Synthesis, photoluminescence and electroluminescence of one iridium complex with 2-(2,4-difluorophenyl)-4-(trifluoromethyl)pyrimidine and tetraphenylimidodiphosphinate ligands

Article abstract of DOI:10.1016/j.jorganchem.2017.07.039

Using 2-(2,4-difluorophenyl)-4-(trifluoromethyl)pyrimidine as main ligand and tetraphenylimidodiphosphinate as ancillary ligand, one iridium complex was synthesized. This complex shows a green emission peaking at 543 nm with a quantum efficiency yield as

Full text of DOI:10.1016/j.jorganchem.2017.07.039

Accepted Manuscript
Synthesis, photoluminescence and electroluminescence of one iridium
complex with 2-(2,4-difluorophenyl)-4-(trifluoromethyl)pyrimidine and
tetraphenylimidodiphosphinate ligands
Yong-Hui Zhou, Jing Xu, Zheng-Guang Wu, You-Xuan Zheng
PII:
S0022-328X(17)30481-3
10.1016/j.jorganchem.2017.07.039
JOM 20054
DOI:
Reference:
To appear in: Journal of Organometallic Chemistry
Received Date: 2 April 2017
Revised Date: 18 July 2017
Accepted Date: 21 July 2017
Please cite this article as: Y.-H. Zhou, J. Xu, Z.-G. Wu, Y.-X. Zheng, Synthesis, photoluminescence and
electroluminescence of one iridium complex with 2-(2,4-difluorophenyl)-4-(trifluoromethyl)pyrimidine
and tetraphenylimidodiphosphinate ligands, Journal of Organometallic Chemistry (2017), doi: 10.1016/
j.jorganchem.2017.07.039.
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ACCEPTED MANUSCRIPT
100
2
Maximum luminance: 42 898 cd/m
Maximum current efficiency: 54.29 cd/A
10
1
0.1
0
2000
4000
6000
8000
10000
2
Luminance (cd/m )

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Synthesis, photoluminescence and electroluminescence of one iridium
complex with 2-(2,4-difluorophenyl)-4-(trifluoromethyl)pyrimidine
and tetraphenylimidodiphosphinate ligands
a
a
b
b,*
Yong-Hui Zhou, Jing Xu, Zheng-Guang Wu, You-Xuan Zheng
a
Jiangsu Key Laboratory of Atmospheric Environment Monitoring and Pollution Control,
Collaborative Innovation Center for atmospheric environment and equipment technology, College
of Environmental Science and Engineering, Nanjing University of Information Science &
Technology, Nanjing 210044, P. R. China
b
State Key Laboratory of Coordination Chemistry, School of Chemistry and Chemical
Engineering, Nanjing University, Nanjing 210093, P. R. China, e-mail: yxzheng@nju.edu.cn
Abstract: Using 2-(2,4-difluorophenyl)-4-(trifluoromethyl)pyrimidine as main ligand
and tetraphenylimidodiphosphinate as ancillary ligand, one iridium complex was
synthesized. This complex shows a green emission peaking at 543 nm with a quantum
efficiency yield as 48% in deareated CH Cl solution. It also has good thermal
2
2
o
stability with a decomposition temperature of 351 C. Using this material as emitter,
the organic light-emitting diode shows good performances with a maximum
2
luminance 42 898 cd/m , a maximum current efficiency up to 54.29 cd/A with low
2
efficiency roll-off. At the luminance of 5000 and 10 000 cd/m , the current
efficiencies are still kept at 46.70 and 40.63 cd/A, respectively.
Keywords: pyrimidine; tetraphenylimidodiphosphinate; organic light-emitting diode;
efficiency; efficiency roll-off
1
. Introduction
Organic light-emitting diodes (OLEDs) have attracted enormous interest in last
decades as are expected to be used in large-size, flexible-panel display technologies
and solid-state lighting [1-16] Notably, iridium(III) complexes are the most widely

