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.