Pure red electrophosphorescence from polymer light-emitting diodes doped with highly emissive bis-cyclometalated iridium(III) complexes

Article abstract of DOI:10.1016/j.jorganchem.2010.05.011

In order to develop highly emissive red phosphorescent materials for OLED application, novel bis-cyclometalated iridium(III) complexes were developed using the 1-(dibenzo[b,d]furan-4-yl)isoquinolinato-N,C^3′ (dbfiq) cyclometalating ligand. When 1,3-bis(3,4-dibutoxyphenyl)propane-1,3- dionate (bdbp) is employed as an ancillary ligand, Ir(dbfiq)₂(bdbp) 1 exhibits red photoluminescence (PL) at 640 nm with a quantum yield (Φ_PL) of 0.61 (in toluene, 298 K). Replacement of bdbp to dipivaloylmethanate (dpm) and acetylacetonate (acac) (Ir(dbfiq)₂(dpm) 2 and Ir(dbfiq)₂(acac) 3, respectively) does not affect the PL spectrum, but reduces Φ_PL to 0.55 and 0.49 for 2 and 3, respectively. Similar tendency is also found in the doped poly(methyl methacrylate) (PMMA) film, and 1 is more emissive (Φ_PL = 0.17) than 2 and 3 (Φ_PL = 0.08 and 0.06, respectively). Using 1 as a phosphorescent dopant, polymer light-emitting diodes (PLEDs) were fabricated, of which structure was ITO/PEDOT:PSS (40 nm)/PVCz:1:PBD (100 nm)/CsF (1 nm)/Al (250 nm). Pure red electroluminescence (EL) is obtained from the fabricated PLEDs, affording a CIE chromaticity coordinate of (0.68, 0.31). When 0.51 mol% of 1 is incorporated in the PVCz-based emitting layer, the PLED shows maximum luminance of 7270 cd m^-2 at 16.5 V, power efficiency of 1.4 lm W ^-1 at 7.5 V, and external quantum efficiency of 6.4% at 9.0 V. PLEDs with the same structure and components were also fabricated using 2 and 3, and their device characteristics were investigated. In proportion to the PL quantum yields, 1 affords better device performance than 2 and 3. Owing to four butoxy groups introduced to the bdbp ligand, 1 exhibits high solubility in organic solvents such as chloroform and toluene, and thus, is an excellent red phosphorescent dopant for solution-processed OLEDs. A highly emissive red phosphorescent bis-cyclometalated iridium(III) complex, bis[1-(dibenzo[b,d] furan-4-yl)isoquinolinato-N,C^3′]iridium(III) [1,3-bis(3,4-dibutoxyphenyl)propane-1,3-dionate-O,O] 1 was developed. It exhibits red photoluminescence with Φ_PL of 0.61 (toluene, 298 K). The electrophosphorescence from the polymer light-emitting diode containing 1 affords a CIE chromaticity coordinate of (0.68, 0.31) shifting to pure red over the NTSC standard.

Full text of DOI:10.1016/j.jorganchem.2010.05.011

Journal of Organometallic Chemistry 695 (2010) 1972e1978
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Pure red electrophosphorescence from polymer light-emitting diodes doped
with highly emissive bis-cyclometalated iridium(III) complexes
,
a
a
a
a
Hidetaka Tsujimoto a, Shigeyuki Yagi a , Hotaka Asuka , Yuji Inui , Shigeru Ikawa , Takeshi Maeda ,
*
Hiroyuki Nakazumi a, Yoshiaki Sakurai b
a Department of Applied Chemistry, Graduate School of Engineering, Osaka Prefecture University, 1-1 Gakuen-cho, Naka-ku, Sakai, Osaka 599-8531, Japan
b Environment and Chemistry Department, Technology Research Institute of Osaka Prefecture, 2-7-1 Ayumino, Izumi, Osaka 594-1157, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
In order to develop highly emissive red phosphorescent materials for OLED application, novel bis-
cyclometalated 0iridium(III) complexes were developed using the 1-(dibenzo[b,d]furan-4-yl)iso-
quinolinato-N,C3 (dbfiq) cyclometalating ligand. When 1,3-bis(3,4-dibutoxyphenyl)propane-1,3-dionate
(bdbp) is employed as an ancillary ligand, Ir(dbfiq)2(bdbp) 1 exhibits red photoluminescence (PL) at
640 nm with a quantum yield (FPL) of 0.61 (in toluene, 298 K). Replacement of bdbp to dipivaloylme-
thanate (dpm) and acetylacetonate (acac) (Ir(dbfiq)2(dpm) 2 and Ir(dbfiq)2(acac) 3, respectively) does not
affect the PL spectrum, but reduces FPL to 0.55 and 0.49 for 2 and 3, respectively. Similar tendency is also
found in the doped poly(methyl methacrylate) (PMMA) film, and 1 is more emissive (FPL ¼ 0.17) than 2
and 3 (FPL ¼ 0.08 and 0.06, respectively). Using 1 as a phosphorescent dopant, polymer light-emitting
diodes (PLEDs) were fabricated, of which structure was ITO/PEDOT:PSS (40 nm)/PVCz:1:PBD (100 nm)/
CsF (1 nm)/Al (250 nm). Pure red electroluminescence (EL) is obtained from the fabricated PLEDs,
affording a CIE chromaticity coordinate of (0.68, 0.31). When 0.51 mol% of 1 is incorporated in the PVCz-
based emitting layer, the PLED shows maximum luminance of 7270 cd mꢀ2 at 16.5 V, power efficiency of
1.4 lm Wꢀ1 at 7.5 V, and external quantum efficiency of 6.4% at 9.0 V. PLEDs with the same structure and
components were also fabricated using 2 and 3, and their device characteristics were investigated. In
proportion to the PL quantum yields, 1 affords better device performance than 2 and 3. Owing to four
butoxy groups introduced to the bdbp ligand, 1 exhibits high solubility in organic solvents such as
chloroform and toluene, and thus, is an excellent red phosphorescent dopant for solution-processed
OLEDs.
