Synthesis and electrical properties of novel oligothiophenes partially containing 3,4-ethylenedioxythiophenes

Article abstract of DOI:10.1039/c3ra44129f

Five sorts of soluble oligothiophenes (trimer to undecamer) containing 3,4-ethylenedioxythiophene (EDOT) were synthesized, and their optical and electrochemical properties were investigated in relation to the chain length of oligothiophenes and the number of EDOT units. The introduction of the EDOT unit into a main oligothienylene unit induced a red shift of absorption bands and a negative shift of oxidation potentials. The conductivity of an electrochemically oxidized film of undecamer was found to be around 1 S cm^-1. A thin-film field effect transistor was preliminary fabricated with neutral undecamer films and the hole mobility was determined.

Full text of DOI:10.1039/c3ra44129f

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PAPER
Synthesis and electrical properties of novel
oligothiophenes partially containing
,4-ethylenedioxythiophenes†
Cite this: RSC Adv., 2014, 4, 2501
3
Ichiro Imae,* Saki Imabayashi, Kenji Komaguchi, Zhifang Tan, Yousuke Ooyama
and Yutaka Harima*
Five sorts of soluble oligothiophenes (trimer to undecamer) containing 3,4-ethylenedioxythiophene (EDOT)
were synthesized, and their optical and electrochemical properties were investigated in relation to the chain
length of oligothiophenes and the number of EDOT units. The introduction of the EDOT unit into a main
oligothienylene unit induced a red shift of absorption bands and a negative shift of oxidation potentials.
Received 3rd August 2013
Accepted 28th October 2013
ꢀ1
The conductivity of an electrochemically oxidized film of undecamer was found to be around 1 S cm
.
DOI: 10.1039/c3ra44129f
A thin-film field effect transistor was preliminary fabricated with neutral undecamer films and the hole
www.rsc.org/advances
mobility was determined.
can be synthesized, the optoelectrical properties superior to
PEDOT will be expected due to their uniform structure.
Introduction
21,22
23,24
25,26
Since 3,4-ethylenedioxythiophene (EDOT) has two electron-
From this viewpoint, dimer,
trimer,
tetramer,
and
27
donating groups at the b-positions of the thiophene ring, pentamer of EDOT were synthesized and their optical and
poly(3,4-ethylenedioxythiophene) (PEDOT) is electrochemically electrochemical properties were investigated. However, EDOT
stable in the oxidation process and can show excellent electrical oligomers longer than hexamer have not been synthesized yet,
1
conductivity. A water dispersion of its composite with poly- because of their poor solubility and instability in air. “Hybrid”
(
styrene sulfonate) (PEDOT-PSS) is commercially available and oligothiophenes composed of EDOT and thiophene were
widely used in organic electronic devices such as organic photo- synthesized to overcome this problem, and their optical, elec-
voltaic cells and electroluminescent displays. PEDOT is known to trochemical, and electrical properties were investigated, but the
be a mixture of oligomers ranging from six to eighteen units and longest oligothiophene which could be purely isolated was
2
28,29
has a poor solubility to common organic solvents. In the p- pentamer.
Very recently, Nesterov et al. succeeded in the
conjugated systems, it is known that a mixing of oligomers with synthesis of hybrid octamer containing EDOT, but their research
different conjugation lengths causes unfavourable properties interest was not focused on the properties of oligothiophene
3
such as formation of trap-site for the charge transport, and dull themselves, but the electrochemical synthesis of EDOT-con-
4
30
color due to absorption of light over a wide wavelength region.
taining polythiophenes using oligothiophenes as monomers.
Oligothiophenes have been studied as model compounds of
In this study, we synthesize a series of EDOT-containing
polythiophenes and optical, electrochemical, and electrical oligothiophenes (EnTs, n ¼ 3, 6, 7, 9, 11) and investigate their
properties of conducting polymers have been claried through optical and electrochemical properties in solution and the
5–9
intensive studies with oligothiophenes. Because of their prop- electrical properties in the solid lm.
erties superior to those of polythiophenes due to the well-dened
10–12
structures, numerous oligothiophenes have been synthesized
and applied to the photo- and electroactive materials such as
Experimental
Materials

eld-effect transistors, electroluminescence and electrochromic
13–20
displays, photovoltaic cells, and non-linearoptics.
