Novel quinoxaline derivatives containing arylaminated aceanthrylene for organic red-light emitting diodes

Article abstract of DOI:10.1016/j.dyepig.2010.04.013

Novel quinoxaline derivatives containing arylaminated aceanthrylene, N,N-diphenylaceanthryleno[2,1-b]quinoxalin-5-amine and N-(naphthalen-2-yl)-N- phenylaceanthryleno[2,1-b]quinoxalin-5-amine, were synthesized as red light emitting materials for use in or

Full text of DOI:10.1016/j.dyepig.2010.04.013

Dyes and Pigments 88 (2011) 44e49
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Novel quinoxaline derivatives containing arylaminated aceanthrylene
for organic red-light emitting diodes
Jae-Wan Jang b, Hyuntae Park a, Min-Ki Shin b, Hyeon Hui Kang a, Dae-Hwan Oh a,
,
a,
*
Sung Ouk Jung c, Soon-Ki Kwon b , Yun-Hi Kim
*
a Department of Chemistry and RINS, Gyeongsang National University, Jinju 660-701, South Korea
b School of Materials Science and Engineering and ERI, Gyeongsang National University, Jinju 660-701, South Korea
c SFC Co., LTD., Cheongwon 363-883, South Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
Novel quinoxaline derivatives containing arylaminated aceanthrylene, N,N-diphenylaceanthryleno[2,1-b]
quinoxalin-5-amine and N-(naphthalen-2-yl)-N-phenylaceanthryleno[2,1-b]quinoxalin-5-amine, were
synthesized as red light emitting materials for use in organic light emitting diodes. The synthesized
materials showed good thermal stability with glass transition temperatures of 142 ꢀC for N,N-diphe-
nylaceanthryleno[2,1-b]quinoxalin-5-amine and 163 ꢀC for N-(naphthalen-2-yl)-N-phenylaceanthryleno
[2,1-b]quinoxalin-5-amine. The dipolar characteristic of the quinoxaline derivatives resulted in prom-
inent solvatochromic effects. The fabricated devices based on N,N-diphenylaceanthryleno[2,1-b]qui-
noxalin-5-amine showed maximum brightness of 17 523 cd/m2 and current efficiency of 4.55 cd/A with
chromaticity coordinates of 0.56 and 0.43. Devices based on N-(naphthalen-2-yl)-N-phenyl-
aceanthryleno[2,1-b]quinoxalin-5-amine containing N-naphthyl-N-phenyl amine displayed red emission
(x, y ¼ 0.60, 0.39) with maximum brightness of 4239 cd/m2 and current efficiency of 2.76 cd/A. The
results show that the synthesized quinoxaline derivatives offer promise as materials for stable and
efficient red emitting organic light emitting diodes.
Received 26 November 2009
Received in revised form
26 April 2010
Accepted 26 April 2010
Available online 4 May 2010
Keywords:
Quinoxaline derivatives
Arylaminated aceanthrylene
Organic light emitting diodes
Red emission
Thermal stability
Efficiency
Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction
lowest unoccupied molecular orbital (LUMO) and display low effi-
ciency due to the non-radiative relaxation of the excited state [1].
During the past two decades, much research and development
has attended organic light emitting diodes (OLEDs). Owing to their
wide range of emission colours, high brightness, low cost and low
power consumption, OLEDs have potential as ‘next generation’ full-
color flat-panel displays. To realize a vivid full-color display, pure red,
green and blue organic light emitters of high efficiency, appropriate
chromaticity coordinates and luminescence are required [1]. Among
the various organic light emitters, blue and green light emitting
materials have been developed in order to meet the requirements for
commercialization of OLEDs. Numerous electro-phosphorescent and
electrofluorescent blue and green light emitting materials have been
reported to date [2e9]. However, the red-light emitting OLEDs
remain inefficient for the realization of such next generation full-
color displays because of their low efficiency, low stability and low
color purity. Red emitting organic materials have a narrow band gap
between the highest occupied molecular orbital (HOMO) and the
Moreover, they have strong intermolecular dipoleedipole interac-
tions or intermolecular -stacking characteristics owing to their
polar or extensively -conjugated structures. This results in
a tendency to aggregate, giving rise to concentration quenching
[1,10e12]. The synthesis of many red emitting materials is a compli-
cated process of low yield [13].
