Solvent reorganization in optical and thermal electron-transfer processes

Abstract

Dielectric continuum models used to calculate the solvent reorganization energy in thermal and photochemical electron-transfer reactions are discussed. Expressions are derived for the reorganization energy for charge redistribution within a single ellipsoidal or spherical solvent cavity. The expressions for the ellipsoidal cavity model differ in important respects from those derived previously. The Marcus two-sphere and the ellipsoidal cavity model differ in important respects from those derived previously. The Marcus two-sphere and the ellipsoidal cavity models can satisfactorily predict the band maximum for the metal-to metal charge-transfer transition in a number of ion-paired and bridged binuclear systems, provided careful consideration is given to the cavity dimensions. In complexes containing 2,2'-bipyridine ligands the solvent can approach close to the charge centers by filling the space between the ligands: this results in a small effective radius and relatively large values for the solvent reorganization energy. On the other hand, in complexes containing ammine ligands the effective radius is increased due to the effects of dielectric saturation resulting from specific ligand interactions with the solvent. Despite the limitations of the two-sphere model the reorganization energies calculated are, in general, not very different from those obtained with the ellipsoidal cavity model.