Bio-based chiral dopants having an isohexide skeleton for cholesteric liquid crystal materials

Abstract

Chiral dopants were synthesized from bio-based epimeric isohexides (glucose-derived isosorbide and mannose-derived isomannide) and their phase transition behaviors and abilities for developing cholesteric liquid crystal (CLC) films were examined with a consideration of the core structure. In spite of lower reactivity of the endo hydroxy group of isomannide caused by the steric hindrance and intermolecular hydrogen bonding, final synthetic yields of chiral dopants bearing an isomannide core (64.5% for IH-2 and 65.0% for IH-4) did not show conspicuous difference compared with chiral dopants bearing isosorbide (68.4% for IH-1 and 74.0% for IH-3). On the other hand, in phase transition behaviors, chiral dopants bearing an isomannide core showed lower crystalline and melting temperatures than IH-1, IH-3 despite of the same substituents. The helical twisting power (HTP) of chiral dopants bearing isosorbide (IH-1 and IH-3) was higher than that of chiral dopants bearing isomannide (IH-2 and IH-4). The calculated HTPs of IH-1 and IH-3 were 26.6 and 42.1 μm⁻¹, respectively. In the case of IH-3, the helical pitch length of CLC could be adjusted to reflect visible light by controlling its amount and showed best performance in the range 5.0 to 7.0 mol%. In contrast to IH-1 and IH-3, it was found that IH-2 and IH-4 could not induce CLC films that reflect visible light.