Color-tunable thiazole-based iridium(III) complexes: Synthesis, characterization and their OLED applications

Article abstract of DOI:10.1016/j.jorganchem.2016.11.018

With respect to the commonly used electron-deficient pyridyl group in the benchmark dopant Ir(ppy)₃, incorporating the electron-rich thiazolyl group with different chromophores have not been extensively studied. In this paper, some iridium(III) complexes bearing functional ligands with the thiazolyl moiety were synthesized and characterized by¹H and¹³C NMR, UV–Vis absorption and photoluminescence spectroscopy. The emission color of these thiazole-based Ir(III) complexes can be tuned from yellow to red and the best phosphorescent organic light-emitting device exhibited the maximum external quantum efficiency of 11.1%, current efficiency of 35.8 cd/A and power efficiency of 21.9 lm/W.

Full text of DOI:10.1016/j.jorganchem.2016.11.018

Accepted Manuscript
Color-tunable thiazole-based iridium(III) complexes: Synthesis, characterization and
their OLED applications
Nga-Yuen Chau, Po-Yu Ho, Cheuk-Lam Ho, Dongge Ma, Wai-Yeung Wong
PII:
S0022-328X(16)30522-8
10.1016/j.jorganchem.2016.11.018
JOM 19703
DOI:
Reference:
To appear in: Journal of Organometallic Chemistry
Received Date: 17 October 2016
Revised Date: 8 November 2016
Accepted Date: 9 November 2016
Please cite this article as: N.-Y. Chau, P.-Y. Ho, C.-L. Ho, D. Ma, W.-Y. Wong, Color-tunable thiazole-
based iridium(III) complexes: Synthesis, characterization and their OLED applications, Journal of
Organometallic Chemistry (2016), doi: 10.1016/j.jorganchem.2016.11.018.
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ACCEPTED MANUSCRIPT
New thiazole-based iridium(III) phosphors were synthesized. Their emission colors could be
tuned from yellow to red. The best OLED exhibited a maximum external quantum efficiency of
11.1%, current efficiency of 35.8 cd/A and power efficiency of 21.9 lm/W.

ACCEPTED MANUSCRIPT
Color-tunable thiazole-based iridium(III) complexes: synthesis,
characterization and their OLED applications
Nga-Yuen Chaua, Po-Yu Hoa,b, Cheuk-Lam Hoa,b*, Dongge Mac*, Wai-Yeung Wonga,b,d
*
a
Institute of Molecular Functional Materials, Department of Chemistry and Institute of
Advanced Materials, Hong Kong Baptist University, Waterloo Road, Kowloon Tong, Hong Kong,
PR China
b
HKBU Institute of Research and Continuing Education, Shenzhen Virtual University Park,
Shenzhen, 518057, PR China
c
State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied
Chemistry, Chinese Academy of Sciences, Changchun 130022, PR China
d
Department of Applied Biology and Chemical Technology, The Hong Kong Polytechnic
University, Hung Hom, Hong Kong, PR China
* Corresponding authors. Tel.: +852 3411 7074; fax: +852 3411 7348.
E-mail addresses: clamho@hkbu.edu.hk (C.-L. Ho), mdg1014@ciac.jl.cn (D. Ma),
rwywong@hkbu.edu.hk; wai-yeung.wong@polyu.edu.hk (W.-Y. Wong).
Keywords: Organic light-emitting diodes; Iridium; Thiazole; Phosphorescence; Color tuning
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ABSTRACT
With respect to the commonly used electron-deficient pyridyl group in the benchmark dopant
Ir(ppy)3, incorporating the electron-rich thiazolyl group with different chromophores have not
been extensively studied. In this paper, some iridium(III) complexes bearing functional ligands
1
with the thiazolyl moiety were synthesized and characterized by H and 13C NMR, UV-Vis
absorption and photoluminescence spectroscopy. The emission color of these thiazole-based
Ir(III) complexes can be tuned from yellow to red and the best phosphorescent organic light-
emitting device exhibited the maximum external quantum efficiency of 11.1%, current efficiency
of 35.8 cd/A and power efficiency of 21.9 lm/W.
