Photoluminescence of a Uranium(IV) Alkoxide Complex.

In this report, we describe the photoluminescence of a homoleptic uranium(IV) alkoxide complex. Excitation of [Li(THF)](2)[U(IV)(O (t) Bu)(6)] leads to the first example of photoluminescence from a well-defined actinide complex originating from an f-f excitation, supported by second order multiconfigurational electronic structure calculations including spin-orbit coupling. These calculations show strong spin-orbit coupling between the excited triplet and singlet states for the 5f-orbital manifold, which leads to a long-lived excited state lifetime of 0.85 s at low temperature. The photophysical properties of homoleptic uranium(V) and uranium(VI) tertbutoxide complexes are also presented; we find that oxidation of the uranium(IV) alkoxide results in quenching of luminescence in [Li(THF)][U(V)(O (t) Bu)(6)] and [U(VI)(O (t) Bu)(6)]. This is attributed to competing ligand to metal charge transfer absorption processes shifted to lower energy upon oxidation of the actinide center, which mask the relevant f-f transitions in the visible region of the electronic absorption spectrum.