Bidentate Aromatic Thiols on Gold: New Insight Regarding the Influence of Branching on the Structure, Packing, Wetting, and Stability of Self-Assembled Monolayers on Gold Surfaces

A series of 2-phenylpropane-1,3-dithiol derivatives with single (<b>R1ArDT</b>), double (<b>R2ArDT</b>), and triple (<b>R3ArDT</b>) octadecyloxy chains substituted at the 4-, 3,5-, and, 3,4,5-positions, respectively, on the aromatic ring were synthesized and used to form self-assembled monolayers (SAMs) on gold. Insight into the relationship between the surface chain and headgroup packing densities was investigated by varying the number of surface chains for the bidentate adsorbates in these monolayers. Characterization of the resulting SAMs using ellipsometry, X-ray photoelectron spectroscopy, polarization modulation infrared reflection–absorption spectroscopy, and contact angle goniometry revealed that the tailgroups become more comformationally ordered and more densely packed as the number of alkyl chains per adsorbate was increased. Conversely, the molecular packing density (i.e., number of molecules per unit area) decreased as the number of alkyl chains per adsorbate was increased. Of particular interest, the desorption profiles obtained in isooctane at 80 °C suggested that the bidentate adsorbate with the most densely packed alkyl chains, <b>R3ArDT</b>, was significantly more stable than the other SAMs, producing the following relative order for thermal stability for the dithiolate SAMs: <b>R3ArDT</b> > <b>R2ArDT</b> > <b>R1ArDT</b>

Inhibiting Reductive Elimination as an Intramolecular Disulfide Dramatically Enhances the Thermal Stability of SAMs on Gold Derived from Bidentate Adsorbents

The bidentate aromatic adsorbate, 5-(octadecyloxy)-1,3-benzenedimethanethiol (<b>R1ArmDT</b>), with a specific design of extended S–S distance and a geometric constraint to resist cyclic disulfide formation was synthesized. The film formation and thermal stability of self-assembled monolayers (SAMs) derived from <b>R1ArmDT</b> were investigated and compared to those of SAMs derived from an analogous bidentate dithiol 2-(4-(octadecyloxy)-phenyl)­propane-1,3-dithiol (<b>R1ArDT</b>), in which the two sulfur atoms can readily form a cyclic disulfide upon reductive elimination from the surface. Although the SAMs derived from <b>R1ArmDT</b> were less densely packed than those derived from <b>R1ArDT</b>, as judged by the data obtained by X-ray photoelectron spectroscopy and polarization modulation infrared reflection absorption spectroscopy, the SAMs derived from <b>R1ArmDT</b> were markedly more thermally stable than those derived from <b>R1ArDT</b>. The greater thermal stability of the <b>R1ArmDT</b> SAMs can be rationalized on the basis of the structure of the bidentate <b>R1ArmDT</b> headgroup, in which the two pendant sulfur atoms cannot access each other intramolecularly to form a cyclic disulfide upon reductive elimination from the surface