Preparation, spectral properties, and electron affinity of bis(thiadiazolo)quinoxaline and bis(thiadiazolo)phenanthroquinoxaline as n-type semiconductors

Article abstract of DOI:10.1246/cl.2011.1252

Bis(thiadiazolo)phenanthroquinoxaline and bis(thiadiazolo)- quinoxaline derivatives were prepared as a new candidate of n-type semiconductors and their electronic properties were investigated by spectroscopic and electrochemical measurements. The bis(thia

Full text of DOI:10.1246/cl.2011.1252

doi:10.1246/cl.2011.1252
1252
Published on the web October 15, 2011
Preparation, Spectral Properties, and Electron Affinity of Bis(thiadiazolo)quinoxaline
and Bis(thiadiazolo)phenanthroquinoxaline as n-Type Semiconductors
Tsutomu Ishi-i,*1 Taro Nakanishi,1 Yuuki Teshima,1 Takeshi Yasuda,*2 and Liyuan Han2
1Department of Biochemistry and Applied Chemistry, Kurume National College of Technology,
1-1-1 Komorino, Kurume, Fukuoka 830-8555
2Organic Thin-Film Solar Cells Group, Photovoltaic Materials Unit, National Institute for Materials Science (NIMS),
Tsukuba, Ibaraki 305-0047
(Received August 17, 2011; CL-110687; E-mail: ishi-i@kurume-nct.ac.jp)
Bis(thiadiazolo)phenanthroquinoxaline and bis(thiadiazolo)-
O
O
S
N
S
N
N
N
N
N
quinoxaline derivatives were prepared as a new candidate of
n-type semiconductors and their electronic properties were
investigated by spectroscopic and electrochemical measure-
ments. The bis(thiadiazolo)phenanthroquinoxaline having the
trifluoromethyl groups indicates n-type characteristic in an
organic field-effect transistor.
NH2
NH2
N
N
N
S
N
S
2
1
AcOH
3 (Y. 52%)
R
O
O
S
N
N
N
N
N
Organic semiconductors have been of much interest in
recent years in view of their application to carrier-transporting
materials in organic light-emitting diodes, organic field-effect
transistors (OFETs), and photovoltaics.1 p-Type semiconductor
has been extensively studied and has excellent characteristics.
In contrast, n-type semiconductor is relatively rare and has
unsatisfactory performance.2 Electron-deficient heteroaromatics
have been used as a central core of the n-type semiconductor
because of their high electron affinity.3-5 Thus, creation of a new
heteroaromatic as an n-type semiconductor is one of the most
important tasks in organic material science.
Recently, we have studied n-type semiconductor hexaaza-
triphenylenes, which are composed of three electron-deficient
pyrazine rings.6 We noticed that a new n-type semiconductor can
be created by replacement of the pyrazine ring with another
electron-deficient heteroaromatic ring. In this new strategy, we
have designed new heteroaromatics bis(thiadiazolo)quinoxaline
(BTQ) and bis(thiadiazolo)phenanthroquinoxaline (BTPQ), in
which two thiadiazole rings are used instead of the pyrazine
rings. Electron affinity of the thiadiazole ring is comparable to
that of the pyrazine ring.4,5,7 In the synthetic strategy, we use the
accumulated knowledge in our hexaazatriphenylene chemistry,
because the BTQ and BTPQ molecules would be obtained from
diaminobenzobis(thiadiazole) that is a synthetic intermediate of
the hexaazatriphenylenes.6 In this letter, we report the prepara-
tion of BTPQs and BTQ, and their spectral properties, electron
affinity, and OFET characteristics.8
The BTQ 3 with phenyl groups was prepared by condensa-
tion reaction of benzil (2) with diaminobenzobis(thiadiazole) 1
(Scheme 1). Similarly, the BTPQs 5a bearing octyl groups, 5b
bearing 4-octylphenyl groups, and 5c bearing 4-(trifluoro-
methyl)phenyl groups were obtained from the corresponding
phenanthrenequinones 4a, 4b, and 4c, respectively (Scheme 1).
The BTQ and BTPQs were identified on the basis of spectro-
scopic methods and elemental analysis.
High electron affinity as well as electrochemical stability are
studied by cyclic voltammetry (Figure 1a).9,10 In 5a-5c, two or
three quasi-reversible reduction potentials were observed around
¹1.6 to ¹2.2 V (vs. Fc/Fc+). The first reduction potentials
N
S
R
R
4a−4d
R
1
5a (Y. 69%): R =
(CH2)7CH3
AcOH
5b (Y. 63%): R =
(CH2)7CH3
CF3
5c (Y. 48%): R =
Scheme 1. Preparation of BTQ and BTPQs.
40
30
20
10
0
(a)
(b)
5c
3
0.6
5b
5a
5b
5c
5a
0.4
0.2
ε
-10
-0.5
0
-1
-1.5
-2
-2.5
-3
250
300
350
400
450
500
Voltage/V vs. Fc/Fc+
Wavelength/nm
Figure 1. (a) Cyclic voltammograms of 5a, 5b, and 5c in
dichloromethane (5 © 10¹4 M) in the presence of tetrabutyl-
ammonium tetrafluoroborate (0.1 M). (b) UV-vis spectra of
3, 5a, 5b, and 5c in dichloromethane (1.0 © 10¹5 M).
(¹1.59 to ¹1.70 V) in 5a-5c shift more positive compared
to those (¹1.7 to ¹1.8 V) of hexaazatriphenylene derivatives6e
bearing the three pyrazine rings, indicating the good electron
affinity in the BTPQ system bearing the two thiadiazole rings
and the one pyrazine ring. An enhancement in the electron
affinity is observed in 5c bearing the electron-withdrawing
trifluoromethyl groups: the first reduction potential (¹1.59 V) in
5c shifts more positive compared to that (¹1.70 V) in 5b bearing
the electron-donating octyl groups. In 5a-5c, no oxidation
potential was observed in the cyclic voltammogram (µ1.5 V),
indicating the n-type semiconducting nature of BTPQs.
In MALDI-TOFMS of 3 and 5a-5c, dimeric aggregate
species were detected.9 In addition to the parent ions of 3
Chem. Lett. 2011, 40, 1252-1253
© 2011 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

