Indolo[3,2-b]carbazole/benzimidazole hybrid bipolar host materials for highly efficient red, yellow, and green phosphorescent organic light emitting diodes

Article abstract of DOI:10.1039/c2jm30207a

By incorporating electron-accepting benzimidazole and electron-donating indolo[3,2-b]carbazole into one molecule, two novel donor-acceptor bipolar host materials, TICCBI and TICNBI, have been synthesized. The photophysical and electrochemical properties of the hybrids can be tuned through the different linkages (C- or N-connectivity) between the electronic donor and acceptor components. The promising physical properties of these two new compounds made them suitable for use as hosts doped with various Ir or Os-based phosphors for realizing highly efficient phosphorescent organic light emitting diodes (PhOLEDs). PhOLEDs using TICCBI and TICNBI as hosts incorporated with Ir-based emitters such as green (PPy)₂Ir(acac), yellow (Bt) ₂Ir(acac), and two new red emitters (35dmPh-6Fiq)₂Ir(acac) (i3) and (4tBuPh-6Fiq)₂Ir(acac) (i6) accomplished high external quantum efficiencies ranging from 14 to 16.2%. Nevertheless, the red PhOLED device incorporating TICNBI doped with the red emitter osmium(ii) bis[3-(trifluoromethyl)-5-(4-tert-butylpyridyl)-1,2,4-triazolate] dimethylphenylphosphine [Os(bpftz)₂(PPhMe₂)₂] achieved a maximum external quantum efficiency, current efficiency, and power efficiency of 22%, 28 cd A^-1, and 22.1 lm W^-1, respectively, with CIE coordinates of (0.65,0.35). The external quantum efficiency remained high (20%) as the brightness reached to 1000 cd m ^-2, suggesting balanced charge fluxes within the emitting layer, rendering devices with limited efficiency roll-off. The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c2jm30207a

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PAPER
Indolo[3,2-b]carbazole/benzimidazole hybrid bipolar host materials for highly
efficient red, yellow, and green phosphorescent organic light emitting diodes†
a
a
b
b
Hao-Chun Ting, You-Ming Chen, Hong-Wei You, Wen-Yi Hung, Sheng-Hsun Lin,c Atul Chaskar,a
*
a
a
c
c
Shu-Hua Chou, Yun Chi, Rai-Hsung Liu and Ken-Tsung Wong
*
*
Received 12th January 2012, Accepted 13th February 2012
DOI: 10.1039/c2jm30207a
By incorporating electron-accepting benzimidazole and electron-donating indolo[3,2-b]carbazole into
one molecule, two novel donor–acceptor bipolar host materials, TICCBI and TICNBI, have been
synthesized. The photophysical and electrochemical properties of the hybrids can be tuned through the
different linkages (C- or N-connectivity) between the electronic donor and acceptor components. The
promising physical properties of these two new compounds made them suitable for use as hosts doped
with various Ir or Os-based phosphors for realizing highly efficient phosphorescent organic light
emitting diodes (PhOLEDs). PhOLEDs using TICCBI and TICNBI as hosts incorporated with Ir-
based emitters such as green (PPy)2Ir(acac), yellow (Bt)2Ir(acac), and two new red emitters (35dmPh-
6Fiq)2Ir(acac) (i3) and (4tBuPh-6Fiq)2Ir(acac) (i6) accomplished high external quantum efficiencies
ranging from 14 to 16.2%. Nevertheless, the red PhOLED device incorporating TICNBI doped with the
red emitter osmium(II) bis[3-(trifluoromethyl)-5-(4-tert-butylpyridyl)-1,2,4-triazolate]
dimethylphenylphosphine [Os(bpftz)2(PPhMe2)2] achieved a maximum external quantum efficiency,
current efficiency, and power efficiency of 22%, 28 cd Aꢀ1, and 22.1 lm Wꢀ1, respectively, with CIE
coordinates of (0.65,0.35). The external quantum efficiency remained high (20%) as the brightness
reached to 1000 cd mꢀ2, suggesting balanced charge fluxes within the emitting layer, rendering devices
with limited efficiency roll-off.
and triplet–triplet annihilation of phosphorescent emitters.3
1. Introduction
Nevertheless, poor carrier mobility and unbalanced charges in
the emitting layer have been observed to hamper the power
efficiency of OLEDs.4 The host materials should cope with some
intrinsic properties like higher triplet energies than that of the
phosphorescent emitter,5 balanced charge flux, appropriate
energy levels aligned with the neighboring functional layers6 and
good thermal and morphological stabilities. In the initial
endeavours of PhOLED research, the unipolar hosts, namely,
hole-transporting material consisting electron-donating moiety
and electron-transporting material possessing electron-with-
drawing moiety have been employed. The unbalanced charge
flux of using unipolar hosts which shifts the recombination zone
near to HTL or ETL, leading to a narrow charge recombination
zone, resulted in possible triplet–triplet annihilation and exciton
diffusion. The main objective of the present PhOLED research is
the development of tailor-made bipolar host materials by proper
selection and linking of hole- and electron-transporting moieties
which could promote balanced charge density for the supply of
balanced flux of electrons and holes to electroluminescent (EL)
materials, leading to the realization of highly efficient PhOLEDs.
