Synthesis, photo-and electro-luminescence of 3-benzoxazol-2-yl-coumarin derivatives

Article abstract of DOI:10.1007/s10895-011-0999-8

Two coumarin derivatives containing electrontransporting benzoxazolyl moiety, 7-(diethylamino)-3- (benzoxazol-2-yl)coumarin (DABOC) and 3-(benzoxazol- 2-yl)benzo[5,6]coumarin (BOBC), were synthesized and characterized. The photoluminescence and electroluminescence of the compounds were investigated detailedly. The compounds exhibited strong blue-green emissions in both solution and solid states, but the devices with DABOC as the emitting layer exhibited orange emission and maximum luminous efficiency of 2.8 cd/A and maximum luminance of 8,800 cd/m², and the devices with BOBC displayed orange-white emission and maximum luminous efficiency of 0.13 cd/A and maximum luminance of 540 cd/m². Springer Science+Business Media, LLC 2011.

Full text of DOI:10.1007/s10895-011-0999-8

J Fluoresc (2012) 22:631–638
DOI 10.1007/s10895-011-0999-8
ORIGINAL PAPER
Synthesis, Photo- and Electro-Luminescence
of 3-Benzoxazol-2-yl-Coumarin Derivatives
Yuling Zhao & Tianzhi Yu & Youzhi Wu & Hui Zhang &
Duowang Fan & Zunwei Gan & Liangliang Yang &
Xiaoqian Han & Yumei Zhang
Received: 30 June 2011 /Accepted: 14 October 2011 /Published online: 19 October 2011
#
Springer Science+Business Media, LLC 2011
Abstract Two coumarin derivatives containing electron-
transporting benzoxazolyl moiety, 7-(diethylamino)-3-
(benzoxazol-2-yl)coumarin (DABOC) and 3-(benzoxazol-
2-yl)benzo[5,6]coumarin (BOBC), were synthesized and
characterized. The photoluminescence and electrolumi-
nescence of the compounds were investigated detailedly.
The compounds exhibited strong blue-green emissions in
both solution and solid states, but the devices with
DABOC as the emitting layer exhibited orange emission
and maximum luminous efficiency of 2.8 cd/A and
maximum luminance of 8,800 cd/m2, and the devices
with BOBC displayed orange-white emission and maximum
luminous efficiency of 0.13 cd/A and maximum luminance
of 540 cd/m2.
Introduction
Organic light-emitting diodes (OLEDs) has been studied by
many research groups througthout the world due to its high
efficiency, low power consumption, fast response time,
wide viewing angle and so on [1–4]. Organic electrolumi-
nescent (EL) devices or organic light-emitting diodes using
fluorescent molecules have attracted great interest in flat-
panel displays and may have the potential to replace liquid
crystal technology [5–7].
Coumarin and its derivatives are well known not only for
their remarkable biological and medical activities [8–10],
but also for their super thermal stability and outstanding
optical properties including an extended spectral response,
high quantum yields, large Stokes shifts and good photo-
stability [11–16]. Coumarin derivatives can be applied to
laser dyes, fluorescent whiteners, fluorescent probes,
nonlinear optical chromophores, polymer science, optical
recording and solar energy collectors [17, 18]. More
important, coumarin dyes were used as blue, green and
red dopants in organic light-emitting diodes (OLEDs) [19–
21]. Coumarin derivatives substituted at 7-position with an
electron-donating group and at 3-position with an electron-
withdrawing group are a typical class of D-A structure
molecules and exhibit strong fluorescence [12, 22–25].
Following our interest on designing and synthesising
new coumarin derivatives with high quantum yield of
fluorescence and greater stability [12, 13, 22, 26–28],
two coumarin derivatives with benzoxazolyl moiety, 7-
(diethylamino)-3-(benzoxazol-2-yl)coumarin (DABOC)
and 3-(benzoxazol-2-yl)benzo[5,6]coumarin (BOBC),
were synthesized and structurally characterized in this
paper. The UV-vis and emission spectra of the compounds
were investigated. We fabricated the electroluminescence
.
