A phosphonated copillar[5]arene: Synthesis and application in the construction of pH-responsive supramolecular polymer in water

Article abstract of DOI:10.1016/j.tetlet.2018.06.058

A phosphonated-functionalized copillar[5]arene was designed and synthesized. ¹H NMR, ¹³C NMR, ESI-MS and HR-MS were performed to provide converging evidences of the structure of obtained copillar[5]arene. Driven by hydrophobic intera

Full text of DOI:10.1016/j.tetlet.2018.06.058

Tetrahedron Letters xxx (2018) xxx–xxx
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
A phosphonated copillar[5]arene: Synthesis and application in the
construction of pH-responsive supramolecular polymer in water
⇑
Zhihua Zhang, Li Shao, Jie Yang
Department of Chemistry, Zhejiang University, Hangzhou 310027, PR China
a r t i c l e i n f o
a b s t r a c t
A phosphonated-functionalized copillar[5]arene was designed and synthesized. 1H NMR, 13C NMR, ESI-
MS and HR-MS were performed to provide converging evidences of the structure of obtained copillar
Article history:
Received 21 April 2018
Revised 22 June 2018
Accepted 25 June 2018
Available online xxxx
[5]arene. Driven by hydrophobic interactions and C–Hꢀꢀꢀp interactions between the decyl moiety and
the pillar[5]arene cavity, a pH-responsive linear supramolecular polymer was fabricated from this phos-
phonate-functionalized AB type monomer in water. Concentration-dependent 1H NMR, NOESY, DOSY,
and specific viscometry were carried out to study the supramolecular polymerization process.
Ó 2018 Elsevier Ltd. All rights reserved.
Keywords:
Copillararenes
Water–solubility
Hydrophobic interactions
C–Hꢀꢀꢀ
p interactions
Supramolecular polymers
Supramolecular self-assembly, which makes the use of mole-
cules rather than atomic units, offers a bottom-up approach for
the construction of new materials on multiple length scales by
sophisticated supramolecular self-assembled systems [11]. It is
worth noting that substituents on both rims of pillararenes can sig-
nificantly influence their physical properties such as solubility,
conformational structure, and corresponding hostꢁguest proper-
ties. Therefore, how to efficiently functionalize pillararenes is a
very important research topic in pillararene chemistry. So far,
two strategies have been reported for the preparation of
functionalized pillararenes: (i) deprotection and functionalization
of preformed pillararenes [6b]; (ii) co-cyclization of different
monomers with functional groups [6a]. With these two
approaches, various functionalized pillararenes have been
prepared. Most of them are only soluble in organic solvents [11],
and the syntheses of functionalized water-soluble pillararenes
are still full of challenges. To date, several cationic copillar[5]are-
nes bearing trimethylammonium salt groups have been reported,
while due to the high toxicity of the cationic groups [12], their
bio-related applications were limited to a large extent. Hence,
the design and synthesis of biocompatible water-soluble copil-
lararenes is of great importance for the broadening of the
pillararene-based toolbox for bio-related applications. It is well-
known that phosphonate motifs are biocompatible groups, which
are usually associated with various biological functions [13].
Consequently, we expect that the modification of pillararenes with
phosphonate groups will not only increase their water-solubility
but also greatly improve their biocompatibility. Herein, a novel
phosphonate-functionalized copillar [5]arene was designed and
synthesized, which would pave a new way for the preparation of
functionalized water-soluble pillararenes. Meanwhile, we further
employing various noncovalent interactions [1], such as p–p stack-
ing interactions, hydrogen bonding, metal–ligand coordination and
host–guest interactions [2]. Owing to the dynamic nature of non-
covalent interactions, supramolecular self-assembled systems
have exhibited peculiar properties including reversibility, self-
healing and stimuli-responsiveness, so they have drawn consider-
able attention for their prospective applications in smart materials
especially in biotechnology and drug-delivery systems [3,4]. Com-
pared with other noncovalent interactions, hostꢁguest interac-
tions possess more stimuli-responsive properties and higher
thermal stability which endowed them with multitudinous appli-
cations. Therefore, the construction of supramolecular self-assem-
bled systems through macrocycle-based host–guest recognition is
of particular interest and significant importance [5].
Pillararenes [6], an emerging generation of macrocyclic hosts,
are linked by methylene (ꢁCH2ꢁ) bridges at para-positions of
1,4-dialkoxybenzene rings. Compared with other macrocyclic
hosts such as crown ethers [7], cyclodextrins [8], calixarenes [9]
and cucurbiturils [10], pillararenes, bearing attractive properties
including facile synthesis, easy functionalization, good solubility
and symmetrical frameworks, are qualified to be prominent
candidates for the construction of threaded complexes and more
⇑
Corresponding author.
E-mail address: jieyang@zju.edu.cn (J. Yang).
https://doi.org/10.1016/j.tetlet.2018.06.058
0040-4039/Ó 2018 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Z. Zhang et al., Tetrahedron Lett. (2018), https://doi.org/10.1016/j.tetlet.2018.06.058

