2-D lanthanide–organic complexes constructed from 6,7-dihydropyrido(2,3-d)pyridazine-5,8-dione: synthesis, characterization and photoluminescence for sensing small molecules

Abstract

Four isostructural lanthanide–organic complexes [Ln(HPDH)(ox)(H₂O)]_n (Ln = Eu³⁺1, Tb³⁺2, Sm³⁺3, Gd³⁺4; H₂PDH = 6,7-dihydropyrido(2,3-d)pyridazine-5,8-dione; H₂ox = oxalic acid) have been synthesized successfully under hydrothermal conditions and characterized by means of elemental analysis, powder XRD, TG-DTA, IR and UV-vis spectroscopy. Complexes 1 and 2 were structurally characterized by single crystal X-ray diffraction analysis. All complexes are composed of 2-D layers, which further construct a 3-D supramolecular network. Each 2-D layer in complexes 1–4 is composed of 1-D Eu–ox infinite chains, which are further connected by biconnected HPDH⁻ ligands. The photoluminescent properties of complexes 1–4 were studied in the solid state at room temperature. The singlet energy level (34 722 cm⁻¹) and the lowest triplet energy level (21 786 cm⁻¹) for the ligand H₂PDH were calculated on the basis of its UV-vis absorbance edges (1 × 10⁻⁴ M in ethanol) and phosphorescence spectrum of complex 4 at 77 K, respectively. The complexes 1, 2, and 3 exhibit metal centered luminescence with characteristic red, green and pink emission, respectively. The energy transfer mechanism and photoluminescence properties were investigated. Complex 1, in which the energy transition from the triplet energy level (³ππ^*) of ligand HPDH⁻ to Eu³⁺ cation is more effective, has been selected as a representative to examine the potential for sensing small molecules by its luminescence properties in different emulsions. EtOH was found to be an excellent enhancing while DMF a highly quenching solvent in this study.