Characterisation of the dimer formed by hydrolysis of uranyl in aqueous solution and its role in the biexponential luminescence decay

The biexponential luminescence decay of aqueous uranyl was investigated through pH, lifetime and steady state absorption and emission spectral studies. The luminescence above 5 × 10−4 M could be attributed to the hydrated monomer (UO22+)aq and the dimer (UO2)2(OH)22+ formed by the hydrolysis of uranyl giving lifetimes of 1.37 ± 0.04 and 8.71 ± 0.60 μs respectively with Keq = (2.0 ± 1.3) × 10−7 M. The dimer absorbs more strongly and has a higher quantum yield than the monomer and its absorption and emission spectra are red-shifted. Adding more uranyl to an aqueous solution or perchloric acid to a fixed concentration of uranyl resulted in a decrease in the amount of dimer relative to monomer, with the luminescence eventually becoming monoexponential with a lifetime of 1.37 μs. At high pH or low uranyl concentrations, the trimer (UO2)3(OH)5+ formed through further hydrolysis of uranyl was thought to cause the change observed in the steady state emission spectrum and it has a similar lifetime to the dimer.