Supramolecular self-assembly of chiral polyimides driven by repeat units and end groups

Article abstract of DOI:10.1039/c7nj02451g

Pyromellitic diimides (PMDIs) are effective building blocks for the construction of supramolecular systems but are infrequently used in comparison with other electron-deficient aromatic systems. We report PMDI-based chiral polyimides that form polymeric supramolecular systems with unique self-assembly features that show time-dependent spectroscopic behaviour. Extensive investigations revealed the driving forces for the self-assembly of the polyimides. One is the complementary aromatic π-π stacking between electron-accepting PMDI and electron-donating phenyl ring in the polymer backbones, and another is the hydrogen bonding interactions of the end groups. The self-assembly is readily disrupted by guest molecules with strong associations with the PMDI and the end groups. The introduction of flexible arylether diimides into the PMDI-based copolymer backbones and the sequence of PMDIs and arylether diimides in the copolymer backbones significantly influence the self-assembly of the polyimides. The results elucidate the mechanisms of polymeric self-assembly of chiral polyimides, providing important information for the development of materials based on polymeric supramolecular systems with properties and functions regulated by composition, sequence and end groups.

Full text of DOI:10.1039/c7nj02451g

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ISSN 1144-0546
PAPER
Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
diarylethene-based metal–organic frameworks
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Supramolecular self-assembly of chiral polyimides driven by
repeat units and end groups
Jijun Yan,a, b Chuanqing Kang,*a Zheng Bian,a Rizhe Jin,a Xiaoye Maa and Lianxun Gaoa
Received 00th January 20xx,
Accepted 00th January 20xx
Pyromellitic diimides (PMDIs) are effective building blocks for the construction of supramolecular systems but are
infrequently used in comparison with other electron-deficient aromatic systems. We report PMDI-based chiral polyimides
that form polymeric supramolecular systems with unique self-assembly features that show time-dependent spectroscopic
behaviour. Extensive investigations revealed the driving forces for the self-assembly of the polyimides. One is the
complementary aromatic π–π stacking between electron-accepting PMDI and electron-donating phenyl ring in the
polymer backbones, and another is the hydrogen bonding interactions of the end groups. The self-assembly is readily
disrupted by guest molecules with strong associations with the PMDI and the end groups. The introduction of flexible
arylether diimides into the PMDI-based copolymer backbones and the sequence of PMDIs and arylether diimides in the
copolymer backbones significantly influence the self-assembly of the polyimides. The results elucidate the mechanisms of
polymeric self-assembly of chiral polyimides, providing important information for the development of materials based on
polymeric supramolecular systems with properties and functions regulated by composition, sequence and end groups.
DOI: 10.1039/x0xx00000x
www.rsc.org/
emission of circularly-polarised light.13-20 Furthermore, I. Huc
demonstrated an oligoamide that was able to encapsulate
fructose with high selectivity through the formation of helical
foldamer.21 Importantly, the selective recognition of
monosaccharides had been a challenge before the publication
of that work. Consequently, the self-assembly of synthetic
chiral polymers is expected to attract increasing research
attention.
Introduction
Biological systems are extremely complicated but highly
efficient. For example, spliceosome, a molecular machine
assembled by snRNAs and protein complexes, has the capacity
to selectively remove non-coding introns from transcribed pre-
mRNAs with high accuracy and efficiency.1 Most advanced
biochemical functions are performed by controlling the precise
self-assembly of chiral biomacromolecules and their resultant
superstructures.2 Some basic features of proteins can be
mimicked by the self-assembly of synthetic foldamers.3
However, unlike the vast majority of natural systems, synthetic
polymeric supramolecular systems are typically constructed
with achiral entities.
