Luminescence and Energy Transfer in Solutions of Rare-Earth Complexes. I. Enhancement of Fluorescence by Deuterium Substitution

Abstract
Fluorescence yields and lifetimes of solutions of europium and terbium salts in solution are greatly enhanced by using deuterated solvents. The magnitude of increase varies from an 18‐fold enhancement in yield and lifetime for an aqueous solution of europium nitrate to a twofold enhancement for europium ethylenediaminetetra‐acetate dissolved in methanol. The enhancement is proportional to the fraction of deuterated solvent present. Results lead to conclusions that the dominant mode of radiationless deactivation of a rare‐earth ion in solution is via loss of energy to hydrogen vibrations. Further, such loss is proportional to the number of hydrogen vibrations in the local rare‐earth environment. Values for the rate constant for radiationless loss of energy to hydrogen vibrations are determined for the salts studied. The influence upon radiationless processes of complexing between the rare earth and anion is discussed. Likewise the dependence of the enhancement factor upon the rare‐earth ion is considered. There is no evidence that the lifetime from the 5D1 level of europium nitrate is enhanced by deuterium substitution. This lifetime is ≤5 μsec in both D2O and H2O solutions.