# Switchable Redox and Thermo-Responsive Supramolecular Polymers Based on Cyclodextrin-Polyoxometalate Tandem

Soumaya Khlifi, Sa Yao, Clément Falaise, Pierre Bauduin, Vincent Guerineau, Nathalie Leclerc, Mohamed Haouas, Hanène Salmi-Mani, Philippe Roger, Emmanuel Cadot

► To cite this version:

Soumaya Khlifi, Sa Yao, Clément Falaise, Pierre Bauduin, Vincent Guerineau, et al.. Switchable Redox and Thermo-Responsive Supramolecular Polymers Based on Cyclodextrin-Polyoxometalate Tandem. Chemistry - A European Journal, 2024, 30 (18), pp.e202303815. 10.1002/chem.202303815. hal-04418306

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Switchable Redox and Thermo-Responsive
Supramolecular Polymers Based on
Cyclodextrin-Polyoxometalate Tandem

Soumaya Khlifi, Sa Yao, Clément Falaise, Pierre Bauduin, Vincent Guerineau, Nathalie Leclerc, Mohamed Haouas, Hanène Salmi-Mani, Philippe Roger, Emmanuel Cadot

► To cite this version:

Soumaya Khlifi, Sa Yao, Clément Falaise, Pierre Bauduin, Vincent Guerineau, et al.. Switchable Redox and Thermo-Responsive Supramolecular Polymers Based on Cyclodextrin-Polyoxometalate Tandem. Chemistry - A European Journal, 2024, 30 (18), pp.e202303815. 10.1002/chem.202303815. hal-04418306

HAL Id: hal-04418306

https://hal.science/hal-04418306v1

Submitted on 4 Jun 2024

**HAL** is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire **HAL**, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

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Distributed under a Creative Commons Attribution 4.0 International License
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Hot Paper

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# Switchable Redox and Thermo-Responsive Supramolecular Polymers Based on Cyclodextrin-Polyoxometalate Tandem

Soumaya Khlifi,*[a] Sa Yao,[a] Clément Falaise,[a] Pierre Bauduin,[b] Vincent Guérineau,[c] Nathalie Leclerc,[a] Mohamed Haouas,[a] Hanene Salmi-Mani,[d] Philippe Roger,[d] and Emmanuel Cadot*[a]

Supramolecular polymers built from stimuli-responsive host-guest interactions represent an attractive way of tailoring smart materials. Herein, we exploit the chaotropic effect of polyoxometalates and related host-guest properties to design unconventional polymer systems with reversible redox and thermo-responsive sol-gel transition. These supramolecular networks result from the association of cyclodextrin-based oligomers and Keggin-type POMs acting as electro-active crosslinking agents.

The supramolecular approach for designing smart materials has been identified as a relevant and robust strategy for merging synergistic functionalities within highly flexible and tunable soft matrices.[1] In this respect, research attention has moved from basic principles of supramolecular chemistry that govern molecular recognition and self-assembly to non-covalent polymers.[2-6] Such a class of materials is made up of arrays of small building units self-assembled by a conglomerate of weak and reversible attractive forces arising from electrostatic interactions, hydrogen bonding, dipole-dipole or ion-dipole interactions, and dispersion forces.[7,8] Furthermore, the formation of supramolecular systems can be reinforced to a large extent by solvent effects. In context, the well-known hydro-

The structure and the dynamics of such self-assembly systems have been investigated using a multiscale approach involving MALDI-TOF, viscosity measurements, cyclic voltammetry, ¹H-NMR (1D and DOSY), and Small-Angle X-ray Scattering. Our results reveal that the chaotropic effect corresponds to a powerful and efficient force that can be used to induce responsiveness in hybrid supramolecular oligomeric systems.

phobic effect has been identified as the main contributor to the self-assembly processes of micelle-like aggregates in aqueous solution. Recently, the chaotropic effect has been shown to be highly effective in a range of supramolecular phenomena involving low charge density anions such as dodecaborates, octahedral metal clusters, or polyoxometalates (POMs).[9,11] This effect arises from high-energy water molecules involved in the hydration shell of the chaotropic ion and from the consecutive desolvation process of the interacting units concomitant to the aggregation mechanism. The magnitude of this solvent effect, driven by a water structure recovery process can be extremely strong, leading to binding constants whose values can exceed 10⁶ in water.[9,12,13] Moreover, such an effect in action may even be at the origin of counter-intuitive phenomena such as anion-anion recognition processes that overcome the electrostatic repulsions.[14,15] Furthermore, the solvation mode of these chaotropic ions gives rise to clear thermochemical features of the supramolecular processes in water, highlighted both by enthalpy gains and entropy penalties.[10,12,13] This chaotropic effect is now recognized as one of the main driving forces behind supramolecular chemistry.[16] In context, some POM specimens have been classified as superchaotropes,[17-27] and have been identified as being capable of forming robust host-guest complexes with γ-cyclodextrin (γCD), a natural macro-cyclic oligosaccharide built with eight glucopyranose units. The resulting systems can benefit from the unique properties of POMs encompassing many functionalities such as redox,[28] catalytic,[29,30] electrochromic and biological activities,[31-33] opening relevant perspectives in materials science.[34] Recent studies demonstrate that Keggin-type POM anions are chaotropic anions and that their chaotrope character is inversely dependent upon the overall POM charge density.[35,36] Thus, modulating the redox state of POM represents a promising means of designing redox-sensitive supramolecular systems. However, this strategy is so far limited to discrete host-guest systems.[11]

[a] Dr. S. Khlifi, Dr. S. Yao, Dr. C. Falaise, Dr. N. Leclerc, Dr. M. Haouas, Prof. Dr. E. Cadot
Institut Lavoisier de Versailles, CNRS UMR 8180, UVSQ, Université Paris-Saclay, 78035 Versailles Cedex (France)
E-mail: soumaya.khlifi@uvsq.fr
emmanuel.cadot@uvsq.fr

[b] Dr. P. Bauduin
Institut de Chimie Séparative de Marcoule, CNRS UMR 5257, CEA, Université de Marcoule, ENSCM, F-30207 Bagnols-sur-Cèze Cedex (France)

[c] Dr. V. Guérineau
Institut de Chimie des Substances Naturelles, CNRS UPR 2301, Université Paris-Saclay, 91198 Gif-sur-Yvette Cedex (France)

[d] Dr. H. Salmi-Mani, Prof. Dr. P. Roger
Institut de Chimie Moléculaire et des Matériaux d'Orsay, CNRS UMR 8182, Université Paris-Saclay, 91405 Orsay Cedex (France)

<figure><img src="image_4.png" /></figure>

Supporting information for this article is available on the WWW under
https://doi.org/10.1002/chem.202303815

<figure><img src="image_5.png" /></figure>

© 2023 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.
<figure><img src="image_2.png" /></figure>

<figure><img src="image_3.png" /></figure>

Hot Paper

Switchable Redox and Thermo-Responsive Supramolecular Polymers Based on Cyclodextrin-Polyoxometalate Tandem

Soumaya Khlifi,*[a] Sa Yao,[a] Clément Falaise,[a] Pierre Bauduin,[b] Vincent Guérineau,[c] Nathalie Leclerc,[a] Mohamed Haouas,[a] Hanene Salmi-Mani,[d] Philippe Roger,[d] and Emmanuel Cadot*[a]

Supramolecular polymers built from stimuli-responsive host-guest interactions represent an attractive way of tailoring smart materials. Herein, we exploit the chaotropic effect of polyoxometalates and related host-guest properties to design unconventional polymer systems with reversible redox and thermo-responsive sol-gel transition. These supramolecular networks result from the association of cyclodextrin-based oligomers and Keggin-type POMs acting as electro-active crosslinking agents.

The structure and the dynamics of such self-assembly systems have been investigated using a multiscale approach involving MALDI-TOF, viscosity measurements, cyclic voltammetry, ¹H-NMR (1D and DOSY), and Small-Angle X-ray Scattering. Our results reveal that the chaotropic effect corresponds to a powerful and efficient force that can be used to induce responsiveness in hybrid supramolecular oligomeric systems.