Non-covalent interactions, such as π–π interactions,
hydrogen bonding and metal–ligand interactions, are common
driving forces in the construction of polymeric supramolecular
systems.22 For example, complementary π–π stacking can
endow a gel material with enhanced mechanical properties
and facile self-healing characteristics,23 and hydrogen bonding
has been used to drive the association of small molecules or
oligomers containing amide or urea groups to form
supramolecular polyamide or polyurethane structures with
rubber-like properties.24, 25 However, as a class of polymer with
excellent performance, polyimides have seldom been applied
in the construction of supramolecular systems, despite the
polymer typically having planar aromatic monomers that can
act as π–donors or acceptors.26 A limited number of functional
polyimide-based supramolecular systems have been
composed of structures that form hydrogen bonding or metal
coordination,27, 28 which bring additional complexity to the
design and preparation of polymers. In addition, the excellent
thermostability and chemical stability of polyimides can endow
The introduction of chirality into the polymer-based
supramolecular system not only imparts it with optical activity
but can also may lead to unique hierarchical structures and
advanced functions.4-6 Furthermore, supramolecular systems
based on chiral polymers are emerging as promising functional
materials.7-12 In recent years, M. Suginome et al have
extensively studied the self-assembly of poly(quinoxaline-2, 3-
diyl)s with tuneable main-chain helical sense, and these
polymers have been applied in catalysis for organic
conversions and as functional films in selectively reflecting or
polyimide-based polymeric self-assembly with
application scope and prospects.
a wider
Several recent studies have demonstrated the self-assembly
of small molecular pyromellitic diimide (PMDI) derivatives and
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aromatic electron donors via charge-transfer π–π stacking.29-32
The self-assembly of PMDI derivatives has been used to
prepare unique supramolecular architectures with advanced
functions, though they are used for construction of
supramolecular systems much less frequently in comparison
with other electron-deficient aromatic compounds such as
naphthalenediimides.
Consequently, in the current study, we have investigated
the self-assembly of PMDI-based chiral polymers, and
polyimides with PMDIs and chiral centres in the main chain.
Most recently, we presented a series of chiral polyimides
derived from L-phenylalanine derivatives that showed slow
self-assembly in dimethylsulfoxide (DMSO) to form gels and
exhibited excellent performance in the recognition of
enantiomers of chiral acids and weak Lewis basic anions.33-35
Chiral polyimides containing PMDIs in the backbone have
demonstrated unique sequence-dependence in self-assembly
and molecular recognition.35 We presumed that the
complementary π–π interactions between the polymer chains
result in the self-assembly, and the disturbance of the self-
assembly by guest molecules was the intrinsic driving force for
molecular recognition. We have, therefore, carried out
extensive studies on the self-assembly of the chiral polyimide
PMDI-Phe to gain deeper insight into how π–π stacking drives
the self-assembly of chiral polyimide (Scheme 1). Chiroptical
spectroscopy and NMR methods were used in this study. The
contributions of the π–π stacking of the repeat units and the
interactions of the end-groups to the polymeric self-assembly
were investigated by performing comparative experiments
using polyimides with different backbone compositions.
Investigations into the association of the model molecules of
the repeat units and end groups using a tweezer molecule
confirmed the driving forces for the polymer self-assembly.
Herein, we present the details of these studies. The results
reveal the mechanism of the self-assembly of chiral polyimides
and demonstrate a simple strategy for the design of chiral-
polyimide-based supramolecular systems with tunable self-
assembly behaviours.
Scheme 1 Preparation of chiral polyimides. TEA = triethylamine, DMAc = N,N-dimethyl
acetamide, PMDA = pyromellitic dianhydride, ODPA = 4,4'-oxydiphthalic anhydride,
HFDA = 4,4'-(hexafluoroisopropylidene)diphthalic anhydride.
Experimental
Materials
All reagents were purchased from commercial resources and
used without further purification unless specific stated. N,N-
Dimethylacetamide (DMAc) was dried over CaH2 and purified
by distillation under reduce pressure prior to use. Pyromellitic
dianhydride (PMDA), 4,4'-oxydiphthalic anhydride (ODPA) and
4,4'-(hexafluoroisopropylidene)diphthalic anhydride (HFDA)
were purified by sublimation before use. External details on
the synthesis of monomers, model molecules and the tweezer
molecule were enclosed in the electronic supplementary
information. Number-averaged molecular weights of the chiral
polyimides were determined to be in range from 10,000 to
15,000 based on end group analysis with 1H NMR.
