Enhanced performance of benzothieno[3,2-b]thiophene (BTT)-based bottom-contact thin-film transistors

Article abstract of DOI:10.1002/chem.201204110

Three new benzothieno[3,2-b]thiophene (BTT; 1) derivatives, which were end-functionalized with phenyl (BTT-P; 2), benzothiophenyl (BTT-BT; 3), and benzothieno[3,2-b]thiophenyl groups (BBTT; 4; dimer of 1), were synthesized and characterized in organic thin-film transistors (OTFTs). A new and improved synthetic method for BTTs was developed, which enabled the efficient realization of new BTT-based semiconductors. The crystal structure of BBTT was determined by single-crystal X-ray diffraction. Within this family, BBTT, which had the largest conjugation of the BTT derivatives in this study, exhibited the highest p-channel characteristic, with a carrier mobility as high as 0.22 cm² V^-1 s^-1 and a current on/off ratio of 1×10 ⁷, as well as good ambient stability for bottom-contact/bottom-gate OTFT devices. The device characteristics were correlated with the film morphologies and microstructures of the corresponding compounds. Copyright

Full text of DOI:10.1002/chem.201204110

FULL PAPER
DOI: 10.1002/chem.201204110
Enhanced Performance of Benzothieno
ACHTUNGTNER[NUNG 3,2-b]thiophene (BTT)-Based
Bottom-Contact Thin-Film Transistors
Peng-Yi Huang,[a] Liang-Hsiang Chen,[b] Yu-Yuan Chen,[a] Wen-Jung Chang,[a]
Juin-Jie Wang,[a] Kwang-Hwa Lii,[a] Jing-Yi Yan,[b] Jia-Chong Ho,[b] Cheng-Chung Lee,[b]
Choongik Kim,*[c] and Ming-Chou Chen*[a]
Abstract: Three new benzothieno
[3,2-
of new BTT-based semiconductors. The
crystal structure of BBTT was deter-
mined by single-crystal X-ray diffrac-
tion. Within this family, BBTT, which
had the largest conjugation of the BTT
derivatives in this study, exhibited the
highest p-channel characteristic, with a
carrier mobility as high as
b]thiophene (BTT; 1) derivatives,
which were end-functionalized with
phenyl (BTT-P; 2), benzothiophenyl
0.22 cm2 VÀ1 sÀ1 and a current on/off
ratio of 1ꢀ107, as well as good ambient
stability for bottom-contact/bottom-
gate OTFT devices. The device charac-
teristics were correlated with the film
morphologies and microstructures of
the corresponding compounds.
(BTT-BT; 3), and benzothienoACTHNUTRGENUG[N 3,2-
b]thiophenyl groups (BBTT; 4; dimer
of 1), were synthesized and character-
ized in organic thin-film transistors
(OTFTs). A new and improved syn-
thetic method for BTTs was developed,
which enabled the efficient realization
Keywords: conjugation · semicon-
ductors · thin films · thiophenes ·
transistors
Introduction
vironment, which limits their application in solution process-
es.[5] Hence, the development of new heteroacenes with high
electrical performance and ambient stability has been of
great recent interest. To this end, fused-thiophene-based ma-
terials with extensive conjugation, high ambient stability,
and strong intermolecular interactions are among the most-
interesting candidates. Recently, several fused-thiophene de-
rivatives with good electrical performance have been report-
ed (Figure 1).[6–10] Among the available p-channel semicon-
ductors, DP-DTT (A),[7a] DP-TTA (B),[7b] DBTDT (C),[7c]
and BTDT (D)[8] are representative examples with hole mo-
bilities (mh) of up to 0.42, 0.14, 0.51, and >1.0 cm2 VÀ1 sÀ1, re-
spectively. For n-channel semiconductors,[9] we have devel-
oped DFP-DTT (E)[9b] and DFP-TTA (F)[9c] with electron
mobilities (me) as high as 0.07 and 0.3 cm2 VÀ1 sÀ1, respective-
Over the past decade, organic thin-film transistors (OTFTs)
have received considerable interest for various applications
in flexible electronics, such as flexible displays, RF-ID
(radio-frequency identification) tags, and e-papers.[1,2]
Among the three essential components in OTFTs, that is,
semiconductors, insulators, and conductors, there has been
huge progress in the development of organic semiconductors
with performance that is comparable to (or surpassing) that
of amorphous silicon.[3] Among organic semiconductors,
pentacene and its derivatives are representative examples
that exhibit high electrical performance with carrier mobili-
ties of >0.1 cm2 VÀ1 sÀ1.[4] However, pentacene derivatives
exhibit photoinduced oxidative instability in an ambient en-
ly. Recently, a few benzoACTHNUTRGNE[NUG d,d’]thieno[3,2-b;4,5-b’]dithiophene
derivatives have been reported and, in particular, its phenyl-
end-capped derivative (P-BTDT; G) exhibited excellent
electrical performance, with a hole mobility of up to
0.70 cm2 VÀ1 sÀ1.[7d] Furthermore, good electrical performance
has been reported in optimized DTBTE (H)-based OTFTs,
with which a carrier mobility of 0.50 cm2 VÀ1 sÀ1 was ach-
ieved, as compared to <0.01 cm2 VÀ1 sÀ1 with un-optimized
BBTT (4).[10] More recently, a DBTDT derivative (I) dem-
onstrated excellent electrical performance with a hole mobi-
lity of up to 8.8 cm2 VÀ1 sÀ1.[7f] In addition, good electrical
performance, with a mobility of 0.15 cm2 VÀ1 sÀ1, was ach-
ieved for DBTTA (J)-based thin-film transistors.[7g]
[a] Dr. P.-Y. Huang,+ Y.-Y. Chen, W.-J. Chang, J.-J. Wang, Dr. K.-H. Lii,
Prof. M.-C. Chen
Department of Chemistry
National Central University, Chung-Li, Taiwan (Republic of China)
E-mail: mcchen@ncu.edu.tw
[b] Dr. L.-H. Chen,+ Dr. J.-Y. Yan, Dr. J.-C. Ho, Dr. C.-C. Lee
Process Technology Division, Display Technology Center
Industrial Technology Research Institute
Hsinchu, Taiwan (Republic of China)
[c] Prof. C. Kim
Department of Chemical & Biomolecular Engineering
Sogang University, 1 Shinsoo-Dong, Mapo-Gu
Seoul 121-742 (Republic of Korea)
E-mail: choongik@sogang.ac.kr
Herein, we explore new semiconductors that are based on
[+] These authors contributed equally to this work.
the simple fused-thiophene, benzothienoACHTNUTRGNEUNG[3,2-b]thiophene
(BTT). Firstly, we explored a new, simple synthetic method
for the preparation of the BTT core. Secondly, the molecu-
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201204110.