The supramolecular approach for designing smart materials has been identified as a relevant and robust strategy for merging synergistic functionalities within highly flexible and tunable soft matrices.[1] In this respect, research attention has moved from basic principles of supramolecular chemistry that govern molecular recognition and self-assembly to non-covalent polymers.[2-6] Such a class of materials is made up of arrays of small building units self-assembled by a conglomerate of weak and reversible attractive forces arising from electrostatic interactions, hydrogen bonding, dipole-dipole or ion-dipole interactions, and dispersion forces.[7,8] Furthermore, the formation of supramolecular systems can be reinforced to a large extent by solvent effects. In context, the well-known hydro-

phobic effect has been identified as the main contributor to the self-assembly processes of micelle-like aggregates in aqueous solution. Recently, the chaotropic effect has been shown to be highly effective in a range of supramolecular phenomena involving low charge density anions such as dodecaborates, octahedral metal clusters, or polyoxometalates (POMs).[9,11] This effect arises from high-energy water molecules involved in the hydration shell of the chaotropic ion and from the consecutive desolvation process of the interacting units concomitant to the aggregation mechanism. The magnitude of this solvent effect, driven by a water structure recovery process can be extremely strong, leading to binding constants whose values can exceed 10⁶ in water.[9,12,13] Moreover, such an effect in action may even be at the origin of counter-intuitive phenomena such as anion-anion recognition processes that overcome the electrostatic repulsions.[14,15] Furthermore, the solvation mode of these chaotropic ions gives rise to clear thermochemical features of the supramolecular processes in water, highlighted both by enthalpy gains and entropy penalties.[10,12,13] This chaotropic effect is now recognized as one of the main driving forces behind supramolecular chemistry.[16] In context, some POM specimens have been classified as superchaotropes,[17-27] and have been identified as being capable of forming robust host-guest complexes with γ-cyclodextrin (γCD), a natural macro-cyclic oligosaccharide built with eight glucopyranose units. The resulting systems can benefit from the unique properties of POMs encompassing many functionalities such as redox,[28] catalytic,[29,30] electrochromic and biological activities,[31-33] opening relevant perspectives in materials science.[34] Recent studies demonstrate that Keggin-type POM anions are chaotropic anions and that their chaotrope character is inversely dependent upon the overall POM charge density.[35,36] Thus, modulating the redox state of POM represents a promising means of designing redox-sensitive supramolecular systems. However, this strategy is so far limited to discrete host-guest systems.[11]

<figure><img src="image_4.png" /></figure>

<footnote>[a] Dr. S. Khlifi, Dr. S. Yao, Dr. C. Falaise, Dr. N. Leclerc, Dr. M. Haouas, Prof. Dr. E. Cadot
Institut Lavoisier de Versailles, CNRS UMR 8180, UVSQ, Université Paris-Saclay, 78035 Versailles Cedex (France)
E-mail: soumaya.khlifi@uvsq.fr
emmanuel.cadot@uvsq.fr</footnote>

<footnote>[b] Dr. P. Bauduin
Institut de Chimie Séparative de Marcoule, CNRS UMR 5257, CEA, Université de Marcoule, ENSCM, F-30207 Bagnols-sur-Cèze Cedex (France)</footnote>

<footnote>[c] Dr. V. Guérineau
Institut de Chimie des Substances Naturelles, CNRS UPR 2301, Université Paris-Saclay, 91198 Gif-sur-Yvette Cedex (France)</footnote>

<footnote>[d] Dr. H. Salmi-Mani, Prof. Dr. P. Roger
Institut de Chimie Moléculaire et des Matériaux d'Orsay, CNRS UMR 8182, Université Paris-Saclay, 91405 Orsay Cedex (France)</footnote>

<footnote>Supporting information for this article is available on the WWW under
https://doi.org/10.1002/chem.202303815</footnote>

<footnote>© 2023 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.</footnote>
Herein, we exploit the host-guest recognition process between γ-CD-based oligomers (notated hereafter γ-POL) and Keggin-type POMs, namely α-[SiW<sub>12</sub>O<sub>40</sub>]⁴⁻ or α-[SiW<sub>11</sub>MoO<sub>40</sub>]⁴⁻, to form responsive supramolecular crosslinked hydrogels. The resulting hybrid system offers the possibility to tune the POM crosslinking ability either through redox or thermal stimuli, inducing reversible switchable sol-gel phase transitions, characterized by a drastic change in the physical-chemical properties. Such a recognition process has been deciphered as resulting from two successive steps leading to the 1:1 and 1:2 host-guest adducts, characterized respectively by their stability constant K<sub>1:1</sub> and K<sub>1:2</sub>, as written in equations (1) and (2). Besides, the global process associated with the global constant, namely β can be also expressed (see equation 3).

$$POM + \gamma-CD \rightleftharpoons POM@\gamma CD$$
$$K_{1:1} = \frac{[POM@\gamma CD]}{[POM] \cdot [\gamma CD]} \qquad (1)$$

$$POM@\gamma CD + \gamma-CD \rightleftharpoons POM@(\gamma CD)_2$$
$$K_{1:2} = \frac{[POM@(\gamma CD)_2]}{[POM@\gamma CD] \cdot [\gamma CD]} \qquad (2)$$

$$POM + 2 \gamma-CD \rightleftharpoons POM@(\gamma CD)_2$$
$$\beta = K_{1:1} \times K_{1:2} \qquad (3)$$

The synthesis of γ-POL, inspired by the literature, was achieved by the addition of γCD onto 1,6-hexamethylene diisocyanate (HDI) in a 1:1 molar ratio.[37] The supplementary information (SI) section gives full details of the optimized and reproducible synthesis procedure (see SI, section II). The resulting γ-POL obtained with a yield of about 70%, is colorless and highly soluble in water (up to 500 g/L), about twice as many as native γCD (230 g/L). Besides, the γCD molar content resulting from γ-POL solution was determined by quantitative ¹H-NMR experiments using maleic acid as an internal standard in D₂O (full details are given in SI, section III). The average HDI/γCD ratio in the final polymer is 1.2±0.1 corresponding to 0.78 mmol of γCD per gram of oligomeric compound. MALDI-TOF analysis reveals the distribution of oligomeric species containing from 1 to 5 γCD per oligomer (see scheme 1 and SI, Figure S1). Then, γ-POL refers to γCD based oligomers distribution with averaged formula (γ-CD-HDI<sub>1,2</sub>)n with n ranging between 1 and 5.

$$\{(C_{48}H_{80}O_{40}) \cdot [C_{8}H_{14}O_{2}N_{2}]_{1.2}\}_{n} = \gamma-\text{POL}$$

The chemical-physical properties of γ-POL were investigated in aqueous solution in the presence of two Keggin-type electro-active POMs, namely α-[SiW<sub>12</sub>O<sub>40</sub>]⁴⁻ and α-[SiW<sub>11</sub>MoO<sub>40</sub>]⁴⁻, selected for their fair hydrolytic and redox stability under aerobic aqueous conditions and for their contrasting affinity towards native γ-CD upon one-electron transfer.[36]

<figure><img src="image_6.png" /><figcaption>**Scheme 1.** Schematic representation of the γ-POL behavior in the presence of oxidized and one-electron reduced Keggin-type polyoxometalates K₄SiW₁₁MoO₄₀. While the γ-POL system is composed of a distribution of short oligomers (see insert, left), hydrogel formation as a yellow phase occurs by mixing the oxidized POM component. The gel converts into a fluid violet solution through chemical reduction using dithionite (Na₂S₂O₈). The hydrogel state can be restored by oxidation of one-electron-reduced POM using chemical oxidants such as peroxodisulfate (Na₂S₂O₈). Physical-chemical characterizations evidence that the supramolecular polymerization arises from a cross-linking process resulting from the host-guest supramolecular recognition between the POM unit and γ-cyclodextrin cavity of the γ-POL. As schematically shown, the assembly-disassembly process that causes gel rupture is controlled by a redox stimulus.</figcaption></figure>
Herein, we exploit the host-guest recognition process between γ-CD-based oligomers (notated hereafter γ-POL) and Keggin-type POMs, namely α-[SiW<sub>12</sub>O<sub>40</sub>]⁴⁻ or α-[SiW<sub>11</sub>MoO<sub>40</sub>]⁴⁻, to form responsive supramolecular crosslinked hydrogels. The resulting hybrid system offers the possibility to tune the POM crosslinking ability either through redox or thermal stimuli, inducing reversible switchable sol-gel phase transitions, characterized by a drastic change in the physical-chemical properties. Such a recognition process has been deciphered as resulting from two successive steps leading to the 1:1 and 1:2 host-guest adducts, characterized respectively by their stability constant K<sub>1:1</sub> and K<sub>1:2</sub>, as written in equations (1) and (2). Besides, the global process associated with the global constant, namely β can be also expressed (see equation 3).

