Highly efficient and spectrally stable white organic light-emitting diodes using new red heteroleptic Iridium(III) complexes

Article abstract of DOI:10.1016/j.dyepig.2017.10.022

Highly efficient red phosphorescent iridium(III) complexes, (MPQ)₂Ir(Pppy) and (MPQ)₂Ir(TMSppy) based on 4-methyl-2-phenylquinoline (MPQ) as a cyclometalated main ligand and 2-[(1,1′-biphenyl)4-yl]pyridine (Pppy) or 2-[4-(trimethylsi

Full text of DOI:10.1016/j.dyepig.2017.10.022

Accepted Manuscript
Highly efficient and spectrally stable white organic light-emitting diodes using new red
heteroleptic Iridium(III) complexes
Hee Un Kim, Jae-Ho Jang, Hea Jung Park, Byung Jun Jung, Wook Song, Jun Yeob
Lee, Do-Hoon Hwang
PII:
S0143-7208(17)31774-6
10.1016/j.dyepig.2017.10.022
DYPI 6320
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 18 August 2017
Revised Date: 12 October 2017
Accepted Date: 12 October 2017
Please cite this article as: Kim HU, Jang J-H, Park HJ, Jung BJ, Song W, Lee JY, Hwang D-H, Highly
efficient and spectrally stable white organic light-emitting diodes using new red heteroleptic Iridium(III)
complexes, Dyes and Pigments (2017), doi: 10.1016/j.dyepig.2017.10.022.
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to
our customers we are providing this early version of the manuscript. The manuscript will undergo
copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please
note that during the production process errors may be discovered which could affect the content, and all
legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT
Highly Efficient and Spectrally Stable White Organic Light-Emitting
Diodes Using New Red Heteroleptic Iridium(III) Complexes
Hee Un Kim,a Jae-Ho Jang,a Hea Jung Park,a Byung Jun Jung,b Wook Song,c Jun Yeob Lee,c
and Do-Hoon Hwang*,a
a Department of Chemistry and Chemistry Institute for Functional Materials,
Pusan National University, Busan 609-735, Republic of Korea
b Department of Materials Science and Engineering, The University of Seoul, Seoul 130-743,
Republic of Korea
c School of Chemical Engineering, Sungkyunkwan University, Suwon, Gyeonggi 440-746,
Republic of Korea
*To whom correspondence should be addressed.
*E-mail: dohoonhwang@pusan.c.kr
1

ACCEPTED MANUSCRIPT
Abstract
Highly efficient red phosphorescent iridium(III) complexes, (MPQ)2Ir(Pppy) and
(MPQ)2Ir(TMSppy) based on 4-methyl-2-phenylquinoline (MPQ) as a cyclometalated main
ligand and 2-[(1,1'-biphenyl)4-yl]pyridine (Pppy) or 2-[4-(trimethylsilyl)phenyl]pyridine
(TMSppy) as a second cyclometalated ligand were synthesized for use in organic light-
emitting diodes (OLEDs). The iridium(III) complex with MPQ and acetylacetone (acac) as an
ancillary ligand, (MPQ)2Ir(acac), was also synthesized for comparison. The geometrical
configuration of the heteroleptic complex (MPQ)2Ir(acac)is N,N-trans, whereas
(MPQ)2Ir(Pppy) and (MPQ)2Ir(TMSppy)show all-facial geometries. Among the iridium(III)
complexes, red OLEDs using the (MPQ)2Ir(Pppy) exhibited significantly more improved
luminance and external quantum efficiency with markedly prevented concentration self-
quenching at high doping concentration by introduction of Pppy. Moreover, a high maximum
external quantum efficiency (EQEmax) of 28.0% with a high color-rendering index (CRI) of
81 was achieved in the white OLEDs (WOLED) fabricated using the bis[2-(4,6-
difluorophenyl)pyridinato-C2,N](picolinato)iridium(III)
[FIrpic],
tris(2-
phenylpyridine)iridium(III) [Ir(ppy)3], and (MPQ)2Ir(Pppy), and it is one of the highest
values among the reported three primary color component WOLED. Also, the WOLED
fabricated using the (MPQ)2Ir(Pppy) showed excellent spectral stability at different voltages.
Keywords: iridium complex; organic light-emitting diodes; phosphorescence
2

ACCEPTED MANUSCRIPT
1. Introduction
Since the successful commercialization of organic light-emitting diodes (OLEDs) using three
primary color emissions in smart phone displays, the market for products such as OLEDs
television sets using white OLEDs (WOLED) has expanded [1-5]. Recently, in the fields of
WOLED techniques, lighting using WOLED have been attractive due to potentially cheap
energy costs and eco-friendly for lighting and various designs as efficient solid-state lighting
applications. Also, the use of WOLED techniques in automotive brake light and traffic signal
lights is also increasing [6-8].
Generally, WOLED are emitted by combination the two or three (red, green, and blue)
primary color components as all-fluorescent, fluorescent-phosphorescent hybrid, and all-
phosphorescent methods. Among them, all-phosphorescent three primary color component
WOLED give broad white emission and high color-rendering index (CRI) [9-14]. Three
primary color component WOLED are desirable in applications such as WOLED televisions
because of the high color quality from small light loss after three color filters [15]. The
emissive layers in all-phosphorescent WOLED based on three primary color mixing use
typical phosphors of blue emitter such as bis[2-(4,6-difluorophenyl)pyridinato-
C2,N](picolinato)iridium(III)
[FIrpic],
green
and
emitter
red emitter
[(pq)2Ir(acac)]
such
as
such
or
fac-tris(2-
as bis(2-
phenylpyridine)iridium(III)
[Ir(ppy)3],
phenylquinoline)(acetylacetonate)iridium(III)
bis(1-
phenylisoquinoline)(acetylacetonate)iridium(III) [(piq)2Ir(acac)] [16-20]. In particular,
(pq)2Ir(acac) and (piq)2Ir(acac) are currently widely used as red emitters in OLEDs because
they have suitable red emission regions [21,22]. However, the use of acetylacetone (acac) as
an ancillary ligand in (pq)2Ir(acac) or (piq)2Ir(acac) causes various problems, such as poor
thermal stability, low photoluminescence (PL) quantum efficiency (ФPL), and concentration
3

ACCEPTED MANUSCRIPT
self-quenching at high doping concentrations of organic light-emitting device [23,24]. New
red emitter materials with high thermal stabilities, high ФPL values, and suppressed self-
quenching effects are needed to solve these problems for OLEDs applications. Among the
previous reports, Lee et. al. Reported an iridium(III) complex with a bulky pinene-substituted
cyclometalated main ligand to reduce concentration self-quenching at very high dopant
concentrations [25].
In this study, we designed new iridium(III) complexes containing a 2-phenylpyridine-based
second cyclometalated ligand instead of acac to give high thermal stability and improved
quantum efficiency for red and white phosphorescent OLEDs. We identified their
geometrical configurations using X-ray crystallography. In general, iridium(III) complexes
containing the 2-phenylpyridine ligand have been known to exhibit a broad full width at
half maximum (FWHM) and high phosphorescent quantum efficiencies [26]. We synthesized
new iridium(III) complexes based on 4-methyl-2-phenylquinoline (MPQ) as the main
cyclometalated
ligand
and
2-[(1,1'-biphenyl)-4-yl]pyridine
(Pppy)
or
2-[4-
(trimethylsilyl)phenyl]pyridine (TMSppy) as the second cyclometalated ligand. Two
iridium(III) complexes containing Pppy or TMSppy as the second cyclometalated ligand
showed significantly improved thermal stabilities and ФPL values compared with those of the
corresponding iridium(III) complex containing an acac ancillary ligand. We also found that
the second cyclometalated ligands are important in preventing self-quenching at high dopant
concentrations in red and white phosphorescent OLEDs.
2. Experimental Section
2.1. Materials
4

