Rotational reorientation dynamics of nile blue A and oxazine 720 in protic solvents

The temperature dependence of the orientational relaxation times of the cationic oxazine dyes nile blue A and oxazine 720 have been determined in water, methanol, ethanol and ethyleneglycol, by measuring the picosecond decays of the polarization dichroism of transient absorption signals. It has been shown that the observed orientational relaxation times are attributed to the ground state solutes. The equilibrium geometries of the two ground state cations, their dipole moments and their atomic charge distributions have been computed in accurate density functional calculations. The hydrodynamic and dielectric contributions to the orientational relaxation times have been estimated on the bases of generalized Stokes-Einstein-Debye theory and of point charge dipole approach, using the molecular parameters obtained in the quantum chemical calculations. The results indicate that the solvent shells around the solutes significantly enhance the orientational relaxation times. The temperature dependence of this effect is negligible in water and methanol, small, but observable in ethanol, and strong in ethyleneglycol. (C) 2002 Elsevier Science B.V. All rights reserved.