Substituent effects on the photophysical and electrochemical properties of iridium(III) complexes containing an arylcarbazolyl moiety

Article abstract of DOI:10.1016/j.dyepig.2014.04.039

A series of new heteroleptic cyclometalated iridium(III) complexes containing a 9-(4-(pyridin-2-yl)phenyl)-9H-carbazole molecular framework have been successfully synthesized and characterized. All of the iridium(III) complexes are thermally stable solids and highly efficient electrophosphors. Different substituents, electron-donating group (-CH₃) and electron-withdrawing groups (-F, -CF₃), were introduced into either the 4- or 5-position of the pyridyl ring in the ligands to verify their influence on the photophysical, electrochemical, photo- and electrophosphoresence properties of these iridium phosphors. Electrophosphorescent OLEDs with outstanding device performance can be fabricated based on the complex bearing a 5-CF₃ unit which exhibited a maximum luminance efficiency of approximately 43.2 cd A^-1, corresponding to an external quantum efficiency of approximately 12.6% and a power efficiency of 27.1 lm W^-1.

Full text of DOI:10.1016/j.dyepig.2014.04.039

Dyes and Pigments 109 (2014) 13e20
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Substituent effects on the photophysical and electrochemical
properties of iridium(III) complexes containing an arylcarbazolyl
moiety
*
Chun Liu , Xiaofeng Rao, Xin Lv, Jieshan Qiu, Zilin Jin
State Key Laboratory of Fine Chemicals, Dalian University of Technology, Linggong Road 2, Dalian 116024, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of new heteroleptic cyclometalated iridium(III) complexes containing a 9-(4-(pyridin-2-yl)
phenyl)-9H-carbazole molecular framework have been successfully synthesized and characterized. All of
the iridium(III) complexes are thermally stable solids and highly efficient electrophosphors. Different
substituents, electron-donating group (eCH3) and electron-withdrawing groups (eF, eCF3), were
introduced into either the 4- or 5-position of the pyridyl ring in the ligands to verify their influence on
the photophysical, electrochemical, photo- and electrophosphoresence properties of these iridium
phosphors. Electrophosphorescent OLEDs with outstanding device performance can be fabricated based
on the complex bearing a 5-CF3 unit which exhibited a maximum luminance efficiency of approximately
43.2 cd Aꢀ1, corresponding to an external quantum efficiency of approximately 12.6% and a power ef-
Received 11 December 2013
Received in revised form
18 April 2014
Accepted 25 April 2014
Available online 9 May 2014
Keywords:
Iridium complex
Carbazole
Phosphorescent material
OLEDs
ficiency of 27.1 lm Wꢀ1
.
Ó 2014 Elsevier Ltd. All rights reserved.
Substituent effect
Ligand-free Suzuki reaction
1. Introduction
even the electro-luminescence performance of the resultant
iridium complexes [13e16]. In other words, for both homoleptic
and heteroleptic iridium complexes, the chemical structure of the
cyclometalating ligand is usually the fundamental aspect to deter-
mine the emission energy and quantum efficiency of the iridium
complexes. In most cases, the cyclometalating ligands are involved
in the lowest-energy triplet excited state, either the ligand-
centered transition or the metal-to-ligand-charge-transfer (MLCT)
triplet state, which is responsible for the phosphorescence. At the
same time, the structure of the ligand and even the position of the
functional group on the ligand framework play important roles in
tuning these physical parameters and in the electroluminescent
(EL) behavior [17e21].
Up to now, among the green-emitting iridium complexes, Ir(p-
py)2(acac) (ppy ¼ 2-phenylpyridine) is one of the most important
examples [11,22,23]. To improve the charge injection and transport
in Ir(ppy)2(acac), the incorporation of the bulky and hole-
transporting carbazole group, as a core part in the ligand frame in
the iridium complexes, is anticipated to help to reduce the energy
barrier height for hole injection and to decrease the tripletetriplet
annihilation [24e28]. Recently, our group reported that a carbazole
group could be easily incorporated as a substituent in the phenyl
ring of the ppy-type ligand [29]. Since the carbazole group is a
strong electron-donating group, the incorporation modifies the
Organic light-emitting diodes (OLEDs) have attracted increasing
interest in both the scientific and industrial fields due to their ap-
plications in high-resolution, multicolor displays and solid-state
lighting [1e3]. In particular, phosphorescent metal complexes
have been the main focus because they can harvest both singlet and
triplet excitons when electrons and holes recombine, reaching up
to 100% internal quantum efficiency [4]. Generally, luminescent
complexes are based on heavy metals such as copper(I) [5], ruth-
enium(II) [6], osmium(II) [7], platinum(II) [8], or iridium(III) [9,10].
