Picosecond solvation dynamics of coumarin 153: The importance of molecular aspects of solvation

Solvation dynamics in polar liquids have been examined using the probe molecule coumarin 153 (Cu153) and picosecond spectroscopic techniques. Steady-state absorption and fluorescence spectra of Cu153 as a function of solvent show that the frequency of the electronic spectrum of this probe provides a convenient measure of solvation energetics. Both nonspecific dipolar and to a smaller degree H-bonding solute–solvent interactions are involved. Time-correlated single photon counting was used to observe time-dependent shifts of the fluorescence spectrum of Cu153 in a variety of alcohols, propylene carbonate, and N-methylpropionamide solvents as a function of temperature. These time-dependent spectral shifts provide a direct measure of the time dependence of the solvation process. Theoretical models that treat the solvent as a dielectric continuum do not adequately account for the observed solvation dynamics. In the solvents studied, such theories predict a single exponential shift of the fluorescence spectrum with a time constant equal to the longitudinal relaxation time (τL) of the solvent. We find that solvation is nonexponential in time and that the average time constant observed is, in general, greater than τL. As the dielectric constant of the solvent increases the disagreement between the observed and predicted solvation times becomes more pronounced. For N-methylpropionamide (ε0∼300 at 245 K) solvation is observed to occur 15 times slower than predicted. These results are discussed in terms of the importance of general molecular aspects of solvation not included in a continuum description. The rotational dynamics of Cu153 in a number of polar solvents has also been examined using time-resolved fluorescence anisotropy measurements. In addition to the ‘‘normal’’ diffusive rotational dynamics we observe a much faster component of the anisotropy decays in polar solvents. The latter rotational component is correlated to the observed solvation time and appears to be due to rapid rotation of the transition dipole of Cu153 in response to changes in the solvent environment.