Synthesis and Characterization of Highly Efficient Solution-Processable Green Ir(III) Complexes with High Current Efficiency and Very Low Efficiency Roll-Off

Article abstract of DOI:10.1002/adfm.201804714

Three new highly efficient green-emitting heteroleptic phosphorescent iridium(III) complexes are designed and synthesized for the fabrication of solution-processable phosphorescent organic light-emitting diodes (PHOLEDs). Their photophysical, thermal, and electroluminescent (EL) properties are systematically investigated. The Ir(III) complexes comprise an amide-bridged trifluoromethyl (?CF₃)-substituted phenylpyridine unit as the main ligand and picolinic acid (pic) and tetraphenylimidodiphosphinate (tpip) as ancillary ligands. In addition, the 2-ethoxyethnol (EO₂?) solubilizing group is attached to the 4-position of pic ancillary ligand via tandem reaction, which improved the absolute photoluminescence quantum yields (PLQYs) and EL performance. The high-performance solution-processable PHOLEDs based on the bis[5-methyl-8-trifluoromethyl-5H-benzo(c)(1,5)naphthyridin-6-one](4-(2-ethoxyethoxy picolinate) iridium(III) (Ir1) complex exhibit a maximum external quantum efficiency (EQE) of 24.22% and a maximum current efficiency (CE) of 92.44 cd A^?1, with the latter being among the best reported CEs achieved though solution processing. In contrast, PHOLEDs with the bis[5-hexyl-8-trifluoromethyl-5H-benzo(c)(1,5)naphthyridin-6-one] (tetraphenylimidodiphosphinato)iridium (Ir3) complex show extremely low efficiency roll-off, with an EQE_max of 19.40% and an EQE of 19.29% at 10 000 cd m^?2.

Full text of DOI:10.1002/adfm.201804714

FULL PAPER
Organic Light-Emitting Diodes
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Synthesis and Characterization of Highly Efficient
Solution-Processable Green Ir(III) Complexes with
High Current Efficiency and Very Low Efficiency Roll-Off
Vijaya Gopalan Sree, Athithan Maheshwaran, Hyein Kim, Ho-Yeol Park, Youngkwang Kim,
Jae Chol Lee, Myungkwan Song,* and Sung-Ho Jin*
1. Introduction
Three new highly efficient green-emitting heteroleptic phosphorescent
iridium(III) complexes are designed and synthesized for the fabrication
In recent years, phosphorescent organic
of solution-processable phosphorescent organic light-emitting diodes
(PHOLEDs). Their photophysical, thermal, and electroluminescent (EL)
properties are systematically investigated. The Ir(III) complexes comprise
light-emitting diodes (PHOLEDs) have
attracted extensive academic and com-
mercial attention for display and lighting
devices. As a display technology that has
matured over the recent decades, vacuum-
processed organic light-emitting diodes
(OLEDs) have achieved external quantum
efficiencies (EQEs) over 30%.[1,2] Although
multiayered device architecture with
notable efficiencies has been fabricated,
the low material utilization and easy pro-
cessing of solution-processed OLEDs have
made them more attractive than vacuum-
processed counterparts.[3,4] However, the
prerequisite is to design solution-process-
able materials with the integrated proper-
ties achievable with vacuum-processed
OLEDs. Among the three primary
components of white light sources, effi-
cient electroluminescent (EL) green color
emitters are critical for achieving efficient
white light, as human eyes are sensitive
to green light (555 nm).[5,6] Solution-pro-
cessable green emitters with excellent EL

an amide-bridged trifluoromethyl ( CF3)-substituted phenylpyridine unit as
the main ligand and picolinic acid (pic) and tetraphenylimidodiphosphinate

(tpip) as ancillary ligands. In addition, the 2-ethoxyethnol (EO2 ) solubilizing
group is attached to the 4-position of pic ancillary ligand via tandem reaction,
which improved the absolute photoluminescence quantum yields (PLQYs)
and EL performance. The high-performance solution-processable PHOLEDs
based on the bis[5-methyl-8-trifluoromethyl-5H-benzo(c)(1,5)naphthyridin-
6-one](4-(2-ethoxyethoxy picolinate) iridium(III) (Ir1) complex exhibit a
maximum external quantum efficiency (EQE) of 24.22% and a maximum
current efficiency (CE) of 92.44 cd A−1, with the latter being among the best
reported CEs achieved though solution processing. In contrast, PHOLEDs
with the bis[5-hexyl-8-trifluoromethyl-5H-benzo(c)(1,5)naphthyridin-6-one]
(tetraphenylimidodiphosphinato)iridium (Ir3) complex show extremely
low efficiency roll-off, with an EQEmax of 19.40% and an EQE of 19.29%
at 10 000 cd m−2.
properties are the optimal solution for attaining cost-effective
large-scale manufacturing and high-performance products.
Heteroleptic Ir(III) complexes [(C^N)2Ir(LX)] bearing two
2-arylpyridinate(C^N) and one ancillary (LX) ligands are
among the most promising phosphorescent emitters for opto-
electronics and lighting applications.[7–10] With the strong
spin–orbit coupling of heavy metal phosphors, they have high
quantum efficiencies[11,12] and easier synthetic accessibility than
their homoleptic Ir(III) complex [Ir(C^N)3] counterparts,[13,14]
along with an emission wavelength that is conveniently tun-
able by systematic design and functionalization of the organic
ligands.[15–19] From the perspective of the molecular design, an
efficient phosphorescent emitter with the ability to harvest both
singlet and triplet excitons could be designed using heavy metals
by achieving 100% internal quantum efficiency.[20–22] Recent
reports have indicated that further enhancing the charge injec-
tion and transporting characteristics of phosphorescent emit-
ters will require the introduction of additional special functional
V. G. Sree, A. Maheshwaran, H. Kim, H.-Y. Park, Prof. S.-H. Jin
Department of Chemistry Education
Graduate Department of Chemical Materials
Institute for Plastic Information and Energy Materials
Pusan National University
Busandaehakro 63-2, Busan 46241, Republic of Korea
E-mail: shjin@pusan.ac.kr
Y. Kim, Dr. J. C. Lee
LG Display
E6 Block LG Science Park
30, Magokjungang 10-ro, Gangseo-gu, Seoul 07796, Republic of Korea
Dr. M. Song
Advanced Functional Thin Films Department Surface Technology
Division
Korea Institute of Materials Science (KIMS)
797 Changwondaero, Changwon, Gyeongnam 51508, Republic of Korea
E-mail: smk1017@kims.re.kr
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/adfm.201804714.
DOI: 10.1002/adfm.201804714
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groups to the cyclometalated ligands of these emitters in order to
shorten the excited-state lifetime and/or increase the quantum
yield.[23,24] Moreover, it should be considered that attaching
functional units on the phosphorescent emitters as steric hin-
drance could efficiently suppress intermolecular interaction
and thus reduce self-quenching effects, thereby further opti-
mizing these phosphorescent emitters for high-performance
nondoped PHOLEDs.[25–27] For instance, Wong and co-workers
introduced a phenoxy group on the cyclometalated ligand of a
2. Results and Discussion
2.1. Synthesis and Characterization
Recently, our group has first reported Ir(III) complexes with
a solubilizing electron-donating ethylene oxide group intro-
duced into the ancillary ligand in order to improve device
performance without compromising the color purity.[35,36] Here,
in order to develop highly efficient solution-processable green
PHOLEDs and to prove the enhancing effect of the solubilizing
group on the device performance, three Ir(III) complexes,
bis[5-methyl-8-trifluoromethyl-5H-benzo(c)(1,5)naphthyridin-
6-one](4-(2-ethoxyethoxy picolinate) iridium(III) (Ir1), bis[5-eth-
ylhexyl-8-trifluoromethyl-5H-benzo(c)(1,5)naphthyridin-6-one]
(picolinate)iridium(III) (Ir2), and bis[5-hexyl-8-trifluoromethyl-
green Ir(III) complex-based emitter and achieved a maximum
−1 [23]
power efficiency (PE) of 77.6 lm W
.
Park et al. achieved
a maximum PE of 25.6 lm W−1 by using side chain–bearing
host and guest units in solution-processed PHOLEDs.[24] By
using carbazole and diphenylphosphoryl-functionalized Ir(III)
complexes as active components, Su and co-workers fabricated
nondoped PHOLEDs with a maximum current efficiency (CE)
and PE of 29.7 cd A−1 and 31.1 lm W−1, respectively.[28] In most
of the reported the solution-processed PHOLEDs, the Ir(III)
complexes with oligomeric or polymeric ligand system or den-
drimers are extensively used in PHOLEDs to enhance the device
performance. Moreover, it is the greatest challenge to introduce
this macromolecular system into Ir(III) complexes, and addi-
tional multistep reaction sequences are required to achieve this
macromolecular system.[29] To overcome this issue, the Ir(III)
5H-benzo(c)(1,5)naphthyridin-6-one]
(tetraphenylimidodi-
phosphinato)iridium(III) (Ir3), were designed and successfully
synthesized according to the route shown in Scheme 1 and
Scheme S1 (Supporting Information). Most reported solution-
processable Ir(III) complexes have been synthesized by mul-
tiple steps, and complex synthetic processes with additional
purification techniques are required.[35] Here, the solubilizing
group in Ir1 was attached through a tandem reaction, which
eliminates the isolation and complicated purification steps.
The three Ir(III) complexes, Ir1, Ir2, and Ir3, were purified
by column chromatography and/or recrystallized using suit-
able solvents. The structures of these Ir(III) complexes were

