3-Fluoro-4-hexylthiophene as a building block for tuning the electronic properties of conjugated polythiophenes

Article abstract of DOI:10.1021/jo302571u

3-Fluoro-4-hexylthiophene has been prepared by a synthetic route involving perbromination of 3-hexylthiophene followed by protection of the 2- and 5-positions of thiophene by trimethylsilyl groups and bromine/fluorine exchange. As expected, 3-hexyl-4-fluorothiophene oxidizes at a higher potential than 3-hexylthiophene; however, all attempts to electropolymerize this new thiophenic monomer have remained unsuccessful. Three terthienyls containing 3-hexylthiophene, 3-fluoro-4-hexylthiophene, and 3-bromo-4-hexylthiophene as the median group have been synthesized and used as substrates for electropolymerization. The electronic properties of the starting terthienyls and the resulting polymers have been analyzed by cyclic voltammetry and UV-vis spectroscopy, and the effects of substitution of the median thiophene ring are discussed.

Full text of DOI:10.1021/jo302571u

Article
pubs.acs.org/joc
3‑Fluoro-4-hexylthiophene as a Building Block for Tuning
the Electronic Properties of Conjugated Polythiophenes
Frederic Gohier, Pierre Frere,* and Jean Roncali
́
́
̀
́
LUNAM Universite, University of Angers, MOLTECH-Anjou UMR CNRS 6200, SCL Group, 2 boulevard Lavoisier, Angers, France
S
* Supporting Information
ABSTRACT: 3-Fluoro-4-hexylthiophene has been prepared by a synthetic route
involving perbromination of 3-hexylthiophene followed by protection of the
2- and 5-positions of thiophene by trimethylsilyl groups and bromine/fluorine
exchange. As expected, 3-hexyl-4-fluorothiophene oxidizes at a higher potential
than 3-hexylthiophene; however, all attempts to electropolymerize this new
thiophenic monomer have remained unsuccessful. Three terthienyls containing
3-hexylthiophene, 3-fluoro-4-hexylthiophene, and 3-bromo-4-hexylthiophene as
the median group have been synthesized and used as substrates for electro-
polymerization. The electronic properties of the starting terthienyls and the
resulting polymers have been analyzed by cyclic voltammetry and UV−vis
spectroscopy, and the effects of substitution of the median thiophene ring are
discussed.
insufficient coverage of the solar irradiation spectrum and hence
limits the efficiency of the resulting OPV cells.
INTRODUCTION
■
The control of the band gap of conjugated polymers has been
subject to intensive research efforts aiming at the development
of optimized donor materials for donor−acceptor bulk hetero-
junction solar cells.1 The band gap of π-conjugated systems can
be controlled through different synthetic approaches, including
rigidification of the conjugated backbone, quinoidization, or
realization of alternating donor (D) and acceptor (A) groups or
synergistic combinations of these various approaches.2
A possible solution to this problem would involve the introduction
of an electron acceptor group at the 4-position of P3HT, but this
approach has been scarcely developed. For example, poly(3-
cyano-4-hexyl)thiophene, synthesized by direct bromination of
regioregular P3HT followed by substitution of the bromine
atoms by cyano groups, has been described, but a band gap of
2.5 eV is reported.10 In fact, this approach is subject to stringent
steric restrictions that drastically limit the size of the possible
candidate withdrawing substituents. Indeed, bulky withdrawing
substituents induce distortions in the conjugated backbone that
compensate or annihilate the expected gap reduction by elec-
tronic effects.11 In this context, the fluorine atom with a van der
Waals radius of 1.35 Å and a Pauling electronegativity of 4.0 is the
smallest electron-withdrawing group.12 The introduction of
fluorine atoms is expected to stabilize the HOMO and LUMO
energy levels and thus to help the stability toward oxidative
degradation.5,13 Introduction of fluorine atoms on the acceptor
part of D−A polymers has recently been realized to give materials
reaching PCE up to 7% in bulk heterojunction solar cells.14
As a further step in our approach of developing D/A con-
jugated polymers with D and A groups on the same thiophene
unit,15,16 we report here the synthesis of 3-fluoro-4-hexylth-
iophene (1) and the effects of the fluorine atoms on the elec-
tronic properties of polythiophenes are discussed on the basis of
a comparative analysis of the electronic properties of the polymers
obtained by electropolymerization of terthienyls 3T-H, 3T-F, and
3T-Br (Chart 1).
