New polycyclic aromatic hydrocarbon dopants for red organic electroluminescent devices

Article abstract of DOI:10.1039/b110153f

Two new anthracene derivatives with red emission have been synthesized. The materials were characterized with photoluminescence spectroscopy, and showed emission peaking at over 600 nm. Organic light-emitting devices were fabricated by using these derivatives as dopants. The devices have a maximum efficiency of 0.6 cd A^-1 and an emission peak at 617 nm. The results of color tuning and fluorescent yield enhancement for anthracene derivatives are presented.

Full text of DOI:10.1039/b110153f

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New polycyclic aromatic hydrocarbon dopants for red organic
electroluminescent devices
B. X. Mi,a Z. Q. Gao,a M. W. Liu,a K. Y. Chan,b H. L. Kwong,b N. B. Wong,a,b
C. S. Lee,a L. S. Hunga and S. T. Lee*a
aCenter of Super-Diamond & Advanced Films (COSDAF) and Department of Physics &
Materials Science, City University of Hong Kong, Hong Kong SAR, China.
E-mail: apannale@cityu.edu.hk; Fax: 852-27887830
bDepartment of Biology & Chemistry, City University of Hong Kong, Hong Kong SAR, China
Received 7th November 2001, Accepted 12th February 2002
First published as an Advance Article on the web 18th March 2002
Two new anthracene derivatives with red emission have been synthesized. The materials were characterized with
photoluminescence spectroscopy, and showed emission peaking at over 600 nm. Organic light-emitting devices
were fabricated by using these derivatives as dopants. The devices have a maximum efficiency of 0.6 cd A21
and an emission peak at 617 nm. The results of color tuning and fluorescent yield enhancement for anthracene
derivatives are presented.
the operating power is consumed by red channels.11 In order to
reduce the power consumption of OLED panels, synthesis of
red materials with high efficiency and color purity remains
challenging.
Introduction
Organic light-emitting diodes (OLEDs) have been considered
as the next generation of flat panel displays (FPD) because of
their considerable merits, such as light weight, thin structure,
wide viewing angle, high contrast ratio, low power consump-
tion, video-rate response time, and availability of full color.
The new technology has stimulated intensive research activities
during the past decade after the report by Tang and VanSlyke.1
Recently, dot matrix OLED panels with blue and green colors
became commercially available for automotive entertainment,
and a full color QVGA (quarter video graphics array, i.e. 320
dots 6 240 dots) panel was also demonstrated.2 In spite of the
fast developments of OLEDs, red emission from OLEDs with
high color purity and good stability is badly needed.3 Red
materials for OLEDs with a high fluorescent quantum yield are
not as common as blue or green materials. Up to now, several
organic compounds with red emission have been reported, such
as pyran-containing compounds,4 porphyrin compounds,5 and
europium metal complexes.6 Because of the instability of rare
earth metal complexes during thermal deposition,7 no europium
metal complexes show a practical operation lifetime even though
they have a sharp emission peak with high color purity. The
most promising red dopant among the porphyrin compounds
is platinum octaethylporphyrin (PtOEP)8 with its phosphore-
scent quantum yield of about 0.5. Although high-efficiency
phosphorescent emission using tris(8-quinolinolato)aluminium
(AlQ3) doped with PtOEP has been achieved at a low current
density, the brightness and chromaticity of this device for practical
operation are not acceptable. Pyran-containing compounds,
such as 4-dicyanomethylene-2-methyl-6-(p-dimethylamino-
styryl)-4H-pyran (DCM),9 4-(dicyanomethylene)-2-methyl-6-
(1,1,7,7-tetramethyljulolidin-9-yl)-4H-pyran (DCJT),10 and
4-(dicyanomethylene)-2-tert-butyl-6-(1,1,7,7-tetramethyljulolidin-
9-yl)-4H-pyran (DCJTB){4 have been widely used for dopant
emitters in red OLEDs. These dyes possess a highly concentra-
tion dependent emission. A desirable red color (CIE: x ~ 0.64,
y ~ 0.36) can only be obtained at high concentration, however,
which dramatically reduces luminance efficiencies. In an active
matrix (AM)-OLED full color display, a significant portion of
The emission wavelength of a fluorescent dye depends on the
energy gap between the highest occupied molecular orbital
(HOMO) and lowest unoccupied molecular orbital (LUMO) of
the molecule. Materials with red emission generally have a
narrow energy gap. It is well known that the energy gap of
polycyclic aromatic hydrocarbon (PAH) compounds depends
on the conjugation length of the molecules: the longer the
conjugation length, the lower the energy gap of the PAH
compounds.12 On the other hand, many PAH compounds have
a very high fluorescent quantum yield, and stable OLEDs
have been fabricated based on these PAH compounds, such as
rubrene13 and perylene.14 It is believed that OLEDs based on
PAH compounds should possess stable operating properties.