Methods
1H and 13C NMR spectra were recorded at 25 °C using a Bruker
AV300 or AV400 spectrometer with DMSO signal (δH = 2.50
ppm, δC = 39.52 ppm) as internal reference of chemical shifts.
2D NOESY experiments were carried out at a mixing time of
600 ms. Fourier transform infrared (FTIR) spectra were
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recorded on a Bruker Vertex70 Win-IR instrument. Specific 1785 cm−1 (imide C = O, symmetrical stretching), 1726 cm−1
rotations were measured on a Perkin Elmer 341LC polarimeter (imide C = O, asymmetrical stretching), 722 cm−1 (imide ring
in a 1 dm tube cell. Circular dichroism (CD) spectra were deformation) and 1380 cm−1 (C-N stretching).
recorded on a BioLogic MOS-450 spectrometer at 25 °C using a
PMDI-Lys. Not determined. Yield: 79.8%.1H NMR (300 MHz,
0.1 mm cuvette (volume 60 μL, Starna Scientific Ltd). Alongside DMSO-d6): δ 8.62–7.60 (m, 2H), 4.92 (s, 1H), 3.63 (s, 3H), 3.55
CD measurements, linear dichroism (LD) spectra were (s, 2H), 2.34-1.92 (m, 2H), 1.83-1.48 (m, 2H), 1.47-1.08 (m, 2H).
recorded to confirm that no LD contributions to CD signals (the 13C NMR (101 MHz, DMSO-d6): δ 165.41, 158.50, 158.19,
LD intensities of all samples were very weak (<0.003) and 139.48, 137.11, 131.59, 127.56, 118.00, 49.67, 20.55. FT-IR
completely negligible).
(KBr): ν 1775 cm−1 (imide C = O, symmetrical stretching), 1719
Unless otherwise specified, the concentration of the cm−1 (imide C = O, asymmetrical stretching), 727 cm−1 (imide
polymer used for all spectroscopic measurements was 5 ring deformation) and 1388cm−1 (C-N stretching).
mg/mL in DMSO. Polymer solution was prepared by dissolving
PMDI-Phe-ODPI. Molecular weight 10060 Da. Yield:
pre-weighed amount of polymer in DMSO by heating in oil 94.6%.1H NMR (300 MHz, DMSO-d6): δ 8.27 (s, 2H), 7.91 (d, J =
bath at 185 °C for 2 min. The first measurement was 7.8 Hz, 2H), 7.71-7.47 (m, 4H), 7.34 (s, 8H), 5.34 (s, 2H), 3.69 (s,
performed just after the newly-prepared polymer solution was 6H), 3.58 (d, J = 12.5 Hz, 2H), 3.41 (d, J = 12.5 Hz, 2H).
13C NMR
cooled down to 25 °C within about 13 min. Time-dependent (101 MHz, DMSO-d6): δ 168.95, 166.21, 166.07, 165.29,
specific rotation and CD spectra were recorded at selected or 160.87, 137.11, 136.92, 133.46, 130.02, 129.30, 126.92,
random intervals. All samples in the cell or cuvette were 126.18, 125.18, 117.74, 114.31, 52.81, 52.51, 33.67. FT-IR
stored in a constant temperature oven at 25 °C during the (KBr): ν 1778 cm−1 (imide C = O, symmetrical stretching), 1716
intervals of the time-dependent measurements to avoid the cm−1 (imide C = O, asymmetrical stretching), 726 cm−1 (imide
influence of temperature to the self-assembly of the polymers. ring deformation) and 1380 cm−1 (C-N stretching).
Polymerizations
PMDI-Phe-HFDI. 15930 Da. Yield: 90.1%.1H NMR (300 MHz,
DMSO-d6): δ 8.27 (s, 2H), 8.03 (d, J = 8.0 Hz, 2H), 7.93–7.63 (m,
4H), 7.35 (d, J = 6.5 Hz, 7H), 5.37 (s, 2H), 3.69 (s, 6H), 3.60 (d, J
= 14.5 Hz, 2H), 3.40 (d, J = 14.5 Hz, 2H). 13C NMR (101 MHz,
DMSO-d6): δ 168.77, 166.00, 165.93, 165.29, 137.75, 136.93,
131.85, 131.42, 130.04, 129.32, 126.95, 124.55, 123.87,
117.71, 52.84, 38.89, 33.71. FT-IR (KBr): ν 1746 cm−1 (imide C =
All the polyimides were prepared through a similar procedure.
