A new control knob for tuning intermolecular halogen bonds

Halogen bonds are ideally suited for designing molecular assemblies and emerge in ‎numerous applications because of their ‎strong directionality and tunability. Here, we establish the ‎surface as a control knob for tuning molecular assemblies by ‎reversing the binding selectivity.‎
A new control knob for tuning intermolecular halogen bonds

Our paper published in Nature Communications 11, 5630 (2020) can be read from https://www.nature.com/articles/s41467-020-19379-4

The self-assembly of molecular building blocks into complex molecular structures plays a fundamental role for many growth processes in nature. To mimic such processes and employ them systematically for designing new materials with custom-made properties we need to understand the mechanisms behind these processes and gain the ability to control them. Intermolecular halogen bonds are particularly well suited for this purpose since they exhibit high directionality and tunable strength. This is a main distinctive feature in comparison to other intermolecular interaction types such as hydrogen bonding or dispersion interactions.

In our study we present an approach for tuning the strength and the directionality of intermolecular halogen bonds by adsorption of halogenated model compounds to reactive vs. inert metal surfaces. This approach relies on adjusting the charge distribution of the halogens by molecule-substrate interactions. In particular, it allows to tune the strength of the so-called σ-hole, a positive region at the caps of the halogens. Therewith, the anticipated binding selectivity can be either retained by adsorption on a relative inert Au(111) substrate or it can be even reversed by adsorption on a relative reactive Cu(111) substrate (see Figure 1).

Figure 1: (left panel) Charge distribution of iodobenzene (C6H5I) and bromobenzene (C6H5Br) in the gas phase. The positive σ-holes appear as blue regions at the caps of the halogens (see blue arrows). Iodine develops a stronger σ-hole than Bromine. As a result, I∙∙∙I connections are preferred over Br∙∙∙Br connections in the gas phase. (right panel) On the inert Au(111) substrate, I∙∙∙I connections between the studied 4-bromo-3"-iodo-p-terphenyl (BrparaImeta-TP) molecules are strongly preferred. On the relative reactive Cu(111) substrate, however, Br∙∙∙Br connections predominate. Therefore, the anticipated bond selectivity is retained on the inert Au(111) surface and reversed on the reactive Cu(111) surface.

We used low temperature atomic force microscopy with CO-functionalized tips, which allows to perform a visual inspection of the molecular assemblies with single bond resolution. Therewith, we determined the adsorption positions and orientations, the bonding angles, and the binding selectivity of the halogenated molecules on different substrates. The corresponding molecular charge distributions and intermolecular energies have been calculated by density functional theory (DFT). Our findings offer an alternative approach for designing molecular assemblies, which is interesting for the fields of supramolecular chemistry, catalysis, drug design, and on-surface chemistry.  Furthermore, the conceptual idea of tuning halogen bonds via reactive atoms in their close vicinity paves the way for future studies that address the general applicability of this phenomenon for different molecular systems and environments.

This work is an international collaboration between the Institutes of Applied Physics, Physical Chemistry, and Organic Chemistry from the Justus Liebig University in Giessen and the Departamento de Física Teórica de la Materia Condensada from the Universidad Autónoma de ‎Madrid.

Original article: Jalmar Tschakert, Qigang Zhong, Daniel Martin-Jimenez, Jaime Carracedo-Cosme, ‎Carlos Romero-Muñiz, Pascal Henkel, Tobias Schlöder, Sebastian Ahles, Doreen ‎Mollenhauer, Hermann A. Wegner, Pablo Pou, Rubén Pérez, André Schirmeisen, ‎and Daniel Ebeling‎, “Surface-Controlled Reversal of the Selectivity of Halogen Bonds”. Nature Communications 11, 5630 (2020) (open access)


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