2
Z. Zhang et al. / Tetrahedron Letters xxx (2018) xxx–xxx
Scheme 1. Synthetic route to water-soluble phosphonate-based copillar[5]arene 1.
employed the obtained copillar [5] arene as a classical AB type
monomer to construct a linear supramolecular polymer and inves-
tigated its pH-responsive behavior in water.
The target water-soluble copillar[5]arene 1 was synthesized
through four steps in total (Scheme 1). Firstly, we obtained the
neutral copillar[5]arene 4 in 29% yield by co-cyclization of 1 equiv
of 1-methoxy-4-decyloxybenzene and 4 equiv of 1,4-bis(2-bro-
moethoxy)benzene according to
a
previous report [11b].
Secondly, pillararene-based phosphonate ester 3 was prepared by
means of the classic Arbuzov reaction between triethyl phosphite
and bromosubstituted pillararene derivative 4. Thirdly, copillar[5]
arene
2
was generated by Mckenna reaction between
bromotrimethylsilane (TMSBr) and copillar[5]arene 3. At last, on
the treatment of 2 with excess ammonium hydroxide for two
hours, the target compound 1 with eight phosphonate groups at
upper and lower rims was collected in a quantitative yield. 1H
NMR spectroscopy, 13C NMR spectroscopy, electrospray
ionization mass spectrum (ESI-MS) and high-resolution mass
spectrometry (HRMS) provide convincing evidences to support
the structure of compound 1 (Fig. S11–16, ESIy).
Fig. 1. COSY NMR spectrum (500 MHz, D2O, 298 K) of 1 at a concentration of
50.0 mM.
Inspired by mother nature, stimuli-responsive water-soluble
supramolecular polymers have drawn considerable attention from
materials community in last decade due to their various applica-
tions [2]. The phosphonate-functionalized copillar[5]arene synthe-
sized here, as a classical AB type monomer, is an ideal candidate for
the construction of stimuli-responsive supramolecular polymers in
water due to its good solubility and acid-responsive property in
aqueous media. Hence, a water- soluble supramolecular polymer
According to previous reports [11a,11b], it is well known that
the alkyl chain can thread into the cavity of the pillar[5]arene to
form quadruple CAHꢀ ꢀ ꢀp interactions. Concentration-dependent
1H NMR experiments with concentrations ranging from 2.00 mM
to 200 mM (Fig. 2) were performed to investigate the self-
assembly behaviors of the AB type monomer 1 in deuterium
oxide. As shown in Fig. 2, only one set of peaks was observed,
reflecting the involvement of fast-exchanging noncovalent
interactions in solution on the proton NMR time scale. Unlike the
protons on common alkyl chains whose chemical shifts locate
around 1 ppm, here the chemical shifts of protons Hb, Hc, Hd, He
and Hf on the decyl moiety exhibited significant upfield shifts,
even below 0 ppm both at low and high concentrations, which
indicated that the alkyl chain threaded into and got shielded by
the electron-rich pillar[5]arene cavity. In Fig. 2 and Fig. S11, we
driven by hydrophobic interactions and CAHꢀ ꢀ ꢀ
p interactions
was efficiently constructed.
In order to make the explanation of the supramolecular poly-
merization more clearly, the attribution of the protons on mono-
mer 1 is quite necessary. Partial HAH COSY spectra of monomer
1 at 50.0 mM in water are shown in Fig. 1 and Fig. S18. Clear
correlations between the protons were observed to validate the
proton assignments shown in Scheme 2.
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Z. Zhang et al. / Tetrahedron Letters xxx (2018) xxx–xxx
3
Scheme 2. Chemical structure of monomer 1 and cartoon representation of the formation of a pH-responsive water-soluble supramolecular polymer.
Fig. 2. 1H NMR spectra (500 MHz, D2O, 298 K) of 1 at different concentrations: (a) 2.0 mM; (b) 5.0 mM; (c) 10.0 mM; (d) 20.0 mM; (e) 60.0 mM; (f) 100.0 mM; (g) 150.0 mM;
(h) 200.0 mM.
observed that two Hi methylene protons on the decyl moiety have
two peaks, meaning the different chemical environments of these
two methylene protons. This is mainly because of the inclusion-
induced restriction on the rotation of the alkyl chain.
In addition, 2D NOESY NMR spectroscopy was performed to
investigate the relatively spatial positions of the components in
hostꢁguest inclusion complex. According to the resultant spec-
trum, nuclear overhauser effect (NOE) correlations were clearly
observed (Fig. 3) between protons H1–3 on the pillararene unit
and protons Ha–f at the end of the alkyl chain. This phenomenon
further confirmed the encapsulation of the alkyl chain by the
pillar[5]arene cavity.
Furthermore, according to the concentration dependent NMR
spectra, we noticed the signals of aromatic protons H1 on 1
exhibited substantial upfield shifts (Fig. 2 and Fig. S17) with the
concentration of monomer 1 increasing. Besides, the peak splitting
of all protons disappeared along with broadening, indicating that
