The luminescence of excited uranyl ions between pH 1 and 12 in the presence of /3-cyclodextrin was studied using conventional fluorometry and at pH 1 and low uranyl concentration
(5 X 10e4 mol dmv3) using laser flash photolysis.
A monoexponential luminescence decay is observed and pcyclodextrin gives a good Stern-Volmer plot with a constant Ksv = 208 + 20 mol-' dm3. This result, together with the linear variation in the decay rate with /3-cyclodextrin concentration,
shows that quenching of uranyl luminescence by fl-cyclodextrin is not static but dynamic, with a quenching rate constant k, = (9.8 f 0.5) X lo7 mol-' dm3 s-l. At pH 3, the luminescence undergoes a b&exponential decay. The decay rate of the longer lifetime component depends on pH. Laser flash photolysis studies reveal that the bi-exponential decay can be interpreted in terms of the deactivation of the excited uranyl aquo-ions and the hydrolysis product of Uvl which is soluble in Hz0 in the presence of /3-cyclodextrin.
At pH 10, the signal of the fluorescence decay disappears (k > 10' s-l). In this case, the quenching of the fluorescence is due to an intramolecular process.
Resume
La fluorescence de l'etat excite de l'ion uranyle *UOz2+ entre pH 1 et 12 en presence de P-cyclodextrine a 6th 6tudiCe & la fois par spectrofluorimetrie et par photolyse laser. A pH 1 et pour des concentrations en U02'+ de 5 X 1O-4 mol dmh3, le declin de la fluorescence est monoexponentiel et suit une cinetique de Stem-Volmer de constante Ksv = 208 f 20 mol-l dm3. Ce resultat ainsi que la variation linkire de la vitesse de disparition de *U02'+ en fonction de la concentration de /3-cyclodextrine, montrent que l'inhibition de la fluorescence est purement dynamique de constante de vitesse bimoleculaire k, = (9,8 4 0,5) X lo7 mol-' dm3 s-r. A partir de pH 3 le declin de la fluorescence n'est plus monocinetique mais se compose de deux cinetiques du premier ordre dont l'une depend du pH. Celle-ci est attri-b&e i la d&activation de l'etat excite de composes hydrolyses de l'uranyle