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

Investigating hydrophobic environment in alkyl-group-functionalized silica particle with various chain lengths using absorption microspectroscopy

A well-known solvatochromic dye, Reichardt's dye (R-dye), was used to evaluate the hydrophobicity of alkyl-group-functionalized silica particles (ASPs) with different chain lengths. The absorption spectra of R-dye were measured in a single ASP in a mixed solution of water and an organic solvent (methanol (MeOH), ethanol (EtOH), acetonitrile (ACN), tetrahydrofuran (THF), or N,N-dimethylformamide (DMF)) using absorption microspectroscopy. The polarity parameter in the ASPs (ET), determined by the absorption maximum, was observed to be smaller than those in bulk solutions, indicating that R-dye was present in a more hydrophobic environment. In EtOH, THF, and DMF, R-dye was distributed within the alkyl chain layer including the organic solvent. An increase in the organic solvent content of the bulk solution led to a higher organic solvent concentration in the alkyl chain layer, resulting in a decrease in ET. In MeOH and ACN, the R-dye was distributed within the alkyl chain layer and concentrated phase. Moreover, with the increase in the organic molecule content, the distribution of R-dye in the concentrated phase became dominant in MeOH and ACN system, leading to an increase in the ET value. The findings presented in this paper are expected to attract the attention of a wide range of researchers in chromatography.

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.

Solvation Effects in Organic Chemistry: A Short Historical Overview

This short overview describes the historical development of the physics and chemistry of organic solvents and solutions from the alchemist era until the present time based on some carefully selected examples that can be considered landmarks in the history of solution chemistry.

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.

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.