Thus, if the n-Hexane, toluene, tetrahydrofuran(THF), dichloromethane, and
EDOT-containing oligothiophenes with well-dened structure acetonitrile were puried by standard methods and used imme-
diately aer purication. Tetraethylammonium perchlorate
(
TEAP) and N-bromosuccinimide (NBS) were puried by recrys-
Department of Applied Chemistry, Graduate School of Engineering, Hiroshima
University, 1-4-1 Kagamiyama, Higashi-Hiroshima 739-8527, Japan. E-mail: imae@
hiroshima-u.ac.jp; harima@mls.ias.hiroshima-u.ac.jp; Fax: +81-82-424-5494; Tel:
tallization from ethanol and benzene, respectively, and dried
under vacuum. 3,4-Ethylenedioxythiophene (EDOT) and 3-hexyl-
2
-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)thiophene (HT-Bpin)
+
81-82-424-7688
were purchased from Tokyo Chemical Industry and used without
†
Electronic supplementary information (ESI) available: Cyclic voltammograms of
EnT. See DOI: 10.1039/c3ra44129f
further purication. 2,5-Dibromo-3,4-ethylenedioxythiophene
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0
0
0
00 00
ꢁ
(
(
DBrEDOT) and 3 ,4 -ethylenedioxy-2,2 :5 ,2 -terthiophene added in one portion. The reaction mixture was stirred at 0 C
31
E3T) were synthesized according to our previous report.
for 1 h, then at room temperature for 1 h. Aer the solvent was
removed by a rotary evaporator, n-hexane (50 mL) was added
and the solution was stirred for 1 h. Aer ltration of the
solution, the solvent was removed by a rotary evaporator. A dark
Synthesis
The synthetic routes of EnTs are shown in Scheme 1 and the
detailed synthetic processes are described below.
red liquid was obtained upon solvent removal. Product was
1
used without further purication. H NMR (500 MHz, (CD
3
)
2
CO,
), 1.28 (t with ne coupling,
), 1.35 (tq, J ¼ 7.40, 7.40 Hz, 6H,
CH CH ), 1.67 (tt, J ¼ 7.40, 8.10 Hz, 6H, SnCH CH ), 4.45 (s, 4H,
d, ppm): 0.89 (t, J ¼ 7.40 Hz, 9H, CH
3
0
0
0
0
0
00 00
5
(
,5 -Dibromo-3 ,4 -ethylenedioxy-2,2 :5 ,2 -terthiophene
DBrE3T)
To a CHCl /CH COOH (20 mL/20 mL) solution of E3T (0.44 g,
/CH COOH (30 mL/30 mL) solution of NBS
3 3
0.51 g, 2.84 mmol) was slowly added at room temperature. The
reaction mixture was stirred at room temperature for 3 h. The
reaction mixture was poured into water, and extracted two times
3
(50 mL each). The combined organic extracts were
washed with Na CO aq. and water and dried over Na SO .
J ¼ 8.10 Hz, 6H, SnCH
2
2
3
2
2
OCH CH O), 7.05 (dd, J ¼ 3.61, 5.14 Hz, 1H, thienyl-H), 7.15 (d,
2
2
3
3
J ¼ 3.43 Hz, 1H, thienylene-H), 7.26 (dd, J ¼ 1.11, 3.61 Hz, 1H,
thienyl-H), 7.38 (dd, J ¼ 1.11, 5.14 Hz, 1H, thienyl-H), 7.39 (d, J ¼
1
(
.43 mmol), a CHCl
3.43 Hz, 1H, thienylene-H).
0
0
0000 0000
0
0
00 00 000 000 0000 0000 00000
3
,4 :3 ,4 -Bis(ethylenedioxy)-2,2 :5 ,2 :5 ,2 :5 ,2 :5 ,2
-
with CHCl
sexithiophene (E6T)
2
3
2
4
The solvent was removed by a rotary evaporator. DBrE3T (0.59 g,
.27 mmol) was obtained as a yellow powder aer column
chromatography (SiO
, n-hexane/toluene ¼ 1/1 (v/v)), followed
by recrystallization from n-hexane/dichloromethane. Yield:
To a DMF (10 mL) solution of E3T-Sn, Pd(PPh
3
)
4
(37 mg, 0.03
1
mmol) was added at room temperature. The reaction mixture
ꢁ
2
aer being stirred at 80 C for 20 h was poured into distilled
water (30 mL) and extracted with dichloromethane. The organic
extract was washed with distilled water and dried over Na SO .