ACCEPTED MANUSCRIPT
Phenyl boronic acid, 2-chloro-4-methylquinoline, 2-(4-bromophenyl)pyridine, acetylacetone,
iridium(III) chloride hydrate, tetrakis(triphenylphosphine)palladium(0), chlorotrimethylsilane,
and silver trifluoromethanesulfonate were purchased from Aldrich and Alfa Asear. All
chemicals were used without further purification.
2.2. General procedure for iridium(III) complexes
2.2.1. Synthesis of 4-methyl-2-phenylquinoline (MPQ)
2-Chloro-4-methylquinoline (5.0 g, 28.1mmol), phenyl boronic acid (4.12 g, 33.8 mmol), and
[Pd(PPh3)4] (0.97 g, 0.84 mmol) were dissolved in tetrahydrofuran. A solution of 2M K2CO3
and Aliquat 336 (1.1 g, 2.72mmol) was added, and the mixture was refluxed with stirring for
12 h under a nitrogen atmosphere. The reaction mixture was cooled to room temperature, the
mixture was extracted with ethyl acetate, and the organic layer was washed with water. The
organic layer was dried over MgSO4. The solvent was removed under reduced pressure to
give a crude residue. The crude product was purified by column chromatography on silica gel
(ethyl acetate/hexane, 1/3, v/v) to obtain MPQ (4.73 g, 76.8%). 1H NMR (300 MHz, CDCl3,
δ): 8.18 (m, 3H), 7.97 (d, 1H), 7.78 (t, 1H), 7.70 (s, 1H), 7.53 (m, 4H), 2.75 (s, 3H, CH3); 13C
NMR (75 MHz, CDCl3, δ): 157.05, 148.14, 144.83, 139.84, 130.29, 129.36, 129.23, 128.82,
127.58, 127.27, 126.05, 123.66, 119.77, 19.02.
2.2.2. Synthesis of 2-(4-(phenyl)phenyl)pyridine (Pppy)
Pppy was prepared from 2-(4-bromophenyl)pyridine using the same procedure as was used to
synthesize MPQ (2.06 g, 69.6%).1H NMR (300 MHz, acetone-d6, δ): 8.70 (d, 1H), 8.25 (d,
13
2H), 7.98 (d, 1H), 7.90 (t, 1H), 7.78 (m, 4H), 7.49 (t, 2H), 7.41 (d,1H), 7.33 (m, 1H); C
5

ACCEPTED MANUSCRIPT
NMR (75 MHz, CDCl3, δ): 157.01, 149.73, 141.70, 140.56, 138.26, 136.82, 128.87, 127.57,
127.49, 127.34, 127.12, 122.17, 120.49.
2.2.3. Synthesis of 2-[4-(trimethylsilyl)phenyl]pyridine (TMSppy)
Under the protection of nitrogen, n-BuLi (7.7 mL, 15.4 mmol, 2.0 M solution in hexane) was
added dropwise via a syringe to 2-(4-bromophenyl)pyridine (3.0 g, 12.8 mmol) in
tetrahydrofuran at -78 oC. The mixture was stirred at -78 oC for 1 h, and chlorotrimethylsilane
(2.45 mL, 19.2 mmol) was added in one portion. The reaction mixture was allowed to warm
to room temperature overnight. After the reaction mixture was quenched with methanol, the
mixture was extracted with ethyl acetate and the organic layer was washed with water. The
organic layer was dried over MgSO4. The solvent was removed under reduced pressure to
give a crude residue. The crude product was purified by column chromatography on silica gel
1
(ethyl acetate/hexane, 1/3, v/v) to obtain TMSppy (1.86 g, 63.7%). H NMR (300 MHz,
CDCl3, δ): 7.75 (d, 1H), 7.99 (d, 2H), 7.73 (m, 2H), 7.65 (d, 2H), 7.22 (m, 1H), 0.30 (s, 9H,
CH3); 13C NMR (75 MHz, CDCl3, δ): 157.45, 149.72, 141.41, 139.70, 136.71, 133.80, 126.13,
122.16, 120.58, 1.08.
2.2.4. Synthesis of iridium(III) µ-chloro bridged dimer [Ir(MPQ)2Cl]2
A mixture of iridium(III) chloride trihydrate (2.0 g, 6.70 mmol) and MPQ (3.23 g, 14.7
mmol) in 2-ethoxyethanol/water (3/1, v/v) was stirred at reflux under a nitrogen atmosphere
for 20 h. The mixture was cooled to room temperature and the precipitate was collected and
washed with water and petroleum ether several times to afford a dark-red powder. The solid
was dried in vacuo to give the cycolmetalated Iridium(III) µ-chloro bridged dimer complex
[Ir(MPQ)2Cl]2, which was used directly in the next step, without purification.
6

ACCEPTED MANUSCRIPT
2.2.5.
Synthesis
of
bis(4-methyl-2-phenylquinoline)irdium(III)(acetylacetonate)
(MPQ)2Ir(acac)
The [Ir(MPQ)2Cl]2 dimer complex (0.5g, 0.38 mmol), sodium carbonate (0.39g, 3.76 mmol),
and acetylacetone (0.23 g, 2.26 mmol) were dissolved in 2-ethoxyethanol and refluxed under
nitrogen for 12 h. After cooling to room temperature, the crude solution was poured into
water, and extracted with dichloromethane/brine. The organic layer was dried over MgSO4,
and concentrated in vacuo. The residue was purified using column chromatography on silica
gel with dichloromethane/hexane as the eluent, and then recrystallized in dichloromethane
1
and methanol to afford the product as an orange-red solid (0.17 g, 62.8%). H NMR (300
MHz, CDCl3, δ): 8.52 (d, 2H), 7.97 (d, 2H), 7.86 (s, 2H), 7.78 (d, 2H), 7.47 (m, 4H), 6.85 (t,
2H), 6.58 (t, 2H), 7.54 (d, 2H), 4.62 (s, 1H), 2.92 (s, 6H, ArCH3), 1.54 (s, 3H, CH3), 1.49 (s,
13
3H, CH3); C NMR (75 MHz, CDCl3, δ): 185.31, 169.63, 150.88, 147.20, 145.82, 136.04,
129.94, 128.50, 127.21, 127.06, 125.56, 125.53, 123.74, 120.69, 117.40, 99.99, 97.77, 28.32,
19.31; Anal. calcd for C37H31IrN2O2: C; 61.05, H; 4.29, N; 3.85, O; 4.40. Found: C; 61.04, H;
4.29, N; 3.64; MALDI-TOF (M+1, C37H31IrN2O2): calcd for 728.20, found; 728.67; HPLC
purity : 99.6%.
2.2.6.
Synthesis
of
bis(4-methyl-2-phenylquinoline)irdium(III)[2-(4-
(phenyl)phenyl)pyridine] (MPQ)2Ir(Pppy)
A solution of silver trifluoromethanesulfonate (0.21 g, 0.83 mmol) in 2-propanol (30 mL)
was added to a solution of [Ir(MPQ)2Cl]2 dimer complex (0.5 g, 0.38 mmol) in
dichloromethane (100 mL). The mixture was stirred at room temperature for overnight. The
deep-red solution was filtered with Celite 545 to remove insoluble material and the crude
+
solution was concentrated in vacuo. The resulting solid, i.e., [Ir(MPQ)2(CH3CN)2 triflate]
(0.45 g, 0.50mmol) and Pppy (0.17 g, 0.75 mmol) were dissolved in ethanol (200 mL) and
7

