Dibenzoannelated tetrathienoacene: Synthesis, characterization, and applications in organic field-effect transistors

Article abstract of DOI:10.1021/ol3012748

Two structural isomers of six-fused-ring sulfur-containing molecules were synthesized as active materials for p-type organic field-effect transistors, and their optical and electrochemical properties were characterized. Field-effect transistors based on t

Full text of DOI:10.1021/ol3012748

ORGANIC
LETTERS
2012
Vol. 14, No. 13
3300–3303
Dibenzoannelated Tetrathienoacene:
Synthesis, Characterization, and
Applications in Organic Field-Effect
Transistors
Jianyao Huang,†,‡ Hao Luo,†,§ Liping Wang,*,§ Yunlong Guo,† Weifeng Zhang,†
Huajie Chen,†,‡ Minliang Zhu,†,‡ Yunqi Liu,† and Gui Yu*,†
Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese
Academy of Sciences, Beijing 100190, P. R. China, Graduate University of Chinese
Academy of Sciences, Beijing 100049, P. R. China, and School of Materials Science and
Engineering, University of Science and Technology Beijing, Beijing 100083, P. R. China
yugui@iccas.ac.cn; lpwang@mater.ustb.edu.cn
Received May 8, 2012
ABSTRACT
Two structural isomers of six-fused-ring sulfur-containing molecules were synthesized as active materials for p-type organic field-effect
transistors, and their optical and electrochemical properties were characterized. Field-effect transistors based on these compounds were
fabricated to investigate the relationships between structures and semiconductor properties.
In the course of development of suitable π-conjugated
molecules used as p-channel organic semiconductor
materials, many ladder-type molecules have been inten-
sively studied, particularly acene- and thienoacene-based
molecules.1 Recently, different isomers of thienoacenes
have received considerable attention because of their
topology of planar frameworks and highly delocalized
electronic structures, which make them good candidates
for applications in organic field-effect transistors (OFETs).2
Structure variations of these molecules by changing sub-
stituents or extending rings would be an effective way to
build a series of sulfur-containing π-conjugated systems.3 As
derivatives of thienoacenes, benzoannelated fused oligothio-
phenes aroused our interest because of the synthetic chal-
lenge and strong intermolecular interactions (CꢀH π,
3 3 3
† Institute of Chemistry.
π
π, and S S interactions, etc.). Dialkylated or
3 3 3
3 3 3
‡ Graduate University of Chinese Academy of Sciences.
diphenyl[1]benzothieno[3,2-b]benzothiophene,4 dibenzo-
[d,d0]thieno[3,2-b:4,5-b0]dithiophene,5 and benzoannelated
pentathienoacene,6 which contain four, five, and seven fused
rings, respectively, have been explored fairly good perfor-
mances as p-channel semiconductors in previous works.
However, six-fused-ring systems, dibenzotetrathienoacenes
§ University of Science and Technology Beijing.
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r
10.1021/ol3012748
Published on Web 06/12/2012
2012 American Chemical Society

