Two-way chromic interconversion of the 2,2'-biphenol-6,6'-diyl dication with 5H,10H-dioxapyrene or 9H,10H-4,5-dihydroxyphenanthrene.

Two-proton or two-electron transfer of the title biphenolic dication proceeds nearly simultaneously to induce 2,6'/2',6- or 6,6'-bond formation to give dioxapyrene or dihydrophenanthrene derivatives, respectively, with vivid changes in color (halochromism and electrochromism).

Wurster's Blue-type cation radicals framed in a 5,10-dihydrobenzo[a]indolo[2,3-c]carbazole (BIC) skeleton: dual electrochromism with drastic changes in UV/Vis/NIR and fluorescence.

Electron-donating dihydrobenzindolocarbazoles (BICs) 1 a-c, which adopt planar disk-shaped geometries, were prepared by gold(I)-catalyzed cyclization as a key step. Due to the presence of a 1,4-phenylenediamine (PD) moiety in the framework, they undergo reversible one-electron oxidation to the corresponding Wurster's Blue (WB)-type species that exhibits NIR absorptions up to λ=1200 nm. In the case of the N,N'-dimethyl derivative, cation radical 1 c(+.) is stable enough to be isolated as a salt and X-ray analysis indicated paraquinoid-type bond alternation in the WB core unit, whereas the bond lengths in the peripheral benzene rings are identical to those in the neutral donor. Upon electrochemical interconversion, the redox pairs of 1 a-c and 1 a-c(+.) exhibited an electrochromic response in the UV/Vis/NIR region, which was accompanied by a drastic change in the fluorescence spectrum because only neutral donors 1 a-c are highly emissive (Φ(F) : 0.7-0.8).

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.