Highly efficient phosphorescent iridium (III) diazine complexes for OLEDs: Different photophysical property between iridium (III) pyrazine complex and iridium (III) pyrimidine complex

Article abstract of DOI:10.1016/j.jorganchem.2009.05.037

The synthesis and luminescence of four new iridium (III) diazine complexes (1-4) were investigated. HOMO and LUMO energy levels of the complexes were estimated according to the electrochemical performance and the UV-Vis absorption spectra, showing the pyr

Full text of DOI:10.1016/j.jorganchem.2009.05.037

Journal of Organometallic Chemistry 694 (2009) 3050–3057
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Journal of Organometallic Chemistry
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Highly efficient phosphorescent iridium (III) diazine complexes for OLEDs: Different
photophysical property between iridium (III) pyrazine complex and iridium (III)
pyrimidine complex
c
c,
Guoping Ge a, Jing He b, Haiqing Guo a, , Fuzhi Wang , Dechun Zou
*
*
a Beijing National Laboratory for Molecular Sciences, State Key Laboratory of Rare Earth Materials Chemistry and Applications, College of Chemistry and Molecular Engineering,
Peking University, 5 Yiheyuan Road, Haidian District, Beijing 100871, China
b College of Materials Science and Technology, Beijing Forestry University, Beijing 100083, China
c Key Laboratory of Polymer Chemistry and Physics of Education Ministry, Department of Polymer Science and Engineering, College of Chemistry and Molecular Engineering,
Peking University, 5 Yiheyuan Road, Haidian District, Beijing 100871, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 7 April 2009
Received in revised form 25 May 2009
Accepted 26 May 2009
Available online 6 June 2009
The synthesis and luminescence of four new iridium (III) diazine complexes (1–4) were investigated.
HOMO and LUMO energy levels of the complexes were estimated according to the electrochemical per-
formance and the UV–Vis absorption spectra, showing the pyrimidine complexes have a larger increase
for the LUMO than the HOMO orbital in comparison with the pyrazine complexes. Several high-efficiency
yellow and green OLEDs based on phosphorescent iridium (III) diazine complexes were obtained. The
devices emitting yellow light based on 1 with turn-on voltage of 4.1 V exhibited an external quantum
efficiency of 13.2% (power efficiency 20.3 lm/W), a maximum current efficiency of 37.3 cd/A. The electro-
luminescent performance for the green iridium pyrimidine complex of 3 is comparable to that of the irid-
ium pyridine complex (PPY)2Ir(acac) (PPY = 2-phenylpyridine), which is among the best reported.
Ó 2009 Elsevier B.V. All rights reserved.
Keywords:
Phosphorescence
Iridium diazine complex
3MLCT
OLED
1. Introduction
complexes [15–18] and reported a polymer-based blue electroph-
osphorescent light-emitting diodes based on an iridium (III) pyrim-
Organic light-emitting diodes (OLEDs) based on phosphorescent
materials have significantly improved electroluminescence (EL)
performance because both singlet and triplet excitons can be har-
vested for light emission. Theoretically, the internal quantum effi-
ciency of phosphorescent emitters can approach 100% [1,2].
Among the research of phosphorescent materials, rationally tuning
the emission wavelength of heavy-metal phosphorescent emitters
over the entire visible range has emerged as an important ongoing
research task [3]. Iridium (III) complexes are highly tunable in the
emission color [4]. Modifying the skeletal arrangement as well as
the substituent groups of the cyclometalating ligand (C^N), where
C^N is usually a C-2 metalated 2-phenylpyridine (PPY) ligand or
analogs, afford significant tuning of phosphorescence [4–7]. Using
appropriate ancillary ligands also cause shifting of the emission
from the red to the blue region [8–10]. Despite OLEDs with iridium
(III) diazine complexes as phosphor have high-efficiency and long
luminance half-life [3,11–16], investigations focused on using dia-
zine as a ligand for iridium complex for OLEDs are still insufficient.
We got succeeded in preparation of several efficient red and
yellow OLEDs based on the phosphorescent iridium (III) pyrazine
idine complex recently [19]. The results inspired us to initiate a
systematic study on the different photophysical property between
iridium (III) pyrazine complex and iridium (III) pyrimidine com-
plex. In this paper, we report the synthesis and characterization
of four new iridium (III) diazine complexes. Among of them, two
are iridium (III) pyrazine complexes, the other two are iridium
(III) pyrimidine complexes. Their structure-photophysical property
relationships were investigated. Several high-efficiency yellow and
green OLEDs based on phosphorescent iridium (III) diazine com-
plexes were obtained.
2. Experimental
2.1. Measurement
1H NMR spectra were measured on a Mercury Plus 300 spec-
trometer in CDCl3 using tetramethylsilane as an internal reference.
