A redox reaction triggered by hydrostatic pressure in dicationic cyclophanes†

Moto Kikuchi^a, Tomoya Kuwabara^b, Gaku Fukuhara*^bc, Takanori Suzuki^a and Yusuke Ishigaki*^a
aDepartment of Chemistry, Faculty of Science, Hokkaido University, Sapporo 060-0810, Japan. E-mail: yishigaki@sci.hokudai.ac.jp
^bDepartment of Chemistry, Institute of Science Tokyo, Meguro-ku, Tokyo 152-8551, Japan
^cInstitute for Materials Chemistry and Engineering, Kyushu University, Fukuoka 819-0395, Japan. E-mail: gaku@ms.ifoc.kyushu-u.ac.jp

First published on 18th July 2025

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Introduction

Stimuli-responsive materials have attracted wide interest in a variety of fields, such as supramolecular polymers, nanoparticles, and organic light-emitting diodes, because they can modulate various properties in response to external stimuli such as mechanical force, heat, light, and electric potential.^1–4 Hydrostatic pressure, one type of mechanical isotropic stimulus, has been investigated since the early 1960s.^5–7 Given that hydrostatic pressurisation of a solution can control the thermodynamic equilibrium and reaction kinetics,^7–9 various reactions in solution under an applied hydrostatic pressure have been investigated. For example, hydrostatic pressure has been shown to disturb the equilibrium of a reaction so that it behaves differently compared to at ambient pressure,^10–18 and thus some reactions are drastically accelerated under high pressure.^19,20 Recently, unique behaviour and systems, such as the dynamic control of chiral recognition induced by hydrostatic pressure²¹ and ratiometric chemosensors that are capable of quantifying the degree of hydrostatic pressure,²² have been reported. Therefore, the study of the effects of hydrostatic pressure has regained significant attention.²³

−RTlnK = ΔG

ΔG = ΔH − TΔS	(1)

ΔG = ΔF + PΔV	(2)

When analysing chemical reactions or equilibria in solution, eqn (1) and (2) (where R is the gas constant, T is the temperature, K is the equilibrium constant, ΔF is the change in the Helmholtz energy, ΔV is the change in the volume, and ΔG is the change in the Gibbs energy) are typically used and the reaction proceeds spontaneously when ΔG is negative. Under conditions of constant pressure, e.g., atmospheric pressure, ΔG is determined by two mutually correlated parameters,²⁴ i.e., the changes in the enthalpy (ΔH) and the entropy (ΔS) (eqn (1)). The ΔS term is mainly affected by the solvation, but it is difficult to predict due to the vast number of solvating molecules in the solution. Therefore, it is not easy to predict the values of ΔH and ΔS to design a system with a negative ΔG term. On the other hand, under conditions of constant temperature, the ΔG term is determined by two different parameters, i.e., ΔF and ΔV. If the pressure (P) is sufficiently large and the ΔV term is negative, the ΔG term would be negative. Compared to the ΔS term, the ΔV term is more predictable because it is based on a change in the structure and the solvation/desolvation process (eqn (2)). A change in the van der Waals volume during the reaction also contributes to the ΔV term, eventually enabling us to exert dynamic control of a targeted solution-state system through the application of pressure.

Cyclophanes that can exhibit flexible change in conformation have attracted considerable attention due to their ability to modulate physical properties through the inclusion and exclusion of guest molecules.^25–32 As these processes are accompanied by changes in the van der Waals volume, their potential responsiveness to hydrostatic pressure is of particular interest. In addition, there have been pioneering studies^33–36 on cationic cyclophanes, such as Stoddart's “blue box,” the properties of which can also be controlled by an electric potential.^37–39 However, the effect of hydrostatic pressure on physical properties as well as redox behaviour remains elusive. Only a few studies on pressure-controlled redox reactions have been reported so far,⁴⁰ probably due to the small change in the van der Waals volume upon electron transfer in reversible organic redox systems. Here, we envisaged that cyclophane-type dication 2²⁺,⁴¹ which should undergo a large change in molecular structure that depends on its redox state, could be a suitable candidate to study how pressure affects a redox reaction. This is because the change in the apparent reaction volume at a certain constant temperature of T can be expected to change upon electron transfer. The term for 2²⁺ can be expressed as shown in eqn (3) and (4).

