A New Series Of 3d Lanthanide Phenoxycarboxylates: Synthesis, Crystal Structure, Magnetism And Photoluminescence Studies

A new series of 3D lanthanide (Ln) frameworks has been synthesized with the general formula [{Ln(L)(H2O)}center dot H2O](n) (where Ln = La(1), Tb(2) and Dy(3); L = (HO)OC6H2(CH2COO)(2); H3L = 2,5-dihydroxy-1,4-benzenediacetic acid = DHBDA). All the crystals were analyzed by single crystal X-ray diffraction. The Ln ions are octa-coordinated having the square antiprism geometry with the D-4d symmetry and connected along the crystallographic a-axis forming a zigzag chain. These chains are further connected by the ligands in three-dimensional crystallographic space. Compounds 2 and 3 were subjected to variable temperature magnetic susceptibility measurements and photoluminescence studies. Magnetic studies reveal that compound 2 possesses antiferromagnetic coupling whereas compound 3 shows ferromagnetic interaction at temperatures above ca. 40 K. The emission spectra of compounds 2 and 3 combine the intra-4f emission of the Ln ions with that of the ligand, depending on the selected temperature, pointing out that the energy transfer processes are thermally activated. In addition, compound 2 presented an invariant D-5(4) lifetime behaviour at temperatures between 15 and 300 K. Theoretical analysis showed that the long-lived lifetimes of the free ligand's excited states and the intramolecular energy transfer rate invariability are responsible for such abnormal behaviour.