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investigated phosphorescent emitting materials for highly efficient OLEDs owing to
their encouraging properties including thermal stability, flexible color tunability, short
excited lifetime and high quantum yields [17-21]. Furthermore, the phosphorescence
of Ir(III) complexes originates from the metal-to-ligand charge transfers (MLCT) and
ligand-centered (LC) transition [17-21], so that the energy level of the excited state
can be controlled by tuning the energy levels of the ligands through substituent effects,
resulting a wide flexible emission color range.
However, for many OLEDs based on Ir(III) complexes, the device efficiency
roll-off ratios are serious, which can mainly be attributed to the deterioration of
charge carrier balance and the increase of nonradioactive quenching processes,
including triplet-triplet annihilation (TTA), triplet-polaron annihilation (TPA), and
electric field induced dissociation of excitons at high current density. At present,
because the hole mobility of most hole transport materials is roughly 2-3 orders of
magnitude higher than the electron mobility of the electron transport materials and
most hosts are also hole transport materials, the efficiency and efficiency roll-off of
OLEDs rely on the capability of electron transport. Thus, it is necessary to use the
ambipolar host materials and synthesize Ir(III) dopants with outstanding electron
mobility to obtain phosphorescent OLEDs with low efficiency roll-off.
In this work, we synthesized
a
green Ir(III) complexes using
2
-(2,4-difluorophenyl)-4-(trifluoromethyl)pyrimidine (dfptfmpm) as the main ligand
with two fluorine atoms substituted on phenyl ring and trifluoromethyl on pyrimidine.
Fluorination can enhance the electron mobility of the complexes resulting in a better
balance of charge injection and transfer, and the lower vibrational frequency of C-F
bond can reduce the rate of nonradiative deactivation and the bulky CF substituents
3
can affect the molecular packing and the steric protection around the metal can
suppress the self-quenching behavior [23-26]. In addition, nitrogen heterocycle will
increase the electron affinity and a more negative framework of ligand C^N may
improve the electron mobility. Furthermore, Htpip (tetraphenylimidodiphosphinate
acid) derivatives reported by our group previously were introduced as ancillary
ligands, which have two diphenyl phosphoryl (Ph P=O) groups [27-32]. Thus, not
2

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only the electron transport ability of the complex be improved, the four bulky phenyl
groups also lead to a larger spatial separation of the neighboring molecules of the
complexes [24-27], resulting high device efficiency and low efficiency roll-off.
2
. Experimental Section
2
.1 General Information
High resolution mass spectra (HR EI-MS) were recorded from Agilent 6540 UHD
Accurate-Mass Q-TOF LC/MS. TGA measurements were carried out on STA 449F3
NETZSCH). Absorption and emission spectra were measured on a Shimadzu
(
UV-3100 and a Hitachi F-4600 luminescence spectrophotometer, respectively. The
thermogravimetric analysis (TGA) curve of the complex was carried out with an STA
4
1
49F3 instrument (NETZSCH) under nitrogen with a temperature increasing rate of
o
0 C/min. Cyclic Volta metric experiments were carried out with an CHI 600E
system (Chenhua, Shanghai) using three electrode cell assemblies in deaerated
dichloromethane solution with tetrabutylammoniumperchlorate as supporting
electrolyte at a scan rate of 100 mV/s. Each oxidation potential was calibrated with
ferrocene as a reference.
2
.2 Synthesis of dfptfmpm and Ir(dfptfmpm) tpip complex
2
All reactions were performed under nitrogen. Solvents were carefully dried and
distilled from appropriate drying agents prior to use for syntheses of ligands.
Tetraphenylimidodiphosphinate (tpip) and its potassium (Ktpip) were synthesized
according to our former publications.
2
-Chloro-4-(trifluoromethyl)pyrimidine (20 mmol), 2,4-difluorophenylboronic acid
(
30 mmol), Na CO (36 mmol) and Pd(PPh ) (0.20 mmol) were dissolved in toluene :
2
3
3 4
ethanol : water (30 mL, 2 : 1 : 2, v / v). The mixture was refluxed for 24 h and
extracted twice with CH Cl at room temperature. The mixed organic solution was
2
2
washed with brine and the solid obtained was purified with column chromatography
on SiO using ethyl acetate and petroleum ether (v : v = 10: 1) as eluant to afford
2
white solid 2-(2,4-difluorophenyl)-4-(trifluoromethyl)pyrimidine (dfptfmpm, yield:

