Poly(thiaheterohelicene): A stiff conjugated helical polymer comprised of fused benzothiophene rings

Article abstract of DOI:10.1021/ol047768z

(Chemical Equation Presented) A novel helical aromatic polymer comprised of fused benzothiophene rings, poly(thiaheterohelicene), was synthesized via an intramolecular ring-closing reaction with a controlled helicity to one-sided bias. The synthetic helic

Full text of DOI:10.1021/ol047768z

ORGANIC
LETTERS
2005
Vol. 7, No. 5
755-758
Poly(thiaheterohelicene): A Stiff
Conjugated Helical Polymer Comprised
of Fused Benzothiophene Rings
Tomokazu Iwasaki, Yusaku Kohinata, and Hiroyuki Nishide*
Department of Applied Chemistry, Waseda UniVersity, Tokyo 169-8555, Japan
nishide@waseda.jp
Received October 30, 2004
ABSTRACT
A novel helical aromatic polymer comprised of fused benzothiophene rings, poly(thiaheterohelicene), was synthesized via an intramolecular
ring-closing reaction with a controlled helicity to one-sided bias. The synthetic helicity control involved the induction of the helical conformation
and its fixation. The ladder polymer showed both an extended
π
-conjugation and planarity and a very stiff helical structure.
Helical macromolecules have a precisely ordered stereo-
structure and have many potential applications such as chiral
separation and sensing using molecular recognition and liquid
crystalline formation through molecular ordering.1-5 π-Con-
jugated helical molecules comprised of fused aromatic rings
are called helicenes and have often been characterized by a
unique optical activities caused by their helical conjugation
and by a stiff structure.6 On the other hand, π-conjugated
macromolecules with thiophene rings possess a stable doped
state due to an excess π-electron conjugation on the
thiophene ring and are often highly electron conductive in
comparison with other π-conjugated aromatic polymers.7,8,11c
For this work, a thiaheterohelicene, which is a helical and
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10.1021/ol047768z CCC: $30.25
© 2005 American Chemical Society
Published on Web 02/08/2005