While the quest for materials showing metallic character and
intrinsic conductivity has been a major motivation for gap reduc-
tion,3 the recent emergence of organic photovoltaics (OPV) at
the forefront of research on π-conjugated systems has provided a
new impetus to the field.4,5 However, the design of active
materials for OPV has also imposed specific prerequisites on the
electronic properties of conjugated systems, and in this regard
the smallest gap is no longer the best since, in addition to a
relatively reduced band gap (1.40−1.50 eV seems to be the
optimum),6 a precise control of the energy levels of the valence
and conduction band of the final material and hence of the
HOMO and LUMO levels of the basic individual molecules or
polymer chains is required to produce donor−acceptor hetero-
junction cells with optimal photon/electron conversion efficiency.2,6,7
Poly(3-hexylthiophene) (P3HT) has been widely investigated
as a donor material in OPV cells, and bulk heterojunction solar
cells based on P3HT have reached power conversion efficiencies
in the range of 5−6%.8 However, these values are lower than the
efficiencies of the best cells based on D−A low band gap
conjugated polymers that have reached values exceeding 8.0%.9
In comparison to D−A low band gap copolymers, P3HT presents
the advantage of a homogeneous structure based on a single
repeating basic unit and hence of a more straightforward syn-
thesis. However, the band gap of P3HT (∼1.90 eV) leads to
Received: November 24, 2012
Published: January 23, 2013
© 2013 American Chemical Society
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2-(tributylstannyl)thiophene (8) yielded terthiophene 3T-H in
95% yield.24 The silyl groups of 6 can be easily replaced by
bromine atoms by treatment with bromine at room temperature
to give 2,5-dibromo-3-fluoro-4-hexylthiophene (9) in 95% yield.23
Attempts to obtain 3T-F by a direct coupling between 8 and 9 led
to a mixture of 3T-F and bithiophene, which are difficult to separate.
Indeed, due to the low reactivity of the dibromo compound 9,
homocoupling of the stannic derivative gave bithiophene as the
major product. A second route toward 3T-F starting from 3T-Br,
in which the fluorination step took place after the Stille coupling,
has been explored (Scheme 2). A Stille cross-coupling reaction
between tribromo derivative 4 and the stannic reagent 8 proceeded
regioselectively at 40 °C to give terthiophene 3T-Br in 73% yield.
Only a small amount of compound 10 (<5%) corresponding to
the tris coupling was detected. It can be noted that when the
temperature was raised to 80 °C, the tetrathiophene 10 was
isolated in 81% yield.
Chart 1
As in the case of compound 1, clean bromine−fluorine exchange
requires the protection of the terminal α-positions with tri-
methylsilyl groups. A simplified procedure has been used for the
preparation of terthiophene 11. As trimethylsilyl chloride does
not react with LDA at −78 °C, it can be present when the base is
added on the terthiophene 3T-Br. The formed dilithiated derivative
is trapped immediately with TMSCl to give terthiophene 11 in 73%
yield. Then bromine of terthiophene 11 was exchanged with lithium
by treatment with n-BuLi at low temperature and then by fluorine
using (PhSO2)2NF to give 12 in 58% yield. The target compound
3T-F was finally obtained in 68% yield as a pale yellow liquid after
removal of silyl groups with TBAF. All new compounds were
fully characterized.
Electronic Properties. The CV traces of 1 and 3-hexylthio-
phene (3) present an irreversible oxidation wave with an anodic
peak potential Epa increasing from 2.22 V for 3 to 2.37 V for 1
upon replacement of hydrogen by the more electronegative
fluorine atom (Figure S1, Supporting Information). Although
successive potential scan to a solution of 3-hexylthiophene (3)
leads to the classical growth of a polymer film on the electrode, all
attempts to electropolymerize 1 with various concentrations of
monomer, solvents, electrolytes, or temperatures remained
unsuccessful.
RESULTS AND DISCUSSION
■
Synthesis. The synthetic routes explored for the synthesis
of 1 are shown in Scheme 1. First we tried to prepare 3-fluoro-
4-hexylthiophene (1) from 3-bromo-4-hexylthiophene (2) by
direct bromine−fluorine exchange. After perbromination of
3-hexylthiophene 3 by bromine in acetic acid to give 1,2,5-
tribromo-3-hexylthiophene (4),17,18 the bromine atoms at the
2- and 5-positions were selectively removed by the sequence
butyllithium−water to give 3-bromo-4-hexylthiophene (2) in
quantitative yield. Although the synthesis of the fluorinated
derivative 1 by direct fluorination of the 3-lithiated thiophene
derivative was described in a patent,19 numerous attempts remained
unsuccessful and afforded miscellaneous products. Proton and
fluorine NMRs showed the formation of several unidentified
compounds but also the insertion of fluorine atom in the
α-position of the thiophene. The stability of the 3-lithiated thiophene
is low above −78 °C, leading to a migration of lithium atoms to
the α-position of thiophene.20 Therefore, in order to avoid the
formation of byproducts, the 2- and 5-positions were blocked by
trimethylsilyl groups in compound 5, easily obtained in 88% yield
by trapping the dilithiated intermediates of 4 by chlorotrime-
thylsilane.21,22 Then bromine−lithium exchange with n-butyllithium
followed by reaction with (PhSO2)2NF proceeded smoothly to
afford 6 in moderate yield.21 The silyl groups were removed with
tetrabutylammonium fluoride to give 1 in 67% yield.19,23
The single occupied molecular orbital (SOMO) of the cation
radicals of 1•+ presents a strong dissymmetry, showing that the
α-position next to the fluorine atom is deactivated, preventing
the coupling reactions involved in the polymerization process.16
Taking into account these negative results and in order to
obtain some information on the effect of fluorine on the elec-
tronic properties of more extended thiophenic conjugated systems,
The synthesis of the terthiophenes depicted in Scheme 2
was based on a 2-fold Stille coupling. Thus, the coupling
between 2,5-dibromothiophene 7 and a slight excess of
a
Scheme 1. Synthesis of 3-Fluoro-4-hexylthiophene (1)
a
Reagents and conditions: (i) Br2, acetic acid, 60°C; (ii) n-BuLi, THF, −78 °C then H2O; (iii) n-BuLi, THF, −78 °C then (PhSO2)2NF; (iv) n-BuLi,
THF, −78 °C then TMSCl; (v) TBAF, CH2Cl2, room temperature.