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1
7
5%). H NMR (400 MHz, CDCl ) δ 9.01 (d, J = 5.0 Hz, 1H), 8.14 (td, J = 8.7, 6.6
3
1
9
Hz, 1H), 7.48 (d, J = 5.0 Hz, 1H), 6.98 – 6.83 (m, 2H). F NMR (376 MHz, CDCl ) δ
3
-70.12 (s, 3F), -105.06 (d, J = 11.0 Hz, 1F), -108.14 (d, J = 11.0 Hz, 1F). MS (ESI)
m/z calcd for C H F N H [M+H]: 261.05, found: 260.95.
11
5
5
2
A mixture of IrCl ⋅3H O (1.8 mmol) and dfptfmpm (4 mmol) in 2-ethoxyethanol
3
2
and water (6 mL, 3 : 1, v / v) was refluxed for 24 h. After cooling, the yellow solid
precipitate was filtered to give the crude cyclometalated Ir(III) chloro-bridged dimer.
MS (ESI) m/z calcd for C H Cl F Ir N H [M+H]: 1492.99, found: 1492.95.
4
4
16
2
20
2
8
Then, the slurry of crude chloro-bridged dimer (0.5 mmol) and Ktpip (1.4 mmol)
in 2-ethoxyethanol (6 mL) was refluxed for 24 h. The solvent was evaporated at low
pressure and the crude product was washed with water, and then chromatographed
with CH Cl gave complex Ir(dfptfmpm) tpip, which were further purified by
2
2
2
1
sublimation in vacuum (yield: 56%). H NMR (400 MHz, CDCl ) δ 9.25 (d, J = 6.0
3
Hz, 2H), 7.83 (ddd, J = 12.4, 7.6, 1.7 Hz, 4H), 7.48 – 7.30 (m, 10H), 7.25 – 7.18 (m,
2
2
3
H), 7.04 (td, J = 7.7, 3.1 Hz, 4H), 6.71 (d, J = 6.0 Hz, 2H), 6.45 (ddd, J = 11.4, 9.5,
19
.2 Hz, 2H), 5.51 (dd, J = 8.6, 2.2 Hz, 2H). F NMR (376 MHz, CDCl ) δ -69.77 (s,
3
3
1
F), -103.00 (d, J = 13.1 Hz, 1F), -105.19 (d, J = 13.1 Hz, 1F). P NMR (162 MHz,
CDCl ) δ 24.32 (s). MS (ESI) m/z calcd for C H F IrN O P H [M+H]: 1128.13,
3
46 28 10
5
2 2
found: 1128.00. HR-MS (m/z) Calcd for C H F IrN O P H [M+H]: 1128.1261.
4
6
28 10
5
2 2
+
Found: 1128.1260 [M+H] .
.3 OLEDs fabrication and measurement
OLEDs with the emission area of 0.1 cm were fabricated on the pre-patterned
2
2
-1
ITO-coated glass substrate with a sheet resistance of 15 Ω sq . Substrate was cleaned
by ultrasonic in organic solvents followed by ozone treatment for 20 min. All
-4
materials used for EL devices were sublimed in vacuum (2.2 × 10 Pa) prior to use.
The 40 nm hole transport material of 1,1-bis(4-(di-p-tolyl-amino)phenyl) cyclohexane
(
TAPC) film was first deposited on the ITO glass substrate. The phosphor (10 wt%)
and host (mCP, N,N’-dicarbazolyl-3,5-benzene) were co-evaporated to form 20 nm
emitting layer from two separate sources. Successively,

