Optically active, amphiphilic poly(meta -phenylene ethynylene)s: Synthesis, hydrogen-bonding enforced helix stability, and direct AFM observation of their helical structures

Article abstract of DOI:10.1021/ja303204m

Optically active, amphiphilic poly(meta-phenylene ethynylene)s (PPEa) bearing l- or d-alanine-derived oligo(ethylene glycol) side chains connected to the backbone via amide linkages were prepared by microwave-assisted polycondensation. PPEa's exhibited an intense Cotton effect in the π-conjugated main-chain chromophore regions in various polar and nonpolar organic solvents due to a predominantly one-handed helical conformation stabilized by an intramolecular hydrogen-bonding network between the amide groups of the pendants. The stable helical structure was retained in the bulk and led to supramolecular column formation from stacked helices in oriented polymer films as evidenced by X-ray diffraction. Atomic force microscopy was used to directly visualize the helical structures of the polymers in two-dimensional crystalline layers with molecular resolution, and, for the first time, their absolute helical senses could unambiguously be determined.

Full text of DOI:10.1021/ja303204m

Article
pubs.acs.org/JACS
Optically Active, Amphiphilic Poly(meta-phenylene ethynylene)s:
Synthesis, Hydrogen-Bonding Enforced Helix Stability, and Direct
AFM Observation of Their Helical Structures
Motonori Banno,† Tomoko Yamaguchi,† Kanji Nagai,†,§ Christian Kaiser,‡ Stefan Hecht,*,‡
and Eiji Yashima*,†
†Department of Molecular Design and Engineering, Graduate School of Engineering, Nagoya University, Chikusa-ku, Nagoya,
464-8603, Japan
‡Department of Chemistry, Humboldt-Universitat zu Berlin, Brook-Taylor-Strasse 2, 12489 Berlin, Germany
̈
S
* Supporting Information
ABSTRACT: Optically active, amphiphilic poly(meta-phenyl-
ene ethynylene)s (PPEa) bearing L- or D-alanine-derived
oligo(ethylene glycol) side chains connected to the backbone
via amide linkages were prepared by microwave-assisted
polycondensation. PPEa’s exhibited an intense Cotton effect
in the π-conjugated main-chain chromophore regions in
various polar and nonpolar organic solvents due to a
predominantly one-handed helical conformation stabilized by
an intramolecular hydrogen-bonding network between the
amide groups of the pendants. The stable helical structure was
retained in the bulk and led to supramolecular column formation from stacked helices in oriented polymer films as evidenced by
X-ray diffraction. Atomic force microscopy was used to directly visualize the helical structures of the polymers in two-dimensional
crystalline layers with molecular resolution, and, for the first time, their absolute helical senses could unambiguously be
determined.
and co-workers in 1997,8 who prepared a series of amphiphilic
INTRODUCTION
■
The design and synthesis of artificial helical polymers1 and
oligo(m-phenylene ethynylene)s bearing chiral and achiral
oligomers (foldamers)2 with a controlled helix-sense has
attracted significant interest in the past two decades because
polar side chains (for example, OPEe and OPEe-Me, Chart 1)
and discovered a strong tendency to fold into a helical
conformation in polar solvents, such as acetonitrile and polar
of their potential applications as chiral materials for asymmetric
organic solvents driven by noncovalent, solvophobic inter-
synthesis, separation of enantiomers, and also as chiral building
actions.8 A preferred-handed helicity can be biased by
diastereoselective inclusion complexation with chiral guests9
or chiral substituents attached either in the side chains10 or at
the termini,11 thus exhibiting an optical activity as evidenced by
an appearance of strong bisignated circular dichroism (CD)
signals.
blocks for self-assembled nanomaterials and devices.1c,q,r,t,3 The
elucidation of their helical structures, in particular their
handedness (right- or left-handed helix),4 is quite important
not only to understand the mechanism of their folding
processes, but also to develop novel helical polymers and
foldamers with specific functions. In contrast to proteins,5
oligonucleotides,6 and synthetic uniform oligomers7 whose
helical structures were unambiguously determined by their
single-crystal X-ray crystallographic analyses, the exact helical
structures of most of helical polymers prepared to date and
some foldamers remain unknown4 mainly due to the difficulty
in obtaining crystals suitable for X-ray analysis. We now report
the synthesis and chiroptical properties of a pair of novel
amphiphilic poly(meta-phenylene ethynylene)s, carrying L- and
D-alanine-derived chiral side chains and reminiscent of a
foldamer-based helical polymer, and the direct observation of
their helical structures by high-resolution atomic force
microscopy (AFM).