Chem. Eur. J. 2013, 19, 3721 – 3728
ꢁ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3721

results revealed that bottom-contact/bottom-gate TFTs that
were based on BBTT films exhibited good electrical per-
formance, with a carrier mobility as high as 0.22 cm2 VÀ1 sÀ1,
a current on/off ratio of 1ꢀ107, and good environmental sta-
bility.
Results and Discussion
Synthesis: The benzo[b]thienoACHTUNGTRNEUNG
[2,3-d]thiophene (BTT)[11,16]
moiety has been synthesized by using various synthetic pro-
cedures in various yields. To efficiently obtain the BTT core,
we developed a new, practically simple synthetic procedure
from commercially available benzothiophene in an overall
yield of 36% over three steps. As shown in Figure 3, 2,3-di-
bromobenzothiophene, which was obtained from the bromi-
nation of benzothiophene with Br2 in 90% yield, was alky-
nylated with trimethylsilylacetylene in a Sonogashira cou-
pling reaction to give 1-(3-bromobenzo[b]thiophen-2-yl)-2-
trimethylsilylacetylene (5) in 80% yield. Annulation and de-
silylation with Na2S in N-methyl-2-pyrrolidone (NMP) at
1908C gave BTT (1) in 52% yield. Markedly, our new syn-
thetic route is the fastest and shortest synthetic approach for
the preparation of the asymmetric fused ring.
Figure 3. Synthetic route to the BTT core.
Figure 1. Examples of fused-oligothiophene semiconductors that were
employed in OTFTs.
The synthesis of functionalized BTTs (2–4) was achieved
as shown in Figure 4. Deprotonation of BTT with nBuLi and
subsequent alkylstannylation afforded the corresponding 2-
lar-structure–property relationships were elucidated by in-
troducing phenyl (BTT-P; 2), benzothiophenyl (BTT-BT; 3),
À
or benzothieno
[3,2-b]thiophenyl groups (BBTT; 4; dimer of
SnR3 BTT derivative in situ, which was coupled with the
corresponding aryl bromide in Stille coupling reactions to
BTT) onto the BTT core (Figure 2). Thirdly, the crystal
structure of BBTT was determined by single-crystal X-ray
diffraction, which showed ideal molecular-stacking arrange-
ments for charge transport. Finally, the film morphology and
microstructure were investigated and correlated with the
film-growth conditions as well as device performance. Our
Figure 2. Chemical structures of the benzothienoACTHNUGRTNEUNG[3,2-b]thiophene (BTT;
2–4) derivatives that were employed in this study.
Figure 4. Synthetic routes to the new BTT derivatives.
3722
www.chemeurj.org
ꢁ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2013, 19, 3721 – 3728

BenzothienoACHTUNGTRENNUNG[3,2-b]thiophene-Based Thin-Film Transistors
FULL PAPER
Table 1. Thermal, optical absorption/emission, and electrochemical data for BTTs 2–4.
Compound
DSC
Tm [8C]
TGA
(5%) [8C]
UV/Vis[a]
lmax [nm]
Reductive
Oxidative
DEgap [eV]
G
potential [V][b]
potential [V][b]
UV[a]
DPV[b]
2
3
4
2’
3’
4’
160
230
334
258
341
461
189
253
313
255
309
471
330
366
388
358
378
411
À1.95
À1.91
À1.89
À1.93
À1.86
À1.75
1.61
1.48
1.27
1.33
1.30
1.14
3.32
3.04
2.91
3.17
2.99
2.74
3.56
3.39
3.16
3.26
3.16
2.89
[a] In o-C6H4Cl2. [b] By DPV in o-C6H4Cl2 at 508C.
give compounds 2–4 in crude yields of 70, 75, and 72%, re-
spectively. These BTTs were further purified by gradient
sublimation at a pressure of about 1ꢀ10À5 Torr at 190–
2558C to give materials that were suitable for the fabrica-
tion of OTFT devices in a yield of about 50%.
Thermal and optical properties: For three new BTT deriva-
tives, differential scanning calorimetry (DSC) exhibited
sharp endotherms above 1608C, and thermogravimetric
analysis plots only showed weight loss (about 5%) upon
heating above 1898C (Table 1). Although the melting tem-
peratures of these BTTs are lower than those of BTDTs
(Figure 5),[7d] both BTTs and BTDTs show excellent thermal
stability among heteroacenes.
Figure 6. Optical spectra of the BTT (2–4) and BTDT derivatives (2’–4’)
in o-C6H4Cl2.
BTTs exhibit much lower HOMOs (>5.47 eV) as well as
significantly larger HOMO–LUMO gaps (>3.16 eV) in
comparison with those of pentacene (about 5.0 eV; 1.70–
2.09 eV) or anthradithiophene (ADT) derivatives (about
5.1 eV; 2.09–2.57 eV),[12] which suggests that the BTT deriv-
atives are not easily oxidized and may have better stability
under ambient conditions compared to pentacene and ADT
derivatives. Hence, the photo-oxidative stability of the BTT
derivatives was investigated by monitoring the absorbance
decay at the lmax of solutions of BTTs in aerated o-C6H4Cl2
that were exposed to the white light of a fluorescent lamp at
room temperature. Under these conditions, no decomposi-
tion was observed for all of the BTTs after five days, thus
demonstrating the good ambient stability of these materials.
Figure 5. Chemical structures of the BTDT derivatives (2’–4’); see
ref. [7d].
