Odd–even effects of an asymmetric dimer on the double-twist structure in an amorphous blue phase

Abstract

We prepared conventional nematic mixtures doped with asymmetric dimers I-n in which phenylpyrimidine and cyanobiphenyl units are connected via a flexible spacer. A chiral dopant was added to the nematic liquid crystal to produce a blue phase material. We investigated the phase transition behaviour depending on the linking chain parity. Among chiral mixtures doped with a concentration lower than 5 wt% of I-n, those doped with an even-membered dimer show a higher upper-limit temperature of an amorphous blue phase (BPIII) than those with an odd-membered dimer. Some even-membered dimers were found to induce a metastable chiral nematic phase above BPIII. The application of an electric field in the high-temperature chiral nematic (N*_H) phase changed it to the nematic phase, which did not reverse to the N*_H phase, but changed to BPIII upon removal of the electric field. A considerable difference was found between the electro-optical behaviours in the induced BPIII and the original BPIII. Among chiral mixtures mixed with a concentration higher than 60 wt% of I-n, mixtures containing an odd-membered dimer exhibit BPIII with wide temperature ranges without a chiral nematic (N*) phase. However, the mixtures containing an even-membered dimer show an N* phase without BPIII. We examine the mechanism by which the asymmetric dimers affect the double-twist structure in BPIII.