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1
,3,5-tri(m-pyrid-3-yl-phenyl)benzene (TmPyPB, 30 nm) as electron transport
material, LiF (1 nm) as electron injection material, and Al (100 nm) as the cathode
-5
were evaporated. The vacuum was less than 1×10 Pa during all materials deposition.
The device characteristics were measured with a computer controlled KEITHLEY
2
400 source meter with a calibrated silicon diode in air without device encapsulation.
The electroluminescence (EL) spectra were measured with a Hitachi F-4600
photoluminescence spectrophotometer. On the basis of the uncorrected PL and EL
spectra, the Commission Internationale de l’Eclairage (CIE) coordinates were
calculated using a test program of the Spectra Scan PR650 spectrophotometer.
3
. Results and discussion
3
.1 Thermal stability
The thermal stability of emitter is very important for efficient OLEDs. If a complex
is suitable for application, the decomposition temperature (T ) should be high enough
d
to guarantee that the complex could be deposited onto the solid face without any
decomposition on sublimation. The thermogravimetric analysis (TGA) curve of
Ir(dfptfmpm) tpip is listed in Figure 2. It can be observed that the decomposition
2
o
temperature (5% loss of weight) is 351 C, indicating that the complex is suitable for
application in OLEDs.
3
.2 Photophysical property
The absorption spectrum of Ir(dfptfmpm) tpip complex was recorded at room
2
-5
temperature in CH Cl at 1 × 10 mol/L (Fig. 3). Like other heteroleptic iridium
2
2
complexes, this complex exhibits two major absorption regions. The intense band at
high energy before 300 nm is assigned to the spin-allowed ligand-centered (π→π*)
transition of dfptfmpm and tpip ligands. The relatively weak absorption bands in the
1
3
range 350 - 450 nm are due to the mixing of MLCT and MLCT (metal-to-ligand
charge-transfer) states, or LLCT (ligand-to-ligand charge-transfer) transition through
strong spin-orbit coupling of iridium atom. [17-33].

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For phosphorescent emitters used in OLEDs, significant mixing of the lowest triplet
and higher-lying singlet excited states caused by efficient SOC favors high
phosphorescent quantum efficiency. At room temperature, Ir(dfptfmpm) tpip complex
2
shows broad emission spectrum in deareated CH Cl solution with maxima at 543 nm,
2
2
generated by the electronic 0-0 transition between the lowest triplet excited state and
the ground state, which makes it green phosphors. In general, the emission band from
MLCT states are broad and featureless, while the highly structured emission band
3
mainly originates from the π-π* state. Accordingly, the complex emit sfrom a mixture
3
of MLCT states and the dominant ligand-based π-π* state. The emission quantum
efficiency yield of Ir(dfptfmpm) tpip complex is 48%, referenced to fac-(Ir(ppy) ) (Φp
2
3
=
0.4) [33]. Furthermore, the phosphorescence excited state lifetime τ of an Ir(III)
p
complex is a crucial factor that determines the rate of TTA and TPA in OLEDs. The
lifetime of Ir(dfptfmpm) tpip complex is 2.66 µs measured in degassed CH Cl with
2
2
2
excitation at 370 nm at room temperature, is indicative of a phosphorescent origin of
the excited state.
3
.3 Theoretical and Electrochemical Property
The frontier molecular orbitals, especially the HOMO and LUMO, are crucial for
the photophysical properties and also important in designing the structure of OLEDs.
To determine the HOMO and LUMO energy levels, cyclic voltammetry (CV) and
theoretical calculations were carried out.
To calculate the HOMO and LUMO energy levels of the Ir(dfptfmpm) tpip
2
complex, cyclic voltammetry experiment was carried out using ferrocene as the
internal standard (Figure 4). During the anodic scan in CH CN, the complex exhibited
3
an obvious oxide peak with oxidation potential attributed to the metal-centered
Ir(III)/Ir(IV) oxide couple in accordance with reported cyclometallated Ir(III) systems.
The HOMO level of the complex was calculated from the oxidation potential and the
LUMO level was estimated from the HOMO and the band gap obtain from UV-vis
absorption spectrum [34]. Therefore, HOMO/LUMO energy levels of the
Ir(dfptfmpm) tpip complex can be estimated as –5.55/–3.21 eV, from the empirical
2

