Organometallic Ir(III) Phosphors Decorated by Carbazole/Diphenylphosphoryl Units for Efficient Solution-Processable OLEDs with Low Efficiency Roll-Offs

Article abstract of DOI:10.1021/acs.inorgchem.9b01601

Recently, solution-processable PhOLEDs have been attracting great interest for their low cost and high productivity relative to the vacuum-deposited devices. Similar to vacuum-deposited OLEDs, however, they usually suffer from serious efficiency roll-offs, especially in high brightness. Finding a feasible way and/or designing novel materials to increase efficiencies and reduce roll-offs simultaneously are highly desired. Herein, a new family of solution-processable cyclometalated iridium(III) phosphors with carbazole (Cz) and/or diphenylphosphoryl (Ph₂PO) units functionalized main ligands has been designed. Owing to Cz and Ph₂PO moieties possessing bulky steric effects, they can suppress the intermolecular strong packing and then decrease TTA effects and emission quenching. Meanwhile, the resulting OLEDs based on the designed phosphors exhibit considerable efficiencies and relatively small efficiency roll-offs. The device based on 4 containing both Cz and Ph₂PO units realized a maximum current efficiency of 21.3 cd A^-1, accompanied by a small roll-off. By optimization of the configuration of OLEDs, the device performance can be further enhanced, demonstrating their potential for high-performance solution-processable PhOLEDs.

Full text of DOI:10.1021/acs.inorgchem.9b01601

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Organometallic Ir(III) Phosphors Decorated by Carbazole/
Diphenylphosphoryl Units for Efficient Solution-Processable OLEDs
with Low Efficiency Roll-Offs
Wei-Lin Song,†,⊥ Hui-Ting Mao,†,⊥ Guo-Gang Shan,*,† Ying Gao,† Gang Cheng,*,‡,§
and Zhong-Min Su*,†,∥
†Institute of Functional Material Chemistry and National & Local United Engineering Lab for Power Battery, Faculty of Chemistry,
Northeast Normal University, Changchun, Jilin 130024, People’s Republic of China
‡State Key Laboratory of Synthetic Chemistry, HKU-CAS, Joint Laboratory on New Materials, and Department of Chemistry, The
University of Hong Kong, Pokfulam Road, Hong Kong SAR, People’s Republic of China
§HKU Shenzhen Institute of Research and Innovation, Shenzhen 518053, People’s Republic of China
∥Changchun University of Science and Technology, Changchun, Jilin 130022, People’s Republic of China
S
* Supporting Information
ABSTRACT: Recently, solution-processable PhOLEDs have been
attracting great interest for their low cost and high productivity relative
to the vacuum-deposited devices. Similar to vacuum-deposited OLEDs,
however, they usually suffer from serious efficiency roll-offs, especially in
high brightness. Finding a feasible way and/or designing novel materials
to increase efficiencies and reduce roll-offs simultaneously are highly
desired. Herein, a new family of solution-processable cyclometalated
iridium(III) phosphors with carbazole (Cz) and/or diphenylphosphoryl
(Ph2PO) units functionalized main ligands has been designed. Owing
to Cz and Ph2PO moieties possessing bulky steric effects, they can
suppress the intermolecular strong packing and then decrease TTA
effects and emission quenching. Meanwhile, the resulting OLEDs based
on the designed phosphors exhibit considerable efficiencies and
relatively small efficiency roll-offs. The device based on 4 containing
both Cz and Ph2PO units realized a maximum current efficiency of 21.3 cd A−1, accompanied by a small roll-off. By
optimization of the configuration of OLEDs, the device performance can be further enhanced, demonstrating their potential for
high-performance solution-processable PhOLEDs.