(DBTTAs), have not been reported yet. In view of under-
standing that superacid-induced intramolecular cyclization
reaction of aromatic methyl sulfoxides7 would be accessible
to various fused thiophene derivatives, we herein report a
facile and effective way to synthesize two isomeric dibenzoan-
nelated tetrathienoacenes: a linear ladder-type [1]benzothieno-
[200,300:40,50]thieno[20,30:4,5]thieno[3,2-b][1]benzothiophene
(L-DBTTA) and an S-shaped [1]benzothieno[300,200:40,50]-
thieno[20,30:4,5]thieno[2,3-b][1]benzothiophene (S-DBTTA)
(Figure 1). Compared with the axisymmetric odd-num-
bered fused oligothiophene rings, even-numbered ones with
centrosymmetry might lead to interesting semiconductor
properties.8 To investigate structureꢀproperty relation-
ships, molecular electronic structures and other physico-
chemical properties of both isomers were described.
5-positions resulted in complex byproducts. Instead, we
tried to prepare 2,5-bis (methylthio)-3,6-diphenylthieno-
[3,2-b]thiophene (4). The synthesis of 3,6-dibromo-2,5-
bis(methylthio) thieno[3,2-b]thiophene (3) was previously
reported,10 while a modified method was employed here.
To avoid the byproducts, 2,5-bis(methylthio)thieno[3,2-b]
thiophene (2) was synthesized using n-butyllithium and
dimethyl disulfide, followed by ready bromination to
obtain the desired compound 3 with a high yield. Then, 4
was readily synthesized via Suzuki reaction of the com-
pound 3 with phenylboronic acid. Unlike the correspond-
ing precursor of L-DBTTA, 4 is insoluble in common
solvents including ether, ethanol, acetone, and dichloro-
methane. Poor solubility led to an increase of reaction time
and temperature of the next oxidation step. The optimized
conditions were stirred at 40 °C for over 24 h. S-DBTTA
was obtained via a similar method with L-DBTTA with a
yield of over70% (twosteps). BothL-and S-DBTTA show
a poor solubility in common solvents because of their rigid
structures and strong intermolecular interactions. They
were fully characterized with spectroscopic and elemental
analyses (see the experimental procedures, Supporting
Information).
Figure 1. Molecular structures of compounds L- and S-DBTTA.
Thermogravimetric analysis (TGA) data indicate weight
loss (5%) only on heating above 340 °C (Figure S1,
Supporting Information), suggesting good thermostabil-
ities for both DBTTAs. Figure 2 shows the UVꢀvis
absorptionspectra of the two compounds indilute solution
of dichloromethane and in thin films. In solution, S-
DBTTA has an absorption peak at 312 nm, and L-DBTTA
shows three absorption peaks at 334, 348, and 366 nm,
which contribute to the πꢀπ* transitions. Compared with
the spectra recorded in solution, a slight red shift was
observed in the thin films of S-DBTTA (δλ = 34 nm).
Similar phenomenon was found in the film of hexathie-
noacenene (HTA), suggesting a typical absorption of
sulfur-rich linear thienoacenes.3e For solution and thin
film of L-DBTTA, absoption spectra show longer absorp-
tion maxima than those of the compound S-DBTTA. The
optical energy gaps are 3.18 and 3.35 eV for L- and
S-DBTTA, respectively, determined by extrapolating the
long-wavelength absorption edge in film phase with the
equation Eg = 1240/λonset eV. This suggests the linear
isomer has better conjugation.
Cyclic voltammograms (CV) of L- and S-DBTTA were
investigated at room temperatiure using vacuum-depos-
ited thin films on indium tin oxide (ITO)-coated glass as a
working electrode and Ag/AgCl as a reference electrode in
dichloromethane solution containing 0.1 M tetrabutylam-
monium hexafluorophosphate (TBAPF6) as a supporting
electrolyte. Their CV curves are shown in Figure 2b.
The onset of the oxidation peaks were 1.22 and 1.30 eV
versus Ag/AgCl for L- and S-DBTTA, respectively. The
corresponding HOMO energy levels were estimated to
According to Takimiya’s synthetic approach to obtain
dinaphtho[2,3-b:20,30-f]thieno[3,2-b]thiophene (DNTT),
thienothiophene could be formed by treating 1,2-bis-
(o-(methylthio)aryl)ethane with an excess of iodine.9 In
analogy, L-DBTTA could be synthesizd through a deriva-
tive of benzo[b]thiophene. However, this route was not
readily carried out because of the difficulty of separating
the intermediate 3-(methythio)benzo[b]thiophene and cor-
responding aldehyde 3-(methylthio)benzo[b]thiophene-2-
carbaldehyde. Therefore, we developed an effective proce-
dure from the commercially available starting material
thieno[3,2-b]thiophene (Scheme 1). 2,5-Bis(2-(methylthio)-
phenyl)thieno[3,2-b]thiophene (1) was obtained with a
moderate yield by Suzuki cross-coupling reaction before
oxidation to 2,5-bis(2-(methylsulfinyl)phenyl)thieno[3,2-
b]thiophene using 30% aqueous hydrogen peroxide. Cy-
clization of the methyl sulfoxide gave the target product
with a yield of 75%. The synthesis of S-DBTTA was
attempted to employ the similar method from 3,6-
dibromothieno[3,2-b]thiophene initially, which was not
successful because the highly reactive hydrogens at 2,
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Commun. 2008, 1548. (c) Gao, P.; Beckmann, D.; Tsao, H. N.; Feng, X.;
€
Enkelmann, V.; Baumgarten, M.; Pisula, W.; Mullen, K. Adv. Mater.
2009, 21, 213. (d) Du, C.; Ye, S.; Chen, J.; Guo, Y.; Liu, Y.; Lu, K.; Liu,
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(8) Shinamura, S.; Osaka, I.; Miyazaki, E.; Nakao, A.; Yamagishi,
M.; Takeya, J.; Takimiya, K. J. Am. Chem. Soc. 2011, 133, 5024.
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2224. (b) Kang, M. J.; Doi, I.; Mori, H.; Miyazaki, E.; Takimiya, K.;
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3430.
be ꢀ5.62 and ꢀ5.70 eV using the equation EHOMO
=
ꢀe(4.40 þ Eonset) eV. Their optical and electrochemical
(10) Fuller, L., S.; Iddon, B.; A. Smith, K. J. Chem. Soc., Perkin
Trans. 1 1997, 3465.
Org. Lett., Vol. 14, No. 13, 2012
3301

