Multidentate Alkanethiols with Cyclohexyl and Pyridyl Frameworks Forming Self-Assembled Monolayers on Gold
Singhana, Burapol 1976-
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Self-assembled monolayers (SAMs) generated from adsorption of the tridentate adsorbate having a cyclohexane ring as a platform were studied. The first generation of these SAMs was derived form cyclohexane-cis,cis-1,3,5-triyltrimethanethiol (CyTSH) and (cis,cis-1,3,5-trimethylcyclohexane-1,3,5-triyl)trimethanethiol (3C1CyTSH). Thin films prepared by the adsorption of these molecules onto the surface of gold were characterized by ellipsometry, contact angle goniometry, polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS), and X-ray photoelectron spectroscopy (XPS). CyTSH was found to generate multilayer thin films with many unbound thiol species and oxidized sulfur moieties (e.g., disulfides and sulfone). In contrast, 3C1CyTSH was found to generate monolayer films in which ~90% of the thiols were bound to the surface of gold (~10% unbound), and there were no oxidized sulfur species. In comparing CyTSH and 3C1CyTSH, the methyl groups of 3C1CyTSH impart rigidity to the structure, which significantly enhances the chemisorption of sulfur to the surface of gold. Ellipsometric measurements and analysis by XPS indicate that the thickness of the SAM formed from 3C1CyTSH is ~5 Å. The second generation of these SAMs generated from the analogs of the R3C6H6(CH2SH)3 (abbreviated as 3CnCyTSH) where R = -(CH2)nH and n = 1, 3, 8 and 13 were also examined. Characterization of the SAMs revealed that all sulfur atoms of these tridentane adsorbates are bound to the surface of gold and generated loosely packed monolayer thin films with lower packing densities of the alkyl chains than a normal alkanethiolate SAM (C18SH). The trends in packing density and conformational order for these SAMs decreased as follows: C18SH >> 3C13CyTSH > 3C8CyTSH > 3C3CyTSH. A preliminary study of the thermal stability of this series of SAMs by XPS measurements has shown that the tridentate adsorbates (3CnCyTSH) are more stable than the normal alkanethiolate SAM (C18SH), a characteristic that is attributable to the chelating effect. Finally, a new pyridyl-based adsorbate (C18OPDSH) was synthesized and used to form SAMs. This SAM was characterized and the data was compared to those generated from the normal alkanethiol C18SH and C18OBDSH. XPS spectra display that a lone electron pair of pyridyl nitrogen atom of C18OPDSH interacts weakly with gold surface with some unbound thiols on the film. Both C18OPDSH and C18OBDSH adsorbates generate loosely and disordered packed thin films when compared to a normal alkanethiol (C18SH). The evaluation of the thermal stability using XPS measurement indicates that the C18OPDSH SAM is more stable when compared to both C18OBDSH and C18SH SAMs.