Anisotropy of pi-pi Stacking as Basis for Superlubricity

When two surfaces move against each other, often high adhesive strength with low friction is desired. Then, the interfacial interaction requires a strong directional anisotropy, as provided by pi-pi stacking. However, a detailed understanding of the direction-dependent strength of pi-pi stacking is lacking because such direction-dependent molecular force measurements are challenging. We use atomic force microscopy-based stereographic single molecule force spectroscopy to perform direction-dependent pulling of a pi-pi stacking model system in solution, namely, a single pyrene molecule on highly oriented pyrolytic graphite. There, the friction force is at least 3 orders of magnitude lower than the adhesive force, indicating superlubricity at the single molecule level. This is in clear contrast to the thiol-Au coordination bond, where we reveal high friction forces. Our measurements quantify and highlight the importance of the directional anisotropy of p-p stacking, which provides a bottom-up understanding of adhesion and friction at material interfaces based on its molecular interactions.