Synthesis, crystal structure and photo- and electro-luminescence of the coumarin derivatives with benzotriazole moiety

Article abstract of DOI:10.1016/j.orgel.2009.09.023

Two new coumarin derivatives containing an electron-transporting moiety (benzotriazole), 7-N,N-diethylamino-3-(benzotriazol-1-yl)coumarin (DABTC-1) and 7-N,N-diethylamino-3-(benzotriazol-2-yl)coumarin (DABTC-2), were synthesized and characterized by element analysis, ¹H NMR, FT-IR and UV-vis absorption spectra. Their structures were determined by X-ray crystallography single crystal analysis. The photoluminescent behaviors of the compounds in both solution and solid states were observed, they exhibit strong blue and blue-green emissions under ultraviolet light excitation, respectively. The energy levels of the HOMO and LUMO of the compounds have been calculated with density functional theory (DFT) and time-dependent density functional theory (TD-DFT) at B3LYP/6-31G(d) level. The electroluminescent devices with the compounds as the emitters were fabricated.

Full text of DOI:10.1016/j.orgel.2009.09.023

Organic Electronics 11 (2010) 41–49
Contents lists available at ScienceDirect
Organic Electronics
journal homepage: www.elsevier.com/locate/orgel
Synthesis, crystal structure and photo- and electro-luminescence
of the coumarin derivatives with benzotriazole moiety
Tianzhi Yu a, , Peng Zhang a,b, Yuling Zhao b, Hui Zhang a, Jing Meng a, Duowang Fan a,
*
Lili Chen c, Yongqing Qiu c
a Key Laboratory of Opto-Electronic Technology and Intelligent Control (Lanzhou Jiaotong University), Ministry of Education, Lanzhou 730070, China
b School of Chemical and Biological Engineering, Lanzhou Jiaotong University, Lanzhou 730070, China
c Institute of Functional Material Chemistry, Faculty of Chemistry, Northeast Normal University, Changchun 130024, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Two new coumarin derivatives containing an electron-transporting moiety (benzotriazole),
7-N,N-diethylamino-3-(benzotriazol-1-yl)coumarin (DABTC-1) and 7-N,N-diethylamino-3-
(benzotriazol-2-yl)coumarin (DABTC-2), were synthesized and characterized by element
analysis, 1H NMR, FT-IR and UV–vis absorption spectra. Their structures were determined
by X-ray crystallography single crystal analysis. The photoluminescent behaviors of the
compounds in both solution and solid states were observed, they exhibit strong blue and
blue–green emissions under ultraviolet light excitation, respectively. The energy levels of
the HOMO and LUMO of the compounds have been calculated with density functional the-
ory (DFT) and time-dependent density functional theory (TD-DFT) at B3LYP/6-31G(d) level.
The electroluminescent devices with the compounds as the emitters were fabricated.
Ó 2009 Elsevier B.V. All rights reserved.
Received 15 July 2009
Received in revised form 22 September
2009
Accepted 23 September 2009
Available online 9 October 2009
Keywords:
Synthesis
Coumarin derivative
Benzotriazole
Photo- and electroluminescence
1. Introduction
dyes were used as blue, green and red dopants in organic
light-emitting diodes (OLEDs) [13–15]. However, coumarin
Organic light-emitting diodes (OLEDs) based on organic
materials are known to be highly efficient and are capable
of producing a wide range of colors, which have attracted
considerable interest and constitute a rapidly developing
field due to their potential use in the development of en-
ergy-efficient, low-cost, full-color, flat-panel displays and
other emissive products [1–4].
Coumarin derivatives exhibit not only excellent biolog-
ical and medical activities [5], but also have super thermal
stability and outstanding optical properties including an
extended spectral response, high quantum yields, superior
photostability. Optical applications of these compounds,
such as laser dyes, nonlinear optical chromophores, fluo-
rescent whiteners, fluorescent probes, polymer science,
optical recording and solar energy collectors, have been
widely investigated [6–12]. More important, coumarin
dyes are easily self-quenched in high concentration due to
the intermolecular interactions and aggregations, so as the
light-emitting materials they are always doped in the host
materials at appropriate concentration to fabricate OLEDs
with reasonable, luminant efficiency [16–19]. 7-N,N-dieth-
ylaminocoumarin dyes substituted at 3-position with an
electron-withdrawing group are a typical class of D-A
structure molecules and exhibit strong fluorescence [20–
22]. So some of these coumarin derivatives have been used
in the fabrication of OLEDs through doping in polymer ma-
trix or Alq3 thin layer as guest-emitting materials [23].