1
8
9%. H NMR (500 MHz, (CD
OCH CH
O), 7.04 (d, J ¼ 4.00 Hz, 2H, thienyl-H), 7.12 (d, J ¼ 4.00
Hz, 2H, thienyl-H). MS (m/z): 461.80 (M ).
3 2
) CO, d, ppm): 4.49 (s, 4H,
2
4
2
2
The solvent was removed by a rotary evaporator. E6T (40 mg,
.07 mmol) was obtained as a red solid aer a column chro-
+
0
1
matography (SiO
NMR (500 MHz, CD
.57 Hz, 8H, OCH
thienyl-H), 7.12 (d, J ¼ 3.88 Hz, 2H, thienylene-H), 7.14 (d, J ¼
.88 Hz, 2H, thienylene-H), 7.25 (dd, J ¼ 1.10, 3.68 Hz, 2H,
thienyl-H), 7.26 (dd, J ¼ 1.10, 5.07 Hz, 2H, thienyl-H). MS (m/z):
2
, n-hexane/toluene (1/2, v/v)). Yield: 40%. H
Cl
, d, ppm): 4.42 (d with ne coupling, J ¼
CH
O), 7.05 (dd, J ¼ 3.68, 5.07 Hz, 2H,
0
0
0
00 00
5
(
-Tributylstannyl-3 ,4 -ethylenedioxy-2,2 :5 ,2 -terthiophene
E3T-Sn)
2
2
2
2
2
To a THF (10 mL) solution of E3T (0.10 g, 0.33 mmol), 0.2 mL of
3
n-butyl lithium (1.65 M in n-hexane, 0.33 mmol) was added at
ꢁ
0
C. Aer 1 h, tributyltin chloride (0.14 g, 0.42 mmol) was
+
6
10.96 (M ).
0
0
0
000 000 00000 00000
0
0
00 00 000 000 0000
3
5
,4 :3 ,4 :3 ,4 -Tris(ethylenedioxy)-2,2 :5 ,2 :5 ,2 :5 ,2 :
000 00000 00000 000000
,2 :5 ,2 -septithiophene (E7T)
E3T-Sn was prepared using E3T (0.50 g, 1.63 mmol). To a DMF
30 mL) solution of E3T-Sn, DBrEDOT (0.16 g, 0.53 mmol) and
(
Pd(PPh
3
)
4
(83 mg, 0.07 mmol) were added at room temperature.
ꢁ
The reaction mixture aer being stirred at 100 C for 16 h was
poured into Na
2 3
CO aq. The formed precipitate was collected by

ltration, and washed by reuxed toluene (10 mL) to remove the
starting materials and by-products. The remaining crude
product was puried by recrystallization from distilled THF,
and E7T (016 g, 0.21 mmol) was obtained as a red solid. Yield:
1
4
0%. H NMR (500 MHz, THF-d
8
, d, ppm): 4.41 (d with ne
O), 4.43 (s, 4H, OCH CH O),
coupling, J ¼ 3.42 Hz, 8H, OCH
2
CH
2
2
2
7
.00 (dd, J ¼ 3.60, 5.04 Hz, 2H, thienyl-H), 7.16 (s, 4H, thieny-
lene-H), 7.23 (dd, J ¼ 0.86, 3.60 Hz, 2H, thienyl-H), 7.30 (dd, J ¼
+
0
.86, 5.04 Hz, 2H, thienyl-H). MS (m/z): 749.94 (M ).
0
0
0000 0000 0000000 0000000
0
0
00 00 000 000 0000
3
,4 :3 ,4 :3
,4
-Tris(ethylenedioxy)-2,2 :5 ,2 :5 ,2 :5 ,2 :
0
000 00000 00000 000000 000000 0000000 0000000 00000000
5
,2 :5 ,2 :5 ,2 :5 ,2 -novithiophene (E9T)
E9T was prepared according to the same procedure as described
1
for E7T by using E3T and DBrE3T. Yield: 31%. H NMR (500
MHz, (CD ) SO, d, ppm): 4.46 (d with ne coupling, J ¼ 6.90 Hz,
3
2
Scheme 1 Synthesis of EnTs (n ¼ 3,6,7,9,11). (i) NBS/THF-CH
ii) Pd(PPh Na CO aq./THF; (iii) NBS/CHCl –CH COOH; (iv)
n-BuLi, Bu SnCl/THF; (v) Pd(PPh /DMF.
