Self-host blue-emitting iridium dendrimer with carbazole dendrons: Nondoped phosphorescent organic light-emitting diodes

Article abstract of DOI:10.1002/anie.201307311

A blue-emitting iridium dendrimer, namely B-G2, has been successfully designed and synthesized with a second-generation oligocarbazole as the dendron, which is covalently attached to the emissive tris[2-(2,4-difluorophenyl)- pyridyl]iridium(III) core through a nonconjugated link to form an efficient self-host system in one dendrimer. Unlike small molecular phosphors and other phosphorescent dendrimers, B-G2 shows a continuous enhancement in the device efficiency with increasing doping concentration. When using neat B-G2 as the emitting layer, the nondoped device is achieved without loss in efficiency, thus giving a state-of-art EQE as high as 15.3 % (31.3 cd A^-1, 28.9 lm W^-1) along with CIE coordinates of (0.16, 0.29). Out of the blue: The title dendrimer has been developed by covalently attaching a second-generation carbazole dendron to an emissive tris[2-(2,4-difluorophenyl)-pyridyl] iridium(III) core through a nonconjugated linker to form an efficient self-host system (see figure). Unlike small molecular phosphors and other phosphorescent dendrimers known to date, the nondoped phosphorescent organic light-emitting diodes herein are realized without a loss in efficiency. Copyright

Full text of DOI:10.1002/anie.201307311

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DOI: 10.1002/anie.201307311
Supramolecular Chemistry
Self-Host Blue-Emitting Iridium Dendrimer with Carbazole Dendrons:
Nondoped Phosphorescent Organic Light-Emitting Diodes**
Debin Xia, Bin Wang, Bo Chen, Shumeng Wang, Baohua Zhang, Junqiao Ding,*
Lixiang Wang,* Xiabin Jing, and Fosong Wang
Abstract: A blue-emitting iridium dendrimer, namely B-G2,
has been successfully designed and synthesized with a second-
generation oligocarbazole as the dendron, which is covalently
attached to the emissive tris[2-(2,4-difluorophenyl)-
pyridyl]iridium(III) core through a nonconjugated link to
form an efficient self-host system in one dendrimer. Unlike
small molecular phosphors and other phosphorescent den-
drimers, B-G2 shows a continuous enhancement in the device
efficiency with increasing doping concentration. When using
neat B-G2 as the emitting layer, the nondoped device is
achieved without loss in efficiency, thus giving a state-of-art
EQE as high as 15.3% (31.3 cdAÀ1, 28.9 lmWÀ1) along with
CIE coordinates of (0.16, 0.29).
modulated to reduce or eliminate the quenching of the
luminance by the site isolation effect.[6] Early in 2005, P. L.
Burn et al. firstly reported blue iridium (Ir) dendrimers with
biphenyl dendrons, and fabricated their corresponding non-
doped PhOLEDs, but there was a loss in device performance
with an external quantum efficiency (EQE) of 3.8%.[7] It was
further improved to 7.9% by using a different triplet core in
2008.[8] Since then little progress has been made because of
the scarcity of functional dendrons with high triplet energy
and good charge transport.[9]
Herein, we report the blue Ir dendrimer B-G2, having
a radius of 23 ꢀ, which can be used for the fabrication of high-
performance nondoped PhOLEDs. As illustrated in Figure 1,
T
he exploration of suitable phosphors for nondoped phos-
phorescent organic light-emitting diodes (PhOLEDs),[1]
which can be used alone as the emitting layer (EML) without
the need to be dispersed into an additional host matrix, has
become an increasingly exciting area of research. Compared
with conventional doped PhOLEDs,[2] the careful work on
precise control over the doping content of phosphors and
inevitable phase segregation[3] are both avoided to simplify
the device fabrication process, and improve device stability
and reproducibility. Most importantly, this nondoped tech-
nology seems especially attractive for blue emission since the
appropriate hosts, having large band gaps for blue emitters,
are still mostly elusive.[4] There is therefore a need to develop
host-free blue-emitting phosphors.
