Synthesis, polymerization, and unusual properties of new star-shaped thiophene oligomers

Article abstract of DOI:10.1021/ol901127q

Terthienobenzene (TTB, 6) was prepared through a new, high yield route along with π-extended derivatives 10 and 11. Electropolymerization of trs-EDOT derivative 11 results in a highly stable cross-linked conjugated polymer that shows polaron confinement b

Full text of DOI:10.1021/ol901127q

ORGANIC
LETTERS
2009
Vol. 11, No. 15
3230-3233
Synthesis, Polymerization, and Unusual
Properties of New Star-Shaped
Thiophene Oligomers
Tyler Taerum, Olena Lukoyanova, Ryan G. Wylie, and Dmitrii F. Perepichka*
Department of Chemistry, McGill UniVersity, Montreal, Quebec H3A 2K6, Canada
dmitrii.perepichka@mcgill.ca
Received May 21, 2009
ABSTRACT
Terthienobenzene (TTB, 6) was prepared through a new, high yield route along with π-extended derivatives 10 and 11. Electropolymerization
of tris-EDOT derivative 11 results in a highly stable cross-linked conjugated polymer that shows polaron confinement between the TTB units
as confirmed by UV-vis-NIR spectroelectrochemistry and EPR.
Thiophene-based π-conjugated oligomers with complex
molecular architectures such as star-shaped,1 X-shaped2 and
dendritic3 oligothiophenes have been the focus of recent
research4 as promising organic semiconductors. Their in-
creased dimensionality (2D or 3D), different from the linear
structure of most conjugated oligomers and polymers, offers
new opportunities to control the morphology of the material
in thin film devices5,6 and to build new nanostructured
materials.7
Star-shaped oligothiophenes have been constructed with
benzene,1a triarylamine,8 truxene9 and several other cores,
and in every instance, the steric constraints of the core
resulted in rotation of the linked (oligo)thiophene arms out
of the plane. Roncali’s group has previously reported
synthesis1d and remarkable performance in photovoltaic
devices10 of star-shaped oligothiophenes based on ter-
thienobenzene (TTB, 6) core. The improved semiconducting
(5) Cravino, A.; Roquet, S.; Aleveque, O.; Leriche, P.; Frere, P.; Roncali,
J. Chem. Mater. 2006, 18, 2584.
(1) (a) Che´rioux, F.; Guyard, L. AdV. Funct. Mater. 2001, 11, 305. (b)
Ponomarenko, S.; Kirchmeyer, S.; Elschner, A.; Huisman, B.-H.; Karbach,
A.; Drechsler, D. AdV. Funct. Mater. 2003, 13, 3043. (c) Pappenfus, T. M.;
Mann, K. R. Org. Lett. 2002, 4, 3044. (d) Nicolas, Y.; Blanchard, P.;
Levillain, E.; Allain, M.; Mercier, N.; Roncali, J. Org. Lett. 2004, 6, 273.
(2) Sun, X.; Liu, Y.; Chen, S.; Qiu, W.; Yu, G.; Ma, Y.; Qi, T.; Zhang,
H.; Xu, X.; Zhu, D. AdV. Funct. Mater. 2006, 16, 917.
(6) (a) Cho, M. Y.; Kim, S. J.; Han, Y. D.; Park, D. H.; Kim, K. H.;
Choi, D. H.; Joo, J. AdV. Funct. Mater. 2008, 18, 2905. (b) Sun, Y.; Xiao,
K.; Liu, Y.; Wang, J.; Pei, J.; Yu, G.; Zhu, D. AdV. Funct. Mater. 2005,
15, 818.
(7) Tour, J. M. Chem. ReV. 1996, 96, 537.
(8) Chahma, M.; Gilroy, J. B.; Hicks, R. G. J. Mater. Chem. 2007, 17,
4768.
(3) Xia, C.; Fan, X.; Locklin, J.; Advincula, R. C. Org. Lett. 2002, 4,
2067. (b) Ma, C.-Q.; Mena-Osteritz, E.; Debaerdemaeker, T.; Wienk, M. M.;
Janssen, R. A.; Ba¨uerle, P. Angew. Chem., Int. Ed. 2007, 46, 1679.
(4) Mishra, A.; Ma, C.-O.; Ba¨uerle, P. Chem. ReV. 2009, 109, 1141.
(9) Pei, J.; Wang, J.-L.; Cao, X.-Y.; Zhou, X.-H.; Zhang, H.-B. J. Am.
Chem. Soc. 2003, 125, 9944.
(10) De Bettignies, R.; Nicolas, Y.; Blanchard, P.; Levillain, E.; Nunzi,
J.-M.; Roncali, J. AdV. Mater. 2003, 15, 1939.
10.1021/ol901127q CCC: $40.75
Published on Web 07/02/2009
 2009 American Chemical Society

