Photoluminescence\nof a Uranium(IV) Alkoxide Complex

In\nthis report, we describe the photoluminescence of\na homoleptic\nuranium(IV) alkoxide complex. Excitation of [Li(THF)]<sub>2</sub>[U<sup>IV</sup>(O<sup><i>t</i></sup>Bu)<sub>6</sub>] leads to\nthe first example of photoluminescence from a well-defined actinide\ncomplex originating from an f–f excitation, supported by second\norder multiconfigurational electronic structure calculations including\nspin–orbit coupling. These calculations show strong spin–orbit\ncoupling between the excited triplet and singlet states for the 5f-orbital\nmanifold, which leads to a long-lived excited state lifetime of 0.85\ns at low temperature. The photophysical properties of homoleptic uranium(V)\nand uranium(VI) tertbutoxide complexes are also presented; we find\nthat oxidation of the uranium(IV) alkoxide results in quenching of\nluminescence in [Li(THF)][U<sup>V</sup>(O<sup><i>t</i></sup>Bu)<sub>6</sub>] and [U<sup>VI</sup>(O<sup><i>t</i></sup>Bu)<sub>6</sub>]. This is attributed to competing ligand to metal\ncharge transfer absorption processes shifted to lower energy upon\noxidation of the actinide center, which mask the relevant f–f\ntransitions in the visible region of the electronic absorption spectrum.