Switching of macromolecular helicity of optically active poly(phenylacetylene)s bearing cyclodextrin pendants induced by various external stimuli

Article abstract of DOI:10.1021/ja060858+

A series of novel phenylacetylenes bearing optically active cyclodextrin (CyD) residues such as α-, β-, and γ-CyD and permethylated β-CyD residues as the pendant groups was synthesized and polymerized with a rhodium catalyst to give highly cis-transoidal

Full text of DOI:10.1021/ja060858+

Published on Web 05/06/2006
Switching of Macromolecular Helicity of Optically Active
Poly(phenylacetylene)s Bearing Cyclodextrin Pendants
Induced by Various External Stimuli
Katsuhiro Maeda,† Hiroaki Mochizuki,† Masaki Watanabe, and Eiji Yashima*,†,‡
Contribution from the Department of Molecular Design and Engineering, Graduate School of
Engineering, Nagoya UniVersity, Chikusa-ku, Nagoya 464-8603, Japan, and Super-structured
Helix Project, ERATO, Japan Science and Technology Agency (JST), 101 Creation Core
Nagoya, 2266-22 Moriyama-ku, Nagoya 463-0003, Japan
Received February 5, 2006; E-mail: yashima@apchem.nagoya-u.ac.jp
Abstract: A series of novel phenylacetylenes bearing optically active cyclodextrin (CyD) residues such as
R-, â-, and γ-CyD and permethylated â-CyD residues as the pendant groups was synthesized and
polymerized with a rhodium catalyst to give highly cis-transoidal poly(phenylacetylene)s, poly-1r, poly-
2â, poly-3γ, and poly-2â-Me, respectively. The polymers exhibited an induced circular dichroism (CD) in
the UV-visible region of the polymer backbones, resulting from the prevailing one-handed helical
conformations. The Cotton effect signs were inverted in response to external chiral and achiral stimuli,
such as temperature, solvent, and interactions with chiral or achiral guest molecules. The inversion of the
Cotton effect signs was accompanied by a color change due to a conformational change, such as inversion
of the helicity of the polymer backbones with a different twist angle of the conjugated double bonds, that
was readily visible with the naked eye and could be quantified by absorption and CD spectroscopies. The
dynamic helical conformations of poly-2â showing opposite Cotton effect signs in different solvents could
be further fixed by intramolecular cross-linking between the hydroxy groups of the neighboring â-CyD units
in each solvent. The cross-link between the pendant CyD units suppressed the inversion of the helicity;
therefore, the cross-linked poly-2âs showed no Cotton effect inversion, although the polymer backbones
were still flexible enough to alter their helical pitch with the same handedness, resulting in a color change
depending on the degree of intramolecular cross-linking.
tion with specific guests;7 their color changes are ascribed to a
Introduction
change in their effective π-conjugation lengths of the polymer
backbones or a conformational change due to supramolecular
π-stacked aggregations.
Recently, we reported a conceptually new colorimetric
detection system based on a unique helical macromolecule
consisting of a chromophoric polyacetylene backbone and a
â-cyclodextrin (CyD) as the side group (poly-2â in Scheme 1),
which exhibited a visible color change due to a change in the
twist angle of the conjugated double bonds induced by inclusion
The development of molecular sensors for the direct colori-
metric detection of achiral and chiral chemical species, in
particular organic neutral molecules including enantiomers, as
well as solvent polarity and temperature has recently been
attracting significant interest with implications for their potential
applications in chiral and sensing materials.1 A number of
receptor molecules and supramolecules have been developed
for this purpose.2-4 In general, they have employed dyes,
transition metals, or chromophoric macrocycles as color indica-
tors combined with selective binding components for target
guests.1-4 π-Conjugated polymers also exhibit a color change
in response to external physical and chemical stimuli5 such as
temperature, light, pH, and solvent composition6 or by interac-
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‡ ERATO, JST.
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10.1021/ja060858+ CCC: $33.50 © 2006 American Chemical Society
J. AM. CHEM. SOC. 2006, 128, 7639-7650
7639

A R T I C L E S
Maeda et al.
Scheme 1. Synthesis and Structures of Poly(phenylacetylene)s Bearing CyD Pendants
complexation with neutral, chiral, and achiral guest molecules
into the chiral cyclodextrin cavity as well as by solvent and
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7640 J. AM. CHEM. SOC. VOL. 128, NO. 23, 2006

Switching of Macromolecular Helicity
A R T I C L E S
macromolecular helicity inversion were then investigated by
means of absorption and circular dichroism (CD) spectroscopies.
Although we previously proposed the inversion of helicity
of the poly-2â backbone as a plausible conformational change
for the visual color change by external stimuli based on the
Cotton effect inversion together with the calculation results of
a model polymer,8 there may be another possibility to explain
the changes in the CD patterns; that is a change in the helical
pitch of poly-2â with the same-handedness rather than the helix
inversion (Figure 1B). This possibility could not be ruled out.
To obtain more direct evidence for inversion of the helicity,
the dynamic helical conformations of poly-2â showing opposite
Cotton effect signs in different solvents were fixed by intramo-
lecular cross-linking between the hydroxy groups of the
neighboring â-CyD units in each solvent, and changes in their
chiroptical properties after cross-linking were investigated in
detail. We anticipated that the intramolecular cross-linking
would suppress conformational changes in the polymer back-
bones to fix the helix-senses in each solvent. On the basis of
these results, the origin of the inversion of the Cotton effect
signs of CyD-bound helical poly(phenylacetylene)s by external
stimuli accompanied by a visible color change was discussed.
Figure 1. Schematic illustrations of a possible conformational change of
poly-1r, -2â, -3γ, and -2â-Me accompanied by a visible color change and
inversion of the Cotton effect sign.
pH, temperature, solvent, salt concentration, or by irradiation
with light. These switchable helical materials have potential
applications in data storage, optical devices, and liquid crystals
for displays, but switching of the macromolecular and supramo-
lecular helicity by chiral stimuli still remains rare,1c,d,f,4,8,11d,ï
although such chiral materials can be used to sense the chirality
of chiral guests.
