Design, synthesis, characterization and application of a novel electron-deficient moiety 1,5-diazacarbazole in high triplet energy host materials

A novel electron deficient, high triplet energy moiety 1,5-diazacarbazole (NCz) was designed, synthesized and firstly introduced for the construction of phosphorescent host materials. And the two synthesized NCz derivatives NCzBPhCz and NCzBPhNCz possessed high triplet energy (E-T = 2.85 eV) and bipolar transporting ability. The structure and nitrogen atom position in the diazacarbazole unit were verified by crystallographic analysis of their single crystals and by 2D-NMR. The frontier molecular orbital distribution, electrochemical, thermal and photophysical properties, and electroluminescence of the two new host materials were systematically investigated. Finally, the bis(3,5-difluoro-2-(2-pyridyl) phenyl-(2-carboxypyridyl) iridium(III)) (FIrpic)-doped NCzBPhCz based blue device exhibited high efficiencies with a eta(ZEQE,max) of 24.0%, a eta(C,max) of 47.8 cd A(-1) and a eta(P,max) of 45.6 lm W-1. Furthermore, a two-color, all-phosphor single layer white device displayed a eta(EQE,max) of 20.7%, a eta(C,max) of 62.1 cd A(-1) and a eta(P,max) of 49.5 lm W-1. This study definitely demonstrates that the novel electron deficient unit NCz has great potential for the fabrication of host materials and provides a useful strategy in the design of new high triplet energy and good charge-transporting hosts.