Solid-State Luminescence of an Americium(III) β-Diketonate Complex: Structure and Spectroscopic Characterization of...

Luminescence spectroscopy is a powerful method for probing the chemical and physical properties of metal ions, as perturbation of electronic states due to ligand-field effects yields changes in emissive behavior. This sensitivity is particularly valuable for the trivalent actinides, whose chemical bonding is still not well understood. Here, we report the synthesis and characterization of an americium β-diketonate, [Am(acac)2(μ-acac)(H2O)]2 (Am2); acac = acetylacetonate. Am2 was prepared via slow evaporation of a methanolic americium-acac solution and characterized via single-crystal X-ray diffraction, Raman, UV-vis-NIR absorbance, and luminescence spectroscopies. The structure consists of acac-bridged Am dimers with both Am(III) metal centers adopting a distorted square antiprismatic coordination geometry. Am2 represents a rare example of ligand-sensitized Am(III) emission in the solid state, with ligand excitation at 320 nm resulting in characteristic 5f-5f emission bands at ca. 699, 850, and 1060 nm that correspond to the 5D1' → 7F1-3' transitions. The absence of the 5D1' → 7F0' transition is consistent with the C1 symmetry about the metal center established from structural studies. The observed peak splitting and bathochromic shifting can be attributed to crystal-field effects and a limited degree of covalency in Am-acac bonding.