In particular, BTT-capped BBTT (4) exhibited the highest
melting point (3348C) and highest weight-loss temperature
(3138C) among the BTT series. The optical absorption spec-
tra of compounds BTT-P (2; lmax ꢀ330 nm), BTT-BT (3;
lmax ꢀ366 nm), and BBTT (4; lmax ꢀ388 nm) were signifi-
cantly blue-shifted compared to those of their corresponding
BTDT derivatives (2’, 3’, and 4’; lmax ꢀ358, 378, and 411 nm,
respectively) in o-C6H4Cl2 (Figure 6). Compared to the
phenyl- or benzothiophenyl-capped BTTs, greater p-elec-
tron delocalization (between the two BTT moieties) was ob-
Electrochemical characterization: Differential pulse voltam-
metry (DPVs) of the BTT derivatives was performed in di-
chlorobenzene at 508C and the resulting oxidation and re-
duction potentials are summarized in Table 1. The DPVs of
BTT-P and BTT-BT exhibited oxidation peaks at about
+1.61 and +1.48 V, respectively (versus ferrocene/ferroceni-
um as an internal standard at +0.6 V). In comparison, the
oxidation potential of the more-conjugated BBTT (4) was
shifted to a less-positive value than those for compounds 2
and 3 (Eox =+1.27 V). Similarly, the Eox values of the BTT
derivatives (+1.61 to +1.27 V) were shifted to more-positive
values than those of their corresponding BTDTs (+1.33 to
+1.14 V), which can be attributed to the lower degree of
molecular conjugation of the BTT core. The electrochemi-
cally derived HOMO–LUMO energy gaps that were ob-
tained from the DPV data were in the order: BBTT
served for the benzothienoACTHNURGTNE[NUG 3,2-b]thiopheyl-substituted
BBTT (4), which gave the latter compound the lowest
HOMO–LUMO energy gap among the BTT derivatives
(Table 1).
The HOMO–LUMO energy gaps that were calculated
from the onset of the experimental optical absorption (2.91–
3.32 eV) increased in the order: BBTT (4)<BTT-BT (3)<
BTT-P (2), which was consistent with the electrochemically
derived HOMO–LUMO gaps (see below). Notably, the
Chem. Eur. J. 2013, 19, 3721 – 3728
ꢁ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
3723

C. Kim, M.-C. Chen et al.
tween the edge-to-face BBTT molecules is approximately
À
3.17 ꢂ (not shown), with a shortest intermolecular S S dis-
tance of 3.97 ꢂ (Figure 8G). The planar molecular structure
and short stacking distance afford ideal conditions for ex-
tended p-orbital interactions of the corresponding mole-
cules.
Figure 7. DPV-derived HOMO and LUMO energy levels of BTTs 2–4
and pentacene (Pen).
Thin-film transistors: Bottom-contact/bottom-gate OTFT
devices were fabricated by the thermal evaporation of a
gold source and drain contacts (channel length: 100 mm,
width: 500 mm), followed by vacuum deposition (1.6ꢀ
10À6 Torr) of each compound onto OTS-treated SiO2/Ag
(100 nm/100 nm) substrates at preset deposition tempera-
tures (TD). The TFT properties were tested under ambient
conditions to explore the device performance. All TFT data,
including carrier mobility, current on/off ratio, and threshold
voltage, are summarized in Table 2 and representative trans-
fer and output plots are shown in Figure 9 (for compound 4)
and in the Supporting Information, Figure S2 (for com-
pounds 2 and 3).
(3.16 eV)<BTT-BT (3.39 eV)<BTT-P (3.56 eV; assuming
ferrocene/ferrocenium oxidation at 4.8 eV; Figure 7), which
was consistent with the values that were obtained from opti-
cal spectroscopy, that is, BBTT (2.91 eV)<BTT-BT
(3.04 eV)<BTT-P (3.32 eV; Table 1). The significantly
lower HOMO energy levels and the larger band-gaps of
these three new BTTs compared to pentacene suggest that
these newly developed fused thiophenes are environmental-
ly very stable (see above).
Devices that were fabricated from these BTT derivatives
all exhibited TFT activity, thus performing as p-channel sem-
iconductors. Of the compounds that were developed in this
study, compound 4 (BBTT), which had the largest degree of
conjugation, exhibited the highest device performance, with
Single-crystal X-ray diffraction: The diffraction-derived
single-crystal structure of BBTT is shown in Figure 8 and
the Supporting Information, Figure S1, and detailed crystal-
lographic data are summarized in the Supporting Informa-
tion, Table S1. As shown, BBTT crystallizes in monoclinic
space group P21/c, with unit-cell parameters of a=
14.1293(6), b=3.9653(2), c=15.0241(6) ꢂ; a=90.00, b=
111.527(2), g=90.008; Z=2.
Table 2. Carrier mobilities (m), current on/off ratios (Ion/Ioff), and thresh-
old voltages (VT) for OTFTs that were fabricated by the vacuum deposi-
tion of compounds 2–4 on OTS-treated substrates at the indicated depo-
sition temperatures (TD).
Compound
TD [8C]
m [cm2 VÀ1 sÀ1
]
Ion/Ioff
VT [V]
2
30
50
80
30
50
80
30
50
80
0.001
0.008
0.012
0.01
0.05
0.10
0.08
0.13
0.22
106
107
107
106
107
107
106
107
107
À5.4
À6.5
À9.5
À1.8
À1.2
À2.4
À5.4
À2.5
À4.2
3
4
Figure 8. Crystal structure of BBTT: A), B) two orthogonal views; dis-
tance [ꢂ] and the dihedral angle [8] of two BTT moieties are shown in
(A). C) Molecular stacking of BBTT molecules, which adopted an edge-
to-face packing with a herringbone angle of 54.28. D)–F) Three orthogo-
À
À
nal views of the space-filling model; selected S S/C C distances [ꢂ] are
indicated.
The fused-phenylene moiety is almost coplanar with the
two fused thiophenes and the two BTT moieties in a single
molecule are coupled in a completely flat geometry with a
separation of 1.45 ꢂ and a dihedral angle of 08 (Figure 8A
and B). The intermolecular distance between adjacent
BBTT planes is 3.53 ꢂ (Figure 8C). Similar to other fused
thiophenes,[7] the unit cell of BBTT exhibits a commonly ob-
served edge-to-face herringbone packing motif with an inter-
molecular packing angle of approximately 54.28 (Figure 8C).
The molecular sheets are stacked along the b axis and the
Figure 9. Transfer (VDS =À16 V) and output plots of an OTFT device
that was fabricated from BBTT (4) films that were grown on an OTS-
coated SiO2 substrate (TD =808C, channel length: 100 mm, channel width:
500 mm). Inset: structure of the OTFT device.
