A new iridium(III) complex with efficient photo- and electro-luminescent properties

Article abstract of DOI:10.1016/j.inoche.2011.05.030

A new complex of Ir(pi)₂(acac) (L = 2-(4-bromophenyl)-1-ethyl- 1H-phenanthro[9,10-d]imidazole) was designed and synthesized. Its molecular structure was determined by a single-crystal X-ray diffraction analysis. The Ir(III) complex showed chara

Full text of DOI:10.1016/j.inoche.2011.05.030

Inorganic Chemistry Communications 14 (2011) 1393–1395
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Inorganic Chemistry Communications
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A new iridium(III) complex with efficient photo- and electro-luminescent properties
a,
Wang Shimin a, Duan Yun b, Du Chen-Xia a, , Wu Yang-Jie
⁎
⁎
a
Department of Chemistry, Zhengzhou University, Zhengzhou 450052, PR China
b
Henan Institute of Metrology, Zhengzhou 450052, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
A new complex of Ir(pi)2(acac) (L=2-(4-bromophenyl)-1-ethyl-1H-phenanthro[9,10-d]imidazole) was designed
and synthesized. Its molecular structure was determined by a single-crystal X-ray diffraction analysis. The Ir(III)
complex showed characteristic phosphorescence with an emission of 570 nm and a high quantum efficiency of 28%.
A high performance yellow OLEDs was successfully fabricated by using this Ir(III) complex as dopant.
© 2011 Elsevier B.V. All rights reserved.
Received 8 March 2011
Accepted 25 May 2011
Available online 1 June 2011
Keywords:
Iridium(III) complexes
Phosphorescence
Crystal structure
Electroluminescence
Cyclometalated iridium(III) complexes have attracted considerable
attention in material research due to their outstanding performance
in organic light-emitting diodes (OLED) [1–4]. The strong spin–orbit
The 2-(4-bromophenyl)-1H-Phenanthro[9,10-d]imidazole ligand
was easily obtained from the reaction of phenanthrequinone, NH4Ac
and the p-bromobenzaldehyde in HAc solvent. Additional alkylation
by EtBr afforded 2-(4-bromophenyl)-1-ethyl-1H-Phenanthro[9,10-d]
imidazole (pi) (Scheme 1). Ir(pi)2(acac) was obtained in moderate
condition from the above ligand by a conventional two-step sequence
[14]. In the first step a chloro-bridged dimmer was formed by the
reaction of pi with IrCl3. Then, this dimmer was cleaved by treatment
with acac in the presence of Na2CO3 to produce the complex Ir(pi)2
(acac). Yellow solid, yield: 24%. Anal. Calc. for C51H39Br2IrN4O2 (%): C,
56.10; H, 3.60; N, 5.13. Found: C, 56.08; H, 3.66; N, 5.15. 1H NMR
(CDCl3, 400 MHz): δ (ppm)=8.84−8.77 (m, 4H), 8.59 (d, J=8.3 Hz,
2H), 8.47 (d, J=8.2 Hz, 2H), 7.71 (t, J=8.4 Hz, 2H), 7.69−7.62 (m,
4H), 7.48 (t, J=7.8 Hz, 2H), 7.34 (t, J=7.8 Hz, 2H), 7.20 (s, 2H), 7.17
(d, J=8.4 Hz, 2H), 5.37−5.16 (m, 4H), 4.21 (s, 1H), 2.05 (t, J=7.1 Hz,
6H), 1.13 (s, 6H). MS (FAB): m/e 1090.3 (M+).
coupling induced by
