SURFACE-ENHANCED RAMAN-SPECTROSCOPY OF HALOGENATED AROMATIC THIOLS ONGOLD ELECTRODES

Surface-enhanced Raman spectroscopy (SERS) has been used to characteri ze monolayers of p-substituted benzenethiols (XBTs) and p-substituted benzenemethanethiols ((XBMTs) (substituent = X = F, Cl, Dr) on gold el ectrodes. Detailed vibrational assignments have been made for the Rama n and SER spectra of all six compounds. All of these molecules exist o n 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 th e gold surface at similar to +800 mV vs SCE and the reduction of water at ca. -1000 mV at neutral pH. Monolayers of ClBT and BrBT can be par tially reduced electrochemically to form mixed monolayers of the halog enated BT and benzenethiol itself. The reductive elimination of the ha lide occurs at potentials more positive than are required for reductio n of the same molecules in solution. FBT, FBMT, and ClBMT cannot be re duced 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 bet ween 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 fabricatin g monolayers with prescribed stabilities and electroactivities.