Optical, third order non-linear optical and electrochemical properties of dipolar, centrosymmetric and C2v organoimido polyoxometalate derivatives.

A family comprising seven arylimido-polyoxometalate (POM) hybrid chromophores (three of which are new), with linear dipolar, C2v and linear centrosymmetric geometries have been synthesised and studied by electronic absorption spectroscopy, electrochemistry, Z-scans (two photon absorption, TPA) and computation (DFT/TD-DFT). These reveal that POM acceptor units are an effective basis for TPA materials: the centrosymmetric bis-POM chromophores produce significant cross sections (δ up to 82 GM) from a single aryl bridge, a similar performance to larger dipolar π-systems combining carbazole or diphenylamino donors with the imido-POM acceptor. DFT/TD-DFT calculations indicate strong communication between POM and organic components is responsible for the linear and non-linear optical behaviour of these compounds, while electrochemical measurements reveal class II mixed valence behaviour resulting from an interplay of through-bond and through-space effects.

Luminescence Vapochromism of a Dynamic Copper Iodide Mesocate.

A copper iodide complex coordinated by three phosphine ligands with the formula [Cu2 I2 (Ph2 PC2 (C6 H4 )C2 PPh2 )3 ] exhibits solvatochromic and vapochromic luminescence properties. A mechanism based on solvent-dependent molecular motion appears to occur. The highly contrasted response observed upon THF solvent exposure makes this complex an appealing candidate for chemical sensor applications.

A Heptanuclear Copper Iodide Nanocluster.

Nanoscale molecular clusters are attractive for the design of materials exhibiting original functions and properties. In particular, copper iodide clusters of high nuclearity are well-known for their stimuli-responsive luminescence properties. The synthesis and characterization of an unprecedented copper(I) iodide molecular cluster based on an original heptanuclear inorganic core are reported. This nanometer-size cluster is formulated as [Cu7I7(P(C6H4CF3)3)6(CH3CN)] and its novel structure has been characterized by X-ray diffraction and multinuclear solid-state 63Cu, 31P, 13C, 19F, and 1H NMR spectroscopy. The photoluminescence properties of this cluster have been studied at variable temperature. Density functional theory calculations have been performed on this large molecular structure and allow one to rationalize the observed luminescence properties. This study highlights the crucial role of cuprophilic interactions in molecular copper iodide clusters for exhibiting photoactive properties.

Evaluation of Ligands Effect on the Photophysical Properties of Copper Iodide Clusters.

Luminescent materials based on copper complexes are currently receiving increasing attention because of their rich photophysical properties, opening a wide field of applications. The copper iodide clusters formulated [Cu4I4L4] (L = ligand), are particularly relevant for the development of multifunctional materials based on their luminescence stimuli-responsive properties. In this context, controlling and modulating their photophysical properties is crucial and this can only be achieved by thorough understanding of the origin of the optical properties. We thus report here, the comparative study of a series of cubane copper iodide clusters coordinated by different phosphine ligands, with the goal of analyzing the effect of the ligands nature on the photoluminescence properties. The synthesis, structural, and photophysical characterizations along with theoretical investigations of copper iodide clusters with ligands presenting different electronic properties, are described. A method to simplify the analysis of the 31P solid-state NMR spectra is also reported. While clusters with electron-donating groups present classical luminescence properties, the cluster bearing strong electron-withdrawing substituents exhibits original behavior demonstrating a clear influence of the ligands properties. In particular, the electron-withdrawing character induces a decrease in energy of the unoccupied molecular orbitals, that consequently impacts the emission properties. The modification of the luminescence thermochromic properties of the clusters are supported by density functional theory (DFT) calculations. This study demonstrates that the control of the luminescence properties of these compounds can be achieved through modification of the coordinated ligands, nevertheless the role of the crystal packing should not be underestimated.

Organoimido-Polyoxometalate Nonlinear Optical Chromophores: A Structural, Spectroscopic, and Computational Study.

