Planarized Star-Shaped Oligothiophenes with Enhanced π-Electron Delocalization

Article abstract of DOI:10.1021/ol0362764

(Matrix presented) Planarized star-shaped oligothiophenes 1 have been synthesized by connecting short-chain oligothiophenes on a benzo[1,2-b:3,4- b′:5,6-b″]-trithiophene central core. Their electrochemical and optical properties have been characterized by cyclic voltammetry and UV-visible spectroscopy, respectively. These results associated with theoretical calculations show the advantage of benzotrithiophene as a central core in terms of π-electron delocalization.

Full text of DOI:10.1021/ol0362764

ORGANIC
LETTERS
2
004
Vol. 6, No. 2
73-276
Planarized Star-Shaped Oligothiophenes
2
with Enhanced π-Electron Delocalization
Yohann Nicolas, Philippe Blanchard,* Eric Levillain, Magali Allain,†
†
Nicolas Mercier, and Jean Roncali*
Groupe Syst e` mes Conjugu e´ s Lin e´ aires, IMMO, UMR CNRS 6501, UniVersit e´ d’Angers,
2
Bd LaVoisier, F-49045 Angers, France, and IMMO, UMR CNRS 6501,
UniVersit e´ d’Angers, 2 Bd LaVoisier, F-49045 Angers, France
philippe.blanchard@uniV-angers.fr; jean.roncali@uniV-angers.fr
Received November 21, 2003
ABSTRACT
Planarized star-shaped oligothiophenes 1 have been synthesized by connecting short-chain oligothiophenes on a benzo[1,2-b:3,4-b′:5,6-b′′]-
trithiophene central core. Their electrochemical and optical properties have been characterized by cyclic voltammetry and UV−visible spectroscopy,
respectively. These results associated with theoretical calculations show the advantage of benzotrithiophene as a central core in terms of
π-electron delocalization.
1
Thiophene-based π-conjugated oligomers are the subject of
considerable current interest due to their use as organic
semiconductors for the realization of devices such as field-
on the design and synthesis of new compounds with
electrochemical, optical, and electronic properties specifically
tailored for each type of application.
In this context, we report here the synthesis and prelimi-
nary characterizations of the first members of a new series
of star-shaped oligothiophenes in which three short linear
oligothiophene chains are connected to a central rigid
trithienobenzene core.
2
3
effect transistors, light-emitting diodes, and photovoltaic
4
cells.
Despite the huge amount of work already invested in these
various applications, it is clear that further progress in these
fields implies an intensification of the research effort focused
Star-shaped oligothiophenes with three or more oligoth-
iophene chains attached to a central benzenic core have
†
IMMO.
5
(
1) B a¨ uerle, P. In Electronic Materials: The Oligomer Approach; M u¨ llen,
K., Wegner, G., Eds.; Wiley-VCH: Weinheim, 1998; Chapter 2, pp 105-
already been reported by several groups. However, as shown
by molecular models and confirmed by experimental data,
1
97.
(
2) (a) Dimitrakopoulos, C. D.; Malenfant, P. AdV. Mater. 2002, 14, 99-
1
1
17. (b) Katz, H. E.; Lovinger, A. J.; Laquindanum, J. G. Chem. Mater.
998, 10, 457-459. (c) Garnier, F.; Yassar, A.; Hajlaoui, R.; Horowitz,
G.; Deloffre, F.; Servet, B.; Ries, S.; Alnot, P. J. Am. Chem. Soc. 1993,
15, 8716-8721. (d) Garnier, F.; Hajlaoui, R.; El Kassmi, A.; Horowitz,
Laigre, L.; Porzio, W.; Armanini, M.; Provasoli, F. Chem. Mater. 1998,
(5) (a) Ch e´ rioux, F.; Guyard, L.; Audebert, P. Chem. Commun. 1998,
2225-2226. (b) Kotha, S.; Chakraborty, K.; Brahmachary, E. Synlett 1999,
10, 1621-1623. (c) Bras, J.; Guillerez, S.; P e´ pin-Donat, B. Chem. Mater.
2000, 12, 2372-2384. (d) Ch e´ rioux, F.; Guyard, L. AdV. Funct. Mater.
2001, 11, 305-309. (e) Geng, Y.; Fechtenk o¨ tter, A.; M u¨ llen, K. J. Mater.
Chem. 2001, 11, 1634-1641. (f) Pappenfus, T. M.; Mann, K. R. Org. Lett.
2002, 4, 3043-3046. (g) Ponomarenko, S. A.; Kirchmeyer, S.; Elschner,
A.; Huisman, B.-H.; Karbach, A.; Drechsler, D. AdV. Funct. Mater. 2003,
13, 591-596. (h) Inoue, S.; Nischiguchi, S.; Murakami, S.; Aso, Y.; Otsubo,
T.; Vill, V.; Mori, A.; Ujiie, S. J. Chem. Res., Synop. 1999, 596-597.
1
1
0, 3334-3339.
(
(
3) (a) Mitschke, U.; B a¨ uerle, P. J. Mater. Chem. 2000, 10, 1471-1507.
4) (a) Videlot, C.; El Kassmi, A.; Fichou, D. Solar Energy Mater. Solar
Cells 2000, 63, 69. (b) Fichou, D. J. Mater. Chem. 2000, 10, 571-588. (c)
Noma, N.; Tsuzuki, T.; Shirota, Y. AdV. Mater. 1995, 7, 647-648.
1
0.1021/ol0362764 CCC: $27.50 © 2004 American Chemical Society
Published on Web 12/19/2003

