A highly efficient red electrophosphorescent iridium(iii) complex containing phenyl quinazoline ligand in polymer light-emitting diodes

Article abstract of DOI:10.1039/c2jm15160j

A novel iridium(iii) complex (PQxD)₂Ir(pic) (HPQxD, N,N,2-triphenylquinazolin-4-amine; pic, 2-picolinic acid) was synthesized and used as a triplet emitter doped in a polymeric host in solution-processed electrophosphorescent light-emitting diodes. Polymer light-emitting devices (PLEDs) with the structure of ITO/PEDOT:PSS/(70% PVK + 30% PBD):x% (PQxD) ₂Ir(pic)/TPBI/CsF/Al were fabricated with high external quantum efficiency. The device based on (PQxD)₂Ir(pic) furnishes a maximum external quantum efficiency of 18.44% and a luminance of 7107 cd m^-2 at 8% doping concentration. The Commission International de L'Eclairage (CIE) coordinates of the complex were (0.619,0.376). The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c2jm15160j

View Article Online / Journal Homepage / Table of Contents for this issue
Dynamic Article Links <
C
Journal of
Materials Chemistry
Cite this: J. Mater. Chem., 2012, 22, 6878
www.rsc.org/materials
PAPER
A highly efficient red electrophosphorescent iridium(III) complex containing
phenyl quinazoline ligand in polymer light-emitting diodes†
a
Qunbo Mei, Lingxia Wang, Yuanhui Guo, Jiena Weng, Fang Yan, Bo Tian and Bihai Tong
a
a
a
a
a
b
*
*
Received 12th October 2011, Accepted 22nd January 2012
DOI: 10.1039/c2jm15160j
A novel iridium(III) complex (PQxD)2Ir(pic) (HPQxD, N,N,2-triphenylquinazolin-4-amine; pic,
2-picolinic acid) was synthesized and used as a triplet emitter doped in a polymeric host in solution-
processed electrophosphorescent light-emitting diodes. Polymer light-emitting devices (PLEDs) with
the structure of ITO/PEDOT:PSS/(70% PVK + 30% PBD):x% (PQxD)2Ir(pic)/TPBI/CsF/Al were
fabricated with high external quantum efficiency. The device based on (PQxD)2Ir(pic) furnishes
a maximum external quantum efficiency of 18.44% and a luminance of 7107 cd mꢀ2 at 8% doping
concentration. The Commission International de L’Eclairage (CIE) coordinates of the complex were
(0.619,0.376).
with poor solubility is doped in polymers. Generally, sterically
hindered aromatic substituents are considered to be better
solubilizing groups. Pyrimidine is expected to be an electron-
deficient unit which has been used to construct novel cyclo-
metalated organic ligands and electron-transporting materials.
Their high electron affinities with pyrimidine-containing cyclo-
metalated organic ligands may possibly afford iridium(III)
complexes which are color tunable and capable of transporting
electrons.10 In the past decade, iridium(III) pyrimidine complexes
have attracted much attention for their high device
performances.11–15
Introduction
Organic light-emitting diodes (OLEDs) have attracted great
attention in the past few decades due to their potential applica-
tions in organic flat-panel displays with advantages of low
driving voltage, high contrast, fast response, and wide viewing
angle.1–5 As phosphorescent emitters, the heavy metal complexes,
such as iridium(III), osmium(II), platinum(II), and ruthenium(II)
complexes, play a critical role because they can fully utilize both
singlet and triplet excitons through the strong spin–orbital
coupling caused by heavy metal ions in complexes.6 Theoreti-
cally, the internal quantum efficiency of the phosphorescent
OLEDs (PhOLEDs) can reach up to 100%. Among these heavy
metal complexes for PhOLEDs, iridium(III) complexes are the
most promising candidates because of their relatively short
excited state lifetime, high photoluminescence efficiency and
excellent color tunability. Recently, great efforts have been
devoted to the design, synthesis and characterization of novel
iridium(III) complexes.7 Highly efficient PhOLEDs have been
reported by doping blue, green, and red iridium(III) complexes
used as phosphors in the small molecule host matrix. However,
the external quantum efficiency (hext) values of iridium(III)
complex-doped PLEDs are still much lower than the iridium(III)
complex-doped OLEDs.8,9 This is because the homogeneous
molecular dispersions cannot form when the iridium(III) complex
In this paper, a highly efficient phosphorescent iridium(III)
complex (PQxD)2Ir(pic) of a 4-phenyl quinazoline derivative was
successfully designed and synthesized. In order to optimize the
molecular structure, a sterically hindered diphenylamine unit was
introduced into the iridium(III) complex to improve its solubility.
