Blue light-emitting, electron-transporting materials based on ethynyl-linked D-A systems

Article abstract of DOI:10.1016/j.cplett.2006.03.095

Two ethynyl-linked π-conjugated compounds with bright blue photoluminescence property were designed, synthesized and fully characterized. The main structures of them contain a pyrene as electron acceptor, a carbazole as electron donor linked by an ethynyl unit. This D-A system has increased electron mobility and high quantum yields. One of the compounds was sandwiched as luminescent in single- and double-layer devices. By incorporating Alq as electron transport layer, the double-layer device exhibits dramatically improved performances compared with the single-layer device. The compounds are promising blue light-emitting and electron-transport materials.

Full text of DOI:10.1016/j.cplett.2006.03.095

Chemical Physics Letters 423 (2006) 293–296
www.elsevier.com/locate/cplett
Blue light-emitting, electron-transporting materials based on
ethynyl-linked D–A systems
a
b
a
a
a,
b,
*
Zujin Zhao , Peng Zhang , Fang Wang , Zixing Wang , Ping Lu *, Wenjin Tian
a
Department of Chemistry, College of Science, Yugu Road No. 38, Zhejiang University, Hangzhou 310027, PR China
Key Laboratory for Supramolecular Structure and Materials of Ministry of Education, Jilin University, Changchun 130012, PR China
b
Received 29 December 2005; in final form 14 February 2006
Available online 7 April 2006
Abstract
Two ethynyl-linked p-conjugated compounds with bright blue photoluminescence property were designed, synthesized and fully char-
acterized. The main structures of them contain a pyrene as electron acceptor, a carbazole as electron donor linked by an ethynyl unit.
This D–A system has increased electron mobility and high quantum yields. One of the compounds was sandwiched as luminescent in
single- and double-layer devices. By incorporating Alq as electron transport layer, the double-layer device exhibits dramatically improved
performances compared with the single-layer device. The compounds are promising blue light-emitting and electron-transport materials.
Ó 2006 Elsevier B.V. All rights reserved.
1. Introduction
are required [9]. Herein we report new bright blue light-
emitting materials, comprise of a pyrene (strong fluophor)
as electron acceptor, a carbazole (hole-transport material)
as electron donor and an ethynyl linkage. These compounds
enhance the electron mobilities, thereby high quantum
yields are obtained. These compounds exhibit red-shifted
emission with an increase of solvent polarities. Further,
EL performance of 3 as the luminescent material was inves-
tigated based on single- or double-layer OLED architecture.
Electroluminescence (EL) devices have been extensively
studied due to their potential application to full-color flat
panel displays [1]. Compared to the display technologies,
organic light emitting diodes (OLEDs) show unique advan-
tages, for example, low drive voltage, wide viewing angle,
easy processing, high brightness and ability to be film-cast
ultra-thin [2]. Organic compounds with aromatic groups
that are linearly conjugated through acetylene linkages
have been the center of much recent attention [3]. Many
organic compounds with different structures have been
designed in order to improve the quantum efficiency, life-
time, emission colors [4–8]. Interest in these compounds
is derived primarily from potential applications where effi-
cient charge transport, fast energy transfer, and good lumi-
nescence properties are presented [3].
2. Experimental section
2.1. Synthesis
Synthesis of D–A compounds is outlined in Scheme 1.
Intermediates 2 [10], 6 [10] and ethynyl pyrene [11] were
synthesized according to references. The key coupling step
(f) is a Pd/Cu-catalyzed Sonogashira reaction.
For OLEDs, the imbalance of electron-hole recombina-
tion in the active layer plays a role in diminishing the quan-
tum efficiency of the OLED device. Compared with organic
hole-transport materials, electron-transport materials with
low electron-injection barriers and high electron mobilities
3. Results and discussion
3.1. Photoluminescence properties
*
Compounds 3 and 7 are highly fluorescent both in solu-
Corresponding authors. Fax: +86 571 87952543 (P. Lu).
tion and the solid state. Quantum yields are 0.96 and 0.95
E-mail address: pinglu@zju.edu.cn (P. Lu).
0009-2614/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.cplett.2006.03.095

