Photochemical mechanistic study of hexafluorobenzene involving the low-lying states

The photochemical isomerization and nonradiative decay processes of hexafluorobenzene (HFB) were investigated theoretically to gain insights into its photochemical mechanism and the perfluoro effect. A complete mechanistic scheme is presented through the characterization of all the possible minima and transition states of the S₀, S₁, and S₂ states at the CASPT2/6-311G**//CAS(6,7)/6-31G* level. On the S₀ potential energy surface, HFB could isomerize to three different products [Dewar-HFB (S₀-P₁), benzvalene-HFB (S₀-P₂), and fulvene-HFB (S₀-P₃)]. Following excitation to the S₂ state with the perpendicular π → σ* transition, a chair-type minimum with C_s symmetry was found on the S₂ potential energy surface. The adjacent S₂/S₁ conical intersection was immediately accessible from the S₂ minimum. The nature of the S₁ state was confirmed to have a π → π* character. Both the S₂ and S₁ photochemistries of HFB yielded Dewar-HFB via the S₁/S₀ conical intersection. The regeneration of the S₀ state from the S₁ and T₂ states via intersystem crossing or internal conversion was also revealed.