Stimuli-responsive helical poly(phenylacetylene)s bearing cyclodextrin pendants that exhibit enantioselective gelation in response to chirality of a chiral amine and hierarchical super-structured helix formation

Article abstract of DOI:10.1021/ma200537p

Novel poly(phenylacetylene)s bearing a β-cyclodextrin (CyD) residue connected to the phenyl ring through an ester (poly-2β) and an ether linkage (poly-3β) as well as an amide linkage (poly-1β) were synthesized and their chiroptical properties were investi

Full text of DOI:10.1021/ma200537p

ARTICLE
pubs.acs.org/Macromolecules
Stimuli-Responsive Helical Poly(phenylacetylene)s Bearing
Cyclodextrin Pendants that Exhibit Enantioselective Gelation
in Response to Chirality of a Chiral Amine and Hierarchical
Super-Structured Helix Formation
Katsuhiro Maeda,† Hiroaki Mochizuki, Keiko Osato, and Eiji Yashima*
Department of Molecular Design and Engineering, Graduate School of Engineering, Nagoya University, Chikusa-ku,
Nagoya 464-8603, Japan
S Supporting Information
b
ABSTRACT: Novel poly(phenylacetylene)s bearing a β-cyclo-
dextrin (CyD) residue connected to the phenyl ring through an
ester (poly-2β) and an ether linkage (poly-3β) as well as an
amide linkage (poly-1β) were synthesized and their chiroptical
properties were investigated with circular dichroism (CD) and
absorption spectroscopies. The chiroptical studies demon-
strated that the linkage groups play an important role in the
conformational change induced by external chiral and achiral
stimuli, such as temperature, solvent, and interactions with a
chiral amine. Poly-1β and poly-2β showed a unique enantioselective gelation in response to the chirality of a chiral amine, and the
polymers further formed hierarchical superstructured helical assemblies on a micrometer scale with a controlled helix sense, as
evidenced by the scanning electron microscopy observations.
’ INTRODUCTION
Chart 1 ) and permethylated β-CyD residues and investigated
the effect of the ring size of the CyD residues and the hydroxy
groups in the CyD unit on their chiroptical properties includ-
ing the macromolecular helicity inversion accompanied by a
color change.5,7
Supramolecular self-assembly that results in the formation
of helical aggregates, such as twisted ribbons, wires, fibers,
and tubes, has been one of the emerging research topics that
not only provides imagination to understanding and mimick-
ing biological helices and superstructures but also creates
functional nanomaterials for optoelectronic devices and
biomaterials.1 Although a huge number of helical assemblies
have been constructed from small molecules through non-
covalent bonding interactions,2 only a few studies have
reported supramolecular helical assemblies based on syn-
thetic polymers, in particular, helical polymers except for
biological polypeptides.3,4
We recently reported a unique helical polymer consisting of
a chromophoric stereoregular (cisꢀtransoidal) polyacetylene
backbone and an optically active β-cyclodextrin (CyD) as the
pendant groups linked through an amide bond (poly-1β in
Chart 1), which showed a characteristic induced circular
dichroism (ICD) in the π-conjugated polymer backbone
regions.5 The polymer also exhibited a visible color change
due to a change in the twist angle of the conjugated polymer
backbone induced by various external stimuli, such as the
temperature, solvent, shape of the molecules, and chirality.
5,6 This visible color change was accompanied by inversion of
the macromolecular helicity of the polymer backbone. We also
synthesized a series of cisꢀtransoidal poly(phenylacetylene)s
bearing R- and γ-CyD (poly-1r and poly-1γ, respectively;
Here we designed and synthesized novel CyD-bound poly-
(phenylacetylene)s bearing a β-CyD residue connected to the
phenyl group through an ester (poly-2β) and an ether (poly-3β)
linkage (Chart 1) instead of an amide linkage in poly-1β and
investigated their chiroptical properties in solution by circular
dichroism (CD) and absorption spectroscopies to evaluate the
effects of the hydrogen-bonding ability and polarity of the
linkages on their helical conformational changes responding to
the external stimuli. During the course of our study, we un-
expectedly discovered that poly-1β and poly-2β exhibited a
unique enantioselective gelation in response to the chirality of
a chiral amine, 1-phenylethylamine (4), and further hierarchically
self-assembled to form a micrometer-scale, right-handed super-
structured helix, as visualized by scanning electron microscopy
(SEM). The effects of the ring size of the CyD residues and the
linkage on the enantioselective gelation and supramolecular self-
assembly of other CyD-bound poly(phenylacetylene)s were also
investigated.
