A dual-responsive hyperbranched supramolecular polymer constructed by cooperative host-guest recognition and hydrogen-bond interactions

Article abstract of DOI:10.1039/C8CC08226J

A homotritopic pillar[5]arene (H₃) containing adenine units was synthesized and employed to interact with a uracil derivative (6-(2,4-dioxo-3,4-dihydropyrimidin-1(2H)-yl)hexanenitrile, G) to form a hyperbranched supramolecular polymer. The hype

Full text of DOI:10.1039/C8CC08226J

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A dual-responsive hyperbranched supramolecular
polymer constructed by cooperative host–guest
recognition and hydrogen-bond interactions†
Cite this: Chem. Commun., 2018,
54, 13821
Received 14th October 2018,
Accepted 13th November 2018
Yu-Qin Jiang, Kai Wu, Qian Zhang, * Ke-Qing Li, Yan-Yan Li, Peng-Yang Xin,
Wei-Wei Zhang and Hai-Ming Guo*
DOI: 10.1039/c8cc08226j
rsc.li/chemcomm
A homotritopic pillar[5]arene (H3) containing adenine units was
Nucleobases are important supramolecular motifs because
synthesized and employed to interact with a uracil derivative of their famous base-pair interactions including hydrogen
(6-(2,4-dioxo-3,4-dihydropyrimidin-1(2H)-yl)hexanenitrile, G) to form a bondings, p–p stacking and the hydrophobic effect.13 Several
hyperbranched supramolecular polymer. The hyperbranched supra- supramolecular polymers have been constructed through base-pair
molecular polymer showed a dual stimulus response both to heat and interactions and have shown excellent stimulus responsiveness or
acid/base. The cooperative host–guest binding and hydrogen-bond efficient drug delivery ability.14 Furthermore, nitrile guests have
interactions play a key role in the supramolecular polymerization.
been excellent motifs for the development of supramolecular
systems based on pillar[5]arenes.15 We have reported a four-unit
Supramolecular polymers,1 the combination of supramolecular [c2] daisy chain constructed using an adenine monofunctionalized
chemistry and polymer science, have aroused considerable pillar[5]arene and a nitrile guest G through cooperative host–guest
research interest and have shown a broad range of potential binding and hydrogen-bond interactions.16 Here, we report the
applications in many fields.2 Various non-covalent interactions, formation of a hyperbranched supramolecular polymer based on a
such as host–guest binding,3,4 multiple hydrogen bonds,5 homotritopic pillar[5]arene H3 constructed by cooperative host–
p-stacking interactions,6 and metal–ligand coordination,4,7 have guest binding and hydrogen-bond interactions (Scheme 1). To the
been used for the construction of supramolecular polymers. best of our knowledge, this is the first report of pillararene-based
Because of the reversibility and adjustability of these weak inter- supramolecular polymerization through cooperative non-covalent
actions, supramolecular polymers show special functions such as interactions.
degradability, self-healing properties and stimuli-responsiveness.8
H3, which is composed of three adenine mono-functionalized
Pillararenes (PAs), which are made up of (substituted) hydro- pillar[5]arene (H) groups, was synthesized (Scheme S1, ESI†) and its
quinone units linked by methylene bridges, possess rigid and structure was characterized by 1H NMR, 13C NMR and MALDI-TOF
p-rich cavities and could bind guests to construct various novel MS (see the ESI†). Guest G, which contains both a uracil group
supramolecular systems. Among them, supramolecular polymers and a nitrile binding site, was also synthesized and carefully
based on pillararenes are a popular research topic and many characterized.
