However in most systems this results in a non-radiative relaxation scheme akin to a tunneling effect that generates a fairly large decay time in the picosecond time domain allowing for photochemistry. Only recently, a direct mechanism has been observed experimentally which can allow an extremely fast relaxation in molecules: it involves a crossing between surfaces, named conical intersection. This intersection or funnel connects directly the relevant excited states. This process provides a direct descent to the lowest energy surface guided by the gradients along these surfaces. In order to maintain a generally fast decay rate, as observed in a variety of molecules such as ethylene–like compounds, a complementary, general mechanism must be added to increase the relaxation. We propose here that such a mechanism exists and involves mediating states or surfaces allowing a more direct connection between the relevant excited surfaces. We have identified here these states in the case of a specifically selected ethylenic molecule.
Ethylene has long been recognized as displaying a conical intersection between the valence ΠΠ* state (V of C•-C• biradical type) and a Zwitterionic state (Z, C+-C-), the latter intersects with the ground state, N and causes the dissipation of the electronic energy. The V-Z intersection is caused by the different symmetry of the two states, in planar configuration of the ethylene, this is 1B1u for the valence (ΠΠ*) and 1Ag for the ((Π*)2) configuration that correlates with the Zwitterionic state. We have chosen here a case molecule, a substituted ethylene, tetrakis dimethylamino ethylene TDMAE, known for its low ionization potential (5.4 eV), due to its dimethylamino groups.
J. Am. Chem. Soc. 127(10), 2005, 16529 - 16534