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a
Scheme 2. Syntheses of Terthiophenes 3T-H, 3T-Br, and 3T-F
a
Reagents and conditions: (i) Pd(PPh3)4, DMF, 80°C; (ii) Br2, CH2Cl2, room temperature; (iii) 8 (excess), Pd(PPh3)4, DMF, room temperature or
40 °C; (iv) 8 (excess), Pd(PPh3)4, DMF, 40 °C; (v) LDA, TMSCl, THF, −78 °C; (vi) n-BuLi, THF, −78 °C then (PhSO2)2NF; (vii) TBAF,
CH2Cl2, room temperature.
Figure 1. Calculated HOMOs and LUMOs and energy levels for 3T-X (X = H, Br or F).
we have carried out a comparative analysis of the electronic properties
of trimers 3T-X (X = H, Br, F) using theoretical calculations, UV−vis
absorption spectroscopy, and cyclic voltammetry.
Theoretical calculations were performed at the ab initio density
functional level with the Gaussian package by using Becke’s
three-parameter gradient corrected functional (B3LYP) with a
polarized 6-31G(d,p) for the HOMO and LUMO level deter-
minations. Hexyl chains were replaced by propyl chains to limit
the computational time. The optimized structures of the trimers
3T-X show a fully planar conformation with the thiophene units
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Table 1. HOMO and LUMO Energy Levels and Energy Gap ΔEth Calculated by DFT Methods, Experimental Absorption Data, and
Cyclic Voltammetric Data of Trimers 3T-X (X = H, Br, F)
a
a
a
b
c
d
e
compd
HOMO (eV)
LUMO (eV)
ΔEth (eV)
λmax (nm)
ΔEopt (eV)
Epa (V)
HOMO (eV)
3T-H
3T-Br
3T-F
−5.03
−5.19
−5.13
b
−1.61
−1.75
−1.70
3.42
3.44
3.43
345
345
350
3.10
3.10
3.06
0.98
1.12
−5.69
−5.82
−5.78
1.08
a
c
d
B3LYP/6.31G(d,p). Conditions: 10−4 M in CH2Cl2. Optical band gap calculated from the edge of the absorption band. Conditions: 5 × 10−4
M
in 0.1 M Bu4NPF6/CH3CN, scan rate 100 mV s−1, vs SCE. Calculated from EHOMO = −Eox,onset − 5.1.27
e
Figure 2. (left) Electropolymerization of 3T-F (top), 3T-Br (middle), and 3T-H (bottom) at 5 × 10−3 M in 0.10 M Bu4NPF6/CH3CN, reference SCE,
v = 50 mV s−1. (right) CV of the resulting electrodeposited materials poly(3T-F) (top), poly(3T-Br) (middle), and poly(3T-F) (bottom) in 0.10 M
Bu4NPF6/CH3CN, reference SCE, v = 50 mV s−1.
in a δ-trans conformation (Figure 1). Replacement of hydrogen
by the bulky bromine atom does not provoke any torsion of the
molecule. The short intramolecular Br- - -S distance (dBr‑ ‑ ‑S =
3.20 Å), inferior to the sum of the van der Waals radii of sulfur
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and bromine (3.65 Å), is an indication of a possible
intramolecular interaction which stabilizes the planar geometry.
In the planar conformation observed for bithiophenes bearing
only one bromine atom, it has been indicated that Br- - -S intra-
molecular interactions could contribute to self-rigidify the con-
jugated system.25 A planar conjugated backbone has also been
reported for oligomers based on the 3,4-difluorothiophene
building block.21 Examination of the electron density of molec-
ular orbitals shows that, for the three terthiophenes, the HOMO
and LUMO are delocalized over the whole conjugated system.