$$POM + \gamma-CD \rightleftharpoons POM@\gamma CD$$
$$K_{1:1} = \frac{[POM@\gamma CD]}{[POM] \cdot [\gamma CD]} \qquad (1)$$

$$POM@\gamma CD + \gamma-CD \rightleftharpoons POM@(\gamma CD)_2$$
$$K_{1:2} = \frac{[POM@(\gamma CD)_2]}{[POM@\gamma CD] \cdot [\gamma CD]} \qquad (2)$$

$$POM + 2 \gamma-CD \rightleftharpoons POM@(\gamma CD)_2$$
$$\beta = K_{1:1} \times K_{1:2} \qquad (3)$$

The synthesis of γ-POL, inspired by the literature, was achieved by the addition of γCD onto 1,6-hexamethylene diisocyanate (HDI) in a 1:1 molar ratio.[37] The supplementary information (SI) section gives full details of the optimized and reproducible synthesis procedure (see SI, section II). The resulting γ-POL obtained with a yield of about 70%, is colorless and highly soluble in water (up to 500 g/L), about twice as many as native γCD (230 g/L). Besides, the γCD molar content resulting from γ-POL solution was determined by quantitative ¹H-NMR experiments using maleic acid as an internal standard in D₂O (full details are given in SI, section III). The average HDI/γCD ratio in the final polymer is 1.2±0.1 corresponding to 0.78 mmol of γCD per gram of oligomeric compound. MALDI-TOF analysis reveals the distribution of oligomeric species containing from 1 to 5 γCD per oligomer (see scheme 1 and SI, Figure S1). Then, γ-POL refers to γCD based oligomers distribution with averaged formula (γ-CD-HDI<sub>1,2</sub>)n with n ranging between 1 and 5.

$$\{(C_{48}H_{80}O_{40}) \cdot [C_{8}H_{14}O_{2}N_{2}]_{1.2}\}_{n} = \gamma-\text{POL}$$

The chemical-physical properties of γ-POL were investigated in aqueous solution in the presence of two Keggin-type electro-active POMs, namely α-[SiW<sub>12</sub>O<sub>40</sub>]⁴⁻ and α-[SiW<sub>11</sub>MoO<sub>40</sub>]⁴⁻, selected for their fair hydrolytic and redox stability under aerobic aqueous conditions and for their contrasting affinity towards native γ-CD upon one-electron transfer.[36]

<figure><img src="image_5.png" /><figcaption>**Scheme 1.** Schematic representation of the γ-POL behavior in the presence of oxidized and one-electron reduced Keggin-type polyoxometalates K₄SiW₁₁MoO₄₀. While the γ-POL system is composed of a distribution of short oligomers (see insert, left), hydrogel formation as a yellow phase occurs by mixing the oxidized POM component. The gel converts into a fluid violet solution through chemical reduction using dithionite (Na₂S₂O₈). The hydrogel state can be restored by oxidation of one-electron-reduced POM using chemical oxidants such as peroxodisulfate (Na₂S₂O₈). Physical-chemical characterizations evidence that the supramolecular polymerization arises from a cross-linking process resulting from the host-guest supramolecular recognition between the POM unit and γ-cyclodextrin cavity of the γ-POL. As schematically shown, the assembly-disassembly process that causes gel rupture is controlled by a redox stimulus.</figcaption></figure>
<figure><img src="image_7.png" /></figure>

As preliminary qualitative and demonstrative results, experi-
ments showing hydrogelation of an initially fluid γ-POL aqueous
solution undergoing a gel-to-fluid conversion through a one-
electron reduction process are depicted in scheme 1. The
300 g/L γ-POL aqueous solution initially fluid converts into a
strong hydrogel in the presence of [SiW<sub>11</sub>MoO<sub>40</sub>]<sup>4-</sup> introduced
as potassium salt with a POM/γCD ratio of 0.5:1. Then, when
the hydrogel is brought into contact with sodium dithionite,
used as reducing agent, a rapid color change from light yellow
to deep violet occurs, accompanied by rupture of the hydrogel,
which becomes fluid (see scheme 1). This process proved to be
fully reversible since the hydrogel could be reproducibly
restored from the reduced hybrid polymer by using an oxidizing
agent such as sodium peroxodisulfate. Herein, such a redox
response was examined under the prism of the host-guest
interactions between POM and γ-CD using various methods
such as viscometry, NMR (including DOSY), electrochemistry,
and small-angle X-ray scattering (SAXS).

Dynamic viscosity measurements showed that the viscosity of aqueous solution containing POM free oligomer γ-POL appears nearly independent upon the concentration over a wide range of concentrations up to [γCD<sub>POL</sub>] = 500 mM and remains lower than 5 cp at 25 °C and close to that corresponding to aqueous solution (see Figure 1A). On the contrary, the viscosity profile shows a strong concentration dependency in the presence of POMs such as H<sub>4</sub>[SiW<sub>12</sub>O<sub>40</sub>] or monosubstituted K<sub>4</sub>[SiW<sub>11</sub>MoO<sub>40</sub>]. The POMs were introduced with a molar ratio POM:γCD of 0.5:1 and in such a condition, the viscosity rises sharply from 300 mM (see Figure 1A). Then, above 500 mM in hybrid POM@γ-POL concentration, the viscosity becomes too high to be accurately determined. The macroscopic gelation process starts at 490 mM with the formation of a low-viscosity gel that thickens at higher concentrations. The POM@γ-POL hydrogels are transparent and homogenous (see scheme 1 or Figure S2) and they have excellent long-term stability retaining their macroscopic properties for more than a month in sealed flasks.

Furthermore, these hybrid POM@γ-POL hydrogels were
found to be highly thermo-responsive. Then, the dynamic
viscosity of diluted hydrogels containing the mixed metal
[SiW<sub>11</sub>MoO<sub>40</sub>]<sup>4-</sup> ion (or [SiW<sub>12</sub>O<sub>40</sub>]<sup>4-</sup>) was measured from room
temperature up to 60 °C revealing that the gels convert into
fully fluid sols around 40 °C (see Figure 1B). The addition of
sodium dithionite (0.5 eq. par POM) as a reducing agent to the
POM@γ-POL hydrogel leads to the one-electron POM reduction,
inducing a phase transition from gel to fluid sol with a color
change to deep blue for [SiW<sub>12</sub>O<sub>40</sub>]<sup>5-</sup> (see Scheme S1), and to
deep purple for [SiW<sub>11</sub>MoO<sub>40</sub>]<sup>5-</sup> (Scheme 1). It should be worth
mentioning that due to fast oxidation in air of the one-electron
reduced [SiW<sub>12</sub>O<sub>40</sub>]<sup>5-</sup> anion, the viscosity of the corresponding
reduced POM@γ-POL system cannot be measured. Under the
applied conditions, the initial dark blue fluid sol reverts to a
colorless gel within a few minutes. However, the one-electron
reduced [SiW<sub>11</sub>MoO<sub>40</sub>]<sup>5-</sup> anion is fairly stable in air and can be
easily prepared as potassium salt from its oxidized parent
through controlled potential electrolysis (see SI, section II).
Then, in such conditions, the resulting viscosity of the reduced

<figure><img src="image_8.png" /><figcaption>**Figure 1.** A) Dynamic viscosity of POM-free γ-POL (black rhombus) and H₄[SiW₁₂O₄₀@γ-POL for POM:γCD = 0.5:1 (blue triangle) as a function of the γCDPOL concentration, in aqueous solution at 25 °C. B) Dynamic viscosity of diluted aqueous hydrogel solution ([γCDPOL] = 415 mM) as a function of temperature before (black rhombus) and after introduction of POM: H₄[SiW₁₂O₄₀] (blue triangle), K₄[SiW₁₁MoO₄₀] (orange circle), and 1 e⁻ reduced K₅[SiW₁₁MoO₄₀] (purple square).</figcaption></figure>