scientists have conducted a variety of studies in this field.3,4,9 To
The binding stoichiometry of H and G in CDCl3 was first
the best of our knowledge, all pillararene-based supramolecular determined to be 1 : 1 (Fig. S19, ESI†). Then, the host–guest
polymers have been constructed through single or orthogonal complexation was investigated by 1H NMR spectroscopy
non-covalent interactions1g,3,4,9 but cooperative supramolecular (Fig. 1). As shown in Fig. 1, the addition of H resulted in a
polymerization based on pillararenes has not been reported. significant upfield shift and broadening of the methylene
0
0
Cooperative non-covalent interactions are very important in protons (H1 –5 ) on the guest, suggesting that G entered into
biosystems10 and functional supramolecular systems.11,12
the cavity of H to form [2]pseudorotaxane as a result of a slow
exchange process on the NMR spectroscopy time scale.16,17
According to the reports by Li and co-workers,18 dipole–dipole,
Henan Key Laboratory of Organic Functional Molecule and Drug Innovation,
Collaborative Innovation Center of Henan Province for Green Manufacturing of Fine C–Hꢀ ꢀ ꢀp and C–Hꢀ ꢀ ꢀO interactions usually exist between
Chemicals, Key Laboratory of Green Chemical Media and Reactions, Ministry of
pillar[5]arenes and nitrile guests. Therefore, similar interactions
between H and G were probably present. A DFT study showed that
Education, School of Chemistry and Chemical Engineering, Henan Normal
University, Xinxiang, Henan 453007, China. E-mail: zhangqian1980@163.com,
pꢀꢀꢀp interactions between A–U base pairs were present in every
guohm518@hotmail.com; Tel: +86 373 3329276
[2]pseudorotaxane.19 Further investigation showed that the signal of
† Electronic supplementary information (ESI) available: Experimental details,
1H NMR, 13C NMR and DOSY analysis. See DOI: 10.1039/c8cc08226j
imide N–H (3-position) shifted downfield and split into a doublet
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with the addition of H, indicating that hydrogen-bond interactions
formed between the A–U base pairs of the neighboring [2]pseudo-
rotaxanes in a rapid association–dissociation process on the NMR
time scale. Such hydrogen-bond interactions can make two adjacent
[2]pseudorotaxanes dimerize together.16 The association constant
between H and G was determined to be as high as (7.2 ꢁ 0.2) ꢂ
104 Mꢃ1 in CDCl3. The high binding ability resulted from the
cooperative hydrogen-bond interactions and host–guest bind-
ing, which involved dipole–dipole, C–Hꢀ ꢀ ꢀp, C–Hꢀ ꢀ ꢀO and pꢀ ꢀ ꢀp
interactions.
Then, the aggregation behavior of H3 and G to form hyper-
branched supramolecular polymer HSP-H3G was investigated
by 1H NMR spectroscopy. In CDCl3, three equivalents of G were
mixed with one equivalent of H3 to form some pseudorotaxanes.
When the concentration of H3 was 3.3 mM (Fig. 2c), proton signals
uc
50
H
for uncomplexed G could be observed. When the concentration
of H3 increased from 3.3 mM to 20.0 mM (Fig. 2d–g), the NMR peak
uc
50
of H disappeared. At the same time, the signals of the imide N–H
on G (3-position) shifted downfield from 11.39/10.20 ppm to
12.37/11.67 ppm. The chemical shift of the amide N–H on H3
also shifted from 1.77 to 2.77/1.65 ppm, accompanied with the
broadening of the peaks. These significant shifts and broad-
ening indicated the formation of hydrogen-bond interactions
and the formation of a hyperbranched supramolecular polymer.
According to the well-defined method of Gibson and Li,20 the
maximum possible polymerization degree (nmax) was calculated
(Table S1, ESI†). With the increase of the concentrations of H3
and G, the calculated sizes of the aggregate increased to large values.
The formation of supramolecular polymers is often accompanied
by a sharp decrease of the diffusion coefficient. Therefore, two
dimensional diffusion-ordered NMR spectroscopy (DOSY) experi-
ments were performed to study the aggregation of H3 and G. As
shown in Fig. S21–S27 (ESI†) and Fig. 3, as the concentrations of H3
increased from 1.7 mM to 20.0 mM (the concentrations of the P5A
cavity increased from 5.1 mM to 60.0 mM), the value of the weight
Scheme 1 (a) Chemical structures of H, H3 and G. (b) The illustration of
the construction of hyperbranched supramolecular polymer HSP-H3G from H3
and G through cooperative host–guest recognition and hydrogen-bond
interactions.
Fig. 2 Partial 1H NMR spectra (400 MHz, CDCl3, 298 K) of H3 ((a), 3.3 mM),
G ((b), 3.3 mM) and H3 upon addition of three equivalents of G at various
concentrations: (c) 3.3 mM, (d) 8.3 mM, (e) 11.6 mM, (f) 16.7 mM, and
(g) 20.0 mM. uc: peaks of uncomplexed guests; sp: peaks of the supra-
molecular polymer (for proton designations, see Scheme 1a).
Fig. 1 Partial 1H NMR spectra (400 MHz, CDCl3, 298 K) of G (a) at a
concentration of 10.0 mM upon addition of equimolar H (b). uc: peaks of
uncomplexed guests.
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6.0 ꢂ 10ꢃ10 m2 sꢃ1 (Fig. S29, ESI†), indicating that the aggregate
reassembled through the removal of acid by the addition of excess
Et3N. In addition, upon increasing the temperature of the 20.0 mM
HSP-H3G solution to 323 K, the value of D increased obviously from
6.1 ꢂ 10ꢃ10 to 1.0 ꢂ 10ꢃ8 m2 sꢃ1 (Fig. S30, ESI†), indicating the
disaggregation of the supramolecular polymers. This could be
explained by the fact that heating could result in the decomplexation
of the host–guest complexes and the dissociation of the hydrogen-
bond interactions, which results in the destruction of the supra-
molecular polymer network. After the mixed solution was
cooled to room temperature, the value of D decreased to
5.8 ꢂ 10ꢃ10 m2 sꢃ1 (Fig. S31, ESI†), suggesting the reassembly
of the supramolecular polymers.
