Molecular switches based on
hydrogen tautomerization

Overview

The concept of using single molecules as electronic components is well established. There are numerous examples of a few or even individual molecules serving as memory elements, diodes, transistors, or switches. However, constructing more complex molecular devices requires that components be brought together and electronically coupled in a controlled manner. Most molecular switches are based on drastic conformational changes in the molecule; this is not compatible with the aim of controlling the coupling between the molecules. The development of molecular logic devices will also require single-molecule switches that can be coupled without compromising their function and that do not involve changes in the molecular frame.

We have investigated a new type of single-molecular switch based on hydrogen tautomerization that meets these requirements. The molecule is naphthalocyanine; the inner core of the molecule shown in Fig. 1. Tautomerization involves the transfer of the inner hydrogen atoms from the horizontal to the vertical orientation. In this experiment we operated and characterized the switch by low-temperature STM. The lowest unoccupied molecular orbital (LUMO) of a free-base naphthalocyanine (Fig. 2) has twofold symmetry and can therefore have two orientations, depending on the position of the two inner hydrogen atoms in the central cavity of the molecule (arrows in Fig. 1).

By increasing the bias voltage between the tip and the sample, a hydrogen tautomerization reaction could be induced by the tunnelling electrons in the STM junction. Although the molecule itself does not rotate, this change is formally equivalent to the rotation of the molecule by 90° and causes a significant change in the tunnelling current measured at the STM tip positioned over the molecule. We have also demonstrated that switching can be even induced by injecting electrons into adjacent molecules (Fig. 3). As the switching is well-defined, highly localized, reversible, intrinsic to the molecule, and does not involve changes in the molecular frame, this class of molecules can be used as building blocks for more complex molecular devices such as logic gates.

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References

[1] P. Liljeroth, J. Repp, G. Meyer, Science, 317, 1203-1206 (2007).

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H tautomerization reaction Figure 1. Schematic of the hydrogen tautomerization reaction responsible for switching. Only the central core of the naphthalocyanine molecule is shown.

Single naphthlocyanine molecule Figure 2. Switching of a single naphthalocyanine molecule by the tunnelling current. Orbital images showing the orientation of the LUMO before and after switching.

Schematic view Figure 3. Schematic view of two coupled naphthalocyanine molecules probed by the tip of a low-temperature scanning tunneling microscope.