Two novel bipolar Ir(III) complexes based on 9-(4-(pyridin-2-yl)phenyl)-9H-carbazole and N-heterocyclic ligand

Article abstract of DOI:10.1016/j.dyepig.2017.07.012

Two novel bipolar Ir(III) complexes named (Czppy)₂Ir(tfmptz) and (Czppy)₂Ir(pptz) were synthesized and characterized, where the Czppy is 9-(4-(pyridin-2-yl)phenyl)-9H-carbazole, tfmptz refers to 2-(3-(trifluoromethyl)-1H-1,2,4-triazol-5-yl)pyridine and Hpptz represents 2-(1H-tetrazol-5-yl)pyridine, respectively. Their molecular structures were confirmed by ¹H NMR and photophysical were investigated. The performances in phosphorescent organic light emitting devices (PhOLEDs) were characterized by a heavy doping concentration of 20 wt% in host as emitting layer. The results suggest that the number of nitrogen atoms in ancillary has no obvious effects on their photophysical properties. Both (Czppy)₂Ir(tfmptz) and (Czppy)₂Ir(pptz) display the blue-green emission with the peaks at 492, 526 nm. The performances of complexes (Czppy)₂Ir(tfmptz) in PhOLEDs superior to that of the (Czppy)₂Ir(pptz) due to the poor thermal stability.

Full text of DOI:10.1016/j.dyepig.2017.07.012

Accepted Manuscript
Two novel bipolar Ir(III) complexes based on 9-(4-(pyridin-2-yl)phenyl)-9H-carbazole
and N-heterocyclic ligand
Xu Huixia, Wang Fang, Wang Kexiang, Sun Peng, Li Jie, Yang Tinting, Wang Hua,
Xu Bingshe
PII:
S0143-7208(17)31191-9
10.1016/j.dyepig.2017.07.012
DYPI 6101
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 23 May 2017
Revised Date: 4 July 2017
Accepted Date: 7 July 2017
Please cite this article as: Huixia X, Fang W, Kexiang W, Peng S, Jie L, Tinting Y, Hua W, Bingshe X,
Two novel bipolar Ir(III) complexes based on 9-(4-(pyridin-2-yl)phenyl)-9H-carbazole and N-heterocyclic
ligand, Dyes and Pigments (2017), doi: 10.1016/j.dyepig.2017.07.012.
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ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT
Two Novel Bipolar Ir(III) Complexes based on
9-(4-(pyridin-2-yl)phenyl)-9H-carbazole and N-heterocyclic ligand
Xu Huixia a, b*, Wang Fang a, b, Wang Kexiang a, b, Sun Peng a, b, Li Jie a, b, Yang Tinting a, b, Wang
Hua a, b, Xu Bingshe a, b
a
Key Laboratory of Interface Science and Engineering in Advanced Materials, Ministry of Education, Taiyuan
University of Technology, Taiyuan Shanxi, 030024, P. R. China
b Shanxi Research Center of Advanced Materials Science and Technology, Taiyuan Shanxi, 030024, P. R. China
Abstract Two novel bipolar Ir(III) complexes named (Czppy)2Ir(tfmptz) and (Czppy)2Ir(pptz)
were synthesized and characterized, where the Czppy is 9-(4-(pyridin-2-yl)phenyl)-9H-carbazole,
tfmptz refers to 2-(3-(trifluoromethyl)-1H-1,2,4-triazol-5-yl)pyridine and Hpptz represents
1
2-(1H-tetrazol-5-yl)pyridine, respectively. Their molecular structures were confirmed by H NMR
and photophysical were investigated. The performances in phosphorescent organic light emitting
devices (PhOLEDs) were characterized by a heavy doping concentration of 20 wt% in host as
emitting layer. The results suggest that the number of nitrogen atoms in ancillary has no obvious
effects on their photophysical properties. Both (Czppy)2Ir(tfmptz) and (Czppy)2Ir(pptz) display
the blue-green emission with the peaks at 492, 526 nm. The performances of complexes
(Czppy)2Ir(tfmptz) in PhOLEDs superior to that of the (Czppy)2Ir(pptz) due to the poor thermal
stability.
Key words: N-heterocyclic ligand, Photophysical properties, Electroluminescent performance
Introduction:
Phosphorescent organic light emitting devices (PhOLEDs) , which the emitter can harvest the single
and triplet excitons to achieve the internal quantum efficiency of 100%, have attracted huge interests
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for application in large-scale display and solid-state lighting due to the outstanding advantages [1-5].
Iridium(III) complexes are the most widely investigated for phosphorescent emitting materials owing
to the obvious merits, such as color tunability, short excited lifetime and high quantum yields [6-9].
