Dual memory of enantiomeric helices in a polyacetylene induced by a single enantiomer

Article abstract of DOI:10.1021/ja0509634

We report the dual memory of both the enantiomeric right- and left-handed helical conformations induced in a polyacetylene based on the temperature-stimulated helicity inversion of the polymer. The polyacetylene folds into a one-handed helix induced by no

Full text of DOI:10.1021/ja0509634

Published on Web 03/19/2005
Dual Memory of Enantiomeric Helices in a Polyacetylene Induced by a Single
Enantiomer
Toyoharu Miyagawa,† Akira Furuko,‡ Katsuhiro Maeda,‡ Hiroshi Katagiri,† Yoshio Furusho,† and
Eiji Yashima*,†,‡
Yashima Super-structured Helix Project, Exploratory Research for AdVanced Technology (ERATO), Japan Science
and Technology Agency (JST), Creation Core Nagoya 101, 2266-22 Anagahora, Shimoshidami, Moriyama-ku,
Nagoya 463-0003, Japan, and Department of Molecular Design and Engineering, Graduate School of Engineering,
Nagoya UniVersity, Chikusa-ku, Nagoya 464-8603, Japan
Received February 15, 2005; E-mail: yashima@apchem.nagoya-u.ac.jp
The helix is a central structural motif for biological macromol-
ecules such as DNA and proteins that adopt a right-handed double
helix and an R-helix, respectively, because of the homochirality of
their components. However, the right-handed helices are not uni-
versal and can be transformed into an opposite left-handed helix
regulated by external stimuli, as observed in natural and artificial
DNAs and polypeptides.1 Since the discovery of the biological
helicity inversion, chemists have been interested in controlling and
switching the helicity of artificial (macro)molecules not only to
mimic biological helices but also for attractive applications in
materials science, chiral sensing, and molecular devices.2 Inversion
of the helical chirality has been thermally and photo- or electrochem-
ically achieved in some optically active synthetic polymers3 and
supramolecules.4 The reversibly interconverting helices are not
Figure 1. Schematic illustration of induced one-handed helicity in optically
mirror images (enantiomers), but diastereomers because optically
inactive poly-1, helix inversion with temperature, and subsequent memory
active components are covalently bonded to the molecules, so that
one of the helices may be predominant under certain conditions.
Here, we report an unexplored approach which produces both
mirror-image enantiomeric helices from the interconverting dia-
stereomeric helices of a polyacetylene, based on the noncovalent
“helicity induction and chiral memory” concept5 assisted by inver-
sion of the macromolecular helicity with temperature (Figure 1).
We previously reported that a right- or left-handed helicity
induced in optically inactive poly(phenylacetylene)s bearing a
carboxy or a phosphonate as the pendant by an optically active
amine can be “memorized” when the amine is replaced by achiral
amines in dimethyl sulfoxide (DMSO).6 Accordingly, the opposite
enantiomeric helicity memory requires the opposite enantiomeric
amine, followed by replacement with achiral amines. The chiral
memory of the self-assembled supramolecular structures has also
been achieved, but one of the enantiomers can be memorized from
diastereomeric supramolecules.7
We synthesized a novel cis-transoidal poly(phenylacetylene)
having a bulky phenyl phosphonate group at the pendant (poly-1;
Figure 1),8 which formed a one-handed helix upon complexation
with chiral amines, such as (R)-1-(1-naphthyl)ethylamine ((R)-2)
([(R)-2])/[poly-1] ) 2) in DMSO at 25 °C, thus showing an induced
circular dichroism (ICD) in the polymer backbone region (Figure
2a). CD titration experiments indicated that 1 equiv of (R)-2 is
enough to induce an almost single-handed helix in poly-1, showing
a full ICD. However, the ICD pattern dramatically changed with
temperature and was inverted at 65 °C (Figure 2b and inset),
accompanied by a change in the absorption spectra (Figure 2e,f).
These spectral changes are completely reversible (Figure 2, inset),
indicating inversion of the helicity of poly-1 by temperature, despite
of the diastereomeric macromolecular helicity at different temperatures. A
left-handed helical conformation of poly-1 induced by (R)-2 at low
temperature reversibly changes into the opposite right-handed helix at high
temperature (A), and these diastereomeric helices of poly-1 are memorized
at different temperatures by replacement of the (R)-2 with achiral 3, resulting
in the formation of enantiomeric mirror image helices of poly-1 (B). The
helix-senses of poly-1 are tentatively assigned on the basis of the Cotton
effect signs of the induced CDs of analogous helical polyacetylenes.6a
the noncovalent bonding interactions (Figure 1A). A similar
temperature-dependent inversion of helicity has been observed in
the covalent systems of helical polymers.3
The right- and left-handed helices of poly-1 induced at 25 and
65 °C are not enantiomers, but diastereomers because of the
presence of the chiral amine complexed with poly-1; therefore, their
CD and absorption spectra differ from one another. When (S)-2 is
used instead, poly-1 with the opposite helicity is induced, which
undergoes a reversible inversion of the helicity by temperature
(Figure S1 and Table S2 in the Supporting Information).9 We
assume that this specific inversion of the helicity may be governed
by a difference in the chiral twisting power between the chiral amine
and the pendant phosphonate chirality, which forces the poly-1 into
either a right- or left-handed helix depending on the temperature.
Other chiral (R)-amines, such as (R)-1-phenylethylamine ((R)-4),
did not show such a temperature-induced inversion of the helicity
in poly-1, and the complexes exhibited the same positive second
