Effect of the bipyridine ligand substituents on the emission properties of phosphorescent Ir(III) compounds

Advanced Optical Materials

Lee

et al. 2019

Self Cite

electrochemical stability, easily tunable energy gaps, and high photoluminescence (PL) quantum efficiency. [8] Toward blue phosphorescent OLEDs (PHOLEDs), there has been significant recent research effort focused on the development of new blue phosphorescent complexes with horizontally oriented emitting dipoles in order to achieve high external quantum efficiency (EQE). The theoretical limit of EQE in OLEDs is 25-30%; without any extra light extraction structures and under the presence of randomly oriented emitting dipoles. [9] In comparison to the EQE of green to red phosphorescent iridium complexes reported in previous studies, [10][11][12][13][14][15][16] blue phosphorescent iridium complexes with over 30% of EQE are still rare. [17][18][19][20] For blue phosphorescent OLEDs, the use of 3,3-di(9H-carbazol-9-yl) biphenyl (mCBP) and diphenylphosphineoxide-4-(triphenylsilyl)phenyl (TSPO1), with large triplet energy as a mixed host, has been reported recently for increasing the EQE up to 31.9%. [21] Therefore, the employment of a suitable host in PHOLEDs is considered an important factor for increasing the EQE. In our earlier research, blue PHOLEDs with 22.5% of EQE were successfully developed using a bipolar host material (9-[3-(9H-carbazol-9-yl)phenyl]-9H-carbazol-3-yl) diphenylphosphine oxide, mCPPO1), and an exciton-blocking material (TSPO1) with high triplet energy. [22] The resulting high efficiency is thought to be attributed to the improvement of the charge balance in the emitting layer (EML), and effective triplet exciton confinement by the TSPO1 in the emitter. Device optimization is still needed, depending on the varied materials including a host or common layer materials combined with the development of phosphorescent triplet emitters. Both the device engineering for optimization and the development of a new host play a key role in the enhancement of EQE; yet the development of phosphorescent emitters with high PL quantum yield (PLQY) is still necessary because the two factors are directly proportional to each other. [23] Consequently, we have prioritized developing bipyridine-based iridium complexes for the creation of efficient blue phosphorescent materials over the past 10 years. [8,[24][25][26][27][28][29][30] Bipyridine ligand, especially 2,3′-bipyridine, possesses larger triplet energy (T 1 = 2.70-2.82 eV) than phenylpyridine (ppy), and also provides an appropriate C^N chelating coordination mode to the metal ion. [31] Moreover, bipyridine-based blue phosphorescent iridium complexes have shown high PLQY and EQE when used in PHOLEDs. [32] This

Section: Introductionmentioning

confidence: 99%

Blue Phosphorescent Ir(III) Complexes Achieved with Over 30% External Quantum Efficiency

Zaen

Advanced Optical Materials

Lee

et al. 2019

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“…Therefore, a ligand characterized by a large triplet energy is needed to develop blue phosphorescent emitting materials. Recently, we reported the evidence that the triplet energy of the bipyridine ligand is greater than that of phenylpyridine (ppy) . In addition, we reported a difference in emission maxima greater than 50 nm and a significant efficiency change depending on the nature of the electron‐donating substituent group in the bipyridine ligand.…”

Section: Methodsmentioning

confidence: 94%

“…1 It is well known that the phosphorescent emission of iridium(III) complexes based on the C,Nchelating bipyridine ligand mostly originates from the triplet ligand-centered ( 3 LC) transition combined with a metalto-ligand charge transfer transition (MLCT). 3 In addition, we reported a difference in emission maxima greater than 50 nm and a significant efficiency change depending on the nature of the electron-donating substituent group in the bipyridine ligand. Recently, we reported the evidence that the triplet energy of the bipyridine ligand is greater than that of phenylpyridine (ppy).…”

mentioning

confidence: 82%

“…10 However, the emission maxima of both complexes appeared to be slightly shifted to shorter wavelengths (by a few nm) compared to that of Ir(III)bis(4,6-(difluorophenyl)pyridinato-N,C 2' )picolinate (FIrpic). Both the acetylacetonate and picolinate ancillary ligands cause destabilization of both the HOMO and LUMO levels compared to that of the homoleptic counterpart Ir(OR 2 pypy) 3 . 10 Triplet (T 1 ) energy estimated from the emission spectra at 77 K are 2.62 for 1 and 2.67 for 2, respectively.…”

mentioning

confidence: 99%

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Blue Phosphorescent Iridium(III) Compounds with the 2′,6′‐Diisopropoxy‐2,3′‐Bipyridine Ligand

