9-(9-Alkylcarbazol-3-yl)-3-(methoxypyridin-3-yl)carbazoles as host materials for very efficient OLEDs

Four derivatives of 9-(9-alkylcarbazol-3-yl)-3-(methoxypyridin-3-yl)carbazoles (HM1-HM4) have been synthesized from key starting compounds: 9-alkyl-3-iodocarbazoles and corresponding 3-(methoxypyridin-3-yl)-9H-carbazoles by using Ullmann coupling reactions. The objective materials have very high thermal stabilities (temperatures of 5 % weight loss 371–387 °C) and can form amorphous layers, also having rather high glass transition temperatures in the region of 89–97 °C. Triplet energy gaps of the four compounds were about 2.7–2.8 eV, making them appropriate for use as host materials in green phosphorescent OLEDs with Ir(ppy)₃ guest. Additionally, a composite host system incorporating a synthesized compound of HM series and bis-4,6-(3,5-di-3-pyridylphenyl)-2-methylpyrimidine was developed. The devices with HM2 or HM4 demonstrated the best characteristics whether the emitting layer was a single host or a co-host system, indicating that both compounds could facilitate ideal energy transfer and achieve carrier balance in the device architectures. Notably, the device using 9-(9-butylcarbazol-3-yl)-3-(2-methoxypyridin-3-yl)carbazole (HM2) host outperformed the other devices, achieving peak efficiencies of 16.9 % (58.3 cd/A and 65.0 lm/W) with maximum luminance exceeding 241100 cd/m².