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1.
Chem Commun (Camb) ; 60(34): 4549-4552, 2024 Apr 23.
Article in English | MEDLINE | ID: mdl-38577743

ABSTRACT

Quaternary ammonium salts of metal derivatives of polyoxometalates [XW11O39M(H2O)]n- (X = P, Si; M = Cr, Mn, Co, Ni, Zn) were successfully tested instead of quaternary ammonium halides as catalysts in the cycloaddition of CO2 to styrene oxide. Remarkably, they gave very satisfactory yields of styrene carbonate at moderate temperature (80 °C).

2.
Nanoscale ; 15(32): 13233-13238, 2023 Aug 17.
Article in English | MEDLINE | ID: mdl-37540202

ABSTRACT

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.

3.
Nanoscale ; 15(25): 10634-10641, 2023 Jun 30.
Article in English | MEDLINE | ID: mdl-37310406

ABSTRACT

Polyoxometalates are nanoscale molecular oxides with promising properties that are currently explored for molecule-based memory devices. In this work, we synthesize a series of Preyssler polyoxometalates (POMs), [Na⊂P5W30O110]14-, stabilized with four different counterions, H+, K+, NH4+, and tetrabutylammonium (TBA+). We study the electron transport properties at the nanoscale (conductive atomic force microscopy, C-AFM) of molecular junctions formed by self-assembled monolayers (SAMs) of POMs electrostatically deposited on the ultraflat gold surface prefunctionalized with a positively charged SAM of amine-terminated alkylthiol chains. We report that the electron transport properties of P5W30-based molecular junctions depend on the nature of the counterions; the low-bias current (in the voltage range [-0.6 V; 0.6 V]) gradually increases by a factor of ∼100 by changing the counterion in the order: K+, NH4+, H+ and TBA+. From a statistical study (hundreds of current-voltage traces) using a simple analytical model for charge transport in nanoscale devices, we show that the energy position of the lowest unoccupied molecular orbital (LUMO) of P5W30 with respect to the Fermi energy of the electrodes increases from ∼0.4 eV to ∼0.7 eV and that the electrode coupling energy also increases from ∼0.05 to 1 meV in the same order from K+, NH4+, H+ to TBA+. We discuss several hypotheses on the possible origin of these features, such as a counterion-dependent dipole at the POM/electrode interface and counterion-modulated molecule/electrode hybridization, with, in both cases, the largest effect in the case of TBA+ counterions.


Subject(s)
Gold , Electrodes , Electron Transport , Microscopy, Atomic Force/methods
4.
J Am Chem Soc ; 145(22): 12136-12147, 2023 Jun 07.
Article in English | MEDLINE | ID: mdl-37216360

ABSTRACT

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.

5.
Nanoscale ; 15(1): 387, 2022 Dec 22.
Article in English | MEDLINE | ID: mdl-36475550

ABSTRACT

Correction for 'Redox-controlled conductance of polyoxometalate molecular junctions' by Cécile Huez et al., Nanoscale, 2022, 14, 13790-13800, https://doi.org/10.1039/D2NR03457C.

6.
Nanoscale ; 14(37): 13790-13800, 2022 Sep 29.
Article in English | MEDLINE | ID: mdl-36102689

ABSTRACT

We demonstrate the reversible in situ photoreduction of molecular junctions of a phosphomolybdate [PMo12O40]3- monolayer self-assembled on flat gold electrodes, connected by the tip of a conductive atomic force microscope. The conductance of the one electron reduced [PMo12O40]4- molecular junction is increased by ∼10, and this open-shell state is stable in the junction in air at room temperature. The analysis of a large current-voltage dataset by unsupervised machine learning and clustering algorithms reveals that the electron transport in the pristine phosphomolybdate junctions leads to symmetric current-voltage curves, controlled by the lowest unoccupied molecular orbital (LUMO) at 0.6-0.7 eV above the Fermi energy with ∼25% of the junctions having a better electronic coupling to the electrodes than the main part of the dataset. This analysis also shows that a small fraction (∼18% of the dataset) of the molecules is already reduced. The UV light in situ photoreduced phosphomolybdate junctions systematically feature slightly asymmetric current-voltage behaviors, which is ascribed to the electron transport mediated by the single occupied molecular orbital (SOMO) nearly at resonance with the Fermi energy of the electrodes and by a closely located single unoccupied molecular orbital (SUMO) at ∼0.3 eV above the SOMO with a weak electronic coupling to the electrodes (∼50% of the dataset) or at ∼0.4 eV but with a better electrode coupling (∼50% of the dataset). These results shed light on the electronic properties of reversible switchable redox polyoxometalates, a key point for potential applications in nanoelectronic devices.

7.
Phys Chem Chem Phys ; 24(26): 16156-16166, 2022 Jul 06.
Article in English | MEDLINE | ID: mdl-35748666

ABSTRACT

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.

8.
Inorg Chem ; 61(20): 7700-7709, 2022 May 23.
Article in English | MEDLINE | ID: mdl-35549467

ABSTRACT

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.

9.
Nanoscale Horiz ; 6(3): 271-276, 2021 03 01.
Article in English | MEDLINE | ID: mdl-33507203

ABSTRACT

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.

10.
Angew Chem Int Ed Engl ; 60(15): 8419-8424, 2021 Apr 06.
Article in English | MEDLINE | ID: mdl-33448550

ABSTRACT

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.