POM@γ-POL system has been compared to the oxidized analog
from RT to 60 °C. As shown in Figure 1B, the POM@γ-POL
system containing the one-electron reduced [SiW<sub>11</sub>MoO<sub>40</sub>]<sup>5-</sup>
remains fluid over the temperature range showing viscosity
with very close values to those found for the POM-free γ-POL
oligomers. This redox-induced viscometry change must be
directly linked to the stability constant which mainly depends
on the charge density of the POM. For the oxidized Keggin-type
[SiW<sub>11</sub>MoO<sub>40</sub>]<sup>4-</sup> anion, the formation of the 1:2 sandwich
complex with the native γCD is characterized by an overall
stability constant β<sub>ox</sub> = 8·10<sup>6</sup> which drops towards a value close
to zero for the one-electron reduced derivative [SiW<sub>11</sub>MoO<sub>40</sub>]<sup>5-</sup>
(β<sub>red~0</sub>).<sup>[36]</sup> Such a contrasting situation between both redox
states well reflects the macroscopic behavior of the POM@γ-
POL system depicted in scheme 1. The formation of the 1:2
sandwich adducts ensures the gelation through POM-induced
cross-linking between the oligomers of the γ-POL, while the
one-electron reduction of the POM leads to the complete
dissociation process observed as a gel-to-fluid transition (see
<figure><img src="image_6.png" /></figure>

As preliminary qualitative and demonstrative results, experi-
ments showing hydrogelation of an initially fluid γ-POL aqueous
solution undergoing a gel-to-fluid conversion through a one-
electron reduction process are depicted in scheme 1. The
300 g/L γ-POL aqueous solution initially fluid converts into a
strong hydrogel in the presence of [SiW<sub>11</sub>MoO<sub>40</sub>]<sup>4-</sup> introduced
as potassium salt with a POM/γCD ratio of 0.5:1. Then, when
the hydrogel is brought into contact with sodium dithionite,
used as reducing agent, a rapid color change from light yellow
to deep violet occurs, accompanied by rupture of the hydrogel,
which becomes fluid (see scheme 1). This process proved to be
fully reversible since the hydrogel could be reproducibly
restored from the reduced hybrid polymer by using an oxidizing
agent such as sodium peroxodisulfate. Herein, such a redox
response was examined under the prism of the host-guest
interactions between POM and γ-CD using various methods
such as viscometry, NMR (including DOSY), electrochemistry,
and small-angle X-ray scattering (SAXS).

Dynamic viscosity measurements showed that the viscosity of aqueous solution containing POM free oligomer γ-POL appears nearly independent upon the concentration over a wide range of concentrations up to [γCD<sub>POL</sub>] = 500 mM and remains lower than 5 cp at 25 °C and close to that corresponding to aqueous solution (see Figure 1A). On the contrary, the viscosity profile shows a strong concentration dependency in the presence of POMs such as H<sub>4</sub>[SiW<sub>12</sub>O<sub>40</sub>] or monosubstituted K<sub>4</sub>[SiW<sub>11</sub>MoO<sub>40</sub>]. The POMs were introduced with a molar ratio POM:γCD of 0.5:1 and in such a condition, the viscosity rises sharply from 300 mM (see Figure 1A). Then, above 500 mM in hybrid POM@γ-POL concentration, the viscosity becomes too high to be accurately determined. The macroscopic gelation process starts at 490 mM with the formation of a low-viscosity gel that thickens at higher concentrations. The POM@γ-POL hydrogels are transparent and homogenous (see scheme 1 or Figure S2) and they have excellent long-term stability retaining their macroscopic properties for more than a month in sealed flasks.

Furthermore, these hybrid POM@γ-POL hydrogels were
found to be highly thermo-responsive. Then, the dynamic
viscosity of diluted hydrogels containing the mixed metal
[SiW<sub>11</sub>MoO<sub>40</sub>]<sup>4-</sup> ion (or [SiW<sub>12</sub>O<sub>40</sub>]<sup>4-</sup>) was measured from room
temperature up to 60 °C revealing that the gels convert into
fully fluid sols around 40 °C (see Figure 1B). The addition of
sodium dithionite (0.5 eq. par POM) as a reducing agent to the
POM@γ-POL hydrogel leads to the one-electron POM reduction,
inducing a phase transition from gel to fluid sol with a color
change to deep blue for [SiW<sub>12</sub>O<sub>40</sub>]<sup>5-</sup> (see Scheme S1), and to
deep purple for [SiW<sub>11</sub>MoO<sub>40</sub>]<sup>5-</sup> (Scheme 1). It should be worth
mentioning that due to fast oxidation in air of the one-electron
reduced [SiW<sub>12</sub>O<sub>40</sub>]<sup>5-</sup> anion, the viscosity of the corresponding
reduced POM@γ-POL system cannot be measured. Under the
applied conditions, the initial dark blue fluid sol reverts to a
colorless gel within a few minutes. However, the one-electron
reduced [SiW<sub>11</sub>MoO<sub>40</sub>]<sup>5-</sup> anion is fairly stable in air and can be
easily prepared as potassium salt from its oxidized parent
through controlled potential electrolysis (see SI, section II).
Then, in such conditions, the resulting viscosity of the reduced

<figure><img src="image_7.png" /><figcaption>**Figure 1.** A) Dynamic viscosity of POM-free γ-POL (black rhombus) and H₄SiW₁₂O₄₀@γ-POL for POM:γCD = 0.5:1 (blue triangle) as a function of the γCDPOL concentration, in aqueous solution at 25 °C. B) Dynamic viscosity of diluted aqueous hydrogel solution ([γCD<sub>POL</sub>] = 415 mM) as a function of temperature before (black rhombus) and after introduction of POM: H₄[SiW₁₂O₄₀] (blue triangle), K₄[SiW₁₁MoO₄₀] (orange circle), and 1 e⁻ reduced K₅[SiW₁₁MoO₄₀] (purple square).</figcaption></figure>

POM@γ-POL system has been compared to the oxidized analog
from RT to 60 °C. As shown in Figure 1B, the POM@γ-POL
system containing the one-electron reduced [SiW<sub>11</sub>MoO<sub>40</sub>]<sup>5-</sup>
remains fluid over the temperature range showing viscosity
with very close values to those found for the POM-free γ-POL
oligomers. This redox-induced viscometry change must be
directly linked to the stability constant which mainly depends
on the charge density of the POM. For the oxidized Keggin-type
[SiW<sub>11</sub>MoO<sub>40</sub>]<sup>4-</sup> anion, the formation of the 1:2 sandwich
complex with the native γCD is characterized by an overall
stability constant β<sub>ox</sub> = 8·10<sup>6</sup> which drops towards a value close
to zero for the one-electron reduced derivative [SiW<sub>11</sub>MoO<sub>40</sub>]<sup>5-</sup>
(β<sub>red~0</sub>).[36] Such a contrasting situation between both redox
states well reflects the macroscopic behavior of the POM@γ-
POL system depicted in scheme 1. The formation of the 1:2
sandwich adducts ensures the gelation through POM-induced
cross-linking between the oligomers of the γ-POL, while the
one-electron reduction of the POM leads to the complete
dissociation process observed as a gel-to-fluid transition (see
<figure><img src="image_9.png" /></figure>

scheme 1). Then, the cross-linking behavior of the mixed-metal
[SiW<sub>11</sub>MoO<sub>40</sub>]<sup>4/5−</sup> Keggin type POMs toward the γ-POL oligomers
was investigated through titration experiments carried out on
dilute systems and monitored by cyclic voltammetry, <sup>1</sup>H-NMR,
and SAXS.

