Functionalization of 'kite' shaped styryl end-capped benzodithiophene with ketone groups: Synthesis, characterization and properties

Article abstract of DOI:10.1016/j.tet.2011.01.006

We report the synthesis of a new bridged end-capped distyryldithiophene with ketone groups on the bridge: E,E-2,7′-bis((2-phenyl)-ethenyl)- benzo[2,1-b:3,4-b′]dithiophene-4,5-dione 1. Optical and electrochemical properties of 1 in solution were investigat

Full text of DOI:10.1016/j.tet.2011.01.006

Tetrahedron 67 (2011) 1628e1632
Contents lists available at ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Functionalization of ‘kite’ shaped styryl end-capped benzodithiophene
with ketone groups: synthesis, characterization and properties
,
Yahia Didane a, Atsufumi Kumagai b, Noriyuki Yoshimoto b, Christine Videlot-Ackermann a
*
,
,
Hugues Brisset c
*
a
ꢀ
Centre Interdisciplinaire de Nanoscience de Marseille (CINaM), CNRS UPR 3118, Campus Luminy, Aix Marseille Universite, Case 913, 13288 Marseille Cedex 09, France
b Graduate School of Engineering, Iwate University, 4-3-5 Ueda, Morioka 020-8551, Japan
c
ꢀ
ꢁ
ꢀ
Laboratoire Materiaux Polymeres-Interfaces-Environnement Marin (MAPIEMdEA 4323), Universite du Sud Toulon-Var, ISITV, BP 56, 83162 La Valette du Var, France
a r t i c l e i n f o
a b s t r a c t
Article history:
We report the synthesis of a new bridged end-capped distyryldithiophene with ketone groups on the
bridge: E,E-2,70-bis((2-phenyl)-ethenyl)-benzo[2,1-b:3,4-b0]dithiophene-4,5-dione 1. Optical and elec-
trochemical properties of 1 in solution were investigated by UVevis absorption and cyclic voltammetry
and compared to the unsubstituted parent molecule (KDS2T). Morphology of 1-based thin films was
investigated by scanning electron microscopy (SEM) and the crystalline structure characterization by
X-ray diffraction (XRD). Thin films were implemented as active layers into organic thin-film transistors
(OTFT) in top contact configuration to evaluate the charge transport properties.
Received 1 December 2010
Received in revised form 4 January 2011
Accepted 5 January 2011
Available online 12 January 2011
Keywords:
Thiophene
Transistors
Semiconductor
Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction
We report here the synthesis and the characterizations of the
distyryl end-capped benzo[2,1-b:3,4-b0]dithiophenes-4,5-dione 1
Molecular modification of oligothiophene is a topic of intensive
investigations for the development of active components in organic
field effect transistors (OFETs).1e3 We have shown the exceptional
stability of p-channel OFETs based on distyryloligothiophenes4,5
(Chart 1). As expected structural modifications of KDS2T core in 1
lead to a considerable positive shift of the oxidation and reduction
potentials in good agreement with spatial representations of the
HOMO and LUMO frontier orbitals obtained by density functional
theory (DFT). Furthermore, the scanning electron microscopy
(SEM) and the crystalline structure characterization by X-ray dif-
fraction (XRD) on 1-based thin films reveal the drastic change on
the morphology with great impact on transport OTFT activity in
solid state.
6
7
ꢀ
recently confirmed by Frechet et al. and Ong et al. Based on these
considerations we have developed synthetic strategies to modify the
structure of the distyryldithiophene (DS2T) to increase: (i) its elec-
troaffinity to turn this compound into an n-type semiconductor,8,9
(ii) the solubility for investigation of solution-processed organic
thin-film transistors or sensors,10e12 (iii) its charge carrier mobility
by bridging the bithiophene core.13 In the latter case, the bridged
distyryldithiophene (KDS2T, for ‘kite’ DS2T) presented in OTFT de-
vices excellent performances in air (
m
¼0.1 cm2/V s, Ion/Ioff>106)
S
S
higher by a factor 5 to the unbridged parent semiconductor (DS2T).
