Subpicosecond fluorescence anisotropy measurements are used to characterize the rotational dynamics of coumarin 153 (C153) in 35 common solvents and eight solvent mixtures at room temperature. The rotational anisotropy decays of C153 are generally nonexponential as a result of the non-Markovian nature of the friction on its rotational motion. Rotational correlation times are observed to be larger in polar solvents than in nonpolar solvents of the same viscosity. This difference is examined in the context of theories of dielectric friction, which relate the extra friction in polar solute/solvent systems to long-range dipole -dipole interactions. Since the latter interactions have been thoroughly characterized via dynamic Stokes shift measurements for the same solute/solvent combinations studied here, the present data provide a unique opportunity to test general concepts of dielectric friction. Contrary to expectations, the departures from simple hydrodynamic behavior cannot be modeled using only theories of rotational dielectric friction. More important than dielectric friction is the role that the relative solute/solvent size plays in determining the extent of solute -solvent coupling. Once this size dependence is approximately accounted for, the remaining departures from simple hydrodynamic behavior are relatively small in all solvents. In polar aprotic solvents, solvation data indicate that dielectric friction effects should be rather modest (10-20% of the total friction). In these solvents no clear correlation is found between dielectric friction predictions and the observed solute -solvent coupling. However, in normal alcohol solvents the effects of dielectric friction are predicted to be large and well beyond the scatter in the experimental data. No evidence for such an important dielectric friction contribution is observed in these solvents, in spite of the fact that long-time components of the solvation dynamics do appear to be present in the rotational friction.

Rotational Dynamics of Coumarin 153: Time-Dependent Friction, Dielectric Friction, and Other Nonhydrodynamic Effects

The fluorescence depolarization dynamics of organic fluorescent dye probes (nile red, cresyl violet, DODCI, rhodamine B, and rhodamine DPPE) were studied in cationic, anionic, and neutral micelles by picosecond time-resolved single-photon-counting technique. The fluorescence anisotropy decay of the dye intercalated inside the micelle is a two-exponential function. The anisotropy decay was interpreted by using a model of rotational (wobbling) and translational diffusion of the dye in the micelle coupled with the rotational motion of the micelle as a whole. The rotational and translational diffusion coefficients of the dye, the order parameter, and the semicone angle for the wobbling diffusion in the micelle were determined. The concept of “microviscosity” in the micelle was critically discussed in the light of the rotational and translational diffusion coefficients and their temperature dependence.

Fluorescence Dynamics of Dye Probes in Micelles

Off‐resonant transient birefringence measurements are analyzed using a reduced equation of motion for the ground state density matrix, which is expanded using an effective Hamiltonian. Assuming that the pump field is weak, we express the polarization relevant for the birefringence signal in terms of a convolution of the tensorial polarizability response function with the external fields. The homodyne‐detected birefringence signal is directly compared with the coherent Raman signal. The relationship between off‐resonant birefringence and spontaneous Raman experiments is discussed. By expanding the polarizability in powers of the nuclear coordinates and applying the Brownian oscillator model to the coordinate response function, we separate the birefringence signal into intra‐ and intermolecular coordinate response functions. Off‐resonant transient birefringences of acetonitrile, chloroform, dimethylsulfoxide, and a series of alcohols were measured. The data are transformed to the frequency domain by using a model independent analysis method. The spectra are discussed in the context of various models for the distribution of intermolecular modes (spectral density) in liquids.

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Off-resonant transient birefringence in liquids

The ultrafast optically-heterodyne-detected optical Kerr effect (OHD-OKE) is established as a relatively simple tool for recording the ultrafast dynamics of liquids with high temporal resolution and excellent signal-to-noise ratios. The principles and practice of the OHD-OKE method are outlined. Its application in recording the dynamics of several molecular liquids is described. The data are discussed in terms of the underlying microscopic molecular motions. Orientational motion--both librational and diffusive--is responsible for a significant fraction of the dynamics. Other potential contributions are discussed, but these are less readily assigned. The application of OHD-OKE measurements in interpreting ultrafast studies of the optical dynamics of solutions is discussed. Finally the extension of OHD-OKE methods to record the dynamics of more complex, heterogeneous, media is described.