The calculated HOMO and LUMO levels and the gap of 3T-X
show a moderate effect of the halogen atoms (Table 1). Thus,
the replacement of hydrogen by fluorine in 3T-F stabilizes the
HOMO and LUMO levels by about 0.10 eV. For 3T-Br
the bromine atom produces a slight decrease (∼0.05 eV) of the
HOMO and LUMO levels in comparison to 3T-F. For both the
HOMO and LUMO, the electron density on the halogen atoms
is relatively low. However, the density on fluorine is larger than
on bromine, suggesting a stronger mesomeric donor effect of
fluorine. The larger positive mesomeric effect of fluorine (σR =
−0.34) in comparison to bromine (σR = −0.16)26 could explain
the small destabilization of the HOMO and LUMO levels
observed for 3T-F.
The spectrum of the neutral poly(3T-H) presents a λmax value at
478 nm and a band gap of 1.99 eV. Replacement of the hydrogen by
halogen atoms leads to a slight decrease of the band gap (ca. 0.10 eV).
As shown in Figure 3 for poly(3T-F), the application of positive
Figure 3. UV−vis absorption spectra of a thin film of poly(3T-F) on
ITO at various applied potentials.
The UV−vis absorption spectra (Figure S2, Supporting
Information) show that the three compounds present similar
absorption maxima (λmax) with, however, a small bathochromic
shift of λmax for 3T-F.
The CV traces of the trimers 3T-X (Figures S3−S5,
Supporting Information) show an irreversible oxidation peak at
0.98 V for 3T-H, while a ca. 100 mV positive shift is observed for
3T-Br and 3T-F, reflecting the electron-withdrawing effect of
halogen substituents.
Electropolymerization. The terthiophenes 3T-X have been
electropolymerized (Figure 2), and the electronic properties of
the resulting polymers, named poly(3T-X), have been studied
(Table 2). Application of recurrent potential scans to acetonitrile
potentials produces a decrease in the intensity of the band cor-
responding to the neutral state and the emergence of a broad band
around 800 nm which could be attributed to the polaron state.
CONCLUSION
■
3-Fluoro-4-hexylthiophene has been synthesized from 3-hexyl-
thiophene by fluorine−bromine exchange reactions performed
on a thiophene derivative protected at the terminal positions by
trimethylsilyl groups. The fluorine atom at the β-position of the
thiophene ring seems to inhibit the electropolymerization, pre-
sumably because of the deactivating effect of the fluorine atom. In
contrast, terthiophenes containing a central 3-halogeno-4-hexyl-
thiophene undergo straightforward electropolymerization sim-
ilarly to their nonhalogenated analogues. Theoretical, spectro-
scopic, and electrochemical data show that the halogen atoms
exert a small influence on the electronic properties of the terthio-
phenes and of the corresponding polymers. The electron-with-
drawing effect of the halogen atom stabilizes the HOMO and
thus leads to an increase of the oxidation potentials of the
terthiophenes and polymers. However, for fluorine, the negative
inductive effect is counterbalanced by a positive mesomeric effect.
On the other hand, the larger steric hindrance of the bromine atom
is limited by S- - -Br intramolecular interactions that stabilize the
planarity of the conjugated backbone. On the whole, the two
halogen atoms have rather similar effects on the electronic
properties of the conjugated system, with a slightly lower gap for
the fluoro compounds and slightly higher oxidation potential for
the bromo compounds.
a
b
Table 2. Cyclic Voltammetry and UV−Vis Absorption Data
for poly(3T-X) (X = H, Br, F)
c
polymer
Epa (V)
λmax (nm)
Eg (eV)
poly(3T-F)
poly(3T-Br)
poly(3T-H)
0.82
0.83
0.75
479
485
478
1.87
1.89
1.99
a
Polymer film deposited on Pt disk (d = 1 mm) CV recorded in 0.10 M
b
Bu4NPF6/CH3CN. Polymer film deposited on ITO and electro-
chemically undoped. Calculated from the foot of the absorption band.
c
solutions of 3T-X leads to the progressive emergence of a new
redox system at lower potential associated with the electro-
deposition of a polymer. The CV of the polymers recorded in a
monomer-free electrolytic medium presents a broad reversible
oxidation wave corresponding to the p-doping of the conjugated
backbone, with Epa increasing from 0.75 V for poly(3T-H) to
0.83 V for poly(3T-Br). These Epa values are clearly correlated
with those of the starting trimers, indicating that the halogen
atoms exert similar electron-withdrawing effects on the for-
mation of the radical cation for the precursor and on the polaron
state of the resulting polymer. As expected, only a small dif-
ference is observed between conjugated systems bearing fluorine
and bromine atoms.
EXPERIMENTAL SECTION
■
1H and 19F NMR spectra were obtained at 300.1 MHz, and 13C spectra
were obtained at 75.7 MHz. The spectra were referenced against the
internal NMR solvent standard. Chemical shifts are expressed in parts
per million (ppm) and are reported as s (singlet), d (doublet), t (triplet),
td (doublet triplet), and m (multiplet); coupling constants J are given
in Hz.
