Reversible achiral-to-chiral switching of single Mn--phthalocyanine molecules by thermal hydrogenation and inelastic electron tunneling dehydrogenation.

Induction of chirality in planar adsorbates by hydrogenation of phthalocyanine molecules on a gold surface is demonstrated. This process merely lowers the molecular symmetry from 4- to 2-fold, but also breaks the mirror symmetry of the entire adsorbate complex (molecule and surface), thus rendering it chiral without any realignment at the surface. Repositioning of single molecules by manipulation with the scanning tunneling microscope (STM) causes interconversion of enantiomers. Dehydrogenation of the adsorbate by means of inelastic electron tunneling restores the mirror symmetry of the adsorbate complex. STM as well as density functional theory (DFT) calculations show that chirality is actually imprinted into the electronic molecular system by the surface, i.e., the lowest unoccupied orbital is devoid of mirror symmetry.