Development of β-linked quaterthiophene and tetrathiafulvalene dimers as new organic semiconductors

Article abstract of DOI:10.1016/j.physb.2009.10.058

A series of β-linked quaterthiophene and tetrathiafulvalene dimers 1-4 has been prepared. Their redox and optical properties are almost the same as the parent monomers indicative of very little interaction between the monomers. Crystal structures of 1, 2, and 4 are highly one-dimensional, in which molecular structure of 1 is planar whereas those of 2 and 4 are in a bent form.

Full text of DOI:10.1016/j.physb.2009.10.058

ARTICLE IN PRESS
Physica B 405 (2010) S373–S377
Contents lists available at ScienceDirect
Physica B
journal homepage: www.elsevier.com/locate/physb
Development of
b
-linked quaterthiophene and tetrathiafulvalene dimers as
new organic semiconductors
Ã
Minoru Ashizawa , Yan Yu, Takuro Niimura, Kazuma Tsuboi, Hidetoshi Matsumoto,
Akihiko Tanioka, Takehiko Mori
Department of Organic and Polymeric Materials, Tokyo Institute of Technology, 2-12-1 O-okayama, Meguro-ku, 152-8552, Tokyo, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of b-linked quaterthiophene and tetrathiafulvalene dimers 1–4 has been prepared. Their redox
Received 1 October 2009
Received in revised form
30 October 2009
and optical properties are almost the same as the parent monomers indicative of very little interaction
between the monomers. Crystal structures of 1, 2, and 4 are highly one-dimensional, in which
molecular structure of 1 is planar whereas those of 2 and 4 are in a bent form.
& 2009 Elsevier B.V. All rights reserved.
Accepted 31 October 2009
Keywords:
Organic semiconductors
Oligothiophene
Tetrathiafulvalene
X-ray structure analysis
Organic field effect transistor
1. Introduction
in common organic solvents. In this study, we report syntheses,
redox and optical properties, and crystal structures of quaterthio-
phene and TTF dimers 1–4 (Scheme 1).
High performance organic semiconductors have been widely
investigated in recent years because of their practical use for
flexible, low-cost, and large-area coverage electronic devices [1].
In organic field effect transistors (OFETs), linear
p-conjugated
molecules are considered to be desirable to make densely packed
molecular arrangement [2–4]. The best charge carrier mobility
of OFETs has already approached and even surpassed that
of amorphous Si devices. However, a simple increase in the
2. Experimental detail
All chemicals and solvents are of reagent grade unless
otherwise indicated. All reactions were carried out under an
argon atmosphere. THF was purified by distillation over sodium
benzophenone kethyl under argon prior to use. NMR data were
obtained on a JEOL JNN-AL300 spectrometer. The melting points
were determined with a Yanako MP micromelting point appara-
tus. Microanalyses were performed at Microanalysis Center,
Tokyo Institute of Technology. Cyclic voltammetry (CV) was
performed with a Yanako electrochemistry analyzer VMA-010.
MS spectra were obtained with a SHIMADZU/KRATOS AXIMA-CFR
for MALDI-TOF MS. UV-vis spectra were collected on a HITACHI
U2800, and photoluminescence spectra were collected on a JASCO
FP-750. Crystal structures were determined with single-crystal
X-ray diffraction data. Measurements were made on a RIGAKU
Mercury CCD system with for 1 and 4 and a RIGAKU Vari Max
RAPID-II system for 2. Thermal gravimetric analysis (TGA) was
conducted on a Seiko instruments TG/DTA6200 (temperature rate
10 1C minꢀ1 under N2).
p-conjugation length in, for example, linear oligothiophenes
reduces the solubility considerably. In order to improve the
solubility by maintaining the desirable molecular packing, a
solution of a processable swivel cruciform oligothiophene has
been investigated to show excellent OFET performance [5].
In order to investigate electronic properties and molecular
packing motifs of twisted molecules, we have developed new
soluble quaterthiophene dimers linked at the
thiophene rings and tetrathiafulvalene (TTF) dimer whose TTF
units are linked at the -position of thiophene ring with ethynyl
spacer. Molecular flexibility based on rotational freedom at the
b-position of the
b
b
-linkage of thiophene rings would be advantageous for solubility
Ã
Corresponding author. Tel.:+81357342426; fax: +8135734876.
