Conical Intersections and Charge Localization in a Minimal Model of a Monomethine Dye System: Degenerate Case

We investigate a minimal valence-bond-based model Hamiltonian for photoisomerization of monomethine dyes. These include a broad class of large conjugated organic molecules including diarylmethane and cyanine dyes, and the chromophores of fluorescent proteins. The model fulfills the most basic expectations for a double bond photoisomerization model: 1) it describes favorable twisting in the excited state and 2) it describes conical intersections which can mediate radidationless decay. We examine the topology, topography and extent of the conical intersection seams. Important properties of the model include 1) two regimes of the coupling ratio where the first excited state intersects the ground and a higher excited state along orthogonal twisting coordinates or along the same coordinate 2) first and higher excited state intersections occurring in regions of little or no twisting and 3) charge localization connected with excited-state twisting which changes discontinuously across a conical intersection. The model is promising as a simple extension of empirical force fields in order to describe photoisomerization processes in methine-bridged systems and as a starting point for more elaborate models which may include antisymmetric internal couplings or which may be interfaced with environmental or quantum dynamical models.