Energy transfer study of the formation, composition, and size of nanostructures of metal ion complexes in aqueous solutions

Abstract

We studied the luminescence intensity (I lum) of the ions Eu(III) and Sm(III) in relation to the concentrations of ions Ln(III) and Al(III) in water at pH 7 at an excess of such beta-diketones as p-methoxybenzoyltrifluoroacetone (MBTA), dibenzoylmethane (DBM), and tenoyltrifluoroacetone (TTA) and in the presence of 1,10-phenanthroline (phen) used as a synergistic agent. Both the enhancement of I lum (Eu(III)) upon addition of Gd(III) (co-luminescence) and the effect of the third ion are found to depend on the order of addition of the ions to the solution and, therefore, on the sequence of formation of nanostructures of complexes of these ions in the solution, in which the transfer of the triplet energy of the organic part of complexes takes place, leading to an enhancement in I lum (Eu(III)). The intensity I lum (Eu(III)) is shown to increase equally rapidly upon addition of either Gd(III) or Al(III) to solutions with DBM + phen. In solutions of all the three beta-diketones studied, the Eu(III) ions incorporate better into nanostructures of triply charged ions whose radius is similar to or smaller than the radius of the Eu(III) ions. Our study of the effect that the replacement of H2O with D2O exerts of I lum of 5 × 10−8 M Eu(III) at different concentrations of ligands shows that, at [Ln(III)] < [OH−] and at a concentration of beta-diketones smaller than 3 × 10−5 M, the deuteration affects I lum(Eu(III)) and, therefore, the first coordination sphere of Eu(III) contains OH groups. It is shown that, in aqueous solutions with 3 × 10−5 M TTA + 10−5 M phen, the increase in I lum(Eu(III)) caused by the introduction of Gd(III) ions results from two processes occurring in the nanostructures of these complexes: the energy transfer from Gd(III) complexes to Eu(III) complexes and the increase of I lum of Eu(III) itself under the conditions in the solution where the total concentration [Ln] ≫ [OH−] and both the photochemical deactivation of Eu(III) and the exchange of its excitation energy for vibrations of the OH groups are suppressed. The reliability of the size estimation of nanostructures of metal complexes is discussed in terms of the effect of these nanostructures on I lum of chelates of Eu(III).