Orientational relaxation times of rhodamine 700 in glycerol-water mixtures

We determined orientational relaxation times for rhodamine 700 dye in glycerol-water mixtures using time-resolved fluorescence depolarization. It appears that the orientational relaxation time varies linearly with the viscosity of the solvent between $1$ and $\text{60\hspace{0.5em}}\mathrm{cP}$ , in accordance with the Perrin-Stokes-Einstein model with stick boundary conditions. Previously others have found that for two anionic dyes in glycerol-water and a cationic dye in glycerol-ethylene glycol mixtures, the orientational relaxation time becomes less sensitive to the viscosity at very high viscosities $\text{(>25\hspace{0.5em}}\mathrm{cP}$ at least). We discuss the influence of dye and solvent on the relation between orientational relaxation time and viscosity, which suggests that the relaxation time as a function of viscosity can be scaled on a common curve.