3
COOH;
8H, OCH
2
CH
2
O), 4.49 (s, 4H, OCH
2
CH
2
O), 7.09 (dd, J ¼ 3.61,
(
3
)
4
,
2
3
3
3
3
3
)
4
5.08 Hz, 2H, thienyl-H), 7.19 (d, J ¼ 3.92 Hz, 2H, thienylene-H),
2502 | RSC Adv., 2014, 4, 2501–2508
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.20 (d, J ¼ 3.92 Hz, 2H, thienylene-H), 7.25 (dd, J ¼ 1.04, 3.61 generated by electrochemical oxidation (doping) and reduction
Hz, 2H, thienyl-H), 7.31 (d, J ¼ 3.92 Hz, 2H, thienylene-H), 7.32 (dedoping) of the E11T lm were measured with a coulometer
(
2
d, J ¼ 3.92 Hz, 2H, thienylene-H), 7.52 (dd, J ¼ 1.04, 5.08 Hz, (home made), where the potential was stepped from ꢀ0.5 V to a
+
H, thienyl-H). MS (m/z): 914.93 (M ).
desired potential and back to ꢀ0.5 V, respectively. Doping
levels, dened as the number of charges per thiophene ring,
were estimated from the doping/dedoping charges, weight of
E11T lm, and molecular weight of the monomer unit.
Apparent mobilities (m) of charge carriers in E11T at various
doping levels (electrode potentials) were calculated from the
following relation, m ¼ s/ne, where s, n, and e denote the elec-
trical conductivity at an electrode potential, the density of
charge carriers estimated from the doping/dedoping charges,
and the elementary charge, respectively. Thicknesses and
morphology of oligothiophene lms were evaluated by a 3D
laser microscope (Keyence Corp., VK-9700).
0
000
00 00
0
0
00 00 000 000 0000
3,4 -Dihexyl-3 ,4 -ethylenedioxy-2,2 :5 ,2 :5 ,2 :5 ,2 -
quinquethiophene (E5T)
E5T was prepared according to the similar procedure to our
previous report for E3T by using HT-Bpin and DBrE3T. Yield:
1
2
6
4%. H NMR (500 MHz, (CD
H, CH
3
)
2
CO, d, ppm): 0.87 (t, J ¼ 7.21 Hz,
), 1.28–1.35 (m, 8H, (CH CH ), 1.35–1.42 (m, 4H,
CH
), 1.66 (tt, J ¼ 7.21, 7.82 Hz, 4H, thienyl-
), 2.80 (t, J ¼ 7.82 Hz, 4H, thienyl-CH ), 4.50 (s, 4H,
OCH CH O), 7.03 (d, J ¼ 5.20 Hz, 2H, thienyl-H), 7.12 (d, J ¼ 3.91
3
2
)
2
3
thienyl-(CH
CH CH
2
)
2
2
2
2
2
2
2
Hz, 2H, thienylene-H), 7.26 (d, J ¼ 3.91 Hz, 2H, thienylene-H),
Field-effect transistors using E11T were fabricated in a
bottom-gate/top-contact conguration. An n-type heavily doped
+
7
.37 (d, J ¼ 5.20 Hz, 2H, thienyl-H). MS (m/z): 638.15 (M ).
Si wafer with a SiO layer of 200 nm and a capacitance of 14.1 nF
cm was used as the gate electrode and dielectric layer. Before
the deposition of E11T lm, the gate dielectrics were placed in
2
0
0
000
00 00
3
2
,4 -Dihexyl-5-tributylstannyl-3 ,4 -ethylenedioxy-
ꢀ2
0
00 00 000 000 0000
,2 :5 ,2 :5 ,2 :5 ,2 -quinquethiophene (E5T-Sn)
E5T-Sn was prepared according to the same procedure as toluene solution of octyltrichlorosilane (OTS) to form an OTS
1
described for E3T-Sn by using E5T. H NMR (500 MHz, self-assembled monolayer (SAM). Then the chloroform solution
(
CD
coupling, 6H, J ¼ 8.06 Hz, SnCH
CH (CH CH and Sn(CH CH
thienyl-CH CH and Sn-CH CH
), 2.80 (t, J ¼ 0.98 Hz, 4H, was calculated in the saturation regime by using the equation,
thienyl-CH ), 4.50 (s, 4H, OCH CH
thienyl-H), 7.11 (s, 1H, thienylene-H), 7.12 (d, J ¼ 3.93 Hz, 1H, current, m the eld-effect mobility, W is the channel width
thienylene-H), 7.12 (d, J ¼ 3.93 Hz, 1H, thienylene-H), 7.26 (d, (1 mm), L the channel length (100 mm), C the capacitance per
3
)
2
CO, d, ppm): 0.85–0.93 (m, 15H, CH
), 1.27–1.46 (m, 18H, thienyl- The device characteristics were measured in air and Ar by using
CH ), 1.58–1.70 (m, 10H, Keithley 2400 and 2611 sourcemeters. The eld-effect mobility
3 2
), 1.17 (t with ne of E11T was spin-coated on the OTS-modied SiO /Si substrate.
