Surface-Enhanced Raman Spectroscopy of Halogenated Aromatic Thiols on Gold Electrodes

Abstract

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.

Virus Templated Gold Nanocube Chain for SERS Nanoprobe

“…The full Raman spectra of the various particles indicated in Figure a are shown in Figure b. Prominent characteristic peaks of 4‐CBT at 1063, 1083, 1100 and 1569 cm −1 were observed in the V‐probe spectra (red lines), consistent with the literature . On the contrary, no discernible SERS signal was detected from a single AuNC (black lines), indicating that the signal generation in the V‐probes was originated from the assembled AuNCs.…”

A study of pyridinethiolate derivative complexes adsorbed on gold by surface-enhanced Raman scattering

“…In fact, we have previously reported [10][11][12] that the stability of the monolayer formed by the 4-mercaptopyridine (pyS) aromatic thiolate is remarkable increased upon coordination to metal centers of the type [M(CN) 5 ] 3À , M = Fe and Ru. This effect has been assigned to the p-back-bonding interaction from the metal centers to the pyS moiety, pp * (pyS) (M II )dp, thus reinforcing the Szafranski [8,9] studies concerning the dependence of the monolayers stability on the electronic properties of the para substituent of benzene rings.…”

“…For these adsorbates, the shifts dependence on applied potential were observed to be smaller than those observed for p systems [5,7]. The investigation of the electronegativity of the substituent group on the electronic density of the sulfur atom of aromatic halo-substituted thiols indicates a stability dependence of the monolayers of these species on the electronic properties of the benzene ring substituents [8,9]. In fact, we have previously reported [10][11][12] that the stability of the monolayer formed by the 4-mercaptopyridine (pyS) aromatic thiolate is remarkable increased upon coordination to metal centers of the type [M(CN) 5 ] 3À , M = Fe and Ru.…”

Electrochemical and structural properties of self-assembled monolayers of 2-methyl-3-(ω-mercaptoalkyl)-1,4-naphthoquinones on gold

“…The new band, absent in the Raman spectrum of solid 2b, emerges in the surface spectra at 281-272 cm À1 . We assign this band to the Au-S stretching vibration based on the argument that the SERS features at similar frequencies 275 [38], 260 [39], and 279-285 cm À1 [9] were attributed to the Au-S stretch. Thus, our spectroscopic data indicate that chemisorption of compound 2b on Au surface proceeds via the S atom.…”