Synthesis and characterization of squaraine dyes and their application to green color filters = 스쿠아레인 염료의 합성 및 특성 분석과 녹색 컬러필터에의 적용에 대한 연구

다국어 초록 (Multilingual Abstract)

Recent advances in VR/AR technology, autonomous vehicles, and wearable devices have increased the demand for high-performance display devices. Among next-generation display technologies, microLED displays have received considerable attention because they can utilize micro-sized LEDs as individual pixels without additional packaging processes, thereby overcoming several limitations of conventional LCD and OLED displays. However, the mass transfer of individual red, green, and blue pixels onto a single wafer substrate remains highly challenging and expensive, and the efficiencies of red and green microLEDs are still significantly lower than those of blue microLEDs. To address these limitations, the introduction of color-conversion layers or color filters based on blue or UV microLEDs has been actively developed to fabricate red and green sub-pixels.
Thus, this study explores the development of high-performance green color filters suitable for integration with microLED displays. Considering the extremely small pixel size of microLEDs, dye-based colorants were judged more appropriate than pigment-based materials. Among various dye structures, squaraine dyes were selected for their high molar absorption coefficients, sharp absorption bands, and excellent stability.
To investigate the applicability of squaraine dyes to optical films, a series of squaraine derivatives was synthesized by introducing various substituents onto the nitrogen atom of the indolenine ring of the squaraine dye core. The synthesized dyes were characterized using UV–Vis spectroscopy, thermogravimetric analysis, nuclear magnetic resonance spectroscopy, and computational methods. Polymer-based films incorporating these dyes were fabricated using different polymer binders. Their optical properties were analyzed by UV–Vis spectroscopy, and thermal and photostability tests were conducted to evaluate changes in spectral properties and reliability of the dyes when applied to polymer matrices.
Furthermore, a squaraine dye exhibiting an appropriate absorption band in the green wavelength region was synthesized to develop a green color filter. Because substituents on the nitrogen atom of the indolenine ring had little effect on the dye’s photophysical properties, a nitro group was introduced at the 5-position of the indolenine ring to induce a sufficient bathochromic shift in the absorption wavelength. A decrease in photostability in the solution state was observed for the modified dye. The triplet-state behavior of the dye was examined through computational calculations and singlet-oxygen generation measurements using DPBF. It was confirmed that the nitro group facilitates intersystem crossing of the squaraine dyes, thereby increasing singlet oxygen generation. However, unlike the solution state, films incorporating the squaraine dye maintained excellent thermal and photostability, enabling the preparation of a green dye mixture with suitable spectral properties for green color filter applications.
Finally, green color filters were fabricated using the optimized green dye mixture along with two yellow compensation dyes. Their photophysical properties were analyzed, and the color coordinates were measured to determine color-gamut coverage within the BT.2020 and sRGB color spaces. The squaraine-based color filters developed in this study exhibited narrow transmission bands and high color-gamut coverage, while maintaining excellent optical properties due to their high reliability. In summary, this study demonstrates the potential of squaraine dyes as promising colorants for next-generation display color filter applications.

국문 초록 (Abstract)

