Induced preference for axial chirality in a triarylmethylium o,o-dimer upon complexation with natural γ-cyclodextrin: strong ECD signaling and fixation of supramolecular chirality to molecular chirality.

The ratio of the easily interconverting rotational isomers of biphenyl-2,2'-diylbis[bis(4-dimethylaminophenyl)methylium] (R)/(S)-1a(2+) can be biased to prefer an R configuration upon 1:1 complexation with γ-cyclodextrin in water. Through the reaction with Na(2)S, the preference of 1a(2+)@γ-CyD for an axial chirality of R can be fixed as the M-helicity of dihydrothiepin 2.