# Functionalized Anti-Cryptophane-A as a Platform for Type II Porous Ionic Liquids

Cintia M Correa, Frederik Philippi, Thierry Brotin, Patrick Berthault, Sebastian Alberti, Sophie Fourmentin, Margarida Costa Gomes

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Cintia M Correa, Frederik Philippi, Thierry Brotin, Patrick Berthault, Sebastian Alberti, et al., Functionalized Anti-Cryptophane-A as a Platform for Type II Porous Ionic Liquids. Advanced Materials Interfaces, 2026, pp.e01000. 10.1002/admi.202501000. hal-05518406

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RESEARCH ARTICLE

OPEN ACCESS

# Functionalized Anti-Cryptophane-A as a Platform for Type II Porous Ionic Liquids

Cintia M. Correa<fnref n="1" />,² | Frederik Philippi² | Thierry Brotin² | Patrick Berthault³ | Sebastian Alberti²,<fnref n="4" /> |
Sophie Fourmentin<fnref n="1" /> | Margarida Costa Gomes²

**Correspondence:** Margarida Costa Gomes (margarida.costa-gomes@ens-lyon.fr)

**Received:** 2 November 2025 | **Revised:** 19 December 2025 | **Accepted:** 12 January 2026

**Keywords:** cryptophanes | gas absorption | ionic liquids | porous liquids

## ABSTRACT

Type II porous ionic liquids could be prepared at room temperature by dissolving up to 5 wt.% of *anti*-cryptophane-A derivatives functionalized with alkyl or fluorinated alkyl chains in phosphonium- and pyrrolidinium-based ionic liquids. In this study, we provide the first direct experimental evidence that these molecular cages retain their internal free volume when dissolved in ionic liquids. At 303 K and 1 bar, the porous ionic liquids prepared as solutions of *anti*-cryptophane-A grafted with a ten-carbon alkyl chain in the ionic liquids [P<sub>6,6,6,14</sub>][NTf<sub>2</sub>] (1:36), [P<sub>6,6,6,14</sub>][DCA] (1:52) and [C<sub>1</sub>C<sub>3</sub>pyrr][NTf<sub>2</sub>] (1:50) absorb 3.0, 3.5 and 11 times more methane than the pure ionic liquid, respectively. The solution of *anti*-cryptophane-A grafted with a fluorinated alkyl chain in [P<sub>6,6,6,14</sub>][NTf<sub>2</sub>] (1:73) absorbs in turn 1.8 times more methane than the pure ionic liquid. We show that 97(2)% of the cryptophane dissolved in [P<sub>6,6,6,14</sub>][NTf<sub>2</sub>] can encapsulate one molecule of methane, whereas only 60(1)% of the cavities in solid cryptophane are available for gas encapsulation. The presence of permanent porosity in the liquid solution is further supported by controlled atmosphere <sup>1</sup>H NMR spectroscopy that allows to distinguish between free and caged methane, thus providing molecular-level evidence that the gas is encapsulated in the free cryptophane cavities.

## 1 | Introduction

Porous liquids are versatile materials that combine the structural features of porous solids with the fluidity of liquids [1]. They can be designed in various forms, including neat viscous liquids (Type I), suspensions of porous solids (Type III), molten porous frameworks (Type IV), and solutions of discrete porous molecules in size-excluded solvents (Type II) [2]. While each type offers unique advantages, they also present specific challenges: Type I systems often suffer from high viscosity, Type III suspensions may undergo phase separation or sedimentation, and Type IV liquids require high temperatures to remain molten. Type II porous liquids are composed of rigid cage molecules dissolved in solvents

that are too bulky to enter the internal cavities and so offer the benefit of permanent porosity in solution. However, they can be limited by the low solubility of the cage molecules and the need for precise molecular design to prevent the filling of the cavities by the solvent.

Cryptophanes are bowl-shaped macrocyclic compounds with a well-defined internal cavity whose size is mainly defined by the size of the three linkers connecting the two cyclotribenzy-lene (CTB) units [3, 4]. Different structural parameters, such as the syn- or anti-relative configuration, characterize these compounds, and they play a key role in their ability to trap in the solid, liquid or gas phases different substrates. It has been

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.

