Synthesis and properties of a solution-processable truxene derivative for oled devices

Article abstract of DOI:10.1166/jnn.2010.2963

A soluble truxene derivative (TR1) attached with triphenylamine at the peripheral position was designed and synthesized. The structure and purity of TR1 were carefully characterized by 1H NMR, UV/vis and photoluminescent spectroscopy, mass spectroscopy, a

Full text of DOI:10.1166/jnn.2010.2963

Copyright © 2010 American Scientific Publishers
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Printed in the United States of America
Journal of
Nanoscience and Nanotechnology
Vol. 10, 6916–6919, 2010
Synthesis and Properties of a Solution-Processable
Truxene Derivative for OLED Devices
Kyu Min Choi1, Kye-Young Kim1, Tae-Dong Kim1, Rupasree Ragini Das2, Byoung Ki Choi2,
Jong Jin Park2ꢀ∗, Jong-Min Kim2, and Kwang-Sup Lee1ꢀ∗
1Department of Advanced Materials, Hannam University, Daejeon 305-811, Korea
2Samsung Advanced Institute of Technology, Gyeonggi-do 449-712, Korea
A soluble truxene derivative (TR1) attached with triphenylamine at the peripheral position was
designed and synthesized. The structure and purity of TR1 were carefully characterized by 1H NMR,
UV/vis and photoluminescent spectroscopy, mass spectroscopy, and thermal analyses. It exhibited
good solubility in common organic solvents and good film forming properties. The maximum absorp-
tion and emission peaks in THF solution were shown at 358 nm and 415 nm, respectively. Bright
blue emission was observed in both solution and solid states under UV excitation. The fluorescent
quantum efficiency was 0.46. The best luminous efficiency was found to be 3.65 cd/A with CIE
coordinates of (0.163, 0.260) in electroluminescence devices.
Keywords: Truxene, Blue Emitter, Solution Processed OLEDs, Electroluminescence, Brightness.
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1. INTRODUCTION
arene stacking behavior leading to amorphous nature,
which is much desirable in thin film devices. It has
also been reported that trialkylated truxenes show self
association property which could be useful for device
applications.11
The development of economically viable large-area flat
panel displays depends largely on the technologies that
would replace the traditional vacuum deposition tech-
niques which are now the industry standard for fabrication
of such devices. The past decade has seen many researches
on solution-processable devices as possible ways to sub-
stitute traditional techniques.1–3 Solution processability is
an old concept in organic devices and its reemergence
owes largely to the development of bilayer devices and the
developments there after. In recent years there have been
a number of studies aimed at investigating the structure-
property relationship regarding truxene derivatives.4–7
This could be regarded as a logical extension of the
studies that were carried out in fluorenes and their deriva-
tives owing to efficient blue emitting properties.8–10
Fluorene and its derivatives are among the most promis-
ing candidates for blue electroluminescent (EL) materi-
als because of their high photoluminescence efficiency,
good charge transport and easy tailorability of proper-
ties. Truxene and its derivatives provide properties simi-
lar to fluorene molecules on the two dimensional domain
and show many interesting attributes from their own
properties. For example, the two dimensional structure
of the truxene derivatives enables them to have a good
The optical properties of blue truxene derivatives
can be tuned by incorporating oligothiophene and
polyphenylene motifs to their peripheral position.8ꢀ12
Fluorene-functionalized truxene derivatives were found to
be thermally and electro-chemically stable towards both p
and n doping.9 These excellent characteristics for truxene-
type molecules make them good candidates for blue
organic light emitting materials.
In this paper, we report the detailed synthetic method,
characterization and optical properties including EL effi-
ciency of new type of truxene derivative as a promising
candidate for blue-light emitting devices.
2. EXPERIMENTAL DETAILS
2.1. Materials
All the starting materials were purchased from Aldrich Co.,
LTD. and used without further purification unless other-
wise noted. 2,7,12-Tribromo-5,5ꢀ,10,10ꢀ,15,15ꢀ-hexahexyl-
truxene (1) and triphenylamine boronic acid (2) were
prepared according to literature procedures.13ꢀ14
∗Authors to whom correspondence should be addressed.
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J. Nanosci. Nanotechnol. 2010, Vol. 10, No. 10
1533-4880/2010/10/6916/004
doi:10.1166/jnn.2010.2963

