Rotational reorientation dynamics of ionic dye solutes in polar solvents with the application of a general model for the solvation shell

The rotational reorientation times of two cationic dyes, nile blue A and oxazine 720 have been determined in various protic and aprotic polar solvents from the picosecond decay of their transient absorption. The results have shown a good agreement with those obtained from theoretical calculations based on Stokes-Einstein-Debye theory, using a simple model for the rotating species. In this model these large closely planar solutes are treated as oblate symmetric tops, their solvent shells are represented by layers with uniform thickness of γ · 2rS, where 2rS is the diameter of the solvent molecule, and γ is a common fitting parameter for all types of solvent.