© 2026 The Author(s). *Advanced Materials Interfaces* published by Wiley-VCH GmbH

<footnote n="1">¹Unité de Chimie Environnementale et Interactions sur le Vivant, UCEIV, UR 4492, Université du Littoral-Côte d'Opale, Dunkerque, France | ²Laboratoire de Chimie de l'ENS Lyon, CNRS and Université de Lyon, Lyon, France | ³NIMBE, CEA, CNRS, Université de Paris Saclay, CEA Saclay, Gif-sur-Yvette, France |</footnote>

<footnote n="4">⁴Department of Physics and Technology,UiT The Arctic University of Norway, Tromsø, Norway</footnote>
clearly established that *anti*-cryptophanes have superior binding
properties, as the helical arrangement of the three linkers enables
them to adapt their conformation to maximize interactions with
the guest in the cavity. *Syn*-cryptophanes cannot change their
conformation as easily as their *anti*-congeners. However, their
more rigid structure can sometimes represent an advantage as
they are less prone to collapse [5, 6].

Since their discovery, *anti*-cryptophane-A congeners, whose
skeleton is made of two CTB units connected by three ethylene-
dioxy linkers, have been studied for different purposes [7]. These
compounds have shown remarkable binding properties toward
small neutral species such as methane or xenon that interact only
weakly with their surrounding environment [8, 9]. In addition,
the possibility of grafting identical or different substituents of
various kinds onto the six benzene rings makes it possible to
easily prepare a wide range of derivatives that can be dissolved
in different media [10, 11]. For example, the introduction of long
alkyl or fluorinated chains, such as those reported in this work,
makes it possible to modify the solubility of these cages and
to study their binding properties in new environments such as
ionic liquids.

Type III porous ionic liquids have been prepared by combining
selected ionic liquids with porous solids, such as metal-organic
frameworks, to form stable suspensions [12]. The virtually unlim-
ited structural diversity of ionic liquids offers broad opportunities
to tailor solvent properties, enabling suspension stability as
well as other task-specific functionalities. Owing to the bulky
nature of their constituent ions, ionic liquids are often unable
to enter the pores of the dispersed solids, thereby preserving
the intrinsic porosity of the solid phase, while their relatively
high viscosities further contribute to suspension stability. Ionic
liquids have also been employed to prepare porous solutions,
or type II porous liquids, in which discrete cage molecules are
dissolved in the solvent while their intrinsic cavities remain
inaccessible to the ionic liquid. Examples include solutions of
metal-organic polyhedra or zirconocene-derived metallocages in
specifically designed bulky dicationic ionic liquids incorporating
long, multi-ether linkers [13, 14], as well as solutions of metal-
organic cages, specifically anionic organic-inorganic doughnut-
like nanostructures, dissolved in trihexyltetradecylphosphonium
chloride ([P<sub>6,6,6,14</sub>][Cl]) [15].

To the best of our knowledge, no experimental studies have been
reported on the preparation and characterization of porous ionic
liquids based on cryptophanes. However, the encapsulation and
host-guest behavior of cryptophanes in conventional organic sol-
vents such as CHCl₃ and CH₂Cl₂ have been previously examined
[16]. Although not originally named porous liquids, solutions
of cryptophanes in bulky solvents like 1,1,2,2-tetrachloroethane
exhibit porous-liquid behavior, as cryptophane cavities remain
empty until gas exposure (e.g., xenon or methane) [8]. Recent
molecular dynamics studies have confirmed this behavior, lead-
ing to formally designating these solutions as porous liquids
[17–19]. In those studies, the authors investigated the encapsula-
tion of gases (H₂, CO₂, SO₂) and their mixtures in cryptophane-
111 dissolved in dichloromethane, with particular emphasis on
the influence of temperature on the encapsulation dynamics.
Cryptophane-111 features a small inner cavity accessible through a
narrow window that excludes solvent molecules such as CH₂Cl₂.

The results reveal that in H₂/CO₂ mixtures, H₂ initially occupies
the cryptophane cavities, but over time, it is displaced by CO₂,
which becomes the preferred guest. In contrast, SO₂ nearly
saturates the cavities on its own, and in CO₂/SO₂ mixtures, CO₂
is first encapsulated but is subsequently replaced by SO₂.