2

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1. Introduction
With organic light-emitting diode (OLED) already making a large footprint in the market
of flat panel displays and solid-state lighting, true red, green and blue emission of sufficient
luminous efficiencies and proper chromaticity are required [1, 2]. At present, many red organic
dyes in use do not show a good compromise between device efficiency and color purity [3-5]. By
consideration of the strong spin-orbit coupling induced by central heavy metal atom, iridium(III)
complexes have been considered as a good phosphor with high quantum efficiency [6-10]. The
substituent effect can notably afford the versatile phosphorescence color tuning of the complexes
from blue to red [11-13]. To our knowledge, the thiazole moiety is a heterocyclic molecule
featuring one nitrogen and one sulfur atom in the aromatic ring [14-16]. It is uncommonly used
in organic semiconducting materials [17]. With respect to the commonly used electron-deficient
pyridyl group in the benchmark dopant Ir(ppy)3, the thiazolyl group with different chromophores
such as carbazole and fluorene moieties has not been extensively studied.
In our present study, thiazole-based iridium(III) complexes with different hole-transporting
1
moieties were synthesized and characterized by H and 13C NMR spectroscopy (Fig. 1). The
photophysical and electrochemical properties of these complexes were also investigated. It is
known that the substitution of pyridine by thiazole as an electron-accepting unit would lower the
frontier molecular orbital energy levels of donor-acceptor alternating conjugated materials [18].
A slight downshift of HOMO and LUMO levels was observed as the energy bandgap was
narrowed. On account of different hole-transporting mobility, complexes 1−5 with fluorene or
carbazole component can exhibit a diverse color emission. For the compounds 1 and 3, the hole-
transporting fluorene moiety is attached to the thiazole ring in its cyclometalating ligand L1. The
introduction of two ethyl groups at 9-position of fluorene ring is beneficial to the solubility of the
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phosphor.
Nowadays, carbazole unit has been widely used as a hole-transporting group in the
chromophore [19-22]. The substitution position of carbazole moiety can vary the emission color
dramatically [23, 24]. From our metal complexes 3, 4 and 5, the emission color could be easily
tuned from yellow to red. Compared with the 3’-substituted one, iridium(III) complex bearing
2’-substituted carbazole ligand is more red-shifted in emission due to the degree of π-electron
conjugation extension.
Fig. 1. Molecular structures of compounds 1−5.
2. Results and discussion
2.1. Synthesis of cyclometalated thiazole-based iridium(III) complexes
The synthetic pathway of L1 is outlined in Scheme 1. 2-Bromo-9,9-diethylfluorene was
firstly prepared by the reaction of 2-bromofluorene and bromoethane in dimethyl sulfoxide
solution at room temperature. After the addition of n-butyllithium and trimethyl borate, the
brominated compound was changed into boronic acid. It was then allowed to react with 2-
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bromothiazole to obtain the 2-(9,9-diethyl-9H-fluoren-2-yl)thiazole ligand L1. The synthetic
pathway of ligand L2 is displayed in Scheme 2. 9-Phenyl-9H-carbazole was synthesized through
Ullmann coupling of carbazole and iodobenzene in the presence of copper. Following the
bromination, 3-bromo-9-phenyl-9H-carbazole was converted into boronic acid and subsequently
treated with 2-bromothiazole to get 2-(9-phenyl-9H-carbazol-3-yl)thiazole L2. As for 2-(9-
phenyl-9H-carbazol-2-yl)thiazole ligand L3, its synthetic procedure is different from that of L2.
As reported in the literature [25, 26], 1,4-dibromo-2-nitrobenzene was firstly reacted with
phenylboronic acid under the Suzuki coupling reaction condition. The 4-bromo-2-nitro-1-
phenylbenzene was formed, which was subjected to reflux in triethylphosphite to yield 2-
bromocarbazole. After consecutive reactions with n-butyllithium and trimethylborate, 2-
bromocarbazole was changed to its boronic acid derivative. It was then reacted with 2-
bromothiazole to obtain the ligand L3.
nBuLi
B(OMe)3
-78 oC
CH3CH2Br
Br
Br
-78 oC
DMSO, NaOH
r.t., overnight
123
123
N
S
Br
S
2 M HCl
B(OH)2
N
Pd(PPh3)4
2 M Na2CO3, THF
reflux
L1
123
Scheme 1. Synthetic route of ligand L1.
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Scheme 2. Synthetic route of ligand L2.
Scheme 3. Synthetic route of ligand L3.
With the ligands L1−L3 in hand, thiazole-based Ir(III) complexes 1−5 were synthesized by
the reported procedures [17]. Shown in Scheme 4, homoleptic complexes 1 and 2 were
synthesized by refluxing the ligand with Ir(acac)3 in glycerol under nitrogen. On the other hand,
heteroleptic complexes 3−5 were prepared by reacting IrCl3·nH2O with the corresponding ligand,
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followed by the deprotonation of acetylacetone. All these compounds were purified through
column chromatography with silica gel and preparative TLC plates. Attempts to prepare the
homoleptic analogue of 5 has met with little success due to the purification problem. All the
1
thiazole-based ligands and their respective complexes were characterized by H and 13C NMR
spectroscopy.
