High-efficiency tris(8-hydroxyquinoline)aluminum (Alq3) complexes for organic white-light-emitting diodes and solid-state lighting

Article abstract of DOI:10.1002/chem.201100707

Combinations of electron-withdrawing and -donating substituents on the 8-hydroxyquinoline ligand of the tris(8-hydroxyquinoline)aluminum (Alq ₃) complexes allow for control of the HOMO and LUMO energies and the HOMO-LUMO gap responsible for emission from the complexes. Here, we present a systematic study on tuning the emission and electroluminescence (EL) from Alq₃ complexes from the green to blue region. In this study, we explored the combination of electron-donating substituents on C4 and C6. Compounds 1-6 displayed the emission tuning between 478 and 526 nm, and fluorescence quantum yield between 0.15 and 0.57. The compounds 2-6 were used as emitters and hosts in organic light-emitting diodes (OLEDs). The highest OLED external quantum efficiency (EQE) observed was 4.6 %, which is among the highest observed for Alq₃ complexes. Also, the compounds 3-5 were used as hosts for red phosphorescent dopants to obtain white light-emitting diodes (WOLED). The WOLEDs displayed high efficiency (EQE up to 19 %) and high white color purity (color rendering index (CRI≈85). Whiter than white: Electron-withdrawing and -donating substituents on the tris(8-hydroxyquinoline) Al^III (Alq₃) complexes allow for control of the emission and electroluminescence from green (526 nm) to the blue region (487 nm), and of the fluorescence quantum yield (0.15-0.57). The compounds used as emitters in OLEDs show a maximum external quantum efficiency (EQE) of 4.6 %, which is among the highest observed for Alq₃ complexes. The complexes were used in white-light emitting OLEDs achieving high efficiency (EQE ≈19 %) and high white color purity (CRI≈85).

Full text of DOI:10.1002/chem.201100707

DOI: 10.1002/chem.201100707
High-Efficiency Tris(8-hydroxyquinoline)aluminum (Alq3) Complexes for
Organic White-Light-Emitting Diodes and Solid-State Lighting
Cꢀsar Pꢀrez-Bolꢁvar, Shin-ya Takizawa, Go Nishimura, Victor A. Montes, and
Pavel Anzenbacher, Jr.*[a]
9076
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2011, 17, 9076 – 9082

FULL PAPER
Abstract: Combinations of electron-
withdrawing and -donating substituents
on the 8-hydroxyquinoline ligand of
the tris(8-hydroxyquinoline)aluminum
(Alq3) complexes allow for control of
the HOMO and LUMO energies and
the HOMO–LUMO gap responsible
for emission from the complexes. Here,
electron-donating substituents on C4
and C6. Compounds 1–6 displayed the
emission tuning between 478 and
526 nm, and fluorescence quantum
yield between 0.15 and 0.57. The com-
pounds 2–6 were used as emitters and
hosts in organic light-emitting diodes
(OLEDs). The highest OLED external
quantum efficiency (EQE) observed
was 4.6%, which is among the highest
observed for Alq3 complexes. Also, the
compounds 3–5 were used as hosts for
red phosphorescent dopants to obtain
white light-emitting diodes (WOLED).
The WOLEDs displayed high efficien-
cy (EQE up to 19%) and high white
color purity (color rendering index
(CRIꢀ85).
we present
a systematic study on
Keywords: aluminum · electrolumi-
nescence · fluorescence · hydroxy-
quinoline · organic light-emitting
diodes
tuning the emission and electrolumi-
nescence (EL) from Alq3 complexes
from the green to blue region. In this
study, we explored the combination of
Introduction
Electroluminescent organic semiconductors are widely in-
vestigated due to their applications in organic light-emitting
diodes (OLEDs) aimed at full-color flat panel or flexible
display technologies,[1] as well as their potential use in or-
ganic white-light-emitting diodes (WOLEDs) for general
lighting applications.[2] To achieve high performance
OLEDs, judicious molecular design is essential as each or-
ganic layer must satisfy stringent requirements regarding
their carrier mobility, desired emission wavelength, thermal
stability, aligned HOMO–LUMO energies, and so forth.
