Reversible Anion‐Driven Switching of an Organic 2D Crystal at a Solid–Liquid Interface

Abstract

Ionic self-assembly of charged molecular building blocks relies on the interplay between long-range electrostatic forces and short-range, often cooperative, supramolecular interactions, yet has been seldom studied in two dimensions at the solid-liquid interface. Here, we demonstrate anion-driven switching of two-dimensional (2D) crystal structure at the Au(111)/octanoic acid interface. Using scanning tunneling microscopy (STM), three organic salts with identical polyaromatic cation (PQPC ) but different anions (perchlorate, anthraquinonedisulfonate, benzenesulfonate) are shown to form distinct, highly ordered self-assembled structures. Reversible switching of the supramolecular arrangement is demonstrated by in situ exchange of the anion on the pre-formed adlayer, by changing the concentration ratio between the incoming and outgoing anion. Density functional theory (DFT) calculations reveal that perchlorate is highly mobile in the adlayer, and corroborate why this anion is only resolved transiently in STM. Surprisingly, the templating effect of the anion persists even where it does not become part of the adlayer 2D fabric, which we ascribe to differences in stabilization of cation conformations by the anion. Our results provide important insight into the structuring of mixed anion-cation adlayers. This is essential in the design of tectons for ionic self-assembled superstructures and biomimetic adaptive materials and valuable also to understand adsorbate-adsorbate interactions in heterogeneous catalysis.

Electrostatically Driven Guest Binding in Self-Assembled Molecular Network of Hexagonal Pyridine Macrocycle at the Liquid/Solid Interface: Symmetry Breaking Induced by Coadsorbed Solvent Molecules

“…15,57 In this respect, the use of an ionic species as guests is particularly interesting, because the position of the guest may not only be determined by direct electrostatic interactions between a host and guest but also by external long range perturbations such as magnitude of change of the substrate bias and voltage pulses to the STM tip 58 or electrical interfacial potential. 59,60 Therefore, dynamic behavior is expected if suitable organic cations are used as a guest in 2D host-guest systems.…”

Graphite and Graphene Fairy Circles: A Bottom-Up Approach for the Formation of Nanocorrals

“…Graphene and graphite are carbon-based surfaces that are receiving a lot of attention because of their relevance in materials sciences. − Multiple molecule-based functionalization protocols have been developed. − Among those that result in nanopatterning, supramolecular self-assembly plays a prominent role, as it often leads to two-dimensional (poly)­crystalline films. − , However, the noncovalent nature of the molecule–substrate interactions provides the layers with only limited stability.…”

Ambient Bistable Single Dipole Switching in a Molecular Monolayer

“…The system we propose is based on an organic salt that consists of a large discoid polyaromatic cation (PQPC 14 + ) and a small inorganic anion (perchlorate, ClO 4 − ) Figure a. In recent work, we demonstrated the rich supramolecular behavior of this compound class, and were able to show transformations between two‐ and three‐dimensional self‐assemblies and reversible electrocompression under electrochemical conditions, and the effect of anions on supramolecular structure formation . The key difference between the molecular liquid‐solid interface and the electrochemical environment is the presence of the electrochemical double layer, where typically a field on the order of up to 10 9 V m −1 exists.…”

“…Strikingly, contact with the gold surface lowers the energy of the ClO 4 − frontier electrons in the side position (see Figure S4), and more so for trapping ClO 4 − between the alkyl chains. The energy landscape shown in Figure c is very shallow as long as the perchlorate is on top of the fused ring system of the PQPC 14 + cation, leading to high mobility of the anion . During normal imaging (not switching), the perchlorate will follow the positive tip within the periphery of the PQP + fused ring system, enhancing the brightness of the latter.…”