properties were believed to result from increased planarity
of such derivatives (compared to benzene-core derived
oligomers). In our search for high-dimensional organic
semiconductors, we turned our attention to TTB derivatives
as tridentate monomers for creating two-dimensional con-
jugated polymers.11
Scheme 2. Synthesis of Oligothiophenes 10 and 11
Here, we report the synthesis of new star-shaped oligoth-
iophenes composed of the TTB core and three symmetrically
positioned thiophene or 3,4-ethylenedioxythiophene (EDOT)
end groups, their electroxidative polymerization and spec-
troelectrochemical studies. In addition, we describe an
improved route for synthesis of TTB 6 in five simple steps
from 1,3,5-trichlorobenzene. An alternative procedure12,1d
relied on a more expensive 2,3-dibromothiophene and
involved two metallorganic couplings, oxidative aromatiza-
tion and photooxidative cyclization affording 6 with overall
34% yield. The scale up of this procedure is hampered by
the final photooxidative coupling which proceeds in low and
variable yield (20-50% in our hands) and requires high
dilution conditions (to prevent polymerization).
In this work, the known trialdehyde precursor 3 was
prepared from 1,3,5-trichlorobenzene 1 through Friedel-Crafts
alkylation13 with chloroform and subsequent hydrolysis in
H2SO4,14 with an improved yield (overall 83%, Scheme 1).
thiophene 8 or tributyltin-EDOT 9 provided access to
monomers 10 and 11, with 48 and 69% yields, respectively.
Moderate yields of these reactions are likely due to low
solubility of planar TTB derivatives.
UV-vis absorption of TTB 6 at λmax 260 nm (4.77 eV)
with a weaker vibronic peak at 288 nm can be attributed
Scheme 1. Synthesis of Terthienobenzene
mostly to HOMO-LUMO transition (DFT calculated gap
calcd
Eg
) 4.89 eV) (Figure 1, Table 1). Extending the
A ring closing reaction with ethyl mercaptoacetate gave TTB-
triester 4, which was previously reported only in a patent.15
Saponification of 4 quantitatively afforded TTB-triacid 5,
which was efficiently decarboxylated by simple heating in
inert atmosphere, to yield parent TTB 6 in 82% yield. The
described approach involved simple isolation procedures (no
chromatography/ recrystallization), was cost-effective, and
afforded TTB 6 in 60-68% overall yield from 1.16
Figure 1. UV-vis spectra (solid line) and fluorescence spectra
(dotted line) of compounds 6 (black), 10 (blue), and 11 (red).
conjugation results in a predictable bathochromic shift to 343
nm (3.62 eV) for thiophene-capped 10 and even a larger shift
(355 nm, 3.49 eV) for EDOT-capped 11. These can still be
ascribed to their HOMO-LUMO transitions (calculated gaps
of 3.69 and 3.62 eV for 10 and 11, respectively). While TTB
6 shows only weak fluorescence in the UV part of the
spectrum, the oligomers 10 and 11 emit blue light at λmax
427 and 434 nm respectively, with photoluminescence
quantum yields of ΦPL ≈ 3.5%. This is somewhat unexpected
since introducing EDOT moieties most often results in
fluorescence quenching.17 The observed Stokes shifts were
quite large (0.71 eV for 10 and 0.63 eV 11), which is typical
for oligothiophenes (0.61 eV for quarterthiophene18).
Bromination of 6 with NBS yielded tribromo-TTB 7 (65%)
(Scheme 2). Subsequent Stille couplings with tributyltin-
(11) Perepichka, D. F.; Rosei, F. Science 2009, 323, 216.
(12) Jayasuriya, N.; Kagan, J.; Owens, J. E.; Kornak, E. P.; Perrine,
D. M. J. Org. Chem. 1989, 54, 4203
.
(13) Veciana, J.; Rovira, C.; Ventosa, N.; Crespo, M. I.; Palacio, F.
J. Am. Chem. Soc. 1993, 115, 57.
(14) Wallenfels, K.; Witzler, F.; Friedrich, K. Tetrahedron 1967, 23,
1845.
(15) Roos, E.; Wagner, K. U.S. Patent 3 663 568, 1972.
Org. Lett., Vol. 11, No. 15, 2009
3231