Choi et al.
Synthesis and Properties of a Solution-Processable Truxene Derivative for OLED Devices
2.1.1. Tr1
emitting layer under vacuum below 5 × 10−7 torr. The
OLED devices were encapsulated with a glass lid by using
Compound 1 (6.0 g, 5.5 mmol), compound 2 (9.2 g, 25.0
mmol) and Pd(PPh3ꢁ4 (1.2 g, 1.1 mmol) were taken in a
flask, degassed and added anhydrous toluene (200 mL).
The mixture was stirred at room temperature under N2 for
30 min. 2 M Na2CO3 aqueous solution was added and
a UV curable epoxy resin. Current-voltage-luminescence
(I–V –L) characteristics were obtained with a Keithley 238
source-measure unit and a PR650 spectrophotometer.
ꢁ
stirred at 80 C for 96 hrs. The mixture was cooled and
3. RESULTS AND DISCUSSION
extracted with CH2Cl2 and aqueous ammonium chloride.
The organic extracts were dried over anhydrous MgSO4
and the solvent removed under reduced pressure. The mix-
ture was purified by column chromatography on silica gel
with petroleum ether: CH2Cl2 (3:1) to afford Tr1 (3.1 g,
The structure and synthetic route leading to TR1 are sum-
marized in Scheme 1. The preparation of TR1 involves
multistep synthesis including 3-fold Suzuki coupling reac-
tions. Specifically, tribrominated truxene core 1 was
coupled with 3 equiv. of boronic acid pinacol ester triph-
enylamine 2 in the presence of Pd(0) and aqueous Na2CO3
to afford TR1. It is readily soluble in common organic sol-
vents as desired. Triphenylamine-based truxene molecules
with a stilbene linkage were reported by Xia et al.10
Although their photophysical properties indicated that the
materials are good candidates for the OLED applications,
they can be thermally unstable at high temperatures due
to the degradation of double bonds. Thus, we designed the
TR1 molecule without double bonds. Thermal properties
of TR1 were investigated by thermogravimetric analysis
(TGA) and differential scanning calorimetry (DSC). The
DSC analysis did not yield the glass transition tempera-
1
36%). H NMR (300 MHz, CDCl3, ppm) ꢂ 8.41 (d, 1H,
J = 8ꢃ1 Hz), 7.62 (d, 2H, J = 6ꢃ0 Hz), 7.31–7.02 (m,
14H), 3.05–2.95 (m, 2H), 2.18–2.09 (m, 2H), 0.93–0.87
(m, 14H), 0.70–0.44 (m, 10H); MALDI-TOF m/z: 1577.3
(M+ꢁ, 1493.1 ([M-C6H13]); Anal. Calcd. for C117H129N3
(1577.3): C, 89.09; H, 8.24; N, 2.66. Found: C, 89.09; H,
8.24; N, 2.66.
2.2. General Instrumentation
1H NMR spectra were recorded on a Varian Oxford 300
NMR Spectroscopy. Chemical shifts were reported in ppm
relative to trimethylsilane. Mass Spectra were carried out
on a Voyager DE™ STR Workstation/Mass Spectrome-
ter. Elemental analysis was performed by SNU Basic Sci-
ence Center. UV/Vis absorption spectra were recorded on
a Perkin Elmer Lambda 14 Spectrophotometer. PL spec-
tra were recorded on an Edinburgh FS920 Steady-State
Fluorometer with 450W Xe-lamp. Fluorescence quan-
tum yields were measured by relative actinometry using
quinine sulfate (ꢄf = 0ꢃ546 in 0.05 M H2SO4ꢁ as an
actinometer.15 Thermal properties were measured by TA
Instruments DSC 2091 Analyzer and Shimadzu TGA 50
Thermogravimetricꢁ analyzer under N2 atmosphere at a
heating rate of 10 C/min. The onset degradation temper-
ature (Tdꢁ was measured where 5% weight loss occurs.
ture (T ꢁ and the melting temperature (T ꢁ, showing that
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the material possesses amorphous nature. As we expected,
ꢁ
Copyright: American Scientific Publishers
the decomposition temperature (Tdꢁ was found at 395 C
indicative of good thermal stability of the material.
The absorption and photoluminescence spectra were
measured in THF solution (Fig. 1). The results are also
reported in Table I. The absorption trace of TR1 exhibits
a main absorption band at wavelength maximum 358 nm.
The main absorption band arises due to ꢅ–ꢅ∗ transi-
tion from conjugated truxene backbones. Compared to the
truxene part that absorbs at around 310 nm, the absorp-
tion band of TR1 is significantly red-shifted indicating
that the conjugation is very well localized throughout the
molecule. The emission spectrum of TR1 is dominated by
a strong and broad fluorescence band centered at 415 nm.
The fluorescence quantum efficiency is 0.46. The absorp-
tion and emission spectra in solid states are almost similar
to those observed in solution. The absorption and emission
2.3. Fabrication of OLED Devices
Poly(3,4-ethylene dioxythiophene) (PEDOT) doped with
polystyrene sulfonate (PSS) (Baytron P VP AI 4083,
H. C. Starck GmbH.) was spin-coated onto indium-tin-
ꢁ
oxide (ITO: work function 4.9 eV) and baked at 200 C
in N2 atmosphere for 5 min. TR1 dissolved in a mixed
solvent of toluene and cyclohexanone (8:2 v/v) as an
active blue emitting layer was spin-coated on the top of
a PEDOT/PSS layer. A blue emitting dopant, 4,4-bis(2,2-
diphenylvinyl)-1,1-biphenyl (DPAVBi), can be doped into
TR1 host.16 After baking of the film at 100 ꢁC for
15 min, aluminum (III) bis(2-methyl-8-quninolinato)-4-
phenylphenolate (BAlq) was deposited as an electron
transporting and hole blocking layer. 1 nm of LiF and
200 nm of Al cathode layers were then deposited on the
N
N
Br
Br
O
B
N
3
+
O
Pd(PPh3)4
Na2CO3
Toluene
Br
1
2
TR1
N
Scheme 1. Chemical structure and synthesis of TR1.
J. Nanosci. Nanotechnol. 10, 6916–6919, 2010
6917

