Tris-heteroleptic cyclometalated iridium(III) complexes with ambipolar or electron injection/transport features for highly efficient electrophosphorescent devices

Article abstract of DOI:10.1002/asia.201403057

A series of tris-heteroleptic phosphorescent cyclometalated Ir^III complexes with two different cyclometalating ligands and one ancillary ligand was synthesized. Through manipulation of the electronic features of individual cyclometalating ligan

Full text of DOI:10.1002/asia.201403057

DOI: 10.1002/asia.201403057
Full Paper
PHOLEDs
tris-Heteroleptic Cyclometalated Iridium(III) Complexes with
Ambipolar or Electron Injection/Transport Features for Highly
Efficient Electrophosphorescent Devices
Xianbin Xu,[a] Xiaolong Yang,[a] Yong Wu,[a] Guijiang Zhou,*[a] Chao Wu,*[b] and Wai-
Yeung Wong*[c]
Abstract: A series of tris-heteroleptic phosphorescent cyclo-
metalated IrIII complexes with two different cyclometalating
ligands and one ancillary ligand was synthesized. Through
manipulation of the electronic features of individual cyclo-
metalating ligands, not only the emission color and electro-
chemical properties of these IrIII complexes can be tuned,
but also unique charge carrier injection and transport traits
can be conferred to them. Owing to the enhanced electron
injection/transport and ambipolar characters associated with
these tris-heteroleptic structures, these IrIII emitters showed
very impressive electroluminescent performance with a maxi-
mum external quantum efficiency of 20.20%, luminance effi-
ciency of 69.41 cdAꢀ1 and power efficiency of 35.15 LmWꢀ1.
These results provide a new outlet to design and synthesize
highly efficient electrophosphors for phosphorescent organic
light-emitting diodes.
Introduction
light-emitting diodes (OLEDs) have attracted numerous re-
search efforts both in academics and industry, and have been
widely recognized as next-generation light sources[1] and dis-
plays.[2] Among all the light-emitting materials used in OLEDs,
phosphorescent (triplet) emitters bearing heavy transition
metals, especially RuII, OsII, IrIII and PtII,[3–6] are widely investigat-
ed. Due to the high spin-orbit coupling constant of the transi-
Owing to their inherent merits of self-emitting nature, high
contrast, fast response, wide viewing angle, low weight, low
power consumption, and ease of thin-film formation, organic
[a] X. Xu, X. Yang, Dr. Y. Wu, Prof. G. Zhou
!
tion metal ions, the spin-forbidden nature of the T1 S0 radia-
MOE Key Laboratory for Nonequilibrium Synthesis and Modulation of Con-
densed Matter, State Key Laboratory for Mechanical Behavior of Materials,
Institute of Chemistry for New Energy Materials
Department of Chemistry, Faculty of Science, Xi’an Jiaotong University
Xi’an 710049 (China)
tive relaxation is eliminated, and an efficient intersystem cross-
ing from singlet states to triplet states takes place. Eventually,
the devices incorporating these organometallic phosphores-
cent emitters can reach nearly 100% internal quantum efficien-
cy by harnessing both singlet and triplet excitons.[2b,c] In view
of their outstanding features including high phosphorescent
quantum yield (Fp), relatively short triplet lifetime (tp), and
tunable emission color, IrIII ppy-type (Hppy=2-phenylpyridine)
complexes show distinct advantages over other triplet emitters
in the field of OLEDs.
Fax: (+86)29-82663914
E-mail: zhougj@mail.xjtu.edu.cn
[b] Prof. C. Wu
Frontier Institute of Science and Technology, Xi’an Jiaotong University
Xi’an 710049 (China)
Fax: (+86)29-83395364
E-mail: chaowu@mail.xjtu.edu.cn
[c] Prof. W.-Y. Wong+
Institute of Molecular Functional Materials, Department of Chemistry
Partner State Key Laboratory of Environmental
Biological Analysis and Institute of Advanced Materials
Hong Kong Baptist University
Waterloo Road, Kowloon Tong, Hong Kong (China)
Fax: (+852)3411-7348
Recently, it was found that ligand functionalization would be
a good strategy to enhance the electroluminescence (EL) per-
formance of IrIII ppy-type phosphorescent emitters or furnish
high-performance complex host materials. Introduction of a tri-
phenylamine, thiazole or carbazole unit into the ppy ligand
can afford improved hole-injection/transport (HI/HT) ability to
such IrIII phosphorescent emitters,[7] while 1,3,4-oxadiazole, or-
ganoboryl, diarylsulfone, and triarylphosphine oxide moieties
in the organic ligands would furnish their enhanced electron-
injection/transport (EI/ET) features.[6d,4e,7a,8] Furthermore, ZnII
and BeII complexes have been developed as high-performance
ambipolar host materials based on ligand functionalization.[6h]
Importantly, such functionalization also works well in PtII ppy-
type phosphorescent emitters.[8c] In the area of electrolumines-
E-mail: rwywong@hkbu.edu.hk
Homepage: http://dx.doi.org/10.1002/chemv.201400130
[+] This author is one of the most prolific contributors to
Chemistry–An Asian Journal. In celebration of the 10th volume of
Chemistry–An Asian Journal in 2015, a short profile of the author in
the series “10 Years Ago and Now” is featured on ChemistryViews at:
http://dx.doi.org/10.1002/chemv.201400130.
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/asia.201403057. It contains the synthetic details
for the organic ligands, single crystal X-ray data for Ir-1 and Ir-4, molecular
orbital patterns for Ir-btN, and the EL data.
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cence (EL), it has been well accepted that balanced injection/
transport for both kinds of charge carriers are very critical to
give high EL efficiencies of the devices by improving the re-
combination efficiency of the charge carriers. Thus, the design
and synthesis of phosphorescent emitters possessing both HI/
HT and EI/ET properties, that is, ambipolar features, represents
an effective way to improve the EL performance of phosphor-
escent OLEDs (PHOLEDs). In this regard, IrIII and PtII ppy-type
phosphors showing ambipolar features have been obtained by
integrating both HI/HT and EI/ET moieties into their individual
organic ligands.[8a,b,d–g] However, these reported phosphores-
cent emitters with ambipolar character might not attain the
desired enhancement in EL effi-
tocols are shown in Scheme 1. As key compounds, the organic
cyclometalating ligands L0–L4 (Scheme S1 in the Supporting
Information) were prepared by Stille cross-coupling of 2-(tribu-
tylstannyl)pyridine with the appropriate aryl halides.[7a,8c] For
L5, 5-bromo-2-phenylpyridine was first synthesized by the
Suzuki cross-coupling reaction of 5-bromo-2-iodopyridine with
phenylboronic acid. Further reaction of 5-bromo-2-phenylpyri-
dine with (Mes)2BF in the presence of nBuLi at ꢀ788C in dry
THF gave L5.[10] Ligand L6 was prepared by the cyclization re-
action of 4-(diphenylamino)benzaldehyde, obtained by the re-
action of triphenylamine with DMF and POCl3 in 1,2-dichloro-
ethane,[11] followed by the reaction with 2-aminobenzenethiol
ciency of PHOLEDs. Accordingly,
there is still much room for de-
veloping other approaches to
endow phosphorescent emitters
with ambipolar properties that
can enhance their EL properties.
