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

Abstract

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 η_EQE,max of 24.0%, a η_C,max of 47.8 cd A⁻¹ and a η_P,max of 45.6 lm W⁻¹. Furthermore, a two-color, all-phosphor single layer white device displayed a η_EQE,max of 20.7%, a η_C,max of 62.1 cd A⁻¹ and a η_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.