However, the Ir(III) complexes tend to possess the electron-transport ability, an example of the
classical green-emitting material of fac-tris(2-phenylpyridine)iridium (Ir(ppy)3). This may result in
an imbalance transportation between hole and electron; consequently reducing the efficiency in
PhOLEDs. Hence the electron-donating groups, such as carbozole, triphenylamine usually were
linked with the phenylpyridine to construct bipolar ligands in order to improve the balance of the
carrier transport [10, 11, 12, 13, 14]. Though this kind of bipolar Ir(III) complexes can be used to
fabricate the PhOLEDs by solution-processed approach, their efficiencies are still far behind the
vacuum-evaporated devices [15, 16].
In addition, Ir(III) complexes based on the N-heterocyclic ancillary ligands as emitter in PhOLED
have received extensive investigations because of their good electron mobility, excellent thermal
stability and flexible modification ability [1, 17, 18]. We have reported Ir(III) complexes based on
the 2-phenylpyridine and 2-(5-(trifluoromethyl)-2H-1, 2,4-triazol-3-yl)pyridine (Htfmptz), donated
as (ppy)2Ir(tfmptz) [19]. Its emission is observed at 485 nm, while it is very difficult to dissolve in
the common organic solution. In this paper, we designed and synthesized two novel bipolar Ir(III)
complexes of (Czppy)2Ir(tfmptz) and (Czppy)2Ir(pptz). In order to obtain the bipolar
cyclometalated ligand and improve the solubility of complex (ppy)2Ir(tfmptz), the electron-donating
carbazole was connected to the electron-withdrawing phenylpyridine. The electron-accepting
ancillary ligands
of
2-(3-(trifluoromethyl)-1H-1,2,4-triazol-5-yl)pyridine
(tfmptz) and
2-(1H-tetrazol-5-yl)pyridine (pptz) were employed. The characteristics of PhOLEDs based on these
compounds are also discussed.
Synthesis of 9-(4-(pyridin-2-yl)phenyl)-9H-carbazole (Czppy)
9-(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)-9H-carbazole:
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9-(4-bromophenyl)-9H-carbazole (3.2 g, 10 mmol), bis(pinacolato)diboron (3.3 g, 13 mmol),
PdCl2(dppf)·CH2Cl2 (244.8 mg, 0.3 mmol) and KOAc (3.0 g, 30 mmol) were dissolved in
N,N-dimethylformamide (50 mL), and were heated to 100 ℃ under the nitrogen atmosphere for 6 h.
The mixture was washed with great deal of distilled water after cooling to the room temperature. The
organic phase was dried with anhydrous sodium sulfate, filtered and concentrated in vacuum. The
crude product was subjected to silica gel column chromatography using a 5:1 (V/V) mixture of
1
petroleum ether and dichloromethane as the eluent to afford the white solid (yield 75 %). H NMR
(600 MHz, CDCl3, δ): 8.14 (d, 2H, J = 7.8 Hz), 8.06 - 8.03 (m, 2H), 7.61 - 7.57 (m, 2H), 7.45 - 7.43
(m, 2H), 7.40 (d, 2H, J = 8.3 Hz), 7.28 (d, 2H, J = 8.0 Hz), 1.40 (s, 12H). Anal. Calcd for
C24H24O2NB: C, 78.26; H, 6.52; N, 3.80. Found: C, 78.92; H, 6.306; N, 3.74.
2-bromopyridine (0.8 g, 5 mmol), 1 (2.3 g, 6 mmol), Pd(PPh3)4 (90 mg, 0.08mmol), Na2CO3 (2
mol·L-1, 8 mL, 15 mmol) and tetrahydrofuran (30 mL) were heated to reflux for 24 h under nitrogen
atmosphere. The water was added after cooling to the room temperature. The organic phase was
purified by chromatography using a 5:1 (V/V) of petroleum ether and dichloromethane as the eluent
1
to obtain white solid of 9-(4-(pyridin-2-yl)phenyl)-9H-carbazole (Czppy) (yield, 82%). H NMR
(600 MHz, CDCl3, δ): 8.76 (d, 1H, J = 4.8), 8.25 - 8.22 (m, 2H), 8.16 (d, 2H , J = 7.7 Hz), 7.85 - 7.80
(m, 2H), 7.71 - 7.67 (m, 2H), 7.48 (d, 2H, J = 8.2 Hz), 7.45 - 7.41 (m, 2H), 7.32 - 7.28 (m, 3H). Anal.
Calcd for C23H16N2: C, 86.25; H, 5.0; N, 8.75. Found: C, 86.87; H, 4.971; N, 8.48.