Recently, we synthesized two new coumarin deriva-
tives, 7-N,N-diethylamino-3-(benzotriazol-1-yl)coumarin
(DABTC-1) and 7-N,N-diethylamino-3-(benzotriazol-2-yl)-
coumarin (DABTC-2), using the Knoevenagel type of con-
densation reaction (Scheme 1). The compounds exhibit
strong blue and blue-green emissions under ultraviolet
light excitation, respectively. Here, the photophysical
properties of the compounds in diluted chloroform
* Corresponding author. Tel.: +86 931 4956935; fax: +86 931 4938756.
E-mail address: Ytz823@hotmail.com (T. Yu).
1566-1199/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.orgel.2009.09.023

42
T. Yu et al. / Organic Electronics 11 (2010) 41–49
N
N
N
CHO
N
N
OH
N
N
N
O
N
O
CN
N
DABTC-1
ClCH2CN
1
N
CHO
OH
N
H
TEA, DMF
110 oC
N
N
N
CN
N
N
N
O
N
O
2
DABTC-2
Scheme 1. Synthetic routes of 7-N,N-diethylamino-3-(benzotriazol-1-yl)coumarin and 7-N,N-diethylamino-3-(benzotriazol-2-yl)coumarin.
solutions and thin films were investigated in detail. We
fabricated the electroluminescence devices by vacuum
vapor-deposited film with doped with the compounds as
emission materials and investigated the electrolumines-
cent properties.
at reflux in the presence of triethylamine according to
the method reported by Sanna et al. [24].
2.2.2. 7-N,N-diethylamino-3-(benzotriazol-1-yl)coumarin
(DABTC-1)
1.93 g (10 mmol) of 4-(N,N0-diethylamino) salicylalde-
hyde, 1.67 g (10.6 mmol) 2-(1H-benzotriazol-1-yl)acetoni-
trile (1) were dissolved in 16 mL of anhydrous alcohol and
treated with piperidine (0.6 mL). The reaction mixture was
refluxed with stirring for 49 h, treated with HCl (3.5%), and
boiled for 5 h. After the reaction was finished, the resulting
mixture was poured into 50 mL of water and extracted
with dichloromethane (3 ꢁ 50 mL). The organic phase
was washed with water (2 ꢁ 50 mL) and dried over anhy-
drous MgSO4. After filtering, the filtrate was evaporated
to dryness under reduced pressure. The crude was purified
by chromatography on silica gel using ethyl acetate/petro-
leum ether (1:6, v/v) as the eluent to give 7-N,N-diethyl-
amino-3-(benzotriazol-1-yl)coumarin (DABTC-1) (1.12 g,
33.5%). m.p. 164–165 °C. 1H NMR: (500 MHz, CDCl3, d,
ppm): 8.116 (d, J = 4.8 Hz, 1H, Aryl-H), 8.049 (s, 1H, 4-H),
7.619–7.594 (m, 1H, Aryl-H), 7.565–7.554 (m, 1H, Aryl-
H), 7.540–7.516 (m, 1H, Aryl-H), 7.434–7.388 (m, 1H,
Aryl-H), 6.678 (d, J = 7.9 Hz, 1H, Aryl-H), 6.608 (s, 1H,
Aryl-H), 3.503–3.451 (m, 4H, –CH2–), 1.261 (t, J = 6.4 Hz,
6H, –CH3). IR (KBr pellet cmꢀ1): 3060 (Aryl-CH), 2971,
2929, 2891 (Alkyl-CH), 1728 (C@O, lactone), 1628 (C@C),
1590, 1524, 1421, 1250, 1121, 752. Anal. Calc. for
C19H18O2N4 (%): C, 68.25; H, 5.43; N, 16.76. Found: C,
68.53; H, 5.57; N, 16.84.
2. Experimental details
2.1. Materials and methods
4-(N,N0-diethylamino) salicylaldehyde from Zhejiang
Huadee Dyestuff Chemical Co. Ltd. (China) was recrystal-
lized from ethanol. 1H-benzotriazole (BTA) from Aldrich
was used without further purification. Triethylamine,
piperidine and chloroacetonitrile were analytical grade
reagents from Tanjin Fuchen Chemical Reagent Factory.