Since Tang and VanSlyke reported the first OLED with
tris(8-hydroxyquinoline)aluminum (Alq3, 1),[3] Alq3 has been
studied as an electron-transporting and -emissive materi-
Alq3 emission, as shown on the example of complex 2.[5]
These optical changes can be interpreted by theoretical cal-
culations[6] and used for the design of new Alq3 derivatives.
To further increase the HOMO–LUMO gap and achieve
strongly blue-shifted emission, we combined the electron-
withdrawing groups, such as fluoro, trifluoromethyl, and
cyano groups at the C6 position with an electron-donating
methyl group at the C4 (4–6) of the quinolinolate ligand. It
is conceivable that such blue emitters could, after doping
with an orange phosphor, yield new Alq3 derivatives useful
as electron-transporting and -emitting hosts for WOLEDs
with a simpler configuration and better color rendering
index (CRI).[7]
A series of new blue-emitting Alq3 complexes 4–6 were
synthesized and their properties were investigated. The
emission color tuning towards the blue was achieved by
combining electron-withdrawing and -donating groups on
the ligand. Preliminary OLEDs using 2–4 and corresponding
WOLEDs were tested as potential sources of white light.
ACHTUNGTRENNUNG
ants in phosphorescent OLEDs (PHOLEDs),[1c] while main-
taining high charge mobility, is another challenge that could
be addressed by developing high-energy Alq3 derivatives.
However, few studies were focused on optimization of the
Alq3 to yield blue emission.[3b,4] Recently, 6-fluoro-Alq3 de-
rivatives (3 and 4), in which the 6-F substituent acts as an
electron-withdrawing group to lower the HOMO level, were
found to yield green-blue emission,[4e,f] while others, such as
triazine and naphthyridine derivatives, showed blue electro-
luminescence (EL).[4g]
Alternatively, the electron-donating methyl at the pyri-
dine ring (positions C2 or C4) also causes a blue-shift of the
Results and Discussion
[a] Dr. C. Pꢁrez-Bolꢂvar, Dr. S.-y. Takizawa, Dr. G. Nishimura,
Dr. V. A. Montes, Prof. Dr. P. Anzenbacher, Jr.
Department of Chemistry and
The ligands 13, 14, and 19 were synthesized from the corre-
sponding nitrophenols 7, 8, and 15 by SnCl2 reduction fol-
lowed by Skraup quinoline synthesis[8] with methyl vinyl
ketone in the presence of HCl (Scheme 1). The AlIII com-
plexes were prepared by reaction with tri(isopropoxide)
AlIII. Synthetic details for the new Alq3 complexes and devi-
ces are shown in Scheme 2 and the detailed synthesis is de-
scribed in the Supporting Information.
Center for Photochemical Sciences
Bowling Green State University, Bowling Green
Ohio 43403 (USA)
Fax : (+1)419-372-9809
E-mail: pavel@bgsu.edu
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201100707.
Chem. Eur. J. 2011, 17, 9076 – 9082
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
9077

P. Anzenbacher, Jr. et al.
Scheme 1. Procedure used for the synthesis of ligands 13, 14 and 19. Inset: X-ray structure of 14.
complexes 1–6. All complexes exhibited irreversible oxida-
tion waves and the HOMO and LUMO levels were ob-
tained from the oxidation and reduction peaks using DPV
following a method from literature.[9] The redox potentials
are listed in Table 1and the corresponding HOMO–LUMO
levels of complexes 1–6 are schematically shown in Fig-
ure 2and Table 1.
Table 1 shows that the introduction of 4-CH3 and 6-F in 4
significantly affects the HOMO–LUMO levels. Compared
to the parent Alq3 (1), 4-CH3 in 2 raises both the HOMO
Scheme 2. Procedure for synthesis the AlIII quinolinolates 4, 5 and 6.