Synthesis and Properties of a Solution-Processable Truxene Derivative for OLED Devices
Choi et al.
2.25 eV
TR1
2.8 eV
LiF (1nm) / Al (200 nm)
BAlq (20 nm)
LiF/Al
2.7 eV
0%, 1%, 3% DPAVBi
in TR1 (50 nm)
DPAVBi
BAlq
PEDOT
5.2 eV
PEDOT : PSS
5.3 eV
ITO
5.28 eV
Substrate
6.5 eV
Fig. 2. An OLED device structure and HOMO and LUMO energy level
diagram of the materials.
300
350
400
450
500
550
600
Wavelength (nm)
respectively. Turn-on voltage of all the devices was below
5 V. Among the series, the device with neat TR1 showed
best I–V response followed by 3% of DPVBi in the
TR1 system. Maximum current density from neat TR1
was 167 mA/cm2(@10 V) with the turn on voltage of
4.0 V. 1% of DPVBi:TR1 exhibited highest brightness
over 6000 cd/m2.
Fig. 1. Absorption (solid) and photoluminescence (dash) spectra of
TR1 in THF solution.
maximum value are about 370 nm and 465 nm, respec-
tively. Electrochemical measurements were carried out to
calculate the frontier orbital energies of TR1. The results
are listed in Table I. The HOMO and LUMO levels of
TR1 are calculated to be −5.28 and −2.25 eV, respec-
tively. Thus, the band gap is found to be 3.03.
Figure 3(d) has the luminous efficiency versus applied
voltage characteristics for the three optimized OLED
A series of OLED devices were fabricated by varying
the doping concentration. The emitting layer was prepared
by solution-processing as described in the experimental
(a)
(b)
180
TR1 (neat)
1% DPAVBi:TR1
3% DPAVBi:TR1
TR1
1% DPAVBi:TR1
3% DPAVBi:TR1
1.0
150
120
90
60
30
0
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section. A dopant DPAVBi was added into host TR1 with
0, 1, and 3%. The DPAVBi is a well known material for
IP: 146.185.205.121 On: Tue, 07 0J.u6 n 2016 16:25:21
Copyright: A1m7ꢀe18rican Scientific Publishers
blue emitting EML as well as a fluorescent dopant.
In
addition, BAlq was used for the electron transporting and
hole blocking layer. Kwong et al. reported that it had excel-
lent hole blocking characteristics in small molecule phos-
phorescent devices.18 As shown in Figure 2, the devices
were constructed with ITO/PEDOT:PSS/TR1:DPAVBi
(50 nm)/BAlq (20 nm)/LiF (1 nm)/Al (200 nm). The energy
levels of the materials were well matched for OLED device
operation.
0.4
0.2
0.0
400
500
600
0
2
4
6
8
10
Wavelength/nm
Voltage(V)
(c)
(d)
TR1
1% DPAVBi:TR1
3% DPAVBi:TR1
TR1
6000
5000
4000
3000
2000
1000
0
4
3
2
1
0
1% DPAVBi:TR1
3% DPAVBi:TR1
Figure 3(a) displays the electroluminescence spectra
obtained from TR1 devices with 0, 1 and 3% of DPAVBi.
In the neat TR1, a strong emission was observed at
482 nm. However in the presence of a dopant, the spec-
trum was shifted to blue wavelength with a sharper band
which seems to increase at higher doping levels. The elec-
troluminescence spectra clearly show that TR1 is in the
blue emitting region and addition of DPAVBi increases
the blue properties. Figures 3(b) and (c) exhibit the
I–V characteristics and brightness of the OLED devices,
4
0
2
6
8
10
0
2
4
6
8
10
Voltage (V)
Voltage(V)
Fig. 3. (a) Electroluminescence spectra, (b) I–V curves, (c) L–V
curves, and (d) luminance efficiency of OLED devices based on TR1
with various dopant ratios.
Table I. Optical and electrochemical properties of TR1.
Table II. Electroluminescence properties of TR1.
ꢆabs
Sample (nm)
ꢆPL
(nm) (%)
ꢄfc
HOMO LUMO Band gap
ꢇb
(eV)
(eV)
(eV)
Turn on
voltage (V)
CIE(x,y)
@100nit
Max. efficiency
(cd/A)
Host
Dopant
TR1a
358
16.9
415 0.46
5.28
2.25
3.03
TR1
0% (neat)
1% DPAVBi
3% DPAVBi
4.0
4.5
4.0
(0.193, 0.312)
(0.169, 0.260)
(0.163, 0.260)
2.65
3.44
3.65
aAll the measurements were taken in THF solution. bMolar extinction coefficient
(×104 M−1 cm−1ꢁ. cFluorescence quantum yield measured relative to quinine sul-
fate (ꢄf = 0ꢃ546 in 0.05 M H2SO4ꢁ.
6918
J. Nanosci. Nanotechnol. 10, 6916–6919, 2010