and the second is a spin-coating solution process.17 Although
the OLEDs can achieve high performance, serious efficiency
INTRODUCTION
■
As time goes on, more electrical energy is largely consumed to
roll-offs still exist for a long time during the device operation,
meet the needs of people’s lives in lighting, luminaire, displays,
which hinder their widespread use in practical applications.18,19
and so on. It is thus obligatory to develop high-performance
In addition, the vacuum deposition process requires intricate
energy-conserving lighting devices. Owing to the superiorities
technological means and throws away a large amount of
of low energy consumption, wide viewing angle, and flexible
organic materials. Furthermore, it is also highly expensive to
display, organic light-emitting diodes (OLEDs) have been
applied in displays and energy-saving lighting.1−4 Phosphor-
maintain the chamber under vacuum, leading to expensive
fabrication costs. Alternatively, a more low-cost method, i.e., a
escent transition-metal materials possess strong spin−orbit
coupling and fast intersystem crossing rates, and they can
solution-processable technique, has been developed re-
cently.20−22 Therefore, it is important to design novel
utilize almost 100% of the excitons to realize high electro-
luminescence (EL) efficiencies.5−7 Among the phosphorescent
iridium(III) phosphors or develop new strategies to optimize
the device performances.
Many groups have devoted great efforts to the development
of a variety of iridium(III) phosphors to fabricate PhOLEDs
with low efficiency roll-offs.23−26 One of the effective ways to
materials, iridium(III) phosphors have drawn much attention
on account of the high emission efficiencies, excellent
photothermal stability, and tunable emission color.7−12 Some
representative works on iridium(III)-based OLEDs have been
summarized in recent reviews.13−16
To date, two main processing techniques are used to
Received: May 29, 2019
fabricate the OLEDs: the first is a vacuum-deposition process,
© XXXX American Chemical Society
A
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Figure 1. Chemical structures of 1−4.
Table 1. Photophysical and Electrochemical Data of 1−4
a
a c
−
ac
ac
,
d
e
f
g
,
phosphor
λabs (nm)
λPL,max
(nm)
Φp (%)
τ
(μs)
Eg (eV)
Eox,onset (V)
HOMO (eV)
LUMO (eV)
1
292
338
412
298
368
409
292
365
403
292
354
413
581, 555, 582
565, 553, 579
569, 555, 577
577, 557, 581
25.2, 10.1
1.32, 0.48
1.09, 0.70
1.24, 0.61
1.24, 0.74
2.18
2.24
2.23
2.23
0.39
0.38
0.41
−5.19
−5.18
−5.21
−3.01
−2.94
−2.98
2
3
4
29.7, 24.3
32.1, 16.1
27.1, 17.4
0.41
−5.21
−2.98
a
b
c
d
e
Measured in dichloromethane solution at 298 K. Measured in dichloromethane solution at 77 K. Spin-coated thin film. Optical band gap. The
f
g
onset oxidation potential. Calculated from the onset oxidation potentials of the complexes. Calculated using the equation ELUMO = EHOMO + Eg.
achieve high-performance solution-processable OLEDs is
designing dendrimers,27,28 which possess good processability
to ensure the formation of homogeneous thin films. In
addition, such a strategy can suppress self-quenching to
improve the efficiency and weaken the efficiency roll-offs
simultaneously. In 2017, Ding and co-workers synthesized red
iridium(III) dendrons containing oligocarbazole and doped
devices with a current efficiency (CE) of 25.7 cd A−1 were
realized, whose CE still remained as high as 23.9 cd A−1 at a
brightness of 1000 cd m−2.29 Although such a strategy is
effective, the synthetic procedures require cumbersome steps
with relatively low total yields. Recent works have shown that
attaching appropriate functional groups with intrinsic elec-
tron-/hole-transporting characteristics can effectively adjust
the carrier-transporting properties and improve the corre-
sponding device performances.27,30−33
concentration of luminescent materials, the device based on
4 exhibited eminent performance, displaying a maximum CE of
21.3 cd A−1 and low efficiency roll-off of 11.2%. The EL
performances for such devices, in fact, could be further
optimized by changing the device structure.
EXPERIMENTAL SECTION
■
General Information. The experimental details and synthetic
routes for the cyclometalated and ancillary ligands as well as the
studied complexes can be found in the Supporting Information.