carbonate, but slowly degraded upon multiple CV scans in
a stronger solvent (benzonitrile). In contrast, the EDOT-based
poly-11 exhibited remarkable stability upon oxidation in both
propylene carbonate and benzonitrile. 1400 Complete CV
cycles of poly-11 on ITO at 100 mV s-1, in air-saturated
electrolyte (Bu4NPF6 in propylene carbonate, in open air
atmosphere) resulted in less than 5% loss of the redox
activity. The improved electrochemical robustness of an
EDOT derived polymer is not unusual,19 albeit the very high
stability of poly-11 even in air atmosphere could also be
due to the cross-linked architecture of this polymer. While
expected planarity of 11 might potentially lead to planar 2D
conjugated polymer sheets, no evidence of such a structure
has yet been obtained. Further studies of electropolymeriza-
tion on atomically flat substrates20 are currently underway.
For optical studies, monomers 10 and 11 were electropo-
lymerized on an indium tin oxide (ITO) transparent electrode.
UV-vis spectra of the prepared films reveal vibronically
structured bands at λmax of 444 nm (2.79 eV) and 480 nm
(2.58 eV) for poly-10 and poly-11, respectively. Interestingly,
DFT calculations (see Supporting Information) predict the
HOMO-LUMO gap to reduce significantly upon dimeriza-
tion (from 3.64 to 2.72 eV for 11 vs di-11) but almost no
effect from the continuing polymerization (2.68 eV for tri-
11, 2.66 eV for tetra-11). The limited effective conjugation
length in these polymers is clearly due to a “meta-type”
connection through the TTB core. As a result, the HOMO
and LUMO are localized between the benzene rings (see
Supporting Information) and are not significantly affected
by the increasing polymerization number.
Table 1. Electronic Properties of TTB Monomers 6, 10 and 11
6
10
11
calcd
EHOMO
(eV)a
-5.92
4.89
1.39
-6.19
260
-5.50
3.69
1.12
-5.92
343
-5.05
3.63
0.78
-5.58
355
calcd
Eg
(eV)a
Epa (V)b
ox
EHOMO (eV)c
exp
λmaxabs(nm)d
Eg (eV)e
4.79
353
3.62
427
3.49
434
opt
λmaxPL(nm)f
Φfl (%)
<1g
3.6h
3.5h
Stokes shift (eV)
1.15
0.71
0.63
a B3LYP/6-311G(d,p) calculations. b In DCM, 0.1 M nBu4NPF6 as
supporting electrolyte, V vs. Ag/AgCl. c Calculated as -4.8-Eox (vs.
Fc/Fc+). d In DCM. e Calculated as 1240/λmax f In DCM, upon excitation
.
at 373 nm. g Benzene (ΦPL ) 5%) as the standard. h 9,10-diphenylanthracene
(ΦPL ) 90%) as the standard.
Cyclic voltammetry (CV) experiments with 6, 10, and 11
revealed their irreversible oxidation at +1.39, +1.12 and
+0.78 V vs Ag/AgCl, respectively (Table 1). For all three
monomers, multiple potential scans lead to progressive
growth of a visible polymer film on platinum electrode
(Figure 2). The onset p-doping potential of the polymers
The stable p-doped poly-11 films were then characterized
by EPR. The doped polymer shows an intense symmetric
signal with a g-factor of 2.006, which can be ascribed to a
polaron (Figure 3). Increasing the doping level at a higher
Figure 2. (left column) Electropolymerization of 6 (0.02 M in
DCM), 10 (0.01 M in DCM), and 11 (7 mM in DCM); (right
column) CVs of the corresponding polymer films in propylene
carbonate.
Figure 3. X-band EPR spectra of electrochemically doped poly-
11 (solid line) and PEDOT (dashed line) films on Pt wire. The
intensities are normalized by the doping charge.
(1.14 V for poly-6, 0.6 V for poly-10, 0.0 V for poly-11)
follows the trend of the HOMO energies of corresponding
monomers. Also, the electrochemical stability of the polymer
depends drastically on the structures of the constituting
monomers. Films of poly-6 were found to be rather unstable,
and completely and rapidly degraded during multiple CV
scans (in either propylene carbonate or benzonitrile solvents).
Poly-10 film was electrochemically stable in propylene
oxidative potential creates more polarons in the polymers
leading to higher intensity of the EPR signal, with little
change of the fwhm. In contrast, the films of PEDOT
prepared under identical conditions show a decrease of the
EPR intensity at higher doping level. These changes in
3232
Org. Lett., Vol. 11, No. 15, 2009

