Emission intensity of terbium(III) bound to benzene-carboxylic acid derivatives

The emission intensity of Tb³⁺ bound to phenol, catechol, resorcinol, benzoic acid, pyridine, nicotonic acid, phthalic acid, salicylic acid, and picolinic acid has been studied. Relative intensities of Tb³⁺ emission indicate that formation of a chelate ring is necessary to observe enhancement of the emission relative to that of Tb³⁺ alone in aqueous solution. The pH dependence of Tb³⁺ emission when bound to phthalic, salicylic, and picolinic acids were examined in detail and evidence was found which indicated that the geometry of the Tb³⁺ ion undergoes several changes as the solution pH is raised. The emission intensities and pH dependences are also found to depend on the ratio of ligand to metal, and this observation support the presence of polynuclear lanthanide complexes existing in solution.