Synthesis of oligomers having a pendant dithienosilole unit and their applications to EL device materials

Article abstract of DOI:10.1016/j.jorganchem.2004.09.039

Oligomers bearing dithienosilole as a pendent were prepared by radical polymerization of the corresponding dithienosilole monomer using AIBN as an initiator. The resulting oligomers exhibited high solubility in common organic solvents and strong photofluorescences (Φ = 0.72-0.76) in THF which are a little higher than that of monomeric dithienosilole DTS(TMS)₂. Applications of the oligomers to EL devices as hole transporting materials were examined and the maximum electroluminance of 2140 cd/m² was obtained from a device composed of ITO/oligomer/Alq₃/Mg-Ag.

Full text of DOI:10.1016/j.jorganchem.2004.09.039

Journal of Organometallic Chemistry 690 (2005) 333–337
www.elsevier.com/locate/jorganchem
Synthesis of oligomers having a pendant dithienosilole unit and
their applications to EL device materials
a,
a,
a
b
Kwang-Hoi Lee *, Joji Ohshita *, Keisuke Kimura , Yoshihito Kunugi ,
a,
*
Atsutaka Kunai
a
Department of Applied Chemistry, Graduate School of Engineering, Hiroshima University, 1-4-1 Kagamiyma, Higashi-Hiroshima 739-8527, Japan
b
Division of Materials Science, Faculty of Integrated Arts and Science, Hiroshima University, Higashi-Hiroshima 729-8521, Japan
Received 2 August 2004; accepted 16 September 2004
Abstract
Oligomers bearing dithienosilole as a pendent were prepared by radical polymerization of the corresponding dithienosilole mono-
mer using AIBN as an initiator. The resulting oligomers exhibited high solubility in common organic solvents and strong photoflu-
orescences (U = 0.72–0.76) in THF which are a little higher than that of monomeric dithienosilole DTS(TMS)2. Applications of the
oligomers to EL devices as hole transporting materials were examined and the maximum electroluminance of 2140 cd/m2 was
obtained from a device composed of ITO/oligomer/Alq3/Mg–Ag.
ꢀ 2004 Elsevier B.V. All rights reserved.
Keywords: Dithienosilole; Electroluminescence; Polyethylene; Radical polymerization
1. Introduction
approach to develop soluble polymeric EL device mate-
rials is obviously the construction of a flexible backbone
bearing p-conjugated functional groups like poly(vinyl
carbazole) that is known as a common hole-transporting
material for EL devices [2].
Since Burroughes et al. [1] introduced poly(phenylene
vinylene) (PPV) to organic light emitting diodes
(OLEDs), various p-conjugated polymers have been
developed and applied to electroluminescent (EL) device
materials. An advantage of using polymeric materials in
EL devices may arise from their readily processability
for the preparation of uniform, large area, and flexible
thin films by spin- or cast-coating. In addition, better
morphological stability of the amorphous phase is often
noted for polymeric materials as compared with mono-
meric compounds. However, p-conjugated polymer usu-
ally exhibited poor solubility in organic solvent unless
long side chains are introduced to the polymers. This
is due to low flexibility of their backbone. An alternative
We have synthesized dithienosilole (DTS, Chart 1) as
a novel p-conjugated moiety, in which b,b0-positions of
2,20-bithiophene are intramolecularly linked by a silyl-
ene bridge. The DTS derivatives have been studied as
either electron or hole transporting materials in EL de-
vices [3]. For example, when DTS substituted with elec-
tron deficient pyridyl groups (DTS(Pyridyl)2) was
employed in an EL device with the structure of ITO/
TPD/Alq3/DTS(Pyridyl)2/Mg–Ag, where TPD, Alq3,
and DTS(Pyridyl)2 are hole transporting, emissive, and
electron transporting materials, respectively, the maxi-
mum luminance reached 16 000 cd/m2. It was also found
that polymers containing DTS and organosilanylene
units in the backbone as repeating components worked
as efficient hole transporting materials [4].
*
Corresponding authors. Tel.: +81 82 482 7743; fax: +81 82 424
5494.
E-mail address: akunai@hiroshima-u.ac.jp (A. Kunai).