Scheme 4. Synthetic route of cyclometalated iridium(III) complexes 1−5.
2.2. Materials characterization
2.2.1. Photophysical properties
The absorption spectra of ligands L1−L3 and their corresponding metal complexes 1−5
were performed in the CH2Cl2 solution at room temperature and shown in Fig. 2 and 3,
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respectively. The intense absorption bands in the UV region for both complexes were attributed
to the π−π* transition in the ligand, which was also located in the organic ligand. The lower
energy bands in the visible region were generally ascribed to the metal-to-ligand charge transfer,
spin-allowed and spin-forbidden ligand-based π−π* transitions. These absorption peaks were
observed in the metal complexes 1−5 only.
L1
L2
1.0
L3
0.8
0.6
0.4
0.2
0.0
250 300 350 400 450 500 550 600 650 700
Wavelength (nm)
Fig. 2. UV-Vis absorption spectra of ligands L1−L3 in CH2Cl2 at 298 K.
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1.0
0.8
0.6
0.4
0.2
0.0
1
2
3
4
5
250 300 350 400 450 500 550 600 650 700
Wavelength (nm)
Fig. 3. UV-Vis absorption spectra of complexes 1−5 in CH2Cl2 at 298 K.
The emission features of these thiazole-based ligands L1−L3 were studied in
dichloromethane solution at room temperature (Fig. 4). They generally emitted blue light. With
respect to the metal complexes 1−5, their emission spectra were investigated in CH2Cl2 solution
at 298 K and 77 K, respectively (Figs. 5 and 6). It is known that the emission in the organic
ligand results from π−π* transition while the emission in iridium(III) complexes arises from
metal-to-ligand charge transfer commonly. All these compounds emitted phosphorescence from
yellow to red at room temperature. After cyclometalation with iridium, their emissions were
remarkably red-shifted. The broad and structureless emission bands were observed at 298 K. It is
3
believed that these compounds possess spin-forbidden MLCT excited states rather than ligand-
based π−π transitions at room temperature[27]. As to 77 K, the slightly blue-shifted and more
structured emission spectra were identified. The vibronic structures found in the emission spectra
signify that the emissions considerably originate from the mixing of 3MLCT excited state as well
as ligand-based 3π−π* transition.
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On changing the hole-transporting unit from fluorene to carbazole in the ligands, a
noticeable shift of the emission peak of the complexes at 298 K and 77 K was perceived. The
emission peak of complex 5 containing 2’-substituted carbazole appears at 611 nm, which falls in
the red region. By adjusting the carbazole substitution from 2’-position to 3’-position, the
emission band showed a blue shift of 64 nm.
L1
L2
L3
1.0
0.8
0.6
0.4
0.2
0.0
300
350
400
450
500
550
Wavelength (nm)
Fig. 4. Photoluminescence spectra of ligands L1−L3 in CH2Cl2 at 298 K.
1.0
0.8
0.6
0.4
0.2
0.0
At 298 K
1
2
3
4
5
500
550
600
650
700
750
Wavelength (nm)
Fig. 5. Photoluminescence spectra of complexes 1−5 in CH2Cl2 at 298 K.
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1.0
0.8
0.6
0.4
0.2
0.0
At 77K
1
2
3
4
5
550
600
650
700
750
Wavelength (nm)
Fig. 6. Photoluminescence spectra of complexes 1−5 in CH2Cl2 at 77 K.
The phosphorescence quantum yields of these complexes were determined in degassed
dichloromethane solution at room temperature and fac-Ir(ppy)3 was used as the reference
standard (Φp = 0.40) (Table 1). Their quantum yields range between 0.26 and 0.49. For the
phosphorescence lifetime measurements, they were performed in degassed toluene solution
(Table 1). These excited state lifetimes in the order of microseconds are the recognition of
phosphorescence emission.
