Molecular Engineering of Phenylbenzimidazole-Based Orange Ir(III) Phosphors toward High-Performance White OLEDs

Article abstract of DOI:10.1021/acs.inorgchem.8b00527

To develop B-O complementary-color white organic light-emitting diodes (WOLEDs) exhibiting high efficiency and low roll-off as well as color stability simultaneously, we have designed two orange iridium(III) complexes by simply controlling the position of

Full text of DOI:10.1021/acs.inorgchem.8b00527

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Molecular Engineering of Phenylbenzimidazole-Based Orange Ir(III)
Phosphors toward High-Performance White OLEDs
Li-Li Wen,† Chun-Xiu Zang,‡ Ying Gao,† Guo-Gang Shan,*,† Hai-Zhu Sun,† Tong Wang,§
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 130024, People’s Republic of China
‡State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, Changchun,
Jilin 130012, People’s Republic of China
§Army Armor Academy NCO Institute, Changchun 130017, People’s Republic of China
S
* Supporting Information
ABSTRACT: To develop B-O complementary-color white organic light-emitting
diodes (WOLEDs) exhibiting high efficiency and low roll-off as well as color stability
simultaneously, we have designed two orange iridium(III) complexes by simply
controlling the position of the methoxyl group on the cyclometalated ligand. The
obtained emitters mOMe-Ir-BQ and pOMe-Ir-BQ show good photophysical and
electrochemical stabilities with a broadened full width at half-maximum close to 100
nm. The corresponding devices realize highly efficient electrophosphorescence with a
maximum current efficiency (CE) and power efficiency (PE) of 24.4 cd A−1 and 15.3
lm W−1 at a high doping concentration of 15 wt %. Furthermore, the complementary-
color all-phosphor WOLEDs based on these phosphors exhibit good performance with
a maximum CE of 31.8 cd A−1, PE of 25.0 lm W−1, and external quantum efficiency of
15.5%. Particularly, the efficiency of this device is still as high as 29.3 cd A−1 and 14.2%
at the practical brightness level of 1000 cd m−2, giving a small roll-off. Meanwhile,
extremely high color stability is achieved by these devices with insignificant chromaticity variation.
Dichromatic WOLEDs exhibiting a truly high color-rendering
index (CRI) remain a fascinating challenge. Aside from that, the
low CRI (<70) is a major benchmark for the development of
two-element WOLEDs, mainly limited by the restricted spectral
bandwidth of orange and blue phosphors.8 Meanwhile, the
INTRODUCTION
■
Since the pioneering study by Kido and his co-workers,1 many
researchers have paid considerable attention to white organic
light-emitting diodes (WOLEDs).2 WOLEDs have been widely
recognized as crucial candidates for display and lighting because
of their striking features, including high electroluminescence
(EL) efficiency, long lifetime, and low production cost, which
are superior to the performance of incandescent lamps and
fluorescent tubes.3 Substantial improvements in their perform-
ance have been realized by employing all-phosphorescent
materials in the emissive layers, due to their inherent ability of
utilizing all electron−hole pairs including singlet and triplet
excitons for radiative decay.4 In consideration of their practical
use in solid-state light, WOLEDs require not only a simple
configuration but also a high EL performance. Although white-
emitting devices comprising three emission units (blue, green,
and red) or more can be effective in accomplishing high-quality
illumination, their complicated structures inevitably result in
high driving voltage and processing complexity.5 Alternatively,
more simply structured WOLEDs composed of two comple-
mentary emitters with satisfactory performance have been
highlighted to make fabrication cost-effective for widespread
commercialization.6 However, there is much room for
improvement in device efficiency, since the theoretical value
of power efficiency for WOLEDs is about 240 lm W−1.7
́
variation of Commission Internationale de l’Eclairage (CIE)
coordinates with increasing operating voltage requires control.9
The stable and efficient orange phosphors along with the
broadened full width at half-maximum (fwhm) are particularly
indispensable to improve the performance of WOLEDs,10
calling for the requirement of commercialized applications.
In our previous works, a series of iridium(III)-based
phosphors bearing 1,2-diphenyl-1H-benzoimidazole (Hpbi)
and its derivatives as the main ligand successfully achieved
excellent optical and electrical stabilities.11 The monochromatic
devices based on these complexes can be fabricated by a high
dopant concentration of 20 wt %, showing high EL efficiency
and low roll-off.12 Remarkably, the white devices based on these
complexes exhibited high EL performance with a maximum
current efficiency of 22.1 cd A−1. Extremely low luminance,
serious efficiency roll-off, and poor spectral stability, however,
were observed.13 Hence, rational cyclometalated ligand design
Received: February 28, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.8b00527
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is still urgently needed for fine-tuning of the photophysical
properties, consequently further enhancing the performance of
white OLEDs.
respectively, which are comparable or even superior to the
stability reported in previous work.15
As previously studied, the luminescence characteristics of
iridium(III) complexes strongly depend on the chemical
structures of aromatic ligands. In this regard, the variation in
emission color can be achieved by modifying cyclometalated
ligands with electron-withdrawing and electron-donating
groups as well as simply adjusting substituent locations.2e,14
With the purpose of developing high-performance white
devices, we selected Hpbi as the parent ligand to construct
the orange emitters, as seen in Figure 1. The methoxyl group
EXPERIMENTAL SECTION
■
General Information. The photophysical characteristics of
mOMe-Ir-BQ and pOMe-Ir-BQ in dichloromethane (1.0 × 10−5
M) were determined with an FL-4600 fluorescent spectrophotometer
and a Cary 500 UV−vis−NIR spectrophotometer for photo-
luminescence (PL) spectra and absorption spectra, respectively.
