The construction of rigid supramolecular polymers in water through the self-assembly of rod-like monomers and cucurbit[8]uril

Article abstract of DOI:10.1039/c4cc02971b

Two new types of supramolecular polymers have been constructed via the self-assembly of rigid rod-like monomers and cucurbit[8]uril (CB[8]) in water. These supramolecular polymers possessed rigid backbones and further aggregated into stick-like bunched fi

Full text of DOI:10.1039/c4cc02971b

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The construction of rigid supramolecular
polymers in water through the self-assembly of
rod-like monomers and cucurbit[8]uril†
Cite this: Chem. Commun., 2014,
50, 7982
Received 22nd April 2014,
Accepted 3rd June 2014
Feng Lin,‡ Tian-Guang Zhan,‡ Tian-You Zhou, Kang-Da Zhang, Guang-Yu Li,
Jian Wu and Xin Zhao*
DOI: 10.1039/c4cc02971b
www.rsc.org/chemcomm
Two new types of supramolecular polymers have been constructed backbones than those reported previously,8 which were visua-
via the self-assembly of rigid rod-like monomers and cucurbit[8]uril lized by TEM and AFM microscopies.
(CB[8]) in water. These supramolecular polymers possessed rigid
backbones and further aggregated into stick-like bunched fibres.
It was reported that BP units could also adopt head-to-head
stacking in the cavity of CB[8].10 Therefore, in order to prevent
the monomers from forming [2+2] head-to-head dimers with
Synthetic polymers are some of the most widely produced CB[8], two isopropyl groups were introduced into the skeleton
materials made by human beings. With the development of of monomer T1 to provide steric hindrance (T1), and in the
supramolecular chemistry, supramolecular polymers, in which structure of monomer T2 a viologen segment was incorporated
the monomers are connected by noncovalent interactions such as to increase electrostatic repulsion of the monomer (Scheme 1).
hydrogen bonding,1 aromatic stacking,2 C–Hꢀꢀꢀp interactions,3 Both strategies should facilitate the formation of supramolecular
metal–ligand bonds,4 donor–acceptor interactions,5 and host–guest polymers with linear backbones.
interactions6 instead of covalent bonds as found in classic
The binding behaviour between monomers T1–T2 and CB[8]
polymers, have drawn considerable attention in the past few was investigated by the 1H NMR titration experiment. As can be
decades because of their important applications in fabricating seen in Fig. 1, successive addition of CB[8] into a solution of T1
smart materials. Although currently a myriad of supramolecular in D2O resulted in a decrease of the intensities of the peaks
polymers have been fabricated by means of molecular self- corresponding to free T1 and growth of a new set of signals
assembly, their structural diversity is quite limited compared which were assigned to CB[8]-encapsulated T1 on the fact of the
with traditional polymers whose morphologies are much more upfield shifts of the signals. The peaks of free T1 significantly
diversified. For example, very recently a new type of polymeric diminished when the molar ratio of T1 and CB[8] reached 1 : 1.
structures named rod–rod block copolymers which possess rigid
backbones have been fabricated that display unique properties
owing to their rigid conformation.7 With respect to their supra-
molecular counterparts, however, there were just very few examples
reported for supramolecular polymers with rigid backbones.8
In this context, this niche remains to be further explored.
We herein report the construction of two types of stick-like
supramolecular polymers in water driven by cucurbit[8]uril
(CB[8])-encapsulation-enhanced stacking of 4,40-bipyridin-1-ium
(BP) units.9 These supramolecular polymers have even more rigid
Laboratory of Synthetic and Self-Assembly Chemistry for Organic Functional
Molecules, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences,
345 Lingling Road, Shanghai, 200032, China. E-mail: xzhao@mail.sioc.ac.cn;
Fax: +86-21-64166128; Tel: +86-21-54925023
† Electronic supplementary information (ESI) available: Synthesis and characteriza-
tions, additional 1H NMR spectra, ITC profiles, 2D NOESY spectra, DOSY profiles and
AFM and SEM images. CCDC 997000. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c4cc02971b
Scheme 1 Chemical structures of monomers T1–T2 and CB[8].