Synthesis of 2-(1H-tetrazol-5-yl)pyridine (Hpttz) 2-Cyanopyridine (10.4 g, 0.1 mol), sodium
azide (7.3 g, 0.12 mol), glacial acetic acid (8.0 g, 0.12mol) and butyl alcohol (40 mL) was put into a
reactor. The mixture was heated to reflux for 4 day. Then adding sodium azide (1.8 g, 0.03 mol),
glacial acetic acid (4.0 g, 0.06 mol) into reactor again, the reaction continues reacting 2 days in
reflux. After cooling to room temperature, the mixture was poured into a large amount of water. Then
driped hydrochloric acid with concentrated, until all the crystal precipitation. By filtering and
vacuum drying, get white crystal, Yield 95%,1H NMR (600 MHz, Chloroform-d, δ) 13.40 (s, 1H),
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8.82 (d, J = 4.9 Hz, 1H), 8.44 (d, J = 7.9 Hz, 1H), 8.01 (t, J = 7.7 Hz, 1H), 7.58-7.54 (m, 1H). Anal.
Calcd for C6H5N5: C, 48.98; H, 3.40; N, 47.62. Found: C, 48.99; H, 3.384; N, 47.35
Synthesis of complex (Czppy)2Ir(tfmptz)
Czppy (800 mg, 2.5 mmol), IrCl3·H2O (350 mg，1 mmol), and 2-ethoxyethanol/H2O (3:1, V/V,
20 mL) were was stirred for 120 ℃ for 24 h under nitrogen atmosphere. After cooling to room
temperature, the mixture was poured into water and the formed precipitate was filtered, washed by
water, ethanol and diethyl ether to obtain the intermediate, assumed to be a chloro-bridged dimer.
Without further purification, a mixture of chloro-bridged dimer (530 mg, 0.3 mmol),
2-(3-(trifluoromethyl)-1H-1,2,4-triazol-5-yl)pyridine（Htfmptz）[19] (64 mg, 0.3 mmol), Na2CO3
(640 mg, 6 mmol), 2-ethoxyethanol (20 mL) were heated to refluxed at 140 ℃ for 24 h, then
extracted with CH2Cl2 and dried over anhydrous Na2SO4. The crude product was subjected to silica
gel column chromatography with CH2Cl2/ ethyl acetate (10:1, V/V). Finally, the light yellow solid
was obtained (yield 62%) 1H NMR (600 MHz, CDCl3, δ): 8.36 (d, J = 7.9 Hz, 1H), 8.08 (t, J = 7.2
Hz, 3H), 8.04 (d, J = 7.7 Hz, 2H), 7.95 - 7.89 (m, 4H), 7.87 (d, J = 8.1 Hz, 1H), 7.70 - 7.68 (m, 1H),
7.58 - 7.53 (m, 2H), 7.48 (d, J = 5.6 Hz, 1H), 7.42 - 7.29 (m, 9H), 7.26 - 7.22 (m, 4H), 7.20 (t, J = 7.5
Hz, 2H), 6.80 (dddd, J = 47.7, 7.1, 5.7, 1.3 Hz, 2H), 6.58 (dd, J = 5.6, 2.1 Hz, 2H). Anal. Calcd: C,
62.13; H, 3.26; N, 10.74. Found: C, 60.99; H, 3.529; N, 10.01. MALDI-TOF-MS (m/z): calcd for
IrC54H34N8F3, 1044.25; found, 1043.80
Synthesis of complex (Czppy)2Ir(pptz)
The procedure is the same as the synthesis of (Czppy)2Ir(tfmptz) by using Hpptz as ancillary ligand.
Yield: 61%. 1H NMR (600 MHz, CDCl3, δ): 8.52 (d, J = 7.9 Hz, 1H), 8.12 (d, J = 5.5 Hz, 1H), 8.08
(d, J = 7.7 Hz, 2H), 8.04 (d, J = 7.7 Hz, 2H), 7.99 (td, J = 7.8, 1.6 Hz, 1H), 7.93 - 7.87 (m, 3H), 7.84
(d, J = 8.1 Hz, 1H), 7.69 (dd, J = 5.9, 1.4 Hz, 1H), 7.57 - 7.52 (m, 2H), 7.44 (dd, J = 5.9, 1.4 Hz, 1H),
7.40 - 7.29 (m, 10H), 7.24 (dq, J = 7.9, 4.2, 3.6 Hz, 3H), 7.20 (t, J = 7.4 Hz, 2H), 6.81 (ddd, J = 7.3,
5.8, 1.3 Hz, 1H), 6.74 (ddd, J = 7.3, 5.8, 1.3 Hz, 1H), 6.63 (d, J = 2.1 Hz, 1H), 6.55 (d, J = 2.0 Hz,
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1H). Anal. Calcd: C, 63.92; H, 3.51; N, 12.90 Found: C, 65.13; H, 4.75; N, 13.10. MALDI-TOF-MS
(m/z): calcd for IrC52H34N9, 977.29 ; found, 977.26.
