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 thermodynamic stability. Because the tetraarylbutadiene subunit contains electron-donating alkoxy groups, 1 undergo reversible two-electron oxidation to 2²⁺, 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 2 b²⁺ and 2 c²⁺, 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 2²⁺ revealed π-π stacking interaction of the diarylmethylium moieties, which is also present in solution. The stacking geometry is the key contributor to the chirosolvatochromic response.