An Optically Active Polymer for Broad-Spectrum Enantiomeric Recognition of Chiral Acids

Article abstract of DOI:10.1002/chem.201700617

Recognition of enantiomers of chiral acids by anion–π or lone pair–π interactions has not yet been investigated but is a significant and attractive challenge. This study reports an optically active polymer-based supramolecular system with capabilities of discriminating enantiomers of various chiral acids. The polymer featuring alternate π-acidic naphthalenediimides (NDIs) and methyl l-phenylalaninates in the backbone exhibits an unprecedented slow self-assembly process that is susceptible to perturbation by various chiral acids. Thus, the combination of anion–π or lone pair–π interactions and sensitivity of the polymeric self-assembly process to external chiral species endows the system with recognition capabilities. This is the first time that anion–π or lone pair–π interactions have been applied in the recognition of enantiomers of various chiral acids with a single system. The results shed light on new strategies for material design by integrating π-acidic aromatic systems and chiral building blocks to afford relevant advanced functions.

Full text of DOI:10.1002/chem.201700617

A Journal of
Accepted Article
Title: An optically active polymer for broad-spectrum enantiomeric
recognition of chiral acids
Authors: Chuanqing Kang, Jijun Yan, Zheng Bian, Xiaoye Ma, Rizhe
Jin, Zhijun Du, and Lianxun Gao
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To be cited as: Chem. Eur. J. 10.1002/chem.201700617
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An optically active polymer for broad-spectrum enantiomeric
recognition of chiral acids
Jijun Yan,[a, b] Chuanqing Kang,*[a] Zheng Bian,[a] Xiaoye Ma,[a] Rizhe Jin,[a] Zhijun Du,[a] and Lianxun
Gao[a]
Abstract: Recognition of enantiomers of chiral acids by anion–π or
lone pair–π interactions has not yet been investigated but is a
significant and attractive challenge. This study reports an optically
active polymer-based supramolecular system with capabilities of
discriminating enantiomers of various chiral acids. The polymer
featuring alternate π-acidic naphthalenediimides (NDIs) and methyl
L-phenylalaninates in the backbone exhibits an unprecedented slow
self-assembly process that is susceptible to perturbation by various
chiral acids. Thus, the combination of anion–π or lone pair–π
interactions and sensitivity of the polymeric self-assembly process to
external chiral species endows the system with recognition
capabilities. This is the first time to apply anion-π or lone pair-π
interactions in the recognition of enantiomers of various chiral acids
with a single system. The results shed light on new strategies for
material design by integrating π-acidic aromatic systems and chiral
building blocks to afford relevant advanced functions.
recognition of anionic or neutral carboxylic acids. The anion–π
interactions of π-acidic aromatic systems with the weak basic
–
anions such as AcO– and PhCO2 have been rarely reported,[6]
whereas the recognition of other carboxylic acids or anions
through lone pair–π or anion–π interactions has not been
reported to date, nor that of chiral acids.
We have thus become interested in the feasibility of chiral
NDIs in enantiomeric recognition of various chiral acids through
anion–π or lone pair–π interactions. This idea could be
implemented by the rational design of chiral NDIs that respond
to enantiomers of chiral acids. The planar conjugated structure
of NDIs tends to form their self-assembly via π–π interactions,
which may be disturbed by the introduction of anion– or acid–
NDI interactions. We assumed that a combination of anion– or
acid–NDI interactions and the disturbance of self-assembly
would be a promising strategy for the recognition of chiral acids.
It is noteworthy that, to date, no study of molecular recognition
has addressed the chiral acid recognition within an NDI system.
To achieve this, we hypothesized that multiple NDIs linked
together as a polymer would show enhanced π–π interactions
and acid–π interactions, leading to enhanced sensitivity to
electron-rich guest molecules. Although several NDI-containing
polymers responsive to pH, F–, or hydrazine have been
developed,[5a, 7] the relation between recognition ability and the
self-assembly of NDI segments has not been clarified.