Films composed of only these complexes were prepared using the
spin-coating method with a concentration of 20 mg mL−1 in
dichloromethane. The photoluminescence quantum yields (PLQYs)
of their films were determined in an integrating sphere. Excited-state
lifetimes of these films were determined with an Edinburgh FLSP920
spectrofluorimeter. Cyclic voltammetry (CV) measurements of these
complexes were carried out with an electrochemical workstation (BAS
100W instrument) using tetrabutylammonium hexafluorophosphate
solution (0.1 M in CH2Cl2) as a supporting electrolyte. A three-
electrode method was used with an aqueous saturated calomel
reference electrode, a platinum-wire auxiliary electrode, and a glassy-
carbon working electrode. Meanwhile, ferrocene/ferrocenium (Fc/
Fc+) was selected as an internal reference. Mass spectrometry (MS)
was carried out on a matrix-assisted laser desorption ionization time-
of-flight (MALDI-TOF) mass spectrometer. The characterization of
1H and 13C NMR was performed on a Varian NMR spectrometer
operating at 500 and 125 MHz with tetramethylsilane as the internal
standard.
Synthesis Procedures. Synthesis of 4-OMePBI. N-Phenyl-o-
phenylenediamine (3.68 g, 20.00 mmol) was solubilized with 15 mL of
dry degassed DMA. Then 4-methoxybenzoyl chloride (3.40 g, 20.00
mmol) dispersed in 10 mL of DMA was added slowly to the mixture
with a constant pressure dropping funnel under nitrogen. This solution
was stirred for 2 h and then was poured into water to give the solid.
After being filtered and dried, the pink product was collected. The
obtained mixture with the product suspended in acetic acid (50 mL)
was heated to reflux for 12 h. After it was cooled to room temperature,
water was added, and then the desired product was filtered and
purified by column chromatography on silica gel. White powder (3.60
g, 12.00 mmol). Yield: 60%. 1H NMR (500 MHz, DMSO-d6, δ
[ppm]): 7.86 (d, J = 8.5 Hz, 1H), 7.45−7.52 (m, 5H), 7.30−7.33 (m,
3H), 7.20−7.26 (m, 2H), 6.80−6.83 (m, 2H), 3.79 (s, 3H). 13C NMR
(125 MHz, CDCl3, δ [ppm]): 160.45, 152.31, 142.92, 137.15, 137.11,
130.83, 129.80, 128.44, 127.41, 122.93, 122.80, 122.24, 119.47, 113.69,
110.23, 55.20.
Figure 1. Chemical structures of the studied complexes mOMe-Ir-BQ
and pOMe-Ir-BQ.
was introduced onto the meta (m)/para (p) position of the
phenyl ring in Hpbi, which is expected to alter the electron
distribution in the frontier orbital levels of designed iridium-
(III) complexes, giving rise to a reduction of the energy gap.
Meanwhile, the introduction of the methoxyl group can
increase the steric hindrance to suppress the self-quenching
interactions. The highly conjugated 2-benzo[d]imidazol-2-yl-
quinoline (Hbiq) was adopted as the ancillary ligand to shift
the emission to the long-wavelength region. As expected, the
designed iridium(III) complexes bis(2-(4-methoxyphenyl)-1-
phenyl-1H-benzoimidazole)(2-(benzimidazol-2-yl)quinoline)-
iridium (mOMe-Ir-BQ) and bis(2-(3-methoxyphenyl)-1-phe-
nyl-1H-benzoimidazole)(2-(benzimidazol-2-yl)quinoline)-
iridium (pOMe-Ir-BQ) exhibit good photophysical and
electrochemical properties, accompanied by emission falling
into the orange-red region. Monochromatic devices employing
mOMe-Ir-BQ and pOMe-Ir-BQ as dopant emitters at a high
doping concentration of 15 wt % exhibit maximum current
efficiencies (CE) of 24.4 and 18.6 cd A−1 and maximum power
efficiencies (PE) of 15.3 and 12.9 lm W−1, respectively.
