Study on Structural Design for Organic Light-Emitting Materials

Abstract

Organic luminescent materials in the near-infrared (NIR) region are important for realizing next-generation lightweight and wearable applications in bioimaging, photodynamic therapy, and communications. Therefore, the development of pure organic light-emitting materials is essential. However, the development of high-efficiency NIR light emitting materials using organic materials is still in its infancy. Organometallic complexes, which readily achieve high efficiencies by triplet harvesting of the d-orbital of the central metal, adversely affect organisms and the environment, making them unsuitable for use in deformable devices. Therefore, purely organic small molecules are the most suitable group of materials for the development of high-efficiency NIR light-emitting materials. However, the emission efficiency of NIR wavelengths is much lower than that of visible light because the non-radiative recombination process occurs rapidly according to the energy gap law. Therefore, to overcome these limitations and achieve high-efficiency further wavelength emission, the materials should be designed from the molecular structure level. This is the inspiration and subject of my Ph.D. research, and I have conducted everything from the design, synthesis, photophysical and electrochemical analysis of phosphorescent organometallic molecules that emit visible light to pure organic molecules that emit NIR light. After that, the research was conducted as a process of analyzing the device emission characteristics in collaboration with co-workers to find out the suitability and possibility of molecular design for application to next-generation organic electronic devices. In Chapter 1, the origin of organic light emitting diodes and the design strategy of traditional organic light emitting molecules and the process of improving efficiency were briefly introduced. And the basic concept of visible light and NIR region phosphorescence emission based on iridium and platinum-based organometallic complex was introduced. Strategies for improving the luminous quantum efficiency in visible light and long wavelengths have also been proposed. In addition, the necessity to develop a platinum organometallic complex compared to iridium and strategies for molecular structure design were introduced. Finally, the limitations of these organometallic complex-based molecules and a promising development strategy for organic light emitting materials that can be applied to the next generation of deformable light emitting devices are briefly presented. In addition, this chapter covered in detail organic light-emitting materials in the NIR region, which are important for realizing next-generation lightweight and wearable applications in bioimaging, photodynamic therapy, and communication. Since inorganic and organometallic light emitting materials are expensive and toxic, the development of pure organic light emitting materials with low toxicity is essential, but development of high-efficiency NIR light emitting materials using organic materials is insufficient. Therefore, a molecular design strategy for developing an organic low-molecular NIR light emitting material with high luminous efficiency that can overcome the energy gap law applied to next-generation wearable devices has been described. After a brief review of the basic knowledge required for NIR emission of organic molecules, pure organic molecules with high luminous efficiency reported recently were classified according to their core molecular structures, and molecular design, physical properties, and luminescence properties were analyzed. In addition, promising design strategies and prospects for the development of next-generation high-efficiency NIR pure organic light emitting materials were presented. Chapter 3 ultimately aimed to develop highly efficient NIR organic light-emitting molecules that are less toxic, eco-friendly, and easy to synthesize. To achieve this goal, I first built and tested a laboratory environment such as various equipment for synthesizing and analyzing organic light emitting molecules. I focused on designing an organometallic complex emitting visible light phosphorescence based on platinum as the first target molecular group. After synthesizing reference molecules, I applied a molecular design strategy to overcome the efficiency degradation problem caused by the rectangular planar structure. The luminescence quantum efficiency of molecules was improved by introducing strategies to increase the bulkiness of the ligand, introduce strong electron-withdrawing substituents, and increase the number of coordination bonds between the central metal and the ligand. Based on these research experiences, I designed the structure of a pure organic small molecule that emits high-efficiency NIR light and calculated its properties theoretically. Based on this characteristic, target molecules were synthesized and photophysical and electrochemical analysis of these molecules was carried out. I presented molecular design strategies for improving the luminous efficiency of pure organic-based light emitting materials by analyzing electroluminescence of organic light emitting devices manufactured based on the measured data. Through these studies, I presented a promising strategy for designing high-efficiency pure organic NIR light-emitting materials that can be finally applied to next-generation deformable organic light-emitting devices. In conclusion, in this study, based on an in-depth study on the development of high-efficiency pure organic-based NIR light emitting materials through molecular structure design, the optimal molecular structure and synthesis route were found to overcome the energy gap law. I believe that this result suggests a promising new strategy in the field of organic material research for the commercialization of not only organic light emitting devices but also various organic electronic devices.