Briefly, modified procedures were used to synthesize all cyclo-
metalated ligands in relatively high yield,37 and the ancillary ligand
Hbiq used was prepared in accordance with a previous work.38 The
target complexes 1−4 were then obtained by reacting the
corrsponding μ-chloro-bridged dimer with the Hbiq ligand. All
complexes were characterized by various spectroscopic techniques
(see the Supporting Information).
Recently, many cyclometalated iridium(III) phosphors
based on the 1,2-diphenyl-H-benzimidazole (HPBI) moiety
as the main ligand have been reported.34−36 Various iridium-
(III) phosphors with carbazole (Cz)- or diphenylphosphoryl
(Ph2PO)-functionalized HPBI ligands have been designed.
The resulting vacuum-deposited OLEDs exhibit high EL
performances due to the steric hindrance effect of Cz and
Ph2PO that can effectively weaken self-quenching and control
charge-carrier characters as well.37 To determine whether such
molecular structures were suitable for constructing iridium(III)
phosphors for solution-processable devices, herein, we
successfully synthesized a new family of iridium(III)
phosphors, denoted 1−4, in which the HPBI ligand was
functionalized with Cz and Ph2PO moieties (see Figure 1).
Meanwhile, we employed 2-benzo[d]imidazol-2-ylquinoline
(Hbiq) as an ancillary ligand to achieve long-wavelength
emissions. High-performance solution-processable devices
employing the designed iridium(III) phosphors as emissive
layers have been achieved with considerable efficiencies and
relatively low roll-offs. By manipulation of the doping
Physical Measurements. The photophysical characteristics of
1−4 in CH2Cl2 solution with a concentration of 1.0 × 10−5 M were
detected. Cary 500 UV−vis−NIR and FL-4600 fluorescent
spectrophotometers were used to measure their absorption spectra
and photoluminescence (PL) spectra, respectively. Meanwhile, the
corresponding films for phosphors were acquired via a spin-coating
method with a concentration of 20 mg mL−1 in CH2Cl2. In addition,
the excited-state lifetimes and photoluminescence quantum yields
(PLQYs) were monitored by an Edinburgh FLSP920 spectrofluorim-
eter and an integrating sphere.
Theoretical Calculations. The Gaussian 09 program package was
used to calculate the studied phosphors.39 The closed-shell and spin-
unrestricted open-shell B3LYP methods under gas-phase conditions
were carried out to optimize the geometries in both the ground state
and the lowest triplet state (T1), respectively.40 The LANL2DZ basis
set was used for the Ir atom, and the standard 6-31G* set was
employed for the rest of the atoms. The inner-core electrons located
on the central Ir(III) atom were replaced by effective core potentials,
and the outer-core electrons (5s)2(5p)6 and valence electrons (5d)6
were left to study.41 TD-DFT calculations were then performed using
the CPCM polarizable conductor calculation model (dichloro-
B
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Figure 2. (a) Absorption and emission spectra of 1−4 in dichloromethane (10−5 M) at room temperature. (b) Emission spectra of those phosphors
in neat films.
methane solvent) at the optimized geometry to deeply understand the
excited-state characteristics.
Electrochemical Measurements. Cyclic voltammetry (CV)
29.7% (2) and Φ = 32.1% (3), respectively. Moreover, the
emission spectra of 1−4 at 77 K WERE tested in dichloro-
methane with peak maximA of 555, 553, 555, and 557 nm,
measurements for all phosphors with a concentration of 0.003 M in
respectively (see Figure S5). The hypochromatic shift observed
CH2Cl2 were carried out using an electrochemical workstation
relative to the emission at room temperature indicates that the
(BAS100W instrument), in which ferrocene was used as the internal
PL spectra for all phosphors are sensitive to temperature.43
standard. An aqueous saturated calomel electrode, a platinum-wire
Therefore, it is shown that the emissions of the phosphors
should be of mixed 3MLCT, 3LLCT, or 3ILCT character.44−48
To explore the emission character of all phosphors, the
lifetimes were tested in both solutions and thin films. They
exhibit a microsecond excited-state decay, which obviously
reveals the phosphorescence emission character.46,49,50 Addi-
tionally, the spin-coated neat films PL spectra based on 1−4
had a negligible red shift compared to those in solution,
probably due to the effective medium polarity in films (see
Table1). It is worth noting that, although different functional
groups have been introduced into the phosphors, they exhibit
almost identical emissions in spin-coated films. This result
indicates that the peripheral Cz and Ph2PO units have a slight
effect on their emission in films but can adjust the charge-
transporting characteristics and the solid-state packing as well.
