The time-resolved fluorescence study of kinetics and thermodynamics of Eu(iii) and Tb(iii) complexes with the DO2A macrocyclic ligand

Abstract

The thermodynamics of the interaction of the H₂DO2A (= 1,4,7,10-tetraazacyclododecane-1,7-diacetic acid) macrocyclic ligand with both Eu(III) and Tb(III) ions was studied by means of luminescence spectroscopy in steady-state and time-resolved modes. It was shown that it is possible to calculate stability constants for both complexes that agree with values determined by other experimental techniques. The study of the formation and dissociation of both Ln(III) complexes shows that there is no significant difference in their reactivity if they contain three (Eu) or two (Tb) water molecules in their coordination shell. The two species of ligands present in the formation reaction, HL⁻ and L²⁻, differ in their reactivity, which can be explained by the difference in charge and the presence of a proton in the macrocyclic cavity. A detailed study of the dissociation reaction proved that the reaction mechanism consists of several pathways of intermediates, which differ in their structure and rate of decomposition.