After discovery of this foldamer motif, a variety of helical
polymers and oligomers consisting of m-phenylene ethynylene
or its analogous units have been prepared aiming at further
developments of functional foldamers being applicable to the
fields of chiral and biological materials science,12−14 and their
helical structures in solution and in the solid state have
extensively been investigated by Moore and others using
NMR,8,15c absorption,8,15a,c fluorescence,8,15 CD,9−11 and
ESR16 spectroscopies, X-ray diffraction (XRD),17 X-ray
scattering,18 and electron microscopic17c methods. Molecular
dynamics simulation studies on the folding processes were also
meta-Linked phenylene ethynylene is one of the most
Received: April 3, 2012
popular structural motifs for foldamers developed by Moore
© XXXX American Chemical Society
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Chart 1. Structures of Oligo- and Poly(m-phenylene ethynylene)s
Scheme 1. Synthesis of (S)-PPEa and (R)-PPEa
a
Table 1. Polymerization Results of (S)-PEa and (R)-PEa in Acetonitrile at 60 °C
b
c
c
d
e
e
polymer
yield (%)
Mn × 10−4
Mw/Mn
[α]20
Δεfirst (M−1 cm−1
)
Δεsecond (M−1 cm−1
)
D
(S)-PPEa
(R)-PPEa
58
50
10.4
3.12
2.70
+967
+113
−110
+109
10.8
−992
−118
a
b
Polymerized under microwave heating. Isolated yield after precipitation into Et2O and short silica column chromatography in CH2Cl2.
c
d
Determined by SEC (polystyrene standards) using a UV−visible detector with THF containing TBAB (0.1 wt %) as the eluent. Measured in
e
CHCl3. The first (Δεfirst) and second (Δεsecond) Cotton effects measured in CHCl3.
reported.19 A helix formation with a preferable helical sense of
oligo(m-phenylene ethynylene) foldamers was evidenced by the
appearance of CD signals,9−11 and the helical structures
including helical pitch were postulated by XRD measure-
ments.17b Nevertheless, the absolute helical sense (right- or left-
handed helix) of the oligo- and poly(m-phenylene ethynylene)
foldamers with optical activity still remains unknown because of
difficulty in obtaining optically active crystalline samples.
The direct observation of helical polymers by scanning probe
microscopy is one of the most promising methods for providing
convincing evidence of their helical structures including the
helical pitch, handedness, and excess of helical handedness, but
the visualization of helical structures is still challenging, and
their real images of right- and left-handed helical structures by
microscopy remain difficult to observe.20
Recently, we found that the rigid rodlike helical poly-
(phenylacetylene)s21 and poly(phenyl isocyanide)s22 bearing L-
or D-alanine pendants with a long alkyl chain self-assembled to
form two-dimensional (2D) crystals with regular helix-bundle
structures on highly oriented pyrolytic graphite (HOPG) upon
exposure to organic solvent vapors such as benzene. High-
resolution AFM revealed their helical conformations in the 2D
crystals and enabled us to determine the molecular length and
packing, helical pitch, and handedness as well. This method has
been proved to be versatile to observe helical structures of
other helical polymers that involve a dynamically racemic
helical poly(phenylacetylene) carrying achiral pendants,21c
a
supramolecular helical stereocomplex composed of isotactic-
and syndiotactic poly(methyl methacrylate)s,23 and a double-
stranded helical polymer.24
In this study, we synthesized novel optically active,
amphiphilic poly(m-phenylene ethynylene)s and investigated
their preferred-handed helical structures in solution and in the
solid state by absorption, CD, and IR spectroscopies as well as
XRD and AFM observations. The polar chiral oligo(ethylene
glycol) side chains are readily derived from either L- or D-
alanine and are linked to the aromatic backbone via an amide
linkage ((S)- or (R)-PPEa), thereby replacing the ester linkage
of Moore’s foldamers with an amide one (Chart 1). Because of
these amide residues, the helical structures of (S)- and (R)-
PPEa were anticipated to be stabilized by intramolecular
hydrogen bonds, and the polymer backbones should therefore
be more rigid than those of Moore’s ester counterparts. This
hydrogen-bonding intrastrand stabilization should facilitate
supramolecular organization in the bulk and in particular
formation of 2D crystals on a substrate while maintaining the
polymers’ helical conformations and indeed, for the first time,
enabled us to observe hollow helical structures in oriented
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Figure 1. (A) CD and absorption spectra of (S)-PPEa (a) and (R)-PPEa (b) in CHCl3 (0.5 mM) at ca. 25 °C. (B) Normalized CD and absorption
spectra of (S)-PPEa in acetonitrile (c), ethanol (d), THF (e), and CHCl3 (f) at ca. 25 °C. The spectra were normalized on the basis of the
absorption maxima.
polymer films by XRD and to determine the absolute helical
handedness of the poly(m-phenylene ethynylene)s by high-
resolution AFM.
in CHCl3. In sharp contrast to the ester-bound oligo(m-
phenylene ethynylene)s (OPEe) prepared by the group of
Moore, the polymers exhibited intense, bisignated Cotton
effects in the π-conjugated phenylene ethynylene chromophore
regions (250−365 nm) that are mirror images of each other.