The present work is concerned with the mechanism of the
visible color change of poly-2â, with respect to the inversion
of helicity of the polymer backbone during changes in the Cotton
effect signs. A series of stereoregular helical poly(phenylacety-
lene)s bearing R-, â-, and γ-CyD and permethylated â-CyD
residues as the pendants (poly-1r, poly-2â, poly-3γ, and poly-
2â-Me, respectively; Scheme 1) was synthesized. The effects
of the ring size of the CyD pendants and the hydroxy groups in
the â-CyD unit on their chiroptical properties including the
Results and Discussion
Synthesis and Polymerization of Phenylacetylenes Bearing
CyD Pendants and Structural Characteristics of the Poly-
mers. Four optically active phenylacetylenes bearing CyD
pendants, such as, R- (1r), â- (2â), γ-CyD (3γ), and perm-
ethylated â-CyD (2â-Me), were prepared as outlined in Scheme
1. The reaction of 4-ethynylbenzoyl chloride with the monoamino-
CyD derivatives, R-CyD-NH2, â-CyD-NH2, γ-CyD-NH2, and
Meâ-CyD-NH2, which had been prepared from the correspond-
ing CyDs according to the literature method,13 afforded 1r, 2â,
3γ, and 2â-Me in good yields, respectively. The polymerization
was performed with a rhodium catalyst, [Rh(nbd)Cl]2,14 in the
presence of triethylamine, giving high molecular weight, cis-
transoidal poly(phenylacetylene)s. The polymerization results
are summarized in Table 1.
The polymerization in DMF proceeded rapidly and homo-
geneously, yielding high molecular weight polymers with low
molecular weight oligomers except for 2â-Me (run 6 in Table
1), as evidenced by the size exclusion chromatography (SEC)
measurements, which could be removed by reprecipitation
(Table 1 and Supporting Information). On the other hand, the
polymerization in DMSO and pyridine (runs 2 and 3 in Table
1, respectively) gave low molecular weight polymers and
oligomers.
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H.; Tsukube, H. J. Am. Chem. Soc. 2004, 126, 6524-6525. (e) Fletcher,
S. P.; Dumur, F.; Pollard, M. M.; Feringa, B. L. Science 2005, 310, 80-
82. (f) Miyake, H.; Sugimoto, H.; Tamiaki, H.; Tsukube, H. Chem.
Commun. 2005, 4291-4293.
9
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A R T I C L E S
Maeda et al.
Table 1. Polymerization of Phenylacetylenes Bearing CyD Pendants with [Rh(nbd)Cl]2 at 30 °Ca
polymer
25
[R]D
c
c
run
monomer
solvent
time (h)
sample code
yield (%)b
10-4Mn
Mw/Mn
in DMSO
in alkaline water
1
2
3
4
5
6
7
2â
DMF
DMSO
pyridine
DMF
DMF
DMF
-
17
160
117
17
17
17
poly-2âd
60e
24f
27f
68e
48e
52
16.3
1.9g
2.3g
16.0
11.1
8.5j
n
1.5
-
-1817
-
508
-
2â
poly-2â
2â
poly-2â
-
-
-
1r
poly-1rh
1.2
1.2
1.4j
n
-786
-365
-414k
141
527
234
875l
147
3γ
poly-3γh
2â-Me
-
poly-2â-Mei
poly(CPA0.1-co-2â0.9
m
-
)
-
a Polymerized under nitrogen: [monomer] ) 0.1 M; [triethylamine]/[Rh] ) 100; [monomer]/[Rh] ) 10 (runs 1 and 4-6), 100 (runs 2 and 3). b MeOH
insoluble part (runs 1-5) and EtOH insoluble part (run 6). c Determined by SEC using DMF containing 10 mM LiCl as the eluent (PEO and PEG standards).
d Soluble in DMSO, DMF, and alkaline water, but insoluble in other common organic solvents and neutral and acidic water. e After removal of oligomers.
f Containing oligomers. g Based on the peak top values of the higher molecular weight fractions in the SEC chromatogram. h Soluble in DMSO, DMF, and
alkaline water. i Soluble in various solvents, such as CHCl3, THF, acetone, ethyl acetate, toluene, and water, but insoluble in DMSO. j Determined by SEC
using CHCl3 as the eluent (polystyrene standards). k In DMSO/DMF (1:1, v/v). l In CHCl3. m Prepared by the macromolecular reaction of PCPA with â-CyD-
NH2. See the Supporting Information. n The Mn and Mw/Mn of the original PCPA are 1.8 × 104 and 2.5, respectively.
transoidal high molecular weight polymers (runs 1, 4-6 in Table
1) were used for further chiroptical properties measurements.
Chiroptical Properties of Polymers: Conformational
Changes in Poly-CyDs with Color Change by Temperature
and Solvent. The chiroptical properties of the optically active
poly(phenylacetylene)s bearing CyD pendants were investigated
by means of absorption and CD spectroscopies. Figure 2 shows
the CD and absorption spectra of poly-1r (A in Figure 2), -2â
(B), and -3γ (C) in DMSO and poly-2â-Me in DMSO-DMF
(1:1, v/v) (D). Poly-2â-Me was insoluble in pure DMSO, and
a DMSO-DMF mixture was used. These polymers appear to
have a predominantly one-handed helical conformation induced
by the covalent-bonded chiral CyD pendants so that they exhibit
an intense CD in the long absorption region of the conjugated
polyene backbones at 25 °C. However, the CD patterns
dramatically and sharply changed at high temperatures and the
signs inverted through a transition temperature (Tm), where the
[θ] values of the first Cotton effect became almost zero. These
CD spectral changes were accompanied by remarkable changes
in the absorption spectra; the absorbance maxima (λmax) (514
nm, poly-1r; 516 nm, poly-2â; 517 nm, poly-3γ; 494 nm, poly-
2â-Me) shifted to a shorter wavelength by 60 nm for poly-1r,
33 nm for poly-2â, 36 nm for poly-3γ , and 18 nm for poly-
2â-Me with clear isosbestic points at 475, 487, 484, and 477
nm, respectively, and the solution color changed from red to
bright yellow (poly-1r), red to yellow-orange (poly-2â and poly-
3γ), or orange to yellow (poly-2â-Me) (Figure 2). Because poly-
2â-Me was insoluble in DMSO, the CD and absorption spectral
changes of poly-1r, -2â, and -3γ were also measured in
DMSO-DMF (1:1, v/v) to investigate the effects of solvent
and the size of the CyD units on the CD inversion behavior.
Similar temperature-induced CD and absorption spectral changes
were also observed in the solvent mixture for poly-1r, -2â, and
-3γ, but the Tm's in DMSO-DMF (1:1, v/v) were lower than
those in DMSO, as summarized in Table 2. On the basis of
these results, it was found that the Tm's are remarkably
influenced by the solvent composition and tend to increase with
an increase in the bulkiness of the pendant CyDs in the order
of poly-1r e poly-2â < poly-2â-Me < poly-3γ.
