Organic field-effect transistors based on heterocyclic co-oligomers containing a pyrazine ring

Article abstract of DOI:10.1039/b617573b

New oligomers containing a pyrazine unit have been prepared: the bithienyl derivatives afforded p-type FET devices whereas the trifluoromethylphenyl derivatives showed n-type FET behavior. The Royal Society of Chemistry.

Full text of DOI:10.1039/b617573b

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Organic field-effect transistors based on heterocyclic co-oligomers
containing a pyrazine ring{
Takahiro Kojima,a Jun-ichi Nishida,a Shizuo Tokito,b Hirokazu Tadac and Yoshiro Yamashita*a
Received (in Cambridge, UK) 1st December 2006, Accepted 11th January 2007
First published as an Advance Article on the web 30th January 2007
DOI: 10.1039/b617573b
also prepared.15 The optical properties and redox potentials of co-
New oligomers containing a pyrazine unit have been prepared:
oligomers 1–4 are shown in Table 1. Oxidation and reduction
potentials of 1–4 were measured by cyclic voltammetry.
Replacement of the benzene ring with a pyrazine ring in 2
increases the oxidation potentials a little and a reduction peak
appears. This fact indicates that the introduction of the pyrazine
ring decreases both the HOMO and LUMO levels. Replacement
of the end-thienyl groups with trifluoromethylphenyl groups in 3
positively shifts the reduction potential, indicating that the LUMO
level is lowered. The replacement of the thiophene groups with the
electron-accepting thiazole rings in 4 induces a further positive shift
of the reduction potential, indicating that 4 has the lowest LUMO
level among them.
the bithienyl derivatives afforded p-type FET devices whereas
the trifluoromethylphenyl derivatives showed n-type FET
behavior.
Much attention has recently been focused on organic field-effect
transistors (OFETs) because of their possible applications to low-
cost integrated circuits and flexible displays.1–10 Development of
new organic semiconductors is very important for the progress in
this field. Pentacene and thiophene oligomers are typical
semiconductors affording p-type FETs. However, they are air-
sensitive and more stable materials have been strongly required. In
order to enhance the air-stability, phenylene-thiophene co-
oligomers have been designed.11,12 However, those systems have
non-planar geometry owing to the steric interactions between the
neighboring rings, resulting in decreased carrier mobilities. In order
to overcome this disadvantage, we have now introduced a pyrazine
ring instead of the phenylene ring because it has no hydrogen
atoms for steric repulsion. The pyrazine ring is an electron-
accepting heterocycle, which would be useful to decrease the
HOMO levels and enhance the air-stability in p-type FETs.
Furthermore, we have also developed here novel n-type semi-
conductors containing a pyrazine ring because it decreases the
LUMO levels leading to high electron affinity which is required for
n-type semiconducting behavior.3,13,14
The absorption and emission maxima of the pyrazine derivative
2 are red-shifted compared to those of other compounds. This fact
can be attributed to the intramolecular charge transfer from the
electron-donating bithienyl group to the electron-accepting
pyrazine group. All of the compounds 1–4 exhibited efficient
fluorescence. The emission maxima are observed at longer
wavelength in the solid state than those in solution, indicating
the presence of the strong intermolecular interaction in the solid
state.
Single crystals of 3 suitable for X-ray analysis were obtained by
recrystallization from chloroform.{ There exist two crystallogra-
phically independent molecules (molecules 1 and 2) each of which
lies about an independent inversion centre. The molecules are a
little twisted (Fig. 1), where the dihedral angles between the
thiophene ring and the pyrazine ring are 4.9u (molecule 1) and
14.7u (molecule 2), and those between the thiophene ring and the
trifluoromethylphenyl group are 16.1u (molecule 1) and 7.8u
(molecule 2). It should be noted that the molecules have a definite
geometry, with the nitrogen of the pyrazine ring and the sulfur of
the thiophene located at the same direction. This is attributed to an
electrostatic interaction between the electron-negative N atom and
the electron-positive S atom, and short contacts between the N and
Pyrazine-containing co-oligomers 2–4 were prepared by the
Stille coupling reaction of 2-bromo-5-iodopyrazine with the
corresponding stannyl reagents in the presence of Pd(PPh3)4 in
DMF in 8–25% yields. For comparison, benzene derivative 1 was
˚
S atoms of 2.9 A are observed for both molecules 1 and 2. Such a
aTokyo Institute of Technology, Interdisciplinary School of Science and
Engineering, Department of Electronic Chemistry, 4259 Nagatsuta,
Midori-ku, Yokohama 226-8502, Japan.
Table 1 Optical properties and redox potentials of co-oligomers 1–4
Solutiona
abs/nm
Solid
em/nm
E-mail: yoshiro@echem.titech.ac.jp; Fax: +81-45-924-5489;
Tel: +81-45-924-5571
Compound
l
lem/nm
l
Eoxb/V
Eredb/V
bNHK Science & Technical Research Laboratories, 1-10-11 Kinuta,
Setagaya-ku, Tokyo 157-8510, Japan
1
2
3
4
a
391
426
407
391
445
482
453
430
511, 545
572
508, 542
539
+0.72
+0.78
—
—
cDivision of Materials Physics, Department of Materials Engineering
Science, Graduate School of Engineering Science, Osaka University, 1-3,