complexes with the solubilizing 2-ethoxyethnol (EO2 ) group is
more desirable and is an emerging strategy to improve the per-
formance of solution-processed PHOLEDs. In general, the hole
mobility is ≈2–3 orders of magnitude higher than the electron
mobility in OLED.[30–32] Furthermore, the Ir(III) complexes with
high electron mobility is highly preferred to fabricate efficient
PHOLEDs with a low efficiency roll-off. Herein, the tetraphe-
nylimidodiphosphinate acid (Htpip) was used as the ancillary
1
confirmed by H, mass, and 13C NMR spectroscopic methods
(Figure S1, Supporting Information).
2.2. Photophysical and Electrochemical Properties

ligand which consist of more polar P O and bulky aromatic
groups which may improve the electron mobility and effectively
reduce the triplet–triplet annihilation (TTA) and triplet–polaron
quenching (TPQ) effects.[30–32] Founded on these research
results, herein we design and synthesize three new heteroleptic
green emitting Ir(III) complexes based on an amide-bridged
The emission quantum yields were measured in degassed
dichloromethane (DCM) at room temperature (RT) and the
well-known green emitter fac-Ir(ppy)3 was used as a reference.
Interestingly, Ir1, Ir2, and Ir3 showed higher quantum yield
of 80%, 75%, and 47%, respectively. The quantum yields were
much higher than that of the standard green emitter fac-Ir(ppy)3
(40%).[32] Furthermore, the study results reveal that the addi-
tion of solubilizing group into metal complexes at a suitable
position not only improves the solubility but also increases the
photoluminescence quantum yield (PLQY) compared to those
of previously reported similar molecular structures.[37,38] As
a result, Ir1 showed maximum EQE among the three Ir(III)
complexes as evident from its higher PLQY. The triplet lifetime
(τ) was measured in deaerated DCM (1 × 10−5 m), as shown in
Figure S2 (Supporting Information). The measured τ values
were 0.66, 0.75, and 0.40 µs for Ir1, Ir2, and Ir3, respectively.
The smallest lifetime of Ir3 may have reduced the TTA, TPQ,
and self-quenching, resulting in extremely low efficiency roll-off
at high current densities.[32] The electrochemical behavior of
Ir1, Ir2, and Ir3 was carried out by cyclic voltammetry (CV), as
shown in Figure S3 (Supporting Information). The oxidation
potential (Eox) was calculated by the onset peak at (1.38 eV) with
respect to Fc/Fc+, which could be ascribed to metal-centered
IrIII/IrIV oxidation couple. Therefore, as estimated from the
optical band gap (Eg), the highest occupied molecular orbital
(HOMO) and lowest unoccupied molecular orbital (LUMO)

trifluromethyl ( CF3)-substituted phenylpyridine skeleton with
a solubilizing group. Furthermore, CF3 is introduced in the
main ligand not only to reduce the vibrational frequency, but also
to minimize the radiation-less deactivation and thus improve the