Received: March 9, 2011
Revised:
April 3, 2011
Published: April 11, 2011
r
2011 American Chemical Society
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’ RESULTS AND DISCUSSION
(Figures 1A and 2A, respectively), indicating a preferred-handed
helix formation induced by the covalent-bonded chiral β-CyD
units. Interestingly, the first Cotton effect sign of poly-2β that
reflects the helix-sense of the polymer was opposite to those in
poly-1β (Figure S2A in the Supporting Information) and poly-
3β in DMSO.7 This suggests that the helix-sense of poly-2β is
opposite to that of poly-1β and poly-3β, regardless of the fact
that the polymers have the same β-CyD pendant groups. As
previously reported, poly-1β exhibited a color change from red to
yellow in the presence of an increasing amount of water
accompanied by inversion of the first Cotton effect from the
negative to positive direction (Figure S2A,D in the Supporting
Information).7 Such a visible color change and the Cotton effect
inversion in poly-1β can be ascribed to a change in the twist angle
of the conjugated double bonds and an inversion of the helicity of
the polymer backbone, respectively.7,8
Synthesis and Conformational Changes of Helical Poly-2β
and Poly-3β in Response to Various Chiral and Achiral
Stimuli. Stereoregular (cisꢀtransoidal) poly-2β and poly-3β
were prepared by the polymerization of the corresponding
monomers with a rhodium catalyst using a method similar to
that previously reported for poly-1rꢀpoly-1γ,7 as outlined in
Scheme 1 in the Experimental Section, and their characteristics
including poly-1rꢀpoly-1γ are summarized in Table 1.7 The
cisꢀtransoidal structures of poly-2β and poly-3β were confirmed
by laser Raman spectroscopy (Figure S1 in the Supporting
Information).
Poly-2β and poly-3β showed an intense CD in the long
absorption regions of the polymer backbones at 25 °C in DMSO
Chart 1. Structures of CyD-Bound Poly(phenylacetylene)s
Poly-2β and poly-3β showed a similar solvatochromism in
DMSO in the presence of an increasing amount of water. The
absorbance maximum (λmax) at 510 nm of poly-2β blue-shifted
Table 1. CyD-Bound Poly(phenylacetylene)s Used in This
Study
Mn ꢁ 10ꢀ4a
Mw/Mn
a
sample code
poly-1βb
poly-2β
poly-3β
poly-1rb
poly-1γb
16.3
8.1
1.5
1.2
1.2
1.2
1.2
9.6
16.0
11.1
a Number-average molecular weight (Mn) and its molecular weight
distribution (Mw/Mn) were determined by size exclusion chromatogra-
phy (SEC) using DMF containing 10 mM LiCl as the eluent with
poly(ethylene oxide) (PEO) and poly(ethylene glycol) (PEG) stan-
dards. b Data are cited from ref 7.
Scheme 1. Synthesis of Poly-2β and Poly-3β
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Figure 1. (AꢀC) CD and UV/vis spectra of poly-2β ([poly-2β] = 1.0 mg/mL, 0.79 mM) in water/DMSO (5:5ꢀ0:10, v/v) at 25 °C (A), in DMSO at
various temperatures (B), and in water/DMSO (6:4, v/v) in the presence of (S)-4 (red lines) and (R)-4 (blue lines) ([4] = 0.23 M) at 25 °C (C). The
lines for water/DMSO (4:6 and 5:5, v/v) in (A) and those for water/DMSO (6:4, v/v) (data not shown) are almost overlapped. (DꢀF) Visible color
changes of poly-2β in water/DMSO mixtures at ambient temperature (ca. 25 °C) (D) in DMSO at 90 (left) and 25 °C (right) (E) and in water/DMSO
(6:4, v/v) with (S)- (left) and (R)-4 (right) at ambient temperature (ca. 25 °C) (F).
Figure 2. (AꢀC) CD and UV/vis spectra of poly-3β ([poly-3β] = 1.0 mg/mL, 0.76 mM) in water/DMSO (5:5ꢀ0:10, v/v) at 25 °C (A), in DMSO at
various temperatures (B), and in water/DMSO (5:5, v/v) in the presence of (S)-4 (red lines) and (R)-4 (blue lines) ([4] = 0.76 M) at 25 °C (C). (DꢀF)
Visible color changes of poly-3β in water/DMSO mixtures at ambient temperature (ca. 25 °C) (D), in DMSO at 90 (left) and 25 °C (right) (E), and in
water/DMSO (5:5, v/v) with (S)- (left) and (R)-4 (right) at ambient temperature (ca. 25 °C) (F).
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Figure 3. Enantioselective gelation of poly-2β and poly-1β upon the addition of chiral amine 4. (AꢀD) Photographs of poly-2β (20 mg/mL) in the
absence (A) and presence of (S)-4 (B), (R)-4 (C), and racemic (RS)-4 (D) ([4]:[poly-2β] 40:1, mol/mol) in water/DMSO (6:4, v/v) at room
temperature. (E) AFM height image (1.1 ꢁ 1.1 μm2) of the organogel composed of poly-2β and (S)-4 (B) transferred on mica. (FꢀH) Photographs of
poly-1β (20 mg/mL) in the absence (F) and presence of (S)-4 (G) and (R)-4 (H) ([4]:[poly-1β] 40:1, mol/mol) in water/DMSO (6:4, v/v) at room
temperature.
