Synthesis of thiophene-pyrrole mixed oligomers end-capped with hexyl group for field-effect transistors

Article abstract of DOI:10.1016/j.tetlet.2008.11.061

Mixed thiophene-N-methylpyrrole oligomers composed of the five and six heterocycles and hexyl substituents at both ends were synthesized, and their structural, electronic, and field-effect properties were investigated. These oligomers behaved as good p-ty

Full text of DOI:10.1016/j.tetlet.2008.11.061

Tetrahedron Letters 50 (2009) 555–558
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Synthesis of thiophene–pyrrole mixed oligomers end-capped
with hexyl group for field-effect transistors
*
Mika Fujii, Tohru Nishinaga , Masahiko Iyoda
Department of Chemistry, Graduate School of Science and Engineering, Tokyo Metropolitan University, Hachioji, Tokyo 192-0397, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Mixed thiophene–N-methylpyrrole oligomers composed of the five and six heterocycles and hexyl sub-
stituents at both ends were synthesized, and their structural, electronic, and field-effect properties were
investigated. These oligomers behaved as good p-type semiconductors on FET devices and the mixed hex-
Received 15 October 2008
Revised 17 November 2008
Accepted 18 November 2008
Available online 21 November 2008
ꢁ2
2
ꢁ1 ꢁ1
amer with a nearly linear structure showed higher hole mobility (5.3 ꢀ 10 cm V
s ) than that of the
pentamer with a banana-shaped structure.
Ó 2008 Elsevier Ltd. All rights reserved.
Organic field-effect transistors (OFETs)1 have been of
considerable academic and industrial interest for the development
of organic electronics.2 In the quest for the high-performance
(denoted as S and O, respectively) oligomers,
a
,a
0
7
-dialkylated
a
SOSOS mixed pentamer showed best performance. Thus, we
started with dihexyl pentamer DH-SNSNS 1 where N denotes pyr-
role ring. In addition, not alternately connected type of DH-SNSSNS
hexamer 2 was also synthesized to see the structure–FET perfor-
mance relationship, since the longer and alternately connected
DH-SOSOSOS heptamer showed smaller mobility than DH-SOSOS
in contrast to the case of the corresponding oligothiophenes.10
As shown in Scheme 1, the synthesis of mixed pentamer 1 was
conducted by Paal–Knorr pyrrole synthesis from tetraketone 5.
devices, a number of novel
designed and synthesized as semiconductors for OFETs. Among
p-conjugated molecules have been
3
them, oligothiophenes and their related derivatives and analogues
4
are one of the most extensively investigated class of compounds
owing to the versatility in functionalization of the thiophene ring
and intrinsic stability of the oligomers.
As an oligothiophene analogue, we recently synthesized
5
thienylfuran oligomers, and found that the dimer packs more den-
6
sely in crystals than pentacene and sexithiophene both known as
O
O
O
H13C6
S
S
ZnCl , Et NH, t-BuOH
2 2
excellent semiconductors. Also, it was revealed that the related thi-
ophene-furan mixed oligomers behave as p-type semiconductors
Br
+
benzene, r.t.
4%
7
whose characteristics are comparable to those of oligothiophenes.
3
4
3
These results tempted us to study the FET characteristics of thio-
phene–pyrrole mixed oligomers.
H13C6
C H
6
13
S
S
NH OAc, 140 C
4
In contrast to oligothiophenes, the FET devices of linear oligo-
pyrroles are unknown partly due to the limited synthetic routes
to oligopyrroles. Thus, the FET characteristics of thiophene–pyr-
S
or
S
AcOH, 40%MeNH2aq
benzene,reflux
O
O
O
O
8
8
5%
5
role mixed oligomers also have not attracted attention, even
though several groups synthesized these type of oligomers previ-
H13C6
S
S
9
C H
6 13
ously. However, we found that newly synthesized derivatives hav-
N
R
N
R
ing substituents of methyl group at nitrogen and hexyl group at
both ends showed good FET performance. Herein, we report the
synthesis and structural, electronic, and field-effect properties of
these oligomers.
1
(R=H, Me)
H13C6
O O
S
S
AcOH, 40%MeNH2aq 1) n-BuLi, THF
For both oligothiophenes4 and thiophene–furan oligomers,7
the alkyl substituents at both ends enhance the FET performance.
Taking balance between solubility and semiconductor performance
into consideration, we chose the hexyl capping group for the
present study. In the case of alternately connected thiophene-furan
e,i
benzene,reflux
96%
2
) CuCl2
59%
6
R
N
S
S
C H
6
13
C H
6
13
S
S
N
R
2
(R=Me)
*
Corresponding author. Tel.: +81 42 677 2546; fax: +81 42 677 2525.
E-mail address: nishinaga-tohru@tmu.ac.jp (T. Nishinaga).
Scheme 1. Synthesis of the targeted oligomers 1 and 2.
0
040-4039/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2008.11.061

