Electrons make molecule flip out

Using an electronic process to change the shape of a conducting molecule could be the way to make a single-molecule electrical switch.

Philip Ball

Reprinted with permission from ref. 1. Copyright (2006) by the American Physical Society.

A single molecule can be electrically switched between two different conformations, with different electronic properties, researchers in Korea have shown1.

Young Kuk at Seoul National University and his colleagues think that this behaviour, which is fully reversible, could be used to fashion nanomolecular switches for molecular electronics. Their results show how, in effect, an electrical signal might be used to alter or break the electrically conducting pathway in a molecular circuit.

In one sense, what Kuk and colleagues have achieved is already thoroughly familiar to chemists: the isomerization of a molecule, which results in a change of shape. Moreover, the molecule that they have studied, azobenzene (in which two benzene rings are linked by an azo group, -N=N-), is one of the most extensively studied from the point of view of such shape changes. The molecule can switch between a 'straight' form (trans-azobenzene or tAB) and a 'kinked' form (cis-azobenzene, cAB).

But the Korean team has induced this jack-knifing in a new way. It is usually done with light and/or heat: tAB is converted to cAB by absorption of ultraviolet light, whereas visible-light absorption or slight warming induces the reverse switch. Kuk and colleagues have not relied on light (that is, photoisomerization) at all, however. They have induced the conformational switch with electrons supplied from the needle-like tip of a scanning tunnelling microscope (STM). This enables them to switch a single molecule in a well-defined position on a gold surface, by positioning the STM tip over it.

The apparent switching of single molecules between different conformational states has been observed before using the STM. For example, David Allara of Pennsylvania State University and co-workers saw it in individual linear, conjugated molecules attached to the surface of gold and surrounded by a carpet of insulating molecules. They found that the height and electrical conductance of the molecules, as measured by the STM, switched stochastically between two states on timescales of seconds to minutes, which they attributed to random jumps between two conformational states2.

Moreover, it has been shown to be possible to exert some control over molecular conformations by using the STM to create electronically excited states. In this way, Mathieu Lastapis and colleagues at the Universit?Paris-Sud in Orsay were able to alter the position of an organic molecule relative to the atomic lattice of the silicon surface on which it was adsorbed3.

Kuk and colleagues have now combined these two ideas, producing a conformational change by means of STM-induced excitation of azobenzene adsorbed on gold. At 100 K, the single molecules (in the trans isomeric form) initially appeared as dumbbell-shaped features: two bright spots side by side, corresponding to the two phenyl rings. But when the researchers altered the voltage applied to the STM tip, they found that at a surface potential of -1.5 V relative to the tip the electrical current due to electron tunnelling suddenly jumped, and the image of the molecule changed to a single bright spot.

There were several possible explanations for this ?but a conformational change of the molecule (to the cis isomer) seemed the only likely one. The molecule was clearly not just being fragmented, for example, because the switch was reversible: the dumbbell reappeared at a sample bias of +2.5 V.

To verify this interpretation, Kuk and colleagues first carried out calculations to find the most stable structures of the tAB and cAB molecules on a gold surface. They found that whereas tAB lies flat, giving a predicted dumbbell-shaped STM image, cAB sits with one phenyl ring parallel to the gold surface and the other folded up away from the surface at an angle of about 111? That would account for the single blob in the corresponding STM image.

The researchers found further support for this interpretation from their calculations and measurements of the electronic spectra of the two conformations as a function of the energy of the electrons tunnelling from or to the STM tip. The observations agreed well with the predictions for tAB, but reasonable agreement for cAB could be obtained only by shifting the predicted spectrum by a small amount (0.5 eV) relative to that measured. Kuk and colleagues admit that they are not sure yet why this shift is necessary ?but it may be due to the transfer of an electron from the gold surface to the cAB molecule.

It is also not yet clear precisely how the isomerization is induced. The natural assumption is that the molecule is simply gaining excitation energy from the electrons tunnelling between it and the STM tip, rather than from photons as in the standard photoisomerization. But at the bias voltage used here, the electron energies would seem to be too small, and so that explanation would need to invoke something like a reduction of the energy barrier to isomerization by interactions between the tAB molecule and the substrate. An alternative possibility is that the switch is being induced by vibrational excitation, as has been proposed previously as a possible cause of isomerization4. So further experiments are needed to figure out exactly what is happening here.

References

1. Choi B.-Y. et al. Conformational molecular switch of the azobenzene molecule: a scanning tunneling microscopy study. Phys. Rev. Lett. 96, 156106 (2006)
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2. Donhauser Z. J. et al. Conductance switching in single molecules through conformational changes. Science 292, 2303?307 (2001)
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3. Lastapis M. et al. Picometer-scale electronic control of molecular dynamics inside a single molecule. Science 308, 1000?003 (2005)
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4. Tanaka S., Itoh S. & Kurita N. Possibility of nonequilibrium isomerization of azobenzene triggered by vibrational excitations. Chem. Phys. Lett. 362, 467?75 (2002)
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