Fig. 4. (a and d) In the zwitterionic resonance structure rotation around the former double bonds in the polymethine chain becomes possible leading to conical intersection 1 (e) or 2 (b). (c) CASSCF energies (top) and excited state dipole moments (bottom) in the gas phase along linearly interpolated geometries (in internal coordinates) between the Franck–Condon minimum and the two minimum energy conical intersection as indicated on the left and right hand sides.
