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.

In this paper, we demonstrate that organic thiosulfates (Bunte salts) with the general formula R−SSO3M, where R is either an aliphatic or aromatic group and M a monovalent cation, constitute a novel class of surface-active compounds with a sulfur-containing headgroup. Bunte salts form self-assembled monolayers (SAMs) on gold under anaerobic conditions and chemisorb forming a Au−S bond, in which the chemical nature of sulfur is indistinguishable by X-ray photoelectron spectroscopy (XPS) from gold thiolate formed upon chemisorption of thiols and disulfides. The S−SO3 bond in the thiosulfate is cleaved during adsorption on the gold surface and the sulfite moiety is released. We have prepared one alkyl thiosulfate (sodium S-dodecylthiosulfate, C12SSO3Na) and two aromatic redox-active thiosulfates (potassium S-(2,5-dihydroxyphenyl)thiosulfate, QSSO3K, and dipotassium S,S ‘-(3,6-dihydroxy-1,2-phenylene)bisthiosulfate, Q(SSO3K)2) and compared the formation and properties of the SAMs prepared from these Bunte salts and the corresponding thiols (1-dodecylmercaptan, C12SH, and 1,4-dihydroxy-2-mercaptobenzene, QSH) using XPS, cyclic voltammetry, and ac impedance spectroscopy. The chemisorption of Bunte salts takes place 1−2 orders of magnitude slower than the adsorption of thiols. The SAMs formed from aromatic Bunte salts QSSO3K or Q(SSO3K)2 have lower surface coverage than those prepared using QSH. With aliphatic compounds, the films prepared from Bunte salts are either slightly or relatively well-blocking, although they do not reach the quality achieved with thiol-based SAMs. The differences in the adsorption time scale and surface coverage are attributed to the bulky thiosulfate headgroup. A major advantage of using Bunte salts derives from the general synthetic pathway to organic thiosulfates, generally involving a one-pot synthesis starting from the corresponding halides and yielding the products as odorless crystalline compounds. This offers a synthetically feasible way of introducing a sulfur-containing surface-active headgroup into several redox-active or other functional molecules, allowing their incorporation in SAMs. This facilitates the preparation of functional monolayers for applications in sensor technology and molecular electronics.

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