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 (1-U) gives the uranyl(V) congener [UO₂(L^N6)][BPh₄] (2-U) (L^N6 is a hexadentate N-donor with two connected bis(imino)pyridine groups). In addition to field-induced slow magnetic relaxation, 2-U displays photoluminescence upon excitation at 440 nm, the first time that both phenomena are observed in one uranyl(V) complex. The emission from 2-U is characterized by five well-resolved bands in the region 510-586 nm, in contrast to the broad emission observed for 1-U centered on 650 nm. The relationship between the electronic structure of 1-U and 2-U is emphasized by the photochromic behavior of 1-U, which, upon irradiation at 365 nm, adopts a stable photoexcited state 1-U^* following a ligand-to-metal electron transfer, with the magnetic and spectroscopic properties of 1-U^* being similar to those of 2-U. A computational study of 1-U and 2-U explains their contrasting emission profiles in terms of the uranyl oxo ligands participating in the key transitions for 2-U but not for 1-U, with the stretching vibrations of the L^N6 ligand also contributing to the fine structure.