The majority of organic red emitting electroluminescent
materials were designed on the basis of electron-donating and
electron-withdrawing groups within a conjugated system or Eu3þ
based complexes [14e17]. The widely used red emitting materials
contain e pyran group such as 4-(dicyanomethylene)-2-methyl-
p
p
6-(p-dimethylaminostyryl)-4H-pyran
(DCM1),
4-(dicyano-
methylene)-2-methyl-6-(juloidin-4yl-vinyl)-4H-pyran (DCM2),
4-dicyanomethylene-2-methyl-6-(1,1,7,7-tetramethyljulolidyl-9-
enyle)-4H-pyran (DCJT) and 4-(dicyanomethylene)-2-t-butyl-6-
(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran
(DCJTB).
These
have been used in conjunction with congeners [1] in which
juloidin groups were utilized as electron donors and cyano
groups as electron-withdrawing groups. Although these mate-
rials have been reported as red-emitters, their reported color in
* Corresponding authors.
E-mail addresses: skwon@gnu.ac.kr (S.-K. Kwon), ykim@gnu.ac.kr (Y.-H. Kim).
0143-7208/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2010.04.013

J.-W. Jang et al. / Dyes and Pigments 88 (2011) 44e49
45
terms of the EL spectrum complicates estimations of their color
purity levels [1]. Recently, several groups have reported qui-
noxaline derivatives as electron transporting and/or hole trans-
porting materials in OLEDs [18e22]; however, these have low
efficiency and chemical stability. It has been reported that
anthracene has interesting photo and electroluminescent prop-
erties and good electrochemical properties [23e30].
This paper concerns novel, red emitting materials composed of
quinoxaline and aceanthrylene with triarylamine, QAD-I and QAD-
II. The quinoxaline moiety increased the electron affinity (EA) of the
molecules due to the additional imine nitrogen in the ring and
improved both electron injection and transport properties [18,22].
Moreover, the introduction of aceanthrylene with an arylamine
moiety increased efficiency not only owing to the anthracene unit
having interesting photo and electroluminescent properties and
good electrochemical and thermal properties, but also due to the
arylamine unit that was utilized as an electron-donating group on
the aromatic rings, as these are widely used as a hole-transporting
layer. OLEDs based on synthesized quinoxaline derivatives were
fabricated and the electroluminescent characteristics of devices
based on novel QAD-I and QAD-II were also reported.
the mass spectra of the samples. UVeVis absorption and photo-
luminescence (PL) spectra were recorded using PerkineElmer
LAMBDA-900 UVeviseNIR spectrophotometer and LS-50B lumi-
nescence spectrophotometer respectively. Cyclic voltammograms
of samples were recorded at room temperature using epsilon E3
cyclic-voltmeter in a 0.1 M solution of tetrabutylammonium per-
chlorolate (Bu4NClO4) in acetonitrile under the nitrogen environ-
ment at a scan rate of 50 mV/s. The platinum rod and wire were
used as working and the counter electrode and a Ag/AgNO3 elec-
trode as the reference electrode.
Pre-patterned indium tin oxide (ITO) substrates were cleaned by
sonication in a detergent solution for 2 min and then washed with
a large amount of doubly distilled water. Further sonication in
ethanol for 2 min was done before blowing dry with a stream of
nitrogen. The ITO substrates were then treated with O2 plasma for
1 min before being loaded into the vacuum chamber. The organic
layers were deposited thermally at a rate of 0.1e0.3 nm sꢁ1 under
a pressure of about 10ꢁ6 torr. OLEDs devices were constructed with
20 nm of copper phthalocyanine (CuPc) (as the hole injection layer,
50 nm of 4,40-bis[1-naphtylphenylamino]triphenylamine (NPB) as
the hole-transporting layer, 20 nm of QAD-1 or QAD-II (3 wt% in
Alq3) as the emission layer, 40 nm of Alq3 as the electron trans-
porting emission layer, 1 nm of LiF as the electron injection layer
and 150 nm of Al as the cathode. The EL was measured under
ambient conditions.