À
À
interactions are reinforced by close interplanar S S and S
À
H contacts. The shortest intermolecular S H distance be-
3724
www.chemeurj.org
ꢁ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2013, 19, 3721 – 3728

BenzothienoACHTUNGTRENNUNG[3,2-b]thiophene-Based Thin-Film Transistors
FULL PAPER
hole mobilities of 0.08–0.22 cm2 VÀ1 sÀ1 and current on/off
ratios of 1ꢀ106–1ꢀ107 (Figure 9). Compounds 2 and 3
showed poorer device performance than compound 4, with
carrier mobilities of 0.001–0.012 cm2 VÀ1 sÀ1 and 0.01–
0.12 cm2 VÀ1 sÀ1, respectively. The current on/off ratios of
compounds 2 and 3 were comparable to those of compound
4. Furthermore, the derived carrier mobilities of the BTT
derivatives strongly depended upon the deposition tempera-
ture (TD). Within the range TD =30–808C, enhanced device
performance was observed at higher deposition tempera-
tures. A clear correlation between m and TD was also evident
in the XRD and AFM characterization data of the corre-
sponding semiconductor films (see below).
BBTT), showed similar XRD patterns, with slightly lower
peak intensities than those of BBTT (see the Supporting In-
formation, Figure S3A). Films of BTT-P (2), with poor TFT
mobility, exhibited low XRD peak intensities, thus indicat-
ing low film texturing (see the Supporting Information, Fig-
ure S3B). This low film texturing is possibly due to com-
pound 2 having the lowest degree of molecular conjugation
among the BTT series considered in this study. The d-spac-
ings of the BTT-BT and BTT-P films (at 2q=6.368 and 6.938
for the first reflections) were 13.9 and 12.7 ꢂ, respectively,
which were smaller than that of BBTT.
Because the TFT performance was quite sensitive to the
deposition temperature (see above), TD-dependent XRD
patterns were also examined. There was a clear correlation
between the mobility values and the XRD peak intensity.
As TD increased, the XRD peak intensities for the films of
all of the compounds (with the same thickness, about
50 nm) increased and sharpened (Figure 10 and the Support-
ing Information), as did the TFT carrier mobility (see
above).
Thin-film microstructure and morphology: Vacuum-deposit-
ed film crystallinity and surface morphology are often used
to evaluate device performance. In general, high device per-
formance, in particular in terms of carrier mobility, is ob-
tained for films that show high crystallinity and large grain
size with enhanced interconnectivity.[1,7d] Thus, the thin-film
microstructures and morphologies of this new class of mate-
rials were studied by wide-angle q/2q XRD and AFM. Con-
ventional q/2q XRD scans were performed to obtain out-of-
plane d-spacings in the vacuum-deposited thin films on
OTS-coated SiO2 substrates at various TD values. The thick-
nesses of all of these films were determined to be about
50 nm by using profilometry. Representative XRD scans of
thin films of these BTT derivatives are shown in Figure 10
(for compound 4) and in the Supporting Information (for
compounds 2 and 3).
It is challenging to characterize the dielectric/semiconduc-
tor interface, at which charge transport in the TFTs occurs,
by using conventional experimental techniques. Hence, the
surface morphologies of organic semiconductor thin films
(50 nm), at which information about the underlying micro-
structure at the interface is conveyed, were characterized by
AFM (Figure 11). Films of BTT-P and BTT-BT exhibited
Figure 10. The q/2q XRD scans of BBTT (4) films that were vapor-depos-
ited at the indicated TD values on OTS-coated SiO2 substrates.
Figure 11. A) AFM images (5ꢀ5 mm2) of films of: A) BTT-P (2), B) BTT-
BT (3), and C) BBTT (4) on OTS-coated SiO2 substrates that were
grown at the indicated TD values. Scale bars: 1 mm.
The q/2q XRD scans of films of BBTT (4; Figure 10) indi-
cated the formation of highly textured films with high peak
intensities, which were consistent with good TFT perform-
ance (see above). The first reflection in the XRD spectrum
was observed at 2q=5.638, which corresponded to a d-spac-
ing of 15.7 ꢂ, that is, slightly smaller than the molecular
length of the corresponding molecule (16.6 ꢂ). This result
indicates that the films might be predominantly aligned such
that their long molecular axes are along the substrate
normal, with a certain degree of tilt angle. Films of BTT-BT
(3), with good carrier mobility (slightly lower than that of
small ball-shaped grains, whereas films of BBTT showed
rod-like morphologies. Furthermore, similar to the XRD
patterns, the surface morphologies of the films also showed
a strong dependence on the TD value and correlated with
the device performance (see above). At high substrate tem-
peratures, films of BTT-P and BTT-BT exhibited larger
grain sizes (Figure 11A and B) than at low TD values.[13]
Similarly, films of BBTT also showed rod-like morphologies
with larger lengths and diameters at higher TD. The large
surface features of all of the thin films at the higher deposi-
Chem. Eur. J. 2013, 19, 3721 – 3728
ꢁ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
3725

C. Kim, M.-C. Chen et al.
Under a nitrogen atmosphere, a solution of 1-(3-bromobenzo[b]thio-
tion temperature correlated well with their enhanced TFT
performance.
phen-2-yl)-2-trimethylsilylacetylene
(112 mg,
0.363 mmol)
and
Na2S·9H2O (174.4 mg, 0.726 mmol) in NMP (15 mL) was stirred at
1908C for 12 h. A saturated aqueous solution of NH4Cl was added to the
reaction mixture and the organic fraction was extracted several times
with hot hexanes, washed with a saturated aqueous solution of NaCl, and
dried with anhydrous Na2SO4. The crude product was purified by column
chromatography on silica gel (hexanes) to give the product as a white
solid (36 mg, 52%). 1H NMR (CDCl3, 300 MHz): d=7.87 (m, J=1.5 Hz,
1H), 7.84 (m, J=1.5 Hz, 1H), 7.51 (d, J=5.4 Hz, 1H), 7.4 (m, 2H), 7.32
(d, J=5.4 Hz, 1H).
Conclusion
A new family of benzothienoACTHNURGTNE[UNG 3,2-b]thiophene-based semi-
conductors was synthesized and characterized. Thin-film
transistors that were fabricated from these molecules exhib-
ited decent device performance and excellent air stability.
