Force Fields for “Ultrafast” Photochemistry: The S2(1Bu) → S1(2Ag) → S0(1Ag) Reaction Path forall-trans-Hexa-1,3,5-triene

High-level ab initio quantum chemical computations (MC-SCF and multireference Moller-Plesset perturbation theory) have been used to investigate the composite relaxation path on three different singlet electronic states of an isolated all-trans-hexa-l,3,5-triene (trans-HT) molecule: the spectroscopic B-u (valence-ionic) state, the lower lying dark (i.e. symmetry forbidden covalent) 2A(g) state (S-1), and finally the ground state (S-0). Our results support the hypothesis that IVR (internal vibrational energy redistribution) from totally symmetric to non-totally symmetric modes must control the dynamics of ultrafast decay in short all-trans polyenes. The salient features of the reaction path are as follows: (a) Motion out of the S-2 FC region and the subsequent relaxation along the S-1 energy surface lies within the space of totally symmetric deformations of the trans-HT molecular backbone. (b) The triggering of fast S-1 -->S-0 radiationless decay requires a non-totally symmetric deformation of the molecular backbone along a nearly barrierless (+/-2 kcal mol(-1)) path. (c) The molecular structure at the S-1-->S-0 decay channel (i.e. at the S-1/S-0 crossing point) and its subsequent evolution on the relaxation path which develops along the S-0 energy surface indicate that reactant back-formation must be the favored process.