.
Keywords Synthesis UV-vis Absorption
.
.
Photoluminescence Electroluminescence
.
Coumarin derivative Benzoxazolyl derivative
:
:
:
:
:
Y. Zhao (*) T. Yu Z. Gan L. Yang X. Han Y. Zhang
School of Chemical and Biological Engineering,
Lanzhou Jiaotong University,
Lanzhou 730070, China
e-mail: zhaoyl66@hotmail.com
:
:
T. Yu H. Zhang D. Fan
Key Laboratory of Opto-Electronic Technology and Intelligent
Control (Lanzhou Jiaotong University), Ministry of Education,
Lanzhou 730070, China
Y. Wu
School of Materials Science & Engineering,
Lanzhou University of Technology,
Lanzhou 730050, China

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J Fluoresc (2012) 22:631–638
devices by vacuum vapor-deposited film with the compounds
as emission materials and investigated the electroluminescent
properties.
aminophenol (1.92 g, 17.6 mmol), ethyl cyanoacetate
(2.00 g, 17.6 mmol) and benzoic acid (1.10 g, 8.5 mmol)
was dissolved in 1-butanol (50 mL). The mixture solution
was refluxed for 8 h till the reaction was completed
(monitored by TLC). After the solvent was partly evaporated
under reduced pressure, the reaction mixture was cooled
to room temperature. 100 mL NaOH (2%) was added
into the mixture and stirring about 0.5 h. The crude product
was obtained by filtering and then recrystallized from
methanol to afford a yellow powder. Yield: 65% (3.82 g). m.
p.: 185–186 °C; IR (KBr pellet, cm−1): 3063 (Aryl-CH),
2971, 2929, 2891 (Alkyl-CH), 1748 (C=O, lactone), 1624
(C=N), 1606 (C=C), 1588, 1508, 1451, 772, 740 cm−1;
1H NMR (500 MHz, CDCl3, δ, ppm): 8.65 (s, 1H, 4-H),
7.81 (t, J=4.8 Hz, 1H, Aryl-H), 7.60 (t, J=4.4 Hz, 1H, Aryl-
H), 7.42 (d, J=9.2 Hz, 1H, Aryl-H), 7.36–7.32 (m, 2H, Aryl-
H), 6.66 (d, J=2.4 Hz, 1H, Aryl-H), 6.54 (s, 1H, Aryl-H),
3.47 (q, J=7.2 Hz, 4H, N-CH2-CH3), 1.26 (t, J=7.2 Hz, 6H,
N-CH2-CH3). Anal. Calcd. for C20H18N2O3: C, 71.84; H,
5.43; N, 8.38. Found: C, 71.81; H, 5.45; N, 8.40.
Experimental
Materials and Methods
4-(N,N′-diethylamino)salicylaldehyde from Zhejiang Huadee
Dyestuff Chemical Co. Ltd. (China) was recrystallized from
ethanol. 2-Hydroxy-1-naphthaldehyde was obtained from
Acros Organics. Ortho-aminophenol and ethyl cyanoacetate
were purchased from AstaTech (Chengdu) Pharmaceutical
Co., Ltd. All other chemicals were analytical grade reagents.
IR spectra (400–4,000 cm−1) were measured on a
1
Shimadzu IRPrestige-21 FT-IR spectrophotometer. H NMR
spectra were obtained on Unity Varian-500 MHz. C, H, and
N analyses were obtained using an Elemental Vario-EL
automatic elemental analysis instrument. Melting points were
determined on a X-4 microscopic melting point apparatus
made in Beijing Taike Instrument Limited Company and the
quoted temperatures were uncorrected. UV-vis absorption
and photoluminescent spectra were recorded on a Shimadzu
UV-2550 spectrometer and on a Perkin-Elmer LS-55
spectrometer, respectively. The electroluminescent spectra
were measured on a PR-650 SpectraScan Colorimeter.