the monomers self-assembled into high molecular weight
aggregates at high concentration [14]. To further investigate
the supramolecular polymerization process, specific viscometry,
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Z. Zhang et al. / Tetrahedron Letters xxx (2018) xxx–xxx
Fig. 5. Photographs of aqueous solutions: (a) 150 mM 1; (b) after adjusting solution
pH by acid treatment; (c) further adjusting solution pH by base treatment.
In the end, we turned to study the pH-responsive property
of monomer 1 in water. It is well-known that phosphonate
group exhibits pH-responsiveness in water since it can be
protoned to turn into a netural group after acidification. There-
fore, it is conceivable that negatively charged monomer 1 and
the corresponding neutral monomer 2 can be interconverted by
changing the solution pH. Evidence for this conversion was
provided in Fig. 5, which was directly observed by naked eyes.
When adjusting the solution pH by acid treatment, the
transparent solution of monomer 1 disappeared immediately
Fig. 3. Partial NOESY NMR spectrum (500 MHz, D2O, 298 K) of 1 at a concentration
of 50.0 mM.
another convenient method to probe the propensity of monomers
to self-assemble into large aggregates, was performed in water by
using a Cannon Ubbelohde semi-microdilution viscometer. In
Fig 4a, a double logarithmic representation of specific viscosity
versus the concentration of monomer 1 in water was obtained. In
the low concentration range, the slope of the curve was 1.02,
demonstrating a linear relationship between specific viscosity
and concentration, which is characteristic for non-interacting
assemblies with constant size, indicating the main presence of cyc-
lic oligomers in dilute solution [5a]. However, a sharp increase in
the viscosity for monomer 1 (slope = 2.02) was observed when
the monomer concentration exceeded the critical polymerization
concentration (CPC, approximately 150 mM). This stronger
concentration dependence indicates that the monomers self-
assembled into supramolecular polymers with increasing size [14].
Supramolecular assemblies formed by monomer 1 were also
probed by two-dimensional diffusion order NMR (DOSY), a com-
mon and effective method to calculate the size of aggregates. As
the concentration of monomer 1 increased from 20.0 mM to 100
mM, the measured weight-average diffusion coefficients (D) grad-
ually decreased from 1.40 ꢂ 10ꢁ10 m2 sꢁ1 to 0.56 ꢂ 10ꢁ10 m2 sꢁ1
(Fig. 4b), implying that the increase of average aggregation size
led to the transition from supramolecular oligomer species to
extended and high-molecular-weight aggregates. When the con-
centration was up to 150 mM, it was unable to obtain the diffusion
coefficient due to the power limitation of the spectrophotometer.
Therefore, we can get the conclusion that monomer 1 formed
extended and high-molecular-weight polymeric structures at high
concentration in water.
along with
a brown precipitate. After adjusting the pH by
the addition of NaOH solution into the solution, the transpar-
ent solution reformed as expected by shaking the tube for
several minutes. These phenomena indicated that the assembly
and disassembly of mononer
acid/base treatment.
1 can be easily controlled by
In conclusion, we designed and synthesized a new phospho-
nate-functionalized copillar[5]arene. The introduction of eight
phosphonate groups on the pillar[5]arene framework can not only
enhance its water-solubility but also endow it with stimuli-
responsiveness. Driven by hydrophobic interactions and CAHꢀ ꢀ ꢀ
p
interactions between the decyl moiety and the pillar[5]arene
cavity, a linear supramolecular polymer was fabricated from this
AB type monomer in water, and various characterization
techniques including concentration-dependent 1H NMR, NOESY,
DOSY, and specific viscometry were carried out to study the
supramolecular polymerization process. Additionally, we demon-
strated that the assembly and disassembly of this supramolecular
polymer can be reversibly controlled by adjusting the solution
pH. Future work will be focused on the design and synthesis of
different functional groups modified copillararenes and apply
them in the fabrication of supramolecular block copolymers,
hydrogels and other self-assembly systems.
Fig. 4. (a) Specific viscosity of the supramolecular polymer versus the monomer concentration in water at 293 K. (b) Concentration dependence of weight average diffusion
coefficient D (500 MHz, D2O, 295 K) of monomer 1.
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Z. Zhang et al. / Tetrahedron Letters xxx (2018) xxx–xxx
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Please cite this article in press as: Z. Zhang et al., Tetrahedron Lett. (2018), https://doi.org/10.1016/j.tetlet.2018.06.058