2
(
2
)
2
2
)
3
3
2
)
2
2
3
2
2
2
2
2
O), 7.03 (d, J ¼ 5.20 Hz, 1H,
I
DS ¼ (WmC /2L)(VGS ꢀ Vth) , where IDS denotes the drain-source
i
2
2
2
i
J ¼ 3.93 Hz, 1H, thienylene-H), 7.26 (d, J ¼ 3.93 Hz, 1H, thie- unit area of the gate dielectric layer, VGS the gate voltage, and Vth
nylene-H), 7.37 (d, J ¼ 5.20 Hz, 1H, thienyl-H).
is the threshold voltage.
0
0
0
00 00000 00000 00000000 00000000
0000 000000
-Tris(ethylenedioxy)-3,3 ,4 ,
3
3
,4 :3 ,4 :3
,4
Results and discussion
Synthesis of hybrid EDOT-containing oligothiophenes
000000000
0
0
00 00 000 000 0000 0000 00000 00000 000000
-tetrahexyl-2,2 :5 ,2 :5 ,2 :5 ,2 :5 ,2 :5 ,2
-
undecithiophene (E11T)
A series of oligothiophenes partially containing EDOT was
synthesized as described in Scheme 1. Terthiophene having one
EDOT unit (E3T) was synthesized by the Suzuki–Miyaura
coupling from 2-thiophene boronic acid and 2,5-dibromo-3,4-
ethylenedioxythiophene in the reasonable yield. Using E3T as
a key compound, hexamer (E6T), heptamer (E7T), and nonamer
E11T was prepared according to the same procedure as
described for E7T by using E5T and DBrEDOT. Yield: 34%. H
1
NMR (500 MHz, CD
2
Cl
2
, d, ppm): 0.89 (t, J ¼ 7.15 Hz, 6H, CH
3
3
),
),
0
1
.91 (t, J ¼ 7.15 Hz, 6H, CH
3
), 1.30–1.50 (m, 24H, (CH CH
2 3
)
31
.60–1.75 (m, 8H, thienyl-CH CH ), 2.79 (t, J ¼ 7.83 Hz, 8H,
2
2
thienyl-CH ), 4.43 (s, 12H, OCH CH O), 6.96 (d, J ¼ 5.19 Hz, 2H,
2
2
2
(E9T) were synthesized by the Stille-coupling. The solubility of
thienyl-H), 7.05 (d, J ¼ 3.79 Hz, 2H, thienylene-H), 7.06–7.12 (m,
H, thienylene-H), 7.18–7.22 (m, 6H, thienyl-H and thienylene-
EDOT-containing oligothiophenes was much higher than those
of unsubstituted oligothiophenes, although the solubility
decreased with the increase of the chain length of oligothio-
phenes due to the enhancement of their p–p interactions.
When nonamer (E11T) was synthesized, four alkyl chains were
introduced to increase the solubility. All compounds were
4
+
H). MS (m/z): 1415.27 (M ).
Measurements
UV-Vis absorption spectra were taken on a spectrophotometer
(Shimadzu, UV-3150). Cyclic voltammetry and spectroelec-
1
characterized by H NMR and mass spectroscopies.
trochemistry were made in TEAP (0.1 M)–acetonitrile using a
potentiostat/galvanostat (Hokuto Denko, HAB-151) and an X–Y
recorder (Riken Denshi, F-57). In situ conductivity measure-
Geometrical effect of EDOT
ments were made, also in TEAP (0.1 M)–acetonitrile, by using To investigate the effect of the introduction of EDOT units into
the two-probe method with a micro-array Pt electrode (ALS Co., oligothiophenes, optical properties of E3T were compared with
0
0
00
Ltd, 65 lines, separation distance ¼ 5 mm, total width ¼ 260 mm) those of 2,2 :5 ,2 -terthiophene (3T). Fig. 1 shows the electronic
or with a two-band Pt electrode (homemade, separation absorption and emission spectra of E3T and 3T. The absorption
31–38
distance ¼ 100 mm, width ¼ 7 mm).
