FLUORESCENCE ANISOTROPY DECAYS AND VISCOUS BEHAVIOR OF 2-METHYLTETRAHYDROFURAN

The decay of the fluorescence anisotropy has been measured for solutions in 2-methyltetrahydrofuran (MTHF) between -50 and -150 degrees C. Solutes include both neutral species (perylene, tetracene, carbazole) and ion pairs (of carbazolyl(-), 1,3-diphenylallyl(-) and its vinylogues). The results lie between values calculated for ellipsoids using the 'stick' and 'slip' boundary conditions. In some cases, notably perylene, two-exponential decay is predicted but not observed, suggesting that the solvent-solute interaction is comparable with solvent-solvent forces, contrasting with the hydrogen-bonded solvents used by other workers. The data are used to discuss cis-trans photo-isomerisation of diphenylallyl ions and to estimate the viscosity of MTHF, for which measurements cover eleven orders of magnitude. The Williams-Landel-Ferry (WLF) equation is obeyed well over the whole range with T-0 = 81 K.