by 32 nm, and the solution color changed from red to yellow-
orange accompanied by the inversion of the first Cotton effect
sign from the positive to negative direction at around water/
DMSO 3:7 (v/v) (Figure 1A,D). Poly-3β bearing an ether linker
maintained its Cotton effect signs in the mixed solvents despite a
similar blue shift in its absorption spectra, resulting in a visible
color change (red to yellow) (Figure 2A,D). The blue shifts in
the absorption spectra suggest that the polymers may have a
more tightly twisted helical conformation in solution, whereas
the red polymers may have an extended π-conjugation. More-
over, poly-2β also exhibited a clear thermochromism, as obser-
ved for poly-1β (Figure S2B,E in the Supporting Information);
the λmax significantly blue-shifted, and the solution color changed
from red at 25 °C to yellow-orange at 90 °C (Figure 1B,E).
However, the first Cotton effect sign of poly-2β remained
positive even at high temperatures, although inversion of the
Cotton effect took place for poly-1β accompanied by a change in
the solution color (Figure S2B,E in the Supporting Information).
In sharp contrast, the CD and absorption spectra of poly-3β, in
which the amide or ester linkage of poly-1β and poly-2β was
replaced by an ether, in DMSO hardly changed upon heating; the
solution remains red with a negative Cotton effect at 460 nm
(Figure 2B,E). These results demonstrated that the linkage
groups of the CyD-bound helical poly(phenylacetylene)s play
an important role in the conformational change induced by
external stimuli, which may be dictated by intramolecular hydro-
gen-bonding networks and dipoleꢀdipole interactions of the
pendant amide and ester groups of poly-1β and poly-2β,
respectively.
of (R)-4 ([(R)-4]/[poly-2β] = 300), however, the Cotton effect
signs and absorption spectra did not change (Figure 1C). We
note that the Cotton effect inversion of poly-2β with (S)-4 was
observed only in water/DMSO 6:4 (v/v) but did not take place
in water/DMSO mixtures (5:5ꢀ0:10, v/v), indicating that the
helical sense inversion appears to be governed by a delicate
balance of specific interactions between poly-2β and 4 and also
solvation with water. In contrast with poly-1β (Figure S2F in the
Supporting Information), no apparent difference in the solution
color was observed for poly-2β in the presence of either (S)-4 or
(R)-4 (Figure 1F). Poly-3β bearing the same β-CyD units
through an ether linkage showed negligible changes in the CD
and absorption spectra independent of the configuration of 4 in
water/DMSO mixtures (5:5ꢀ0:10) (for water/DMSO 5:5, see
Figure 2C,F). These results suggest that a preferential chiral
solvation with 4 on the polar amide and ester linkers rather than
an enantioselective inclusion complexation of 4 in the β-CyD
residues may be the driving force for the observed changes in the
absorption and CD spectra and the solution color.
Enantioselective Gelation and Hierarchical Super-Struc-
tured Helix Formation. During the course of our study on
chirality sensing, we unexpectedly discovered that poly-2β en-
antioselectively gelled only in the presence of (S)-4 when the
concentration was higher than 20 mg/mL (Figure 3).10 Upon the
addition of (S)-4 ([(S)-4]/[poly-2β] = 40), a solution of poly-2β
(20 mg/mL) in a water/DMSO mixture (6:4, v/v) gradually
gelled in a few minutes (Figure 3B), whereas the solution never
gelled even in the presence of an excess amount of (R)-4 (80
equiv) (Figure 3C). An atomic force microscopy (AFM) image
of the ultrathin layer of the poly-2β gel with (S)-4 obtained by
transfer from the bulk gel surface onto highly oriented pyrolytic
graphite (HOPG)11 showed well-developed fibrous nanonet-
work structures (Figure 3E), although the helically twisted
structures could not be observed. Quite interestingly, the same
solution also became a gel when the racemic 4 and nonracemic 4
rich in the (S)-enantiomer of 50% ee ([4]/[poly-2β] = 40) were
added (Figure 3D). These results suggest that (R)-4 hardly
As previously reported, poly-1β discriminated the chirality of
1-phenylethylamine (4) and exhibited a CD inversion in the
presence of (S)-4 accompanied by a visible color change, whereas
the poly-1β solution remained unchanged with a positive first
Cotton effect (Figure S2C,F in the Supporting Information).5,7,9
Poly-2β was also sensitive to the chirality of 4, and (S)-4 ([(S)-
4]/[poly-2β] = 300) induced an inversion in the Cotton effect
signs in a mixture of water and DMSO (6/4, v/v). In the presence
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Figure 4. CD and absorption spectral changes of poly-1β (A) and poly-2β (B) with (S)-4 ([(S)-4]/[poly-2β] 40:1, mol/mol, [polymer] = 20 mg/mL)
in water/DMSO (6:4, v/v). Photographs of poly-1β (C) and poly-2β (D) with (S)-4 at room temperature before (red lines) and after (blue lines)
heating at 50 (poly-1β) and 60 °C (poly-2β) (green lines).
contributes to the gelation, but poly-2β exclusively interacts with
(S)-4, resulting in the gel. We also noted that (S)-4 at >10 equiv is
required for the gelation.