0022-328X/$ - see front matter ꢀ 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2004.09.039

334
K.-H. Lee et al. / Journal of Organometallic Chemistry 690 (2005) 333–337
spectra of the oligomers displayed generally broad peaks
for aromatic and aliphatic groups probably due to the
effects of end-groups and diastereomeric environment
1
of the backbone, H NMR integration ratios and the
Ph
S
Ph
S
DTS R = H
Si
DTS(TMS)2 R = SiMe3
DTS(SMe)2 R = SMe
R
R
data of combustion elemental analysis well agree with
the proposed structures shown in Scheme 1.
Chart 1.
In this paper, we report the synthesis of novel oligo-
2.2. Optical and electrochemical properties of oligo(vinyl-
dithienosilole)s
mers having a DTS unit as a pendent, which show high
solubility in common organic solvents. The optical and
electrochemical properties of the resulting oligomers
and their applications to hole transporting materials in
double-layer EL devices are described.
Some properties of the oligomers are summarized in
Table 1. Molecular weights (Mw) of the oligomers deter-
mined by GPC were 4500 (Mw/Mn = 1.12) for oligomer
1 and 3700 (Mw/Mn = 1.07) for oligomer 2, respectively.
Both oligomers showed good solubility in common or-
ganic solvents such as THF, chloroform, ether, and ben-
zene. The UV absorption (356 nm) and emission kmax
(425 nm) of oligomer 1 in THF are coincident with those
of monomeric DTS(TMS)2 (Chart 1). The kmax of
absorption and emission for oligomer 2 shifted to higher
energy regions only slightly by 3 and 5 nm from those of
oligomer 1 and DTS(TMS)2, respectively, indicating
that substituents on silole silicon did not exert consider-
able influence on the electronic states of the DTS-con-
taining oligomers (see Fig. 1).
Both oligomers showed strong fluorescence in THF
when irradiated with a UV lamp. Fluorescence quan-
tum yields of the oligomers in solution phase were cal-
culated using a 9,10-diphenylnaphthalene solution as a
standard to be 0.76 for oligomer 1 and 0.72 for oligo-
mer 2, respectively, which are a little higher than that
of DTS(TMS)2 (U = 0.69) under the same conditions
(see Table 1).
2. Results and discussion
2.1. Preparation ofDTS-substituted oligoethylenederivatives
Scheme 1 shows the synthetic route to oligomers 1
and 2. The monomers, vinyl-DTSs 4 and 5 were pre-
pared in 24% and 80% yields, respectively, from the
reactions between 3,30-dilithio-5,50-bis(trimethylsilyl)-
2,20-bithiophene with the corresponding dichlorosilanes,
dichlorophenyl(p-vinylphenyl)silane and dichlorometh-
ylvinylsilane. Oligomerization of vinyl monomers 4
and 5 was carried out in chlorobenzene using AIBN as
a radical initiator at 90 ꢁC. The terminal point of the
polymerization was determined by disappearance of
1
the monomers via H NMR analysis or thin layer chro-
matography (TLC). The resulting mixtures were repre-
cipitated from ethanol for oligomer 1 and methanol
for oligomer 2 to afford the corresponding oligomers
in 60% and 26% yields, respectively. The lower yield of
oligomer 2 would result from the high solubility of lower
molecular weight products even in methanol. Character-
ization of the resulting oligomers was carried out by the
Electrochemical properties of oligomers were also
studied by cyclic voltammetry (CV). For both of the
oligomers, the first CV cycle revealed a sharp, irreversi-
ble oxidation peak in an anodic region, but the peak dis-
appeared in the second cycle, showing that the oligomer
1
1H NMR and elemental analysis. Although H NMR
R
X
Br
X
Si
S
2 n-BuLi
Cl2SiR
SiMe3
Me3Si
Me3Si
SiMe3
S
S
S
Br
4 R = Ph, X = p-phenylene (24%)
5 R = Me, X = none (80%)
3
R
X
n
Si
AIBN
Me3Si
SiMe3
S
S
1 R = Ph, X = p-phenylene (60%)
2 R = Me, X = none (26%)
Scheme 1.

