Synthesis and characterization of novel iridium complexes with ligands of 2-phenylimidazo[1,2-a]pyridine derivatives and application to organic light-emitting diode

Article abstract of DOI:10.1246/cl.2005.1222

Novel iridium complexes with ligands of 2-phenylimidazo[1,2-a]pyridines were prepared, and the emission maxima were found to be unusually dependent on the substituents on the phenyl ring. An OLED using the derivative was fabricated to show white emission.

Full text of DOI:10.1246/cl.2005.1222

1222
Chemistry Letters Vol.34, No.9 (2005)
Synthesis and Characterization of Novel Iridium Complexes
with Ligands of 2-Phenylimidazo[1,2-a]pyridine Derivatives
and Application to Organic Light-emitting Diode
Shin-ya Takizawa, Jun-ichi Nishida, Toshimitsu Tsuzuki,y Shizuo Tokito,y and Yoshiro Yamashitaꢀ
Department of Electronic Chemistry, Interdisciplinary Graduate School of Science and Engineering,
Tokyo Institute of Technology, Nagatsuta, Midori-ku, Yokohama 226-8502
yNHK Science and Technical Research Laboratories, Kinuta, Setagaya-ku, Tokyo 157-8510
(Received June 13, 2005; CL-050759)
Novel iridium complexes with ligands of 2-phenyl-
of the dimer complexes with acetylacetone in the presence of so-
dium carbonate.
imidazo[1,2-a]pyridines were prepared, and the emission maxi-
ma were found to be unusually dependent on the substituents on
the phenyl ring. An OLED using the derivative was fabricated to
show white emission.
In the absorption spectrum of the complex 1,7 the band,
which can be assigned to the singlet metal to ligand charge-
transfer (1MLCT) transition, was observed at 387 nm. The band
3
at around 440 nm can be attributed to a mixture of MLCT and
ligand-centered triplet ꢁ–ꢁꢀ transition.
The emission maxima and oxidation potentials are summa-
rized in Table 1. Figure 1 depicts the emission spectra of
complexes. The complex 1 exhibits two main peaks at 516 and
537 nm. On the other hand, the emission maximum of 2 is
red-shifted by 31 nm compared to that of 1 owing to the effect
of extended ꢁ conjugation without a remarkable change of the
HOMO level, as supported by the electrochemical data (see
Recently, phosphorescent materials such as iridium (Ir) and
platinum (Pt) complexes have attracted great attention because
of their applications to organic light-emitting diodes (OLEDs).1,2
Particularly, the devices using Ir complexes show high external
quantum efficiencies and the emission wavelengths greatly de-
pend on the organic ligands.3,4 For instance, the effects of sub-
stituents on the ppy (2-phenylpyridine) ligand have been exten-
sively studied to afford blue or red phosphors so far.4 However,
Ir complexes based on new ligands besides ppy, as well as the
relationship between ligand structures and photophysical proper-
ties, have not been well explored. For the development of new
phosphors, 2-phenylimidazo[1,2-a]pyridine seems to be a prom-
ising ligand because of the following advantages. First, the LU-
MO energy level of 2-phenylimidazo[1,2-a]pyridine is relatively
high,5 so it is considered to be useful for making the emission
blue-shifted. Second, various derivatives can be easily prepared
by a mild one-step reaction from the corresponding 2-aminopyri-
dines and 2-bromoacetophenones,6 leading to easy functionali-
zation and tuning of the emission color. In this context, we have
now prepared and characterized a series of Ir complexes with
these ligands. In addition, an OLED using one of the complexes
as a dopant emitter was fabricated here.
2-Phenylimidazo[1,2-a]pyridine and its derivatives were
prepared in good yields (62–95%) from the corresponding 2-
aminopyridines and 2-bromoacetophenones (Scheme 1). The Ir
complexes containing acetylacetonate as an ancillary ligand
were prepared by two-step reactions via Ir(III)-ꢀ-chloro-bridged
dimer complexes according to a conventional procedure.4b The
dimer complexes could be obtained from iridium(III) chloride
trihydrate and the free ligands in moderate yields. The final Ir
complexes 1–4 were prepared in 20–29% yields by treatment
Table 1). This red shift is smaller than that observed in bis(2-
0
phenylpyridinato-N,C2 )acetylacetonate (Ir(ppy)2(acac))4b and
Ir(NaPy)2(acac) [NaPy: 2-(1-naphthalenyl)pyridine]4e (84 nm).
This difference may be explained by the fact that the LUMO
of the 2-phenylimidazopyridine is almost localized in the imida-
zopyridine side in contrast to ppy whose LUMO is extended to
the phenyl group as revealed by the MO caluculations.8 This fact
is supported by the following investigation of the electron-with-
drawing substituent effects in the complexes 3 and 4. In the cor-
responding Ir complexes with ppy ligands, fluorine substitution
at the meta position to Ir on the phenyl ring is the most effective
to lower the HOMO level based on a mixture of phenyl–ꢁ and
Ir–d orbitals, whereas trifluoromethyl and perfluorophenyl
groups at the ortho or para position make the HOMO level
lower.4a,4c The similar effect was observed in our study with
regard to fluorine substitution: the emission maximum of the
complex 3 (497 nm) was shorter by 19 nm than that of the non-
substituted complex 1. This can be explained by considering that
the HOMO level is lowered since the oxidation potential of 3 is
much higher than that of 1 (Table 1). In addition, these shifts of
N
N
N
N
O
O
O
O
Ir
Ir
R4
R3
O
NaHCO3
EtOH
N
N
+
Br
Ar
Ar
R1
2
N
NH2
R2
2
2
Ar: phenyl (86 %)
naphthyl (62%)
1: R1 = R2 = R3 = R4 = H
3: R1 = H, R2 = F, R3 = H, R4 = F
4: R1 = CF3, R2 = H, R3 = CF3, R4 = H
2,4-difluorophenyl (95%)
3,5-bis(trifluoromethyl)phenyl (72%)
Scheme 1. Synthesis of 2-phenylimidazo[1,2-a]pyridines.
Scheme 2. Structures of iridium complexes.
Copyright ꢀ 2005 The Chemical Society of Japan