4,40-bis(9-carbazolyl) biphenyl (CBP), 2-(4-biphenylyl)-5-
(4-tert-butylphenyl)-1,3,4-oxadiazole (Bu-PBD), 4,40,400-
tris-N-naphthyl-N-phenylamino-triphenylamine (2-TNAT-
A) and N,N0-bis-(naphthyl)-N,N0-diphenyl-1,10-biphenyl-
4,40-diamine (NPB) were purchased from Electro-Light
Technology Corp., Beijing. The other solvents were analyt-
ical grade reagents.
IR spectra (400–4000 cmꢀ1) were measured on a Shima-
dzu IRPrestige-21 FT-IR spectrophotometer. 1H NMR spec-
tra were obtained on Unity Varian-500 MHz. C, H, and N
analyses were obtained using an Elemental Vario-EL auto-
matic elemental analysis instrument. Melting points were
measured by using an X-4 microscopic melting point appa-
ratus made in Beijing Taike Instrument Limited Company,
and the thermometer was uncorrected. UV–vis absorption
and photoluminescent spectra were recorded on a Shima-
dzu UV-2550 spectrometer and on a Perkin–Elmer LS-55
spectrometer, respectively. The electroluminescent spectra
were measured on a Hitachi MPF-4 spectrometer.
2.2.3. 7-N,N-diethylamino-3-(benzotriazol-2-yl)coumarin
(DABTC-2)
0.49 g (2.53 mmol) of 4-(N,N0-diethylamino) salicylal-
dehyde, 0.40 g (2.53 mmol) 2-(1H-benzotriazol-2-yl)aceto-
nitrile (2) were dissolved in 12 mL of absolute alcohol, and
then piperidine (0.2 mL) was dropped under ice bath. The
mixture was stirred for 12 h at room temperature, treated
with HCl (3.5%) and boiled for 5 h. After the reaction was
finished, the resulting mixture was poured into 20 mL of
water and extracted with dichloromethane (3 ꢁ 20 mL).
The organic phase was washed with water and dried over
anhydrous MgSO4. After filtering, the filtrate was evapo-
rated to dryness under reduced pressure. The crude was
purified by chromatography on silica gel using ethyl ace-
tate/petroleum ether (1:6, v/v) as the eluent to give 7-
2.2. Synthesis and characterization of 7-N,N-diethylamino-3-
(benzotriazol-1-yl)coumarin (DABTC-1) and 7-N,N-
diethylamino-3-(benzotriazol-2-yl)coumarin (DABTC-2)
The synthetic routes were shown in scheme 1.
2.2.1. 2-(1H-benzotriazol-1-yl)acetonitrile (1) and 2-(1H-
benzotriazol-2-yl)acetonitrile (2)
The intermediates 1 and 2 were synthesized by conden-
sation of 1H-benzotriazole with chloroacetonitrile in DMF

T. Yu et al. / Organic Electronics 11 (2010) 41–49
43
N,N-diethylamino-3-(benzotriazol-2-yl)coumarin (DABTC-
2) (0.63 g, 74.8%). m.p. 156–158 oC. 1H NMR: (500 MHz,
CDCl3, d, ppm): 8.280 (s, 1H, 4-H), 7.954 (d, J = 4.8 Hz, 2H,
Aryl-H), 7.441–7.264 (m, 3H, Aryl-H), 6.682 (d, J = 8 Hz,
1H, Aryl-H), 6.608 (s, 1H, Aryl-H), 3.484–3.440 (m, 4H, –
CH2–), 1.255 (t, J = 5.2 Hz, 6H, –CH3). IR (KBr pellet
cmꢀ1): 3067 (Aryl-CH), 2975, 2925, 2871 (Alkyl-CH),
1731 (C@O, lactone), 1624 (C@C), 1573, 1517, 1421,
1233, 1125, 745. Anal. Calc. for C19H18O2N4 (%): C, 68.25;
H, 5.43; N, 16.76. Found: C, 68.51; H, 5.72; N, 16.37.
els. The structure optimization and energy calculations
were performed with the GAUSSIAN 98 program.
