Molecular design and synthesis for conformation-modulated organic light-emitting materials = 유기 발광 소재의 형태 변형을 위한 분자 설계 및 합성 연구

다국어 초록 (Multilingual Abstract)

Organic light-emitting materials hold significant promise for optoelectronic applications due to their tunable optoelectronic properties, multifunctional emissive behavior, and cost efficiency. These materials have enabled significant advancements in various fields, including organic light-emitting diodes (OLEDs), photodynamic therapy (PDT), bio-imaging, sensors, information encryption, and anti-counterfeiting technologies.
Despite these advancements, achieving high quantum efficiency in the solid state remains challenging, primarily due to non-radiative decay driven by molecular reorganization energy during structural relaxation in electronic transitions. Thus, minimizing reorganization energy is critical for enhancing photoluminescence quantum yield (PLQY) by suppressing exciton quenching. Conventional strategies, such as employing fused and π-extended structures, reduce reorganization energy by enforcing geometric rigidity. However, these approaches face limitations, including aggregation-caused quenching (ACQ), poor solubility, and synthetic complexity. Furthermore, highly distorted donor–acceptor (D–A) structures designed for efficient triplet harvesting often increase molecular flexibility, exacerbating non-radiative losses. Thus, alternative molecular design strategies are required to overcome these limitations and enable efficient emission across various wavelengths.
This challenge motivates my Ph.D. research, which aims to develop conformation-modulated light-emitting materials by substituent engineering and leveraging non-covalent interactions (NCIs) as a novel design strategy. My work encompasses the design, synthesis, and characterization of organic light-emitting materials, including organometallic complexes and metal-free organic small molecules, emitting covering emission from the deep-red/near-infrared (NIR) to deep-blue regions. Additionally, collaborative studies assess the potential of these materials for next-generation optoelectronic applications, with a focus on universal applicability across the visible spectrum.
Chapter 1 provides a comprehensive overview of the molecular design principles and emission mechanisms of organic light-emitting materials. It examines the fundamentals of emissive processes, with a focus on the role of reorganization energy and non-radiative decay at the molecular level. The concept of conformation-modulated rigidity is introduced as a strategy to address the limitations of conventional fused-ring systems. By optimizing molecular structures and leveraging NCIs, such as intra- and intermolecular hydrogen bonding, this approach enhances rigidity and stabilizes flexible single bonds, reducing reorganization energy and suppressing non-radiative decay. The chapter also presents a state-of-the-art molecular library of light-emitting small molecules, categorized by emission color, molecular structure, photophysical properties, and device performance. Finally, it outlines perspectives for advancing next-generation organic light-emitting materials.
Chapters 2 and 3 detail the development of conformation-modulated materials with enhanced emissive properties and multifunctionality for diverse applications. These chapters describe the systematic design of molecular structures to achieve desired optoelectronic properties, validated through computational studies. Promising candidates are synthesized using efficient retrosynthetic pathways, followed by rigorous purification and experimental characterization. Initially, the focus is on organometallic iridium(III) complexes incorporating rigid ancillary ligands to suppress molecular motion. These complexes exhibit efficient phosphorescence in the deep-red/NIR region and, with morpholine-functionalized ligands, demonstrate potential for PDT applications. Subsequently, the research shifts to metal-free pure organic emitters to address the cost and intrinsic stability challenges associated with organometallic systems. By incorporating NCIs, particularly intramolecular hydrogen bonding (IHB), these materials achieve stable and efficient emissions across the red to deep-blue spectrum. IHBs enhance solid-state emissions and enable unique packing modes that induce multifunctional properties, such as aggregation-induced emission (AIE), mechanochromic luminescence (MCL), and room-temperature phosphorescence (RTP). These findings underscore the versatility of NCIs in modulating radiative characteristics, as demonstrated through photoluminescence (PL) and electroluminescence (EL) analyses. Consequently, the reliable performance of OLEDs is realized.
In conclusion, my Ph.D. research highlights the efficacy of conformation-modulated molecular design strategies, emphasizing the role of structural rigidity in enhancing emission efficiency and multifunctionality across a broad range of light-emitting materials. These findings will contribute to the development of a comprehensive molecular design strategy, providing a blueprint for next-generation organic optoelectronic technologies.