EL performance. In addition, the electron-accepting CF3 sub-
stituents in the cyclometalated ligand prevent the suppression of
self-quenching action in phosphorescence emitters as they may
produce a steric factor around the metal and thereby improve the
device performance.[33,34] Moreover, the introduction of a longer
alkyl chain into the metal complexes affords some interesting
properties. We achieved Ir(III) complexes with extremely high
solubility toward organic solvents and reduced intermolecular
interaction, which are major factors necessary to enhance the
device performance of solution-processed PHOLEDs. The syn-
thesized Ir(III) complexes showed better quantum yield than
did the well-known fac-tris(2-phenylpyridine)Ir(III), Ir(ppy)3. Via
solution processing, the resulting PHOLEDs achieved a remark-
ably high CEmax of 92.44 cd A−1 with a corresponding EQEmax of
24.22%. One of the other Ir(III) complexes also achieved respect-
able CEmax and EQEmax results of 85.31 cd A−1 and 19.40%,
respectively, with a very low efficiency roll-off.
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Scheme 1. Synthetic route of metal complexes.
were −5.7 and −3.2 eV, respectively (Table 1). The Ir3 complex
showed a reversible oxidation with a redox potential, which is
very similar that for previously reported tpip-based Ir(III) com-
plexes.[30,37] Figure 1 shows the molecular orbital distribution
from the density functional theory (DFT) calculation of the
three green Ir(III) complexes, Ir1, Ir2, and Ir3. The HOMOs of
Ir1, Ir2, and Ir3 were mainly distributed over Ir(III) d-orbitals
and from a π-orbital localized on the –CF3-substituted ppy
ligand. For all three complexes, LUMO was mainly located on
their ancillary ligand and Ir atom. HOMO-1 of Ir1 and Ir3 is
distributed over main ligand and Ir atom while HOMO-1 for
Ir2 is distributed mainly in main ligand and Ir atom, and little
in ancillary ligand. LUMO-1 for Ir1 and Ir2 is distributed over
ancillary, and one of the main ligands while LUMO-1 of Ir3 is
mainly located in ancillary ligand and minorly in metal atom
and main ligand. Moreover, our computation study was in good
agreement with our experimentally calculated energy levels.
Figure 2 shows the UV–vis absorption spectra and photolumi-
nescence (PL) spectra measured in DCM solution [10−5 m] and
neat film of Ir1, Ir2, and Ir3 at RT, while the low-temperature
spectra were measured in 2-methyltetrahydrofuran matrix at
77 K. The detailed photophysical, electrochemical, and thermal
properties are summarized in Table 1. The absorption spectra
of Ir1, Ir2, and Ir3 showed intense absorption peaks at 334 and
1
345 nm, which was assigned to the spin-allowed π–π* transi-
tions of cyclometalated ligands. The next long tail absorption in
the region of 400–460 nm was reasonably assigned to a mixed
state of metal-to-ligand charge transfer 1MLCT and 3MLCT
resulting from spin–orbital coupling. The introduction of the