These results prompted us to associate the rigidification of conju-
gated system with the introduction of withdrawing groups as car-
bonyl groups into the bridge in order to increase the electroaffinity
and induce a potential n-type character in solid state.
KDS2T
O
O
S
S
1
* Corresponding authors. Tel.: þ33 4 94 14 67 24; fax: þ33 4 94 14 27 86 (H.B.);
tel.: þ33 6 17 24 81 93; fax: þ33 4 91 82 95 80 (C.V.-A.); e-mail addresses: videlot@
cinam.univ-mrs.fr (C. Videlot-Ackermann), brisset@univ-tln.fr (H. Brisset).
Chart 1. Structure of KDS2T and diketone 1.
0040-4020/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2011.01.006

Y. Didane et al. / Tetrahedron 67 (2011) 1628e1632
1629
2. Results and discussion
cation and radical anion at E1/2(ox1)¼0.58 V and E1/2(red1)¼ꢀ2.30 V
versus Fc/Fcþ, respectively.13 As expected for the diketone 1 com-
pared to KDS2T positive shifts are observed for E1/2(red1) and E1/
2(ox1) (ꢀ1.00 and 0.82 V versus Fc/Fcþ, respectively) due to the
electro withdrawing effect of carbonyl groups (Fig. 2). However the
effect is more important for E1/2(red1) with a positive shift of 1.30 V
compared to only 0.24 V for E1/2(ox1). Moreover the CV of 1 shows
a second redox system in oxidation and reduction corresponding to
the formation of dication (E1/2(ox2)¼1.00 V) and dianion
(E1/2(red2)¼ꢀ1.73 V). The reduction wave associated with the first
oxidation wave appeared as a sharp cathodic peak characteristic of
the adsorption of the oxidized species followed by cathodic de-
sorption from the electrode surface.17 Contrary as expected, a com-
parison of the E1/2(ox2)ꢀE1/2(ox1) and E1/2(red2)ꢀE1/2(red1) values
(0.18 and 0.73 V, respectively) for 1 shows that the dication state is
obtained more easily than dianion state after the first redox process
indicating that the charges are more separated in the dication state.
2.1. Synthesis of the diketone 1
The synthetic route toward 1 is outlined in Scheme 1. The dia-
ldehyde 2 was synthesized as described previously for the syn-
thesis of KDS2T.13 The diketone 1 is obtained by using of potassium
cyanide in ethanol.20 Addition of tetrabutylamonium bromide in
the mixture increases the yield of reaction from 3 to 54%.
Scheme 1. Synthetic way of diketone 1.
2.2. UVevis spectra of diketone 1
S
S
20
10
0
As expected for a rigid conjugated system UVevis spectra of 1 in
solution shows a well resolution of the vibronic structure with
maxima at 367, 386, and 404 nm together with an absorption band
in the low energy range (650 nm) attributed to an intramolecular
charge transfer (Fig. 1).14 UVevis spectra comparison with un-
bridged DS2T reveals a hypsochromic shift maxima of absorption of
39 nm for diketone 1 indicating a reduction in the effective conju-
gation length. As for KDS2T, this hypsochromic shift can be attrib-
uted to the kite shape of bisthienobenzene core, which penalizes the
O
S
O
S
delocalization of the
p electrons to form an extended conjugated
pathway between both styryl units.13 Furthermore in the case of 1
the electronic interaction between carbonyl groups and conjugated
oligomer backbone can decrease the effective conjugation. A similar
observation was reported for copolymers based on cyclopen-
-10
tadithiophenone.15,16 By extrapolation of the high
l range edge of
-2000
-1000
0
1000
the absorption spectra a value of 2.60 eV is obtained for the optical
bandgap (Eg(opt)), which is 0.23 eV lower compared to KDS2T.
+
E (mV) vs Fc/Fc
Fig. 2. CV of KDS2T (dotted line) and 1 (straight line) at 10ꢀ3 M in 0,1 M n-Bu4NPF6/
1,2-dichlorobenzene, v¼250 mV.sꢀ1
.
