Ln(III) complexes of a bis(5-(pyridine-2-yl)-1,2,4-triazol-3-yl)methane ligand: synthesis, structure and fluorescent properties

Abstract

The new tetradentate ligand H2L bis(5-(pyridine-2-yl)-1,2,4-triazol-3-yl)methane ligand H2L has been used to prepare corresponding Ln(III) complexes [Ln(L)(HL)H2O]∙6H2O where Ln(III) is Tb(III), Sm(III), Eu(III), Gd(III), Nd(III). The resulting complexes were investigated by various means including elemental- and thermogravimetric analysis and IR-,UV-,VIS spectroscopy. The complexes form an isostructural series in the solid state according to X-ray diffraction. The optimal value of triple state energy of the ligand sensitizes both fluorescent visible and near-IR emission of the lanthanides. However, the overall emission is moderate as result of the quenching action of coordinated water molecules. The displacement of water molecules from the inner coordination sphere both in solid samples and the solution results in a signivicant increase of the efficiency of the luminescence. The best value of the quantum yield was obatained for the Tb(III) complex with 41% . Luminescence titrations demonstrate that [Tb(L)(HL)H2O] is a pH-sensitive emitter which is able to detect pH values at both near neutral pH and acidic pH ranges.