The amounts of charges spectrum of 3T showed a structureless peak due to the
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Optical properties of EnTs
Electronic absorption spectra of other oligothiophenes, E6T,
E7T, E9T and E11T are shown in Fig. 3. In all cases, broad bands
with vibronic structure were observed similarly to E3T.
Although the bands showed a red-shi with increasing the
length of oligothiophene chains reecting the expansion of p-
conjugation systems, the absorption maximum of E11T was
similar to that of E9T because the effective p-conjugation may
be disturbed by the steric hindrance of the four alkyl chains
introduced at b-positions of thiophene rings. Previously, we
have synthesized soluble thiophene hexamer and decamer (6T
37,38,44
and 10T, Fig. 4) which do not contain EDOT units.
Compared with them, the absorption bands of oligothiophenes
containing EDOT were remarkably red-shied due to the elec-
tron-donating effect of oxygen atoms of EDOT unit and the
enhancement of coplanarity to expand the effective p-conjuga-
tion length.
Fig. 1 Electronic absorption (solid) and emission (dashed) spectra of
E3T (purple) and 3T (black) in dichloromethane solution. For emission
spectra, excitation at 354 nm.
Electrochemical stabilities of EnT
Results of cyclic voltammetries of EnTs are summarised in Table
1
and Fig. 1S (see ESI).† E3T showed an oxidation peak at 0.50 V
+
vs. Fc/Fc , but the current of the corresponding reduction peak
rotational freedom of thiophene rings, while the absorption
band of E3T showed a well-resolved vibronic structure. The
absorption maximum of E3T was observed at 374 nm, remark-
ably red-shied from that of 3T (lmax ¼ 354 nm), whereas
emission spectra of E3T and 3T were similar in shape to each
other.
was very small. When the redox reaction was repeated, another
For understanding these results, we estimated the molec-
ular structures of E3T and 3T by ab inito MO method using
Gaussian 09 (HF/6-31*, C
dral angle of 3T was around 30 degrees, that of E3T was almost
degree. Very recently, it is reported that the intramolecular
2
-symmetry) (Fig. 2). While the dihe-
0
interaction between chalcogen atoms affects the geometry of
molecules, although the origin of the interaction is still not
39–43
clear.
For example, it is reported that the crystal structure
of EDOT dimer shows the planar structure, and that the
distances between sulfur and oxygen atoms is signicantly
shorter than the sum of their van der Waals radii, which
evidences the occurrence of the above-mentioned strong
39
interaction. Such a rigid conjugated system will cause the
quinoid-like planar geometry even in ground state of E3T,
which is likely to give the vibronic ne structure in the
absorption spectrum and the decrease in the Stokes shi of
the emission spectrum as observed in Fig. 1.
Fig. 3 Electronic absorption spectra of EnTs (n ¼ 6, 7, 9 and 11) and
nTs (n ¼ 6 and 10) in dichloromethane solution.
Fig. 2 Molecular structures of (a) E3T and (b) 3T estimated by ab initio
MO method (Gaussian09), HF/6-31G*, C
2
-symmetry.
Fig. 4 Chemical structures of 6T and 10T (R ¼ alkyl groups).
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a
Table 1 Redox potentials estimated from cyclic voltammetry
1
b
1 b
2 b
pa /V
2 b
Oligothiophenes
E
pa /V
E
pc /V
E
E
pc /V
c
E3T
E6T
E7T
E9T
E11T
0.50
0.08
ir
c
ꢀ0.05
ꢀ0.36
ꢀ0.26
0.57
0.33
0.40
ir
ir
c
ꢀ0.16
ꢀ0.09
0.13
d
d
d
d
Fig. 5 Laser microscopic images of E7T film obtained by drop-cast
br
br
br
br
method. (a) 2D and (b) 3D images.
