Estudo da absorção eletrônica do complexo de európio u(bzac)3.2H2O pelo método TDDFT com uso da aproximação da carga pontual para o íon Eu(III)

Abstract

In this work, using quantum chemistry methods based on density functional theory (DFT), spectroscopic properties such as electronic absorption in the ultraviolet region of coordination compounds with Ln(III) lanthanide ions were studied; in particular, tris-betadiketonates compounds with Eu(III) ion with two water molecules completing the coordination sphere with the general formula Eu(bzac)3.2H2O (bzac=bezoylacetone), resulting in a coordination number equal to eight . Molecular structures were optimized using the MOPAC 2016 program with the Sparkle/AM1 method. The approximation used in the Sparkle model simulates the lanthanide ion by a point charge, considered adequate due to the essentially ionic behavior of the ion-ligand bond, resulting from the shielding of the 4f valence orbitals by the filled 5s and 5p shells. The optimized structures were submitted to the TDDFT (Time Dependent DFT) method implemented in Orca 4.2.1, with the B3LYP/6-31G(d) protocol, to obtain the excited states, located in the ligands, of singlet and triplet spin multiplicities. In these calculations, the Eu(III) ion was replaced by a point charge which, based on Batista and Longo's (2002) model, was varied between +3.0 and 3.5e. Using the excited states wave functions and the transition oscillator strengths, the absorption spectra were simulated by means of a Lorentzian shape, for comparison with available experimental data. To obtain the spectral curves, the Avogadro program was used. The TDDFT calculation was carried out at two levels: complete TDDFT and TDDFT-TDA (Tamm-Dancoff approximation), with 100 roots, a suitable value for the studied spectral region and with a moderate computational cost. The bands of interest in the luminescence of the complexes are those of lower energy (low lying states), and for those the full TDDFT method provided better results than the TDA method, when compared to the experimental data. Regarding the use of the point charge approximation, the value of +3.0e provided the best prediction than larger values, contrary to what was proposed by Batista and Longo with the use of semi-empirical methods instead of DFT, indicating that this the latter method does not show the same behavior as the INDO/S approximation upon increasing the point charge value, suggesting additional studies on the nature of the excited states described by DFT methods for such compounds. All the conclusions about the results obtained by the full and TDA-TDDFT methods and the trend of band displacements according to the variation of the point charge, referring to the model compound Eu(bzac)3.2H2O are in line with calculations carried out for compounds similar to this one, with betadiketone ligand btfa (benzoylacetone) in place of bzac and with the ligand bipy (bipyridine) replacing the two water molecules in the coordination sphere.