Photoluminescence of uranium(VI): quenching mechanism and role of uranium(V)

The photoluminescence of uranium(VI) is observed typically in the wavelength range 400 - 650 nm with the lifetime of several hundreds μs and is known to be quenched in the presence of various halide ions (case A) or alcohols (case B). Here, we show by density functional theory (DFT) calculations that the quenching involves an intermediate triplet excited state which exhibits uranium(V) character. The DFT results are consistent with previous experimental findings suggesting the presence of photo-excited uranium(V)–radical pair during the quenching process. In the ground state of uranyl(VI) halides, the ligand contributions to the highest occupied molecular orbitals increase with the atomic number (Z) of halide ion allowing larger ligand-to-metal charge transfer (LMCT) between uranium and halide ion. Consequently, larger quenching effect is expected as Z increases. The quenching mechanism is essentially the same in case A and B, and is driven by an electron transfer from the quencher to the UO22+ entity. The relative energetic stabilities of the triplet excited state define the "fate" of uranium so that in case A uranium(V) is oxidized back to uranium(VI), while in case B uranium remains as pentavalent.