The scientific community was perplexed in the nineteenth century as to how the atoms in the mystery molecule benzene were ordered. The structure of this “aromatic” molecule was quickly discovered to be remarkably simple: six-carbon and six hydrogen atoms.
But how might these twelve atoms build a chemically stable entity by arranging themselves in space? Friedrich August Kekulé, a chemist who subsequently became a professor at the University of Bonn, shone a light into the darkness.
In the winter of 1861, he sat napping by the hearth, according to legend. Kekulé was instantly struck by the image of a snake eating its own tail. He discovered that benzene’s carbon atoms had to be arranged in a circle, like a tiny wagon wheel.
“This dream ultimately laid the foundation for the massive expansion of the chemical industry toward the end of the 19th century,” says Prof. Sigurd Höger of the Kekulé Institute of Organic Chemistry and Biochemistry at the University of Bonn, who is a member of the Transdisciplinary Research Area “Building Blocks of Matter and Fundamental Interactions” at the University of Bonn. Benzene is an important building block for paints, pharmaceuticals, and plastics, for example.
Hundreds of benzene rings in the shape of a ladder
Although the wheel is frequently referred to as humanity’s first innovation, the ladder is actually far older. Kekulé’s successors at the University of Bonn had long dreamed about molecules with hundreds of benzene rings in the shape of a ladder.
This dream ultimately laid the foundation for the massive expansion of the chemical industry toward the end of the 19th century.Prof. Sigurd Höger
Researchers from the University of Bonn’s Kekulé Institute and Mulliken Center for Theoretical Chemistry, as well as a team lead by Prof. John Lupton of the University of Regensburg’s Institute of Experimental and Applied Physics, have now built such a molecular ladder.
This is a molecule having two tracks of “conjugated polymers,” in which the carbon atoms alternate between double and single bonds. They make up the handrails that you use to climb up a regular ladder.
The researchers created a flexible “snake” out of a precursor molecule that comprised only a single polymer chain and connected polymerizable groups. The second rail of the ladder was made in a subsequent stage for some of the material using a zipper reaction, similar to closing an anorak.
In this method, the team was able to create a rigid “ladder” polymer with two conjugated rails in addition to the polymer with a single conjugated rail. Both polymers were the same length, thus they could now be compared: What effect would it have on the material’s characteristics if a snake was transformed into a ladder?
A scanning tunneling microscope was used to study the structure. One nanometer (a millionth of a millimeter) high, two nanometers broad, and one hundred nanometers long, the molecular ladder is small.
Extensive computer simulations utilizing a novel theory that predicts the specific motions of all atoms within the molecule further validated the shape and unusual rigidity of the ladders compared to the snakes.
Potential building block for electronics
“The ladder structure is retained not only when the molecules are placed on a surface, but also when they are dissolved in a liquid,” says Prof. Lupton of the University of Regensburg. He goes on to say that this property allows energy to travel through the molecule in space, making it a possible building block for optical networks, circuits, and sensors.
In theory, such polymers can be utilized to create novel displays based on organic light-emitting diodes (OLEDs) or to turn light into electricity in a solar cell. When light strikes such a molecule, it is absorbed and a little packet of energy is produced.
The researchers were able to see how these packages moved around the ladder almost unhindered, almost like they were on a zipline. On the other hand, the open snake-like polymers do not exhibit this effect. The packages move down the “snakes” and lose energy, similar to regular polymer molecules.
Kekulé’s shattered dream
“While old Kekulé ‘saw’ the single-molecule as a ring, he certainly never dreamed that there would one day be giant molecules of such rigidity that they are unable to bite their own tails,” says Höger, summarizing the result with a wink.