The detail synthetic process has been shown in Scheme 1.
Materials and Measurements
1H-NMR spectra were confirmed using a Switzerland Bruker DR×600. C, H and N microanalysis
were carried out with an Elemental Vario EL elemental analyzer. Mass spectra were obtained on
SHIMADZU matrix-assisted laser desorption/ionization time-of-flight mass pectrometer
(MALDI-TOF-MASS). UV-vis absorption spectra were recording using Lambda Bio 40. The
Photoluminescence (PL) spectra were measured by HORIBA FluouoMax-4 spectrophotometer. The
low-temperature phosphorescence spectra at 77 K in 2-methyltetrahydrofuran (2-MeTHF) were
performed on an Edinburgh F-980 spectrometer. Thermogravimetric analysis curves (TGA) were
undertaken on a Netzsch TG 209F3 under dry nitrogen atmosphere heating at a rate of 10 ℃/min
from room temperature to 800 ℃.
Cyclic voltammetry (CV) was carried out in CH2Cl2 solution with chromatographic purity at room
temperature using a CHI 660E voltammetry analyzer. Tetrabutylammonium hexafluorophosphate
(TBAPF6) (0.1 M) was used as the supporting electrolyte. The platinum wire is used as working
electrode. The platinum electrode is the counter and a calomel electrode is the reference with
ferrocenium-ferrocene (Fc+/Fc) as the internal standard. The scan rate for CV curves is 100 mV/s.
The highest occupied molecular orbital level (ꢀꢁꢂꢃꢂ) was calculated according to the equation
ꢀꢄꢅꢆꢅ = −4.8 − e(ꢀꢇꢈꢉ − ꢀꢊꢈꢉ)V where ꢀꢇꢈꢉ was the first oxidation peaks measured from CV
curves and ꢀꢊꢈꢉ was the oxidation peak of ferrocene. The lowest unoccupied molecular orbital level
(ꢀꢋꢌꢃꢂ) was calculated from the equation of Eꢍꢎꢆꢅ = Eꢄꢅꢆꢅ + Eꢏ, and ꢀꢐis obtain from the
absorption and emission spectra intersection (λꢑꢒꢓ), that is ꢀꢐ = ꢔꢕꢖꢗ
.
ꢘ
ꢙꢚꢛ
Theoretical calculations were performed using the Gaussian 03 package. Geometry optimization
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was performed by density functional theory (DFT) in B3LYP/6-31G(d) basis sets. The molecular
orbitals were visualized using Gauss view 5.0.
OLED fabrication and measurements
PhOLEDs with area of 3 × 3 mm2 were fabricated by vacuum deposition onto the indium tin oxide
(ITO) glass substrate. ITO glass substrate was cleaned in order with deionized water, acetone and
ethanol. The electroluminescent (EL) spectra and CIE coordinates were measured by PR-655
spectrophotometer. The current density – voltage – luminance (J−V−L) characteristics of PhOLEDs
and OLEDs were recorded using Keithley 2400 Source Meter and ST-900M Spot Brightness Meter.
The external quantum efficiency (EQE) values were calculated according to previously reported
methods [20]. All measurements were carried out at room temperature under ambient conditions.
Results and discussion
Molecular structures and thermal stability
Scheme 1 shows the chemical structures and synthetic routes of the ligands and Ir(III) complexes.
Cyclometalated ligand of Czppy are obtained from the reaction between 2-bromopyridine and
9-(4-(4, 4, 5, 5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl) -9H-carbazole. The ancillary ligands of
Htfmptz and Hpptz are synthesized according to the methods in reference [19]. The
electron-donating group was introduced in cyclometalated ligand to improve the hole transportation.
Furthermore, the ancillary ligands of tetrazol and trizole have been proved to have the excellent
electron withdrawing abilities. As a result, the bipolar Ir(III) complexes, (Czppy)2Ir(tfmptz) and
(Czppy)2Ir(pptz), were synthesized. Their molecular structures are defined by 1H NMR and element
analyses.
Their thermal stabilities were measured by thermogravimetric analysis (TGA) under a nitrogen
flow, as shown in Figure 1. Both complexes show good thermal stabilities with the decomposition
temperature (Td, the temperature at the loss weight 5%) around 388 and 334 ℃, respectively. The
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increase in the number of nitrogen atoms in ancillary led to a lower Td for (Czppy)2Ir(pptz).