Encouragingly, our recent results
have indicated that the structur-
al factor can also play a critical
role in determining the EL per-
formance of the IrIII phosphores-
cent complexes.[7l]
We present here new exam-
ples of cyclometalated IrIII elec-
trophosphors with two different
main-group-bearing ppy-type li-
gands and one acetylacetonate
ancillary ligand. By introducing
HI/HT and EI/ET units into differ-
ent ppy-type ligands, the phos-
phorescent emitters can show
tris-heteroleptic structures as
well as unique ambipolar or EI/
ET characteristics. The tris-heter-
oleptic phosphorescent emitters
not only exhibit unique photo-
physical behavior but also an
ambipolar character that can
furnish these complexes with
excellent EL performance. Our
results clearly indicate that such
a molecular design represents
another approach to afford
high-performance triplet emit-
ters.
Results and Discussion
Synthetic strategies and chem-
ical characterization
Chemical structures of the new
tris-heteroleptic IrIII complexes
and the detailed synthetic pro-
Scheme 1. Synthesis of the tris-heteroleptic IrIII cyclometalated complexes.
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in the presence of concentrated hydrochloric acid and hydro-
gen peroxide in ethanol.[12] The cyclometalation of IrCl3·nH2O
with 1.0 equivalent of L0 and 1.0 equivalent of each of the cor-
responding ligands (L1–L6) led to the chloride-bridged dimers,
which upon treatment with acetylacetone (Hacac) in the pres-
ence of Na2CO3 would give the desired tris-heteroleptic IrIII
complexes. These air-stable complexes were isolated in high
purity by thin layer chromatography on silica gel with the
proper eluent. Theoretically, the highest yield for these com-
plexes should be about 33%. However, addition of the two
kinds of ligands in two individual batches in preparing chlo-
ride-bridged dimers should potentially solve this problem.
The chemical structures of all these new tris-heteroleptic IrIII
cyclometalated complexes have been fully characterized. Two
sets of proton signals assigned to the two different cyclometa-
1
lating ligands from their H NMR spectra have indicated their
tris-heteroleptic structures, as the conventional symmetric
counterparts typically give just one set of signals for one type
of ligand. Furthermore, the 13C NMR spectra support their tris-
heteroleptic structures as well. Even more, their tris-heterolep-
tic structures have been confirmed by X-ray crystallography.
For Ir-1 and Ir-4, their single crystals were successfully ob-
tained by slow diffusion of their CHCl3 solution into hexane.
The X-ray data revealed that the central iridium center was co-
ordinated by two different ppy-type cyclometalating ligands
and one chelating acac ligand. The coordination around the Ir
center is distorted octahedral, with the cis-O,O, cis-C,C, and
trans-N,N chelating disposition (Figure 1, Table 1 and Table S1,
Supporting Information).
Figure 1. ORTEP drawings of a) Ir-1 and b) Ir-4 with thermal ellipsoids drawn
at 10% probability level. Labels on the carbon atoms (except for those
bonded to Ir) and hydrogen atoms are omitted for clarity.
tion of PHOLEDs by vacuum thermal deposition. No crystalliza-
tion or melting process was observed in the DSC curves of
these complexes. The glass-transition behavior of these com-
plexes appeared at high temperatures of over 1508C, which
can be attributed to the branched and distorted configuration
of the substituted electron-donating/-withdrawing groups.
All of the tris-heteroleptic IrIII complexes show two kinds of
absorption bands in their UV/Vis spectra according to their ori-
gins of transition (Figure 2A and Table 2). The intense absorp-
tion bands can be assigned to
Thermal and photophysical properties
The thermal properties of the tris-heteroleptic IrIII complexes
were studied by thermogravimetric analysis (TGA) and differen-
tial scanning calorimetry (DSC) under a nitrogen atmosphere
(Table 2). The TGA data show that the 5% weight loss tempera-
ture (DT5%) of all the complexes ranges from 325 to 3548C, in-
dicating their good thermal stabilities required for the fabrica-
!
the spin-allowed S1 S0 transi-
tions of the ligands, such as p–
p* transitions, which are located
Table 1. Selected structural parameters of Ir-1 and Ir-4.
below 350 nm for all the com-
Compound
Bond angles [8]
Bond lengths [ꢁ]
plexes except Ir-2 and Ir-6. The
S1 S0 transitions associated
C(11)-Ir(1)-C(28) 95.2(7)
C(28)-Ir(1)-N(1) 80.8(7)
C(28)-Ir(1)-N(2) 98.4(7)
C(28)-Ir(1)-O(3) 87.0(7)
N(1)-Ir(1)-O(3) 92.4(6)
C(28)-Ir(1)-O(4) 173.4(6)
N(1)-Ir(1)-O(4) 88.9(5)
O(3)-Ir(1)-O(4) 88.5(5)
C(11)-Ir(1)-C(28) 92.2(5)
C(28)-Ir(1)-N(1) 97.4(5)
C(28)-Ir(1)-N(2) 79.9(5)
C(28)-Ir(1)-O(3) 95.2(4)
N(1)-Ir(1)-O(3) 91.2(4)
C(28)-Ir(1)-O(4) 173.2(5)
N(1)-Ir(1)-O(4) 89.4(4)
O(3)-Ir(1)-O(4) 87.9(4)
C(28)-Ir(1)-N(1) 96.1(7)
C(28)-Ir(1)-N(2) 79.5(7)
N(1)-Ir(1)-N(2) 175.4(6)
C(11)-Ir(1)-O(3) 173.0(6)
N(2)-Ir(1)-O(3) 88.6(5)
C(11)-Ir(1)-O(4) 89.9(6)
N(2)-Ir(1)-O(4) 95.6(6)
Ir(1)ꢀC(11) 1.97(2)
!
Ir(1)ꢀC(28) 1.95(2)
with Ir-2 and Ir-6 induce absorp-
Ir(1)ꢀN(1) 2.03(2)
Ir(1)ꢀN(2) 2.06(1)
Ir(1)ꢀO(4) 2.15(1)
Ir(1)ꢀO(3) 2.14(1)
tion bands of lower energy,
about 388 nm for Ir-2 and about
414 nm for Ir-6, which can be at-
tributed to the strong electron-
donating property of the NPh2
group. The weaker and low-
energy absorption features of
Ir-1
C(11)-Ir(1)-N(1) 80.8(5)
C(11)-Ir(1)-N(2) 93.0(5)
N(1)-Ir(1)-N(2) 173.1(4)
C(11)-Ir(1)-O(3) 171.7(4)
N(2)-Ir(1)-O(3) 95.2(4)
C(11)-Ir(1)-O(4) 89.8(4)
N(2)-Ir(1)-O(4) 93.4(4)
Ir(1)ꢀC(11) 1.99(1)
Ir(1)ꢀC(28) 2.01(1)
Ir(1)ꢀN(1) 2.05(1)
Ir(1)ꢀN(2) 2.07(1)
Ir(1)ꢀO(3) 2.16(1)
Ir(1)ꢀO(4) 2.16(1)
!
