Complexos de isotiocianatos de lantanídeos com quelantes aromáticos: supressão da luminescência mediada pela transferência de energia tripleto-estado de transferência de carga

Abstract

The photoluminescence efficiency in compounds of lanthanide ions is a result of the complex balance between absorption of photons, energy transfer and quenching mechanisms, among which the multiphonon decay and suppression mediated by low energy ligand-to-metal charge transfer states (LMCT) which might be generated by ligands that have a low oxidation potential, for instance the NCS–. Less than a decade ago, this process was described mainly in terms of the energy transfer between 4f states and LMCT states, not being considered the participation of intraligands states. In the present study, we investigated the photoluminescent properties of new lanthanide isothiocyanates coordination complexes with N-acetyl-2-aminopyridine, N-benzoyl-2-aminopyridine and N-benzoyl-2- aminopyrimidine as ligands. We also synthesized the well-known complexes of formulas [Ln(NCS)3(phen)3] (H2O), and [Ln(NO3)3(Phen)2] with phen = 1,10-phenanthroline. The complexes were characterized via CHN elemental analysis, vibrational spectroscopy, complexometric titration and thermogravimetric analysis. Their spectroscopic properties were investigated via molecular diffuse reflectance, excitation and emission spectroscopy along with time decay of the emitting states. The results from the analysis of emission spectroscopy and photoluminescence excitation, as well as the lifetimes measurements of the emitting states of the investigated complexes provided what we believe to be the most expressive experimental demonstration of the luminescence quenching via triplet-LMCT energy transfer process in Eu(III) complex so far.