Evaluating the Effect of Extended Conjugation and Regioisomerism on the Optoelectronic Properties and Device Efficiencies of Blue Light-Emitting Benzobisoxazoles

Four new blue light-emitting materials based on benzo[1,2-d:4,5-d ']bisoxazole (BBO) have been synthesized, characterized, and fabricated into organic light-emitting diode (OLED) devices. Using a combination of theoretical and experimental methods, we investigated the effect of conjugation by comparing bulky alkyl groups and planar aromatic groups along the 2,6-axis. Two of these molecules, PB2Cz and PB3Cz, are cross-conjugated cruciform-type BBOs with phenyl and carbazole groups along the 2,6 and 4,8 axes, respectively. The other two molecules, AB2Cz and AB3Cz, have extended conjugation via the carbazole groups along the 4,8-axis and bulky adamantyl groups along the 2,6-axis. Concurrently, we explored the effect of regioisomerism on optoelectronic and device properties arising from attaching carbazole at the 2- (2Cz) or 3- (3Cz) position along the 4,8-axis. The materials' geometric and electronic properties were predicted using time-dependent density functional theory (TD-DFT) calculations at the mPW3PBE/SV level. The molecules' photoluminescent properties were measured in solution and film states. The BBO molecules were used as dopants in mixed host/guest OLED devices, producing teal to deep blue emission. Specifically, the AB2Cz and AB3Cz, with adamantyl on the 2,6-axis, exhibit blue to deep-blue emissions of 414-422 nm (CIE x < 0.20, CIE y < 0.10). In comparison, PB2Cz and PB3Cz have slightly longer emission wavelengths of 472-476 nm (CIE x < 0.16, CIE y < 0.28) and high brightness of 2700-3500 cdm(-2). The BBOs with 2Cz resulted in more efficient devices with EQEs of similar to 2.8-3.2%, while the 3Cz BBOs had EQEs of similar to 1.1-1.5%. This work provides insight into designing efficient, purely organic blue-fluorescent OLED materials based on the BBO moiety.