Toward n-channel organic thin film transistors based on a distyryl-bithiophene derivatives

Article abstract of DOI:10.1016/j.tet.2012.04.020

Solution and solid-state properties of two new perfluoroalkyl end-substituted analogues of distyryl-bithiophene (CF₃-DS2T and diCF₃-DS2T) are presented. Vacuum deposited thin films were investigated by atomic force microscopy, X-ray diffraction, and implemented as active layers into organic thin film transistors. While physicochemical measurements in solution suggest a preferential hole injection and transport inside CF₃-DS2T and diCF₃-DS2T films, electrical measurements performed under high vacuum show that CF₃-DS2T behaves as n-type semiconductor while no charge transport was measured in diCF ₃-DS2T. The results highlighted the importance of substituents on conjugated backbone and on the resulting fine ordering in solid state to control the charge transport.

Full text of DOI:10.1016/j.tet.2012.04.020

Tetrahedron 68 (2012) 4664e4671
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Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Toward n-channel organic thin film transistors based on a distyryl-bithiophene
derivatives
Yahia Didane a, Rocio Ponce Ortiz b y, Jian Zhang a, Keijyu Aosawa c, Toshinori Tanisawa c,
, ,
*
a
a
a
c
a
, ,
z
*
ꢀ ꢀ
Christine Videlot-Ackermann a
€
Hecham Aboubakr , Frederic Fages , Jorg Ackermann , Noriyuki Yoshimoto , Hugues Brisset
,
,
*
a
ꢀ
Centre Interdisciplinaire de Nanoscience de Marseille (CINaM), CNRS UMR 7325, Aix Marseille Universite, 13288 Marseille Cedex 09, France
b Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, IL 60208, USA
c Graduate School of Engineering, Iwate University, 4-3-5 Ueda, Morioka 020-8551, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Solution and solid-state properties of two new perfluoroalkyl end-substituted analogues of distyryl-
bithiophene (CF3-DS2T and diCF3-DS2T) are presented. Vacuum deposited thin films were in-
vestigated by atomic force microscopy, X-ray diffraction, and implemented as active layers into organic
thin film transistors. While physicochemical measurements in solution suggest a preferential hole in-
jection and transport inside CF3-DS2T and diCF3-DS2T films, electrical measurements performed under
high vacuum show that CF3-DS2T behaves as n-type semiconductor while no charge transport was
measured in diCF3-DS2T. The results highlighted the importance of substituents on conjugated backbone
and on the resulting fine ordering in solid state to control the charge transport.
Received 23 January 2012
Received in revised form 29 March 2012
Accepted 4 April 2012
Available online 17 April 2012
Keywords:
Organic semiconductor
Transistor
Ó 2012 Elsevier Ltd. All rights reserved.
n-Channel
Thin film morphology
1. Introduction
electrochemical data obtained for a range of chemical structures.1,5e9
Accordingly, semiconductors with a first reduction potential ER1
During the last few years several studies have reported organic
thin film transistors (OTFTs) based on n-channel semiconductors
designed by various synthetic strategies. While initial n-type organic
semiconductors suffered from two principal drawbacks, such as low
carrier mobilities and poor environmental stability,1e4 new com-
pounds as small molecules or polymers have emerged enabling air-
stable n-channel OTFTs.1,5e21 Despite lower mobilities than conven-
tional inorganic semiconductors, the development of n-type mate-
rials is essential to enable complementary metal oxide
semiconductor (CMOS) technology. Compounds with strong electron
affinity are usually found to enable electron transport in the solid
state. In order to predict the O2/H2O sensitivity of organic n-type
materials, studies on arylene bisimides and other semiconductors
derived an empirical method from the consideration of the
ranging from ꢀ0.4 to 0.0 V versus SCE should result in OTFTs exhib-
iting stable electron transport in air, provided that an optimized thin
film morphology is obtained. This indeed contributes to favor the
generation of the anion-radical species responsible for negative
charge carrier transport in the solid as electron injection into the
lowest unoccupied molecular orbital (LUMO) of the semiconductor
from the metal can be facilitated.22 As small molecules, oligomers of
thiophene are widely known as excellent p-type semiconductors.
