No abstract

No abstract

No abstract

In this study the optical spectroscopy, the excited state dynamics, and in particular the Tb3+ → Eu3+ energy transfer, have been investigated in detail both from the theoretical and experimental point of view in eulytite double phosphate hosts A3Tb­(PO4)3 (A = Sr, Ba) doped with Eu3+. It has been found that the energy transfer is strongly assisted by fast migration in the donor Tb3+ subset. Moreover, the transfer rates and efficiencies depend significantly on the nature of the divalent elements present in the structure and hence on the distances between Tb3+-Eu3+ nearest neighbors. It is shown that the competition between quadrupole–quadrupole and exchange interaction is crucial in accounting for the transfer rates.

No abstract

Much is known about the photoluminescence of lanthanide-containing systems, particularly amorphous silicates or organic-inorganic hybrids and crystalline metal-organic frameworks. Comparatively, stoichiometric microporous Ln-silicates are poorly studied. Here, we report the exceptional photoluminescence of microporous AV-24, K(7)[Ln(3)Si(12)O(32)] x xH(2)O (Ln(3+) = Sm(3+), Eu(3+), Gd(3+), Tb(3+)), the first silicate possessing Ln(3+)-O-Ln(3+) dimers (inter-Ln distance ca. 3.9 A), i.e., two edge-sharing {LnO(6)} octahedra embedded in a crystalline matrix. It is totally unprecedented that in AV-24 Eu(3+)-O-Eu(3+) dimers behave like discrete entities, i.e., molecules: they (i) have a unique emission signature, with pseudopoint group symmetry (C(i)), different from the symmetry (C(1)) of each individual constituent Eu(3+) ion, and (ii) exhibit the unusually long (5)D(0) lifetime of 10.29 ms (12 K). In accord with the experimental evidence, a molecular orbital model shows that the Eu(3+)-O-Eu(3+) dimers are energetically more stable than the individual metal ions.

Seven-coordinate, pentagonal-bipyramidal (PBP) complexes [Ln(bbpen)Cl] and [Ln(bbppn)Cl], in which Ln = Tb 3+ (products I and II), Eu 3+ (III and IV), and Gd 3+ (V and VI), bbpen 2− = N,N'-bis(2-oxidobenzyl)-N,N'bis(pyridin-2-ylmethyl)ethylenediamine, and bbppnpropanediamine, were synthesized and characterized by single-crystal X-ray diffraction analysis, alternating current magnetic susceptibility measurements, and photoluminescence (steady-state and time-resolved) spectroscopy.Under a static magnetic field of 0.1 T, the Tb 3+ complexes I and II revealed single-ion magnet (SIM) behavior. Also, upon excitation at 320 nm at 300 K, I and II presented very high absolute emission quantum yields (0.90 ± 0.09 and 0.92 ± 0.09 respectively), while the corresponding Eu 3+ complexes III and IV showed no photoluminescence.Detailed theoretical calculations on the intramolecular energy transfer (IET) rates for the Tb 3+ products indicated that both excited singlet and triplet ligand states contribute efficiently to the overall emission performance. The

ABSTRACT:The polarizability of the charge density in the overlap region of a chemical bond has already been correlated to its covalency in diatomic molecules and in lanthanide compounds. This concept has been successfully extended in the present contribution to alkali halide MX (M ¼ Li, Na, K and X ¼ F, Cl, Br) crystals. For more covalent compounds such as silicon, aluminum, and tin oxides the proposed treatment of the crystalline environment cannot be considered quantitative. However, for MX systems, good correlations between the chemical bond overlap polarizabilities and macroscopic properties such as molar polarizabilities and energy band gaps were established. In addition, the overlap region is regarded as a localized plasmon-like charge distribution (chemical bond overlap plasmon-CBOP), thus leading to absorption and Raman scattering processes by the overlap region, and electron energy-loss spectroscopy. The processes associated with CBOP excitations and energy-loss have been used to assign structures and shoulders present in spectra of MX crystals that have either not been assigned or ambiguously assigned as excitons or surface plasmons excitations.