ACCEPTED MANUSCRIPT
refluxed under nitrogen atmosphere for 16 h. After cooling to room temperature, the crude
solution was poured into water, and extracted with dichloromethane/brine; the organic layer
was dried over MgSO4 and concentrated in vacuo. The residue was purified using column
chromatography on silica gel with dichloromethane/hexane as the eluent, and then
recrystallized in dichloromethane and methanol to afford the product as a red solid (0.29 g,
69.1%). 1H NMR (300 MHz, CDCl3, δ): 8.21 (d, 1H), 8.07 (s, 1H), 8.03 (s, 1H), 7.88 (m, 2H),
7.81 (t, 2H),7.72 (t, 2H), 7.63 (d, 1H), 7.52 (t, 1H), 7.44 (d,1H), 7.29 (m, 3H), 6.99 (m, 3H),
13
6.92 (m, 3H), 6.75 (m, 9H), 2.82 (s, 3H, ArCH3), 2.76 (s, 3H, ArCH3); C NMR (75 MHz,
CDCl3, δ): 166.9, 166.5, 166.2, 163.8, 160.4, 157.3, 148.6, 148.4, 147.3, 146.9, 145.8, 144.5,
144.3, 144.1, 143.7, 141.8, 141.4, 140.8, 139.6, 138.3, 137.6, 136.8, 130.2, 129.6, 129.5,
129.1, 128.5, 127.9, 127.6, 127.1, 126.2, 125.6, 125.2, 124.8, 124.5, 124.3, 123.6, 122.1,
121.9, 121.1, 119.8, 119.3, 118.9, 118.5, 20.1, 19.8; Anal. Calcd for C49H36IrN3: C; 68.51, H;
4.22, N; 4.89. Found: C; 68.56, H; 4.14, N; 4.68; MALDI-TOF (M+1, C49H36IrN3): calcd for
859.25, found; 859.29. HPLC purity : 99.7%.
2.2.7. Synthesis of bis(4-methyl-2-phenylquinoline)irdium(III)(2-biphenyl-4-yl)[2-(4-
(trimethylsilyl)phenyl)pyridine] (MPQ)2Ir(TMSppy)
(MPQ)2Ir(TMSppy) was prepared from the[Ir(MPQ)2Cl]2 dimer complex by the same
1
procedure as was used to synthesize(MPQ)2Ir(Pppy) (0.28 g, 67.8%). H NMR (300 MHz,
CDCl3, δ): 8.27 (d, 1H), 8.09 (s, 1H), 7.95 (s, 1H), 7.84 (m, 5H), 7.68 (d, 2H), 7.62 (d, 1H),
7.48 (t, 1H), 7.23 (m, 2H), 6.94 (m, 2H), 6.82 (m, 2H), 6.74 (m, 4H), 6.67 (m, 3H) 2.83 (s,
13
3H, ArCH3), 2.74 (s, 3H, ArCH3), 0.04 (s, 9H, CH3). C NMR (75 MHz, CDCl3, δ): 166.5,
166.4, 166.1, 163.5, 160.9, 156.9, 148.7, 148.0, 147.8, 146.6, 145.1, 144.8, 144.3, 144.2,
143.8, 141.2, 141.0, 139.2, 137.0, 136.4, 135.5, 129.8, 129.1, 129.0, 128.2, 127.6, 127.4,
125.8, 125.7, 125.4, 124.6, 124.5, 124.1, 123.6, 121.9, 121.8, 120.0, 119.6, 118.9, 118.7,
8

ACCEPTED MANUSCRIPT
118.6, 19.3, 19.1, 1.25. Anal.Calcd for C46H40IrN3Si: C; 64.61, H; 4.71, N; 4.91. Found: C;
65.38, H; 4.71, N; 4.82. MALDI-TOF (M+1, C46H40IrN3Si): calcd for 855.26, found; 855.73
(HPLC purity : 99.6%.)
2.3. Fabrication of red and white OLEDs
2.3.1. Red Phosphorescent OLEDs Fabrication
The device structure of the red OLEDs was ITO (50 nm)/PEDOT:PSS (40 nm)/TAPC (20
nm)/mCP (10 nm)/TCTA:TPBi:red dopant (12.5:12.5:x%)/TSPO1 (35 nm)/LiF (1 nm)/Al
(200 nm). In these devices, the red dopants were (MPQ)2Ir(acac), (MPQ)2Ir(Pppy), and
(MPQ)2Ir(TMSppy); the doping ratio of TCTA to TPBi was 1:1, and the doping
concentrations of the red dopants (x%) were 5%, 10%, and 15%. Prior to use, the ITO
substrate was cleaned by sonication in distilled water and isopropanol followed by UV/ozone
treatment for 10 min. After treatment with oxygen plasma, poly(3,4-ethylenedioxythiophene)
doped with poly(styrenesulfonic acid) (PEDOT:PSS, Bayer AG, Germany) was spin-coated
onto the cleaned ITO substrates. 1,1-Bis[(di-4-tolyamino)phenyl]cyclohexane (TAPC) and
1,3-bis(N-carbazolyl)benzene (mCP), which serves as the hole-transport layer (HTL), were
deposited onto the PEDOT:PSS layer. The red emissive layer was prepared by co-
evaporating host TCTA and TPBi with x% red dopant. Next, 4-
(triphenylsilyl)phenyldiphenylphosphine oxide (TSPO1), which acts as a high triplet-energy
exciton blocking layer (HBL) with electron transport properties, was deposited on the
emissive layer. Finally, lithium fluoride (LiF) was deposited as an electron injecting layer
(EIL), and aluminum was deposited by vacuum evaporation on top of the film through a
mask at less than 2.0 × 10-6Torr.
2.3.2. White phosphorescent OLEDs Fabrication
9

ACCEPTED MANUSCRIPT
The structure of the WOLED was ITO (50 nm)/PEDOT:PSS (40 nm)/TAPC (20 nm)/mCP
(10 nm)/mCP:FIrpic (x%):Ir(ppy)3 (y%)/TCTA:TPBi:red dopant (z%)/TSPO1 (5 nm)/TPBi
(30 nm)/LiF (1 nm)/Al (200 nm). The blue, green, and red dopants in W1, W2, and W3 were
FIrpic, Ir(ppy)3, and red dopant, respectively. The optimal doping concentrations of Ir(ppy)3
(0.5%) and red dopant (15%) in W1, W2, and W3 were fixed and the doping concentration of
FIrpic in each device was controlled from 5% to 15%. The ITO treatment procedure was the
same as that for the red phosphorescent OLEDs. PEDOT:PSS was spin-coated onto the
cleaned ITO substrates and then TAPC and mCP, which act as a HTL, were deposited onto
the PEDOT:PSS layer. A blue and green light emissive layer was prepared by co-evaporating
mCP, as the host, and FIrpic and Ir(ppy)3, as blue and green dopants, respectively. The red
emissive layer was prepared by co-evaporating host TCTA and TPBi, and red dopant. Next,
TSPO1 and TPBi, which acts as a high triplet-energy exciton blocking layer with electron
transport properties, were deposited on the emissive layer. Finally, LiF was deposited as an
EIL, and aluminum was deposited by vacuum evaporation on top of the film through a mask
at less than 2.0 × 10-6Torr.
2.4. Measurements
1H and 13C NMR spectra were recorded in CDCl3 using a Varian Mercury 300 MHz (75
MHz) spectrometer. MALDI-TOF mass spectra were obtained using a ZMS-DX303 mass
spectrometer (JEOL Ltd.). HPLC was performed using a WatersTM 600 Controller with a
WatersTM 486 tunable absorbance detector. TGA was performed using an SDT Q600 V20.9
o
Build 20 instrument under a nitrogen atmosphere at a heating rate of 10 C/min. UV-visible
absorption spectra were recorded using a Shimadzu UV-3600 spectrophotometer, while PL
spectra were recorded on a Shimadzu RF 5301 PC fluorometer. CV was performed using a
CH Instruments 600D voltammetric analyzer at a potential scanning rate of 50-100 mV/s at
10

