Homoleptic tris-cyclometalated iridium complexes with 2-phenylbenzothiazole ligands for highly efficient orange OLEDs

Article abstract of DOI:10.1039/c1jm10757g

Homoleptic tris-cyclometalated iridium(iii) complexes containing 2-phenylbenzothiazole derivatives as ligands have been successfully synthesized and characterized for the first time. Electron-donating (CH₃, OCH₃) and -withdrawing groups (F) were introduced into the 6-position of the benzothiazole moiety in the ligands to verify their influence on the optical and electronic properties of the complexes. Organic light-emitting diodes using these iridium complexes as doped emitters exhibited orange electrophosphorescence with excellent performances. An extremely high brightness of 95800 cd m^-2 and a maximum luminance efficiency of 87.9 cd A ^-1 (46.0 lm W^-1) were achieved for the pristine complex without any substituent in the ligand. These performances represent a significant improvement for vacuum-deposited orange OLEDs and the new record of the efficiencies for orange OLEDs reported so far. The substituents in the ligand were observed to be rather unimportant to influence the performance of this series of iridium complexes.

Full text of DOI:10.1039/c1jm10757g

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PAPER
Homoleptic tris-cyclometalated iridium complexes with
2-phenylbenzothiazole ligands for highly efficient orange OLEDs†
a
Renjie Wang, Di Liu, Huicai Ren, Ting Zhang, Xinzeng Wang and Jiuyan Li
b
b
a
b
a
*
Received 20th February 2011, Accepted 27th July 2011
DOI: 10.1039/c1jm10757g
Homoleptic tris-cyclometalated iridium(III) complexes containing 2-phenylbenzothiazole derivatives as
ligands have been successfully synthesized and characterized for the first time. Electron-donating (CH3,
OCH3) and -withdrawing groups (F) were introduced into the 6-position of the benzothiazole moiety in
the ligands to verify their influence on the optical and electronic properties of the complexes. Organic
light-emitting diodes using these iridium complexes as doped emitters exhibited orange
electrophosphorescence with excellent performances. An extremely high brightness of 95 800 cd mꢀ2
and a maximum luminance efficiency of 87.9 cd Aꢀ1 (46.0 lm Wꢀ1) were achieved for the pristine
complex without any substituent in the ligand. These performances represent a significant improvement
for vacuum-deposited orange OLEDs and the new record of the efficiencies for orange OLEDs
reported so far. The substituents in the ligand were observed to be rather unimportant to influence the
performance of this series of iridium complexes.
yields and excellent device performance, finally for WOLEDs
Introduction
applications. 2-Phenylbenzothiazole is one typical ligand
Organic light-emitting diodes (OLEDs) have attracted great
interest in both the scientific and industrial fields due to their
applications in low-cost, more-efficient flat-panel displays and
solid-state lighting.1,2 In particular, heavy metal complexes based
electroluminescent materials have been the main focus because
they can harvest both singlet and triplet excitons to realize
a theoretical internal quantum efficiency of 100%.3,4 Among
them, cyclometallated iridium complexes, which have relatively
short excited-state lifetime, high phosphorescence efficiency and
flexible color tunability, are regarded as the most promising
phosphors in organic light-emitting devices. To date, various
phosphorescent iridium complexes including their tris-cyclo-
metalated derivatives have been synthesized and thoroughly
studied.5–7 In addition to the three primary color emitting iridium
complexes, the orange- or yellow-emitting iridium phosphors are
of great interests as well since they can be used in combination
with blue emitters to fabricate two-emitting-component white
organic light-emitting diodes (WOLEDs), which has definitely
become one of the most important directions of OLEDs study in
recent years.8–10
framework to construct orange-yellow iridium complexes. Bis(2-
0
phenylbenzothiozolato-N,C2 )iridium(acetylacetonate)
[bt2Ir
(acac)]11 and a number of derivatives with substituents on the
ligands have been developed for OLED applications.12 Recently,
we also demonstrated that the introduction of an electron-
withdrawing substituent in the 6-position of the benzothiazole
ring resulted in a dramatic ascent of the OLED performances for
their orange-emitting iridium complexes.13 The two-emitting-
component WOLEDs containing these orange phosphors
accordingly exhibited excellent performances that outweigh the
literature data of WOLEDs. We are enlightened by the excellent
0
performance of the bis(2-phenylbenzothiozolato-N,C2 )iridium
(acetylacetonate) derivatives to prepare their homoleptic tris-
cyclometalated iridium complexes for OLED applications.
