Solution processible imidazole-based iridium dendrimers with oligocarbazole for nondoped phosphorescent OLEDs

Article abstract of DOI:10.1016/j.orgel.2019.02.017

By introducing t-butyl- or methoxyl-containing oligocarbazole into the periphery of tris(mesityl-2-phenyl-1H-imidazole)iridium (III) [Ir (mpim)₃], two novel imidazole-based Ir dendrimers ^tBu-D2-Ir (mpim)₃ and MeO-D2-Ir (mpim)₃ have been designed and synthesized through a convenient post-dendronization route. Due to the effective encapsulation, the intermolecular interactions and thus luminescence quenching in solid states is found to be gradually reduced following a sequence of Ir (mpim)₃ > MeO-D2-Ir (mpim)₃ > ^tBu-D2-Ir (mpim)₃. Compared with the bare Ir (mpim)₃ core (Φ_PL = 0.38 and τ = 0.26 μs), accordingly, the film photoluminescence quantum yield and excited state lifetime are improved to 0.54 and 0.39 μs for MeO-D2-Ir (mpim)₃ and 0.88 and 1.09 μs for ^tBu-D2-Ir (mpim)₃. When these developed Ir dendrimers are adopted as the emitting layer alone, ^tBu-D2-Ir (mpim)₃ achieves an excellent nondoped device performance, revealing a maximum current efficiency as high as 35.7 cd/A (13.2%, 37.4 lm/W) together with Commission Internationale del_Eclairage coordinates of (0.21, 0.45). Even at a high luminance of 1000, 5000 and 10000 cd/m², it still remains to be 27.8, 24.7 and 21.8 cd/A, respectively, indicative of the gentle efficiency roll-off. The result clearly demonstrates the great potential of imidazole-based Ir dendrimers used for efficient nondoped phosphorescent organic light-emitting diodes.

Full text of DOI:10.1016/j.orgel.2019.02.017

Accepted Manuscript
Solution processible imidazole-based iridium dendrimers with oligocarbazole for
nondoped phosphorescent OLEDs
Mingming Zhang, Xuefei Li, Shumeng Wang, Junqiao Ding, Lixiang Wang
PII:
S1566-1199(19)30086-2
DOI:
https://doi.org/10.1016/j.orgel.2019.02.017
ORGELE 5123
Reference:
To appear in: Organic Electronics
Received Date: 2 November 2018
Revised Date: 21 January 2019
Accepted Date: 11 February 2019
Please cite this article as: M. Zhang, X. Li, S. Wang, J. Ding, L. Wang, Solution processible imidazole-
based iridium dendrimers with oligocarbazole for nondoped phosphorescent OLEDs, Organic
Electronics (2019), doi: https://doi.org/10.1016/j.orgel.2019.02.017.
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ACCEPTED MANUSCRIPT
Graphic Abstract
Solution processible imidiazole-based Ir dendrimers with oligocarbazole have
been demonstrated for nondoped PhOLEDs, revealing a maximum current efficiency
of 35.7 cd/A accompanied by a small efficiency roll-off at high luminance.

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Solution Processible Imidazole-Based Iridium
Dendrimers with Oligocarbazole for Nondoped
Phosphorescent OLEDs
Mingming Zhang,a,b Xuefei Li,a,b Shumeng Wang,a Junqiao Ding,*,a,b Lixiang
Wang*,a,b
a State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of
Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, P. R. China
b University of Science and Technology of China, Hefei 230026, P. R. China

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Abstract
By introducing t-butyl- or methoxyl-containing oligocarbazole into the periphery of
tris(mesityl-2-phenyl-1H-imidazole)iridium(III)
[Ir(mpim)3],
two
novel
t
imidazole-based Ir dendrimers Bu-D2-Ir(mpim)3 and MeO-D2-Ir(mpim)3 have been
designed and synthesized through a convenient post-dendronization route. Due to the
effective encapsulation, the intermolecular interactions and thus luminescence
quenching in solid states is found to be gradually reduced following a sequence of
t
Ir(mpim)3 > MeO-D2-Ir(mpim)3 > Bu-D2-Ir(mpim)3. Compared with the bare
Ir(mpim)3 core (ΦPL = 0.38 and τ = 0.26 µs), accordingly, the film photoluminescence
quantum yield and excited state lifetime are improved to 0.54 and 0.39 µs for
t
MeO-D2-Ir(mpim)3 and 0.88 and 1.09 µs for Bu-D2-Ir(mpim)3. When these
t
developed Ir dendrimers are adopted as the emitting layer alone, Bu-D2-Ir(mpim)3
achieves an excellent nondoped device performance, revealing a maximum current
efficiency as high as 35.7 cd/A (13.2%, 37.4 lm/W) together with Commission
Internationale del_Eclairage coordinates of (0.21, 0.45). Even at a high luminance of
1000, 5000 and 10000 cd/m2, it still remains to be 27.8, 24.7 and 21.8 cd/A,
respectively, indicative of the gentle efficiency roll-off. The result clearly
demonstrates the great potential of imidazole-based Ir dendrimers used for efficient
nondoped phosphorescent organic light-emitting diodes.
Keywords: PhOLEDs; imidazole; Ir dendrimer; oligocarbazole; nondoped device

