Solvation dynamics around a solute dissolved in water have been measured for the first time on the femtosecond time scale. The measurements were made by the fluorescence time-dependent Stokes shift technique, which employs polar fluorescent probes and ultrafast fluorescence spectroscopy to characterize the solvation dynamics of electronically excited molecules.The probe used in this study is the 7-(dimethylamino)coumarin-4-acetate ion. The observed microscopic solvation dynamics are well modeled by a biexponential decay with lifetimes ( ,, t2) and amplitudes (A¡, A2) as follows: t¡ = 0.16 ps (A¡ = 0.33) and r2 = 1.2 ps (A2 = 0.67). The experimental results have been compared to predictions from the dielectric continuum theory, the linearized MSA theory, and recently published molecular dynamics simulations of water solvation.

in part to a change in configuration to a tetrahedral geometry cannot be determined from these results. Such evidence may be gleaned from future high-temperature spectrophotometric, Raman, NMR, or neutron-scattering experiments.Another consideration for establishing the configuration of the acetate complexes concerns whether this ligand is mono-or bidentate in character. Numerous X-ray crystallographic and spectroscopic data indicate that in the solid state acetate may serve as a bridging ligand,29•30 a bidentate ligand,31-34 a monodentate ligand with partial secondary bonding,35 and a mondentate ligand.29,36•37 However no corresponding results were found for ferrous complexes, and it cannot be assumed that the same bonding will necessarily be preserved in solution. Certainly at 25 °C the formation constants for ferrous complexes with known bidentate ligands such as oxalate and malonate (log X, = 3.02 and 2.24, respectively)38 are somewhat larger than for acetate (1.439), but this cannot be construed as definitive evidence for either mono-

The transient solvation of a polar fluorescent probe has been studied by the time resolved Stokes shift technique with roughly five times shorter time resolution than previously reported. New shorter time components in the solvation relaxation function C(t) have been discovered for methanol, propionitrile, and propylene carbonate; the C(t) function for acetonitrile is singly exponential within the limitations of the instrument. The observed C(t) has been compared to theoretical calculations using the dielectric continuum (DC) model for each solvent, with non-Debye expressions for the solvent dielectric response. For methanol the DC model predictions agree closely with experiment. For the polar aprotic solvents propylene carbonate and propionitrile, the shape of the experimental decay is different from the DC predictions, but the average decay times 〈τs〉 are closer to the DC predictions than previously reported. The comparison of theory and experiment is clearly limited by the inconsistencies and limited frequency range of the dielectric relaxation data found in the literature. The dynamic solvation measurements have also been compared to predictions of the mean spherical approximation as applied to solvation dynamics, which appear to give slower solvation rates than are observed experimentally.

It has been found that the electron-transfer time ret of the intramolecular electron-transfer reaction of electronically excited bianthryl is not equal to the longitudinal relaxation time of the solvent in various polar aprotic solvents. This demonstrates that the recently observed agreement of ret and r¡, in specific cases, is not general. In addition, microscopic solvation times ts have been measured for the solvents, and it has been observed that t, is very similar to for a broad range of polar aprotics.

Surface science model studies of Cs solvation by protic and aprotic solvents J. Vac. Sci. Technol. A 4, 1307 (1986); 10.1116/1.573598Snowballing effect for electron solvation in dilute solutions of polar molecules in nonpolar solvents Subpicosecond fluorescence spectroscopy of the polar dye molecules coumarin 311 and coumarin 102 has been used to measure the microscopic solvation dynamics of several polar aprotic solvents. The measured solvation times are significantly longer than the longitudinal relaxation times 1"1 ofthe solvents. 1"1 is the predicted time for solvation according to dielectric continuum theory. The experiments were made with a newly constructed subpicosecond ultraviolet emission apparatus that takes advantage of recent advances in ultrafast laser technology. The newly developed, time saving procedure [Nagarajan et al., J. Chem. Phys. 86, 3183 (1987)] for measuring the microscopic solvation response functions was used in this research.2372