Elemental analyses were performed on an Elementary Vario EL
instrument. Mass spectra were measured on a Bruker BIFLEX III
mass spectrophotometer or Bruker Apex IV FTMS. UV–Vis absorp-
tion spectra were recorded using a Shimadzu UV-2550 spectropho-
tometer. Emission spectra were measured on a Hitachi F-4500
fluorescence spectrophotometer.
* Corresponding authors. Fax: +86 10 62755702 (H. Guo).
E-mail address: guohq@pku.edu.cn (H. Guo).
0022-328X/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2009.05.037

G. Ge et al. / Journal of Organometallic Chemistry 694 (2009) 3050–3057
3051
Cyclic voltammetry was performed with a computer-controlled
EG&G potentiostat/galvanostat model 283 in a one-compartment
electrolysis cell consisting of a platinum bottom working electrode,
a platinum wire counter electrode, and an Ag/AgCl reference elec-
trode. Cyclic voltammograms were monitored at scan rate of
50 mV sꢀ1 and recorded in HPLC-grade dichloromethane. The con-
centration of the complex was maintained at 1.0 mM and each
solution contained 0.1 M of tetrabutylammonium hexafluorophos-
phate (TBAP) as the electrolyte. The highest occupied molecular
orbital (HOMO) and the lowest unoccupied molecular orbital
(LUMO) energy level were calculated by assuming the energy level
of ferrocene/ferrocenium to be ꢀ4.8 eV.
17.98%. 1H NMR (CDCl3, 300 MHz) d: 8.81(d, 2H), 8.46(m, 2H),
7.51(m, 3H), 7.18(m,1H). EI-MS: Calc. for C10H8N2 156; Found 156.
2.2.1.6. DFPPM. Yellow powders. Yield: 79%. Anal. Calc. for
C10H6F2N2: C, 62.50; H, 3.15; N, 14.58. Found: C, 62.68; H, 2.97;
N, 14.36%. 1H NMR (CDCl3, 300 MHz) d: 8.86(d, 2H), 8.13(m, 1H),
7.31(m, 1H), 6.98(m, 2H). EI-MS: Calc. for C10H6F2N2 192; Found
192.
2.2.1.7. 4-PPM. Pale yellow powders. Yield: 51%. Anal. Calc. for
C10H8N2: C, 76.90; H, 5.16; N, 17.94. Found: C,77.03; H, 5.03; N,
18.27%. 1H NMR (CDCl3, 300 MHz) d: 9.29(s, 1H), 8.79(d, 1H),
8.11(m, 2H), 7.74(m, 1H), 7.54(m, 3H). EI-MS: Calc. for C10H8N2
156; Found 156.
2.2. Synthesis
2.2.1. General procedure for the synthesis of diazine ligands
2.2.1.8. 4-DFPPM. Pale yellow powders. Yield: 33%. Anal. Calc. for
C10H6F2N2: C, 62.50; H, 3.15; N, 14.58. Found: C, 62.75; H, 2.94;
N, 14.53%. 1H NMR (CDCl3, 300 MHz) d: 9.30(s, 1H), 8.79(d, 1H),
8.27(m, 1H), 7.84(d, 1H), 7.02(m, 2H). EI-MS: Calc. for C10H6F2N2
192; Found 192.
Diazine ligands (abbreviated with the general formula da) 2-
phenylpyrazine (PPZ), 2-(2,4-difluorophenyl)-pyrazine (DFPPZ),
2-methyl-3-phenylpyrazine (MPPZ), 2-(2,4-difluorophenyl)-3-
methylpyrazine (DFMPPZ), 2-phenylpyrimidine (PPM) and
2-(2,4-difluorophenyl)-pyrimidine (DFPPM) were obtained from
the reaction of chlorodiazines and the corresponding arylboronic
acids by a similar procedure [20], so that a detailed description is
provided only for MPPZ. 4-Phenylpyrimidine (4-PPM) and 4-
(2,4-difluorophenyl)-pyrimidine (4-DFPPM) were prepared
according to the similar procedure previously reported [21].
2.2.2. General procedure for the synthesis of the iridium complexes
All synthetic procedures involving IrCl3ꢂ3H2O and other Ir(III)
species were carried out in inert gas atmosphere despite the air
stability of the compounds, the main concern being the oxidative
stability of intermediate complexes at the high temperatures used
in the reactions [6]. Cyclometalated Ir(III)
l-chloro-bridged dimers
2.2.1.1. Synthesis of MPPZ. 3.23 g of 2-chloro-3-methylpyrazine
(1 equiv.; 25 mmol), 3.66 g of phenylboronic acid (1.2 equiv.;
30 mmol) and 0.66 g of triphenylphosphine (0.1 equiv.; 2.5 mmol)
were dissolved in 1,2-dimethoxyethane (25 mL). 34 mL of a 2 M
K2CO3 (2.7 equiv.; 67.5 mmol) aqueous solution were added and
the mixture was purged with nitrogen. Palladium acetate (0.14 g;
0.025 equiv.) was added and the mixture was refluxed for 18 h.
