Rotational reorientation dynamics of polar dye molecular probes by picosecond laser spectroscopic technique

Abstract

Fluorescence lifetimes and rotational reorientation times for four structurally similar dye molecules-three monocations: cresyl violet, nile blue, and oxazine 720 and one neutral but polar: nile red-have been measured by picosecond time-resolved fluorescence depolarization spectroscopy using the single-photon counting technique, in a number of solvents, which included a wide range of alcohols, other hydrogen-bonding liquids, and a few aprotic liquids. The rotational reorientation of the dye probes (assumed to be oblate ellipsoids) are sought to be explained in terms of the Stokes-Einstein-Debye theory and dielectric friction. The individual contributions to the rotational friction due to the above two factors were calculated using reasonable values for the molecular volume and dipole moment of the solute. The rotational behavior of all the four dyes in amides and aprotic solvents is reasonably well explained in terms of the simple stick hydrodynamic model with the "molecular volume" obtained by using the measured rotational reorientation time in water. On the other hand, in order to describe the rotational reorientation dynamics of all the dye molecules in n-alcohols, it is necessary to include the friction contribution due to the dielectric properties of the solvent. It appears that a change in boundary condition, something intermediate between stick and slip or close to slip, is required to satisfactorily explain the rotational reorientation times of the dye molecules in polyalcohols like ethylene glycol and glycerol. Investigation of the rotational behavior of all the four dyes as a function of viscosity by varying the temperature has been carried out in three solvents: 1-heptanol, 1-undecanol, and ethylene glycol. While the rotational reorientation times had a good linear η/T dependence, it was found that at a particular macroscopic viscosity value the rotational reorientation times obtained by the solvent variation and temperature variation are different. From the temperature variation study it was found that there is a satisfactory agreement between the solvent viscosity activation energy and the activation energy obtained for the reorientation rate of the dye probe molecules.