Efficient yellow OLEDs based on bis-tridentate iridium(III) complexes with two C∧N∧N-coordinating ligands

Article abstract of DOI:10.1016/j.ica.2019.119168

Four bis-tridentate iridium(III) complexes with two different C^∧N^∧N-coordinating ligands have been synthesized and characterized. All complexes exhibited yellow/orange emission peaking at 580–604 nm with short lifetimes of 1.4–1.6 μs

Full text of DOI:10.1016/j.ica.2019.119168

Journal Pre-proofs
Efficient yellow OLEDs based on bis-tridentate iridium(III) complexes with two
C ∧ N ∧ N-coordinating ligands
Yuan-Yuan Hu, Wei Luo, Yan Wang, Bi-Hai Tong, Man-Keung Fung, Qian-
Feng Zhang
PII:
S0020-1693(19)31154-5
https://doi.org/10.1016/j.ica.2019.119168
ICA 119168
DOI:
Reference:
Inorganica Chimica Acta
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6 August 2019
17 September 2019
22 September 2019
Please cite this article as: Y-Y. Hu, W. Luo, Y. Wang, B-H. Tong, M-K. Fung, Q-F. Zhang, Efficient yellow OLEDs
based on bis-tridentate iridium(III) complexes with two C ∧ N ∧ N-coordinating ligands, Inorganica Chimica Acta
(2019), doi: https://doi.org/10.1016/j.ica.2019.119168
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Efficient yellow OLEDs based on bis-tridentate iridium(III)
complexes with two C∧N∧N-coordinating ligands
Yuan-Yuan Hu,†a Wei Luo,†b Yan Wang,a Bi-Hai Tong,*a Man-Keung Fung*b and Qian-Feng Zhang*a
aKey Laboratory of Metallurgical Emission Reduction & Resources Recycling, Ministry of Education, Institute of
Molecular Engineering and Applied Chemistry, School of Metallurgy Engineering, Anhui University of Technology,
Maanshan, 243002, Anhui, China. E-mail: tongbihai@163.com, zhangqf@ahut.edu.cn
bJiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Institute of Functional Nano & Soft
Materials (FUNSOM) & Collaborative Innovation Center of Suzhou Nano Science and Technology, Soochow
University, Suzhou 215123, P. R. China. E-mail: mkfung@suda.edu.cn
† Authors contributed equally.
Abstract
Four bis-tridentate iridium(III) complexes with two different C∧N∧N-coordinating ligands have
been synthesized and characterized. All complexes exhibited yellow/orange emission peaking at
580–604 nm with short lifetimes of 1.4–1.6 μs and medium photoluminescence quantum yields
(PLQYs) of 31%–53%. These complexes exhibit good thermal stability with decomposition
temperature (Td) exceeding 310 ℃. The luminescence efficiency and thermal stability of iridium
complexes were improved by the m-substituted CF3 groups. As a result, complex 2 exhibited the
highest PLQYs of 0.53 and Td of 348 ℃. The OLEDs based on complex 2 showed good
performances. The device at 2% doping concentration showed a maximum current efficiency of 36.4
cd A-1, power efficiency of 29.1 lm W-1 and external quantum efficiency (EQE) of 14.4%. These
results suggest that these complexes have potential for the application in high-efficiency OLEDs.
Keywords: Bis-tridentate iridium(III) complex, Cyclometalation, C∧N∧N-coordinating ligands,
OLEDs
1. Introduction
1