‡ These authors contributed equally to this work.
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An attempt to grow single crystals of supramolecular polymers
fabricated from a 1 : 1 mixture of T1 or T2 and CB[8] for X-ray
crystallographic analysis was not successful. However, the crystals
of monomer T1 were successfully grown by slow evaporation of a
solution of T1 in ethanol. Crystallographic analysis revealed that
two BP units were stacked upon each other in a head-to-tail manner
in the solid state which further led to one dimensional extension
of T1 molecules (Fig. 2, top).12 The BP units adopted an offset
face-to-face stacking with an average distance being 3.73 Å, a
typical distance for aromatic stacking. Thus, in the presence of
CB[8], the encapsulation of two stacked BP units in the cavity of a
CB[8] molecule, just as that found in the crystal structure of
complex 1-phenyl-BP-CB[8] we reported previously,9 could be
expected. It would lead to the formation of linear supramolecular
polymers (Fig. 2, bottom). This expectation was confirmed by 2D
1H NMR NOESY of a 1 :1 mixture of T1 and CB[8] in D2O. The
spectrum displayed intermolecular NOE connections between Ha
and Hd, and Hb and Hd, clearly indicating that two BP units were
stacked upon each other and aligned in a head-to-tail manner in
Fig. 1 Partial 1H NMR spectra of (a) T1, (b) T1 + CB[8] (1 : 0.2), (c) T1 +
CB[8] (1 : 0.4), (d) T1 + CB[8] (1 : 0.6), (e) T1 + CB[8] (1 : 0.8), and (f) T1 +
CB[8] (1 : 1) in D2O at 25 1C. The concentration of T1 was 2.0 mM.
Thus a 1 : 1 binding stoichiometry was suggested for them and the cavity of CB[8] (Fig. S6, ESI†). Similar NOE contacts were also
further corroborated by Job’s plot (Fig. S1, ESI†). Furthermore, observed for a solution of T2 and CB[8] (Fig. S7, ESI†), suggesting
it was also observed that the resolution of the peaks decreased a similar host–guest binding between CB[8] and BP units of T2.
upon the addition of CB[8], suggesting the formation of poly- These results clearly indicated that the rigid monomers and CB[8]
meric aggregates. In the case of 1H NMR titration of T2 with self-assembled into linearly polymeric structures in water. Since
CB[8], the peaks became broad and shifted upfield upon the the monomers are fully rigid, the resulting supramolecular polymers
addition of CB[8], also indicating the formation of polymeric could be expected to possess a rigid rod-like conformation.
structures (Fig. S2, ESI†). A 1 : 1 binding stoichiometry for T2
The formation of supramolecular polymers were further
and CB[8] was also confirmed by Job’s plot (Fig. S3, ESI†). evidenced by 2D 1H NMR diffusion ordered spectroscopy
The binding behaviour between CB[8] and T1 and T2 were (DOSY), which is a technique widely used to characterize
further investigated by the isothermal titration calorimetry supramolecular structures in solution by correlating chemical
(ITC) experiment (Fig. S4 and S5, ESI†), which again revealed resonances with diffusion coefficients (D).13 While a solution of
a 1 : 1 binding model for the monomers and CB[8] and generated T1 alone in D2O (1.0 mM) gave a D value of 2.9 ꢂ 10ꢃ10 m2 sꢃ1, the
apparent binding constants to be (2.1 ꢁ 0.36) ꢂ 107 and D value of the solution of a mixture of T1 and CB[8] (1 : 1, 1.0 mM)
(8.3 ꢁ 1.8) ꢂ 105 Mꢃ2 for CB[8] and T1, and CB[8] and T2, was determined to be 1.6 ꢂ 10ꢃ10 m2 sꢃ1 and further decreased to
respectively. The lower binding constant of the latter might be 8.5 ꢂ 10ꢃ11 m2 sꢃ1 when the concentration of the mixture was
attributed to the competitive binding resulting from the inter- increased to 6.0 mM (Fig. S8–S10, ESI†). For T2, the D values were
action between CB[8] and the viologen unit of T2.11
determined to be 4.0 ꢂ 10ꢃ10, 1.6 ꢂ 10ꢃ10, and 1.1 ꢂ 10ꢃ10 m2 sꢃ1
Fig. 2 The packing structure of T1 in the single crystal, highlighting the linearly extending of T1 molecules through stacking of BP units (top), and
illustration of the formation of rigid linear supramolecular polymers after the encapsulation of the stacked BP units in the cavity of CB[8] generated by
Hartree–Fock/3-21G on the basis of the crystal structure of T1 (bottom). The hydrogen atoms and counter anions were omitted for clarity.
