Bent-core Units Research Articles

Ferroelectricity of a bent-core material with cholesteryl terminal chain

We have studied the phase sequence and physical properties of an asymmetric bent-core material with a chiral rigid cholesteryl moiety, and a flexible achiral alkyl side chain at the other end. The combination of the achiral bent core with the chiral cholesteryl unit results in properties different from those of both the usual calamitic and bent-core materials. We find that the presence of the bent core unit induces a wide-temperature-range optically isotropic (probably BPIII) mesophase between the isotropic and cholesteric (chiral nematic) phases. Below the cholesteric phase a ferroelectric smectic-C* structure occurs in which the bent-core units do not seem to form a polar close packing.

Liquid-Crystalline Dimers Composed of Bent-Core Mesogenic Units

Systematic studies on symmetric twin molecules composed of two bent-core mesogenic units are reported for the first time. It is shown that the formation of mesophases mainly depends on the chemical structure of the spacer. A sufficiently flexible spacer containing dimethylsiloxane units allows a decoupling of the mesogenic moieties; the related compounds exhibit a ferroelectric SmCP phase proved by electro-optical measurements. On the basis of X-ray data, a plausible structure model is presented. The connection of two bent-core mesogenic units by means of a tetraethylene glycol spacer results in dimers, which are able to form columnar phases. For twins containing a simple hydrocarbon chain as spacer, in most cases liquid-crystalline behavior could not be detected. These general tendencies remain valid if the bent-core unit is changed, e.g., by increasing the number of aromatic rings, by lateral substitution, and by inversion of the direction of one of the connecting groups between the aromatic rings, resp...

End functionalised liquid crystalline bent-core molecules and first DAB derived dendrimers with banana shaped mesogenic units

New series of non-symmetric bent-core compounds with a terminal carboxylic function as well as its benzyl esters and pentafluorophenyl esters have been synthesised. For most pentafluorophenylesters and for a benzylester with fluorine atoms in the ortho positions to the alkoxy chain different types of columnar ribbon phases (ColrPA and ColobPA) and a polar smectic SmCPA phase have been observed. Both the columnar and the smectic phases show antiferroelectric switching behaviour. Based on these functionalised banana-shaped molecules the first example of an amphiphilic bent-core molecule with a terminal 1-acylaminopropane-2,3-diol unit and DAB derived dendrimers of the first as well as the second generation have been synthesised and their physical properties have been investigated. Neither the diol-terminated bent-core molecule nor the dendrimers show any electrooptical response. Dendrimers with long aliphatic chains as well as the amphiphile form non-polar bilayer smectic phases, whereas dendrimers with shorter chains organise into a mesophase with a 2D lattice (Col). It seems that hydrogen bonding at the periphery of the bent-core units is unfavourable for the formation of polar smectic phases.