Controlling the Stereochemistry and Regularity of Butanethiol Self-Assembled Mono layers on Au(111)

The rich stereochemistry of the self-assembled monolayers (SAMs) of four butanethiols on Au(111) is described, the SAMs containing up to 12 individual C, S, or Au chiral centers per surface unit cell. This is facilitated by synthesis of enantiomerically pure 2-butanethiol (the smallest unsubstituted chiral alkanethiol), followed by in situ scanning tunneling microscopy (STM) imaging combined with density functional theory molecular dynamics STM image simulations. Even though butanethiol SAMs manifest strong headgroup interactions, steric interactions are shown to dominate SAM structure and chirality. Indeed, steric interactions are shown to dictate the nature of the headgroup itself, whether it takes on the adatom-bound motif RS(center dot)Au(0)S(center dot)R or involves direct binding of RS(center dot) to face-centered-cubic or hexagonal-close-packed sites. Binding as RS(center dot) produces large, organizationally chiral domains even when R is achiral, while adatom binding leads to rectangular plane groups that suppress long-range expression of chirality. Binding as RS(center dot) also inhibits the pitting intrinsically associated with adatom binding, desirably producing more regularly structured SAMs.