Excited state reactions in studies of complex formation between actinides and organic ligands with laser induced methods

In a former study we analyzed the complex formation between uranium and 2,3-dihydroxybenzoic acid using the fluorescence properties of the ligand. The formation constant was found to be log K11 = -3.11 ± 0.16.We have validated this complex formation using the fluorescence properties of uranium. The formed complex has no fluorescence properties. This is confirmd by a monoexponetial fluorescence decay and also the fluorescence spectra show no shift in the emission maxima.However the fluorescence lifetime of the non-complexed uranium in the solution depends on the concentration of the added ligand. This is due to the dynamic quench effect of the ligand molecule. This quench effect is strongly correlated to the pH of the solution. This leads to the conclusion that only the protonated ligand effects this quench process. Taking this effect into account we are able to calculate the fluorescence intensities of the non-complexed uranium and by use of these data the complex formation between uranium and 2,3-dihydroxybenzoic acid. We obtained a one to one complex formation accompanied by the release of two protons from the ligand, as also found in the study of the fluorescence properties of the ligand. However the derived complex formation constants differ between both methods. The formation constant obtained from the uranium study was found to be log K = -3.99 ± 0.44. This is about 0.9 orders of magnitude lower compared to the constant derived from the study of the ligand. The confidence limits of the two data sets overlap nevertheless the deviation of the both constants leads to the conclusion that other reactions may be involved. From the uranium fluorescence we do not expect such reactions. But the non-complexed ligand may show separate reactions in the excited state, which lead to a change of the fluorescence intensity of the ligand and resulting in an increase of the calculated formation constant. The consequence of the inclusion of these reactions in the examination leads to an exact determination of the ligand concentration accompanied by a much better agreement of the assigned formation constants.