Rotational diffusion of the 7-diethylamino-4-methylcoumarin C1 dye molecule in polar protic and aprotic solvents

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.

Five wave mixing: surface-specific transient grating spectroscopy as a probe of low frequency intermolecular adsorbate motion

Surface-specific five wave mixing spectroscopy is used to examine adsorbate dynamics at the fused silica/air interface. Signals whose temporal response is significantly broader than the instantaneous fourth-order electronic polarizability are attributed to low frequency, adsorbate nuclear motion. Based on the dependence of these dynamics on the moment of inertia and their comparison with dynamics observed in liquids, the five wave mixing temporal response has been assigned to adsorbate intermolecular librational motion. Demonstration of this new fourth-order technique allows one, in principle, to extend all third-order nonlinear bulk-phase spectroscopies to surfaces and interfaces with specificity.