Structures and manifestation of ortho-, meta-, and para-NH2-substitution in the optical spectra of europium and terbium aminobenzoates

Four pairs of isostructural europium and terbium ortho-, meta, and para-aminobenzoates ([Ln(2-ABenz)(3)(H2O)], [Ln(3-ABenz)(3)(H2O)(3)], [Ln(3-ABenz)(3)(H2O)(3)]center dot 3H(2)O, [Ln(4-ABenz)(3)(H2O)], (Ln = Eu, Tb; ABenz - aminobenzoate anion)) were investigated using X-ray diffraction and optical spectroscopy (luminescence and luminescence excitation spectra, as well as vibrational IR and Raman spectra). The crystal structures of [Tb(2-ABenz)(3)(H2O)], [Eu(3-ABenz)(3)(H2O)(3)], [Eu(3-ABenz)(3)(H2O)(3)]center dot 3H(2)O, and [Eu(4-ABenz)(3)(H2O)] were determined by single crystal X-ray analysis. The [Eu(3-ABenz)(3)(H2O)(3)]structure constitutes a new type of lanthanide m-aminobenzoate in the R3 space group. The Ln(3+) coordination polyhedron formed by three bidentate chelate carboxylic groups and three terminal water molecules in both structures of m-aminobenzoate can be described as a distorted three-capped trigonal prism. The influence of the incorporation of three solvate outer-sphere H2O molecules in the crystal lattice of m-aminobenzoate on the Eu3+ luminescence center was analyzed. Restructuring of the LnO(9) coordination polyhedron results in a decrease in the distortions of the crystal field that appears as a loss of extra splitting of the Ln(3+) electronic levels in [Ln(3-ABenz)(3)(H2O)(3)]center dot 3H(2)O. The effect of the electron-donating -NH2 group located in different positions on the benzene ring on process of the excitation energy transfer to Ln3+ ion is examined. The hypsochromic shift of an intense interligand charge transfer (ILCT) band in the Tb3+ excitation spectra of the sequence of o-, m-, and p-NH2-substituted compounds is observed. This shift is the consequence of the different electron density distribution depending on the NH2 position in the ligands. The predominant role of low-energy ligand-metal charge transfer (LMCT) states in the quenching of the luminescence of the europium o- and p-aminobenzoates in contrast with the europium m-aminobenzoates is discussed. Compounds including a supplementary -Cl substituent on the benzene ring [Ln(2-A-5-Cl-Benz)(3)(H2O)] (Ln = Eu, Tb) and [Eu(4-A-2-Cl-Benz)(3)(H2O)] are also under consideration. (C) 2014 Elsevier B.V. All rights reserved.