Therefore, we conjecture that devices based on these materials
would exhibit similar EL emission profiles but distinct different
EL performances caused by their different charge-transporting
properties and intermolecular interactions in the films.
Theoretical Calculations. To deepen the understanding
of the influence of functional groups on the photophysical and
electrochemical properties for all phosphors, the corresponding
theoretical calculations were implemented. The highest
occupied molecular orbital (HOMO) and the lowest
unoccupied molecular orbital (LUMO) with corresponding
energy levels are displayed in Figure 3. The LUMOs for 1−4
are mainly distributed on the ancillary ligand. The HOMOs of
all complexes are mainly composed of a mixture of the d
orbitals for the Ir(III) atoms and some distributions on the
ancillary ligands. The T1 states of 1−4 originate from HOMO
→ LUMO (73%), HOMO → LUMO (89%), HOMO →
LUMO (70%), HOMO → LUMO (90%) transitions,
respectively (see Table S5,). Therefore, it is speculated that
all phosphors should possess 3MLCT/3ILCT character in
different proportions. Spin densities calculated by unrestricted
DFT methods are more localized on iridium(III) atoms as well
as on the ancillary ligands, which matches well with the
topology of the HOMO → LUMO excitations for these
complexes (see Figure S6 in the Supporting Information).
These results are consistent with the observed phosphorescent
emission profiles are both room temperature and 77 K. In
addition, according to triplet excited state calculations obtained
electrode, and a glassy-carbon electrode were selected as the reference
electrode, the auxiliary electrode, and the working electrode,
respectively. Tetrabutylammonium hexafluorophosphate (1 × 10−1
M) was used as the supporting electrolyte.
Device Fabrication and Characterization. The devices were
fabricated on patterned indium−tin oxide (ITO) coated glass
substrates with a sheet resistance of 30 Ω/sqr. All glass substrates
were cleaned with Decon 90 and rinsed with deionized water. Then,
they were dried in an oven, following a treatment in an ultraviolet−
ozone chamber. PEDOT:PSS was spin-coated onto the glass
substrates as the hole-transporting layer. Then, a mixture of the
iridium(III) complex with PVK and OXD-7 was prepared as a 10 mg
cm−3 solution in dichloromethane and spin-coated onto the
PEDOT:PSS layer to form the emissive layer. Next, TPBI was
selected as the electron-transporting layer. Finally, LiF/Al was
vacuum-deposited as the metal cathode. An Ocean Optics Maya
2000-PRO spectrometer, Minolta LS-110 luminance meter, and
Keithley 2400 source meter were used to test the EL spectra and
luminance efficiency−voltage characteristics of the resulting devices.
RESULTS AND DISCUSSION
■
Photophysical Properties. UV−vis absorption and PL
spectra of 1−4 were recorded in CH2Cl2 at room temperature,
and the numerical data are summarized in Table 1. As for most
iridium(III) phosphors, the absorptions below 380 nm are
1
assigned to allowed π−π* transitions in the center of the
ligands, as shown in Figure 2a. Weak and broad absorptions
1
are found at a longer wavelength (>400 nm) due to MLCT
(metal to ligand charge transition), 3MLCT, and 3π−π*
transitions.8,42 To better comprehend the features of the
excited states involved in the main transitions, we simulated
the absorption spectra by performing TDDFT calculations
(see Tables S1−S4 and Figures S1−S4). With complex 4 as an
example, the absorption bands below 380 nm from the
excitation of S4, S16, and S19 are mainly ascribed to π−π*
transitions of both the main ligands and ancillary ligands. The
1
long absorption bands have obvious MLCT characters. The
calculated UV−vis absorption spectra matched well with the
experimental spectra. The corresponding assignments for
complexes 1−3 can be also obtained from the TD-DFT results.
