Electron time-dependent fluorescence

In the case of electron transfers in solution there appears to be a greater cohesiveness of views, and the need for vibrational assistance is well established for reactions accompanied by vibrational changes (e.g., changes in bond lengths). A detailed analysis of the experiments could be made because of the existence of independent data, which include X-ray crystallography, EXAFS, resonance Raman spectra, time-dependent fluorescence Stokes shifts, among others. 

One may inquire as to what this experience with solutions suggests for the study of reactions in clusters. In the case of electron transfers supplementary information, such as time-dependent fluorescence Stokes shift in clusters, would again be helpful. Equation (2.3) can be modified to include a D(t), as in the isothermal case, if needed from the results of such data. For isomerizations, also, it would be useful to have, for solutions or clusters, detailed analogous data such as the above Stokes shift. However, because of the low intensity of such a fluorescence in this case, such data appear to be absent or scarce. 

Time-resolved fluorescence spectroscopy of polar fluorescent probes that have a dipole moment that depends upon electronic state has recently been used extensively to study microscopic solvation dynamics of a broad range of solvents. Section II of this paper deals with the subject in detail. The basic concept is outlined in Figure 1, which shows the dependence of the nonequilibrium free energies (Fg and Fe) of solvated ground state and electronically excited probes, respecitvely, as a function of a generalized solvent coordinate. Optical excitation (vertical) of an equilibrated ground state probe produces a nonequilibrium configuration of the solvent about the excited state of the probe. Subsequent relaxation is accompanied by a time-dependent fluorescence spectral shift toward lower frequencies, which can be monitored and analyzed to quantify the dynamics of solvation via the empirical solvation dynamics function C(t), which is defined by Eq. (1). 

In the following subsections we will review simulation studies we have performed aiming at investigating the dielectric behavior of SCW and solvation dynamics of excess electrons, making a close connection to recent pulse radiolysis and time-dependent fluorescence experiments. 

Dielectric friction is the measure of the dynamic interaction of a charged or dipolar solute molecule with the surrounding polar solvent molecules. This concept has been applied, by Hynes et al. [339] and others [486], to solvent- and time-dependent fluorescence shifts resulting from the electronic absorption by a solute in polar solvents. If the solvent molecules are strongly coupled to the charge distribution in ground- and excited-state molecules, the relatively slow solvent reorientation can lead to an observable time evolution of the fluorescence spectrum in the nano- to picosecond range. This time-dependent fluorescence (TDF) has been theoretically analysed in terms of dynamic... 

In view of the great importance of chemical reactions in solution, it is not surprising that basic aspects (structure, energetics, and dynamics) of elementary solvation processes continue to motivate both experimental and theoretical investigations. Thus, there is growing interest in the dynamical participation of the solvent in the events following a sudden redistribution of the charges of a solute molecule. These phenomena control photoionization in both pure liquids and solutions, the solvation of electrons in polar liquids, the time-dependent fluorescence Stokes shift, and the contribution of the solvent polarization fluctuations to the rates of electron transfer in oxidation-reduction reactions in solution. 

Time dependent fluorescence depolarization is influenced by the exciton annihilation which occurs in confined molecular domains . Photoemission results from singlet exciton fusion as shown by the excitation intensity dependence which occurs in anthracene crystals. Reabsorption of excitonic luminescence is an effect which has been shown to occur in pyrene crystals. The dynamics of exciton trapping in p-methylnaphthalene doped naphthalene crystals involves phonon assisted detrapping of electronic energy. Ps time resolved spectroscopy was the experimental technique used in this work. 

FIGURE 3.10 Time-dependent fluorescence spectra upon the reaction of photoisomerization of pentadiene-l,3into 3-methylcyclobutene corresponding to transitions from the resonant level to the vibronic levels of the ground electronic states of pentadiene-1,3 (upper spectrum) and 3-methylcyclobutene (lower spectmm).Each spectrum is normalized to the maximum intensity value. The frequency co is counted from the frequency of the 0-0 transition. 

Functionality preferences for metathesis of Ru carbenes to alkenes and alkynes with electronic and steric diversity were studied using time-dependent fluorescence quenching (Scheme 116). ... 

Hydrogen transfer in excited electronic states is being intensively studied with time-resolved spectroscopy. A typical scheme of electronic terms is shown in fig. 46. A vertical optical transition, induced by a picosecond laser pulse, populates the initial well of the excited Si state. The reverse optical transition, observed as the fluorescence band Fj, is accompanied by proton transfer to the second well with lower energy. This transfer is registered as the appearance of another fluorescence band, F2, with a large anti-Stokes shift. The rate constant is inferred from the time dependence of the relative intensities of these bands in dual fluorescence. The experimental data obtained by this method have been reviewed by Barbara et al. [1989]. We only quote the example of hydrogen transfer in the excited state of... 

The present experiments are mute as to the timescale on which delocalization may occur. EPR results on Ru(bpy)"5 demonstrate localization of the bpy electron density in this Ru(II)(bpy)2 (bpy )+ species on the EPR timescale, but suggest that delocalization may occur on a timescale only slightly longer. It is possible that either time-resolved EPR or temperature dependent fluorescence depolarization experiments may establish the time-scale of localization in Ru(bpy) +. 

Electron tunneling from the first singlet excited state of pentacene to duroquinone in sucrose octaacetate glass was discovered [80] by observing the time dependence of fluorescence emission. The tunneling distance,... 

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