3-(Benzoxazol-2-yl)Benzo[5,6]Coumarin (BOBC) The
preparation of BOBC was similar to that described for
DABOC. The crude product was recrystallized from DMF
to afford yellow floccules. Yield: 87%. m.p.: > 250 °C. IR
(KBr pellet, cm−1): 3065 (Aryl-CH), 1734 (C=O, lactone),
1624 (C=N), 1608 (C=C), 1568, 1457, 1215, 770, 736 cm−1;
1H NMR (500 MHz, CDCl3, δ, ppm): 9.65 (s, 1H, 4-H),
8.73 (d, J=8.8 Hz, 1H, Aryl-H), 8.35 (d, J=9.2 Hz, 1H,
Aryl-H), 8.12 (d, J=8.0 Hz, 1H, Aryl-H), 7.91–7.80 (m, 3H,
Aryl-H), 7.68 (t, J=7.6 Hz, 2H, Aryl-H), 7.53–7.45 (m, 2H,
Aryl-H). Anal. Calcd. for C20H11NO3: C, 76.67; H, 3.54; N,
4.47. Found: C, 76.71; H, 3.56; N, 4.45.
Synthesis and Characterization
of 7-(Diethylamino)-3-(Benzoxazol-2-yl)Coumarin (DABOC)
and 3-(Benzoxazol-2-yl)Benzo[5,6]Coumarin (BOBC)
The synthetic routes were shown in Scheme 1. 7-
(diethylamino)-3-(benzoxazol-2-yl)coumarin (DABOC) was
synthesized according to the method reported by Ye et al. [29]
OLEDs Fabrication
7-(Diethylamino)-3-(Benzoxazol-2-yl)Coumarin
(DABOC) Under nitrogen atmosphere, a mixture of 4-(N,N′-
diethylamino)salicylaldehyde (3.40 g, 17.6 mmol), Ortho-
The multilayer OLEDs were fabricated by vacuum-
deposition method. ITO-coated glass with a sheet resistance
Scheme 1 Synthetic routes
of 7-(diethylamino)-3-
COOH
N
O
CHO
NH2
OH
(benzoxazol-2-yl)coumarin
(DABOC) and 3-(benzoxazol-
2-yl)benzo[5,6]coumarin
(BOBC)
O
NCCH2COOC2H5
+
+
Et2N
OH
OH
Et2N
O
DABOC
COOH
CHO
N
NH2
OH
O
NCCH2COOC2H5
+
+
O
O
BOBC

J Fluoresc (2012) 22:631–638
633
PL of DABOC (Solution)
PL of DABOC (Solid powder)
EL of device I
R□~20 Ω/□ was cut into 3 cm×3 cm plates and etched in
dilute hydrochloric acid for 20 min. Then the ITO
substrates were routinely cleaned by ultrasonic treatment
in solvents and then cleaned by exposure to an UV-ozone
ambient. All organic layers were sequentially deposited
without breaking vacuum (2×10−4 Pa). Thermal deposition
rates for organic materials, LiF and Al were~2Å/s, ~ 1Å/s
and 10Å/s, respectively. The active area of the devices was
12 mm2. The EL spectra were measured on a PR-650
SpectraScan Colorimeter. The characterization of
brightness-current-voltage (B–I–V) were measured with a
3645 DC power supply combined with a 1980A spot
photometer and were recorded simultaneously. All
measurements were done in the air at room temperature
without any encapsulation.
