Photoinduced charge transfer PCT and solvent relaxation

It is important to note that the rate of solvent relaxation depends on the solvent viscosity. If the time required for the reorganization of solvent molecules around 

Finally, if the solvent reorganization time is of the order of the excited-state lifetime, the first emitted photons will correspond to wavelengths shorter than those emitting at longer times. In this case, the fluorescence spectrum observed under continuous illumination will be shifted but the position of the maximum cannot be directly related to the solvent polarity. 

It should be recalled that, in polar rigid media, excitation on the red-edge of the absorption spectrum causes a red-shift of the fluorescence spectrum with respect to that observed on excitation in the bulk of the absorption spectrum (see the explanation of the red-edge effect in Section 3.5.1). Such a red-shift is still observable if the solvent relaxation competes with the fluorescence decay, but it disappears in fluid solutions because of dynamic equilibrium among the various solvation sites. 

The solvent, propanol at —70 °C, is viscous enough to permit observation of solvent relaxation in a time range compatible with the instrument response (FWHM of 5 ns). 

The shift of the fluorescence spectrum as a function of time reflects the reorganization of propanol molecules around the excited phthalimide molecules, whose dipole moment is 7.1 D instead of 3.5 D in the ground state (with a change in orientation of 20°). The time evolution of this shift is not strictly a single exponential. 

In contrast, at room temperature, the reconstructed fluorescence spectra were found to be identical to the steady-state spectrum, which means that solvent relaxation occurs at times much shorter than 1 ns in fluid solution. 

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