Building on these experimental and theoretical studies, sev-
eral authors have highlighted the under-explored potential of
cryptophane-based porous solutions [18, 20–23]. Since 2015,
cryptophanes have been recognized as promising cage molecules
due to their rigid structures and well-defined internal cavities
[1, 24]. However, when dissolved in conventional, stable organic
solvents, cryptophane cavities are blocked by solvent molecules
that compete with guest species. It is therefore unclear if the
cavities remain unoccupied and whether such solutions can be
considered porous liquids [3]. We believe that ionic liquids offer
a promising and more feasible alternative to organic solvents:
their low volatility and bulky molecular structure minimize
solvent intrusion into the cavities, thereby preserving the intrin-
sic porosity of cryptophane and enabling the formation of a
porous liquid.

In this study, we aim to demonstrate for the first time
the formation of porous solutions by incorporating cage
molecules — specifically *anti*-cryptophane-A and its grafted
derivatives — into different families of ionic liquids based on
phosphonium, imidazolium and pyrrolidinium cations and on
bis(trifluoromethylsulfonyl)imide, dicyanamide and chloride
anions. Cryptophane-A congeners were chosen in this study
because they are easier to functionalize and less prone to
crystallization when dissolved in ionic liquids. Methane and
carbon dioxide are used to probe the porosity and are followed
through spectroscopic methods.

2 | Results and Discussion

## 2.1 | Solubility in Different Ionic Liquids

At room temperature and up to 80°C, the solubility of crypt-A in
[P<sub>6,6,6,14</sub>][NTf<sub>2</sub>] and [P<sub>6,6,6,14</sub>][Cl], is below 2.0 wt.% (or 1:58 mole
ratio) and 2.5 wt.% (or 1:70 mole ratio), respectively (Table 1).
These solubilities do not appear to correlate with the hydrophilic-
ity of the ionic liquids as [P<sub>6,6,6,14</sub>][NTf<sub>2</sub>] is more hydrophobic
than [P<sub>6,6,6,14</sub>][Cl]. Similarly, crypt-A shows a very low solubility in
1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide,
[C<sub>4</sub>C<sub>1</sub>im][NTf<sub>2</sub>] — less than 2 wt.% (1:114) — despite the presence
of aromatic rings in the cation structure, which could potentially
interact favorably with the molecular cage.

Grafting crypt-A with alkyl or perfluoroalkyl chains has the
effect of increasing the solubility in most of the tested ionic
liquids. Several grafted versions of *anti*-cryptophane-A cages
are represented in Figure 1 and were prepared and character-
ized using nuclear magnetic resonance (¹H and ¹³C{¹H} NMR)
and high-resolution mass spectrometry (HRMS) as described in
Supporting Information.

