James M. Tour, LeRoy Jones II, Darren L. Pearson, Jaydeep J. S. Lamba, Timothy P. Burgin, George M. Whitesides, David L. Allara, Atul N. Parikh, Sundar Atre, Journal of the American Chemical Society 117, 9529 (1995)

This paper describes studies of the formation of self-assembled monolayers (SAMs) and multilayers on gold surfaces of rigid-rod conjugated oligomers that have thiol, alpha,omega-dithiol, thioacetyl, or alpha,omega-dithioacetyl end groups. The SAMs were analyzed using ellipsometry, X-ray photoelectron spectroscopy (XPS), and infrared external reflectance spectroscopy. The thiol moieties usually dominate adsorption on the gold sites; interactions with the conjugated pi-systems are weaker. Rigid rod alpha,omega-dithiols form assemblies in which one thiol group binds to the surface while the second thiol moiety projects upward at the exposed surface of the SAM. In situ deprotection of the thiol moieties by deacylation of thioacetyl groups using NH4OH permits formation of SAMs without having to isolate the oxidatively unstable free thiols. Moreover, direct adsorption, without exogenous base, of the thioacetyl-terminated oligomers can be accomplished to generate gold surface-bound thiolates. However, in the non-base-promoted adsorptions, higher concentrations of the thioacetyl groups, relative to that of thiol groups, are required to achieve monolayer coverage in a given interval. A,thiol-terminated phenylene-ethynylene system was shown to have a tilt angle of the long molecular axis of less than 20 degrees from the normal to the substrate surface. These aromatic alpha,omega-dithiol-derived monolayers provide the basis for studies leading to the design of molecular wires capable of bridging proximate gold surfaces.