Aceno[2,1,3]thiadiazoles for field-effect transistors: Synthesis and crystal packing

Article abstract of DOI:10.1021/ol200748c

An efficient synthetic approach to a series of aceno[2,1,3]thiadiazole derivatives is described. 2-TIPS and 2-TES molecules exhibited different crystal packings, and 2-TIPS show good device performances with hole mobility up to 0.4 cm² V^-1 s^-1 and an average mobility of 0.15 cm² V^-1 s^-1 as the active material for organic field-effect transistors. All of the results demonstrate these aceno[2,1,3]thiadiazole derivatives as promising materials for optoelectronic devices.

Full text of DOI:10.1021/ol200748c

ORGANIC
LETTERS
2011
Vol. 13, No. 10
2642–2645
Aceno[2,1,3]thiadiazoles for Field-Effect
Transistors: Synthesis and Crystal
Packing
Ting Lei, Yan Zhou, Chu-Yang Cheng, Yue Cao, Yang Peng, Jiang Bian, and Jian Pei*
Beijing National Laboratory for Molecular Sciences, the Key Laboratory of Bioorganic
Chemistry and Molecular Engineering of the Ministry of Education, College of
Chemistry and Molecular Engineering, Peking University, Beijing 100871, China
jianpei@pku.edu.cn
Received March 21, 2011
ABSTRACT
An efficient synthetic approach to a series of aceno[2,1,3]thiadiazole derivatives is described. 2-TIPS and 2-TES molecules exhibited different
crystal packings, and 2-TIPS show good device performances with hole mobility up to 0.4 cm2 Vꢀ1 sꢀ1 and an average mobility of 0.15 cm2 Vꢀ1 sꢀ1
as the active material for organic field-effect transistors. All of the results demonstrate these aceno[2,1,3]thiadiazole derivatives as promising
materials for optoelectronic devices.
Oligoacenes are among the most important families
of organic semiconductors because of their interesting
electronic properties for organic field-effect transistors
(OFETs) and organic photovoltaics (OPVs).1 Recently,
considerable investigations of the synthesis, functionalization,
and device performances of oligoacenes show their abun-
dance of structure diversity and fascinating electronic
properties.1ꢀ3 Figure 1 shows five typical pentacene deri-
vatives, electron-rich tetraceno[2,3-b]thiophene (Thio) and
electron-deficient tetraceno[2,3-c][1,2,5]thiadiazole (Thia),
1,2,3,4-tetrachloropentacene (Chlo), and 1,2,3,4-tetra-
fluoropentacene (Fluo). The syntheses and device perfor-
mances of Thio, Chlo, and Fluo were widely investigated by
Bao and Anthony et al.1,2 However, the synthetic ap-
proach to aceno[2,1,3]thiadiazoles (Thia) and their OFET
performances have not yet been reported because of the
complexity of the synthesis and the characterization.4
(1) (a) Anthony, J. E. Angew. Chem., Int. Ed. 2008, 47, 452. (b)
Anthony, J. E. Chem. Rev. 2006, 106, 5028.
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Z. J. Am. Chem. Soc. 2009, 131, 3733. (c) Tang, M. L.; Reichardt, A. D.;
Miyaki, N.; Stoltenberg, R. M.; Bao, Z. J. Am. Chem. Soc. 2008, 130,
6064. (d) Tang, M. L.; Okamoto, T.; Bao, Z. N. J. Am. Chem. Soc. 2006,
128, 16002.
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T.; Michael Steigerwald, L.; Nuckolls, C. J. Am. Chem. Soc. 2006, 128,
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Nuckolls, C. Adv. Mater. 2005, 17, 407. (c) Miao, Q.; Nguyen, T. Q.;
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r
10.1021/ol200748c
Published on Web 04/21/2011
2011 American Chemical Society