Poly-2β did not form a gel in water/DMSO (6/4, v/v)
containing (S)-4 (([4]/[poly-2β] = 40ꢀ1000) when the poly-
2β concentration was 1 mg/mL. Unprecedentedly, we found a
small amount of precipitate after the solution had been allowed
to stand for 5 days, whereas no precipitation occurred for poly-2β
with (R)-4. AFM images of poly-2β cast from a dilute solution of
poly-2β (0.1 mg/mL) in water/DMSO (6:4, v/v) containing
(S)-4 ([(S)-4]/[poly-2β] = 300) on mica after the sample had
been allowed to stand for hours revealed individual poly-2β
chains after 2 h, but aggregated large particles were observed after
24 h (Figure S3 in the Supporting Information). These results
suggest that poly-2β gradually aggregates with time in the solvent
with (S)-4 and finally forms a fibril-like helical morphology
through hierarchical self-assembly. By measuring the AFM
images of the individual poly-2β chains, we estimated the
number-average molecular length (Ln) and average height to
be 15.7 and 1.8 ( 0.16 nm, respectively, based on an evalua-
tion of ca. 100 molecules. The SEM images of the precipitates
after sputter-coating with gold clearly revealed right-handed
twisted rod-like ribbon structures (Figure 5A,C) with a
diameter of a few micrometers and a length of >100 μm.
Quite interestingly, some of them formed double helices, and
each strand also appeared to be predominantly right-handed
(Figure 5A,B). The expanded SEM image of the open-ended
cross section indicates that the helical aggregates may not be
tubular in morphology (Figure 5D). As a consequence, the
chirality of (S)-4 biased the preference of the helical poly-2β
for the formation of a micrometer-scale, right-handed super-
structured helix.
To gain insight into the mechanism of this unique enantiose-
lective gelation of poly-2β, we examined the effects of the water
content in DMSO (water/DMSO = 0ꢀ100%, v/v) on the gelation
in the presence of a constant concentration of (S)-4 (40 equiv).
Poly-2β solutions with (S)-4 gelled within 0.5 h in the mixed
solvents with water contents of >60 vol %, although a further
increase in the water content (>90 vol %) caused a precipitation of
the polymer (Table S1 in the Supporting Information).
We then investigated the gelation ability of a series of CyD-
bound poly(phenylacetylene)s (poly-1β, poly-3β, poly-1r, and
poly-1γ) in water/DMSO mixed solvents under the same
experimental conditions. A similar enantioselective gelation took
place for poly-1β in response to the chirality of 4, although a
rather longer time (∼2 days) was required for the gelation
(Table S1 in the Supporting Information and Figure 3FꢀH).
However, solutions of poly-3β, poly-1r, and poly-1γ did not
form a gel even after 1 week (Table S1 in the Supporting
Information), suggesting that the chiral solvation through the
cooperative interaction of 4 with the linker residues of the CyD-
bound poly(phenylacetylene)s as well as a favorable enantiose-
lective inclusion complexation of 4 into the β-CyD cavity seem to
be essential for the formation of a gel. However, the enantios-
electivity of 4 with β-CyD derivatives was reported to be
poor12,13 so that the preferred-handed helical array of the β-
CyD units and the polar amide (poly-2β) or ester (poly-1β)
linker residues along the polymer backbones may play a crucial
role for the emergence of such an enantioselective gelation. The
gels of poly-1β and poly-2β prepared in water/DMSO (6:4, v/v)
with (S)-4 ([4]/[polymer] = 40, [polymer] = 20 mg/mL)
exhibited characteristic ICDs, but the ICD intensities decreased
with an increase in the temperature and almost completely
disappeared after melting at 50 and 60 °C, respectively. These
gelꢀsol transitions accompanied by the ICD changes reversibly
took place (Figure 4). These results suggest that a preferred-
handed helical conformation of the polymers may be necessary
for the gelation.
Similar supramolecular aggregates could be obtained when
a solution of poly-2β in water/DMSO mixture (1 mg/mL; 6:4,
v/v) was slowly evaporated under reduced pressure on a tilted
glass plate (Figure 5E), resulting in fibril-like helical aggregates
independent of the presence and absence of (S)-4, and right-
handed twisted aggregates could be partially observed. Other
CyD-bound poly(phenylacetylene)s (poly-1β, poly-3β, poly-1r,
and poly-1γ) also formed fibril-like aggregates (Figure 6), in
which only poly-1β partially formed a twisted aggregate in
a hierarchical fashion, although its helical structure and
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Figure 5. SEM images of self-assembled helical poly-2β aggregates obtained from slow evaporation of a solution of poly-2β (1 mg/mL) in water/
DMSO (6:4, v/v) in the presence (AꢀD) and absence (E) of (S)-4 ([(S)-4]:[poly-2β] 1000:1, mol/mol). The open-ended cross-section of aggregates
(D) and side views of an individual helical poly-2β aggregate (AꢀC) and multiple fibril-like aggregates (E) are shown.
residues. The biased preference for one particular helical sense
may possibly be amplified in the presence of (S)-4.
