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

The excited-state properties of the layered compound hydrogen uranyl phosphate (HUP), HUO2PO4·4H2O and of solids derived therefrom by intercalative ion-exchange reactions have been examined. The reaction exploited are given in eqn. (1)–(3). HUO2PO4 + M+ → MUO2PO4 + H+ (1) M = K, Ag, NH4, NC5H6 (pyridinium), n-C4H9NH3, n-C8H17NH3 HUO2PO4 + 12M2+ → M12OU2PO4 + H+ (2) M = Ca, Zn, Cu, (∼0.4 equivalents incorporated) HUO2PO4 + 13M3+ → M13UO2PO4 + 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 A. 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 A for HUP, NH4UP, Ca12UP, and n-C8H17NH3UP, respectively. All of the samples exhibit electronic absorption spectra characteristics of the UO2+2 chromophore; for substituent cation processing visible absorption bands, these transition appear superimposed in each spectrum. Except for the n-C8H17NH+3, Ag+, Cu2+, and Cr(urea)3+6 salts, the samples all exhibits yellow-green emission characteristic of the UO2+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 ∼ 102–103 μs include HUP and the NH+4 pyridinium+, K+, Ca2+, and Zn2+ derivatives. These solids also have radiative quantum efficiencies. or, approaching unity at 295 K. Values of τ and or have been used to calculate radiative (kr) and nonradiative (knr) are constants for excited-state decay. Values of kr are nearly constants at ∼(1–) × 103s −1 for the samples, whereas knr values span several orders of magnitude. Possible quenching mechanisms for the weakly emissive and nonemissive samples include excited-state energy transfer (CU2+, Eu3+, and Cr(urea)3+6 derivatives), electron transfer (Ag+ and Cu2+ derivatives) and H-atom abstraction (n-C4H9NH+3 and n-C8H17NH+3 derivatives). Partially Eu-substituted HUP provides evidence for excited-state energy transfer: both the UO2+2-based emission and pink Eu3+ luminescence are simultaneously observed excitation of of the UO2+2 chromophore. The relative intensities of the two types of emission vary with Eu content in both this system and Eu-substituted Ca12UP samples. Concentration effects on luminescence have also been investigated with Ag-substituted KUP. The UO2+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 (λmax ∼ 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.