At the same time, the introduction of diphenylamine can effi-
ciently improve the hole-transporting capacity and thus increase
the doping concentration. The devices using the complex as
dopant were fabricated and exhibited a maximum external
quantum efficiency of 18.44%.
Experimental
General details
Commercially available reagents were used as received unless
otherwise stated. N,N-Dimethylacetamide (DMAc), CHCl3, and
N,N-dimethylformamide (DMF) were dried by standard proce-
dures before use. NMR and mass spectroscopy (MS) were in
agreement with the structure. NMR spectra were measured on
a Bruker Ultra Shield Plus 400 MHz spectrometer with TMS as
the internal standard and CDCl3/DMSO-d6 as the solvent. GC-
MS was determined on a Shimadzu GC-MS-QP2010 Plus mass
aKey Laboratory for Organic Electronics
(KLOEID), Institute of Advanced Materials (IAM), Nanjing University
of Posts and Telecommunications (NUPT), Nanjing 210046, China.
E-mail: iamqbmei@njupt.edu.cn
bInstitute of Molecular Engineering and Applied Chemistry, College of
Metallurgy and Resources, Anhui University of Technology, Ma’anshan,
Anhui 243002, P. R. China. E-mail: tongbihai@ahut.edu.cn
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c2jm15160j
& Information Displays
6878 | J. Mater. Chem., 2012, 22, 6878–6884
This journal is ª The Royal Society of Chemistry 2012

View Article Online
spectrometer, and electron spray ionization (ESI) mass spectra
on a Thermo LCQDECAXP mass spectrometer. Melting points
were measured on a SGW X-4 micro-melting point spectrometer.
(10 mL). After being stirred for 1 h at room temperature under
nitrogen as a protective gas, a solution of 4-chloro-2-phenyl-
quinazoline (1.93 g,
8 mmol) in anhydrous N,N-dime-
thylformamide (10 mL) was added dropwise under N2
atmosphere while stirring and the mixture was stirred at room
temperature for 24 h. After reaction, the resulting mixture was
poured into water and extracted with ethyl acetate. The organic
phase was combined, washed with brine, dried over anhydrous
magnesium sulfate and filtered. The solvent was completely
removed and the residue was purified by column chromatog-
raphy over silica gel to give HPQxD as light yellow crystals.
Yield: 56%. Mp: 156–157 ꢁC. 1H NMR (CDCl3, 400 MHz)
d (ppm): 7.13–7.17 (t, 1H), 7.19–7.25 (m, 6H), 7.34–7.42 (m, 8H),
7.65–7.69 (t, 1H), 7.98–8.00 (d, 1H), 8.26–8.29 (m,2H). GC-MS
(m/z): 372 (M+).
Synthesis of 2-phenylquinazolin-4(3H)-one (1)16
A mixture of benzaldehyde (3.18 g, 30 mmol), anthranilamide
(4.08 g, 30 mmol) and NaHSO3 (6.24 g, 60 mmol) in DMAc
(30 mL) was stirred at 110 ꢁC for 8 h under N2 atmosphere. Then
the reaction mixture was cooled to room temperature and poured
into 500 mL water with stirring. The precipitate was collected by
filtration and washed with water and ethanol to give 2-phenyl-
quinazolin-4(3H)-one as white crystals. Yield: 87.3%. Mp: 220–
223 ꢁC. 1H NMR (DMSO-d6, 400 MHz) d (ppm): 7.49–7.58 (m,
4H), 7.72–7.74 (d, 1H), 7.91–7.94 (t, 1H), 8.13–8.18 (t, 3H). 13C
NMR (DMSO-d6, 100 MHz) d (ppm): 121.01, 126.58, 127.75,
128.96, 129.12, 132.78, 135.56, 154.49, 162.03. ESI-MS (m/z):
219 (M+).