Cyclometalated iridium(III) complexes have attracted considerable attention due to their
flexible color tenability, high luminescent efficiency and potential applications in organic
light-emitting diodes (OLEDs).[1] The most common cyclometalated iridium(III) complexes have
three bidentate ligands.[2] However, the multidentate ligands such as pincer tridentate ligand are
potentially more robust against the metal-ligand dissociation compared with bidetate counterpart.[3,4]
Meanwhile, due to the enhanced rigidity imposed by the tridentate ligand, these Ir(III) complexes
usually possess higher luminescent efficiency.[5] For example, the bis-tridentate Ir(III) complexes
have been proved to have higher thermal, photochemical stability and emission efficiency than the
tri-bidentate reference.[6-8] The structural advantages with high symmetry are also beneficial to the
functionalization of bis-tridentate Ir(III) complexes and the purification of products.[9]
The study of bis-tridentate Ir(III) complexes with monoanionic N∧C∧N ligands (i.e., 1,
3-di(2-pyridyl)benzene derivatives) was first carried out by Williams and coworkers.[10] They
systematically studied the preparation and photophysical properties of these complexes, but their
potential for OLED application has rarely been evaluated. Recently, the research of Chi and
coworker advanced the OLED application of bis-tridentate Ir(III) complexes.[3,11] They designed and
synthesized a series of novel charge-neutral bis-tridentate Ir(III) complexes with featured dianionic
tridentate chelates of 2-pyrazolyl-6-phenyl pyridine derivatives. These complexes exhibited adequate
stability and high electroluminescence efficiencies. Classically, the more desirable strategy for the
synthesis of charge-neutral bis-tridentate Ir(III) complexes include the collocation of one
monoanionic tridentate ligand and another dianionic tridentate ligand. For the monoanionic
∧
∧
tridentate ligand, the N
C
N-coordinating ligands, such as the aforementioned 1,
3-di(2-pyridyl)benzene derivatives are often used in the preparation of tridentate Ir(III) complexes.[10,
2

12]
Recently, we reported device based on bis-tridentate Ir(III) complexes with functional 1,
3-di(2-pyridyl)benzene tridentate ligands with a maximum EQE of 13.2% and luminance efficiency
of 41.4 cd A-1.[13] Another common monoanionic ligand is C∧N∧N-coordinating ligands such as
6’-phenyl-2, 2’-bipyridine.[9, 14-19] In theory, these ligands will have fewer side coordination reactions
than the N∧C∧N-coordinating ligands because of the guiding effect of central nitrogen atom. In fact,
no side reactions were found in the cyclometallation reactions of 6-phenyl-2, 2’-bipyridine with
other transition metal.[14] Mamo and coworkers even found the C∧N∧N-coordinating products from a
reaction of IrCl3 with an analogue of terpyridine.[18] Whittle and coworkers reported the synthesis of
cationic bis-tridentate iridium(III) complexes with the reaction of [Ir(dpyx)Cl(μ-Cl)2] (dpyxH =
1,3-di(2-pyridyl)-4,6-dimethylbenzene) with 6-phenyl-2,2′-bipyridine (phbpyH) in refluxing
ethylene glycol.[19] However the charge-neutral bis-tridentate Ir(III) complexes with C ∧ N ∧
N-coordinating monoanionic ligands were barely reported, and the associated chemistry and
optoelectronic applications need to be explored.
As an extension of our research, four charge-neutral bis-tridentate Ir(III) complexes had been
designed and synthesized in this paper. 4, 6-Diphenyl-2, 2'-bipyridine derivatives were employed as
the monoanionic tridentate ligands, and the highly reactive 2-pyrazolyl-6-phenyl pyridine derivatives
were used as the dianionic tridentate ligands. The effect of substituents on the photoelectric
properties of complexes was studied. OLEDs were fabricated to evaluate their application prospects
as efficient OLED emitters.
2. Experimental
2.1. Materials and characterization
3