Among them, compounds based on iridium are regarded as the
most promising phosphors for organic light-emitting devices,
owing to their relatively short excited-state lifetime, high emission
quantum yield, high photo- and thermal stability, and flexible color
tuning through ligand-structure control [11,12]. To date, research
on iridium complexes has focused on the design of new cyclo-
metalating ligands either to tune the emission color of the resulting
complexes or to achieve high-efficiency emission for OLEDs. In
general, both the ligand framework and the substituents thereon
combine to determine the optical and electronic parameters and
* Corresponding author. Tel.: þ86 411 84986182.
E-mail address: chunliu70@yahoo.com (C. Liu).
http://dx.doi.org/10.1016/j.dyepig.2014.04.039
0143-7208/Ó 2014 Elsevier Ltd. All rights reserved.

14
C. Liu et al. / Dyes and Pigments 109 (2014) 13e20
HOMO and/or the LUMO level, as well as the 3MLCT state of the
complex.
was concentrated under vacuum, and the product was isolated by
short-column chromatography on silica gel (200e300 mesh).
L-2: yield 90%, light yellow solid. 1H NMR (400 MHz, CDCl3,
Although iridium phosphors and carbazole derivatives have
been widely investigated in the OLEDs area either as dopants or
host materials [30,31], a systematic investigation of the substituent
at the pyridyl ring on the luminescent properties of (ppy-carba-
zole)2Ir(acac) derivatives is still absent. Herein, we report a sys-
tematic study on the synthesis and properties of a series of (ppy-
carbazole)2Ir(acac) derivatives. Our interest is to reveal how these
simple functional groups in the pyridyl ring affect the photo-
physical and electroluminescent properties of the corresponding
iridium complexes. It is experimentally observed that the incor-
poration of substituent such as 4-CH3, 5-CH3 or 5-F group on the
pyridyl ring of the ligand does not change significantly the phos-
phorescence energy and the emission color of the iridium com-
plexes, while the introduction of a strongly electron-withdrawing
5-CF3 group on the pyridyl ring of the ligand results in a great
bathochromic shift of the emission color of the iridium complex.
25 ꢁC):
d
¼ 8.61 (d, J ¼ 4.8 Hz, 1H), 8.22 (d, J ¼ 8.4 Hz, 2H), 8.15 (d,
J ¼ 8.0 Hz, 2H), 7.67 (t, J ¼ 8.8 Hz, 3H), 7.48 (d, J ¼ 8.0 Hz, 2H), 7.42 (t,
J ¼ 7.6 Hz, 2H), 7.28 (d, J ¼ 7.2 Hz, 2H), 7.13 (d, J ¼ 4.8 Hz, 1H), 2.47 (s,
3H) ppm. 13C NMR (100 MHz, CDCl3, 25 ꢁC):
d
¼ 156.59, 149.72,
148.20, 140.87, 138.66, 138.37, 128.59, 127.30, 126.16, 123.61, 121.73,
120.48, 120.19, 110.00 21.49 ppm. MS (EI): m/z ¼ 334.1474 [M]þ
L-5: yield 96%, yellow solid. 1H NMR (400 MHz, CDCl3, 25 ꢁC):
d
¼ 9.01 (s, 1H), 8.29 (d, J ¼ 8.8 Hz, 2H), 8.16 (d, J ¼ 7.6 Hz, 2H), 8.06
(d, J ¼ 6.8 Hz, 1H), 7.95 (d, J ¼ 8.4 Hz, 1H), 7.74 (d, J ¼ 8.4 Hz, 2H),
7.50 (d, J ¼ 8.0 Hz, 2H), 7.44 (t, J ¼ 7.2 Hz, 2H), 7.32 (t, J ¼ 7.2 Hz, 2H)
ppm. 13C NMR (100 MHz, CDCl3, 25 ꢁC):
d
¼ 159.73, 146.83, 140.55,
139.48, 136.74, 134.20, 128.83, 127.29, 126.11, 124.93, 123.64, 120.43,
120.29, 119.95, 109.81 ppm. MS (EI): m/z ¼ 388.1191 [M]þ.
2.2.2. Synthesis of iridium complexes Ir-1eIr-5
IrCl3$3H2O (1 mmol) and the ligands L-1eL-5 (2.5 mmol) were
added to a mixture of 2-ethoxyethanol and water (v/v ¼ 3:1, 12 mL).
The mixture was refluxed under nitrogen for 24 h. Upon cooling to
room temperature, the yellow precipitate was collected by filtration
and washed with water. The wet solid was completely dried to give
the crude chloro-bridged dimer complex. Without further purifi-
cation, this dimer was added to a mixture of K2CO3 (690 mg,
5 mmol), acetyl acetone (2 mmol), and 2-ethoxyenthane (10 mL).
