A Korean research team has devised a new synthesis technique that can fundamentally reconstruct the molecular skeleton of organic semiconductors.
A joint research team led by Professors Park Young-seok and Min Seung-kyu of the Department of Chemistry at the Ulsan National Institute of Science and Technology (UNIST) announced Tuesday that they have developed an original repetitive synthesis method that sequentially inserts boron and oxygen bonds into the edge of the skeleton of pentacene, an organic semiconductor material.
Organic semiconductors, which replace rigid conventional silicon semiconductors and are used in next-generation flexible and lightweight displays, solar cells and sensors, exhibit characteristics that vary greatly depending on the structure of their molecules and the arrangement of their atoms. Until now, it has been difficult to insert specific elements into the molecular skeleton itself, which is composed of carbon rings, in a desired form. As a result, properties have mainly been modified by attaching side chains or other functional groups to the outside of the skeleton.
However, the research team overcame these existing limitations by directly inserting oxygen and boron atoms into the skeleton of pentacene, which consists of five benzene rings linked side by side, thereby modifying the molecular skeleton itself.
According to Dr. Jung Sung-hwa, who participated as the first author, the synthesis method takes as one cycle an "iodination reaction," which adds an iodine atom at a specific position, followed by the chemical reaction of attaching a boron reagent and then closing the ring. By repeating this cycle to expand the benzene rings one by one, a structure in which boron-oxygen bonds are continuously connected along the edge of the pentacene skeleton can ultimately be completed.
Applying this repetitive synthesis technique, the researchers successfully produced three types of pentacene derivatives with different positions of boron-oxygen bonds. The three newly synthesized substances all absorbed and emitted light at different wavelengths, and their "fluorescence quantum yield" — the efficiency with which absorbed light is re-emitted as light — all exceeded 0.70. Such high luminous efficiency suggests that the materials can be widely applied to high-performance fluorescence sensors, light-emitting organic semiconductors and optoelectronic materials in the future.
"This study presents a method for stepwise synthesis of new acene derivatives with continuous boron-oxygen bonds," Professor Park said. "In that it allows the design of molecules with desired lengths and arrangements through repetitive synthesis, we expect it to broaden the chemical diversity of organic semiconductor molecules and contribute to the development of next-generation organic semiconductor design and synthesis."
The research, supported by the National Research Foundation of Korea (NRF) under the Ministry of Science and ICT and by the Ministry of Trade, Industry and Energy, was published online on April 16 in Angewandte Chemie International Edition, the most authoritative international journal in the field of chemistry.
