Synthesis And Electroluminescence Of Dendritic Thieno-[3, 4-b]-Pyrazine Based Red Light-Emitting Materials

Organic light-emitting diodes (OLEDs) have been receiving more and more research and industry attentions due to practical application in large-area flat-panel displays and solid-state lighting. Among the three primary colors, the red light-emitting materials and devices still lag behind in terms of efficiencies and stability. Red host emitters have currently emerged as a trend since they are capable of fabricating non-doped OLEDs, which are apparently much easier than the doping technique. In addition, to be solution-processible is one important merit for light-emitting materials nowadays. Dendritic molecules are a new type of solution-processible light-emitting materials besides the traditional polymers. Dendrimers combine the merits of both small molecules and polymers and have been accepted as the third type of light-emitting materials.Solution-processible red hose light-emitting material and its non-doped red OLEDs are researched in this paper. Thieno-[3,4-b]-pyrazine and its derivatives are a groups of promising red fluorophores with extra large Stokes' shifts and high fluorescence quantum yields (ΦF). However, thieno-[3,4-b]-pyrazine is a planar chromophore. It is necessary to eliminate the strong intermolecular interaction and the consequent fluorescence-quenching by intelligent structure modification if efficient solid-state emission is desired for thieno-[3,4-b]-pyrazine derivatives. In addition, there have been only few reports on small molecular thieno-[3,4-b]-pyrazine based red emitters that were used for OLEDs by vacuum evaporation technique. The solution-processible thieno-[3,4-b]-pyrazine based red emitting materials and their application in OLEDs are still absent so far. Therefore, it is the aims of this paper to develop novel solution-processible thieno-[3,4-b]-pyrazine molecules and to fabricate their efficient non-doped red OLEDs via solution methods. Three types ten novel thieno-[3,4-b]-pyrazine-cored dendrimers were designed and synthesized. These dendritic materials were used as host emitting layer to fabricate non-doped OLEDs by spin coating technique, and the electroluminescent properties were investigated."Emissive core-charge transporting groups" type, in which the hole transporting arylamino or carbazolyl groups are linked directly to the thieno-[3,4-b]-pyrazine core. C1-TP and C2-TP exhibit saturated red fluorescence and have better solubility than the parent molecule TP. The peripheral carbazole units facilitate the hole transporting ability in the neat films of C1-TP and C2-TP. Non-doped OLEDs are fabricated by spin coating technique using these dendritic materials as host emitting layer. However, both theΦF of C1-TP and C2-TP and their electroluminescent performance were rather than ideal. This is because the intramolecular charge transfer (ICT) from the carbazole or amino groups to the thieno-[3,4-b]-pyrazine core severely quenched the light-emission of the core."Emissive core-spacers" type, in which the polyphenyl or diphenyl-benzopyrene spacers are connected to the thieno-[3,4-b]-pyrazine emissive core. The bulky aromatic spacers groups provide efficient site-isolation effect on the planar emissive core and significant molecular weight. These compounds exhibit pure saturated red fluorescence in solution and solid film. Compared with the diphenyl-benzopyrene based P-DTP and TP-DTP, the polyphenyl based DTP has better solubility and can be synthesized via easier procedure. They exhibited similar EL performance in their non-doped OLEDs. In addition, DTP performed better in OLEDs than its second-generation counterpart D2TP.In particular, DTP has both excellent solubility and thermal stability. Therefore, two types of non-doped OLEDs were successfully fabricated either by spin coating or vacuum evaporating DTP layer. Due to introduction of the hole transporting layer and the improved charge balance in the evaporated device, a maximum brightness of 1753 cd m-2 and a peak current efficiency of 0.74 cd A-1 were obtained, which represents the best efficiency of thieno-[3,4-b]-pyrazine-based red emitting materials reported so far."Emissive core-spacers-charge transporting groups" type molecules were designed and synthesized, in which the hole transporting arylamino or carbazolyl groups are connected through the pentaphenylphenyl spacers to the thieno-[3,4-b]-pyrazine core. In this configuration, the ICT effect was effectively decreased by the bulky spacers and the target dendrimers N-DTP and C-DTP have higherΦF and proper hole transporting ability. The solution processed OLEDs containing C-DTP and N-DTP as non-doped emitting layer exhibited a maximum luminance of 925 cd m-2 and 1020 cd m-2 and a current efficiency of 0.53cd/A and 0.58 cd/A, respectively. These are the best performance for spin coated OLEDs in this paper. N-DTP and C-DTP are already comparable in EL efficiency with the vacuum evaporated thieno-[3,4-b]-pyrazine analogues reported in literatures, however, they are superior in the merit of being solution-processible.In summary, the synthesis and application of the solutin-processible thieno-[3,4-b]-pyrazine based red emitting dendrimers were reported for the first time in this paper. They have merits of large Stokes'shifts, good solubility and saturated red emission in their non-doped OLEDs. They are superior to their reported analogues in terms of solution-processible merit and comparable or better EL performance.