K.-H. Lee et al. / Journal of Organometallic Chemistry 690 (2005) 333–337
335
Table 1
Properties of oligomers 1 and 2
Peak potentialc/V
600
400
200
0
Oligomer Molecular
weighta
Fluorescence
quantum yield (U)b vs. Ag/Ag+
Oligomer 1
Oligomer 2
Mw Mw/Mn
4500 1.12
3700 1.07
1
2
0.76
0.72
0.96
0.90
a
b
c
Determined by GPC, relative to polystyrene standards.
Measured in THF using 9,10-diphenylanthracene as a standard.
First oxidative potential.
10
15
20
Applied voltage (V)
(a)
2500
2000
1500
1000
500
Oligomer 1
Oligomer 2
Oligomer 1
Oligomer 2
0
5
10
15
20
(b)
Applied voltage (V)
300
400
500
Wavelength (nm)
Fig. 2. I–V (a) and L–V (b) plots of oligomers 1 and 2 in the device of
ITO/oligomer/Alq3/Mg–Ag.
Fig. 1. UV and emission spectra of oligomers 1and 2 in THF.
(MeS) groups at the a,a0-positions (DTS(SMe)2 in Chart
1) (maximum luminance = 480 cd/m2) [5]. The external
EL efficiencies of the EL devices were 0.10% for oligo-
mer 1 at 18 V, and 0.04% for oligomer 2 at 17 V, respec-
tively. The EL kmax of the devices with oligomer 1 or 2
were coincident with that of the Alq3 emission (Fig.
3). The single layer EL devices having the present oligo-
mers gave no detectable luminance at all.
films decomposed after the first scan. The first oxidation
potential of oligomer 1 is positively shifted from that of
oligomer 2 but only slightly, again indicating similar
electronic states of the oligomers.
2.3. Application of oligo(vinyldithienosilole)s to EL devices
The performance of double layer EL devices was
demonstrated employing spin-coated films of oligomers
1 and 2 as the hole transporting materials. The plots of
current density–voltage (I–V) and luminance–voltage
(L–V) of the devices with the structure of ITO/oligo-
mer/Alq3/Mg–Ag are summarized in Fig. 2. The maxi-
mum current densities of the devices having oligomers
1 and 2 are 640 mA/cm2 at 20.5 V and 580 mA/cm2 at
21.5 V, respectively. The device with oligomer 2 showed
a little lower EL efficiency than that with oligomer 1, in
respect to the current density. In contrast, the luminance
of the device having oligomer 1 reached 2140 cd/m2 at
18.5 V, which is much stronger than that of the device
having oligomer 2 (500 cd/m2 at 17 V). The lower max-
imum luminance for the device with oligomer 2 substi-
tuted by methyl group on silole silicon would be due
to the low thermal stability of the oligomer film toward
the Joule heat caused by an applied voltage rather than
the carrier mobility of the oligomer film. The maximum
luminances of the EL devices with oligomer 1 or 2 as a
hole transporting layer were larger than that of the de-
vice with monomeric DTS substituted with methylthio
In conclusion, we prepared highly soluble novel
oligomers having DTS as a pendent. The resulting olig-
omers exhibited strong photofluorescences (U =
0.72–0.76) in THF. A strong luminance (2140 cd/m2)
was observed from the EL device with oligomer 1 as a
hole transporting material.
Oligomer 1
Oligomer 2
400
500
600
700
Wavelength/nm
Fig. 3. EL spectra of the devices having oligomer 1 at 18 V or 2 at 17 V
as a hole transporting layer.

336
K.-H. Lee et al. / Journal of Organometallic Chemistry 690 (2005) 333–337
3. Experimental
3.1. General
0.33 (s, 18H, –SiMe3), 5.27 (d, 1H, J = 10.8 Hz, vinyl
proton), 5.77 (d, 1H, J = 17.6 Hz, vinyl proton), 6.70
(dd, 1H, J = 10.8 and 17.6 Hz, vinyl proton), 7.28 (s,
2H, thiophene protons), 7.35–7.44 (m, 5H, m- and p-
Ph, and phenylene protons), 7.60 (d, 2H, J = 8.0 Hz,
phenylene protons), 7.65 (dd, 2H, J = 1.5 and 7.3 Hz,
o-Ph); 13C NMR (d in CDCl3) 0.12, 114.87, 125.87,
128.12, 130.20, 131.66, 132.15, 135.45, 135.72, 136.44,
136.63, 139.29, 141.68, 142.24, 155.56; 29Si NMR (in
CDCl3) ꢀ6.70, ꢀ27.20. Anal. Calc. for C28H32S2Si3: C,
65.06; H, 6.24. Found: C, 65.10; H, 6.11%.
