Largely π-extended thienoacenes with internal thieno[3,2-b]thiophene substructures: Synthesis, characterization, and organic field-effect transistor applications

Article abstract of DOI:10.1021/ol302243t

Two largely π-extended thienoacenes with internal thieno[3,2-b]thiophene substructures, i.e., bis[1]benzothieno[2,3-d;2′,3′-d′] benzo[1,2-b;4,5-b′]-dithiophene (BBTBDT) and bis(naphtho[2,3-b]thieno)[2, 3-d;2′,3′-d′]benzo[1,2-b;4,5-b′]dithiophene (BNTBDT),

Full text of DOI:10.1021/ol302243t

ORGANIC
LETTERS
2012
Vol. 14, No. 18
4914–4917
Largely π‑Extended Thienoacenes
with Internal Thieno[3,2‑b]thiophene
Substructures: Synthesis,
Characterization, and Organic
Field-Effect Transistor Applications
Tatsuya Yamamoto,† Takeshi Nishimura,† Takamichi Mori,† Eigo Miyazaki,†
Itaru Osaka,† and Kazuo Takimiya*,†,‡
Department of Applied Chemistry, Graduate School of Engineering, Hiroshima
University, Higashi-Hiroshima 739-8527, Japan, and Emergent Molecular Function
Research Team, RIKEN Advanced Science Institute, Wako, Saitama 351-0198, Japan
ktakimi@hiroshima-u.ac.jp
Received August 13, 2012
ABSTRACT
Two largely π-extended thienoacenes with internal thieno[3,2-b]thiophene substructures, i.e., bis[1]benzothieno[2,3-d;20,30-d0]benzo[1,2-b;4,5-b0]-
dithiophene (BBTBDT) and bis(naphtho[2,3-b]thieno)[2,3-d;20,30-d0]benzo[1,2-b;4,5-b0]dithiophene (BNTBDT), were synthesized, characterized,
and evaluated as an active layer in thin-film organic field-effect transistors.
Thienoacene-based organic semiconductors have been
intensively investigated during the past decade, and in fact
they have contributed significantly in the development
of high-performance organic field-effect transistors show-
ing mobilities higher than 1.0 cm2 Vꢀ1 sꢀ1.1 Excellent air
stability is another attractive advantage of such thienoacene-
based organic semiconductors as [1]benzothieno[3,2-b]-
[1]benzothiophene (BTBT)-2 and dinaphtho[2,3-b;20,30-f]-
thieno[3,2-b]thiophene (DNTT)-based materials3 (Figure 1).
The key structural feature in these materials is an internal
thieno[3,2-b]thiophene substructure incorporated into a
π-extended molecular framework, which significantly con-
tributes to good stability by lowering the HOMO energy
levels4 and to high mobility in the solid state by effecting the
intermolecular interaction via a nonbonded sulfurꢀ
sulfur interaction, which largely helps intermolecular orbital
overlap.1c These apparent benefits from the internal thieno-
[3,2-b]thiophene substructure in the π-extended systems
have prompted synthetic chemists to develop related or-
ganic semiconductors with the thieno[3,2-b]thiophene sub-
structure in largely extended π-systems with more than
five fused-aromatic rings.5 We have also been interested
in such large π-systems with two thieno[3,2-b]thiophene
† Hiroshima University.
‡ RIKEN Advanced Science Institute.
(1) (a) Anthony, J. E. Chem. Rev. 2006, 106, 5028–5048. (b) Anthony,
J. E. Angew. Chem., Int. Ed. 2008, 47, 452–483. (c) Takimiya, K.;
Shinamura, S.; Osaka, I.; Miyazaki, E. Adv. Mater. 2011, 23, 4347–
4370. (d) Wang, C.; Dong, H.; Hu, W.; Liu, Y.; Zhu, D. Chem. Rev. 2012,
112, 2208–2267.
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Y.; Otsubo, T. J. Am. Chem. Soc. 2006, 128, 3044–3050. (b) Takimiya,
K.; Ebata, H.; Sakamoto, K.; Izawa, T.; Otsubo, T.; Kunugi, Y. J. Am.
Chem. Soc. 2006, 128, 12604–12605. (c) Ebata, H.; Izawa, T.; Miyazaki,
E.; Takimiya, K.; Ikeda, M.; Kuwabara, H.; Yui, T. J. Am. Chem. Soc.
2007, 129, 15732–15733. (d) Izawa, T.; Miyazaki, E.; Takimiya, K.
Chem. Mater. 2009, 21, 903–912.
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2224–2225. (b) Yamamoto, T.; Shinamura, S.; Miyazaki, E.; Takimiya,
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T.; Shinamura, S.; Miyazaki, E.; Takimiya, K. Chem. Sci. 2010, 1, 179–
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Ikeda, M.; Kuwabara, H. Adv. Mater. 2011, 23, 1222–1225. (e) Niimi,
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r
10.1021/ol302243t
Published on Web 09/10/2012
2012 American Chemical Society

