A single-molecule van der Waals compass

Abstract

Single-molecule imaging is challenging but highly beneficial for investigating intermolecular interactions at the molecular level1–6. Van der Waals interactions at the sub-nanometre scale strongly influence various molecular behaviours under confinement conditions7–11. Inspired by the traditional compass12, here we use a para-xylene molecule as a rotating pointer to detect the host–guest van der Waals interactions in the straight channel of the MFI-type zeolite framework. We use integrated differential phase contrast scanning transmission electron microscopy13–15 to achieve real-space imaging of a single para-xylene molecule in each channel. A good correlation between the orientation of the single-molecule pointer and the atomic structure of the channel is established by combining the results of calculations and imaging studies. The orientations of para-xylene help us to identify changes in the van der Waals interactions, which are related to the channel geometry in both spatial and temporal dimensions. This work not only provides a visible and sensitive means to investigate host–guest van der Waals interactions in porous materials at the molecular level, but also encourages the further study of other single-molecule behaviours using electron microscopy techniques.