Fig. 3 2D DOSY (600 MHz, 298 K) plot of solutions of H3 with three
equivalents of G in CDCl3.
As shown in Fig. S33 (ESI†), as the concentrations of the
mixed solution of H3 and three equivalents of G increased from
average diffusion coefficients (D) decreased from 2.2 ꢂ 10ꢃ9 to 1.7 mM to 17.0 mM (the concentration of H3), the specific
6.1 ꢂ 10ꢃ10 m2 sꢃ1 (D1.7/D20 = 3.6), revealing the concentration viscosity increased dramatically from 0.3 to 2.5 mPa S (red dots). As
dependence of the supramolecular polymerization of the H3 and G the concentrations of H3 increased from 1.7 mM to 17.0 mM, the
mixture.
specific viscosity increased from 8.6 ꢂ 10ꢃ3 to 2.4 ꢂ 10ꢃ1 mPa S
The concentration-dependent viscosity changes provided further (blue dots in Fig. S33, ESI†). The completely different phenomena of
convincing evidence of the self-assembly behaviors of the compo- the viscosity change showed that H3 did not aggregate to form a
nents. As shown in Fig. 4, the supramolecular polymer HSP-H3G supramolecular polymer.21 DOSY experiments also supported such
that aggregated from H3 and G showed a viscosity transition that a conclusion. The value of D of 20.0 mM H3 was 1.2 ꢂ 10ꢃ9 m2 sꢃ1
was described by a change in the slope in the double logarithmic (Fig. S32, ESI†), which was significantly bigger than that of
plots of the specific viscosity versus concentration. In the low 20.0 mM HSP-H3G (D = 6.1 ꢂ 10ꢃ10 m2 sꢃ1, Fig. S27, ESI†). The
concentration range, the slope of the curve was 0.7, which suggested possible reason was that only hydrogen-bond interactions
a linear relationship between the specific viscosity and the concen- existed between the adenines in the H3 solution, which were
tration, which is one of the characteristics of non-interacting too weak to form supramolecular polymers. The cooperative
assemblies of a constant size and this demonstrated the predomi- non-covalent interactions between H3 and G were strong
nance of the oligomers in dilute solutions. When the concentration enough to make them assemble.
increased above 7.8 mM, a slope of 1.6 was observed, indicating a
transition from the oligomer to a hyperbranched supramolecular three adenine units. H3 interacted with (6-(2,4-dioxo-3,4-dihydro-
polymer with increasing size.
pyrimidin-1(2H)-yl)hexanenitrile (G)) through cooperative hydrogen-
We synthesized a homotritopic pillar[5]arene H3 containing
Interestingly, the reversible aggregation and disaggregation bond interactions and host–guest binding, which involved
of HSP-H3 could be realized by stimulation with aspirin and dipole–dipole, C–Hꢀ ꢀ ꢀp, C–Hꢀ ꢀ ꢀO and pꢀ ꢀ ꢀp interactions.
heat, respectively. Upon addition of 60.0 mM aspirin to the Thanks to the strong cooperative interactions, H3 and G formed
20.0 mM solution of HSP-H3G, the value of D increased from a hyperbranched supramolecular polymer at a low concen-
6.1 ꢂ 10ꢃ10 (Fig. S27, ESI†) to 1.2 ꢂ 10ꢃ9 m2 sꢃ1 (Fig. S28, ESI†). tration. 1H NMR, viscosity measurements and DOSY experi-
This was because aspirin destroyed the hydrogen-bond inter- ments at various concentrations confirmed the cooperative
actions between the base pairs. Afterwards, upon addition of hyperbranched supramolecular polymerization. The supra-
63.0 mM Et3N to the above solution, the value of D decreased to molecular polymer showed dual-responsiveness to heating
and cooling, or the addition of aspirin and a base. This was
the first report of a supramolecular polymer based on a
pillar[5]arene constructed through cooperative non-covalent
interactions. The present research provides a new method for
the construction of smart supramolecular polymer materials.
This work was supported financially by the Innovative
Talents Programme of Henan Province (174100510025), the Henan
Science and Technology Program (162300410204, 18A150030), and
the Scientific Research Foundation for Doctors (qd16106 and
2016QK09) of Henan Normal University.
Conflicts of interest
Fig. 4 Specific viscosity of HSP-H3G (298 K) in a CHCl3 solution versus
the concentration of H3.
There are no conflicts to declare.
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