4-Methylresorcinol based bent-core liquid crystals with azobenzene wings – a new class of compounds with dark conglomerate phases

Mohamed Alaasar,Marko Prehm,Carsten Tschierske,Marcel Brautzsch

doi:10.1039/c4tc00533c

Mohamed Alaasar, Marko Prehm + Show 2 more

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https://doi.org/10.1039/c4tc00533c

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Abstract

Stochastic achiral symmetry breaking in soft matter systems, leading to conglomerates of macroscopically chiral domains (so-called dark conglomerate = DC phases) is of contemporary interest from a fundamental scientific point of view as well as for numerous potential applications in chirality sensing and non-centrosymmetric materials. Herein we report the synthesis and investigation of first azobenzene containing bent-core mesogens derived from 4-methylresorcinol forming DC phases with a new structure, distinct from the known fluid sponge-like distorted smectic phases as well as from the helical nano-filament phases (HNF phases, B4 phases). The effects of chain length and other structural modifications on achiral symmetry breaking were investigated. Homologues with relatively short alkyl chains form achiral intercalated lamellar LC phases (B6 phases), but on increasing the chains, these are replaced by the chiral and optically isotropic DC phases. Compounds with the longest alkyl chains form low birefringent crystalline conglomerates which represent less distorted versions of the optically isotropic DC-phases. Introducing additional peripheral substituents at both outer rings removes the DC phases. The DC phases were also removed and replaced by modulated smectic phases if the azo groups were replaced by ester units, showing that azo groups favour DC phase formation with new nanostructures, distinct from the previously known types.

Highlights

IntroductionSince the discovery of polar order and chirality in the liquid crystalline (LC) phases of compounds with a bent aromatic core (bent-core liquid crystals 1⁄4 BCLCs)[1] this eld has become a thrust area of liquid crystal research by providing a signi cant impact on the general understanding of molecular selfassembly in so condensed matter.[2,3] these materials are of signi cant interest for numerous practical applications, as ferroelectrica, exoelectrica and pyroelectrica,[4] for command surfaces and sensors,[5] non linear optical applications[6] and in fast switching electrooptical devices.[7] A great fraction of contemporary interest is focussed on the so-called dark conglomerate phases (DC phases).[8,9,10,11,12,13,14,15,16,17,18] The common feature of these DC phases is the absence of birefringence due to a local distortion of long range periodicity and the inherent phase chirality indicated by stochastic achiral symmetry breaking, leading to conglomerates of macroscopically chiral domains
Since the discovery of polar order and chirality in the liquid crystalline (LC) phases of compounds with a bent aromatic core[1] this eld has become a thrust area of liquid crystal research by providing a signi cant impact on the general understanding of molecular selfassembly in so condensed matter.[2,3] these materials are of signi cant interest for numerous practical applications, as ferroelectrica, exoelectrica and pyroelectrica,[4] for command surfaces and sensors,[5] non linear optical applications[6] and in fast switching electrooptical devices.[7]
A great fraction of contemporary interest is focussed on the so-called dark conglomerate phases (DC phases).[8,9,10,11,12,13,14,15,16,17,18]

Summary

Introduction

Since the discovery of polar order and chirality in the liquid crystalline (LC) phases of compounds with a bent aromatic core (bent-core liquid crystals 1⁄4 BCLCs)[1] this eld has become a thrust area of liquid crystal research by providing a signi cant impact on the general understanding of molecular selfassembly in so condensed matter.[2,3] these materials are of signi cant interest for numerous practical applications, as ferroelectrica, exoelectrica and pyroelectrica,[4] for command surfaces and sensors,[5] non linear optical applications[6] and in fast switching electrooptical devices.[7] A great fraction of contemporary interest is focussed on the so-called dark conglomerate phases (DC phases).[8,9,10,11,12,13,14,15,16,17,18] The common feature of these DC phases is the absence of birefringence due to a local distortion of long range periodicity and the inherent phase chirality indicated by stochastic achiral symmetry breaking, leading to conglomerates of macroscopically chiral domains. A broader variety of different molecular structures could lead to an improved understanding of the molecular structural factors governing the formation of distinct subtypes of DC phases, providing rules for the directed design of such materials

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