Synthesis of iridium(III) complex ((PQxD)2Ir(pic))19
HPQxD (1.68 g, 45 mmol) and hydrated iridium(III)chloride
(0.528 g, 15 mmol) were added to a mixture of 2-ethoxyethanol
(15 mL) and distilled water (5 mL). The mixture was stirred at
100 ꢁC for 24 h under nitrogen. After being cooled to room
temperature, the precipitate was collected by filtration and
washed with water and ethanol. Subsequently, the cyclo-
metalated Ir(III) m-chloride bridged dimer was dried under
vacuum to afford a red solid. Cyclometalated Ir(III) m-chloride
bridged dimer (0.7 g, 0.36 mmol), 2-picolinic acid (0.4 g,
3.6 mmol), K2CO3 (1.0 g, 7.2 mmol) and 20 mL dichloromethane
were put into a round bottle, then stirred at room temperature
for 8 h. After reaction, the mixture solution was distilled under
vacuum. The crude product was purified by column chroma-
tography over aluminium oxide using dichloromethane as the
eluent. It was further purified by resettlement from ethanol to
give (PQxD)2Ir(pic) as a red solid. Yield: 68.2%. 1H NMR
(CDCl3, 400 MHz) d (ppm): 6.58–6.62 (t, 2H), 6.67–6.74 (m, 2H),
6.81–6.84 (t, 1H), 6.92–6.94 (m, 1H), 7.04–7.07 (m, 2H), 7.13–
7.17 (t, 1H), 7.21–7.25 (m, 7H), 7.27–7.38 (m, 10H), 7.43–7.52
Synthesis of 4-chloro-2-phenylquinazoline (2)17
2-Phenylquinazolin-4(3H)-one (4.7 g, 21 mmol) was added to
20 mL of CHCl3, then 10 mL of phosphorus oxychloride was
added and the mixture was refluxed for 12 h under N2 atmo-
sphere. After being cooled, the mixture was poured into ice-water
and rendered alkaline by dilute ammonia. The resulting precip-
itate was filtered off and purified by chromatography on silica gel
with a mixture of petroleum ether and ethyl acetate as the eluent
to give 4-chloro-2-phenylquinazoline as yellow solids. Yield:
86%. Mp: 113–115 ꢁC. 1H NMR (DMSO-d6, 400 MHz) d (ppm):
7.55–7.60 (t, 3H), 7.63–7.66 (t, 1H), 7.87–7.92 (m, 2H), 8.14–8.18
(t, 3H). GC-MS (m/z): 240 (M+).
Synthesis of N,N,2-triphenylquinazolin-4-amine (HPQxD)17–19
A 60% suspension of sodium hydride (0.55 g, 15 mmol) in
paraffin oil was added slowly to a solution of diphenylamine
(1.69 g, 10 mmol) in anhydrous N,N-dimethylformamide
Scheme 1 Synthesis of the ligands and (PQxD)2Ir(pic).
This journal is ª The Royal Society of Chemistry 2012
J. Mater. Chem., 2012, 22, 6878–6884 | 6879

View Article Online
(m, 9H), 7.67–7.70 (t, 1H), 7.75–7.77 (d, 1H), 7.90–7.92 (d, 1H),
7.94–7.98 (t, 1H), 8.43–8.45 (d, 1H). ESI-MS (m/z): 1082 (Na+).
Spectroscopy and dynamic measurements
UV-vis absorption spectra were recorded using a UV-3600 Shi-
madzu UV-vis spectrophotometer. Photoluminescence spectra
were measured on a Shimadzu RF-5301PC fluorescence spec-
trophotometer. Cyclic voltammetry (CV) was recorded at a Pt
disk electrode (WE) in HPLC grade dichloromethane solution
containing 0.001 M of the iridium(III) complex and 0.1 M tet-
rabutylammonium hexafluorophosphate as the supporting elec-
trolyte at a scanning rate of 0.1 V sꢀ1 at room temperature on
a CHI660C spectrometer. The Pt wire and Ag/AgNO3 were used
as the counter and reference electrodes, respectively. Ferrocene
was selected as the standard. Thermal gravimetric analysis was
measured on a DTG-60 Shimadzu spectrometer.
Fig.
1
UV-vis absorption and photoluminescence spectra of
(PQxD)2Ir(pic).
solvent according to the literature procedures. The reaction
conditions were mild and the yield of products was satisfactory.
