Rotational dynamics of coumarins in nonassociative solvents: Point dipole versus extended charge distribution models of dielectric friction

Rotational diffusion of three structurally similar coumarins; coumarin 6, c oumarin 7, and coumarin 30 has been studied in two nonassociative solvents; dimethylsulfoxide (DMSO) and octanenitrile using steady-state fluorescence depolarization technique. The experimentally measured reorientation times of these coumarins are almost identical in a given solvent at a particular temperature. However, these coumarins are found to rotate slower in octanen itrile compared to DMSO especially at higher values of viscosity over tempe rature. The mechanical friction experienced by the coumarins has been model ed using the Stokes-Einstein-Debye hydrodynamic theory with slip boundary c ondition and the dielectric friction using the point dipole as well as the extended charge distribution models. Point dipole models of Nee-Zwanzig and van der Zwan-Hynes fail to explain the experimentally observed trend even in a qualitative way, whereas the extended charge distribution model of Ala vi-Waldeck is successful in predicting the observed behavior at least quali tatively in these nonassociative solvents. (C) 2001 American Institute of P hysics.