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Directionality is key for the functionality of molecular machines, which is often achieved by built-in structural chiralities. Here, we present a scanning tunneling microscopy study of achiral H₂Pc and HPc molecules that acquire chirality by adsorption onto a Ag(100) surface. The adsorption-geometry-induced chirality is caused by a −29° (+29°) rotation of the molecules with respect to the [011] substrate direction, resulting in tautomerization processes that preferentially occur in a clockwise (counterclockwise) direction. The directionality is found to be independent of the particular energy and location of charge carrier injection. In contrast to built-in structural chiralities that are fixed by the molecular structure, the direction of proton motion in HPc on Ag(100) can be inverted by a rotation of the molecule on the substrate.