Synthesis and supramolecular organization of regioregular polythiophene block oligomers

Article abstract of DOI:10.1021/jo902490m

Chemical Equation Presented The self-assembly of functional polythiophenes was studied by a bottom-up approach from molecule to polymer . The synthesis and the X-ray structure of 2, 5-dibromo-3-styryl and 32′,3′,4′,5′,6′-pentafluorostyryl-thiophenes revealed a supramolecular arrangement controlled by ν-ν interactions between the aromatic rings. A [2 + 2] photocyclization reaction in the solid state of (E)-1-2ν,5ν-dibromo-3ν-thienyl-2-pentafluorophenylethene triggers the formation of a rare cycloadduct comprising thiophene rings. The X-ray analysis confirmed its rctt stereochemistry. The synthesis of P3HT-b-P3ST and P3HT-b-P3STF block oligomers was achieved by a GRIM method in good yields. An indirect proof of well-defined nanostructured organization was provided by the partial photocyclization of pendant styryl moieties under UV irradiation.

Full text of DOI:10.1021/jo902490m

pubs.acs.org/joc
Synthesis and Supramolecular Organization of Regioregular
Polythiophene Block Oligomers
Sebastien Clement,†,‡ Franck Meyer,† Julien De Winter,†,# Olivier Coulembier,*,†
Christophe M. L. Vande Velde, Matthias Zeller,^ Pascal Gerbaux,# Jean-Yves Balandier,‡
Sergey Sergeyev,‡,z Roberto Lazzaroni,§ Yves Geerts,‡ and Philippe Dubois*,†
ꢀ
ꢀ
†Laboratory of Polymeric and Composite Materials, Center of Innovation and Research in Materials and
Polymers (CIRMAP), University of Mons UMONS, Place du Parc 20, 7000 Mons, Belgium, ‡Laboratoire de
ꢁ
ꢀ
ꢀ
Chimie des Polymeres, CP206/1 Universite Libre de Bruxelles, Boulevard du triomphe, Faculte des Sciences,
1050 Bruxelles, Belgium, §Laboratory for Chemistry of Novel Materials, CIRMAP, University of Mons UMONS,
Place du Parc 20, 7000 Mons, Belgium, Department of Applied Science, Karel de Grote University College,
Department of Applied Engineering, Salesianenlaan 30, 2660 Antwerp, Belgium, ^Department of Chemistry,
Youngstown State University, One University Plaza, Youngstown, Ohio 44555-3663, #Organic Chemistry
Laboratory, Mass Spectrometry Center, University of Mons UMONS, Avenue Maistriau 19, 7000 Mons Belgium,
z
and Organic Synthesis, Department of Chemistry, University of Antwerpen, Groenenborgerlaan 171,
2020 Antwerpen, Belgium
philippe.dubois@umons.ac.be; olivier.coulembier@umons.ac.be
Received November 25, 2009
The self-assembly of functional polythiophenes was studied by a bottom-up approach “from
molecule to polymer”. The synthesis and the X-ray structure of 2,5-dibromo-3-styryl and 3-
20,30,40,50,60-pentafluorostyryl-thiophenes revealed a supramolecular arrangement controlled by
π-π interactions between the aromatic rings. A [2 þ 2] photocyclization reaction in the solid state
of (E)-1-20,50-dibromo-30-thienyl-2-pentafluorophenylethene triggers the formation of a rare cyclo-
adduct comprising thiophene rings. The X-ray analysis confirmed its rctt stereochemistry. The
synthesis of P3HT-b-P3ST and P3HT-b-P3STF block oligomers was achieved by a GRIM method in
good yields. An indirect proof of well-defined nanostructured organization was provided by the
partial photocyclization of pendant styryl moieties under UV irradiation.
Introduction
future developments of photolithography and related
methods. Thus, the “bottom-up” approach has arisen as a
rational alternative to the economic and technological chal-
lenges faced by the semiconductor industry.2 The design of
tridimensional structures could afford new successful path-
ways via the accurate scaling of polymer nanodevices.
During the past few years, π-conjugated polymers and
more specifically polythiophenes have attracted tremendous
interest, stimulated by their potential applications in the deve-
lopment of new systems such as light-emitting diodes (LEDs),
electrochromic displays, field-effect transistors (FETs), and
more recently, photovoltaic devices.3 The performance of
Engineering of advanced materials at the micro- and
nanoscale is a permanent requirement for the construction
of next-generation polymer nanodevices. In this regard, the
microfabrication of integrated circuits is mainly ensured by
photolithography, this conventional method consisting in
the transfer of a geometric shape from a photomask to a
light-sensitive compound. Over the past decades, litho-
graphic techniques have dramatically enhanced the pattern
resolution and reached an unprecedented degree of minia-
turization down to 30-40 nm.1 However, efforts toward
further downscaling are time-consuming and might limit the
(2) Lu, W.; Lieber, C. M. Nat. Mater. 2007, 6, 841–850.
(1) (a) Garcia, R.; Martinez, R. V.; Martinez, J. Chem. Soc. Rev. 2006, 35,
29–38. (b) Gratton, S. E. A.; Williams, S. S.; Napier, M. E.; Pohlhaus, P. D.;
Zhou, Z.; Wiles, K. B.; Maynor, B. W.; Shen, C.; Olafsen, T.; Samulski, E. T.;
DeSimone, J. M. Acc. Chem. Res. 2008, 41, 1685–1695. (c) Lim, Y.-B.; Lee,
M. Angew. Chem., Int. Ed. 2009, 48, 2–6.
(3) (a) McCullough, R. D.; Ewbank, P. C. Handbook of Conducting Poly-
mers; Marcel Dekker: New York, 1997. (b) Handbook of Oligo- and Poly-
thiophenes; Fichou, D., Ed.; Wiley-VCH: Weinheim, 1999. (c) Osaka, I.;
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DOI: 10.1021/jo902490m
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Clement et al.
