CALCULATION OF THE SOLVENT REORGANIZATION FREE-ENERGY IN THE DIELECTRIC CAVITY MODEL

The solvent reorganization free energy, E(s), is an important characte ristic that affects the activation energy and spectral properties of d ifferent charge transfer processes in polar media. Calculation of E(s) implies two steps. The first step is evaluation of the charge redistr ibution 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 ca lculations, based on schemes which incorporate equilibrium solvent eff ects. The concept of transition molecular orbitals (MO) is used which assumes the electronic transition to be a transition of an electron be tween two transition MO: the highest occupied and the lowest unoccupie d MO of the solute. For an electron transfer between different species the transition MO are the highest occupied MO of the donor and the lo west unoccupied MO of the acceptor. The difference between the electro nic densities of the two transition MOs gives the charge redistributio n. The changes of the other MOs upon transition are described as polar ization of an effective electronic continuum of the cavity. The model of a cavity in a dielectric continuum is used to describe a solute or a donor-acceptor complex in its reactive configuration. Our descriptio n of the solute charge redistribution allows the introduction of a 'fi xed charge density' formulation for E(s) in the second step. As an exa mple, application of our formalism to the photoinduced transition betw een the ground and the lowest excited states of an acridine dye is giv en. 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 mode s and environmental polarization on the reorganization energy. We foun d that neglecting any of these effects can result in seriously overest imating the value of the reorganization energy.