Solution-Mediated Annealing Pathways Are Critical for Supramolecular Ordering of Complex Macrocycles at Surfaces

Self-organizations of supramolecular assemblies at surfaces typically achieve highly ordered two-dimensional packing structures due to the negotiation of various intermolecular interactions into a minimum free energy configuration. As we advance these systems toward the long-term goal of achieving programmable surface functionality, however, we consider systems of greater complexity in molecular building-block architecture and in growth conditions. Here, we study the polymorphic self-assembly of tricarbazolo triazolophane macrocycles (tricarb) at the solution/solid interface to determine the underlying pathway that tricarb follows when it transitions between two structures. Tricarb species, depending on peripheral functionalization, self-assemble into kinetically trapped disordered structures or thermodynamically favored ordered honeycomb structures. Experiments varying pre- and post-deposition conditions (solubility, concentration, and temperature) suggest that a solution-mediated annealing pathway, as opposed to an on-surface rearrangement, is key to a transition from disordered structures to honeycomb. Molecular dynamic simulations provide nanoscale insights into the roles of peripheral groups and solvent molecules in self-assembly. Substituents on the exterior of the molecule not only affect solubility but also stabilize tricarb-tricarb hydrogen-bonded structures. Peripheral groups influence the formation/re-formation and strength of adsorbate-adsorbate contacts and also limit solvent interactions about a macrocyclic core. The combination of simulation and experiment demonstrates the role of the solution-mediated annealing pathway in the self-assembly of complex supramolecular systems.