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these semiconducting materials is closely related to the
electronic interactions between the polymer chains in the
solid state, which in turn depends on their supramolecular
organization.4 Indeed, the combination of well-ordered π-
conjugated structures and a homogeneous domain distribu-
tion is required for the formation of highly efficient electro-
nic devices at the macroscopic level. Thus, the rational design
of starting building blocks should impart a highly ordered
pattern to the functional macromolecular organization by a
bottom-up control. Consequently, this process should sig-
nificantly improve the physical properties of new organic
electronic materials. In this context, the supramolecular
concept offers new alternatives for the engineering of such
systems and their integration into cost-effective processes.
The advent of supramolecular chemistry has opened new
perspectives for the fabrication of such functional materials.5
A wide variety of intermolecular noncovalent interactions
(π-π stacking, hydrogen bonding, halogen bonding, or
metal-ligand coordination) has emerged in the supramole-
cular toolbox and represents diverse options at the chemist’s
disposal.6 Extension of these supramolecular concepts to
polymeric materials leads to their self-assembly into con-
trolled supramolecular architectures through recognition
processes.7 Block copolymers are well-known examples of
self-assembling systems, which rely upon microphase separa-
tion of chemically distinct blocks into nanoscale periodic
domains.8 The resulting arrangements depend on the relative
length and incompatibility of the polymer blocks, the con-
nectivity constraints, and the type of interactions.
In the quest for high performance organic materials, the
engineering of well-organized nanodevices is thereby under
development in numerous research groups. In this contribu-
tion, we present the synthesis of 2,5-dibromo-3-styrylthio-
phene and 1-20,50-dibromo-30-thienyl-2-pentafluorophenyl-
ethene and their organization at the molecular scale. π-π
interactions between aromatic rings exert a high level of
control over the crystalline arrangement, which is illustrated
by the conversion of (E)-1-20,50-dibromo-30-thienyl-2-penta-
fluorophenylethene into the corresponding cycloadduct by a
[2 þ 2] photocyclization reaction in the solid state. Subse-
quent polymerizations between the 3-hexylthiophene and
styrylthiophene derivatives have been carried out by the
Grignard metathesis (GRIM) methodology. GRIM is a syn-
thesis method for regioregular aromatic polymers recently
developed by McCullough et al.9 and further modified by
Yokozawa et al.10 One interesting feature of GRIM is its
quasi-living chain growth mechanism,11 which could lead to
the possibility of developing a regioregular block copolymer
with a narrow PDI in one pot. In this study, well-defined dib-
lock structures with self-complementary cores, i.e., poly(3-
hexylthiophene)-b-poly(3-styrylthiophene) (P3HT-b-P3ST)
and poly(3-hexylthiophene)-b-poly(3-20,30,40,50,60-pentafluo-
rostyrylthiophene) (P3HT-b-P3STF), were synthesized via
GRIM polymerization. The incorporation of poly(3-hexyl-
thiophene) (P3HT) blocks is motivated by its remarkable
physical properties, namely, a high hole-mobility supple-
mented by a high solubility in a variety of organic solvents.
These properties have been associated with their molecular
organization in the solid state. Recently, similar π-stacked
molecular assemblies have been reported for polythiophene
with acetylenic side chains or π central systems (fused
thiophene, pyridazine, etc.) leading to a hole mobility larger
than that of P3HT.12 On the basis of these findings, our block
oligomers with the pendant styryl moieties should exhibit a
strong tendency to form a π-stacked molecular arrangement.
This effective supramolecular organization of block oligo-
mers P3HT-b-P3ST and P3HT-b-P3STF by π-π stacking is
demonstrated by concomitant covalent bond formation
under UV irradiation in the solid state, this strategy provid-
ing an indirect evidence of well-ordered structures.
Results and Discussion
Synthesis of the Monomers. 3-Styrylthiophene monomers
2 and 3 were synthesized according to conventional synthetic
routes as illustrated in Scheme 1. A Wittig reaction between
2,5-dibromo-3-thiophenecarboxaldehyde 1 and benzyltri-
phenylphosphonium bromide in the presence of DBU pro-
vides 2,5-dibromo-3-styrylthiophene 2 as a mixture 1.5:1 of
E/Z isomers (Scheme 1).13
1
The amount of Z isomer was determined by H NMR
spectroscopy by integration of peaks at 6.69 and 6.33 ppm
(3JH-H = 12.0 Hz) assigned to the vinylic protons of the cis
isomer, with respect to the doublets at 7.00 and 6.89 ppm
(3JH-H = 16.5 Hz) for the trans double bond. Subsequent
treatment with iodine for 7 days at room temperature
allowed the isomerization into the (E)-2,5-dibromo-3-styryl-
thiophene 2. However, the presence of a residual amount of
Z isomer (95:5 conversion ratio of E/Z olefins 2) required a
further purification by chromatography to give the (E)-
isomer 2 in pure form (Figures S4 and S5 in Supporting
Information). For the fluorinated monomer 3, perfluoro-
benzyltriphenylphosphonium bromide was used as Wittig
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SCHEME 1. Synthesis of 3-Styryl Monomers 2 and 3
reagent. When 2,5-dibromo-3-thiophenecarboxaldehyde 1
reacted with the phosphonium salt in the presence of sodium
hydride, the (E)-1-20,50-dibromo-30-thienyl-2-pentafluoro-
phenylethene 3 was formed in 78% yield as a single isomer.
The facile crystallization of compounds 2 and 3 convinced
us to study their crystal packing by single crystal X-ray
diffraction. The crystallographic data of (E)-2,5-dibromo-
3-styrylthiophene 2 revealed a nearly planar conformation;
the thiophene and phenyl rings are slightly rotated relative to
each other by an angle of 4.5(2)ꢀ.