substructures and focused on two thienoacenes with seven
or nine fused aromatic rings, namely bis[1]benzothieno[2,3-
d;20,30-d0]benzo[1,2-b;4,5-b0]dithiophene (BBTBDT) and
bis(naphtho[2,3-b]thieno)[2,3-d;20,30-d0]benzo[1,2-b;4,5-b0]-
dithiophene (BNTBDT) (Figure 1). Here, we report their
synthesis featured by a double iodine-promoted thienothio-
phene formation reaction, characterization by means of single
crystal X-ray analysis, UVꢀvis absorption spectra, photoelec-
tron spectroscopy in air (PESA), and their thin-film-based
organic field-effect transistor (OFET) characteristics.
present approach to large thienoacenes is quite efficient,
as such highly π-extended thienoacenes as BNTBDT with
nine aromatic rings in a linear manner can be conveniently
synthesized. As expected from the highly π-extended,
rigid, and planar structures without any substituents at
the periphery, BBTBDT and BNTBDT are not soluble in
common organic solvents, and thus their characterizations
were done with mass spectroscopy and combustion ele-
mental analysis. For BBTBDT, single crystal X-ray anal-
ysis affords definite structural confirmation (Figure 2).
Figure 2. Molecular structure of BBTBDT.
Figure 1. Thienoacene-based organic semiconductors with internal
thieno[3,2-b]thiophene substructure(s).
Owing to their poor solubility and good film forming
property, evaluation of electronic structures of BBTBDT
and BNTBDT was done by using their vacuum-deposited
thin films. As depicted in Figure 3a, ionization potentials
(IPs), closely related to the HOMO energy levels, of both
compounds are determined to be 5.2 eV, whereas the
optical band gap estimated from the absorption edge are
2.8 eV for BBTBDT and 2.4 eV for BNTBDT, respectively
(Figure 3b). In general, extension of the π-electron system
tends to elevate the HOMO energy level, i.e., lower the IP,
and reduce the HOMOꢀLUMO gap. The experimental IPs
for the compounds are not the case, and in fact, calculated
HOMO energy levels using the DFT methods7 are 5.51 eV
for BBTBDT and 5.16 eV for BNTBDT, respectively,
below the vacuum level (Figure S3). This can be partially
interpreted by the fact that the present evaluation was done
in the thin film state, where the electronic structure can be
influenced by intermolecular interaction; in this context,
BBTBDT may have strong intermolecular interaction in the
The synthesis of BBTBDT and BNTBDT are outlinedin
Scheme 1, where the iodine-promoted thienothiophene
formation from o-bis(methylthio)stilbenes, originally de-
veloped for the synthesis of DNTT, is employed at the
final step. For the synthesis of appropriate precursors
for BBTBDT and BNTBDT, the WittigꢀHorner reaction
was found to be very useful. Thus, a key intermedi-
ate in the synthesis is 2,5-bis(methylthio)terphthalalde-
hyde (1), which was reacted with a 2-fold amount of di-
methyl 2-(methylthio)benzylphosphonate (2)6 or dimethyl
((3-(methylthio)naphthalen-2-yl)methyl)phosphonate (4)
to give the corresponding tetrakis(methylthio)-1,4-di((E)-
styryl)benzene derivatives (3 or 5) in good yields. The
subsequent iodine-promoted thienothiophene formation
gave the desired thienoacene, BBTBDT or BNTBDT,
respectively, in modest yields. It should be noted that,
although the yields of the final steps are not very high, the
Scheme 1. Syntheses of BBTBDT and BNTBDT
Org. Lett., Vol. 14, No. 18, 2012
4915