국문 초록 (Abstract)

제1장에서는 유기 발광 소재의 우수성을 소개하며, 이들이 유기 발광 다이오드, 광전자 소자, 화학 및 바이오 기술 등 다양한 분야에 응용될 가능성을 논의합니다. 이에 따라 발광 효율 향상이 핵심 과제로 강조되므로, 비발광 프로세스를 억제하고 재배열 에너지에 기반한 이론적 탐구를 진행하였습니다. 이를 바탕으로, 기존 공유 결합 기반 분자 설계 전략의 한계를 보완하기 위해 비공유 상호작용을 활용한 분자 형태 고정 전략을 차세대 고효율 유기 발광 소재 설계 방향으로 제시하였습니다. 또한, 비공유 상호작용의 효과를 분석하고 발광 특성 및 광학 소자 성능에 미치는 영향을 자세히 검토하며, 이를 통해 고효율 및 다기능성을 실현할 수 있는 설계 전략과 응용 가능성을 전망하였습니다.
제2장과 3장에서는 기존 발광 소재의 한계를 극복하고, 양자 효율 향상 및 다기능성 발광 특성을 달성하기 위한 실험적 연구 방법과 결과를 다룹니다. 연구는 다음과 같은 과정으로 진행되었습니다. 먼저, 계산화학적 탐구를 통해 형태 변형이 제어된 분자들의 실현 가능성을 검증하고, 역 합성 분석을 통해 효율적인 합성 경로를 수립하였습니다. 이후, 합성된 분자들을 이론적 및 실험적 방법으로 분석하며, 구조 설계가 발현된 특성에 미치는 영향을 정성적·정량적으로 입증하였습니다.
제3-1장에서는 이리듐 유기금속착화물 구조를 기반으로 한 신규 인광 소재를 설계하였으며, 기존 보조 리간드의 유연성으로 인한 효율 저하 문제를 해결하기 위해 평면성 보조 리간드를 도입하여 비발광 프로세스를 억제하고 양자 효율을 개선하였습니다. 또한, 모폴린 치환체를 도입하여 세포 내 리소좀에 선택적으로 위치할 수 있는 특성을 부여하고, 근적외선 인광 소재를 활용한 광역동치료 응용 가능성을 확인하였습니다.
제3-2, 3-3, 3-4장에서는 유기금속착화물 소재의 한계를 극복하고자 순수 유기물 기반 발광체를 탐구하였습니다. 내부양자효율 개선을 위해 열활성지연형광(TADF)을 나타내는 적색, 녹색, 청색 발광 순수 유기물 소재를 개발하였으며, 다양한 전자 공여체–전자 수용체 조합의 구조를 설계하였습니다. 나아가, 방향족 고리 연결체를 도입함으로써 최적화된 오비탈 겹침 정도를 유도하였으며, 페닐 및 이종 원소가 치환된 피리디닐 치환체의 비공유 상호작용을 비교함으로써 분자 형태 변형과 고정을 유도하였으며 내부 수소 결합을 강화하여 효율 개선 및 다기능성 발광 특성을 실현하였습니다. 그 결과, 우수한 발광 특성을 갖춘 유기 발광 다이오드를 제작하여 합리적인 성능을 구현하였습니다.
종합적으로, 본 연구는 유기 발광 소재의 분자 형태 변형 제어 전략에 대한 이론적 타당성을 제시하고, 이를 효율적인 합성과 특성화에 적용하여 발광 파장에 제한되지 않는 보편적 분자 설계 전략임을 증명하였습니다. 따라서, 형태 변형을 활용한 유기 발광 소재 설계 전략은 효율 개선 및 다기능성 발현에 기여하며, 차세대 유기 광전자 응용 분야 발전에 중요한 통찰을 제공할 것으로 기대됩니다.

