Synthesis of π-conjugated 2,2:6′,2″-terpyridine-substituted oligomers based on 3,4-ethylenedioxythiophene

Article abstract of DOI:10.1021/ol303512f

Dissymmetric π-conjugated monomers and oligomers incorporating 3,4-ethylenedioxythiophene (EDOT) units and bearing terpyridine end groups were synthesized in good yields through Vilsmeyer-Haak formylation followed by a reaction with 2-acetylpyridine in ba

Full text of DOI:10.1021/ol303512f

ORGANIC
LETTERS
2013
Vol. 15, No. 5
1028–1031
Synthesis of π‑Conjugated
2,2:60,200-Terpyridine-Substituted Oligomers
Based on 3,4-Ethylenedioxythiophene
€
Laure Fillaud, Gaelle Trippe-Allard, and Jean Christophe Lacroix*
ꢀ
ꢀ
Nano-Electro-Chemistry group, Univ Paris Diderot, Sorbonne Paris Cite, ITODYS,
UMR 7086 CNRS, 15 rue Jean-Antoine de Baıf, 75205 Paris Cedex 13, France
¨
lacroix@univ-paris-diderot.fr
Received December 21, 2012
ABSTRACT
Dissymmetric π-conjugated monomers and oligomers incorporating 3,4-ethylenedioxythiophene (EDOT) units and bearing terpyridine end groups
were synthesized in good yields through VilsmeyerꢀHaak formylation followed by a reaction with 2-acetylpyridine in basic media or, for the longest
oligomers, direct CꢀH bond arylation. Theyhave a low HOMOꢀLUMO gapand are easily oxidized at low potentials. Uponcomplexationwith cobalt(II)
and iron(II) they yield new hybrid materials that can be used in various applications ranging from photovoltaics to spintronics.
Conjugated polymers and oligomers based on thiophene
(T) are currently used in plastic and organic electronics as
organic semiconducting materials to fabricate various de-
vices such as field-effect transistors, light-emitting diodes,
and organic solar cells.1,2 Poly(3,4-ethylenedioxythiophene)
(PEDOT) and functionalized EDOT oligomers are other
widely used thiophene-based materials, characterized by a
lower oxidation potential and a smaller intrinsic band gap.3
Organometallic compounds incorporating π-conjugated
oligomers as ligands are of considerable interest in various
domains.4 Cobalt complexes have, for instance, been re-
cently used in Gratzel-type solar cells as alternatives to the
usual iodine-based redox shuttle system. Photovoltaic yields
as high as 12% have been reported.5 Molecular spintronics
is another growing research field in which organometallic
compounds incorporating extended π-conjugated systems
as ligands are used.6
In this paper, we will present the synthesis of several
oligomers (and some monomers) combining EDOT and
thiophene units, and bearing a terminal terpyridine (tPy)
moiety. Using suchligands, cobalt(II) andiron(II) complexes
have been obtained and characterized.
(1) (a) Zhu, Z. T.; Rogers, J. A. Organic Thin-Film Transistors:
Fundamental and Applied Aspects. Handbook of Organic Electronics
and Photonics ; American Scientific Publishers: 2008; Vol 1, p 225. (b)
Perepichka, I. F.; Perepichka, D. F. Handbook of Thiophene-Based
Materials; John Wiley & Sons: Chichester, U.K., 2009.
(2) (a) Roncali, J. Chem. Rev. 1997, 97, 173. (b) Amir, E.; Rozen, S.
Angew. Chem., Int. Ed. 2005, 44, 7374. (c) Mishra, A.; Ma, C. Q.;
Bauerle, P. Chem. Rev. 2009, 108, 1141. (d) Roquet, S.; Cravino, A.;
Leriche, P.; Aleveque, O.; Frere, P.; Roncali, J. J. Am. Chem. Soc. 2006,
(4) (a) Mangeney, C.; Lacroix, J. C.; Chane-Ching, K. I.; Jouini, M.;
Villain, F.; Ammar, S.; Jouini, N.; Lacaze, P. C. Chem.;Eur. J. 2001, 7,
5029. (b) Kubo, W.; Nagao, M.; Otsuba, Y.; Homma, T.; Miyata, H.
Langmuir 2009, 25, 13340.
