2800
J. Hu et al. / Journal of Organometallic Chemistry 693 (2008) 2798–2802
gave the almost completely superposed electroluminescence (EL)
3.0
2.5
spectra. It indicated that the EL spectrum did almost not change
when the voltage changed. It means that the EL spectrum of the de-
vice was independent of the applied voltage. The Commission
International de I’Eclairage (CIE) coordinates were calculated to
be (0.18, 0.54) based on the EL spectrum at an applied voltage of
8 V.
2.0
1.5
1.0
0.5
Figs. 5 and 6 show the luminance–voltage–current density
curve and the power efficiency–current density–external EL quan-
tum efficiency curve, respectively. The device showed a quite high
efficiencies and brightness. At a voltage of 8.5 V (where the current
density was 11.4 mA mꢀ2), it gave a maximum external quantum
0.0
-0.5
-1.0
-1.5
-2.0
efficiency of 12.1% and
a maximum current efficiency of
31.7 cd Aꢀ1; at a voltage of 5 V, it gave a maximum power effi-
ciency of 16.5 lm Wꢀ1; and at a voltage of 14 V, it gave a maximum
brightness of 79,640 cd mꢀ2. These performances and the previous
related reports [14,15] favored the supposition that the high rigid-
ity of the ligand framework would significantly reduce the nonra-
diative transition and therefore enhance the emitting efficiency of
the materials [17]. The rigid structure may be one of the factors for
increasing fluorescence efficiency of Ir complexes, so OLEDs based
on this rigid ligand–Ir complex could give a high brightness and
efficiencies. The results may provide a useful reference for the de-
sign of new luminescent materials.
-0.4 -0.2 0.0 0.2
0.4 0.6 0.8 1.0 1.2
+
Potential (V vs Fc /Fc)
Fig. 2. Cyclic voltammogram of the complex (biio)2Ir(acac).
The complex exhibited a one-electron oxidation wave during the
anodic scan in CH2Cl2 solution, with the half-wave oxidative po-
tential of 0.71 V. On the basis of the onset potential of the oxida-
tion, we can roughly estimate the highest occupied molecular
orbital (HOMO) energy level of this Ir complex with regard to the
energy level of the ferrocenium/ferrocene redox couple (being
approximately 4.8 eV negative to the vacuum level) [24]. All the
cyclic voltammetry data are listed in Table 1.
3. Conclusion
Yang and co-workers have reported an Ir complex (tbi)2Ir(acac)
which is also based on a benzoimidazo derivative ligand 1-hexyl-2-
p-tolyl-1H-benzo[d]imidazole [25]. Compared with (tbi)2Ir(acac),
the complex (biio)2Ir(acac) has a decreased HOMO energy level
In summary, we designed and synthesized a new iridium com-
plex (biio)2Ir(acac) with a rigid cyclometalated ligand biio. The
complex has the characters of simply synthetic procedure and
strong phosphorescence. The maximum brightness of the electro-
luminescent device using this complex as dopant can reach
79,640 cd mꢀ2 with an external quantum efficiency of 12.1% and
of ꢀ5.30 eV (Table 1). Though the extension of
p-conjugation in
(biio)2Ir(acac) may bring a decrease of the energy gap and lead to
a red shift of the PL spectrum [21,22], we observed not a red but
a little blue shift emission from the PL spectrum of (biio)2Ir(acac).
We presume that the introduction of carbonyl group into the li-
gand framework may increase the energy gap by decreasing the
a maximum current efficiency of 31.7 cd Aꢀ1
.
4. Experimental
HOMO energy level and therefore bring
wavelength.
a shorter emitting
4.1. Reagents and instruments
Reagents were used as purchased without further purification.
1H NMR spectra was measured on a Bruker AVANCE 400 spectrom-
eter in CDCl3 using TMS as an internal reference. Mass spectra (MS)
were measured on a VG-ZAB-HS spectrometer with electron im-
pact ionization. Elemental analysis was performed on a Perkin–El-
mer 240C elemental analyzer. The UV–Vis spectra were recorded
on VARIAN Cary 5000 spectrometer while PL spectra were re-
corded on Perkin–Elmer LS 50B luminescence spectrophotometer.
CV measurements were carried out with a CHI660C electrochemi-
cal analyzer (CH Instruments) at room temperature. Current, volt-
age, and light-intensity measurements were made simultaneously
using a Keithley 2400 source meter and a Newport 1835-C optical
meter equipped with a Newport 818-ST silicon photodiode.
2.3. Electroluminescent properties
For studying electroluminescent properties of this iridium com-
plex, device using complex (biio)2Ir(acac) as dopant emitter was
fabricated. The device was prepared with the following structure:
ITO/NPB(40 nm)/Ir-complex:
CBP(7%,
30 nm)/BCP(15 nm)/
Alq(30 nm)/LiF(1 nm)/Al(100 nm) in which ITO (indium tin oxide)
was used as the anode, NPB (4,40-bis[N-(1-naphthyl)-N-phenylami-
no]biphenyl) was used as the hole-transporting material, CBP (4,40-
N,N0-dicarbozole biphenyl) as the host, the iridium complex as the
dopant, BCP (2,9-dimethyl-4,7-dipheny-1,10-phenanthroline) as
the hole blocker, Alq (tris(8-hydroxyquinolinato)aluminium) as
the electron transporter, and LiF/Al as the cathode (Fig. 3).
The device had a maximum emission at 496 nm (Fig. 4), which
resembled the PL spectrum of the complex. It showed that the en-
ergy transfer from the host material CBP to the Ir complex emitter
was very efficient. At the voltages of 6, 8, 10 and 12 V, the device
4.2. Synthesis of materials
4.2.1. Synthesis of the cyclomatelated ligand
1,8-Naphthalic anhydride (1.98 g, 10.0 mmol) and benzene-1,2-
diamine (1.08 g, 10.0 mmol) were dissolved in 20 mL of butan-1-ol.
After refluxed for 10 h, the solution was cooled to room tempera-
ture, the precipitate was collected. After recrystallised from bu-
tan-1-ol and ethanol, the pure product benzo[de]benzo[4,5]-
Table 1
The electrochemical behavior of (biio)2Ir(acac) and (tbi)2Ir(acac)a
complex
E1/2 (V)
Eonset (V)
HOMO (eV)b
PL (nm)
(biio)2Ir(acac)
(tbi)2Ir(acac)a
0.71
0.52
0.50
0.39
ꢀ5.30
ꢀ5.19
496
500, 534
imidazo[2,1-a]isoquinolin-7-one was given with the yield of 85%.
Yellow acicular solid. Mp 190–192 °C.1H NMR (CDCl3, 400 MHz)
d: 8.96 (d, J = 9.6 Hz, 1H), 8.86 (d, J = 9.6 Hz, 1H), 8.62 (m, 1H),
8.35 (d, J = 10.8 Hz, 1H), 8.21 (d, J = 11.3 Hz, 1H), 7.84-7.96 (m,
a
The data of the complex (tbi)2Ir(acac) were cited from lit. [25].
Determined from the onset oxidation potential.
b