Compared with bis-cyclometalated iridium complexes with ace-
tylacetone or other groups as ancillary ligand, the tris-cyclo-
metalated derivatives usually have the same emitting energy and
similar quantum yields. Furthermore, the tris-cyclometalated
analogues have one special advantage that certain functional
groups such as charge-transporting dendrons can be grafted on
all three C^N ligands to reach larger dendron density and better
site-isolating effect on the iridium core. This is especially signif-
icant when the resultant dendritic iridium complexes are used as
neat film emitters to make non-doped monochromatic or white
OLEDs.14,15 However, it was reported that the tris-chelate
iridium complex of the 2-phenylbenzothiazole ligand framework
has not been prepared by using the conventional synthetic
methods.16 Fortunately, in 2004, Crutchley et al.17 reported
a modified procedure by treating the chloride-bridged dimer with
Our interest is to design and synthesise novel blue- and orange-
emitting iridium complexes having high luminescent quantum
aState Key Laboratary of Fine Chemicals, School of Chemical Engineering,
Dalian University of Technology, 2 Linggong Road, Dalian, 116024,
China. E-mail: jiuyanli@dlut.edu.cn; Fax: +86 411 84986233
bSchool of Chemistry, Dalian University of Technology, 2 Linggong Road,
Dalian, 116024, China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c1jm10757g
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the third molar equivalent of the C^N ligand in the presence of an
important reagent, silver trifluoromethane sulfonate, to lead to
the successful synthesis of tris-cyclometalated iridium complexes.
More recently, blue phosphorescent dendrimers based on fac-tris
(phenyltriazolyl)iridium(III) cores have also been successfully
synthesized via this method.18 All of these have driven us to
explore the modified method to develop the homoleptic tris-
cyclometalated iridium complexes of 2-phenylbenzothiazole
ligands, which are still vacant up to now.
confirms that it is a facial isomer. As shown in Fig. 1, complex 1
has three cyclometalating ligands in a pseudooctahedral coor-
dination geometry around the metal center, and the Ir–C bonds
ꢀ
are found to be shorter than the Ir–N bonds, with 2.019 A for
ꢀ
Ir–C and 2.180 A for Ir–N in average.
Optical properties
Complexes 1–4 show similar UV-vis electronic absorption and
photoluminescence (PL) spectra at room temperature, as dis-
played in Fig. 2. In common with most IrIII complexes, 1–4 show
two major absorption bands in their UV-vis absorption spectra.
The intense bands in the short wavelength region below 400 nm
appear to be ligand-based transitions which closely resemble the
spectra of the free ligands. The weaker absorptions in the range
of 400–500 nm are likely due to metal-to-ligand charge-transfer
transitions (1MLCT and 3MLCT). Upon photoexcitation at
400 nm at room temperature, complexes 1–4 show intense orange
luminescence with the emission peak at 540–546 nm and
a shoulder at 580–590 nm. From the high energy band of the
phosphorescence spectra, the triplet energies were determined as
ca. 2.30–2.27 eV for 1–4. These energy levels are well below the
level of the widely used CBP (4,40-N,N0-dicarbazolebiphenyl,
2.56 eV) host and the efficient energy transfer process is possible
if CBP is selected as the host matrix for these complexes to
In this paper, we report the synthesis and electroluminescent
properties of the tris-cyclometalated iridium complexes, 1–4
(Scheme 1), with the corresponding 2-phenylbenzothiazole
derivatives as the exclusive ligand. 1–4 are successfully prepared
through transformation from the precursors, bis(2-phenyl-
0
benzothiozolato-N,C2 )iridium(acetylacetonate) derivatives, in
the presence of the important catalyst of silver trifluoromethane
sulfonate. The orange-emitting OLEDs containing these new
complexes as dopant emitters exhibit extremely high brightness
and efficiencies, and relatively high device stability. The best
device performance is achieved for complex 4 with a maximum
forward viewing luminance efficiency of 87.9 cd Aꢀ1 and a power
efficiency of 46.0 lm Wꢀ1, representing a new record of the effi-
ciencies for orange OLEDs reported so far.
Results and discussion
Synthesis
The ligands 1c–4c and the iridium complexes 1–4 were synthe-
sized according to the procedure described in Scheme 1. The
desired tris-cyclometalated complexes 1–4 were successfully
prepared in 65–80% yields by reaction of the corresponding
ligands with bis-cyclometalated iridium complexes in diglyme at
ꢁ
160 C for 20 h in the presence of silver trifluoromethane sulfo-
nate. We have examined the possibility to prepare these tris-
cyclometalated complexes using the traditional methods either
from the chloride-bridged dimer (C^N2Ir(u-Cl)2IrC^N2) or from
the bis-cyclometalated complexes with acac ancillary ligand
(C^N2Ir(acac)). It is observed that the desired compounds could
not be generated in the absence of silver trifluoromethane
sulfonate, implying that this salt indeed plays essential roles in
this reaction. All the obtained complexes were characterized by
1H and 13C NMR, and MALDI-TOF or API-ES mass spec-
trometry. Single-crystal X-ray diffraction analysis of complex 1
Fig. 1 X-Ray crystal structure of 1.
Scheme 1 Synthesis of complexes 1–4.