However, by introducing appropriate electron withdrawing groups
(EWGs), as fluor (eF) or perfluoroalkyl (eCF3), it is possible to switch
their hole-type conductivity to the electron-type one. Chemical
substitution with EWG functionalities is often considered as a versa-
tile approach to lowering the LUMO energy level of conjugated sys-
tems. First n-type derivatives obtained in the oligothiophene family
were
a,u-perfluorohexyl-quaterthiophene (DFH-4T) and a,u-per-
fluorohexyl-sexithiophene (DFH-6T).23,24 Transistors based onvapor-
deposited DFH-4T and DFH-6T thin films on silicon dioxide (SiO2),
hexamethyldisilazane (HMDS), poly(styrene) (PS) and poly(vinyl al-
cohol)(PVA) dielectric layers showedelectronmobility upto 0.06 and
0.02 cm2/V s invacuum, respectively. Despite the presence of electron
withdrawing groups and a dense crystal packing, transistors do not
retain their good activity when exposed to air. An independent work
has shown the air activity of n-channel DFH-4T based transistors by
* Corresponding authors. Tel.: þ34 952131863; fax: þ34 952132000 (R.P.O.); tel.:
þ33 4 94 14 6724; fax: þ33 4 94 14 2786 (H.B.); tel.: þ33 6 17 24 8193; fax: þ33 4
91 41 8916 (C.V.-A.); e-mail addresses: rocioponce@uma.es (R.P. Ortiz), brisset@
univ-tln.fr (H. Brisset), videlot@cinam.univ-mrs.fr (C. Videlot-Ackermann).
y
Department of Physical Chemistry, University of Malaga, Malaga 29071, Spain.
z
ꢀ
ꢁ
Laboratoire Materiaux Polymeres-Interfaces-Environnement Marin (MAPIEM e
ꢀ
EA 4323), Universite du Sud Toulon-Var, ISITV, BP 56, 83162 La Valette du Var,
France.
0040-4020/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2012.04.020

Y. Didane et al. / Tetrahedron 68 (2012) 4664e4671
4665
using polymethylmethacrylate (PMMA) as organic insulating layer.25
On the base of the quaterthiophene conjugated core, a completework
investigated how the placement of the perfluoroarene unit in the
conjugated skeleton could trigger the sign of the majority charge
carrier. With the perfluoroarene units located at both termini of the
quaterthiophene oligomer, the diperfluorophenyl-quaterthiophene
DF-4T was found to be n-type under argon or under vacuum (mod-
erate vacuum w20 mTorr or high vacuum w10ꢀ5 Torr).26,27 Gold
source and drain electrodes-containing transistors exhibited excel-
lent parameters for DF-4T with an electron mobility up to 0.43 cm2/
V s and on/off ratio of10,8 while the two others with fluoroareneunits
incorporating in the oligothiophene chain are p-type with no ambi-
polar property. A theoretical study on such diperfluorophenyl
substituted quaterthiophenes has stressed not only the major role of
the intermolecular contributions (transfer integrals, reorganization
energies, dependencies on intermolecular separation and arrange-
ment)tocontrolthechargetransportpropertiesofmaterials, but they
have also emphasized the critical role played by several macroscopic
properties within the device architecture, which could overwhelm
the electronic effect of the perfluoroarene unit depending on its lo-
cation in the conjugated structure.28 Following the same synthetic
approach perfluoroarene-containing analogues of distyryl-
oligothiophenes (DFSnT, n¼2, 4) were studied.29,30 The in-
corporation of perfluoroarene rings in DSnTs series (n¼2, 3, 4), where
an exceptional stability of p-channel OTFTs has been shown,31 was
found to lower the LUMO energies, consistently with the electron
withdrawing influence of the fluorine atoms. Nevertheless, DFS2T
and DFS4T thin films exhibit a p-type semiconducting behavior as
active layer implemented in OTFT devices under identical conditions
as DF-4T for accurate comparison.30 Such p-type transport was ob-
served not only in air,29 but also under high vacuum.30 In contrast to
DF-4T, the incorporation of double bonds between perfluoroarene
units and oligothiophene cores fails to produce the similar trend in
vacuum. In such perfluoroarene-terminated oligothiophene de-
rivatives, an efficient electron density confinement at the molecular
scale is advanced to affect the final transport properties.30 As efficient
EWGs, eCF3 substitutions on aryl naphthalenetetracarboxylic dii-
mide (NTCDI) enable to lower LUMO level and help to improve
electron charge injection. Resulting mobilities and air stability on n-
channel OTFTs have been considerably increased.32 In order to ex-
pand the series of fluorinated distyryl-bithiophenes, two new per-
fluoroalkyl end-substituted analogues of distyryl-bithiophene (CF3-
DS2Tand diCF3-DS2T) were synthesized by introducing eCF3 groups
on arene end-units with various numbers and position. Scheme 1
gives the chemical structure of DS2T, DFS2T, CF3-DS2T and diCF3-
DS2T.
