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 degrees 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 degrees C, and became a nematic phase giving no helicity in the temperature range of 16-19 degrees C, and then showed a right-handed N* phase giving a decrease in helical pitch when the temperature increased from 20 to 25 degrees 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.