Time-resolved spectroscopic measurements on microscopic solvation dynamics

This paper reinvestigates the use of transient fluorescence spectroscopy of polar aromatics in solution as a method to determine microscopic solvation dynamics. It is shown that the compounds previously employed as polar fluorescent probes tend to fall into three photophysical classes depending upon: (i) whether the photon induced change in μ occurs simultaneously with photon absorption (ii) whether solvent motion subsequent to photon absorption is required to induce the change in μ; or (iii) whether two excited-state isomers with different μ's are present simultaneously. The consequence of the different classes on microscopic solvation dynamic measurements is discussed with a molecular example for each class: (i) 4-aminophthalimide, (ii) 4-(9-anthryl)-N,N-dimethylaniline, and (iii) bianthryl, respectively. In addition, we introduce a new transient fluorescence procedure for the determination of solvation dynamics that has advantages over the traditional transient Stokes-shift method. Finally, for the first time, extensive measurements on the solvation dynamics of a polar aprotic solvent have been made. The observed dynamics of the solvent (glycerol triacetate) are highly nonexponential; this has important implications for chemical reactions in similar solvents. Interestingly, the experimentally observed microscopic dynamics are in qualitative agreement with predictions of the dielectric continuum model.