Design, Synthesis and Electroluminescence Performance of Flexible Fluorenyl Block Delayed Fluorescence Dimers

doi:10.6023/A23020051

Abstract

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

Cite this article

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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Fig. & Tab. 10

Scheme 1. Synthetic route of 2TC-Fu

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

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

Table 1. Physicochemical characterization data of 2TC-Fu

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

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

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

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

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

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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