Chains of fused carbon-containing rings have unique optoelectronic properties that make them useful as semiconductors. These chains, known as acenes, can also be tuned to emit different colors of light, which makes them good candidates for use in organic light-emitting diodes.
The color of light emitted by an acene is determined by its length, but as the molecules become longer, they also become less stable, which has hindered their widespread use in light-emitting applications.
MIT chemists have now come up with a way 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.
“This class of molecules, despite their utility, have challenges in terms of their reactivity profile,” says Robert Gilliard, the Novartis Associate Professor of Chemistry at MIT and the senior author of the new study. “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 appears today in Nature Chemistry.
Colorful molecules
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 (transistors that use an electric field to control the flow of current in a semiconductor).
Recent work has shown that acenes in which some of the carbon atoms are replaced, or “doped,” with boron and nitrogen have even more useful electronic properties. 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.
To try to make acenes more stable, Gilliard decided to use a ligand 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.
For this 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.
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.