Hermi F. Brito

Spectroscopic properties of a new light-converting device Eu(thenoyltrifluoroacetonate)3 2(dibenzyl sulfoxide). A theoretical analysis based on structural data obtained from a sparkle model

Abstract We report the synthesis, characterization and spectroscopic properties of the compound Eu(thenoyltrifluoroacetonate), 2(dibenzyl sulfoxide) which is highly luminescent under UV excitation. Experimental and theoretical results on ligand field parameters, 4f-4f intensities and intramolecular energy transfer processes are described. The characteristic emission spectrum of the Eu 3+ ion shows a very high intensity for the hypersensitive 5 D 0 → 7 F 2 transition, pointing to a highly polarizable chemical environment around the Eu 3+ ion. No emission at low temperature from the organic part of the compound is observed, in contrast to the case of the analog compound with Gd 3+ , indicating that energy transfer from the ligands to the Eu 3+ ion is quite efficient. Lifetime measurements confirm that the Eu 3+ luminescence has a higher efficiency than in the case of the hydrated compound (two water molecules in the place of the dibenzyl sulphoxides). The theoretical calculations are based on an optimized coordination geometry and electronic structure obtained from the SMLC/AM1 (Sparkle Model for the calculation of lanthanide complexes within the Austin Model 1), recently introduced in the literature, and ligand field and 4f-4f intensity models previously developed. A comparison with experiment is made. Considerably high values of intramolecular energy transfer rates are predicted.

Experimental and theoretical emission quantum yield in the compound Eu(thenoyltrifluoroacetonate)3.2(dibenzyl sulfoxide)

Abstract We report the experimental determination and a theoretical calculation of the 4f–4f emission quantum yield in the Eu(thenoyltrifluoroacetonate)3.2(dibenzyl sulfoxide) compound. The experimental procedure for determining the quantum yields is based on a method previously proposed by Bril and coworkers. The theoretical calculations are carried out by solving an appropriate set of rate equations and by using spectroscopic parameters and energy transfer rates recently reported for this compound. For the sake of comparison, the hydrated compound (two water molecules in the place of the dibenzyl sulfoxide) is also studied. Good agreement between theory and experiment is obtained, provided that the lifetime of the lowest triplet level of the donor ligand is ≥10−5 s at room temperature.

Influence of the N-[methylpyridyl]acetamide ligands on the photoluminescent properties of Eu(III)-perchlorate complexes

Abstract This work reports the synthesis, characterization and spectroscopic studies of Eu(III)-perchlorate complexes with amide ligands derived from N -[ x -methylpyridyl]acetamide where x =3, 4 and 6. The Eu(ClO 4 ) 3 3( x -mpa) complexes were characterized by elemental analyses, molar conductance, TG analyses and vibrational spectroscopy. Raman and infrared data show that the perchlorate ion, (ClO 4 − ), is bonded to Eu(III) as a monodentate ligand and that in these three complexes water molecules are not coordinated to the rare earth ion. The profiles of the emission spectra of the complexes with 3- and 4-mpa ligands are very similar but they differ from the complex containing 6-mpa ligand. This spectroscopic behavior can be rationalized in terms of the Ω λ ( λ =2 and 4) and R 02 experimental intensity parameters. The values of Ω 2 (∼6.5×10 −20 cm 2 ) in these complexes are smaller than in Eu(III)-TTA compounds ( Ω 2 ∼35.0×10 −20 cm 2 ), indicating that in the former case the rare earth ion is in a chemical environment less polarizable. Lifetime and emission quantum efficiency measurements for the emitting level 5 D 0 were carried out and the results are discussed.

