Nitrogen introduction of spirobifluorene to form α-, β-, γ-, and δ-aza-9,9′-spirobifluorenes: New bipolar system for efficient blue organic light-emitting diodes

Four aza-9,9′-spirobifluorenes (aza-SBFs) with nitrogen atom at different positions of one fluorene moiety were synthesized to study the structure-properties relationships. α-Aza-SBF and β-aza-SBF possessed almost completely separated the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), while γ-aza-SBF and δ-aza-SBF showed overlapped HOMO and LUMO orbitals. The aza-SBFs showed excellent bipolar features and good thermal stabilities than those of SBFs. The maximum current efficiencies (CE) of α-, β-, γ-, and δ-aza-SBF-based OLEDs were 28.8, 24.9, 25.5, and 27.2 cd/A, respectively. Compared to the SBF, all of four aza-SBFs showed better devices performances. The CE and power efficiency (PE) of OLED based on α-aza-SBF was 28.8 cd/A and 22.6 lm/W, while the SBF-based OLED was only 12.3 cd/A and 8.2 lm/W. The maximum external quantum efficiency of α-aza-SBF-based OLED was 15.4%, which was 2.5 times than that of the SBF-based one (6.6%) due to introduction of nitrogen improving electron transporting. Novel materials based on these components and their potential applications in organic electronics were expected due to their excellent bipolar features.