Spatially Heterogeneous Nature of Self-Catalytic Reaction in Hetero-Double Helix Formation of Helicene Oligomers.

A 1:1 mixture of pseudoenantiomeric aminomethylenehelicene oligomers, (P)-tetramer and (M)-pentamer, in fluorobenzene show a self-catalytic phenomenon in the formation of hetero-double helices from random coils. This study visualizes the spatially heterogeneous nature of the self-catalytic reaction in dilute solution. UV/Vis imaging analysis of the mixture at 70 °C, containing random coils, exhibits a homogeneous bright area. When the solution is cooled from 70 to 30 °C and held at that temperature, dark domains of approximately 1 mm in size appear, which move approximately at a rate of 1 mm min-1 . The dark domains indicate that weaker UV/Vis absorption results from the formation of hetero-double helices, which is supported by circular dichroism (CD) imaging experiments. Then a homogeneous mixture is regenerated upon heating to 55 °C, as shown by CD imaging. Under self-catalytic conditions, a homogeneous solution spontaneously changed to a heterogeneous solution in the process of hetero-double-helix formation.