Intramolecular sensitization of americium luminescence in solution: shining light on short-lived forbidden 5f transitions

Abstract

The photophysical properties and solution thermodynamics of water soluble trivalent americium (Am^III) complexes formed with multidentate chromophore-bearing ligands, 3,4,3-LI(1,2-HOPO), Enterobactin, and 5-LIO(Me-3,2-HOPO), were investigated. The three chelators were shown to act as antenna chromophores for Am^III, generating sensitized luminescence emission from the metal upon complexation, with very short lifetimes ranging from 33 to 42 ns and low luminescence quantum yields (10⁻³ to 10⁻²%), characteristic of Near Infra-Red emitters in similar systems. The specific emission peak of Am^III assigned to the ⁵D₁ → ⁷F₁ f–f transition was exploited to characterize the high proton-independent stability of the complex formed with the most efficient sensitizer 3,4,3-LI(1,2-HOPO), with a log β₁₁₀ = 20.4 ± 0.2 value. In addition, the optical and solution thermodynamic features of these Am^III complexes, combined with density functional theory calculations, were used to probe the influence of electronic structure on coordination properties across the f-element series and to gain insight into ligand field effects.