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Table 1
Photophysical data of the complexes 1−5
λmaxa (nm) λemb (nm) at 293 K
c
d
λemb (nm) at 77 K
ET (eV) at 77 K Φp
τe (ꢀs)
328 (4.7)
344 (4.1)
409 (1.4)
309 (6.2)
344 (4.2)
398 (1.5)
377 (1.9)
404 (4.8)
489 (2.3)
322 (6.0)
390 (1.8)
455 (0.5)
280 (4.2)
326 (4.3)
420 (1.0)
578, 625(sh)
541, 573(sh)
587, 633(sh)
547, 579(sh)
611
578, 601(sh), 631(sh)
2.15
2.33
2.14
2.30
2.02
0.35 1.40
0.49 1.28
0.47 1.47
0.29 1.40
0.26 1.21
1
2
3
4
5
533, 550(sh), 574(sh)
579, 600(sh), 630(sh)
540, 558(sh), 582(sh)
615, 670(sh)
a Absorption measured in CH2Cl2 at 293 K, extinction coefficients (104 M-1 cm-1) are shown in
parentheses.
b Emission measured in CH2Cl2. sh = shoulder peak.
c Triplet energy measured from emission maximum in CH2Cl2 at 77 K.
d
Phosphorescence quantum yield measured in degassed CH2Cl2 at 293 K using fac-Ir(ppy)3 as
the reference standard.
e Phosphorescence lifetime measured in degassed toluene.
2.2.2. Electrochemical properties
To study the electrochemical behavior of these five complexes 1−5, cyclic voltammetry
was employed and ferrocenium/ferrocene (Fc/Fc+) was used as an internal standard. A set of
electrodes consisting a carbon glassy working electrode, a Ag/AgCl reference eletrode as well as
a platinum wire counter electrode was used. All these compounds were measured in
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tetrahydrofuran (THF) solution while tetrabutylammonium perchlorate was used a supporting
electrolyte.
Upon an anodic sweep in THF solution, all compounds showed a reversible one-electron
oxidation process (Table 2). These reversible peaks could be attributed to the Ir(III)/Ir(IV) metal-
based oxidation. Towards a cathodic sweep, all compounds have an irreversible wave. Their
calculated HOMO and LUMO energies are in the range of −4.97 to −5.15 and −2.27 to −2.39,
respectively.
It is found that complex 5 has a narrower HOMO-LUMO band gap than other four
complexes, and has the HOMO and LUMO energies of −4.96 and −2.30 eV, respectively.
Consistent with the photoluminescence measurements, 5 has the most red-shifted emission
among all these iridium(III) complexes. As for complex 4, its respective HOMO and LUMO
energies are located at −5.14 eV and −2.35 eV. In line with the literature report [28], the ligation
of Ir(III) ion to more electron-rich C-3 position of carbazole unit in 5 will increase the energy of
metal d-orbital, which gives rise to a higher HOMO energy for complex 5. As the electron-rich
nitrogen atom of carbazole is at para position to the metal center, electrons will go through the
aromatic ring to the iridium atom. This will raise the HOMO level and lower the oxidation
potential. By contrast, the ligation to the less electron-rich C-2 position will not make a
significant effect on the metal d-orbital energy. Therefore, complex 4 has a more positive
oxidation potential than 5.
From the results, complexes 1 and 3 with fluorene moiety have larger HOMO-LUMO
energy gaps, suggesting that they are more blue-shifted in the PL emission. Compared with their
homoleptic complexes 1 and 2, complexes 3 and 4 containing acetylacetonate ancillary ligand
have narrower energy gaps. In agreement with their PL spectra, the emission wavelengths of 3
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and 4 are more red-shifted than those of 1 and 2.
Table 2
Electrochemical studies of complexes 1−5 at room temperature
d
Eoxa (V)
Ereda (V)
−2.41
−2.53
−2.44
−2.45
−2.50
HOMOb (eV) LUMOc (eV) Eg (eV) Eg,opte (eV)
0.35
2.76
2.84
2.76
2.79
2.67
2.36
2.50
2.34
2.47
2.19
1
2
3
4
−5.15
−5.11
−5.12
−5.14
−4.97
−2.39
−2.27
−2.36
−2.35
−2.30
0.31
0.32
0.34
0.17
5
a
Determined from the onset of oxidation and reduction, and referenced to the ferrocene couple
in dichloromethane (Fc/Fc+, −4.8 eV).
b HOMO = − (4.8 − Eox).
c LUMO = − (4.8 + Ered).
d Eg = − (HOMO − LUMO).
e Optical band gap estimated from the absorption edge.
2.2.3. Thermal analysis
The thermal stabilities of these five compounds 1−5 were examined by thermogravimetric
analysis (TGA). The temperature at 5% weight-loss was recorded and listed in Table 3. The
o
decomposition temperatures of these complexes are in the range of 285 to 382 C. They are
thermally stable, and so they can be applicable for device fabrication using thermal vapor
deposition.
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Table 3
Thermal analysis of complexes 1−5.
a
Complex
Td (oC)
370
1
2
3
4
5
382
325
315
285
a Td is the 5% weight-loss temperature.
2.3. Electroluminescent properties
To study the electroluminescence (EL) properties of complexes 1−5, devices D1−D8 with
different doping concentration were fabricated and deposited by vacuum thermal evaporation.