ACCEPTED MANUSCRIPT
room temperature in a dichloromethane solution containing 0.1 M tetrabutylammonium
perchlorate as the supporting electrolyte. A Ag/AgCl reference electrode, platinum wire
counter electrode, and ferrocene/ferrocenium (Fc/Fc+) internal standard were used. The J-V-L
characteristics and EL spectra of the phosphorescent OLEDs were obtained using a Keithley
2400 source measurement unit and CS 1000 spectrophotometer. All devices were fabricated
by vacuum thermal evaporation and were encapsulated with a glass lid and CaO getter before
device measurements. A Keithley 2400 source measurement unit and CS 1000
spectroradiometer were used to investigate the device performances. A Lambertian
distribution of light emission was assumed in all EQE measurements.
3. Results and Discussion
3.1. Synthesis and Thermal Properties
The heterolepticiridium(III) complexes, (MPQ)2Ir(acac), (MPQ)2Ir(Pppy), and
(MPQ)2Ir(TMSppy) were synthesized according to the procedure shown in Scheme 1. The
MPQ cyclometalated main ligand was prepared via a Suzuki coupling reaction between 2-
chloro-4-methylquinoline and phenylboronic acid. The second cyclometalated ligands, i.e.,
Pppy and TMSppy, were prepared from 2-(4-bromophenyl)pyridine and phenylboronic acid
or chlorotrimethylsilane, respectively, via a Suzuki coupling or silylation reaction.
(MPQ)2Ir(acac) was synthesized using the conventional two-step reaction reported by
Thompson’s group [22]. The Ir(III)-µ-chloro-bridged dimer was formed by reaction of the
cyclometalated main ligand MPQ with IrCl3·H2O in a mixture of 2-ethoxyethanol and water
(3:1, v/v). The Ir(III)-µ-chloro-bridged dimer was then reacted with acac in 2-ethoxyethanol
to afford (MPQ)2Ir(acac) in 63%. In the syntheses of (MPQ)2Ir(Pppy) and
(MPQ)2Ir(TMSppy), a solution of silver trifluoromethanesulfonate in 2-propanol was added
to a solution of the Ir(III)-µ-chloro-bridged dimer in dichloromethane and the mixture was
11

ACCEPTED MANUSCRIPT
o
stirred at room temperature for 12 h. It was then reacted at 80 C for 12 h with 2.2 equiv. of
Pppy or TMSppy in ethanol to give (MPQ)2Ir(Pppy) and (MPQ)2Ir(TMSppy) in yields of
69% and 68%, respectively. The synthesized iridium(III) complexes were fully characterized
1
using H and 13C NMR spectroscopies, elementary analysis, and matrix-assisted laser
desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry (see Figure S1 in the
Supporting Information). They were purified by the vacuum sublimation using a train
sublimator before device fabrication. High-performance liquid chromatography (HPLC)
analysis showed that their purities were greater than 99.6% (Figure S2).
The thermal stabilities of (MPQ)2Ir(acac), (MPQ)2Ir(Pppy), and (MPQ)2Ir(TMSppy) were
investigated using thermogravimetric analysis (TGA). The 5% weight-loss temperatures (Td)
o
of (MPQ)2Ir(acac), (MPQ)2Ir(Pppy), and (MPQ)2Ir(TMSppy) were 367, 415, and 406 C,
respectively (Figure S3). The Td values show that the iridium(III) complexes with Pppy or
TMSppy as the second cyclometalated ligand are thermally more stable than the
corresponding iridium(III) complex containing acac as the ancillary ligand.
Scheme 1. Synthesis and chemical structures of (MPQ)2Ir(acac), (MPQ)2Ir(Pppy),and
(MPQ)2Ir(TMSppy).
12

ACCEPTED MANUSCRIPT
3.2. X-ray Crystallography
The structures of (MPQ)2Ir(acac), (MPQ)2Ir(Pppy), and (MPQ)2Ir(TMSppy)were
investigated using single-crystal X-ray crystallography. Figure 1 and Table 1 shows the
single crystal and crystal packing structure of (MPQ)2Ir(acac), (MPQ)2Ir(Pppy), and
(MPQ)2Ir(TMSppy) as well as the selected bond lengths. The crystal structures of the
iridium(III) complexes with acac or 2-phenylpyridine-based second cyclometalated ligands
show different configurations with distorted octahedral geometries around the iridium(III)
atom. The geometrical configuration of the heteroleptic complex (MPQ)2Ir(acac) is N,N-trans,
whereas (MPQ)2Ir(Pppy) and (MPQ)2Ir(TMSppy) have facial geometries. All the σ-donor
carbon atoms in (MPQ)2Ir(Pppy) (C11, C33, and C49) and (MPQ)2Ir(TMSppy) (C16, C32,
and C40) are located trans to nitrogen atoms (N1, N2, and N3) in both the cyclometalated
main ligand and second cyclometalated ligands. The lengths of Ir-C bonds between the
iridium atom and two cyclometalated main ligands in (MPQ)2Ir(Pppy) [Ir(1)-C(33); 1.985(8)
Å, Ir(1)-C(49); 2.000(8) Å] and (MPQ)2Ir(TMSppy) [Ir(1)-C(16); 2.012(4) Å, Ir(1)-C(32);
2.007(4) Å] are slightly longer than those in (MPQ)2Ir(acac) [Ir(1)-C(16); 1.978(5) Å, Ir(1)-
C(27); 1.973(5) Å]. Also, the lengths of the Ir-N bonds in (MPQ)2Ir(Pppy) [Ir(1)-N(2);
2.181(6) Å, Ir(1)-N(3); 2.224(7) Å] and (MPQ)2Ir(TMSppy) [Ir(1)-N(1); 2.236(3) Å, Ir(1)-
N(2); 2.198(3) Å] are also relatively longer than that in (MPQ)2Ir(acac) [Ir(1)-N(1); 2.063(4)
Å, Ir(1)-N(2); 2.069(4) Å]; this could be because the steric hindrance effects of the second
cyclometalated ligand spacers with relatively bulky Pppy or TMSppy groups are greater than
that of the acac ancillary ligand. The Ir-C and Ir-N bonds between the iridium atom and
second cyclometalated ligands in (MPQ)2Ir(Pppy) [Ir(1)-C(11); 2.015(8) Å, Ir(1)-N(1);
13

ACCEPTED MANUSCRIPT
2.142(6) Å] and (MPQ)2Ir(TMSppy) [Ir(1)-C(40); 2.009(4) Å, Ir(1)-N(3); 2.145(3) Å] were
similar.
The distances between the two nearest iridium atoms in the crystal-packing structures of
(MPQ)2Ir(acac), (MPQ)2Ir(Pppy), and (MPQ)2Ir(TMSppy) are 10.80, 15.32, and 10.94 Å,
respectively. The distances between the two nearest parallel MPQ planes in (MPQ)2Ir(acac),
(MPQ)2Ir(Pppy), and (MPQ)2Ir(TMSppy) are 3.43, 9.74, and 3.49 Å, respectively. In
particular, the distance between the two nearest iridium atoms and two nearest parallel MPQ
planes in (MPQ)2Ir(Pppy) are relatively longer than those in (MPQ)2Ir(acac) and
(MPQ)2Ir(TMSppy). These results indicate that (MPQ)2Ir(Pppy) has weak intermolecular π-π
interactions between the two nearest iridium atoms and two nearest MPQ ligands, which
could minimize bimolecular interactions by sterically hindered Pppy spacers. Minimum
bimolecular interactions between iridium(III) complexes can generally prevent self-
quenching in phosphorescence [25]. Complete crystal data (cif) for (MPQ)2Ir(acac),
(MPQ)2Ir(Pppy), and (MPQ)2Ir(TMSppy) can be found in Supporting Information.
3.3. Photophysical Properties
The photophysical properties of the iridium(III) complexes with second cyclometalated
ligands were investigated using UV-visible absorption and PL spectroscopy in dilute toluene
solution at 298 K. The UV-visible absorption and PL spectra are shown in Figure 2, and the
results are summarized in Table 2. The intense absorption peaks in the short wavelength
region below 340 nm in the spectra of all the synthesized iridium(III) complexes are
attributed to spin allowed π-π* transitions, which closely resembles the corresponding
transition in the MPQ cyclometalated main ligand. (MPQ)2Ir(acac) has absorption bands with
low extinction coefficients in the range 350-600 nm; these are assigned to singlet and triplet
14