Here presented the synthesis of PMDI-Phe as an example. To a
completely dried 100-mL, three-necked flask equipped with a
mechanical stirring device, were charged PMDA (0.692 g, 3.0
mmol), 1a (0.654 g, 3.0 mmol), DMAc (18 mL) and TEA (0.45
mL, 3.2 mmol) under N2 flow. The mixture was mechanically
stirred at room temperature for 22h. Then TEA (0.45 mL, 3.2
mmol) and Ac2O (0.90 mL, 6.4 mmol) was added to reaction
solution and continued stirred for 1 h at room temperature
and 2 h at 80°C. The resultant viscous suspension was poured
into excess MeOH and filtrated to give pale yellow powder
(0.95 g, 84.3%). Molecular weight 12550 Da. 1H NMR (400 MHz,
DMSO-d6): 8.26 (s, 2H), 7.35 (s, 4H), 5.41 (s, 1H), 3.67 (s, 3H),
3.50-2.81 (m, 2H). 13C NMR (101 MHz, DMSO-d6): δ 168.58,
165.21, 165.06, 137.55, 136.89, 136.31, 135.70, 130.05, 129.38,
126.96, 118.72, 118.23, 117.70 , 52.92, 33.63. FT-IR (KBr): ν
1778 cm−1 (imide C = O, symmetrical stretching), 1716 cm−1
(imide C = O, asymmetrical stretching), 726 cm−1 (imide ring
deformation) and 1380 cm−1 (C-N stretching).
O, symmetrical stretching), 1724 cm−1 (imide
C = O,
asymmetrical stretching), 722 cm−1 (imide ring deformation)
and 1377 cm−1 (C-N stretching).
ODPI-Phe-PMDI. 15390 Da. Yield: 72.0%.1H NMR (300 MHz,
DMSO-d6): δ 8.34-8.14 (m, 2H), 7.93 (d, J = 7.9 Hz, 2H), 7.67-
7.41 (s, 4H), 7.40-7.03 (m, 8H), 5.37 (s, 2H), 3.86-3.64 (m, 6H),
3.64-3.49 (m, 2H), 3.48-3.23 (m, 2H). 13C NMR (101 MHz,
DMSO-d6): δ 168.58, 166.03, 165.86, 165.09, 160.67, 136.72,
136.66, 136.31, 134.25, 130.35, 129.30, 126.98, 125.94,
124.83, 118.70, 113.57, 53.07, 52.90, 33.63. FT-IR (KBr): ν 1778
cm−1 (imide C = O, symmetrical stretching), 1716 cm−1 (imide C
=
O, asymmetrical stretching), 726 cm−1 (imide ring
deformation) and 1380 cm−1 (C-N stretching).
ODPI-Phe. Not determined. Yield: 94.3%. 1H NMR (300 MHz,
DMSO-d6): δ 8.09-7.82 (m, 2H), 7.69–7.39 (m, 4H), 7.38-7.13
(m, 4H), 5.32 (s, 1H), 3.78-3.61 (m, 3H), 3.55 (d, J = 10.7 Hz, 1H),
3.38 (d, J = 12.4 Hz, 1H). 13C NMR (101 MHz, DMSO-d6): δ
168.95, 166.24, 166.09, 165.91, 161.36, 160.84, 160.71, 160.19,
136.81, 134.32, 133.46, 130.31, 129.22, 126.98, 126.17, 125.95,
125.12, 124.80, 114.51, 114.30, 113.62, 113.45, 52.79, 52.52,
33.64. FT-IR (KBr): ν 1778 cm−1 (imide C = O, symmetrical
stretching), 1716 cm−1 (imide C = O, asymmetrical stretching),
726 cm−1 (imide ring deformation) and 1380 cm−1 (C-N
stretching).
