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Directed Assembly of Functionalized Carborane Analogs


Controlling molecular building blocks and their placement at the nanoscale is an important issue for assembly from the bottom up. Manipulating single molecules on the surface using self-assembly can be used in creating novel molecular devices. Self-assembled monolayers (SAMs) form when molecules spontaneously assemble on a surface from either solution or vapor deposition. Cage molecules, specifically carboranethiols have many advantages such as rigid three-dimensional structures, high-stability to chemical and heat degradation, symmetry, rigidity, straightforward functionalization and controllable intermolecular interactions. Using the unique properties, we can fine tune SAMs and gain a fundamental chemical and physical understanding at the nanoscale. Assembling carboranethiols onto Au{111}, creates pristine monolayers with minimal defects and are made rigid through intermolecular interactions. Difunctionalized carboranes have gained interest due to the second thiol group. Assembling carboredithiol on Au{111} reveals a hexagonally close packed monolayer with two different intensity protrusions. We attribute these two protrusions as two distinct binding sites on the surface: with both thiols bound or one thiol bound and one unbound. Controlling the directionality of these binding sites is possible through protonation. Using strong acids and bases we can direct binding modalities in either direction. Functionalizing carboranethiols provides even greater tunability over the surface. P-carborane and its functionalized analog, p-mercaptobenzoic acid are analyzed using STM. These assemblies pack in a hexagonally close packed lattice which adsorb primarily as thiolates and thiols. Contact angle measurements confirm the hydrophilic character of p-mercaptobenzoic acid monolayers containing the carboxylic acid group. Mixed monolayers of p-carborane and p-mercaptobenzoic acid provide an excellent foundation for two and three dimensional structures. Using STM’s local barrier height (LBH) mode we can track dipoles on a surface. Assembling various carboranes with different dipoles allows us to visualize how these dipoles align and interact with neighboring molecules. Dipoles align based on intermolecular interactions with surrounding molecules and across different monolayer and surface defects, and locally align at low temperatures. Finally we look at place-exchange reactions involving alkanethiolates and alkaneselenoates through STM. Alkanthiolates are rapidly replaced by alkaneselenoates, as selenol coverage increases. The monolayer structure changes as selenoate coverage increases and with positive sample bias in STM, the selenolate-gold complex becomes labile and exchanges positions with neighboring thiolates.

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