ACCEPTED MANUSCRIPT
3
1
metal-to-ligand charge transfer (1MLCT and MLCT) states. The MLCT/3MLCT bands of
the iridium(III) complexes with second cyclometalated ligands are highly and clearly
separated, at 398/446 nm for (MPQ)2Ir(Pppy) and 402/450 nm for (MPQ)2Ir(TMSppy).
(MPQ)2Ir(Pppy) and (MPQ)2Ir(TMSppy) show a high and prominent absorption band above
350 nm, from efficient MLCT. The energy band gaps (Eg) of (MPQ)2Ir(acac),
(MPQ)2Ir(Pppy), and (MPQ)2Ir(TMSppy) were calculated to be 2.09, 2.14, and 2.12 eV,
respectively, from the UV-visible absorption edge.
The maximum PL emission peaks of (MPQ)2Ir(acac), (MPQ)2Ir(Pppy), and
(MPQ)2Ir(TMSppy) occur at 589, 597, and 597 nm, respectively, in dilute toluene solution at
298 K. The maximum PL emission peaks from (MPQ)2Ir(Pppy) and (MPQ)2Ir(TMSppy) are
red-shifted compared with that of (MPQ)2Ir(acac). Also, red-shifts of the maximum PL
emission peaks from solution to the neat film state were observed for (MPQ)2Ir(acac) (595
nm), (MPQ)2Ir(Pppy) (601 nm), and (MPQ)2Ir(TMSppy) (602 nm); this could be caused by
aggregation effects of the iridium(III) complexes.
The maximum low-temperature PL emission peaks of (MPQ)2Ir(acac), (MPQ)2Ir(Pppy), and
(MPQ)2Ir(TMSppy) in dilute toluene solution at 77 K are at 580, 576, and 578 nm,
respectively (Figure S4). The maximum emissions of (MPQ)2Ir(acac), (MPQ)2Ir(Pppy), and
(MPQ)2Ir(TMSppy) at 77 K are hypsochromically shifted by 9, 19, and 18 nm compared to
those of a solution at 298 K, because of the rigidochromic effect [27-29]. Also, the maximum
emission wavelength at 77 K for (MPQ)2Ir(Pppy) and (MPQ)2Ir(TMSppy) are relatively
more hypsochromically shifted than that of (MPQ)2Ir(acac), because of the steric effects of
the relatively bulky Pppy and TMSppy groups. The triplet energies (T1) of (MPQ)2Ir(acac),
(MPQ)2Ir(Pppy), and (MPQ)2Ir(TMSppy) are 2.14, 2.13, and 2.13 eV, respectively,
determined from their emission spectra at 77 K, suggesting that the T1 energies of the
synthesized iridium(III) complexes mainly depend on the cyclometalated MPQ ligand.
15

ACCEPTED MANUSCRIPT
The PL quantum yields of the synthesized iridium(III) complexes in dilute degassed 2-
methyltetrahydrofuran solution were compared with that of (pq)2Ir(acac) as a standard (ФPL
=
0.10) [30]. The measured ФPL values of (MPQ)2Ir(acac), (MPQ)2Ir(Pppy), and
(MPQ)2Ir(TMSppy) were 0.11, 0.16, and 0.13, respectively. The higher ФPL values of
(MPQ)2Ir(Pppy) and (MPQ)2Ir(TMSppy) compared with those of (MPQ)2Ir(acac) and
(PQ)2Ir(acac) suggest that the second cyclometalated ligands are better than the acac ancillary
ligand at enhancing the PL quantum efficiencies of the red emitters. In particular, Pppy is an
excellent candidate as the second cyclometalated ligand for highly luminescent iridium(III)
complexes.
3.4. Theoretical Calculations
We performed theoretical calculations based on density functional theory (DFT), using the
B3LYP hybrid exchange-correlation functional and 6-31G(d) basis sets, to improve our
understanding of the effects of Pppy and TMSppy second cyclometalated ligands in
iridium(III) complexes. The calculations were conducted using the Gaussian 09 package [31].
Figure 3 shows the electron distributions and calculated energy levels of the highest occupied
molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) for
(MPQ)2Ir(acac), (MPQ)2Ir(Pppy), and (MPQ)2Ir(TMSppy). The electron densities in the
HOMO of (MPQ)2Ir(acac) are mainly distributed over the phenyl ring in MPQ and the
iridium atom, and the LUMO is mostly distributed over the quinoline moiety. In most cases,
the acac ancillary ligand in the iridium(III) complex does not affect the HOMO and LUMO
distributions. However, in the case of (MPQ)2Ir(Pppy) and (MPQ)2Ir(TMSppy), the electron
densities in the HOMOs are mainly localized on the phenyl rings of the MPQ moieties,
iridium atoms, and phenyl units in the second cyclometalated ligands (Pppy and TMSppy). In
contrast, the electron densities in the LUMOs are mainly distributed over the quinoline
16

ACCEPTED MANUSCRIPT
moiety, as in (MPQ)2Ir(acac). The theoretical calculations show that the Pppy and TMSppy
second cyclometalated ligands do not contribute to the LUMOs, but selectively contribute to
the HOMOs of the iridium(III) complexes.
The calculated HOMO/LUMO energy levels of (MPQ)2Ir(acac), (MPQ)2Ir(Pppy), and
(MPQ)2Ir(TMSppy) were -4.84/-1.65, -4.80/-1.59, and -4.79/-1.58 eV, respectively. The
HOMO and LUMO energy levels of the iridium(III) complexes containing second
cyclometalated ligands were slightly higher than those of the iridium(III) complex containing
the acac ancillary ligand.
3.5. Electrochemical Properties
The electrochemical behaviors in dichloromethane of the synthesized iridium(III) complexes
were investigated using cyclic voltammetry; the results are shown in Figure S5.
(MPQ)2Ir(acac), (MPQ)2Ir(Pppy), and (MPQ)2Ir(TMSppy)show reversible electron
oxidations at positive potentials in the region 0.40-0.60 V. The onset potentials of the first
oxidations (Eoxionset) of (MPQ)2Ir(acac), (MPQ)2Ir(Pppy), and (MPQ)2Ir(TMSppy) are at 0.49,
0.44, and 0.41 V (vs Ag/Ag+), respectively. The Eoxi
values of (MPQ)2Ir(Pppy) and
onset
(MPQ)2Ir(TMSppy) are clearly lower than that of the parent complex (MPQ)2Ir(acac). The
HOMO energy levels of (MPQ)2Ir(acac), (MPQ)2Ir(Pppy), and (MPQ)2Ir(TMSppy) are -5.21,
-5.16, and -5.13 eV, respectively. The HOMO energy levels of the iridium(III) complexes
containing second cyclometalated ligands are slightly higher than those of (MPQ)2Ir(acac).
The LUMO energy levels of (MPQ)2Ir(acac), (MPQ)2Ir(Pppy), and (MPQ)2Ir(TMSppy) are -
3.12, -3.02, and -3.01 eV, respectively, based on a combination of the HOMO energy levels
and optical band gaps. These HOMO and LUMO energy levels for the iridium(III) complexes
are consistent with the theoretically calculated results.
17

