Time-resolved spectra, solvent relaxation

Let us first consider that the characteristic time of the solvent relaxation, Tr, is much larger than t. In such a case, no spectral change due to the interaction with the solvent can be observed in the emission spectrum because the de-excitation of the fluorophore occurs prior to the solvent relaxation. In the opposite case when Tr>>t, the emission occurs from the fully relaxed state. The observed red shift of the emission spectrum is independent on the time after excitation and reflects only the strength of the dipole-dipole interaction between the fluorophore and the surrounding solvent molecules. The most interesting situation occurs when the characteristic time of the solvent relaxation is comparable to t. In such a case, the red shift of the spectrum will increase with the time after excitation as it will correspond to the emission from more relaxed states. Hence, the dynamics of the solvent molecules around the fluorophore is translated into the time dependence of the maximum and the half-width of the emission band which can be followed by the time-resolved emission spectroscopy measurements. 

The total transient Stokes shift (v(O)-v(oo)) observed in our time resolved experiments of coumarin in bulk water was 820 cm"1. In the case of C343 adsorbed on Z1O2 it is 340 cm 1. From measurements of the time-zero spectrum, i.e. the emission spectrum of C343 before solvent relaxation, Maroncelli et al. estimated the Stokes shift from solvation to be 1953 cm 1 for C343 in water [8]. Thus the time resolution of our experiments allows to observe about 42% of the total solvation process. Especially the very initial part, containing the inertial response is missed. 

We have performed picosecond time resolved absorption spectroscopy for organic dyes in alcoholic solution and have shown the following results. The recXral shape of the difference spectrum before and after the excitation is expressed as the superposition of the absorption and fluorescence spectra detected under steady state condition when the solvent relaxation time is sufficiently short compared with the time resolution of the experimental equipment and the excited state lifetime. On the other hand, the spectrum in the viscous solvent at low temperature shows slightly sharp in initial and broadens its shape with time. 

Figure 5. Hme-resolved spectra of C1S3 in dioxane and benzene (connected points). The times shown are 0,0.1,0.5,1,2, and 10 ps from tight to left. The dashed curves are the steady-state emission spectra (red curve) and the spectrum expected prior to any solvent relaxation (blue curve see Ref. 17).

A map of the singlet-singlet excitation and photoisomerization potential energy surface for tetraphenylethylene in alkane solvents were prepared using fluorescence and picosecond optical calorimetry (Figure 3.4) [4]. The line shapes of the vertical and relaxed exdted-state emissions at 294 K in methylcyclohexane were obtained from the steady-state emission spectrum, the wavelength dependence of the time-resolved fluorescence decays, and the temperature dependence of the vertical and relaxed state emission quantum yields and of the time-resolved fluorescence decays. 

In Time Resolved Fluorescence (TRF) experiments, depicted schematically in Figure 12, the emission spectrum line shape changes from a peak value of AE(0) to the final AE(oo) peak of the equilibrium emission spectrum as the solvent responds to the new solute electronic state." " " Experimental results in bulk liquids show that the nonequilibrium correlation functions initially exhibit a very fast (less than 50 fs" ) inertial component, which may account for 60-80% of the total relaxation in water. This is followed by a multiexponential relaxation on the subpicosecond to picosecond timescale," corresponding to reorientation and translation of solvent molecules, or, to particular intramolecular solvent modes" " around the solute. Slower dynamics are found in more viscous liquids." ... 

Better separation of poorly resolved signals can obviously be achieved by measuring a spectrum at higher field. However, because increased relaxation-times result in sharper lines,18 the resolution can also be improved by using a low concentration, a high temperature, and a nonviscous solvent (for example, acetone). Besides, the use of a... 

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