Surface-controlled reversal of the selectivity of halogen bonds

Surface-controlled-reversal

Article published in Nature Communications by Pablo Pou and Rubén Pérez, IFIMAC researchers and members of the Theoretical Condensed Matter Physics Department of Universidad Autónoma de Madrid (Spain), in collaboration with the group of Daniel Ebeling and André Schirmeisen at the Justus Liebig University in Giessen (Germany).

Figure 1: (left panel) Electrostatic potential of iodobenzene (C6H5I) and bromobenzene (C6H5Br) in the gas phase. Intermolecular bonds between these molecules are controlled by the positive region at the caps of the halogens (blue arrows), the so-called “σ-hole”. At gas phase, Iodine develops a stronger σ-hole than Bromine, favouring I∙∙∙I connections over Br∙∙∙Br links. (right panel) This behaviour is also observed for molecules adsorbed on an inert Au(111) substrate. However, on the relative reactive Cu(111) substrate, Br∙∙∙Br connections predominate! The surface has tuned the strength of the σ-holes reversing the anticipated bond selectivity.

The self-assembly of molecular building blocks is nowadays one of the most promising approaches to design new materials. 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. Halogen bonds rely on the strongly anisotropic character of the charge distribution around a halogen atom that is covalently bound to an organic molecule.  A comprehensive understanding of the mechanisms controlling this subtle electronic effect would provide us with the ability to tune them in order to develop materials with custom-made properties.

This work, featured as an Editor’s Highlight, presents 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. These findings constitute a new control knob for tuning molecular self-assembly. [Full article].

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