After refluxing under nitrogen for 24 h, the solution was cooled to
room temperature. The crude product was purified by column
chromatography over silica using a CH2Cl2:n-hexane (1:1) as eluent
to yield the pure product of the desired iridium complex.
2. Experimental
2.1. General procedures
All reagents and solvents were obtained from Alfa Aesar or
Avocado, and the solvents were treated as required prior to use.
Other chemicals were purchased from commercial sources and
used without further purification, unless otherwise noted. 1H NMR
and 13C NMR spectra were record on a 400 MHz Varian Unity Inova
spectrophotometer. Mass spectra were taken on MALDI micro MX
and HP1100LC/MSD MS spectrometers. The intensities of the crystal
Ir-1: yield 39%, yellow solid. 1H NMR (400 MHz, CDCl3, 25 ꢁC):
data were collected on a Bruker SMART APEX CCD diffractometer
with graphite-monochromated Mo-K
d
¼ 8.50 (d, J ¼ 4.8 Hz, 2H), 8.02 (d, J ¼ 7.6 Hz, 4H), 7.88e7.83 (m,
ꢀ
a
(l
¼ 0.71073 A) using the
4H), 7.65e7.61 (m, 2H), 7.30e7.25 (m, 6H), 7.20e7.17 (m, 6H), 7.15e
7.12 (m, 2H), 7.06 (d, J ¼ 6.8 Hz, 2H), 6.46 (d, J ¼ 2.0 Hz, 2H), 5,34 (s,
SMART and SAINT programs. Thermogravimetry analyses (TGA)
were carried out using a Perkin-Elmer thermogravimetry analyses
(TGA) at a heating rate of 10 ꢁC minꢀ1 under a nitrogen atmosphere.
IR spectra were recorded on an FTIR NEXUS Spectrometer using KBr
1H), 1.88 (s, 6H) ppm. 13C NMR (100 MHz, CDCl3, 25 ꢁC)
d
¼ 184.92,
167.63, 148.63, 148.26, 143.72, 140.36, 137.57, 137.37, 130.78, 125.51,
124.80, 123.19, 121.71, 119.91, 119.51, 118.70, 118.67, 110.51, 100.79,
28.88 ppm. IR (solid, cmꢀ1): 3412, 3044, 2860, 1605, 1577, 1514,
1450, 1397, 1334, 1261, 1334, 1229, 959, 918, 776, 749, 723, 647, 616.
MALDI-TOF-MS (m/z): 930.2491 [M]þ, 831.2092 [M-acac]þ.
Ir-2: yield 45%, yellow solid. 1H NMR (400 MHz, CDCl3, 25 ꢁC):
disks and wave numbers were given in cmꢀ1
. The photo-
luminescence and UVevis absorption spectra measurements
were performed on a Perkin-Elmer LS55 spectrometer and a
Perkin-Elmer Lambda 35 spectrophotometer, respectively. Phos-
phorescence lifetimes were measured on an Edinburgh FLS920
Spectrometer in degassed CH2Cl2 solutions with excitation wave-
length at 410 nm. The electrochemical measurements of these
iridium complexes were carried out by using a conventional three-
electrode configuration and an electrochemical workstation
(BAS100B, USA) at a scan rate of 100 mV sꢀ1. A glass carbon working
electrode, a Pt-wire counter electrode, and a saturated calomel
electrode (SCE) reference electrode were used. All measurements
were made at room temperature on samples dissolved in CH2Cl2,
with 0.1 M Bu4NPF6 as the electrolyte. Density functional theory
(DFT) calculations using the B3LYP functional were performed. The
basis set used for C, H, O, N and F atoms was 6-31G while LanL2DZ
basis set were employed for iridium atoms. All these calculations
were performed with Gaussian 09.
d
¼ 8.31 (d, J ¼ 5.6 Hz, 2H), 8.02 (d, J ¼ 7.6 Hz, 4H), 7.80 (d, J ¼ 8.0 Hz,
2H), 7.66 (s, 2H), 7.29e7.25 (m, 6H), 7.19e7.16 (m, 6H), 7.12e7.09 (m,
2H), 6.88 (d, J ¼ 6.0 Hz, 2H), 6.46 (d, J ¼ 2.0 Hz, 2H), 5.31 (s, 1H), 2.45
(s, 6H), 1.87 (s, 6H) ppm. 13C NMR (100 MHz, CDCl3, 25 ꢁC)
d
¼ 184.75, 167.03, 148.83, 148.71, 147.64, 143.99, 140.49, 137.32,
131.03, 125.48, 124.51, 123.14, 122.84, 119.83, 119.50, 119.42, 118.54,
110.61, 100.64, 28.86, 21.37 ppm. IR (solid, cmꢀ1): 3432, 3059, 2921,
2852, 1619, 1578, 1514, 1477, 1450, 1397, 1352, 1335, 1311, 1229, 1173,
1121, 919, 813, 750, 724, 648. MALDI-TOF-MS (m/z): 959.2899 [M]þ,
859.2281 [M-acac]þ.