Received 6 February 2010
Received in revised form
8 May 2010
Accepted 10 May 2010
Available online 31 May 2010
Keywords:
Bis-cyclometalated iridium(III) complex
Red phosphorescence
Organic light-emitting diode
Polymer light-emitting diode
Photoluminescence quantum yield
Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction
(C^N)2(O^O) [11,12] complexes (C^N, 2-arylpyridinate or a related
ligand; O^O, a
b-diketonate ligand). In these complexes, strong
Recent development in organic light-emitting diodes (OLEDs)
has brought about considerable progress in electrooptical appli-
cations such as self-emitting flat-panel displays and illumination
devices [1e5]. The promising strategy to obtain high performance
OLEDs is utilization of phosphorescent emitters because they can
overcome the limit of quantum efficiency in fluorescent OLEDs [6].
From this viewpoint, phosphorescent organometallic complexes
with heavy metal centers have received considerable attention as
emitting dopants for OLEDs. Especially, bis- and tris-cyclo-
metalated Ir(III) complexes have been intensely studied, as rep-
resented by homoleptic Ir(C^N)3 [7e10] and heteroleptic Ir
spin-orbit coupling facilitating intersystem crossing from the
singlet excited state to the triplet state leads to high internal
quantum efficiency of phosphorescent electroluminescence (EL),
up to 100% [13,14]. Thus, numerous amounts of Ir(III)-based
phosphorescent emitters for RGB primary colors have so far been
developed. However, the development of red phosphorescent Ir
(III) complexes with high quantum yields is still difficult because
they are intrinsically less emissive than blue and green emitters.
Although Ir(piq)3 [9] and Ir(btp)2(acac) [15] are well known as red
phosphorescent emitters (piq and btp; 1-phenylisoquinolinato-N,
0
30
C2 and 2-(20-benzo[b]thienyl)pyridinato-N,C ligands, respec-
tively), their photoluminescence quantum yields in solution are
not more than 0.26. In order to achieve high device performance
of red emitting OLEDs, highly emissive red phosphorescent
materials are eagerly required.
* Corresponding author. Tel.: þ81 72 254 9324; fax: þ81 72 254 9910.
E-mail address: yagi@chem.osakafu-u.ac.jp (S. Yagi).
0022-328X/$ e see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2010.05.011

H. Tsujimoto et al. / Journal of Organometallic Chemistry 695 (2010) 1972e1978
1973
Scheme 1. Synthesis of bis-cyclometalated Ir(III) complexes 1e3.
Another important matter is applicability of phosphorescent
organometallic complexes as emitting dopants in solution-pro-
cessed OLEDs [16]. For fabrication of OLEDs, solution processing
methods such as spin-coating and ink-jet printing techniques are
intensely investigated because of lots of advantages; efficient use of
constituent materials, facile device fabrication with saved electric
power, and availability of large-area devices. Thus, from the
industrial viewpoint, solution processing is eagerly required rather
than vacuum deposition processing, and the achievement of high
device performance in solution-processed devices is one of main
subjects of development of OLEDs. The phosphorescent organo-
metallic complexes used for OLEDs are, however, often less soluble
in organic solvents and host materials used in fabrication of solu-
tion-processed OLEDs. With this respect, the development of well-
soluble phosphorescent organometallic complexes should provide
lots of opportunities to optimize the device performance.
In the present study, we report a novel, highly emissive red
phosphorescent Ir(III) complex 1, composed of two 1-(dibenzo[b,d]
0
furan-4-yl)isoquinolinato-N,C3 (dbfiq) cyclometalating ligands and
one 1,3-bis(3,4-dibutoxyphenyl)propane-1,3-dionate-O,O (bdbp)
ancillary ligand. We expect that the rigid and extended
p-conju-
gation framework of the dbfiq ligand should yield pure and satu-
rated red emission with high quantum efficiency. Aimed at
solution-processed OLEDs, bdbp is employed as a diketonate
ancillary ligand to improve the solubility in organic media.
Recently, we reported that heteroleptic cycloplatinate(II)
complexes with bdbp as an ancillary ligand, namely Pt(C^N)(bdbp),
exhibited improved solubility in various organic solvents as well as
enhanced photoluminescence in comparison with the complexes
bearing aliphatic ancillary ligands such as dipivaloylmethanate
(dpm) and acetylacetonate (acac) [17]. We here show the EL
performance of the solution-processed polymer light-emitting
diode (PLED), using 1 as a red emitting dopant. We also investigate
2 and 3, the dpm and acac analogues of 1, respectively, to elucidate
how the bdbp ligand affects the PL and EL properties of the dbfiq-
based bis-cyclometalated iridium(III) complex.
