Authors: Latif Ullah Khan | Zahid U. Khan | Lucca Blois | Lubna Tabassam | Hermi F. Brito | Santiago J. A. Figueroa
Source: Inorganic Chemistry (article, 2023-01-30)
Field: Materials Science — Materials Chemistry

Strategy to Probe the Local Atomic Structure of Luminescent Rare Earth Complexes by X-ray Absorption Near-Edge Spectroscopy Simulation Using a Machine Learning-Based PyFitIt Approach Rare earth(III) β-diketonates are highly remarkable luminophores in the visible spectral region among the rare earth compounds, owing to the efficient contribution from the 4f-4f intraconfigurational transitions. To get detailed structural insight into the RE<sup>3+</sup> sites (RE = Eu, Gd, and Sm), X-ray absorption near-edge spectroscopy (XANES) can be very potent in probing the local chemical environment around the RE<sup>3+</sup> ion. In this work, a PyFitIt machine learning approach was employed as a new strategy to simulate the Eu, Gd, and Sm L<sub>3</sub>-edge XANES and thereby determine the local atomic structure of the luminescence RE<sup>3+</sup> β-diketonate complexes, [Eu(tta)<sub>3</sub>(H<sub>2</sub>O)<sub>2</sub>], [C<sub>4</sub>mim][Eu(dbm)<sub>4</sub>], [Gd(tta)<sub>3</sub>(H<sub>2</sub>O)<sub>2</sub>], and [Sm(dbm)<sub>3</sub>(phen)] (tta, 3-thenoyltrifluoroacetonate; dbm, dibenzoylmethane; phen, phenanthroline; and C<sub>4</sub>mim, 1-butyl-3-methylimidazolium bromide). Continuous Cauchy wavelet transform validated the PyFitIt calculated XANES by visualizing very efficiently the coordination geometries, composed of O and O/N backscatterers around the RE<sup>3+</sup> (RE = Eu and Gd) and Sm<sup>3+</sup> ions, respectively, as a pinkish-red color map in the two-dimensional images of the corresponding complexes. Extended X-ray absorption fine structure fit in Artemis also corroborated the three-dimensional structures generated by PyFitIt XANES simulation for all the compounds. Though, relatively slightly higher bond distance values for the Sm<sup>3+</sup> complex are due to the higher atomic radius of the Sm<sup>3+</sup> ion when compared to the Eu<sup>3+</sup> and Gd<sup>3+</sup> complexes. Meanwhile, higher Debye-Waller factor (σ<sup>2</sup>) values for the [C<sub>4</sub>mim][Eu(dbm)<sub>4</sub>] when compared to the [Eu(tta)<sub>3</sub>(H<sub>2</sub>O)<sub>2</sub>] indicated the structure disorder, owing to the distortion in the local geometry. It is noteworthy that the optical properties, described mainly by the Ω<sub>λ</sub> (λ = 2 and 4) 4f-4f intensity parameters, are very sensitive to the local coordination environment around the Eu<sup>3+</sup> ion. Thus, a close agreement between the experimental and theoretically calculated Ω<sub>λ</sub> parameter values confirmed that the PyFitIt calculated square antiprismatic structures are precisely similar to the real structures of the Eu<sup>3+</sup> complexes. XANES ; Chemistry ; Europium ; X-ray absorption spectroscopy ; Spectroscopy ; Absorption (acoustics) ; Luminescence ; Ion ; Absorption spectroscopy ; Analytical Chemistry (journal) ; Crystallography ; Optoelectronics ; Materials science ; Optics Lanthanide and Transition Metal Complexes ; Luminescence Properties of Advanced Materials ; Radioactive element chemistry and processing