Multifunctional homoleptic iridium(III) dendrimers towards solution-processed nondoped electrophosphorescence with low efficiency roll-off

Article abstract of DOI:10.1016/j.orgel.2014.04.008

A series of new iridium dendrimers comprised of bifunctional charge transport peripheral groups have been designed and facilely synthesized. The relationship between the structures and their photophysical, electrochemical and electrophosphorescent performances is investigated. Through the incorporation of rigid electron-transporting phosphine oxide groups and/or hole-transporting arylamine units, the new complexes all have good thermal stabilities with high glass-transition temperature up to 284 °C. Besides, the peripheries sufficiently shield the emissive core from the intermolecular interactions and prevent luminance quenching in neat films. Solution-processed phosphorescent organic light-emitting device (PhOLED) based on bipolar phosphor 2 as neat emitter achieves a maximum current efficiency of 12.4 cd A^-1 with Commission Internationale de l'Eclairage coordinates of (0.57, 0.42), and the value remains at 11.5 cd A^-1 at a practical luminance of 1000 cd m^-2. This low roll-off can be attributed to the bipolar nature of the emitter. This indicates that rational incorporation of charge-transporting moieties into the sphere of iridium(III) core is a simple and effective approach to develop efficient host-free phosphors for solution-processable nondoped PhOLEDs.

Full text of DOI:10.1016/j.orgel.2014.04.008

Organic Electronics 15 (2014) 1598–1606
Contents lists available at ScienceDirect
Organic Electronics
journal homepage: www.elsevier.com/locate/orgel
Multifunctional homoleptic iridium(III) dendrimers towards
solution-processed nondoped electrophosphorescence with low
efficiency roll-off
Minrong Zhu a, Yanhu Li b, Jingsheng Miao b, Bei Jiang a, Chuluo Yang a, , Hongbin Wu b,
⇑
⇑
,
Jingui Qin a, Yong Cao b
a Hubei Collaborative Innovation Center for Advanced Organic Chemical Materials, Hubei Key Lab on Organic and Polymeric Optoelectronic Materials,
Department of Chemistry, Wuhan University, Wuhan 430072, People’s Republic of China
b Institute of Polymer Optoelectronic Materials and Devices, State Key Laboratory of Luminescent Materials and Devices, South China University of Technology,
Guangzhou 510640, People’s Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of new iridium dendrimers comprised of bifunctional charge transport peripheral
groups have been designed and facilely synthesized. The relationship between the struc-
tures and their photophysical, electrochemical and electrophosphorescent performances
is investigated. Through the incorporation of rigid electron-transporting phosphine oxide
groups and/or hole-transporting arylamine units, the new complexes all have good thermal
stabilities with high glass-transition temperature up to 284 °C. Besides, the peripheries suf-
ficiently shield the emissive core from the intermolecular interactions and prevent lumi-
nance quenching in neat films. Solution-processed phosphorescent organic light-emitting
device (PhOLED) based on bipolar phosphor 2 as neat emitter achieves a maximum current
efficiency of 12.4 cd Aꢀ1 with Commission Internationale de l’Eclairage coordinates of
Received 23 October 2013
Received in revised form 2 April 2014
Accepted 8 April 2014
Available online 24 April 2014
Keywords:
Iridium complex
Electrophosphorescence
Charge transport
Dendrimer
(0.57, 0.42), and the value remains at 11.5 cd Aꢀ1 at a practical luminance of 1000 cd mꢀ2
.
This low roll-off can be attributed to the bipolar nature of the emitter. This indicates that
rational incorporation of charge-transporting moieties into the sphere of iridium(III) core is
a simple and effective approach to develop efficient host-free phosphors for solution-pro-
cessable nondoped PhOLEDs.
Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction
considerable interests because of the demonstrated poten-
tial for achieving unity internal quantum efficiency [2].
During the past decade, organic light-emitting diodes
(OLEDs) have emerged as competitive candidates for
solid-state lighting sources and full-color flat-panel dis-
plays [1]. Endowed with the capability to harvest both
singlet and triplet excitons, phosphorescent organic
light-emitting diodes (PhOLEDs) with transition metal
complexes as luminescent materials have attracted
Given this, the search for stable and efficient phosphores-
cent emitters covering the entire visible light spectrum is
particularly crucial for the commercialization of OLEDs
[3]. Since it could act as the complementary color of the
blue one to realize high-performance white light emission,
the development of efficient orange phosphors has
attracted great attention [4].
Till now, most of the efficient PhOLEDs have been fabri-
cated through vacuum thermal evaporation in multilayer
configurations [5]. But the layer-by-layer deposition tech-
nique allowing for complicated device structures to
⇑
Corresponding authors. Tel.: +86 27 68756101.
E-mail addresses: clyang@whu.edu.cn (C. Yang), hbwu@scut.edu.cn
(H. Wu).
http://dx.doi.org/10.1016/j.orgel.2014.04.008
1566-1199/Ó 2014 Elsevier B.V. All rights reserved.

M. Zhu et al. / Organic Electronics 15 (2014) 1598–1606
1599
achieve high performance is
a
rather costly process.
O
O
Tedious and complicated device fabrication processes are
too far cost-effective to compete with other contemporary
flat-display technologies. To simplify the device structure,
many multifunctional materials possessing luminescent
and charge injection/transporting characteristics have
been developed [6]. Though numerous efforts have been
carried out to improve the performance of the solution-
processed PhOLEDs, their efficiencies are still far behind
the vacuum-evaporated devices [7].
P
P
N
N
Ir
Ir
3
3
N
N
Recently, several groups have been interested in inte-
gration of functional units with charge injection and trans-
porting characters into the organic ligands to generate
small-molecule or dendritic heavy metal emitters [8]. We
have developed a series of triphenylamine-based iridium
dendrimers for highly efficient solution-processed elec-
trophosphorescence [9]. Bearing excellent hole-transport-
ing ability, the outer triphenylamine units not only act as
good spacers isolating the iridium core but also participate
straightforwardly in charge injection, transport and energy
transfer. We have demonstrated that the first-generation
of the current Ir dendritic array is structurally enough for
solution-processable host-free phosphors. Up to now, most
of the functional iridium complexes and/or dendrimers
reported have been constructed from hole-type arylamine
groups or polyphenylene dendrons [10]. Generally, elec-
tron mobilities of organic materials are usually two or
three orders of magnitude lower than hole mobilities,
which would lead to reduced device efficiency and lifetime
due to poor carrier balance and an accumulation of positive
charge in the recombination zone [11].
