Linear and H-shaped chiral dimers for applications in photonic liquid crystals

The Optical Kerr effect in blue phases has recently been shown to be of practical use in fast switching display devices. This mode provides a linear response with respect to an applied electric field and thus grey-scale is possible. Fast responses are of prime interest in advanced display technologies for multiscreens and 3D displays. However, blue phases with wide temperature ranges of existence are almost unknown, and polymer stabilisation is used as the preferred option for creating room temperature blue phases, however, these are only kinetically stable. The objective of this study therefore was to seek new material design in the discovery of blue phase systems. The target design for exploration was dimers composed of two rod-like liquid crystal moieties that were either terminally or laterally attached to one another by aliphatic spacer units, in the belief that such systems would provide for low melting points.

Thus, two sets of chiral dimers were synthesized, using different length chiral spacers, in the attempt to produce blue phases. The linear dimers exhibited crystal B phases, while the H-shaped dimers exhibited low temperature chiral nematic phases. The odd-even effect was clearly observed in the transition temperatures for both dimers. Contact mixtures of the dimers in E7 showed induced chiral nematic phases with relatively short pitches, while pitch measurements and helical twisting power studies showed the helical twisting power values of both sets of dimers were much lower than needed in order to observe a blue phase.