Rotational diffusion of rose bengal

Abstract

Picosecond time resolved fluorescence depolarization spectroscopy has been used to measure the rotational reorientation times of rose bengal as a function of viscosity. The variation of viscosity has been effected in two different ways—using different solvents and different compositions of aqueous binary mixtures. While the Stokes–Einstein–Debye (SED) hydrodynamics theory is found to be reasonably satisfactory to explain the rotational diffusion of rose bengal in amides and aprotic solvents, the dielectric friction model provides a better appreciation of its motion in normal alcohols. It is possible to mimic the nonhydrodynamic behaviour of rose bengal in tertiary butanol–water (t-BuOH+H2O) mixture by incorporating the contribution due to dielectric friction, although the solvation effects appear to be important in the t-BuOH-rich zone of the binary mixture. In hexamethylphosphoramide–water (HMPA+H2O) binary mixture the looplike profile of the rotational reorientation time (τr) vs viscosity (η) can be understood only on the basis of significant solvation effects. The variation in the boundary condition as an alternate explanation for understanding the rotational dynamics in binary mixtures was not found to be satisfactory.