The poly-1r, -2â, and -3γ also exhibit a similar color change
(from red to yellow) in the presence of an increasing amount
of water (0 to 50% v/v) accompanied by inversion of the Cotton
effect signs in DMSO at 25 °C. For example, the CD and
absorption spectral changes as well as the visible color changes
of poly-2â in DMSO in the presence of an increasing amount
of water are shown in Figure 3A,C, respectively. The absorbance
maxima (λmax) at 516 nm shifted to a shorter wavelength by 54
nm, and the solution color changed from red to yellow
accompanied by the inversion of the first Cotton effect sign
from the negative to positive direction at around DMSO-water
) 8:2 (v/v). As shown in Figure 3B, the Tm values of poly-1r,
-2â, and -3γ linearly decreased with an increase in the content
of water, demonstrating that we could control the Tm values by
adjusting the solvent compositions. In addition, the specific
rotation ([R]D) of the polymers also changed from negative in
DMSO to positive values in alkaline water as shown in Table
1.
These unique solvatochromisms of poly-1r, -2â, and -3γ
were also observed in DMSO in the presence of an increasing
amount of various alcohols instead of water. The DMSO
solutions of poly-1r, -2â, and -3γ containing simple primary
alcohols such as methanol, ethanol, n-propanol, and n-butanol
showed a color change, depending on the compositions and
temperatures as reflected by their difference in hydrophobicity
(Figure S3 in Supporting Information). These changes were also
accompanied by inversion of the Cotton effect signs. The Tm
values increased in the order of methanol, ethanol, n-propanol,
and n-butanol, but decreased in the order of poly-3γ > poly-
1r > poly-2â (Table 2). These unique solvatochromisms make
these systems effective discriminators of structurally similar
alcohols with the naked eye on the basis of the temperature-
dependent color changes.
(15) (a) Simionescu, C. I.; Percec, V.; Dumitrescu, S. J. Polym. Sci., Part A:
Polym. Chem. 1977, 15, 2497-2509. (b) Simionescu, C. I.; Percec, V. Prog.
Polym. Sci. 1982, 8, 133-214. (c) Matsunami, S.; Kakuchi, T.; Ishii, F.
Macromolecules 1997, 30, 1074-1078. (d) Kishimoto, Y.; Eckerle, P.;
Miyatake, T.; Kainosho, M.; Ono, A.; Ikariya, T.; Noyori, R. J. Am. Chem.
Soc. 1999, 121, 12035-12044.
The cooperative interaction between the adjacent CyD
pendants may play an important role in these solvatochromisms
with the Cotton effect inversion stimulated by temperature and
solvent compositions. To explore this, a copolymer of (4-
carboxyphenyl)acetylene (CPA) and 2â (poly(CPA0.1-co-2â0.9))
bearing 91 mol % â-CyD residues was prepared from cis-
(16) (a) Shirakawa, H.; Ito, T.; Ikeda, S. Polym. J. (Tokyo) 1973, 4, 460-462.
(b) Tabata, M.; Tanaka, Y.; Sadahiro, Y.; Sone, T.; Yokota, K.; Miura, I.
Macromolecules 1997, 30, 5200-5204. (c) Tabata, M.; Sone, T.; Sadahiro,
Y. Macromol. Chem. Phys. 1999, 200, 265-282.
(17) Maeda, K.; Goto, H.; Yashima, E. Macromolecules 2001, 34, 1160-1164.
9
7642 J. AM. CHEM. SOC. VOL. 128, NO. 23, 2006

Switching of Macromolecular Helicity
A R T I C L E S
Figure 2. CD and absorption spectral changes of poly-1r (A), poly-2â (B), and poly-3γ (C) in DMSO and poly-2â-Me (D) in DMSO/DMF (1:1, v/v) (1
mg/mL) with temperature; visible differences in color at 25 °C and higher temperatures are also shown in insets.
Table 2. CD Inversion Temperature (Tm) of Poly(phenylacetylene)s Bearing CyD Pendants in Various Solvent Mixturesa
Tm (°C)
solvent
poly-1r
poly-2
â
poly-3
γ
poly-2â-Me
DMSO
66
35
70
32
29
43
50
55
90
53
56
69
73
77
b
45
b
b
b
DMSO-DMFc
DMSO-MeOHd
DMSO-EtOHd
DMSO-nPrOHd
DMSO-nBuOHd
55e
61e
64e
65e
b
a The concentration of the polymers is 1 mg/mL. b Insoluble in the solvent mixture. c DMSO/DMF ) 1:1 (v/v). d DMSO/alcohol ) 8:2 (v/v). e DMSO/
alcohol (9:1, v/v) was used because of solubility limit.
transoidal poly((4-carboxyphenyl)acetylene) (PCPA) with â-CyD-
NH2 through a macromolecular substitution reaction (Scheme
1). The CD and absorption spectra of a DMSO solution of the
poly(CPA0.1-co-2â0.9) hardly changed upon heating (no ther-
mochromism); the solution remains yellow with a weak positive
Cotton ([θ] ) 4 × 103) at 440 nm. Moreover, the DMSO
solution showed no solvatochromism in the presence of water
and alcohols (MeOH 30 vol %), and the specific rotation sign
of the copolymer in DMSO ([R]25D +141) was the same as that
in alkaline water ([R]25 +147) (Table 1), indicating that the
D
cooperative interaction between the â-CyD pendants might be
essential for the dramatic conformational change as observed
for the poly-2â. However, the intramolecular hydrogen bond
interaction between the â-CyD units may not be the major
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J. AM. CHEM. SOC. VOL. 128, NO. 23, 2006 7643

A R T I C L E S
Maeda et al.
Figure 3. CD and absorption spectral changes of poly-2â in DMSO-water (100:0-50:50, v/v) at 25 °C (A), plots of the CD inversion temperature (Tm)
of poly-1r (■), poly-2â (b), and poly-3γ (2) versus DMSO contents (%, v/v) in mixtures of DMSO and water (B), and visible color changes of poly-2â
in DMSO-water mixtures at ambient temperatures (C). The concentration of the polymers is 1 mg/mL.
driving force for the transition because the poly-2â-Me, where
all of the hydroxy groups of â-CyD residues are replaced with
methoxy groups, also exhibited a similar thermochromism in
DMSO-DMF (1:1, v/v) with a higher Tm of 45 °C (Table 2).