So far, numerous classes of bipolar host materials have been
reported which comprise triaryl amine or carbazole moieties as
hole-transporting group and oxadiazole, triazole, azine, phe-
nanthroline, benzimidazole, etc., as electron-transporting
Organic light-emitting diodes (OLEDs) have aroused consider-
able interest because of their potential applications in low power
consumption displays, lighting and the printed electronics
industry.1 More importantly, 100% internal quantum efficiency
can be achieved by dispersing appropriate transition metal-
centered phosphorescent dopant in a suitable host material
which can harvest both electron-generated singlet and triplet
excitons.2 The selection of host material for the phosphorescent
dopant is one of the key factors in the design of next generation
highly efficient phosphorescent organic light-emitting diodes
(PhOLEDs). The host material must possess the ability to
suppress the pernicious effects such as aggregation quenching
aDepartment of Chemistry, National Taiwan University, Taipei 106,
Taiwan. E-mail: kenwong@ntu.edu.tw; Fax: +886-2-33661667; Tel:
+886-2-33661665
bInstitute of Optoelectronic Sciences, National Taiwan Ocean University,
Keelung 202, Taiwan. E-mail: wenhung@mail.ntou.edu.tw
cDepartment of Chemistry, National Tsing Hua University, Hsinchu 300,
Taiwan
† Electronic supplementary information (ESI) available: Experimental
procedure, spectral analysis, crystal data, AFM images of TICCBI and
TICNBI, and DFT calculation. CCDC reference numbers 864064,
864065. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c2jm30207a
This journal is ª The Royal Society of Chemistry 2012
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group.7 More recently, Ma and Yang et al. reported universal
bipolar hosts incorporating benzimidazole/aryl amine hybrid for
hosting blue, green, orange and white phosphors, achieving
maximum external quantum efficiencies (hext) of 16.1%, 22.7%,
20.5% and 19.1%, respectively.8 In addition, PhOLEDs utilizing
a carbazole/phosphine oxide hybrid bipolar host material BCPO
reported by Cheng and Chou exhibited significantly promising
efficiencies of 23.5%, 21.6% and 17% for blue, green, and red,
respectively.9 We have also used benzimidazole/carbazole hybrid
to realize highly efficient yellow-green PhOLED with the hext of
bipolar transporting characteristics and suitable triplet energy
(ET), consequently enhance the efficiency of PhOLEDs. By
changing the linking topologies between the donor and acceptor,
we are able to manipulate the spatial distributions of molecular
orbital distributions locating at the different structural features
to modulate the photophysical and electrochemical properties.
Along with these two bipolar hosts we also reported the
synthesis of two new red Ir complexes. It is well known that
(piq)2Ir(acac) derivatives,17 where piq is 1-(phenyl)isoquinoline,
can efficiently suppress the triplet–triplet annihilation showing
short phosphorescent lifetime (ꢁ1.2 to 2.5 ms). According to
DuPont group’s theories18 and previous literature,19 the intro-
duction of various substituents (e.g. tBu, F, Me, OMe) on Ir-
complexes can not only alter the HOMO–LUMO levels and
emission wavelength but also minimize molecular packing and
concentration quenching of luminescence. For better utilizing the
advantages of chemical structure tuning, we tried to attach
bulkyl and halo-substituent like F, t-Bu and Me on Ir-phenyl-
isoquinolino complexes to reduce the self-quenching effect which
is believed to cause low device efficiency. Among the substituted
phenylisoquinoline-based iridium complexes, the complexes
associated with (4tBuPh-6Fiq)2Ir(acac) (i6) and (35dmPh-
6Fiq)2Ir(acac) (i3) derivatives of isoquinoline ligand show suit-
able photoluminescence wavelength in dichloromethane (618 nm
and 628 nm, respectively).
19.2% and power efficiency of 62 lm Wꢀ1 10
.
Among the bipolar host materials reported in the literature, we
found that they are associated with low electrochemical stability
due to the easily polymerized hole-transporting moieties such as
triaryl amine/carbazole.11 Taking into account this observation,
we envision that still there is ample scope to develop the novel
bipolar host using electrochemically stable hole-transporting
moiety. Comparing with carbazole, indolo[3,2-b]carbazole
derivatives also demonstrate great hole-transporting ability12
and have been applied in various organic electronics such as hole-
transporting material in light-emitting diodes,13 transistors,14 and
photovoltaic cells.15 However, to the best of our knowledge,
indolo[3,2-b]carbazole has been seldom reported as the host
material in OLEDs.16 In the quest for novel bipolar host, we
show here for the first time that the combination of indolo[3,2-b]-
carbazole as donor and benzimidazole moieties as acceptor in
designing of new bipolar host materials TICCBI and TICNBI
(Scheme 1) can lead to thermal and morphological stabilities,
This study focuses on the tuning of the thermal, photophysical
and electrical properties by different linking topologies between
the donor and acceptor constituents, synthesis of new Ir-based
phosphors ((35dmPh-6Fiq)2Ir(acac) and (4tBuPh-6Fiq)2-
Ir(acac)) as well as the performance of PhOLEDs hosted by
TICCBI and TICNBI. Green devices hosted by TICCBI and
TICNBI along with (PPy)2Ir(acac) emitter have efficiency values
of 14% and 15%, respectively. While, the yellow devices of the
same hosts with (Bt)2Ir(acac) emitter reveal efficiency of 15.4%
and 16.2%. The red PhOLEDs of these hosts show efficiency
values of 14.4% and 15.6% in conjugation with (35dmPh-
6Fiq)2Ir(acac) emitter whereas the values are 15.3% and 15.5%
for (4tBuPh-6Fiq)2Ir(acac) emitter. Noteworthily, the device
adopting TICNBI as the host and Os(bpftz)2(PPhMe2)2 as the
emitter provided a saturated red electrophosphorescence with
CIE coordinates (x,y) of (0.65,0.35) and remarkably high effi-
ciencies of 22% (28 cd Aꢀ1) and 22.1 lm Wꢀ1
.
2. Results and discussion
2.1 Synthesis
Scheme 1 depicts the synthesis of two indolo[3,2-b]carbazole-
cored bipolar host materials TICCBI and TICNBI, and two red
Ir-centered phosphors (35dmPh-6Fiq)2Ir(acac) and (4tBuPh-
6Fiq)2Ir(acac). TICCBI and TICNBI were readily prepared by
Ullmann coupling reaction of indolo[3,2-b]carbazole20 with 2-(4-
iodophenyl)-1-phenyl benzimidazole (2) and 1-(4-iodophenyl)-2-
phenyl benzimidazole (3),21 respectively. The red emitters
(35dmPh-6Fiq)2Ir(acac) and (4tBuPh-6Fiq)2Ir(acac) are syn-
thesised from 4-fluoro-2-((trimethyl silyl) ethynyl) benzaldehyde
oxime. The cyclization followed by Suzuki coupling with
substituted phenyl boronic acid yielded 5b and 5a which on
further reaction with iridium trichloride and acetyl acetone
Scheme
1 Synthetic routes and chemical structures of TICCBI,
TICNBI, bipolar hosts and (4tBuPh-6Fiq)2Ir(acac), (35dmPh-6Fiq)2-
Ir(acac) emitters.
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offered (35dmPh-6Fiq)2Ir(acac) and (4tBuPh-6Fiq)2Ir(acac),
respectively. The products were purified by column chromatog-
raphy and further purification was carried out by sublimation.
1H and 13C NMR spectra, mass spectral data (FAB and HRMS),
and elemental analysis of TICCBI, TICNBI, (35dmPh-
6Fiq)2Ir(acac) and (4tBuPh-6Fiq)2Ir(acac) are in agreement with
the structural identities and purities.
