Color tunable phosphorescent light-emitting diodes based on iridium complexes with substituted 2-phenylbenzothiozoles as the cyclometalated ligands

Article abstract of DOI:10.1016/j.jorganchem.2004.07.003

Several iridium complexes {iridium(III)bis[2-(3-methoxyphenyl)-1,3- benzothiozolato-N,C^2′] acetylacetonate (MeO-BT) ₂Ir(acac), iridium(III)bis[2-(2,4-difluorophenyl)-1,3- benzothiozolato-N,C^2′] acetylacetonate (2F-BT) ₂Ir(acac), and iridium(III)bis[2-(2,4-difluorophenyl)-6-fluoro-1,3- benzothiozolato-N,C^2′] acetylacetonate (3F-BT) ₂Ir(acac)} having different substituents on 2-phenylbenzothiazole have been synthesized. Several iridium complexes {iridium(III)bis[2-(3- methoxyphenyl)-1,3-benzothiozolato-N,C^2′] acetylacetonate (MeO-BT)₂Ir(acac), iridium(III)bis[2-(2,4-difluorophenyl)-1,3- benzothiozolato-N,C^2′] acetylacetonate (2F-BT) ₂Ir(acac), and iridium(III)bis[2-(2,4-difluorophenyl)-6-fluoro-1,3- benzothiozolato-N,C^2′] acetylacetonate (3F-BT) ₂Ir(acac)} having different substituents on 2-phenylbenzothiazole have been synthesized. The phosphorescent light emitting diodes (PHOLEDs) using these iridium complexes as dopant emitters were fabricated. The experimental results revealed that the emissive colors of PHOLEDs could be finely tuned by suitable modification of the substituents on the 2-phenylbenzothiazole ligands. Furthermore, these iridium complexes show better emissive properties than the known iridium(III)bis(2-phenylbenzothiozolato-N,C^2′) acetylacetonate (BT)₂Ir(acac).

Full text of DOI:10.1016/j.jorganchem.2004.07.003

Journal of Organometallic Chemistry 689 (2004) 4882–4888
www.elsevier.com/locate/jorganchem
Color tunable phosphorescent light-emitting diodes based on
iridium complexes with substituted 2-phenylbenzothiozoles as the
cyclometalated ligands
a
Wei-Chieh Chang , Andrew Teh Hu , Jiun-Pey Duan , Dinesh Kumar Rayabarapu ,
a,*
b
b
b,
*
Chien-Hong Cheng
a
Department of Chemical Engineering, National Tsing-Hua University, Hsinchu 300, Taiwan
b
Department of Chemistry, National Tsing-Hua University, Hsinchu 300, Taiwan
Received 7 June 2004; accepted 1 July 2004
Available online 11 November 2004
Abstract
0
Several iridium complexes {iridium(III)bis[2-(3-methoxyphenyl)-1,3-benzothiozolato-N,C ] acetylacetonate (MeO-BT)
2
2
Ir(acac),
0
2
2
iridium(III)bis[2-(2,4-difluorophenyl)-1,3-benzothiozolato-N,C ] acetylacetonate (2F-BT) Ir(acac), and iridium(III)bis[2-(2,4-di-
0
fluorophenyl)-6-fluoro-1,3-benzothiozolato-N,C ] acetylacetonate (3F-BT) Ir(acac)} having different substituents on 2-phenyl-
2
2
benzothiazole have been synthesized. The phosphorescent light emitting diodes (PHOLEDs) using these iridium complexes as
dopant emitters were fabricated. The experimental results revealed that the emissive colors of PHOLEDs could be finely tuned
by suitable modification of the substituents on the 2-phenylbenzothiazole ligands. Furthermore, these iridium complexes show better
0
2
emissive properties than the known iridium(III)bis(2-phenylbenzothiozolato-N,C ) acetylacetonate (BT)
ꢀ
2
Ir(acac).
