Fluorescence anisotropy decays and viscous behaviour of 2-methyltetrahydrofuran

Abstract

The decay of the fluorescence anisotropy has been measured for solutions in 2-methyltetrahydrofuran (MTHF) between –50 and –150 °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 T₀= 81 K.