The device configuration used in this work was ITO/MoO3 (10 nm)/NPB (60 nm)/TCTA (8
nm)/TCTA:Ir(III) complex (10 nm)/TPBi (35 nm)/LiF (1 nm)/Al (Fig. 7), where doping
concentrations of metal complexes were 5% and 8% and TCTA was a hole-transporting host
material.
Fig. 7. (a) Device architecture and molecular structures of the key compounds involved in OLED
devices D1−D8 and (b) energy levels of the device configuration.
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(b)
(a)
600
500
400
300
200
100
0
13
Device D3
Device D4
104
103
Device D3
Device D4
10
8
5
102
3
101
0
100
14
2
4
6
8
10
12
0
100
200
300
400
500
600
2
Voltage (V)
Current Density
(
mA/cm )
Fig. 8. (a) The current density-voltage-luminance (J-V-L) characteristics and (b) the external
quantum efficiencies (EQE) as a function of current density for devices D3 and D4.
The current and power efficiencies (ηL and ηP) of devices doped with the same complex
were generally increased with the doping concentration used[29, 30]. For devices D1 and D2, an
increase on doping concentration will raise the external quantum efficiencies (ηext) from 4.69%
to 5.62%. Complex 2 showed the best electroluminescence performance (Fig. 8). Device D4 with
8 wt% 2 showed a maximum ηL of 35.8 cd/A, a maximum ηext of 11.1% and a maximum ηP of
21.9 lm/W. Its emission peak was observed at 544 nm, resulting in an orange emission. As shown
in Fig. 9, the EL spectra of D4 doped with 8 wt% 2 was in line with its PL spectrum. This
implied that these two devices displayed pure EL emission of dopant without any emission from
TCTA host material. The EL emission is derived from the triplet excited state of the
metallophosphors. Besides, the energy transfer from TCTA host to the dopant was sufficient [31].
As displayed in Fig. 10, the EL emission of device D4 doped with 8 wt% complex 2 is not
affected by varying the driving voltage from 4 V to 11 V.
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1.0
0.8
0.6
0.4
0.2
0.0
EL
PL
450
500
550
600
650
700
750
Wavelength (nm)
Fig. 9. EL spectra of device D4 doped with complex 2 at 8 wt%, along with the PL spectrum of
complex 2 for comparison
.
1.0
0.8
0.6
0.4
0.2
0.0
4 V
5 V
6 V
7 V
8 V
9 V
10 V
11 V
400 450 500 550 600 650 700 750
Wavelength (nm)
Fig. 10. EL spectra of D4 at various driving voltage.
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D1
D2
D3
D4
D5
D6
D7
D8
1.0
0.8
0.6
0.4
0.2
0.0
400
450
500
550
600
650
700
750
Wavelength (nm)
Fig. 11. EL spectra of devices D1−D8 at 8 V.
Among these, device D8 doped with 8 wt% 5 exhibited red photoluminescence, with the
emission peak at 616 nm. However, the major emission peak was found at 424 nm from its EL
spectrum. In view of this phenomenon, a device using TCTA was also fabricated for comparison
(Fig. 12). It indicated that the emission from device D8 is mainly due to the host materials.
EL of TCTA
EL of D8
PL of 5
1.0
0.8
0.6
0.4
0.2
0.0
400
450
500
550
600
650
700
750
Wavelength (nm)
Fig. 12. EL and PL spectra of device D8 and TCTA.
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From the electroluminescent data, complexes 2 and 4 with 3’-substituted carbazole
moieties are more efficient electrophosphors than 1 and 3 with fluorene moieties (Fig. 13 and
Table 4). As carbazole has good hole-transporting property, it may better help charge
transportation and exciton formation [19].
40
D1
D2
D3
D4
D5
30
D6
D7
D8
20
10
0
0
2000
4000
6000
8000
10000
2
Luminance (cd/m )
Fig. 13. Plots of luminous efficiencies as a function of luminance for D1−D8.
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Table 4
Electroluminescence data of complexes 1−5 doped phosphorescent OLEDs
Device Complex EQEa (%)
(wt%)
Current
Power
Brightnessa
(cd/m2)
b
EL λem (nm)
efficiencya efficiencya
(cd/A)
8.82
(lm/W)
4.63
4.69
5.62
9.24
11.08
2.64
3.27
4.20
2.61
9608
6795
19905
24923
4037
5486
4765
1977
584
(0.55, 0.39)
584
(0.57, 0.40)
544
(0.44, 0.52)
544
(0.44, 0.52)
592
(0.56, 0.35)
592
(0.57, 0.36)
462, 544
(0.46, 0.53)
422, 616
(0.46, 0.43)
D1
D2
D3
D4
D5
D6
D7
D8
1
(5%)
1
(8%)
2
(5%)
2
(8%)
3
(5%)
3
(8%)
4
(8%)
5
(8%)
10.64
30.32
35.82
4.98
7.11
19.47
21.92
2.52
4.03
2.85
10.95
3.03
11.34
2.88
a Maximum values obtained from the devices.
b CIE coordinates (x, y) obtained at 8 V.