Films that were deposited onto OTS-coated SiO2 substrates
under properly adjusted substrate temperatures achieved an
efficacious compromise between high film crystallinity and
good film-grain interconnectivity, thereby resulting in high
OTFT performance, with mobilities as high as
0.22 cm2 VÀ1 sÀ1 and current on/off ratios as high as 1ꢀ107.
In particular, our study with optimized device-fabrication
conditions showed a 30–70-fold increase in carrier mobility
for BBTT films, in comparison to the results (carrier mobili-
ties of 0.003–0.007 cm2 VÀ1 sÀ1) in the literature.[10]
Synthesis of 2-biphenylbenzothienoACTHNUTRGNEUGN[3,2-b]thiophene (BTT-P; 2): Under a
nitrogen atmosphere and anhydrous conditions at 08C, nBuLi (0.75 mL,
2.5m in hexanes, 1.86 mmol) was slowly added to a solution of BTT
(354 mg, 1.86 mmol) in THF (30 mL) and the mixture was stirred for
40 min. Next, tri-n-butyltin chloride (0.67 g, 2.05 mmol) was added and
the mixture was stirred at 08C for 30 min, warmed to RT, and stirred for
a further 8 h. After simple filtration under a nitrogen atmosphere, THF
was removed under vacuum and toluene (40 mL) was added. This solu-
tion was transferred into
1.86 mmol) and tetrakis(triphenylphosphine)palladium
a
solution of 4-bromobenzene (292 mg,
(129 mg,
0.11 mmol) in toluene (40 mL) and the mixture was heated at reflux
(1408C) for 2 days. After cooling to RT, the mixture was filtered through
a column of celite; toluene was used to extract the product from the
celite pad. The combined toluene solution was evaporated and the de-
sired product was recrystallized from hexanes to give the crude product
in 75% yield. The product was further purified by gradient sublimation
at a pressure of about ꢀ10À5 Torr at 1908C, thus giving the product as a
bright-yellow solid (336 mg, 68%). M.p. 1608C; 1H NMR (CDCl3,
300 MHz): d=7.85 (dd, J=7.5, 3.6 Hz, 2H), 7.69 (d, J=7.2 Hz, 2H), 7.54
(s, 1H), 7.46–7.32 ppm (m, 5H); 13C NMR (75 MHz; CDCl3): 147.09,
142.20, 138.56, 134.48, 133.81, 132.77, 129.04, 128.03, 125.86, 124.73,
124.39, 123.80, 120.86, 116.11 ppm; elemental analysis calcd for C16H10S2:
C 72.14, H 3.78; found: C 72.09, H 3.85; HRMS (EI): m/z calcd for
C16H10S2 266.0224 [M]+; found: 266.0219.
Experimental Section
Materials and methods: All chemicals and solvents (Aldrich, Arco, or
TCI Chemical Co.) were of reagent grade. The reaction solvents (toluene,
diethyl ether, and THF) were distilled under a nitrogen atmosphere from
sodium/benzophenone ketyl and halogenated solvents were distilled from
CaH2. 1H and 13C NMR spectra were recorded on Bruker 300 or 500 in-
struments and were referenced to solvent signals. Differential scanning
calorimetry (DSC) was performed on a Mettler DSC 822 instrument at a
scan rate of 10 KminÀ1. Thermogravimetric analysis (TGA) was per-
formed on a Perkin–Elmer TGA-7 thermal analysis system by using dry
nitrogen as the carrier gas at a flow rate of 10 mLminÀ1. UV/Vis absorp-
tion and fluorescence spectra were obtained in the indicated solvents at
RT on JASCO V-530 and Hitachi F-4500 spectrometers, respectively. IR
spectra were obtained on a JASCO FT/IR-4100 spectrometer. Differen-
tial pulse voltammetry (DPV) experiments were performed with a con-
ventional three-electrode configuration (platinum-disk working electrode,
an auxiliary platinum-wire electrode, and a non-aqueous silver reference
electrode) with a supporting electrolyte of 0.1m tetrabutylammonium
Synthesis of 2-benzothiophenylbenzothienoACTHNUTRGNE[UNG 3,2-b]thiophene (BTT-BT;
3): The synthetic procedure was similar to that for BTT-P (2), except that
2-bromobenzothiophene (397 mg, 1.86 mmol) was used instead of 4-bro-
mobenzene. After similar work-up steps, the desired product was purified
by gradient sublimation at a pressure of about ꢀ10À5 Torr at 2408C, thus
giving the product as a green–yellow solid (312 mg, 52%). M.p. 2308C;
1H NMR (CDCl3, 300 MHz): d=7.87–7.75 (m, 4H), 7.52 (s, 1H), 7.50 (s,
1H), 7.44–7.33 ppm (m, 4H); this material was insufficiently soluble to
obtain a useful 13C NMR spectrum; elemental analysis calcd for C18H10S3:
C 67.04, H 3.13; found: C 67.09, H 3.08; HRMS (EI): m/z calcd for
C18H10S3: 321.9945 [M]+; found: 321.9940.
hexafluorophosphate (TBAPF6) in the specified dry solvent on
a
Synthesis of bisbenzothienoACTHNUGRTENUNG[3,2-b]thiophene (BBTT; 4): At 08C, a solu-
CHI621C Electrochemical Analyzer (CH Instruments). All electrochemi-
cal potentials were referenced to an Fc+/Fc internal standard (at +0.6 V).
Elemental analysis was performed on a Heraeus CHN-O-Rapid elemen-
tal analyzer. Mass spectrometry was performed on a JMS-700 HRMS in-
strument. 3-Bromobenzo[b]thiophene[14] and 2,3-dibromobenzo[b]thio-
phene[15] were prepared according to literature procedures. Benzothieno-
tion of BTT (5.30 g, 0.028 mol) and N-bromosuccinimide (NBS, 5.45 g,
0.031 mol) in THF (175 mL) was stirred for 30 min, then warmed to RT
and stirred for a further 12 h. NaHSO4 was added to the reaction mix-
ture, followed by a saturated aqueous solution of NaCl. The organic frac-
tion was extracted with diethyl ether and dried with anhydrous MgSO4.
[3,2-b]thiophene[16] was prepared according the following new routes:
2-BromobenzothienoACTHNUTRGNEU[GN 3,2-b]thiophene was obtained after column chroma-
tography on silica gel (hexanes) as a white solid (7.24 g, 96%).[18] The
product was used in the next step without further purification. 1H NMR
(CDCl3, 300 MHz): d=7.84 (d, J=8.1 Hz, 1H), 7.77 (d, J=6.3 Hz, 1H),
7.44–7.33 (m, 2H), 7.32 ppm (s, 1H).
