TY - JOUR T1 - Chiral heliconical ground state of nanoscale pitch in a nematic liquid crystal of achiral molecular dimers JF - Proceedings of the National Academy of Sciences JO - Proc Natl Acad Sci USA SP - 15931 LP - 15936 DO - 10.1073/pnas.1314654110 VL - 110 IS - 40 AU - Chen, Dong AU - Porada, Jan H. AU - Hooper, Justin B. AU - Klittnick, Arthur AU - Shen, Yongqiang AU - Tuchband, Michael R. AU - Korblova, Eva AU - Bedrov, Dmitry AU - Walba, David M. AU - Glaser, Matthew A. AU - Maclennan, Joseph E. AU - Clark, Noel A. Y1 - 2013/10/01 UR - http://www.pnas.org/content/110/40/15931.abstract N2 - The appearance of new nematic liquid crystal (LC) equilibrium symmetry (ground state) is a rare and typically important event. The first and second nematics were the helical phase and blue phase of chiral molecules, both found in 1886 in cholesteryl benzoate by Reinitzer, discoveries that marked the birth of LC science. The third nematic, the achiral uniaxial phase, also found in the 19th century, ultimately formed the basis of LC display technology and the portable computing revolution of the 20th century. Despite this achievement, the 20th can claim only the fourth nematic, the lyotropic biaxial phases found by Saupe. Now, early in the 21st, the heliconical structure of the fifth nematic is observed, an exotic chiral helix from achiral molecules.Freeze-fracture transmission electron microscopy study of the nanoscale structure of the so-called “twist–bend” nematic phase of the cyanobiphenyl (CB) dimer molecule CB(CH2)7CB reveals stripe-textured fracture planes that indicate fluid layers periodically arrayed in the bulk with a spacing of d ∼ 8.3 nm. Fluidity and a rigorously maintained spacing result in long-range-ordered 3D focal conic domains. Absence of a lamellar X-ray reflection at wavevector q ∼ 2π/d or its harmonics in synchrotron-based scattering experiments indicates that this periodic structure is achieved with no detectable associated modulation of the electron density, and thus has nematic rather than smectic molecular ordering. A search for periodic ordering with d ∼ in CB(CH2)7CB using atomistic molecular dynamic computer simulation yields an equilibrium heliconical ground state, exhibiting nematic twist and bend, of the sort first proposed by Meyer, and envisioned in systems of bent molecules by Dozov and Memmer. We measure the director cone angle to be θTB ∼ 25° and the full pitch of the director helix to be pTB ∼ 8.3 nm, a very small value indicating the strong coupling of molecular bend to director bend. ER -