Crypt-F (structure depicted in Figure 1b) is soluble at room
temperature in both [P<sub>6,6,6,14</sub>][NTf<sub>2</sub>] and [C<sub>1</sub>C<sub>3</sub>pyrr][NTf<sub>2</sub>] up to
at least 4.0 wt.% (or 1:44 molar ratio) and 4.4 wt.% (or 1:74 molar
<table><caption>**TABLE 1** | Solutions prepared with ionic liquids and anti-cryptophane-A derivatives, including the weight of each component, mass percentage, and molar ratio. Homogeneity of the solutions is reported here at room temperature of 295 ± 2 K.</caption><thead><tr><th>N.º</th><th>Ionic liquid</th><th>Solid</th><th>m<sub>solid</sub>/mg</th><th>m<sub>liquid</sub>/mg</th><th>%wt</th><th>n<sub>solid</sub>:n<sub>solvent</sub></th><th>Solution?</th></tr></thead><tbody><tr><td>1</td><td>[P<sub>6,6,6,14</sub>][NTf<sub>2</sub>]</td><td>crypt-A</td><td>22.23</td><td>1107.5</td><td>2.0</td><td>1:58</td><td>no</td></tr><tr><td>2</td><td>[P<sub>6,6,6,14</sub>][Cl]</td><td>crypt-A</td><td>7.180</td><td>292.79</td><td>2.5</td><td>1:70</td><td>no</td></tr><tr><td>3</td><td>[C<sub>4</sub>C<sub>1</sub>im][NTf<sub>2</sub>]</td><td>crypt-A</td><td>14.36</td><td>673.98</td><td>2.1</td><td>1:114</td><td>no</td></tr><tr><td>4</td><td>[P<sub>6,6,6,14</sub>][NTf<sub>2</sub>]</td><td>crypt-F</td><td>14.15</td><td>586.69</td><td>2.5</td><td>1:73</td><td>yes</td></tr><tr><td>5</td><td>[P<sub>6,6,6,14</sub>][NTf<sub>2</sub>]</td><td>crypt-F</td><td>34.81</td><td>867.53</td><td>4.0</td><td>1:44</td><td>yes</td></tr><tr><td>6</td><td>[C<sub>1</sub>C<sub>3</sub>pyrr][NTf<sub>2</sub>]</td><td>crypt-F</td><td>7.550</td><td>171.10</td><td>4.4</td><td>1:74</td><td>yes</td></tr><tr><td>7</td><td>[P<sub>6,6,6,14</sub>][NTf<sub>2</sub>]</td><td>crypt-(C<sub>10</sub>)</td><td>11.16</td><td>301.77</td><td>3.7</td><td>1:36</td><td>yes</td></tr><tr><td>8</td><td>[P<sub>6,6,6,14</sub>][NTf<sub>2</sub>]</td><td>crypt-(C<sub>10</sub>)</td><td>27.44</td><td>976.10</td><td>2.1</td><td>1:47</td><td>yes</td></tr><tr><td>9</td><td>[P<sub>6,6,6,14</sub>][DCA]</td><td>crypt-(C<sub>10</sub>)</td><td>13.19</td><td>373.96</td><td>3.5</td><td>1:52</td><td>yes</td></tr><tr><td>10</td><td>[P<sub>6,6,6,14</sub>][Cl]</td><td>crypt-(C<sub>10</sub>)</td><td>10.16</td><td>244.96</td><td>4.1</td><td>1:47</td><td>yes</td></tr><tr><td>11</td><td>[C<sub>1</sub>C<sub>3</sub>pyrr][NTf<sub>2</sub>]</td><td>crypt-(C<sub>10</sub>)</td><td>12.48</td><td>254.00</td><td>4.9</td><td>1:50</td><td>yes</td></tr><tr><td>12</td><td>[P<sub>6,6,6,14</sub>][NTf<sub>2</sub>]</td><td>crypt-(C<sub>10</sub>)<sub>6</sub></td><td>25.16</td><td>468.76</td><td>5.4</td><td>1:40</td><td>no</td></tr><tr><td>13</td><td>[P<sub>6,6,6,14</sub>][NTf<sub>2</sub>]</td><td>crypt-(C<sub>10</sub>)<sub>6</sub></td><td>26.30</td><td>224.74</td><td>12</td><td>1:18</td><td>no</td></tr><tr><td>14</td><td>[C<sub>1</sub>C<sub>3</sub>pyrr][NTf<sub>2</sub>]</td><td>crypt-(C<sub>10</sub>)<sub>6</sub></td><td>7.400</td><td>127.72</td><td>5.8</td><td>1:70</td><td>no</td></tr></tbody></table>

<figure><img src="image_3.png" /><figcaption>**FIGURE 1** | Cryptophane and ionic liquid chemical structures and short names used in this work: (a) not modified anti-cryptophane-A [crypt-A], anti-cryptophane-A grafted with (b) a fluorinated eleven-carbon chain (-C<sub>11</sub>H<sub>6</sub>F<sub>17</sub>) [crypt-F], (c) a ten-carbon chain (-C<sub>10</sub>H<sub>21</sub>) [crypt-(C<sub>10</sub>)], (d) six ten-carbon chains (-C<sub>10</sub>H<sub>21</sub>) = R [crypt-(C<sub>10</sub>)<sub>6</sub>]. Ionic liquids: (e) Trihexyltetradecylphosphonium bis(trifluoromethylsulfonyl)imide [P<sub>6,6,6,14</sub>][NTf<sub>2</sub>], chloride [P<sub>6,6,6,14</sub>][Cl], dicyanamide [P<sub>6,6,6,14</sub>][DCA], (f) 1-Methyl-1-propylpyrrolidinium bis(trifluoromethylsulfonyl)imide [C<sub>1</sub>C<sub>3</sub>pyrr][NTf<sub>2</sub>], and (g) 1-Butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [C<sub>4</sub>C<sub>1</sub>im][NTf<sub>2</sub>].</figcaption></figure>

