Unraveling the Nanostructure of Supramolecular Assemblies of Hydrogen-Bonded Rosettes on Graphite: An Atomic Force Microscopy Study

The self-organization of multicomponent tetrarosette assemblies into ordered nanostructures on graphite surfaces has been studied by atomic force microscopy (AFM). Real-space information on the level of individual molecules allowed us to analyze the underlying structure in unprecedented detail. In highly ordered nanorod domains, tetrarosettes 13⋅(DEB)12 arrange in the form of parallel rows with a spacing of 4.6 ± 0.1 nm. High resolution AFM revealed the internal packing of the tetrarosette assemblies in these rows, which can be described by an oblique lattice with a = 2.5 ± 0.3 nm, b = 5.0 ± 0.1 nm, and γ = 122 ± 3°. The results, together with recent improvements in synthetic approaches, contribute to the development of a general strategy to develop H-bonding-based nanostructures with molecular precision. The self-assembly of small molecular building blocks into supramolecular aggregates by using non-covalent interactions is anticipated to open the path toward the realization of molecular devices and well-defined nanometer-scale structures and objects (1). Expanding on the profound knowledge in the fields of supramolecular chemistry (2–5) and intermolecular (surface) forces (6, 7), recent progress in nanostructuring has enabled several groups to arrange supramolecular aggregates in two dimensions on surfaces by clever design of the interactions and by using scanning probe microscopy approaches at variable temperatures (8–11). Other promising approaches include molecular beam epitaxy (12). As shown very recently, supramolecular assemblies in three dimensions serve as potentially valuable templates for the formation of, e.g., single crystal silver nanowires (13). In this article, we report on our recent progress in the formation and structural analysis of nanometer-scale aggregates by using self-assembled rosette structures based on hydrogen-bonding (14). Our previous work on molecular boxes derived from the corresponding double rosettes showed that nanorod structures can be obtained under certain conditions on graphite surfaces (15). Because the synthetic pathways for selective functionalization of these molecules have been developed fully in recent years, it is possible to attach functional units, such as receptors and reporters, at virtually any preselected location in the molecules. If the lateral assembly of higher order structures, such as nanorods or crystals, can be controlled similarly well in two dimensions, it will be possible to position and pattern functional nanostructures by spontaneous self-assembly processes. These nanostructures possess, hence, considerable potential as templates, receptor arrays, versatile molecular print-boards, etc. Here, we focus on the necessary molecular level investigation of the structure, organization, and two-dimensional morphology of novel tetrarosette supramolecular nanostructures and their evolution in monolayers on graphite. The ultimate aim is the development of a generally applicable approach toward positioning of functional nanostructures with submolecular precision in well-defined arrays.