Cyclic voltammetry was carried out using a platinum disk as the
working electrode, platinum wire as the counter electrode, and saturated
calomel as the reference electrode.
The optical properties of the polymers have been analyzed on
thin films electrodeposited on indium tin oxide (ITO) electrodes.
2,3,5-Tribromo-4-hexylthiophene (4) was prepared from 3-hexylth-
iophene by treatment with bromine as described in literature.18
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3-Bromo-4-hexyl-2,5-bis(trimethylsilyl)thiophene (5). 4 (2.94 g,
7.26 mmol) was dissolved in THF (25 mL), and the solution was cooled
to −78 °C. A solution of butyllithium (1.6 M in cyclohexane, 2 equiv,
9.0 mL) was dropped in slowly, and the solution was stirred for 15 min.
Chlorotrimethylsilane (2.2 mL, 17.5 mmol) was slowly added, and the
solution was stirred overnight with warming to room temperature. After
addition of water, the phases were separated and the aqueous phase was
extracted with Et2O (2 × 20 mL). The organic phases were dried over
MgSO4 and then evaporated under vacuum. The crude product was
chromatographed with petroleum ether as eluent to afford a light yellow
oil in 88% yield (2.50 g). 1H NMR (CDCl3): 2.66 (t, 2H, J = 7.7 Hz);
1.53 (m, 2H); 1.34 (m, 6H); 0.91 (m, 3H); 0.39 (s, 9H); 0.34 (s, 9H)
ppm. 13C NMR (CDCl3): 150.1; 139.2; 138.3; 121.7; 31.6; 31.1; 30.9;
29.8; 22.7; 14.1; 0.1; −0.8 ppm. Anal. Calcd for C16H31BrSSi2: C, 49.08;
H,7.98. Found: C, 48.73; H, 7.67.
3-Fluoro-4-hexyl-2,5-bis(trimethylsilyl)thiophene (6). 5 (0.37 g,
0.94 mmol) was dissolved in THF (5 mL), and the solution was cooled
to −78 °C. A solution of butyllithium (1.6 M in cyclohexane, 1.1 equiv,
0.64 mL) was dropped in slowly, and the solution was stirred for 20 min.
A solution of N-fluorobenzenesulfonimide (0.439 g, 1.39 mmol) in THF
(3 mL) was slowly added, and the solution was stirred for 2 h at −78 °C
and then stirred overnight with warming to room temperature. After
addition of water, the organic phase was separated and the aqueous
phase was extracted with Et2O (2 × 20 mL). The organic phases were
dried over MgSO4 and then evaporated under vacuum. The crude
product was chromatographed with petroleum ether as eluent to afford a
light yellow oil in 51% yield (0.16 g). 1H NMR (CDCl3): 2.54 (t, 2H, J =
8.0 Hz); 1.53 (m, 2H); 1.34 (m, 6H); 0.89 (m, 3H) ; 0.37 (s, 9H); 0.31
(s, 9H) ppm. 19F NMR (CDCl3): −124.3 ppm. 13C NMR (CDCl3):
163.4 (d, J = 262 Hz); 139.1 (d, J = 24 Hz); 138.5 (d, J = 3 Hz); 119.4 (d,
J = 30 Hz); 31.6; 30.7; 29.6; 28.2; 22.6; 14.1; −0.1; −0.5 ppm. Anal.
Calcd for C16H31FSSi2: C, 58.12; H,9.45; S, 9.70. Found: C, 57.80; H,
9.37; S, 9.24.
The crude product was chromatographed with petroleum ether as
eluent, and a pale yellow oil was obtained in 73% yield (0.37 g). 1H NMR
(CDCl3): 7.45 (dd, 2H, J = 3.7 and 1.2 Hz); 7.36 (m, 2H); 7.15 (dd, 2H,
J = 3.7 and 1.2 Hz); 7.09 (m, 2H); 2.80 (t, 2H, J = 8.3 Hz); 1.64 (2H, m);
1.43 (m, 2H); 1.34 (4H, m); 0.91 (t, 3H, J = 7 Hz) ppm. 13C NMR
(CDCl3): 139.5; 135.1; 134.5; 130.6; 129.3; 127.5; 127.2; 126.7; 126.4;
126.1; 126.0; 111.8; 31.5; 29.6; 29.5; 29.3; 22.6; 14.1 ppm. Anal. Calcd
for C18H19BrS3: C, 52.55; H, 4.65. Found: C, 52.42 ; H, 4.69.
