Double-Helical Tiled Chain Structure of the Twist-Bend Liquid Crystal Phase in CB7CB

• Published in: Crystals (Basel) 2024-07, Vol.14 (7), p.583
• Main Authors: Tuchband, Michael R., Shuai, Min, Graber, Keri A., Chen, Dong, Zhu, Chenhui, Radzihovsky, Leo, Klittnick, Arthur, Foley, Lee, Scarbrough, Alyssa, Porada, Jan H., Moran, Mark, Yelk, Joseph, Hooper, Justin B., Wei, Xiaoyu, Bedrov, Dmitry, Wang, Cheng, Korblova, Eva, Walba, David M., Hexemer, Alexander, Maclennan, Joseph E., Glaser, Matthew A., Clark, Noel A.
• Format: Article
• Language: English
• Subjects: Bend radius; Brickwork; Contours; Cooling; Crystal structure; dimers; heliconical; Liquid crystals; Materials; Molecular chains; Molecular structure; Nematic crystals; Phase transitions; Polymers; RSoXS; Self-assembly; Sequences; Shape; Structure; Temperature; Tiling; twist-bend; Twisted nematic liquid crystal displays
• ISSN: 2073-4352; 2073-4352
• DOI: 10.3390/cryst14070583

The twist-bend nematic liquid crystal phase is a three-dimensional fluid in which achiral bent molecules spontaneously form an orientationally ordered, macroscopically chiral, heliconical winding of a ten nanometer-scale pitch in the absence of positional ordering. Here, the structure of the twist-bend phase of the bent dimer CB7CB and its mixtures with 5CB is characterized, revealing a hidden invariance of the self-assembly of the twist-bend structure of CB7CB, such that over a wide range of concentrations and temperatures, the helix pitch and cone angle change as if the ground state for a pitch of the TB helix is an inextensible heliconical ribbon along the contour formed by following the local molecular long axis (the director). Remarkably, the distance along the length for a single turn of this helix is given by 2πRmol, where Rmol is the radius of bend curvature of a single all-trans CB7CB molecule. This relationship emerges from frustrated steric packing due to the bent molecular shape: space in the fluid that is hard to fill attracts the most flexible molecular subcomponents, a theme of nanosegregation that generates self-assembled, oligomer-like correlations of interlocking bent molecules in the form of a brickwork-like tiling of pairs of molecular strands into duplex double-helical chains. At higher temperatures in the twist-bend phase, the cone angle is small, the director contour is nearly along the helix axis z, and the duplex chains are sequences of biaxial elements formed by overlapping half-molecule pairs, with an approximately 45° rotation of the biaxis between each such element along the chain.