2.2 Thermal analysis
TICCBI and TICNBI exhibited excellent morphological stabili-
ties with distinct glass transition temperatures (Tg) of 174 ꢂC and
168 ꢂC and melting points (Tm) of 385 ꢂC and 406 ꢂC respec-
tively, determined by using differential scanning calorimetry
(DSC). TICCBI and TICNBI also showed to have high heat
tolerance with decomposition temperatures (Td) of 466 ꢂC for
ꢂ
Fig. 1 Room-temperature UV-vis and PL spectra of TICCBI and
TICNBI in various solutions and in neat films, as well as phosphores-
cence (Phos.) spectra recorded from their EtOH solutions at 77 K.
TICCBI and 475 C for TICNBI, corresponding to 5% weight
loss, which were determined by using thermogravimetric analysis
(TGA). The data are summarized in Table 1. Furthermore, these
compounds exhibit very good film stability as investigated by
atomic force microscopy (AFM). As shown in Fig. S4 in the
ESI†, the homogeneous films showed very smooth surface upon
annealing at 100 ꢂC for 30 min under nitrogen, giving a root-
mean-square (RMS) roughness of 0.279 nm for TICCBI, and
0.339 nm for TICNBI film. The rigid molecular configuration of
TICCBI and TICNBI enhances their thermal and morphological
stabilities. These morphological and thermal stabilities are
desirable for the host material in PhOLEDs as they can suppress
the aggregation formation and render amorphous characteristics
in the solid film state.
the charge transfer phenomenon in the ground state and excited
state, we have measured the UV-vis and PL spectra of TICCBI
and TICNBI in various organic solvents. The absorption spectra
of these two molecules in cyclohexane, toluene, dichloromethane
and acetonitrile are nearly identical. The solvent-independent
absorption spectra indicate that the Franck–Condon transitions
are subjected to rather small dipole moment change in the
ground state.22 In contrast, significant solvent-dependent PL
emission spectra of TICCBI were observed, showing different
behaviour of the excited state as compared to those of TICNBI.
The short wavelength region around 410 nm with distinct
vibronic features stays intact while the long wavelength tailing of
the emission spectra shifts continuously along with the solvent
polarity, indicating fluorescence solvatochromism in TICCBI.
This result suggests the generation of a highly polar charge-
separated excited state of TICCBI due to D–A charge transfer
upon excitation, giving the PL highly sensitive to the dielectric
environment. In contrast, TICNBI only displayed solvent-inde-
pendent emission with locally excited emission from donor
moiety (indolocarbazole). The neat films’ PL spectra of TICCBI
and TICNBI were red-shifted by 5–10 nm as compared to those
in CH2Cl2 solution (Fig. S1 in the ESI†), revealing different
dielectric surrounding in the solid state, which is usually
encountered in typical organic solids. Apparently, a stronger
charge transfer emission peak centered at 450 nm was detected
for the thin film of TICCBI. Notably, the triplet energies (ET) of
TICCBI and TICNBI, determined from the highest energy peaks
of the phosphorescence spectra, are identical as 2.61 eV. The
values of ET of TICCBI and TICNBI are sufficiently high for
them to host the green, yellow, and red phosphorescent emitters.
2.3 Photophysical properties
Fig. 1 displays electronic absorption (UV-vis) and photo-
luminescence (PL) spectra (recorded at room temperature) of
TICCBI and TICNBI in various solutions and in the form of
solid thin films, as well as their phosphorescence spectra in EtOH
at 77 K; Table 1 summarizes the data. In the UV-vis spectra of
TICCBI and TICNBI, the solvent polarity independent absorp-
tion peaks from 330 to 340 nm are ascribed to the p–p* transi-
tion and peaks around 400 nm are owing to n–p* transition of
the indolocarbazole moiety. In addition, we observed a broad
peak from 350 to 380 nm as the charge transfer absorption in
TICCBI. While the p–p* absorption spectra of TICCBI and
TICNBI thin films centered at 345 and 344 nm, respectively, were
slightly red-shifted relative to those of solutions (Fig. 1). Since
bipolar molecules TICCBI and TICNBI are composed of indo-
locarbazole as the central donor and phenyl benzimidazole as the
peripheral acceptors, it is reasonable to anticipate the occurrence
of intramolecular charge transfer. Therefore, in order to explore
Table 1 Physical properties of TICCBI and TICNBI
(Tg/Tm/Td)/ꢂC
(E1/2 a/E1/2 b)/V
labs solc/nm
labs film/nm
lPL solc/nm
lPL film/nm
ET/eV
(HOMO/LUMO/Egd)/eV
Ox
Red
TICCBI
TICNBI
174/385/466
168/406/475
0.48/ꢀ2.63
277 338 405
268 337 401
279 345 409
270 344 409
413 439
411 435
420 449
418 441
2.61
2.61
ꢀ5.3/ꢀ2.3/3.0
ꢀ5.5/ꢀ2.5/3.0
0.51/—
a
b
c
d
In CH2Cl2 containing 0.1 M TBAPF6. In DMF containing 0.1 M TBAP. In CH2Cl2. Determined through AC-2 measurements; LUMO ¼
HOMO+ Eg; the value of Eg was calculated from the absorption onset of the solid film.
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2.4 Theoretical study
numbers in Fig. S2 (ESI†). The geometrical variations between
B3LYP/6-31G(d) S0 and CIS/6-31G(d) S1 in the dihedral angles
F(16,1,17,18) of TICCBI increased from 52ꢂ to 55ꢂ, whereas
TICNBI showing a greater rotation from 53.8ꢂ to 68.4ꢂ was
observed. These results indicate that the molecular configuration
of indolocarbazole (D) and benzimidazole (A) in TICNBI twists
in S1 with poor coplanarity, which agrees with the lack of D to A
charge transfer behaviour in the previous solvent-dependent
emission experiments.
Quantum mechanical calculations were further conducted to
understand the interesting photophysical properties of TICCBI
and TICNBI at the molecular level using the GAUSSIAN 09
suite program. We try to characterize the first singlet excited
states (S1), aiming at realizing the geometries and molecular-
orbital contributions for the fluorescent electronic transitions.23
The calculations of excited-state properties significantly require
more computational efforts than needed for the ground state. To
reach this goal, most quantum-mechanical excited-state studies
employ the configuration interaction with all singly excited
determinants (CIS)24 as a cost-effective approach for optimizing
the S1 structures. Geometry optimizations of singlet ground
states (S0) and the HOMO/LUMO orbitals for TICCBI and
TICNBI were processed by density functional theory (DFT)
using Beck’s three-parameterized Lee–Yang–Parr exchange
functional (B3LYP) with 6-31G(d) basis sets as shown in Fig. 2.
