Rotational diffusion of coumarins in alcohols: a dielectric friction study.

The rotational dynamics of three structurally similar polar molecules viz., coumarin 440, coumarin 151 and coumarin 450 has been studied in alcohols at room temperature using steady-state fluorescence depolarization method and time correlated single photon counting technique. The observed reorientation times of all the three probes are found to be longer than those predicted by Stokes-Einstein-Debye (SED) hydrodynamic theory with stick boundary condition and a deviation towards super-stick behavior is noted. Dielectric friction theories of Nee-Zwanzig and van der Zwan-Hynes, which treat the solute as a point dipole, overestimate the dielectric friction contribution exhibited by all the three coumarins in alcohols. Results are discussed in the light of theoretical models and the possibility of hydrogen bonding between the amino group of the probe molecules and the hydroxyl group of the alcohols.