Ir-3: yield 26%, yellow solid. 1H NMR (400 MHz, CDCl3, 25 ꢁC):
d
¼ 8.31 (s, 2H), 8.02 (d, J ¼ 8.0 Hz, 4H), 7.81e7.74 (m, 4H), 7.43 (d,
J ¼ 8.4 Hz, 2H), 7.29e7.25 (m, 6H), 7.19e7.15 (m, 6H), 7.12e7.09 (m,
2H), 6.41 (d, J ¼ 6.0 Hz, 2H), 5.34 (s, 1H), 2.31 (s, 6H), 1.90 (s, 6H)
ppm. 13C NMR (100 MHz, CDCl3, 25 ꢁC)
d
¼ 184.80, 164.91, 148.02,
2.2. Preparation of materials
147.80, 143.99, 140.38, 138.17, 137.03, 131.54, 130.77, 125.44, 124.22,
123.12, 119.86, 119.42, 118.53, 118.24, 110.54, 100.79, 28.96,
18.51 ppm. IR (solid, cmꢀ1): 3436, 3050, 3954, 1578, 1514, 1487,
1450, 1399, 1334, 1313, 1230, 1016, 811, 749, 723, 680. MALDI-TOF-
MS (m/z): 958.2781 [M]þ, 859.2336 [M-acac]þ.
2.2.1. Synthesis of ligands L-1eL-5 [29]
A mixture of the N-heteroaryl halide (1.0 mmol), 4-(9H-carba-
zol-9-yl)-phenylboronic acid (1.5 mmol), K2CO3 (2.0 mmol),
Pd(OAc)2 (1.5 mol%), ethanol (6 mL) and distilled water (2 mL) was
stirred at 80 ꢁC in air for indicated time (<30 min). The reaction was
monitored by TLC. The reaction mixture was added to brine (30 mL)
and extracted four times with ethyl acetate (4 ꢂ 30 mL). The solvent
Ir-4: yield 40%, yellow solid. 1H NMR (400 MHz, CDCl3, 25 ꢁC):
d
¼ 8.39 (m, 2H), 8.03 (d, J ¼ 8.0 Hz, 4H), 7.89e7.85 (m, 2H), 7.79 (t,
J ¼ 8.8 Hz, 2H), 7.51e7.47 (m, 2H), 7.31e7.16 (m, 14H), 6.46 (d,
J ¼ 2.8 Hz, 2H), 5.41 (s, 1H), 1.90 (s, 6H) ppm. 13C NMR (100 MHz,

C. Liu et al. / Dyes and Pigments 109 (2014) 13e20
15
CDCl3, 25 ꢁC)
d
¼ 185.55, 147.49, 142.84, 140.22, 139.99, 137.52,
compounds, the organic cyclometalating ligands were prepared by
a palladium-catalyzed Suzuki reaction of 4-(9H-carbazol-9-yl)-
phenylboronic acid with the appropriate aryl halides in aqueous
ethanol under ligand-free, aerobic conditions in high yields of over
90% [29]. Our method provides a much higher yield than earlier
reported traditional convergent strategies [33,34]. All of the het-
eroleptic complexes were synthesized through the traditional two-
step procedure [35]. First, the cyclometalation of the IrCl3$3H2O
with each cyclometalating ligand generated the corresponding
chloro-bridged dimer [Ir(C^N)2Cl]2. Then the target heteroleptic
iridium(III) complexes Ir-1eIr-5 were obtained by refluxing the
corresponding chloro-bridged dimer with acetyl acetone (acac) in
2-ethoxyethanol in the presence of K2CO3. These new complexes all
exhibited good solubility and were sufficiently stable in common
organic solvents such as dichloromethane, enabling them to be
purified by column chromatography on silica gel.
136.71, 130.25, 127.66, 125.77, 125.64, 125.30, 125.11, 124.81, 123.37,
123.26, 119.96, 119.76, 118.92, 110.14, 100.83, 28.36 ppm. IR (solid,
cmꢀ1): 3483, 3050, 2966, 1578, 1515, 1484, 1450, 1395, 1332, 1234,
1174, 859, 810,784, 749, 722, 680, 592. MALDI-TOF-MS (m/z):
966.2284 [M]þ, 867.2361 [M-acac]þ.
