Energy transfer between Ln(III) ions in aqueous solutions upon formation of labile binuclear fluoride and nitrate complexes of these ions

The influence of anions on nonradiative electronic energy transfer between lanthanide ions in aqueous solutions is studied. The rate constant k _t of energy transfer was found to increase by three orders of magnitude upon addition of salts of hydrofluoric acid to the solution. This effect is caused by the formation of bridge labile binuclear complexes, an excited energy donor-fluorine anion-acceptor, which increase the encounter time between a donor and an acceptor, resulting in the increase in the probability of energy transfer. The independence of k _t of the Förster overlap integral in the bridge complex containing F^? was explained by the fact that energy transfer occurs in each event of the binuclear complex formation, the maximum rate constant of energy transfer being equal to the rate constant of the binuclear complex association. Bridge complexes formed via the NO ₃ ^? anion are also considered. These binuclear complexes are unstable, and k _t for them is proportional to the Förster overlap integral. In this case, energy transfer occurs not during each event of the complex formation. This allows us to estimate the lower limit of the dissociation constant of the binuclear complex and its stability constant. Thus, the study of influence of various anions on energy transfer represents a new efficient luminescent method for analysis of properties of labile binuclear complexes in solutions.