A central idea in electron transfer theories is the coupling of the electronic state of a molecule to its structure. Previous studies used mechanical forces to induce structural and chemical changes in large biological molecules, but mechanically control of the electron transfer reaction of a small molecule without bond rupture, and determining the relationship between force and distribution of redox equilibria have not been reported.
Here we show experimentally that fine changes to molecular structures by mechanically stretching a single metal complex molecule via changing the metal-ligand bond length can shift its electronic energy levels and predictably guide electron transfer reactions, leading to the changes in redox state. We monitor the redox state of the molecule by tracking its characteristic conductance, determine the shift in the redox potential due to mechanical stretching of the metal-ligand bond, and perform model calculations to provide insights into the observations. The work demonstrates that a mechanical force can prompt the molecule into a geometry that favors the electron transfer reaction, shift its redox potential, change its redox state, and thus allow the manipulation of single molecule conductance. This observation reveals experimentally the role of electronic-nuclear coupling in the electron transfer reactions at the single molecule level, and its potential application in mechanically controllable molecular switches.