A TRLFS study of curium(III) naphthalene and hydroxyquinoline complexes in aqueous solution

The unknown complex formation of Cm(III) with two naphthalene ligands, 2,3­dihydroxynaphthalene (23NAP) and 2­naphthalene (2NAP) as well as 6­hydroxyquinoline (6HQ) was studied by time-resolved laser-induced fluorescence spectroscopy (TRLFS). Aromatic molecules with hydroxyl groups have the potential to enhance the solubility and mobility of metals by forming complexes. We explored both the luminescence (lifetimes and individual emission spectra) and the excitation properties (excitation spectra) of the formed Cm(III) species. The experiments were performed at a fixed total Cm(III) concentration of 0.3 µM by varying the ligand concentrations (0.03-3.0 mM) and the pH (1.9-12.4) at an ionic strength of 0.1 M (NaClO4). The spectroscopic speciation indicates the formation of CmpLqHr species. In more detail, Cm(III) forms 1:1 and 1:2 complexes with 23NAP, a 1:2 complex with 2NAP and a 1:1 complex with 6HQ. Independent from the ligand and at pH values above 9.5 strong indications were found for ternary complexes with OH- with a 1:2:-1 stoichiometry. The Cm(III)-23NAP complexes exhibited the largest red shift of the Cm3+ emission maximum at 593.7 nm to: 598.3 nm for Cm(23NAP)H2+, 604.4 nm for Cm(23NAP)+, 608.4 nm for Cm(23NAP)2-, and 616 nm for Cm(23NAP)2OH2-. Based on the factor analysis of the emission data the stability constants were calculated to be: (a) for 23NAP log ß111 = 20.2 ± 0.8, log ß110 = 12.7 ± 0.4, log ß120 = 20.5 ± 0.4, and log ß12-1 = 9.7 ± 0.4; (b) for 2NAP log ß120 = 19.4 ± 0.2, and log ß12-1 = 10.3 ± 0.1; and (c) for 6HQ log ß110 = 8.8 ± 0.3, and log ß12-1 = 5.1 ± 0.9 at an ionic strength of 0.1 M (NaClO4). The stability constants were compared to those of natural pyoverdins.