Electroluminescent properties of novel fluorene derivatives with aromatic amine moieties

Article abstract of DOI:10.1080/15421400500365292

2,7-Bis[(4-diphenylamino)phenylvinylene]-9,9-diethylfluorene(BDPDF, 1) and 2,7-bis[(9-ethyl-3-carbazolylvinylene)-9,9-diethylfluorene(BECDF, 2) have been synthesized and characterized. To evaluate electroluminescent performance, compound 1 and 2 were used

Full text of DOI:10.1080/15421400500365292

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Electroluminescent Properties
of Novel Fluorene Derivatives
with Aromatic Amine Moieties
Kwang-Yol Kay a , Jung-Hoon Kim a , Sun-Ho Kim b
Ho-Cheol Park b & Jong-Wook Park c
,
a Department of Molecular Science and Technology ,
Ajou University , Suwon, Korea
b Electronic Device Division , Vistorm, Seongnam,
Korea
c Department of Chemistry, Center for
Nanotechnology Research , The Catholic University ,
Pucheon, Korea
Published online: 20 Aug 2006.
To cite this article: Kwang-Yol Kay , Jung-Hoon Kim , Sun-Ho Kim , Ho-Cheol Park &
Jong-Wook Park (2006) Electroluminescent Properties of Novel Fluorene Derivatives
with Aromatic Amine Moieties, Molecular Crystals and Liquid Crystals, 444:1, 121-128,
DOI: 10.1080/15421400500365292
To link to this article: http://dx.doi.org/10.1080/15421400500365292
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Mol. Cryst. Liq. Cryst., Vol. 444, pp. 121–128, 2006
Copyright # Taylor & Francis LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400500365292
Electroluminescent Properties of Novel Fluorene
Derivatives with Aromatic Amine Moieties
Kwang-Yol Kay
Jung-Hoon Kim
Department of Molecular Science and Technology, Ajou University,
Suwon, Korea
Sun-Ho Kim
Ho-Cheol Park
Electronic Device Division, Vistorm, Seongnam, Korea
Jong-Wook Park
Department of Chemistry, Center for Nanotechnology Research,
The Catholic University, Pucheon, Korea
2,7-Bis[(4-diphenylamino)phenylvinylene]-9,9-diethylfluorene(BDPDF, 1) and
2,7-bis[(9-ethyl-3-carbazolylvinylene)-9,9-diethylfluorene(BECDF, 2) have been
synthesized and characterized. To evaluate electroluminescent performance, com-
pound 1 and 2 were used as emitting materials and dopant materials, respectively
and the basic device structure was ITO=m-MTDATA=a-NPD=1 or 2 or doped
DPVBi with 1 or 2=Alq3=LiF=Al.
The devices using BDPDF(1) and BECDF(2) show EL peak at 470 nm and 486 nm.
The device showed a blue light-emission with the CIE chromaticity coordinates of
(0.177, 0.268).
Keywords: aromatic amine; doping agent; electroluminescence; fluorene
1. INTRODUCTION
Since the electroluminescent devices based on small organic molecules
[1] were first developed, the search for new electroluminescent materials
The author (K. Y. K.) wishes to acknowledge the financial support from BK21 project
in 2003, and this work was supported by the Medium-range Point Project from Indus-
trial Technolocy Development Fund of the MOCIE, Korea.
Address correspondence to Jong-Wook Park, Department of Chemistry, Center for
Nanotechnology Research, The Catholic University, Pucheon 420-734, Korea. E-mail:
hahapark@catholic.ac.kr
121

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K.-Y. Kay et al.
and the study on the optimization of device structure have expanded
rapidly. In this regard, there has been a considerable interest in
developing efficient and stable organic blue light-emitting device to
achieve full-color display application. Many blue electroluminescent
materials have been synthesized [2–5]. The blue doped emitter in
OLED often necessitates the judicious selection or design of an
appropriate blue host material which has a wide enough band gap
energy and a set of matching LUMO=HOMO level to effect the sen-
sitization. In commercial circles, one of the best blue emitters used in
OLED is believed to have been patented by Idemitsu Kosan Co.
based on the basic structure of distyrylarylene (DSA) host doped with
a hole transporting amine-substituted DSA Dopant [6]. This doped blue
emitter could achieve a luminous efficiency of >6 lm=W at 100 cd=m2
with a lifetime of continuous DC operation of >30,000 h, but blue emit-
ters and dopants still remain the most sought-after material.
In this paper, we report the synthesis and characterization of
new fluorene derivatives with triphenylaminovinylene and carbazolyl-
vinylene moieties, i.e., 2,7-bis[(4-diphenylamino)phenylvinylene]-9,9-
diethylfluorene(BDPDF, 1) and 2,7-bis(9-ethyl-3-carbazolylvinylene)
-9,9-diethylfluorene(BECDF, 2). These two compounds are used as
emitting materials, and then as dopant materials to the host com-
pound, DPVBi in the basic device structure; ITO=m-MTDATA=
a-NPD=1 or 2=Alq3=LiF=Al.
2. EXPERIMENTS
BDPDF(1) and BECDF(2) were synthesized by Witting-Horner
reaction of the precursor phosphonate 8 with aldehydes 9 and 10,
respectively, as described in Scheme 1.
2.1. 2,7-Bis[(4-diphenylamino)phenylvinylene]-9,9-
diethylfluorene(BDPDF, 1)
A solution of 2,7-bis(methyldiethoxyphosphonate)-9,9-diethylfluorene
(1.33 g, 2.50 mmol) and 4-diphenylaminobenzaldehyde (2.40 g,
8.80 mmol) in THF (30 ml) was stirred at room temperature for 3 h,
and then NaH (0.30 g, 12.5 mmol) was added. The reaction mixture
was stirred for 20 h, diluted with water and then extracted with
CH2Cl2. After evaporation of solvent in vacuum, the crude product
was chromatographed on silica gel with CH2Cl2=hexane (1:2) and then
recrystallized from MeOH=CHCl3 (1:75) to give a pure solid product
(0.97 g, 51%).

