Color tuning based on a six-membered chelated iridium(III) complex with aza-aromatic ligand

Article abstract of DOI:10.1246/cl.2005.1668

A six-membered chelated iridium complex containing aza-aromatic ligand is prepared. Emission color tuning from yellow to deep red is observed for electroluminescent devices with different doping concentrations. Copyright

Full text of DOI:10.1246/cl.2005.1668

1668
Chemistry Letters Vol.34, No.12 (2005)
Color Tuning Based on a Six-membered Chelated Iridium(III) Complex
with Aza-aromatic Ligand
Rui Zhu, Jian Lin, Gui-An Wen, Shu-Juan Liu, Jun-Hua Wan, Jia-Chun Feng, Qu-Li Fan,
Gao-Yu Zhong, Wei Wei, and Wei Huangꢀ
Institute of Advanced Materials (IAM), Fudan University, Shanghai, 200433, P. R. China
(Received July 12, 2005; CL-050899)
A six-membered chelated iridium complex containing
aza-aromatic ligand is prepared. Emission color tuning from
yellow to deep red is observed for electroluminescent devices
with different doping concentrations.
Cu, Na2CO3
NH + Br
N
Nitrobenzene
refluxed
N
Ligand 1
N
In the past few years, iridium(III) complexes as electrophos-
phorescent material were becoming increasingly important due
to their high quantum yields of photoluminescence (PL) and
electroluminescence (EL).1 Most of the ligands reported in the
iridium(III) complexes are based on the derivatives of o-pyridyl-
arene, o-pyridylheterocycle, or 2-phenyloxazole.2 Among them,
phenylpyridine (ppy) structures (Figure 1a) are the most popu-
lar.1 Recently, alkenylpyridines (Figure 1b) as organic ligands
for phosphorescent iridium complexes were reported.2,3 All
these iridium complexes were prepared in a five-membered che-
lated framework. Iridium complexes with six-membered chelat-
ed framework were rarely prepared.4 In this letter, a six-mem-
bered chelated iridium(III) complex containing aza-aromatic li-
gand as shown in Figure 1c is reported. The pyridine and carba-
zole units in the ligand are connected through a nitrogen atom.
The conjugated electron pair on the nitrogen atom acts as a
bridge and electron donor to form the ꢀ-type conjugation with
the aryl groups. Thus, this molecule is also a conjugated struc-
ture like the alkenylpyridine ligand,5 which forms a six-mem-
bered chelated framework with metal center. This iridium com-
plex possesses some interesting properties which are different
from those of the common iridium complexes for organic
light-emitting diodes (OLEDs).
IrCl3
Cl
N
N
2-ethoxyethanol
refluxed
Ir
Ir
Cl
N
N
2
2
IrIII
-µ-chloro-bridged dimer
picolinic acid
N
N
N
N
Na2CO3
2-ethoxyethanol
refluxed
Ir
N
O
O
(cpy)2Ir(pico)
Scheme 1. The synthetic route to (cpy)2Ir(pico).
was carried out by nuclear magnetic resonance (NMR) and
MALDI-TOF mass spectroscopy.6 The MALDI-TOF mass spec-
troscopy shows a molecular ion peak at m=z 801, corresponding
to [(cpy)2Ir(pico)]þ, with fragment at m=z 679, corresponding to
[(cpy)2Ir]þ. The 5% decomposition temperature is at 360 ꢁC
under N2 atmosphere.
Figure 2 displays the UV–vis absorption and emission spec-
tra of the complex in the solid state and degassed CH2Cl2 solu-
tion (5 ꢂ 10ꢃ6 M). The complex possesses the similar absorption
spectra in the solid state and solution. The strong absorption
bands at ca. 300 nm (" ¼ 8:7 ꢂ 104 Mꢃ1 cmꢃ1) is assigned to
the spin-allowed 1ꢀ–ꢀꢀ transition of the cyclometallated aza-ar-
omatic ligand. The next broad absorption bands around 360 nm
(" ¼ 4:0 ꢂ 104 Mꢃ1 cmꢃ1) are ascribed to the contributions of
the typical spin-allowed metal-to-ligand charge transfer 1MLCT
transition, the spin-forbidden 3MLCT transitions and the 3ꢀ–ꢀꢀ
transitions. PL spectra of (cpy)2Ir(pico) was measured in
degassed CH2Cl2 solution at room temperature giving very
weak emission. Similarly, very weak emission was observed in
other solvents. In sharp contrast, highly intense luminescence
was obtained with ꢂmax at 538 nm in the recrystallized solid
(ꢀPL ¼ 5%), and the PL spectrum in CH2Cl2 solution shows
redshift of about 10 nm, compared with that of the solid state.
This solvent quenching phenomenon can be attributed to bipolar
structure constructed with the electron-rich nitrogen atom and
the electron-deficient metal center, so that rapid energy-transfer
quenching, possibly incorporating internal conversion and sol-
vent collision deactivation, takes places.9 Few iridium com-
plexes for OLEDs show the similar solvent quenching effect.1
The complex iridium(III) bis[2-(N-carbazolyl)pyridinato-
0
N,C3 ] picolinate ((cpy)2Ir(pico)) was prepared according to
the synthetic procedures shown in Scheme 1.6 Ligand 1 was pre-
pared through the copper-catalyzed Ullmann reaction7 between
carbazole and 2-bromopyridine. The IrIII-ꢁ-chloro-bridged di-
mer was prepared according to a similar procedure in the litera-
ture.4,8 Picolinic acid was chosen as the third ligand to produce
the target complex. Although 2,4-pentanedione (Hacac) had also
been tried as the third ligand, the precipitate possesses poor sol-
ubility in common solvents. The identification of the complex
N
N
N
R
N
a
b
c
Figure 1. Ligands for the iridium complexes based on a)
phenylpyridine ligand, b) alkenylpyridine ligand, and, c) aza-
aromatic ligand.
Copyright Ó 2005 The Chemical Society of Japan

