Molecules rarely reemit from the initially excited level after the absorption of a photon and if the molecule is sufficiently complex, light emission, a slow process, is not the main decay channel for the excited state. This relaxation is essential to maintaining life, since our biological memory, DNA is continuously exposed to light and photochemistry can be harmful to it, in general it is then very rapid. The electronic energy is rapidly degraded and thermalized within the ground electronic state of these systems, preventing most of the photochemistry through dissipation of the energy in the medium. These properties arise from the coupling (essentially non adiabatic) between the electronic configurations of the molecular systems accessible by optical excitation, and the other energetically accessible configurations.
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.
• Interaction laser-matière › Physico-Chimie et Chimie-Physique
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