The two phases were then separated and the aqueous phase was
extracted with ethyl acetate (4 ꢁ 60 mL). The combined organic
phases were washed with water (80 mL) and brine (80 mL) and
were dried over MgSO4. After evaporation of the solvent, a yellow
powder was obtained. The crude product was chromatographed
using a dichloromethane:acetone = 20:1 column to yield 65% of
the pure MPPZ as pale yellow crystals after solvent evaporation
and drying. Anal. Calc. for C11H10N2: C, 77.62; H, 5.92; N, 16.46.
Found: C, 77.41; H, 6.01; N, 16.41%. 1H NMR (CDCl3, 300 MHz) d:
8.51(d, 1H), 8.46(d, 1H), 7.62–7.52(m, 2H), 7.53-7.48(m, 3H), 2.65
(s, 3H). EI-MS: Calc. for C11H10N2 170; Found 170.
of general formula (da)2Ir( -Cl)2Ir(da)2, where da represents the
l
cyclometalated diazine, were synthesized by the same method re-
ported by Nonoyama [22]. The crude products of these dimers
were used for subsequent preparation of (da)2Ir(acac). Only the
syntheses of (MPPZ)2Ir(acac) (4) will be described in detail.
2.2.2.1. Synthesis of (MPPZ)2Ir (acac). Cyclometalated Ir(III)-chloro-
bridged dimers [(MPPZ)2IrCl]2 were synthesized according to the
Nonoyama route, by refluxing IrCl3ꢂ3H2O with 2.2 equiv. of MPPZ
in a 3:1 mixture of 2-ethoxyethanol and water [22]. The reaction
mixture was refluxed under nitrogen atmosphere for 24 h. After
cooling to room temperature, a precipitate was collected by suc-
tion and washed with ethanol, acetone, and then dried in vacuum
to give crude [(MPPZ)2IrCl]2 as an orange powder. The crude
[(MPPZ)2IrCl]2 (0.4 mmol), 1.2 mmol of acetyl acetone and 0.33 g
of sodium carbonate were refluxed under nitrogen atmosphere in
20 mL of 2-ethoxyethanol for 15 h. After the reaction mixture
was cooled to room temperature, the precipitate was filtered off
and washed with water, ethanol, and ether to afford orange pow-
dery (MPPZ)2Ir(acac) (1) in 61% yield. Anal. Calc. for C27H25IrN4O2:
C, 51.50; H, 4.00; N, 8.90. Found: C, 51.27; H, 4.17; N, 8.73%. 1H
NMR (CDCl3, 300 MHz) d: 8.45(d, 2H), 8.22(d, 2H), 7.92(d, 2H),
6.91(t, 2H), 6.74 (t, 2H), 6.25(d, 2H), 5.23(s, 1H), 3.10(s, 6H),
1.80(s, 6H). EI-MS: Calc. for C27H25IrN4O2 630; Found 630.
2.2.1.2. PPZ. Yellow powders. Yield: 57%. Anal. Calc. for C10H8N2: C,
76.90; H, 5.16; N, 17.94. Found: C,76.87; H, 5.04; N, 18.20%. 1H
NMR (CDCl3, 300 MHz) d: 9.04(s, 1H), 8.64(d, 1H), 8.51(d, 1H),
8.03(m, 2H), 7.50(m, 3H). EI-MS: Calc. for C10H8N2 156; Found 156.
2.2.1.3. DFPPZ. Pale yellow powders. Yield: 45%. Anal. Calc. for
C10H6F2N2: C, 62.50; H, 3.15; N, 14.58. Found: C, 62.66; H, 3.34;
N, 14.22%. 1H NMR (CDCl3, 300 MHz) d: 9.08(s, 1H), 8.69(d, 1H),
8.54(d, 1H), 8.04(m, 1H), 7.01(m, 2H). EI-MS: Calc. for C10H6F2N2
192; Found 192.
2.2.2.2. (DFMPPZ)2Ir(acac)
(2), Iridium (III) bis(2-(2,4-difluoro-
0
phenyl)-3-methylpyrazine-N,C2 ) (acetylacetonate). Yellow powders.
Yield: 67%. Anal. Calc. for C27H21F4IrN4O2: C, 46.22; H, 3.02; N, 7.98.
Found: C, 45.95; H, 3.05; N, 8.04%. 1HNMR (CDCl3, 300 MHz) d:
8.34(d, 2H), 8.24(d, 2H), 6.42(m, 2H), 5.60(m, 2H), 5.26(s, 1H),
2.91(s, 3H), 2.85(s, 3H), 1.81(s, 6H). EI-MS: Calc. for C27H21F4IrN4O2
702; Found 702.
