Temperature dependence of the luminescence of two-center REE complexes with N-heterocyclic ligands at an excess of metal ions with respect to the ligand

The temperature dependence of the spectral-luminescent characteristics of rare earth element (REE) complexes with an N-heterocyclic ligand based on 2,2'-bipyridyldicarboxamide in the simultaneous presence of two emitting centers (europium + terbium) was studied. Measurements were conducted in a physiological temperature range (20–60$^\circ$C). Solutions of the ligand, europium nitrate hexahydrate and terbium nitrate pentahydrate in acetonitrile (with concentrations of 3 $\cdot$ 10$^{-5}$ mol/l) were mixed in the ratio of 1 : 1 : 1. In order to reveal the features of energy transitions in such compounds, the results obtained were compared with similar results for solutions of the europium or terbium complex with the same ligand. For the studied compounds, the lifetimes of europium and terbium ions in solutions at different temperatures were obtained. The temperature dependences of the luminescence quantum efficiency of the compounds are obtained. The ratios of the integrated luminescence intensities of europium and terbium ions are calculated as functions of temperature. It has been found that the luminescence lifetime of europium and terbium ions in solution at the recording wavelengths of europium and terbium (615 and 545 nm) does not depend on the mixing order of the ligand, europium, and terbium. Also, based on the experimental data, a conclusion is made that the luminescence lifetime remains unchanged in the temperature range from 20 to 60$^\circ$C.