Fluorescence anisotropy of ketocyanine dyes in pure and mixed binary solvents

Fluorescence anisotropies (r) of two ketocyanine dyes have been determined in neat alcohols, nitriles and their binary mixtures. For a particular clas s of solvents the values of r is given by Perrin's equation. The lifetimes (tau(f)) of the solutes in the S-1 state have been determined from a study of variation of r with the viscosity of the solvent. The calculated tau(f) values for the alcohols are higher than that of the nitriles and the result s agree well with those obtained by independent measurements in picosecond domain. In alcohol+nitrile binary mixture the solute exhibits preferential solvation by alcohol molecules. The limiting fluorescence anisotropies of t he solutes in all the solvents come as similar to 0.38 indicating that abso rption and emission dipole moments are parallel. (C) 1999 Elsevier Science S.A. All rights reserved.