Macroscopic chirality of twist-bend nematic phase in bent dimers confirmed by circular dichroism

Abstract

Many achiral bent molecules and some polymers with such repeat units exhibit a liquid crystal phase transition between a conventional nematic (N), and a second nematic (N_x) with periodically modulated orientation. Theory predicts several possible structures for the N_x phase, some of which are chiral (helical), and others achiral (e.g. zigzag). Experimentally distinguishing which modulation type is present is non-trivial and the structure is often assumed to be helical despite the other possibilities. Here we use circular dichroism (CD) spectroscopy to directly and unambiguously identify chirality in the N_x phase of two different dimer series, “DTC5Cn” and “CBCnCB”, which vary in (CH₂)_n spacer length (n = 5, 7, 9). In doing so we have determined that the modulation type is helical in all samples. The temperature dependence of the CD signal was also investigated, suggesting that the helical structure progressively unwinds on approach to the high temperature N phase.