1
2
O
P
N
N
N
Ir
Ir
3
3
O
P
O
P
4
3
Scheme 1. The chemical structures of complexes 1–4.
and o-dichlorobenzne proportionally at 230 °C. The chem-
ical structures of the iridium complexes were verified by
1H NMR, 13C NMR spectroscopy, and matrix-assisted laser
desorption–ionization time-of-flight (MALDI-TOF) mass
spectrometry, and elemental analysis.
To address this issue, we have designed and synthesized
four first-generation iridium dendrimers (Scheme 1) fea-
tured with electron-transporting phosphine oxide groups
and/or hole-transporting arylamine units to facilitate
charge injection/transport, and explored their applications
as nondoped emitters for solution-processed electrophos-
phorescence. Phosphine oxide derivatives have been the
subject of electron-transport materials for optoelectronic
devices [12]. We anticipate that the charge balance of the
self-host iridium dendrimers could be manipulated at
molecular level through the integration of the electron-
donor and -acceptor.
2.2. Thermal properties
The thermal properties of the new phosphors were
investigated by using differential scanning calorimetry
(DSC) and thermogravimetric analysis (TGA). With molec-
ular weights above 1900 g molꢀ1, the four phosphors show
thermal-decomposition temperatures (Td, corresponding
to 5% weight loss, Fig. 1) around 500 °C, which are much
higher than those of small-molecule phosphors Ir(ppy)3
(413 °C) and Ir(piq)3 (384 °C) [14]. Furthermore, they all
exhibit very high glass-transition temperatures (Tg) in the
ranges from 246 °C to 284 °C. Among them, complex 4
has the highest Tg value (284 °C) due to the presence of
two bulky and rigid phosphine oxide groups, which is con-
sistent to the findings that the presence of arylphosphine
oxide moieties incorporated into polymers imparts high
thermal stability [15]. Compared with those of 1 and 3
bearing triphenylamine unit, 2 with carbazole as elec-
tron-donor reveals higher Tg value since carbazole is more
rigid than triphenylamine [16]. The excellent thermal
properties of the phosphorescent emitters should favor
the morphological stability of thin films in nondoped elec-
troluminescent devices.
2. Results and discussion
2.1. Synthesis and characterization
Scheme 2 illustrates the synthesis of the new first-
generation iridium(III) dendrimers 1–4. For L1, L2 and L3,
different charge-transporting aryl groups were attached
to the 2,4-dibromopyridine through stepwise Suzuki–
Miyaura cross-coupling reactions to form unsymmetrically
functionalized ligands [13]. L4 bearing two electron-
withdrawing aryl phosphine oxide was prepared in one
step using the same coupling reaction. The preparation of
homoleptic complexes was accomplished with a procedure
previously reported by us, which involved treating
Ir(acac)3 (acac = 2,4-pentanedionate) with 3.2 equiv of free
ligand in glycerol mixed with 2-(2-methoxyethoxy)ethanol
2.3. Photophysical properties
The absorption and PL spectra of the complexes
recorded in toluene and films are presented in Fig. 2. The
intense absorptions in the range of 340–380 nm can be

1600
M. Zhu et al. / Organic Electronics 15 (2014) 1598–1606
O
P
O
P
O
O
O
P
Br
B
HO
OH
N
B
N
N
N
N
N
N
Br
Br
Ir
Ir
Ir
6
3
i
ii
N
N
N
5
L1
1
O
O
P
P
Br
O
O
O
P
B
N
N
N
O
O
B
N
Br
Br
6
3
N
i
N
ii
N
Ir(acac)3
7
N
L2
iii
2
N
N
Br
O
O
OH
OH
N
N
B
B
N
Br
Br
3
O
P
O
P
O
P
O
P
i
ii
L3
6
8
3
O
O
P
P
N
O
O
B
P
Br
Br
N
N
Ir
O
ii
3
O
P
O
P
6
L4
4
Scheme 2. Synthetic routes of bipolar Ir(III) complexes. Reagents and conditions: (i) Pd(PPh3)4, 2 M Na2CO3, THF, reflux, 48 h; (ii) Pd(PPh3)4, 2 M Na2CO3,
toluene, ethanol, reflux, 48 h; (iii) Ir(acac)3, o-dichlorobenzene, 2-(2-methoxyethoxy)ethanol, glycerol, 230 °C, 36 h.
solution exhibits a featureless emission peak at 551 nm,
Tg = 247 oC
Tg = 268 oC
Tg = 246 oC
1
2
3
4
which falls in the yellowish green region. The color tuning
of the two complexes results from the influence on the
electronic properties of the appended charge-transporting
moieties on the complexes [18]. The geometry were
optimized at B3LYP/6-31G(d) level, and the electronic
properties of the compounds were calculated at
tHCTHhyb/6-311++G(d, p) level (for details, see Section 4).
Tg = 284 oC
As shown in Fig. S2, the HOMOs consist of a mixture of p-
orbitals of phenyl ring in 2-phenylpyridine (ppy) ligand
and d-orbitals of Ir, whereas the LUMOs delocalize on the
ligand with predominant contribution from the pyridyl
150
200
250
300
350
p-orbitals, which coincide with the literature results [19].
Temperature (oC)
For compound 1 and 2, the HOMO orbitals extend to the
electron-donating diphenylamine/carbazole groups on
the phenyl ring. In general, the introduction of electron-
donating or -withdrawing groups will increase or decrease
the HOMO level, respectively; while the electron-acceptors
will stabilize the LUMO more than the electron-donors. In
addition, the extended conjugation of the ligand will nar-
row the band gap to some extent. The above-mentioned
three factors contribute to color-tuning of the phosphors.