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1. Introduction
Phosphorescent organic light-emitting diodes (PhOLEDs) capable of wet
preparation have attracted much attention in recent years due to their good
compability with low-cost, large-area and flexible flat-panel displays [1-4]. Besides
small molecules [5, 6] and polymers [7, 8], dendrimers containing transition-metal
complexes are believed to be a promising class of electroluminescent materials for
solution processed PhOLEDs [9-16]. Such phosphorescent dendrimers have both the
well-defined structures of small molecules and the excellent solution processibility of
polymers. Most importantly, they can not only maintain the inherent emissive
properties from cores, but also realize prohibited intermolecular interactions to reduce
triplet-triplet annihilation (TTA) in neat films because of the characteristic shielding
effect [17, 18]. Therefore, they are able to be independently used as the emitting layer
(EML) to fabricate efficient nondoped devices without any additional host. In this
case, the tedious doping technology and potential phase segregation would be avoided
to enhance device performance including efficiency, lifetime and reproducibility
[19-21].
Nowadays blue- [12-14, 22, 23], green- [16, 18, 24-29], yellow- [30] and
red-emitting [31, 32] phosphorescent dendrimers with a self-host feature have been
developed for high-performance nondoped PhOLEDs, where the outer dendrons act
as the hosts and the inner core plays the same role as dopant. For example, with
tris[2-(2,4-difluorophenyl)-pyridyl]iridium(III) [Ir(dfppy)3] as the core and
oligocarbazole as the dendron, a self-host blue Ir dendrimer B-G2 was demonstrated

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to reveal a continuous enhancement in the device efficiency with increasing doping
concentration [10]. And the corresponding nondoped device was achieved without
loss in efficiency, thus giving a state-of-art external quantum efficiency (EQE) of 15.3%
(31.3 cd/A, 28.9 lm/W) and Commission Internationale del_Eclairage (CIE)
coordinates of (0.16, 0.29). Furthermore, solution processed white OLEDs were
realized
by
simply
blending
a
yellow
phosphor
(iridium(III)[5-trifluoromethyl-2-(9,9-diethylfluoren-2-yl)pyridine]) [Ir(Flpy-CF3)3]
into B-G2 [33]. Unlike the traditional host-based devices, the host-induced power
efficiency losses could be eliminated, leading to an improved power efficiency of 58.8
lm/W along with CIE coordinates of (0.44, 0.45).
Albeit the successes, the adopted core in B-G2 consists of the electron-withdrawing
F atom, which is proved to be readily cleaved to shorten the device lifetimes [34]. On
the other hand, imidazole-based Ir complexes in absence of F, such as
tris(mesityl-2-phenyl-1H-imidazole)iridium(III) [Ir(mpim)3], turn out to be more
stable than the F-containing counterparts [35, 36]. Meanwhile, they show red-shifted
emissions towards a greenish-blue region, suitable for the fabrication of low driving
voltage and power-efficient white OLEDs [37, 38]. With these considerations, here
we further report imidazole-based Ir dendrimers tBu-D2-Ir(mpim)3 and
MeO-D2-Ir(mpim)3 by introducing different oligocarbazole dendrons into the
periphery of Ir(mpim)3 via a nonconjugated linkage (Figure 1). On the basis of a
nondoped device configuration, a promising current efficiency as high as 35.7 cd/A
(13.2%, 37.4 lm/W) is obtained. Even at a high luminance of 1000, 5000 and 10000

