Multiconfigurational Dynamics Explain Photochemical Reactivity and Torquoselectivity Towards Fluorinated Polyacetylenes

The discovery of the conductivity of polyacetylene ignited the field of organic electronic materials. Functionalizing polyacetylenes with electron withdrawing groups (e.g., fluorine), has theoretically been shown to increase the air-stability of PAs and open new avenues in organic electronics. Burns and coworkers recently reported a novel synthetic route to fluorinated polyacetylenes which utilizes as a key step the completely stereoselective photochemical electrocyclic ring-closing of hexafluorinated dienes. This photochemical torquoselective (photo-torquoselective) reaction is, to our knowledge, the first of its kind. While the torquoselectivity model (Houk and co-workers) describes the stereospecificity of thermal electrocyclic reactions, no such reactivity model exists for their photochemical counterpart. We have used multiconfigurational quantum chemical calculations and ab initio molecular dynamics simulations to describe this reaction and to determine the origin of its stereoselectivity. We show that the reaction proceeds through the S1 excited state with a lifetime of 988 fs. This reaction lies along an energetically unfavorable pathway which results in a reaction quantum yield of approximately 0.9%. We predict that the reaction pathway to the unobserved product lies 0.2 eV (4.6 kcal mol−1) higher in energy than the pathway to the observed isomer. The unobserved isomer brings adjacent fluorine substituents to within 2.67 and 2.60 Å of each other at the S1/S0 crossing point, resulting in a closed-shell repulsion. This repulsion is responsible for increasing the energy of the unobserved pathway which directs the reaction exclusively to the observed product.