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

Abstract

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 = 2phenylpyridine) 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. Timedependent 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 (T 1 ) 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 lightemitting 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.

Novel Emission Color‐Tuning Strategies in Heteroleptic Phosphorescent Ir(III) and Pt(II) Complexes

“…Our recent results should indicate this color‐tuning strategy by devising asymmetric heteroleptic Ir(III) complexes with two different cyclometalating ppy‐type ligands . Since the emission character of the ppy‐type Ir(III) complexes is dominated by the nature of cyclometalating ppy‐type ligands, incorporation of ligands with different electrochemical properties into one asymmetric heteroleptic Ir(III) complex will furnish a flexible modulation of the energy level of the lowest excited state T 1 .…”

“…It is well accepted that balanced injection/transport for both kinds of charge carriers can play a crucial role in furnishing high EL performance of the devices by improving the recombination efficiency of the charge carriers . Introduction of OPh, NPh 2 unit into the ppy ligands can enhance HI/HT ability to these Ir(III) phosphors, while B(Mes) 2 unit, 9‐Phenyl‐9‐phosphafluorene oxide (PFO) moiety, SO 2 Ph group in organic ligands can furnish their improved EI/ET features . Therefore, more importantly, introduction of hole injection/transport (HI/HT) and electron injection/transport (EI/ET) units into different cyclometalating ligands of asymmetric heteroleptic Ir(III) complexes can endow these phosphors with ambipolar or enhanced EI/ET features, which is beneficial to give high EL performance.…”

“…On the basis of these considerations aforementioned, our group has successively developed new ppy‐type Ir(III) complexes 62 – 75 with the aim of not only tuning phosphorescence emission color but also achieving high EL performance (Figures , , , and ) . By judicious combination of two ppy‐type ligands with different electrochemical properties into one asymmetric heteroleptic structure, the broad range of color tuning from yellow‐green (complex 67 , λ =544 nm) to red (complex 71 , λ =623 nm) is achieved (Figures and ).…”

“…Complexes 67 and 68 also consist of one relative electron‐rich ppy‐type ligand with OPh and NPh 2 moieties and another electron‐deficient ppy‐type ligand bearing PFO moiety (Figures ) . Their emissive T 1 excited states exhibit LLCT and MLCT characters as well.…”

“…Complexes 67 and 68 show ambipolar features, which is helpful to improve the recombination efficiency of the charge carriers in OLEDs. Owing to this unique electronic character, the yellow‐green‐emitting OLED with dopant 67 emits at 3.6 V and achieves maximum efficiencies of 19.3 % (EQE), 82.5 cd A −1 (CE), and 57.3 lm W −1 (PE) …”