1253
(a)
8.0
6.0
4.0
2.0
0.0
(b)
1.E-08
1.E-09
1.E-10
1.E-11
1.E-12
1.2E-04
8.0E-05
4.0E-05
0.0E+00
We thank Professor Dr. Teruo Shinmyozu and Mr. Masaru
Konno (Kyushu University) for the CV measurement, and Mr.
Tomoyuki Ishikawa (Fukuoka Industrial Technology Center) for
the measurement of MALDI-TOFMS. This work was partially
supported by Grant-in-Aid for Scientific Research from the
Ministry of Education, Culture, Sports, Science and Technology
of Japan.
Gate voltage/V
50
45
40
35
30
25
20
References and Notes
0
10
20
30
40
50
0
10 20 30 40 50
Gate voltage/V
1
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Drain voltage/V
Figure 2. (a) Drain current-voltage relationship within the
gate voltage range from 0 to 50 V for an OFET based on 5c. (b)
Transfer characteristics (at a drain voltage of 50 V) of the OFET.
2
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(m/z 398), 5a (m/z 620), 5b (m/z 772), and 5c (m/z 684),
significant peaks are seen at a multiple of the parent ion to 796,
1240, 1544, and 1368, respectively. The self-assembling
aggregative nature is suitable for the electron carrier-transport-
ing.11
In dichloromethane, 5a, 5b, and 5c show absorption bands
at 414, 424, and 415 nm, respectively (Figure 1b). The bands in
5a-5c shift bathochromically compared to the band (357 nm) of
3 because of the expanding of the ³-electron system. Emission
bands were observed in the blue light region at 431 nm for 3,
433 nm for 5a, 472 nm for 5b, and 434 nm for 5c.9 Moderate
fluorescence quantum yields (ΦFL) were provided from the
BTPQ system: ΦFL of 0.19 in 5a, 0.53 in 5b, and of 0.36 in 5c.
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emitting materials.12
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going from solution to thin film state was observed in 3 and
5a-5c.9 For example, in 5c the band shifts from 415 nm in the
solution state to 427 nm in the film state. The bathochromic
shifts found in the film state indicate that the BTQ and BTPQ
molecules are self-assembled to form an aggregate, as suggested
in MALDI-TOFMS.
HOMO and LUMO levels in the film state were estimated
from the absorption tails and the ionization potentials.9 The
energy gap between HOMO and LUMO levels in 5b is smaller
than that of 5a, indicating that in 5b the introduced phenyl
groups decrease the LUMO level (¹3.26 eV) and increase the
HOMO level. In 5c, both the HOMO and LUMO levels decrease
because of the introduction of the trifluoromethyl groups. The
lowering LUMO level (¹3.70 eV) in 5c is suitable for the
electron injection and the subsequent electron-transport in
OFET.
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nature, we fabricated an OFET devise of 5c, expected to be the
most effective example. The drain current-voltage relationship
indicates an n-channel behavior (Figure 2a). From the plot of the
square root of the drain current versus the gate voltage, the
The preparation of the parent BTPQ and BTQ was reported without
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Supporting Information is available electronically on the CSJ-
Journal Web site, http://www.csj.jp/journals/chem-lett/index.html.
¹5
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cm2 V¹1 s¹1 with a threshold voltage of 11 V and an on/off ratio
of 3.3 © 103 (Figure 2b).
In conclusion, we have created new heteroaromatics
bis(thiadiazolo)quinoxaline and bis(thiadiazolo)phenanthroquin-
oxaline having the high electron affinity, which are new
candidates for electron-transporting materials.
Chem. Lett. 2011, 40, 1252-1253
© 2011 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/