6
1
T
0T
0.35
0.20
0.28
0.13
0.35
0.28
a
b
ꢀ1
Scan rate: 50 mV s , electrolyte: Bu
4
NClO
4 2 2
(0.1 M), solvent: CH Cl .
+
c
d
Vs. Fc/Fc . ir ¼ irreversible. br ¼ broad peaks.
Absorption spectra of E11T lms biased at different poten-
tials were measured in TEAP (0.1 M)–acetonitrile to identify
chemical species formed at different oxidation states (Fig. 6). At
the neutral state, E11T showed only one sharp peak at around
+
oxidation peak was newly observed at around 0 V vs. Fc/Fc , and
the current of the redox peaks increased with increasing the
number of redox cycles. We have already found that E3T can be
electrochemically polymerized to form the polymer. Thus, this
new peak can be ascribed to the redox reaction of the formed
5
00 nm due to a p–p* transition. When the E11T was oxidized
+
at 0.0 V vs. Ag/Ag , two peaks were observed at around 700 and
1500 nm, and they were ascribed to the one-electron oxidized
species of E11T. The intensity of these peaks increased with
increasing the electrode potential, and reached a maximum at
31
polymer. In the case of E6T, two redox reactions were observed at
0
+
0.3 V. When the E11T was oxidized at higher potentials than 0.3
.08 and 0.57 V vs. Fc/Fc . Among them, the rst redox reaction
V, the intensity of these peaks decreased and one broad peak
due to the two-electron oxidized species of E11T was newly
observed at 1200 nm. These results suggest that E11T can be
was reversible, but the second one was irreversible and formed a
polymeric material on the working electrode. E7T and E9T
showed two reversible redox processes due to the stabilization of
the formed charges by delocalization of p-electron systems. E11T
showed broad peaks, but these peaks were found to contain two
reversible redox processes from the spectroelectrochemical
investigation (vide infra).
one- and two-electron oxidized in the potential range from 0 to
+
0
.8 V vs. Ag/Ag . These spectral changes were reversible due to
the electrochemical stability, and E11T showed the excellent
electrochromic behaviour (Fig. 7).
Fig. 8 depicts semilogarithmic plots of doping level and
conductivity against potential for E11T. In concert with the
electrochemical oxidation, the doping level gradually increased
Film forming property
For the application of EnTs to solid state devices such as and nally reached around 20% at 0.75 V. This value corre-
transparent conductive lms and eld-effect transistors, we sponds to 200% doping per E11T, suggesting that E11T is
tried to fabricate thin lms of EnTs by casting and/or spin- almost completely two-electron oxidized, which is consistent
coating techniques. When the spin-coatings of E3T, E6T, E7T with the results of above-mentioned spectroelectrochemistry
and E9T were tried onto substrates of SiO
2
, ITO and OTS- (Fig. 6). The electrical conductivities also increased with the
modied SiO /Si, smooth and homogeneous lms were not increase in potential, but showed a maximum at 0.55 V vs. Ag/
2
+
ꢀ1
obtained. A plausible reason may be that these oligothiophenes Ag . The maximum value of doped E11T was 0.73 S cm , which
are highly crystalline. Previously, we successfully obtained is slightly higher than that of dodecathiophene without EDOT
smooth lms of oligothiophenes combined with electrically
inert polymer backbones, for example, polyethylenes having
4
,45–47
pendant oligothiophenes,
linear polymers including oli-
48–51
gothienylene bridged by alkylenes or silylenes,
and poly-
Thus, smooth
lms of EnTs will be obtained by the introduction of inert
36–38,44,52
silsesquioxanes having oligothiophenes.

polymer backbones, which is under investigation. We also tried
to fabricate the lm of E7T by a drop-cast method and found a
unique morphology as shown in Fig. 5, where hexagonally
ordered protuberances were observed. The mechanism is
currently under investigation. On the other hand, smooth lms
of E11T were successfully obtained by a spin-coating method
ꢀ
1
with a chloroform solution of E11T (10 mg mL ).
Electrical properties of solid-state E11T
Since smooth and thin lms of E11T were obtained on SiO2,
ITO, and OTS-modied SiO /Si substrates, electrochemical and
2
electrical properties of E11T lms could be investigated.
Fig. 6 Spectroelectrochemistry of E11T film.
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Fig. 7 Electrochromic behaviour of E11T film. (a) Neutral and (b)
oxidized films.