2004 Elsevier B.V. All rights reserved.
Keywords: Iridium complexes; Phosphorescent; PHOLED; Light-emitting diode; Color-tunable
1
. Introduction
complexes, facilitating the population accumulation of tri-
plet state. Since singlet and triplet excitons can be fully uti-
lized, the PHOLEDs thus can theoretically attain 100%
internal quantum yield and ca. 20% external quantum
yield. As a result, the PHOLEDs with fac-tris(2-phenylpy-
Recently, organic light emitting diodes (OLEDs) have
attracted worldwide attention for their high emission effi-
ciency and low operating voltage in flat panel displays.
An important breakthrough is phosphorescent light emit-
ting diodes (PHOLEDs) with the use of the heavy metal
complexes, particularly iridium complexes [1–8]. The
PHOLEDs have four-fold theoretical efficiency than the
fluorescent light emitting diodes, assuming electron–hole
recombination is statistically controlled. The highly effi-
cientemissionofPHOLEDsresultsfromstrongspin–orbit
coupling effect of the electronic states in the heavy metal
ridine)iridium (Ir(ppy) ) exhibit green emission with an ex-
3
ternal quantum efficiency (g ) of 15% [9,10].
ext
Both the electroluminescent (EL) efficiency and the
emissive wavelength of devices based on iridium com-
plexes are affected greatly by the organic ligands. Thomp-
son et al. [11] reported a series of neutral iridium(III)
complexes that exhibit different emissive colors. Recently,
GrushinÕs group synthesized a series of iridium(III)
complexes with fluorinated 2-arylpyridines and showed
that the emissive colors of the materials can be finely
tuned by systematic control of the nature and position
of the substituents on the ligands [12,13]. These results
*
Corresponding authors. Tel.: +886-3-5713830; fax: +886-3-
5715408.
E-mail address: athu@che.nthu.edu.tw (A.T. Hu).
0
022-328X/$ - see front matter ꢀ 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2004.07.003

W.-C. Chang et al. / Journal of Organometallic Chemistry 689 (2004) 4882–4888
4883
prompted us to systematically study the substitution on
-phenylbenzothiazole and their effect on emissive colors.
Synthesis of 2-aminothiophenol: 2-Aminobenzothiaz-
ole 1a (27.8 mmol) and 10 N KOH were refluxed in
ethylene glycol (6.0 ml) under nitrogen at 125 ꢁC for
15 h. The reaction mixture was then cooled to room
temperature, and neutralized with 37.8 wt% hydrochlo-
ric acid (1 g). The reaction mixture was extracted with
ethyl acetate and dried over Na SO prior to concentra-
tion in vacuum. The resulting solid was recrystallized in
methanol to give the desired yellow solid yellow 2a
(yield = 78%).
2
In this paper, we report the synthesis and characterization
of a series of iridium complexes having different substi-
tuted 2-phenylbenzothiazole ligands. The experimental
results reveal that the devices based on these iridium com-
plexes exhibit different emissive colors with high bright-
ness, external quantum efficiency and power efficiency.
2
4
2
. Experimental details
Synthesis of 2-phenylbenzothiazole (3a): 2-Amino-
thiophenol 2a (16 mmol), benzaldehyde (16 mmol) and
p-toluenesulfonic acid monohydrate (PTSA) (1.6 mmol)
were refluxed in chloroform (20 ml) under nitrogen for
24 h. After cooling, the mixture was extracted with
water as well as ether and dried with Na SO . After
2
.1. Synthesis of iridium complexes
The synthetic protocol for the cyclometallated irid-
ium(III) complexes is essentially identical to that report-
ed previously [14] and outlined in Scheme 1.