5
56
M. Fujii et al. / Tetrahedron Letters 50 (2009) 555–558
First we attempted to synthesize 1(R = H), but we failed to obtain
the product pure enough for FET application due to the instability
of the products. Thus, 1(R = Me) were synthesized from 5.
Similarly, the synthesis of hexamer 2(R = Me) was conducted by
Paal–Knorr reaction of asymmetric diketone 6 followed by homo-
coupling of the resultant trimer. For the synthesis of the precursors
of asymmetric 1,4-diketones 5 and 6, we chose cross-coupling of
Table 1
Absorption maxima and oxidation potentials of the oligomers
Compd
(R = Me)
Absmax (nm)
E1/2ox1a (V)
E1/2ox2a (V)
370b
0.06c,d
1
0.20
0.51
0.59
0.16
0.55
e,f
c,f,g
DH-SOSOS
DH-SSSSS
421
426
0.20
e,h
d,e,h
0.37
0.08
405b
c,d
2(R = Me)
e,h
d,e,h
DH-SSSSSS
440
0.33
methyl ketone and
a-bromomethyl ketone in the presence of
NH1 instead of Stetter’s procedure. The advan-
1
9f
a
V versus Fc/Fc+.
ZnCl –t-BuOH–Et
2
2
b
c
d
e
f
In benzene.
In dichloromethane.
tages of the former method are the facile access to the precursors
and avoiding the use of toxic sodium cyanide.
These oligomers 1(R = Me) and 2(R = Me) are yellow and orange
crystals, respectively, soluble in common organic solvents, and sta-
ble in the solid and solution phases. Thus, they can be purified by
normal recrystallization from dichloromethane–hexane, even
0.1 M n-Bu
4
NPF
6
.
In THF.
Ref. 7a.
0.1 M n-Bu
Ref. 12.
g
h
4 4
NClO .
though the solubility of 2(R = Me) (ca. 2 mg/ml in CH
than that of 1(R = Me) (ca. 20 mg/ml in CH Cl ). The moderate sol-
ubility of 2(R = Me) is in contrast to the case of longer DH-SOSOSOS
2 2
Cl ) is less
2
2
views. Thus, the linearity of 2(R = Me) may cause greater van der
Waals contact in solid states and hence decrease the solubility.
The electronic absorption maxima and oxidation potentials of
7a
2 2
heptamer (ca. 10 mg/ml in CH Cl ), and can be rationalized by
the difference in the molecular structures as described below.
We tried to obtain single crystals suitable for X-ray structural
analysis, but all attempts have not succeeded so far. Thus, the
structures of 1(R = Me) and 2(R = Me) were estimated by DFT cal-
culations at the B3LYP/6-31G(d) level (see Supplementary data
for details). As shown in Figure 1, the thiophene and pyrrole rings
are twisted from coplanarity for both cases due to the presence of
the steric repulsion between methyl group and the adjacent thio-
phene rings. The averaged dihedral angles at S–C–C–N connections
are 147° for both cases. As concerns the molecular shapes, a top
view of 1(R = Me) shows a banana-shaped structure. This deforma-
tion takes place because of the difference in the bond angles of
S–C–C and N–C–C connections between thiophene and pyrrole
rings which stems from the different lengths between the S–C
1
(R = Me) and 2(R = Me) are summarized in Table 1, together with
the data of the corresponding dihexyl-oligothiophenes (DH-SSSSS,
DH-SSSSSS) and mixed pentamer (DH-SOSOS). The absorption
maxima of 1(R = Me) and 2(R = Me) show a large hypsochromic
1
2
7a
shift due to the out-of-plane distortion of the
p-system as
predicted by the DFT calculations. Nevertheless, the oxidation
potentials of 1(R = Me) and 2(R = Me) are much lower than those
of DH-SSSSS, DH-SSSSSS, and DH-SOSOS owing to the presence
of the electron-rich pyrrole moiety. This idea is consistent with
the fact that the p-extension by the insertion of one thiophene ring
from 1(R = Me) to 2(R = Me) does not affect much the oxidation
potentials.
The FET devices of 1(R = Me) and 2(R = Me) were fabricated by
vacuum deposition of the oligomers on bare or octadecyltrichloros-
ilane (OTS) treated Si/SiO
and N–C bonds. Similar deformations were also observed in the
alternately connected thiophene–furan oligomers.7a On the other
2
substrates and then source and drain Au
hand, a top view of 2(R = Me) shows a nearly linear structure with
a gentle S-shape, though the side views of both oligomers show
bent structures. These bending structures on the side views may
be flattened by packing force in solid states, but the flattening of
2
8
7
6
5
4
3
2
1
0
10
1
-
6
Vg=
40
a
b
-5
the
p-systems essentially does not alter the shape on the top
-4
35
-3
30
25
-2
-2
10
10
2
1
1
0
5
0
-1
-4
0
2
0
0
-20
-40
0
-10 -20 -30 -40
Drain Voltage (V)
Gate Voltage (V)
Figure 2. (a) Output characteristic and (b) transfer characteristic at ꢁ40 V of drain
voltage for 2(R = Me) with OTS treatment. Gate dielectric layer is a thermally
oxidized 200 nm thick SiO
respectively.
2
and the channel length and width are 0.05 and 5 mm,
Table 2
Field-effect characteristics of the oligomers
(cm2
V
ꢁ1
s
ꢁ1
)
I
on/Ioff
Vth (V)
Compd SiO treatment
2
l
3
103
1
1
(R = Me)
(R = Me)
Bare
OTS
OTS
Bare
Bare
OTS
Bare
7.9 ꢀ 10ꢁ
17
ꢁ
3
4
9.1 ꢀ 10
10
ꢁ5.4
a
ꢁ2
ꢁ2
3
DH-SOSOS
3.6 ꢀ 10
5.1 ꢀ 10
1.2 ꢀ 10
5.3 ꢀ 10
4.4 ꢀ 10
10
ꢁ2
DH-SSSSSb
10
10
4
ꢁ1
ꢁ
ꢁ
2
4
2
2
(R = Me)
(R = Me)
ꢁ1
2
5
10
ꢁ2.3
ꢁ2
b
ꢁ2
3
DH-SSSSSS
2 ꢀ 10
a
Figure 1. Top and side views of the optimized structures of (a) (b) 1(R = Me) and (c)
d) 2(R = Me).
Ref. 7a.
Ref. 4f.
b
(

