Asymmetric tris-Heteroleptic Iridium^III Complexes Containing a 9-Phenyl-9-phosphafluorene Oxide Moiety with Enhanced Charge Carrier Injection/Transporting Properties for Highly Efficient Solution-Processed Organic Light-Emitting Diodes

A cyclometalating ligand containing a 9-phenyl-9-phosphafluorene oxide (PhFlPO) moiety has been synthesized and used to construct asymmetric tris-heteroleptic cyclometalating Ir^III complexes in combination with other ppy-type (Hppy = 2-phenylpyridine) ligands containing a functional group with a different charge carrier injection/transporting character. Their photophysical properties, electrochemical behaviors, and electroluminescent (EL) performances have been characterized in detail. Time-dependent density functional theory (TD-DFT) and natural transition orbital (NTO) calculation were carried out to gain insight into the photophysical properties of these complexes. The NTO results show that the characters of the lowest triplet excited states (T₁) can be delicately manipulated through the combination of different cyclometalating ligands. In addition, the strong electron injection/transporting (EI/ET) ability associated with the PhFlPO moiety can confer EI/ET properties to the asymmetric tris-heteroleptic cyclometalating Ir^III complexes. Consequently, the solution-processed organic light-emitting diodes/devices (OLEDs) based on these asymmetric tris-heteroleptic Ir^III phosphorescent complexes can exhibit outstanding electroluminescent (EL) performances with the maximum external quantum efficiency (η_ext) of 19.3%, current efficiency (η_L) of 82.5 cd A^-1, and power efficiency (η_P) of 57.3 lm W^-1 for the yellow-emitting device. These results show the great potential of a PhFlPO moiety in developing phosphorescent emitters and functional materials with excellent EI/ET properties.