The HOMO orbitals are mostly distributed over the indolo-
carbazole unit of TICCBI and TICNBI. In addition, the phe-
nylene linker between the donor and acceptor units also
contributes to the HOMO orbitals. This indicates that the
HOMO levels of TICCBI and TICNBI are mainly determined by
the indolocarbazole and phenylene linker with considerable
contribution from the former. Particularly noteworthy, the
LUMO orbital of TICNBI is located on indolocarbazole,
whereas the LUMO of TICCBI is dispersed on whole phenyl
benzimidazole moiety. The complete spatial separation of
HOMO and LUMO energy levels in TICCBI suggests that the
HOMO–LUMO excitation would shift the electron density
distribution from the central indolocarbazole to the peripheral
phenyl benzimidazole moieties, leading to a polarized excited
state which supports our observation of red-shift emission in
polar solvents.
The calculated first electronic transitions, emission wave-
lengths, oscillator strengths (f), composition in terms of molec-
ular orbital configurations and experimental emissions are listed
in Table 2. Decisive evidence was given combining the results of
Table 2 and Fig. 2, the emission peak with larger oscillator
strength of TICCBI at 396.8 nm was assigned as a D–A charge
transfer character, arising from the indolocarbazole-centered
HOMO to the benzimidazole-located LUMO. In contrast, the
smaller oscillator strength of HOMO / LUMO transition of
TICNBI indicates no relevance to the D–A charge transfer.
Computational analyses for the excited-state geometries and
electronic transitions confirmed experimental emission spectra
where obvious D–A charge transfer character in TICCBI was
shown. We are convinced with theoretical tools, it is possible to
reasonably investigate the electronic characteristics of structure-
similar systems and provide insights to clearly define the struc-
ture–properties relation.
2.5 Electrochemical properties
The electrochemical properties of TICCBI and TICNBI were
examined by cyclic voltammetry (CV) (Fig. 3). The observed
reversible oxidation potential and quasi-reversible reduction
potential in TICCBI can be ascribed to the indolocarbazole and
N-phenylbenzimidazole, respectively. It is interesting to see the
difference in the oxidation potentials of TICCBI and TICNBI,
where, TICCBI exhibited a lower oxidation potential owing to
the slightly extended p-conjugation of indolocarbazole through
C-connectivity to N-phenylbenzimidazole. As compared to
TICCBI, the reduction of TICNBI shows poor reversibility,
which is because the LUMO is localized on the electron-rich
central indolocarbazole as shown in the theoretical calculation. It
is intriguing to note that electropolymerization did not occur
during multiple-cycle CV scans (Fig. S3 in the ESI†), implying
superior electrochemical stability of TICCBI and TICNBI as
compared to those of carbazole-based materials. We believe that
the radical cations of TICCBI and TICNBI upon electrochemical
oxidation are localized on the central phenylene ring of the
indolocarbazole core, impeding efficient electropolymerization.
Thus, the appending of the electron-transporting N-phenyl-
benzimidazole units onto the indolocarbazole core renders
TICCBI and TICNBI with promising electrochemical stability,
bipolar characteristics and their potential for efficient hole and
electron transport. The HOMO energy levels of TICCBI and
TICNBI were determined using photoelectron yield spectroscopy
(Riken AC-2), and the LUMO energy levels were calculated from
the HOMO values by using the equation LUMO ¼ HOMO + Eg,
where Eg is the optical band gap determined from the onset
wavelength of the absorption band. The calculated HOMO/
LUMO energies are summarized in Table 1.
In order to predict the excited states, optimized ground state
geometries of TICCBI and TICNBI were optimized in the S1 by
CIS methods with 6-31G(d) basis sets. After the optimizations of
S1, time dependent density functional theory (TD-DFT)25 is
adopted to obtain the computation of emission energies, oscil-
lator strengths, and composition of excited states in terms of
singly excited Slater determinants. Bond lengths and dihedral
angles obtained from B3LYP/6-31G(d) S0 and CIS/6-31G(d) S1
are summarized in Table S1 (ESI†), with the atoms denoted by
Fig. 2 S0 frontier molecular orbitals (HOMO and LUMO) of TICCBI
(left) and TICNBI (right), calculated with DFT at a B3LYP/6-31G (d)
level.
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Table 2 Calculated emission data derived from TDDFT//B3LYP/6-31G(d) according to the CIS/6-31G(d) optimized geometries
Host
Electronic transition
lem/nm
f
MO configuration
Coefficient
Exp. lem/nm
TICCBI
TICNBI
—
S1 / S0
S1 / S0
—
396.8
377
—
0.69
0.13
—
HOMO / LUMO
HOMO / LUMO
HOMO ꢀ 1 / LUMO
0.7
0.61
0.32
439
435
—
Fig. 3 Cyclic voltammograms of TICCBI and TICNBI; 0.1 M TBAP
(reduction) in DMF and 0.1 M TBAPF6 (oxidation) in CH2Cl2 were used
as supporting electrolytes. A glassy carbon electrode was used as the
Fig. 4 Crystal packing of (a) TICCBI and (b) TICNBI. The solvent
working electrode; scan rate: 100 mV sꢀ1
.
molecules are omitted for clarity.
a cryostat under vacuum. Fig. 5(a) and (b) show the represen-
tative TOF transient for holes. Both transient photocurrent
signals were dispersive, suggesting the presence of hole traps. The
transit time, tT, can be evaluated from the intersection point of
two asymptotes in the double-logarithmic representation of the
TOF transient. It needed to determine the carrier mobilities
according to the equation m ¼ d2/VtT, where d is the sample
thickness and V is the applied voltage. Fig. 5(c) shows the field
dependence of the hole mobility follows the nearly universal
Poole–Frenkel relationship and the values are in the range of 4 ꢃ
10ꢀ6 to 2 ꢃ 10ꢀ5 cm2 Vꢀ1 sꢀ1 for TICCBI and 1 ꢃ 10ꢀ5 to 4 ꢃ
10ꢀ5 cm2 Vꢀ1 sꢀ1 for TICNBI at fields varying from 2.6 ꢃ 105 to
5.5 ꢃ 105 V cmꢀ1. In contrast, the electron transient photocurrent
signals were too weak, which cannot be used to determine the
electron mobility by the TOF technique.