ACCEPTED MANUSCRIPT
These HOMO and LUMO energy levels are well matched with those of tris(4-carbazoyl-9-
ylphenyl)amine (TCTA; HOMO:–6.0 eV, LUMO: –2.70 eV) and 1,3,5-tris(1-phenyl-1H-
benzimidazol-2-yl)benzene (TPBi; HOMO: –6.70 eV, LUMO: –2.70 eV) as host materials.
The HOMO and LUMO energy levels of the iridium(III) complexes are summarized in Table
2.
3.6. Electrophosphorescent OLEDs
The performances of the synthesized iridium(III) complexes in red phosphorescent OLEDs
were evaluated by fabricating multi-layered devices with the structure ITO (50
nm)/PEDOT:PSS (40 nm)/TAPC (20 nm)/mCP (10 nm)/TCTA:TPBi:red dopant
(12.5:12.5:x%)/TSPO1 (35 nm)/LiF (1 nm)/Al (200 nm). We fabricated devices using TCTA
and TPBi as the mixed-hosts in the emissive layer for efficient carrier injection and charge
balance [32]. Doping concentrations of 5% for (MPQ)2Ir(acac), 15% for (MPQ)2Ir(Pppy),
and 15% for (MPQ)2Ir(TMSppy) gave the optimal performances. The doping concentration
of iridium(III) complexes with Pppy or TMSppy second cyclometalated ligands were higher
than that of the iridium(III) complex containing an acac ancillary ligand. It has been known
that the thermally evaporated iridium complex host would exist as an aggregated form rather
than a discrete molecule in the host-guest thin film even at low doping concentration [33]. So
the above results suggest that the introduction of relatively bulky Pppy or TMSppy groups
into the iridium(III) complex effectively suppresses concentration self-quenching especially
at high doping concentrations. The various doping concentrations and detailed device
characteristics of (MPQ)2Ir(acac), (MPQ)2Ir(Pppy), and (MPQ)2Ir(TMSppy) are summarized
in Table 3.
18

ACCEPTED MANUSCRIPT
Figure 4a shows that the optimized red phosphorescent OLEDs fabricated using
(MPQ)2Ir(acac), (MPQ)2Ir(Pppy), and (MPQ)2Ir(TMSppy) have electroluminescent (EL)
maximum emission peaks at 592, 600, and 600 nm, respectively. The maximum emission
peaks of the (MPQ)2Ir(acac), (MPQ)2Ir(Pppy), and (MPQ)2Ir(TMSppy) devices red-shift
slightly as the doping concentration increases from 5% to 15%; this may be the result of
increased dopants aggregation and strong intermolecular interactions at high doping
concentration (Figure S6). The red-shifts of the maximum emission peaks at high doping
concentrations of all the synthesized iridium(III) complexes are consistent with the
aggregation effect observed for neat films, described in photophysical properties. The
Commission Internationale de L’Eclairage (CIE) coordinates of the (MPQ)2Ir(acac),
(MPQ)2Ir(Pppy), and (MPQ)2Ir(TMSppy) devices at the optimal doping concentration are
observed to be (0.59,0.41), (0.60,0.40), and (0.58,0.41), respectively, at 1,000 cd/m2.
Figure 4b shows the current density-voltage-luminance (J-V-L) curves of the red
phosphorescent OLEDs. The current density of the OLEDs increased with increases doping
concentration of (MPQ)2Ir(Pppy) and (MPQ)2Ir(TMSppy), due to improved charge hopping
at high doping concentrations [34]. The turn-on voltage (Vturn-on; defined as the bias at a
luminance of 1 cd/m2) and maximum luminance (Lmax) of the (MPQ)2Ir(acac) device are 4.5
V and 5,235 cd/m2, respectively. On the other hand, (MPQ)2Ir(Pppy) and (MPQ)2Ir(TMSppy)
dramatically change the device performances of red phosphorescent OLEDs. The Vturn-on and
Lmax values of the iridium(III) complexes with second cyclometalated ligands are 4.0 V and
14,690 cd/m2, respectively, for the (MPQ)2Ir(Pppy) device, and 3.5 V and 12,220 cd/m2 for
the (MPQ)2Ir(TMSppy) device.
Figure 4c shows the external quantum efficiency-L-current efficiency (EQE-L-CE) curves of
red phosphorescent OLEDs. The maximum external quantum efficiency (EQEmax)/maximum
current efficiency (CEmax) of the (MPQ)2Ir(acac) device is 11.1%/20.4 cd/A, and those of the
19

ACCEPTED MANUSCRIPT
(MPQ)2Ir(Pppy) and (MPQ)2Ir(TMSppy) devices are 21.4%/35.5 cd/A and 18.4%/31.8 cd/A,
respectively. In particular, the EQEmax and CEmax values of the (MPQ)2Ir(Pppy) device are
higher than those of (MPQ)2Ir(acac) and (MPQ)2Ir(TMSppy). These results show that the
iridium(III) complex with a Pppy second cyclometalated ligand significantly improves the
EQE and CE compared with the corresponding acac ancillary ligand and TMSppy second
cyclometalated ligand. These results can be explained by significant weak intermolecular π-π
interactions for (MPQ)2Ir(Pppy), as described in the X-ray crystallography measurement. The
weak intermolecular π-π interactions of (MPQ)2Ir(Pppy) can effectively prevent
concentration self-quenching at high doping concentration and suppress triplet-triplet
annihilation in the device [25]. Relatively high ФPL of the (MPQ)2Ir(Pppy) emitter compared
with those of the other emitters is another factor for the high EQE of the (MPQ)2Ir(Pppy)
device. The device characteristics at other doping concentrations of (MPQ)2Ir(acac),
(MPQ)2Ir(Pppy), and (MPQ)2Ir(TMSppy) are shown in Figure S7 and S8.
The (MPQ)2Ir(Pppy) emitter, which displayed broad FWHM and high EQE values in red
phosphorescent OLEDs, is an appropriate red emitter for the fabrication of all-
phosphorescent three primary color component WOLED with high EQE and high CRI. We
developed WOLED that incorporated FIrpic, Ir(ppy)3, and (MPQ)2Ir(Pppy) as emissive
dopants at different doping concentrations. The three device structures, i.e., W1, W2, and W3,
were ITO (50 nm)/PEDOT:PSS (40 nm)/TAPC (20 nm)/mCP (10 nm)/mCP:FIrpic (12.5 nm,
x = 5% for W1, 10% for W2, 15% for W3):Ir(ppy)3 (0.5%)/TCTA:TPBi:(MPQ)2Ir(Pppy)
(7.25 nm, 15%)/TSPO1 (5 nm)/TPBi (30 nm)/LiF (1 nm)/Al (200 nm). The doping
concentrations of Ir(ppy)3 (0.5%) and (MPQ)2Ir(Pppy) (15%) in all white devices were fixed,
and the doping concentration of FIrpic in each device was controlled at 5%, 10% and 15% for
W1, W2 and W3, respectively. Figure 5a shows the EL spectra of the WOLED fabricated
with (MPQ)2Ir(Pppy). The white EL spectra show blue, green, and red emission peaks at 473,
20

