Spontaneous helix formation in non-chiral bent-core liquid crystals with fast linear electro-optic effect

Abstract

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.

Polar Order, Mirror Symmetry Breaking, and Photoswitching of Chirality and Polarity in Functional Bent‐Core Mesogens

“…On the other hand, chirality is not visible in planar alignment and in freely suspended films, meaning that the chiral conglomerate structure is stabilized in the homeotropic cells, similar to the case of conglomerate formation in nematic phases of BCLCs . We assume that the basis of chirality in the investigated smectic phases is the formation of a heliconical structure with the helix axis parallel to the layer normal, owing to the escape from a growing polarization in the clusters . This might be combined with a synchronization of helical molecular conformations of the transiently chiral 4‐cyanoresorcinol based bent‐core mesogens, allowing a denser packing .…”

“…Compounds with molecular shapes at the cross‐over between linear (rod‐like) and bent shapes (BCLCs), such as mesogenic dimers with odd spacers, hockey stick molecules, and molecules with reduced length of the bent core or with a reduced bending angle have received significant attention, because they provide new LC phase structures. 4‐Cyanoresorcinol is a very useful bent‐core unit with reduced bending angle, which has led to a number of new and unique mesophases, for example, the nontilted antiferroelectric SmAP A phase and a short‐pitch heliconical smectic phase designated as SmC s P F hel . In previous work, the 4‐cyanoresorcinol core was combined with two identical wings, either azobenzenes or phenyl benzoates …”

“…4‐Cyanoresorcinol is a very useful bent‐core unit with reduced bending angle, which has led to a number of new and unique mesophases, for example, the nontilted antiferroelectric SmAP A phase and a short‐pitch heliconical smectic phase designated as SmC s P F hel . In previous work, the 4‐cyanoresorcinol core was combined with two identical wings, either azobenzenes or phenyl benzoates …”

Stereochemical Rules Govern the Soft Self‐Assembly of Achiral Compounds: Understanding the Heliconical Liquid‐Crystalline Phases of Bent‐Core Mesogens

“…Synthesis : The synthesis of the compounds was conducted as shown in Scheme by acylation of 4‐cyanoresorcinol with the properly substituted benzoyl chlorides in CH 2 Cl 2 in the presence of pyridine . The synthetic procedures and analytical data of the new compounds are collated in the Supporting Information, together with the structural analysis data.…”

“…The synthetic procedures and analytical data of the new compounds are collated in the Supporting Information, together with the structural analysis data. The synthesis and analytical data of compounds 1/6 , 1/12 , 1/16 and 1/18 have been reported in previous work …”