Self-assembled monolayers (SAMs) are a facile tool to tune surface properties and thereby to modify materials. By functionalizing SAM molecules, the interface between the surface and the environment can be manipulated, opening new avenues for surface chemistry and reactivity. Here, we examined SAMs composed of carborane cage molecules with a carboxylic acid functional group and tethered to a Au{111} surface via a thiol moiety. In previous work, using scanning tunneling microscopy (STM), the para-functionalized carboxylic acid carboranethiol was visualized in the form of mixed monolayers of functionalized and unfunctionalized molecules, with the functionalized molecule adopting the same nearest-neighbor spacing on the surface as the unfunctionalized para-carboranethiol, approximately 7.2 Å. In contrast, in our current study of carboranethiols with a carboxylic acid functional group attached in the meta position, we could image pure functionalized monolayers via STM with nearest-neighbor spacings over 1 Å larger than the unfunctionalized carboranethiols, approximately 8.4 Å. Results were the same for the two isomers of carboranethiols studied. Furthermore, using contact angle titration, we probed the acidity of the carboxylic acid functional group on the surface to explore the effects of changing dimensionality from a 3D solvent system to a 2D thin film. Our titration results demonstrated a shift from a solution pKa of 3.01-3.23, to an apparent pKa on the surface of approximately 6.95. Density functional theory calculations on the desolvation effects of two-dimensional confinement reproduced the experimentally observed pKa shift. Additional STM results of SAMs deposited in ethanol versus in benzene suggest that reactivity changes are more pronounced than just the acidity of the carboxylic group. These results support the hypothesis that the confinement of the molecule in a 2D environment induces a significant change in reactivity as observed by the pKa shift.

Reactivity of confined carboxylic acids in thin films

255th National Meeting (2018)

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