Femtosecond concerted rotation of molecules on a 2D material interface

Abstract Function is dynamic and originates at atomic interfaces. Combining the degrees of freedom of molecules (1,2) with the peculiar properties of 2D quantum materials (3,4,5) can create novel functionality (6,7). Here, we report the manipulation and ultrafast imaging of a unidirectional gearing motion in molecules on a 2D quantum material. To visualize and disentangle the intertwined structural and electronic dynamics of such a hybrid interface, we record a ‘full molecular movie’ (8) by imaging the atomic positions (9,10), the evolution of the molecular orbital wavefunctions (11,12) and the modification of electronic states of the substrate (13). In a multimodal investigation in a single setup (14), we disentangle dynamics in valence and core electrons of both the molecule and the surface with femtosecond and sub-ångström precision. The ultrafast rotational motion is fueled by the transfer of hot holes into the molecules that results in “supercharging” of the film. As hot carriers move through the interface, we track a transient modification of the frontier molecular orbitals and observe a chiral symmetry breaking associated with local structural rearrangements. Our calculations show that the “supercharging” changes the interfacial potential energy landscape and triggers the gearing motion. The experiment offers all-in-one imaging of the electronic, molecular orbital, chemical and structural dynamics during the flow of charge and energy across the hybrid interface. Our approach provides detailed dynamical information on the mechanism underlying surface-adsorbed molecular gears and enables tailoring novel functionalities in hybrid active matter.