Variations of Diffusion Coefficients of Redox Active Molecules in Room Temperature Ionic Liquids upon Electron Transfer

• Published in: The journal of physical chemistry. B Vol. 112; no. 47; pp. 14952 - 14958
• Main Authors: Zigah, Dodzi, Ghilane, Jalal, Lagrost, Corinne, Hapiot, Philippe
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 27-11-2008
• Subjects: B: Statistical Mechanics, Thermodynamics, Medium Effects; Chemical Sciences
• ISSN: 1520-6106; 1520-5207
• DOI: 10.1021/jp8055625

In ionic liquids, the diffusion coefficients of a redox couple vary considerably between the neutral and radical ion forms of the molecule. For a reduction, the inequality of the diffusion coefficients is characterized by the ratio γ = D red /D ox, where D red and D ox are the diffusion coefficients of the electrogenerated radical anion and of the corresponding neutral molecule, respectively. In this work, measurements of γ have been performed by scanning electrochemical microscopy (SECM) in transient feedback mode, in three different room temperature ionic liquids (RTILs) sharing the same anion and with a series of nitro-derivative compounds taken as a test family. The smallest γ ratios were determined in an imidazolium-based RTIL and with the charge of the radical anion localized on the nitro group. Conversely, γ tends to unity when the radical anion is fully delocalized or when the nitro group is sterically protected by bulky substituents. The γ ratios, standard potentials of the redox couple measured in RTILs, and those observed in a classical organic solvent were compared for the investigated family of compounds. The stabilization energies approximately follow the γ ratios in a given RTIL but change considerably between ionic liquids with the nature of the cation.