Study of lanthanide aromatic acid complexes in silica gels by photoacoustic spectroscopy

Lanthanide complexes Ln(p-ABA)3·H2O (p-ABA: p-aminobenzoic acid; Ln3+:La3+, Tb3+ and Er3+) have been incorporated into silica gels via a sol–gel method. Upon heat treatment at 120 °C, photoacoustic (PA) intensity of the ligand increases for Tb3+, La3+ and Er3+ complexes in silica gels, respectively, while this difference cannot be observed for the samples without heat treatment. Different PA intensities of the samples are interpreted by comparison with their luminescence spectra. The nephelauxetic parameters and PA branching vectors of Er3+ complex in silica gel have been calculated. Spectral results indicate that p-ABA does not coordinate with lanthanide ions in silica matrix without a suitable heat treatment. For the co-doped samples, it is shown that the emissions of Tb3+ are enhanced with addition of La(p-ABA)3·H2O and remarkably quenched with the addition of Er(p-ABA)3·H2O. The possible mechanisms for these phenomena are proposed.