a heavy-metal ion promotes an efficient
intersystem crossing (ISC) between the singlet and the triplet excited
state manifold. Therefore, both singlet and triplet excitons can be
harnessed and then strong electroluminescence with an internal
efficiency theoretically approaching to 100% can be achieved [5–8].
The Ir(III) complexes reported generally contain two cyclometalated
ligands and a bidentate, monoanionic ancillary ligand, or with three
cyclometalated ligands. Both the luminescent efficiency and emission
colors of Ir(III) complexes can be tuned by introduction of sub-
stituents with different electronic effects or variations of the
conjugation system on ligands [9–13]. Thus, manipulation of the
skeletal arrangement as well as the substituent groups of the cy-
clometalating ligand may represent
a promising venue for the
development of highly phosphorescent Ir(III) complexes. Although a
wide range of Ir(III) materials have been reported, the number of
highly phosphorescent imidazole-based cyclometalated Ir(III) com-
plexes is still rare [13,14]. 1,3,5-tris(N-phenylbenz-imidazole-2-yl)-
benzene (TPBI), a commonly used electron transporter and hole
blocker, is a derivative of benzoimidazole compounds. Therefore,
benzoimidazole(bi)-based cyclometalated iridium complexes may
have good electron transporting ability, which is highly desirable in
designing high efficiency OLEDs [15]. In this paper, a rigid ligand 2-
phenylphenanthro[9,10-d]imidazole (pi) was synthesized by a very
simple method, and the OLED device based on this cyclometalated
complex Ir(pi)2(acac) gave good brightness and efficiency.
The structure of Ir(pi)2(acac) was confirmed by single crystal X-ray
diffraction study (Fig. 1). The iridium resides in an approximately
octahedral environment and the two nitrogen atoms of pi ligands
exhibit cis–C–C and trans-N, N chelated dispositions. The Ir―C bonds
1.987(2) Å and Ir―O bonds 2.150(2) Å are similar to those of (fbi)2Ir
(acac) (Ir–C, 2.006(4) Å; Ir–O, 2.145(6) Å) [13]. However, the Ir―N
bonds 2.076(2) Å are a little longer than the Ir―N bonds (2.038(3) Å) in
(fbi)2Ir(acac) possibly due to the more steric congestion of pi ligand. The
Phenanthro[9,10-d]imidazolyl ring is approximately coplanar with the
phenyl ring (the dihedral angle between the two planes is 4.0(0.1)°).
The dihedral angles between the chelating ring of Ir–C–C–C–N and
phenyl ring is 5.2°.
The absorption and emission spectra of Ir(pi)2(acac) recorded in
dichloromethane at room-temperature are shown in Fig. 2. In the
absorption spectrum, the intense bands around 250–350 nm can be
assigned to spin-allowed 1π–π* transitionsof theligand, which correlate
⁎
Corresponding authors. Tel.: +86 371 67767896; fax: +86 371 67763390.
E-mail address: dcx@zzu.edu.cn (C.-X. Du).
1387-7003/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2011.05.030

