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 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 ion. In this work, a PyFitIt machine learning approach was employed as a new strategy to simulate the Eu, Gd, and Sm L-edge XANES and thereby determine the local atomic structure of the luminescence RE β-diketonate complexes, [Eu(tta)(HO)], [Cmim][Eu(dbm)], [Gd(tta)(HO)], and [Sm(dbm)(phen)] (tta, 3-thenoyltrifluoroacetonate; dbm, dibenzoylmethane; phen, phenanthroline; and Cmim, 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 (RE = Eu and Gd) and Sm 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 complex are due to the higher atomic radius of the Sm ion when compared to the Eu and Gd complexes. Meanwhile, higher Debye-Waller factor (σ) values for the [Cmim][Eu(dbm)] when compared to the [Eu(tta)(HO)] indicated the structure disorder, owing to the distortion in the local geometry. It is noteworthy that the optical properties, described mainly by the Ω (λ = 2 and 4) 4f-4f intensity parameters, are very sensitive to the local coordination environment around the Eu ion. Thus, a close agreement between the experimental and theoretically calculated Ω parameter values confirmed that the PyFitIt calculated square antiprismatic structures are precisely similar to the real structures of the Eu complexes.