All the commercially available reagents were used without further purification. All the reactions
were conducted under a N2 atmosphere. 6-Phenyl-4-(p-tolyl)-2, 2'-bipyridine (L1H) and
4-(4-bromophenyl)-6-phenyl-2, 2'-bipyridine (L2H) were purchased from Energy Chemical
Company. 2-Phenyl-6-(3-(trifluoromethyl)-1H-pyrazol-5-yl) pyridine (L3H2) was prepared as
previously described[13]. NMR spectra were measured with a Bruker AV400 spectrometer. High
resolution mass spectra (HRMS) were recorded on a TOF 5600plus instrument. Cyclic voltammetry
were investigated on a CHI1140B Electrochemical Analyzer. Thermogravimetric analysis (TGA)
was carried out on a METTLER TGA 1 instrument. Photoluminescence (PL) spectra were measured
with a PerkinElmer LS-55 fluorescence spectrophotometer. UV/Vis spectra were measured on a
Purkinje General TU-1901 spectrophotometer. PL quantum efficiency and lifetime were measured
with an Edinburgh FL920 instrument.
2.2. Preparation of ligands
1-(6-(3-(Trifluoromethyl)phenyl)pyridin-2-yl)ethanone:
A
mixture
of
1-(6-bromopyridin-2-yl)ethanone (2.0 g, 10.0 mmol), 3-(trifluoromethyl)phenylboronic acid (1.9 g,
10.0 mmol), K2CO3 (2.8 g, 20.0 mmol), PdCl2(dppf) (0.2 g, 0.3 mmol), THF (30 mL) and H2O (20
mL) was stirred at 100 °C for 12 h under N2 atmosphere. After cooling to room temperature, the
resulting mixture was poured into water and extracted with ethyl acetate. The ethyl acetate extract
was washed with water, dried over Na2SO4, and concentrated to yield a solid. Chromatography
(Vpetroleum ether/Vethyl acetate, 7:1) afforded the 1-(6-(3-(trifluoromethyl)phenyl)pyridin-2-yl)ethanone as
a white solid (1.5 g, 56%). 1H NMR (400 MHz, CDCl3) δ 8.38 (s, 1H), 8.31 (d, J = 7.8 Hz, 1H), 8.06
(dd, J = 6.7, 2.1 Hz, 1H), 7.99 – 7.95 (m, 2H), 7.74 (d, J = 8.4 Hz, 1H), 7.66 (t, J = 7.8 Hz, 1H), 2.85
(s, 3H). 19F NMR (376 MHz, CDCl3) δ -62.70 (s). MS ((+)-ESI): m/z = 266.11 (calcd. 266.08 for
[C14H11F3NO] [M+H]+).
2-(3-(Trifluoromethyl)-1H-pyrazol-5-yl)-6-(3-(trifluoromethyl)phenyl)pyridine (L4H2): A THF
4

solution of 1-(6-(3-(trifluoromethyl)phenyl)pyridin-2-yl)ethanone (2.7 g, 10.0 mmol) was added to a
suspension of NaH (1.2 g, 50.0 mmol) in anhydrous THF (40 mL) in ice bath. After stirred for 30
min, ethyl trifluoroacetate (2.4 mL, 20.0 mmol) was injected and the mixture was refluxed overnight.
After then, the mixture was neutralized with aqueous HCl. The organic solvent was removed in
vacuo. The residue was extracted with ethyl acetate and washed with water. The organic layer was
dried over Na2SO4 and concentrated to yield the crude 1, 3-dione intermediate. Without further
purification, this intermediate and N2H4 (80%, 3 mL) was dissolved in ethanol (50 mL). After
refluxing for 12 h, the solvent was removed in vacuo. The residue was purified by silica gel column
chromatography (Vpetroleum ether/Vethyl acetate, 3:1). The pure L4H2 was obtained as a white solid in 33%
yield (1.2 g).
1HNMR (400 MHz, CDCl3) δ 8.05–8.00 (m, 2H), 7.95 (t, J = 7.9 Hz, 1H), 7.76 (d, J = 8.0 Hz,
1H), 7.65 (d, J = 7.8 Hz, 1H), 7.57–7.48 (m, 3H), 7.03 (s, 1H). 19FNMR (376 MHz, CDCl3) δ -60.32
(s, 3F), -60.78 (s, 3F). MS ((+)-ESI): m/z = 358.13 (calcd. 358.08 for [C16H10F6N3] [M+H]+).
2.3. Synthesis of Ir complexes
A mixture of the corresponding ligands (L1H or L2H, 1.8 mmol), IrCl3·3H2O (0.6 g, 1.8
mmol), 2-ethoxyethanol (30 mL) and H2O (10 mL) was stirred at 120 °C for 12 h. After the solution
was cooled to room temperature, the crude dimer intermediates were collected by filtration and
washed successively with water and ethanol. Then the dimers were used in the subsequent reactions
without further purification. A mixture of the respective dimers (0.1 mmol), 0.5 equivalents of L3H2
or L4H2, and 10 equivalents of NaOAc in decalin (20 mL) was refluxed for 12 h. After cooling to
room temperature, the mixture was purified by column chromatography on silica (petroleum ether,
then petroleum ether/CH2Cl2 =1:1) to afford the respective products.
1
[Ir(L1)(L3)] (1), red solid, yield: 34%. HNMR (400 MHz, DMSO-d6) δ 8.90 (d, J = 7.7 Hz,
2H), 8.66 (s, 1H), 8.23 (d, J = 7.5 Hz, 2H), 8.14–7.92 (m, 5H), 7.84 (d, J = 10.6 Hz, 2H), 7.74–7.63
5

