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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.

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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-

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Preferential solvation of solvatochromic probe coumarin 153 has been studied in toluene−acetonitrile and toluene−methanol solvent mixtures. Solvatochromic shifts of absorption and fluorescence spectra of coumarin 153 in these nonideal systems are analyzed as a function of solvent polarity. There is no apparent preferential solvation of coumarin 153 in the ground state. For the excited-state molecule preferential solvation is observed in toluene−acetonitrile mixtures and it is confirmed by a theoretical model. For the excited C153 in toluene−methanol mixtures strong nonlinearity in the fluorescence solvatochromic shifts is attributed mainly to formation of a hydrogen bond between methanol and C153. IR spectra of C153 in solvent mixtures containing methanol confirm formation of a hydrogen bond between methanol and C153. The presence of a hydrogen-bonded C153 in toluene−methanol mixtures causes an unusual dependence of the C153 fluorescence lifetime on the mole fraction of methanol in solution.

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