Ten organoimido polyoxometalate (POM)-based chromophores have been synthesized and studied by hyper-Rayleigh scattering (HRS), Stark and Resonance Raman spectroscopies, and density functional theory (DFT) calculations. HRS ß0 values for chromophores with resonance electron donors are significant (up to 139 × 10-30 esu, ∼5 times greater than that of the DAS+ cation), but systems with no donor, or the -NO2 acceptor show no activity, in some cases, despite large DFT-predicted ß-values. In active systems with short (phenyl) π-bridges, ß0 values comfortably exceed that of the purely organic structural analogue N,N-dimethyl-4-nitroaniline (DMPNA), and intrinsic ß-values, ß0/N3/2 (where N is the number of bridge π-electrons) thus appear to break empirical performance limits (ß0/N3/2 vs λmax) for planar organic systems. However, ß0 values obtained for extended systems with a diphenylacetylene bridge are comparable to or lower than that of their nitro analogue, N,N-dimethyl-4-[(4-nitrophenyl)ethynyl]-aniline (DMNPEA). Resonance Raman spectroscopy confirms the involvement of the POM in the electronic transitions, whether donor groups are present or not, but Stark spectroscopy indicates that, in their absence, the transitions have little dipolar character (hence, NLO inactive), consistent with DFT-calculated frontier orbitals, which extend over both POM and organic group. Stark and DFT also suggest that ß is enhanced in the short compounds because the extension of charge transfer (CT) onto the POM increases changes in the excited-state dipole moment. With extended π-systems, this effect does not increase CT distances, relative to a -NO2 acceptor, so ß0 values do not exceed that of DMNPEA. Overall, our results show that (i) the organoimido-POM unit is an efficient acceptor for second-order NLO, but an ineffective donor; (ii) the nature of electronic transitions in arylimido-POMs is strongly influenced by the substituents of the aryl group; and (iii) organoimido-POMs outperform organic acceptors with short π-bridges, but lose their advantage with extended π-conjugation.

Luminescence Mechanochromism Induced by Cluster Isomerization.

Luminescent mechanochromic materials exhibiting reversible changes of their emissive properties in response to external mechanical forces are currently emerging as an important class of stimuli-responsive materials because of promising technological applications. Here, we report on the luminescence mechanochromic properties of a [Cu4I4(PPh3)4] copper iodide cluster presenting a chair geometry, being an isomer of the most common cubane form. This molecular cluster formulated [Cu4I4(PPh3)4]·2CHCl3 (1) exhibits a highly contrasted emission response to manual grinding, and, interestingly, the optical properties of the ground phase present striking similarities with those of the cubane isomer. In order to understand the underlying mechanism, a comparison with two related compounds has been conducted. The first one is a pseudopolymorph of 1 formulated as [Cu4I4(PPh3)4]·CH2Cl2 (2), which exhibits luminescent mechanochromic properties as well. The other one is also a chair compound but with a slightly different phosphine ligand, namely, [Cu4I4(PPh2C6H4CO2H)4] (3), lacking mechanochromic properties. Structural and optical characterizations of the clusters have been analyzed in light of previous electronic structure calculations. The results suggest an unpreceded mechanochromism phenomenon based on a solid-state chair → cubane isomer conversion. This study shows that polynuclear copper iodide compounds are particularly relevant for the development of luminescent mechanochromic materials.

Increasing p-type dye sensitised solar cell photovoltages using polyoxometalates.

Lindqvist polyoxometalate (POM) additives increase VOC in p-type DSSCs by up to 140%, yielding substantial efficiency gains for poorly matched dyes and redox mediators. For better dye/electrolyte combinations, these gains are typically outweighed by losses in JSC. Charge lifetime and transient IR measurements show that this is due to retardation of both recombination and electron transfer to the mediator, and a positive shift in the NiO valence band edge. The POMs also show their own, limited sensitizing effect.

Hedgehog-shaped {Mo368} cluster: unique electronic/structural properties, surfactant encapsulation and related self-assembly into vesicles and films.

The hedgehog-shaped {Mo368} cluster shows unique electronic (extremely high extinction coefficient) and structural features, especially regarding its size, the high number of delocalized electrons which allows to measure the surface enhanced Raman scattering (SERS) spectrum and the option for coordination chemistry inside the cavity. Its relative instability in aqueous solution can be overcome by embedment in a hydrophobic shell of dimethyldioctadecylammonium cations. The resulting hybrid self-assembles into spherical vesicles in acetone-water mixtures, according to a process directed by hydrophobic-hydrophilic interactions. It also forms rather stable Langmuir monolayers while a second layer evolves under higher surface pressure, in accordance with a rather low alkyl surface density.