FIGURE 2. View down the crystallographic a axis of the non-
centrosymmetric organization of compound 2. Colors are as fol-
lows: gray, carbon; white, hydrogren; brown, bromine; orange,
sulfur atoms.
groups possess an opposite dipole moment with respect to
their hydrogenated counterparts, and consequently the mo-
lecules self-assemble through strong π-donor/π-acceptor
interactions. The slow evaporation of a chloroform solution
of compound 3 provides suitable material for a single crystal
X-ray analysis. The molecules are almost planar (3.1(1)ꢀ
between the aromatic rings) and the stronger π-π interac-
tions between the thiophene and pentafluorophenyl groups,
as well as the smaller difference in width between the two
groups as compared to 2, now pairs the molecules in a head-
to-tail mode. At the supramolecular level, the pairs of
molecules again stack in columns with distances between
FIGURE 1. Columnar arrangement in a head-to-head mode of
compound 2. Colors are as follows: gray, carbon; white, hydrogen;
brown, bromine; orange, sulfur atoms.
From the supramolecular point of view, the small differ-
ence in polarity combined with the large difference in width
of the two aromatic groups induces a columnar organization
in a head-to-head fashion in the crystals of 2 dominated by
π-π interactions, as shown in Figure 1. The efficient π-π
stacking leads to a close parallel packing of the olefins with a
repeat intermolecular distance between the double bond
˚
double bond centers of 3.646(3) A within pairs and
˚
3.733(3) A between pairs (Figure 3). This supramolecular
arrangement is well illustrated in the molecular packing
diagram of 3 in Supporting Information (Figure S6).
This arrangement results in a packing that is similar to that
of 2 with every second molecule in the columns inverted,
leading to the space group P21/c. In this structure, a number
of contacts shorter than the van der Waals radii exist (i.e.,
˚
centers of 3.94 A. Moreover, Br S intermolecular dis-
3 3 3
˚
tances of 3.82 A are also observed and contribute to the
stabilization of the supramolecular scaffold.
Interestingly, the centroid-centroid separation between the
˚
phenyl and thiophene rings is exactly the same (3.94 A), which
˚
two F Br intermolecular distances of 3.22 and 3.33 A),
3 3 3
confirms the perfect superposition of successive molecules.
A second relevant feature lies in an unusual supramolecular
arrangement, namely, a noncentrosymmetric order and macro-
scopic polarity; adjacent molecular columns adopt a nearly
perpendicular orientation in herringbone geometry (Figure 2).
In (E)-1-20,50-dibromo-30-thienyl-2-pentafluorophenyleth-
ene 3, the presence of the perfluorinated ring was expec-
ted to change the supramolecular organization dramati-
cally. Indeed, it is well-established that pentafluoroaromatic
indicating both the strength of the π-π interactions and the
less ideal fit of the molecules in the plane due to the larger size
of the fluorinated ring.
Thus, supramolecular scaffolds formed by the thiophene
units 2 and 3 confirm their effective association owing to
π-π interactions. The incorporation of styrylthiophene
units in the oligomer backbone is thereby expected to be
a reliable approach for controlling the self-assembly of
oligomers via π-stacked groups. Moreover, the introduction
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SCHEME 2. Photocyclization Reaction in the Solid State of Olefin 3
of well-adjusted pending double bonds along the oligomeric
chain represents an original strategy to lock the system
through covalent bond formation mediated by photoirradia-
tion. This methodology would ensure a fine supramolecular
control of the architecture and prevent any segregation of
block co-oligomers.
between the π-stacked groups, which fits with the topochemical
18
˚
postulate, i.e., a typical cutoff distance of 3.9 A.
As mentioned before, the structural organization of styryl-
thiophene derivatives 2 and 3 exhibits a distance between
˚
the double bonds of less than 4.2 A, which fits the require-
˚
ment of Schmidt’s rules (i.e., 3.94 A in sample 2, 3.65 and
˚
3.73 A in sample 3). Accordingly, it has been decided to study
the ability of olefin 3 to undergo a [2 þ 2] cycloaddition
reaction in the solid state under UV irradiation. Finely
powdered crystals of 3 were placed between two glass plates
and photoirradiated with a UV lamp at 340 nm for 24 h
(Scheme 2).
1
The H NMR analysis of the resulting yellowish powder
has revealed unequivocally a nearly quantitative photocycli-
zation by the vanishing of the peaks initially present at 7.32
and 6.77 ppm and the appearance of a new peak at 4.81 ppm
corresponding to the cyclobutane protons (Figure 4).19 This
unique signal confirmed the creation of one single stereoiso-
mer 4 with the cis, trans, trans (rctt) geometry.
X-ray diffraction analysis of a single crystal of cyclo-
adduct 4 (grown by slow evaporation of a CHCl3 solution)
has provided a clear confirmation of the rctt arrangement of
the cyclobutane ring, according to the topochemical expec-
tation for the E isomer of the carbon-carbon double bond.
Consistent with this geometry, the pairs of pentafluoroben-
zene and thiophene groups point to opposite directions with
respect to the plane of the cyclobutane ring (Figure 5).
FIGURE 3. Columnar arrangement in a head-to-tail fashion of
compound 3. Colors are as follows: gray, carbon; white, hydrogen;
brown, bromine; yellow, fluorine; orange, sulfur atoms.
