The Congress of Young Scientists: St Petersburg University chemists learn to control the luminescence of crystals
Scientists at St Petersburg University and Tomsk Polytechnic University have developed a method to control the emission of organometallic compounds. This approach will increase the efficiency of light-emitting materials through the targeted use of non-covalent chemical interactions. The model of the luminophore molecule was presented at the exhibition of the 2nd Congress of Young Scientists. This year, the 2nd Congress of Young Scientists is a key event within the framework of the Decade of Science and Technology.
The constant increase in electricity consumption on our planet means developing modern energy-saving technologies. Their development therefore is among the most important areas of research. Today, organic light emitting diodes (OLED) are considered the most energy-efficient light source. They are used in most displays of most popular electrical appliances from fitness bracelets to smartphones, cameras, and TVs.
The research findings are published in the international journal ACS Omega. The research project is supported by a grant from the Russian Science Foundation.
Among the most promising light-emitting materials used in manufacturing process of OLED devices are organometallic compounds. Organometallic compounds are chemical compounds where the metal atom is directly bonded to one or more carbon atoms. Such compounds have been used in the light-emitting layers of OLED devices; luminescent sensors; and biological labels in medicine. Yet, lifetime of blue diodes in such devices is less than lifetime of red and green diodes. So, chemists are actively engaged in developing new efficient sources of visible luminescence.
The scientists at St Petersburg University, together with the chemists at Tomsk Polytechnic University, synthesised a new type of the luminescent material. Additionally, they identified a way to control its luminescence. The optical properties of organometallic compounds are often determined not only by their molecular structure, but also by the system of bonds between molecules, said Mikhail Kinzhalov, Principal Investigator of the project and Associate Professor in the Department of Physical Organic Chemistry at St Petersburg University. Thus, the molecules of the substance are combined into supramolecular structures. Change in the mutual arrangement of molecules in this structure can lead to change in the properties of the entire material and make it more efficient.
To synthesise new molecules, the scientists used organic compounds of platinum. In these organic compounds, the metal atom and the organic fragment are a single system. This improves optical properties. To increase the strength of the luminescence, it is necessary to increase the rigidity of the substance. To this end, the researchers used luminophore molecules in the supramolecular structure developed by intermolecular halogen bonds of iodine and chlorine atoms.
The development of the method for controlling the luminous abilities of crystals of organometallic compounds was carried out within the framework of a grant from the Russian Science Foundation. The studies were carried out on the equipment of the resource centres of the Research Park at St Petersburg University. Among these resource centres are: the Magnetic Resonance Research Centre; the Centre for X-Ray Diffraction Studies; the Chemical Analysis and Materials Research Centre; and the Centre for Optical and Laser Materials Research.
‘As it became clear during the study, replacing one organic solvent with another solvent during the crystallisation process enabled us to obtain luminophore with different optical properties. For example, if we replace the solvent chloroform with acetonitrile, we obtain orange luminophore instead of green luminophore. In this case, both substances will have the same composition and molecular structure. The difference in the colour of the radiation is explained by the different mutual arrangement of molecules in crystals and their interaction with each other,’ said Mikhail Kinzhalov.
The X-ray diffraction method enabled the scientists to discover that in the crystals of the orange luminophore there is an intermolecular interaction of platinum atoms of two neighbouring molecules, and the green luminophore lacks such "relationships". Additionally, the chemists at St Petersburg University found that the efficiency of the orange luminescence is 24 times higher than the green luminescence. The experiments confirmed that the increase in the luminescence efficiency is systematic and is due to the interaction between platinum atoms. Thus, by changing the conditions of crystallisation of the substance, the chemists can control the luminescence of given substances.
Yet, there is no universal way to obtain luminophore crystals with desired optical characteristics, said Mikhail Kinzhalov, Principal Investigator of the project. This requires more experimental data on the effect of certain non-covalent bonds on the properties of the substances. The scientists at St Petersburg University are planning to develop a technology to control the luminescence of substances and, by doing so, ensure the transition to resource-saving energy through the use of organic light sources. Earlier, the researchers at St Petersburg University synthesised luminescent nanoparticles to conduct medical diagnostic testing with contrast.