7,7,8,8-Tetraaryl-o-quinodimethane Stabilized by Dibenzo Annulation: A Helical π-Electron System That Exhibits Electrochromic and Unique Chiroptical Properties

When two benzene rings are fused to a tetraaryl-o-quinodimethane skeleton, sterically hindered helical molecules 1 acquire a high thermo-dynamic stability. Because the tetraarylbutadiene subunit contains electrondonating alkoxy groups, 1 undergo reversible two-electron oxidation to 2(2+), which can be isolated as deeply colored stable salts. Intramolecular transfer of the point chirality (e. g., sec-butyl) on the aryl groups to helicity induces a diastereomeric preference in dications 2b(2+) and 2c(2+), which represents an efficient method for enhancing circular-dichroism signals. Thus, those redox pairs can serve as new electrochiroptical response systems. X-ray analysis of dication 2(2+) revealed pi-pi stacking interaction of the diarylmethylium moities, which is also present in solution. The stacking geometry is the key contributor to the chirosolvatochromic response.