A meta-molecular tailoring strategy towards an efficient violet-blue organic electroluminescent material

In this paper, an efficient violet-blue emitter 4,4′′-bis(1-(4-(tert-butyl)phenyl)-1H-phenanthro[9,10-d]imidazol-2-yl)-1,1′:3′,1′′-terphenyl (m-BBTPI) was designed and synthesized by linking two phenanthroimidazole units via the meta position of a freely rotatable phenyl bridge. The present design provides a suitable level of conjugation between the two phenanthroimidazole units such that fluorescence is strengthened over the single unit while a violet-blue emission can be maintained by limiting the amount of redshift. The new emitter m-BBTPI is also found to have good thermal stability, strong violet-blue emission and bipolar charge transporting properties. An electroluminescent device using m-BBTPI as a non-doped emission layer shows a low turn-on voltage (3.2 V), good colour purity (0.16, 0.06) as well as high current and power efficiencies (1.99 cd A^-1, 1.81 lm W^-1). These performance parameters are comparable to the state-of-the-art non-doped violet-blue OLEDs.