POMs exhibit a sensitive electrochemical response to their
local environment in solution (nature of the solvent, ion pairing,
protonation, etc).[38] In this context, previous reports have
shown that POM complexation processes with CDs in aqueous
solution significantly modify the electrochemical characteristics
evidenced by simultaneous changes in redox potentials or
current intensity.[10,36]

Cyclic voltammograms of a 1 mM oxidized POM (H₄[SiW₁₂O₄₀] or K₄[SiW₁₁MoO₄₀]) in 0.025 mol L⁻¹ HClO₄ aqueous solution were recorded by increasing the γ-POL oligomers concentration up to 10 eq. of γCD unit. As shown in Figures 2A and 2B, the first one-electron reduction wave undergoes a significant gradual negative shift extending over –130 mV for both anions while the half-wave potential of the second one-electron transfer remains quasi-constant whatever the γ-POL concentration (see Figure S3). The use of the Nernst equation to quantitatively analyze electrochemical behavior has been successfully applied to determine the stability constants of the

POM-γCD supramolecular adducts in both POM redox states,
giving reliable data consistent with those previously found by
using ITC or NMR.[10,36] A similar analysis allowed the extraction
of K₁:₁ and K₁:₂ associated to equations (1) and (2), and then the
resulting global stability constant β of equation (3) led to
satisfactory fits between experimental and calculated data (see
SI, section IV). Comparison of the stability constants reveals that
the intrinsic stability constants K₁:₁ for the oxidized POMs is
significantly higher with γ-POL than for native γCD. Such a
result can be straightforwardly attributed to the heightened
hydrophobic nature of the host matrix, which results from the
introduction of HDI (hexyl diisocyanate) arms. These arms
facilitate a more favorable desolvation process, thereby initiat-
ing the supramolecular interactions with the superchaotropic
ions. Moreover, the global constant β<sub>ox</sub>, (2.2·10<sup>6</sup> and 6.9·10<sup>6</sup> for
H₄[SiW₁₂O₄₀] and K₄[SiW₁₁MoO₄₀], respectively) decreases dra-
matically down to β<sub>red</sub> ~0 as a consequence of the one-electron
transfer process. This outcome suggests nicely that the overall
charge density of the Keggin-type POMs has a dramatic
influence on the host-guest interaction strength within the γ-
POL.[36] In the same way, a severe decrease of the first peak
currents occurs upon the addition of the γ-POL. This intensity
current variation reflects the POM complexation by γ-POL that
manifests by the concomitant decrease of the diffusion
coefficient of the POM units.

Then, the diffusion coefficients value can be estimated from
the Randles-Sevcik equation (see SI, section V). Besides, the
decrease in intensity measured in the presence of the γ-POL
oligomers is compared to that observed with the native γCD
(see Figure 3A). The comparison strikingly shows that large
POM@γ-POL hybrid aggregates are formed in solution, diffusing
much more slowly than the corresponding adducts containing
discrete native γCD units. In addition, it is worth to mention
that in such a concentration range, the viscosity of the
electrolytic solution is not altered upon the γ-POL addition. As
shown in Figure 3A, the variation of the diffusion coefficient
reaches a limit value for γ-CD:POM ratio greater than 4,
corresponding to 150 ± 10 µm² s⁻¹ and 40 ± 10 µm² s⁻¹ for native
γCD and γ-POL, respectively. Hence, by using the Stokes-
Einstein equation, the limit size of the formed aggregates can
be estimated. In the presence of native γCD, the hydrodynamic
radius R<sub>h</sub> of the aggregates reaches 1.45 ± 0.05 nm, which is
slightly higher than the expected dimension of the 1:2
supramolecular adduct, whose crystallographic radius was
measured to be 1.05 nm.<sup>[18,20]</sup> Under similar conditions, assum-
ing POM@γ-POL as a spherical object, the Stoke-Einstein
analysis agrees with a significantly larger average radius R<sub>h</sub> of
about 3.7 nm (see SI, section VI).

Furthermore, NMR has been shown highly informative in
studying qualitatively and quantitatively γCDs and related
supramolecular behaviors.[39,40] As widely reported, these versatile
macrocycles provide local NMR probes sensitive to through-
space interactions with guest molecules. Besides, the ¹H-NMR
study gives useful complementary information to those
obtained from cyclic voltammetry. Therefore, a 2 mM γ-POL
aqueous solution was titrated with oxidized POM anions
H₄[SiW₁₁O₄₀] and K₄[SiW₁₁MoO₄₀], or reduced K₅[SiW₁₁MoO₄₀].

<figure><img src="image_10.png" /><figcaption>**Figure 2.** Cyclic voltammetry of A) H₄[SiW₁₂O₄₀] and B) K₄[SiW₁₁MoO₄₀] ([POM] = 1 mM, glassy carbon working electrode, Ag/AgCl reference electrode, and scan rate of 50 mV/s) in the presence of increasing amount of γ-POL (from 0 to 10 eq. in γ-CD). The experiments have been performed in 0.025 M HClO₄ aqueous solution.</figcaption></figure>
<figure><img src="image_8.png" /></figure>

scheme 1). Then, the cross-linking behavior of the mixed-metal
[SiW<sub>11</sub>MoO<sub>40</sub>]<sup>4/5−</sup> Keggin type POMs toward the γ-POL oligomers
was investigated through titration experiments carried out on
dilute systems and monitored by cyclic voltammetry, <sup>1</sup>H-NMR,
and SAXS.

POMs exhibit a sensitive electrochemical response to their
local environment in solution (nature of the solvent, ion pairing,
protonation, etc).[38] In this context, previous reports have
shown that POM complexation processes with CDs in aqueous
solution significantly modify the electrochemical characteristics
evidenced by simultaneous changes in redox potentials or
current intensity.[10,36]

Cyclic voltammograms of a 1 mM oxidized POM (H₄[SiW₁₂O₄₀] or K₄[SiW₁₁MoO₄₀]) in 0.025 mol L⁻¹ HClO₄ aqueous solution were recorded by increasing the γ-POL oligomers concentration up to 10 eq. of γCD unit. As shown in Figures 2A and 2B, the first one-electron reduction wave undergoes a significant gradual negative shift extending over –130 mV for both anions while the half-wave potential of the second one-electron transfer remains quasi-constant whatever the γ-POL concentration (see Figure S3). The use of the Nernst equation to quantitatively analyze electrochemical behavior has been successfully applied to determine the stability constants of the

POM-γCD supramolecular adducts in both POM redox states,
giving reliable data consistent with those previously found by
using ITC or NMR.[10,36] A similar analysis allowed the extraction
of K₁:₁ and K₁:₂ associated to equations (1) and (2), and then the
resulting global stability constant β of equation (3) led to
satisfactory fits between experimental and calculated data (see
SI, section IV). Comparison of the stability constants reveals that
the intrinsic stability constants K₁:₁ for the oxidized POMs is
significantly higher with γ-POL than for native γCD. Such a
result can be straightforwardly attributed to the heightened
hydrophobic nature of the host matrix, which results from the
introduction of HDI (hexyl diisocyanate) arms. These arms
facilitate a more favorable desolvation process, thereby initiat-
ing the supramolecular interactions with the superchaotropic
ions. Moreover, the global constant β<sub>ox</sub>, (2.2·10<sup>6</sup> and 6.9·10<sup>6</sup> for
H₄[SiW₁₂O₄₀] and K₄[SiW₁₁MoO₄₀], respectively) decreases dra-
matically down to β<sub>red</sub> ~0 as a consequence of the one-electron
transfer process. This outcome suggests nicely that the overall
charge density of the Keggin-type POMs has a dramatic
influence on the host-guest interaction strength within the γ-
POL.[36] In the same way, a severe decrease of the first peak
currents occurs upon the addition of the γ-POL. This intensity
current variation reflects the POM complexation by γ-POL that
manifests by the concomitant decrease of the diffusion
coefficient of the POM units.

Then, the diffusion coefficients value can be estimated from
the Randles-Sevcik equation (see SI, section V). Besides, the
decrease in intensity measured in the presence of the γ-POL
oligomers is compared to that observed with the native γCD
(see Figure 3A). The comparison strikingly shows that large
POM@γ-POL hybrid aggregates are formed in solution, diffusing
much more slowly than the corresponding adducts containing
discrete native γCD units. In addition, it is worth to mention
that in such a concentration range, the viscosity of the
electrolytic solution is not altered upon the γ-POL addition. As
shown in Figure 3A, the variation of the diffusion coefficient
reaches a limit value for γ-CD:POM ratio greater than 4,
corresponding to 150 ± 10 µm² s⁻¹ and 40 ± 10 µm² s⁻¹ for native
γCD and γ-POL, respectively. Hence, by using the Stokes-
Einstein equation, the limit size of the formed aggregates can
be estimated. In the presence of native γCD, the hydrodynamic
radius R<sub>h</sub> of the aggregates reaches 1.45 ± 0.05 nm, which is
slightly higher than the expected dimension of the 1:2
supramolecular adduct, whose crystallographic radius was
measured to be 1.05 nm.<sup>[18,20]</sup> Under similar conditions, assum-
ing POM@γ-POL as a spherical object, the Stoke-Einstein
analysis agrees with a significantly larger average radius R<sub>h</sub> of
about 3.7 nm (see SI, section VI).