E-mail address: ashizawa.m.aa@m.titech.ac.jp (M. Ashizawa).
0921-4526/$ - see front matter & 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.physb.2009.10.058

ARTICLE IN PRESS
S374
M. Ashizawa et al. / Physica B 405 (2010) S373–S377
Scheme 1. Dimeric TTF and quaterthiophene molecules.
2.1. Material Syntheses
2.2 mmol) in 1,2-dimethoxyethane (15 ml) was added dropwise.
The mixture was heated under reflux overnight. The reaction
mixture was poured into water and extracted with CS2. After
evaporation, the residue was purified by column chromatography
on silica gel with CS2-hexane (1:1, v/v), and recrystallization from
toluene afforded a yellow solid 4 (0.05 g, 8%). Mp=216–217 1C.
2.1.1. TTF dimer 1
Into a solution of TTF iodide 9 (0.50 g, 1.51 mmol), Pd(PPh3)4
(0.10 g, 0.08 mmol), and copper (I) iodide (0.03 g, 0.15 mmol) in
toluene (15 ml) was added triethylamine (15 ml). After the
mixture was stirred at room temperature for 5 min, diethynyl-
bithiophene 8 (0.25 g, 0.77 mmol) in toluene (10 ml) was added
dropwise. The resulting mixture was stirred at 60 1C overnight
and poured into 2 M HCl (50 ml). After the insoluble materials
were removed by filtration through celite, the filtrate was
extracted with CS2. The extract was successively washed with a
saturated NaHCO3 solution and water, and dried over MgSO4.
After evaporation, the residue was purified by column chromato-
graphy on silica gel with CS2-hexane (1:1, v/v), and recrystalliza-
tion from toluene gave a red solid (0.14 g, 33%). Mp=176–177 1C
(decomp.). Ms (MALDI-TOF) m/z 731.5 (M+ +H); 1H NMR
Ms (MALDI-TOF) m/z 768.0(M+); 1H NMR (300 MHz, CDCl3)
d 0.94
(t, J=7.3 Hz 6 H), 1.41–1.51(m, 4 H), 1.66–1.73 (m, 4 H), 2.82
(t, J=7.3 Hz, 4 H), 6.61 (s, 2 H), 6.62 (d, J=3.5 Hz, 2 H), 6.92
(d, J=4.6 Hz, 2 H), 7.13 (d, J=3.5 Hz, 2 H), 7.16 (d, J=4.4 Hz, 2 H).
Anal. Calcd for C40H34S8: C, 62.29; H: 4.44; S: 33.26. Found:
C, 62.07; H: 4.37; S: 33.57.
2.1.4. Single-crystal structure analyses
Slow evaporation of a CS2-hexane (1:1, v/v) solution of 1 gave
red rods for X-ray structure analysis. Crystal data: C32H26S10
,
Mw=729.92, monoclinic, space group P21/c, a=14.420(6),
(300 MHz, CS2-CDCl3)
1.71–1.76 (m, 4 H), 2.81 (t, J=6.9 Hz, 4 H), 6.32 (s, 4 H), 6.49
(s, 2 H), 6.70 (s, 2 H).
d 1.00 (t, J=6.9 Hz 6 H), 1.40–1.50 (m, 4 H),
3
˚
˚
b=99.163(11)1, V=1628.3(12) A ,
b=5.235(2), c=21.850(9) A,
Z=2, 17219 reflections measured, 4396 unique (Rint=0.248). Final
R indices [I 4 2 (I)]: R1=0.0708, wR2=0.0986.
s
Vapor diffusion of hexane into a CS2 solution of 2 afforded
yellow needles for X-ray structure analysis. Crystal data:
2.1.2. Quaterthiophene dimers 2 and 3
Quaterthiophene dimer 2: Dibromo-dibutylbithiophene 14
(0.25 g, 0.58 mmol), tri(n-butyl)stannyl-butylterthiophene 15
(0.71 g, 1.20 mmol), and Pd(PPh3)4 (0.05 g, 0.04 mmol) were
dissolved in 20 ml of toluene. The mixture was refluxed for 48 h.