Results and discussion
All polyimides were prepared by polycondensation of amino
acid-derived diamines and dianhydrides followed by chemical
imidisation with acetic anhydride and triethylamine (Scheme
1). PMDI-Phe, ODPI-Phe and HFDI-Phe were prepared from
polycondensation of L-phenylalanine (Phe)-derived diamine 1a
with PMDA, ODPA and HFDA, respectively, whereas PMDI-Lys
was prepared from methyl L-lysinate (1b) and PMDA. The
asymmetric diamines 1a and 1b were arranged in random
directions along the main chain. Copolymers PMDI-Phe-ODPI
and PMDI-Phe-HFDI were prepared from diimide-derived
diamine 1c and ODPA and HFDA, respectively, whereas ODPI-
Phe-PMDI was prepared from 1d and PMDA. The methods
used for the synthesis of the copolymers ensure well-defined
1
HFDI-Phe. Not determined. Yield: 89.1%. H NMR (300 MHz,
DMSO-d6): δ 8.16– 7.95 (m, 2H), 7.94–7.72 (m, 3H), 7.68 (d, J =
10.0 Hz, 1H), 7.35 (d, J = 8.3 Hz, 2H), 7.26 (d, J = 7.7 Hz, 2H),
5.35 (s, 1H), 3.67 (s, 3H), 3.56 (s, 1H), 3.35 (s, 1H). FT-IR (KBr): ν
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chain structures. PMDI-Phe-ODPI and ODPI-Phe-PMDI have structural alterations, demonstrating the critical role of both
different diimide sequences based on the connection of PMDI PMDI and Phe segments in the slow self-assembly of PMDI-
to the aromatic or aliphatic carbon of the diamine monomer.
Phe. The results are in agreement with the presumption that
The PMDI-based chiral polyimide PMDI-Phe is capable of the self-assembly of the chiral polyimide is driven by the
thermo-reversibly gelling DMSO during long-term standing.32 complementary π–π stacking of planar PMDI segments and the
Sustained dynamic changes of the specific rotations and phenyl ring of the Phe segments.35
circular dichroism (CD) spectra were observed during and after
Further investigation using the copolymers PMDI-Phe-ODPI,
the gelation, strongly indicating the slow self-assembly of the PMDI-Phe-HFDI and ODPI-Phe-PMDI confirmed the
polymer (Fig. 1a and 2a). In time-dependent CD spectra, the association of PMDI and Phe segments. In PMDI-Phe-ODPI and
CD band at 272 nm changes from minimum negative to PMDI-Phe-HFDI, PMDI segments are connected to an aromatic
maximum positive within 170 hours (Fig. 2a). The phenomena carbon of the Phe segment; in ODPI-Phe-PMDI, the PMDI
may be derived from the formation of supramolecular chirality segments are conneted to an aliphatic carbon at the chiral
that is opposite to the initial chirality of the polymer chains centre. In comparison with PMDI-Phe, PMDI-Phe-ODPI shows
before aggregation.5, 36 The reverse of the chirality before and less change in specific rotations and CD spectra with time,
after self-assembly results in persistent changes in the time- whereas the other two copolymers show almost constant
dependent optical rotation and CD spectra. Subsequently, we specific rotations except for small changes in the initial stage
tracked the time-dependent specific rotations of PMDI-Phe at within 2 hours (Fig. 1d and S3). Moreover, PMDI-Phe-ODPI
different concentrations in DMSO. According to the tracking gels DMSO after a longer period of standing compared to
experiments, the variation range and rate of the CD spectra at PMDI-Phe, while the other two copolymers do not gel DMSO
272 nm significantly decrease upon dilution of the polymer at all. Therefore, the interactions between the PMDI and Phe
solution from 11.7 to 8.0 mM and below (Fig. 1b and Fig. S1), segments are the key drivers for the slow stacking of the
suggesting the dominative role of interchain stacking in the polymer chains, which can be suppressed to a certain degree
polymeric self-assembly.
by the introduction of flexible and non-planar ODPI and HFPI
segments. Moreover, the subtle differences in the time-
dependent specific rotations and gelation properties of PMDI-
Phe-ODPI and ODPI-Phe-PMDI are probably a result of the
different diimide sequences in the polymer backbones (Fig. 1d).