Table 1. Electrochemical properties of the AlIII complexes.
Eox
[V]
Ered
HOMO
[eV]
LUMO
[eV]
HOMO–LUMO
[eV]
The structure of the AlIII complexes was confirmed by
NMR spectroscopy and an X-ray structure of complex 4 is
shown in Figure 1.
To investigate the effects of the substituents on the
HOMO–LUMO energy levels, cyclic voltammetry (CV) and
differential pulse voltammetry (DPV) were carried out for
[V][a]
1
2
3
4
5
6
0.82
0.73
1.02
1.01
0.98
1.14
À2.37
À2.48
À2.20
À2.36
À2.41
À2.24
À5.62
À5.53
À5.82
À5.81
À5.78
À5.94
À2.43
À2.32
À2.60
À2.44
À2.39
À2.56
3.19
3.21
3.22
3.37
3.39
3.39
[a] Determined by DPV in 1.0 mm solutions in CH2Cl2 containing tetra-
butylammonium perchlorate and referenced against Fc/Fc+ (0.360 V vs.
Ag/AgNO3).
Figure 2. HOMO–LUMO levels and magnitudes of the HOMO–LUMO
Figure 1. X-ray structure of complex 4.
gap in complexes 1–6.
9078
www.chemeurj.org
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2011, 17, 9076 – 9082

Organic Light-Emitting Diodes
FULL PAPER
Table 2. Photophysical properties of the AlIII complexes recorded in
CH2Cl2 at 228C.
and LUMO levels due to its electron-donating properties,
whereas the fluorine atom lowers the energy levels because
of its electron-withdrawing property in complex 3. The
energy gaps of 2 and 3 are slightly larger than that of Alq3,
since 4-CH3 particularly affects the LUMO level, while 6-
fluorine atom influences the HOMO level. The 6-fluorine
substituent leads to a larger decrease in the HOMO level
than LUMO level, thus widening the HOMO–LUMO gap.
This is rationalized by the HOMO level being mostly locat-
ed on the ligand phenoxide moiety, while LUMO is on the
pyridyl.[6] While these aspects were known,[4e,5] the tedious
syntheses of disubstituted quinolinols hampered the studies
of such materials.
[a]
[b]
[c]
Abs labs
FL lsol
FL lfilm
Fsol
Ffilm
tF
ET
[nm]
[nm]
[nm]
[ns]
[eV]
1
2
3
4
5
6
387
382
373
367
374
382
526
501
499
478
482
478
508
489
484
482
462
471
0.15
0.38
0.35
0.57
0.46
0.32
0.22
0.35
0.26
0.41
0.48
0.60
15.4
23
26.5
23
23
20.5
2.03
2.19
2.11
2.21
2.20
2.21
[a] Fluorescence quantum yield—determined by using quinine sulfate
(0.1m H2SO4) as a standard. [b] Determined in an absolute quantum
yield setup, with excitation at 375 nm from a laser diode. [c] Triplet
energy—estimated from phosphorescence spectrum in 2-MeTHF glass at
77 K.
As expected, complex 4, with a combination of 4-CH3-
and 6-F substituents, gave a larger HOMO–LUMO gap
(3.37 eV) than complexes 2 and 3 (3.21 and 3.22 eV). This is
explained by the combined effect of methyl and fluorine
groups on the LUMO and HOMO levels: the LUMO level
remains unchanged due to the methyl group, whereas the
fluorine atom lowered the HOMO level. Complex 5 with
the electron-withdrawing 6-CF3 group shows a trend similar
to complex 4 in the energy levels, and its energy gap
(3.39 eV) is also similar. Complex 6 with the 6-cyano group
also displayed an identical energy gap (3.38 eV), even
though both the HOMO and LUMO levels are decreased.