(m, 2H), 7.40–7.32 (m, 1H), 7.13 (s, 1H), 6.77 (dt, J = 14.2, 7.4 Hz, 2H), 6.69–6.62 (m, 1H), 6.58
(dd, J = 11.4, 4.0 Hz, 1H), 5.94 (d, J = 6.9 Hz, 1H), 5.90 (d, J = 7.0 Hz, 1H), 1.20 (s, 3H) . 19FNMR
(376 MHz, DMSO-d6) δ -58.23 (s, 3F). HRMS ((+)-ESI): m/z = 802.1548 (calcd. 802.1770 for
[C38H26N5F3Ir] [M+H]+).
[Ir(L1)(L4)] (2), red solid, yield: 23%. 1HNMR (400 MHz, DMSO-d6) δ 8.91 (s, 2H), 8.65 (s,
1H), 8.22 (dd, J = 26.1, 3.8 Hz, 3H), 8.03 (d, J = 16.6 Hz, 4H), 7.71 (d, J = 1.5 Hz, 1H), 7.47 (d, J =
7.1 Hz, 2H), 7.35 (s, 1H), 7.16 (s, 1H), 6.95 – 6.74 (m, 2H), 6.66 (d, J = 7.4 Hz, 1H), 6.18 (d, J = 7.6
Hz, 1H), 5.91 (d, J = 4.7 Hz, 1H), 5.73 (s, 1H), 1.20 (d, J = 0.5 Hz, 3H). 19FNMR (376 MHz,
DMSO-d6) δ -58.29 (s, 3F), -60.20 (s, 3F). HRMS ((+)-ESI): m/z = 870.1637 (calcd. 870.1643 for
[C39H25N5F6Ir] [M+H]+).
1
[Ir(L2)(L3)] (3), red solid, yield: 28%. HNMR (400 MHz, DMSO-d6) δ 8.90 (d, J = 7.7 Hz,
2H), 8.66 (d, J = 2.3 Hz, 1H), 8.23 (d, J = 8.5 Hz, 2H), 8.08–7.93 (m, 5H), 7.86 (t, J = 7.3 Hz, 2H),
7.74–7.68 (m, 2H), 7.39–7.31 (m, 1H), 7.13 (s, 1H), 6.84–6.71 (m, 2H), 6.69–6.62 (m, 1H),
6.59–6.53 (m, 1H), 5.92 (dd, J = 17.0, 6.3 Hz, 2H). 19FNMR (376 MHz, DMSO-d6) δ -58.27 (s, 3F).
HRMS ((+)-ESI): m/z = 866.0471 (calcd. 866.0718 for [C37H23N5BrF3Ir] [M+H]+).
1
[Ir(L2)(L4)] (4), red solid, yield: 29%. HNMR (400 MHz, DMSO-d6) δ 8.93 (d, J = 4.4 Hz,
1H), 8.89 (d, J = 8.4 Hz, 1H), 8.68 (s, 1H), 8.27–8.23 (m, 2H), 8.21 (s, 1H), 8.09 (d, J = 8.8 Hz, 1H),
8.06 (s, 1H), 8.04–8.01(m, 2H), 7.99 (d, J = 8.0 Hz, 1H), 7.83 (t, J = 6.7 Hz, 2H), 7.72 (d, J = 6.8 Hz,
1H), 7.40–7.31 (m, 1H), 7.17 (s, 1H), 6.87 (d, J = 7.4 Hz, 1H), 6.81 (t, J = 7.9 Hz, 1H), 6.67 (t, J =
7.1 Hz, 1H), 6.15 (d, J = 8.2 Hz, 1H), 5.91 (d, J = 7.6 Hz, 1H). 19FNMR (376 MHz, DMSO-d6) δ
-58.48(s, 3F), -60.71 (s, 3F). HRMS ((+)-ESI): m/z = 934.0580 (calcd. 934.0592 for
6