294
Z. Zhao et al. / Chemical Physics Letters 423 (2006) 293–296
N
a
b
N
N
f
H
3
I
1
2
N
H
N
N
I
I
I
N
e
c
d
N
N
N
f
7
4
5
6
Scheme 1. Synthesis of 3 and 7: (a) (CH3)3CCl, AlCl3; (b) K2CO3, Cu, 18-crown-6, 1,4-diiodobenzene, DMF, 170 °C, N2, 12 h; (c) K2CO3, Cu, 18-crown-
6, 1-iodo-4-methylbenzene, DMF, N2, 170 °C, 12 h; (d) KI, KIO3, acetic acid, refluxed, 70 min; (e) K2CO3, Cu, 18-crown-6, carbazole, DMF, N2, 170 °C,
12 h; (f) CuI, Pd(PPh3)2Cl2, Ph3P, NEt3/benzene: 3/5 (v/v), 100 °C, N2, 4 h.
in solution, respectively. Emission spectra of 3 and 7 are
shown in Fig. 1. In cyclohexane, 3 exhibits bright blue light
emission at 408 nm with a shoulder at 432 nm. As the ratio
of THF increased, the emission peak is red-shifted and the
shoulder disappears. In pure THF, 3 emits at 447 nm. Mean-
while, the quantum yield decreases. Compound 7 shows the
similar emission phenomena. The red-shifted emission can
be attributed to intramolecular charge transfer (ICT) transi-
tion between the carbazole and pyrene [12], while the
quenching phenomena comes from the dipolar nature of 3
or 7 that can interact with THF [13], which is probably
due to the non-emissive charge-transfer. (See Table 1).
were performed with a Bioanalytical Systems BAS 100B/
W electrochemical work station, using Pt working elec-
trode, an auxiliary Pt electrode, and an Ag/Ag+ reference
electrode, standardized against ferrocenium/ferrocene
redox couple. Cyclic voltammetric studies of 3 were carried
out in DMF containing 0.1 M TBAP (tetra-n-butylammo-
nium perchlorate) as the supporting electrolyte. Results
with 3 are shown in Fig. 2. In the negative region 3 shows
a reversible reduction potential at E1/2 of À2.23 V (vs. Ag/
Ag+). This is due to the contribution of the electron accep-
tor pyrene in the D–A system. This potential is lower than
that (ꢀ À2.4 eV) of 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-
1,3,4-oxadiazole (PBD), one of the most widely used elec-
tron injection/transport or hole-blocking materials [14],
and comparable with that of CN-PPV (À3.10 eV), which
shows good electron-transporting ability in OLEDs [15].
This energy level may provide a closer match to the work
function of Al when used as electron-transporting material.
Moreover a reversible oxidation process (E1/2 = 1.21 V (vs.
Ag/Ag+)) is observed which can be attributed to the elec-
tron-donating carbazole segment. The LUMO level of 3
is estimated to be À2.84 eV and the HOMO level is about
À5.91 eV [16,17]. The electrochemical bandgap of 3.07 eV,
is close to the value obtained from the absorption spectra
(2.89 eV). The HOMO energy level of 3 is much lower than
those of most currently known EL materials [18]. Thus 3
can be employed as a hole-blocking material in OLEDs.
3.2. Electrochemical properties
The electrochemical behavior of 3 was investigated by
cyclic voltammetry (CV) to obtain information about the
energy level structures. Electrochemical measurements
0%THF
20%THF
40%THF
60%THF
80%THF
100%THF
3
3.3. Electroluminescent properties
We selected 3 as an example to investigate EL perfor-
mance in fabricated OLEDs. Fig. 3 shows the EL emission
spectra of 3 in a single-layer device (ITO/PEDOT/PVK:3
(1:1 W/W)/LiF/Al) with respect to the PL emission spectra
in solid film. Note the EL emission peaks coming from 3 at
464 nm and 491 nm in a single-layer device, while PL emis-
sion peaks are observed at 465 nm and 493 nm in a solid
film. The single-layer device (ITO/PEDOT/PVK:3 (1:1 W/
W)/LiF/Al) shows a turn-on voltage at 5.5 V, maximum
7
350
400
450
500
550
600
Wavelength (nm)
Fig. 1. Emission spectra of 3 and 7 in THF/cyclohexane with different
volume fractions.

