Photo and electroluminescence of 2-anilino-5-phenylpenta-2,4-dienenitrile derivatives

Article abstract of DOI:10.1039/b103508h

Three derivatives of 2-anilino-5-phenylpenta-2,4-dienenitrile, i.e. 1-3, were assessed as the emitting materials in organic electroluminescent devices. A single hetero-junction device was fabricated using 2 (or 3) as the emitter, which was doped 3% by wei

Full text of DOI:10.1039/b103508h

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Photo and electroluminescence of 2-anilino-5-phenylpenta-2,4-
dienenitrile derivatives
Tahsin J. Chow,* Richard Lin, Chung-Wen Ko and Yu-Tai Tao*
Institute of Chemistry, Academia Sinica, Taipei, Taiwan 115
Received 19th April 2001, Accepted 8th October 2001
First published as an Advance Article on the web 13th November 2001
Three derivatives of 2-anilino-5-phenylpenta-2,4-dienenitrile, i.e. 1–3, were assessed as the emitting materials in
organic electroluminescent devices. A single hetero-junction device was fabricated using 2 (or 3) as the emitter,
which was doped 3% by weight into 1,1’,1@-triphenyl-2,2’,2@-(benzene-1,3,5-triyl)tri-1H-benzimidazole (TPBI). It
can be turned on at voltage 6 V, with luminance up to y6000 Cd m22 (y2000 Cd m22 for 3) and an external
quantum efficiency y0.75% (0.25% for 3). The performance was improved considerably by inserting an
additional layer of 4,4’-dicarbazolyl-1,1’-biphenyl (CBP) between the hole-transporting layer and TPBI. A more
than doubled luminance and external quantum efficiency were obtained for the revised configuration. In
another design, the emitting layer was constructed by spin-coating a mixture of 1 doped into
polycarbazolylethylene (PVK). The device could be turned on at 12 V and reached a luminance of 650 Cd m22
at 22 V. The physical properties of these compounds as well as the devices’ characteristics were discussed.
Experimental
Introduction
Device fabrication
Designing organic materials for the fabrication of light
emitting diodes (OLEDs) has been a research area of current
interest since the ground-breaking work of Tang et al.1 and
Friend et al.2 The OLED devices can be used for making low-
cost display products. Continuing efforts have been spent not
only on the development of new efficient materials, but also on
ingenious device fabrications.3 In particular, the fabrication of
multilayer structures and harvesting the light energy using
dopants are attractive for high luminescent efficiency, high
stability, and color tunability.4 A better understanding of the
correlation between the energetics of charge carriers/interlayer
barriers and the device performance will certainly aid the
strategy for material choice and device configuration.
In our previous study on the photochemistry of
2-(N-methylanilino)-5-phenylpenta-2,4-dienenitrile (1),5 it was
found that some derivatives showed intense fluorescence upon
photoexcitation. Molecular orbital calculations indicated that
the highest occupied molecular orbital (HOMO) of 1 lies on the
aniline–diene chromophore, whereas the lowest unoccupied
molecular orbital (LUMO) lies along the cyano–diene part. In
the excited state the molecule rotates along the N–C(diene)
bond to form a twist intramolecular charge transfer state
(TICT)6 with a high dipole moment (6.86 D). Adding electron-
withdrawing groups on the aniline ring caused a blue shift of
the emission, whereas adding the same groups on the phenyl
ring on the other end caused a red shift.5 Such color-tuning
properties may be valuable in designing electroluminescent
materials in OLED applications. In this report, the electro-
luminescence properties of three related structures, i.e.
