Homodinuclear lanthanide {Ln2} (Ln = Gd, Tb, Dy, Eu) complexes prepared from an o-vanillin based ligand: luminescence and single-molecule magnetism behavior

Four dinuclear lanthanide complexes [Gd-2 (H2L)(2) (mu-piv)(2) (piv)(2)]center dot 2CHCl(3) (1), [Tb-2 (H2L)(2) (mu-piv)(2) (piv)(2)]center dot 2CHCl(3) (2), [Dy-2 (H2L)(2) (mu-piv)(2) (piv)(2)]center dot 2CHCl(3) (3) and [Eu-2 (H2L)(2) (mu-piv)(2) (piv)(2)]center dot 2CHCl(3) (4) were synthesized by the reaction of appropriate Ln(III) chloride salts and a multidentate ligand, 2,2'-(2-hydroxy-3-methoxy-5-methylbenzylazanediyl) diethanol (H3L) in the presence of pivalic acid. 1-4 are neutral and are held by two monoanionic, [H2L](-) ligands. The two lanthanide ions are doubly bridged to each other via two phenolate oxygen atoms. Both the lanthanide ions are nine coordinated and possess a distorted capped square antiprism geometry. Photophysical studies reveal that Tb3+ (2) and Dy3+ (3) complexes display strong ligand-sensitized lanthanide-characteristic emission. The Tb3+ complex (2) shows a very high overall quantum yield of 76.2% with a lifetime of 1.752 ms. Magnetic studies reveal single-molecule magnet behavior for 3 which shows in its ac susceptibility studies a two-step slow relaxation yielding two effective relaxation energy barriers of Delta E = 8.96 K and 35.51 K.