Scheme 1
Figure 1.
π-conjugated molecule comprised of fused sulfur-containing
aromatic rings, was in this study selected as the target
molecule bearing an electronic system with a helical
structure. Although helical oligomers consisted of fused
aromatic rings, including benzene and thiophene rings, have
already been synthesized via stepwise synthetic routes,6,9,10
there have been no reports of a helicene, including thiahet-
erohelicene, with a high molecular weight or a sufficient
number of repeating units to form a helical structure.
An efficient synthetic route to produce poly(thiaacene)s,
which are planar and ladder-type molecules, comprised of
fused sulfur-containing aromatic rings, has been developed
and reported by our laboratory.11 The key step to form the
poly(thiaacene)s is a quantitative intramolecular ring-closing
reaction of the alkyl sulfoxide-substituted aromatic polymers.
For example, ring-closing within poly[1,4-phenylene-alt-2,5-
bis(methylsulfinyl)-1,4-phenylene] yielded poly(phenylene-
2,5-dithia-1,4-diyl).11a This poly(thiaacene) has the fused
benzothiophene units that are alternatively linked, or fused,
up and down and has a significantly developed π-electron
conjugation.11
In this study, we successfully used this synthetic method
to design and synthesize a poly(thiaheterohelicene), poly-
[phenylene-4,6-bis(alkylsulfonio)-1,3-diyl triflate] (Poly-S+
in Figure 1). We also present, for the first time, a “synthetic”
method for controlling the helicity. Helical conformation of
the chiral alkylsulfoxide-substituted poly(1,3-phenylene)
(Poly-SO) was induced using a specific solvent with one-
side bias and then was fixed to form a stiff helical structure
by covalent bonds through the ring-closing reaction.
The pre-polymer (Poly-S) poly[4,6-bis((S)-(+)-2-methyl-
butylthio)-1,3-phenylene-alt-2-methyl-1,3-phenylene] was
synthesized as shown in Scheme 1, based on the following
molecular design. (i) Polycondensation of 2-methyl-1,3-
phenylenebis(pinacol borate) (4) and 1,5-dibromo-2,4-bis-
((S)-(+)-2-methylbutylthio)benzene (6) lead to both a high
molecular weight pre-polymer and the introduction of two
alkylthio groups for the subsequent ring formation reaction.
(ii) The presence of the 2-methyl group of 4 (or one of every
other phenylene ring in Poly-SO) results in the selective
intramolecular and electrophilic attack of the pendant sul-
foxides on the favored 4 and 6 positions of adjacent
phenylene rings (see the Supporting Information).
2,6-Dibromotoluene (3) was prepared from p-nitro-
toluene in reasonable yield12 and converted to 2-methyl-1,3-
phenylenebis(pinacol borate) (4). The Williamson reaction
with subsequent bromination of 1,3-benzenedithiol gave
another monomer, 1,5-dibromo-2,4-bis[(S)-(+)-2-methylbu-
tylthio]benzene (6), in high yield. Pd-catalyzed polyconden-
sation of 4 and 613 yielded the chiral alkylthio-substituted
poly(1,3-phenylene), poly[4,6-bis((S)-(+)-2-methylbutylthio)-
1,3-phenylene-alt-2-methyl-1,3-phenylene] (Poly-S), which
is soluble in the common solvents. The number averaged
molecular weight (Mn) of Poly-S14 was 1.1 × 104 (Mw/Mn
) 1.2, degree of polymerization ) 84 based on the phenylene
rings), which was sufficient to form 12 helical pitches in
the targeted helical polymer.
(12) Preparation of 3 is shown in the Supporting Information.
(13) The pendant sulfides of 6 were oxidized. The obtained dibromoben-
zene bearing alkylsulfoxide was enantiopure (>97%) proved by 1H NMR.
The sulfoxide derivative was polymerized with 4 by the Suzuki coupling
reaction, to afford a low molecular weight oligomer (Mw < 1000).
(14) 13C NMR of Poly-S showed eight signals in the aromatic region.
(11) (a) Haryono, A.; Miyatake, K.; Natori, J.; Tsuchida, E. Macromol-
ecules 1999, 32, 3146. (b) Oyaizu, K.; Mikami, T.; Mitsuhashi, F.; Tsuchida,
E. Macromolecules 2002, 35, 67. (c) Oyaizu, K.; Iwasaki, T.; Tsukahara,
Y.; Tsuchida, E. Macromolecules 2004, 37, 1257. (d) Tsuchida, E.; Oyaizu,
K. Bull. Chem. Soc. Jpn. 2003, 76, 15.
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Org. Lett., Vol. 7, No. 5, 2005