Upon photoexcitation, 1 and 4 exhibit identical emission
maxima of 581 and 577 nm, respectively, accompanied by Φ =
25.2% (1) and Φ = 27.1% (4). In addition, 2 and 3 exhibit
strong emission with peaks of 565 and 569 nm as well as Φ =
C
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−5.18, −5.21, and −5.21 eV for 1−4, respectively, from the
onset oxidation potentials in CV curves. On the basis of EHOMO
and optical band gaps (Eg), the ELUMO values of 1−4 are
calculated to be −3.01, −2.94, −2.98, and −2.98 eV (see
Figure S8 in the Supporting Information).51,52
Device Performance. Taking into full consideration of the
good solubility of the iridium(III) phosphors, we fabricated
solution-processable devices with the configuration of ITO/
PEDOT:PSS/PVK:OXD-7:iridium(III) phosphors (10:1:x)/
TPBI (40 nm)/LiF (1.2 nm)/Al (100 nm) (see Figure 4a).
The HOMO/LUMO diagrams and molecular structures of
used materials in the devices are displayed in Figure 5. Here,
ITO and LiF/Al were employed as the anode and cathode,
respectively, poly(3,4-ethylene-dioxythiophene)-poly-
(styrenesulfonate) (PEDOT:PSS) acted as the hole-injecting
layer (HIL), and 1,3,5-tris(1-phenyl-1H-benzimidazol-2-yl)-
benzene (TPBI) was chosen as the electron-transporting layer.
The emissive layer was prepared by spin-coating a solution of a
poly(N-vinylcarbazole) (PVK):2,2′-(1,3-phenylene)bis[5-(4-
tert-butylphenyl)-1,3,4-oxadiazole] (OXD-7):iridium(III)
blend at different concentrations. Figure 4c−f displays the
normalized EL spectra of the solution-processable OLEDs
based on 1−4. The electroluminescent profiles of the devices
are almost similar to their emission spectra, which indicate that
the electroluminescence and photoluminescence should
Figure 3. HOMO and LUMO distributions of 1−4.
by TD-DFT, their excited state characters were further
confirmed by electron density difference analysis (see Figure
S7 in the Supporting Information).
Electrochemical Properties. As shown in Figure S8 and
Table 1, the electrochemical behaviors of Ir(III) phosphors
were studied. The EHOMO values are evaluated to be −5.19,
Figure 4. (a) The OLED device structure. (b) EQE curves of all devices. EL spectra (inset) and J−V−L characteristics of devices based on 1 (c), 2
(d), 3 (e), and 4 (f).
D
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Figure 5. Energy level diagram of doped devices and the chemical structures of employed materials.