2.2.1.4. DFMPPZ. Yellow powders. Yield: 63%. Anal. Calc. for
C11H8F2N2: C, 64.08; H, 3.91; N, 13.59. Found: C, 63.82; H, 3.82;
N, 13.24%. 1H NMR (CDCl3, 300 MHz) d: 8.51(s, 2H), 7.48(m, 1H),
7.08–6.93(m, 2H), 2.52(s, 3H). EI-MS: Calc. for C11H8F2N2 206;
Found 206.
2.2.2.3. (PPM)2Ir(acac) (3), Iridium (III) bis(2-phenylpyrimidine-
0
N,C2 ) (acetylacetonate). Yellow powders. Yield: 20%. Anal. Calc. for
C25H21IrN4O2: C, 49.91; H, 3.52; N, 9.31. Found: C, 49.58; H, 3.69; N,
8.97%. 1HNMR (CDCl3, 300 MHz) d: 8.78(m, 2H), 8.66(m, 2H),
7.96(m, 2H), 7.14(t, 2H), 6.88(m, 2H), 6.81(m, 2H), 6.33(d, 2H),
2.2.1.5. PPM. Pale yellow crystals. Yield: 61%. Anal. Calc. for
C10H8N2: C, 76.90; H, 5.16; N, 17.94. Found: C,76.80; H, 5.25; N,

3052
G. Ge et al. / Journal of Organometallic Chemistry 694 (2009) 3050–3057
5.23(s, 1H), 1.80(s, 6H). EI-MS: Calc. for C25H21IrN4O2 602; Found
602.
3. Results and discussion
3.1. Synthesis of the materials
2.2.2.4. (DFPPM)2Ir(acac) (4), Iridium (III) bis(2-(2,4-difluorophenyl)-
0
pyrimidine-N,C2 ) (acetylacetonate). Yellow powders. Yield: 56%.
Standard procedure was followed to synthesize diazine ligands
from chlorodiazines and appropriate arylboronic acids, as illus-
trated in (a) of Scheme 1, using MPPZ as the representative exam-
ple. The syntheses of the bis-cyclometalated iridium complexes of
diazines are shown in Scheme 1, using (MPPZ)2Ir (acac) as the rep-
resentative example: (b) reaction of IrCl3ꢂ3H2O with diazine ligand
Anal. Calc. for C25H17F4IrN4O2: C, 44.57; H, 2.54; N, 8.32. Found:
C, 44.28; H, 2.73; N, 8.11%. 1HNMR (CDCl3, 300 MHz) d: 8.90(m,
2H), 8.59(m, 2H), 7.10(t, 2H), 6.43(m, 2H), 5.75(m, 2H), 5.24(s,
1H), 1.83(s, 6H). ESI-MS: Calc. for C25H18F4IrN4O2 675.09896;
Found m/z 675.10118 [M+H+].
to form a chloride-bridged dimer, (da)2Ir(l-Cl)2Ir(da)2 (da = cyclo-
2.3. OLEDs fabrication and measurements
metalated diazine); (c) replacement of bridging chlorides with
bidentate b-diketonate ligands to give the desired products,
(da)2Ir(acac) (acac = acetylacetonate); Chart 1 shows the com-
plexes (da)2Ir(acac) synthesized in this study.
The device was fabricated with a standard vacuum–vapor depo-
sition process. The emitting area was 2 ꢁ 2 mm and all OLEDs were
formed on the same glass substrate. 4,4-bis[N-(1-naphthyl)-N-
phenyl-amino]biphenyl (NPB) – the hole transporter – was first
deposited on the indium tin oxide (ITO)-coated glass. The NPB
layer was followed by a layer of 4,40-N,N0-dicarbazole-biphenyl
(CBP) doped with Iridium complexes. Layers of TPBI – to serve as
the hole-blocker and electron-transporter; tri-(8-hydroxyquino-
line) aluminum(III) (Alq3) – as the electron-transporter; and
Mg:Ag/Ag – as the cathode, were successively deposited. The EL
spectra were measured with a spectrofluorometer FP-6200 (JAS-
In addition, reactivity of diazine ligands with iridium ion in syn-
thesis of iridium complex was investigated. MPPZ and DFMPPZ
can react with Ir(III) successfully to form corresponding iridium
complex, but PPZ and DFPPZ cannot. These results suggest that
steric influence of methyl group in MPPZ and DFMPPZ may play
an important role in the cyclometalating procedure. Ir(III) can
selectively attack on the 1-position nitrogen atom in the presence
of methyl group, whereas 4-position nitrogen atom in PPZ and
DFPPZ may also react with Ir(III) that cause many by-products
(Fig. 1).