A comparative study in solution and in solid state based on
DFS2T, CF3-DS2T and diCF3-DS2T reveals the great influence of
perfluorination by alkyl chains on the arene moieties of distyryl-
bithiophene skeleton on electronic structure, physical properties,
and device efficiencies. As expected, the incorporation of per-
fluoroalkyl chains was found to lower the LUMO energies but not
dramatically to predict a preferential electron injection from gold
metal electrodes to the LUMO level. While any obvious electron
density confinement occurred in both CF3-DS2T and diCF3-DS2T, n-
channel OTFTs can be observed only for CF3-DS2T in high vacuum.
In the series of fluorinated oligomers, a particular attention was
taken to realize and measure the OTFT devices in the same condi-
tions for a more accurate discussion. Classical dielectrics, bare and
HMDS-treated Si/SiO2 substrates instead of PS, PVA or BCB, were
chosen as n-channel OTFTs were observed with DF-4T under vac-
uum.26,27 The main result coming out of this comparative study is
the influence of the high electronic effect of eCF3 groups together
with a fine molecular arrangement in CF3-DS2T-based thin films to
control the charge carrier type, i.e., electrons, transported in or-
ganic active layers.
Scheme 1. Chemical structure of DS2T, DFS2T, CF3-DS2T and diCF3-DS2T.
2. Results and discussion
To understand and quantify the effects of the perfluoroalkyl
substitutions on the aromatic backbone of distyryl-bithiophene
derivatives, absorption spectroscopy and cyclic voltammetry mea-
surements in solution were carried out. All data were compared to
those obtained for previous reported distyryl-bithiophenes (DS2T
and DFS2T).29e31 Optical and electrochemical data of DS2T, DFS2T,
CF3-DS2T and diCF3-DS2T based solutions are reported in Table 1
and the energy diagram of LUMO and HOMO (highest orbital mo-
lecular orbital) levels for the four oligomers is shown in Fig. 1.
In a simple Schottky-type injection barrier model, as presented
on Fig. 1, the relative carrier injection barriers should be described
as the difference between the molecular HOMO energy and the
metal electrode work function, here gold with Ef w4.9 eV, for the
hole injection (
LUMO energy and the metal work function for the electron in-
jection ( Ee). Compounds designated to give rise to air-stable n-
DEh), and the difference between the molecular
D
channel OTFTs may possess a LUMO level lying near the metal work
function to facilitate the electron injection into the semiconductor
from the metal.22 The overall picture resulting from the physico-
chemical measurements in solution of CF3-DS2T and diCF3-DS2T
compared to DFS2T is the similar influence of perfluorination by

4666
Y. Didane et al. / Tetrahedron 68 (2012) 4664e4671
Table 1
Optical and electrochemical data of distyryl-bithiophene (DS2T) and its derivatives (DFS2T, CF3-DS2T and diCF3-DS2T)
R'
R''
lmax (nm)a
Eg (eV)
E1/2(ox1) (V)
E1/2(red1) (V)
HOMO (eV)b
LUMO (eV)b
Ref
R
opt
R''
R'
R
R0
R00
DS2T
DFS2T
CF3-DS2T
diCF3-DS2T
H
F
CF3
H
H
F
H
CF3
H
F
H
H
425
430
429
430
2.60
2.55
2.55
2.52
0.48
0.59
0.63
0.62
ꢀ2.26
ꢀ2.10
ꢀ2.10
ꢀ2.02
ꢀ5.18
ꢀ5.29
ꢀ5.29
ꢀ5.34
ꢀ2.58
ꢀ2.74
ꢀ2.74
ꢀ2.82
33,34
29,30
This ref.
This ref.
a
In CH2Cl2.
b
Estimated from experimental data with LUMO (eV)¼ꢀ4.84 eVꢀE1/2 (red1) and HOMO (eV)¼LUMOꢀEg.
Fig. 2. DFT-optimized geometries, frontier orbitals, and orbital energies (eV vs vac-
uum) of CF3-DS2T and diCF3-DS2T.
transport to confer a p-channel activity in air and in vacuum. With
a lower LUMO level, diCF3-DS2T should be more appropriated for
an electron injection from gold electrodes although a high
DEe
value of 2.08 and 2.2 eV, obtained by, respectively, the experimental
data and DFT computations, is predicted as a limiting factor. In
order to classify a compound as n-type organic semiconductor,
transport properties in solid state have to be investigated to iden-
tify the majority charge carriers (holes or electrons) involved.