	(3)

	(4)

Here, the subscripts 2˙⁺ and 2²⁺ represent the van der Waals volume of the radical cation and dication, while 2˙⁺, sol and 2²⁺, sol refer to the solvated molar volume of the radical cation and dication, respectively. Given that dication 2²⁺ contains two xanthylium units linked by flexible alkylene spacers, both the distance between the tricyclic π skeletons and the overall molecular conformation can change dynamically in solution, something which cannot be realised by reference monocation 1⁺ with a monomeric structure (Fig. 1).⁴¹ During a one-electron (1e) reduction of 2²⁺, the electrostatic repulsion between the tricyclic π skeletons can be expected to be reduced so that the stacking distance between the π units would decrease. In other words, the term in eqn (4) can be expected to be negative for the reduction of 2²⁺. In addition, the term in eqn (4) can be expected to be negative due to desolvation caused by the smaller charge on the radical cation. Thus, the value should be negative and the 1e reduction of 2²⁺ is expected to be facilitated when hydrostatic pressure is applied. Here, we report a serendipitous discovery that cyclophane-type dication 2²⁺ can be easily reduced in a water-containing solvent by applying hydrostatic pressure. We also reveal that the reduction behaviour changes for different salts of the same dicationic cyclophane because the counter anions affect the degree of solvation of the water molecules. This study is the first demonstration showing that the redox behaviour of cationic π-conjugated compounds changes upon applying hydrostatic pressure.

Results and discussion

Initial considerations

The cyclophane structure of dication 2²⁺ is an ideal platform to study how pressure affects the redox behaviour of a cationic π-conjugated compound as the reversible two-stage 1e reduction that gives an isolable neutral biradical state 2^2• via radical cation state 2˙⁺ can be examined (Fig. 1). In addition, despite the presence of only one kind of cationic chromophore, 2˙⁺ and 2²⁺ are distinguishable based on their UV-Vis spectra. Although 2˙⁺ could not be isolated, the spectral features and the molar extinction coefficients of the 2˙⁺ salts were revealed using UV-Vis spectroscopy of an equimolar mixed solution of 2²⁺ and 2²˙ (Fig. S8, ESI†). 4-Methoxyphenyl and 5-(4-methoxyphenyl)thienyl groups were selected as the aryl groups at the 9-position of the xanthylium unit to investigate the influence that the conjugation of the π system has on the redox behaviour. We selected three anions, i.e., BF₄⁻, PF₆⁻, and NTf₂⁻, as the counterions to gain insights into the effect that their polarisation has on the redox behaviour. As shown in Scheme 1, the salts of 2²⁺(PF₆⁻)₂ and 2²⁺(NTf₂⁻)₂ were newly prepared from the corresponding diols using a procedure similar to the formation of 2²⁺(BF₄⁻)₂.⁴¹

Pressure-dependent UV-Vis spectra

Before investigating how the application of pressure affects the UV-Vis spectra for 2²⁺, the counterion dependency of the UV-Vis spectra for 2²⁺ under atmospheric pressure conditions was examined. For that purpose, the UV-Vis spectra of the BF₄⁻, PF₆⁻, and NTf₂⁻ salts of 2²⁺ were measured in dehydrated CH₂Cl₂ (c = 0.81–1.75 × 10⁻⁵ M; Fig. S7, ESI†). All macrocyclic dications exhibit strong absorptions in the visible region which correspond to the two xanthylium chromophores [λ_max/nm (logε): 420 (5.00) for 2a²⁺(BF₄⁻)₂, 422 (4.93) for 2a²⁺(PF₆⁻)₂, 425 (5.05) for 2a²⁺(NTf₂⁻)₂, 434 (4.79) for 2b²⁺(BF₄⁻)₂, 435 (4.86) for 2b²⁺(PF₆⁻)₂, and 437 (4.93) for 2b²⁺(NTf₂⁻)₂]. There is almost no difference in the λ_max and logε values of 2²⁺(BF₄⁻)₂, 2²⁺(PF₆⁻)₂, and 2²⁺(NTf₂⁻)₂, which means that the UV-Vis spectra of 2²⁺ do not depend on the counter anions, and thus, under these highly dilute conditions, ion pairing between the cyclophane dication and counter anion can be expected to be negligible.