Optically-heterodyne-detected optical Kerr effect (OHD-OKE): Applications in condensed phase dynamics

We report a comprehensive study of the UV photolysis of I3− in ethanol solution, using femtosecond time resolved transient transmission experiments. We interpret our results to indicate that with high probability, photoexcitation leads to direct formation of di‐iodide ions within 300 fs, which are vibrating coherently. Through our experiments we have been able to determine that the time scales for vibrational dephasing, vibrational relaxation, and reorientation of the fragment ions are 400 fs, 4 ps, and 5 ps, respectively. Transmission signals at 620 nm and at 880 nm, which are above and below the λmax of the known absorption of I2−, oscillate at a precisely opposite phase. This and other results presented indicate that through the oscillations we are observing coherent vibration of the I2− photofragment. UV transient transmission experiments have been conducted in order to characterize the time scales for recombination. Preliminary results show that recombination takes place on several time scales. A fas...

Ultrafast photodissociation of I3. Coherent photochemistry in solution

Rotational diffusion times for three mechanically similar phenoxazine dyes with differing electronic properties were measured. These studies probe the importance of dipole/dipole and ion/dipole couplings to the friction. The experimental results are compared with the predictions of available continuum theories of dielectric friction. Future directions for a more realistic model of dielectric friction are discussed.

A test of continuum models for dielectric friction. Rotational diffusion of phenoxazine dyes in dimethylsulfoxide

An experimental test of dielectric friction models using the rotational diffusion of aminoanthraquinones

The rotational diffusion of three mechanically similar phenoxazine dyes possessing distinct electrical properties was studied in isopropanol. The results, along with previously presented results from other polar solvents, were analyzed in terms of continuum theories for rotational dielectric friction. It was found that a continuum based theory can account for the observed rotational relaxation dynamics, but only with realistic modeling of the solute charge distribution (not a point dipole), and by accounting for both frequency and wave vector dependences of the solvent dielectric properties.

The influence of wave vector dependent dielectric properties on rotational friction. Rotational diffusion of phenoxazine dyes

Polarization spectroscopy has been developed as a useful method for the investigation of molecular reorientation in both liquid phase solutions and in the gas phase. This technique has the advantage of measuring a single particle orientational correlation function directly but the disadvantage of averaging over rotation in all electronic states. Described and characterized herein is a variant of this technique, optically heterodyned polarization spectroscopy, which is able to disentangle various contributions to the signal and determine the rotational relaxation of the solute molecule in different electronic states independently. This work also demonstrates the measurement of the normalized value of the orientational correlation function at time zero, r(0), without extensive normalization of laser parameters. Lastly, various technical advantages of the optically heterodyned method are discussed.

Optically heterodyned polarization spectroscopy. Measurement of the orientational correlation function

The measurement of the rotational relaxation time of seven fluorene derivatives in dimethyl sulfoxide and paraffin oil is used to assess the generality of a recently proposed model of dielectric friction. This work shows that the friction experienced by these solute molecules is reproduced quite well at a qualitative level. Although agreement between the experiment and the model can also be captured by adjusting certain parameters, these adjustments are not always clearly correlated with molecular properties

R. S. Hartman

Papers

A test of continuum models for dielectric friction. Rotational diffusion of phenoxazine dyes in dimethylsulfoxide

An experimental test of dielectric friction models using the rotational diffusion of aminoanthraquinones

The influence of wave vector dependent dielectric properties on rotational friction. Rotational diffusion of phenoxazine dyes

Optically heterodyned polarization spectroscopy. Measurement of the orientational correlation function

A Test of Dielectric Friction Models. Rotational Diffusion of Fluorenes in Dimethyl sulfoxide