Green phosphorescent homoleptic iridium(III) complexes for highly efficient organic light-emitting diodes

Article abstract of DOI:10.1016/j.dyepig.2018.04.025

Homoleptic Ir(III) complexes, Ir(ppyTMS)₃ and Ir(mPppyTMS)₃, based on 2-phenyl-5-(trimethylsilyl)pyridine (ppyTMS) and 2-(1,1′-biphenyl-3′-yl)-5-(trimethylsilyl)pyridine (mPppyTMS) as cyclometalated ligands, respectively, were synthesized for highly efficient green phosphorescent organic light-emitting diodes (OLEDs). The trimethylsilyl and phenyl groups introduced on the 2-phenylpyridine ligand suppressed the intermolecular interactions and the triplet-triplet annihilation process taking place via molecular aggregation, which otherwise decrease the OLED efficiency. The green phosphorescent OLEDs doped with Ir(ppyTMS)₃ and Ir(mPppyTMS)₃ as green emitters exhibited maximum electroluminescent wavelengths of 525 and 529 nm, respectively, at an optimized doping concentration of 5%. The Commission Internationale de L'Eclairage coordinates of these OLEDs were (0.35,0.62) and (0.37,0.61), respectively, at a luminance of 1000 cd m^?2. The maximum external quantum efficiency and maximum power efficiency (PE_max) were 16.6%/66.1 lm W^?1 for the Ir(ppyTMS)₃ device and 18.1%/70.3 lm W^?1 for the Ir(mPppyTMS)₃ device, which were higher than those of Ir(ppy)₃ without substituents on the 2-phenylpyridine ligand. Moreover, the PE_max value of the Ir(mPppyTMS)₃ device is one of the highest values among the reported devices fabricated using homoleptic Ir(III) complexes for green phosphorescent OLEDs.

Full text of DOI:10.1016/j.dyepig.2018.04.025

Accepted Manuscript
Green phosphorescent homoleptic iridium(III) complexes for highly efficient organic
light-emitting diodes
Hee Un Kim, Hea Jung Park, Jae-Ho Jang, Wook Song, In Hwan Jung, Jun Yeob
Lee, Do-Hoon Hwang
PII:
S0143-7208(18)30416-9
10.1016/j.dyepig.2018.04.025
DYPI 6686
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 24 February 2018
Revised Date: 12 April 2018
Accepted Date: 13 April 2018
Please cite this article as: Kim HU, Park HJ, Jang J-H, Song W, Jung IH, Lee JY, Hwang D-H, Green
phosphorescent homoleptic iridium(III) complexes for highly efficient organic light-emitting diodes, Dyes
and Pigments (2018), doi: 10.1016/j.dyepig.2018.04.025.
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
Green Phosphorescent Homoleptic Iridium(III) Complexes for
Highly Efficient Organic Light-Emitting Diodes
Hee Un Kim,a,† Hea Jung Park,a,† Jae-Ho Jang,a Wook Song,b In Hwan Jung,c
Jun Yeob Lee, *,b and Do-Hoon Hwang*,a
aDepartment of Chemistry and Chemistry Institute for Functional Materials, Pusan National
University, Busan 609-735, Republic of Korea
bSchool of Chemical Engineering, Sungkyunkwan University 2066, Seobu-ro, Jangan-gu,
Suwon, Gyeonggi 440-746, Republic of Korea
cDepartment of Chemistry, Kookmin University, 77 Jeongneung-ro, Seongbuk-gu, Seoul
02707, Republic of Korea
*To whom correspondence should be addressed.
*E-mail: dohoonhwang@pusan.c.kr
†H.U.K. and H.J.P. contributed equally

ACCEPTED MANUSCRIPT
Abstract
Homoleptic Ir(III) complexes, Ir(ppyTMS)3 and Ir(mPppyTMS)3, based on 2-phenyl-5-
(trimethylsilyl)pyridine (ppyTMS) and 2-(1,1'-biphenyl-3'-yl)-5-(trimethylsilyl)pyridine
(mPppyTMS) as cyclometalated ligands, respectively, were synthesized for highly efficient
green phosphorescent organic light-emitting diodes (OLEDs). The trimethylsilyl and phenyl
groups introduced on the 2-phenylpyridine ligand suppressed the intermolecular interactions
and the triplet-triplet annihilation process taking place via molecular aggregation, which
otherwise decrease the OLED efficiency. The green phosphorescent OLEDs doped with
Ir(ppyTMS)3 and Ir(mPppyTMS)3 as green emitters exhibited maximum electroluminescent
wavelengths of 525 and 529 nm, respectively, at an optimized doping concentration of 5%.
The Commission Internationale de L’Eclairage coordinates of these OLEDs were (0.35,0.62)
and (0.37,0.61), respectively, at a luminance of 1000 cd m–2. The maximum external quantum
efficiency and maximum power efficiency (PEmax) were 16.6%/66.1 lm W–1 for the
Ir(ppyTMS)3 device and 18.1%/70.3 lm W–1 for the Ir(mPppyTMS)3 device, which were
higher than those of Ir(ppy)3 without substituents on the 2-phenylpyridine ligand. Moreover,
the PEmax value of the Ir(mPppyTMS)3 device is one of the highest values among the reported
devices fabricated using homoleptic Ir(III) complexes for green phosphorescent OLEDs.
Keywords: green phosphorescence; iridium(III) complex; organic light-emitting diodes;
power efficiency

ACCEPTED MANUSCRIPT
1. Introduction
Iridium(III) complexes have attracted considerable attention as emitters in phosphorescent
organic light-emitting diodes (OLEDs) because of their advantages such as 100% theoretical
internal quantum efficiency using both singlet and triplet excitons, high external quantum
efficiency (EQE), and high power efficiency (PE) [1,2]. In particular, Ir(III) complexes have a
short triplet lifetime, good thermal stability, high photoluminescence quantum yields (ФPL),
and can exhibit additional emission wavelengths by ligand tuning in comparison with those
of other emitters such as platinum(II), osmium(II), and ruthenium(II) complexes [3-5].
Among the most widely used RGB emitters, the green emitter, Ir(ppy)3, was first reported by
Watts and co-workers and is a homoleptic Ir(III) complex formed from 2-phenylpyridine
using Ir(III) acetylacetonate [Ir(acac)3] [6]. Subsequently, Ir(ppy)3 and Ir(ppy)3-derivatives
have been prepared and utilized as efficient green phosphorescent emitters with well-defined
highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital
(LUMO) energy levels [7-11]. However, the Ir(ppy)3 moiety suffers from some drawbacks as
a green dopant in phosphorescent OLEDs, including severe self-quenching because of strong
bimolecular interactions and triplet-triplet annihilation at high doping concentration arising
from their small sizes. These factors result in the diminution of device performances such as
luminance and quantum efficiency in phosphorescent OLEDs [12-15]. It has been previously
reported that endowing bulkiness to the cyclometalated ligand in Ir(III) complexes can
minimize the bimolecular interaction between the molecules, and thus hinder the triplet-
triplet annihilation in phosphorescent OLEDs [16-20].
In our previous studies, we found that a device comprising a heteroleptic iridium complex
containing a 2-phenylpyridine-based ancillary ligand showed improved performances than
that comprising the corresponding iridium complex with the acetylacetonate (acac) ancillary

