Carbazole-based coplanar molecule (CmInF) as a universal host for multi-color electrophosphorescent devices

Article abstract of DOI:10.1039/c1jm13794h

The synthesis, isomerism, photophysics and electrophosphorescent characterization of some functional cyclometallated iridium(iii) complexes containing 2-[2-(N-phenylcarbazolyl)]pyridine and 2-[3-(N-phenylcarbazolyl)] pyridine molecular frameworks are described. A carbazole-based coplanar molecule (CmInF) obtained through the intramolecular ring closure of aryl substitutions at the C3 and C6 positions exhibits a high triplet energy (E_T = 2.77 eV), morphological stability (T_g = 195 °C) and hole mobility in the range of up to 5 × 10^-3 cm² V^-1 s^-1. Highly efficient multi-color electrophosphorescent devices have been successfully achieved employing CmInF as the universal host material doped with phosphorescent dopants of various colors under the same device configuration of ITO/PEDOT:PSS (300 A)/TCTA (250 A)/CmInF: dopant (250 A)/TAZ (500 A)/LiF/Al (PEDOT:PSS = poly(ethylene dioxythiophene):polystyrene sulfonate; TCTA = 4,4′,4′′-tri(N- carbazolyl)triphenylamine; TAZ = 3-(4-biphenylyl)-4-phenyl-5-(4-tert- butylphenyl)-1,2,4-triazole). Through the mixing of two phosphorescent dopants of complementary colors, we also fabricated a two-color white organic light-emitting device (WOLED) with the same device structure consisting of 12 wt% FIrpic and 0.3 wt% (Mpg)₂Ir(acac) co-doped into CmInF as a single-emitting-layer, which exhibits peak WOLED efficiency of 13.4% (23.4 cd A^-1) and 11.2 lm W^-1 with the Commission Internationale de L'Eclairage (CIE) coordinates of (0.33, 0.37). In addition, the use of such device structure in full-color OLEDs has the advantages of simplifying manufacturing process and reducing production cost that are the critical issues of commercialization.

Full text of DOI:10.1039/c1jm13794h

View Article Online / Journal Homepage / Table of Contents for this issue
Dynamic Article Links <
C
Journal of
Materials Chemistry
Cite this: J. Mater. Chem., 2012, 22, 215
www.rsc.org/materials
PAPER
Carbazole-based coplanar molecule (CmInF) as a universal host for
multi-color electrophosphorescent devices
b
c
Cheuk-Lam Ho,ae Liang-Chen Chi, Wen-Yi Hung, Wei-Jiun Chen,c Yu-Cheng Lin,c Hao Wu,ae
*
b
Ejabul Mondal, Gui-Jiang Zhou, Ken-Tsung Wong and Wai-Yeung Wong
d
b
ae
*
*
Received 6th August 2011, Accepted 22nd September 2011
DOI: 10.1039/c1jm13794h
The synthesis, isomerism, photophysics and electrophosphorescent characterization of some functional
cyclometallated iridium(III) complexes containing 2-[2-(N-phenylcarbazolyl)]pyridine and 2-[3-(N-
phenylcarbazolyl)]pyridine molecular frameworks are described. A carbazole-based coplanar molecule
(CmInF) obtained through the intramolecular ring closure of aryl substitutions at the C3 and C6
positions exhibits a high triplet energy (ET ¼ 2.77 eV), morphological stability (Tg ¼ 195 ꢀC) and hole
mobility in the range of up to 5 ꢁ 10ꢂ3 cm2 Vꢂ1 sꢂ1. Highly efficient multi-color electrophosphorescent
devices have been successfully achieved employing CmInF as the universal host material doped with
phosphorescent dopants of various colors under the same device configuration of ITO/PEDOT:PSS
ꢀ
ꢀ
ꢀ
ꢀ
(300 A)/TCTA (250 A)/CmInF: dopant (250 A)/TAZ (500 A)/LiF/Al (PEDOT:PSS ¼ poly(ethylene
dioxythiophene):polystyrene sulfonate; TCTA ¼ 4,40,400-tri(N-carbazolyl)triphenylamine; TAZ ¼ 3-(4-
biphenylyl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole). Through the mixing of two phosphorescent
dopants of complementary colors, we also fabricated a two-color white organic light-emitting device
(WOLED) with the same device structure consisting of 12 wt% FIrpic and 0.3 wt% (Mpg)2Ir(acac) co-
doped into CmInF as a single-emitting-layer, which exhibits peak WOLED efficiency of 13.4% (23.4 cd
Aꢂ1) and 11.2 lm Wꢂ1 with the Commission Internationale de L’Eclairage (CIE) coordinates of (0.33,
0.37). In addition, the use of such device structure in full-color OLEDs has the advantages of
simplifying manufacturing process and reducing production cost that are the critical issues of
commercialization.
emitters of heavy-metal complexes are normally used as the
Introduction
emitting guests in a host material to reduce the quenching effect
associated with the relatively long excited-state lifetimes of triplet
emitters as well as the triplet–triplet annihilation, etc. Conse-
quently, the selection of a suitable host material dispersed with
various phosphorescent guest dopants is of equal importance to
achieve high device efficiency. The singlet and triplet levels of the
host material should be higher than those of the phosphorescent
guest dopants to prevent the reverse energy transfer from guest to
host and also confine triplet excitons within the emitting layer.4–7
For blue PHOLEDs, the design of such host material becomes
a big challenge, particularly for a blue triplet emitter, in which
the conjugation length of the host molecule must be extremely
confined to achieve a triplet energy level higher than the photon
energies of blue light ($2.70 eV). In contrast, a host material with
triplet energy higher than that of the blue phosphor may not be
suitable for red, green, yellow emitters, etc., because of the
unbalanced charge carrier transport in the emitting layer.
Organic light-emitting diodes (OLEDs) have drawn tremendous
attention due to their potential applications in flat-panel displays
and lighting sources.1,2 For full-color OLED display applica-
tions, it is essential to deliver a set of primary red-green-blue
(RGB) emitters with sufficiently high luminous efficiency.
Notably, the use of phosphorescent materials, in which the
strong spin–orbit coupling due to the heavy-atom effect leads to
singlet–triplet state mixing, can boost the electroluminescence
(EL) efficiency.3 In phosphorescent OLEDs (PHOLEDs), triplet
aInstitute of Molecular Functional Materials and Department of Chemistry
and Centre for Advanced Luminescence Materials, Hong Kong Baptist
University, Waterloo Road, Kowloon Tong, Hong Kong, P.R. China.