CO),
a
source-measure unit R6145(Advantest), multimeter
2000(Keithley) and luminance was directly detected by using a
multifunctional optical meter 1835-C(Newport). All the measure-
ments were automatically controlled by a computer system, and
only about 400 ms are needed to get one L –J–V curve by using
our measuring system. The quantum efficiency was calculated by
using the luminescence, emission spectra and current density data.
Furthermore, it was also found that symmetry of the ligand
structure may also play an important role. We got succeeded in
preparation of the iridium complexes with PPM or DFPPM as li-
gand that has two symmetrical nitrogen atoms beside the phenyl
ring. However, we got failed in preparation of the iridium com-
plexes with 4-PPM or 4-DFPPM as ligand at the same condition.
N
HO
OH
N
Cl
B
DME,H2O, K2CO3
PPh3, Pd(OAc)2
(a)
(b)
(c)
N
+
N
MPPZ
N
N
N
N
N
N
Cl
Ir
IrCl3.3H2O
2-ethoxyethanol and water
Ir
Cl
2
2
(MPPZ)2Ir(µ-Cl)2Ir(MPPZ)2
MPPZ
N
N
O
acetylacetone, Na2CO3
2-ethoxyethanol, reflux
(MPPZ)2Ir(µ-Cl)2Ir(MPPZ)2
Ir
O
2
1
Scheme 1.

G. Ge et al. / Journal of Organometallic Chemistry 694 (2009) 3050–3057
3053
N
N
N
N
N
O
O
O
O
N
N
O
O
N
Ir
Ir
O
O
Ir
2
F
F
Ir
2
2
2
F
F
(DFPPM)2 Ir(acac)
(MPPZ)2 Ir(acac)
1
(DFMPPZ)2 Ir(acac)
(PPM)2 Ir(acac)
4
2
3
Chart 1.
1-Position nitrogen atom in 4-PPM and 4-DFPPM may react with
Ir(III) resulting in many by-products (Fig. 1).
minescence (PL) spectra of the complexes in dichloromethane at
room temperature. All the complexes exhibit bands in the UV
and in the visible region similar to those found for iridium phenyl-
pyridine-type complexes. The bands below 350 nm are assigned to
3.2. Photophysical data
the spin-allowed 1p p* transition of diazine ligands, in the visible
–
The absorption and emission data of the complexes are summa-
rized in Table 1. Fig. 2 shows the UV–Vis absorption and photolu-
region weaker bands can be assigned to singlet and triplet MLCT
transitions and
3
(p–p
*) transitions.
The low-energy MLCT band in complex 3 (460 nm) is signifi-
cantly blue-shifted compared to that of complex 1 (496 nm)
(Fig. 2). The iridium pyrazine complex 1 emits yellow light with
a maximum peak at 575 nm, whereas the iridium pyrimidine com-
plex 3 emits green light with a maximum peak at 527 nm (Fig. 2)
that is in good consistent with their absorption spectra. The small
Stokes shift between emission signal and the lowest energy
absorption band in the iridium complexes, in combination with a
structureless spectral feature, suggest that the phosphorescence
originate primarily from the 3MLCT state, together perhaps with
N
4
N
N
N
1
3
a lesser contribution from the 3p p* excited states [3]. Similarly,
–
compared with iridium pyrazine complex 2, iridium pyrimidine
complex 4 also shows a 50 nm blue-shift of the emission band.
Fig. 1. The molecular structures of PPZ and 4-PPM.
The results imply that the orientation of
p conjugation would exert
a large effect on the photophysical properties of the phosphores-
cent complexes. Replacing the phenyl fragment with a 2,4-difluo-
rophenyl substituent in the cyclometalating ligand blue-shifted
the emission wavelengths of the complexes from 575 and
527 nm to 546 and 496 nm, respectively (Table 1), by lowering
the HOMO energy level of the cyclometalated ligand using two
electronegative F atoms [3]. DFT calculations indicate that the
highest occupied molecular orbital (HOMO) is mainly contributed
Table 1
Photophysical and electrochemical data for (da)2Ir(acac).
(da)2Ir(acac)
kabs (nm)
kem(fwhm) (nm)
(MPPZ)2Ir (acac) (1)
(DFMPPZ)2Ir(acac) (2)
(PPM)2Ir(acac) (3)
258, 322, 410, 496
258, 327, 410, 468
261, 341, 406, 460
256, 330, 396, 463
575(61)
546(63)
527(55)
496(62)
(DFPPM)2Ir(acac) (4)
from d
p
states of the iridium atom and the relatively electron rich
fwhm, full width at half-maximum.
1.2
1.1
1.2
1.1
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
-0.1
1
2
3
4
-0.1
250
300
350
400
450
500
550
600
650
700
Wavelength (nm)
Fig. 2. UV–Vis absorption and PL spectra of (da)2Ir (acac) in CH2Cl2.