Persistent luminescence mechanisms: human imagination at work

The present status and future progress of the mechanisms of persistent luminescence are critically treated with the present knowledge. The advantages to be achieved by a further need as well as the pitfalls of the excessive use of imagination are shown. As usual, in the beginning of the present era of persistent luminescence since the mid 1990s, the imagination played a more important role than the sparse solid experimental data and the chemical common sense and knowledge was largely ignored. Since some five years, the mechanistic studies seem to have reached the maturity and – perhaps deceivingly – it seems that there are only details to be solved. However, the development of red emitting nanocrystalline materials poses a challenge also to the more fundamental studies and interpretation. The questions still luring in the darkness include the problems how the increased surface area affects the defect structure and how the “persistent energy transfer” really works. There is still some light to be thrown onto these matters starting with agreeing on the terminology: the term phosphorescence should be abandoned altogether. The long lifetime of persistent luminescence is due to trapping of excitation energy, not to the forbidden nature of the luminescent transition. However, the technically well-suited term “afterglow” should be retained for harmful, short persistent luminescence.

Intermolecular energy transfer and photostability of luminescence-tuneable multicolour PMMA films doped with lanthanide–β-diketonate complexes

It is reported in this work the preparation, characterisation and photoluminescence study of poly(methylmethacrylate) (PMMA) thin films co-doped with [Eu(tta)3(H2O)2] and [Tb(acac)3(H2O)3] complexes. Both the composition and excitation wavelength may be tailored to fine-tune the emission properties of these Ln3+–β-diketonate doped polymer films, exhibiting green and red primary colours, as well as intermediate colours. In addition to the ligand–Ln3+ intramolecular energy transfer, it is observed an unprecedented intermolecular energy transfer process from the 5D4 emitting level of the Tb3+ ion to the excited triplet state T1 of the tta ligand coordinated to the Eu3+ ion. The PMMA polymer matrix acts as a co-sensitizer and enhances the overall luminescence intensity of the polymer films. Furthermore, it provides considerable UV protection for the luminescent species and improves the photostability of the doped system.

Luminescence investigation of the Sm(III)-β-diketonates with sulfoxides, phosphine oxides and amides ligands

Abstract In this paper we report on a photoluminescent investigation of complexes involving Sm 3+ -β-diketonates with sulfoxides, phosphine oxides and amides ligands. In the synthesis of the coordination compounds we used samarium tris(thenoyltrifluoroacetonate) dihydrated precursor with the following ligands (L): DBSO and PTSO sulfoxides; TPPO phosphine oxide and (PHA) N -phenylacetamide. They have shown high orange luminescence characteristic of the Sm 3+ ion. The emission spectra of the Sm 3+ -complexes present narrow bands arising from the 4 G 5/2 → 6 H J ( J =5/2, 7/2, 9/2, 11/2) transitions with the hypersensitive 4 G 5/2 → 6 H 9/2 transition as a prominent group. It is observed an efficient intramolecular energy transfer from the triplet state (T) of the ligands to the emitting 4 G 5/2 state of the Sm 3+ ion. The experimental intensity parameters ( η Sm and η Eu ) for the Sm and Eu complexes have been determined and compared. The lifetimes ( τ ) of the emitting level 4 G 5/2 of the Sm-complexes are approximately 10 times higher than in the precursor compound [Sm(TTA) 3 ·(H 2 O) 2 ] indicating that radiative processes are operative in all the compounds due to the absence of multiphonon relaxation by coupling with the OH oscillators.

Polymer matrix sensitizing effect on photoluminescence properties of Eu3+-β-diketonate complex doped into poly-β-hydroxybutyrate (PHB) in film form

In this work is reported the sensitization effect by polymer matrices on the photoluminescence properties of diaquatris(thenoyltrifluoroacetonate)europium(III), [Eu(tta)3(H2O)2], doped into poly-β-hydroxybutyrate (PHB) with doping percentage at 1, 3, 5, 7 and 10% (mass) in film form. TGA results indicated that the Eu3+ complex precursor was immobilized in the polymer matrix by the interaction between the Eu3+ complex and the oxygen atoms of the PHB polymer when the rare earth complex was incorporated in the polymeric host. The thermal behaviour of these luminescent systems is similar to that of the undoped polymer, however, the Tonset temperature of decomposition decreases with increase of the complex doping concentration. The emission spectra of the Eu3+ complex doped PHB films recorded at 298 K exhibited the five characteristic bands arising from the 5D0 → 7FJ intraconfigurational transitions (J = 0–4). The fact that the quantum efficiencies η of the doped film increased significantly revealed that the polymer matrix acts as an efficient co-sensitizer for Eu3+ luminescent centres and therefore enhances the quantum efficiency of the emitter 5D0 level. The luminescence intensity decreases, however, with increasing precursor concentration in the doped polymer to greater than 5% where a saturation effect is observed at this specific doping percentage, indicating that changes in the polymeric matrix improve the absorption property of the film, consequently quenching the luminescent effect.

