Multifunctional Luminescent and Proton-Conducting Lanthanide Carboxyphosphonate Open-Framework Hybrids Exhibiting Crystalline-to-Amorphous-to-Crystalline Transformations

Abstract

The chemistry of metal phosphonates has been progressing fast with the addition of new materials that possess novel structural features and new properties, occasionally in a cooperative manner. In this paper, we report a new family of functional lanthanide-carboxyphosphonate materials. Specifically, the lanthanide is La, Ce, Pr, Sm, Eu, Gd, Tb, or Dy and the carboxyphosphonate ligand is 2-hydroxyphosphonoacetic acid (H 3 HPA). All reported LnHPA compounds, Ln 3 (H 0.75 O 3 PCHOHCOO) 4 · x H 2 O ( x = 15–16), crystallize in the orthorhombic system. Two types of structures were isolated: series I and II polymorphs. For both series, the three-dimensional (3D) open frameworks result from the linkage of similar organo-inorganic layers, in the ac -plane, by central lanthanide cations, which yield trimeric units also found in other metal-HPA hybrids. Large oval-shaped 1D channels are formed by the spatial separation of the layers along the b -axis and filled with lattice water molecules. LnHPA materials undergo remarkable crystalline-to-amorphous-to crystalline transformations upon dehydration and rehydration cycles, as confirmed by thermodiffraction and NMR spectroscopy. The highest proton conductivity was observed for GdHPA (series II), 3.2 × 10 –4 S cm –1 at 98% RH and T = 21 °C. The dehydration–rehydration chemistry was also followed by photoluminescence spectroscopy. It was shown that loss and reuptake of water molecules are accompanied by clear changes in the photoluminescence spectra and lifetimes of the Eu analog (series II). Our present results reveal a wide family of well-characterized, multifunctional lanthanide-based phosphonate 3D-structured metal–organic frameworks (MOFs) that show reversible crystalline-to-amorphous-to-crystalline transformations and, at the same time, exhibit high proton conductivity.