E-mail: rwywong@hkbu.edu.hk
bDepartment of Chemistry, National Taiwan University, Taipei, 106,
Taiwan. E-mail: kenwong@ntu.edu.tw
cInstitute of Optoelectronic Sciences, National Taiwan Ocean University,
Keelung, 202, Taiwan. E-mail: wenhung@mail.ntou.edu.tw
dMOE Key Laboratory for Nonequilibrium Synthesis and Modulation of
Condensed Matter and Department of Chemistry, Xi’an Jiaotong
University, Xi’an, 710049, P.R. China
eAreas of Excellence Scheme, University Grants Committee of HKSAR,
P.R. China
Recently, efficient green and red PhOLEDs with 100% internal
quantum efficiency have been disclosed,8–10 but highly efficient
and stable blue PHOLEDs still remain a demanding issue. On the
other hand, white light emission can be obtained by the mixing of
This journal is ª The Royal Society of Chemistry 2012
J. Mater. Chem., 2012, 22, 215–224 | 215

View Article Online
several guest materials that give rise to RGB colors or two
complementary colors (blue and red; blue and orange/yellow)
through adoption of either the mixed fluorophore/phosphor or
all-phosphor system. The design and synthesis of a host material
that can work in various PHOLEDs are therefore important not
only for blue, green and red phosphors but also for the realiza-
tion of white OLEDs (WOLEDs) based on their suitable
combinations. Up to now, reported literature examples of
universal host materials for PHOLEDs are limited. For example,
Cheng and Chou recently synthesized a bipolar host material bis-
4-(N-carbazolyl)phenyl)phenylphosphine oxide (BCPO) con-
taining a phenylphosphine oxide and two carbazole groups,
which exhibits a high triplet energy (ET) of 3.01 eV and a high
glass transition temperature (Tg) of 137 ꢀC.11 By using BCPO as
the universal host, high external quantum efficiencies (hext) of
23.5%, 21.6% and 17% were achieved for blue, green and red
phosphorescent devices, respectively, using simple device archi-
tectures. Cheng et al. also reported three triptycene derivatives
2,6,14-tris-(diphenylamino)triptycene (TATP), 2,6,14-tris-car-
bazoyltriptycene (TCTP) and 2,6,14-tris-(diphenylphosphoryl)
triptycene (TPOTP) with remarkably high ET of 2.94–3.15 eV
and Tg of 178–238 ꢀC.12 TATP can be employed as a general host
for blue, green and red PHOLEDs, providing high EL efficien-
cies of 7.8%, 11.5% and 9.8%, respectively. Shu et al. reported the
fluorene/triarylamine hybrid based host material tris[4-(9-phe-
nylfluoren-9-yl)phenyl]amine (TFTPA) that exhibits a high ET of
2.89 eV and a Tg of 186 ꢀC.13 By employing TFTPA as the host,
the devices display hext as high as 12.3% for blue, 11.8% for green
and 9.2% for red. Yang et al. reported a silicon-bridged
compound p-BISiTPA, combining p-type triphenylamine and
n-type benziꢀmidazole groups, showing a high ET of 2.69 eV and
a Tg of 102 C accompanied by the bipolar charge transporting
feature.14 Devices with p-BISiTPA as the host show excellent
performance, with hext as high as 14.2% for blue, 18.9% for
orange and 17.4% for white OLEDs at a practical brightness of
1000 cd mꢂ2. Most recently, the same research group has devel-
oped three new silicon-bridged compounds (m-BISiTPA,
p-OXDSiTPA and m-OXDSiTPA) by integrating electron-
donating arylamine and electron-transporting benzimidazole/
oxadiazole functionalities into one molecule via a silicon-bridged
linkage mode.15 By using these compounds as the universal hosts,
they achieved multi-color PHOLEDs for blue, green, orange and
all-phosphor white-light electrophosphorescence, exhibiting
the intramolecular ring closure of aryl substituents at the C3 and
C6 positions and its applications in PHOLEDs based on various
phosphorescent dopants in appropriate concentrations. The
molecular design endows CmInF with several desirable proper-
ties: (i) the adoption of a coplanar chromophore, in which
carbazole was fused to the neighboring phenylene rings through
intramolecular annulation via sp3-hybridised carbon atoms
bearing two p-tolyl groups as the peripheral substituents, makes
the molecule rigid and bulky, and introduces molecular
constraint; (ii) the carbazole unit can simultaneously possess
sufficiently large ET and carrier-transport properties, which is
often used as the host material; and (iii) the introduction of aryl
groups as peripheral substituents of the coplanar backbone also
impedes the intermolecular interactions between the molecules
leading to their exhibition of amorphous morphologies. We
anticipate that the combination of carbazole and fluorene
moieties in a coplanar p-conjugated backbone and the presence
of two tolyl groups at sp3 C9 position in CmInF could prevent
efficiently the intermolecular interactions between molecules,
thereby increasing high morphological and thermal stability
(Tg ¼ 195 ꢀC). Moreover, a moderate ET value of 2.77 eV makes
it a suitable host for PHOLEDs of various emission colors. The
application of CmInF as the universal host doped with various
phosphorescent emitters in the common device structure shows
excellent device performance with hext of 9.2% for blue, 15.1% for
green, 13.7% for red and 17.5% for yellow. A dual complemen-
tary emission based all-phosphor WOLED, hosted by CmInF,
was achieved with a peak hext of 13.4%, a current efficiency (hc)
of 23.4 cd Aꢂ1 and a power efficiency (hp) of 11.2 lm Wꢂ1
.
Results and discussion
Synthesis and chemical characterization
Scheme 1 lists the structures of CmInF and various color-tunable
phosphorescent iridium(III) complexes used in this study. The
compound 4-bromo-2-nitrobiphenyl was obtained from the
Suzuki–Miyaura coupling of phenylboronic acid with
2,5-dibromonitrobenzene in the presence of catalytic Pd
(PPh3)4.20 It was then heated in triethylphosphine under the
Cadogan’s reductive cyclization conditions to afford 2-bromo-
carbazole.21 Ullmann condensation between 2-bromocarbazole
and iodobenzene gave 2-bromo-N-phenylcarbazole in good
yield,22 and the synthesis of the cyclometallating ligand was then
completed by Stille coupling of 2-bromo-N-phenylcarbazole with
2-(tributylstannyl)pyridine (Scheme 2).23
high hext
.
Carbazole-based host materials have been used extensively in
PHOLEDs due to their large ET and good hole-transporting
properties.16–18 Well-known carbazole derivatives are 4,40-N,N0-
dicarbazolebiphenyl (CBP; ET ¼ 2.56 eV)4 and N,N0-dicarba-
zolyl-3,5-benzene (mCP; ET ¼ 2.9 eV),5,19 but both of them suffer
from low thermal and morphological stability (Tg z 65 ꢀC),
which are the essential properties for high performance OLED
fabrication. To address these issues, many carbazole-based host
materials with high ET have been reported for blue PHOLEDs.
Therefore, there is still a great demand for new universal host
materials which can produce multi-color PHOLEDs including
WOLEDs with proper doping concentrations of the emitters.14,15
In this paper, we report an isomeric family of functional carba-
zole-derived organometallic phosphors and a carbazole-based
coplanar host molecule (CmInF, see Scheme 1) obtained through
Following the ligand synthesis, 2-[2-(N-phenylcarbazolyl)]
pyridine was easily converted to the new homoleptic iridium(III)
complex Ir(2-Cz)3 via a one-step cyclometallation with [Ir(acac)3]
(Hacac ¼ acetylacetone) in refluxing glycerol at high temperature
(>200 ꢀC).24 Here, the reaction was run with an excess of ligand,
which can be easily recovered and reused. On the other hand, the
iridium-m-chloro-bridged dimer was synthesized by the reaction
of IrCl3$xH2O with 2-[2-(N-phenylcarbazolyl)]pyridine accord-
ing to a conventional procedure.25 The diiridium(III) complex was
then converted to the mononuclear iridium(III) complex (2-Cz)2Ir
(acac) by replacing the two bridging chlorides with a bidentate
monoanionic acac ligand. The synthetic pathways of the metal
complexes are summarized in Scheme 3. Spectroscopic and
216 | J. Mater. Chem., 2012, 22, 215–224
This journal is ª The Royal Society of Chemistry 2012

View Article Online
Scheme 1 Chemical structure of CmInF and various color-tunable phosphorescent iridium(III) complexes used in this study.
Scheme 2 Synthesis of 2-[2-(N-phenylcarbazolyl)]pyridine.
Scheme 3 Synthesis of new phosphorescent complexes Ir(2-Cz)3 and (2-Cz)2Ir(acac).
This journal is ª The Royal Society of Chemistry 2012
J. Mater. Chem., 2012, 22, 215–224 | 217

View Article Online
elemental analyses of both complexes are consistent with the
expected formulations of their structures. The facial geometry of
1
the homoleptic complex is confirmed by the simplicity of H
analogy with previously reported IrIII complexes.29 The weakest
MLCT absorptions are red-shifted from the homoleptic to the
corresponding heteroleptic complexes owing to the strong ligand
field of acac. By comparing each of the isomeric complexes,
different absorption profiles were also observed.