PEDOT can be rationalized by formation of low-spin
bipolarons,21 a well-known effect for linearly conjugated
polymers at a high doping level.22
The sole polaron nature of the charge carriers in poly-11
(throughout the tested potential window) was confirmed in
a UV-vis-NIR spectroelectrochemical experiment. A distinct
electrochromic switching between the neutral (red) and doped
(blue) polymer was observed (Supporting Information).
Gradual increase of the potential from 0 to +1 V extenuates
the absorption band at 480 nm and two new bands at λmax
680 and 1316 nm appear indicating formation of a polaron
state. The doped polymer became the major species at 0.8
V and the only species at 1.0 V. A characteristic isosbestic
point at 548 nm indicates that a clear transition between the
two distinctive species (neutral and radical cation/polaron)
takes places during the doping. At higher voltages, a gradual
decrease of all absorption bands started to occur, but no new
species with NIR absorbance (cf. bipolaron) were seen. This
observation is also consistent with the EPR data.
The observed special behavior of poly-11 is a result of
the topology of the TTB core which precludes direct
resonance conjugation between the arms. As a result, the
polarons are isolated on the bis-EDOT arms and cannot
recombine into a bipolaron. This is in sharp contrast to other
polythiophenes (and most conjugated polymers) where no
isosbestic point is observed in spectroelectrochemistry due
to bipolaron formation at higher potentials.23,24
Figure 4. Spectroelectrochemistry of the poly-11 film on ITO.
to terthienobenzene 6 and its π-extended star-shaped deriva-
tives 10 and 11. All three compounds and particularly EDOT-
functionalized 11 can be used as monomers to prepare high-
dimensional conjugated polymers by electrooxidation. The
spectroelectrochemical studies of poly-11 showed switching
between red- and blue-colored stable states and revealed a
distinctive isosbestic point characteristic of a clear transition
between a neutral and polaron state. Complete suppression
of the bipolaron formation (also confirmed by EPR spec-
troscopy) is interesting for the design of high-spin materials.
In summary, we demonstrated an efficient synthetic route
(16) During the preparation of this manuscript, an alternative synthesis
of TTB from trichlorotris(trimethylsilylethynyl)benzene was reported:
Kashiki, T.; Shinamura, S.; Kohara, M.; Miyazaki, E.; Takimiya, K.; Ikeda,
M.; Kuwabara, H. Org. Lett. 2009, 11, 2473.
Acknowledgment. This work was supported by NSERC,
CFI and Centre for Self-Assembled Chemical Structures.
D.F.P. acknowledges DuPont Young Professor award. O.L.
is thankful to FQRNT for PBEEE fellowship. We thank
Mykola Kondratenko and Dr. Fred Morin (McGill Univer-
sity) for initial experiments towards synthesis of 6 and EPR
measurements, respectively.
(17) Perepichka, I. F.; Perepichka, D. F.; Meng, H.; Wudl, F. AdV. Mater.
2005, 17, 2281.
(18) Zhang, X.; Matzger, A. J. Org. Chem. 2003, 98, 9813.
(19) Groenendaal, L.; Zotti, G.; Aubert, P.-H.; Waybright, S. M.;
Reynolds, J. R. AdV. Mater. 2003, 15, 855.
(20) Sakaguchi, H.; Matsumura, H.; Gong, H.; Abouelwafa, A. M.
Science 2005, 310, 1002.
(21) Domagala, W.; Pilawa, B.; Lapkowski, M. Elecrochim. Acta 2008,
53, 4580.
Supporting Information Available: Experimental details;
(22) Bre´das, J. L.; Street, G. B. Acc. Chem. Res. 1985, 18, 309.
(23) Sotzing, G. A.; Reynolds, J. R.; Steel, P. J. AdV. Mater. 1997, 9,
795.
1
variable temperature H NMR of 2; movie file showing
electrochromic switching of poly-11; EPR spectra; results
of DFT calculations. This material is available free of charge
via the Internet at http://pubs.acs.org.
(24) Suppression of bipolaron formation was recently shown in 3D cross-
linked polythiophenes with conjugation length (4 thiophene units) limited
by chain distortion. However, no isosbestic point was observed in the
spectroelecrtochemical experiment: Piron, F.; Leriche, P.; Mabon, G.; Grosu,
I.; Roncali, J. Electrochem. Commun. 2008, 10, 1427.
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