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)2Ir­(dipba) (fIr1) and (f2piq)2Ir­(dipba) (dfIr2), comprising two cyclometaling ligands of fluorophenyl-isoquinoline
derivatives (fpiq and f2piq) 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.