Photodimerization. The [2 þ 2] photocyclization reaction
in the solid state represents a distinctive mode of covalent
bond formation in a regio- and stereoselective manner.14
This solventless approach affords highly valuable products
in high yields with low impact on the environment, also as a
result of the reduced formation of byproducts. Specific
geometric criteria postulated by Schmidt have outlined the
requirements that two double bonds should meet for under-
going such a reaction,15 and further investigations have
depicted the topochemical principles for evaluating the
possible occurrence of such dimerizations.16 Thus, a pair of
olefinic bonds should be arranged approximately parallel to
Five H F intramolecular contacts involving the fluorine
3 3 3
atoms in ortho position of the pentafluorobenzene group
and hydrogen atoms belonging to the cyclobutane ring,
˚
˚
namely, H6 F9 (2.35 A), H7 F13 (2.40 A), H27
3 3 3 3 3 3
3 3 3
˚
˚
˚
F35 (2.33 A), H27 F9 (2.44 A), and H26 F29 (2.53 A),
3 3 3 3 3 3
are observed. This peculiar conformation of the molecule
results in a dumbbell shape that is notoriously difficult to
pack. As a result, shape requirements take precedence, and no
π-π interactions between the thiophene and pentafluorophe-
nyl rings are observed; a sharp contrast with respect to the
starting building block 3. The molecular packing diagram of 4
is provided for comparison with 3 in Supporting Information
(Figure S7). To the best of our knowledge, this resulting 4,40-
[(1R,2R,3β,4β)-1,3-bis(2,5-dibromo-3-thienyl)-2,4-bis(penta-
fluoro-phenyl)]cyclobutane 4is the first 3-thiophene-substituted
cyclobutane reported to date and constitutes a very rare type of
cyclobutane derivative containing a thiophene heterocycle.20
For monomer 2, UV irradiation was performed under the
same conditions. After 24 h, the white compound 2 turned
brown but the crystalline morphology remained consistent.
The 1H NMR analysis of the crude material confirms a
˚
one another at a distance in the range of 3.5-4.2 A, as
observed in compounds 2 and 3. Interestingly, π-π interac-
tions occurring between a pair of aromatic rings have found
some applications as a driving force for potentially photoreac-
tive structures.17 This functional aspect lies in the distance
(14) Nagarathinam, M.; Peedikakkal, A. M. P.; Vittal, J. J. Chem.
Commun. 2008, 5277–5288.
(15) Schmidt, G. M. J. Pure Appl. Chem. 1971, 27, 647–678.
(16) Ramamurthy, V.; Venkatesan, K. Chem. Rev. 1987, 87, 433–481.
(17) Vedernikov, A. I.; Kuzmina, L. G.; Sazonov, S. K.; Lobova, N. A.;
Loginov, P. S.; Churakov, A. V.; Strelenko, Y. A.; Howard, J. A. K.;
Alfimov, M. V.; Gromov, S. P. Russ. Chem. Bull., Int. Ed. 2007, 56, 1860–
1883.
€
(19) Nattinen, K. I.; Rissanen, K. Cryst. Growth Des. 2003, 3, 339–353.
(20) Amato, M. E.; Musumarra, G.; Scarlata, G.; Lamba, D.; Spagna, R.
J. Crystallogr. Spectrosc. Res. 1989, 19, 791–808.
(18) Coates, G. W.; Dunn, A. R.; Henling, L. M.; Ziller, J. W.; Lobkovsky,
E. B.; Grubbs, R. H. J. Am. Chem. Soc. 1998, 120, 3641–3649.
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FIGURE 5. X-ray structure of cycloadduct 4 obtained by photo-
cyclization of olefin 3; the structure is locked by steric hindrance.
Colors are as follows: gray, carbon; white, hydrogen; brown,
bromine; yellow, fluorine; orange, sulfur atoms. Blue dotted lines
represent short H F contacts.
3 3 3
Recently, a similar approach has been developed to attach
electroactive donor-acceptor polymeric blocks, using inter-
molecular π-π interactions in a concerted manner.21 On the
basis of this concept, it was decided to exploit the styryl
monomers 2 and 3 as building units for the formation of self-
complementary supramolecular oligomers. We selected to
synthesize low molecular weight block co-oligomers based
on poly(3-styrylthiophene) (P3ST) or poly(3-20,30,40,50,60-pen-
tafluorostyrylthiophene) (P3STF) linked to a poly(3-hexyl-
thiophene) (P3HT) sequence in a 1:1 ratio of P3HT/P3ST
and P3HT/P3STF. As previously stated, the incorporation
of the P3HT segment was motivated by a combination of
physical properties such as good solubility, chemical stabi-
lity, and excellent electronic properties, which make P3HT
one of the most attractive building blocks for elaborating
conjugated block co-oligomers.22 The preparation of poly(3-
hexylthiophene)-b-poly(3-styrylthiophene) (P3HT-b-P3ST)
5 and poly(3-hexylthiophene)-b-poly(3-20,30,40,50,60-penta-
fluorostyrylthiophene) (P3HT-b-P3STF) 6 has been carried
out by GRIM-type polymerization. This synthetic method
represents an efficient strategy for the construction of re-
gioregular polythiophenes combining soft and safe operating
conditions.9,10,23 Moreover the “living” chain growth me-
chanism of the GRIM reaction might afford straightforward
access to regioregular block co-oligomers according to a one-
pot procedure.22c,24
FIGURE 4. 1H NMR spectra of olefin 3 (upper) and cycloadduct 4
(lower) in CDCl3 at room temperature.
cycloaddition reaction by the appearance of a peak at 4.4
ppm (cyclobutane protons), but the photoreactive olefin 2
was converted into the corresponding cycloadduct in low
yields (about 20%, determined by 1H NMR). This decrease
in the rate of the photoreaction of 2 is consistent with the
increase in the distance between the two ethylenic bonds in
˚
˚
the crystal structure of 2 (3.94 A in 2 vs 3.65 and 3.73 A in 3)
and might be assigned to a change in the intermolecular
distance as the reaction proceeds.19 The variation of the
experimental parameters such as the wavelength and the
reaction time did not induce any higher yield, but investiga-
tions aiming at the optimization of this reaction are far
beyond the scope of this article. However, this partial [2 þ 2]
photocyclization reaction of unit 2 also confirmed the ability
of 3-styrylthiophene to react under UV irradiation.
