Tracking Confined Reaction Based on Host-Guest Interaction Using Single-Molecule Conductance Measurement.

The host-guest interaction acts as an essential part of supramolecular chemistry, which can be applied in confined reaction. However, it is challenging to obtain the dynamic process during confined reactions below micromolar concentrations. In this work, a new method is provided to characterize the dimerization process of the guest 1,2-bis(4-pyridinyl) ethylene in host cucurbit[8]curil using scanning tunneling microscope-break junction (STM-BJ) technique. The guest reaction kinetics is quantitatively by nuclear magnetic resonance (NMR) and in situ single-molecule junctions. It is found that in the single-molecule conductance measurements, the electrical signals of the reactants with a concentration as low as 5 × 10-6  m are clearly detected, and the reaction kinetics at micromolar concentrations are further obtained. However, in NMR measurements, the characteristic peak signal of the reactants is undetectable when the concentration of the reactants is lower than 0.5 × 10-3  m and it cannot be quantified. In addition, the strong electric field from the nanogap accelerates the reaction. This work reveals that single-molecule STM-BJ techniques are more sensitive for tracking confined reactions than that by NMR techniques and can be used to study effect of extremely strong electric field on kinetics.

Selective Fabrication of Single-Molecule Junctions by Interface Engineering.

Recent progress in addressing electrically driven single-molecule behaviors has opened up a path toward the controllable fabrication of molecular devices. Herein, the selective fabrication of single-molecule junctions is achieved by employing the external electric field. For molecular junctions with methylthio (-SMe), thioacetate (-SAc), amine (-NH2 ), and pyridyl (-PY), the evolution of their formation probabilities along with the electric field is extracted from the plateau analysis of individual single-molecule break junction traces. With the increase of the electric field, the SMe-anchored molecules show a different trend in the formation probability compared to the other molecular junctions, which is consistent with the density functional theory calculations. Furthermore, switching from an SMe-anchored junction to an SAc-anchored junction is realized by altering the electric field in a mixed solution. The results in this work provide a new approach to the controllable fabrication and modulation of single-molecule junctions and other bottom-up nanodevices at molecular scales.