Laterally π-Extended Dithia[6]helicenes with Heptagons: Saddle-Helix Hybrid Molecules

A laterally π-extended dithia[6]helicene 1, representing an interesting saddle-helix hybrid molecule containing an unusual heptagon, has been synthesized by MoCl₅-mediated oxidative stitching of tetrakis(thienylphenyl)naphthalene precursor 2 involving reactive-site capping by chlorination and subsequent Pd-mediated dechlorination of tetrachlorinated intermediate 1-Cl₄. Highly distorted, wide helical structures of dithia[6]helicenes (1 and 1-Cl₄) were clarified by single-crystal X-ray diffraction analyses where heterochiral slipped π-π stacking was displayed in a one-dimensional fashion. Notably, theoretical studies on the thermodynamic behavior of 1 predicted an extraordinarily high isomerization barrier of 49.7 kcal·mol^-1, which enabled optical resolution and chiroptical measurements. Electronic structures of these huge helicenes were also examined by photophysical and electrochemical measurements.