Spontaneous Helix Formation in Non-Chiral Bent-Core Liquid Crystals with Fast Linear Electro-Optic Effect

Liquid crystals (LCs) represent one of the foundations of modern communication and photonic technologies. Present display technologies are based mainly on nematic LCs, which suffer from limited response time for use in active colour sequential displays and limited image grey scale. Herein we report the first observation of a spontaneously formed helix in a polar tilted smectic LC phase (SmC phase) of achiral bent-core (BC) molecules with the axis of helix lying parallel to the layer normal and a pitch much shorter than the optical wavelength. This new phase shows fast (∼30 μs) grey-scale switching due to the deformation of the helix by the electric field. Even more importantly, defect-free alignment is easily achieved for the first time for a BC mesogen, thus providing potential use in large-scale devices with fast linear and thresholdless electro-optical response. les

Polyphilic hydrogen bonded block molecules involving semiperfluorinated and silylated molecular fragments

2,4-Diamino-6-phenyl-1,3,5-triazines grafted with one or two semiperfluorinated chains at the phenyl substituent have been investigated in binary mixtures with one-chain and two-chain oligo(dimethylsiloxy) terminated 4-alkoxy- and 3,4-dialkoxybenzoic acids. Equimolar mixtures of the triazines with the complementary acids form discrete double hydrogen-bonded heterodimers with an elongated central core. Mesomorphic properties are observed only if at least two terminal fluoroalkyl chains are attached to the triazine entity. Three to four heterodimers generate aggregates with preferably parallel adjusted rod-like H-bonded cores. The space requirement of the segregated peripheral fluoroalkyl and oligosiloxane segments considerably exceeds that of the central core region, leading to an overall circular geometry of the associates which organize in columnar LC phases. The chains initiate a local segregation which is however not long range and retains a time and space averaged two-dimensional hexagonal lattice

Chirality synchronization of hydrogen-bonded complexes of achiral N-heterocycles

2,4-Diamino-6-phenyl-1,3,5-triazines carrying a single oligo(ethylene oxide) (EO) chain form an optically isotropic mesophase composed of a conglomerate of macroscopic chiral domains with opposite sense of chirality even though the constituent molecules are achiral. This mesophase was proposed to result from the helical packing of hydrogen-bonded triazine aggregates, providing long-range chirality synchronization. The results provide first evidence for macroscopic achiral symmetry breaking upon conglomerate formation in an amorphous isotropic phase formed by hydrogen-bonded associates of simple N-heterocycles that are related to prebiotic molecules

Chirality synchronization of hydrogen-bonded complexes of achiral N-heterocycles

2,4-Diamino-6-phenyl-1,3,5-triazines carrying a single oligo(ethylene oxide) (EO) chain form an optically isotropic mesophase composed of a conglomerate of macroscopic chiral domains with opposite sense of chirality even though the constituent molecules are achiral. This mesophase was proposed to result from the helical packing of hydrogen-bonded triazine aggregates, providing long-range chirality synchronization. The results provide first evidence for macroscopic achiral symmetry breaking upon conglomerate formation in an amorphous isotropic phase formed by hydrogen-bonded associates of simple N-heterocycles that are related to prebiotic molecules

A self-assembled liquid crystal honeycomb of highly stretched (3-1-1)-hexagons

A new liquid crystalline honeycomb phase is reported, containing highly stretched giant hexagonal cells with two opposing walls spanned by three consecutive end-to-end H-bonded rods, the (3-1-1) hexagons

Effects of chain branching and chirality on liquid crystalline phases of bent-core molecules: blue phases, de Vries transitions and switching of diastereomeric states

Bent-core molecules based on a resorcinol bisbenzoate core with a series of distinct substituents in different positions at the central resorcinol core have been synthesized and characterized. The focus is on the effect of branched terminal groups in the racemic and chiral forms on the mesomorphic properties. These were investigated by differential scanning calorimetry, optical polarizing microscopy, X-ray diffraction, electro-optic and dielectric methods. Only bent-core mesogens derived from 4- cyanoresorcinol exhibit liquid crystalline phases and the mesophases of these compounds are strongly influenced by the branching and enantiomeric composition of the terminal chains. Depending on the structure of the rod-like wings and the enantiomeric composition, cybotactic nematic phases (NcybC), BPIII-like isotropic mesophases (BPIIIcybC*) and various polar and apolar smectic phases (SmA, SmC, SmC*, SmCsPA, SmCsPA*) are formed. For one compound, a de Vries type smectic phase is observed and it appears that with decreasing temperature, order develops in two steps. First, at the SmA to SmC transition, the tilt direction becomes long range ordered and in a second step a long range ordering in bend direction takes place. Hence, for the optically active compound a transition from chirality induced polar switching to bend induced (shape induced) antiferroelectricity takes place. In this SmCsPA* phase a homogeneous layer chirality is induced under an applied electric field which interacts with the fixed molecular chirality leading to the energetically favoured diastereomeric state and giving rise to a field direction dependent uniform tilt director orientation. Field reversal induces a flipping of the layer chirality, which first leads to the less favorable diastereomeric state, and then this slowly relaxes to the more stable one by a spontaneous reversal of the tilt direction