2. Experimental
All reagent and solvents used in this study were purchased from
Aldrich Chemical Co. and Fluka. Only analytical grade quality
chemicals were used for the experiments. Spectroscopic grade
CHCl3 (Aldrich) was used to measure UVeVisible absorption and
emission spectra of the samples. Infrared (IR) spectra of materials
synthesized were recorded using a Genesis II FT-IR spectrometer. 1H
NMR spectra were recorded using DRX 300 MHz NMR Bruker
spectrometers and chemical shifts in the spectra are reported in
units of parts per million with tetramethylsilane as internal stan-
dard. Thermogravimetric analyzer (TGA) of the samples were
carried out using TA instrument 2050 thermogravimetric analyzer
under nitrogen with heating rate of 10 ꢀC/minute from 50 to 800 ꢀC.
Differential scanning calorimetry (DSC) study was carried out for
the samples using TA Instruments 2100 differential scanning
calorimeter under nitrogen with heating rate of 10 ꢀC/min from 30
to 300 ꢀC. A Jeol JMS-700 mass spectrometer was utilized to obtain
2.1. Synthesis of aceanthrylene-1,2-dione (1)
This compound was prepared from oxalyl chloride and anthra-
cene by the reported procedure (mp ¼ 268 ꢀC) [31].
2.2. Synthesis of 6-bromo-aceanthrylene-1,2-dione (2)
A solution of 1 (5.4 g, 23.25 mmol) in nitrobenzene was heated
to 145 ꢀC and stirred while a solution of bromine (3.90 g,
24.40 mmol) in nitrobenzene was added dropwise. The ensuing
mixture was stirred and heated for 15 min, then cooled and diluted
with ethanol to facilitate filtration. After washing with alcohol and
drying, the yellow solid was obtained by crystallization from
o-dichlorobenzene as reported (dp gt; 260 ꢀC) [32].
Fig. 1. Synthetic scheme of the quinoxaline derivatives of QAD-I and QAD-II: (i) oxalyl chloride, AlCl3, CS2, (ii) Br2, nitrobenzene, (iii) o-phenylenediamine, EtOH and (iv) Pd(OAc)2,
P(t-Bu)3, NaO-t-Bu.

46
J.-W. Jang et al. / Dyes and Pigments 88 (2011) 44e49
a
100
80
60
40
20
0
0
100
200
300
400
500
600
700
800
Temperature
b
100
0.04
0.00
80
60
40
20
0
-0.04
-0.08
-0.12
-0.16
-0.20
20
40
60
80
100 120 140 160 180 200 220
0
100
200
300
400
500
600
700
o
Temperature ( C)
Temperature
Fig. 2. The TGA and DSC curves of (a) QAD-I and (b) QAD-II.
2.3. Synthesis of quinoxaline derivative compound (QD)
8.7Hz), 7.82e7.77 (m, 3H), 7.72e7.70 (m, 1H), 7.58e7.49 (m, 1H),
7.28e7.21 (m, 5H), 7.17e7.14 (m, 4H), 6.99e6.95 (m, 2H). FT-IR (KBr,
cmꢁ1): 3030 (aromatic CeH), 1587 (C]N), 1542, 1495 (C]C), 1335
(CeN). Mass spectrometry (GCeMS): calcd for C34H21N3: 473.19,
found 473.19. Anal. calcd for C34H21N3: C, 86.60; H, 4.49; N, 8.91,
found: C, 86.56; H, 4.52. N, 8.83.
This compound was prepared by the rapid condensation of
o-phenylenediamine with 2 in boiling pyridine to give the qui-
noxaline derivative which was crystallized using chlorobenzene to
provide a bright yellow solid (mp ¼ 289 ꢀC) [32].