Fig. 1. UVevis absorption and photoluminescence spectra of 1 (a) and 2 (b) in toluene
solutions at 298 K.

1974
H. Tsujimoto et al. / Journal of Organometallic Chemistry 695 (2010) 1972e1978
Table 1
Table 2
UVevis absorption and photoluminescence spectral data of 1e3.
Electrochemical properties of 1e3.
Complex UVevisa,b
PL in
PL in
Compd
Eo1x/2/Va
Er1e/2d/Vb
HOMO/eVc
LUMO/eVc
solutiona,c
PMMA filme
1
2
3
0.82
0.97
0.93
ꢀ1.62
ꢀ1.58
ꢀ1.53
5.21
5.36
5.32
2.77
2.81
2.86
labs/nm (log
e
)
lPL/nm sPL
/m
sd FPL
lPL/nm FPL
1
2
3
342 (4.96), 494 (3.86) 640
337 (4.69), 492 (3.77) 639
336 (4.73), 492 (3.78) 639
1.07
1.04
0.93
0.61 635
0.55 634
0.49 638
0.17
0.08
0.06
a
The oxidation potential vs. SCE.
The reduction potential vs. SCE.
The HOMO and LUMO levels are represented versus vacuum.
b
c
a
b
c
Obtained in toluene at 298 K.
[Ir complex] ¼ 10
m
m
M.
M.
[Ir complex] ¼ 1.0
and 3 were identified by 1H NMR and MALDI-TOF mass spectra as
well as elemental analyses. One can see that there are geometric
isomers for the structure of the Ir(C^N)2(O^O)-type complex.
According to the literatures, X-ray crystallographic analyses indi-
cate that cis-C,C and trans-N,N configuration is more favored for
this type of complexes [7,11,20,21]. Although we tried to clarify the
configuration of 1e3 by X-ray crystallography, suitable single
crystals were not obtained. However, the elemental analyses
afforded satisfactory data to guarantee the purity of the present Ir
(III) complexes, and the 1H NMR spectrum, showing one signal set,
d
e
The
c
2 value for each data is 1.000e1.003.
Obtained for 1.5 wt%-doped PMMA spin-coated films (film thickness: ca.
100 nm) at ambient temperature.
2. Results and discussion
2.1. Synthesis of red phosphorescent Ir(III) complexes 1e3
The synthesis of Ir(III) complex 1 is shown in Scheme 1. The
preparation of diketone bdbp-H was previously reported [17]. The
cyclometalating ligand dbfiq-H was prepared by the Suzu-
kieMiyaura cross-coupling reaction of commercially available
dibenzo[b,d]furan-4-ylboronic acid and 1-chloroisoquinoline. The
indicated that each complex exists as
Supplementary data).
a single isomer (see
obtained dbfiq-H was subsequently reacted with IrCl3 to afford a
m-
chloro-bridged Ir(III) dimer (dbfiq)2Ir( -Cl)2Ir(dbfiq)2, which was
m
reacted with bdbp-H in 2-ethoxyethanol in the presence of Na2CO3
to afford 1 [15]. The yield of 1 significantly depended on the reac-
tion time, and the optimization of the reaction time was carried out
using an automatic synthesizer to obtain uniform conditions (see
Experimental and 4.3). The yield of 1 increased with the increase of
the reaction time, and the maximum yield of 55% was obtained at
3 h. Further extension of the reaction time led to the decrease in the
yield down to 15% at 24 h. The addition of Ag2O as a halide absorber
was not effective to improve the yield of 1 [18,19]. Under the
optimized reaction conditions, the yield of 1 was finally improved
up to 60% in a manual way (see Experimental and 4.2.3).
The Ir(III) complexes 2 and 3 with dpm and acac ancillary
ligands, respectively, were prepared as the reference compounds in
a similar way to the preparation of 1 (65 and 57% yields for 2 and 3,
respectively, from (dbfiq)2Ir(m-Cl)2Ir(dbfiq)2). The structures of 1, 2
Fig. 2. Time-resolved photoluminescence decay of 1 (1.0 mM) in deaerated toluene at
ambient temperature; excitation wavelength, 388 nm. Data were collected for
photoluminescence in the region of >550 nm.
Fig. 3. Luminanceevoltage (a) and current densityevoltage (b) characteristics of
PLEDs containing varying concentrations of 1 (x ¼ 0.13, 0.26, 0.51, 1.0, and 1.5 mol%).

H. Tsujimoto et al. / Journal of Organometallic Chemistry 695 (2010) 1972e1978
1975
observed PL was phosphorescence. Such a short phosphorescence
lifetime is effective to suppress tripletetriplet annihilation in
phosphorescent OLEDs [22]. The PL spectra of 2 and 3 were almost
identical to that of 1, showing the lPLs at 639 nm. This indicates that
the O^O ligand did not significantly affect the lPL. It is notable that
the present red phosphorescent Ir(III) complexes are very emissive:
the PL quantum yield (FPL) of 1 is 0.61 (in toluene at 298 K), and
much more emissive than well-known red phosphorescent mate-
rials such as Ir(piq)3 (FPL; 0.26 in toluene at 298 K) [9] and Ir
(btp)2(acac) (FPL; 0.21 in 2-methyltetrahydrofuran at 298 K) [15].