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fabricated using 1–4 as phosphorescent emitters. The device
structures are as follows: ITO/NPB (40 nm)/1–4 : CBP (5%,
30 nm)/TPBI (45 nm)/LiF (1 nm)/Al (device I), and ITO/
PEDOT : PSS (40 nm)/1–4 : CBP (5%, 30 nm)/TPBI (45 nm)/LiF
(1 nm)/Al (device II) (NPB ¼ 4,40-bis[N-(1-naphthyl)-N-phenyl-
amino]-biphenyl, PEDOT ¼ poly(3,4-ethylenedioxythiophene),
PSS ¼ poly(styrene sulfonate), CBP ¼ 4,40-N,N0-dicarbazolebi-
phenyl, and TPBI
¼
1,3,5-tris[N-(phenyl)benzimidazole]-
benzene), where NPB was used as the hole-transporting layer,
PEDOT : PSS as hole-injecting and -transporting layer, CBP as
the host material and TPBI as an electron-transporting/hole-
blocking material. Different hole-transporting layers were
utilized for these two types of devices in order to verify their
influence on the device performances. All the devices exhibited
intense orange emission at the doping concentration of 5 wt%.
For each complex, the EL spectra under a given voltage from
devices I and II are identical to each other, and they are both
independent of the driving voltage (Fig. S1 in the ESI†), indi-
cating the excellent spectral stability. As an example, the EL
spectra of these complexes in device II are illustrated in Fig. 3.
The EL spectra of these complexes are located in the same
wavelength region and have identical profiles to their PL, indi-
cating that the EL emission originates from the triplet excited
state of these complexes. A red shift of about 10 nm is observed
in the EL spectra if compared with their PL, which is a frequently
observed phenomenon and is probably caused by the additional
effect of the electric field on the excited states of the emitting
species in OLEDs. Additionally, there is no residual emission
from CBP or TPBI, even at high drive voltages, implying
a complete energy transfer from the CBP host to the IrIII complex
in the emissive layer and the effective hole-blocking function of
the TPBI layer in addition to its electron-transporting role. The
important EL performance data of 1–4 in different devices are
Fig. 2 The UV-vis absorption and photoluminescence spectra of 1–4.
fabricate OLEDs. Relative to fac-Ir(ppy)3 as the standard, the
phosphorescent quantum yields of these complexes were
measured in degassed toluene solution and they all show high
quantum efficiencies ranging from 0.29 to 0.37 (Table 1).
Apparently, the phosphorescent efficiency of these complexes,
especially for 1, 2 and 4, are comparable with that of fac-Ir(ppy)3
(0.4). The good electroluminescent behaviors can be expected for
these complexes based on the proper triplet energy levels and the
high photoluminescent quantum yields.
Electrochemical properties
The electrochemical behavior of these IrIII complexes was
measured in CH2Cl2 by cyclic voltammetry. During the anodic
scan at the rate of 100 mV sꢀ1, all these complexes show one
reversible oxidation wave in the range of 0.74–0.88 V vs. a satu-
rated calomel electrode (SCE). It is found that introducing the
electron-withdrawing group (F) in the benzothiazolyl unit makes
the oxidation process shift slightly to more positive potential for
these complexes, while electron-donating groups (CH3, OCH3)
lead to less positive values. The highest occupied molecular
orbital (HOMO) energy levels of these complexes were deter-
mined by the onset potential of the oxidation wave according to
ox
the empirical equation: HOMO (eV) ¼ ꢀ(4.4 + E ). The
onset
optical band gaps of these complexes are determined by the
absorption edge technique. Finally, their lowest unoccupied
molecular orbital (LUMO) energy levels were estimated from the
HOMO energy levels and the band gaps. All the electrochemical
and electronic data of these complexes are summarized in
Table 1.
Phosphorescent organic light-emitting diodes
In order to investigate the electroluminescence (EL) properties of
the new IrIII complexes, two types of light-emitting devices were
Fig. 3 The EL spectra of complex 1–4 in device II at 10 V.
Table 1 Photophysical and electrochemical data for complexes 1–4
Compound
labs/nma
lem/nmb
HOMO/eV
LUMO/eVc
Eg/eVd
b
Fp
1
2
3
4
323(4.66), 384(4.11), 430(3.79), 476(3.36)
326(4.58), 384(4.00), 432(3.63), 477(3.15)
331(4.62), 384(4.16), 431(3.81), 478(3.33)
324(4.57), 388(4.07), 431(3.79), 475(3.51)
540, 581 sh
543, 584 sh
546, 590 sh
540, 580 sh
0.34
0.35
0.29
0.37
ꢀ5.28
ꢀ5.16
ꢀ5.14
ꢀ5.22
ꢀ3.11
ꢀ3.04
ꢀ2.99
ꢀ3.09
2.17
2.12
2.15
2.13
a
Measured in CH2Cl2 with a concentration of 5 ꢂ 10ꢀ6 M and log 3 values are shown in parentheses. b In degassed toluene, Fp is shown relative to fac-[Ir
(ppy)3], lexc ¼ 400 nm. c LUMO ¼ HOMO + Eg. d Estimated from the absorption edge (ledge) of solid films by equation of Eg ¼ 1240/ledge
.
