  In the pursuit of sustainable lubricant materials, the conversion of common
organic molecules into graphitic material has been recently shown to
effectively reduce friction of metallic interfaces. Aromatic molecules are
perfect candidates due to their inertness and possibility to form carbon-based
tribofilms. Among many promising possibilities, we selected a group of common
aromatic compounds and we investigated their capability to reduce the adhesion
of iron interface. Ab initio molecular dynamic simulations of the sliding
interface show that hypericin, a component of St. John's wort, effectively
separates the mating iron surfaces better than graphene. This phenomenon is due
to the size of the molecule, the reactivity of the moieties at its edges and
the possibility to stack several of these structures that can easily slide on
top of each other. The decomposition of the lateral groups of hypericin
observed in the dynamic simulations suggests that the clustering of several
molecules is possible, offering innovative paths to lubricate sliding contacts
with compounds not typically employed in tribology.