’ CONCLUSIONS
In summary, we found that the linkage groups of the β-CyD-
bound helical poly(phenylacetylene)s play a crucial role in the
conformational change induced by external stimuli, such as the
temperature, solvent, and chirality. In addition, we demonstrated
that poly-1β and poly-2β with a β-CyD pendant connected
through polar amide and ester linkages, respectively, showed a
unique enantioselective gelation in response to the chirality of
the chiral amine (4), and the polymers further formed hierarch-
ical superstructured helical assemblies on a micrometer scale with
a controlled helix-sense, as evidenced by the SEM observations.
We note that hierarchical superstructured helix formation
through self-assemblies of synthetic helical polymers is quite
rare,4 although a large number of helical aggregates with a well-
defined structure has been constructed from small molecules
based on self-assembly, supramolecular assembly through non-
covalent interactions, or both.1,2 The detailed mechanism for
such a specific gelation and micrometer-scale superstructured
helix formation observed for the particular CyD-bound poly-
(phenylacetylene)s is not completely understood at present, but
these helical systems will contribute to the design and synthesis
of new stimuli-responsive helical polymers with greater sensitiv-
ity accompanied by a visible6 or fluorescence color change14 and
also provide intriguing potentials for applications in future smart
chiral materials for chiral recognition8,15 and enantioselective
Figure 6. SEM images of self-assembled poly-1β (A), poly-3β (B),
poly-1r (C), and poly-1γ (D) aggregates prepared from a DMSO/
water (4:6, v/v) solution (1 mg/mL) onto a tilted glass plate by slowly
catalysis.8,16
evaporating the solvent under reduced pressure.
’ EXPERIMENTAL SECTION
handedness were not completely uniform and one-handed.
These SEM observations suggest that the CyD-bound poly-
(phenylacetylene)s tend to form unique micrometer-scale fibril-
like arrays, and poly-1β and poly-2β with a β-CyD pendant
connected through a polar amide or ester linkage self-aggregate
to form twisted ribbon structures biased by the chiral CyD
Instruments. NMR spectra were measured on a Varian Mercury-
300 spectrometer operating at 300 MHz for 1H with TMS (for CDCl3)
or a solvent residual peak (for DMSO-d6) as the internal standard. IR
spectra were recorded using a JASCO Fourier transform IR-620 spectro-
photometer (Hachioji, Japan). Optical rotation was measured on a
JASCO P-1030 polarimeter in a 5 or 1 cm quartz cell at 25 °C equipped
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with a temperature controller (EYELA NCB-2100, Tokyo, Japan).
Absorption and CD spectra were measured in a 0.1 cm quartz cell
unless otherwise noted using a JASCO V-570 spectrophotometer and
JASCO J-725 or JASCO J-820 spectropolarimeter, respectively. Tem-
perature was controlled with a JASCO PTC-348WI or PTC-423 L and
an ETC-505T apparatus for CD and absorption measurements, respec-
tively. The concentrations of polymers were calculated based on the
monomer units and were 1 mg/mL unless otherwise stated. SEC was
performed using a JASCO PU-980 liquid chromatograph equipped with
a UVꢀvisible detector (254 nm; JASCO UV-970), an RI detector
(JASCO RI-930), and a column oven (JASCO CO-965). The molecular
weights of polymers were determined at 40 °C using Tosoh TSKgel R-
3000 (30 cm) and R-5000 (30 cm) SEC columns connected in series,
and DMF containing 10 mM LiCl was used as the eluent at a flow rate of
0.5 mL/min. The molecular weight calibration curve was obtained with
poly(ethylene oxide) (PEO) and poly(ethylene glycol) (PEG) stan-
dards (Tosoh). Laser Raman spectra were measured on a JASCO RMP-
200 spectrophotometer. Matrix-assisted laser desorption/ionization
time-of-flight mass (MALDI-TOF MS) spectra were recorded on a
Voyager-DE STR (PerSeptive Biosystems) using R-cyano-4-hydroxy-
cinnamic acid or glycerol as the matrix. SEM measurements were
performed on a JEOL JMS-5600 or a HITACHI S-5000 SEM with
the accelerating voltage of 10 kV and emission current of 10 μA. AFM
measurements were performed using a Nanoscope IIIa microscope
(Veeco Instruments, Santa Barbara, CA) in air at ambient temperature
with standard silicon cantilevers (NCH, Nanoworld, Neuch^atel,
Switzerland) in the tapping mode. The AFM images were measured at
the resonance frequency of the tips with 125 μm long cantilevers
(200ꢀ300 Hz) and a spring constant of ∼40 N/m. All images were
collected with the maximum available number of pixels (512) in each
direction. The scanning speed was at a line frequency of 1.0 Hz.