Ir-5: yield 52%, orange solid. 1H NMR (400 MHz, CDCl3, 25 ꢁC):
d
¼ 8.80 (s, 2H), 8.03 (d, J ¼ 8.0 Hz, 4H), 7.93 (d, J ¼ 8.4 Hz, 2H),
7.87e7.84 (m, 4H), 7.29e7.25 (m, 8H), 7.22e7.18 (m, 6H), 6.48 (d,
J ¼ 2.0 Hz, 2H), 5.39 (s, 1H), 1.93 (s, 6H) ppm. 13C NMR (100 MHz,
CDCl3, 25 ꢁC)
d
¼ 185.78, 171.01, 150.47, 145.28, 141.64, 140.08,
139.00,134.64,130.70,126.52,125.71,124.53,123.44,120.06,119.94,
119.19, 118.44, 110.26, 101.15, 28.66 ppm. IR (solid, cmꢀ1): 3429,
3065, 1616, 1578, 1488, 1446, 1396, 1331, 1265, 1138, 1087, 919, 815,
750, 723,673. MALDI-TOF-MS (m/z): 1065.2180 [MꢀH]þ, 965.1568
[M-acac]þ.
All the ligands and complexes were fully characterized by NMR
spectroscopy and MALDI-TOF mass spectrometry. Due to the
characteristic singlet resonance peaks located at 5.34 and 1.88 ppm
in the 1H NMR spectrum and 100.79 and 28.88 ppm in the 13C NMR
spectrum the presence of the auxiliary acac in Ir-1 was indicated. In
each case, the MALDI-TOF mass spectrum reveals the respective
parent ion peak clearly. Furthermore, crystal structures of Ir-1 and
Ir-4 have been obtained by single crystal X-ray diffraction analyses,
which provide insight on intermolecular interactions in the solid
state and can also present valuable data for the computational
studies of the ground-state electronic properties of the molecule.
Perspective drawings of Ir-1 and Ir-4 are shown in Fig. 1. As
confirmed by X-ray crystallography, each of the molecular struc-
tures reveals the central iridium atom to be coordinated by two
anionic C^N ligands and one chelating acac anion. Under a nitrogen
atmosphere, the thermal properties of these iridium(III) complexes
were characterized by thermogravimetric analysis (TGA), and the
results are summarized in Table 1. The TGA data reveal that all the
complexes have excellent thermal stability and their 5% weight-
2.3. OLEDs fabrication and measurements
The pre-cleaned ITO glass substrates (30
U
ꢁsqꢀ1) were treated
by UV-ozone for 20 min. Then, a 40 nm thick NPB (4,40-bis[N-(1-
naphthyl)-N-phenylamino]-biphenyl) was deposited on the ITO
glass substrates. The emitting layers were then deposited by co-
evaporation of the iridium complex phosphor and the CBP (4,40-
N,N0-dicarbazolebiphenyl) host. Successively, TPBI (1,3,5-tris[N-
(phenyl)-benzimidazole]benzene), LiF and Al were evaporated at a
base pressure less than 10ꢀ6 torr. The EL spectra, CIE coordinates
and currentevoltageeluminance characteristics of the devices
were measured with a PR705 photometer and a source-measure-
unit Keithley236 under ambient conditions. The forward viewing
external quantum efficiency (hext) was calculated using the lumi-
nance efficiency, EL spectra and human photopic sensitivity.
3. Results and discussion
reduction temperatures (D
T5%) range from 367 to 385 ꢁC, which
3.1. Synthesis and characterization
are necessary for high-performance OLEDs.
Chemical structures and the detailed synthetic protocols
of these new iridium(III) complexes are shown in Scheme 1. The
9-(4-bromophenyl)carbazole was synthesized from carbazole as
the starting material by a modified Ullman reaction and the
3.2. Photophysical properties
The absorption and photoluminescence (PL) spectra of all the
iridium(III) complexes in CH2Cl2 solutions are depicted in Fig. 2 and
the data are summarized in Table 1. Similar UVevis electronic
4-(9H-carbazol-9-yl)phenylboronic acid was synthesized in
a
similar procedure to that described previously [32]. As the key
Scheme 1. Synthetic routes to the cyclometalated iridium(III) complexes Ir-1eIr-5.

16
C. Liu et al. / Dyes and Pigments 109 (2014) 13e20
Fig. 2. Absorption spectra (a) and photoluminescence spectra (b) for the iridium(III)
complexes Ir-1eIr-5 in CH2Cl2 at 293 K.
Fig. 1. Crystal structure of Ir-1 (a) and Ir-4 (b) with thermal ellipsoids drawn at 30%
probability level. Labels on carbon atoms (except for those bonded to the Ir center) and
hydrogen atoms are omitted for clarity.