2.5. OLEDs fabrication
The multilayer OLEDs were fabricated by vacuum-depo-
sition method. ITO-coated glass with a sheet resistance
Rh ꢂ 20
X
/h was cut into 3 cm ꢁ 3 cm plates and etched
in dilute hydrochloric acid for 20 min. Then the ITO sub-
strates 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 ꢁ 10ꢀ4 Pa). Thermal deposi-
tion rates for organic materials, LiF and Al were ꢂ2 Å/s,
ꢂ1 Å/s and 10 Å/s, respectively. The active area of the
devices was 12 mm2. The EL spectra were measured on a
Hitachi MPF-4 fluorescence spectrometer. The character-
ization of brightness–current–voltage (B–I–V) were mea-
sured with a 3645 DC power supply combined with a
1980A spot photometer and were recorded simulta-
neously. All measurements were done in the air at room
temperature without any encapsulation.
2.3. Crystallography
Suitable crystals of DABTC-1 and DABTC-2 were ob-
tained by evaporation of ethyl acetate solutions, respec-
tively. The diffraction data were collected with a Bruker
Smart Apex CCD area detector using a graphite-monochro-
mated Mo Ka radiation (k = 0.71073 Å) at 20 °C. The struc-
tures of DABTC-1 and DABTC-2 were solved by using the
program SHELXL and Fourier difference techniques, and re-
fined by full-matrix least-squares method on F2. All hydro-
gen atoms were added theoretically. The crystals and
experimental data of DABTC-1 and DABTC-2 are shown in
Table S1. The selected bond lengths and bond angles of
DABTC-1 and DABTC-2 are listed in Tables S2 and S3,
respectively.
3. Results and discussion
3.1. X-ray crystal structures of DABTC-1 and DABTC-2
2.4. Quantum chemical calculations
The crystal structures and packing diagrams of DABTC-1
and DABTC-2 are given in Figs. 1–4, respectively. The crys-
tal data and experimental details are shown in Table S1.
The selected bond lengths and bond angles of DABTC-1
and DABTC-2 are listed in Tables S2 and S3, respectively.
The structure of DABTC-1 and DABTC-2 were optimized
by semiempirical density functional theory (DFT) using a
B3LYP/6-31G (d) basis set. The structural energies of DAB-
TC-1 and DABTC-2 were calculated at B3LYP/6-31G(d) lev-
Fig. 1. Crystal structure of DABTC-1.

44
T. Yu et al. / Organic Electronics 11 (2010) 41–49
Fig. 2. Packing diagram of molecules of DABTC-1 along a-axis. H atoms are omitted for clarity.
Fig. 3. Crystal structure of DABTC-2.
Acceptable crystals of DABTC-1 for X-ray analysis were
The packing diagram for unit cell of DABTC-1 was
shown in Fig. 2. There is cross ABAB-type stacking between
the benzotriazole group of one DABTC-1 molecule and the
coumarin ring of other DABTC-1 molecule, the interplanar
(Aꢃ ꢃ ꢃB) distance is approximately 3.6 Å, which means weak
obtained by slow evaporation of ethyl acetate solution, and
the structure was measured by X-ray crystallography. The
crystal of DABTC-1 belongs to the monoclinic space group
P21/c, a = 15.920(2) Å, b = 12.8933(18) Å, c = 17.083(3) Å,
b = 106.299(3)°, U = 3365.7(9) Å3, Z = 8, Dc = 1.320 g cmꢀ3
,
intermolecular p–p stacking interaction between DABTC-1
l
= 0.089 mmꢀ1. As shown in Fig. 1, there is an asymmetric
molecules in crystal lattice.
unit consisted of two molecules in crystal structure of
DABTC-1 due to the different space configuration of two
ethylic groups of diethylamine in 7-position of coumarin
ring. These two molecules are space conformers, the ben-
zotriazole skeleton of one DABTC-1 molecule cross-paral-
leled and superposed with the coumarin ring of other
DABTC-1 molecule, and the distance of the planes is about
3.6 Å. In addition, the benzotriazole skeletons are not
coplanar with the coumarin rings in these two molecules,
the dihedral angles are 65.40° and 48.75°, respectively.
Acceptable crystals of DABTC-2 for X-ray analysis were
also obtained by slow evaporation of ethyl acetate solu-
tion. The crystal of DABTC-2 belongs to the monoclinic
space group P21/c, a = 17.416(5) Å, b = 12.743(4) Å, c =
15.707(5) Å, b = 102.527(5)°, U = 3403.1(18) Å3, Z = 8, Dc =
1.305 g cmꢀ3 = 0.088 mmꢀ1. Similarly, there is also an
, l
asymmetric unit consisted of two molecules in crystal
structure of DABTC-2 due to the different space configura-
tion of two ethylic groups of diethylamine in 7-position of
coumarin ring (Fig. 3). Compared with DABTC-1, these

T. Yu et al. / Organic Electronics 11 (2010) 41–49
45
Fig. 4. Packing diagram of molecules of DABTC-2 along c-axis. H atoms are omitted for clarity.
space conformers did not neither parallel nor superpose
each other. As shown in Fig. 4, there is no intermolecular
p–p stacking interaction between DABTC-2 molecules in
crystal lattice. From the crystal structure of DABTC-2, we
can see that the benzotriazole skeletons are not also copla-
nar with the coumarin rings in these two molecules, the
dihedral angles are 56.92° and 47.26°, respectively.
