A Hexagonal Bipyramidal Uranyl(V) Single-Ion Magnet Showing Finger-Type Photoluminescence

The occurrence of uranyl(V) compounds in nuclear fuel cycles and radioactive waste necessitates a deep understanding of the electronic structure of these 5f¹ species. Characteristic properties of pentavalent uranium can include single-molecule magnet (SMM) behavior and finger-like luminescence. However, both properties have not previously been observed in the same uranyl(V) complex. Here, we show that one-electron reduction of the hexagonal bipyramidal uranyl(VI) compound [UO₂(L^N6)][BPh₄]₂ (<b>1-U</b>) gives the uranyl(V) congener [UO₂(L^N6)][BPh₄] (<b>2-U</b>) (L^N6 is a hexadentate <i>N</i>-donor with two connected <i>bis</i>(imino)pyridine groups). In addition to field-induced slow magnetic relaxation, <b>2-U</b> displays photoluminescence upon excitation at 440 nm, the first time that both phenomena are observed in one uranyl(V) complex. The emission from <b>2-U</b> is characterized by five well-resolved bands in the region 510-586 nm, in contrast to the broad emission observed for <b>1-U</b> centered on 650 nm. The relationship between the electronic structure of <b>1-U</b> and <b>2-U</b> is emphasized by the photochromic behavior of <b>1-U,</b> which, upon irradiation at 365 nm, adopts a stable photoexcited state <b>1-U</b>^<b>*</b> following a ligand-to-metal electron transfer, with the magnetic and spectroscopic properties of <b>1-U</b>^<b>*</b> being similar to those of <b>2-U</b>. A computational study of <b>1-U</b> and <b>2-U</b> explains their contrasting emission profiles in terms of the uranyl oxo ligands participating in the key transitions for <b>2-U</b> but not for <b>1-U</b>, with the stretching vibrations of the L^N6 ligand also contributing to the fine structure.