Derivatives of hydrogen uranyl phosphate: excited-state properties of a family of lamellar solids

Abstract

The excited-state properties of the layered compound hydrogen uranyl phosphate (HUP), HUO 2PO 4·4H 2O and of solids derived therefrom by intercalative ion-exchange reactions have been examined. The reaction exploited are given in eqn. (1)–(3). HUO 2PO 4 + M + → MUO 2PO 4 + H + (1) M = K, Ag, NH 4, NC 5H 6 (pyridinium), n-C 4H 9NH 3, n-C 8H 17NH 3 HUO 2PO 4 + 1 2 M 2+ → M 1 2 OU 2PO 4 + H + (2) M = Ca, Zn, Cu, (∼0.4 equivalents incorporated) HUO 2PO 4 + 1 3 M 3+ → M 1 3 UO 2PO 4 + H + (3) M = Cr(urea) 6, Eu (∼0.07 equivalents incorporated) The products of these reactions have all been characterized by elemental analysis, IR spectroscopy, and X-ray powder diffraction. The latter reveals that all compounds retain the lamellar structure of HUP and can be indexed in tetragonal symmetry, using c lattice values derived from 001 data and a lattice values of ∼6.99 Å. Although the a values are roughly constants, the interlamellar spacings (distance from the middle of one layer to the middle of the adjacent layer) vary widely; typical values are 8.69, 9.01, 10.34 and 18.76 Å for HUP, NH 4UP, Ca 1 2 UP, and n-C 8H 17NH 3UP, respectively. All of the samples exhibit electronic absorption spectra characteristics of the UO 2+ 2 chromophore; for substituent cation processing visible absorption bands, these transition appear superimposed in each spectrum. Except for the n-C 8H 17NH + 3, Ag +, Cu 2+, and Cr(urea) 3+ 6 salts, the samples all exhibits yellow-green emission characteristic of the UO 2+ 2 moiety when excited with blue or near-UV light at 295 K. Emission decay curves are exponential for all of the emissive solids and yields lifetimes, τ, ranging from ∼ 1–450 μs. Samples having τ values of ∼ 10 2–10 3 μs include HUP and the NH + 4 pyridinium +, K +, Ca 2+, and Zn 2+ derivatives. These solids also have radiative quantum efficiencies. ø r, approaching unity at 295 K. Values of τ and ø r have been used to calculate radiative (k r) and nonradiative (k nr) are constants for excited-state decay. Values of k r are nearly constants at ∼(1–) × 10 3s −1 for the samples, whereas k nr values span several orders of magnitude. Possible quenching mechanisms for the weakly emissive and nonemissive samples include excited-state energy transfer (CU 2+, Eu 3+, and Cr(urea) 3+ 6 derivatives), electron transfer (Ag + and Cu 2+ derivatives) and H-atom abstraction (n-C 4H 9NH + 3 and n-C 8H 17NH + 3 derivatives). Partially Eu-substituted HUP provides evidence for excited-state energy transfer: both the UO 2+ 2-based emission and pink Eu 3+ luminescence are simultaneously observed excitation of of the UO 2+ 2 chromophore. The relative intensities of the two types of emission vary with Eu content in both this system and Eu-substituted Ca 1 2 UP samples. Concentration effects on luminescence have also been investigated with Ag-substituted KUP. The UO 2+ 2-based emission is partially quenched in samples prepared from solution in which the Ag:K ratio is as little as ∼ 1:10,000. Some evidence for an exciton mechanism has been obtained in this studies. Derivatives of HUP based on cationic transition metal complexes such as Cr(urea) 3+ 6 afford interesting comparisons of excited-state properties with solution environments. To illustrate, the Cr complex exhibits fluorescence and phosphorescence at 77 K in an EPA glass. Absorption bands due to this complex are relatively unaffected when the complex is incorporated into the HUP lattice (λ max ∼ 635 nm), but no emission is observed from the solid at 77 K. These observation illustrate the role of environment on excited-state and the versatility of HUP as a host lattice for evaluating environmental effects.