LAUSR

DOI: 10.1002/smtd.201900464 The construction of artificial molecularlevel devices and machines, both of which function with external stimuli, has attracted considerable attention in the areas of nanoscience and nanotechnology. Rotaxanes and pseudorotaxanes, well-known for their unique shuttling motion in mechanically interlocked molecular systems, have proved to be versatile as molecular switches or molecular machines. Since the first molecular shuttle was reported,[1] synthetic chemists have developed a family of artificial molecular systems with various topology architectures and responsive modes to external stimuli,[2–8] which has extremely broadened the range of potential applications. However, formidable challenges still remain as follows: 1) molecular machines can hardly work independently and cooperate like real machines, so integration at the single-molecule level into an electrical nanocircuit should be an inevitable technical route; 2) even though submolecular movements in molecular machines that are usually inaccessible in ensemble experiments can be detected by classical spectrum analysis methods (such as NMR, circular dichroism, etc.), both controlling individual molecular machines and understanding the intrinsic principle at the molecular level are still elusive because of the lack of efficient single-molecule analytical tools. In comparison with single-molecule optical approaches, electrical approaches, which have been developed to realize label-free single-molecule electrical monitoring of molecular interactions by using different device architectures, such as single-molecule break junctions,[9–14] nanotubes,[15,16] nanowires,[17,18] and nanopores,[19] are more practical and easily readable. Among these approaches, single-molecule techniques,[20,21] in particular graphene–molecule–graphene single-molecule junctions (GMG-SMJs),[22] are attractive because they are able to covalently integrate individual molecular systems tested as the conductive channel into electrical nanocircuits, which might solve the above-mentioned critical issues. Previous reports[23] have proven that these techniques are the robust platforms of single-molecule electrical detection that is capable Single-molecule detection is able to reveal rich spatial/temporal information and elucidate intrinsic mechanisms of inter or intramolecular interactions that are not accessible in ensemble experiments, which is of fundamental importance to solve the key issues in physical, chemical, and biological sciences. Here, a robust, label-free single-molecule electrical approach capable of directly exploring the detailed dynamic process of stochastic movement of alkyl chains bearing different charges through a macrocycle in a molecular machine at the single-event level is represented by using stable graphene–molecule–graphene single-molecule junctions (GMG-SMJs). These junctions are built by covalently sandwiching a pseudorotaxane featuring a permethylated α-cyclodextrin between nanogapped graphene electrodes. In situ long-term single-molecule electrical measurements unambiguously show reproducible large-amplitude multiple-level fluctuations that are highly dependent on charge and temperature. Both theoretical simulations and experimental data prove that this observation originates from random motions of alkyl chains along the axle within a time scale of a few milliseconds. Observation of transient changes in current signals by gating effects allows direct real-time monitoring of the submolecular translational process. These investigations conceptualize the capability of GMG-SMJs to probe fast single-molecule chemical reactions or behaviors. Molecular Physics