NMR spectral pattern, which indicates that the number of
coupled spins is equal to that of the protons on one ligand
because the three ligands are magnetically equivalent due to the
3-fold symmetry of the molecule.26 On the other hand, the
preparation of Ir(3-Cz)3 (a geometrical linkage isomer of
Ir(2-Cz)3) was reported previously.27 The homoleptic series
appears to show higher thermal stability than their heteroleptic
analogue as revealed from the onset decomposition temperature
(Tdec). Moreover, improvement in the thermostability was noted
as we change the cyclometallating ligand from 2-[2-(N-phenyl-
carbazolyl)]pyridine to 2-[3-(N-phenylcarbazolyl)]pyridine in
these complexes.
All of the iridium(III) complexes emit strong phosphorescence
arising from the lowest energy 3MLCT transition at room
temperature in CH2Cl2. The substitution position of the pyridyl
ring on N-phenylcarbazole has a great influence on the photo-
luminescence (PL) spectra of the heavy-metal complexes.
Clearly, the PL maximum of the 2-substituted homoleptic
derivative is more red-shifted relative to the 3-substituted coun-
terpart (from 515 nm for Ir(3-Cz)3 to 578 nm for Ir(2-Cz)3) due to
the more conjugated structure of the former group over the latter
one (Fig. 1). This is also the case for the heteroleptic species,
(3-Cz)2Ir(acac) versus (2-Cz)2Ir(acac) (from 515 to 591 nm).
Here, a prominent color-tuning of phosphorescence emission
from green to orange/red is achieved by simply altering the
ligation of metal with carbon atom at 2- or 3-position of
carbazole unit in these linkage isomers. While the carbon atoms
at 2-/7- and 3-/6-position of carbazole have different electronic
density, where the 3-/6-position can be activated by the nitrogen
atom and hence more electron-rich than the 2-/7-site, the energy
level of carbazole-containing complexes can be changed by
substitution at the 3-/6- or 2-/7-position. The more electron-
donating 3-position on carbon of carbazole pushes up the energy
of metal d orbital when it is ligated to the metal ion, giving rise to
a higher energy level of the highest occupied molecular orbital
(HOMO) than that of the less electron-donating 2-position and
this is accompanied by significant bathochromic shifts in lem for
these iridium(III) complexes.30 This approach is different from
those commonly used for other complexes in which the emission
wavelength can be tuned by varying the electronic nature and/or
position of the substituents on the cyclometallated ligands.31
The HOMO and the lowest unoccupied molecular orbital
(LUMO) energy levels of Ir(2-Cz)3 and (2-Cz)2Ir(acac) were
determined electrochemically (HOMO/LUMO: ꢂ4.81/–2.31 eV
for Ir(2-Cz)3 and ꢂ4.83/–2.43 eV for (2-Cz)2Ir(acac)). As
compared to [Ir(PPy)3] and (PPy)2Ir(acac) (PPy ¼ 2-phenyl-
pyridyl), the HOMOs of Ir(2-Cz)3 and (2-Cz)2Ir(acac) are
elevated when the electron-rich and electron-donating carbazolyl
units are attached to the pyridyl rings, signaling that both
complexes have a lower ionization potential than their PPy
congeners. This leads to a better hole injection/hole transport
ability and hence these triplet emitters are potentially dual
functional in nature.
The synthesis of the host CmInF was carried out following our
previously reported literature.28 From the X-ray crystallographic
analysis,28 we observed the coplanar structure of CmInF.
Differential scanning calorimetry (DSC) and thermogravimetric
analysis (TGA) were used to characterize the morphological
property and thermal stability of CmInF. The results show a high
ꢀ
onset decomposition temperature (Td ¼ 392 C, corresponding
to 5% weight loss) in the TGA curꢀve and a distinctly high glass
transition temperature (Tg) at 195 C and melting point (Tm) at
ꢀ
346 C from the DSC study. Therefore, this material can form
homogeneous and stable amorphous films, which are the two
critical factors for its successful use in thin film devices. The
amorphous behavior and high Tg value of CmInF reveal that the
rigidity of the conjugated backbone and the presence of the
peripheral p-tolyl substituents effectively suppress intermolecular
interaction.
Photophysical and charge transport properties
In this study, we have prepared two new cyclometallated iridium
(III) complexes containing 2-[2-(N-phenylcarbazolyl)]pyridine
and their photophysical properties were compared to their
isomeric congeners derived from 2-[3-(N-phenylcarbazolyl)]
pyridine (Table 1). The strongest absorption bands in the ultra-
1
violet region are assigned to the spin-allowed intraligand p–p*
transitions. The next lower energy in the shoulder region of the
1p–p* transitions and the weaker broad shoulders extending into
the visible region with appreciable intensity are associated with
an admixture of the typical spin-allowed metal-to-ligand charge
transfer 1MLCT, spin–orbit coupling enhanced 3p–p* and spin-
3
forbidden MLCT transitions. These assignments are made by
Table 1 Photophysical and thermal data of Ir(2-Cz)3, (2-Cz)2Ir(acac), Ir(3-Cz)3 and (3-Cz)2Ir(acac)
Emission (293 K)
Absorption (293 K)
labsa/nm
b
d
lema/nm
FP
sPc/ms
Td /ꢀC
Ir(2-Cz)3
(2-Cz)2Ir(acac)
Ir(3-Cz)3
(3-Cz)2Ir(acac)27b
266, 324, 359sh, 432
287, 341, 510sh
291sh, 316, 398sh
301, 325, 358sh, 435sh
578
591
515
515
0.32
0.29
0.43
0.41
0.18
0.21
0.46
0.42
458
388
477
404
27a
a
b
c
Measured in CH2Cl2. Phosphorescent quantum yield measured in degassed CH2Cl2 relative to fac-[Ir(PPy)3] (FP ¼ 0.40), lex ¼ 400 nm. Triplet
emission lifetime measured in degassed CH2Cl2. d Onset decomposition temperature at 5% weight reduction. sh ¼ shoulder peak.
218 | J. Mater. Chem., 2012, 22, 215–224
This journal is ª The Royal Society of Chemistry 2012

View Article Online
Fig. 3 Cyclic voltammogram of CmInF.
Fig. 1 Absorption and PL spectra of the iridium(III) complexes with 2-
[2-(N-phenylcarbazolyl)]pyridine and 2-[3-(N-phenylcarbazolyl)]pyridine
groups.
multiple cycles of CV scanning. It was previously reported that
without blocking the active sites C3 and C6 of carbazole, the
oxidation process is not reversible.35 The observed reversible
oxidation potential of CmInF indicates that carbazole is
p-conjugated with fluorene at 3 and 6 positions. Thus, the rigid
peripheral fluorene moieties conjugated to the carbazole core not
only emerge with a significant impact on the oxidation potential
of the whole molecule, but their addition also efficiently blocks
the electrochemically active sites (C3 and C6) of carbazole and
may improve its electrochemical stability in the EL devices.
Moreover, the existence of stable radical cationic species suggests
that it can be useful for potential hole-transporting material. The
HOMO energy level (ꢂ5.4 eV) of CmInF was estimated by
atmospheric photoelectron spectroscopy. From the equation
LUMO ¼ HOMO + DEg, where DEg (3.1 eV) is the optical band
gap determined from the absorption threshold, we estimated the
LUMO energy level to be ꢂ2.3 eV.
Fig. 2 Absorption and PL spectra of CmInF in solution and neat film, as
well as the corresponding phosphorescence (Phos) spectrum recorded
from its EtOH solution at 77 K.
