixtures, which were examined by scanning electron microscopy (SEM). Molecular dynamics (MD) simulations for the POM-amino acid species revealed that the hydrophobic forces are mainly responsible for the initial aggregation into incipient micelle type structures, in which the organic arms are buried inside the aggregate while POM polar heads are more exposed to the solvent with tetrabutyl-ammonium counter cations acting as linkers.

Charge Effect on the Formation of Polyoxometalate-Based Supramolecular Polygons Driven by Metal Coordination.

The metal-driven self-assembly of a Keggin-based hybrid bearing two remote pyridine units was investigated. The resulting supramolecular species were identified by combination of 2D diffusion NMR spectroscopy (DOSY) and electrospray ionization mass spectrometry (ESI-MS) as a mixture of molecular triangles and squares. This behavior is different from that of the structural analogue Dawson-based hybrid displaying a higher charge, which only led to the formation of molecular triangles. This study highlights the decisive effect of the charge of the POMs in their self-assembly processes that disfavors the formation of large assemblies. An isothermal titration calorimetry (ITC) experiment confirmed the stronger binding in the case of the Keggin hybrids. A correlation between the diffusion coefficient D and the molecular mass M of the POM-based building block and its coordination oligomers was also observed. We show that the diffusion coefficient of these compounds is mainly determined by their occupied volume rather than by their shape.

Photochromism and Dual-Color Fluorescence in a Polyoxometalate-Benzospiropyran Molecular Switch.

The photophysical properties of a Keggin-type polyoxometalate (POM) covalently bounded to a benzospiropyran (BSPR) unit have been investigated. These studies reveal that both closed and open forms are emissive with distinct spectral features (λem (closed form)=530 nm, λem (open form)=670 nm) and that the fluorescence of the BSPR unit of the hybrid is considerably enhanced compared to BSPR parent compounds. While the fluorescence excitation energy of the BSPR reference compounds (370 nm) is close to the intense absorption responsible of the photochromic character (350 nm), the fluorescence excitation of the hybrid is shifted to lower energy (400 nm), improving the population of the emissive state. Combined NOESY NMR and theoretical calculations of the closed form of the hybrid give an intimate understanding of the conformation adopted by the hybrid and show that the nitroaryl moieties of the BSPR is folded toward the POM, which should affect the electronic properties of the BSPR.

Tailor-made Covalent Organic-Inorganic Polyoxometalate Hybrids: Versatile Platforms for the Elaboration of Functional Molecular Architectures.

Post-functionalization of organically modified polyoxometalates (POMs) is a powerful synthetic tool to devise functional building blocks for the rational elaboration of POM-based molecular materials. In this personal account we focus on iodoaryl-terminated POM platforms, describe reliable routes to the synthesis of covalent organic-inorganic POM-based hybrids and their integration into advanced molecular architectures or multi-scale assemblies as well as their immobilization onto surfaces. Valorisation of the remarkable redox properties of POMs in the fields of artificial synthesis and molecular electronic is especially considered.