The Bottom-Gate Top-Contact (BGTC) configuration was used
for the OTFT devices based on CF3-DS2T and diCF3-DS2T. Au-based
source and drain electrodes were deposited on top of the semi-
conducting layer after the latter was evaporated onto bare or
HMDS-treated Si/SiO2 substrates at different substrate tempera-
tures (Tsub¼30 and 80 ꢁC). While no transistor activity was observed
in air or in moderate vacuum (270e300 mTorr) for both CF3-DS2T
and diCF3-DS2T, the only one operating channel is observed in high
vacuum (below 6ꢂ10ꢀ6 Torr) with CF3-DS2T as active layer in OTFTs
based on SiO2 insulator layer coated with HDMS for either Tsub¼30
or 80 ꢁC. In comparison with bare Si/SiO2 substrates, HMDS-treated
ones posses less electron trapping sites at the semiconductor/in-
sulator interface to be filled before the carriers become mobile.35
Unexpectedly, by application of positive drain and gate voltages
OTFTs operate in the accumulation mode demonstrating that CF3-
DS2T behaves as n-type semiconductor in high vacuum. Any
ambipolar property was observed in either air or vacuum. For an
identical BGTC configuration, DFS2T behaved as a p-type semi-
conductor directly in air on bare SiO2 dielectric.30 Typical output
characteristics of OTFT devices fabricated with CF3-DS2T at Tsub¼30
and 80 ꢁC on HMDS-treated Si/SiO2 substrates are shown in Fig. 3a
and b, respectively.
Fig. 1. Energy diagram of LUMO and HOMO levels for distyryl-bithiophene (DS2T) and
its derivatives (DFS2T, CF3-DS2T and diCF3-DS2T). The horizontal thick line corre-
sponds to Fermi level of gold electrode (w4.9 eV). LUMO and HOMO energies are
estimated from experimental data with LUMO (eV)¼ꢀ(4.84 eVþE1/2 (red)) and HOMO
(eV)¼LUMOꢀEg where Eg optical band gap.
alkyl groups (eCF3) than by fluorine atoms (eF) on arene end-units
of DS2T
p-core. It is worth noting that the LUMO and HOMO en-
ergies are identical for DFS2T and CF3-DS2T, while those of diCF3-
DS2T are even w0.05e0.08 eV lower. The molecular orbital plots of
CF3-DS2T and diCF3-DS2T obtained by DFT reveal highly delo-
calized HOMOs and LUMOs having aromatic and quinoidal con-
figurations, respectively (Fig. 2).
It is nevertheless essential to mention that high drain-source
(Vd) and gate (Vg) voltages up to 200 V have to be applied in or-
der to observe a drain current (Id) as high as 0.3
mA on heated
Lowest HOMO and LUMO energies for diCF3-DS2T than for CF3-
DS2T underline the results obtained by experimental data. Based on
the strictly schematic representation of orbital energies, CF3-DS2T
involved as active layer in OTFT devices with Au-based electrodes
should operate identically as DFS2T i.e., both hole injection and
substrates. At this stage of the study, it remains consistent to sug-
gest that a contact resistance should have an effect on the measured
data as high voltages were applied to observe an accumulation
mode for a high drain-source voltage (Vd>25 V). Nevertheless, such
contact resistance should not have a direct influence on the active

Y. Didane et al. / Tetrahedron 68 (2012) 4664e4671
4667
Fig. 3. Output characteristics of CF3-DS2T based-OTFTs on HMDS-treateꢀd6 Si/SiO2
substrate at Tsub¼30 ꢁC (a) and 80 ꢁC (b) measured in vacuum (below 6ꢂ10 Torr).
layer behavior i.e., p- or n-type. An electron mobility up to
8.1ꢂ10ꢀ4 cm2/V s together with an Ion/Ioff ratio of w107 were
measured for CF3-DS2T films deposited at 80 ꢁC as against
5.2ꢂ10ꢀ4 cm2/V s and 106 for CF3-DS2T films deposited at 30 ꢁC.
The average mobilities and standard deviations are 5.6ꢂ10ꢀ4 (cm2/
V s)ꢃ2.5ꢂ10ꢀ4 and 4.1ꢂ10ꢀ4 (cm2/V s)ꢃ1.1ꢂ10ꢀ4 for OTFTs with
CF3-DS2T deposited on HMDS-treated Si/SiO2 substrates at 80 and
30 ꢁC, respectively. A good reproducibility of measured data was
obtained as five individual OTFTs were tested for each substrate
temperature. Despite modest mobility values, the most significant
result is the observation for the first time of n-channel OTFTs in the
distyryl-oligothiophene series where any obvious electron density
confinement occurs compared to DSF-2T.29 By changing the sub-
stituents by eCF3 on arene end-units, the electron density is
delocalized on the overall molecular backbone without any hin-
dering due to the presence of double bonds. The electronic effect of
such eCF3 groups has been demonstrated in arene-end substituted
NTCDI compounds where the electron mobility increased with the
number of eCF3. However, such results were associated with
unique morphological features, including plate-like and rod-like
morphologies.32
Fig. 4. AFM pictures and corresponding AFM profiles of CF3-DS2T and diCF3-DS2T thin
films deposited at Tsub¼30 ꢁC (a, b) and 80 ꢁC (c, d) on HMDS-treated Si/SiO2 substrates
with a nominal thickness of 50 nm.
