Electronegative oligothiophenes having difluorodioxocyelopentene-annelated thiophenes as solution-processable n-Type OFET materials

Article abstract of DOI:10.1246/cl.2009.460

Solution-processable electronegative oligothiophenes bearing difluorodioxocyclopentene-annelated thiophenes have been synthesized. The electronic properties of the oligomers were investigated by absorption spectroscopy and electrochemical measurements. Th

Full text of DOI:10.1246/cl.2009.460

460
Chemistry Letters Vol.38, No.5 (2009)
Electronegative Oligothiophenes Having Difluorodioxocyclopentene-annelated Thiophenes
as Solution-processable n-Type OFET Materials
Yutaka Ie,1 Makoto Okabe,1 Yoshikazu Umemoto,1 Hirokazu Tada,2 and Yoshio Asoꢀ1
1The Institute of Scientific and Industrial Research, Osaka University, 8-1 Mihogaoka, Ibaraki, Osaka 567-0047
2The Graduate School of Engineering Science, Osaka University, 1-3 Machikaneyama, Toyonaka, Osaka 560-8531
(Received February 26, 2009; CL-090202; E-mail: aso@sanken.osaka-u.ac.jp)
Solution-processable electronegative oligothiophenes bear-
ing difluorodioxocyclopentene-annelated thiophenes have been
synthesized. The electronic properties of the oligomers were
investigated by absorption spectroscopy and electrochemical
measurements. Their films fabricated by spin-coating showed
typical n-channel OFET behavior.
Figure 2. Chemical structures of 1 and 2.
Much attention has been focused on the development of su-
perior materials for organic field-effect transistors (OFETs) due
to their potential for low cost, large area, and flexible electronic
devices.1 The majority of such research has been devoted to
p-type organic semiconductors, and oligo- and polythiophenes
are one of the most studied ꢀ-conjugated systems.1a,1b,2 On the
other hand, the investigation of n-type ꢀ-conjugated semicon-
ductors, which are an essential component for the realization
of organic complementary circuits, has been falling behind.3–7
We have recently reported that difluorodioxocyclopenta[c]thio-
phene (C) and difluorodioxocyclopenta[b]thiophene (B) units
effectively contribute to increasing the electronegative character
of oligothiophenes, and hence the vacuum-deposited film of
BTTB (Figure 1) exhibited high field-effect electron mobility.8
Another important aspect is the development of OFET materials
sufficiently soluble in organic solvents, because their active
layers can be fabricated by solution techniques such as spin-
coating and drop-casting.9 However, solution-processable n-
type-OFET materials are still rare owing to the difficulty of
adjusting the electron-withdrawing nature, solubility, and film
morphology.7b,10,11 In this communication, we report on the syn-
thesis, properties, and FET performance of electronegative
oligothiophenes having difluorodioxocyclopentene-annelated
thiophenes (B and C units) as well as 3-hexylthiophene (H unit)
to ensure solution processability (Figure 1).
stannyl compounds 1 and 2 (Figure 2) with the corresponding
central units by Stille couplings followed by acidic ketal depro-
tection. Unlike BTTB, these oligomers are soluble in common
organic solvents such as THF, chloroform, and toluene. Thus,
the products could be purified by silica-gel column chromatog-
raphy and preparative gel-permeation liquid chromatography.
Detailed synthetic procedures are given in Supporting Informa-
tion.12
The electronic properties of the oligothiophenes were inves-
tigated by UV–vis absorption spectra and cyclic voltammetry
(CV) measurements (Figure S1),12 and the data were summariz-
ed in Table 1. Compared with the absorption maximum of BTTB
(463 nm in THF),8 the introduction of hexyl groups causes a
slight blue shift of 5 nm for BHHB. On the other hand, the re-
placement of the thiophene unit in BHTHB with the C unit re-
sults in 36-nm red shift for BHCHB. As we have previously re-
ported, this is attributable to the enhancement of a donor (H unit)
and acceptor (B and C units) alternative configuration.8 In CV
measurements, all the oligomers showed a reversible reduction
wave, reflecting electrochemical stability of their radical anions.
Moreover, the reduction potential of BHCHB is positively shift-
ed largely compared with those of BHHB and BHTHB, indicat-
ing that the B unit as well as the C unit effectively lowers the
LUMO energy levels (Table 1)13 and thus enhances the electro-
negativity of oligothiophenes.
Novel oligothiophenes BHHB, BHTHB, and BHCHB were
synthesized, similarly to the recently reported BTTB,8 from
Bottom-contact OFET devices were fabricated on the gate
electrode of p-doped silicon substrate. Source and drain gold
electrodes with W=L of 294 mm/25 mm were prepatterned on
a layer of SiO2 dielectrics (300 nm). After HMDS treatment
of the substrate, films of the oligomers with a thickness of
100 nm were prepared by spin-coating from 1.0 wt % chloroform
solution at 1500 rpm for 1 min onto the substrate. Subsequently,
under high vacuum, the OFET devices were annealed at 150 ꢁC
Table 1. Absorptiona and electrochemical datab of the oligo-
mers
Oligomer
ꢁmaxa/nm
Ered1=2/V
Ep.a/V
LUMOc/eV
BHHB
BHTHB
BHCHB
458
482
518
ꢂ1:72
ꢂ1:70
ꢂ1:26
+0.99
+0.96
+1.28
ꢂ3:08
ꢂ3:10
ꢂ3:54
aIn THF. bIn C6H5F, 0.1 M n-Bu4NPF6, V vs. Fc/Fcþ. cLUMO ¼
Figure 1. Chemical structures of BTTB, BHHB, BHTHB, and
BHCHB.
ꢂðEred1=2 þ 4:8Þ.
Copyright ꢀ 2009 The Chemical Society of Japan

