Achieving High Performances of Nondoped OLEDs Using Carbazole and Diphenylphosphoryl-Functionalized Ir(III) Complexes as Active Components

Article abstract of DOI:10.1021/acs.inorgchem.7b01516

Nondoped electroluminescent devices offer advantages over their doped counterparts such as good reproducibility, reduced phase separation between host and guest materials, and potential of lower-cost devices. However, low luminance efficiencies and signif

Full text of DOI:10.1021/acs.inorgchem.7b01516

Article
pubs.acs.org/IC
Achieving High Performances of Nondoped OLEDs Using Carbazole
and Diphenylphosphoryl-Functionalized Ir(III) Complexes as Active
Components
Hui-Ting Mao,† Chun-Xiu Zang,‡ Guo-Gang Shan,*,† Hai-Zhu Sun,† Wen-Fa Xie,*,‡
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, P. R. China
‡State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, Changchun,
Jilin 130012, P. R. China
S
* Supporting Information
ABSTRACT: Nondoped electroluminescent devices offer advantages over their
doped counterparts such as good reproducibility, reduced phase separation between
host and guest materials, and potential of lower-cost devices. However, low
luminance efficiencies and significant roll-off values are longstanding issues for
nondoped devices, and a rational design strategy for the preparation of efficient
phosphors is highly desired. In this work, cyclometalated Ir(III) complexes
3CzIr(mtpy), 4CzIr(mtpy), 3POIr(mtpy), and 4POIr(mtpy) bearing carbazole
(Cz) or diphenylphosphoryl (Ph2PO) groups substituted at different positions of
1,2-diphenyl-H-benzimidazole (HPBI) were designed and synthesized. Owing to
the steric effects induced by these groups, a significant intermolecular interaction
was avoided, thereby reducing self-quenching and triplet−triplet annihilation
(TTA) at high brightness. Simultaneously, attached functional moieties manipulate
the charge-carrier character and enhance the EL performance of the complexes.
Device N3-10, based on 3POIr(mtpy), successfully realized excellent performance
and improved efficiency stability, rendering a turn-on voltage of 2.5 V, a maximum current efficiency of 29.7 cd A−1, and a
maximum power efficiency of 31.1 lm W−1, which are all almost 3-fold higher than that of the control device N-10 based on
parent complex. Inspiringly, all of the devices showed reduced efficiency roll-off as luminance increased. To the best of our
knowledge, these are good results for green-emitting PHOLEDs using vacuum evaporation techniques, and they provide
fundamental insights into the future realization of efficient phosphorescent Ir(III) complexes and corresponding nondoped
devices.
limit the performance of devices for practical applications.2b,4
Consequently, phosphorescent emitters normally have to be
blended into suitable host materials to keep the emissive
INTRODUCTION
■
Owing to the electron spin−orbit coupling (SOC) and fast
intersystem crossing (ISC), phosphorescent transition-metal
complexes provide high electroluminescence (EL) efficiencies
by utilizing all singlet and triplet excitons to achieve internal
chromophores separated, thus reducing the concentration
quenching and/or triplet−triplet annihilation (TTA).2b,4
quantum efficiencies (IQEs) of nearly 100%.1 Phosphorescent
OLEDs (PhOLEDs) show a better future as the potential
candidates for full-color flat-panel displays and efficient solid-
state lighting. In particular, cyclometalated iridium(III)
complexes are considered to be the most promising
phosphorescent dyes due to their merits of a high quantum
yield and a relatively short excited-state lifetime as well as
flexibility in color tuning, and thermal and electrochemical
stability.1c,2 Also, its ambipolar character, with oxidation mainly
involving the metal and reduction mainly involving the
coordinating ligands, allows the charge recombination to
occur on the metal complex with formation of a luminescent
excited state.3 However, luminescence self-quenching in the
solid state and poor charge-transport capability significantly
Nevertheless, host−guest systems have a few obstacles in
their commercialization, such as phase separation upon heating
and accurate control of doping concentration, despite improved
EL efficiency.5 Alternatively, nondoped devices are more
attractive because they are easier to employ for the preparation
of reliable and reproducible commercial devices.6
To date, many efforts have been made to design iridium(III)
dyes to fabricate efficient nondoped phosphorescent devices. In
2012, the Lai group synthesized phenanthroimidazole deriva-
tives TPA-BPI, with a structure of donor−linker−acceptor, and
deep-blue emitters were obtained.7 The resulting nondoped
Received: June 14, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.7b01516
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
devices exhibited excellent performance, accompanied by a
favorable current efficiency up to 2.63 cd A−1 and an external
quantum efficiency up to 3.08%. Wang and co-workers
reported that an amidinate-ligated iridium(III) bis(2-pyridyl)-
phenyl complex, which endowed the nondoped device with a
rather low turn-on voltage (2.3 cd m−2 at 2.4 V) and a
respectable power efficiency as high as 32.5 lm W−1, which is
superior to some reported iridium(III)-based doped devices.8
Recently, our group and others have demonstrated that 1,2-
diphenyl-H-benzimidazole (HPBI) could serve as an excellent
cyclometalated ligand for phosphorescent iridium(III) com-
plexes, giving rise to significant enhancement of the emitting
properties such as improved nondoped EL efficiency.9 More
importantly, the nondoped devices exhibit reduced efficiency
roll-offs at higher brightnesses. We chose 2-(3-methyl-1H-1,2,4-
triazol-5-yl)pyridine (mtpy) as an ancillary ligand because of
the deprotonation of the azole ring upon coordination, leading
to a neutral iridium complex. It also indicates that the
introduction of small substituents to HPBI ligands, such as
methyl and tert-butyl moieties, results in much more efficient
heteroleptic iridium(III) complexes, which achieve higher EL
performances and improved efficiency stability compared to
that of the parent complex.10 From a chemical structure
standpoint, such small alkyl groups should have little effect on
their charge-transporting abilities, although the charge-trans-
porting ability is crucial to the EL efficiency, in particular for
nondoped devices. Therefore, it is believed that EL perform-
ances can be further enhanced if the charge-transporting
abilities of employed materials are modified judiciously.
Recent reports have shown that the charge injection and
transporting characters for phosphorescent dyes could be
modified through the integration of functional moieties, for
instance, main-group moieties.11 Moreover, it should be
considered that attaching functional units on the phosphor-
escent emitters as steric hindrance could efficiently suppress
intermolecular interaction and hence, reduce self-quenching
effects, making them impressive as phosphorescent dyes for
high-performance nondoped PhOLEDs.11a,12 Therefore, there
is much room to design efficient phosphors with HPBI ligands
for high-performance nondoped devices by molecular engineer-
ing. To prove this concept, we attempt to simultaneously attach
relatively large functional units on HPBI ligand as steric
hindrance and manipulate its charge-transporting abilities.
On the basis of the above considerations, herein, carbazole
(Cz) and diphenylphosphoryl (Ph2PO) groups are incorpo-
rated into different positions of the HPBI ligands to design a
class of phosphorescent iridium complexes, namely 3CzIr-
(mtpy), 4CzIr(mtpy), 3POIr(mtpy), and 4POIr(mtpy) (see
Figure 1). As the steric hindrance units, Cz and Ph2PO
effectively suppressed intermolecular interactions between the
emissive Ir cores and manipulated their charge-carrier
characters. In addition, the parent complex (pbi)2Ir(mtpy)
was also prepared for the control device. Consequently, these
functional complexes, with favorable charge mobilities, realize
the yellow-green emission at 511, 504, 498, and 523 nm,
respectively. High-performance nondoped devices based on
functionalized iridium(III) complexes are obtained, achieving
improved EL efficiencies and lower efficiency roll-off than that
of the parent-complex-based device. For example, the non-
doped device using 3POIr(mtpy) as the emitting layer exhibits
outstanding performance with a maximum current efficiency
(CE) of 29.7 cd A−1 and a maximum power efficiency (PE) of
Figure 1. Chemical structures of (pbi)2Ir(mtpy), 3CzIr(mtpy),
4CzIr(mtpy), 3POIr(mtpy), and 4POIr(mtpy).
