Solution Chemistry of Europium(III) Aqua Ion at Micromolar Concentrations as Probed by Direct Excitation Luminescence Spectroscopy

Direct excitation europium(III) luminescence spectroscopy is used to study the speciation of aqueous europium(III) ions at micromolar concentrations and near neutral pH. The pH and concentration dependence of the europium(III) ⁷F₀→⁵D₀ excitation peak is consistent with the formation of both mononuclear and dinuclear europium(III) hydroxide complexes at pH 6.5. Luminescence intensity and lifetime quenching studies in the presence of Nd^III at pH 5.0 and 6.5 support the formation of a dinuclear complex at pH 6.5. Steady state excitation and time‐resolved luminescence spectroscopy are consistent with the formation of innersphere nitrate and fluoride complexes, but outersphere perchlorate and chloride complexes at pH 6.5 and 5.0.     

Luminescent properties of Ba_3(PO_4)_2:Eu,Tb phosphors

Luminescence of europium (Ⅲ),europium(Ⅱ) and terbium(Ⅲ) has been observed in Bas(PO4)2:Eu,Tb phosphors which are synthesized in air atmosphere.The valence state of europium is influenced by amount of terbium.It is notable that the relative intensity of the emission spectra peaks corresponding to Eu2+ is increased if the amount of Tb3+ is increased.These phenomena can be explained by an electron transfer mechanism.We predict a new kind of two-rare-earth codoped trichromatic phosphors in Ba3(PO4)2 matrix.     

Sorption of a complex of europium (III) with acetylacetone on hydrophobized hexadecyl silica gel and hyper crosslinked polysterene

The sorption of a complex of europium (III) with acetylacetone on silica gel chemically modified with hexadecyl groups (SiO₂-C₁₆) and hyper crosslinked polystyrene (HLPS) was studied. Maximum extraction was observed at pH 5–7 when SiO₂-C₁₆ was used as the sorbent and at pH 4–7 in the case of crosslinked polystyrene. The partition coefficients for HLPS and silica gel were calculated as 7 × 10³ and 1 × 10² cm³/g, respectively. Quantitative extraction of the europium (III) complex was possible in dynamic conditions using a microcolumn (length, 10 mm; internal diameter, 3mm) packed with HLPS at pH 5 (10–50 mL sample volume). Desorption of europium using solutions of nitric acid at different concentrations was investigated. Quantitative desorption was achieved using 5 mL of 1 M HNO₃. A linear range of detection was observed at an amount of europium from 5 to 25 μg in a 10-mL sample (650 nm).     

Cathodic europium(III) electroluminescence at an oxide-coated aluminum electrode

Electroluminescence is produced at an oxide-coated aluminum electrode during cathodic polarization of the electrode in an acetate solution containing nitrate and traces of europium(III). The europium(III) ion in the ⁵D₀ state is the emitter. Linear log-log calibration plots are obtained for the range 1 × 10^?8?1 × 10^?5 M europium(III). A mechanism for the cathodic luminescence is proposed.     

Crystal Structure and Luminescence of a Europium Nitrate Complex with Furancarboxylic Acid and 2,2′-Bipyridine

Abstract

A quaternary mixed ligand europium complex, [Eu(FA)₂NO₃bipy]₂, has been synthesized, where FA = α-furancarboxylic acid anion and bipy=2,2′-bipyridine. The europium complex crystallizes in the triclinic system, space group P1. Its structure was determined by X-ray diffraction methods. The two europium ions in the dimer are held together by four carboxylate groups of furancarboxylic acid and each europium ion is further bonded to one chelated bidentate nitrate and one 2,2′-bipyridine molecule. The coordination modes of the four carboxylate groups are divided into two types, bidentate bridging and tridentate bridging, making a coordination number of 9. Excitation and luminescence spectra observed at 77 K show that the europium ion site in the crystal has low symmetry and emission 5D ₁→⁷ F_J of the Eu³⁺ ion disappears after 20 μs.     