2.6 Crystal structure
In order to get better idea of intermolecular interactions of
TICCBI and TICNBI, suitable single crystals of TICNBI were
obtained by sublimation, whereas crystals of TICCBI were
obtained by slow evaporation of the DMSO solvent. X-Ray
diffraction analyses indicate that we get the crystals of TICCBI
and TICNBI packed in the monoclinic crystal structure with space
group P21/n and triclinic crystal structure with space group P1,
respectively (Table S2 in the ESI†). In TICCBI, the dihedral
angles between indolocarbazole/phenylene-linker and phenylene-
linker/benzimidazole were calculated as 52.8ꢂ and 56.3ꢂ, respec-
tively, whereas in TICNBI, the dihedral angles were calculated as
32.6ꢂ and 59.6ꢂ, respectively. The highly twisted conformations
render the molecules rather bulky and in noncoplanar confor-
mation as shown in Fig. 4. The noncoplanar molecular confor-
mation prevents the intermolecular interaction which enables
them to form amorphous thin film via vacuum deposition. The
crystal packing of TICCBI indicates the absence of intermolecular
p–p interactions, in which the closest plane-to-plane distance
between benzimidazole and phenylene linker was estimated to be
For the assessment of electron transport property, the elec-
tron-only device was fabricated with a device structure of ITO/
BCP (30 nm)/TICCBI or TICNBI (50 nm)/TPBI (20 nm)/LiF/Al.
Here, BCP serves as the hole-blocking layer to impede the hole
current in the device. Fig. 6 depicts the current vs. voltage
characteristics of electron-only devices. The introduction of BCP
contacting to ITO is for limiting the injection of hole carrier, as
shown in the inset of Fig. 6. Apparently, different linking
topologies (C- or N-connection of benzimidazole) have
pronounced effect on the electron transport and the magnitude
of current follows the order: TICCBI > TICNBI.
ꢀ
ca. 4.3 A. In contrast, the crystal packing of TICNBI shows
interesting intermolecular D–A interactions, in which the closest
plane-to-plane distance between benzimidazole and central
ꢀ
indolocarbazole was estimated to be ca. 3.8 A (Fig. 4).
2.7 Charge carrier mobility and electron only devices
2.8 OLED device
In order to confirm the hypothesis that the appending of elec-
tron-accepting N-phenylbenzimidazole units onto the electron-
donating indolo-carbazole core renders TICCBI and TICNBI
with promising bipolar transporting nature, we used time-of-
flight (TOF) techniques to measure carrier mobility.26 The device
used for TOF measurement was prepared through vacuum
deposition with the structure ITO–glass/TICCBI (1.8 mm) or
TICNBI (2.3 mm)/Ag (150 nm) and then it was placed inside
To investigate the new indolocarbazole–benzimidazole hybrid
materials (TICCBI and TICNBI) as hosts in organic light emit-
ting diodes, we fabricated the emitting device with typical
multi-layer architecture of indium-tin-oxide (ITO)/polyethylene
dioxythiophene:polystyrene sulfonate (PEDOT:PSS, 30 nm)/
4,40-bis[N-(1-naphthyl)-N-phenyl-amino]biphenyl (NPB, 20 nm)/
4,40,400-tri(N-carbazolyl)triphenylamine (TCTA, 5 nm)/emitter
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electron-transporting layer, and LiF and Al were used as the
electron injecting layer and cathode, respectively. Moreover,
a thin layer (ca. 5 nm) of TCTA was inserted between NPB and
the emitting layer (EML) to reduce the excessive energy barrier
between NPB and the host material. Scheme 2 shows chemical
structures of the heavy metal containing complexes used in this
study.
The current density–voltage–luminance (J–V–L) characteris-
tics, external quantum efficiency as well as power efficiency
versus luminance curve, and EL spectra of the devices are shown
in Fig. 7 and 8. All the device performances are summarized in
Table 3.
All turn-on voltages of the TICCBI-based devices (2 eV) are
lower than those of the TICNBI-based devices (2.5 eV), which
could be attributed to the HOMO level of TICCBI (ꢀ5.3 eV)
which is 0.2 eV higher than that of TICNBI (ꢀ5.5 eV). Taking
the difference in the J–V performance caused by using different
emitting dopant into account, we could make a conclusion that
the emitting dopants could have a strong influence on the charge
carrier transport of the emission layer. We initially examined the
performance of (PPy)2Ir(acac)-based green devices (devices A1
and B1). Fig. 7(a) shows that the J–V–L characteristics of devices
A1 and B1 are nearly identical, indicating that hosts TICCBI and
TICNBI have very little influence on the charge transport char-
acteristics and the driving voltage is approximately 5.6 and 5.1 V
at 1000 cd mꢀ2, respectively. As shown in Fig. 7(b), the device B1
using TICNBI host exhibited the higher external quantum effi-
ciency (hext) of 15%, current efficiency (hc) of 55.8 cd Aꢀ1 and
power efficiency (hp) of 56 lm Wꢀ1 as compared with that (14%,
52.4 cd Aꢀ1, and 31.2 lm Wꢀ1) of device A1 using TICCBI as the
host. It is worth noting that the hext of device B1 still remains as
high as 14.9% at a brightness of 1000 cd mꢀ2 without significant
decay, which can be explained by the balance of charge carriers in
the emitting layer. Next, we fabricated yellow phosphorescent
devices (devices A2 and B2) using (Bt)2Ir(acac) as the dopant. As
the green devices, the device B2 incorporating TICNBI as
the host exhibited higher efficiencies (16.2%, 45.7 cd Aꢀ1, and
37 lm Wꢀ1) relative to that of device A2 (15.4%, 42.2 cd Aꢀ1, and
31.6 lm Wꢀ1). These values are comparable with those of yellow
phosphorescent devices in the literature,30 which may serve as the
potential emitting element for generating white light in combi-
nation with a complementary blue phosphor.31
Fig. 5 Representative TOF transients for (a) TICCBI (1.8 mm thickness;
E ¼ 3.3 ꢃ 105 V cmꢀ1); (b) TICNBI (2.3 mm thickness; E ¼ 3.9 ꢃ
105 V cmꢀ1). Inset: double-logarithmic plot. (c) Hole mobilities of
TICCBI and TICNBI plotted with respect to E1/2
.
