Monitoring structural transformations in crystals. 7. 1‐Chloro­anthracene and its photodimer

Crystals of the 1‐chloro­anthracene photodimer, viz. trans‐bi(1‐chloro‐9,10‐di­hydro‐9,10‐anthracenediyl), C₂₈H₁₈Cl₂, were obtained from the solid‐state 4+4]‐photodimerization of the monomer, C₁₄H₉Cl, followed by recrystallization. The symmetry of the product mol­ecules is defined by the orientation of the reactant mol­ecules in the crystal. The mutual orientation parameters calculated for adjacent monomers explain the reactivity of the compound. The mol­ecules in the crystal of the monomer and the recrystallized photodimer pack differently and the photodimer has crystallographically imposed inversion symmetry.