Considering their broad fwhm values over 100 nm, we have
succeeded in the fabrication of two-component WOLEDs with
bis(4,6-(difluorophenyl)-pyridinato-N,C2)picolinate iridium-
(III) (FIrpic) as the bluish emitter and m/pOMe-Ir-BQ as
orange emitting, respectively. Device W2 based on pOMe-Ir-
BQ exhibits promising performance with a maximum CE of
31.8 cd A−1, PE of 25.0 lm W−1, EQE of 15.5%, and these
values are 29.3 cd A−1, 17.7 lm W−1, and 14.2% at 1000 cd m−2,
respectively, revealing very low roll-off. Both WOLEDs possess
a high CRI of 74 for device W1 and 77 for device W2. More
importantly, when the driving voltage was increased from 6 to 8
V, devices W1 and W2 displayed extremely high color stability
with ΔCIE = (0.0019, 0.0003) and (0.0238, 0.0045),
Synthesis of 3-OMePBI. The synthesis of 3-OMePBI is similar to
that of 4-OMePBI by using 3-methoxybenzoyl chloride as the starting
1
material. White powder (3.78 g, 12.59 mmol). Yield: 63% H NMR
(500 MHz, DMSO-d6, δ [ppm]): 7.90 (d, J = 8.0 Hz, 1H), 7.47−7.53
(m, 3H), 7.24−7.36 (m, 5H), 7.15−7.21 (m, 2H), 7.11 (d, J = 8.0 Hz,
1H), 6.89−6.91 (m, 1H), 3.69 (s, 3H). 13C NMR (125 MHz, CDCl3,
δ [ppm]): 159.25, 152.14, 142.81, 137.16, 136.98, 131.01, 129.81,
129.24, 128.51, 127.37, 123.33, 122.95, 121.87, 119.78, 116.17, 113.91,
110.40, 55.14.
Synthesis of mOMe-Ir-BQ. The starting materials 4-OMePBI (1.08
g, 3.60 mmol) and IrCl3·3H2O (0.53 g, 1.50 mmol) were dissolved in
40 mL of degassed 2-ethoxyethanol/water (3/1, v/v). The mixture was
reacted for 24 h under reflux with an argon environment before it was
cooled to room temperature. The colored solid was obtained by
filtration, washed afterward with water and ethanol, and dried. A
mixture of the dimer [Ir(4-OMePBI)2(μ-Cl)]2 (0.50 g, 0.30 mmol),
Hbiq (0.17 g, 0.66 mmol), and aqueous K2CO3 (0.11 g, 0.72 mmol) in
45 mL of dichloromethane/methanol (5/4, v/v) was refluxed under an
argon atmosphere in dark. After the mixture was reacted for 16 h, the
solvent was completely removed under the condition of reduced
pressure. Purification of the remaining solid on silica gel (300−400
mesh) using dichloromethane/ethyl acetate (1/2, v/v) as the eluent
1
was adopted to give the pure product as a powder. Yield: 57% H
NMR (500 MHz, DMSO-d6): δ [ppm] 8.59−8.65 (m, 2H), 8.09 (d, J
= 9.0 Hz, 1H), 8.02 (d, J = 8.0 Hz, 1H), 7.70−7.77 (m, 6H), 7.63 (d, J
B
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Scheme 1. Synthetic Routes of Main Ligands and Iridium(III) Complexes
= 8.5 Hz, 1H), 7.50−7.53 (m, 4H), 7.30 (d, J = 6.0 Hz, 1H), 7.15−
7.18 (m, 1H), 7.08 (t, J = 8.5 Hz, 1H), 6.95−7.04 (m, 3H), 6.90 (d, J =
8.0 Hz, 1H), 6.73−6.76 (m, 1H), 6.68 (t, J = 8.0 Hz, 1H), 6.61 (t, J =
7.5 Hz, 1H), 6.53−6.57 (m, 2H), 6.45−6.48 (m, 1H), 6.38−6.40 (m,
1H), 6.05 (d, J = 2.0 Hz, 1H), 5.97 (d, J = 8.0 Hz, 1H), 5.93 (d, J = 8.0
Hz, 1H), 5.85 (d, J = 2.0 Hz, 1H), 5.41 (d, J = 8.5 Hz, 1H), 3.45 (d, J
= 20.0 Hz, 6H). MS (MALDI-TOF, m/z): 1036.3 [M + H]+. Anal.
Calcd for C56H40IrN7O2: C, 64.97; H, 3.89; N, 9.47. Found: C, 64.90;
H, 4.05; N, 9.41.
RESULTS AND DISCUSSION
Syntheses. The cyclometalated ligands 4-OMePBI and 3-
■
OMePBI were obtained via the reaction of the commercially
available precursors with the synthetic method shown in
Scheme 1. The extended π-conjugated ancillary ligand Hbiq
was synthesized according to previously reported routes, which
is also important in tuning the emission wavelength.18 The
targeted complexes mOMe-Ir-BQ and pOMe-Ir-BQ were
prepared by a two-stage route generally used for most
iridium(III) complexes. The chloride-bridged dimers were
formed by cyclometalated ligands reacted with IrCl3·3H2O
according to the Nonoyama route.19 The chemical structures of
phosphorescent iridium(III) complexes were fully identified by
various techniques such as mass spectrometry, 1H NMR
spectroscopy, and elemental analysis.