The FPLs of 2 and 3 are 0.55 and 0.49 (in toluene at 298 K),
respectively, a little smaller than that of 1. We also investigated the
PL properties of 1e3 in polymer matrices. The PL spectra of 1e3
were obtained for 1.5 wt%-doped poly(methyl methacrylate)
(PMMA) thin films, each of which was identical to that obtained in
solution. The FPLs were significantly reduced to 0.17, 0.08, and 0.06
for 1e3, in comparison to those obtained for the solutions.
However, 1 was more emissive than 2 and 3 even in the polymer
matrix. Thus, replacing dpm and acac to bdbp is beneficial to
enhance phosphorescence quantum efficiency of the present bis-
cyclometalated Ir(III) complex system. Also, owing to the bdbp
ancillary ligand, 1 is very soluble in organic solvents such as toluene
and chloroform. For example, the solubility of 1 in toluene is
28 mM, much larger than that of 3 (14 mM). This property is also
helpful to tune the concentration of the phosphorescent dopant in
a polymer host upon fabrication of PLED.
Fig. 4. Normalized electroluminescence spectra from PLEDs containing varying
concentrations of 1 (x ¼ 0.13, 0.26, 0.51, 1.0, and 1.5 mol%). Each spectrum was obtained
at the voltage that provides the maximum luminance.
2.2. UVevis absorption and photoluminescent properties
In Fig. 1 are shown UVevis absorption and photoluminescence
(PL) spectra of 1 and 2 in toluene at 298 K. The spectral data for 1e3
in solutions and polymer matrices are also summarized in Table 1,
accompanied by PL quantum yields and PL lifetimes. In the UVevis
spectrum of 1, strong structured absorption bands at <400 nm are
2.3. Electrochemical properties
In order to determine the HOMO-LUMO band gaps of the Ir(III)
complexes, the electrochemical properties were investigated. The
cyclic voltammograms were obtained for DMF solutions of 1e3
(1.0 mM), using 0.1 M tetrabutylammonium perchlorate as a sup-
porting electrolyte [11,23,24]. The redox potentials were recorded
relative to the Ag/AgCl reference electrode with Pt wire used for
both working and counter electrodes. Ferrocene (Cp2Fe) was used
as an internal reference (Cp2Fe/Cp2Feþ ¼ 0.41 V vs. SCE in DMF).
assigned to C^N ligand-centered (LC)
pe
p* transitions. The next
low-energy transitions at 400e500 nm can be assigned to the spin-
allowed metal-to-ligand charge transfer (1MLCT). The absorption
bands with comparable intensities to the 1MLCT transitions are also
observed at 500e600 nm, which are assigned to the spin-forbidden
3MLCT transitions. The relatively large intensities of the 3MLCT
transition bands indicate effective mixing of the 3MLCT level with
the 1MLCT due to the strong spin-orbit coupling caused by the Ir(III)
center [7,15]. In comparison to the spectrum of 1, the reduced molar
absorptivity was obtained in the near-UV region (300e400 nm) for
The complexes 1e3 showed reversible oxidation and reduction
ox
with half-wave potentials (E and Er1e/2d, respectively). The obtained
1/2
data are summarized in Table 2. The HOMO and LUMO levels were
estimated from the oxidation and reduction potentials, respec-
tively, assuming that the HOMO of Cp2Fe is 4.8 eV versus vacuum.
The HOMO of 1e3 ranged from 5.21 to 5.36 eV, and the LUMO did
from 2.77 to 2.86 eV. Thus, the HOMO and LUMO levels are inde-
pendent on the diketonate ancillary ligand.
2. This spectral difference arises from the intense pe
p* transition in
the extensively conjugated bdbp ligand of 1 masking the LC (C^N)
transition bands. The 3MLCT transition band was also observed for
2 in the same region as 1. The reference complex 3 exhibited
a similar absorption spectrum to 2.
The PL spectrum of 1 was obtained for a deaerated toluene
solution, and the lPL was observed at 640 nm, emitting pure red. As
2.4. Electroluminescent properties of PLEDs
shown in Fig. 2, the PL lifetime of 1.07
m
s (c2 ¼ 1.000), well fitted to
Using 1 as a phosphorescent dopant, the PLED based on poly(N-
vinylcarbazole) (PVCz) was fabricated [25,26]. The configuration of
a single-exponential decay, was obtained for 1, indicating that the
Table 3
EL device performance of PLED containing 1 as a phosphorescent dopant.
Devicea
lEL/nmb
Lmax/cd mꢀ2c
hc/cd Aꢀ1c
hp/lm Wꢀ1c
hext/%c
CIE (x, y)b
1 (x ¼ 0.13 mol%)
2 (x ¼ 0.26 mol%)
3 (x ¼ 0.51 mol%)
4 (x ¼ 1.0 mol%)
5 (x ¼ 1.5 mol%)
636
636
637
640
641
3620 (14.5)
5430 (15.0)
7270 (16.5)
5220 (17.5)
4620 (17.5)
2.2 (7.5)
2.9 (9.0)
3.9 (9.0)
1.9 (13.5)
1.7 (12.5)
0.93 (7.5)
0.99 (8.5)
1.4 (7.5)
0.45 (11.5)
0.47 (10.5)
3.5 (7.5)
4.9 (9.0)
6.4 (9.0)
3.5 (13.5)
3.3 (12.5)
(0.63, 0.29)
(0.68, 0.31)
(0.68, 0.31)
(0.69, 0.31)
(0.69, 0.31)
a
The emitting layer consists of PVCz, PBD and 1 (PVCz, 84.6e85.8 mol%; PBD, 13.8e14.0 mol%; 1, x mol%). The molar ratio of PVCz was represented by the concentration of
the vinylcarbazole monomer unit.
b
Obtained at the voltage where the maximum luminance was observed.
