Enhancing luminescence properties of lanthanide(iii)/pyrimidine-4,6-dicarboxylato system by solvent-free approach

Abstract

The solvent-free melt reactions between lanthanide(III) nitrates and pyrimidine-4,6-dicarboxylic acid (H₂pmdc) led to three-dimensional (3D) coordination polymers with formula [Ln(μ₄-pmdc)(NO₃)(H₂O)]_n (1-Ln) [Ln(III) = La, Ce, Pr, Nd, Sm, and Eu] and {[Ln(μ₄-pmdc)(NO₃)(H₂O)]·H₂O}_n (2-Ln) [Ln(III) = Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu]. The crystal structure of compounds 1-Ln is built up from the junction of centrosymmetric Ln(III)-pmdc dimers, where pmdc ligand adopts an unusual μ₄–κ²N,O:κ²N’,O’:κ²O,O′′:κO′′′ coordination mode, rendering a metal coordination number of 10. The smaller ionic radii of the second half of the series prevents the latter coordination mode in 2-Ln, giving rise to a framework based on triangular shaped motifs. A common structural feature is the coordination of the nitrate anion as terminal chelating ligand which limits the presence of a unique coordination water molecule. Magnetic properties have been analysed for all compounds, through the variable temperature susceptibility and field dependent magnetization. Photoluminescence measurements were performed at different temperatures and excitation wavelengths upon Sm³⁺, Eu³⁺, Tb³⁺, and Dy³⁺ containing compounds which exhibit strong characteristic emissions in the visible region. Decay lifetime measurements indicate long-lived excitation states in the compounds by achieving values in the range of millisecond for the terbium compound. The absolute quantum yields of the polycrystalline samples result in 20% and 18% for 1-Eu and 2-Tb, respectively, and in almost negligible response for 1-Sm and 2-Dy. Illumination of single crystals of these compounds revealed waveguiding behaviour along the edges of the main axis of the crystals.