Synthesis and Electroluminescence Properties of Two Iridium（Ⅲ） Complexes with Nitrogen Heterocycle Structures

doi:10.7503/cjcu20210415

Abstract

Abstract:

Two main ligands were synthesized based on 2-（3，5-bis（trifluoromethyl）phenyl）-5-fluoropyrimidine （tfmphfppm） and 2-（2，6-bis（trifluoromethyl）pyridin-4-yl）-5-fluoropyrimidine（tfmpyfppm） cores with trifluoromethyl（CF₃） units. Then， two iridium（III） complexes［（Ir（tfmphfppm）₂（pop） and Ir（tfmpyfppm）₂（pop）］ were prepared using these two main ligands and 2-（5-phenyl-1，3，4-oxadiazol-2-yl）phenol（pop） ancillary ligand. The two Ir（III） complexes show blueish-green and green emissions peaking at 484 and 504 nm with photoluminescence quantum yields（PLQYs） of 76% and 89%， respectively. Due to the nitrogen heterocycle and oxadiazol units， both Ir（III） complexes have low LUMO levels and good electron mobilities. Using the two Ir（III） complexes as emitters， the organic light-emitting diodes（OLEDs） show good performances with a maximum luminance（L_max）， a maximum current efficiency（η_c，max）， a maximum power efficiency（η_p，max） and a maximum external efficiency（EQE_max） of 33379 cd/m²， 76.55 cd/A， 31.59 lm/W and 26.7%， respectively. Furthermore， both devices show low efficiency roll-off. And at the brightness of 1000 cd/m²， the η_c can still keep at 72.71 cd/A. The research results suggest that the nitrogen heterocycle， 2，5-phenyl-1，3，4-oxadiazol and CF₃ units in the ligands can improve the luminescence property， electron mobility of the Ir（III） complexes and their corresponding device performances.

Key words: Iridium（III） complex, Nitrogen heterocycle, 2, 5-Phenyl-1, 3, 4-oxadiazol, Organic light-emitting diode, Current efficiency

CLC Number:

O614

TrendMD:

ZHOU Yonghui, HUANG Rujun, YAN Jianyang, LI Yajun, QIU Huanhuan, YANG Jinxuan, ZHENG Youxuan. Synthesis and Electroluminescence Properties of Two Iridium（Ⅲ） Complexes with Nitrogen Heterocycle Structures[J]. Chem. J. Chinese Universities, 2022, 43(1): 20210415.

Figures/Tables 10

Scheme 1 Synthetic routes of complexes Ir(tfmphfppm)2(pop) and Ir(tfmpyfppm)2(pop)

Fig.1 Crystal diagrams of Ir(tfmphfppm)2(pop)(CCDC No.1830860)(A) and Ir(tfmpyfppm)2(pop) (CCDC No.1950738)(B) shown at a 30% probability levelThe hydrogen atoms are omitted for clarity.

Fig.2 TG curves of Ir(tfmphfppm)2(pop)(a) and Ir(tfmpyfppm)2(pop)(b)

Fig.3 UV?Vis absorption and emission spectra at room temperature(A) and emission spectra at 77 K(B) of Ir(tfmphfppm)2(pop)(a， a′) and Ir(tfmpyfppm)2(pop)(b, b′) complexes in degassed CH2Cl2 solutions(5×10-5 mol/L)

Table 1 Physical properties of Ir(tfmphfppm)2(pop) and Ir(tfmpyfppm)2(pop) complexes

Complex	T_d^a/℃	λ_abs^b/nm	λ_em^c/nm	CIE coordinate^d	PLQY^e（%）	HOMO/LUMO^f/eV
Ir（tfmphfppm）₂（pop）	320	272/313	484	（0.17， 0.42）	76	-5.73/-3.30（-5.59/-2.15）
Ir（tfmpyfppm）₂（pop）	344	274/314	504	（0.22， 0.57）	89	-5.75/-3.51（-5.67/-2.45）

Fig.4 Cyclic voltammetry curves in CH3CN solutions(5×10-4 mol/L)(A) and contour plots(B) of Ir(tfmphfppm)2(pop) and Ir(tfmpyfppm)2(pop)

Fig.5 Transient electroluminescence(A) and electric field dependence of electron mobility in the thin films of Ir(tfmpyfppm)2(pop) and Alq3(B)（A） td： Delay time， determined by the arrival of the slower charge carrier of the injected carriers at the emission zone.

Fig.6 Energy level diagram of HOMO and LUMO levels(A) and molecular structures of materials used in this study(B)

Fig.7 Characteristics of devices D1（a) and D2(b)（A） electroluminescence spectra；（B） luminance-voltage-current density curves；（C） current efficiency-luminance curves；（D） power efficiency-luminance curves.

Table 2 Electroluminescence data of the devices D1 and D2

Device	V_turn?on^a/V	L_max^b/（cd·m^-2）	η_{c， max}^c/（cd·A^-1）	η_c，L1000^d/（cd·A^-1）	EQE^e（%）	η_p，max^f/（lm·W^-1）	CIE coordinate
D1	4.4	17533	47.27	43.36	20.9	20.37	（0.161， 0.396）
D2	4.3	33379	76.55	72.71	26.7	31.59	（0.205， 0.554）

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