Arrested Handedness and Disordered Stacking in Crystals of the Pre-helical Molecule 7,8-Dioxa[6]helicene

A characteristic feature of many molecules of biological origin is their 'handedness' or enantiomorphism. L. Pasteur prepared and separated enantiomeric crystals in 1848.(1) Since then, manifestations of symmetry breaking have attracted much attention.(2,3) Simple models of the new horseshoe shaped molecule, 7,8-dioxa[6]helicene ( 1), indicated a low inversion barrier between helical enantiomers. A quasi-planar pi system capable of stack formation was also observed. The molecule in question is a so-called helicene. Aromatic helicenes and heteroaromatic helicenes constitute a class of molecules renowned for their characteristic helical structure and their high specific optical rotation of the separated C-2 symmetric individual enantiomers.(4) With the aim of investigating how the conflict between the tendency to stack and the pre-helicity of the molecules influences the solid state structure, we prepared single crystals of compound 1 and investigated the crystal structure by synchrotron X-ray diffraction.