The monomers showed negligible CD at wavelengths greater
than 275 nm. These results indicate that (S)-PPEa and (R)-
PPEa possess a preferred-handed helical conformation with an
opposite helix-sense induced by the covalent-bonded chiral
alanine pendants even in CHCl3, in which the ester-bound
OPEe exhibited no CD at all in the presence of chiral guests,
independent of the chain length.9 In CHCl3, the helical
conformation of PPEa’s is most likely stabilized by intra-
molecular hydrogen bonds between the nonadjacent amide
residues, so that the CD and absorption spectra hardly changed
in CHCl3 even at 55 °C. The IR spectra of PPEa measured in
CHCl3 supported the intramolecular hydrogen-bond networks;
sharp signals assigned to the amide NH and carbonyl stretching
(amide I) bands appeared at approximately 3274 and 1633
cm−1, respectively,26 while those of the corresponding
monomer shifted to higher wavenumbers at 3439 and 1655
cm−1, respectively (Table S1 and Figure S1).
(S)-PPEa also displayed similar CDs in their patterns but
with more intense Cotton effect intensities in polar solvents,
such as acetonitrile, ethanol, and THF and their absorption
maxima slightly shifted to a shorter wavelength as compared to
that in CHCl3 (Figure 1B), indicative of the polymer adopting
an excess one-handed helical conformation in these polar
solvents. The magnitude of the Cotton effects that reflects the
helical sense excess of the polymer increased with an increase in
the polarity of the solvents;11c the observed CD intensity
increased in the order CHCl3 < THF < ethanol < acetonitrile.
In THF and ethanol, however, such hydrogen bonding would
be weakened. Nevertheless, the polymer maintained its helical
structure because of favorable solvophobic interactions between
the polar side chains of PPEa and solvents, which is capable of
strengthening the aromatic π−π interactions, resulting in the
folded helical conformation.8
RESULTS AND DISCUSSION
■
Synthesis and Polymerization of Optically Active m-
Phenylene Ethynylenes. The optically active monomers, m-
trimethylsilylethynyl iodobenzenes bearing an L- or D-alanine
residue with a tri(ethylene glycol) as the pendant through an
amide linkage ((S)- or (R)-PEa, respectively), were synthesized
as outlined in Schemes S1−S4, and these were polymerized
according to the previously reported method using the
microwave-assisted polycondensation25 in acetonitrile at 60
°C (Scheme 1). The polymerization results are summarized in
Table 1. The polymerization of (S)-PEa and (R)-PEa
homogeneously proceeded, yielding high molecular weight
polymers as an orange-brown solid in moderate yield (58% and
50%, respectively). The number-average molecular weight (Mn)
and its distribution (Mw/Mn) were estimated to be 10.4 × 104
and 3.12 for (S)-PPEa as well as 10.8 × 104 and 2.70 for (R)-
PPEa, as determined by size exclusion chromatography (SEC)
with polystyrene standards using tetrahydrofuran (THF)
containing 0.1 wt % tetra-n-butylammonium bromide
(TBAB) as the eluent.