31.1 lm W−1, which is 3-fold higher compared to that of the
reference device (11.1 cd A−1, 11.6 lm W−1).
EXPERIMENTAL SECTION
■
General Information. All reagents and solvents used for the
experiment were purchased from commercial sources. All solvents for
chemical synthesis were dried and distilled from appropriate drying
agents prior to use. The experiments were prepared under a nitrogen
1
atmosphere. H NMR spectra were recorded with a Bruker Avance
500 MHz spectrometer, and chemical shifts are reported with
tetramethylsilane (TMS) as the internal standard. An electrospray
ionization mass spectrometer was used to record mass spectra for
these complexes. The high-resolution mass spectral (HRMS) analysis
was recorded on Bruker Daltonics flex Analysis. Elemental analyses
were performed on a Perkin-Elemer 240C elemental analyzer.
Synthesis of Ligands. 9-(3-(1-phenyl-1H-benzimidazol-2-yl)-
phenyl)-9H-carbazole (3CzPBI), 9-(4-(1-phenyl-1H-benzimidazol-2-
yl)phenyl)-9H-carbazole (4CzPBI), diphenyl(3-(1-phenyl-1H-benzi-
midazol-2-yl)phenyl)phosphine oxide (3POPBI), diphenyl(4-(1-phe-
nyl-1H-benzimidazol-2-yl)phenyl)phosphine oxide (4POPBI) (see
Scheme S1),9b,13 and 2-(3-methyl-1H-1,2,4-triazol-5-yl)pyridine
(mtpy) were conveniently synthesized according to early reports.14
These complexes were prepared according to our previous reports.9a,d
Synthesis of Iridium(III) Complexes. See Scheme1 for a
schematic describing the synthesis of the iridium(III) complexes.
3CzIr(mtpy). A mixture of IrCl3·3H2O (0.35 g, 1.00 mmol) and
3CzPBI (0.95 g, 2.20 mmol) in 2-ethoxyethanol and water (3:1, v/v)
was refluxed for 24 h under argon. Then, the reaction mixture was
cooled down to room temperature. After filtered, the residue was
washed with water (30 mL) and ethanol (30 mL). The μ-chloro-
Scheme 1. Synthetic Routes of Iridium(III) Complexes
B
DOI: 10.1021/acs.inorgchem.7b01516
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Figure 2. (a) UV−vis absorption spectra of the complexes in CH2Cl2 (10−5 M). (b) PL spectra at 298 K. (c) PL spectra at 77 K. (d) PL spectra of
iridium complexes in neat film.
bridged dimer complex (0.64 g, 0.29 mmol), [Ir(3CzPBI)2Cl]2, was
collected without further purification after it was dried out. A solution
of mtpy (0.07 g, 0.45 mmol) and the μ-chloro-bridged dimer complex
[Ir(3CzPBI)2Cl]2 (0.40 g, 0.18 mmol) in CH2Cl2 (30 mL) and
ethanol (10 mL) was refluxed for 12 h under argon in the dark. After
cooling to room temperature, the mixture was filtered. The crude
product was purified by silica gel column chromatography (eluent:
ethyl acetate/dichloromethane = 1:1, v/v) to afford white powder in
63% yield (0.27 g, 0.23 mmol). 1H NMR (500 MHz, CDCl3, δ
[ppm]): 8.15 (d, J = 7.5 Hz, 4H), 8.08−8.12 (m, 2H), 7.77−7.81 (m,
4H), 7.65−7.73 (m, 3H), 7.55−7.59 (m, 2H), 7.45−7.48 (m, 2H),
7.39 (t, J = 7.0 Hz, 4H), 7.20−7.31 (m, 11H), 7.11−7.18 (m, 4H),
7.07−7.10 (m, 2H), 6.84−6.88 (m, 4H), 6.28 (d, J = 8.0 Hz, 1H), 5.93
(d, J = 8.0 Hz, 1H), 2.40 (s, 3H). MS [m/z]: Calcd for C70H47IrN10
1220.4, Found 1221.4 (M + H+). Anal. Calcd for C70H47IrN10: C,
68.89; H, 3.88; N, 11.48. Found: C, 68.93; H, 3.82; N, 11.39.
4CzIr(mtpy). According to the above-mentioned typical procedure,
the main ligand 3CzPBI of 3CzIr(mtpy) was replaced by 4CzPBI.
The crude product was purified by silica gel column chromatography
(eluent: ethyl acetate/dichloromethane = 1:1, v/v) to afford white
powder in 67% yield (0.29 g, 0.24 mmol). 1H NMR (500 MHz,
CDCl3, δ [ppm]): 8.23 (d, J = 5.0 Hz, 1H), 8.11−8.13 (m, 5H), 8.07
(d, J = 7.5 Hz, 1H), 7.81−7.84 (m, 1H), 7.75−7.77 (m, 4H), 7.65−
7.71 (m, 3H), 7.56 (s, 1H), 7.18−7.20 (m, 12H), 7.11−7.17 (m, 5H),
7.02−7.08 (m, 3H), 6.93−6.95 (m, 2H), 6.88 (d, J = 8.5 Hz, 2H),
6.77−6.79 (m, 2H), 6.25 (d, J = 8.0 Hz, 1H), 5.91 (d, J = 8.5 Hz, 1H),
2.39 (s, 3H). MS [m/z]: Calcd for C70H47IrN10 1220.4, Found 1221.4
(M + H+). Anal. Calcd for C70H47IrN10: C, 68.89; H, 3.88; N, 11.48.
Found: C, 68.87; H, 3.86; N, 11.51.
3POIr(mtpy). According to the above-mentioned typical procedure,
the main ligand 3CzPBI of 3CzIr(mtpy) was replaced by 3POPBI.
The crude product was also purified by silica gel column
chromatography (eluent: methanol/dichloromethane = 1:20, v/v) to
afford white powder in 53% yield (0.23 g, 0.18 mmol). 1H NMR (500
MHz, CDCl3, δ [ppm]): 8.01−8.09 (m, 2H), 7.81 (d, J = 5.5 Hz, 1H),
7.47−7.60 (m, 11H), 7.38−7.46 (m, 13H), 7.36−7.37 (m, 8H), 7.21−
7.24 (m, 2H), 6.97−7.06 (m, 4H), 6.90−6.92 (m, 1H), 6.81−6.88 (m,
1H), 6.70−6.78 (m, 2H), 6.62−6.64 (m, 1H), 6.09 (d, J = 8.5 Hz,
1H), 5.77 (d, J = 8.5 Hz, 1H), 2.32 (s, 3H). MS [m/z]: Calcd for
C70H51IrN8O2P2 1290.3, Found 1291.3 (M + H+). Anal. Calcd for
C70H51IrN8O2P2: C, 65.16; H, 3.98; N, 8.68. Found: C, 65.18; H, 4.03;
N, 8.72.
4POIr(mtpy). According to the above-mentioned typical procedure,
the main ligand 3CzPBI of 3CzIr(mtpy) was replaced by 4POPBI.