2
4
O
H
R1
R2
R3
NH2
N
S
N
1
0N KOH
R4
reflux, 24h
,
PTSA/ CHCl3
NH2
R2
R3
R1
ethylene glycol
5h, 125 C
R1
S
o
SH
1
1
R = H (1a)
1
R = H (2a)
3
1
R4
R = F (1b)
1
R = F (2b)
R1
S
R1
IrCl 3H O
3
2
N
Cl
N
S
2
-ethoxyethanol:H O=3:1
2
Ir
2
Ir
o
R2
R3
R2
R3
1
10 C, 12h, N2
Cl
2
R4
R4
4
R1
OO
,
Na CO
S
N
Ir
O
2
3
R2
R3
2
-ethoxyethanol
O
o
1
10 C, 6h, N
2
R4
2
1
2
3
4
R = R = R = R = H
(BT) Ir(acac)
2
1
2
4
3
R = R = R = H, R = OMe (MeO-BT) Ir(acac)
2
1
3
2
4
R = R = H, R = R = F
(2F-BT) Ir(acac)
2
1
2
4
3
R = R = R = F, R = H
(3F-BT) Ir(acac)
2
Scheme 1.

4884
W.-C. Chang et al. / Journal of Organometallic Chemistry 689 (2004) 4882–4888
removal of solvent, the crude product was recrystallized
with methanol to give a yellow solid 3a.
Synthesis of l-chloro-bridged iridium dimer (4a): 2-
Phenylbenzothiazole 3a (2.2 mmol), iridium trichloride
hydrate (1.0 mmol), 2-methoxyethanol/water solution
C H F N S O Ir. Elemental analysis: Found: C,
31 19 2 2 2 2
47.90%; H, 2.97%; N, 3.63%. Calc. C, 47.42%; H,
2.42%; N, 3.57%.
Iridium(III)bis[2-(2,4-difluorophenyl)-6-fluoro-1,3-
0
2
benzothiozolat-N,C ]–acetylacetonate
(3F-BT) Ir(a-
2
1
(
v:v = 3:1) were heated and stirred at 110 ꢁC for 12 h un-
cac): Yield: 58%. H NMR (500 MHz, CDCl3)
d = 1.76 (s, 6H), 5.15 (s, 1H), 5.74 (dd, J = 1.5 Hz,
J = 9.0 Hz), 6.37–6.42 (m, 2H), 7.16–7.20 (m, 2H),
7.62 (dd, J = 2.5 Hz, J = 7.5 Hz, 2H), 7.95 (dd, J = 1.5
Hz, J = 8.0 Hz, 2H); GC-MS, m/z: 819;
C H F N S O Ir. Elemental analysis: Found: C,
der nitrogen. After cooling to room temperature, the
crude solution was filtrated to give an orange–yellow
precipitate. Then the precipitate was washed with etha-
nol and hexane for several times. After dried in vacuum,
a cyclometalated Ir(III) l-chloro-bridged dimer (4a) was
obtained.
3
1
17
6
2
2
2
45.23%; H, 2.01%; N, 3.34%. Calc. C, 45.41%; H,
2.07%; N, 3.42%.
Synthesis of iridium acetylacetone complex: Dimer 4a
0.78 mmol), 2,4-pentanedione (2.0 mmol), and Na CO3
(
2
(
5 mmol) were refluxed in the presence of 2-methoxyeth-
2.2. Physical measurements and PHOLED fabrication
anol under nitrogen for 6 h. After cooling to room tem-
perature, the crude product was filtrated to give an
orange–yellow precipitate. Then the precipitate was
washed with water, ethanol, and hexane for several
times. Finally, the crude product was purified by column
chromatography (silica, dichloromethane) and dried to
UV–Vis absorption spectra were recorded using a
Hitachi U-3300 spectrophotometer. Photoluminescent
give the desired iridium complex (BT) Ir(acac)
2
(
yield = 72%).
The same procedure was adopted for the preparation
N
N
N
N
of (MeO-BT) Ir(acac), (2F-BT) Ir(acac), and (3F-
2
2
H C
3
CH3
BT) Ir(acac). The yields of (MeO-BT) Ir(acac), (2F-
2
2
BCP
NPB
BT) Ir(acac), and (3F-BT) Ir(acac) were 64%, 60%,
2
2
and 58%, respectively. All the Ir complexes were sub-
limed at 280–300 ꢁC. The spectral data, yields and ele-
mental analytical data of these iridium complexes are
listed below.
