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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.
References
P. Liljeroth, J. Repp, G. Meyer, Science, 317, 1203-1206 (2007).
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IBM Research