Thus, according to information gleaned from previous
reports and our assumptions, we designed and prepared a novel
chiral poly(naphthalenediimide) (PNDI) by alternatively arranging
NDIs and chiral centers from methyl L-phenylalanate in the
backbone (Scheme 1). We then studied the self-assembly of
PNDI into a polymeric supramolecular system that endowed the
polymer with the capacity to recognize enantiomers of various
chiral acids. Mechanisms of the self-assembly and the
enantiomeric recognition were also discussed. The advantages
in the broad-spectrum enantiomeric recognition and the use of
polymeric self-assembly demonstrate a new approach to the
design of NDI-based functional materials. The results of this
work are presented herein.
Introduction
Electron-poor π-acidic naphthalenediimides (NDIs) with large
quadrupole moments have been used extensively as models to
study anion–π interactions.[1] The functional relevance of anion–
π interactions has been demonstrated by the wide application of
NDIs in anion recognition and transport, catalysis and smart
materials.[2] The development of NDI-based anion recognition
with thermal anion-to-NDI electron transfer (ET) has attracted
considerable attention;[1-2] however, sensitivity to neutral
molecules remains elusive and challengable.
The enantiomeric recognition of chiral acids and their anions
is an important field owing to the ubiquity of this functional group
in biological and pharmaceutical molecules.[3] Sensors for the
recognition of chiral acids through hydrogen bonding or ion
pairing usually work exclusively for a limited number of chiral
acids with specific structural features.[4] Thus, the chiral
discrimination of various acids using a single sensing system
remains a challenge. This promoted us to study whether NDIs
could be applied for chiral acid recognition. NDIs have been
used for the recognition of neutral amines via lone pair–NDI
interactions;[5] however, they have not been used for the
[a]
J. Yan, Dr. C. Kang, Dr. Z. bian, Dr. X. Ma, Dr. R. Jin, Dr. Z. Du,
Prof. Dr. L. Gao
State Key Laboratory of Polymer Physics and Chemistry
Changchun Institute of Applied Chemistry, Chinese Academy of
Sciences
5625 Renmin Street, Changchun 130022, China
E-mail: kangcq@ciac.ac.cn
[b]
J. Yan
University of Chinese Academy of Sciences
Beijing 100049, China
Scheme 1. Preparation of PNDI.
Supporting information for this article is given via a link at the end of
the document.
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Results and Discussion
confirms the formation of chiral supramolecular architecture,
which is in agreement with the results from the CD spectra.
PNDI was prepared through polycondensation of chiral diamine
methyl L-4‘-aminophenylalaninate hydrochloride (MAPA) and
1,4,5,8-naphthalenetetracarboxylic dianhydride (NDA) followed
by chemical diimidization with triethylamine and acetic anhydride
(Scheme 1). The polymerization and imidization were confirmed
by NMR and FT-IR spectra. The molecular weight of PNDI was
1
determined to be 4925 by H NMR spectra. PNDI is soluble in
polar organic solvents such as dimethyl sulfoxide (DMSO), N,N-
dimethylacetamide (DMAc), and N,N-dimethylformade (DMF).
Initial tests revealed the formation of a thermo-reversible gel
from DMSO solution (5 mg/mL) upon long-term standing,
indicating the occurence of polymeric self-assembly. Thermal
treatment of the gel showed that the gel-sol transition
temperature is 140 °C. SEM images of the xerogel showed the
formation of stacked spherical particles responsible for the
gelation (Figure 1).
Figure 1. SEM image of the xerogel of PNDI. The bar represents 1 μm.
