Immobilization of lanthanide ions in a pillared layered double hydroxide

A Zn-Al layered double hydroxide (LDH) pillared by 2,2'-bipyridine-5,5'-dicarboxylate (BDC) anions was used as a porous matrix to intercalate LnCl(3) (Ln = Eu, Gd). Metal loadings of 9.0 wt% for Eu and 11 wt% for Gd were achieved, which correspond to about two lanthanide ions for every three BDC ions. The fitting of the room-temperature Eu L-3-edge EXAFS for the europium-containing material revealed one shell of 7 +/- 1 oxygen/nitrogen atoms at 2.41 angstrom. No evidence for a Eu-Cl bond was found. The emission spectra for this material display the typical Eu3+ red emission and a large broad band peaking around 460 nm, which was attributed to the emission arising from the triplet levels of free ligands not coordinated to Eu3+ ions. Measurement of the emission spectra under different excitation wavelengths and also at low temperature showed the existence of only one type of Eu3+ binding site. The D-5(0) quantum efficiency was estimated to be low (7.7%), due to a relatively high nonradiative transition probability caused by the presence of water molecules in the first coordination shell. The number of water molecules was calculated as 3.6 +/- 1, suggesting that the incorporated europium ions were 6-coordinate with four oxygen atoms from water molecules and two nitrogen atoms from a bidentate bipyridyl ligand.