Regulation of excitation and luminescence efficiencies of europium and terbium benzoates and 8-oxyquinolinates by modification of ligands

The opportunities of optimisation of luminance of the lanthanide compounds by modification of ligands are discussed. Variations of the excitation and luminescence efficiencies at introduction of nitro- (NO₂), sulfo- (SO₃⁻), hydroxy- (OH), amino- (NH₂), and phenylamino- (NHC₆H₅) groups in the aromatic ligands were studied. Investigation of luminescence and luminescence excitation spectra of europium and terbium compounds with 10 derivatives of benzoic acid, 2-furancarboxylic acid and their adducts with 1,10-phenanthroline and 2,2′-bipyridine was undertaken. Study of the spectra of lanthanide 8-oxyquinolinates was carried out also. Luminescence efficiencies were measured at 77 and 300 K. Paths of the energy transfer from the ligands to Ln³⁺ ion were examined. Influence of radicals on the energies of the ligand triplet states and on the energies of the ligand–metal charge transfer states (LM CTS) of europium compounds was analysed. High luminescence efficiencies of europium and terbium benzoates, and terbium anthranylates and salicylates were obtained. Effect of increasing the luminescence efficiencies of europium salicylates and 8-oxyquinolinates at introduction of acceptor nitro-and sulfo-groups in the ligand was revealed. Channel of dissipation of the excitation energy through the ligand π^*–n transition of europium and terbium nitro- and dinitrobenzoates was found. Influence of relative positions of the lowest triplet levels of two non-equivalent ligands of compound on the energy transfer to Eu³⁺ and Tb³⁺ ions was considered.     

The Zintl Phase Eu2Si

The Zintl phase Eu₂Si was synthesized from elemental europium and silicon in a sealed tantalum tube in a high‐frequency furnace at 1270 K and subsequent annealing at 970 K. Investigation of the sample by X‐ray powder and single crystal techniques revealed: Co₂Si (anti‐PbCl₂) type, space group Pnma, a = 783.0(1), b = 504.71(9), c = 937.8(1) pm, wR2 = 0.1193, 459 F² values and 20 variables. The structure contains two europium and one silicon site. ¹⁵¹Eu Mössbauer spectroscopic data show a single signal at an isomer shift of −9.63(3) mm/s, compatible with divalent europium. Within the Zintl concept electron counting can be written as (2Eu²⁺)⁴⁺Si⁴⁻, in agreement with the absence of Si‐Si bonding. Each silicon atom has nine europium neighbors in the form of a tri‐capped trigonal prism. The silicon coordinations of the Zintl phases Eu₂Si, Eu₅Si₃, EuSi, and EuSi₂ are compared.     

Catalytic activity of some organolanthanoid derivatives in styrene and propene polymerization

Organolanthanoids of several classes were examined as potential styrene and propene polymerization catalysts. They are: molecular hydrides of divalent lanthanoids (samarium, europium, ytterbium); naphthalene and stilbene complexes of neodymium(III), samarium(II), europium(II), ytterbium(II), lutetium(III); amides and alkoxides (including heterobifunctional derivatives) of praseodymium(III), neodymium(III), samarium(II), europium(II), thulium(III), ytterbium(II, III); thiolate of samarium(III); phenyl and phenylethinyl derivatives of europium(II), thulium(III), ytterbium(II); methylytterbium cluster Yb₈ (μ‐CH₃)₁₄(μ‐CH₂)(THF)₆; heterobimetallic samarium(II), ytterbium(II, III) complexes; diazabutadiene ytterbium(III) derivatives; metallic praseodymium and ytterbium, activated by iodine. The highest activity in styrene polymerization revealed hydrides, naphthalene and stilbene complexes of samarium(II), europium(II) and ytterbium(II). In the propene polymerization only [(η⁵‐C₅H₄)CH₂CH­(CH₂OBu)(η¹‐O)]YbMe(THF) displayed noticeable activity.Copyright © 2001 John Wiley & Sons, Ltd.     

