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 xCO2 > 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.