Synthesis of a high-efficiency red phosphorescent emitter for organic light-emitting diodes

Article abstract of DOI:10.1039/b313843g

Four novel red phosphorescent emitter compounds bis(1-phenylisoquinolinato- N,C^2′)iridium(acetylacetonate), (piq)₂Ir(acac), bis(1-(1′-naphthyl)isoquinolinato-N,C^2′) iridium(acetylacetonate), (1-niq)₂Ir(acac), bis(1-(2′-naphthyl) isoquinolinato-N,C^2′)iridium(acetylacetonate), (2-niq) ₂Ir(acac) and bis(1-phenyl-5-methylisoquinolinato-N,C ^2′)iridium(acetylacetonate), (m-piq)₂Ir(acac), have been synthesized and fully characterized. Electroluminescent devices with a configuration of ITO/NPB/CBP:dopant/BCP/AlQ₃/Al were fabricated. All devices emitted in the red region with an emission ranging from 624 to 680 nm. (m-piq)₂Ir(acac) shows a maximum brightness of 17 164 cd m ^-2 at a current density of J = 300 mA cm^-2 and the best luminance efficiency of 8.91 cd A^-1 at a current density of J = 20 mA cm^-2. (1-niq)₂Ir(acac) exhibits pure-red emission with 1931 CIE (Commission International de L'Eclairage) chromaticity coordinates x = 0.701, y = 0.273.

Full text of DOI:10.1039/b313843g

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A R T I C L E
Synthesis of a high-efficiency red phosphorescent emitter for
organic light-emitting diodes
Cheng-Hsien Yang, Chia-Cheng Tai and I-Wen Sun
Department of Chemistry, National Cheng Kung University, Tainan, Taiwan, 701,
Republic of China
Received 31st October 2003, Accepted 24th December 2003
First published as an Advance Article on the web 11th February 2004
Four novel red phosphorescent emitter compounds bis(1-phenylisoquinolinato-N,C2’)iridium(acetylacetonate),
(piq)2Ir(acac), bis(1-(1’-naphthyl)isoquinolinato-N,C2’)iridium(acetylacetonate), (1-niq)2Ir(acac), bis(1-(2’-
naphthyl)isoquinolinato-N,C2’)iridium(acetylacetonate), (2-niq)2Ir(acac) and bis(1-phenyl-5-
methylisoquinolinato-N,C2’)iridium(acetylacetonate), (m-piq)2Ir(acac), have been synthesized and fully
characterized. Electroluminescent devices with a configuration of ITO/NPB/CBP:dopant/BCP/AlQ3/Al were
fabricated. All devices emitted in the red region with an emission ranging from 624 to 680 nm. (m-piq)2Ir(acac)
shows a maximum brightness of 17 164 cd m22 at a current density of J ~ 300 mA cm22 and the best
luminance efficiency of 8.91 cd A21 at a current density of J ~ 20 mA cm22. (1-niq)2Ir(acac) exhibits
pure-red emission with 1931 CIE (Commission International de L’Eclairage) chromaticity coordinates
x ~ 0.701, y ~ 0.273.
iridium complexes: (piq)2Ir(acac), (1-niq)2Ir(acac), (2-niq)2Ir-
(acac) and (m-piq)2Ir(acac). The iridium complexes were
prepared from the isoquinoline ligands and iridium trichloride
Introduction
In the past decade, great progress has been made in organic light-
emitting diodes (OLEDs).1–3 Both red-emitting fluorescent and
phosphorescent dopants have been investigated. Fluorescent
dyes, including [2-methyl-6-[2,3,6,7-tetrahydro-1H,5H-benzo[ij]-
quinolizin-9-yl]-4H-pyran-4-ylidene]propanedinitrile (DCM2)
and 4-(dicyanomethylene)-2-tert-butyl-6-(1,1,7,7-tetramethyl-
julolidyl-9-enyl)-4H-pyran (DCJTB) series, usually exhibit a
maximum external quantum efficiencies limited to 1–2%.4,5
However, by employing triplet-based phosphorescent dyes in
OLEDs where both singlet and triplet excited states participate,
the external quantum efficiency can reach as high as 5–6%.6,7
Recently, the photophysics of cyclometalated metal complexes
has been the subject of extensive studies. OLEDs based on the
triplet emitters such as Ir(III),8,9 Pt(II),10 Ru(II),11 and Os(II)12
have been demonstrated with high efficiency. The best
performing phosphorescent dopants have been shown to be
those based on iridium complexes.10,13 These iridium com-
plexes are highly suitable for OLED applications due to their
relatively short excited state lifetime, high photoluminescence
efficiencies and excellent color tunability.13
Scheme 1 Synthesis of 1-phenyl-5-methylisoquinoline (m-piq).
