Aqueous uranyl benzoate species characterized by different spectroscopic techniques

In this study UV-vis spectroscopy as well as RT and cryo time-resolved laser-induced fluorescence spectroscopy (TRLFS) were applied to reinvestigate the complexation of uranium(VI) by benzoic acid (BA). For the first time in the liquid phase the existence of a U(VI)-BA complex with a 1:2 stoichiometry could be proven.
RT TRLFS measurements revealed a static as well as a dynamic ligand-initiated quench process in the U(VI)-BA system. At these conditions no luminescence emission resulting from complex formation was found. Consequently cryo TRLFS was applied to increase the maximum detect-able BA:U(VI) ratio. By this for the first time a lumines-cence spectrum of the 1:2 U(VI)-BA complex could be determined. This species is characterized by emission bands at 467, 485, 505, 526, and 550 nm which are blue-shifted compared to the ones of the uranyl ion. The luminescence lifetime of the 1:2 complex amounts to 9.21 ± 0.01 µs at -18°C compared to 150.4 ± 0.5 µs for uranyl.
Stability constants of both, the 1:1 and the 1:2 species, have been calculated to be log β110 = 2.66 ± 0.18 and log β120 = 4.48 ± 0.24, respectively. UV-vis spectroscopy combined with factor analysis yielded the molar absorption spectrum of the 1:2 U(VI)-BA species which is characterized by absorption bands at 406, 418, 432.5, 447, and 461 nm and a molar absorption coefficient of 22 L•mol-1•cm-1.