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

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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.