Except for poly-1r, the CD and absorption spectral changes
of these CyD-bound helical poly(phenylacetylene)s in DMSO,
DMSO-DMF, and DMSO-alcohol mixed solvents with tem-
perature were completely reversible and independent of the
polymer concentrations (0.05-4.0 mg/mL) and time, indicating
that the formation of aggregates depending on the temperature
could be excluded. SEC and dynamic light scattering (DLS)
analyses of the poly-2â in solution also support this conclusion
(Supporting Information). In the SEC analysis using DMSO as
the eluent at temperatures below and above Tm (70 °C) (30 and
80 °C), the elution time and peak shape of the poly-2â were
independent of the temperatures. We then tried to measure the
DLS of the poly-2â in DMSO, but the Tm value of poly-2â
was too high to measure the DLS.18 We then used a DMSO-
methanol mixture (8:2, v/v) as the solvent because poly-2â
exhibited a similar thermochromism in DMSO-methanol (8:
2, v/v) with a lower Tm of 29 °C. The DLS data were analyzed
by the cumulant method to give the translational diffusion
coefficients (Ds). The corresponding hydrodynamic radius (Rh)
was calculated using the Stokes-Einstein equation. The esti-
mated Rh values for the poly-2â in DMSO-methanol (8/2) at
20 and 40 °C were almost the same, 12 and 14 nm, respec-
tively.19 Consequently, these polymers may undergo a confor-
mational transition including a helix-inversion with a different
helical pitch by changing the temperature and solvent composi-
tions, and these conformational changes lead to the thermo-
chromism and solvatochromism. We will discuss in more detail
the conformational changes in these polymers later.
CD and absorption spectral changes with temperature in DMSO
and DMSO-DMF (1:1, v/v) (Figure S4 in Supporting Informa-
tion). Poly-1r also exhibited inversion of the Cotton effect sign
at Tm ) 66 and 35 °C in DMSO and DMSO-DMF (1:1, v/v)
upon heating, accompanied by a blue-shift of their absorption
maxima at high temperatures (Figure 2A and Table 2). However,
the CD and absorption spectra of the poly-1r did not change
in the cooling process even after the temperature became below
the Tm during the heating process, and a further cooling was
required to recover the original state before heating. These
hysteresis changes occurred reversibly, indicating that poly-1r
may form an aggregate at high temperatures. Filtration experi-
ments support the formation of aggregates of poly-1r at high
temperatures in DMSO and DMSO-DMF (1:1, v/v); for
example, the poly-1r solution in DMSO-DMF (1:1, v/v) (B
in Figure S4) could not pass through a membrane with a pore
size of 0.2 µm at room temperature (ca. 20 °C) after heating
the solution above the Tm, whereas it passed through the
membrane before heating and after cooling to -10 °C, once
heated above the Tm. This aggregate formation of poly-1r is
considered to be due to the low solubility of poly-1r in DMSO
and DMSO-DMF (1:1, v/v) compared with the other polymers.
These results imply that the helix-sense of dynamic helical poly-
1r can be “frozen” simply by heating the polymer solution
above the Tm followed by cooling to an appropriate temperature.
The visible color changes in these polymers can be ascribed
to a change in the twist angle of the conjugated double bonds.
The blue-shifts of the absorption spectra of these polymers in
the high temperature ranges suggest that the polymers may have
a more tightly twisted helical conformation at high temperatures
in solution, whereas the red-colored polymers may have an
extended π-conjugation. Molecular mechanics calculations with
the Dreiding force field for the model polymer of poly-2â (20-
mer) were then conducted to obtain information regarding
possible helical conformations of poly-2â (Supporting Informa-
tion).20 The optimized right- and left-handed helical structures
of poly-2â are shown in Figure 4. The averaged dihedral angles
Among the CyD-bound helical poly(phenylacetylene)s, poly-
1r was found to show an interesting hysteresis effect on the
(18) In the DLS measurements, we used a poly-2â with different molecular
weight (Mn ) 10.0 × 104 and Mw/Mn ) 1.5).
(19) The DLS measurements of poly-2â in DMSO/H2O (9:1, v/v) were also
performed at above (60 °C) and below (25 °C) the Tm (55 °C). The estimated
Rh value at 25 °C (13 nm) was almost the same as that at 60 °C (19 nm),
supporting that the formation of aggregates in the DMSO/H2O mixed
solvent depending on the temperature could be also excluded.
(20) Yashima, E.; Matsushima, T.; Okamoto, Y. J. Am. Chem. Soc. 1997, 119,
6345-6359.
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Switching of Macromolecular Helicity
A R T I C L E S
Figure 4. Calculated right- (A, φ ) 10 ( 4°) and left-handed (B, φ ) -31 ( 5°) helical conformations of poly-2â (20-mer). Shown are space-filling
models in top view (A, B; left) and cylinder models in side view (A, B; right): scale bar, 1.5 nm. The main chain carbon atoms are shown in red (A) and
yellow (B) using the space-filling model for clarity (side view). Schematic illustration of interconvertible right- and left-handed helices of poly-2â is also
shown in C. The pendant CyDs arrange in left- and right-handed helical array along the right- and left-handed poly-2â backbones, respectively.
of adjacent double bonds from planarity (φ) for the favorable
conformations in the right- and left-handed helices are 10 ( 4°
(Figure 4A) and -31 ( 5° (Figure 4B), respectively. Therefore,
it seems plausible that the red-colored poly-2â may have a right-
handed helix and the yellow-colored poly-2â may have a left-
handed helix (Figure 4C).
The right- and left-handed helices of the CyD-bound helical
poly(phenylacetylene)s are not exactly enantiomers but are
diastereomers because of the presence of chiral CyD pendants;
therefore, their CD and absorption spectra differ from one
another. The observed changes in the CD spectra (inversion of
the Cotton effect signs) due to temperature and solvent
compositions together with the calculation results strongly
indicate the inversion of helicity of these polymers' backbones
(Figure 1A). However, there may be another possibility to
explain the changes in the CD patterns; that is a change in the
helical pitch of these polymers with the same handedness rather
than the helix inversion (Figure 1B), and this possibility could
not be thoroughly ruled out. Further experimental results and
discussions on the helix inversion will be described later.
CD Inversion Accompanied by Color Change on Com-
plexation with Small Molecules. CyDs possess a chiral
hydrophobic cavity to form inclusion complexes with a variety
of organic molecules that fit the cavity size of CyDs, and they
have been widely used as building components to construct
various supramolecular assemblies21 and as chiral selectors for
chromatographic enantioseparations.22 We anticipated that the
Figure 5. Visible color changes in poly-2â in DMSO-water (8:2, v/v)
(A) and poly-3γ in DMSO-water (75:25, v/v) (B) at 25 °C induced by the
CyD-bound poly(phenylacetylene)s could also form such inclu-
addition of guest molecules (4s7 for poly-2â, 6 and 8s10 for poly-3γ)
sion complexes, which might induce a change in their helical
conformations in the polymer backbones, resulting in a color
change. When 1-adamantanol (4) or (-)-borneol (5) was added
to the poly-2â solution (20 equiv to monomer units of the
(20 equiv to monomer units of the polymers) and structures of the guests.