!
3 MLCT S0
1 MLCT S0
and
Ir-4
(MLCT=metal-to-ligand charge
transfer) transitions for the com-
plexes Ir-1, Ir-3, Ir-4, and Ir-5 are
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ca. 605 nm and Ir-B1 ca. 607 nm), but display a batho-
chromic shift with respect to that of the symmetric
complex Ir-SO2 (ca. 556 nm).
Table 2. Photophysical, electrochemical and thermal data for the tris-heteroleptic IrIII
complexes.
[b]
[c]
Compd Absorption (293 K)[a]
Emission Fp tp
tr[d] DT5%
/
In order to interpret the photophysical behavior
associated with these tris-heteroleptic phosphores-
cent IrIII complexes, we turned our attention to theo-
retical computations. It is well known that the phos-
phorescence of ppy-type IrIII complexes can be as-
labs/nm
lem [nm]
[ms] [ms] Tg [8C]
293 K
Ir-1
Ir-2
Ir-3
Ir-4
Ir-5
Ir-6
268 (4.62), 284 (4.63), 358 (3.86), 397
556
564
582
560
581
575
0.52 0.99 1.90 344/
(3.65), 430 (3.47), 490 (3.17), 520 (2.60)
259 (4.67), 284 (4.66), 359 (4.31), 388
(4.31), 436 (3.93), 497 (3.22), 523 (2.82)
262 (4.58), 285 (4.60), 324 (4.46), 339
(4.44), 410 (3.57), 505 (3.39), 530 (2.96)
264 (4.67), 282 (4.67), 368 (3.78), 410
(3.59), 462 (3.41), 495 (3.34), 522 (2.89)
263 (4.56), 285 (4.59), 321 (4.44), 338
(4.41), 413 (3.65), 506 (3.60), 531 (3.32)
263 (4.65), 281 (4.64), 301 (4.56), 414
(4.45), 467 (4.13), 495 (3.57)
153
0.35 0.81 2.31 334/
!
cribed to the triplet excited states formed by T1 S0
153
0.39 1.25 3.20 336/
transitions with MLCT characters. Importantly, the
time-dependent density functional theory (TD-DFT)
calculation results have clearly indicated that the
168
0.48 1.10 2.29 332/
151
0.95 0.84 0.88 325/
150
!
T1 S0 transitions for the tris-heteroleptic IrIII com-
!
plexes can be assigned to the LUMO HOMO or ad-
0.25 0.94 3.76 354/
157
jacent orbitals due to the large coefficient in the CI
expansion of their T1 states (Table 3). Hence, we fo-
cused on the molecular orbital (MO) patterns ob-
tained by DFT calculations to gain insight into their
phosphorescent emission behavior (Figure 3). For Ir-
[a] Measured in CH2Cl2 at a concentration of 10ꢀ5 m, and log e values are shown in pa-
rentheses.[b] In degassed CH2Cl2 relative to fac-[Ir(ppy)3] (Fp =0.40), lex =360 nm.
[c] Measured in degassed CH2Cl2 solutions at a sample concentration of about 10ꢀ5 m,
and the excitation wavelength was set at 370 nm for all the samples at 293 K. [d] The
triplet radiative lifetimes (tr) were deduced from tr =tP/FP.
!
1, Ir-2, and Ir-4, the T1 S0 transition can be ascribed
!
to LUMO HOMO. It can be clearly seen that the
found beyond 350 nm (Support-
ing
Information,
Figure S1).
Those for Ir-2 and Ir-6 are locat-
ed above 420 nm (Figure S1).
Upon irradiation with 360 nm
UV light, all of these complexes
emit an intense yellow to
orange-red
The maximum emission wave-
lengths (lem for these com-
phosphorescence.
)
plexes range from 556 to
582 nm (Figure 2 B and Table 2).
The structureless pattern of the
photoluminescence (PL) spectra
for all the complexes indicates
the predominantly MLCT charac-
ter for the lowest triplet excited
states in the complexes at 293 K.
The lem for Ir-1 (ca. 556 nm), Ir-2
(ca. 564 nm), Ir-4 (ca. 560 nm),
and Ir-6 (ca. 575 nm) all experi-
ence a bathochromic shift with
respect to that of their hetero-
leptic IrIII analogues with sym-
metric structure (Ir-SO2 ca.
556 nm, Ir-O ca. 505 nm, Ir-N ca.
533 nm, Ir-Si ca. 536 nm, and Ir-
btN ca. 572 nm) (Supporting In-
formation, Figure S2).[6d] Howev-
er, the lem for Ir-3 (ca. 582 nm)
and Ir-5 (ca. 581 nm) show a hyp-
sochromic shift relative to that
of the symmetric analogues con-
taining the B(Mes)2 group (Ir-B
Figure 2. A) UV/Vis and B) PL spectra of the tris-heteroleptic IrIII complexes in CH2Cl2 at 293 K.
Table 3. Contribution of the metal dp orbitals to the corresponding MOs of the tris-heteroleptic IrIII complexes
together with the TD-DFT results.
Complex Contribution Contribution The largest coefficient in The largest coefficient in The oscillator
of metal dp
orbitals to
HOMO [%]
of metal dp
orbitals to
LUMO [%]
the CI expansion of the T1 the CI expansion of the S1 strength (f) of
!
state (S0!T1 excitation
state (S0!S1 excitation
the S1 S0
transition
energy)[a]
energy)[a]
Ir-1
Ir-2
Ir-3
Ir-4
Ir-5
Ir-6
40.77
4.74
3.41
3.41
2.14
3.36
1.43
3.44
H!L: 0.65298
(518.7 nm)
H!L: 0.51536
(522.9 nm)
H!L: 0.66447
(553.0 nm)
H!L: 0.66581
(520.4 nm)
H!L: 0.66440
(548.5 nm)
H!L+1: 0.6593
(558.2 nm)
H!L: 0.68708
(465.9 nm)
H!L: 0.63732
(477.4 nm)
H!L: 0.69060
(483.5 nm)
H!L: 0.69134
(467.2 nm)
H!L: 0.69233
(498.0 nm)
H!L: 0.63853
(470.1 nm)
0.0210
0.0121
0.0303
0.0204
0.0803
0.0138
44.67
44.51
43.28
6.21
[a] H!L represents the HOMO to LUMO transition. CI stands for configuration interaction.
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pattern to that of its symmetric congener Ir-btN (Supporting
Information, Figure S3), which indicates the negligible influ-
ence of the cyclometalating ligand with SO2Ph on the phos-
phorescent behavior of Ir-6. Hence, they emit phosphores-
cence with similar lem
.