Chemistry Letters Vol.34, No.12 (2005)
1669
ing was observed for the complex. The devices based on the
complex also possess solid-state solvation effect. Tuning of
emission color is obtained with doped and undoped devices.
Further study of photophysical properties and the optimization
device based on the complex are under investigation.
This work was financially supported by the National Natural
Science Foundation of China under Grants Nos. 60325412,
90406021, and 50428303 as well as the Shanghai Commission
of Science and Technology under Grants Nos. 03DZ11016 and
04XD14002 and the Shanghai Commission of Education under
Grant No. 2003SG03. RZh would also like to extend his grati-
tude to Fudan University and IAM for the 4th Graduate Creation
Foundation (No. CQH2203001) and scholarships offered.
Figure 2. Normalized UV–vis absorption and PL spectra in the
solid state (– –) and degassed CH2Cl2 (– –) at room tempera-
ture. All spectra were normalized at their respective peak
maximum.
References and Notes
1
M. A. Baldo, D. F. O’Brien, Y. You, A. Shoustikov, S. Sibley,
M. E. Thompson, and S. R. Forrest, Nature, 395, 151 (1998);
M. A. Baldo, M. E. Thompson, and S. R. Forrest, Nature,
403, 750 (2000); M. Ikai, S. Tokito, Y. Sakamoto, T. Suzuki,
and Y. Taga, Appl. Phys. Lett., 79, 156 (2001); X. Gong,
M. R. Robinson, J. C. Ostrowski, D. Moses, G. C. Bazan,
and A. J. Heeger, Adv. Mater., 14, 581 (2002); J. C. Ostrowski,
M. R. Robinson, A. J. Heeger, and G. C. Bazan, Chem.
Commun., 2002, 784; T. Karatsu, T. Nakamura, S. Yagai, A.
Kitamura, K. Yamaguchi, Y. Matsushima, T. Iwata, Y. Hori,
and T. Hagiwara, Chem. Lett., 32, 886 (2003); H. Konno and
Y. Sasaki, Chem. Lett., 32, 252 (2003); A. J. Sandee, C. K.
Williams, N. R. Evans, J. E. Davies, C. E. Boothby, A. Kohler,
R. H. Friend, and A. B. Holmes, J. Am. Chem. Soc., 126, 7041
(2004); F. I. Wu, H. J. Sua, C. F. Shu, L. Luo, W. G. Diau,
C. H. Cheng, J. P. Duan, and G. H. Lee, J. Mater. Chem.,
15, 1035 (2005).
B. M. J. S. Paulose, D. K. Rayabarapu, J. P. Duan, and C. H.
Cheng, Adv. Mater., 16, 2003 (2004).
D. K. Rayabarapu, B. M. J. S. Paulose, J. P. Duan, and C. H.
Cheng, Adv. Mater., 17, 349 (2005).
H. C. Li, P. T. Chou, Y. H. Hu, Y. M. Cheng, and R. S. Liu,
Organometallics, 24, 1329 (2005).
K. S. Song, L. Liu, and Q. X. Guo, Tetrahedron, 60,
9909 (2004); L. E. Nitzshe, C. Chabalowshi, and R. E.
Christoffersen, J. Am. Chem. Soc., 98, 4794 (1976); F. G.
Bordwell, X. Zhang, and J. P. Cheng, J. Org. Chem., 56,
3216 (1991).
Doped and undoped light-emitting devices based on the
complex were fabricated.6 In device A, CBP was chosen as host