Upon photoexcitation at 400 nm at room temperature, com-
plexes Ir-1eIr-4 show similar photoluminescence with emission
peak at 513e519 nm, while the complex Ir-5 is a yellowegreen
emitting with a peak at 543 nm, exhibiting a red shift 25 nm
relative to Ir-1. Evidently, the incorporation of electron-
withdrawing group (5-CF3) on the pyridyl ring of the cyclo-
metalating ligand results in a universal bathochromic shift of the
phosphorescence of the iridium complexes. The phosphorescent
lifetimes of complexes Ir-1eIr-5 are determined in the range of
absorption spectra was exhibited by complexes Ir-1eIr-4. In com-
mon with most iridium(III) complexes, Ir-1eIr-5 show two major
absorption bands in their UVevis absorption spectra. The intense
bands in the ultraviolet region (below 400 nm) are assigned to the
spin-allowed intraligand 1
p
ep
* transitions, which closely resemble
the spectra of the free ligands. The next lower energy in the
shoulder region of 1
* transitions and the weak broad shoulders
pep
extending into the visible region (400e500 nm) with appreciable
intensity are likely due to metal-to-ligand charge-transfer (1MLCT),
1.77e2.18
ms at room temperature. Relative to Ir(ppy)2(acac)
spin-orbit coupling enhanced 3p *, and spin-forbidden (3MLCT)
ep
(Vp ¼ 0.34) as the standard [36], the phosphorescent quantum
transitions.
yields of these were measured in degassed CH2Cl2 and they all
Table 1
Photophysical and electrochemical data for complexes Ir-1eIr-5.
labs [nm]a
lem [nm]b
Vp
sp
[m
s]c
Eg [eV]d
Eoxonset [V]
HOMO/LUMO [eV]e
D
T5% [ꢁC]
b
Compound
Ir-1
Ir-2
Ir-3
Ir-4
Ir-5
259 (0.58), 293 (0.28), 345 (0.29), 444 (0.033)
257 (0.75), 294 (0.49), 345 (0.44), 445 (0.027)
257 (0.66), 294 (0.32), 335 (0.31), 445 (0.027)
257 (0.92), 284 (0.49), 345 (0.44), 444 (0.054)
260 (1.31), 290 (0.79), 369 (0.54), 478 (0.081)
517
513
519
518
543
0.18
0.19
0.27
0.14
0.31
1.92
1.78
1.77
1.84
2.18
2.45
2.51
2.41
2.44
2.26
0.83
0.79
0.81
0.94
0.88
ꢀ5.23/ꢀ2.78
ꢀ5.19/ꢀ2.68
ꢀ5.21/ꢀ2.80
ꢀ5.34/ꢀ2.90
ꢀ5.28/ꢀ3.02
378
385
388
367
382
a
Measured in CH2Cl2 with a concentration of 10ꢀ5 M and ε values (ꢂ105) are shown in parentheses.
In degassed CH2Cl2, Vp is shown relative to fac-[Ir(ppy)3] (Vp ¼ 0.40).
b
c
Emission lifetime in degassed CH2Cl2 with a concentration of 10ꢀ5 M at 293 K, lexc ¼ 410 nm.
d
e
Estimated from the absorption edge (ledge) of solid films by equation of Eg ¼ 1240/ledge
.
ox
HOMO ¼ ꢀe(4.4 þ E
), LUMO ¼ HOMO þ Eg.
onset

C. Liu et al. / Dyes and Pigments 109 (2014) 13e20
17
show high quantum efficiencies ranging from 0.14 to 0.31. Appar-
ently, the phosphorescent efficiency of Ir-5 is comparable with that
of (ppy)2Ir(acac). The good electroluminescent behavior can be
expected for these complexes based on the proper triplet energy
levels and the high photoluminescent quantum yields.
B3LYP hybrid functional theory with Gaussion 09. Fig. 4 illustrates
the HOMO and LUMO distributions for complexes Ir-1 and Ir-5.
Those for Ir-2, Ir-3 and Ir-4 are provided in Fig. S1 in the Supporting
Information. For all present complexes, the HOMOs are localized on
the iridium center and the phenyl and carbazole of the cyclo-
metalating ligands as described in the literature. On the other hand,
the LUMO orbitals of these iridium(III) complexes are distributed
on both the aryl and pyridyl rings of the cyclometalating ligands. It
was noteworthy that Ir-5 with the 5-CF3 on the pyridyl ring, the
LUMO of the novel complex is more intensively shifted onto the
pyridyl ring in comparison with Ir-1. The shifting of the LUMO
distribution and the lower levels are probably because that the
strong electron-withdrawing eCF3 substituent further enhances
the electron-deficient feature of the pyridyl moiety.