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onset
formulas EHOMO = -(Eox +4.8) and ELUMO = EHOMO + Eg [34].
In the theoretical calculation, the density functional theory (DFT) calculations for
Ir(III) complexes are conducted to gain insights into the electronictates and the orbital
distribution employing the Gaussian09 software12 with B3LYP functional; the
accurate energy and location of HOMO/LUMO were calculated with QMForge
program [35-37]. The basis set used for C, H, O, N, F and P atoms was 6-31G(d,p),
while the LanL2DZ basis set were employed for Ir atoms [38]. According to the
Figure 4, it is obvious that the HOMO is mainly located on dfptfmpm main ligand
(38.07%) and the d orditals of iridium atom (55.14%), while the LUMO is mainly
located on the cyclometalated main ligands (91.73%). Both HOMO and LUMO are
rarely located on ancillary ligand, which are only 6.79% and 2.29%, respectively.
3
.4 OLEDs Performance
To investigate the possibility of Ir(dfptfmpm) tpip as emitter, PhOLED with the
2
structure of TAPC (1,1-bis[4-(di-p-tolylamino)phenyl]cyclohexane, 40 nm) /
Ir(dfptfmpm) tpip (10 wt%): mCP (1,3-bis(9H-carbazol-9-yl)benzene, 20 nm) /
2
TmPyPB (1,3,5-tri(m-pyrid-3-yl-phenyl)benzene, 30 nm) / LiF (1 nm) / Al (100 nm)
was fabricated. The molecule structures and energy level diagrams of the materials
and devices are shown in Figure 5. TAPC and TmPyPB were used as the hole and
electron transport materials due to their good carrier mobility [39,40], respectively,
and mCP was used as the host material. Apparently, the HOMO/LUMO levels of the
Ir(III) complex are within those of mCP, thus, carriers are expected to transport easily
between layers and excellent carrier trapping would be the main mechanism in the
devices. Notably, carriers would be well confined and recombined within the EML
and the TTA/TPA effects would be effectively avoided. The EL spectra,
luminance-voltage, current efficiency-luminance and power efficiency-luminance
characteristics of all the devices are shown in Fig. 6.
The device exhibits typical emission with the peak maxima at 536 nm in
accordance with the PL spectrum of Ir(dfptfmpm) tpip in CH Cl solution (Fig. 2)
2
2
2
indicating that the EL emissions of the devices originate from the triplet excited states

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of the phosphors. The Commission Internationale de 1’Eclairage (CIE) color
coordinates operated at 8 V are (0.35, 0.62), corresponding to the green region. And
the emission spectra are almost invariant of the current density and also do not show
any concentration dependence. No emission from TAPC, mCP and TmPyPB
suggested that the exciton was only formed in the emissive layer and the complete
energy transfer from host to dopant.
2
The device shows good performances with a maximum luminance 42 898 cd/m at
1
3.0 V. And the maximum current efficiency, external quantum efficiency and power
efficiency are up to 54.29 cd/A, 14% and 25.44 lm/W, respectively, at a luminance of
2
1
186 cd/m . Furthermore, the device efficiency roll-off is not serious. For example, at
2
the luminance of 5000 cd/m , the current efficiency is 46.70 cd/A. Even at the
2
luminance of 10 000 cd/m , the current efficiency is still remain as 40.63 cd/A. From
the good device performances it can be conclude that the introduction of
2
-(2,4-difluorophenyl)-4-(trifluoromethyl)pyrimidine
and
tetraphenylimidodiphosphinate ligands has definitely improved the electron mobility
of the iridium complex. As a result, the recombination chance of electrons and holes
has been strengthened, which is beneficial to the improvement of OLEDs’
performances.
4
. Conclusion.
In conclusion,
-(trifluoromethyl)pyrimidine as main ligand and tetraphenylimidodiphosphinate as
one
iridium
complex
with
2-(2,4-difluorophenyl)-
4
ancillary ligand was reported. This green material has an emission peak at 543 nm
with a quantum efficiency yield as 48% in deareated CH Cl solution. The OLED
2
2
using this complex as emitter shows good performances with a maximum luminance
2
4
2 898 cd/m , a maximum current efficiency up to 54.29 cd/A with low efficiency
2
roll-off. At the luminance of 5000 and 10 000 cd/m , the current efficiencies are still
kept at 46.70 and 40.63 cd/A, respectively.
Acknowledgements
This work was supported by the National Natural Science Foundation of China