But these useful PAH compounds emit light mainly in the blue
and yellow regions with only a few PAH compounds reported
to have emission above 600 nm. Moreover, PAH compounds
with more fused carbon rings have a high sublimation
temperature. This limits the application of red-emission PAH
compounds in OLEDs.
The conjugation length of a fluorescent PAH compound can
be enlarged to give a red shift in two ways: one way is to
increase the fused benzene rings linearly such as naphthalene,
anthracene, tetracene and pentacene. The other way is to add
benzene rings or fused benzene rings in two dimensions to form
PAH peri-compounds. In this work, we attempted to obtain
red-emitting PAH compounds based on the anthracene
skeleton, which can be modified to form PAH peri-compounds.
Experimental
Materials
All the starting materials and solvents for synthesis of red
materials were purchased from Aldrich or Acros companies.
Tetrahydrofuran (THF) was dried by refluxing and distil-
ling with sodium and monitored using benzophenone. AlQ3,
N,N’-bis (1-naphthyl)-N,N’-diphenyl-1,1’-biphenyl-4,4’-diamine
(NPB), and a trimer of N-arylbenzimidazoles (TPBI) were
synthesized in our laboratory. AlQ3 was purified further by
{The IUPAC name for julolidine is 2,3,6,7-tetrahydro-1H,5H-
pyrido[3,2,1-ij]quinoline.
DOI: 10.1039/b110153f
J. Mater. Chem., 2002, 12, 1307–1310
This journal is # The Royal Society of Chemistry 2002
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train sublimation under nitrogen several times. Patterned
indium tin oxide (ITO) coated glasses with a sheet resistance of
30 V/% were used as substrates.
introduced. The temperature was lowered to 278 uC. 8.9 ml
(14.2 mmol) of 1.6 M n-BuLi hexane solution was added
dropwise via an addition funnel. The dark orange suspension
obtained was stirred at this temperature for half an hour. Then
1.5 g (6.2 mmol) 1-chloro-9,10-anthraquinone in 30 ml dry
THF was added via an addition funnel. The reaction mixture
was allowed to warm up to room temperature at constant
stirring for 1 hour, and was then hydrolyzed with 30 ml 2 M
Na2CO3 under N2 for another hour. The organic layer was
separated and the water layer was extracted three times with
THF. The solvent was removed, and the organic layers were
collected, combined with the first separated organic layer, and
washed with ether to obtain a pale yellow product, 1-chloro-
Apparatus
Proton NMR (1H, 300 MHz) spectra of samples in solutions
were recorded at room temperature on a Varian instru-
ment. Photoluminescence (PL) and absorption spectra were
measured with a Perkin-Elmer LS50 fluorescence spectro-
photometer and a Perkin-Elmer Lambda 2S UV–Visible
spectrophotometer respectively in dilute solutions of 1 6
1025 M. OLEDs were fabricated using a modified Edward
AUTO 306 vacuum chamber at a base pressure of 5 6 1026
mbar. All the materials were deposited in one pump-down.
Two shadow masks were used to define the deposition area,
respectively, for organic and metal layers. The current–
voltage–luminance characteristics and EL spectra of the
9,10-di(9’-anthryl)-9,10-dihydroanthracene-9,10-diol.
Then,
similar to the preparation of DpNA, the resulting diol was
reduced with hydrogen iodide to the corresponding anthracene.
By heating the corresponding anthracene from step two with
potassium hydroxide, the pAAA crude product was obtained.
This dark violet solid was very unstable in solution, due to its
sensitivity to light and oxygen. However, it could be purified by
column chromatography using (1 : 3) benzene and petroleum
ether as carrier. Melting point: 156–160; 1H NMR (chloro-
form-d, 300 MHz) d: 8.72(s, 1 H), 8.46(s, 1 H), 8.25(d, 1 H),
8.18(d, 3 H), 8.06(dd, 2 H), 7.68(dd, 2 H), 7.48(m, 2 H),
7.30(dd, 6 H), 7.22(d, 2 H), 7.13(dd, 4 H); MS: m/z 528.1 (M1);
calc. for C42H24: C: 95.4%, H: 4.5%, found: C: 96.7%, H: 4.3%;
IR (KBr disk, n/cm21): 3048 (s), 1442 (w), 1307 (w), 883 (m),
842 (m), 798 (m), 759 (s), 733 (s); UV–vis [l/nm (e/cm21 M21)]:
CH2Cl2, 572 (12050), 532 (7670), 297 (18840).
devices were measured with
a computer-controlled DC
power supply and a Spectrascan PR650 photometer at room
temperature. The emission area of the devices is 0.1 cm2,
defined by the overlapping area of the anode and cathode.