The crude product was also purified by silica gel column
chromatography (eluent: methanol/dichloromethane = 1:15, v/v and
then methanol/dichloromethane = 1:10, v/v) to afford white powder
1
in 58% yield (0.25 g, 0.20 mmol). H NMR (500 MHz, CDCl3, δ
[ppm]): 8.09 (t, J = 7.5 Hz, 1H), 8.03 (d, J = 7.0 Hz, 1H), 7.87 (d, J =
5.5 Hz, 1H), 7.77−7.81 (m, 2H), 7.74 (s, 3H), 7.71−7.72 (m, 1H),
7.63−7.67 (m, 2H), 7.40−7.46 (m, 5H), 7.26−7.29 (m, 3H), 7.21−
7.24 (m, 7H), 7.15−7.18 (m, 2H), 7.10−7.14 (m, 2H), 7.03−7.08 (m,
4H), 6.70−7.01 (m, 2H), 6.95−6.98 (m, 2H), 6.88−6.92 (m, 2H),
6.77 (d, J = 8.5 Hz, 1H), 6.64 (d, J = 8.0 Hz, 1H), 6.53−6.59 (m, 4H),
6.06 (d, J = 7.0 Hz, 1H), 5.71 (d, J = 8.5 Hz, 1H), 2.32 (s, 3H). MS
[m/z]: Calcd for C70H51IrN8O2P2 1290.3, Found 1291.3 (M + H+).
Anal. Calcd for C70H51IrN8O2P2: C, 65.16; H, 3.98; N, 8.68. Found: C,
65.08; H, 3.88; N, 8.62.
Physical Measurements. UV−vis absorption spectra were
measured on Cary 500 UV−vis-NIR spectrophotometer. The
photoluminescence (PL) spectra of these complexes were reported
on the FL-4600 FL spectrophotometer. The excited-state lifetimes (τ)
and Φp in solution were measured on a transient spectrofluorimeter
(Edinburgh FLSP920) using a time-correlated single-photon counting
system. The luminescence quantum efficiencies (Φp) in neat films
were measured in an integrating sphere. Emission spectra were
recorded using the F-7000 FL spectrophotometer.
Theoretical Calculations. The calculations reported here were
carried out by using the Gaussian 09 software package.15 The
geometrical structures of the singlet ground state and the lowest triplet
state for iridium(III) complexes were fully optimized with C1
symmetry constraints by using the restricted closed-shell and open-
shell PBE1PBE methods with the LANL2DZ basis set for the Ir atom
and 6-31G* for the rest of the atoms.16 The effective core potential on
Ir replaced the inner-core electrons, leaving the outer core (5s)2(5p)6
electrons and the (5d)6 valence electrons of iridium(III).17 On the
basis of the optimized S0, Tamm−Dancoff Approximation (TDA) in
time-dependent DFT (TD-DFT) calculations were performed in
order to determine the vertical transition characters and simulate the
UV−vis absorption property simultaneously.18 The solvent effect was
also considered with the polarized continuum model.
Electrochemical Measurements. Cyclic voltammetry (CV)
measurements were carried out with a BAS 100 W instrument with
a scan rate of 100 mV s−1 in CH2Cl2 (10−3 M) with three electrode
C
DOI: 10.1021/acs.inorgchem.7b01516
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Table 1. Photophysical and Electrochemical Characteristics of Iridium(III) Complexes
a,
b,
c
a
,
c
a
,
c
d
e
f
g
complexes
λPL,max
(nm)
ϕp
τ
Eg
Eox
HOMO
(eV)
LUMO
(%)
(μs)
(eV)
(V)
(eV)
(pbi)2Ir(mtpy)
3CzIr(mtpy)
4CzIr(mtpy)
3POIr(mtpy)
4POIr(mtpy)
501, 493, 520
511, 500, 525
504, 500, 515
498, 486, 509
523, 517, 534
48, 24
0.33, 0.19
0.38, 0.28
0.87, 0.48
1.07, 0.87
1.24, 1.08
2.67
2.43
2.59
2.58
2.53
0.83
0.47
0.54
0.64
0.53
−5.15
−5.27
−5.34
−5.44
−5.33
−2.48
−2.84
−2.75
−2.86
−2.80
12.7, 10.3
10.3, 52.8
27.1, 9.2
45.5, 58.4
a
b
c
d
Measured in CH2Cl2 solution (10−5 M) at 298 K. Measured in CH2Cl2 solution (10−5 M) at 77 K. Measured in the neat film. Optical band gap.
Measured by CV with ferrocene as the standard. Calculated from the onset oxidation potentials of the complexes. Estimated by using the empirical
e
f
g
equation of ELUMO = EHOMO + Eg.
cell assemblies. A silver wire was used as an Ag/Ag+ quasi reference
electrode, a platinum wire was used as the counter electrode, and a
glassy carbon electrode was used as a working electrode. Tetra-
butylammonium-hexafluorophosphate (0.1 M) was selected as the
supporting electrolyte. At the end of each experiment, ferrocene was
selected as the internal standard.
Device Fabrication and Characterization. Organic layers were
fabricated by vacuum evaporation onto patterned indium−tin-oxide
(ITO)-coated glass substrates with a sheet resistance of 20 Ω per
square. All of the complexes were vacuum-deposited without any
decomposition. A shadow mask was used to define the cathode and
make four 10 mm2 devices on each substrate. The thickness of the
organic layers and metal was measured in situ with a quartz oscillator.
The EL spectra of the devices have been measured on an Ocean
Optics Maya 2000-PRO spectrophotometer. All of the measurements
were performed at room temperature under ambient conditions.
involved in the excited state of these complexes.22 In view of
the above analysis, it is believed that their emission should
originate from the admixture of MLCT, LLCT, and LC
characters.
The emission lifetimes of the complexes are measured to be
0.38, 0.87, 1.07, and 1.24 μs, respectively, which fall into the
microsecond timescale. The relative shorter lifetimes of the
complexes were expected to increase spin-state mixing, leading
to effective intersystem crossing from the singlet to the triplet
state.20b,23 In the PL spectra of spin-coated neat films, all
complexes exhibit obvious red-shifts by ca. 12−15 nm
compared to those in solution, probably as a result of the
molecular aggregation and TTA in the solid state.24
Theoretical Calculations. Figure 3 shows orbital distribu-
tions of the highest occupied molecular orbital (HOMO) and
3
3
3
RESULTS AND DISCUSSION
■
Photophysical Properties. The UV−vis absorption and
PL spectra of these complexes in CH2Cl2 (10−5 M) are
depicted in Figure 2a, and the corresponding data are showed
in Table 1. In common with most iridium(III) complexes, the
absorption spectra mainly consist of two regions: the shorter
wavelength (<380 nm) absorptions, which are primarily due to
the spin-allowed 1π−π* transitions of the ligands, and the long
wavelength (>400 nm) absorptions in the low-energy region,
which can be assigned to mixing among the typical spin-allowed
metal-to-ligand charge transitions (1MLCT) and ligand-to-
ligand charge transitions (1LLCT), together with spin-
forbidden 3MLCT, 3LLCT, 3LC transitions facilitated by
spin−orbit coupling.2a,19 The spin−orbit coupling is enhanced
caused not only by the presence of π−π* and MLCT states but
also by the heavy-atom effect of Ir center.20
The PL spectra of these complexes measured in CH2Cl2 at
room temperature are depicted in Figure 2b. All of the
complexes display a distinct vibronic structure, indicating that
the 3LC excited state contributed to the emissive excited states.
3CzIr(mtpy) and 4CzIr(mtpy) exhibit nearly identical
patterns, with emission peaks at 511 and 504 nm, respectively,
with Φ = 12.7% (3CzIr(mtpy)) and 10.3% (4CzIr(mtpy)).
Additionally, 3POIr(mtpy) and 4POIr(mtpy) are intensely
luminescent, with maxima at 498 and 523 nm, respectively, and
Φ = 27.1% (3POIr(mtpy)) and 45.5% (4POIr(mtpy)).
