When I was in graduate school, a good friend of mine (who was studying under the same mentor as I) decided to take on a research project involving pentacene. Pentacene is a wonderfully simply organic molecule that has promising electronic behavior. So named because of the five (penta-) aromatic benzene rings that are fused together, the molecule resembles a flat rod of hexagons lined up in a row. Due to the overlap of molecular orbitals present in pentacene, the center ring becomes very electron rich. This array of overlapping pi orbitals is what makes pentacene so attractive for use in electronics; the material is being used for transistors as well as molecular wires. However, the very thing that makes pentacene attractive (the electron-rich central ring) is also pentacenes greatest weakness.
In organic chemistry most bond formation begins with electrons present on an atom reaching out and “attacking” another atom; the electrons then become the bond between the two atoms. As you might expect, atoms that have an excess of electrons make better “attackers” or nucleophiles, as they are called. Pentacene is so rich in electrons on its central ring that it becomes a little too reactive. It drove my friend in graduate school to distraction – he had to keep all of his materials in the freezer, and had to shield the material from oxygen present in the air. Oxygen is rapidly attacked by the electrons from the central ring, oxidizing the pentacene and destroying the desirable electronic properties. This makes it a rather difficult compound to use in organic synthesis as its temperature sensitivity limits the number of possible reactions that can be used, and the oxygen sensitivity makes it a pain to handle.
Adding to pentacenes limitations is the fact that although the molecule is flat and long, like a rigid block, it doesn’t pack into a very attractive crystalline structure. For electronics to be successful using this molecule, the crystal structure has to consist of overlapping arrays of the molecules; that way the electrons can jump from one molecule to another, which provides the avenue of charge conduction across the bulk of the material. Pentacene stubbornly refuses to crystallize in an overlapping array, instead precipitating in an alternating arrangement that chemists call “edge to face”. So, despite the attractive electronic behavior of a single molecule, pentacene in bulk is hard to work with and often gives you an undesirable crystalline conformation.
Last night I read an article published by a group of Chinese chemists which makes a slight change to the molecular structure of pentacene, and as a result, they have solved the numerous drawbacks inherent to the fully hydrocarbon structure. They published their results in the Journal of Materials Chemistry. Their technique is to replace the center six-atom ring of pentacene with a five-membered ring containing nitrogen. A few nitrogens and sulfurs are sprinkled amongst the rest of the structure, all of which are still made up of six-membered rings. The resulting molecule is called pyrrolobisbenzothiazine. This molecule has a higher stability than pentacene while still offering comparable electronic properties.
One important and exciting difference between this new molecule and pentacene is that the nitrogen-containing compound stacks in the more desirable face-to-face arrangement, forming an extended 3-D array of molecular rods. This allows the pi orbitals of adjacent molecules to overlap, providing a route for electrons to pass through the crystal. The synthesis of this new molecule is not complicated and the final molecule is stable to both heat and oxygen, very much unlike pentacene. I have to chuckle when I think about this new molecule, as I’ve read so many articles in the past where chemists have struggled mightily to convince pentacene to behave in the way that they desire. It turns out that a simple swap of atoms in the molecule solves all of the problems. This material should be put to use in transistors and flat-screen displays very soon.
This source of this article can be found at: http://www.rsc.org/Publishing/Journals/JM/article.asp?doi=b809486a