Different from the other aforementioned complexes, the tris-
heteroleptic compounds Ir-3 and Ir-5 possess two electron-
withdrawing/-accepting groups, that is, SO2Ph and B(Mes)2,
which are bonded to the two ppy ligands, respectively. The
MO patterns for Ir-3 and Ir-5 are quite similar to those of their
symmetrical counterparts (Ir-B and Ir-B1) bearing the B(Mes)2
group with the HOMO contributed by both the p orbitals of
the phenyl ring chelated with metal center and the dp orbitals
of the Ir atom as well as the LUMO located on the p orbitals of
the cyclometalating ligand possessing the B(Mes)2 group, espe-
cially with substantial contribution of the “empty” pp orbitals
of the boron atom (Figure 3).[13] Based on our previous investi-
gations of the photophysical behavior of Ir-B and Ir-B1,[13] the
MO patterns for Ir-3 and Ir-5 have shown that the electrons in
their MLCT processes are mainly accepted by the B(Mes)2
moiety instead of the pyridyl unit due to the strongly electron-
accepting ability of the B(Mes)2 moiety. It has also been shown
that the SO2Ph group can host an electron in the MLCT pro-
cess of Ir-SO2, in spite of its weaker electron-accepting ability
than that of the B(Mes)2 moiety.[6d,14] Due to the weaker elec-
tron-accepting ability of the SO2Ph group as compared to that
of the B(Mes)2 group, the electrons in the MLCT processes of
Ir-3 and Ir-5 are notably hosted by the B(Mes)2 moiety
(Figure 3). Owing to the stronger electron-accepting ability of
the B(Mes)2 moiety, more stable MLCT states than those in Ir-
SO2 will be formed. As a result, Ir-3 and Ir-5 show bathochrom-
ically-shifted phosphorescence with respect to Ir-SO2. However,
the existence of the electron-withdrawing SO2Ph group in Ir-3
and Ir-5 will definitely restrain the electron-hosting of the
B(Mes)2 moiety in their MLCT processes, which affords a higher
energy level to the MLCT states in Ir-3 and Ir-5 than that of
the MLCT states formed in Ir-B and Ir-B1. Hence, the lem for Ir-
3 and Ir-5 exhibit a hypsochromic effect as compared to that
of Ir-B and Ir-B1 (Table 2 and Supporting Information, Fig-
ure S2).
Figure 3. Plots of the LUMO or LUMO+1 (top) and HOMO (bottom) for the
tris-heteroleptic IrIII complexes.
HOMOs for Ir-1, Ir-2, and Ir-4 are mainly located on both the p
orbitals of the cyclometalating ligands bearing OPh, NPh2, and
SiPh3 groups and the dp orbitals of the Ir center, while their
LUMOs are dominantly contributed by the p orbitals of the cy-
clometalating ligand bearing the SO2Ph moiety. Together with
the different contribution of dp orbitals of the Ir center to the
HOMO and LUMO (Table 3), it can be concluded that the emis-
sive T1 excited states involved in Ir-1, Ir-2, and Ir-4 exhibit
both MLCT and LLCT characters. However, their symmetric
counterparts Ir-O, Ir-N, Ir-Si, and Ir-SO2 possess T1 excited
states with dominating MLCT features.[6d] Obviously, the tris-
heteroleptic structures for Ir-1, Ir-2, and Ir-4 with the electron-
withdrawing SO2Ph group in one ligand will definitely facilitate
both the MLCT and LLCT processes and hence produce more
stable T1 excited states. As a result, Ir-1, Ir-2, and Ir-4 show
a
bathochromic effect in their phosphorescent emission
!
maxima and absorption bands assigned to T1 S0 transitions
with respect to that of the symmetric counterparts (Table 2
and Supporting Information, Figure S2). Owing to the much
stronger electron-donating ability of the NPh2 group than that
of OPh and SiPh3 units, Ir-2 displays a longer phosphorescent
emission wavelength than that of Ir-1 and Ir-4 (Figure 2B and
Table 2). For Ir-2, its lem (ca. 564 nm) exhibits a red-shift of
about 31 nm as compared with that of its symmetric counter-
part Ir-N. Despite having an NPh2 group like Ir-2, Ir-6 only
shows a small red-shift of about 3 nm in its lem (ca. 575 nm)
with respect to the symmetric Ir-btN counterpart. This result
can also be supported from the TD-DFT calculations. Different
Thus, it can be clearly seen that the photophysical behavior
and energy level for the emissive MLCT excited state can be di-
versely adjusted by judiciously tuning the electronic structure
and the combination of the different organic ligands. The re-
sults have also shown that such a molecular design can fur-
thermore provide a good approach for manipulating the opto-
electronic properties of tris-heteroleptic IrIII complexes without
tedious chemical modification of the organic ligands as com-
pared to the traditional symmetric counterparts.
The phosphorescent quantum yields (Fp) for all of the tris-
heteroleptic IrIII triplet emitters were also measured in de-
gassed CH2Cl2 using fac-Ir(ppy)3 as a standard (Table 2). All
complexes showed decent Fp ranging from 0.25 to 0.95 and
relatively short phosphorescent lifetimes (tp) with a magnitude
of about 0.81–1.25 ms. The high Fp and short tp render these
complexes promising for the development of high-per-
formance PHOLEDs.
!
from Ir-1, Ir-2, and Ir-4, the T1 S0 transition in Ir-6 is assigned
to LUMO+1 HOMO which mainly exhibits intra-ligand
!
charge transfer (ILCT) or ligand-centered feature (Figure 3). The
ILCT character for the emissive T1 excited states in Ir-6 is also
indicated by the more structured line-shape of its emission
spectrum (Figure 2B). Furthermore, the MO corresponding to
the formation of T1 excited states in Ir-6 exhibits a very similar
Chem. Asian J. 2015, 10, 252 – 262
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Redox properties
phorescent complexes afforded by the tris-heteroleptic struc-
tures should result in an improved EL performance (see
below).
The electrochemical properties of these tris-heteroleptic IrIII
complexes were investigated by cyclic voltammetry (CV) with
ferrocene as an internal standard. The results are listed in
Table 4 which includes the assignment of the redox processes.
The common redox procedures for all the complexes are the
oxidation of the iridium(III) center at about 0.5 V and the re-
Fabrication and characterization of PHOLEDs
As mentioned above, an ambipolar ability can effectively im-
prove the EL performance through realizing a more balanced
charge injection/transport for
both kinds of charge carriers in
Table 4. Redox properties of the tris-heteroleptic IrIII complexes.
the devices. Clearly, doping the
phosphorescent emitters with an
ox
red
Complex E1/2 [V]
E1/2 [V]
HOMO [eV] LUMO [eV]
ambipolar ability in the emission
Main-group moiety Metal center Main-group moiety Pyridyl moiety
layer (EML) should benefit the
charge injection/transport bal-
ance in the EML to broaden the
charge recombination zone and
improve the EL performance.
