Connectivity and twist: engineering high-performance green phosphorescent OLEDs using unipolar symmetric bicarbazole regioisomers

Abstract

Molecular structural differences by position can be crucial in developing promising materials for device applications. We synthesized four regioisomeric symmetric bicarbazoles (BCzPh) with distinct dihedral angle twists using oxidative C–C coupling or transition-metal catalyzed Suzuki coupling methods. The structural differences in connectivity manifested in fine-tuning of the photophysical, thermal, and electrochemical properties of the materials, as well as the device performance. Particularly, understanding the balance between the resonance and conjugation effects seems crucial for manipulation of triplet energy levels. Our findings indicate that bicarbazoles with greater twist angles exhibit larger singlet–triplet/HOMO–LUMO energy gaps, and improved power and luminance efficiencies, benefiting phosphorescent organic light-emitting diode (PhOLED) devices. The external quantum efficiencies of PhOLEDs were over 23.4% and 23.9% for BCzPh-based devices B and D, with device C reaching a maximum brightness of 203 490 cd m⁻², followed by device A at 96 953 cd m⁻². Notably, all BCzPh compounds served as excellent host materials, demonstrating stable, high-purity green-color emission in devices that turned on at voltages as low as 2.2 V.