ꢁ
128, 3459. (e) Lacroix, J. C.; Chane-Ching, K. I.; Maquere, F.; Maurel,
F. J. Am. Chem. Soc. 2006, 128, 7264.
(5) Yella, A.; Lee, H.-W.; Tsao, H. N.; Yi, C.; Chandiran, A. K.;
Nazeeruddin, M; K.; Diau, E. W.-G.; Yeh, C.-Y.; Zakeeruddin, S. M.;
Gratzel, M. Science 2011, 334, 629.
(6) (a) Tang, J.; Wang, Y.; Klare, J. E.; Tulevski, S.; Wind, S. J.;
Nuckolls, C. Angew. Chem. 2007, 179, 3966. (b) Parks, J. J.; Champagne,
A. R.; Costi, T. A.; Shum, W. W.; Pasupathy, A. N.; Neuscamman, E.;
Flores-Torres, S.; Cornaglia, P. S.; Aligia, A. A.; Balseiro, C. A.; Chan,
G. K. ꢀL.; Abruna, H. D.; Ralph, D. C. Science 2010, 328, 1370.
(3) (a) Akoudad, S.; Roncali, J. Chem. Commun. 1998, 2081. (b)
Huchet, L.; Akoudad, S.; Levillain, E.; Emge, A.; Bauerle, P.; Roncali, J.
J. Phys. Chem. B 1998, 102, 7776. (c) Thomas, C. A.; Zong, K.; Abboud,
K. A.; Steel, P. J.; Reynolds, J. R. J. Am. Chem. Soc. 2004, 126, 16440. (d)
Nielsen, C. B.; Angerhofer, A.; Abboud, K. A.; Reynolds, J. R. J. Am.
Chem. Soc. 2008, 130, 9734. (e) Trippe-Allard, G.; Lacroix, J. C.
Tetrahedron 2013, 69, 861.
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10.1021/ol303512f
Published on Web 02/19/2013
2013 American Chemical Society

One of the most simple methods of preparing tPy
oligomers is to convert a formyl oligomer by a one-pot
reaction with 2-acetylpyridine in the presence of base.7
Another possibility is to start from a short tPy-oligomer
and then to increase the conjugation length.8 We have
employed both methodologies to prepare various oligo-
mers bearing tPy.
Scheme 2. Synthesis of Formyl Oligomer from Dissymmetric
Oligomer Based on Thiophene and EDOT
Scheme 1. Formylation of Mixed Oligomer by VilsmeyerꢀHaak
Reaction
They are listed in Table 1 along with their UVꢀvisible
spectroscopic properties and oxidation potentials.
Several formyl oligomers were thus synthesized by
VilsmeyerꢀHaak formylation of the corresponding un-
substituted oligomers with phosphoryl chloride in DMF
after adaptation of a reported procedure.9 This method is
the most useful since it is totally selective for the R-position
ofthiophene. Themixedand symmetricterthiopheneTET,
incorporating two thiophenes and one central EDOT unit,
was converted in this manner to TET-CHO 1 in 91% yield
(Scheme 1). Bithiophene, EDOT, and bi-EDOT were also
transformed to their corresponding aldehydes by this
method.
Table 1. Mixed T/E with Terminal Aldehyde Function
In order to study the influence of the position of EDOT
in a terthienyl oligomer, we also prepared TTE-CHO 6 and
ETT-CHO 7. However, because of the lack of selectivity of
the VilsmeyerꢀHaak reaction, we were unable to obtain
such dissymmetric formyl terthienyl oligomers by this ap-
proach. Another two-step strategy, depicted in Scheme 2,
was therefore used. First, a short formyl oligomer was halo-
genated, and then this was coupled with another short
oligomer via a Stille or Suzuki cross-coupling reaction. Thus,
to prepare TTE-CHO 6, bithienyl stannane, TT-SnBu3,10
and 2-bromo-5-formyl-EDOT 8,11 previously prepared ac-
cording to reported procedures, were coupled by a Stille
reaction in poor yield (17%). For ETT-CHO 7, 2-formyl-
bithiophene 3 was first iodinated by mercury oxide and
iodine in 80% yield. Then, the resulting compound 9 gave
ETT-CHO 7 by Suzuki reaction with boronic EDOT in
satisfactory yield (63%).
a 10ꢀ5 M in ACN. b 10ꢀ3 M in 0.1 M LiClO4/ACN, 100 mV sꢀ1
.
