Environmental effects on a conical intersection: A model study

Excited-state processes at conical intersections (CIs) involving charge transfer phenomena can depend sensitively on the influence of a polar and polarizable environment. We propose here a formulation to describe the chromophore - environment interaction for such situations. In a model study, we focus on an extension of the two-electron two-orbital model by V. Bonacic-Koutecky, J. Koutecky, and J. Michl [Angew. Chem., Int. Ed. Engl., 1987, 26, 170], which yields a diabatic model for the S-1-S-0 CI in protonated Schiff bases and related systems, and describes the charge properties and charge translocation phenomena associated with this CI. The electrostatic effects of the environment, which are expected to strongly affect the CI topology, are accounted for by a dielectric continuum model. This translates to the image of free energy surfaces for the coupled chromophore - environment system represented by molecular coordinates plus a solvent coordinate. The environment's impact on the location and character of the CI is investigated. The limiting situations of "frozen'' and equilibrium solvation effects are examined.