Fig. 5 also shows the photoluminescent spectra of DAB-
TC-1 and DABTC-2 in dilute chloroform solutions and the
solid films. It is shown that DABTC-1 compound exhibits
bright blue emission with peak at 447 nm and DABTC-2
compound exhibits bright blue–green emission at
485 nm in dilute solution, and the emission peak of DAB-
TC-2 was red-shifted by about 38 nm compared with that
of DABTC-1 due to the larger conjugation of molecule in
DABTC-2. Compared with the emission spectra in the solu-
tions, the emission peaks of the solid films of DABTC-1 and
DABTC-2 are observed at 462 and 485 nm, respectively.
The emission peak of DABTC-1 in the solid film was batho-
chromically shifted by 15 nm with respect to that in the
solution, and much broader emission band of the solid film
brought about the compact piling of the solids was oc-
curred, suggesting that the compound in the solid film
has more intermolecular interaction than that in solution
situation, and thus may lead to change of the emission
band. The emission spectrum of the film of DABTC-2 was
similar to that of the solution, the emission peaks were
at 485 nm, but the former was little broader than the lat-
ter. The differences between the films and the solutions
were explained by the crystal structures of DABTC-1 and
3.2. Absorption and fluorescence of DABTC-1 and DABTC-2
UV–vis absorption spectra of DABTC-1 and DABTC-2 in
diluted chloroform solutions are given in Fig. 5. It is shown
that the absorption spectrum of DABTC-1 exhibits absorp-
tions at 253 nm and 414 nm, respectively. The absorption
spectrum of DABTC-2 is similar to that of DABTC-1, there
are two absorption at 253 nm and 424 nm. An important
feature of the absorption spectrum of DABTC-2 is shown
that the second absorption band at 424 nm was bathochro-
mically shifted by about 10 nm with respect to that of
DABTC-1 in which the second absorption band is at
414 nm, it can be generally deduced that the conjugative
effect of the 2-substituted benzotriazole derivative (i.e.
DABTC-2) is larger than that of the 1-benzotriazolyl isomer
(i.e. DABTC-1).
DABTC-2. There was weak intermolecular
interaction between DABTC-1 molecules in crystal lattice,
whereas no intermolecular stacking interaction be-
p–p stacking
p-p
UV (DABTC-1)
UV (DABTC-2)
tween DABTC-2 molecules existed in crystal lattice.
UV–vis absorption and photoluminescent spectra of
coumarin 6 (C6) and 7 (C7) in diluted chloroform solutions
show the maximum peaks at 456 nm (C6), 437 nm (C7),
490 nm (C6) and 491 nm (C7), respectively [25]. Compared
with commercial coumarin 6 and 7, although the structure
of DABTC-2 is similar to that of coumarin 6 and 7, UV–vis
absorption and photoluminescent spectra of DABTC-2 shift
to blue due to the electron-withdrawing benzotriazole
substituents at 3-position.
1.5
1.0
0.5
0.0
1.5
1.0
0.5
0.0
PL (DABTC-1, Solution)
PL (DABTC-1, Film)
PL (DABTC-2, Film)
PL (DABTC-2, Solution)
The photoluminescence behaviors of DABTC-1 and
DABTC-2 vs. the concentrations in chloroform solutions
were shown in Fig. 6. It was found that the emission spec-
tral features of DABTC-1 and DABTC-2 changed with the
solution concentrations. The concentration required for
300
400
500
600
Wavelength (nm)
Fig. 5. UV–Vis absorption and photoluminescent spectra of the com-
pounds in dilute chloroform solutions and the solid films.