2.4. Synthesis of quinoxaline derivative of QAD-I
2.5. Synthesis of quinoxaline derivative of QAD-II
A mixture of QD (2 g, 5.22 mmol), diphenylamine (1.06 g,
6.16 mmol), Pd(OAc)2 (0.07 g,0.1 mmol), tri-tert-butylphosphine
(0.05 g, 0.26 mmol) and sodium tert-butoxide (0.75 g, 7.83 mmol)
in toluene(30 mL) was stirred under nitrogen at 120 ꢀC for 16 h. The
ensuing mixture was allowed to cool to room temperature, filtered
through cellite and washed with dichloromethane. The filtrate
and washings were collected and washed with water and then
dried over MgSO4. The solvent was removed and the solid purified
using silica gel column chromatography employing dichloro-
methane and hexane (3:1). Yield: 1.88 g (76.4%). 1H NMR (300 MHz,
A mixture of QD (2 g, 5.22 mmol), naphthylphenylamine (1.34 g,
6.16 mmol), Pd(OAc)2 (0.07 g,0.1 mmol), tri-tert-butylphosphine
(0.05 g, 0.26 mmol) and sodium tert-butoxide (0.75 g, 7.83 mmol)
in toluene (30 mL) under a nitrogen atmosphere was stirred at
120 ꢀC for 10e16 h. The mixture was allowed to cool to room
temperature, filtered through cellite and washed with dichloro-
methane. The filtrate and washings were collected and washed
with water and then dried over MgSO4. The solvent was removed
and the solid purified with silica gel column chromatography using
dichloromethane and hexane (3:1). Yield: 1.96 g (72.2%). 1H NMR
CDCl3):
d[ppm] ¼ 8.46(d, 1H, 6.6Hz), 8.43e8.31 (m, 3H), 8.19 (d, 1H,
(300 MHz, CDCl3)
d
[ppm] ¼ 8.39e8.34 (m, 3H), 8.19e8.17 (m, 2H),
Table 1
Physical properties of quinoxaline derivatives.
Compound
Tm
Tg
HOMO
(eV)b
LUMO
(eV)b
CH2Cl2
labs max (nm)
n-Hexane
lem max (nm)
Toluene
lem max (nm)
EA lem max
(nm)
THF lem max
(nm)
CH2Cl2 lem max
(nm)
(ꢀC)a
(ꢀC)a
QAD-I
QAD-II
280
e
142
163
5.4
5.38
3.24
3.26
514
522
555
564
591
602
610
633
616
633
623
638
a
Melting and glass transition temperature (Tm, Tg) obtained from DSC measurement.
HOMO and LUMO measured in a CH2Cl2 solution. EA: Ethylacetate, THF: Tetrahydrofuran.
b

J.-W. Jang et al. / Dyes and Pigments 88 (2011) 44e49
47
Compared with well known pyran-containing red dyes, the
synthesized QAD-1 and QAD-II are more promising for the red
emitting OLEDs because of their simple synthesis with high yield.
The synthesized QAD-I and QAD-II were confirmed by various
spectroscopic studies such as 1H NMR, FT-IR, MS and elemental
analysis. They exhibit good solubility in common organic solvents
of dichloromethane, chloroform, toluene and o-dichlorobenzene
despite the fact they have a fused planar system of aceanthryl-
quinoxaline. The good solubility of QAD-1 and QAD-II could be due
to the introduction of arylamine moiety.
The thermal behavior of QAD-1 and QAD-II was determined by
thermogravimetric analyzer (TGA) and differential scanning calo-
rimetry (DSC). QAD-1 and QAD-II show good thermal stability with
the glass transition temperature (Tg) of 142 ꢀC for QAD-I and 163 ꢀC
for QAD-II (Fig. 3). QAD-1 and QAD-II containing an aceanthryl
group have increased Tg when compared to those containing an
acenaphtyl group [18]. QAD-II with the N-naphtyl-N-phenyl amine
shows a higher Tg than QAD-I with the N,N-diphenyl amine.
The increased stability of the glassy state in these molecules can be
due to the rigid fused aromatic structure and hence this can
increase device longevity. In TGA studies, the 5% weight loss was
observed at 405 ꢀC for QAD-I and 453 ꢀC for QAD-II (Fig. 3).
Fig. 2 shows UVeVis absorption and PL spectra of QAD-1 and
QAD-II in CH2Cl2 solution. The electronic spectra of the molecules
a
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
250 300 350 400 450 500 550 600 650 700 750 800
W avelength (nm)
b
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
reveal several bands arising from
p/p* and charge-transfer
transitions. In particular, * transition of the characteristic
p
/p
250 300 350 400 450 500 550 600 650 700 750 800
W avelength (nm)
anthracene and quinoxaline appeared at 335, 360, and 405 nm. The
lower energy transition representing positive solvatochromism was
considered mainly to be charge-transfer transition character. As the
polarity of the solvent increased, the transition became red shifted.
