Electrochemical Behavior of Aromatic Diamines and the Growth of Conducting Films

Abstract

Aromatic diamine compounds have p-conjugated organic systems based on a phenylene and two amino groups.1 They also have been used commercially as precursors for dyes, pigments, curing agents and high performance polymer materials; however, electrochemistry of the aromatic diamine compounds have not been adequately established, despite the fact that the aromatic diamine compounds contain electrochemically active aniline units in their chemical structures. On the other hand, we have designed and proposed electropolymerization systems of p-conjugated aromatic compounds such as pyrrole, thiophene and aniline in highly conductive ionic liquid media, electrochemically and morphologically characterizing the polymerized conducting films grown on the electrode surface.2 In this work, we report the voltammetric behavior of several aromatic diamine compounds containing a biphenyl framework (Fig. 1), investigating the conducting film formation on the electrode surface. Four aromatic diamines (Bzd(m-Me)2, Bzd(m-CF3)2, Bzd(o-Me)2 and Bzd(o-OMe)2) were supplied by SEIKA Corporation. The electrooxidative behavior of the aromatic diamine compounds was evaluated by cyclic voltammetry (CV) method using a conventional three-electrode cell equipped with a Pt disk working electrode (0.020 cm2), a Pt wire counter electrode and an Ag/Ag+ reference electrode. A 0.1 mol dm-3 tetrabutylammonium hydrogen sulfate (TBA-HS) / MeCN was used as an electrolytic media for the CV measurement. All electrochemical measurements were carried out under dry argon atmosphere at ambient temperature. The electrical conductivity of the films was estimated by the resistivity measured by a dc 4-prove method.Fig. 2 depicted the cyclic voltammograms for the electrochemical oxidation of Bzd(m-Me)2 and Bzd(m-CF3)2. It was found that an irreversible oxidation peak was observed in each aromatic diamine compound; however, the oxidation peak of Bzd(m-CF3)2 drastically shifted to the potential region more positive than that of Bzd(m-Me)2 because of the electron withdrowing by the trifluoromethyl group in Bzd(m-CF3)2. It should be noted that no film formation on the working electrode surface was observed in the case of Bzd(m-Me)2 and Bzd(m-CF3)2 whereas both Bzd(o-Me)2 and Bzd(o-OMe)2 significantly formed the films on the electrode surface, giving the relatively large voltammetric responses. From this result the position of the substituents seems likely to play an important role in the film growth. The electrical conductivity of the film formed in the electrochemical oxidation of Bzd(o-OMe)2 was 1.2 S cm-1, which was similar to the conductivity value previously found in the electropolymerized polypyrrole film.2 The electrochemical activity and morphology of the films obtained will be discussed.