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 5f 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 [UO(L)][BPh] () gives the uranyl(V) congener [UO(L)][BPh] () (L is a hexadentate -donor with two connected (imino)pyridine groups). In addition to field-induced slow magnetic relaxation, displays photoluminescence upon excitation at 440 nm, the first time that both phenomena are observed in one uranyl(V) complex. The emission from is characterized by five well-resolved bands in the region 510-586 nm, in contrast to the broad emission observed for centered on 650 nm. The relationship between the electronic structure of and is emphasized by the photochromic behavior of which, upon irradiation at 365 nm, adopts a stable photoexcited state following a ligand-to-metal electron transfer, with the magnetic and spectroscopic properties of being similar to those of . A computational study of and explains their contrasting emission profiles in terms of the uranyl oxo ligands participating in the key transitions for but not for , with the stretching vibrations of the L ligand also contributing to the fine structure.