Self-assembly of supramolecular polymers in water from tetracationic and tetraanionic monomers in water through cooperative electrostatic attraction and aromatic stacking

Article abstract of DOI:10.1016/j.cclet.2018.10.016

The cooperative electrostatic attraction and π-π aromatic stacking interactions between tetrahedral tetrapyridinium TP and three tetraanionic tetraphenylethylene derivatives TPE-1～3 led to the formation of a new kind of supramolecular polymer networks in water, which have been confirmed by ¹H NMR, fluorescence, isothermal titration calorimetric (ITC) and dynamic light scattering (DLS) experiments. ITC studies show that the contributions of enthalpy and entropy were comparable, reflecting the importance of hydrophobicity in driving the intermolecular aromatic stacking. DLS experiments indicate that the linear supramolecular polymers formed by these tetratopic monomers further aggregated into networks of 10²-nm size.

Full text of DOI:10.1016/j.cclet.2018.10.016

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CCLET 4680 No. of Pages 4
Chinese Chemical Letters xxx (2018) xxx–xxx
Contents lists available at ScienceDirect
Chinese Chemical Letters
journal homepage: www.elsevier.com/locate/cclet
Communication
Self-assembly of supramolecular polymers in water from tetracationic
and tetraanionic monomers in water through cooperative electrostatic
attraction and aromatic stacking
*
*
Ya-Kun Zhao, Zhong-Zheng Gao, Hui Wang, Dan-Wei Zhang , Zhan-Ting Li
Department of Chemistry, Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials and Collaborative Innovation Center of Chemistry for
Energy Materials (iChEM), Fudan University, Shanghai 200438, China
A R T I C L E I N F O
A B S T R A C T
Article history:
The cooperative electrostatic attraction and
p-p aromatic stacking interactions between tetrahedral
Received 15 September 2018
Received in revised form 11 October 2018
Accepted 16 October 2018
Available online xxx
tetrapyridinium TP and three tetraanionic tetraphenylethylene derivatives TPE-1ꢀ3 led to the formation
of a new kind of supramolecular polymer networks in water, which have been confirmed by 1H NMR,
fluorescence, isothermal titration calorimetric (ITC) and dynamic light scattering (DLS) experiments. ITC
studies show that the contributions of enthalpy and entropy were comparable, reflecting the importance
of hydrophobicity in driving the intermolecular aromatic stacking. DLS experiments indicate that the
linear supramolecular polymers formed by these tetratopic monomers further aggregated into networks
of 102-nm size.
Keywords:
Supramolecular polymers
Electrostatic attraction
Aromatic stacking
Self-assembly
© 2018 Chinese Chemical Society and Institute of Materia Medica, Chinese Academy of Medical Sciences.
Published by Elsevier B.V. All rights reserved.
Aqueous phase
In the past decades, supramolecular polymers have been an
attractive topic in supramolecular chemistry due to their dynamic
nature and high responsiveness to outside stimuli [1]. Currently,
hydrogen bonding [2], metal coordination [3], donor-acceptor
interaction [4], and hydrophobicity [5] have been extensively used
for driving the generation of supramolecular polymers. Neverthe-