Scheme 1. Synthesis of Aceno[2,1,3]thiadiazole Derivatives
Using a Mukaiyama Aldol Reaction
Figure 1. Five typical pentacene derivatives compared in discussion.
To understand the electronic properties of the aceno-
[2,1,3]thiadiazoles, the investigation of density functional
theory (DFT) calculations for these five model oligoacene
derivatives is summarized in Table S1 (Supporting Informa-
tion). The calculations showed that compound Thia exhib-
ited the lowest LUMO levels and the smallest bandgap among
these five compounds, which might be attributed to its quinone
form of electronic structure. Interestingly, aceno2,1,3]thia-
diazole derivatives also had the lowest positive charge trans-
port reorganization energy. Therefore, it is believed that
aceno[2,1,3]thiadiazole derivatives might show very promis-
ing charge transport properties as organic semiconducting
materials.
Herein, we present a facile synthetic route and charac-
terization of four aceno[2,1,3]thiadiazole derivatives for
OFETs. The investigations demonstrate that these mole-
cules easily self-assemble into single crystal nano/micro-
wires (NWs/MWs) through πꢀπ stacking. The crystal
structures of 2-TIPS and 2-TES show different crystal
packing compared to the reported pentacene counterparts.
Finally, these MWs are employed as the active materials
to directly fabricate OFET devices by solution process.
The good performance with a hole mobility up to
0.4 cm2 Vꢀ1 sꢀ1 is achieved from 2-TIPS, which is quite
high among the donorꢀacceptor pentacene derivatives.2
Scheme 1 presents two synthetic approaches to aceno-
[2,1,3]thiadiazole derivatives. Route 1 is ineffective for the
construction of the aceno[2,1,3]thiadiazole skeleton fused
with five or more rings becaue of the synthetic difficulty of
some synthons.4 Herein, we adopted a Mukaiyama aldol
reaction to synthesize larger acenes as shown in route 2.
Synthon 1 was synthesized from commercial available 1,
4-dimethoxybenzene in ten gram scales with very high yields
as illustrated in Scheme 1. The Mukaiyama aldol reaction
between 1 and thiophene-2,3-dicarbaldehyde were em-
ployed to afford quinone 3, which was directly used in
the next step after filtration and washing. An alkynylation
reaction of 3 with a mixture of n-BuLi and TIPSCCH or
TESCCH following by a reduction with NaH2PO2 af-
forded the desired 1-TIPS or 1-TES. Following the same
procedure, we also obtained 2-TIPS and 2-TES in good
yields. We introduced triisopropylsilyl (TIPS) or triethyl-
silyl (TES) substituted acetylene units to enhance the solu-
bility of the molecules and further ensure their good crystal
packing.1 Such a synthetic approach to aceno[2,1,3]thiadiazole
derivatives is widely applicable because many 1,2-dicarbal-
dehyde compounds are available to replace thiophene-2,
3-dicarbaldehyde. These four molecules showed good solu-
bility in common solvents, so it provided us convenience
to purify and characterize their structures through 1H and
13C NMR and MS. The thermal decomposition tempera-
ture of 350 °C under nitrogen atmosphere revealed a good
thermal stability of these four oligoacene derivatives.
The photophysical and the electrochemical properties of
these four aceno[2,1,3]thiadiazoles were investigated in
detail and summarized in Table S2 (Supporting Informa-
tion). The absorption features of 1-TIPS and 1-TES
peaked at 611 nm with an onset at 650 nm; however,
2-TIPS and 2-TES displayed absorption maximum λmax at
730 nm with an onset at 810 nm, which showed obvious
red-shift relative to 1-TIPS and 1-TES due to the increase
of the effective conjugation length. The cyclovoltammetry (CV)
measurement showed that 1-TIPS and 1-TES had
HOMO/LUMO levels at ꢀ5.67/ꢀ3.65 eV with a bandgap
of 2.02 eV, and 2-TIPS and 2-TES had HOMO/LUMO
Org. Lett., Vol. 13, No. 10, 2011
2643