Fig. 9 Apparent mobilities of charge carriers in E11T film plotted
against doping level.
Table 2 Performances of FET using E11T
Rotational
speed /rpm
FET
m /
cm V
a
b
2
ꢀ1 ꢀ1
Run
SAM
None
OTS
Atmosphere
s
Ion/Ioff
ꢀ5
ꢀ5
ꢀ4
ꢀ4
1
2
3
4
5
6
7
8
2000
Air
Ar
Air
Ar
Air
Ar
Air
Ar
3.5 ꢂ 10
4.3 ꢂ 10
6.6 ꢂ 10
3.4 ꢂ 10
4
10
9
24
c
c
3000
None
OTS
—
—
ꢀ5
ꢀ4
ꢀ4
1.3 ꢂ 10
3.9 ꢂ 10
1.2 ꢂ 10
4
8
33
Fig. 8 Doping level and electrical conductivity of E11T film plotted
against electrode potential.
a
Rotational speed of spin-coating for preparation of E11T lm.
Measurement atmosphere. Not measured.
b
c
ꢀ
1
37,38
(
0.4 S cm ).
This result indicates that the introduction of
it was expected that the eld-effect transistor (FET) using E11T
as a semiconducting layer will show a good performance. To
conrm it, FET devices using E11T were preliminarily fabri-
cated with different device preparation conditions and
measured in Ar and air atmospheres (Table 2). Among them, the
EDOT enhances the charge transport properties of oligothio-
phenes due to the stabilization of introduced positive charges.
To get an insight into transport properties of the E11T,
apparent mobilities of charge carriers were estimated by
combining doping level and conductivity data. The mobilities
are plotted in Fig. 9 as a function of doping level. The mobilities
device using E11T lm prepared on OTS-modied SiO /Si
2
ꢀ7
2
ꢀ1 ꢀ1
substrate by spin-coating with 3000 rpm showed the highest
at low doping levels below 3% are 4–7 ꢂ 10 cm V
s ,
ꢀ
4
ꢀ4
2
ꢀ1 ꢀ1
mobilities (3.4 ꢂ 10 and 6.6 ꢂ 10 cm V
respectively). Also, the device measured in Ar showed the better
on/Ioff ratio, because E11T will be oxidized in air. For the
s
in Ar and air,
which can be explained by the interchain hopping transport of
50,51
monocation radicals (polarons).
The mobility plots showed a
I
maximum at a doping level of 13%, in which one- and two-
electron oxidized states coexist. When the doping level was
increased beyond 15%, the mobility slightly decreased. A
plausible reason for this difference may be explained as follows:
almost all E11T was two-electron oxidized at a doping level of
improvement of FET performances, the optimization of device
fabrication is under investigation.
Conclusions
20%, so that it becomes difficult for the positive charge to move
because of a Coulombic repulsion. This result was observed in EDOT-containing oligothiophenes were newly synthesized and
36–38
the oligothiophenes having silsesquioxane networks.
their optical and electrochemical properties were investigated.
Since E11T was found to have the good electrical conduc- It was found that the introduction of EDOT caused the
tivity from the experiment of in situ conductivity measurement, remarkable red-shi of absorption bands. Oxidation potentials
2506 | RSC Adv., 2014, 4, 2501–2508
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common organic solvents and could form smooth lms by spin- 22 S. Akoudad and J. Roncali, Synth. Met., 1998, 93, 111.
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Acknowledgements
2
This work was supported in part by grants from the Hitachi
Metals/Materials Science Foundation (I.I.), the Hiroshima Bank
(
I.I.), and grants-in-aid for scientic research from the Japan 26 M.Turbiez, P.Fr `e reandJ.Roncali, J. Org. Chem., 2003, 68, 5357.
Society for the Promotion of Science (JSPS) (no. 22550198, I.I. 27 D. Wasserberg, S. C. J. Meskers, R. A. Janssen, E. Mena-
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Osteritz and P. B ¨a uerle, J. Am. Chem. Soc., 2006, 128, 17007.
Amimoto and Dr Daisuke Kajiya, the Natural Science Center for 28 R. G. Hicks and M. B. Nodwell, J. Am. Chem. Soc., 2000, 122,
Basic Research and Development (N-BARD), Hiroshima
University for the measurement of mass spectroscopy.
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29 M. Turbiez, P. Fr `e re, M. Allain, C. Videlot, J. Ackermann and
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