Typical of other similar Ir(III) complexes upon cooling to 77
K, the spectra become more well-resolved, and the emission
maxima blue-shift to 500, 500, 486, and 517 nm. The
hypsochromic shifts observed mainly originated from the
solvent reorganization in a fluid solution at low temperature,
which stabilized the charge-transfer states prior to emis-
sion.11b,20b,21 Considering the apparent shift in 77 K, we
speculate that both the 3MLCT and 3LLCT characters are also
Figure 3. Contours and contributions of the frontier molecular orbitals
of studied iridium(III) complexes.
lowest unoccupied molecular orbital (LUMO) energy levels
and optimized molecular structures of the two different series.
In the case of Cz-based series, the HOMOs contribute mainly
from the d orbitals of the iridium atom and the cyclometalated
ligand, while the LUMOs distribute mainly at one of the
phenylbenzimidazole moieties. For Ph2PO-based series, the
HOMOs contribute mainly from the d orbitals of the iridium
atom and the phenylbenzimidazole of one main ligand, with
there being a small distribution from the ancillary ligand. As
shown in Table S1, the T1 states of 3CzIr(mtpy) and
4CzIr(mtpy) originate from HOMO → LUMO (80%) and
HOMO → LUMO (86%), while 3POIr(mtpy) and 4POIr-
(mtpy) both originate from HOMO → LUMO (87%). It is
3
3
3
conjectured that the mixture of the MLCT, LC, and LLCT
D
DOI: 10.1021/acs.inorgchem.7b01516
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Figure 4. (a) Energy level diagram of nondoped devices N1-20−N4-20 and the chemical structures of employed materials. (b) Current density−
voltage−luminance characteristics and EL spectra (inset) of devices N1-20−N4-20. (c) Efficiency−luminance curves of devices N1-20−N4-20
based on 3CzIr(mtpy), 4CzIr(mtpy), 3POIr(mtpy), and 4POIr(mtpy).
transition characters contributed to their emissions. In addition,
the differences in electron density between T1 and the ground
state S0 for these complexes obviously confirm the transition
characters mentioned above (see Figure S5).
4,4′,4″-tris(N-carbazolyl)triphenylamine (TCTA) and the
emitters (about 0.26−0.43 eV), but TCTA mainly acts as a
buffer gradient to facilitate hole injection and block electrons,
resulting in a better confinement of excitons in the emission
layers and thus improving device performance. The electro-
luminescence (EL) spectra of devices N1-20−N4-20 at a
driving voltage of 5 V are depicted in Figure 4b (inset). The
maxima EL emission of the nondoped devices using 3CzIr-
(mtpy), 4CzIr(mtpy), 3POIr(mtpy), 4POIr(mtpy), and
(pbi)2Ir(mtpy), appear at 513, 517, 496, 532, and 511 nm,
respectively. In each case, the EL spectrum is almost coincident
with the PL spectrum of the corresponding complex and
indicates that the EL and PL spectra originate from the decay of
singlet excitons, and the formation of detrimental excimers or
exciplexes is effectively suppressed.
The current density−voltage−luminance (J−V−L) charac-
teristics are shown in Figure 4b. The devices exhibit low turn-
on voltages (estimated at the brightness of 1 cd m−2) of 3.0 V
(N1-20), 2.5 V (N2-20), 2.6 V (N3-20), and 2.1 V (N4-20). In
addition, they show very low driving voltages at the practical
brightness of 100 cd m−2 and 1000 cd m−2 for display and
lighting. The driving voltages for 100 cd m−2 and 1000 cd m−2
were as low as 3.5/4.4, 2.8/3.6, 3.5/4.5, 2.8/3.6 V, respectively.
The maxima brightness (Lmax) of devices N1-20−N4-20 can be
as high as 23 258, 30 263, 21 464, and 49 026 cd m−2, which are
far higher than that of control device N-20 (7331 cd m−2).
All of the devices exhibit attractive performance with low
driving voltages and enhanced luminescence in each case. We
can surmise that probably the Cz/Ph2PO groups not only act
as bulky building units to suppress the TTA effect but also play
a role in modification of charge-carrier transport characters, so
we investigated the J−V characteristics of single-carrier devices
to estimate their intrinsic charge-transport process in EML. The
configurations of hole-only and electron-only devices are ITO/
MoO3 (3 nm)/NPB (40 nm)/Ir complexes (20 nm)/NPB (40
nm)/MoO3 (3 nm)/Ag (120 nm) and ITO/LiF (0.5 nm)/
Bphen (40 nm)/Ir complexes (20 nm)/Bphen (40 nm)/LiF
(0.5 nm)/Ag (120 nm), respectively. For the hole-only devices,
Electrochemical Properties. The electrochemical proper-
ties of these complexes are probed by the cyclic voltammetry
(CV), and the relevant electrochemical dates are listed in Table
1. The HOMO energy levels of the complexes were measured
by the onset oxidation potentials as −5.27, −5.34, −5.44, and
−5.33 eV, respectively. The LUMO energy levels of −2.84
(3CzIr(mtpy)), −2.75 (4CzIr(mtpy)), −2.86 (3POIr(mtpy)),
and −2.80 eV (4POIr(mtpy)) were estimated according to the
HOMO energy levels and optical band gaps (Eg).4 Because of
the electron-withdrawing ability of the diphenylphosphoryl
group (Ph2PO) in ligand HPBI, 3POIr(mtpy) and 4POIr-
(mtpy) showed lower HOMO levels of −5.44 and −5.33 eV
relative to 3CzIr(mtpy) (−5.27 eV) and 4CzIr(mtpy) (−5.34
eV).
Device Performance. To understand the electrolumines-
cent properties of these complexes, we fabricated nondoped
devices N1−20 (3CzIr(mtpy)), N2−20 (4CzIr(mtpy)), N3−
20 (3POIr(mtpy)), and N4−20 (4POIr(mtpy)) with 20 nm
thickness of EML, using these carbazole (Cz)- and
diphenylphosphoryl (Ph2PO)-based iridium complexes as the
phosphors. A nondoped device with a parent Ir(III) complex
was also fabricated as a reference device, namely, N-20. The
structure of the fabricated devices as follows: ITO/MoO3 (3
nm)/NPB (35 nm)/TCTA (5 nm)/Ir complexes (20 nm)/
Bphen (40 nm)/LiF (0.5 nm)/Mg:Ag (120 nm, 15:1), in which
MoO3 acts as the hole-injecting layer (HIL), and LiF as an
electron-injecting layer (EIL). 4,4-Bis (N-(1-naphthyl)-N-
phenylamino)-biphenyl (NPB) was used as the hole-trans-
porting layer (HTL), and 4,7-diphenyl-1,10-phenanthroline
(Bphen) was used as the electron-transporting layer (ETL),
while the present complexes alone were used as the EML. The
energy level diagrams of the device and molecular structures of
materials employed are displayed in Figure 4a. From the energy
level diagrams, there is a large energy-injection barrier between
E
DOI: 10.1021/acs.inorgchem.7b01516
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
MoO3 (LUMO = −2.3 eV) was used to block the electron
injection from Ag (−4.7 eV), corresponding to a large energy-
injection barrier of 2.4 eV. In the electron-only devices, LiF
(HOMO = −5.9) was used to block the hole injection from
ITO (−4.8 eV), corresponding to a large energy-injection
barrier of 1.1 eV.25 As shown in Figure 5, all of the single-carrier
POIr(mtpy)-based devices achieved lower driving voltages,
higher current density, and luminescence.
Figure 4c revealed the CE and PE as functions of luminance
for these devices. Devices N1-20 and N2-20, based on Cz-
containing complexes, displayed good performance, with
maxima CE of 19.9 and 25.4 cd A−1, and maxima PE of 17.9
and 26.6 lm W−1. Ph2PO-containing complexes endowed
devices N3-20 and N4-20 with better performance, maxima CE
of 25.2 and 28.7 cd A−1, and maxima PE of 22.6 and 30.1 lm
W−1. These start-of-the-art efficiencies are almost 2-fold greater
than those of the reference device (13.1 cd A−1, 13.7 lm W−1).