Fig. 6 Current density–voltage (I–V) characteristics of electron-only
devices.
(25 nm)/1,3,5-tris(N-phenylbenzimidizol-2-yl)benzene (TPBI,
50 nm)/LiF (0.5 nm)/Al (100 nm), for which the hosts are
TICCBI for device A, or TICNBI for device B. Among the
numerous possible heavy-metal containing complexes, we chose
the
bis(2-phenylpyridinato)
iridium(III)
acetylacetonate
(PPy)2Ir(acac)27 for green emission, bis(2-phenyl benzothiozo-
0
lato-N,C2 ) iridium (acetylacetonate) (Bt)2Ir(acac)28 for yellow
emission and osmium(II) bis(3-(trifluoromethyl)-5-(4-tert-butyl-
pyridyl)-1,2,4-triazolate) dimethylphenylphosphine Os(bpftz)2-
(PPhMe2)2,29 as well as (35dmPh-6Fiq)2Ir(acac) (i3)
and (4tBuPh-6Fiq)2Ir(acac) (i6) for red emissions. The doping
concentration of the dopants is 10 wt%. NPB was used
as the hole-transporting layer, TPBI was used as the
Scheme 2 Chemical structures of the heavy metal complex used in this
study.
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Fig. 7 (a) L–V–J characteristics, (b) plots of EL efficiency versus
brightness and (c) EL spectra for devices incorporating TICCBI and
TICNBI doped with 10 wt% PPy2Ir(acac) and Bt2Ir(acac).
Fig. 8 (a) L–V–J characteristics, (b) plots of EL efficiency versus
brightness and (c) EL spectra for devices incorporating TICCBI and
TICNBI doped with 10 wt% Os1, (35dmPh-6Fiq)2Ir(acac) (i3) and
(4tBuPh-6Fiq)2Ir(acac) (i6).
Finally, we demonstrated highly efficient red phosphorescent
devices using the emitters composed of the third-row Os(II)
(devices A3 and B3) and phenylisoquinolinyl chelated Ir(III)
(devices A4, A5, B4 and B5) complexes. The device characteris-
tics are shown in Fig. 8 and data are summarized in Table 3. The
devices exhibited relatively pure red emission with the corre-
respectively. The best EL performance is achieved for device B3
with Os(bpftz)2(PPhMe2)2 doped in TICNBI, in which very high
hext of 22%, hc of 28 cd Aꢀ1, and hp of 22.1 lm Wꢀ1 were obtained,
whereas the hext still remains 20.8% at 1000 cd mꢀ2. Compared to
contemporary results, device B3 is the best of Os-based
devices.29,32 For device A3 using TICCBI as host, the device
ꢁ
sponding CIE (Commission Internationale de L’eclairage)
coordinates x ¼ 0.65, y ¼ 0.35 for Os(bpftz)2(PPhMe2)2 and
x ¼ 0.67–0.70, y ¼ 0.30–0.32 for phenylisoquinolinyl-Ir(III),
performance also achieved 21%, 26.6 cd Aꢀ1, and 25 lm Wꢀ1
,
Table 3 Electroluminescent properties of the devices
a
/
Von
V
At 1000 nitb/
V, %
Lmax
/
Imax
/
hext max/
%, cd Aꢀ1
hp max/
lm Wꢀ1
CIE1931
(x,y)
Device
Host: 10% dopant
cd mꢀ2
mA cmꢀ2
A1
A2
A3
A4
A5
B1
B2
B3
B4
B5
TICCBI: PPy2Ir(acac)
TICCBI: Bt2Ir(acac)
TICCBI: Os1
TICCBI: (35dmPh-6Fiq)2Ir(acac)
TICCBI: (4tBuPh-6Fiq)2Ir(acac)
TICNBI: PPy2Ir(acac)
TICNBI: Bt2Ir(acac)
TICNBI: Os1
TICNBI: (35dmPh-6Fiq)2Ir(acac)
TICNBI: (4tBuPh-6Fiq)2Ir(acac)
2
2
2
2
5.6, 13.5
5.7, 15.2
6.6, 20
8.7, 11
7.6, 11
5.1, 14.9
6, 15.7
6, 20.8
7.2, 13.6
6.4, 14.2
218 400 (12 V)
132 600 (12.5 V)
76 900 (12.5 V)
16 900 (15.5 V)
18 100 (13.5 V)
225 600 (12 V)
142 300 (12.5 V)
81 600 (12.5 V)
28 300 (14.5 V)
48 300 (13 V)
2260
1740
1400
1440
1710
1770
1800
1990
1300
1940
14, 52.4
15.4, 42.2
21, 26.6
14.4, 7.9
15.3, 12
15, 55.8
16.2, 45.7
22, 28
31.2
31.6
25
8
13
56
37
22.1
8.4
10.5
0.35,0.62
0.52,0.48
0.65,0.35
0.70,0.30
0.68,0.32
0.34,0.62
0.51,0.48
0.65,0.35
0.69,0.30
0.67,0.32
2
2.5
2.5
2.5
2.5
2.5
15.6, 9.6
15.5, 13.4
a
b
Turn-on voltage at which emission became detectable. The values of driving voltage and hext of device at 1000 cd mꢀ2
.
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whereas the hext still remains 20% at 1000 cd mꢀ2. Considering
the out coupling factor, this is reaching nearly 100% internal
efficiency. The high efficiency and low efficiency roll-off at high
luminance for devices A3 and B3 can be attributed to the use of
the bipolar hosts TICCBI and TICNBI, which may have resulted
in balanced charge fluxes and a broad distribution of recombi-
nation region within the emitting layer. For devices A4 and B4
doped with (35dmPh-6Fiq)2Ir(acac), they displayed deep-red
emission with CIE coordinates of (0.70,0.30) and the device
performances (14.4–15.6%). For devices A5 and B5 doped with
(4tBuPh-6Fiq)2Ir(acac), they displayed deep-red emission with
CIE coordinates of (0.68,0.32) and the device performances
(15.3–15.5%) are significantly better than those of phenyl-
isoquinolinyl-iridium complexes using CBP as the host.33
By altering the linking topology (C- or N-connectivity) of the
benzimidazole substituents at the central indolo-carbazole core,
the efficiencies of the devices based on TICNBI as host are
typically higher than those based on TICCBI, which may be due
to the superior carrier balance that caused greater degree of
energy transfer in the emissive layer. These findings show that
even small structural variations may change the charge-carrier
balance in the device or the chemical compatibility of host and
emitter. This information could be useful for the design of an
efficient host.
development of next generation amorphous bipolar host mole-
cules for PhOLEDs.