For example, complex 3 emits yellowish green light due
to the decreased HOMO energy level by phosphine oxide,
Fig. 1. DSC traces of 1–4 recorded at a heating rate of 10 °C minꢀ1
.
assigned to the spin-allowed
p–
p* transitions of cyclo-
metalating ligands. The weak absorption shoulders in
lower-energy region can be attributed to a mixture of
spin-allowed singlet metal-to-ligand charge transfer
(1MLCT) and spin-forbidden 3MLCT transitions [17].
Complex 1 emits orange–red light in toluene solution
with an emission maximum at 592 nm, while 3 in toluene

M. Zhu et al. / Organic Electronics 15 (2014) 1598–1606
1601
the increased LUMO energy level by triphenylamine unit
and the increased conjugation.
(a)1.0
L1
L2
L3
L4
Solid state photoluminescence quantum yields (PLQYs)
of the complexes were measured using an integrating
sphere apparatus on the quartz plate and the data are
listed in Table 1. The PLQYs of the four complexes after
doped into poly(methyl methacrylate) (PMMA) matrix at
10% by weight are in the range of 43–55% [20]; it is note-
worthy that the PLQYs in neat films is not significantly
dropped, remaining in the range of 21–33%. This indicates
that the iridium complexes appended with bulky electron-
acceptor arylphosphine oxide and different electron-
donors provide good encapsulation for emissive core and
effectively suppress the intermolecular interactions [21].
0.8
0.6
0.4
0.2
0.0
300
350
400
450
500
Wavelength (nm)
(b) 1.0
1
2
3
4
2.4. Electrochemical properties
0.8
0.6
0.4
0.2
0.0
Cyclic voltammetry (CV) was performed to investigate
the electrochemical behaviors of 1–4 (Fig. 3). The HOMO
energy levels of the compounds were determined from
the half-wave potentials with regard to the energy level
of ferrocene (ꢀ4.8 eV below vacuum); no reduction data
was observed, and thus the LUMO levels were deduced
from HOMO energy levels and optical band gaps deter-
mined by the onset of absorption. As mentioned above,
the strong donor will destabilize the metal d-orbital, lead-
ing to the high HOMO energy level. Complex 1 with the
strongest electron-donating group has the highest HOMO
energy level, which is close to the work function of com-
mon anode buffer layer PEDOT:PSS (ꢀ4.80 eV) and thus
beneficial for the facile charge injection into emission layer
(EML). The HOMO level decreased in the following order:
1 > 2 > 3 ꢁ 4 (Table 1), which coincides well with the order
of the electron-donating abilities of the substitutes on the
ligand.
300
350
400
450
500
550
600
Wavelength (nm)
(c) 1.0
1
2
3
4
0.8
0.6
0.4
0.2
0.0
2.5. Electrophosphorescence
To evaluate the phosphors as self-host phosphorescent
emitters, nondoped PhOLEDs were fabricated with typical
xdouble-layer configuration as follows: ITO/poly(ethylene-
450 500 550 600 650 700 750 800
Wavelength (nm)
dioxythiophene):poly(styrenesulfonate)
(PEDOT:PSS,
50 nm)/iridium complexes (80 nm)/1,3,5-tris(N-phen-
ylbenzimidazol-2-yl)benzene (TPBI, 25 nm)/CsF (1.5 nm)/
Al (150 nm). In this sandwich geometry, TPBI and CsF act
Fig. 2. (a) UV–vis absorption spectra of the ligands in toluene solution, (b)
UV–vis absorption spectra and (c) PL spectra of the phosphors 1–4
recorded both in toluene (open) and films (solid).
Table 1
Photophysical, electrochemical and thermal data of 1–4.
1
2
3
4
Tg/Td (°C)
kabs (nm)
kabs (nm)
247/499
288/375
295/379
592
615
43 (21)
ꢀ4.92/ꢀ2.56
268/505
287/364
293/365
562
585
46 (33)
ꢀ5.20/ꢀ2.60
246/486
293/377
305/404
551
560
55 (29)
ꢀ5.35/ꢀ2.40
284/500
291/401
312/418
572
586
51 (24)
ꢀ5.33/ꢀ2.51
a
b
a
kem,max (nm)
b
kem,max (nm)
c
U
(%)
d
HOMO/LUMO (eV)
a
b
c
Measured in toluene with a concentration of 10ꢀ5 M.
Neat film data (measured with the excitation wavelength of 420 nm for the PL spectra).
Solid state quantum yields using an integration sphere were measured at 298 K in PMMA at 10 wt.%, values in neat films are in parenthesis. All PLQYs
are 5%.
d
The HOMO energies were estimated from the half-wave potentials and the LUMO energies were deduced from the energy gap (Eg) and HOMO.

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M. Zhu et al. / Organic Electronics 15 (2014) 1598–1606
(a) 1.0
1
1
2
3
4
0.8
0.6
0.4
0.2
0.0
2
3
4
-0.5
0.0
0.5
1.0
1.5
450 500 550 600 650 700 750
Wavelength (nm)
Potential vs. Fc/Fc+(V)
Fig. 3. Oxidation scan curves of cyclic voltammograms for the complexes
in CH2Cl2.
(b)
101
100
104
103
102
101
100
10-1
10-2
10-3
10-4
10-5
10-6
10-7
as electron-transporting layer (ETL) and electron-injecting
layer (EIL), respectively. Fig. 4 shows the EL spectra, cur-
rent density–voltage–brightness (J–V–L) characteristics,
and efficiency versus current density curves of the devices,
and Table 2 summarizes the EL data. The electrolumines-
cence (EL) devices display identical spectra with their PL
counterparts without additional emission from the aggre-
gation or the ligands. This further proves that these first-
generation functional iridium dendrimers could be used
alone as EML. The driving voltage decreased in the follow-
ing order: 1 > 2 > 3 ꢁ 4, which coincides well with the
sequence of the HOMO levels. The low turn-on voltage
for 1 (3.5 V) can be attributed to the small energy barrier
between PEDOT:PSS and EML. Low-lying HOMO levels
caused by the electron-withdrawing phosphine oxide
group in 3 and 4 partially induced large turn-on voltages
of the devices due to the retarded hole-injection from
anode buffer layer to EML. Meanwhile, we note that the
devices for 2–4 exhibit relative low leakage current in
the small voltage region (0–4 V). For instance, the leakage
current before turn-on for the device based on 2 is around
10ꢀ3 mA cmꢀ2, which suggests good surface morphology of
the EML. However, the leakage current for the device based
on 1 is as high as 10ꢀ1 mA cmꢀ2, which may leads to the
fair device performance.