Synthesis of pOMe-Ir-BQ. The procedure for synthesis of complex
pOMe-Ir-BQ was similar to that of mOMe-Ir-BQ by using 3-
1
OMePBI as the starting material. Yield: 52% H NMR (500 MHz,
DMSO-d6): δ [ppm] 8.64 (d, J = 8.5 Hz, 1H), 8.59 (d, J = 8.5 Hz,
1H), 8.18 (d, J = 9.5 Hz, 1H), 8.02 (d, J = 8.0 Hz, 1H), 7.71−7.83 (m,
6H), 7.61−7.68 (m, 3H), 7.49−7.53 (m, 2H), 7.34 (d, J = 6.5 Hz,
1H), 7.10−7.18 (m, 2H), 7.04−7.07 (m, 2H), 6.93−6.97 (m, 2H),
6.76 (t, J = 7.5 Hz, 1H), 6.67−6.70 (m, 1H), 6.56−6.64 (m, 3H), 6.47
(d, J = 8.5 Hz, 1H), 6.26 (d, J = 8.5 Hz, 1H), 6.12−6.16 (m, 2H), 5.99
(d, J = 8.0 Hz, 1H), 5.90 (d, J = 8.5 Hz, 1H), 5.39 (d, J = 8.5 Hz, 1H),
3.36 (d, J = 10.0 Hz, 6H). MS (MALDI-TOF, m/z): 1036.3 [M + H]+.
Anal. Calcd for C56H40IrN7O2: C, 64.97; H, 3.89; N, 9.47. Found: C,
65.03; H, 3.82; N, 9.39.
Photophysical Properties. The absorption and PL spectra
of mOMe-Ir-BQ and pOMe-Ir-BQ, as recorded in CH2Cl2 at
room temperature, are shown in Figure 2, and the
Theoretical Calculations. The Gaussian 09 software package was
employed in all calculations of studied complexes.16 The structures of
mOMe-Ir-BQ and pOMe-Ir-BQ were fully optimized with C1
symmetry constraint using restricted closed-shell B3LYP for ground
states (S0) and spin-unrestricted open-shell B3LYP for the lowest
triplet states (T1), respectively.17 The standard 6-31G* basis set on H,
C, O, and N atoms and LANL2DZ on the iridium(III) atom were used
in these calculations. The inner-core electrons in the iridium(III) atom
were displaced by relativistic effective core potentials, while the outer-
core electrons (5s)2(5p)6 and valence electrons (5d)6 remain to be
considered. To further understand excited-state electronic properties
of both complexes, time-dependent density functional theory (TD-
DFT) calculations were carried out in consideration of the solvent
environment. The expectation values calculated for S2 were smaller
than 2.05 in spin-unrestricted calculations.
Figure 2. Room-temperature absorption and PL spectra of mOMe-Ir-
BQ and pOMe-Ir-BQ. Inset: luminescence photographs of mOMe-Ir-
BQ and pOMe-Ir-BQ in CH2Cl2 solution.
OLED Fabrication and Measurements. All devices in this work
were prepared on indium tin oxide (ITO) glass substrates with a sheet
resistance of 20 Ω/sqr. A standard precleaning process for ITO
composed of rinsing in Decon 90 and deionized water was adopted,
followed by treatment in a UV−ozone chamber. The thermal
deposition of the functional layer was operated under a high vacuum
degree of ∼5.0 × 10−4 Pa. The definition of the cathode was carried
out with a shadow mask, and two 10 mm2 devices were made. A quartz
oscillator was adopted to measure the thickness of the electrodes and
organic layers. Luminance efficiency−voltage characteristics and EL
spectra for the unpackaged devices were measured with a computer-
controlled Minolta LS-110 luminance meter, Keithley 2400 source
meter, and Ocean Optics Maya 2000-PRO spectrometer.
corresponding data are presented in Table 1. Both phosphor-
escent complexes exhibit high-intensity absorption bands
located at 270−320 nm in their UV−vis spectra, which are
ascribed to allowed π−π* transitions of the cyclometalated
ligands. The absorption bands occurring from 320 to 400 nm
correspond to an admixture of a spin-allowed metal-to-ligand
charge transfer (1MLCT) transition and a ligand-to-ligand
charge transfer (1LLCT) transition. The weaker absorption
bands above 400 nm are mainly attributed to a spin-forbidden
metal-to-ligand charge transfer (3MLCT) transition.
Upon photoexcitation, two phosphors exhibit intense orange
and orange-red emission with peaks at 593 and 606 nm for
C
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Table 1. Photophysical and Electrochemical Properties for mOMe-Ir-BQ and pOMe-Ir-BQ
a c
−
ac
,
d
e
f
g
c
h
h
complex
λPL,max
(nm)
τ
(μs)
Eox onset (V)
Eg (eV)
HOMO /LUMO (eV)
Φp (%)
kr (105 s−1
)
knr (106 s−1
)
mOMe-Ir-BQ
pOMe-Ir-BQ
593, 580, 602
606, 595, 625
1.78, 0.57
1.53, 0.21
0.32
0.28
2.23
− 5.12/− 2.89
− 5.08/− 2.90
30.9
11.4
5.42
1.21
4.22
2.18
5.43
a
b
c
Measured in deaerated CH2Cl2 (10−5 M) at ambient temperature. Measured in 10−5 M CH2Cl2 solution at 77 K. Measured as a neat film at
d
e
ambient temperature. The onset oxidation potentials versus the ferrocenium/ferrocene (Fc+/Fc) couple. Deduced according to the UV−vis
f
g
h
absorption spectra. Calculated from the onset oxidation potentials. Calculated from Eg and HOMO. Calculated according to the equations τ = 1/
(kr + knr) and Φ = kr/(kr + knr).