3
In all, seven formyl oligomers were generated as inter-
mediate compounds in the synthesis of tPy oligomers.
All compounds show a strong absorption band between
290 and 450 nm due to πꢀπ* transitions. This band
position is strongly influenced by several structural param-
eters such as the conjugation length. Indeed, formyl-
EDOT 2 shows an absorption maximum below 300 nm;
those incorporating two units (2T-CHO 3, TE-CHO 4, 2E-
CHO 5) have absorption maxima between 300 and 400 nm
whereas those with three thiophene units absorb above
400 nm. The number of EDOT units is another important
factor. When it increases (from zero for 2T-CHO 3 to one
for TE-CHO 4, and two for 2E-CHO 5), a bathochromic
shift is observed (20 nm for one EDOT and 45 nm for two
EDOT). This behavior is due to the electron-donating
(7) For examples, see: (a) Cargill Thompson, A. M. W. Coord. Chem.
Rev. 1997, 160, 1. (b) Encinas, S.; Flamigni, L.; Barigelletti, F.;
Constable, E. C.; Housecroft, C. E.; Schofield, E. R.; Figgemeier, E.;
Fenske, D.; Neuburger, M.; Vos, J. G. Chem.;Eur. J. 2002, 8, 137.
(8) For examples, see: (a) Goodall, W.; Wild, K.; Arm, K. J.; Gareth
Williams, J. A. J. Chem. Soc., Perkin Trans 2 2002, 1669. (b) Houarner-
Rassin, C.; Blart, E.; Buvat, P.; Odobel, F. J. Photochem. Photobiol. A:
Chem. 2007, 186, 135. (c) Andreiadis, E. S.; Demadrille, R.; Imbert, D.;
ꢀ
Pecaut, J.; Mazzanti, M. Chem.;Eur. J. 2009, 15, 9458.
(9) Raimundo, J. M.; Blanchard, P.; Gallego-Planas, N.; Mercier, N.;
Ledoux-Rak, I.; Hierle, R.; Roncali, J. J. Org. Chem. 2002, 67, 205.
(10) Bair, J. S.; Harrison, R. G. J. Org. Chem. 2007, 72, 6653.
(11) Jessing, M.; Brandt, M.; Jensen, K. J.; Christensen, J. B.; Boas,
U. J. Org. Chem. 2006, 71, 6734.
Org. Lett., Vol. 15, No. 5, 2013
1029

Table 2. Mixed T/E Oligomer-tPy
Scheme 3. Direct CꢀH Arylation of Thiophene-EDOT
ability of the ethylenedioxy group on the thiophene ring,
which decreases the HOMOꢀLUMO gap of the mole-
cules. The EDOT position on the formylterthiophene also
affects the absorption maximum. When EDOT is located
in the center of the terthiophene, its electron-donating ability
is stronger than when it is far from the aldehyde function. For
TTE-CHO 6 and ETT-CHO 7, intramolecular S O inter-
3 3 3
actions are less numerous than for TET-CHO 1. Because of
this, the TET-CHO 1 oligomer possesses a more rigid
structure, inducing a higher bathochromic effect, than
TTE-CHO 6 or ETT-CHO 7.12 An electrochemical study
was also performed on these formyl oligomers. All com-
pounds were oxidized with an irreversible peak whose
position changed with the structure. The electrochemical
behavior depends on the same structural parameters as the
UVꢀvisible spectroscopic properties. Eox decreases when
the conjugation chain increases: the irreversible oxidation
peak is at 1.66 V/SCE for E-CHO 2, whereas it is around
1 V for formyl-terthiophene 6 (TTE-CHO) and decreases
to 0.9 V when the distance between the EDOT and the
formyl groups increases (TET-CHO 1 and ETT-CHO 7).
Moreover, the number of EDOT units exerts a marked
effect. Eox decreases by 0.21 V when the number of EDOTs
goes from one (TE-CHO 4) to two (2E-CHO 5).