Device with complex 1 as EML exhibits the lowest cur-
rent efficiency of 2.4 cd Aꢀ1 with red light emission. For the
complex 2, the device achieves the maximum current effi-
ciency of 12.4 cd Aꢀ1 and maximum external quantum effi-
ciency of 8.8% at a luminance of 250 cd mꢀ2, which are
comparable to the best performances of the solution-pro-
cessed orange–red emitting PhOLEDs [22]. Even at a prac-
tical luminance of 1000 cd mꢀ2, the values are maintained
as high as 11.5 cd Aꢀ1 and 8.2%. The small efficiency roll-off
values should be attributed to the sufficiently depressed
concentration quenching of emissive core and the bipolar
charge-transporting abilities of the ligands containing elec-
tron-donating carbazole and electron-accepting phosphine
oxide unit. The devices based on complexes 3 and 4 show
fair efficiencies in comparison with the device based on
complex 2. To get insight into the origin of lower EL
efficiencies in the devices based on complexes 3 and 4,
charge transporting properties of the complexes were
1
2
3
4
0
2
4
6
8 10 12 14 16 18 20 22 24 26 28 30
Voltage (V)
103
102
101
100
10-1
(c)
101
100
10-1
1
2
3
4
0
10 20 30 40 50 60 70 80
Current Density (mA cm-2)
Fig. 4. (a) EL spectra at applied voltage of 10 V; (b) current density–
voltage–luminance characteristics; (c) current efficiency and power
efficiency versus current density curves for devices.
investigated. We measure J–V characteristics of the elec-
tron-only devices and hole-only devices, with device struc-
tures of ITO/Al (40 nm)/emitting layer (100 nm)/CsF
(1.5 nm)/Al (100 nm) and ITO/PEDOT4083 (40 nm)/
emitting layer (100 nm)/MoO3 (10 nm)/Al (100 nm),
respectively. The deduced electron mobilities using the
space-charge-limited-current (SCLC) model from the J–V
characteristics of single charge carrier devices for 1–4 are
1.6 ꢂ 10ꢀ5
,
1.0 ꢂ 10ꢀ5
,
7.8 ꢂ 10ꢀ6
,
and 1.8 ꢂ 10ꢀ5 cm2
Vꢀ1 sꢀ1
, respectively; while the hole mobilities are
1.5 ꢂ 10ꢀ7
,
1.5 ꢂ 10ꢀ8
,
1.1 ꢂ 10ꢀ8
,
and 3.7 ꢂ 10ꢀ9 cm2
Vꢀ1 sꢀ1, respectively. On the basis of these observations,
we concluded that relatively lower efficiencies in the
devices based on complexes 3 and 4 can be partially

M. Zhu et al. / Organic Electronics 15 (2014) 1598–1606
1603
Table 2
Electroluminescence data of the devices.
Lmax b/V (cd mꢀ2
)
gc (cd Aꢀ1
c
)
gp (lm Wꢀ1
d
)
gEQE (%)
CIE (x, y)
a
e
f
EML
Von (V)
1
2
3
4
3.5
3.7
7.0
7.0
541/13.4
2.4
12.4
5.2
1.0
6.6
1.8
2.2
3.0
8.8
3.1
5.4
0.64, 0.35
0.57, 0.42
0.54, 0.45
0.59, 0.40
2900/12.0
1016/24.8
1937/29.0
6.6
a
b
c
d
e
f
Turn-on voltage.
Maximum luminance.
Maximum current efficiency.
Maximum power efficiency.
Maximum external quantum efficiency.
Measured at 10 V.
attributed to the more severe carrier imbalance due to the
electron-withdrawing substitutes on the phenyl ring,
which has little contribution to hole injection to the EML
and hole transportation therein. On the other side, it is
not clear at this stage why complexes 1 showed inferior
devices than complexes 2 even the former exhibited better
charge transporting properties.
thermal stability of the samples under a nitrogen atmo-
sphere was determined by measuring their weight loss
while heating at a rate of 15 °C minꢀ1 from 25 to 800 °C.
Cyclic voltammetry (CV) was carried out in nitrogen-
purged dichloromethane (oxidation scan) at room
temperature with a CHI voltammetric analyzer. Tetrabutyl-
ammonium hexafluorophosphate (TBAPF6) (0.1 M) was
used as the supporting electrolyte. The conventional
three-electrode configuration consists of a platinum work-
ing electrode, a platinum wire auxiliary electrode, and an
Ag wire pseudo-reference electrode with ferrocenium–
ferrocene (Fc+/Fc) as the internal standard. Cyclic
3. Conclusions
In summary, we have designed and synthesized a series
of new homoleptic Ir(III) complexes by integrating with
different charge-transporting moieties onto the emissive
core to achieve charge balance in the EML while maintain
the self-host ability of the phosphors. For the device using
2 as neat emitter, a maximum external efficiency of 8.8% is
realized with small value (6.8%) of efficiency roll-off at
high brightness. This indicates that rational incorporation
of charge-transporting moieties into the sphere of irid-
ium(III) core is a simple and effective approach to develop
efficient host-free phosphors for solution-processable
nondoped PhOLEDs. We believe that this strategy of bipo-
lar phosphors would be applicable to generate novel phos-
phorescent materials that have emission colors covering
the whole visible region for use in multicolor OLEDs.
voltammograms were obtained at scan rate of 100 mV sꢀ1
.