Phosphorescent dendrimers present one pathway toward
this end.[5] With a triplet emitter located at the central core,
the strong interactions between emissive cores can be well
Figure 1. The designed self-host blue-emitting iridium dendrimer B-G2
and model compound B-G0.
[*] D. Xia,[+] B. Wang,[+] B. Chen, S. Wang, Dr. B. Zhang, Dr. J. Ding,
Prof. L. Wang, Prof. X. Jing, Prof. F. Wang
State Key Laboratory of Polymer Physics and Chemistry, Changchun
Institute of Applied Chemistry, Chinese Academy of Sciences
Changchun 130022 (P. R. China)
tris[2-(2,4-difluorophenyl)-pyridyl]iridium(III) [Ir(dfppy)3] is
employed as the triplet core owing to the intense room-
temperature blue phosphorescence.[10] Meanwhile, the
second-generation oligocarbazole is selected as the dendron
because the carbazole unit possesses a triplet energy of about
3.0 eV, as well as an excellent hole-transporting capability,
and has been widely applied as the building block for the
design of wide-band-gap host materials and multifunctional
phosphors.[11] In addition, an ether linkage is incorporated
between the core and dendron for convenient synthesis while
simultaneously not altering the coreꢁs emission. Here the
four-carbon-atom chain is preferentially utilized because the
carbazole-containing alkyl bromide with shorter chains is
unstable, and thus undergoes elimination reactions and forms
inactive olefins.[12] Although the chain-length of the spacer
E-mail: junqiaod@ciac.ac.cn
lixiang@ciac.ac.cn
B. Chen, S. Wang
University of the Chinese Academy of Sciences
Beijing 100049 (P. R. China)
[+] These authors contributed equally to this work.
[**] We are grateful to the 973 Project (2009CB623601 and
2009CB930603), National Natural Science Foundation of China
(Nos. 20923003, 21174144 and 21074130), and Science Fund for
Creative Research Groups (No. 20921061) for financial support of
this research.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201307311.
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Table 1: The photophysical, electrochemical, and thermal properties of the dendrimers.
labs(loge)[a]
[nm]
ls[b]
lf[c]
FPL
Eg
tav
[ms]
HOMO[f]
[eV]
LUMO[f]
[eV]
Tg/Td
[8C]
[b]
[d]
[e]
[nm]
[nm]
[eV]
B-G0
B-G2
236 (4.8), 264 (4.8), 320 (4.0), 348 (4.1),
381 (3.9), 419 (3.4), 447 (2.9)
241 (5.7), 268 (5.4), 297 (5.3), 335 (4.8),
350 (4.7), 381 (4.1), 420 (3.5), 442 (3.3)
467
467
484
468
0.57
0.69
2.69
0.11
À5.23
À5.26
À2.54
À2.58
n.d./355
271/437
2.68
0.59
[a] Measured in 10À5 m dichloromethane solution. [b] Measured in N2-saturated toluene solution with a concentration of 10À5 m, and [Ir(ppy)3]
(FPL =0.40) is used as the reference. [c] Measured in neat films. [d] The optical band gap estimated from the absorption onset. [e] Measured in neat
onset
films under N2 excited at l=355 nm- [f] HOMO=Àe(Eox + 4.8 V), LUMO=HOMO + Eg.
may have some influence on device performance and further 300 nm, which are assigned to the spin-allowed ligand-
experiments should be performed, they are beyond the scope centered (LC) transitions, and weaker metal-to-ligand
of this work.