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listed in Table 2, and Fig. 4 and 5 depict the current density–
voltage–luminance (J–V–L) characteristics and current efficiency
of these devices.
brightness, and EL efficiencies. For example, the turn-on volt-
ages for device II are 4.2–5.2 V, about 1 V lower than those (5.4–
6.5 V) for the NPB-based device I. The decreased driving volt-
ages in device II will be definitely favorable to higher power
efficiencies, as proved by the following discussion and the data in
Table 2. The performance of 1 and 2 in device II is substantially
enhanced, with peak hext ¼ 19.1%, hL ¼ 55.2 cd Aꢀ1 and hp ¼
22.9 lm Wꢀ1 for 1 and hext ¼ 20.7%, hL ¼ 56.7 cd Aꢀ1 and hp ¼
25.4 lm Wꢀ1 for 2. The performance enhancement for 3 in
device II is not as notable as for other derivatives. The EL
performances are further boosted for 4. In device II, complex 4
exhibited a turn-on voltage of 4.2 V, a maximum brightness
In device I with NPB as hole transporter which is deposited
directly on the ITO anode without a layer of hole injecting
material in between, all complexes exhibited excellent device
performances. For example, the performance of the 1-based
device is quite remarkable with a high luminance of 70 300 cd
mꢀ2 at 15 V and a peak luminance efficiency (hL) of 44.3 cd Aꢀ1 at
4.51 mA cmꢀ2, corresponding to a peak external quantum effi-
ciency (hext) of 15.5%, and a peak power efficiency (hp) of 13.9 lm
Wꢀ1. Notably, complex 4 exhibited a maximum luminance (Lmax
)
of 68 780 cd mꢀ2 and higher efficiencies of hL ¼ 53.8 cd Aꢀ1
,
of 95 800 cd mꢀ2, and peak efficiencies of hext ¼ 29.6%, hL
¼
hext ¼ 18.3% and hp ¼ 19.9 lm Wꢀ1 at 0.56 mA cmꢀ2. As illus-
trated by the efficiency–current density curves in Fig. 4b, all these
devices witness a gradual efficiency decay at the higher current
density region, which are typically attributed to a combination of
triplet–triplet annihilation and field-induced quenching effects.
However, at a high current density of 100 mA cmꢀ2, the effi-
ciencies still remain high at 8–15% (Table 2) for 1–4 based
devices. These data correspond to a loss of below 30% from the
maximum hext values and to a much lower loss of 15–25% from
the hext values at 20 mA cmꢀ2. This efficiency roll-off was
frequently observed especially for electrophosphorescent
OLEDs19,20 and the high hext losses of 40–45% from 20 to 100 mA
cmꢀ2 have been reported for orange iridium OLEDs.19 The
present orange OLEDs based on the iridium complexes 1–4 are
advantageous in the relatively slow efficiency decay. Wong
et al.21 reported in 2008 an orange phosphorescent iridium(III)
87.9 cd Aꢀ1 and hp ¼ 46.0 lm Wꢀ1 at 6 V, which represents
a tremendous increase of about 63% in both hext and hL values
with reference to those in 4-doped device I. Likewise, the device
II witnesses the efficiency roll-off with increasing current density.
It seems that the efficiencies decay more rapidly in PEDOT : PSS
based device II than in NPB based device I, especially in the
region close to the efficiency maxima. Nevertheless, the effi-
ciencies at 20 and 100 mA cmꢀ2 for each complex in device II are
still substantially higher than those in device I, except for 1 at
high current density region (Table 2). The better EL character-
istics of device II probably result from a more balanced electron
and hole transportation and more efficient recombination in the
emitting layer that is brought by using PEDOT : PSS as the hole-
injecting layer. Evidently, the EL efficiencies of device II derived
from 1 and 2 are better than those of the previously reported
orange-emitting OLEDs, and the extraordinary values from 4
clearly outweigh by ꢃ2 times those of the best results reported in
the literature.21,22 The energy level diagram of these OLEDs is
illustrated in Fig. S2 in the ESI†. The HOMO and LUMO
orbitals of all these iridium complexes are located within those of
the CBP host, implying that direct charge trapping is one
important excitation mechanism for these iridium dopants.
Therefore, both direct charge trapping and energy transfer
combine to be responsible for the high EL efficiencies of these
OLEDs.
complex with outstanding luminance efficiency of 40.2 cd Aꢀ1
,
Kim et al.22 then reported an orange emitting device with a very
high peak external quantum efficiency of 14.4%, both of which
represented the highest efficiencies for orange electro-
phosphorescence up to now. Apparently, the efficiencies of our
1–4 based orange devices I are superior to or comparable with
those highest reported so far.
