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)3·3H2O, 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→7F0-4 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 Ωλ (λ=2 and 4), maximum splitting of the 7F1 state (ΔE) and the ratio between the 5D0→7F0 and 5D0→7F2 transition intensities (R02) 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 Eu3+ ion was also determined.