Novel Thieno-[3,4-b]-Pyrazines Cored Dendrimers with Carbazole Dendrons: Design, Synthesis, and Application in Solution-Processed Red Organic Light-Emitting Diodes

A series of novel red-emitting thieno-[3,4-b]-pyrazine-cored molecules containing oligo-carbazole dendrons (called C1-TP, C2-TP) are synthesized. Their photophysical, electrochemical, and electroluminescent properties are investigated. The peripheral carbazolyl units facilitate the hole transporting ability and inhibit the intermolecular interactions, but quench the fluorescence of the thieno-[3,4-b]-pyrazine core through Intramolecular Charge Transfer (ICT). Introduction of a polyphenyl spacer between the core and the first generation carbazole dendrons, i.e., C-DTP, decreases the ICT efficiency. In addition to providing the site-isolation effect on the planar emissive core, these bulky dendrons enable these molecules to be solution processable. As a result, efficient OLEDs with saturated red emission are fabricated by spin coating technique using these dendritic materials as nondoped emitting layer. C-DTP exhibits much better device performance than C1-TP and C2-TP, while the small molecular reference compound containing neither the spacer nor the carbazole dendrons (TP) fails to transmit pure red emission under identical conditions. A brightness of 925 cd m(-2) and a luminous efficiency of 0.53 cd A(-1) are obtained for C-DTP, which are comparable with OLEDs fabricated from thieno-[3,4-b]-pyrazine-based counterparts by the vacuum deposition method or those assembled with other red fluorescent dendrimers via the solution processing method.