Helical Polyacetylenes Synthesized in Helical Sense and Pitch Controllable Chiral Nematic Liquid Crystal with Unprecedented Temperature Dependence

We have synthesized two types of novel chiral compounds, D-1 and D-2−D-5, where D-1 is an asymmetric center-containing chiral compound of ( S )-configuration, and D-2−D-5 are so-called double chiral compounds consisting of axially chiral binaphthyl moiety of ( S )-configuration and asymmetric center moieties of ( S )-, ( S )-, ( R )- and racemic configuration, linked with methylene spacers of 6, 12, 12 and 12, respectively. These chiral compounds were adopted as chiral dopants to induce chiral nematic liquid crystals (N*-LCs). The N*-LC including the double chiral dopant D-3, abbreviated as N*-LC with D-3 (system 2), showed an extraordinary increase in helical pitch (decrease in helical twisting power) in polarizing optical microscope (POM) when the temperature increased from 0 to 30 °C. This is due to a substantial cancellation in twisting power between the double chiral moieties having opposite screw directions, although both of them have the same ( S )-configuration. Meanwhile, the N*-LC including the two types of chiral dopants D-1 and D-3, abbreviated as N*-LC with both D-1 and D-3 (system 3), exhibited a temperature-dependent helical inversion; It showed a notable increase in left-handed helical pitch when the temperature increased from 0 to 15 °C, and became a nematic phase giving no helicity in the temperature range of 16−19 °C, and then showed a right-handed N* phase giving a decrease in helical pitch when the temperature increased from 20 to 25 °C. The changes in helical sense and helical pitch in the N*-LC were found to be thermally reversible. The helical pitch-controllable N*-LC (system 2) enabled us to control the screw structure and the spiral morphology of the helical polyacetylene by changing only the polymerization temperature.