In studying two-dimensional self-assemblies at interfaces, great strides have been gained in understanding how supramolecular interactions affect assembled structure to produce functional materials. However, there is a great challenge in characterizing these systems at the critical nanosecond / nanometer scales, where insight into pre-equilibrium stages and fundamental steps of self-assembly is needed to advance supramolecular design. We have studied the self-assembly of two prototypical molecular systems at the solution-graphite interface, using scanning tunneling microscopy (STM) with atomistic molecular dynamic (MD) simulations. These studies allow new insight into the fundamental steps of molecular self-assembly. The first system is an alkoxybenzonitrile that contains competing van der Waals, electrostatic, and steric interactions. STM was used to determine packing structure and to benchmark force fields. MD simulations revealed the atomistic and sub-nanosecond stages of desorption, reabsorption, and on-surface motion; this provided new insight into molecular level assembly dynamics. Second, the self-assembly of a series of π-conjugated macrocycles and their flexible precursors were compared. Peripheral functionalization with alkyl and glycol chains resulted in comparable solubility, but self-assembly can be tuned between a tight packing of aromatic cores with desorbed glycol chains or a looser packing of aromatic cores separated by interdigitated alkyl chains. While the macrocycles self-assemble immediately and spontaneously, their flexible linear precursors exhibit slower self-assembly kinetics, due to the large conformational space available. STM tip bias or aromatic co-solutes reduced that conformational space and, remarkably, significantly accelerated self-assembly. The observations and from these studies provide insights into the dynamics and design rules of small molecule self-assembly.

Molecular self-assembly at surfaces: Dynamics, interactions, and design

2019 Spring National Meeting

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