The fact that the polyimide PMDI-Phe-ODPI displays more
significant self-assembly than ODPI-Phe-PMDI indicates that
the contributions to the polymeric self-assembly of the diaryl-
substituted PMDI segments are more important than those of
the dialkyl-substituted segments.38
The polymerisation of PMDI-Phe will leave a monoester of
phthalic diacid (MPA) at one or both ends of the polymer chain
upon quenching with methanol (Scheme 2). The end group
MPA as a hydrogen bond donor and acceptor may influence
the self-assembly of the polymer. However, the comparison
between FT-IR spectra of the solution and gel of PMDI-Phe
does not show clear evidence of hydrogen bonding or other
intermolecular interactions (Fig. S4). Thus, the end-capped
polyimide PMDI-Phe-EC was prepared to clarify the role of the
MPA end group on the self-assembly of the polymer. The end-
capped polyimide shows a negative Cotton signal at 256 nm,
which slowly fades along with the formation of a positive
Cotton signal at 282 nm; this is attributed to a longitudinally
polarised imide transition (Fig. 2b).39 In comparison with
Fig. 1 Time-dependent specific rotations and CD spectra of chiral polyimides. a) Time-
dependent specific rotation of PMDI-Phe; b) Plots of CD spectra of PMDI-Phe at 272
nm versus time at different concentrations in DMSO; c) Constant specific rotations of
PMDI-Lys, ODPI-Phe and HFDI-Phe; d) Time-dependent specific rotations of
copolyimides PMDI-Phe-ODPI, ODPI-Phe-PMDI and HFDI-Phe-PMDI.
Amino-acid-based polyimides have not shown the formation
of polymeric supramolecular systems before.26, 37 Therefore,
we designed the chiral polyimides PMDI-Lys
HFDI-Phe to distinguish the key structural factors that drive
the polymeric self-assembly. Unlike the structure of PMDI-Phe
, ODPI-Phe and
PMDI-Phe, PMDI-Phe-EC shows a similar but slower change of
CD spectra at the specified wavelength with time and far
slower gelation (Fig. 2c), indicating the important role of the
end groups in the polymeric self-assembly. The time-
dependent specific rotation of PMDI-Phe-EC gradually
decreases linearly with time; this is a significantly different
behaviour from that of PMDI-Phe, which is consistent with the
trend in the later stages (Fig. 2d). This consistency suggests
comparable or identical kinetics of self-assembly and resultant
supramolecular architecture. In the time-dependent specific
rotation of PMDI-Phe, the initial rapid stage is proposed to be
,
that of PMDI-Lys contains a long aliphatic chain. In ODPI-Phe
and HFDI-Phe, the rigid and planar PMDI segment is replaced
with flexible, non-planar 4,4'-oxydiphthalic diimide (ODPI) and
4,4'-(hexafluoroisopropylidene)diphthalic
diimide
(HFDI)
segments. Notably, unlike PMDI-Phe, three new polyimides
show constant specific rotations and CD spectra and no
capability to induce gelation of DMSO (Fig. 1c and S2). The
significant difference in chiroptical behaviour results from
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a result of the synergistic effects of the interchain π–π stacking Consequently, the NOESY experiments confirm our assumption
of PMDI and Phe segments and the hydrogen bonding from that the slow self-assembly of PMDI-Phe is driven by the
the MPA end groups, whereas the subsequent slow and steady complementary π–π interactions of the PMDI and Phe
stage is a result of π–π stacking alone.
segments, similarly to other diimide molecules (Fig. 3c).
end-capped with 1e
O
O
O
O
MeO2C
CO2Me
N
N
N
N
NO2
O
O
O
O
MeO2C
NH2
1e
NO2
O
polyimide before quenching
O
O
O
MeO2C
N
O
N
N
O
O
O
O
O
O
MeOH
Fig. 3 NOESY spectra of PMDI-Phe (a) and interchain short-range spatial coupling of
protons (b). Cross peaks in red rectangles in the NOESY spectra are derived from the
spatial proximity of Ha in the PMDI segment and the protons of the Phe segment.