compounds 1–3, are examined in order to explore their
potential usage as light emitting materials in OLED devices.1
Prepatterned indium–tin oxide (ITO) substrates with an
effective individual device area of 3.14 mm2 were cleaned by
sonication in a detergent solution for 3 min and then washed
with a large amount of doubly distilled water. Further
sonication in ethanol for 3 min was done before the substrate
was blown dry with a stream of nitrogen. The ITO substrates
were then treated with oxygen plasma for 1 min before being
loaded into the vacuum chamber. The organic layers were
deposited thermally at a rate of 0.1–0.3 nm s21 under a pre-
ssure of 2 6 1025 Torr. Devices with a configuration of ITO/
NPB/TBPI : 2(or 3)/TBPI/Mg : Ag, where NPB and TPBI are
4,4’-bis[N-(1-naphthyl)-N-phenylamino]biphenyl and 1,1’,1@-
triphenyl-2,2’,2@-(benzene-1,3,5-triyl)tri-1H-benzimidazole
respectively, were assembled with a thickness of 40, 20, and
20 nm for NPB, TPBI doped with 3% of 2 (or 3), and pure
TPBI, respectively. Devices of the configuration ITO/NPB/
CBP/TBPI : 2(or 3)/TBPI/Mg : Ag, where CBP is 4,4’-dicar-
bazolyl-1,1’-biphenyl, were assembled with 20, 20, 20, and
20 nm thickness of NPB, CBP, TPBI doped with 3% of 2 (or 3),
and pure TPBI, respectively. A 50 nm cathode layer was
deposited by co-evaporation of a Mg : Ag alloy (ca. 10 : 1
ratio). The device was completed by capping a Ag layer with
100 nm thickness (at rate 0.1–0.4 nm s21).
Instrumentation
1H and 13C spectra were obtained on a Bruker APX-400
spectrometer. Chemical shifts of H were measured downfield
1
from TMS in d units, while those of 13C were recorded with the
central peak of CDCl3 at d 77.00 as an internal reference. Mass
spectra were obtained on a VG70-250S spectrometer or a VG
Trio-2000 spectrometer. Infrared spectra were recorded on a
Perkin-Elmer 682 infrared spectrophotometer. Absorption and
emission spectra were recorded on a Hewlett-Packard 8453
absorption and a Hitachi F-4500 fluorescence spectrophoto-
meter, and emission quantum yields were measured with
reference to 9,10-diphenylanthracene. Elemental analyses were
obtained on a Perkin-Elmer 2400 CHN instrument. Melting
points were measured on a Thomas-Hoover mp apparatus and
42
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This journal is # The Royal Society of Chemistry 2002
DOI: 10.1039/b103508h

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were uncorrected. Current–voltage–light intensity (I–V–L) of
LEDs were measured using a Keithley 2400 source meter and a
Newport 1835C multifunction optical meter, equipped with a
silicon photodiode (Newport, model 818-ST). The measure-
ments of I–V–L were performed simultaneously through an
IEEE interface to a computer.
400 MHz] 7.37 (t, J ~ 8 Hz, 2H), 7.40–7.60 (m, 7H), 7.63 (d,
J ~ 8 Hz, 2H), 7.71–7.80 (m, 3H), 8.21 (d, J ~ 8 Hz, 2H); dC
[CDCl3; 100 MHz; 1H-decoupled] 108.11, 111.07, 114.82,
121.43, 122.21, 122.77, 124.79, 127.54, 128.67, 130.00, 130.81,
136.69, 140.79, 144.10, 145.62; m/z (EI) 320 (100%), 291 (10),
242 (20), 229 (20), 163 (30), 202 (8).
2-Diphenylamino-5-phenylpenta-2,4-dienenitrile (2)
Results and discussion
A round-bottomed flask fitted with a condenser was charged
with diphenylamine (5.07 g, 30 mmol), aqueous formaldehyde
solution (2.7 mL 37%, 33 mmol), and NaCN (1.47 g, 30 mmol)
in acetic acid (20 mL). The solution was heated at 45 uC for 2 h,
then was cooled and neutralized with dilute NaOH solution. It
was extracted several times with ether. The combined ether
layer was washed with saturated NaHCO3 and brine, dried
over anhydrous MgSO4, and concentrated in vacuo. Diphenyl-
aminoacetonitrile (2.50 g, 12 mmol) was collected by distilla-
tion at reduced pressure. Mp 50–53 uC; dH [CDCl3; 400 MHz]
4.51 (s, 2H), 7.03–7.12 (m, 6H), 7.31–7.37 (m, 4H); dC [CDCl3;
100 MHz; 1H-decoupled] 41.29, 106.00, 121.23, 123.51, 129.71,
146.37. Cinnamaldehyde (1.15 mL, 10.0 mmol) and diphenyl-
aminoacetonitrile (2.08 g, 10.0 mmol) were dissolved in ben-
zene (25 mL) containing K2CO3 (2.67 g) and KOH (0.56 g).