Chiroptical Properties in Dilute Solution. Moore and
co-workers previously reported that oligo(m-phenylene
ethynylene)s bearing electron-withdrawing ester residues with
polar tri(ethylene glycol) chains as the pendants (OPEe, Chart
1) solvophobically fold into helical conformations in polar
solvents, such as acetonitrile and aqueous organic solutions.8
This strong tendency of helical folding is attributed to the
intramolecular π−π stacking between nonadjacent electron-
poor phenylene ethynylene residues in conjunction with the
favorable interactions between the polar side chains and the
polar solvent, while the nonpolar folded backbone is hidden
from the polar solvent. On the contrary, in less polar “good”
solvents for both backbone and side chains, such as CHCl3, the
oligomers adopt a random-coil conformation. These conforma-
tional states are also dependent on the chain-length8c,15a and
temperature8a,15b as well as solvent polarities.11c
Addition of 2,2,2-trifluoroethanol (TFE) as a hydrogen-bond
competitor to a CHCl3 solution of (S)-PPEa caused a
significant decrease in the Cotton effects, and the CD
completely disappeared in the presence of a small amount of
TFE (2.5 vol %) (Figure 2). These sudden changes in the CD
spectra were accompanied by a large red-shift in the absorption
up to ca. 13 nm along with an appearance of a new peak at 307
On the basis of these precedents, we first investigated the
solvent effects on the chiroptical properties of PPEas in a dilute
solution by measuring the absorption and CD spectra of (S)-
PPEa and (R)-PPEa in various solvents. Figure 1A shows the
typical CD and absorption spectra of (S)-PPEa and (R)-PPEa
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intermolecular hydrogen bonds to the amides (as it is present
in large excess as compared to the polymers’ amide residues).
When such a system is subsequently heated, the TFE−amide
contacts are broken, and the intramolecular hydrogen-bonding
network is reinstalled leading to folding that is thermodynami-
cally driven by the release of TFE. Hence, the “inverse melting”
behavior appears to be entropic in origin.
The importance of the intramolecular hydrogen-bonding
network between the amide residues is perhaps most apparent
when comparing the folding behavior of (S)-PPEa (vide supra)
with its N-methylated derivative (S)-PPEa-NMe (Chart 1),
which can readily be prepared by direct N-methylation using
excess methyl iodide in the presence of excess sodium hydride
as the base (see the Supporting Information). Almost complete
1
N-methylation was clearly visible from the H NMR spectra
Figure 2. (A) CD (top) and absorption (bottom) spectral changes of
(S)-PPEa with the increasing volume fraction of TFE in CHCl3 at 25
°C. (B) Changes in the CD intensities at the first (Δεfirst) and second
(Δεsecond) Cotton effects and the intensity ratio of the absorption
maxima at 279 and 307 nm (Abs307/Abs279) versus volume % of TFE
in CHCl3. The spectra were normalized on the basis of the absorption
maxima.
(Figure S2) that furthermore showed much narrower peaks,
indicating the absence of π−π stacking interactions and hence
an unfolded random coil conformation. Much more impres-
sively, the absence of any CD signal as well as the occurrence of
transoid absorption bands unambiguously demonstrate that
(S)-PPEa-NMe adopts a random coil conformation, regardless
of the solvent (Figure S3). Even in the folding-promoting
solvent CH3CN, (S)-PPEa-NMe is not folding, demonstrating
the importance of the hydrogen-bonding for helix formation in
(S)-PPEa and the slightly more electron-rich character of the
amide-substituted m-phenylene ethynylene units, leading to
weaker π−π stacking contacts (as compared to their ester
counterparts, which fold under these conditions).15c
Helical Structure of PPEa. To obtain structural
information of PPEa in the solid state, wide-angle X-ray
diffraction (WAXD) measurements of oriented PPEa films
were performed. Samples for the WAXD measurements were
prepared from a concentrated CHCl3 solution in a magnetic
field (11.75 T overnight) by slow evaporation,22b and the films
exhibited a birefringence as observed by polarizing optical
microscopy (Figure 4G). Figure 4A−D shows the WAXD
patterns of the magnetically oriented PPEa films, which
displayed diffuse, but apparent equatorial and near-meridional
reflections. The four equatorial reflections, 27.29, 15.89, 13.62,
and 10.37 Å for (S)-PPEa and 27.22, 15.52, 13.64, and 10.20 Å
for (R)-PPEa, can be indexed with a 2D hexagonal lattice of a =
31.51 and 31.43 Å, respectively, and the observed d-spacings are
listed in Table 2. The strong meridional reflections at 3.56 Å for
(S)-PPEa and 3.55 Å for (R)-PPEa most likely correspond to
the spacing between the π-stacked benzene units,27 which were
in accordance with those reported for the endo-methyl
substituted m-phenylene ethynylene oligomers (OPEe-Me,
Chart 1) (Table S2). Moore and co-workers previously
reported that the introduction of methyl groups into the
nm (Figure 2A) derived from a transoid conformation of the m-
phenylene ethylene units, leading to a random coil induced by
TFE,8,15a which hampered the intramolecular hydrogen-bond
networks of PPEa chains. This interpretation is supported by
the fact that the NH and amide I bands of PPEa in a CHCl3−
TFE mixture (100/5 (v/v)) significantly shifted to higher
wavenumbers accompanied by band broadening and/or
splitting in its IR spectrum (Table S1 and Figure S1).