Lately, Okada and coworkers demonstrated a high efficiency
red OLED,14 Ir(piq)3 [tris(1-phenylisoquinolinato-N,C2)iridium-
(III)], which exhibits a maximum emission peak at 623 nm, and
efficiency of the electroluminescence device is 8.0 lm W21
9.3 cd A21 at 100 cd m22, 6.3 lm W21, 8.4 cd A21 at 300 cd m22
,
;
its CIE-coordinates are (0.68, 0.33). In this paper, we report a
series of high efficiency red phosphorescent iridium complexes
based on the isoquinoline derivatives.
Results and discussion
Isoquinoline ligands were synthesized from 3,4-dihydroisoquino-
line as shown in Scheme 1.15 We prepared 1-phenylisoquinoline
(piq), 1-(1’-naphthyl)isoquinoline (1-niq), 1-(2’-naphthyl)iso-
quinoline (2-niq) and 1-phenyl-5-methylisoquinoline (m-piq)
for metal complexes. Only the synthesis of 1-phenyl-5-
methylisoquinoline (m-piq) is described in the Experimental
section.
Scheme 2 Synthesis of the iridium complexes.
J . M a t e r . C h e m . , 2 0 0 4 , 1 4 , 9 4 7 – 9 5 0
Scheme 2 outlines the synthetic process for red phosphorescent
T h i s j o u r n a l i s ß T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4
9 4 7

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Fig. 3 Configuration of the device.
Fig. 1 PL Spectra of the Ir complexes in CH2Cl2.
blocking layer, tris-(8-hydroxyquinoline)aluminium(III) (AlQ3)
as an electron transport layer, 4,4’-bis(N-carbazolyl)biphenyl
(CBP) as the host material, and iridium complexes as the
dopant. The corresponding CIE coordinates are x ~ 0.679,
y ~ 0.318 for (piq)2Ir(acac), x ~ 0.701, y ~ 0.273 for
(1-niq)2Ir(acac), x ~ 0.697, y ~ 0.299 for (2-niq)2Ir(acac) and
x ~ 0.677, y ~ 0.321 for (m-piq)2Ir(acac) (as shown in Fig. 4)
All four devices show red to deep-red emissions, these were
close to the National Television Standards Committee
recommended red for a video display.
to form a dimer, [(C‘N)2Ir(m-Cl)2Ir(C‘N)2], followed by the
reaction with acetylacetone in the presence of sodium carbonate.8
All procedures involving Ir(III) species were carried out under
nitrogen gas atmosphere. All these materials were characterized
by 1H and 13C NMR as well as mass spectrometry.
The signals in the 1H NMR spectrum could be exactly assigned
to the various hydrogen atoms when the iridium was chelated
with ligands (see Experimental section). In the mass spectrum,
all these compounds show the corresponding ion peaks of
acetylacetonate and ligand. After using vacuum sublimation to
purify these complexes, we found that (1-niq)2Ir(acac) has poor
thermal stability. Therefore, we only report the mass spectral
data of this complex.
The photoluminescence (PL) and electroluminescence (EL)
spectra are shown in Fig. 1 and Fig. 2, respectively. The PL
and EL spectra show the same trends. The PL spectrum of
(piq)2Ir(acac) in CH2Cl2 shows an emission band at 618 nm.
