Measurements of the solute dependence of solvation dynamics in 1-propano. The role of specific hydrogen-bonding interactions

Time-resolved Stokes shift measurements (approximately 20 ps effective resolution) are used to measure the solvation times of 16 different probe solutes in a single solvent, I-propanol (253 K). The solutes were chosen so as to provide a wide range of chemical structures and physical properties consistent with the requirements of strong fluorescence and high solvent sensitivity. Rather than exhibiting a continuous range of solvation times, the dynamics observed with these solutes indicated a division into two distinct classes. The majority (11) of the solutes were observed to fall into a 'normal' category. The solvation times of these solutes span a relatively narrow range, 0.14-0.22 ns, times consistent with expectations based on nonspecific theories of solvation dynamics. The remaining five solutes, all simple aromatic amines, form a distinct group whose solvation dynamics is at least 2-fold faster than those of the 'normal' solutes. The difference between these two classes of solutes appears to reflect differences in the nature of their hydrogen-bonding interactions with the solvent.