Ultrasmall Superparamagnetic Iron Oxide Nanoparticles with Europium(III) DO3A as a Bimodal Imaging Probe

A new prototype consisting of ultrasmall superparamagnetic iron oxide (USPIO) nanoparticles decorated with europium(III) ions encapsulated in a DO3A organic scaffold was designed as a platform for further development of bimodal contrast agents for MRI and optical imaging. The USPIO nanoparticles act as negative MRI contrast agents, whereas the europium(III) ion is a luminophore that is suitable for use in optical imaging detection. The functionalized USPIO nanoparticles were characterized by TEM, DLS, XRD, FTIR, and TXRF analysis, and a full investigation of the relaxometric and optical properties was conducted. The typical luminescence emission of europium(III) was observed and the main red emission wavelength was found at 614 nm. The relaxometric study of these ultrasmall nanoparticles showed r₂ values of 114.8 mm ^?1_Fes^?1 at 60 MHz, which is nearly double the r₂ relaxivity of Sinerem^®.     

Ion beam induced emissions from solid europium compounds

Impact of energetic heavy particles on europium compound surfaces gives rise to radiative optical emission from reflected and sputtered particles and from the excited states of the solid compounds. In the present paper we discuss the optical spectrum and the sputtered secondary ion mass spectrum observed when solid europium oxide (Eu₂O₃) and europium chloride (EuCl₃) are bombarded with 90 keV Ar⁺ ions from an ion accelerator. We observe the reduction reaction in solid europium chloride (EuCl₃) by bombardment with a 20 A/cm² beam of 90 keV Ar⁺ ions.     

Photokinetics of gas phase europium(III) chelated with hexafluoroacetylacetonate

The europium chelate salt, triethylammonium tetrakis-hexafluoroacetylacetone europium(III), was evaporated into the gas phase and the luminescence properties of the gas phase material were examined. The rate of decay of luminescence of the gas phase europium chelate was observed to increase with temperature. An activation energy of 2650 cm⁻¹ was measured for the barrier to non-radiative decay from the ⁵D₀ state. The quantum yield for formation of the ⁵D₀ state showed a slight temperature dependence at temperatures above 220°C.     

Complexes of Rare Earths and Dipicolinato Ions Encapsulated in X- and Y-zeolites: Luminescence Properties

The complexes {Ln(DP)}Z where Ln³⁺ = La³⁺ or Eu³⁺, DP is the dipicolinate ion (2,6)-pyridine dicarboxylate: C₅H₃N(COO^-)₂ and Z = one of the faujasite-type X or Y zeolites have been synthesized and investigated by XRD, Raman and IR spectroscopy, and Eu³⁺ luminescence spectroscopy. The rare earth complexes are synthesized inside the super-cages of the zeolites; the degree of complexation never exceeds1DP/1Ln. Only the Ln ions, which are in the super-cages, may be complexed. Luminescent europium complexes encapsulated in zeolite matrices were obtained. The ⁵D₀ F_J luminescence observed under excitation into the lowest-energy ligand-centered absorption band (275 nm)indicates that a DP to Eu energy transfer occurs in these systems. The complex versus the zeolite framework conformations influence the europium emission characteristics: the transfer is more efficient when the complexed europium [Eu(DP)]⁺ is directly bonded to the framework oxygen atoms rather than to a residual water molecule.     

Lanthanide‐to‐Lanthanide Energy‐Transfer Processes Operating in Discrete Polynuclear Complexes: Can Trivalent Europium Be Used as a Local Structural Probe?

This work, based on the synthesis and analysis of chemical compounds, describes a kinetic approach for identifying intramolecular intermetallic energy‐transfer processes operating in discrete polynuclear lanthanide complexes, with a special emphasis on europium‐containing entities. When all coordination sites are identical in a (supra)molecular complex, only heterometallic communications are experimentally accessible and a Tb→Eu energy transfer could be evidenced in [TbEu( L5 )(hfac)₆] (hfac=hexafluoroacetylacetonate), in which the intermetallic separation amounts to 12.6 Å. In the presence of different coordination sites, as found in the trinuclear complex [Eu₃( L2 )(hfac)₉], homometallic communication can be induced by selective laser excitation and monitored with the help of high‐resolution emission spectroscopy. The narrow and non‐degenerated character of the Eu(⁵D₀?⁷F₀) transition excludes significant spectral overlap between donor and acceptor europium cations. Intramolecular energy‐transfer processes in discrete polynuclear europium complexes are therefore limited to short distances, in agreement with the Fermi golden rule and with the kinetic data collected for [Eu₃( L2 )(hfac)₉] in the solid state and in solution. Consequently, trivalent europium can be considered as a valuable local structural probe in discrete polynuclear complexes displaying intermetallic separation in the sub‐nanometric domain, a useful property for probing lanthanido‐polymers.     