More critically, ambipolar phos-
phorescent emitters can effec-
tively trap both kinds of charge
Ir-1
Ir-2
Ir-3
Ir-4
Ir-5
Ir-6
0.44
0.34
0.49
0.46
0.56
0.46
0.55
0.48
ꢀ2.30
ꢀ2.63
ꢀ2.66
ꢀ2.71
ꢀ2.51
ꢀ2.68
ꢀ2.58
ꢀ5.24
ꢀ5.14
ꢀ5.36
ꢀ5.26
ꢀ5.35
ꢀ5.14
ꢀ2.50
ꢀ2.49
ꢀ2.85
ꢀ2.60
ꢀ2.82
ꢀ2.52
ꢀ2.31
ꢀ1.95, ꢀ2.33
ꢀ2.25
ꢀ1.98, ꢀ2.36
ꢀ2.28
0.34
duction process for both the pyridyl moiety from about ꢀ2.5 V
to ꢀ2.7 V and the SO2Ph group (ca. ꢀ2.3 V) (Table 4). With the
electron-donating moieties, Ir-1, Ir-2, and Ir-6 show another
carriers in the EML to improve their recombination efficiency
and hence increase the EL efficiency. In order to confirm the
ambipolar character of the concerned complexes, both hole-
and electron-only devices have been fabricated and their J–V
curves have been obtained. Without loss of generality, the rep-
resentative complexes Ir-1 and Ir-2 have been selected for
constructing the single-carrier devices. The J–V characteristics
of these devices indicated that the complexes are capable of
transporting both electrons and holes with significant currents
of similar magnitude (Supporting Information, Figure S4), con-
firming their good ambipolar features.
ox
oxidation wave at a lower potential (E1/2 ca. 0.44 V, 0.34 V, and
0.34 V for Ir-1, Ir-2, and Ir-6, respectively), which should be as-
signed to the oxidation event for the electron-rich OPh and
NPh2 moieties (Table 4). The electron-accepting B(Mes)2 moiety
should induce the reduction process at about ꢀ2.0 V in Ir-3
(ca. ꢀ1.95 V) and Ir-5 (ca. ꢀ1.98 V) due to its stronger electron-
accepting ability than that of SO2Ph (Table 4).
Owing to the much stronger electron-donating ability of
NPh2 than that of OPh, the first oxidation process for Ir-2 and
Ir-6 (ca. 0.34 V) occurs at a lower potential than that of Ir-1 (ca.
0.44 V) (Table 4). Compared with that of Ir-1, Ir-2, Ir-4, and Ir-6,
the higher oxidation potential for the IrIII center in Ir-3 and Ir-5
can be ascribed to the strongly electron-withdrawing/-accept-
ing ability of SO2Ph and B(Mes)2 moieties, which reduces the
electron density on the IrIII center to some extent. Without
a group possessing an obvious electron-donating character, Ir-
4 just shows one oxidation procedure at about 0.46 V, attribut-
able to the oxidation of the metal center.
From the obtained CV data for all the tris-heteroleptic IrIII
complexes, it can be clearly seen that Ir-1, Ir-2, and Ir-6 have
a higher HOMO level as well as a lower LUMO level, which fa-
cilitates the injection for both kinds of charge carriers. Further-
more, the structural features reveal the potential of the ambi-
polar character.[8d,e] On the contrary, Ir-3 and Ir-5 with only
electron-withdrawing groups possess much lower LUMO
levels, which should ensure their dominant EI/ET abilities. Com-
pared with all the other complexes and its symmetric counter-
parts,[6d] Ir-4 might show some ambipolar character as well,
since the oxidation potential for the IrIII center is not too high.
It is well known that a balanced injection and transport for
both kinds of charge carriers is very critical for attaining high
EL efficiencies through enhancing their recombination efficien-
cy. Hence, the ambipolar properties of the concerned phos-
Inspired by their ambipolar properties, the phosphorescent
IrIII complexes with tris-heteroleptic structure have been em-
ployed to fabricate PHOLEDs with the configuration of ITO/
MoO3 (10 nm)/HT-1 (40 nm)/x wt-% Ir:CBP (25 nm)/TPBI
(45 nm)/LiQ (2 nm)/Al (100 nm). 4,4’-N,N’-Dicarbazolebiphenyl
(CBP) was incorporated as the host material for the phosphor-
escent emitters, while 2,2’,2’’-(1,3,5-phenylene)-tris(1-phenyl-
1H-benzimidazole) (TPBI) was employed to construct the func-
tional layer serving the purpose of hole blocking and electron
transport. The hole injection layer and electron injection layer
were made from MoO3 and (8-hydroxyquinolinato)lithium
(LiQ), respectively. The hole-transporting layer was constructed
by using carbazole-based HT-1 developed in our laboratory.[7j]
In order to optimize the EL performance, devices with different
doping levels were fabricated (Figure 4).
All the devices emitted yellow to orange-red electrophos-
phorescence when a proper driving voltage was applied
(Figure 5). The EL spectra showed high resemblance to the PL
spectra of the phosphorescent emitters used in the devices
(Figure 2B and Figure 5), indicating that the same triplet excit-
ed states induced both the EL and PL processes. With increas-
ing driving voltage (V), the emitted luminance (L) was intensi-
fied together with an increase in the current density (J)
(Figure 6 and Supporting Information, Figure S5). The devices
made with the tris-heteroleptic phosphorescent emitters show-
Chem. Asian J. 2015, 10, 252 – 262
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257
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
Figure 5. EL spectra for the best devices at about 8 V.
performance (10.66%, 36.52 cdAꢀ1 and 17.59 LmWꢀ1 for
device D3) inferior to that of the analogues Ir-1, Ir-2, and Ir-6
showing typical ambipolar features. Bearing two kinds of EI/ET
groups, Ir-3 and Ir-5 showed obviously an EI/ET ability rather
than HI/HT feature. Accordingly, Ir-3 and Ir-5 should suffer
more from an unbalanced charge carrier injection/transport
problem than Ir-4. Thus, Ir-3 and Ir-5 resulted in lower EL effi-
ciencies than the rest of the tris-heteroleptic triplet emitters
(Table 5). The best device C2 doped with Ir-3 just showed an
EL efficiency with hext of 9.95%, hL of 29.50 cdAꢀ1, and hp of
15.97 LmWꢀ1, while the device E2 with Ir-5 as emitter exhibit-
ed an even poorer EL performance. Taking all the EL results
into consideration, the critical role played by both the ambipo-
lar property of the phosphorescent emitter and the balanced
charge carrier injection/transport in enhancing EL efficiencies
can be clearly seen.