3054
G. Ge et al. / Journal of Organometallic Chemistry 694 (2009) 3050–3057
4
2
ꢀ5.27 eV for 3; ꢀ5.64 eV for 2 and ꢀ5.65 eV for 4; respectively).
(MPPZ)2Ir (acac)
(DFMPPZ)2 Ir(acac)
(PPM)2Ir(acac)
As a result, the pyrimidine complexes have a larger increase for
the LUMO than the HOMO orbital, resulting in a widening of the
HOMOꢀLUMO gap in comparison with the pyrazine complexes.
(DFPPM)2Ir(acac)
0
-2
-4
-6
-8
Ag
Mg:Ag
Alq3
TPBi
CBP:x% of 1
NPB
400
600
800
1000 1200 1400 1600 1800 2000
Potential [mV vs. Ag/AgCl]
ITO
Fig. 3. Cyclic voltammograms of 1, 2, 3 and 4 in 0.1 M tetrabutylammonium
hexafluorophosphate (TBAP) at a scan rate of 50 mV sꢀ1
Glass substrate
.
phenyl group of the cyclometalated ligand, whereas the lowest
unoccupied molecular orbital (LUMO) is predominantly located
at the N-heterocyclic part of the cyclometalated ligand [3,23].
When electron withdrawing groups were incorporated onto the
phenyl ring, it will decrease the HOMO energy and thereby in-
crease the HOMOꢀLUMO energy gap [24].
Device A: ITO/NPB(40 nm)/Ir:CBP(10%, 30 nm)/AlQ(50 nm)/
Mg:Ag(150 nm, 10:1)/Ag(10 nm). Devices B–D: ITO/NPB(40 nm)/
Ir:CBP(x%, 30 nm)/TPBi(15 nm)/AlQ(50 nm)//Mg:Ag(150 nm, 10:1)
/Ag(10 nm) (x = 7.8, B; x = 10, C; x = 18, D).
3.3. Electrochemical properties
Cyclic voltammetry (CV) was employed to investigate the elec-
trochemical behavior of the complexes (da)2Ir(acac). The highest
occupied molecular orbital (HOMO) and lowest unoccupied molec-
ular orbital (LUMO) energy levels of the materials were estimated
according to the electrochemical performance and the UV–Vis
absorption spectra. As shown in Fig. 3, the potentials for oxidation
were observed to be 1.04, 1.37, 1.00 and 1.38 V, respectively. The
oxidation potential, measured relative to a ferrocenium/ferrocene
reference (Fc+/Fc), are listed in Table 2. The HOMO energy values
for these iridium complexes were calculated based on the value
of ꢀ4.8 eV for Fc with respect to zero vacuum level [25–27]. The
electrochemical data and energy levels of the complexes are sum-
marized in Table 2. The oxidation of the two fluorinated complexes
occurred at significant more positive potentials (1.37 and 1.38 V)
than the unfluorinated complex 1 (1.04 and 1.00 V). These values
are similar to the reported bis-cyclometalated iridium complexes
[3,28,29].
The HOMOꢀLUMO level of these Ir (III) complexes are also af-
fected by the diazine group. This was revealed by the fact that sub-
stitution of the MPPZ ligand by the PPM ligand markedly raised
the LUMO energy from ꢀ3.13 eV of 1 to ꢀ2.93 eV of 3, while
replacement of the DFMPPZ ligand with DFPPM led to a shift from
ꢀ3.39 eV for 2 to ꢀ3.12 eV for 4. It should be noted that minor
changes at the HOMO were also observed (ꢀ5.31 eV for 1 and
Ag
Mg:Ag
Alq3
TPBi
CBP:10%Ir
NPB
ITO
Glass substrate
Device E: ITO/NPB(40 nm)/CBP:(PPM)2Ir(acac)(10%, 30 nm)/
TPBi(15 nm)/Alq3(50 nm)//Mg:Ag(150 nm, 10:1)/Ag(10 nm). Device
F: ITO/NPB(40 nm)/CBP:(PPY)2Ir(acac) (10%, 30 nm)/TPBi (15 nm)/
AlQ(50 nm)//Mg:Ag(150 nm, 10:1)/Ag(10 nm).
3.4. Electrophosphorescent properties
To illustrate the EL properties of the new Ir(III) complexes, mul-
tilayer OLED devices using complexes 1 and 3 as dopants in the
Table 2
Electrochemical potentials and energy levels of the iridium complexes.
Eox vs. Ag/AgCl (V)
Eox vs. Fc (V)
Eg (eV)
HOMOb (eV)
LUMOc (eV)
a
Iridium complex
1
2
3
4
1.04
1.37
1.00
1.38
0.51
0.84
0.47
0.85
2.18
2.25
2.34
2.53
ꢀ5.31
ꢀ5.64
ꢀ5.27
ꢀ5.65
ꢀ3.13
ꢀ3.39
ꢀ2.93
ꢀ3.12
a
Bandgap estimated from the UV–Vis absorption spectra.