Acknowledgements
We greatly appreciate the financial support from National
Science Council of Taiwan (NSC 98-2119-M-002-007-MY3 and
100-2112-M-019-002-MY3) and Ministry of Economic Affairs
(100-EC-17-A-08-S1-042).
Notes and references
1 (a) C. W. Tang and S. A. VanSlyke, Appl. Phys. Lett., 1987, 51, 913;
(b) M. A. Baldo, M. E. Thompson and S. R. Forrest, Nature, 2000,
403, 750.
2 M. A. Baldo, D. F. O’Brien, Y. You, A. Shoustikov, S. Sibley,
M. E. Thompson and S. R. Forrest, Nature, 1998, 395, 151.
3 S.-J. Su, H. Sasabe, T. I. Takeda and J. Kido, Chem. Mater., 2008, 20,
1691.
4 H. H. Fong, C. H. Wallace Choy, K. N. Hui and Y. J. Liang, Appl.
Phys. Lett., 2006, 88, 113510.
5 (a) C. Adachi, R. C. Kwong, P. Djurovich, V. Adamovich,
M. A. Baldo, M. E. Thompson and S. R. Forrest, Appl. Phys. Lett.,
2001, 79, 2082; (b) R. J. Holmes, S. R. Forrest, Y.-J. Tung,
R. C. Kwong, J. J. Brown, S. Garon and M. E. Thompson, Appl.
Phys. Lett., 2003, 82, 2422; (c) S. Tokito, T. Iijima, Y. Suzuri,
H. Kita, T. Tsuzuki and F. Sato, Appl. Phys. Lett., 2003, 83, 569;
(d) K. Goushi, R. Kwong, J. J. Brown, H. Sasabe and C. Adachi, J.
Appl. Phys., 2004, 95, 7798.
6 (a) M. A. Baldo, S. Lamansky, P. E. Burrows, M. E. Thompson and
S. R. Forrest, Appl. Phys. Lett., 1999, 75, 4; (b) M. H. Lu,
M. S. Weaver, T. X. Zhou, M. Rothman, R. C. Kwong, M. Hack
and J. J. Brown, Appl. Phys. Lett., 2002, 81, 3921.
7 (a) A. Chaskar, H.-F. Chen and K.-T. Wong, Adv. Mater., 2011, 23,
3876; (b) S. O. Jeon, S. E. Jang, H. S. Son and J. Y. Lee, Adv. Mater.,
2011, 23, 1436; (c) Y. Tao, Q. Wang, C. Yang, C. Zhong, J. Qin and
D. Ma, Adv. Funct. Mater., 2010, 20, 2923.
8 (a) S. Gong, Y. Chen, J. Luo, C. Yang, C. Zhong, J. Qin and D. Ma,
Adv. Funct. Mater., 2011, 21, 1168; (b) S. Gong, Y. Chen, C. Yang,
C. Zhong, J. Qin and D. Ma, Adv. Mater., 2010, 22, 5370.
9 H.-H. Chou and C.-H. Cheng, Adv. Mater., 2010, 22, 2468.
10 W.-Y. Hung, L.-C. Chi, W.-J. Chen, Y.-M. Chen, S.-H. Chou and
K.-T. Wong, J. Mater. Chem., 2010, 20, 10113.
Conclusions
Two new bipolar host materials TICCBI and TICNBI
composed of indolo[3,2-b]carbazole donor core and N-phenyl
benzimidazole acceptor peripherals were designed and synthe-
sized. The physical properties of the hosts can be modulated
through the different linkages between the electronic donor and
acceptor components. The promising properties include
reversible redox behavior, high morphological stability, and
suitable energy levels, rendering them as potential good candi-
dates for realizing highly efficient eletrophosphorescence
devices. PhOLEDs incorporating TICCBI as the host doped
with Ir-based emitters such as green (PPy)2Ir(acac), yellow
(Bt)2Ir(acac), and two new red emitters (35dmPh-6Fiq)2Ir(acac)
(i3) and (4tBuPh-6Fiq)2Ir(acac) (i6) accomplished high external
quantum efficiencies ranging from 14 to 15.3%. In contrast, the
devices hosted by TICNBI achieved the best EL performance,
with a maximum current efficiency (hc) of 28 cd Aꢀ1 and
11 Y. Tao, C. Yang and J. Qin, Chem. Soc. Rev., 2011, 40, 2943.
12 Y. Wu, Y. Li, S. Gardner and B. S. Ong, J. Am. Chem. Soc., 2005, 127,
614.
13 (a) N. X. Hu, S. Xie, Z. Popovic, B. Ong and A. M. Hor, J. Am. Chem.
Soc., 1999, 121, 5097; (b) H. P. Zhao, X. T. Tao, P. Wang, Y. Ren,
J. X. Yang, Y. X. Yan, C.-X. Yuan, H. J. Liu, D. C. Zou and
M. H. Jiang, Org. Electron., 2007, 8, 673; (c) S. Lengvinaite,
J. V. Grazulevicius, S. Grigalevicius, R. Gu, W. Dehaen,
V. Jankauskas, B. Zhang and Z. Xie, Dyes Pigm., 2010, 85, 183.
14 (a) P. T. Boudreault, S. Wakim, M. L. Tang, Y. Tao, Z. Bao and
M. Leclerc, J. Mater. Chem., 2009, 19, 2921; (b) Y. Guo, H. Zhao,
G. Yu, C. Di, W. Liu, S. Jiang, S. Yan, C. Wang, H. Zhang,
X. Sun, X. Tao and Y. Liu, Adv. Mater., 2008, 20, 4835.
15 (a) J. Lu, F. Liang, N. Drolet, J. Ding, Y. Tao and R. Movileanu,
Chem. Commun., 2008, 5315; (b) E. Zhou, S. Yamakawa, Y. Zhang,
K. Tajima, C. Yanga and K. Hashimoto, J. Mater. Chem., 2009,
19, 7730.
16 S. Lengvinaite, J. V. Grazulevicius, S. Grigalevicius, R. Gu,
W. Dehaen, V. Jankauskas, B. Zhang and Z. Xie, Dyes Pigm.,
2010, 85(3), 183.