The mixture was heated at 70 uC in a round-bottomed flask
fitted with a condenser for 2 h. The reaction was quenched by
the addition of water, and was extracted several times with
ethyl acetate. The combined ethyl acetate solution was washed
with brine, dried over anhydrous MgSO4, and concentrated
in vacuo. Compound 2 (2.8 g, 9.0 mmol) was recrystallized
from hexane as yellowish needles. Mp 147–149 uC (decomp.)
(Found: C, 85.81; H, 5.72. C23H18N2 requires C, 85.68; H,
5.63%); nmax(KBr)/cm21 3028, 2214, 1586, 1569, 1487, 1335; dH
[CDCl3; 400 MHz] 6.55 (d, J ~ 17 Hz, 1H), 6.58 (d, J ~
24 Hz, 1H), 7.11–7.16 (m, 6H), 7.18–7.25 (m, 2H), 7.25–7.38
(m, 6H), 7.42–7.46 (m, 2H); dC [CDCl3; 100 MHz; 1H-
decoupled] 115.10, 119.28, 123.53, 124.32, 124.69, 126.81,
128.50, 128.79, 129.56, 132.22, 136.06, 136.40, 144.79; m/z (EI)
322 (80%), 321 (100), 245 (6), 219 (40).
The absorption and emission spectra
The synthesis of compound 1 has been described previously,
and a similar procedure was followed in preparing 2 and 3.5
They are yellowish solids with green fluorescence, and their
absorption and emission spectra in cyclohexane are shown in
Fig. 1. There are two major absorption bands in the range of
280–420 nm, one at lmax 360–390 nm and another at 300–
320 nm. The lmax values of the low energy bands of 2 and 3 are
red-shifted with respect to that of 1, as a result of extended
delocalization of the nitrogen lone pair onto the additional
phenyl rings. The trend is consistent with semiempirical
estimations shown in Fig. 2. Irradiation of 1 at 360 nm gave
rise to a broad emission at lmax 480 nm (Wf ~ 0.07), while
irradiation of 2 and 3 at 390 nm yielded emissions at lmax 475
(Wf ~ 0.47) and 435 nm (Wf ~ 0.39), respectively. Fluores-
cence quantum yields (Wf) were measured in cyclohexane using
9,10-diphenylanthracene as a standard.7 The emission inten-
sities of 2 and 3 are much stronger than that of 1.
The emission spectrum of 3 showed a smaller Stokes shift
than those of 1 and 2, since the degree of conformational
change of the former during excitation is expected to be less
than those of the latter. The carbazole nitrogen in 3 is locked
coplanar with the rings, yet the N-phenyl moieties of 1 and 2
cannot be totally coplanar with the diene due to the steric
hindrance. The calculated frontier orbitals of 1–3 by AM1,8
with full geometrical optimization, are shown in Fig. 2. The
results simulate their electronic configuration in the ground
state in the gas phase. In methylene chloride, the oxidation
potentials of 1, 2, and 3 were measured by cyclic voltametry to
be 1.06, 1.09 and 1.12 V, respectively. Since the oxidation
waves are not completely reversible, structural changes
occurred during the electrochemical processes. These oxidation
potentials can be correlated to the HOMO and LUMO levels
by the equation IP ~ Eox z 4.40 eV,9 and were estimated to be
2-(Carbazol-9-yl)-5-phenylpenta-2,4-dienenitrile (3)
A round-bottomed flask fitted with a condenser was charged
with carbazole (5.01 g, 30 mmol), bromoacetonitrile (2.75 mL,
36 mmol), K2CO3 (6.2 g), and KOH (1.68 g) in benzene
(300 mL). The solution was heated to reflux for 18 h, then
was cooled and neutralized with dilute HCl solution. It was
extracted several times with ethyl acetate. The combined
organic layer was washed with saturated NaHCO3 and brine,
dried over anhydrous MgSO4, and concentrated in vacuo.