Affected by nitrogen atoms, the chlorine atom on quinazoline
behaves very actively and is easily modified and functionalized.
The synthesis of the iridium(III) complex (PQxD)2Ir(pic)
included two steps. First, the cyclometalated Ir(III) m-chloride
bridged dimer was synthesized by reacting IrCl3$3H2O and the
OLED fabrication and measurement
The devices based on the iridium(III) complex with the structure
of
ITO/PEDOT:PSS/(70%
PVK
+
30%
PBD):x%
(PQxD)2Ir(pic)/TPBI/CsF/Al were fabricated. The doping
concentrations of (PQxD)2Ir(pic) were 1%, 2%, 4%, 8% and 12%,
respectively; and the mass ratio of PBD was 30% in the PVK–
PBD blend. The molecular structure of (PQxD)2Ir(pic) and the
device configurations are shown in Scheme 1 and Fig. 4. Poly(N-
vinylcarbazole) (PVK), 2-tert-butylphenyl-5-biphenyl-1,3,4-
ꢁ
cyclometalated ligand at 100 C for 24 h under nitrogen. Then,
the chloride-bridged dimer was reacted with 2-picolinic acid in
dichloromethane (CH2Cl2) solution to afford target products in
68% yield. The structures of these products were confirmed by 1H
NMR and mass spectrometry.
oxadiazole
benzene (TPBI), and CsF were purchased from Aldrich and
poly(ethylenedioxythiophene)–poly(styrenesulfonic acid)
(PBD),
1,3,5-tris(N-phenylbenzimidazol-2-yl)
UV-vis absorption and photoluminescence properties
(PEDOT:PSS) (Baytron P4083) was from Bayer. ITO (indium tin
oxide) glass substrates were washed in turn with acetone,
substrate cleaning detergent, deionized water and isopropyl
alcohol, then oven dried. The cleaned substrates were treated
with oxygen plasma to improve the work function of ITO. After
that, a 40 nm thick PEDOT/PSS film was spin-coated onto the
pre-cleaned and dried ITO glass and baked in a vacuum oven at
80 ꢁC for 8 h. Then the PVK, PBD and (PQxD)2Ir(pic) with
different ratios were dissolved separately in chlorobenzene and
spin-coated on top of the dried PEDOT:PSS layer to obtain an
emitting layer (70 nm) inside a glove box under vacuum. The
assembly was then annealed for 20 min at 100 ꢁC. Subsequently,
a layer of CsF (1.5 nm) and a layer of Al (120 nm) were vacuum-
evaporated at 3 ꢂ 10ꢀ4 Pa on the top of the EL polymer layer.
The efficient light-emitting device area is 0.19 cm2. Except for the
PEDOT:PSS that was spin-coated in air, other processes were
performed in the glove box. EL spectra and current density–
voltage–luminance (J–V–L) characteristics were collected by
a PR705 photometer and a Keithley 236 source meter, respec-
tively. The external quantum efficiency was calibrated in an
integrating sphere (Labsphere, IS080).
The absorption and the photoluminescence spectra of the
complex (PQxD)2Ir(pic) in CH2Cl2 solution at room temperature
are shown in Fig. 1. The strong absorption bands between
220 nm and 340 nm in the ultraviolet region areas were assigned
to the spin-allowed p–p* transitions on the cyclometalated
ligand. In addition, the weak and broad absorption bands
spanning from 350 to 550 nm can be attributed to the typical
spin-allowed and spin-forbidden metal-to-ligand charge transfer
bands (MLCT) and were clearly observed. It can be found that
the MLCT absorption bands of (PQxD)2Ir(pic) were mostly
covered by the emission band of the host material (PVK–PBD
Results and discussion
Synthesis and characterization
As shown in Scheme 1, the cyclometalated ligand HPQxD was
conveniently prepared by N-alkylation of diphenylamine with 4-
chloro-2-phenylquinazoline in the presence of alkali in a polar
Fig. 2 TGA data of (PQxD)2Ir(pic) under N2 atmosphere.