The self-assembly of PNDI was then explored by long-term
tracking of the spectroscopic behavior of the polymer solution,
which revealed an unprecedented slow self-assembly of the
polymer. Solutions of PNDI in DMSO (5 mg/mL, 10.2 mM based
on repeat unit) for spectroscopic studies were freshly prepared
before measurements. The UV-Vis spectra of PNDI show
characteristic absorption bands for the π-π* transitions of the
NDI cores at 362 and 382 nm (Figure 2a and 2b).[1a] The
hypochromic effect of the bands reveals an organized
aggregation of the NDI segments, which takes hundreds of
hours to reach balance. The circular dichroism (CD) spectra
display a negative Cotton effect at 380 nm with a bathochromic
shift and a new positive Cotton effect at 338 nm; both undergo
enhancement in magnitude over time (Figure 2c and 2d). The
CD spectra indicate the occurrence of chiral aggregation with
neighbouring chromophores arranged in M-type helicity.[8] The
fluorescence analysis shows the excimer emission at 455 nm
(λex 360 nm) with a hyperchromic effect and long fluorescent
lifetime (1.56 ns) as a result of aggregation of the NDIs (Figure
2e, 2f and 2g).[9] We also observed that the optical rotation of
PNDI changed slowly from -454° to -306° and kept stable at -
306° and returned to -454° by thermal treatment at 140 °C, the
temperature for gel-sol transition of PNDI gel (Figure 2h). The
continuous but thermally reversible change of optical rotation
Figure 2. Dynamic spectroscopic behaviours of PNDI in DMSO (5 mg/mL). (a)
time-dependent UV-Vis spectra; (b) plots of the π-π* transitions bands at 362
and 382 nm versus time; (c) the time-dependent fluorescence spectra; (d)
hyperchromic effect of the maximum emission against time; (e) fluorescence
lifetime decay profile; (f) changes of optical rotation against time and thermal
treatment, in which the optical rotation returned to the original value after
heating to 140 °C for 2 minutes (measured after cooling to room temperature).
The slow self-assembly and resultant chiral supramolecular
architectures enable PNDI to discriminate chiral acids or anions
by external responses. We thus explored the use of PNDI to
recognize enantiomers of chiral acids. The solutions for the
investigations were prepared by heating a suspension of PNDI
and an enantiomer of chiral acids or anions in an oil bath at
185 °C for 2 minutes. Surprisingly, we observed the formation of
a series of stable NDI˙ˉ absorption bands at 476, 606, 700 and
776 nm in the UV-Vis spectra of PNDI in the presence of one or
both of the enantiomers of all the chiral acids tested (Scheme 2
and Figure 3).[6a, 6b] The formation of NDI˙ˉ was confirmed with
the addition of oxidant NOBF4 into PNDI solution with NDI˙ˉ
bands triggered by (R)-2 or enantiomers of other chiral acids,
which resulted in complete disappearance of the characteristic
NDI˙ˉ bands in the UV-Vis spectra owing to the oxidation of the
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6b]
radical anion to neutral NDI (Figure 3).[6a,
The thermal
The tests of the enantiomeric recognition by PNDI with both
sodium mandelate (1·Na) and mandelic acid (1) demonstrate the
capacity of PNDI to discriminate D- and L-enantiomers of both
the anion and the acid. Both the D- and L-enantiomers induce
NDI˙ˉ in the early stage, but NDI˙ˉ from the L-enantiomers
gradually decreases and disappears (Figure 4a-4d). The NDI˙ˉ
bands from 5 and 12·Na exhibit the same trends as those from 1.