Figure 3. Calculated HOMO and LUMO distributions of complexes mOMe-Ir-BQ and pOMe-Ir-BQ. All MO surfaces correspond to an isocontour
value of |Ψ| = 0.02.
mOMe-Ir-BQ and pOMe-Ir-BQ, respectively. Evidently, the
introduction of a methoxyl group to the iridium(III) complexes
causes a bathochromic shift, in comparison with the reference
complex (pbi)2Ir(biq) reported in previous work (emission
peak at 573 nm). The spectra reveal similar features of
unstructured and broad emission bands, an indication of
emission from 3MLCT states,20 which was further confirmed by
quantum chemical calculations (see below). Additionally, the
lifetime of pOMe-Ir-BQ is shorter than that of mOMe-Ir-BQ,
arising from the fact that p-OCH3 modification can induce
greater electron donation into the Ir−C bond and consequently
result in the increment of radiative decay rate. Additionally, the
emission spectra of mOMe-Ir-BQ and pOMe-Ir-BQ at low
temperature were measured in CH2Cl2 with peaks of 580 and
595 nm (as shown in Figure S1 in the Supporting Information).
The obvious rigidochromic shift in comparison with emission at
room temperature indicates that the lowest excited states
complexes in both the solid state and degassed solution are in
the microsecond regime, supporting the typical emission from
the phosphorescence. The PLQY values for mOMe-Ir-BQ and
pOMe-Ir-BQ were estimated to be 30.9 and 11.4% in their neat
films, respectively. In order to explore the origin of PLQY
variation for mOMe-Ir-BQ and pOMe-Ir-BQ, the radiative (kr)
and nonradiative decay rate constants (knr) of neat films were
calculated on the basis of their lifetimes and PLQYs. As given in
Table 1, both iridium(III) complexes exhibited comparable kr
values of 5.42 × 105 s−1 for mOMe-Ir-BQ and 5.43 × 105 s−1
for pOMe-Ir-BQ. A knr value of 4.22 × 106 s−1 for pOMe-Ir-
BQ was observed, which is nearly 3 times higher than that of
mOMe-Ir-BQ, estimated as 1.21 × 106 s−1. The nearly
unchanged kr and increased knr for pOMe-Ir-BQ resulted in
its relatively low PLQY.
Theoretical Calculations. To verify the effect of
substituent position on the photophysical and electrochemical
observations, theoretical calculations of mOMe-Ir-BQ and
pOMe-Ir-BQ were undertaken. The highest occupied molec-
ular orbital (HOMO) and lowest unoccupied molecular orbital
(LUMO) levels are plotted in Figure 3, while the selected
computational data are described in Table S1 in the Supporting
Information. Both complexes exhibit similar LUMO distribu-
tions, which are mainly located over the whole ancillary ligands
and a small portion on the iridium(III) atom. The HOMO of
3
possess a mixing of MLCT/3LLCT characters. The neat films
based on mOMe-Ir-BQ and pOMe-Ir-BQ give significantly red
shifted, broadened, and featureless emission with peaks at 602
and 625 nm (as shown in Figure S2 in the Supporting
Information) with lifetimes of 0.57 and 0.21 μs. These films
display much more broadened spectra in comparison to
(pbi)2Ir(biq) (fwhm 58 nm), which is definitely critical for
improving the color quality of WOLEDs. The lifetimes of all
D
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Figure 4. EL characteristics of monochromatic devices based on the studied emitters mOMe-Ir-BQ and pOMe-Ir-BQ: (a) architecture; (b) EL
spectra measured at 7 V; (c) current density−voltage−luminance characteristics; (d) current, power, and external quantum efficiency curves.
mOMe-Ir-BQ is chiefly distributed on the iridium(III) and
ancillary ligand, but slightly on the main ligands. Differing from
that, the HOMO of pOMe-Ir-BQ primarily resides on the main
ligands and iridium(III) atom and partially on the ancillary
ligand. Detailed discussions on orbital distributions reveal that
the phenyl rings with substituent groups in pOMe-Ir-BQ have
considerable electron distribution in their HOMO while the
rings in mOMe-Ir-BQ possess little electron distribution in
their HOMO. As previously reported, a substituent attached to
a position with significant electron density would generally give
rise to a strong influence on the MO, and vice versa.21
Therefore, the methoxyl groups on the para site in pOMe-Ir-
BQ should display more effect on the HOMO level in
comparison to mOMe-Ir-BQ, leading to the various photo-
physical and electrochemical properties, which is consistent
with the experimental results. Moreover, the calculated results
shown in Table S1 indicate that designed complexes possess
3MLCT/3LLCT character with 90% contribution of HOMO →
LUMO for mOMe-Ir-BQ and 76% contribution of HOMO →
LUMO for pOMe-Ir-BQ.
Electrochemical Properties. CV measurements were
utilized to further probe the electrochemical properties of
both complexes, and the results are given in Table 1.