(3′-Bromo-4′-hexyl-[2,2′:5′,2″-terthiophene]-5,5″-diyl)bis-
(trimethylsilane) (11). 3T-Br (1.0 g, 2.43 mmol) and chlorotrime-
thylsilane were dissolved in THF (8 mL), and the solution was cooled to
−78 °C. A solution of lithium diisopropylamide (prepared from BuLi
(2.5 M, 2.2 equiv, 2.1 mL) and diisopropylamine (0.69 mL, 2.2 equiv) in
THF (4 mL)) was added slowly at −78 °C. The solution was stirred for
1 h at −78 °C and warmed to room temperature overnight. After
addition of water, the phases were separated and the aqueous phase was
extracted with Et2O (2 × 20 mL). The organic phases were dried over
MgSO4 and then evaporated under vacuum. The crude product was
chromatographed with petroleum ether as eluent to afford a light yellow
oil in 73% yield (0.98 g). 1H NMR (CDCl3): 7.54 (d, 1H, J = 3.5 Hz);
7.24 (m, 3H); 2.86 (t, 2H, J = 8.2 Hz); 1.67 (2H, m); 1.44 (m, 6H); 0.40
(t, 3H, J = 7.0 Hz) ppm. 13C NMR (CDCl3): 141.3; 141.2; 140.3; 139.7;
139.2;, 134.3; 134.0; 130.5; 129.4; 127.6; 127.2; 111.7; 31.4; 29.6; 29.3;
29.2; 22.6; 14.1; −0.1 ppm. HRMS: calcd for C24H36BrS3Si2 555.0696,
found 555.0691.
(3′-Fluoro-4′-hexyl-[2,2′:5′,2″-terthiophene]-5,5″-diyl)bis-
(trimethylsilane) (12). 11 (0.68 g, 1.22 mmol) was dissolved in
THF (3 mL), and the solution was cooled to −78 °C. A solution of
butyllithium (1.6 M in cyclohexane, 1.3 equiv, 1.1 mL) was dropped
in slowly, and the solution was stirred for 20 min. A solution of
N-fluorobenzenesulfonimide (0.5 g, 1.58 mmol) in THF (2 mL) was
slowly added, and the solution was stirred for 2 h at −78 °C and then
stirred overnight with warming to room temperature. After addition of
water, the organic phase was separated and the aqueous phase was
extracted with Et2O (2 × 20 mL). The organic phases were dried over
MgSO4 and then evaporated under vacuum. The crude product was
chromatographed with petroleum ether as eluent to afford a light yellow
oil in 58% yield (0.35 g). 1H NMR (CDCl3): 7.29 (d, 1H, J = 3.5 Hz);
7.20 (m, 3H); 2.75 (t, 2H, J = 7.8 Hz); 1.65 (2H, m); 1.36 (m, 6H); 0.90
(t, 3H, J = 7.0 Hz); 0.36 (s, 9H); 0.35 (s, 9H) ppm. 19F (CDCl3): −127.4
ppm. 13C NMR (CDCl3): 152.0 (d, J = 263 Hz); 141.3, 140.8 (d, J =
2 Hz); 140.0 (d, J = 4 Hz); 137.6 (d, J = 5 Hz); 134.3 (d, J = 2 Hz); 128.9
(d, J = 22 Hz); 127.0 (d, J = 8 Hz); 124.4 (d, J = 3 Hz); 114.2 (d, J = 15 Hz);
31.5; 29.4; 29.1; 26.2; 22.6; 14.1, −0.1 ppm. HRMS: calcd for C24H35FS3Si2
494.1418, found 494.1413.
3′-Fluoro-4′-hexyl-2,2′:5′,2″-terthiophene (3T-F). 12 (4.0 g,
8.1 mmol) was dissolved in THF (50 mL), and the solution was cooled
to 0 °C. A solution of tetrabutylammonium fluoride (1 M in cyclohexane,
2.2 equiv, 17.8 mL) was dropped in slowly, and the solution was stirred for
30 min at room temperature. After addition of water (50 mL) and CH2Cl2
(40 mL), the phases were separated and the aqueous phase was
extracted with dichloromethane (2 × 20 mL). The organic phases were
dried over MgSO4 and then evaporated under vacuum. The crude
product was chromatographed with petroleum ether as eluent to afford a
colorless oil in 68% yield (1.93 g). 1H NMR (CDCl3): 7.34 (dd, 1H, J =
5.1 and J = 1.1 Hz); 7.28 (dd, 1H, J = 5.1 and J = 1.1 Hz); 7.24 (dd, 1H,
J = 3.6 and J = 1.1 Hz); 7.15 (dd, 1H, J = 3.6 and J = 1.1 Hz); 2.72 (t, 2H,
J = 7.8 Hz); 1.62 (m, 2H); 1.32 (m, 8H); 0.88 (m, 3H) ppm. 19F NMR
(CDCl3): −121.7 ppm. 13C NMR (CDCl3): 151.8 (d, J = 263 Hz);
135.5 (d, J = 2 Hz); 132.4 (d, J = 5 Hz); 129.2 (d, J = 22 Hz); 126.7
(d, J = 8 Hz); 125.7 (d, J = 13 Hz); 124.7 (d, J = 4 Hz); 124.2 (d, J = 3
Hz); 114.2 (d, J = 15 Hz); 31.5; 29.5; 29.2; 26.2; 22.5; 14.1 ppm.
MALDI-TOF: 350.0770. Anal. Calcd for C18H19FS3: C, 61.67; H, 5.46;
S, 27.44. Found: C, 61.27; H, 5.44; S, 27.48.
3-Fluoro-4-hexylthiophene (1). 6 (0.66 g, 2.0 mmol) was
dissolved in THF (6 mL), and the solution was cooled to 0 °C. A
solution of tetrabutylammonium fluoride (1 M in cyclohexane, 3 equiv,
3 mL) was added slowly, and the solution was stirred for 30 min at room
temperature. After addition of water, the phases were separated and the
aqueous phase was extracted with dichloromethane (2 × 20 mL). The
organic phases were dried over MgSO4 and then evaporated under
vacuum. The crude product was chromatographed with petroleum ether
1
as eluent to afford a colorless oil in 67% yield (0.25 g). H NMR
(CDCl3): 6.82 (dd, 1H, J = 3.8 and J = 3.8 Hz); 6.64 (d, 1H, J = 3.8 Hz);
2.52 (t, 2H, J = 7.7 Hz); 1.61 (m, 2H) ; 1.32 (m, 6H); 0.90 (m, 3H) ppm.
19F NMR (CDCl3): −134.7 ppm. 13C NMR (CDCl3): 156.9 (d, J = 258
Hz); 131.7 (d, J = 23 Hz); 119.6 (d, J = 8 Hz); 102.7 (d, J = 21 Hz); 31.6;
29.2; 28.9; 26.8; 22.6; 14.0. Anal. Calcd for C10H15FS: C, 64.47; H, 8.12.
Found: C, 64.35; H, 8.07.
2,5-Dibromo-3-fluoro-4-hexylthiophene (9). 6 (0.10 g, 0.3 mmol)
was dissolved in CH2Cl2 (3 mL). To this solution, at 0 °C, was slowly
added bromine (0.03 mL, 0.6 mmol). The solution was then stirred for
30 min at room temperature. A saturated thiosulfate solution (3 mL)
was added to reduce nonreacted bromine. The aqueous phase was
extracted with CH2Cl2 (2 × 20 mL). The combined organic phases were
dried over magnesium sulfate and then evaporated under vacuum. The
crude oil was chromatographed with cyclohexane to afford a light yellow
oil in 96% yield (0.10 g). 1H NMR (CDCl3): 2.52 (t, 2H, J = 7.6 Hz);
1.52 (m, 2H); 1.29 (m, 6H); 0.89 (m, 3H); 0.37 (s, 9H); 0.31 (s, 9H)
ppm. 19F NMR (CDCl3): −126.7 ppm. 13C NMR (CDCl3): 153.4 (d,
J = 263 Hz); 132.2 (d, J = 23 Hz); 107.3 (d, J = 10 Hz); 89.3 (d, J = 3 Hz);
31.5; 28.7; 28.5; 27.0; 22.5; 14.1 ppm. Anal. Calcd for C10H13Br2FS: C,
34.91; H, 3.81. Found: C, 35.12; H, 3.83.
3′-Bromo-4′-hexyl-2,2′:5′,2″-terthiophene (3T-Br). 4 (0.50 g,
1.23 mmol) and tributylstannylthiophene (8; 1.41 g, 3.69 mmol) were
dissolved in dimethylformamide (2 mL). Argon was bubbled for 10 min
in the solution. Then Pd(PPh3)4 (70 mg, 0.06 mmol) was added and the
solution was stirred for one night at 40 °C. A solution of KF (1 M, 2 mL)
was added to the mixture, and the solution was stirred for 1/2 h. Brine
(3 mL) was added, and the mixture was extracted with ethyl acetate
(3 × 10 mL). The organic phases were dried over MgSO4 and evaporated.
4′-Hexyl-5′-(thiophen-2-yl)-2,2′:3′,2″-terthiophene (10). 4
(0.50 g, 1.23 mmol) and tributylstannylthiophene (1.88 g, 5.03
mmol) were dissolved in dimethylformamide (3 mL). Argon was
bubbled for 10 min into the solution. Then Pd(PPh3)4 (70 mg, 0.06
mmol) was added and the solution was stirred for one night at 80 °C. A
solution of KF (1 M, 2 mL) was added to the mixture, and the solution
1502
dx.doi.org/10.1021/jo302571u | J. Org. Chem. 2013, 78, 1497−1503

The Journal of Organic Chemistry
Article
1
was stirred for /2 h. Brine (3 mL) was added, and the mixture was
(12) Reichenbacher, K.; Suss, H. I.; Hulliger, J. Chem. Soc. Rev. 2005,
34, 22.