Table 2. Summary of EL Performances of Studied Devices
b
c
d
CE (cd A−1
)
PE (lm W−1
)
EQE (%)
(EQE roll-off (%))
(CE roll-off (%))
(PE roll-off (%))
a
device
doping concn (wt %)
L (cd m−2
)
max
at 1000 cd m−2
max
at 1000 cd m−2
max
at 1000 cd m−2
CIE (x, y)
1
10
20
10
20
10
20
10
20
10
15200
16400
8750
15.8
15.6
5.5
14.7 (6.9)
14.7 (5.8)
5.5 (0.0)
8.4
8.2
2.4
1.8
7.1
8.3
12.8
9.0
6.6
7.7 (8.3)
6.9 (15.8)
2.3 (2.5)
1.7 (4.1)
6.5 (8.4)
7.0 (15.6)
9.9 (22.6)
7.1 (21.1)
5.1 (22.7)
5.8
5.8
2.0
1.6
4.8
6.8
7.6
5.9
4.8
5.3 (8.6)
5.5 (5.2)
2.0 (0.0)
1.6 (0.0)
4.5 (6.2)
5.3 (22.1)
6.6 (13.1)
5.3 (10.1)
(0.50, 0.49)
(0.51, 0.49)
(0.48, 0.50)
(0.50, 0.49)
(0.49, 0.50)
(0.50, 0.49)
(0.49, 0.50)
(0.50, 0.49)
(0.48, 0.49)
2
3
4
7470
4.4
4.3 (2.3)
15400
17880
12800
14850
13.5
15.9
21.3
16.2
13.7
12.8 (4.9)
14.3 (9.4)
18.9 (11.2)
14.5 (10.5)
12.2 (10.9)
e
2
11140
4.3 (10.4)
a
b
c
Maximum luminance of the devices. The maximum CE and the values measured at 1000 cd m−2 at different concentrations. The maximum PE
d
and the values measured at 1000 cd m−2 at different concentrations. The maximum EQE and the values measured at 1000 cd m−2 at different
concentrations. The device configuration ITO/PEDOT:PSS/PVK:2 (10:1:x)/TPBI (40 nm)/LiF (1.2 nm)/Al (100 nm).
e
originate from the decay of the same excitons and without any
unfavorable excimeric emission. What is more, the photo-
luminescence spectra of doped films show absolute energy
transfer between the host and guest materials (see Figures S9−
S11 in the Supporting Information). In addition, the
luminescence quantum yields of all phosphors doped in the
PVK and OXD-7 thin films were also measured. All of the
doped films exhibit relatively high PLQYs, and the
corresponding data are given in Table S6. It is clear that the
4-based doped film with a doping concentration of 10 wt %
exhibits the best PLQY, as high as 65.1%, showing good energy
transfer from the host to guest as well as weak emission
quenching in the doping film. Therefore, we speculate the
device employing 4 as emitting layer would show better EL
performances.
cyclometalated ligands as well as the color purity of the
OLEDs.53
As shown in Figure 4c−f, the devices achieve favorable
luminance; for example, the maximum brightnesses (Lmax) of
3-based devices are as high as 15400 cd m−2 (10 wt %) and
17880 cd m−2 (20 wt %). The maximum brightnesses (Lmax) of
4-based devices are 12800 cd m−2 (10 wt %) and 14850 cd
m
−2 (20 wt %). Table 2 reveals the device performances. 3 and
4 endow their devices with promising efficiencies, with
maximum CEs of 13.5 cd A−1 (10 wt %) and 15.9 cd A−1
(20 wt %) and and 21.3 cd A−1 (10 wt %) and 16.2 cd A−1 (20
wt %), respectively. Significantly, they all exhibit low efficiency
roll-offs. At 1000 cd m−2, the current efficiencies for 3- and 4-
based devices are slightly decreased to 12.8 and 18.9 cd A−1 at
a doping concentration of 10 wt %, corresponding to efficiency
roll-offs of only 4.9% and 11.2%, respectively. As expected, the
4-based doped OLED displays excellent performance, showing
the highest efficiency among all studied devices, which is
probably due to its higher PLQY in doped film as mentioned
above and better charge carrier balance in the device. The
Ph2PO/Cz groups introduced into the complex can effectively
reduce the intermolecular aggregation and suppress self-
quenching effects, leading to the favorable stability of EL
efficiency. Moreover, the more balanced carrier transport
characteristic probably enlarges the recombination zone and
efficiently confines the triplet excitons generated within the
EML.54,55 In general, it is believed that carbazole and
diphenylphosphoryl groups could increase the overall device
efficiency and decrease the efficiency roll-offs to a certain
degree.