This significant enhancement should be attributed mainly to its
balanced carrier transport induced by the functional units, as
revealed by the characteristics of their single-carrier devices.
Inspiringly, at a practically high brightness of 1000 cd m−2, their
efficiency roll-offs are only 7.0% (N1-20), 4.7% (N2-20), 17.9%
(N3-20), and 7.3% (N4-20) for CE, all of which are much
lower than that of reference device (31.3%). The results
indicate that introducing the Cz/Ph2PO moieties could
effectively suppress the intermolecular aggregation due to the
large steric hindrance. In addition, its more balanced carrier
transport result in a broader distribution of the recombination
zone and efficient confinement of the triplet excitons generated
within the EML. These therefore increase the overall device
efficiency in many cases. To the best of our knowledge, these
are good results for green-emitting PHOLEDs by use of
vacuum-evaporation techniques (see Table S2).
Figure 5. Current density−voltage characteristics of the single-carrier
devices (solid for hole-only and hollow for electron-only) based on
3C-3CzIr(mtpy), 4C-4CzIr(mtpy), 3P-3POIr(mtpy), and 4P-
4POIr(mtpy) in comparison with that of R-(pbi)2Ir(mtpy).
devices show significant currents, indicating superiority in
charge-carrier mobility. It is also noted that the electron-
transporting ability of Cz/Ph2PO-containing complexes is
stronger than that of the parent complex (pbi)2Ir(mtpy). It
is known to be a plausible deduction that conventional devices
are almost hole-predominant devices.26 The enhancement of
electron transport will facilitate the injection and transport of
electrons, broaden the recombination zone, and balance charge
flux, particularly for the nondoped devices, which is beneficial
for suppressing TTA and TPQ (triplet−polaron quenching)
inside the EML through reducing exciton density and excess
charges.27 These could explain why the CzIr(mtpy) and
To shed light on further optimization of device performance,
we changed the thickness of the EML from 20 to 10 nm for the
purpose of reducing the self-quenching effect. Nondoped
devices N1-10−N4-10 were fabricated with a conventional
configuration of ITO/MoO3 (3 nm)/NPB (35 nm)/TCTA (5
nm)/Ir complexes (10 nm)/Bphen (40 nm)/LiF (0.5 nm)/
Mg:Ag (120 nm, 15:1) (as shown in Figure 6a), where the
materials employed here are kept unchanged from those used
in the analysis of 20 nm thick EML. For comparison, the
control device N-10 with parent Ir(III) complex as the EML
Figure 6. (a) Schematic diagram of EL device configurations. (b) EL spectra of devices with 10 nm thickness of EML (at 5 V). (c) Current density−
voltage−luminance characteristics of devices N1-10−N4-10. (d) Efficiency−luminance curves of devices N1-10−N4-10.
F
DOI: 10.1021/acs.inorgchem.7b01516
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Table 2. Summary of the EL Performance of the Devices
a,
b,
c
d
d
,
c
d,c
PE (lm W−1
device
Vturn‑on
(V)
Lmax (cd m−2, V)
CE (cd A−1
)
)
EQE (%)
λEL (nm)
CIE[(x, y), V]
N-20
2.6, 3.3, 4.3
3.0, 3.5, 4.4
2.5, 2.8, 3.6
2.6, 3.5, 4.5
2.1, 2.8, 3.6
2.5, 3.1, 4.1
2.5, 3.6, 4.7
2.5, 3.0, 3.7
2.5, 3.0, 3.7
2.2, 2.9, 3.6
7331, 7.5
23 258, 8.5
30 263, 7.5
21 646, 8.5
49 026, 8.0
5193, 6.5
13.1, 9.0
19.9, 18.5
25.4, 24.2
25.2, 20.7
28.7, 26.6
11.1, 6.3
13.7, 6.5
17.9, 13.5
26.6, 21.2
22.6, 14.4
30.1, 22.7
11.6, 4.8
4.1
6.0
7.8
8.3
8.3
3.5
7.1
7.7
9.6
7.5
511
513
517
496
532
511
515
514
496
532
(0.35, 0.57), 5
(0.35, 0.58), 5
(0.38, 0.58), 5
(0.31, 0.55), 5
(0.42, 0.56), 5
(0.35, 0.57), 5
(0.36, 0.58), 5
(0.37, 0.58), 5
(0.30, 0.55), 5
(0.42, 0.56), 5
N1-20
N2-20
N3-20
N4-20
N-10
N1-10
N2-10
N3-10
N4-10
29 697, 7.5
31 106, 8.0
20 761, 7.5
23.3, 22.7
25.7, 24.5
29.7, 25.6
20.9, 18.9
26.9, 20.4
31.1, 20.9
25.8, 19.9
36 701, 8.0
24.7, 23.3
a
b
c
d
The voltages estimated at 1 cd m−2. Measured at 100 cd m−2. Measured at 1000 cd m−2. Maximum values of the devices.
was also adopted under the same conditions. As shown by the
EL spectra in Figure 6b, all of the devices exhibited yellow-
green light with an emission peak at 515 nm and CIE
coordinates of (0.36, 0.58) for N1-10, an emission peak at 514
nm and CIE coordinates of (0.37, 0.58) for N2-10, an emission
peak at 496 nm and CIE coordinates of (0.30, 0.55) for N3-10,
and an emission peak at 532 nm and CIE coordinates of (0.42,
0.56) for N4-10.
induced by functional moieties, manifesting the merits of the
synthetically versatile strategy. Although significant advances
have been achieved in this field, it is believed that better EL
data could be obtained through rational molecular design and
further device optimization. Herein, we provided a general
design strategy to enhance EL performance. We anticipate that
the good relationship between molecular structures and device
performances obtained will offer excellent support for designing
efficient phosphorescence Ir(III) complexes containing an
HPBI-cyclometalated ligand and corresponding nondoped
devices.
The J−V−L characteristics and efficiencies versus luminance
curves of these devices are depicted in Figure 6, and the key EL
data are summarized in Table 2. All devices display low turn-on
voltage, for instance, 2.5 V for N1-10, 2.5 V for N2-10, 2.5 V
for N3-10, and 2.2 V for N4-10. Also, similar to the results
mentioned above, they show very low driving voltages at high
luminances of 100 cd m−2 and 1000 cd m−2. Devices N1-10
and N2-10, based on Cz-containing complexes, show a much
better performance (Lmax = 29 697 cd m−2, CE = 23.3 cd A−1,
and PE = 20.9 lm W−1 for N1-10, Lmax = 31 106 cd m−2, CE =
25.7 cd A−1, and PE = 26.9 lm W−1 for N2-10) than that of the
control device (Lmax = 5193 cd m−2, CE = 11.1 cd A−1, and PE
= 11.6 lm W−1). Similarly, devices N3-10 and N4-10, based on
Ph2PO-containing complexes, exhibit excellent efficiencies with
Lmax = 20 761 cd m−2, CE = 29.7 cd A−1, and PE = 31.1 lm W−1
for N3-10, Lmax = 36 701 cd m−2, CE = 24.7 cd A−1, and PE =
25.8 lm W−1 for N4-10, which are almost 3 times of those of
the control device. Owing to the low driving voltages, the
maximum PE of Ph2PO-based devices are even higher than
their CE, revealing the more effective injection and transport of
CONCLUSIONS
■
In summary, we have designed and synthesized four yellow-
green phosphorescent emitters, 3CzIr(mtpy), 4CzIr(mtpy),
3POIr(mtpy), and 4POIr(mtpy), consisting of functional
moieties (Cz and Ph2PO) substituted at different positions of
HPBI and investigated their applications in nondoped OLEDs.
