Eva Korblova

Chiral heliconical ground state of nanoscale pitch in a nematic liquid crystal of achiral molecular dimers

Significance 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.

A Twist-Bend Chiral Helix of 8nm Pitch in a Nematic Liquid Crystal of Achiral Molecular Dimers

Significance 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.

Helical Nanofilament Phases

Packing Bananas and Boomerangs Assembling achiral molecules typically generates achiral domains. However, odd things can happen when the molecules are banana-or boomerang-shaped—their cores can twist out of plain to form left- or right-handed helices, which can then pack into chiral domains that will polarize light (see the Perspective by Amabilino). Hough et al. (p. 452) show that if you make the situation even more complex by frustrating the packing of adjacent layers, you can create a material that appears to be macroscopically isotropic with only very local positional and orientational ordering of the molecules but still shows an overall chirality. In a second paper, Hough et al. (p. 456) also show that if you change the chemistry of the molecules to allow for better overall packing, you can create a situation where helical filaments form that also tend to pack in layered structures. However, the frustration between the two types of packing leads to macroscopically chiral and mesoporous structures. Molecules lacking handedness can form layered, mesoporous helical structures. In the formation of chiral crystals, the tendency for twist in the orientation of neighboring molecules is incompatible with ordering into a lattice: Twist is expelled from planar layers at the expense of local strain. We report the ordered state of a neat material in which a local chiral structure is expressed as twisted layers, a state made possible by spatial limitation of layering to a periodic array of nanoscale filaments. Although made of achiral molecules, the layers in these filaments are twisted and rigorously homochiral—a broken symmetry. The precise structural definition achieved in filament self-assembly enables collective organization into arrays in which an additional broken symmetry—the appearance of macroscopic coherence of the filament twist—produces a liquid crystal phase of helically precessing layers.

Chiral Isotropic Liquids from Achiral Molecules

Packing Bananas and Boomerangs Assembling achiral molecules typically generates achiral domains. However, odd things can happen when the molecules are banana-or boomerang-shaped—their cores can twist out of plain to form left- or right-handed helices, which can then pack into chiral domains that will polarize light (see the Perspective by Amabilino). Hough et al. (p. 452) show that if you make the situation even more complex by frustrating the packing of adjacent layers, you can create a material that appears to be macroscopically isotropic with only very local positional and orientational ordering of the molecules but still shows an overall chirality. In a second paper, Hough et al. (p. 456) also show that if you change the chemistry of the molecules to allow for better overall packing, you can create a situation where helical filaments form that also tend to pack in layered structures. However, the frustration between the two types of packing leads to macroscopically chiral and mesoporous structures. Banana-shaped molecules lacking handedness form a macroscopically isotropic fluid that still has overall chirality. A variety of simple bent-core molecules exhibit smectic liquid crystal phases of planar fluid layers that are spontaneously both polar and chiral in the absence of crystalline order. We found that because of intralayer structural mismatch, such layers are also only marginally stable against spontaneous saddle splay deformation, which is incompatible with long-range order. This results in macroscopically isotropic fluids that possess only short-range orientational and positional order, in which the only macroscopically broken symmetry is chirality—even though the phases are formed from achiral molecules. Their conglomerate domains exhibit optical rotatory powers comparable to the highest ever found for isotropic fluids of chiral molecules.

Spontaneous ferroelectric order in a bent-core smectic liquid crystal of fluid orthorhombic layers.

The ferroelectric properties of bent-core liquid crystalline molecules emerge from ordering within the smectic layers. Macroscopic polarization density, characteristic of ferroelectric phases, is stabilized by dipolar intermolecular interactions. These are weakened as materials become more fluid and of higher symmetry, limiting ferroelectricity to crystals and to smectic liquid crystal stackings of fluid layers. We report the SmAPF, the smectic of fluid polar orthorhombic layers that order into a three-dimensional ferroelectric state, the highest-symmetry layered ferroelectric possible and the highest-symmetry ferroelectric material found to date. Its bent-core molecular design employs a single flexible tail that stabilizes layers with untilted molecules and in-plane polar ordering, evident in monolayer-thick freely suspended films. Electro-optic response reveals the three-dimensional orthorhombic ferroelectric structure, stabilized by silane molecular terminations that promote parallel alignment of the molecular dipoles in adjacent layers.

Athermal photofluidization of glasses.

Azobenzene and its derivatives are among the most important organic photonic materials, with their photo-induced trans-cis isomerization leading to applications ranging from holographic data storage and photoalignment to photoactuation and nanorobotics. A key element and enduring mystery in the photophysics of azobenzenes, central to all such applications, is athermal photofluidization: illumination that produces only a sub-Kelvin increase in average temperature can reduce, by many orders of magnitude, the viscosity of an organic glassy host at temperatures more than 100 K below its thermal glass transition. Here we analyse the relaxation dynamics of a dense monolayer glass of azobenzene-based molecules to obtain a measurement of the transient local effective temperature at which a photo-isomerizing molecule attacks its orientationally confining barriers. This high temperature (T(loc)~800 K) leads directly to photofluidization, as each absorbed photon generates an event in which a local glass transition temperature is exceeded, enabling collective confining barriers to be attacked with near 100% quantum efficiency.