Furthermore, NMR has been shown highly informative in
studying qualitatively and quantitatively γCDs and related
supramolecular behaviors.[39,40] As widely reported, these versatile
macrocycles provide local NMR probes sensitive to through-
space interactions with guest molecules. Besides, the ¹H-NMR
study gives useful complementary information to those
obtained from cyclic voltammetry. Therefore, a 2 mM γ-POL
aqueous solution was titrated with oxidized POM anions
H₄[SiW₁₁O₄₀] and K₄[SiW₁₁MoO₄₀], or reduced K₅[SiW₁₁MoO₄₀].

<figure><img src="image_9.png" /><figcaption>**Figure 2.** Cyclic voltammetry of A) H₄[SiW₁₂O₄₀] and B) K₄[SiW₁₁MoO₄₀] ([POM] = 1 mM, glassy carbon working electrode, Ag/AgCl reference electrode, and scan rate of 50 mV/s) in the presence of increasing amount of γ-POL (from 0 to 10 eq. in γ-CD). The experiments have been performed in 0.025 M HClO₄ aqueous solution.</figcaption></figure>
<figure><img src="image_11.png" /></figure>

<figure><img src="image_12.png" /><figcaption>**Figure 3.** A) Relative diffusion coefficients of POM as a function of γCD/POM molar ratio, measured from cyclic voltammetry of a solution of 1 mM of POM with various γCD content, using Randles-Sevcik equation. B) Diffusion coefficients of γCD as a function of POM/γCD molar ratio, measured from ¹H-DOSY NMR (400 MHz, D₂O, 298 K) of a solution of 2 mM of γCD with various POM content. Filled dots correspond to the native γCD while the empty dots are related to the γCD within the γ-POL: H₄[SiW₁₂O₄₀] (blue triangle), K₄[SiW₁₁MoO₄₀] (orange circle). C) ¹H-NMR titration of 2 mM γ-POL with K₄[SiW₁₁MoO₄₀] (ox.) and K₅[SiW₁₁MoO₄₀] (red.) Keggin anions. The ¹H-NMR spectra (400 MHz, D₂O, 298 K) highlight the H2, H3, H4, H5, and H6 resonances of the γ-POL oligomers.</figcaption></figure>

The resulting ¹H-NMR spectra of γ-POL with [SiW₁₁MoO₄₀]⁴⁻/³⁻ are given in Figure 3C (see Figure S4 for [SiW₁₂O₄₀]⁴⁻) showing the main resonances between 3.5 and 4.2 ppm of the γ-POL protons H2, H3, H4, H5, and H6. As expected, the NMR signals are broad due to the restricted molecular motion and the polydispersity of the γ-POL oligomers. In the presence of diamagnetic [SiW₁₂O₄₀]⁴⁻ and [SiW₁₁MoO₄₀]⁴⁻ anions, substantial chemical shift variations involving the H3, H5, and H6 nuclei are observed up to 4 eq. in POM, while these resonances undergo simultaneously a slight but significant line broadening (see SI, Table S1). These results provide evidence that the diamagnetic Keggin anions interact mainly with the primary face of the γCD units through a labile complexation mode (see Scheme 1 or Figure S5). However, titration with the paramagnetic one-electron reduced [SiW₁₁MoO₄₀]⁵⁻ species causes a large gradual line broadening until the quasi-disappearance of the NMR signals. Such an efficient relaxation process results from dipolar interactions between nuclear spin and free electrons due to some extent to the spatial proximity of the ¹H-NMR probes and the paramagnetic POM.

¹H-NMR diffusion-ordered spectroscopy (DOSY) was also carried out to investigate the supramolecular aggregation of the diamagnetic Keggin anions with γ-POL. As a starting point,

it should be noted that the γCD covalently trapped in the oligomeric unit exhibits a lower self-diffusion coefficient ($D_{γ-POL}$=198 µm²s⁻¹) than that observed for native γCD ($D_{γCD}$=254 µm²s⁻¹) in similar concentration conditions (see Figure 3B). In presence of POMs, the average self-diffusion coefficient of γ-POL drops abruptly due to the host-guest recognition process. DOSY self-diffusion coefficient ($D_{obs}$) profile as a function of POM concentration is still consistent with the formation of much bigger aggregates in the presence of the oligomeric γ-POL than native γCD (see Figure 3B). Importantly, the $D_{obs}$ values reach a minimum for 0.5 eq. of POM corresponding to $D_{γ-POL}$ = ~145 µm²s⁻¹ for γ-POL and $D_{γCD}$ = ~220 µm²s⁻¹ for native γCD. As previously reported, this specific 0.5:1 ratio corresponds to the formation of the POM@(γCD)₂ “sandwich” complex,[10] where the encapsulated POM is expected to act as a cross-linking node within the γ-POL oligomeric chains. This behavior is probably at the origin of the hydrogel formation observed in concentrated media. Furthermore, these results align with cyclic voltammetry study, even though the self-diffusion coefficients observed are consistently lower when measured through electrochemical methods (see Figure 3A). This apparent discrepancy likely stems from the distinct media used in both methods, which consist of electrolytic HClO₄ aqueous solutions or water. The presence of electrolyte protons likely favours ion pairing with POM, thereby reducing POM···POM electrostatic repulsions. Under these conditions, the formation of aggregates involving POM and γ-POL is more probable than in an electrolyte-free aqueous solution. Moreover, it is worth to mention that DOSY NMR exclusively provides an average diffusion coefficient value, covering the range from the most aggregated to the most labile part of the polymer, while the electrochemical method probes the heavier POM species.

At last, the thermally activated fluid-to-gel phase transition has been studied by variable-temperature ¹H-NMR experiments. In agreement with the viscosity measurement, the POM@γ-POL system exhibits thermoreversible behavior between room temperature and 80 °C evidenced either through ¹H-NMR chemical shift or line broadening variations (see Figure S6). Then, ¹H-NMR spectroscopic features of the H6 proton signal were monitored showing a perfect reversibility over several cycles of heating from RT to 80 °C. Note that no polymer degradation was detected (see Figure S7).

Lastly, this specific POM@γ-POL oligomeric system is especially favorable for a SAXS study, due to the extremely high electron density of POM-based aggregates, which produce high contrast with water or organic matter. Therefore, the POM-induced cross-linking process has been also investigated by the SAXS method, providing information i) on the electronic density distribution within the POM@γ-POL aqueous solutions at a molecular and supramolecular scale, ranging typically from the Å up to several tens of nm and ii) on the distribution of the oligomeric species in the chemical space. SAXS spectra were collected at a constant H₄[SiW₁₂O₄₀] concentration of 4 mM upon the increasing concentration of γ-POL corresponding to POM/γCD<sub>POL</sub> molar ratio of 4, 2, 1, 0.5, and 0.25 respectively (see Figure 4). At [γ-POL]=0 mM, the scattering of H₄[SiW₁₂O₄₀] shows the typical scattering profile of spherical objects that
<figure><img src="image_10.png" /></figure>

<figure><img src="image_11.png" /><figcaption>**Figure 3.** A) Relative diffusion coefficients of POM as a function of γCD/POM molar ratio, measured from cyclic voltammetry of a solution of 1 mM of POM with various γCD content, using Randles-Sevcik equation. B) Diffusion coefficients of γCD as a function of POM/γCD molar ratio, measured from ¹H-DOSY NMR (400 MHz, D₂O, 298 K) of a solution of 2 mM of γCD with various POM content. Filled dots correspond to the native γCD while the empty dots are related to the γCD within the γ-POL: H₄[SiW₁₂O₄₀] (blue triangle), K₄[SiW₁₁MoO₄₀] (orange circle). C) ¹H-NMR titration of 2 mM γ-POL with K₄[SiW₁₁MoO₄₀] (ox.) and K₅[SiW₁₁MoO₄₀] (red.) Keggin anions. The ¹H-NMR spectra (400 MHz, D₂O, 298 K) highlight the H2, H3, H4, H5, and H6 resonances of the γ-POL oligomers.</figcaption></figure>