Choi et al.
Synthesis and Properties of a Solution-Processable Truxene Derivative for OLED Devices
devices. The neat TR1 device had a lower turn-on voltage
for light emission, which is consistent with the current
density characteristics. However, maximum efficiency of
the TR1 device showed 2.65 cd/A of the lower value
than that of the 1% and 3% doped DPAVBi in the
TR1 host. Table II summarizes the I–V –L characteris-
tics of the devices. The luminous efficiency of the devices
varies depending on the doping concentration. The CIE
coordinates of (0.163, 0.260) reveals that TR1:DPAVBi
blending system can be used as blue emitting materials.
References and Notes
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4. CONCLUSION
A truxene derivative, TR1, having strong electron donat-
ing group at the periphery of the truxene core was
synthesized as a promising candidate for blue emit-
ting OLED devices. The UV-vis absorption and emis-
sion maxima for TR1 were observed at 358 nm and
415 nm, respectively. It shows good solubility in com-
mon organic solvents, which enable solution casting of the
material on OLED devices. The structure of the device
was constructed with ITO/PEDOT:PSS/TR1:DPAVBi
(50 nm)/BAlq (20 nm)/LiF (1 nm)/Al (200 nm). The EL
band of TR1 was observed at around 482 nm, exhibiting
the blue emitting properties. The best EL efficiency of the
devices obtained by TR1 was found to be 3.65 cd/A. The
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CIE coordinates of the corresponding devices were (0.163,
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0.260).
Copyright: American Scientific Publishers
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Acknowledgment: This work was mainly supported by
the Samsung Research Grant. One of authors (Kwang-Sup
Lee) gives thanks to the Active Polymer Center for Pat-
tern Integration (NRF ERC 2009-0063224) and the Basic
Science Research Fund of Hannam University.
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Received: 31 December 2008. Accepted: 30 September 2009.
J. Nanosci. Nanotechnol. 10, 6916–6919, 2010
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