The reaction mixture was poured into 1 M HCl (50 ml) and
extracted with CS2. The organic phase was subsequently washed
with a saturated NaHCO3 solution and water, and dried over
MgSO4. After evaporation, the residue was purified by column
chromatography on silica gel with CS2-hexane (1:1, v/v), and
recrystallization from toluene afforded a yellow solid (0.13 g,
33%). Mp=202–203 1C. Ms (MALDI-TOF) m/z 882.7 (M+); 1H NMR
C48H50S8, Mw=883.40, orthorhomic, space group Aea2, a=
3
˚
˚
16.4463(3), b=49.9261(9), c=5.35235(10) A, V=4394.82(14) A ,
Z=4, 11780 reflections measured, 4011 unique (Rint=0.160). Final
R indices [I 4 2s(I)]: R1=0.1101, wR2=0.1759.
Vapor diffusion of hexane into a CS2 solution of 4 afforded
yellow needles for X-ray structure analysis. The obtained crystal
was basically isostructural to 2. Crystal data: C40H40S8,
3
˚
˚
a=16.433(18), b=39.68(4), c=5.475(6) A, V=3570(7) A , Z=4.
As for the single crystal of 3, vapor diffusion of hexane into a
CS2 solution of 3 afforded very thin yellow needles, which were
not suitable for X-ray structure analysis.
(300 MHz, CDCl3)
d 0.94 (t, J=7.2 Hz, 12 H), 1.36–1.47 (m, 8 H),
1.60–1.72 (m, 8 H), 2.75–2.83 (m, 8 H), 6.57 (s, 2 H), 6.62(d,
J=3.5 Hz, 2 H), 6.81 (d, J=3.9 Hz 2 H), 6.84–6.89 (m, 8 H). Anal.
Calcd for C48H50S8: C, 65.26; H: 5.70; S: 29.04. Found: C, 64.99; H:
5.51; S: 29.13.
3. Results and discussions
Quaterthiophene dimer 3 was prepared in a similar manner as
described for 2. Mp=186, 187 1C. Ms (MALDI-TOF) m/z 995.8
3.1. Synthesis of TTF dimer 1
(M+); 1H NMR (300 MHz, CDCl3)
d 0.88 (m, 12 H), 1.30 (m, 24 H),
The synthesis of TTF dimer 1 is outlined in Scheme 2.
1.61–1.73 (m, 8 H), 2.69–2.83 (m, 8 H), 6.60 (s, 2 H), 6.64(d,
J=3.0 Hz, 2 H), 6.84 (d, J=3.9 Hz, 2 H), 6.84–6.93 (m, 8 H). Anal.
Calcd for C56H66S8: C, 67.55; H: 6.68; S: 25.76. Found: C, 67.46; H:
6.40; S: 26.04.
Dibromide
5 is readily accessible by three steps from the
starting -butylthiophene [6]. The conversion of dibromide 5
a
to diiodide 6 is accomplished via lithium–halogen exchange,
followed by quenching with iodine. Subsequent Sonogashira
coupling of 6 with (trimethylsilyl)acetylene (TMSA), followed by
removal of the TMS groups, and then the cross-coupling with TTF
iodide 9, affords 1 by three steps in 16% yield. Unfortunately, we
2.1.3. Quaterthiophene dimer 4
A
mixture of dibromo-dibutylbithiophene 14 (0.39 g,
0.90 mmol), Na2CO3 (0.5 g), and Pd(PPh3)4 (0.12 g, 0.11 mmol) in
1,2-dimethoxyethane (20 ml) and water (30 ml) was heated
under reflux. A solution of terthiopheneboronic acid 15 (0.64 g
have failed to prepare directly b-linked dimer 11 by Stille
coupling to date, in which an unidentified byproduct was
obtained.

ARTICLE IN PRESS
M. Ashizawa et al. / Physica B 405 (2010) S373–S377
S375
Scheme 2. Synthesis of dimeric TTF.
Scheme 3. Synthesis of dimeric quaterthiophenes.