charge-transfer (MLCT) transitions in the l = 300-450 nm
With this design strategy, an efficient self-host system was region. With respect to B-G0, the intensity of the absorption
formed in one dendritic molecule (B-G2), where the sur- at around l = 240 nm, characteristic of the carbazole moiety
rounding carbazole dendrons are expected to function as the in B-G2, is found to be strengthened after the introduction of
host for the emissive core. Therefore, unlike small molecular oligocarbazole. The photoluminescent (PL) spectra of B-G2
phosphors and other phosphorescent dendrimers known to and B-G0 both in toluene and in the solid state are shown in
date, the nondoped B-G2 device has been achieved without Figure 2. Both B-G0 and B-G2 exhibit the same emission
the expense of efficiency for the first time, thus generating maxima at l = 467 nm with an identical spectral profile in
a
state-of-art EQE as high as 15.3% (31.3 cdAÀ1
,
toluene solutions, similar to that of the [Ir(dfppy)3] core.[10]
28.9 lmWÀ1) along with Commission Internationale de
lꢁEclairage (CIE) coordinates of (0.16, 0.29).
The synthesis of B-G2 is depicted in Scheme S1 of the
Supporting Information). n-Butyl bromide terminated
second-generation carbazole dendron underwent a nucleo-
philic substitution to afford the ether-linked product in a high
yield (69%), and was then reacted with 2,4-difluorophenyl-
boronic acid by Suzuki–Miyaura cross-coupling to give the
target dendritic ligand. Finally, given the poor solubility of
dendritic ligand in glycerol, a modified two-step procedure
was adopted to synthesize B-G2. That is, a m-chloride-bridged
dimer was initially formed, followed by the further ligation
with the dendritic ligand in mesitylene and catalyzed by
AgSO3CF3. Under these reaction conditions, the facial isomer
was directly obtained and isolated in an acceptable total yield
of 26%, which was confirmed by the 1H NMR spectrum of B-
G2 (see Figure S4 in the Supporting Information) showing
only one set of ligand signals.[11] As a prototype, B-G0,
without dendrons was also prepared to reveal the self-host
nature of B-G2.
Figure 2. The PL spectra in toluene (solid lines) and neat film (dashed
lines) of B-G0 and B-G2.
This similarity is reasonable since the carbazole dendrons are
Thermogravimetric analysis (TGA) and differential scan- linked to the emissive core through a nonconjugated spacer.
ning calorimetry (DSC) were used to examine the thermal In neat films, however, different PL behaviors are
properties of B-G2. It is thermally stable with a high observed for B-G0 and B-G2. On going from solution to the
decomposition temperature (Td: at a 5% weight loss) of solid state, B-G0 shows a bathochromic shift of about 17 nm,
4378C. Furthermore, even in the presence of flexible alkoxy which is associated with the appearance of a long tail in the
units, B-G2 displays an unexpected glass-transition temper- range of l = 520-650 nm, and indicative of the existence of
ature (Tg) as high as 2718C, which is far superior to those of strong intermolecular interactions for B-G0. Unlike B-G0,
biphenyl-based blue Ir dendrimers (70–1508C).[7–8,9a] The the PL spectrum of B-G2 (film) matches well with that in
enhanced Tg of B-G2 might arise from a higher degree of solution, thus suggesting that the interactions between
dendron rigidity derived from carbazole relative to that of emissive Ir cores in B-G2 are almost completely suppressed
biphenyl, and it is highly desirable to attain effective by the attached carbazole dendrons. This feature is further
encapsulation for the Ir core.
verified by the film transient PL spectra of B-G0 and B-G2.
The photophysical properties of B-G0 and B-G2 were As can be clearly seen in Figure S10 in the Supporting
firstly explored, and the corresponding data are summarized Information, B-G2 decays much more slowly than B-G0.
in Table 1. As depicted in Figure S9 (see the Supporting Accordingly, the average lifetime, estimated by a biexponen-
Information), B-G2 has intense absorption bands below l = tial fit of emission decay curves, increases from 0.11 ms for B-
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G0 to 0.59 ms for B-G2 (Table 1 and S1 in the Supporting
Information).
