Highly efficient organic light-emitting diodes (OLEDs) based on an iridium complex with rigid cyclometalated ligand

A red-emitting iridium complex (DMBA)2Ir(acac) (DMBA = 5,6-dihydro-9,10-methylenedioxy benzo[c]acridine, acac = acetylacetone) containing rigid cyclometalated ligand DMBA was synthesized and its photophysical, electrochemical and electroluminescent properties were characterized. Organic light emitting devices utilizing the complex as dopant emitter were fabricated. When double hole-transporting layers were adopted, the brightness and efficiencies of electroluminescent devices were improved greatly. Based on the excimer emission, a maximum brightness of 66,312 cd m−2 at 15 V, together with a maximum external quantum efficiency of 17.7% at 5.0 V was achieved at a doping concentration of 5%. The result suggests that the introduction of a methylenedioxy group into the N-heterocycle of the cyclometalated ligand leads to higher-energy MLCT transitions and a blue shift of the photoluminescence spectrum.