Overview of Highly Fluorescent Materials Used in Organic Electronics

Organic electronics are establishing a new foundation for low-cost microelectronics technology. From vacuum tubes through the rise of organic devices, transistors, light-emitting diodes and photovoltaic cells, the development of microelectronics represents a historical evolution. Alan J. Heeger, Alan G. MacDiarmid and Hideki Shirakawa were awarded the Nobel Prize in 2000 for their work in "the discovery and development of conductive polymers." Particularly following this development, the importance placed on conductive and semiconducting polymers that could be used not only in Light Technology but in many other fields has increased further. The field of "Organic Electronics" is used to create a range of technologies from solar cells that you print and use to rolled-up screen displays you can put in your pocket. "Organic Electronics" takes its name from the use of "organic" semiconductors made with carbon-based materials instead of silicon, as in traditional electronics. While optoelectronic coatings are still being developed, organic electronics are already transforming the technology we purchase. Organic semiconductor materials typically appear in two forms: small molecules containing tens or hundreds of atoms, or long chains containing thousands of repeating molecules (plastics).

For a long time, semiconducting plastics and small molecules lagged behind the inorganic (carbon-free) semiconductors used in modern computer chips.

However, following recent research and developments, organic semiconductors are demonstrating sufficiently good performance and are being commercialized for use in new applications. Modifications impossible to achieve with silicon derivatives are being made with organic semiconductors. The most striking of these modifications is converting organic material into ink. This means electronic circuits can be printed with the potential to produce components as quickly as newspaper printing. Plastic-based, these circuits can also be made flexible and no longer need to be housed in rigid boxes. OLEDs (Organic Light Emitting Diodes), which hold an important place within organic electronics, are a technology first developed by Kodak in 1987. These devices are presented as an alternative to Liquid Crystal Display (LCD) technology. Due to their low energy consumption and thin, lightweight nature, OLEDs have become increasingly common in flat screens and, more recently, in smartphones and tablet computers. However, feedback has been received that these materials lose brightness over time. Hundreds of research studies are being conducted to address these and similar shortcomings and to provide better technology. With the acceleration of research and improvement efforts, OLED technology has become a developing and promising technology.

The latest type of light-emitting diodes (LEDs) is an abbreviation with the full form "Organic Light Emitting Device" or "Organic Light Emitting Diode." It is also referred to as "Organic Electroluminescent Device" (OEL).

It typically consists of a series of organic thin film layers that emit light between two electrical connections (electrodes). OLEDs are composed of low-molecular-weight organic materials (SM-OLED—small molecule OLED) or polymer-based materials (PLED, LEP). Unlike LCDs with different layers and flexible light-emitting diodes (FED), OLEDs are monolithic (single-layer). This is because during manufacture, each layer is coated over another to create a single, seamless structure. Initially developed for indicator applications, OLEDs have enabled the production of bright, colorful screens that provide wide viewing angles at low power. Unlike LCD screens, these do not require backlighting. OLEDs are generally produced on glass but can also be made on plastic and flexible materials. The flexible OLED model "FOLED (flexible-OLED)" made by Universal Display is an example of this. It is clear that the future production of displays such as a pen with a rolled screen will revolutionize portable devices. OLED technology, which is developing worldwide, represents a true revolution for both lighting and display technology sectors. OLED technology is also listed as a priority technological activity area in TÜBİTAK's "Vision 2023" report. Like LEDs, OLEDs are solid-state semiconductor devices. At 100 to 500 nanometers (nanometer: one billionth of a meter) thick, or approximately 200 times thinner than a human hair. OLEDs have two or three organic layers; in a three-layer design, the final layer facilitates the transport of electrons from the cathode to the light-emitting layer. Considering a two-layer design, an OLED consists of the following components:

Figure 1. OLED Components

Substrate: The layer that supports and underlies the OLED. Anode: Creates positively charged holes to allow electron passage when current flows through the device, and is transparent. Organic Layers: These layers are made from organic molecules or polymers. • Conduction layer: This layer is made from organic plastic (polyaniline, etc.) and carries electron holes from the anode. • Emission layer: Made from a different organic plastic than the conduction layer (polyfluorene, etc.), it carries electrons from the cathode; this is where light is actually produced. Cathode: Can be transparent depending on the type of OLED. The cathode is the layer that provides electrons to the device when voltage is applied. It is known that small organic molecules were used in the first OLED studies conducted in the 1980s. Although small molecules emit bright light, scientists experienced some difficulties when coating these molecules on the substrate surface to form organic layers (such as the vacuum condensation method). From the 1990s onwards, researchers began using large polymer molecules as light emitters due to their lower cost and ability to spread over large areas. Fluorescent polymers play an important role in laser, nonlinear optics, sensor and LED applications. Among the many fluorescent polymers examined, fluorene derivatives attracted attention. The inherently high emission efficiency of these polymers indicates their suitability for use in OLEDs.