3.3. Electrochemical characterization
The electrochemical properties of these new iridium complexes
were measured in deoxygenated CH2Cl2 solutions containing 0.1 M
tetra(n-butyl)ammonium hexafluorophospate (n-Bu4NPF6) as the
supporting electrolyte by cyclic voltammetry. The cyclic voltam-
mograms are shown in Fig. 3. During the anodic scan at the rate of
100 mV sꢀ1, all these complexes show one reversible oxidation
wave in the range of 0.79e0.94 V vs. a saturated calomel electrode
(SCE), which should be assigned to the metal centered IrIII/IrIV
oxidation couple and/or the possible oxidation of the aryl part or
the electron-donating carbazole group on the cyclometalating
ligand. Compared to the complex (ppy)2Ir(acac) (0.72 V), the
oxidation of these new iridium complexes become a little more
difficult. It is found that introducing the electron-donating groups
(4-CH3, 5-CH3) in the pyridine unit makes the oxidation process
shift slightly to more positive potential for these complexes, while
electron-withdrawing groups (5-F, 5-CF3) lead to less positive
3.4. Electrophosphorescent OLEDs
In order to investigate the EL properties of these iridium com-
plexes, Ir-1 and Ir-5 were used as doped emitters to fabricate
monochromatic OLEDs, as shown in Fig. 5. From the high energy
band of the phosphorescence spectra, the triplet energies were
determined as ca. 2.26e2.47 eV for Ir-1eIr-5. These energy levels
are well below the level of the widely used CBP (2.56 eV) host and
the efficient energy transfer process is possible if CBP is selected as
the host matrix for these complexes to fabricate OLEDs. Two types
of light-emitting devices were fabricated using Ir-1 or Ir-5 as
phosphorescent emitters. The device structures are as follows: ITO/
NPB (40 nm)/CBP: iridium complex (6%, 30 nm)/TPBI (45 nm)/LiF
(1 nm)/Al (device I), and ITO/NPB (40 nm)/TCTA (10 nm)/CBP:
iridium complex (6%, 30 nm)/TPBI (45 nm)/LiF (1 nm)/Al (device II),
where NPB was used as the hole-transporting layer, TCTA (4,40,400-
tris(carbazol-9-yl)-triphenylamine) as the electron/exciton-
blocking layer, CBP as the host material and TPBI as an electron-
transporting/hole-blocking material. Different hole-transporting
layers were utilized for these two devices in order to verify their
influence on the device performances.
As an example, the EL spectra of Ir-1 in device I and its PL at
room temperature and at 77 K are illustrated in Fig. 6. The EL
spectrum of Ir-1 is consistent with the PL spectrum at room tem-
perature and indicates that the EL originates from the triplet states
of the phosphor. Typical of other similar iridium complexes, the
spectra experience a blue-shift of about 16 nm in glass matrices at
77 K as a consequence of rigidification. This blue-shift is a
frequently observed phenomenon and is mainly due to the solvent
reorganization in a fluid solution at low temperature that can
values. On the basis of the onset potential of the first oxidation
ox
ðE
Þ and the absorption edge data, the HOMO and LUMO energy
onset
levels can be estimated from the empirical formulae:
EHOMO ¼ ꢀe(Eoonxset þ 4.4) and ELUMO ¼ EHOMO þ Eg (where Eg is the
optical energy gap obtained from the absorption threshold of the
film samples). All the electrochemical and electronic data for these
novel iridium complexes are summarized in Table 1. For complex Ir-
1, both the HOMO and LUMO decreased relative to those of the
parent (ppy)2Ir(acac). However, the larger downwards shift of the
LUMO than HOMO similarly leads to a smaller HOMOeLUMO band
gap in comparison with the (ppy)2Ir(acac). A small decrease of
0.24 eV of the LUMO level was detected for Ir-5 in comparison with
Ir-1, in combination with the unchanged HOMO level, finally
resulting in a similar decrease of the HOMOeLUMO band gap due
to incorporation of the 5-CF3 substituent.
In order to understand the impact of the modification groups (e
CH3, eF and eCF3) on the photophysical and electrochemical
behavior of these iridium complexes, density functional theory
(DFT) calculations were performed for complexes Ir-1eIr-5 using
Fig. 3. Cyclic voltammograms of Ir-1eIr-5 measured in CH2Cl2 at a scan rate of
100 mV sꢀ1
.
Fig. 4. Contour plots of frontier molecular orbitals of Ir-1 and Ir-5 in ground state.

18
C. Liu et al. / Dyes and Pigments 109 (2014) 13e20
Fig. 5. The general structure for OLED devices and the molecular structures of the relevant.
the Ir-1-based device I is quite remarkable with a high luminance of
56,140 cd mꢀ2 at 14 V and a peak luminance efficiency (hL) of
36.7 cd Aꢀ1, corresponding to a peak power efficiency (hP) of
19.2 lm Wꢀ1. As illustrated by the efficiency-current density curves
in Fig. 7, both of the devices witness gradual efficiency decay at the
Fig. 6. The PL and EL spectra for Ir-1.
stabilize the charge transfer states prior to emission [37]. No re-
sidual emission from CBP or TPBI is observed, which suggests an
efficient energy transfer from the host exciton to the phosphor
molecule upon electrical excitation and the effective hole-blocking
function of the TPBI layer in addition to its electron-transporting
role. The EL performance data of Ir-1 and Ir-5 in different devices
are listed in Table 2. In devices I and II, both complexes exhibited
excellent device performances. For example, the performance of
Table 2
EL data of the different OLED devices based on Ir-1 and Ir-5.