Reduction of Eu3 + to Eu2 + in Al-Codoped Silica Glasses Fabricated by the Sol-Gel Technique and CO2-Laser Processing

The spectroscopic properties of europium in aluminium codoped silica glasses produced by the sol-gel technique have been studied with respect to the dopant concentrations and the thermal processing applied to the samples. After thermal annealing at temperatures up to 950_°C the bright red fluorescence around 613 nm characteristic for the trivalent europium ions (Eu^{3 +}) has been observed. The lifetime was measured to be 0.1–2.4 ms depending on dopant concentrations and thermal treatment. Subsequent CO₂-laser processing in air (short time remelting) gave rise to a bright blue fluorescence consisting of two broad bands, lying around 450 and 490 nm, with their peak position depending on the ratio between the aluminium and europium concentrations. The fluorescence lifetimes were found to be shorter than 1 s. This blue fluorescence is attributed to the divalent europium ion (Eu^{2 +}), leading to the conclusion that the CO₂-laser processing of europium doped alumina-silica glasses resulted in the reduction of the trivalent to the divalent europium ion. Laser processing could therefore be a valid alternative to conventional thermal annealing for the generation of Eu^{2 +} in alumina-silica glasses.     

Formation and dissociation kinetics of Eu(III) complexes with H5do3ap and similar dota-like ligands

Complexation kinetics of the europium(III)-H₅do3ap complex (1,4,7,10-tetraazacyclododecane-1,4,7-triacetic-10-(methylphosphonic acid)) was studied at 25 °C and I = 0.1 M KCl and compared with the analogous H₄dota-like ligands. The mechanism of the formation reactions between the europium(III) ion and H₃do3a, H₄dota and H₄dota analogs having one methylphosphonic (H₅do3ap) or phosphinic acid pendant arms with a propionate side chain (H₅do3ap^PrA) was proposed on the basis of TRLIFS measurements. The experimental data of the dissociation kinetics of the europium(III) complexes with H₅do3ap and H₅do3ap^PrA ligands show that there is no change in reactivity of the complexes due to the side chain of the phosphinic acid bifunctional chelate. The TRLIFS study of the proton-assisted decomplexation reaction of the europium(III) complexes with H₃do3a, H₄dota, H₅do3ap and H₅do3ap^PrA demonstrates different behaviour influenced by a change in the number of water molecules during the reaction. The advantages and disadvantages of the TRLIFS methodology as a new experimental technique for simultaneous evaluation of kinetics and reaction mechanisms are discussed.     

Pressure‐induced Changes of the Crystal Structure of Eu4P3

Pressure‐induced structural changes and electronic properties of rhombohedral Eu₄P₃ were characterised by means of X‐ray powder diffraction and X‐ray absorption spectroscopy at the Eu L_III threshold. The measurements at low pressures indicate oxidation states of the europium atoms which are compatible with a composition Eu₃²⁺Eu³⁺P₃. At a pressure of 8(1) GPa, Eu₄P₃ undergoes a structural phase transition. The cubic high pressure modification with (anti‐)Th₃P₄ type crystal structure is also identified as a compound with a non‐integer average oxidation state of the europium atoms.     

