In-situ label-free single-molecule dynamic detection of thermal-reversible reactions

Direct monitoring of chemical reactions under external thermal control at the single-molecule scale remains highly challenging. Herein we report the first example of in-situ label-free single-molecule detection of a thermal-reversible Diels-Alder (DA) reactions using the scanning tunneling microscopy break junction (STM-BJ) technique equipped with a thermocouple mounted under the substrate with feedback control. The initial and final conductance states in single-molecule junctions based on the reaction can be reversibly switched in-situ between two different temperature levels. Furthermore, quantitative statistical analysis of the single-molecule reaction dynamics reveals that orientated external electric fields can selectively accelerate the forward DA reaction more than three times while having no impact on the reverse reaction. This finding emphasizes the potential of electric fields in selectively manipulating chemical reactions. The combined theoretical calculations suggest that the applied electric fields stabilize the dipole of the transition state and therefore reduce the energy barrier of the forward reaction. The integration of reversible reactions into nanocircuits not only provides an efficient strategy to manipulate chemical reactions but also offers new opportunities for designing functional molecular-scale devices such as transistors and switches.