Synthesis of bridged oligothiophenes: Toward a new class of thiophene-based electroactive surfactants

Article abstract of DOI:10.1021/ol034398q

(Matrix presented) A series of bridged oligothiophenes have been synthesized. Their novel molecular architecture, comprising two oligothiophenes linked together by a bridge at one α-extremity and one binding group at the other α-extremity, is expected to improve their utility as electroactive surfactants for semiconducting nanoparticles or organic electronics.

Full text of DOI:10.1021/ol034398q

ORGANIC
LETTERS
2003
Vol. 5, No. 11
1879-1882
Synthesis of Bridged Oligothiophenes:
Toward a New Class of
Thiophene-Based Electroactive
Surfactants
,†
Carine Edder and Jean M. J. Fre´chet*
Department of Chemistry, UniVersity of California-Berkeley,
Berkeley, California 94720-1460, and Lawrence Berkeley National Laboratory,
Materials Science DiVision, Berkeley, California 94720
frechet@cchem.berkeley.edu
Received March 6, 2003
ABSTRACT
A series of bridged oligothiophenes have been synthesized. Their novel molecular architecture, comprising two oligothiophenes linked together
by a bridge at one r-extremity and one binding group at the other r-extremity, is expected to improve their utility as electroactive surfactants
for semiconducting nanoparticles or organic electronics.
Polythiophene-based solar cells currently attract much at-
tention.1 Indeed, their electronic properties such as relatively
large hole mobilities and absorption in the visible region2
make them good candidates as hole conductors and/or
sensitizers for such systems. Recently, the blend of the
conjugated polymer poly(3-hexylthiophene) (P3HT) with
CdSe nanoparticles was successfully used in an organic-
inorganic hybrid solar cell.1d Further improvements of this
system are likely to result if the interaction of the components
at the nanocrystal-polymer interface can be enhanced. A
first step in this direction was recently accomplished when
penta(3-hexylthiophene)-2-phosphonic acid was used as a
surfactant for CdSe nanoparticles.3 The strong affinity of the
phosphonic acid group for the CdSe surface allowed efficient
coating of the nanocrystals, resulting in enhanced solubility
in nonpolar solvents. Moreover, excitation to the CdSe
nanoparticles induced an efficient charge separation at the
interface, revealing the potential of an oligothiophene surf-
actant as a mediator component in polymer-nanocrystal
devices.
† www.frechet.com
(1) Selected references: (a) Gramstro¨m, M.; Petritsch, K.; Arias, A. C.;
Lux, A.; Andersson, M. R.; Friend, R. H. Nature 1998, 395, 257. (b)
Gebeyehu, D.; Brabec, C. J.; Sariciftci, N. S.; Vangeneugden, D.; Kiebooms,
R.; Vanderzande, D.; Kienberger, F.; Schindler, H. Synth. Met. 2002, 125,
279. (c) Grant, C. D.; Schwartzberg, A. M.; Smestad, G. P.; Kowalik, J.;
Tolbert, L. M.; Zhang, J. Z. J. Electroanal. Chem. 2002, 522, 40. (d) Huynh,
W. U.; Dittmer, J. J.; Alivisatos, A. P. Science 2002, 295, 2425. (e)
Camaioni, N.; Garlaschelli, L.; Geri, A.; Maggini, M.; Possamai, G.; Ridolfi,
G. J. Mater. Chem. 2002, 12, 2065.
During routine preparation,4 CdSe nanoparticles are com-
monly capped with organic ligands such as trioctyphosphine
oxide. Therefore, surfactants with a stronger affinity for the
(2) Handbook of Oligo- and Polythiophenes; Fichou, D., Ed; Wiley-
CH: Weinheim, Germany, 1999.
(3) Milliron, D. J.; Alivisatos, A. P.; Pitois, C.; Edder, C.; Fre´chet, J.
M. J. AdV. Mater. 2003, 15, 58.
10.1021/ol034398q CCC: $25.00 © 2003 American Chemical Society
Published on Web 05/08/2003