Meanwhile, the electrochemical stabilities were investigated
by multicycle tests. As shown in Figure S3 in the Supporting
Information, only a slight current change for the first oxidation
peaks and nearly unchanged second peaks were observed after
25 cycles for the two complexes, indicating good electro-
chemical stabilities. Both mOMe-Ir-BQ and pOMe-Ir-BQ
complexes produce first oxidation waves with onset oxidation
potentials (Eox onset) of 0.32 and 0.28 V, respectively, which
are attributed to the oxidation of the metal-centered iridium-
(III). The Eox onset of pOMe-Ir-BQ shows a slightly cathodic
shift of 0.04 V relative to mOMe-Ir-BQ, revealing that the
electronic characteristics of the iridium(III) metal could be
finely tuned by adjusting the position of the methoxyl group on
the cyclometalated ligand. The energy gaps (Eg) of mOMe-Ir-
BQ and pOMe-Ir-BQ are calculated as 2.23 and 2.18 eV
according to their absorption onset values of the solutions,
respectively. The HOMO levels of mOMe-Ir-BQ and pOMe-
Ir-BQ are −5.12 and −5.08 eV, estimated from their onset
oxidation potential with regard to ferrocene/ferrocenium,
which has an energy level of 4.80 eV under vacuum. The
LUMO levels of mOMe-Ir-BQ and pOMe-Ir-BQ are −2.89
and −2.90 eV, deduced from corresponding the HOMO and
Eg. Consequently, the above behaviors clearly indicated that the
excited-state properties were affected by the position of
methoxyl bonded on Hpbi ligands, resulting in a variation of
photophysical and electrochemical performance.
Monochromatic OLED Characterization. The mono-
chromatic devices D1 and D2 were constructed using mOMe-
Ir-BQ and pOMe-Ir-BQ as the dopants with the same
configuration as follows: ITO/MoO3 (3 nm)/TAPC (35
nm)/TCTA (5 nm)/DCzPPy:emitter (15 wt %, 20 nm)/
TmPyPB (40 nm)/LiF (0.5 nm)/Ag:Mg (1:15, 120 nm),
depicted in Figure 4a. The molecular structures and HOMO/
LUMO diagrams of materials involved in these devices are
provided in Figure S4 in the Supporting Information. MoO3
and LiF are chosen as hole- and electron-injecting layers, with
an indium tin oxide (ITO) anode and Ag:Mg cathode. Bis[4-
(N,N-ditolylamino)phenyl]cyclohexane (TAPC) serves as the
hole-transporting layer (HTL) with a high mobility of ∼10−2
22
cm2 V−1 s−1
.
Tris(4-(9H-carbazol-9-yl)phenyl)amine
(TCTA) is used as an exciton-blocking layer because of its
high triplet energy (2.85 eV), which is higher than that of the
host and designed emitters.23 Additionally, TCTA can also act
as a buffer layer with the appropriate HOMO level (5.7 eV)
between hole-transporting (5.3 eV) and host materials (6.1
eV). The bipolar host 2,6-bis(3-(9H-carbazol-9-yl)phenyl)-
pyridine (DCzPPy) with a high triplet energy of 2.71 eV is
selected to balance charge transport and confine triplets in the
emitting layer (EML).24 1,3,5-Tris(m-pyrid-3-ylphenyl)benzene
(TmPyPB) is introduced as an electron-transporting as well as
hole-blocking material with a mobility of ∼10−3 cm2 V−1 s−1
and a deep HOMO of 6.7 eV, respectively.25
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Inorganic Chemistry
Article
The phosphors mOMe-Ir-BQ and pOMe-Ir-BQ were
individually doped in the DCzPPy at concentrations as high
as 15 wt %. The detailed characteristics of the doped devices are
shown in Figure 4, and key EL data are summarized in Table 2.
and S0 exhibit an obvious increase in comparison with that from
T1 to S0, which is prone to an electron transition from higher
excited states to the ground state. Moreover, the calculated
general trend of pOMe-Ir-BQ is almost the same as that of
mOMe-Ir-BQ. Considering the strong SOC of mOMe-Ir-BQ
and pOMe-Ir-BQ, the phosphorescent emissions from higher
excited states may be realized, possibly leading to the broad
fwhm values.26 The interactions between iridium(III) com-
plexes and the materials used in the devices under an electric
field might be another reason for the broad fwhm values of
these complexes.13,27 Furthermore, device D1 comprising
mOMe-Ir-BQ exhibits excellent performance with a maximum
luminance (Lmax) of 23471 cd m−2 at 12 V, a maximum CE of
24.4 cd A−1, a maximum PE of 15.3 lm W−1, and a maximum
external quantum efficiency (EQE) of 9.9%. The EQE still
remains as high as 8.3% while the brightness is up to the
practical level of 1000 cd m−2. Device D2 comprising pOMe-Ir-
BQ exhibits slightly lower performance along with peak CE,
PE, and EQE values of 18.6 cd A−1, 12.9 lm W−1, and 9.1%,
respectively, which can be attributed to the lower PLQY of
pOMe-Ir-BQ.
