Charge transfer dynamics in photophysical systems: A case of saddle point avoidance

The simple bidimensional model of a reactive coordinate coupled to a harmonic mode is known to display varied dynamical behaviour, due to the interplay of energetic and frictional factors. Molecular systems which exhibit the photophysical process of intramolecular charge transfer generally are characterized by an internal coordinate coupled to the solvent polarization. They therefore provide an appealing application of the dynamic model, by identifying a solute variable with an intrinsic timescale tau(R), and a macroscopic solvent variable with relaxation time tau(S). In this paper, a theoretical analysis based on a bidimensional diffusion equation suggests that time-dependent Stokes shifts observed in system undergoing intramolecular charge transfer processes may provide evidence of a kinetic pathway which implies saddle point avoidance.