Luminescent properties of diketonates of trivalent europium with dimethyl sulfoxide

Abstract Photoluminescent properties of the tris(thenoyltrifluoroacetonate)europium(III) dihydrate with dimethyl sulfoxide (DMSO), in the solid state, are reported. The compound Eu(TTA)3·2DMSO and the precursor salt were characterized by IR spectroscopy, differential scanning calorimetry and elemental analysis. The emission data indicate that the substitution of the two water molecules by DMSO ligand in the complex causes an intensification of luminescence corresponding to the 5D0→7FJ (J=0–4) transitions associated with one of the site symmetries. Large values of the Ω 2 intensity parameter (35.2×10−20 cm2) were obtained, reflecting the hypersensitive character of the 5D0→7F2 transition and indicating that Eu3+ ion is in a highly polarizable chemical environment. This is consistent with systems containing sulfoxide ligands DBSO and PTSO. The DMSO complex also shows a higher value for the Ω 4 parameter (6.8×10−20 cm2) as a consequence of the difference in the basicity of the oxygen donor ligand. Lifetime measurements (τ=0.72 ms) confirm that the Eu3+ luminescence has a higher efficiency than in the case of the hydrated compound.

Emission quantum yield of europium (III) mixed complexes with thenoyltrifluoroacetonate and some aromatic ligands

Abstract We report the experimental determination and theoretical calculation of the 4f–4f emission quantum yield (q) and 5D0 excited state lifetime in the Eu (TTA)3.nL compounds (where TTA=thenoyltrifluoroacetonate, L denotes 1,10-phenantroline orp-tolyl sulfoxide and n=1 or 2). The experimental q values for these compounds vary significantly by changing the L ligand. The relation between the emission quantum yield and photophysical characteristics of the compounds is discussed. The calculations are carried out by using theoretical models for ligand–rare earth ion energy transfer processes and numerical solutions of the rate equations. Pathways for the intramolecular energy transfer process between the ligands and the rare earth ion are proposed. The theoretical results agree well with the experimental data for the compounds analyzed.

Luminescence properties of the layered niobate KCa2Nb3O10 doped with Eu3+ and La3+ ions

Abstract In the present work the layered perovskite phase KCa 2 Nb 3 O 10 was doped with rare earth ions through a solid-state reaction, producing compounds of general formula K 1− x Ln x Ca 2− x Nb 3 O 10 (where Ln=La 3+ or Eu 3+ and x =0.02). The photoluminescence properties of the layered oxide doped with rare earth ions in the perovskite-type layers were investigated based on the luminescence data. The KCa 2 Nb 3 O 10 phase does not exhibit emission bands when excited in the UV region. On the other hand the matrices doped with Ln 3+ ions show a blue emission ( λ exc =270 nm) at 298 and 77 K, indicating that the electronic properties of the layered materials are modified by the lanthanide insertion. In the case of K 0.98 Eu 0.02 Ca 1.98 Nb 3 O 10 the characteristic red emission of the trivalent europium ion is also observed under excitation at 270 and 394 nm. The luminescence spectra of the Eu 3+ -compound show the 5 D 0 → 7 F 0 transition split into two peaks, indicating that the Eu 3+ ions are located in at least two distinct site symmetries. The results also suggest the occurrence of an energy transfer process between the niobate matrix and the Eu 3+ ion. The luminescence decays exhibit biexponential curves corroborating the presence of Eu 3+ ions in more than one chemical environment.