Circular-Polarization-Induced Enantiomeric Excess in Liquid Crystals of an Achiral, Bent-Shaped Mesogen

摘要

Achiral, bent-shaped mesogens have opened up a new era in liquid-crystal science with respect to polarity and chirality. Bent-shaped mesogens are known to segregate into two self-assembling, chiral domains in which the molecules are believed to have chiral conformations. In this sense the system is regarded as a racemate, despite being composed of achiral molecules. Hence, the numbers of the two enantiomers are equivalent and the enantiomeric excess is zero in an unperturbed state. However, the system is not necessarily racemic because an imbalance of chirality can be produced by an external chiral stimulus. At least two methods have been elucidated for this: 1) a small amount of chiral dopant is effective in inducing almost 100 % ee, and 2) a chiral surface has been successfully used to induce a finite ee. Herein we demonstrate another method that does not involve chiral molecular species. Novel, achiral, bent-shaped dimers with photochromic moieties on both side wings form large, chiral domains enantioselectively when the samples are cooled from the conventional anticlinic SmCA phase to a low-temperature BX phase similar to B4 while being irradiated with circularly polarized light (CPL). The enantioselectivity can be controlled by the handedness of the CPL.