Аннотация:The luminescent properties of three novel water-soluble europium complexes with organic ligands based on 2,2'-bipyridyl substituted with phosphonic acids were studied in water and heavy water at temperatures from 293 to 333 K. With an increase in temperature, a decrease in the intensity of europium luminescence was observed both in water and heavy water. The luminescence intensity in the studied temperature range is well described by the van’t Hoff plot for each solution. For all complexes, the luminescence lifetime in heavy water was much longer than in water due to effective quenching of the excited states by the OH oscillators of water molecules located in the first coordination sphere of the europium. The number of water molecules in the first coordination sphere of the europium at different temperatures was calculated; it did not vary with temperature. We conclude that the main mechanism of luminescence quenching is the enhanced energy back transfer from the excited state of europium to the triplet level of the ligand or the charge transfer state. The energy back transfer probability rises simultaneously with a displacement of the position of the europium ion relative to the inverse center in the complex upon solution heating.