The resulting ¹H-NMR spectra of γ-POL with [SiW₁₁MoO₄₀]⁴⁻/⁻ are given in Figure 3C (see Figure S4 for [SiW₁₂O₄₀]⁴⁻) showing the main resonances between 3.5 and 4.2 ppm of the γ-POL protons H2, H3, H4, H5, and H6. As expected, the NMR signals are broad due to the restricted molecular motion and the polydispersity of the γ-POL oligomers. In the presence of diamagnetic [SiW₁₂O₄₀]⁴⁻ and [SiW₁₁MoO₄₀]⁴⁻ anions, substantial chemical shift variations involving the H3, H5, and H6 nuclei are observed up to 4 eq. in POM, while these resonances undergo simultaneously a slight but significant line broadening (see SI, Table S1). These results provide evidence that the diamagnetic Keggin anions interact mainly with the primary face of the γCD units through a labile complexation mode (see Scheme 1 or Figure S5). However, titration with the paramagnetic one-electron reduced [SiW₁₁MoO₄₀]⁵⁻ species causes a large gradual line broadening until the quasi-disappearance of the NMR signals. Such an efficient relaxation process results from dipolar interactions between nuclear spin and free electrons due to some extent to the spatial proximity of the ¹H-NMR probes and the paramagnetic POM.

¹H-NMR diffusion-ordered spectroscopy (DOSY) was also carried out to investigate the supramolecular aggregation of the diamagnetic Keggin anions with γ-POL. As a starting point,

it should be noted that the γCD covalently trapped in the oligomeric unit exhibits a lower self-diffusion coefficient ($D_{γ-POL}$=198 µm²s⁻¹) than that observed for native γCD ($D_{γCD}$=254 µm²s⁻¹) in similar concentration conditions (see Figure 3B). In presence of POMs, the average self-diffusion coefficient of γ-POL drops abruptly due to the host-guest recognition process. DOSY self-diffusion coefficient ($D_{obs}$) profile as a function of POM concentration is still consistent with the formation of much bigger aggregates in the presence of the oligomeric γ-POL than native γCD (see Figure 3B). Importantly, the $D_{obs}$ values reach a minimum for 0.5 eq. of POM corresponding to $D_{γ-POL}$ = ~145 µm²s⁻¹ for γ-POL and $D_{γCD}$ = ~220 µm²s⁻¹ for native γCD. As previously reported, this specific 0.5:1 ratio corresponds to the formation of the POM@(γCD)₂ “sandwich” complex,[10] where the encapsulated POM is expected to act as a cross-linking node within the γ-POL oligomeric chains. This behavior is probably at the origin of the hydrogel formation observed in concentrated media. Furthermore, these results align with cyclic voltammetry study, even though the self-diffusion coefficients observed are consistently lower when measured through electrochemical methods (see Figure 3A). This apparent discrepancy likely stems from the distinct media used in both methods, which consist of electrolytic HClO₄ aqueous solutions or water. The presence of electrolyte protons likely favours ion pairing with POM, thereby reducing POM···POM electrostatic repulsions. Under these conditions, the formation of aggregates involving POM and γ-POL is more probable than in an electrolyte-free aqueous solution. Moreover, it is worth to mention that DOSY NMR exclusively provides an average diffusion coefficient value, covering the range from the most aggregated to the most labile part of the polymer, while the electrochemical method probes the heavier POM species.

At last, the thermally activated fluid-to-gel phase transition has been studied by variable-temperature ¹H-NMR experiments. In agreement with the viscosity measurement, the POM@γ-POL system exhibits thermoreversible behavior between room temperature and 80 °C evidenced either through ¹H-NMR chemical shift or line broadening variations (see Figure S6). Then, ¹H-NMR spectroscopic features of the H6 proton signal were monitored showing a perfect reversibility over several cycles of heating from RT to 80 °C. Note that no polymer degradation was detected (see Figure S7).

Lastly, this specific POM@γ-POL oligomeric system is especially favorable for a SAXS study, due to the extremely high electron density of POM-based aggregates, which produce high contrast with water or organic matter. Therefore, the POM-induced cross-linking process has been also investigated by the SAXS method, providing information i) on the electronic density distribution within the POM@γ-POL aqueous solutions at a molecular and supramolecular scale, ranging typically from the Å up to several tens of nm and ii) on the distribution of the oligomeric species in the chemical space. SAXS spectra were collected at a constant H₄[SiW₁₂O₄₀] concentration of 4 mM upon the increasing concentration of γ-POL corresponding to POM/γCD<sub>POL</sub> molar ratio of 4, 2, 1, 0.5, and 0.25 respectively (see Figure 4). At [γ-POL]=0 mM, the scattering of H₄[SiW₁₂O₄₀] shows the typical scattering profile of spherical objects that
<figure><img src="image_13.png" /></figure>

<figure><img src="image_14.png" /><figcaption>**Figure 4.** Small angle X-ray scattering profiles of H₄[SiW₁₂O₄₀]/γ-POL mixtures containing 4 mM H₄[SiW₁₂O₄₀] and variable concentration of γ-POL ranging from 0 to 4 eq. in γCDPOL. The increase of the scattering intensity for q<3 nm⁻¹ is consistently analyzed as the formation of hybrid POM@γ-POL aggregates of constant size (around 6 nm in diameter) whose concentration increases with that of the γ-POL oligomers.</figcaption></figure>

undergo intermolecular electrostatic repulsions.[41] Then, upon increasing the γ-POL content, the scattering intensity increases continuously for q < 3 nm⁻¹, i.e. in a space window of 2 to 20 nm corresponding to the expected supramolecular distances. Interestingly, the scattering diagrams exhibit an upturn around 1 nm⁻¹ consistent with the formation of POM@γ-POL aggregates with an average size of around 6 nm. Furthermore, the negligible excess scattering observed over the γ-POL composition indicates that the formed aggregates maintain an almost constant size. It should be worth mentioning that this average diameter of 6 nm lies perfectly with the size determined by the electrochemical method (~6.4 nm) and is also in relatively good agreement with those determined by DOSY NMR.

In summary, here we report a redox and thermo-responsive supramolecular polymer exploiting the dynamic properties of association between cyclodextrin-based oligomers and Keggin-type POM that acts as a switchable cross-linker. The supramolecular polymerization process and properties of the resulting material are directly related to the chaotic character of the Keggin-type POM that can be tuned by its redox state. In its oxidized state, the global host-guest binding constant β<sub>ox</sub> is high enough to ensure quantitative complexation and the subsequent crosslinking process between the oligomeric chains. For concentrations higher than 300 g/L, the γ-POL oligomers lead to the formation of hydrogel in the presence of oxidized POM. Conversely, the reduction of the POM provokes the dramatic decrease of the β<sub>ox</sub> global constant at the origin of the dissociation of the host-guest complexes and the consecutive phase transition from gel to sol. This perfectly reversible assembly/disassembly process could also be observed through heating/cooling cycles. However, the role of cations could also be of some importance in promoting the cross-linking process through ion pairing and electrostatic

charge screening, or even in directing the supramolecular gelation process at multiple length scales.[42]

In conclusion, these supramolecular host-guest hybrid oligomers offer a promising means of immobilizing POMs in versatile and flexible matrices for designing reactive materials such as active components for smart membranes or for highly selective extraction applications.

## Acknowledgements

The authors thank the financial support of the LabEx CHARMM-MAT (grant number ANR-11-LBX-0039). S. Y. thanks the China Scholarship Council (201904910419).

## Conflict of Interests

The authors declare no conflict of interest.

## Data Availability Statement

The data that support the findings of this study are available in the supplementary material of this article.

**Keywords:** polymer · supramolecular assembly · cyclodextrins · polyoxometalates · redox stimuli

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[7] S. Polarz, S. Landsmann, A. Klaiber, *Angew. Chem. Int. Ed.* **2014**, *53*, 946–954.

[8] Y. Yu, J. Rebek, *Acc. Chem. Res.* **2018**, *51*, 3031–3040.

