Stoddart−Heath [2]Rotaxane Molecular Switch Made Simple: A Density Functional Theory Study on Model Junction Devices

The essential components of the Stoddart−Heath-type [2]rotaxane molecular switch tunnel junction devices are the aromatic shuttle and stations, which are attached to other components, such as linkers, stoppers, and anchors. In this study, we explored a possibility of whether the molecular switch can be made simple by leaving only the π-stacked aromatic key components between two metal electrodes. The current−voltage (I−V) characteristics of these simplest model devices were calculated from density functional theory using the nonequilibrium matrix Green’s function formalism. When the aromatic components are in direct contact with the surfaces, the I−V characteristics depend dramatically on the orientation of the π stacks, and spatial or temporal changes in the orientation can decrease the device robustness. The robustness can be restored by introducing a buffer layer or a covalent bond (say bulky stoppers and anchors as well as titanium adhesion layers) between the π stack and the electrodes, and this is, indeed, what has been done in the actual fabrication of the working devices. We also propose an alternative strategy to build a simple switch that uses controlled orientation change as the basis for switching.