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K. Cui, K. Mali, D. Wu, X. Feng, K. Müllen, M. Walter, S. De Feyter, S. Mertens:
"Reversible Anion-Driven Switching of an Organic 2D Crystal at a Solid-Liquid Interface";
Small, 2017 (2017), 17023791 - 17023798.

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 (PQPC6
+) but different anions (perchlorate, anthraquinonedisulfonate,
benzenesulfonate) are shown to form distinct, highly ordered selfassembled
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.