Bipolar 9-linked carbazole-π-dimesitylborane fluorophores for nondoped blue OLEDs and red phosphorescent OLEDs

Three dimesitylborane-containing fluorophores with various π-conjugated systems attached at the 9th position of carbazole, namely, 9-(4′-bromobiphenyl-4-yl)-9H-carbazole (Cz9Ph2B), 9-(4-(5-(dimesitylboryl)thiophen-2-yl)phenyl)-9H-carbazole (Cz9ThPhB), and 9-(4-(4-(dimesitylboryl)styryl)phenyl)-9H-carbazole (Cz9SB) were synthesized and their photophysical and electroluminescent properties were investigated for application in nondoped blue OLEDs as well as red phosphorescent OLEDs (PhOLEDs). The electron-accepting dimesitylboryl group and various π-conjugated segments appended to the electron-donating carbazole moiety impart the three fluorophores with bipolar transporting ability, and their energy levels are matched with those of the adjacent carrier-transporting layers. These bulky fluorophores are thermally stable with glass transition temperatures and degradation temperatures reaching up to 105 and 383 °C, respectively. In addition, efficient nondoped Cz9PhThB- and Cz9SB-based blue OLEDs with maximum currents of 1.51 and 4.03 cd A−1 and external quantum efficiencies (EQE) of 2.30 and 4.72% were achieved, respectively. Notably, the Cz9Ph2B-based red PhOLEDs exhibits a relatively low turn-on voltage (3.3 V) and high electroluminescence efficiencies (maximum current = 23.12 cd A−1 and EQE = 14.10%). Their performance is superior to that of the corresponding device using conventional 4,4′-N,N′-dicarbazolbiphenyl as the host material. Moreover, a maximum brightness of 39700 cd m−2 was also achieved.