University of Silesia in Katowice

Organic compounds in electronics – future or already present?

| Sławomir Kula, PhD |

Throughout the past years, the popularity of electronic devices based on organic light-emitting diodes has increased rapidly. The amount of these types of devices on the market is higher each year. Interestingly, OLED diodes are used in creating displays and screens of various sizes and use. Moreover, prices of these products go lower which increases their availability and competitiveness. Currently, the income from the devices based on OLED is counted in millions of dollars. Its popularity results from a great number of advantages such as a full spectrum of colours, incredible contrast, smooth movement, and a deeper black level. Additionally, their stable time of work has significantly increased. The dynamic development of OLED can be observed in the gigantic number of patents and publications published each year. Recently, it is hard to list even a part of research problems regarding OLED dealt with in the literature. However, great attention is given to the emitters, in particular blue light emitters. They face high-level standards which aim at providing high efficiency, proper quality of colours as well as long and stable work. Blue emitters are additionally used to produce light of different colours and efficient decrease in a device’s energy use. In literature, there are multiple examples of blue light emitters. Many of them require a costly and many-stage synthesis. It significantly restricts their possibilities of practical use. Thus, there is a need for designing cheap and stable chemical compounds, which greatly improve the devices’ effectiveness, chromaticity coordinates (colour purity), and durability.

An interesting alternative for many compounds studied in the aspect of blue light emitters are derivatives of phenanthro[9,10-d]imidazole. They can be successfully obtained with a one-stage condensation reaction. Importantly, it does not require any catalytic system or hazardous reagents. The formula of phenanthro[9,10-d]imidazole derivatives can be properly shaped by choosing substrates. By the use of aldehyde, we introduce a substituent, i.e. another group of atoms, on the position of carbon atom (C2) in the central imidazoline ring. On the other hand, a properly selected amine is responsible for a substituent located at the nitrogen atom (N1). In the case there is a hydrogen atom (N-H) in the N1 position, we can change it to another substituent (in particular an alkyl chain) in an independent reaction. Additionally, we can also modify the core of the phenanthro[9,10-d]imidazole using the Suzuki coupling, the Ullmann reaction, or Buchwald-Hartwig reaction. Interestingly, the mentioned core of the phenanthro[9,10-d]imidazole shows high thermal stability, which is often increased by the introduced substituents. Moreover, due to its electron deficiency, it is an acceptor, i.e. can receive electrons. Furthermore, phenanthro[9,10-d]imidazole are characterised by a high level of carrier motion and good film formation. Blue OLED diodes produced with the use of such compounds are high-performance and often meet the criteria for colour purity.

Within the research realised thanks to the “Freedom of Research” call for proposals, four new chemical compounds from the group of phenanthro[9,10-d]imidazole derivatives have been obtained. The formula of synthesised compounds has been confirmed by the NMR spectroscopy and elemental analysis. What is more, the mechanism of the condensation reaction enabling the synthesis of the derivatives has been analysed in detail. Next, the obtained compounds have been compared in terms of thermal stability as well as optical and electrochemical properties. The results were compared with the DFT calculations. Eventually, the electroluminescence ability of the prepared compounds has been studied. The realisation of particular physical and chemical research has enabled the evaluation of the considered derivatives in the context of further applied research and possibilities for the practical use of the obtained materials.

The research has been published in the article: S. Kula, P. Ledwon, A. M. Maroń, M. Siwy, J. Grzelak, M. Szalkowski, S. Maćkowski, E. Schab-Balcerzak, Synthesis, photophysical properties and electroluminescence characterization of 1-phenyl-1H-phenanthro[9,10-d]imidazole derivatives with N-donor substituents, „Dyes and Pigments”, 2021, 192, 109437, IF= 4.889, points according to the Ministry: 100.