The onset potential was determined from the intersection
of two tangents drawn at the rising and background
current of the cyclic voltammogram.
4.2. Computational details
The geometrical and electronic properties were per-
formed with the Gaussian 09 program package. The calcu-
lation was optimized by means of the B3LYP (Becke three
parameters hybrid functional with Lee–Yang–Perdew cor-
relation functionals) with the 6-31G(d) atomic basis
set.32. Then the electronic structures were calculated at
tHCTHhyb/6-311++G(d, p) level. Molecular orbitals were
visualized using Gaussview.
4. Experimental
4.1. General information
4.3. Device fabrication and measurement
1H NMR and 13C NMR spectra were measured on a
MECUYR-VX300 spectrometer. Elemental analyses of car-
bon, hydrogen, and nitrogen were performed on a Vario
EL III microanalyzer. Melting points of the solids were
measured on X-4 melting point apparatus and uncorrected.
Mass spectra were measured on a ZAB 3F-HF mass spectro-
photometer or Bruker BIFLEX III TOF mass spectrometer.
UV–vis absorption spectra were conducted on a Shimadzu
UV-2500 recording spectrophotometer. Photoluminescent
(PL) spectra were recorded on a Hitachi F-4500 fluores-
cence spectrophotometer. The solid state PL quantum
efficiencies (PLQYs) were measured using an integration
sphere. Differential scanning calorimetry (DSC) was per-
formed on a NETZSCH DSC 200 PC unit at a heating rate
of 10 °C minꢀ1 from room temperature to 400 °C under a
flow of nitrogen. Thermogravimetric analysis (TGA) was
undertaken with a NETZSCH STA 449C instrument. The
Patterned indium tin oxide (ITO)-coated glass sub-
strates with a sheet resistance of 15–20
X/square under-
went wet-cleaning course in an ultrasonic bath,
a
beginning with acetone, followed by detergent, deionized
water, and isopropanol. After oxygen-plasma treatment, a
50 nm thick anode buffer layer of PEDOT:PSS (Baytron P
4083, Bayer AG) film was spin-cast on the ITO substrate
and dried by baking in vacuum oven at 80 °C overnight.
The emitting layer was prepared by spin-coating from
chlorobenzene solution of the dendrimers on top of the
PEDOT layer, then annealed at 100 °C for 20 min. TPBI
(25 nm), Ba (4 nm) and Al (150 nm) were evaporated with
a
shadow mask successively at a base pressure of
3 * 10ꢀ4 Pa. The thickness of the evaporated TPBI and cath-
ode was monitored by a quartz-crystal thickness/ratio
monitor (Sycon model STM-100/MF). The cross-sectional
area between the cathode and anode defined the pixel size

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M. Zhu et al. / Organic Electronics 15 (2014) 1598–1606
of 19 mm2. Except for the spin coating of the PEDOT layer,
all the processes were carried out in the controlled atmo-
sphere of a nitrogen dry-box (Vacuum Atmosphere Co.)
containing less than 1 ppm oxygen and moisture. All mea-
surements were carried out at room temperature under
ambient conditions.
2H), 7.48–7.45 (m, 6H), 1.33 (s, 12H); 13C NMR (75 MHz,
CDCl3): d 134.85, 134.69, 132.30, 132.19, 131.44, 131.32,
128.80, 128.64, 82.27, 24.79; MS (EI): m/z 403 [M]+. Anal.
Calcd. for C24H26BO3P: C 71.31, H 6.48; found: C 71.62, H
6.76.
L1: degassed toluene (45 mL) and ethanol (15 mL) were
added to a mixture of 5 (3.11 g, 7.76 mmol), 6 (3.76 g,
9.30 mmol), Na2CO3 (2 M, 15 mL, 30.00 mmol) and
Pd(PPh3)4 (0.18 g, 0.16 mmol). The resulting mixture was
stirred and heated to reflux for 48 h under argon atmo-
sphere. After cooling to room temperature, the solvent
was evaporated under reduced pressure and taken up with
CH2Cl2 (120 mL). The organic layer was washed with brine
(30 mL) and water (30 mL) sequentially and dried over
anhydrous Na2SO4. After filtered, the solvent was evapo-
rated to dryness and subjected to column chromatography
on silica gel (eluent: CH2Cl2/methanol = 30:1, v/v) to give
L1 as yellow solid (2.32 g, 50%). mp: 104–110 °C. 1H NMR
(300 MHz, CDCl3): d 8.71 (d, J = 5.4 Hz, 1H), 7.91 (s, 1H),
7.89 (d, J = 5.1 Hz, 1H), 7.85 (d, J = 7.5 Hz, 2H), 7.78 (d,
J = 7.8 Hz, 2H), 7.71–7.67 (m, 4H), 7.58 (d, J = 7.2 Hz, 2H),
7.51–7.49 (m, 4H), 7.30–7.28 (m, 6H), 7.15–7.08 (m, 6H),
7.08–7.03 (m, 2H); 13C NMR (75 MHz, CDCl3): d 157.37,
149.73, 148.53, 146.91, 141.71, 132.50, 132.38, 131.69,
131.57, 130.95, 128.95, 128.37, 128.18, 127.51, 126.90,
126.74, 124.37, 122.98, 122.54, 119.18, 117.62; MS (EI):
m/z 598 [M]+; Anal. Calcd. for C41H31N2OP: C 82.26, H
5.22, N 4.68; found: C 82.03, H 4.93, N 4.57.
4.4. Synthesis of materials
Starting chemicals and reagents were purchased from
commercial sources and used as received without further
purification. Solvents for synthesis were purified according
to standard procedures prior to use. All reactions were
performed under an inner argon atmosphere. Synthesis of
4-(diphenylamino)phenylboronic acid, 9-(4-(4,4,5,5-tetra-
methyl-1,3,2-dioxaborolan-2-yl)phenyl)-9H-carbazole and
4-bromophenyldiphenylphosphine oxide were proceeded
following the reported procedures [23–25].
