Molecular electronics where single molecules perform basic functionalities of digital circuits is a fascinating concept that one day may augment or even replace nowadays semiconductor technologies. The tautomerization of molecules, i.e., the bistable functional position of hydrogen protons within an organic frame, has recently been intensively discussed as a potential avenue towards nano-scale switches. It has been shown that tautomerization can be triggered locally or non-locally, for example by a scanning tunneling microscope (STM) tip positioned directly above or in close vicinity to the molecule. Whereas consensus exists that local switching is caused by inelastic electrons which excite vibrational molecular modes, the detailed processes responsible for non-local tautomerization switching and —even more important in the context of this work— methods to control, engineer, and potentially utilize this process are largely unknown.
Here, we demonstrate for H2Pc molecules on Ag(111) that the tautomerization processes are mediated by surface state electrons with a well-defined dispersion relation. We are able to controllably decrease or increase the probability of non-local, hot electron-induced tautomerization by atom-by-atom–designed Ag nanostructures. We show that Ag atom walls act as potential barriers which exponentially damp the hot electron current between the injection point and the molecule, reducing the switching probability by up to 83% for a four-atom wide wall. By placing the molecule in one and the STM tip in the other focal point of an elliptical nanostructure we could coherently focus hot electrons onto the molecule which led to an almost tripled switching probability. Finally, we will discuss to what extent interference phenomena may be used to control tautomerization. Our results demonstrate that the tautomerization switching of single molecules can remotely be controlled by the utilizing suitable nanostructures and may pave the way for designing new tautomerization-based switches.