[9] A. A. Ivanov, C. Falaise, D. Landy, M. Haouas, Y. V. Mironov, M. A. Shestopalov, E. Cadot, *Chem. Commun.* **2019**, *55*, 9951–9954.

[10] S. Yao, C. Falaise, A. A. Ivanov, N. Leclerc, M. Hohenschutz, M. Haouas, D. Landy, M. A. Shestopalov, P. Bauduin, E. Cadot, *Inorg. Chem. Front.* **2021**, *8*, 12–25.

[11] K. I. Assaf, W. M. Nau, *Org. Biomol. Chem.* **2023**, *21*, 6636–6651.

[12] K. I. Assaf, M. S. Ural, F. Pan, T. Georgiev, S. Simova, K. Rissanen, D. Gabel, W. M. Nau, *Angew. Chem. Int. Ed.* **2015**, *54*, 6852–6856.

[13] K. I. Assaf, D. Gabel, W. Zimmermann, W. M. Nau, *Org. Biomol. Chem.* **2016**, *14*, 7702–7706.

[14] M. A. Moussawi, M. Haouas, S. Floquet, W. E. Shepard, P. A. Abramov, M. N. Sokolov, V. P. Fedin, S. Cordier, A. Ponchel, E. Monflier, J. Marrot, E. Cadot, *J. Am. Chem. Soc.* **2017**, *139*, 14376–14379.

[15] C. Falaise, S. Khlifi, P. Bauduin, P. Schmid, W. Shepard, A. A. Ivanov, M. N. Sokolov, M. A. Shestopalov, P. A. Abramov, S. Cordier, J. Marrot, M. Haouas, E. Cadot, *Angew. Chem. Int. Ed.* **2021**, *60*, 14146–14153.

[16] K. I. Assaf, W. M. Nau, *Angew. Chem. Int. Ed.* **2018**, *57*, 13968–13981.

[17] Z. Y. Wei, Z. K. Wu, S. Ru, L. B. Ni, W. Wei, *Chem. J. Chin. Univ.* **2022**, *43*, 20210665.

[18] S. Khlifi, J. Marrot, M. Haouas, W. E. Shepard, C. Falaise, E. Cadot, *J. Am. Chem. Soc.* **2022**, *144*, 4469–4477.

[19] G. Izzet, M. Ménand, B. Matt, S. Renaudineau, L.-M. Chamoreau, M. Sollogoub, A. Proust, *Angew. Chem. Int. Ed.* **2012**, *51*, 487–490.
<figure><img src="image_12.png" /></figure>

<figure><img src="image_13.png" /><figcaption>**Figure 4.** Small angle X-ray scattering profiles of H₄[SiW₁₂O₄₀]/γ-POL mixtures containing 4 mM H₄[SiW₁₂O₄₀] and variable concentration of γ-POL ranging from 0 to 4 eq. in γCDPOL. The increase of the scattering intensity for q<3 nm⁻¹ is consistently analyzed as the formation of hybrid POM@γ-POL aggregates of constant size (around 6 nm in diameter) whose concentration increases with that of the γ-POL oligomers.</figcaption></figure>

undergo intermolecular electrostatic repulsions.[41] Then, upon increasing the γ-POL content, the scattering intensity increases continuously for q < 3 nm⁻¹, i.e. in a space window of 2 to 20 nm corresponding to the expected supramolecular distances. Interestingly, the scattering diagrams exhibit an upturn around 1 nm⁻¹ consistent with the formation of POM@γ-POL aggregates with an average size of around 6 nm. Furthermore, the negligible excess scattering observed over the γ-POL composition indicates that the formed aggregates maintain an almost constant size. It should be worth mentioning that this average diameter of 6 nm lies perfectly with the size determined by the electrochemical method (~6.4 nm) and is also in relatively good agreement with those determined by DOSY NMR.

In summary, here we report a redox and thermo-responsive supramolecular polymer exploiting the dynamic properties of association between cyclodextrin-based oligomers and Keggin-type POM that acts as a switchable cross-linker. The supramolecular polymerization process and properties of the resulting material are directly related to the chaotic character of the Keggin-type POM that can be tuned by its redox state. In its oxidized state, the global host-guest binding constant β<sub>ox</sub> is high enough to ensure quantitative complexation and the subsequent crosslinking process between the oligomeric chains. For concentrations higher than 300 g/L, the γ-POL oligomers lead to the formation of hydrogel in the presence of oxidized POM. Conversely, the reduction of the POM provokes the dramatic decrease of the β<sub>ox</sub> global constant at the origin of the dissociation of the host-guest complexes and the consecutive phase transition from gel to sol. This perfectly reversible assembly/disassembly process could also be observed through heating/cooling cycles. However, the role of cations could also be of some importance in promoting the cross-linking process through ion pairing and electrostatic

charge screening, or even in directing the supramolecular gelation process at multiple length scales.[42]

In conclusion, these supramolecular host-guest hybrid oligomers offer a promising means of immobilizing POMs in versatile and flexible matrices for designing reactive materials such as active components for smart membranes or for highly selective extraction applications.

## Acknowledgements

The authors thank the financial support of the LabEx CHARMM-MAT (grant number ANR-11-LBX-0039). S. Y. thanks the China Scholarship Council (201904910419).

## Conflict of Interests

The authors declare no conflict of interest.

## Data Availability Statement

The data that support the findings of this study are available in the supplementary material of this article.

**Keywords:** polymer · supramolecular assembly · cyclodextrins · polyoxometalates · redox stimuli

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Manuscript received: November 16, 2023

Accepted manuscript online: December 26, 2023

Version of record online: February 9, 2024
<figure><img src="image_14.png" /></figure>

[20] Y. Wu, R. Shi, Y.-L. Wu, J. M. Holcroft, Z. Liu, M. Frasconi, M. R. Wasielewski, H. Li, J. F. Stoddart, *J. Am. Chem. Soc.* **2015**, *137*, 4111–4118.

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[23] P. Su, A. J. Smith, J. Warneke, J. Laskin, *J. Am. Soc. Mass Spectrom.* **2019**, *30*, 1934–1945.

[24] P. Yang, W. Zhao, A. Shkurenko, Y. Belmabkhout, M. Eddaoudi, X. Dong, H. N. Alshareef, N. M. Khashab, *J. Am. Chem. Soc.* **2019**, *141*, 1847–1851.

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[29] C. T. Buru, O. K. Farha, *ACS Appl. Mater. Interfaces* **2020**, *12*, 5345–5360.

[30] S.-S. Wang, G.-Y. Yang, *Chem. Rev.* **2015**, *115*, 4893–4962.

[31] S.-M. Wang, J. Hwang, E. Kim, *J. Mater. Chem. C* **2019**, *7*, 7828–7850.

[32] L. S. Van Rompuy, T. N. Parac-Vogt, *Curr. Opin. Biotechnol.* **2019**, *58*, 92–99.

[33] A. Blazevic, A. Rompel, *Coord. Chem. Rev.* **2016**, *307*, 42–64.

[34] Y. Martinetto, B. Pegot, C. Roch-Marchal, B. Cottyn-Boitte, S. Floquet, *Eur. J. Inorg. Chem.* **2019**, *21*.

[35] A. Sole-Daura, J. M. Poblet, J. J. Carbo, *Chem. Eur. J.* **2020**, *26*, 5799–5809.

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[37] A. H. Karoyo, L. D. Wilson, *Langmuir* **2016**, *32*, 3066–3078.

[38] T. Ueda, *ChemElectroChem* **2018**, *5*, 823–838.

[39] H.-J. Schneider, F. Hacket, V. Rüdiger, H. Ikeda, *Chem. Rev.* **1998**, *98*, 1755–1786.

[40] M. Haouas, C. Falaise, N. Leclerc, S. Floquet, E. Cadot, *Dalton Trans.* **2023**, *52*, 13467–13481.

[41] M. Hohenschutz, J.-F. Dufrêche, O. Diat, P. Bauduin, *J. Phys. Chem. Lett.* **2023**, *14*, 3602–3608.

[42] A. Misra, K. Kozma, C. Streb, M. Nyman, *Angew. Chem. Int. Ed.* **2020**, *59*, 596–612.

Manuscript received: November 16, 2023

Accepted manuscript online: December 26, 2023

Version of record online: February 9, 2024