WOLED Characterization. Encouraged by the good EL
performances of the monochrome devices and their broad
fwhm values over 100 nm, complementary-color all-phosphor
WOLEDs were designed and fabricated with a similar device
configuration. With the introduction of FIrpic as a sky blue
emitter, the two-color WOLEDs were fabricated with the
detailed configuration ITO/MoO3 (3 nm)/TAPC (35 nm)-
TCTA (5 nm)/DCzPPy:FIrpic [10 wt %, (20 − x) nm]/
DCzPPy:emitter (15 wt %, x nm)/TmPyPB (40 nm)/LiF (0.5
nm)/Ag:Mg (1:15, 120 nm), where the thickness x of the
doping layers incorporating mOMe-Ir-BQ and pOMe-Ir-BQ
was controlled at 14 and 13 nm for devices W1 and W2,
respectively. The designed emitters were heavily doped in host
materials with a concentration of 15 wt %, rarely reported in
previous work. In general, phosphorescent emitters usually
were dispersed into an appropriate host material to inhibit
Table 2. Performance of Monochromatic Devices D1 and
D2
a
ab
,
ab
,
Lmax
(cd
m−2
CE
PE
(cd
(lm
ab
,
device λEL (nm)
)
A−1
)
W−1
)
EQE
CIE [(x, y), V]
D1
581
23471 24.4,
20.7
15.3,
9.5
9.9, 8.3 (0.52, 0.47),
7.0
D2
593
35587 18.6,
15.7
12.9,
7.5
9.1, 7.2 (0.56, 0.44),
7.0
a
Maximum values of the monochromatic devices for current efficiency
(CE), power efficiency (PE), and external quantum efficiency (EQE),
b
respectively. Values of the devices at 1000 cd m−2 for CE, PE, and
EQE, respectively.
Strong yellow and orange emissions are obtained for devices
D1 and D2 with CIE coordinates of (0.52, 0.47) and (0.56,
0.44), in which no EL components from the host or adjacent
materials can be observed. The large fwhm values of 108 and
105 nm for D1 and D2 did not arise from excimer emission,
which is conducive to achieving a stable color quality of the
WOLEDs. To further understand the possible reasons for such
large fwhm values, density functional theory was carried out to
study the excited state properties. The spin−orbital coupling
(SOC) strength between the ground state (S0) and triplet
excited states (Tn) have been calculated, and related data are
summarized in Table S2 in the Supporting Information. In
principle, the strong SOC between T1 and S0 is important to
ensure efficient phosphorescent emission. However, in some
cases, the exciton at higher excited states can also be used to
emit light. The SOC between T1 and S0 of mOMe-Ir-BQ is 110
cm−1, which is favorable to realizing phosphorescent emission.
As shown in Table S2, the SOC intensities between T3/T5/T6
Figure 5. EL characteristics of two-element WOLEDs based on the studied emitters mOMe-Ir-BQ and pOMe-Ir-BQ: (a) proposed energy diagram;
(b) EL spectra; (c) current density−voltage−luminance characteristics; (d) current, power, and external quantum efficiency curves. Insert in (b): EL
image of W2.
F
DOI: 10.1021/acs.inorgchem.8b00527
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Inorganic Chemistry
Article
efficiency roll-off caused by strong intermolecular interactions.
However, the extremely low doping concentration (less than 10
wt %) is not controlled precisely, limiting further application in
WOLEDs. Surprisingly, devices W1 and W2 successfully
realized composed high-concentration orange emitters, which
is favorable to reproduce and control in the evaporation
process. Meanwhile, high EL efficiencies and low efficiency roll-
off were obtained (see below), which is probably due to the
steric effects induced by the methoxy group. The single-host
strategy is employed to avoid unnecessary energy barriers,
which is beneficial for carrier injection and transport among
different emissive layers.28
The EL characteristics and relative data of the WOLEDs can
be found in Figure 5 and Table 3, respectively. As shown in
Table 3. Summary of EL Performance of White OLEDs
ab
Figure 6. Normalized EL spectra of for W1 based on the designed
emitter mOMe-Ir-BQ at various voltages.
,
PE
a
ab
,
Lmax (cd
CE
(lm
device m−2, V) (cd A−1
)
W−1
)
EQE
11.5,
10.3
15.5,
14.2
CRI CIE [(x, y), V]
CONCLUSIONS
■
W1
W2
30843,
11.5
25.4,
23.2
17.5,
11.2
74 (0.36, 0.40),
6.5
In this work, we have synthesized the two stable and efficient
iridium(III) complexes mOMe-Ir-BQ and pOMe-Ir-BQ by
modifying the substituent position on the main ligand,
successfully fine-tuning crucial photophysical and electronic
properties. The resulting complexes exhibit orange and orange-
red emission with fwhm values close to 100 nm, which is
beneficial to improve the color quality of the WOLEDs. Using
mOMe-Ir-BQ and pOMe-Ir-BQ as dopant emitters at the high
concentration of 15 wt %, monochromatic devices D1/D2
display good performance with maximum CE, PE, and EQE
values of 24.4/18.6 cd A−1, 15.3/12.9 lm W−1, and 9.9/9.1%,
respectively. When FIrpic was employed as the blue emitter, all-
phosphor white devices W1 and W2 realize encouraging
performance: maximum brightnesses of 30843 and 60161 cd
m−2, CE of 25.4 and 31.8 cd A−1, PE of 17.5 and 25.0 lm W−1,
and EQE of 11.5 and 15.5%, respectively. More importantly,
both devices exhibit high-quality emission with CRI values of
up to 74 for W1 and 77 for W2. Remarkably, device W1 shows
excellent color stability with a negligible variation of (0.0019,
0.0003) between 6 and 8 V. Our preliminary investigations
should give important guidance to the design of efficient
phenylbenzimidazole-based phosphors used for high-perform-
ance OLEDs.