N
O
O
0
2
Al
O
N
N
Iridium(III)bis(2-phenylbenzothiozolato-N,C )–ace-
1
tylacetonate, (BT) Ir(acac): Yield: 72%. H NMR (500
N
N
2
MHz, CDCl ) d = 1.73 (s, 6H), 5.11 (s, 1H), 6.38 (d,
3
J = 7.5 Hz, 2H), 6.61 (td, J = 1.5 Hz, J = 8.0 Hz, 2H),
CBP
6.83 (td, J = 1.5 Hz, J = 8.0 Hz), 7.39–7.42 (m, 4H),
7.62 (dd, J = 1.0 Hz, J = 7.5 Hz, 2H), 7.87–7.89 (m,
2H), 8.06–8.08 (m, 2H); GC-MS, m/z: 711;
Alq3
C H N S O Ir. Elemental analysis: Found: C,
3
1
23
2
2
2
5
3
2.45%; H, 3.74%; N, 3.97%. Calc. C, 52.30%; H,
.23%; N, 3.94%.
Iridium(III)bis[2-(3-methoxyphenyl)-1,3-benzothi-
0
2
ozolato-N,C ]–acetylacetonate, (MeO-BT) Ir(acac):
2
5
5 nm-Mg/Ag
40 nm-Alq3
1
Yield: 64%. H NMR (500 MHz, CDCl ) d = 1.74 (s,
3
6H), 3.70 (s, 6H), 5.10 (s, 1H), 6.25 (d, J = 9.0 Hz, 2H),
6.35 (d, J = 7.0 Hz, 2H), 7.18 (s, 2H), 7.40–7.41 (m,
4H), 7.87–7.88 (m, 2H), 8.05–8.07 (m, 2H); GC-MS,
1
0 nm-BCP
m/z: 771; C H N S O Ir. Elemental analysis: Found:
3
3
27
2
2
4
3
0 nm-Ir complex(7%):CBP
30 nm-NPB
C, 51.06%; H, 3.63%; N, 3.39%. Calc. C, 51.26%; H,
3
.50%; N, 3.62%.
Iridium(III)bis[2-(2,4-difluorophenyl)-1,3-benzo-
0
thiozolato-N,C ]–acetylacetonate, (2F-BT) Ir(acac):
2
ITO
2
1
Yield: 60%. H NMR (500 MHz, CDCl ) d = 1.76 (s,
3
Glass substrate
6H), 5.13 (s, 1H), 5.76 (d, J = 1.5 Hz, 9.5 Hz, 2H),
6.38 (t, J = 7.5 Hz, 2H), 7.43–7.48 (m, 4H), 7.92–
7.93 (m, 2H), 8.01–8.02 (m, 2H); GC-MS, m/z: 784;
Fig. 1. The general structure for PHOLEDs and the molecular
structures of the compounds used in these devices.

W.-C. Chang et al. / Journal of Organometallic Chemistry 689 (2004) 4882–4888
4885
35000
30000
25000
20000
15000
10000
(
(
(
(
BT) Ir(acac)
2
MeO-BT) Ir(acac)
2
2F-BT) Ir(acac)
2
3F-BT) Ir(acac)
2
5000
0
300
400
500
600
700
Wavelength (nm)
ꢀ4
Fig. 2. The UV–Vis absorption spectra of iridium complexes in CH
2
Cl
2
(concentration = 1 · 10 ).
1
1
0
0
0
0
0
.2
.0
.8
.6
.4
.2
.0
(
(
(
BT) Ir(acac)
2
MeO-BT) Ir(acac)
2
2F-BT) Ir(acac)
2
(3F-BT) Ir(acac)
2
450
500
550
600
650
700
750
Wavelength (nm)
ꢀ4
2 2
Fig. 3. The photoluminescent spectra of iridium complexes in CH Cl (concentration = 1 · 10 ).