The maximum values of luminance (Lmax), current efficiency (hc), power efficiency (hp), and external quantum efficiency (hext) for the devices. The values in parentheses
c
are the voltages at which they were obtained.

1976
H. Tsujimoto et al. / Journal of Organometallic Chemistry 695 (2010) 1972e1978
Table 4
EL device performance of PLEDs containing 2 and 3 as a phosphorescent dopant.a
Ir complex
lEL/nmb
Lmax/cd mꢀ2c
hc/cd Aꢀ1c
hp/lm Wꢀ1c
hext/%c
CIE (x, y)b
2
3
636
639
4109 (16.5)
4575 (16.5)
1.9 (10.5)
1.6 (10.5)
0.56 (10.5)
0.49 (10.5)
3.3 (10.5)
2.9 (10.5)
(0.68, 0.31)
(0.68, 0.31)
a
The device structure was the same as that of Device 3 (x ¼ 0.51 mol%) in Table 4.
b
c
Obtained at the voltage where the maximum luminance was observed.
The maximum values of luminance (Lmax), current efficiency (hc), power efficiency (hp), and external quantum efficiency (hext) for the devices. The values in parentheses
are the voltages at which they were obtained.
the device is ITO (transparent anode, 150 nm)/PEDOT:PSS (40 nm)/
emitting layer (EML, 100 nm)/CsF (1.0 nm)/Al (cathode, 250 nm),
where the EML consists of PVCz (84.6e85.7 mol%, based on the
vinylcarbazole monomer unit), PBD (13.9e14.0 mol%), and 1
(0.13e1.5 mol%). We especially focused on the concentration of 1 to
optimize the ratio of the constituent materials of the EML. The
PEDOT:PSS and PVCz emitting layers were successively spin-coated
on ITO, and the CsF layer and the Al cathode were placed by the
vacuum deposition. In Fig. 3 are shown luminanceevoltage (LeV)
and current densityevoltage (JeV) relationships of the PLED at
different concentrations of 1 (x mol%). The EL spectra of the devices
are shown in Fig. 4. The device performance is also summarized in
Table 3. As the increasing voltage was applied, the electric current
emerged at 4.5e6.0 V along with red EL. The emission maximum
was slightly red-shifted from 636 to 642 nm when the concentra-
tion of 1 increased. This red shift can be explained by excimer
formation of 1 that partially contributes to the EL spectrum
[11,24,27]. The maximum luminance (Lmax) was also affected by the
concentration of 1. At x ¼ 0.13 mol%, Lmax was just 3620 cd mꢀ2 at
14.5 V, corresponding to 1.3 cd Aꢀ1, and the EL spectra was
accompanied with weak emission at 436 nm assigned to the PVCz-
PBD exciplex [28], indicating that the excitons of PVCz were not
sufficiently consumed for the electrophosphorescence from 1. The
addition of the dopant at the ratio of x ¼ 0.51 mol% led to the
increase in Lmax up to 7270 cd mꢀ2 (@16.5 V) corresponding to
2.6 cd Aꢀ1. This dopant concentration also afforded better PLED
performance; hj max ¼ 3.9 cd Aꢀ1 (@9.0 V), hp max ¼ 1.4 lm Wꢀ1
(@7.5 V), and hext max ¼ 6.4% (@9.0 V). Further doping led to the
decrease in Lmax as well as the device performance. It is worthy to
note that the present device exhibits the Commission Inter-
nationale de L’Eclairage (CIE) chromaticity coordinate of (0.68,
0.31), shifting to the pure red region over the National Television
System Committee (NTSC) standard of red, (0.67, 0.33).
respectively), both of which gave the CIE chromaticity coordinate of
(0.68, 0.31). The device characteristics are summarized in Table 4 and
obviously inferior to PLED containing 1 (see Table 4, x ¼ 0.51 mol%).
This tendency corresponds to that for the PL quantum yields in
solutions and PMMA films. In Fig. 5 is shown the HOMO/LUMO levels
and the triplet energy of the materials used in the present PLED. The
HOMO and LUMO levels of 1e3 are almost similar to one another and
embedded between the HOMO of PVCz (5.80 eV) and the LUMO of
PBD (2.40 eV) [29]. Thus, efficient hole and electron transfer to the Ir
(III) complex should occur in the emitting layer. Also, as the triplet
energy of the iridium complexes (1.94e1.95 eV) are lower than those
of PVCz (2.46 eV) [30] and PBD (2.46 eV) [31], the back energy transfer
should be suppressed. Taking this into consideration, the PLED
performance in luminance should be determined by the intrinsic
quantum efficiency of 1e3 in radiative decay.
3. Conclusions
In conclusion, we developed novel pure red phosphorescent bis-
cyclometalated Ir(III) complexes 1e3, employing dbfiq C^N ligand.
