Author: Madeleine Jepsen
Institution: Hillsdale College
A micrograph of the crystal of a tellurazole oxide developed at McMaster University. Credit: Vargas Laboratory, McMaster University
Imagine a jigsaw puzzle that solves itself when someone takes it out of its box. Researchers at McMaster University have synthesized a molecule able to self-assemble into ring structures. These molecules are the chemical equivalents of self-assembling puzzle pieces.
The molecules, known as tellurazole oxides, have unique properties that allow them to assemble into cyclic molecules of their own accord when placed in solution. These findings mark a major advance in supramolecular chemistry, a field that examines intermolecular forces, or the bonds formed between molecules.
The cyclic tellurazole oxide compounds rely on a type of bonding similar to the hydrogen bonding between molecules of water. In hydrogen bonding, a partially positive hydrogen atom is attracted to the partially negative oxygen atom of another water molecule. Collectively, the many hydrogen bonds that form in a body of water strengthen the connections between the individual molecules, determining the physical and chemical properties of water.
Crystal engineers take advantage of a similar phenomenon known as halogen bonding as a way to manipulate the compound into cyclic structures. Dr. Ignacio Vargas Baca’s laboratory at McMaster University has expanded upon this halogen bonding, using larger, heavier atoms known as chalcogens to bring about the same bonding effect. Bonds between tellurazole oxides depend on the forces of attraction between partially positive chalcogens and partially negative oxygen atoms, both of which are found within each molecule.
“Essentially, these molecules, because they have these two regions of positive charge and negative charge, tend to associate with each other,” says Dr. Vargas, associate professor in McMaster's Department of Chemistry and Chemical Biology. “But they do so in this particular case, making rings.”
To synthesize tellurazole oxides, the research team started with a molecule containing the chalcogen tellurium and a partially negative oxygen atom in its structure. After synthesizing the tellurazole oxides, which required a number of unstable intermediate compounds, the team produced surprisingly stable compounds capable of self-assembling into six- and four-membered rings.
Unlike other molecules that include tellurium, these rings have bonds capable of withstanding atmospheric oxygen and room-temperature conditions.
“What is really remarkable in this case is that when you put these rings into solution, they survive — they exist in solution,” Vargas says. “That is something that my colleagues working in the field of halogen bonding have been trying to do for a while.”
This chemistry has a wide array of potential applications, including communication technology, gas storage, and the acceleration of other types of chemical reactions.
“The point is that the molecules that we have made — these rings that we have made — we have shown that they're able to bind to metal ions, and by doing that, they give us the means to create a special environment around metal ions that could not be made before,” Vargas says. “When you do that, you change the chemical properties of the metal ion. When you do that, one can expect naturally that there will be some eventual application in catalysis.
The attraction between the tellurium and oxygen atoms allows electrons to move through a series of connected molecules, making the material a semiconductor and thus a potential candidate for use in electronic applications.
“They also change refractive index when they get exposed to an electric field, and it is precisely this change of refractive index with application of electric field that can be used to support, modulate, or create pulses of light,” Vargas says.
The rings, when organized into a crystal structure with large enough cavities, can also be used to safely store gasses, giving the compounds further potential in the field of crystal engineering.
Now that the team has crystallized these compounds and examined their composition through X-ray crystallography, they hope to further understand the forces behind the molecular self-assembly in future research.
"This is a seed we have found — one we have never seen. It has sprouted, and now we need to see how tall the tree will grow and what kind of fruit it will bear," Vargas says. "Once we understand the properties of these new materials, we can look at their potential applications."
Sources:
Interiew with Dr. Vargas Barca
Press release: http://www.eurekalert.org/pub_releases/2016-04/mu-mra041816.php
Published article: http://www.nature.com/ncomms/2016/160419/ncomms11299/pdf/ncomms11299.pdf