ACCEPTED MANUSCRIPT
ligand [18,21,22]. Additionally, substitution of a phenyl group on the 5-position of the phenyl
ring of the 2-phenylpyridine ancillary ligand efficiently suppressed concentration self-
quenching at high doping concentration [22]. Furthermore, it was reported that the
introduction of a TMS group on the 5-position of the pyridine ring of the 2-phenylpyridine
ligand in heteroleptic Ir(III) complex could be another strategy to improve OLED device
performances [23]. The maximum current efficiency and power efficiency of OLED device
were increased from 18.0 cd A-1/11.1 lm W-1 to 24.5 cd A-1/16.5 lm W-1 by the TMS
substitution. The TMS substituent on the pyridine moiety could impart hindered
intermolecular interaction and prevented exciton quenching during OLED operation.
In a continuation of our research efforts toward developing efficient phosphorescent OLEDs,
bulky substituents were introduced on the 2-phenylpyridine ligands in homoleptic green
phosphorescent Ir(III) complexes to achieve three-dimensional hindered structures by triply
bulky cyclometalated ligands. The homoleptic Ir(III) complexes were found to be thermally
more stable than the acac-containing heteroleptic complexes, and were thus suitable for
fabricating OLED devices via vacuum deposition methods [24]. In this work, we successfully
synthesized two green phosphorescent homoleptic Ir(III) complexes, Ir(ppyTMS)3 and
Ir(mPppyTMS)3, containing 2-phenyl-5-(trimethylsilyl)pyridine (ppyTMS) and 2-(1,1'-
biphenyl-3'-yl)-5-(trimethylsilyl)pyridine (mPppyTMS) as ligands, respectively. Ir(ppyTMS)3
and Ir(mPppyTMS)3 showed improved thermal stabilities, ФPL, and OLED device
performances compared to Ir(ppy)3. The effect of endowing bulkiness on the ligands has been
described in detail in the results and discussion section.

ACCEPTED MANUSCRIPT
Si
N
OH
B
OH
Ir
Ir(acac)3
N
Si
Si
N
Glycerol
N
Si
ppyTMS
Br
Si
Ir(ppyTMS)3
Pd(PPh3)4, 2M K2CO3
THF, Aliquat
n-BuLi
N
N
TMSCl, Ether
Si
Br
Br
N
1
OH
B
Ir(acac)3
Glycerol
OH
Ir
Si
N
N
Si
N
mPppyTMS
Si
Ir(mPppyTMS)3
Scheme 1. Synthesis of Ir(ppyTMS)3 and Ir(mPppyTMS)3.
2. Experimental
2.1. Materials
2,5-Dibromopyridine, phenylboronic acid, 3-biphenylboronic acid, n-butyllithium (2.5 M
solution in hexane), trimethylsilyl chloride, tetrakis(triphenylphosphine)palladium(0), and
Ir(III) acetylacetonate were purchased from Aldrich and Alfa Aesar. All chemicals were used
without further purification.
2.2. Synthesis of Green Phosphorescence Iridium(III) Complexes
2.2.1. Synthesis of 2-bromo-5-(trimethylsilyl)pyridine (1)
Under a nitrogen atmosphere, n-BuLi (33.8 mL, 84.4 mmol, 2.5 M solution in hexane) was
added dropwise to 2,5-dibromopyridine (20.0 g, 84.4 mmol) in anhydrous diethyl ether (200

ACCEPTED MANUSCRIPT
mL) at –78 °C. After stirring the mixture at –78 °C for 1 h, trimethylsilyl chloride (12.8 mL,
101.4 mmol) was added. Subsequently, the reaction mixture was allowed to warm to room
temperature and stirred for another 12 h. Distilled water was added to the reaction mixture
and the organic layer was extracted with ethyl acetate. The combined organic layers were
dried over MgSO4. After filtration to remove MgSO4, the solvent was removed under reduced
pressure to obtain a crude residue. The crude product was purified by vacuum distillation.
The fraction boiling at 87 °C (1.5 mmHg) (1) was collected as a colourless oil (18.5 g,
95.3%). 1H NMR (300 MHz, CDCl3) δ (ppm): 8.40 (s, 1H), 7.62 (d, J = 7.8 Hz, 1H), 7.45 (d,
13
J = 7.8 Hz, 1H), 0.28 (s, 9H). C NMR (75 MHz, CDCl3) δ (ppm): 154.85, 154.19, 143.34,
127.57, 127.40, 0.09.
2.2.2. Synthesis of 2-phenyl-5-(trimethylsilyl)pyridine (ppyTMS)
Compound 1 (6.0 g, 26.0 mmol), phenylboronic acid (3.2 g, 26.0 mmol), and
tetrakis(triphenylphosphine)palladium(0) (0.9 g, 0.8 mmol) were dissolved in anhydrous
tetrahydrofuran (25 mL). An aqueous solution of 2 M K2CO3 (100 mL) and Aliquat 336 (1.1
g, 2.6 mmol) was added, and the mixture was refluxed overnight with stirring under a
nitrogen atmosphere. Subsequently, the reaction mixture was cooled to room temperature and
extracted with ethyl acetate. The organic layer was washed with water and dried over MgSO4.
After filtration, the solvent was removed under reduced pressure to obtain a crude residue.
The crude product was purified by silica gel column chromatography (ethyl acetate/hexane,
1:3 v/v) to obtain ppyTMS (3.9 g, 65.7%). 1H NMR (300 MHz, CDCl3) δ (ppm): 8.78 (s, 1H),
8.29 (d, J = 7.8 Hz, 2H), 7.86 (m, 2H), 7.52 (m, 3H), 0.33 (s, 9H). 13C NMR (75 MHz, CDCl3)
δ (ppm): 155.61, 153.76, 140.5, 138.9, 128.5, 129.1, 127.6, 127.3, 121.8, 0.02.

