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.