In further tests, PNDI unambiguously exhibits sensitivity to (R)-2
and complete inertness to (S)-2 over the observed duration
(Figure 4e and 4f). Further evaluation reveals the recognition of
enantiomers of 3, 4 and 6–11 by the enantiospecific formation of
NDI˙ˉ, similarly to that of 2. Interestingly, the inverse
configurations of the enantiomers are recognized for 6–8, which
have limited structural variation. For 13–16 and 17·Na, PNDI
recognizes enantiomers through the difference of the changes in
the NDI˙ˉ band intensities (Figure 4g and 4h). One enantiomer of
13–16 or 17·Na leads to faster enhancement of the NDI˙ˉ bands
than another.
treatment of the chiral acid-PNDI system is crucial to trigger the
formation of NDI˙ˉ. At the temperature of the thermal treatment,
lone-pair electrons of the carboxyl groups have the capacity to
induce thermal ET from the chiral acids to the NDIs, which has
not been reported previously. However, this inference is
reasonable because the lone-pair electrons of oxygen and
nitrogen atoms in neutral molecules can drive lone pair–NDI
interactions according to X-ray crystallography.[5, 10]
Scheme 2. Chiral acids tested for enantiomer recognition by PNDI. (a) Only
one enantiomer (configuration symbol is shown in bracket) induced the stable
NDI˙ˉ bands. (b) Both enantiomers induced the NDI˙ˉ bands but in different
change rate of the intensity (the rate comparison is shown by configuration
symbols in bracket). (c) Both enantiomers induced the same NDI˙ˉ bands but
the different π-π* transition bands. (d) Both enantiomers induced NDI˙ˉ bands
and π-π* transition bands indistinguishable.
Figure 4. NDI˙ˉ bands of PNDI in the presence of enantiomers of chiral acids
or anions after standing for the specified time and corresponding plots of the
NDI˙ˉ band at 476 nm against time. (a, b) both D- and L-1·Na induce NDI˙ˉ
bands, but the NDI˙ˉ bands from L-1·Na are slowly quenched; (c, d) both D-
and L-1 induce NDI˙ˉ bands, but the NDI˙ˉ bands from L-1 are weak at early
stage and quickly quenched; (e, f) (R)-2 induces NDI˙ˉ bands but (S)-2 not; (g,
h) both D- and L-13 induce NDI˙ˉ bands but in different rate in enhancement of
the band intensity.
Figure 3. The formation of NDI˙ˉ induced by (R)-2 and its oxidation with
NOBF4. (a) time-dependent UV-Vis spectra of PNDI in the presence of (R)-2,
showing hyperchromic NDI˙ˉ bands at 476, 606, 700 and 776 nm; (b) complete
disappearance of NDI˙ˉ bands after addition of NOBF4 to above solution; (c)
the intensity of NDI˙ˉ bands at 476 nm versus time before and after the
addition of NOBF4.
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Figure 5. Typical time-dependent UV-Vis spectra and CD spectra with difference in the π-π* transition bands in the presence of chiral acids. (a, b) UV-Vis spectra
of PNDI in the presence of enantiomers of 18; (c) plots of the specified bands of UV-Vis spectra against time; (d, e) CD spectra of PNDI in the presence of
enantiomers of 19·Na; (f) plots of the specified bands of CD spectra against time.
Enantiomers of 17, 18, 19·Na and 20 are not distinguishable
by the NDI˙ˉ bands, but presents difference in intensities of the
π-π* transition bands (at 362 and 382 nm), which may be
discriminated in the time-dependent UV-Vis or CD spectra of
PNDI. For example, in the presence of D- or L-18, PNDI exhibits
same time-dependent NDI˙ˉ bands but different intensities of the
π-π* transition bands during whole period of the measurements
(Figure 5a-5c). Chiral acid sodium salt 19·Na result in decay of
the CD signal at 386 nm corresponding to the π-π* transition
bands, which is opposite to that of PNDI itself (Figure 5d, 5e and
2c). Enantiomers of 19·Na lead to different function of the CD
signal versus time (Figure 5f). Enantiomers of only three (12, 19
and 21) of the 25 chiral acids and sodium salts are completely
indistinguishable by PNDI.
