A key question concerning optically active molecules is what is dynamics of the excited states?
Specifically, what are the predominant non-radiative decay mechanisms. The schematic below shows several options for the energy of the potential energy surfaces versus some configurational co-ordinate. On the left both S1 and S2 excited states decay to a conical intersection with the ground state. In contrast, on the right they have distinctly different decay paths.
But how does one go beyond such schematics. It turns out that for a broad class of dyes one can justify from high level quantum chemistry calculations a description in terms of just three valence bond states (see below).
The description in terms of the three diabatic states allow us to consider a somewhat "generic" or minimal model which exhibits a number of significant features, including:
- Conical intersections between the S1 and S0 surfaces only occur for large twist angles.
- In contrast, S2/S1 intersections can occur near the Franck-Condon region.
- When the molecule has left-right symmetry, all intersections are associated with con- or dis-rotations and never with single bond twists.
- For asymmetric molecules (i.e. where the bridge couples more strongly to one end) then the S2 and S1 surfaces bias torsion about different bonds.
- Charge localization and torsion pathway biasing are correlated.
BTW, if you look at the paper look at the great job Seth did at writing Informative section headings!