Recognition tunneling (RT) is an emerging technique for investigating single molecules in a tunnel junction. We have previously demonstrated its capability of single molecule detection and identification, as well as probing the dynamics of intermolecular bonding at the single molecule level. Here by introducing cucurbituril as a new class of recognition molecule, we demonstrate a powerful platform for electronically investigating the host-guest chemistry at single molecule level. In this report, we first investigated the single molecule electrical properties of cucurbituril in a tunnel junction. Then we studied two model guest molecules, aminoferrocene and amantadine, which were encapsulated by cucurbituril. Small differences in conductance and lifetime can be recognized between the host-guest complexes with the inclusion of different guest molecules. By using a machine learning algorithm to classify the RT signals in a hyper dimensional space, the accuracy of guest molecule recognition can be significantly improved, suggesting the possibility of using cucurbituril molecule for single molecule identification. This work enables a new class of recognition molecule for RT technique and opens the door for detecting a vast variety of small molecules by electrical measurements.

Beyond π–π stacked benzene rings, non-bonded conducting channels are also confirmed in non-strict face-to-face aligned thiophenes or phenyl-thiophene in BDT derivatives.

Amides are essential in the chemistry of life. Detecting the chemical bond states within amides could unravel the nature of amide stabilization and planarity, which is critical to the structure and reactivity of such molecules. Yet, so far, no work has been reported to detect or measure the bond changes at the single-molecule level within amides. Here, we show that a transition between single and double bonds between N and C atoms in an amide can be monitored in real time in a nanogap between gold electrodes via the generation of distinctive conductance features. Density functional theory simulations show that the switching between amide isomers proceeds via a proton transfer process facilitated by a water molecule bridge, and the resulting molecular junctions display bimodal conductance states with a difference as much as nine times.

We introduce a versatile recognition tunneling technique using doubly cucurbit [7]uril-functionalized electrodes to form supramolecular junctions that capture analytes dynamically by host-guest complexation. This results in characteristic changes in their single-molecule conductance. For structurally related drug molecules (camptothecin, sanguinarine, chelerythrine, and berberine) and mixtures thereof, we observed distinct current switching signals related to their intrinsic conductance properties as well as pH-dependent effects which can be traced back to their different states (protonated versus neutral). The conductance variation of a single molecule with pH shows a sigmoidal distribution, allowing us to extract a pK a value for reversible protonation, which is consistent with the reported macroscopic results. The new electronic method allows the characterization of unmodified drug molecules and showcases the transfer of dynamic supramolecular chemistry principles to single molecules.

Creating single‐molecule junctions with a long‐lived lifetime at room temperature is an open challenge. Finding simple and efficient approaches to increase the durability of single‐molecule junction is also of practical value in molecular electronics. Here it is shown that a flexible gold‐coated nanopipette electrode can be utilized in scanning tunneling microscope (STM) break‐junction measurements to efficiently enhance the stability of molecular junctions by comparing with the measurements using conventional solid gold probes. The stabilizing effect of the flexible electrode displays anchor group dependence, which increases with the binding energy between the anchor group and gold. An empirical model is proposed and shows that the flexible electrode could promote stable binding geometries at the gold‐molecule interface and slow down the junction breakage caused by the external perturbations, thereby extending the junction lifetime. Finally, it is demonstrated for the first time that the internal conduit of the flexible STM tip can be utilized for the controlled molecule delivery and molecular junction formation.