ratio), respectively. Similar quantities of crypt-(C<sub>10</sub>) were successfully dissolved in [P<sub>6,6,6,14</sub>][NTf<sub>2</sub>], [P<sub>6,6,6,14</sub>][DCA], [P<sub>6,6,6,14</sub>][Cl] and [C<sub>1</sub>C<sub>3</sub>pyrr][NTf<sub>2</sub>] as reported in Table 1, reaching concentrations as high as 4.9 wt.% (or 1:50 molar ratio) in this last case. For crypt-(C<sub>10</sub>)<sub>6</sub>, the solubility at room temperature in [P<sub>6,6,6,14</sub>][NTf<sub>2</sub>] and [C<sub>1</sub>C<sub>3</sub>pyrr][NTf<sub>2</sub>] is below 5.4 wt.% (1:40 molar ratio) and 5.8 wt.% (1:70 molar ratio), respectively. At these concentrations, the cryptophanes precipitate from the ionic liquids. In both cases,

crypt-(C<sub>10</sub>)<sub>6</sub> dissolves at the listed concentrations upon heating to 50°C (Table 1).

For solutions of crypt(C<sub>10</sub>) dissolved in [P<sub>6,6,6,14</sub>][NTf<sub>2</sub>], SAXS measurements confirmed that the solutions remained homogeneous, with no precipitation of solid cryptophane. As expected, the observed structural pattern was typical of the pure ionic liquid, with no evidence of crystalline precipitate (Figure S11) [25].
Surprisingly, the room-temperature solubility of cryptophanes grafted with six ten-carbon alkyl chains (Figure 1d) was similar to, or lower than, that of the cages with only one alkyl chain. This behaviour might be explained either by the stability of the solid or by the intermolecular interactions with the ionic liquids. Raising the temperature to 50°C was sufficient to obtain homogeneous solutions in the ionic liquids studied. Mixing six-chain cages with single-chain analogues offers an alternative strategy to reduce symmetry, disrupt crystallization, and enhance solubility [2, 26]. Predictive tools such as molecular simulations or thermodynamic models could further guide the rational selection of ionic liquids, reducing trial-and-error and enabling systematic design of cryptophane-based porous ionic liquids.

## 2.2 | Probing the Porosity of the Solutions

The porosity of the cryptophane solutions in ionic liquids was investigated by measuring gas absorption in the solid solute, the pure ionic liquid, and the prepared homogeneous solutions under identical temperature and pressure conditions [27]. If the cryptophane cavities remain unoccupied in the solution, the amount of gas absorbed should increase proportionally to the amount of dissolved cryptophane. As in previous studies [28], we employed a gravimetric method to measure the uptake of a selected gas at constant temperature and at pressures ranging from 0–5 bar as described in Supporting Information.

Figure 2 shows the methane absorption in the pure ionic liquids and in solutions containing 3.7 wt.% and 3.5 wt.% of crypt-(C₁₀) in [P₆,₆,₆,₁₄][NTf₂] and [P₆,₆,₆,₁₄][DCA], respectively. At 1 bar and 303 K, pure [P₆,₆,₆,₁₄][NTf₂] absorbs 0.012mmol g⁻¹ of CH₄ while the solution of 3.7 wt.% of crypt-(C₁₀) absorbs 0.041 mmol g⁻¹ — a relative increase of more than 240% that points toward a porous solution. At 1 bar and 303 K, the measured CH₄ uptake indicates that 0.8 mol of gas per mole of crypt-(C₁₀) are being absorbed. This value increases with pressure, reaching a maximum of 0.94 mol of CH₄ per mol of crypt-(C₁₀) at 5 bar and 303 K, as listed in Table S4. The methane absorption is fully reversible, as shown in Figure 2 — the absorption (ABS) isotherms overlap with the desorption (DES) after the solution is put under vacuum. We observed, nevertheless, that the solution absorbs slightly less CH₄ after the first desorption while still taking up significantly more gas than the pure ionic liquid — 0.56 mol of CH₄ per mol of crypt-(C₁₀) at 1 bar and 0.74 mol mol⁻¹ at 5 bar, both at 303 K.

The adsorption isotherms of CH₄, CO₂, and N₂ in solid crypt-(C₁₀) (Figure S14) exhibit a pressure-dependent trend similar to that observed for cryptophane solutions: a sharp increase in gas uptake up to approximately 1 bar, followed by a moderate, linear increase at higher pressures. This behavior contrasts with that of the pure ionic liquid, in which gas uptake increases linearly with pressure even at partial pressures of gas below 1 bar.

