| Abstract
| - In recent work, we have demonstrated that the excited-state paths for radiationless deactivation andtrans → cis isomerization for a series of polyenes involve a point of conical intersection between the covalentexcited and ground states. In this paper we show how motion through this point can “trigger” the productionof a transient π-diradical species (on the ground state) featuring an inverted double bond/single bond pattern.The computations have been carried out using an ab initio CAS−SCF approach and a hybrid molecularmechanics−valence bond (MM−VB) force field on three polyenes of increasing chain length: hexa-1,3,5-triene, octa-1,3,5,7-tetraene, and dodeca-1,3,5,7,9,11-hexaene (C12H14). Our computations show that productionof the π-diradical must be a general feature of photoexcited polyenes. The investigation of the geometricaland electronic structure of these species indicates that they correspond to a pair of neutral solitons carryingunpaired electrons. Semiclassical trajectory computations on the “minimal” polyacetylene model C12H14 areused to model the creation, evolution, and annihilation dynamics of these transient entities.
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