Properties of a tunable multinuclear nickel polyoxotungstate platform.

A series of hybrid nickel bisphosphonate (BP) polyoxometalates (POMs) has been isolated. The complexes NaK-Ni7-Ale2 (Ale = [H2O3PC(C3H6NH2)(OH)PO3H2]) and NaNH4-Ni7-Ale2 are both made of two {PW9O34} fragments enclosing a heptanuclear Ni(II) core connected to two alendronate ligands. By pre-functionalising the alendronate moiety through the amino group, the naphthalene (napht) derivative Ni7-(AleNapht)2 and the compounds Ni7-(AlePy2 Ni)2 (py = pyridyl) and Ni7-(AleAc2Ni)2 (Ac = acyl) have been obtained. Compared with the NaK-Ni7-Ale2 species, these last two complexes contain two additional Ni(II) centres connected through two bis(2-pyridylmethyl)amine or two iminodiacetate groups, respectively. This results show that it is possible to functionalise the heptanuclear POM in a controlled manner. Quantification of the magnetic interactions in NaK-Ni7-Ale2 revealed that in the {Ni7} core, ferromagnetic interactions are predominant, with a S = 5 ground state. Magnetisation versus dc field sweeps on a single crystal of NaK-Ni7-Ale2 exhibited hysteresis at low temperature. (1)H and (31)P NMR studies in aqueous solution performed on NaK-Ni7-Ale2 and Ni7-(AleNapht)2 evidenced that the PW9/Ni7/bisphosphonate assembly is stable in solution. This was completed by (31)P magic angle spinning (MAS) investigations and confirmed by (1)H DOSY experiments. The electrochemistry of these compounds proceeds through two well-defined four-electron chemically reversible waves in a medium at pH 6. NaK-Ni7-Ale2 proved to be efficient for the electrocatalytic reduction of nitrate, nitrite and nitrous oxide. Remarkably, its electrocatalytic efficiency for nitrate reduction is approximately three times higher than those previously reported for POMs in a medium at pH >4 under the same potential. The catalytic properties of two representatives of the hybrid family were also examined. It is shown that these nickel bisphosphonate polyoxotungstates are pre-catalysts for the oxidation of alcohols into ketones or carboxylic acids, depending on the classes of alcohols considered, the stoichiometric oxidant used being H2O2. Noticeably, it has been found that an analogous cobalt bisphosphonate polyoxotungstate complex does not present any related activity, highlighting the crucial role of the 3d cations on the catalytic process.

Sulfonium polyoxometalates: a new class of solid-state photochromic hybrid organic-inorganic materials.

For the very first time, sulfonium polyoxometalate (POM) assemblies are shown to develop efficient solid-state photochromism in ambient conditions. The optical properties of the already known Rb(0.75)(NH(4))(5.25)[(Mo(3)O(8))(2)O(O(3)PC(CH(2)S(CH(3))(2))OPO(3))(2)]·8H(2)O (1) and a new material (Me(3)S)(4)[Mo(8)O(26)] (2) under UV excitation are investigated by diffuse reflectance spectroscopy, revealing that the color change effect is highly tunable playing with the nature of the POM. A mechanism involving the photoreduction of Mo(6+) cations associated with electron transfers from the sulfonium cations toward the POMs is proposed.

New Epoxy and Hardener System Based on an Imidazolium Ionic Liquid as an Anticorrosive Coating for Steel in the Marine Environment.

The large sizes of cations and anions of organic salts are the driving force for the application of ionic liquids (organic salts) in harsh salty conditions. Moreover, the formation of crosslinked ionic liquid networks as anti-rust and anticorrosion protective films on the substrate surfaces repels seawater salt and water vapor from their surface to prevent corrosion. In this respect, an imidazolium epoxy resin and polyamine hardener as ionic liquids were prepared by the condensation of either pentaethylenehexamine or ethanolamine with glyoxal and p-hydroxybenzaldehyde or formalin in acetic acid as a catalyst. The hydroxyl and phenol groups of the imidazolium ionic liquid were reacted with epichlorohydrine in the presence of NaOH as a catalyst to prepare polyfunctional epoxy resins. The chemical structure, nitrogen content, amine value, epoxy equivalent weight, thermal characteristics, and stability of the imidazolium epoxy resin and polyamine hardener were evaluated. Moreover, their curing and thermomechanical properties were investigated to confirm the formation of homogeneous, elastic, and thermally stable cured epoxy networks. The corrosion inhibition and salt spray resistance of the uncured and cured imidazolium epoxy resin and polyamine as coatings for steel in seawater were evaluated.