quenched with MeOH
CO2H
O
N
MeO2C
N
N
CO2Me
Since the π–π acceptor-donor interactions of the PMDI and
Phe segments with the assistance of the hydrogen bonding of
the end groups drive the self-assembly of PMDI-Phe, a guest
molecule with a stronger affinity for the PMDI segments or the
end groups will inhibit the polymeric self-assembly. Previously,
we utilised similar tactics in molecular recognition, in which
the original self-assembly process of polyimides was disturbed
by guest anions and chiral acids.33-35 Recent reports have
demonstrated the recognition of pyromellitic diimide through
complementary π–π stacking and hydrogen-bondings using a
pyrenyl-based tweezer molecule.43, 44 Hence, we designed the
novel isophthalic diamide-based tweezer IPDA to investigate
the inhibition of the self-assembly (Scheme 3). As expected,
the time-dependent specific rotation of PMDI-Phe is constant
in the presence of IPDA (Fig. 4a). The introduction of IPDA into
PMDI-Phe also leads to invariable CD spectrum with significant
decrease of the intensity of CD band at 272 nm (Fig. S5), which
is attributed to the destruction of both original polymer
conformation and supramolecular architectures. An enhanced
absorption at 305 nm in UV-Vis spectra is observed along with
the colour change from light yellow to deep red (Fig. S6). Thus,
the competitive association of IPDA with the PMDI segments
and MPA end groups prevents the original interactions
between the polymer backbones.
O
O
O
MPA end group
PMDI segment
Scheme 2 Structure of the end group of PMDI-Phe and end-capped polymer
Fig. 2 Comparison of the chiroptical spectra of PMDI-Phe with and without end-capping.
a) Time-dependent CD spectra of PMDI-Phe; b) Time-dependent CD spectra of PMDI-
Phe-EC; c) Plots of the CD of PMDI-Phe and PMDI-Phe-EC at 272 nm versus time; d)
Plots of specific rotations of PMDI-Phe and PMDI-Phe-EC versus time.
The interchain π–π stacking of PMDI and Phe segments was
confirmed by NMR analysis. The 2D NOESY spectrum of PMDI-
Phe in DMSO shows unambiguous cross peaks attributed to
the short-range spatial coupling of Ha with Hb, Hc, Hd and He
(Fig. 3a and 3b). Ha is located on the PMDI segment, and the
others are located on the Phe segment. The results strongly
suggest the short-range spatial proximity of the PMDI and Phe
O
O
MeO2C
a
NHBoc
N
N
BocHN
b
c
a'
CO2Me
O
O
2a
O
OMe
NH
O
N
e
d
CO2H
MeO2C
segments
in
the
polymer
backbones.
Several
IPDA
i
Pr
CO2Me
naphthalenediimide-derived small molecules and polymers
have been assembled through intermolecular π–π interactions,
in which the electron-deficient diimides and electron-rich aryls
are packed through their electrostatic complementation.33, 40-42
NH
O
2b
O
OMe
Scheme 3 Models of PMDI segment and end group and tweezer molecule.
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Accordingly, we prepared compounds 2a and 2b as models (Fig. 4d). Moreover, the NOESY spectra of the mixture does not
for the repeat unit and end group of PMDI-Phe, respectively show the original cross peaks of the PMDI proton Ha with the
(Scheme 3). The interactions between IPDA and PMDI-Phe Phe protons corresponding to the interchain interactions of
were simulated with the model compounds. The introduction PMDI-Phe as shown in the NOESY spectra of the polymer itself
of IPDA into a solution of 2a leads to a colour change from (Fig. 3a and 5a), suggesting that IPDA prevents the interactions
light yellow to deep red in the same way as introduction of of the polymer backbones. Through the complementary π–π
IPDA into a solution of PMDI-Phe, as a result of a charge- stacking and hydrogen bonding, the backbones of PMDI-Phe
transfer absorption from the diimide-tweezer’s π–π stacking.45 are entangled to form micelles that are responsible for the
Hydrogen bonding between the amides of IPDA and carbonyl gelation (Fig. 5b). However, in the presence of IPDA, the
of 2a are also driving forces for the association.43, 44 The 1H backbones are thought to be embedded in the tweezer
NMR spectrum of 2a becomes complicated owing to molecule to form necklace-like chains with local sandwich
association with IPDA
,
which breaks the symmetric structures at the repeat units and end groups, which disrupt
microenvironment of 2a (Fig. 4b). The proton signal at 8.36 the associations of the polymer chains (Fig. 5b). The
ppm (Ha of 2a) shifts upward to 7.39 ppm, and the signals for destruction of the original micelle networks results in sol-gel
two protons on the Phe aryls at 7.42 ppm (protons Hb and Hc) conversion, whereas each of the ‘necklaces’ is free in DMSO.