4.4.1. 4-(4-Bromopyridin-2-yl)-N,N-diphenylaniline (5)
Degassed THF (81 mL) was added to a mixture of 4-
(diphenylamino)phenylboronic acid (5.23 g, 18.00 mmol),
2,4-dibromopyridine (5.12 g, 21.60 mmol), Na2CO3 (2 M,
27 mL, 54.00 mmol) and Pd(PPh3)4 (0.21 g, 0.18 mmol).
The mixture was refluxed for 24 h under argon. After cool-
ing to room temperature, the solvent was evaporated
under reduced pressure. The residue was poured into
water (40 mL) and then extracted with CH2Cl2 (100 mL
*3). The organic extracts were collected and dried with
anhydrous Na2SO4. After filtered and removal of the sol-
vent, the residue was purified by column chromatography
on silica gel (eluent: petroleum/ethyl acetate = 5:1, v/v) to
give 5 as white solid (5.06 g, 70%). mp: 108–110 °C. 1H
NMR (300 MHz, CDCl3): d 8.45 (d, J = 4.5 Hz, 1H), 7.85 (s,
1H), 7.82 (d, J = 7.2 Hz, 2H), 7.34 (d, J = 4.5 Hz, 1H), 7.28
(d, J = 6.9 Hz, 4H), 7.14–7.09 (m, 6H), 7.06 (d, J = 7.2 Hz,
2H); 13C NMR (75 MHz, CDCl3): d 158.18, 149.99, 149.08,
147.05, 133.19, 131.04, 129.20, 127.68, 124.76, 124.27,
123.34, 122.81, 122.49; MS (EI): m/z 401 [M]+. Anal. Calcd.
for C23H17BrN2: C 68.84, H 4.27, N 6.98; found: C 69.01, H
4.15, N 6.67.
4.4.3. 9-(4-(4-Bromopyridin-2-yl)phenyl)-9H-carbazole (7)
According to the similar procedure for the preparation
of 5, degassed THF (100 mL) was added to a mixture of
9-(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phe-
nyl)-9H-carbazole (8.20 g, 22.15 mmol) and 2,4-dibromo-
pyridine (5.25 g, 22.15 mmol), Na2CO3 (2 M, 33 mL,
66.00 mmol) and Pd(PPh3)4 (0.25 g, 0.22 mmol). The mix-
ture was refluxed for 24 h under argon. After cooling to
room temperature, the solvent was evaporated under
reduced pressure. The residue was poured into water
(40 mL) and then extracted with CH2Cl2 (100 mL *3). The
organic extracts were collected and dried with anhydrous
Na2SO4. After filtered and removal of the solvent, the resi-
due was purified by column chromatography on silica gel
(eluent: petroleum/ethyl acetate = 5:1, v/v) to give 7 as
white solid (5.65 g, 64%). mp: 137–139 °C. 1H NMR
(300 MHz, CDCl3): d 8.57 (d, J = 5.4 Hz, 1H), 8.22 (d,
J = 8.1 Hz, 2H), 8.17 (d, J = 7.8 Hz, 2H), 7.99 (s, 1H), 7.71
(d, J = 8.4 Hz, 2H), 7.49 (d, J = 5.7 Hz, 1H), 7.45–7.40 (m,
4H), 7.33 (d, J = 7.2 Hz, 2H); 13C NMR (75 MHz, CDCl3): d
157.40, 150.10, 140.23, 138.65, 136.35, 133.33, 128.18,
126.71, 125.81, 125.15, 123.42, 123.31, 120.12, 119.98,
109.52; MS (EI): m/z 400 [M]+. Anal. Calcd. for C23H15BrN2:
C 69.19, H 3.79, N 7.02; found: C 69.28, H 3.66, N 7.17.
L2: according to the similar procedure for the prepara-
tion of L1, degassed toluene (30 mL) and ethanol (10 mL)
were added to a mixture of 7 (2.00 g, 5.00 mmol), 6
(2.83 g, 7.00 mmol), Na2CO3 (2 M, 10 mL, 20.00 mmol)
and Pd(PPh3)4 (0.11 g, 0.10 mmol). The resulting mixture
was stirred and heated to reflux for 48 h under argon
atmosphere. After cooling to room temperature, the sol-
vent was evaporated under reduced pressure and taken
4.4.2. (4-(4,4,5,5-Tetramethyl-1,3,2-dioxaborolan-2-yl)
phenyl)diphenyl phosphine oxide (6)
4-Bromophenyldiphenylphosphine
oxide
(6.18 g,
17.30 mmol), bis(pinacolato)diborane (4.83 g, 19.03 mmol),
and KOAc (5.94 g, 60.55 mmol) were mixed together in a
150 mL flask. After degassing, dry dioxane (80 mL) was
added to the mixture under flow of argon. Afterward,
[Pd(dppf)Cl2] (150 mg, dppf = 1,10-bis(diphenylphospha-
nyl)ferrocene) was added. The reaction mixture was kept
at 85 °C overnight and then cooled to room temperature.
