Synthesis and Characteristics of Organic Red-Emissive Materials Based on Phenanthro[9,10-d]imidazole

Red emission is one of the three primary colors that are essential for the realization of full-color displays and solid-state lightings. A high solid-state efficiency is a crucial factor for the applications in organic light-emitting diodes (OLEDs). In this work, two new donor-acceptor-donor type phenanthro[9,10-d]imidazole (PIM)-based derivatives, (2Z,2'Z)-2,2'-(1,4-phenylene)bis(3-(4-(1-phenyl-1H-phenanthro[9,10-d]imidazol-2-yl)phenyl)acrylonitrile) (PIDSB) and 2,3-bis(4'-(1-phenyl-1H-phenanthro[9,10-d]imidazol-2-yl)-[1,1'-biphenyl]-4-yl)fumaronitrile (PIDPh), are designed and synthesized. Both of them possess high thermal stabilities. PIDPh shows typical characteristics of aggregation-induced emission enhancement, while PIDSB displays an aggregation-caused quenching effect. They both exhibit significant red-shifted emissions compared with PIM owing to intramolecular charge transfer. In the film state, the emission peaks of PIDSB and PIDPh are located at 538 nm and 605 nm with high photoluminescent quantum yields of 63.82 % and 41.26 %, respectively. The non-doped OLED using PIDPh as the active layer shows the maximum external quantum efficiency of 2.06 % with a very low efficiency roll-off, and exhibits the electroluminescent peak at 640 nm with a Commission Internationale de l'Éclairage coordinate of (0.617,0.396), meeting well the criteria of red OLEDs.