New synthetic route to benzothieno[3,2-b]thiophene (BTT; 1): Under a
AHCTUNGTRENNUNG
nitrogen atmosphere and anhydrous conditions, a solution of 2,3-dibro-
mobenzo[b]thiophene (1.0 g, 3.43 mmol) in triethylamine (50 mL), [Pd-
ACHTUNGTRENNUNG(PPh3)2Cl2] (120.2 mg, 0.171 mmol), CuI (65.2 mg, 0.343 mmol), and tri-
methylsilylacetylene (344 mg, 3.43 mmol) was heated at reflux for 19 h.[17]
Water was added to quench the reaction and the solvent was removed.
The organic fraction was extracted with diethyl ether, filtered through a
short column of celite, and then dried with anhydrous Na2SO4. The de-
sired product was distilled at 808C under vacuum (10À3 Torr) to give 1-
(3-bromobenzo[b]thiophen-2-yl)-2-trimethylsilylacetylene as a white solid
(790 mg, 75%). 1H NMR (CDCl3, 300 MHz): d=7.75 (m, 2H), 7.44 ppm
(m, 2H); 13C NMR (CDCl3, 75 MHz): 138.09, 137.26, 126.65, 125.45,
123.77, 122.22, 120.24, 114.35, 106.00, 96.06, À0.23 ppm; elemental analy-
sis calcd for C13H13S2BrSSi: HRMS (FAB): m/z calcd for C13H13S2BrSSi:
307.9691 [M]+; found: 307.9692 [M]+ (79Br), 309.9609 [M]+ (81Br).
The subsequent synthetic procedure was similar to that for BTT-P, except
that 2-bromobenzothieno
benzene. After similar work-up steps, the desired product (72% crude
yield) was purified by gradient sublimation at pressure of about
ACHTUNGTREN[NUNG 3,2-b]thiophene was used instead of 4-bromo-
a
ꢀ10À5 Torr at 2558C, thus giving the product as a bright-yellow solid
(254 mg, 51%). M.p. 3348C; this material was insufficiently soluble to
obtain useful 1H or 13C NMR spectra; elemental analysis calcd for
C20H10S4: C 63.46, H 2.66; found: C 63.40, H 2.73; HRMS (EI): m/z calcd
for C20H10S4: 377.9665 [M]+; found: 377.9655.
X-ray crystal-structure determination of BBTT: Yellow crystals that were
suitable for X-ray diffraction were crystallized from a hot solution of
3726
www.chemeurj.org
ꢁ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2013, 19, 3721 – 3728

BenzothienoACHTUNGTRENNUNG[3,2-b]thiophene-Based Thin-Film Transistors
FULL PAPER
BBTT in toluene. The chosen crystals were mounted onto a glass fiber.
Data collection for BBTT was performed on a Bruker Smart Apex2
CCD diffractometer with CuKa radiation (l=1.54178 ꢂ) at 293(2) K.
After data collection, the frames were integrated and absorption correc-
tions were applied. The initial crystal structure was solved by using direct
methods, the structure solution was expanded through successive least-
squares cycles, and the final solution was determined. All of the non-hy-
drogen atoms were refined anisotropically. Hydrogen atoms that were at-
tached onto carbon atoms were fixed at calculated positions and refined
by using a riding mode. Crystal data, data collection, and refinement pa-
rameters are summarized in the Supporting Information, Table S1
(CCDC-911203 (BBTT) the supplementary crystallographic data for this
paper. These data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
2008, 130, 17681–17683; d) S. Subramanian, S. K. Park, S. R. Parkin,
V. Podzorov, T. N. Jackson, J. E. Anthony, J. Am. Chem. Soc. 2008,
130, 2706–2707; e) M. Ashizawa, K. Yamada, A. Fukaya, R. Kato,
K. Hara, J. Takeya, Chem. Mater. 2008, 20, 4883–4890; f) E. Yagod-
kin, Y. Xia, V. Kalihari, C. D. Frisbie, C. J. Douglas, J. Phys. Chem.
C 2009, 113, 16544–16548; g) R. P. Ortiz, J. Casado, V. Hernandez,
J. T. L. Navarrete, J. A. Letizia, M. A. Ratner, A. Facchetti, T. J.
Marks, Chem. Eur. J. 2009, 15, 5023–5039; h) I. Osaka, T. Abe, S.
Shinamura, E. Miyazaki, K. Takimiya, J. Am. Chem. Soc. 2010, 132,
5000–5001.
[3] a) A. Facchetti, Mater. Today 2007, 10, 28–37; b) H. Sirringhaus,
Proc. IEEE 2009, 97, 1570–1579; c) A. C. Arias, J. D. Mackenzie, I.
McCulloch, J. Rivnay, A. Salleo, Chem. Rev. 2010, 110, 3–24; d) L.
Huang, M. Stolte, H. Burckstummer, F. Wurthner, Adv. Mater. 2012,
24, 5750–5754; e) H. Minemawari, T. Yamada, H. Matsui, J. Tsutsu-
mi, S. Haas, R. Chiba, R. Kumai, T. Hasegawa, Nature 2011, 475,
364–367; f) K. Kuribara, H. Wang, N. Uchiyama, K. Fukuda, T.
Yokota, U. Zschieschang, C. Jaye, D. Fischer, H. Klauk, T. Yamamo-
to, K. Takimiya, M. Ikeda, H. Kuwabara, T. Sekitani, Y.-L. Loo, T.
Someya, Nat. Commun. 2012, 3, 723–729.
[4] a) T. W. Kelley, L. D. Boardman, T. D. Dunbar, D. V. Muyres, M. J.
Pellerite, T. Y. P. Smith, J. Phys. Chem. B 2003, 107, 5877–5881;
b) H. Klauk, M. Halik, U. Zschieschang, F. Eder, G. Schmid, C.
Dehm, Appl. Phys. Lett. 2003, 82, 4175–4177; c) O. L. Griffith, J. E.
Anthony, A. G. Jones, Y. Shu, D. L. Lichtenberger, J. Am. Chem.
Soc. 2012, 134, 14185–14194; d) J. E. Anthony, D. L. Eaton, S. R.