Scheme 1a
a Reagents and conditions (yield): (i) (a) NaH, THF, 0 °C; (b) 1-bromobutane, ∆ (84%). (ii) NBS, CH3CO2H (66%). (iii) (a) nBuLi,
THF, -78 °C, 90 min; (b) Me3SnCl (91%). (iv) (a) nBuLi, THF, -78 °C, 2 h; (b) ClP(O)(OEt)2 (63%). (v) (a) LDA, THF, -78 °C, 90
min; (b) Me3SnCl (89%). (vi) 1,9-Dibromononane, K2CO3, 18C6, THF, ∆, 3 days (89%). (vii) NBS, DMF, CHCl3 (8, 95%; 10, 96%; 12,
81%). (viii) PdCl2(PPh3)2, DMF, THF, ∆ (9, 68%; 11, 70%; 13, 34%; 14, 10%).
nanocrystals are necessary to allow an efficient displacement
of this insulating organic layer. Inspired by metal-ligand
coordination chemistry, several groups have begun investi-
gating the use of dithiol5 or diazaperylene6 surfactants,
expecting a chelate effect to favor the surfactant-CdSe
interaction. We have now designed a bidentate ligand
composed of two tethered oligothiophenes, each containing
one binding group. This molecular architecture is expected
to facilitate binding to and organization around the nano-
crystal. In particular, we expect favorable effects from the
intramolecular π-stacking interaction between the two oligo-
thiophene units of the bidentate molecule. The new design
of thiophene-based electroactive surfactants links two oligo-
thiophene backbones containing at least five thiophene units
by a bridge located at one R-extremity of each oligothiophene
moiety. Binding groups with a strong affinity for CdSe
nanoparticles are then introduced at the other R-extremities,
in direct conjugation with the oligothiophene moieties.
Finally, the solubility of the entire assembly is adjusted by
introducing one or more substituents on the oligothiophene
backbone.
Taking into account all these design features, compound
13 was selected as our first target (Scheme 1). A long (C9)
alkyl chain was chosen for the bridge as it adds both
solubility and flexibility to the molecule. The ester group at
the R-position not only plays the role of a linker to the bridge,
but also acts as a protecting group at this position, allowing
the oligomer chain to grow exclusively from the other
R-position.7 The other R-extremity was functionalized
with a diethylphosphonate group, useful in its own right
but also a potential precursor of the strongly binding
phosphonic acid moiety.8 Finally, instead of the alkyl chains
commonly chosen for solubility,2 we substituted alkoxy
chains since their higher polarity facilitated chromatographic
purification.
Our synthetic approach relies upon the synthesis of three
building blocks: the bridged bis(bithiophene) (Scheme 1,
7), the â-substituted monothiophene (1), and the bithiophene
bearing the diethylphosphonate at one of its R-positions (4).
(4) (a) Peng, X.; Wickham, J.; Alivisatos, A. P. J. Am. Chem. Soc. 1998,
120, 5343. (b) Manna, L.; Scher, E. C.; Alivisatos, A. P. J. Am. Chem.
Soc. 2000, 122, 12700.
(5) (a) Mattoussi, H.; Mauro, J. M.; Goldman, E. R.; Anderson, G. P.;
Sundar, V. C.; Mikulec, F. V.; Bawendi, M. G. J. Am. Chem. Soc. 2000,
122, 12142-50. (b) Pathak, S.; Choi, S.-Y.; Arnheim, N.; Thompson, M.
E. J. Am. Chem. Soc. 2001, 123, 410. (c) Aldana, J.; Wang, Y. A.; Peng,
X. J. Am. Chem. Soc. 2001, 123, 8844.
(7) (a) Malenfant, P. R. L.; Groenendaal, L.; Fre´chet, J. M. J. J. Am.
Chem. Soc. 1998, 120, 10990. (b) Kirschbaum, T.; Azumi, R.; Mena-
Osteritz, E.; Ba¨uerle, P. New J. Chem. 1999, 241.
(6) Schmelz, O.; Mews, A.; Basche´, T.; Herrmann, A.; Mu¨llen, K.
Langmuir 2001, 17, 2861.
(8) McKenna, C. E.; Higa, M. T.; Cheung, N. H.; McKenna, M.-C.
Tetrahedron Lett. 1977, 2, 155.
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Org. Lett., Vol. 5, No. 11, 2003

