Electrochemical study of tris(4-bromophenyl)amine and 2,2,6,6- tetramethylpiperidine-1-oxyl in room-temperature ionic liquids

The electrochemical behavior of tris(4-bromophenyl)amine (TBPA) and 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) was comparatively studied in room temperature ionic liquids (RTILs) containing 1-butyl-3-methylimidazolium cation [BMIm]⁺ and the anions BF₄^-, PF ₆^-and CF₃SO₃^-. TEMPO showed a well-defined electrochemical reversibility with anodic to cathodic peak current ratio (I_a/I_c) equals to unity in all RTILs, at a glassy carbon electrode. In contrast, I_a/I_c ratio greater than unity was observed at all scan rates employed (10-1000 mV s^-1) in cyclic voltammograms (CV) of TBPA in RTILs. Controlled-potential bulk electrolysis of TBPA in RTILs yielded a dark blue colored solution of monocation TBPA⁺, which is stable under nitrogen. Diffusion coefficient of TBPA⁺ cation was determined in all three ILs by chronoamperometry and found to be at least one order of magnitude less than the calculated value of TBPA. This effect, in conjunction with the deviation of the I_a/I _c ratio from unity could be ascribed to ion-pairing interaction between the TBPA⁺ cation and the anion of the RTILs.