Triangulene. No matter how hard you try, you can’t put double bonds on the middle rings without having a carbon atom form five bonds, which it refuses to do. So you end up with unpaired electrons instead.
A lot of organic chemistry feels like an episode of Mythbusters, if a bit of an undramatic one. Imagine a couple of chemists sitting at a white board, asking each other, “Is it actually possible to build this thing?” Getting 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 won’t react with anything. Sometimes, the challenge is that the products will react with everything, often with explosive consequences. But clearing these hurdles is usually more than an intellectual curiosity; in many cases these odd molecules can tell us about basic principles of chemistry. The molecules may also have useful properties that we’d like to study in the hope that we can figure out how to make a stable molecule that behaves the same way.
In the latest triumph, a Swiss-UK team has managed to make a molecule called triangulene. It’s a strange beast: a flat triangle of carbon that has an odd combination of bonds that leave a couple of electrons free. These electrons are expected to give it magnetic properties, but we haven’t been able to confirm this because the molecule also reacts with everything it comes in contact with. The trick to making it was crafting individual molecules by hand—a hand that operated a scanning-tunneling microscope.
At a gross level, triangulene doesn’t seem that special. In many ways, it looks like what you’d get if you took a sheet of graphene and trimmed it down. Graphene is a sheet of linked rings made of alternating single and double bonds. Triangulene is simply a triangle-shaped sheet 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 in high school chemistry, you might have discussed the basic individual unit of graphene, a six-carbon ring called benzene. With six bonds between those carbons, single and double bonds (made of one or two electron pairs) can alternate. This produces the neatly symmetric structure shown here. But it’s also possible to view these electrons as the diffuse cloud that quantum mechanics tells us exists. In this view, there aren’t any real single and double bonds; instead, the electrons are evenly distributed among the orbitals of all the atoms in the ring.
On some levels, both views are right, and it takes some special circumstances to create a situation where they come into conflict. Triangulene presents one of those circumstances. Because of the geometry, it’s impossible to draw a diagram of single and double bonds that allows them to alternate. Instead, you end up with two unpaired electrons. And in trying to make triangulene, it becomes very clear that these unpaired electrons don’t simply hide away in a diffuse cloud of bonds. Instead, they angrily react with everything that comes close enough, destroying triangulene shortly after it’s made.
So far, the solution has been to make chemical cousins of triangulene that have bulky carbon chains attached. These effectively block off anything that might react, which lets us study some of the properties of the molecule. But they’re not the real thing, 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 so, they started with a relative of triangulene, one where the critical electrons were safely involved in chemical bonds with hydrogen. They then deposited these molecules on a flat, nonreactive surface (a sodium chloride—aka salt—crystal in this case) and pulled a vacuum on the surface. After locating them, the researchers used a scanning tunneling electron microscope to hit the locations of the hydrogen bonds with a precise jolt of energy, which popped the hydrogen loose, leaving an unpaired electron behind. They ended up with a molecule of triangulene (except in cases where a rare oxygen molecule made it into 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 also worked, which is a bit surprising given that metals have many reactive electrons. Computer modeling using density functional theory indicates that the electrons are held in orbitals that don’t contact the metal surface, preventing any reactions from taking place. The same computations also do a good job of predicting the location of all the molecule’s electrons, which can be located using an atomic force microscope.
The one thing the authors don’t do much of is characterize the molecule (they call it “beyond the scope of this paper”). Triangulene’s free electrons are expected to make it magnetic and to give it access to some interesting electron spin states. But now that it turns out to be relatively painless to make some, those sorts of studies can’t be long in coming.