By Xinjun Xu Shiyan Chen Gui Yu* Chong抋n Di Han You Dongge Ma and Yunqi Liu*

Since organic light-emitting diodes (OLEDs) were developed by Tang and Van Slyke electroluminescent (EL) materials have attracted significant attention due to their potential application in flat panel displays and illuminations. High-performance red- green- and blue-emitting materials and devices are essential to achieve full color displays. High-efficiency red and green OLEDs have already been described whereas highly efficient blue-light emission remains a challenge. A number of blue-emitting materials have been explored.  However most of these materials exhibit large energy gaps and low electron affinities (EA) leading to inefficient electron injection into the blue emitters. Therefore it is necessary to synthesize blue-emitting materials with high EA values facilitating injection and transport of electrons. It is well known that introducing strong electron-accepting cyanic groups into organic molecules would enhance the EA values resulting in efficient electron injection. Nevertheless application of organic molecules with strong electron-withdrawing groups as an emissive layer in OLEDs is very limited due to formation of exciplexes with the electron-donating arylamine compounds which are widely used as the hole-transporting materials (HTMs). Consequently it is a major challenge to eliminate formation of exciplexes between the HTMs and blue-emitting acceptors and to obtain blue emission originating from the electron acceptors.

We report the synthesis and characterization of 23-dicyano-56-di-(4-(2345-tetraphenylphenyl)phenyl)pyrazine (CPP chemical structure is shown in Fig. 1) and 67-dimethyl-23-di-(4-(2345-tetraphenylphenyl)phenyl)quinoxaline (MPQ). The blue light-emitting material CPP contains strong electron-accepting groups which can form exciplex with electron-donating arylamines widely used as hole-transporting materials. In the organic light-emitting diodes inserting the MPQ layer between the hole-transporting and CPP layers can effectively suppress formation of exciplexes and give high-efficiency blue-light emission (Fig. 2) from the CPP layer with a maximum luminance of 6230 cd/m² and a maximum luminous efficiency of 5.2 cd/A (Fig. 3). Our results provide a possibility to obtain high-efficiency blue electroluminescence from the compounds with strong electron-accepting groups.

Adv. Mater. (2007 19(9) p.1281-1285)

Figure 1. Chemical structure of CPP molecule and schematic of electroluminescent devices.

Figure 2. Electroluminescence spectra of ITO/NPB/MPQ/CPP/Al (dotted line) and ITO/NPB/CBP/MPQ/CPP/Al (solid line).

Figure 3. The luminous efficiency-current density curves of the devices ITO/NPB/MPQ/CPP/Al (? and ITO/NPB/CBP/MPQ/CPP/Al (▪).