Prior to any copolymerization, some preliminary homo-
polymerization reactions of 2 and 3 have been attempted.
Despite the efficiency of the GRIM process, the resulting
poly(3-styrylthiophene) (P3ST) and poly(3-20,30,40,50,60-
pentafluorostyrylthiophene) (P3STF) proved highly insolu-
ble even for polymerization degrees (DP) lower than 20,
making any spectroscopic and chromatographic analysis
impossible. This insolubility led us to associate P3ST and
Synthesis of Block Oligomers. The crystal structures of
olefins 2 and 3 have demonstrated that a π-π interaction
between the aromatic and thiophene rings is capable of self-
assembling the compounds in columnar systems with great
reliability and predictability. The columnar arrangement of
compounds 2 and 3 and their systematic face-to-face orga-
nization raise another challenge. It could be assumed that
identical supramolecular arrangements may occur at the
macromolecular level for polythiophenes flanked by styryl
substituents along the main chain. The interactions between
individual π-π stacked groups should cooperate and exert a
high level of control over the supramolecular organization.
(22) (a) Yokozawa, T.; Adachi, I.; Miyakoshi, R.; Yokoyama, A. High
Perform. Polym. 2007, 19, 684–699. (b) Ohshimizu, K.; Ueda, M. Macro-
molecules 2008, 41, 5289–5294. (c) Zhang, Y.; Keisuke, T.; Kouske, H.;
Kazuhito, H. J. Am. Chem. Soc. 2008, 130, 7812–7813.
(23) Adachi, I.; Miyakoshi, R.; Yokoyama, A.; Yokozawa, T. Macro-
molecules 2006, 39, 7793–7795.
ꢁ
(24) (a) Ouhib, F.; Khoukh, A.; Ledeuil, J.-B.; Martinez, H.; Desbrieres,
J.; Dagron-Lartigau, C. Macromolecules 2008, 41, 9736–9743. (b) Wu, P.-T.;
Ren, G.; Li, C.; Mezzenga, R.; Jenekhe, S. A. Macromolecules 2009, 42,
2317–2320.
(21) Fernandez, G. E.; Perez, M.; Sanchez, L.; Martin, N. Angew. Chem.,
Int. Ed. 2008, 47, 1094–1097.
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SCHEME 3. One-Pot Synthesis of Block Cooligomers 5 and 6
P3STF (of low molecular weight) into soluble poly(3-
hexylthiophene) sequences. Block co-oligomers with low
degrees of polymerization (DP ∼20) were prepared. Such
an oligomeric structure should be much easier to characterize
than the polymeric substance and could be considered as a
model for the polymer physical behavior, as well.
the incorporation of the 3-20,30,40,50,60-pentafluorostyryl
fragment in 6. Finally, both diblock topologies were assessed
by GPC analyses by the appearance of monomodal traces
(P3HT-b-P3ST 5, Mn =1800 g mol-1, PDI = 1.58; and
P3HT-b-P3STF 6, Mn = 3200 g mol-1, PDI = 1.38) recor-
ded at lower elution volume with respect to the starting
P3HT macroinitiatior.
Matrix-assisted laser desorption ionization time-of-flight
(MALDI-ToF) mass spectrometry analyses of 5 and 6 con-
firmed both the covalent association of ST(F) and HT units
by the combination of x units of 3-hexylthiophene with
y units of 3-20,30,40,50,60-pentafluorostyrylthiophene or 3-
styrylthiophene and exhibited a set of peaks corresponding
to Br/H chain ends (see Figures S12 and S13 available in
Supporting Information). Maximum molecular weights of
about 3000-3100 g mol-1 (i.e, DP ∼17-18) were observed
in the MALDI-TOF spectra. Such values are in perfect
agreement with that expected (DP ∼20).
The synthetic route to P3HT-b-P3ST 5 and P3HT-b-
P3STF 6 starts with the formation of a controlled P3HT
fragment from 2-bromo-3-hexyl-5-iodothiophene (B3HT),
as shown in Scheme 3. For this purpose, the GRIM polym-
erization of B3HT was performed with Ni(dppp)Cl2 catalyst
in THF for an initial monomer-to-initiator ratio of 10.11
After 2 h at rt, a few drops of the crude mixture were
withdrawn to confirm complete monomer conversion (DP
∼9 as estimated by 1H NMR). Subsequently, the
preformed Grignard reagent of styrylthiophene 2 or 20,30,40,
50,60-pentafluorostyrylthiophene 3 were allowed to react for
2 h with the living P3HT oligomer chains, leading in a one-
pot procedure to compounds 5 and 6 (after quick acidic
termination reaction). As expected, the covalent association
of low molecular weight P3ST or P3STF to a few units of
3HT allowed for recovery of soluble P3HT-b-P3ST and
P3HT-b-P3STF co-oligomers.
The UV-vis absorption spectra of P3HT-b-P3ST 5 and
P3HT-b-P3STF 6 in CHCl3 show two major bands at 305 nm
and in the 400-450 nm spectral region (Figure S14 in Support-
ing Information). The high energy band at 305 nm is associated
with the pendant styryl moieties, whereas the HOMO-LUMO
transitions corresponding to the polythiophene backbones 5
and 6appear at 425 and 440 nm, respectively.25 In the solid state
(Figure S15 in Supporting Information), the λmax in the
400-450 nm of the two co-oligomers 5 and 6 are red-shifted
respectively by about 40 nm, as compared to the values in
solution. This band attributed to the π-π* interband transition
of unsaturated bonds in polythiophenes is very sensitive to the
effective conjugation length.26 Indeed, the confinement of the
polymer chains in the condensed phase is anticipated to support
a planar arrangement of the adjacent thiophene rings and to
enhance the delocalization of electrons within phenyl groups
and the polythiophene backbone. However, this red shift
obtained for co-oligomers 5 and 6 in the solid state is less
significant than that previously obtained for P3HT (∼100 nm),
indicating a lower increase in the planarity of P3HT-b-P3ST
and P3HT-b-P3STF chains when going to the solid state. The
thermal properties of these polythiophene-based oligomers
were also studied by TGA analysis (Figure S16 in Supporting
Information). Both compounds 5 and 6 show an identical
behavior characterized by a weight loss above 400 ꢀC due to
the decomposition of P3HT segments, as previously described
The IR spectrum of 5 allowed to identify the characteristic
νC-H absorption at 2925 cm-1 associated with the hexyl
chain and the intense νCdC band at 3057 cm-1 assigned to the
phenyl group. As far as the fluorinated compound 6 is
concerned, the same νC-H band is observed at 2928 cm-1
and the νC-F band at 1493 cm-1 supports the presence of a
fluorinated group in the molecular structure.