Their electrochemical properties were also investigated
by cyclic voltammetry (CV) with the ferrocene/ferrocenium
(Fc/Fc+) couple as the reference. During the anodic sweeping
in dichloromethane, multiple oxidation waves were observed
for B-G2. Based on the CV curve of B-G0, the first p-doping
process in B-G2 can be ascribed to one-electron oxidation of
the Ir-phenyl center, while the others located at more positive
potentials are related to the surrounding carbazole dendrons
(see Figure S11 in the Supporting Information). The corre-
sponding HOMO level of B-G2 is determined to be À5.26 eV,
close to that of B-G0. According to the optical band gap (Eg)
estimated from the absorption onset, the LUMO levels of B-
G0 and B-G2 are calculated to be À2.54 and À2.58 eV,
respectively. The similar energy-level alignment of B-G2 and
B-G0 further illustrates that the nonconjugated linkage is
beneficial to maintain the properties of the emissive Ir core.
PhOLEDs of B-G2 were fabricated with a configuration
of ITO/PEDOT:PSS (40 nm)/H2:B-G2 (40 nm)/SPPO13
(50 nm)/LiF (1 nm)/Al (100 nm), where H2 and SPPO13
(2,7-bis(diphenylphosphoryl)-9,9’-spirobifluorene) acted as
the host and electron-transporting materials, respectively
(see Figure S12 in the Supporting Information).[11a,13] For
comparison, devices with H2:B-G0 as the EML were also
prepared under the same conditions. The ratio of B-G2 in H2
was firstly tuned to evaluate its potential as a host-free
phosphor. As presented in Figure 3, the luminous efficiency
continuously increases with the increasing doping content,
and reaches the maximum value when B-G2 is used in its
undiluted form. As a result, unlike B-G0, whose best
Figure 3. The doping concentration dependence of the luminous
~
&
efficiency (hL) for B-G0- ( ) and B-G2-based ( ) PhOLEDs.
quenching in B-G0, whereas this quenching turns out to be
negligible in B-G2 because of the shielding effect from
periphery dendrons.
It is thus understandable that, the peak luminous effi-
ciency, power efficiency, and EQE are prominently enhanced
from 1.6 cdAÀ1, 1.2 lmWÀ1, and 0.7% for B-G0 to
31.3 cdAÀ1, 28.9 lmWÀ1, and 15.3% for B-G2, respectively
(Table 2). The resulting EQE for B-G2 is about 22-fold that of
the nondoped device of B-G0. And this value is even 29%
higher than that of the optimized doped device H2:B-G0 (20
wt.%), which reveals an EQE of 11.9% (23.2 cdAÀ1
18.2 lmWÀ1; Figure 4b). Together with the fact that there is
,
device performance is obtained at a low-doping
Table 2: Summary of electroluminescence data for PhOLEDs.