less, in most cases, supramolecular polymers can exist only in
organic solvents. Further application of supramolecular polymers
as biomaterials would require high stability and solubility in water
to enable good biocompatibility [5,6]. In this context, hydropho-
bicity-driven encapsulation of cyclodextrins and cucurbiturils for
hydrophobic monomeric or dimeric species has been used to
produce various cross-linked supramolecular polymers in aqueous
solution [5]. Another more straightforward approach involves the
use of ion-pair electrostatic attraction, which can provide both
good water-solubility and biocompatibility. However, single ion-
pair interaction is not able to allow for efficient binding due to the
competitiveness of water. Schmuck et al. has successfully
combined ion-pair electrostatic attraction and multiple hydrogen
bonds to generate switchable supramolecular systems in polar
solvents [7]. Previously, we have developed a novel motif of
mutually promoting electrostatic (ion-pair) attraction and
aromatic stacking in water [8], which has been used to assemble
supramolecular polymers in water from tetrapyridinium and disk-
shaped coronene multicarboxylate monomers [9]. Because it has
been well-established that multitopic building blocks exhibit
enhanced binding through multivalency [10], we become inter-
ested in the self-assembly of two discrete multitopic monomers
into supramolecular networks through this cooperative binding
motif. We herein report the generation of
a new kind of
supramolecular polymers in water from tetrapyridinium TP and
tetraphenylethylene derivatives TPE-1ꢀ3 (Fig. 1).
Highly water-soluble tetraphenylethylene derivatives TPE-1ꢀ3
were chosen as aromatic multianionic monomers for their
studying electrostatic and stacking interactions with water-soluble
tetracationic monomer TP [10] in water. For comparison,
dipyridinium DP [11] was also prepared as bromide salt. The
synthesis and characterization details for TPE-1ꢀ3 are provided in
the Supporting information.
1H NMR titration experiments were first performed by adding
increasing amounts of TP or DP to the solution of TPE-1ꢀ3 in D2O.
Adding TP (0–2.0 mmol/L) to the solution of TPE-1 (1.0 mmol/L) in
D2O caused dramatic upfield shifting of the H-1 and H-2 signals of
TPE-1 (Fig. 2a). Notably, the addition of 1.0 equiv. of TP could cause
the maximum shifting, about –0.33 ppm for H-1 and –0.52 ppm for
H-2, respectively. Further increase of TP to 2.0 equiv. only led to
small shifting (< –0.05 ppm). This result supported that the
* Corresponding authors.
binding between the two compounds occurred in
a 1:1
E-mail addresses: zhangdw@fudan.edu.cn (D.-W. Zhang), ztli@fudan.edu.cn
(Z.-T. Li).
https://doi.org/10.1016/j.cclet.2018.10.016
1001-8417/© 2018 Chinese Chemical Society and Institute of Materia Medica, Chinese Academy of Medical Sciences. Published by Elsevier B.V. All rights reserved.
Please cite this article in press as: Y.-K. Zhao, et al., Self-assembly of supramolecular polymers in water from tetracationic and tetraanionic
monomers in water through cooperative electrostatic attraction and aromatic stacking, Chin. Chem. Lett. (2018), https://doi.org/10.1016/j.
cclet.2018.10.016