Carbazol-9-ylacetonitrile (3.0 g, 15 mmol) was collected from a
silica gel column chromatograph eluted with a mixture of
hexane and ethyl acetate. Mp 137–138 uC (Found: C, 81.47; H,
5.07. C13H10N2 requires C, 81.53; H, 4.89%); nmax(KBr)/cm21
3058, 2939, 1600, 1485, 1454, 1416, 1325; dH [CDCl3; 400 MHz]
7.33 (t, J ~ 7 Hz, 2H), 7.42 (d, J ~ 8 Hz, 2H), 7.53 (t,
J ~ 7 Hz, 2H), 8.10 (d, J ~ 8 Hz, 2H); dC [CDCl3; 100 MHz;
1H-decoupled] 31.11, 108.26, 114.23, 120.76, 120.78, 123.74,
126.50, 139.44. Cinnamaldehyde (1.15 mL, 10.0 mmol) and
carbazol-9-ylacetonitrile (1.03 g, 5.0 mmol) were dissolved in
THF (15 mL) containing K2CO3 (1.04 g) and KOH (0.28 g).
The mixture was heated at 70 uC in a round-bottomed flask
fitted with a condenser for 2 h. The reaction was quenched by
the addition of water, and was extracted several times with
ethyl acetate. The combined ethyl acetate solution was washed
with brine, dried over anhydrous MgSO4, and concentrated in
vacuo. Compound 3 (0.8 g, 2.6 mmol) was collected as an
orange solid. Mp 149–152 uC (decomp.) (Found: C, 85.94; H,
5.42. C23H16N2 requires C, 86.22; H, 5.03%); nmax(KBr)/cm21
3025, 2925, 2209, 1620, 1598, 1480, 1449 cm21; dH [CDCl3;
Fig. 1 The absorption (solid lines) and emission (dashed lines) spectra
of compounds 1–3 in cyclohexane. Their relative intensities are not
normalized.
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Fig. 2 The calculated energy levels of compounds 1, 2 and 3, where the
geometry of the molecules is fully optimized by AM1.8
5.46 and 2.45 eV for 1, 5.49 and 2.67 eV for 2, and 5.52
and 2.56 eV for 3. The HOMO and LUMO values are
useful in understanding/designing the device characteristics/
configuration.
Fabrication of LED devices
Electroluminescent devices were attempted with compounds 1,
2, and 3 as emitting dyes. Two types of devices were fabricated.
In one the dye was doped into the electron-transporting
material (ETL) of a single hetero-junction device using NPB as
the hole-transporting layer (HTL) and TPBI as the ETL. TPBI
was chosen as the ETL since it is a well-known electron
transporting material10 and a significant spectral overlap is
found between the emission spectrum of TPBI and the
absorption spectra of the dyes examined, which fulfills the
requirement of a Forster type energy transfer. In the other type
of device, the dye was doped into a polymer layer poly-
carbazolylethylene (PVK) by spin-coating. The molecular
structures of several film-forming materials used in the study
are as shown.
Fig. 3 The I–V plots for the devices made with 2 and 3. The
compositions are shown in the inset.