Photophysical properties
Two compounds exhibit the similar absorption and emission spectra in CH2Cl2 solution (Figure
2a), and the relative data are summarized in Table 1. The absorption bands blow 300 nm are assigned
to π-π* transitions from ligands. The weak absorption peaks in the range of 300-400 nm are
attributes to the typical spin-allowed singlet metal-to-ligand charge-transfer transition (1MLCT). The
others, which can be found at about 412 nm, are mainly origin from the spin-orbit coupling triplet
MLCT transitions. Their emission of (Czppy)2Ir(tfmptz) and (Czppy)2Ir(pptz) are observed at the
493, 526 nm.
The absorption bands and emission spectra of (Czppy)2Ir(tfmptz) appears the significant blue
shift relative to the free-carbazole (ppy)2Ir(tfmptz) [19]. The geometry were optimized at
B3LYP/6-31G(d) level, and the electronic properties of two compounds were calculated (in Figure 3).
The HOMO orbitals extend to the electron-donating carbazole and d-orbitals of Ir, while the LUMO
are distributed on the ancillary ligand. Therefore, the introduction of the electron-donating groups
will increase the HOMO level. The HOMO and LUMO of (Czppy)2Ir(tfmptz) and (Czppy)2Ir(pptz)
are located at the different groups, this may be conductive to promote bipolar charge-transporting
ability [21, 22].
The energy gaps (ꢀꢐ ) of (Czppy)2Ir(tfmptz) and (Czppy)2Ir(pptz) are estimated by the
intersection of the absorption and emission spectra in CH2Cl2 to be 2.84 and 2.86 eV, respectively.
The low-temperature phosphorescent spectra were measured in 2-methyltetrahydrofuran (2-MeTHF)
matrixes at 77 K showed fine vibronic structures (in Figure 2b). The triplet energies are calculated to
about 2.55 eV. These results indicate that the small changes in ancillary ligand, that the
electron-withdrawing trifluoromethyl was replaced by a nitrogen atom, have no obvious effects on
the ground and excited states of Ir(III) complex.
The electrochemical properties were measured by cyclic voltammetry using ferrocene as the
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internal standard (Figure 4). The oxidation potentials (ꢀꢇꢈꢉ) of complexes (Czppy)2Ir(tfmptz),
(Czppy)2Ir(pptz) are 1.25 and 1.18 V, respectively. Relative energy levels of ꢀꢁꢂꢃꢂ/ꢀꢋꢌꢃꢂ are
-5.67/-2.83 and -5.55/-2.69 eV by the formulas of ꢀꢁꢂꢃꢂ = −4.8 − e(ꢀꢇꢈꢉ − ꢀꢊꢈꢉ)V and Eꢋꢌꢃꢂ
=
ꢀꢁꢂꢃꢂ + ꢀꢏ. The change from the electron-withdrawing trifluoromethyl to nitrogen results in a
stablized LUMO level and an increased HOMO level.
Electroluminescent Performance
The electroluminescent (EL) were characterized by fabricating devices 1 and 2, in which
(Czppy)2Ir(tfmptz) and (Czppy)2Ir(pptz) were used as dopants and the synthesized TAZ-2Cz in
our laboratory as host [20]. The device structure is ITO / MoO3 (3 nm) / NPB (30 nm)/TCTA( 10 nm)
/TAZ-2Cz: 20 wt% (Czppy)2Ir(tfmptz) or (Czppy)2Ir(pptz（) 20 nm）/ TPBi (50 nm)/ LiF (1 nm)/ Al
(100 nm) ((Czppy)2Ir(tfmptz) in device 1 and (Czppy)2Ir(pptz) in device 2), in which
N’-bis(naphthalen)-N,N’-bis(phenyl)-benzidine (NPB) and 1, 3, 5-tris(2-nhenylbenzimidazolyl)
-benzene (TPBi) can take the hole-transport layer and electron-transfer layer roles, respectively;
tris(4-carbazoyl- 9-ylphenyl)amine (TCTA) is used to reduce hole injection barrier and block the
electron; MoO3 (molybdenum oxide) and LiF (lithium fluoride) serve as the hole- and
electron-injecting layers. The bipolar TAZ-2Cz was chosen due to the excellent carrier transporting
nature and high triplet level (2.62 eV). It is proved that the performances of TAZ-2Cz in PhOLEDs
superior to the classical host of 4, 4’-N, N’-dicarbazolebiphenyl (CBP) with the same device
structure. The EL characteristics of devices are displayed in Figures 5, 6 7 and the relative data are
summarized in Table 2.