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cd/m2, it still remains to be 27.8, 24.7 and 21.8 cd/A, respectively, indicative of the
gentle efficiency roll-off.
2. Results and discussion
2.1 Synthesis and characterization
A post-dendronization method similar to our previous work [16] is utilized for the
t
convenient synthesis of the imidazole-based Ir dendrimers Bu-D2-Ir(mpim)3 and
MeO-D2-Ir(mpim)3. As depicted in Scheme 1, 4-bromo-2,6-dimethylaniline was
firstly
amidated
with
benzoyl
chloride
to
afford
N-(4-bromo-2,6-dimethylphenyl)benzamide (1), followed by a dehydro-cyclization.
The resultant 1-(4-bromo-2,6-dimethylphenyl)-2-phenyl-1H-imidazole (2) was then
converted to 1-(4-hydroxy-2,6-dimethylphenyl)-2-phenyl-1H-imidazole (4) after a
successive bromide-to methoxyl conversion and demethylation under BBr3.
Subsequently, through a modified two-step complexation, the key intermediate
HO-Ir(mpim)3 functionalized with three reactive hydroxyl groups was successfully
prepared in an acceptable yield of 38%. Finally, with the alkyl bromide oligocarbazole
dendrons (tBu-D2-C4-Br and MeO-D2-C4-Br) in hand, a Williamson reaction was
t
performed to easily produce the desired dendrimers Bu-D2-Ir(mpim)3 and
MeO-D2-Ir(mpim)3 in a high yield of 83-87%. Their molecular structures were well
1
characterized using H NMR, MALDI-TOF spectra and elemental analysis (Figure
1
S1-S5). The total number of resonances in the H NMR spectra of HO-Ir(mpim)3,
tBu-D2-Ir(mpim)3 and MeO-D2-Ir(mpim)3 are equal to the number of resonances in a

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single C^N ligand, indicative of the inherent C3 symmetry and facial isomer.
t
Moreover, both Bu-D2-Ir(mpim)3 and MeO-D2-Ir(mpim)3 are thermally stable,
whose decomposition temperature (Td, corresponding to a 5% weight loss) is detected
to be 409 and 384 oC, respectively (Table 1 and Figure S6).
2.2 Electrochemical properties
t
The electrochemical properties of Bu-D2-Ir(mpim)3 and MeO-D2-Ir(mpim)3 were
investigated by cyclic voltammetry (CV) with ferrocene/ferrocenium (Fc/Fc+) as the
reference. During the sweeping in dichloromethane, they both show multiple
oxidation processes with no reduction ones (Figure 2). Compared with Ir(mpim)3, the
first oxidation wave located at about -0.08 V can be assigned to the central Ir core,
while the others at positive potentials are from the outer oligocarbazole dendrons.
Accordingly, the highest occupied molecular orbital (HOMO) and lowest unoccupied
molecular orbital (LUMO) energy levels of tBu-D2-Ir(mpim)3 and
MeO-D2-Ir(mpim)3 are estimated to be -4.72 eV and -2.10 eV, respectively, close to
those of Ir(mpim)3. The observation suggests that the incorporation of oligocarbazole
dendrons does not affect the electrochemical behavior of the Ir core. In addition, we
t
note that the second oxidation wave occurs at 0.53 V for Bu-D2-Ir(mpim)3 and 0.40
V for MeO-D2-Ir(mpim)3. The ease oxidation of the methoxyl-containing
oligocarbazole in MeO-D2-Ir(mpim)3 relative to t-butyl-containing oligocarbazole in
tBu-D2-Ir(mpim)3 is understandable when considering the stronger electron donating
ability of methoxyl than t-butyl. According to the literature [13], this is favorable for