To evaluate the carrier mobility of CmInF, we carried out time-
of-flight (TOF) measurements using the device structure of
Fig. 2 presents the UV-Vis absorption and PL spectra of
CmInF in dilute solution (CH2Cl2) and neat film, as well as the
corresponding phosphorescence (Phos) spectrum recorded from
its EtOH solution at 77 K. The absorption spectra of CmInF in
solution and neat film are very similar with peaks appearing at
328, 366 and 385 nm, which imply that no significant intermo-
lecular interactions occur in the ground state. The absorption
peaks at 366 and 385 nm are attributed to the p–p* transition of
the extended conjugation of coplanar carbazole–fluorene chro-
mophore of CmInF. Its emission spectra in solution and neat film
are also similar with peaks appearing at 394 nm and 415 nm. The
phosphorescence spectrum of CmInF in EtOH at 77 K indicates
the high-energy phosphorescence emission at 448 nm, corre-
sponding to the ET value of 2.77 eV. The decrease in triplet
energy agrees with the extended p-conjugation at C3 and C6
positions on the central carbazole core and reveals that CmInF
has a lower ET value as compared to CzSi (ET ¼ 3.02 eV),32 but
a higher value than those in the earlier reported host materials
DFC (ET ¼ 2.53 eV)33 and CPhBzIm (ET ¼ 2.48 eV).34 This ET
value is sufficiently high to prevent the reverse triplet energy
transfer from the dopant back to CmInF. Thus, CmInF should be
a suitable host for various phosphorescent devices from blue to
red.
We also studied the electrochemical properties of CmInF using
cyclic voltammetry (CV) and its cyclic voltammogram is shown
in Fig. 3. CmInF exhibits one reversible oxidation process at
1.09 eV (vs. Ag/AgCl). No reduction peak was observed. It is
worth mentioning that no electropolymerization occurred during
Fig. 4 (a) Representative TOF transient for CmInF (hole, 3 mm thick-
ness, E ¼ 2.3 ꢁ 105 V cmꢂ1). Inset: double logarithmic plot. (b) Hole
mobility vs. E1/2
.
This journal is ª The Royal Society of Chemistry 2012
J. Mater. Chem., 2012, 22, 215–224 | 219

View Article Online
ITO/CmInF (3 mm)/Ag (100 nm). Fig. 4(a) shows a representative
TOF transient for holes which reveals the nondispersive hole-
transport characteristics and this implies that the carriers move
with constant velocity across the organic film. To determine the
carrier mobilities, the transit time, tT, can be clearly evaluated
from the intersection point of two asymptotes in the double log-
arithmic representation of the TOF transient as shown in the inset
of Fig. 4(a). The hole mobilities of CmInF thus determined as
a function of the electric field are shown in Fig. 4(b) and the hole
mobilities are in the range from 2 ꢁ 10ꢂ3 to 5 ꢁ 10ꢂ3 cm2 Vꢂ1 sꢂ1 for
fields varying from 105 to 3.3 ꢁ 105 V cmꢂ1. It is worth noting that
a hole mobility of up to 5 ꢁ 10ꢂ3 cm2 Vꢂ1 sꢂ1 is very high among the
carbazole-based oligomers reported in the literature.
transfer from the host exciton to the phosphor upon electrical
excitation (see Fig. 5).
Fig. 6 shows the current density–voltage–luminance (J–V–L)
characteristics, and external quantum and power efficiencies as
a function of brightness for each of the PHOLED devices. The
pertinent EL properties analyzed from these graphs are
summarized in Table 2. All of the devices in this study exhibited
low turn-on voltages (3–3.5 V) and the difference in the J–V
performance is presumably because of the different emitting
dopants that influence the charge carrier transport of EML. We
initially examined the performance of the FIrpic-based blue
device with Commission Internationale de L’Eclairage (CIE)
coordinates of (0.17, 0.37). The hext can reach 9.2% photons/
electron with the hc and hP values of 20.2 cd Aꢂ1 and 11.8 lm Wꢂ1
.
These values are higher than those obtained using the same
triplet emitter in CBP (6.1%, 7.7 lm Wꢂ1) and mCP (7.5%, 8.9 lm
Wꢂ1) hosts.5 The values are moderate among FIrpic-based
PHOLEDs, presumably because the ET of CmInF is not high
enough relative to FIrpic. However, this is a worthy sacrifice to
acquire a compromise among the ET of RGB phosphors.
Next, we employed CmInF as a green host for (PPy)2Ir(acac),
Ir(3-Cz)3 and Ir(TPA)3 dopants with the same device
Electrophosphorescent properties
To evaluate the performance of CmInF as a universal host
material for multi-color emission, several PHOLED devices have
been fabricated with the same device configuration of ITO/
ꢀ
PEDOT:PSS (300 A)/TCTA (250 A)/CmInF doped with various
ꢀ
ꢀ
ꢀ
ꢀ
phosphor dyes (250 A)/TAZ (500 A)/LiF (5 A)/Al. The con-
ducting polymer poly(ethylene dioxythiophene):polystyrene
sulfonate (PEDOT:PSS) was used as the hole-injection layer,36
4,40,400-tri(N-carbazolyl)triphenylamine (TCTA) as the hole-
transport layer as well as an exciton blocker (ET: 2.76 eV,
HOMO/LUMO: ꢂ5.7/–2.4 eV) to confine triplet excitons within
the emitting layer (EML).37,38 The 1,2,4-triazole-based material,
3-(4-biphenylyl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole
(TAZ) (ET: 2.7 eV, HOMO/LUMO: ꢂ6.3/–2.7 eV), was used as
the electron-transport and hole-blocking layer to provide exciton
and carrier confinement.39 The EML consists of the large-gap
CmInF host doped with the blue iridium(III) bis[(4,6-di-fluo-
rophenyl)-pyridinato-N,C2 ]picolinate FIrpic,4 green bis(2-phe-
0
nylpyridine)iridium(III) acetylacetonate (PPy)2Ir(acac),40 red
(Mpq)2Ir(acac),41 and a series of our tailor-made dopants
(Ir(3-Cz)3, Ir(TPA)3,42 Ir(2-Cz)3 and (2-Cz)2Ir(acac), see Scheme
1) emitting from green to red. The green dopant (3-Cz)2Ir(acac)
was not tested for PHOLED here since it has very similar
emission features as Ir(3-Cz)3. The ET estimated from the highest
energy peaks of the phosphorescence spectra for light from blue
to red colors are 2.65–2.00 eV, which are lower than CmInF
(ET ¼ 2.77 eV) and can be confined in EML. There are no
residual emissions from the host and/or adjacent layers, even at
high drive currents, indicating that the EL emission originates
from the dopant and there is complete energy and/or charge
Fig. 6 (a) J–V–L characteristics, (b) external quantum efficiency and (c)
power efficiency versus brightness for PHOLED devices with different
dopants.
Fig. 5 EL spectra of devices incorporating the host CmInF doped with
various dopants.