split into two signals at 7.64 and 7.20 ppm (Fig. 4b). These
changes result from differences in the chemical
microenvironment of the protons because of the shielding and
deshielding effect exerted by IPDA
, which leads to
downshifting and upshifting of the originally overlapping
proton signals. A mixture of 2b and IPDA exhibits upshifts of
the signals for Hd and He in 2b from 8.18 and 8.15 ppm to 8.09
and 7.90 ppm, respectively, owing to shielding from IPDA (Fig.
4c). Job analyses based on the 1H NMR spectra for the
complexations of both 2a and 2b with IPDA clearly show a
preference for 1:1 binding, and the binding constants were
determined to be 60.0 and 115.9 M-1 (Fig. S7-10). The binding
tests demonstrated that IPDA is capable of binding with not
only the PMDI segment but also the end group, which is the
essential factor for inhibiting the polymeric self-assembly.
Fig. 5 NOESY spectra (regional) of a mixture of PMDI-Phe and IPDA (a) and a schematic
illustration of the inhibition of the self-assembly of PMDI-Phe by IPDA (b).
It is remarkable that the integration of multiple PMDI and
Phe segments in PMDI-Phe afford the capacity to self-
assemble, while the model of the repeat unit, 2a, does not
show the capability to self-assemble. Thus, these
investigations also provide potential techniques for the
regulation or amplification of polymer properties and their
potential functions through the integration of building blocks
to obtain group effects in the polymeric self-assembly. As we
have described above, the properties of the polyimide with
rigid and planar aromatic diimides and Phe segments are also
regulated by the introduction of flexible and non-planar
structures, which changes the original interactions. Another
remarkable phenomenon is the spectroscopic difference
between PMDI-Phe-OPDI and OPDI-Phe-PMDI, which reveals
the importance of defined sequence for the polymeric self-
assembly.13-15, 35 We also found the influence of the end groups
upon the self-assembly of PMDI-Phe, which may present a
Fig. 4 Inhibition of the polymeric self-assembly of PMDI-Phe and simulations with
model compounds. a) Time-dependent specific rotation of PMDI-Phe in the absence
and presence of tweezer molecules; b), c) and d) 1H NMR spectra (regional) of 2a, 2b
and PMDI-Phe, respectively, in the absence (bottom) and presence (top) of IPDA. One
of the Ha protons of 2a overlaps with a proton of IPDA at 8.36 ppm according to the
integration.
tactic to manipulate the self-assembly or other properties of
the polymer.46,
47
According to these results, the rationale
1H NMR analysis of a mixture of IPDA and PMDI-Phe
revealed similar splits and upshifts of the proton signals with
the appearance of new peaks at 8.02 (overlapped with the
protons of IPDA), 7.49 and 7.13 ppm, suggesting similar
interactions of the polymer with IPDA to those of the models
design of compositions, sequences and even end groups may
be effective strategies for regulating the properties and
functions of supramolecular systems and materials based on
chiral polyimides.
6 | J. Name., 2012, 00, 1-3
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Acknowledgements
This work was financially supported by the National Natural
Science Foundation of China (21574126 and 21274142) and
the National Basic Research Program of China
(2014CB643603).
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New Journal of Chemistry
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Complementary aromatic π–π stacking and hydrogen bonding induce time-dependent chiroptical
spectroscopic behaviours of L-phenylalaninate-derived chiral polyimide.