Subsequently, to investigate the response of 2²⁺ to hydrostatic pressure, the UV-Vis spectra of 2²⁺(BF₄⁻)₂ were measured under various hydrostatic pressures. Here, 2²⁺ is expected to exhibit a response to hydrostatic pressure when the value of is negative and it is known that the value of depends on the solvent. Upon measuring the pressure-dependent UV-Vis spectra of 2²⁺(BF₄⁻)₂ in dehydrated and water-containing solvents, we found that 2²⁺(BF₄⁻)₂ exhibits unique behaviour in water-containing solvents. In the case of 2a²⁺(BF₄⁻)₂ in CH₂Cl₂/H₂O (v/v = 2000/3; c = 2.90 × 10⁻⁵ M; 2870 equiv. of H₂O molecules per molecule of 2a²⁺), the UV-Vis spectrum of 2a²⁺(BF₄⁻)₂ continuously changed with increasing pressure and a decrease in the absorption band at ∼420 nm was observed. Meanwhile, a new peak appeared at ∼450 nm, which was assigned to the absorption of 2a˙⁺BF₄⁻ (Fig. 2a). It is likely that a minute amount of hydroxide ions in the solution acts as a reductant.⁴² Similar pressure-dependent behaviour was observed for 2b²⁺(BF₄⁻)₂ in CH₂Cl₂/H₂O (v/v = 2000/3; c = 1.71 x 10⁻⁵ M; 4870 equiv. of H₂O molecules per molecule of 2b²⁺) (Fig. S9a, ESI†). Thus, the 1e reduction of 2²⁺(BF₄⁻)₂ leading to the formation of radical cationic species was observed in both cases. On the other hand, the UV-Vis spectra of 2²⁺(BF₄⁻)₂ in dehydrated CH₂Cl₂ were remarkedly different. Here, a gradual increase of the absorbance at ∼430 nm was observed upon applying hydrostatic pressure, and only a slight red shift was observed without any significant changes in the spectral shape (Fig. 2b and Fig. S9b, ESI†). This behaviour is typical for π-conjugated compounds,^5,7 indicating that 2²⁺(BF₄⁻)₂ maintains its dicationic state upon being subjected to hydrostatic pressurisation in dehydrated solvents. Moreover, the monotonic hyperchromic effect observed only occurred because of the compression of the solution upon hydrostatic pressurisation. These results suggest that the presence of a small amount of water is essential for the observed 1e reduction of 2²⁺(BF₄⁻)₂ upon hydrostatic pressurisation. It should also be noted here that when the pressure-dependent UV-Vis spectrum of monocation 1a⁺BF₄⁻ was measured in CH₂Cl₂/H₂O (v/v = 100/1; c = 4.65 × 10⁻⁵ M; 12800 equiv. of H₂O per molecule of 1a⁺) (Fig. 2c), only a gradual increase in the absorbance and a red shift were observed, as is the case for 2²⁺(BF₄⁻)₂ in dehydrated CH₂Cl₂, indicating that 1⁺BF₄⁻ maintains its cationic state upon hydrostatic pressurisation, even in water-containing solvents. Therefore, the cyclophane structure is essential for 1e reduction and the generation of the radical cationic state of 2²⁺(BF₄⁻)₂ under exposure to hydrostatic pressure in water-containing solvents. The reversibility of redox reactions was confirmed by the one-electron oxidation of as-generated radical cationic species (Fig. S11, ESI†).

Reaction-volume changes

To investigate the response of 2²⁺ to hydrostatic pressure in detail, we calculated according to eqn (5), where K is the abundance ratio of 2˙⁺/2²⁺ for the 1e reduction.

