Recognition and Ordering at Surfaces: The Importance of Handedness and Footedness

The expression of chirality at surfaces, arising from the adsorption of chiral molecules, is usually discussed in terms of the molecular handedness. However, the adsorption process often leads to a new manifestation of chirality in the form of the adsorption footprint. Therefore, in order to fully define the chirality of the interface we propose that both the handedness and the footedness of the system must be considered. To illustrate this point, we describe the ordering behavior of the molecules tartaric acid, succinic acid, proline, and 3-pyrroline-2-carboxylic (PCA) on a Cu(110) surface using deconvolution maps separating the arrangement of enantiomers, conformers, adsorption footprints and rotamers within an organized assembly. Tartaric and succinic acid mimic the behavior of a conventional 3D conglomerate and racemic compound in terms of both the handedness and footedness, respectively. However, racemic PCA and proline, while expressing a random solid solution of enantiomers and conformers, both display unexpected degrees of order when adsorbate footprint and orientation are considered.