Synthesis of red dopant materials
Two red dopant materials, tetrabenzo[de,hi,op,st]pentacene
(DpNA) and 7-(9-anthryl)dibenzo[a,o]perylene (pAAA), with
the chemical structures shown in Fig. 1, were synthesized
according to the procedure described in the literature.15 In the
synthesis of DpNA, 1,5-dichloroanthraquinone and 1-bromo-
naphthalene were used as starting materials. First, the
1,5-dichloroanthraquinone was condensed with 1-naphthyl-
magnesium bromide in dry THF, which was freshly prepared
from 1-bromonaphthalene and magnesium turnings. Second,
the resulting diol was reduced with hydrogen iodide to the
corresponding anthracene. Finally, by heating the product
from step two with (1 mol : 30 mol) potassium hydroxide in
quinoline solution, DpNA was obtained. This crude product
was purified by recrystallization in xylene, then by sublimation
under vacuum. Melting point: 333–334 uC; 1H NMR (benzene-
d6, 300 MHz) d: 8.52(d, 2 H), 8.12(d, 2 H), 8.06(d, 2 H), 7.07(d,
2 H), 7.55(d, 2 H), 7.48(d, 2 H), 7.40(s, 2 H), 7.30(m, 1 H),
7.22(m, 3 H); calc. for C34H18: C: 95.7%, H: 4.2%, found: C:
95.3%, H: 4.3%; IR (KBr disk, n/cm21): 3047 (s), 1599 (w), 1570
(w), 1500 (w), 1420 (w), 1383 (w), 1345 (w), 836 (s), 795 (s), 775
(s), 760 (s), 748 (s); UV–vis [l/nm (e/cm21 M21)]: CH2Cl2, 623
(12930), 577 (7510), 290 (11290).
Preparation of OLED devices
The chemical structures of the organic materials other than the
red dopants used in the device fabrication are shown in Fig. 2.
Several device configurations were utilized in order to evaluate
the performance of the red materials. The ITO glass substrate
In the synthesis of pAAA, a lithium reagent was used instead
of magnesium turnings. This was due to the thermal instability
of the starting material 9-bromoanthracene. In a 100 ml flask,
3.2 g (12.4 mmol) 9-bromoanthracene and 8 ml dry THF were
Fig. 2 Chemical structure of organic materials used in the study other
than the red dopants.
Fig. 1 Chemical structures of the red dopants reported.
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was cleaned with detergents and deionized water, and dried in
an oven for about two hours. Then, it was treated with UV–
ozone for 25 minutes prior to loading into a deposition
chamber. Organic materials were sequentially deposited onto
the ITO substrate at a pressure around 1.0 6 1025 mbar to
form the desired device configurations. The pressure during
metal deposition was below 9.0 6 1026 mbar.
Results and discussion
1. DpNA devices
Fig. 3 shows the absorption spectra of a DpNA film and a
DpNA solution in toluene, and the PL spectrum is shown in the
inset. The PL emission is around 647 nm in toluene solution.
Due to the rigid structure of this material, the overlap of the
absorption and emission spectra is very marked.16 The PL
yield is low because the luminance suffers severely from self-
absorption in this compound which can act as an inner
filter.16 It may also be attributed to a small difference in energy
levels between the excited singlet state and triplet state of
DpNA, as observed in anthracene solids.17 Intersystem cross-
ing could result in internal quenching of excited singlet states.16
The first absorption peaks of the DpNA film and solution are
667 nm and 628 nm respectively. The absorption peak of the
DpNA film shifts to red by 40 nm as compared to the DpNA
solution, and the width of the absorption peak broadens,
implying strong intermolecular interactions in DpNA. This is
expected from the planar structure of DpNA, which causes
concentration quenching.16 It is clear from the results that this
material can only be used as a dopant in OLEDs.
Because the absorption peak of DpNA in diluted solution is
around 628 nm, it is very difficult to find a host material to
match the resonance energy transfer conditions. We used
different host materials to optimise the performance of OLED
devices. The best result was obtained with a device of structure
ITO/NPB–1% DpNA (70 nm)/TPBI–1% DpNA (50 nm)/Mg–
Ag (200 nm). The EL and current–voltage characteristics of
this device are shown in Fig. 4. The maximum luminance is
only 22 cd m22, and the emission of TPBI cannot be completely
extinguished. The poor performance is attributed to both the
planar structure of DpNA and the poor energy matching
between the host and the dopant.
