Rare earth metal-containing polymers. 4. Energy transfer from uranyl to europium ions in ionomers

Sensing Uranyl(VI) Ions by Coordination and Energy Transfer to a Luminescent Europium(III) Complex

The release of uranyl(VI) is a hazardous environmental issue, with limited ways to monitor accumulation in situ. Here, we present a method for the detection of uranyl(VI) ions through the utilization of a unique fluorescence energy transfer process to europium(III). Our system displays the first example of a "turn-on" europium(III) emission process with a small, water-soluble lanthanide complex triggered by uranyl(VI) ions.

An investigation of the interactions of Eu³⁺ and Am³⁺ with uranyl minerals: implications for the storage of spent nuclear fuel

The reaction of a number of uranyl minerals of the (oxy)hydroxide, phosphate and carbonate types with Eu(iii), as a surrogate for Am(iii), have been investigated. A photoluminescence study shows that Eu(iii) can interact with the uranyl minerals Ca[(UO2)6(O)4(OH)6]·8H2O (becquerelite) and A[UO2(CO3)3]·xH2O (A/x = K3Na/1, grimselite; CaNa2/6, andersonite; and Ca2/11, liebigite). For the minerals [(UO2)8(O)2(OH)12]·12H2O (schoepite), K2[(UO2)6(O)4(OH)6]·7H2O (compreignacite), A[(UO2)2(PO4)2]·8H2O (A = Ca, meta-autunite; Cu, meta-torbernite) and Cu[(UO2)2(SiO3OH)2]·6H2O (cuprosklodowskite) no Eu(iii) emission was observed, indicating no incorporation into, or sorption onto the structure. In the examples with Eu(3+) incorporation, sensitized emission is seen and the lifetimes, hydration numbers and quantum yields have been determined. Time Resolved Laser Induced Fluroescence Spectroscpoy (TRLFS) at 10 K have also been measured and the resolution enhancements at these temperatures allow further information to be derived on the sites of Eu(iii) incorporation. Infrared and Raman spectra are recorded, and SEM analysis show significant morphology changes and the substitution of particularly Ca(2+) by Eu(3+) ions. Therefore, Eu(3+) can substitute Ca(2+) in the interlayers of becquerelite and liebigite and in the structure of andersonite, whilst in grimselite only sodium is exchanged. These results have guided an investigation into the reactions with (241)Am on a tracer scale and results from gamma-spectrometry show that becquerelite, andersonite, grimselite, liebigite and compreignacite can include americium in the structure. Shifts in the U[double bond, length as m-dash]O and C-O Raman active bands are similar to that observed in the Eu(iii) analogues and Am(iii) photoluminescence measurements are also reported on these phases; the Am(3+) ion quenches the emission from the uranyl ion.

A case study of energy transfer mechanism from uranium to europium in ZnAl₃O₄ spinel host by photoluminescence spectroscopy

Zinc aluminate (ZAO), a member of spinel class of inorganic compounds has been of much interest of late due to its wide range of use in catalysis, optical, electronic and ceramic industries. When doped with several lanthanides, this material has proved to be a potential host matrix for phosphors. As lanthanides suffer from poor (direct) excitation and emission cross sections, the use of a co-dopant ion can help to circumvent this and extract better emission from a lanthanide doped ZAO system. In this connection, energy transfer mechanism from uranium to europium in the ZAO host was investigated by photoluminescence spectroscopic technique. It was seen that uranium gets stabilized in the hexavalent state as UO6(6-) (octahedral uranate) where as the lanthanide ion, Eu is stabilized in its trivalent state in the ZAO host. In the co-doped system, an efficient energy transfer pathway from the uranate to europium ion was observed. Based upon emission and life time data a suitable mechanism was proposed for the energy transfer (quenching) process. It was proposed that after excitation by photons, the uranate ions transfer their energy to nearby (5)D1 level of Eu(3+) ions which non-radiatively de-excites to the corresponding lower levels of (5)D0. Further this (5)D0 level decays in a radiative mode to the (7)F manifold giving the characteristic emission profile of trivalent Eu. It was proposed that both static and dynamic types of energy transfer mechanism were responsible for this process.

Flux synthesis, crystal structure, and photoluminescence of a heterometallic uranyl-europium germanate with U═O-Eu linkage: K4[(UO2)Eu2(Ge2O7)2]

Crystals of a uranyl-europium germanate, K4[(UO2)Eu2(Ge2O7)2], have been grown at high temperature from a KF-MoO3 flux and structurally characterized by single-crystal X-ray diffraction. The structure contains PaCl5-type chains formed of edge-sharing EuO7 pentagonal bipyramids that are connected by digermanate groups such that layers of europium germanate are formed. Neighboring layers are further linked by UO6 tetragonal bipyramids through uranyl ion oxygen atoms to generate a 3D framework with 6-ring channels where the K(+) cations are located. The structure contains an unusual heterometallic U═O-Eu linkage. Photoluminescence studies show strong red emission at room temperature. The relative intensities of the (5)D0 → (7)F1 and (5)D0 → (7)F2 transitions confirm that the europium site lacks an inversion center. All of the emission lines show similar decay curves, which can be well-fitted by a single exponential decay function with radiative lifetimes of 0.53 ± 0.03 ms. No emission is observed in the region from 450 to 550 nm typical of the uranyl cation, indicating that, upon uranyl excitation, the energy is either transferred to the Eu(3+) centers or lost to nonradiative processes.

Uranyl sensitization of samarium(III) luminescence in a two-dimensional coordination polymer

Heterometallic carboxyphosphonates UO(2)(2+)/Ln(3+) have been prepared from the hydrothermal reaction of uranyl nitrate, lanthanide nitrate (Ln = Sm, Tb, Er, Yb), and phosphonoacetic acid (H(3)PPA). Compound 1, (UO(2))(2)(PPA)(HPPA)(2)Sm(H(2)O)·2H(2)O (1) adopts a two-dimensional structure in which the UO(2)(2+) metal ions bind exclusively to the phosphonate moiety, whereas the Ln(3+) ions are coordinated by both phosphonate and carboxylate functionalities. Luminescence studies of 1 show very bright visible and near-IR samarium(III)-centered emission upon direct excitation of the uranyl moiety. The Sm(3+) emissive state exhibits a double-exponential decay with lifetimes of 67.2 ± 6.5 and 9.0 ± 1.3 μs as measured at 594 nm, after excitation at both 365 and 420 nm. No emission is observed in the region typical of the uranyl cation, indicating that all energy is either transferred to the Sm(3+) center or lost to nonradiative processes. Herein we report the synthesis, crystal structure, and luminescent behavior of 1, as well as those of the isostructural terbium, erbium, and ytterbium analogues.