Table 1 shows that the HOMO–LUMO energies are in good
correlation with the optical properties of 1–6.
Figure 3 shows the fluorescence of the complexes and
their fluorescence spectra in CH2Cl2. The data are summar-
ized in Table 2. Complexes 1–6 exhibit quinolinolate p–p*
transition peaks in the range of 367–387 nm. Clearly, the
fluorescence maxima of complexes 2–6 are blue-shifted
compared to the parent Alq3 because of the substituent-
mediated increase in HOMO–LUMO gaps. The complexes
4–6 exhibit significant blue fluorescence. Complex 2 with 4-
CH3 and 3 with 6-F show the emission maxima of 500 nm,
whereas Alq3 has green emission with lmax =526 nm. The
combination of the two substituents in 4 blue-shifted the
fluorescence by 48 nm relative to Alq3, thereby affording
much bluer fluorescence (lmax =478 nm). This is because
complex 4 displays a larger HOMO–LUMO gap. Complexes
5 and 6 also exhibited blue emission with similar peaks
(lmax =482, 478 nm, respectively) as 4. As shown in the Sup-
porting Information the shift in absorption spectra is also in
agreement with these results; the absorption peaks of com-
plexes 4–6 appeared at a shorter wavelength compared to
Alq3. It is worth noting that the quantum yields of the new
blue AlIII complexes (F=0.46-0.57) in solution are consider-
ably higher than that of Alq3 (F=0.15).
Also, the triplet energies of 4–6 determined from time-
gated phosphorescence spectra recorded at 77 K in MeTHF
were blue-shifted relative to 1–3. This confirms that this ap-
proach also enables triplet energy tuning in the Alq3 deriva-
tives. Importantly, the triplet energies of complexes 2, 4–6
are higher than those of orange dopants such as bis(2-
phenyl-1-quinoline)iridium acetylacetonate ([Ir(pq)2ACHTUNGTRENNUNG(acac)];
ET=2.10 eV), which makes 4–6 suitable as host materials
for orange phosphorescent emitters for WOLED applica-
tions.[7]
To demonstrate the viability of the new complexes 3–5 (3
had not been previously tested) as blue emitters, we first
fabricated OLEDs using complexes 1–5 as electron-trans-
porting and -emitter materials by using simple device config-
urations (A and B) (Table 3). Configuration A also provides
an insight into the effect of the different LUMO levels on
the efficiency of the electron injection directly from the
electrode. Complex 6 was prepared in a spin-coated device
and compared to the other complexes also in spin-coated
OLEDs as guests in polyvinylcarbazole (PVK; 10 mgmLÀ1
in toluene) as a host (See Supporting Information). The EL
from the OLEDs (Device B, Table 3) fabricated with the
new Alq3 derivatives displayed strong blue-shifted EL with
respect to 1 and 2 (see Supporting Information). Complexes
4 and 5 were tested also in a more ellaborate architecture
(C): indium tin oxide (ITO)/poly(3,4-ethylene-dioxythio-
Figure 3. Fluorescence of complexes 1–6 and corresponding fluorescence
spectra recorded in dichloromethane.
Chem. Eur. J. 2011, 17, 9076 – 9082
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
9079

P. Anzenbacher, Jr. et al.
Table 3. Electroluminescence characterization for OLEDs with configu-
rations A–D.