Chirality-Preserving Growth of Helical Filaments in the B4 Phase of Bent-Core Liquid Crystals

The growth of helical filaments in the B4 liquid-crystal phase was investigated in mixtures of the bent-core and calamitic mesogens NOBOW and 8CB. Freezing-point depression led to nucleation of the NOBOW B4 phase directly from the isotropic phase in the mixtures, forming large left- and right-handed chiral domains that were easily observed in the microscope. We show that these domains are composed of homochiral helical filaments formed in a nucleation and growth process that starts from a nucleus of arbitrary chirality and continues with chirality-preserving growth of the filaments. A model that accounts for the observed local homochirality and phase coherence of the branched filaments is proposed. This model will help in providing a better understanding of the nature of the B4 phase and controlling its growth and morphology for applications, such as the use of the helical nanophase as a nanoheterogeneous medium.

Organization of the polarization splay modulated smectic liquid crystal phase by topographic confinement

Recently, the topographic patterning of surfaces by lithography and nanoimprinting has emerged as a new and powerful tool for producing single structural domains of liquid crystals and other soft materials. Here the use of surface topography is extended to the organization of liquid crystals of bent-core molecules, soft materials that, on the one hand, exhibit a rich, exciting, and intensely studied array of novel phases, but that, on the other hand, have proved very difficult to align. Among the most notorious in this regard are the polarization splay modulated (B7) phases, in which the symmetry-required preference for ferroelectric polarization to be locally bouquet-like or “splayed” is expressed. Filling space with splay of a single sign requires defects and in the B7 splay is accommodated in the form of periodic splay stripes spaced by defects and coupled to smectic layer undulations. Upon cooling from the isotropic phase this structure grows via a first order transition in the form of an exotic array of twisted filaments and focal conic defects that are influenced very little by classic alignment methods. By contrast, growth under conditions of confinement in rectangular topographic channels is found to produce completely new growth morphology, generating highly ordered periodic layering patterns. The resulting macroscopic order will be of great use in further exploration of the physical properties of bent-core phases and offers a route for application of difficult-to-align soft materials as are encountered in organic electronic and optical applications.

Self-assembled monolayers for liquid crystal alignment: simple preparation on glass using alkyltrialkoxysilanes

A simple procedure for the preparation of octadecylsiloxane self-assembled monolayers (SAMs) on float glass substrates is described. The method utilizes commercial octadecyltriethoxysilane, OTE: n-C18H37Si(OCH2CH3)3, as the SAM precursor, with deposition accomplished in toluene solution using n-butylamine as catalyst. This synthetic approach obviates the use of the problematic trichlorosilanes typically required for the preparation of high quality SAMs, and is characterized by a wide ‘process window,’ utilizing off-the-shelf reagents without special handling.

Orientation of a Helical Nanofilament (B4) Liquid‐Crystal Phase: Topographic Control of Confinement, Shear Flow, and Temperature Gradients

The liquid-crystal (LC) phases of bent-core molecules have been of great interest because of their unusual polar and chiral properties, as well as their exotic collective behavior, including the formation of macroscopic chiral structures from achiral molecules. [ 1–9 ] The bent-core molecules strongly nanosegregate, leading to the formation of polar and chiral arrangements of the bent cores into planar or modulated layers. Subtle interactions between layers can be manipulated to produce a variety of ferroor anti-ferroelectric phases. Among the most mysterious and interesting of these new phases is the B4, a locally layered phase with hexatic or semicrystalline ordering of the layers, showing large optical rotation and light scattering which suggests a strongly chiral local structure. Recently, it has been shown that the B4 is a phase of spontaneously self-assembling helical nanofi laments (HNFs), the morphology of which is driven by a tendency for local saddle splay deformation of the semicrystalline layers. [ 7 ] These nanofi laments, shown in Figure 1 b for the bent-core material NOBOW (benzoic acid, 4-[(E)-([4-(nonyloxy)phenyl]imino) methyl]-,1,1’-(1,3-phenylene)ester) (Figure 1 a) grow from the higher temperature isotropic or fl uid bent-core phases with a well-defi ned pitch of the helical twist and lateral structure and dimensions. The bulk B4 is a close packing of HNFs in which the fi lament axes are parallel to one another and their layer twist becomes macroscopically coherent. In the fi lament growth process a nucleation event establishes the handedness of a fi lament and this handedness is transferred to subsequently growing fi laments to form large ( ≈ 100μ m dimension) homochiral domains with either right-handed and left-handed characteristics. Atomic force microscopy (AFM) imaging of the air/LC surface of the B4 reveals this bulk structure,