It was shown that, as steric-hindrance-inducing groups, Cz and
Ph2PO could efficiently suppress intermolecular interaction
and reduce self-quenching effects. By introducing the functional
groups, charge-carrier transportation characters were nicely
modified. As a result, all nondoped devices exhibited the state-
of-the-art performance and reduced efficiency roll-off, which are
much higher than that of the control device. Nondoped device
N3−10, based on 3POIr(mtpy), showed excellent EL
efficiencies, including the maximum CE of 29.7 cd A−1 and
the maximum PE of 31.1 lm W−1, which are almost 3 times
than that of the unfunctionalized counterpart. This work
manifested the great potential of these functional materials as
high performance EMLs for nondoped OLEDs, and provided
an easy molecular design strategy to develop efficient
phosphors for optical devices.
charges.22 28 Alternatively, it is worth noting that all of the
b,
devices showed little efficiency roll-off as the luminance
increased. At a high luminance of 1000 cd m−2, their CE
remained as 22.7, 24.5, 25.6, and 23.3 cd A−1, corresponding to
reduced efficiency roll-offs of 2.6, 4.7, 13.8, and 5.7%,
respectively. The maximum efficiency of the reference device
is remarkably decreased to 6.3 cd A−1, accompanied by worse
roll-off of 43.2% at 1000 cd m−2. The excellent EL efficiencies
and the improved stability of efficiency should be mainly due to
the large steric hindrance caused by the functional moieties,
which could partly prevent the formation of excimers or
exciplexes and avoid significant TTA and nonemissive pathways
caused by intermolecular excited-state interactions.2c Also, the
remarkably improved electron mobility of the CzIr(mtpy) and
POIr(mtpy) can facilitate carrier flux balance in EML with
respect to hole-predominant devices, which further reduced
TTA-induced efficiency roll-off.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.7b01516.
Supplemental schemes, figures, and calculated energy
levels of the lower-lying transitions of all complexes
(PDF)
AUTHOR INFORMATION
Corresponding Authors
*G.-G.S.: Fax: +86-431-85684009; Tel.: +86-431-85099108; E-
mail: shangg187@nenu.edu.cn
■
The success of Cz/Ph2PO-containing complexes in yellow-
green emitting devices with superior EL performance is
ascribed to large steric hindrance and balanced charge transport
G
DOI: 10.1021/acs.inorgchem.7b01516
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Fluorescence Emitter with Quantum-Well Structure. ACS Appl.
Mater. Interfaces 2016, 8, 20955−20961.
*W.-F.X.: Tel: +86-13844907598; E-mail: xiewf@jlu.edu.cn
*Z.-M.S.: E-mail: zmsu@nenu.edu.cn
(6) (a) Huang, T. H.; Lin, J. T.; Chen, L. Y.; Lin, Y. T.; Wu, C. C.
Dipolar Dibenzothiophene S,S-Dioxide Derivatives Containing Diaryl-
amine: Materials for Single-Layer Organic Light-Emitting Devices.
Adv. Mater. 2006, 18, 602−606. (b) Shi, H. P.; Xin, D. H.; Gu, X. G.;
Zhang, P. F.; Peng, H. R.; Chen, S. M.; Lin, G. W.; Zhao, Z. J.; Tang,
B. Z. The synthesis of novel AIE emitters with the triphenylethene-
carbazole skeleton and para-/meta-substituted arylboron groups and
their application in efficient non-doped OLEDs. J. Mater. Chem. C
2016, 4, 1228−1237.
(7) Zhang, Y.; Lai, S.-L.; Tong, Q.-X.; Lo, M.-F.; Ng, T.-W.; Chan,
M.-Y.; Wen, Z.-C.; He, J.; Jeff, K.-S.; Tang, X.-L.; Liu, W.-M.; Ko, C.-
C.; Wang, P.-F.; Lee, C.-S. High Efficiency Nondoped Deep-Blue
Organic Light Emitting Devices Based on Imidazole-π-triphenylamine
Derivatives. Chem. Mater. 2012, 24, 61−70.
ORCID
Hai-Zhu Sun: 0000-0002-5113-8267
Zhong-Min Su: 0000-0002-3342-1966
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors gratefully acknowledge the financial support from
National Natural Science Foundation of China (21303012,
21273030, 51203017, 21131001, and 61474054), 973 Program
(2013CB834801), and the Science and Technology Develop-
ment Planning of Jilin Province 20130204025GX.
(8) Liu, Y.; Ye, K.; Fan, Y.; Song, W.; Wang, Y.; Hou, Z. Amidinate-
ligated iridium(III) bis(2-pyridyl)phenyl complex as an excellent
phosphorescent material for electroluminescence devices. Chem.
Commun. 2009, 3699−3701.
REFERENCES
■
(1) (a) Baldo, M. A.; O’Brien, D. F.; You, Y.; Shoustikov, A.; Sibley,
S.; Thompson, M. E.; Forrest, S. R. Highly efficient phosphorescent
emission from organic electroluminescent devices. Nature 1998, 395,
151−154. (b) Reineke, S.; Lindner, F.; Schwartz, G.; Seidler, N.;
Walzer, K.; Lussem, B.; Leo, K. White organic light-emitting diodes
with fluorescent tube efficiency. Nature 2009, 459, 234−238. (c) Fan,
C.; Li, Y.; Yang, C.; Wu, H.; Qin, J.; Cao, Y. Phosphoryl/Sulfonyl-
Substituted Iridium Complexes as Blue Phosphorescent Emitters for
Single-Layer Blue and White Organic Light-Emitting Diodes by
Solution Process. Chem. Mater. 2012, 24, 4581−4587.
(2) (a) Lamansky, S.; Djurovich, P.; Murphy, D.; Abdel-Razzaq, F.;
Lee, H.-E.; Adachi, C.; Burrows, P. E.; Forrest, S. R.; Thompson, M. E.
Highly Phosphorescent Bis-Cyclometalated Iridium Complexes:
Synthesis, Photophysical Characterization, and Use in Organic Light
Emitting Diodes. J. Am. Chem. Soc. 2001, 123, 4304−4312. (b) Ding,
J.; Wang, B.; Yue, Z.; Yao, B.; Xie, Z.; Cheng, Y.; Wang, L.; Jing, X.;
Wang, F. Bifunctional green iridium dendrimers with a ″self-host″
feature for highly efficient nondoped electrophosphorescent devices.
Angew. Chem., Int. Ed. 2009, 48, 6664−6666. (c) You, Y.; Nam, W.
Photofunctional triplet excited states of cyclometalated Ir(III)
complexes: beyond electroluminescence. Chem. Soc. Rev. 2012, 41,
7061−7084.
(9) (a) Cao, H.; Shan, G.; Wen, X.; Sun, H.; Su, Z.; Zhong, R.; Xie,
W.; Li, P.; Zhu, D. An orange iridium(III) complex with wide-
bandwidth in electroluminescence for fabrication of high-quality white
organic light-emitting diodes. J. Mater. Chem. C 2013, 1, 7371−7379.
(b) Shan, G. G.; Li, H. B.; Sun, H. Z.; Cao, H. T.; Zhu, D. X.; Su, Z. M.
Enhancing the luminescence properties and stability of cationic
iridium(III) complexes based on phenylbenzoimidazole ligand: a
combined experimental and theoretical study. Dalton Trans. 2013, 42,
11056−11065. (c) Cao, H.-T.; Shan, G.-G.; Yin, Y.-M.; Sun, H.-Z.;
Wu, Y.; Xie, W.-F.; Su, Z.-M. Modification of iridium(III) complexes
for fabrication of high-performance non-doped organic light-emitting
diode. Dyes Pigm. 2015, 112, 8−16. (d) Mao, H.-T.; Zang, C.-X.; Wen,
L. L.; Shan, G.-G.; Sun, H.-Z.; Xie, W.-F.; Su, Z.-M. Ir(III) Phosphors
Modified with Fluorine Atoms in Pyridine-1,2,4-triazolyl Ligands for
Efficient OLEDs Possessing Low-Efficiency Roll-off. Organometallics
2016, 35, 3870−3877.
