Rotational dynamics study of structurally similar (medium sized) coumarin dyes in aqueous mixtures of DMSO and DMA

Abstract

The rotational diffusion dynamics of four medium sized, structurally similar, polar Coumarin dyes namely, Coumarin-478 (C478), Coumarin-480 (C480), Coumarin-519 (C519), and Coumarin-523 (C523) has been studied at room temperature in aqueous DMSO and aqueous DMA by using steady state and time resolved fluorescence depolarization techniques. All the probes exhibited faster rotation in organic solvent rich zone than in water rich zone that resulted in a downward bent “hook profile” of rotational reorientation time as a function of viscosity. A faster rotational diffusion is observed for C523 in contrast to the other three solutes in both the aqueous solvent mixtures. The difference between τr values at iso-viscous points is smaller in case of aqueous DMA than in aqueous DMSO which reveals the importance of dielectric friction. Since both solutes and solvents are polar in nature, the Nee-Zwanzig (NZ) and van der Zwan and Hynes (ZH) dielectric friction theories have been used along with Stokes-Einstein-Debye (SED) hydrodynamic theory to assess the experimental results and understand the microfriction experienced by the solute. An important aspect of this work is to establish a relation between viscosity and hydrogen bond strengths at varying compositions of solvent mixtures as well as hydrogen bond strength between solute and solvents with the help of Gaussian 09 software. The H-bond strength thus calculated is found to be higher at a composition where the viscosity is the maximum. It provides an additional support to the experimental results.