Calculated from the oxidation potentials.
Deduced from the HOMO and Eg.
b
c

G. Ge et al. / Journal of Organometallic Chemistry 694 (2009) 3050–3057
3055
N
O
O
Al
N
N
N
N
O
Alq3
NPB
N
N
N
N
N
N
N
N
CBP
TPBi
Fig. 4. The general structure for devices A–F and the molecular structures of the compounds used in these devices.
active emitting layer have been fabricated. Fig. 4 shows the general
structure for device A–F and the molecular structures of the com-
pounds used in the device. We have used CBP as the high triplet
energy host for the iridium complexes because of its proven excel-
lency as the host for iridium complexes and theoretical confirma-
tion of favorable triplet energy [30]. Key characteristics of these
devices are list in Table 3.
In devices B–D, TPBi was used as the electron-transporting/
hole-blocking material and the CBP layer was doped with 7.8%,
10.0%, and 18.0% of 1, respectively. All of these devices emitted or-
ange-yellow light with an emission maximum at 580 nm. There are
no characteristic emission peaks from CBP or Alq3 even at high cur-
rent density, indicating an effective energy transfer from the host
exciton to the dopant. Meanwhile, there is no exciton decay in
the Alq3 layer due to the hole-blocking action of the TPBi layer.
Correspondingly, the emission from Alq3 was observed in the EL
spectrum of device A without TPBi indicating that device A has
excitons combined in the Alq3 layer due to no effective hole-block-
ing action in the absence of TPBi (Fig. 5). In addition, device A
shows a relative lower device efficiencies (Table 3), indicating TPBi
has key role in the balance of electron and hole transport, giving
highly efficient charge capture inside the device.
iridium phosphors under the electric field, which cause the charac-
teristic of 3p p* increases in EL excitation process compared to PL
excitation process [31].
Comparison of the performances of devices B–D indicates that
device B, with 7.8% of 1 shows the highest device efficiencies (Ta-
ble 3). A turn-on voltage of 4.1 V, with a maximum brightness of
11565 cd/m2 at 19.8 V, external quantum efficiency of 13.2% at
0.392 mA/cm2, current efficiency of 37.3 cd/A at 0.392 mA/cm2,
–
1.0
A
B
C
D
0.8
0.6
0.4
0.2
0.0
-0.2
The EL spectrum of device B does not change significantly with
variation of the applied voltage as show in Fig. 5. But compared
with its PL spectrum peaking at 575 nm, the EL spectra of the de-
vice posed a major peak locating at 580 nm with a weak shoulder
at 620 nm (Fig. 6). A similar phenomena was seen on (tbt)2Ir(acac)
and (tpbi)2Ir(acac) reported by Wei et al. [31]. These phenomena
may be assigned to the complex interactions among the CBP and
300
400
500
600
700
800
900
Wavelength (nm)
Fig. 5. The EL spectra of devices A–D with 1 as phosphor.
Table 3
Performance characteristics for OLEDs based on iridium complexes 1 and 3.
Device
kmax (nm)
Von (V)
Lmax (cd/m2)
gext,max (%)
gmax (lm/W)
gImax (cd/A)
100 cd/m2
gext (%)
g
(lm/W)
gI (cd/A)
A
B
C
D
E
F
580
580
580
580
527
522
4.1
4.1
4.1
3.7
3.2
3.1
69548 (17.8)
5.4
5.4
14.6
37.3
33.8
27.1
58.2
56.0
4.0
5.1
11.0
36.9
31.6
27.1
53.7
55.2
111565 (19.8)
105774 (21.3)
96675 (17.8)
120450 (15.0 V)
153991 (18.5 V)
13.2
12.4
10.3
13.9
14.4
20.3
14.7
18.5
48.6
44.4
13.1
11.6
10.3
12.9
14.2
16.0
13.2
13.9
35.5
31.5

3056
G. Ge et al. / Journal of Organometallic Chemistry 694 (2009) 3050–3057
1.2
1.1
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
-0.1
A
B
C
D
6V
8V
10V
12V
14V
16V
10
10
PL
A
B
C
D
1
1
0.01
0.1
1
10
100
1000
400
500
600
700
800
Current Density (mA/cm2 )
Wavelength (nm)
Fig. 8. The current efficiency–current– power efficiency characteristics of devices
A–D.