These results thoroughly demonstrate the capacity of PNDI
for the enantiomeric recognition of an extraordinary wide range
of chiral acids, including amino acids, protected amino acids and
well-known pharmaceutical molecules like ibuprofen (3) and
penicillamine (16).[4] The specific interactons between acid or its
anion and NDIs generate the characteristic NDI˙ˉ absorption
bands at long wavelengths, which are highly identifiable owing to
their separation from the absorption bands of chiral acid,
mitigating overlap. The chiral self-assembly of PNDI enhances
the sensitivity of the recognition. It should be noted that all
tested chiral acids trigger changes in the dynamic spectroscopic
behavior of PNDI owing to the response of the self-assembly
process to the association of the NDIs with external chiral acids.
The difference in the response to enantiomers is determined by
whether the stereochemistry of an enantiomer matches with the
chirality of PNDI and its assembly.
suggesting the short-range contact of the NDI moiety and the
phenylalanine-derived moiety between the polymer backbones
(Figure 6a and 6c). Small molecular NDIs have been reported to
form assemblies through intermolecular π-π interactions in a
coplanar or herringbone motif, in which the electron deficient
NDI cores and electron-rich phenyl side chains were packed
through their electrostatic complementation.[11] Consequently,
we propose that the short-range contacts derived from the
NOESY of PNDI are similar to those of small molecular NDIs.
The π-π interactions between the naphthalene core and the
phenyl of PNDI drive the slow self-assembly of main chains of
the polymer.
Then, the NOESY of PNDI in the presence of chiral acid 2
confirmed our assumption that the introduction of chiral acids
disturbs the polymeric self-assembly. In comparison with PNDI,
the introduction of 2 does not lead to disappearance of the cross
peaks but unambiguously decreases the intensity of the cross
peaks (Figure 6b). In NOESY, the volume integration of cross
peak is in reciprocal proportion to the spatial distance between
related protons.[12] The integration of the cross peak attributed to
the protons H-1 and H-2 and the protons H-3 and H-4 decrease
significantly when chiral acid is introduced into the system,
indicating that the space between the NDIs and the phenyls of
the chiral moiety of the polymeric backbones becomes larger
(Figure 6d and 6e).[13] The enantioselective association of the
NDIs with chiral acids weakens the original interactions between
the NDIs and the phenyls and triggers the evolvement of the
self-assembly and chiral supramolecular architecture of PNDI
that results in the spectroscopic changes of the π-π* transition
bands (Figure 6e). An enantiomer of a chiral acid is superior to
another enantiomer to be embeded into the assemblies of PNDI
through anion– or acid–NDI interactions. In this sense, the
combination of anion– or acid–NDI interactions and the
disturbance of the self-assembly of PNDI is an effective method
for the recognition of chiral acids.
Deep insight into the mechanisms of the self-assembly of
PNDI and the recognition to enantiomers of chiral acids were
carried out with 2D NOESY. The NOESY of PNDI shows
unambiguous cross peaks attributed to the short-range contacts
of the protons H-1 and H-2 with the protons from H-3 to H-7,
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Figure 6. Regional 2D NOESY and proposed mechanisms. (a) NOESY of PNDI in the absence of (R)-2; (b) NOESY of PNDI in the presence of (R)-2; (c) short-
range contacts between main chains of PNDI; (d) Proposed model of acid-NDI interactions; (e) Proposed disturbance of the assembly of PNDI by (R)-2. In each
NOESY spectra, the cross peaks A, B, C, and D are attributed to the close contacts of the protons on the NDI (H-1 and H-2) with the protons on the phenyls (H-3
and H-4), the proton on the chiral center (H-6), the protons on the methyl (H-7), and the protons on the methylene (H-5), respectively. Peak C may be overlapped
with another part of peak D. The cross peak E is attributed to the close contacts of the protons on the phenyl (H-3 and H-4) with the protons on the methylene (H-
5), which is used as standard for the volume integrations of other cross peaks because the distance between the adjacent protons keeps nearly constant in the
absence or presence of the chiral acid. The numbers are the volume integrations with the cross peak E as standard.