Tuning the photochromic properties of molybdenum bisphosphonate polyoxometalates.

Seven hybrid organic-inorganic bisphosphonate molybdenum(VI) polyoxometalate complexes with the general formula [(Mo(3)O(8))(4)(O(3)PC(C(m)H(2m)NRR'R″)(O)PO(3))(4)](8-) (m = 3; R, R', and R″ = H or CH(3)) and [(Mo(3)O(8))(2)(O)(O(3)PC(C(m)H(2m)NRR'R″)(O)PO(3))(2)](6-) (m = 3 or 4; R, R', and R″ = H or CH(3)) have been synthesized and their structures solved using single-crystal X-ray diffraction. These compounds are made of a {Mo(12)} or a {Mo(6)} inorganic core functionalized by various alkylammonium bisphosphonates, with these ligands differing by the length of their alkyl chains and the number of methyl groups grafted on the N atom. The nature of the counter-cations (Na(+), K(+), Rb(+), Cs(+), and/or NH(4)(+)) constituting these materials has also been modulated. (31)P NMR spectroscopic studies in aqueous media have shown that all the dodecanuclear complexes reported here are stable in solution, whereas for the hexanuclear compounds, a dynamic equilibrium between two isomers has been evidenced, and the corresponding standard thermodynamic parameters determined for one of them. The electrochemical properties of six representative compounds of this family have been investigated. It has been found that the Mo(6+)/Mo(5+) reduction potential is similar for all the polyoxometalates studied. Besides, it is shown that electrochemical cycling is an efficient method for the deposition of these compounds on a surface. The photochromic properties of all the complexes reported herein have been studied in the solid state. Under irradiation in the near ultraviolet (UV), the {Mo(12)} systems shift from white to reddish-brown, while the {Mo(6)} compounds develop a purple coloration. The coloration kinetics has been systematically quantified and the optical band gaps, the salient coloration kinetic parameters and the coloration kinetic half-life times have been determined. This has evidenced that several of these materials develop very strong and rapid UV-induced color changes, with remarkable coloration contrasts. Finally, the optical properties of these systems are discussed in light of several salient parameters as the POM topology, the nature of the grafted bisphosphonate ligand, and the design of the hydrogen-bonding network at the organic-inorganic interface.

Polyoxometalates functionalized by bisphosphonate ligands: synthesis, structural, magnetic, and spectroscopic characterizations and activity on tumor cell lines.

We report the synthesis and characterization of eight new Mo, W, or V-containing polyoxometalate (POM) bisphosphonate complexes with metal nuclearities ranging from 1 to 6. The compounds were synthesized in water by treating Mo(VI), W(VI), V(IV), or V(V) precursors with biologically active bisphosphonates H(2)O(3)PC(R)(OH)PO(3)H(2) (R = C(3)H(6)NH(2), Ale; R = CH(2)S(CH(3))(2), Sul and R = C(4)H(5)N(2), Zol, where Ale = alendronate, Sul = (2-Hydroxy-2,2-bis-phosphono-ethyl)-dimethyl-sulfonium and Zol = zoledronate). Mo(6)(Sul)(2) and Mo(6)(Zol)(2) contain two trinuclear Mo(VI) cores which can rotate around a central oxo group while Mo(Ale)(2) and W(Ale)(2) are mononuclear species. In V(5)(Ale)(2) and V(5)(Zol)(2) a central V(IV) ion is surrounded by two V(V) dimers bound to bisphosphonate ligands. V(6)(Ale)(4) can be viewed as the condensation of one V(5)(Ale)(2) with one additional V(IV) ion and two Ale ligands, while V(3)(Zol)(3) is a triangular V(IV) POM. These new POM bisphosphonates complexes were all characterized by single-crystal X-ray diffraction. The stability of the Mo and W POMs was studied by (31)P NMR spectroscopy and showed that all compounds except the mononuclear Mo(Ale)(2) and W(Ale)(2) were stable in solution. EPR measurements performed on the vanadium derivatives confirmed the oxidation state of the V ions and evidenced their stability in aqueous solution. Electrochemical studies on V(5)(Ale)(2) and V(5)(Zol)(2) showed reduction of V(V) to V(IV), and magnetic susceptibility investigations on V(3)(Zol)(3) enabled a detailed analysis of the magnetic interactions. The presence of zoledronate or vanadium correlated with the most potent activity (IC(50)~1-5 µM) against three human tumor cell lines.

