Conical intersections, charge localization, and photoisomerization pathway selection in a minimal model of a degenerate monomethine dye.

We propose a minimal model Hamiltonian for the electronic structure of a monomethine dye, in order to describe the photoisomerization of such dyes. The model describes interactions between three diabatic electronic states, each of which can be associated with a valence bond structure. Monomethine dyes are characterized by a charge-transfer resonance; the indeterminacy of the single-double bonding structure dictated by the resonance is reflected in a duality of photoisomerization pathways corresponding to the different methine bonds. The possible multiplicity of decay channels complicates mechanistic models of the effect of the environment on fluorescent quantum yields, as well as coherent control strategies. We examine the extent and topology of intersection seams between the electronic states of the dye and how they relate to charge localization and selection between different decay pathways. We find that intersections between the S(1) and S(0) surfaces only occur for large twist angles. In contrast, S(2)/S(1) intersections can occur near the Franck-Condon region. When the molecule has left-right symmetry, all intersections are associated with con- or disrotations and never with single bond twists. For asymmetric molecules (i.e., where the bridge couples more strongly to one end) the S(2) and S(1) surfaces bias torsion about different bonds. Charge localization and torsion pathway biasing are correlated. We relate our observations with several recent experimental and theoretical results, which have been obtained for dyes with similar structure.