Lanthanide-containing Polyoxometalates Research Articles

Lanthanide-Containing Polyoxometalate Crystallized with Bolaamphiphile Surfactants as Inorganic–Organic Hybrid Phosphors

Lanthanide elements such as europium exhibit distinctive emissions due to the transitions of inner-shell 4f electrons. Inorganic materials containing lanthanide elements have been widely used as phosphors in conventional displays. The hybridization of lanthanide ions with organic components enables to control of the material’s shapes and properties and broadens the possibility of lanthanide compounds as inorganic–organic materials. Lanthanide ion-containing polyoxometalate anions (Ln-POM) are a promising category as an inorganic component to design and synthesize inorganic–organic hybrids. Several inorganic–organic Ln-POM systems have been reported by hybridizing with cationic surfactants as luminescent materials. However, single-crystalline ordering has not been achieved in most cases. Here, we report syntheses and structures of inorganic–organic hybrid crystals of lanthanide-based POM and bolaamphiphile surfactants with two hydrophilic heads in one molecule. An emissive decatungstoeuropate ([EuW10O36]9−, EuW10) anion was employed as a lanthanide source. The bolaamphiphile counterparts are 1,8-octamethylenediammonium ([H3N(CH2)8NH3]2+, C8N2) and 1,10-decamethylenediammonium ([H3N(CH2)10NH3]2+, C10N2). Both hybrid crystals of C8N2-EuW10 and C10N2-EuW10 were successfully obtained as single crystals, and their crystal structures were unambiguously determined using X-ray diffraction measurements. The photoluminescence properties of C8N2-EuW10 and C10N2-EuW10 were investigated by means of steady-state and time-resolved spectroscopy. The characteristic emission derived from the EuW10 anion was retained after the hybridization process.

Highly luminescent soft aggregates and films assembled by amphiphilic polyoxometalate complex in a polymerizable aprotic ionic liquid

Hybrid co-assembly from lanthanide-containing polyoxometalates (POMs) and surfactants could efficiently optimize the formed aggregate luminescent properties and structural diversity. Meanwhile, compared to conventional solvents, aprotic ionic liquids (ILs) are not only excellent assembly media but also ideal solvents for luminescent lanthanide compounds. Therefore, in this work, the luminescent amphiphilic POM complex, namely surfactant-encapsulated POM hybrid (SEP) was initially prepared mainly through electrostatic interaction from polyanionic lanthanide-containing POM, Na9(EuW10O36)·32H2O (EuW10) and cationic surfactant, 1-octadecyl-3-methylimidazolium bromide ([C18mim]Br, OB). Then, highly luminescent lamellar micro-fiber aggregates were assembled by SEP in a polymerizable imidazolium-IL solvent, i.e. 1-vinyl-3-butyl imidazolium terafluoroborate ([Vbim]BF4). After morphology and structure characterizations for soft aggregates in detail, a lamellar aggregation arrangement mechanism was proposed. Further through in situ photopolymerization for such aggregate matrices, the flexible and organized films with enhanced luminescent properties were obtained. Our work indicates that the rational design of polymerizable organized aggregate matrices in ILs could greatly improve the photophysical properties of such luminescent soft materials and also expand their practical applications.

α-AsW9O33]9- bridged hexagonal clusters of Ln(III) showing field induced SMM behavior: experimental and theoretical insight.

Polyoxometalates (POM), as inorganic polydentate oxygen donors, provide binding opportunities for oxophilic lanthanide metal centers to construct novel Ln-substituted POM materials with exciting structures and attractive properties. Herein, we have reported four arsenotungstate [α-AsW9O33]9- based lanthanide-containing polyoxometalates [CsxK36-x{Ln6(H2O)12(α-AsW9O33)6}]·yH2O (Ln = Er (1), Gd (2), Ho (3), and Tb (4)), which are synthesized in an alkaline medium. Complexes 1-3 are the dimeric structures of [Ln3(H2O)6(α-AsW9O33)3]18- polyanions, whereas complex 4 is a hexamer of the polyanion [Tb (H2O)2(α-AsW9O33)]6- as a building unit. In all the complexes, [α-AsW9O33]9- units are staggered up and down and give rise to the chair conformation, where one [α-AsW9O33]9- unit bridges two Ln(III) centers through four μ2-oxygen and two terminal oxygen atoms, resulting in the hexagonal arrangement of lanthanides. The dynamic magnetic measurement indicates that only complex 1 exhibits slow relaxation of magnetization with an applied dc field (1500 Oe). To gain insight into the slow relaxation of magnetization in complex 1, the ligand-field parameters and the splitting of the ground-state multiplet of the Er(III) ions have been estimated. The ab initio calculation results confirm that the ground state wave function of these molecules (1, 3, and 4) is mainly composed of a mixture of mJ states, and the non-axial crystal field (CF) terms are more predominant than the axial CF term. The solid-state fluorescence spectra of 1-4 reveal that the photoexcitation O → M ligand-to-metal charge-transfer (LMCT) of arsenotungstate fragments is effectively quenched due to the spatial coordination environment around the Ln(III) ion.

