Finely-tuned blue-phosphorescent iridium complexes based on 2-phenylpyridine derivatives and application to polymer organic light-emitting device

Article abstract of DOI:10.1246/cl.2006.748

We have developed new blue-phosphorescent iridium complexes based on 2-phenylpyridine derivatives with pertinent substituents and an ancillary ligand to accomplish selective tuning of the HOMO and LUMO levels toward high energy gap: A polymer light-emitti

Full text of DOI:10.1246/cl.2006.748

748
Chemistry Letters Vol.35, No.7 (2006)
Finely-tuned Blue-phosphorescent Iridium Complexes Based on 2-Phenylpyridine Derivatives
and Application to Polymer Organic Light-emitting Device
Shin-ya Takizawa,1 Hidenori Echizen,1 Jun-ichi Nishida,1 Toshimitsu Tsuzuki,2 Shizuo Tokito,2 and Yoshiro Yamashitaꢀ1
1Department of Electronic Chemistry, Interdisciplinary Graduate School of Science and Engineering,
Tokyo Institute of Technology, Nagatsuta, Midori-ku, Yokohama 226-8502
2NHK Science and Technical Research Laboratories, Kinuta, Setagaya-ku, Tokyo 157-8510
(Received February 9, 2006; CL-060171; E-mail: yoshiro@echem.titech.ac.jp)
We have developed new blue-phosphorescent iridium com-
R
R
R
F
plexes based on 2-phenylpyridine derivatives with pertinent sub-
stituents and an ancillary ligand to accomplish selective tuning
of the HOMO and LUMO levels toward high energy gap: A
polymer light-emitting device doped with the blue phosphor
was fabricated to show blue emission.
N
N
N
LDA/I
THF
Me SiCF /CuI/KF
2
3
3
F
F
I
NMP
F C
3
F
F
F
6a: R = H
6b: R = Me
7a : R = H (83%)
7b: R = Me (56%)
8a: R = H (32%)
8b: R = Me (33%)
Recently, phosphorescent platinum or iridium complexes
have become important emitting-materials for organic light-
emitting diodes (OLEDs) because their efficient phosphores-
cence which originates from strong spin–orbit coupling leads
to high internal quantum efficiencies in devices.1–3 In particular,
Ir(ppy)3 (ppy: 2-phenylpyridine) and its derivatives have attract-
ed much attention, and systematic control of the emission color
has been carried out by introduction of substituents on the li-
gands or extension of ꢀ-conjugation.3 Among them, blue-phos-
phorescent materials have been extensively investigated because
these are strongly required for applications in full-color display.2
The concept of molecular design for blue-phosphorescent
complexes with 2-phenylpyridines is based on the fact that the
HOMO locates on the phenyl-ꢀ and iridium-d orbitals, while
the LUMO is localized on the pyridine ring.4 In this regard, high-
energy phosphorescence has been achieved by introducing elec-
tron-withdrawing groups on the phenyl ring to lower the HOMO
level. For example, fluorine substitution at the meta position to Ir
on the phenyl ring is effective to lower the HOMO level,3a,3d,3f
and a trifluoromethyl (CF3) group at the para position has the
similar effect.3a,3f However, the color purities and quantum effi-
ciencies of blue-phosphorescent materials are still unsatisfacto-
ry. In this context, we presumed that if both fluorine and CF3
groups are introduced at the suitable positions on the phenyl
ring, the HOMO energy level would be largely lowered to obtain
the desired deep-blue color purity of phosphorescence. In this
paper, we clearly demonstrated that the proposed method is
greatly effective to blue-shift the emission compared to the re-
spective introduction of these substituents on the ppy ligand.
Scheme 2. Synthesis of the ligands.
Furthermore, combination with the ancillary ligand of 2-pyridyl-
triazolate and a methyl substitution on the pyridine ring resulted
in rare deep-blue phosphorescence at room temperature, whose
emission maximum (447 nm) is remarkably shorter by 21 nm
than that of well-known blue emissive iridium bis(4,6-difluoro-
phenylpyridinato)picolate (FIrpic) (1).2a,2b We report here the
synthesis and characterization of new blue-phosphorescent iridi-
um complexes 2–5 (Scheme 1) along with the polymer light-
emitting device (PLED) doped with the blue phosphor.
The new ligands 8a and 8b were readily prepared via
iodination of difluorophenylpyridine derivatives 6 as shown in
Scheme 2. The iodides 7a and 7b were obtained in 83 and
56% yields, respectively, by the reaction with iodine after treat-
ment of 6 with LDA. The reaction of the iodides 7a and 7b with
(trifluoromethyl)trimethylsilane in the presence of CuI and KF
gave the CF3-substituted ligands 8a and 8b in 32 and 33% yields,
respectively. The biscyclometalated Ir complexes 2–5 were pre-
pared by a conventional two-step reaction3b via dichloro-bridged
dimer complexes in moderate total yields (20–58%).
Figure 1 shows the photoluminescence (PL) spectra of the
complexes 1–5, and the photophysical and electrochemical data
are listed in Table 1. In the absorption spectrum of CF3-substi-
tuted complex 2,5 the band, which can be assigned to the singlet
metal to ligand charge-transfer (1MLCT) transition, was ob-
1.2
1
1.0
2
0.8
3
R
2
R
2
N
0.6
0.4
0.2
0.0
4
5
N
N
O
N
N
Ir
Ir
F
F
O
N
N
R
1
R
1
CF
3
2
F
2
F
1: R =H, R = H
2: R = CF , R = H
3: R = CF , R = Me
4: R =CF , R = H
5: R = CF , R = Me
1
2
1
1
3
2
2
400
450
500
550
600
650
700
1
3
2
3
Wavelength /nm
1
3
2
Scheme 1. Structures of the Ir complexes.
Figure 1. PL spectra of the Ir complexes in CH2Cl2.
Copyright ꢀ 2006 The Chemical Society of Japan