with better defined grain edges. On the contrary, increasing sub-
strate temperature leads to an opposite effect for diCF3-DS2T films
as the high number of small needles like grains observed at 30 ꢁC
give way to 5 mm-long grains unconnected to each other and ran-
domly dispersed on the surface. Based on AFM images observed at
30 ꢁC for both oligomers, CF3-DS2T and diCF3-DS2T, the macro-
scopic morphology is not sufficient to explain the lack of charge
transport in diCF3-DS2T-based thin films as continuous films were
observed.
In order to understand how the solid-state ordering in the
semiconductor affects thin film microstructure and charge trans-
port properties, X-ray diffraction experiments were carried out. X-
ray diffraction (XRD) was performed on CF3-DS2T and diCF3-DS2T
thin films as 50 nm thick vacuum deposited layers at 30 and 80 ꢁC
on bare and HMDS-treated Si/SiO2 substrates (Fig. 6). The XRD
scans show that thin films present different textures depending on
AFM pictures of 50 nm thick-vapor-deposited CF3-DS2T and
diCF3-DS2T films grown on either HMDS-treated or bare Si/SiO2
substrates at 30 and 80 ꢁC are presented on Figs. 4 and 5, re-
spectively. CF3-DS2T exhibits plate-like small grains with some
high out-of-plane grains at 30 ꢁC regardless of the Si/SiO2 substrate
nature, bare or treated-HMDS. By increasing the substrate tem-
perature during the organic vacuum deposition up to 80 ꢁC, the
same trends are observed but with an increased grain size together

4668
Y. Didane et al. / Tetrahedron 68 (2012) 4664e4671
Fig. 6.
q/2q mode of X-ray diffraction patterns of CF3-DS2T and diCF3-DS2T based thin
films deposited at Tsub¼30 ꢁC (a) and 80 ꢁC (b) on bare and HMDS-treated Si/SiO2
substrates with a nominal thickness of 50 nm.
nature of the dielectric seems to be a relevant parameter as p-
channel DFS2T-based OTFTs operated on bare or HMDS-treated Si/
SiO2 substrates and PMMA,29 a charge transport activity in CF3-
DS2T-based thin films is observed only on HMDS-treated Si/SiO2
substrates. Another main difference between CF3-DS2T and DFS2T
is the observation of a second polymorphism on Fig. 6 as de-
termined by the dominate diffraction peak (001) and its shoulder
(0010) corresponding to a principal d-spacing of 2.25 nm together
with an additional d0-spacing of 2.03 nm, respectively. Some spe-
cific intermolecular interactions in CF3-DS2T-based thin films give
rise to a fine ordering in the solid state where the occurrence of
(00l) progressions means that the ab-planes of the grains are ori-
ented parallel to the substrate surface with a small tilted angle for
Fig. 5. AFM pictures and corresponding AFM profiles of CF3-DS2T and diCF3-DS2T thin
films deposited at Tsub¼30 ꢁC (a, b) and 80 ꢁC (c, d) on bare Si/SiO2 substrates with
a nominal thickness of 50 nm.
the position of perfluoroalkyl groups and the nature of the di-
electric surface. No significant peak reflections are recorded for
diCF3-DS2T at either Tsub¼30 or 80 ꢁC on both bare and HMDS-
treated Si/SiO2 substrates. The lack of n-channel behavior of
diCF3-DS2T may be due to unfavorable molecular packing in thin
the second polymorphism. Such ordering is favorable to a
pep
stacking in solid film for an efficient electron transport in OTFT
devices.
solid film, which generates in particular a less-effective
pep
stacking as already observed for a NTCDI with fluorinated phenyl-
ethyl groups.36 On the contrary, textured patterns are observed
with CF3-DS2T but only for films deposited on HMDS-treated Si/
SiO2 at either Tsub¼30 or 80 ꢁC agreeing with an effective charge
transport activity observed for a dielectric surface containing less
electron trapping sites as HMDS-treated Si/SiO2. Despite compa-
rable XRD patterns,29 DFS2T and CF3-DS2T give rise to drastic dif-
ferent electrical responses. While DFS2T behaved as a p-type
semiconductor in air and under vacuum,29,30 CF3-DS2T presents an
electron transport activity but exclusively under high vacuum.
