Supramolecular colorimetric pressure sensing: ratiometric quantification based on pressure-modulated association

Abstract

Supramolecular colorimetric pressure sensing in solution was achieved using silicon-bridged diazulenylmethyl cation 1_PF6, whose monomer–dimer equilibrium shifts under hydrostatic pressure. Pressurization promotes desolvation of the strongly solvated dimer to produce the monomer. The resulting pressure-induced equilibrium shift enables colorimetric sensing for quantitative pressure readout.

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Colorimetric molecular probes and indicators are chemosensors that translate target analytes or external stimuli into visually detectable responses that can be interpreted either qualitatively or quantitatively.^1–3 A wide variety of colorimetric systems has been developed, and their high sensitivity and selectivity, rapid response, and direct readout by the naked eye have enabled their extensive applications in chemical sensing of metal ions,^4,5 volatile organic compounds,^6,7 hazardous substances,^8,9 and biomolecules.^10–12 In addition, colorimetric approaches have been applied for quantitative detection of external stimuli such as temperature,^13–15 pH,^16–18 magnetic fields,^19–21 light,^22–24 and pressure^24–26 in diverse environments.

Incorporation of molecular structures that recognize target species or external stimuli and translate this recognition information into measurable spectral changes plays an essential role in typical colorimetric sensor design.¹ For organic dye-based sensors or chemosensors, such signal transduction is frequently achieved through weak noncovalent interactions with energies on the order of a few kcal mol⁻¹, such as Coulombic interactions, van der Waals interactions, hydrogen bonding, π–π interactions, and hydrophobic interactions. These interactions often modulate molecular solvation and organization, which are in turn strongly correlated with the entropic terms (rather than enthalpic terms) in the fundamental thermodynamic expression for the free energy described by eqn (1).

−RT ln K = ΔG

= ΔH − TΔS (1)

= ΔF + PΔV (2)

where R is the gas constant, K is the equilibrium constant, T is the absolute temperature, and ΔG, ΔH, ΔS, and ΔF are the changes in the Gibbs free energy, enthalpy, entropy, and Helmholtz free energy, respectively. As indicated by eqn (2), pressure (P) also perturbs the equilibrium through the reaction volume change (ΔV). Because weak interactions also induce pressure-specific changes including conformational changes, molecular recognition, and solvation, chemosensors that exhibit spectral changes driven by such interactions offer promising molecular platforms for detecting pressure stimuli in solutions.

The highly sensitive detection of hydrostatic pressure acting isotropically in solution is an emerging challenge in the rapidly developing fields of mechanobiology^27–30 and mechanochemistry.^31–33 In these research fields, there is a strong demand for methods with high spatial and temporal resolutions that can visualize where, when, and to what extent mechanical stimuli act inside living tissues or complex materials. Although recent efforts have focused on the development of pressure-responsive fluorescence probes to enable highly sensitive pressure detection,^34–38 operation of such probes typically requires external excitation which in practice can give rise to issues such as localized illumination, light scattering, and photobleaching. In this context, a colorimetric pressure chemosensor that operates without excitation light and allows readout via simple transmission, reflection, or even direct visual inspection is highly desirable for application in a practical and robust pressure detection method.

Herein, we report the serendipitous discovery that the silicon-bridged diazulenylmethyl cation 1_PF6 (Fig. 1a)³⁹ functions as a colorimetric pressure chemosensor via pressure-triggered supramolecular association that can drastically alter the detected wavelengths. This chromophoric compound is a carbocation featuring one-dimensional π-conjugation, with the positive charge delocalized across the entire framework, endowing it with thermal stability and an intense absorption band in the visible red region. In solution, the vertically oriented substituents attached to the silicon atom, together with the nearby counter anion, promote the formation of J-type aggregates, giving rise to a new absorption band at longer wavelengths. As reported previously,³⁹ the choice of the counter anion is crucial for promoting the formation of the ordered J-type aggregate. In particular, PF₆⁻ was selected because fluorine-containing anions can fit into the electron-deficient concave region of the curved π-framework through electrostatic interactions, thereby facilitating the formation of the pressure-responsive dimeric species. In this study, the hydrostatic pressure response of the aggregates was investigated, and the system was analyzed by assuming a simplified monomer–dimer equilibrium, which provided a reasonable approximation for the dominant spectral changes observed experimentally. The dimer is more strongly polarized than the monomer and thus is solvated more strongly. Therefore, desolvation was promoted under hydrostatic pressure, shifting the equilibrium from the dimer toward the monomer (Fig. 1b). The absorption-spectrum changes caused by the pressure-induced equilibrium shift in solvents with different polarities were systematically investigated as a function of the hydrostatic pressure. These measurements demonstrated that the resulting wavelength change enables quantitative readout of the pressure in the solution. This study revealed that pressure can modulate ground-state monomer–dimer equilibrium, providing fundamental basis for colorimetric sensing and imaging of local mechanical stimuli in microenvironments relevant to mechanobiology and mechanochemistry.