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the hole injection and transport in a dendritic Ir complex, which will be discussed
below. Moreover, the oxidation processes of the methoxyl-containing oligocarbazole
dendron and the Ir(mpim)3 core may interact with each other (Figure S7), leading to
the inferior reversibility and electrochemical stability of MeO-D2-Ir(mpim)3
compared with tBu-D2-Ir(mpim)3.
2.3 Photophysical properties
Figure 3 shows the UV-Vis absorption spectra in dichloromethane and
photoluminescence (PL) spectra in toluene and films for the imidazole-based Ir
dendrimers compared with Ir(mpim)3. As can be clearly seen, both tBu-D2-Ir(mpim)3
and MeO-D2-Ir(mpim)3 exhibit two distinct bands including the weak absorption in
the range of 325-450 nm and intense absorption below 325 nm. The first one is
attributed to the metal-to-ligand charge-transfer (MLCT) transitions from the inner Ir
core. And the second one is assigned to the ligand-centered (LC) transitions from the
inner Ir core together with the π-π* transitions from the outer dendrons, whose
absorbance is found to be greatly increased after the introduction of oligocarbazole.
With respect to Ir(mpim)3, additionally, their PL spectra in toluene remain nearly
unchanged with a 0-0 emission at 470 nm and 0-1 emission at 496 nm. Given the
nonconjugated linkage between dendron and core, the observed similarity is
reasonable, suggesting that the inherent emission of the Ir core is independent of the
peripheral dendrons in solutions.

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However, different PL behaviors are observed ongoing from solutions to solid
states. As for Ir(mpim)3 without any dendrons, the PL spectrum moves to a longer
wavelength accompanied by a significant enhancement of 0-1 and 0-2 emissions. In
contrast, no obvious variation of the spectral profile and only a bathochromic shift of
8 and 14 nm are observed for tBu-D2-Ir(mpim)3 and MeO-D2-Ir(mpim)3, respectively.
Because of the encapsulation from the outer dendrons, the intermolecular interactions
and thus luminescence quenching in neat films are found to be gradually decreased
following a sequence of Ir(mpim)3 > MeO-D2-Ir(mpim)3 > tBu-D2-Ir(mpim)3. This is
further verified by the transient PL spectra (Figure 4), in which the excited state
lifetimes are determined to be 0.26, 0.39 and 1.09 µs for Ir(mpim)3,
MeO-D2-Ir(mpim)3 and tBu-D2-Ir(mpim)3, respectively. And the film
photoluminescence quantum yield (PLQY) is correspondingly up from 0.38 of
Ir(mpim)3 to 0.54 of MeO-D2-Ir(mpim)3 and 0.88 of tBu-D2-Ir(mpim)3 (Table 1).
It should be noted that the encapsulation from methoxyl-containing oligocarbazole
in MeO-D2-Ir(mpim)3 is not as effective as that from t-butyl-containing
t
oligocarbazole in Bu-D2-Ir(mpim)3. Besides the different steric hindrance between
methoxyl and t-butyl, the triplet energy arrangement is tentatively responsible for the
above phenomenon (Figure 5). Although they are both higher than that of the
Ir(mpim)3 core (2.63 eV), the triplet energy is reduced from 2.83 eV of
t-butyl-containing oligocarbazole to 2.75 eV of methoxyl-containing oligocarbazole
owing to the stronger electron donating ability of methoxyl than t-butyl. Thereby the
possibility of the back triplet energy transfer from core to dendron may be increased

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[39], leading to the reduced lifetime and PLQY of MeO-D2-Ir(mpim)3 relative to
tBu-D2-Ir(mpim)3.
2.4 Electroluminescence properties
t
To investigate the electroluminescence (EL) properties of Bu-D2-Ir(mpim)3 and
MeO-D2-Ir(mpim)3, nondoped PhOLEDs were fabricated with a configuration of
ITO/PEDOT:PSS (40 nm)/Ir dendrimer (40 nm)/TSPO1 (5 nm)/TmPyPB (45 nm)/LiF
(1
nm)/Al
(Figure
S8).
Herein,
PEDOT:PSS
[poly(3,4-ethylenedioxythiophene:poly(styrenesulfonate)] serves as the hole-injection
layer, whereas TSPO1 [diphenyl(4-(triphenylsilyl)phenyl)phosphine oxide] and
TmPyPB [1,3,5-tri(m-pyrid-3-yl-phenyl)benzene] act as the exciton blocking layer
and the electron-transporting layer, respectively. Similar to their PL counterparts, two
developed imidazole-based Ir dendrimers both give bright greenish-blue EL solely
from the central Ir core, and no emission residue from the peripheral dendrons is
detected (Figure 6a). The related CIE coordinates are (0.21, 0.45) for
tBu-D2-Ir(mpim)3 and (0.22, 0.47) for MeO-D2-Ir(mpim)3.
As mentioned above, a better hole injection and transport can be anticipated in
MeO-D2-Ir(mpim)3 than in tBu-D2-Ir(mpim)3 because methoxyl-containing
oligocarbazole has a shallow HOMO level compared to t-butyl-containing
oligocarbazole. Therefore, the current density-voltage and luminance-voltage curves
t
distinctly shift towards a lower driving voltage from Bu-D2-Ir(mpim)3 to
MeO-D2-Ir(mpim)3 (Figure 6b). For instance, the turn-on voltage at 1 cd/m2 is down