ACCEPTED MANUSCRIPT
2.2.3. Synthesis of 2-[(1,1'-biphenyl)-3-yl]-5-(trimethylsilyl)pyridine (mPppyTMS)
mPppyTMS (4.9 g, 62.1%) was prepared from 3-biphenylboronic acid (5.2 g, 26.0 mmol) and
compound 1 using the same procedure as described for ppyTMS. 1H NMR (300 MHz, CDCl3)
δ (ppm): 8.83 (s, 1H), 8.31 (s, 1H), 8.01 (d, J = 7.5 Hz, 1H), 7.88 (d, J = 7.5 Hz, 1H), 7.71 (m,
4H), 7.49 (m, 4H), 0.35 (s, 9H). 13C NMR (75 MHz, CDCl3) δ (ppm): 157.36, 153.93, 142.01,
141.75, 141.05, 139.97, 133.36, 129.23, 128.79, 127.84, 127.53, 127.44, 127.32, 125.83,
120.07, –1.53.
2.2.4. Synthesis of Ir(ppyTMS)3
ppyTMS (2.8 g, 12.3 mmol) and Ir(acac)3 (1.0 g, 2.0 mmol) were dissolved in glycerol (150
mL), and the mixture was stirred at reflux under a nitrogen atmosphere for 25 h. The reaction
mixture was then cooled to room temperature and 1 N HCl solution was added. Subsequently,
the mixture was filtered to obtain a crude product, which was then purified by silica gel
column chromatography (ethyl acetate/hexane, 1:3 v/v) to obtain Ir(ppyTMS)3 (0.8 g, 46%).
1H NMR (300 MHz, CDCl3) δ (ppm): 7.84 (d, J = 7.5 Hz, 3H), 7.67 (m, 6H), 7.45 (s, 3H),
6.99 (m, 3H), 6.93 (m, 6H), 0.05 (s, 27H). 13C NMR (75 MHz, CDCl3) δ (ppm): 166.95,
161.76, 150.44, 144.00, 140.75, 137.28, 132.67, 129.99, 124.04, 119.63, 118.18, -1.208. Anal.
Calcd. for C42H48IrN3Si3: C, 57.89; H, 5.55; N, 4.82; Si, 9.67. Found: C, 57.93; H, 5.51; N,
4.63. MALDI-TOF (M+, C42H48IrN3Si3): Calcd. 871.28, found 871.65. HPLC purity: 99.7%.
2.2.5. Synthesis of Ir(mPppyTMS)3
Ir(mPppyTMS)3 (1.0 g, 43%) was prepared from mPppyTMS (3.7 g, 12.3 mmol) using the
same procedure as described for Ir(ppyTMS)3. 1H NMR (300 MHz, CDCl3) δ (ppm): 7.93 (m,
6H), 7.72 (d, J = 7.8 Hz, 3H), 7.66 (d, J = 7.5 Hz, 6H), 7.52 (s, 3H), 7.42 (t, J = 7.5 Hz, 6H),
13
7.28 (m, 6H), 7.16 (d, J = 7.8 Hz, 3H), 0.07 (s, 27H). C NMR (75 MHz, CDCl3) δ (ppm):

ACCEPTED MANUSCRIPT
166.81, 161.10, 150.56, 144.68, 142.22, 140.98, 137.64, 133.13, 132.57, 129.04, 128.65,
126.46, 126.03, 122.62, 118.40, –1.211. Anal. Calcd. for C60H60IrN3Si3: C, 65.54; H, 5.50; N,
3.82; Si, 7.66. Found: C, 65.65; H, 5.47; N, 3.78. MALDI-TOF (M+, C60H60IrN3Si3): Calcd.
1099.37, found 1099.74. HPLC purity: 99.8%.
2.3. Measurements
1H and 13C NMR spectra were recorded in CDCl3 using a Varian Mercury 300 (1H: 300 MHz,
13C: 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 Build 20 instrument under nitrogen atmosphere at a heating rate of 10 °C min−1. UV-
vis 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 analyser at a potential scanning rate of 50–100 mV s–1
at room temperature in a dichloromethane solution containing 0.1 M tetrabutylammonium
perchlorate as the supporting electrolyte. 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 Lambertian distribution of light emission was assumed in all EQE
measurements.
2.4. Device Fabrication of Green Phosphorescent OLEDs
In order to investigate the EL properties of Ir(ppyTMS)3 and Ir(mPppyTMS)3 as green

ACCEPTED MANUSCRIPT
emitters, phosphorescent OLEDs were fabricated. The structure of the phosphorescent
OLEDs was ITO (50 nm)/PEDOT:PSS (60 nm)/TAPC (30 nm)/TCTA:TPBi:emitter (25 nm,
x%)/TSPO1 (5 nm)/TPBi (30 nm)/LiF (1 nm)/Al (200 nm). In the device, the ratio of TCTA
to TPBi in the mixed host was 1:1 and the concentration of the emitter (x%) was either 3%,
5%, or 10%. The ITO substrate was cleaned by sonication in distilled water and isopropyl
alcohol, followed by UV/O3 treatment for 15 min. The PEDOT:PSS layer was spin-coated
onto the cleaned ITO substrates. TAPC served as the hole injection (HIL) and transport layer
(HTL) and was deposited on the PEDOT:PSS layer. The green emissive layer was prepared
by co-evaporation of TCTA, TPBi, and x% of the green emitter. Next, TSPO1 and TPBi,
which acted as the high triplet-energy HBL with electron-transport properties and the electron
injection (EIL) layer, respectively, were deposited on the emissive layer. Finally, lithium
fluoride (LiF) was deposited as an EIL, and aluminium (Al) was deposited by vacuum
evaporation on top of the film through a mask at less than 2.0 × 10−6 Torr.
3. Results and Discussion
3.1. Synthesis and Thermal Properties
The two homoleptic green phosphorescent Ir(III) complexes, Ir(ppyTMS)3 and
Ir(mPppyTMS)3, were synthesized according to the procedure shown in Scheme 1. The
trimethylsilyl-substituted cyclometalated ligands, ppyTMS and mPppyTMS were prepared by
the Suzuki coupling reaction between either phenylboronic acid or (1,1'-biphenyl)-3-
ylboronic acid and 2-bromo-5-(trimethylsilyl)pyridine. Next, ppyTMS and mPppyTMS were
reacted with Ir(III) acetylacetonate [Ir(acac)3] in glycerol to afford Ir(ppyTMS)3 and
Ir(mPppyTMS)3 in 46 and 43% yields, respectively. The prepared complexes were purified
1
by the train sublimation method under vacuum and then fully characterized using H- and

