LUMINESCENCE OF EUROPIUM(II) COMPLEXES WITH CROWN COMPOUNDS

The fluorescence properties of divalent europium complexes with several kinds of crown compounds such as crown ethers, azacrown ethers, N-pivot-lariat azacrown ethers, and cryptands have been systematically investigated under UV irradiation. The most intense emission is yielded by a methanol solution of the Eu2+-15-crown-5 complex, the intensity of which is 690 times greater than that of a methanolic EuCl2 solution with the same Eu2+ concentration. The enhancement of the emission intensity can be attributed to ''insulation'' of the Eu2+ ion from the solvent molecules based on the encapsulation effect and ''restriction'' of the expansion or vibration of the Eu2+ ion. The peak of the Eu2+ emission located at 489 nm for the Eu2+ ion solvated by methanol and in general tend to appear at the shorter wavelength side (417-488 nm) as the complexes form. The stoichiometries of several Eu2+ complexes were determined by using the emission intensity molar ratio method and by monitoring the shift and splitting of the proton magnetic resonances for the Sr2+ analogues. Radiative and nonradiative rate constants for the excited states of the complexes were estimated from quantum yields and luminescence lifetimes. The luminescence enhancement owing to complexation was found to be, not due to an increase in the radiative rate constants, but due to a decrease in the nonradiative ones. The former was almost the same value of order of 10(5) s-1, while the latter changed from 10(5) to 10(8) s-1. Furthermore, we found that the crown ethers having a bromoalkyl side chain were induced to form complexes with Eu2+ by UV irradiation and that the emission intensity of Eu2+ in the complexes increased significantly. Methacrylate polymers containing cyclic polyether structures, such as 15-crown-5, 18-crown-6 and cryptand[2. 2. 2], have been prepared and formed complexation with Eu2+ and their luminescence properties studied in a solid state. They generally gave blue bright emissions in the region of ca. 420-445 nm under UV irradiation. For the Eu2+ polymer containing cryptand[2.2.2], the peak of the emission spectra was located at 455 nm. The Eu2+ polymer containing the 15-crown-5 ring exhibited the largest intensity of luminescence and its emission intensity was ca. 20% of that for CaWO4 : Pb(NBS 1026). There are supposed to be two types of quenching mechanisms, one is responsible for the Eu2+-Eu2+ distance and another is due to a structural change of complexes. The Eu2+ ions in the solid polymer ligands exhibited fairly better stability to air than the Eu2+ ions due to complexing with the monomeric polyethers in a methanol solution. We have studied effects of the coexistence of the several divalent ions with Eu2+ on the luminescence properties and found out the Eu2+-Zn2+ doubly-doped polymer containing the 15-crown-5 structure to be twice stronger phosphor than the Eu2+ only doped one.