levels at ꢀ5.42/ꢀ3.78 eV with a bandgap of 1.64 eV.
Compared to Chlo (HOMO/LUMO: ꢀ5.38/ꢀ3.45 eV)2b
and Fluo (HOMO/LUMO: ꢀ5.39/ꢀ3.35 eV)2c counter-
parts, the absorption bands of these molecules were much
more red-shifted, which can be attributed to the much
lowered LUMO levels induced by the [2,1,3]thiadiazole
unit. Computational results indicated that the HOMO
levels of these aceno[2,1,3]thiadiazoles were located on
sulfur atom of the thiophene ring and their LUMO levels
are located on the sulfur atom of the thiadiazole unit.
Therefore, these molecules are typical donorꢀacceptor
systems Figure S3 (Supporting Information).
unsuitable for charge transport because of the lack of a
long-distance charge-transporting channel.6
Figure 3. (a) SEM image, (b) TEM image, (c) electron diffrac-
tion pattern of 2-TES, and (d) the theoretically predicted crystal
habit using BFDH method.
Organic single crystal nano/microwires (NWs/MWs) have
shown promising applications in the fabrication of high
performance OFETs.7,8 Recently, the large-area alignment
of solution processed nanowires is realized by Bao and co-
workers through filtration and transfer method.9 Single crys-
tal MWs of 2-TIPS and 2-TES were grown by a phase
transfer procedure of injecting methanol into their CH2Cl2
(DCM) solution, and MWs of 2-TIPS and 2-TES were both
formed as suspensions. As shown in Figure 3a, 2-TIPS
formed “microbelts” (Figure S7, Supporting Information)
hundreds of micrometers long; in contrast, 2-TES formed
“microwires” with a relatively high regularity in width (2.72 (
0.72 μm) (Figure 3). To correlate the observed nanowires with
the molecular single crystal structure, selected-area elec-
tron diffraction (SAED) and crystallographic packing
calculation were performed. The diffraction patterns were
Figure 2. Single-crystal structure and packing of (a) TIPS-PEN,
(b) iso-TIPS-PEN, (c) 1-TIPS, and (d) 2-TIPS.
The obtained single crystals of 1-TIPS, 2-TIPS, and
2-TES exhibited unconventional crystal packing com-
pared with other pentacene derivatives. TIPS-PEN and
its derivatives always had well-ordered two-dimensional
crystal packing (Figure 2a).1,2 Iso-TIPS-PEN, whose 5,14
positions were substituted with TIPS-acetylenyls, showed
a two-pair herringbone packing (Figure 2b).5 However,
2-TIPS and 2-TES did not follow such packing models,
which formed unconventional one-dimensional column
packing (Figure 2c and Figure S6, Supporting Informa-
tion). For 2-TIPS and 2-TES, in one column, the mole-
cules were packed head-to-tail to form a long column, and
the columns were packed together to form the crystals.
Inside the column, both 2-TIPS and 2-TES exhibited good
ꢀ ^
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Silbey, R.; Bredas, J.-L. Chem. Rev. 2007, 107, 926.
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W.; Liu, C.; Chen, K.; Wang, C.; Liu, Y.; Zhu, D. Adv. Mater. 2006, 18,
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Wang, X.; Zhu, D. Adv. Mater. 2006, 18, 3010. (e) Xiao, S.; Tang, J.;
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Mannsfeld, S. C. B.; Lu, X.; Xiong, Y.; Jenekhe, S. A.; Bao, Z.; Xia, Y.
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Cao, Y.; Wang, J.; Pei, J. J. Am. Chem. Soc. 2007, 129, 12386. (h) Kim,
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˚
πꢀπ stacking with a distance about 3.42 and 3.37 A,
respectively, both of which were smaller than that of
5
TIPS-PEN (3.47 A). The reduced packing distance might
˚
be caused by the strong donorꢀacceptor interactions.
However, as shown in Figure 2d, the crystal structure of
1-TIPS was very similar to that of iso-TIPS-PEN; such
two-pair herringbone packing makes the molecule
(9) Oh, J. H.; Lee, H. W.; Mannsfeld, S.; Stoltenberg, R. M.; Jung, E.;
Jin, Y. W.; Kim, J. M.; Yoo, J.-B.; Bao, Z. Proc. Natl. Acad. Sci. U.S.A.
2009, 106, 6065.
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Chem. Soc. 2001, 123, 9482.
2644
Org. Lett., Vol. 13, No. 10, 2011