The phosphors 1−4 were individually doped into PVK and
OXD-7 at different concentrations of 10 and 20 wt %,
respectively. Figure 4b−f shows the external quantum
efficiency (EQE) and current density−voltage−luminance
(J−V−L) characteristics of EL devices. Bright yellow electro-
luminescence can be recorded with Commission Internationale
́
de l’Eclairage (CIE) coordinates of (0.50, 0.49) (10 wt %) and
(0.51, 0.49) (20 wt %) for 1, (0.48, 0.50) (10 wt %) and (0.50,
0.49) (20 wt %) for 2, (0.49, 0.50) (10 wt %) and (0.50, 0.49)
(20 wt %) for 3, and (0.49, 0.50) (10 wt %) and (0.50, 0.49)
(20 wt %) for 4. The CIE coordinates of the devices are almost
consistent at different doping concentrations, which are
extraordinarily important for the precise control of the
emission energies of the iridium(III) phosphors via modifying
E
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Figure 6. (a) Optimized device configuration for 2. (b) EQE curves of the device. (c) EL spectra of the device. (d) Current density−voltage−
luminance characteristics.
As can be seen in Figure 4d and Table 2, the efficiencies of
the devices based on 2 are much lower than those of its
counterparts having the same device structure of ITO/
PEDOT:PSS/PVK:OXD-7:iridium(III) phosphors (10:1:x)/
TPBI (40 nm)/LiF (1.2 nm)/Al (100 nm). The phenomenon
may be ascribed to the introduction of two Ph2PO units into
the cyclometalated ligands, leading to imbalanced charge
carriers in the EML. To further optimize the device
performance, we removed the electron-transporting material
OXD-7 in the EML for 2-based devices with the aim of
manipulating charge-carrier transportation. The corresponding
device configuration and is displayed in Figure 6a. By
optimization, the maximum CE reaches 13.7 cd A−1, which
is an increase of about 3-fold over the previous data. In
addition, the optimized device also exhibits a low efficiency
roll-off of only 10.9% for CE at 1000 cd m−2 (see Figure 6b−d
and Table 2).
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.9b01601.
■
S
Supplemental schemes and photophysical and theoreti-
cal data of all complexes (PDF)
AUTHOR INFORMATION
Corresponding Authors
*Email for G.-G.S.: shangg187@nenu.edu.cn.
*Email for G.C.: ggcheng@hku.hk.
*Email for Z.-M.S.: zmsu@nenu.edu.cn.
■
ORCID
Gang Cheng: 0000-0001-8081-8205
Zhong-Min Su: 0000-0002-3342-1966
Author Contributions
⊥W.-L.S. and H.-T.M. contributed equally to this work.
CONCLUSIONS
Notes
■
The authors declare no competing financial interest.
In general, a new family of iridium(III) phosphors named 1−4
were designed and synthesized, in which the cyclometalated
ligands were substituted by Cz and/or Ph2PO groups
simultaneously. The intermolecular interaction and emission
quenching could be suppressed to some extent, due to the
strong steric effects caused by the Cz and Ph2PO units.
Meanwhile, the charge-carrier transporting characters of the
complexes were well manipulated by controlling the number
and position of such large functional groups. Therefore, the
corresponding phosphors displayed low efficiency roll-offs even
when the doping concentration was high. The doped device
based on 4 displayed eminent performance with a CE of 21.3
cd A−1. This work emphasizes the significance of large
functional groups in optimizing cyclometalated iridium(III)
phosphors used in solution-processable OLEDs. What is more,
it provides a feasible strategy for the future development of
iridium(III) phosphors and relevant optical devices.
ACKNOWLEDGMENTS
■
The authors gratefully acknowledge financial support from the
National Natural Science Foundation of China (21303012,
21273030, 51203017, 21131001, and 61474054), the 973
Program (2013CB834801), the fundamental Research Funds
for the Central Universities (2412019FZ012 and
2412019QD007), the Basic Research Program of Shenzhen
(JCYJ20170818141858021), and the Telecommunications
Scientific Foundation (NY215061).
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