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

• Pairs of isostructural Eu3+ and Tb3+o-, m-, and p-aminobenzoates were examined.
• The position of NH2 in the benzene ring defines an efficiency of Ln3+ excitation.
• LMCT states in Eu3+o- and p-aminobenzoates are luminescence quenchers.
• Three outer-sphere H2O reduce distortions of Ln3+ luminescence center.

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]·3H2O, [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]·3H2O, 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 Ln3+ 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 LnO9 coordination polyhedron results in a decrease in the distortions of the crystal field that appears as a loss of extra splitting of the Ln3+ electronic levels in [Ln(3-ABenz)3(H2O)3]·3H2O. 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)n] (Ln = Eu, Tb) and [Eu(4-A-2-Cl-Benz)3(H2O)n] are also under consideration.

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