17 (a) Y.-J. Su, H.-L. Huang, C.-L. Li, C.-H. Chien, Y.-T. Tao,
P.-T. Chou, S. Datta and R.-S. Liu, Adv. Mater., 2003, 15, 884; (b)
C. L. Li, Y. J. Su, Y. T. Tao, P. T. Chou, S. Datta and R. S. Liu,
Adv. Funct. Mater., 2005, 15, 387.
maximum external quantum efficiency (hext
) of 22% for
Os-based red electrophosphorescence, 55.8 cd Aꢀ1 and 15%
for green electrophosphorescence, and 45.7 cd Aꢀ1 and 16.2%
for yellow electrophosphorescence. The hext remains as high as
20% for red and 14.9% for green electrophosphorescence at
a brightness of 1000 cd mꢀ2. The high efficiency and low effi-
ciency roll-off at high luminance can be attributed to the
balanced charge fluxes, a broad distribution of recombination
region within the emitting layer, the well-matched energy levels
between the host and hole-transport layer, and complete spatial
separation of HOMO and LUMO energy levels. The EL
performances of the devices closely correlate to the molecular
structural features, and substantial gains in the EL perfor-
mances can be made by subtle changes in the design of the host
material. Our study presents a new guideline for the molecular
design of amorphous bipolar materials, paving the way for the
18 (a) V. V. Grushin, N. Herron, D. D. LeCloux, W. J. Marshall,
V. A. Petrov and Y. Wang, Chem. Commun., 2001, 1494; (b)
Y. Wang, N. Herron, V. V. Grushin, D. LeCloux and V. Petrov,
Appl. Phys. Lett., 2001, 79, 449.
8406 | J. Mater. Chem., 2012, 22, 8399–8407
This journal is ª The Royal Society of Chemistry 2012

View Article Online
19 (a) I. R. Laskar and T.-M. Chen, Chem. Mater., 2004, 16, 111; (b)
K.-H. Fang, L.-L. Wu, Y.-T. Huang, C.-H. Yang and I.-W. Sun,
Inorg. Chim. Acta, 2006, 359, 441; (c) C.-H. Yang, C.-C. Tai and
I.-W. Sun, J. Mater. Chem., 2004, 14, 947; (d) P. J. Hay, J. Phys.
Chem. A, 2002, 106, 1634.
20 L. N. Yudina and J. Bergman, Tetrahedron, 2003, 59, 1265.
21 Z. Ge, T. Hayakawa, S. Ando, M. Ueda, T. Akiike, H. Miyamoto,
T. Kajita and M.-A. Kakimoto, Adv. Funct. Mater., 2008, 18, 584.
22 F.-M. Hsu, C.-H. Chien, C.-F. Shu, C.-H. Lai, C.-C. Hsieh,
K.-W. Wang and P.-T. Chou, Adv. Funct. Mater., 2009, 19, 2834.
23 Y. L. Liu, J. K. Feng and A. M. Ren, J. Phys. Chem. A, 2008, 112,
3157.
29 Y.-L. Tung, S.-W. Lee, Y. Chi, Y.-T. Tao, C.-H. Chien, Y.-M. Cheng,
P.-T. Chou, S.-M. Peng and C.-S. Liu, J. Mater. Chem., 2005, 15,
460.
30 (a) C.-L. Ho, W.-Y. Wong, Q. Wang, D. Ma, L. Wang and Z. Lin,
Adv. Funct. Mater., 2008, 18, 928; (b) Y.-S. Park, J.-W. Kang,
D. M. Kang, J.-W. Park, Y.-H. Kim, S.-K. Kwon and J.-J. Kim,
Adv. Mater., 2008, 20, 1957; (c) W.-Y. Hung, L.-C. Chi,
W.-J. Chen, Y.-M. Chen, S.-H. Chou and K.-T. Wong, J. Mater.
Chem., 2010, 20, 10113; (d) S.-L. Lai, S.-L. Tao, M.-Y. Chan,
M.-F. Lo, T.-W. Ng, S.-T. Lee, W.-M. Zhao and C.-S. Lee, J.
Mater. Chem., 2011, 21, 4983; (e) R. Wang, D. Liu, H. Ren,
T. Zhang, H. Yin, G. Liu and J. Li, Adv. Mater., 2011, 23,
2823.
24 R. J. Cave, K. Burke and E. W. Castner, Jr, J. Phys. Chem. A, 2002,
106, 9294.
25 M. E. Casida, C. Jamorski, K. C. Casida and D. R. Salahub, J. Chem.
Phys., 1998, 108, 4439.
31 H. Sasabe, J. Takamatsu, T. Motoyama, S. Watanabe,
G. Wagenblast, N. Langer, O. Molt, E. Fuchs, C. Lennartz and
J. Kido, Adv. Mater., 2010, 22, 5003.
26 M. Borsenberger and D. S. Weiss, in Organic Photoreceptors for
Imaging Systems, New York, 1993, and references therein.
27 C. Adachi, M. A. Baldo, M. E. Thompson and S. R. Forrest, J. Appl.
Phys., 2001, 90, 5048.
28 (a) S. Lamansky, P. Djurovich, D. Murphy, F. Abdel-Razzaq,
H.-E. Lee, C. Adachi, P. E. Burrows, S. R. Forrest and
M. E. Thompson, J. Am. Chem. Soc., 2001, 123, 4304; (b)
W.-C. Chang, A. T. Hu, J.-P. Duan, D. K. Rayabarapu and
C.-H. Cheng, J. Organomet. Chem., 2004, 689, 4882.
32 (a) C.-H. Chien, F.-M. Hsu, C.-F. Shu and Y. Chi, Org. Electron.,
2009, 10, 871; (b) S.-Y. Ku, W.-Y. Hung, C.-W. Chen, S.-W. Yang,
E. Mondal, Y. Chi and K.-T. Wong, Chem.–Asian J., 2012, 7, 133.
33 (a) H.-C. Li, P.-T. Chou, Y.-H. Hu, Y.-M. Cheng and R.-S. Liu,
Organometallics, 2005, 24, 1331; (b) Y.-J. Su, H.-L. Huang,
C.-L. Li, C.-H. Chien, Y.-T. Tao, P.-T. Chou, S. Datta and
R.-S. Liu, Adv. Mater., 2003, 15, 884; (c) C.-L. Li, Y.-J. Su,
Y.-T. Tao, P.-T. Chou, C.-H. Chien, C.-C. Cheng and R.-S. Liu,
Adv. Funct. Mater., 2005, 15, 387.
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