220 | J. Mater. Chem., 2012, 22, 215–224
This journal is ª The Royal Society of Chemistry 2012

View Article Online
Table 2 Device characteristics of PHOLEDs with different phosphorescent dopants
Voltage and hext
Imax/mA
cmꢂ2
hc/cd
Aꢂ1
hp/lm
Wꢂ1
Dopant (%)
Vona/V
at 1000 nitb/V, %
Lmax/cd mꢂ2
hext (%)
CIE (x, y)
(PPy)2Ir(acac), 9%
FIrpic, 12%
(Mpq)2Ir(acac), 9%
Ir(3-Cz)3, 9%
Ir(2-Cz)3, 9%
Ir(TPA)3, 8%
(2-Cz)2Ir(acac), 8%
FIrpic, 12% + (Mpq)2Ir(acac) 0.3%
3
3
3
3.5
3
3.5
3
3
5.5, 14.5
8.3, 6
8.3, 10
122 000 (13 V)
19 800 (14.5 V)
21 300 (14.5 V)
130 000 (15 V)
80 200 (14.5 V)
95 000 (16 V)
51 000 (15 V)
15 900 (14.5 V)
700
704
820
910
880
860
1240
570
15.1
9.2
13.3
12.4
17.5
12
55.6
20.2
14.1
43.6
44
45
25.6
23.4
42.2
11.8
9.7
27
29
24
0.34, 0.62
0.17, 0.37
0.65, 0.35
0.30, 0.62
0.56, 0.44
0.38, 0.60
0.60, 0.39
0.33, 0.37
7, 11.8
7.5, 16.2
7.45, 11.1
6.5, 10.8
9, 7.7
13.7
13.4
19
11.2
a
b
Turn-on voltage at which emission became detectable. The values of driving voltage and hext of device at 1000 cd mꢂ2
.
configuration. The (PPy)2Ir(acac)-based device exhibited
a maximum brightness (Lmax) of 122 000 cd mꢂ2 at 13 V and
attractive EL efficiencies (15.1%, 55.6 cd Aꢂ1, 42.2 lm Wꢂ1) that
are superior to those of the Ir(3-Cz)3-based device (12.4%, 43.6
based device exhibited a Lmax of 21 300 cd mꢂ2 at 14.5 V and very
good EL efficiencies (13.3%, 14.1 cd Aꢂ1, 9.7 lm Wꢂ1) with the
CIE coordinates of (0.65, 0.35).
Through the mixing of two dopants showing complementary
colors,46 we also fabricated a two-color white OLED (WOLED)
under the same device structure consisting of 12 wt% FIrpic and
0.3 wt% (Mpg)2Ir(acac) co-doped into CmInF as a single-emit-
ting-layer. The WOLED exhibited a Lmax of 15 900 cd mꢂ2 at
14.5 V, with maximum efficiencies (hext, hc, and hp) of 13.4%,
23.4 cd Aꢂ1, and 11.2 lm Wꢂ1 (Fig. 7). Although a gradual
decrease in efficiency with an increase of current density was
observed, the EL efficiency of WOLED remained at 7.7% at the
practical brightness of 1000 cd mꢂ2. The inset of Fig. 7 displayed
the EL spectra of the WOLED device recorded at various
brightness levels. The relative intensity of the red emission
decreased slightly when the brightness increased, leading to
a slight shift in the CIE coordinates from (0.36, 0.37) at a value of
L ¼ 1000 cd mꢂ2 to (0.33, 0.37) at 11 300 cd mꢂ2. This color shift
originated from the shifting of the recombination zone upon
increasing the applied voltage and from the easier formation of
high-energy excitons at higher voltage.47,48 Although the
performance of these devices has yet to be optimized, it is our
intention to highlight the potential merits of CmInF as
a universal host towards color-tunable OLED applications.
cd Aꢂ1, 27 lm Wꢂ1) and Ir(TPA)3-based device (12%, 45 cd Aꢂ1
,
24 lm Wꢂ1). It is worth noting that these green-emitting devices
show a low roll-off in hext (Fig. 6(b)). When the brightness rea-
ches 1000 cd mꢂ2, hext still remains as high as 14.5% with a low
efficiency roll-off value of 4% for the (PPy)2Ir(acac)-based
device. The Ir(3-Cz)3- and Ir(TPA)3-based devices also exhibited
low efficiency roll-off values of only 4.8% and 7.5%, respectively,
at a practical brightness of 1000 cd mꢂ2. To further evaluate the
suitability of CmInF as the host material for low-energy triplet
emitters, we fabricated PHOLED devices by using Ir(2-Cz)3
(orange), (2-Cz)2Ir(acac) (red) and (Mpq)2Ir(acac) (red) as
phosphorescent dopants. The Ir(2-Cz)3-based device showed an
impressive performance with a low turn-on voltage of 3 V, Lmax
of 80 200 cd mꢂ2 at 14.5 V, and peak EL efficiencies of 17.5%, 44
cd Aꢂ1, and 29 lm Wꢂ1 with the CIE coordinates at (0.56, 0.44). It
is also worth mentioning that the Ir(2-Cz)3-based device gave
better performance than other recently reported PHOLEDs in
the orange chromaticity region.43–45 For the red emitters, the (2-
Cz)2Ir(acac)-based device exhibited a Lmax of 51 000 cd mꢂ2 at 15
V and respectable EL efficiencies (13.7%, 25.6 cd Aꢂ1, 19 lm Wꢂ1
)
with the CIE coordinates of (0.60, 0.39) while (Mpq)2Ir(acac)-
Conclusions
In summary, we have investigated the effect of the position of
pyridyl ring substituent at C2 and C3 sites of the carbazole ring
on the emission color, photophysics and electrophosphorescent
properties of some iridium(III) cyclometallates bearing N-phe-
nylcarbazole moieties. An interesting color-tuning strategy was
developed by a change of the chelating carbon atom by
a different binding site of the carbazolyl group. The origin of the
wavelength shift arises from the striking difference of electron
density on the carbon atom bound to the iridium centre. We also
reported nicely a prominent multi-color spectrum employing
CmInF as a universal host material doped with various phosphor
dyes with the same device configuration to reach the ultimate
target for highly efficient, color-switchable OLEDs. The present
work furnished OLED colors spanning from sky-blue to red
(475–620 nm) with high EL efficiencies (9.2–17.5%) which have
great potential for application in multi-color displays and
possibly in white light illumination sources through the mixing of
two phosphorescent dopants of complementary colors.
Fig. 7 EL efficiencies for two-color WOLED. Inset: EL spectra of
WOLED device at various brightness levels.
This journal is ª The Royal Society of Chemistry 2012
J. Mater. Chem., 2012, 22, 215–224 | 221

View Article Online
polarity of the applied bias, the selected carriers (holes or elec-
trons) were swept across the sample with a transit time of tT. For
an applied bias V and a sample thickness D, the applied electric
field E is equal to V/D; the carrier mobility is then given by the
equation m ¼ D/(tT E) ¼ D2/(VtT) from which the carrier transit
times, tT, can be extracted from the intersection points of the two
asymptotes to the plateau and tail sections in the double loga-
rithmic plot.
Experimental
Photophysical and thermal measurements
All reactions were carried out under nitrogen atmosphere with
the use of standard Schlenk techniques. Glassware was oven-
dried at about 120 ꢀC. Analytical grade solvents were purified by
distillation over appropriate drying agents under an inert
nitrogen atmosphere prior to use. All reagents and chemicals,
unless otherwise stated, were purchased from commercial sour-
ces and used without further purification. The positive-ion fast
atom bombardment (FAB) mass spectra were recorded in m-
nitrobenzyl alcohol matrices on a Finngin-MAT SSQ710 mass
spectrometer. NMR spectra were measured in deuterated
solvents as the lock and reference on a JEOL JNM-EX270 FT
NMR system or a Varian INOVA 400 MHz FT-NMR spec-
OLED device fabrication
All chemicals were purified through vacuum sublimation prior to
use. The OLEDs were fabricated through vacuum deposition of
the materials at 10ꢂ6 Torr onto ITO-coated glass substrates
having a sheet resistance of 15 U sqꢂ1. The ITO surface was
cleaned ultrasonically—sequentially with acetone, methanol, and
deionized water—and then it was treated with UV-ozone. The
1
trometer, with H and 13C NMR chemical shifts quoted relative
to Me4Si standard. Steady-state spectroscopic measurements
were conducted both in solution and solid films prepared by
vacuum (2 ꢁ 10ꢂ6 Torr) deposition on a quartz plate. Absorption
spectra were recorded with a U2800A spectrophotometer
(Hitachi). Fluorescence spectra at 300 K and phosphorescent
spectra at 77 K were measured on a Hitachi F-4500 spectro-
photometer upon excitation at the absorption maxima. The
differential scanning calorimetry (DSC) analyses were performed
on a Universal V2,6D TA Instrument at a heating rate of 10 ꢀC
minꢂ1 from 50 to 400 ꢀC under a nitrogen atmosphere. Ther-
mogravimetric analysis (TGA) was undertaken with a Universal
V2,6D TA Instrument or a Perkin-Elmer TGA6 thermal
analyzer. The thermal stability of the samples under a nitrogen
atmosphere was determined by measuring their 5% weight ꢀloss
ꢂ1
ꢀ
deposition rate of each organic material was ca. 1–2 A s
.
