The luminescent properties of divalent europium complexes of crown ethers and cryptands

The luminescent properties of divalent europium complexes with crown ether, azacrown ether, N-pivot-azacrown ether, and cryptand in methanol or water have been systematically investigated under UV irradiation. These divalent europium complexes show greatly enhanced emission from 417 nm to 488 nm in the visible blue region in comparison with that of the methanol solution of EuCl2. The aqueous solution of EuCl2 is non-luminescent. This obvious distinction in luminescent properties between the macrocyclic ligand-coordinated divalent europium and uncoordinated divalent europium is attributed to the "insulation effect" of Eu2+ ion from the solvent molecules of CH3OH and H2O by the macrocyclic crown ether or cryptand encapsulation to divalent europium. Moreover, these macrocyclic ligands provide an additional restriction to the electronic charge expansion of the excited Eu2+. This also contributes to the enhancement of the Eu2+ luminescence. Among all the investigated macrocyclic ligands, 15-crown-5 (15C5) affords the largest enhancement to the Eu2+ emission. The intensity of the Eu2+-15C5 complex is 690 times that of the EuCl2 methanol solution with the same Eu2+ concentration. This special emission enhancement effect is related to the particular complex composition of 1:3 (Eu2+:15C5) and corresponding configuration of Eu2+-15C5 complex in methanol. Concerning the mechanism, the luminescence enhancement of divalent europium by complexation with these macrocyclic crown ether or cryptand ligands is found to be initiated from the decrease in non-radiative rate constant rather than from the increase in radiative one. The divalent europium complexes of methacrylate polymeric polyether derivatives such as 15C5-, 18-crown-6- (18C6), and cryptand [2.2.1]- or [2.2.2]-containing polymer and copolymer have also been prepared. Their luminescent properties in solid state have been studied to aim for practical application. As a similar situation to the simple polyether complexes, the divalent europium complex with 15C5-containing polymer or copolymer shows the largest luminescent enhancement effect. Its emission intensity reaches about 20% that of the commercial inorganic luminescence product CaWO4:Pb (NBS 1026). In addition, the doping effect of several divalent ions, namely Mg, Sr, Ba and Zn in polymeric complexes, has also been investigated according to the luminescence concentration quenching mechanism in solid state luminescence materials. The emission intensity of 15C5-containing polymer europium(II) complex is raised to twice stronger by doping of Zn2+ ion. (C) 1998 Elsevier Science S.A.