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from 2.8 V to 2.4 V, which is even smaller than previously reported self-host blue Ir
dendrimers.11 In spite of this, a maximum current efficiency of 35.7 cd/A and 21.1
cd/A, a maximum power efficiency of 37.4 lm/W and 25.5 lm/W, and a peak EQE of
t
13.2% and 7.3% are realized for the nondoped devices of Bu-D2-Ir(mpim)3 and
MeO-D2-Ir(mpim)3, respectively (Figure 6c and 6d, Table 2). Compared with
t
MeO-D2-Ir(mpim)3, the superior device efficiency for Bu-D2-Ir(mpim)3 can be
explained by the higher film PLQY originating from the effective encapsulation.
t
Furthermore, Bu-D2-Ir(mpim)3 reveals a gentle efficiency roll-off, and its current
efficiency slightly decays to 27.8, 24.7 and 21.8 cd/A at 1000, 5000 and 10000 cd/m2,
t
respectively. In view of the obtained promising performance, Bu-D2-Ir(mpim)3 may
be also suitable for power-efficient white OLEDs without an additional host [33],
which will be reported by our group in due course.
3. Conclusion
In summary, two novel imidazole-based Ir dendrimers have been designed and
synthesized with Ir(mpim)3 as the core and t-butyl- or methoxyl-containing
oligocarbazole as the dendron. Benefiting from the reduced intermolecular
interactions in solid states caused by the effective encapsulation, their film PLQYs
and excited state lifetimes are improved obviously compared with the bare core.
When a nondoped configuration is adopted, as a result, a promising current efficiency
as high as 35.7 cd/A is realized associated with a gentle efficiency roll-off at high

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luminance. The result clearly demonstrates the great potential of imidazole-based Ir
dendrimers used for efficient nondoped PhOLEDs.
4. Experimental section
General information: 1H NMR spectra were measured on a Bruker Avance 400 NMR
spectrometer. Elemental analysis was performed using a Bio-Rad elemental analysis
system. MALDI/TOF (Matrix assisted laser desorption ionization/Time-of-flight)
mass spectra were performed on AXIMA CFR MS apparatus (COMPACT) using
2-[(2E)-3-(4-tert-butylphenyl)-2-methylprop-2-enylidene]malononitrile (DCTB) as
the matrix. Thermal properties of the dendrimers were analyzed with a
o
Perkin-Elmer-TGA 7 instrument under nitrogen at a heating rate of 10 C min-1.
UV-vis absorption and PL spectra were measured with a Perkin-Elmer Lambda 35
UV-vis spectrometer and a Perkin-Elmer LS 50B spectrofluorometer, respectively.
Phosphorescence spectra of the oligocarbazole dendrons were measured at 77 K in a
toluene solvent. The film PLQY was measured using an integrating sphere (Binson
C9920-2) under N2. The transient PL spectra were measured under a N2 atmosphere
and excited at a 375 nm with Edinburgh fluorescence spectrometer (FLS920).
Following a biexponential fitting, the corresponding average lifetimes were estimated
2
2
according to the equation: τav = (A1τ1 + A2τ2 )/(A1τ1 + A2τ2). CV measurements were
carried out in dichloromethane with a conventional three-electrode system consisting
of a platinum working electrode, a platinum counter electrode, and an Ag/AgCl
reference electrode. The supporting electrolyte was 0.1 M tetrabutylammonium