3. Concluding remarks
Five thiazole-based iridium(III) complexes were synthesized and characterized. The
emission color was tuned from yellow to red by varying the aromatic moiety in the ligand. In this
study, we used thiazole in substitution for the typical pyridine ring, which could give a fair
performance on the OLED application. Complexes 3 and 4 with a thiazole ring attached to the
3’-substituted carbazole unit exhibited more brilliant OLED performance than compounds 1 and
2, in which the fluorene was used instead. Device D4 doped with 8 wt% 2 showed the maximum
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ηext of 11.1%, ηL of 35.8 cd/A and ηP of 21.9 lm/W. Seemingly, for 3 and 4, the charge transport
between carbazole and thiazole moieties in the ligand as well as the charge transfer with iridium
metal atom might be more effective, leading to the better device efficiencies.
4. Experimental
4.1. Synthesis of L1
2-Bromo-9,9-diethylfluorene was firstly prepared by the reaction of 2-bromofluorene (2.0
g, 8.160 mmol) and bromoethane (2.4 ml, 0.033 mol) in dimethyl sulfoxide solution at room
temperature. After treatment with n-butyllithium (2.4 M, 7.180 mmol) and trimethyl borate (2.2
ml, 0.020 mol), the brominated compound (1.97 g, 6.528 mmol) was changed into its boronic
acid derivative. A mixture of 2-bromothiazole (0.58 g, 3.513 mmol), the respective boronic acid
(1.33 g, 5.018 mmol) and Pd(PPh3)4 (184 mg, 0.159 mmol) was refluxed in THF solution (40 ml)
and 2 M Na2CO3. The ligand 2-(9,9-diethyl-9H-fluoren-2-yl)thiazole L1 was obtained as a white
1
solid (68%). Spectral data: MS (MALDI-TOF): m/z 305.44 (M+). H NMR (CDCl3): δ (ppm)
8.00 (s, 1H, Ar), 7.93−7.91 (m, 1H, Ar), 7.89 (d, J = 4.0 Hz, 1H, Ar), 7.77−7.73 (m, 2H, Ar),
7.36−7.35 (m, 3H, Ar), 7.33 (d, J = 4.0 Hz, 1H, Ar), 2.16−2.04 (m, 4H, CH2), 0.33 (t, J = 7.4 Hz,
6H).
4.2. Synthesis of L2
9-Phenyl-9H-carbazole was prepared through Ullmann coupling of carbazole (2.0 g, 0.012
mol) and iodobenzene (2.0 ml, 0.018 mol) with a catalytic amount of copper iodide (1.59 g,
8.372 mmol) and 1,10-phenanthroline (1.66 g, 8.372 mmol). 9-Phenyl-9H-carbazole (2.04 g,
8.372 mmol) reacted with NBS (1.56g, 8.791 mmol) to give 3-bromo-9-phenyl-9H-carbazole
21

ACCEPTED MANUSCRIPT
(2.67 g, 8.287 mmol) which was then converted into boronic acid (0.95 g, 3.315 mmol) and
subsequently reacted with 2-bromothiazole (0.24 ml, 2.652 mmol). The 2-(9-phenyl-9H-
carbazol-3-yl)thiazole ligand L2 was formed as a pale yellow powder (56%). Spectral data: MS
1
(MALDI-TOF): m/z 326.41 (M+). H NMR (CDCl3): δ (ppm) 8.78 (d, J = 1.4 Hz, 1H, Ar), 8.22
(d, J = 6.9 Hz, 1H, Ar), 8.03−8.01 (m, 1H, Ar), 7.88 (d, J = 3.2 Hz, 1H, Ar), 7.65−7.61 (m, 2H,
Ar), 7.59−7.56 (m, 2H, Ar), 7.52−7.48 (m, 1H, Ar), 7.45−7.40 (m, 3H, Ar), 7.35−7.32 (m, 1H,
Ar), 7.30 (d, J = 3.2 Hz, 1H, Ar).