Chemistry Letters Vol.34, No.9 (2005)
1223
Table 1. Photophysical and electrochemical properties of 1–4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
Complex
ꢅ
em/nma
ꢀb
E1=2ox/Vc
1
2
3
4
1
2
3
4
516; 537
547; 587
497; 518
487; 517
0.024
0.019
0.035
0.032
0.17
0.18
0.50
0.68
aIn CH2Cl2 at room temperature. bIn CH2Cl2 using fac-
c
Ir(ppy)3 (ꢀ ¼ 0:40)9 as the reference. In DMF containing
0.1 mol dmꢁ3 n-Bu4NPF6, vs Cp2Fe/Cp2Feþ.
400
450
500
550
600
650
700
Wavelength /nm
emission maxima for the complex 1, 2, and 3 are consistent with
shifts of end-absorption wavelengths in their absorption spectra.7
On the other hand, a recent research by Coppo et al. reveals that
an Ir complex with a 3,5-CF3-substituted ppy ligand exhibits
longer emission maxima (466 and 499 nm) than a 2,4-F-substi-
tuted complex (461 and 491 nm), even though the HOMO level
of the CF3-substituted complex is lower than that of the F-sub-
stituted complex.4f This can be attributed to a simultaneous low-
ering of the LUMO level due to a stronger electron-inductive na-
ture of CF3 than F. In contrast, our 3,5-CF3-substituted complex
4 exhibits a further blue shift (487 nm) by 10 nm compared to the
2,4-F-substituted complex 3. The electrochemical data shows
that the HOMO level of 4 is lower than that of 3. Thus, introduc-
tion of CF3 groups to the phenyl ring makes the HOMO–LUMO
gap slightly large without a notable change of the LUMO level.
This is a unique characteristic which is different from that of ppy
derivatives.
The phosphorescence life time of 3 was also measured in
polycarbonate thin film. The emission decayed double exponen-
tially with long life times (ꢂ1 ¼ 5:8 ms, ꢂ2 ¼ 15:5 ms). In addi-
tion, a vibronic fine structure clearly appeared in the emission
spectra of all complexes, and large Stokes shifts were observed.
These results may be interpreted by considering that this kind of
complexes emits primarily from the ligand-based 3(ꢁ–ꢁ) exited
states.4b,4d
A preliminary OLED using complex 3 as a dopant emitter
was fabricated by a high-vacuum thermal deposition method on-
to a clean glass substrate. The device structure is as follows: in-
dium tin oxide (ITO)/4,40-bis[N-(1-naphthyl)-N-phenylamino]-
biphenyl (ꢃ-NPD) (40 nm)/3% complex 3 doped in 4,40-N,N0-
dicarbazolylbiphenyl (CBP) (35 nm)/2,9-dimethyl-4,7-diphen-
ylphenanthroline (BCP) (10 nm)/tris(8-hydroxyquinoline)alu-
minium (Alq3) (35 nm)/LiF (0.5 nm)/Al (100 nm). The resulting
EL spectra were found to be dependent on the luminance.7 Inter-
estingly, the Commission Internationale de l’Eclairage (CIE)
coordinate at 2862 cd/m2 is near a white region (x ¼ 0:28,
y ¼ 0:41) owing to the broad emission from 3 and a slight mix-
ing of the blue emission from ꢃ-NPD. The emission maximum
of 3 is not so changed from that in the PL spectrum. The external
quantum efficiency ꢄext is 1.2% at 146 cd/m2, J ¼ 4:6 mA/cm2,
where the luminance efficiency and power efficiency are 3.2 cd/
A and 1.4 lm/W.
In conclusion, we prepared novel Ir complexes with ligands
of 2-phenylimidazo[1,2-a]pyridine derivatives, and succeeded in
fabrication of the OLED using 3. The complex 1 was found to
exhibit green phosphorescence, whose emission maximum could
be changed to shorter wavelengths by introduction of strong
electron-withdrawing groups into the phenyl rings: the behavior
is different from that of known ppy derivatives because of the
Figure 1. Emission spectra of Ir complexes in CH2Cl2.
difference in distribution of their frontier orbitals. These Ir com-
plexes would be modified to give new blue phosphorescent ma-
terials by combination with electron-donating substituents on the
imidazopyridine side as well as use of other ancillary ligands
such as picolinic acid.2
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
4
5
This fact is confirmed by the reduction potentials of 2-phenylimidazo-
red
[1,2-a]pyridine (Ep ¼ ꢁ3:28 V and ꢁ3:10 V vs Cp2Fe/Cp2Feþ) and
red
ppy (Ep ¼ ꢁ3:13 V). In addition, the LUMO of free 2-phenylimi-
dazo[1,2-a]pyridine ligand was found to be almost localized in the imi-
dazopyridine side by the PM3 calculations.
6
7
8
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9
Published on the web (Advance View) July 30, 2005; DOI 10.1246/cl.2005.1222