Materials. DMF and DMSO were dried over calcium hydride and
distilled under reduced pressure. Triethylamine (Et3N) was distilled and
dried over KOH pellets under nitrogen. [(Norbornadiene)rhodium(I)
chloride]2 ([Rh(nbd)Cl]2) was purchased from Aldrich and used as
received. β-CyD and p-toluenesulfonylchloride were obtained from
Kishida (Osaka, Japan). (4-Carboxyphenyl)acetylene (CPA)17 and 4-
[2-(trimethylsilyl)ethynyl]phenol18 were prepared according to the
previously reported methods. (R)- and (S)-1-Phenylethylamine (4)
were kindly supplied from Yamakawa Chemical (Tokyo, Japan). Two
novel phenylacetylenes, [(6-β-CyD)carboxyphenyl]acetylene (2β) and
[(6-β-CyD)oxyphenyl]acetylene (3β), were synthesized according to
Scheme 1. CyD-bound poly(phenylacetylene)s (poly-1βꢀpoly-3β,
poly-1r, and poly-1γ) were synthesized according to the previously
reported method,5,7 and their Mn and Mw/Mn values are summarized in
Table 1.
filtration, washed with water, and dried in vacuo at room temperature
overnight to give pure 2β (5.89 g, 4.67 mmol) in 59% yield as a white
25
powder. Mp > 300 °C. [R]D þ130.9 (c 0.5, DMSO). IR (nujol, cmꢀ1):
2106 (CtC), 1718 (CdO of ester). 1H NMR (300 MHz, DMSO-d6):
δ 3.20ꢀ3.90 (m, 36H; CH and CH2), 4.31ꢀ4.95 (m, 12H; OH and
tCH), 5.37ꢀ5.63 (m, 12H; OH), 7.62 (d, J = 8.1 Hz, 2H; aromatic),
7.98 (d, J = 8.4 Hz, 2H; aromatic). MALDI-TOF MS: m/z = 1263 [2β þ
H]þ. Anal. calcd (%) for C51H74O36 4H2O: C, 45.88; H, 6.19. Found:
3
C, 45.84; H, 6.32.
[(6-β-CyD)oxyphenyl]acetylene (3β). To a solution of β-Tos-
CyD (3.38 g, 2.62 mmol) in DMF (70 mL) was added 4-
[2-(trimethylsilyl)ethynyl]phenol (0.50 g, 2.62 mmol) and potassium
carbonate (0.36 g, 2.60 mmol), and the mixture was stirred at 80 °C.
After 14 h, the reaction mixture was allowed to cool to room temperature
and then poured in a large amount of acetone. The resulting precipitate
was collected by filtration, washed with water, and dried in vacuo at room
temperature overnight. The crude 3β was purified by a column
chromatography charged with Diaion HP-20 with water/methanol
(10:0 to 6:4, v/v) as the eluent to give pure 3β (0.65 g, 0.52 mmol)
25
in 20% yield as a white solid. Mp > 300 °C. [R]D þ142.2 (c 0.5,
DMSO). IR (nujol, cmꢀ1): 2101 (CtC). 1H NMR (300 MHz, DMSO-
d6): δ 3.20 ꢀ 3.78 (m, 36H; CH and CH2), 4.31ꢀ4.95 (m, 12H; OH
and tCH), 5.37ꢀ5.63 (m, 12H; OH), 6.94 (d, J = 9.3 Hz, 2H;
aromatic), 7.37 (d, J = 8.7 Hz, 2H; aromatic). MALDI-TOF MS:
m/z = 1236 [3β þ H]þ. Anal. calcd (%) for C50H74O35 3H2O: C,
3
46.58; H, 6.25. Found: C, 46.53; H, 6.31.
Polymerization. Polymerization was carried out in a dry ampule
under a dry nitrogen atmosphere using [Rh(nbd)Cl]2 as a catalyst
according to Scheme 1. A typical polymerization procedure is des-
cribed below.