Scheme 1. Synthesis of Dibenzoannelated Tetrathienoacenes
characteristics suggest both linear and kinked symmetrical
thiophene-based fused-ring molecules have low-lying HOMO
energy levels and relatively large energy gaps, which indicate
good stability against oxygen under ambient conditions.
Both DBTTAs have complete or partially phene-like elec-
tronic structures which would be stable delocalized at the
neutral state, whereas injection of positive charge requires
large deformation of molecular structure to result in a larger
reorgnization energy (λ) than the acene-like structures.1a,11
The kinked S-DBTTA was more obvious to have such effect
due to the nonlinear structure. Although having similar
HOMO energy levels, L- and S-DBTTA with different
molecular structures show different energy gaps and energy
levels of the lowest unoccupied molecular orbitals (LUMO).
The LUMO energy levels were estimated to be about ꢀ2.44
eV for L-DBTTA and ꢀ2.35 eV for S-DBTTA from the
optical band gap and HOMO energy level. These experi-
mental data are in reasonable agreement with the theoretical
studies (Table S1, Supporting Information).
substrates under different substrate temperatures with
bottom-gate-top-contact configuration showed typical
p-channel FET characteristics (Figure 3). The field-effect
characteristics are summarized in Table 1. The best perfor-
mance (a mobility of 0.15 cm2 Vꢀ1 sꢀ1 and a current on/off
ratio of 106) was found for L-DBTTA at Tsub = 40 °C. This
behavior correlates well with high crystallinity, as indicated
by the relatively higher intensity in the corresponding
X-ray diffraction (XRD) patterns and larger interconnected
crystalline grains observed in the atomic force microscope
(AFM) images. For the compound S-DBTTA, the best
performance was obtained at Tsub = 50 °C with a mobility
of 0.047 cm2 Vꢀ1 sꢀ1 and an on/off ratio of 106. Though
AFM images showed larger grains were formed in the thin
films of S-DBTTA deposited at 70 °C, their mobilities
reduced to 0.032 cm2 Vꢀ1 ꢀ1. One possibility is that, with
s
the increase of the temperature, the morphologies of the films
became worse owing to the increase of grain boundaries.
XRD measurements of the DBTTAs-based thin films at
different substrate temperatures exhibit diffraction peaks
at 2θ = 5.70, 11.42, and 23.06° (Figure S2, Supporting
Information), which correspond to the primary, second-,
and fourth-order diffraction peaks, respectively. The pri-
mary peak shows strong diffraction and determines the
corresponding d-spacing of 1.55 nm, which is very close to
Table 1. OFET Characteristics of the DBTTA-Based Devices
Fabricated on OTS-Treated Si/SiO2 Substrates under Different
Substrate Temperatures, L = 80 μm, W/L = 110
Figure 2. (a) UVꢀvis absorption spectra of L- and S-DBTTA in
dilute dichloromethane solution and in thin films at room
temperature. (b) Cyclic voltammograms of compounds L- and
S-DBTTA in dichloromethane.
compd
temp (°C)
μ (cm2 Vꢀ1 sꢀ1
)
Ion/Ioff
Vth (V)
L-DBTTA
30
40
0.040
0.15
9.6 ꢁ 105
5.9 ꢁ 106
1.0 ꢁ 106
7.9 ꢁ 106
1.9 ꢁ 106
5.6 ꢁ 106
1.1 ꢁ 106
1
9
50
0.075
0.029
0.047
0.032
0.014
7
S-DBTTA
30
ꢀ8
ꢀ7
ꢀ11
ꢀ12
All devices fabricated by vapor deposition of DBTTAs on
octadecyltrichlorosilane (OTS)-treated Si/SiO2 (300 nm)
50
70
100
(11) Yamamoto, T.; Shinamura, S.; Miyazaki, E.; Takimiya, K.
Bull. Chem. Soc. Jpn. 2010, 83, 120.
3302
Org. Lett., Vol. 14, No. 13, 2012

(Figure S3, Supporting Information), indicating d-spacing
of 1.38 and 1.11 nm. These experimental data suggest a
polymorphism in solid state which results in the formation
of carrier trap sites and thus lowers the mobility.1a
In summary, we developed a successful strategy of
synthesizing a new family of dibenzoannelated tetrathie-
noacenes as typical p-type organic semiconductors. Mobi-
lities of 0.15 and 0.047 cm2 Vꢀ1 sꢀ1 were obtained for
L- and S-DBTTA, respectively. For both C2-symmetrical
isomers, the linear one is a better candidate thanthe kinked
one for fabricating organic field-effect transistors. Further
investigation of the device optimization of DBTTAs is
underway.
Acknowledgment. This work was supported by the Na-
tional Natural Science Foundation of China (20825208
and 21021091), the Major State Basic Research Develop-
ment Program (2011CB808403 and 2011CB932303), and
the Chinese Academy of Sciences.
Figure 3. Electrical characteristics of the devices: (a) output and (b)
transfer characteristics of the L-DBTTA-based OFETs; (c) output
and (d) transfer characteristics of the S-DBTTA-based OFETs.
Supporting Information Available. Experimental de-
tails, thermogravimetric analysis data and XRD for the
synthesis and characterization of the compounds L- and
S-DBTTA, device fabrication, and field-effect character-
istics of the devices. This material is available free of
charge via the Internet at http://pubs.acs.org.
the length of the L-DBTTA molecule (1.498 nm, Support-
ing Information), indicating that the molecule was nearly
perpendicular to the substrate and formed a perfectly
packed structure. Being consistent with the direction of
current flow, this orientation has been found favorable for
achieving high mobility. However, the diffraction peaks
of S-DBTTA show two strong peaks at 6.38 and 7.98°
The authors declare no competing financial interest.
Org. Lett., Vol. 14, No. 13, 2012
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