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perchlorate (n-Bu4NClO4). All potentials were calibrated against the Fc/Fc+ couple.
onset
The HOMO levels were calculated according to the equation HOMO = -e(Eox
+
onset
4.8 V), where Eox
is the onset value of the first oxidation wave. And the LUMO
levels were calculated according to the equation LUMO = HOMO + Eg, where Eg is
the optical bandgap estimated from the absorption onset.
Device fabrication and testing: To fabricate nondoped PhOLEDs, a 40 nm-thick
PEDOT:PSS film was firstly deposited on the pre-cleaned and UVO-treated ITO-glass
substrates (20 Ω per square). After baked at 120 oC for 40 min, then solutions of the Ir
dendrimers in chlorobenzene were filtered through a filter (0.45 µm) and spin coated
on PEDOT:PSS as the EML. The thickness of the EML was about 40 nm after
o
annealing at 120 C for 30 min. Subsequently, the substrate was transferred to a
vacuum thermal evaporator and a 5 nm-thick film of TSPO1 and a 45 nm-thick film
of TmPyPB was evaporated on top of the EML at a base pressure less than 10-6 Torr (1
Torr = 133.32 Pa). Finally, 1 nm LiF and 100 nm Al were deposited successively as
the cathode through a shadow mask with an array of 14 mm2 openings. The current
density-voltage-luminance characteristics were measured using a Keithley source
measurement unit (Keithley 2400 and Keithley 2000) calibrated by a silicon
photodiode. The EL spectra were measured using a SpectraScan PR650
spectrophotometer. All the measurements were carried out at room temperature under
ambient conditions. EQE was calculated from the luminance, current density and EL
spectra assuming a Lambertian distribution.

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Synthesis: All chemicals and reagents used in this work were received from
commercial sources without further purification. Solvents for chemical synthesis were
t
purified according to the standard procedures. Bu-D2-C4-Br and MeO-D2-C4-Br
were prepared according to our previous work [10, 40].
N-(4-bromo-2,6-dimethylphenyl)benzamide
(1):
The
mixture
of
4-bromo-2,6-dimethylaniline (5.0 g, 25 mmol) and pyridine (4.0 g, 50 mmol) in 30
o
mL CH2Cl2 was stirred, and cooled to 0 C under ice bath. To this solution, benzoyl
chloride (3.5 g, 25 mmol) in 10 mL CH2Cl2 was added dropwisely under argon
atmosphere, followed by a stirring overnight at room temperature. After the reaction
completed, the solvent was removed by vacuum distillation. Then the residue was
washed by petroleum ether, and dried in vacuum to give the crude product 1, which
was directly used for the next step without further purification.
1-(4-bromo-2,6-dimethylphenyl)-2-phenyl-1H-imidazole (2): 1 (7.6 g, 25 mmol)
dissolved in 50 mL xylene was added PCl5 (7.8 g, 37.5 mmol) slowly. The mixture
was refluxed overnight under argon. When the reaction completed, the solvent was
removed by vacuum distillation, followed by adding 50 mL THF and cooling to 0 oC.
Then 2,2-dimethoxyethan-1-amine (6.6 g, 62.5 mmol) in THF(10 mL) was added
dropwise under argon atmosphere, and the mixture was stirred for 8 h at room
temperature. The mixture was added 30 mL 6 M HCl, and further heated to reflux
overnight. After cooling to room temperature, the mixture was poured into saturated

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NaHCO3 solution, and extracted with CH2Cl2. The organic phase was washed with
water and dried over anhydrous sodium sulfate. Finally, the residue was purified by
column chromatography on silica gel with petroleum ether: ethyl acetate = 4:1 as the
eluent to give the product 2 (4.7 g, 58 %). 1H NMR (400 MHz, d6-DMSO): δ = 7.50
(s, 2H), 7.29 (s, 6H), 7.28 (d, J = 1.2 Hz, 1H), 7.26 (d, J = 1.2 Hz, 1H), 1.88 (s, 6H).
1-(4-methoxy-2,6-dimethylphenyl)-2-phenyl-1H-imidazole (3): Methanol (20 mL) in a
flask was cooled to 0 oC with ice bath before sodium (2.1 g, 92 mmol) was added. The
ice water bath was removed, and the mixture was stirred until the sodium disappeared.
Then DMF (20 mL), CuI (3.5 g, 18.4 mmol) and 2 (3.0 g, 9.2 mmol) were added into
this sodium methoxide solution. The resulting mixture was heated to reflux for 4 h
under argon atmosphere. The hot mixture was rapidly filtered, and the filtrate was
poured into water and extracted with CH2Cl2. The combined organic layers were
neutralized with 1 M HCl, followed by washing with water and brine, drying with
Na2SO4. After solvent removal, the residue was purified by silica gel column
chromatography using petroleum ether:ethyl acetate = 10:1 as eluent to give the
product 3 (1.8 g, 72%). 1H NMR (400 MHz, d6-DMSO): δ = 7.31 (ddd, J = 8.4, 5.6,
2.5 Hz, 2H), 7.27 (dd, J = 6.6, 3.3 Hz, 3H), 7.22 (s, 1H), 7.20 (s, 1H), 6.81 (s, 2H),
3.78 (s, 3H), 1.88-1.81 (m, 6H).