Figure 2. Image of a fluorene-based fluorescent polymer under 365 nm ultraviolet light

The Gilch, Horner-Emmons, Suzuki, Yamamoto and Stille methods are used in the synthesis of polyfluorenes. Although these methods allow polymerization to be carried out with high efficiency when used, they require quite demanding conditions. Additionally, the obtained materials must pass through important purification steps to achieve high color purity. Work conducted with polyazomethines has easier reaction conditions. These reactions do not require catalysts. A single by-product is formed as a result of the reaction, and extensive purification is not needed to remove by-products. The electronic properties of conjugated azomethines can be tuned by selecting appropriate electron-withdrawing or electron-donating groups. Extended benzodifuran-thiophene systems attached to the azomethine group were synthesized by Frere and colleagues. The effect of the thiophene group on the electronic properties of electron-withdrawing groups on the electron-donating benzodifuran, obtained as a result of cyclic voltammetry and UV-vis spectroscopy, was examined. They developed benzodifuran (BDF)-based donor-acceptor systems. In addition to cyclic voltammetry and UV-vis spectroscopy analyses of the synthesized BDF-based azomethine compounds, theoretical calculations were performed using the B3LYP/6-31G(d,p) (B3LYP—Becke-3-Lee-Yang-Parr) method. As the number of electron-withdrawing groups on the BDF increased, the HOMO-LUMO energy levels were found to stabilize. On the other hand, when an electron-donating group was attached to the thiophene portion, it was observed to cause a dicationic state, and the stability of the positively charged oxidized form increased. Conjugated thienazomethines with different polymerization degrees were also synthesized. Their molecular weights and polydispersity indices were consistent with stepwise growth rather than dynamic component amine transfer. The absorbance of neutral compounds was found to be adjustable between 450 nm and 630 nm depending on their degree of conjugation. The oxidized absorbance of the compounds ranged between 490 nm and 1111 nm. Thus, the prepared azomethine compounds were found to change color in the visible and near-IR regions. Furthermore, the electrochromic properties of conjugated azomethines in solution were investigated and electrochromic devices were prepared. According to this, blue and colorless states were obtained in the range of +2V to -2V.

Figure 3. An azomethine compound exhibiting electrochromic properties

Organic solar cells, like OLEDs, are among the best alternatives to silicon-based solar cells due to their low cost, partial transparency, flexibility and light weight. For some time, fullerene derivatives have been used as acceptors in organic solar cells. When these derivatives are used together with electron-conducting polymers and small molecules, the efficiency of organic solar cells has exceeded 11 percent. However, fullerenes have disadvantages such as weak absorption in the visible region, inability to adjust HOMO-LUMO band energy gaps, and unstable morphology. For this reason, fullerenes are hindering the development of organic solar cells. In contrast, fullerene-free acceptors have high absorption in the visible and near-red regions, adjustable energy levels, device durability, ease of synthesis and purification. Therefore, research into fullerene-free acceptors has accelerated. The synthesis and applications of thienothiophene (TT) and its derivatives cover a broad area in the literature. TT is the smallest rigid molecule with a bicyclic structure from the group of fused thiophenes. Compared to other heterocyclic compounds (furan, selenophene, etc.), the thiophene ring has advantages such as high chemical and thermal stability, allowing structural modifications, and being electron-rich. If you also wish to synthesize molecules with the properties mentioned above in your laboratory, some basic steps you should follow are outlined below: For molecules intended to be synthesized for use in organic electronics, molecular conformations and HOMO-LUMO band energy gaps should first be examined through computer-aided chemical calculations. Planarity in conformations is important to avoid disruption of conjugation. Disruptions in conjugation increase the HOMO-LUMO band energy gap. This is an undesirable situation. For OLED applications, the band gap of materials to be used should be 3 eV, and in organic solar cells, this value should be below 2 eV. Following the calculations, synthesis of molecules with the desired band energy gap is performed, and then electrochemical analyses of these molecules are conducted. If a reversible voltammogram is obtained as a result of electrochemical analyses, the electrochromic properties of the molecules can be examined. Spectroelectrochemical analyses are performed on molecules exhibiting electrochromic properties. If the intensities of absorbances present in the spectrum decrease and new absorption curves are obtained when voltage is applied, fluorescence studies of the materials are conducted. Finally, fluorescence quantum yields are measured. The closer the fluorescence quantum yield is to 1, simple single-layer device studies can be performed for potential use of these materials in OLEDs or organic solar cells.