Using TPBI as the host
A device of structure ITO/NPB/TPBI : 2(or 3)/TPBI/Mg : Ag
was constructed. A 40 nm of NPB film was deposited on ITO
glass followed by 20 nm of TPBI doped with 3% by weight of
the dye. Another 20 nm layer of pure TPBI was deposited to
isolate the doped layer from the electrode contact. The
performances of the devices are presented in Fig. 3–6. The
devices for both 2 and 3 gave a turn-on voltage around 6 V
and a luminance of y6000 Cd m22 for a 2-doped and
y2000 Cd m22 for a 3-doped device at a drive voltage of
15 V. External quantum efficiencies of 0.75% and 0.25% were
achieved for 2- and 3-doped devices respectively. The EL
spectrum, shown in Fig. 8, indicates some blue shift of the
emission from the photoluminescence spectrum of the same
doped film. It was reported that the TPBI may serve as a hole-
blocker when used in conjunction with NPB in a two-layer
device configuration.11–13 A CBP layer inserted between the
NPB and TPBI layers could effectively promote the transport-
ing of holes into the TPBI layer because of the reduction of the
barrier to crossing by introduction of an intermediate level
(HOMO of CBP 5.5 eV) between the HOMO of NPB (5.2 eV)
and that of TPBI (6.2 eV) (Fig. 7). Thus a device configura-
tion of ITO/NPB/CBP/TPBI : 2(or 3)/TPBI/Mg : Ag was
Fig. 4 The L–V plots for the devices made with 2 and 3. The
luminescence intensity was higher in the presence of CBP in both cases.
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Fig. 8 Emission spectra of
3
in three different forms, i.e. the
photoluminescence of a thin film (dashed line), the electrolumines-
cences of LED devices without CBP (dotted line) and with CBP (solid
line). The weak band at lmax 375 nm is derived from TPBI.
Fig. 5 The L–I plots for the devices made with 2 and 3 fabricated with
and without CBP.
where the influence of CBP can be further seen. The
photoluminescence of 3 doped in TPBI (at 3%) is shown by
a dashed line as a standard. The weak band at l 375 nm is a
residual emission of TPBI, as a result of incomplete energy
transfer. In the presence of CBP, the EL spectrum overlaps
nicely with the PL spectrum. The small shoulder at 375 nm
vanished, probably because an alternative emission mechanism
operates in addition to the Forster energy transfer mechanism.
In contrast, the EL in the absence of CBP layer showed a slight
‘‘blue shift’’ (dotted line in Fig. 8) compared to the PL. The
‘‘blue shift’’ became more apparent at higher applied voltages,
e.g. 14 V vs. 8 V, when more electrons were pushed into the
NPB layer. A very similar behavior was observed in the case of
the 2-doped device.
constructed. While the I–V characteristics showed that a lower
current was obtained at the same voltage under this con-
figuration than in the absence of a CBP layer, the luminance as
well as external quantum efficiency were much improved.
A
maximum luminance of w9000 Cd m22 for 2- and
y5000 Cd m22 for 3-doped device was obtained respectively.
The external quantum efficiency was also enhanced, with 1.7%
for 2 and 0.9% for 3-doped devices (Fig. 3–6). While these
devices are not yet optimized, their performances are already
comparable with those typical blue- and green-light emitting
devices of similar fabrications.12–14
Fig 8 shows the emission spectra obtained for the devices,
Using PVK as host
Compound 1 was known to undergo photodimerization upon
UV irradiation, therefore it was blended into PVK polymer to
reduce molecular aggregation.5 The PVK polymer served both
as a hole transporting material and as an energy donor. A
concentration analysis on the fluorescence of 1 indicated that
energy transfer from PVK to 1 occurred effectively in solutions
with concentrations higher than 3% by weight. The carbazole
emission at 410 nm was quenched completely upon irradiation
at 340 nm, while only the emission of 1 at 480 nm was observed.
Three different weight ratios (1%, 3%, and 5%) of the
blended material in CH2Cl2 were spin-coated onto the surface
of ITO glass to form films of ca. 100 nm thick. It was covered
with a thin layer of TPBI (as the ETL) by vapor deposition.