Electroluminescence, Fluorene, Doping agent, Aromatic Amine
123
SCHEME 1 Synthesis of BDPDF(1) and BECDF(2).
1H-NMR(CDCl3, 400 MHz): d ¼ 7.40 ꢀ 7.00 (m, 38H, Ar-H), 2.07
(q, 4H, CH2), 0.37 (t, 6H, CH3). FT-IR(KBr): 2962, 2917, 1591, 1508,
1492, 1280 cmꢀ1.
2.2. 2,7-Bis(9-ethyl-3-carbazolylvinylene)-9,9-
diethylfluorene(BECDF, 2)
BECDF was synthesized with same procedure as BDPDF’s (Final step
yield: 51%).
1H-NMR(CDCl3, 400 MHz): d ¼ 8.30 ꢀ 7.10 (m, 24H, Ar-H), 4.40
(q, 4H, CH2), 2.10 (q, 4H, CH3), 0.35 (t, 6H, CH3). FT-IR(KBr): 3020,
2962, 1596 cmꢀ1.
3. RESULT AND DISCUSSION
Figures 1 and 2 show the UV-absorption and photoluminescence (PL)
spectra of the two emission layers in thin films on quartz plates. The

124
K.-Y. Kay et al.
maximum absorption of BDPDF(1) appeared at 410 nm, and the
absorption edge of BDPDF(1) at 475 nm. For BECDF(2), the maximum
absorption appeared at 404 nm and the absorption edges at 460 nm.
The energy band gap of BDPDF and BECDF was 2.67 eV and
2.74 eV according to UV absorption edge calculation. The HOMO
energy level of these materials was calculated from the onset potential
of cyclic voltammogram by assuming the absolute energy level of fer-
rocene(FOC) is 4.80 eV [7] and it gave 5.90 eV (BDPDF) and 6.00 eV
(BECDF), respectively.
We measured I-V characteristics and luminance-current-voltage
characteristics of related EL devices. The device structure was ITO
150 nm=m-MTDATA 42 nm=a-NPD 12 nm=BDPDF or BECDF or
doped DPVBi 24 nm=Alq324 nm=LiF 1 nm=Al 200 nm and doped
DPVBi was doped with BDPDF or BECDF.
Figure 3 shows forward-biased current-voltage (I-V) intensity
curves for the undoped organic EL device. These curves were obtained
when the devices were operated in a continuous direct current (dc)
mode under forward bias (positive voltage applied to the ITO contact).
These device exhibit diode characteristics, that is, forward current
increases with increasing forward bias voltage. The turn-on voltage
of these devices is about 4.50 V at a current density of approximately
FIGURE 1 UV-visible (dot line) and photoluminescence (solid line) spectra of
BDPDF thin film on quartz substrate (excitation wavelength 410 nm).

Electroluminescence, Fluorene, Doping agent, Aromatic Amine
125
FIGURE 2 UV-visible (dot line) and photoluminescence (solid line) spectra of
BECDF thin film on quartz substrate (excitation wavelength 404 nm).
FIGURE 3 Current-voltage (I-V) characteristics of undoped EL devices.

126
K.-Y. Kay et al.
FIGURE 4 EL spectra of devices using BDPDF as an emissive layer (dot line)
and doped into DPVBi (solid line).
FIGURE 5 EL spectra of devices using BECDF as an emissive layer (dot line)
and doped into DPVBi (solid line).

Electroluminescence, Fluorene, Doping agent, Aromatic Amine
127
TABLE 1 Luminescent Properties of the Devices Using BDPDF(1) and
BECDF(2) as an Emitting Layer
Efficiency
[l m=W]
@ 120 cd=m2
CIE coordinate
Emitting
material
EL peak
[nm]
Efficiency Voltage [V]
[cd=A]
@ 120 cd=m2
x
y
(1)
470
464
486
451
0.15
0.83
0.56
0.24
0.45
1.91
1.30
0.68
9.6
7.2
7.3
8.6
0.238
0.177
0.222
0.183
0.404
0.268
0.380
0.213
(1)-doped DPVBi
(2)
(2)-doped DPVBi
500 mA= cm2, depending upon the total thickness of the organic EL
devices.
Figures 4 and 5 show the EL spectra of the devices using BDPDF,
BECDF as emitting layers, together with those of the devices using
BDPDF, BECDF as a guest material to DPVBi, respectively. In Figure
4, the EL spectrum of the device using BDPDF as an emitting layer
peaks at 470 nm. As a result, the device shows a greenish blue emis-
sion with the CIE chromaticity coordinates of (0.238, 0.404) and lumi-
nous efficiency of 0.45 cd=A. When the BDPDF doped into DPVBi, the
EL peak of the device is shifted to shorter wavelength, which is at
464 nm, and the device emits a sky blue light with the CIE chroma-
ticity coordinates of (0.177, 0.268). The maximum luminous efficiency
of doped device with BDPDF shows 1.91 cd=A (as shown in Table 1).
In Figure 5, the EL peak of BECDF shows at 486 nm, which is
almost same wavelength as compared to the PL spectrum, as illustrat-
ing in Figure 2. The device also shows a greenish blue emission with
the CIE chromaticity coordinates of (0.222, 0.380) and luminous
efficiency of 1.3 cd=A. However, the BECDF-doped device using DPVBi
as a host material shows lower luminous efficiency compared to doped
device with BDPDF, which is 0.68 cd=A. The EL characteristics of
these blue-emitting devices are summarized in Table 1.
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