Computational Evidence in Favour of a Two-State Two-Mode Model of the Retinal Chromophore Photoisomerization

In this paper we use ab initio multiconfigurational second-order perturbation theory to establish the intrinsic photoisomerization path model of retinal chromophores. This is accomplished by computing the ground state (S0) and the first two singlet excited-state (S1, S2) energies along the rigorously determined photoisomerization coordinate of the rhodopsin chromophore model 4-cis-γ-methylnona-2,4,6,8-tetraeniminium cation and the bacteriorhodopsin chromophore model all-trans-hepta-2,4,6-trieniminium cation in isolated conditions. The computed S2 and S1 energy profiles do not show any avoided crossing feature along the S1 reaction path and maintain an energy gap >20 kcal·mol-1. In addition, the analysis of the charge distribution shows that there is no qualitative change in the S2 and S1 electronic structure along the path. Thus, the S1 state maintains a prevalent ionic (hole-pair) character whereas the S2 state maintains a covalent (dot-dot) character. These results, together with the analysis of the S1 reaction coordinate, support a two-state, two-mode model of the photoisomerization that constitutes a substantial revision of the previously proposed models.