Low-Percentage Ln Doping in a Tetranuclear Lanthanum Polyoxometalate Assembled from [MoO] Polyanions Yielding Visible and Near-Infrared Luminescence.

A rare case of low-percentage trivalent lanthanide doping in multinuclear lanthanide polyoxometalates (LnPOMs) was investigated. The [La(MoO)(HO)(MoO)] polyanion was chosen as the host material for this study. In this polyanion the central [La(MoO)] core is coordinated by four heptamolybdate groups as well as 16 water molecules. The tetranuclear lanthanum POM was doped with 5% of Eu, Tb, Sm, Dy, Nd, Er, and Yb (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 [MoO] heptamolybdate polyanions with Eu, Tb, Sm, Dy, and Nd instead of La were synthesized, and a detailed analysis revealed that the tetranuclear clusters formed monomers or dimers linked through oxygen bridges. The smaller lanthanide ions, namely, Er and Yb, 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.