[Study on spectroscopic properties of Eu and Tb mixed solid complexes with a diamide ligand]

In the present work, Eu(NO3)3 and Tb(NO3)3 complexes with a diamide ligand 1,6-bis[(2'-benzylaminoformyl)phenoxyl]hexane (L) were prepared in the solution of chloroform and ethyl acetate. Their mixed complexes with different molar ratio also synthesized by coprecipitation. Eu and Tb complexes were mixed with different molar ratio, mechanically ground, and a series of mixed solid complexes were obtained. These mixed complexes were characterized by elemental analysis, UV-Vis, IR and XPS spectra. The analytical data were obtained by a Vario EL CHN and indicated that Eu and Tb complexes formed a 2:3 metal-to-ligand stoichiometries 2RE(NO3)3 x 3L x 4H2O. Their IR spectra were recorded on a Bruke FTS66V/S spectrophotometer. The results indicate that all complexes have similar IR spectra, of which the characteristic bands have similar shifts, suggesting that they have a similar coordination structure. UV-Vis spectra were recorded on a Hitachi U-3010 spectrophotometer and showed that under the influence of the mixed ions, the absorbance of the mixed complexes is not identical with that of the pure complexes. XPS spectra were analyzed on a PHI-5702 X-ray photoelectron spectroscope (XPS) operating with monochromatic Mg K alpha irradiation at pass energy of 29.4 eV. The binding energies of O (1s), Eu (3d) and Tb (4d) in the two kinds of mixed complexes were changed compared with Eu-L and Tb-L complexes. This indicates that these two synthetic methods were not a simple physical mixing process, but there was some chemical effect between the mixed Eu-L and Tb-L complexes. The fluorescence spectra of the mixed complexes were obtained on a Hitachi F-4500 spectrophotometer at room temperature. The excitation and emission slit widths was 1.0 nm. It was concluded from the excitation spectra that the best excitation wavelengths for Eu and Tb complexes are 396 and 320 nm respectively. For the convenience of comparing the fluorescence intensities with each other, the excitation wavelengths were set to 320 nm. For the mixed complexes prepared by coprecipitation, the peak positions of the 5 D4-->7 F6 and 5 D4-->7 F5 transitions were not changed. The peak at 590 nm was assigned to the 5D0-->7 F1 and 5 D4-->7 F4 transitions. Its position is dependent on the content of Eu and Tb complexes. When the content of Eu complex is large, this peak is near to the position of the 5 D0-->7 F1 transition, but when the content of Tb complex is large, it is near to the position of the 5 D4-->7 F4 transition. The peak at 620 nm is a combined peak of the 5 D0-->7 F2 and 5 D4-->7 F3 transitions. It has a similar change with the peak at 590 nm. The change of these peak positions could indicate that there was interaction between Eu and Tb complexes. The fluorescence intensities of the mixed solid complexes were changed obviously as compared with the pure Eu and Tb complexes. The fluorescence intensities of their 5 D4-->7 F6 and 5 D4-->7 F5 transitions were lower than those of the Tb complex as well as the theoretical values calculated by the molar ratio of Tb complex, and decreased with the increase in the content of Eu complex, which shows that the fluorescence intensities of terbium ions were quenched by europium ions. The fluorescence intensities of the two combined peaks at 590 and 620 nm are higher than that of Eu complex but lower than that Tb complex and they increased with the increase in the content of Tb complex, which indicates that the fluorescence intensities of Eu3+ were sensitized by Tb3+. In the mixed complexes prepared by grinding, the fluorescence intensities of Eu3+ were also sensitized by Tb3+ and the fluorescence intensities of Tb3+ are also quenched by Eu3+. Under the excitation of UV light, the mixed complexes resulted by coprecipitation exhibit different fluorescence color.