Indirect photometric determination of europium(II) in some inorganic materials containing aliovalent europium species

A procedure is proposed for the indirect spectrophotometric determination of trace europium(II) in poorly soluble inorganic materials. The approach based on sample dissolution in the presence of excess vanadium(V) salts followed by the determination of vanadium(IV), equivalent to the concentration of europium(II), is used. The photometric determination of V(IV) is based on its oxidation with a potassium permanganate solution followed by the measurement of absorbance of excess MnO₄⁻. The procedure is applied to the analysis of EuF_{3 − x} with low europium(II) concentrations in the study the dissolution products of europium fluorides in salt melts and in the study the products of sorption europium(II) by strontium fluorapatite from salt melts. The proposed procedure is rapid and simple. It possesses satisfactory performance characteristics.     

CRYSTAL STRUCTURE AND LUMINESCENCE OF (4, 4′ -DIMETHYL-2, 2′ -BIPYRIDINE)-TRIS(BENZOATE)EUROPIUM(III)

Abstract

A mixed ligand europium complex, [Eu(BA)₃dmbpy]₂, has been prepared, where BA = benzoate and dmbpy = 4, 4′-dimethyl-2, 2′-bipyridine. The complex crystallizes in the tri-clinic system, space group P ^? ₁. Its structure has been determined using X-ray diffraction methods. The two europium ions in the molecule are held together by four carboxylate groups of benzoic acid and each europium ion is further bonded to one bidentate carboxylate group and one 4, 4′-dimethyl-2, 2′-bipyridine molecule. Excitation and luminescence spectra observed at 77 K show that the europium site in the crystal has low symmetry and changes of the chemical surroundings of the europium ion in the molecule depend mainly on the flexibility of 4, 4′-dime-thyl-2, 2′-bibyridine.     

Metal‐Ion Sensing Europium(III) Complexes with Bidentate Phosphine Oxide Ligands Containing a 2,2′‐Bipyridine Framework

Novel Eu^III complexes with bidentate phosphine oxide ligands containing a bipyridine framework, i.e., [3,3′‐bis(diphenylphosphoryl)‐2,2′‐bipyridine]tris(hexafluoroacetylacetonato)europium(III) ([Eu(hfa)₃(BIPYPO)]) and [3,3′‐bis(diphenylphosphoryl)‐6,6′‐dimethyl‐2,2′‐bipyridine]tris(hexafluoroacetylacetonato)europium(III) ([Eu(hfa)₃(Me‐BIPYPO)]), were synthesized for lanthanide‐based sensor materials having high emission quantum yields and effective chemosensing properties. The emission quantum yields of [Eu(hfa)₃(BIPYPO)] and [Eu(hfa)₃(Me‐BIPYPO)] were 71 and 73%, respectively. Metal‐ion sensing properties of the Eu^III complexes were also studied by measuring the emission spectra of Eu^III complexes in the presence of Zn^II or Cu^II ions. The metal‐ion sensing and the photophysical properties of luminescent Eu^III complexes with a bidentate phosphine oxide containing 2,2′‐bipyridine framework are demonstrated for the first time.     

Green Luminescence of Divalent Europium in the Hydride Chloride EuHCl

Luminescence properties of divalent europium in the mixed‐anion hydride chloride EuHCl were studied for the first time. Olive‐green single crystals of EuHCl (PbFCl‐type structure: tetragonal, P4/nmm, a = 406.58(3) pm, c = 693.12(5) pm, c/a = 1.705, Z = 2) resulted from the reaction of elemental europium (Eu), sodium hydride (NaH) and sodium chloride (NaCl), while powder samples were prepared from the binary components europium dihydride (EuH₂) and dichloride (EuCl₂). Low temperature X‐ray powder diffraction proved the absence of phase transitions for 12(2) K ≤ T ≤ 295(2) K. Bright green emission was observed under UV‐excitation and assigned to the 4f⁶5d¹–4f⁷ transition of divalent europium. Temperature‐dependent luminescence absorption and emission, as well as lifetime measurements were carried out on single crystal and powder samples. Surprisingly, only limited concentration quenching was found. Additionally, two emission bands (485 and 510 nm) are observed, whose intensity ratio depends strongly on temperature. In order to explain this behavior for a single Eu²⁺ site, we suggest either a dynamical Jahn–Teller effect in the excited 5d¹ state or emission from both a 4f⁶5d¹ state and a trapped exciton state.