(
PL) and electroluminescent (EL) spectra were measured
on the ITO glass. A layer of CBP doped with 7%
Ir complex followed the NPB layer. Layers of
BCP (2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline)
to serve as the hole blocker, Alq (tris[8-hydroxyquino-
line]) as the electron transporter and Mg:Ag/Ag as the
cathode were successively then deposited. Current,
voltage, and light-intensity measurements were made
simultaneously using a Keithley 2400 source meter
and a Newport 1835-C optical meter equipped with
a Newport 818-ST silicon photodiode. All chemicals
on a Hitachi F-4500 fluorescence spectrophotometer.
The PHOLEDs were fabricated by vacuum deposi-
tion of the materials at 10 Torr onto a clean glass
that was pre-coated with a layer of indium tin oxide
with a sheet resistance of 25 X square . Fig. 1 shows
the general structure for PHOLEDs and the molecular
structures of the compounds used in these devices.
NPB (4,4-bis[N-(1-naphthyl)-N-phenlamino]biphenyl)
was used as a hole transporter and was first deposited
ꢀ
6
ꢀ
1

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W.-C. Chang et al. / Journal of Organometallic Chemistry 689 (2004) 4882–4888
Table 1
Performance of PHOLEDs
a
2
L (cd/m , V)
Dopant phosphor
Turn on
voltage (V)
gext (%, V)
g
c
(cd/A, V)
g
p
CIE, 8V [X, Y]
Emissive
wavelength (kmax)
b
(lm/W, V)
(
(
(
(
BT)
2
Ir(acac)
Ir(acac)
3.8
3.2
4.0
4.2
9.28, 9.5
6.50, 8.5
11.2, 10.5
12.4, 10
101643, 14.5
37876, 14
96317, 15
34.0, 9.5
11.0, 8.5
45.0, 9.0
50.3, 10.5
11.4, 9
4.32, 8
(0.47, 0.52)
(0.63, 0.37)
(0.37, 0.60)
(0.37, 0.61)
560
610
530
530
MeO-BT)
2
2F-BT)
3F-BT)
a
2
Ir(acac)
Ir(acac)
13.9, 10
16.0, 9.5
2
125348, 15
The data for external quantum efficiency (gext), brightness (L), current efficiency (g
c
), and power efficiency (g
p
) are the maximum values of the
device.
b
2
Brightness = 1 cd/m .
(
BT) Ir(acac)
2
1.0
0.5
0.0
(MeO-BT) Ir(acac)
2
(
2F-BT) Ir(acac)
2
(
3F-BT) Ir(acac)
2
400
500
600
700
800
Wavelength (nm)
Fig. 4. The electroluminescent spectra of PHOLEDs.
1
1
1
1
1
1
1
1
e+6
e+5
e+4
e+3
e+2
e+1
e+0
e-1
1e+5
1e+4
1e+3
(
(
(
(
BT)
2
Ir(acac)
MeO-BT)
2
Ir(acac)
2F-BT)
3F-BT)
2
Ir(acac)
Ir(acac)
2
1
1
1
e+2
e+1
e+0
1e-1
0
0
6
9
12
15
Voltage /(V)
Fig. 5. The current–voltage–luminance (I–V–L) characteristics of PHOLEDs.

W.-C. Chang et al. / Journal of Organometallic Chemistry 689 (2004) 4882–4888
4887
1
4
2
0
8
6
4
2
0
1
1
(
(
(
(
BT) Ir(acac)
2
MeO-BT) Ir(acac)
2
2F-BT) Ir(acac)
2
3F-BT) Ir(acac)
2
0
20
40
Current Density(mA/cm )
Fig. 6. The external quantum efficiency of PHOLEDs.
60
80
100
2
used for PHOLEDs were sublimed in vacuum before
use.
3.2. Electroluminescence
As shown in Fig. 4, PHOLEDs based on (BT) Ir-
2
(
(
acac), (MeO-BT) Ir(acac), (2F-BT) Ir(acac), and
2
2
3
. Results and discussion
3F-BT) Ir(acac) emit yellow light (k
= 560 nm),
max
2
orange–red (kmax = 610 nm), yellow–green (kmax = 530
nm), and yellow–green (kmax = 530 nm), respectively.