An intriguing phenomenon was observed when the CD
spectra of (S)-PPEa in CHCl3 solutions containing minor
amounts of TFE were investigated as a function of temperature.
Unexpectedly, partial denaturation caused by small amounts (2
vol %) of TFE in CHCl3 at room temperature was not
completed by additional heating of the sample, but, on the
contrary, refolding was observed at elevated temperatures
(Figure 3A). The folding transition temperatures are highly
sensitive to the amount of TFE present, showing a strong
increase with the increasing TFE content ranging from 1.0 to
2.5 vol % in CHCl3 (Figure 3B). At first glance, this anomalous
“inverse melting” behavior seems to contradict classical
temperature-induced denaturation experiments with biopoly-
mers, such as polypeptides/proteins or DNA, in which higher
temperatures provide the enthalpy necessary to break the
noncovalent interactions responsible for folding.2b However, in
the present case, the polymer is denatured in CHCl3 at low
temperatures because the TFE successfully interrupts the
intramolecular amide hydrogen-bonding network by forming
Figure 3. (A) CD spectral changes of (S)-PPEa with varying temperature (5−55 °C) in CHCl3 containing 2 vol % TFE. (B) Changes in the CD
intensities at the first (Δεfirst) and second (Δεsecond) Cotton effects versus temperature at varying amounts of TFE (1.0−2.5 vol %) in CHCl3.
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Figure 4. X-ray diffraction patterns of oriented (S)-PPEa (A) and (R)-PPEa (C) films prepared from concentrated CHCl3 solutions (ca. 16 wt %).
The magnified X-ray diffraction patterns corresponding to the area indicated by the white squares in (A) and (C) are shown in (B) and (D),
respectively. (E) Optimized right-handed helical structure of (S)-PPEa on the basis of XRD structural analyses followed by molecular mechanics
calculations (see the Supporting Information). The main-chain carbon atoms are shown using the ball and stick model, and the pendant tri(ethylene
glycol) chains are omitted for clarity. The helical pitch (3.56 Å) corresponding to π−π stacking is also shown. (F) Schematic illustration of a possible
hexagonal packing of (S)-PPEa. (G) Polarized (top) and nonpolarized (bottom) optical micrographs of an (R)-PPEa film.
reasonable by taking into account the longer side chain of PPEa
as compared to OPEe-Me. Figure 4E and F illustrates the
molecular structures of the right-handed helical (S)-PPEa
model (60-mer) and its hexagonal packing, respectively,
estimated on the basis of the XRD data. It should be
emphasized that PPEa’s carrying no methyl groups at the
endo-positions fold into a helical conformation in a variety of
polar and nonpolar solvents and further self-assemble into
columnar nanotubules as stabilized by the intramolecular
hydrogen-bonding networks. In strong contrast, the corre-
sponding ester-linked OPEe’s that form a helical conformation
in specific polar solvents cannot maintain their helical structure
in the bulk and adopt a zigzag conformation to pack into a
lamellar arrangement.17
Table 2. X-ray Diffraction Data of Oriented (S)- and (R)-
PPEa Films
(S)-PPEa
(R)-PPEa
a
b
c
a
b
c
dobs/Å
dcal/Å
hkl
Iobs
dobs/Å
dcal/Å
hkl
Iobs
27.29
15.89
13.62
10.37
27.29
15.76
13.65
10.31
100
110
200
210
vs
w
s
27.22
15.52
13.64
10.20
27.22
15.72
13.61
10.29
100
110
200
210
vs
m
s
m
w
a
Spacings observed in X-ray diffraction patterns of (S)- and (R)-PPEa
b
oriented films. Spacings calculated and indexed on the basis of
hexagonal unit cells with parameters a = 31.51 Å for (S)-PPEa and
31.43 Å for (R)-PPEa. Observed intensities: vs = very strong, s =
c
strong, m = medium, and w = weak.
Although m-phenylene ethynylene oligomers have been
considered to have a ∼6 units per turn16 helical conformation
in polar solvents such as acetonitrile and in the solid state for
OPEe-Me, their exact helical structures, in particular, the helical
sense, have not yet been elucidated unambiguously, most likely
because of the difficulty in obtaining a single-crystal suitable for
X-ray crystallographic analysis.17b To gain further insight,
theory employing molecular mechanics simulations was used
for postulating the helical sense of foldamer-based helical
oligomers and polymers, such as m-phenylene ethynylene
oligomers containing an optically active helicene unit,11d m-
endo-positions of m-phenylene ethylene oligomers (OPEe-Me)
resulted in the formation of helical nanotubules with a similar
hexagonal arrangement in the solid state, whereas the
corresponding m-phenylene ethylene oligomers packed into a
lamellar arrangement, presumably to avoid formation of hollow
interior channels.17b Comparison of helical structures of PPEa
with those of Moore’s oligomers OPEe-Me revealed that the X-
ray diffraction patterns are very similar to one another.
