Aminomethylenehelicene Oligomers Possessing Flexible Two‐Atom Linker Form a Stimuli‐Responsive Double‐Helix in Solution

Aminomethylenehelicene oligomers up to the ( M )‐heptamer were synthesized by reductive amination from a formylhelicene building block. The oligomers containing more than three helicenes formed a double helix in 1,3‐dilfuorobenzene. The ( M )‐tetramer and ( M )‐pentamer unfolded into a random coil by heating to 60 °C, whereas the ( M )‐hexamer only slightly unfolded at the same temperature. A two‐sided thermal hysteresis was detected in the structural change of the ( M )‐tetramer and ( M )‐pentamer during cooling and heating. The ( M )‐pentamer and ( M )‐hexamer unfolded with the addition of trifluoroacetic acid and regenerated a double helix with the addition of triethylamine.