Reactivity Of Self-assembled Monolayers Research Articles

Cooperativity at the Solution/Solid Interface: Formation and Reactivity of Self-Assembled Monolayers

Cooperativity is a molecular phenomenon, in which multiple interactions participate synchronously to significantly affect the activity and/or selectivity of the interacting components by accelerating (or impeding) their chemical or physical processes. Cooperative effects are prevalent in biology and can be observed in chemistry and materials science. Synthesis of functional materials frequently requires surfaces and interfaces where cooperative interactions can propagate with substrate assistance. In this context, of great interest is the characterization and control of ligands binding to metal centers, given their rich chemistry and prevalence in functional materials. In addition, self-assembly of compounds on surfaces can be steered by cooperative effects, and supramolecular polymerization can also proceed in a cooperative manner. Essential for these kinds of chemistries is the availability of highly resolving analysis methods to investigate reaction and adsorption outcomes directly at the surface. Scanning tunneling microscopy (STM) is particularly well suited to the study of such processes on surfaces and interfaces by offering both molecular resolution and real time and space information regarding changes in the electronic structure of materials upon adsorption and/or reaction.In this contribution, we present quantitative STM experimental results and supporting theoretical calculations of the binding dynamics of neutral ligands (heteroaromatics, O2) to metal porphyrins adsorbed on solid substrates as well as self-assembly of polyaromatics on metallic supports. Single molecule level STM imaging was accomplished in-situ under ambient solution conditions. We measured binding affinities and uncovered surface modulated cooperativity both in coordination reactions and in molecular self-assembly -- results essential for understanding adsorbate-surface interactions. Computations corroborate our experimental findings. The high-resolution, quantitative knowledge gained at the molecular level combined with theoretical studies offer not only insights into chelation, adsorption, and cooperativity but also into control parameters for surface-driven synthesis, tunable surface functionalization, and catalysis selectivity control.

Reactivity of Self-Assembled Monolayers: Local Surface Environment Determines Monolayer Erosion Rates

We use scanning tunneling microscopy (STM) to study octanethiol self-assembled monolayers (SAMs) on Au(111) exposed to atomic hydrogen. While the overall net reaction is to remove octanethiol molecules from the underlying gold surface, the monolayer structure heavily influences the rate of this reaction and molecules located along surface defects are preferentially removed before those located in close-packed areas. Octanethiol molecules remaining on the gold surface can go through significant rearrangement: domain boundaries can change both size and structure, annealing into surrounding close-packed domains; film defects diffuse to the edge of close-packed areas; and molecules located along the edge of close-packed domains shift position, changing the size and shape of the remaining close-packed features. Monolayer reactivity increases with increasing hydrogen-atom exposure, and we compare the experimental results with kinetic Monte Carlo simulations. We find that the edges of defect sites are potentiall...

Reactivity of self-assembled monolayers: formation of organized amino functionalities

The reactivity of self-assembled monolayers (SAMs) of mercaptodocosanol (MDO) on gold has been screened by a variety of wet chemical reactions. Changes in thickness and composition were monitored using X-ray photoelectron spectroscopy (XPS). The order and tilt angles were studied using near-edge X-ray absorption fine structure spectroscopy (NEXAFS). Sulfonyl chlorides could not be reacted to form sulfonates, and p-toluenesulfonyl chloride (TosCl) at longer reaction times even resulted in a partial desorption of the alkanethiol film. A coupling reaction of silanes to the hydrophilic surface was very much dependent on the reagent. Whereas chlorodimethylphenylsilane (CPS) at elongated reaction times leads to an almost complete removal of the monolayer, p-aminophenyltrimethoxysilane (APTS) attaches to the OH terminated surface. From the orientation analysis a broad distribution of tilt angles of the aromatic moiety is concluded. Attachment of the APTS moieties induces disorder in the thiol layer. The average tilt angle of the hydrocarbon chains of the MDO layer changes from 30° for the unperturbed, well-ordered layer to 44° for the reacted film and indicates an increasing number of gauche conformations.