Integration of functionalized 2D materials into structured devices requires in-depth understanding of the structure, reactivity, and acid-base properties of the adsorbed molecules at the junction. Self-assembled monolayers (SAMs) have been used extensively to probe and to tune the properties of these interfaces. Alkanethiols or other functionalized alkyl chains with acidic moieties have been used to probe interfacial acid-base behavior. Here, we examine self-assembled monolayers (SAMs) of carboranethiolate molecules functionalized with carboxylic acid group.s Due to the high axial symmetry and robust nature of these molecules, many of the common defects observed n-alkanethiolate monolayers are eliminated symmetry, enabbling controlled platforms to study the properties at substrate/environment interfaces. Self-assembled monolayers of carboranethiolates with carboxylic acids attached in the meta position have pKas ~2 pH units higher on the surface. In this study, we explore the effects of the position of the carboxylic acid functional group and the curvature of the surface on both structure and surface pKa of the functionalized monolayer. By exploring these different conditions with contact angle titration and scanning tunneling microscopy (STM) we elucidate the roles of environmental conditions and their effects on the underlying mechanisms of surface pKa shift. We hypothesize that the position of the functional group results in surface pKa shifts of lesser magnitude the closer it is to the substrate/environment interface. This shift is enhanced with greater curvature of the surface as the acidic moieties become more exposed. This work creates platforms to tailor the acid-base properties of different monolayers in addition to the surface wettability of substrates. This work also has applications in functionalizing surfaces of multilayer devices with solution-processable techniques that have interfaces modulated to control chemical reactivity.

Exploration of variables involved in surface pKa shifts of carboxylated carboranethiol self-assembled monolayers

2019 Fall National Meeting

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.