Machikaneyama, Toyonaka 560-8531, Japan
21.96
21.86
21.76
—
b
In CH2Cl2. 0.1 M nBu4NPF6 in DMF, Pt electrode, scan rate
{ Electronic supplementary information (ESI) available: Synthesis of co-
oligomers, FET device fabrication and performance data, and molecular
structure. See DOI: 10.1039/b617573b
100 mV/s, V vs Fc/Fc+.
1430 | Chem. Commun., 2007, 1430–1432
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Table 3 Field-effect characteristics of 2 and 4 on top contact
geometry
Mobility/
On/off
Compound Type Tsub/uC cm2 V21 s21 ratio
Threshold/V
2
4
p
n
50
50
0.01
0.04
1.4 6 104 235
5.1 6 103 +36
Fig. 1 A view of the crystal packing of compound 3.
Table 2 Field-effect characteristics of 1–4 films on bottom contact
geometry at different temperatures
Mobility/
On/off
Compound Type Tsub/uC cm2 V21 s21 ratio
Threshold/V
1
2
3
4
p
p
n
n
r.t
50
r.t
50
r.t
50
r.t
50
3.1 6 1026 4.0 6 103 2141
1.1 6 1024 5.1 6 105 2109
3.6 6 1024 9.3 6 107 269
1.0 6 1023 4.6 6 106 2126
2.9 6 1024 1.6 6 106 +108
8.3 6 1024 2.8 6 106 +105
1.2 6 1023 2.7 6 107
3.0 6 1023 2.7 6 107
+62
+41
definite geometry is considered to be favorable for an ordered
molecular arrangement. Those molecules are packed in a
herringbone manner as shown in Fig. 1. On the other hand, the
benzene derivative 1 is considered to have a disordered structure
since the direction of the thienyl groups cannot be controlled.
FET devices based on 1–4 were fabricated on SiO2/Si substrates
by vapor-deposition with bottom contact geometry. Gold electro-
des forming channels of 25 mm length (L) and 6 mm width (W)
were photolithographically defined. The semiconductor layer was
evaporated on the electrode/dielectric surface, where the SiO2 layer
was 600 nm thick. The FET performances are summarized in
Table 2.
Fig. 2 Output characteristics (a) and transfer characteristics (b) of
oligomer 4 FET.
50 nm of semiconductor deposition at 50 uC by using shadow
masks with W/L of 1.0 mm/100, 50 mm. The SiO2 gate electric was
200 nm thick and treated with hexamethyldisilazane (HMDS). The
FET measurements were carried out at room temperature in a
high vacuum chamber (1025 Pa). The mobilities were improved as
expected (2: p-type: 0.01 cm2 V21 s21, 4: n-type; 0.04 cm2 V21 s21).
At the same time, the threshold voltages were reduced (Table 3).
The output characteristics of 4 with the top contact geometry are
shown in Fig. 2.
Bithienyl derivatives 1 and 2 showed p-type FET characteristics.
The hole mobility and on/off ratio of 2 are higher than those of 1.
In addition, the air-stability of the device of 2 was enhanced
compared to that of 1. These improved performances can be
attributed to the introduced pyrazine ring as expected. On the
other hand, the trifluoromethylphenyl derivatives 3 and 4 showed
n-type FET behavior. The electron mobility was improved by
replacing the thienyl unit with the thiazole one. This replacement
also induces the reduction of the threshold voltage. This is
considered to be due to the higher electron affinity of the thiazole
ring as deduced from the higher reduction potential of 4. Higher
FET performances are generally observed with top contact
geometry than with bottom contact one because of better contacts
between the electrodes and semiconductors in the former.
Therefore, top-contact devices using 2 and 4 were fabricated to
improve the performances. Gold electrodes were defined after
The films of the co-oligomers 1–4 deposited on SiO2/Si
substrates were investigated by X-ray diffraction in reflection
mode (XRD) (Fig. 3). Sharp reflections were observed in the 1–3
films, indicating the formation of lamella ordering and crystallinity
on the substrate. However, the thin film of 4 which showed the
highest electron mobility did not show clear XRD peaks. This
point is still puzzling.
In summary, we have developed new co-oligomers containing a
pyrazine unit. The bithienyl derivatives afforded p-type FET
devices, where the introduction of the pyrazine ring was found to
enhance the FET performance as well as the air-stability. The
trifluoromethylphenyl derivatives showed n-type FET behavior
and the introduction of the electron-accepting unit was effective to
increase the electron mobility and decrease the threshold voltage.
This work was supported by The 21st Century COE program, a
Grant-in Aid for Scientific Research on Priority Areas
This journal is ß The Royal Society of Chemistry 2007
Chem. Commun., 2007, 1430–1432 | 1431

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(SIR97) and refined by the full-matrix least-squares method on F2 with
anisotropic temperature factors for non-hydrogen atoms. Hydrogen atoms
were located at calculated positions. Absorption correction was applied
using an empirical procedure. C26H14N2S2F6 M = 532.52, crystal
¯
dimensions 0.70 6 0.60 6 0.05 mm, triclinic, space group P1, a =
˚
6.176(6), b = 9.17(1), c = 20.01(3) A, a = 99.68(5), b = 89.06(5), c =
105.91(4)u, V = 1074(7) A , Z = 2, Dc = 1.646 g cm23, 9653 reflections
3
˚
collected, 4641 independent (Rint = 0.019), GOF = 1.31, R1 = 0.044, wR2 =
0.148 for all reflections. CCDC reference number 629009. For crystal-
lographic data in CIF or other electronic format see DOI: 10.1039/
b617573b
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1432 | Chem. Commun., 2007, 1430–1432
This journal is ß The Royal Society of Chemistry 2007