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 followin g thiols onto the surface of gold: 1,2-bis(mercaptomethyl)-4,5-dihexadecylb enzene (1), 1-mercaptomethyl-3,4-dihexadecyl-benzene (2), hexadecanethiol ( 4a), and and eicosanethiol (4b). This study also compares the rates of adso rption of these adsorbates to those obtained for the aromatic disulfide ana logue of 1 (2,3-dithia-6,7-dihexadecyltetralin, 3) and the normal dialkyl d isulfide analogues of 4a and 4b (hexadecyl disulfide, 5a, and eicosyl disul fide, 5b, respectively). The adsorption behavior was monitored using ex sit u ellipsometry and polarization modulation infrared reflection absorption s pectroscopy (PM-IRRAS). The adsorption profiles suggest that monolayer form ation proceeds via two distinct kinetic regimes: (1) a fast initial adsorpt ion, where ca. 80-90% of the monolayer forms during the first few minutes o f immersion, followed by (2) a slower orientational ordering lasting severa l hours. Comparison of the rates of adsorption of the aromatic dithiols 1 t o those of aromatic monothiol 2 and normal alkanethiols 4a and 4b reveals t hat the structure of the adsorbate plays a substantial role during the init ial stages of thiol adsorption. The impact of structural and/or chemical Va riations is further illustrated by comparing the poor quality of the monola yer generated from the aromatic disulfide 3 to those of monolayers generate d from 1, 2, 4, and 5.