Multiphase Behavior of Tetraphenylethylene Derivatives with Different Polarities at High Pressures

Although both pressure and temperature are essential parameters governing thermodynamics, the effects of the pressure on solution-phase equilibria have not been well studied compared to those of temperature. Here, we demonstrate the interesting pressure-dependent behavior of tetraphenylethylene (TPE) derivatives in multiphase systems composed of an organic phase and an aqueous phase in the presence and absence of γ-cyclodextrin (γ-CD). In this system, tetraphenylethylene monocarboxylic acid (TPE1H) and its dicarboxylic acid (TPE2H₂) are distributed in the aqueous phase and dissociated into the corresponding anions, that is, TPE1^– and TPE2^2–, when the pH is sufficiently high. The distribution ratios of TPE1H/TPE1^– and TPE2H/TPE2^2– show opposing pressure dependencies: the distribution of the former in the organic phase increases with increasing pressure, whereas that of the latter decreases. The 1:1 complexation constants of TPE1^– and TPE2^2– with γ-CD, which can be determined from the distribution ratios in the presence of γ-CD, also show opposing pressure dependencies: the former shows a positive pressure dependence, but the latter exhibits a negative one. These pressure effects on the distribution and complexation of TPE derivatives can be interpreted based on the differences in the molecular polarity of these solutes. The water permittivity is enhanced at high pressure, thus stabilizing the more polar TPE2^2– in the aqueous phase to a larger extent than TPE1^– and, as a result, reducing its distribution in the organic phase, as well as its complexation with γ-CD. Fluorescence spectra in the aqueous phase suggest that the TPE derivatives form aggregates with γ-CD molecules, as detected by the specific fluorescence. In addition, the fluorescence intensities of the γ-CD complexes are enhanced at high pressures because of the restricted rotation of the phenyl rings in the TPE molecules. This study provides new perspectives for multiphase partitioning and an attractive alternative to conventional extraction methods.