Chirality inversion in cholesteric phases of bent-shaped liquid crystal dimers with chiral dopants

Abstract

Incorporating molecular chirality into liquid crystals (LCs) yields nanoscale one-handed helical structures. The helical pitch of a cholesteric (Ch) phase (i.e., a chiral nematic phase) is typically slightly temperature-dependent, and the sense of helical rotation (i.e., chirality) rarely inverts. This study reports helical pitch divergence and chirality inversion in the Ch phase of bent-shaped LC dimers added with a chiral dopant (CD). We prepared four chiral LC mixtures containing equal proportions of two ester-linked 4-(trans-4-pentylcyclohexyl)phenyl-based bent-shaped LC dimers with odd-numbered spacers, each doped with 0.25, 0.50, 1.0, or 2.0 wt% of an isosorbide-based CD, ISO-(6OBA)₂. All mixtures exhibited enantiotropic chiral twist-bend nematic and Ch phases near room temperature. Polarized optical microscopy revealed that in the Ch phases of the dimer mixtures with 0.25, 0.50, and 1.0 wt% CD, the helix diverged and reformed with opposite chirality just above the phase transition temperature. The high- and low-temperature Ch phases were identified as right- and left-handed, respectively, using circular dichroism spectroscopy and the contact preparation method. The dimer mixture with 2.0 wt% CD exhibited only helix divergence of the right-handed Ch phase. These phenomena were also observed in each nonblended dimer, but not in other bent-shaped cyanobiphenyl-based twist-bend nematogenic dimers with different linkages. Our findings offer new insights into chirality inversion in LCs and have promising potential for temperature-tunable chiral-switching systems based on bent-shaped LC dimers.