2-ethoxyethnol (EO2 ) group in Ir1 did not affect the optical

properties, possibly because the EO2 group was attached to the
ancillary ligand by ether linkage.[35,36] Thus, this additional solu-
bilizing group may have enhanced the solubility of the Ir(III)
complexes, resulting in homogeneous and tightly packed films.
The optical band gaps (Egopt) of the three Ir(III) complexes were
measured from the UV–vis absorption edge (Table 1). The three
new Ir(III) complexes emitted bright green light upon UV irra-
diation. Upon photoexcitation at corresponding MLCT absorp-
tion band, the Ir1, Ir2, and Ir3 complexes exhibit almost fea-
tureless emission spectra with maximum PL (PLmax) at 514,
510, and 534 nm, respectively, which clearly reveals that the
emissions of the three Ir(III) complexes originated from the
Table 1. Thermal, photophysical, and electrochemical properties of the Ir(III) complex.
Complexes
PLQYb) [%]
CH2Cl2
80%
HOMO [eV]
LUMO [eV]
Egc) [eV]
Tg/Td [°C]
λPLa) [nm]
Neat film
λ
PL [nm]
77 K
502
T [µs]
CH2Cl2
0.66
CH2Cl2
514
Ir1
Ir2
Ir3
112/353
147/364
161/340
523
515
536
2.5
2.5
2.3
−5.7
−5.7
−5.6
−3.2
−3.2
−3.3
510
501
75%
0.75
530
516
47%
0.40
a)Measured in CH2Cl3 solution at RT (1 × 10−5 m); b)Relative PLQY; c)HOMO is calculated from the onset oxidation potentials from CV, LUMO = Eg (optical) + HOMO.
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complexes in thin films. At low-temperature emission spectra
at 77 K, both Ir(III) complexes show much structured emission
spectra, which display rigidochromic blueshifts relative to RT
3
emission spectra ensuring that LC π–π* nature of the emis-
sive excited state.[39] As shown in Figure S4 (Supporting Infor-
mation), the UV–vis spectrum of the guests and the PL spectra
of the mixed hosts show a large overlapping area, and thus the
host materials can effectively transfer their energy to the green
emitters, leading to a higher device efficiency.[40] The transient
PL results revealed that the triplet lifetimes (τ) of the host films
doped with the three Ir(III) complexes decreased in the order,
Ir2 > Ir1 > Ir3, which is in agreement with those measured in
solution and nondoped film.
2.3. Thermal Properties
The thermal properties of Ir1, Ir2, and Ir3 were investigated
under nitrogen atmosphere using thermogravimetric analysis
(TGA) and differential scanning calorimetry (DSC) as shown
in Figures S5 and S6 (Supporting Information). The decom-
position temperatures of Ir1, Ir2, and Ir3 corresponding to 5%
weight loss were 353, 364, and 340 °C, respectively, indicating
their good thermal stability of the Ir(III) complexes, which is
essential for the efficient fabrication of OLEDs. Compared to
Ir1 and Ir3, the DSC analysis of Ir2 revealed a higher glass
transition temperature (Tg) in the range of 112–161 °C, which
Figure 1. Contours and contributions of the frontier molecular orbitals of
the studied Ir(III) complexes.
3MLCT or 3ligand to ligand charge transfer (LLCT) state and not
3
from ligand center (LC) π–π*, which always show vibronically
structured emission spectra.[39] Furthermore, a small PL redshift
was observed in the neat film, which revealed the aggregation
and/or strong intermolecular interactions between the Ir(III)
(a)
(b)
UV-solution
PL-solution
PL-neat flim
2-MeTHF matrix at 77K
UV-solution
PL-solution
PL-neat flim
2-MeTHF matrix at 77K
1.0
1.0
0.8
0.6
0.4
0.2
0.0
0.8
0.6
0.4
0.2
0.0
300
400
500
600
700
300
400
500
600
700
Wavelength (nm)
Wavelength (nm)
(c)
1.0
UV-solution
PL-solution
PL-neat flim
2-MeTHF matrix at 77K
0.8
0.6
0.4
0.2
0.0
300
400
500
600
700
Wavelength (nm)
Figure 2. UV–vis and PL spectra of a) Ir1, b) Ir2, and c) Ir3 in solution and in film state.
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supported its good film morphological stability at RT (Table 1).
The lower Tg value of Ir1 is due to presence of solubilizing
chain.[36]
(a)
2.4. Electroluminescent Properties of Green PHOLEDs via
Solution Processing
Figure S7 (Supporting Information) shows the atomic force
microscope (AFM) 3D images of single (4,4′-bis(N-carbazolyl)-
1,1′-biphenyl (CBP)) and mixed (4,4′,4″-tris(N-carbazolyl)triph-
enylamine (TCTA) and 1,3,5-tris(N-phenyl-benzimidazol-2-yl)
benzene (TPBi)) host films containing the corresponding Ir(III)
complexes by spin coating. The respective root mean square
(RMS) surface roughness values of films with CBP as host
were 0.62, 0.66, and 0.69 nm for Ir1, Ir2, and Ir3, respectively,
while RMS values for mixed host with the respective complexes
(Ir1, Ir2, and Ir3) were measured to be 0.48, 0.50, and 0.56 nm,
respectively. All six films exhibited a homogeneous and smooth
morphology with barely any significant surface morphological
difference among them. To understand the EL properties of the
three different Ir(III) complexes, we fabricated devices A (Ir1),
B (Ir2), and C (Ir3) with CBP chosen as the host because of its
favorable factors such as low electron injection barrier, effective
host-to-guest energy transfer, ability to generate excitons in both
host and guest, and effective hole confining function.[41] From
Figure 3 (energy level), the dopant molecules favor the minority
carriers (electrons) leading to the generation of excitons in the
guest by the balanced carrier injection. However, increased effi-
ciency requires that sufficiently numerous excitons are generated
in the host. The lower injection barrier for electrons facilitates
the injection of electrons leading to a desirable and balanced car-
rier injection on the host. As the applied bias voltage is increased,
more electrons could be trapped in the host due to the saturating
electrons being trapped by the scarcity of guest molecules. Thus,
the electrons trapped in the CBP host assist in attracting holes,
which increases the number of excitons generated in host and
thus facilitates effective energy transfer from host to guest.
Devices were fabricated with a device structure of indium tin oxide
(ITO)/PEDOT:PSS (40 nm)/EML (30 nm)/TPBi (40 nm)/LiF
(1 nm)/Al (120 nm), while TPBi was the electron transport
layer (ETL). Table 2 summarizes the EL performances of the
PHOLEDs using different dopants in single- and double-host
systems. Figure 4 shows the current density–voltage–luminance
(J–V–L) characteristic of the devices fabricated using CBP as
the host. Devices with the Ir1, Ir2, and Ir3 complexes exhibited
a maximum luminance of 22 083, 34 729, and 16 993 cd m−2,
respectively. Figure 4 shows that device A exhibits a maximum
EQE and CE of 18.51% and 70.18 cd A−1 with a commission
internationale de l’éclairage (CIE) of (0.33, 0.61), respectively,
compared to 17.20% and 61.22 cd A−1 for device B with a CIE
of (0.34, 0.61), 7.19% and 26.31 cd A−1 for device C with a CIE
of (0.38, 0.58), respectively. Devices A and B show ELmax values
of 523 and 522 nm (Figure 4d), respectively, while device C’s
emission peak is at 534 nm.
(b)
Figure 3. a) Energy level diagram and b) chemical structures of the
employed materials.
comprised of TCTA and an electron-transporting host, TPBi in
1:1 weight ratio doped with 15 wt% of Ir1 and Ir2, and 12 wt%
of Ir3. Devices D (Ir1), E (Ir2), and F (Ir3) were fabricated with
a device structure of ITO/PEDOT:PSS (40 nm)/EML (50 nm)/
TPBi (40 nm)/LiF (1 nm)/Al (120 nm). These mixed host emit-
ting layer (EML) provided balanced charge carrier transport in
the EML, improved device lifetime behavior[42] and prevented
charge accumulation at the interfaces.[43] In the J–V–L char-
acteristics shown in Figure 5, devices D, E, and F showed
maximum luminance of 42 249, 35 292, and 29 745 cd m−2
with turn-on voltages (Von) of 5.3, 5.0, and 5.4 V, respec-
tively. Figure 5b,c shows the EQE versus luminance and
the CE–current density–PE curve of devices D–F. Device D
showed an excellent CEmax of 92.44 cd A−1 and a maximum
EQE of 24.22% with a CIE of (0.33, 0.61). This is one of
the best reported CE results for solution-processed green
PHOLEDs. Similarly, PHOLEDs with Ir2 also showed a high
CE of 84.25 cd A−1 and a maximum EQE of 23.53% with a CIE
of (0.34, 0.61). Although device F showed a maximum EQE
Further, a double host with different carrier transport
properties, i.e., one material with better hole-transporting
properties and one with better electron-transporting proper-
ties, was chosen to fabricate the devices. The mixed host was
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Table 2. Device performance of green PHOLEDs using Ir1, Ir2, and Ir3 complexes as guests.
CE [cd A−1
Max., @30 mA cm−2
]
PE [lm W−1
]
Maximum luminance [cd m−2
]
Device
Host
CBP
Guest
OV [V]
EQE [%]
Max., @10 000 cd m−2
18.51, 11.47
17.20, 15.50
7.19, 7.07
CIE [x, y]
ELmax [nm]
@30 mA cm−2
A
B
C
D
E
Ir1
Ir2
Ir3
Ir1
Ir2
Ir3
Ir1
Ir2
Ir3
5.6a), 7.3b)
5.7a), 8.2b)
5.7a), 8.7b)
5.3a), 8.5b)
5.0a), 7.6b)
5.4a), 8.7b)
4.6a), 7.82b)
4.5a), 9.2b)
5.1a), 8.3b)
70.18, 39.52
61.22, 47.40
26.31, 26.20
92.44, 60.53
84.25, 49.56
46.22, 44.24
89.11, 84.79
87.10, 86.77
85.31, 81.91
22.88, 13.94
17.49, 12.34
6.75, 6.49
0.335, 0.615
0.340, 0.612
0.380, 0.587
0.331, 0.617
0.340, 0.612
0.382, 0.587
0.330, 0.617
0.340, 0.612
0.383, 0.596
521
522
534
520
522
534
520
522
534
22 083
34 729
16 993
42 249
35 292
29 745
51 808
39 618
54 914
TCTA:TPBi
24.22, 22.91
23.53, 16.71
13.58, 13.27
25.91, 19.22
25.12, 19.31
19.40, 19.29
26.43, 17.98
33.08, 15.01
13.19, 11.47
32.69, 30.56
37.96, 37.49
20.30, 17.72
F
G
H
I
a)Operation voltage (OV) at 1 cd m−2 b)OV at 10 000 cd m−2
; .
of 13.58%, it exhibited a very low efficiency roll-off of 13.27%
(EQE) at 10 000 cd m−2. Additionally, devices G–I were fabri-
cated using an interlayer 4,4′cyclohexylidenebis[N,N-bis(4-
methylphenyl)benzenamine] (TAPC) at the interface of hole-
injecting layer (HIL) and EML with the dopants Ir1, Ir2, and
Ir3, respectively, with the device structure of ITO/PEDOT:PSS
(40 nm)/TAPC (8 nm)/EML (60 nm)/TPBi (50 nm)/LiF (1 nm)/
Al (120 nm). Figure 6 and Figure S8 (Supporting Information)
show the electro-optical characteristics (J–V–L, CE–current den-
sity–PE, and EQE–luminance) of Ir1, Ir2, and Ir3 with TAPC
interlayer. Devices G and H achieved maximum brightness
values of 51, 808, and 39 618 cd m−2 with EQEmax of 25.91% and
25.12%, respectively. Device I showed a maximum luminance
of 54 914 cd m−2. PHOLEDs with the Ir3 achieved a maximum
EQE of 19.40%, 19.29% at 10 000 cd m−2, and a maximum
CE of 85.31 and 81.91 cd A−1 at 30 mA cm−2. It could be seen
that the EL efficiency roll-off in device I was very low and the
resulting high efficiencies at relatively high current densities
was observed. For device I, the CE values at the practical lumi-
nance of 10 000 and 50 000 cd m−2 were 84.81 and 69.03 cd A−1,
respectively. Device I also achieved EQE of 19.29% and 15.73%
at 10 000 and 50 000 cd m−2, respectively. This efficiency roll-off
of devices with Ir3 was attributed to the application of tpip as