ACCEPTED MANUSCRIPT
13C-NMR, elemental analysis, and matrix-assisted laser desorption/ionization (MALDI-TOF)
mass spectrometry (Figure S1). The purities of Ir(ppyTMS)3 and Ir(mPppyTMS)3 were 99.7
and 99.8%, respectively, as determined by high-performance liquid chromatography (HPLC)
(Figure S2).
The thermal properties of Ir(ppyTMS)3 and Ir(mPppyTMS)3 were investigated using
thermogravimetric analysis (TGA). The 5% weight loss temperature (Td) of Ir(ppyTMS)3 and
Ir(mPppyTMS)3 were 373 and 410 °C, respectively (Figure S3). The Td values of the
trimethylsilyl- and phenyl-substituted homoleptic Ir(III) complexes demonstrated that they
were thermally more stable than the Ir(ppy)3 complex (Td= 353 °C), which has no
substituents on the 2-phenylpyridine ligands.
3.2. X-ray Crystallography
The results of X-ray crystallographic structure determinations of Ir(ppy)3, Ir(ppyTMS)3, and
Ir(mPppyTMS)3 are shown in Figure 1. The single-crystal structures of Ir(ppy)3,
Ir(ppyTMS)3, and Ir(mPppyTMS)3 show identical facial geometrical configurations with
distorted octahedral geometry around the iridium atom, typical for homoleptic Ir(III)
complexes. The three Ir–C bond lengths in Ir(ppyTMS)3 (Ir(1)–C(11) 2.006 Å, Ir(1)–C(25)
2.023 Å, Ir(1)–C(39) 2.013 Å) and Ir(mPppyTMS)3 (Ir(1)–C(10) 2.014 Å, Ir(1)–C(30) 2.010
Å, Ir(1)–C(50) 2.011 Å) were similar to those of Ir(ppy)3 (Ir(1)–C(8A) 2.018 Å, Ir(1)–C(8B)
2.015 Å, Ir(1)–C(8C) 2.016 Å), whereas the three Ir–N bond lengths in Ir(ppyTMS)3 (Ir(1)–
N(1) 2.116 Å, Ir(1)–N(2) 2.109 Å, Ir(1)–N(3) 2.140 Å) and Ir(mPppyTMS)3 (Ir(1)–N(1)
2.116 Å, Ir(1)–N(2) 2.119 Å, Ir(1)–N(3) 2.122 Å) were slightly shorter than those of Ir(ppy)3
(Ir(1)–N(2A) 2.137 Å, Ir(1)–N(2B) 2.128 Å, Ir(1)–N(2C) 2.126 Å). In other words, the Ir–N
bonds of the trimethylsilyl-substituted Ir(III) complexes were stronger than those of Ir(ppy)3,

ACCEPTED MANUSCRIPT
and indicated the LUMO stabilization in the Ir(ppyTMS)3 and Ir(mPppyTMS)3 complexes
[25]. The selected bond lengths (Å) of the single-crystal structures of Ir(ppy)3, Ir(ppyTMS)3,
and Ir(mPppyTMS)3 are summarized in Table 1.
The distances between the two nearest iridium atoms in the crystal-packing structures of
Ir(ppy)3, Ir(ppyTMS)3, and Ir(mPppyTMS)3 are 8.447, 9.651, and 11.626 Å, respectively.
These values for Ir(ppyTMS)3 and Ir(mPppyTMS)3 are relatively longer than that of Ir(ppy)3.
In particular, the Ir(mPppyTMS)3 complex with has an additional phenyl substituent on the 5-
position of the phenyl ring, showed a significantly longer distance between the two nearest Ir
atoms as compared to Ir(ppy)3 and Ir(ppyTMS)3. This result indicates that Ir(ppyTMS)3 and
Ir(mPppyTMS)3 had relatively weaker bimolecular interactions between the neighbouring
iridium complexes compared to Ir(ppy)3 because of the sterically bulky trimethylsilyl and
phenyl spacers. Detailed crystallographic data (cif) of Ir(ppyTMS)3 and Ir(mPppyTMS)3 can
be found in the Supporting Information.
3.3. Photophysical Properties
Figure 2 displays the absorption and emission spectra of the synthesized Ir(III) complexes,
Ir(ppy)3, Ir(ppyTMS)3, and Ir(mPppyTMS)3, and the photophysical data for these materials
are summarized in Table 2. Figure 2(a) shows the UV-visible absorption spectra of Ir(ppy)3,
Ir(ppyTMS)3, and Ir(mPppyTMS)3 in dichloromethane solutions at 298 K with their
absorption maxima at 286, 287, and 292 nm, respectively, which correspond to the spin-
allowed 1LC* (1π–π*) transitions of the cyclometalated ligands. In the region above 380 nm,
the synthesized Ir(III) complexes showed broad absorption bands in the range of 380–397 nm
1
that corresponded to the metal to ligand charge transfer MLCT* (1d–π*) and weak but
3
detectable MLCT* (3d–π*) transitions in the range of 486–501 nm. The optical energy

ACCEPTED MANUSCRIPT
bandgaps (Eopt) of Ir(ppy)3, Ir(ppyTMS)3, and Ir(mPppyTMS)3 were obtained from the
absorption edges as 2.56, 2.52, and 2.49 eV, respectively.
The maximum PL emission peaks (λmax) of Ir(ppy)3, Ir(ppyTMS)3, and Ir(mPppyTMS)3 in
dichloromethane solution were observed at 516, 521, and 527 nm, respectively (Figure 2(b)).
Substitution of the trimethylsilyl group on the 5-position of the pyridine ring in Ir(ppyTMS)3
lead to a bathochromic shift of 5 nm in the emission maximum relative to the λmax of Ir(ppy)3.
In addition, the introduction of phenyl group on the 4-position of the phenyl ring in
Ir(mPppyTMS)3 led to a bathochromic shift of 6 nm in the emission maximum compared to
that of Ir(ppyTMS)3. Furthermore, the emission maxima of Ir(ppy)3, Ir(ppyTMS)3, and
Ir(mPppyTMS)3 in the neat film state, which were observed at 525, 530, and 535 nm,
respectively, were bathochromically shifted compared to those observed in the solution state.
This could be attributed to the aggregation effects between the Ir(III) complexes in the neat
films prepared by drop casting. The maximum low-temperature PL emissions of Ir(ppy)3,
Ir(ppyTMS)3, and Ir(mPppyTMS)3 in dilute dichloromethane solution at 77 K are observed at
513, 516, and 523 nm, respectively (Figure S4). At 77 K, the emission maxima (λmax) of
Ir(ppyTMS)3 and Ir(mPppyTMS)3 were hypsochromically shifted by 4 and 5 nm,
respectively, compared to those of the samples analysed in solution at 298 K, likely owing to
the rigidochromic effect [26,27]. The triplet state energies (T1) of Ir(ppy)3, Ir(ppyTMS)3, and
Ir(mPppyTMS)3 were 2.42, 2.40, and 2.37 eV, respectively, as determined from the emission
spectra at 77 K. The relative PL quantum yields (ФPL) of Ir(ppyTMS)3 and Ir(mPppyTMS)3
in dilute degassed dichloromethane solution (10–5 M) were compared to that of Ir(ppy)3,
which was chosen as a standard (ФPL = 0.40) [28], and determined to be 0.42 and 0.45,
respectively, showing slightly higher values than that of Ir(ppy)3. The improved ФPL of
Ir(ppyTMS)3 and Ir(mPppyTMS)3 could be attributed to the reduced free rotation of the
compounds by the bulky trimethylsilyl and phenyl substituents.

