THEORETICAL-MODEL OF PHOTOINDUCED INTRAMOLECULAR CHARGE-TRANSFER PROCESSES

Molecular systems which undergo upon photoexcitation intramolecular ch arge transfer processes accompanied by conformational changes are know n to exhibit complex spectroscopic and dynamical behaviour. Typical sp ectroscopic effects are dual fluorescence and pronounced solvatochromi sm. Dynamical features are revealed by the temperature dependence of f luorescence emission intensities and shifts under stationary condition s; the precursor-successor behaviour for the observed decay elf the hi gh-frequency Sand and the rise of the low-frequency band, after pulse excitation in correspondence of the high-frequency absorption; the flu orescence depolarization effects; the dynamic Stokes shifts measured i n time-dependent experiments. All these effects can be rationalized by a stochastic model in which coupling of the fluorescent probe with th e polar environment is explicitly taken in account. Ingredients of the model are the potential energy surface for the probe internal coordin ates, and hydrodynamic and dielectric properties of the solvent. The m odel reduces to the simple kinetic scheme based on two-state interconv ersion process only in the limiting case of a relatively high energy b arrier separating the two emitting states, and rapid equilibration of the polar solvent with respect to the instantaneous probe dipole momen t. (C) 1997 Elsevier Science S.A.