Solution-processible Thieno-[3,4-b]-pyrazine Derivatives with Large Stokes Shifts for Non-doped Red Light-emitting Diodes

A group of novel thieno‐[3,4‐b]‐pyrazine‐cored molecules containing polyphenyl dendrons with or without arylamino or carbazolyl surface groups (DTP, N‐DTP and C‐DTP) are synthesized and investigated. They are characterized by extra large Stokes shifts of over 250 nm. In addition, to provide the site‐isolation effect on the planar emissive core, the bulky dendrons enable these molecules to be solution processible. The peripheral carbazolyl or arylamino units facilitate the hole transporting ability in the neat films of these molecules. These dendritic materials are used as a non‐doped emitting layer to fabricate organic light‐emitting diodes (OLEDs) using a spin coating technique and saturated red emission is obtained. The dendritic molecules with arylamino or carbazolyl surface groups (N‐DTP and C‐DTP) exhibit a brightness of 1020 cd m−2 and a luminous efficiency of 0.6 cd A−1, both higher than the dendritic analog without the surface functional groups (DTP), even superior to the small molecular reference compound which fails to transmit pure red emission under identical conditions. This performance is also comparable with that from vacuum deposited thieno‐[3,4‐b]‐pyrazine‐based counterparts and that for some other solution processible red fluorescent dendrimers. This is the first example of solution processible thieno‐[3,4‐b]‐pyrazine derivatives for OLED applications. A design strategy is applied to the planar thieno‐[3,4‐ b]‐pyrazine emissive core by attaching bulky polyphenyl dendrons with charge transporting surface groups to form solution processible dendritic derivatives. The red OLEDs fabricated by spin coating the solutions of these derivatives as non‐doped emitting layers exhibit saturated red electroluminescence with better or comparable performance to vacuum deposited counterparts or other solution processible red fluorescent dendrimers.