Cotton effect (∆ꢀsecond) over the temperature range (25-100 °C);
the sign was the same as that of the poly-1-(R)-2 complex at high
temperature (Table S1).9
The vibrational CD (VCD) and IR spectra provide strong
evidence of the pendant phosphonate chirality (Figure 3). The poly-
1-(R)-2 complex exhibits a positive couplet VCD in DMSO-d6 at
ca. 25 °C in the PdO stretching band region (1180-1260 cm-1
)
† ERATO, JST.
‡ Nagoya University.
(Figure 3a, left), whose intensity significantly diminished at 70 °C,
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C O M M U N I C A T I O N S
from those before the memory because the absorption and CD
spectra differ from one another; the memory efficiency estimated
on the ICD values of the poly-1-(R)-2 or -(S)-2 complex at 25
and 65 °C as the base values was ca. 90% (Table S2).9 The memory
lasted for the extremely long time of over 1 month at ambient
temperature with a decrease in the CD intensity of ca. 6%.
Because of the unique feature of the dynamic helical poly-1,
even with a low enantiomeric excess (ee), 2 can induce a
predominantly one-handed helix in poly-1.2d,6b For example, a 35%
ee of 2 induced an intense ICD in poly-1 similar to that of the
100% ee at 25 °C and the event at 65 °C after inversion of the
helicity (Figure S2).9 The replacement of the nonracemic amine
with achiral 3 produced the enantiomeric helices of poly-1 with a
high optical activity (Table S2). Consequently, both enantiomeric
helices can be produced from the dynamically diastereomeric helical
polyacetylenes induced by a single enantiomer with a low optical
activity. The present results not only demonstrate this new
phenomenon but also will provide new approaches for the rational
design of novel switchable helical architectures and the construction
of new chiral materials in areas such as liquid crystals, chiral
selectors, and chiral sensors.2b,c,5
Figure 2. Macromolecular helicity inversion of poly-1-(R)-2 complex with
temperature in DMSO, and the memory of the diastereomeric helices at
different temperatures. Shown are the CD spectra of poly-1 (1 mg/mL)
with (R)-2 ([(R)-2]/[poly-1] ) 2) at 25 (a, blue line) and 65 °C (b, green
line), and the isolated poly-1 from (a) (c, red solid line) and (b) (d, red
dotted line) by SEC fractionation using a DMSO solution of 3 (0.08 M) as
the mobile phase. Five equivalents of 3 was added to the poly-1-(R)-2
solution at 65 °C before the SEC fractionation for (d). Absorption spectra
of poly-1 with (R)-2 in DMSO at 25 and 65 °C and the memorized poly-1
from (a) and (b), in DMSO at 25 °C, are also shown in (e) (blue line), (f)
(green line), (g) (red line), and (h) (black dotted line), respectively. Inset
shows the temperature-dependent CD intensity (368 nm) changes of the
poly-1-(R)-2 complex; heating and cooling rates ) 1 °C/min.
Acknowledgment. We thank H. Onouchi for his preliminary
experimental support.
Supporting Information Available: Experimental details (PDF)
and X-ray crystallographic data (CIF). This material is available free
of charge via the Internet at http://pubs.acs.org.
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Figure 3. VCD (a and b) and FT-IR spectra (c and d) of poly-1-(R)-2
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accompanied by a slight change in the IR spectra (Figure 3b,d).
These results indicate that the pendant phosphonate of poly-1 is
quite optically active rich in either the (R)- or (S)-enantiomer with
(R)-2, which may determine the helix-sense of the poly-1 at low
temperature. Upon heating, however, the pendant phosphonate may
racemize or exist as the achiral form due to the resonance effect of
the P-OH and PdO groups. Therefore, the chirality of the amine
(R)-2 will, in turn, determine the helix-sense of the poly-1, resulting
in inversion of the helicity and the Cotton effect signs at high
temperature. The corresponding monomer 1 complexed with (R)-2
showed no VCD in the region, indicating no asymmetric induction
in the monomer level.10 Moreover, the fact that a positive couplet
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VCD in the main-chain related vibration region (1125-1150 cm-1
)
(9) For more details, see Supporting Information.
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phenyl phenylphosphonate, a model compound of poly-1 with (R)-2,
showed that 1 chirally exists as a racemic mixture in the crystal, but the
model phosphonate becomes achiral when complexed with (R)-2 through
proton transfer.9 Similar achiral phosphonates in the crystal have been
reported,11 but the chirality of the phosphonates in solution has not yet
been considered.12 Consequently, asymmetric induction in the phosphonate
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as poly-1, and may not occur in small molecular systems.
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of poly-1 at 25 °C completely changed to the opposite sign, a
negative couplet at 70 °C (Figure 3, right), also supports the
inversion of the helicity of poly-1 due to temperature.
The resulting diastereomeric right- and left-handed helices of
poly-1 obtained at 25 and 65 °C were successfully memorized when
the chiral amine was replaced by an achiral diamine such as 3 at
those temperatures (Figure 1B).6,9 The perfect mirror image Cotton
effects and identical absorption spectra of the isolated poly-1s by
size exclusion chromatography (SEC) indicate that the separated
species indeed correspond to enantiomeric forms13 (Figure 2c,d).
The memorized poly-1 may have a different helical conformation
(12) Schug, K. A.; Lindner, W. Chem. ReV. 2005, 105, 67-114.
(13) Nilsson, K. P. R.; Olsson, J. D. M.; Konradsson, P.; Ingana¨s, O.
Macromolecules 2004, 37, 6316-6321.
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