We have correlated the experimental data on CH₄ absorption in pure [P₆,₆,₆,₁₄][NTf₂] and in the solution of 3.7%(w/w) crypt-(C₁₀) in [P₆,₆,₆,₁₄][NTf₂] considering that the absorption of the gas in the pure ionic liquid follows Henry's law, while an additional host-guest interaction with crypt-(C₁₀) is expected in the solutions. Details, equations and fits of the corresponding isotherms can be found in the Figures S16–S19.

<figure><img src="image_4.png" /><figcaption>**FIGURE 2** | Absorption/desorption cycles of CH₄ at 303 K and up to 5 bar using an intelligent gravimetric analyzer (IGA) in (top) pure [P₆,₆,₆,₁₄][NTf₂] in black and in the porous solution containing 3.7 %wt of crypt(C₁₀) (1st ABS scan followed by DES in blue and 2nd scan in red). (bottom) Same data set for [P₆,₆,₆,₁₄][DCA] (black) and its porous solution (3.5 %wt) (1st ABS/DES in orange, 2nd scan in green). Absorption is represented by solid symbols and desorption by empty symbols.</figcaption></figure>

The equilibrium constants, K_c, for the host-guest interaction of cryptophane in these two very different environments are in excellent agreement, K_c = 5.9(6) bar⁻¹ for the neat solid and K_c = 5.7(8) bar⁻¹ for the solution, thus validating the fitting of our data using the equations reported in the Supporting Information. The difference lies in the asymptotic value, i.e., the fraction of cryptophane molecules available as a host for CH₄.

As depicted in Figure 3, the active site fraction, α, was found to be 60(1)% in the neat solid crypt-(C₁₀), and increases to 97(2)% when it is dissolved in [P₆,₆,₆,₁₄][NTf₂]. Thus we observe that the cryptophane is much more available for absorption or the gas enters more easily the cages when the host is dissolved in the ionic liquid.
<figure><img src="image_5.png" /><figcaption>**FIGURE 3** | CH₄ capacity of solid and dissolved cryptophane expressed in amount of gas per cage at 303 K. Gray dashed line: in solid crypt-(C₁₀) (α₅bar ≈ 50 %); solid blue line: in crypt-(C₁₀) dissolved in [P₆,₆,₆,₁₄][NTf₂] (α₅bar ≈ 95 %); solid black line: in crypt-(C₁₀) dissolved in [P₆,₆,₆,₁₄][DCA] (α₅bar ≈ 99 %); solid red line: in crypt-(C₁₀) dissolved [C₁C₃pyrr][NTf₂] (α₅bar ≈ 70 %). Hollow symbols are the experimental data points and the lines are the fits described in Supporting Information.</figcaption></figure>

At pressures below 3 bar, the absolute amount of CH₄ absorbed in the dissolved cryptophane exceeds that dissolved in the liquid, despite the latter being present in large excess, as shown in Figure S19. Direct analysis of the experimental data further reveals that, consistent with previous studies, the quantity of gas absorbed per cryptophane cage decreases in the order CO₂ > CH₄ > N₂ [29]. This enhanced uptake by cages dissolved in the ionic liquid is likely related to the fact that dissolved methane is more readily transferred into the cages when compared with gaseous methane when it is adsorbed in the solid [30].

Permanent porosity is also observed when cryptophane grafted with a fluorinated chain (Figure 1b) is dissolved in [P<sub>6,6,6,14</sub>][NTf<sub>2</sub>], as shown by the increase of gas absorption in the solution when compared with the pure ionic liquid (Figure S12). The lower methane absorption of this porous solution compared to those shown in Figure 2 is solely due to its reduced cryptophane concentration (1:73 n<sub>solid</sub>:n<sub>solvent</sub>). The amount of CH₄ contained in the dissolved cages at 303 K is similar for all the cryptophanes studied, ranging from 0.48–0.74 mol of CH₄ per mol of cryptophane at 1 and 5 bar, respectively.

