New peptide-containing alkanethiol self-assembled monolayers (SAMs) on gold have been prepared. These monolayers exhibit extended interchain hydrogen bonding and have well-ordered alkane chains. Elemental composition and thickness of the monolayer are obtained by X-ray photoelectron spectroscopy. External reflective IR spectroscopy shows that the SAMs are uniaxial and possess microcrystalline, densely packed methylene chains with hydrogen bonding between neighboring amide moieties. These highly ordered monolayers form excellent electrochemical spacers as characterized by electrochemical blocking studies and double-layer capacitance measurements. The well-defined structure makes this system a promising prototype for systematic investigations of long-range electron transfer through amide bonds.

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 (C n -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.

We report the phase separation of a self-assembled monolayer formed from a binary mixture of adsorbates, n-decanethiol, and an amide-containing alkanethiol of similar length (3-mercapto-N-nonylpropionamide), as studied by scanning tunneling microscopy. While mixtures of n-alkanethiols of similar length (i.e., n-decanethiol and n-dodecanethiol) show no phase separation, the introduction of a hydrogen-bonding functionality buried deep within the film induces the formation of single-component domains on the nanometer scale. Phase separation occurs at all relative compositions studied, and for these molecules maintains the same exposed terminal functionality across the entire film. In nonequimolar concentrations of adsorbates, we observe that the solution component present in greater concentration will dominate the composition of the adsorbed monolayer in super proportion to that in solution, consistent with enthalpic contributions from both the solvent and intermolecular interactions of adsorbates.

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Hybrid self-assembled monolayers (SAMs) containing well-defined strata of different polarity enable insight into how fundamental interactions lead to higher order structure and may provide useful analogies for self-assembled multilayers, new hybrid materials, and functional biological interfaces. We report amide-containing alkanethiol SAMs with internal polar sublayers that are two amide groups thick and nonpolar overlayers comprising either dodecyl or hexadecyl chains. The assemblies have been characterized by X-ray photoelectron spectroscopy (XPS), contact angle goniometry, and external reflective infrared spectroscopy (FTIR-ERS). XPS demonstrates the SAMs are of monolayer thickness, chemisorbed to the gold substrate, and anisotropically oriented. Contact angle data show the methyl surface for n = 16 is highly ordered, but the surface for n = 12 is less well ordered. FTIR-ERS reveals that the alkyl chains for n = 16 are close packed, but that those for n = 12 are disordered. FTIR-ERS also shows that, although the two-amide sublayers are compositionally identical, they are well ordered and assume polyglycine-II-like conformations for n = 16, but they are poorly ordered for n = 12. Comparison of these two SAMs to each other in the context of previously reported one- and three-amide SAMs leads to two conclusions. (1) The threshold n for alkyl chain length ordering in two-amide SAMs is 12 ≤ n ≤ 16. Thus, in SAMs with internal amide sublayers both one and two amide groups thick, the threshold number of methylenes required to form ordered alkyl regions is significantly increased compared to alkanethiol SAMs, demonstrating destructive interference of the amide region with the hydrocarbon ordering process. (2) In two-amide SAMs the formation of a well-ordered amide region depends on the ordering of an overlying hydrocarbon region. This behavior differs with that previously demonstrated for one- and three-amide SAMs, in which the amide groups assume characteristic conformations regardless of hydrocarbon region thickness and order. For two-amide SAMs, the apparent dependence of amide ordering on complementary ordering in the alkyl region provides evidence of an energetic interplay between the two sublayers, manifested as a “reverse ordering” effect. The previously unobserved elastic−elastic character of the buried interface in two-amide SAMs is contrasted with the rigid−elastic interface found in the one-amide SAMs.

We have investigated the role of internal functionality in self-assembled monolayers of a family of amidecontaining alkanethiol molecules on Au{111} using scanning tunneling microscopy. In addition to van der Waals interactions that are present within n-alkanethiol self-assembled monolayers, hydrogen bonding between adjacent buried amide groups contributes to the stability of the amide-containing molecules on the surface and causes spontaneous phase separation upon coadsorption with an n-alkanethiol. A deposition solution concentration dependence study reveals that this is an observed trend across a range of examined solution compositions. Additionally, hydrogen bonding affects the packing structure of the amide-containing alkanethiol self-assembled monolayers. Although they adopt the same ( 3× 3)R30°base lattice as n-alkanethiolate self-assembled monolayers, the amide-containing molecules form superlattice structures that are more linear than n-alkanethiol monolayers due to the hydrogen bonds they form. The internal functionality of monolayers can be used to control their formation and stability.

Self-assembled monolayers of 1-(10-mercaptodecyl)imidazole on Au electrodes were used to bind cobalt “picket fence” porphyrin (cobalt 5,10,15,20-tetrakis(α,α,α,α-2-pivalamidophenyl)porphyrin) to the electrode surface. The binding involved coordination of the cobalt center of the porphyrin to the pendant imidazole groups in the monolayer coating. Attempts to coordinate the Co(II) oxidation state of the porphyrin to the coatings were not successful. However, with the Co(III) oxidation state, substantial binding was achieved which persisted even when the Co(III) was reduced to Co(II). Absorption spectra of the attached porphyrin were obtained for both oxidation states of the cobalt center. The remaining axial coordination site on the attached cobalt porphyrin is accessible to ligands, for example, imidazole, in aqueous solution.

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