Figure 4. General configuration of the PHOLED devices and the molecular
structures of the relevant compounds used in these devices.
ing an ambipolar character displayed very attractive EL per-
formances at the optimized doping level (Table 5). The opti-
mized device A4 with Ir-1 as emitter can be turned on at 4.0 V
and showed excellent EL performance with a maximum bright-
ness (Lmax) of 43522 cdmꢀ2 at 14.0 V, a peak external quantum
efficiency (hext) of 20.20%, a luminance efficiency (hL) of
69.41 cdAꢀ1, and a power efficiency (hp) of 35.15 LmWꢀ1. The
device B1 made from Ir-2 also furnished an attractive EL ability,
as indicated by hext of 13.09%, hL of 44.36 cdAꢀ1, hp of
23.20 LmWꢀ1, Lmax of 48759 cdmꢀ2 at 14.0 V, and Vturnꢀon of
4.4 V. With Ir-6 as triplet emitter, the best device F2 gave Lmax
of 24948 cdmꢀ2 at 12.0 V and re-
Phosphorescent emitters with either HI/HT property or EI/ET
feature afforded by the same functional group, that is, Ir-O, Ir-
N, and Ir-SO2, have been successfully developed. Unfortunate-
ly, they generally showed hext of about 10%, hL of about
35 cdAꢀ1
,
and hp of 22 LmWꢀ1 (Supporting Information,
Table S2). Thus, their EL performance was poorer than those
associated with the tris-heteroleptic IrIII complexes possessing
ambipolar character with the same device configuration. The
symmetric heteroleptic ppy-type IrIII complex bearing the main
sulted in a peak EL efficiency hext
of 11.31%, hL of 34.59 cdAꢀ1
,
Table 5. The performance of the optimized PHOLEDs doped with tris-heteroleptic IrIII complexes.
and hp of 20.89 LmWꢀ1. Despite
the fact that Ir-1, Ir-2, and Ir-6
all possess ambipolar properties,
Ir-1 exhibited a much better EL
performance than Ir-2 and Ir-6.
This result might be ascribed to
the notion that the EI/ET ability
of the SO2Ph group cannot
match the high HI/HT ability of
the NPh2 group, which has in-
duced a relatively unbalanced
charge carrier injection and
transport ability of Ir-2 and Ir-6.
Showing less ambipolar charac-
ter, Ir-4 generally afforded an EL
Device Phosphorescent
dopant
Vturnꢀon
[V]
Lmax
hext
%
hL
hp
lmax [nm][b]
[a]
[a]
[a]
[cdmꢀ2
]
[cdAꢀ1
]
[LmWꢀ1
]
A4
B1
C2
D3
E2
F2
Ir-1 (9 wt%)
Ir-2 (6 wt%)
Ir-3 (6 wt%)
Ir-4 (12 wt%)
Ir-5 (6 wt%)
Ir-6 (3 wt%)
4.0
4.4
4.2
3.8
4.8
4.0
43522 (14.0) 20.20
69.41 (6.2) 35.15 (6.2)
44.34 (6.0) 23.20 (6.4)
556 (0.45,
0.52)
564 (0.49,
0.50)
588 (0.55,
0.44)
564 (0.48,
0.50)
580 (0.53,
0.46)
575 (0.52,
0.45)
(6.2)
48759 (14.0) 13.09
(6.0)
41352 (16.0) 9.95 (7.8) 29.50 (7.8) 15.97 (7.8)
52952 (12.0) 10.66
(6.6)
44401 (15.0) 7.41 (9.6) 25.24 (9.6) 8.42 (8.8)
36.52 (6.8) 17.59 (6.6)
24948 (12.0) 11.31
(5.2)
34.59 (5.2) 20.89 (5.2)
[a] Maximum values of the devices. Values in parentheses are the voltages at which they were obtained. [b] CIE
coordinates (x, y) are shown in parentheses.
Chem. Asian J. 2015, 10, 252 – 262
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258
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
Conclusion
Introducing different functional
moieties with charge carrier in-
jection/transport properties to
the two cyclometalating ligands,
a class of tris-heteroleptic ppy-
type IrIII phosphorescent emitters
has been successfully developed.
The functional groups not only
affect the basic photophysical
properties but also afford unique
charge carrier injection/transport
abilities, including ambipolar
and EI/ET features. Encouraging-
ly, the ambipolar character af-
forded by the tris-heteroleptic
structures can furnish phosphor-
escent IrIII complexes with im-
pressive EL efficiencies, such as
hext of 20.20%, hL of 69.41 cdAꢀ1
,
and hp of 35.15 LmWꢀ1. Given
the unique ambipolar abilities
and attractive device per-
formance, these phosphorescent
complexes hold significant po-
tential for their application in
PHOLEDs. More importantly, this
study might represent a new
avenue for the development of
highly efficient phosphorescent
emitters for improving the EL
performance of PHOLEDs.
Figure 6. The J–V–L and the efficiency versus current density relationship for devices A4, B2, and F2.
ligands integrated with both POPh2 and NPh2 groups has been
developed. The more complicated orange emitter with the
symmetric structure led to a reasonable EL performance with
Experimental Section
NL of 29.6 cdAꢀ1, Next of 8.8%, and NP of 15.5 LmWꢀ1.[8d] The
ppy-type PtII complex bearing both B(Mes)2 and NPh2 groups
in one ppy-type ligand has also been obtained showing
General information: All reactions were conducted under a nitro-
gen atmosphere and no special precautions were required during
a good EL efficiency with NL of 35.0 cdAꢀ1, Next of 10.1%, and NP the workup. Solvents were carefully dried and distilled from appro-
of 36.6 LmWꢀ1 [8e]
However, by comparing these reported EL
.
priate drying agents prior to use. Commercially available reagents
were used without further purification unless otherwise stated. All
reactions were monitored by TLC with Merck precoated aluminum
plates. Flash column chromatography was carried out using silica
gel from Shenghai Qingdao (200–300 mesh).
results with the corresponding data obtained in this study, the
advantages associated with the tris-heteroleptic structure to
afford an ambipolar feature to the IrIII phosphorescent emitters
become apparent. Different from the reported symmetric am-
bipolar phosphorescent emitters with both electron-donating
and electron-withdrawing moieties in the same ligand, the
newly developed IrIII phosphorescent emitters bear electron-
Physical measurements: UV/Vis spectra were recorded on a Shi-
madzu UV-2250 spectrophotometer. The photoluminescent proper-
ties of the tris-heteroleptic complexes were characterized using an
donating and electron-withdrawing moieties in different li- Edinburgh Instruments FLS920 spectrophotometer. The phosphor-
escence quantum yields (FP) were determined in CH2Cl2 solutions
at 293 K against fac-[Ir(ppy)3] standard (FP =0.40). The lifetimes
were measured by using an Edinburgh Instruments FLS920 single
photon counting spectrometer with a nano-LED as the excitation
source. The data analysis was conducted by iterative convolution
of the luminescence decay profile with the instrument response
function using the software package provided by Edinburgh Instru-
ments. Fast atom bombardment (FAB) mass spectra were collected
with a Finnigan MAT SSQ710 system. The thermal gravimetric anal-
gands. The tris-heteroleptic structure can separate the distribu-
tion of HOMO and LUMO more effectively, which has been
shown to enhance the ambipolar features of the concerned
complexes.[6h] Thus, these complexes showed improved EL per-
formances as compared with those reported previously. Hence,
such a design of ambipolar tris-heteroleptic phosphorescent
emitters might represent a new platform to develop high-per-
formance emitters for PHOLEDs.
