Photoluminescent microporous lanthanide silicate AV-21 frameworks

The hydrothermal synthesis and structural characterization of the lanthanide silicate system Na(6)Ln(2)Si(12)O(30).x H(2)O] (Ln=La(3+), Sm(3+), Eu(3+), Gd(3+), and Tb(3+)), named AV-21, has been reported. Structural elucidation of the Sm(3+) analogue (isomorphous with the Eu(3+), Gd(3+), and Tb(3+) 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 Ln(3+)-Ln(3+) distances and the maximum distance over which energy transfer is active. Though the majority of Ln(3+) 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.