	(5)

The values of K were assumed to be the concentration ratio of 2˙⁺/2²⁺. We calculated the values of K using a simultaneous equation for the absorbance at λ_max of radical cation 2˙⁺ and dication 2²⁺. As shown in Fig. 3, the natural logarithm of the value of the ratio of 2a˙⁺/2a²⁺ was plotted as a function of the pressure, which furnished a good linear correlation. The line fitted to this data showed an excellent linear relationship [r = 0.977 for 2a˙⁺BF₄⁻/2a²⁺(BF₄⁻)₂; the value of ln K at 280 MPa was excluded because it was almost saturated]. The values of obtained from the slope of the plot were relatively large negative values [ for 2a²⁺(BF₄⁻)₂], indicating not only that the desolvation of H₂O molecules occurs due to the decrease of cationic moieties caused by redox reaction, but also that a conformational change caused by the reduction of the electrostatic repulsion between the xanthylium units of the cyclophane-type dications occurred.

Theoretical studies

To gain insight into the conformational changes of the cyclophane-type dication 2²⁺ and radical cation 2˙⁺, the intramolecular π⋯π contacts were investigated using density functional theory (DFT) calculations at the (U)ωB97X-D/6-31G* level. Identical values were obtained when calculated under pressure conditions of 280 MPa (Fig. S13–S16, ESI† and Table 1). As shown in Fig. 4, the shortest C⋯C distance between the π units for the radical cations is 0.07–0.08 Å shorter than for the dications (3.25 Å for 2a²⁺; 3.17 Å for 2a˙⁺; 3.24 Å for 2b²⁺; 3.17 Å for 2b˙⁺), suggesting that 2˙⁺ has a smaller van der Waals volume than 2²⁺. This result indicates that the electrostatic repulsion between the cationic π units of 2²⁺ decreased upon injection of an electron into 2²⁺. In addition, to investigate the effect of solvation on the π⋯π distance in 2²⁺ and 2˙⁺, we also conducted DFT calculations at the (U)ωB97X-D/6-31G* level using the self-consistent reaction field (SCRF) method⁴³ (Fig. S13–S16, ESI†). With regard to the polarity, regardless of the solvent, the calculated C⋯C distance between the π units of 2²⁺ remained almost unchanged. A similar trend was observed for 2˙⁺. Since the bulk solvent effect on the distance between the π units was negligible in both 2²⁺ and 2˙⁺, the difference in the intramolecular π⋯π contacts can be inferred from the number of positive charges on the molecular unit. Thus, the value of when going from the dication to the radical cation can be attributed to desolvation and a decrease in electrostatic repulsion.

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Pressure-dependent UV-Vis spectra of 2²⁺ with different counter anions

Next, to investigate the effect that the counter anion has on the redox behaviour of the dications, pressure-dependent UV-Vis spectra of 2²⁺(PF₆⁻)₂ and 2²⁺(NTf₂⁻)₂ were measured in dehydrated CH₂Cl₂ and CH₂Cl₂/H₂O (v/v = 2000/3) (Fig. 5). Upon hydrostatic pressurisation in dehydrated CH₂Cl₂, gradual increases in the absorbance at ∼420 nm and a red shift were observed for both 2a²⁺(PF₆⁻)₂ and 2a²⁺(NTf₂⁻)₂ similar to those seen in 2a²⁺(BF₄⁻)₂, which is typical behaviour for normal π-conjugated molecules. Upon hydrostatic pressurisation in CH₂Cl₂/H₂O (v/v = 2000/3), the UV-Vis spectrum of 2a²⁺(PF₆⁻)₂ changed, i.e., the absorption band at ∼420 nm decreased and a new peak emerged at ∼450 nm derived from the generation of 2a˙⁺PF₆⁻. This finding indicates that a radical cation was formed similar to the case of the BF₄⁻ salt. Conversely, 2a²⁺(NTf₂⁻)₂ exhibited typical behaviour for a π-conjugated compound under pressure, even in CH₂Cl₂/H₂O (v/v = 2000/3). These results indicate that the redox behaviour of 2a²⁺ under hydrostatic pressure was clearly affected by the counter anion. Similar anion-dependent behaviour was observed in 2b²⁺(PF₆⁻)₂ and 2b²⁺(NTf₂⁻)₂ using pressure-dependent UV-Vis spectroscopy (Fig. S9, ESI†).