Complex Vturn-on Lmax [cd
hext
hL [cd
hP [lm
lem
CIE (x, y)
[V]
mꢀ2, V]a
[%, V]a Aꢀ1, V]a Wꢀ1, V]a [nm] at 9 V
Ir-1 (I)
Ir-5 (I)
Ir-1 (II)
Ir-5 (II)
4.7
4.5
4.4
4.2
38,400, 10 10.7, 5 36.7, 5 19.2, 5
56,140, 14 11.0, 6 39.1, 6 24.5, 6
45,650, 10 11.4, 5 40.7, 5 25.5, 5
30,430, 11 12.6, 5 43.2, 5 27.1, 5
516 0.28, 0.62
542 0.40, 0.56
516 0.27, 0.61
543 0.40, 0.56
a
The maximum values of the devices. The data in the parentheses are the volt-
ages (V) at which the data are obtained.
Fig. 7. JeVeL characteristics and current efficiencyecurrent density curves for Ir-1 and
Ir-5 in device I.

C. Liu et al. / Dyes and Pigments 109 (2014) 13e20
19
higher current density region, which are typically attributed to a
combination of tripletetriplet annihilation and field-induced
quenching effects.
In recent years, white organic light-emitting diodes have been of
considerable interest due to their potential applications in solid-
state lighting and full-color displays. For display applications, it is
important that the generated white light can be separated into
three primary colors with equal emission intensity after passing
through the color filters [40,41]. Chen et al. reported that in order to
generate the white spectrum with three evenly separated red,
green and blue peaks, the central emission wavelength of the green
emitter should be located at 545 nm [42]. We particularly note that
in this work the complex Ir-5 is easily synthesized as a new
By introducing TCTA as the electron/exciton-blocking layer, the
current density over the whole detected voltage range is always
higher in device II than in device I for each complex, as indicated by
the data under an example voltage of 9 V (Table 2). The better
performance for device II should be resulting from a more balanced
electron and hole transportation and more efficient recombination
in the emitting layer that is brought by using TCTA as the electron/
exciton-blocking layer. As a result, the device II exhibits overall
improvements in the turn-on voltage, brightness, and EL effi-
ciencies. For example, the turn-on voltages of device II are 4.2e
4.4 V, which are lower than 4.5e4.7 V for device I. The decreased
driving voltages in device II will be definitely favorable to higher
power efficiencies, as shown by the efficiency-current density
curves in Fig. 8 and the data in Table 2. The performance of Ir-1
and Ir-5 in device II is substantially enhanced, with peak
hL ¼ 40.7 cd Aꢀ1 and hP ¼ 25.5 lm Wꢀ1 for Ir-1 and hL ¼ 43.2 cd Aꢀ1
and hP ¼ 27.1 lm Wꢀ1 for Ir-5. We particularly note that the overall
performances of Ir-5 are better than Ir-1 in both devices I and II,
with lower driving voltages and higher luminance and especially
remarkably higher efficiencies. The introduction of the electron-
withdrawing eCF3 group contributes to the outstanding OLEDs
performance. As reported in the literature, fluorination can
enhance the electron mobility and result in a better balance of
charge injection and transfer. In addition, the lower vibrational
frequency of the CeF may reduce the rate of radiationless deacti-
vation [38,39].
yellowish-green emitter, with
a central electroluminescence
wavelength of 543 nm. We also expect that the EL performance of
Ir-5 could be further optimized by choosing other novel host
materials.
4. Conclusions
In conclusion, a series of heteroleptic cyclometalated iridium(III)
complexes containing 9-(4-(pyridin-2-yl)phenyl)-9H-carbazole
molecular framework have been synthesized and fully character-
ized. All of the complexes are thermally stable solids and highly
efficient electrophosphors. This work presents a systematic inves-
tigation of the substituent effects on the luminescent properties of
(ppy-carbazole)2Ir(acac) derivatives. It is noteworthy that Ir-5 is an
excellent greeneyellow phosphorescent material with a maximum
luminance efficiency of 43.2 cd Aꢀ1 and a power efficiency of
27.1 lm Wꢀ1, which has potential applications in white OLEDs.
Acknowledgment
Financial support by the National Natural Science Foundation of
China (21276043) and the Ministry of Education (the Program for
New Century Excellent Talents in University NCET-10-0283) is
acknowledged. We thank Dr. Jiuyan Li (Dalian University of Tech-
nology) for OLEDs measurements.
Appendix A. Supplementary data
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.dyepig.2014.04.039.
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