Cyclophane-Based Fluorescence Tuning Induced by Hydrostatic Pressure Changes

Solution-State Hydrostatic Pressure Chemistry: Application to Molecular, Supramolecular, Polymer, and Biological Systems

ConspectusPressure (P), as one of the most inherent state quantities, has become an academic subject of study and has attracted attention for a long time for the minute control of reaction equilibria and rates, not only in the gas phase, based on the gas state equation, but also in the solution state. In the latter case, the pressure applied to the solutions is classified as hydrostatic pressure, which is a type of isotropic mechanical force. For instance, deep-sea organisms are exposed to hydrostatic pressure environments of up to 100 MPa, implying that hydrostatic pressurization plays a role in homeostatic functions at physiological levels. The pressure control of such complicated biological behavior can be addressed by thermodynamics or kinetics. In fact, the spontaneity (ΔG) of a reaction that is governed by weak interactions (approximately 10 kcal/mol), such as electrostatic, van der Waals, hydrophobic, hydrogen bonding, and π-π stacking, is determined by the exquisite balance of enthalpy (ΔH) and entropy changes (ΔS), in accordance with the fundamental thermodynamic equation ΔG = ΔH - TΔS. The mutually correlated ΔH-ΔS relationship is known as the enthalpy-entropy compensation law, in which a more negative enthalpic change (more exothermic) causes further entropic loss based on a more negative entropy change. Namely, changing the temperature (T) as the state quantity, except for P, is highly likely to be equal to controlling the entropy term. The solution-state entropy term is relatively vague, mainly based on solvation, and thus unpredictable, even using high-cost quantum mechanical calculations because of the vast number of solvation molecules. Hence, such entropy control is not always feasible and must be demonstrated on a trial-and-error basis. Furthermore, the above-mentioned equation can be rearranged as ΔG = ΔF + PΔV, enabling us to control solution-state reactions by simply changing P as hydrostatic pressure based on the volume change (ΔV). The volume term is strongly relevant to conformational changes, solvation changes, and molecular recognition upon complexation and thus is relatively predictable, that is, volumetrically compact or not, compared to the complicated entropy term. These extrathermodynamic and kinetic observations prompted us to use hydrostatic pressure as a controlling factor over a long period. Hydrostatic pressure chemistry in the solution phase has developed over the past six decades and then converged and passed the fields of mechanochemistry and mechanobiology, which are new but challenging and current hot topics in multidisciplinary science. In this Account, we fully summarize our achievements in solution-state hydrostatic pressure chemistry for smart/functional molecular, supramolecular, polymer, and biological systems. We hope that the phenomena, mechanistic outcomes, and methodologies that we introduced herein for hydrostatic-pressure-controlling dynamics can provide guidance for both theoretical and experimental chemists working in supramolecular and (bio)macromolecular chemistry, mechanoscience, materials science, and technology.

A pressure-induced ratiometric signalling chemosensor: a case of helical anthracenes

One of the helical anthracenes, [4]HA, in which two fused anthracene ends are spatially arranged top and bottom, exhibits a ratiometric fluorescence response due to the hydrostatic pressure-dependent intramolecular [4+4] photocyclodimerization. This ratiometric signalling comes from the formation of an intramolecular stacked species and its subsequent photoreaction upon hydrostatic pressurization. The ratiometric indexes as a function of hydrostatic pressure may enable us to quantify an unknown pressure in solutions.

Hydrostatic Pressure-Controllable Chiroptical Properties of Chiral Perylene Bisimide Dyes: A Chiral Aggregation Case

Hydrostatically pressurized spectroscopic and lifetime decay analyses of optically active perylene bisimides were demonstrated in the pressure range of 0.1-320 MPa to show a π-stacked aggregation. The hydrostatic pressure-induced excitation and circular dichroism spectral changes of the fluorescence perylene dye enabled us to differentiate the slight pressure-sensitive aggregates. This work will lead to a new strategy for creating a pressure-responsive supramolecular polymerization material.