[C38H22N5BrF6Ir] [M+H]+).
2.4. OLED fabrication and testing
OLEDs were fabricated by successively vacuum deposition of molecular materials on ITO glass
which was treated by a standard process.[13] MoO3 was served as the hole-injection material.
Di-[4-(N, N-ditolyl-amino)-phenyl] cyclohexane (TAPC) acted as the hole-transport material and 1,
3-bis(N-carbazolyl)benzene (mCP) as the host material. N, N, N-tris(4-(9-carbazolyl)phenyl)amine
(TCTA) acted as the electron-blocking material. 1, 3, 5-Tri[(3-pyridyl)-phen-3-yl]benzene (TmPyPB)
acted as the electron-transporting materials and 8-hydroxyquinolinolato-lithium (Liq) as the
electron-injection materials. Aluminum was used as the cathode. Inficon thickness monitor was used
to monitor the film thickness. The electroluminescence (EL) spectra were measured using a Spectra
Scan PR650. The current-voltage (I-V) and luminance-voltage (L-V) characteristics were measured
using a Keithley 2400 Sourcemeter unit with a calibrated silicon diode.
3. Results and Discussion
3.1. Synthesis, characterization and thermal properties
In order to obtain good volatility of iridium(III) complexes, it is necessary for complexes to
maintain charge neutrality. So, two kind of tridentate ligands, i.e. the monoanionic C∧ N∧
N-coordinating chelates and dianionic C∧N∧N-coordinating chelates, were used to balance the
charge of trivalent iridium atoms. In view of the fact that some cyclometallated transition-metal
complexes can be prepared by the directly reaction of the monoanionic C∧N∧N-coordinating
chelates and transition metal salts (such as K2[PdCl4], K2[PtCl4], RhCl3)[14], while the tridentate
ligand already present in the coordination sphere of the Ir(III) atom may be inhibiting the
7

cyclometallation of the incoming monoanionic C∧N∧N-coordinating chelates[9], the monoanionic C∧
N∧N-coordinating chelates reacted with iridium trichloride firstly to ensure the cyclometallation in
our experiments. Then the further coordination will be smoothly carried out with the dianionic
chelates with pyrazolyl fragments which have strong coordination ability. Therefore, the synthesis of
the bis-tridentate Ir(III) complexes includes two steps (Scheme 1). In the first step, the monoanionic
chelates (L1H or L2H) reacted with equivalent amounts of IrCl3·3H2O in 2-ethoxyethanol/H2O
mixed solution by refluxing for 12 h, forming the red precipitates of cyclometalate Ir(III) dimer
intermediates. Then, the crude intermediates reacted with equivalent amounts of dianionic chelates
(L3H2 or L4H2) in the presence of NaOAc and in high boiling solvent decalin by refluxing for 12 h,
forming the desirable products in moderate yields. All complexes are quite stable under air and were
fully characterized by proton NMR and HRMS.
Thermogravimetric analysis (TGA) showed that all these iridium(III) complexes have good
thermal stability (Table 1 and Fig. 1). The 5% weight loss temperature (Td) of complex 1 is 323 ℃,
The Td of complexes 2 is the highest, reaching 348 ℃. Complexes 3 and 4 have the relatively lower
Td of 312 and 314 ℃, respectively. The excellent thermal stability of these complexes is conducive to
sublimation and improvement of the service life of OLEDs based on them.
3.2 UV–vis absorption and PL properties
The UVvis absorption and PL spectra are exhibited in Fig. 2. All complexes exhibit similar
absorption spectra. The two strong absorption bands (ε > 8 × 104 M-1 cm-1) below 310 nm are
1
attributed to the π-π* transitions of two different tridentate ligands. The intense absorption bands
(ε > 2 × 104 M-1 cm-1) around 380 nm are belong to the singlet metal-to-ligand charge transfer
8