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1-(4-hydroxy-2,6-dimethylphenyl)-2-phenyl-1H-imidazole (4): A solution of 3 (2.7 g,
9.7 mmol) in dry CH2Cl2 (50 mL) was cooled to 0 oC, and BBr3 (2.7 mL 1 M solution
in CH2Cl2, 29 mmol) was added dropwise. After stirring for 4 h at room temperature,
the reaction was carefully quenched with methanol. The solvent was removed by
vacuum distillation. Then the residue was recrystallization by acetone to give the pure
1
product 4 as white solid (2.3 g, 90%). H NMR (400 MHz, d6-DMSO): δ = 9.65 (s,
1H), 7.35-7.31 (m, 2H), 7.27 (dd, J = 6.5, 2.8 Hz, 3H), 7.20 (d, J = 1.2 Hz, 1H), 7.17
(d, J = 1.1 Hz, 1H), 6.60 (s, 2H), 1.79 (s, 6H).
tris(2-(1-(4-hydroxy-2,6-dimethylphenyl)-1H-imidazol-2-yl)phenyl)iridium
(HO-Ir(mpim)3): IrCl3·3H2O (1.0 g, 2.83 mmol) and 4 (1.87 g, 7.08 mmol) were
added in a 30 mL mixture of 2-methoxyethanol (15 mL) and water (15 mL). The
mixture was refluxed for 24 h and then poured into water. The solid was collected by
filtration and dried in vacuum to give the chloro-bridged dimer. Next, the crude dimer,
silver trifluoroacetate (1.25 g, 5.66 mmol), 4 (1.5 g, 5.66 mmol), ethelene glycol
monophenyl ether (15 mL) and ethylene glycol (15 mL) were heated to 120 oC for 24
h under argon atmosphere. After cooling to room temperature, the solvent was
removed by vacuum distillation. The pure product HO-Ir(mpim)3 (1.1 g) was obtained
in a total yield of 38% by chromatography on silica gel using petroleum ether : ethyl
acetate = 4:1 as eluent. 1H NMR (400 MHz, d6-DMSO): δ = 9.75 (s, 1H), 7.12 (s, 1H),
6.72 (d, J = 7.5 Hz, 1H), 6.68 (d, J = 2.7 Hz, 2H), 6.57 (s, 1H), 6.47 (t, J = 7.3 Hz,
1H), 6.37 (t, J = 7.4 Hz, 1H), 6.13 (d, J = 7.7 Hz, 1H), 1.93 (s, 3H), 1.74 (s, 3H).

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t
General Procedure for the Synthesis of Bu-D2-Ir(mpim)3 and MeO-D2-Ir(mpim)3: A
t
mixture of Bu-D2-C4-Br or MeO-D2-C4-Br (3.5 equiv.), HO-Ir(mpim)3 (1 equiv.)
and cesium carbonate (5 equiv.) in DMF was heated to 80 oC under argon atmosphere
for 24 h. The mixture was cooled to room temperature, poured into water and
extracted with ethyl acetate. The combined organic layers were washed with brine and
water, followed by drying with Na2SO4. After the solvent removal, the desired
dendrimers were obtained by silica gel column chromatography using petroleum ether:
t
ethyl acetate = 6:1 as eluent for Bu-D2-Ir(mpim)3 and toulene : ethyl acetate = 10:1
as eluent for MeO-D2-Ir(mpim)3.
1
tBu-D2-Ir(mpim)3 (530 mg, 87%): H NMR (400 MHz, C6D6): δ = 8.49 (s, 4H), 8.01
(d, J = 1.8 Hz, 2H), 7.65-7.47 (m, 10H), 7.25-7.18 (m, 3H), 6.99 (s, 1H), 6.85 (t, J =
6.4 Hz, 1H), 6.72 (d, J = 6.3 Hz, 1H), 6.69 (d, J = 7.2 Hz, 1H), 6.64 (d, J = 1.9 Hz,
1H), 6.57 (s, 1H), 6.34 (s, 1H), 3.41 (d, J = 5.5 Hz, 2H), 2.03 (s, 3H), 1.73 (s, 4H),
1.65 (s, 5H), 1.45 (s, 36H). MALDI-TOF (m/z): 3308.8 [M+]. Anal. calcd. For
C219H228IrN15O3: C, 79.46; H, 6.94; N, 6.35; Found: C, 79.40; H, 7.20; N, 6.21.
1
MeO-D2-Ir(mpim)3 (485 mg, 83%): H NMR (400 MHz, C6D6): δ = 7.89 (s, 2H),
7.74 (s, 4H), 7.53 (s, 1H), 7.46 (d, J = 8.6 Hz, 2H), 7.36 (d, J = 8.8 Hz, 4H), 7.19 (d, J
= 8.9 Hz, 6H), 6.97 (s, 1H), 6.83 (d, J = 6.9 Hz, 1H), 6.69 (t, J = 9.6 Hz, 2H), 6.61 (s,
1H), 6.55 (s, 1H), 6.33 (s, 1H), 3.92 (s, 2H), 3.58 (s, 12H), 3.43 (s, 2H), 2.01 (s, 3H),
1.76 (d, J = 6.5 Hz, 2H), 1.71 (s, 3H), 1.51 (s, 2H). MALDI-TOF (m/z): 2996.2 [M+].