ACCEPTED MANUSCRIPT
505, and 600 nm, respectively. These three emission peaks well match the peak wavelengths
of FIrpic, Ir(ppy)3, and (MPQ)2Ir(Pppy), respectively. The CIE coordinates of W1, W2, and
W3 are (0.47,0.42), (0.46,0.43), and (0.44,0.44), respectively, at 1,000 cd/m2, and are close to
the CIE coordinates of standard warm white light (0.44,0.40). The correlated color
temperature (CCT) values of W1, W2, and W3 are 2605, 2932, and 3283 K, respectively, at
1,000 cd/m2. Devices W1, W2, and W3 showed intense and broad white emissions with CRI
of 78, 81, and 81, respectively. The CRI values of the WOLED device with (MPQ)2Ir(Pppy)
are therefore greater than the required CRI value of at least 75 for a WOLED to be used in
general solid-state lighting application [35]. The J-V-L and EQE-L-power efficiency (EQE-
L-PE) characteristics of the WOLED are shown in Figure 5b and 5c, and details of the
performances of the EL devices are summarized in Table 4. The Vturn-on values of the W1,
W2, and W3 devices are 6.0, 5.5, and 5.5 V, and the Lmax values are 28,040, 34,500, and
38,320 cd/m2, respectively. The EQE and PE values are the most important requirements of
WOLED for lighting applications. The EQEmax and maximum power efficiency (PEmax
)
values of W1, W2, and W3 are 22.6%/19.5 lm/W, 26.1%/23.2 lm/W, and 28.0%/26.0 lm/W,
respectively. An EQEmax value of 28.0% was obtained for the W3 device. Table 5 shows the
EQEmax and CIE coordinates of this work as well as those of other WOLEDs reported in the
literature [17, 36-43]. The EQEmax of the W3 device is the highest value among the reported
WOLEDs fabricated using three primary color components.
The WOLED color stability is also important in real applications. We investigated the color
stability, by observing the EL spectra and CIE coordinates of W1-W3 at various driving
voltages from 8 to 12 V. As can be seen from Figure S10, the white EL spectra of the
WOLED were not significantly different at different driving voltages. Changes in the CIE
coordinates of W1-W3 at various driving voltages were also investigated, as shown in Figure
6. The CIE-X and CIE-Y values of W1-W3 with increasing driving voltage from 8 to 12 V
21

ACCEPTED MANUSCRIPT
were similar. Especially, the CIE coordinates of W3 (0.44, 0.44) were unchanged at different
driving voltages.
4. Conclusions
In conclusion, red phosphorescent iridium(III) complexes, (MPQ)2Ir(Pppy) and
(MPQ)2Ir(TMSppy), were designed and successfully synthesized to investigate the effect of
second cyclometalated Pppy and TMSppy ligands on their use in phosphorescent OLEDs.
The devices fabricated using (MPQ)2Ir(Pppy) and (MPQ)2Ir(TMSppy) exhibited EQE of
21.4% and 18.4%, respectively, at 15% doping concentration. Especially, the second
cyclometalated Pppy ligand prevented concentration self-quenching and suppressed triplet-
triplet annihilation at high doping concentration. The WOLED fabricated using
(MPQ)2Ir(Pppy) exhibited the highest EQE of 28.0%. Especially, CIE-X and CIE-Y values at
different driving voltages of the WOLED fabricated with (MPQ)2Ir(Pppy) showed excellent
spectral stability.
Acknowledgements
This work was supported by the Ministry of Trade, Industry and Energy (MOTIE, Korea)
under Industrial Technology Innovation Program (No. 10067715), and National Research
Foundation
(NRF)
grants,
funded
by
the
Korean
government
(NRF-
2014R1A2A2A01007318 and No. 2011-0030013 through GCRC SOP).
References
[1] Forrest SR. The path to ubiquitous and low-cost organic electronic appliances on plastic.
Nature 2004;428:911-8.
22

ACCEPTED MANUSCRIPT
[2] Baldo MA, O’Brien DF, You Y, Shoustikov A, Sibley S, Thompson ME, Forrest SR.
Highly efficient phosphorescent emission from organic electroluminescent devices.
Nature 1988;395:151-4.
[3] Ouyang X, Li XL, Ai L, Mi D, Ge Z, Su SJ. Novel “hot exciton” blue fluorophores for
high performance fluorescent/phosphorescent hybrid white organic light-emitting diodes
with superhigh phosphorescent dopant concentration and improved efficiency roll-off.
ACS Appl Mater Interfaces 2015;7;7869-77.
[4] Li XL, Ouyang X, Liu M, Ge Z, Peng J, Cao Y, Su SJ. Highly efficient single- and multi-
emission-layer fluorescent/phosphorescent hybrid white organic light-emitting diodes
with ∼20% external quantum efficiency. J Mater Chem C 2015;3;9233-9.
[5] 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;3259-302.
[6] Farinola GM, Ragni R. Electroluminescent materials for white organic light emitting
diodes. Chem Soc Rev 2011;40:3467-82.
[7] Sasabe H, Kido J. Development of high performance OLEDs for general lighting. J
Mater Chem C 2013;1:1699-707.
[8] D’Andrade BW, Forrest SR. White organic light-emitting devices for solid-state lighting.
Adv Mater 2004;16:1585-95.
[9] Wang Q, Ding J, Ma D, Cheng Y, Wang L, Jing X, Wang F. Harvesting excitons via two
parallel channels for efficient white organic LEDs with nearly 100% internal quantum
efficiency: fabrication and emission-mechanism analysis. Adv Funct Mater 2009;19: 84-
95.
[10]Wu H, Ying L, Yang W, Cao Y. Progress and perspective of polymer white light-
emitting devices and materials. Chem Soc Rev 2009;38:3391-400.
23

ACCEPTED MANUSCRIPT
[11]Zou J, Wu H, Lam CS, Wang C, Zhu J, Zhong C, Hu S, Ho CL, Zhou GJ, Wu H, Choy
WCH, Peng J, Cao Y, Wong WY. Simultaneous optimization of charge-carrier balance
and luminous efficacy in highly efficient white polymer light-emitting devices. Adv
Mater 2011;23:2976-80.
[12]Sun Y, Forrest SR. High-efficiency white organic light emitting devices with three
separate phosphorescent emission layers. Appl Phys Lett 2007;91:263503/1-3.
[13] Han C, Zhu L, Li J, Zhao F, Zhang Z, Xu H, Deng Z, Ma D, Yan P. Highly efficient
multifluorenyl host materials with unsymmetrical molecular configurations and localized
triplet states for green and red phosphorescent devices. Adv Mater 2014;26;7070-7.
[14] Han F, Zhang X, Zhang J, Wei Y, Zhang X, Huang W, Xu H. 3D-Encapsulated iridium-
complexed nanophosphors for highly efficient host-free organic light-emitting diodes.
Chem Commun 2016;52;5183-6.
[15]Chang CH, Cheng HC, Lu YJ, Tien KC, Lin HW, Lin CL, Yang CJ, Wu CC. Enhancing
color gamut of white OLED displays by using microcavity green pixels. Org Electron
2010;11:247-54.
[16]Chang YL, Song Y, Wang Z, Helander MG, Qiu J, Chai L, Liu Z, Scholes GD, Lu Z.
Highly efficient warm white organic light-emitting diodes by triplet exciton conversion.
Adv Funct Mater 2013;23:705-12.
[17]Zhao Y, Chen J, Ma D. Ultrathin nondoped emissive layers for efficient and simple
monochrome and white organic light-emitting diodes. ACS Appl Mater Interfaces
2013;5:965-71.
[18]Kim YH, Cheah KW, Kim YY. High efficient white organic light-emitting diodes with
single emissive layer using phosphorescent red, green, and blue dopants. Appl Phys Lett
2013;103:053307/1-4.
24