(sexithiophene) 14, we were interested in modifying com-
ponents of our system, i.e., backbone and bridge, to obtain
a variety of bidentate systems displaying different electronic
and geometric properties. For this purpose, a benzene ring
was introduced in the conjugated backbone, as in 18 (Scheme
2), and the â-substituent was modified, as in 22 (Scheme
3). Only four additional steps were necessary to obtain 18
Scheme 2a
Scheme 3a
a Reagents and conditions (yield): (i) (a) Mg (1.1 equiv), THF;
(b) ClP(O)(OEt)2 (22%). (ii) 5-(Trimethylstannyl)-2,2′-bithiophene,
PdCl2(PPh3)2, DMF, THF, ∆ (69%). (iii) (a) LDA (2 equiv),
TMEDA, THF, -78 °C, 90 min (99%); (b) Me3SnCl. (iv) 10,
PdCl2(PPh3)2, DMF, THF, ∆ (10%).
A Stille-type coupling reaction was chosen to connect the
thiophene subunits, because of its compatibility with both
carbo- and phosphoester groups. Side reactions such as
homocoupling and loss of functionalization are inherent to
metal-catalyzed cross-coupling reactions; therefore, isolation
of pure oligothiophenes is often tedious and requires
extensive chromatographic purification.2 Our synthetic strat-
egy takes advantage of the polarity differences of the three
building blocks to simplify purification. Thus, the bis-
(pentathiophene) 13 was prepared according to the strategy
described in Scheme 1. Regioselective bromination of 1 using
N-bromosuccinimide (NBS), followed by transmetalation
with n-butyllithium (nBuLi) and subsequent quenching with
trimethyltin chloride, afforded 3. The phosphonate 4 was
obtained by lithiation of 2,2′-bithiophene and quenching of
the anion with diethylchlorophosphate. The tin reagent 5 was
similarly prepared by lithiation of 4 with lithium diiso-
propylamide (LDA), followed by quenching with trimethyltin
chloride. The bridged bis(bithiophene) 7 was easily obtained
by reaction of 2,2′-bithiophene carboxylic acid 67a with 1,9-
dibromononane in the presence of 18-crown-6. Bromination
of 7 at the R-position using NBS gave the dibrominated
compound 8, which was then coupled with the tin reagent 3
to afford 9. Bromination of this compound, followed by
coupling with 5, finally gave the bis(pentathiophene) 13. The
bis(sexithiophene) 14 was prepared from 10 using a second
sequence of coupling-bromination reactions as applied for
10, followed by a final Stille-coupling reaction with 5.
Having demonstrated the feasibility of our synthetic
strategy with 13 and its extension to the longer bis-
a Reagents and conditions (yield): (i) (a) nBuLi, THF, -78 °C
to rt, 1 h; (b) Me3SnCl (89%). (ii) PdCl2(PPh3)2, DMF, THF (70%).
(iii) NBS, DMF, THF (98%). (iv) 5, PdCl2(PPh3)2, DMF, THF, ∆
(57%).
from 10 and p-dibromobenzene. The phosphonate 15 was
prepared by reacting the Grignard reagent of p-dibromo-
benzene with diethylchlorophosphate. Cross-coupling reac-
tion of 15 with 5-(trimethylstannyl)-2,2′-bithiophene9 gave
16. The stannate 17 was prepared by quenching the LDA-
generated anion of 16 with trimethyltin chloride. Coupling
with 10 finally gave the bis(pentathiophene) 18. The low
yield associated with this reaction, as observed for 14,
suggests either a solubility problem or an intramolecular
interaction between the two oligomer units that precludes
an efficient heterocoupling reaction and leads to the forma-
tion of a significant amount of byproducts.
Replacement of the alkoxy chain in 13 by the strong donor
group present in 22 is expected to significantly affect the
(9) Groenendaal, L.; Bruining, M. J.; Hendrickx, E. H. J.; Persoons, A.;
Vekemans, J. A. J. M.; Havinga, E. E.; Meijer, E. W. Chem. Mater. 1998,
10, 226.
Org. Lett., Vol. 5, No. 11, 2003
1881

lithiation of 3,4-ethylenedioxythiophene with nBuLi and
subsequent quenching with trimethyltin chloride. It was then
coupled with 8 to afford 20. Bromination, followed by
coupling reaction with 5, finally gave the new bridged
molecule 22.
Scheme 4a
A more rigid bridge may be necessary to avoid the
intermolecular cross-linking of two nanoparticles by the same
surfactant. The m-xylene linker used in 27 is more predis-
posed to provide the desired conformation to the molecular
system than the long alkyl chain previously used, without
being too rigid, as in the bis(quaterthiophene) systems based
on 1,8-naphthalene linker.10 The new molecule 27 was
prepared by esterification of 6 with R,R′-dibromo-m-xylene,
followed by two successive bromination/coupling reactions,
according to the same approach as described for 13 (Scheme
4).
In summary, the synthetic strategy described in this paper
has been successfully used to obtain a variety of oligothio-
phenes, displaying a novel molecular architecture based on
the linkage of twin oligothiophenes in the same molecule.
Structural modifications of the backbone or the bridge group
enables the tuning of the electronic and geometric properties
of these bridged molecules. Further investigation of these
systems involves their application as surfactants for nano-
crystals and electroactive components for organic-based
electronics and photovoltaic systems.
Acknowledgment. Support of this work by the U.S.
Department of Energy under grant DE-AC03-76SF00098 is
gratefully acknowledged. C.E. also thanks the Swiss National
Science Foundation for partial fellowship support.
Supporting Information Available: Complete details for
the synthesis and characterization of compounds 1-5 and
7-27. This material is available free of charge via the
Internet at http://pubs.acs.org.
a Reagents and conditions (yield): (i) 6, K2CO3, 18C6, THF, ∆,
3 days (82%). (ii) NBS, DMF, CHCl3 (24, 93%; 26, 93%). (iii) 3,
PdCl2(PPh3)2, DMF, THF, ∆ (75%). (iv) 5, PdCl2(PPh3)2, DMF,
THF, ∆ (11%).
OL034398Q
electronic properties of this molecule, which is prepared as
(10) Kuroda, M.; Nakayama, J.; Hoshino, M.; Furusho, N.; Kawata, T.;
shown in Scheme 3. The stannate 19 was obtained by
Ohba, S. Tetrahedron 1993, 49, 3735.
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Org. Lett., Vol. 5, No. 11, 2003