The UVeVis absorption spectra of QAD-I and QAD-II show an
absorption maximum at 514 nm for QAD-I and 522 nm for QAD-II.
The fluorescence emission of QAD-I and QAD-II in various solvents
are summarized in Table 1. Bathochromic shifts in the emission (for
QAD-I and QAD-II) were observed in n-hexane (555 nm, 564 nm),
toluene (591 nm, 602 nm), EA (610 nm, 633 nm) and THF (616 nm,
633 nm) as given in Table 1. QAD-1 and QAD-II containing
aceanthryl group show more red-shift in UVeVis absorption and
emission when compared to those with an acenaphtyl group [18].
The emission maximum (lmax) of QAD-II with an N-naphthyl-N-
phenyl amine group is red-shifted when compared to that of QAD-I
with an N,N-diphenyl amine group.
Fig. 3. UVeVis absorption and photoluminescence spectra of (a) QAD-I and (b) QAD-II
in CH2Cl2.
7.79e7.74 (m, 5H), 7.62 (m, 1H), 7.52e7.50 (m, 3H), 7.35e7.25 (m,
8H), 7.04e6.98 (m, 1H). FT-IR (KBr, cmꢁ1): 3030 (aromatic CeH),
1585 (C]N), 1540, 1495 (C]C), 1338 (CeN). M ass spectrometry
(GCeMS): calcd for C38H25N3: 523.20, found: 523.20. Anal. calcd for
C38H25N3: C, 87.16; H, 4.81; N, 8.02, found: C, 87.20; H, 4.83; N, 8.11.
3. Result and discussion
The synthetic scheme of the novel organic materials, QAD-1 and
QAD-II, is shown in Fig. 1. QAD-1 and QAD-II were easily synthe-
sized by N-arylation of naphtylphenylamine or diphenylamine with
60-bromo-aceanthrylene-1,2-quinoxaline obtained by the dehy-
dration of 60-bromoaceanthylene-1,2-dione and phenylenedi-
amine. After purification of the product by silica gel column
chromatography, QAD-1 and QAD-II were obtained in the form of
an orange solid with good yields of 76% and 72% respectively.
The electrochemical properties of the QAD-1 and QAD-II were
investigated by cyclic voltammetric studies and the redox poten-
tials of the compounds were assessed as shown in Table 1. QAD-I
and QAD-II show reversible oxidation potentials characterized by
Eonset value at 1.0 V and 0.98 V respectively. This is attributed to the
a
b
0.000025
0.000010
QAD-1
QAD-2
0.000020
0.000015
0.000010
0.000005
0.000000
-0.000005
-0.000010
0.000008
0.000006
0.000004
0.000002
0.000000
-0.000002
-0.000004
-2.0
-1.5
-1.0
-0.5
0.0
-2.0
-1.5
-1.0
-0.5
0.0
Experiment Type : Cyclic Voltammetry (CV) (Title : CV Run for BAS-Epsilon)
Experiment Type : Cyclic Voltammetry (CV) (Title : CV Run for BAS-Epsilon)
Fig. 4. The cyclic voltammetry of (a) QAD-I and (b) QAD-II.

48
J.-W. Jang et al. / Dyes and Pigments 88 (2011) 44e49
oxidation of triphenylamine moiety. The estimated ionization
potentials (IP ¼ Eonset þ4.4 eV ¼ HOMO level) are about 5.4 eV for
QAD-I and 5.38 eV for QAD-II. The higher HOMO energy levels of
QAD-I and QAD-II can be efficient for the hole injection and
transport. QAD-I and QAD-II also show the reduction potential
(Fig. 4). The LUMO levels were approximately 3.24 and 3.26 eV for
QAD-I and QAD-II respectively. QAD-I and QAD-II with the qui-
noxaline moiety show the lower LUMO levels, which will be much
effective for the efficient electron injection and transport. These
results suggest that QAD-I and QAD-II are more promising mate-
rials for the efficient red emitting OLED devices.