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for the solution of T2 alone (1.0 mM) and a mixture of T2 and CB[8]
(1 : 1) at 1.0 and 5.0 mM in D2O, respectively (Fig. S11–S13, ESI†).
These results strongly suggested the formation of supramolecular
polymers in solution. In addition to DOSY, the dynamic light
scattering (DLS) experiment also revealed the existence of
supramolecular polymers in solution. Upon the addition of
1 equiv. of CB[8] to the solutions of monomers in water, the
hydrodynamic diameters (Dh) of the aggregates formed were
found to increase with an increase in the concentration of
mixtures of the monomers and CB[8] (1 : 1), which was attributed to
a higher degree of polymerization of the supramolecular polymers
at higher concentrations (Fig. S14 and S15, ESI†). Furthermore, no
1H NMR signals of free monomers were observed when concen-
trated solutions of the supramolecular polymers were diluted,
suggesting that no significant dissociation of the supramolecular
polymers occurred even in very dilute solutions, indicating a
high stability of the supramolecular polymers (Fig. S16 and S17,
ESI†). The variable-temperature 1H NMR experiment was performed
for the supramolecular polymers. No apparent disassociation
of the supramolecular polymers was observed even at 75 1C,
suggesting again that the polymeric structures were highly
stable (Fig. S18 and S19, ESI†).
The morphology of the as-prepared supramolecular polymers
was investigated by transmission electron microscopy (TEM). In
the TEM images straight stick-like objects can be observed
(Fig. 3), which is consistent with the expectation for rigid linear
polymeric chains. Their widths were estimated to be dozens of
nanometers while their lengths were several hundred nanometers.
The atomic force microscopy (AFM) study also revealed the
formation of stick-like structures (Fig. S20, ESI†). Since the
diameter of the backbone of a single supramolecular polymer
chain should be close to the diameter of CB[8] (ca. 1.75 nm),14
the stick-like objects observed under the microscopes should
be bundles of individual polymers produced by the aggregation
of the linear chains, as illustrated in Fig. 4. The aggregation of
the polymer chains was attributed to outer-surface interactions
occurring through the convex face of CB[8].15 In contrast,
monomers T1 and T2 generated ill-defined aggregates under
similar conditions, as revealed by scanning electron micro-
scopy (SEM) (Fig. S21, ESI†). These results further confirmed
that the stick-like objects were generated from supramolecular
polymers.
Fig. 4 Cartoon representation of the formation of rigid linear supramo-
lecular polymers and their further aggregation into bundles.
In summary, two new types of supramolecular polymers with
rigid backbones have been constructed in water through the
self-assembly of rigid rod-like monomers and CB[8], driven by
CB[8]-encapsulation-enhanced dimerization of 4,40-bipyridin-1-
ium units. The use of rigid supramolecular monomers has
some advantages over their flexible counterparts. Firstly, it
removes the obstacle of cyclization of flexible monomers, which
seriously hampers the polymerization of supramolecular polymers.
Furthermore, the resulting supramolecular polymers with a rigid
conformation offer accurate control over the spatial distance of
substituents on their backbones because random coiling or
folding of the backbones can be eliminated for these rigid
linear polymers. This should be crucial for the fabrication of
functional materials which requires fine control over the spatial
distance of functional units. The potential of these advantages
is currently being explored.
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