46
T. Yu et al. / Organic Electronics 11 (2010) 41–49
500
400
300
200
100
0
8 x 10-7 mol/L
2 x 10-6 mol/L
4 x 10-6 mol/L
6 x 10-6 mol/L
8 x 10-6 mol/L
1 x 10-5 mol/L
4 x 10-5 mol/L
8 x 10-5 mol/L
1 x 10-4 mol/L
2.0 x 10-7 mol/L
a
b
8.0 x 10-7 mol/L
600
4.0 x 10-6 mol/L
8.0 x 10-6 mol/L
1.5 x 10-5 mol/L
400
3.0 x 10-5 mol/L
6.0 x 10-5 mol/L
8.0 x 10-5 mol/L
1.0 x 10-4 mol/L
200
0
450
500
550
600
400
450
500
550
600
Wavelength (nm)
Wavelength (nm)
Fig. 6. The emission spectra of DABTC-1 (a) and DABTC-2 (b) in dilute chloroform solutions at room temperature.
the maximum emission intensity was different in DABTC-1
and DABTC-2, which implies that concentration plays an
important role in the different dye molecules. There is
the strongest blue emission at about concentration of
3.0 ꢁ 10ꢀ5 mol/L for DABTC-1 and 8.0 ꢁ 10ꢀ6 mol/L for
DABTC-2, respectively. When the concentrations were
lower than these concentrations, the relative emission
intensities increased gradually with increasing the concen-
trations. When the concentrations were higher than these
concentrations, the emission intensities were decreasing.
3.4. Quantum chemical calculations
From the crystal structures of DABTC-1 and DABTC-2
(Figs. 1 and 3), it was found that there were two molecules
in the crystal structures. For the sake of quantum chemical
calculations, we chose one molecule of them to optimize
DABTC-1 and DABTC-2 geometrical structures, respec-
tively. B3LYP/6-31G(d) optimized structures of DABTC-1
and DABTC-2 are close to their X-ray crystallographic data.
The HOMO and LUMO levels of DABTC-1 and DABTC-2
were deduced using the DFT method as shown in Figs. S1
and S2, respectively. It can be seen that the levels of HOMO
and LUMO of DABTC-1 were ꢀ5.510 eV and ꢀ1.589 eV, and
the energy gap between HOMO and LUMO is about
3.921 eV. For DABTC-2, the data were ꢀ5.487 eV,
ꢀ1.594 eV and 3.893 eV, respectively. It can be also found
that the electron density distributions of the HOMO orbi-
tals were different from that of the HOMO orbitals for DAB-
TC-1 and DABTC-2.
The UV–vis absorption spectra of DABTC-1 and DABTC-
2 were also calculated by the time-dependent density
functional theory (TD-DFT) at the same level. The experi-
mental and calculated data for the lower-lying singlet
states of DABTC-1 and DABTC-2 were listed in Table 1. As
shown in Table 1, there are some discrepancies between
theoretical and experimental data due to several influenc-
ing factors, such as solvent effect and intermolecular inter-
action. It has been found that the lowest energy absorption
at 414 nm for DABTC-1 or 424 nm for DABTC-2 was an
excitation from the HOMO to the LUMO. The transition
occurring at 253 nm was attributed to the electronic
3.3. Fluorescence quantum yield of the compound 3 (UF,3
)
Fluorescence quantum yields of DABTC-1 and DABTC-2
in chloroform solution were checked with anthracene,
respectively. The quantum yield (UF) were calculated
according to the relationship [26,27]:
!
R1
S
1 ꢀ 10ꢀA
n
nS
ꢀ
ꢁ
2
~
~
IFð
v
IFð
Þd
v
Þd
0
UF
¼
UFS
R1
S
1 ꢀ 10ꢀA
~
v
~
v
0
where USF is the quantum yield of anthracene as standard,
which was assumed to be 0.25 for all environments [27].
R1
R1
S
~
IFðv
~
Þdv
~
IFðv
~
Þdv
The integral
and
are the areas under
0
0
the emission curves of the investigated and standard com-
pound, respectively. A and AS are the absorbances at the
wavelength of excitation, n and nS are the refractive indices
for the investigated and standard, respectively. In the re-
search, the UF, DABTC-1 and UF, DABTC-2 are 0.53 and 0.69,
respectively. Fluorescence quantum yields of Coumarin 6
(UF, Coumarin 6) in acetonitrile is 0.63 [28]. Fluorescence
p ? p* transition from the HOMO to the LUMO+2 for DAB-
quantum yields of DABTC-1 and DABTC-2 are comparative
to that of coumarin 6.
TC-1 or from the HOMOꢀ2 to the LUMO+1 for DABTC-2,
respectively.