Kim

Bulletin Korean Chem Soc

et al. 2018

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Cyclometallated iridium(III) complexes, especially those bearing the C,N-chelated bipyridine ligand, have attracted enormous attention for more than a decade due to their high efficiency in phosphorescent organic-light emitting diodes (PHOLEDs). 1 It is well known that the phosphorescent emission of iridium(III) complexes based on the C,Nchelating bipyridine ligand mostly originates from the triplet ligand-centered ( 3 LC) transition combined with a metalto-ligand charge transfer transition (MLCT). 2 Therefore, a ligand characterized by a large triplet energy is needed to develop blue phosphorescent emitting materials. Recently, we reported the evidence that the triplet energy of the bipyridine ligand is greater than that of phenylpyridine (ppy). 3 In addition, we reported a difference in emission maxima greater than 50 nm and a significant efficiency change depending on the nature of the electron-donating substituent group in the bipyridine ligand. Therefore, by incorporating proper substituents into the bipyridine ligand, significant emission and efficiency changes can be expected. However, due to its high reactivity and low selectivity, the introduction of other functionalized substituents into the pyridine ring is very difficult and limited. 4 There is a recent increasing interest in the introduction of ancillary ligand into ligand frameworks because it can improve charge trapping and electron mobility in OLEDs as well as cause emission energy changes of dopant molecules. 5 From the perspective of an increasing demand for developing high efficiency and stable phosphorescent iridium(III), especially blue iridium(III) phosphor, the study of the effect on ancillary ligand should be continuous. Herein, we report the crystal structures of two blue phosphorescent Ir(III) compounds bearing bulky isopropoxy substituents of the bipyridine and ancillary ligands. Moreover, the photophysical and electrochemical properties of these compounds were systematically investigated for the purpose of determining the possibility of PHOLED application.According to our previous reports, the 2 0 ,6 0 -diisopropoxy-2,3 0 -bipyridine ligand (L) and its Ir(III) compounds (1 and 2) were synthesized in moderate to high yields (Scheme 1). 6,7 The structures of both 1 and 2 were confirmed by X-ray diffraction analysis and their structures and packing diagrams are shown in Figures 1 and 2, respectively. In particular, there are three possible geometric isomers C,N-trans, N,N-trans, and C,C-trans for 1, while four geometric isomers, namely facial (fac), meridional (mer), N,N-trans-meridional, and C,C-trans-meridional configurations, are possible for 2. 8 Single crystal X-ray analysis indicates that both 1 and 2 exhibit distorted octahedral geometries around the iridium metal center, as shown in Figures 1 and 2. However, there is a difference in their structural features. In the case of 1, only the N,N-trans configuration was observed, while 2 shows the N,N-trans-meridional configuration. This can be due to the strong backing-bonding between t...

“…460 nm, which clearly supports the fact that the triplet energy (T 1 ) of bipyridine is greater than that of fluorinated phenypyridine(dfppy). 15 For all compounds, the emission in a CH 2 Cl 2 glassy matrix at 77 K was measured to evaluate triplet energy. As shown in Figure 2, well-resolved emission was observed in all cases, which could be due to the 3 LC transition.…”

Section: Mlctmentioning

confidence: 99%

Homoleptic Iridium(III) Compounds Bearing Bulky Bipyridine Ligand for Potential Application to Organic Light‐emitting Diodes

Kim

Bulletin Korean Chem Soc

et al. 2017

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Two sky-blue phosphorescent iridium compounds, fac-tris(2 0 ,6 0 -diisopropoxy-2,3 0 -bipyridinato)-N,C 4 )Ir (III) (1) and fac-tris(2 0 ,6 0 -diisopropoxy-4-tert-butyl-2,3 0 -bipyridinato)-N,C 4 )Ir(III) (2), were synthesized by a one-pot reaction of a reactive Ir(I) compound with a corresponding bipyridine ligand, to investigate their photophysical and electrochemical properties for potential application to white organic light-emitting diodes (WOLEDs). Their structures were confirmed by varied spectroscopic methods. Results indicated that both compounds possess facial geometry. The absorption, emission, thermal stability, and electrochemical properties were also investigated systematically. The two compounds showed sky-blue emission with λ max = 462-463 nm; their photoluminescence quantum efficiencies relative to that of FIrpic werẽ 0.3-0.4. Compound 2 with a bulky substituent displayed the suppression of concentration quenching at high concentration, and its emission was less shifted to longer wavelengths compared to compound 1. Owing to the bulky substituent, the quantum efficiency of 2 is higher than that of its nonsubstituted counterpart. Therefore, the introduction of the bulky substituent in the ligand framework is an important strategy for developing high-efficiency blue phosphorescent materials. The two compounds developed in this study show high thermal and electrochemical stability, making them suitable candidates for OLED applications.