Hanns-Walter Tunger

Re-entrant and induced mesophases: Mixed systems showing re-entrant TGBA and re-entrant cholesteric phases

Abstract We have investigated the behaviour of chiral carbohydrate-based trioxadecalines with a cyano group in mixed systems. The mixture of a trioxadecaline having a Ch phase and a trioxadecaline having a Ch, SA and a S∗Cphase sequence induces a re-entrant TGBA and Ch phase. Below the stabilized S∗C phase, a re-entrant Ch phase is observed. In the mixture of the cholesteric trioxadecaline with CCH7, itself only showing a nematic phase, a TGBA phase and a broad range SA phase are induced from two non-smectic compounds. Both mixtures show a blue phase with a pitch ranging from the UV to red depending on the molar fraction.

Sign inversion of the helical pitch in carbohydrate-based liquid crystals

Abstract A four-step synthesis starting from tri-O-acetyl-D-glucal leads to chiral trioxadecalin compounds containing a cyano group. The mesogenic properties of a homologous series of liquid crystals are reported; for two derivatives a helix inversion of the cholesteric phase is observed.

Structural variation of liquid crystalline trioxadecalins

Synthesis and mesogenic properties of new liquid crystals bearing a chiral trioxadecalin system are described. Boron-containing three-ring systems with a lateral methoxy group show cholesteric, TGBA and smectic A phases. Molecules containing four or five rings show mostly smectic C* phases. The insertion of a triple bond leads to ferroelectric smectic C* phases, but compounds with a flexible spacer between the rings show only monotropic smectic A phases. Lateral fluorination of the aromatic rings leads, depending on the position of the fluorine, either to stabilised smectic phases with lower transition temperatures or to cholesteric phases with complete suppression of all smectic phases.

Cholesteric helix inversion: Novel nitro compounds showing unusual changes of the cholesteric helical pitch

Abstract We present a homologeous series of liquid crystalline trioxadecalin compounds having a terminal alkoxy chain and a nitro group. The (1S, 3R, 6R,8R)-3-(4″-nitrophenyl)-8-(4′-octoxyphenyl)-2,4, 7-trioxabicyclo [4.4.0]decane 2 c shows a temperature-dependent inversion of the cholesteric helix at lower temperatures. For higher temperatures, the reciprocal helical pitch reaches a minimum, then it increases, tending to a second inversion point just above the clearing point. An additional chiral centre in the side chain leads for nitro compounds 3 to non-mesomorphic behaviour. For the cyano compounds 4, the change in the cholesteric helix is suppressed, for the R configuration, but for the S configuration helical inversion occurs at high temperatures and selective reflection above the transition to the TGBA phase.

Carbohydrate-based liquid crystals: new compounds showing re-entrant TGBA and cholesteric phases and dopant-induced TGBA, SA and SC* phases

Re-entrant TGBA and cholesteric phases are observed in a homologous series of pure chiral carbohydrate-based compounds, and contact preparation induces SA phases between cholesteric and nematic compounds as well as re-entrant and Sc* phases in two substances which in pure form only show cholesteric and SA phases.

Ferroelectric liquid crystals based on carbohydrates

Abstract New ferroelectric liquid crystals derived from carbohydrates are presented and their mesomorphic properties are discussed.

Use of glucose as a trifunctional building block for chiral liquid crystals

Glucose is used as a trifunctional chiral central core in laterally substituted liquid crystals. The synthesis of 1′-(4-O-(4″-octyloxybenzoyl)-2,3,6-trideoxy-β-D-erythrohexopyranosyl)-4′-hexyloxybenzene and its related 6-halogeno derivatives is described as an example for rod-like molecules with small lateral substituents. All the compounds obtained exhibit a cholesteric mesophase. It is shown that there is a correlation between the size of the substituent at the 6-position and the transition temperature from the cholesteric to the isotropic phase. Also, the synthesis of Y-mesogenic compounds containing spaced and/or unspaced substituents in the 4- and 6-positions is described, followed by a qualitative discussion of the results obtained.