목차 (Table of Contents)

• Abstract i
• Abbreviations vii
• List of Figures xi
• List of Tables xviii
• Chapter 1
• Introduction
• 1-1. General background on organic light-emitting materials 1
• 1-2. Light-emission mechanisms in organic materials 2
• 1-2-1. Fluorescence 2
• 1-2-2. Phosphorescence 3
• 1-2-3. Thermally activated delayed fluorescence (TADF) 4
• 1-2-4. Other emission mechanisms 6
• 1-3. Fundamentals of light-emission efficiency 7
• 1-3-1. Radiative and non-radiative electronic transition 10
• 1-3-2. Role of the reorganization energy 11
• 1-4. Conformational modulation leveraging NCIs 13
• 1-4-1. Overview of NCIs 13
• 1-4-2. Conformation-modulated molecular design 14
• 1-4-3. Characterization techniques for NCIs 15
• 1-5. Progress in molecular design 17
• 1-5-1. Functioning mechanism of NCIs 18
• 1-5-2. Blue light-emitting organic small molecules 20
• 1-5-3. Green light-emitting organic small molecules 27
• 1-5-4. Red light-emitting organic small molecules 35
• 1-5-5. Deep-red/NIR light-emitting organic small molecules 41
• 1-6. Research motivation 49
• Chapter 2
• Experimental
• 2-1. Introduction to experimental methods 51
• 2-2. Computational investigation 53
• 2-3. Materials and basic characterization 53
• 2-4. Electrochemical analysis 54
• 2-5. Thermal analysis 55
• 2-6. Photophysical characterization 55
• 2-7. Single crystal X-ray diffraction analysis 56
• 2-8. Morphological analysis 56
• 2-9. Detection of singlet oxygen 56
• 2-10. Detection of superoxide radical anion 57
• 2-11. OLED device fabrication 57
• 2-12. Doped polymer matrix film preparation 58
• Chapter 3
• Materials Synthesis and Results
• 3-1. Conformation-modulated deep-red/NIR light-emitting organometallic Ir(III) complexes 59
• 3-1-1. Synthetic procedures 61
• 3-1-2. Synthesis and basic characterization 66
• 3-1-3. Theoretical calculations 68
• 3-1-4. Single crystal analysis 69
• 3-1-5. Photophysical properties 70
• 3-1-6. Characterization on application 72
• 3-2. Conformation-modulated orange/red light-emitting organic small molecules 75
• 3-2-1. Synthetic procedures 76
• 3-2-2. Synthesis and basic characterization 81
• 3-2-3. Theoretical calculations 83
• 3-2-4. Single crystal analysis 88
• 3-2-5. Photophysical properties 89
• 3-2-6. Morphological analysis 95
• 3-2-7. Characterization on application 96
• 3-3. Conformation-modulated green/yellow light-emitting organic small molecules 101
• 3-3-1. Synthetic procedures 103
• 3-3-2. Synthesis and basic characterization 107
• 3-3-3. Theoretical calculations 118
• 3-3-4. Single crystal analysis 111
• 3-3-5. Photophysical properties 112
• 3-3-6. Morphological analysis 120
• 3-3-7. Characterization on application 121
• 3-4. Conformation-modulated deep-blue light-emitting organic small molecules 123
• 3-4-1. Synthetic procedures 125
• 3-4-2. Synthesis and basic characterization 129
• 3-4-3. Theoretical calculations 131
• 3-4-4. Photophysical properties 135
• 3-4-5. Characterization on application 138
• Chapter 4
• Summary and Conclusion
• 4-1. Summary 140
• 4-2. Conclusion 142
• Chapter 5
• References 144
• Outlook and Perspectives 154
• Summary in Korean 156
• Curriculum Vitae 158