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TPE-1ꢀ3 by DP were also performed (Fig. S6), which revealed
continuous quenching of the fluorescence of TPE-1. However, no
quenching saturation was observed after 4.0 equiv. of DP was
added, reflecting relatively lowered binding strength. Adding 8.0
equiv. of MP to the solution of TPE-1 or TPE-3 caused nearly no
quenching for their fluorescence, indicating that at low concen-
tration, no binding occurred.
Concentration-varying fluorescence experiments showed that
the three tetraphenylethylene tetraanion did not exhibit aggrega-
tion-induced emission [12], probably due to the electrostatic
repulsion of the anions. UV–vis absorption spectra were further
recorded for the mixtures of TPE-1 and TP as well as DP by keeping
a constant total concentration (Fig. S7 in Supporting informaiton).
Job’s plot was thus obtained for the first mixture by recording the
absorbance change of the TPE unit at 295 nm (Fig. S7), which
further supported the 1:1 binding stoichiometry. Job’s plot could
not be obtained from similar UV–vis experiments for the mixture
of TPE-1 and DP because of limited absorbance change of the TPE
unit. We thus adopted the fluorescence approach and Job’s plot
Fig. 1. The structures of ionic compounds TPE-1ꢀ3, TP, MP and DP.
1
Fig. 2. Partial H NMR (400 MHz) spectra of the mixtures of a) TPE-1 (1.0 mmol/L) and TP (0–2 mmol/L) and b) TPE-1 (1.0 mmol/L) and DP (0–4 mmol/L) in D2O at 25 ꢁC.
stoichiometry. Similar shifting tendency was also observed when
adding TP to the solution of TPE-2 or TPE-3 in D2O. For TPE-2, 1.0
equiv. of TP caused its H-1 and H-2 signals to shift upfield by –0.37
and –0.54 ppm, respectively, and for TPE-3, the values were –0.45
and –0.49 ppm, respectively (Figs. S1 and S2 in Supporting
information). 1H NMR titration experiments were also conducted
for TPE-1 with the addition of DP in D2O (Fig. 2b). It can be found
that 2.0 equivalent of DP could cause maximum upfield shifting
(–0.24 and –0.45 ppm, respectively) for the H-1 and H-2 signals of
TPE-1, and further addition of another 2.0 equiv. led to very small
shifting (< 0.06 ppm), supporting their binding was in a 1:2
stoichiometry. As expected, 1H NMR experiments for the mixture
of DP with TPE-2 or TPE-3 gave rise to similar results (Figs. S3 and
S4 in Supporting inforamtion). In contrast, adding 8.0 equiv. of MP
to the solution of TPE-1 or TPE-3 (1.0 mmol/L) in D2O caused only
up to –0.09 ppm of upfield shifting for their H-1 and H-2 signals in
the 1H NMR spectra (Fig. S5 in Supporting information), indicative
of much weaker binding.
Fluorescence emission experiments were then performed for
the three mixtures of TP and TPE-1ꢀ3. Significant fluorescence
quenching was found for all three tetraphenylethylene derivatives
by TP (Fig. S6 in Supporting information). Again, 1.0 equiv. of TP
could lead to the maximum quenching, which was consistent with
the above 1H NMR experiments and again supported the 1:1
stoichiometry. Given the low concentration of both samples, this
observation reasonably evidenced high binding strength between
the two multitopic samples. Fluorescence titration experiments for
obtained supported the 1:2 binding stoichiometry (Fig. S8 in
Supporting information). As expected, Job’s plots obtained using
similar absorption and fluorescence methods supported 1:1
stoichiometry for the mixture of TP with TPE-2 or TPE-3 and
1:2 stoichiometry for the mixture of DP with either of them
(Figs. S9-S14 in Supporting inforamtion). Because TPE-1ꢀ3 and TP
have four ionic aromatic units, whereas DP has two, the above
binding stoichiometry clearly pointed to a one-to-one ion-pair
binding motif.
Isothermal titration calorimetric (ITC) experiments were then
carried out to quantitatively evaluate the binding behavior of TPE-
1ꢀ3 with TP or DP (Fig. 3). The data of the apparent association
constants (Ka), Gibbs energy changes (DG) and associated enthalpy
and entropy changes are summarized in Table 1. Because the above
fluorescence experiments revealed that at low concentrations, no
important binding occurred for monotopic MP, the quite high
association constants exhibited by ditopic DP supported that the
two pyridinium units of DP cooperated in binding the TPE
derivatives, reasonably through folding into a cleft conformation.
This folding conformation might interact with one or two
appended benzene rings of the TPE monomers, even though
currently we could differentiate them. Such a binding motif
suggested that the mixtures of DP with the TPE derivatives did not
produce any kind of supramolecular polymers. By assuming that
the two cationic molecules adopted the identical binding motif, we
might expect that tetratopic TP would form linear supramolecular
polymers with the tetraanions. For all the three tetraanions, TP
Please cite this article in press as: Y.-K. Zhao, et al., Self-assembly of supramolecular polymers in water from tetracationic and tetraanionic
monomers in water through cooperative electrostatic attraction and aromatic stacking, Chin. Chem. Lett. (2018), https://doi.org/10.1016/j.
cclet.2018.10.016

