Polarized Luminescence of Anisotropic LaPO₄:Eu Nanocrystal Polymorphs

Lanthanide elements exhibit highly appealing spectroscopic properties that are extensively used for phosphor applications. Their luminescence contains precise information on the internal structure of the host materials. Especially, the polarization behavior of the transition sublevel peaks is a fingerprint of the crystal phase, symmetry, and defects. However, this unique feature is poorly explored in current research on lanthanide nanophosphors. We here report on a detailed investigation of the evolution of Eu³⁺ luminescence during the thermally induced phase transition of LaPO₄ nanocrystal hosts. By means of <i>c</i>-axis-aligned nanocrystal assemblies, we demonstrate a dramatic change of the emission polarization feature corresponding to the distinct Eu³⁺ site symmetries in different LaPO₄ polymorphs. We also show that changes of the nanocrystal structure can be identified by this spectroscopic method, with a much higher sensitivity than the X-ray diffraction analysis. This new insight into the nanostructure-luminescence relationship, associated with the unprecedented polarization characterizations, provides a new methodology to investigate phase transitions in nanomaterials. It also suggests a novel function of lanthanide emitters as orientation-sensing nanoprobes for innovative applications such as in bioimaging or microfluidics

Photo–Click Chemistry to Design Highly Efficient Lanthanide β‑Diketonate Complexes Stable under UV Irradiation

Europium (<i><b>t</b></i><b>-Eu</b>) and gadolinium (<i><b>t</b></i><b>-Gd</b>) β-diketonate complexes with photoactive <i>t</i>-bpete ligand, [Ln­(btfa)₃(<i>t</i>-bpete)­(MeOH)] (Ln = Eu, Gd), where btfa^– and <i>t</i>-bpete are 4,4,4-trifluoro-1-phenyl-1,3-butanedionate and <i>trans</i>-1,2-bis­(4-pyridyl)­ethylene, respectively, were synthesized, characterized by vibrational, absorption (reflectance) and photoluminescence spectroscopies and their crystal structure was determined using single-crystal X-ray diffraction. B3LYP calculations were performed to support the interpretation and rationalization of the experimental results. The complexes, under UV irradiation, do not display the typical photodegradation of the β-diketonate ligands exhibiting, in turn, an unprecedented photostability during, at least, 10 h. During UV-A exposure (>330 nm), the emission intensities of both complexes increase drastically (∼20 times), whereas for <i><b>t</b></i><b>-Eu</b> the emission quantum yield is enhanced at least 30-fold. A mechanism based on a photoclick trans-to-cis isomerization of both <i>t</i>- and <i>c</i>-bpete moieties was proposed to explain the abnormal photostability of these compounds, either in solid state or in solution. The experimental and computational results are consistent with a photostationary state involving the trans-to-cis isomerization of the bpete ligand under continuous UV-A exposure, which thus diverts the incident radiation from other deleterious photochemical or photophysical processes that cause the typical photobleaching behavior of chelate lanthanide complexes. This shielding mechanism could be extended to other ligands permitting the design of new lanthanide-based photostable systems under UV exposure for applications in lighting, sensing, and displays

Effects of Dopant Addition on Lattice and Luminescence Intensity Parameters of Eu(III)-Doped Lanthanum Orthovanadate

A series of La_1–<i>x</i>Eu_<i>x</i>VO₄ samples with a different Eu³⁺ content was synthesized via a hydrothermal route. An increase in the dopant content resulted in a decrease in lattice constants of the materials. Plane-wave DFT calculations with PBE functional in CASTEP confirmed this trend. Next, CASTEP calculations were used to obtain force constants of Eu–O bond stretching, using a novel approach which involved displacement of the Eu³⁺ ion. The force constants were then used to calculate charge donation factors <i>g</i> for each ligand atom. The chemical bond parameters and the geometries from DFT calculations were used to obtain theoretical Judd–Ofelt intensity parameters Ω_λ. The effects of geometry changes caused by the dopant addition were analyzed in terms of Ω_λ. The effects of distortions in interatomic angles of the Eu³⁺ coordination geometry on the Ω_λ were analyzed. Effects of distortions of atomic positions in the crystal lattice on the Ω_λ and photoluminescence intensities of Eu³⁺ 4f–4f transitions were discussed. It was shown that the ideal database geometry of LaVO₄ corresponds to the highly symmetric coordination geometry of Eu³⁺ and very low Ω₂. On the contrary, experimental intensities of the ⁵D₀ → ⁷F₂ transition and the corresponding Ω₂ parameters were high. Consequently, distortions of crystal structure that reduce the symmetry play an important role in the luminescence of the LaVO₄:Eu³⁺ materials and probably other Eu³⁺-doped phosphors based on zircon-type rare earth orthovanadates

Red-Green Emitting and Superparamagnetic Nanomarkers Containing Fe₃O₄ Functionalized with Calixarene and Rare Earth Complexes

The design of bifunctional magnetic luminescent nanomaterials containing Fe₃O₄ functionalized with rare earth ion complexes of calixarene and β-diketonate ligands is reported. Their preparation is accessible through a facile one-pot method. These novel Fe₃O₄@calix-Eu­(TTA) (TTA = thenoyltrifluoroacetonate) and Fe₃O₄@calix-Tb­(ACAC) (ACAC = acetylacetonate) magnetic luminescent nanomaterials show interesting superparamagnetic and photonic properties. The magnetic properties (M-H and ZFC/FC measurements) at temperatures of 5 and 300 K were explored to investigate the extent of coating and the crystallinity effect on the saturation magnetization values and blocking temperatures. Even though magnetite is a strong luminescence quencher, the coating of the Fe₃O₄ nanoparticles with synthetically functionalized rare earth complexes has overcome this difficulty. The intramolecular energy transfer from the T₁ excited triplet states of TTA and ACAC ligands to the emitting levels of Eu³⁺ and Tb³⁺ in the nanomaterials and emission efficiencies are presented and discussed, as well as the structural conclusions from the values of the 4f–4f intensity parameters in the case of the Eu³⁺ ion. These novel nanomaterials may act as the emitting layer for the red and green light for magnetic light-converting molecular devices (MLCMDs)

Red-Green Emitting and Superparamagnetic Nanomarkers Containing Fe₃O₄ Functionalized with Calixarene and Rare Earth Complexes

The design of bifunctional magnetic luminescent nanomaterials containing Fe₃O₄ functionalized with rare earth ion complexes of calixarene and β-diketonate ligands is reported. Their preparation is accessible through a facile one-pot method. These novel Fe₃O₄@calix-Eu­(TTA) (TTA = thenoyltrifluoroacetonate) and Fe₃O₄@calix-Tb­(ACAC) (ACAC = acetylacetonate) magnetic luminescent nanomaterials show interesting superparamagnetic and photonic properties. The magnetic properties (M-H and ZFC/FC measurements) at temperatures of 5 and 300 K were explored to investigate the extent of coating and the crystallinity effect on the saturation magnetization values and blocking temperatures. Even though magnetite is a strong luminescence quencher, the coating of the Fe₃O₄ nanoparticles with synthetically functionalized rare earth complexes has overcome this difficulty. The intramolecular energy transfer from the T₁ excited triplet states of TTA and ACAC ligands to the emitting levels of Eu³⁺ and Tb³⁺ in the nanomaterials and emission efficiencies are presented and discussed, as well as the structural conclusions from the values of the 4f–4f intensity parameters in the case of the Eu³⁺ ion. These novel nanomaterials may act as the emitting layer for the red and green light for magnetic light-converting molecular devices (MLCMDs)

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