ACCEPTED MANUSCRIPT
3.4. Theoretical Calculation
The HOMO and LUMO distributions of Ir(ppy)3, Ir(ppyTMS)3, and Ir(mPppyTMS)3 were
determined by density functional theory (DFT) calculations using B3LYP/6-31G(d) basis sets
[29]. As shown in Figure 3, the electron densities in the HOMO of Ir(ppy)3 and Ir(ppyTMS)3
were similarly distributed and primarily localized over the iridium metal and all phenyl rings
of the ppy and ppyTMS ligands, while the HOMO of Ir(mPppyTMS)3 was distributed over
the iridium metal and the two phenyl rings of the ppy-backbone in the mPppyTMS ligands.
For all Ir(III) complexes, the LUMOs were distributed over the pyridine rings and some
phenyl rings of the ppy-backbone in ppy, ppyTMS, and mPppyTMS. The calculated
HOMO/LUMO energy levels of Ir(ppy)3, Ir(ppyTMS)3, and Ir(mPppyTMS)3 were –4.90/–
1.41, –4.87/–1.46, and –4.85/–1.57 eV, respectively. The presence of the trimethylsilyl group
on the 5-position of the pyridine ring decreased its LUMO level and increased the HOMO
level; consequently, the HOMO-LUMO energy band gap was reduced. The calculated
HOMO energy level of Ir(mPppyTMS)3, which has an additional phenyl substituent on the 4-
position of the phenyl ring in the mPppyTMS ligand compared to ppyTMS, was slightly
higher than that of Ir(ppyTMS)3.
3.5. Electrochemical Properties
The electrochemical behaviour of the synthesized Ir(III) complexes was studied using cyclic
voltammetry (CV) in dichloromethane solution. Figure 4(a) displays the reversible oxidation
waves in the cyclic voltammograms of Ir(ppy)3, Ir(ppyTMS)3, and Ir(mPppyTMS)3. The
measured HOMO energy levels of Ir(ppy)3, Ir(ppyTMS)3, and Ir(mPppyTMS)3 were –5.35, –

ACCEPTED MANUSCRIPT
5.34, and –5.32 eV (vs. Fc/Fc+), respectively (Table 2). The LUMO energy levels calculated
from the differences between the HOMO energy level and the optical band gap (∆Eband gap
=
E
LUMO - EHOMO) were found to be –2.79, –2.81, and –2.83 eV for Ir(ppy)3, Ir(ppyTMS)3, and
Ir(mPppyTMS)3, respectively. The decreasing tendency of the HOMO and LUMO energy
levels is consistent with those that were calculated theoretically by DFT calculation. These
HOMO and LUMO energy levels are well-matched with the energy levels of the host
materials of tris(4-carbazoyl-9-ylphenyl)amine (TCTA, HOMO: –6.0 eV, LUMO: –2.70 eV)
and 2,2',2''-(1,3,5-benzinetriyl)-tris(1-phenyl-1H-benzimidazole) (TPBi, HOMO: –6.70 eV,
LUMO: –2.70 eV) used for the device fabrication (Figure 4(b)).
3.6. Electrophosphorescent OLEDs
Green phosphorescent OLEDs were fabricated using Ir(ppy)3, Ir(ppyTMS)3, and
Ir(mPppyTMS)3 as green emitters with a device configuration of ITO (50 nm)/PEDOT:PSS
(60 nm)/ TAPC (30 nm)/TCTA:TPBi:green emitter (25 nm, x%)/ TSPO1 (5 nm)/TPBi (30
nm)/LiF (1.5 nm)/Al (200 nm) [ITO= indium tin oxide; PEDOT:PSS= poly(3,4-
ethylenedioxythiophene) doped with poly(styrenesulfonic acid; TAPC= 4,4'-
cyclohexylidenebis(N,N-bis(4-methylphenyl)benzenamine);
TSPO1=
diphenyl-(4-
(triphenylsilyl)phenyl)phosphine oxide]. TAPC served as both the hole-transport layer (HTL)
and an electron-blocking layer (EBL). Both TSPO1 and TPBi acted as high triplet-energy
hole-blocking layer (HBL) and electron-transporting layer (ETL). The TCTA:TPBi (1:1)
mixture was used as a mixed-host system for efficient carrier injection and charge balance
[30]. The electroluminescence (EL) performances of the fabricated green phosphorescent
OLEDs were evaluated by the controlling the concentration of the Ir(ppyTMS)3 and
Ir(mPppyTMS)3 dopants in the range of 3–10%. The optimal doping concentration of the

ACCEPTED MANUSCRIPT
fabricated devices was 5% of both Ir(ppyTMS)3 and Ir(mPppyTMS)3. The EL performances
of the devices fabricated using Ir(ppyTMS)3 and Ir(mPppyTMS)3 as green emitters were
significantly improved compared to that of the device with Ir(ppy)3 at 5% doping
concentration. The detailed device characteristics using Ir(ppyTMS)3 and Ir(mPppyTMS)3 in
various doping concentrations are summarized in Table 3.
Figure 5 shows the EL spectra of the Ir(ppyTMS)3 and Ir(mPppyTMS)3 devices with
increasing doping concentrations. As the doping concentration increased from 3% to 10%,
the maxima of the EL peaks of Ir(ppyTMS)3 and Ir(mPppyTMS)3 devices were red-shifted by
1–4 nm because of dopant aggregation and intermolecular interactions between the Ir(III)
complexes. Additionally, the increasing of dopant concentration led to a gradual
improvement in EL intensity of emission shoulder at ~570 nm for both Ir(ppyTMS)3 and
Ir(mPppyTMS)3 devices. It could be originated by the direct charge injection from the
electrode to the dopant due to the increased doping concentration at the electrode interfaces
[31]. The increase of the relative EL intensity around 570 nm caused the movement of
Commission Internationale de L’Eclairage (CIE) coordinates of Ir(ppyTMS)3 and
Ir(mPppyTMS)3 devices toward yellow region as shown in Table 3. The emission peak
maxima of the Ir(ppyTMS)3 and Ir(mPppyTMS)3 devices at 5% doping concentration were
observed at 525 and 529 nm, respectively. The CIE coordinates of the Ir(ppyTMS)3 and
Ir(mPppyTMS)3 devices at the optimized doping concentration and 1000 cd m–2 were
(0.35,0.62) and (0.37,0.61), respectively.
The current density-voltage-luminance (J-V-L) curves for the Ir(ppyTMS)3 and
Ir(mPppyTMS)3 devices with 5% doping concentration are shown in Figure 6(a). The other J-
V-L curves of Ir(ppyTMS)3 and Ir(mPppyTMS)3 devices at 3% and 10% doping
concentrations are shown in Figure S5. The turn-on voltages of the devices were 2.7 and 2.6

