RESUMO
The odd-even effect in luminescent [Eu2 (L)3 (H2 O)x ]â y(H2 O) complexes with aliphatic dicarboxylate ligands (L: OXA, MAL, SUC, GLU, ADP, PIM, SUB, AZL, SEB, UND, and DOD, where x=2-6 and y=0-4), prepared by the precipitation method, was observed for the first time in lanthanide compounds. The final dehydration temperatures of the Eu3+ complexes show a zigzag pattern as a function of the carbon chain length of the dicarboxylate ligands, leading to the so-called odd-even effect. The FTIR data confirm the ligand-metal coordination via the mixed mode of bridge-chelate coordination, except for the Eu3+ -oxalate complex. XRD results indicate that the highly crystalline materials belong to the monoclinic system. The odd-even effect on the 4 f-4 f luminescence intensity parameters (Ω2 and Ω4 ) is explained by using an extension of the dynamic coupling mechanism, herein named the ghost-atom model. In this method, the long-range polarizabilities ( α* ) were simulated by a ghost atom located at the middle of each ligand chain. The values of α* were estimated using the localized molecular orbital approach. The emission intrinsic quantum yield ( QLnLn ) of the Eu3+ complexes also presented an the odd-even effect, successfully explained in terms of the zigzag behavior shown by the Ω2 and Ω4 intensity parameters. Luminescence quenching due to water molecules in the first coordination sphere is also discussed and rationalized.
RESUMO
Persistent luminescence materials Lu2O3:R(3+),M (Pr,Hf(IV); Eu; or Tb,Ca(2+)) were successfully and rapidly (22 min) prepared by microwave-assisted solid-state synthesis (MASS) using a carbon microwave susceptor and H3BO3 as flux. Reaction times are reduced by up to 93% over previous synthetic methods, without special gases application and using a domestic microwave oven. All materials prepared with H3BO3 flux exhibit LuBO3 impurities that were quantified by Rietveld refinement from synchrotron radiation X-ray powder diffraction patterns. The flux does not considerably affect the crystalline structure of the C-Lu2O3, however. Scanning electron micrographs suggest low surface area when H3BO3 flux is used in the materials' synthesis, decreasing the amount of surface hydroxyl groups in Lu2O3 and improving the luminescence intensity of the phosphors. The carbon used as the susceptor generates CO gas, leading to complete reduction of Tb(IV) to Tb(3+) and partial conversion of Pr(IV) to Pr(3+) present in the Tb4O7 and Pr6O11 precursors, as indicated by X-ray absorption near-edge structure data. Persistent luminescence spectra of the materials show the red/near-IR, reddish orange, and green emission colors assigned to the 4f(n) â 4f(n) transitions characteristics of Pr(3+), Eu(3+), and Tb(3+) ions, respectively. Differences between the UV-excited and persistent luminescence spectra can be explained by the preferential persistent luminescence emission of R(3+) ion in the S6 site rather than R(3+) in the C2 site. In addition, inclusion of Hf(IV) and Ca(2+) codopants in the Lu2O3 host increases the emission intensity and duration of persistent luminescence due to generation of traps caused by charge compensation in the lattice. Photonic materials prepared by MASS with H3BO3 flux show higher persistent luminescence performance than those prepared by the ceramic method or MASS without flux. Color tuning of persistent luminescence in Lu2O3:R(3+),M provides potential applications in bioimaging as well as in solar cell sensitizers.