Table 1
Absorption spectra data of DABTC-1 and DABTC-2.
Compound
DABTC-1
Transition character
OSCa
kmax, cal. (nm)
kmax, exp. (nm)b
Transition feature
*
HOMO ? LUMO
HOMO ? LUMO+2
HOMO ? LUMO
HOMOꢀ2 ? LUMO+1
0.7397
0.1583
0.9473
0.1778
369.32
243.25
378.24
233.37
414
253
424
253
p
p
p
p
?
?
?
?
p
p
p
p
*
*
*
DABTC-2
a
Oscillator strength coefficients (f).
kmax in dichloromethane solvent.
b

T. Yu et al. / Organic Electronics 11 (2010) 41–49
47
3.5. OLED performance
2.0%
1.0%
100
80
60
40
20
0
2.0%
1.0%
50
40
30
20
10
0
The molecular structures of the materials and the de-
vices with the configuration of ITO/2-TNATA (5 nm)/NPB
(40 nm)/CBP: DABTC-1 or CBP: DABTC-2 (ꢁ wt%, 30 nm)/
PBD (30 nm)/LiF (1 nm)/Al (100 nm) were depicted in
Fig. 7. DABTC-1: CBP or DABTC-2: CBP was employed as
the emitter, 2-TNATA, NPB, and PBD were used as hole
injection, hole transport and electron transport materials,
respectively. LiF was used as the electron–injection layer.
Fig. 8 shows the EL spectra of the devices with DABTC-1
and DABTC-2 doped at concentrations of 1.0 wt% at differ-
ent driving voltages. The intensities of EL emissions in-
crease gradually with increasing the driving voltages, but
the emission spectral features do not change. The devices
of DABTC-1 exhibit blue emissions with the maximum
peaks at ca. 462 nm, the devices of DABTC-2 exhibit
blue–green emissions with the maximum peaks at ca.
485 nm. The EL spectra of DABTC-1 and DABTC-2 are iden-
tical to the PL spectra of them in the solid films, respec-
tively. Such results indicate that these emissions take
place from DABTC-1 and DABTC-2 molecules.
0
5
10
15
20
Voltage (V)
-5
0
5
10
15
20
25
Voltage (V)
Fig. 9. The luminance vs. voltage and current density vs. voltage (inset)
curves of the devices with different DABTC-1 concentrations.
age of the device is about 8.5 V, and the maximal lumi-
nance reaches 48 cd/m2 at 15 V.
Fig. 10 shows the representative luminance-current
density–voltage characteristics of the devices doped DAB-
TC-2 with different concentrations. The turn-on voltages
of the devices at the doped concentrations of 1.0 wt% and
2.0 wt% were 10 V and 6 V, respectively. The maximal
luminances were 371 cd/m2 at 23 V for the device doped
Fig. 9 shows the representative luminance-current den-
sity–voltage characteristics of the devices doped DABTC-1
with different concentrations. At the doped concentration
of 1.0 wt%, the device shows a turn-on voltage about
12 V, and the luminance reaches as high as 40 cd/m2 at
21 V. At the doped concentration of 2.0 wt%, a turn-on volt-
N
N
N
NPB
N
O
N
N
N
N
PBD
N
N
2-TNATA
CBP
Fig. 7. The molecular structures of materials and the structure of EL devices.
3.5
13 V
14 V
15 V
16 V
17 V
18 V
19 V
20 V
b
17 V
a
2.0
1.5
1.0
0.5
0.0
18 V
19 V
20 V
21 V
22 V
23 V
3.0
2.5
2.0
1.5
1.0
0.5
0.0
400
450
500
550
600
650
400
450
500
550
600
650
Wavelength (nm)
Wavelength (nm)
Fig. 8. EL spectra of the devices used DABTC-1 (a) and DABTC-2 (b) as dopants at different applied voltages. The devices with the configuration of ITO/2-
TNATA (5 nm)/NPB (40 nm)/CBP: dopant (1.0 wt%, 30 nm)/PBD (30 nm) / LiF (1 nm)/Al (100 nm).

48
T. Yu et al. / Organic Electronics 11 (2010) 41–49
200
150
100
50
Table 2
2.0%
1.0%
2.0%
1.0%
EL performances of DABTC-1 and DABTC-2 doped devices.