concentration of 20 wt%, the nondoped device
efficiency of B-G2 is significantly higher than those
of corresponding doped devices. To the best of our
knowledge, this is the first report that nondoped
PhOLEDs are realized without sacrificing the effi-
ciency. In this respect, B-G2 differs from blue Ir
dendrimers with biphenyl dendrons.[7–8,9a] Further-
more, it should be noted that the interesting doping
concentration dependence of B-G2 is also quite
different from that previously reported for green-
and red-emitting Ir dendrimers,[14] even if the same
size carbazole dendron is used to encapsulate the Ir
core. The reason lies in that, apart from the carbazole
dendron, the F substituents in B-G2 may also
contribute to modifying the molecular packing and
thereby minimizing the self-quenching behavior.[15]
Figure S13 in the Supporting Information com-
pares the electroluminescent (EL) spectra between
Brightness[b] hL
hp
EQE[b] CIE[c]
[a]
[b]
[b]
EML
Von
[V]
[cdmÀ2
]
[cdAÀ1
]
[lmWÀ1
]
[%]
[x, y]
H2:B-G0 (10 wt%) 3.5
H2:B-G0 (20 wt%) 3.6
H2:B-G0 (30 wt%) 3.4
4940
6050
6700
1140
2940
3720
4420
5190
6160
6530
6800
16.8
23.2
21.8
1.6
11.4
13.8
19.0
19.8
23.0
26.8
31.3
14.4
18.2
20.0
1.2
7.8
(0.16, 0.32)
(0.15, 0.26)
(0.16, 0.27)
(0.18, 0.32)
(0.16, 0.29)
(0.15, 0.23)
(0.15, 0.26)
(0.15, 0.29)
(0.15, 0.29)
(0.15, 0.29)
(0.16, 0.29)
11.9
10.9
0.7
5.8
7.0
10.1
9.7
11.2
13.1
15.3
B-G0 (100 wt%)
3.2
H2:B-G2 (10 wt%) 4.2
H2:B-G2 (20 wt%) 3.7
H2:B-G2 (30 wt%) 3.5
H2:B-G2 (40 wt%) 3.1
H2:B-G2 (60 wt%) 3.2
H2:B-G2 (80 wt%) 3.2
7.2
10.7
15.6
17.0
19.4
23.3
28.9
B-G2 (100 wt%)
3.6
[a] Turn-on voltage at a brightness of 1 cdmÀ2. [b] The maximum data for brightness,
luminous efficiency (hL), power efficiency (hp), and external quantum efficiency
(EQE). [c] CIE at 7 V.
the nondoped devices of B-G0 and B-G2. Noticeably, the EL
spectrum of B-G2 is nearly identical to its PL counterpart,
thus showing blue triplet emission with CIE coordinates of
(0.16, 0.29). In contrast, the l = 490 nm emission related to 0–
1 transition dominates the whole EL spectrum of B-G0, thus
leading to red-shifted CIE coordinates of (0.18, 0.32).
Consistent with the difference between the film PL spectra
of B-G0 and B-G2 as discussed above, these observations
indicate that there exists severe aggregation-induced self-
no phase separation, no tedious work on the control of doping
ratios, and no requirement for wide-band-gap hosts in this
case, the efficiency improvement powerfully demonstrates
the advantage of a self-host system over a physical blend.
Additionally, in Figure 4a, we note that the maximum
brightness of B-G2 is comparable to that of H2:B-G0
(20 wt.%), which may be attributed to the two favorable
charge-transport routes in a neat B-G2 EML. On one hand,
the effective charge injection distance into the Ir core is below
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In conclusion, we have demonstrated that an efficient self-
host system can be successfully constructed in blue Ir
dendrimers on the basis of the second-generation carbazole
dendron. The designed dendrimer B-G2 has a different
doping concentration dependence from both small molecular
phosphors and other phosphorescent dendrimers, and exhib-
its a promising nondoped device EQE of 15.3% (31.3 cdAÀ1,
28.9 lmWÀ1). As B-G2 represents the first example whose
nondoped device performance outperforms the correspond-
ing doped devices, we believe that this work will shed light on
the development of self-host phosphorescent dendrimers for
applications in nondoped PhOLEDs.
Received: August 20, 2013
Revised: September 24, 2103
Published online: December 6, 2013
Keywords: dendrimers · heterocycles · iridium ·
.
supramolecular chemistry · synthesis design
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(a) and luminous efficiency (hL) and EQE as a function of current
~
density (J) (b) for the nondoped B-G2 ( ) and optimized doped
*
device of H2:B-G0 (20 wt%; ).
30 ꢀ according to the literature.[16] Therefore, intramolecular
charge transport can happen from the carbazole dendron to
the core because B-G2 has a moderate molecular size of 23 ꢀ.
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could be envisioned to control the intermolecular interactions
in B-G2 because of the aforementioned better rigidity of
carbazole dendron in comparison to biphenyl dendron.
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