Olefin-Bridged Bidentate Adsorbates for Generating Self-Assembled Monolayers on Gold.

A series of custom-designed olefin-bridged bidentate adsorbates composed of an olefin group linking symmetrical hydrocarbon moieties of varying chain lengths was synthesized and used for the preparation of self-assembled monolayers (SAMs) on gold. The structures of the adsorbates are in the form Z-CH(CH)[CHSH] () where = 12-15 and = + 3 (, , , and ). The influence of the olefin linker on the structural and interfacial properties of the SAMs was investigated and compared to SAMs formed from analogous -alkanethiols. Characterization techniques included ellipsometry, X-ray photoelectron spectroscopy (XPS), polarization modulation-infrared reflection-adsorption spectroscopy (PM-IRRAS), and contact angle measurements. The SAMs exhibited ellipsometric thicknesses that were similar to their monodentate counterparts, suggesting that the new olefin-bridged adsorbates pack similarly to the monodentate analogs. Characterization by PM-IRRAS revealed that the SAMs were as conformationally ordered as those derived from the reference -alkanethiols with the exception of the adsorbate with the shortest chain length , which exhibited low coverage and a liquid-like structure. Unlike the SAMs derived from the -alkanethiols, the SAMs failed to exhibit "odd-even" effects. However, the SAMs displayed similar hexadecane contact angles as their -alkanethiol counterparts with the exception of , which exhibited markedly diminished hexadecane contact angles. The similar structural and interfacial properties of the SAMs, when compared to analogous -alkanethiol SAMs, render the molecular architecture of the olefin-bridged dithiol as a robust platform for the synthesis of adsorbates with two chemically distinct tailgroups for use in the preparation and study of phase-incompatible "conflicted" interfaces.