Fig. 6. The PL spectra of 1 and the electroluminescence spectra of device B at
different voltages.
and power efficiency of 20.3 lm/W at 0.00708 mA/cm2, were
achieved (Figs. 7–9). Furthermore, the efficiency of this device re-
mained as high as external quantum efficiency of 7.4%, brightness
of 21416 cd/m2, current efficiency, 21 cd/A, power efficiency of
4.7 lm/W under driven current density of 100 mA/cm2. The device
A
B
C
D
10
showed a gradual decrease in gext with increasing current density,
which is attributed to increasing triplet–triplet annihilation of the
phosphor-bound excitons and field-induced quenching effects [6].
The performance of device B is comparable with those of
(fbi)2Ir(acac) (brightness, 21105 cd/m2; external quantum effi-
ciency, 7.3%; current efficiency, 21 cd/A; power efficiency, 4.7 lm/
W at a current density of 100 mA/cm2) reported by Lin and co-
workers [32] and favorably with those of (bt)2Ir(acac)(brightness,
2500 cd/m2 at a current density of 10 mA/cm2; external quantum
efficiency, 5.5% at a current density of 100 mA/cm2) reported by
Thompson and co-workers [6].
The performance of device C and device D at 10 wt% and 18%
concentration, respectively, are also very remarkable (Figs. 7–9).
Device C with turn-on voltage of 4.1 V has its highest brightness
of 105774 cd/m2 at 21.3 V, a maximum external quantum effi-
ciency of 12.4%, a highest power efficiency of 14.7 lm/W and a
highest current efficiency of 33.8 cd/A. Device D has lowest turn-
on voltage of 3.7 V of the three devices, and its highest brightness
of 96675 cd/m2 at 17.8 V, a maximum external quantum efficiency
1
0.1
1
10
100
1000
Current Density(mA/cm2 )
Fig. 9. The external quantum efficiency–current characteristics of devices A–D.
of 10.3%, a highest power efficiency of 18.5 lm/W and a highest
current efficiency of 27.1 cd/A were achieved.
We have also fabricated two electroluminescent (EL) devices
using 3 (device E) and (PPY)2Ir(acac) (device F) as dopants in the
emitting layers. To compare the relative electroluminescent prop-
erties, the device structure and the thickness of the layers have
been kept constant. Device F emits green light with an emission
maximum at 522 nm, whereas replacement of one CH group at
the pyridyl fragment by a nitrogen atom of the pyrimidine ligand
in complex 3 makes the emission redshift. The performances of
green emitting device E appear to be very promising, and its per-
formance parameters are comparable with those of device F.
2600
100000
2400
2200
A
B
C
D
2000
1800
1600
1400
1200
1000
800
10000
1000
100
10
4. Conclusion
A
B
C
D
In summary, we got succeeded in preparation of four phospho-
rescent iridium (III) diazine complexes emitting yellow or green
colors. The results suggest that steric influence of substituent
group or symmetry of the ligand play a very important role in
the cyclometalated procedure. Secondly, the iridium pyrimidine
complexes show a significantly blue-shift of the emission band,
compared with iridium pyrazine complexes. The iridium pyrazine
complex 1 emits yellow light with a maximum peak at 575 nm,
whereas the iridium pyrimidine complex 3 emits green light with
600
400
200
0
1
-2
0
2
4
6
8
10
12
14
16
18
20
22
Voltage (V)
Fig. 7. The current–voltage– luminance characteristics of devices A–D.

G. Ge et al. / Journal of Organometallic Chemistry 694 (2009) 3050–3057
3057
[11] A. Tsuboyama, H. Mizutani, S. Okada, T. Takiguchi, S. Miura, T. Moriyama, S.
Igawa, J. Kamatani, M. Furugori, Eur. Pat. Appl. (2002) EP1191612.
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Peng, G.H. Lee, Adv. Funct. Mater. 14 (2004) 1221.
a maximum peak at 527 nm. Finally, very highly efficient OLEDs
using the complexes 1 and 3 as the phosphorescent dopants have
been demonstrated.
[14] Z.W. Liu, G. Min, Z.Q. Bian, D.B. Nie, Z.L. Gong, Z.B. Li, C.H. Huang, Adv. Funct.
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Acknowledgements
[15] G.L. Zhang, H.Q. Guo, Y.T. Chuai, D.C. Zou, Acta Chim. Sinica 63 (2005) 143.
[16] G.L. Zhang, Z.H. Liu, H.Q. Guo, Y.T. Chuai, C.G. Zhen, D.C. Zou, Chem. J. Chinese.
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[17] G.L. Zhang, H.Q. Guo, Y.T. Chuai, D.C. Zou, Mater. Lett. 59 (2005) 3002.
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doi:10.1016/j.ica.2008.10.001.
The works were supported by the National Nature Science
Foundation of China (No. 20474003 and No. 90401028) and 973
Program, PR China (2002CB613405).
[19] G.P. Ge, X.H. Yu, H.Q. Guo, F.Z. Wang, D.C. Zou, Synthetic Met. (2009),
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