1.0
0.5
0.0
400
500
600
700
800
900
Wavelength (nm)
Fig. 2 Photoluminescence spectra of DABOC in acetonitrile solution
and solid powder, and electroluminescence spectrum of the device I
Results and Discussion
From Fig. 1, it is shown that the compound DABOC
exhibits bright blue–green emission with peak at 490 nm
and the compound BOBC shows bright blue emission at
466 nm in diluted acetonitrile solutions. The maximum
emission wavelength of DABOC was bathochromically
shifted by about 24 nm compared with that of BOBC
due to an electron repelling diethylamine in the 7-
position in DABOC molecule. From the excitation and
emission maxima of DABOC and BOBC, the Stoke’s
shifts of DABOC and BOBC are 2,475 and 4,182 cm−1,
respectively. The compound BOBC has a larger Stoke’s
shift.
Absorption and Fluorescence of DABOC and BOBC
UV-vis absorption and photoluminescent spectra of the
compound DABOC and BOBC in diluted acetonitrile
solutions are shown in Fig. 1. It is shown that the absorption
spectrum of DABOC exhibits absorption maxima at 220,
279 and 437 nm, respectively. The absorption spectrum of
BOBC is similar to that of DABOC, there are three
absorption bands at 220, 260 nm and 390 nm. Compared
with DABOC, the second and third absorption bands of
BOBC were blue-shifted. It is suggesting that the
compound DABOC has an electron repelling diethylamine
in the 7-position which enhances the electron density of
coumarin ring, however, the compound BOBC has a
benzocoumarin skeleton which disperses the electron density
of coumarin ring, so the absorption bands of BOBC were
blue-shifted compared to that of DABOC.
Fluorescence quantum yield of DABOC and BOBC in
acetonitrile were measured with anthracene as reference
( ΦSF ¼ 0:25 [30]). The quantum yields (ΦF) were calculated
PL of BOBC (Solution)
PL of BOBC (Solid powder)
EL of device II
UV-vis of DABOC
EL of device III
1.0
1.0
0.5
0.0
1.0
0.5
0.0
UV-vis of BOBC
PL of DABOC
PL of BOBC
0.5
0.0
400
500
600
700
800
900
Wavelength (nm)
200
300
400
500
600
700
Wavelength (nm)
Fig. 3 Photoluminescence spectra of BOBOC in acetonitrile solution
and solid powder, and electroluminescence spectra of the device I and
device III
Fig. 1 UV-vis absorption and photoluminescence spectra of DABOC
and BOBC in acetonitrile solutions (C ¼ 1:0 ꢀ 10ꢁ6mol=L)

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J Fluoresc (2012) 22:631–638
Fig. 4 The molecular structures
of materials and the structures
of EL devices
N
O
N
N
Et2N
O
O
NPB
N
N
DABOC
N
N
N
N
O
O
Al
N
N
O
N
O
N
O
O
BOBC
TPBI
Alq3
Al (100 nm)
Al (100 nm)
Al (100 nm)
LiF (0.5 nm)
Alq3 (30 nm)
LiF (0.5 nm)
TPBI (30 nm)
LiF (0.5 nm)
Alq3 (30 nm)
BOBC (50 nm)
BOBC (50 nm)
DABOC (50 nm)
NPB (50 nm)
MoO3
NPB (50 nm)
MoO3
NPB (50 nm)
MoO3
ITO Glass
ITO Glass
ITO Glass
Device I
Device II
Device III
according to the literatures [30, 31]. Fluorescence quantum
yields of DABOC (ΦF, DABOC) and BOBC (ΦF, BOBC) in
acetonitrile solutions were measured to be 0.62 and 0.45,
respectively.
The photoluminescence spectra of DABOC and
BOBC in acetonitrile solution and solid powder were
compared as shown in Figs. 2 and 3, respectively. Due
to more intermolecular interaction in the solid state,
the photoluminescence spectrum of DABOC in solid
powder is much broader than that in acetonitrile
solution, and the emission peak is red-shifted from
490 nm in solution to 556 nm in solid powder (Fig. 2).
