Effects of Dimensional Confinement on the Reactivity of Carboxylic-Acid-Functionalized Molecules

Self-assembled monolayers (SAMs) provide a system to explore the effects of dimensionality on chemical functional groups. We probed interactions within carboxylic-acid-functionalized carboranethiol SAMs as well as the interactions between the monolayer and the environment. Monolayers composed entirely of meta-functionalized carboranethiols were imaged via scanning tunneling microscopy (STM). Unlike para-functionalized carboranethiols, which adopted the same nearest-neighbor spacing as unfunctionalized carboranes (7.2 Å), meta-functionalized carboranethiols have a larger spacing of 8.4 Å. Studies with two isomers of meta-carboranethiols yielded similar results. To examine the impact of dimensionality on the carboxylic acid group’s acidity, we employed contact angle titration. We found a pK_a shift of nearly four pH units from 3.01-3.23 in solution to approximately 6.95 on the surface. Density functional theory calculations tested the effect of desolvation by two-dimensional (2D) confinement, showing a shift in pK_a consistent with experimental results. STM-observed differences in SAMs deposited in ethanol versus benzene provided further evidence for profound changes in carboxylic acid reactivity when placed in the 2D confinement imposed by the thin-film configuration.