ACCEPTED MANUSCRIPT
[19]Wang Q, Ma D, Leo K, Ding J, Wang L, Qiao Q, Jia H, Gnade BE. Using interlayer
step-wise triplet transfer to achieve an efficient white organic light-emitting diode with
high color-stability. Appl Phys Lett 2014;104:193303.
[20]Ye J, Chen Z, An F, Sun M, Mo HW, Zhang X, Lee CS. Achieving highly efficient
simple-emission layer fluorescence/phosphorescence hybrid white organic light-emitting
devices via effective confinement of triplets. ACS Appl Mater Interfaces 2014;6:8964-70.
[21]Su SJ, Gonmori E, Sasabe H, Kido J. Highly efficient organic blue-and white-light-
emitting devices having a carrier- and exciton-confining structure for reduced efficiency
roll-off. Adv Mater 2008;20:4189-94.
[22]Lamansky S, Djurovich P, Murphy D, Abdel-Razzaq F, Lee HE, Adachi C, Burrows PE,
Forrest SR, Thompson ME. Highly phosphorescent bis-cyclometalated iridium
complexes:ꢀ synthesis, photophysical characterization, and use in organic light emitting
diodes. J Am Chem Soc 2001;123:4304-12.
[23]Reineke S, Schwartz G, Walzer K, Falke M, Leo K. Highly phosphorescent organic
mixed films: The effect of aggregation on triplet-triplet annihilation. Appl Phys Lett
2009;94:163305/1-3.
[24] Baldo MA, Adachi C, Forrest SR. Transient analysis of organic electrophosphorescence.
II. Transient analysis of triplet-triplet annihilation. Phys Rev B 2000;62:10967-77.
[25]Xie HZ, Liu MW, Wang OY, Zhang XH, Lee CS, Hung LS, Lee ST, Teng PF, Kwong
HL, Zheng H, Che CM. Reduction of self-quenching effect in organic
electrophosphorescence emitting devices via the use of sterically hindered spacers in
phosphorescence molecules. Adv Mater 2001;13:1245-48.
[26]Baldo MA, Lamansky S, Burrows PE, Thompson ME, Forrest SR. Very high-efficiency
green organic light-emitting devices based on electrophosphorescence. Appl Phys Lett
1999;75:4/1-4.
25

ACCEPTED MANUSCRIPT
[27]Frin KM, Iha NYM. Photoinduced isomerization and luminescence of fac-
[Re(CO)3(ph2phen)(bpe)]+. J Braz Chem Soc 2006;17:1664-71.
[28]Chen P, Meyer TJ. Medium effects on charge transfer in metal complexes. Chem Rev
1998;98:1439-77.
[29]Karatsu T, Takahashi M, Yagai S, Kitamura A. Photophysical properties of substituted
homoleptic and heteroleptic phenylimidazolinato Ir(III) complexes as a blue
phosphorescent material. Inorg Chem 2013;52:12338-50.
[30] Kim DH, Cho NS, Oh HY, Yang JH, Jeon WS, Park JS, Suh MC, Kwon JH. Highly
efficient red phosphorescent dopants in organic light-emitting devices. Adv Mater
2011;23:2721-6.
[31]Frisch MJ, Trucks GW, Schlegel HB, Scuseria GE, Robb MA, Cheeseman JR, Scalmani
G, Barone V, Mennucci B, Petersson GA, Nakatsuji H, Caricato M, Li X, Hratchian HP,
Izmaylov AF, Bloino J, Zheng G, Sonnenberg JL, Hada M, Ehara M, Toyota K, Fukuda
R, Hasegawa J, Ishida M, Nakajima T, Honda Y, Kitao O, Nakai H, Vreven T,
Montgomery JJA, Peralta JE, Ogliaro F, Bearpark M, Heyd JJ, Brothers E, Kudin KN,
Staroverov VN, Kobayashi R, Normand J, Raghavachari K, Rendell A, Burant JC,
Iyengar SS, Tomasi J, Cossi M, Rega N, Millam JM, Klene M, Knox JE, Cross JB,
Bakken V, Adamo C, Jaramillo J, Gomperts R, Stratmann RE, Yazyev O, Austin AJ,
Cammi R, Pomelli C, Ochterski JW, Martin RL, Morokuma K, Zakrzewski VG, Voth
GA, Salvador P, Dannenberg JJ, Dapprich S, Daniels AD, Farkas Ö, Foresman JB, Ortiz
JV, Cioslowski J, Fox DJ, Gaussian 09, Gaussian, Inc. Wallingford CT 2009.
[32]Seo JA, Gong MS, Lee JY. High efficiency yellowish green phosphorescent emitter
derived from phenylbenzothienopyridine ligand. Org Electron 2014;15: 2068-72.
26

ACCEPTED MANUSCRIPT
[33] Reineke S, Schwartz G, Walzer K, Falke M, Leo K. Highly phosphorescent organic
mixed films: The effect of aggregation on triplet-triplet annihilation. Appl Phys Lett
2009;94:163305/1-3.
[34]Park J, Oh H, Oh S, Kim J, Park HJ, Kim OY, Lee JY, Kang Y. Deep blue
phosphorescent organic light-emitting diodes with excellent external quantum efficiency.
Org Electron 2013;14:3228-33.
[35]Son YH, Kim SH, Kwon JH. All-phosphorescent three-color two-stack tandem white
organic light emitting diodes with high-color-rendering index values. J Inf Disp
2014;15:185-9.
[36]Gong S, Chen Y, Luo J, Yang C, Zhong C, Qin J, Ma D. Bipolar tetraarylsilanes as
universal hosts for blue, green, orange, and white electrophosphorescence with high
efficiency and low efficiency roll-off. Adv Funct Mater 2011;21:1168-78.
[37] Chou HH, Li YK, Chen YH, Chang CC, Liao CY, Cheng CH. New iridium dopants for
white phosphorescent devices: enhancement of efficiency and color stability by an
energy-harvesting layer. ACS Appl Mater Interfaces 2013;5:6168-75.
[38] Pan B, Wang B, Wang Y, Xu P, Wang L, Chen J, Ma D. A simple carbazole-N-
benzimidazole bipolar host material for highly efficient blue and single layer white
phosphorescent organic light-emitting diodes. J Mater Chem C 2014;2:2466-9.
[39] Wang R, Liu D, Ren H, Zhang T, Yin H, Liu G, Li J. Highly efficient orange and white
organic light-emitting diodes based on new orange iridium complexes. Adv Mater
2011;23:2823-7.
[40] Cui LS, Liu Y, Liu XY, Jiang ZQ, Liao LS. Design and synthesis of pyrimidine-based
iridium(III) complexes with horizontal orientation for orange and white phosphorescent
OLEDs. ACS Appl Mater Interfaces 2015;7:11007-14.
27

ACCEPTED MANUSCRIPT
[41] Guo LY, Zhang XL, Wang HS, Liu C, Li ZG, Liao ZJ, Mi BX, Zhou XH, Zheng C, Li
YH, Gao ZQ. 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:
5412-8.
[42] Xie YM, Cui LS, Liu Y, Zu FS, Li Q, Jiang ZQ, Liao LS. Efficient blue/white
phosphorescent organic light-emitting diodes based on a silicon-based host material via a
direct carbon-nitrogen bond. J Mater Chem C 2015;3:5347-53.
[43] Kim HU, Jang JH, Song W, Jung BJ, Lee JY, Hwang DH. Improved luminance and
external quantum efficiency of red and white organic light-emitting diodes with
iridium(III) complexes with phenyl-substituted 2-phenylpyridine as a second
cyclometalated ligand. J Mater Chem C 2015;3:12107-15.
28

ACCEPTED MANUSCRIPT
Graphical Abstract
29