OLEDs based on QAD-I and QAD-II were fabricated using the
configuration: indium tin oxide (ITO)/copper pthalocyanine
(CuPc)/N,N-bis-(1-naphthalenyl)-N,N-bis-phenyl-(1,1-biphenyl)-
4,4-diamine (NPB)/tris-8-hydroxyquinoline)aluminum (Alq3):
QAD-I (3 wt%) or QAD-II (3 wt%)/LiF/Al. CuPc, NPB, Alq3, LiF and
Al layers were used as hole injection, hole transporting, electron
transporting, electron injection and cathode layers respectively.
QAD-I (3 wt%) doped in Alq3 (host) for Device I and QAD-II
(3 wt%) doped in Alq3 for Device II were used as emissive layer
(EML). As the color of fabricated devices with red dopant
depends on the concentration of the dopants employed, OLEDs
with red dopant (QAD-I for device I and QAD-II for device II)
1.0
0.8
0.6
0.4
0.2
0.0
400
500
600
700
Wavelength (nm)
Fig. 6. Electroluminescence (EL) spectra of (ꢁ) device I and (ee) device II.
performances. The voltage-luminance-efficiency characteristics
are shown in Fig. 3. The maximum brightness and efficiency
of device I were observed to be 17 523 cd/m2 (7.78 V) and
4.55 cd/A (4.02 V) respectively. In the case of device II,
the maximum brightness and efficiency were observed as
4239 cd/m2 (8.6 V) and 2.76 cd/A (4.27 V) respectively. Hence,
it is reported that the diphenylamino moiety in QAD-I gives
higher brightness and efficiency in device I when compared to
those of QAD-II with naphthyl phenyl amine in device II. The
results suggest that the electron injection into QAD-I containing
N,N-diphenyl amine moiety in device I can be improved when
compared to that into QAD-II containing N-naphthyl-N-phenyl
amine moiety in device II. The electroluminescence (EL) spectra
of the devices I and II are shown in Fig. 5. In the EL spectra, it
is confirmed that the emission comes from QAD-I and QAD-II
due to efficient energy transfer from Alq3 host although slight
shoulder emission is represented in the 510 nm. It confirms that
the introduction of aceanthryl-quinoxaline derivatives causes
concentration of
3
wt% were fabricated for the improved
a
5
4
3
2
1
0
20000
17500
15000
12500
10000
7500
5000
2500
0
a
bathochromic shift in the EL spectrum. The observed EL
spectra show a peak at 600 nm (CIE x,y: 0.56, 0.43) for device I
and a peak at 616 nm (CIE x,y: 0.60, 0.39) for device II (Fig. 6).
4
5
6
7
8
Voltage (V)
3
2
1
0
5000
4000
3000
2000
1000
0
b
4. Conclusions
Novel quinoxaline derivatives containing arylaminated acean-
thrylene, QAD-I and QAD-II, were designed and synthesized
successfully as red emitting materials in OLEDs. The quinoxaline
moiety and arylamine group were utilized as the electron-with-
drawing group and electron-donating group respectively in these
derivatives. The introduction of anthracene leads to the improved
color purity and thermal stability. When OLEDs based on these
derivatives were fabricated, the characteristic electroluminescence
(EL) spectrum of the device based on QAD-I containing N,N-
diphenyl amine is observed at 600 nm (CIE: 0.56, 0.43) with
a maximum brightness of 17 523 cd/m2 (7.78 V) and efficiency of
4.55 cd/A (4.02 V). Devices based on QAD-II containing N-naphthyl-
N-phenyl amine show the red emission at 616 nm (CIE: 0.60, 0.39)
with the maximum brightness of 4239 cd/m2 (8.6 V) and efficiency
of 2.76 cd/A (4.27 V). It is proposed that the optimized device
structures based on these novel quinoxaline derivatives, QAD-I and
QAD-II, can be useful for the stable and efficient red emitting device
applications.
4
5
6
7
8
9
Voltage (V)
Fig. 5. Efficiency (C) e voltage e EL brightness (-) characteristics of (a) device I:
ITO/CuPc/NPB/Alq3: QAD-I (3 wt%)/LiF/Al and (b) device II: ITO/CuPc/NPB/Alq3: QAD-II
(3 wt%)/LiF/Al.

J.-W. Jang et al. / Dyes and Pigments 88 (2011) 44e49
49
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