Solute and Solvent Dependencies of Intermolecular Interactions in Different Density Regions in Supercritical Fluids. A Generalization of the Three‐Density‐Region Solvation Mechanism

Pyrene, 6‐propionyl‐2‐( N,N ‐dimethylamino)naphthalene (PRODAN) and ethyl p‐( N,N ‐diethylamino)benzoate (DEAEB) are used as fluorescent probes to study solute‐solvent interactions in supercritical fluoroform, carbon dioxide, and ethane as functions of density and temperature. The Py values, solvatochromic shifts of fluorescence spectra, and dual fluorescence from the locally excited singlet state and twisted intramolecular charge transfer state are strongly fluid density dependent. While the intensities of solvent effects are different for different probes and different fluids, the density dependencies of spectroscopic responses of the probes follow a characteristic pattern. The results are in strong support of the three‐density‐region solvation mechanism.