Volume change during thermal [4 + 4] cycloaddition of [2.2] (9,10)anthracenophane

We show that the reaction path connecting the tethered bi(anthracene-9,10-dimethylene) and its photodimer proceeds stepwise via a diradicaloid transition state where one σ-bond is made before a second. The newly found transition state (TS) has a smaller molecular volume than either the reactant or the product giving an atomistic explanation to the recently found pressure catalyzed barrier lowering and rate enhancement. The density functional methods used include long-range contributions as required in a system where the dispersion interactions are significant. We discuss this transformation in the context of the diamond-to-graphite transition owing to the similarity of σ-bond breakage into a delocalized π-system. We also comment on the controversy surrounding the equilibrium geometries of photoisomerized cyclophanes, concluding that D2h symmetry in the photoisomer of the title molecule is a transition structure connecting a pair of degenerate ground state D2 geometries.