Fig. 1 (a) Molecular structure of 1_PF6. (b) Conceptual representation of the pressure-induced monomer–dimer equilibrium shift of 1_PF6.

The absorption properties of the monomeric and dimeric forms of 1_PF6 were evaluated in solvents with different polarities. In 1,2-dichloroethane (DCE), the absorption maximum was observed at λ_max = 647 nm at 0.1 MPa (Fig. 2a). For clarity, full spectra and expanded views of the absorption peak maxima are shown in Fig. S3 and S4, respectively. Upon application of hydrostatic pressure, the absorption band underwent a bathochromic shift accompanied by an increase in the intensity. These changes can be attributed to the changes in the dielectric constant of the solvent^40,41 and the effective increase in the concentration⁴² of 1_PF6 under pressure, respectively. As shown by green lines in Fig. 2a, the spectrum recorded after depressurization from 280 MPa recovered to that observed at an ambient pressure (0.1 MPa). These observations clearly indicate that the pressure-dependent changes in the spectroscopic properties of the 1_PF6 monomer are reversible. In mixed solvents of DCE and methylcyclohexane (MCH) with a DCE/MCH volume ratio of 4/6, a new band appeared at approximately 800 nm, which can be assigned to the formation of the J-type dimer (Fig. 2b).³⁹ Similarly, in the mixed solvents with DCE/MCH ratios of 3/7 (v/v) (Fig. 2c) and 2/8 (v/v) (Fig. 2d), two distinct absorption bands were observed at shorter and longer wavelengths, indicating the coexistence of monomeric and dimeric species in the mixed solvents. Deconvolution of the absorption spectra into the monomer and dimer contributions (see S5 in the SI for details) allowed estimation of the mole fractions of both species, from which the equilibrium constant for dimerization (K_dim) was calculated based on the monomer (M)–dimer (D) equilibrium 2M ⇌ D. It was found that K_dim increased with decreasing solvent polarity (Table 1), indicating that the population of the dimer increased in less polar environments.

Fig. 2 UV/vis/NIR absorption spectra of 1_PF6 in (a) DCE (39 µM), (b) DCE/MCH = 4/6 (v/v) (200 µM), (c) DCE/MCH = 3/7 (v/v) (100 µM), and (d) DCE/MCH = 2/8 (v/v) (50 µM) at 0.1, 40, 80, 120, 160, 200, 240, and 280 MPa (from black to blue) at room temperature, measured in a high-pressure cell. The green line indicates the spectrum at 0.1 MPa depressurized from 280 MPa.

Table 1 Solvent composition and K_dim for the monomer–dimer equilibrium of 1_PF6 at atmospheric pressure

Solvent composition (DCE/MCH)	K dim/mol^−1
4/6	0.0225
3/7	0.323
2/8	17.7

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The pressure-dependent shift in the monomer–dimer equilibrium was quantified to evaluate the performance of 1_PF6 as a colorimetric pressure sensor. For all examined mixed solvent compositions, hydrostatic pressure induced a reduction in the dimer-associated absorption band at approximately 800 nm, while simultaneously enhancing the monomer-derived band at approximately 600 nm (Fig. 2b–d). For each solvent composition, deconvolution of the absorption spectra recorded at different pressures was used to deduce the populations of the monomeric and dimeric species and the corresponding pressure-dependent K_dim values (Fig. 3a–c). A pronounced decrease in K_dim was observed upon increasing the pressure to 120 or 160 MPa, indicating a reduction in the dimer population. Upon further pressurization, K_dim nearly saturated. This saturation can be attributed to an exhaustion of the pressure-responsive contributions, such as solvation layer reorganization and compression of structural cavities^43,44 which lead to a diminished ΔV and consequently to a reduced sensitivity of K_dim to further pressure increases. The pressure dependence of K_dim was reproducible, as confirmed by multiple measurements of 1_PF6 in DCE/MCH = 2/8 (v/v) (50 µM), showing a consistent trend within the experimental error (Fig. S5–S7). In addition, as indicated by the green lines in Fig. 2b–d, the spectral profiles measured after returning the pressure from 280 MPa to ambient pressure nearly coincided with the original spectra. These results demonstrate that the supramolecular colorimetric response arising from the pressure-induced shift in monomer–dimer equilibrium is reversible. Therefore, 1_PF6 functions as a supramolecular pressure chemosensor that reads hydrostatic pressure colorimetrically through the pressure-dependent equilibrium between the monomer and dimer in the low-pressure regime at approximately 100–160 MPa.