λ
max (nm), [θ]max of the first Cotton effect (104 degree cm2 dmol-1), and K
(M-1) are also shown.
polymer) in DMSO-H2O (8:2, v/v) at 25 °C, the solution color
immediately changed from yellow-orange to red accompanied
by the inversion of the Cotton effect signs and a large red-shift
of λmax by 25 and 10 nm with clear isosbestic points at 335 and
493 nm for 4 and 5, respectively (Figure 5A) (see also Figures
S5A and S5B in Supporting Information). In contrast, cyclooc-
tanol (6) and cyclohexanol (7) brought about neither such a
dramatic color change in the solution nor the Cotton effect
inversion. The changes in the absorbance intensity at 479 nm
of poly-2â as a function of the concentration of the guests
showed a sigmoidal curvature, indicating that the guest coop-
eratively entered in the â-CyD cavity of poly-2â to induce a
(21) For reviews, see: (a) Wenz, G. Angew. Chem., Int. Ed. Engl. 1994, 33,
803-822. (b) Philp, D.; Stoddart, J. F. Angew. Chem., Int. Ed. Engl. 1996,
35, 5, 1155-1196. (c) Nepogodiev, S. A.; Stoddart, J. F. Chem. ReV. 1998,
98, 1959-1976. (d) Shimomura T.; Yoshida, K.; Ito, K. Hayakawa, R.
Polym. AdV. Technol. 2000, 11, 837-839. (e) Harada, A.; Kawaguchi, Y.;
Hoshino, T. J. Inclusion Phenom. Macrocyclic Chem. 2001, 41, 115-121.
(f) Harada, A. Acc. Chem. Res. 2001, 34, 456-464. (g) Ito, K.; Shimomura,
T.; Okamura, Y. Macromol. Symp. 2003, 201, 103-110. (h) Engeldinger,
E.; Armspach, D.; Matt, D. Chem. ReV. 2003, 103, 4147-4173. (i) Easton,
C. J.; Lincoln, S. F.; Barr, L.; Onagi, H. Chem.-Eur. J. 2004, 10, 3120-
3128.
(22) (a) Schurig, V.; Nowotny, H.-P. Angew. Chem., Int. Ed. Engl. 1990, 29,
939-957. (b) Li, S.; Purdy, W. C. Chem. ReV. 1992, 92, 1457-1470. (c)
Terabe, S.; Otsuka, K.; Nishi, H. J. Chromatogr., A 1994, 666, 295-319.
(d) Chankvetadze, B. J. Chromatogr., A 1997, 792, 269-295. (e) Lebrilla,
C. B. Acc. Chem. Res. 2001, 34, 653-661.
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A R T I C L E S
Maeda et al.
conformational change (Figure S6 in Supporting Information).
A Hill plot analysis23 of the absorption titration data resulted
in the apparent binding constants (Ks) (Figure 5A and Figures
S5 and S6 in Supporting Information). The increasing order of
Ks is in good agreement with that reported for the modified
â-CyD derivative,24 but the K values are smaller than those
during the complexation of the modified â-CyD in pure water,
because organic solvents such as DMSO cause a decrease in
the stability of the inclusion complexes relative to water.
Therefore, in pure DMSO, the poly-2â showed no color change
with 4-7. On the other hand, poly(CPA0.1-co-2â0.9) showed no
change in the CD and absorption spectra upon the addition of
4 and 5 in DMSO-H2O (8:2, v/v). These results suggest that
the cooperative interaction between the adjacent CyD pendants
may play an important role in the conformational change
induced by inclusion complexation with the guest molecules.
Even if the change of the interaction between the neighboring
pendants upon inclusion complexation would be very small for
each monomer unit, the effect can be significantly amplified
by the cooperative interaction, resulting in the change in the
backbone conformation.
Poly-3γ also showed a similar CD inversion accompanied
by a color change in response to the specific guest molecules
capable of interacting with γ-CyD25 as evidenced by the large
K values (Figure 5B and Figures S5 and S7 in Supporting
Information). The addition of cyclododecanone (8), cyclode-
canone (9), and cyclododecanol (10) to the poly-3γ solution in
DMSO-H2O (75:25, v/v) at 25 °C brought about a color change
from yellow-orange to red or orange accompanied by the
inversion of the Cotton effect signs (Figure 5B and Figure S5C
in Supporting Information) and a large red-shift of λmax (Figure
S5D in Supporting Information). On the other hand, the addition
of 8 and 9 to the poly-2â solution in DMSO-H2O (8:2, v/v) at
25 °C did not bring about such a color change. These results
indicate that poly-2â and poly-3γ can respond to molecular
recognition events that occurred at the remote CyD pendants
through inclusion complexation with specific organic molecules,
which can be observed as a color change with the naked eye.26
Color Change by Complex Formation with Polymers. It
is known that CyDs can selectively form inclusion complexes
(rotaxanes) with polymers27 as well as small molecules, depend-
ing on their cavity sizes. For example, â-CyD can be selectively
threaded onto a poly(propylene glycol) (PPG) chain in water
to form an insoluble, crystalline polyrotaxane, whereas it cannot
form a complex with poly(ethylene glycol) (PEG) and poly-
(methyl vinyl ether) (PMVE). Instead, these are selectively
complexed with R- and γ-CyDs, respectively.27 We then
investigated if poly-2â bearing â-CyD pendants could form
inclusion complexes with PPG, thus showing a conformational
change. PPG (20 equiv to monomer units of poly-2â) with
different molecular weights (Mn ) 400, 1000, 2000, and 4000)
was then added to the alkaline solution of poly-2â. The
absorption and CD spectral changes were highly dependent on
the molecular weight of PPGs and were not dramatic for PPGs
of Mn ) 400 and 2000 and almost no change was observed
with the addition of PPG of Mn ) 4000, whereas the λmax and
[θ]max showed a large blue-shift (44 nm) with a dramatic
decrease in the absorbance intensity with the addition of PPG
of Mn ) 1,000 (PPG1000) (Figure S8). The solution color
changed from orange-yellow to bright yellow, but the solution
was a nonviscous liquid, and neither gelation nor precipitation
was observed. A similar molecular weight-dependent inclusion
complexation was reported for â-CyD, which could most
27d
efficiently form such a complex with PPG1000
.
On the other
hand, the addition of PEG (Mn ) 1,000) and PMVE (Mn )
60,000) did not bring about any absorption and CD spectral
changes.27d Therefore, these observations suggest the possibility
of a polyrotaxane formation between the poly-2â and PPG. The
1H NMR spectrum of poly-2â with PPG1000 ([PPG1000]/[poly-
2â] ) 1) also supports such a polyrotaxane formation, at least
in part, because the peak due to the methyl group of the PPG1000
became considerably broadened in the presence of poly-2â
(Figure S9 in Supporting Information).27d
CD Inversion Accompanied by Color Change on Com-
plexation with Chiral Molecules. Although a number of
molecular sensors for visual detection of chemical species have
been developed,1-7 the direct colorimetric detection of enanti-
omers remains difficult and rare.1c,d,f,4,8 However, the visual
detection of chirality is among the most powerful and convenient
methods to assign the absolute configuration of chiral com-
pounds, and such molecular sensors can also be applied to
separate enantiomers as chiral stationary phases in HPLC.28 We
then investigated whether the present CyD-bound poly(pheny-
lacetylene)s could respond to the chirality of chiral guest
molecules and exhibit an enantioselective visible color change.
