Expanding the Transuranic Metal-Organic Framework Portfolio: The Optical Properties of Americium(III) MOF-76

Reported is the synthesis, crystal structure, and solid-state characterization of a new americium containing metal-organic framework (MOF), [Am(C9H3O6)(H2O)], MOF-76(Am). This material is constructed from Am3+ metal centers and 1,3,5-tricarboxylic acid (BTC) ligands, forming a porous three-dimensional framework that is isostructural with several known trivalent lanthanide (Ln) analogs (e.g., Ce, Nd, and Sm-Lu). The Am3+ ions have seven coordinates and assume a distorted, capped trigonal prismatic geometry with C-1 symmetry. The Am3+-O bonds were studied via infrared spectroscopy and compared to several MOF-76(Ln) analogs, where Ln = Nd3+, Eu3+, Tb3+, and Ho3+. The results show that the strength of the ligand carboxylate stretching and bending modes increase with Nd3+ < Eu3+ < Am3+ < Tb3+ < Ho3+, suggesting the metal-oxygen bonds are predominantly ionic. Optical absorbance spectroscopy measurements reveal strong f-f transitions; some exhibit pronounced crystal field splitting. The photoluminescence spectrum contains weak Am3+-based emission that is achieved through direct and indirect metal center excitation. The weak emissive behavior is somewhat surprising given that ligand-to-metal resonance energy transfer is efficient in the isoelectronic Eu3+ (4f(6)) and related Tb3+ (4f(8)) analogs. The optical properties were explored further within a series of heterometallic MOF-76(Tb1-xAmx) (x = 0.8, 0.2, and 0.1) samples, and the results reveal enhanced Am3+ photoluminescence.