Resorcin[4]arene-derived mono-, bis- and tetra-imidazolium salts as ligand precursors for Suzuki-Miyaura cross-coupling.

Eleven resorcinarene cavitands bearing either one, two or four (3-R-1-imidazolylium)-methyl substituents (R = (n)Bu, Ph, Mes, (i)Pr(2)C(6)H(3)) anchored at resorcinolic "ortho" positions have been synthesised from the appropriate bromomethylated precursor. Combining the imidazolium salts with palladium acetate and Cs(2)CO(3) gave active Suzuki-Miyaura cross coupling catalysts. The highest activities were observed with the doubly functionalised cavitands, which all have the imidazolylium groups attached to proximal resorcinol units.

Synthesis and characterization of a new meso-tetrakis (2,4,6-trimethylphenyl) porphyrinto) zinc(II) supported sodium alginate gel beads for improved adsorption of methylene blue dye.

Chemical modification of a biopolymer offers a simple strategy to realize new materials with added benefits. In this paper, meso-tetrakis(2,4,6-trimethylphenyl) porphyrinto) zinc(II) supported sodium alginate gel beads were successfully prepared, characterized, and used for the removal of a cationic dye from aqueous solution. The chemical structure of the prepared composite beads was confirmed by FT-IR, SEM, XRD and TGA/DTG techniques. Analytical data suggested that the sodium alginate beads and meso-tetrakis(2,4,6-trimethylphenyl) porphyrinto) zinc(II) complex interacted via non-covalent interaction (H-bonding) mode. The morphological micrographs showed spherical and smooth composite beads. The crosslinking of hydroxyl and carboxylate groups of the biopolymer with calcium ions and the incorporation of meso-tetrakis(2,4,6-trimethylphenyl) porphyrinto) zinc(II) into alginate matrix resulted in a small decrease of residual mass. The maximum adsorption capacities of methylene blue, at 20 °C, were found to be 52.3 mg/g and 34.8 mg/g for sodium alginate composite beads (3%) and plain sodium alginate beads, respectively. The adsorption process followed Freundlich isotherm and pseudo second order kinetics. The thermodynamic study displayed an exothermic and non-spontaneous process.

Polyoxometalate-based metal organic frameworks (POMOFs): structural trends, energetics, and high electrocatalytic efficiency for hydrogen evolution reaction.

The grafting of the triangular 1,3,5-benzene tricarboxylate linkers (denoted trim) on tetrahedral ε-Keggin polyoxometalates (POMs) capped by Zn(II) ions, formed in situ under hydrothermal conditions, has generated three novel POM-based metal organic frameworks (POMOFs). (TBA)(3)[PMo(V)(8)Mo(VI)(4)O(36)(OH)(4)Zn(4)][C(6)H(3)(COO)(3)](4/3)·6H(2)O (ε(trim)(4/3)) is a 3D open-framework built of molecular Keggin units connected by trim linkers, with channels occupied by tetrabutylammonium (TBA) counterions. ε(trim)(4/3) is a novel (3,4)-connected net, named ofp for open-framework polyoxometalate, and computer simulations have been used to evaluate its relative stability in comparison with ctn- and bor-like polymorphs, showing the stability of this novel phase directly related to its greatest density. A computational study was also undertaken with the aim of locating TBA molecules, the positions of which could not be deduced from single crystal X-ray diffraction, and further rationalizes their structure directing role. In (TBA)(3)[PMo(V)(8)Mo(VI)(4)O(37)(OH)(3)Zn(4)][C(6)H(3)(COO)(3)] (ε(2)(trim)(2)), the building unit is not the molecular Keggin but a dimerized form of this POM. Their connection via trim linkers generates a 3D framework with channels filled by TBA cations. In (TBA)(3)[PMo(V)(8)Mo(VI)(4)O(37)(OH)(3)Zn(4)][C(6)H(3)(COO)(3)]·8H(2)O ([ε(trim)](∞)), zigzag chains are connected via the organic linkers, forming 2D grids. Modified electrodes were fabricated by direct adsorption of the POMOFs on glassy carbon or entrapment in carbon paste (CPE). A remarkable electrocatalytic hydrogen evolution reaction (HER) was detected with a yield greater than 95%, and a turnover number as high as 1.2 × 10(5) was obtained after 5 h. The reported POMOF-based electrodes are more active than platinum, with a roughly 260 mV anodic shift. Finally, the electrocatalytic activities of ε(trim)(4/3)/CPE electrodes in various XCl (X = Li, Na, K, Cs) media have been studied. This allowed us to detect a cation effect and propose an electrocatalytic mechanistic pathway for the HER.