Figure 3. CD spectra of Poly-SO in CHCl3/CH3CN (1/1) (dotted
line) and of Poly-S+ (obtained from intramolecular ring-closing
reaction in CHCl3/CH3CN) in CHCl3 (solid line in the given
temperature range).
Figure 2. Molecular structure of Tri-S and Tri-S+. Ellipsoids
showed 30% probability levels. Dihedral angles (deg) for selected
phenyl and phenylene planes: 40.3 between the C(1)-C(2)-C(3)-
C(4)-C(5)-C(6) plane and the C(8)-C(9)-C(10)-C(18)-C(19)-
C(20) plane for Tri-S; 2.05 between the C(1)-C(2)-C(3)-C(4)-
C(5)-C(6) plane and the C(9)-C(10)-C(11)-C(20)-C(21)-
C(22) plane for Tri-S+.
which were prepared via the same reactions for the corre-
sponding polymers (Figure 2).19 The dihedral angle between
the terminal planes and the central phenyl, or phenylene, ring
was estimated to be 40.3° and 2.05° for Tri-S and Tri-S+,
respectively. The ring-closing in Tri-S+ caused a highly
planar structure on the fused benzothiophene moieties.20
The ladder polymer Poly-S+ did not show any Cotton
effects in the circular dichroism (CD) when it was dissolved
in dichloromethane, DMF, DMSO, or a mixture of three
solvents with ether or hexane. This suggested that the
polymer was involved equally in a mixture with a right- and
left-handed helical sense. On the other hand, three Cotton
effect-based CD maxima appeared at 256 (positive), 283
(negative), and 298 (positive) nm for Poly-SO in a mixture
of chloroform and acetonitrile (a poor solvent), used as a
solvent, although no Cotton effects were observed when only
chloroform (a good solvent) solution was used as a solvent.
CD intensity significantly decreased with solution temper-
ature in the range from 25 to 50 °C. These results indicated
that solvophobic forces induced the formation of a helix for
Poly-SO (Figure 3).
The pendant alkyl sulfide of Poly-S was quantitatively
oxidized using peracetic acid to afford the alkyl sulfoxide
derivative.15 Intramolecular ring-closing condensation with
excess pure triflic acid or electrophilic attack of the pendant
sulfoxide on the 4 or 6 position of the adjacent phenylene
unit proceeded slowly in comparison with the previous ladder
polymers which were prepared by research group at
Waseda.11a-c After one week, ring closure gave a ladder
polymer in which the tetrahydrothiophenium bridge between
the benzene rings was formed without any structural defects
based on the NMR spectroscopy.16 Poly-S+ is soluble in
chloroform, acetonitrile, and DMF and insoluble in diethyl
ether and hexane.
The UV-vis spectrum17 of Poly-S+ showed an absorption
maximum (λmax) at 315 nm with a shoulder at 343 nm, which
was bathochromically shifted compared to those of the
nonring-closed pre-polymer Poly-SO (λmax ) 281 nm,
absorption shoulder ) 318 nm), which suggested extended
π-conjugation. The planarity in the structure of Poly-S+ after
ring-closure was studied by an X-ray crystallographic
analysis of the trimer model compounds, Tri-S and Tri-S+,18
Methods for controlling helical bias, or helicity, in helical
polymers have been studied. For example, a discussion
recently focused on the helical conformation of aromatic
conjugated polymers with pendant chiral groups in a mixture
of so-called good and poor solvents.21,22 We intended in this
(15) The oxidation conditions such as reaction concentration, temperature,
and time were optimized. The sulfide completely disappeared in the 1H
NMR spectrum. Any peroxidation of the sulfide, such as to the sulfone,
did not occur, evidenced by the absence of a peak attributed to the sulfone
bond (1150, 1350 cm-1) in the IR spectrum (see the Supporting Informa-
tion).
(18) Layering the chloroform solution of Tri-S with hexane afforded
colorless needlelike crystals suitable for an X-ray diffraction study. The
colorless needlelike crystals of Tri-S+ were obtained from an acetonitrile
solution layered with diethyl ether. The details of the X-ray crystallographic
data are shown in the Supporting Information.
(19) Tri-S was synthesized by Suzuki coupling of 1,5-dibromo-4,6-bis-
(methylthio)benzene (1 equiv) and phenylboronic acid (2 equiv) with Pd-
(PPh3)4 (1/50 equiv) in a mixture of THF and aqueous Na2CO3 solution
(for details, see the Supporting Information).
(16) Ring-closing or the fused-ring formation of Poly-S+ was proved
by 1H NMR, as follows. The signal of the methylene units adjacent to the
sulfur atoms shifted to a lower magnetic field (2.99 ppm) from that of the
precursor sulfoxides (2.57 ppm) due to the electron-withdrawing by the
sulfonio group. Integral values of the protons derived from the phenyl ring
(3H), the aryl methyl group (3H), and the methylene units on the sulfonio
bridges (4H) completely agreed with those calculated for the repeating units
of Poly-S+.
(20) λmax ) 272 nm (for Tri-S), 299 nm (for Tri-S+).
(21) Fiesel, R.; Neher, D.; Shcerf, U. Synth. Met. 1999, 102, 1457.
(22) (a) Nelson, J. C.; Saven, J. G.; Moore, J. S.; Wolynes, P. G. Science
1997, 277, 1793. (b) Prince, R. B.; Saven, J. G.; Wolynes, P. G.; Moore,
J. S. J. Am. Chem. Soc. 1999, 121, 3114. (c) Prince, R. B.; Moore, J. S.;
Brunsveld, L.; Meijer, E. W. Chem. Eur. J. 2001, 19, 4150. (d) Hill, D. J.;
Moore, J. S. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 5053.
(17) All of the UV-vis and CD measurements were carried out in a 0.2
mM solution.
Org. Lett., Vol. 7, No. 5, 2005
757