ACCEPTED MANUSCRIPT
V for Ir(ppyTMS)3 and Ir(mPppyTMS)3, respectively, at the optimized doping concentration.
The Ir(mPppyTMS)3 device showed a higher current density as compared to the Ir(ppy)3 and
Ir(ppyTMS)3-based devices, indicating better carrier hopping and charge balance of the
Ir(mPppyTMS)3 device [32]. The maximum luminances (Lmax) of the Ir(ppyTMS)3 and
Ir(mPppyTMS)3 devices were 42213 and 67809 cd m–2 at 5% doping concentration,
respectively, while the Lmax of Ir(ppy)3 device was 40637 cd m–2 (Figure S7). The highest
Lmax of the Ir(mPppyTMS)3 device could be attributed to its better recombination efficiency
with high ФPL as compared to the Ir(ppy)3 and Ir(ppyTMS)3-based devices.
The EQE-L-PE curves of the Ir(ppyTMS)3 and Ir(mPppyTMS)3 devices at optimized doping
concentration are shown in Figure 6(b). The maximum external quantum efficiency (EQEmax
of the Ir(ppyTMS)3 and Ir(mPppyTMS)3 devices were 16.6 and 18.1% at 5% doping
concentration, respectively, which are higher than that of the Ir(ppy)3 device (EQEmax
)
=
16.3%) (Figure S7). It has been known that the thermally evaporated iridium complex guest
would exist as an aggregated form rather than a discrete molecule in the host-guest thin film
even at low doping concentration [14]. Therefore, the highest EQEmax of Ir(mPppyTMS)3
among the synthesized Ir(III) complexes can be explained by the greatest distance between
the neighbouring iridium centres, which leads to a small diffusion length of the triplet exciton
by the bulky substituents on the ppy ligands, consequently suppressing the triplet-triplet
annihilation in the OLEDs [33]. Furthermore, the high current density of the Ir(mPppyTMS)3
device at low voltage seemingly affected the efficient charge balance in the emitting layer,
leading to high EQE. At a luminance of 1000 cd m–2, the EQEs were 14.8% for Ir(ppy)3, 15.3%
for Ir(ppyTMS)3, and 15.8% for Ir(mPppyTMS)3. The devices with the trimethylsilyl- and
phenyl-substituted homoleptic Ir(III) complexes also showed low efficiency roll-off with
increasing luminance as compared to the Ir(ppy)3 device. The maximum power efficiency
(PEmax) of the Ir(ppyTMS)3 and Ir(mPppyTMS)3 devices at 5% doping concentration were

ACCEPTED MANUSCRIPT
66.1 and 70.3 lm W-1, respectively, while the PEmax of Ir(ppy)3 was 57.5 lm W–1. The
Ir(mPppyTMS)3 device showed a higher PEmax than the Ir(ppyTMS)3 and Ir(ppy)3 devices,
which is one of the highest PE values reported to date among homoleptic Ir(III) complexes
for green phosphorescent OLEDs [34-39]. The driving voltage of the devices were 3.9, 3.8,
and 3.6 V for Ir(ppy)3, Ir(ppyTMS)3, and Ir(mPppyTMS)3, respectively. The low driving
voltage and high EQE of the Ir(mPppyTMS)3 device improved the PE value [40].
4. Conclusions
In conclusion, we successfully developed trimethylsilyl- and phenyl-substituted homoleptic
Ir(III) complexes, Ir(ppyTMS)3 and Ir(mPppyTMS)3, for use in highly efficient green
phosphorescent OLEDs. The introduction of the bulky substituents, trimethylsilyl and phenyl,
on the 2-phenylpyridine (ppy) ligand in these complexes led to a long distance between the
neighbouring molecules, suppressing triplet-triplet annihilation in the green phosphorescent
OLEDs. Ir(mPppyTMS)3, which had both trimethylsilyl at the 5-position of pyridine ring and
phenyl at the 4-position of the phenyl ring in the ppy ligand, showed the best device
performances with the highest EQE, Lmax, and PEmax. The high current density of
Ir(mPppyTMS)3 device at low voltage indicated better carrier hopping and charge balance,
which led to an efficient recombination of excitons in the emitting layer and consequently, a
high external quantum efficiency. The Ir(mPppyTMS)3 complex achieved the highest PEmax
in the green phosphorescent OLED among the synthesized Ir(III) complexes owing to the
comprehensive effects of the low efficiency roll-off, high EQE, and low driving voltage.
Acknowledgements

ACCEPTED MANUSCRIPT
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] Chi Y, Chou P-T. Transition-metal Phosphors with Cyclometalating Ligands:
Fundamentals and Applications. Chem Soc Rev 2010; 39; 638-55.
[2] Baldo M A, Thompson M E, Forrest S R. Phosphorescent Materials for Application to
Organic Light Emitting Devices. Pure Appl Chem 1999;71;2095-106.
[3] Baldo M A, Lamansky S, Burrows P E, Thompson M E, Forrest S R. Very High-
efficiency Green Organic Light-emitting Devices Based on Electrophosphorescence. Appl
Phys Lett 1999;75;4-6.
[4] Adachi C, Kwong R C, Djurovich P, Adamovich V, Baldo M A, Thompson M E, Forrest S
R. Endothermic Energy Transfer: A Mechanism for Generating Very Efficient High-energy
Phosphorescent Emission in Organic Materials. Appl Phys Lett 2001;79;2082-84.
[5] Tsuzuki T, Shirasawa N, Suzuki T, Tokito S. Color Tunable Organic Light-emitting
Diodes Using Pentafluorophenyl-substituted Iridium Complexes. Adv Mater 2003;15;1455-
58.
[6] Dedeian K, Djurovich P I, Garces F O, Carlson G, Watts R J. A New Synthetic Route to
the Preparation of a Series of Strong Photoreducing Agents: fac-Tris-ortho-metalated
Complexes of Iridium(III) with Substituted 2-Phenylpyridines. Inorg Chem 1991;30;1685-87.

ACCEPTED MANUSCRIPT
[7] Hofbeck T, Yersin H. The Triplet State of fac-Ir(ppy)3. Inorg Chem 2010;49;9290-99.
[8] Yang C–H, Fang K–H, Chen C–H, Sun I–W. High Efficiency mer-Iridium Complexes for
Organic Light-emitting Diodes. Chem Commun 2004;19;2232-33.
[9] Jung S, Kang Y, Kim H–S, Kim Y–H, Lee C–L, Kim J–J, Lee S–K, Kwon S–K. Effect of
Substitution of Methyl Groups on the Luminescence Performance of IrIII Complexes:
Preparation, Structures, Electrochemistry, Photophysical Properties and Their Applications in
Organic Light-Emitting Diodes (OLEDs). Eur J Inorg Chem 2004;2004;3415-23.
[10] Grushin V V, Herron N, LeCloux D D, Marshall W J, Petrov V A, Wang Y. New,
Efficient Electroluminescent Materials Based on Organometallic Ir Complexes. Chem
Commun 2001;16;1494-95.
[11] Namdas E B, Ruseckas A, Samuel D W. Photophysics of fac-Tris(2-Phenylpyridine)
Iridium(III) Cored Electroluminescent Dendrimers in Solution and Films. J Phys Chem B
2004;108;1570-77.
[12] Song D, Zhao S, Luo Y, Aziz H. Causes of Efficiency Roll-off in Phosphorescent
Organic Light Emitting Devices: Triplet-triplet Annihilation Versus Triplet-polaron
Quenching. Appl Phys Lett 2010;97;243304/1-3.
[13] Kawamura Y, Brooks J, Brown J J, Sasabe H, Adachi C. Intermolecular Interaction and a
Concentration-quenching Mechanism of Phosphorescent Ir(III) Complexes in a Solid Film.
Phys Rev Lett 2006;96;017404/1-4.
[14] 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.

