Relaxation paths and dynamics of photoexcited polyene chains: Evidence for creation and annihilation of neutral soliton pairs

In recent work, we have demonstrated that the excited-state paths for radiationless deactivation and trans --> cis isomerization for a series of polyenes involve a point of conical intersection between the covalent excited and ground states. In this paper we show how motion through this point can "trigger" the production of a transient pi-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 molecular mechanics-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 production of the pi-diradical must be a general feature of photoexcited polyenes. The investigation of the geometrical and electronic structure of these species indicates that they correspond to a pair of neutral solitons carrying unpaired electrons. Semiclassical trajectory computations on the "minimal" polyacetylene model C12H14 are used to model the creation, evolution, and annihilation dynamics of these transient entities.