Chem. Asian J. 2015, 10, 252 – 262
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259
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
ysis (TGA) was conducted on a NETZSCH STA 409C instrument
under nitrogen with a heating rate of 208C minꢀ1. Differential scan-
ning calorimetry (DSC) was performed on a NETZSCHDSC 200 PC
unit under a nitrogen flow at a heating rate of 108C minꢀ1. Electro-
chemical measurements were made using a Princeton Applied Re-
search model 2273A potentiostat at a scan rate of 100 mVsꢀ1. A
conventional three-electrode configuration consisting of a glassy
carbon working electrode, a Pt-sheet counter electrode, and a Pt-
wire reference electrode was used. The supporting electrolyte was
0.1m [Bu4N]PF6 in MeCN. Ferrocene was added as a calibrant after
each set of measurements, and all potentials reported are quoted
with reference to the ferrocene–ferrocenium (Fc/Fc+) couple. The
oxidation (Eox) and reduction (Ered) potentials were used to deter-
mine the HOMO and LUMO energy levels using Equations (1) and
(2):
5.24 (s, 1H, acac), 1.83 (s, 3H, acac), 1.78 ppm (s, 3H, acac);
13C NMR (100 MHz, CDCl3): d=184.74, 184.68, 167.71, 166.31,
149.98, 148.93, 148.25, 147.99. 147.68, 147.20, 146.43, 142.09,
139.35, 137.90, 137.08, 136.67, 132.41, 131.29, 128.94, 128.78,
128.75, 127.34, 125.20, 125.05, 124.72, 124.51, 123.52, 122.85,
122.70, 120.40, 119.58, 119.51, 117.92, 114.56 (Ar), 100.56, 28.74,
28.71 ppm (acac); FAB-MS (m/z): 907 [M]+; elemental analysis calcd
(%) for C45H36IrN3O4S: C, 59.59; H, 4.00; N, 4.63; found: C, 59.37; H,
3.89; N, 4.56.
1
Ir-3: Eluent: CH2Cl2/Et2O (35:1, v/v). Yield: 24%. H NMR (400 MHz,
CDCl3): d=8.48 (d, J=5.6 Hz, 1H, Ar), 8.34 (d, J=5.6 Hz, 1H, Ar),
7.89 (d, J=8.0 Hz, 1H, Ar), 7.80 (t, J=7.6 Hz, 1H, Ar), 7.55 (d, J=
8.4 Hz, 2H, Ar), 7.50–7.46 (m, 3H, Ar), 7.28–7.20 (m, 6H, Ar), 6.98 (t,
J=6.4 Hz, 1H, Ar), 6.88 (d, J=8.0 Hz, 1H, Ar), 6.65 (s, 1H, Ar), 6.62
(s, 4H, Ar), 5.91 (s, 1H, Ar), 5.25 (s, 1H, acac), 2.30 (s, 6H, Mes), 1.82
(s, 3H, acac), 1.78 (s, 3H, acac), 1.61 ppm (s, 12H, Mes); 13C NMR
(100 MHz, CDCl3): d=184.90, 184.63, 167.76, 165.97, 148.55, 148.22,
142.25, 142.04, 141.31, 140.50, 139.18, 137.58, 137.34, 136.24,
132.15, 131.22, 128.88, 128.62, 127.55, 127.06, 123.54, 122.92,
122.62, 122.44, 119.68, 119.45, 119.34 (Ar), 100.53, 28.74, 28.68
(acac), 23.00, 21.27 ppm (Mes); FAB-MS (m/z): 988 [M]+; elemental
analysis calcd (%) for C51H48BIrN2O4S: C, 62.00; H, 4.90; N, 2.84;
found: C, 61.87; H, 4.78; N, 2.66.
EHOMO ¼ ꢀðEoxþ4:8Þ eV
and
ð1Þ
ELUMO ¼ ꢀðEredþ4:8Þ eV
ð2Þ
which were calculated using the internal standard ferrocene value
of 4.8 eV with respect to the vacuum level.
1
Ir-4: Eluent: CH2Cl2/Et2O (40:1, v/v). Yield: 23%. H NMR (400 MHz,
CDCl3): d=8.45 (d, J=6.0 Hz, 1H, Ar), 8.33 (d, J=5.6 Hz, 1H, Ar),
7.85 (d, J=8.0 Hz, 1H, Ar), 7.78 (t, J=8.0 Hz, 1H, Ar), 7.59–7.57 (m,
3H, Ar), 7.53–7.43 (m, 4H, Ar), 7.39 (t, J=7.6 Hz, 2H, Ar), 7.34–7.28
(m, 6H, Ar), 7.24 (s, 4H, Ar), 7.20 (d, J=7.2 Hz, 6H, Ar), 7.17 (s, 1H,
Ar), 6.98–6.93 (m, 2H, Ar), 6.70 (s, 1H, Ar), 6.22 (s, 1H, Ar), 5.24 (s,
1H, acac), 1.80 (s, 1H, acac), 1.78 ppm (s, 3H, acac); 13C NMR
(100 MHz, CDCl3): d=184.74, 148.33, 144.71, 140.71, 137.39, 136.98,
136.20, 134.59, 132.35, 131.19, 129.01, 128.85, 128.74, 127.49,
127.19, 123.60, 122.95, 122.71, 122.09, 119.62, 119.33, 118.88 (Ar),
100.54, 28.70 ppm (acac); FAB-MS (m/z): 998 [M]+; elemental analy-
sis calcd (%) for C51H41IrN2O4SSi: C, 61.36; H, 4.14; N, 2.81; found: C,
61.28; H, 4.10; N, 2.69.
General procedure for the synthesis of tris-heteroleptic iridiu-
m(III) complexes: Under a N2 atmosphere, L0, L (L=L1–L6, 1.0
equivalent each), and IrCl3·nH2O were heated to 1108C in a mixture
of ethoxyethanol and water (3:1, v/v) for 18 h. The reaction mixture
was then cooled to room temperature and water was added. The
chloride-bridged precursor complexes were formed as colored pre-
cipitates, which were collected by filtration and dried in vacuum.
The precursor complex, acetylacetone (5.0 equiv), and Na2CO3
(10.0 equiv) were added to ethoxyethanol under N2. The reaction
mixture was heated to 1108C for 16 h before it was cooled to
room temperature. Water was added, and the crude product was
collected. After purification with thin layer chromatography on
silica gel with the appropriate eluent, the complexes were ob-
tained in approximately 20% overall yield.
1
Ir-5: Eluent: CH2Cl2/Et2O (35:1, v/v). Yield: 20%. H NMR (400 MHz,
CDCl3): d=8.54 (d, J=5.6 Hz, 1H, Ar), 8.45 (s, 1H, Ar), 7.77 (t, J=
8.0 Hz, 1H, Ar), 7.61–7.54 (m, 5H, Ar), 7.44 (t, J=8.0 Hz, 1H, Ar),
7.35–7.31 (m, 3H, Ar), 7.26–7.21 (m, 3H, Ar), 6.84–6.77 (m, 6H, Ar),
6.64 (t, J=7.8 Hz, 1H, Ar), 6.11(d, J=8.0 Hz, 1H, Ar), 5.10 (s, 1H,
acac), 2.23 (s, 6H, Mes), 2.11 (s, 12H, Mes), 1.78 (s, 3H, acac),
1.23 ppm (s, 3H, acac); 13C NMR (100 MHz, CDCl3): d=185.10,
184.44, 170.57, 166.74, 155.89, 149.94, 149.08, 148.66, 148.62,
144.87, 144.07, 142.31, 140.93, 139.51, 139.33, 137.20, 132.78,
132.39, 131.51, 129.88, 128.77, 128.53, 127.30, 125.10, 123.79,
123.04, 122.75, 121.22, 120.11, 119.58, 118.02 (Ar), 100.74 (acac),
28.52, 27.99 (Mes), 23.58, 21.29 ppm (acac); FAB-MS (m/z): 988 [M]+
; elemental analysis calcd (%) for C51H48BIrN2O4S: C, 62.00; H, 4.90;
N, 2.84; found: C, 61.83; H, 4.88; N, 2.76.