the ancillary ligand (presence of more polar P O) and bulky

substituents ( CF3) in the main ligand, which decreased the
self-quenching and improved the hole–electron balance in
charge injection. This balanced charge transport facilitated the
injection and broadened the recombination zone.
Roll-off in PHOLEDs is usually resulted due to the possible
quenching process: TTA and TPQ.[44] To figure out this issue,
the TTA model was employed to fit J–EQE curves using the
equation
(a)
(b)
105
1000
100
10
200
,
,
,
A
B
C
,
,
,
A
B
C
104
103
102
101
100
160
120
80
100
10
1
40
1
0
0
40
80
120
160
0
3
6
9
12
15
Current density (mA/cm2)
Voltage (V)
(c)
(d)
100
10
1
1.0
0.8
0.6
0.4
0.2
0.0
A
B
C
A
B
C
0.1
0
10k
20k
30k
40k
400
500
600
700
Luminance (cd/m2)
Wavelength (nm)
Figure 4. a) J–V–L characteristics, b) CE–luminance–PE curves, c) EQE versus luminance, and d) EL spectra of devices A–C.
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Lifetime of PHOLEDs with the synthesized complexes was
tested and is shown in Figure S10 (Supporting Information).
Device structure of devices D–F was used for the stability meas-
urement. Lifetime measurements were carried out for devices
after encapsulation under continuous electrical stress. With the
initial luminance of 6000 cd m−2, devices with Ir3 showed better
stability while Ir1 and Ir2 exhibited a similar lifetime stability.
These study results demonstrate that the addition of a solu-
bilizing group to the Ir(III) complex at a suitable position not
only improves the overall solubility of the Ir(III) complex, but
also increases the emission quantum yield.[38] In particular,
the balanced charge injection and broadened recombination
zone suppress TTA and TPQ, and improve the recombination
probability with low EL efficiency roll-off.[43]
(a)
105
104
103
102
101
100
250
,
,
,
D
E
F
200
150
100
50
0
0
0
0
4
8
12
16
Voltage (V)
(b)
1000
100
10
1000
100
10
3. Conclusion
,
,
,
D
E
F
In summary, we designed and synthesized three new highly
efficient green emitting Ir(III) complexes for high-performance
solution-processed PHOLEDs. The introduction via tandem

reaction of an EO2 solubilizing group into the ancillary ligand
in the Ir(III) complexes did not affect the optical and electrical
properties of the Ir(III) complexes and improved the PLQY and
EL performance of solution-processed PHOLEDs without com-
promising the color characteristics. Photophysical, thermal,
electrochemical, and electro-optical studies were conducted
to understand the characteristics of the Ir(III) complexes. The
Ir1 complex showed a high PLQY of 80%. PHOLEDs were
fabricated using CBP as the single host and TCTA:TPBi (1:1)
as the mixed host to evaluate their PHOLED performances. An
impressively high CEmax of 92.44 cd A−1 and an EQE of 24.22%
were achieved using Ir1 and the mixed host system. These
are among the best reported CE results for solution-processed
green PHOLEDs. PHOLEDs incorporating the complex Ir3
achieved an EQEmax of 19.40% and a CEmax of 85.31 cd A−1,
while still retaining exceptionally low efficiency roll-off, with CE
and EQE of 84.81 cd A−1 and 19.29% at 10 000 cd m−2, and
69.03 cd A−1 and 15.73% at 50 000 cd m−2, respectively. This
low-efficiency roll-off effect is exceptionally favorable for the
commercial fabrication of solution-processed devices.
1
1
40
80
120
160
200
Current density (mA/cm2)
(c)
100
10
1
D
E
F
0.1
10k
20k
30k
40k
50k
4. Experimental Section
Luminance (cd/m2)
Synthesis of 2-Chloro-N-methylpyridin-3-amine (a):
A
solution
Figure 5. a) J–V–L characteristics, b) CE–luminance–PE curves, and
c) EQE versus luminance curves of devices D–F.
of 3-amino-2-chloropyridine (2.46 g, 19.2 mmol) in anhydrous
tetrahydrofuran (THF) (50 mL) was added to a suspension of sodium
hydride (NaH) (0.78 g, 19.2 mmol, 60% dispersion in oil) and THF
(50 mL) under nitrogen atmosphere. The reaction mixture was stirred
at RT for 3 h. Afterward, iodomethane (1.2 mL, 19.2 mmol) was added,
and the reaction mixture was heated to 40 °C for 9 h. After cooling to
RT, the solvent was removed and the crude material was dissolved in
DCM and filtered to remove any salt. The filtrate was purified by column
chromatography on silica gel using ethyl acetate (EtOAc): n-hexane
(10:90% as an eluent (v/v) to afford compound a as a pale-yellow liquid
(1.52 g). (yield: 62%). 1H NMR (300 MHz, CDCl3,δ): 7.72 (d, 1H),
7.18–7.0 (m, 1H), 6.8 (d, 1H), 4.2 (s, 1H), 3.16 (d, 3H). 13C NMR
(75 MHz, CDCl3, δ): 141.8, 137.0, 136.2, 123.6, 116.9, 30.0.


J0
J
η
=
1 + 8
− 1
(1)


