柔性芴基嵌段型延迟荧光二聚体的设计、合成及电致发光性能

doi:10.6023/A23020051

摘要/Abstract

热激活延迟荧光(TADF)材料虽然可以通过反向系间窜越(RISC)利用三线态激子, 但紧密堆积状态下的三重态激子湮灭会严重降低器件效率. 本工作采用二苯基芴作为嵌段骨架, 三嗪-咔唑(TRZ-Cz)作为TADF发光单元合成了嵌段型二聚体材料2TC-Fu. 通过柔性烷基链将二苯基芴桥连在TADF的中间, 不仅有效避免了发光核的聚集猝灭, 还提高了其在常用溶剂中的溶解性, 使其可以通过溶液法制备有机发光二极管(OLEDs). 光物理和热力学测试表明, 2TC-Fu的三线态为2.89 eV, 且具有较小的ΔE_ST(0.27 eV)和较高的玻璃化转变温度(142 ℃). 以2TC-Fu作为非掺杂发光层, 溶液法器件的最大电流效率(CE)和外量子效率(EQE_max)分别为8.3 cd/A和4.4%, 几乎是未嵌段发光单元的5倍, 充分证明嵌段单元可以有效抑制激子猝灭. 将其作为主体材料, 掺杂器件实现了13.1%的EQE_max, 这在溶液法OLEDs中属于较高水平. 上述研究结果表明, 柔性芴基嵌段策略可以有效提高分子的电致发光效率, 为可溶液加工型TADF分子的开发提供了新的途径.

关键词: 热激活延迟荧光, 二苯基芴, 嵌段, 有机发光二极管

Organic light-emitting diodes (OLEDs) have a series of advantages such as high resolution, wide viewing angle, lightweight, and flexible solution processing that can realize a large area of self-illumination display. As the third generation of luminescence materials, thermally-activated delayed fluorescence (TADF) material can harvest 100% of triplet excitons through reverse intersystem crossing (RISC). However, the triplet excitons exhibit severe aggregation-caused quenching (ACQ) behavior in the tightly stacked state, which undoubtedly reduces the device’s performance. In this work, we synthesized the block dimer TADF material 2TC-Fu, among the molecular structure, diphenylfluorene serves as the block framework and triazinocarbazole (TRZ-CZ) serves as the TADF emissive unit. First, carbazole is connected to the end of 1,6-dibromohexane, and then the symmetric structure is synthesized from nucleophilic substituted bisphenol fluorene, after, monofluorotriazine is attached to both ends of the symmetric structure by a nucleophilic substitution reaction to generate the target product 2TC-Fu finally. Because the addition of diphenyl fluorene obviously shows a spatial torsion angle, the irregular distribution of the emitting unit is realized by bridging the flexible alkyl chain in the middle of TADF, thus effectively avoiding the ACQ of the TADF unit and improving its solubility in the solvent. Thermodynamic properties show that 2TC-Fu has a high glass transition temperature (142 ℃) and a thermal decomposition temperature of 442 ℃, proving that it can be sufficient to withstand the heat treatment procedures during device preparation. In addition, the device 2TC-Fu based exhibits a high CE_max and EQE_max of 8.3 cd/A and 4.4% respectively, which exceeds 3 times the efficiency of non-block TADF unit-based, demonstrating the feasibility of the flexible fluorenyl block strategy. Moreover, the devices 2TC-Fu doped with TADF material 5CzBN achieved 13.1% of EQE_max and 3489 cd/m² of brightness. In summary, the design of a flexible fluorenyl block strategy to avoid large-scale aggregation of luminescent nuclei significantly improves the device efficiency, which provides a new way for the development of TADF molecules. Although the aggregation-induced fluorescence quench-ing was effectively inhibited by increasing the molecular spacing between luminescent units and increasing the solubility of molecules by using large fluorenyl groups and alkoxy chains, we will not stop researching the blocking strategy. In the future, we will continue to explore the factors that affect the performance of TADF devices in all aspects, such as the type of block groups and the steric hindrance of substituents.

Key words: thermally-activated delayed fluorescence, diphenylfluorene, block, organic light-emitting diodes

引用此文

葛凤洁, 张开志, 曹清鹏, 徐慧, 周涛, 张文浩, 班鑫鑫, 张晓波, 李娜, 朱鹏. 柔性芴基嵌段型延迟荧光二聚体的设计、合成及电致发光性能[J]. 化学学报, 2023, 81(9): 1157-1166.

Fengjie Ge, Kaizhi Zhang, Qingpeng Cao, Hui Xu, Tao Zhou, Wenhao Zhang, Xinxin Ban, Xiaobo Zhang, Na Li, Peng Zhu. Design, Synthesis and Electroluminescence Performance of Flexible Fluorenyl Block Delayed Fluorescence Dimers[J]. Acta Chimica Sinica, 2023, 81(9): 1157-1166.

