Electrochemical oxidation of a self-assembled monolayer (SAM) of 4-aminothiophenol on polycrystalline gold electrodes leads to a complex voltammetric behavior characterized by an initial irreversible oxidation at ∼+0.77 V versus SSCE (sodium saturated calomel electrode) and the formation of a pseudostable surface redox couple at +0.53 V. The oxidized form of this couple is hydrolyzed in acidic solutions to another redox pair with the formal redox potential of ∼+0.3 V. We show that the oxidation leads to a radical−radical coupling reaction between two adjacent aminothiophenol molecules, yielding an electrode surface modified with 4‘-mercapto-4-aminodiphenylamine, the thiol derivative of a head-to-tail aniline dimer. The oxidized form of the dimer, quinone diimine, undergoes hydrolysis to the corresponding quinone monoimine and, eventually, to the original surface-bound 4-aminothiophenol and benzoquinone. The mechanism of the monolayer oxidation reaction has been elucidated by a variety of electrochemical and spectroelectrochemical techniques together with electrochemical data obtained with a soluble model compound, 4-(methylthio)aniline. In addition, X-ray photoelectron spectroscopy (XPS) characterization of the 4-aminothiophenol (Au−SPhNH2), the 2-(4‘-mercaptophenylamino)benzoquinone (Au−SPhNH−BQ), and the oxidized 4-aminothiophenol SAMs is reported. The formation of an electrode surface modified with aniline dimers explains the beneficial effect that 4-aminothiophenol SAM exhibits in the electrochemical polymerization of aniline. We suggest that it favors the direct addition of aniline monomers to the oligomer chains on the surface, which results in a more ordered structure compared with the deposition of oligomers from the solution. This result is very important for the preparation of highly ordered polyaniline films for advanced applications in molecular electronics and sensor technology. The results also show that after the initial dimerization step, aniline polymerization can proceed through coupling of the neutral monomer to the oxidized oligomer.

Photocurrent spectroscopy, where the photocurrent response of the electrode is measured as a function of the illumination wavelength, has proven a powerful technique for the characterization of ultrathin conductive polymer films on semiconductor and metal electrodes. In electropolymerization, the initiation of the growth of poly(3-methylthiophene) film is dependent on the nature of the electrode surface. O n clean, native indiumtin oxide (ITO) electrodes the polymerization commences with the deposition of long oligomeric chains on the surface. When the surface-reagent interactions are enhanced, favoring adsorption, e.g., on platinum or silylated I T 0 electrodes, short chains are deposited on the surface, and the initial step may be a direct reaction with surface-bound species. This is especially true with IT0 electrodes covalently modified with thiophene-containing groups. The addition of small amounts of oligomeric species in the polymerization medium results in the electrochemical modification of the electrode surface, and further deposition can take place on the organic surface so formed. The electropolymerized films do not have a controlled structure at the molecular level and consist of a mixture of species with different conjugation lengths, which hampers their utilization in molecular electronics and other sophisticated applications. The surface reactions leading to a more ordered film could be favored under carefully controlled conditions.

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