with a doping concentration of 10 wt %. In device B, no host
material was used. Figure 3 shows the EL spectra of the doped
and undoped light-emitting devices at 12 V, together with the
PL spectrum in the solid state. Compared with the PL spectrum,
the EL spectrum of the undoped device B (ꢂmax ¼ 652 nm) has a
significant red shift in electric field. In device A, by doping the
complex to the CBP host, a significant blue shift occurs (ꢂmax
¼
570 nm) compared with the undoped device A. These results
may be referred to as the solid-state solvation effect (SSSE),
which has been reported for other bipolar organic molecules in
doped light-emitting devices.9 The conjugated electron pair on
the nitrogen atom and metal center form a bipolar framework
in the complex. By doping the polar complex into the triaryl-
amine host, the local electric field in the film is changed. The in-
termolecular dipole–dipole interactions of host and guest result
in these spectral shifts.9,10 Device A give a maximum brightness
of 138 cd mꢃ2 at J ¼ 282 mA cmꢃ2; and device B shows a max-
2
3
4
5
imum brightness of 2080 cd mꢃ2 at J ¼ 445 mA cmꢃ2
.
In conclusion, a six-membered chelated iridium complex
containing aza-aromatic ligands is demonstrated. The complex
possesses some different properties in comparison with the com-
mon iridium complexes for OLEDs. Significant solvent quench-
6
The synthesis, NMR data, MALDI-TOF mass spectrum, and
properties of devices of the complex are described in detail
in the Supporting Information.
7
8
J. Pang, Y. Tao, S. Freiberg, X. P. Yang, M. D’Iorio, and
S. N. Wang, J. Mater. Chem., 12, 206 (2002).
S. Lamansky, P. Djurovich, D. Murphy, F. Abdel-Razzaq, R.
Kwong, I. Tsyba, M. Bortz, B. Mui, R. Bau, and M. E.
Thompson, Inorg. Chem., 40, 1704 (2001); J. P. Duan, P. P.
Sun, and C. H. Cheng, Adv. Mater., 15, 224 (2003); T. Tsuzuki,
N. Shirasawa, T. Suzuki, and S. Tokito, Adv. Mater., 15, 1455
(2003).
9
Y. L. Tung, S. W. Lee, Y. Chi, L. S. Chen, C. F. Shu, F. I. Wu,
A. J. Carty, P. T. Chou, S. M. Peng, and G. H. Lee, Adv.
Mater., 17, 1059 (2005).
Figure 3. The PL spectrum in the solid state (– –) and electro-
luminescent spectra of device A (– –) and B (– –) at 12 V: A.
ITO/NPB (70 nm)/10 wt % complex in CBP (60 nm)/BCP
(30 nm)/Mg–Ag (250 nm); B. ITO/NPB (30 nm)/complex with-
out host (25 nm)/BCP (30 nm)/Mg–Ag (250 nm). All spectra
were normalized at their respective peak maximum.
10 V. Bulovic, R. Deshpande, M. E. Thompson, and S. R. Forrest,
Chem. Phys. Lett., 308, 317 (1999); C. F. Madigan and V.
Bulovic, Phys. Rev. Lett., 91, 247403 (2003).
Published on the web (Advance View) November 19, 2005; DOI 10.1246/cl.2005.1668