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Anal. calcd. For C183H156IrN15O15: C, 73.33; H, 5.25; N, 7.01; Found: C, 73.24; H,
5.45; N, 6.98.
Author Information:
Corresponding authors: junqiaod@ciac.ac.cn; lixiang@ciac.ac.cn
Acknowledgements
The authors acknowledge the financial support from the National Key Research and
Development Program (2016YFB0401301), the 973 Project (2015CB655001) and the
National Natural Science Foundation of China (Nos. 51573183 and 51873205).
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Scheme 1. Synthetic route of the imidazole-based Ir dendrimers Bu-D2-Ir(mpim)3
and MeO-D2-Ir(mpim)3. Reagents and conditions: (i) pyridine, CH2Cl2, 0 oC; (ii) PCl5,
o
xylene, reflux, and then 2,2-dimethoxyethan-1-amine, THF, 0 C; (iii) CH3OH, Na,
CuI, DMF, reflux; (iv) BBr3, CH2Cl2, 0 oC;
(v) IrCl3·3H2O, water,
2-methoxyethanol, reflux, and then silver trifluoroacetate, 4, ethelene glycol
monophenyl ether, ethylene glycol, 120 oC; (vi) Cs2CO3, DMF, 80 oC.

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Figure 1. Molecular structures of the imidazole-based Ir dendrimers
tBu-D2-Ir(mpim)3 and MeO-D2-Ir(mpim)3 together with the core Ir(mpim)3.

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Ir(mpim)3
tBu-D2-Ir(mpim)3
MeO-D2-Ir(mpim)3
-0.2 0.0
0.2
0.4
0.6
0.8
1.0
1.2
Potential (V vs Fc/Fc+)
t
Figure 2. CV plots for Bu-D2-Ir(mpim)3 and MeO-D2-Ir(mpim)3 compared with
Ir(mpim)3.

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Ir(mpim)3
0.5
0.4
0.3
0.2
0.1
absorbtion
solution PL
film PL
1.0
0.8
0.6
0.4
0.2
0.0
3.0
0.0
1.0
tBu-D2-Ir(mpim)3
absorbtion
solution PL
film PL
2.5
2.0
1.5
1.0
0.5
0.8
0.6
0.4
0.2
0.0
3.0
0.0
1.0
absorbtion
solution PL
film PL
MeO-D2-Ir(mpim)3
2.5
2.0
1.5
1.0
0.5
0.0
0.8
0.6
0.4
0.2
0.0
300
400
500
600
700
Wavelength (nm)
Figure 3. UV-Vis absorption spectra in dichloromethane and PL spectra in both
toluene and films for Ir(mpim)3, tBu-D2-Ir(mpim)3 and MeO-D2-Ir(mpim)3.

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100
10-1
10-2
10-3
Ir(mpim)3
MeO-D2-Ir(mpim)3
tBu-D2-Ir(mpim)3
2
4
6
8
10
12
Time(us)
t
Figure 4. Transient PL spectra for thin films of Ir(mpim)3, Bu-D2-Ir(mpim)3 and
MeO-D2-Ir(mpim)3.