used without further purification. DMAc was redistilled over CaH2 under
reduced pressure. 1,4,5,8-Naphthalenetetracarboxylic dianhydride (NDA)
was purified by sublimation before use. 1H NMR spectra were recorded
on a Bruker AV300 or AV400 spectrometer at room temperature with
TMS (δ = 0.000 ppm) as the internal standard. 13C NMR spectra were
recorded on a Bruker AV400 spectrometer at room temperature with
Conclusions
In conclusion, we have successfully developed the NDI-based
chiral polymer PNDI as an enantiomer sensor for a broad range
of chiral acids. The enantiomeric recognition by PNDI was
achieved through the NDI˙ˉ absorption bands generated by
anion–π or lone pair–π interactions and the π-π* transition
bands of the polymer. The association of carboxyl groups with
NDIs of PNDI triggered changes in the original self-assembly
process of the polymer. In this sense, a combination of the acid–
or anion–NDI interactions and the dynamic self-assembly
provided the response of PNDI to acids and facilitated their
recognitions. This is the first time to apply anion-π or lone pair-π
interactions in enantiomeric recognition of chiral acids. Though
the recognition takes long time, the discovery of carboxylic acid-
induced NDI˙ˉ provides unprecedented insight into the
exploration of acid–NDI interactions and applications. With these
results, we have demonstrated a novel strategy for designing
polymeric materials with unique intermolecular interactions and
self-assembly by integrating π-acidic aromatic systems and
chiral building blocks to achieve the relevant advanced functions.
DMSO (δ = 39.520 ppm) as the internal standard. 2D NOESY spectra
was recorded on a Bruker AV500 spectrometer at room temperature with
mixing time at 600 ms. The NOE volume integration was carried out with
Bruker TopSpin2.1 using the intensity of the cross peak of H-3/4 and H-5
as internal standard because of the nearly constant distance of the
protons. Optical rotations were measured on a Perkin Elmer 341LC
polarimeter in a 1 dm or 0.1 dm tube at 0.5 g/dL in DMSO. Scanning
electron microscopy (SEM) images were taken using an XL 30 ESEM
FEG field emission scanning electron microscope (Micrion FEI PHILIPS).
Samples were prepared by drop a small amount of gel from DMSO onto
small glass followed by natural evaporation of solvent. Fourier transform
infrared (FTIR) spectra were recorded on a Bruker Vertex70 Win-IR
instrument. UV-Vis spectra were recorded on a Shimadzu UV-2500 UV-
Vis spectrophotometer at 25 °C using a 0.1 mm cuvette (obtained from
Starna Scientific Ltd). Circular dichroism (CD) spectra were recorded on
a BioLogic MOS-450 spectropolarimeter using a 0.1 mm demountable
quartz cuvette at 25 °C. Linear dichroism (LD) spectra were recorded
alongside CD measurements to confirm the generation of CD signals
from chiral PNDI and its assemblies but not from the macroscopic
anisotropy in the gel. LD spectra showed that the optical densities of all
sample were very weak (<0.003), and their contributions to the CD
spectra were completely negligible. Fluorescence experiment was
performed using Hitachi F-7000 fluorescence spectrophotometer using a
fluorescence cell with 10 mm path. All measurements were performed at
25 °C and with air as reference in UV-Vis spectrometry and with DMSO
Experimental Section
Materials and methods. Chemicals and solvents were purchased from
commercial resources. Unless otherwise specified, all reagents were
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as reference in CD spectrometry. The samples in the quartz cuvette were
stored at 25 °C during the whole period of the spectroscopic
investigations. Time-dependent optical rotation, UV-Vis, CD and
fluorescence spectra were recorded at selected or random intervals.
Unless otherwise specified, the concentration of the polymer used for all
spectroscopic measurements was 5 mg/mL (10.2 mM) based on repeat
unit. PNDI solution was prepared by dissolving pre-weighed amount of
the polymer in DMSO by heating in oil bath at 185 °C for 2 min. The first
spectroscopic measurement was performed just after the newly-prepared
polymer solution was cooled down to 25 °C within about 13 min.
