Theoretical model of photoinduced intramolecular charge transfer processes

Molecular systems which undergo upon photoexcitation intramolecular charge transfer processes accompanied by conformational changes are known to exhibit complex spectroscopic and dynamical behaviour. Typical spectroscopic effects are dual fluorescence and pronounced solvatochromism. Dynamical features are revealed by the temperature dependence of fluorescence emission intensities and shifts under stationary conditions; the precursor-successor behaviour for the observed decay of the high-frequency band and the rise of the low-frequency band, after pulse excitation in correspondence of the high-frequency absorption; the fluorescence depolarization effects; the dynamic Stokes shifts measured in time-dependent experiments. All these effects can be rationalized by a stochastic model in which coupling of the fluorescent probe with the polar environment is explicitly taken in account. Ingredients of the model are the potential energy surface for the probe internal coordinates, and hydrodynamic and dielectric properties of the solvent. The model reduces to the simple kinetic scheme based on two-state interconversion process only in the limiting case of a relatively high energy barrier separating the two emitting states, and rapid equilibration of the polar solvent with respect to the instantaneous probe dipole moment.