Control of Monolayer Assembly Structure by Hydrogen Bonding Rather Than by Adsorbate−Substrate Templating

Stratified amide-containing self-assembled monolayers (SAMs) provide opportunities for investigating the fundamental dependence of supramolecular structure upon molecular constitution. We report a series of amide-containing alkanethiol SAMs (Cn-1AT/Au, n = 9, 11−16, 18) in which the hydrophobic overlayer thickness is systematically varied and the thickness of the polar region is held constant. The results from X-ray photoelectron spectroscopy, contact angle goniometry, reflective IR spectroscopy, and electrochemical measurements provide a consistent structural picture of the series. The amide underlayers in all the SAMs are well-ordered and extensively hydrogen bonded. However, the alkyl chains are disordered below n = 15. Comparison of the assembly structures shows that the chain length threshold for alkyl ordering is several methylenes higher than in n-alkanethiol SAMs. This indicates that alkyl chains adjacent to an amide underlayer are destabilized as compared to n-alkanethiols and that the amide underlayer destructively interferes with alkyl close packing as compared to the Au(111)−sulfur template. However, the amide regions of the SAMs are all well-ordered, showing that the amide sublayer acts as a “template” that is independent of alkyl chain length. The amide region dominates over gold−sulfur epitaxy in establishing the structure of these assemblies, and the amide−alkyl boundary provides an example of a “rigid−elastic” buried organic interface. Implications of these studies for molecular control of bulk properties, lipid-linked protein structure and function, buried organic interfaces in other systems, rationally designed ordered multilayers, and hybrid supramolecular systems are discussed.