Picosecond reorientational dynamics of polar dye probes in binary aqueous mixtures

Abstract

Picosecond time dependent fluorescence method has been used to measure the rotational reorientation times (τr) of three kinds of dye probes—oxazine 720 (a monocation), nile red (neutral but polar), and resorufin (a monoanion)—in a series of binary mixtures of water–amides, water–dipolar aprotics, and water–alcohols at 298 K. Most of the binary mixtures are characterized by the fact that at a particular composition (between 25% to 40% of the organic solvent in water), the viscosity (η) of the solution reaches a maximum value that is higher than the viscosities of the two solvents. As a consequence, the viscosity profile of the solution as a function of the organic solvent exhibits a bivaluedness, the extent of which is more, if the liquids are nearly isoviscous. The dielectric properties of the solution also change across the composition range. All the dyes show a near linear behavior of τr vs η in formamide–water, N-methylformamide–water, and methanol–water mixtures. A dual-valued profile for τr vs η is obtained for the cation oxazine 720 in the three dipolar aprotic mixtures (N, N-dimethylformamide–water, N, N-dimethylacetamide–water, and dimethylsulphoxide–water), with the rotational reorientation times being higher in the organic solvent-rich zone, compared to the corresponding isoviscous point in the water-rich zone. However, the anion resorufin shows a bivalued profile of τr vs η only in dimethylsulphoxide–water mixtures, while the neutral nile red shows a linear behavior in all the dipolar–aprotic water mixtures. A hook-type profile of τr vs η is seen for the anion resorufin in ethanol–, 1-propanol–, and 2-propanol–water mixtures and for the cation oxazine 720 in 1-propanol– and 2-propanol–water mixtures; but nile red shows no departure from linear behavior even in alcohol–water mixtures. The rotational dynamics of the cation oxazine 720 in dipolar aprotic–water mixtures is explained in terms of solvation since the dielectric friction is minimal in these solutions. While the concept of dielectric friction model seems to be adequate to understand the nonhydrodynamic behavior of the oxazine 720, the additional contribution due to solvation effects has to be necessarily taken into account to explain the rotational dynamics of the anion resorufin in alcohol–water mixtures.