Synthesis of Blue-Emitting Materials with Sterically Induced Twisted Structures and Their Application in Blue Organic Light-Emitting Diode (OLED)

doi:10.6023/A25060223

Abstract

Organic light-emitting diode (OLED) technology has been widely applied in the fields of full-color flexible displays and solid-state lighting. As one of the three primary colors, blue light plays a crucial role in OLED technology, making blue-emitting materials a focal point of research in this domain. To achieve blue light emission with high color purity, blue-emitting materials need to emit light within a narrow wavelength range. However, this narrow-band emission often comes at the cost of reduced luminescence efficiency, as precise regulation of molecular energy levels to realize narrow-band emission is highly challenging in practical material design. Considering this, this work proposes an innovative molecular design strategy. Through the Suzuki-Miyaura coupling reaction of pyridine units with corresponding boronate esters, three bipyridine derivatives with different substitution sites were successfully synthesized. Subsequently, 3,6-di-tert-butylcarbazole groups were introduced to the ortho, meta, and para positions of the pyridine nitrogen atoms in the bipyridine core via C—N coupling reactions, yielding three target compounds with distinct twisted structures. In these materials, the 3,6-di-tert-butyl- carbazole unit acts as the donor and the pyridine unit as the acceptor. A significant dihedral angle exists between the carbazole substituents and the bipyridine core. This steric-induced twisted structure effectively restricts the extension of the molecular conjugated system and suppresses the energy loss due to intramolecular vibrations, thereby ensuring high color purity of blue light. Thermogravimetric analysis indicates that all three materials exhibit excellent thermal stability, with decomposition temperatures exceeding 310 ℃. Photophysical tests reveal that 2,2'-bis(3,6-di-tert-butyl-9H-carbazol-9-yl)-3,3'-bipyridine (2TCzOBPy) and 4,4'-bis(3,6-di-tert-butyl-9H-carbazol-9-yl)-3,3'-bipyridine (2TCzPBPy) show deep blue light emission at 440 nm and 422 nm, respectively, in doped films. OLED devices fabricated with these two compounds as the emissive layers have CIE coordinates of (0.152, 0.063) and (0.172, 0.098), respectively, with y values of the color coordinates both less than 0.1, indicating deep blue light emission with high color purity. Additionally, devices based on 3,3'-bis(3,6-di-tert-butyl- 9H-carbazol-9-yl)-4,4'-bipyridine (2TCzMBPy) exhibit blue-green light emission, achieving a maximum external quantum efficiency (EQE) of 3.6% and a maximum brightness of 5423 cd•m⁻².

Key words: organic light-emitting diode (OLED), twisted structure, bipyridine, blue light, high color purity

Cite this article

Yan An, Xu Shipan, Du Xuyang, Liu Wenping, Zhao Kunxiang, Li Shengli, Zhong Daokun, Zhou Guijiang, Yang Xiaolong, Sun Yuanhui. Synthesis of Blue-Emitting Materials with Sterically Induced Twisted Structures and Their Application in Blue Organic Light-Emitting Diode (OLED)[J]. Acta Chimica Sinica, 2025, 83(11): 1317-1323.

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Fig. & Tab. 9

Scheme 1. Synthetic routes of the three compounds

Figure 1. TGA curves of the three compounds

Compound	Absorption^a/nm	Emission^b/nm	τ^b/ns	PLQY^b/%	T_d/℃
2TCzOBPy	243, 295, 328, 342	440	2.8	50.7	337
2TCzMBPy	239, 297, 345, 391	487	11.6	94.4	317
2TCzPBPy	241, 296, 344	422	3.2	38.0	340

Table 1. Photophysical properties data of the three compounds

Figure 2. UV-Vis absorption spectra in dichloromethane solution

Figure 3. Fluorescence emission spectra in films

Figure 4. Optimized molecular structures and frontier molecular orbital distributions of the three compounds

Figure 5. Chemical structures of organic materials and simplified configuration of OLED device

Figure 6. (a) EL spectra and CIE coordinate diagram, (b) current density-voltage-luminance curves, (c) maximum external quantum efficiency-luminance curves, and (d) current efficiency-luminance-power efficiency curves based on the three materials

Device	V_on^a/V	L_max^b/(cd•m⁻²)	EQE^c/%	CE^d/(cd•A⁻¹)	PE^e/(lm•W⁻¹)	λ_EL/nm	CIE (x, y)
2TCzOBPy	4.5	2888	2.4/2.3	5.4	2.9	437	(0.172, 0.098)
2TCzMBPy	4.6	5423	3.6/2.4	11.2	6.3	496	(0.224, 0.399)
2TCzPBPy	5.4	71	0.5/—	0.3	0.2	416	(0.152, 0.063)

Table 2. Electroluminescence properties of three materials

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位阻诱导扭曲结构的蓝光材料合成及其在蓝色有机发光二极管(OLED)中的应用