The Elusive Heptacene Isolated Seventy-Five Years After First Coming into Limelight

Heptacene is isolated after 75 years of research. Image Source: The Royal Society of Chemistry

On March 20, 2017, a team of scientists from Germany and the US isolated heptacene. A compound that holds immense potential for the electronic appliances industry. With this, scientists have finally put an end to a seventy-five year-long quest to isolate the compound, which was first claimed to be synthesized in 1942.

UNDERSTANDING HEPTACENE

Heptacene is a polycyclic, aromatic hydrocarbon compound that is made of seven linearly-fused benzene rings. It is the longest acene to be made as a pure solid. It has long been pursued for its applications in light-emitting diodes or solar cells.

Structure of Heptacene
Chemical structure of Heptacene

It increases the efficiency with which these cells capture light, by enabling a process called singlet fission, where two molecules of the cell capture one quantum of solar light to generate two charged carriers. This enhanced efficiency could tremendously impact the functioning of the solar energy industry, which is rapidly gaining momentum as one of the most viable and cost-effective alternatives to conventional fossil fuels.

In doing so, heptacene could potentially reduce the planet’s carbon footprint and help reverse climate change.

THE SEARCH FOR HEPTACENE

Heptacene first shot into the spotlight in 1942, when the German chemist – Erich Clar announced the synthesis of two of the longest acenes, hexacene and heptacene.

However, it was subsequently found that he had only synthesized the related non-linear hydrocarbons. The quest to isolate heptacene reached its next milestone in 2006, when a team spearheaded by Dr. Douglas Necker – a chemist and founder of the Center for Photochemical Science at Bowling Green State University synthesized heptacene and hexacene, the other acene that had been claimed to have been synthesized by Dr. Clar nearly four decades ago.

Unfortunately, this team too met failure as both these hydrocarbons were found to decompose in only a few hours’ time unless they were stabilized in a polymer.

In 2017, a team of German and American scientists found that heptacene could be isolated by simply heating the X-shaped dimers to 300°C for a few minutes. Heating the compound helped to release the free compound which could be stored at room temperature for several weeks.

“It wasn’t clear if heptacene could exist outside of a stabilising matrix,” says Holger Bettinger from the University of Tübingen. “We were thinking so much about the stability issues that we wouldn’t do the obvious for a long time,” he adds.

Synthesis mechanism of Heptacene
Chemical route employed by Bettinger’s group. Image Source: American Chemical Society

This finding was especially significant since heptacene’s inherent polyradical character makes it quite unstable. Being able to isolate heptacene also presents an opportunity for researchers to better understand the nature and properties of longer-chain acenes which, some researchers believe, may function as semi-conductors or insulators.

“Acenes are models for very narrow graphene nanoribbons, but it is not clear what their properties would be, as nothing is known beyond nonacene,” says Bettinger. “People have argued they might be metals, other say they might be semiconductors or even insulators.”

The team has also worked on isolating octacene and nonacene, but has only been able to embed them in solid argon at the temperature of -243.15 degrees Celsius.

CLOSING THOUGHTS

The immense potential of heptacene notwithstanding, scientists have yet to devise a method to include this compound in electronic devices owing to its inherent instability, which could affect the longevity of these devices.

However, that does not detract from the significance of this milestone, which is sure to pave the way for a deeper understanding and newer applications of long-chain acenes.

Source American Chemical Society The Royal Society of Chemistry

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