Surface-enhanced Raman spectroscopy (SERS) has been used to characterize monolayers formed from benzenethiol (BT), benzenemethanethiol (BMT), p-cyanobenzenemethanethiol (CBMT), diphenyl disulfide (DPDS), and dibenzyl disulfide (DBDS) on roughened gold electrodes. All five species adsorb dissociatively as the corresponding thiolates. The charge transfer and electrostatic interactions between the adsorbates and the surface depend strongly on the applied potential. The aromatic ring in BT is tilted relative to the mean surface plane at all applied potentials. The rings in BMT and CBMT also are tilted, but stepping the potential to positive or negative extremes causes a reduction in surface coverage and permits the BMT and CBMT molecules to lie more flat on the surface. Full coverage can be restored by reimmersing the electrode in thiol solution. Monolayers formed from symmetric disulfides are exactly like those formed from the corresponding thiols. If the gold electrode is immersed for prolonged periods in disulfide solution and not rinsed with fresh ethanol, multilayers form, which can be electrochemically reduced (S−S bond cleavage). The monolayers formed from all five compounds are robust over the potential window for the gold electrode; complete desorption occurs between +800 and +1000 mV vs SCE, coincident with oxidation of the surface, and between −1000 and −1200 mV due to hydrogen generation (reduction of water) at the interface.

Surface-enhanced Raman spectroscopy (SERS) has been used to characterize monolayers of p-substituted benzenethiols (XBTs) and p-substituted benzenemethanethiols (XBMTs) (substituent = X = F, Cl, Br) on gold electrodes. Detailed vibrational assignments have been made for the Raman and SER spectra of all six compounds. All of these molecules exist on the surface as thiolates, with the aromatic ring tilted relative to the surface normal. Monolayers of the XBTs and XBMTs remain intact on the surface throughout the potential range between the oxidation of the gold surface at ∼ +800 mV vs SCE and the reduction of water at ca. −1000 mV at neutral pH. Monolayers of ClBT and BrBT can be partially reduced electrochemically to form mixed monolayers of the halogenated BT and benzenethiol itself. The reductive elimination of the halide occurs at potentials more positive than are required for reduction of the same molecules in solution. FBT, FBMT, and ClBMT cannot be reduced at the surface, and the BrBMT monolayer is only slightly reduced. The electrochemical reactivities of the XBT and XBMT monolayers are explained in terms of facilitated electron transfer from the metal to the adsorbed thiolate, the properties of the leaving group (halogen), and the electronic consequences of having a methylene spacer group between the sulfur and the aromatic ring. This work shows the feasibility of modifying aromatic self-assembled monolayers in situ to form mixed monolayers. It also provides a framework for designing and fabricating monolayers with prescribed stabilities and electroactivities.

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