Development of a synthetic route to polyfluoroacetylene – a conductive, organic polymer - has been a persistent challenge since it was first predicted to be a more stable and, potentially, more conductive alternative to polyacetylene. A recent revelation has exploited the mechanochemical unzipping of ladderenes which are in turn formed by photochemical 4π-electrocyclic ring-closing reactions at 255 nm: chemistry which is, under these mild conditions, unique to perfluorocarbons.
Using a combination of ultrafast time-resolved spectroscopy, multiconfigurational calculations and non-adiabatic dynamics simulations we have studied the photochemical isomerization of hexafluorobenzene into Dewar-hexafluorobenzene – the prototypical ring closing reaction in ladderene formation. We have identified that the stabilization of σ type orbitals by the electron-withdrawing character of the fluorines - the so called “perfluoro effect” - reduces the energy gap between the ¹B_2u (ππ*) and ¹E_1g (πσ*) electronically excited states. These two states vibronically couple along vibrational modes of e_2u symmetry softening the ¹B_2u state along this coordinate. This perturbation of the potential energy surface creates a barrier towards formation of the hexafluoro-benzvalene isomer and stabilizes the conical intersection which leads to Dewar-hexafluorobenzene formation thus promoting the photochemically productive pathway and enforcing chemoselectivity.

Jordan Cox

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