(10) Wen, L.-L.; Yu, J.; Sun, H.-Z.; Shan, G.-G.; Zhao, K.-Y.; Xie, W.-
F.; Su, Z.-M. Simple molecular structure design of iridium(III)
complexes: Achieving highly efficient non-doped devices with low
efficiency roll-off. Org. Electron. 2016, 35, 142−150.
(11) (a) Xu, H.; Chen, R.; Sun, Q.; Lai, W.; Su, Q.; Huang, W.; Liu,
X. Recent progress in metal-organic complexes for optoelectronic
applications. Chem. Soc. Rev. 2014, 43, 3259−3302. (b) Zhou, G.; Ho,
C.-L.; Wong, W.-Y.; Wang, Q.; Ma, D.; Wang, L.; Lin, Z.; Marder, T.
B.; Beeby, A. Manipulating Charge-Transfer Character with Electron-
Withdrawing Main-Group Moieties for the Color Tuning of Iridium
Electrophosphors. Adv. Funct. Mater. 2008, 18, 499−511. (c) Yang, C.
L.; Zhang, X. W.; You, H.; Zhu, L. Y.; Chen, L. Q.; Zhu, L. N.; Tao, Y.
T.; Ma, D. G.; Shuai, Z. G.; Qin, J. G. Tuning the Energy Level and
Photophysical and Electroluminescent Properties of Heavy Metal
Complexes by Controlling the Ligation of the Metal with the Carbon
of the Carbazole Unit. Adv. Funct. Mater. 2007, 17, 651−661. (d) Zhu,
M.; Li, Y.; Miao, J.; Jiang, B.; Yang, C.; Wu, H.; Qin, J.; Cao, Y.
Multifunctional homoleptic iridium(III) dendrimers towards solution-
processed nondoped electrophosphorescence with low efficiency roll-
off. Org. Electron. 2014, 15, 1598−1606.
(3) Orselli, E.; Kottas, G. S.; Konradsson, A. E.; Coppo, P.; Frohlich,
R.; De Cola, L.; van Dijken, A.; Buchel, M.; Borner, H. Blue-Emitting
Iridium Complexes with Substituted 1,2,4-Triazole Ligands: Synthesis,
Photophysics, and Devices. Inorg. Chem. 2007, 46, 11082−11093.
(4) (a) Qin, T.; Ding, J.; Wang, L.; Baumgarten, M.; Zhou, G.;
Mullen, K. A Divergent Synthesis of Very Large Polyphenylene
̈
Dendrimers with Iridium(III) Cores: Molecular Size Effect on the
Performance of Phosphorescent Organic Light-Emitting Diodes. J. Am.
Chem. Soc. 2009, 131, 14329−14336. (b) Wang, Y.; Lu, Y.; Gao, B.;
Wang, S.; Ding, J.; Wang, L.; Jing, X.; Wang, F. Self-Host Blue-
Emitting Iridium Dendrimer Containing Bipolar Dendrons for
Nondoped Electrophosphorescent Devices with Superior High-
Brightness Performance. ACS Appl. Mater. Interfaces 2016, 8,
29600−29607.
(5) (a) Xue, K.; Sheng, R.; Duan, Y.; Chen, P.; Chen, B.; Wang, X.;
Duan, Y.; Zhao, Y. Efficient non-doped monochrome and white
phosphorescent organic light-emitting diodes based on ultrathin
emissive layers. Org. Electron. 2015, 26, 451−457. (b) Tsai, W. L.;
Huang, M. H.; Lee, W. K.; Hsu, Y. J.; Pan, K. C.; Huang, Y. H.; Ting,
H. C.; Sarma, M.; Ho, Y. Y.; Hu, H. C.; Chen, C. C.; Lee, M. T.;
Wong, K. T.; Wu, C. C. A versatile thermally activated delayed
fluorescence emitter for both highly efficient doped and non-doped
organic light emitting devices. Chem. Commun. 2015, 51, 13662−
13665. (c) Tang, C.; Liu, F.; Xia, Y.-J.; Lin, J.; Xie, L.-H.; Zhong, G.-Y.;
Fan, Q.-L.; Huang, W. Fluorene-substituted pyrenes-Novel pyrene
derivatives as emitters in nondoped blue OLEDs. Org. Electron. 2006,
7, 155−162. (d) Meng, L.; Wang, H.; Wei, X.; Liu, J.; Chen, Y.; Kong,
X.; Lv, X.; Wang, P.; Wang, Y. Highly Efficient Nondoped Organic
Light Emitting Diodes Based on Thermally Activated Delayed
(12) (a) Ding, J.; Lu, J.; Cheng, Y.; Xie, Z.; Wang, L.; Jing, X.; Wang,
̈
F. Solution-Processible Red Iridium Dendrimers based on Oligocarba-
zole Host Dendrons: Synthesis, Properties, and their Applications in
Organic Light-Emitting Diodes. Adv. Funct. Mater. 2008, 18, 2754−
2762. (b) Pu, Y.-J.; Iguchi, N.; Aizawa, N.; Sasabe, H.; Nakayama, K.-i.;
Kido, J. fac-Tris(2-phenylpyridine)iridium (III)s, covalently sur-
rounded by six bulky host dendrons, for a highly efficient solution-
processed organic light emitting device. Org. Electron. 2011, 12, 2103−
2110.
(13) Kan, W. J.; Zhu, L. P.; Wei, Y.; Ma, D. G.; Sun, M. Z.; Wu, Z. B.;
Huang, W.; Xu, H. Phosphine oxide-jointed electron transporters for
the reduction of interfacial quenching in highly efficient blue
PHOLEDs. J. Mater. Chem. C 2015, 3, 5430−5439.
H
DOI: 10.1021/acs.inorgchem.7b01516
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
(14) Chen, T.-R. Synthesis, structure, and field-effect property of 2-
(benzimidazol-2-yl) quinoline. Mater. Lett. 2005, 59, 1050−1052.
(15) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci,
B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H.
P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.;
Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima,
T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, J. A, Jr.;
Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin,
K. N.; Staroverov, V. N.; Keith, T.; Kobayashi, R.; Normand, J.;
Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.;
Cossi, M.; Rega, N.; Millam, J. M.; Klene, M.; Knox, J. E.; Cross, J. B.;
Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.;
Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.;
Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador,
P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, O.;
Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian 09,
Revision A.01; Gaussian, Inc.: Wallingford, CT, 2009.
(16) Francl, M. M.; Pietro, W. J.; Hehre, W. J.; Binkley, J. S.; Gordon,
M. S.; Defrees, D. J.; Pople, J. A. Self-consistent molecular orbital
methods. XXIII. A polarization-type basis set for second-row elements.
J. Chem. Phys. 1982, 77, 3654−3665.
(17) Hay, J. P.; Wadt, W. R. Ab initio effective core potentials for
molecular calculations. Potentials for K to Au including the outermost
core orbitals. J. Chem. Phys. 1985, 82, 299−310.
(18) Peach, M. J.; Williamson, M. J.; Tozer, D. J. Influence of Triplet
Instabilities in TDDFT. J. Chem. Theory Comput. 2011, 7, 3578−3585.
(19) Nazeeruddin, M. K.; Humphry-Baker, R.; Berner, D.; Rivier, S.;
Zuppiroli, L.; Graetzel, M. Highly Phosphorescence Iridium
Complexes and Their Application in Organic Light-Emitting Devices.