M. Fujii et al. / Tetrahedron Letters 50 (2009) 555–558
557
electrodes with top-contact configuration.13 The measurements
were conducted under air. As shown in Figure 2 and Table 2, these
oligomers behave as p-type semiconductors. In spite of the dis-
torted structure in solution, 1(R = Me) showed good hole mobility
characteristics
V
(l
= 5.3 ꢀ 10 cm V
ꢁ2
2
ꢁ1 ꢁ1
s
,
I
on/Ioff = 105, and
th = ꢁ2.3 V with OTS treatment) than those of 1(R = Me) with a
banana-shaped structure, suggesting that the enhanced molecular
linearity causes the better FET performance.
ꢁ3
2
ꢁ1 ꢁ1
(
l
= 9.1 ꢀ 10 cm V
s
with OTS treatment), which is compa-
To investigate the thin film structures, the XRD measurements
were performed. As shown in Table 3 and Figure S21, the overall
structure of the film of 2(R = Me) is layered with the d-spacing ob-
tained from the first reflection peak being 3.15 nm. Since the
molecular length estimated by the DFT calculations (2(R = Me)
rable to those of more planar DH-SSSSS and DH-SOSOS. Further-
more, 2(R = Me) with nearly linear structure exhibited better FET
Table 3
3
.7 nm) is slightly larger than the d-spacing value, the molecule
Observed d-spacings and molecular lengths for films of the oligomers
is slightly tilted from a vertical orientation to the substrate. The tilt
angle is similar to those of oligothiophenes, although these angles
can not be directly compared due to the difference in the estima-
tion method of the molecular length. In contrast, the XRD pattern
of the film of 1(R = Me) (Fig. S20) appears to be assigned to micro-
crystalline phase. Thus, the films were also investigated by atomic
force microscope (AFM) to compare the film morphologies of
1(R = Me) and 2(R = Me). As shown in Figure 3, many small grains
with similar size were observed in both films but the grains of
2(R = Me) appeared to be denser, which is considered to cause
the better FET performance of 2(R = Me).
Compd
(R = Me)
d Spacing (nm)
Molecular length (nm)
a
1
—
3.06
3.0
3.15
3.54
3.3
b
3.15c
DH-SOSOS
d
c,e
DH-SSSSS
(R = Me)
3.74
a
2
3.7
d
c,e
DH-SSSSSS
3.92
a
Estimated by DFT calculations.
Ref. 7a.
Determined by X-ray analysis.
Ref. 4f.
b
c
d
e
Both lengths include standard van der Waals radii for carbon (1.70 Å) or
hydrogen atoms (1.20 Å).
In summary, we have newly synthesized thiophene–methylpyr-
role mixed oligomers 1(R = Me) and 2(R = Me), and revealed for the
first time that this type of oligomers showed p-type FET behavior
as good as the corresponding oligothiophenes. In comparison
between 1(R = Me) and 2(R = Me), the p-extension by the incorpo-
ration of one thiophene ring does not affect the HOMO levels. How-
ever, the hole mobility of 2(R = Me) is significantly enhanced due to
the nearly linear molecular structure of 2(R = Me) which seems to
cause the greater intermolecular interaction and better film
morphology.
Acknowledgments
This work was supported by Grant-in-Aid for Scientific
Research (C) (No. 19550049) from the Ministry of Education,
Culture, Sports, Science and Technology of Japan. We thank Mr.
Katsura Hirai and Mr. Tatsuya Tanaka (Konica MinoltaTechnology
Center, Inc.) for preparation of thin films and measurements of
field-effect properties, AFM, and XRD of these films.
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tetlet.2008.11.061.
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