Non-radiative deactivation of the excited states of europium, terbium and ytterbium complexes by proximate energy-matched OH, NH and CH oscillators: an improved luminescence method for establishing solution hydration states

The radiative rate constants for depopulation of the excited states of clos ely-related series of anionic, neutral and cationic europium, terbium and y tterbium complexes have been measured in H2O and D2O. With the aid of selec tive ligand deuteriation, the relative contributions of OH, NH (both amide and amine) and CH oscillators have been measured and critically assessed. Q uenching of the Eu D-5(0) excited state by amine NH oscillators is more tha n twice as efficient as OH quenching. The importance of the distance betwee n the excited Ln ion and the XH oscillator is described with recourse to pu blished crystallographic information. The general equation, q = A'(Delta k( H2O)-k(D2O))(corr) is presented and revised values of A' for Eu (1.2 ms), T b (5 ms) and Yb (1 mu s) given, which allow for the quenching contribution of closely diffusing OH oscillators. The relevance of such studies to the h ydration state of certain gadolinium complexes is described and clear evide nce provided for a break in hydration at gadolinium.