ACCEPTED MANUSCRIPT
[15] Takayasu S, Suzuki T, Shinozaki K. Intermolecular Interactions and Aggregation of fac-
Tris(2-phenylpyridinato-C2,N)iridium(III) in Nonpolar Solvents. J Phys Chem B
2013;117;944-56.
[16] Xie H Z, Liu M W, Wang O Y, Zhang X H, Lee C S, Hung L S, Lee S T, Teng P F,
Kwong H L, Zheng H, Che C M. 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.
[17] Liang A, Huang G, Chen S, Hou H. A Novel Cyclometalated Iridium(III) Complex
Containing Dibenzo-24-crown-8: Synthesis, Luminescence and Application in Highly
Efficient Green Phosphorescent OLEDs. RSC Adv 2015;5;49466-70.
[18] Kim H U, Jang J–H, Song W, Jung B J, Lee J Y, Hwang D–H. Improved Luminance and
External Quantum Efficiency of Red and White Organic Light-emitting Diodes with
Iridium(III) Complexes with Phenylsubstituted 2-Phenylpyridine as a Second Cyclometalated
Ligand. J Mater Chem C 2015;3;12107-15.
[19] Hwang M C, Thangaraju K, So K H, Shin S–C, Kwon S–K, Kim Y–H. A New Bulky
Trimethylsilylxylene Substituted Iridium(III) Complex with Picolinic Acid as Ancillary
Ligand: Synthesis; Characterization and Applications for Efficient Yellow-green Emitting
Phosphorescent Organic Light Emitting Diodes. Synth Met 2012;162;391-7.
[20] Rehmann N, Hertel D, Meerholz K, Becker H, Heun S. Highly Efficient Solution-
processed Phosphorescent Multilayer Organic Light-emitting Diodes Based on Small-
molecule Hosts. Appl Phys Lett 2007;91;103507/1-3.
[21] Kim H U, Jang J-H, Park H J, Lee J Y, Hwang D-H. Synthesis of New Heteroleptic
Iridium(III) Complex Consisting of 2-Phenylquinoline and 2-[4-(Trimethylsilyl)

ACCEPTED MANUSCRIPT
phenyl]Pyridine for Red and White Organic Light-Emitting Diodes. J Nanosci Nanotechnol
2017;17;5587-92.
[22] Kim H U, Jang J-H, Park H J, Jung B J, Song W, Lee J Y, Hwang D-H. Highly Efficient
and Spectrally Stable White Organic Light-Emitting Diodes Using New Red Heteroleptic
Iridium(III) Complexes. Dyes Pigments 2018;149;363-72.
[23] Lee J, Park C H, Kwon J, Yoon S C, Do L-M, Lee C. Improved Performance of
Solution-processable OLEDs by Silyl Substitution to Phosphorescent Iridium Complexes.
Synth Met 2012;162;1961-7.
[24] Tsuboyama A, Iwawaki H, Furugori M, Mukaide T, Kamatani J, Igawa S, Moriyama T,
Miura S, Takiguchi T, Okada S, Hoshino M, Ueno K. Homoleptic Cyclometalated Iridium
Complexes with Highly Efficient Red Phosphorescence and Application to Organic Light-
Emitting Diode. J Am Chem Soc 2003;125;12971-9.
[25] Kim J J, You Y, Park Y–S, Kim J–J, Park S Y. Dendritic Ir(III) Complexes
Functionalized with Triphenylsilylphenyl Groups: Synthesis, DFT Calculation and
Comprehensive Structure-property Correlation. J Mater Chem 2009;19;8347-59.
[26] Frin K M, Iha N Y M. Photoinduced Isomerization and Luminescence of fac-
[Re(CO)3(ph2phen)(bpe)]+. J Braz Chem Soc 2006;17;1664-71.
[27] Chen P, Meyer T J. Medium Effects on Charge Transfer in Metal Complexes. Chem Rev
1998;98;1439-77.
[28] Tamayo A B, Alleyne B D, Djurovich P I, Lamansky S, Tsyba I, Ho N N, Bau R,
Thompson M E. Synthesis and Characterization of Facial and Meridional Tris-cyclometalated
Iridium(III) Complexes. J Am Chem Soc 2003;125;7377-87.

ACCEPTED MANUSCRIPT
[29] 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 J A, Peralta J E, Ogliaro F, Bearpark M, Heyd J J, Brothers E, Kudin K N,
Staroverov V N, 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 Ö, Foresman J B, Ortiz J V, Cioslowski J,
Fox D. J. Gaussian 09; Gaussian, Inc.: Wallingford, CT, 2009.
[30] Lee J, Lee J–I, Lee J Y, Chu H Y. Enhanced Efficiency and Reduced Roll-off in Blue and
White Phosphorescent Organic Light-emitting Diodes with a Mixed Host Structure. Appl
Phys Lett 2009;94;193305/1-3.
[31] Ouyang X, Chen D, Zeng S, Zhang X, Su S, Ge Z. Highly Efficient and Solution-
processed Iridium Complex for Single-layer Yellow Electrophosphorescent Diodes. J Mater
Chem 2012; 22;23005-11.
[32] Yook K S, Jeon S O, Joo C W, Lee J Y. High Efficiency Deep Blue Phosphorescent
Organic Light-emitting Diodes. Org Electron 2009;10;170-3.
[33] Namdas E B, Ruseckas A, Samuel I D W, Lo S C, Burn P L. Triplet Exciton Diffusion in
fac-Tri(2-phenylpyridine) Iridium(III)-cored Electroluminescent Dendrimers. Appl Phys Lett
2005;86;091104/1-3.
[34] Ulbricht C, Beyer B, Friebe C, Winter A, Schubert U S. Recent Developments in the