η0 4 J
J0


where η and η0 refer to EQE with and without TTA and J0 is the
critical current density at η = η0/2. EQE curves fitted for complex
Ir1 and Ir2 were in agreement with the TTA model (Figure S9,
Supporting Information), indicating that the TTA process is
responsible for efficiency roll-off at high current densities. J0
values, defined as the current density at η = η0/2 for devices
with Ir1 and Ir2, were 91.86 and 84.2 mA cm−2, respectively.
Synthesis of (N-(2-Chloropyridin-3-yl)-N-methyl-3-(trifluoromethyl)
benzamide) (b): NaH (2.8 g, 7.1 mmol) was added to anhydrous
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(a)
(b)
1000
105
104
103
102
101
100
100
1000
100
10
,
I
,
I
80
60
40
20
100
10
1
0
0
1
3
6
9
12
15
0
20
40
60
80
100
Current density (mA/cm2)
Voltage (V)
(c)
(d)
100
1.0
0.8
0.6
0.4
0.2
0.0
I
10
1
I
0.1
0
10k 20k 30k 40k 50k 60k
Luminance (cd/m2)
400
500
600
700
800
Wavelength (nm)
Figure 6. a) J–V–L characteristics, b) CE–luminance–PE curves, c) EQE versus luminance, and d) EL spectra of device I.
THF (30 mL) and the mixture was cooled to 0 °C under nitrogen
atmosphere. Compound (a) (5 g, 3.5 mmol) was added into the reaction
mixture and stirred for 1 h. After that, 3-(trifluoromethyl)benzoyl
chloride (10.97 g, 52.59 mmol) was added and stirred for 8 h at RT.
After reaction completion, water (50 mL) was added into the reaction
mixture to quench the reaction. Finally, the organic layer was extracted
with DCM, washed with brine solution, and dried over anhydrous
Na2SO4. The solvent was evaporated off, and the solid residue was
purified by column chromatography on silica gel using EtOAc:n-hexane
(20:80% v/v) as an eluent to afford b as a white solid (3.5 g,). (yield:
70%). 1H NMR (300 MHz, CDCl3,δ): 8.25 (s, 1H), 7.63 (s, 1H), 7.54
(d, 3H), 7.38–7.29 (s, 1H), 7.18 (s, 1H) 3.33 (s, 3H). 13C NMR (75 MHz,
CDCl3, δ): 169.6, 149.5, 138.8, 139.4, 134.5, 132.3, 131.1, 130.8, 129.1,
123.8, 123.7, 123.6, 128.5, 37.5.
Synthesis of (5-Methyl-8-(trifluoromethyl)benzo[c][1, 5]naphthyridin-
6(5H)-one) (c): The mixture of compound (b) (2 g, 6.0 mmol), Pd(PPh3)4
(0.7 g, 0.59 mmol), Na2CO3 (3.18 g, 30 mmol), and 2-(dimethylamino)
ethyl methacrylate (DMA) (30 mL) was stirred at 150 °C for 4 h. After
that, the reaction mixture was cooled to RT and water (50 mL) was added.
The resulting mixture was extracted with DCM (100 mL), and the organic
phase was dried over anhydrous Na2SO4. The solvent was evaporated
off, and the solid residue was purified by column chromatography
on silica gel using EtOAc:n-hexane (8:92% v/v) as an eluent to afford
c as a white solid (1.5 g). (yield: 75%). 1H NMR (300 MHz, CDCl3,δ):
8.96 (s, 1H), 8.80 (s, 1H), 8.62 (s, 1H), 8.05 (s, 1H), 7.77 (d, 1H),
7.58 (s, 1H) 3.88 (s, 3H). 13C NMR (75 MHz, CDCl3, δ): 161.1, 146.5,
144.9, 138.4, 137.8, 132.9, 130.7, 129.0, 128.2, 124.7, 124.3, 122.7,
122.6, 38.0.
δ): 7.64–7.63 (d, 1H), 7.06–7.02 (m, 1H), 6.84–6.81 (d, 1H), 4.33
(s, 1H), 3.01 (t, 2H), 1.57–1.55 (d, 1H), 1.41–1.28 (m, 8H), 0.87 (m,
6H). 13C NMR (75 MHz, CDCl3, δ): 146.8, 140.5, 139.8, 124.7, 123.6,
55.1, 40.8, 32.0, 29.3, 26.0, 23.2, 14.1, 12.6.
Synthesis of N-(2-Chloropyridin-3-yl)-N-(2-ethylhexyl)-3-(trifluoromethyl)
benzamide (e): NaH (1.39 g, 58.14 mmol) was added to anhydrous
THF (30 mL) and the mixture was cooled to 0 °C under nitrogen
atmosphere. Compound (d) (5 g, 29.07 mmol) was added to the
reaction mixture and stirred for 1 h. After that, 3-(trifluromethyl)benzoyl
chloride (9.09 g, 43.61 mmol) was added and stirred for 8 h at RT. After
completion of the reaction, the reaction mixture was quenched by the
addition of water (50 mL). Finally, the organic layer was extracted with
DCM, washed with brine solution, and dried over anhydrous Na2SO4.
The solvent was evaporated off, and the solid residue was purified by
column chromatography on silica gel using EtOAc:n-hexane (10:40%
v/v) as an eluent to afford e as a white solid (3.5 g). (yield: 70%). 1H
NMR (300 MHz, CDCl3,δ): 8.25 (s, 1H), 7.45–7.56 (d, 4H), 7.32 (d, 1H),
7.18 (s, 1H), 7.18 (s, 1H), 4.15 (s, 1H), 3.25–3.23 (t, 1H), 1.32–1.22
(d, 8H), 0.98 (s, 6H). 13C NMR (75 MHz, CDCl3, δ): 169.3, 149.5, 138.8,
139.4, 134.5, 132.3, 131.1, 130.8, 129.1, 128.5, 124.8, 123.7, 123.6, 49.2,
37.1, 32.0, 29.3, 26.0, 23.0, 14.1, 11.6.
Synthesis of 5-(2-Ethylhexyl)-8-(trifluoromethyl)benzo[c][1,5]naphthy-
ridin-6(5H)-one ( f ): The mixture of compound (e) (4 g, 10.62 mmol),
Pd(PPh3)4 (0.6 g, 0.53 mmol), Na2CO3 (3.18 g, 53.10 mmol), and of
DMA (30 mL) was stirred at 150 °C for 4 h. Then the reaction mixture
was cooled to RT and water (50 mL) was added. The resulting mixture
was extracted with DCM (100 mL) and the organic phase was dried
over anhydrous Na2SO4. The solvent was evaporated off, and the solid
residue was purified by column chromatography on silica gel using
EtOAc:n-hexane (10:60% v/v) as an eluent to afford f as a white solid
(3 g). (yield: 75%). 1H NMR (300 MHz, CDCl3, δ): 8.99–8.96 (d, 1H),
8.79 (s, 1H), 8.61 (s, 1H), 8.03–8.01 (d, 1H), 7.73–7.70 (d, 1H), 7.51
(s, 1H), 4.38–4.27 (d, 2H), 1.90 (s, 1H), 1.40–1.29 (m, 8H), 0.94–0.87