An example of compound was reported to show such
differences of PL spectra between solution and solid
powder [27]. The similar phenomenon was also found in
photoluminescence spectra of BOBC in acetonitrile
Fig. 5 The luminance versus
voltage and current density
versus voltage (inset) curves
of the device I
10000
8000
6000
4000
2000
0
500
400
300
200
100
0
5
10
15
Voltage (V)
Device I
5
10
15
Voltage (V)

J Fluoresc (2012) 22:631–638
635
electron transport materials, respectively. LiF was used as
the electron-injection layer.
3.0
2.5
2.0
The EL spectrum of the device I using DABOC as
emitter, as shown in Fig. 2, has an orange emission band
centered at 580 nm. Compared to the photoluminescence
spectrum of DABOC measured in the solid powder, the
emission peak of the device I is red-shifted by 24 nm and
the EL spectrum is much broader. It is well known
that the EL spectrum of the device was obtained from
the compact amorphous thin film while the PL
spectrum of the solid powder was measured from the
polycrystalline powder, the intermolecular interaction
in the amorphous thin film is larger than that in solid
powder of the compound, and thus it may lead to the
different emission. Another reason should be reminded
here that the EL spectrum was obtained from the
multiple layer structures, thus some interactions may
occur between NPB and DABOC and/or between
DABOC and Alq3 in OLEDs, yielding different PL and
EL spectra.
0
100
200
300
400
2
500
Current Density (mA/cm )
Fig. 6 Current efficiency-current density characteristic of the device I
solution and solid powder, the emission peaks of solution
and solid powder appear at 466 and 518 nm, respectively
(Fig. 3).
More intriguing spectral differences between PL and EL
were observed in the device II and device III containing the
emitting layer of BOBC (Fig. 3). The maximum PL of
BOBC in the solid powder occurred at 518 nm while the
EL peaks of the device II were observed at 530 nm with a
shoulder of 625 nm. Originally, the emission band at
530 nm was suspected to originate from Alq3 used as
electron transport material in the device II because the
emission of Alq3 generally appears at 530 nm. Thus, the
device III with TPBI as electron transport material was
fabricated, and the EL spectrum of the device III also
revealed two peaks of 530 nm and 625 nm. Though the
OLED Performance of DABOC and BOBC
The molecular structures of the materials and the devices
with the configuration of ITO / MoO3 / NPB (50 nm) /
DABOC (50 nm) / Alq3 (30 nm) / LiF (0.5 nm) / Al
(100 nm), ITO / MoO3 / NPB (50 nm) / BOBC (50 nm) /
Alq3 (30 nm) / LiF (0.5 nm) / Al (100 nm) and ITO / MoO3
/ NPB (50 nm) / BOBC (50 nm) / TPBI (30 nm) / LiF
(0.5 nm) / Al (100 nm) were depicted in Fig. 4. DABOC or
BOBC was employed as the emitter. MoO3, NPB, and Alq3
or TPBI were used as hole injection, hole transport and
Fig. 7 The luminance versus
voltage and current density
versus voltage (inset) curves
600
500
400
300
200
100
0
of the device II
500
400
300
200
100
5
10
15
Voltage (V)
Device II
0
5
10
15
Voltage (V)

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J Fluoresc (2012) 22:631–638
Device II
Device III
intensity of the emission at 625 nm of the device III was
much larger compared to that of the device II, the EL
spectra of the two devices were found to exhibit similar
emitting patterns, indicating that the emission of 530 nm is
attributed to the intrinsic emission from BOBC. In addition,
we measured the PL spectra of the thin films of NPB and
TPBI, they exhibit emission peaks at 450 nm and 385 nm,
respectively. It was indicated that the emission at 625 nm of
the device II or device III was originated from NPB, Alq3
and TPBI. Compared to the device II, TPBI was not only
used as electron transport material but also used as hole
blocking layer in device III, which prohibited the holes
from diffusing to TPBI layer. The results were suggested
that the origin of the emission of the device II or device III
at 625 nm is attributed to an exciplex formed between NPB
and BOBC.