Lanthanide-containing polyoxometalate as luminescent down-conversion material for improved printable perovskite solar cells

Full utilization of the incident light is an effective strategy to enhance photovoltaic performance of perovskite solar cells (PSCs). Here, we used lanthanide-containing polyoxometalate Na9[EuW10O36] (EuW10) as a luminescent down-conversion material incorporated in mesoporous TiO2 layer for boosting the power conversion efficiency (PCE) of the printable hole transport material-free PSCs. The EuW10 can convert the high energy photons into the low energy visible photons matched well with the absorption of perovskite, so that photogenerated current carriers increase substantially. This research represents the first example of using lanthanide-containing polyoxometalate as down-conversion materials to effectively improve PCE in solar cells. Compared with pristine TiO2 based PSCs, the EuW10-TiO2 based PSCs showed an enhanced power conversion efficiency (PCE) from 11.42% to 14.36% and obtained a remarkable photocurrent density of 23.46 mA/cm2. Besides, the EuW10-TiO2 based device exhibited a better long-term stability, which could maintain 98% initial efficiency after storage for 14 days. Since the efficiency of the printable perovskite solar cells has been a key factor to achieve commercial implementations, we consider that the results from this work demonstrate a new kind of down-conversion materials to boost PCE and stability.

New Robust Luminescent Supramolecular Assemblies Based on [Ln(Mo8O26)2]5- (Ln = Eu, Sm) Polyoxometalates.

This work highlights for the first time the photoluminescence (PL) properties of two new [Ln(Mo8O26)2]5- (Ln = Eu, Sm) lanthanide-containing polyoxometalates. Stable crystals of their tetrabutylammonium salts were synthesized, and their structures were confirmed by single-crystal X-ray diffraction. The robustness of the [Ln(Mo8O26)2]5- complexes in an acetonitrile solution has been evidenced by Fourier transform Raman and PL spectroscopies. The tetraphenylphosphonium derivatives were obtained by a salt metathesis reaction. The two series exhibit high thermal stability in air and are efficient phosphors at room temperature.

Enhancing electrofluorochromic efficiency through C30H31N6+-sensitized layer-by-layer polyoxometalate films

The inability to use organic cations in layer-by-layer (LBL) films stringently limits the efficiencies of most LBL films, such as luminescence-switched lanthanide-containing polyoxometalate (Ln-POM) functional composite films. Here, selecting luminescent organic cations such as C30H31N6+ (JGB) to facilitate energy transfer is proposed as a strategy to alleviate the fluorescence resonance energy transfer (FRET) efficiency problem. In typical Ln-POM ([ErSiW9/JGB]9) films, the FRET process occurs from luminescent species to electro-reduced POM, resulting in fluorescence quenching. In contrast, the organic cations (C30H31N6+) are able to transfer the harvested energy to the luminescent species (Sm), entailing a ~677-fold improvement over the overall luminescence efficiencies of [SmSiW9/JGB]9 films. Incorporating C30H31N6+ into the SmSiW9-contained LBL films elevates the electrofluorochromic efficiency to 93%, enhancing the power conversion efficiency by about 21.7%. The FRET contribution in [SmSiW9/JGB]9 was significantly higher than that in the [ErSiW9/JGB]9 films because of the structural change of SmSiW9 in the multilayer film. This strategy has immediate implications for using fluorescent organic cations to improve the performance of luminescence-switched POM functional devices.