The other complexes exhibit bathochromic shifts at 664.8, 633
and 623.4 nm. The EL spectra of these devices based on these
four iridium complexes were recorded at a current density of
J ~ 50 mA cm22. The EL spectrum of (piq)2Ir(acac) and
(m-piq)2Ir(acac) show the same emission band at 624 nm,
(2-niq)2Ir(acac) shows the band at 633 nm, and (1-niq)2Ir(acac)
shows the maximum bathochromic shift at 680 nm. AC-2 mea-
surements showed that these compounds have high HOMO
values in the range 25.11 to 25.35 eV.
Electrophosphorescence data for the iridium complexes are
summarized in Table 1. In these complexes, we change the
substituent at C-1 of isoquinoline from phenyl to 1-naphthyl
and 2-naphthyl. The data show that (1-niq)2Ir(acac) and
(2-niq)2Ir(acac) as the dopant give better bathochromic shifts
(680 and 634 nm, respectively), but have poor brightness and
luminance efficiency. In contrast, when we retain the isoquino-
line C-1 substituent group as phenyl and change the C-5 sub-
stituent group from hydrogen to methyl, a better performance
in brightness and luminance can be achieved. (m-piq)2Ir(acac)
showed a better performance than (piq)2Ir(acac) in both bright-
ness and luminance efficiency. When (m-piq)2Ir(acac) was used
as the dopant, a maximum brightness of 17 164 cd m22 was
achieved at a current density of J ~ 300 mA cm22
.
In summary, we have designed and synthesized cyclometa-
lated iridium complexes suitable for use as red dopant materials
in OLEDs. In comparison to the known derivatives of this
Devices were fabricated by high vacuum (1026 Torr) thermal
evaporation on pre-cleaned indium–tin oxide (ITO) glass
substrates as shown in Fig. 3. With a base pressure of y1 6
1026 Torr, the organic and metal cathode layers were grown
successively. In this device, 4,4’-bis[N-(1-naphthyl)-N-phenyl-
amino]biphenyl (NPB) acted as a hole transport layer, 2,9-
Dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) as a hole
Fig. 4 CIE coordinates of (piq)2Ir(acac) (0.679, 0.318), (1-niq)2Ir-
(acac) (0.701, 0.273), (2-niq)2Ir(acac) (0.697, 0.299) and (m-piq)2Ir-
(acac) (0.677, 0.321).
Fig. 2 EL Spectra of iridium complexes at J ~ 50 mA cm22
J . M a t e r . C h e m . , 2 0 0 4 , 1 4 , 9 4 7 – 9 5 0
.
9 4 8

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Table 1 Electrophosphorescence data for iridium complexes
(2 6 100 ml). The organic layer was washed with water (3 6
100 ml) and dried over MgSO4.
Brightness/ LE/
cd m22
Voltage/ CIE HOMO/
V coordinates eV
Compound
cd A21
1-Phenyl-5-methylisoquinoline
(piq)2Ir(acac)
1566a
3505b
7.83
7.01
6.43
5.68
5.21
8.7
x ~ 0.679; 25.25
y ~ 0.318
All of the organic layer above was reacted for 16 h together
with active manganese dioxide (100 g). After the reaction, the
manganese dioxide was removed by filtration, and the benzene
was distilled off. Recrystallization of the resulting crystals from
cyclohexane afforded 1-phenyl-5-methylisoquinoline as yellow
crystals (11.94 g, 54.5 mmol).
9.8
11.0
12.4
13.6
6431c
11362d
15621e
(1-niq)2Ir(acac)
(2-niq)2Ir(acac)
48a
106b
210c
415d
591e
0.240
0.218
0.210
0.207
0.197
11.9
13.2
14.4
16.1
17.1
x ~ 0.701; 25.11
y ~ 0.273
1-Phenylisoquinoline (piq) (yield 33%)
Tm ~ 95 uC, EIMS: m/z 205, [M]1; 1H NMR (acetone-d6,
400 MHz): d 8.57 (d, J ~ 5.6 Hz, 1H), 8.09 (d, J ~ 8.5 Hz, 1H),
8.01 (d, J ~ 8.2 Hz, 1H), 7.79–7.69 (m, 4H), 7.62 (t, J ~ 8 Hz,
1H), 7.58–7.50 (m, 4H).