Lanthanide polyoxocationic complexes: experimental and theoretical stability studies and Lewis acid catalysis.

The [ε-PMo(V)(8)Mo(VI)(4)O(36)(OH)(4){Ln(III)(H(2)O)}(4)](5+) (Ln=La, Ce, Nd, Sm) polyoxocations, called εLn(4), have been synthesized at room temperature as chloride salts soluble in water, MeOH, EtOH, and DMF. Rare-earth metals can be exchanged, and (31)P NMR spectroscopic studies have allowed a comparison of the affinity of the reduced {ε-PMo(12)} core, thus showing that the La(III) ions have the highest affinity and that rare earths heavier than Eu(III) do not react with the ε-Keggin polyoxometalate. DFT calculations provide a deeper insight into the geometries of the systems studied, thereby giving more accurate information on those compounds that suffer from disorder in crystalline form. It has also been confirmed by the hypothetical La→Gd substitution reaction energy that Ln ions beyond Eu cannot compete with La in coordinating the surface of the ε-Keggin molybdate. Two of these clusters (Ln=La, Ce) have been tested to evidence that such systems are representative of a new efficient Lewis acid catalyst family. This is the first time that the catalytic activity of polyoxocations has been evaluated.

Functional Materials Based on Cyclometalated Platinum(II) ß-Diketonate Complexes: A Review of Structure-Property Relationships and Applications.

Square planar organoplatinum(II) complexes have garnered immense interest in the area of materials research. The combination of the Pt(II) fragment with mono-, bi- tri- and tetradentate organic ligands gives rise to a large variety of complexes with intriguing properties, especially cyclometalated Pt(II) complexes in which ligands are connected through covalent bonds demonstrate higher stability, excellent photoluminescence properties, and diverse applications. The properties and applications of the Pt(II)-based materials can be smartly fine-tuned via a judicious selection of the cyclometalating as well as ancillary ligands. In this review, attempts have been made to provide a brief review of the recent developments of neutral Pt(II) organometallic complexes bearing bidentate cyclometalating ligands and ß-diketonate ancillary ligands, i.e., (C^N)Pt(O^O) and (C^C)Pt(O^O) derivatives. Both small (monomeric, dimeric) and large (polymeric) materials have been considered. We critically assessed the role of functionalities (ligands) on photophysical properties and their impact on applications.

Synthesis of a resorcinarene-based tetraphosphine-cavitand and its use in Heck reactions.

A resorcinarene cavitand substituted by four -CH(2)PPh(2) pendant arms was synthesised starting from a generic C(5)-resorcinarene. Combining this tetraphosphine with palladium acetate and Cs(2)CO(3) gave an active Heck catalyst. The highest activities were observed by using a tetraphosphine/Pd ratio of ca. 1:1.

Fine-tuning polyoxometalate non-linear optical chromophores: a molecular electronic "Goldilocks" effect.

A new aryl-imido polyoxometalate non-linear optical chromophore (POMophore) with a diphenylamino donor group attains the highest ßzzz, 0 value (196 × 10-30 esu by Hyper-Rayleigh Scattering, HRS), and best transparency/non-linearity trade off yet for such materials. Stark spectroscopic and DFT investigation of this compound, plus NMe2 and carbazole analogues, show that its high performance results from a combination of strongly dipolar electronic transitions, and strong electronic communication across the π-system.