최근 시장을 선도하고 있는 VR/AR 기기, 자율주행 자동차, 웨어러블 디바이스 등의 등장에 따라 디스플레이 장치의 성능 향상에 대한 요구가 높아지고 있다. 차세대 디스플레이 기술 중 하나인 마이크로LED 디스플레이는 별도의 패키징 과정 없이 초소형 LED 자체를 픽셀로 활용함으로써 기존의 LCD 및 OLED 디스플레이의 한계를 극복할 수 있어 많은 주목을 받고 있다. 그러나, 각 R/G/B 픽셀을 동일한 웨이퍼 기판에 전사하는 공정이 매우 까다로워 제작 단가를 높이며 적색과 녹색 마이크로LED의 효율이 청색에 비해 매우 낮다는 단점을 가지고 있다. 이를 해결하기 위해 청색 및 UV 영역을 발광하는 마이크로LED를 기반으로 컬러필터를 도입하여 나머지 적색과 녹색을 담당하는 픽셀을 구현하는 기술이 개발되고 있다.
따라서 본 논문에서는 마이크로LED 디스플레이에 적용 가능하며 우수한 성능을 나타내는 녹색 컬러필터를 개발하기 위해 다양한 연구를 진행하였다. 마이크로LED는 크기가 매우 작기 때문에 컬러필터에 사용되는 색재는 안료보다는 염료를 사용하는 것이 적합하다고 판단하였으며, 다양한 염료군 중 높은 몰흡광계수와 날카로운 흡수밴드, 그리고 높은 안정성을 가지는 squaraine 염료를 선정하였다.
우선, squaraine 염료가 흡수용 필름에 적용 가능한 지 알아보기 위해, 다양한 치환기를 squaraine 염료 모체의 인돌닌 고리의 질소 원자에 도입한 squaraine 염료를 합성하였다. 합성된 염료들은 UV-Vis 분광법, 열중량분석법, 핵자기공명분광법, 시뮬레이션 계산 등을 통해 분석하였으며, 이 염료들과 여러 고분자 바인더를 사용하여 필름을 제작하였다. 제작된 필름은 UV-Vis 분광법을 통해 광특성을 분석하였으며, 추가적으로 내열성과 내광성 분석을 진행하였다. 이를 통해 squaraine 염료가 고분자 필름에 적용되었을 때 분광특성 및 내구성이 어떻게 변화하는지 확인하였다.
또한, 녹색 컬러필터의 제작을 위해 적절한 흡수 파장대를 가지는 squaraine 염료를 합성하였다. 인돌닌 고리의 질소 원자에 도입된 치환기는 염료의 광특성에 큰 영향을 주지 않았기 때문에, 여기서는 indolenine 고리의 5번 자리에 니트로 치환기를 도입함으로써 흡수 파장대를 크게 변화시키고자 하였다. 이 과정에서 예상하지 못한 squaraine 염료의 용액 내광성 감소 현상을 확인하였으며, 분자 시뮬레이션 및 DPBF를 이용한 단일항 산소 생성량 분석을 통하여 squaraine 염료가 가지는 삼중항 특성을 분석하였다. 그 결과, 니트로 치환기가 squaraine 염료의 단일항-삼중항 에너지 차이를 감소시키고 spin-orbit coupling을 증가시킴으로써 계간 전이를 촉진시키고 단일항 산소의 생성을 증가시키는 것을 확인하였다. 한편, 필름으로 제작한 squaraine 염료는 우수한 내열성 및 내광성을 유지하는 것을 확인하였으며, 이를 통해 녹색 컬러필터에 사용되기에 적절한 분광특성을 가지는 녹색 염료 혼합물을 제조할 수 있었다.
최종적으로, 녹색 염료 혼합물과 황색 보정 염료 2종을 사용하여 녹색 컬러필터를 제조하여 이들의 분광특성을 분석하고, 색좌표를 측정하여 현재 사용되고 있는 BT2020 및 sRGB 색 공간에서의 색 재현율을 확인하였다. 그 결과, 본 논문에서 제작한 squaraine 염료 기반 컬러필터는 좁은 투과 반치폭과 높은 색 재현율을 나타내었으며, 높은 내구성을 바탕으로 우수한 광특성을 유지할 수 있었다. 결과적으로 본 연구에서는 squaraine 염료가 차세대 디스플레이용 컬러필터 색재로서 충분한 가능성을 지님을 보여주었다.

목차 (Table of Contents)

