Electrochromic and unique chiroptical properties of helically deformed tetraarylquinodimethanes generated from less-hindered dicationic precursors upon reduction

Electron-donating 1,1,4,4-tetraarylbutadiene is a representative electrochromic dye, and the same chromophore can be found in 9,10-bis(diarylmethylene)-9,10-dihydrophenanthrene (dibenzo-oQD) albeit in a fixed s-cis geometry. Unlike thermodynamically unstable 7,7,8,8-tetraaryl-o- quinodimethane, spontaneous electrocyclization is prohibited by dibenzo-annulation. Several derivatives of dibenzo-oQD were successfully generated despite highly strained geometry caused by steric hindrance between the bulky diarylmethylene units. Their precursors are phenanthrene-9,10-diyl bis(diarylmethylium) dyes (PDM²⁺) stabilized by four electron-donating alkoxy groups. The redox pairs of dibenzo-oQD/PDM²⁺ exhibit vivid change in color upon redox interconversion (electrochromism). Both dibenzo-oQD and PDM²⁺adopt a helical conformation, whose configuration is unstable. When a chiral alkoxy substituent is attached on each of the aryl groups, the point chirality is successfully transmitted to helicity of PDM²⁺. Resulting diastereomeric biasing is the key for dibenzo-oQD/PDM²⁺to exhibit the two-way-output response (e.g., UV-vis and CD). In addition, much more strained quinodimethane derivatives, 1,2-bis(diarylmethylene)acenaphthene and 1,16-diaryldibenzo[b,n]perylene, were also generated from the dicationic precursors, demonstrating that the reductive transformation can serve as a useful protocol to generate severely deformed π-conjugated systems.