All reactions were carried out under a dry nitrogen
atmosphere. THF and ether were distilled from so-
dium–potassium alloy before use. Acetonitrile was dried
over P2O5. NMR spectra were recorded on JEOL Model
JNM-EX 270 and JEOL Model JNM-LA 400 spectrom-
eters. Mass spectra were measured on a Hitachi M80B
spectrometer. UV–Vis spectra were measured with a
Hitachi U-3210 spectrophotometer, and emission spec-
tra were recorded on a Shimadzu RF5000 spectropho-
tometer. IR spectra were measured with a Shimadzu
FTIR Model 8700 spectrometer.
3.4. Preparation of 5
Compound 5 was prepared as described for 4 by
using dichloromethylvinylsilane as a reactant: m.p. 61–
1
3.2. Preparation of dichlorophenyl(p-vinylphenyl)silane
63 ꢁC; MS m/z 378 (M+); H NMR (d in CDCl3) 0.32
(s, 18H, –SiMe3), 0.50 (s, 3H –SiMe), 5.95 (s, 1H,
J = 4.4 and 19.3 Hz, vinyl proton), 6.11 (1H, J = 4.4
and 14.4 Hz, vinyl proton), 6.20 (1H, J = 14.4 and
19.3 Hz, vinyl proton), 7.16 (s, 2H, thiophene protons);
13C NMR (d in CDCl3) ꢀ5.65, 0.10, 133.68, 135.27,
136.35, 141.64, 143.05, 154.97; 29Si NMR (d in CDCl3)
ꢀ6.87, 19.80. Anal. Calc. for C17H26S2Si3: C, 53.91; H,
6.92. Found: C, 53.92; H, 6.82%.
To a mixture of 1.05 g (43.20 mmol) of magnesium in
15 mL of THF was added 5.78 g (41.70 mmol) of 4-chlo-
rostyrene slowly at room temperature. After refluxed for
3 h the resulting solution containing the Grignard rea-
gent was transferred to a solution of 8.82 g (41.70 mmol)
of PhSiCl3 in 30 mL of ether at 0 ꢁC. After the mixture
was stirred overnight at room temperature, 100 mL of
dry n-hexane was added to the mixture, then the mixture
was filtered to remove MgCl2. After organic solvents
were removed with rotary evaporation, the residue was
distillated to give 47.60 g (41%) of dichlorosilane as col-
3.5. Preparation of oligomer 1
To a solution of 0.100 g (0.193 mmol) of 4 in 0.10 mL
of chlorobenzene was added 8.70 mg (0.053 mmol) of
AIBN. After stirred at 90 ꢁC for 20 h, the resulting mix-
ture was reprecipitated twice from ethanol to give 60 mg
(60%) of 1 as gray powder; m.p. 190–194 ꢁC; 1H NMR (d
in CDCl3) 0.18 (s, 18H, –SiMe3), 0.43–1.80 (br, 3H, ali-
phatic protons), 6.28–7.73 (br, 11H, thiophene, phenyl,
and phenylene protons). Anal. Calc. for (C28H32S2Si3)n:
C, 65.06; H, 6.24. Found: C, 64.62; H, 6.46%.
1
orless liquid. MS m/z 278 (M+); H NMR (d in CDCl3)
5.36 (d, 1H, J = 10.8 Hz, vinyl proton), 5.85 (d, 1H,
J = 17.5 Hz, vinyl proton), 6.74 (dd, 1H, J = 10.8 and
17.5 Hz, vinyl proton), 7.44–7.53 (m, 5H, m- and p-Ph,
and phenylene protons), 7.70–7.75 (m, 4H, o-Ph and
phenylene protons); 13C NMR (d in CDCl3) 116.18,
126.03, 128.33, 131.11, 131.75, 131.98, 134.05, 134.39,
136.16, 140.77 [6].
