Fluorescence anisotropy decays of the 7-diethylamino-4-methylcoumarin C1 in various polar solvents of different viscosities and hydrogen bond donor/acceptor character have been recorded by means of the fluorescence upconversion and time-correlated single photon counting techniques. The resulting characteristic times for the rotational diffusion fall into two classes with regards to the viscosity-dependency : n-alcohols and other solvents. This deviation from the simple Stokes–Einstein–Debye model may be interpreted in terms of rotation of the coumarin molecule under two different hydrodynamic boundary-conditions (stick or slip) in the two solvent classes. Possible explanations for this behaviour are discussed, and in particular solvent attachment and additional dielectric friction. Both these phenomena may in fact, under certain conditions, explain our findings. Our opinion, however, is that the dielectric friction model offers a more realistic picture of the additional rotational friction experienced by C1 in n-alcohols.
