Preparation, crystal structure and optical spectroscopy of the rare earth complexes (RE³⁺=Sm, Eu, Gd and Tb) with 2-thiopheneacetate anion

Rare earth complexes with the formulae Sm(TPAC)₃·3H ₂O, Eu₂(TPAC)₆·5.25H₂O and RE(TPAC)₃·3.5H₂O (where RE=Gd and Tb), and TPAC=2-thiopheneacetate) have been synthesized and characterized by complexometric titration, elemental analyses, infrared spectroscopy, and X-ray crystallography. Infrared data suggested the presence of both bridging and chelating TPAC anions. The crystal structure of the [Eu₂(TPAC) ₆·(H₂O)₃]·2.25H₂O complex in the solid state, determined by X-ray diffraction, revealed that it crystallizes in the orthorhombic crystal system (space group Aba2), with two crystallographically independent Eu³⁺ centers (Eu1 and Eu2). These europium centers are held together by one bidentate bridging and two tridentate bridging carboxylate groups. The existence of two Eu³⁺ centers was also supported by the emission spectrum. The luminescence properties of the RE-TPAC complexes were investigated by measuring the excitation and emission spectra, and the intramolecular ligand-to-rare earth energy transfer mechanisms were discussed. The emission spectra of the Eu³⁺ and Tb³⁺ ions displayed only narrow bands arising from ⁵D₀→ ⁷F₀ and ⁵D₀→⁷F _0-4 transitions, respectively, indicating an efficient luminescence sensitization of these ions by the TPAC 'antenna'. On the other hand, the emission spectrum of the Sm³⁺-complex displayed a broad band from the phosphorescence of the TPAC ligand which overlapped the 4f⁵- intraconfigurational transitions. The theoretical intensity parameters Ω_λ (λ=2 and 4), maximum splitting of the ⁷F₁ state (ΔE) and the ratio between the ⁵D₀→⁷F₀ and ⁵D ₀→⁷F₂ transition intensities (R ₀₂) were calculated based on the X-ray crystalline structure for the Eu³⁺-complex, and a comparison with experimental data were made. The emission quantum efficiency (η) of the D5 emitting level of the Eu ³⁺ ion was also determined.