Among the four CyD-bound helical poly(phenylacetylene)s
tested, we found that poly-2â can discriminate the chirality of
the enantiomers of 1-phenylethylamine (11) and exhibited a
color change (Figure 6). In a mixture of DMSO and alkaline
water (pH 11.7) (3:7, v/v), poly-2â exhibited a negative first
Cotton effect in the presence of excess (S)-11 ([(S)-11]/[poly-
2] ) 990), and the solution color immediately changed from
red to yellow; whereas poly-2â and the poly-2â-(R)-11 in the
same solvent remained yellow with a positive first Cotton effect.
Consequently, the chirality of 11 can be discriminated with the
naked eye. However, the fact that a large amount of (S)-11 was
required for these changes in the CD and the solution color
suggests that a chiral solvation effect rather than an enantiose-
lective inclusion complexation in the â-CyD residues may be
the driving force for the observed color change.29 Poly-1r and
poly-3γ did not show any difference in their solution colors in
the presence of (S)- and (R)-11 under the same experimental
conditions. We then investigated the changes in the Cotton effect
pattern and absorption spectra accompanied by a color change
(23) Connors, K. A. Binding Constants; John Wiley: New York, 1987.
(24) (a) Hamasaki, K.; Ueno, A.; Toda, F.; Suzuki, I.; Osa, T. Bull. Chem. Soc.
Jpn. 1994, 67, 516-523. (b) Liu, Y.; Han, B.-H.; Sun, S.-X.; Wada, T.;
Inoue, Y. J. Org. Chem. 1999, 64, 1487-1493.
(25) Rekharsky, M. V.; Inoue, Y. Chem. ReV. 1998, 98, 1875-1917.
(26) We also investigated the inclusion complexation of poly-1r in DMSO-
H2O (85:15, v/v) with small guest molecules, such as indole, anthranilic
acid, and 1, 10-decanediol, which are known to form inclusion complexes
with R-CyD in water.24 However, poly-1r showed no apparent CD and
absorption spectral changes with these guests. The reason is not clear at
the present time, but the poly-1r formed aggregates in the solvent mixture,
which may prevent a further conformational change induced by the inclusion
complexation with the guest molecules.
(27) (a) Harada, A.; Kamachi, M. Macromolecules 1990, 23, 2821-2823. (b)
Harada, A.; Kamachi, M. J. Chem. Soc., Chem. Commun. 1990, 1322-
1323. (c) Harada, A.; Li, J.; Suzuki, S.; Kamachi, M. Macromolecules 1993,
26, 5267-5268. (d) Harada, A.; Okada, M.; Li, J.; Kamachi, M.
Macromolecules 1995, 28, 8406-8411.
(28) (a) Hirose, K.; Nakamura, T.; Nishioka, R.; Ueshige, T.; Tobe, Y.
Tetrahedron Lett. 2003, 44, 1549-1551. (b) Hirose, K.; Yongzhu, J.;
Nakamura, T.; Nishioka, R.; Ueshige, T.; Tobe, Y. J. Chromatogr., A 2005,
1078, 35-41. (c) Hirose, K.; Yongzhu, J.; Nakamura, T.; Nishioka, R.;
Ueshige, T.; Tobe, Y. Chirality 2005, 17, 142-148.
9
7646 J. AM. CHEM. SOC. VOL. 128, NO. 23, 2006

Switching of Macromolecular Helicity
A R T I C L E S
Figure 6. (A) CD and absorption spectral changes of poly-2â in alkaline water (pH 11.7)-DMSO (7:3, v/v) in the presence of 0-100% ee of 11 at 25 °C.
(B) Changes in CD intensity ([θ]first) (red 9)and λmax (blue b) of poly-2â versus the % ee of 11 and (C) visible difference of the poly-2â with (R)- (left)
and (S)-11 (right).
Figure 7. Effect of cross-linking of poly-2â in alkaline water. CD and absorption spectra before (A: poly-2â) and after (B: CLH O-poly-2â) cross-linking
2
in alkaline water (pH 11.7) (a, c, e, and g) and DMSO (b, d, f, and h) at 25 °C. The concentration of polymers is 1 mg/mL.
in the poly-2â solution in the presence of 11 with a different
enantiomeric excess (ee) of 11 (0, 50, and 100% ee of 11 (R-
or S-rich)) (Figure 6B). Interestingly, the racemic 11 and (R)-
rich 11 of 50% ee could not induce a conformational change in
poly-2â, resulting in almost no change in their absorption and
CD spectra (Figure 6A), while the poly-2â is sensitive to the
chirality of (S)-11, and (S)-rich 11 of 50% ee showed a
significant change in the CD and absorption spectra as well as
100% ee of (S)-11. The CD intensity and the λmax of poly-2â
seem to have a linear relationship with the ee of 11 ((S)-rich)
(Figure 6B). Under the same conditions, the poly(CPA0.1-co-
2â0.9) did not show any spectroscopic and color change,
indicating that the cooperative interaction between the fully
substituted, neighboring â-CyD units might be indispensable
for this phenomenon. Other chiral compounds such as pheny-
lalaninol and 1-phenylethyl alcohol were also tested, but similar
changes in the CD and absorption spectra depending on the
difference in their configuration could not be observed.
Fixation of Interconvertible Helical Structures of Poly-
2â by Intramolecular Cross-Linking. As described above, the
CyD-bound poly(phenylacetylene)s show inversion of the Cotton
effect signs accompanied by a visible color change by respond-
ing to various external stimuli, such as temperature and solvent.
For example, poly-2â showed ICDs with opposite Cotton effect
signs in the polymer backbone regions in DMSO and alkaline
water (Figure 7A). The Cotton effect inversion, in particular,
the first Cotton effect accompanied by a dramatic change in
their absorption spectra, was assumed to originate from inversion
of the helicity of the polymer backbone. If this is the case, poly-
2â may have a helical conformation with an opposite helical
sense in DMSO and in alkaline water. The molecular mechanics
calculation results suggested that the predominant helix-sense
(29) Although the enantioselective binding of 11 with â-CyD derivatives has
been investigated, the selectivity was poor, indicating that a chiral solvation
effect rather than an enantioselective inclusion complexation may be the
driving force for the observed color change. (a) Keim, W.; Ko¨hnes, A.;
Meltzow, W.; Ro¨mer, H. J. High Resol. Chromatogr. 1991, 14, 507-527.
