Rotational diffusion kinetics of polar solutes in hexamethylphosphoramide–water systems

The rotational reorientation times (τr) of three kinds of dyes—cresyl violet (a monocation), nile red (neutral but polar), and resorufin (a monoanion) were measured in a series of binary mixtures of hexamethylphosphoramide (HMPA)–water at 298 K using the picosecond fluorescence depolarization spectroscopy. A hook-type profile of τr vs η was observed for all three dyes. The contribution due to the dielectric friction is minimal and is irrelevant in explaining the rotational dynamics of nile red. The dielectric friction correction is modest for cresyl violet and resorufin. The solvation effects in the binary mixture are dominant for understanding the rotational dynamics of cresyl violet. The microscopic structure features of the binary mixture are brought out by the study of resorufin.