Formation of a homo double helix of a conjugated polymer with carboxy groups and amplification of the macromolecular helicity by chiral amines sandwiched between the strands

Article abstract of DOI:10.1002/anie.201301005

Sandwiched amines: A π-conjugated polymer with carboxy groups self-associates to form a racemic double helix. In contrast, with chiral amines it forms a unique one-handed double helix, in which the strands sandwich pairs of chiral amines through cyclic hydrogen-bonding networks (see picture). The chiral information of the amines is transferred to the polymer backbones with remarkable amplification of the helical chirality. Copyright

Full text of DOI:10.1002/anie.201301005

Angewandte
Chemie
DOI: 10.1002/anie.201301005
Helical Structures
Formation of a Homo Double Helix of a Conjugated Polymer with
Carboxy Groups and Amplification of the Macromolecular Helicity by
Chiral Amines Sandwiched between the Strands**
Wataru Makiguchi, Shinzo Kobayashi, Yoshio Furusho, and Eiji Yashima*
Since the discovery of the double-
helical structure of DNA, enormous
efforts have been made to mimic its
structure based on synthetic and
supramolecular approaches.[1] Until
today, a few structural motifs have
been proposed for the rational design
and synthesis of artificial double heli-
ces[2] based on attractive interactions,
such as metal coordinations,[2a,c,i] inter-
strand hydrogen-bonding,[2b,d,j,3] and
aromatic–aromatic interactions.[2f,4]
We recently proposed a modular
strategy to construct a series of artifi-
cial double-helical oligomers that are
connected by various linkages. In this
approach, complementary strands
Figure 1. a) Formation of a double helix (poly-(R)-1·poly-2) from complementary strands of
with crescent-shaped m-terphenyl-
poly-(R)-1 and poly-2. b) Formation of a racemic homo double helix of poly-2 in the solid state.
c) Formation of a preferred-handed homo double helix of poly-2 in solution induced by optically
based rigid backbones that bear ami-
dine and carboxy groups, respectively,
assembled to preferred-handed
active amines through inclusion complexation. Each strand of the homo double helix is drawn in
different colors for clarity.
a
double-helical structure through the
formation of amidinium–carboxylate
salt bridges.[2e,h,5] Based on this modular strategy, a fully
organic double-helical polymer (poly-(R)-1·poly-2) was also
synthesized by employing complementary homopolymers
composed of optically active amidine and achiral carboxylic
acid units (Figure 1a).[6] The double-helical structure was
directly elucidated by high-resolution atomic force microsco-
py (AFM) of the two-dimensional (2D) crystals of the
polymer deposited on a graphite substrate, which disclosed
the helical pitch as well as its handedness.[6]
Here, we describe the synthesis and structure of a double-
helical homopolymer (poly-2), which consists of identical
carboxylic acid strands, with controlled helicity through
noncovalent chiral acid–base interactions. We found that the
achiral poly-2 self-associated into a racemic double helix
through hydrogen-bonding interactions in the solid state
(Figure 1b), while it formed a predominantly one-handed
double helix with a chiral amine sandwiched between the
strands, thus forming a unique cyclic hydrogen-bonding
network (Figure 1c).[7] The amplification of the helical
chirality of the poly-2 duplex was also investigated in terms
of “the sergeants and soldiers”[8] principle and “majority
rule”.[9] For comparison, model carboxylic acid monomers
and dimers were also synthesized.
After exposure to vapors of toluene at around 258C for
12 h, poly-2 deposited from a dilute toluene/tetrahydrofuran
(THF; 30/1, v/v) solution (ca. 0.0025 mgmLÀ1) onto a highly
oriented pyrolytic graphite (HOPG) self-associated into well-
defined 2D helix bundles (Figure 2a). The high-resolution
AFM image (Figure 2b) shows a number of periodic oblique
stripes, which originate from the double-stranded helical
structure of poly-2, consisting of right- and left-handed
double-helical strands (green and red lines, respectively)
with a helical pitch of around 1.6 nm.[10] The observed helical
pitch is in good agreement with that of the hetero-double-
[*] W. Makiguchi, Dr. S. Kobayashi, Dr. Y. Furusho,[+]
Prof. Dr. E. Yashima
Department of Molecular Design and Engineering
Graduate School of Engineering, Nagoya University
Chikusa-ku, Nagoya 464-8603 (Japan)
E-mail: yashima@apchem.nagoya-u.ac.jp
[+] Present address: Molecular Engineering Institute
Kinki University
Kayanomori, Iizuka, Fukuoka 820-8555 (Japan)
[**] This work was supported in part by a Grant-in-Aid for Scientific
Research (S) from the Japan Society for the Promotion of Science
(JSPS), Japan Science and Technology Agency (JST), and for
Scientific Research on Innovative Areas, “Emergence in Chemistry”
(21111508) from the MEXT.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201301005.
