Calculation of the solvent reorganization free energy in the dielectric cavity model

Abstract

The solvent reorganization free energy, E s, is an important characteristics that affects the activation energy and spectral properties of different charge transfer processes in polar media. Calculation of E s implies two steps. The first step is evaluation of the charge redistribution upon transition. The second step is calculation of E s for a given charge redistribution. We developed a formalism that allows one to compute the charge redistribution from data of quantum chemical calculations, based on schemes which incorporate equilibrium solvent effects. The concept of transition molecular orbitals (MO) is used which assumes the electronic transition to be a transition of an electron between two transition MO: the highest occupied and the lowest unoccupied MO of the solute. For an electron transfer between different species the transition MO are the highest occupied MO of the donor and the lowest unoccupied MO of the acceptor. The difference between the electronic densities of the two transition MOs gives the charge redistribution. The changes of the other MOs upon transition are described as polarization of an effective electronic continuum of the cavity. The model of a cavity in dielectric continuum is used to describe a solute or a donor-acceptor complex in its reactive configuration. Our description of the solute charge redistribution allows the introduction of a ‘fixed charge density’ formulation for E s in the second step. As an example, application of our formalism to the photoinduced transition between the ground and the lowest excited states of an acridine dye is given. We estimated how strong the influence of the solute wave function's modulation through non-equilibrium environmental polarization is and studied the effect of the coupling between solute polar inertial modes and environmental polarization on the reorganization energy. We found that neglecting any of these effects can results in seriously overestimating the value of the reorganization energy.