Scheme 1. Synthesis of Trithienobenzene Derivativesa
Scheme 2. Synthesis of Star-Shaped Molecules 1a
a
Reagents and conditions: (i) n-BuLi (1 equiv), THF, -70 °C;
(
(
ii) tetrahydrothiophene-3-one, THF, -70 °C to reflux, then aq HCl;
iii) chloranil, ethyleneglycol, reflux; (iv) 2-thienylmagnesium
bromide, NidpppCl
n-BuLi (6 equiv), THF, 0 °C and then 20 °C; (vii) Bu
2
, Et
2
O, reflux; (v) hν, cat. I
2
, O
2
, toluene; (vi)
SnCl.
3
a
Reagents and conditions: (i) NBS (1.05 equiv), DMF, -20
°
5 4 4 3 2
C; (ii) C H11COCl, SnCl , benzene, 0 °C; (iii) LiAlH , AlCl , Et 0,
2
0 °C; (iv) NBS (1.05 equiv), DMF, 20 °C; (v) cat. Pd(PPh
3 4
) ,
the steric interactions associated with the grafting of a
thiophene moiety on a benzene ring produce a dihedral angle
that results in a limitation of the effective conjugation.
toluene, reflux.
5
a-d,g
To solve this problem we describe here the use of a rigid
and planar central core involving three thiophene rings fused
to the benzenic ring with the aim of building a planar
conjugated molecule of C3h symmetry.
tion time from 11 to 2 h and improvement of the reaction
1
0
yield from 34 to 60%. This reaction appeared to be highly
regioselective since only one regioisomer was obtained. The
presence of only two doublets at 7.54 and 7.64 ppm,
reflecting a high degree of molecular symmetry and the
6
As generally observed in the oligothiophene series, the
dihedral angle between two consecutive thiophene rings is
close to 0° in the solid state. Therefore, the trithienobenzene
core offers the possibility of building planar star-shaped
oligothiophenes with enhanced π-electron delocalization.
The target compounds 1 have been synthesized according
to the procedure depicted in Schemes 1 and 2. Commercially
available 2,3-dibromothiophene 3 was subjected to a regi-
oselective lithium-bromine exchange at the 2-position of
the thiophene ring using 1 equiv of n-BuLi at low temper-
ature. After reaction of this lithium salt with tetrahy-
drothiophene-3-one and subsequent acidification, the result-
ing carbinol was directly dehydrated and oxidized in the
presence of chloranil in refluxing ethylene glycol to afford
3
typical value of the thiophenic R,â-coupling constant ( J )
1
5
.4 Hz) observed in the H NMR spectrum, are in good
agreement with the structure of 6.
Single crystals, grown by slow evaporation of a solution
of 6 in a chloroform/ethanol mixture, have been analyzed
by X-ray diffraction. The obtained crystallographic structure
definitely confirms the regular head-to-tail orientation of the
thiophene rings and the expected fully planar geometry of
molecule 6 (Figure 1).
Regioselective lithiation of compound 6 at the R-position
of each thiophene ring with 6 equiv of n-BuLi and
subsequent quenching with tributylstannyl chloride gave the
corresponding tristannyl reagent 7.
7
bithiophene 4 in good yield. A nickel-catalyzed Kumada
coupling reaction between compound 4 and freshly prepared
2-Bromo-5-hexylthiophene 9 and 5-bromo-5′-hexyl-2,2′-
bithiophene 12 were prepared by treatment of 2-hexylth-
iophene 8 and 5-n-hexyl-2,2′-bithiophene 11, respectively,
2
-thienylmagnesium bromide gave terthiophene 5 in excellent
8
yield. The shamrock-shaped key central core benzo[1,2-b:
,4-b′:5,6-b′′]trithiophene 6 was then prepared by oxidative
1
1
3
with NBS in N,N-dimethylformamide (Scheme 2). Com-
pound 11 was preferably synthesized by successive acylation
and reduction from 2,2′-bithiophene 10 in order to circum-
vent the difficulties associated with the purification of 11
when prepared by direct alkylation of the lithium salt of 2,2′-
photocyclization by irradiation of a diluted toluene solution
of 5 under aerobic conditions in the presence of a catalytic
amount of iodine.
9
A modification of the literature procedure by using a 400
1
2
bithiophene.
W high-pressure Hg lamp allowed shortening of the irradia-
(
6) (a) van Bolhuis, F.; Wynberg, H.; Havinga, E. E.; Meijer, E. W.;
(10) A previous route to compound 6 involving more drastic conditions
has been reported: Proetzsch, R.; Bieniek, D.; Korte, F. Tetrahedron Lett.
1972, 6, 543-544.
Staring, E. G. J. Synth. Met. 1989, 30, 381. (b) Chaloner, P. A.; Gunatunga,
S. R.; Hitchcock P. B. Acta Crystallogr., Sect. C 1994, 50, 1941. (c) Pelletier,
M.; Brisse, F. Acta Crystallogr., Sect. C 1994, 50, 1942.
(11) B a¨ uerle, P.; W u¨ rthner, F.; G o¨ tz, G.; Effenberger, F. Synthesis 1993,
1099-1103.
(
7) Wynberg, H.; Heeres, G. J.; Jordens, P.; Sinnige, H. J. M. Recl. TraV.
Chim. 1970, 89, 545-552.
(12) Improved purification of 11 was recently reported by: Sotgiu, G.;
Zambianchi, M.; Barbarella, G.; Botta, C. Tetrahedron 2002, 58, 2245-
2251.
(
(
8) Jayasuriya, N.; Kagan, J. Heterocycles 1986, 24, 2901-2904.
9) Jayasuriya, N.; Kagan, J. J. Org. Chem. 1989, 54, 4203-4205.
274
Org. Lett., Vol. 6, No. 2, 2004