11.
Angew Chem Int Ed Engl ; 60(12): 6518-6525, 2021 Mar 15.
Article in English | MEDLINE | ID: mdl-33350554

ABSTRACT

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.

12.
ACS Appl Mater Interfaces ; 12(42): 48109-48123, 2020 Oct 21.
Article in English | MEDLINE | ID: mdl-32986397

ABSTRACT

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.

13.
Inorg Chem ; 59(4): 2458-2463, 2020 Feb 17.
Article in English | MEDLINE | ID: mdl-31977199

ABSTRACT

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.

14.
Chem Sci ; 11(40): 11072-11080, 2020 Sep 18.
Article in English | MEDLINE | ID: mdl-34123198

ABSTRACT

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.

15.
Front Chem ; 7: 830, 2019.
Article in English | MEDLINE | ID: mdl-31850318

ABSTRACT

The binary assembly DDA-{Mo132}/OA-γ-Fe2O3 (DDA = didodecyldimethylammonium, {Mo132} = [Mo132O372(CH3COO)30(H2O)72]42-, OA = oleic acid) constitutes one of the two examples in the literature of binary superlattices made of a mixing of nanocrystals and oxo-clusters. In a precedent work, we reported in details the preparation of such magnetic binary systems and studied the effect of the nature of the polyoxometalates (POMs) on the magnetic properties. In the present paper, we study the stability of this model binary assembly under heating at various temperatures. Indeed, especially if magnetic and/or transport properties are targeted, an annealing can be essential to change the phase of the nanocrystals in a more magnetic one and/or to desorb the organic capping of the nano-objects that can constitute an obstacle to the electronic communication between the nano-objects. We gave evidence that the maghemite organization in the binary assembly is maintained until 370°C under vacuum thanks to the presence of the POMs. This latter evolve in the phase MoO3, but still permits to avoid the aggregation of the nanocrystals as well as preserve their periodical arrangement. On the contrary, an assembly made of pure γ-Fe2O3 nanocrystals displays a clear aggregation of the nano-objects from 370°C, as attested by transmission and scanning electronic microscopies and confirmed by magnetic measurements. The stability of the magnetic nanocrystals in such POMs/nanocrystals assemblies opens the way to (i) the elaboration of new binary assemblies from POMs and numerous kinds of nanocrystals with a good control on the magnetic properties and to (ii) the investigation of new physical properties as exchange coupling, or magneto-transport in such systems.

16.
J Am Chem Soc ; 140(44): 14903-14914, 2018 Nov 07.
Article in English | MEDLINE | ID: mdl-30362733

ABSTRACT

Here we report on the use of a silanol-decorated polyoxotungstate, [SbW9O33( tBuSiOH)3]3- (1), as a molecular support to describe the coordination of a vanadium atom at a single-site on silica surfaces. By reacting [V(Mes)3·thf] (Mes = 2,4,6-trimethylphenyl) with 1 in tetrahydrofuran, the vanadium(III) derivative [SbW9O33( tBuSiO)3V(thf)]3- (2) was obtained. Compound 2 displays the paramagnetic behavior expected for a d2-VIII high spin complex (SQUID measurements) with a triplet electronic ground state (ca. 30 kcal·mol-1 more stable than the singlet, from DFT calculations). Compound 2 proves to be a reliable model for reduced isolated-vanadium atom dispersed on silica surfaces [(≡Si-O)3VIII(OH2)], an intermediate that is often proposed in a Mars-van Krevelen type mechanism for partial oxidation of light alcohols. Oxidation of 2 under air produced the oxo-derivative [SbW9O33( tBuSiO)3VO]3- (3). In compound 2, the d2-electrons are localized in degenerated d(V) orbitals, whereas in the electronically analogous bireduced-[SbW9O33( tBuSiO)3VO]5-, 3·(2e), one electron is localized on d(V) orbital and the second one is delocalized on the polyoxotungstic framework, leading to a unique case of a bireduced heteropolyanion derivative with completely decoupled d1-V(IV) and d1-W(V). Our body of experimental results (EPR, magnetic measurements, spectroelectrochemical studies, Raman spectroscopy) and theoretical studies highlights (i) the role of the apical ligand coordination, i.e., thf (σ-donor) vs oxo (π-donor), in destabilizing or stabilizing the d(V) orbitals relative to the d(W) orbitals, and (ii) a geometrical distortion of the O3VO entity that causes a splitting of the degenerated orbitals and the stabilization of one d(V) orbital in the bireduced compound 3·(2e).

17.
Nanoscale ; 10(36): 17156-17165, 2018 Sep 20.
Article in English | MEDLINE | ID: mdl-30187072

ABSTRACT

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.

18.
Proc Natl Acad Sci U S A ; 115(36): 8895-8900, 2018 09 04.
Article in English | MEDLINE | ID: mdl-30131428

ABSTRACT

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.

19.
Chem Sci ; 9(25): 5578-5584, 2018 Jul 07.
Article in English | MEDLINE | ID: mdl-30061989

ABSTRACT

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.

20.
Rapid Commun Mass Spectrom ; 32(19): 1703-1710, 2018 Oct 15.
Article in English | MEDLINE | ID: mdl-29989245

ABSTRACT

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.

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