Although the hexagonal lattice of PPEa (31−32 Å) appears
to be slightly larger than those of OPEe-Me (30 Å),17b it seems
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Figure 5. (A) AFM height (left) and phase (right) images of (S)-PPEa on HOPG (scale = 300 × 300 nm) prepared by casting a dilute CHCl3
solution (0.02 mg/mL). The cross-section profile denoted by the white dashed line and the zoomed image corresponding to the area indicated by
the square are also shown. (B,D) High-resolution AFM phase images of (S)-PPEa (B) and (R)-PPEa (D) on HOPG (scale = 40 × 20 nm).
Schematic representations of the right-handed helical (S)-PPEa and left-handed helical (R)-PPEa structures with periodic oblique stripes (yellow
lines), which denote a one-handed helical array of the pendants, are also shown. For wider AFM images of 2D self-assembled (S)-PPEa and (R)-
PPEa, see Figures S6 and S7, respectively. (C,E) Optimized helical structures of (S)-PPEa (C, 60-mer) and (R)-PPEa (E, 60-mer) calculated on the
basis of XRD structural analyses followed by molecular mechanics calculations (see the Supporting Information). Each structure is represented by
space-filling models, and six sets of hydrogen-bonded helical arrays (n and n + 6) of the pendants are shown in different colors for clarity. (F,G) The
detailed helical structures of (S)-PPEa (12-mer) taken from (C) are also shown by a ball and stick model in (F) (top view) and (G) (side view). In
these models, the long oligo(ethylene glycol) side groups were replaced by terminal methyl groups for clarity.
ethynylpyridine oligomers bearing saccharide moieties as the
terminal,13d and azobenzene-bound poly(m-phenylene ethyny-
lene) containing D-amino acid residues.12j Optically active
poly(m-phenylene ethynylene-alt-p-phenylene ethynylene)s
bearing chiral pendants were also suggested to adopt a
left14b- or right14c,d-handed helix in solution judging from the
Cotton effect patterns in their CD spectra. However, these
computational methods certainly involve a significant un-
certainty, and hence an experimental approach for reliable
structure elucidation is highly desirable.
Recently, we developed a facile method to directly observe
the helical structures of several synthetic helical polymers
including poly(phenylacetylene)s bearing L- or D-alanine
residues with a long alkyl chain as the pendants.21 Flat
poly(phenylacetylene) monolayers were first formed epitaxially
on the basal plane of the graphite, on which helical
poly(phenylacetylene) further self-assembled into chiral 2D
helix-bundled crystals upon exposure to organic solvent
vapors.21a The surface of the 2D crystals was extremely smooth
and flat, which enabled us to observe high-resolution AFM
images and to determine the molecular packing, helical pitch,
and handedness when combined with XRD analysis. Hence, we
applied this procedure to visualize the helical structures of (S)-
and (R)-PPEa’s on HOPG.
Figure 5A shows a typical AFM image of (S)-PPEa deposited
on HOPG from a dilute CHCl3 solution (0.02 mg/mL) under a
nitrogen atmosphere28 at ambient temperature. (S)-PPEa self-
assembled into well-defined 2D helix-bundles with a constant
height of 2.6 nm, indicating the polymer maintains its folded
structure on HOPG. Careful observation of the AFM image
together with an additional experiment (Figure S4)29 suggests
that a regular monolayer composed of nonhelical (S)-PPEa
chains with a planar conformation (an average height of 0.4 nm
(first layer); see Figures 5A and S4) was first formed on the
graphite substrate, on which the 2D helix-bundled (S)-PPEa
chains with controlled helicity and lateral packing were
generated, as observed for helical poly(phenylacetylene)s.21a
The bundle structures were clearly resolved into individual
polymer chains packed parallel to each other with a chain-to-
chain spacing of ca. 3.0 nm, which is in good agreement with
that deduced from the XRD measurements (3.15 nm).