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S. Wang et al. / Inorganic Chemistry Communications 14 (2011) 1393–1395
Scheme 1. General synthetic route of ligand.
with the transition observed for the free pi. The absorption in the range
of 365–400 nm corresponds to spin-allowed S°→1MLCT transition. The
broad band at 430 nm can be assigned to S°→ 3MLCT transition and
gains intensity by mixing with the higher lying 1MLCT transition
through the spin–orbit coupling of iridium(III). This mixing is strong
enough in this complex that the formally spin forbidden 3MLCT has an
extinction coefficient that is almost equal to the spin allowed 1MLCT
transition [13].
previously by electrochemical studies and theoretical calculations, the
oxidation occurred mainly at the Ir metal cationic site, together with a
minor contribution from the cyclometalated phenyl fragment [18].
Variations of cyclometalated ligands, including replacement of pyridine
by other heteroaromatic rings and incorporation of substituents on the
rings, were shown to affect the oxidation potential of the iridium ion in
congeners of (ppy)2Ir(acac) [19]. Accordingly, replacing the benzoimi-
dazole fragment with a phenanthro[9,10-d]imidazolyl moiety would
significantly affect the electrochemical property of this complex.
However, the oxidization potential of Ir(pi)2(acac) appears to be only
slightly higher (10 mV) than that of Ir(bi)2(acac), showing that the
metal localized HOMOs are at very similar energies. According to the
The iridium complex showed characteristic phosphorescence with
a high quantum efficiency of 28% in degassed CH2Cl2 solutions upon
irradiation with 430 nm light at an ambient temperature. The
maximum emission peak was found to be dependent on the polarity
of the solvent, which suggests the 3MLCT→π transition of the
emissive state. The complex shows broad and featureless transition
with maximum at 570 nm in the photoluminescence (PL) spectrum
(Fig. 2). It has been confirmed that the relative contribution of 3MLCT
vs ligand-based 3π–π* transitions may affect the Stokes shifts
between the 3MLCT and emission bands as well as the phosphores-
cence spectral feature [16]. The significant Stokes shifts (140 nm)
assist us in assigning the 3π–π* states as the dominant emitting state
at room temperature in equilibrium with the other neighboring states.
The excited state lifetime of Ir(pi)2(acac), measured at 298 K in
degassed CH2Cl2 solution, is 0.57 μs, comparable with that of
benzoimidazole-based iridium complexes reported previously [13].
Furthermore, the emission band of complex Ir(pi)2(acac) is red-
shifted about 50 nm compared with that of Ir(bi)2(acac) owing to the
enlarged π-conjugation of Phenanthro[9,10-d]imidazolyl ring. The
short excited state lifetime together with the high phosphorescence
yield should be advantageous to highly efficient OLED devices.
The complex Ir(pi)2(acac) undergoes a reversible one-electron
oxidation wave of 0.91 V vs Fc+/Fc [17] in CH2Cl2, which can be
attributed to metal-centered IrIII/IrIV oxidation process. As revealed
equation Eg =ELUMO- EHOMO=1240/λonset (eV) and EHOMO=− EOX
+
4.74 eV [20], the HOMO and LUMO levels of Ir(pi)2(acac) were
calculated to be −5.65 and −2.89 eV respectively.
To demonstrate the basic electrophosphorescent properties of Ir
(pi)2(acac), we fabricated the device with a multilayer configuration
ITO/NPB (30 nm)/Ir(pi)2(acac): CBP (5%) (30 nm)/BCP (10 nm)/AlQ
(30 nm)/LiF (1 nm)/Al (200 nm) and investigated its electrolumines-
cent (EL) performances. Fig. 3 (inset) shows the EL spectrum of the
device at 10 V bias. The device had a maximum emission at 566 nm,
which resembled the PL spectrum of the complex. It showed that the
energy transfer from the host material CBP to the Ir complex emitter
was very efficient. Moreover, the EL spectrum of the device was
independent of the applied voltage. Fig. 3 shows the current density–
voltage–brightness (I–L–V) characteristics of this device. A turn-on
voltage of 4 V and a maximum brightness of 11232 cd/m2 at 12 V
were obtained. The calculated maximum current efficiency and power
efficiency were 4.6 cd/A and 1.82 lm/W, respectively.
Fig. 2. UV–vis and PL spectrum of Ir(pi)2(acac) in CH2Cl2 solution at 298 K
(c=1.0×10−5 mol/L).
Fig. 1. Crystal structure of Ir(pi)2(acac).

S. Wang et al. / Inorganic Chemistry Communications 14 (2011) 1393–1395
1395
Appendix A. Supplementary material
CCDC 768352 contains the supplementary crystallographic data for
this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif. Supplementary data associated with this article can
be found, in the online version, at doi:10.1016/j.inoche.2011.05.030.
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Fig. 3. The I–L–V characteristics of the device base on Ir(pi)2(acac). Inset: EL spectrum at
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In conclusion, a new iridium complex Ir(pi)2(acac) with a rigid
cyclometalated ligand 2-(4-bromophenyl)-1-ethyl-1H-phenanthro
[9,10-d]imidazole was designed and synthesized. This complex has
the characteristics of simply synthetic procedure and strong phospho-
rescence. Furthermore, the iridium complex could be used as anefficient
dopant for fabricating yellow organic light-emitting diodes and gave a
maximum brightness of 11232 cd/m2 at 12 V.
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This work was funded by a grant from Chinese National Natural
Science Foundation (Grant no. 20771093).