Additionally to the measurement conditions (air or vacuum), the
Recent work reported in the literature the great effect of the
halogenated moieties on the packing of molecules with SeF in-
teractions involved in the solid state assembly of organic semi-
conducting materials.37,38 Based on structural parameters in thin
film phase obtained for the three fluorinated compounds DFS2T,
CF3-DS2T and diCF3-DS2T, the major difference is the great impact
of the fine crystalline environment, which is achieved in the solid
state. While substitutions with eCF3 lowered LUMO level in both
CF3-DS2T and diCF3-DS2T compounds, the lack of n-channel be-
havior of diCF3-DS2T thin films even in vacuum is due to the less-
effective
pep stacking. Contrary to DFS2T, any efficient electron

Y. Didane et al. / Tetrahedron 68 (2012) 4664e4671
4669
R'
O
OCH3
OCH3
density confinement at the molecular scale occurred confirming
the high electronic effect of such eCF3 groups on conjugated
backbone. From specific intermolecular interactions resulting in
high ordering in CF3-DS2T-based thin films emerge n-channel
OTFTs on HMDS-treated Si/SiO2 substrates. CF3-DS2T is the first
electron transporting derivative observed in the distyryl-
oligothiophene series. Indeed, more extensive studies demon-
strated that different chemical substitutions on the distyryl-
oligothiophene core with EWGs as fluoro-atoms (eF),29,30 cyano
groups (eCN)39 or carbonyl groups (eC]O)40 failed to produce n-
type organic semiconductors. Therefore, this approach aiming at
switching the sign of the majority carriers in oligothiophenes using
EWGs cannot be considered as general and the particular structure
of the conjugated backbone as well as the resulting solid state
microstructure has to be taken in account.
P
R
1a: R = CF3, R' = H
1b: R= H, R' = CF3
R'
O
S
R'
S
O
R
R'
S
S
R'
CF3-DS2T: R = CF3, R' = H
diCF3-DS2T: R= H, R' = CF3
R
3. Conclusion
R'
Scheme 2. Synthetic scheme for the perfluoroalkyl-DS2T oligomers (CF3-DS2T and
diCF3-DS2T).
In conclusion, two novel distyryl-bithiophenes with per-
fluoroalkyl groups (eCF3) were synthesized: E,E-5,50-(bis((4-
trifluoromethyl)phenyl)-ethenyl)-2,20-bithiophene (CF3-DS2T) and
E,E-5,50-bis(2-(3,5-di-(trifluoromethyl)phenyl)-ethenyl)-2,20-bithio-
phene (diCF3-DS2T). A comparative study with the perfluoroarene-
containing distyryl-bithiophenes analogue (DFS2T) underlines the
influence of perfluorination by either alkyl groups (eCF3) or by
6 h. The mixture was then distilled under reduced pressure to
eliminate the excess of phosphite and the oily residue purified by
column chromatography using chloroform/methanol (1/1) as the
eluent. 1H NMR (250 MHz, CDCl3)
d
: 3.10 (d, 2H, J¼21.75 Hz, CH2-P),
fluorine atoms (eF) on arene end-units of DS2T
p-conjugated core.
3.63 (s, 3H, OCH3), 3.64 (s, 3H, OCH3), 7.34 (d, 2H, J¼8.25 Hz, Hbenz),
While DFS2T implemented as active layer into OTFTs behaves as a p-
type organic semiconductor, CF3-DS2T leads to n-channel OTFTs.
With a fine microstructure observed by AFM and XRD of CF3-DS2T-
based thin films deposited on HMDS-treated Si/SiO2 substrates
heated to 80 ꢁC, electron injection can occur from gold electrodes to
LUMO level generating an electron transport. A mobility up to
9.8ꢂ10ꢀ4 cm2/V s together with an Ion/Ioff ratio of w107 were mea-
sured under high vacuum. The absence of a fine solid-state ordering
in diCF3-DS2T thin films ateither 30 or 80 ꢁC revealed by XRD leads to
a total absence of charge transport in such films. Analyses of these
materials reveal a direct relationship between molecular structure,
solid state microstructure and electrical properties. The results of this
study indicate that the molecular geometry and intermolecular in-
teractions in the crystalline state govern the electrical properties of
OTFTs by directly influencing key factors as interface states and
morphology of organic thin films.
7.48 (d, 2H, J¼8.25 Hz, Hbenz). 13C NMR (67.5 MHz, CDCl3)
d: 31.50
(CH2), 33.70 (CH2), 52.77 (CH3), 52.88 (CH3), 122.00 (CF3), 125.45
(Cbenz), 129.92 (Cbenz), 130.03 (Cbenz), 137.70 (Cbenz). MS (FABþ) m/z:
calcd 268, found 269.