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Figure 2. a) AFM phase image of self-associated double-stranded heli-
cal poly-2 on HOPG. b) Magnification of (a). c) Crystal structure of 32.
helical poly-(R)-1·poly-2 (1.7 nm).[6] The wide-angle X-ray
Figure 3. a) CD (left axis) and absorption (right axis) spectra of poly-2
in the presence of varying amounts of (R)-4 in CHCl3/THF (98/2, v/v)
at ambient temperature (ca. 23–258C). b) Plots of the CD signal
intensity (left axis) at 367 nm (De367) and molar absorptivity (right
axis) at 343 nm (e343) versus the molar ratio of (R)-4 to poly-2.
[poly-2]=1.37 mm per monomer unit.
diffraction (WAXD) measurements of uniaxially oriented
1
films of poly-2 and H NMR studies of a dimeric model 5
(Scheme S1 and Figure S3 in the Supporting Information) in
solution further support the self-associated double-helical
structure. The structural features of poly-2 resemble those
observed for the poly-(R)-1·poly-2 heteroduplex (Fig-
ure S1c)[6] and an apparent nuclear Overhauser effect
(NOE) cross-peak was observed between the interstrand
protons of 5 in CDCl3 (Figures S3–S6). In addition, the X-ray
single-crystal analysis of 3, a unit model for poly-2, showed
the self-association of 3 to an entwined double helix through
the hydrogen bonds between the two carboxy groups (Fig-
ure 2c).
We anticipated that if poly-2 could self-associate in
solution, forming a racemic double helix, the sense of the
helix in the double-helical poly-2 could be biased in the
presence of a chiral amine through acid–base interactions.[7]
This possibility was first investigated using circular dichroism
(CD) and absorption spectroscopies. Figure 3a shows the CD
and absorption spectra of poly-2 in the presence of an
increasing amount of (R)-1-phenylethylamine ((R)-4) in
a mixture of CHCl3/THF (98/2, v/v). A small amount of
THF was used, because poly-2 did not completely dissolve in
CHCl3 in the absence of THF. Poly-2 exhibited a characteristic
induced CD (ICD) signal in the absorption regions of the p-
diethynylphenylene linkage chromophore (300–380 nm). The
CD signal intensity gradually increased with an increased
amount of (R)-4 and reached a plateau value at around [(R)-
4]/[poly-2] = 1 (Figure 3b), thus indicating that poly-2 formed
a chiral main-chain structure, for instance, a preferred-handed
double helix, induced by (R)-4.
diameter (dH) of (243 Æ 71) nm (Figure S2), whereas a solution
of poly-2 did not show any significant light scattering, thus
suggesting that poly-2 exists as a single strand in the absence
of (R)-4 in CHCl3/THF (98/2, v/v); polar THF appears to
hamper the interstrand hydrogen bonds.
Fortunately, we obtained single crystals from a solution of
an equimolar mixture of 3 and a racemic amine 6 in CCl4
suitable for X-ray analysis, while 5 produced only amorphous
solids. The crystal structure of the 3·6 complex showed
a unique double-stranded inclusion structure. The two
carboxylate anions and the two R and S ammonium cations
assemble to form a 2 + 2 ion-pair and the two m-terphenyl
units sandwich a pair of the enantiomeric amines 6 through
a cyclic hydrogen-bonding network with an average N···O
distance of 2.67 ꢀ (Figure 4). Sada et al. reported similar
hydrogen-bonded network complexes based on primary or
secondary ammonium salts of a bulky carboxylic acid in
nonpolar solvents and in the crystalline state.[11]
The formation of 2 + 2 ion-paired inclusion complexes
(32·42 and 32·62 as well as 132·42 and 132·62) between the
monomer models of poly-2 (3 and 13) and chiral amines (4
and 6) in solution were also supported by the positive-mode
cold-spray ionization mass spectrometry (CSI-MS; Fig-
ure S7). These results combined with those of the CD titration
indicated that poly-2 likely forms a predominantly one-
handed double-helical structure with a chiral amine in a cyclic
hydrogen-bonding network.