Fig. 3 Pressure dependence of K_dim for the monomer–dimer equilibrium of 1_PF6 in (a) DCE/MCH = 2/8 (v/v) (50 µM), (b) DCE/MCH = 3/7 (v/v) (100 µM), (c) DCE/MCH = 4/6 (v/v) (200 µM) at 0.1, 40, 80, 120, 160, 200, 240, and 280 MPa at room temperature, measured in a high-pressure cell. Linear fits were applied to the low-pressure regime, specifically from 0.1 to 120 MPa for (a) and (c), and from 0.1 to 160 MPa for (b), showing correlation coefficients (r) of 0.938 for (a), 0.840 for (b), 0.963 for (c), and (d) schematic representation of pressure-induced desolvation and the resulting shift in the monomer–dimer equilibrium.

Next, the pressure dependence of K_dim was quantitatively analyzed in the low-pressure regime below the saturation region. In this range, analysis of K_dim using eqn (3) afforded ΔV associated with dimer formation, with the obtained ΔV values summarized in Table 2.

Table 2 Solvent composition and ΔV for the monomer–dimer equilibrium of 1_PF6

Solvent composition (DCE/MCH)	ΔV/cm^3 mol^−1
2/8	9.3 ± 2.4
3/7	4.2 ± 1.6
4/6	3.2 ± 0.6

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In all examined solvent mixtures, ΔV was positive, indicating that dimerization leads to an overall increase in the system volume and that the application of hydrostatic pressure therefore shifts the equilibrium consistently toward the monomer. This behavior reflects the stronger solvation of the dimer relative to the monomer, and the pressure-induced desolvation that drives the equilibrium toward the monomer (Fig. 3d). The magnitude of ΔV decreases with increasing solvent polarity, indicating that the extent of solvation for the monomer and the dimer becomes more similar in more polar environments, thereby reducing the difference in their effective volumes. Consequently, this quantitative analysis clarifies how solvent polarity systematically governs the reaction volume change and pressure sensitivity of the monomer–dimer equilibrium, providing a clear design principle for tuning colorimetric pressure sensing in solution.

Finally, the origin of the positive reaction volume change associated with the monomer–dimer equilibrium of 1_PF6 was examined in detail. The monomer structure was obtained by geometry optimization using density functional theory (DFT) calculations (Fig. 4a and Table S1), whereas the dimer structure was extracted from the crystal packing (Fig. 4b and Table S2).³⁹ The calculation results revealed that the combined molecular volume of the two isolated monomer units (313.18 ų) was smaller than that of the dimer (418.51 ų). In addition, the dimer adopted a slipped arrangement imposed by steric repulsion between the methyl groups (Fig. 4b), which extended the distribution of positive charges across the dimer. This geometry resulted in a highly polarized structure, as evidenced by the markedly larger dipole moment of the dimer (35.76 D) compared to that of the monomer (18.18 D). Increased polarity leads to stronger solvation of the dimer. This interpretation is experimentally supported by the larger bathochromic shift of the dimer absorption band compared to that of the monomer under pressurization (Fig. S8), indicating a higher sensitivity of the dimer to the pressure-induced changes in the dielectric environment. These observations collectively justify a model in which hydrostatic pressure promotes desolvation, thereby shifting the equilibrium toward the less solvated monomer. This pressure-induced equilibrium shift is the fundamental operating principle for the use of 1_PF6 as a colorimetric pressure sensor in solution.

Fig. 4 (a) Optimized structure of 1_PF6 obtained from DFT calculations and (b) the dimer structure of 1_PF6 extracted from the crystal packing.

In conclusion, this study established the silicon-bridged diazulenylmethyl cation 1_PF6 as an effective supramolecular colorimetric pressure chemosensor in solution based on the pressure-dependent equilibrium between its monomeric and J-type dimeric forms. Hydrostatic pressure induces a reversible shift in this ground-state equilibrium, resulting in distinct spectral changes that enable quantitative pressure readout. The pressure response originates from the positive reaction volume associated with dimer formation, indicating that due to its stronger polarity and solvation, the dimer possesses a larger effective volume than the monomer. Because hydrostatic pressure promotes desolvation, the equilibrium consistently shifts toward the less solvated monomer at higher pressures. The sensitivity of this colorimetric response depends on the solvent polarity, highlighting the critical role of solvation effects. Overall, the pressure-sensing function of 1_PF6 is governed by the pressure-modulated differences in the polarity and solvation between the monomeric and dimeric species in the ground state. By translating hydrostatic pressure directly into a visible color change in the absence of external excitation, this system provides a simple and robust molecular platform for colorimetric pressure sensing in solution-phase environments relevant to mechanobiology and mechanochemistry.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article have been included as part of supplementary information (SI). Supplementary information is available. See DOI: https://doi.org/10.1039/d6cc00561f.

Acknowledgements

This study was supported by Grants-in-Aid (No. JP23H04023 to M. M. and No. JP23H04020, JP24K01536, and JP24K21791 to G.F.) from the Japan Society for the Promotion of Science (JSPS), FOREST (JPMJFR232I) from the Japan Science and Technology Agency (to M.M.), and Mitsubishi Foundation (to G.F.).

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