A Eu<sup>III</sup> Tetrakis(β-diketonate) Dimeric
Complex: Photophysical Properties, Structural Elucidation by Sparkle/AM1
Calculations, and Doping into PMMA Films and Nanowires
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Abstract
Reaction of Ln<sup>III</sup> with
a tetrakis(diketone) ligand H<sub>4</sub>L [1,1′-(4,4′-(2,2-bis((4-(4,4,4-trifluoro-3-oxobutanoyl)
phenoxy)methyl)propane-1,3-diyl)bis(oxy)bis(4,1-phenylene))bis(4,4,4-trifluorobutane-1,3-dione)]
gives new podates which, according to mass spectral data and Sparkle/AM1
calculations, can be described as dimers, (NBu<sub>4</sub>[LnL])<sub>2</sub> (Ln = Eu, Tb, Gd:Eu), in both solid-state and dimethylformamide
(DMF) solution. The photophysical properties of the Eu<sup>III</sup> podate are compared with those of the mononuclear diketonate (NBu<sub>4</sub>[Eu(BTFA)<sub>4</sub>], BTFA = benzoyltrifluoroacetonate),
the crystal structure of which is also reported. The new Eu<sup>III</sup> dimeric complex displays bright red luminescence upon irradiation
at the ligand-centered band in the range of 250–400 nm, irrespective
of the medium. The emission quantum yields and the luminescence lifetimes
of (NBu<sub>4</sub>[EuL])<sub>2</sub> (solid state: 51% ± 8%
and 710 ± 2 μs; DMF: 31% ± 5% and 717 ± 1 μs)
at room temperature are comparable to those obtained for NBu<sub>4</sub>[Eu(BTFA)<sub>4</sub>] (solid state: 60 ± 9% and 730 ±
5 μs; DMF: 30 ± 5% and 636 ± 1 μs). Sparkle/AM1
calculations were utilized for predicting the ground-state geometries
of the Eu<sup>III</sup> dimer. Theoretical Judd–Ofelt and photoluminescence
parameters, including quantum yields, predicted from this model are
in good agreement with the experimental values, proving the efficiency
of this theoretical approach implemented in the LUMPAC software (http://lumpac.pro.br). The kinetic scheme for modeling energy
transfer processes show that the main donor state is the ligand triplet
state and that energy transfer occurs on both the <sup>5</sup>D<sub>1</sub> (44.2%) and <sup>5</sup>D<sub>0</sub> (55.8%) levels. Furthermore,
the newly obtained Eu<sup>III</sup> complex was doped into a PMMA
matrix to form highly luminescent films and one-dimensional nanowires
having emission quantum yield as high as 67%–69% (doping concentration
= 4% by weight); these materials display bright red luminescence even
under sunlight, so that interesting photonic applications can be foreseen