(MLCT) transitions. The weak absorption from 460 nm extended to the visible region are likely due
3
to a combination of triplet MLCT transitions and π-π* transitions.[13] Compared with complexes 1
and 3, complexes 2 and 4 show a hypsochromic shift in absorption due to the introduction of the
electron-withdrawing trifluoromethyl substituents.
As show in Fig. 2, all bis-tridentate Ir(III) complexes exhibit strong yellow-to-orange emission
with maxiumum emitting wavelength between 580 nm and 604 nm in CH2Cl2 at RT. Unlike our
previously reported bis-tridentate Ir(III) complexes,[13] these complexes exhibit broad and featureless
3
emission. It is suggested that their luminescence may mainly originate from the MLCT and ligand
-to-ligand charge transfer (LLCT) transitions.[19, 20] The yellow/orange phosphors are essential
components to realizing high efficiency, economical two-color white OLEDs solid-state light
sources.[2] Compared with the emission wavelength (567 nm) of homoleptic bis-tridentate Ir(III)
complex of ligand L3,[13] the emission wavelengths of these titled complexes were red shifted by
more than 13 nm, which confirms the dominance of ligands L1 and L2 in emission because of their
more large conjugated system. Similarly, the electro-withdrawing trifluoromethyl substituents also
cause a blue shift in the peak of the emission. For example, compared with complexes 1 and 3, the
introduction of trifluoromethyl substituents in complexes 2 and 4 blue shifts the PL peaks from 604
and 601 nm to 586 and 580 nm, respectively. Meanwhile, compared with complexes 1 and 2,
replacing the electron donating methyl substituent in L1 with the electron-withdrawing bromo
substituent in L2, forming complexes 3 and 4, leads to a slightly blue shifts in PL peaks from 604
and 586 nm to 601 and 580 nm, respectively. To eliminate the interference of oxygen, the
photoluminescence quantum yields (QYs) of these complexes doped in poly(methyl methacrylate)
(PMMA) at a concentration of 1 wt% were determined. As shown in Table 1, complexes 1 and 3
9

had the lower QYs of 0.31 and 0.37, while complexes 2 and 4 had the higher QYs of 0.53 and 0.46.
This might be because introducting of trifluoromethyl group on benzene unit of L4 can effectively
prevent molecular aggregation and concentration quenching is reduced. Indeed, the low vibrational
frequency C-F bond can reduce the radiationless deactivation rate and the electron-withdrawing
nature of trifluoromethyl group can also enhance the electron affinity and electron mobility of the
iridium complexes.[13] The transient photoluminescence decay curves (Fig. 3, Table 1) displayed
single exponential decay with fitting lifetimes between 1.4 and 1.7 μs for titled complexes, together
with the oxygen sensitivity confirming their triplet-emitting nature.[7] Complexes 3 and 4 have
shorter luminescence lifetimes than complexes 1 and 2, due to the presence of heavy atoms of
bromine, which introduced faster radiative decay rates (1.94 × 105 and 3.12 × 105s-1 for 1 and 2, 2.64
× 105 and 3.28 × 105 s-1 for 3 and 4, respectively ).
3.3 Electrochemical properties
Cyclic voltammetry measurement shows all these complexes exhibited reversible oxidation
(Fig. 4). The oxidation potential of complex 2 was 645 mV, which was higher than that of complex
1 (545 mV) due to the influence by the electron-withdrawing trifluoromethyl substituent on benzene
unit of L4 in complex 2. The oxidation potential of complex 4 (739 mV) was higher than that of
complex 3 (573 mV) for the same reasons. Replacing the electron donating methyl substituent in L1
with the electron-withdrawing bromo substituent in L2 has also led to the increase of oxidation
potential of complexes, i.e., the oxidation potential of complex 3 (or 4) was slightly higher than that
of complex 1 (or 2). The reason for greater influence of trifluoromethyl substituent than that of
bromo substituent can be attributed to the stronger electron pulling capability of the trifluoromethyl
10