3.3. Preparation of 4
3.6. Preparation of oligomer 2
To a solution of 3,30-dilithio-5,50-bis(trimethylsilyl)-
2,20-bithiophene prepared from the reaction of 1.00 g
(2.13 mmol) of 3 and 2.70 mL (2.27 mmol) of a 1.58 M
n-butyllithium/hexane solution in 15 mL of ether at
ꢀ80 ꢁC was added 0.59 g (2.13 mmol) of dichlorophe-
nyl(p-vinylphenyl)silane at the same temperature. After
the reaction temperature was raised to room tempera-
ture, 10 mL of THF was added. The reaction mixture
was heated to reflux for 3 h, then hydrolyzed with water.
The organic layer was separated and the aqueous layer
was extracted with ether. The organic extracts were com-
bined and dried with MgSO4. The solvent was removed
with rotary evaporation, and the reside was chromato-
graphed on a silica gel column with n-hexane as an eluent
to afford 0.26 g (24%) of 4 as pale yellow powder: m.p.
Oligomer 2 was synthesized by the same method as
described for oligomer 1 (26% yield): gray powder;
1
m.p. 212–217 ꢁC; H NMR (d in CDCl3) ꢀ0.60–0.43
(br, 21H, –SiMe and –SiMe3), 0.59–1.65 (br, 3H, ali-
phatic protons), 6.55–7.35 (br, 2H, thiophene protons).
Anal. Calc. for (C17H25S2Si3)n: C, 53.91; H, 6.92.
Found: C, 53.94; H, 6.98%.
3.7. CV measurements
The CV measurements were carried out in acetonit-
rile using Ag/Ag+ as a reference electrode, an oligo-
mer-coated ITO glass as a working electrode, and a Pt
wire as the counter electrode, respectively. The cur-
rent–voltage curve was recorded at room temperature
on a Hokuto Denko HAB-151 potentiostat/galvanostat.
1
72–75 ꢁC; MS m/z 516 (M+); H NMR (d in CDCl3)

K.-H. Lee et al. / Journal of Organometallic Chemistry 690 (2005) 333–337
337
3.8. Fabrication of EL devices
References
[1] J.H. Burroughes, D.D.C. Bradley, A.N. Marks, K. Mackay, R.H.
Friend, P.L. Burns, A.B. Holmes, Nature 347 (1990) 539.
[2] (a) W. Zhu, H. Tian, A. Elschner, Chem. Lett. (1999) 501;
(b) S. Shirai, J. Kido, Chem. Lett. (2002) 386.
An oligomer solution (3.6 g/L in benzene) was spin-
coated on an ITO electrode at the rate of 3000 rpm to
give the film thickness of 30 nm, and then Alq3 (an emiss-
ive layer) and Mg–Ag (a cathode) were deposited at
1 · 10ꢀ5 Torr on the spin-coated oligomer layer step by
step with the thicknesses of 60 and 170 nm respectively.
[3] (a) J. Ohshita, M. Nodono, H. Kai, T. Watanabe, A. Kunai, M.
Shiotani, A. Adachi, K. Okita, Y. Harima, K. Yamashita, M.
Ishikawa, Organometallics 18 (1999) 1453;
(b) J. Ohshita, H. Kai, A. Takata, T. Iida, A. Kunai, N. Ohta, K.
Komaguchi, M. Shiotani, A. Adachi, K. Sakamaki, K. Okita,
Organometallics 20 (2001) 4800.
Acknowledgements
[4] (a) J. Ohshita, M. Nodono, A. Takata, H. Kai, A. Adachi, K.
Sakamaki, K. Okita, A. Kunai, Macromol. Chem. Phys. 201
(2000) 851;
This work was supported by a Grant-in-Aid Scientific
Research (B) (No. 16350102) from the Ministry of Edu-
cation, Science, Sports, and Culture of Japan. We thank
Sankyo Kasei Co. Ltd, and Tokuyama Co. Ltd. for
financial support.
(b) J. Ohshita, T. Sumida, A. Kunai, Macromolecules 33 (2000)
8890.
[5] K.-H. Lee, J. Ohshita, A. Kunai, Organometallics (in press).
[6] E.A. Chernyshev, T.L. Krasnova, S.A. Petrukhina, Zhurnal
Prikladnoi Khimii 40 (1967) 1871, CA 67:116912.