[1,2,4]Triazolo[1,5-a]pyridine-Based Host Materials for Green Phosphorescent and Delayed-Fluorescence OLEDs with Low Efficiency Roll-Off

Abstract

Herein, a series of universal bipolar host materials, 9,9'-([1,2,4]triazolo[1,5- a]pyridine-2,6-diylbis(4,1-phenylene))bis(9 H-carbazole) (TP26Cz1), 3-(2-(4-(9 H-carbazol-9-yl)phenyl)-[1,2,4]triazolo[1,5- a] pyridine-6-yl)-9-phenyl-9 H-carbazole (TP26Cz2), 9,9'-([1,2,4]triazolo[1,5- a]pyridine-2,7-diylbis(4,1-phenylene))bis(9 H-carbazole) (TP27Cz1), and 3-(2-(4-(9 H-carbazol-9-yl)phenyl)-[1,2,4]triazolo[1,5- a]pyridin-7-yl)-9-phenyl-9 H-carbazole (TP27Cz2), using [1,2,4]triazolo[1,5- a]pyridine (TP) as electron-transporting moiety and carbazole as hole-transporting moiety, were designed and synthesized. All four compounds possess remarkable carrier-transporting properties and excellent thermal stability with high glass-transition temperature ( T) in the range of 136-144 °C. The hole- and electron-transporting abilities could be regulated by adjusting the linkage mode between the carbazole and TP units, and balanced charge-transporting properties were realized in TP26Cz2 and TP27Cz2. The phosphorescent and thermally activated delayed-fluorescence (TADF) organic light-emitting diodes (OLEDs) based on these host materials exhibit superior performance with high efficiency and low roll-off. For example, TP26Cz2-hosted phosphorescent OLED (PhOLED) exhibits the maximum external quantum efficiency (η) of 25.6%, and at the high luminance of 5000 cd m, the η still remained at 25.2%. TP27Cz1-hosted TADF device exhibits the maximum η of 15.5% and only dropped to 15.4% at the luminance of 1000 cd m. Moreover, the influence of linking mode of carbazole unit and TP units in these hosts on their photophysical and carrier-transporting properties as well as the electroluminescence (EL) performance of devices was discussed.

Carbazole/Benzimidazole-Based Bipolar Molecules as the Hosts for Phosphorescent and Thermally Activated Delayed Fluorescence Emitters for Efficient OLEDs

“…In addition, the PLQY of the 9% 4CzIPN-doped o -CbzBiz film was 93.9%, which also explains the high performance of the o -CbzBiz-hosted device. The performance of device F1 is either slightly higher than or comparable to that of other devices using carbazole-based hosts reported in the previous literature, ,− suggesting that the carbazole/benzimidazole-based hosts developed in this work are also promising hosts for TADF emitters.…”

“…All three compounds were obtained in good yields (68–84%) by using 2-(4-bromophenyl)-1-phenyl-1 H -benz­[ d ]­imidazole to react with the corresponding carbazole-containing boronic acid intermediates via palladium-catalyzed Suzuki coupling reactions. These carbazole-containing boronic acid intermediates were synthesized according to the previous literature . All target compounds were soluble in common organic solvents such as toluene, dichloromethane, and chloroform, so they were characterized by 1 H NMR spectroscopy, 13 C NMR spectroscopy, and high-resolution mass spectrometry.…”

“…However, both molecular structures of CBP and mCP are constructed by benzene and an electron-donating moiety, i.e., carbazole, such that they are classified as unipolar molecules. This feature would lead to unbalanced charge-transporting in the emitting layer (EML) when CBP and mCP are applied as the host of OLEDs. − As a result, the charge recombination zone of such devices is closer to the interface between EML and electron-transporting layer (ETL) or hole-transporting layer (HTL), leading to a narrower recombination zone and inferior performance at higher applied voltage and brightness of the device. , Compared with unipolar molecules, bipolar molecules have also been applied as the host materials for OLEDs due to their balanced hole- and electron-transporting capability that could be beneficial to improve the device performance. − For example, Wang et al reported that, when electron-withdrawing moieties such as pyridine, benzimidazole, and triazine were introduced to the molecular structure of CBP as the host materials of phosphorescent OLEDs using Ir­(ppy) 3 as the emitter, the EQEs of the devices were 17.1–18.9%, which were significantly higher than that of the CBP-based device (EQE = 13.8%) . On the other hand, Song et al reported that electron-withdrawing [1,2,4]­triazolo­[1,5- a ]­pyridine can be integrated with carbazole to construct the host materials for phosphorescent and TADF OLEDs with high EQE .…”

“…13−15 As a result, the charge recombination zone of such devices is closer to the interface between EML and electron-transporting layer (ETL) or hole-transporting layer (HTL), leading to a narrower recombination zone and inferior performance at higher applied voltage and brightness of the device. 16,17 Compared with unipolar molecules, bipolar molecules have also been applied as the host materials for OLEDs due to their balanced hole-and electron-transporting capability that could be beneficial to improve the device performance. 18−20 For example, Wang et al reported that, when electron-withdrawing moieties such as pyridine, benzimidazole, and triazine were introduced to the molecular structure of CBP as the host materials of phosphorescent OLEDs using Ir(ppy) 3 as the emitter, the EQEs of the devices were 17.1−18.9%, which were significantly higher than that of the CBP-based device (EQE = 13.8%).…”

1,3-Dipolar cycloaddition of cyanopyridines to heterocyclic N-imines – combined experimental and theoretical studies

“…1) are highly selective inhibitors of Janus kinases type 1 and 2 respectively and are regarded as promising drug candidates. Also this heterocyclic unit was shown to be useful in design of host materials for high-performance red [6], white and RGB [7] or green [8] PhOLEDs. Similarly, [1,2,4]triazolo [1,5-a]pyrimidines, which chemistry recently was reviewed, have a broad spectrum of biological activity [9].…”