group compared with bromine atom and their different substitution positions. The HOMO and
LUMO levels were deduced from the CV data and UV absorption edges. The results showed the
electron-withdrawing substituents have concurrently dragged down the HOMO and LUMO levels,
but have a greater impact on the HOMO levels. The overall result is the widening of the energy gap,
which is consistent with the change of PL wavelength.
3.4 Electrophosphorescent properties
Because complex 2 exhibited the highest QY, its EL properties were studied with the device
structure shown in Fig. 5. The EL characteristics of devices D1D4 (complex 2, 28%) were
displayed in Fig. 6 and Table 2. As displayed in the EL spectra (Fig. 6a), devices show strong
yellow light emissions at 572580 nm with a shoulder around 612 nm. The structured emission
profiles suggest a neglectable contribution from the π-π* transitions under the electric excitation.
With the increase of doping concentration, the EL peaks red shift, which can attributed to the
aggregation of complex at high concentrations. The EL spectra reveal 614 nm blue shift compared
with the PL spectra in CH2Cl2, which is probably due to the different media polarity and restricted
molecular vibration in rigid medium. Except the EL emissions originate from complex 2, no other
emission peak was observed, implying an efficient energy transfer from the host to phosphor. The
Commission Internationale de L’Eclairage (CIE) coordinates were slightly changed from (0.54, 0.46)
to (0.55, 0.44) from devices D1 to D4.
Fig. 6b shows the current density-voltage-luminance properties. They display turn-on voltages
of 3.5–3.9 V. The maximum brightness of devices is between 6878 and 9472 cd m-2. The brightness
decreases with increasing doping concentration, and is highest at 2% low doping concentration. As
11

shown in Fig 6c and Fig 6d, device D1 (2%) exhibits a maximum current efficiency of 36.4 cd A-1,
power efficiency of 29.1 lm W-1 and external quantum efficiency (EQE) of 14.4%. With the increase
of doping concentration, the device efficiency decreases. Especially when the doping concentration
is exceed 4%, the efficiency decreases obviously, for example, the maximum EQE of device D2 (4%)
is 97% of D1, while the maximum EQEs of D3 (6%) and D4 (8%) reduce to 80% and 60% of D1,
respectively, which indicates that the aggregation of complexes is serious under this condition. This
is due to the large conjugate plane of tridentate ligands, which leads to the strong aggregation
tendency of these bis-tridentate iridium(III) complexes.[3]
4. Conclusions
Four novel bis-tridentate iridium(III) complexes with two different C∧N∧N-coordinating
ligands have been successfully synthesized. The effects of different substituents on the
photophysical properties of the bis-tridentate iridium(III) complexes were studied. These complexes
exhibited good thermal stability with decomposition temperature (Td) exceeding 310 ℃. These
complexes exhibited bright yellow phosphorescence with PL quantum yields from 0.31 to 0.53 in
PMMA films. The OLEDs based on complex 2 showed good performances. The device at 2%
doping concentration showed a maximum current efficiency of 36.4 cd A-1, power efficiency of 29.1
lm W-1 and external quantum efficiency (EQE) of 14.4%. Our results suggest these bis-tridentate
iridium(III) complexes are promising for high-efficiency OLED applications.
Acknowledgements
This work is supported by the National Natural Science Foundation of China (21572001 and
61875144).
12

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Scheme 1 Synthetic routes of the iridium(III) complexes.
100
90
80
70
60
50
40
1
2
3
4
100
200
300
400
500
600
700
800
Temperature (C)
Fig. 1 Thermal gravimetric spectra of as-prepared iridium (III) complexes.
15

1
2
3
4
120
100
80
60
40
20
0
1.0
0.8
0.6
0.4
0.2
0.0
250 300 350 400 450 500 550 600 650 700
Wavelength (nm)
Fig. 2 UV–vis absorption (a) and photoluminescence spectra (b) of as-prepared iridium complexes in
CH2Cl2 solution.
1
2
3
1000
4
800
600
400
200
0
0
2000
4000
6000
8000
10000
Time (ns)
Fig. 3 Emission decay curves of as-prepared irdium(III) complexes in PMMA films at a conc. of 1 wt%.
16