• Abstract i
• List of tables xi
• List of figures xiii
• List of schemes xvii
• Chapter 1 1
• Introduction 1
• 1.1 MicroLED display 1
• 1.2 Color-conversion microLED display 2
• 1.3 Color filters for color-conversion microLED display 4
• 1.4 Squaraine dyes 5
• 1.5 References 7
• Chapter 2 11
• Synthesis and Characterization of Indolenine-Based Squaraine Dyes and Their Photophysical Properties in Polymer Thin Films 11
• 2.1 Introduction 11
• 2.2 Experimental 13
• 2.2.1 Materials 13
• 2.2.2 Instruments 13
• 2.2.3 Synthesis 14
• 2.2.3.1 Synthesis of indolium salts 14
• 2.2.3.2 Synthesis of squaraine dyes 14
• 2.2.4 Electrochemical analysis of squaraine dyes 16
• 2.2.5 Fabrication of squaraine thin films 16
• 2.3 Results and discussion 17
• 2.3.1 Photophysical properties of the squaraine dyes 17
• 2.3.2 Computational calculations 20
• 2.3.2.1 Optimized ground state geometries of the squaraine dyes 20
• 2.3.2.2 Transition dipole moments of the squaraine dyes 21
• 2.3.3 Electrochemical analysis of the squaraine dyes 23
• 2.3.4 Thermal properties of the squaraine dyes 25
• 2.3.5 Photostability of the squaraine dyes 26
• 2.3.6 Characterization of the squaraine thin films 29
• 2.3.6.1 Optical properties of the squaraine thin films 29
• 2.3.6.2 Thermal properties of the squaraine thin films 31
• 2.3.6.3 Photostability of the squaraine thin films 34
• 2.4 Conclusion 35
• 2.5 References 37
• Chapter 3 41
• Photophysical Properties and Excited-State Dynamics of Nitro-Substituted Squaraines for Green Color Filter Applications: An Experimental and Computational Study 41
• 3.1 Introduction 41
• 3.2 Experimental 43
• 3.2.1 Materials 43
• 3.2.2 Instruments 43
• 3.2.3 Synthesis 44
• 3.2.3.1 Synthesis of 1-(3-methoxy-3-oxopropyl)-2,3,3-trimethyl-5-nitro-3H-indol-1-ium bromide (InEsN, 2b) 44
• 3.2.3.2 Synthesis of (Z)-4-((1-(3-methoxy-3-oxopropyl)-3,3-dimethyl-5-nitro-3H-indol-1-ium-2-yl)methylene)-2-(((Z)-1-(3-methoxy-3-oxopropyl)-3,3-dimethyl-5-nitroindolin-2-ylidene)methyl)-3-oxocyclobut-1-en-1-olate (EsSQN, 3b) 45
• 3.2.4 Electrochemical analysis of squaraine dyes 45
• 3.2.5 Fabrication of squaraine thin films 46
• 3.3 Results and discussion 46
• 3.3.1 Photophysical properties of the squaraine dyes 46
• 3.3.2 Computational calculations 48
• 3.3.2.1 Optimized ground state geometries of the squaraine dyes 48
• 3.3.2.2 Transition dipole moments of the squaraine dyes 50
• 3.3.2.3 Average local ionization energy (ALIE) calculations of the squaraine dyes 51
• 3.3.3 Electrochemical analysis of squaraine dyes 54
• 3.3.4 Thermal properties of the squaraine dyes 55
• 3.3.5 Photostability of the squaraine dyes 56
• 3.3.5.1 Photostability in solution state 56
• 3.3.5.2 Singlet oxygen generation and the triplet-state properties of the squaraine dyes 58
• 3.3.6 Characterization of the squaraine thin films 61
• 3.4 Conclusion 64
• 3.5 References 66
• Chapter 4 73
• Development of Green Color Filter Using Indolenine-Based Squaraine Dyes for Color-Conversion Micro-LED Displays 73
• 4.1 Introduction 73
• 4.2 Experimental 75
• 4.2.1 Materials 75
• 4.2.2 Instruments 75
• 4.2.3 Synthesis 76
• 4.2.3.1 Synthesis of 1-(3-methoxy-3-oxopropyl)-2,3,3-trimethyl-3H-indol-1-ium bromide (InEs) 76
• 4.2.3.2 Synthesis of (Z)-4-((1-(3-methoxy-3-oxopropyl)-3,3-dimethyl-3H-indol-1-ium-2-yl)methylene)-2-(((Z)-1-(3-methoxy-3-oxopropyl)-3,3-dimethyl-indolin-2-ylidene)methyl)-3-oxocyclobut-1-en-1-olate (EsSQ) 76
• 4.2.3.3 Synthesis of 1-(3-methoxy-3-oxopropyl)-2,3,3-trimethyl-5-nitro-3H-indol-1-ium bromide (InEsN) 77
• 4.2.3.4 Synthesis of (Z)-4-((1-(3-methoxy-3-oxopropyl)-3,3-dimethyl-5-nitro-3H-indol-1-ium-2-yl)methylene)-2-(((Z)-1-(3-methoxy-3-oxopropyl)-3,3-dimethyl-5-nitroindolin-2-ylidene)methyl)-3-oxocyclobut-1-en-1-olate (EsSQN) 77
• 4.2.4 Preparation of squaraine dye mixture and thin films 78
• 4.2.5 Preparation of green color filters 78
• 4.3 Result and Discussion 79
• 4.3.1 Characterization of thin films with squaraine dye mixture 79
• 4.3.2 Photophysical properties of color filters 81
• 4.3.3 Chromatic properties of color filters 84
• 4.4 Conclusion 86
• 4.5 References 87
• Summary 89
• 초 록 91
• List of publications 95