Chemists at the Massachusetts Institute of Technology (MIT) have come up with a way to make molecules known as acenes more stable. These molecules, could be useful as organic light-emitting diodes (LEDs) or solar cells, among other possible applications.
Acenes are chains of fused carbon-containing rings that have unique optoelectronic properties that make them useful as semiconductors. Optoelectronics is the study and application of electronic devices and systems that find, detect and control light.
According to an MIT News release, acenes can also be tuned to emit different colors of light, which makes them good candidates for use in organic LEDs. The color of light emitted by an acene is determined by its length, the news release explains. However, as the molecules become longer, they also become less stable, which has hindered their widespread use in light-emitting applications.
Now chemists at MIT have devised a method to make these molecules more stable, allowing them to synthesize acenes of varying lengths. “Using their new approach, they were able to build molecules that emit red, orange, yellow, green, or blue light, which could make acenes easier to deploy in a variety of applications," the release adds.
“This class of molecules, despite their utility, have challenges in terms of their reactivity profile,” Robert Gilliard, the Novartis Associate Professor of Chemistry at MIT and the senior author of the new study, says in the news release. “What we tried to address in this study first was the stability problem, and second, we wanted to make compounds where you could have a tunable range of light emission.”
MIT research scientist Chun-Lin Deng is the lead author of the paper, which was published in Nature Chemistry on 5 December.
According to the release, acenes consist of benzene molecules — rings made of carbon and hydrogen — fused together in a linear fashion. Because they are rich in sharable electrons and can efficiently transport an electric charge, they have been used as semiconductors and field-effect transistors, the release explains.
Recent studies have also shown that acenes in which some of the carbon atoms are replaced (or “doped”) with boron and nitrogen have even more useful electronic properties. The release adds: “However, like traditional acenes, these molecules are unstable when exposed to air or light. Often, acenes have to be synthesized within a sealed container called a glovebox to protect them from air exposure, which can lead them to break down. The longer the acenes are, the more susceptible they are to unwanted reactions initiated by oxygen, water, or light.”
In an effort to make the acenes more stable, Gilliard decided to use a ligand (an ion or molecule which donates a pair of electrons to the central metal atom or ion to form a coordination complex) that his lab has previously worked with, known as carbodicarbenes. In a study published last year, they used this ligand to stabilize borafluorenium ions, organic compounds that can emit different colors of light in response to temperature changes, the release says.
For this new study, Gilliard and his co-authors developed a new synthesis that allowed them to add carbodicarbenes to acenes that are also “doped” with boron and nitrogen. With the addition of the new ligand, the acenes became positively charged, which improved their stability and also gave them unique electronic properties, the release adds.
“Using this approach, the researchers created acenes that produce different colors, depending on their length and the types of chemical groups attached to the carbodicarbene. Until now, most of the boron, nitrogen-doped acenes that had been synthesized could emit only blue light,” the release explains.
These acenes, the release says, also remain stable in both air and water, which could lend to useful applications in imaging and medical use cases.
“Red emission is very important for wide-ranging applications, including biological applications like imaging,” Gilliard says in the release. “A lot of human tissue emits blue light, so it’s difficult to use blue-fluorescent probes for imaging, which is one of the many reasons why people are looking for red emitters.”
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