In the cyclic voltammetry measurement, 1 shows two pairs of
3.2. Syntheses of quaterthiophene dimers 2–4
reversible waves like the parent TTF under the same condition:
E11=2 ¼ 0:23l V, E12=2 ¼ 0:57 V for 1 and E11=2 ¼ -0:07 V, E12=2 ¼ -0:48 V
for TTF (vs Ag/AgNO3 in PhCN with 0.1 M n-Bu4NPF6, grassy
carbon working electrode, and scan rate 100 mV sꢀ1), indicating
that two TTF units of 1 are practically independent. The first redox
potential of 1 is significantly larger than that of TTF because an
electron-withdrawing acetylene unit is attached to the TTF
moiety. The E21=2-E11=2 value (0.34 V) for 1, which is electrochemical
estimation of the on-site Coulomb repulsion, is smaller than
Scheme 3 illustrates syntheses of quaterthiophene dimers 2–4.
The key compounds 13 were obtained by the Kumada coupling of
b-iodothiophene 12 which was prepared via lithium–halogen
dance reaction of -iodothiophene [7]. The NBS bromination of 13
a
gave dibromide 14, in which treatment with 1 equiv. and 2 equiv.
of NBS afforded mono- and dibromide, respectively. The Stille
coupling of 14 with tributylstannyl butyl- and hexylterthiophenes
15 gave quaterthiophene dimers 2 in 33% yield and 3 in 20% yield,
respectively. On the other hand, 4 was prepared by the Suzuki
0.55 V for TTF, reflecting the extended p-conjugation of 1.

ARTICLE IN PRESS
S376
M. Ashizawa et al. / Physica B 405 (2010) S373–S377
Table 1
Redox and optical properties of dimers 2–4 are summarized
in Table 1. Quaterthiophene dimers 2–4 show two irreversible
Redoxa and optical propertiesb
oxidation waves, which are almost the same as those of the
E1ox
E2ox
labs,
(nm)
lem,
(nm)
max
max
parent monomer
a,o-dibuthylquaterthiophene (DB4T). The
HOMO levels of 2–4 are estimated to be ꢀ5.20ꢁ ꢀ5.29 eV
from the onset oxidation potentials. Dimers 2–4 exhibit UV-Vis
absorption maxima at 408–418 nm and photoluminescence (PL)
maxima at 507–511 nm. Compared with DB4T, these values are
slightly red shifted. These results indicate that there is very little
interaction between the monomers. The thermogravimetric
analysis (TGA) in nitrogen of 2 shows the onset decomposition
temperature of 425 1C. In contrast, an analogous oligothiophene
2
0.58
0.56
0.61
0.58
0.31
0.79
0.74
0.80
0.83
418
418
408
400
511
510
507
494
3
4
DB4T
Ferrocene
a
vs. Ag/AgNO3 in PhCN with 0.1 M n-Bu4NPF6, grassy carbon working
electrode, scan rate 100 mV sꢀ1
.
10ꢀ5 M in CHCl3.
b
a,o-dihexylsexythiophene (DH6T) decomposes at 320 1C under
similar conditions [8].
S3
C12
S5
C1
C15
C11
3.3. Crystal structure of TTF dimer 1
C14
C10
C9
C7
C6
C4
C5
S1
C13
C2
TTF dimer 1 crystallizes in the monoclinic system with the
space group P21/c. The molecular and crystal structures of 1
are shown in Fig. 1. A half molecule of 1 is crystallographically
S2
C3
C8
C16
S4
independent, and
a unit cell contains two molecules. In
contrast to the chair form of the neutral TTF [9], the whole
molecule of 1 is almost planar. In the unit cell, TTF dimer 1
˚
makes two kinds of uniform stacks along the b axis (5.325 A),
and the molecular planes are nearly perpendicular to each
other (Fig. 1(b)). Therefore the packing motif of 1 is one-
dimensional.