Poly-2β. 2β (1.51 g, 1.20 mmol) was placed in a dry ampule, which
was then evacuated on a vacuum line and flushed with dry nitrogen, and
a three-way stopcock was attached to the ampule. DMF (7.7 mL) was
added with a syringe, and 2β was dissolved completely and Et3N
(1.7 mL) was added. To this monomer solution was added a solution
of [Rh(nbd)Cl]2 in DMF at 30 °C. The concentrations of the monomer
and the rhodium catalyst were 0.1 and 0.001 M, respectively. The color
of the mixture changed to reddish brown, and the solution became
viscous within 10 min. After 17 h, the resulting polymer was precipitated
in a large amount of methanol, collected by centrifugation, and dried in
vacuo at 50 °C for 2 h (1.64 g, quantitative). The SEC measurement of
the obtained methanol insoluble fraction showed a bimodal SEC trace,
suggesting that the poly-2β consisted of two fractions, a high-molecular-
weight polymeric fraction and a low-molecular-weight oligomeric frac-
tion. To remove the low-molecular-weight fraction, we dissolved the
methanol insoluble part in DMSO (100 mL) and added methanol
(100 mL) as a poor solvent. The resulting yellow precipitate was
collected by centrifugation and washed with methanol. This reprecipita-
tion procedure was repeated two times. The fractionated polymer was
then dried in vacuo at 50 °C for 2 h to give poly-2β showing a unimodal
SEC trace (0.69 g, 45% yield). Poly-2β was soluble in DMSO, DMF, and
water but insoluble in other common organic solvents, such as toluene,
THF, CHCl3, acetonitrile, and acetone. The Mn and Mw/Mn of the poly-
2β were estimated to be 8.1 ꢁ 104 and 1.2, respectively, as determined
by SEC (PEO and PEG standards using DMF containing 10 mM LiCl as
the eluent). The elemental analysis of the polymer agreed satisfactorily
with the calculated values. The stereoregularity of the poly-2β was
investigated by NMR and Raman spectroscopies. However, we could
not estimate the stereoregularity of the poly-2β by its 1H NMR spectrum
because of the broadening of the main chain protons and the overlapping
with some CyD protons even at 80 °C. The Raman spectrum of poly-2β
gave useful information and showed intense peaks at 1547, 1335,
and 963 cmꢀ1, which can be assigned to the CdC, CꢀC, and
CꢀH bond vibrations in the cis-polyacetylenes,20 whereas those in the
Mono-6-deoxy-6-toluenesulfonyl-β-cyclodextrin (β-Tos-
CyD). This compound was prepared from β-CyD and p-toluenesulfo-
nylchloride according to the literature method.19 The yield was 14.7%
1
based on β-CyD. H NMR (300 MHz, DMSO-d6): δ 2.43 (s, 3H;
ArꢀCH3), 3.19ꢀ3.78 (m, 42H; CH and CH2), 4.14ꢀ4.58 (m, 6H;
OH), 4.72ꢀ4.94 (m, 7H; CH), 5.60ꢀ5.88 (m, 14H; OH), 7.24 (d, 2H;
aromatic), 7.77 (d, 2H; aromatic).
Sodium Salt of CPA (CPA-Na). To a suspension of CPA (2.2 g,
15.1 mmol) in water (275 mL) was added 1.0 N NaOH aqueous
solution (13.5 mL, 13.5 mmol), and the mixture was stirred at room
temperature overnight. After filtration, the filtrate was lyophilized to give
CPA-Na (1.87 g, 11.2 mmol) in 74% yield. This compound was used for
the next reaction without further purification.
[(6-β-CyD)carboxyphenyl]acetylene (2β). To a solution of β-
Tos-CyD (10.3 g, 7.97 mmol) in DMSO (150 mL) was added CPA-Na
(1.71 g, 10.2 mmol), and the mixture was stirred at 80 °C. After 46 h, the
reaction mixture was allowed to cool to room temperature and poured in
a large amount of acetone. The resulting precipitate was collected by
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ARTICLE
trans-polyacetylene were not observed, indicating that the poly-2β
possesses a highly cisꢀtransoidal structure (Figure S1 in the Supporting
Information).21
(6/4, v/v) containing (S)-4 ([(S)-4]/[poly-2β] = 300) was prepared.
After the sample had been allowed to stand for 2 and 24 h, a 10 μL
aliquot of the stock solution was dropped on a freshly cleaved mica, the
solution was simultaneously blown off with a stream of nitrogen, then
the mica substrate was dried in vacuo overnight to measure the AFM
images in the tapping mode (Figure S3 in the Supporting Information).
SEM Observations of Helically Assembled Poly-2β Pre-
pared in Solution with (S)-4. A stock solution of poly-2β (1 mg/
mL) in water/DMSO (6:4 v/v) containing an excess amount of (S)-4
([(S)-4]/[poly-2β] = 1000) was prepared in a vessel with a screw cap.
After the solution had been allowed to stand at room temperature (ca.
20ꢀ25 °C) for 2 days, the solvent was slowly removed by evaporation
under reduced pressure in a desiccator. The residual solid was scratched
out with a spatula and mounted on copper stubs with a double-sided
conductive carbon tape. The sample was then sputter-coated with gold
under an electric current of 20 mA at 8 Pa for 20 s before SEM
measurements.
SEM Observations of Self-Assembled CyD-bound Poly-
(phenylacetylene)s Prepared in Solutions without (S)-4. A
typical procedure is described below. A solution of poly-2β (0.1 mg/
mL) in water/DMSO (6:4, v/v) was prepared in a vessel with a screw
cap. A glass plate was vertically placed in the vessel. and the solvent was
slowly removed by evaporation under reduced pressure in a desiccator.
The film formed on the glass surface after removal of the solvent was
peeled off using a razor and mounted on copper stubs with a double-
sided conductive carbon tape. The sample was then sputter-coated with
gold under an electric current of 20 mA at 8 Pa for 20 s before SEM
measurements. The same procedures were performed in the experi-
ments with poly-1β, poly-3β, poly-1r, and poly-1γ.