ꢂ1
ꢀ
Subsequently, LiF was deposited at 0.1 A s and then capped
with Al (ca. 5 A sꢂ1) through shadow masking without breaking
ꢀ
the vacuum. The J–V–L characteristics of the devices were
measured simultaneously in a glove-box using a Keithley 6430
source meter and a Keithley 6487 picoammeter equipped with
a calibration Si-photodiode. EL spectra were measured using
a photodiode array (OTO SD1200).
Synthesis of 4-bromo-2-nitrobiphenyl
Phenylboronic acid (1.50 g, 12.3 mmol), 2,5-dibromoni-
trobenzene (3.45 g, 12.3 mmol) and Pd(PPh3)4 (1.42 g, 1.42
mmol) were dissolved in a mixture of toluene (250 mL) and 2 M
aq. Na2CO3 (12 mL). The degassed mixture was heated at 90 ꢀC
while heating proceeded at a rate of 10 C minꢂ1 up to 700 C.
ꢀ
ꢀ
for 5.5 h, cooled to 20 C and then diluted with distilled H2O.
Cyclic voltammetry
The organic phase was separated and the organic products were
extracted into diethyl ether. The combined extracts were dried
with MgSO4 and concentrated under reduced pressure to give
a dark brown liquid. The main product was separated by chro-
matography eluting with a mixture of hexane and CH2Cl2 and
characterized by fast-atom bombardment (FAB) mass spec-
trometry. The mixture was used in the next step without further
The oxidation potential was determined by cyclic voltammetry
(CV) in CH2Cl2 solution (1.0 mM) containing 0.1 M tetra-n-
butylammonium hexafluorophosphate (TBAPF6) as a support-
ing electrolyte at a scan rate of 100 mV sꢂ1. A glassy carbon
electrode and a platinum wire were used as the working and
counter electrodes, respectively. The ferrocene/ferrocenium
(Fc/Fc+) redox couple in THF/TBAP occurs at E1/2 ¼ +0.51 V.
The potential value was recorded relative to the oxidation
potential of ferrocene, which was added to the electrolyte as an
internal standard. All potentials were recorded versus Ag/AgCl
(saturated) as a reference electrode.
purification (yield: 70%). MS (FAB): m/z 278 (M+). H NMR
1
(CDCl3): d [ppm] 8.00 (d, J ¼ 2.0 Hz, 1H, Ar), 7.75 (dd, J ¼ 8.2
and 2.0 Hz, 1H, Ar), 7.45–7.41 (m, 3H, Ar), 7.33 (d, J ¼ 8.2 Hz,
1H, Ar), 7.31–7.27 (m, 2H, Ar). 13C NMR (CDCl3): d [ppm]
136.5, 135.6, 135.5, 133.5, 131.8, 129.5, 129.1, 128.8, 128.0, 127.3,
124.2, 121.6 (Ar).
Time-of-flight (TOF) mobility measurements
2-Bromocarbazole
The samples for the TOF measurements were prepared through
vacuum deposition in the configuration of ITO/CmInF (3 mm)/
Ag (100 nm) and then placed inside a cryostat and kept under
vacuum. The thickness of the organic film was monitored in situ
using a quartz crystal sensor and calibrated using a thin film
thickness measurement system (K-MAC ST2000). A nitrogen
laser was used as the excitation source (l ¼ 337 nm) and was
incident on the sample through the ITO. Under an applied dc
bias, the transient photocurrent was swept across the bulk of the
organic film towards the collection electrode (Al); it was then
recorded using a digital storage oscilloscope. Depending on the
4-Bromo-2-nitrobiphenyl (1.50 g, 5.42 mmol) and P(OEt)3 (4.80
g, 37.9 mmol) were heated under reflux for 10 h. The excess P
(OEt)3 was distilled off under vacuum. The remaining residue
was diluted with a 1 : 1 mixture of MeOH and distilled H2O to
give a precipitate which was filtered off and washed several times
with a 1 : 1 mixture of MeOH/H2O, followed by hexane. Column
chromatography using hexane as eluent gave the title compound
as a white solid (500 mg, 38%). MS (FAB): m/z 246 (M+). H
1
NMR (CDCl3): d [ppm] 11.40 (s, 1H, NH), 8.15–8.11 (dd, J ¼
8.0 Hz, 2H, Ar), 7.68 (d, J ¼ 1.7 Hz, 1H, Ar), 7.54–7.19 (m, 4H,
222 | J. Mater. Chem., 2012, 22, 215–224
This journal is ª The Royal Society of Chemistry 2012

View Article Online
Ar). 13C NMR (CDCl3): d [ppm] 140.6, 139.9, 126.1, 121.9, 121.8,
(2-Cz)2Ir(acac). This heteroleptic iridium(III) complex was
synthesized by reacting IrCl3$3H2O (151 mg, 0.51 mmol) with 3
equivalents of 2-[2-(N-phenylcarbazolyl)]pyridine (486 mg, 1.52
mmol) in a mixture of 2-ethoxyethanol and water (6 : 2, v/v). The
mixture was refluxed for 24 h and then cooled to room temper-
ature. A precipitate gradually formed during the reaction. The
precipitate was collected by filtration and washed with ethanol
followed by hexane. The solid was then pumped dry completely
to give the crude iridium(III) dimer for subsequent reaction.
Without further purification, this crude dimer was mixed with 10
equivalents of Na2CO3 (196 mg, 0.185 mmol) in 2-ethoxyethanol
(8 mL) and 3 equivalents of acetylacetone (0.056 mL, 0.554
mmol) were added. The reaction mixture was then refluxed for 16
h. After the mixture was cooled to room temperature, water was
added and extracted with CH2Cl2. The organic layer was
collected and the solvent was subsequently removed. The residue
was then purified using preparative TLC plates eluting with
a mixture of CH2Cl2/hexane to give a reddish orange solid in 41%
yield. MS (FAB): m/z 930 (M+). 1H NMR (CDCl3): d [ppm] 8.67
(d, J ¼ 4.0 Hz, 2H, Ar), 7.93–7.91 (m, 2H, Ar), 7.79–7.54 (m,
15H, Ar), 7.25 (m, 8H, Ar), 7.02–6.93 (m, 3H, Ar), 5.26 (s, 1H,
acac), 1.83 (s, 6H, CH3). 13C NMR (CDCl3): d [ppm] 184.55
(CO), 168.96, 148.36, 142.67, 141.33, 138.21, 137.68, 136.50,
134.94, 129.70, 127.05, 126.78, 125.63, 123.05, 122.57, 121.41,
120.45, 118.92, 118.62, 109.73, 109.02, 105.88 (Ar), 100.49 (CH),
28.85 (CH3). Anal. Calcd for C51H37N4O2Ir: C, 65.86; H, 4.01;
N, 6.02; found: C, 66.02; H, 3.92; N, 6.20%.
121.6, 121.3, 120.3, 119.1, 118.2, 113.5, 111.2 (Ar).
