Reactivity control of carboxylic acid-terminated self-assembled monolayers on gold: Acid fluoride versus interchain carboxylic anhydride

Reactions that occur at interfaces often show different behaviors from their solution analogues. In this paper, we demonstrated how proximity effect, one of the unique phenomena at interfaces, could control the product distributions of interfacial reactions. Self-assembled monolayers (SAMs) of 16-mercaptohexadecanoic acid on gold surfaces were treated with cyanuric fluoride and pyridine, which are generally used for forming acid fluorides from carboxylic acids in the solution-based reaction. After the treatment, two different products, acid fluorides (AFs) and interchain carboxylic anhydrides (ICAs), were controllably obtained at surfaces under different reaction conditions with keeping the reagents the same. Various factors, such as the concentrations of reagents, reaction time, and additives, affected the product distribution (or the reaction pathway) at surfaces. We found that one of the key factors in controlling the reaction pathway was a relative contribution from the proximity effect of adjacent carboxylic acid chains in the SAMs (kinetic control) and the equilibrium shift (thermodynamic control). The relative reactivity of AF- and ICA-presenting surfaces toward primary amines, such as undecylamine and [((6-aminohexyl)amino)carbonyl]ferrocene, was also investigated, in terms of the number and the ordering of the amines coupled onto the surfaces.