A lot of organic chemistry feels like an episode of Mythbusters, albeit a bit undramatic. Imagine a couple of chemists sitting at a white board asking each other, “Is it really possible to build this thing?” Earning a PhD can often depend on figuring out how to overcome the challenges of constructing a molecule.
Sometimes the challenges come because the starting materials don’t react with anything. Sometimes the challenge is that the products will respond everything, often with explosive consequences. But clearing these hurdles is usually more than an intellectual curiosity; in many cases, these strange molecules can tell us something about the basics of chemistry. The molecules may also have useful properties that we want to study, hoping to figure out how to make a stable molecule that behaves in the same way.
In the latest triumph, a Swiss-British team has succeeded in creating a molecule called triangulene. It’s a strange beast: a flat triangle of carbon with a strange combination of bonds that release a few electrons. These electrons are expected to give it magnetic properties, but we have not been able to confirm this because the molecule also reacts with everything it comes into contact with. The trick to making it was making individual molecules by hand – a hand operating a scanning tunneling microscope.
On a gross level, triangulene doesn’t seem all that special. In many ways, it’s similar to what you get when you take a sheet of graphene and crop it. Graphene is a layer of linked rings made of alternating single and double bonds. Triangulene is simply a triangular leaf with three rings on each side. But at that size, two different ideas about chemical bonds have a head-on collision.
If you paid attention to chemistry in high school, you may have discussed graphene’s individual basic unit, a six-carbon ring called benzene. With six bonds between those carbon atoms, single and double bonds (made of one or two electron pairs) can alternate. This yields the neatly symmetrical structure shown here. But it is also possible to think of these electrons as the diffuse cloud that exists according to quantum mechanics. In this view, there are no true single and double bonds; instead, the electrons are evenly distributed among the orbitals of all atoms in the ring.
On some levels, both views are right, and it takes some special circumstances to create a situation where they conflict. Triangulene presents one of those conditions. Because of the geometry, it is impossible to draw a diagram of single and double bonds that allow them to alternate. Instead, you get two unpaired electrons. And in trying to make triangulene, it becomes very clear that these unpaired electrons aren’t just hiding in a diffuse cloud of bonds. Instead, they react angrily to anything that comes close enough, destroying triangulene shortly after it is created.
Until now, the solution has been to make chemical relatives of triangulene with thick carbon chains attached. These effectively block anything that might react, allowing us to study some of the properties of the molecule. But they’re not the real ones, and that apparently bothered the researchers.
While they didn’t come up with a way to make the molecule in bulk, the researchers figured out how to make molecules one at a time. To do this, they started with a relative of triangulene, one in which the critical electrons were safely involved in chemical bonds with hydrogen. They then deposited these molecules on a flat, non-reactive surface (in this case, a sodium chloride or salt crystal) and applied a vacuum to the surface. After locating them, the researchers used a scanning tunneling electron microscope to hit the locations of the hydrogen bonds with a precise energy blast, which dislodged the hydrogen and left behind an unpaired electron. They ended up with a molecule of triangulene (except in cases where a rare oxygen molecule reached the vacuum chamber, in which case it reacted with the molecule).
The authors then tried the same approach with the precursor on a metal surface. This tack worked too, which is a bit surprising given that metals have a lot of reactive electrons. Computer modeling using density functional theory indicates that the electrons are held in orbitals that do not contact the metal surface, preventing reactions from taking place. The same calculations also do a good job of predicting the location of all of the molecule’s electrons, which can be located using an atomic force microscope.
The only thing the authors don’t do much about is characterizing the molecule (they call it “outside the scope of this article”). Triangulene’s free electrons are expected to make it magnetic and give access to some interesting electron spin states. But now that it’s proving to be relatively painless to make a few, that kind of research can’t be long in coming.
Nature Nanotechnology2017. DOI: 10.1038/NNANO.2016.305 (About DOIs).