Adsorption profiles of chelating aromatic dithiols and disulfides: Comparison to those of normal alkanethiols and disulfides

This study provides a comparison of the rates of adsorption of the following thiols onto the surface of gold: 1,2-bis(mercaptomethyl)-4,5-dihexadecylbenzene (1), 1-mercaptomethyl-3,4-dihexadecyl-benzene (2), hexadecanethiol (4a), and and eicosanethiol (4b). This study also compares the rates of adsorption of these adsorbates to those obtained for the aromatic disulfide analogue of 1 (2,3-dithia-6,7-dihexadecyltetralin, 3) and the normal dialkyl disulfide analogues of 4a and 4b (hexadecyl disulfide, 5a, and eicosyl disulfide, 5b, respectively). The adsorption behavior was monitored using ex situ ellipsometry and polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS). The adsorption profiles suggest that monolayer formation proceeds via two distinct kinetic regimes: (1) a fast initial adsorption, where ca. 80-90% of the monolayer forms during the first few minutes of immersion, followed by (2) a slower orientational ordering lasting several hours. Comparison of the rates of adsorption of the aromatic dithiols 1 to those of aromatic monothiol 2 and normal alkanethiols 4a and 4b reveals that the structure of the adsorbate plays a substantial role during the initial stages of thiol adsorption. The impact of structural and/or chemical Variations is further illustrated by comparing the poor quality of the monolayer generated from the aromatic disulfide 3 to those of monolayers generated from 1, 2, 4, and 5.