From Foldable Open Chains to Shape-Persistent Macrocycles: Synthesis, Impact on 2D Ordering, and Stimulated Self-Assembly.

ABSTRACT

Small molecule self-assembly at surfaces offers an efficient route to highly ordered organic films that can be programmed for a variety of chemical and electronic applications. The success of these materials depends on the ability to program intermolecular interactions to guide precise structural ordering. Toward this objective, we have designed and synthesized a series of bis(triazolo)benzene-based π-conjugated molecules. Our synthesis exploits a last-stage C-C cross-coupling reaction to close up zigzag-shaped linear precursors to cyclized products, so that direct side-by-side comparisons can be made for their structure-dependent self-assembly behavior at surfaces and response to external stimuli. Indeed, scanning tunneling microscopy (STM) analysis revealed distinct differences as the conformational flexibility of the molecular backbone and the chemical structure of the peripheral groups are varied. Specifically, alkyl chains adsorb and form interdigitated structures, whereas oligo ethylene glycol (OEG) chains remain desorbed and thus shift self-assembly to more densely packed π-conjugated cores. While the macrocycles self-assemble immediately and spontaneously, their linear precursors exhibit slower self-assembly kinetics, which could be attributed to the difference in the degree of conformational freedom. We also found that perturbation by the STM tip and the addition of cosolutes profoundly impacted the kinetics of self-assembly and surface patterning. This highly unusual behavior highlights the importance of noncovalent interactions that are inherently weak in solution but can be made strong for symmetric and conformationally restricted molecules confined within 2D surfaces.