Time-resolved laser-induced fluorescence spectroscopy (TRLFS) of aqueous Am(III) complexes at ambient and elevated temperature

Time-resolved laser-induced fluorescence spectroscopy (TRLFS) has been extensively used as a sensi-tive and selective technique to analyze actinide complexation with inorganic and organic ligands in trace metal concentrations. However, the application of TRLFS onto Am(III) complexation systems was up to now limited because of the much lower luminescence intensity and much shorter lifetime of Am(III) in comparison to U(VI) or Cm(III).
We investigated the complexation behavior of Am(III) complexes with lactate (Lac) and substituted benzoic acids like pyromellitic acid (1,2,4,5-benzenetetracarboxylic acid, BTC) at ambient and ele-vated temperatures with TRLFS.
Using the emission of the 5D1-7F1 transition at around 691 nm, spectral data like luminescence life-times, luminescence maxima and complex stability constants were calculated. Temperature dependent stability constants were determined to estimate thermodynamic data (reaction enthalpy, reaction en-tropy).
The Am(III) aquo ion shows at pH 4-6 a luminescence lifetime of 23 ns, corresponding to approxi-mately 9 coordinating water molecules. Complexation with BTC shows no change of the excitation and emission maximum but an increase of the luminescence intensity and lifetime. The luminescence lifetime was prolonged to 27 ns, corresponding to 8 remaining water molecules in the first coordina-tion shell. This indicates an exchange of 1 water molecule with 1 coordination site of the ligand, re-sulting in an Am-BTC 1:1 complex [1]. In contrast, complexation with lactate causes a red shift of the excitation wavelength of Am(III) (Fig. 1), resulting in a red shift of the luminescence emission maxi-mum of about 5 nm. The luminescence lifetime is prolonged up to 37 ns which corresponds to 5-6 re-maining water molecules. This indicates an exchange of about 3-4 water molecules with coordination sites of ligand molecules which implies the formation of 1:1, 1:2 and 1:3 complexes. The stability con-stants increase slightly with rising temperature which is consistent with an endothermic complexation reaction.