Although the O^O ancillary ligand does not affect the emission
wavelength, the employment of the bdbp ligand, in place of acac
and dpm, gives rise to the increase in FPL up to 0.61 in toluene at
298 K. The bdbp ligand enhanced the solubility of the dbfiq-based
bis-cyclometalated Ir(III) complexes in organic solvents. Using
these red phosphorescent complexes as emitters, PVCz-based
PLEDs were fabricated. Pure red EL with the CIE chromaticity
coordinate of (0.68, 0.31) is obtained from 1e3, and the PLED with 1
shows better performance (Lmax ¼ 7270 cd mꢀ2 at 16.5 V, hj
¼ 3.9 cd Aꢀ1 at 9.0 V, hp
¼ 1.4 lm Wꢀ1 at 7.5 V, hexp
max
max
¼ 6.4% at 9.0 V), in comparison to those with 2 and 3. Taking
max
into consideration that the HOMO-LUMO levels and triplet energy
of 1e3 are similar (HOMO, 5.21e5.36 eV; LUMO, 2.77e2.86 eV;
triplet energy, 1.94e1.95 eV), the PLED performance predominantly
depends on the phosphorescent quantum efficiency of the emitter.
From these results, 1 is an excellent red phosphorescent material
for solution-processed OLEDs.
We also prepared PLEDs using the reference complexes 2 and 3.
The device structure was the same as that for 1, and the concentration
of the Ir(III) complex was adjusted to 0.51 mol%. The EL spectra from 2
and 3 were almost identical to that of 1 (lEL ¼ 639 and 636 nm,
4. Experimental
4.1. General
1H NMR spectra were obtained on a Jeol JNM-LA300 (300 MHz),
a Jeol JNM-LA400 (400 MHz), or a Jeol JNM-ECX400 (400 MHz)
spectrometer, using TMS as an internal standard (0.00 ppm).
Matrix-assisted laser desorption/ionization time-of-flight (MALDI-
TOF) mass spectra were measured on a Shimadzu Kratos Compact
MALDI 2 using shinapinic acid as a matrix. Elemental analyses were
carried out on a Yanako CHN CORDER MT-3 analyzer.
Cyclic voltammograms were recorded on a Hokuto Denko HZ-
5000 electrochemical measurement system at a scanning rate of
100 mV/s. The measurements were performed for the Ir(III)
complexes in freshly distilled DMF, where 0.1 M tetrabutylammo-
nium perchlorate (Bu4NþClO4ꢀ) was used as a supporting electro-
lyte. The potentials were recorded relative to a Ag/AgCl reference
Fig. 5. Energy diagram of the present PLED. The red and blue solid lines represent the
HOMO and LUMO levels, respectively, and the green dashed lines represent the T1
levels (For interpretation of the references to colour in this figure legend, the reader is
referred to the web version of this article).

H. Tsujimoto et al. / Journal of Organometallic Chemistry 695 (2010) 1972e1978
1977
electrode with Pt wire used for both working and counter elec-
trodes. Ferrocene was used as an internal reference (Cp2Fe/
Cp2Feþ ¼ 0.41 V vs. SCE in DMF).
ethoxyethanol (83 mL) was added a solution of IrCl3$3H2O (740 mg,
2.10 mmol) in water (27 mL), and the mixture was heated on the oil
bath for 24 h, where the bath temperature was kept at 105 ꢂC. After
cooling, water (400 mL) was added, and the precipitate was collected
by filtration. The obtained precipitate was washed with ethanol
(100 mL) and hexane (100 mL) to obtain the titled compound as a red
solid (1.45 g, 0.89 mmol, 85%). This material was highly insoluble, and
thus, used in the next reaction without further purification.
UVevis and photoluminescence (PL) spectra were measured on
a Shimadzu UV-3100 and a Jasco FP-6600 spectrophotometer,
respectively, using a quartz cell. PL lifetimes were obtained on
a Horiba Jobin Yvon FluoroCube spectroanalyzer using a 388 nm
nanosecond-order LED light source. PL quantum yields were
obtained on a Hamamatsu Photonics C9920 PL quantum yield
measurement system. The sample solutions for optical and pho-
tophysical measurements were deaerated by argon bubbling fol-
lowed by complete sealing, and the analyses were carried out just
after preparation of the samples.
4.2.3. Preparation of bis[1-(dibenzo[b,d]furan-4-yl)isoquinolinato-
0
N,C3 ]iridium(III) [1,3-bis(3,4-dibutoxyphenyl)propane-1,3-dionate-
O,O] (1)
A mixture of (dbfiq)2Ir(m-Cl)2Ir(dbfiq)2 (500 mg, 0.306 mmol),
For the preparation of dbfiq-H, dibenzo[b,d]furan-4-ylboronic
acid and 1-chloroisoquinoline were purchased from SigmaeAldrich
Co.. As a catalyst for the SuzukieMiyaura cross-coupling reaction,
Pd(PPh3)4 was purchased from Wako Pure Chemical Industries, Ltd.
1,2-Dimethoxyethane was purchased from Wako Pure Chemical
Industries, Ltd. For the preparation of 1e3, 2-ethoxyethanol was
purchased from Wako Pure Chemical Industries, Ltd and used as
freshly opened. The preparation of 1,3-bis(3,4-dibutoxyphenyl)
propane-1,3-dione (bdbp-H) was previously reported [17].
For fabrication of PLEDs, poly(N-vinylcarbazole) (PVCz,
Mw ¼ 25,000e50,000) was purchased from SigmaeAldrich Co., and
used after purification by reprecipitation from THF to methanol.
Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:
PSS, Clevios P VP AI.4083) was purchased from G.H. Starck Clevios
GmbH, and 2-(4-biphenylyl)-5-(4-tert-butylphenyl-1,3,4-oxadia-
zole) (PBD) was purchased from Tokyo Chemical Industry Co., Ltd.
Cesium fluoride and Al wires were purchased from Wako Pure
Chemical Industries, Ltd and The Nilaco Corporation, respectively.
Pre-patterned indium-tin-oxide (ITO, 150 nm thickness) glass
bdbp-H (512 mg, 1.00 mmol) and Na2CO3 (200 mg, 1.89 mmol) in
ethoxyethanol (40 mL) was heated at 130 ꢂC for 3 h under nitrogen.
After cooling, the reaction mixture was poured into water (200 mL)
and extracted by chloroform (150 mL ꢁ 2). The organic layer was
combined and washed sat. brine (300 mL ꢁ 3), and then dried over
anhydrous Na2SO4. The solvent was removed on a rotary evapo-
rator, and the residue was purified by silica gel column chroma-
tography using chloroform as eluent. Further purification was
carried out by recrystallization from hexane/ethanol to obtain 1 as
a red powder (475 mg, 0.367 mmol, 60%); 1H NMR (400 MHz,
CDCl3)
d
0.76 (t, J ¼ 7.6 Hz, 6H), 0.91 (t, J ¼ 7.6 Hz, 6H), 1.15e1.24 (m,
4H), 1.37e1.55 (m, 8H), 1.68e1.75 (m, 4H), 3.49e3.67 (m, 4H), 3.93
(t, J ¼ 6.6 Hz, 4H), 6.44e6.46 (m, 3H), 6.69 (d, J ¼ 8.3 Hz, 2H), 7.11 (d,
J ¼ 1.8 Hz, 2H), 7.21e7.36 (m, 8H), 7.49 (d, J ¼ 8.3 Hz, 2H), 7.53 (d,
J ¼ 6.3 Hz, 2H), 7.70e7.79 (m, 6H), 7.92 (d, J ¼ 7.6 Hz, 2H), 8.58 (d,
J ¼ 6.3 Hz, 2H), 9.12 (d, J ¼ 8.3 Hz, 2H); MALDI-TOF MS m/z 1292
([M]þ). Anal. Calcd for C73H67IrN2O8: C 67.83; H 5.22, N 2.17, Found:
C 67.80; H 5.27, N 2.12.
substrates with sheet resistance of ca. 10
Sanyo Vacuum Industries Co., Ltd.
U
/, were purchased from
4.2.4. Preparation of bis[1-(dibenzo[b,d]furan-4-yl)isoquinolinato-
0
N,C3 ]iridium(III) (2,2,6,6-tetramethylheptane-3,5-dionate-O,O) (2)
A mixture of (dbfiq)2Ir(m-Cl)2Ir(dbfiq)2 (200 mg, 0.123 mmol),
4.2. Preparation of materials
dipivaloylmethane (50.0 mg, 0.271 mmol) and Na2CO3 (100 mg,
0.720 mmol) in 2-ethoxyethanol (40 mL) was heated at 130 ꢂC for
1.5 h under nitrogen. After cooling, water (150 mL) was added, and
the precipitate was collected by filtration. The obtained solid was
dissolved in chloroform, and the organic was washed with water
(100 mL ꢁ 2) and sat. brine (100 mL), and then dried over anhy-
drous Na2SO4. The solvent was removed on a rotary evaporator, and
the residue was purified by alumina column chromatography using
dichloromethane as eluent to obtain 2 as a red powder (153 mg,
4.2.1. Preparation of 1-(dibenzo[b,d]furan-4-yl)isoquinoline (dbfiq-
H)
To a mixture of 1-chloroisoquinoline (1.74 g, 10.1 mmol) and Pd
(PPh3)4 (403 mg, 0.349 mmol) in 1,2-dimethoxyethane (50 mL) were
added a solutionofdibenzofuran-4-ylboronicacid(2.30g,10.9 mmol)
in ethanol (50 mL), followed by addition of 2.6 M sodium carbonate
(50 mL). Then, the mixture was heated at reflux for 19 h under
nitrogen. Aftercooling, ethylacetate(50mL)andwater(100mL)were
added, and the insoluble materials were removed by filtration. The
filtrate was divided into two layers, and the upper organic layer was
separated using a separation funnel. The lower was extracted with
ethyl acetate (30 mL ꢁ 2), and all the organic layers were combined,
washed with water (100 mL ꢁ 3) and sat. brine (100 mL), and dried
over anhydrous Na2SO4. The solvent was removed on a rotary evap-
orator, and the residue was purified by silica gel column chromatog-
0.159 mmol, 65%); 1H NMR (400 MHz, CDCl3)
d 0.76 (s,18H), 5.40, (s,
1H), 6.52 (d, J ¼ 7.9 Hz, 2H), 7.20e7.35 (m, 6H), 7.49 (d, J ¼ 7.9 Hz,
2H), 7.54 (d, J ¼ 6.2 Hz, 2H), 7.70e7.81 (m, 6H), 7.94 (d, J ¼ 7.9 Hz,
2H), 8.37 (d, J ¼ 6.2 Hz, 2H), 9.14 (d, J ¼ 8.4 Hz, 2H); MALDI-TOF MS
m/z 964 ([M]þ). Anal. Calcd for C53H43IrN2O4: C 66.02; H 4.50, N
2.91. Found: C 65.66; H 4.69, N 2.82.
4.2.5. Preparation of bis[1-(dibenzo[b,d]furan-4-yl)isoquinolinato-
0
raphy using chloroform as eluent to afford
a
yellow solid.
