Formation reactions and photophysical parameters of highly luminescent lanthanoids(III) complexes with 4-hydroxypyridine-2,6-dicarboxylic acid

For the purpose of finding ligand designing strategy for lanthanoids energy-transfer luminescence, formation reactions of 4-hydroxypyridine-2,6- dicarboxylic acid (chelidamic acid, CDA) with terbium(III) and europium(III) ions in aqueous solution were investigated. A luminescent complex species having 1:3 stoichiometry was formed at pH 8.4-11.6. Those complexes were hydrolyzed by more alkaline solution. The relative quantum yield of Tb(III)-CDA complex at pH 11.6 was 0.636 (λ_em = 544 nm) on a quinine sulfate basis. The luminescence lifetimes of Tb(III)-CDA were calculated as 1.02 ms at pH 8.4 and 1.55 ms at pH 11.6. The luminescence intensities of Tb(III) and Eu(III)-CDA complexes increased from pH 9 to 11 with the deprotonation degree of the 4-hydroxyl group on the complexes. The deprotonated Ln(III)-CDA complex species possibly has a resonance structure of the pyridone type, which would contribute to the highly enhanced luminescence intensity. Detailed photophysical data in terms of photoexcitation and the triplet energy level (low temperature phosphorescence) are given.