Synthesis, crystal structure and luminescent properties of lanthanide extended structure with asymmetrical dinuclear units based on 2-(methylthio)benzoic acid

The extended structures [Ln(2)(L)(6)(OH2)(4)] with L=2-(methylthio)benzoato (2-CH3S-C6H4COO-) and Ln=Tb (1), Eu (2) and Gd (3) were successfully synthesized and characterized. The single crystal structure of compound 1 was determined and showed an extended structure made up of asymmetrical dinuclear units with the formula catena-poly[{Tb(H2O)(4)}-(mu-L-1 kappa O:2 kappa O')(2)-{Tb(L-kappa O,O')2}-(mu-L-1 kappa O:2 kappa O')(2)]. In the molecule of 1, there are two distinct metal sites. The Tb atom in site 1 is bound to four coordinated water molecules and four oxygen atoms from four different benzoate ligands, two of which bridge to site 2 Tb atoms on one side and two to site 2 Tb atoms on the other side. The site 2 Tb atom is bound to four oxygen atoms from two chelating benzoate ligands and four oxygen atoms from four different benzoate ligands, two of which bridge to site 1 Tb atoms on one side and two to site 1 Tb atoms on the other side. The bridging benzoate ligands extend the framework in one-dimension with alternating site 1/site 2 Tb atoms. The luminescent properties of these asymmetric dinuclear extended structures are quite peculiar and showed a single emitting lanthanide center. The quantum yields of 1 (ca. 50-55%) is practically independent of the excitation energy, whereas those of 2 are vanishing small (<1%) when excited at the ligand states and become sizable (ca. 10-20%) upon excitation at the intra-4f manifold. To reconcile these experimental observations in conjunction with the spectral data for compounds 1 and 3, a strong interaction between the lanthanide emitting states at sites 1 and 2 was proposed. For compound 1, the numerical solutions of the rate equations provided evidences that when the transition rates between the emitting states at both sites are larger than the highest decaying rate of these states, the system becomes an effective single emitter. This establishes, for the first time, quantitative relationships between the transition rates between the lanthanide centers and their decaying rates. In the case of compound 2, the same strong interactions are present and, additionally, a ligand-tometal charge transfer (LMCT) state was invoiced to account for the dependence of the quantum yields with the excitation wavelength. Hydrothermal syntheses were also performed; however, X-ray powder diffraction and luminescence spectra showed more asymmetric structures around the lanthanide ion when compared to the crystalline compounds. (C) 2015 Published by Elsevier B.V.