Synthesis, characterization and photoluminescence properties of a benzoic modified lanthanide-containing polyoxometalate

One benzoic modified lanthanide-containing polyoxometalate (LCP) [N(CH3)4]3K2[Ho(C7H5O2)(H2O)2(α-PW11O39)]·11H2O (1) was successfully synthesized and further characterized by single crystal X-ray diffraction analyses, elemental analyses, powder X-ray diffraction (PXRD), thermogravimetric analyses (TGA) and IR spectroscopy. The single crystal X-ray diffraction analyses indicate that both benzoic acid ligand and [α-PW11O39]7− group directly coordinate Ho3+ center for the formation of a ternary system in 1. In addition, the polyanion of 1 can further form a 3D framework via the bridging K1 and K2 ions. The investigation of photoluminescence of 1 shows that 1 exhibits orange emission due to the characteristic f-f transitions of Ho3+ ion in the emission spectrum of 1.

Zwitterionic Surfactant Micelle-Directed Self-Assembly of Eu-Containing Polyoxometalate into Organized Nanobelts with Improved Emission and pH Responsiveness.

Recently, hybrid coassembly between polyoxometalates (POMs) and cationic building blocks provides an efficient strategy to greatly optimize POMs' functionality as well as their aggregate structural diversity. Adaptive hybrid supramolecular materials with enhanced luminescence have then been obtained from lanthanide-containing POMs. In this work, a commercially available and pH-switchable zwitterionic surfactant, tetradecyldimethylamine oxide (C14DMAO), was chosen to coassemble with a lanthanide-containing anionic POM [Na9(EuW10O36)·32H2O, abbreviated as EuW10] in water. The much improved red-emitting luminescent nanobelts at a C14DMAO/EuW10 molar ratio ( R) of 20 were obtained, which exhibited longer luminescence lifetime and higher quantum yield compared with EuW10 aqueous solution. After careful characterization of morphology and structure of nanobelts, an unusual axial lamellar aggregation arrangement mechanism was proposed. It was the partial protonation of C14DMAO at the solution pH of about 6.5 that led to positively charged micelles, being bridged by anionic EuW10 clusters to aggregate into such novel nanobelts under the synergetic effects of appropriate electrostatic, hydrogen-bonding, and hydrophobic interactions. The resulted pH-responsive luminescent nanobelts and their aggregation model should offer attractive references for preparing smart optical supramolecular materials.

Synthesis, characterization, and photoluminescence properties of three two-dimensional lanthanide-containing Dawson-type polyoxometalates.

Three [α2-P2W17O61]10--based lanthanide-containing polyoxometalates (LCPs) [N(CH3)4]3H8{Ln(H2O)7[Ln(H2O)3Ln(H2O)4(α2-P2W17O61)2]}·48H2O [Ln = La3+ (1), Ce3+ (2), and Pr3+ (3)] were successfully obtained by the reaction of a trivacant Dawson-type precursor [P2W15O56]12- and lanthanide ions under hydrothermal conditions. The compounds 1-3 were fully characterized by single-crystal X-ray diffraction and a series of analytical methods, including elemental analyses, powder X-ray diffraction (PXRD), IR spectroscopy, Raman spectroscopy, UV/vis diffuse reflectance spectroscopy, and thermogravimetric analysis (TGA). Single-crystal X-ray diffraction analyses indicated that 1-3 were isostructural and exhibited a 2D network architecture through the alternative connection of [α2-P2W17O61]10- and lanthanide ions. The photoluminescence properties of 1-3 were investigated at room temperature, and 3 displayed the characteristic emissions of a Pr3+ emitting center, indicating that POMs can transfer energy and sensitize Pr3+ emissions. The reason for the weak emission intensity of Pr3+ emitting ions was because the radiationless losses via a charge-transfer Pr → W state negatively affected the emission of the Pr3+ ions in LCPs.

Self-Assembly of Europium-Containing Polyoxometalates/Tetra-n-alkyl Ammonium with Enhanced Emission for Cu2+ Detection.

