Theoretical and Experimental Investigation of the Tb3+-Eu3+ Energy Transfer Mechanisms in Cubic A3Tb0.90Eu0.10({PO}4)3 (A = Sr, Ba) Materials

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 A(3)Tb(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

Electron energy-loss cross sections for the chemical bond overlap plasmon Of the hydrogen molecule

We examine the possibility of detecting the chemical bond overlap plasmon (CBOP) of the hydrogen molecule by electron inelastic scattering. The CBOP has been predicted to efficiently absorb and scatter electromagnetic radiation above the molecular ionization threshold in the cases of alkali halides. For the hydrogen molecule the quadrupole nature of the CBOP energy-loss cross section leads to cross section values with impacting electron energy dependence and an angular behavior which are totally distinguishable from the usual ionization, inter-band transitions and dissociation processes. Previously established relationships between the CBOP and the polarizability of the overlap region suggest this an a promising theoretical tool for quantifying covalency in the chemical bond

How minor structural changes generate major consequences in photophysical properties of RE coordination compounds; resonance effect, LMCT state

Lanthanide coordination compounds of the formula Na[Ln(L)4] (1Ln), where Ln ¼ La3þ, Eu3þ, Gd3þ, Tb3þ, L ¼ [L] and HL ¼ dimethyl(4-methylphenylsulfonyl)amidophosphate, were synthesized. Their structural and spectroscopic properties were discussed in detail based on X-ray diffraction measurements, IR spectroscopy, absorption and emission spectroscopy at 293 and 77 K and theoretical calculations of the intramolecular energy transfer (IET) rates. DFT calculations were used to investigate the 1Ln electronic properties required to calculate the transition rates. 30 and 22 pathways of intramolecular nonradiative energy transfer were examined in the case of 1Eu and 1Tb, respectively. It is shown that the main pathway for sensitization of the lanthanide emission is either the triplet (1Eu) or singlet (1Tb) transfer, occurring mainly through the exchange mechanism. The energy rates for energy transfer from S1 and T1 equal WS ¼ 1:53 105 s 1 (1Eu), WT ¼ 5:14 106 s 1 (1Eu) and WS ¼ 4:09 107 s 1 (1Tb), WT ¼ 6:88 105 s 1 (1Tb). The crucial role of the 7F5 level in the energy transfer process of 1Tb and the participation of the LMCTstate in the depopulation of the ligand singlet state of 1Eu were demonstrated. The influence of the resonance effect on the splitting of the 7F1 level in 1Eu was analyzed. By comparing the properties of 1Ln with the properties of 2Ln coordination compounds, sharing the same ligand and crystallizing in the same crystallographic system (monoclinic), but with a different space group, it is demonstrated how slight structural changes can affect the photophysical properties of Ln compounds.publishe

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

Novel trivalent europium β-diketonate complexes with N-(pyridine-2-yl)amides and N-(pyrimidine-2-yl)amides as ancillary ligands: photophysical properties and theoretical structural modeling

Eighteen new Eu3+ complexes and their Gd3+ analogues with 1,3-diketonate as main ligands and N-(pyridine-2-yl)amides or N-(pyrimidine-2-yl)amides as ancillary ligands were synthesized. The replacement of water molecules by those amides in the Eu3+ complexes increase the intrinsic quantum yields of luminescence, making them comparable or even more efficient than Eu3+ complexes with standard ancillary ligands such as 2,2′-bipyridine. The luminescence spectra of Gd3+ complexes in comparison with the Eu3+ ones show that efficient ligand-to-metal intramolecular energy transfer processes take place. In most cases the experimental Judd-Ofelt intensity parameters (Ω2 and Ω4) for the Eu3+ complexes show variations as a function of the temperature (77 and 300 K) that overall apparently does not follow clearly any trend. For this reason, geometric variations (on the azimuthal angle φ and ancillary ligands distances) were carried out in the coordination polyhedron for simulating thermally induced structural changes. It has been observed that, in this way, the Ω2 and Ω4 can be satisfactorily reproduced by in silico experiments. It was concluded that, at low-temperature, the ancillary ligands become closer to the Eu3+ ion and the angular variations affect more Ω2 than Ω4, in agreement to the theoretical calculations. The use of N-(pyridine-2-yl)amides or N-(pyrimidine-2-yl)amides as ancillary ligands in Eu3+ 1,3-diketonates looks to be a good strategy for obtaining highly luminescent complexes.publishe

Visible and Near-Infrared Luminescence of Lanthanide-Containing Dimetallic Triple-Stranded Helicates: Energy Transfer Mechanisms in the SmIII and YbIII Molecular Edifices

The photophysical properties of the triple-stranded dimetallic helicates [Ln2(LC−2H)3]·H2O (Ln = Nd, Sm, Dy, Yb) are determined in water and D2O solutions, and energy transfer processes are modeled for SmIII. The luminescence of NdIII, SmIII, and YbIII is sensitized by (LC−2H)2-, but the energy transfer from the ligand to the LnIII ions is not complete, resulting in residual ligand emission. The luminescence of the NdIII helicate is very weak due to nonradiative de-excitation processes. On the other hand, the YbIII and SmIII helicates exhibit fair quantum yields, 1.8% and 1.1% in deuterated water, respectively. The energy transfer rates between (LC−2H)2- and SmIII levels are calculated by direct and exchange Coulomb interaction models. This theoretical modeling coupled to numerical solutions of the rate equations leads to an estimate of the emission quantum yields in H2O and D2O, which compares favorably with experimental data. The main component of the ligand-to-metal energy transfer (97.5%) goes through a 3ππ* → 5G5/2(1) path, and the operative mechanism is of the exchange type. For the YbIII helicate, minor effects of oxygen on the sensitization of YbIII and nanosecond time-resolved spectroscopy point to the energy transfer mechanism being consistent with a recently proposed pathway involving fast electron transfer and YbII. No up-conversion process could be identified. Ligand-field splitting of the 2F5/2 (3E1/2 + E3/2) and 2F7/2 (2E1/2 + E3/2) levels of YbIII is consistent with D3 symmetry

Efficient and tuneable photoluminescent boehmite hybrid nanoplates lacking metal activator centres for single-phase white LEDs

White light-emitting diodes (WLEDs) are candidates to revolutionize the lighting industry towards energy efficient and environmental friendly lighting and displays. The current challenges in WLEDs encompass high luminous efficiency, chromatic stability, high colour-rending index and price competitiveness. Recently, the development of efficient and low-cost downconverting photoluminescent phosphors for ultraviolet/blue to white light conversion was highly investigated. Here we report a simple route to design high-efficient WLEDs by combining a commercial ultraviolet LED chip (InGaAsN, 390 nm) and boehmite (gamma-AlOOH) hybrid nanoplates. Unusually high quantum yields (eta(yield) = 38-58%) result from a synergic energy transfer between the boehmite-related states and the triplet states of the benzoate ligands bound to the surface of the nanoplates. The nanoplates with eta(yield) = 38% are able to emit white light with Commission International de l'Eclairage coordinates, colour-rendering index and correlated colour temperature values of (0.32, 0.33), 85.5 and 6,111 K, respectively; overwhelming state-of-the-art single-phase ultraviolet-pumped WLEDs phosphors