This journal is ª The Royal Society of Chemistry 2012
6880 | J. Mater. Chem., 2012, 22, 6878–6884

View Article Online
€
blend lmax ¼ 430 nm). It indicated that the Forster energy can
transfer from the singlet-excited states of the host to the MLCT
states of the iridium(III) complex (PQxD)2Ir(pic) effectively. The
PL spectrum of (PQxD)2Ir(pic) in CH2Cl2 solution showed
a strong red phosphorescent emission at 599 nm with a PL effi-
ciency of 11.6% (Fig. S1†), indicating the efficient emissive decay
from the lowest excited triplet state to the singlet ground state.
Based on the observed optical properties and comparison with
those typical iridium(III) complexes,20,21 it was obvious that the
observed emission of the iridium(III) complex (PQxD)2Ir(pic) was
phosphorescent in nature. The absolute emission quantum effi-
ciencies (FPL) of (PQxD)2Ir(pic) in solid and in doping films were
also measured by an integrating sphere. It is interesting to note
that (PQxD)2Ir(pic) revealed a high solid PL efficiency of 5.3%.
The PL quantum efficiency increased from 31.2% to a maximum
of 54.87% going from low to intermediate doping concentrations
(1–4%) and then decreased with increasing concentration from
4% to 12% (Fig. S3†).
Fig. 4 General structures of the devices and molecular structures of the
relevant compounds.
Thermal analysis
with respect to the zero vacuum level.22–24 The lowest unoccupied
molecular orbital (LUMO) energy level was calculated to be
ꢀ2.96 eV from the HOMO energy level value and the energy gap
derived from the UV-vis absorption band. It is clear that the
HOMO and LUMO levels of the complex are exactly between
the HOMO and LUMO levels of PVK (HOMO ¼ ꢀ5.8 eV and
LUMO ¼ ꢀ2.2 eV) and PBD (HOMO ¼ ꢀ6.2 and LUMO ¼
ꢀ2.4 eV). So the iridium(III) complex (PQxD)2Ir(pic) could trap
both electrons and holes, and this resulted in high device
efficiency.25
In order to investigate the thermal stability of the iridium(III)
complex (PQxD)2Ir(pic), thermal gravimetric analysis (TGA)
measurement was made using a TGA instrument under N2
atmosphere at a heating rate of 10 ꢁC minꢀ1 from room
temperature to 700 ꢁC. As shown in Fig. 2, the iridium(III)
complex was very stable under N2 atmosphere and the 5 wt%
weight loss temperature was 272 ꢁC. The iridium(III) complex was
decomposed and gave a mass loss of 74.9% at 500 ꢁC. It was
suitable to fabricate the device by either spin coating or vacuum
vapor deposition method.
Electrochemical properties
Electrophosphorescent properties
The electrochemical properties of the iridium(III) complex
(PQxD)2Ir(pic) were investigated by cyclic voltammetry in
CH2Cl2 solution at room temperature using ferrocene as the
internal standard. During the anodic scan, the cyclic voltam-
mogram showed a reversible oxidation wave which was due to
the IrIV/IrIII oxidation with the half-wave oxidation potential
(E1/2ox) of 0.576 V vs. Ag/AgNO3 as shown in Fig. 3. The highest
occupied molecular orbital (HOMO) energy level was calculated
to be ꢀ5.29 eV relative to the value of ꢀ4.8 eV for ferrocene (Fc)
To investigate the EL properties of the iridium(III) complex
(PQxD)2Ir(pic), the PLEDs using (PQxD)2Ir(pic) as dopant with
the device structure of ITO/PEDOT:PSS (40 nm)/(70% PVK +
30% PBD):x% (PQxD)2Ir(pic) (70 nm)/TPBI (30 nm)/CsF
(1.5 nm)/Al (120 nm) were fabricated. Fig. 4 shows the general
Fig. 5 Relative energy-level diagram of the iridium(III) complex utilized
Fig. 3 Cyclic voltammogram of (PQxD)2Ir(pic) and ferrocene.
This journal is ª The Royal Society of Chemistry 2012
in PLEDs.
J. Mater. Chem., 2012, 22, 6878–6884 | 6881

View Article Online
Fig. 8 Current density–voltage–luminance curves of the devices.
Fig. 6 Electroluminescence spectra of the devices at 10.5 mA cmꢀ2
.