758a
1709b
3043c
5664d
7895e
3.79
3.42
3.04
2.83
2.63
10.7
12.1
13.4
15.2
16.6
x ~ 0.697; 25.14
y ~ 0.299
(m-piq)2Ir(acac) 1781a
8.91
7.91
7.25
6.19
5.72
9.6
11.1
12.8
15.1
16.9
x ~ 0.677; 25.35
y ~ 0.321
1-(1’-Naphthyl)isoquinoline (1-niq) (yield 30%)
3959b
7246c
1
Tm ~ 106.4 uC, EIMS: m/z 255, [M]1; H NMR (acetone-d6,
12375d
17164e
400 MHz): d 8.66 (d, J ~ 5.6 Hz, 1H), 8.08–8.02 (m, 3H), 7.89
(d, J ~ 5.9 Hz, 1H), 7.75 (td, J ~ 8, 1.3 Hz, 1H), 7.69 (td,
J ~ 7.3, 1.38 Hz, 1H), 7.58–7.45 (m, 4H), 7.35–7.32 (m, 2H).
For each parameter, the data in different rows correspond to those
measured at different current density: a 20 mA cm22 b 50 mA cm22
c 100 mA cm22 d 200 mA cm22 e 300 mA cm22
,
,
.
,
.
1-(2’-Naphthyl)isoquinoline (2-niq) (yield 41%)
Tm ~ 163.3 uC, EIMS: m/z 255, [M]1; H NMR (acetone-d6,
400 MHz): d 8.62 (d, J ~ 5.6 Hz, 1H), 8.23–8.18 (m, 2H), 8.09–
8.01 (m, 4H), 7.87 (dd, J ~ 8.4, 1.6 Hz, 1H), 7.82–7.76 (m, 2H),
7.65–7.58 (m, 3H).
1
class,16 (m-piq)2Ir(acac) shows better efficiency and equivalent
CIE coordinates data. Although (2-niq)2Ir(acac) can not give
brightness and luminance efficiency as high as (piq)2Ir(acac), it
produces a more pure red color than (m-piq)2Ir(acac) and can
be used in special applications as indicated by its CIE data. We
believe the efficiency can be further improved by using a more
suitable host material for the emitting layer, or a more suitable
device structure.7
1-Phenyl-5-methylisoquinoline (m-piq) (yield 54.5%)
1
Tm ~ 61.2 uC, EIMS: m/z 219, [M]1; H NMR (acetone-d6,
400 MHz): d 8.60 (d, J ~ 5.6 Hz, 1H), 7.91 (d, J ~ 8.5 Hz, 1H),
7.87 (dd, J ~ 5.95, 0.69 Hz, 1H), 7.68–7.66 (m, 2H), 7.60 (d,
J ~ 7.0 Hz, 1H), 7.57–7.47 (m, 4H).
Experimental
Instrumentation
Preparation of red phosphorescence iridium complexes, (piq)2-
Ir(acac), (1-niq)2Ir(acac), (2-niq)2Ir(acac) and (m-piq)2Ir(acac)
1H NMR and 13C NMR spectra were measured using a Bruker
AMX-400 (400 MHz), FAB mass spectra were collected with a
Bruker APEX II, and Photoluminesence spectra were recorded
with a HITACHI F-4500. The melting point data were
measured with a PYRIS Diamond TG/DTA-10 at a heating
All procedures involving IrCl3?H2O were carried out in nitro-
gen gas atmosphere. Cyclometalated Ir(III) m-chloro-bridged
dimers were synthesized by the method reported by Nono-
yama.17 IrCl3?H2O (Next Chimica) and 2.5 equiv. of ligand
were heated in a 3:1 mixture of 2-ethoxyethanol and water.
This slurry was heated at 100 uC for 24 h. After cooling to room
temperature, the precipitate was filtered off and washed with
water. The obtained solid was placed in a flask and dispersed in
2-ethoxyethanol. Acetylacetone and sodium carbonate were
added to the solution and the mixture were heated at 120 uC for
12–16 h. After cooling to room temperature, the crude product
was filtered off and washed with water, followed by two
portions of n-hexane and ether. The solid was dried in vacuum
and zone sublimed to give pure product which was used for
advanced analysis and device fabrication.
rate of 20 uC min21
.