Parkin, Org. Lett. 2002, 4, 15–18; e) S. K. Park, T. N. Jackson, J. E.
Anthony, D. A. Mourey, Appl. Phys. Lett. 2007, 91, 063514–063517.
[5] The degradation of pentacene derivatives under ambient conditions
is due to photoinduced oxidative decomposition at the C6/C13 posi-
tions and the formation of dimeric Diels–Alder adducts on the elec-
tron-rich central ring. Hence, the introduction of substituents onto
the pentacene C6/C13 positions has been found to be effective at
enhancing stability, as well as improving carrier mobility; see:
a) J. E. Anthony, J. S. Brooks, D. L. Eaton, S. R. Parkin, J. Am.
Chem. Soc. 2001, 123, 9482–9483; b) H. Yamada, Y. Yamashita, M.
Kikuchi, H. Watanabe, T. Okujima, H. Uno, T. Ogawa, K. Ohara, N.
Ono, Chem. Eur. J. 2005, 11, 6212–6221; c) A. Maliakal, K. Ragha-
vachari, H. Katz, E. Chandross, T. Siegrist, Chem. Mater. 2004, 16,
4980–4986; d) P. Coppo, S. G. Yeates, Adv. Mater. 2005, 17, 3001–
3005; e) H. Meng, M. Bendikov, G. Mitchell, R. Helgeson, F. Wudl,
Z. Bao, T. Siegrist, C. Kloc, C. H. Chen, Adv. Mater. 2003, 15, 1090–
1093.
Device fabrication and characterization: For the fabrication of bottom-
contact/bottom-gate OTFTs, silver (100 nm) was deposited by sputtering
onto a glass substrate as a bottom-gate electrode, as patterned by a
shadow mask. Then, a SiO2 layer (100 nm) was thermally deposited as a
gate dielectric onto the bottom-gate electrode. For treatment with OTS
(octadecyltrichlorosilane), the SiO2 surface was exposed to a solution of
OTS in toluene in a self-assembly chamber for 10 h to give a hydrophobic
surface with an advancing aqueous contact angle of about 1058. A gold
layer (thickness: 100 nm) was thermally evaporated through a shadow
mask to define the source and drain contacts with a channel length and
width of 100 and 500 mm, respectively. Subsequently, an organic semicon-
ductor layer (50 nm) was thermally evaporated through a shadow mask
onto substrates that were held at a predetermined temperature of 30, 50,
or 808C at a rate of 0.03 nmsÀ1. All of the vacuum-deposition processes
were performed at a pressure of about 1.6ꢀ10À6 Torr. The film thickness-
es were measured on a Dektak 3030 profilometer. The capacitance of the
gate insulators were measured on a HP 4284A Precision LCR meter. The
I–V characteristics of the OTFTs were measured in the dark on a semi-
conductor parameter analyzer (HP 4155A) for at least 10 different devi-
ces. All of the films were examined by X-ray diffraction (XRD, Siemens
D5000) with CuKa radiation. The surface morphology was characterized
by using atomic force microscope (AFM, Digital Instruments Nanosco-
pe III).
Acknowledgements
This work was supported by the National Science Council, Taiwan, Re-
public of China (NSC100-2628M-008-004 and NSC100-2627-E-006-001),
by the Industrial Technology Research Institute of Taiwan
(B361A51500), by the National Research Foundation of Korea (2011-
0007730), and by a grant from the Center for Advanced Soft Electronics
under the Global Frontier Research Program (2011-0031628) of the Min-
istry of Education, Science and Technology, Korea.
[6] a) K. Takimiya, S. Shinamura, I. Osaka, E. Miyazaki, Adv. Mater.
2011, 23, 4347–4370; b) L. Tan, L. Zhang, X. Jiang, X. D. Yang, L. J.
Wang, Z. Wang, L. Q. Li, W. P. Hu, Z. G. Shuai, L. Li, D. B. Zhu,
Adv. Funct. Mater. 2009, 19, 272–276; c) Y. Liu, C. A. Di, C. Y. Du,
Y. Q. Liu, K. Lu, W. F. Qiu, G. Yu, Chem. Eur. J. 2010, 16, 2231–
2239.
[7] a) Y. Sun, Y. Ma, Y. Liu, Y. Lin, Z. Wang, Y. Wang, C. Di, K. Xiao,
X. Chen, W. Qiu, B. Zhang, G. Yu, W. Hu, D. B. Zhu, Adv. Funct.
Mater. 2006, 16, 426–432; b) Y. Liu, Y. Wang, W. Wu, Y. Liu, H. Xi,
L. Wang, W. Qui, K. Lu, C. Du, G. Yu, Adv. Funct. Mater. 2009, 19,
772–778; c) J. Gao, R. Li, L. Li, Q. Meng, H. Jiang, H. Li, W. Hu,
Adv. Mater. 2007, 19, 3008–3011; d) J. Youn, M.-C. Chen, Y.-J.
Liang, H. Huang, R. P. Ortiz, C. Kim, C. Stern, T.-S. Hu, L.-H.
Chen, J.-Y. Yan, A. Facchetti, T. J. Marks, Chem. Mater. 2010, 22,
5031–5041; e) P.-Y. Huang, L.-H. Chen, C. Kim, H.-C. Chang, Y.-j.
Liang, C.-Y. Feng, C.-M. Yeh, J.-C. Ho, C.-C. Lee, M.-C. Chen, ACS
Appl. Mater. Interfaces 2012, 4, 6992–6998; f) Y. Miyata, E. Yoshika-
wa, T. Minari, K. Tsukagoshi, S. Yamaguchi, J. Mater. Chem. 2012,
22, 7715–7717; g) J. Huang, H. Luo, L. Wang, Y. Guo, W. Zhang, H.
Chen, M. Zhu, Y. Liu, G. Yu, Org. Lett. 2012, 14, 3300–3303.
[8] a) H. Ebata, T. Izawa, E. Miyazaki, K. Takimiya, M. Ikeda, H. Ku-
wabara, T. Yui, J. Am. Chem. Soc. 2007, 129, 15732–15733; b) K. Ta-
kimiya, H. Ebata, K. Sakamoto, T. Izawa, T. Otsubo, Y. Kunugi, J.