intensity of Poly-S+ (at 278 nm) and PBG (at 224 nm)25 in
several solvents and in the temperature range of -30 to +80
°C were examined and are shown in Figure 4. The CD
intensity and profile of Poly-S+ did not change even at high
temperatures and did not depend on the solvent, Figure 4.26
In summary, a novel poly(thiaheterohelicene) Poly-S+
comprised of fused benzothiophene rings, which has π-con-
jugated ladder structure, was synthesized. The helical
structure was maintained over a wide range of temperatures
and in a variety of solvents. We have simulated the electronic
conduction of poly(thiaheterohelicene) based on a nanometer-
sized electronic device,27 and it suggested not only that
electron transmittance occurs along the helical π-conjugation
under a lower bias voltage (around the Fermi level of Au)
but also that a solenoid-like induced magnetism occurs. Poly-
(thiaheterohelicene) is a good candidate for studying the one-
dimensional wire in an organic molecular-based electromag-
net.
Figure 4. CD intensity of PolyS+ (at 278 nm, solid column) and
PBG (at 224 nm, open column) in various solvents; (inset) -30 to
+80 °C temperature dependence of the CD absorbance for PolyS+
in DMF (at 278 nm, solid circle). 1,2-DCE ) 1,2-dichloroethane.
Acknowledgment. This work was partially supported by
a Grant-in-Aid for the COE Research Programs “Practical
Nano-Chemistry” and “Molecular Nano-Engineering” from
MEXT, Japan.
study to develop synthetic helicity-fixation. The helical
conformation of Poly-SO was first induced in a mixture of
chloroform and acetonitrile (v/v ) 1/1), and its helicity was
fixed in Poly-S+ through the ring-closing reaction. The
experimentally obtained Poly-S+ gave four CD maxima
based on the Cotton effects at 251, 275, 302, and 325 nm
(negative, positive, positive, and positive, respectively) not
only in the chloroform and acetonitrile mixture but also in
pure chloroform. This indicated that the helicity of Poly-S+
is maintained with a one-side bias and that Poly-S+ possesses
a stiff helical structure.
Supporting Information Available: Synthesis and char-
acterization of the monomers and polymers and details of
the X-ray crystallographic analysis. This material is available
free of charge via the Internet at http://pubs.acs.org.
OL047768Z
(24) (a) Block, H. Poly(γ-benzyl-L-glutamate) and Other Glutamic Acid
Containing Polymers. In Polymer Monographs; Gordon and Breach Science
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(25) The CD absorptions at 278 nm (for Poly-S+) and 224 nm (for PBG)
correspond to their λmax values in the UV-vis absorptions.
The stiffness of the π-conjugated helical structure of Poly-
S+ was studied using poly(γ-benzyl-L-glutamate) (PBG)23
as a control example of a stiff helical polymer.3,24 The CD
(26) Permittivity; 1,4-dioxane ) 2.24, CHCl3 ) 4.90, 1,2-DCE ) 10.45,
DMF ) 36.71, and DMSO ) 48.90 (from Chemical Safety Data Sheets;
Royal Society of Chemistry: London, 1990; Vol. 1).
(23) Poly(γ-benzyl-L-glutamate) was purchased from the Aldrich Co.,
and its molecular weight was Mw ) 37 000.
(27) Tagami, K.; Tsukada, M.; Wada, Y.; Iwasaki, T.; Nishide, H. J.
Chem. Phys. 2003, 119, 7491.
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Org. Lett., Vol. 7, No. 5, 2005