It is observed that the both devices emit brighter green-blue emission with maximum peaks
500/527 for device 1 and 500 nm for device 2. Compared with the PL spectra in CH2Cl2 solution, the
EL spectra have a little red shift, while the shoulder at 527 nm becomes weaker in device 2 dopant
by (Czppy)2Ir(pptz) in Figure 5. This indicates that the emissions are dominated by the dopants and
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the effective energy transports are achieved between host and dopants. As shown in Figure 6a, the EL
spectra of device 1 remain almost unchanged under the different driving voltages from 5 to 10 V.
while with the increasing of voltage from 5 to 10 V, a new weak emission peak at 433 nm was
appeared in Figure 6b due to the poor stability and carrier-transport properties of (Czppy)2Ir(pptz).
Both devices have the similar turn-on voltages (ꢜ , the voltage at 1 cd/m2) at 4.0 and 3.9 V. The
ꢈꢝ
totally different of current density - voltage - Luminance (J-V-L), current efficiency- current
density- power efficiency curves (ηꢓ − J − ηꢞ) and luminance - external quantum efficiency (L-EQE)
are exhibited in Figure 7. The device 1 based on (Czppy)2Ir(tfmptz) with trifluoromethyl group and
three nitrogen atoms in ancillary ligand achieves a good EL performance with the maximum
luminance (Lmax), maximum ηꢓ, ηꢞ and EQE of 17 415 cd/m2, 44.5 cd/A, 27.7 lm/W, and 14.4 %,
respectively. While the device 2 employing (Czppy)2Ir(pptz) with four nitrogen atom without
trifluoromethyl in ligand exhibits an inferior EL performance with Lmax of 6 791 cd/m2, the
luminance efficiencies of 23.2 cd/A, 15.7 lm/W and EQE of 7.6 %. In addition, the device 2 displays
a serious roll-off in efficiencies, as can be seen in Fig. c, due to the weak thermal stability to some
extent resulted from the increase of the number of nitrogen atom in ancillary ligand.
Conclusion
In summary, we have been designed and synthesized two Ir(III) complexes (Czppy)2Ir(tfmptz) and
(Czppy)2Ir(pptz), which have the same bipolar cyclometalated ligand and the different
N-heterocyclic derivatives as ancillary ligands modified with different number of nitrogen atom. The
heavy doping PhOLEDs were fabricated to study their EL performances. The similar UV-vis
absorption and emission in solution were observed regardless of the different ancillary ligands. Two
Ir(III) complexes exhibit the brighter green-blue emission with the double peaks at 493, 526 nm. On
the other hand, the EL performance of (Czppy)2Ir(tfmptz) in PhOLED with the maximum EQE of
14.4 % has the superior properties to the (Czppy)2Ir(pptz).
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Acknowledgements
This work was financially supported by the International Science and Technology Cooperation
Program of China (2012DFR50460); National Natural Scientific Foundation of China (21071108);
Natural Science Foundation of Shanxi Province (2015021070); Program for Changjiang Scholar and
Innovation Research Team in University (IRT0972); and Shanxi Provincial Key Innovative Research
Team in Science and Technology (201513002-10).
References
[1] Huang S-F, Sun H-Z, Shan G-G, Li F-S, Zeng Q-Y, Zhao K-Y, Su Z-M. Rational design and
synthesis of cationic Ir(III) complexes with triazolate cyclometalated and ancillary ligands for
multi-color tuning. Dyes Pigments 2017;139: 524-532
[2] Jing Y-M, Wang F-Z, Zheng Y-X, Zuo J-L. Efficient deep red electroluminescence of iridium(III)
complexes with 2,3-diphenylquinoxaline derivatives and tetraphenylimidodiphosphinate. J.
Mater. Chem. C 2017; 5: 3714-3724
[3] Ma Y, Liu S, Yang H, Zeng Y, She P, Zhu N, Ho C-L, Zhao Q, Huang W, Wong W-Y.
Luminescence Color Tuning by regulating electrostatic interaction in light-emitting devices and
two-photon excited information decryption. Inorg. Chem. 2017; 56: 2409-2416
[4] Zhao J-H, Hu Y-X, Lu H-Y, Lü Y-L, Li X. Progress on benzimidazole-based iridium(III)
complexes for application in phosphorescent OLEDs. Org. Electron. 2017; 41: 56-72
[5] Jayabharathi J, Jayamoorthy K, Thanikachalam V. Synthesis, photophysical and
electroluminescent properties of green organic light emitting devices based on novel iridium
complexes containing benzimidazole ligands. J. Organomet. Chem. 2014; 761: 74-83.