25
Spectroscopic Data of Poly-2β. [R]D þ1102 (c 0.1, DMSO), þ165
(c0.1, water). IR (KBr, cmꢀ1): 1718 cmꢀ1 (CdO of ester). Anal. calcd (%)
for (C51H74NO36 5H2O)n: C, 45.27; H, 6.26. Found: C, 45.18; H, 6.25.
3
Poly-3β. This polymer was prepared according to an analogous
method for the synthesis of poly-2β. The obtained methanol-insoluble
fraction also consisted of a high-molecular-weight polymer and a low-
molecular-weight oligomer. To remove the low-molecular-weight oli-
gomer, we dissolved the methanol-insoluble part in DMSO and added
methanol as a poor solvent. The resulting yellow precipitate was
collected by centrifugation and washed with methanol. This procedure
was repeated twice. The fractionated polymer was dried in vacuo at
50 °C for 2 h to give poly-3β showing a unimodal SEC trace (0.31 g, 61%
yield). Poly-3β was soluble in DMSO, DMF, and alkaline water (pH
>10). The Mn and Mw/Mn of the poly-3β were estimated to be 9.6 ꢁ 104
and 1.2, respectively, as determined by SEC (PEO and PEG standards
using DMF containing 10 mM LiCl as the eluent).
25
Spectroscopic Data of Poly-3β. [R]D ꢀ585 (c 0.1, DMSO), ꢀ25 (c
0.1, alkaline water (pH 10.5)). Anal. calcd (%) for (C50H74O35
7H2O)n: C, 44.12; H, 6.52. Found: C, 44.27; H, 6.61.
3
CD Measurements: Effect of H2O on CD of Poly-2β and
Poly-3β. A typical experimental procedure for poly-2β is described
below. A solution of poly-2β (2 mg/mL) in DMSO was prepared in six
1 mL flasks equipped with stopcocks. To each flask was added deionized,
degassed H2O (50, 100, 150, 200, and 300 μL) using a Hamilton
microsyringe, respectively, and the solutions were diluted with DMSO
so as to keep the poly-2β concentration to be 1.0 mg/mL. Absorption
and CD spectra were then taken for each flask at various temperatures
controlled with a thermocontroller. In a similar manner, effect of H2O
on CD of poly-3β was investigated.
Effect of Chiral Amine (4) on CD of Poly-2β and Poly-3β. A
typical experimental procedure for poly-2β is described below. A stock
solution of poly-2β (2.5 mg/mL) in DMSO was prepared in a 1 mL flask
equipped with a stopcock. A 400 μL aliquot of the stock solution of
poly-2β was transferred to two 1 mL flasks equipped with stopcocks,
and 600 μL of deionized, degassed water was added to each flask using
a Hamilton microsyringe so as to keep the poly-2β concentration to be
1.0 mg/mL. To each flask was added 30 μL of (S)- and (R)-4, and the
absorption and CD spectra were then taken for each flask at 25 °C. The
same procedure was performed in the experiments with poly-3β using
(S)- and (R)-4.
’ ASSOCIATED CONTENT
S
Supporting Information. Raman spectra of poly-2β and
b
poly-3β, CD and UV/vis spectral changes of poly-1β with visible
color changes induced by external stimuli, solvent effects on
gelation of CyD-bound poly(phenylacetylene)s, and AFM
images of poly-2β with (S)-4 deposited on mica. This material
is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: yashima@apchem.nagoya-u.ac.jp.
Enantioselective Gelation. A typical experimental procedure is
described below. Poly-2β (ca. 4.0 mg, 3.2 μmol) was placed in ten vessels
and a 20 μL aliquot of DMSO was then added to each vessel. After
complete dissolution of the poly-2β, the solutions were diluted with
appropriate amounts of DMSO then water so as to keep the water
contents (vol %) (0, 10, 20, 30, 40, 50, 60, 70, 80, and 90) and the poly-
2β concentration at 20 mg/mL. To each vessel was added 16 μL of (S)-
or (R)-4 (128 μmol) with a Hamilton microsyringe, and the solutions
were immediately mixed with a vibrator. The same procedures were
performed in the experiments with poly-1β, poly-3β, poly-1r, and poly-
1γ (Table S1 in the Supporting Information).
Present Addresses
†Graduate School of Natural Science and Technology, Kanazawa
University, Kakuma-machi, Kanazawa 920-1192, Japan.
’ ACKNOWLEDGMENT
This work was partially supported by Grant-in-Aid for Scien-
tific Research (S) from the Japan Society for the Promotion of
Science (JSPS).
AFM Measurements of Gel. An organogel of poly-2β was
prepared in the same way as that described above in water/DMSO
(6:4 v/v) in the presence of (S)-4 ([poly-2β] = 20 mg/mL, [(S)-
4]/[poly-2β] = 40). A freshly cleaved HOPG was placed on the gel;
then, surface layers were transferred by peeling off the HOPG
substrate.11 After the sample was dried under vacuum for 2 h, the
AFM measurement was performed in air at ambient temperature in the
tapping mode (Figure 3E).
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