Formyl oligomers were then used as reactants to achieve
a terpyridine chelating group. Starting from formyl oligo-
mers, 2,20:60,200-terpyridine (tPy) derivatives were synthe-
sized by a one-pot synthesis reported by Constable.7b,13
The formyl oligomer was heated at reflux for at least 16 h
with a slight excess of 2-acetylpyridine (2.03 equiv) in a
basic medium (KOH/NH3/ethanol). The corresponding
terpyridine was readily obtained by filtration. Thus, E-tPy
10, TE-tPy 11, 2E-tPy 12, TTE-tPy 13, TET-tPy 14, and
ETT-tPy 15 were obtained starting from E-CHO 2, TE-
CHO 4, 2E-CHO 5, TTE-CHO 6, TET-CHO 1, and ETT-
CHO 7, respectively, with yields between 28% and 66%.
An alternative to the use of formyl oligomers to synthe-
size terpyridine oligomers is to couple a terpyridine unit
with an aryl or a heteroaryl group. In this context, exam-
ples with Stille14 and Suzuki15 reactions are the most
reported. In the past few years, direct CꢀH arylation has
a 10ꢀ5 M in CH3CN. b 10ꢀ3 M in 0.1 M LiClO4/CH3CN, scan rate
100 mV sꢀ1
.
3
became more and more popular,16 and Borghese17 devel-
oped a new method to prepare oligomers based on EDOT.
In this method, EDOT or its derivatives react with a
halogeno-arene in the presence of palladium acetate. Thus,
the synthesis of organometallic precursors, required in the
usual cross-coupling reactions, is avoided. Among its
numerous advantages, direct CꢀH arylation allows, de-
pending on the reagent in excess, the synthesis of both
symmetrical and unsymmetrical oligomers. Nevertheless,
the few reported examples of monoarylation leading to
ꢁ
(12) Turbiez, M.; Frere, P.; Roncali, J. Tetrahedron 2005, 61, 3045.
(13) Constable, E. C.; Figgemeier, E.; Housecroft, C. E.; Kokatam,
S. L.; Medlycott, E. A.; Neuburger, M.; Schaffner, S.; Zampese, J. A.
Dalton Trans. 2008, 6752.
(14) Aspley, C. J.; Gareth Williams, J. A. New J. Chem. 2001, 25,
1136.
(16) (a) You, S. L.; Xia, J.-B. Top. Cur. Chem. 2010, 292. (b)
McGlacken, G. P.; Bateman, L. M. Chem. Soc. Rev. 2009, 38 (8), 2447.
(17) Borghese, A.; Geldhof, G.; Antoine, L. Tetrahedron Lett. 2006,
47, 9249.
(15) (a) Goodall, W.; Wild, K.; Arm, K. J.; Gareth Williams, J. A.
J. Chem. Soc., Perkin Trans. 2 2002, 1669. (b) Svoboda, J.; Stenclov, P.;
Uhlık, F.; Zednık, J.; Vohlıdal, J. Tetrahedron 2011, 67, 75.
(18) (a) Amaladass, P.; Clement, J. A.; Mohanakrishnan, A. K.
Tetrahedron 2007, 63, 10363. (b) Liu, C.-Y.; Zhao, H.; Yu, H.-H. Org.
Lett. 2011, 13, 4068.
1030
Org. Lett., Vol. 15, No. 5, 2013

ranging from 0.74 to 1.52 V/SCE, which depends on the
structure and the number of EDOT moieties. Each tPy
oligomer has an oxidation potential lower than that of the
corresponding formyl oligomer, with the tPy group being less
electron-withdrawing than the aldehyde functional group.
As an example of new hybrid materials with metallic
cores incorporating tPy oligomers, we have synthesized two
complexes by reacting E-tPy 10 with CoCl2 6H2O and
FeCl2 6H2O, followed by anion exchange, to yield [Co-
3
3
(E-tPy)2][PF6]2 20 and [Fe(E-tPy)2][PF6]2 22, respectively.
The Co(E-tPy)2][PF6]2 20 paramagnetic complex has been
studied using NMR (see Supporting Information (SI)). The
terpyridine signal appears before 100 ppm, which indicates
that this complex is low-spin at rt.22 Similar results were
obtained with [Co(2EB-tPy)2][PF6]2 21 (see SI).
Figure 1 compares the electrochemical properties of
Co(E-tPy)2 20 and [Fe(E-tPy)2] 22 with that of E-tPy 10.