[6] Jayabharathi J, Thanikachalam V, Sathishkumar R. Highly phosphorescent green emitting
iridium(III) complexes for application in OLEDs. New J. Chem. 2015; 39: 235-245.
10

ACCEPTED MANUSCRIPT
[7] Yin D, Feng J, Jiang NR, Ma R, Liu YF, Sun HB. Two-dimensional stretchable organic
light-emitting devices with high efficiency. ACS Appl. Mater. Interfaces 2016; 8: 31166
[8] Yang XL, Zhou GJ, Wong WY. Functionalization of phosphorescent emitters and their host
materials by main-group elements for phosphorescent organic light-emitting devices. Chem. Soc.
Rev. 2015; 44: 8484-8575
[9] Tao P, Li W-L, Zhang J, Guo S, Zhao Q, Wang H, Wei B, Liu S-J, Zhou X-H, Yu Q, Xu B-S,
Huang W. Facile synthesis of highly efficient lepidine-based phosphorescent iridium(III)
complexes for yellow and white organic light-emitting diodes. Adv. Funct. Mater. 2016; 26:
881-894
[10]Zhu M, Li Y, Miao J, Jiang B, Yang C, Wu H, Qin J, Cao Y. Multifunctional homoleptic
iridium(III) dendrimers towards solution-processed non-doped electrophosphorescence with low
efficiency roll-off. Org. Electron. 2014; 15: 1598-1606
[11]Cho W, Yulhee Ki, Lee C, Park J, Gal Y-S, Lee JW, Jin S-H. Single emissive layer white
phosphorescent organic light-emitting diodes based on solution-processed iridium complexes.
Dyes Pigments. 20141; 8: 115-120
[12]Cao S, Hao L, Lai W-Y, Zhang H, Yu Z, Zhang X, Liu X, Huang W. Distinct phosphorescence
enhancement of red-emitting iridium(III) complexes with formyl-functionalized phenylpyridine
ligands, J. Mater. Chem. C, 2016; 4: 4709-4718
[13]Lai W-Y, Levell JW, Jackson AC, Lo S-C, Bernhardt PV, Samuel IDW, Paul LB. A
phosphorescent poly(dendrimer) containing iridium(III) complexes: synthesis and light-emitting
properties. Macromolecules 2010; 43: 6986–6994
[14]Lai W-Y, Balfour MN, Levell JW, Bansal AK, Burn PL, Lo S-C, Samuel IDW. Poly(dendrimers)
with phosphorescent iridium(III) complex-based side chains prepared via ring-opening
metathesis polymerization, Macromolecules 2012; 45: 2963−2971
[15]Xu H, Sun P, Wang K, Zhang X, Ren J, Yang T, Zhang X, Hao Y, Wang H, Xu B, Wong W-Y.
11

ACCEPTED MANUSCRIPT
Solution-processed blue and blue-green phosphorescent organic light-emitting devices using
iridium(III) complexes based on 9-(6-(4-phenyl-1H-1,2,3-triazol-1-yl)hexyl)-9Hecarbazole
ligand. Dyes Pigments. 2016; 134: 148-154
[16]Ho C-L, Wong W-Y. Small-molecular blue phosphorescent dyes for organic light-emitting
devices. New J. Chem. 2013; 37: 1665-1683
[17]Mao H-T, Zang C-X, Wen L-L, Shan G-G, Sun H-Z, Xie W-F, Su Z-M. Ir(III) phosphors
modified with fluorine atoms in pyridine-1,2,4-triazolyl ligands for efficient OLEDs possessing
low-efficiency roll-off. Organometallics. 2016; 35: 3870-3877
[18]Enrico O, Gregg SK, Asgeir EK, Paolo C, Roland F, Luisa D C, Addyvan D, Michael B, Herbert
B. Blue-emitting iridium complexes with substituted 1,2,4-triazole ligands: synthesis,
photophysics and devices. Inorg. Chem. 2007; 46: 11082-11093
[19]Liu H, Bai Q, Yao L, Zhang H, Xu H, Zhang S, Li W, Gao Y, Li J, Lu P, Wang H, Yang B, Ma Y.