1
2.4. DFT of diketone 1
S
S
DS2T
The spatial representations of the HOMO and LUMO frontier
orbitals have been investigated by ab initio quantum calculations
with the Gaussian 03 package of programs at a hybrid density
functional theory (DFT) level with B3LYP/6-31G(d,p) procedure.21
HOMO and LUMO levels of neutral molecules KDS2T and 1 are
shown in Fig. 3. The HOMO of both compounds is located over the
S
S
KDS2T
O
O
S
S
0
300
400
500
600
700
800
(nm)
Fig. 1. Absorption spectra of DS2T, KDS2T and 1 in methylene chloride.
2.3. Cyclic voltammetry of diketone 1
Cyclic voltammogram (CV) of KDS2T reveals two reversible one
electron redox systems corresponding to the formation of radical
Fig. 3. Spatial representations of HOMO and LUMO levels for KDS2T and diketone 1.

1630
Y. Didane et al. / Tetrahedron 67 (2011) 1628e1632
all conjugated system. As contrary distributions of LUMO levels are
very different between KDS2T and 1. While LUMO is localized along
the entire conjugated system for KDS2T, the LUMO level for 1 is
mainly localized on the central dithiopheneediketone core. This is
in good agreement with electrochemical data and the high value for
E1/2(red2)ꢀE1/2(red1) indicating that negative charges are locating
on the dithiophene core.
EHOMO is estimated from E1/2(ox1) and ELUMO from the relation
ELUMO¼EHOMOþEg(opt).18,19 HOMO/LUMO energies levels are esti-
mated as follows for KDS2T (ꢀ5.42/ꢀ2.94) and diketone 1 (ꢀ5.66/
ꢀ3.06) versus DS2T (ꢀ5.32/ꢀ2.72 eV) and reported on an energy
diagram (Fig. 4). The electrochemically derived LUMO energies
decrease continuously from DS2T, KDS2T to 1, which is consistent
with an increase of electroaffinity due to the rigidification and in-
troduction of electron-withdrawing groups on the conjugated core.
2.5. Thin films characteristics
q/2q X-ray diffraction spectra of 1-based thin films vacuum-
deposited at 80 ꢁC, with a nominal thickness of 50 nm reveal that
the films are characterized by sharp and small reflections (Fig. 5).
The peaks can be indexed from the (001) to (004) reflection, in-
dicating that the ab-planes of the grains are oriented parallel to the
substrate surface. To compare to the peaks indexed up to 14th order
in highly oriented polycrystalline KDS2T based film,13 the low oc-
currence of (00l) progressions until only four reflections suggests
a low microstructural order of 1-based thin films. However, the
interplanar d(001)-spacing of 1.97 nm is similar to the value found
for KDS2T based film (1.98 nm), which corresponds to the molec-
ular length of KDS2T determined by the single-crystal X-ray anal-
ysis.13 This result suggests that the shape of 1 is identical to KDS2T
and molecules are nearly perpendicular onto the substrate with
a molecular tilt angle of w18ꢁ to the normal of the substrate.
Characterization of the thin film microstructure at the in-
terface with the substrate, the active region for charge transport
in OTFTs, is challenging by conventional techniques, therefore it
is often evaluated by scanning electronic microscopy (SEM).
Fig. 6 shows the surface morphology of 1-based thin films as
50 nm thick vacuum-deposited layers on Si/SiO2 substrates
heated at 80 ꢁC. Numerous grains appear with identical sizes,
height and shape homogeneously deposited on the surface to-
gether with no well defined grain boundary. The order observed
by XRD is located in the bulk of such grains as no terraces are
observed on top of grains.
