Visible and near-infrared emission from lanthanoid β-triketonate assemblies incorporating cesium cations

The reaction of the β-triketonate ligands tris(4-methylbenzoyl)methanide and tribenzoylmethanide with the trivalent lanthanoid Eu³⁺, Er³⁺, and Yb³⁺ in the presence of Cs⁺ afforded polymeric structures where the repeating units are represented by bimetallic tetranuclear assemblies of formulation {[Ln(Cs)(β-triketonate)₄]₂}_n. The only exception is the structure formed by reaction of tris(4-methylbenzoyl)methanide, Yb³⁺, and Cs⁺, which yielded a polymeric assembly where the repeating units are mononuclear Yb³⁺ complexes bridged by Cs⁺ cations. Photophysical measurements on the obtained materials confirmed efficient sensitization from the ligand excited states to the 4ƒ* excited stated of the three lanthanoids. According to transient absorption data, Er³⁺ and Yb³⁺ are sensitized via energy transfer from the triplet state of the β-triketonate ligands. On the other hand, energy transfer to Eu³⁺ seems to occur via an alternative pathway, possibly directly via the singlet state or through ligand-to-metal charge transfer states. The emission measurements confirm efficient sensitization for all three lanthanoids and bright near-infrared emission for Er³⁺ and Yb³⁺, a characteristic that seems to be linked to the specific chemical structure of the β-triketonate ligands.