Novel Schiff-base derivatives and corresponding Eu(III) complexes: multiple-color, pH-responsive mechanism and fluorescence property

Abstract

Abstract Great attention has been paid to synthesize organic ligands with good coordination ability to Eu(III) ion, however, in rare earth coordination field, it's still a challenge to prepare multifunctional organic ligands with good performance and potential application. In this work, we designed and synthesized novel methoxypicolinohydrazide Schiff-base derivatives (MPDS) and their corresponding Eu(III) complexes. Multiple-color emission in the solid state and pH response in alkaline environment of MPDS was investigated. The fluorescence intensity, color purity and fluorescence lifetime of Eu(III) complexes were studied by comparing different substituent groups. Density functional theory (DFT) calculations suggest that MPDS exhibit diverse fluorescence emission from green to orange due to the different electronegativity of substituent groups on the salicylaldehyde, and the maximum emission wavelengths are 512 nm, 534 nm and 582 nm, respectively. Besides, because of intramolecular charge transfer mechanism (ICT), MPDS exhibit the unique response to pH 7.0–12.0 by emitting green fluorescence in different linear ranges. All complexes emit excellent red fluorescence of Eu(III) ion with good thermal stability and high color purity. The effect of different substituent groups on fluorescence intensity and fluorescence lifetime of Eu(III) complexes is ordered: [EuL3 (NO3)3]·2H2O > [EuL1 (NO3)3]·2H2O > [EuL2 (NO3)3]·2H2O. Besides, the color purity of [EuL3 (NO3)3]·2H2O is 97.33%, which is the highest in all title Eu(III) complexes. These results indicate that [EuL3(NO3)3]·2H2O containing electron-donating substituent possesses the best fluorescence property. Based on the above results, both MPDS and Eu(III) complexes have a promising application prospect as luminescent functional material in the optical field.