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(21301095, 21371093, 91433113) and the Natural Science Foundation of Jiangsu
Province (BK20130054).
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Figure captions
Fig. 1. The synthetic route of the ligands and Ir(dfptfmpm) tpip complex.
2
Fig. 2. The TGA curve of Ir(dfptfmpm) tpip complex.
2
Fig. 3. The absorption and emission spectra of Ir(dfptfmpm) tpip complex in CH Cl
2
2
2
-5
at 1 × 10 mol/L at room temperature.
Fig. 4. The Cyclic voltammogram in CH CN and spatial distributions of the
3
HOMO/LUMO levels of the Ir(dfptfmpm) tpip complex.
2
Fig. 5. Energy level diagrams of HOMO/LUMO levels (relative to vacuum level) for
device and molecular structures of materials investigated in this study.
Fig. 6. Device characteristics: (a) normalized EL spectra at 8 V; (b)
luminance-voltage (L-V), (c) current efficiency-luminance (η -L) and (d) power
c
efficiency-luminance (η -L) curves.
p

ACCEPTED MANUSCRIPT
NH
iii
P
P
i
ii
O
O
O
O
Si
Si
NH
NK
+
P
P
P
Cl
Htpip
Ktpip
F
F
F
F
Cl
N
N
iv
F
Cl
Cl
F
F
+
v
Ir
Ir
F
F3C
N
N
N
N
N
N
B(OH)2
F3C
F3C
CF3
2
2
vi Ktpip
o
(
i) toluene, 110 C, 6 h;
(
ii) H O , THF;
2
2
F
(iii) KOH, methanol;
(
iv) Pd(PPh ) , Na CO , toluene, ethanol,
3
o
4
2
3
H O, 90 C, 24 h;
O
Ir
P
P
2
F
o
(v) IrCl , EtOCH CH OH, H O, 140 C, 24 h;
N
3
2
2
2
o
(vi) EtOCH CH OH, 140 C, 24 h;
N
N
O
2
2
F3C
2
Ir(dfptfmpm) tpip
2
Fig. 1. The synthetic route of the ligands and Ir(dfptfmpm) tpip complex.
2

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100
90
80
70
60
50
40
30
20
10
0
100
200
300
400
500
600
700
o
Temperature ( C)
Fig. 2. The TGA curve of Ir(dfptfmpm) tpip complex.
2

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(a)
Ir(dftfmpm) tpip
4
2
1.2x10
(b)
dfptfmpm
tpip
4
1
8
6
4
2
.0x10
3
.0x10
3
.0x10
3
.0x10
3
.0x10
0.0
300
400
500
600
300
400
500
600
700
Wavelength (nm)
Wavelength (nm)
Fig. 3. (a) The normalized absorption spectra of the complex Ir(dfptfmpm) tpip and ligands
2
dfptfmpm, tpip; (b) the absorption (with molar extinction coefficient) and emission spectra of
-5
complex Ir(dfptfmpm) tpip in CH Cl at 1 × 10 mol/L at room temperature.
2
2
2

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3
3
2
2
1
1
0
0
.5
.0
.5
.0
.5
.0
.5
.0
LUMO
-
0.5
1.0
1.5
-
HOMO
-
0.6
0.8
1.0
1.2
E (V)
Fig. 4. The Cyclic voltammogram in CH CN and spatial distributions of the HOMO/LUMO levels
3
of the Ir(dfptfmpm) tpip complex.
2

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2
.00
2
.30
.21
F
2
.73
3
O P
O P
F
LiF/Al
Ir
N
N
N
3
F C
ITO
2
Ir(dfptfmpm) tpip
2
5
.50 5.55
5
.80
N
6
.68
N
N
N
N
N
N
mCP
TmPyPB
TAPC
Fig. 5. Energy level diagrams of HOMO/LUMO levels (relative to vacuum level) for device and
molecular structures of materials investigated in this study.

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(
a)
(b)
4
4x10
4
4
4
3
x10
2x10
1x10
0
300
400
500
600
700
800
0
3
6
9
12
Wavelength (nm)
Voltage (V)
100
(c)
(d)
10
10
1
0.1
1
0
2000
4000
6000
8000
10000
0
2000
4000
6000
8000
10000
-2
2
Luminance (cd m )
Luminance (cd/m )
Fig. 6. Device characteristics: (a) normalized EL spectra at 8 V; (b) luminance-voltage (L-V), (c)
current efficiency-luminance (η -L) and (d) power efficiency-luminance (η -L) curves.
c
p

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Highlights
•
•
•
One efficient green iridium(III) complex was synthesized.
The complex shows good thermal stability.
The OLED based on this complex displays maximum current efficiency of 54.29
cd/A.
•
The device shows low efficiency roll-off.