Chemistry Letters Vol.35, No.7 (2006)
749
Table 1. Photophysical and electrochemical properties of 1–5
em/nma
5 wt % 5 (100 nm)/Ba (3 nm)/Al (100 nm). The device showed
clear electroluminescent characteristics.5 It exhibited blue emis-
sion whose spectrum is similar to the PL spectrum of 5 in solu-
tion, and no emission from PVK was observed even at high cur-
rent densities. The maximum external quantum efficiency and
luminance efficiency were 1.2% and 2.3 cd/A, respectively.
The Commission Internationale de l’Eclairage (CIE) coordinates
at 100 cd/m2 are in a blue region (x ¼ 0:20, y ¼ 0:25).
Complex
ꢁ
ꢀb
E1=2ox/Vc
1
2
3
4
5
468, 495
457, 485
454, 481
451, 480
447, 476
0.11
0.27
0.33
0.28
0.19
0.90
1.14
1.10
1.31
1.27
aIn CH2Cl2. bIn CH2Cl2 using 9,10-diphenylanthracene (ꢀ ¼
0:81) as a reference. cIn MeCN containing 0.1 mol dmꢁ3 n-
Bu4NPF6, vs Cp2Fe/Cp2Feþ.
In conclusion, a series of new blue-phosphorescent iridium
complexes based on ppy derivatives were prepared, and the sys-
tematic control of the HOMO and LUMO levels was carried out
to achieve high energy gap by combining substituents and ancil-
lary ligand effects. We succeeded in preparing complex 5 which
shows deep-blue phosphorescence (447 nm) at room tempera-
ture. The introduction of both fluorine and CF3 groups on the
phenyl ring was found to be effective to lower the HOMO level.
This methodology would provide a new approach to obtain high-
energy phosphorescence, and be applicable to other systems
containing metal–phenyl fragments. Our preliminary EL study
demonstrated that complex 5 would be a potential candidate
for deep-blue emitter in PLEDs, and the optimization of device
structure is currently underway.
served at 374 nm. The shoulder at a longer wavelength around
400 nm can be attributed to a mixture of 3MLCT and ligand
centered triplet ꢀ–ꢀꢀ transition. These bands were apparently
blue-shifted compared to those of FIrpic (1), although the shape
is similar to each other. Interestingly, complex 2 exhibited blue
emission at room temperature and its emission maximum
(457 nm) is shorter by 11 nm than that of 1. This can be attributed
to the lower HOMO level of 2 since the oxidation potential of 2
is much higher than that of 1 (see Table 1). This result indicates
that the CF3-substitution at that position greatly affects the
HOMO energy level. On the other hand, complex 3 having a
methyl group at the para position with respect to Ir on the
pyridine ring showed a further 3 nm blue shift (454 nm). This
may be explained by considering that the LUMO energy level,
whose orbital is localized on the pyridine ring, is raised. This
is supported by the fact that the oxidation potential is almost
insensitive to the methyl substitution (2: 1.14 V; 3: 1.10 V) even
though the blue shift was observed. The similar blue shift by
methyl-substitution on the pyridine ring has been also observed
in Ir(ppy)3.3g
Further blue shifts were achieved by replacement of the
picolate ligand with 3-trifluoromethyl-5-(2-pyridyl)-1,2,4-tri-
azolate.2d Thus, the emission maximum (451 nm) of complex 4
became shorter by 6 nm than that of complex 2. This is ascribed
to the lower HOMO energy level since complex 4 showed a
higher oxidation potential (1.31 V) than 2 (1.14 V). In addition,
methyl-substitution on the pyridine rings of complex 4, leading
to complex 5, afforded the bluest emission (447 nm). This
hypsochromic shift (4 nm) is consistent with the shift observed
in complexes 2 and 3 (3 nm). The absorption spectrum provides
an additional evidence for the largest HOMO–LUMO gap of
complex 5, where the absorption bands appear in shorter wave-
length region compared with other complexes.5 In addition, the
emission lifetime of 5 (2.93 ms) suggests that its blue emission
originates from triplet excited state. Such a deep-blue phospho-
rescence is rare to be observed at room temperature, although
there are some iridium and platinum complexes exhibiting pure
blue emission only at low temperatures.1c,3d
This work was supported by The 21st Century COE program
and a Grant-in-Aid for Scientific Research on Priority Areas
(No. 15073212) from the Ministry of Education, Culture, Sports,
Science and Technology, Japan.
References and Notes
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2
3
The PLED doped with complex 5 was fabricated by a wet
process. PLEDs fabricated using wet processes have been impor-
tant for the practical applications in large-area devices owing to
their simple and low-cost fabrication processes.6 Poly(N-vinyl-
carbazole) (PVK) was selected as a host material because it is
known to have high triplet energy along with high hole mobili-
ty.7 The simple device structure is as follows: indium tin oxide
(ITO)/poly(ethylenedioxythiophene) doped with poly(stylene-
sulfonate) (PEDOT/PSS) as a hole-injecting layer (30 nm)/
PVK and electron-transporting 1,3-bis[(4-tert-butylphenyl)-
1,3,4-oxadiazolyl]phenylene (OXD-7) blend layer doped with
4
5
P. J. Hay, J. Phys. Chem. A 2002, 106, 1634.
The absorption spectra, detailed experimental procedure, and PLED
performance data are available in Electronic Supporting Informa-
tion.
6
7
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Published on the web (Advance View) June 3, 2006; doi:10.1246/cl.2006.748