Daniel Krüerke

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.

Light shutters from antiferroelectric liquid crystals of bent-shaped molecules

We present novel scattering-type displays using antiferroelectric smectic phases of liquid crystals of bent-shaped molecules. There can be two distinct states (‘racemic’ and ‘chiral’) that work in opposite ways. The racemic structure is scattering in the OFF state and is optically clear under electric fields. The chiral structure is transparent at zero fields and is scattering in the field ON state. These two structures may be reversibly interchanged, implying their use in devices that consume energy only during switching from one stable state to the other.

Investigations on electrooptical effects in chiral discotic columnar mesophases

Abstract Investigations on the nature of both linear and nonlinear electrooptic effects in columnar mesophases of chiral discotic dibenzopyrene and triphenylene derivatives and their mixtures are presented. We specially focus on the influence of chirality on the switching process.

Phases, phase transitions and confinement effects in a series of antiferroelectric liquid crystals

Using a variety of optical and electro-optical techniques, as well as dielectric spectroscopy, we have investigated three homologues in the chiral liquid crystal series nF1M7, where n denotes the length of the unbranched terminal chain. The main focus of the study is the series of smectic C subphases, i.e. SmC* α, SmC* 1/3 and SmC* ¼. During switching in the SmC* α phase, a peculiar redirection of the plane of biaxiality, distinguishing this phase from SmA* and SmC*, was observed. We present a simple explanation for this behaviour which correlates well with the clock model description of the SmC* α phase. We found a zero mesoscopic polarization for the SmC* ¼ phase and a non-zero mesoscopic polarization for SmC* 1/3, observations which are consistent with a distorted clock model. The dielectric spectroscopy investigations, performed at several different cell gaps, clearly show that the dielectric response in these materials is easily dominated by surface-induced structures if the cell gap is reduced, thus reflecting that the bulk thermodynamic phase exists in very thick cells only.

Surface-imaging of frozen blue phases in a discotic liquid crystal with atomic force microscopy

Discotic cholesteric phases with extremely small pitches were obtained with cellobiose derivatives as chiral dopants. These binary mixtures tend to form up to three distinct blue phases. An interesting property of these mixtures is that the blue phases can be supercooled to a glass-like state. Microscopic studies, reflection spectra, and Kossel diagrams all indicate that the three discotic blue phases BPDI, BPDII and BPDIII are analogous to the well-known calamitic modifications. In addition to the optical studies, we investigated the free surfaces of the frozen blue phases using atomic force microscopy.

New chiral discotic triphenylene derivativesexhibiting a cholesteric blue phase and a ferroelectrically switchable columnarmesophase

We have synthesized a number of new achiral and chiral triphenylene derivatives bearing laterally substituted phenylene units in the side chains. Their polymorphism includes the first reported discotic blue phase of a pure compound and a ferroelectrically switchable columnar mesophase.

Ferroelectric switching of chiral discotic lyomesophases

Abstract By diluting a ferroelectric columnar liquid crystal with dodecane as apolar organic solvent, soft columnar phases as well as the chiral columnar nematic phase (Nc*) were achieved as lyomesomorphic systems. All obtained lyomesophases exhibited polar switching over a wide temperature range, even down to room temperature. We found an unusual concentration dependence for the selective reflection observed in the highly twisted lyotropic columnar nematic phase. Different possible models are proposed which could explain the observed optical and electro-optical properties of the studied lyomesophases.

Chiral radial pentaynes exhibiting cholesteric discotic phases

The synthesis of some new chiral radial pentaynes exhibiting enantiotropic cholesteric discotic (ND*) phases at moderate temperatures is presented. By alteration of the lateral substituents we were able to obtain the phenomenon of selective reflection as well as a temperature induced helix inversion. In the latter, dilution with a nematic discotic compound has no influence on the inversion temperature, indicating that the phenomenon arises from a molecular property of the chiral compound. By adding another cholesteric radial pentayne with an almost temperature independent pitch, the inversion temperature is shifted linearly with composition in the cholesteric mixture confirming the additivity of the helical twisting power for chiral compounds in a cholesteric discotic system.

First observation of electromechanical effects in a chiral ferroelectric columnar liquid crystal

Electromechanical (converse piezoelectric) responses of an electrically switchable chiral ferroelectric columnar liquid crystal 1,2,5,6,8,9,12,13-octakis\[(S)-2-heptyloxy] dibenzo\[e,l] pyrene, were studied under a.c. electric fields. The liquid crystal phase has a C2 rotational symmetry, the same as that of SmC* liquid crystals or of Rochelle Salt, but the responses are orders of magnitude weaker. The possible physical reasons for the observed weak mechanical responses and, in view of the results, the switching mechanism are discussed.

STORAGE OF LASER-INDUCED HOLOGRAPHIC GRATINGS IN DISCOTIC LIQUID CRYSTALS

We demonstrate that low molar mass discotic liquid crystals can be used for holographic storage. Grating structures are optically induced and stored in the glassy state of the investigated liquid crystal. The material is suitable for multiple storage processes since the stored information can be erased by heating the sample above the glass transition temperature.