Diverse helical structures made of achiral mesogenic dimers

The formation of chiral structures from achiral building blocks, and the propagation of chirality across length-scales in soft matter are still poorly understood phenomena. Here, we have studied a system revealing an exceptional diversity of spontaneously chiral phases formed by achiral mesogenic dimers linked by spacers with an odd number of atoms. Depending on terminal chain length and temperature, these compounds form a well-known heliconical nematic (N_TB) phase and distinct helical smectic phases - ranging from the nanoscale-pitch SmC_TB-SH, through the SmC_TB-DH phase with ~ 50 nm periodicity, to the newly identified SmC_TB-C phase featuring a micron-scale helix. In the SmC_TB-C phase heliconical order is preserved even though the interlayer molecular intercalations strongly suppress the azimuthal rotation of the director. Resonant soft X-ray scattering (RSoXS) measurements were performed for the first time across an entire homologous series. Results confirmed the double-helical structure of the SmC_TB-DH phase in which a longer helix is superimposed on the short one. The intensity of the resonant signals revealed an anomaly: the non-monotonic temperature evolution is due to the transient passage of the structure from a four-layer helix, through a nearly perfect three-layer clock-like helix, before decoupling of the short and long helices at lower temperatures.