(m, 6H). 13C NMR (75 MHz, CDCl3, δ): 161.8, 146.5, 144.9, 138.4, 137.8,
132.9, 130.7, 129.0, 128.0, 124.7, 124.3, 122.7, 122.6, 50.7, 37.1, 32.0,
29.3, 26.0, 23.0, 14.1, 12.6.
Synthesis of 2-Chloro-N-(2-ethylhexyl)pyridin-3-amine (d): A solution
of 3-amino-2-chloropyridine (5 g, 38.89 mmol) in anhydrous THF
(50 mL) was added to a suspension of NaH (1.65 g, 38.89 mmol, 60%
dispersion in oil) and THF (50 mL) under nitrogen atmosphere. After
the reaction mixture was stirred at RT for 3 h, 2-ethylhexyl bromide
(5.78 mL, 38.89 mmol) was added, and the reaction mixture was heated
to 40 °C for 9 h. After cooling to RT, the solvent was removed and the
crude material was dissolved in DCM and filtered to remove any salt.
The filtrate was purified by column chromatography on silica gel using
EtOAc:n-hexane (10:60% v/v) as an eluent to afford compound d as
a pale-yellow liquid (3.1 g). (yield: 62%). 1H NMR (300 MHz, CDCl3,
Synthesis of 2-Chloro-N-hexylpyridin-3-amine (g): A solution of 3-amino-
2-chloropyridine (5 g, 38.9 mmol) in anhydrous THF (50 mL) was added
to a suspension of sodium hydride (NaH) (1.5 g, 38.9 mmol, 60%
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dispersion in oil) and THF (50 mL) under nitrogen atmosphere. The
reaction mixture was stirred at RT for 3 h. Afterward, 1-bromohexane
(5.4 mL, 38.9 mmol) was added and the reaction mixture was heated
to 40 °C for 9 h. After cooling to RT, the solvent was removed and the
crude material was dissolved in DCM and filtered to remove any salt.
The filtrate was purified by column chromatography on silica gel using
EtOAc:n-hexane (10:90% v/v) as an eluent to afford compound g as a
pale-yellow liquid (3.1 g). (yield: 62%). 1H NMR (300 MHz, CDCl3,δ):
7.62–7.61 (d, 1H), 7.04–6.99 (m, 1H), 6.81–6.78 (d, 1H), 4.27 (s, 1H),
3.09–302 (q, 2H), 1.61–1.54 (m, 2H), 1.36–1.26 (6H, m), 0.86–0.82
(t, 3H). 13C NMR (75 MHz, CDCl3, δ): 146.8, 140.5, 139.8, 124.7, 124.6,
51.8, 32.7, 31.5, 27.7, 23.7, 14.1.
130.8, 130.7, 126.7, 126.4, 125.5, 125.4, 124.7, 124.5, 123.1, 123.0,
121.9, 121.8, 118.6, 118.6, 118.0, 118.0, 116.7, 113.7, 68.9, 68.1, 67.1,
30.0, 29.9, 15.2. FAB+ MS (m/z): [M + H]+ calcd. for C38H28F6IrN5O6,
957.16.32; found 958.16.
Synthesis of bis[5-Ethylhexyl-8-trifluoromethyl-5H-benzo(c)(1,5)naphthy-
ridin-6-one] (picolinate)iridium(III) Ir2: A solution of compound (k)
(1 g, 0.63 mmol), 2-picolinic acid (0.504 g, 3.19 mmol), and Na2CO3
(0.67 g, 6.39 mmol) in 2-ethoxyethanol (25 mL) was stirred at 120 °C
under nitrogen atmosphere for 12 h. After cooling to RT, the reaction
mixture was poured into water and extracted with DCM. The organic
layer was dried over anhydrous Na2SO4 and concentrated under vacuum,
giving a residue that was purified by column chromatography silica gel
using MeOH:EtOAc (40:60% v/v) as an eluent to afford compound Ir2
as a yellow solid (0.5 g). (yield: 50%). 1H NMR (300 MHz, CDCl3,δ):
8.72 (d, J = 5.4 Hz, 1H), 8.38 (d, J = 7.7 Hz, 1H), 8.19 (s, 1H), 8.12
(s, 1H), 7.99 (t, J = 7.7 Hz, 1H), 7.78–7.72 (m, 3H), 7.58–7.53 (m, 1H),
7.46–7.42 (m, 1H), 7.38–7.30 (m, 2H), 6.82 (s, 1H), 6.53 (s, 1H), 4.49–
4.24 (m, 4H), 2.02–1.86 (m, 2H), 1.60–1.25 (m, 17H), 1.00–0.82 (m,
11H). 13C NMR (75 MHz, CDCl3, δ): 173.0, 167.0, 161.5, 161.4, 153.5,
150.3, 149.2, 148.8, 147.1, 144.9, 142.9, 142.0, 135.3, 134.8, 132.4, 132.0,
131.9, 131.5, 131.2, 131.2, 130.8, 130.7, 126.7, 126.4, 125.5, 125.4, 124.7,
124.5, 123.1, 123.0, 121.9, 121.8, 118.6, 118.6, 118.0, 118.0, 116.7, 113.7,
68.9, 68.1, 67.1, 30.0, 29.9, 15.2. FAB+ MS (m/z): [M + H]+ calcd. for
C48H48F6IrN5O4, 1065.32; found, 1066.33.
Synthesis
of
N-(2-Chloropyridin-3-yl)-N-hexyl-3-(trifluoromethyl)
benzamide (h): NaH (1.88 g, 47.41 mmol) was added to anhydrous THF
(30 mL) and the mixture was cooled to 0 °C under nitrogen atmosphere.
Compound (g) (5 g, 23.50 mmol) was added to the reaction mixture
and stirred for 1 h. After that, 3-(trifluoromethyl)benzoyl chloride
(7.353 g, 35.25 mmol) was added and stirred for 8 h at RT. After
completion of the reaction, the reaction mixture was quenched by the
addition of water (50 mL). Finally, the organic layer was extracted with
DCM, washed with brine solution, and dried over anhydrous Na2SO4.
The solvent was evaporated off, and the solid residue was purified by
column chromatography on silica gel using EtOAc:n-hexane (10:40 v/v)
1
as an eluent to afford h as a white solid (3.5 g). (yield: 70%). H NMR
(300 MHz, CDCl3,δ): 8.24 (s, 1H), 7.54–7.46 (d, 4H), 7.27–7.25 (t, 1H),
7.17 (s, 1H), 4.12–4.07 (s, 1H), 3.59–3.49 (m, 1H), 1.59–1.56 (d, 2H),
1.27 (s, 6H), 0.84 (s, 3H). 13C NMR (75 MHz, CDCl3, δ): 169.3, 149.5,
139.8, 139.4, 134.5, 132.3, 131.8, 131.1, 129.1, 128.5, 124.8, 124.7, 124.6,
44.0, 31.5, 27.4, 27.7, 22.7, 14.1.
Synthesis of bis[5-Hexyl-8-trifluoromethyl-5H-benzo(c)(1,5)naphthyridin-
6-one](Tetraphenylimidodiphosphinato)Iridium(III). Ir3:
A solution of
compound (l) (1 g, 0.54 mmol) and ktpip[32] (0.617 g, 1.35 mmol)
in 2-ethoxyethanol (25 mL) was stirred at 140 °C under nitrogen
atmosphere for 24 h. After cooling to RT, the reaction mixture was
poured into water and extracted with DCM. The organic layer was