solid state. In any case, these IPs in the thin film state are
thought to be suitable as a p-channel organic semiconductor
for OFET application in terms of both facile hole injection
from the source electrode and good stability in air.8
in which a typical molecular lamella structure along the crys-
tallographic a-axis is observed (Figure 5a). In the BBTBDT
layer in the crystallographic bc-plane (Figure 5b), π-stacking
columns with face-to-face molecular overlap exist, which
is in sharp contrast to related thienoacene-based organic
semiconductors such as BTBT2 and DNTT derivatives3 as
well as acenedithiophene derivatives,9 which generally
crystallize into the herringbone packing structure afford-
ing strongly interactive, two-dimensional electronic struc-
tures. To gain further insight into the intermolecular
interaction, we calculated intermolecular transfer integrals
(orbital coupling) of the HOMO (tHOMO) in the bc-plane.10
The calculated tHOMOs are relatively small (∼22 meV)
compared to the ones observed for DNTT and BTBT deri-
vatives(up to 80 meV),1c which qualitatively agrees well with
the lower mobility of the present BBTBDT-based OFETs.
Figure 3. Characterization of evaporated thin films: (a) ioniza-
tion potentials (IPs) determined by photoelectron yield spectro-
scopy in air and (b) UVꢀvis absorption spectra.
OFET characteristics of BBTBDT and BNTBDT were
evaluated by using vapor-deposited thin films on Si/SiO2
substrates modified with octyltrichlorosilane (OTS)- or
octadecyltrichlorosilane (ODTS)-SAM with a top-contact,
bottom-gate device configuration. Typical output and
transfer curves of the BBTBDT- and BNTBDT-based
devices are shown in Figure 4, and the mobility extracted
from the saturation regime withother device characteristics
are summarized in Table S1.
It is interesting to note that the extracted mobilities of
BBTBDT-based devices are not significantly affected by
the device fabrication conditions; regardless of the surface
treatment and substrate temperature during deposition
(Tsub,), the mobilities are on the order of 10ꢀ2 to 10ꢀ1
cm2 Vꢀ1 sꢀ1 and the maximum mobility obtained was
0.14 cm2 Vꢀ1 ꢀ1. Compared to the mobilities of other
s
Figure 4. Transfer and output characteristics of OTFTs fabri-
cated on ODTS-treated Si/SiO2 substrate: (a) BBTBDT (Tsub
60 °C) and (b) BNTBDT (Tsub = 150 °C).
related π-extended thienoacene-based OFETs, the present
values are rather modest. This could be partially attributed
to the packing structure in the solid state. XRD patterns of
the BBTBDT thin film for both out-of-plane and in-plane
measurements indicate that the film is nicely crystalline,
and all the XRD peaks that appeared are indexed by the
bulk single crystal structure (Figure S4). This means that
packing structures both in the thin film state and in the
bulk single crystals are identical, and thus the electronic
structure in the solid state can be discussed by using the
crystal structure elucidated by the single crystal structural
analysis. Figure 5 shows the crystal structure of BBTBDT,
=
In contrast to the modest OTFT characteristics of the
BBTBDT-based devices, the BNTBDT-based ones
showed a much enhanced mobility up to 0.94 cm2
V
characteristics are largely influenced by Tsubs: at Tsub
ꢀ1 sꢀ1. It is interesting to note, however, that the device
s
higher than 100 °C, the mobility exceeds 0.1 cm2 Vꢀ1 sꢀ1
whereas, at lower Tsubs, the mobilities are lower by
more than 1 order of magnitude (Table S1). These results
,
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od at the B3LYP/6-31g(d) level using the Gaussian 03 program package.
Frisch, M. J. et al. Gaussian 03, revision C.02; Gaussian, Inc.: Wallingford,
CT, 2004. See Supporting Information for full reference.
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4916
Org. Lett., Vol. 14, No. 18, 2012