1
Ir-1: Eluent: CH2Cl2/Et2O (35:1, v/v). Yield: 20%. H NMR (400 MHz,
CDCl3): d=8.47 (d, J=6.0 Hz, 1H, Ar), 8.38 (d, J=5.6 Hz, 1H, Ar),
7.80–7.75 (m, 3H, Ar), 7.69 (d, J=7.6 Hz, 1H, Ar), 7.65 (d, J=8.0 Hz,
2H, Ar), 7.55 (d, J=8.0 Hz, 1H, Ar), 7.48–7.42 (m, 2H, Ar), 7.37 (d,
J=7.6 Hz, 2H, Ar), 7.32 (t, J=8.0 Hz, 1H, Ar), 7.20–7.14 (m, 4H, Ar),
7.00 (t, J=7.2 Hz, 1H, Ar), 6.77 (d, J=8.0 Hz, 2H, Ar), 6.73 (s, 1H,
Ar), 6.36 (dd, J=2.0 Hz, 8.4 Hz, 1H, Ar), 5.68 (d, J=2.4 Hz, 1H, Ar),
5.24 (s, 1H, acac), 1.82 (s, 3H, acac), 1.77 ppm (s, 3H, acac);
13C NMR (100 MHz, CDCl3): d=184.83, 184.75, 167.63, 157.56,
156.45, 149.95, 148.65, 148.49, 148.05, 141.94, 139.58, 139.54,
137.34, 137.21, 132.46, 131.12, 129.31, 128.77, 127.42, 125.19,
123.67, 122.99, 121.99, 121.08, 119.78, 119.62, 119.47, 118.27, 110.71
(Ar), 100.58, 28.70, 28.65 ppm (acac); FAB-MS (m/z): 832 [M]+; ele-
mental analysis calcd (%) for C39H31IrN2O5S: C, 56.30; H, 3.76; N,
3.37; found: C, 56.52; H, 3.48; N, 3.16.
1
Ir-6: Eluent: CH2Cl2/Et2O (45:1, v/v). Yield: 20%. H NMR (400 MHz,
CDCl3): d=8.24 (d, J=6.4 Hz, 1H, Ar), 7.88–7.86 (m, 2H, Ar), 7.60
(d, J=8.0 Hz, 1H, Ar), 7.56–7.46 (m, 4H, Ar), 7.43–7.34 (m, 5H, Ar),
7.28–7.24 (m, 2H, Ar), 7.14 (m, 3H, Ar), 6.90 (d, J=8.0 Hz, 4H, Ar),
6.70 (s, 1H, Ar), 6.48 (d, J=8.4 Hz, 1H, Ar), 5.50 (s, 1H, Ar), 5.17 (s,
1H, acac), 1.78 (s, 3H, acac), 1.75 ppm (s, 3H, acac); 13C NMR
(100 MHz, CDCl3): d=185.22, 184.97, 178.43, 166.86, 150.88, 149.94,
148.63, 148.52, 148.31, 148.08, 146.75, 141.40, 140.12, 136.95,
134.27, 132.77, 132.46, 131.26, 128.91, 128.67, 127.58, 126.97,
1
Ir-2: Eluent: CH2Cl2/Et2O (40:1, v/v). Yield: 18%. H NMR (400 MHz,
CDCl3): d=8.35 (d, J=7.2 Hz, 2H, Ar), 7.73–7.64 (m, 4H, Ar), 7.59
(d, J=8.0 Hz, 1H, Ar), 7.53–7.44 (m, 3H, Ar), 7.39–7.33 (m, 3H, Ar),
7.29 (s, 1H, Ar), 7.10–7.06 (m, 5H, Ar), 7.00 (t, J=2.0 Hz, 1H, Ar),
6.92 (t, J=7.2 Hz, 2H, Ar), 6.80 (d, J=8.0 Hz, 4H, Ar), 6.76 (s, 1H,
Ar), 6.44 (dd, J=2.4 Hz, 8.8 Hz, 1H, Ar), 5.48 (d, J=2.0 Hz, 1H, Ar),
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126.58, 125.93, 124.34, 123.75, 123.48, 123.42, 122.91, 122.31,
119.39, 119.17, 119.14, 114.07 (Ar), 101.14, 28.59, 28.35 ppm (acac);
FAB-MS (m/z): 963 [M]+; elemental analysis calcd (%) for
C47H36IrN3O4S2: C, 58.61; H, 3.77; N, 4.36; found: C, 58.53; H, 3.83;
N, 4.42.
P-03/08) and Hong Kong Baptist University (FRG2/12-13/083)
for financial support. The work was also supported by Partner
State Key Laboratory of Environmental and Biological Analysis
(SKLP-14-15-P011) and Strategic Development Fund of HKBU.
Keywords: ambipolar · heteroleptic · iridium(III) complexes ·
organic light-emitting diodes · phosphorescence
OLED fabrication and measurements: The pre-cleaned ITO glass
substrates were treated with ozone for 20 min. Then, a 10 nm-thick
layer of MoO3 was deposited on the ITO glass substrates before
a 40 nm-thick HT-1 film was deposited. The corresponding IrIII
phosphorescent emitter and CBP host were co-evaporated to form
a 25 nm emitting layer. Successively, TPBI, LiQ, and Al were evapo-
rated at a base pressure of less than 10ꢀ6 Torr. The EL spectra and
CIE coordinates were measured with a PR650 Spectra colorimeter.
The J–V–L curves of the devices were recorded using a Keithley
2400/2000 source meter and the luminance was measured using
a PR650 SpectraScan spectrometer. All of the experiments and
measurements were carried out at room temperature under ambi-
ent conditions.
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Acknowledgements
This work was supported by Tengfei Project from Xi’an Jiao-
tong University, National Basic Research Program of China (973
Project 2013CB834702), the Key Creative Scientific Research
Team in Shannxi Province (2013KCT-05), the Fundamental Re-
search Funds for the Central Universities, the National Natural
Science Foundation of China (nos. 20902072 and 21171098),
the Program for New Century Excellent Talents in University,
the Ministry of Education of China (NECT-09–0651) and Funda-
mental Research Funds for the Central Universities. We also
thank Hong Kong Research Grants Council (HKBU203011 and
HKUST2/CRF/10), Areas of Excellence Scheme of HKSAR (AoE/
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Published online on December 15, 2014
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