The electrode reaction of tris(2,2′-bipyridine)ruthenium complexes ([Ru(bpy)3]n+, n = 2 and 3) has been investigated in 1-ethyl-3- methylimidazolium tetrafluoroborate (EMI[BF4]) ionic liquid. The formal potential of [Ru(bpy)3]3+/2+ couple was 0.85 V vs. ferrocene/ferrocenium at 25°C. The diffusion coefficients of [Ru(bpy)3]2+ and [Ru(bpy)3]3+ were (7.8 ± 0.1) × 10-8 and (3.3 ± 0.1) × 10-8 cm2 s-1, respectively, at 25°C. The diffusion coefficients in EMI[BF4] were smaller than those reported in EMITFSA (TFSA- = bis(trifluoromethylsulfonyl)amide), of which the viscosity is close to that of EMI[BF4]. In addition, the ratio of the diffusion coefficient of [Ru(bpy)3]2+ to that of [Ru(bpy)3]3+ in EMI[BF4] was smaller than that in EMITFSA, indicating the diffusivity of the cationic species is strongly affected by the charge density of the anion of ionic liquids. The rate constant for [Ru(bpy)3]3+/2+ couple at 25°C was (6.7 ± 0.2) × 10-5 cm s-1, which was smaller by two orders of magnitude than that in EMITFSA, suggesting the difference in the charge densities between [BF4]- and TFSA- also affects the outer-sphere component of reorganizing energy of the complexes.
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