Other ionic liquids, such as [P<sub>6,6,6,14</sub>][DCA], were also used as solvents for crypt-(C<sub>10</sub>). In this case, as shown in the bottom graph of Figure 2, the amount of methane absorbed by the solution is also significantly higher than by pure [P<sub>6,6,6,14</sub>][DCA]. Contrary to [P<sub>6,6,6,14</sub>][NTf<sub>2</sub>], the gas uptake increases slightly after the first desorption. This behavior suggests that, in solutions of crypt-(C<sub>10</sub>) in [P<sub>6,6,6,14</sub>][DCA], the cages are more thoroughly degassed as the pressure swing absorption/desorption cycles progress.

Both crypt-F and crypt-(C<sub>10</sub>) can be dissolved in [C<sub>1</sub>C<sub>3</sub>pyrr][NTf<sub>2</sub>], a low-viscosity ionic liquid [31, 32], to form porous solutions, as illustrated in Figure S13 where an

increase in gas absorption — reversible over at least three absorption-desorption cycles — confirms that the cages remain accessible in solution.

At 1 bar and 303 K, a 1:50 (4.9 wt.%) solution of crypt-(C₁₀) in [C<sub>1</sub>C<sub>3</sub>pyrr][NTf₂] absorbs over eleven times more CH₄ than the neat ionic liquid (at 303 K and 1 bar the absorption increases from 0.0022 mmol g⁻¹ in the neat liquid to 0.0268 mmol g⁻¹ in the solution). This enhanced uptake suggests that cryptophane-based porous [C<sub>1</sub>C<sub>3</sub>pyrr][NTf₂] could serve as an effective electrolyte — if electrochemically stable — for electrochemical gas conversion [33, 34], the readily accessible encapsulated gases enabling more efficient reactions under milder conditions [35, 36].

The highest amount of methane absorbed per mole of crypt-(C₁₀) dissolved in [C<sub>1</sub>C<sub>3</sub>pyrr][NTf₂] is relatively low — 0.68 mol of CH₄ per mole of cryptophane at 5 bar partial pressure of gas — compared with the other two solutions in [P<sub>6,6,6,14</sub>][NTf₂] and [P<sub>6,6,6,14</sub>][DCA]. This observation can be related with the larger non-polar domains in the ionic liquids based on phosphonium cations whose alkyl chains can surround the dissolved cages [37].

## 2.3 | Free and Reversibly Caged CH₄

We have followed by ¹H NMR the absorbed CH₄ at 2 bar and temperatures ranging from 238–318 K in pure [P<sub>6,6,6,14</sub>][NTf₂] and in two porous solutions of crypt-F and crypt-(C₁₀) in [P<sub>6,6,6,14</sub>][NTf₂] at concentrations of 1:44 and 1:47 n<sub>solid</sub>:n<sub>solvent</sub>, respectively. The results obtained are depicted in Figure 4 where in blue are all the control samples — neat ionic liquid and cryptophane solutions in the absence of CH₄ — and in black, the same liquid samples in equilibrium with CH₄ at 2 bar. The control samples show no signals in the region where methane would be expected. As anticipated, all samples containing CH₄ exhibit characteristic signals confirming its presence. For pure [P<sub>6,6,6,14</sub>][NTf₂], the CH₄ signal appears as a sharp peak at approximately δ<sub>f</sub> = −0.8 ppm, in agreement with previous reports on methane dissolved in organic solvents [8]. This peak corresponds to methane dissolved in the ionic liquid, hereafter referred to as free methane. The signal remains essentially unchanged across the tested temperatures of 238, 268, 288, 298, 308, and 318 K.

In the crypt-F and crypt-(C₁₀) solutions in [P<sub>6,6,6,14</sub>][NTf₂], a broader and high-field-shifted signal for methane appears at approximately δ<sub>c</sub> = −2.6 ppm, indicative of the presence of CH₄ inside the cryptophane cavities, herein referred to as reversibly caged CH₄, showing a fast exchange regime occurring between free and complexed molecules. Garel et al. [8] reported a signal for reversibly bound CH₄ ranging from −3.0 to −2.0 ppm depending on the host/guest ratio (0.33 and 1.35 equivalents, respectively) when cryptophane-A is dissolved in dichloromethane at 298 K. Contrary to what was found in pure [P<sub>6,6,6,14</sub>][NTf₂], variations in temperature significantly affect the methane signal in the cryptophane solutions — as the temperature rises from 288 to 308 K, the signal becomes sharper and more intense.