(BNTBDT), with two internal thieno[3,2-b]thiophene sub-
structures by featuring the iodine-promoted thienothio-
phene formation reaction. As expected from the molecular
structures, these thienoacenes have suitable HOMO energy
levels as p-channel organic semiconductors for OFET
application. Although the device performances of the
BBTBDT-based OTFTs were modest (up to 0.14 cm2
V
ꢀ1 sꢀ1), the BNTBDT-based OFETs gave relatively high
mobilities close to 1.0 cm2 Vꢀ1 ꢀ1. The present experi-
s
mental results indicate that the largely π-extended thienoa-
cenes cannot always afford good performances as organic
semiconductors. Nevertheless, the BNTBDT-based OFETs
showing mobilities close to 1.0 cm2 Vꢀ1 sꢀ1 are quite attrac-
tive, since recent OFET applications require many different
material characteristics, not only high mobility and air
stability but also thermal stability, for device integration
and medical applications.12 For the realization of thermally
stable OTFTs, a largely π-extended organic semiconductor
seems to be advantageous, since the thermal stability of the
thin film state is generally good for large molecules. From
this viewpoint, further optimization and application of
BNTBDT-based OTFTs are now underway in our group.
Figure 5. Packing structure of BBTBDT: (a) molecular lamella
structure along the crystallographic a-axis direction, (b) π-stacking
structure in the bc-plane. Calculated transfer integrals (tHOMOs)
are tb = 12 meV and tp = 22 meV.
can be qualitatively explained by considering the mechan-
ism of crystal growth: during vacuum deposition, highly
extended molecules such as BNTBDT tend to readily form
crystal cores on the substrate at low temperature resulting in
crystalline thin films with small crystallites.11 Such a thin
film thus has many grain boundaries, which in fact limit the
charge transport in thin film transistors. In contrast, a
higher Tsub can regulate crystal nucleation, resulting in thin
films with larger grains. This speculation is supported by the
surface observation by AFM (Figure S6): the BNTBDT
thin film deposited at Tsub = rt has a surface without clear
texture or consisting of very small grains less than 100 nm in
size, whereas the film deposited at 150 °C clearly shows
larger grains around 500 nm in size.
Acknowledgment. This work was financially supported
by Grants-in-Aid for Scientific Research (No. 23245041)
from MEXT, Japan and a Founding Program for World-
Leading R&D on Science and Technology (FIRST),
Japan. HRMSs were carried out at the Natural Science
Center for Basic Research and Development (N-BARD),
Hiroshima University.
Supporting Information Available. Synthetic details,
characterizations data, and NMR spectra of materials,
crystallographic information files (CIF) for BBTBDT,
XRD patterns and AFM images of evaporated thin films,
detailed FET characteristics, complete author list for
ref 7. This material is available free of charge via the Internet
at http://pubs.acs.org.
In summary, we have successfully synthesized highly
π-extended thienoacenes, bis[1]benzothieno[2,3-d;20,30-d0]-
benzo[1,2-b;4,5-b0]dithiophene (BBTBDT) and bis(naphtho-
[2,3-b]thieno)[2,3-d;20,30-d0]benzo[1,2-b;4,5-b0]dithiophene
(12) Kuribara, K.; Wang, H.; Uchiyama, N.; Fukuda, K.; Yokota,
T.; Zschieschang, U.; Jaye, C.; Fischer, D.; Klauk, H.; Yamamoto, T.;
Takimiya, K.; Ikeda, M.; Kuwabara, H.; Sekitani, T.; Loo, Y.-L.;
Someya, T. Nat. Commun. 2012, 3, 723–1ꢀ7.
(11) (a) Sirringhaus, H.; H. Friend, R.; Wang, C.; Leuninger, J.;
Mullen, K. J. Mater. Chem. 1999, 9, 2095–2101. (b) Niimi, K.; Shinamura,
S.; Osaka, I.; Miyazaki, E.; Takimiya, K. J. Am. Chem. Soc. 2011, 133, 8732–
8739.
The authors declare no competing financial interest.
Org. Lett., Vol. 14, No. 18, 2012
4917