Time dependent Stokes’ shift

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.

Here, S is the Huang-Rhys factor [63], which is related to the intensity of the 0-n vibronic transition, Io-n = e sSn/n, and reflects the time-dependent Stokes shift associated with a given type of vibrational mode (e.g., S 0.6 for the high-frequency C=C stretch modes [61,64,65]). For the class of systems studied here, two types of phonon modes are considered per monomer unit, i.e., high-frequency C=C stretch modes and low-frequency ring-torsional modes. 

F. Ingrosso, B. Mennucci and J. Tomasi, Quantum mechanical calculations coupled with a dynamical continuum model for the description of dielectric relaxation time dependent Stokes shift of coumarin Cl53 in polar solvents, J. Mol. Liq., 108 (2003) 21 -6. 

C. P. Hsu, X. Y. Song and R. A. Marcus, Time-dependent Stokes shift and its calculation from solvent dielectric dispersion data, J. Phys. Chem. B, 101 (1997) 2546-51. 

One of the applications of the TDPCM model is the calculation of experimental observable the time dependent Stokes shift S(t) (TDSS). In experiments, the time evolution of the solvent orientational response is evaluated from the time dependent shift of the solute maximum fluorescence signal v(t) with respect to its equilibrium value v (oo) [45] ... 

We used transient hole burning to measure the solvation dynamics (Fig. 1).[3-S] These experiments yield both a time-dependent Stokes shift and a time-dependent width, both of which are related to the solvation dynamics. Here we consider the time-dependent Stokes shift. The time-dependent widths will be discussed elsewhere.[9]... 

It is clear from these results that there is an approximate correlation between the value of the average relaxation time Xs and the longitudinal relaxation time Xl (see section 4.7). However, careful examination of the time-dependent Stokes shift (TDSS) data reveals that C(t) is described by more than one relaxation process. This is not difficult to understand, considering that many aprotic solvents are found to have more than one relaxation process. This can be attributed to the formation of dimers and other aggregates in these liquids. This is especially the case in aprotic solvents such as DMSO and PC, which have very high dipole moments. Alcohols also have multiple relaxation processes due to the presence... 

Table 7.5 Average Relaxation Times from Time-Dependent Stokes Shifts (TDSS) for Coumarin Cl 52 in Various Solvents Together with the Longitudinal Relaxation Time Determined by Dielectric Relaxation Spectroscopy [20]...

Figure 3 (a) Change in energy of the solute due to solvation, with increase in time ( h> h). (b) Time-resolved emission spectra displaying time-dependent Stokes shift. 

It is observed that in the nematic phase of a liquid crystal, the solvation dynamics of coumarin 503 are biexponential [184a]. The slowest time constant decreases from 1670 ps at 311.5 K to 230 ps at 373 K. The solvation time is not affected by the nematic-isotropic phase transition. Thus, it appears that the local environment and not the long-range order controls the time-dependent Stokes shift. A theoretical model has been developed to explain the experimental findings. This model takes into account the reorientation of the probe as well as the fiuctuation of the local solvent polarization. Similar results are also obtained for rhodamine 700 in the isotropic phase of octylcyanobiphenyl [184b]. 

For monitoring the relaxation process, we used two parameters of the time-resolved emission spectrum (a) the full width in the half-maximum (fwhm) at the time t after excitation, 5(t), and (b) the correlation function, C(f), corresponding to the normalized time-dependent Stokes shift [130] ... 

Middelhoek ER, Vandermeulen P, Veihoeven JW, Glasbeek M (1995) Picosecond time-dependent stokes shift studies of fluoroprobe in liquid solution. Chem Phys 198(3) 373-380. doi 10.1016/0301-0104<95)00219-e... 

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