1
In both cases, the H NMR spectra of 5 and 6 showed
four signals in the range 0.8-2.9 ppm corresponding to the
methylene and methyl protons of the 3-hexylthiophene units.
The presence of the vinylic protons in the region between
6.5 and 7.4 ppm and the 4-proton on the aromatic thiophene
ring at 6.98 ppm (a typical value for regioregular polythio-
phene) was confirmed for 5and 6.9a A broad signal was observed
at 7.30 ppm, consistent with phenyl rings in compound 5.
Integration parameters of appropriate signals (vinylic
protons of styryl groups and methylene ones of hexyl chains)
brought relevant information about the ratio of each block.
Indeed, the P3HT/P3ST ratio in block co-oligomer 5 was
around 56:44, and a nearly equimolar quantity (49:51) was
observed for P3HT-b-P3STF 6, which is very similar to the
target amount (50/50) for each polymerization reaction. It is
worth noting that 19F NMR spectra of 6 exhibits three
multiplets between -142 and -163 ppm, which is in good
agreement with values previously found for 3, proving
(25) Clarke, T. M.; Gordon, K. C.; Officer, D. L.; Grant, D. K. J. Phys.
Chem. A 2005, 109, 1961–1973.
(26) He, J.; Su, Z.; Yan, B.; Xiang, L.; Wang, Y. J. Macromol. Sci. Pure
Appl. Chem. 2007, 44, 989–993.
1566 J. Org. Chem. Vol. 75, No. 5, 2010

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Clement et al.
JOCArticle
in the literature.27 A second weight loss is also observed from
250 to 400 ꢀC for 6, which may be assigned to the degradation
of the 20,30,40,50,60-pentafluorostyryl moieties. Finally, the DSC
thermograms of polythiophenes 5 and 6 exhibit a melting point
transition around 200 ꢀC (Figure S17 in Supporting Infor-
mation).
identified by a photochemical C-C bond formation of
superimposed styryl groups for systems 5 and 6.
Conclusion
In conclusion, we have demonstrated that the supramole-
cular organization of polythiophene chains can be controlled
owing to the rational design of the constitutive units. The
synthesis of new styryl and 20,30,40,50,60-pentafluorostyryl-
functionalized thiophene monomers and the characteriza-
tion of their crystal structures are reported. The mole-
cular arrangement of those molecules by π-π aromatic
interactions has allowed a [2 þ 2] photocyclization reac-
tion in the solid state, and the first X-ray structure of a
3-thiophene-substituted cyclobutane was obtained. The for-
mation of regioregular block polythiophene co-oligomers
with hexyl and styryl or 20,30,40,50,60-pentafluorostyryl sub-
stituents was then performed and characterized by 1H NMR,
19F NMR, and MALDI-TOF mass spectroscopies. The
incorporation of phenyl and 20,30,40,50,60-pentafluorophenyl
groups is expected to control the oligomeric chain orien-
tation by π-π interactions. Therefore, the evidence of
precise nanostructured organizations was obtained by an
original approach, namely the photoirradiation in the solid
state of oligothiophene compounds. As a result, a UV-
induced [2 þ 2] cycloaddition reaction was clearly evidenced
by the appearance of new signals assigned to cyclobutane
protons, which confirmed the persistence of the π-π inter-
actions at the macromolecular level and the remarkable
arrangement of the oligomeric chains relative to each other.
This “bottom-up” construction of polythiophenes offers
a reliable and highly efficient strategy for the elabora-
tion of well-defined polymeric structures of higher molecular
weight. The wide range of noncovalent interactions thereby
suggests further promising results for the engineering of
optoelectronic devices by recognition processes in a near
future.
Photoirradiation of Oligothiophenes. Taking into account
the ease of the [2 þ 2]photocyclization reaction involving
the compounds 2 and 3, we suggested that the π-π interac-
tions would persist at the macromolecular level and self-
assemble the co-oligomers 5 and 6 with the same accuracy. In
a similar manner, the π-stacked aromatic groups should
place the styryl double bonds according to Schmidt’s rules
˚
(distance between the double bonds in the range 3.5-4.2 A)
and allow for a photoreaction in the solid state under UV
irradiation. Consequently, we used an irradiation procedure
that has mirrored the experimental proceedings applied for
compounds 2 and 3. Thus, the irradiation of finely powdered
5 and 6 was performed with a UV lamp at 340 nm for 18 h.
Analysis of irradiated samples 5 and 6 has shown a sharp
change in the composition upon irradiation, as evidenced by
1H NMR spectroscopy. For both compounds, the 1H NMR
spectra show a new signal in the 5.15-5.30 ppm region
assigned to a cyclobutane ring formation; a slight variation
in the chemical shift values remains consistent with different
structures and chemical environments as well. Thus, a lim-
ited amount of P3HT-b-P3ST 5 underwent a UV-induced
[2 þ 2] cycloaddition as highlighted by the new triplet
recorded at 5.18 ppm (Figure S20 in Supporting
Information). We assume that the strength of interaction
involving the phenyl and thiophene rings is able to pack the
oligomeric chains and consequently the double bonds at a
˚
distance spanning 3.5-4.2 A. However, the conversion rate
into cyclobutane units remains at a low level, closely related
to the poor reactivity of 2,5-dibromo-3-styrylthiophene 2
under the same conditions.