By replacing the hole-transporting NPB layer with the hole-
injecting PEDOT : PSS 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 10 V (Table 2). This should result from the excellent
hole injecting ability and the superior hole transporting mobility
of the PEDOT : PSS layer as compared to NPB. As a result, the
device II exhibits overall improvements in the turn-on voltage,
It should be noted that the overall performances of the parent
complex 4 are better than those of other three derivatives 1–3 in
both devices I and II, with lower driving voltages and higher
luminance and especially remarkably higher efficiencies. This
probably results from the slightly higher luminescent quantum
yield of 4 as compared to 1–3. The result suggests that the
Table 2 Performance of IrIII complexes-doped electrophosphorescent OLEDsa
Lbmax/cd mꢀ2
hext (%)c
hext (%)b
b
hL /cd Aꢀ1
b
hP /lm Wꢀ1
Complex
lem/nm
CIE, 10 V (x, y)
Von/V
J10V/mA cmꢀ2
1
2
3
4
554, 590 sh
554, 590 sh
558, 594 sh
558, 594 sh
558, 600 sh
558, 600 sh
554, 590 sh
554, 590 sh
(0.50, 0.50)
(0.49, 0.50)
(0.51, 0.48)
(0.51, 0.48)
(0.52, 0.48)
(0.52, 0.47)
(0.49, 0.50)
(0.49, 0.50)
6.5
5.2
6.3
5.1
5.4
4.5
5.5
4.2
1.44
11.8
5.83
19.0
6.84
13.8
5.60
21.0
70 300 (15)
31 370 (13)
44 190 (14)
45 960 (14)
37 430 (16)
35 040 (14)
68 780 (15)
95 800 (14)
14.3, 11.5
15.9, 10.1
11.3, 9.62
14.8, 11.2
10.9, 8.22
11.3, 8.52
17.3, 14.8
23.7, 18.3
15.5 (11)
19.1 (8)
13.5 (9)
20.7 (7)
14.5 (7)
14.2 (6)
18.3 (10)
29.6 (6)
44.3 (11)
55.2 (8)
37.1 (9)
56.7 (7)
37.4 (7)
35.8 (6)
53.8 (10)
87.9 (6)
13.9 (9)
22.9 (7)
12.9 (9)
25.4 (7)
16.8 (7)
18.8 (6)
19.9 (8)
46.0 (6)
a
For each compound the first row gives the data for NPB-based device I, and the second row is for PEDOT : PSS-based device II. b Maximum values of
the devices. Values in parentheses are the voltages at which they were obtained. c Values collected at 20 mA cmꢀ2 and 100 mA cmꢀ2, respectively. sh ¼
shoulder.
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Fig. 4 J–V–L characteristics (a) and current efficiency–current density (b) curves for complex 1–4 in device I.
Fig. 5 J–V–L characteristics (a) and current efficiency–current density (b) curves for complex 1–4 in device II.
modification groups (CH3, OCH3, and F) on the ligands are rather
unimportant here in optimizing the EL performance for tris(2-
with 2-phenylbenzothiazole ligands. By using these orange-
emitting IrIII complexes as dopants, the OLEDs had very high
brightness and efficiencies, in which a brightness of 95 800 cd mꢀ2
and a maximum luminance efficiency of 87.9 cd Aꢀ1 (46.0 lm
Wꢀ1) were achieved, and these good performances represent
a significant improvement for vacuum-deposited orange OLEDs.
Given the ease of synthesis and performance advantages of
iridium complexes based on a 2-phenylbenzothiazole framework,
we are now working on the synthesis of the dendritic iridium
complexes based on the ligand framework and their application
in solution-processable and non-doped monochromatic or white
OLEDs.
phenylbenzothiozolato)iridium(III)
complexes.
This
has
happened in some other cases,23,24 although it is common that
introduction of simple groups as substituents usually has
a significant influence on the properties and performance of either
[Ir(C^N)3]- or [Ir(C^N)2(acac)]-type complex21,25,26 based OLEDs.
We particularly note that the external quantum efficiencies
(EQE) of 4-doped device II in our study is are high as 29.6%,
which is the highest ever reported for orange OLEDs, and even
comparable with the hext of 29% for green OLED reported in the
literature.27 It is generally accepted that the theoretical limit of
the forward viewing external quantum efficiency of OLEDs is
20%.28 However, devices doped with iridium complexes have
been reported to have external quantum efficiencies in the range
20–29% even without the use of any outcoupling tech-
niques,9,27,29–32 and it seems that these devices are operating at an
internal quantum efficiency (IQE) close to 100%. Importantly, it
is predicted that the EQE of OLEDs can be further improved to
30–50% considering that outcoupling efficiency in excess of 50%
is possible.33 In the present study the high phosphorescent
quantum yields of these iridium complexes should be the first
factor responsible for the excellent EL performance. In addition,
the utilization of TPBI as the hole/exciton blocking layer helps to
confine all the excitons in the emitting layer. Furthermore, the
good alignment of the triplet energy levels of iridium dopant and
the CBP host definitely facilitates the desired energy transfer
from the host to the dopant and eliminates the reverse energy
transfer back to the host, finally well confining the excitons on
the dopant emitters and leading to efficient electroluminescence.
Experimental
Instruments and methods
1H NMR and 13C NMR spectra were recorded 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 data were collected on a Bruker
SMART APEX CCD diffractometer with graphite-mono-
ꢀ
chromated Mo-Ka (l ¼ 0.71073 A) using the SMART and
SAINT programs. The photoluminescence and UV-vis absorp-
tion spectra measurements were performed on a Perkin-Elmer
LS55 spectrometer and a Perkin-Elmer Lambda 35 spectrapho-
tometer, respectively. Cyclic voltammograms of the these iridium
complexes were recorded on an electrochemical workstation
(BAS100B, USA) at room temperature in a 0.1 M [Bu4N]PF6
solution under nitrogen gas protection.