Z. Zhao et al. / Chemical Physics Letters 423 (2006) 293–296
295
Table 1
Optical and thermal properties of 3 and 7
a
a
Absorbance (nm)
THF
Emission (nm)
THF
U
Tm (°C)
Td (°C)
Cyclohexane
Powder
3
7
373, 398
380, 399
408, 432
406, 430
447
437
496
466
0.96
0.95
259.1
299.5
385.5
477.4
a
Measured by DSC and TGA analysis in N2 at heating rate of 10 °C/min.
5.3 V, maximum luminance of 585 cd mÀ2 at 15 V, and
the efficiency of 0.65 cd mÀ2 at 13 V (Table 2). It is obvious
that the performance is greatly improved by adding an elec-
tron-transport layer (Alq). The maximum luminance and
efficiency of the double-layer device is almost 10 times of
those of the single-layer. Analysis of HOMO and LUMO
energy levels of layers for the single- and double-layer
devices are shown in Fig. 5. HOMO energy level of 3
(5.91 eV) lies between those of PEDOT (5.4 eV) or PVK
(5.78 eV) and that of Alq (5.95 eV), while the LUMO
energy level of 3 lies between those of PVK (2.32 eV) and
Alq (3.0 eV). The barrier of 0.1 eV is reduced for electrons
to migrate from the negative electrode to PVK layer, while
the barrier of 0.17 eV is increased for holes to transport
from the PVK layer to the cathode. Thus with the present
configuration (ITO/PEDOT/PVK:3 (1:1 W/W)/Alq/LiF/
Al), Alq acts as an electron-transporter and a hole-blocker.
Charge recombination is confined to one PVK layer. On the
other hand, an efficiency energy transfer occurs from the
host PVK (404 nm) [19] to the guest 3 that has a strong
absorption at 398 nm (Fig. 1). Such an energy transfer is
consistent with the relative HOMO and LUMO energy lev-
els [10,20] of 3.
20
3
0
-20
-40
-60
-3
-2
-1
0
1
2
Potential (V)
Fig. 2. Cyclic voltammograms (CVs) of 3.
(s)
PL
EL
300
400
500
600
700
800
Wavelength (nm)
Fig. 3. The EL spectra of 3 in single-layer device (ITO/PEDOT/PVK:3
(1:1 W/W)/LiF/Al) and PL spectra in solid film.
12
10
8
600
500
400
300
200
100
0
luminance of 56 cd mÀ2 at 13 V, and the efficiency of
0.06 cd mÀ2 at 14 V (Table 2). It is known that in PVK,
holes are dominant and electrons are minor, leading to
the unbalance of electrons and holes. Therefore, the perfor-
mance of single-layer devices is unsatisfying. To improve
device performances, a double-layer device (ITO/PEDOT/
PVK:3 (1:1 W/W)/Alq/LiF/Al) was fabricated containing
Alq as electron-transport layer. The current–voltage–lumi-
nance (I–V–L) characteristics of the double-layer device
(ITO/PEDOT/PVK:3 (1:1 W/W)/Alq/LiF/Al) is shown in
Fig. 4. The double-layer device exhibits turn-on voltage of
6
4
2
0
0
2
4
6
8
10 12 14 16
Voltage (V)
Fig. 4. I–V–L characteristics of double-layer device (ITO/PEDOT/PVK:3
(1:1 W/W)/Alq/LiF/Al).
Table 2
Summary of device performances
b
Device
PL
EL
Efficiencya
Efficiencymax
(cd mÀ2, V)
Onset voltage
(V)
Lmax
(nm)
(nm)
(cd mÀ2, 100 cd mÀ2
)
(cd mÀ2, V)
Single
Double
a
465, 493
465, 493
464, 491
466, 500
–
0.47
0.06, 14
0.65, 13
5.5
5.3
56, 13
585, 15
Efficiency obtained when the brightness is 100 cd mÀ2
Maximum efficiency obtained in the experiments.
.
b

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Z. Zhao et al. / Chemical Physics Letters 423 (2006) 293–296
2.32
2.84
2.32
2.84
2.84
3.0
3.1
3.1
3.1
4.8
4.8
4.8
3
5.78
3
5.78
3
5.4
5.4
5.4
5.91
5.91
5.91
5.95
Alq
PVK
(c)
LiF/Al
ITOP EDOT
PVKL iF/Al
ITO PEDOT
(b)
LiF/Al
ITO PEDOT
(a)
Fig. 5. Relative energy alignments in ITO/PEDOT/3/LiF/Al (a), ITO/PEDOT/PVK:3/LiF/Al (b), and ITO/PEDOT/PVK:3/Alq/LiF/Al (c).
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In conclusion, two bright blue light-emitting materials
have been synthesized and characterized. Compounds 3
and 7 have good photoluminescence properties and ther-
mal stabilities. Compound 3 has been used to fabricate
the single- and double-layer OLED devices. The latter
exhibits better performances. Compounds 3 and 7 are
promising bright blue emitters and/or electron-transport-
ing materials for OLED application.
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P. Lu thanks National Nature Science Foundation of
China (20374045), Nature Science Foundation of Zhejiang
Province (R404109) and Ministry of Education of China for
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Appendix A. Supporting information available
Synthetic and characterization details for 3 and 7. The
detailed device fabrication and characterization procedures
(PDF).
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