Chemistry Letters Vol.38, No.5 (2009)
461
Table 2. Field-effect characteristics of the oligomers
ꢂ/cm2 Vꢂ1 sꢂ1
lated thiophenes and lipophilic hexyl groups into a ꢀ-conjugated
oligomeric backbone. Our preliminary studies demonstrated that
the combination of the B and C units is considerably improve-
ment for n-channel OFET performance. It is important to note
that this finding is in marked contrast to recently reported
BCB-based OFET devices fabricated by a vacuum-deposition
method,8 where the nonstacking manner in the crystal has sug-
gested the disturbance of its OFET behavior. Further design of
related soluble ꢀ-conjugated systems for the application to the
active layers of n-type OFET devices and photovoltaic devices
is underway in our laboratory.
Oligomer
Ion=Ioff
Vth/V
BHHB
BHTHB
BHCHB
no FET
6:7 ꢃ 10ꢂ7
1:1 ꢃ 103
7:5 ꢃ 104
30
20
1:9 ꢃ 10ꢂ4
This work was supported by Grants-in-Aid for Scientific
Research (Nos. 20027011 and 20108004) from the Ministry of
Education, Culture, Sports, Science and Technology, Japan,
and the Industrial Technology Research Grant Program in
2008 from the New Energy and Industrial Technology Develop-
ing Organization (NEDO) of Japan, and also supported by the
Cooperative Research with Sumitomo Chemical Co., Ltd.
Figure 3. (a) Output characteristics of bottom-contact OFET of
BHCHB and (b) transfer characteristics of the same device at
100 V of drain voltage.
References and Notes
1
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with low field-effect electron mobility. Moreover, the OFET per-
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force microscope (AFM) measurements. As shown in
Figure 4a, unconnected islands were observed for the BHHB
film, which clearly visualized the lack of charge-transporting
pathways. The BHTHB film displayed needle-like isolated crys-
tals, whereas the observation of the BHCHB film revealed that
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12 Supporting Information is available electronically on the CSJ-Journal
Web site, http://www.csj.jp/journals/chem-lett/index.html.
13 The LUMO levels are calculated under the premise that the energy
level of ferrocene/ferrocenium (Fc/Fcþ) is ꢂ4:8 eV below the vacuum
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14 X-ray diffraction measurement of BHCHB films did not show clear
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are formed on the SiO2 substrates.
Figure 4. AFM images of thin films of BHHB (a), BHTHB (b),
and BHCHB (c).
Published on the web (Advance View) April 11, 2009; doi:10.1246/cl.2009.460