Figure 2. Cyclic voltammograms of 0.5 mM solutions of 1b (solid
Figure 1. Crystallographic structure of compound 6.
4 6 2 2
line) and 2b (dashed line,) in 0.2 M Bu NPF /CH Cl , Pt electrodes,
scan rate ) 100 mV/s.
The target compounds 1a and 1b were then assembled by
a threefold Stille reaction between the tris(stannyl) derivative
stannyl chloride led to the corresponding stannyl derivative.
This intermediate compound was directly engaged in the final
Stille reaction with 2-bromo-5-n-hexylthiophene, affording
7
2
and 2-bromo-5-n-hexylthiophene 9 or 5-bromo-5′-n-hexyl-
,2′-bithiophene 12, respectively. Thus, treatment of com-
pound 7 with 4 equiv of bromo derivatives 9 or 12 in the
presence of a catalytic amount of Pd(PPh led to the
formation of the target compounds 1a and 1b.
2b. Derivative 16 was obtained by a Stille coupling reaction
3 4
)
either between 2-iodobenzo[b]thiophene 15 and 2-tributyl-
stannylthiophene or between the stannyl derivative of
thianaphthene 14 and 2-bromothiophene.
The synthesis of the reference linear compounds 2a and
b is described in Scheme 3. Compound 2a was prepared
2
The electronic properties of the star-shaped compounds 1
and of the corresponding linear reference compounds 2 have
been analyzed by cyclic voltammetry and UV-vis spectros-
copy. The cyclic voltammograms (CV) of 1 and 2 exhibit a
one-electron oxidation wave corresponding to the formation
of the cation radical. An analysis of the electro-oxidation of
compound 1b by thin-layer cyclic voltammetry using 2,3-
dichloro-1,4-naphthoquinone as an internal reference defini-
tively showed that the oxidation process involves only one
electron per molecule.
by Suzuki coupling between thianaphthene-2-boronic acid
3 and bromo derivative 9. On the other hand, a regiose-
lective lithiation reaction at the free R-position of the
thiophene ring of 16 and subsequent reaction with tributyl-
1
Scheme 3. Synthesis of the Linear Molecules 2a
At a scan rate of 100 mV/s, the oxidation process, which
is irreversible for 1a and 2a, becomes reversible for 1b and
quasi-reversible for 2b, respectively (Figure 2). An analysis
of the scan rate dependence of the CV of compound 2b
shows that full reversibility is reached above 1 V/s. The
different behaviors of 1b and 2b clearly show that the
assembly of the benzo-oligothiophenes in a star-shaped
system considerably stabilizes the cation radical. It is also
worth noting that the difference in intensity between the
voltammograms of 1b and 2b at the same concentration (0.50
mM (see Figure 2) or 0.25 mM (not shown)) may be
correlated to the difference of the related diffusion coef-
ficients in solution, the larger star-shaped molecule probably
exhibiting a lower coefficient.
a
Reagents and conditions: (i) 9 (1.1 equiv), DME, Pd(PPh
, H O, reflux; (ii) n-BuLi, THF, -50 °C then I , -50 to
0 °C; (iii) 2-tributylstannylthiophene, cat. Pd(PPh , toluene,
reflux; (iv) n-BuLi (1 equiv), THF, 0 °C then 20 °C; (v) Bu SnCl,
0 °C; (vi) 2-bromothiophene or 2-bromo-5-hexylthiophene, Pd-
PPh , toluene, reflux.
3
)
4
,
As shown in Table 1, for both series of compounds, the
lengthening of the oligothiophene chain induces a negative
shift of the anodic peak potential (Epa) due to the extension
of the conjugation length. In addition, comparison of the data
for compounds 1 and 2 reveals a significant negative shift
of Epa for the star-shaped molecules (140 and 220 mV for
Ba(OH)
2
2
2
2
3 4
)
3
2
(
3 4
)
Org. Lett., Vol. 6, No. 2, 2004
275