60161,
10.0
31.8,
29.3
25.0,
17.7
77 (0.37, 0.38),
8.5
a
Maximum values of the monochromatic devices for current efficiency
(CE), power efficiency (PE), and external quantum efficiency (EQE),
b
respectively. Values of the devices at 1000 cd m−2 for CE, PE, and
EQE, respectively.
Figure 5b, the EL spectra of devices contain two main emission
peaks at 470 and 573 nm for W1 and 469 and 586 nm for W2,
corresponding to emissions from FIrpic and designed emitters.
The broad EL emissions were realized by W1 and W2 with CRI
values as high as 74 and 77, which stem from the well-matched
EL wavelengths between blue and orange emitters. These high
CRI values are very close to the standard value (80) required
for lighting applications, outperforming the most values of two-
color WOLEDs previously reported.15b,29 The CIE color
coordinates of W1 and W2 are (0.36, 0.40) and (0.37, 0.38)
at 6.5 and 8.5 V, respectively, together with correlated color
temperatures of 4583 and 4371 K. Moreover, device W1
realizes good EL performance with Lmax of 30843 cd m−2 at
11.5 V, a maximum CE of 25.4 cd A−1, a maximum PE of 17.5
lm W−1, and a maximum EQE of 11.5%. Meanwhile, device W1
achieves extremely stable color with ΔCIE = (0.0019, 0.0003)
accompanied by an operating voltage range of 6−8 V, as seen in
Figure 6. Device W2 exhibits better performance with Lmax of
60161 cd m−2 at 10.0 V, a maximum CE of 31.8 cd A−1, a
maximum PE of 25.0 lm W−1, and a maximum EQE of 15.5%.
More importantly, device W2 exhibits low efficiency roll-off
with the CE still remaining as high as 29.3 cd A−1 up to the
practical brightness level of 1000 cd m−2. As shown in Figure
S5 in the Supporting Information, device W2 displays good
chromatic stability with ΔCIE = (0.0238, 0.0045) between 6
and 8 V. The rather stable tradeoff is ascribed to the vital role of
the bipolar host, which can extend the exciton recombination
zone, leading to effective suppression of the triplet−triplet
annihilation.30 Additionally, the excellent confining structure is
imperative to boost the chromaticity stability.31 We believe that
the performance of these WOLEDs can be further improved by
optimizing the device configuration and doping concentration
of iridium(III) complexes.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.8b00527.
■
S
Figures and tables as described in the text and calculated
energy levels of the lower-lying transitions of all
complexes (PDF)
AUTHOR INFORMATION
Corresponding Authors
*Email for G.-G.S.: shangg187@nenu.edu.cn.
*Email for W.-F.X.: xiewf@jlu.edu.cn.
*Email for Z.-M.S.: zmsu@nenu.edu.cn;.
■
ORCID
Hai-Zhu Sun: 0000-0002-5113-8267
Wen-Fa Xie: 0000-0003-0912-8046
Zhong-Min Su: 0000-0002-3342-1966
G
DOI: 10.1021/acs.inorgchem.8b00527
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Phosphorescent Ir(III) Complexes with Light-Harvesting Functional
Moieties for Efficient Blue and White PhOLEDs in Solution-Process.
Adv. Funct. Mater. 2017, 27, 1701002. (c) Wang, J.; Chen, J.; Qiao, X.;
Alshehri, S. M.; Ahamad, T.; Ma, D. Simple-Structured Phosphor-
escent Warm White Organic Light-Emitting Diodes with High Power
Efficiency and Low Efficiency Roll-off. ACS Appl. Mater. Interfaces
2016, 8, 10093−10097. (d) Guo, L. Y.; Zhang, X. L.; Wang, H. S.; Liu,
C.; Li, Z. G.; Liao, Z. J.; Mi, B. X.; Zhou, X. H.; Zheng, C.; Li, Y. H.;
Gao, Z. Q. New homoleptic iridium complexes with C∧NN type
ligand for high efficiency orange and single emissive-layer white
OLEDs. J. Mater. Chem. C 2015, 3, 5412−5418. (e) Xue, K.; Chen, B.;
Han, G.; Duan, Y.; Chen, P.; Yang, Y.; Duan, Y.; Wang, X.; Zhao, Y.
Efficient simplified orange and white phosphorescent organic light-
emitting devices with reduced efficiency roll-off. Org. Electron. 2015,
22, 122−126.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors gratefully acknowledge financial support from the
NSFC (21303012, 21273030, 21131001, and 61377027), the
973 Program (2013CB834801), and the Science and
Technology Development Planning of Jilin Province
(20130522167JH). This work was a China Postdoctoral
Science Foundation funded project (2013M540239 and
2014T70269).
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DOI: 10.1021/acs.inorgchem.8b00527
Inorg. Chem. XXXX, XXX, XXX−XXX