The three systems exhibit an irreversible oxidation peak
corresponding to E-tPy 10 oxidation at 1.5 V/SCE. Re-
versible signals corresponding to the Co(III)/Co(II) and
Fe(III)/Fe(II) redox couples are observed at 0.15 and 0.95
V/SCE, respectively. The 0.15 V value for [Co(E-tPy)2]-
[PF6]2 20 is close to the standard redox potential of several
cobalt-based shuttles used in Gratzel-type solar cells. Note
also that the redox potential corresponding to Co(III)/
Co(II) in [Co(2EB-tPy)2][PF6]2 is around 0.2 V and does
not seem to be strongly affected by the oligomer length.
Suchcompoundscould alsopresent spintransition proper-
ties around or above rt, as seen with analogous cobalt(II)
complexes.23
In the present paper, we have obtained, in good yields,
dissymmetric π-conjugated oligomers incorporating 3,4-
ethylenedioxythiophene units and bearing terpyridine.
They have a low HOMOꢀLUMO gap and are oxidized
at low potentials. The shortest oligomers have been com-
plexed with cobalt(II) and iron(II). They yield new hybrid
materials that can be used in various applications ranging
from photovoltaics to spintronics.
Figure 1. (a) General scheme for complexation of cobalt(II) with
π-conjugated 2,2:60,200-terpyridine oligomers. (b) Electrochemi-
cal response of EtPy 10 (black), Co(EtPy)2(PF6ꢀ) 20 (blue), and
Fe(EtPy)2(PF6ꢀ) 22 (red). C = 1.0 ꢁ 10ꢀ3 M in 0.1 M LiClO4/
CH3CN, scan rate 100 mV sꢀ1
.
3
unsymmetrical derivatives gave poor to moderate yields
(35 to 55%) after 8 to 12 h of reaction.18 Catellani et al.19
achieved satisfactory yields (72ꢀ82%) by increasing the
reaction time (24 h). Terpyridine oligomers 2ETtPy 16 and
2EBtPy 17 were synthesized according to this methodology
(Scheme 3). First, 2-bromothienylterpyridine BrTtPy13 18
and 2-bromophenylterpyridine BrBtPy20 19 were prepared
according to reported procedures. A slight excess of bi-
EDOT (1.3 equiv) with the bromo-arene-terpyridine in
DMF in the presence of KOAc and Pd(OAc)2 was heated
at 80 °C for 1 h. Thus, 2ETtPy 16 and 2EBtPy 17 were
1
isolated in yields around 70%, and H NMR revealed no
trace of symmetrical compounds.
The UVꢀvisible spectra of the various molecules show a
strong absorption band between 288 and 458 nm due to
πꢀπ* transitions (Table 2). The maximum absorption
position is influenced by the same parameters as in the
formyl oligomers. The conversion of aldehyde to tPy has
an additional bathochromic effect (λmax = 368 and 398 nm
for TE-CHO 4 and TE-tPy 11, respectively), and band
Acknowledgment. This work was supported by the
ANR-08-blan-0186-01. I. Buchet-Maulien and Dr. J. S.
Lomas are warmly thanked for their help in this work.
Supporting Information Available. Synthesis proce-
dures and characterization data. This material is available
free of charge via the Internet at http://pubs.acs.org.
multiplicity appears due to the terpyridine function.21
A
preliminary electrochemical study was performed on these
tPy oligomers. They present anirreversible oxidation peak,
(22) Constable, E. C.; Housecroft, C. E.; Kulke, T.; Lazzrini, C.;
Schofield, E. R.; Zimmerman, Y. Dalton Trans. 2001, 2864.
(23) Hayami, S.; Komatsu, Y.; Shimizu, T.; Kamihata, H.; Lee, Y. H.
Coord. Chem. Rev. 2011, 255, 1981.
(19) Ca’, N. D.; Maestri, G.; Catellani, M. Chem.;Eur. J. 2009, 15,
7850.
(20) Han, F. S.; Higuchi, M.; Kurth, D. G. Org. Lett. 2007, 9 (4), 559.
(21) Schubert, U. S.; Hien, O.; Eschbaumer, C. Macromol. Rapid
Commun. 2000, 21, 1156.
The authors declare no competing financial interest.
Org. Lett., Vol. 15, No. 5, 2013
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