Fig. 4 EL and current–voltage characteristics of a device with the
configuration ITO/NPB–1% DpNA (70 nm)/TPBI–1% DpNA (50 nm)/
Mg–Ag (200 nm).
conjugation length. Therefore, we designed another PAH
compound, pAAA.
2. pAAA devices
Fig. 5 shows the absorption and PL spectra of pAAA in dilute
solution. The PL peak of pAAA is at 608 nm. The chemical
structure of this compound is not as rigid as DpNA. It has a
rotatable anthryl group. For this reason, the Stokes shift of
pAAA is larger than that of DpNA. The reduced self-
absorption of pAAA molecules can increase the PL quantum
efficiency of this compound compared to DpNA. Furthermore,
similar to DpNA, the PL quantum efficiency can also be
improved by decreasing the intersystem crossing from the
excited singlet to excited triplet because of the rotatable anthryl
substituted group.17 The width of the PL peak is only 40 nm.
This narrow emission is very attractive because high color
purity can be expected using this material. The first absorption
peak is around 568 nm, and the second absorption peak is
around 530 nm. The vibronic spacing of 1262 cm21 between
568 and 530 nm may correspond to a number of C–H aromatic
in-plane deformation bands.19 The fluorescence quantum yield
(Qy) is 0.1 measured with Rhodamine B (Qy ~ 0.66) as
standard, using non-degassed spectrophotometric grade
CH2Cl2 as solvent. It should be noted that the fluorescence
of aromatic molecules is strongly oxygen quenched20 and this
will dramatically reduce the measured value. The red emission
can be clearly seen under 365 nm UV lamp excitation. Also, in
Fig. 5, the EL spectrum of AlQ3 is shown. There were three
vibration peaks in the pAAA absorption spectrum, of which
the second and the third ones were fully overlapped by the
We tried to modify the structure of DpNA to enhance the PL
yield18 by breaking down one peri-bond of DpNA, and a
compound with a very high PL yield was obtained. However,
the emission shifted to green because of the reduction in the
Fig. 5 Absorption and PL spectra of pAAA in dilute solution, and
AlQ3 electroluminescence of a device with the configuration ITO/NPB
(70 nm)/AlQ3 (70 nm)/Mg–Ag(200 nm).
Fig. 3 Absorption spectra of the DpNA film and solution (in toluene),
and the PL spectrum (inset).
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Fig. 7 Luminance efficiency of a device with the configuration ITO/
NPB (70 nm)/AlQ3–2% pAAA (35 nm)/AlQ3 (35 nm)/Mg–Ag(200 nm).
Fig. 6 The EL spectrum of a device with the configuration ITO/NPB
(70 nm)/AlQ3–2% pAAA (35 nm)/AlQ3 (35 nm)/Mg–Ag(200 nm).
References
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emission spectrum of AlQ3 emission. However the strongest
peak of pAAA absorption did not overlap much with the
emission of AlQ3. As a result, the energy transfer from AlQ3 to
pAAA is not efficient and the AlQ3 emission cannot be totally
suppressed in the presence of pAAA, as shown in Fig. 6.
OLEDs with the structure ITO/NPB (70 nm)/AlQ3–2%
pAAA (35 nm)/AlQ3 (35 nm)/Mg–Ag (200 nm) were prepared.
The EL spectrum of the devices in Fig. 6 reveals an emission
peak at about 616 nm. The CIE coordinates are x ~ 0.625, y ~
0.358. It can be seen that the emission of the device is broader
than the PL of the pAAA. The intensity of this peak increases
with increasing dopant concentrations of the device. This
might be attributed to the dimer or trimer emission of pAAA
molecules. It is evident from Fig. 6 that the AlQ3 emission
cannot be completely converted to pAAA emission, indicating
that a better host material is needed in order to achieve higher
efficiency and purer red emission.
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The plot of luminance efficiency vs. current density is shown
in Fig. 7. The efficiency is almost constant over a large range of
current density with a maximum value of about 0.6 cd A21. The
constant efficiency will facilitate the design of driving circuits in
OLED displays.
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Conclusions
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Two red materials for OLEDs were designed and synthesized.
OLEDs were fabricated using these red materials as dopants.
The devices with pAAA dopants can achieve a current
efficiency 0.6 cd A21 with CIE coordinates of x ~ 0.625, y ~
0.358.
Acknowledgement
This work is supported by the Research Grants Council of
Hong Kong SAR via a grant [no. Cityu 1/98c (8730009)].
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J. Mater. Chem., 2002, 12, 1307–1310