OLED fabrication and measurements
Conclusions
In conclusion, we have demonstrated for the first time the
synthesis of homoleptic tris-cyclometalated iridium complexes
The pre-cleaned ITO glass substrates (30 U ,ꢀ1) were treated
by UV-Ozone for 20 min. For device II, a 40 nm thick
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PEDOT : PSS film was first deposited on the ITO glass
substrates, and baked at 120 ꢁC for 30 min in air. All the organic
layers were deposited by vacuum evaporation in a vacuum
chamber with a base pressure less than 10ꢀ6 torr. The EL spectra,
CIE coordinates, and current–voltage–luminance characteristics
were measured with computer-controlled Spectrascan PR 705
photometer and a source-measure-unit Keithley 236 under
ambient conditions. The forward viewing external quantum
efficiency (hext) was calculated by using the luminance efficiency,
EL spectra and human photopic sensitivity.
Compound 1d: yield 65%. 1H NMR (400 MHz, CDCl3) d 8.03
(d, J ¼ 8.0 Hz, 2H), 7.61 (d, J ¼ 4.0 Hz, 2H), 7.58 (d, J ¼ 8.0 Hz,
2H), 7.18–7.13 (m, 2H), 6.88–6.84 (m, 2H), 6.66–6.62 (m, 2H),
6.39 (d, J ¼ 8.0 Hz, 2H), 5.14 (s, 1H), 1.75 (s, 6H). 13C NMR (100
MHz, CDCl3): d 186.1, 180.3, 161.8, 159.3, 148.1, 147.6, 141.9,
135.1, 130.3, 125.9, 121.4, 116.1, 109.0, 101.8, 53.6, 28.5.
MALDI-TOF-MS (m/z): 748.0 [M]+.
Compound 2d: yield 76%. 1H NMR (400 MHz, CDCl3) d 7.96
(d, J ¼ 8.4 Hz, 2H), 7.67 (s, 2H), 7.60 (d, J ¼ 7.6 Hz, 2H), 7.23 (d,
J ¼ 7.2 Hz, 2H), 6.85–6.79 (m, 2H), 6.63–6.59 (m, 2H), 6.40 (d,
J ¼ 7.6 Hz, 2H), 5.12 (s, 1H), 2.51 (s, 6H), 1.75 (s, 6H). 13C NMR
(100 MHz, CDCl3): d 185.7, 179.4, 149.1, 148.2, 142.1, 135.5,
135.1, 131.8, 129.9, 129.0, 125.7, 122.2, 121.3, 119.9, 101.6, 28.5,
21.6. MALDI-TOF-MS (m/z): 740.0 [M]+.
Compound 3d: yield 63%. 1H NMR (400 MHz, CDCl3) d 7.98
(d, J ¼ 12.0 Hz, 2H), 7.58 (d, J ¼ 8.0 Hz, 2H), 7.36 (s, 2H), 7.00
(d, J ¼ 8.0 Hz, 2H), 6.85–6.81 (m, 2H), 6.63–6.59 (m, 2H), 6.40
(d, J ¼ 8.0 Hz, 2H), 5.13 (s, 1H), 3.91 (s, 6H), 1.76 (s, 6H). 13C
NMR (100 MHz, CDCl3): d 185.8, 177.3, 157.0, 146.1, 144.6,
141.5, 133.4, 132.9, 129.5, 125.1, 121.8, 121.3, 116.2, 114.2, 104.9,
56.1, 28.5. MALDI-TOF-MS (m/z): 772.1 [M]+.
Materials synthesis
1a–3a, 1b–3b, and 4c were conveniently synthesized according to
the literature methods.34,35
General procedure for synthesis of 2-phenyl-6-substituted ben-
zothiazole (1c–3c). To compounds 1b–3b wetted with a small
amount of ethanol beforehand was added 30% aqueous sodium
hydroxide (8 mol equiv.). The mixture was diluted by water to
provide a final suspension of 10% aqueous sodium hydroxide.
The diluted sample was added within 5 minutes to a stirred
solution of potassium ferricyanide (4 equiv.) in water (20 w%) at
80–90 ꢁC. After reaction for 3 h and then cooling to room
temperature, the reaction solution was poured into water and
neutralized with dilute aqueous HCl. Extraction of the solution
with dichloromethane followed by evaporation of solvent
produced the crude product residue, which was isolated by
column chromatography over silica using petroleum ether (30–
60 ꢁC) and dichloromethane (4 : 1) as eluent to yield the pure
product 1c–3c as white solid.
Compound 1c: yield 84%. 1H NMR (400 MHz, CDCl3) d 8.07–
8.04 (m, 2H), 8.03–8.00 (m, 1H), 7.58 (d, J ¼ 8.0 Hz, 1H), 7.51–
7.49 (m, 3H), 7.22 (d, J ¼ 8.8 Hz, 1H). APCI-MS (m/z): 230
[M + H]+.