indexed according to the single crystal data. For 2-TES,
the strong diffraction of (100), (020), and (040) was
determined according to the single crystal data, which
indicated that the single crystal nanowire were grown
along the (100) direction. These results are in agreement
with the Wulff plot of the crystal shape (Figure 3d) that
was calculated using the Bravais-Fredel-Donnay-Harker
(BFDH) method.10 Similar results were also obtained for
2-TIPS and shown in Figure S7 (Supporting Information).
As shown by the Wulff plots, the growing direction of the
two nanowires is consistent with the column direction,
which is the πꢀπ stacking direction.
20 devices), but 2-TES only had a hole mobility
of 0.05 cm2 Vꢀ1 sꢀ1 (average 0.028 cm2 Vꢀ1 sꢀ1 from
20 devices), which was an order of magnitude lower.
Figure 5. Intracolumn packing of (a) 2-TIPS and (b) 2-TES crystals.
A first-principle calculation based on the hopping model
was employed to estimate the hole and electron mobility of
1-TIPS and 1-TES (Supporting Information).6,12 The
computational results show that the main transporting
channels of hole or electron were along the column. The hole
mobility was estimated to be 1.23 cm2 Vꢀ1 sꢀ1 for 2-TIPS and
0.02 cm2 Vꢀ1 sꢀ1 for 2-TES, which is consistent with our
experimental results. This large difference of the hole mobility
can be attributed to the different intracolumn packings. As
shown in Figure 5, 2-TIPS has two types of intermolecular
distances and overlaps, and 2-TES has only one intermolecular
distance and overlap in the column. In addition, the electron
mobilities of 2-TIPS and 2-TES were also calculated and up to
0.65 and 1.28 cm2 Vꢀ1 sꢀ1, respectively. However, we did not
observe the electron transport in the nanowires, which may be
attributed to oxygen trapping or injection barrier.13
Figure 4. (a, c) Transfer and (b, d) output curves of the FET
devices fabricated from (a, b) 2-TIPS and (c, d) 2-TES micro wires.
In summary, we have developed a series of [2,1,3]thia-
diazole-based oligoacenes 1-TES, 1-TIPS, 2-TES, and 2-
TIPS through facile and convergent preparations. Such an
approach provides a convenient route to develop various
thiadiazole-fused oligoacene derivatives. These molecules
exhibit unique crystal packing, largely different from other
pentacene derivatives. The good performance with a hole
mobility up to 0.4 cm2 Vꢀ1 sꢀ1 is achieved from 2-TIPS,
which is quite high among the donorꢀacceptor pentacene
derivatives. Theoretical prediction and OFET perfor-
mances demonstrate that [2,1,3]thiadiazole-based oligoacenes
are very promising for organic semiconducting materials.
We also employed these MWs to fabricate the OFET
devices. Single organic microwire transistors were fabricated
by directly spin-coating the suspensions of the microwires
onto the n-octadecyltrimethoxylsilane (OTS)-treated SiO2/
nþþ-Si substrate. The OTS self-assembled monolayer
(SAM) was modified according to the method reported by
Bao and co-workers.11 The polyethylene (PE) fibers with 20
μm diameter as a shadow mask were mounted on the wafer,
and a gold layer with a thickness of 80 nm was deposited onto
˚
the substrate by thermal evaporation with the rate of 0.05 A
s
ꢀ1. Devices fabricated using 1-TIPS and 1-TES as the active
materials did not show obvious field-effect properties; this
might be attributed to their unsuitable crystal packing. Both 2-
TIPS and 2-TES molecules owned identical optical and
electrochemical properties and even had similar crystal pack-
ing; however, they showed very different device performances.
The transfer and output curves of both molecules are shown in
Figure 4. Both 2-TIPS and 2-TES showed typical p-type
charge-transport characteristics. 2-TIPS exhibited a hole
mobility up to 0.4 cm2 Vꢀ1 sꢀ1 (average 0.15 cm2 Vꢀ1 sꢀ1 from
Acknowledgment. This work was supported by the
Major State Basic Research Development Program (No.
2009CB623601) from MOST and fundings from NSFC.
Supporting Information Available. Experimental details
and 1H and 13C NMR spectra. This material is available free
of charge via the Internet at http://pubs.acs.org.
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S.; Bredas, J.-L. J. Am. Chem. Soc. 2006, 128, 9882. (b) Wang, L.; Nan,
(10) Donnay, J. D. H.; Harker, D. Am. Mineral. 1937, 22, 463.
Calculation details are described in the Supporting Information.
(11) Ito, Y.; Virkar, A. A.; Mannsfeld, S.; Oh, J. H.; Toney, M.;
Locklin, J.; Bao, Z. J. Am. Chem. Soc. 2009, 131, 9396.
G.; Yang, X.; Peng, Q.; Li, Q.; Shuai, Z. Chem. Soc. Rev. 2010, 39, 423.
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Marder, S. R.; Zhan, X. Adv. Mater. 2010, 22, 3876.
Org. Lett., Vol. 13, No. 10, 2011
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