Previous studies have shown that ionic liquids with pronounced non-polar domains, such as [P<sub>6,6,6,14</sub>][NTf₂], exhibit distinct stages of dynamic slowing as temperature decreases [37, 38]. Initially, the charged network slows, while the surrounding alkyl
<figure><img src="image_6.png" /><figcaption>**FIGURE 4** | 400 MHz ¹H NMR spectra of pure [P<sub>6,6,6,14</sub>][NTf₂] and its solutions with crypt-F (1:44) and crypt-(C₁₀) (1:47 *n*<sub>solid</sub>:*n*<sub>solvent</sub>). In blue, the control samples in absence of CH₄ and in black containing approximately 2 bar of CH₄ at temperatures from 298 to 318 K.</figcaption></figure>

chains remain mobile. Our NMR results support these previous findings: in [P<sub>6,6,6,14</sub>][NTf₂], dissolved CH₄ resides mostly in the non-polar regions near the cation's long alkyl chains and is largely unaffected by temperature variations within the studied range. Even at 238 K, when all other ¹H NMR signals collapse into a single broad resonance, a distinct sharp signal attributed to the hydrogens of CH₄ remains visible. In contrast, porous solutions show pronounced temperature effects, likely due to rapid exchange between free and caged methane, which is more temperature-sensitive. This exchange causes slight peak shifts and progressive broadening of the CH₄ signal until it disappears at lower temperatures.

**3 | Conclusion**

This study concludes that homogeneous solutions formed by solubilizing grafted *anti*-cryptophane-A cages in ionic liquids can form porous solutions capable of absorbing significantly more gas than the pure ionic liquid itself. To the best of our knowledge, this is the first report of type II porous ionic liquids prepared from these components. We demonstrate that, consistent with

earlier studies on cryptophane solutions in organic solvents where ¹H NMR distinguished between free and caged methane, the same differentiation can also be achieved in ionic liquids with only minor experimental adjustments to account for their higher viscosities.

Several advantages emerge from using ionic liquids over conventional organic solvents for preparing type II porous liquids based on cryptophanes. Most notably, ionic liquids are non-volatile and can be easily tailored to enhance the solubility of cryptophanes and their derivatives. Since the intrinsic porosity is preserved, the gas content in solution is substantially increased, globally and also per cryptophane dissolved. Furthermore, the gas capacity of the solutions rises as temperature increases. This behavior highlights the potential of cryptophane-ionic liquid solutions as reservoirs for gases of interest for a variety of applications, such as methane, carbon dioxide and hydrogen.

Our research will focus on using tailored ionic liquids and functionalized cryptophanes for applications involving gaseous hosts, with an emphasis on characterizing the solutions and understanding how their properties are influenced by gas dis-
solution thermodynamics, dynamics, and mass transport in the porous medium.

While this study focused on preparing and characterizing porous ionic liquids based on *anti*-cryptophane-A, we anticipate that *syn*-cryptophane-B stereoisomers could enable more stable porous solutions, less prone to cage collapse under elevated gas pressures. Although their synthesis is more complex and challenging [39–41], these structures may offer significant advantages. Another promising route is the use of cryptophane-based porous liquids for enantiomeric separations. Since inherent chirality has been a defining feature of cryptophane derivatives from the first synthesis, porous ionic liquids incorporating these cages could provide a chiral environment for guest molecules, facilitating selective separations [42]. This approach is particularly attractive for liquid–liquid separation processes, as ionic liquids can be designed to both dissolve cryptophane cages and remain immiscible with either aqueous or organic phases.

## Acknowledgments

C.M.C thanks the project GreeNH₃ (PEPR Hydrogene ANR-22-PEHY-0015) and CDeePL (ANR-23-CE06-0010) for the financial support. F.P. acknowledges funding by the European Union. The authors thank B. Abécassis for providing access and training to the SAXS facilities at ENS Lyon supported by the LABEX iMUST of the University of Lyon (ANR-10-LABX-0064), created within the program *Investissements d'Avenir* set up by the French government and managed by the French National Research Agency (ANR).

## Conflicts of Interest

The authors declare no conflicts of interest.

## Data Availability Statement

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

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**Supporting Information**

Additional supporting information can be found online in the Supporting Information section.

**Supporting File:** admi70411-sup-0001-SuppMat.pdf.