The dynamic light scattering (DLS) measurements of the
poly-2·(R)-4 complex in CHCl3/THF (98/2, v/v) indicated that
the complex formed a chiral aggregate with a hydrodynamic
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2 more or less induced the CD signals, but their spectral
patterns and intensities significantly depended on the struc-
tures of the tested amines as anticipated (see the Supporting
Information for more details).
We next investigated if the macromolecular helicity of the
homoduplex of poly-2 could be amplified in solution in the
presence of nonracemic amines, such as 4, by following the
changes in the ICD signal intensity of poly-2 with respect to
the enantiomeric excess (ee) of 4 (R-rich; “majority rule”,[9]
Figure 6a). The formation of the complex of poly-2 with
nonracemic 4 displayed a positive nonlinear relationship
(amplification of the helical chirality), that is, the observed
Cotton effect intensity at 367 nm (De367) was nonlinearly
increased with respect to the ee value of 4 during the homo-
double-helix formation. The excess of enantiomers that are
randomly sandwiched between the polymer strands might
induce an excess of a one-handed double helix, in spite of its
proportion, thus resulting in a more intense ICD signal than
that expected from the ee value of 4.
The changes in the ICD signal intensity of poly-2 induced
by (R)-4 in the presence of achiral benzylamine at different
molar ratios (“the sergeants and soldiers” effect)[8] were also
investigated (Figure 6b). The formation of the complex of
poly-2 with (R)-4 and benzylamine displayed a large positive
nonlinear relationship between the molar fractions of (R)-4
and the observed intensities of the Cotton effect at 330 nm
(De330) and 366 nm (De366) (Figure 6b), thus indicating the
amplification of the helical chirality because of the strong
cooperativity during the homoduplex formation. In other
Figure 4. Complexation of 3 with 6 and the crystal structure of 32·62
complex.
As a model study, the carboxylic acid dimer 5 was also
subjected to the CD, MS, and 2D NMR experiments in the
presence of (R)-4 and/or (S)-6. Dimer 5 exhibited an ICD
signal with (R)-4 in the p-diethynylphenylene linkage chro-
mophore region (300–370 nm), the signs of the Cotton effects
of which were similar to those of poly-2 complexed with (R)-
4. The CD signal intensity gradually
increased with an increase in the amount
of (R)-4 and reached a plateau value at
around [(R)-4]/[COOH] = 2 (Figure 5a
and b), thus indicating that dimer 5 also
formed a similar preferred-handed double-
stranded helical inclusion complex induced
by (R)-4. The formation of such inclusion
complexes of 5 with (R)-4 and (S)-6 was
supported by CSI-MS (Figure 5c) and
1H NMR spectra (Figures S8–S11 for
5·(R)-4 and Figures S12–S15 for 5·(S)-6
complexes). Clear interstrand and inter-
molecular NOE cross-peaks were observed
for the aromatic protons of 5 with the
terminal TMS groups, and the aromatic
protons of 5 with the aromatic or aliphatic
protons of (R)-4 and (S)-6 (Figures S11 and
S15, respectively), thus indicating the
homoduplex structure induced by the
chiral amine (Figure 5d).
The effects of the structures of the
chiral amines (primary (4, 9, and 10),
secondary (6 and 7), and tertiary amines
(8) with a different bulkiness, and primary
amino alcohols (11 and 12)) on the induc-
tion of the preferred-handed helicity in
poly-2 (Figure S16) and its model dimer 5
(Figure S17) were then investigated. All
Figure 5. a) CD and absorption spectra of 5 in the absence and presence of (R)-4 in CHCl3 at
ambient temperature (ca. 23–258C). b) Plots of the CD signal intensity at 349 nm (De349
versus the molar ratio of (R)-4 to the carboxy groups of 5. [5]=2.0 mm. c) Positive-mode
CSI-MS spectrum of 52·62 (CHCl3/MeOH=4/1 (v/v) at À208C). d) Energy-minimized
)
the amines that were complexed with poly- structures of the 52·(piperidine)2 inclusion duplex.