The solventwas concentrated and the inorganicsalt was dis-
solved completely after addition of water (50 mL). After
extracted with CH2Cl2 (100 mL *3), the combined organic
layer was washed with brine (30 mL) and then dried over
anhydrous Na2SO4. The crude product was purified by silica
gel column chromatography (eluent: CH2Cl2/metha-
nol = 30:1, v/v) to yield the desired compound as colorless
oil (6.43 g, 92%). 1H NMR (300 MHz, CDCl3): d 7.89 (d,
J = 7.2 Hz, 2H), 7.66 (d, J = 7.8 Hz, 4H), 7.52 (d, J = 7.2 Hz,

M. Zhu et al. / Organic Electronics 15 (2014) 1598–1606
1605
up with CH2Cl2 (120 mL). The organic layer was washed
with brine (30 mL) and water (30 mL) sequentially and
dried over anhydrous Na2SO4. After filtered, the solvent
was evaporated to dryness and subjected to column chro-
matography on silica gel (eluent: CH2Cl2/methanol = 30:1,
v/v) to give L2 as yellow solid (2.63 g, 88%). mp: 275–
277 °C. 1H NMR (300 MHz, CDCl3): d 8.82 (d, J = 5.7 Hz,
1H), 8.34 (s, 1H), 8.27 (d, J = 7.2 Hz, 2H), 8.19 (d,
J = 5.1 Hz, 1H), 8.15 (d, J = 8.2 Hz, 2H), 8.02 (d, J = 7.5 Hz,
2H), 7.88–7.83 (m, 4H), 7.73–7.70 (m, 6H), 7.65–7.50 (m,
8H), 7.31–7.28 (m, 2H); 13C NMR (75 MHz, CDCl3): d
156.74, 149.95, 147.76, 141.37, 140.13, 138.09, 137.51,
132.54, 131.80, 131.55, 130.91, 128.33, 126.92, 126.76,
126.62, 125.64, 123.06, 120.08, 119.89, 119.76, 118.29,
109.38; MS (EI): m/z 596 [M]+; Anal. Calcd. for C41H29N2OP:
C 82.53, H 4.90, N 4.70; found: C 82.30, H 5.12, 4.57.
124.50, 123.21, 122.32, 119.74, 117.89; MS (EI): m/z 598
[M]+; Anal. Calcd. for C41H31N2OP: C 82.26, H 5.22, N
4.68; found: C 82.30, H 4.89, 4.54.
L4: according to the similar procedure for the prepara-
tion of L1, degassed toluene (72 mL) and ethanol (24 mL)
were added to a mixture of 2,4-dibromopyridine (1.90 g,
8.00 mmol) and 6 (8.08 g, 20.00 mmol), Na2CO3 (2 M,
24 mL, 48.00 mmol) and Pd(PPh3)4 (0.37 g, 0.32 mmol).
The resulting mixture was stirred and heated to reflux for
48 h under argon atmosphere. After cooling to room tem-
perature, the solvent was evaporated under reduced pres-
sure and taken up with CH2Cl2 (120 mL). The organic layer
was washed with brine (30 mL) and water (30 mL) sequen-
tially and dried over anhydrous Na2SO4. After filtered, the
solvent was evaporated to dryness and subjected to col-
umn chromatography on silica gel (eluent: CH2Cl2/metha-
nol = 20:1, v/v) to give L4 as yellow solid (2.70 g, 53%). mp:
4.4.4. 4-Bromo-2-(4-(diphenylphosphoryl)phenyl)pyridine
(8)
According to the similar procedure for the preparation
of 5, degassed THF (45 mL) was added to a mixture of 6
246–247 °C. 1H NMR (300 MHz, CDCl3):
d 8.79 (d,
J = 5.4 Hz, 1H), 8.12 (d, J = 7.5 Hz, 2H), 7.95 (s, 1H), 7.84–
7.79 (m, 5H), 7.70–7.67 (m, 8H), 7.56–7.50 (m, 14H); 13C
NMR (75 MHz, CDCl3): d 156.21, 149.89, 147.61, 141.90,
140.90, 133.53, 133.10, 132.35, 132.01, 131.89, 131.32,
130.97, 128.07, 126.60, 126.45, 120.47, 118.60; MS (EI):
m/z 630 [M]+; Anal. Calcd. for C41H31NO2P2: C 77.96, H
4.95, N 2.22; found: C 77.76, H 4.89, 2.08.
(4.04 g,
10.00 mmol),
2,4-dibromopyridine
(2.84 g,
12.00 mmol), Na2CO3 (2 M, 15 mL, 30.00 mmol) and
Pd(PPh3)4 (0.12 g, 0.10 mmol). The mixture was refluxed
for 24 h under argon. After cooling to room temperature,
the solvent was evaporated under reduced pressure. The
residue was poured into water (40 mL) and then extracted
with CH2Cl2 (100 mL *3). The organic extracts were col-
lected and dried with anhydrous Na2SO4. After filtered
and removal of the solvent, the residue was purified by col-
umn chromatography on silica gel (eluent: CH2Cl2/metha-
nol = 30:1, v/v) to give 8 as white solid (2.34 g, 54%). mp:
4.4.5. Preparation of the homoleptic complex 1
L1 (1.02 g, 1.70 mmol) and Ir(acac)3 (0.25 g, 0.50 mmol)
were added to a 50 mL round-necked flask. Thereafter, dis-
tilled o-dichlorobenzene (5 mL), 2-(2-methoxyethoxy)eth-
anol (10 mL) and glycerol (15 mL) were to the flask
sequentially. The mixture was heated to 230 °C for 36 h.
After completion, o-dichlorobenzene was removed under
reduced pressure. The mixture was poured into H2O and
extracted with CH2Cl2. The organic phase was washed with
brine and dried over anhydrous Na2SO4. The solvent was
evaporated under reduced pressure and the residue was
purified through column chromatography with CH2Cl2/
methanol (20:1, v/v) as eluent to afford 1 (0.95 g) as red
powder with a yield of 95%. 1H NMR (300 MHz, CDCl3): d
7.88 (s, 3H), 7.74–7.71 (m, 18H), 7.55–7.49 (m, 15H),
7.36 (d, J = 7.8 Hz, 6H), 7.12–7.09 (m, 12H), 6.85–6.82 (m,
18H), 6.68–6.63 (m, 12H), 6.27–6.20 (m, 6H); 13C NMR
134–136 °C. 1H NMR (300 MHz, CDCl3):
d 8.53 (d,
J = 5.1 Hz, 1H), 8.45 (d, J = 5.1 Hz, 1H), 8.07 (d, J = 7.2 Hz,
2H), 7.93 (s, 1H), 7.83 (d, J = 7.8 Hz, 2H), 7.72 (d,
J = 6.6 Hz, 4H), 7.57–7.53 (m, 6H); 13C NMR (75 MHz,
CDCl3): d 157.70, 151.00, 141.62, 134.05, 132.90, 129.07,
128.02, 127.32, 126.30, 124.66, 121.32; MS (EI): m/z 434
[M]+. Anal. Calcd. for C23H17BrNOP: C 63.61, H 3.95, N
3.23; found: C 63.87, H 4.12, N 3.09.
