TY - JOUR T1 - Influence of Terminal Carboxyl Group on Structure and Reactivity of Functionalized m-Carboranethiolate Self-Assembled Monolayers JF - Chemistry of Materials Y1 - 2020 A1 - Dominic P Goronzy A1 - Jan Stanek A1 - Erin Avery A1 - Han Guo A1 - Zdenek Bastl A1 - Michal Dusek A1 - Nathan M Gallup A1 - Saliha Gun A1 - Monika Kucerakova A1 - Brian J Levandowski A1 - Jan Machacek A1 - Vaclav Sicha A1 - John C Thomas A1 - Adem Yavuz A1 - K N Houk A1 - M Fatih Danisman A1 - Ersen Mete A1 - Anastassia N Alexandrova A1 - Tomas Base A1 - Paul S Weiss KW - acidity KW - carborane KW - carboxyl KW - monolayer KW - SAM KW - self-assembly AB -

The structure and function of self-assembled monolayers (SAMs) at the nanoscale are determined by the steric and electronic effects of their building blocks. Carboranethiol molecules form pristine monolayers that provide tunable two-dimensional systems to probe lateral and interfacial interactions. Additional ω-functionality, such as carboxyl groups, can be introduced to change the properties of the exposed surfaces. Here, two geometrically similar isomeric m-carborane analogs of m-mercaptobenzoic acid, 1 COOH-7-SH-1,7-C2B10H10 and racem-1-COOH-9-SH-1,7-C2B10H10, are characterized and their SAMs on Au{111} are examined. The latter isomer belongs to the rare group of chiral cage molecules and becomes, to our knowledge, the first example assembled on Au{111}. Although different in symmetry, molecules of both isomers assemble into similar hexagonal surface patterns. The nearest neighbor spacing of 8.4 ± 0.4 Å is larger than that of non-carboxylated isomers, consistent with the increased steric demands of the carboxyl groups. Computational modeling reproduced this spacing and suggests a tilt relative to the surface normal. However, tilt domains are not observed experimentally, suggesting the presence of strong lateral interactions. Analyses of the influence of the functional groups through the pseudo-aromatic m carborane skeleton showed that the thiol group attached to either carbon or boron atoms increases the carboxyl group acidity in solution. In contrast, the acidity of the exposed carboxyl group in the SAMs decreases upon surface attachment; computational analyses suggest that the driving force of this shift is the dielectric of the environment in the monolayer as a result of confined intermolecular interactions, proximity to the Au surface, and partial desolvation.

VL - 32 UR - https://pubs.acs.org/doi/10.1021/acs.chemmater.0c02722 ER -