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

摘要

Subpicosecond time-resolved fluorescence of trans-4-dirnethylarnino-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 S_0.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.