3.4. Crystal structure of quaterthiophene dimers 2 and 4
Quaterthiophene dimer 2 crystallizes in the orthorhombic
system with the space group Aea2. Although the crystal quality of
4 is poor, compound 4 seems to be also isostructural to 2. A half
molecule of 2 is crystallographically independent, and a unit cell
contains four molecules (Fig. 2). The molecule has a twisted
V-form centered at the
b-linked bithiophene part. The twisted
b
form agrees with the electronically independent redox and optical
properties. The quaterthiophene groups form uniform stacks
along the c axis embedded in the herringbone-like molecular
packing (Fig. 2(b)). There are some intermolecular CH-p contacts
o
c
shorter than the van der Waals distance in the stack. However, the
largely twisted form of 2 breaks the electronic through-bond
and through-space interactions between the quaterthiophene
units. The calculated intrastack transfer integrals are by one order
magnitude larger than the interstack integrals. Accordingly, the
molecular packing is highly one-dimensional.
FET devices of 2–4 were fabricated by spin-coating from
chloroform solutions, although 2–4 were not vacuum evaporated.
However, the FET performance is not observed. This may be
related to the one-dimensional molecular packing of these
molecules.
a
4. Conclusions
We have prepared a series of
b-linked quaterthiophene and
TTF dimers. Their redox and optical properties reflect the
c
inherent properties of the parent monomers, indicating that
o
these monomers work independently, and the
b-linkage does
Fig. 1. Crystal structure of 1. (a) Molecular structure and (b) (c) molecular packing.
not connect the two units electronically. The crystal structures of
1, 2, and 4 are highly one-dimensional, in which 1 is in the planar
form while 2 and 4 are in the twisted V-form. The present
synthetic approach allows stepwise introduction of functional
groups such as electron-donating and withdrawing units into the
twisted bithiophene part. Along with this line, development of
coupling because the Stille coupling gave only a trace of the
product. Dimers 2–4 are moderately soluble in common organic
solvents.

ARTICLE IN PRESS
M. Ashizawa et al. / Physica B 405 (2010) S373–S377
S377
C23
C21
C15
C7
C8
S4
C6
C14
S3
C22
C24
C1
S2
C20
C16
C17
C3
C12
C11
C9
C5
C13
C19
C18
S1
C4
C10
C2
b
o
CH- contacts
π
a
Fig. 2. Crystal structure of 2. (a) Molecular structure and (b) molecular packing viewed along the c axis. Arrows indicate short CH-p intermolecular contacts.
new class of functional organic semiconductors is currently in
progress.
References
[1] H. Klauk (Ed.), Organic Electronics, Wiley-VCH, Weinheim, 2006.
[2] S. Lee, B. koo, J. Shin, E. Lee, H. Park, H. Kim, Appl. Phys. Lett. 88 (2006) 162109.
[3] T. Yamamoto, K. Takimiya, J. Amer. Chem. Soc. 129 (2007) 2224.
[4] M. Mas-Torrent, M. Durkut, P. Hadley, X. Ribas, C. Rovira, J. Amer. Chem. Soc.
126 (2004) 984.
Acknowledgments
[5] A. Zen, A. Bilge, F. Galbecht, R. Alle, K. Meerholz, J. Grenzer, D. Neher, U. Scherf,
T. Farrell, J. Amer. Chem. Soc. 128 (2006) 3914.
[6] M.J. Marsella, I.T. Kim, F. Tham, J. Amer. Chem. Soc. 122 (2000) 974.
[7] L.M. Schnu¨rch, M. Spina, F.A. Kahn, D.M. Mihovilovic, P. Stanetty, Chem. Soc.
Rev. 36 (2007) 1046.
[8] H.-S. Kim, Y.-H. Kim, T.-H. Kim, Y.-Y. Noh, S. Pyo, M.H. Yi, D.-Y. Kim, S.-K. Kwon,
Chem. Mater. 19 (2007) 3561.
[9] F.W. Cooper, C.N. Kenny, W.J. Edmonds, A. Nagel, F. Wudl, P. Cooper, Chem.
Comm. (1971) 889.
We thank Prof. Uekusa (Tokyo Institute of Technology) for
helping the single-crystal X-ray diffraction measurements. Thanks
are also due to the Instrumental Center in the Institute for
Molecular Science for the assistance in single-crystal X-ray
diffraction measurements. This work was in part financially
supported by
a
Grant-in-Aid for Scientific Research (B)
(no. 17340114) from MEXT.