Thus, in addition to the cyclophane-type structure, the involvement of H₂O molecules in the solvation is crucial for the observed unique behaviour. In the case of the less polar anions BF₄⁻ and PF₆⁻, both of which are less solvated by H₂O than the dication, the H₂O molecules preferentially solvate the cyclophane-type dication 2²⁺ to facilitate the reduction (Fig. 6a). Meanwhile, in the case of the more strongly polar NTf₂⁻ anion, the reduction does not proceed for 2²⁺(NTf₂⁻)₂ even in a water-containing solvent (Fig. 6b), as the H₂O molecules would preferentially solvate the counter anions rather than the cyclophane-type dication 2²⁺, which indicates that high hydrostatic pressurisation itself does not cause the 1e reduction of 2²⁺. The desolvation of the H₂O molecules from the H₂O-solvated cyclophane-type dications occurs upon hydrostatic pressurisation in water-containing solvents. As a result, this desolvation process can trigger the 1e reduction of 2²⁺(BF₄⁻)₂ and 2²⁺(PF₆⁻)₂ because their terms can be expected to be negative due to the cyclophane structure. This study suggests that the solvation of H₂O molecules is involved in the 1e reduction under high hydrostatic pressure, and furthermore that the reduction of 2²⁺ is also facilitated by the cyclophane structure.

Conclusions

The reduction of 2²⁺(BF₄⁻)₂ and 2²⁺(PF₆⁻)₂, i.e., dications with a cyclophane structures, proceeded when hydrostatic pressure was applied in water-containing solvents. The key factor driving this reduction is the large negative value for the change in the apparent reaction volume at a certain constant temperature T . DFT calculations suggested that the large negative value of is due to desolvation of the H₂O molecules and the change in the proximity between the π units caused by a 1e reduction accompanied by a decrease in electrostatic repulsion. Indeed, monocation 1⁺BF₄⁻ did not undergo a 1e reduction upon hydrostatic pressurisation, even in water-containing solvents, indicating that the redox behaviour is made possible by the cyclophane structure. In addition, 2²⁺(NTf₂⁻)₂ did not exhibit the reduction behaviour, even when hydrostatic pressure was applied in water-containing solvents, indicating that the reduction behaviour can be adjusted and controlled via judicious selection of the counter anion. Therefore, this study provides valuable guidelines for the rational design of molecules with redox behaviour that can be modulated using hydrostatic pressure, whilst it opens the doors to the development of novel pressure- and redox-responsive molecules.

Author contributions

Y. I. developed the concept of this study. M. K. conducted the synthetic and spectroscopic experiments as well as the theoretical calculations. T. K. and G. F. contributed to the spectroscopic studies under hydrostatic pressure. Y. I., G. F., and T. S. supervised the project. M. K., Y. I., and G. F. prepared the manuscript with feedback from all authors.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article have been uploaded as part of the ESI.†

Acknowledgements

This work was supported by Grant-in-Aid from MEXT and JSPS (No. JP25KJ0481 to M. K., JP23H04020, JP24K01536, and JP24K21791 to G. F., JP25K08604 to T. S., and JP23H04011, JP25H00873, and JP25H01259 to Y. I.) and JST PRESTO (JPMJPR23Q1 to Y. I). Y. I. is grateful for the Toyota Riken Scholar Program. This work was also supported by the Research Program of “Five-star Alliance” in “NJRC Mater. & Dev.” MEXT.

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Footnote

† Electronic supplementary information (ESI) available: Photographs for experimental apparatus; Experimental sections; hydrostatic pressure spectroscopic results; DFT calculations. CCDC 2446622. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d5qm00426h

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