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) center dot 3H(2)O, Eu-2(TPAC)(6) center dot 5.25H(2)O and RE(TPAC)(3) center dot 3.5H(2)O (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 [Eu-2(TPAC)(6) center dot (H2O)(3)] center dot 2.25H(2)O 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 5D(0) -> 7 F-0 and 5D(0) -> F0-4 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 4f(5)-intraconfigurational transitions. The theoretical intensity parameters Omega(lambda) (lambda = 2 and 4), maximum splitting of the F-7(1) state (Delta E) and the ratio between the D-5(0) -> F-7(0) and D-5(0) -> F-7(2) 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 (eta) of the 5 D emitting level of the Eu3+ ion was also determined. (c) 2005 Elsevier B.V. All rights reserved.