Photoluminescent Microporous Lanthanide Silicate AV-21 Frameworks

The hydrothermal synthesis and structural characterization of the lanthanide silicate system [Na6Ln2Si12O30x H2O] (Ln=La3+, Sm3+, Eu3+, Gd3+, and Tb3+), named AV-21, has been reported. Structural elucidation of the Sm3+ analogue (isomorphous with the Eu3+, Gd3+, and Tb3+ frameworks) using single-crystal synchrotron X-ray diffraction and solid-state NMR spectroscopy reveal disorder in the Si(1) second coordination sphere. La-AV-21 presents a distinct framework. These materials combine microporosity and interesting photoluminescence features with structural flexibility that allows the introduction of a second or third type of lanthanide center. Room-temperature lifetime decay dynamics have been used to estimate the Ln3+Ln3+ distances and the maximum distance over which energy transfer is active. Though the majority of Ln3+ centers occupy regular framework positions, the Ln(2) defect centers are disordered over the Na(1) sites in the pores and greatly influence the energy-transfer process, providing a unique opportunity for studying the relationship between structural disorder and photoluminescence properties in framework solids.