Self-assembled monolayers of 1,4-benzenedimethanethiol on polycrystalline silver and gold films: an investigation of structure, stability, dynamics, and reactivity

Abstract

1,4-Benzenedimethanethiol (HS-CH₂-C₆H₄-CH₂-SH, BDMT) adsorbs dissociatively on silver and gold surfaces yielding self-assembled monolayers with the thiolate structure. Whereas the molecule adsorbs flat on silver as a result of the loss of two thiol protons, it adsorbs with the molecular plane perpendicular to the gold surface with the loss of one thiol proton. This is manifested by the presence of the ring C-H and S-H stretches and the S-C-H bend for the Au monolayer and the absence of these for the Ag monolayer in the surface-enhanced Raman spectra. The difference in adsorbate geometry is presumably due to the changes in interaction and not to differences in the lattice constants of the two surfaces, which is rather small. In both cases, metal-adsorbate p bonding is weak, resulting in only small shifts in the ring modes. BDMT monolayers are more stable than alkanethiol monolayers and desorb only at a fairly high temperature of 423 K in air, whereas alkanethiols desorb below 373 K. An increase in temperature leads to structural changes in the Au monolayer and the molecules begin to lie flat on the surface, and desorption occurs from this state. The Ag monolayer is less stable thermally and desorption is eventless. Difference in the desorption temperatures point to the importance of energetics of self-assembly in determining the stability. The thiol proton on gold surfaces can be removed easily by exposing the monolayer to basic solutions. The gold monolayer upon exposure to thiols leads to the formation of disulfides, suggesting the formation of a prototypical bilayer. A completely new S-S stretching frequency is seen upon reaction with 4-methoxybenzenethiol (MOBT) with the complete absence of the S-H stretch. Other spectral features of the bilayer can be attributed to BDMT and MOBT subunits. The respective thiols in solution, however, do not react, leading to disulfide. Reaction with silver monolayer leads to the displacement of BDMT for MOBT and no reaction is observed. With n-alkanethiols, the reactivity decreases with the alkane chain length. The alkanethiol part of the spectrum resembles that of the corresponding self-assembled monolayer. Surface-enhanced reactivity of the type observed here has not been reported hitherto. The MOBT part of the bilayer desorbs first after cleaving the S-S bond and BDMT leaves the surface subsequently. X-ray exposure of the monolayers leads to beam-induced damage which is manifested in the Raman spectra. Whereas the damage is severe for Ag, part of the Au monolayer survives X-ray exposure.