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

Abstract

Metrics & MoreArticle Recommendations CONSPECTUS: Pressure (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 su...

Control of intramolecular singlet fission in a pentacene dimer by hydrostatic pressure

“…† These values appear to be very similar to those observed in other p systems such as anthracene, pyrene, and perylene. 28 Hence, these results conrm that the intramolecular stacking behavior (vide supra) and substantial conformational relaxation around the biphenyl linker (particularly in the S 1 transition) may not be affected by hydrostatic pressure. Parallel experiments in methylcyclohexane (MCH) and THF showed similar slopes from −0.588 to −0.755 cm −1 MPa −1 , verifying the occurrence of no signicant conformational changes even in a wide variety of dipole moments (0.00-1.75 (Table S1 and Fig.…”

“…Steady-state UV/vis absorption/excitation fluorescence spectroscopy and fluorescence lifetime decay measurement were conducted using a custom-built high-pressure apparatus. 28,45 As the process has previously reported in detail in a previous study, here, we briefly describe this. A quartz inner cell was filled with the sample solution, and then the cell was set into the outer cell, where sapphire or quartz windows were fitted.…”

“…A distinct difference in the negative DV ‡ was observed for the different solvents: −2.5 cm 3 mol −1 in toluene and −3.5 cm 3 mol −1 in THF, compared to the almost zero DV ‡ (−0.2 cm 3 mol −1 ) in MCH (see Table 1). Considering that DV ‡ is the degree of compactness in a transition state depending on not only structural changes, such as differences in bond lengths and angles, 26 but also solvation, 28,45 in this case relating to the intramolecular SF system, the latter solvation contribution seems to be preferable. Hence, these results can be explained in terms of the intramolecular charge density change in the correlated TT.…”

“…Hydrostatic pressure is a mechanical isotropic stimulus and one of the most significant state quantities, enabling scientists to precisely control thermodynamic equilibria and kinetic rates, particularly in solutions in both ground and excited states. [26][27][28] Thus, studies on hydrostatic pressure have been widely conducted since the early 1960s. 29 In the 1980s, advances were made in the understanding of photophysical and photochemical characteristic processes, such as excimer/exciplex formation, 30,31 photoinduced electron transfer, 32 and twisted intramolecular charge transfer 33 upon hydrostatic pressurization.…”

Sumanene-stacked supramolecular polymers. Dynamic, solvation-directed control

“…To date, we have revealed the hydrostatic-pressure-induced spectral responses of functional molecules, supramolecules, polymers, and biomacromolecules. 57 This hydrostatic pressure-control concept can be accounted for using eqn (1), in which the volume change term (DV1), related to solvation/desolvation and conformational changes, plays a decisive role in modulating dynamic equilibrium systems. Thus, this approach was applied in the present sumanene polymerization case.…”