results obtained for the two systems shows that in addition
to the full planarization of the central core, in agreement
with the X-ray data, connection to the fused trithienobenzene
system results in a decrease of the dihedral angle between
the central core and the first thiophene ring from ca. 29 to
Table 1. Electrochemical, Optical, and Thermal Data for
Compounds 1 and 2
compd
Epa/Va
λmax/nmb
λmax/eV
Td/°Cc
1
1
2
2
6
a
b
a
b
1.16
0.93
1.30
1.15
1.55
357
404
326
379
3.47
3.07
3.80
3.27
405
430
185
232
1
1°, thus confirming that the use of 6 as a central core leads
to a more planar and hence more conjugated system.
The analysis of the thermal stability of 1a and 1b by TGA
shows that the star-shaped molecules begin to decompose
only above 400 °C, that is, more than 200 °C above the
decomposition temperature of their linear analogues.
To summarize, the first members of a new class of star-
shaped oligothiophenes have been synthesized. Comparison
of the electronic properties of these compounds to those of
their linear analogues and other already known star-shaped
oligothiophenes shows that the use of a centrally fused
trithienobenzene core allows development of a new class of
planar star-shaped conjugated systems with enhanced π-elec-
tron delocalization.
288, 267
a
Performed with 0.1 mM of compound in 0.2 M Bu4NPF6/CH2Cl2, Pt
b
electrodes, Ag/AgCl as a reference, scan rate ) 100 mV/s. In CH2Cl2.
Temperature of decomposition corresponding to 5% weight loss from TGA
c
analysis under N2 with a heating rate of 10 °C/min.
series a and b, respectively), indicative of an increase of the
HOMO levels for compounds 1.
On the other hand, UV-vis data show that the passage
from a linear to a star-shaped system results in a 25-31 nm
bathochromic shift of λmax corresponding to a significant
decrease of the HOMO-LUMO energy gap (Table 1).
Comparison of the UV-vis data of 1b to those of a
recently described star-shaped oligothiophene based on a
benzenic core, namely, 1,3,5-tris[5-(5′′-decyl-2,2′:5′2′′-ter-
thienyl)]benzene (λmax ) 405 nm)5g shows that, while both
compounds absorb at the same wavelength, the trithienothio-
As a first illustration of the potentialities of these new
compounds, we have recently shown that heterojunction solar
cells based on vacuum-sublimed thin films of the star-shaped
compound 1b exhibit power conversion efficiencies ca. 20
times larger than those obtained with the cells based on the
1
3
linear compound 2b.
2
Acknowledgment. We thank the SCAS of the University
of Angers for analytical characterizations and the city of
Angers for financial support for Y. N.
phene core allows an economy of six sp carbon atoms, thus
underlining the advantage of the fused central core 6 over a
simple benzene ring in terms of π-electron delocalization.
To gain more insight into the relationship between
structure and electronic properties, the structure of the parent
compound of 1b without hexyl substituents and that of the
Supporting Information Available: Synthetic proce-
dures, structural characterization for all compounds, and
X-ray data for compound 6. This material is available free
of charge via the Internet at http://pubs.acs.org.
1,3,5-tris[5-(2,2′-dithienyl)]benzene were modelized by theo-
retical calculations based on the density functional method
using Gaussian 98 (see Supporting Information). In both
cases, optimization was carried out by imposing an all-anti
conformation to the thiophene rings. Comparison of the
OL0362764
(13) De Bettignies, R.; Nicolas, Y.; Blanchard, P.; Levillain, E.; Nunzi,
J.-M.; Roncali, J. AdV. Mater. 2003, 15, 1939-1943.
276
Org. Lett., Vol. 6, No. 2, 2004