4.3 Synthesis of L3
1,4-Dibromo-2-nitrobenzene (4.60 g, 0.016 mol) and phenylboronic acid (2.0 g, 0.016 mol)
was refluxed in toluene (40 ml) with a catalytic amount of Pd(PPh3)4 (568 mg, 0.492 mmol). 4-
Bromo-2-nitro-1-phenylbenzene (3.17 g, 0.011 mol) was formed and then refluxed in
triethylphosphite to yield 2-bromocarbazole. 2-Bromocarbazole (1.27 g, 5.162 mmol) and
iodobenzene (0.87 ml, 7.741 mmol) were refluxed to yield 2-bromophenylcarbazole. After
consecutive reactions with n-butyllithium and trimethylborate, boronic acid (0.622g, 2.167 mmol)
prepared would react with 2-bromothiazole (0.16 ml, 1.734 mmol). The ligand L3 was obtained
as a white powder (60%). Spectral data: MS (MALDI-TOF): m/z 326.36 (M+). 1H NMR (CDCl3):
δ (ppm) 8.17 (t, J = 8.8 Hz, 2H, Ar), 8.05 (s, 1H, Ar), 7.87−7.85 (m, 2H, Ar), 7.66−7.52 (m, 2H,
Ar), 7.52−7.48 (m, 2H, Ar), 7.46−7.38 (m, 2H, Ar), 7.33−7.29 (m, 2H, Ar).
4.4 Synthesis of cyclometalated iridium(III) complexes
4.4.1. General procedures for the synthesis of homoleptic cyclometalated iridium(III) complexes
According to the literature report [17], the homoleptic cyclometalated Ir(III) complexes
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were synthesized by a one-pot reaction. A mixture of the corresponding cyclometalating ligand
and [Ir(acac)3] was dissolved in degassed glycerol and heated at 220 oC for 24 h under nitrogen.
After cooling to room temperature, the reaction mixture was diluted with D.I. water and
extracted with CH2Cl2 three times. Subsequently, the organic layer was dried over Na2SO4 and
concentrated under reduced pressure. The complex was purified by column chromatography over
silica gel eluting with a mixture of CH2Cl2/hexane solution.
1
1: Orange powder (36%). Spectral data: MS (MALDI-TOF): m/z 1105.38 (M+). H NMR
(CDCl3): δ (ppm) 7.47 (s, 3H, Ar), 7.22 (d, J = 8.0 Hz, 3H, Ar), 7.17−7.13 (m, 9H, Ar), 7.11 (d, J
= 4.0 Hz, 3H, Ar), 7.06 (d, J = 4.0 Hz, 3H, Ar), 7.01−6.97 (m, 3H, Ar), 2.06−1.99 (m, 12H, CH3),
0.49 (t, J = 8.0 Hz, 9H, CH2), 0.37 (t, J = 8.0 Hz, 9H, CH2). 13C NMR (CDCl3): δ (ppm) 178.46,
150.53, 143.69, 142.01, 141.57, 139.86, 128.40, 126.55, 126.22, 122.24, 120.39, 118.64, 115.76
(Ar), 55.35, 33.57, 32.63, 8.95, 8.55 (fluorene).
1
2: Yellow powder (29%). Spectral data: MS (MALDI-TOF): m/z 1168.16 (M+). H NMR
(CDCl3): δ (ppm) 8.19 (s, 3H, Ar), 8.02 (d, J = 7.6 Hz, 3H, Ar), 7.33−7.32 (m, 6H, Ar),
7.24−7.21 (m, 3H, Ar), 7.13−7.10 (m, 6H, Ar), 6.91 (d, J = 7.6 Hz, 6H, Ar), 6.70−6.65 (m, 9H,
Ar), 6.48 (t, J = 7.4 Hz, 3H, Ar).
4.4.2. General procedures for the synthesis of heteroleptic cyclometalated iridium(III) complexes
Heteroleptic cyclometalated Ir(III) complexes were prepared via two reaction steps. 2.5
Equivalents of the respective cyclometalating ligand reacted with IrCl3·nH2O in a mixture of 2-
o
ethoxyethanol:H2O (3:1, v/v). The mixture was stirred at 80 C overnight to yield the chloro-
bridged dimer. Precipitates were formed gradually. The crude Ir(III) dimer was dried and used
for the subsequent reaction without further purification. Acetylacetone and sodium carbonate
23

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were mixed with Ir(III) dimer in 2-ethoxyethanol solution. The reaction mixture was refluxed for
16 h under nitrogen and then cooled to room temperature. The mixture was washed with D.I.
water and extracted with CH2Cl2. The organic layer was dried over Na2SO4 and concentrated
under reduced pressure. The complex was purified by column chromatography over silica gel.
