Determining the Relative Structural Relevance of Halogen and Hydrogen Bonds in Self-Assembled Monolayers

Although hydrogen bonds have long been established as a highly effective intermolecular interaction for controlling the formation of self-assembled monolayers, the potential utility of the closely related halogen bonds has only recently emerged. The synergistic use of both halogen and hydrogen bonds provides a unique, multitiered strategy toward controlling the morphology of self-assembled structures. However, the interplay between these two interactions within monolayer systems has been little studied. Here, we have systematically investigated this interplay in self-assembled monolayers formed at the solid–liquid interface, with a specific attention on determining the structural relevance of the two interactions in the formation of 2D supramolecular structures. A single molecule that can simultaneously act as both a halogen and a hydrogen bond donor was paired with molecules that are effective acceptors for both of these interactions. The bimolecular networks that result from these pairings were studied by using scanning tunneling microscopy coupled with density functional theory calculations. Additional measurements on similar networks formed by using structural analogues in which halogen-bonding interactions are no longer possible give significant insight into the structure-determining role of these interactions. We find that in some monolayer systems the halogen bonds serve no significant structure-determining role, and the assembly is dominated by hydrogen bonding; however, in other systems, effective cooperation between the two interactions is observed. This study gives clear insight into the synergistic and competitive balance between halogen and hydrogen bonds in self-assembled monolayers. This information is expected to be of considerable value for the future design of monolayer systems using both halogen and hydrogen bonds.