dried over anhydrous Na2SO4 and concentrated under vacuum, giving
a residue that was purified by column chromatography silica gel using
MeOH:EtOAc (20:80% v/v) as an eluent to afford compound, Ir3 as
a yellow solid (0.3 g). (yield: 40%) 1H NMR (300 MHz, CDCl3) δ 8.83
(d, J = 5.1 Hz, 2H), 8.06 (s, 2H), 7.78–7.72 (m, 4H), 7.41–7.30 (m, 7H),
7.26–7.24 (m, 1H), 7.19–7.08 (m, 6H), 7.06–6.99 (m, 2H), 6.91–6.82
(m, 4H), 6.52 (s, 2H), 4.40–4.25 (m, 4H), 1.90–1.80 (m, 4H), 1.62–1.36
(m, 13H), 0.95 (t, J = 6.4 Hz, 5H). 13C NMR (75 MHz, CDCl3, δ): 161.4,
149.5, 148.2, 143.5, 140.4, 139.9, 138. 4, 133.2, 130.8, 130.1, 130.6,
130.6, 130.5, 129.9, 128.3, 128.1, 127.5, 127.3, 125.7, 123.2, 122.1, 117.5,
117.4, 43.0, 31.8, 27.1, 26.9, 22.8, 14. 4. FAB+ MS (m/z): [M + H]+ calcd.
for C62H56F6IrN5O4P2, 1303.33; found, 1304.34.
Synthesis of 5-Hexyl-8-(trifluoromethyl)benzo[c][1,5]naphthyridin-6(5H)-
one (i): The mixture of compound (h) (4 g, 10.39 mmol), Pd(PPh3)4 (0.6
g, 0.519 mmol), Na2CO3 (3.18 g, 51.97 mmol), and DMA (30 mL) was
stirred at 150 °C for 4 h. Then the reaction mixture was cooled to RT and
water (50 mL) was added. The resulting mixture was extracted with DCM
(100 mL) and the organic phase was dried over anhydrous Na2SO4. The
solvent was evaporated off, and the solid residue was purified by column
chromatography on silica gel using EtOAc:n-hexane (10:60 v/v) as an
eluent to afford i as a white solid (3 g). (yield: 75%). 1H NMR (300 MHz,
CDCl3, δ): 8.99–8.96 (d, 1H), 8.78 (s, 1H), 8.61 (s, 1H), 8.03–8.01 (d,
1H), 7.72–7.69 (d, 1H), 7.53 (m, 1H), 4.34–4.32 (m, 2H), 1.77–1.68 (m,
2H), 1.47–1.35 (m, 6H), 0.90 (s, 3H). 13C NMR (75 MHz, CDCl3, δ):
161.8, 146.5, 144.9, 138.4, 137.8, 132.9, 130.7, 129.0, 128.0, 124.7, 124.3,
122.7, 122.6, 45.5, 32.5, 27.7, 27.4, 23.7, 14.1.
Synthesis of Iridium Dimer Complex: Compounds (c, f, or i)
(1.5 g, 53.91 mmol) and IrCl3·3H2O (0.76 g, 134.77 mmol) were added
to a mixture of 2-ethoxyethanol and water (40 mL, 3:1 v/v). The reaction
mixture was stirred at 140 °C for 20 h and a yellow precipitate was
obtained and cooled to RT. The precipitate was filtered and washed with
water (60 mL) and methanol (10 mL) and subsequently dried under
vacuum to afford a compound as yellow solid.
Synthesis of bis[5-Methyl-8-trifluoromethyl-5H-benzo(c)(1,5)napht-
hyridin-6-one](4-(2-ethoxyethoxy picolinate)iridium(III), Ir1 via Tandem
Reaction: A solution of compound (j) (1 g, 0.63 mmol), 4-chloropicolinic
acid (0.504 g, 3.19 mmol), and Na2CO3 (0.67 g, 6.39 mmol) in
2-ethoxyethanol (25 mL) was stirred at 120 °C under nitrogen
atmosphere for 12 h. After cooling to RT, the reaction mixture was
poured into water and extracted with DCM. The organic layer was
dried over anhydrous Na2SO4 and concentrated under vacuum, giving
a residue that was purified by column chromatography silica gel using
methanol (MeOH):EtOAc (30:70% v/v) as an eluent to afford compound
Ir1 as a yellow solid (0.5 g). (yield: 50%). 1H NMR (300 MHz, CDCl3,δ):
8.70 (d, J = 5.4 Hz, 1H), 8.18 (s, 1H), 8.12 (s, 1H), 7.86 (d, J = 2.6
Hz, 1H), 7.76 (d, J = 8.6 Hz, 2H), 7.57–7.52 (m, 1H), 7.44 (d, J = 5.6
Hz, 2H), 7.38–7.33 (m, 1H), 6.93–6.90 (m, 1H), 6.83 (s, 1H), 6.62 (s,
1H), 4.25 (d, J = 2.9 Hz, 2H), 3.84 (s, 6H), 3.79–3.76 (m, 2H), 3.56
(q, J = 7.0 Hz, 2H), 1.20 (t, J = 7.0 Hz, 3H). 13C NMR (75 MHz, CDCl3,
δ): 173.0, 167.0, 161.5, 161.4, 153.5, 150.3, 149.2, 148.8, 147.1, 144.9,
142.9, 142.0, 135.3, 134.8, 132.4, 132.0, 131.9, 131.5, 131.2, 131.2,
Theoretical Calculation: The calculations reported here were carried
out by using the Gaussian 09[45] software package. Ground state DFT
calculations were performed to understand the molecular distributions
of the Ir(III) complexes. The DFT calculations of the Ir(III) complexes
were performed using the B3LYP functional with the 6–31G (d,p)
basis set for C, H, N, O, and F, and LANL2DZ for the Ir atom. To
understand the spatial distribution of the frontier molecular orbitals
clearly, the contour plots of LUMO + 1 to HOMO − 1 of Ir1, Ir2, and
Ir3 were simulated, and the calculated energies are provided in Table S1
(Supporting Information).
Device Fabrication and Measurement: The HIL, poly(3,4-ethylened
ioxythiophene):polystyrenesulfonate (PEDOT:PSS) was spin-coated
onto the UV–ozone-treated ITO-coated glass substrate, followed by
annealing in air for 15 min at 150 °C, respectively. The EML was then
spin-coated onto the HILs and annealed at 100 °C for 30 min in nitrogen
atmosphere. ETL, LiF, and Al were sequentially deposited over the EML
layer in vacuum at a pressure of 5 × 10−6 Torr. All measurements of
devices were carried out under ambient condition at RT. The thickness
of each layer was measured by an Alpha-step IQ surface profiler (KLA
Tencor, San Jose, CA).
Supporting Information
Supporting Information is available from the Wiley Online Library or
from the author.
©
Adv. Funct. Mater. 2018, 1804714
1804714 (9 of 10)
2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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©
Adv. Funct. Mater. 2018, 1804714
1804714 (10 of 10)
2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim