Rigid oligomers based on the combination of 3,6-dimethoxythieno[3,2-b]thiophene and 3,4-ethylenedioxythiophene

Article abstract of DOI:10.1016/j.tetlet.2009.10.021

New oligomers based on the combination of the 3,6-dimethoxythieno[3,2-b]thiophene and 3,4-ethylenedioxythiophene (EDOT) moieties have been prepared and studied by UV-vis spectroscopy and cyclic voltammetry. The use of the intrinsically rigid thienothiophene units in addition to the S?O intramolecular interactions leads to planar conformation of the conjugated chains. While the radical cations of oligomers end capped by n-hexyl chains show a tendency to the dimerization, those substituted by n-hexylsulfanyl chains present a high stability.

Full text of DOI:10.1016/j.tetlet.2009.10.021

Tetrahedron Letters 50 (2009) 7148–7151
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Rigid oligomers based on the combination of 3,6-dimethoxy-
thieno[3,2-b]thiophene and 3,4-ethylenedioxythiophene
Mathieu Turbiez a,b, Noémie Hergué a, Philippe Leriche a, Pierre Frère a,
*
a University of Angers, Laboratory CIMA-CNRS, Group Linear Conjugated Systems, 2 Bd Lavoisier 49045, Angers, France
b BASF Klybeckstrasse 141, 4002 Basel, Switzerland
a r t i c l e i n f o
a b s t r a c t
Article history:
New oligomers based on the combination of the 3,6-dimethoxythieno[3,2-b]thiophene and 3,4-ethylene-
dioxythiophene (EDOT) moieties have been prepared and studied by UV–vis spectroscopy and cyclic vol-
tammetry. The use of the intrinsically rigid thienothiophene units in addition to the Sꢀ ꢀ ꢀO intramolecular
interactions leads to planar conformation of the conjugated chains. While the radical cations of oligomers
end capped by n-hexyl chains show a tendency to the dimerization, those substituted by n-hexylsulfanyl
chains present a high stability.
Received 1 September 2009
Revised 29 September 2009
Accepted 2 October 2009
Available online 8 October 2009
Keywords:
Ó 2009 Elsevier Ltd. All rights reserved.
Thienothiophene
Ethylenedioxythiophene
Stille reaction
Sigma-dimers
Thiophene-based conjugated oligomers represent one of the
most widely investigated class of functional -conjugated systems,
interactions which rigidify the conjugated backbone. In this con-
text, we have shown that poly-3,6-dimethoxythieno[3,2-b]thio-
phene presents a self-rigidification through Sꢀ ꢀ ꢀO intramolecular
interactions of the conjugated chain.16 Electropolymerized or
chemically generated poly-3,6-dialkoxythieno[3,2-b]thiophenes
present a moderate band gap around 1.7–1.8 eV.16,17
We report here on the synthesis and the characterization of hy-
brid EDOT–thienothiophene oligomers 1 and 2 (Chart) in which
thieno[3,2-b]thiophene and 3,6-dimethoxythieno[3,2-b]thiophene
units have been inserted between the two EDOT moieties.
p
motivated by their potential technological applications as organic
semi-conducting materials for the fabrication of (opto)electronic
devices such as field-effect transistors (OFET), electroluminescent
diodes (OLED) and photovoltaic cells.1–3 Careful modifications on
the oligothiophene backbone allow to control the energy levels
of the HOMO and LUMO frontier orbitals and thus to tune crucial
electronic properties such as ionization potential, electron affinity,
absorption and emission properties.4 For example, structural mod-
ifications, consisting of rigidifying planar-conjugated frameworks,
have been developed to limit the rotational disorder between the
thiophene rings which affects the band gap in the solid state.4,5 Re-
cently many works based on the incorporation of the intrinsically
rigid thieno[3,2-b]thiophene units in conjugated oligomers or
polymers have been developed to improve their electronic proper-
ties and optimize the performances of the corresponding devices.6–11
However, the extension of the conjugated systems is always lim-
ited by the twist from planarity between adjacent thienothiophene
units. Substitution on the b-position with alkyl chain for enhancing
the solubility increases steric hindrances leading to the materials
with higher gap.10,11 Fully fused oligothiophene derivatives have
recently been described but the lengthening of the conjugated
chain is rapidly limited by the synthetic pathways.12–15 An alterna-
tive consists in associating rigid units by sigma bonds and addi-
tionally in limiting the rotational disorder by intramolecular
O
O
O
O
S
SC6H13
S
S
C6H13
S
S
C6H13
S
S
S
C6H13
S
1b
O
O
1a
O
O
O
O
O
O
MeO
S
SC6H13
S
MeO
O
S
S
C6H13
C6H13
S
S
S
OMe
S
C6H13
S
OMe
O
O
2b
O
2a
The comparison of the electronic properties of these two series
of oligomers shows the role of the methoxy groups both for
strengthening the rigidification of the conjugated systems and for
increasing the donor character of the molecules.
* Corresponding author.
E-mail address: pierre.frere@univ-angers.fr (P. Frère).
0040-4039/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2009.10.021

M. Turbiez et al. / Tetrahedron Letters 50 (2009) 7148–7151
7149
Table 1
The syntheses of compounds 1 and 2 are based on a twofold Stille
Electrochemical and UV–vis spectroscopic data of compounds 1, 2 and 9
coupling reaction between the dibromo-thienothiophene units 5 or
6 and the tributylstannyl-EDOT derivatives 7 (Scheme 1). Treatment
of thienothiophene cores 3 or 4 with 2 equiv of N-bromosuccinimide
(NBS) in CHCl3 at ꢁ20 °C afforded dibromo derivatives 5 or 6 in 95%
or 90% yields, respectively. Then the coupling reactions of 5 and 6
with a slight excess of stannyl derivative 7a18 in the presence of
Pd(PPh3)4 led to oligomers 1a and 2a in 54% and 25% yields, respec-
tively.19 The substitution of the EDOT core by the hexylsulfanyl
chains has been achieved by the sequence BuLi/S8 leading to thiola-
to-EDOT derivative which was immediately treated with bromohex-
ane to give compound 8 in 78% yield (Scheme 2). The stannyl
derivative 7b, obtained by the sequence BuLi/Bu3SnCl on 8, was cou-
pled with 5 or 6 to lead to oligomers 1b and 2b in 45% and 30% yields,
respectively.19
a
a
b
b
Compounds
kmax (nm)
k0–0 (nm)
E1 (V)
E2 (V)
DE (mV)
1a
2a
398
402
422
410
414
431
418
426
446
430
438
455
0.65
0.45
0.57
0.74
0.56
0.65
1.02
0.80
0.84
0.85
0.66
0.68
370
350
270
110
100
30
9a20
1b
2b
9b
a
kmax (main absorption band) and k0–0 (lowest energy band) in CH2Cl2 10ꢁ5 M.
b
Versus Ag/AgCl, 100 mV sꢁ1
.
O
O
The optical and electrochemical properties of 1 and 2 are gath-
ered in Table 1 and compared to their analogues 920,21 incorporat-
ing a 2,20-bithiophene core in the median position (Scheme 3).
The comparison of the UV–vis absorption spectra of oligomers 1
and 2 shows the role of the methoxy group for strengthening the
self-rigidification of the conjugated chain (Fig. 1). Compared to com-
pounds 9, the spectra of compounds 1 present a blue shift of the
absorption band as expected for shorter oligomers. For 1, the shape
of the band is narrowed and the vibronic fine structure is more
accentuated with the apparition of second maxima (k0–0). Such
effects are the indication of a higher rigidification of the conjugated
chain due to the intrinsically rigid thienothiophene core in the
median position. Additionally, the insertion of the methoxy groups
in oligomers 2 provokes a refinement of the fine structure and a
slight bathochromic shift of the absorption band. The first effect sug-
gests that the methoxy groups increase the self-rigidification of the
S
S
R
R
S
S
O
O
9
9a, R = -C6H13
9b, R = -SC6H13
Scheme 3. Structure of compounds 9.
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
2b
1b
9b
R
R
S
A
S
Br
Br
NBS, CHCl3
S
S
R
R
3, R =H
4, R =OMe
5, R =H
6, R =OMe
O
O
350
400
450
500
550
Hex
Bu3Sn
S
7a
λ (nm)
1a or 2a
1b or 2b
Figure 1. UV–vis spectra of compounds 1b, 2b and 9b in CH2Cl2.
Pd(PPh3)4
5 or 6
6
100 mV/s
500 mV/s
1000 mV/s
Pd(PPh3)4
4
2
0
O
O
SHex
Bu3Sn
S
7b
Scheme 1. Synthesis of oligomers.
-2
-4
O
O
O
O
O
O
1) BuLi
2) S8
1) BuLi
SHex
Bu3Sn
SHex
S
2) ClSnBu3
3) Br-C6H13
S
S
-0.5
0.0
0.5
1.0
EDOT
7b
E (V) / Ag/AgCl
8
Scheme 2. Synthesis of 2-hexylsulfanyl-3,4-ethylenedioxythiophene.
Figure 2. CV of 2a in 0.1 M Bu4NPF6/CH2Cl2.

7150
M. Turbiez et al. / Tetrahedron Letters 50 (2009) 7148–7151
E2
- 1e-
E1
- 1e-
E2
- 1e-
E1
- 1e-
2b2+
.
2b+
2a2+
.
2b
2a+
2a
+ 1e-
+ 1e-
+ 1e-
fast
+ 1e-
fast
Dimerization slow
E3
- 1e-
.
3+
2+
(2a)2
(2a)2
fast
σ-dimer
Scheme 4.
conjugated chains by the multiplication of Sꢀ ꢀ ꢀO intramolecular
interactions between the median thienothiophene unit and the
external EDOT moieties.22 Thus, the synergy of the structural and
electronic effects of the methoxy groups leads to a higher rigidity
of the conjugated systems and to a lower HOMO–LUMO gap.
In conclusion, we have synthesized a series of hybrid oligomers
combining EDOT and thienothiophene units. These oligomers pres-
ent a planar conformation due to the association of the intrinsically
rigid thieno[3,2-b]thiophene unit together with the self-rigidifica-
tion of the conjugated chain by Sꢀ ꢀ ꢀO intramolecular interactions.
The radical cations of oligomers substituted with n-hexyl chains
present a tendency to dimerize while with hexylsulfanyl group as
substituent the stability of the radical cations is strongly improved.
The electrochemical properties of oligomers have been analyzed
by cyclic voltammetry (CV) and the oxidation potentials are gath-
ered in Table 1. The CV traces of oligomers 1a and 2a are strongly
dependent on the scan rates (Fig. 2 for 2a). Thus at 100 mV sꢁ1 scan
rate, the CV of 1a and 2a shows a first partially reversible oxidation
peak followed by a second reversible peak and then by a third irre-
versible peak. By increasing the rate of the scan in potential, the
reversibility of the first peak increases while the intensity of the
third peak strongly decreases. From scan rate of 1 V sꢁ1, the two
first oxidation waves are fully reversible while the third oxidation
peak is quasi vanished. At high scan rate the CV corresponds to the
subsequent formation of the radical cation 1a+Å or 2a+Å at potential
E1 and then a dication 1a2+ or 2a2+ at potential E2. At lower scan
References and notes
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rate the CV data can be interpreted by the formation of a dimeric
2þ
dication ð1aÞ2 or ð2aÞ22þ, named
r-dimer, by coupling of the rad-
ical cation (Scheme 4). Such dimerization processes have already
been described for bithiophene derivatives.23,24
The kinetic of the dimerization process of the radical is not very
fast and an amount of radical cation is not dimerized when the po-
tential reaches E2 allowing the formation of dication 1a2+ or 2a2+
.
The third oxidation potential (E3) corresponds to the oxidation step
of the -dimer involving the transfer of one electron followed by
another chemical reaction. The intensity of this oxidation step al-
lows evidencing the formation of the -dimer. By increasing the
scan rate, the time scale allowing the coupling reaction becomes
shorter, thus limiting the formation of -dimer and by conse-
r
r
15. Navacchia, M. L.; Melucci, M.; Favaretto, L.; Zanelli, A.; Gazzano, M.; Bongini,
A.; Barbarella, G. Org. Lett. 2008, 10, 3665.
16. Turbiez, M.; Frère, P.; Leriche, P.; Mercier, N.; Roncali, J. Chem. Commun. 2005,
r
1161–1163.
quence decreasing the intensity of the third oxidation peak.
The CV of compounds 1b and 2b end capped by hexylsulfanyl
chains shows two reversible close oxidation waves, even for scan
rate inferior to 100 mV sꢁ1, corresponding to the successive forma-
tion of radical cation and dication. Thus the presence of the sulfur
atoms on the external positions strongly stabilizes the radical cat-
ion which does not present the dimerization process.25,26
Compared to tetrathiophene derivatives 9a and 9b, the first oxi-
dation potential of 1a and 1b is positively shifted as expected by
the shortening of the conjugated chain. Moreover, the higher dif-
17. DeCremer, L.; Verbiest, T.; Koeckelberghs, G. Macromolecules 2008, 41, 568–
578.
18. Turbiez, M.; Frère, P.; Roncali, J. J. Org. Chem. 2003, 68, 5357–5360.
19. All new compounds exhibited spectral properties consistent with the assigned
structures: Compound 1a: orange solid, mp 142 °C, 1H NMR (500 MHz) 0.89 (t,
3J = 6.8 Hz, 6H), 1.33 (m, 12H), 1.61 (m, 4H), 2.64 (m, 4H, 3J = 7.2 Hz), 4.23 (m, 4H),
4.32 (m, 4H), 7.28 (s, 2H). 13C NMR (125.7 Hz) 14.1, 22.6, 25.7, 28.8, 30.3, 31.5,
64.5, 65.1, 108.4, 114.1, 116.6, 136.0, 137.5, 137.6, 137.7. MS Maldi-tof calcd for
C30H36O4S4: 588.15; found: 588.14. Compound 2a: orange solid, mp 202 °C, 1
H
NMR (500 MHz) 0.89 (t, 3J = 6.8 Hz, 6H), 1.33 (m, 12H), 1.63 (m, 4H), 2.65 (m, 4H,
3J = 7.1 Hz), 4.09 (s, 6H), 4.23 (m, 4H), 4.33 (m, 4H). 13C NMR (125.7 Hz) 14.1, 22.5,
25.8, 28.8, 30.4, 31.5, 59.4, 64.5, 65.2, 106.1, 117.3, 126.2, 136.8, 136.9, 144.7. MS
Maldi-tof Calcd for C32H40O6S4: 648.17; found: 648.09. Compound 1b: orange
solid, mp 140 °C, 1H NMR (500 MHz) 0.90 (t, 3J = 6.8 Hz, 6H), 1.28 (m, 8H), 1.40
(m, 4H), 1.62 (m, 4H), 2.73 (t, 4H, 3J = 7.5 Hz), 4.34 (m, 8H), 7.34 (s, 2H). 13C NMR
(125.7 Hz) 13.9, 22.5, 28.1, 29.3, 31.3, 38.5, 64.7, 64.9, 109.3, 115.1, 118.6, 136.5,
138.5, 139.0, 140.6. MS Maldi-tof calcd for C30H36O4S6: 652.09; found: 652.14.
Compound 2b: orange solid, mp 136 °C, 1H NMR (500 MHz) 0.88 (t, 3J = 7.0 Hz,
6H), 1.25 (m, 8H), 1.41 (m, 4H), 1.63 (m, 4H), 2.75 (t, 4H, 3J = 7.5 Hz), 4.11 (s, 6H),
4.25 (m, 4H), 4.36 (m, 4H). 13C NMR (125.7 Hz) 14.1, 22.4, 28.0, 29.5, 32.3, 38.9,
59.9, 64.7, 64.9, 109.3, 115.1, 118.6, 136.5, 138.5, 140.6, 146.3. MS Maldi-tofcalcd
for C32H40O6S6: 712.12; found: 712.08.
ference
D
E = E2 ꢁ E1 observed for 1a and 1b corresponds to an in-
crease of the coulombic repulsion between the two positive
charges in the dication state. It can be noted that the lower differ-
ence
DE observed for 9b, 1b and 2b compared to 9a, 1a and 2a is an
indication of the strong participation of the terminal sulfur atoms
of the hexylsulfanyl chains for stabilizing the positive charge of
the dication in the external position. Finally, by comparison with
1a and 1b, the introduction of the methoxy group on the central
thienothiophene unit provokes a negative shift of the oxidation
potentials for 2a and 2b due to the mesomeric donor effect of
the ether groups.
20. Turbiez, M.; Frère, P.; Allain, M.; Videlot, C.; Ackermann, J.; Roncali, J. Chem. Eur.
J. 2005, 11, 3742–3752.
21. Compound 9b has been synthesized by Stille coupling between
dibromobithiophene and 2.5 equiv of 7b. Orange solid, mp = 118 °C, 1H NMR
(500 MHz) 0.90 (t, 3J = 6.8 Hz, 6H), 1.28 (m, 8H), 1.40 (m, 4H), 1.62 (m, 4H), 2.73

M. Turbiez et al. / Tetrahedron Letters 50 (2009) 7148–7151
7151
(t, 4H, 3J = 7.5 Hz), 4.34 (m, 8H),7.05 (d, 3J = 3.8 Hz, 2H), 7.10 (d, 3J = 3.8 Hz, 2H).
13C NMR (125.7 Hz) 14.0, 22.5, 28.1, 29.4, 31.3, 38.0, 64.7, 64.9, 105.0, 114.4,
123.4, 123.7, 133.0, 135.8, 137.0, 143.9. MS Maldi-tof calcd for C32H38O4S6:
678.11; found: 677.96.
23. Heinze, J.; John, H.; Dietrich, M.; Tschuncky, P. Synth. Met. 2001, 119, 49–52.
24. Tschuncky, P.; Heinze, J.; Smie, A.; Engelmann, G.; Kossmehl, G. J. Electroanal.
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