Fluorescence anisotropy of nile red and oxazine 725 in an isotropic liquid crystal

Steady-state and time-resolved fluorescence anisotropy for nile red and oxazine 725 in the isotropic phase of the liquid crystal p-methoxybenzylidene-n-butylaniline (MBBA) are measured in the temperature range 50-80°C. The fluorescence characteristics of the dye molecules in the isotropic MBBA agree well with the Perrin equation over the investigated temperature range. The decay of the fluorescence anisotropy of the dye molecules in the isotropic liquid crystal follows the simple hydrodynamic (Debye-Stokes-Einstein) model rather than the Landau-de Gennes model for the orientational dynamics of neat isotropic liquid crystals. These results imply that the microscopic anisotropy, which is important in the dynamics of neat isotropic liquid crystals, has no effects on the reorientation dynamics of the dopant dye molecules. Attractive interactions among MBBA molecules are so strong that the dopant molecules are not embedded in the pseudonematic domains of the isotropic phase. The dielectric friction affects the reorientation dynamics to give small deviations from the simple hydrodynamic model.