L3: according to the similar procedure for the prepara-
tion of L1, degassed toluene (30 mL) and ethanol (10 mL)
were added to a mixture of 8 (2.44 g, 5.60 mmol) and 4-
(diphenylamino)phenylboronic acid (2.44 g, 8.40 mmol),
Na2CO3 (2 M, 10 mL, 20.00 mmol) and Pd(PPh3)4 (0.13 g,
0.11 mmol). The resulting mixture was stirred and heated
to reflux for 48 h under argon atmosphere. After cooling
to room temperature, the solvent was evaporated under
reduced pressure and taken up with CH2Cl2 (120 mL).
The organic layer was washed with brine (30 mL) and
water (30 mL) sequentially and dried over anhydrous
Na2SO4. After filtered, the solvent was evaporated to
dryness and subjected to column chromatography on silica
gel (eluent: CH2Cl2/methanol = 30:1, v/v) to give L3 as
yellow solid (1.11 g, 33%). mp: 107–114 °C. 1H NMR
(300 MHz, CDCl3): d 8.82 (d, J = 5.4 Hz, 1H), 8.30 (d, J =
7.2 Hz, 2H), 8.17 (d, J = 7.5 Hz, 2H), 8.03 (s, 1H), 7.88–
7.83 (m, 5H), 7.72 (d, J = 6.9 Hz, 6H), 7.57–7.43 (m, 12H),
7.33–7.31 (m, 2H); 13C NMR (75 MHz, CDCl3): d 156.13,
149.73, 148.55, 146.63, 142.46, 132.59, 132.13, 131.58,
130.28, 128.97, 128.16, 128.00, 127.27, 126.69, 126.53,
(75 MHz, CDCl3):
d 166.97, 162.22, 148.82, 147.87,
147.69, 146.84, 142.00, 137.21, 133.12, 132.68, 132.42,
132.27, 131.16, 130.12, 128.85, 127.33, 127.26, 125.14,
124.98, 122.66, 119.18, 115.76, 114.89; MALDI-TOF: Calcd.
for C123H90IrN6O3P3 1985.2; found 1985.2; Anal. Calcd. for
C123H90IrN6O3P3: C 74.42, H 4.57, N 4.23; found: C 74.50, H
4.63, N 4.02.
4.4.6. Preparation of the homoleptic complex 2
90% 1H NMR (300 MHz, CDCl3): d 8.17 (s, 3H), 7.94 (d,
J = 6.9 Hz, 6H), 7.87 (s, 3H), 7.82–7.79 (m, 12H), 7.72–
7.66 (m, 18H), 7.57 (d, J = 6.3 Hz, 6H), 7.50–7.48 (m,
12H), 7.07–7.01 (m, 18H), 6.85–6.82 (m, 6H); 13C NMR
(75 MHz, CDCl3):
d 165.22, 159.36, 149.76, 148.42,
148.11, 147.54, 147.22, 134.36, 132.02, 131.89, 131.79,
131.34, 129.78, 128.67, 128.52, 128.28, 128.12, 127.97,
125.50, 124.21, 122.64, 120.58, 116.68; MALDI-TOF: Calcd.
for C123H84IrN6O3P3 1979.2; found 1979.1; Anal. Calcd. for

1606
M. Zhu et al. / Organic Electronics 15 (2014) 1598–1606
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C123H84IrN6O3P3: C 74.64, H 4.28, N 4.25; found: C 74.27, H
4.25, N 4.02.
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4.4.7. Preparation of the homoleptic complex 3
90% 1H NMR (300 MHz, CDCl3): d 7.95 (s, 3H), 7.67 (d,
J = 5.4 Hz, 3H), 7.55 (d, J = 7.8 Hz, 12H), 7.39 (s, 3H), 7.35–
7.28 (m, 24H), 7.18 (d, J = 7.8 Hz, 24H), 7.12 (d, J = 6.6 Hz,
12H), 7.02–7.00 (m, 6H), 6.65–6.60 (m, 3H); 13C NMR
(75 MHz, CDCl3): d 167.49, 163.11, 149.11, 148.77, 143.01,
142.17, 141.27, 140.16, 134.71, 134.04, 133.36, 133.13,
132.50, 129.90, 128.35, 126.66, 124.33, 120.82, 119.42,
118.13, 111.53; MALDI-TOF: Calcd. for C123H90IrN6O3P3
1985.2; found 1985.4; Anal. Calcd. for C123H90IrN6O3P3:
C 74.42, H 4.57, N 4.23; found: C 74.66, H 4.56, N 4.05.
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691 (2006) 3519.
4.4.8. Preparation of the homoleptic complex 4
82% 1H NMR (300 MHz, CDCl3): d 7.97 (s, 3H), 7.84 (d,
J = 8.4 Hz, 6H), 7.75–7.72 (m, 18H), 7.58–7.50 (m, 18H),
7.36–7.28 (m, 18H), 7.21–7.16 (m, 18H), 7.01–6.98 (m,
6H), 6.65–6.61 (m, 3H); 13C NMR (75 MHz, CDCl3): d
165.70, 158.92, 148.11, 147.00, 140.89, 139.58, 134.80,
133.33, 132.51, 132.30, 131.98, 131.76, 128.97, 128.65,
128.28, 127.43, 124.49, 121.76, 117.78; MALDI-TOF: Calcd.
for C123H90IrN3O6P6 2084.1; found 2084.2; Anal. Calcd. for
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Chem. 22 (2012) 11128;
C
123H90IrN3O6P6: C 70.88, H 4.35, N 2.02; found: C 70.63, H
(d) C. Fan, L. Zhu, B. Jiang, Y. Li, F. Zhao, D. Ma, J. Qin, C. Yang, J. Phys.
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The manuscript was written through contributions of
all authors. All authors have given approval to the final ver-
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Appendix A. Supplementary material
Supplementary data associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/
j.orgel.2014.04.008.
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