Carbazole-benzonitrile based organic semiconductors: Synthesis, characterization and electroluminescent property

Abstract

In order to explore the relationship between molecular structure and the potential of being host materials, two carbazole-benzonitrile based molecules (DCzBNPh-1 and DCzBNPh-2) with possible high triplet energy and small energy gap (△ E ST ) between lowest singlet state (S 1 ) and lowest triplet state (T 1 ) were synthesized, forming the core moiety of terphenylene. Their initial difference is the positions of cyano (CN) groups. Studies on single crystal analysis, mechano-responsiveness, density functional theory (DFT) calculations, cyclic voltammetry (CV) measurements, thermal tests and solution photophysical properties verify their qualification of being host materials and clarify the CN units related difference. Then DCzBNPh-1 and DCzBNPh-2 were applied as host materials, matching tris(2-phenylpyridine)iridium (Ir(ppy) 3 ) as the dopant to construct phosphorescent organic light-emitting diodes (PhOLEDs) with the same optimal structure. Despite their unbalanced carrier transporting character, for DCzBNPh-2 based device, the maximum emission brightness reaches 61,760 cd m −2 (weaker than DCzBNPh-1), while the maximum external quantum efficiency (EQE max ) of 22% and the turn-on voltage of 2.5 V surpass those of DCzBNPh-1. The satisfying result manifests DCzBNPh-2 should be the best host of this series. • Two host materials with high triplet energy has been designed and synthesized. • Terphenylene-cored CzBN isomers fill in the vacancy of multicore based CzBN derivatives. • Green PhOLEDs were prepared with the maximum external quanmtum efficiency about 22% and low turn-on voltage of 2.5 V.