The high-resolution AFM images of (S)- and (R)-PPEa’s
revealed a number of periodic oblique stripes (yellow lines in
Figure 5B and D), probably originating from a one-handed
helical array of the pendants that were tilted clockwise or
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Figure 6. AFM height (A) and phase (B) images of (S)-PPEa prepared by casting a dilute CHCl3/TFE solution (96/4, v/v, 0.02 mg/mL) on
HOPG (scale = 500 × 500 nm). The cross-section profile denoted by the white dashed line in (A) is also shown. (C) Typical 2D fast Fourier
transform of the phase image in (B) indicating the periodicity of 6.2 nm. (D) Schematic representation of the PPEa chains lying flat on the substrate
with a planar conformation aligned parallel to the graphite lattice.
counterclockwise at +52° and −56° with respect to the main-
chain axis for (S)-PPEa and (R)-PPEa, respectively. This
remarkable 2D mirror-image relationship suggests that (S)-
PPEa and (R)-PPEa have enantiomeric right- and left-handed
helical structures with a helical pitch of 1.03 0.12 and 1.03
0.11, respectively, as estimated from the average distance
between each stripe. The helical pitch obtained from the AFM
images is almost identical to the helical pitches of the pendant
helical arrangements as estimated from the molecular model
constructed on the basis of the XRD structural analyses
followed by molecular mechanics calculations (Figure 5C and
E). The detailed structures (12-mer) taken from Figure 5C are
also shown in Figure 5F and G (see also the Supporting
Information). The polymer models appear to have six sets of
hydrogen-bonded helical arrays linking n and (n + 6) pendants
with the one-sixth helical pitch of 1.05 nm.30 On the basis of an
evaluation of more than 300 individual polymer chains in the
2D helix-bundles, the number-average molecular length (Ln)
and the length distribution (Lw/Ln) were also estimated to be
21.5 12.2 nm and 1.34 for (S)-PPEa as well as 18.9 8.51
nm and 1.20 for (R)-PPEa. The degree of polymerization (DP)
was then calculated using the helical parameter, 5.7 units per
turn (3.6 Å), to be 340 and 299 for (S)-PPEa and (R)-PPEa,
respectively (Figure S5). These values correspond to Mn = 11.8
× 104 for (S)-PPEa and Mn = 10.4 × 104 for (R)-PPEa, in good
agreement with the values determined by SEC (vide supra).
The effect of TFE on the self-assembled PPEa structure on
HOPG was also investigated. Figure 6 shows the AFM image of
(S)-PPEa prepared by casting a dilute CHCl3 solution
containing 4 vol % of TFE on HOPG. The PPEa molecules
spontaneously self-assembled into a highly ordered monolayer
as evidenced by parallel stripes with a periodicity of ca. 6.2 nm.
The 2D fast Fourier transform of this monolayer showed 3-fold
symmetry (Figure 6C), reflecting the crystallographic structure
of the HOPG substrate. The average height of the monolayer
was ca. 0.7 nm. These results indicate that the PPEa molecules
lie flat on the substrate with a planar conformation aligned
parallel to the graphite lattice, probably because of the strong
G
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Figure 7. AFM height (A) and phase (B) images of onion-like assemblies of (S)-PPEa on HOPG (scale = 310 × 310 nm) prepared by casting a
dilute CHCl3 solution (0.02 mg/mL). The cross-section profile denoted by the white dashed line in (A) is also shown. (C) Schematic illustration for
onion-like assemblies corresponding to the area indicated by the squares in (A). A cyclic chain is shown in red. (D) AFM height and phase images of
cyclic (S)-PPEa chains and its molecular model (E,F). The cross-section height profile measured along the white dashed line is also shown in (D).
and epitaxial adsorption of the long pendant chains on HOPG
(Figure 6B and D).
