Rational Design and Characterization of Heteroleptic Phosphorescent Complexes for Highly Efficient Deep-Red Organic Light-Emitting Devices.

Two new deep-red iridium(III) complexes, (fpiq)2 Ir(dipba) (fIr1) and (f2 piq)2 Ir(dipba) (dfIr2), comprising two cyclometaling ligands of fluorophenyl-isoquinoline derivatives (fpiq and f2 piq) and a N-heterocyclic carbene (NHC)-based ancillary ligand of N,N'-diisopropylbenzamidinate (dipba) are designed, synthesized, and characterized. Given the unique four-membered Ir-N-C-N backbone built by the metal center and the ancillary ligand, both phosphors achieve significant improvement for their comprehensive optoelectronic characteristics. Density function theory (DFT) calculations and electrochemical measurements support the genuine pure red phosphorescent emission of fIr1 and dfIr2 based on their clearly distinct electron density distributions of the HOMO/LUMO orbitals compared with other red-emitting Ir(III) derivatives. Both new phosphors show deep-red emission with λmax values in the region of 650-660 nm with high PLQYs and short excited-state lifetimes. The phosphorescent organic light emitting diodes (PhOLEDs) based on fIr1 and dfIr2 realize deep-red EL with the stable CIEx,y coordinates of (0.70, 0.30) and (0.69, 0.31), the peak EQE/PE values of 15.4%/9.3 lm W-1 and 16.7%/10.4 lm W-1 , respectively, which maintain such high levels as 10.6%/3.5 lm W-1 and 10.8%/3.6 lm W-1 at the practical luminance of 1000 cd m-2 . They are the highest EL values reported for the OLEDs with such deep-red CIE coordinates.