1
3: Orange powder (41%). Spectral data: MS (MALDI-TOF): m/z 900.25 (M+). H NMR
(CDCl3): δ (ppm) 7.78 (d, J = 3.6 Hz, 2H, Ar), 7.43 (d, J = 3.6 Hz, 2H, Ar), 7.40 (s, 2H, Ar),
7.37−7.35 (m, 2H, Ar), 7.19−7.17 (m, 6H, Ar), 6.58 (s, 2H, Ar), 5.26 (s, 1H, acac), 1.97−1.92 (m,
4H, fluorene), 1.88−1.82 (m, 10H, acac and fluorene), 0.34 (t, J = 8.0 Hz, 3H, fluorene), 0.06 (t,
J = 8.0 Hz, 3H, fluorene). 13C NMR (CDCl3): δ (ppm) 185.02 (acac), 178.70, 151.02, 143.74,
142.76, 142.31, 141.31, 140.14, 139.67, 126.88, 126.19, 124.55, 122.4, 119.97, 118.60, 115.35
(Ar), 100.49 (acac), 55.21 (CH), 32.96 (CH2), 32.47 (CH2), 28.51 (acac), 8.81 (CH3), 8.45 (CH3).
1
4: Yellow powder (26%). Spectral data: MS (MALDI-TOF): m/z 948.18 (M+). H NMR
(CDCl3): δ (ppm) 8.13 (s, 2H, Ar), 7.87 (d, J = 7.6 Hz, 2H, Ar), 7.50 (d, J = 4.0 Hz, 2H, Ar), 7.39
(t, J = 7.8 Hz, 4H, Ar), 7.28−7.18 (m, 6H, Ar), 7.17−7.14 (m, 4H, Ar), 7.11 (d, J = 4.0 Hz, 4H,
Ar), 7.07−7.03 (m, 2H, Ar), 6.15 (s, 2H, Ar), 5.16 (s, 1H, acac), 1.74 (s, 6H, acac). 13C NMR
(CDCl3): δ (ppm) 184.98 (acac), 178.71, 143.50, 142.43, 139.98, 139.33, 137.58, 134.15, 129.19,
126.77, 126.62, 124.53, 120.02, 118.97, 118.07, 116.36, 114.57, 113.07, 109.74 (Ar), 100.59
(acac), 28.48 (acac).
1
5: Red powder (19%). Spectral data: MS (MALDI-TOF): m/z 948.16 (M+). H NMR
(CDCl3): δ (ppm) 7.81 (d, J = 4.0 Hz, 2H, Ar), 7.73 (d, J = 8.0 Hz, 2H, Ar), 7.58−7.54 (m, 6H.
Ar), 7.49−7.47 (m, 4H, Ar), 7.45 (d, J = 4.0 Hz, 2H, Ar), 7.44−7.40 (m, 2H, Ar), 7.22−7.17 (m,
4H, Ar), 7.07−7.03 (m. 2H, Ar), 6.94 (s, 2H, Ar), 5.26 (s, 1H, acac), 1.84 (s, 6H, acac). 13C NMR
(CDCl3): δ (ppm) 181.88 (acac), 178.76, 141.58, 139.81, 139.05, 138.1, 137.44, 132.66, 129.76,
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127.03, 126.92, 125.99, 125.85, 123.92, 122.74, 120.59, 119.37, 115.95, 109.14, 106.11 (Ar),
100.47 (acac), 28.53 (acac).
Acknowledgements
C.-L. Ho thanks Hong Kong Baptist University (FRG1/15-16/043 and FRG2/15-16/074), the
Science,
Technology
and
Innovation
Committee
of
Shenzhen
Municipality
(JCYJ20160531193836532) and the National Natural Science Foundation of China (project
number 21504074) for their financial support. W.-Y. Wong thanks the Areas of Excellence
Scheme of the HKSAR (AoE/P-03/08), Hong Kong Research Grants Council (HKBU 12304715),
the National Basic Research Program of China (2013CB834702), Hong Kong Baptist University
(FRG1/14-15/084) and the Hong Kong Polytechnic University for the financial support. The
present work was also supported by the Inter-institutional Collaborative Research Scheme (RC-
ICRS/14-15/02).
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Iridium complexes incorporate with the electron-rich thiazolyl group were
designed and synthesized
-
-
-
The emission color of these thiazole-based Ir(III) complexes can be tuned from
yellow to red
The best phosphorescent organic light-emitting device exhibited the maximum
external quantum efficiency of 11.1%, current efficiency of 35.8 cd/A and power
efficiency of 21.9 lm/W