(b) Nie, M. Y.; Zhou, L. M.; Wang, Q. H.; Zhu, D. Q. Chromatographia
2000, 51, 736-740. (c) Rekharsky, M.; Inoue, Y. J. Am. Chem. Soc. 2000,
122, 4418-4435.
9
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A R T I C L E S
Maeda et al.
support the intramolecular cross-linking (see below). The ECH
content introduced in the CLH O-poly-2â was estimated to be
2
8.5 residues per CyD unit on the basis of the molar absorptivity
at 466 nm (ꢀ466 ) 5475) of the original poly-2â in 2 N NaOH
solution. This value satisfied the elemental analysis (see
Supporting Information). The cross-linked polymer maintained
its cis-transoidal structure, which was confirmed by laser
Raman spectroscopy. The CD and absorption spectra of CLH O
-
2
poly-2â were then measured in DMSO and alkaline water
(Figure 7B). Although the absorbance maximum of CLH O
-
2
poly-2γ in DMSO exhibited a considerable red-shift as observed
for poly-2â (Figure 7A), the first Cotton effect sign of the
CLH O-poly-2â corresponding to the polymer backbone region
2
remained positive in DMSO as well as in alkaline water,
although the CD intensity of the first Cotton effect ([θ]first
)
decreased from 1.88 × 104 to 0.69 × 104 in alkaline water after
cross-linking.31 In sharp contrast, poly-2â showed an opposite,
negative first Cotton effect in DMSO in the same region.
Moreover, the CLH O-poly-2â showed almost the same CD
2
Figure 8. Schematic illustrations of intramolecular cross-linking of the
pattern in the shorter wavelength region irrespective of the
solvents. These results indicate that the cross-linking reaction
between the pendant CyD residues significantly influences the
dynamic nature of the helical conformation of poly-2â; the helix-
â-CyD pendants in poly-2â in alkaline water and DMSO; CLH O-poly-2â
2
and CLDMSO-poly-2â stand for the intramolecularly cross-linked poly-2âs
in water and DMSO, respectively.
of poly-2â may be right-handed with an extended helical
conformation in DMSO, but in alkaline water, poly-2â may have
an opposite left-handed helix with a tight helical conformation
(Figure 4). These changes in their helical senses with a different
helical pitch can lead to the solvatochromism and thermo-
chromism. However, a change in the helical pitch of poly-2â
with the same-handedness rather than the helix inversion cannot
be ruled out to explain the observed changes in their CD patterns
and the solution color as shown in Figure 1. To obtain more
direct evidence for inversion of the helicity of dynamic helical
poly-2â in DMSO and alkaline water, the helical structures of
poly-2â showing opposite Cotton effect signs were intramo-
lecularly cross-linked between the hydroxy groups of the
neighboring â-CyD units in each solvent. We anticipated that
if the helix-sense of the poly-2â could be fixed by the
intramolecular cross-linking reaction, the resulting cross-linked
poly-2â could not show such changes in the Cotton effect signs
and absorption spectra, resulting in no thermo- and solvato-
chromisms. The cross-linking reactions were performed in
alkaline water and DMSO using epichlorohydrin (ECH) and
4-methyl-1,3-phenylene diisocyanate (TDI) as the cross-linking
reagents, respectively (Figure 8).
sense of the CLH O-poly-2â appears to be fixed to that in
2
alkaline water before the cross-linking.
The cross-linking reaction of poly-2â (10 mM) was also
performed in DMSO using an excess amount of TDI ([TDI]/
[poly-2â] ) 180) as a cross-linker at 50 °C for 14 h. A DMSO-
soluble, intramolecularly cross-linked polymer (CLDMSO-poly-
2â) was obtained after removal of a DMSO insoluble part (ca.
35%) by filtration. The TDI content introduced in the CLDMSO
-
poly-2â was estimated to be 3.5 residues per one CyD unit using
the ꢀmax value of poly-2â in DMSO (ꢀ514 ) 5380). The obtained
CLDMSO-poly-2â was insoluble in alkaline water, and therefore,
the CD and absorption spectra were measured in DMSO at
various temperatures to determine if the helix-sense could be
fixed by the cross-linking reaction (Figure 9A). The CD spectra
of CLDMSO-poly-2â hardly changed upon heating (25-90 °C)
while maintaining the negative first Cotton effect and the
magnitude of the CD was almost constant;32 the results were
completely different from those of poly-2â (see Figure 2B).
Furthermore, the addition of water to the DMSO solution of
CLDMSO-poly-2â did not bring about an inversion of the Cotton
effect sign, although a large blue-shift in the absorption spectra
was observed in the presence of increasing amounts of water
(Figure 9B). A similar blue-shift also occurred for poly-2â, but
which was accompanied by inversion of the Cotton effect sign
(see Figure 3A).33 These results indicate that the right- and left-
handed helical conformations of poly-2â in different solvents
In alkaline water, poly-2â (10 mM monomer units) was
allowed to react with an excess amount of ECH ([ECH]/[poly-
2â] ) 176) at ambient temperature for 4 h.30 Most of the
obtained cross-linked polymer (CLH O-poly-2â) was soluble
2
in water, DMF, and DMSO but contained an insoluble part (ca.
30%) in these solvents, which was removed by filtration after
(31) The specific rotation [R]25 of CLH O-poly-2â in DMSO also changed
D
2
from a negative (-1817°) to a positive (+168°) value after cross-linking
in alkaline water, whereas the specific rotation sign remained a positive
value in alkaline water after cross-linking ([R]25 +185°).
dissolving the polymer in water. The soluble part (CLH O-poly-
2
(32) The specific rotation [R]25D of CLDMSO-poly-2â iDn DMSO also maintained
a negative value in the temperature range of 25-80 °C ([R]25D -521° and
[R]80 -357°).
2â) appears to consist mainly of intramolecularly cross-linked
poly-2â as supported by SEC analysis; the elution time of the
(33) One Dmay think that CLDMSO-poly-2â contained phenylcarbamate groups
on the CyD units, which may affect the chiroptical property of the polymer.
Poly-2â was then allowed to react with phenyl isocyanate (PI) to produce
a noncross-linked model polymer, whose hydroxy groups were partially
modified with phenylcarbamoyl groups. The content of the introduced PI
units was 3.2 residues per CyD unit, and this value was almost the same
as that of CLDMSO-poly-2â. This model polymer also showed a similar
inversion of the Cotton effect signs with temperature as did poly-2â in
DMSO. Therefore, the effect of the phenylcarbamate groups on the CyD
pendants on the CD spectrum can be neglected.
peak top of CL -poly-2â was almost the same as that of
H2O
poly-2â before cross-linking, although the polymer contained
a higher molecular weight fraction in part due to the intermo-
lecular cross-linking reaction (Figure S10 in Supporting Infor-
mation). Atomic force microscopy (AFM) measurements also
(30) Harada, A.; Li, J.; Kamachi, M. Nature 1993, 364, 516-518.