Inset: CIE plots of different doping concentrations.
structure and molecular structure of the compounds used in the
devices. ITO (indium tin oxide) on a substrate of the electrolu-
minescent device acts as the anode and CsF/Al as the cathode.
PEDOT/PSS as the hole injection layer can smooth the ITO
surface, reduce the probability of electrical shorts, decrease the
turn-on voltage and prolong the operation lifetime of the device.
The PVK–PBD blend can be used as host material, and TPBI as
the hole-blocking material can improve electron injection and
electron transport. In the emitting layer, the doping concentra-
tions of (PQxD)2Ir(pic) were 1%, 2%, 4%, 8% and 12%, respec-
tively. The doping concentrations of (PQxD)2Ir(pic) were
according to the host and the mass ratio of PBD was 30% in the
PVK–PBD blend. The HOMO/LUMO levels of the materials
used in PLEDs are described in Fig. 5. The PVK–PBD blend
material with high HOMO levels acts as electron-donating and
hole-transport media, while TPBI with low LUMO levels as
electron-accepting and electron-transport media. Holes were
injected from the anode to HOMO levels of host material and
electrons were injected into LUMO levels. The HOMO and
LUMO levels of (PQxD)2Ir(pic) were embedded between the
HOMO of PVK (5.80 eV) and the LUMO of PBD (2.40 eV).
Thus, efficient energy transfer to (PQxD)2Ir(pic) occurred in the
emitting layer (EML). The PLED performance in luminance
Fig. 9 Luminance efficiency–luminance of the devices.
should be determined by the intrinsic quantum efficiency of
(PQxD)2Ir(pic) in radiative decay. Because the HOMO level of
(PQxD)2Ir(pic) was 0.51 eV higher than that of PVK, it behaved
as an effective trapping site for holes; meanwhile, the LUMO
level of (PQxD)2Ir(pic) was 0.56 eV lower than that of PBD,
electrons injected from the TPBI layer into the emitting layer
were mostly transported through the PBD layer and were
composited in the iridium phosphor. Thus, varying the concen-
tration of (PQxD)2Ir(pic) in the EML should significantly alter
the degree of charge injection in PVK–PBD-based devices.
Fig. 7 Electroluminescence spectra of the device (8%) at different
current densities.
Fig. 10 External quantum efficiency–luminance of the devices.
This journal is ª The Royal Society of Chemistry 2012
6882 | J. Mater. Chem., 2012, 22, 6878–6884

View Article Online
Table 1 Performances of the electrophosphorescent devices
Maxima of quantum efficiency
Turn-on
voltage/V
Lmax
/
Roll-off (%)
at 1000 cd mꢀ2
Device
cd mꢀ2
EQE (%)
LE/cd Aꢀ1
Voltage/V
CD/mA cmꢀ2
L/cd mꢀ2
CIE(x,y)
1(1%)
2(2%)
3(4%)
4(8%)
5(12%)
4.2
4.8
6.1
7.1
8.2
4196
6226
6485
7107
4082
9.93
16.26
14.84
18.44
15.45
11.98
16.10
19.03
20.73
13.17
25.63
12.78
8.56
19.92
23.69
8.0
8.8
10.8
13.5
13.4
1.35
0.82
0.77
0.91
0.59
162
132
146
188
77.8
(0.5899,0.3812)
(0.6016,0.3831)
(0.6126,0.3801)
(0.6192,0.3756)
(0.6194,0.3744)
Therefore, the current density of the PVK–PBD-based devices
exhibited a strong dependence on the doping concentration.
