Inverse thermoresponse of a water-soluble helicene oligomer in aqueous-organic mixed solvent systems

A water-soluble ethynylhelicene (M)-tetramer with TEG groups exhibited the inverse thermoresponse in water/triethylamine/acetone, in which heating induced the formation of double helix and cooling dissociation. In this study, we examined the solvent effect with regard to water content at temperatures between 5 and 40 °C, and showed that the inverse thermoresponse occurred in different aqueous solution systems. In acetone, the oligomer exhibited the ordinary thermoresponse: Cooling induced the formation of double helix and heating dissociation. The increase in water content to approximately 10% caused the switch to the inverse thermoresponse. Further increase inhibited the association, and did not induce the thermoresponse. When the water content was increased to approximately 50%, the inverse thermoresponse appeared as observed previously study. Two types of inverse thermoresponses, termed type I and II, appeared at high and low water contents, respectively.