SELF-ASSEMBLED MONOLAYERS AND MULTILAYERS OF CONJUGATED THIOLS, ALPHA,OMEGA-DITHIOLS, AND THIOACETYL-CONTAINING ADSORBATES - UNDERSTANDING ATTACHMENTS BETWEEN POTENTIAL MOLECULAR WIRES AND GOLD SURFACES

This paper describes studies of the formation of self-assembled monola yers (SAMs) and multilayers on gold surfaces of rigid-rod conjugated o ligomers that have thiol, alpha,omega-dithiol, thioacetyl, or alpha,om ega-dithioacetyl end groups. The SAMs were analyzed using ellipsometry , X-ray photoelectron spectroscopy (XPS), and infrared external reflec tance 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 gr oup 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 forma tion of SAMs without having to isolate the oxidatively unstable free t hiols. Moreover, direct adsorption, without exogenous base, of the thi oacetyl-terminated oligomers can be accomplished to generate gold surf ace-bound thiolates. However, in the non-base-promoted adsorptions, hi gher concentrations of the thioacetyl groups, relative to that of thio l groups, are required to achieve monolayer coverage in a given interv al. A,thiol-terminated phenylene-ethynylene system was shown to have a tilt angle of the long molecular axis of <20 degrees from the normal to the substrate surface. These aromatic alpha,omega-dithiol-derived m onolayers provide the basis for studies leading to the design of molec ular wires capable of bridging proximate gold surfaces.