Excited-State Charge Transfer at a Conical Intersection:  Effects of an Environment

The influence of a polar and polarizable environment on charge transfer processes at a conical intersection (CI) can be described by a diabatic free energy model yielding coupled surfaces as a function of both molecular coordinates and a solvent coordinate. We extend and apply this model for the S₁−S₀ CI in protonated Schiff bases, representing a model for retinal isomerization (Faraday Discuss. 2004, 127, 395, 2004). A dielectric continuum description of the solvent is combined with a minimal, two-electron-two-orbital electronic structure model according to Bonačić-Koutecký, Koutecký, and Michl (Angew. Chem. 1987, 26, 170), which characterizes the charge translocation effects at the CI. The model predicts that the nonequilibrium solvent state resulting from the S₀ → S₁ Franck−Condon transition can entail the disappearance of the CI, such that solvent motion is necessary to reach the CI seam. The concerted evolution of the intramolecular coordinates and the solvent coordinate is illustrated by an excited-state minimum energy path.