Turn on
voltage [V]
Max luminance
[cdmÀ2
Max EQE
[%]
Max current
G
]
efficiency [cdAÀ1
]
A: PEDOT:PSS/NPD/complex/CsF/Al
1
2
3
4
5
5.5
3.5
4.0
9.3
52300
15300
3640
980
1.4
1.9
0.5
0.4
0.06
4.6
5.2
1.1
1.2
0.05
14
45
B: PEDOT:PSS/MoO3/TPD/TCTA/complex/Yb/Ag
3
4
5
5.0
3.5
7.0
4940
31040
1640
0.2
4.0
1.9
0.5
10.4
4.5
C: PEDOT:PSS/MoO3/TPD/TCTA/complex/TPBI/Yb/Ag
3
4
5
4.0
3.6
4.2
8570
16700
16800
0.2
4.0
4.1
0.6
10.8
10.1
D: PEDOT:PSS/MoO3/TPD/TCTA/complex/BPhen/Yb/Ag
4
5
3.6
4.2
9770
18580
4.6
3.6
12.3
9.2
A
(PEDOT:PSS)
(30 nm)/
MoO3 (1.5 nm)/N,N’-diphenyl-N,N’-bis(3-methylphenyl)-l,1’-
biphenyl-4,4’diamine (TPD) (40 nm)/tris(4-carbazol-9-yl-
phenyl)amine (TCTA) (5 nm)/complex 3–5 (50 nm)/1,3,5-
tris(N-phenylbenzimidazol-2-yl)benzene (TPBI) (10 nm)/
metal cathode (100 nm).
For example, the device based on the complex 4 as an
emitter showed maximum external quantum efficiency
(EQE) of 4.0% and maximum current efficiency of
10.8 cdAÀ1. The turn-on voltage was 3.6 V and the maximum
luminance observed was 16700 cdmÀ2. The same device
using complex 5 as the emitter yielded maximum luminance
of 16800 cdmÀ2, with a turn-on voltage of 4.2 V, current effi-
ciency of 10.1 cdAÀ1, and maximum EQE 4.1%. Another
set of OLEDs (configuration D, Table 3) employing batho-
phenanthroline (BPhen) instead of TPBI was prepared
using complexes 4 and 5. For the OLED based on the com-
plex 4, the maximum luminance observed peaked at
9770 cdmÀ2, the maximum EQE was 4.6%, and the current
efficiency was 12.3 cdAÀ1. In the case of complex 5 and
Bphen, the best OLED exhibited a maximum luminance of
18580 cdmÀ2, maximum EQE of 3.6% and maximun cur-
rent efficiency of 9.2 cdAÀ1. The EQE for complex 4 is
among the highest ever reported for an OLED based on an
Alq3 derivative.[4g] Furthermore, the current efficiency for
both complexes is one of the highest for undoped Alq3-
based OLEDs.[10] On the other hand, complex 6 decomposes
during thermal deposition and must be processed from solu-
tion in a mixture with a polyvinyl carbazole (PVK) host. As
expected, the reference devices with 2 and 3 confirmed that
new AlIII complexes 4 and 5 show blue-shifted EL compared
to 2 and 3 (Figure 4). A presence of a shoulder at longer
wavelengths (ꢀ550 nm) in the EL spectra is possibly due to
the electron-deficient character of these complexes. It is
Figure 4. Top: Electroluminescence of complexes 3–5 and a white-light
emitting OLED based on complex 4. Bottom: Operating principle of the
WOLED emissive layer: Complex 4 acts as a host but also emits blue
fluorescence. The non-emissive triplet excitons travel by diffusion and
are captured by a spatially separated red dopant, which then emits red
light. The combination of blue and red light furnishes white light (CRI>
80).[7,2b]
known that highly electron-deficient materials can form an
exciplex with adjacent triarylamine compounds.[11,4g] It
should be noted that the high current efficiency and EQE
observed for the OLEDs with BPhen (Device C, Table 3)
stems also from the fact that the electron transport layer
(BPhen, LUMO ꢀÀ2.9 eV) is a better match with LUMO
energy of the complexes 4 and 5 when used as a host for
WOLEDs (Table 4).
Table 4. Electroluminescence characterization for WOLEDs with host
complexes 4 and 5.[a]
Turn on
Maximum
EQE Luminous
[%]
CRI
voltage [V] luminance [cdmÀ2
]
efficiency [cdAÀ1
]
4
5
4.5
3.3
45120
18420
18.8
15.6
30.3
23.2
77
85
[a] ITO/P:P/MoO3/TPD/TCTA/complex/complex:[Ir
BPhen/Alq3/cathode.