Ground- and excited-state dynamic control of an anion receptor by hydrostatic pressure

Stimulus-responsive supramolecular architectures have become an attractive alternative to conventional ones for many applications in sensing, drug-delivery and switchable memory systems. Herein, we used an anion receptor (H: host) as a hydrostatic-pressure-manipulatable fluorescence foldamer and halide anions as chiral (binaphthylammonium) and achiral (tetrabutylammonium) ion pairs (SS or RR ·X and TBA·X; X = Cl, Br), and then investigated their (chir)optical properties and molecular recognition behavior under hydrostatic pressures. The conformational changes and optical properties of H in various organic solvents were revealed by UV/vis absorption and fluorescence spectra and fluorescence lifetimes upon hydrostatic pressurization. The anion-recognition abilities of H upon interactions with SS or RR·X and TBA·X at different pressure ranges were determined by hydrostatic-pressure spectroscopy to quantitatively afford the binding constant (K anion) and apparent reaction volume changes . The results obtained indicate that hydrostatic pressure as well as solvation plays significant roles in the dynamic control of the present supramolecular system in the ground and excited states. This work will provide a new guideline for further developing hydrostatic-pressure-responsive foldamers and supramolecular materials.

Pressure-driven, solvation-directed planar chirality switching of cyclophano-pillar[5]arenes (molecular universal joints)

Planar chiral cyclophanopillar[5]arenes with a fused oligo(oxyethylene) or polymethylene subring (MUJs), existing as an equilibrium mixture of subring-included (in) and -excluded (out) conformers, respond to hydrostatic pressure to exhibit dynamic chiroptical property changes, leading to an unprecedented pressure-driven chirality inversion and the largest ever-reported leap of anisotropy (g) factor for the MUJ with a dodecamethylene subring. The pressure susceptivity of MUJs, assessed by the change in g per unit pressure, is a critical function of the size and nature of the subring incorporated and the solvent employed. Mechanistic elucidations reveal that the in-out equilibrium, as the origin of the MUJ's chiroptical property changes, is on a delicate balance of the competitive inclusion of subrings versus solvent molecules as well as the solvation of the excluded subring. The present results further encourage our use of pressure as a unique tool for dynamically manipulating various supramolecular devices/machines.

Hydrostatic Pressure-Regulated Cellular Calcium Responses

Hydrostatic pressure control has attracted much attention and presents a still challenging objective from mechanobiological viewpoints. Herein, we reveal the calcium entry processes in HeLa cells by means of hydrostatic pressure spectroscopy. The steady-state fluorescence spectral data comprehensively elucidated the factors controlling the outcomes of the hydrostatic pressure-stimulated calcium entry behavior. The present work leads to a new perspective on ion regulations in living cells and an attractive alternative to conventional mechanostimuli.

Hydrostatic Pressure-Induced Spectral Variation of Reichardt's Dye: A Polarity/Pressure Dual Indicator

The famous solvatochromic Reichardt's dye was applied to quantify hydrostatic pressure in media. The UV/vis spectra of the dye in various organic solvents are shifted bathochromically or hypsochromically at the shorter- or longer-wavelength band, respectively, upon hydrostatic pressurization. The E _T value, determined by an absorption maximum, in ethyl acetate increases from 38.5 kcal mol^-1 at 0.1 MPa to 39.2 kcal mol^-1 at 300 MPa, which is mostly equal to the one in chloroform at 0.1 MPa. These spectroscopic origins were supported by the time-dependent density functional theory (TD-DFT) calculations. The concept and approach proposed in this paper, i.e., a dual indicator, should attract the attention of a broad spectrum in multidisciplinary science.

A 1,6-Diphenylpyrene-Based, Photoluminescent Cyclophane Showing a Nematic Liquid-Crystalline Phase at Room Temperature

Photoluminescent nematic liquid crystals have been an attractive research target for decades, because of their potential applications in optoelectrical devices. Integration of luminescent motifs into cyclic structures is a promising approach to induce low-ordered liquid-crystalline phases, even though relatively large and rigid luminophores are used as emitters. Here, we demonstrate a 1,6-diphenylpyrene-based, unsymmetric cyclophane showing a stable nematic phase at room temperature and exhibiting strong photoluminescence from the condensed state. The observed sky-blue photoluminescence was dominated by the emission species ascribed to assembled luminophores rather than monomers.

Pressure-dependent guest binding and release on a supramolecular polymer

Pressurization on a supramolecular host–guest system induces the compression of binding pockets, discharging the guest molecules.