Structure and optical properties of 3-bromo-4-methylthio-2,6-lutidine N-oxide and its eight-coordinate europium(III) and terbium(III) aqua complexes

New europium and terbium [LnCl3L3(H2O)2] polycrystalline complexes were synthesized with the ligand L = 3-bromo-4-methylthio-2,6-lutidine N-oxide. IR, Raman, electron absorption UV-VIS and emission spectra measurements were carried out for these compounds. Re-crystallization of these materials led to formation of single crystals with another composition [LnL(H2O)7]Cl3∙L3, they were characterized by X-ray diffraction technique and spectroscopic measurements. Both types of the materials consist of eight coordinated LnCl3O3O’2 or LnOO’7 polyhedrons containing O - oxygen atoms of the N-oxide group, chlorine atoms or O’ - oxygen atoms of the water units. The crystal structure was solved for two complexes of the formula [Ln(L)(H2O)7]Cl3∙(L)3 where Ln = Eu and Tb. X-ray diffraction analysis reveals that these complexes are isostructural with P21/m space group. The optimization of the molecular system and frequency calculations were carried out using density functional theory (DFT). The DFT quantum chemical calculations of the vibrational wavenumbers were performed for the ligands used for the Eu3+ and Tb3+ ions complexation. The emission measurements were carried out with the femtosecond excitation. The effective ligand-to-metal energy transfer was discovered in the [LnCl3L3(H2O)2] complexes formed in the polycrystalline state.