Lanthanide-containing polyoxometalates (POMs) can be used to detect various materials, but their luminescence in water has suffered enormous limitations due to the strong fluorescence quenching. Herein, to resolve this problem, three-dimensional nanoparticles built by mixed Weakley-type europium-containing POMs (Na9[EuW10O36]·32H2O, abbreviated to EuW10) and tetra-n-alkyl ammonium (TA) with enhanced fluorescent properties have been designed in aqueous solution using an ionic self-assembly (ISA) technique, which is mainly driven by the electrostatic interaction between EuW10 and TA. The morphology and fluorescent properties of the system as well as some influencing factors (alkyl chain length, amino group, and inorganic salt concentration) were systematically investigated. The results indicated that the fluorescent intensity of EuW10/tetramethylammonium bromide (TMAB) composite increased about 14 times, whereas the extent of increase of fluorescence for EuW10/tetraethylammonium bromide (TEAB) and EuW10/tetrabutylammonium bromide (TMAB) composites gradually decrease due to the bulkier steric hindrance of the longer alkyl chain. Besides, the luminescence of EuW10/TMAB nanoparticles is pH responsive, and the reversibility of their structures and luminescence can be realized upon the addition of NaOH/HCl. Moreover, the EuW10/TMAB system also shows great fluorescence-sensing behavior, which could detect Cu2+ with a detection limit of 0.15 μM. Our work provides a facile construction strategy for a functional fluorescent complex via POMs-based supramolecular self-assembly in aqueous solution, which will be further used in biomarkers and sensors.

Multicolor luminescent hybrid assembled materials based on lanthanide-containing polyoxometalates free from energy transfer crosstalk

Multicolor luminescent hybrid materials of a co-assembly of Ln-POMs, quinine, and block copolymer behave as molecular pixels without energy transfer crosstalk.

The application of spontaneous flocculation for the preparation of lanthanide-containing polyoxometalates intercalated layered double hydroxides: highly efficient heterogeneous catalysts for cyanosilylation

The lanthanide-containing polyoxometalates (POMs) have been intercalated into layered double hydroxides (LDHs) successfully through the spontaneous flocculation method, resulting in the formation of new heterogeneous LDHs–POMs catalysts Mg3Al–LnW10 (Ln=Eu, Tb and Dy). The synthetic method can prevent Mg2+/Al3+ cations leaching out of the LDHs during intercalation and successfully control the final Mg2+/Al3+ ratio in the corresponding Mg3Al–LnW10. Most importantly, the as-prepared Mg3Al–LnW10 can be obtained without the formation of the impurity phase. The heterogeneous Mg3Al–EuW10 exhibits highly efficient catalytic reactivity for cyanosilylation of various aldehydes and ketones under solvent-free conditions. The Mg3Al–EuW10 can be easily recovered and reused for at least 10 times without obvious decrease of catalytic activity and the composition and the structure of the Mg3Al–EuW10 catalyst remain stable. Moreover, the Mg3Al–EuW10-catalyzed cyanosilylation of benzaldehyde and hexanal gives the highest TON of 119,950 and 119,906, respectively.

From a versatile arsenotungstate precursor to a large lanthanide-containing polyoxometalate–carboxylate hybrid

From the versatile precursor [As2W19O67(H2O)]14− through combined disassembly and reassembly processes, the large, POM–carboxylate hybrid K14Li4H20[As6W58O206Ce4(pydc)2(H2O)6]·44H2O was obtained in conventional aqueous solution.

An efficient heterogeneous catalyst based on highly dispersed Na7H2LaW10O36·32H2O nanoparticles on mesoporous silica for deep desulfurization

A lanthanide-containing polyoxometalate of Na7H2LaW10O36·32H2O (LaW10) have been chemically anchored to the amino-modified mesoporous silica through an organic linker. The catalysts have been characterized by ICP-ES, FT-IR, N2 sorption measurements, UV–vis, MAS-NMR, and HR-TEM. It is found that the LaW10 nanoparticles are highly dispersed on mesoporous silica, and the resulting catalysts show highly efficient deep desulfurization in only 35min with almost 100% conversion of dibenzothiophene by using (SiO2/NH3+/LaW10)/[bmim]BF4 (bmim=1-butyl-3-methylimidazolium) in the presence of H2O2 as oxidant. Furthermore, deep desulfurization proceeds smoothly in model oil with S content as low as 100ppm. A scaled-up experiment, in which the volume of model oil increases from 5 to 1000mL with S content of 1000ppm, indicates that almost 100% sulfur removal can be achieved in 45min. Additionally, the catalyst could be recycled and reused at least ten times without any decrease of the catalytic efficiency. To the best of our knowledge, the (SiO2/NH3+/LaW10)/[bmim]BF4 catalyst system with H2O2 as oxidant exhibits great potential for practical application as desulfurization systems reported so far.