2-Bromo-N-phenylcarbazole
The N-arylation of 2-bromocarbazole (2.50 g, 10.2 mmol) was
carried out by the Ullmann condensation of 2-bromocarbazole
with iodobenzene (3.12 g, 15.3 mmol) in the presence of CuI
(1.35 g, 7.14 mmol), KOH (4.00 g, 71.4 mmol) and 1,10-phe-
nanthroline (1.42 g, 7.14 mmol) in p-xylene (50 mL) and the
ꢀ
mixture was stirred at 140 C for two days. The cooled mixture
was extracted with CH2Cl2. The organic layer obtained was
washed with water and dried over anhydrous MgSO4 and
evaporated. The product was purified by flash chromatography
on silica gel using hexane as eluent to give a pure white solid
1
(90%, 2.95 g). MS (FAB): m/z 322 (M+). H NMR (CDCl3):
d [ppm] 8.10–8.08 (d, 1H, J ¼ 7.6 Hz, Ar), 7.98–7.96 (d, 1H,
J ¼ 8.4 Hz, Ar), 7.63–7.59 (m, 2H, Ar), 7.53–7.48 (m, 4H, Ar),
7.42ꢂ7.35 (m, 3H, Ar), 7.30–7.27 (m, 1H, Ar). 13C NMR
(CDCl3): d [ppm] 141.68, 141.08, 137.03, 130.05, 127.91, 127.11,
126.34, 123.05, 122.71, 122.25, 121.44, 120.36, 120.26, 119.50,
112.78, 109.95 (Ar).
Synthesis of 2-[2-(N-phenylcarbazolyl)]pyridine
2-Bromo-N-phenylcarbazole (1.43 g, 4.45 mmol) and 2-(tribu-
tylstannyl)pyridine (2.15 g, 6.68 mmol) were mixed in dry toluene
(50 mL) and Pd(PPh3)4 (0.51 g, 0.45 mmol) was then added to the
solution. The resulting mixture was stirred at 110 ꢀC for 2 days.
After cooling to room temperature, the reaction mixture was
poured into a separating funnel and CH2Cl2 was added followed
by washing with water. The organic phase was dried over
MgSO4. Solvent was then removed and the residue was purified
by column chromatography eluting with CH2Cl2/hexane (1 : 1,
v/v). The title product was obtained as a white solid (1.36 g, 95%).
MS (FAB): m/z 320 (M+). 1H NMR (CDCl3): d [ppm] 8.66–8.65
(d, J ¼ 4.4 Hz, 1H, Ar), 8.22–8.14 (m, 2H, Ar), 8.04 (s, 1H, Ar),
7.87 (m, 1H, Ar), 7.76 (m, 2H, Ar), 7.72 (m, 4H, Ar), 7.47 (m, 3H,
Ar), 7.30 (m, 1H, Ar), 7.19 (m, 1H, Ar). 13C NMR (CDCl3):
d [ppm] 158.09, 149.60, 141.74, 141.42, 137.50, 136.68, 129.97,
127.59, 127.34, 126.24, 123.96, 122.97, 121.83, 120.99, 120.52,
120.48, 120.06, 119.09, 109.87, 108.34 (Ar).
Acknowledgements
W.-Y. Wong acknowledges the financial support from Hong
Kong Baptist University (FRG2/09-10/091), Hong Kong
Research Grants Council (HKBU202709 and HKUST2/CRF/
10) and Areas of Excellence Scheme, University Grants
Committee of HKSAR, P.R. China (Project No. [AoE/P-03/08]).
W.-Y. Hung and K.-T. Wong are grateful for the financial
support from the National Science Council (NSC 98-2119-M-
002-007-MY3, 100-2112-M-019 -002 -MY3) of Taiwan.
References
1 C. W. Tang and S. A. Van Slyke, Appl. Phys. Lett., 1987, 51, 913.
2 M. A. Baldo, M. E. Thompson and S. R. Forrest, Nature, 2000, 403,
750.
Synthetic procedures of new iridium(III) compounds
3 M. A. Baldo, D. F. O’Brien, Y. You, A. Shoustikov, M. E. Thompson
and S. R. Forrest, Nature, 1998, 395, 151.
Ir(2-Cz)3. This homoleptic iridium(III) complex was synthe-
sized by heating a solution of [Ir(acac)3] (262 mg, 0.535 mmol)
and 3.5 molar equivalents of 2-[2-(N-phenylcarbazolyl)]pyridine
(600 mg, 1.87 mmol) in glycerol (15 mL) and the mixture was
heated to 200 ꢀC under a N2 atmosphere. After 18 h, the reaction
mixture was then cooled down to room temperature and water
(20 mL) was added. The mixture was then filtered followed by
repetitive washing with CH2Cl2. The reddish orange product was
obtained in 43% yield. Its UV and PL spectra have confirmed the
absence of any excess ligand after purification. 1H NMR (THF-
d8): d [ppm] 8.00–7.98 (d, J ¼ 8.28 Hz, 3H, Ar), 7.92 (m, 3H, Ar),
7.71–7.62 (m, 24H, Ar), 7.76 (m, 3H, Ar), 7.55–7.53 (m, 6H, Ar),
6.87–6.83 (m, 6H, Ar). MS (FAB): m/z 1150 (M+). Anal. Calcd
for C69H45N6Ir: C, 72.04; H, 3.94; N, 7.31; found: C, 72.12; H,
4.18; N, 7.45%.
4 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.
5 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.
6 S. Tokito, T. Iijima, Y. Suzuri, H. Kita, T. Tsuzuki and F. Sato, Appl.
Phys. Lett., 2003, 83, 569.
7 K. Goushi, R. Kwong, J. J. Brown, H. Sasabe and C. Adachi, J. Appl.
Phys., 2004, 95, 7798.
8 S.-J. Su, C. Cai and J. Kido, Chem. Mater., 2011, 23, 274.
9 Y. Tao, Q. Wang, C. Yang, C. Zhong, K. Zhang, J. Qin and D. Ma,
Adv. Funct. Mater., 2010, 20, 304.
10 Y. Tao, Q. Wang, L. Ao, C. Zhong, J. Qin, C. Yang and D. Ma, J.
Mater. Chem., 2010, 20, 1759.
11 H.-H. Chou and C.-H. Cheng, Adv. Mater., 2010, 22, 2468.
12 H.-H. Chou, H.-H. Shih and C.-H. Cheng, J. Mater. Chem., 2010, 20,
798.
13 P.-I. Shih, C.-H. Chien, F.-I. Wu and C.-F. Shu, Adv. Funct. Mater.,
2007, 17, 3514.
This journal is ª The Royal Society of Chemistry 2012
J. Mater. Chem., 2012, 22, 215–224 | 223

View Article Online
14 S. Gong, Y. Chen, C. Yang, C. Zhong, J. Qin and D. Ma, Adv.
Mater., 2010, 22, 5370.
15 S. Gong, Y. Chen, J. Luo, C. Yang, C. Zhong, J. Qin and D. Ma, Adv.
Funct. Mater., 2011, 21, 1168.
16 S. J. Lee, J. S. Park, K. J. Yoon, Y. I. Kim, S. H. Jin, S. K. Kang,
Y. S. Gal, S. Kang, J. Y. Lee, J. W. Kang, S. H. Lee, H. D. Park
and J. J. Kim, Adv. Funct. Mater., 2008, 18, 3922.
Chem. Rev., 2006, 250, 2093; (i) P. L. Burn, S.-C. Lo and
I. D. W. Samuel, Adv. Mater., 2007, 19, 1675; (j) Y. Chi and
P.-T. Chou, Chem. Soc. Rev., 2007, 36, 1421; (k) J. A. G. Williams,
S. Develay, D. L. Rochester and L. Murphy, Coord. Chem. Rev.,
2008, 252, 2596; (l) H. Wu, L. Ying, W. Yang and Y. Cao, Chem.
Soc. Rev., 2009, 38, 3391; (m) Y. Chi and P.-T. Chou, Chem. Soc.
Rev., 2010, 39, 638.
17 F. M. Hsu, C. H. Chen, P. I. Shih and C. F. Shu, Chem. Mater., 2009,
21, 1017.