800
600
400
200
Compound
Maximum efficiency
(cd/A)
Efficiency at
20 mA/cm2 (cd/A)
DABTC-1
DABTC-2
1.0 wt%
2.0 wt%
1.0 wt%
2.0 wt%
0.33 @ 0.22 mA/cm2
0.60 @ 0.10 mA/cm2
1.81 @ 0.16 mA/cm2
1.63 @ 0.40 mA/cm2
0.09
0.15
0.97
1.05
0
0
2
4
6
8
10 12 14 16 18 20 22
Voltage (V)
increasing, the conductive characteristics of the devices
was considerably improved, and the turn-on voltages of
the devices were reduced. It was shown that benzotriazole
group had excellent electron-transporting ability in these
EL devices. From Figs. 9 and 10, it was found that the lumi-
nance of DABTC-2 devices was very stronger than that of
DABTC-1 devices due to the larger conjugation of molecule
in DABTC-2. Similarly, it was found that the current effi-
ciency of DABTC-2 devices was higher than that of DAB-
TC-1 devices (Fig. 11).
0
0
5
10
15
20
25
Voltage (V)
Fig. 10. The luminance vs. voltage and current density vs. voltage (inset)
curves of the devices with different DABTC-2 concentrations.
concentration of 1.0 wt% and 823 cd/m2 at 17 V for the de-
vice doped concentration of 2.0 wt%, respectively.
From above results, it was indicated that the perfor-
mance of the DABTC-2 doped devices is be superior to that
of the DABTC-1 doped devices.
Fig. 11 shows the relationships between current effi-
ciency and the current density in the devices with various
DABTC-1 and DABTC-2 doping concentrations. It is shown
that all doped devices have higher efficiencies at low cur-
rent densities, and then the efficiencies fall off at higher
current densities. The above finding indicates that the elec-
tron hole recombination may not be effective under high
current density, revealing that the present host, CBP, might
not be an optimal host material. The present device perfor-
mance could be further improved by substituting other
host candidates. Table 2 summarized the performances of
the devices with various DABTC-1 and DABTC-2 doping
concentrations in CBP host. It was shown that the 2 wt%
DABTC-1 and DABTC-2 doped devices had better
efficiencies.
It was interesting to note that the turn-on voltages of
the devices decreased with the increasing of compounds
(DABTC-1 or DABTC-2) doping concentration in the CBP
host from Figs. 9 and 10. In view of the structures of the
two compounds, they were substituted by the electron-
withdrawing benzotriazoles regarded as accptors [29]
and used as electron-transporting materials [30]. Further-
more, the host material CBP can provide good hole-trans-
porting properties [31]. With DABTC concentration
3.6. Conclusions
We designed and prepared two new coumarin deriva-
tives containing an electron-transporting moiety (benzotri-
azole), 7-N,N-diethylamino-3-(benzotriazol-1-yl)coumarin
(DABTC-1) and 7-N,N-diethylamino-3-(benzotriazol-2-
yl)coumarin (DABTC-2). The photoluminescent behaviors
of the compounds in both solution and solid states were
investigated and discussed. The doped devices fabricated
from DABTC-1 or DABTC-2 doped in CBP show the EL spec-
tra of DABTC-1 or DABTC-2 is identical to the PL spectra of
them in the solid films, respectively. The ITO/2-TNATA
(5 nm)/NPB (40 nm)/CBP: DABTC-1 (2.0 wt%, 30 nm)/PBD
(30 nm)/LiF (1 nm)/Al (100 nm) device displayed the pure
blue emission, a maximum luminous efficiency of 0.15 cd/
A at the current density of 20 mA/cm2, and maximum lumi-
nance of 48 cd/m2 at 15 V. At the same condition, the DAB-
TC-2 doped devices exhibited the brighter blue–green
emission, a maximum luminous efficiency of 1.05 cd/A at
the current density of 20 mA/cm2, and maximum lumi-
nance of 823 cd/m2 at 17 V.
a
b
1
0.1
0.01
1E-3
0.1
0.01
2.0%
1.0%
2.0%
1.0%
1
10
100
1
10
100
Current Density (mA/cm2)
Current Density (mA/cm2)
Fig. 11. Current efficiency-current density characteristics of the doped devices with various DABTC-1 (a) and DABTC-2 (b) concentrations.

T. Yu et al. / Organic Electronics 11 (2010) 41–49
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Acknowledgements
This work was supported by the National Natural Sci-
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for Changjiang Scholars and Innovative Research Team in
University (IRT0629) and also supported by ‘Qing Lan’ tal-
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