The Interplay of Lateral and Tiered Interactions in Stratified Self-Organized Molecular Assemblies

Abstract

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.

Replacing −CH2CH2– with −CONH– Does Not Significantly Change Rates of Charge Transport through AgTS-SAM//Ga2O3/EGaIn Junctions

“…We are thus constrained to use relatively small R groups, where the molecular order of these groups in the SAM is not established, but is almost certainly less than in n-alkanethiolates. [51][52][53][54]77 Whatever the reason, the data in Figures 1 and 2 rates of tunneling. 56 The theory of tunneling through junctions containing SAMs is not sufficiently developed at present to give any guidance to these studies, and it is not clear whether this low sensitivity is expected or not.…”

“…The amide group (-CONH-) is one of the best understood in organic chemistry, 49,50 and while not isostructural with -CH 2 CH 2 -is similar in size, and is known, from prior work, to be compatible with the formation of SAMs. [51][52][53][54][55] ii) Perhaps more importantly, we wished to develop a system of SAMs to use in studies of charge transport that was more easily modified structurally than are derivatives of n-alkanethiolates. Preparing compounds of the structure HS(CH 2 ) n R, where n is 10 -20, and R is a group that we might wish to select, with as few restrictions as possible, from the full range of organic and organometallic groups, can be synthetically arduous.…”

Gradients of Rectification: Tuning Molecular Electronic Devices by the Controlled Use of Different‐Sized Diluents in Heterogeneous Self‐Assembled Monolayers

“…[9] Some of these include intermolecular hydrogen bonds; [8a,10] electrostatic interactions between dipolar moieties; [11] and control of van der Waals (vdW) interactions between n-alkyl chains. [12] Fort his study,t he vdW-based strategy was employed to change the supramolecular structure of monolayers because it avoids loss of rectification ratio resulting from additional structural modification of HSC 11 BIPY.T he chain length of diluent (HSC n )w as varied while keeping the length of rectifier (HSC 11 BIPY) constant.…”

Self-Assembled Monolayers on Gold for the Fabrication of Radioactive Stents

“…[30] This particular orientation of the adsorbate molecules might be aided by the amide± amide interactions that are known to help in the lateral stabilization of monolayer structures. [31] After having been exposed to a basic solution of rhenium gluconate for a period of 3 h, the layers of adsorbate 7 were characterized again. CA measurements showed that the surface had become more hydrophilic (h a = 78 ± 3), when compared to the original layer of 7, and an increase in hysteresis (Dh = 49) indicates an increased disorder.…”