Am. Chem. Soc. 2006, 128, 12604–12605; c) K. Takimiya, Y. Kunugi,
Y. Konda, H. Ebata, Y. Toyoshima, T. Otsubo, J. Am. Chem. Soc.
[1] a) H. H. Choi, W. H. Lee, K. Cho, Adv. Funct. Mater. 2012, 22,
4833–4839; b) R. Li, J. W. Ward, D.-M. Smilgies, M. M. Payne, J. E.
Anthony, O. D. Jurchescu, A. Amassian, Adv. Mater. 2012, 24, 5553–
5558; c) C. Kim, A. Facchetti, T. J. Marks, Science 2007, 318, 76;
d) A. R. Murphy, J. M. J. Frechet, Chem. Rev. 2007, 107, 1066–1096;
e) B. A. Jones, A. Facchetti, M. R. Wasielewski, T. J. Marks, J. Am.
Chem. Soc. 2007, 129, 15259–15278; f) J. E. Anthony, Chem. Rev.
2006, 106, 5028–5048; g) H. Yan, Z. H. Chen, Y. Zheng, C.
Newman, J. R. Quinn, F. Dotz, M. Kastler, A. Facchetti, Nature
2009, 457, 679–687; h) M. L. Tang, S. C. B. Mannsfeld, Y. S. Sun,
H. A. Becerril, Z. Bao, J. Am. Chem. Soc. 2009, 131, 882–883.
[2] a) R. J. Kline, S. D. Hudson, X. Zhang, D. J. Gundlach, A. J. Moad,
O. D. Jurchescu, T. N. Jackson, S. Subramanian, J. E. Anthony, M. F.
Toney, L. J. Richter, Chem. Mater. 2011, 23, 1194–1203; b) A. Mar-
rocchi, M. Seri, C. Kim, A. Facchetti, A. Taticchi, T. J. Marks, Chem.
Mater. 2009, 21, 2592–2594; c) Y. Didane, G. H. Mehl, A. Kumagai,
N. Yoshimoto, C. Videlot-Ackermann, H. Brisset, J. Am. Chem. Soc.
Chem. Eur. J. 2013, 19, 3721 – 3728
ꢁ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
3727

C. Kim, M.-C. Chen et al.
2006, 128, 3044–3050; d) K. Takimiya, Y. Kunugi, Y. Konda, N. Nii-
hara, T. Otsubo, J. Am. Chem. Soc. 2004, 126, 5084–5085.
[9] a) M.-C. Chen, Y.-J. Chiang, C. Kim, Y.-J. Guo, S.-Y. Chen, Y.-J.
Liang, Y.-W. Huang, T.-S. Hu, G.-H. Lee, A. Facchetti, T. J. Marks,
Chem. Commun. 2009, 1846–1848; b) C. Kim, M.-C. Chen, Y.-J.
Chiang, Y.-J. Guo, J. Youn, H. Huang, Y.-J. Liang, Y.-J. Lin, Y.-W.
Huang, T.-S. Hu, G.-H. Lee, A. Facchetti, T. J. Marks, Org. Electron.
2010, 11, 801–813; c) J. Youn, P.-Y. Huang, Y.-W. Huang, M.-C.
Chen, Y.-J. Lin, H. Huang, R. P. Ortiz, C. Stern, M.-C. Chung, C.-Y.
Feng, L.-H. Chen, A. Facchetti, T. J. Marks, Adv. Funct. Mater. 2012,
22, 48–60.
[10] H. Chen, Q. Cui, G. Yu, Y. Guo, J. Huang, M. Zhu, X. Guo, Y. Liu,
J. Phys. Chem. C 2011, 115, 23984–23991.
[11] a) V. V. Ghaisas, B. D. Tilak, J. Sci. Ind. Res. 1957, 16b, 345; b) N. B.
Chapman, C. G. Hughes, R. M. Scrowston, J. Chem. Soc. C 1970,
2431–2435.
[12] a) C. Kim, P.-Y. Huang, J.-W. Jhuang, M.-C. Chen, J.-C. Ho, T.-S.
Hu, J.-Y. Yan, L.-H. Chen, G.-H. Lee, A. Facchetti, T. J. Marks, Org.
Electron. 2010, 11, 1363–1375; b) M.-C. Chen, C. Kim, S.-Y. Chen,
Y.-J. Chiang, M.-C. Chung, A. Facchetti, T. J. Marks, J. Mater. Chem.
2008, 18, 1029–1036.
[13] As the substrate temperature is increased, semiconductor molecules
diffuse more rapidly, thus facilitating grain growth. Therefore, films
that are deposited at higher substrate temperatures generally exhibit
larger grain sizes.
[14] J. Fournier Dit Chabert, L. Joucla, E. David, M. Lemaire, Tetrahe-
dron 2004, 60, 3221–3230.
[15] V. Lyaskovskyy, R. Frohlich, E.-U. Wurthwein, Chem. Eur. J. 2007,
13, 3113–3119.
[16] a) I. Fouad, Z. Mechbal, K. I. Chane-Ching, A. Adenier, F. Maurel,
J.-J. Aaron, P. Vodicka, K. Cernovska, V. Kozmik, J. Svoboda, J.
Mater. Chem. 2004, 14, 1711–1721; b) S. K. Balenko, N. I. Makarova,
O. G. Karamov, V. P. Rybalkin, I. V. Dorogan, L. L. Popova, E. N.
Shepelenko, A. V. Metelitsa, V. V. Tkachev, S. M. Aldoshin, V. A.
Bren, V. I. Minkin, Russian Chemical Bulletin. 2007, 56, 2400–2406;
c) A. Ricci, D. Balucani, M. Bettelli, Gaz. Chim. Ital. 1971, 101,
774–786.
[17] This synthetic method is similar to the synthesis of benzothiophene,
see: T. Kashiki, S. Shinamura, M. Kohara, E. Miyazaki, K. Takimiya,
M. Ikeda, H. Kuwabara, Org. Lett. 2009, 11, 2473–2475. Because
the crystal structure of BBTT clearly showed two BTT fragments,
the selectivity of the Sonogashira coupling at the 2-position was con-
firmed.
[18] A. Machara, M. Pojarova, J. Svoboda, Collection of Czechoslovak
Chemical Communications 2007, 72, 952–964. Because this article is
not commonly available, the synthetic procedure and NMR spectra
are provided in this study.
Received: November 17, 2012
Published online: January 29, 2013
3728
www.chemeurj.org
ꢁ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2013, 19, 3721 – 3728