1
2
3
4
0.0
0.2
0.4
0.6
0.8
+
1.0
1.2
Potential (V) vs Fc /Fc
Fig. 4 Cyclic voltammetry curves of as-prepared iridium (III) complexes in CH2Cl2.
Fig. 5 General structure for the devices and their energy level diagram.
104
103
102
(a)
(b)
2%
4%
6%
8%
1.0
0.8
0.6
0.4
0.2
0.0
2%
4%
6%
8%
40
30
20
10
0
3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5 8.0
Voltage(V)
500
550
600
650
700
750
Wavelength (nm)
17

50
40
30
20
10
16
14
12
10
8
40
30
(C)
(d)
2%
4%
6%
8%
20
10
2%
4%
6%
8%
6
4
100
1000
-2
10000
100
1000
Luminance (cd m
10000
-2
)
Luminance (cd m
)
Fig. 6 (a) EL spectra of complex 2 under different doping level. (b) Current
density-voltage-luminance (J-V-L) characteristics. (c) Power efficiencies/luminance efficiencies vs.
luminance. (d) External quantum efficiency vs. luminance.
Table 1 Physical properties for the iridium(III) complexes
ηb
(%)
τb
HOMOd
(eV)
LUMOe
(eV)
Eg
Td
a
opt
f
λmax
complex
(nm)
(μs)
1.6
(eV)
2.06
2.11
2.07
2.12
(℃)
323
348
312
314
604
586
601
580
31
53
37
46
-5.35
-3.29
1
2
3
4
1.7
1.4
1.4
-5.45
-5.37
-5.54
-3.34
-3.30
-3.42
[a] Photoluminescence spectra were recorded in CH2Cl2 at a conc. of 10-5 M. [b] Lifetime and quantum yields were
recorded in PMMA at a conc. of 1 wt%. [c] HOMO = –4.8 – E1/2ox. [e] LUMO levels were calculated from HOMO and
Egopt. Egopt was estimated from the absorption edge. [f] Temperature corresponding to 5% weight loss to the initial
weight in TGA analyses.
Table 2 Summary of device luminescence and efficiency data of complex 2
turn-on
votage
(V)
current
Power
ηEQE
CIEx, y
device
λEL,max
brightness
(cd m-2)
efficiency efficiency
(%) coordinates
(concentration)
(nm)
(cd A-1)
36.4
(lm W-1)
29.1
D1 (2%)
D2 (4%)
D3 (6%)
D4 (8%)
572
576
580
580
3.9
9472
9446
8402
6878
14.4
13.9
11.5
8.7
0.54, 0.46
0.54, 0.46
0.55, 0.45
0.55, 0.44
3.9
34.0
27.5
3.7
27.0
18.6
3.5
20.5
13.4
18

1, Unusual neutral Ir(III) complexes with two C∧N∧N ligands were synthesized.
2, The structure-property relationship in these complexes had been revealed.
3, The CF3 substituted complex 2 exhibited the best PLQYs of 0.53 and Td of 348 ℃.
4, The yellow device based on 2 gave a peak external quantum efficiency of 14.4%.
19

For Table of Contents
Efficient yellow OLEDs based on bis-tridentate iridium(III)
complexes with two C∧N∧N-coordinating ligands
Yuan-Yuan Hu,†a Wei Luo,†b Yan Wang,a Bi-Hai Tong,*a Man-Keung Fung*b and Qian-Feng Zhang*a
Abstract
Complex 2 exhibited the highest PLQYs of 0.53, Td of 348 ℃ and peak EQE
14.4%.
15
14
13
12
N
N
11
Ir
N
N
F3C
N
CF3
10
9
100
1000
10000
Luminance (cd m-2)
20

Conflict of interest statement
We declare that we have no financial and personal relationships with other people or
organizations that can inappropriately influence our work, there is no professional or other personal
interest of any nature or kind in any product, service and/or company that could be construed as
influencing the position presented in, or the review of, the manuscript entitled, “Efficient yellow
OLEDs based on bis-tridentate iridium(III) complexes with two C∧N∧N-coordinating ligands”！
21