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Synthesis of PNDI through polycondensation of NDA and MAPA. To
a flame-dried 100-mL three-necked flask equipped with a mechanical
stirrer, was charged NDA (1.341 g, 5.0 mmol), MAPA (1.154 g, 5.0
mmol), DMAc (30 mL), and triethylamine (0.75 mL, 5.0 mmol) under N2
flow. The mixture was mechanically stirred at 80 °C for 5 h. Then,
triethylamine (0.75 mL, 5.0 mmol) and Ac2O (1.5 mL, 10 mmol) was
added to the reaction solution followed by stirring for another 2 h. The
resulting solution was poured into excess methanol and filtrated to give
pale yellow powder (1.91 g), yield 82.7%. Mn = 4925, DP = 11.5
(calculated from 1H NMR). PNDI was soluble in H2SO4 at room
temperature and in polar aprotic solvents NMP, DMSO, DMF, and DMAc
on heating. However, PNDI was insoluble in solvents like THF, MeCN,
dichloromethane, and methanol. 1H NMR (300 MHz, DMSO-d6): δ8.92–
8.31 (m, 4H), 7.58–6.92 (m, 4H), 6.09 (s, 0.8H), 5.97 (s, 0.2 H), 3.70 (s,
3H), 3.56–3.17 (m, 2H). 13C NMR (100 MHz, DMSO-d6): δ168.44, 161.63,
137.29, 133.14, 130.44, 129.63, 128.78, 128.34, 128.01, 118.71, 53.81,
51.55, 33.82. FT-IR (KBr): ν 1713 cm−1 (imide C = O, symmetrical
stretching), 1673 cm−1 (imide C = O, asymmetrical stretching), 770 cm−1
(imide ring deformation) and 1343 cm−1 (C-N stretching).
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Preparation of samples for enantiomeric recognition of chiral acids.
The DMSO solutions of chiral acids tested were prepared by dissolving
an enantiomer of a chiral acid in DMSO at 58.7 mM unless otherwise
specified. The concentration of chiral acid 9, 11 and 12 or corresponding
salts were set at 29.3 mM since these compounds were diacids. The
concentration of chiral acid 15 and 21 were set at 46.8 mM because of
their limited solubility in DMSO. The solutions of chiral acid sodium salts
were prepared by neutralizing corresponding acid in DMSO with
equivalent NaHCO3. Samples for acid recognition were prepared by
dissolving 10 mg of PNDI in 2mL of DMSO solution of chiral acid or its
sodium salt in a glass vial by heating in oil bath at 185 °C for 2 min. The
resultant concentration of PNDI was 10.2 mM based on repeat unit. The
first spectroscopic measurement was performed just after the newly-
prepared polymer solution was cooled down to 25 °C within about 13 min.
The sample (0.06 mL) was carefully filled in a demountable quartz
cuvette with 0.1 mm path length for spectroscopic measurements.
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This work was financially supported by the National Natural
Science Foundation of China (21574126 and 21274142) and the
National Basic Research Program of China (973 Program)
(2014CB643603).
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Keywords: molecular recognition • self-assembly • anion–π
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interactions • naphthalenediimides • chiral acids
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10.1002/chem.201700617
Chemistry - A European Journal
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Jijun Yan, Chuanqing Kang,* Zheng
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An optically active polymer for broad-
spectrum enantiomeric recognition of
chiral acids
Grouping up π-acidic aromatic systems and chiral building blocks in a polymer
backbone affords unique self-assembly behaviour and the capability of recognizing
enantiomers of various chiral acids. The anion– or lone pair–π interactions between
anions or neutral acids with naphthalenediimides (NDIs) perturb the self-assembly
process of the polymer, which provides a novel strategy for designing polymeric
materials to achieve advanced functions.
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