N,C3 ]iridium(III) (pentane-2,4-dionate-O,O) (3)
Recrystallization from chloroformehexane gave awhite crystal of the
A mixture of (dbfiq)2Ir(m-Cl)2Ir(dbfiq)2 (210 mg, 0.129 mmol),
titled compound in 73% yield (2.19 g, 7.43 mmol); 1H NMR (400 MHz,
acetylacetone (29.0 mg, 0.284 mmol) and Na2CO3 (105 mg,
0.991 mmol) in 2-ethoxyethanol (40 mL) was stirred at 130 ꢂC for
3 h under nitrogen. After cooling, chloroform (200 mL) was added,
and the organic layer was washed with water (200 mL ꢁ 3) and sat.
brine (200 mL), and then dried over anhydrous Na2SO4. The solvent
was removed on a rotary evaporator, and the red compound was
obtained. And recrystallization from hexane/ethanol gave a red
powder of 3 in 57% yield (129 mg, 0.147 mmol); 1H NMR (400 MHz,
CDCl3)
d 7.35e7.39 (m, 1H), 7.42e7.44 (m, 2H), 7.48e7.56 (m, 2H),
7.69e7.75 (m, 2H), 7.78 (d, J ¼ 7.8 Hz,1H), 7.84 (dd, J ¼ 0.9 and 8.7 Hz,
1H), 7.95 (d, J ¼ 8.7 Hz, 1H), 8.03 (dd, J ¼ 1.5 and 7.8 Hz, 1H), 8.12 (dd,
J ¼ 1.5 Hz and 7.8 Hz,1H), 8.73 (d, J ¼ 5.9 Hz,1H); MALDI-TOF MS m/z
296 ([M þ H]þ). Anal. Calcd for C21H13NO: C 85.40; H 4.44, N 4.74,
Found: C 85.26; H 4.64, N 4.70.
4.2.2. Preparation of (dbfiq)2Ir(
This compound was prepared according to the conventional
procedure [11,32]. To a solution of dbfiq-H (1.25 g, 4.32 mmol) in 2-
m
-Cl)2Ir(dbfiq)2
CDCl3)
d
1.74 (s, 6H), 5.21 (s, 1H), 6.37 (d, J ¼ 7.8 Hz, 2H), 7.19e7.34
(m, 6H), 7.47 (d, J ¼ 8.4 Hz, 2H), 7.63 (d, J ¼ 6.0 Hz, 2H), 7.70e7.84
(m, 6H), 7.98 (d, J ¼ 7.3 Hz, 2H), 8.50 (d, J ¼ 6.3 Hz, 2H), 9.12 (d,

1978
H. Tsujimoto et al. / Journal of Organometallic Chemistry 695 (2010) 1972e1978
J ¼ 8.4 Hz, 2H); MALDI-TOF MS m/z 880 ([M]þ). Anal. Calcd for
C47H31IrN2O4$2H2O: C 61.63; H 3.85, N 3.06. Found: C 61.72; H 3.58,
N 3.13.
Appendix. Supplementary data
Supplementary data associated with this article can be found in
the online version, at doi:10.1016/j.jorganchem.2010.05.11.
4.3. Optimization of preparation of 1
References
The investigation of optimized reaction time for the preparation
of 1 was carried out using Metler Malti-Max 750 automatic
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The pre-patterned ITO glass substrate was routinely cleaned by
ultrasonic treatment in detergent solution, distilled water, meth-
anol, acetone, chloroform, and hexane, and then treated in boiled
2-propanol, followed by drying with gentle flow of dry air. PEDOT:
PSS (40 nm) was spin-coated on the ITO layer pre-treated with
UV-O3, and then dried at 120 ꢂC for 30 min under Ar. For fabri-
cation of EML, a mixture of PVCz, PBD, and the Ir(III) complex in
toluene (PVCz; 10 mg/0.7 mL of toluene) was filtered through
a 0.2 mm Millex-FG filter (Millipore Co.). Then, this stock solution
was spin-coated onto the PEDOT:PSS layer under Ar. As the
complexes 1e3 are different in molecular weight, the ratio of
the components of EML was defined by molar percentage, where
the concentration of PVCz was represented by the vinylcarbazole
unit. The fabricated EML contained 84.6e85.8 mol% of the vinyl-
carbazole unit, 13.8e14.0 mol% of PBD, and 0.13e1.5 mol% of Ir(III)
complex. Thereafter, CsF (1.0 nm) and Al (250 nm) layers were
successively deposited onto the organic layers by vacuum depo-
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covered with a glass cap and encapsulated by a UV-curing epoxy
resin under argon atmosphere to prevent oxidation of the cathode
and the organic layer. The area of the emitting part was 10 mm2.
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dry argon. The configuration of PLED was ITO (anode)/PEDOT:PSS
(40 nm)/PVCz:PBD:Ir(III) complex (100 nm)/CsF (1.0 nm)/Al
(cathode, 250 nm).
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Acknowledgement
The authors express their thanks to Horiba, Ltd for helpful
advice in the photoluminescence lifetime measurement.
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