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(C 9 H 3 O 6 )(H 2 O)], MOF-76(Am) . This material is constructed from Am 3+ 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 Am 3+ ions have seven coordinates and assume a distorted, capped trigonal prismatic geometry with C 1 symmetry. The Am 3+ -O bonds were studied via infrared spectroscopy and compared to several MOF-76(Ln) analogs, where Ln = Nd 3+ , Eu 3+ , Tb 3+ , and Ho 3+ . The results show that the strength of the ligand carboxylate stretching and bending modes increase with Nd 3+ < Eu 3+ < Am 3+ < Tb 3+ < Ho 3+ , 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 Am 3+ -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 Eu 3+ (4f 6 ) and related Tb 3+ (4f 8 ) analogs. The optical properties were explored further within a series of heterometallic MOF-76(Tb 1- x Am x ) ( x = 0.8, 0.2, and 0.1) samples, and the results reveal enhanced Am 3+ photoluminescence.