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7648 J. AM. CHEM. SOC. VOL. 128, NO. 23, 2006

Switching of Macromolecular Helicity
A R T I C L E S
Figure 9. Effect of cross-linking of poly-2â in DMSO. CD and absorption spectral changes of CLDMSO-poly-2â (1 mg/mL) (A) in DMSO at various
temperatures and (B) in DMSO-water mixtures (100:0-50:50, v/v) at 25 °C.
of CLDMSO-poly-2â, the change in the helical pitch could not
be suppressed due to its low degree of cross-linking, although
such a low degree of intramolecular cross-linking was sufficient
to fix the helix-sense. Consequently, the cross-linking between
the CyD pendants suppresses the inversion of the helicity, but
the polymer backbones are still flexible and can alter their helical
pitch, resulting in a color change.
AFM analyses of poly-2â before and after the cross-linking
on a freshly cleaved mica surface were then conducted to
observe changes in morphology and conformation of the
polymer main chains (Figure S11 in Supporting Information).
Individual polymer chains can be directly visualized on mica
prepared from dilute solutions of the CLDMSO-poly-2â and
CLH O-poly-2â as well as poly-2â in DMSO (0.05 mg/mL).
2
These results also support the belief that the observed Cotton
Figure 10. (A) Changes in λmax of poly-2â, CLH O-poly-2â, and CLDMSO
-
effect inversions accompanied by a color change are not due to
the aggregation of the polymers. The average height of the poly-
2â was determined to be 2.0 ( 0.3 from ca. 100 cross-section
profiles, which was shorter than the molecular diameter of a
helical poly-2â model; the computer-generated molecular
diameters of helical poly-2âs having left- and right-handed
helical conformations are 4.1 and 4.0 nm, respectively (Figure
4). The strong interaction of the polymer chains with the mica
surface and the decompression of the polymer due to solvent
vaporization as well as a tip-induced deformation of the samples
should be taken into consideration for the reduced heights of
poly-2â versus the changes in DMSO-water2 ratio (100:0-50:50, v/v)
at 25 °C. (B) Visible color changes of the polymers in DMSO and
DMSO-water (50:50, v/v) at 25 °C. The concentration of polymers is
1 mg/mL.
can be fixed at each handedness by the intramolecular cross-
linking reactions in each solvent. Consequently, the inversion
of the Cotton effect signs might originate not from a change in
the helical pitch of poly-2â with the same-handedness but from
the inversion of the helix-sense of the polymer backbones.
Figure 10 shows the changes in the absorbance maxima of
poly-2â, CLH O-poly-2â, and CLDMSO-poly-2â in DMSO-
the poly-2â on mica.34 The average height value of the CLH O
-
2
2
water mixed solvents relative to the DMSO content (100 to 50
v/v%) of the solvents. Poly-2â exhibited a large blue-shift in
the presence of an increasing amount of water, resulting in a
change in the solution color from red to yellow accompanied
by an inversion of the Cotton effect signs. After cross-linking,
poly-2â slightly increased to 2.6 ( 0.2 nm compared with that
of the poly-2â, and the polymer chains appear to become thicker,
although this value is still shorter than those of the computer-
generated molecular diameters of the helical poly-2â. Similar
AFM images were also observed for CLDMSO-poly-2â. These
results indicate that the intramolecular cross-linking between
the CyD units may contribute to retention of their cylindrical
CLH O-poly-2â and CLDMSO-poly-2â more or less showed a
2
similar blue-shift of the λmax, leading to a visible color change.
However, these cross-linked polymers maintained their Cotton
effect signs in the mixed solvents. In particular, the change in
the solution color of CLDMSO-poly-2â was remarkable. The
difference in the absorption spectral changes and the solution
(34) (a) Prokhorova, S. A.; Sheiko, S. S.; Mo¨ller, M.; Ahn, C.-H.; Percec, V.
Macromol. Rapid Commun. 1998, 19, 359-366. (b) Uchihashi, T.; Choi,
N.; Tanigawa, M.; Ashino, M.; Sugawara, Y.; Nishijima, H.; Akita, S.;
Nakayama, Y.; Tokumoto, H.; Yokoyama, K.; Morita, S.; Ishikawa, M.
Jpn. J. Appl. Phys. 2000, 39, L887-L889. (c) Gao, S.; Chi, L.; Lenhert,
S.; Anczykowski, B.; Niemeyer, C. M.; Adler, M.; Fuchs, H. ChemPhy-
sChem 2001, 6, 384-388.
color between CLDMSO-poly-2â and CLH O-poly-2â may arise
2
from the difference in the degree of cross-linking; in the case
9
J. AM. CHEM. SOC. VOL. 128, NO. 23, 2006 7649

A R T I C L E S
Maeda et al.
structure on a mica surface during the AFM observation.
Although the helical structures of diastereomeric helical poly-
2â could not be visualized by the AFM, a recently developed
high-resolution AFM technique based on ordered two-dimen-
sional helix-bundle formation of helical poly(phenylacetylene)s
bearing a long alkyl pendant35 may be useful for the direct
visualization of the diastereomeric helical poly-2â after intro-
ducing similar long alkyl chains in the CyD pendants. The work
along this line is now in progress.
cross-linking,37 which will be further used as novel chiral
materials for enantiomer separations and catalysts.
Experimental Section
Full experimental details are available in Supporting Infor-
mation.
Acknowledgment. We thank S. Ohsawa, Dr. S. Sakurai, and
Dr. T. Nishimura for their help in the AFM measurements and
computer simulations. This work was partially supported by
Grant-in-Aid for Scientific Research from Japan Society for the
Promotion of Science and the Ministry of Education, Culture,
Sports, Science, and Technology, Japan, and the 21st Century
COE Program “Nature-Guided Materials Processing” of the
Ministry of Education, Culture, Sports, Science and Technology.
Conclusions
In summary, we have demonstrated a conceptually new direct
colorimetric detection system for neutral chemical species
including enantiomers as well as solvent and temperature based
on a change in the tunable helical pitch (“helical spring”)36 with
the macromolecular helicity inversion. In addition, such a
switchable dynamic helical conformation can be, for the first
time, fixed in each handedness as a result of the intramolecular
Supporting Information Available: Experimental details
(PDF). This material is available free of charge via the Internet
at http://pubs.acs.org.
JA060858+
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(36) (a) Jung, O.-S.; Kim, Y. J.; Lee, Y.-A.; Park, J. K.; Chae, H. K. J. Am.
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