J. Am. Chem. Soc. 2003, 125, 8790−8797.
(20) (a) Ho, C. L.; Wong, W. Y.; Zhou, G. J.; Yao, B.; Xie, Z.; Wang,
L. Solution-Processible Multi-component Cyclometalated Iridium
Phosphors for High-Efficiency Orange-mitting OLEDs and Their
Potential Use as White Light Sources. Adv. Funct. Mater. 2007, 17,
2925−2936. (b) Ho, C.-L.; Wong, W.-Y.; Wang, Q.; Ma, D.; Wang, L.;
Lin, Z. A Multifunctional Iridium-Carbazolyl Orange Phosphor for
High-Performance Two-Element WOLED Exploiting Exciton-Man-
aged Fluorescence/Phosphorescence. Adv. Funct. Mater. 2008, 18,
928−937. (c) Wang, Y.; Herron, N.; Grushin, V. V.; LeCloux, D.;
Petrov, V. Highly efficient electroluminescent materials based on
fluorinated organometallic iridium compounds. Appl. Phys. Lett. 2001,
79, 449−451. (d) Lee, S.-J.; Park, J.-S.; Song, M.; Shin, I. A.; Kim, Y.-I.;
Lee, J. W.; Kang, J.-W.; Gal, Y.-S.; Kang, S.; Lee, J. Y.; Jung, S.-H.; Kim,
H.-S.; Chae, M.-Y.; Jin, S.-H. Synthesis and Characterization of Red-
Emitting Iridium(III) Complexes for Solution-Processable Phosphor-
escent Organic Light-Emitting Diodes. Adv. Funct. Mater. 2009, 19,
2205−2212.
4041−4046. (b) Ho, C. L.; Wang, Q.; Lam, C. S.; Wong, W. Y.; Ma,
D.; Wang, L.; Gao, Z. Q.; Chen, C. H.; Cheah, K. W.; Lin, Z.
Phosphorescence color tuning by ligand, and substituent effects of
multifunctional iridium(III) cyclometalates with 9-arylcarbazole
moieties. Chem. - Asian J. 2009, 4, 89−103.
(24) (a) He, L.; Duan, L.; Qiao, J.; Wang, R.; Wei, P.; Wang, L.; Qiu,
Y. Blue-Emitting Cationic Iridium Complexes with 2-(1H-Pyrazol-1-
yl)pyridine as the Ancillary Ligand for Efficient Light-Emitting
Electrochemical Cells. Adv. Funct. Mater. 2008, 18, 2123−2131.
(b) Shan, G.-G.; Li, H.-B.; Cao, H.-T.; Sun, H.-Z.; Zhu, D.-X.; Su, Z.-
M. Influence of alkyl chain lengths on the properties of iridium(III)-
based piezochromic luminescent dyes with triazole-pyridine type
ancillary ligands. Dyes Pigm. 2013, 99, 1082−1090.
(25) Wang, K.; Zhao, F.; Wang, C.; Chen, S.; Chen, D.; Zhang, H.;
Liu, Y.; Ma, D.; Wang, Y. High-Performance Red, Green, and Blue
Electroluminescent Devices Based on Blue Emitters with Small
Singlet-Triplet Splitting and Ambipolar Transport Property. Adv.
Funct. Mater. 2013, 23, 2672−2680.
(26) (a) Antoniadis, H.; Abkowitz, M. A.; Hsieh, B. R. Carrier deep-
trapping mobility-lifetime products in poly(p-phenylene vinylene).
Appl. Phys. Lett. 1994, 65, 2030−2032. (b) Kepler, R. G.; Beeson, P.
M.; Jacobs, S. J.; Anderson, R. A.; Sinclair, M. B.; Valencia, V. S.;
Cahill, P. A. Electron and hole mobility in tris(8-hydroxyquinolinolato-
N1,O8) aluminum. Appl. Phys. Lett. 1995, 66, 3618−3620.
(27) (a) Wu, C.; Djurovich, P. I.; Thompson, M. E. Study of Energy
Transfer and Triplet Exciton Diffusion in Hole-Transporting Host
Materials. Adv. Funct. Mater. 2009, 19, 3157−3164. (b) Han, T. H.;
Choi, M. R.; Woo, S. H.; Min, S. Y.; Lee, C. L.; Lee, T. W. Molecularly
controlled interfacial layer strategy toward highly efficient simple-
structured organic light-emitting diodes. Adv. Mater. 2012, 24, 1487−
1493. (c) Wang, Z. B.; Helander, M. G.; Liu, Z. W.; Greiner, M. T.;
Qiu, J.; Lu, Z. H. Controlling carrier accumulation and exciton
formation in organic light emitting diodes. Appl. Phys. Lett. 2010, 96,
043303.
(28) (a) He, G.; Pfeiffer, M.; Leo, K.; Hofmann, M.; Birnstock, J.;
Pudzich, R.; Salbeck, J. High-efficiency and low-voltage p-i-n
electrophosphorescent organic light-emitting diodes with double-
emission layers. Appl. Phys. Lett. 2004, 85, 3911−3913. (b) Zhang, J.;
Ding, D.; Wei, Y.; Han, F.; Xu, H.; Huang, W. Multiphosphine-Oxide
Hosts for Ultralow-Voltage-Driven True-Blue Thermally Activated
Delayed Fluorescence Diodes with External Quantum Efficiency
beyond 20%. Adv. Mater. 2016, 28, 479−485. (c) Li, W.; Li, J.; Liu, D.;
Li, D.; Wang, F. Cyanopyridine Based Bipolar Host Materials for
Green Electrophosphorescence with Extremely Low Turn-On
Voltages and High Power Efficiencies. ACS Appl. Mater. Interfaces
2016, 8, 21497−21504.
(21) (a) Tamayo, A. B.; Garon, S.; Sajoto, T.; Djurovich, P. I.; Tsyba,
I. M.; Bau, R.; Thompson, M. E. Cationic Bis-cyclometalated
Iridium(III) Diimine Complexes and Their Use in Efficient Blue,
Green, and Red Electroluminescent Devices. Inorg. Chem. 2005, 44,
8723−8732. (b) Ho, C. L.; Wong, W. Y.; Gao, Z. Q.; Chen, C. H.;
Cheah, K. W.; Yao, B.; Xie, Z. Y.; Wang, Q.; Ma, D. G.; Wang, L. X.;
Yu, X. M.; Kwok, H. S.; Lin, Z. Y. Red-Light-Emitting Iridium
Complexes with Hole-Transporting 9-Arylcarbazole Moieties for
Electrophosphorescence Efficiency/Color Purity Trade-off Optimiza-
tion. Adv. Funct. Mater. 2008, 18, 319−331. (c) Zhao, Q.; Li, F.; Liu,
S.; Yu, M.; Liu, Z.; Yi, T.; Huang, C. Highly Selective Phosphorescent
Chemosensor for Fluoride Based on an Iridium(III) Complex
Containing Arylborane Units. Inorg. Chem. 2008, 47, 9256−9264.
(22) Wong, W.-Y.; Ho, C.-L. Functional metallophosphors for
effective charge carrier injection/transport: new robust OLED
materials with emerging applications. J. Mater. Chem. 2009, 19,
4457−4482.
(23) (a) Zhu, Y. C.; Zhou, L.; Li, H. Y.; Xu, Q. L.; Teng, M. Y.;
Zheng, Y. X.; Zuo, J. L.; Zhang, H. J.; You, X. Z. Highly efficient green
and blue-green phosphorescent OLEDs based on iridium complexes
with the tetraphenylimidodiphosphinate ligand. Adv. Mater. 2011, 23,
I
DOI: 10.1021/acs.inorgchem.7b01516
Inorg. Chem. XXXX, XXX, XXX−XXX