Photoirradiation of the co-oligomer 6 has then been
investigated. In this case, the UV light session of 18 h
interestingly led to a significant response of cycloaddition
Experimental Section
1
reaction by H NMR, namely, a clear peak at 5.30 ppm
2,5-Dibromo-3-styrylthiophene (2). Compound 2 was ob-
tained according to a slightly modified literature procedure.10
A 1.34 g (5 mmol) portion of 1 and benzyltriphenylphospho-
nium bromide (2.16 g, 5 mmol) were dispersed in THF (100 mL).
To the resulting suspension was added DBU (1.53 mL, 10.23
mmol), and the resulting mixture was refluxed overnight under
argon. The resulting reaction mixture was diluted with dichloro-
methane and washed with 1 mol L-1 hydrochloric acid. The
organic layer was separated and dried over magnesium sulfate,
and the solvents removed with a rotary evaporator. The residue
was purified on silica using ethyl CH2Cl2/hexane (1:1) as eluent,
affording 2 as a mixture of E/Z isomers in 78% yield. 1H
NMR(300 MHz, CDCl3) δ: 7.70-7.22 (m, 11 H), 7.00 (d, 1 H,
Jtrans= 16.5 Hz), 6.89 (d, 1 H, Jtrans = 16.5 Hz), 6.69 (d, 1 H,
Jcis = 12.0 Hz), 6.60 (s, 1 H), 6.33 (d, 1 H, Jcis = 12.0 Hz).
Isomerization Procedure. Compound E/Z 2 (2 mmol) was
dissolved in dichloromethane (50 mL), and iodine (1.52 g, 6
mmol) was added. The resulting mixture was stirred at room
temperature for 7 days and then washed with a saturated
solution of sodium thiosulfate and water. The organic layer
was dried over magnesium sulfate and evaporated to dryness,
and the residue was purified on silica using CH2Cl2/hexane (1:1)
as eluent. Slow evaporation of a solution of 2 in CH2Cl2/hexane
attributed to new cyclobutane protons (Figure S21 in Sup-
porting Information). As seen for monomer 3 with respect to
2, a higher conversion into the photoproduct is observed
with co-oligomer 6, which confirms a good superposition of
pendant olefins with a small intermolecular distance. This
results from stronger interactions between the thiophene and
pentafluorobenzene rings packing the polythiophene back-
bones in the desirable arrangement. As far as the amount
of cyclized styryl units is concerned, we can expect that
only a limited number of double bonds is able to photoreact
under UV irradiation. Indeed, the deformation of the
polythiophene backbone due to the cyclobutane formation
is expected to separate successive double bonds (distance
˚
>4.2 A) and consequently prevent subsequent reactivity.
Because of the relatively low solubility of photoirradiated
1
cross-linked oligomers and the poor resolution of the H
NMR spectra, the double bond conversion into cyclobutane
ring was roughly estimated to be in the range of 2-6% for
compound 5 and 5-10% for 6. Finally, the self-assembly of
polythiophenes driven by π-π interactions was clearly
1
(27) Higashihara, T.; Ueda, M. React. Funct. Polym. 2009, 69, 457–462.
afforded white crystals in 92% yield. E-2: mp 58-60 ꢀC. H
J. Org. Chem. Vol. 75, No. 5, 2010 1567

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Clement et al.
JOCArticle
NMR (300 MHz, CDCl3) δ: 7.70-7.22 (m, 5 H), 7.00 (d, 1 H,
the same manner (solution B). After stirring solution A for 3 h,
solution B was added to the previous mixture via a syringe, and
the resulting solution was stirred overnight. The reaction was
quenched quickly by pouring HCl aq (5 M) into the solution and
stirring for 0.5 h. Then, the mixture was precipitated in cold
MeOH and filtered. The product was washed well with MeOH
and hexane to afford a red solid. 5: yield 0.66 g (63%). IR
J
trans= 16.5 Hz), 6.89 (d, 1 H, Jtrans = 16.5 Hz). 13C{1H} NMR
(125 MHz, CDCl3) δ: 139.1, 138.1, 131.3, 128.2, 127.8, 127.4,
120.0, 110.1. UV-vis (CH2Cl2): λmax = 310 nm (ε = 42000).
Anal. Calcd for C12H8Br2S: C 41.89; H, 2.34; S, 9.32. Found: C
41.63; H 2.26, S 9.08
2,5-Dibromo-3-20,30,40,50,60-pentafluorostyrylthiophene (3). Un-
der nitrogen, to a solution of 4-20,30,40,50,60-pentafluorobenzyltri-
phenylphosphonium bromide (2.61 g, 5 mmol) in DMF (50 mL)
was added slowly sodium hydride (0.24 g, 6 mmol, 60% dispersion
in mineral oil). The reaction mixture was stirred during 45 min.
Next, 1.34 g (5 mmol) of 1 was slowly added, and the mixture was
heated at 60 ꢀC overnight. Then, after addition of water (250 mL),
a precipitate was obtained that was isolated by filtration. The
residue was dissolved in CH2Cl2, dried under MgSO4, filtrated, and
evaporated. The crude product was purified by column chroma-
tography on silica gel with hexane/CH2Cl2 (9:1) as eluent. Slow
evaporation of a solution of 3 in CH2Cl2 afforded white crystals.
Yield: 1.69 g (78%); mp 103-105 ꢀC. 1H NMR (300 MHz, CDCl3)
(υ~C-H): 3057, 3025, 2953, 2925, 2854, 1494, 954 cm-1 1H
.