The normalized electroluminescence (EL) spectra of
(PQxD)2Ir(pic) doped PVK–PBD devices in the doping
concentrations from 1% to 12% are shown in Fig. 6. We noticed
that there are weak emission peaks at about 438 nm from the host
PVK–PBD at 1% and 2% doping concentrations. Then the peak
decreased with an increase in doping concentration and finally
disappeared with increasing doping concentration to 4%. It can
be speculated that the energy is completely transferred from host
to (PQxD)2Ir(pic) at higher doping concentration. But in the PL
spectra for different doping concentrations (Fig. S2†), even at the
doping concentration of 12%, complete quenching of the PVK–
PBD emission is not observed. This implies that energy transfer
from the PVK–PBD blend to the iridium complex is inefficient
under photo-excitation. The phenomenon with complete
quenching of the host EL emission at higher doping concentra-
tions (4% to 12%) indicates that excitons in the red dopants
formed both by direct charge trapping in the dopant molecules
and energy transfer from the host to (PQxD)2Ir(pic) are efficient
under electrical excitation.26 So the doping concentrations have
a significant influence on the mixing of single and triplet emis-
sions of MLCT. Red-emitting EL spectra with peaks at about
616 nm are observed in all of the devices. Meanwhile, the elec-
troluminescent spectra and CIE coordinate variation of different
doping concentration were shown in Fig. 6, the electrolumines-
cent spectral stability was very good and the CIE plots were
almost identical. The Commission International de l’Eclairage
(CIE) color coordinates are (0.62,0.38) which are quite close to
the standard red demanded by the National Television System
Committee (NTSC).
19.92% of its maximum upon 1000 cd mꢀ2 (Table 1). The extra-
high efficiency is an indication of balanced electron and hole
recombination in the host matrix, and complete energy and
charge transfer from the host material to iridium(III) complex
upon electrical excitation.
The devices showed a gradual increase in turn-on voltage with
increasing doping concentrations at the given current density
from 1% to 12%, which is attributed to the significant carrier-trap
of (PQxD)2Ir(pic).27 In order to investigate the performances of
these electrophosphorescent devices with different doping
concentrations, the parameters of these devices are summarized
in Table 1. The best device performance was obtained in the
device at 8% doping concentration.
Conclusions
A novel red-emitting iridium(III) complex containing phenyl
quinazoline ligand and 2-picolinic acid was synthesized. Its
photophysical, electrochemical and electrophosphorescent
properties were investigated. The device based on (PQxD)2Ir(pic)
at 8% doping concentration had an excellent performance. A
maximum EL peak at 616 nm and a high-efficiency emission with
a maximum external quantum efficiency as high as 18.44% were
obtained at 8% doping concentration. We expect that with
optimization of the molecular and device structures, efficient
phosphorescent devices could be achieved.
Acknowledgements
This work was financially supported by the National Basic
Research Program of China (973 Program, 2009CB930601), the
National Natural Science Foundation of China (project no.
50803027, no. 50903001, no. 20905038), and the Natural Science
Fund for Colleges and Universities in Jiangsu Province (grant
no. 08KJD430020). The authors are grateful to Prof. Jun-Biao
Peng, Prof. Yong Cao and Dr Bao-Feng Zhao (Institute of
Polymer Optoelectronic Materials and Devices, South China
University of Technology) for OLED tests.
Fig. 7 presents electroluminescence spectra of the device (8%)
at different current densities. With the change of current density
from 10 mA cmꢀ2 to 200 mA cmꢀ2, the spectrum is almost
unchanged and there are no residual emissions from the host
even at high current density. It can be concluded that the device
had a good light-stability.
Fig. 8 shows current density and luminance vs. voltage curves
of (PQxD)2Ir(pic)-doped OLEDs at 1% to 12% doping concen-
trations. The device at 8% doping concentration achieves
a maximum luminance of 7107 cd mꢀ2 at a current density of
78.2 mA cmꢀ2 and the turn-on voltage is 13.5 V. The maximum
external quantum efficiency and the maximum luminous effi-
ciency of the device at 8% doping concentration are 18.44%
(Fig. 10) and 20.73 cd Aꢀ1 (Fig. 9), respectively. The EL device,
a traditional EL structure, shows an obvious efficiency roll-off at
high luminance, and the luminance efficiency decreases by
Notes and references
1 Z. L. Shen, P. E. Burrows, V. Bulovic, S. R. Forrest and
M. E. Thompson, Science, 1997, 276, 2009–2011.
2 J. D. Slinker, A. A. Gorodetsky, M. S. Lowry, J. J. Wang, S. Parker,
R. Rohl, S. Bernhard and G. G. Malliaras, J. Am. Chem. Soc., 2004,
126, 2763–2767.
3 S. C. Lo, E. B. Namdas, P. L. Burn and I. D. W. Samuel,
Macromolecules, 2003, 36, 9721–9730.
This journal is ª The Royal Society of Chemistry 2012
J. Mater. Chem., 2012, 22, 6878–6884 | 6883

View Article Online
4 M. A. Baldo, M. E. Thompson and S. R. Forrest, Nature, 1998, 395,
151–154.
