Making molecules stand up

It’s a longstanding dream of chemists to be able to manipulate individual molecules, orientating them in three dimensions and bringing them together in specific ways. In a new study published in the journal Nature, a German team reveals it has done just that: lifting a single flat molecule up onto its edge.

Synthesising complex molecules is sometimes likened to building structures using Lego. Chemists take smaller molecules and “click” them together to make larger ones with particular properties.

In reality, though, the process is more like building with Lego blocks that you can’t hold. All you can do is mix together bags of different pieces, relying on their natural preference to join together in specific ways. In many ways, it’s a wonder that chemists are able to make the complex molecules that form our drugs and materials as efficiently as they do.

Since the 1980s, it has been possible to image and even manipulate individual atoms and molecules on a surface, using scanning probe microscopy. Unlike light microscopy and even electron microscopy, scanning probe microscopy is not limited by diffraction, and can achieve much higher resolutions. This is because the microscope uses a tiny physical probe, often with a single-molecule tip, to scan the surface of an object.

In a landmark 1990 study, IBM scientists were able to spell out their company logo using individual xenon atoms. But xenon atoms are large and spherical, so the task was easy compared to the daunting task of controlling organic molecules, which often have complex three-dimensional shapes.

In recent years, scientists have made great progress in manipulating individual molecules on a surface, but controlling their 3D orientation remains elusive.

Now, a group from the Jülich Research Centre in Germany, led by Ruslan Temirov, has managed to flip a single, flat organic molecule on to its edge using the tip of a scanning probe microscope.

The small organic molecule, essentially a tiny fragment of graphene, normally lies flat on a silver surface. But by anchoring one end of it to the surface, and manipulating the other end with the microscope probe, the molecule was moved into an upright position.

“Until now, it was assumed that the molecule would revert back to its favoured position and lie flat on the surface,” says first author Taner Esat.

“But that is not the case. The molecule is surprisingly stable in the upright orientation. Even when we push it with the tip of the microscope, it does not fall over; it simply swings back up again. We can only speculate as to the reason for this.”

The team was able to show that the molecule displayed different electronic properties when upright, compared to its flat orientation. This could have applications in nano-scale electronics or in the generation of holograms.

The field of single-molecule fabrication still has a long way to go before we see complex nanostructures being assembled one atom at a time. But as this work shows, scientists, and molecules, are rising to the task.

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