The two organic layers were covered by a layer of Mg–Ag
(10 : 1) alloy, followed by a pure silver layer as the anode. L–V
plots showed that the device began to emit green light at 13 V at
blending ratios of 3% and 5% (Fig. 9). A closer look at the
emission spectra showed that at 1% weight ratio the light was
emitted entirely from the carbazole moieties of PVK (lmax at
400 nm), while at higher concentrations the emission of 1 at
lmax 500 nm began to increase. It was clear that the energy
transfer from carbazole to 1 was not satisfactory even at 5%
weight ratio. The luminance is, nevertheless, rather low.
In an attempt to increase the current density and the
efficiency of energy transfer, the weight ratio of 1 was increased
to 10% and NPB was added into the blends. In a similar
process, a device was fabricated using a mixture of compound
1, NPB, and PVK in a weight ratio 10 : 40 : 100. The device
started emitting at 12 V, and the brightness reached
100 Cd m22 at 20 V. The spectra matched that of 1, indicating
a complete energy transfer from PVK to 1. However, the
external quantum efficiency was still too low (0.015%) to be of
any practical value.
Fig. 6 External quantum efficiencies for the devices made with 2 and 3.
A better performance was clearly shown in the presence of a CBP layer.
Fig. 7 Relative HOMO/LUMO energy levels in the single (left) and
double (right) hetero-junction designs. In the absence of CBP, the
transportation of holes from NPB to TPBI was slow due to a higher
energy barrier (ca. 1.0 eV). In the presence of CBP, the flow of holes
was faster with reduction of the barriers (right).
2-(Biphenyl-4-yl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole
(PBD) has been widely used as an efficient electron
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Fig. 10 I–V–L plots for a single layer LED device using a blend of
compounds containing 1 : NPB : PBD : PVK in weight ratio
3 : 50 : 50 : 100.
Fig. 9 Intensity versus voltage plots for LED devices fabricated by
blending compound 1 into PVK at three different weight percentages.
At 1% doping ratio, the emission derived totally from PVK.
Acknowledgement
transporting material and it was chosen to be another additive
to the blend. A mixture of compound 1 : NPB : PBD : PVK
in weight ratios 3 : 50 : 50 : 100 was used to assemble a single
layer device. The material was spin-coated on ITO, followed by
the deposition of Mg : Ag electrode as described before. The
device began to emit green light at about 12 V. The emission
spectrum with lmax at 500 nm was slightly red-shifted com-
pared with the PL of 1. A maximum luminescence of
650 Cd m22 and a current density of 460 mA cm22 were
obtained at 23 V (Fig. 10). The optimal external quantum
efficiency was much better than the one without PBD (ca. 0.10
%). Nevertheless, the performance of this device was much
poorer than the double hetero-junction devices fabricated for 2
and 3 earlier, presumably due to the intrinsic nature of the
compounds itself (low Wf) as well as the method of fabrication
for the devices (multi-layer by vapor deposition vs. single-layer
by spin-coating).
Financial supports from Academia Sinica and the National
Science Council of Republic of China are gratefully acknow-
ledged. Some helpful comments by the editors of this journal
through the reviewing processes are also appreciated.
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Conclusion
It was demonstrated that the derivatives of 2-anilino-5-
phenylpenta-2,4-dienenitrile (1–3) may be used as electro-
luminescent materials in organic LED fabrications. Better
performance was achieved with a device configuration of ITO/
NPB/CBP/TPBI : 2(or 3)/TPBI/Mg : Ag, where two layers of
hole-transporting materials NPB/CBP was used. A turn-on vol-
tage of y6 V for both 2- and 3-doped devices and a maximum
luminance of w9000 Cd m22 and y5000 Cd m22, respectively,
were observed. The optimal external quantum efficiencies were
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thermal, photochemical and electrochemical stability of these
devices, both by improved fabrication methods and by struc-
tural modification of the compounds, which is also aimed at
further improving the efficiency and colour-tuning.
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