Deep Desulfurization by Amphiphilic Lanthanide‐Containing Polyoxometalates in Ionic‐Liquid Emulsion Systems under Mild Conditions

Amphiphilic lanthanide-containing polyoxometalates (POMs) were prepared by surfactant encapsulation. Investigation of these lanthanide-containing POMs in oxidative desulfurization (ODS) showed that highly efficient deep desulfurization could be achieved in only 14 min with 100% conversion of dibenzothiophene under mild conditions by using (DDA)(9)LaW(10)/[omim]PF(6) (DDA=dimethyldioctadecylammonium, omim=1-octyl-3-methyl-imidazolium) in the presence of H(2) O(2) . Furthermore, deep desulfurization proceeds smoothly in model oil with an S content as low as 50 ppm. A scaled-up experiment in which the volume of model oil was increased from 5 to 1000 mL with S content of 1000 ppm indicated that about 99% sulfur removal can be achieved in 40 mins in an ionic-liquid emulsion system. To the best of our knowledge, the (DDA)(9)LaW(10)/[omim]PF(6) catalyst system with H(2)O(2) as oxidant is one of the most efficient desulfurization systems reported so far.

A high nuclear lanthanide-containing polyoxometalate aggregate synthesized in choline chloride/urea eutectic mixture

A high nuclear lanthanide-containing polyoxometalate aggregate {[(CH3)3N(CH2)2OH]2(NH4)12}[Ce4(Mo4)(H2O)16(Mo7O24)4]·8H2O has been successfully synthesized in choline chloride/urea eutectic mixture. This method in synthesizing this anti-HIV activity aggregate has the advantages of convenience, facility, saving time and absence of additional noxious organic reagents. It has been characterized by XPS, IR, UV–vis spectra, TG analyses, power X-ray diffraction and single crystal X-ray diffraction. The CV property of the compound has also been tested.

Highly Efficient Extraction and Oxidative Desulfurization System Using Na7H2LaW10O36⋅32 H2O in [bmim]BF4 at Room Temperature

Highly efficient, deep desulfurization of model oil containing dibenzothiophene (DBT), benzothiophene (BT), or 4,6-dimethyldibenzothiophene (4,6-DMDBT) has been achieved under mild conditions by using an extraction and catalytic oxidative desulfurization system (ECODS) in which a lanthanide-containing polyoxometalate Na(7)H(2)LnW(10)O(36)⋅32 H(2)O (LnW(10); Ln = Eu, La) acts as catalyst, [bmim]BF(4) (bmim = 1-butyl-3-methylimidazolium) as extractant, and H(2)O(2) as oxidant. Sulfur removal follows the order DBT>4,6-DMDBT>BT at 30 °C. DBT can be completely oxidized to the corresponding sulfone in 25 min under mild conditions, and the LaW(10)/[bmim]BF(4) system could be recycled for ten times with only slight decrease in activity. Thus, LaW(10) in [bmim]BF(4) is one of the most efficient systems for desulfurization using ionic liquids as extractant reported so far.

ChemInform Abstract: Recent Advances in Lanthanide‐Containing Polyoxotungstates

AbstractReview: 42 refs.

Lanthanopolyoxotungstates in silica nanoparticles: multi-wavelength photoluminescent core/shell materials

Photoluminescent lanthanopolyoxotungstate core/shell nanoparticles are prepared by the encapsulation of lanthanide-containing polyoxometalates (POMs) with amorphous silica shells. The preparation of morphological well-defined core/shell nanoparticles is achieved by the hydrolysis of tetraethoxysilane in the presence of POMs using a reverse microemulsion method. The POMs used are decatungstolanthanoates of [Ln(W5O18)2]9− type (Ln(III) = Eu, Gd and Tb). Photoluminescence studies show that there is efficient emission from the POM located inside the SiO2 shells, through excitation paths that involve O → Eu/Tb and O → W ligand-to-metal charge transfer. It is also shown that the excitation of the POM containing europium(III) may be tuned towards longer wavelengths via an antenna effect, by coordination of an organic ligand such as 3-hydroxypicolinate. The POM/SiO2 nanoparticles form stable suspensions in aqueous solution having the advantage of POM stabilization inside the core and the possibility of further surface grafting of chemical moieties via well known derivatization procedures for silica surfaces. These features together with the possibility of tuning the excitation wavelength by modifying the coordination sphere in the lanthanopolyoxometalate, make this strategy promising to develop a new class of optical bio-tags composed of silica nanobeads with multi-wavelength photoluminescent lanthanopolyoxometalate cores.