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图/表 10

图式1. 2TC-Fu的合成路线图

Scheme 1. Synthetic route of 2TC-Fu

图1. 10 ℃/min升温速率下2TC-Fu的TGA (a)和DSC (b)曲线

Figure 1. TGA (a) and DSC (b) curves of 2TC-Fu recorded at a heating rate of 10 ℃/min

图2. 基态分子构型的DFT计算、前线轨道的计算与单三线态的垂直激发态能

Figure 2. DFT calculations of the molecular configurations in the ground states, calculation of frontier orbit and vertical excited state energy of single triaxial state

λ_abs^a/nm	λ_fl^b/nm	(S₁/T₁^c)/eV	(HOMO/LUMO^d)/eV	(ΔE_g/ΔE_ST)/eV	(T_d^e/T_g)/℃
285/336/368	432	3.16/2.89	–5.56/–2.51	3.05/0.273	442/142

表1. 2TC-Fu的物理化学表征数据

Table 1. Physicochemical characterization data of 2TC-Fu

图3. 室温下2TC-Fu在甲苯中的紫外-可见吸收和荧光光谱以及在77 K下2TC-Fu在甲苯中的归一化磷光光谱(a)、室温下2TC-Fu的固体荧光和延迟荧光曲线(b)、2TC-Fu的瞬态曲线(c)、2TC-Fu归一化的紫外溶剂化(d)和荧光溶剂化(e)、2TC-Fu在各种溶剂中的Lippert-Magada图(f)

Figure 3. UV-Vis absorption and fluorescence spectra (a) of 2TC-Fu in toluene at room temperature, and normalized phosphorescence spectra (a) of 2TC-Fu at 77 K, film fluorescence and delayed fluorescence spectra (b) of 2TC-Fu at room temperature, transient spectra (c) of 2TC-Fu measured at 77 K, normalized UV solvation curves (d) and FL solvation curves (e) of 2TC-Fu, Lippert-Magada plot (f) of 2TC-Fu in various solutions

Solvent	Δf(ε, n)^a	λ_abs^b/nm	λ_f ^c/nm	Δv^d/cm^-1
Toluene	0.013	371	432	3806
EA	0.200	365	463	5799
DMF	0.274	364	477	6508
DMSO	0.286	363	480	6715

表2. 2TC-Fu在不同溶剂中的光谱特性

Table 2. Spectral properties of 2TC-Fu in different solvents

图4. 溶液处理的TADF-OLEDs器件能级图、结构图(a)和所用的有机材料(b)

Figure 4. The energy level diagram, structure diagram (a) and organic materials used in the solution-processed TADF-OLEDs devices (b)

图5. (a)非掺杂器件的电流密度-电压-亮度(J-V-L)曲线、(b)非掺杂器件的电流效率(CE)和功率效率(PE)、(c)非掺杂器件的外量子效率(EQE)与相对电流密度曲线、(d)非掺杂器件的电致发光(EL)光谱

Figure 5. (a) Current density-voltage-luminance (J-V-L) curves of undoped OLEDs, (b) current efficiency (CE) and power efficiency (PE) curves of undoped OLEDs, (c) external quantum efficiency (EQE) versus current density of undoped OLEDs, (d) electroluminescent (EL) spectra of undoped OLEDs

Device	V_on^a/V	CE_max^b/(cd•A^-1)	PE_max^c/(lm•W^-1)	EQE_max^d/%	L_max^e/(cd•m^-2)	CIE^f (x, y)	EL_max/nm
TRZ-Cz	5.4	1.7	0.8	0.9	220	(0.17, 0.26)	477
2TC-Fu	5.0	8.3	5.2	4.4	1378	(0.19, 0.31)	479
2TC-Fu:10% (w) 5CzBN	3.8	15.9	12.5	7.2	2862	(0.18, 0.42)	494
2TC-Fu:20% (w) 5CzBN	4.4	33.9	21.3	13.1	3195	(0.22, 0.47)	501
2TC-Fu:30% (w) 5CzBN	4.6	26.9	16.9	10.2	3489	(0.24, 0.53)	511

表3. 2TC-Fu器件的电致发光性能

Table 3. EL performances of the devices using 2TC-Fu-based devices

图6. (a)掺杂器件的J-V-L曲线、(b)掺杂器件的EQE曲线、(c)掺杂器件的EL光谱和(d)能量传递图

Figure 6. (a) J-V-L curves of doped OLEDs, (b) EQE curves of doped OLEDs, (c) EL spectra of doped OLEDs and (d) energy transfer diagram

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