Figure 4. Images of dithienothiophene compounds exhibiting fluorescence properties under 365 nm radiation and device studies

One day, the latest technological device you purchase will not be a phone in your pocket or a smartwatch on your wrist. You are very close to having optoelectronic skin with an ultra-thin, flexible LED display that sticks to your skin like a transparent serum and can be worn over your hand. Researchers at Tokyo University are the latest group working on this subject. If you are also curious about these optoelectronic skins, I recommend taking a look. I thank Prof. Dr. Turan Öztürk and Prof. Dr. William Skene for the photographs used in this article. Assistant Professor Dr. Sultan Funda Ekti Anadolu University Faculty of Science Department of Chemistry

References A. Faccinetto, S. Mazzucato, D. Pedron, R. Bozio, S. Destri and W. Porzio, ChemPhysChem, 2008, 9, 2028. M. Yildirim and I. Kaya, Synth. Met., 2011, 161, 13 A. C. Grimsdale, K. Leok Chan, R. E. Martin, P. G. Jokisz and A. B. Holmes, Chem. Rev., 2009, 109, 897. Szarko, J. M.; Guo, J.; Rolczynski, B. S.; Chen, L. X. J. Mater. Chem. 2011, 21, 7849. Wei, W.; Yu, N.; Zhu, R.; Peng, B.; Huang, W. J. Appl. Polym. Sci. 2008, 108, 2438. Jin, S.-H.; Kang, S.-Y.; Kim, M.-Y.; Chan, Y. U.; Kim, J. Y.; Lee, K.; Gal, Y.-S. Macromolecules 2003, 36, 3841. Phanichphant, S.; Anuragudom, P.; Newaz, S. S.; Lee, T. R. Macromolecules 2006, 39, 3494. Leclerc, M.; Ranger, M.; Rondeau, D. Macromolecules 1997, 30, 7686. Scherf, U.; Nothofer, H. G.; Meisel, A.; Miteva, T.; Neher, D.; Forster, M.; Oda, H.; Lieser, G.; Sainova, D.; Yasuda, A.; Lupo, D.; Knoll, W. Macromol. Symp. 2000, 154, 139. Miller, R. D.; Kreyenschmidt, M.; Klaerner, G.; Fuhrer, T.; Ashenhurst, J.; Karg, S.; Chen, W. D.; Lee, V. Y.; Scott, J. C. Macromolecules 1998, 31, 1099. Frere, P., Gohier, F. and Moussalem C. (2015), Tetrahedron Letters, 56, 5116. Barik S., Navarathne D., LeBorgne M. and Skene W. G. (2013c), Journal of Materials Chemistry C, J. Mater. Chem. C, 1, 5508. Yokota T., Zalar P., Kaltenbrunner M., Jinno H., Matsuhisa N., Kitanosako H., Tachibana Y., Yukita W., Koizumi M., Someya T, Sci. Adv. 2016; 2: e1501856 Lu L., Zheng T., Wu Q., Schneider A.M., Zhao D., Yu L., Chem. Rev., 115, 12666-12731, 2015. Lin Y., Zhan X., Acc. Chem. Res., 49, 175-183, 2016. Li S., Zhang Z., Shi M., Li C.-Z., Chen H., Phys.Chem.Chem.Phys., 19, 3440-3458, 2017 Cinar M.E., Ozturk T., Chem. Rev., 115, 3036-3140, 2015. http://www.oled-info.com http://electronics.howstuffworks.com/oled1.htm