Low-Percentage Ln³⁺ Doping in a Tetranuclear Lanthanum Polyoxometalate Assembled from [Mo₇O₂₄]^6- Polyanions Yielding Visible and Near-Infrared Luminescence

A rare case of low-percentage trivalent lanthanide doping in multinuclear lanthanide polyoxometalates (LnPOMs) was investigated. The [La₄(MoO₄)(H₂O)₁₆(Mo₇O₂₄)₄]^14- polyanion was chosen as the host material for this study. In this polyanion the central [La₄(MoO₄)]¹⁰⁺ core is coordinated by four heptamolybdate groups as well as 16 water molecules. The tetranuclear lanthanum POM was doped with 5% of Eu³⁺, Tb³⁺, Sm³⁺, Dy³⁺, Nd³⁺, Er³⁺, and Yb³⁺ (according to synthesis), and the structures and luminescence properties of the x%Ln:LaPOMs were investigated. Additionally a series of tetranuclear lanthanide POMs built from [Mo₇O₂₄]^6- heptamolybdate polyanions with Eu³⁺, Tb³⁺, Sm³⁺, Dy³⁺, and Nd³⁺ instead of La³⁺ were synthesized, and a detailed analysis revealed that the tetranuclear clusters formed monomers or dimers linked through oxygen bridges. The smaller lanthanide ions, namely, Er³⁺ and Yb³⁺, did not form tetranuclear clusters, but instead mononuclear sandwich-type POMs were obtained. The obtained structures were shown to be lanthanide-specific, and not a result of different synthetic/crystallization conditions. The luminescence properties of the x%Ln:LaPOMs were compared with the luminescence properties of the LnPOMs.