Highly efficient near ultraviolet organic light-emitting diode based on ameta-linked donor–
acceptor molecule. Chem. Sci. 2015; 6: 3797-3804
[20]Xu H, Sun P, Wang K, Li J, Wang F, Miao Y, Wang H, Xu B, Wong W-Y, Bipolar hosts and
non-doped
deep-blue
emitters
(CIEy=0.04)
based
on
phenylcarbazole
and
2-(2-phenyl-2H-1,2,4-triazol-3-yl)pyridine groups. J. Mater. Chem. C 2017; 5: 445 -4462
[21] Peng T, Li G, Ye K, Wang C, Zhao S. Liu Y, Hou Z, Wang Y. Highly efficient phosphorescent
OLEDs with host independent and concentration-insensitive properties based on a bipolar
iridium complex. J. Mater. Chem. C. 2013; 1: 2920–2926.
[22]Tao YT, Wang Q, Yang CL, Zhong C, Zhang K, Qin JG, Ma DG. Tuning the Optoelectronic
Properties of Carbazole/Oxadiazole Hybrids through Linkage Modes: Hosts for Highly Efficient
Green Electrophosphorescence Adv. Funct. Mater. 2010; 20: 304-311
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Scheme 1 Synthetic routes of the ligand and complexes
Figure 1 The TGA curves of (Czppy)2Ir(tfmptz) and (Czppy)2Ir(pptz)
Figure 2 UV-vis absorption (a) and emission (b) spectra in CH2Cl2 solution at room temperature
Figure 3 Orbitals distributions of HOMO/ LUMO for (Czppy)2Ir(tfmptz) and (Czppy)2Ir(pptz)
Figure 4 Cyclic voltammetry curves of (Czppy)2Ir(tfmptz) and (Czppy)2Ir(pptz)
Figure 5 EL spectra of device 1 and 2
Figure 6 The EL spectra of device 1 and 2 at the different voltages
Figure 7 (a) J-V-L characteristics, (b) current efficiency- current density- power efficiency curves
(ηꢓ − J − ηꢞ) and (c) external quantum efficiency (EQE) versus luminance for devices 1 and 2.
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Scheme 1
Br
N
O
O
O
O
O
O
B
N
B
B
N
Br
N
KOAC, [PdCl2(dppf)]CH2Cl2, DMF, 6h
Pd(PPh3)4, 2M Na2CO3
THF, reflux, 24h
N
Czppy
N
N
N
N
Ir
2
N
HN
N
N
N
CF3
CF3
N
N
Cl
N
(Czppy)2Ir(tfmptz)
Ir
Ir
2-ethoxyethanol
K2CO3, 140oC, 24h
Cl
2
2
N
N
N
N
Ir
2
N
N
N
N
N
HN
N
N
N
N
(Czppy)2Ir(pptz)
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Figure 1
Figure 2a
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Figure 2b
Figure 3
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Figure 4
Figure 5
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Figure 6a
Figure 6b
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Figure 7a
Figure 7b
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Figure 7c
Table 1. Photophysical and electrochemical properties of (Czppy)2Ir(tfmptz) and (Czppy)2Ir(pptz)
c
a
a
b
ꢣꢤ
ꢀꢐ
λꢟꢠꢑ
λꢒꢡ
ꢀꢢ
HOMO/LUMO
Complexes
(nm)
(nm)
(eV)
(℃)
(eV)
(eV)
(Czppy)2Ir(tfmptz)
(Czppy)2Ir(pptz)
345, 372, 421
345, 372, 419
493, 526
2.55
2.55
388
375
-5.67/-2.83
-5.55/-2.69
2.84
2.86
493, 526
a
b
Recorded in CH2Cl2 solution at room temperature; Triplet level (T1) calculated based on low-temperature PL
spectra at 77K in 2-MeTHF solution; c Energy gap (ꢀꢐ) was estimated from the intersection of UV-vis and emission
spectra
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Table 2. EL performances of devices 1 and 2
e
CIEf
ꢀꢫꢀꢦꢧꢉ
(%)
a
b
c
d
g
ꢩꢪ
ꢥꢦꢧꢉ
(nm)
(Czppy)2Ir(tfmptz) 500, 527
(Czppy)2Ir(pptz) 500
ꢜ
ꢨꢦꢧꢉ
ꢩꢇ
ꢈꢝ
Device
Dopant
(V) (cd/m2) (eV) (eV)
(x, y)
1
4.0
3.9
17 415 44.5 27.7 (0.25, 0.58)
14.4
2
6 791 23.2 15.7 (0.26, 0.56)
7.6
a
c
d
the EL peaks collected at 8 V, b turn-on voltage at 1 cd/m2, the maximum luminance, the
maximum current efficiency, e the maximum power efficiency, f the CIE coordinate at voltage of 8 V,
g the maximum external quantum efficiency
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Highlight
The bipolar Ir(Ⅲ) complexes were designed and synthesized.
The green-blue emission was observed.
The performances in PhOLEDs were characterized by the heavy dope concentration.