4.1.2. Dimethyl 3,5-di(trifluoromethyl)benzylphosphonate 1b. The
suitable benzylbromide (1.00 g, 3.26 mmol) was mixed with tri-
methylphosphite (2.79 mL, 16.28 mmol) and refluxed at 112 ꢁC for
6 h. The mixture was then distilled under reduced pressure to
eliminate the excess of phosphite and the oily residue purified by
column chromatography using chloroform and methanol (1/1) as
the eluent. 1H NMR (250 MHz, CDCl3)
CH2-P), 3.65 (s, 3H, OCH3), 3.68 (s, 3H, OCH3), 7.70 (s, 3H, Hbenz). 13
NMR (67.5 MHz, CDCl3) : 31.30 (CH2), 33.51 (CH2), 52.83 (CH3),
d
: 3.10 (d, 2H, J¼22.50 Hz,
C
d
52.94 (CH3),120.88 (CF3),125.20 (Cbenz),129.84 (Cbenz),131.5 (Cbenz),
134.5 (Cbenz). MS (FABþ) m/z: calcd 336, found 337.
4. Experimental section
4.1. Synthesis
4.1.3. Typical procedure WittigeHorner. Into a flask of 100 mL,
0.9 mmol of 2,20-bithienyl-5,50-dicarboxaldehyde and 2.07 mmol of
phosphonate were dissolve in 60 mL of anhydrous tetrahydrofur-
ane under argon. At 0 ꢁC 4.14 mmol of t-BuOK are added. After 10 h
of agitation at room temperature the solvent was evaporated in
vacuo. The crude product was dissolved in methylene chloride and
water was added. The mixture is extracted with methylene chlo-
ride. The combined organic phases were washed with water, dried
(MgSO4), and evaporated in vacuo. The product was purified on
a silica gel column. The desired fractions were pooled and con-
centrated to afford E isomers.
Perfluoroalkyl-distyryl-bithiophene derivatives were prepared
by WittigeHorner olefinations between adequate phosphonate and
2,20-bithienyl-20,50-dicarboxaldehyde (Scheme 2).
Methylenechloride, methanol, tetrahydrofurane (THF), chloro-
form, were purchased from CarloErba. Tetrabutylamonium hexa-
fluorophosphate (TBHP) was purchased from Fluka. Silica gel
(240e400 mesh) was obtained from Merck. Trimethylphosphite,
butyllithium in hexane, p-trifluoromethylbenzylbromide and
3,5-di(trifluoromethyl)benzylbromide were purchased from
SigmaeAldrich. 2,20-bithienyl-5,50-dicarboxaldehyde were pre-
pared as described in the literature.41 Melting points are un-
corrected and were obtained from an Electrothermal 9100
apparatus. 1H NMR and 13C NMR spectra were recorded on Bruker
AC 250 at, respectively, 250 MHz and 62.5 MHz. UV spectra were
obtained on Varian Cary 50.
4.1.4. E,E-5,50-(Bis((4-trifluoromethyl)phenyl)-ethenyl)-2,20-bithio-
phene (CF3-DS2T). Yield: 55%. Mp 224e226 ꢁC; 1H NMR (250 MHz,
CDCl3)
d
: 6.83 (d, 2H, J¼16.25 Hz, Heth), 6.97 (d, 2H, J¼3.75 Hz, Hthio),
7.05 (d, 2H, J¼3.75 Hz, Hthio), 7.21 (d, 2H, J¼16.50 Hz, Heth), 7.49 (d,
4H, J¼8.75 Hz, Hbenz), 7.54 (d, 4H, J¼9.00 Hz, Hbenz). HMRS: calcd
506.0598, found 506.0609. Elemental Anal. calcd for C26H16F6S2: C,
61.65; H, 3.18; S, 12.66. Found: C, 61.60; H, 2.98; S, 12.94.
4.1.1. Dimethyl p-trifluoromethylbenzylphosphonate 1a. The suit-
able benzylbromide (3.00 g, 12.55 mmol) was mixed with trime-
thylphosphite (10.76 mL, 62.75 mmol) and refluxed at 112 ꢁC for
4.1.5. E,E-5,50-Bis(2-(3,5-di-(trifluoromethyl)phenyl)-ethenyl)-2,20-
bithiophene (diCF3-DS2T). Yield: 21%. Mp 222e224 ꢁC; 1H NMR
(250 MHz, CDCl3)
d
: 6.85 (d, 2H, J¼16.25 Hz, Heth), 6.97 (d, 2H,

4670
Y. Didane et al. / Tetrahedron 68 (2012) 4664e4671
ðIDÞsat ¼ ðW=2LÞCi ðVG ꢀ VtÞ2
m
(1)
J¼3.75 Hz, Hthio), 7.05 (d, 2H, J¼3.75 Hz, Hthio), 7.21 (d, 2H,
J¼16.50 Hz, Heth), 7.65 (s, 2H, Hbenz), 7.80 (s, 4H, Hbenz). 1HMRS:
calcd 642.0345, found 642.0367. Elemental Anal. calcd for
C28H14F12S2: C, 52.34; H, 2.20; S, 9.98, F, 35.48. Found: C, 52.79; H,
2.00; S, 10.07.
where Ci is the capacitance per unit area of the gate insulator layer,
VG is the gate voltage, Vt is the threshold voltage, and is the field-
effect mobility. All the data were obtained by randomly measuring
m
five individual OTFTs for each substrate temperature.
