Stokes shift, time dependence

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... 

Figure 3.17 presents ps-TR spectra of the olehnic C=C Raman band region (a) and the low wavenumber anti-Stokes and Stokes region (b) of Si-rra i-stilbene in chloroform solution obtained at selected time delays upto 100 ps. Inspection of Figure 3.17 (a) shows that the Raman bandwidths narrow and the band positions up-shift for the olehnic C=C stretch Raman band over the hrst 20-30 ps. Similarly, the ratios of the Raman intensity in the anti-Stokes and Stokes Raman bands in the low frequency region also vary noticeably in the hrst 20-30 ps. In order to better understand the time-dependent changes in the Raman band positions and anti-Stokes/Stokes intensity ratios, a least squares htting of Lorentzian band shapes to the spectral bands of interest was performed to determine the Raman band positions for the olehnic... 

All these features were observed experimentally for solutions of 3-amino-/V-methylphthalimide, 4-amino-/V-methylphthalimide, and for nonsubstituted rhoda-mine. The results were observed for cooled, polar solutions of phthalimides, in which the orientational relaxation is delayed. Exactly the same spectral behavior was observed [50] by picosecond spectroscopy for low viscosity liquid solutions at room temperature, in which the orientational relaxation rate is much higher. All experimental data indicate that correlation functions of spectral shifts Av-l(t), which are used frequently for describing the Time Dependent Stokes Shift, are essentially the functions of excitation frequency. 

Apart from pure benzene and pure polar solvents, either acetonitrile or methanol, we have considered xp = 0.2 and xp = 0.7 molar fractions of the polar solvent. Systems ranging from 256 (pure benzene) to 512 (pure polar solvents) molecules were used. From well equilibrated (1 ns) simulations with the coumarin in the ground state So, one to two hundred equally distant configurations were selected. In these configurations the coumarin state was switched to the Si state and the solvent was let to relax in a series of 10 ps long NVE simulations. The solvent response was monitored using the normalized time-dependent stokes-shift function ... 

In order to gain some microscopic insight into the mechanisms giving rise to the observed time-dependent Stokes-shift, we have calculated the benzene center-of-mass and acetonitrile... 

In this work we presented the results of Molecular Dynamics simulations performed to study the solvatochromism and the dynamic stokes-shift of coumarin 153 in mixtures of solvents. We showed the ability of MD to reproduce available data of the time-dependent Stokes-shifts. Moreover, MD allowed us to interpret these dynamics in benzene-acetonitrile mixtures in terms of motions of benzene around the coumarin or rotation of acetonitrile. The role of benzene in the solvation process of Cl53 seems to be more important than usually assumed. 

Figure 3 Normalized time dependent Stokes shift (TDSS) and NO ground and excited states transition energy correlation functions (ECF) for the low and room temperature state points.

The method is based on the ability of the excited state of suitable chromophores to both drive and respond to motions in their immediate environment. For example, coumarin excited states have a large dipole moment that causes nearby charged groups to move to stabilize the dipole. As these groups move and reduce the energy of the excited state, the fluorescence shifts toward the red. In simple solutions, this time-resolved Stokes shift (TRSS) experiment measures the time-dependent polarity of the solvent surrounding the coumarin [9]. 

Thus we see that the first moment of the spectral density multiplied by h is the reorganization energy (i.e., one half of the Stokes shift magnitude), whereas the time dependence of the first moment of p(w) corresponds to the fluorescence Stokes shift. Thus the time dependence of S t) is determined entirely by the spectral density. At high temperature [i.e., when p(w) contains frequencies less than 2kBT], S(t) becomes the classical correlation function [36] used by many previous authors [7-10], This follows from... 

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. 

The Practical Determination of C(0- The time-dependent fluorescence Stokes shift of the spectrum should manifest itself as (i) a rapid decay in the fluorescence intensity on the blue edge of the fluorescence spectrum, (ii) a... 

TABLE 2 Low Temperature Solvation Dynamics Determined from Time-Dependent Fluorescence Stokes Shift Measurements... 

Because of the nonlinearity of CRS, conventional wisdom states that high input field peak amplitudes, which are readily provided by ultrashort laser pulses, are required to induce a strong nonlinear polarization (cf. (6.1)— (6.3)). The use of transform-limited femtosecond pulses, however, results in a broad distribution of Raman shifts, uy, — ujs, that by far exceeds the line width of a typical Raman resonance ( —-10 r-rn ) in condensed-phase samples at room temperatures. This is schematically depicted in Fig. 6.2A where the frequency-time dependences of a transform-limited pump and Stokes... 

Fig. 6.2. Illustration of the frequency-time dependences of pump and Stokes pulses in three different CRS excitation pulse schemes and their corresponding spectral resolution of Raman shifts. A Using a pair of transform-limited femtosecond pulses of broad spectral and narrow temporal widths results in a broad bandwidth of Raman shifts that exceeds the line width of a single Raman resonance. B Using transform-limited picosecond pulses of broad temporal and narrow spectral width readily provides high spectral resolution matching the Raman resonance line width to be probed. Selection of a Raman resonance shifted by AQr is achieved by tuning the frequency of one of the laser beams by the same amount. C Spectral focusing of a pair of identically linear chirped pump and Stokes femtosecond pulses results in a narrow instantaneous frequency difference in the CRS process, thus also providing narrow-bandwidth CRS excitation. Selection of a Raman resonance shifted by AQr is achieved by adjusting the time delay At between the pulses. Shifted pulses in (B) and (C) are depicted hatched...

Absorption maxima of the open-ring form in benzene, THF, and acetonitrile were observed in the wavelengths ranging from 335 to 340 nm. Although the maximum showed a small hypsochromic shift in hexane, the solvent shift in the absorption spectrum was rather small. On the other hand, the fluorescence spectra showed remarkable Stokes shifts depending on the solvent polarity. The maximum at 488 nm in hexane shifted to 560 nm in THF. At the same time, the intensity decreased. The fluorescence intensity in acetonitrile was <1% of the intensity in hexane. The results indicate that the excited state of the open-ring form has a polar structure with a large dipole moment. 

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