Fig. 4. Energy diagram of LUMO and HOMO energies for DS2T, KDS2T and 1 based on
experimentalandDFT. ThehorizontalthicklinecorrespondstoFermilevelofgoldelectrode
(4.9 eV).
electrodes were deposited on top of the semiconducting layer
after the latter was evaporated onto untreated and HMDS-treated
Si/SiO2 substrates at temperature (Tsub) of either 30 ꢁC or 80 ꢁC. All
measurements were performed in air at room temperature. Any
field-effect activity either under positive or negative voltages is
measured probably due to the low microstructural order of thin
films.
Top-contact thin-film transistors were fabricated in the man-
ner described in the Experimental part. Drain and source Au
200
100
In plane
Out of plane
001
150
100
d = 0.335 nm
50
003
002
d = 1.97 nm
004
0
0
0
5
10
15
2
20
25
30
35
0
5
10
15
2
20
(deg.)
25
30
35
(deg.)
Fig. 5.
q
/2
q
Mode of X-ray diffraction patterns of 1-thin film deposited at Tsub¼80 ꢁC on Si/SiO2 with a nominal thickness of 50 nm.

Y. Didane et al. / Tetrahedron 67 (2011) 1628e1632
1631
Fig. 6. SEM pictures of diketone 1 thin films as 50 nm thick vacuum-deposited layers on Si/SiO2 substrates heated at 80 ꢁC.
3. Conclusion
used as internal standard. UVevis absorption spectra were obtained
on a Varian Cary 1E spectrophotometer.
In conclusion, a novel bridged distyryldithiophene with ketone
groups on the bridge was prepared. As expected an important
positive shift of reduction potential was observed indicating an
increase of the electroaffinity. Unfortunately 1-based films reveal
a low microstructural order in solid state preventing any efficient
charge transport in air.
4.3. Theoretical details
Spatial representations of the HOMO and LUMO frontier orbitals
have been investigated by ab initio quantum calculations with the
Gaussian 03 package of programs at a hybrid density functional
theory (DFT) level with B3LYP/6-31G(d,p) procedure.21
4. Experimental
4.1. Synthesis
4.4. Film characterizations
Scanning electronic microscopy (SEM) pictures were realized by
a JEOL field emission gun scanning electron microscope (FEG-SEM,
model JSM 6320F). Thin films of 1, analyzed by X-ray film diffrac-
tometry (XRD), were fabricated by vacuum deposition in a pressure
of 5ꢂ10ꢀ5 Pa using K-cell type crucible. Si wafer (covered by SiO2
layer 300 nm thick) was used as substrates. The substrate tem-
perature (Tsub) was controlled to be 80 ꢁC by heating the block on
which the substrates are mounted. The deposition rate was 6 nm/
min. The final film thickness was 50 nm. The as-deposited thin
films were characterized using X-ray diffraction in air using an
X-ray diffractometer (Regaku Co., ATX-G), which was specially
designed for characterization of thin films. Both in-plane and out-
of-plane diffractions could be measured, because the goniometer
Ethanol, methylene chloride, toluene were purchased from Car-
loErba. Tetrabutylamonium bromide and hexafluorophosphate
(TBHP) were purchased from Fluka and silica gel (240e400 mesh)
from Merck. 1,2-Dichlorobenzene was purchased from Sigmae
Aldrich. Compound 2 was prepared as described previously.13
Melting point is uncorrected and was obtained from an Electro-
thermal 9100 apparatus. 1H NMR and 13C NMR spectra were recor-
ded on Bruker AC 250 at, respectively, 250 MHz and 62.5 MHz.
4.1.1. E,E-2,70-Bis((2-phenyl)-ethenyl)-benzo[2,1-b:3,4-b0]dithio-
phene-4,5-dione 1. To a refluxing mixture of 0.10 g (0.24 mmol) of 2 in
10 mL of ethanol is added 0.04 g (0.60 mmol) of potassium cyanide20
in 5 mL of water tetrabutylammonium bromide (3%). After 12 h of
stirring under reflux, the mixture is cooled before adding methylene
chloride. After extraction, drying, filtering and solvent evaporation
under vacuum, the crude product is purified by chromatography un-
der silica gel with toluene to give 0.06 g (54%) of blue solid corre-
sponding tothe titlecompound. Mp¼253e254 ꢁC.1H NMR (250 MHz,
has not only conventional q/2q axes but also in-plane q/2qc axes.