N-(2-o-Tolylethyl)benzamide
Benzoic acid (12.2 g, 100 mmol) was dissolved in 100 ml of dry
dimethylformamide. While the solution was cooled with ice
with stirring, triethylamine (11.5 g) and ethyl chloroformate
(11 g) were added. After a lapse of 30 min, 2-methylphenethyl-
amine (14 g, 104 mmol) was added dropwise. The mixture was
stirred at room temperature for 1 h and then at 50–60 uC for 1 h,
and poured into ice-water. The crystallized precipitates were
washed in water, dried, and recrystallized from cyclohexane to
afford N-(2-o-tolylethyl)benzamide (14.66 g, 61.3 mmol).
Bis(1-phenylisoquinolinato-N,C2’)iridium(acetylacetonate) (piq)2-
Ir(acac) (yield 53.8%)
5-Methyl-1-phenyl-3,4-dihydroisoquinoline
Tm ~ 372.9 uC, FAB MS: m/z 700, [M 1 1]1; 1H NMR
(CD2Cl2, 400 MHz): d 9.01 (d, J ~ 6.4 Hz, 2H), 8.51 (dd, J ~
6.4,1.2 Hz, 2H), 8.25 (d, J ~ 8 Hz, 2H), 7.99 (m, 2H), 7.76 (m,
4H), 7.57 (d, J ~ 6.4 Hz, 2H), 6.97 (t, J ~ 7.6 Hz, 2H), 6.69 (t,
J ~ 7.6 Hz, 2H), 6.37 (d, J ~ 7.6 Hz, 2H), 5.35 (s, 1H), 1.81 (s,
6H). 13C NMR (CD2Cl2, 400 MHz) d: 175.1, 158.9, 141.8,
137.0, 130.6, 127.4, 123.8, 120.9, 119.9, 118.9, 118.0, 117.5,
116.8, 116.5, 110.6, 110.4, 90.7, 18.5.
N-(2-o-Tolylethyl)benzamide (14.66 g, 61.3 mmol) was heated
with stirring at 130 uC for 4 h together with 150 ml of xylene,
50 ml of phosphorus oxychloride and 50 g of phosphorus
pentoxide. The reaction mixture was decanted to remove
the solvent. The residue was carefully decomposed with ice-
water, and made weakly alkaline with 45% sodium hydroxide.
The crystals which precipitated were extracted with benzene
J . M a t e r . C h e m . , 2 0 0 4 , 1 4 , 9 4 7 – 9 5 0
9 4 9

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Bis(1-(1’-naphthyl)isoquinolinato-N,C2’)iridium(acetylacetonate) -
(1-niq)2Ir(acac) (yield 34.5%)
Acknowledgements
This work was supported by the National Science Council of
the Republic of China, Taiwan.
Tm ~ 369.8 uC, FAB MS: m/z 800, [M 1 1]1.
Bis(1-(2’-naphthyl)isoquinolinato-N,C2’)iridium(acetylacetonate)
(2-niq)2Ir(acac) (yield 64.4%)
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m ~ 370 uC, FAB MS: m/z 800, [M 1 1]1; 1H NMR (CD2Cl2,
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8
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OLED Fabrication and measurement
Pre-patterned ITO glass with an effective device of area
0.16 cm2 was cleaned in detergent for 10 min, and then washed
with a large amount of doubly distilled water. After sonication
in pure water for 5 min, the glass was dried in an oven at 180 uC
for 90 min. The organic layers were deposited thermally at a
rate of 0.1 nm s21 and pressure of y1 6 1026 Torr in a
deposition system. Aluminium was deposited as the cathode.
The HOMO value of iridium complexes were measured with a
RIKEN Photoelectron Spectrometer AC-2. The electrophos-
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These devices were fixed in the device holder and collected the
light from the front face.
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standard while (m-piq)2Ir(acac) shows better luminance efficiency
and brightness. We predict that (m-piq)2Ir(acac) should exhibit a
better gext than (piq)2Ir(acac). See: Y. J. Su, H. L. Huang, C. L. Li,
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9 5 0
J . M a t e r . C h e m . , 2 0 0 4 , 1 4 , 9 4 7 – 9 5 0