4.1.6. Physicochemical measurements in solution. UVevisible ab-
sorption spectra were obtained on a Varian Cary 1E spectropho-
tometer. The electronic absorption maximum (lmax) and the optical
band gap (Eg) are directly extracted from absorption spectra of CF3-
DS2T and diCF3-DS2T based solution. Cyclic voltammetry (CV) data
were acquired using a BAS 100 Potentiostat (Bioanalytical Systems)
and a PC computer containing BAS100W software (v2.3). A three-
electrode system based on a platinum (Pt) working electrode (di-
ameter 1.6 mm), a Pt counter electrode and an Ag/AgCl (with 3 M
NaCl filling solution) reference electrode was used. Tetrabuty-
lammonium hexafluorophosphate (TBHP) (Fluka) was used as re-
ceived and served as supporting electrolyte (0.1 M). All experiments
were carried out in anhydrous 1,2-dichlorobenzene (electronic
grade purity) at 20 ꢁC. Ferrocene was used as internal standard.
Electrochemical reduction/oxidation potential versus Fc/Fcþ
(E1/2(red1) and E1/2(ox1)) values are determined from the cyclic
voltammogram at a concentration of 1ꢂ10ꢀ3 M with a scan rate of
4.1.8. Film characterizations. Atomic force microscopy (AFM) mea-
surements were done on thin films in air with a Nanoscope III
Multimode (Instrument, Inc.), operating in the tapping mode. Fur-
thermore, thin films were analyzed by X-ray film diffractometry
(XRD) where thin films of CF3-DS2T and diCF3-DS2T were fabri-
cated 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, which were kept at room temperature (Tsub¼RT)
or heated to 80 ꢁC (Tsub¼80 ꢁC). The deposition rate and final film
thickness were 1.2 nm/min and 50 nm, respectively. 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. The used
wavelength of X-ray in the experiments was 0.1542 nm.
4.1.9. Computational Methodology. DFT calculations were carried
out using the Gaussian 03 program.42 Becke’s three-parameter
exchange functional combined with the LYP correlation functional
(B3LYP) was employed.43 We also made use of the standard
6e31G** basis set.44
50 mV sꢀ1
.
4.1.7. OTFTs fabrication. The Bottom-Gate Top-Contact (BGTC)
configuration was used for the OTFT devices based on CF3-DS2T
and diCF3-DS2T derivatives. Highly n-doped silicon wafers (gate),
ꢂ
covered with thermally grown silicon oxide SiO2 (3000 A, in-
Acknowledgements
sulating layer), were purchased from Vegatec (France) or WRS
materials (USA) and used as device substrates. Trimethylsilation of
the Si/SiO2 surface was carried out by either exposing the silicon
wafers to hexamethyldisilazane (HMDS) vapor at room tempera-
ture in a closed air-free container under nitrogen for greater than
one week or by immersing the Si/SiO2 in a pure solution HMDS at
room temperature overnight. The capacitance per unit area of
either bare or HMDS deposition silicon dioxide dielectric layers
was 1.0e1.3ꢂ10ꢀ8 F/cm2. The semiconductor layer was vacuum
deposited onto the insulating layers to a nominal thickness of
50 nm as determined with an in situ quartz crystal monitor.
Substrate temperature (Tsub) during deposition was controlled by
heating the block on which the substrates are mounted. The de-
position control occurred in a three steps’ process: (i) during the
evaporator reached the right pressure, the substrates were cov-
ered by a shuttle to prevent deposition in the first stages of
evaporation, (ii) once the evaporation rate was constant at about
0.1e0.2 A/s, and (ii) the shuttle was opened and the deposition on
the substrates started. The first two steps are to prevent the de-
position of a high amount of impurities in the deposited layers.
The Au source and drain electrodes were evaporated on top of the
organic thin film through a shadow mask with specific channel
length and channel width W.
R.P.O. acknowledges funding from the European Community’s
Seventh Framework Programme through
a Marie Curie In-
ternational Outgoing Fellowship (Grant Agreement 234808).
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