Spectroscopic and electrochemical properties of europium(III) ion in hydrophobic ionic liquids under controlled condition of water content

Abstract

In order to elucidate coordination states of europium(III) ion in ionic liquids (ILs) and its physicochemical behavior, spectroscopic and electrochemical properties of Eu(III) were studied as a function of water content in hydrophobic ILs with bis(trifluoromethanesulfonyl)imide (tfsi) anion, and N,N-diethyl- N-methyl- N-(2-methoxyethyl)ammonium (demma) or 1-butyl-3-methylimidazolium (bmim) cations. Luminescence lifetime and spectrum of Eu(III) were measured by using time-resolved laser induced fluorescence spectroscopy to determine the inner-sphere hydration number, N H 2 O and intensity ratio of 5D 0 → 7F 2 to 5D 0 → 7F 1 transitions, I 2/ I 1, respectively. The I 2/ I 1 was independent of kinds of the cations of ILs, and increased from 0.6 to 3.5 with the decrease in N H 2 O of 9 to 0. These indicate that Eu(III) acts spectroscopically as aqua ion in the water-saturated ILs, and that at N H 2 O < 9 tfsi anion involve in asymmetry of the coordination sphere of Eu(III) and in the quenching of photo-excited Eu(III). Cyclic voltammogram of Eu ion in the dehydrated and water-saturated ILs was measured to determine electrochemical parameters such as redox potential, E 0 and diffusion coefficient, D o of Eu(III), dependent on kinds of the cations and on the water content. The E 0 in the dehydrated demma-tfsi salt was shifted to more positive 0.75 V than that in aqueous solution, indicating that divalent Eu(II) is more stable in the IL. The D o was much lower than estimated from the viscosity of IL, leading to the effective inhibition of motion of Eu(III) by the matrix of IL.