ELECTROCHEMICAL POST-SELF-ASSEMBLY TRANSFORMATION OF 4-AMINOTHIOPHENOL MONOLAYERS ON GOLD ELECTRODES

Electrochemical oxidation of a self-assembled monolayer (SAM) of 4-ami nothiophenol on polycrystalline gold electrodes leads to a complex vol tammetric behavior characterized by an initial irreversible oxidation at similar to +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 similar to +0.3 V. We show that the oxidation leads to a radical-radical coupling reac tion between two adjacent aminothiophenol molecules, yielding an elect rode 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 quin one monoimine and, eventually, to the original surface-bound 4-aminoth iophenol and benzoquinone; The mechanism of the monolayer oxidation re action has been elucidated by a variety of electrochemical and spectro electrochemical 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-aminoth iophenol (Au-SPhNH2), the 2-(4'-mercaptophenylamino)benzoquinone (Au-S PhNH-BQ), and the oxidized 4-aminothiophenol SAMs is reported. The for mation of an electrode surface modified with aniline dimers explains t he beneficial effect that 4-aminothiophenol SAM exhibits in the electr ochemical polymerization of aniline. We suggest that it favors the dir ect addition of aniline monomers to the oligomer chains on the surface , which results in a more ordered structure compared with the depositi on of oligomers from the solution. This result is very important for t he preparation of highly ordered polyaniline films for advanced applic ations in molecular electronics and sensor technology. The results als o show that after the initial dimerization step, aniline polymerizatio n can proceed through coupling of the neutral monomer to the oxidized oligomer.