When Aromaticity Falls Short in Molecule-Surface Interactions

Aromaticity is one of the most important concepts in organic chemistry. There are cases in which a molecule undergoes changes to increase its aromaticity. This higher aromaticity comes with an energetic gain and is commonly referred to as aromatic stabilization. Previously, it has been reported that some molecules undergo such a stabilization when adsorbing on a surface, which has been identified as the reason for charge transfer into the molecular π-system. Utilizing photoemission orbital tomography and density functional theory, we investigate changes in the molecular π-system upon adsorption and elucidate the influence on the aromaticity. We demonstrate how the energetic gain from an aromatic stabilization on surfaces can be outweighed by hybridization. Uncovering a mechanism in which the molecular π-system forms dative bonds with the surface, our study reveals that the concept of aromatic stabilization on surfaces has been incomplete so far.