18 T. J. Park, W. S. Jeon, J. J. Park, S. Y. Kim, Y. K. Lee, J. Jang and
J. H. Kwon, Thin Solid Films, 2008, 517, 896.
19 V. Adamovich, J. Brooks, A. Tamayo, A. M. Alexander,
P. I. Djurovich, B. W. D’Andrade, C. Adachi, S. R. Forrest and
M. E. Thompson, New J. Chem., 2002, 26, 1171.
20 M. Tavasli, S. Bettington, M. R. Bryce, A. S. Batsanov and
A. P. Monkman, Synthesis, 2005, 10, 1619.
21 (a) J. I. G. Cadogan and M. Cameron-Wood, J. Chem. Soc., 1962,
361; (b) J. I. G. Cadogan, M. Cameron-Wood, R. K. Mackie and
R. J. G. Searle, J. Chem. Soc., 1965, 4831; (c) J.-F. Morin and
M. Leclerc, Macromolecules, 2001, 34, 4680; (d) F. Dierschke,
30 (a) G. Zhou, W.-Y. Wong and X. Yang, Chem.–Asian J., 2011, 6,
1706; (b) C. Yang, X. Zhang, H. You, L. Zhu, L. Chen, L. Zhu,
Y. Tao, D. Ma, Z. Shuai and J. Qin, Adv. Funct. Mater., 2007, 17,
651; (c) K. Zhang, Z. Chen, C. Yang, X. Zhang, Y. Tao, L. Duan,
L. Chen, L. Zhu, J. Qin and Y. Cao, J. Mater. Chem., 2007, 17,
3451; (d) S. Bettington, M. Tavasli, M. R. Bryce, A. Beeby, H. Al-
Attar and A. P. Monkman, Chem.–Eur. J., 2006, 13, 1423.
31 W.-Y. Wong and C.-L. Ho, Coord. Chem. Rev., 2009, 253, 1709.
32 M.-H. Tsai, H.-W. Lin, H.-C. Su, T.-H. Ke, C. Wu, F.-C. Fang,
Y.-L. Liao, K.-T. Wong and C.-I. Wu, Adv. Mater., 2006, 18, 1216.
33 K.-T. Wong, Y.-M. Chen, Y.-T. Lin, H.-C. Su and C.-C. Wu, Org.
Lett., 2005, 7, 5361.
34 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.
€
A. C. Grimsdale and K. Mullen, Synthesis, 2003, 2470.
22 (a) M. Sonntag and P. Strohriegl, Chem. Mater., 2004, 16, 4736; (b)
F. Ullmann and M. Zlokasoff, Chem. Ber., 1905, 38, 2211.
23 J. K. Stille, Angew. Chem., Int. Ed. Engl., 1986, 25, 508.
24 K. Dedeian, P. I. Djurovich, F. O. Garces, G. Carlson and
R. J. Watts, Inorg. Chem., 1991, 30, 1685.
35 J. He, H. Liu, Y. Dai, X. Ou, J. Wang, S. Tao, X. Zhang, P. Wang and
D. Ma, J. Phys. Chem. C, 2009, 113, 6761.
36 A. Elscher, F. Bruder, H.-W. Heuer, F. Jonas, A. Karbach,
S. Kirchmeyer, S. Thurm and R. Wehrmann, Synth. Met., 2000,
111, 139.
25 S. Lamansky, P. Djurovich, D. Murphy, F. Abdel-Razzaq, H. F. Lee,
C. Adachi, P. E. Burrows, S. R. Forrest and M. E. Thompson, J. Am.
Chem. Soc., 2001, 123, 4304.
37 S.-J. Su, E. Gonmori, H. Sasabe and J. Kido, Adv. Mater., 2008, 20,
4189.
38 Y.-Y. Lyu, J. Kwak, W. S. Jeon, Y. Byun, H. S. Lee, D. Kim, C. Lee
and K. Char, Adv. Funct. Mater., 2009, 19, 420.
€
26 M. G. Colombo, T. C. Brunold, T. Riedener, H. U. Gudel,
€
€
M. Fortsch and H. Burgi, Inorg. Chem., 1994, 33, 545.
39 J. Kido, M. Kimura and K. Nagai, Science, 1995, 267, 1332.
40 C. Adachi, M. A. Baldo, M. E. Thompson and S. R. Forrest, J. Appl.
Phys., 2001, 90, 5048.
41 W.-Y. Hung, T.-C. Tsai, S.-Y. Su, K.-T. Wong and L.-C. Chi, Phys.
Chem. Chem. Phys., 2008, 10, 5822.
42 G.-J. Zhou, Q. Wang, C.-L. Ho, W.-Y. Wong, D. Ma, L. Wang and
Z. Lin, Chem.–Asian J., 2008, 3, 1830.
43 C.-H. Lin, Y. Chi, M.-W. Chung, Y.-J. Chen, K.-W. Wang,
G.-H. Lee, P.-T. Chou, W.-Y. Hung and H.-C. Chiu, Dalton
Trans., 2011, 40, 1132.
44 C.-L. Ho, W.-Y. Wong, Q. Wang, D. Ma, L. Wang and Z. Lin, Adv.
Funct. Mater., 2008, 18, 928.
45 W.-Y. Wong, G.-J. Zhou, X.-M. Yu, H.-S. Kwok and B. Tang, Adv.
Funct. Mater., 2006, 16, 838.
46 G.-J. Zhou, W.-Y. Wong and S. Suo, J. Photochem. Photobiol., C,
2010, 11, 133.
47 Y. Shao and Y. Yang, Appl. Phys. Lett., 2005, 86, 073510.
48 W.-Y. Hung, Z.-W. Chen, H.-W. You, F.-C. Fan, H.-F. Chen and
K.-T. Wong, Org. Electron., 2011, 12, 575.
27 (a) W.-Y. Wong, C.-L. Ho, Z.-Q. Gao, B.-X. Mi, C.-H. Chen,
K.-W. Cheah and Z. Lin, Angew. Chem., Int. Ed., 2006, 45, 7800;
(b) C.-L. Ho, Q. Wang, C.-S. Lam, W.-Y. Wong, D. Ma, L. Wang,
Z.-Q. Gao, C.-H. Chen, K.-W. Cheah and Z. Lin, Chem.–Asian J.,
2009, 4, 89.
28 K.-T. Wong, T.-C. Chao, L.-C. Chi, Y.-Y. Chu, A. Balaiah,
S.-F. Chiu, Y.-H. Liu and Y. Wang, Org. Lett., 2006, 8, 5033.
29 (a) H. Z. Xie, M. W. Liu, O. Y. Wang, X. H. Zhang, C. S. Lee,
L. S. Hung, S. T. Lee, P. F. Teng, H. L. Kwong, H. Zhen and
C. M. Che, Adv. Mater., 2001, 13, 1245; (b) Y. H. Song, S.-J. Yeh,
C.-T. Chen, Y. Chi, C.-S. Liu, J.-K. Yu, Y.-H. Hu, P.-T. Chou,
S.-M. Peng and G.-H. Lee, Adv. Funct. Mater., 2004, 14, 1221; (c)
W.-Y. Wong and C.-L. Ho, J. Mater. Chem., 2009, 19, 4457; (d)
H. Yersin, Top. Curr. Chem., 2004, 241, 1; (e) P.-T. Chou and
Y. Chi, Chem.–Eur. J., 2007, 13, 380; (f) B. M. W. Langeveld and
U. S. Schubert, Adv. Mater., 2005, 17, 1109; (g) M. A. Baldo,
M. E. Thompson and S. R. Forrest, Pure Appl. Chem., 1999, 71,
2095; (h) R. C. Evans, P. Douglas and C. J. Winscom, Coord.
224 | J. Mater. Chem., 2012, 22, 215–224
This journal is ª The Royal Society of Chemistry 2012