ACHTUNGTREN(NUNG piq)acac]/complex/
9080
www.chemeurj.org
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2011, 17, 9076 – 9082

Organic Light-Emitting Diodes
FULL PAPER
Recently, the white light-emitting OLEDs aimed at poten-
tial application as sources of ambient light using blue fluo-
rescence became a focus of OLED research. Here, the blue
EL can be mixed with orange or red emission to obtain
white light.[7] Hence, the complexes 4 and 5 were employed
as blue-fluorescent hosts in WOLED architectures with
sults in strong blue electroluminescence and increased exter-
nal efficiency of the corresponding OLEDs. Specifically, the
OLED based on complex 4 showed the maximum EQE of
4.6%, and the current efficiency was 12.3 cdAÀ1, which is
among the highest ever reported for an OLED based on an
Alq3 derivative. Complexes 4 and 5 were successfully tested
as hosts for fluorescent-phosphorescent WOLEDs. The
device based also on complex 4 doped with orange-red light
[Ir(pq)2ACHTUNGTRENNUNG(acac)] as the phosphorescent dopant was same as
above except for the emissive layer. The emissive layer was
composed of a blue-emitting sublayer of complex 4 (5 nm)
followed by a sublayer consisting of complex 4 doped with
emitting dopant [Ir(pq)2ACTHNUTRGNEU(GN acac)] showed and showed high ef-
ficiency (EQEꢀ19%) and stable high-CRI (CRIꢀ80).
7% of [Ir(pq)2ACHTUNGTRENNUNG(acac)] (24 nm), followed with a sublayer of
complex 4 (10 nm). The same approach was used for com-
plex 5. Here, the sublayer of a neat complex 4 or 5 acts as a
blue emitter while in the same complexes in the doped sub-
layer it acts as a host for the phosphorescent dopant
(Table 4).
Experimental Section
Commercially available solvents and reagents were used as received. Di-
chloromethane was distilled from CaH2 under argon. All reactions were
monitored using Whatman K6F Silica Gel 60 ꢃ analytical TLC plates by
UV detection (254 and 365 nm). Silica gel (60 ꢃ, 32–63 mm) from EMD
Science was used for column chromatography. 1H and 13C APT NMR
spectra were recorded using a Bruker DRX-300 (300 MHz) spectrome-
ter, and chemical shifts were referenced to the residual resonance signal
of the deuterated solvent. Melting points (uncorrected) were measured
using Thomas Hoover capillary melting point apparatus. EI-MS spectra
were recorded using a Shimadzu QP5050 A. MALDI-TOF/MS spectra
were recorded using a Bruker Daltonics Omniflex spectrometer.
The WOLED with complex 4 displayed maximum lumi-
nance of 45120 cdmÀ2. The EQE was 18.8% and the current
efficiency was 30.3 cdAÀ1 and power efficiency 14.2 LmWÀ1
(CRI=77). This device architecture utilizes a combination
of sky-blue fluorescence and diffusive triplet transfer to a
spatially separated red dopant. The long-range diffusion of
the triplet excitons makes it possible for triplets to travel a
longer distance to reach the red triplet-emitting dopant.[7]
Thus, the blue fluorescence is emitted together with green
and red light to generate white light. The WOLED device
with complex 5 displayed a turn-on voltage of 3.3 V and the
driving voltages for 100 and 1000 cdmÀ2 were 6 and 8.8 V,
respectively. The maximum luminance observed was
18420 cdmÀ2. The maximum EQE obtained was 15.6% and
the maximum current efficiency was 23.2 cdAÀ1 and power
efficiency was 14.6 LmWÀ1 (CRI=85).