NMR (300 MHz, CDCl3) δ: 7.63-6.86 (m, 9H) 2.78 (m, 2 H),
1.68 (m, 2 H), 1.36 (m, 6 H), 0.88 (t, 3 H, 3JH-H = 6.0). UV-vis
(CHCl3): λmax = 438 nm. 6: yield 0.89 g (68%). IR (υ~C-H):
1
2928, 2857, 1512, 1493, 1004, 959 cm-1; H NMR (300 MHz,
CDCl3): δ 7.41 (m, 1 H), 6.98 (s, br., 2 H), 6.83 (m, 1H), 2.80 (m, 2
H), 1.71 (m, 2 H), 1.35 (m, 6 H), 0.92 (m, 3 H). 19F NMR (282
MHz, CDCl3): δ -143.1 (2F), -155.9 (1F), -163.0 (2F). UV-vis
(CHCl3): λmax = 425 nm.
Acknowledgment. This work was supported by “the Re-
vetement Fonctionnels” program (SMARTFILM project)
δ: 7.32 (d, 1 H, 3JH-H = 16.5), 7.23 (s, 1 H), 6.77 (d, 1 H, 3JH-H
=
^
of the Region Wallonne and the European Commission
16.5). 13C{1H} NMR (125 MHz, CDCl3) δ: 146.4, 143.1, 138.5,
128.1, 126.9, 126.8, 115.0, 113.0, 112.6. 19F NMR (282 MHz,
CDCl3) δ: -143.4 (2F), -157.4 (1F), -163.5 (2F). EI-MS (m/z):
434 (54, Mþ), 355 (66, Mþ - Br), 336 (14, Mþ - BrF), 274 (100,
Mþ - 2 Br). UV-vis (CH2Cl2): λmax = 308 nm (ε = 21600). Anal.
Calcd for C12H3Br2F5S: C 33.21; H, 0.70; S, 7.39. Found: C 33.02;
H 0.44, S 7.10
ꢀ
(FEDER, FSE). CIRMAP is also very grateful for their
general financial support in the form of Objectif 1-Hainaut:
Materia Nova, as well as to the Belgian Federal Government
Office of Science Policy (PAI 6/27). We thank the Labor-
ꢁ
atoire de Physico-chimie des Polymeres et des Interfaces
(University of Cergy Pontoise, France) for 19F NMR spec-
tra. The diffractometer was funded by NSF grant 0087210,
by Ohio Board of Regents grant CAP-491, and by YSU. O.
C. and P.G. (Research Associates) and J.D.W. (Research
Fellow) are grateful to the Belgian National Fund for
Scientific Research (FRS-FNRS).
Photodimerization of Monomer 3. A 50 mg portion of
finely powdered crystals of 3 was placed between two glass
plates (76 ꢀ 26 mm) and was irradiated for 24 h. Compound 4
was isolated without further purification. Slow evaporation of 4
in CHCl3 afforded yellowish crystals. Yield: 98%; mp 188-190
ꢀC. 1H NMR (300 MHz, CDCl3) δ: 7.00 (s, 2 H), 4.81 (m, 4 H).
13C{1H} NMR (125 MHz, CDCl3) δ: 146.9, 139.4, 135.8, 128.3,
112.2, 111.6, 110.2, 40.8, 38.0. 19F NMR (282 MHz, CDCl3)
δ: -140.9 (4F), -154.4 (2F), -161.8 (4F). Anal. Calcd for
C24H6Br4F1S2: C 33.21; H, 0.70; S, 7.39. Found: C 32.94; H
0.48, S 7.12
Typical Procedure for the Synthesis of Poly(3-hexylthiophene)-
b-poly(3-styrylthiophene) (5) and Poly(3-hexylthiophene)-b-poly-
(3-20,30,40,50,60-pentafluorostyrylthiophene) (6). Two round-bot-
tomed flasks (100 mL) equipped with three-way stopcocks were
dried by heating under reduced pressure and cooled to room
temperature. B3HT(1.12 g, 3 mmol) was driedby three successive
azeotropic distillations with toluene, and then dry THF (15 mL)
was added. A 2 mol/L solution of i-PrMgCl in THF (1.50 mL)
was added via a syringe, and the mixture was stirred at 0 ꢀC for 30
min(solutionA). SolutionA was addedinone portion tothe right
amount of Ni(dppp)Cl2 catalyst in THF (10 mL). In the other
flask, 3 mmol of 2 (1.03 g) or 3 (1.30 g), previously dried by
azeotropic distillation, was reacted with i-PrMgCl (1.50 mL) in
Supporting Information Available: Materials and appara-
tus. Crystal and structure refinement data for compounds 2-4.
Thermal ellipsoid plot for compounds 2-4. Atomic coordinates
(ꢀ 104) and equivalent isotropic displacement parameters (ꢀ
3
˚
10 ) for 2-4. Bond lengths (A) and angles (deg) for for 2-4.
Anisotropic displacement paremeters (ꢀ 103) for 2-4. 1H NMR
and 13C{1H} spectra of 2 in CDCl3. Molecular packing diagram
of 3 and 4. 19F and 13C{1H} NMR spectra of 3 and 4 in CDCl3.
MALDI-TOF spectra for P3HT-b-P3ST 5 and P3HT-b-P3STF
6. UV-vis spectra of oligomers 5 and 6 in CHCl3 and in solid
state. TGA and DSC diagrams for P3HT-b-P3ST 5 and P3HT-
b-P3STF 6. 1H NMR spectrum in CDCl3 of co-oligomer 5 and
after 3 days of co-oligomer 6 in the range of 4-8 ppm. 1H NMR
spectrum of co-oligomer 5 and 6 after 18 h of UV irradiation in
the range of 4-8 ppm. This material is available free of charge
via the Internet at http://pubs.acs.org.
1568 J. Org. Chem. Vol. 75, No. 5, 2010