assembled (Figure 7C). Figure 7D shows a double-ring
assembly with a diameter of 37 and 45 nm for an inner ring
and outer ring, respectively, which correspond to 186 and 234
monomer units (Figure 7E and F). The mechanism for such
ring-like assembly formations is not clear at present, but may be
ascribed to a template effect of the cyclic polymers that
probably nucleate the assemblies of linear polymers.32
Interestingly, besides 2D-assembled helical PPEa chains
(Figure 5), cyclic molecules (Figure 7D) and onion-like
assembled PPEa chains (Figure 7A and B) with a similar
height of ca. 0.4 nm were also observed during the high-
resolution AFM observations when casting a dilute CHCl3
solution (0.02 mg/mL) of (S)-PPEa on HOPG. Such cyclic
polymers could be inevitably generated during the poly-
condensation reaction of the monomers, followed by the
termination.31 The onion-like assemblies seem to consist of a
few cyclic polymers around which linear polymers may be
CONCLUSIONS
■
We have successfully synthesized optically active, amphiphilic
poly(m-phenylene ethynylene)s carrying chiral tri(ethylene
glycol) side chains derived from L- or D-alanine and attached
H
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to the aromatic backbone via an amide linkage. In sharp
contrast to the analogous ester-linked oligo(m-phenylene
ethynylene)s, the amide-linked polymers adopt remarkably
stable, excess one-handed helical conformations in a variety of
polar and nonpolar solvents. The helical structure is stabilized
by six sets of intramolecular hydrogen-bonding helical arrays
between the pendants as characterized by absorption, CD, and
FT-IR spectroscopies, as well as comparison with the
corresponding N-methylated derivative. Because of its excep-
tional stability, the helix was retained even in the solid state,
allowing its unprecedented structural characterization. There-
fore, for the first time, the helical structures of m-phenylene
ethynylene foldamers including the helical pitch and helical
sense could unambiguously be determined by high-resolution
AFM observations combined with X-ray diffraction measure-
ments. The present study implies that previously unsolved
helical structures of other helical polymers and oligomers,1r,4
and π-stacked supramolecular helical assemblies derived from
small molecules,1j may be determined by high-resolution AFM
if 2D self-assembled helix-bundled crystals are available on
substrates. From a more applied perspective, the finding that
during the hierarchical supramolecular organization of our
polymers their hollow helical structure is retained in the solid
state offers the prospect of engineering chiral nanoporous
materials, suitable for separation and sensing of (chiral)
compounds and analytes, respectively.
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ASSOCIATED CONTENT
* Supporting Information
Experimental details. This material is available free of charge via
the Internet at http://pubs.acs.org.
■
S
(4) Yashima, E. Polym. J. 2010, 42, 3−16.
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AUTHOR INFORMATION
Corresponding Author
sh@chemie.hu-berlin.de; yashima@apchem.nagoya-u.ac.jp
■
(7) (a) Ute, K.; Hirose, K.; Kashimoto, H.; Nakayama, H.; Hatada,
K.; Vogl, O. Polym. J. 1993, 25, 1175−1186. (b) Ito, Y.; Ohara, T.;
Shima, R.; Suginome, M. J. Am. Chem. Soc. 1996, 118, 9188−9189.
(c) Tanatani, A.; Yokoyama, A.; Azumaya, I.; Takakura, Y.; Mitsui, C.;
Shiro, M.; Uchiyama, M.; Muranaka, A.; Kobayashi, N.; Yokozawa, T.
J. Am. Chem. Soc. 2005, 127, 8553−8561. (d) Dolain, C.; Jiang, H.;
Present Address
§Department of Applied Chemistry, Graduate School of
Engineering, Nagoya University, Chikusa-ku, Nagoya,
464−8603, Japan.
Notes
The authors declare no competing financial interest.
́
Leger, J.-M.; Guionneau, P.; Huc, I. J. Am. Chem. Soc. 2005, 127,
12943−12951. (e) Suginome, M.; Noguchi, H.; Murakami, M. Chem.
Lett. 2007, 36, 1036−1037. (f) Kendhale, A. M.; Poniman, L.; Dong,
Z.; Laxmi-Reddy, K.; Kauffmann, B.; Ferrand, Y.; Huc, I. J. Org. Chem.
2011, 76, 195−200.
ACKNOWLEDGMENTS
■
This work was supported in part by a Grant-in-Aid for Scientific
Research (S) from the Japan Society for the Promotion of
Science (JSPS) and the Global COE Program “Elucidation and
Design of Materials and Molecular Functions” of the Ministry
of Education, Culture, Sports, Science, and Technology, Japan.
M.B. expresses his thanks for the JSPS Research Fellowship for
Young Scientists (No. 9164).
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Journal of the American Chemical Society
Article
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were also observed for (S)-PPEa, we could not characterize these
reflections, which may be related to the electron densities along with
the main-chain axis.
(28) When the sample was exposed to organic solvent vapors
according to a previously reported method,21,22,24 almost similar AFM
images were obtained, indicating that organic solvent vapors may not
be essential for obtaining 2D assembled helix-bundles at least for
PPEa.
(29) Casting a more dilute solution (0.01 mg/mL) of (S)-PPEa on
HOPG produced the isolated monolayers with an average height of 0.4
nm (first layer, Figure S4), which was much less than the molecular
diameter of a helically folded PPEa model (3.2 nm). Further
deposition of the same dilute solution on the monolayers on HOPG
resulted in the formation of the 2D helix-bundled (S)-PPEa assemblies
(second layer) whose AFM images were almost identical to those
shown in Figure 5A.
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pendants, the main-chain has a 5.7 units per turn right-handed helical
structure.
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