(13) Babudri, F.; Farinola, G. M.; Naso, F.; Ragni, R. Chem. Commun.
extracted with ethyl acetate (3 × 10 mL). The organic phases were dried
over MgSO4 and evaporated. The crude product was chromatographed
with a mixture of petroleum ether and CH2Cl2 (1/1) as eluent, and a
2007, 1003.
1
pale yellow solid was obtained in 81% yield (0.41 g). Mp: 95 °C. H
(14) (a) Liang, Y.; Feng, D.; Wu, Y.; Tsai, S.-T.; Li, G.; Ray, C.; Yu, L. J.
Am. Chem. Soc. 2009, 131, 7792. (b) Liang, Y.; Xu, Z.; Xia, J.; Tsai, S.-T.;
Wu, Y.; Li, G.; Ray, C.; Yu, L. Adv. Mater. 2010, 22, E135. (c) Liang, Y.;
Yu, L. Acc. Chem. Res. 2010, 43, 1227. (d) Son, H. J.; Wang, W.; Xu, T.;
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(e) Price, S. C.; Stuart, A. C.; Yang, L.; Zhou, H.; You, W. J. Am. Chem.
Soc. 2011, 133, 4625.
NMR (CDCl3): 7.50 (dd, 1H, J = 1.1 and 5.1 Hz); 7.37 (dd, 1H, J = 1.1
and 5.1 Hz); 7.23 (dd, 1H, J = 1.1 and 3.6 Hz); 7.17 (m, 2H); 7.13 (dd,
1H, J = 3.6 and 5.1 Hz); 7.06 (m, 2H); 6.94 (dd, 1H, J = 3.6 and 5.1 Hz);
2.64 (m, 2H); 1.49 (m, 2H); 7.21 (m, 6H); 0.87 (t, 3H, J = 6.5 Hz). 13C
NMR (CDCl3): 141.1; 136.3; 135.9; 135.8; 134.7; 131.8; 129.1; 128.8;
127.4; 127.2; 127.1; 126.8; 126.1; 125.9; 125.6; 31.2; 30.5; 29.2; 28.4;
22.4; 14.0. IR (cm−1): 3207, 2925, 1456, 1220, 693. HRMS: calcd for
C22H23S4 415.0677, found 415.0676.
(15) (a) Hergue,
Roncali, J. Org. Lett. 2011, 13, 1762. (b) Mallet, C.; Savitha, G.; Allain,
M.; Kozmik, V.; Svoboda, J.; Frere, P.; Roncali, J. J. Org. Chem. 2012, 77,
2041. (c) Savitha, G.; Hergue, N.; Guilmet, E.; Allain, M.; Frere, P.
Tetrahedron Lett. 2011, 52, 1288. (d) Yassin, A.; Savitha, G.; Leriche, P.;
Frere, P.; Roncali, J. New J. Chem. 2012, 36, 2412.
(16) Hergue, N.; Mallet, C.; Frere, P.; Allain, M.; Roncali, J.
Macromolecules 2009, 42, 5593.
(17) Blanchard, P.; Verlhac, P.; Michaux, L.; Frer
Eur. J. 2006, 12, 1244.
́ ̀
N.; Mallet, C.; Savitha, G.; Allain, M.; Frere, P.;
̀
ASSOCIATED CONTENT
* Supporting Information
́
̀
■
S
̀
Figure S1, presenting the CVs of 1 and 3, Figure S2, presenting
the UV−vis absorption spectra of 3T-X, Figures S3−S5,
presenting the CVs of 3T-X, tables giving computational data
for 3T-X, and figures giving NMR spectra of 1−12, 3T-Br, and
3T-F. This material is available free of charge via the Internet at
http://pubs.acs.org.
́
̀
̀
e, P.; Roncali, J. Chem.
(18) He, M.; Zhang, F. J. Org. Chem. 2007, 72, 442.
(19) Heeney, M.; Farrand, L.; Gilles, M.; Thompson, M.; Tierney, S.;
Shkunov, M.; Sparrowe, D.; McCulloch, I. Eur. Pat. 1279690, 2003
(20) Doadt, E. G.; Snieckus, V. Tetrahedron Lett. 1985, 26.
(21) Sakamoto, Y.; Komatsu, S.; Suzuki, T. J. Am. Chem. Soc. 2001, 123,
4643.
(22) Frey, J.; Bond, A. D.; Holmes, A. B. Chem. Commun. 2002, 2424.
(23) Nicolas, Y.; Blanchard, P.; Roncali, J.; Allain, M.; Mercier, N.;
Deman, A. L.; Tardy, J. Org. Lett. 2005, 7, 3513.
AUTHOR INFORMATION
Corresponding Author
*E-mail: pierre.frere@univ-angers.fr.
■
Notes
The authors declare no competing financial interest.
(24) The synthesis of compound 3T-H has been described in 40%
yield by Kumada coupling: Chaloner, P. A.; Gunatunga, S. R.;
Hitchcock, P. B. J. Chem. Soc., Perkin Trans. 2 1997, 1597.
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