The WOLED with complex 4 as a host shows higher
EQE and maximum current efficiency than the one using
complex 5. From the perspective of a potential use as light
sources, these devices emit both blue and orange-red light,
which combined gives warm white light with the CRI value
of 80–85. CRI is a quantitative measure of the light source
ability to reproduce colors in comparison with an ideal light
source (black-body radiation, CRI=100). CRI>80 is more
than sufficient for general illumination. For example, the
new energy saving fluorescent lamps display CRIꢀ80.
Figure 4 shows the EL spectra and photographs of the white
OLEDs.
Phosphorescence spectra were recorded using a single-photon-counting
spectrofluorimeter from Edinburgh Analytical Instruments (FLSP 920)
equipped with a pulsed xenon flash-lamp (mF920H, 200–900 nm, 10–
100 Hz) for time-gated experiments. For phosphorescence studies at
77 K, the samples were dissolved in spectroscopic grade 2-methyltetrahy-
drofuran with optical densities around 0.1 at the wavelength of excita-
tion. The samples were placed in quartz EPR tubes (Norrell) and im-
mersed in a Dewar with liquid nitrogen. The signal acquisition of the
photomultiplier tube was electronically gated to avoid saturation of the
detector by fluorescence.
OLEDs were fabricated on ITO-coated glass substrates (100–150 nm
thick, R& ꢀ15 W per &). The ITO-coated substrates were degreased by
detergent and organic solvents and then UV-ozone cleaned to increase
the ITO work function. Organic layers were deposited by thermal evapo-
ration in a high-vacuum chamber (Angstrom engineering) (10À7 Torr).
The layers consisted of a 1.5 nm-thick molybdenum trioxide (MoO3),
40 nm-thick N,N’-diphenyl-N,N’-bis(3-methylphenyl)-l,1’-biphenyl-4,4’-di-
amine (TPD), a 5 nm-thick tris(4-cabazol-9-yl-phenyl)amine (TCTA), a
50 nm-thick complexes 3–5, and
a 10 nm-thick bathophenanthroline
(BPhen) and a metal cathode (100 nm) were also deposited by thermal
evaporation through a shadow mask (0.04 cm2). For all the devices the
hole injection layer, poly(3,4-ethylenedioxythiophene):poly(styrene-sulfo-
nate) (PEDOT:PSS, PVP AL 4083 Clevios) was spin-coated over a
cleaned ITO substrate at 3000 rpm and heated at 1408C for 10 min.
Acknowledgements
Conclusion
This work was supported by AFOSR(FA9550-05-1-0276 to P.A.), the
State of Ohio (WCI-PVIC to P.A.), NSF(DMR-1006761 to P.A.) and
BGSU.
In summary, new blue-emitting AlIII complexes with substi-
tuted 8-hydroxyquinoline ligands aimed at application in
OLEDs were synthesized. It was found that an appropriate
combination of electron-donating (position C4) and elec-
tron-withdrawing substituents (position C6) on Alq3 allows
for effective control of the HOMO–LUMO gap, while
tuning the emission color from the green to the blue region.
The fluorescence quantum yields of the new complexes
were 2–3 times higher than that of a parent Alq3, which re-
[1] a) Organic Light-Emitting Devices:
A Survey (Ed.: J. Shinar),
Springer, Heidelberg, 2003; b) Organic Light-Emitting Materials and
Devices (Eds.: Z. Li, H. Meng), CRC Press, Boca Raton, 2007;
c) Highly Efficient OLEDs with Phosphorescent Materials (Ed.: H.
Yersin), Wiley-VCH, Weinheim, 2008.
[2] a) B. W. D’Andrade, S. R. Forrest, Adv. Mater. 2004, 16, 1585–1595
and references therein; b) Y. Sun, N. C. Giebink, H. Kanno, B. Ma,
Chem. Eur. J. 2011, 17, 9076 – 9082
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
9081

P. Anzenbacher, Jr. et al.
M. E. Thompson, S. R. Forrest, Nature 2006, 440, 908–912; c) G.
Schwartz, S. Reineke, K. Walzer, K. Leo, Appl. Phys. Lett. 2008, 92,
053311.