The used wavelength of X-ray in the experiments was 0.1542 nm.
4.5. OTFTs fabrication
The top contact configuration was used for the OTFT devices
based on 1. Highly n-doped silicon wafers (gate), covered with
thermally grown silicon oxide SiO2 (300 nm, insulating layer),
were purchased from Vegatec (France) and used as device sub-
strates. Hexamethyldisilazane (HMDS) treatment was carried out
by immersing the Si/SiO2 substrate in a pure HMDS solution at
room temperature overnight. The capacitance per unit area of ei-
ther untreated or HMDS-treated silicon dioxide dielectric layers
was 1.2e1.3ꢂ10ꢀ8 F/cm2. The semiconductor layer was vacuum-
deposited onto the insulating layers, using an Edwards Auto 306
apparatus, at a rate of 4e7 nm/min under a pressure of 1e2ꢂ
10ꢀ6 mbar to a nominal thickness of 50 nm as determined with a in
situ quartz crystal monitor. Substrate temperature (Tsub) during
deposition was controlled by heating or not, the block on which the
substrates are mounted. The substrate temperature was fixed to
30 ꢁC or 80 ꢁC. The Au source and drain electrodes (channel length
CDCl3)
d
: 6.91 (d, 3J¼15.95 Hz, 2H, Heth), 7.10 (d, 3J¼16.12 Hz, 2H, Heth),
7.34 (s, 2H, Hthio), 7.30e7.47 (m, 10H, Hphe).13C NMR (67.5 MHz, CDCl3)
d: 119.88,125.30, 126.69,128.68, 128.89,131.50,135.71, 141.75,143.45,
174.17, 181.64 (C]O). UV (CH2Cl2) (log 3): 228 (4.73), 265 (4.67), 367
(4.96), 367 (4.97), 386(5.05), 404 (4.94), 635 (4.13). SM (ESIþ) m/z: 425
[MþH]þ, 442 [MþNH4]þ, 447 [MþN]þ.
4.2. Solution measurements
Cyclic voltammetric (CV) data were acquired using a BAS 100
Potentiostat (Bioanalytical Systems) and a PC computer containing
BAS 100W software (v2.3). A three-electrode system with a Pt
working electrode (diameter 1.6 mm), a platinum counter electrode,
and an Ag/AgCl (with 3 M NaCl filling solution) reference electrode
was used. Tetrabutylammonium hexafluorophosphate (TBHP)
(Fluka) was used as received. Anhydrous 1,2-dichlorobenzene has an
electronic grade purity. Each compound was at 1ꢂ10ꢀ3 M in 1,2-di-
chlorobenzene/TBHP 0.1 M. E1/2 redox values are determined from
the cyclic voltammogram at a scan rate of 50 mV sꢀ1. Ferrocene was
L¼50 m, channel width W¼1 mm) were evaporated on top of the
m
organic thin film through a shadow mask. Hewlett-Packard 4140B
pico-amperemetereDC voltage source is used to apply an in-
creasing drain-source voltage (VD) under a constant gate voltage

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Y. Didane et al. / Tetrahedron 67 (2011) 1628e1632
11. Didane, Y.; Diallo, A. K.; Fiorido, T.; Suzuki, A.; Yoshimoto, N.; Bernardini, S.;
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(VG). Positive or negative voltages can be ranged from 0 to j100j V.
All measurements were performed in air.
12. Fiorido, T.; Bendahan, M.; Aguir, K.; Bernardini, S.; Martini, C.; Brisset, H.; Fages, F.;
Videlot-Ackermann, C.; Ackermann, J. Sens. Actuators B: Chem. 2010, 151, 77e82.
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Acknowledgements
€
The authors acknowledge S. Nitsche and D. Chaudanson from
CINaM for SEM pictures. Pr F. Fages is gratefully acknowledged for
his fruitful discussions.
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