Optical Spectroscopy of Ca₉Tb_1–<i>x</i>Eu_<i>x</i>(PO₄)₇ (<i>x</i> = 0, 0.1, 1): Weak Donor Energy Migration in the Whitlockite Structure

The luminescence spectroscopy of Tb³⁺ and Eu³⁺ has been studied in the Ca₉Tb­(PO₄)₇, Ca₉Eu­(PO₄)₇, and Ca₉Tb_0.9Eu_0.1(PO₄)₇ materials having a whitlockite structure, by using excitation in the near UV, vacuum UV and X-ray regions. The Eu³⁺ ion in Ca₉Eu­(PO₄)₇ is located mainly in two cationic sites, as evidenced by the fine structure of the ⁵D₀ → ⁷F₀ transition at 5 K. In the case of Ca₉Tb_0.9Eu_0.1(PO₄)₇, weak Tb³⁺ → Eu³⁺ energy transfer is observed upon excitation in the UV bands of Tb³⁺. The low efficiency of the transfer appears to be due to slow energy migration in the ⁵D₄ subset of the Tb³⁺ ions. The overall behavior is strongly affected by the multisite and disordered nature of the Tb-based whitlockite host

Circularly Polarized Luminescence from an Eu(III) Complex Based on 2‑Thenoyltrifluoroacetyl-acetonate and a Tetradentate Chiral Ligand

A new chiral complex {[Eu<b>L</b>(tta)₂(H₂O)]­CF₃SO₃; <b>L</b> = <i>N</i>,<i>N</i>′-bis­(2-pyridylmethylidene)-1,2-(<i>R</i>,<i>R</i> + <i>S</i>,<i>S</i>)-cyclohexanediamine; tta = 2-thenoyltrifluoroacetyl-acetonate} has been synthesized and characterized from a structural and spectroscopic point of view. The molecular structure in the solid state shows the presence of one chiral <b>L</b>, two tta, and one water molecules bound to the metal center. <b>L</b> and tta molecules can efficiently harvest and transfer to Eu­(III) the UV light absorbed in the 250–400 nm range. The forced electric-dipole ⁵D₀ → ⁷F₂ emission band dominates the Eu­(III) emission spectra recorded in the solid state and in solution of acetonitrile or methanol and the calculated intrinsic quantum yield of the metal ion is around 40–50%. The light emitted by the enantiopure complex shows a sizable degree of polarization with a maximum value of the emission dissymmetry factor (<i>g</i>_lum) equal to 0.2 in methanol solution. If compared with the complex in the solid state or in acetonitrile solution, then the first coordination sphere of Eu­(III) when the complex is dissolved in methanol is characterized by the presence of one CH₃OH molecule instead of water. This fact is related to different Eu­(III) CPL signatures in the two solvents

Dynamics of the Energy Transfer Process in Eu(III) Complexes Containing Polydentate Ligands Based on Pyridine, Quinoline, and Isoquinoline as Chromophoric Antennae

In this work, we investigated from a theoretical point of view the dynamics of the energy transfer process from the ligand to Eu(III) ion for 12 isomeric species originating from six different complexes differing by nature of the ligand and the total charge. The cationic complexes present the general formula [Eu(L)(H2O)2]+ (where L = bpcd2– = N,N′-bis(2-pyridylmethyl)-trans-1,2-diaminocyclohexane N,N′-diacetate; bQcd2– = N,N′-bis(2-quinolinmethyl)-trans-1,2-diaminocyclohexane N,N′-diacetate; and bisoQcd2– = N,N′-bis(2-isoquinolinmethyl)-trans-1,2-diaminocyclohexane N,N′-diacetate), while the neutral complexes present the Eu(L)(H2O)2 formula (where L = PyC3A3– = N-picolyl-N,N′,N′-trans-1,2-cyclohexylenediaminetriacetate; QC3A3– = N-quinolyl-N,N′,N′-trans-1,2-cyclohexylenediaminetriacetate; and isoQC3A3– = N-isoquinolyl-N,N′,N′-trans-1,2-cyclohexylenediaminetriacetate). Time-dependent density functional theory (TD-DFT) calculations provided the energy of the ligand excited donor states, distances between donor and acceptor orbitals involved in the energy transfer mechanism (RL), spin-orbit coupling matrix elements, and excited-state reorganization energies. The intramolecular energy transfer (IET) rates for both singlet-triplet intersystem crossing and ligand-to-metal (and vice versa) involving a multitude of ligand and Eu(III) levels and the theoretical overall quantum yields (ϕovl) were calculated (the latter for the first time without the introduction of experimental parameters). This was achieved using a blend of DFT, Judd–Ofelt theory, IET theory, and rate equation modeling. Thanks to this study, for each isomeric species, the most efficient IET process feeding the Eu(III) excited state, its related physical mechanism (exchange interaction), and the reasons for a better or worse overall energy transfer efficiency (ηsens) in the different complexes were determined. The spectroscopically measured ϕovl values are in good agreement with the ones obtained theoretically in this work