The structure of the obtained fluorine-containing furan-phenylenes and a light transistor based on a molecule of a partially fluorinated derivative.
Russian scientists have obtained new organic materials that can simultaneously emit light and conduct charges. This unusual property will allow in the future to use them to create more advanced and cheaper displays. All this is due to the presence of fluorine atoms in certain parts of the molecule. The results of the research, supported by grants from the Russian Science Foundation (RSF), are published in the journal Advanced Functional Materials.
OLED displays are widespread in modern technology: they are installed in TVs, computers, laptops, in concert and sports halls. Such screens provide high brightness and contrast, but have one significant drawback: the matrix that creates the image consists of many LEDs, turning on and off each of which is controlled by a separate transistor, transmitting electric current to the emitters. Hundreds and thousands of such chains greatly complicate the matrix production technology, so scientists are developing special devices – light transistors – that combine the ability to emit light and switch current.
It is problematic to make materials that combine both necessary properties, since they are partly mutually exclusive: for a substance to conduct charges well, the molecules in it must be located very close to each other. At the same time, dense “packing” often hinders luminescence: neighboring molecules “extinguish” each other. Therefore, researchers from the Novosibirsk Institute of Organic Chemistry named after N.N.Vorozhtsov (Novosibirsk), the Institute of Synthetic Polymeric Materials named after N. S. Enikolopov and Moscow State University named after M.V. Lomonosov (Moscow) started looking for the optimal material.
As the basis for the new molecule, the scientists took furan-phenylene co-oligomers – organic compounds containing chains of aromatic rings made of carbon, oxygen and hydrogen atoms. Moreover, three rings in these molecules are six-membered, like a honeycomb, and two are five-membered. Earlier studies have shown that furan-phenylenes emit bright light, therefore they can be used in optoelectronics, they also have good solubility and molecular rigidity. In addition, some “building blocks” for their synthesis can potentially be obtained from natural raw materials. However, these compounds are not capable of conducting negative charges (electrons), and therefore cannot be used in light transistors. To solve this problem, chemists synthesized a series of furan-phenylene derivatives by selectively replacing hydrogen atoms with fluorine atoms. It was chosen as a substitute because more than other chemical elements it “pulls” electrons from neighboring atoms, thereby creating better conditions for the redistribution of charges in the molecule.
The synthesized compounds differed in the number of fluorine atoms (from four to fourteen) and their position (the substituents were located on different phenylene rings). It turned out that fluorine-containing molecules were significantly more stable to oxidation than conventional furan-phenylenes, since they had a more energetically favorable electronic structure. Among the other advantages of furan-phenylene co-oligomers, bright luminescence, molecular rigidity, and solubility are retained with the correct arrangement of substituents.
Scientists have obtained crystals and thin films from synthesized molecules and investigated their properties. Some samples, where the arrangement of fluorine atoms and the crystal structure were most favorable, carried out both positive and negative charges well due to the fact that the molecules created a kind of “tunnels” for their movement. Such transport of charges of both signs allowed researchers to manufacture unique samples of light transistors based on thin films: the light generation efficiency reached 0.6%, which corresponds to the level of the best world developments.
“Our research has allowed us to obtain unique molecules that combine the ability to efficiently photo- and electroluminescence and charge transport. Materials based on them will make it possible to create light transistors for modern displays and other light-emitting devices, which will reduce the cost of their production, increase reliability and, possibly, will allow finding new applications for organic electronics devices, ”says Maxim Kazantsev, project manager for a grant from the Russian Science Foundation, Ph.D. , Head of the Laboratory of Organic Electronics, NIOCh SB RAS.
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