ACCEPTED MANUSCRIPT
Application of Phosphorescent Iridium(III) Complex Systems. Adv Mater 2009;21;4418-41.
[35] Yang X, Zhou G, Wong W-Y. Functionalization of Phosphorescent Emitters and Their
Host Materials by Main-group Elements for Phosphorescent Organic Light-emitting Devices.
Chem Soc Rev 2015;44;8484-575.
[36] Zhao J, Yu Y, Yang X, Yan X, Zhang H, Xu X, Zhou G, Wu Z, Ren Y, Wong W-Y.
Phosphorescent Iridium(III) Complexes Bearing Fluorinated Aromatic Sulfonyl Group with
Nearly Unity Phosphorescent Quantum Yields and Outstanding Electroluminescent
Properties. ACS Appl Mater Interfaces 2015;7;24703-14.
[37] Wu Z-G, Zhou J, Yu L, Karotsis G, Wang Y, Zheng Y-X, Pan Y. Novel Phosphine Oxide-
based Electron-transporting Materials for Efficient Phosphorescent Organic Light-emitting
Diodes. J Mater Chem C 2017;5;8579-85.
[38] Liu B, Dang F, Feng Z, Tian Z, Zhao J, Wu Y, Yang X, Zhou G, Wu Z, Wong W-Y.
Novel Iridium(III) Complexes Bearing Dimesitylboron Groups with Nearly 100%
Phosphorescent Quantum Yields for Highly Efficient Organic Light-emitting Diodes. J Mater
Chem C 2017;5;7871-83.
[39] Wang S, Zhang B, Wang Y, Ding J, Xie Z, Wang L. Solution-processed Multilayer Green
Electrophosphorescent Devices with Self-host Iridium Dendrimers as the Nondoped Emitting
Layer: Achieving High Efficiency While Avoiding Redissolution-induced Batch-to-batch
Variation. Chem Commun 2017;53;5128-31.
[40] Jang S E, Yook K S, Lee J Y. High Power Efficiency in Simplified Two Layer Blue
Phosphorescent Organic Light-emitting Diodes. Org Electron 2010;11;1154-7.

ACCEPTED MANUSCRIPT
Table captions
Table 1. Selected bond lengths (Å) of Ir(ppy)3, Ir(ppyTMS)3, and Ir(mPppyTMS)3.
Table 2. Summary of photophysical and electrochemical properties for trimethylsilyl-
substituted homoleptic iridium(III) complexes.
Table 3. Summary of performances of green phosphorescent OLEDs.

ACCEPTED MANUSCRIPT
Table 1.
Ir(ppy)3
Ir(ppyTMS)3
Ir(mPppyTMS)3
Ir(1)-C(10)
Ir(1)-C(8A)
Ir(1)-C(8B)
Ir(1)-C(8C)
Ir(1)-N(2A)
Ir(1)-N(2B)
Ir(1)-N(2C)
2.018 Å
2.015 Å
2.016 Å
2.137 Å
2.128 Å
2.126 Å
Ir(1)-C(11)
Ir(1)-C(25)
Ir(1)-C(39)
Ir(1)-N(1)
Ir(1)-N(2)
Ir(1)-N(3)
2.006 Å
2.023 Å
2.013 Å
2.116 Å
2.109 Å
2.140 Å
2.014 Å
2.010 Å
2.011 Å
2.116 Å
2.119 Å
2.122 Å
Ir(1)-C(30)
Ir(1)-C(50)
Ir(1)-N(1)
Ir(1)-N(2)
Ir(1)-N(3)

ACCEPTED MANUSCRIPT
Table 2.
Photophysical
Electrochemical
a/b
d
Dopant
λabs
λem
Eopt
T1
HOMO LUMO
c
ФPL
(nm)
(nm)
(eV) (eV)
(eV)
-5.35
-5.34
-5.32
(eV)
-2.79
-2.81
-2.83
Ir(ppy)3
286, 380, 454, 486 516/520 0.40 2.56 2.42
287, 390, 460, 492 521/530 0.42 2.52 2.40
Ir(ppyTMS)3
Ir(mPppyTMS)3 292, 397, 467, 501 527/535 0.45 2.49 2.37
aMaximum PL emission wavelength, measured in dichloromethane solution.
bMaximum PL emission wavelength, measured in neat film state.
cMeasured in degassed dichloromethane solution (10-5 M) relative to Ir(ppy)3 (ФPL = 0.40)
dCalculated from the absorption edge, Eg = 1240/λabs, onset.

ACCEPTED MANUSCRIPT
Table 3.
λmax
CIEa
(x,y)
Vt
Lmax
EQEb
(%)
PEb
LEb
Device
x%
(nm)
(V) (cd m-2)
(lm W-1)
(cd A-1)
3
5
523 (0.34,0.62) 2.7 38330 16.4/15.0 62.9/46.3 60.1/55.0
525 (0.35,0.62) 2.7 42210 16.6/15.3 66.1/47.6 60.9/55.9
Ir
(ppyTMS)3
10 526 (0.36,0.61) 2.7 43740 15.6/14.0 54.7/45.3 58.1/51.1
3
5
528 (0.36,0.61) 2.6 60680 17.9/15.6 69.8/47.7 66.7/58.3
529 (0.37,0.61) 2.6 67810 18.1/15.8 70.3/46.7 67.2/58.7
Ir
(mPppyTMS)3
10 532 (0.38,0.60) 2.6 71060 16.4/14.8 60.6/47.3 63.5/54.8
512 (0.32,0.61) 3.0 40640 16.3/14.8 57.5/40.8 54.9/50.3
Ir(ppy)3
5
a Value measured at a luminance of 1000 cd m-2.
b Values measured at maximum efficiency and luminance of 1000 cd m-2.

ACCEPTED MANUSCRIPT
Figure captions
Figure 1. (a) Single-crystal and (b) crystal packing structures of Ir(ppy)3, Ir(ppyTMS)3, and
Ir(mPppyTMS).
Figure 2. (a) UV-visible absorption (in CH2Cl2, inset; expanded spectra from 450 to 550 nm)
and (b) PL spectra (left; in CH2Cl2, right; in film) of Ir(ppy)3, Ir(ppyTMS)3, and
Ir(mPppyTMS)3 at 298 K.
Figure 3. Frontier molecular orbitals HOMO and LUMO of Ir(ppy)3, Ir(ppyTMS)3, and
Ir(mPppyTMS)3.
Figure 4. (a) Cyclic voltammograms and (b) energy diagrams of Ir(ppy)3, Ir(ppyTMS)3, and
Ir(mPppyTMS)3.
Figure 5. EL spectra of green phosphorescent OLEDs with Ir(ppyTMS)3 and Ir(mPppyTMS)3
at 5% doping concentration.
Figure 6. (a) Current density-voltage-luminance (J-V-L) and (b) external quantum efficiency-
L-power efficiency (EQE-L-PE) curves of the Ir(ppy)3, Ir(ppyTMS)3, and Ir(mPppyTMS)3
devices at 5% doping concentration.

ACCEPTED MANUSCRIPT
Figure 1.