Compound 2c: yield 82%. 1H NMR (400 MHz, CDCl3) d 8.10–
8.08 (m, 2H), 7.97 (d, J ¼ 8.0 Hz, 1H), 7.70 (s, 1H), 7.51–7.48 (m,
3H), 7.31 (d, J ¼ 8.0 Hz, 1H), 2.51 (s, 3H). APCI-MS (m/z): 226
[M + H]+.
Compound 4d: the characterization spectroscopy data are
identical to those in the literature.6
General procedure for synthesis of complexes 1–4. A mixture of
1c (or 2c, 3c, 4c) (1.3 mmol), 1d (or 2d, 3d, 4d) (0.65 mmol), silver
trifluoromethane sulfonate (335 mg, 1.3 mmol), and diglyme
ꢁ
(0.6 mL) was stirred under a nitrogen atmosphere at 160 C for
20 h. The reaction was allowed to cool to room temperature, and
the mixture was purified by column chromatography over silica
gel using the mixture of dichloromethane and light petroleum
(1 : 3 to 1 : 1) as eluent to give pure 1 (or 2, 3, 4) as an orange
powder.
Tris(2-phenyl-6-fluorobenzothiozolato)iridium(III) (1). Yield
318 mg (76%). 1H NMR (400 MHz, CDCl3) d 7.64–7.62 (d, J ¼
8.0 Hz, 3H), 7.50–7.48 (d, J ¼ 8.0 Hz, 3H), 6.94–6.90 (m, 3H),
6.84–6.80 (m, 3H), 6.71–6.66 (m, 3H), 6.59–6.55 (m, 6H). 13C
NMR (100 MHz, CDCl3): d 179.2, 161.2, 158.7, 157.2, 148.7,
140.4, 136.2, 133.6, 131.4, 126.1, 120.8, 115.4, 109.2. MALDI-
TOF-MS (m/z): 877.0 [M]+. Anal. calcd for C39H21F3IrN3S3: C,
53.41; H, 2.41; N, 4.79; found: C, 53.14; H, 2.39; N, 4.51%.
Compound 3c: yield 88%. 1H NMR (400 MHz, CDCl3) d 8.05–
8.03 (m, 2H), 7.95 (d, J ¼ 8.8 Hz, 1H), 7.49–7.46 (m, 3H), 7.34
(s, 1H), 7.09 (d, J ¼ 8.8 Hz, 1H), 3.88 (s, 3H). APCI-MS (m/z):
242 [M + H]+.
Tris(2-phenyl-6-methylbenzothiozolato)iridium(III) (2). Yield
470 mg (80%). 1H NMR (400 MHz, CDCl3) d 7.62–7.60 (d, J ¼
8.0 Hz, 3H), 7.55 (s, 3H), 6.90–6.86 (m, 3H), 6.81–6.77 (m, 3H),
6.73–6.71 (d, J ¼ 8.0 Hz, 3H), 6.61–6.59 (d, J ¼ 8.0 Hz, 3H),
6.54–6.52 (d, J ¼ 8.0 Hz, 3H), 2.32 (s, 9H). 13C NMR (100 MHz,
CDCl3): d 178.1, 157.8, 150.4, 140.9, 136.2, 134.7, 132.7, 130.9,
128.4, 125.7, 122.1, 120.4, 119.2, 21.2. API-ES-MS (m/z): 866.0
[M + H]+. Anal. calcd for C42H30IrN3S3: C, 58.31; H, 3.50; N,
4.86; found: C, 58.09; H, 3.41; N, 4.69%.
General procedure for synthesis of the iridium complexes (1d–
4d). IrCl3$3H2O (1 mmol) and the ligands 1c–4c (2.3 mmol) were
added in a 16 mL of mixture of 2-ethoxyethanol and water (v/v ¼
3 : 1). The mixture was refluxed under nitrogen for 24 h and
cooled to room temperature. The orange precipitate was
collected by filtration and washed with water, ethanol and
hexane. The solid was then pumped dry completely to give the
crude chloro-bridged dimer complex. Without further purifica-
tion, the dimer was added to a mixture of Na2CO3 (5 mmol),
acetyl acetone (1.5 mmol), and 2-ethoxyethano (10 mL). After
refluxing under nitrogen for 24 h, the solution was cooled to
room temperature. The orange precipitate was filtered off and
washed with water, ethanol and hexane. The crude product was
purified by chromatography on silica gel using petroleum ether/
dichloromethane as mobile phase.
