Novel Reaction System to Produce Useful Semiconductor Molecules

In the electronics field, organic solar cells find a range of applications, particularly to develop novel electronic devices such as wearable devices.

Such batteries are often composed of organic semiconductor molecules, which are strong yet light. Therefore, researchers worldwide have aimed to find innovative schemes for the growth of such semiconductor molecules.

However, synthesizing such molecules generally involves the use of valuable rare metal catalysts. Apart from leading to high manufacturing costs, the chances of contamination of the metal catalysts make the process complicated.

For this purpose, in a new study reported in Angewandte Chemie International Edition, a team of researchers from Okayama University, including Professor Seiji Suga and Associate Professor Koichi Mitsudo, designed an innovative reaction system to produce thienoacene derivatives—essential building blocks in the synthesis of organic semiconductors.

The focus of the researchers was to build carbon-sulfur (C-S) bonds via organic electrolysis, which is an eco-friendly reaction.

We focused on C-S bonds, as they are an abundant and significant in the field of pharmaceuticals and materials science, such as in certain antidepressant and antifungal medications.

Seiji Suga, Professor, Okayama University

The C-S bonds are traditionally built through a technique known as “transition metal-catalyzed cross-coupling,” which involves the use of rare metal catalysts. This makes the reaction costly and, therefore, impossible.

The researchers of this study, therefore, sought a different method, known as “electrochemical carbon-heteroatom bond formation,” which is an environment-friendly reaction that needs only mild conditions.

Despite the fact that researchers have developed various novel electrochemical carbon-heteroatom coupling reactions in the past, to date, such reactions had never been utilized to synthesize thienoacenes.

Over the past several years, we were interested in the development of new methods for thienoacene synthesis, acene derivatives that have a good track record in organic electrochemistry and are attractive candidates for useful organic materials.

Seiji Suga, Professor, Okayama University

After finding the foundation of their study, the researchers went in-depth to determine new electrochemical techniques for the synthesis of thienoacenes. They discovered that the preferred C-S bond formation happened smoothly when a “bromide” ion was present, which serves as a strong promoter of the reaction.

With this tactic, the researchers were successful in synthesizing a kind of thienoacene derivatives known as the “π-expanded thienoacene derivatives.” Fascinatingly, this study is the first to realize successful C-S bond formation for producing thienoacene derivatives.

Our study was the first to report an electro-oxidative dehydrogenative reaction to produce C-S bonds for thienoacenes. We found that bromide ion, which catalytically promotes the reaction as a halogen mediator, is essential to the reaction.

Seiji Suga, Professor, Okayama University

This study lays the foundation for synthesizing future organic semiconductor molecules using an economical method, without using high-cost metal catalysts.

“The key to this research lies in the method of 'electrochemical synthesis,' which is a clean and renewable source of energy,” concluded Suga.

Using these results, the Okayama University researchers hope to realize sustainable organic synthesis with the least effect on the environment. The study is also a crucial step toward reaching the UN sustainable development targets and thus promoting an improved future for humanity.

Journal Reference:

Mitsudo, K., et al. (2020) Electrochemical Synthesis of Thienoacene Derivatives: Transition-Metal-Free Dehydrogenative C−S Coupling Promoted by a Halogen Mediator. Angewandte Chemie International Edition. doi.org/10.1002/anie.202001149.