Thioaromatic monolayers on gold: a new family of self-assembling monolayers

Self-assembling (SA) monolayers of the aromatic compounds thiophenol (TP), p-biphenyl mercaptan (BPM), and p-terphenyl mercaptan (TPM) were prepared on gold electrodes. Contact angle (CA) measurements and ellipsometry indicate that TP forms poorly defined layers, while BPM and TPM form monolayers with reproducible CAs and ellipsometric thicknesses. BPM and TPM films are substantially more stable than TP layers toward various electrochemical conditions and replacement by alkanethiols as shown by the combined CAs, ellipsometric thicknesses, and the electrochemical measurements of the corresponding monolayer-covered electrodes. The surface coverages of the three monolayer systems, determined from the electrochemical responses of the respective electrodes, show that the blocking efficiency increases with the number of phenyl rings in the molecule. Three electrochemical techniques, namely gold oxide formation/removal, cyclic voltammetry (CV), and ac impedance were used to determine the monolayer surface coverage. The ac impedance technique proves to be superior for surface coverage determination, mainly due to the milder conditions applied. Molecular mechanics calculations show that BPM and TPM might form (√3 x √3)R30° overlayers on Au(llI) with the molecules nearly perpendicular to the gold surface. Such overlayers would have favorable intermolecular interactions and retain the usual structure for sulfur on Au(llI).