Tris(2-phenyl-6-methoxybenzothiozolato)iridium(III) (3). Yield
430 mg (72%). 1H NMR (400 MHz, CDCl3) d 7.76 (s, 3H), 7.65–
7.63 (d, J ¼ 8.0 Hz, 3H), 6.88–6.84 (m, 3H), 6.74–6.71 (m, 3H),
6.67–6.65 (d, J ¼ 8.0 Hz, 3H), 6.52–6.50 (d, J ¼ 8.0 Hz, 3H), 6.42–
6.40 (d, J ¼ 8.0 Hz, 3H), 3.74 (s, 9H). 13C NMR (100 MHz,
CDCl3): d 176.6, 157.3, 152.8, 146.7, 136.3, 134.0, 133.0, 130.4,
This journal is ª The Royal Society of Chemistry 2011
J. Mater. Chem., 2011, 21, 15494–15500 | 15499

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11 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.
12 I. R. Laskar and T.-M. Chen, Chem. Mater., 2004, 16, 111.
13 R. Wang, D. Liu, H. Ren, T. Zhang, H. Yin, G. Liu and J. Li, Adv.
Mater., 2011, 23, 2823.
14 J. Li and D. Liu, J. Mater. Chem., 2009, 19, 7584.
15 P. L. Burn, S.-C. Lo and I. D. W. Samuel, Adv. Mater., 2007, 19, 1675.
16 S. Lamansky, P. Djurovich, D. Murphy, F. Abdel-Razzaq,
R. Kwong, I. Tsyba, M. Bortz, B. Mui, R. Bau and
M. E. Thompson, Inorg. Chem., 2001, 40, 1704.
17 M. C. DeRosa, D. J. Hodgson, G. D. Enright, B. Dawson,
C. E. B. Evans and R. J. Crutchley, J. Am. Chem. Soc., 2004, 126,
7619.
125.2, 121.1, 120.2, 115.5, 105.6, 55.8. API-ES-MS (m/z): 914.0 [M
+ H]+. Anal. calcd for C42H30IrN3O3S3: C, 55.24; H, 3.31; N, 4.60;
found: C, 55.12; H, 3.19; N, 4.44%.
Tris(2-phenylbenzothiozolato)iridium(III) (4). Yield 336 mg
(65%). 1H NMR (400 MHz, CDCl3) d 7.80–7.78 (d, J ¼ 8.0 Hz,
3H), 7.67–7.65 (d, J ¼ 8.0 Hz, 3H), 7.20–7.16 (m, 3H), 6.92–6.86
(m, 6H), 6.83–6.79 (m, 6H), 6.68–6.66 (d, J ¼ 8.0 Hz, 3H), 6.61–
6.59 (d, J ¼ 8.0 Hz, 3H). 13C NMR (100 MHz, CDCl3): d 179.2,
157.8, 152.3, 140.7, 136.2, 132.5, 131.2, 127.0, 126.1, 124.6, 122.4,
120.5, 119.7. MALDI-TOF-MS (m/z): 822.8 [M]+. Anal. calcd
for C39H24IrN3S3: C, 56.91; H, 2.94; N, 5.11; found: C, 56.78; H,
2.79; N, 5.02%.
18 S.-C. Lo, R. N. Bera, R. E. Harding, P. L. Burn and I. D. W. Samuel,
Adv. Funct. Mater., 2008, 18, 3080.
19 W.-Y. Wong, G.-J. Zhou, X.-M. Yu, H.-S. Kwok and B.-Z. Tang,
Adv. Funct. Mater., 2006, 16, 838.
20 H. Wu, G. Zhou, J. Zou, C.-L. Ho, W.-Y. Wong, W. Yang, J. Peng
and Y. Cao, Adv. Mater., 2009, 21, 4181.
Acknowledgements
21 C.-L. Ho, W.-Y. Wong, Q. Wang, D. Ma, L. Wang and Z. Lin, Adv.
Funct. Mater., 2008, 18, 928.
22 D.-S. Leem, S. O. Jung, S.-O. Kim, J.-W. Park, J. W. Kim, Y.-S. Park,
Y.-H. Kim, S.-K. Kwon and J.-J. Kim, J. Mater. Chem., 2009, 19,
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23 D. M. Kang, J.-W. Kang, J. W. Park, S. O. Jung, S.-H. Lee,
H.-D. Park, Y.-H. Kim, S. C. Shin, J.-J. Kim and S.-K. Kwon,
Adv. Mater., 2008, 20, 2003.
We thank the National Natural Science Foundation of China
(U0634003, 20704002, 20923006, and 21072026), the Ministry of
Education for the New Century Excellent Talents in University
(Grant NCET-08-0074), the NKBRSF (2009CB220009), and the
Fundamental Research Funds for the Central Universities
(DUT10LK16) for financial support of this work.
24 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.
25 V. V. Grushin, N. Herron, D. D. LeCloux, W. J. Marshall,
V. A. Petrov and Y. Wang, Chem. Commun., 2001, 1494.
26 S. Jung, Y. Kang, H.-S. Kim, Y.-H. Kim, C.-L. Lee, J.-J. Kim,
S.-K. Lee and S.-K. Kwon, Eur. J. Inorg. Chem., 2004, 3415.
27 D. Tanaka, H. Sasabe, Y.-J. Li, S.-J. Su, T. Takeda and J. Kido, Jpn.
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15500 | J. Mater. Chem., 2011, 21, 15494–15500
This journal is ª The Royal Society of Chemistry 2011