16 Y. Imai, S. Sato, R. Takasawa and M. Ueda, Communications, 1981,
35–36.
5 C. Adachi, A. Baldom and S. R. Forrest, et al., Appl. Phys. Lett.,
2000, 77, 904–907.
17 Y. Fang, S. J. Hu, Y. Z. Meng, J. B. Peng and B. Wang, Inorg. Chim.
Acta, 2009, 362, 4985–4990.
6 M. A. Baldo, S. Lamanskv and P. E. Burrows, Appl. Phys. Lett., 1999,
75, 4.
18 Y. Fang, B. H. Tong, S. J. Hu, S. J. Wang, Y. Z. Meng, J. B. Peng and
B. Wang, Org. Electron., 2009, 10, 618–622.
7 X. J. Zhang, Y. H. Xu and H. H. Shi, Chin. J. Lumin., 2007, 28, 44–48.
8 L. Wang, B. Liang, F. Huang, J. B. Peng and Y. Cao, Appl. Phys.
Lett., 2006, 89, 151115.
9 Y. Zhang, Y. H. Xu, Q. L. Niu, J. B. Peng, W. Yang, X. H. Zhu and
Y. Cao, J. Mater. Chem., 2007, 17, 992.
10 C. F. Lin, W. S. Huang, H. H. Chou and J. T. Lin, J. Organomet.
Chem., 2009, 694, 2757–2769.
11 Y. H. Song, S. J. Yeh, C. T. Chen, Y. Chi, C. S. Liu, J. K. Yu,
Y. H. Hu, P. T. Chou, S. M. Peng and G. H. Lee, Adv. Funct.
Mater., 2004, 14, 1221–1226.
12 Z. W. Liu, M. Guan, Z. Q. Bian, D. B. Nie, Z. L. Gong, Z. B. Li and
C. H. Huang, Adv. Funct. Mater., 2006, 16, 1441.
13 J. P. Duan, P. P. Sun and C. H. Cheng, Adv. Mater., 2003, 15, 224.
14 F. M. Hwang, H. Y. Chen, P. S. Shen, C. S. Liu, Y. Chi, C. F. Shu,
F. L. Wu, P. T. Chou, S. M. Peng and G. H. Lee, Inorg. Chem.,
2005, 44, 1344.
19 J. S. Parka, M. K. Songa, Y. S. Gal, J. W. Lee and S. H. Jin, Synth.
Met., 2011, 161, 213–218.
20 M. G. Colombo, T. C. Brunold, T. Riedener, H. U. Gudel,
M. Fortsch and H. B. Burgi, Inorg. Chem., 1994, 33, 545.
21 M. G. Colombo, A. Hauser and H. U. Gudel, Inorg. Chem., 1993, 32,
3088.
22 J. Pommerehne, H. Vestweber, W. Guss, R. F. Mahrt, M. Porsch and
J. Daub, Adv. Mater., 1995, 7, 551.
23 M. Thelakkat and H. W. Schmidt, Adv. Mater., 1998, 10, 219.
24 X. W. Zhang, J. Gao, C. L. Yang, L. N. Zhu, Z. A. Li, K. Zhang,
J. G. Qin, H. You and D. G. Ma, J. Organomet. Chem., 2006, 691,
4312.
25 D. R. Whang, Y. You, S. H. Kim, W. I. Jeong, Y. S. Park, J. J. Kim
and S. Y. Park, Appl. Phys. Lett., 2007, 91, 233501.
26 X. Gong, J. C. Ostrowski, D. Moses, G. C. Bazan and A. J. Heeger,
Adv. Funct. Mater., 2003, 13, 439.
15 G. P. Ge, J. He, H. Q. Guo, F. Z. Wang and D. C. Zou, J. Organomet.
Chem., 2009, 694, 3050.
27 Y. F. Wang, H. Tan, Y. Liu, C. X. Jiang, Z. Y. Hu, M. X. Zhu,
L. Wang, W. G. Zhu and Y. Cao, Tetrahedron, 2010, 66, 1483–1488.
6884 | J. Mater. Chem., 2012, 22, 6878–6884
This journal is ª The Royal Society of Chemistry 2012