0.14
0.12
0.10
0.08
0.06
0
100
200
300
400
500
Current density (mA/cm2)
Fig. 9 Current efficiency-current density characteristics of the device
II and the device III
Figure 5 shows the representative luminance-current
density-voltage characteristics of the device I. The turn-on
voltage of the device I was observed at 4.7 V, and the
maximum luminance was 8,800 cd/m2 at 16.6 V. Figure 6
shows the relationship between the current efficiency and
the current density in the device I. The device I exhibits a
maximum luminous efficiency of 2.8 cd/A at the current
density of 20 mA/cm2 and the voltage of 7.7 V.
Figures 7 and 8 show the representative luminance-
current density-voltage characteristics of the device II and
the device III. As shown in Figs. 7 and 8, the turn-on
voltages of the device II and the device III were observed at
6 V and 8 V, the maximal luminances were 540 cd/m2 at
15.5 V for the device II and 170 cd/m2 at 17 V for the
device III. Figure 9 shows the relationships between the
current efficiency and the current density in the device II
and the device III. The device II exhibits a maximum
luminous efficiency of 0.13 cd/A at the current density of
20 mA/cm2 and the voltage of 8.5 V. The device III has a
maximum luminous efficiency of 0.10 cd/A at the current
density of 20 mA/cm2 and the voltage of 13 V. Both the
devices show the orange-white emission. In terms of
luminance and efficiency, the device II is much better than
the device III.
To contrast the device II with the device I, although
they had same structural patterns, the luminance and
efficiency of the device I are much higher than that of
the device II.
From the absorption spectra of DABOC and BOBC and
the EL spectrum of NPB [32], it was found that the
Fig. 8 The luminance versus
voltage and current density
versus voltage (inset) curves
200
100
0
150
100
50
of the device III
5
10
15
Voltage (V)
Device III
0
5
10
15
Voltage (V)

J Fluoresc (2012) 22:631–638
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Conclusions
We designed and synthesized two coumarin derivatives
containing electron-transporting benzoxazolyl moiety, 7-
(diethylamino)-3-(benzoxazol-2-yl)coumarin (DABOC)
and 3-(benzoxazol-2-yl)benzo[5,6]coumarin (BOBC). The
photoluminescent and electroluminescent behaviors of the
compounds were investigated and discussed. Fluorescence
quantum yield of DABOC is higher than that of BOBC.
The devices of ITO / MoO3 / NPB (50 nm) / DABOC
(50 nm) / Alq3 (30 nm) / LiF (0.5 nm) / Al (100 nm)
displayed the orange emission (580 nm), a maximum
luminous efficiency of 2.8 cd/A at the current density of
20 mA/cm2, and maximum luminance of 8,800 cd/m2 at
16.6 V. The ITO / MoO3 / NPB (50 nm) / BOBC (50 nm) /
Alq3 (30 nm) / LiF (0.5 nm) / Al (100 nm) device showed
the orange-white emission (530 and 625 nm), a maximum
luminous efficiency of 0.13 cd/A at the current density of
20 mA/cm2, and maximum luminance of 540 cd/m2 at
15.5 V. The ITO / MoO3 / NPB (50 nm) / BOBC (50 nm) /
TPBI (30 nm) / LiF (0.5 nm) / Al (100 nm) device
exhibited the orange-white emission (530 and 625 nm), a
maximum luminous efficiency of 0.1 cd/A at the current
density of 20 mA/cm2, and maximum luminance of
170 cd/m2 at 17 V. In terms of luminance and efficiency,
the device of DABOC is much better than the device of
BOBC.
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Acknowledgements This work was supported by the Natural
Science Foundation of Gansu Province (096RJZA086) and the
National Natural Science Foundation of China (Grant 60776006),
and also supported by ‘Qing Lan’ talent engineering funds (QL-05-23A)
by Lanzhou Jiaotong University.
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