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208C, and even at low temperatures (À10–108C; Figures S18
and S19). These results demonstrate that the right- and left-
handed double helices of 5 assisted by the chiral and/or
achiral amines are dynamic in nature and the excess of the
handedness is completely governed by the chirality of the
amines that are sandwiched between the strands under
thermodynamic equilibrium.[5d,12]
The unique features of the amplification of the macro-
molecular helicity are now available in a variety of polymeric
and supramolecular helical systems through covalent and
noncovalent bonding interactions,[13,14] but the present results
may be the first example of the amplification of the helical
chirality in double-helical polymers through noncovalent
acid–base interactions.[15]
In summary, we have found that a m-terphenyl-based
conjugated polymer that bears carboxy groups self-associates
to form a racemic double helix through interstrand hydrogen
bonds between the carboxy groups in the solid state and in
solution. With chiral amines in solution, the polymer forms
a unique double helix in which the two m-terphenyl units
sandwich a pair of chiral amines through a cyclic hydrogen-
bonding network, resulting in a preferred-handed double-
helical inclusion complex. An excess of the one-handed
double helix was induced in the presence of nonracemic
amines and also chiral/achiral amine mixtures during the
duplex formation, thus resulting in the amplification of the
helical chirality. This result suggests that the chiral informa-
tion of chiral amines, which are trapped between the polymer
strands through acid–base interactions, can be transferred to
the main chain and further amplified in the double-helical
polymer backbone with a great cooperativity as an excess of
a one-handed helical sense. The present findings of the
formation of a metal-free homoduplex imply that more
sophisticated supramolecular homoduplexes assisted by
metal coordination may also be formed by taking advantage
of the recently developed and emerging concept of metal–
organic frameworks (MOFs), because carboxylic acids have
been extensively utilized as one of the most popular building
blocks for constructing supramolecular MOF materials with
specific functions.[16] Work along this line is now ongoing in
our laboratory.
Figure 6. a) CD (left axis) and absorption (right axis) spectra of poly-2
in the presence of 4 (1 equiv) with different ee values ((R)-rich) in
CHCl3/THF (98/2, v/v) at ambient temperature (ca. 23–258C). Under-
neath, the changes in CD signal intensity (De367) of poly-2 versus the
ee value of 4 are shown. [Poly-2]=1.37 mm per monomer unit. b) CD
(left axis) and absorption (right axis) spectra of poly-2 in the presence
of (R)-4 and benzylamine in different molar ratios (([(R)-4]+[benzyla-
mine])/[poly-2]=1) in CHCl3/THF (98/2, v/v) at ambient temperature
(ca. 23–258C). Underneath, the changes in CD signal intensities (De330
and De366) of poly-2 versus mole fractions of (R)-4 are shown.
[poly-2]=1.37 mm per monomer unit.
Received: February 4, 2013
Published online: April 8, 2013
Keywords: chiral amplification · chirality · double helix ·
.
helical structure · hydrogen bonds
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[15] We attempted to visualize the helical structure of poly-2
complexed with (R)-4 or benzylamine by high-resolution AFM
under various conditions, according to the procedure for visual-
izing the helical structure of the poly-2 duplex on HOPG
(Figure 2). However, we could not obtain 2D crystals of the poly-
2·amine complexes on HOPG suitable for the observation of the
helical structures, because the poly-2 duplex most likely expands
upon complexation of the amines between the strands as
anticipated from the crystal structure of the 32·62 and
32·(piperidine)2 inclusion complex (Figure 4). Therefore, we
could not determine the helical structure of the poly-2·amine
duplex (see Figures S21 and S22 in more detail), although we
presume that the poly-2·(R)-4 duplex may possess a right-
handed double-helical structure, by comparing the first Cotton
effect sign to that of the right-handed double-helical poly-(R)-
1·poly-2, the helical sense of which was determined by direct
AFM observation.[6]
[16] For reviews, see: a) M. Eddaoudi, D. B. Moler, H. Li, B. Chen,
T. M. Reineke, M. OꢄKeeffe, O. M. Yaghi, Acc. Chem. Res. 2001,
34, 319 – 330; b) S. Beer, O. B. Berryman, D. Ajami, J. Rebek, Jr.,
Chem. Sci. 2010, 1, 43 – 47; c) J.-R. Li, J. Sculley, H.-C. Zhou,
Chem. Rev. 2012, 112, 869 – 932.
Angew. Chem. Int. Ed. 2013, 52, 5275 –5279
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
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