[5] a) J. Kido, Y. Iizumi, Chem. Lett. 1997, 27, 963–964; b) L. S. Sapo-
chak, A. Padmapermura, N. Washton, F, Endrino, G. T. Schmett, J.
Marshall, D. Fogarty, P. E. Burrows, S. R. Forrest, J. Am. Chem. Soc.
2001, 123, 6300–6307.
[6] P. E. Burrows, Z. Shen, V. Bulovic, D. M. McCarty, S. R. Forrest,
J. A. Cronin, M. E. Thompson, J. Appl. Phys. 1996, 79, 7991–8006.
[7] P. Anzenbacher Jr. , V. A. Montes, S. Takizawa, Appl. Phys. Lett.
2008, 93, 163302.
[3] a) C. W. Tang, S. A. VanSlyke, Appl. Phys. Lett. 1987, 51, 913; b) C.
Chen, J. Shi, Coord. Chem. Rev. 1998, 171, 161–174; c) M. Cçlle, J.
Gmeiner, W. Milius, H. Hillebrecht, W. Brutting, Adv. Funct. Mater.
2003, 13, 108–112.
[4] a) T. A. Hopkins, K. Meerholz, S. Shaheen, M. L. Anderson, A.
Schmidt, B. Kippelen, A. B. Padias, H. K. Hall, Jr., N. Peyghambari-
an, N. R. Armstrong, Chem. Mater. 1996, 8, 344–351; b) V. A.
Montes, G. Li, R. Pohl, J. Shinar, P. Anzenbacher, Jr., Adv. Mater.
2004, 16, 2001–2003; c) V. A. Montes, R. Pohl, J. Shinar, P. Anzen-
bacher, Jr., Chem. Eur. J. 2006, 12, 4523–4535; d) C. Pꢁrez-Bolꢂvar,
V. A. Montes, P. Anzenbacher, Jr., Inorg. Chem. 2006, 45, 9610–
9612; e) Y.-W. Shi, M.-M. Shi, J.-C. Huang, H.-Z. Chen, M. Wang,
X.-D. Liu, Y.-G. Ma, H. Xu, B. Yang, Chem. Commun. 2006, 1941–
1943; f) M.-M. Shi, J.-J. Lin, Y.-W. Shi, M. Ouyang, M. Wang, H.-Z.
Chen, Mater. Chem. Phys. 2009, 115, 841–845; g) S.-H. Liao, J.-R.
Shiu, S.-W. Liu, S.-J. Yeh, Y.-H. Chen, C.-T. Chen, T. J. Chow, C.-I.
Wu, J. Am. Chem. Soc. 2009, 131, 763–777.
[8] Z. H. Skraup, Chem. Ber. 1880, 13, 2086.
[9] J. Pommerehne, H. Vestweber, W. Guss, R. Mahrt, H. Bꢄssler, M.
Porsch, J. Daub, Adv. Mater. 1995, 7, 551–554.
[10] a) S. Chen, X. Li, W. Xie, Y. Zhao, C. Li, S. Liu, Thin Solid Films
2008, 516, 3364–3374; b) A. Benor; S. Takizawa; C. Perez-Bolivar,
P. Anzenbacher, Jr., Org. Electron. 2010, 11, 938–945; S. Takizawa;
C. Perez-Bolivar, P. Anzenbacher, Jr., Org. Electron. 2010, 11, 938–
945.
[11] a) K. Itano, H. Ogawa, Y. Shirota, Appl. Phys. Lett. 1998, 72, 636;
b) L. H. Chan, R. H. Lee, C. F. Hsieh, H. C. Yeh, C. T. Chen, J. Am.
Chem. Soc. 2002, 124, 6469–6479.
Received: March 8, 2011
Published online: July 21, 2011
9082
www.chemeurj.org
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2011, 17, 9076 – 9082