Moreover, their EL spectra are very close to their
PL spectra, as shown in Figs. 3 and 4. The substitu-
tent on benzothiazole group of the iridium complexes
shows substantial effect on their PL value. The emiss-
ive wavelength of PHOLEDs with the iridium com-
plexes can be tuned from 530 to 610 nm by the
3
.1. UV–Vis absorption and photoluminescence
The UV–Vis absorption and PL spectra of iridium
complexes are shown in Figs. 2 and 3. The characteristic
absorption peaks of all the complexes are as follows:
(
(
BT) Ir(acac) at 314, 327, 403, 442 nm, (MeO-BT) Ir-
2 2
acac) at 323, 420, 522 nm, (2F-BT) Ir(acac) at 321, 384,
2
4
tense absorptions of 320–325 nm for iridium complexes
are assigned to the ligand-based p–p transitions, whereas
the other absorptions come from singlet and triplet metal
to ligand charge transfer ( MLCT and MLCT) transi-
tion. The absorptions at 442, 522, 415, and 416 nm for
15 nm, and (3F-BT) Ir(acac) at 321, 380, 416 nm. The in-
2
change of substituents. In case of (MeO-BT) Ir(acac)
2
and (2F-BT) Ir(acac), their substitution effect leads
2
*
to 45 nm red-shift and 35 nm blue-shift, respectively
with respect to (BT) Ir(acac), which exhibits yellow
2
1
3
EL (kmax = 560 nm).
The current–voltage–luminance (I–V–L) characteris-
tics of PHOLEDs with iridium complexes and their de-
vice performances are shown in Fig. 5 and Table 1,
(
and (3F-BT) Ir(acac), respectively, were assigned as the
BT) Ir(acac), (MeO-BT) Ir(acac), (2F-BT) Ir(acac),
2 2 2
2
3
MLCT bands for the iridium complexes.
respectively. Among the iridium complexes, (3F-BT) -
2
2
Ir(acac) has the highest brightness (125348 cd/m at 15
These iridium complexes showed strong photolumi-
nescence in the green to orange region. For example, ir-
radiation of (BT) Ir(acac) with 320-nm light gives strong
V). Furthermore, fluorinated iridium complexes (2F-
BT) Ir(acac) and (3F-BT) Ir(acac) exhibit higher
2
2
2
photoluminescence in dichloromethane at 557 nm (Table
1
quantum efficiency (gext, g , and g ) compared with
c
p
). Similarly, irradiation of dichloromethane solutions of
MeO-BT) Ir(acac), (2F-BT) Ir(acac) and (3F-BT) Ir-
(BT) Ir(acac) as shown in Fig. 6 and Table 1, and their
turn on voltages are comparable.
2
(
(
2
2
2
acac) led to strong photoluminescence at 605, 530, and
30 nm, respectively. The PL results show that the pres-
5
ence of electron-donating group such as OMe caused red
shift whereas electron-withdrawing substituents such as
F on the cyclometalated benzothiazole group of the Ir
complex cause blue-shift of their PL spectra.
4. Conclusion
We have successfully synthesized four kinds of irid-
ium complexes ((BT) Ir(acac), (MeO-BT) Ir(acac),
2
2

4888
W.-C. Chang et al. / Journal of Organometallic Chemistry 689 (2004) 4882–4888
(
2F-BT) Ir(acac) and (3F-BT) Ir(acac)) and fabricated
2
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2
the PHOLEDs with them as the emissive layers. The
experimental results show that the emissive colors of
the PHOLEDs iridium complexes can be modulated
by the nature of substituents and their performances
are better than that of iridium complex without
[
[
3] C. Adachi, M.A. Baldo, S.R. Forrest, M.E. Thompson, Appl.
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2
2
the highest brightness of 125348 cd/m at 15 V and
8
84–888.
a maximum external quantum efficiency of 12.4% at
1
[
[
[
7] Y.J. Su, H.L. Huang, C.L. Li, C.H. Chien, Y.T. Tao, P.T. Chou,
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Acknowledgement
[
We thank professor Chien-Hong Cheng, National
Tsing Hua University, for instrumental support.
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