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 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 2 b(2+) and 2 c(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 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.

First stable 7,7,8,8-tetraaryl-o-quinodimethane: isolation, X-ray structure, electrochromic response of 9,10-bis(dianisylmethylene)-9,10-dihydrophenanthrene.

Oxidative cyclization of 2,2'-bis(dianisylethenyl)biphenyl yielded the dicationic salt of phenanthrene-9,10-diylbis(dianisylmethylium), which in turn afforded the severely congested title molecule as the first stable tetraaryl-o-quinodimethane derivative upon reduction.