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3
Fig. 3. ITC measurements for the titration of TP (1 mmol/L in water) into water solution of TPE-1 (0.1 mmol/L) (a), TPE-2 (0.1 mmol/L) (c), TPE-3 (0.1 mmol/L) (e). DP (3 mmol/
L in water) into water solution of TPE-1 (0.1 mmol/L) (b), TPE-2 (0.1 mmol/L) (d), TPE-3 (0.1 mmol/L) (e).
Table 1
Summary of the apparent association constants and related thermodynamic data in water.
Entry
Complex
Ka (L/mol)
D
G (kcal/mol)
D
H
ÀT
DS
(kcal/mol)
(kcal/mol)
1
2
3
4
5
6
TPE-1/TP
TPE-2/TP
TPE-3/TP
TPE-1/DP
TPE-2/DP
TPE-3/DP
(3.9 Æ 0.48)Â105
(1.3 Æ 0.12)Â105
(2.2 Æ 0.24)Â105
(2.0 Æ 0.28)Â104
(1.7 Æ 0.28)Â104
(1.1 Æ 0.37)Â104
À7.62
À6.98
À7.27
À5.86
À5.78
À5.49
À3.06
À3.22
À3.33
À1.57
À1.46
À1.76
À4.56
À3.75
À3.94
À4.29
À4.32
À3.73
displayed considerably higher binding affinity than DP, which
showed that the supramolecular polymers formed by TP might
further undergo aggregation to allow for binding enhancement.
comparable. However, the DH of the mixture of TP was all
pronouncedly larger than that of DP, which we tentatively
attributed to the enhanced electrostatic attraction due to the
further aggregation of the supramolecular polymers formed by TP.
Finally, dynamic light scattering (DLS) experiments were
conducted in water for all the above six mixtures, with the molar
ratio being controlled to be consistent with the binding
stoichiometry. The three mixtures of TPE/DP (1:2, [DP] = 1.0
mmol/L) gave rise to an average hydrodynamic diameter (DH) of
For all the six mixtures, the contribution of ÀT
DS was larger than
H, which reflected that the release of the “confined” water
D
molecules exposed to the hydrophobic surface of the aromatic
units as a result of stacking produced the largest driving force for
the binding of the opposite ions [8,13]. For the mixtures of DP and
TP with the identical TPE derivative, the ÀTDS values were
Please cite this article in press as: Y.-K. Zhao, et al., Self-assembly of supramolecular polymers in water from tetracationic and tetraanionic
monomers in water through cooperative electrostatic attraction and aromatic stacking, Chin. Chem. Lett. (2018), https://doi.org/10.1016/j.
cclet.2018.10.016

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1.5, 1.6 and 2.0 nm (Fig. S15 in Supporting inforamtion),
respectively, which was consistent with the formation of simple
1:2 complexes. In contrast, the three mixtures of TPE/TP (1:1,
[TP] = 0.5 mmol/L) afforded much larger DH of 255, 295 and
342 nm, respectively. The values clearly supported the formation of
large supramolecular entities. As expected, with the increase of the
concentration, the DH values increased pronouncedly (Fig. S16 in
Supporting information). The above fluorescence and ITC experi-
ments supported that these three mixtures formed linear
supramolecular polymers, even though they should be random-
arranged and other forms of supramolecular entities could not be
excluded. Because the apparent association constant of these three
systems was all considerably higher than that of the corresponding
complex of DP, we propose that the linear supramolecular
polymers underwent further aggregation to produce 3D networks,
even though the refined binding motif cannot be established due to
the dynamic and low directionality of electrostatic and stacking
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Please cite this article in press as: Y.-K. Zhao, et al., Self-assembly of supramolecular polymers in water from tetracationic and tetraanionic
monomers in water through cooperative electrostatic attraction and aromatic stacking, Chin. Chem. Lett. (2018), https://doi.org/10.1016/j.
cclet.2018.10.016