Solvation and Solvatochromism in CO₂-Expanded Liquids. 3. The Dynamics of Nonspecific Preferential Solvation

Subpicosecond time-resolved fluorescence of trans-4-dimethylamino-4′-cyanostilbene (DCS) is used to measure solvation dynamics in the gas-expanded liquid (GXL) system CH₃CN + CO₂ at 25 °C along the liquid−vapor coexistence curve. These measurements are supplemented by measurements of the steady-state solvatochromism of DCS and of its rotation and isomerization times. Molecular dynamics computer simulations and a semiempirical spectral model that reproduces the observed solvatochromism in this system are used to interpret the experimental results. Simulations indicate that at compositions of x_CO2 > 0.5, the cybotactic region surrounding DCS is enriched in CH₃CN molecules, and the extent of this enrichment is greater in S₁ than that in S₀. Solvation dynamics are dominated by the CH₃CN component. These dynamics are biphasic, consisting of a subpicosecond inertial component, followed by a slower picosecond component, related to the redistribution of CH₃CN molecules between the cybotactic region and the bulk solvent.